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The role of surface energy of boron nitride on gross melt fracture elimination of polymers Seth, Manish 2011

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T H E R O L E O F S U R F A C E E N E R G Y O F B O R O N N I T R I D E O N G R O S S M E L T F R A C T U R E E L I M I N A T I O N O F P O L Y M E R S by Manish Seth B.Tech Chemical Engineering, Harcourt Butler Technological Institute, India, 1997 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF T H E REQUIREMENTS FOR T H E D E G R E E OF MASTER OF APPLIED SCIENCE In The Faculty of Graduate Studies Department of Chemical and Biological Engineering We accept this thesis as conforming to the required standard T H E UNIVERSITY OF BRITISH COLUMBIA August 2001 © 2 0 0 1 Manish Seth In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. The University of British Columbia Vancouver, Canada Department of DE-6 (2/88) THE ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS Abstract Experiments were carried out to measure the surface energy of boron nitride (BN) powders. A reliable procedure has been developed for measurement of surface energy of powders using the capillary rise technique based on Washburn's equation. It is measured by finding the contact angle from liquid penetration experiments with polar and non-polar liquids. Both the dispersive and non-dispersive components of surface energy are determined. The results of the surface energy of BN powders has been found to correlate well with the critical shear rate for the onset of melt fracture, indicating the importance of surface energy in the procedure of selecting an effective processing aid. In addition, experiments were carried out in both an Instron capillary rheometer equipped with a special annular die (Nokia Maillefer wire coating cross-head) and a parallel-plate rheometer to investigate the effect of a new processing additive (boron nitride powder in combination with a fluoroelastomer) on the rheology and processability of molten polymers. Metallocene polyethylenes with and without boron nitride (BN) and fluoroelastomer are tested in extrusion. First, it is demonstrated that B N is a superior processing aid compared to conventional fluoropolymer ones. Secondly, it is found that the combination of B N powders with a small amount of a fluoroelastomer improves even further the processability of molten polymers. They can essentially be processed at higher shear rates without exhibiting gross melt fracture. DSC and T G A experiments using polymer specimens with and without BN were also carried out to investigate the mechanism by which gross melt fracture is being eliminated in the presence of boron nitride. ABSTRACT ii THE ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS Table of Contents A B S T R A C T ii T A B L E O F C O N T E N T S iii LIST O F FIGURES vi LIST O F T A B L E S ix A C K N O W L E D G E M E N T S x 1 INTRODUCTION 1 2 L I T E R A T U R E R E V I E W 10 2.1 CHEMICAL STRUCTURE AND PHYSICAL PROPERTIES OF PE 10 2.2 RHEOLOGICAL MEASUREMENTS 12 2.2.1 Parallel Plate Rheometer - Linear Viscoelasticity 12 2.2.2 Sliding Plate Rheometer - Simple Shear 15 2.2.3 Capillary Rheometer 17 2.3 T H E GENERAL FLOW C U R V E OF LINEAR POLYMERS 23 2.4 M E L T FRACTURE 25 2.5 MECHANISMS TO EXPLAIN M E L T FRACTURE 28 2.5.1 Die exit effects 29 2.5.2 Die entry effects 31 2.5.3 Wall slip in polymer melts 32 2.6 POLYMER PROCESSING AIDS 35 2.7 FACTORS AFFECTING POLYMER FLOW 40 2.7.1 Pressure effects 40 2.7.2 Temperature effects - Time-temperature superposition 41 2.8 SURFACE CHEMISTRY 42 2.8.1 Contact angle 42 2.8.2 Measurement of Contact angle 44 2.8.3 Wettability 46 2.8.4 Contact angle of powders 47 2.8.5 Surface free energy and surface tension 48 3 O B J E C T I V E S 53 TABLE OF CONTENTS iii THE ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 4 MATERIALS 54 4.1 POLYMERS 54 4.2 BORON NITRIDE AND FLUOROEL A S TOMER STUDIED 54 4.3 BLEND PREPARATION 57 4.4 T H E R M A L ANALYSIS (DSC AND T G A EXPERIMENTS) 60 5 MEASUREMENT OF SURFACE ENERGY OF BORON NITRIDE POWDERS 66 5.1 INTRODUCTION 66 5.2 SURFACE ENERGY OF POWDERS 67 5.2.1 Capillary rise method 67 5.2.2 Modified procedure 69 5.2.3 Contact angle and surface energy of powders 72 5.3 EXPERIMENTAL SETUP FOR MEASURING T H E WETTABILITY 74 5.3.1 Test procedure 75 5.3.2 Selection criterion for liquid 77 5.4 RESULTS AND DISCUSSION 78 5.5 EFFECTIVENESS OF B N 84 5.6 SUMMARY 87 6 EFFECT OF COMBINING BORON NITRIDE AND FLUOROELASTOMER: A NEW PROCESSING AID 88 6.1 INTRODUCTION 88 6.2 EXPERIMENTAL 90 6.2.1 Materials 90 6.2.2 Linear viscoelastic measurements 91 6.2.3 Capillary experiments 95 6.3 RESULTS AND DISCUSSION 96 6.3.1 Effect of B N and Teflon® on processing 96 6.3.2 Effect of BN and fluoroelastomer on processing 99 6.3.3 Effect of BN on polymer processing 105 6.3.4 Effect of BN concentration on polymer processing 109 6.4 SUMMARY 112 TABLE OF CONTENTS iv THE ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 7 CONCLUSIONS 113 8 R E C O M M E N D A T I O N S 115 9 R E F E R C E N C E S 116 10 N O M E N C L A T U R E 123 TABLE OF CONTENTS V T H E R O L E O F S U R F A C E E N E R G Y O F B O R O N N I T R I D E O N G R O S S M E L T F R A C T U R E E L I M I N A T I O N O F P O L Y M E R S List of Figures Figure 1-1: Typical picture of m-LLDPE extrudates at different shear rates 6 Figure 2-1: Schematic representation of a linear and a branched polyethylene molecule 11 Figure 2-2: Parralel plate rheometer 13 Figure 2-3: Simple Shear Flow 15 Figure 2-4: Velocity profile in simple shear under no-slip 16 Figure 2-5: Schematic of a capillary rheometer 18 Figure 2-6: Wall pressure distribution for capillary flow 20 Figure 2-7: Bagley plot for determining the end correction 21 Figure 2-8: Crosshead die for wire coating 22 Figure 2-9: A typical apparent flow curve of a linear polymer 24 Figure 2-10: Flow curves under slip condition 33 Figure 2-11: The flow curve of a linear low-density polyethylene with and without the addition of 250ppm fluoropolymer 38 Figure 2-12: The apparent flow curves for PE Exact®3128 without and with boron nitride obtained in a rheometer with Nokia Maillefer crosshead having 3.00 mm die and 1.52 mm tip at 163°C 39 Figure 2-13: Droplet of a liquid on solid substrate at equilibrium 43 Figure 2-14: Traditional method for wettability measurements of powders 48 Figure 4-l:The structure of boron nitride 55 Figure 4-2: SEM pictures of different BN samples 58 Figure 4-3: SEM pictures of the B N and fluoroelatomer particles dispersed in m-LLDPE Exact®3128 59 Figure 4-4: DSC curve for Exceed® 143 with 500 ppm of BN (first heating cycle) 60 Figure 4-5: DSC curve for Exceed®143 with 500 ppm of BN (cooling cycle) 61 Figure 4-6: DSC curve forExceed®143 with 500 ppm of BN (second heating L I S T O F F I G U R E S vi THE ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS cycle) 61 Figure 4-7: T G A curve for extrudate obtained with pure Exceed®143 63 Figure 4-8: T G A curve for extrudate obtained with Exceed® 143 having 500ppm of fluoroelastomer 63 Figure 4-9: T G A curve for extrudate obtained with Exceed® 143 having 2500ppm of boron nitride 64 Figure 5-l:Powder column-packing device 70 Figure 5-2:Penetration-rate measurement system 76 Figure 5-3:Effect of column-packing pressure on the penetration rate of liquid 79 Figure 5-4:Measurement of wettability of different B N powders with methanol 80 Figure 5-5:Measurement of wettability of different B N powders with water 81 Figure 5-6:Measurement of wettability of different B N powders with ct-bromonaphthalene 81 Figure 5-7:Measurement of wettability of different BN powders (AS610-AS614) with methanol 82 Figure 5-8: Measurement of wettability of different BN powders (AS610-AS614) with water 83 Figure 5-9: Measurement of wettability of different B N powders (AS610-AS614) with a-bromonaphthalene 83 Figure 5-10: Relation between ratio of dispersive and non-dispersive components of surface energy of B N powders to critical shear rate for onset of gross melt fracture 86 Figure 6-1: Dynamic moduli, G', G", and complex viscosity rj* of m-LLDPE Exact® 3128 (with and without BN) at 163°C 93 Figure 6-2: Dynamic moduli, G', G", and complex viscosity rj* of m-LLDPE Exact® 3128 (with and without fluoroelastomer) at 163°C 93 Figure 6-3: Dynamic moduli, G', G", and complex viscosity rj* of m-LLDPE Exact®3128 (pure resin and combination of BN and fluoroelastomer) at 163°C 94 Figure 6-4: The effect of combining 0.05% BN and 0.05% Teflon® on the melt fracture performance of PE Exact®3128 obtained in a rheometer with Nokia Maillefer crosshead having 0.122" die and 0.06" tip at 204°C 97 LIST OF FIGURES vii THE ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS Figure 6-5: The effect o f combining 0.1% of B N and 0.05% Teflon® on the melt fracture performance of P E Exceed® 116 obtained in a rheometer with Nokia Maillefer crosshead having 0.122" die and 0.06" tip at 204°C 98 Figure 6-6: The effect o f combining 0.1% of B N and 0.1% fluoroelastomer on the melt fracture performance o f P E Exact®3128 obtained in a rheometer with Nokia Maillefer crosshead having 0.122" die and 0.06" tip at 163°C 100 Figure 6-7: The extrudate samples to illustrate the combined effect of B N and fluoroelastomer in the extrusion of m- L L D P E Exact®3128 obtained at 926 s'1 and 163°C: (a) pure resin; (b) 0.1% B N ; (c) 0.1 % fluoroelastomer; (d) combination of 0.1% B N and 0.1% fluoroelastomer 102 Figure 6-8: The effect o f combining 0.1% & 0.2% of B N and 0.05% fluoroelastomer on the melt fracture performance of P E Exceed® 116 obtained in a rheometer with Nokia Maillefer crosshead having 0.098" die and 0.055" t i p a t 2 0 4 ° C 103 Figure 6-9: The effect o f combining 0.1% B N (differing each other in processing methods) on the melt fracture performance of P E Exact®3128 obtained in a rheometer with Nokia Maillefer crosshead having 0.122" die and 0.06" tip at 163°C :. ; 105 Figure 6-10: Dynamic moduli, G' , G" , and complex viscosity rj* of m - L L D P E Exact®3128 (with and without AS612 B N ) at 163°C 106 Figure 6-11: Dynamic moduli, G ' , G" , and complex viscosity r|* o f m - L L D P E Exact®3128 (with and without AS613 Molybdenum-sulfide) at 163°C 107 Figure 6-12: Dynamic moduli, G' , G" , and complex viscosity r|* of m - L L D P E Exact®3128 (pure resin and AS614 [combination of B N & CTB]) at 163°C 107 Figure 6-13: The effect o f combining different concentrations of B N on the melt fracture performance of P E Exceed® 143 obtained in capillary rheometer with capillary die having L / D = 40 and D = 0.02"at 204°C 109 Figure 6-14: The effect o f combining different concentrations of B N on the melt fracture performance o f P E Exceed® 143 obtained in rheometer with Nokia Maillefer crosshead having 0.098" die and 0.055" tip at 204°C 110 LIST OF FIGURES viii THE ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS List of Tables Table 4-1: Comparison of boron nitride to common fillers 55 Table 4-2: The average particle sizes and states of agglomeration properties of various boron nitride powder 56 Table 4-3: Latent heat of crstallization values for pure resin, with B N and fluoroelastomer 62 Table 4-4: T G A results for pure resin, with BN and fluoroelastomer 64 Table 5-1: Constants of liquids used for finding contact angle 78 Table 5-2: Surface energy components of boron nitride powders 84 Table 6-1: Summary of blends used in extrusion experiments 92 Table 6-2: Effect of BN, fluoroelastomer and their combination on the maximal shear rate yielding a smooth extrudate in extrusion of m-LLDPE Exact®3128 and Exceed® 116 (Nokia Mallifier Crosshead) attached to rheometer at 163°C and 204°C respectively 104 Table 6-3: Effect of AS-series B N on the maximal shear rate yielding a smooth extrudate in extrusion of m-LLDPE Exact®3128 (Nokia Mallifier Crosshead) attached to rheometer at 163°C 1 108 Table 6-4: Effect of B N on the maximal shear rate yielding a smooth extrudate in extrusion of m-LLDPE Exceed® 143 with capillary die L/D = 40 and D = 0.02" and with Nokia Mallifier Crosshead die attached to rheometer at204°C I l l LIST OF TABLES ix T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS A c k n o w l e d g e m e n t s I wish to express my sincere gratitude and appreciation to my supervisor, Prof. Sawas G. Hatzikiriakos, for his skillful guidance, support, and encouragement during the course of this work. Also I would like to thank Dr. J.S.Lasowaski for his continuous support and guidance during the course of this work. I am thankful to Sally Finora for her assistance with the surface energy experiments. I am thankful to Saint-Gobain Advanced Ceramics Corp., Amherst, NY, USA, for providing financial support to this project. I am also thankful to them for preparing and providing all boron nitride samples. I would like to acknowledge Ecole Polytechnique, Montreal, Canada, for generously offering their twin screw extruders for the preparation of polymer blends. I wish to thank, Alfonsius Budi Ariawan and Divya Chopra, Rheolab members for their helpful discussions and exchange of ideas. I thank my parents for their love and continuing support. Most of all, I thank my wife Nisha Seth who has been a source of strength and motivation for success. T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 1 Introduction From last three decades, civilization has experienced two profound revolutions: first is a revolution of the mind - the computer revolution, second is a revolution of matter - the materials revolution, one keyed to replacing traditional materials, such as steel, copper, aluminium, glass, cotton, wool and paper with organic synthetic materials also called as polymers. Polymeric materials are indispensable in our daily life and also for the development o f industrial products and leading technology. Without polymers, today's consumer goods would largely be undesignable, unmanufacturable, unusable, and unaffordable. Polymeric materials have the capability of being molded or shaped usually by the application o f heat and pressure. This property of plasticity, often found in combination with other special properties such as low density, low electrical conductivity, transparency and toughness allows polymers to be made into a great variety o f products. Polyethylene is probably the polymer we see most in daily life. Polyethylene is the most popular plastic in the world. Polyethylene is a significant commodity product with a worldwide manufacture volume in 1994 of around 65 million metric tons. For such a versatile material, it has a very simple structure, the simplest o f all commercial polymers. A molecule of polyethylene is nothing more than a long chain o f carbon atoms, with two hydrogen atoms attached to each carbon atom. Linear low-density polyethylene ( L L D P E ) resins are polyethylene (PE) plastic materials with densities in the range o f 0.915-0.925 gm/cm 3 . L L D P E has numerous applications, ranging from films and extrusion coatings to various household items such as food containers. This is the polymer that makes grocery bags, shampoo bottles, children's toys and even bulletproof CHAPTER 1 - INTRODUCTION 1 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS vests. Two important advantages of polyethylene are its inertness to water and micro-organisms and its low cost, around $0.50 per kilogram. The large number of commodity and specialty resins collectively known as L L D P E are in fact made up of various resins, each different from the other in the type and content of copolymer, compositional and branching uniformity, crystallinity and density, and molecular weight and molecular weight distribution ( M W D ) . Polyethylene was first produced in the laboratories o f Imperial Chemical Industries, L td . ( I d ) , England, in a fortuitous experiment in which ethylene was subjected to 1400 atm of pressure at 170°C. This polymer results from the polymerization o f ethylene, CH2=CH2, and has the following structure: ( - C H 2 - C H 2 - ) „ where n is the polymerization index. L L D P E resins are produced in industry with several classes o f catalysts: Ziegler catalysts based on titanium or vanadium compounds, Kaminsky and D o w catalysts utilizing metallocene complexes, and Phillips catalysts based on chromium oxides. These resins are manufactured either in gas-phase, solution or slurry polymerization process. Traditionally, Ziegler-Natta catalysts have been used since the 1950's for the polymerization of ethylene and propylene to produce polyethylene and polypropylene respectively. With Ziegler-Natta catalysts the polymer is produced at much lower pressures than the previous method. The polyethylene produced is also a much less branched polymer than its predecessor. Polymers produced with Ziegler-Natta catalysts have higher melting points than those produced by the old high-pressure method. This CHAPTER 1 - INTRODUCTION 2 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS makes these polymers much more commercially useful than the previous high pressure ones. A major recent technical advancement in the polyolefins industry is the development of homogeneous metallocene catalysts for polymerisation reactions. This class of catalysts produces polymers with narrow molecular weight and comonomer distributions, which, combined with controlled amounts of long chain branching, it is claimed to lead to both excellent processability and superior mechanical properties (Knight and Lai, 1992; Swogger et al, 1995). It, metallocene catalysts, represents the next generation of polyethylene manufacturing technology that is becoming the new global benchmark. The discovery of new catalytic systems based on metallocenes began a new era in polyolefins technology. Metallocene-based catalysts in comparison with conventional Ziegler-Natta systems offer higher versatility and flexibility both for the synthesis and control of the structures of polyolefins. High-density polyethylene (HDPE), polypropylene (atatic, isotactic, syndiotactic etc.), polystyrene, ethylene-propylene-diene terpolymers (EPDM) are among the most remarkable products obtained. In rotational molding, for example, metallocene PE (m-PE) grades promise a broader processing window, shorter cycle times, improved flow properties and potentially better warpage control. The metallocene market in 2000 has been estimated to be 6 million tons. Metallocene polyethylenes exhibit advantages in process and quality over conventional polyethylene products (N.Rohse, P. Bailey, 1997). The metallocene technology allows the synthesis of chemically more homogeneous compounds than that of Ziegler-Natta catalysts. The metallocene product has a narrower molecular weight distribution than that of conventional LLDPE. As a result, it has lower melt elasticity for CHAPTER 1 - INTRODUCTION 3 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS the same melt index. This leads to lower die swelling in extrusion. Stretch film made from metallocene polyethylene has high tackiness between individual layer and the films adhere well to each other and keep the packaged good firmly together. The low degree of scatter of the strength values permits a higher level of pre-stretching and thus the number of tear decrease. The amount of plastic per packaging unit can be reduced whether through using thinner films or increasing pre-stretching. Films produced from m-LLDPE also show very good mechanical strength value, in particular puncture toughness and puncture resistance by a factor of 6 (N.Rohse, P. Bailey, 1997). A further advantage of stretch films consists in low relaxation in the stretched state, which leads to a greater packaging stability. The m-LLDPE films are also suitable for medical devices. The toughness of m-LLDPE resins permits thinner, lighter-weight films and the lower density of the films give a higher yield than polyvinyl chloride (PVC) providing more film area per kilogram. Films made from m-LLDPE are very stable under radiation and sterilization and offer good low temperature flexibility. Linear low-density polyethylenes (LLDPE) are used in industry to manufacture films, pipes, sheets and profiles. Film is the largest application market for L L D P E resins used in packaging. LLDPE film is also used for nonpackaging applications such as industrial sheeting and agricultural mulch film. High clarity film produced with LLDPE resins manufactured with metallocene catalysts is used for food packaging and medical applications. Injection molded products, mostly for housewares, represent the second largest market for LLDPE. LLDPE resins are also used for blow molding (bottles, drums), for rotational molding (toys, large containers, tanks), for pipe and tubing, and for wire coating in electrical and telephone industry. The physical performance of products CHAPTER 1 - INTRODUCTION 4 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS made with L L D P E resins depends on the molecular weight, molecular weight distribution, comonomer type and content, its distribution in the resin, and processing conditions. However, the processing characteristics of L L D P E resins are primarily influenced by the weight average molecular weight and the molecular weight distribution. To process the various PE's in order to make useful products, various processes are used, among others, profile extrusion, film casting, film blowing and blow moulding. For any process to be economically feasible, the rate of production should be high enough from industrial point of view. However, it is well known that in many commercial polymer processing operations, including extrusion, wire coating, blow molding, film blowing, flow instabilities occur (Petrie and Denn, 1976; Ramamurthy, 1986). In these processes, a polymeric melt emerging from the die often shows surface distortions at throughput rates above a critical value. As a result of these instabilities, the final product becomes unattractive and commercially unacceptable. This effect can range from loss of gloss of extrudate surface to the appearance of gross distortions. The parameters affecting the degree of extrudate distortion include the process temperature, the flow rate, concentration and type of additive, geometrical dimensions of the die, the chemical nature of the polymer, the entrance geometry to the die and many others. These flow instabilities collectively known as melt fracture can manifest themselves in the form of either small amplitude periodic distortions appearing on the surface of extrudates (surface melt fracture or sharkskin) or severe irregular distortions at higher throughput rates (gross melt fracture). C H A P T E R 1 - INTRODUCTION 5 T H E R O L E O F S U R F A C E E N E R G Y O F B O R O N NITRIDE O N GROSS M E L T F R A C T U R E E L I M I N A T I O N OF P O L Y M E R S (a) (b) (c) Figure 1-1: Typical pictures of m-LLDPE extrudates at different shear