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The effect of boron nitride on the rheology and processability of molten polymers Yip, Franky Kam Yin 2000

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THE EFFECT OF BORON NITRIDE O N T H E R H E O L O G Y A N D PROCESSABILITY OF M O L T E N POLYMERS by Franky Kam Yin Yip  Bachelor of Chemical Engineering, University of Minnesota, 1997  A THESIS SUBMITTED IN PARTIAL F U L F I L L M E N T OF T H E REQUIREMENTS FOR T H E D E G R E E OF M A S T E R OF APPLIED SCIENCE in The Faculty of Graduate Studies Department of Chemical and Bio-Resource Engineering  We accept this thesis as conforming to the required standard  T H E UNIVERSITY OF BRITISH C O L U M B I A Nov 1999 © 1999 Franky Kam Yin Yip  In  presenting  degree freely  at  this  the  thesis  in  partial  fulfilment  University  of  British  Columbia, I agree that the  available for  copying  of  department publication  this or of  reference  thesis by  this  for  his  and study. scholarly  or  her  thesis for  of  I further  purposes  requirements  for  Library  agree that permission  may  representatives.  financial  the  It  gain shall not  be is  granted  by  understood be  of  tkt^,\c^[ ^ a  KUyw^Q  The University of British C o l u m b i a Vancouver, Canada  Date  DE-6  (2/88)  Ueo  21^.  iW  for  that  allowed without  T,.^,,.,.^  advanced  shall make  the  permission.  Department  an  it  extensive  head  of  my  copying  or  my  written  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  Abstract Experiments were carried out using capillary, sliding plate, and parallel plate rheometer and a blow moulding machine in order to assess the effect of boron nitride (BN) powders on the rheology and processability of a metallocene polyethylene, a Teflon FEP polymer and a polypropylene. In particular, to assess the processability of these resins in the presence of B N or not, the critical conditions for the onset of melt fracture is determined. The influence of the average size of boron nitride particles, its agglomeration if present, its concentration in the resins and the quality of the attained dispersion on the melt fracture of the metallocene polyethylene was particularly investigated. For this a number of boron nitride powders were examined having various characteristics, i.e. agglomeration, different particle size etc. It was found that for B N to be efficient in eliminating surface and gross meltfracture,the particle size should be about 10-20 microns in size, and free of agglomeration. Concentration was also found to be a critical factor, with the optimum value to fall in the range between 200 and lOOOppm. The mechanism by which B N acts to eliminate melt fracture was also investigated. Six different capillary dies with various entrance angles were used to study the entrance effect. A new apparatus was also designed during the course of this study in order to visualize the flow patterns developed at the entrance of the capillary. It was found that B N changes the entrance flow pattern of a PP by eliminating the onset of unstable flow. This is done by providing the condition for a smoother transfer of momentum from a polymer layer to the next. The effect of B N type on two blow molding grade high-density polyethylene was also studied by using a blow moulding machine. It was found that B N can eliminate the surface melt fracture of blow moulding grade HDPE depending on the properties of resins.  ABSTRACT  ii  THE EFFECT OF BORON NITRIDE TYPE ON THE RHEOLOGY AND PROCESSATLrTY OF MOLTEN POLYMERS  ABSTRACT T A B L E OF CONTENTS LIST OF FIGURE LIST OF T A B L E S ACKNOWLEDGEMENTS  ii iii vi' xi/ xiii  1  1  INTRODUCTION  2. L I T E R A T U R E R E V I E W 2.1  C H E M I C A L S T R U C T U R E A N D PHYSICAL PROPERTIES OF VARIOUS P O L Y M E R S  7  2.1.1  Polyethylene  7  2.1.2  Fluoropolymer  11  2.1.3  Polypropylene  12  2.2  P O L Y M E R PROCESSING AIDS  12  2.3  EQUIPMENTS U S E D  16  2.4  2.5  2.3.1  Capillary Rheometer  17  2.3.2  Parallel Plate Rheometer  23  2.3.3  Sliding Plate Rheometer  26  2.3.4  Flow Visualization Cell  29  2.3.5  Blow Moulding machine  32  M E L T F R A C T U R E AND F L O W C U R V E OF P O L Y M E R 2.4.1  Surface melt fracture  2.4.2  Gross melt  38 40  fracture  E X P L A N A T I O N OF M E L T F R A C T U R E P H E N O M E N O N  42 44  2.5.1  Die Exit Effect: sharkskin  44  2.5.2  Flow instability in the die land: sharkskin  45  2.5.3  Die Entrance Effect: gross melt fracture  46  TABLE OF CONTENTS  iii  THE EFFECT OF BORON NITRIDE TYPE ON THE RHEOLOGY AND PROCESSATLrTY OF MOLTEN POLYMERS  2.5.4 2.6  Wall Slip  FACTORS AFFECTING POLYMER FLOW  47 49  2.6.1  Temperature Effect - Time Temperature Superposition  49  2.6.2  Pressure Effects  50  2.6.3  Viscous heating  51  3. O B J E C T I V E S  53  4. M A T E R I A L S & B L E N D I N G M E T H O D S  54  4.1  P O L Y M E R STUDIED  54  4.2  B O R O N NITRIDE TYPES  56  4.3  P O L Y M E R BLENDS  61  5. R E S U L T S A N D DISCUSSION  63  5.1  INTRODUCTION  63  5.2  M E L T F R A C T U R E P E R F O R M A N C E OF M E T A L L O C E N E P O L Y E T H Y L E N E : C A P I L L A R Y R H E O M E T E R STUDIES  65  T H E E F F E C T OF B O R O N NITRIDE O N T H E R H E O L O G I C A L BEHAVIOR OF T E F L O N FEP 4100 AND M E T A L L O C E N E POLYETHYLENE: PARALLEL PLATE RHEOMETER & SLIDING P L A T E R H E O M E T E R STUDIES  79  5.3  5.3.1  Change of Rheological Behavior  79  5.3.2  Effect of the Die Geometry on the B N Performance  91  5.3.3  Wall Slip  95  5.4  F L O W VISUALIZATION O N P O L Y P R O P Y L E N E  98  5.5  T H E E F F E C T OF B O R O N NITRIDE O N HDPE: B L O W M O L D I N G STUDIES  103  5.5.1  Flow Curves of Resins  103  5.5.2  Rheological Measurement of Resins A and B  107  TABLE OF CONTENTS  iv  THE EFFECT OF BORON NITRIDE TYPE ON THE RHEOLOGY AND PROCESSAILITY OF MOLTEN POLYMERS  5.5.3  Transient Extrusion Experiments  110  5.5.4  Visual Observation of the extrudate  112  6. C O N C L U S I O N S  117  7. R E C O M M E N D A T I O N S F O R F U T U R E W O R K  119  REFERENCE  120  NOTATION  125  TABLE OF CONTENTS  v  THE EFFECT OF BORON NITRIDE TYPE ON THE RHEOLOGY AND PROCESSATLITY OF MOLTEN POLYMERS  List of Figures Figure l-l:Different types of melt fracture. A)smooth extrudate, B)an extrudate showing small amplitude periodic distortions (sharkskin) and C)an extrudate showing gross melt fracture  4  Figure 2-1: Schematic representation of a linear and a branched polyethylene molecule  8  Figure 2-2: The flow curve of linear polyethylene and that of polyethylene containing 250 ppm fluoropolymer (Stewart et al, 1993)  15  Figure 2-3: The effect of the addition of 0.1% of polyethylene on the flow curve of resin FEP 4100 at 350 °C (Rosenbaum, and Hatzikiriakos, 1998) using a capillary rheometer  16  Figure 2-4: A simple schematic of capillary rheometer  18  Figure 2-5: Wall pressure distribution for capillary flow (Dealy, 1982)  19  Figure 2-6: The schematic diagram for Bagley plot  20  Figure 2-7: Crosshead die for wire coating (from Buckmaster et al, 1997)  22  Figure 2-8: Parallel plate rheometer  24  Figure 2-9: Simple schematic of simple shear flow utilized by a sliding plate rheometer Figure 2-10: a) Simple shear flow without slip, b) and with slip occur  26 27  Figure 2-11: Sliding plate rheometer with a flush-mounted shear stress transducer  28  Figure 2-12: The schematic diagram of the flow visualization set up  29  Figure 2-13: The laser and a beam forming system  30  Figure 2-14: Photograph of the quartz capillary and four radiation heaters surrounding the die  31  Figure 2-15: Photograph of the rheometer, microscope and the camera  31  Figure 2-16: The schematic of the quartz capillary die  32  Figure 2-17: A schematic diagram of an extrusion blow moulding machine  34  Figure 2-18a: The procedure to produce the final desired blow mould product  35  Figure 2-18b: The diagram of die and mandrel on the blow moulding unit  36  List of Figures  ^  THE EFFECT OF BORON NITRIDE TYPE ON THE RHEOLOGY AND PROCESSATLITY OF MOLTEN POLYMERS  Figure 2-18c: The diagram of the cross section of blow moulding die and mandrel Figure 2-19: Atypical flow curve for linear polymer  37 39  Figure 2-20: The surface velocity of the melt accelerate from zero inside the die to the extrusion velocity outside  45  Figure 2-21:The flow curve of polymers and rubbers depends on the radii once the wall shear stress exceeds the critical condition  47  Figure 4-1: Polyethylene  54  Figure 4-2: Polypropylene  54  Figure 4-3: DuPont Teflon® fluoro-copolymer of Tetrafluoroethylene/hexafluoropropylene (Teflon® FEP) resin type 4100 Figure 4-4: The structure of boron nitride  55 57  Figure 4-5: Photos of the B N particles (white) dispersed into P E (black). The magnification is 320 times in these pictures. The particle size is 5-10 p:m for CTF5 and more than 40 u,m for CTL40. It can also be seen that CTF5 is well dispersed (a) as opposed to the CTL40 that exhibits a certain degree of agglomeration (b)  58  Figure 4-6: S E M picture of various B N (200 = 200 times magnification, lk = 1000 times and 6k = 6000 times). It can be seen that 431 & CTF5 have uniform particle sizes including no agglomerated particle as opposed to C T U F  59  Figure 4-6 (continued): S E M picture of various B N (200 = 200 times magnification, lk = 1000 times and 6k = 6000 times). It can be seen that 431 & CTF5 is a uniform powder including no agglomerated particle as opposed to C T U F  60  Figure 5-1: Flow curves of the virgin and filled m-LLDPE Exact® 3128 obtained by using the capillary rheometer with a capillary die having D=0.254 mm and L/D=40 at T=163°C  List of Figures  65  vii  THE EFFECT OF BORON NITRIDE TYPE ON THE RHEOLOGY AND PROCESSATLrTY OF MOLTEN POLYMERS  Figure 5-2: The flow curve of m-LLDPE Exact® 3128 by using the capillary rheometer with Nokia Maillefer crosshead having 3.00 mm die and 1.52 mm tip at 163 °C  66  Figure 5-3: The flow curve of m-LLDPE Exact® 3128 with and without boron nitride (CTF5) obtained with the Nokia Maillefer 4/6 crosshead attached at 163°C. Blends were prepared by dry-mix method. It shows that B N has little effect on melt fracture  67  Figure 5-4: The flow curves of pure and filled m-LLDPE Exact® 3128 with B N (type CTF5) at concentrations of 0.02%, 0.1%, and 0.5% obtained at 163°C by using the crosshead die (second mixing technique). It can be seen that B N has a significant effect on the melt fracture.  69  Figure 5-5. The extrudate samples to illustrate the effect of B N (CTF5) on the extrusion of PE Exact 3128 obtained at 617 s" and 163°C: 1) pure 1  resin; 2) 0.02% BN; 3) 0.1% BN; 4) 0.5% of B N (CTF5)  70  Figure 5-6: The flow curve for the m-LLDPE Exact® 3128 with and without BN431 obtained by using the capillary rheometer at 163°C by using second technique  73  Figure 5-7: The flow curve for the m-LLDPE Exact® 3128 with and without BN431 obtained by using the capillary rheometer at 163°C by using second technique  74  Figure 5-8: Transient capillary experiments for 0.1% B N filled m-LLDPE Exact® 3128 at various shear rates. Note that the force at 950 pounds representing the shear rate of 100 s"  74  1  Figure 5-9: The effect of various boron nitride types on the maximum shear rate yielding a smooth extrudate in extrusion of P E Exact 3128 at 163°C for two concentrations of B N (200ppm and lOOOppm)  75  Figure 5-10: The effect of temperature on the melt fracture performance of B N 428,  430 and CTF5 filled m-LLDPE resin at two  temperatures of 163°C and 204°C  List of Figures  different 76  viii  THE EFFECT OF BORON NITRIDE TYPE ON THE RHEOLOGY AND PROCESSATLITY OF MOLTEN POLYMERS  Figure 5-11: The flow curve of pure resin m-LLDPE Exact  3128 and of m-  L L D P E Exact® 3128 resin containing BN427 to BN431 (0.1 weight % in all cases) using the crosshead die at 163 °C  77  Figure 5-12: Linear viscoelastic data of m-LLDPE Exact® 3128 at 163°C with and without B N (type CTF5)  80  Figure 5-13: Linear viscoelastic data ( G \ G") of m-LLDPE Exact® 3128 at 163°C with and without B N (type 431)  81  Figure 5-14: The wall shear stress vs frequency for the virgin m-LLDPE and mL L D P E with BN431 with different levels concentration by using the parallel plate rheometer at 163 °C  82  Figure 5-15: Linear viscoelastic data for Teflon FEP® 4100 at the reference temperature of 300°C with and without B N (type CTF5)  83  Figure 5-16: The dynamic moduli, G ' & G " , of 0.02% CTF5 added Teflon FEP®4100 at reference temperature 300°C  84  Figure 5-17: The dynamic moduli, G ' & G " , of 0.05% CTF5 added Teflon FEP®4100 at reference temperature 300°C  85  Figure 5-18: The dynamic moduli, G ' & G " , of 0.17% CTF5 added Teflon FEP®4100 at reference temperature 300°C  86  Figure 5-19: The effect of B N concentration on the zero shear viscosity of Teflon FEP® 4100 at 300°C  87  Figure 5-20: The effect of B N concentration on the zero shear viscosity of Teflon