rates, (a) Smooth extrudate {fA < 70 s"1), (b) Surface or Sharkskin melt fracture (fA = 70-350 s"1), (c) Gross melt fracture (fA > 350 s"1). Figure 1-1 shows typical extrudates exhibiting surface and gross melt fractures, obtained in the capillary extrusion of m - L L D P E resin. A relevant way to evaluate the processability of polymers, particularly in extrusion applications, is to determine their melt fracture performance, i.e. to determine the critical shear stress and/or shear rate for the onset of the appearance of extrudate irregularities. Ramamurthy (1986) conducted an extensive study of the melt fracture phenomenon using a variety of polyethylene resins. He concluded that the surface melt fracture (loss of gloss, appearance of haziness and small amplitude periodic distortions) occurs at a critical shear stress between 0.1 and 0.14 MPa for all polyethylenes, regardless of molecular structure and temperature. However, the critical shear stress variation is much larger (0.1 - 0.5 MPa) across different families of polyethylene (including metallocene) resins. Furthermore, there is no general correlation reported in the literature between critical operating parameters and molecular characteristics of the resins. It also appears that the critical shear stress for the onset of 6 CHAPTER 1 - INTRODUCTION T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS melt fracture depends on a number of factors including temperature, geometric characteristics o f dies (diameter and length-to-diameter ratio), metal of construction of the die, and thermal and processing history of the resin (Dealy and Wissbrun, 1990; Ramamurthy, 1986; Mounihan etal, 1990; Pearson and Denn, 1997; Cogswell, 1977). In order to eliminate the surface melt fracture and increase the rate o f production, polymer processing aids (PPA's) are used. These processing aids are mainly fluoropolymers that are added into the resin at concentration o f a few hundred ppm (typically 1000 ppm). During polymer flow, they diffuse to the wall and slowly coat it with a thin layer. In turn, the polymer slips over this thin layer. As a result, a significant drop in the shear stress and thus pressure drop is obtained, which obviously improves the extrudate appearance. It is noted that these PA's (fluoropolymers) can only eliminate sharkskin but not gross melt fracture. It has recently been demonstrated that certain Boron Nitride (BN) based compositions may act as effective processing aids in the extrusion o f a number o f fluoropolymers and polyolefins (Buckmaster, 1997; Rozenbaoum and Randa, 1998; Y i p and Hatzikiriakos, 1999). Boron Nitride (BN) is a solid lubricant, whose structure resembles to that o f graphite. In polymer processing, it is used as a foam nucleating agent in most commercial applications for fluoropolymer foams such as heat insulation, foamed tubing, etc. In the presence o f a blowing agent, it initiates the nucleation of voids in polymer extrudate. B N can successfully be used as a processing aid to eliminate not only sharkskin melt fracture but also substantially postpone gross melt fracture to significantly higher shear rates well within the gross melt fracture region. It is noted that conventional fluoropolymers can only eliminate sharkskin; they do not appear to have an effect on the C H A P T E R 1 - I N T R O D U C T I O N 7 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS gross melt fracture phenomena. In addition, it has also been suggested that the surface energy o f Boron Nitride plays a key role in eliminating gross melt fracture in the extrusion o f polyethylenes. However, B N samples containing relatively high amounts of boron oxide (B2O3) does not perform well as processing aids. This has been attributed to their higher surface energy that facilitates polymer chain adsorption at their surface and this has an adverse effect on their ability to act as solid lubricants. Therefore, it is interesting to perform experiments in order to measure the surface energy of various Boron Nitride (BN) powders used in our previous studies. This might also shed light into explaining the mechanism o f B N in eliminating gross melt fracture. In addition to these samples, we analyzed surface energy of three more B N samples prepared by using different processing methods. The aim o f this work is to measure the surface energy o f boron nitride powders and correlate their performance as processing aids with the critical shear rate for the onset o f gross melt fracture. A reliable procedure is been developed for measurement o f surface energy o f powders using the capillary rise technique based on Washburn's equation. The surface energy is resolved into its dispersive (London dispersion and Vander wall) and non-dispersive (dipole-hydrogen bonding) force contributions. It is based on the measurement o f contact angle of the powder with water and a - bromonapthalene. In addition to this, the effect of a new processing additive ( B N in combination with a fluoroelastomer) on the rheology and processability of molten polymers is studied. The equipment used includes an Instron capillary rheometer equipped with a special annular die (Nokia Maillefer wire coating cross-head) and a parallel-plate rheometer. Metallocene polyethylenes with and without B N and fluoroelastomer are tested in CHAPTER 1 - INTRODUCTION 8 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS extrusion. Firs t , it is demonstrated that B N is a superior processing aid compared to conventional fluoropolymer ones. Secondly, it is found that the combinat ion o f B N powders w i t h a small amount o f a fluoroelastomer improves even further the processability o f molten polymers (melt fracture performance). This performance is demonstrated in the continuous extrusion o f two metallocene L L D P E ' s . CHAPTER 1 - INTRODUCTION 9 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 2 Literature Review 2.1 Chemical Structure and Physical Properties of Polyethylene Polyethylene is a very familiar material in the modern world. This polymer has one o f the simplest molecular structure ([-CH2-CH2-] „)• A molecule of polyethylene is nothing more than a long chain o f carbon atoms, with two hydrogen atoms attached to each carbon atom. The simple structure of polyethylene is shown in the following figure: H H H H H H H H H H H 1 1 I I I 1 I I I I I —C—C—C—C—C—C—C—C—C—C—C—****** I I I I I I I I I I I H H H H H H H H H H H Sometimes, some o f the carbons, instead of having hydrogen atoms attached to them, wil l have long chains of polyethylene attached to them. This structure is named branched, or low-density polyethylene (LDPE) . I f no branching occurs, the structure is called linear polyethylene or H D P E (see Figure 2-1). Linear polyethylene is much stronger than branched polyethylene although branched polyethylene is cheaper and easier to make. Linear polyethylene is normally produced with molecular weights in the range o f 100,000 to 500,000 although higher M W ' can be produced. Polyethylene with molecular weights of three to six million is referred to as ultra-high molecular weight polyethylene (TJHMWPE). T J H M W P E can be used to make fibers, which are so strong that they have replaced Kevlar for use in bulletproof vests. Branched polyethylene is made by free radical vinyl polymerization. Linear polyethylene is made by a more complicated procedure called Ziegler-Natta polymerization, while using metallocene catalysis polymerization makes U r T M W P E . CHAPTER 2 - LITERATURE REVIEW 10 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS A molecule of linear polyethylene, or HDPE A molecule of branched polyethylene, or LDPE Figure 2-1: Schematic representation of a linear and a branched polyethylene molecule. The range of molecular weights of commercial LLDPE resins is relatively narrow, usually from 50,000 to 200,000. One accepted parameter that relates to the resin molecular weight is the melt index, a rheological parameter which, broadly defined, is inversely proportional to molecular weight. A typical melt index range for L L D P E resins is from 0.1 to 5.0, but can reach over 30 for specific applications. The vast majority of the experiments described in this work were performed using linear low-density polyethylene (LLDPE). The typical melting temperature of commercial PE resins is about 130°C; however, there are some exceptions. Its glass transition temperature is about -125°C. LLDPEs are predominantly semicrystalline materials at room temperature, exhibiting amorphous and crystalline structures. CHAPTER 2 - LITERATURE REVIEW 11 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 2.2 Rheological Measurements Rheology is the science that deals with the way materials deform when forces are applied to them. Rheological characterization of polymer melts involves several types of experiments: capillary rheometer experiments, in which a molten polymer is forced through a channel of known diameter and length; linear dynamic viscoelastic experiments using a cone-and-plate or a parallel plate configuration; and sliding plate rheometer experiments producing various types of non-linear strain deformations. In this section, the basic flow equations in these types of experiments are presented. This will be followed by a brief review of the melt fracture and wall slip phenomena. 2.2.1 Parallel Plate Rheometer - Linear Viscoelasticity The term viscoelasticity is derived from the merger of two terms viz. viscosity and elasticity. Viscosity relates to the "resistance" of material to flow under deformation (or stress). Elasticity relates to the response of an elastic solid like material subject to stress. Viscosity is an energy dissipative process, while elasticity is an energy storage process. The class of materials, which exhibit both viscous and elastic behavior are termed viscoelastic materials. Polymeric materials are a typical example of this category. The most widely used experiments to determine the linear viscoelastic properties of polymers are small amplitude oscillatory shear tests. Measurements of rheological properties at low shear rates and deformations are usually carried out in rotational rheometers such as a parallel-plate rheometer (Figure 2-2). In this work, a Rheometrics System IV parallel-plate rheometer was used. The two plates are mounted on a common axis of symmetry, and the sample is inserted in the space between them, The upper plate CHAPTER 2 - LITERATURE REVIEW 12 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS is rotated at a specified angular velocity o)(t) and as a result the sample is subjected to shear. The motion of the upper plate is programmed, and the resulting torque, M, is measured (so called constant strain rheometers). Pressure transducer Figure 2-2 Parallel plate rheometer Reproducibility o f such a device lies within ±2%. Another mode of operation is fixing the torque and measuring the displacement (constant-stress rheometers). In this experiment, a sample of material is subjected to a simple shear ring deformation such that the shear strain is a function o f time given by: y(t) = y0 sin(tvt) (2-1) where y0 is the strain amplitude and co is the frequency. The stress is then measured as a function of time. It can be shown that the shear stress is sinusoidal in time and independent o f strain for small enough strains (small linear viscoelastic limit): a(t) = o-0 sin( cot + S) (2-2) where <J0 is the stress amplitude and 8 is a phase shift, or the mechanical loss angle. Using a trigonometric identity, one can rewrite Equation (2-2) in the following form: 13 CHAPTER 2 - LITERATURE REVIEW T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS o-(0 = Yo [G'(fi>) s i n O O + G\a>) cos(cot)} (2-3) where G'(cu) is the storage/elastic modulus and G"(<y) is the loss modulus. The elastic modulus (G ' ) o f a material is defined as the ratio of the elastic (in-phase) stress to strain and relates to the material's ability to store energy elastically. Similarly, the loss modulus ( G " ) o f a material is the ratio o f the viscous (out of phase) component to the strain, and is related to the material's ability to dissipate stress through flow. The ratio o f these moduli (G 'VG' ) is defined as tan 8, and indicates the relative degree of viscous to elastic dissipation, or damping, o f the material. These two quantities can be calculated from the amplitude ratio, Gd = cr0/y0, and the phase shift, 5, as follows: G' = Gd cos(5) (2-4) G" = Gd sin(<?) (2-5) This allows defining a complex modulus, G\a>), as follows: G\a)) = G'(a))+iGK(co) (2-6) Alternatively, the stress can be expressed in terms of two material functions, rf and rf\ having units o f viscosity as follows: o"(0 = / o \nX<°) cos(<y/) + 7]\o)) sin(arf)] (2-7) Thus the complex viscosity can be expressed as: i 7 > ) = ; 7 > ) - i i 7 > ) (2-8) ti' = G"/a (2-9) T}"=G'/co (2-10) CHAPTER 2 - LITERATURE REVIEW 14 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS where n'((o) is the dynamic viscosity, n"(a))\s the in-phase component of the complex viscosity. From the above equation, the tangent of the mechanical loss or phase shift can be calculated as: 2.2.2 Sliding Plate Rheometer - Simple Shear The laboratory procedure that most closely approximates simple shear is to place a thin layer o f fluid between two flat plates, clamp one of the plates in place, and move the second plate at a constant velocity, w, as shown in Figure 2-3. Wetted area, A Figure 2-3 Simple shear flow Under no-slip conditions, the shear strain and shear rate can be written as follows: K') = ^  (2-12) h y(0 = x (2-13) h CHAPTER 2 - LITERATURE REVIEW 15 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS The velocity field is given as: vx=r(<)y v = v z = 0 (2-14) ^XNV.V.VAV.V.V.V.V.V.V.'. u h y=yn = u/h Figure 2-4: Velocity profiles in simple shear under no-slip The components of the rate of deformation tensor are: f„=r(.0 and the stress tensor components are of the form fO 1 6\ 1 0 0 v 0 0 0 , ° ] JCC r , 0 0 (2-15) (2-16) One simple experiment is to perform steady shear and measure the component Txy from the total force needed to move the upper plate. The viscosity can be calculated from r] = T^iy. Repeating this experiment for different y values one may determine the viscosity material function. CHAPTER 2 - LITERATURE REVIEW 16 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 2.2.3 Capillary Rheometer Flow o f molten polymer through a tube or a channel under pressure is commonly encountered in polymer processing, for example in an extrusion die or in the runner feeding o f an injection mold. This type of flow is also used as the basis of capillary rheometer. A typical schematic is depicted in Figure 2-5. In this work, capillary rheometer was primarily used to extrude various blends at high shear rates. The extrudates obtained were visually examined to determine the sharkskin and gross melt fracture taking place at the surface of extrudate. The present section covers the underlying principles of capillary rheometry. The capillary rheometer consists of a melt reservoir, or barrel, for melting the polymer and a plunger or piston that causes the melt to flow through the capillary die of known diameter, D, and length, L. The quantities normally measured are the flow rate, Q, (related to the piston speed) and the driving pressure, Pd, (related to force on the piston that is measured by means o f a load cell). The measured piston force, Fj, is related to Pd as follows: (2-17) where Rb is the radius of the barrel or reservoir. Alternatively, Pd can be measured by mounting a pressure transducer directly in the barrel. Capillary rheometers are used primarily to determine the viscosities in the shear rate range o f 5 to 1000 s"1 (Dealy and Wissburn, 1990). However, in this thesis, shear rates as high as 4000 s'1 were achieved by selecting appropriate dimensions for the die CHAPTER 2 - LITERATURE REVIEW 17 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS (smaller dies result into higher shear rates for the same Q). Reproducibility of capillary rheometers is + 5%. Figure 2-5: Schematic of a capillary rheometer [Baird and Collias (1995)]. To calculate the viscosity, it is necessary to know the wall shear stress and the wall shear rate, and it is therefore necessary to have reliable techniques for evaluating these basic rheological quantities on the basis of experimental data. For steady-state, fully-developed flow of an incompressible Newtonian fluid, the wall shear stress, <JW, can be calculated as: 4L CHAPTER 2 - LITERATURE REVIEW 18 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS The magnitude o f the wall shear rate, yw, for a Newtonian fluid can be calculated as: For the case of a non-Newtonian fluid, this quantity is called the apparent wall shear rate, yA, that is the rate that a Newtonian fluid would have at the same volumetric flow rate Q: Capillary flow o f a Newtonian fluid is a controlable flow, which means that the flow kinematics does not depend on the nature of the fluid. Capillary flow of molten polymers, however, is only a partially controllable flow. This means that the velocity distribution in this flow is governed not only by the boundary conditions but also depends on the nature o f the fluid. To account for this, at least two corrections should be applied to the experimental data. First, the velocity profile in the flow of a polymeric fluid is nonparabolic, and one must correct the wall shear rate, yw, defined by Equation 2-19. This correction, generally known as the Rabinowitch correction, can be calculated as (Dealy and Wissbrun, 1990): It is noted that the correction factor b is a local quantity depending on yA. It can be shown that the true wall shear rate then can be obtained by use of the following equation (Dealy and Wissbrun, 1990): (2-20) b = d(\ogyA) d(\ogaw) (2-21) (2-22) CHAPTER 2 - LITERATURE REVIEW 19 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS For a power-law fluid, the shear stress is given by a = Ky" (2-23) where o~ is the shear stress, y is the shear rate, K is the consistency index, and n is the power law exponent. It can be shown that the wall shear rate for a power law fluid can be expressed as: '3 + l/n 7 A (2-24) Thus, it can be seen from Equations (2-22) and (2-24) that the Rabinowitch correction is equal to Mn for a power law fluid and 1 for a Newtonian fluid. It mainly represents deviation from Newtonian behavior. Second, the pressure drop must be corrected for the additional pressure required for the melt to pass through the contraction between the barrel and the capillary. The wall pressure distribution actually observed for capillary flow of molten polymers is sketched in Figure 2-6. One can see that the pressure drop, Pd is clearly not the pressure drop that one would observe for fully developed flow in a capillary of a length L. There is a B A R R E L C A P I L L A R Y P d - * --> i L i i ^ i i Pd FULLY rent - > DEVELOPED w ENTRANCE LENGTH o 0 0 L Pex Pa Figure 2-6: Wall pressure distribution for capillary flow (from Dealy, 1982) CHAPTER 2 - LITERATURE REVIEW 20 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS significant pressure drop near the entrance o f the die, APent- There may also be residual pressure at the die exit, called the exit pressure, A P ^ , but it is quite small compared to APent. The total pressure correct ion for exit and entrance regions is called the end pressure, A P e n < / , that is, A / L ^ A / ^ + A / L (2-25) The true w a l l shear stress is then obtained as: cr... = <L/D) (2-26) The pressure correction, APe„d , or the Bagley end correct ion can be determined by use o f a scheme proposed by Bag ley (1957). H e suggested to measure the dr iv ing pressure, Pd, at various values o f the f low rate, Q , using a variety o f capillaries having different lengths. F o r each value o f the apparent wa l l shear rate (Equat ion 2-20), he then plotted the dr iv ing pressure versus LID and drew a straight line through the points as shown in Figure 2-7. ei U R Figure 2-7: Bag ley plot for determining the end correct ion. CHAPTER 2 - LITERATURE REVIEW 21 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS Extrapola t ing the lines to the Pd=Q axis, he then obtained an end correct ion, e. Thus, the true w a l l shear stress can then be calculated as fol lows: Pd 4(L/D + e) (2-27) A n alternative wa y to determine the end correct ion is to use orifice capillaries w i th L/D=0. Th i s w i l l directly measure APent and thus equation (2-26) can be used directly to infer the w a l l shear stress. In addi t ion to capillary dies, a crosshead die was also used to assess the processability o f the various resins. The crosshead was a N o k i a Mai l le fer 4/6 that included dies and tips o f various diameters ("tip" is the wire guide) wi th equal entry cone angles o f 6 0 ° and the die land length o f 7.62 mm. The schematic o f the crosshead is shown in F igure 2-8. Figure 2 -8 : Crosshead die for wi re coating (from Buckmaster et al., 1997) The molten polymer enters port 11 to the die 2. Then the polymer is guided through to orifice 8 by the wi re guide 16. The interior and the exterior surface o f the wi re tubular shape is formed by the passage 24 and 4 respectively. The wire guide provides a channel CHAPTER 2 - LITERATURE REVIEW 22 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION O F POLYMERS as a mandrel to produce the tubular shape extrudate (10). The speed of wire reached the orifice 8 and draw down to a thinner cross-section. It makes a thinner polymer coating 26 on the wire. Wire was not used in our study. Therefore, a hollow shape extrudate was obtained during the experiment. The apparent shear rate was calculated by using the formula applied for slit dies having a large aspect ratio (Bird et al., 1987): YA= 6-$ (2-28a) 0.25(D-d)20.5n(D + d) where Q is the volumetric flow rate, d and D are the tip and die diameters, correspondingly. The apparent wall shear stress was estimated as the average of the shear stress at the inner and outer walls by using the following formula, which is based on the assumption of a power-law fluid (Bird et. al., 1987). The general formula for the shear stress distribution is: APDI (2-28b) where is the shear stress at radius r, AP is the pressure drop, L is the length of the die land, and fi is the parameter depending on the geometry and the power law index. 2.3 The General Flow Curve of Linear Polymers All polymer melts and concentrated solutions exhibit instabilities during extrusion when the stresses to which they are subjected become sufficiently high. The term melt fracture is used collectively for all extrusion instabilities. These various flow instabilities observed in the flow of polymeric liquids through capillary, slit and annular dies are reflected in the apparent flow curve, determined by means of a capillary rheometer. This CHAPTER 2 - LITERATURE REVIEW 23 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS is essentially a log-log plot of the wall shear stress, aw, as a function of the apparent shear rate, yA. 1 m CL 0.1 -101 102 103 10" h <s-') Figure 2-9: A typical apparent flow curve of a linear polymer A typical apparent flow curve (wall shear stress vs. apparent shear rate) for a linear polymer such as high-density polyethylene and liner low-density polyethylene is shown in Figure 2-9. One can easily identify five different flow regions. Initially there is a stable region where the extrudate appears smooth and glossy (region 1). In this region, the behavior of the melt resembles that of a Non-Newtonian fluid and the viscosity can be represented by a power-law expression (Equation 2-23). In this region, the no-slip boundary condition is assumed to be valid. Beyond some critical wall shear stress, crci, which is typically of the order of 0.1-0.2 MPa, the first visual manifestation of an extrusion instability appears as a high-frequency, small-amplitude distortion of the extrudate known as sharkskin (region 2). As the name implies, sharkskin is a roughness that usually modulates the extrudate diameter by no more than 1% and consists of 24 CHAPTER 2 - LITERATURE REVIEW T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS semiregular cracks or grooves that run mainly perpendicular to the flow (Piau, 1990; Benbow and Lamb, 1963). The onset of sharkskin appears to coincide with a change in the slope o f the apparent flow curve. A t a second critical value aci and within a certain range o f apparent shear rates, the flow ceases to be stable (region 3). It is the region of oscillating, or stick-slip, melt fracture where the extrudate has the appearance of alternating smooth and distorted portions. In this region, pressure drop oscillates between two extreme values. The periodic variations of the pressure and apparent shear rate define a hysteresis loop that connects the two branches of the apparent flow curve. Despite the scatter in the reported values o f the second critical shear stress, it is generally accepted that gross melt fracture is a basic characteristic of polymer systems. The greater the molecular weight and the narrower its distribution, the greater the amplitude o f the hystersis loop. A t higher throughputs, there is sometimes a transition to a second stable flow regime in which the extrudate again becomes smooth. This is the superextrusion region (region 4). Finally, at still higher shear rates, there is a transition to a wavy chaotic distortion {gross melt fracture), which gradually becomes more severe with increase in apparent shear rate yA (region 5). This typical behavior has been observed in the capillary extrusion o f many linear polymers such as high density and linear low-density polyethylene (Kalika and Denn, 1987), polytetrafluoroethylene (Tordella, 1969), polybutadiene (Vinogradov et al., 1972b), and others. 2.4 Melt Fracture As discussed before, melt fracture is an instability, which occurs beyond a critical shear rate or stress in a capillary, slit or annular dies during extrusion of polymers. The CHAPTER 2 - LITERATURE REVIEW 25 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS term melt fracture was introduced by Tordella (1956) because of the audible tearing noises, which accompanied the distortion of the extrudate. The appearance of a melt-fractured extrudate resembles that of a turbulent Newtonian fluid stream, which prompted by Vinogradov and Ivanova (1968) to describe the same phenomena as elastic turbulence. Melt fracture is a major problem in the extrusion of polyolefins and many other commercial polymeric materials. It depends on various operational and geometric factors, which includes die geometry, the polymer structure and its molecular characteristics and the process temperature. Melt fracture is most easily observed during extrusion at high throughputs through a long die. Below some critical throughput, the surface of the extrudate is smooth and glossy. A t a critical throughput value and higher the surface becomes distorted. The nature o f the surface distortions is dependent on the type of a polymer. The most complete set o f phenomena is observed in the extrusion o f linear polyethylenes (Kalika and Denn, 1987). A t a first critical stress, which is typically o f the order o f 0.1-0.2 M P a , the extrudate surface exhibits a small-amplitude, high-frequency periodic distortion. This is generally known as surface melt fracture or sharkskin. As the name implies, sharkskin is a roughness that usually modulates the extrudate diameter by no more than 1% and consists o f semiregular cracks or grooves that run mainly perpendicular to the flow (Piau et ai, 1990; Benbow and Lamb, 1963). However, Piau and E l Kissi (1992) using a photographic technique pointed out that in the case of highly entangled polymers, the size and spacing of these cracks may be of the same order of magnitude as the diameter of the extrudate. They stated that in this case, it is impossible to define sharkskin as a small-amplitude, high-frequency roughness, and that this CHAPTER 2 - LITERATURE REVIEW 26 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS definition concerns only a secondary phenomenon, based on observations carried out well downstream of the outflow section and hence after relaxation of the stresses and cracks. Surface melt fracture often manifests itself as a change in the slope of the apparent flow curve (Ramamurthy, 1986) as discussed in the previous section. There is a second critical stress at which periodic pressure pulsations are frequently observed. The extrudate surface alternately shows relatively smooth and sharkskin regions. This is known as stick-slip, or spurt flow. The average stress remains approximately constant in the stick-slip region. Note that oscillations in pressure drop are not always observed. Pudjijanto and Denn (1994) as well as Waddon and Keller (1990, 1992) found a stable "island" in the stick-slip region of polyethylene, where pressure oscillations stopped, extrusion pressure significantly dropped, and the extrudate became reasonably smooth. This island exists only in a narrow temperature window and a small variation of temperature, e.g. of the order of 1 °C, can interchange oscillations with a stable response. If the shear rate is increased further, a second stable flow regime may be encountered in which the emerging extrudate is smooth again (Tordella, 1969). This region is generally known as the superextrusion region. Finally at even higher shear rates, the extrudate becomes grossly distorted and is characterized by a wavy, chaotic, and non-periodic appearance; this is so called gross melt fracture. While the origins of extrudate distortions are still in dispute, there is agreement that sharkskin and gross melt fracture are distinguished not only by the appearance of the extrudate, but also by the critical conditions for their onset and by the character of the accompanying flow inside the die. Gross melt fracture occurs when the wall shear stress reaches a critical condition that seems to depend only on the polymeric fluid and little or CHAPTER 2 - LITERATURE REVIEW 27 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS not at all on the characteristics of the die (diameter, length, and the material of construction) (Kalika and Denn, 1987; Piau et al, 1990). Sharkskin, on the other hand, does not occur for all polymers (Denn, 1990), and for those for which it does occur, the onset condition has been found to depend on the shape of the outlet region of the die (Piau et al., 1990), the length of the die (Moynihan et al, 1990) and in some cases on the material of construction of the die or the presence of lubricants or sticking agents at the die surface (Ramamurthy, 1986; Hatzikiriakos and Dealy, 1991b). Most commercial grades of branched polymers, such as low-density polyethylene and branched polysiloxane, do not exhibit sharkskin or spurt flow, but they do exhibit gross melt fracture, usually with a large amplitude periodic distortion that appears to be accompanied by a swirling flow upstream of the entrance to the capillary. Finally, it is generally accepted that sharkskin originates at the die exit (Howells and Benbow, 1962), while the flow within the die is unsteady only when gross melt fracture occurs (Benbow and Lamb, 1963). 2.5 Mechanisms to Explain Melt Fracture The mechanisms or explanations of melt fracture which have been proposed in literature involve one or more of the following features: • die exit effects; • die entry effects; • slip at the die wall. CHAPTER 2 - LITERATURE REVIEW 28 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 2.5.1 Die exit effects: Sharkskin /Surface melt fracture There is universal agreement that sharkskin is initiated at the die exit. The first theory about surface melt fracture (sharkskin) was proposed by Howells and Benbow (1962) and later by Cogswell (1977). They hypothesized that the polymer fractures due to high stretching rates and to high stresses as a result of the abrupt change (shear to free surface flow) in the boundary condition at the exit of the die. The most likely cause of sharkskin is incorporated in a mechanism proposed by Bergem (1976) and Cogswell (1977). According to them, the melt leaving the die in the neighborhood of the wall experiences a large, rapid, tensile deformation as the velocity field adjusts form the no-slip boundary condition to the free-surface condition. Polymer chains are stretched during the tensile deformation, which causes the highly entangled polymer to respond like a rubber. The large stresses on the free surface causes crack to open, probably by an undefined stick-slip mechanism. Piau et al. (1988) showed that cracks on the surface o f extrudate always originate at the exit of the die. It should be noted that the existence of localized stresses at the die exit is confirmed by birefringence photographs (Vinogradov and Malkin , 1980). Tremblay (1991) simulated the flow of a linear polydimethylsiloxane melt. He showed that high stresses at the die exit produce negative hydrostatic pressure. He suggested that cavitation should occur very close to the die lip, thus leading to surface effects. Why there should be a critical condition for this phenomenon is not explained. Kurtz (1992) suggested that two critical conditions are required for sharkskin. First, a critical value of the wall shear stress must be exceeded and, second, the extrudate must be stretched for a sufficient period of time as it leaves the die. Moynihan et al. CHAPTER 2 - LITERATURE REVIEW 29 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS (1990) added to this conclusion that the melt should be first "pre-stressed" critically at the entry region o f the die. Ramamurthy (1986) suggested that the onset of sharkskin was accompanied by the occurrence o f wall slip in the capillary. This suggestion is supported by a noticeable slope change in the flow curve at the onset of sharkskin (Kurtz, 1984) which can be interpreted as slip. However, Piau and E l Kissi (1992) argued that slip in the die cannot explain the origin o f the sharkskin. Hatzikiriakos (1994) carried out numerical simulations o f the flow o f high density and linear low density polyethylenes under slip conditions and showed that slip is not a necessary condition for the occurrence of the sharkskin phenomenon, although it may affect it. Instead, a critical extension rate at the capillary exit and a critical pre-stress o f the polymer at the land region of the die provide the necessary conditions for its occurrence. Wang et al. (1996) speculated that the slope change in the flow curve arises from a combination o f interfacial slip and cohesive failure due to chain disentanglement initiated on the die wall in the exit region. Since the disentanglement state is unstable for the adsorbed chains, it is followed by a consequent re-entanglement, thus producing entanglement-disentanglement fluctuations that cause the sharkskin phenomenon. But perhaps the most significant finding made by Wang et al. (1996) is that the sharkskin dynamics is in good correlation with chain relaxation process. They used capillaries of different diameter and measured the period of surface distortions. Regardless of the capillary geometry, the period of surface distortions was found to be directly proportional to a polymer characteristic relaxation time, which was determined as an inverse of the crossover frequency o f the storage and loss moduli. Recently, the interfacial molecular CHAPTER 2 - LITERATURE REVIEW 30 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS instability mechanism proposed by Barone and coworkers (1998) is a specific manifestation of the conceptual model, with the plausible assumption that a coil-stretch transition accompanied by disentanglement of near wall chains in the exit reason is the local cause of the stick-slip mechanism. 2.5.2 Die entry effects: Gross melt fracture Observations of the flow into the die at extrusion rates above and below that at which gross melt fracture occurs show that in some circumstances there is a clear interaction between the converging flow at the die entry and flow instabilities in the die. Most researchers suggest that the region upstream of the contraction is the site of initiation of the gross melt fracture type of instability. They base their opinion on experimental observations using essentially two methods: observation of flow tracers in the fluid, and flow birefringence. This latter technique for observing and investigating polymer flow through capillaries was intensively studied by Vinogradov and Malkin (1980), and the appearance of melt fracture was characterized by the observation of birefringence patterns before and after the critical regime. However, the precise instability mechanism is not completely clear yet, and it seems to be affected by various properties, such as fluid rheology, capillary die entrance geometry, and thermal effects. One explanation, given by Bagley and Schreiber (1961), is that the liquid polymer is fractured by elongational stresses in the entry region of the die. They gave a critical capillary wall recoverable shear strain criterion for the onset of gross melt fracture. White (1973) gave a different explanation based on experimental observations of the flow of a viscoelastic fluid through a contraction. He argued that a hydrodynamic instability was initiated in the form of a spiral flow when a critical Weissenberg number (We=Av/S, CHAPTER 2 - LITERATURE REVIEW 31 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS where X is the characteristic relaxation time, 8 is the spacing, and v is the relative velocity) was reached. In his opinion, this instability is the initial mechanism of polymer melt extrudate distortions. Most authors agree in claiming that, above a certain extrusion rate, the flow upstream of the contraction becomes unstable. These instabilities occur in the form of sudden pulsations, which were confirmed by visualization (Piau et al., 1990) and birefringence measurements (Tordella, 1969). They showed that such instabilities started along the upstream flow axis owing to the high elongation stresses that develop in this area. These instabilities trigger the phenomenon of gross melt fracture, which is often seen in the form of a regular helix oscillating at the same frequency as that of the pulsations of the upstream elongational flow (Piau et al., 1990). 2.5.3 Wall Slip in Polymer Melts There is preponderance of evidence available in literature that slip occurs above the stick-slip transition in highly entangled polymers and a reasonable amount of evidence that slip accompanies and even precedes sharkskin. It has been commonly observed that the polymer melt loses its adhesion to the wall once the wall shear stress exceeds a critical value, a c and the no-slip boundary condition is no longer valid at such high shear stress values. Mooney and Black (1952) were the first to study the slip phenomena. They used the capillaries of different radii to determine the flow curve of raw rubber and found that the slope of flow curve depends on the radii once the shear stress exceeds the critical value shown in Figure 2-9. CHAPTER 2 - LITERATURE REVIEW 32 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS Dx >D2 >£>3 ow = const J ^ ^ - ^ -YA\ YA2 YAi YA Figure 2-10: F low curves under slip condition Deviations from the no-slip boundary condition were observed by many other investigators (Laun, 1982; Lin , 1985). L i m and Schowalter (1989) reported observations o f slip for a polybutadiene melt using a heat transfer technique to record deviations from fully-developed flow. Kurtz (1984), Ramamurthy (1986) and Kalika and Denn (1987) showed that the flow curves obtained for various polyethylenes were discontinuous and exhibited a change in slope corresponding to the occurrence of surface melt fracture. Ramamurthy (1986), therefore, associated the appearance o f this phenomenon with a loss o f adherence at the interface between the polymer and wall material. He also observed that the onset o f surface distortion was dependent on the material of construction of the die, and it could be delayed or eliminated by changing the material of the die, although this is in contrast to observations for poly(vinyl alcohol)-borax solutions reported by Kraynik and Schowalter, (1981). This is a clear manifestation that the nature of the interface plays a crucial role in this phenomenon. The addition of fluoropolymers to the resin eliminates surface defects in spite of the fact that it promotes wall slip (Rudin et ai, 1985). Thus, it is clear that the wall slip itself is not the primary cause of sharkskin. The same conclusion was drawn by CHAPTER 2 - LITERATURE REVIEW 33 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS Hatzikiriakos and Dealy (1991a,b) who carried out experiments in both a sliding plate and a capillary rheometer to determine the effect of the presence of two fluoropolymers on the slip velocity of a HDPE. They found that in one case slip increased, while in the second case it decreased, although both fluoropolymers eliminated surface defects. Therefore, it can be observed that wall slip is not the only contributor to the cause of surface melt fracture. The mechanism of wall slip is not yet clear. One hypothesis involves the well known theory of Vinogradov (Vinogradov et al, 1972b). He argued for a transition at some critical wall stress from a melt state to a so called "forced high elastic state" in which the polymer melt could be treated as a rubbery solid, and thus adhesive failure could be understood. Another mechanism is based on the ability of polymer molecules to entangle and form a physical network. According to Brochard and de Gennes (1992), there exists a monolayer of a polymer next to a wall where molecules are attached to the wall through several sites along their backbone. These chains are connected with the bulk of the material through entanglements. Under flow, the polymer molecules in the bulk are stretched, and in turn, they apply forces to the molecules at the interface through the entanglements. At some critical shear stress, some of the chains detach from the interface, and as a result, a weak slip boundary condition is obtained. This process depends on the interfacial conditions, e.g. the presence of fluoropolymer coatings which reduce polymer adsorption. If the shear stress is increased further, sudden disentanglement occurs, and a strong slip close to plug flow is obtained. Consequently, the polymer chains relax and entangle again, and this alternating transition between a weak and strong slip keeps on in a continuous fashion resulting in pressure drop oscillations in capillary flow. This flow CHAPTER 2 - LITERATURE REVIEW 34 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS mainly depends on the molecular characteristics of the polymer, e.g. the number of entanglements. 