FEP® 4100 at 320°C  88  Figure 5-21: The effect of B N concentration on the zero shear viscosity of Teflon FEP® 4100 at 340°C  89  Figure 5-22: The relationship between the activation energy for flow, Ea, and the B N (CTF5) concentration in Teflon FEP® 4100  90  Figure 5-23: The end pressure as a function of apparent shear rate of m-LLDPE Exact® for four orifice dies having various entrance angles Figure 5-24: The end pressure as a function of apparent shear rate of 0.05% CTF5 filled m-LLDPE Exact® for four orifice dies having various  List of Figures  92  THE EFFECT OF BORON NITRIDE TYPE ON THE RHEOLOGY AND PROCESSAILrTY OF MOLTEN POLYMERS  entrance angles  93  Figure 5-25: The effect of contraction angle on the wall shear stress for mL L D P E at 163°C  94  Figure 5-26: The flow curve of pure m-LLDPE Exact® 3128 using capillary dies having various diameters dies (0.254, 0.508 and 1.27mm)  95  Figure 5-27: The flow curve of pure m-LLDPE Exact® 3128 with addition of BN431 for capillary dies having various diameters dies (0.254, 0.508 and 1.27mm)  96  Figure 5-28: The flow curve of pure m-LLDPE Exact® 3128 with addition 0.1% of BN431 determined by the sliding plate rheometer using various gap spacing  97  Figure 5-29: The flow curve of polypropylene with and without B N (CTF5) by using the transparent quartz capillary die  98  Figure 5-30a: Pictures of the flow of polypropylene at various apparent shear rates (left 32.4 s", middle 320 s" and bottom right 650 s") at 200°C 100 1  1  1  Figure 5-30b: A schematic diagram to explain the flow pattern development at 650s"  100  1  Figure 5-31: Pictures of the flow of polypropylene with (b) and without (a) the addition of 0.1% B N (CT5) at the shear rate of 650s" at 200°C 1  101  Figure 5-32: PP extrudate at the shear rate of450s" with (a) and without boron 1  nitride (b) obtained with the quartz transparent die. The extrudate of PP with 0.1%BN is smooth (a), while the extrudate of pure PP exhibits gross melt fracture (b)  102  Figure 5-33: The flow curve of Resin A and B by using the crosshead die. The reduction on wall shear stress by addition of B N is cleanly seen  104  Figure 5-34: The head pressure varied with the die gap of the pure H D P E Resin A and Resin B at the screw speed of 30 rpm  105  Figure 5-35: The effect of shear rate at the die exit on the head pressure of Resin A at the die gap of 0.5mm. The screw speed was 30 rpm  106  Figure 5-36 : The effect of shear rate at the die exit on the head pressure of Resin B at the die gap of 0.5 mm. The screw speed was 30rpm  List of Figures  107  x  THE EFFECT OF BORON NITRIDE TYPE ON THE RHEOLOGY AND PROCESSAHJTY OF MOLTEN POLYMERS  Figure 5-37: The dynamic moduli and complex viscosity, r|*, for resin A @190°C  108  Figure 5-38: The dynamic moduli and complex viscosity, T|*, for resin B @185°C  109  Figure 5-39: The transient extrusion experiment for resin A with and without B N . The screw speed was 30 rpm  110  Figure 5-40: The transient experiment for resin B with and without B N  Ill  Figure 5-41: The surface appearance of extrudate (bottle) made by Resin A at the shear rate of2800s" . A) Pure resin A, B) Resin A containing 1  0.025% B N and C) Resin A containing 0.1% B N  112  Figure 5-42: S E M pictures showing the surface appearance of the bottle made by using resin A with and without the addition of B N . A) Pure resin A at the top and B) Resin A containing 0.1% B N at the bottom. It can be seen that the amplitude periodic distortions decrease by the addition of B N into the resin  ...113  Figure 5-43: Surface appearance of extrudate (part of bottle) made by using Resin B at 2800s" A) Pure resin B, B) Resin B containing 0.025% 1  B N and C) Resin B containing 0.1% B N  114  Figure 5-44: The effect of temperature on the bottle surface appearance. The bottle was collected at 190°C (A) exhibits melt fracture while that made at 205°C (B) is free of any defects  115  Figure 5-45: The effect of the induction time on the surface appearance of the bottle. The sample (A) collected at time after 1 minute from the start-up of extrusion isfracturedcompared the smooth one (B), which was collected at 10 minutes after the start-up. A) Sample collected at time = 1 minute (A) and B) Time =10 minutes (B)  List of Figures  116  xi  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  List of Tables Table 2-1: Critical wall shear stress values at the onset of surface melt fracture for various types of polymers  40  Table 2-2: Critical wall shear stress value for the onset of gross melt fracture  42  Table 4-1: The chemical and physical properties of Resins A and B  55.  Table 4-2: Comparison of boron nitride to common  57  fillers  Table 4-3: The average particle sizes and states of agglomeration properties of various boron nitride powders  58  Table 4-4: Summary of the various blends prepared together with the blending methods used  62  Table 5-1: The effect of B N on the melt fracture of m-LLDPE (Exact® 3128) at three temperatures obtained for B N type CTF5 added to resin in a drymixed form  68  Table 5-2: Effect of the B N type (CTUF, CTL40 and CTF5) and concentration on the maximal shear rate yielding a smooth extrudate in extrusion of P E Exact® 3128 (Nokia Maillefer crosshead attached to the rheometer, D=3.0mm, d=1.52mm) at 163°C. The resin and B N are initially preextruded by the second technique to attain good B N dispersion into the resin  List of Table  71  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  Acknowledgements  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 study. His insights and ideas have greatly contributed to the quality of this work. I thank Dr. Robert DiRaddo for his collaboration and assistance in studying the effect of boron nitride on blow moulding of high-density polyethylene. Their hospitality while I was performing experiments at NRC-JML Boucherville, Quebec is greatly appreciated. This work was supported by Carborandum, Amherst, N Y , USA. I am also thankful to them for preparing and providing all boron nitride samples. M y colleagues from Rheolab at U B C helped me in various ways. I wish to thank Igor Kazatchkov, Alfonsius Budi Ariawan, and Eugene Rozenbaoum for their helpful discussions and exchange of ideas. I thank my parents for their love and continuing support. Most of all, I thank my best friend Joyce who has been a source of strength and motivation for success.  ACKNOWLEDEMENTS  xiii  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  1 Introduction Polyethylene (PE) resins have been produced commercially for over fifty years and it is one of the most massively produced commodity polymers. In 1994, the global P E production capacity was 45 million metric tons per year operated at an average rate of 81% to produce almost 37 million metric tons of polyethylene, allocated between high density polyethylene (>40), high pressure-low density polyethylene (<40%) and linear low density polyethylene (20%). North America, Western Europe and Japan were the largest producing areas with almost 70% of the total global capacity: Western Europe was the largest producer of HP-LDPE and the United States was the largest producer of L L D P E and HDPE. Ziegler-Natta catalysts have been used since the 1950's for the polymerization of ethylene and propylene to produce polyethylene and polypropylene respectively. Before the discovery of Ziegler-Natta catalysts in the 1950's, the methods used for the production of these polymers were not commercially viable and a new technology had to be devised. Ziegler-Natta catalyst was first discovered by the German scientist Karl Ziegler. Later, an Italian scientist, Guilio Natta developed the catalyst further. Out of appreciation of their work, they were both awarded the Nobel prize in 1963. This discovery became a new technology for polymerization in that time. 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 makes these polymers much more commercially useful than the previous high pressure ones.  CHAPTER 1 - INTRODUCTION  1  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  Recently, a new family of catalysts has been discovered, that is metallocenes. The new metallocene catalysts, first used commercially in 1991, are rapidly emerging as the primary driving force for the global polyethylene business. 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 ZieglerNatta 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 P E (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 L L D P E . As a result, it has lower melt elasticity for 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 prestretching 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. CHAPTER 1 - INTRODUCTION  2  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  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. To process the various PE's in order to make useful products, various processes are used i.e., profile extrusion, film casting, film blowing and blow moulding. For any process to be economically feasible, the rate of production should be high enough. However, it is well known that the rate of production in many polymer processing operations are limited by the onset of flow instabilities (Petrie and Denn, 1976; Larson, 1992). In particular, once the shear rate exceeds the critical value, distortions appear on the surface of the extrudate. 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). Figure  CHAPTER 1 - INTRODUCTION  3  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  1-1 shows the two different types of melt fracture, namely surface and gross melt fractures, obtained in the capillary extrusion of Teflon® FEP resin.  A Figure 1-1:  B  C  A) smooth extrudate B) an extrudate showing small amplitude periodic distortions (sharkskin) C) an extrudate showing gross melt fracture  In order to eliminate the surface melt fracture and increase the rate of production, polymer processing aids (PA's) are used. These processing aids are mainly fluoropolymers that are added into the resin at concentration of 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 can eliminate sharkskin but not gross melt fracture. CHAPTER 1 - INTRODUCTION  4  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  Recently, it was shown that compositions containing fine boron nitride (BN) particles can be successfully used as processing aids 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 in the extrusion of polyolefins and fluoropolymers (Buckmaster, et al.,1997). Boron nitride is a solid lubricant, whose structure resembles that of 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 of a blowing agent, it initiates the nucleation of voids in polymer extrudate. In this study, B N particles are used without a blowing agent, so that the extruded polymer is unfoamed. During the extrusion of fluoropolymers or polyolefins with B N particles, the maximal shear rate at which the extrudate appears smooth is usually orders-of-magnitude higher than that which can ordinarily be achieved in the absence of this additive. More importantly, this maximal shear rate is usually much higher than that at which the virgin resin exhibits gross melt fracture. This means that B N , unlike fluoropolymers in the extrusion of polyethylene, can eliminate not only surface and stick-slip melt fracture but also significantly delay the onset of gross melt fracture to much higher shear rates (Rosenbaum et al., 1999). A requirement to observe this unique action of B N in the process is that the extrusion experiments be carried out with a special crosshead die of the type used for wire coating (Rosenbaum et al., 1999). The rheometer used by these authors was a standard Instron pistondriven constant-speed capillary unit. The crosshead was a Nokia Maillefer 4/6 that included dies and tips of various diameters ("tip" is the wire guide) with equal entry cone angles of 60° and the die land length of 7.62 mm. The same equipment was also used to carry out the present work.  CHAPTER 1 - INTRODUCTION  5  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  In this study, we use several types of boron nitride differing from each other in the particle size and composition. The objective of this thesis is to study the effect of the boron nitride type and concentration on the rheology and processability of molten polymers as they can be assessed by means of a capillary rheometer equipped with the Nokia Maillefer 4/6 crosshead. This particular die mimics the wire coating process. Furthermore, experiments are also carried out using a blow moulding machine at high shear rates for blow moulding grades of HDPE in order to assess the effect of B N as a processing aid in the blow moulding process.  