2.6 Polymer Processing Aids As discussed before, melt fracture poses a constraint on rate of production, which is undesirable from the industrial point of view. To increase the production, one must eliminate melt fracture or postpone it to high rates. The most common approach to achieve this objective is the use of polymer processing aids (PPA). Polymer processing aids are used together with the resins in order to (i) facilitate extrusion at reduced pressure drops and (ii) to increase the rate of production in extrusion operations by eliminating melt fracture phenomena. There are various characteristics to predict the performance of a PPA. It should be incompatible with the resin (immiscible), have low coefficient of friction with the resin to be processed, coat and adhere to the die surface and it should not react with other additives such as antioxidants and stabilizers that are contained in the polymer to be processed. Processing aids are usually fluoroelastomers that can be added to the resin at concentrations of a few hundred ppm (typically ~ lOOOppm), e.g. at the time of processing or introduced as a masterbatch. These processing aids reduce the pressure required to extrude the resin at a particular flow rate and eliminate or postpone melt fracture to high extrusion rates. Priester and Stika (1992,1994) suggested that the factors that may affect the performance of the processing aid include the level of additive, dispersion quality and the interaction with other ingredients (antioxidants and stabilizers) in the resin. They have also mentioned that a large number of small particles of the additive can give a better dispersion quality than a small number of large particles. Thus, small particles can do a CHAPTER 2 - LITERATURE REVIEW 35 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS better job in coating a die. The ideal particle size of the additive should be compatible with the polymer to be processed, it should have a good affinity for metal surface and should be less than 5 microns. Priester and Stika (1992,1994) have also studied the effect of die composition, surface roughness and surface cleanliness on the appearance of the extrudate. They have found that the higher the percentage of the surface covered with processing aid, the lower the die pressure will be. They have reported that there is also a critical surface roughness of the die beyond which the performance of processing aid started decreasing. Moreover, they have examined various methods of die cleaning. The best cleaning method was found to be using high-pressure water (about 20,000 psi). Finally, they reported that a die surface covered with contaminants would require a much higher level of additive than one that is clean in order to achieve the same performance in terms of melt fracture and extrusion pressure. Rudin ( 1 9 8 5 ) performed experiments for a LLDPE with the addition of Dynamar® (fluoropolymer) as a polymer processing additive. They have found that the presence of Dynamar® at the surface causes the polymer to slip and that the slip velocity increases with increasing the concentration of PA in the resin. They studied the surface of the extrudate samples by using the X-ray photoelectron spectroscopy (ESCA) in order to detect the levels of fluorine. In addition, the extrudate were fractured in liquid nitrogen and the cross section was also analyzed to measure the fluorine concentration. The data indicated that a measurable concentration of fluorine at the surface exists, while the average concentration through the cross section was too low for the ESCA reading. From these data, one may suggest that processing aid such as Dynamar® move to the die/metal interface from the bulk. There the PA functions as a lubricant and as a result reduces the CHAPTER 2 - LITERATURE REVIEW 36 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS driving pressure of extrusion. They have also mentioned that a proper balance of incompatibility and diffusion rate is required to ensure coating of the metal die. This does not affect the efficiency of the extruder. Similar observations have also been reported for other types of processing aids. Stewart (1992) reported that a masterbatching step was required in order to provide a good quality of dispersion of additives into the resin. Athey and Thamm (1986) reported that there is a rheological change of LLDPE, HDPE, HMWHDPE and PP with the addition of a small quantity of F K M - A (Vinylidene fluoride and hexafluoropropylene). The addition of F K M - A changed the effective viscosity of the resin, thus reducing the driving pressure during the extrusion at certain shear rate. In general, processing aids reduce the pressure required to extrude the resin at a particular flow rate and eliminate or postpone melt fracture to higher extrusion rates. Note that these additives can eliminate only sharkskin and the so-called stick-slip (oscillating or cyclic) melt fracture. To the best of our knowledge, they do not appear to have an effect on the extrudate appearance in the gross melt fracture region. A typical example of the action of a fluoropolymeric PA is shown in Figure 2-11. Figure 2-11 shows the. flow curves of a linear low-density polyethylene with and without the addition of 250 ppm of fluoropolymer obtained from an extruder by using an annular die (Stewart et al., 1993). This example shows that the presence of processing aid reduces the required extrusion pressure before the onset of gross melt fracture region. However, it appears that there is no significant change in pressure in the gross melt fracture region. Sharkskin and stick-slip surface instabilities can be eliminated in this case. However, these processing aids have no effect on the gross melt fracture regime. CHAPTER 2 - LITERATURE REVIEW 37 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS Figure 2-11: The flow curve of a linear low-density polyethylene with and without the addition of 250 ppm fluoropolymer (Stewart et al., 1993). Recently, it has been reported that certain boron nitride (BN) based compositions may act as effective processing aids in the extrusion of a number of fluoropolymers and polyolefins (Buckmaster, 1997; Rozenbaoum and Randa, 1998; Yip and Hatzikiriakos, 1999, Lee and Kim, 2000). It has been reported that B N can successfully be used as a processing aid to eliminate not only sharkskin melt fracture but also substantially postpone gross melt fracture to significantly higher shear rates well within the gross melt fracture region. It is noted that conventional fluoropolymers can only eliminate sharkskin; they do not appear to have an effect on the gross melt fracture phenomena. Yip and Hatzikiriakos (1999), reported that BN is an effective processing aid when it possess the following characteristics (i) average particle size of up to about 10u.ni, (ii) no agglomerations (iii) absence of boron oxides in its structure and (iv) good dispersion of B N into the resin under process. Also, boron nitride must be used at its CHAPTER 2 - LITERATURE REVIEW 38 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS optimal concentration depending on the type of polymer and the extrusion temperature. A typical example o f the action o f a Boron nitride as a P A is shown in Figure 2-12. Figure 2-12 shows the flow curves o f a linear low-density polyethylene with and without the addition o f 500 ppm of boron nitride (Rozenbaoum et al., 1998) obtained from a capillary rheometer by using an annular die. It can be observed that the presence o f B N does not *.—-W w CO CU OJ JZ w 75 c CO (0 CL CL < 0.1 — i 1 1 i I I I 1 , , , r -P E Exact 3128 T=163°C Crosshead: D=3.0 mm, d=1.524 mm fi • virgin resin A with 0.05% BN Open symbols correspond to smooth extrudate _ i i • • • . , i 10 2 10 3 Apparent shear rate (j^, s"1 Figure 2-12: The apparent flow curves for P E Exact 3128 without and with boron nitride obtained in a rheometer with Nokia Maillefer crosshead having 3.00 mm die and 1.52 mm tip at 163°C (Rozenbaoum et al., 1998). alter the flow curve during extrusion. It is noted that pressure reduction was observed with traditional processing aids. However, the presence of B N has a significant effect on extrudate appearance well within the gross melt fracture regime. It had also been reported that the additive had no or very little effect on the extrudate appearance in the capillary geometry (Rosenbaum et. al., 1998). The greatest CHAPTER 2 - LITERATURE REVIEW 39 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS influence o f the additive occurred in crosshead dies and tips where the additive particles seemed to enhance melt slippage and relieve internal stresses. L e e and K i m (2000) also reported that the critical apparent shear rate for onset o f melt fracture and the shape o f extrudate are highly dependent on processing temperature, L / D and content o f the B N . They found that the addition o f 0.5 wt % o f B N in L L D P E eliminate or delay sharkskin, stick-slip melt fracture and gross melt fracture to much higher rates, even though there is no difference in the linear viscoelastic and mechanical properties between neat and L L D P E containing B N . In addition to that L e e and K i m (2000), observed that the cri t ical apparent shear rate for the onset o f melt fracture and slip was found to drop considerably wi th increase in the apparent characteristic relaxation time. It was found that the op t imum melt fracture performance is obtained at a specific boron nitride concentration, and is characterized by a min imum relaxation time for the resin defined i n terms o f recoverable shear. These observations are consistent w i th those reported by Rozenbaoum et al. (1998) and Y i p et al. (1999). 2.7 Factors Affecting Polymer Flow 2.7.1 Pressure effects T h e presence o f large pressure gradients are typical in the processing o f molten polymers. The compressibil i ty o f these materials in a molten state is quite high, and the effect o f pressure on the viscosi ty cannot be neglected. It is k n o w n from experiments (Rauwendaal and Fernandez, 1985; K a l i k a and Denn, 1987) that the apparent f low curves do not superpose for capillaries o f different L/D ratios. Instead, the apparent f low curves shift to higher values o f the wal l shear stress wi th increase o f the L/D ratio and therefore, CHAPTER 2 - LITERATURE REVIEW 40 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS pressure. The pressure dependence of viscosity is typically represented by an exponential function (first order approximation) which for a given temperature can be written as 77 = rf exp(aP) (2,2c where rf is the viscosity at ambient pressure, a is the pressure coefficient of viscosity and P is the absolute pressure. Because pressure increases the viscosity of polymers, it results in the formation of a "pre-stress zone" at the die entrance. This might explain the high stresses encountered by the melt as it enters the die and undergoes gross melt fracture. 2.7.2 Temperature effects - time-temperature superposition Rheological properties of molten polymers usually depend on temperature. This means that to obtain a complete picture of the behaviour, experiments must be carried out at several temperatures. It is often found that rheological data measured at several temperatures can be brought together on a single master curve by means of "time-temperature superposition." This greatly simplifies the description of the effect of temperature. Furthermore, it makes possible the display on a single curve of material behaviour covering a much broader range of time or frequency than can ever be measured at a single temperature. Materials whose behaviour can be displayed in this way are said to be "thermorheologically simple" (Dealy and Wissbrun, 1990). It was found that data for different temperatures can often be superposed by introducing a shift factor, ar, determined empirically. Thus, if one makes a plot of a rheological property versus time, ar is obtained from the horizontal shift necessary to bring the data for any temperature T onto the same curve as data for temperature TQ. For CHAPTER 2 - LITERATURE REVIEW 41 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS example, flow curves (shear stress vs. shear rate) w i l l be plotted as shear stress versus f aj. N o t e that no shift factor is required for quantities containing no units o f time. This implies that a plot o f such a quantity versus another, both containing no units o f time, w i l l be temperature independent. The shift factor is a function o f temperature, and the W L F equation has been found useful [Ferry, 1980]: -C?(T-T0) where C° and C ° are constants determined at To for each material. This equation holds at temperatures very close to glass transition temperature, Tg. A t temperatures at least 100 K above Tg, an empirical relationship, the Arrhenius equation, was found to be va l id : l o g ( * r ) = ^ f 1 1 \J Tref J (2-31) where Ea is the flow activation energy, R is the gas constant, and T^f is the reference temperature. Since polyethylenes are processed at temperatures much higher than Tg, this equation is often used by rheologists. 2.8 Surface Chemistry 2.8.1 Contact Angle T h e phenomena o f wett ing or non-wetting o f a sol id by a l iqu id is understood by contact angle. Consider the drop o f a l iquid resting on a solid surface. The drop o f l iquid forming an angle may be considered as resting in equil ibrium by balancing the three CHAPTER 2 - LITERATURE REVIEW 42 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS Y'v Figure 2-13: Droplet of a liquid on solid substrate at equilibrium forces involved - (i) the interfacial tension between solid and liquid fai), (ii) the interfacial tension between solid and vapor (frv) and (iii) the interfacial tension between liquid and vapor (//„). The angle between the liquid and the solid phase is known as the contact angle or wetting angle. It is the angle included between the tangent plane to the surface of the liquid and the tangent plane to the surface of the solid, at any point along their point of contact. Contact angle phenomena are complex and their interpretation is to some extent still controversial. The interest in contact angles is two fold: first, they play a major role in a number of technological, environmental and biological phenomena, and secondly, they are also a manifestation of the surface tension of the solid on which the contact angle is formed. Therefore, in principle the surface tension can be measured. Studies and measurements of contact angles of liquids on solids have great technological importance. This is especially true with water; every action of water on earth is controlled by its wetting behavior with the solid with which it comes into contact. For example, contact angle of water on our skin is about 90 degrees. If it was zero, water could have penetrated the pores of the skin and possibly been absorbed by blood. Most of the polymers like polyethylene, polypropylene and polytetra-fluoroethylene (PTFE) show CHAPTER 2 - LITERATURE REVIEW 43 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS high contact angle behavior with many liquids. An example is the Teflon coated frying pan. Contact angle measurement has shown that the contact angle is about 35 degrees for cooking oil on Teflon; thus oil must not stick to the surface making it easier for cleaning. In the manufacture of printing inks, the contact angle formed by a drop of ink on the paper determines the printing quality of ink. For good quality of paper, contact angle should be between 90° and 110°. If contact angle is less than 90° then ink will spread on paper and if it is greater than 110° then breaks will occur while printing. 2.8.2 Measurement of Contact Angles The most commonly used method for measuring the contact angle involves direct observation of the profile of a liquid drop or an air bubble resting on a plane solid surface (Wu, 1982). The measurement of the contact angle formed at the point where a liquid contacts a substrate (liquid or solid) is called the contact angle. The contact angle formed between a "sessile" drop and its supporting surface is relative to the forces at the liquid/solid or liquid/liquid interface and can be used as a direct test specification or as a quality characteristic. The contact angle is obtained by measuring the angle formed between the tangent to the profile at the point of contact with the solid surface. This can be done on a projected image (Kneen and Benton, 1937), or a photograph of the drop profile (Thiessen and Schoon, 1940), or directly measured by using a telescope fitted with a goniometer eyepiece (Bigelow and Zisman, 1946; Fox and Zisman, 1950; Hewlett and Pollard, 1977). Contact angles accurate up to ± lor 2° can readily be obtained. The uncertainties are higher for small angles (less than 10°) and large angles (larger than 160°), because of the difficulty in locating the point of contact for constructing a tangent. Alternatively, the contact angle may be measured by the reflected light method CHAPTER 2 - LITERATURE REVIEW 44 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS (Langmuir and Schaefer, 1937; Fort and Patterson, 1963). A light beam emitted from a microscope will be reflected back into the microscope when the light beam is shed at right angles to the drop surface. Thus, i f the microscope is focused on a three phase contact point, the angle which microscope makes with the vertical direction when the liquid drop appears bright is equal to the contact angle. The method is, however, restricted only to angles less than 90°; all unwanted reflections from the drop surface must be eliminated. Interference microscopy method is also used for measuring the contact angle. This method is used for contact angles less than about 10° which are generally difficult to measure accurately by the tangent method or the reflected light method. Recently, Sawicki (1978), reported an interference microscopy method capable o f measuring contact angles less than 10° to an accuracy of ± 0.1°. The profile o f the leading edge o f a drop can also be observed. In this method, a monochromatic light (5461 o A - green mercury line) is reflected with a beam splitter onto the reflecting substrate to form interference bands parallel to the drop edge. The contact angle is calculated from the spacing o f the interference bands, the refractive index o f the liquid, and the wavelength o f the light. The contact angles can also be determined by measuring the dimensions o f a liquid drop. Liquids drop suitable for contact angle measurement can readily be placed onto solid surfaces by using a microsyringe. For very small drops, of the order o f 10"4 ml (Mack, 1936; Bartell and Zuidema, 1936), the distorting effect of gravity is negligible, and the drop takes the shape of a spherical segement. For larger drops, the drops shapes are distorted by gravity. The drop profiles are described by the equation o f Bashforth and Adams. The equation has been integrated numerically. Standard graphs are available for calculating the contact angle form the drop geometry. CHAPTER 2 - LITERATURE REVIEW 45 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS On the other hand, polynomial functions have been obtained and used to calculate contact angle from the drop volume and drop base radius (Fisher, 1979). Commercial apparatus for measuring the contact angle is available (Rame-Hart Inc., N J ; Kernco Instruments Co. , N Y ) . Contact angles accurate up to ± lo r 2° can readily be obtained by using these instruments. It is to be noted that these traditional methods can be used only for plane solid surfaces. These methods cannot be used for powdered/porous solid materials because of heavy penetration o f liquid into the powder. Therefore, the contact angle of liquids on powders is measured by the wettability technique based on Washburn's equation. 2.8.3 Wettability The wettability of a liquid is defined as the contact angle between a droplet o f the liquid in thermal equilibrium on a horizontal surface. The wetting angle 9 is given by the angle between the interface of the droplet and the horizontal surface. The liquid is seemed to be wetting when 0 < 9 < 90 degrees and non-wetting when 90 < 9 < 180. 9 = 0° corresponds to the perfect wetting of the liquid on the surface and the drop spreads forming a film on the surface. Actually, the wetting angle 9 is a thermodynamic variable that depends on the interfacial tensions of the surfaces. In thermodynamic equilibrium the wetting angle 9 is given by Young's law ylv cos 6 = ysv - y s l (2-32) which is mainly a force balance at the point of contact of the three phases. CHAPTER 2 - LITERATURE REVIEW 46 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 2.8.4 Contact Angle of Powders The contact angle of powders cannot be directly measured by the sessile and pendant drop methods due to heavy penetration of liquid into the powder. An early attempt to measure the contact angle of a powder was done by Bartell (1927, 1942). In his method, the powder is compressed in a cylinder, at one end of which liquid can penetrate the powder. At the other end of the cylinder the gas pressure is increased until the liquid ceases to penetrate. From the pressure difference over the cylinder (Ap) the cosG can be calculated by means of the following equation: 2y. cos 6 Ap = — (2-33) r where r is the radius of the pores between the particles. This method was disregarded because the technique is exceedingly tedious and that errors associated with the measurement was of more than 10%. A second method for the determination of cosG has been given by Washburn (1921). In this method a powder was compacted into a glass tube by manual tapping. Then compacted powder was held into a liquid bath to measure the height of penetrated liquid into the packing. The rise of liquid height into the powder was observed and recorded manually (see Figure 2-14). The rate of rise of liquid in powder is given by following equation: dh2 = Kyw cos (9 dt 2p where yiv and p are the surface tension and viscosity of liquid, respectively, 9 is the liquid contact angle on the solid surface, r is the capillary radius, K is tortousity constant. CHAPTER 2 - LITERATURE REVIEW 47 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS Figure 2-14: Traditional method for wettability measurements o f powders The disadvantages with this method are; (i) since liquid permeates up unevenly into the powder, precise measurement of height of liquid rise into the powder is difficult to measure; (ii) it is difficult to maintain uniform and reproducible packing by manual tapping; (iii) due to low column-packing pressure the packing density of the column is unstable and inconsistent and its structure changes as wetting proceeds; (iv) non-reproducible experimental data are usually obtained. In the present work, we have done modifications to overcome some of the experimental difficulties. The details o f these modifications are discussed in a later section of this thesis. It is also noted that this method is based on the surface energy and surface tension of many surfaces involved, and thus these are discussed in the next section. 