CHAPTER 1 - INTRODUCTION  6  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  2  Literature Review  2.1  Chemical Structure and Physical Properties of Various Polymers 2.1.1  Polyethylene  Polyethylene is probably the polymer one may see most in daily life. It is the most popular plastic in the world. This is the polymer that makes grocery bags, shampoo bottles, children's toys, and even bulletproof vests. For such a versatile material, it has a very simple structure, the simplest of all commercial polymers. A molecule of polyethylene is nothing more than a long chain of carbon atoms, with two hydrogen atoms attached to each carbon atom. It might be easier to draw it like the picture below, only with the chain of carbon atoms being many thousands of atoms long:  AAAAA*  H H H H H H H H H H H I I I I I I 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  ******  Some of the basic informations about polyethylene are listed below: thermoplastics, fibers ethylene free radical chain polymerization, Zieglar-Natta polymerization and metallocene catalysis polymerization Morphology: highly crystalline (linear), highly amorphous (branched) 137 °C Melting temperature: Glass transition temperature: -30 °C Uses: Monomer: Polymerization:  CHAPTER 2- LITERATURE REVIEW  7  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS Sometimes its structure is a little more complicated. Some of the carbons, instead of having hydrogens, have long chains of polyethylene attached to them. This is called branched, or low-density polyethylene (LDPE). When there is no branching, it is called linear polyethylene, or HDPE (see schematic below). Linear polyethylene is much stronger than branched polyethylene, but branched polyethylene is cheaper and easier to make.  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.  Linear polyethylene is normally produced with molecular weights in the range of 200,000 to 500,000, but it can be made even higher. Polyethylene with molecular weights of three to six million is referred to as ultra-high molecular weight polyethylene, or  CHAPTER 2- LITERATURE REVIEW  8  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  U H M W P E . U H M W P E can be used to make fibers, which are so strong that have replaced Kevlar for use in bulletproof vests. Large sheets of it can be used instead of ice for skating rinks. The new discovery of metallocene technology can be used to produce polymers over the full range of conventional densities. The challenge facing the polymer industry and its converter customer is how this technology can be utilized to obtain a competitive advantage. In the last two years, high-density and linear low-density polyethylene resins produced by metallocene catalysts have become a commercial reality in all regions of the world except Eastern and Central Europe. Polyethylene  manufacturers refer to  metallocenes as the "next generation catalysts". Technological revolutions in metallocene chemistry have allowed polyolefin producers to molecularly engineer their resins, achieving important results in comonomer amounts (including the addition of a third comonomer), molecular weight, molecular weight distribution, and long chain branching. These systems are compatible with the modern polymerization systems so no major modification costs are required. Now, the yields are similar to those from the current Ziegler-Natta and chromium catalysts and manufacturing costs per pound of polymer approximate those of conventional catalysts. Unlike the linear low-density phenomena of the early 80's, metallocene technology is not restricted by polymerization capacity nor by processing capability. Thus far the market penetration has been specialty areas, but metallocene polyolefins have the potential of penetrating very significantly into the L L D P E and even L D P E markets. Metallocene polyolefins are expected to command 1020% of the polyolefins market by 2010. Let us look at what metallocene is. Metallocene  CHAPTER 2- LITERATURE REVIEW  9  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  is a positively charged metal ion sandwiched between two  negatively charged  cyclopentdienyl anions. The diagram below shows cyclopentdienyl anions.  Cyclop entadiene  Cyclopentadienide anion  You may notice that one carbon atom with two hydrogens, whereas the rest have one. These two hydrogens are acidic, that is, they can be removed very easily. When this happens, it leaves its bonding electrons behind. So the carbon it left now has an extra pair of electrons. Six electrons in a ring molecule like this will make the ring aromatic. It can give a very stable complex. These cyclopentadienide ions have a charge of -1, so when a cation comes along, like Fe with a +2 charge, two of the anions will form an iron sandwich. That iron sandwich is called ferrocene. Sometimes a metal with a bigger charge is involved, like zirconium with a +4 charge. To balance the charge, the zirconium will bond to two chloride ions, -1 charge on each, to give a neutral compound.  CI—Zr—CI  CHAPTER 2- LITERATURE REVIEW  10  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  But those chlorines can be removed. When this happens, they take their electrons with them. That zirconium is now lacking electrons. It will take them from wherever it can get them. Good sources of electrons are vinyl monomers. That carbon-carbon double bond is chock full of electrons. So a nearby vinyl monomer, say, ethylene, can come in and donate its electrons to the zirconium. As the process continues, the metallocene polyethylene can be formed.  2.1.2  Fluoropolymer  Fluoropolymer is another class of polymers, which is of interest to the present work. They belong to the class of paraffinic polymers that have some or all of the hydrogen replaced by fluorine. The first known fluoropolymer is Teflon® and it was discovered by DuPont about 50 years ago. Its melting point is very high (about 425°C) and therefore its processing needs special methods such as sintering and pressing. Its structure is shown below:  CF  DuPont  discovered  that  a  CF  2  2  copolymer  of  tetrafluoroethylene  (TFE)  and  hexafluoropropylene (HFP) has the good properties of Teflon but it has a much lower melting — CF  2  CF  2  CF  2  CF— CF  CHAPTER 2- LITERATURE REVIEW  point  that  allows  for  conventional processing. FEP is one of the  fluoropolymers.  It  has  3  11  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  predominantly linear chains and is produced by copolymerization of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP). FEP has a crystalline melting point of about 265°C determined by differential scanning calorimetry (DSC) and density of 2,150 kg/m . It is a soft plastic with tensile strength, wear resistance, and creep resistance lower 3  than those of many other engineering plastics. It is chemically inert with a low dielectric constant (about 2) over a wide range of frequencies and temperatures.  2.1.3  Polypropylene Polypropylene (PP) is a third type of polymer that is of interest to the present  work. It has the following structure:  H  Y?  H x  H  —  CH  3  -  H CH  3  As can be seen, the monomer for PP is propylene. The material mainly produced by the Ziegler-Natta method which discussed in section 2.1.1. Polypropylene has a wide range of applications. It included fiber, filaments to films and extrusion coating. The majority of the polypropylene is in the form of isotactic. The advantages of PP included inertness to water and microorganisms and the cost of the PP is very low (about $0.7 per kg).  2.2  Polymer Processing Aids As discussed before, processing aids are used together with the resins in order to  facilitate extrusion at reduced pressure drops as well as increase rate of production in extrusion operations by eliminating melt fracture phenomena. There are various factors to  CHAPTER 2- LITERATURE REVIEW  12  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  determine the performance of a PA. 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. Priester and Stika (1992,1994) suggested that the factors that can 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 better job in coating a die. The ideal particle size of the additive should be incompatible 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 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 will 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.  CHAPTER 2- LITERATURE REVIEW  13  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  Rudin (1985) performed experiments for a L L D P E with the addition of Dynamar® 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 P A 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 E S C A reading. From these data, one may suggest that processing aid such as Dynamar® move to the die/metal interface from the bulk. There the P A functions as a lubricant and as a result reduces the driving pressure  of extrusion.  They also mentioned that a proper balance  of  incompatibility and diffusion rate is required to ensure in order for the additive to coat 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 L L D P E , HDPE, H M W H D P E and PP with the  addition  of  a  small  quantity  of  FKM-A  (Vinylidene  fluoride  and  hexafluoropropylene). The addition of F K M - A changed the viscosity of the resin, thus reducing the driving pressure during the extrusion at certain shear rate.  CHAPTER 2- LITERATURE REVIEW  14  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS 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.  LLDPE 200°C Gross MF  MPa PE Sharkskin  0.1  'PE+ fluoropolymer  0.01  10  10' YA, s"  J  1  Figure 2-2: The flow curve of a linear low-density polyethylene with and without the addition of 250 ppm fluoropolymer (Stewart et al., 1993).  Typical examples are shown in figure 2-2 and 2-3. Figure 2-2 shows the flow curves of a linear low-density polyethylene with and without the addition of 250 ppm fluoropolymer (Stewart et al., 1993) obtained from an extruder by using an annular die. Figure 2-3 shows the effect of the addition of 0.1% of polyethylene on the flow curve of resin FEP® 4100 at 350 °C (Rosenbaum, and Hatzikiriakos, 1998) obtained by means of a capillary rheometer. Both examples show that the presence of processing aid reduce the  CHAPTER 2- LITERATURE REVIEW  15  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS 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 both cases. However, these processing aids have no effect on the gross melt fracture regime.  ~i  i  i  i i r~r~n  ~i  I  I  I  I  TTT1  ~i  1—i—i  i i i i  T=350°C L/D=40 D=0.762 mm  0.2 CO Q_  CO  £  w  0.1  S 0.08 JC  CO  % 0.06 i_  co  Q. C l  <  0.04 y Teflon F E P 4100 Teflon F E P 4100 + 0.1% P E  0.02 10  -I  I  I  L  1  J  10  2  1  1  I  '  I ' l l  10  Apparent shear rate; s  3  10  4  _1  Figure 2-3: The effect of the addition of 0.1% of polyethylene on the flow curve of resin FEP 4100 at 350 °C (E.E Rosenbaum, S.G. Hatzikiriakos, 1998) on a capillary rheometer.  2.3  Equipment Used in this Study Several pieces of equipment were used to carry out this study. In particular  experimental equipment were used to assess the processability of resins in the presence of B N and additionally to characterize rheology of the resins. These pieces of equipment  CHAPTER 2- LITERATURE REVIEW  16  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS will be described here together with the equations needed in order to calculate relevant fundamental quantities from the raw data.  2.3.1  Capillary Rheometer  The most widely used type of melt rheometer is the capillary rheometer. The property most frequently measured in a capillary rheometer is viscosity, a quantity very useful from the engineering point of view.  A typical capillary rheometer is capable of  measuring the viscosity of polymer over the range of shear rates from 5 to 5,000 s". 1  Moreover, from such a piece of equipment the processability of the polymers may also be assessed. As the polymer is extruded through a die, a visual inspection of the extrudate surface is possible. A simple schematic of capillary rheometer is shown in figure 2-4. This apparatus consists of a melt reservoir or barrel ,which is used to melt the polymer. This is accomplished by means of heating elements surrounding the barrel. A piston is used to push the melt flow through the capillary die, having a pre-specified diameter, D, and length, L . The quantities normally measured from this apparatus are the flow rate, Q, which is related to the speed of the piston and the driving pressure, which is related to the force applied on the piston.  CHAPTER 2- LITERATURE REVIEW  17  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  For steady-state, fully developed flow of an incompressible Newtonian fluid, the wall shear stress and wall shear rate can be calculated from:  Wall shear stress,  cr =  (2-1)  Wall shear rate,  f =  (2-2)  w  where G is the wall shear stress AP is the pressure drop along the rheometer (see figure 2-5) D is the diameter of the capillary die L is the length of the die, and Q is the volumetric flow rate w  For the case of non-Newtonian fluids, the quantity defined by Eq. 2-2, is only the apparent shear rate, y , defined as A  CHAPTER 2- LITERATURE REVIEW  18  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILrTY OF MOLTEN POLYMERS 32£  Apparent shear rate,  YA  (2-3)  -  BARREL CAPILLARY Pd  FULLY DEVELOPED FLOW REGION  H  H  w  Pex Pa  Figure 2-5:  Wall pressure distribution for capillary flow (Dealy, 1982)  It can also be shown that the true wall shear rate and the apparent shear rate are related through a correction factor, known as the Rabinowitsch correction. This can be expressed as (Dealy and Wissbrun, 1990):  V 4  7A  ,  (2-4)  (2-5)  where  For a power law fluid, the shear stress is related to the shear rate through:  cr =  CHAPTER 2- LITERATURE REVIEW  Ky  n  (2-6)  19  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS where K is the consistency index, a is the shear stress and n is the power law exponent. For such a power law fluid, it can be shown that the wall shear rate can be expressed as: (2-7)  It can be seen that, the Rabinowitsch correction ,b, is equal to 1/n for a power law fluid and equal to unity for a Newtonian fluid. The schematic diagram for the pressure drop in capillary flow of a molten polymer is shown in figure 2-5. First, there is a large pressure drops associated with the entrance region, AP . There also appears a small pressure drop at the exit of the capillary ent  known as the exit pressure, APex- The summation of these two pressures (entrance and exit) are known as, A P ^ t h e end pressure. APend =AP + AP, ent er  (2-8)  Pd  0  Figure 2-6: The schematic diagram for Bagley plot.  CHAPTER 2- LITERATURE REVIEW  20  T H E E F F E C T O F B O R O N NITRIDE O N T H E R H E O L O G Y A N D P R O C E S S A B I L r T Y O F M O L T E N POLYMERS T h e end pressure c a n b e determined b y u s i n g a w e l l k n o w n technique c a l l e d B a g l e y t e c h n i q u e (1931). A c c o r d i n g t o this p r o c e d u r e , the d r i v i n g  pressure, Pd, i s  measured u s i n g a variety o f capillaries w i t h different L / D ratio at v a r i o u s f l o w rates. F o r e a c h f l o w rate, the d r i v i n g pressure is p l o t t e d as a f u n c t i o n o f L / D ratio, and extrapolating the lines t o L/D=0. T h e end pressure c a n be obtained as the intercept o n the pressure axis o r as a n end c o r r e c t i o n o n the intercept o n the L / D axis. T h e s c h e m a t i c d i a g r a m f o r a t y p i c a l B a g l e y p l o t is s h o w n i n f i g u r e 2 - 6 . U s i n g the B a g l e y c o r r e c t i o n , t h e true w a l l shear stress c a n n o w be calculated as:  (2-9)  =—T-  Cogswell  (1981) has suggested that the use o f an o r i f i c e die(L/D=0) i s a shortcut  m e t h o d to determine w i t h reasonable a c c u r a c y f o r  APend- I n this case, the w a l l shear stress  c a n be c a l c u l a t e d b y ,  4(L/D) T h e c a p i l l a r y rheometer  is capable o f p r o v i d i n g  (  2  _  1  0  )  m u c h o f t h e data that are  necessary f o r the study o f melt processability. T h e c a p i l l a r y rheometer m i m i c s reasonably w e l l t h e p r a c t i c a l f l o w i n m o u l d s a n d dies a n d also p r o v i d e s extrudate o n w h i c h a subjective assessment o f quality m a y be made. B e s i d e s the c a p i l l a r y  d i e , a crosshead d i e w a s also a p p l i e d t o assess the  p r o c e s s a b i l i t y o f the v a r i o u s resins. T h e crosshead w a s a N o k i a M a i l l e f e r 4 / 6 that i n c l u d e d dies and tips o f v a r i o u s diameters ( " t i p " is the w i r e g u i d e ) w i t h equal entry cone  C H A P T E R 2- L I T E R A T U R E R E V I E W  21  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS angles of 60° and the die land length of 7.62 mm. The schematic of the crosshead is shown in Figure 2-7.  Figure 2-7: Crosshead die for wire coating (from Buckmaster et al., 1997)  The molten polymer enter the port 11 to the die 2. Then the polymer is guided through to orifice 8 by the wire guide 16. The interior and the exterior surface of the wire tubular shape is formed by the passage 24 and 4 respectively. The wire guide provides a channel 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. By using this crosshead, 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 =  0.25(D-d) 0.5n(D+d) 2  (2-11a)  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  CHAPTER 2- LITERATURE REVIEW  22  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS assumption of a power-law fluid (Bird, R.B et a l , 1987). The general formula for the shear stress distribution is:  (2-1 lb)  where x is the shear stress at radius r, AP is the pressure drop, L is the length of n  the die land, and /} is the parameter depending on the geometry and the power law index.  2.3.2  Parallel Plate Rheometer The parallel plate rheometer is used primarily for the measurement of linear  viscoelastic properties. The subjection of a material to a sinusoidal shear history and measurement of the stress response is well established as a very useful tool in order to obtain information about the rheological behavior of this material. In this work, a Rheometrics System IV parallel-plate rheometer was used to characterize the linear viscoelastic behavior of the materials. The upper and lower plate are mounted on the same axis of symmetry. The upper plate is rotated with a certain angular speed co(t), and as a result, the sample is subject to shear. The type of deformation used is small amplitude oscillatory shear. The strain as a function of time is given by: y(0 = 7oSin(ar)  (2-12a)  where y is the strain amplitude and GO is the frequency. 0  This small amplitude oscillatory shear test experiment is essentially used to determine the linear viscoelastic properties of polymeric material. If the strain amplitude is too small, then the shear stress is sinusoidal in time and independent of the magnitude of strain. Thus it can be written as:  CHAPTER 2- LITERATURE REVIEW  23  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  o-(f) = o- sin(a* + 5) 0  (2-12b)  where Ob is the stress amplitude and 5 is the phase shift, or the mechanical loss angle.  transducer Figure 2-8: Parallel plate rheometer  By using the trigonometric identity, the shear stress can be written as: <*($) = 7o [G'(P>) sin(fi*) + G"(co) cosffot)]  (2-13)  where the quantity G'(«a) is the storage modulus and G"(o>) is the loss modulus. These two quantities can be calculated from the amplitude ratio and phase shift.  G' = G cos(S)  (2-14)  G" = G sin(8)  (2-15)  d  d  where G = o" /y is the amplitude ratio . From the storage and the loss modulus, one d  0  0  can calculate the complex modulus, G*(co), which is given as: G*((») = G'(fl))+/G"(fl))  (2-16)  Also, the shear stress can be expressed as follow:  CHAPTER 2- LITERATURE REVIEW  24  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS c ( 0 = Yo[l\ ) °K(ot) a c  + Tj\a)sm{at)]  (2-17)  where the complex viscosity can be expressed as: T;* (o) = j]'(a>)-ir]'(a>)  (2-18)  n' = G"/co  (2-19)  t]"=G'/co  (2-20)  where n' is the dynamic viscosity, 77" is 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:  (2-21)  CHAPTER 2- LITERATURE REVIEW  25  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  2.3.3  Sliding Plate Rheometer The sliding plate rheometer generates shear deformations by the linear motion of  one flat plate relative to another. It has certain advantages over the use of rotational flows. Edge failure is a less problem in this geometry so that higher shear rates can be reached for polymer melts. The type of flow generated by such rheometer is called simple shear. A thin layer of fluid is placed between the two flat plates, where the upper plate is translated over the lower by a constant velocity. A simple schematic of this is shown in Figure 2-9.  Wetted area, A  Figure 2-9: Simple schematic of simple shear flow utilized by a sliding plate rheometer.  When the upper plate is translated by a certain displacement Ax, the shear strain can be defined by:  Y=Ax/h  (2-22)  where h is the gap spacing between the two plates  CHAPTER 2- LITERATURE REVIEW  26  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS By differentiating this with respect to time and defining d(zlx)/dt as u, the velocity of the upper place, then one can define the nominal shear rate, y„, as equal to u/h. The shear stress can also be calculated, if one measures the force required to shear the upper plate, as o~ = F / A where F is the pulling force and A is the wetted area.  If one assumes that the no-slip boundary assumption is valid, then the actual shear rate, y, is equal to the nominal shear rate , y„. When slip is present, the actual shear rate is less than the nominal shear rate and these two cases are shown schematically in figure 2-10.  U  s  Figure 2-10: a) Simple shearflowwithout slip, b) and with slip occurs.  In fact y and f under slip condition are related through the following expression n  y„=y  + 2u /h s  (2-23)  where u is the slip velocity. s  CHAPTER 2- LITERATURE REVIEW  27  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  Moving plate  Beam  Sample  Figure 2-11: Sliding plate rheometer with a flush-mounted shear stress transducer.  For this work a commercial sliding plate rheometer with a flush-mounted shear stress transducer was used (Interlaken). The basic features of the transducer are shown in figure 2-11. A n end plate is acted on by the shear stress generated by the fluid and transmits the resulting moment to the cantilever beam. To avoid melt penetration into the gap around the end plate, the deflection of the latter must be limited to very small levels. That is why a capacitance system was used, where a capacitor is formed by the probe acting as one of the plates, and the beam as the second plate. The shear stress is measured by means of calibrating the transducer by hanging known weight and measuring the displacement. Thus, in real experiments beam deflection is transformed directly into stress.  CHAPTER 2- LITERATURE REVIEW  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESS ABILITY OF MOLTEN POLYMERS 2.3.4  Flow Visualization A new apparatus was designed during the course of this study to visualize the  flow patterns developed at the entrance of the capillary i.e. going from a large reservior to a fine capillary die. Figure 2-12 shows a schematic diagram of the flow visualization experiment.  Figure 2-12: The schematic diagram of the flow visualization set up.  This new apparatus consists of the following parts: •  Helium-Neon laser having output power 10 mW, with a spatial filter and collimator with a focal length of 200 mm;  •  Laser beam chopper, a D C motor with a plastic disc having 5 segments;  •  Cylindrical lens, focal length 150 mm;  •  Sphericalfocusinglens, focal length 600 mm.  CHAPTER 2- LITERATURE REVIEW  29  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  •  Quartz capillary, 1.5 mm diameter, encased in a steel holder, surrounded by 4 radiation heaters;  •  Microscope (Nikon SMZ-2T) and a Nikon FM-2 photographic camera.  Figure 2-13 also shows the equipment set-up for the laser and a beam forming system. The total length of the metal optical support rail is 2 meters. The collimated laser beam is spread into a plane by means of a cylindrical lens and subsequently focused by a spherical lens onto the center axis of the capillary. The resulting thickness of the laser sheet crossing the capillary did not exceed 0.1 mm.  Figure 2-13: The laser and a beam forming system.  Figure 2-14 shows the position of quartz capillary die surrounded by four radiation heaters. Figure 2-15 shows the lower part of the rheometer, microscope and camera.  CHAPTER 2- LITERATURE REVIEW  30  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  Figure 2-14: Photograph of the quartz capillary and four radiation heaters surrounding the die.  Figure 2-15: Photograph of the rheometer, microscope and the camera.  CHAPTER 2- LITERATURE REVIEW  31  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS The heaters were insulated in all sides except the side facing the quartz die. There was also a thermocouple attached to the bottom of the quartz capillary die to control the temperature. Figure 2-15, it shows the combination of the capillary with the microscope. The laser beam should be turned on at all times during video recording and picturing. The quartz die was made by Precision Glass Products and the schematic diagram is shown in figure 2-16. The diameter of the capillary is 1.5 mm and the L / D ratio is 7.5. This quartz capillary die allows observation of the flow at the shear stresses up to 0.4 MPa and temperatures up to 250 °C  015  Figure 2-16: The schematic diagram of the quartz capillary die.  2.3.5  Blow Molding Machine Extrusion blow moulding is one of the main processes in the plastic industry. The  Battenfeld/Fisher 50 mm extrusion blow moulding machine was employed in our experiments in order to examine the effect of B N on the processability of blow moulding  CHAPTER 2- LITERATURE REVIEW  32  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS high-density polyethylene. It mainly consists of an extruder and blow moulder parts. The processing step can be simply summarized as follows:  •  The extruder plasticises and homogenizes the raw plastic material.  •  The parison head shapes the plasticised material into a perform (parison) that  must be suitable for the end product. •  The blow moulder receives a parison by the mould and form the article.  •  The post operating unit trims the article from flash and release the product  completely finished orientated out of the machine.  The blow moulding machine forms hollow parts and containers from plastic. The formation of a hollow tube, so called "Parison" and the mold itself is a hollow cavity in the shape of the part. Extrusion of the parison is controlled by a heated hollowed barrel in which a rotating screw conveys solid feed material, compresses and melts it, and finally pumps the melt through the tubing die to form a parison. The actual screw shape is dependent on the rheological characteristics of the plastic melt. Air is introduced into the parison by the blow pin and blow the parison into the hollow cavity mold. Figure 2-17 shows a schematic diagram for the extrusion blow moulding machine.  CHAPTER 2- LITERATURE REVIEW  33  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  HEAD PLASTIC PELLETS  Plastic pellets are introduced to the extruder. As they travel through the barrel, heat generated from external heater bands PARISON melt the plastic. This molten plastic is then forced through a head 4m wtiichwill make a hollow tub e.  Figure 2-17:  A schematic diagram of an extrusion blow moulding machine.  The material feed section has deep conical grooves on the inside circumference of the barrel. The grooves are getting shallower toward the feed section of the screw. The extrusion screw transports the polymer from the extruder feed zone through the diehead. Extrusion screws is most important part of the extruder. It have three basic sections. 1. feed section. 2. transition section. 3. metering section. In the feed section, the material is taken in and compacted in conjunction with the feeding grooves at the rear of the extruder barrel. It supplies the compression section with a constant polymer flow. In the transition, friction heat is generated. The material is molten and plasticized. Any entrapped air from the feed zone will escape backwards. The metering section mixes and homogenizes the polymer. At the screw tip, the material should be completely homogeneous and ready for the next processing step in the diehead.  CHAPTER 2- LITERATURE REVIEW  34  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  The thickness of the blow molded part is determined by the thickness of the parison, which, in turn, is determined by the length of the die opening and the extrusion speed. After the material passes through the diverging die, it forms a parison and the final desired product can be made. The procedure to produce the final desired blow mould product in the blow molder part can be summarized in figure 2-18.  Figure 2-18 :  The procedure to produce the final desired blow mould product  •  step 1 : The mould and the blow pin are in the original form.  •  step 2 : The parison comes out from the extruder.  CHAPTER 2- LITERATURE REVIEW  35  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS •  step 3 : The mould close and the swing knife move at a high speed in order to  cut the parison. The mould is cooled by water and this assist the plastic to hold the shape of the mould. •  step 4 : The parison is blowed by injection of air through the blow pin. The  plastic is released until the bottle shape was formed.  Figure 2-18b shows the diagram of annular geometry die and mandrel of the blow moulding unit. The inside diameter of the die is 3.8 cm and the diameter of the mandrel is 3.5cm.  Direction of melt flow  Die  Mandrel  Round Die/mandrel assembly Figure 2-18b:  The diagram of die and mandrel of the blow moulding unit.  Figure 2-18c shows the front view of the cross section of blow moulding die and mandrel. The dimension of the die and mandrel are shown in the figure.  CHAPTER 2- LITERATURE REVIEW  36  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  Direction of melt flow  1  i  Zo=3.5cm M  •  Zo=3.8cm Figure 2-18c:  The diagram of the cross section of blow moulding die and mandrel.  In blow moulding extrusion, a die having an L / D ratio of 20 is used as a standard . If one wishes to increase the output rate, a 24:1 ratio can also be employed. The L / D ratio is the screw length divided by its diameter. Screws to process different materials have also different compression ratios. The compression ratio is generally referred to as the depth of the feeding zone flight compared to the depth of the metering zone flight. Blow moulding is an effective way to process hollow parts, and it involves relatively low tooling costs. However, the process requires long cycle times, secondary trimming, and requires high start-up cost.  CHAPTER 2- LITERATURE REVIEW  37  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS 2.4  Melt Fracture and the Flow Curve of Polymers Melt fracture is a major problem in the extrusion of polyolefins and many other  polymeric materials. As originally proposed by Tordella, the transition from a smooth laminar flow to the melt fracture region involves the rupture of streamlines and this is how the term "melt fracture" was originated. Melt fracture depends on several operational and geometric factors. These include the die geometry, the polymer structure and its molecular characteristic, the temperature, and the types and concentration of additives. Melt fracture always appears in extrusion, when the flow rate or shear stress exceeds a critical value [Ramamurty (1986)]. The melt fracture behavior of most commercially significant polymers has been widely investigated by researchers. However, most of the literature is biased towards the polyethylene. This is expectable, since P E exhibits all related phenomena, ranging from loss of gloss to gross melt fracture. Other polymers either show fewer types of distortions or do not show melt fracture at all during processing. For L L D P E , the first appearance of surface roughness known as sharkskin or surface melt fracture is followed by the occurrence of gross melt fracture at the wall shear stress of about 0.3-0.4 MPa. The flow instabilities of polymeric liquids through capillary, slit and annular dies generally known as melt fracture, can also be reflected in the apparent flow curve. Such a curve can typically be determined by means of a capillary rheometer. It is essentially a log-log plot of the wall shear stress, <j , as a function of the apparent shear rate, w  CHAPTER 2- LITERATURE REVIEW  y. A  38  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  A typical flow curve for a linear polymer such as high-density polyethylene and linear low-density polyethylene is shown in figure 2-19. Similar flow curves have been determined in the capillary extrusion of many linear polymers such as high-density and linear low-density polyethylene (Kalika and Denn, (Tordella, 1969), polybutadiene (Vinogradov et al,  1987), polytetrafluoroethylene 1972b), and others. In the first  region, the shear stress is directly proportional to the shear rate to some power n (Eq. 26). The viscosity of the polymer follows such a relation in this region. The extrudate is smooth and the no-slip boundary condition can be assumed to be valid. When the wall shear stress exceeds a critical shear stress value, o i , (region 2) small amplitude periodic c  distortions appear on the surface of the extrudate. This phenomenon is known as "sharkskin" or surface melt fracture. Consequently, when the wall shear stress exceeds the second critical point, a 2, (region 3) the flow ceases to be stable. Instead, pressure C  drop along the die fluctuates between two extreme values. In other words, the flow curve in this region is never stable. The appearance of the extrudate in this flow region is  CHAPTER 2- LITERATURE REVIEW  39  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS referred to as "stick-slip" or oscillating melt fracture. It consists of smooth and melt fracture parts that alternate periodically. The smooth portion of the extrudate corresponds to the descending part of the flow curve and the melt fractured portion corresponds to the ascending one. At higher throughput value (region 4) or wall shear stress values, there is sometimes a transition to a second stable flow regime, in which the extrudate becomes again smooth. This is known as the superextrusion region although not in all polymers such a flow region has been observed to exist. Finally, at even higher shear rate values, there is a transition to a wavy chaotic distortion (gross meltfracture),which gradually becomes more severe with increase of the apparent shear rate, y  A  2.4.1  (region 5).  Surface Melt Fracture Many researchers reported the critical wall shear stress value at the onset of  surface distortion (surface melt fracture). Table 2-1 summarizes representative results: Table 2-1: Critical wall shear stress values at the onset of surface melt fracture of various types of polymer. Authors  a (MPa)  Polymer  Temperature (°C)  Herranen and Savolaninen  0.35  LLDPE  237 and 267  Ramamurthy  0.14  LLDPE  160 and 260  Tordella  0.15  HDPE  150  Kalika and Derm  0.26  LLDPE  215  Hatzikiriakos and Dealy  0.09  HDPE  180  Bartos  0.08  PP  200-260  Bartos  0.07-0.11  LDPE  125-225  Vinogradov et al  0.03-0.097  PP  180-240  Rosenbaum  0.18  TFE-HFP  300-350  CHAPTER 2- LITERATURE REVIEW  c  40  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS Surface melt fracture has been reported to be affected by various parameters. Some of the main parameters are discussed below and are classified as operational, geometrical, surface and chemical structure parameters. Operational Parameters: Kurtz (1992) has reported that the distortion severity increases with increase of the shear rate. The amplitude of the distortion increases linearly with the wall shear stress. Moreover, he reported that both sharkskin frequency and period increase with increase of the flow rate. Furthermore, the severity of distortion decrease and the onset of surface meltfractureis delayed with increase of temperature. Geometrical parameters: In general, the length to diameter ratio of the die is found to be independent of the critical wall shear stress. However, Moynihan (1990) has shown that the critical flow rate decreases with decrease of the die length. Venet (1996) also reported that the surface distortion is more intense in longer dies. Constantin (1984) has found that the wall shear stress is independent of the diameter of the die for LLDPE, but increases with increase of die diameter for HDPE. Dennison (1967) and Kurtz (1984) have shown that there is no effect of the die entry angle on the onset and development of surface distortion. Piau et al (1990) reported that there is a possible die exit shape effect on the critical condition for the onset of surface melt fracture. Surface parameter Ramamurty (1986) and Kurtz (1992) have reported that there is a significant influence of die composition (die material) both on the onset and development of surface melt fracture. However, some researchers have found no influence. The difference may  CHAPTER 2- LITERATURE REVIEW  41  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS be due to the insufficient duration of the flow (induction time). In general, one can say that the extrusion process must be long enough for the equilibrium of chemical exchanges between wall and polymer to be established. Kissi and Piau (1995) promoted polymer slip at the wall by addition of small amounts of fluoropolymers into the resins. Chemical structure of polymer Many researchers have reported that the number and the length of branches of the polymer molecules have strong effects on the development of the instability. For example, the surface distortion of L L D P E is stronger than those of H D P E and PP at certain flow rates. The critical shear stress for the onset of surface distortion for L L D P E is lower than those of HDPE. Karbashewski (1995) found that an increase in the number of short branches (comonomer content) in a linear polymer reduces the degree of sharkskin. Moreover, comonomer type and composition also affect the flow curve and the melt fracture behavior. Venet (1996) found that polymers, which exhibit less strain hardening, result in sharkskin at smaller wall shear values.  2.4.2  Gross melt fracture  Table 2-2 summarizes some of the finding reported by various researchers for critical shear stress values for the onset of gross melt fracture. Gross meltfractureis also affected by various parameters. Some of the main ones are discussed below and are classified again as operational, geometrical and surface parameters.  