2.8.5 Surface free energy and Surface tension A surface is the region between the condensed and gas phases. Similarly, an interface is the region between two adjacent phases. These regions cannot be considered as separated phases because they are only several molecular diameters thick and their properties change with the perpendicular distance from the bulk phase. The molecules in the bulk o f an isotropic condensed phase are in a symmetrical force field i.e. the attractive Compacted Powder Filter Paper >• Liquid bath CHAPTER 2 - LITERATURE REVIEW 48 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS forces between the molecules are the same in all directions. The molecules on the surface are attracted into the bulk by neighborhood molecules o f the same phase and the resultant sum o f all forces points perpendicularly towards the surface plane i.e., the attraction from the gas phase is zero. Th is creates an asymmetric force field w i th a surface excess energy called surface free energy (Kloubek , 1992). The tendency to decrease this asymmetry accounts for adsorption, adhesion and wett ing phenomena. The forces at the interface o f t w o condensed phases are usually unbalanced and give rise to interfacial free energy. B o t h the surface and interfacial free energies are denoted by the symbol y. The higher the surface free energies and the smaller the interfacial free energy o f the adjoining phases, the higher is their w o r k o f adhesion, W a . Th i s is expressed by the Dupre equation which , for the interface between a sol id (s) and a l iqu id (1), is expressed as: K=Y,+7i-Y:, (2-35) A l l the values are g iven in m J / m 2 . A t the interface between t w o liquids there w i l l be interactions between molecules o f different type and the interfacial tension arises due to the attractive forces between the molecules in the different fluids. The interfacial tension o f a g iven l iqu id surface is measured by finding the force across any line on the surface divided by the length o f the line segment. Thus, the interfacial tension becomes a force per unit length, wh ich is equivalent to the energy per unit area. The intermolecular attractions, which cause interfacial tension, result f rom a variety o f w e l l k n o w n intermolecular forces. M o s t o f these forces, such as the metallic bond or the hydrogen bond are a function o f specific chemical nature (Fowkes , 1964). O n the other hand, L o n d o n dispersion forces exists in all types o f matter and always give an CHAPTER 2 - LITERATURE REVIEW 49 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS attraction force between adjacent atoms or molecules no matter h o w dissimilar their chemical nature may be ( L o n d o n , 1937). The L o n d o n dispersion forces arise from the interaction o f fluctuating electronic dipoles wi th induced dipoles in neighboring molecules or atoms. These forces depend on electrical properties o f the volume elements invo lved and the distance between them and are independent o f temperature. In a l iqu id there are t w o main interatomic forces, the metallic bond and the L o n d o n dispersion forces. T h e surface tension o f the l iquid is a result o f t w o types o f forces, namely dispersion forces and non-dispersion forces (metallic bonds). Adhes ion refers to the state in wh ich t w o dissimilar bodies are held together by a intimate interfacial contact such that mechanical force or w o r k can be transferred across the interface (Kemba l l , 1954; E l e y and Tabor , 1961; Sa lmon and H o u w i n k , 1965; B ike rman , 1968; G o o d , 1976; W u , 1982). The interfacial forces helding the t w o phases together may arise from Vande r Waals forces, chemical bonding o r electrostatic attraction. T h e mechanical strength o f the system is determined not only by the interfacial forces but also by the mechanical properties o f the interfacial zone and the t w o bulk phases. W h e n an adhesively bonded structure breaks under l o w applied stress, the structure is often said to have " poor adhesion." Thermodynamic adhesion refers to equi l ibr ium interfacial forces or energies associated wi th reversible processes, such as ideal adhesive strength, w o r k o f adhesion and heat o f wetting. Th is term was first proposed by E l e y (1961). Chemica l adhesion refers to adhesion invo lv ing chemical bonding at the interface. Mechanica l adhesion arises from microscopic mechanical in ter locking over substantial portions o f the interface ( A S T M D 9 0 7 ) . CHAPTER 2 - LITERATURE REVIEW 50 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS Vande r wal ls attraction between t w o planar microscopic bodies diminishes rapidly w i t h distance by z" 6 where z is the distance o f separation. The equil ibrium o interfacial separation is typically 2-5 A. Therefore, intimate molecular contact at the interface is necessary to obtain strong interfacial attractions. Wi thou t intimate molecular contact, interfacial attractions w i l l be very weak and the applied stress that can be transmitted from one phase to the other through the interface w i l l accordingly be very l o w . Therefore, intimate molecular contact at the interface is necessary to fo rm a strong adhesive bond. H o w e v e r , intimate molecular contact alone is not sufficient to give a strong adhesive bond in many cases. L iqu ids and solids o f similar chemical composi t ions have intermolecular forces o f similar magnitudes. Y e t liquids have poor mechanical strength, and solids have g o o d mechanical strength. Furthermore, the mechanical strength o f a polymer drops o f f sharply be low its cri t ical molecular weight for entanglements. Th is is because the mechanical strength o f a polymer derives mainly from molecular entanglements. H i g h l y ordered crystals can have g o o d mechanical strength without molecular entanglements by virtue o f tight molecular packing and lack o f flaws. M o l e c u l a r entanglements are all important in common polymers. Viscoelas t ic dissipation constitutes the major component o f fracture energy. The magnitude o f viscoelastic dissipation depends on the extent o f molecular entanglements as we l l as on the magnitude o f intermolecular force. The volume o f the dissipation zone around the crack tip can be large only i f molecules are entangled. Therefore, it is important to recognize that the interface is a region o f finite thickness wherein the segments o f the macromolecules may interpenetrate. Its mechanical strength w i l l very much depend on its structure. CHAPTER 2 - LITERATURE REVIEW 51 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS It is we l l k n o w n that the l o w surface-energy component o f a polymer blend can preferentially migrate to the surface o f the die wal l . This process is driven by differences in the surface energies o f components in the blend. F o r example, surface enrichment o f po ly (v inyl methyl ether) ( P V M E ) , w h i c h is the lower surface energy component, was detected at all composi t ions o f the blends o f P V M E and polystyrene. This phenomenon has been found to be very useful i n many practical applications such as surface modif icat ion o f polymers, formulation o f adhesives, reduction in friction o f polymer surfaces and improvement o f coating. This mechanism is used to explain the migrat ion o f fluoropolymer, when used as processing additives, to the die wa l l . CHAPTER 2 - LITERATURE REVIEW 52 THE ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 3 Objectives The a im o f this w o r k is a comprehensive study o f the effect o f surface energy o f B N on gross melt fracture behavior o f polyethylenes. Furthermore, a new processing aid ( P A ) is developed by combining B N wi th a conventional P A . M o r e specifically, the objectives are as fo l lows: • T o develop a reliable procedure for measuring the surface energy o f powders using the capil lary rise method based on Washburn 's equation; • T o measure the surface energy o f boron nitride powders and correlate their performance wi th the crit ical shear rate for the onset o f gross melt fracture. • T o conduct a thorough rheological characterization o f metallocene l inear- low density polyethylene ( m - L L D P E ) resin wi th and without the new processing additive (boron nitride powder in combination wi th a fluoroelastomer) as a function o f temperature, pressure and composi t ion. • T o study the processability o f the m - L L D P E wi th the use o f various boron nitride powders (prepared through different processing methods) in extrusion using a crosshead die, wh ich mimics the wire coating process. • T o determine the cri t ical condi t ion for the onset o f melt fracture o f the m - L L D P E w i t h addit ion o f the new processing additive (boron nitride powder in combinat ion wi th a fluoroelastomer) as a function o f temperature, pressure, additive type and composi t ion. CHAPTER - 3 OBJECTIVES 53 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 4 Materials The objective of this chapter is to present the characteristics of the polymers and various type of boron nitride powders and fluoroelastomer used to carry out the present study. 4.1 Polymers The resins studied in this work in order to investigate the effect of various B N powders and their combination with fluoroelastomer in their processability during continuous extrusion are three metallocene catalyzed polyethylenes, namely Exact® 3128, Exceed® 116 and Exceed® 143. ExxonMobil Company provided all of these samples. Exact® 3128 is ethylene butene LLDPE copolymer with p = 0.9 and MI = 1.2 and Exceed® 116 is ethylene hexene L L D P E copolymer with p = 0.917 and MI = 1.0. 4.2 Boron Nitrides & Fluoroelastomers Studied Boron nitride is a white solid lubricant. It has a high thermal conductivity, low dielectric loss modulus, low thermal expansion and high lubricity over a wide temperature range. To the best of the author's knowledge, BN has the highest thermal conductivity of any commercial electrical insulator in the polymer system. Boron nitride powder has been shown to be an excellent additive for coatings and release agents, as well as for oils, potting compounds, friction plates, etc. Also, the powder is white, clean, and safe-to-use directly as a high-temperature lubricant and release agent. The product typically enhances lubricity, chemical resistance, and thermal conductivity. Table 4-1 shows the comparison of boron nitride powder to other common fillers. CHAPTER 3 - MATERIALS AND BLENDING METHODS 54 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS _ Table 4-1: Compar i son o f boron nitride to c o m m o n fillers B N A L 2 0 3 A L N Thermal Conduc t iv i ty ( W / m / K ) - 3 0 0 40 50-170 Die lec t r ic Constant 4 9 9 '•6.0 A 2.5 A Key • Boron O Nitrogen Figure 4-1: The structure o f boron nitride Figure 4-1 shows a typical structure for B N . E a c h boron atom is connected to four nitrogen atoms, and each nitrogen atom is connected to four boron atoms. The structure o f B N is similar to that o f graphite. In this study, w e have examined twelve types o f B N , wh ich includes the six different types o f boron nitride used in previous studies ( Y i p and Hatz ik i r iakos , 1999). The average particle size and its state o f agglomeration o f each type o f boron nitride used in ours studies are summarized in Table CHAPTER 3 - MATERIALS AND BLENDING METHODS 55 THE ROLE OF SURFACE ENERGY OP BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 4-2. These include B N type C T F 5 wi th particle size o f 5-10 p m and C T U F , wh ich contains a fair amount (2%) o f B2O3 compared to C T F 5 . A m o n g all B N used, only B N 4 2 7 and B N 429 exhibit agglomeration. O n the other hand B N 428, and C T F 5 have the smallest particle size and are free o f any agglomerated particles. F o r more details regarding the boron nitride morphology pictures, please refer to ( Y i p and Hatz ik i r iakos , 1999). Table 4 -2 : The average particle sizes and states o f agglomeration properties o f various boron nitride powder . Sample I D Part icle size from S E M , approx. (pm) Agglomerated Agglomera ted Size (pm) C T F 5 10 N o C T U F 3 N o 427 4 Y e s >180 428 1.5 N o 429 3 Y e s >300 430 20 N o A S 6 1 0 8 N o A S 6 1 1 6 N o A S 6 1 2 4 N o A S 6 1 3 25 N o A S 6 1 4 12 N o B N 20 M e d i u m Fluoroelas tomer 180 N o AS-ser ies samples differ by each other due to different processing methods for their manufacturing. Sample A S 6 1 0 is a milled B N powder, whi le sample A S 6 1 1 is a C T F 5 bo ron nitride fiirnaced twice during production. Sample A S 6 1 3 is molybdenum-sulfide (M0S2) whi le sample A S 6 1 4 includes a small amount o f thermally stable inorganic salt C a l c i u m Tetra-Borate ( C T B ) . Figure 4-2 shows the morphology study o f some o f the B N samples by using a Hi tach i S-2300 Scanning Elec t ron M i c r o s c o p e ( S E M ) operating at 5 k e V . The S E M pictures o f boron nitride powder and fluoroelastomer CHAPTER 3 - MATERIALS AND BLENDING METHODS 56 THE ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS used to study the effect o f combining these t w o are shown in Figure 4-3. It was observed that this particular B N powder used had an average particle size o f 15-20 Ltm wi th medium degree o f agglomeration. O n the other hand, the fluoroelastomer had an average particle size o f 180 u.m without any degree o f agglomeration. 4.3 Blend Preparation It is noted that a uniform dispersion is a necessary condi t ion for obtaining a g o o d performance o f processing aids (Rosenbaoum and Randa, 2000; Y i p and Hatz ik i r iakos , 1999). T h e fo l lowing technique was used for blend preparation. The B N and fluoroelastomer powders were mixed wi th appropriate amounts o f the v i rg in resin. Then, a master batch o f 10 wt % B N and 10 wt % fluoroelastomer wi th ground P E was prepared by using a preparation mixer. A desired final concentration o f a particular blend was obtained by mix ing v i rg in ground P E wi th the master batch by means o f a V" extruder equipped w i t h a coo l ing system and a 2 " pelletizer. This procedure results into a uniform dispersion o f B o r o n Ni t r ide and fluoroelastomer into the polymer. Some o f the blends were prepared by means o f a twin screw extruder. B o t h methods resulted into similar degree o f dispersion. CHAPTER 3 - MATERIALS AND BLENDING METHODS 57 THE ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE. ELIMINATION OF POLYMERS CHAPTER 3 - MATERIALS AND BLENDING METHODS 58 T H E ROI.E O F S U R F A C E E N E R G Y O F B O R O N NITRIDE O N GROSS M E L T F R A C T U R E E L I M I N A T I O N OF P O L Y M E R S (e) 00 Figure 4-2: SEM pictures of different BN samples: (a) CTF5; (b) AS610; (c) AS611; (d) AS612;(e) AS613;(f) AS614. Figure 4-3: SEM pictures of the BN and Fluoroelastomer particles dispersed in m-LLDPE Exact® 3128. (a) The particle size of Boron nitride is 15-20um while (b) fluoroelastomer is 180pm. C H A P T E R 3 - M A T E R I A L A N D B L E N D I N G M E T H O D S 59 T H E R O L E O F S U R F A C E E N E R G Y O F B O R O N N I T R I D E O N G R O S S M E L T F R A C T U R E E L I M I N A T I O N O F P O L Y M E R S 4.4 Thermal Analysis (DSC and TGA experiments) To investigate the mechanism by which boron nitride changes the properties of the materials and subsequently helps in eliminating gross melt fracture, we have performed thermal analysis of polymer blends along with boron nitride and fluoroelastomer. We performed D S C experiments to measure the latent heat of crystallization for polymer blends having different concentrations of Boron nitride. Figure 4-4, 4-5 and 4-6 shows o . o T -05-Temperature (*C) Figure 4-4: D S C curve for Exceed® 143 with 500ppm of B N (first heating cycle) the D S C curves (I s t heating, cooling and I I n d heating) for Exceed® 143 with 500ppm of boron nitride (CTF5). We analyzed the latent heat of crystallization for blends having 200ppm, 500ppm and 2500ppm of B N respectively and observed that latent heat of crystallization for polymer increases with increase in boron nitride content in polymer, C H A P T E R 3 - M A T E R I A L S A N D B L E N D I N G M E T H O D S 60 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS which means that presence of boron nitride renders the polymer more crystalline. In other words, the polymer chains are more ordered and there are less entanglement in the 1 . 7 5 1 . 2 S H 0 . 7 S - 0 . 2 S SO 90 110 Temperaiure (*C) 130 Figure 4-5: DSC curve for Exceed® 143 with 500ppm of B N (cooling cycle) -02 H -I i 120.«9"C 50 70 110 130 Temperaiure (*C) Figure 4-6: DSC curve for Exceed® 143 with 500ppm of BN (second heating cycle). CHAPTER 3 - MATERIALS AND BLENDING METHODS 61 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS polymer chains. Th is suppresses the local slip present between the polymer streamlines during extrusion. This mechanism possibly helps in eliminating the melt fracture during the extrusion o f polymers. Table 4-3 provides the latent heat o f crystall ization values for pure resin, w i t h B N and fluoroelastomer along wi th their melting points in different cycles. Table 4-3: Latent heat o f Crystal l izat ion values for pure resin, wi th B N and fluoroelastomer. Sample ID r Heating Cooling 2 n d Heating T m (°Q Energy (J/g) T C C Q Energy (J/g) T r a , rc) Tm 2CC) Energy(J/g) EX143 116.25 80.15 105.16 74.38 110.19 119.5 86.10 EX143 +200ppmBN 117.94 79.81 106.34 73.55 112.74 121.19 86.83 EX143 +500ppmBN 117.38 81.30 106.84 76.19 112.01 120.49 88.23 EX143 +2500ppmBN 116.73 85.73 107.94 76.10 112.0 120.90 88.01 EX350 + 500ppm 119.90 76.15 105.54 72.58 110.15 120.54 80.60 Fluoroelastomer A s discussed before, w e observed in our extrusion experiments wi th blends o f polymer inc luding boron nitride that there is no change in flow curve. W e also performed experiments o n T G A w i t h extrudates o f different concentrations o f boron nitride and fluoroelastomer. F igure 4-7, 4-8 and 4-9 shows T G A analysis for pure polymer extrudate, extrudate w i t h 500ppm o f fluoroelastomer and extrudate having 2500ppm o f boron nitride respectively. First , w e observed from the results that there is no residue left wi th pure polymer and polymer w i th fluoroelastomer. However , there is a 2160ppm o f residue found w i t h a sample o f polymer having an initial concentration o f 2500ppm o f boron CHAPTER 3 - MATERIALS AND BLENDING METHODS 62 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS ap 80 £ 40 I 20 0--20--40 04.38*60-4 IOC (I8.10il«) H.2Q%41CM80C C2.133mol 4.433% 480-610C (0B5T6mfl) 2O0 400 ' 600 Temperature ("Cj 10 9 s 2 1 0 1 1000 Figure 4-7: T G A curve for extrudate obtained with pure Exceed® 143. £ 40 I 20 e i 8 4 X 6 O - « l 0 C <I8.75«101 I4J4K4IIMS0C (2.93Smg) J.9321; 48M10C 10 9 e 7 6 p * a. 4 I 3 il S 1 0 Temperature (°C) Figure 4-8: T G A curve for extrudate obtained with Exceed® 143 having 500ppm of fluoroelastomer. CHAPTER 3 - MATERIALS AND BLENDING METHODS 63 THE ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS Temperature (*C) Figure 4-9: T G A curve for extrudate obtained wi th Exceed® 143 having 2500ppm o f B o r o n nitride. nitride. Th i s shows that some boron nitride remains at the wa l l during extrusion. Table 4-4 shows T G A results for pure resin, w i th B N and fluoroelastomer. Table 4-4: T G A results for pure resin, wi th B N and fluoroelastomer. Sample ID Initial Weight (mg) Residue (% left) EX143 19.087 0 EX143 +200ppmBN 19.817 0.05796 EX143 +500ppmBN 20.365 0.05147 EX143 +2500ppmBN 20.126 0.216 EX350 + 500ppm Fluoroelastomer 20.468 0 Recent ly, D u P o n t Fluoroproducts performed experiments regarding extrusion o f polyethylene using dies and tips o f B N (Private Communicat ions) . They observed only an improvement to 151 s"1 from 50 s"1 when the B N die and tips were used in comparison to the normal metal dies. It shows that gross melt fracture elimination wi th B N is partly CHAPTER 3 - MATERIALS AND BLENDING METHODS 64 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS due to w a l l slip as some o f the B N migrates to the die wa l l . M a j o r contr ibut ion to the elimination o f melt fracture is due to the removal o f local slip between the polymer streamlines ( K a z a t c k o v et al . , 2000). CHAPTER 3 - MATERIALS AND BLENDING METHODS 65 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 5 Measurement of Surface Energy of Boron Nitride Powders 5.1 Introduction A s discussed earlier certain boron nitride ( B N ) based composi t ions may act as effective processing aids in the extrusion o f a number o f f luoropolymers and polyolefins (Buckmaster and Randa, 1996; Rosenbaoum and Hatz ik i r iakos , 1998; Y i p and Hatz ik i r i akos , 1999; L e e and K i m , 2000). B o r o n Ni t r ide ( B N ) is a sol id lubricant, whose structure resembles to that o f graphite. B N can successfully be used as a processing aid to eliminate not only sharkskin melt fracture but also substantially postpone gross melt fracture to significantly higher shear rates we l l wi thin the gross melt fracture region. It is noted that conventional f luoropolymers can only eliminate sharkskin; they do not appear to have an effect o n the gross melt fracture phenomena. In previous studies ( Y i p and Hatz ik i r iakos , 1999), it was demonstrated that important properties o f B N in eliminating gross melt fracture includes particle size, shape and degree o f agglomeration. Y i p et al (2000) have also argued that the presence o f boron oxides into the B N powders might play a significant role since it increases its surface energy. They reported that a B N containing 2 % o f B2O3 d id not perform as w e l l as another B N having the same characteristics. Therefore, it becomes important to assess the surface energy o f B N powders. The aim o f this part o f the w o r k is to measure the surface energy o f boron nitride powders and correlate their performance wi th the critical shear rate for the onset o f gross melt fracture. The method is based on the measurement o f contact angle o f the powder w i t h a polar and a non-polar l iquid in order to resolve its two components, namely the dispersive and non-dispersive contributions. CHAPTER-5 MEASUREMENT OF SURFACE ENERGY 66 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 5.2 Surface Energy of Powders Measur ing the surface energy o f powders has always been critical because direct contact angle measurements cannot be used on finely dispersed solid materials. The capillary rise technique wi th its we l l established theoretical basis offers an alternative for indirect contact angle measurement. Capi l lary rise technique is wide ly applied for finely divided powder incorporated in a matrix (composite materials) where final surface and mechanical propertied depend on the interactions between the t w o entities. This technique is extensively used in pharmaceutical industry for wettabili ty studies o f drug powders (Duncan-Hewi t t and Nisman, 1993; Hansford and Grant, 1980). Such information is important in research area such as rate o f drug release, opt imizat ion o f mult icomponent formulations and in drug manufacturing to assess the adhesion characteristics o f the active component on mix ing surface. M o r e o v e r , l iquid penetration in fibrous/porous materials is applied by many industries such as paper and textile (Pezron and Quere, 1995) as we l l as in process dealing wi th filtration o r geographical matter (Grat toni and Chiot i s , 1995). Howeve r , precise wettability studies o f powders have always been cri t ical in terms o f precision, reproducibil i ty and absolute values obtained. W e have modified the conventional procedure o f measuring the penetration rate by measuring the rate o f rise o f l iquid in terms o f weight instead o f height and thus developed a new reliable experimental procedure based on Washburn 's theory. 5.2.1 Capillary Rise Method A direct measurement o f contact angles, 6, o f liquids on powders is impossible due to heavy penetration o f l iquid into the powder ( W u , 1980). Thus, the obvious choice CHAPTER-5 MEASUREMENT OF SURFACE ENERGY 67 THE ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS is to measure the contact angle by measuring the wettabili ty o f powders. The method used in this w o r k is the capillary rise o f packed powder, a phenomenon governed by the Washburn ' s equation. T o describe the flow behavior o f liquids through a capillary, Washburn (1921) combined the capillary dr iv ing force equation wi th the Poisseulle v iscous flow equation under conditions o f steady flow and obtained the fo l lowing equation, where # v and p. are the surface tension and viscosi ty o f l iquid, respectively, 6 is the l iquid contact angle on the solid surface, r is the capillary radius, Ap is the difference in density between the l iquid and its surrounding medium, g is the gravitational acceleration and h is the l iqu id penetration distance into the capillary at time t. I f the l iquid penetration and capil lary radius are small enough, the gravity term Apgh in equation (5-1) can be neglected and thus equation (5-1) reduces to In the application o f the Washburn equation to a packed powder column, where the capillaries inside the co lumn are tortuous and their radii vary wi th in the column, the overal l penetration process is an average on all these individual processes. Thus, the observed penetration rate dh2/dl, corresponds to an average radius, that is to replace r in equation (5-2). Th is average value is a hypothetical mean radius termed tortuosity constant K ( E l y and Pepper, 1946). Thus, replacing r w i th K, equation (5-2) becomes: (5-1) dh 2 _ ryh cos 9 dt 2 p. (5-2) dh2 Ky hcos0 dt 2p (5-3) CHAPTER-5 MEASUREMENT OF SURFACE ENERGY 68 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS It can be seen from Equat ion 5-3 that the penetration rate dh2/dt is directly related to the l iquid contact angle 6. Thus, Equa t ion 5-3 provides an indirect method in determining the l iquid contact angle o f the powdered solid sample that is by measuring the l iquid penetration rate in its packed column. 