CHAPTER 2- LITERATURE REVIEW  42  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  Table 2-2:  Critical wall shear stress for the onset of gross melt fracture.  Authors  a (MPa) c  Molecular  Polymer  T(°C)  characteristic Utracki and Gendron  0.75  MW=126k  HDPE  190  Ramamurthy  0.435  MFI= 1  LLDPE  220  MW=114,000 El Kissi and Piau  0.25  MW=143,000  LLDPE  190  El Kissi and Piau  0.06  MW=83 5,000  PDMS  23  E l Kissi and Piau  0.06  MW=1,840,000  PDMS  23  Vinogradov et al  0.36  MW=102,000  PB  22  to  580,000  Operational parameters: Uhland (1979) reported that an increase of the temperature reduces the spurt flow rate range for HDPE. Ramamurthy (1986) has also concluded that for L L D P E in the temperature range between 160°C and 260°C, the critical shear stress is constant. Geometrical parameters: Ballenger and Chen (1971) have reported that the spurt zone depends heavily on the die geometry. They have also suggested that the spurt flow rate range increases with increase of the L/D ratio. An orifice die, therefore, makes it totally disappear. Tordella (1963) found that the die entry angle has no effect on the spurt flow for LDPE. Also, the frequency of the volume distortion was three times as high for conical entry dies (60°)  CHAPTER 2- LITERATURE REVIEW  43  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS compared to the flat entry die. Although the amplitude is reduced, the critical shear rate does not change. Surface parameter Ramamurthy (1986) noticed that the die material has an effect on the critical shear stress for the volume distortion. For L L D P E , the shear stress varies between 0.39 MPa for aluminum to 0.43 for carbon steel and bronze. Lim (1970), Piau and Kissi (1995) have reported that coating the die with PTFE makes the spurt oscillations disappear. Chemical structure of polymer The flow curve discontinuity can be reduced by reducing the molecular weight of the polymer or boarding the MWD. A lower Mw shifts the spurt zone to higher flow rates. Before the oscillation zone, the flow curve is sensitive to the molecular parameter. The pressure levels decrease with decrease of the Mw. In the gross meltfractureregion, the flow curve is insensitive to the Mw and M W D .  2.5 Explanation of Melt Fracture Phenomena Several theories have been proposed to explain the melt fracture phenomenon in the literature. In spite of this, no single satisfactory theory exists that provides a reasonable explanation for all observed phenomena. In this section, a summary of some of these theories will be presented. It should be noted that theories which are most relevant to the present work will only be discussed.  CHAPTER 2- LITERATURE REVIEW  44  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS 2.5.1  Die Exit Effect: Sharkskin Melt Fracture  Surface distortions occur under stable flow conditions. There is no measurable barrel instability and the polymer flow is laminar in the die. High tensile stress may created at the exit of the die. The surface of the melt accelerates from velocity zero inside the die to the extrusion velocity outside as shown in figure 2-20. If the developed elongational stress exceeds a critical value, rupture occurs. As a result, a loss of gloss of extrudate (sharkskin) or its more severe form of small amplitude periodic distortion will appear on its surface.  This phenomenon could be avoided if the material is able of  responding elastically so that the skin can stretch and the stress relax becoming less than the critical value for rupture. Therefore, a low elastic modulus and low viscosity material can promote the stress relaxation. Such materials are low molecular weight polymer or polymers with long branches in their backbone. In these types of polymers, sharkskin is never observed.  >  > Rapid stretching of surface Figure 2-20: The surface velocity of the melt accelerate from zero inside the die to the extrusion velocity outside  2.5.2  Flow instability in the die land: sharkskin Weill (1980) has explained the surface and gross distortions with relaxation  oscillation. In the case of surface distortion, the relaxation oscillation is initiated at the die  CHAPTER 2- LITERATURE REVIEW  45  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS entrance and propagate towards the exit where the crack on the extrudate skin appears. There is no variation of pressure in the entrance of the die during the surface melt fracture. Tordella (1963) observed the instability in the die wall but not the converging entrance part during the surface meltfractureregion by using the birefringence technique. He suggested that this observation is a rupture of streamlines by strong elongation (limit of elastic deformation and entanglement of molecules at the wall) and local slip. In spite of the experimental observation, it is still not clear how to relate the flow instability to the surface melt fracture. Therefore, use of the concept of flow instability in relation to melt fracture should be considered with great care.  2.5.3  Die Entrance Effect: Gross melt fracture Many researchers have reported that there is some relation between the  converging flow at the die entry and flow instabilities that manifest itself as gross distortion on the surface of extrudates (Leonov and Prokunin, 1994). They suggested that the region upstream of the contraction is the site of initiation of gross melt fracture type of instability. Vinogradov and Malkin (1980) used the flow birefringence technique for observing and investigating the flow of polymer. They investigated the appearance of the melt fracture before and after the critical regime. Bagley and Schreiber (1961) gave out an explanation that the melt fracture of polymer melts is due to the elongational stresses in the entry region of the die. However, White (1973) gave out a different explanation. He suggested that the hydrodynamic instability was initiated in the form of a spiral flow when the critical Weissenberg number (We=Xv/5, where X is the characteristic relaxation time, v is the relative velocity, 8 is the spacing) was reached.  CHAPTER 2- LITERATURE REVIEW  46  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  The melt fracture mechanism is not completely understood yet and it seems to depend on various parameters, such as the polymer rheology, thermal effect and the die entry geometry. Piau et al (1990) used a visualization technique and Tordella (1969) used a birefringence technique to confirm that above a certain extrusion rate, the flow upstream of the contraction becomes unstable. These instabilities occur in the form of sudden pulsations before the flow entering the contraction. They have shown that the instabilities start along the upstream flow axis due to the high elongation stresses that are developed in this area.  2.5.4  Wall slip It has been commonly observed that the melt loses its adhesion to the wall once  the wall shear stress exceeds a critical value, a . At such high shear stress values, the noc  slip boundary condition is no longer valid. Wall slip of molten polymer has been reported by many researchers. Mooney and Black (1952) were the first to study the slip phenomenon. They used capillaries with various radii to determine the flow curve of raw rubber. They found that the slope of the flow curve depends on the radii once shear stress exceeds the critical value as shown in figure 2-21.  D >D > £> / ^ O ^ X  o  w  = const  2  3  <t > yAI y AI yA?, ?A Figure 2-21: The flow curve of polymers and rubbers depends on the radii once the wall shear stress exceeds the critical condition.  CHAPTER 2- LITERATURE REVIEW  47  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABELITY OF MOLTEN POLYMERS  Moreover, Lim and Schowalter(1989)  have studied the slip behavior of  polybutadiene melts by using a probe to measure heat transfer from the melt to the wall under slip and no slip conditions. Kalika and Denn (1987) reported that the flow curve obtained for various polyethylenes is discontinuous and that the slope of the flow curve changes beyond a critical wall shear stress value. This change of slope has been reported to correspond to the occurrence of the surface melt fracture and therefore it has been related to the onset of slip. Ramamurty (1986) observed that the onset of slip also depends on the material of the capillary die and that improving adhesion to the wall (decrease slip) improves extrudate distortion. Many other authors have proposed the relation between sharkskin and wall slip. However, Rudin et al (1985) has reported that the addition of a fluoropolymer to the resin eliminates the surface melt fracture by promoting slip. Hatzikiriakos and Dealy (1992a,b) also measured the effect of two fluoropolymer on the slip velocity of HDPE. One of the fluoropolymers increased the slip velocity, while the other was decreased it, although both eliminated the occurrence of surface defects. Therefore, it can be shown that wall slip is not the only contributor to the cause of surface melt fracture. Mooney (1931) has derived an expression for determining the slip velocity as a function of wall shear stress. He has assumed that the slip velocity, wall shear stress and pressure gradient are all constant along the length of capillary die. The expression for the case of circular channels is as follows: 7 a =f , +8^A  CHAPTER 2- LITERATURE REVIEW  s  (2-24)  48  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  where y  A  is the apparent shear rate, y  Aj  is the apparent shear rate corrected for the effect  of slip and u is the slip velocity. For a given shear stress, the apparent shear rate is s  linearly proportional to 1/D with a slope equal to 8u . Ramamurthy (1986) has reported a s  set of such lines for linear low-density polyethylene for different wall shear stress values. The slope of each line was set equal to 8u . Therefore, by using capillary dies having s  different radii, slip velocity can be determined as a function of wall shear stress. However, this Mooney technique to measure the slip velocity is indirect and does not account for the viscous heating effects that can be significant. (Rosenbaum et al., 1998)  2.6  Factors Affecting Polymer Flow  There are many factors affecting polymer flow, Some of the most significant ones will be discussed in this section.  2.6.1 The  Time Temperature Superposition: Temperature Effect rheological  properties of molten polymer are highly  depended  on  temperature. Therefore, experiments must be carried out at several temperatures in order to get a complete picture of their rheological behavior. A single master curve can be obtained by shifting the data taken at several temperatures together. This greatly simplifies the description of the temperature effect. Besides, the single master curve can cover a broader range of frequency or time scales compared to the range covered by data obtained at a single temperature. A shift factor, aj, can be calculated in order to superpose the data from different temperatures at a reference one. The amount of shifting on each curve represents the shift factor at that particular temperature. When the temperature, T, is much higher than the  CHAPTER 2- LITERATURE REVIEW  49  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  glass transition temperature, T , usually T +100 K, the shift factors can be represented by g  g  an Arrhenius equation, as follows  a = exp T  where  E  a  R [T  is the activation energy of flow,  T„ )\  (2-25)  f  T f re  is the reference temperature and R is the  universal gas constant. If the activation energy increases, then the viscosity becomes more temperature dependent, as one approaches T . As mentioned, the above equation is g  only valid for temperatures at least 100 K above T . If the temperature is close to glass g  transition temperature, then the W L F equation should be used: -Cl(r-7b)  Z,og(a ) = [C2+(T-To)]  (2-26)  T  where Cj and C are the universal constants and can be determined at T for each 2  a  polymeric material.  2.6.2  Pressure Effect  The presence of large pressure gradients are typical in the processing of molten polymers. The compressibility of these materials in a molten state is not negligible, and the effect of pressure on the viscosity and other rheological material functions cannot be neglected. Rauwendaal and Fernandez (1985) and Kalika and Denn (1987) reported that the apparent flow curves do not superpose for capillaries of different L/D ratios. The apparent flow curves shift to higher values of the wall shear stress, pressure, with increase of the L/D ratio. The pressure dependence of viscosity can be written as follow: 77 = 77 exp(aP) 0  CHAPTER 2- LITERATURE REVIEW  ( 7  _ ^ 7 7  50  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  where a is the pressure coefficient of viscosity, if is the viscosity at ambient pressure, and P is the absolute pressure. It has also been proven that pressure has an effect on the slip velocity. Hatzikiriakos and Dealy (1992a) studied the slip behavior of several high-density polyethylene blends at various pressures and temperatures. They have found that the slip velocity decreases with increase in pressure and this effect saturates at very high pressure values. Therefore, as the pressure drops along the capillary, the slip velocity increases and the fluid accelerates towards the exit of the capillary. This gives rise to a high extensional rate, which may be the primary cause of the surface melt fracture [Hatzikiriakos, 1994].  2.6.3  Viscous heating  Most polymer processing operations give rise to the generation of high deformation rates. Viscous heating cannot be neglected for such high shear rates. Due to the low thermal conductivity of polymer, temperature increases are considerable nonuniform within a flowing polymer melt. Shidara and Denn (1993) have discussed the effect of viscous heating for a molten polystyrene in slit extrusion. To explain their results they assessed this effect to be significant. They pointed out that a numerical solution of the full field in capillary/slit flow incorporating pressure and temperature effects is needed. Cox and Macosco (1974) observed large temperature rises in capillary extrusion of acrylonitrile butadiene styrene (ABS), which can be as high as 70 K for certain apparent shear rates.  CHAPTER 2- LITERATURE REVIEW  51  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS In order to assess viscous heating effects, it is necessary to assume appropriate boundary conditions at the wall of the capillary. Two limiting cases are usually considered. In the isothermal case, the wall is assumed to be at a uniform temperature. In the adiabatic case, it is assumed that there is no heat transfer to the wall. In the isothermal case, the temperature profile asymptotically reaches a fully developed profile, while in the adiabatic case a continuous, infinite temperature rise is predicted for a infinitely long capillary. The real condition is between these limiting cases. It is also important to note that, according to these solutions, the temperature rise is higher for longer capillaries and for those having a larger diameter. Thus, length and height of slits or length and diameter of capillaries are important parameters. If one suspects that viscous heating effects are important, then correction factors should be used. For this, see Rosenbaum and Hatzikiriakos (1995) and references therein to appropriately assess the viscous heating effects.  CHAPTER 2- LITERATURE REVIEW  52  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  3 Objectives The primary objective of this work is a comprehensive study of the effect of boron nitride (BN) type and concentration on the rheology and processability of molten polymers. In particular, the following goals are targeted and these can be summarized as follows:  •  To conduct a thorough rheological characterization of a metallocene linear-low density polyethylene (m-LLDPE) and a Teflon FEP® 4100 resin with and without the addition of boron nitride as a function of temperature, pressure, boron nitride type and composition.  •  To determine the critical condition for the onset of melt fracture of the mL L D P E with addition of B N as a function of temperature, additive type and composition.  •  To study the processability of the m-LLDPE with the use of various boron nitride powders in extrusion using a crosshead die that mimics the wire coating process.  •  To study the effect of presence of B N on the processability of two blow moulding HDPE grades in the blow molding operation.  •  To understand the mechanism by which B N eliminates gross melt fracture. To this direction the visualization experiments and the imaging of the flow pattern development in the die entrance will be very helpful.  CHAPTER 3 - OBJECTIVE  53  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  4  Materials and Blending Methods The objective of this chapter is to present the characteristics of the various types of  polymers and B N powders used to carry out the work. The different preparation methods of the polymer blends are also discussed. 4.1  Polymers Studied  Three resins were studied in this work in order to assess the effect of various B N powders in their processability in continuous extrusion. These are the metallocene catalyzed polyethylene Exact® 3128 (Exxon), a polypropylene resin and a DuPont fluorocopolymer of tetrafluoroethylene/hexafluoropropylene (Teflon® FEP 4100). The chemical structure of polyethylene, polypropylene and Teflon® FEP 4100 are shown in figure 4-1 to 4-3.  H  H  — c  H  — c — c — c  H  H  H  H  polyethylene  Figure 4.1  H  \  •H  Figure 4.2  H  /  Ff  Ff I  H I ,  H  CH  ;c=c  CFf  3  3  Polypropylene  CHAPTER 4 - MATERIALS AND BLENDING METHODS  54  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  CF  2  CF  2  CF  2  CF— CF  Figure 4.3  3  DuPont fluoro-copolymer of tetrafluoroethylene/hexafluoropropylene (Teflon® FEP)  Besides these three resins, two types of high density polyethylene resins were also empolyed in our studies to assess the effect of B N in the extrusion blow moulding. These are labeled as Resin A and Resin B, which are produced by Petromont company, Montreal, Quebec. Some of the chemical and physical properties of resins A and B are listed below in Table 4.1. Table 4.1:  The chemical and physical properties of Resin A and Resin B.  Properties  Typical values, Resin A  Typical values, Resin B  melt index  0.08dg/min  0.35 dg/min  flow index  10 dg/min  35 dg/min  density  0.948 g/cm  processing temperature  190 °C  185 °C  tensile strength at yield  24.1 MPa  26.9 MPa  melting point  140 °C  130 °C  3  CHAPTER 4 - MATERIALS AND BLENDING METHODS  0.953 g/cm  3  55  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS Resin A is a high molecular weight high density polyethylene resin with a broad molecular weight distribution produced using the Union Carbide's TJNTPOL® process. This resin is designed for use in the blow moulding of large parts such as shipping containers in sizes from 5 to 50 gallons. It is also used in automotive fuel tanks, jerrycans, floating docks, etc. Resin B is a high density polyethylene resin with an intermediate molecular weight distribution that is also produced using Union Carbide's UNTPOL® process. This resin is intended primarily for use in intermittent high shear rate blow moulding equipment designed for high speed production of blow moulded containers.  4.2  Types of Boron Nitride  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, B N 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 4-2 shows the comparison of boron nitride powder to other common fillers.  CHAPTER 4 - MATERIALS AND BLENDING METHODS  56  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS Table 4-2:Comparison of boron nitride to common fillers BN  AL2O3  ALN  Thermal Conductivity (W/m/K)  -300  40  50-170  Dielectric Constant  4  9  9  "2.5 Key • Boron O NKrogsn  A'  Figure 4-4: The structure of boron nitride  Figure 4-4 shows a typical structure for BN. Each boron atom is connected to four nitrogen atoms, and each nitrogen atom is connected to four boron atoms. The structure of B N is similar to that of graphite. In this study, we have examined eight types of BN. The average particle size and its state of agglomeration of each type are summarized in table 43. These include B N type CTF5 with particle size of 5-10 urn, CTL40 which is essentially an agglomerated version of CTF5 with particle size of more than 40 urn, and C T U F which contains a fair amount of B2O3 compared to CTF5. Among all B N used, only CTUF, BN427 and B N 429 exhibit agglomeration. On the other hand B N 428, B N 431 and CTF5 have the smallest particle size and are free of any agglomerated particles. Pictures taken under microscope to determine the degree of dispersion of CTF5 and CTL40 into the m-PE  CHAPTER 4 - MATERIALS AND BLENDING METHODS  57  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS are shown in figure 4-5. S E M pictures were also taken for each type of B N as shown in figure 4-6. These pictures allow us to get useful information on the average particle size and whether or not agglomerated particles are present.  Table 4-3: The average particle sizes and states of agglomeration properties of various boron nitride powder. particle size from S E M , approx. (urn) agglomerated agglomerated size (urn) Ctl40 40 no ctf5 10 no ctuf 3 yes >50 427 4 yes >180 428 1.5 no 3 429 yes >300 430 20 no 431 5 no  a) Good dispersion of CTF5 into m-PE  b) Poor dispersion of CTL40 into m-PE  Figure 4-5: Photos of the BN particles (white) dispersed into PE (black). The magnification is 320 times in these pictures. The particle size is 5-10 urn for CTF5 and more than 40 urn for CTL40. It can also be seen that CTF5 is well dispersed (a) as opposed to the CTL40 that exhibits a certain degree of agglomeration (b).  CHAPTER 4 - MATERIALS AND BLENDING METHODS  58  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  Figure 4-6: SEM picture of various BN powders (200 = 200 times magnification, lk = 1000 times and 6k = 6000 times). It can be seen that 428 is a uniform powder having no agglomerated particle. On the other hand, agglomerated particles of BN 429 can be as large as 370 urn.  CHAPTER 4 - MATERIALS AND BLENDING METHODS  59  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  CTUF-lk  Figure 4-6 (continued): SEM picture of various BN (200 = 200 times magnification, lk - 1000 times and 6k = 6000 times). It can be seen that 431 & CTF5 have uniform particle sizes including no agglomerated particle as opposed to CTUF.  CHAPTER 4 - MATERIALS AND BLENDING METHODS  60  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  4.3  Polymer Blends  Three techniques were used to introduce B N into the resins. According to the first technique, the B N particles in a finely divided state were thoroughly dry-mixed with the resin pellets at appropriate concentrations without using any extrusion equipment. According to the second technique, the pure resin pellets were first ground into a fine powder form by using a grinder. Then, a masterbatch of 10 wt % B N with ground mL L D P E was prepared by using a preparation mixer. A desired final concentration of materials were obtained by mixing the pure ground m-LLDPE with the masterbatch by means of a %" single screw extruder equipped with a cooling system. The final material was chopped and collected in the pelletized form by using a 2" pelletizer. According to the third technique, a masterbatch of 10% B N with ground m-LLDPE was also prepared. However, a twin screw extruder was used to blend the B N with metallocene catalyzed polyethylene Exact® 3128 instead of the single screw extruder this time. Using these techniques mentioned above, one can obtain various levels of B N dispersion into the polymer matrix. The blends which were prepared and studied in this work are listed in Table 4-4 along with their composition..  CHAPTER 4 - MATERIALS AND BLENDING METHODS  61  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS Table 4-4: Blend (at  Summary of the various blends prepared together with the blending methods used. 200,  500  Dry-mixed  and lOOOppm)  10% Masterbatch + single  10% Masterbatch + twin  screw extruder  screw extruder  m-PE + CTUF  V  m-PE + CTL40  V  m-PE + CTF5  V  m-PE+ 427 to 431  Resins A and B  +  CTF5  CHAPTER 4 - MATERIALS AND BLENDING METHODS  62  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  5  Results and Discussion  5.1  Introduction It is well known that the rate of production of many polymer processing  operations including fiber spinning, film blowing, extrusion, and various coating flows, is limited by the onset of flow instabilities (Petrie and Denn, 1976; Larson, 1992). In particular in extrusion processes when the throughput exceeds a critical value, small amplitude periodic distortions appear on the surface of extrudates (surface melt fracture or sharkskin) and at higher throughput rates these take a more severe form of larger irregular distortions (gross meltfracture)(Tordella, 1969). To increase the rate of production by eliminating or postponing the melt fracture phenomena to higher shear rates, processing additives/aids must be used. These are mainly fluoropolymers that are widely used in the processing of polyolefins (HDPE, LLDPE) and other commodity polymers. They are added to the base polymer at low concentrations (approximately 0.1%), and they essentially  act as die lubricants,  modifying the properties of the polymer-wall interface (increasing slip of the molten polymers). As a result of this lubrication effect, the onset of instabilities is postponed to much higher output rates and the power requirement for extrusion is significantly reduced. Note that these additives can eliminate only sharkskin and the so-called stickslip (oscillating or cyclic) melt fracture. To the best of author's knowledge, they do not appear to have an effect on the extrudate appearance in the gross meltfractureregion. In this chapter, the effect of boron nitride (BN) based compositions as a processing aid in the continuous extrusion of a number of resins is examined. Its effect on the rheology of these resins is also tested. It has shown previously that compositions CHAPTER 5 - RESULT AND DISCUSSION  63  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  containing B N can be successfully used as processing aids 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 in the extrusion of polyolefins and fluoropolymers (Buckmaster et al., 1997). In this work, we first try to determine the characteristics of the B N powders that point out this unique action and secondly to determine the mechanism by which B N eliminates melt fracture. Finally the use of the boron nitride additives in commercially blow moulding extrusion processes is also demonstrated.  