5.2.2 Modified Procedure In the traditional procedure, the powder is placed into a glass tube and packed manually. T y p i c a l co lumn-pack ing pressures are in the range o f 0.3 M P a . W i t h such a l o w column-packing pressure, the packing density o f the co lumn is non-uniform and it changes as wet t ing proceeds. A s a result, such experimental data is subject to serious statistical error and scatter. T o improve the accuracy and the reproducibil i ty o f the co lumn penetration process, w e have designed a new pressure control led column-packing device. F igure 5-1 provides a simple schematic. Th is design a l lows for a high-pressure control led application o f up to 2 M P a . This pressure is applied to the powder that is init ially placed in the glass tube. The co lumn packing density obtained is about four times higher from that obtained by using conventional manual tapping. M o r e o v e r , w i th the decrease o f co lumn height, the effect o f gravitational force on the penetration rate is been minimized. T h e visual observation by reading the penetration height along the co lumn at different t ime intervals is another factor causing additional experimental error. Th is is due to the fact that the penetration o f the l iquid in the co lumn may not be uniform during the process, especially w i th co lumn packed under the application o f l o w pressures. It is obvious that the automatic weight measurement, used here, has a greater precision than the visual height observation. Therefore, in our modified procedure, the experimental CHAPTER-5 MEASUREMENT OF SURFACE ENERGY 69 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS Load Supporter Penetration Tube Piston Powder Figure 5-1: Powder column-packing device data includes the weight of the column as a function of time. Assuming a constant porosity within the column, the column weight, Wt, can be related to the penetration height, h, in the column by Wt =Wa + p7tR2{\-<l>)h (5-4) CHAPTER-5 MEASUREMENT OF SURFACE ENERGY 70 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS where Wo is the initial weight o f column, p is the l iquid density, R is the inner radius o f tube and <j> is the porosi ty o f the packed powder column. Therefore, the difference o f penetration weight, Aw, can be expressed as, C o m b i n i n g equations (5-3) and (5-5) results into, dt 2p Def in ing K ' = K [liR2 (1-<J))] 2 as a geometric factor, equation (5-6) can be writ ten as The geometric factor, K' is constant as long as the packing and the particle size remains the same. This has to be experimentally determined for each type o f powder and packing. Thus, to calculate the contact angle from either Equa t ion 5-3 or Equa t ion 5-7, the constant K or K' should first be determined. This cannot be directly measured. H o w e v e r , it can be obtained by a capillary penetration experiment w i th a l iquid , wh ich is assumed that totally wets the powder. In this case cos 6 becomes equal to 1, and thus K' can be calculated directly from Equat ions 5-3 or 5-7. H a v i n g determined K' can n o w be used to calculate the unknown contact angle o f the powder wi th different l iquids. The precise and accurate determination o f AT' is essential for a correct application o f Washburn ' s equation. A good packing control wi th a reproducible procedure is essential as one cannot measure in a first step the value o f parameter K ' and reuse the same powder packing for penetration experiments wi th another l iquid. Therefore, w e use a special co lumn-packing device to compress and to have reproducible packing o f the powder . The quantity, d(Aw2)/dt represents better the average properties o f powder Aw = p,7iR2(\-<f>)h (5-5) d(Aw2) _ Kp2ylv cosG dt 2p (5-7) CHAPTER-5 MEASUREMENT OF SURFACE ENERGY 71 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS structure compared to that o f the traditional method. Therefore, d(Aw2)/dt is statistically more representative. G i v e n the val idi ty o f equation (5-7), straight line should be obtained by plot t ing Aw2 versus t ime (/). 5.2.3 Contact Angle and Surface Energy of Powders M o s t important applications o f polymers require that they adhere we l l to other substrates. Adhes ion is a manifestation o f the attractive forces, wh ich are categorized as chemical , hydrogen bond and van der waal forces. It is agreed in theory that attraction due only to hydrogen bonding and van der waal forces is sufficient to produce adhesive joints between polymers o f strength equal to that o f the polymers themselves without the need for chemical bonds (Owens and Wendt , 1969). Since these forces decrease as the inverse o f the sixth power o f the distance between molecules, it is expected that surfaces to be adhered must come into intimate wett ing contact. Thermodynamic wett ing (contact angle 0 between l iquid and solid) is a function o f four parameters, g iven by the Y o u n g equation: COS6Y1V=YSV-YS1 (5-8) where Yiv , Y*> a n d Y>i a r e m e interfacial energies o f the l iquid and sol id against their saturated vapor and o f the interface between l iquid and solid, respectively. It is evident that wet t ing is favored by l o w sol id-l iquid interfacial energy, high sol id surface energy and l o w l iquid surface energy. Theoretically, it has been suggested that the total free energy o f a surface is the sum o f contributions from dispersive force components (superscript D ) and non-CHAPTER-5 MEASUREMENT OF SURFACE ENERGY 72 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS dispersive force components (superscript N D ) (Fowkes , 1964; Owens and Wendt , 1969; W u , 1971). Thus, the surface free energy can be wri t ten as: y = yD+ym (5-9) The interface is composed o f the t w o adjacent interfacial regions (Fowkes , 1964), and the interfacial tension must therefore be the sum o f the tensions in each o f these regions. I f w e consider the interface o f t w o liquids whose subscripts are mentioned as 1 & 2 then in the interfacial region o f the l iquid 1, the molecules are attracted toward the bulk o f l iquid 1 by intermolecular forces, wh ich tend to produce a tension equal to the surface tension o f l iquid 1 ( j i ) . Howeve r , at the interface there is also an attraction by the L o n d o n dispersion forces o f the l iquid 2 for those l iquid 1 molecules in the interfacial region. These molecules are in a different force field than those at the surface o f the l iquid 1 because o f this interaction, and therefore the tension in this layer is a function o f the difference between the surface tension o f the l iquid 1 and the attractive force exerted by the L o n d o n dispersion force interaction between l iquid 1 and l iquid 2. It is expected that the geometric mean o f the dispersion force attractions should predict the magnitude o f the interaction between these dissimilar materials (Gir i fa lco, 1957). T h e effect o f interfacial attraction on the tension in the interface can be predicted by the geometric components o f the surface tension o f the l iquid 1 and o f the l iquid 2 ( V y i d Y 2 d ) . Thus, the tension in the interfacial region o f the l iquid 1 is equal to y i - V y i d Y 2 d . S imilar ly in the interfacial region o f l iquid 2 the attractive force o f bulk l iquid 2 is partially balanced by the attractive force o f the l iquid 1 and tension in this layer is equal to 72 - ^y^Yi- Since the interfacial tension y i 2 is the sum o f tension in these o f two layers CHAPTER-5 MEASUREMENT OF SURFACE ENERGY 73 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS rn=ri+r2 • * m ( 5 - 1 0 ) W h e n both dispersion and non-dispersion forces are in effect then tension in the interfacial region o f a sol id (s) and l iquid (1) is rt-r^+rb-ilrFrf-tJrrrr (5-11) Combin ing Y o u n g ' s equation wi th equation (11), the surface energy components o f a sol id can be wri t ten in the fo l lowing form i+™e = 2^<f? i Y h ) + 2 ^ < ^ irw) ( 5 " 1 2 ) The value o f yiD can be determined from available values o f yu and yf by means o f equation (5-9). B y measuring the contact angle o f two different l iquids against a sol id (powder) , t w o equations can be obtained. These can be solved to determine yf and ysND. Thus, the components o f surface energy o f a powder due to interfacial forces can be found. T h e sum o f these components according to equation (5-9) w i l l y ie ld the total sol id surface energy o f sol id, ys. 5.3 Experimental setup for measuring the wettability A special column-packing device is used to compress the powder into the capillary. Th is is shown in Figure 5-1 as discussed briefly before. The powder is filled into a glass tube (open from both ends) o f 14.1 m m inner diameter, length 100 m m and then compressed wi th load placed on the column-packing device. The co lumn packing density obtained, is about four times higher from that obtained by manual tapping. T o hold the packed bed o f powder in glass tube, filtering w o o l is attached to one end o f the tube. CHAPTER-5 MEASUREMENT OF SURFACE ENERGY 74 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS A s discussed earlier, in the traditional method, the rise o f the height o f the l iquid is measured. H o w e v e r , this is difficult to measure accurately, since the l iquid permeates up unevenly into the powder that causes serious experimental error in some cases. In our experiments, the weight o f the co lumn is recorded as a function o f time. D u r i n g the penetration process, weight o f co lumn is monitored on-line by an electronic balance and is recorded versus time by a computer interfaced wi th an electronic balance. The data acquisi t ion system can be programmed to record data at a min imum rate o f 10 points per second. A schematic diagram o f the penetration rate measurement setup is shown in Figure 5-2. 5.3.1 Test procedure (1) Preparation of glass tubes: Glass tubes are thoroughly cleaned wi th disti l led water and then dried at 1 1 0 ° C for 2 hours. A small amount (O. lg) o f filtering w o o l has to be attached at one end o f the tube in order to maintain the packed bed o f powder intact. (2) Preparation of powder columns: 1 g o f sample powder ( B o r o n Ni t r ide) is placed in the penetration tube. B y using the column-packing device (Figure5-1), the powder is compressed at a specified pressure (1.8 M P a ) . The pressure is calculated by d iv id ing the applied force wi th the contact area o f piston. P o w d e r is compressed for 10 minutes, until no further changes in the height o f the co lumn are observed. A s discussed earlier, the procedure for packing o f powder columns is an important factor to consider in order to ensure reproducibility. The powder should be sufficiently packed so that the rising l iquid does not change the spatial arrangement o f particles. CHAPTER-5 MEASUREMENT OF SURFACE ENERGY 75 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS Moving Platform Electronic Balance Supporting Rake Powder Figure 5-2: Penetration-rate measurement system (3) Measurement of Penetration Rate: The glass tube with the compressed column of sample powder is kept vertically and the penetration liquid bath has to be attached carefully to the bottom of glass tube to measure the mass and rate o f liquid rise in the packed column. The measurements are carried out at room temperature, and the penetrated weight change is recorded by means of a computer that is interfaced with the electronic balance. CHAPTER-S MEASUREMENT OF SURFACE ENERGY 76 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 5.3.2 Selection Criterion for Liquid The l iquids used in the present w o r k are methanol, water and a -bromonapthalene. These liquids were chosen for the fo l lowing reasons: (i) L i q u i d s should not dissolve the B N powder; (i i) L i q u i d s should be as less viscous and as less volati le as possible. The selection o f a-bromonapthalene, a nonpolar l iquid, w i l l a l low the calculat ion o f the dispersive component o f the surface energy. O n the other hand, Water , a polar l iqu id , w i l l a l low the determination o f the non-dispersive component o f surface energy at the sol id / l iquid interface; T h e ideal l iqu id , wh ich can perfectly wet the B N sample powder , is considered to be methyl a lcohol . A l t h o u g h acetone and n-hexane also wets B N readily (cost9 is about equal to 1) were disregarded due to their relatively high volatili t ies. D u r i n g experiments performed w i t h acetone, the weight o f powder was first observed to be increasing. H o w e v e r , after 5-6 minutes, it started decreasing due to significant vaporizat ion. The same problem was encountered wi th hexane and therefore these t w o liquids were rejected. In spite o f this, acetone and n-hexane are considered to be ideal fluids, w h i c h can perfectly wet B N powders ( c o s # = 1). The range o f l iquid selection was also l imited by the unavailability o f surface energy data o f liquids that offers both contributions separately i.e. dispersive and non-dispersive. Experiments using glycerol , presented penetration difficulties due to its high viscosi ty. Th is makes the experiment extremely time consuming and difficult to detect sources o f experimental error. A l l experiments wi th a -bromonapthalene were performed CHAPTER-5 MEASUREMENT OF SURFACE ENERGY 77 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS in a fume hood due to its potential hazardous properties. Surface energies, density and viscosity of all three liquids at 20°C are given in Table 5-1. Table 5-1: Constants o f liquids used for finding Contact Angle Liquid Y (mJ/m 2 ) Y° (mJ/m 2 ) (mJ/m 2 ) P (kg/m 3) ri (mPa.s) Methanol 22.55 22.55 0 791 0.59 Water 72.6 21.6 51 1000 1.002 a-Bromonaphthalene 44.6 44.6 0 1483 4.8 5.4 Results and Discussion Figure 5-3 depicts liquid penetration experiments and the effect o f packing pressure on wettability experiments. We have performed experiments at different pressures ranging from 0.3 M P a to 1.8 M P a in order to investigate how the column-packing pressure affects the experimental accuracy and reproducibility. As it can be seen from the Figure 5-3 increase of packing pressure improves the degree o f linearity o f the penetration curves. Powder columns prepared under low packing pressure is difficult to handle without damaging it. It was observed that during such experiments the liquid penetration through such columns was uneven and that the experimental accuracy and reproducibility becomes poor. At low packing pressures, the particle orientation and the tortuosity vary significantly from one point to another within the column. Moreover, structural changes in the powder also take place during penetration experiments. On the CHAPTER-5 MEASUREMENT OF SURFACE ENERGY 78 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 1.0 0.9 0.8 0.7 c<P o) 0 .6 % 0.5 0.4 0.3 0.2 3 4 5 6 7 8 9 10 11 t (min) Figure 5-3: Effect o f column-packing pressure on the penetration rate o f l iquid . other hand, increase in packing pressure produces uniform density along the packed columns and reduces the possibility o f structural changes o f powder during experiments. Figure 5-4 plots the rate o f increase in weight o f packed columns using several boron nitride samples wi th methanol. These experiments were performed to calculate the tortuosity constant, K , given in Washburn 's equation i.e., considering methanol as an ideal l iquid wh ich can perfectly wet the boron nitride samples, c o s # = l . It can be seen that all data points fall on straight lines as equation 1 implies. It is noted that data points collected during the first three minutes have been neglected due to the fact that initial penetration o f l iquid refers to the l iquid penetration through the filtering glass w o o l . 79 CHAPTER-5 MEASUREMENT OF SURFACE ENERGY T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 0.15 0.30 Figure 5-4: Measurement o f wettability o f different B N powders wi th methanol. Figure 5-5 depicts the rate o f increase in weight o f packed columns using several boron nitride samples w i t h water. F r o m the slope o f the wettability lines o f Figure 5-5, the contact angle o f boron nitride samples wi th water is calculated. Similar ly , from Figure 5-6 the contact angle o f boron nitride samples wi th a-bromonaphthalene can be calculated. A l l experimental runs were performed three times in order to check reproducibil i ty. Reproducibi l i ty for contact angles was found to be wi th in ± 3 ° . After determining the contact angles using the t w o different l iquids (water and a -bromonaphthalene), using Figures 5-5 and 5-6, the surface energy contribution components o f boron nitride samples are found by means o f Equa t ion 5-12. CHAPTER-5 MEASUREMENT OF SURFACE ENERGY 80 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 0.26 0.24 h 0.22 0.20 at CN I 0.18 0.16 0.14 0.12 • 427 • 429 • 430 • 428 • C T F 5 • C T U F 6 7 8 t (min.) 0.44 0.42 0.40 0.38 H 0.36 -2 I 0.34 H 0.32 0.30 0.28 10 11 Figure 5-6: Measurement of wettability of different BN powders with a-bromonaphthalene. CHAPTER-5 MEASUREMENT OF SURFACE ENERGY 81 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS In the similar way, Figures 5-7, 5-8 and 5-9 show the penetration experiments for the second set of boron nitride samples (AS-series) with methanol, water and a -bromonaphthalene respectively. These experiments were done with a similar objective. All values of surface energy components for all samples obtained are tabulated in Table 5-2. 0.162 0.160 0.158 0.156 J> 0.154 0.152 0.150 0.148 0.146 i 1 1 1 1 i 1 1 i 1 1 1 • i ' ' 1 1 i T—i—i—I—i—i—i—r • • • ' • . . . i • AS610 • AS612 • AS614 6 7 8 Time (min.) 0.44 0.42 0.40 0.38 0.36 £ 0.34 O) 0.32 0.30 0.28 0.26 0.24 10 11 Figure 5-7: Measurement of wettability of different B N powders (AS610-AS614) with methanol. CHAPTER-5 MEASUREMENT OF SURFACE ENERGY 82 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS IE 0.018 0.017 0.016 0.015 h 0.014 0.013 i • 1 1 1 i 1 1 1 • i 1 1 1 1 i 1 1 1 1 .1 1 1 1 ' i Time (min.) 0.8 0.7 0.6 0.5 3 0.4 0.3 0.2 10 11 Figure 5-8: Measurement of wettability of different BN powders (AS610-AS614) with water. 0.074 0.072 F-0.070 0.068 0.066 F-3 0.064 0.062 F 0.060 0.058 F-0.056 0.054 i — I — I — I — I — I — i — i — I — I — I — I — I — I — I — I — I — I — I — I — I — i — I — | — i — I — i — i — I — I — i — r - i 4 • AS610 • AS612 A AS614 J — I — I— t - J— I — I I I I I I I I I I I I l _ I . . . . I 6 7 8 Time (min.) i . • • 2 I 10 11 Figure 5-9: Measurement of wettability of different BN powders (AS610-AS614) with a- bromonaphthalene. 83 CHAPTER-5 MEASUREMENT OF SURFACE ENERGY T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 5.5 Effect iveness of BN A s discussed and demonstrated, previously (Rosenbaum et. al. 1998; Y i p et. al. 2000), B N is an effective processing aid when it possess the fo l lowing characteristics (i) average particle size o f up to about 2 0 p m , (ii) no agglomerations (iii) absence o f boron oxides in its structure and (iv) g o o d dispersion into the resin under process. M o s t o f the samples (427-430, C T F 5 ) are similar in terms o f composi t ion but still they have different surface energy. Th i s is due to fact that surface energy o f B N crystals (powder) depends o n particle size, shape and morphology. B o r o n nitride samples (427-430) have surface energy comparable to that o f C T F 5 . H o w e v e r , they do not perform w e l l in terms o f eliminating gross melt fracture due to the presence o f agglomerated particles. Sample C T U F also performs poor ly because it has a very high surface energy that facilitates T a b l e 5-2: Surface Energy components o f B o r o n nitride powders S a m p l e ID y D ( m J / m 2 ) Y N D ( m J / m 2 ) y ( m J / m 2 ) C r i t i c a l shear rate , ( s 1 ) C T F 5 36.89 13.20 50.10 926 C T U F 28.01 33.78 61.80 155 427 16.98 32.95 49.93 70 428 15.40 17.62 33.03 300 429 23.08 16.34 39.43 360 430 20.35 30.10 50.45 155 A S 6 1 0 14.90 9.78 24.68 216 A S 6 1 2 44.36 10.58 54.94 1049 A S 6 1 4 24.03 9.03 33.06 617 84 CHAPTER-5 MEASUREMENT OF SURFACE ENERGY T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS polymer chain adsorption. Adsorbed polymer chains have an adverse effect on the ability o f the powder to provide lubrication. In fact, C T U F contains about 2 % o f B2O3, wh i ch increase its surface energy as can be seen from Table 5-2. H o w e v e r , it is not clear h o w a B N powder w i l l perform based on its total surface energy. F r o m Table 5-2, it can be seen that the dispersive component o f C T F 5 is much higher than that o f the others. C T F 5 was found to be the most effective processing aid from all the other samples. The fact that its dispersive component o f the total surface energy is high, increases its affinity w i th the solid die wa l l . Thus, C T F 5 has the tendency o f remaining at the wa l l p rovid ing proper lubrication to the polymer f lows. A s a result, melt fracture (especially) is eliminated. F r o m our previous studies, the maximum shear rate for producing smooth extrudates when C T F 5 used, was found to be 925 s"1 whi le that for C T U F was only 155 s"1. Th i s shows that for B N to postpone the onset o f melt fracture o f polymers at high shear rates, the dispersive component to the surface energy to be as high as possible, whi le its non-dispersive component to be as l o w as possible. Ex t rus ion studies o f m -L L D P E Exac t® 3128 w i t h boron nitride powders ( A S 6 1 0 - A S 6 1 2 ) demonstrated that A S 6 1 2 is one o f the best samples for postponing the melt fracture for the AS-ser ies . W i t h sample A S 6 1 2 , the max imum shear rate for producing smooth extrudates is 1049 s"1 whi le that for A S 6 1 4 is 617 s' 1 and that for A S 6 1 0 produces smooth extrudates only up to 154 s' 1. Af ter analyzing the surface energy results o f these three samples, w e came to a similar conclusion. That is the dispersive component o f surface energy should be as high as possible whi le the non-dispersive one as l o w as possible in order to obtain a g o o d performance wi th the use o f B N . CHAPTER-5 MEASUREMENT OF SURFACE ENERGY 85 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS We found that the ratio of dispersive to non-dispersive components of surface energy o f all B N powders correlates well with the critical shear rate for onset of gross melt fracture. Figure 5-10 depicts this correlation. A s it is obvious from Figure 5-10 the higher the ratio o f dispersive to non-dispersive component of surface energy, the better the performance of boron nitride as a processing aid. A dispersive component provides good affinity with the wall and thus proper lubrication, while a low non-dispersive component provides possibly good dispersion (lack of agglomeration). These are two necessary B N characteristics for good performance. 1200 — 1000 '</> u. :> o •ft 800 CO c o ro CO CD SI w 600 400 O 200 1 1 1 < I 1 1 1 1 I 1 1 1 I 1 I —1 i i | I 1 1 1 -* / -- ---• 1 1 1 1 1 1 1 1 1 1 I I i i 1 i r 0 1 2 3 4 5 DC/NDC Figure 5-10: Relation between ratio of dispersive and non-dispersive components of surface energy of B N powders to critical shear rate for onset of gross melt fracture. CHAPTER-5 MEASUREMENT OF SURFACE ENERGY 86 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 5 . 6 Summary A new reliable procedure was developed to measure the surface energy of powders. Powders are compacted at high pressures (1.8 MPa) with a special column-packing device to attain uniform packing, which produces results reproducible and statistically representative. Using the surface energy results of the various powders, their performance as processing aids in eliminating melt fracture is explained. To provide good performance, as a processing aid, the B N powder should have a surface energy with a dispersive component as high as possible and a non-dispersive component as low as possible. This will result to a high ratio of the two surface energy contributions, as the critical shear rate for the onset of melt fracture was found to correlate well with this. CHAPTER-5 MEASUREMENT OF SURFACE ENERGY 87 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 6 Effect of Combining Boron nitride and Fluoroelastomer: A new processing aid 6.1 Introduction In this chapter the effect o f a new processing additive (boron nitride powder in combinat ion wi th a fluoroelastomer) on the rheology and processability o f molten polymers is studied. The equipment used includes an Instron capil lary rheometer equipped w i t h a special annular die ( N o k i a Mai l le fer wi re coat ing cross-head) and a parallel-plate rheometer. The three metallocene polyethylenes w i th and without boron nitride ( B N ) and fluoroelastomer are tested in extrusion. First , it is demonstrated that B N is a superior processing aid compared to conventional f luoropolymer ones. Secondly, it is found that the combinat ion o f B N powders w i th a small amount o f a fluoroelastomer improves even further the processability o f molten polymers (melt fracture performance). A s w e discussed earlier, the rate o f product ion o f many commercia l polymer processing operations l ike extrusion, b l o w molding , film b lowing , film casting, fiber spinning and various coat ing flows, is l imited by the onset o f flow instabilities [Petrie and Denn , 1976; Ramamurthy, 1986; L a r s o n 1992). In particular, in extrusion processes when the throughput exceeds a crit ical value, small amplitude periodic distortions appear on the surface o f extrudates (surface melt fracture or sharkskin) and at higher throughput rates these take a more severe form o f larger irregular distortions (gross melt fracture) (Tordel la , 1969). A number o f techniques have been used for eliminating or delaying melt fracture at higher shear rates. F o r example, increasing the processing temperature or using local C H A P T E R 6 - A N E W PROCESSING AID 88 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS heating o f the exit o f the extrusion die or changing the material o f construct ion o f the die in extrusion o r f i lm b l o w i n g processes have been proved to be effective methods in delaying the onset o f surface melt fracture to higher shear rates to a certain extent (Hatz ik i r iakos , 1994). H o w e v e r , to increase the rate o f product ion by eliminating or postponing the surface melt fracture phenomena to even higher shear rates, processing additives/aids should be used. These are mainly f luoropolymers that are wide ly used in the processing o f polyolefins and other commodi ty polymers. They are added to the base polymer at l o w levels (approximately 0.1%). They essentially act as die lubricants, modifying the properties o f the polymer-wal l interface (increase slip o f molten polymers o n die wal l ) . A s a result o f this lubrication effect, the onset o f instabilities is postponed to much higher output rates. M o r e o v e r , the power requirement for extrusion is significantly reduced. It should be emphasized that these additives can eliminate only surface (sharkskin) and stick-slip (oscil lating or cycl ic) melt fracture but not gross melt fracture phenomena. It has recently been demonstrated that certain boron nitride ( B N ) based composi t ions may act as effective processing aids in the extrusion o f a number o f f luoropolymers and polyolefins (Buckmaster , 1997; Rozenbaoum and Randa, 1998; Y i p and Hatz ik i r i akos , 1999). B o r o n Ni t r ide ( B N ) is a solid lubricant, whose structure resembles to that o f graphite. B N can successfully be used as a processing aid to eliminate not only sharkskin melt fracture but also substantially postpone gross melt fracture to significantly higher shear rates wel l wi thin the gross melt fracture region. It is noted that conventional f luoropolymers can only eliminate sharkskin; they do not appear to have an effect o n the gross melt fracture phenomena. CHAPTER 6 - A NEW PROCESSING AID 89 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS In this study, we combine the BN with certain fluoroelastomers to produce enhanced polymer processing additives. First, it is demonstrated that BN is in general a superior processing aid from conventional fluoroelastomers, also known before (Buckmaster, 1997; Yip and Hatzikiriakos, 1999). Secondly, we demonstrate that combinations of B N with conventional fluoroelastomers produce enhanced polymer processing additives that perform better than either of these constituents when are used individually. This performance is demonstrated in the continuous extrusion of two metallocene LLDPE's. 6.2 EXPERIMENTAL 6.2.1 Materials In this study, three metallocene-catalyzed polyethylenes were evaluated in terms of their processability as discussed before. The morphology of boron nitride powders and fluoroelastomer were investigated by using a Hitachi S-2300 Scanning Electron Microscope (SEM) operating at 5keV (for details see chapter 4). It was observed that the B N powder used had an average particle size of 15-20 pm with medium degree of agglomeration, while the fluoroelastomer had an average particle size of 180 pm without any degree of agglomeration. It is noted that a uniform dispersion is a necessary condition for obtaining a good performance of processing aids (Rozenbaoum and Randa, 1998; Yip and Hatzikiriakos, 1999). A twin-screw extruder was used to prepare blends of BN and polymer, (for details see Chapter 4). This procedure results into a uniform dispersion of Boron Nitride and fluoroelastomer into the polymer. The various blends prepared and studied in this work CHAPTER 6 - A NEW PROCESSING AID 90 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS are summarized in Table 6-1. It is noted that in some formulations listed in Table 6-1, Teflon® was used instead o f fluoroelastomer and in some others a small amount o f Ca rbon TetraBorate ( C T B ) was used together wi th B N . The various blends were tested for their melt fracture performance in continuous extrusion using a special annular cross-head die (see Figure 2-8). This was attached to both a capillary rheometer and a single screw extruder. The cross-head die was a N o k i a Mai l le fer 4/6 that included dies and tips o f various diameters ("tip" is the wi re guide) w i th equal entry cone angles o f 60° and the die land length o f 7.62 mm. Results from both the capillary rheometer and single screw extruder were identical. 6.2.2 Linear Viscoelastic Measurements T o study possible effects o f the B N and fluoroelastomer addit ion to the resin on its rheology, linear viscoelastic measurements were carried out for metallocene P E ' s w i th B N and fluoroelastomer individually and combined. This type o f experiment usually serves as a measure o f possible structure formation at certain time scales. The parallel-plate rheometer for these linear viscoelastic experiments was a Rheometr ics System I V ut i l iz ing a parallel-plate geometry wi th plates o f diameter equal to 25 mm. Frequency sweep experiments were performed in the frequency range from 0.02 to 500 rad/s. The w o r k i n g temperature range was 1 6 3 - 2 0 4 ° C . Figures 6-1, 6-2 and 6-3 depict the dynamic modul i , G ' and G " , and complex viscosi ty r j* o f metallocene P E E x a c t ® 3 1 2 8 compared wi th resin filled wi th B N , fluoroelastomer and their combination respectively at 163°C. CHAPTER 6 - A NEW PROCESSING AID 91 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS Table 6-1: Summary of blends used in extrusion experiments Base Resin BN (ppm) Teflon (ppm) Fluoroelastomer (ppm) C T B (ppm) Exactw3128 500 0 Exact®3128 0 500 Exact®3128 500 500 Exact®3128 1000* 0 Exact®3128 0 0 Exact®3128 1000* 0 Exceed® 116 1000 0 Exceed® 116 1000 500 Exceed® 116 2000 0 Exceed®116 1000 0 Exceed® 116 2000 0 Exceed®! 16 1000 0 0 0 0 0 0 0 0 0 1000** 0 1000** 0 0 0 0 0 0 88 0 44 500 88 500 44 *A different type of BN Uian the rest that had a high degree of agglomerated particles. **A different type of fluoroelastomer used in these formulations that did not reduce the shear stress as usually expected for fluoroelastomers. This is due to mismatch of viscosities between the fluoroelastomer and polymer. CHAPTER 6 - A NEW PROCESSING AID 92 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 106 CC 105 I 10* w w _o T J c CD * 103 O ) cc 1 o 55 102 I I 1 I 11 1 1—I—I 1 l l l l T O O pure resin • 0.1% Fluor. 9F O 104 CD OL CO O o w ° > X o. E o O 10" 10° 101 102 10 3 10 3 Frequency (co), rad/s Figure 6-2: Dynamic moduli, G' and G " , and complex viscosity n* of metallocene PE Exact® 3128 (with and without fluoroelastomer) at 163°C. 93 CHAPTER 6 - A NEW PROCESSING AID T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 106 (0 ID | 1 0 * CO in T J c (0 S, 103 o co 102 t-I — I— r " T " I ' l l i I 1 — i — r i i 111 0 0 o o o O pure resin • 0.1% B N + 0.1% Fluor. O 104 (0 CL O O in > X iK CL E o O 10-1 10° 101 102 Frequency (co), rad/s 103 103 Figure 6 - 3 : Dynamic moduli, G ' and G " , and complex viscosity r|* o f metallocene P E Exact® 3128 (pure resin and combination of B N & fluoroelastomer) at 163°C. It can be seen that no significant difference was found in the linear viscoelastic behavior o f the virgin resin compared with those filled with B N , fluoroelastomer and their combination respectively.. Similar results were also observed for virgin resin P E Exceed® 116 compared with the corresponding filled resins at 204°C. Moreover, similar conclusions were also reported by Rosenbaum et al. (Rozenbaoum and Randa, 1998), who also performed a detailed rheological characterization of several resins in order to determine the effect of B N on their rheology. CHAPTER 6 - A NEW PROCESSING AID 94 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 6.2.3 Capillary Experiments Capi l la ry rheometer experiments were used to determine the shear rate at wh ich smooth extrudates can be produced in the continuous extrusion o f the t w o metallocene P E ' s . A n Instron piston-driven constant-speed capillary rheometer bearing a load cell o f 5000 lb, and a non-conventional barrel o f diameter o f 1" was used in order to accommodate the use o f t h e cross-head die. Previous research in our lab has shown that B N had an effect o n the extrudate appearance o f polymers in capillary geometry (both capil lary and orifice dies wi th a different entrance angle) similar to that attained wi th the use o f a conventional processing aid (Rozenbaoum and Randa, 1998; Y i p and Hatz ik i r i akos , 1999). Its greatest influence, however, occurs in cross-head dies and tips where a more streamlined f low is obtained. In such flows, the additive particles seem to enhance melt slippage in the bulk and thus, relieve internal stresses ( K a z a t c k o v and Hatz ik i r i akos , 2000). Th is action, demonstrated in a visualizat ion study ( K a z a t c k o v and Hatz ik i r i akos , 2000) , eliminates surface melt fracture and postpones the cri t ical shear rate for the onset o f gross melt fracture to significantly higher shear rate values. A l l composi t ions listed in Table 6-1 were tested wi th the cross-head die at various shear rates in order to identify the critical shear rates for the onset o f surface and gross melt fracture. Be tween testing the various blends, the rheometer was flushed wi th three full loads o f polypropylene in order to purge B N and fluoroelastomer remains o f f the die surface. The extrudates were collected in a cold-water bath in order to freeze their appearance and avoid sagging effects. CHAPTER 6 - A N E W PROCESSING AID 95 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 6.3 RESULTS AND DISCUSSION 6.3.1 Effect of BN and Teflon® in processing A s already discussed and demonstrated in previous reports, B o r o n Ni t r ide is an effective processing aid to eliminate not only sharkskin melt fracture but also to postpone gross melt fracture to significantly higher shear rates (Rozenbaoum and Randa , 2000; Y i p and Ha tz ik i r i akos , 2000). In this section, B N is combined wi th a Teflon® processing aid in order to study whether or not the processability o f polymers improves even further. Figures 6-4 and 6-5 depict the f low curves o f Exact® 3128 and E x c e e d ® 116 wi th and wi thout B N and Teflon® obtained wi th a cross-head die attached to the rheometer at 2 0 4 ° C . It can be seen that the combination o f B N and Teflon® processing aid improves the processability for polymers more than either o f these t w o constituents may do when used individual ly. F o r example, it can be seen from Figure 6-4 that v i rg in Exac t® 3128 exhibits fracture at a cri t ical apparent shear rate o f about 50 s'1. Th is cri t ical rate increases to about 1300 s*1 w i t h the addit ion o f 0 .05% Teflon®, and to about 1800 s"1 w i th the addit ion o f 0 .05% B N . Consequently, this becomes about 2,400 s"1 by combining 0 .05% B N and 0 .05% Teflon®. CHAPTER 6 - A NEW PROCESSING AID 96 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS CO CL S io° co co CD 1— w l_ CO CD sz co "co •+-» c 2 CO Q-CL < P E Exact 3128 T=204°C (400°F) Rheometer with crosshead d/D=0.06070.122" O virgin O 0.05% Teflon A 0.05% BN A 0.05% BN+0.05% Teflon 0.05% Teflon 0.05% BN 0.05% BN and 0.05% Teflon -10 2 1 0 3 Apparent shear rate (y), s" Figure 6-4: The effect o f combining 0 .05% o f B N wi th 0 .05% Teflon® on the melt fracture performance o f P E Exac t® 3128 in rheometer wi th N o k i a Mai l le fe r crosshead having 0 .122" die and 0 .06" tip at 2 0 4 ° C . Similar ly , it can be seen from Figure 6-5 that the addit ion o f 0 .05% B N increases the cri t ical apparent shear rate for the onset o f melt fracture from about 110 s"1 for the v i rg in E x c e e d ® 116 polymer to about 900 s'1. In the presence o f 0 . 1 % B N and 0 .05% Teflon®, this cri t ical rate becomes about 2,000 s'1. C H A P T E R 6 - A N E W P R O C E S S I N G A I D 97 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS (0 C L P E Exceed 116 • T=204°C (400°F) Crosshead d/D=0.0670.122" < CL Onset of MF: \ PE+BN+Tefloh i . , i 1 0 1 1 0 2 1 0 3 Apparent shear rate (y ), s -1 F i g u r e 6 - 5 : T h e effect o f combining 0 . 1 % o f B N wi th 0 .05% Teflon® o n the melt fracture performance o f P E Exceed® 116 in rheometer wi th N o k i a Mai l le fe r crosshead having 0 .122" die and 0.06" tip at 2 0 4 ° C . T h e use o f Teflon® causes the f low curve to shift significantly towards lower shear stress values (see Figure 6-4 and 6-5) and as a result improves further the processability o f the resin. It has been observed that during extrusion, Teflon® particles that are finely dispersed in polyethylene come into contact wi th metal dies and displace the polyethylene from the surface. D u r i n g extrusion, the Teflon® particles spread as a result o f the shear stress and form a very thin layer, wh ich completely coats the die surface. D u e to poor adhesion characteristics between the polyethylene and the Teflon®, the polyethylene slips over the thin coating o f Teflon®. This enhanced slip significantly reduces the pressure required to extrude the polyethylene at a particular shear rate. O n the 98 CHAPTER 6 - A NEW PROCESSING AID T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS contrary, the use o f B N does not reduce shear stress. In spite o f this, B o r o n Ni t r ide seems to perform better as a processing aid compared to Teflon®. This is due to the fact that B N causes a change in the f low pattern at the entrance o f the die where gross melt fracture occurs (see reference K a z a t c k o v and Hatz ik i r iakos , 2000 for more details). F r o m the results presented in Figures 6-4 and 6-5, it is clear that the best performance in terms o f melt fracture is obtained in the presence o f both B N (eliminates gross fracture) and Teflon® (decreases the pressure drop). This combinat ion forms a superior processing aid that is better than either o f the two constituents, when used individual ly ( U S patent F L - 0 1 8 2 , pending). 6.3.2 Effect of Combining B N with a Fluoroelastomer on processing It has been shown in the previous section that the addit ion o f Teflon® decreases the extrusion pressure drop. Th is is the case for most f luoropolymers. A l s o for B N to be effective as a processing aid, the particle size should be small and free o f any agglomerations ( Y i p and Hatz ik i r iakos , 1999; K a z a t c k o v and Hatz ik i r i akos , 2000) . Figure 6-6 presents an interesting case. The flow curve o f v i rg in m - L L D P E (Exact® 3128) is plotted together w i th its flow curves in the presence o f a B N , a fluoroelastomer and the combinat ion o f the two. CHAPTER 6 - A NEW PROCESSING AID 99 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS Q I i i i i • • • • I i i i i i I ' l l i i i i i • • • I 1 10 100 1000 Apparent S h e a r Ra te y A ( s 1 ) Figure 6-6: The effect of combining 0.1% of BN with 0.1% of a fluoroelastomer on the melt fracture performance of PE Exact® 3128 in rheometer with Nokia Maillefer crosshead having 0.122" die and 0.06" tip at 163°C. First, it can be observed that the presence of B N and fluoroelastomer do not alter the flow curve. While for BN this is expected, for the fluoroelastomer, however, a significant reduction was expected. However, a mismatch in the viscosity of the fluoroelastomer with the resin is the primary cause of this behavior. The critical shear rate for the onset of virgin PE was found to be around 50 s'1 consistent with the value of 42 s"1 reported previously (Yip and Hatzikiriakos, 1999). The presence of BN or fluoroelastomer postpones this critical shear rate to only 216 s"1. For the case of fluoroelastomer is due to the viscosity mismatch as already discussed, while for the case of BN is due to the high degree of agglomeration present in the BN powder used in part CHAPTER 6 - A NEW PROCESSING AID 100 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS o f the study ( K a z a t c k o v and Hatz ik i r iakos , 2000). In spite o f the fact that both processing aids are ineffective when used individually, their combination postpones the cri t ical shear rate for the onset o f extrudate distortion to 1234 s"1. W e believe that this is a significant, interesting and unexpected result, the or ig in o f w h i c h is not clear at this point. A synergetic mechanism is in effect in this case and further studies are needed to clarify this unexpected result. The cri t ical shear rates for all experiments in this study are listed in Table 6-2. Figure 6-7 depicts pictures o f extrudate samples used to illustrate the effect o f combining B N w i t h a fluoroelastomer in the extrusion o f m - L L D P E Exac t® 3128. These extrudates were obtained at the shear rate o f 926 s'1 and temperature o f 163°C. One can see that the combinat ion o f B N and fluoroelastomer results in a smooth extrudate (d) at shear rates where the extrudate w o u l d normally exhibit gross melt fracture (a, b) and surface melt fracture for the case o f B N (c). Simi lar experiments w i th those above, were performed wi th E x c e e d ® 116 using 0 . 1 % and 0 .2% B N w i t h and without 0 .05% fluoroelastomer in order to investigate its melt fracture performance in rheometry wi th the cross-head die having a diameter o f 0 .098" and a tip o f 0 .055" at 2 0 4 ° C . The fluoroelastomer used in these formulations was different from that used to produce the data o f Figure 6-6, as a reduction in the shear stress results (shown below). Thermal ly stable inorganic salt C a l c i u m Tetra-Borate ( C T B ) was also used in combination wi th the B o r o n Ni t r ide . The die and tip combination was changed in order to achieve higher shear rates during extrusion. Figure 6-8 plots the CHAPTER 6 - A NEW PROCESSING AID 101 T H E R O L E O F SURFACE E N E R G Y O F B O R O N NITRIDE O N GROSS M E L T F R A C T U R E E L I M I N A T I O N O F P O L Y M E R S Figure 6-7: The extrudate samples to illustrate the combined effect of BN and fluoroelastomer in the extrusion of m-LLDPE Exact® 3128 obtained at 926s"1 and 163°C: (a) pure resin; (b) 0.1% BN; (c) 0.1% Fluoroelastomer; (d) combination of 0.1% BN and 0.1% Fluoroelastomer. flow curves of all tested formulations. It can be observed that in the case of pure resin, surface melt fracture is initiated at the critical shear rate of about 707 s*. The change in the critical shear rate for the onset of surface melt fracture for virgin resin was due partly to different die dimensions, which was also confirmed by performing experiments with 102 CHAPTER 6 - A NEW PROCESSING AID T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS (44 ppm) renders the onset of extrudate distortion to be about 3534 s'1. All results are listed in Table 6-2. A small reduction in shear stress was observed with all cases that use combinations of B N and fluoroelastomer. CO o_ tn <n <D i _ OT l _ co in CO c (0 O -C L < 10° io-1 i i i i i i 1 i i i i—rr-| 1— T = 204°C, Exceed 116 7 Crosshead die : D=0.098", d= 0.055" open symbols correspond to smooth extrudate v A . V J I A v ft v ft V AftO v o pure resin • 0.2% BN + 88ppm CTB A 0.1% Bn + 44ppm CTB V 0.2% BN + 0.05% Fluor. + 88ppm CTB -o 0.1% BN + 0.05% Fluor. -+ 44ppm CTB -- J I I I I l _ 102 103 104 Apparent wall shear rate y,(s ) Figure 6-8: The effect of combining 0.1 % & 0.2% BN with 0.05% fluoroelastomer on the melt fracture performance of PE Exceed ® 116 in rheometer with Nokia Maillefer crosshead having 0.098" die and 0.055" tip at 204°C. CHAPTER 6 - A N E W PROCESSING AID 103 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS Table 6-2: Effect of BN, fluoroelastomer and their combination on the maximal shear rate yielding a smooth extrudate in extrusion of m-LLDPE Exact® 3128 and Exceed® 116 (Nokia Maillefer Cross-head) attached to rheometer at 163°C and 204°C respectively. Blend ID Max. Shear Rate(s *) Exact® 3128 50 Exact® 3128 + 0.05% Teflon® 1300 Exact® 3128 + 0.05% B N 1800 Exact® 3128 + 0.05% Teflon® + 0.05% BN 2400 Exact®3128 + 0.1%BN* 216 Exact® 3128 + 0.1% Fluoroelastomer * * 216 Exact® 3128 + 0.1% BN* + 0.1% Fluoroelastomer** 1234 Exceed® 116 110 Exceed® 3128 + 0.05% B N 900 Exact® 3128 + 0.05% Teflon® + 0.05% BN 2000 Exceed® 116 +0.2% B N + 88ppmCTB 2120 Exceed® 116 + 0.1 % BN + 44ppm CTB 1273 Exceed® 116 + 0.2% BN + 0.05% Fluor. + 88ppm CTB 4242 Exceed® 116 + 0.1% BN + 0.05% Fluor. + 44ppm CTB 3534 *,**Different types of BN and fluoroelastomer used in these formulations. See explanations in Table 1 and text. CHAPTER 6 - A NEW PROCESSING AID 104 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS It can be concluded once again that the combination of Boron Nitride and fluoroelastomer postpones the onset of extrudate distortion to high shear rates well within the gross melt fracture flow regime. Such high critical shear rates cannot be achieved by using either o f the two constituents ( B N or fluoroelastomer) individually. 6.3.3 Effect of BN on Polymer Processing In addition to the polymer blends discussed earlier, we also have performed extrusion experiments with five different types of boron nitride samples. Sample AS613 is molybdenum sulphide (M0S2), while sample AS614 is having boron nitride along with Calcium Tetraborate (CTB) . As it can be seen from Figure 6-9, there is no change in the shear stress with and without addition of boron nitride as expected (Yip and Hatzikiriakos, 1999) " T ' I ).06 in / O PE (virgin) • 0.1%AS610 A 0.1%AS611 V 0.1%AS612 O 0.1%AS613 O 0.1%AS614 ond to smooth extrudate J _ J . i 10° 10' 10 2 10 3 Apparent Shear Rate y A (s"1) Figure 6-9: The effect of combining 0.1% of B N (differing each other in processing methods) on the melt fracture performance of P E Exact® 3128 in rheometer with Nokia Maillefer crosshead having 0.122" die and 0.06" tip at 163°C. 10° T = 163°C, m-PE Exact3128 Crosshead Die: D=0.12in, d= I a CL. CD W CD w c CD 10-1 Q Open symbols corresp CHAPTER 6 - A NEW PROCESSING AID 105 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS To study possible effects of the BN, molysulfide and combination of BN and CTB addition to the resin on its rheology, linear viscoelastic measurements were carried out for metallocene PE's with BN at 163°C on parallel-plate rheometer. Figures 6-10, 6-11 and 6-12 depict the dynamic moduli, G' and G " , and complex viscosity rj* of metallocene PE Exact®3128 compared to resin filled with different types of BN made through different processing methods at 163°C. Figure 6-10: Dynamic moduli, G' and G " , and complex viscosity rj* of metallocene PE Exact® 3128 (with and without AS612 BN) at 163°C CHAPTER 6 - A NEW PROCESSING AID 106 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 10 6 o' S. 10 5 o E in O 10 4 fc-•O 10 3 \r (0 <D Cn £ o CO 1 0 2 10 1 o pure resin • 0.1% A S 6 1 3 o CO 0. > x m CL. E o O 10- 1 10° 10 1 10 2 Frequency (co), rad/s 10 3 10 3 Figure 6-11: D y n a m i c modul i , G ' and G " , and complex viscosi ty r\* o f metallocene P E Exac t® 3128 (wi th and without A S 6 1 3 Molybdenum-sulf ide) at 163°C. P E E x a c t 3 1 2 8 , T=163°C 10 1 10 4 CO CL VI o o w > X _a> CL E o O 10-1 10° 10' 10 2 Frequency (co), rad/s 10 3 10 3 Figure 6-12: D y n a m i c modul i , G ' and G " , and complex viscosi ty r j* o f metallocene P E Exac t® 3128 (pure resin and A S 6 1 4 [combination o f B N & C T B ] ) at 163°C. 107 CHAPTER 6 - A NEW PROCESSING AID T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS It can be seen that no significant difference was found in the linear viscoelastic behavior of the virgin resin compared with those filled with BN, molysulfide and combination of B N and CTB. Moreover, similar conclusions were also reported by Rosenbaum et al. (1998), Yip et al. (2000) and Seth et al. (2001), who also performed a detailed rheological characterization of several resins in order to determine the effect of B N on their rheology. Table 6 - 3 : Effect of AS-series B N on the maximal shear rate yielding a smooth extrudate in extrusion of m-LLDPE Exact® 3128 (Nokia Maillefer Cross-head) attached to rheometer at 163°C. Blend ID Max. Shear Rate(s"*) Exact® 3128 62 Exact® 3128 + 0.1% AS610 154 Exact® 3128 + 0.1% AS611 93 Exact® 3128 + 0.1% AS612 1049 Exact® 3128 + 0.1% AS613 309 Exact® 3128 + 0.1% AS614 617 Sample AS612 was found to postpone the onset of melt fracture to the shear rate of 1049 s"1, while with the addition of CTB to boron nitride, smooth extrudates could be obtained up to the shear rate of 617s"1. The critical shear rate for onset of melt fracture for all B N samples is provided in Table 6-3. CHAPTER 6 - A N E W PROCESSING AID 108 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 6.3.4 Effect of B N concentration on Polymer Processing W e have performed experiments w i th different concentrations o f boron nitride ( C T F 5 ) using a new resin, Exceed® 143. A l l samples were prepared and provided by D u P o n t Fluoroproducts . First ly, w e carried out our experiments w i t h a capillary die having L / D = 40 and D =0.02" at 2 0 4 ° C up to the shear rate o f 10,000 s'1. Consequently, w e repeated these experiments using the crosshead die at 2 0 4 ° C . Figure 6-13 depicts the effect o f concentration o f boron nitride on the melt 10° CO a. b to in a> <a <u c CD i TO CL Q . < 10"1 I-— 1 1 1 1—I I I I I T = 204°C, LTD = 40, D = 0.02" Exceed 143, B N - C T F 5 0 6 e o pure resin • 0.02% B N A 0.05% B N V 0.25% B N 10 1 10 2 1 0 3 Apparent shear rate y A ( s 1 ) 10 4 Figure 6-13: T h e effect o f combining different concentrations o f B N on the melt fracture performance o f P E E x c e e d ® 1 4 3 in capillary rheometer wi th capillary die having L / D =40 and D = 0 .02" at 2 0 4 ° C . fracture performance o f resin Exceed® 143 using a capillary die having L / D = 40 and D = 0 .02" at 2 0 4 ° C . It is obvious from Figure 6-13 that the presence o f boron nitride does not 109 CHAPTER 6 - A NEW PROCESSING AID T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS alters the flow curve as expected. The critical shear rate of pure resin for onset of sharkskin was found to be at 211 s"1 and that for gross melt fracture was found to be 1640 s*1. In presence of boron nitride, sharkskin was postponed to 469 s"1 irrespective of concentration of boron nitride. N o change was observed for the onset o f gross melt fracture with 200 and 500ppm of B N . However, in the presence of 2500ppm of B N , gross melt fracture was postponed up to the shear rate of 2343 s"1. Figure 6-14 depicts the effect of concentration of boron nitride on melt fracture performance o f resin Exceed®143 using the crosshead die having 0.098" die diameter and 0.055" tip diameter at 204°C. io° V CO OT CO CO CO CO -4—* c CD i CO C L CL. < 10- 1 h-~T 1 1 1—r~ T = 204°C, Exceed 143, BN - C T F 5 Crosshead die D = 0.098", d = 0.055" V S • t 8 o pure resin • 0.02% B N A 0.05 % B N V 0.25 % B N open symbols corresponds to smooth extrudate i i i i i i • t ^ i t i i • 10 2 10 3 10" Apparent shear rate y A ( s 1 ) Figure 6-14: The effect of combining different concentrations of B N on the melt fracture performance of m - L L D P E Exceed® 143 in rheometer with Nokia Maillefer crosshead having 0.098" die and 0.055" tip at 204°C. CHAPTER 6 - A NEW PROCESSING AID 110 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS The above mentioned die and tip combination was chosen in order to obtain very high shear rates up to 4000 s"1 during extrusion. It was found that the onset of sharkskin takes place at 707 s"1 for virgin resin. The presence of 200ppm of BN postpones this to 990 s*1. Increase of boron nitride concentration postpones the onset of sharkskin to much high shear rates i.e. 500ppm of BN postpones it to 1414 s"1 and 2500ppm of BN to 2120 s"1' All results are listed in Table 6-4. Table 6-4: Effect of B N on the maximal shear rate yielding a smooth extrudate in extrusion of m-LLDPE Exceed ® 143 with capillary die L/D = 40 and D = 0.02" and with Nokia Maillefer Cross-head die attached to rheometer at 204°C. Blend ID Capillary die (L/D = 40, D = 0.02") Exceed® 143 Exceed® 143 + 0.02% B N Exceed® 143 + 0.05% B N Exceed® 143 + 0.25% B N Crosshead die (D = 0.098", d = 0.055") Exceed® 143 Exceed® 143 + 0.02% BN Exceed® 143 + 0.05% BN Exceed® 143 + 0.25% BN Max. Shear Rate(s"1) 211 469 469 469 707 990 1414 2120 CHAPTER 6 - A N E W PROCESSING AID 111 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS F r o m these results it can be concluded that the B o r o n Ni t r ide has an effect on the processing o f polymers in capillary extrusion only at high concentrations. The effects in crosshead die becomes more pronounced at even lower concentrations mainly due to streamlined f low, smooth entrance and possibly better mix ing characteristics at the entrance to the die. 6.4 Summary The effect o f combining B o r o n Ni t r ide w i th a fluoroelastomer as a possible processing aid in the continuous extrusion o f metallocene L L D P E ' s was studied. B N combined w i t h a fluoroelastomer can be successfully used as a processing aid since it can eliminate melt fracture phenomena at shear rates not accessible when either o f the t w o constituents o f the additive is used individually. W h i l e the effect o f B N and fluoroelastomer o n the melt fracture m - L L D P E ' s is significant, their rheological behavior changes very little. A better fluoroelastomer that its viscosi ty matches better w i th those o f the resins used in this study, might have improved the polymer processability even further. In such cases, a higher pressure reduction than that observed here, w o u l d be expected to result. CHAPTER 6 - A NEW PROCESSING AID 112 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 7 Conclusions Experiments were carried out to measure the surface energy o f various boron nitride powders varying in particle size, shape, degree o f agglomeration and presence o f boron oxides and correlated them wi th their performance in eliminating gross melt fracture. In this w o r k , a reliable procedure has been developed for measurement o f surface energy o f powders using the capillary rise technique based on Washburn ' s equa t ioa Powder s are compacted at high pressures (1.8 M P a ) wi th a special co lumn-packing device to attain uniform packing. This produces results reproducible and statistically representative. U s i n g the surface energy results o f the various powders, it has been concluded that to provide g o o d performance, as a processing aid, the B N powder should have a surface energy wi th a dispersive component as high as possible and a non-dispersive component as l o w as possible. Th is w i l l result in to a high ratio o f the t w o surface energy contributions, and this ratio was found to correlate w e l l w i th the cri t ical shear rate for the onset o f melt fracture. Experiments were also carried out in parallel and capillary rheometers w i th a N o k i a Mal l i e fe r crosshead for Exac t® 3128 and Exceed® 116 to study the effect o f combining B o r o n Ni t r ide w i th a fluoroelastomer as a possible processing aid in the continuous extrusion o f metallocene L L D P E ' s . B N combined wi th a fluoroelastomer can be successfully used as a processing aid since it can eliminate melt fracture phenomena at shear rates not accessible when either o f the two constituents o f the additive used individually. W h i l e the effect o f B N and fluoroelastomer on the melt fracture m -L L D P E ' s is significant, their rheological behavior changes very little. A better 113 CHAPTER-7 CONCLUSIONS T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS fluoroelastomer that its viscosity matches better with those of the resins used in this study, might have improved the polymer processability even further. In such cases, a higher pressure reduction than that observed here, would be expected to result. CHAPTER-7 CONCLUSIONS 114 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 8 Recommendations Based o n the experience gained during this study, the fo l lowing recommendations can be made for future work : - A d d i t i o n a l experiments should be conducted to measure the surface energy o f the new processing aid (combination o f boron nitride and fluoroelastomer) so as to analyze their combined effect on processing o f polymers. A l s o , surface energy can be measured by Inverse Gas Chromatography ( I G C ) , a new sophisticated technique to measure the dispersive and non-dispersive components o f surface energy for powders , so as to correlate the results w i th the existing method. - It might be useful to study the adsorption o f boron nitride powders on polyethylene, in order to identify the detailed mechanism by which B N affect the extrusion o f mol ten polymers. T h e flow visual izat ion study should be continued in much more detail and cou ld also be used independently to measure the slip veloci ty directly. CHAPTER- 8 RECOMMENDATIONS 115 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS 9 References Adewale , K . P . , L e o n o v , A . I . , Modeling spurt and stress oscillations in flows of molten polymers. R h e o l . A c t a , 36, 110-127 (1997). Bagley , E . B . , End corrections in the capillary flow of polyethylene. J . A p p l . Phys. , 28, 624 (1957). Bagley , E . B . , Schreiber, H . P . , Effect of die entry geometry on polymer melt fracture and extrudate distortion. Trans. Soc. Rheo l . , 5, 341 (1961). B e n b o w , J .J . , L a m b , P . , New aspects of melt fracture. S . P . E . Trans., 3, 7 (1963). B e r g e m , N . , Visualization studies of polymer melt flow anomalies in extrusion. P roc . 8th Int. Congr . Rheo l . , Gothenberg, p. 50 (1976). Binning ton , R . J . , T roup , G . J . , Boge r , D . V . , A low-cost laser-speckle photographic technique for velocity measurement in slow flows. J . N o n - N e w t o n i a n F l u i d M e c h . , 12, 255-267 (1983). B i r d , R . B . , Stewart, W . E . , Lightfoot , E . N . , Transport phenomena. 2nd ed., W i l e y , N . Y . , 1962. B i r d , R . B . , Arms t rong , R . C . ; Hassager, O . , Dynamics of polymeric liquids, vol. 1: Fluid mechanics. W i l e y , N Y , 1987. Brocha rd , F . , de Gennes, P . - G . , Shear-dependent slippage at a polymer/solid interface. Langmui r , 8, 3033-3037 (1992). Buckmaster , M . D . , Henry , D . L . , Randa, S . K . , High speed extrusion. U . S . Pat. N o . 5,688,457 (1997). C o g s w e l l , F . N . , Stretching flow instabilities at the exits of extrusion dies. J . N o n -N e w t o n i a n F l u i d M e c h . , 2, 37-47 (1977). C o g s w e l l , F . N . , Converging flow and stretching flow: a compilation. J . N o n - N e w t o n i a n F l u i d M e c h . , 4, 23 (1978). C o x , H . W . , M a c o s k o , C . W . , Viscous dissipation in die flows. A I C h E J . , 20 (4), 785-795 (1974). Dealy , J . M . , Rheometers for molten plastics. Reinhold , N Y , 1982. REFERENCES 116 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS Dealy , J . M . , Wissbrun, K . F . , Melt rheology and its role in plastics processing: theory and applications. Re inhold , N Y , 1990. D e n n , M . M . , Surface-induced effects in polymer melt flow. P roc . X l t h In t .Congr .on Rheology , Brussels, Be lg ium. In: Moldenaers P . , Keunings R . (eds.) Theretical and applied rheology. Elsevier Science Publishers, 45-49, 1992. Denn , M . M . , Issues in viscoelastic fluid mechanics. A n n u . Rev . F l u i d M e c h . , 22, 13-34 (1990). Denn . M . M . , Polymer flow instabilities: a picaresque tale. Chem. E n g . E d . , 28, 162-166 (1994). Duncan-Hewi t t ,W. , and Ni sman , R . , J . Adhes ion Sc i . Technology,7 ,263 (1993). E l K i s s i , N . , P iau , J . M . , The different capillary flow regimes of entangled polydimethylsiloxane polymers: macroscopic slip at the wall, hysteresis and cork flow. J . N o n - N e w t o n i a n F l u i d M e c h . , 37, 55-94 (1990). E l y , D . D . , and Pepper, D . C . , A dynamical determination of adhesion tension. Trans. Faraday S o c , 42, 697 (1946). Ferry , J . D . , Viscoelastic properties of polymers. Wi l ey , N . Y . , 1980. F o w k e s , F ' . M . , Attractive forces at Interfaces. Ind. E n g . Chem. 56, 40 (1964). Gai t , J . , M a x w e l l , B . , Velocity profiles for polyethylene melts. M o d e r n Plastics, D e c , 115-132(1964) . G i a c o m i n A . J . , Samurkas, T . ; Dealy , J . M . , A novel sliding plate rheometer for molten plastics. P o l y m . E n g . S c i . , 2 9 ( 8 ) (1989). G i r i f a l c o , L . A . , G o o d , R . J . , J . Phys. Chemistry, 61,904 (1957). Gra t toni , C . A . , Chiot is , E . D . , and D a w e , R . A . , J . Chem. Tech . B io - techno logy 64, 17 (1995). Hansford , D . T . , Grant, D . J . W . , and N e w t o n , J . M . , P o w d e r Technology, 26, 119 (1980). Hatz ik i r i akos , S . G . , Dealy , J . M . , Wall slip of molten high density polyethylene. I. Sliding plate rheometer studies. J . Rheo l . , 35 (4), 497-523 (1991a). Hatz ik i r i akos , S . G . , Dealy , J . 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Hatzikiriakos, S.G., A multimode interfacial constitutive equation for molten polymers. J. Rheol., 39(1), 61-71 (1995). Hatzikiriakos, S.G., Hong, P.; Ho, W.; Stewart, C.W., The effect of teflon coatings in polyethylene capillary extrusion. J. Appl. Polym. Sci., 55, 595-603 (1995). Hill, D.A., Hasegawa, T, Denn, M . M . , On the apparent relation between adhesive failure and melt fracture. J. Rheol., 34, 891-918 (1990). Howells, E.R., Benbow, J.J., Flow defects in polymer melts. Trans. Plast. Inst., 30, 240-253 (1962). Kalika, D.S., Denn, M . M . , Wall slip and extrudate distortion in linear low-density polyethylene. J. Rheol., 31 (8), 815-834 (1987). Kazatckov, L B , Yip, F., and Hatzikiriakos, S.G., The effect of boron nitride on the rheology and processing of polyolefins. Rheol. Acta, 39, 583 (2000). 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Piau, J.M., El Kissi, N , Trenblay, B., Low Reynolds number flow visualization of linear and branched silicones upstream of orifice dies. J. Non-Newtonian Fluid Mech., 30, 197-232 (1988). REFERENCES 119 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS Piau, J.M., El Kissi, N., Trenblay, B., Influence of upstream instabilities and wall slip on melt fracture and sharkskin phenomena during silicone extrusion through orifice dies. J. Non-Newtonian Fluid Mech., 34, 145-180 (1990). Pudjijanto, S., Denn, M . M . , A stable "island" in the slip-stick region of linear low-density polyethylene. J. Rheol., 38 (6), 1735-1744 (1994). Ramamurthy, A. V., Wall slip in viscous fluids and influence of materials of construction. J. Rheol., 30 (2), 337-357 (1986). Rauwendaal, C , Fernandez, F., Experimental study and analysis of a slit die viscometer. Polym. Eng. Sci., 25, 765 (1985). 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REFERENCES 121 THE ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS Wang, S.Q., Drda, P.A., Inn, Y.W., Exploring molecular origins of sharkskin, partial slip, and slope change in flow curves of linear low density polyethylene. J. Rheol., 40(5), 875-898 (1996). Washburn, E.W., The dynamics of capillary flow. Physics Review, 17, 273 (1921). Warren, R.C., Viscous heating. Rheological measurement, Elsevier, London, 1988, 119-149. Wu, S., Calculation of Interfacial tension in polymer systems. Journal of Polymer Science, C34, 19, (1971) Wu,S., Polymer Interface and Adhesion, Wiley, NY, 1982 Yip,F., Rozenbaoum,E.E., Randa, S.K., Hatzikiriakos, S.G., and Stewart, C.W., A N T E C '99, Soc. Plastics Eng'rs, Tech. Papers, 45, 1223, NY, USA, (1999) REFERENCES 122 THE ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS Notation a? shift factor b Rabinowi t sch correct ion D capillary diameter, m d tip diameter, m Ea activation energy for flow, J e B a g l e y end correct ion or energy Fd Piston'force, lb g gravitational acceleration, m/s 2 G shear modulus, P a G' storage modulus, P a G" loss modulus, P a G* complex modulus, P a Gd amplitude ratio in oscil latory shear h gap between plates, m h l iquid penetration distance in capillary, m m I melt polydispersity K power- law consistency index, M P a • s" K Tortousi ty constant K' geometric factor L capillary length or length o f sample, m n power- law exponent P absolute pressure, P a Pa ambient pressure, P a dr iv ing pressure, P a ^end Bagley correction, P a NOTATION 123 T H E ROLE OF SURFACE ENERGY OF BORON NITRIDE ON GROSS MELT FRACTURE ELIMINATION OF POLYMERS exit pressure drop, Pa Went entrance pressure drop, Pa Q volumetric flow rate, m 3/s r capillary radius, m Rb Radius o f barrel, in T absolute temperature, K t time, s Tg glass transition temperature, K Tref reference temperature, K U melt velocity, m/s Us slip velocity, m/s W a Work o f adhesion, mJ/m 2 Wo initial weight o f column, gm wt total weight o f column A w difference of penetration weight, gm Ax plate displacement, m Greek Letters a pressure coefficient of viscosity, Pa"1 8 mechanical loss angle r(0 shear strain Y shear rate, s"1 YA apparent shear rate, s"1 Y A,s apparent shear rate, corrected for slip, s Yw wall shear rate, s"1 Yo strain amplitude in oscillatory shear Ys surface energy of solid NOTATION 124 T H E R O L E O F S U R F A C E E N E R G Y O F B O R O N N I T R I D E O N G R O S S M E L T F R A C T U R E E L I M I N A T I O N O F P O L Y M E R S Yi surface tension o f l iquid Ysi interfacial tension between solid and l iquid Yu interfacial tension between l iquid and vapor Ysv interfacial tension between solid and vapor Y° dispersive component o f surface energy, m J / m 2 non-dispersive component o f surface energy, m J / m 2 M viscosi ty o f l iquid, m P a .s P density o f l iquid, k g / m 3 e l iquid contact angle on solid surface * porosi ty o f packed powder co lumn viscosity, P a • s 7o zero-shear viscosity, P a • s TJ* complex viscosity, P a • s 0"c cri t ical shear stress for the onset o f melt fracture, P a C T W wal l shear stress, P a °"o stress amplitude in oscil latory shear, P a CD frequency, rad/s or specific volume, c m 3 / g NOTATION 125 

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