CHAPTER 5 - RESULT AND DISCUSSION  64  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABrLITY OF MOLTEN POLYMER  5.2  Melt Fracture Performance of Metallocene Polyethylene: Capillary rheometer studies  We have already discussed the first technique that was used to prepare blends. According to the first technique, the B N particles in a finely divided state were thoroughly dry-mixed with the resin pellets at appropriate concentrations. Figure 5-1 shows the flow curves of the virgin and 0.5% B N filled (CTF5) m-LLDPE Exact® 3128 obtained by using the capillary rheometer with a capillary die having a diameter, D, of 0.254 mm and a length to diameter ratio, L/D, of 40 at T=163°C. The Bagley correction was applied to the raw data in order to obtain an accurate value for the wall shear stress. It was determined by using an orifice die of L/D=0 having the same die diameter.  CO 1 0°  —i  1—i—i i i i i 1  ~i—i—I I T I I 1  1 1—r~  m-LLDPE ,T = 163°C L/D = 40, D = 0.254m m  » <u  t j c k ?  ^  u  1—  w  O  sharkskin  gros^ 0 O C O O  to  0  smooth a «: O O  <D  D=0.01 ", pure D=0.01 ", C T F 5  ca a.  CL  <  1 o_i  1 o  1  10  2  i i ii i  1o  3  1 0'  1o  5  A p p a r e n t s h e a r rate, s'  Figure 5-1: Flow curves of the virgin and filled Exact 3128 obtained by using the capillary rheometer with a capillary die having D=0.254 mm and L/D=40 at T=163°C.  CHAPTER 5- RESULT AND DISCUSSION  65  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  In the case of pure resin, sharkskin first appears at about a shear rate of 45 s", followed by stick-slip and finally gross melt fracture at higher shear rate. The addition of 0.5% B N shows no effect on flow curve of the resin. The appearance of the CTF5 filled extrudates is almost the same as those of the pure resin one. Figure 5-2 depicts the flow curve of m-LLDPE Exact 3128 (pure resin) obtained by using the Nokia Maillefer crosshead having 3.00 mm diameter and 1.52 mm tip at 163 °C. The formulas for calculating the wall shear stress and apparent shear rate were Equations 2-1 la and 2-1 lb. The onset of surface melt fracture for the virgin resin is about 42 s" that 1  agrees well with the value of 45 s" obtained from a circular die. The stick slip region does 1  not appear in this case of the crosshead die. The surface melt fracture is followed by the gross melt fracture occur at about 500 s". 1  -i—i—i—i i |  o_  T  1  1  I  I  1  I  I  I  T  1  A  >0/  m - L L D P E , T=163°C 10° V- Crosshead: D=3.0mm, d=1.524mm  A  •  CO  •  co  CD co 1_  CO CD  n  co  "co  c CD CO  a. <  O 10  -1  O  10  _J  r  Gross MF  Sharkskin MF  O i  |  | | i I  10  1  10  2  Apparent shear rate, s"  3  1  Figure 5-2: The flow curve of PE Exact 3128 by using the capillary rheometer with Nokia Maillefer crosshead having 3.00 mm die and 1.52 mm tip at 163 °C.  CHAPTER 5- RESULT AND DISCUSSION  66  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABIUTY OF MOLTEN POLYMER  ~i  1—i—i—i—i i i  ~i  1—i—i—r  O  virgin resin  • A V  0.05% BN 0.1% BN 0.25% BN  9  0.1 \-  open symbol corresponds to smooth extrudate _i  10  i  i  i  i  100 A p p a r e n t s h e a r rate, S "  i  t'  1000 1  Figure 5-3: The flow curve of PE Exact* 3128 with and without boron nitride (CTF5) obtained with the Nokia Maillefer 4/6 crosshead attached at 163°C. Blends were prepared by dry-mix method. It shows that B N has little effect on melt fracture. 1  Figure 5-3 shows the flow curve of m-LLDPE Exact® 3128 with and without boron nitride (CTF5) at 163 °C using the crosshead die attached to the capillary rheometer. Blends were prepared by the dry-mix method. The flow curve of pure resin almost coincides with the other three ones that correspond to different B N concentrations. The  CHAPTER 5- RESULT AND DISCUSSION  67  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILrrY OF MOLTEN POLYMER maximum shear rate to yield melt fracture at 163 °C upon the appearance of the extrudate are 62, 62, 77 and 92 s" for the B N concentrations of 0, 0.05, 0.1 and 0.25 weight % 1  respectively.  Experiments at different temperatures were also performed in order to study the effect of temperature on the appearance of the extrudate. Table 5-1 summarizes the results of three sets of experiments at various temperatures obtained for B N type CTF5. The best results were obtained at the maximum B N concentration of 0.25% and the temperature of 204°C.  However, this critical rate is far below the maximal shear rates yielding a smooth  extrudate that was reported previously (Rosenbaum et al., 1998). Similar results were obtained with B N type C T U F and CTL40. In general, one may conclude that the addition of B N in the dry powder form into the resin has little effect on the melt fracture behavior of resins regardless of the B N type and concentration. However, this could be partially explained by the low degree of dispersion of B N particles that can be achieved by this technique of mixing (dry-mixing). Table 5-1: The effect of B N on the melt fracture of m-LLDPE (Exact® 3128) at three temperatures obtained for B N type CTF5 added to resin in a dry-mixed form. Max. shear B N concentration T,°C rate, s" mass % 42 0 163 0.05 42 77 0.1 93 0.25 1  180  0 0.05 0.1 0.25  62 93 93 93  204  0 0.05 0.1 0.25  93 93 113 124  CHAPTER 5- RESULT AND DISCUSSION  68  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  1  CO Q_  1  1  1  1  1  1  1  1  1  1  1  1  •  1  P E Exact 3128+ BN(CTF5), T=163°C Crosshead: D=3.0mm, d=1.524mm  • m  10° r  •  % CO  — I  CO <D sz  1  a  virgin resin  •  0  0.02% B N  •4—'  £  CO Q.  0.1% B N 0.5% B N  V  10I  ®  O p e n s y m b o l s correspond to s m o o t h extrudate \ i  10  t  i  1  i  i  i  i  i  11  10  i  i  i  Apparent shear rate, s"  i  i  i  i  11  10  2  3  1  Figure 5-4: The flow curves of virgin and filled m-LLDPE Exact® 3128 with B N (type CTF5) at concentrations of 0.02%, 0.1%, and 0.5% obtained at 163°C by using the crosshead die (second mixing technique). It can be seen that B N has a significant effect on the melt fracture.  Figure 5-4 compares the flow curves obtained for the pure m-LLDPE Exact® 3128 and filled resins with B N (type CTF5) concentrations of 0.02%, 0.1%, and 0.5% at 163°C by using the crosshead die. The blends were prepared by using the second technique. The maximal shear rate at which the extrudates were smooth exceeded 926 s". This is well 1  above the critical rate for the onset of gross melt fracture of the virgin resin, which is about 500 s" (see Figure 5-2). Experiments have shown that the maximal shear rate yielding a 1  smooth extrudate is also sensitive to the B N concentration. The optimal performance was  CHAPTER 5- RESULT AND DISCUSSION  69  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER obtained at low B N contents, particularly 0.02% and 0.1%. Further increase in the additive concentration resulted in decrease in the maximal shear rate for the onset of flow instabilities. The maximal shear rate of 926 s" was obtained for B N (CTF5) concentrations 1  of 0.02% and 0.1%, while the concentration of 0.5% resulted in the maximum shear rate of 617 s". This means that there seems to exist some critical concentration resulting in 1  optimum performance. This depends on the type of the resin and additive. Figure 5-5 depicts extrudate samples obtained at different B N (CTF5) concentrations at the shear rate of 617 s". It can be seen that at this shear rate, virgin resin extrudate exhibits severe gross 1  melt fracture, while 200 and 1000 ppm of B N eliminate completely the gross melt fracture. On the other hand, 5000 ppm of B N does not seem to eliminate gross melt fracture, although it appears to alter the extrudate appearance to a certain degree.  Figure 5-5. The extrudate samples to illustrate the effect of B N (CTF5) on the extrusion of m-LLDPE Exact® 3128 obtained at 617 s" and 163°C: 1) pure resin; 2) 0.02% BN; 3) 0.1% BN; 4) 0.5% of BN (CTF5) 1  Table 5-2 summarizes the effect of the B N type (CTUF, CTL40 and CTF5) and concentration on the maximal shear rate yielding a smooth extrudate in the extrusion of m-  CHAPTER 5- RESULT AND DISCUSSION  70  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESS ABILITY OF MOLTEN POLYMER L L D P E Exact® 3128 (Nokia Maillefer crosshead attached to the rheometer, D=3.0mm, d=1.52mm).  Table 5-2:Effect of the B N type (CTUF, CTL40 and CTF5) and concentration on the maximal shear rate yielding a smooth extrudate in extrusion of m-LLDPE Exact® 3128 (Nokia Maillefer crosshead attached to the rheometer, D=3.0mm, d=1.52mm) at 163°C. The resin and B N are initially pre-extruded by the second technique to attain good B N dispersion into the resin.  BN concentration, mass % 0 0.02 0.1 0.5  Max. shear rate, s"  CTL40  0.02 0.1 0.5  62 93 77  CTF5  0.02 0.1 0.5  926 926 617  BNtype  Pure P E CTUF  1  42 155 155 155  From the results of Table 5-2, it can be seen that the B N CTF5 appears to be the best B N compared to that of C T U F and CTL40. One may naturally ask why this is the case. In order to answer this question, let's review the morphological characteristics of these three different B N powders. CTF5 has an average particle size of 5-10 um, CTL40 which is essentially an agglomerated version of CTF5 has an average particle size of more than 40 urn, and C T U F has an average particle size of 5-10 \im, but contains a fair amount of B2O3 (about 2%) compared to CTF5. This modifies its surface energy so that becomes higher to those of CTF5 and CTL40. As was shown in figure 4-5, the dispersion of the CTF5 is superior compared to that of CTL40 in the pure resin. Thus, one may conclude  CHAPTER 5- RESULT AND DISCUSSION  71  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER that smaller average particle size may result into a better dispersion of B N into m-LLDPE and this results into a better performance in terms of eliminating surface and gross melt fracture.  The average particle size of C T U F is similar to CTF5's. The only difference is that C T U F contain about 2 % of  B2O3.  This shows that surface energy plays an important role  here. The presence of B203 may cause stronger polymer adsorption and the creation of a stronger structure that is not desirable to improve melt fracture performance.  Further studies on the effect of type and concentration of B N were also carried out. Five additional types of B N were evaluated by using the capillary rheometer equipped with the Nokia Maillefer 4/6 crosshead mentioned above. Various B N concentrations and temperatures were used. The method used to introduce B N into m-LLDPE is by using twin screw extruder (third technique described in section 4.3). The various blends of B N were prepared with the metallocene catalyzed polyethylene m-LLDPE Exact® 3128. The particle size and agglomerated properties of the various B N powders are listed in Table 4-3.  Figure 5-6 shows the flow curve for the m-LLDPE Exact with and without BN431 filled by using the capillary rheometer with a capillary die having L/D=40 and D=1.27 mm. The flow curve for BN431 filled resin is lower than that compared to the pure one. The addition of 0.1% of BN431 does not have any significant effect on the appearance of the extrudate except that the stick-slip oscillating region does not appear in the presence of BN431.  CHAPTER 5- RESULT AND DISCUSSION  72  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  10°  "T  1  1—I  I I  ~T  1  1  1  1—I  I I  ~l  1  1  1  1—I  I I  m-LLDPE, T=163°C isharkskin 3 V  V  V  CO  smooth  V  to to  CD  V  — I -*—' to  CO CD  sz to  V 10-  1  A  A  gross  AA stick-slip only in pure resin  A  v v  "co  v  c  V  A A  A  CD  L_ CO CL Q<  10'  V  D=0.05", virgin resin  A  D=0.05 , 0 . 1 % B N 4 3 1 M  :  10  10  1  10  2  Apparent shear rate, s"  3  1  Figure 5-6: The flow curve for the m-LLDPE Exact 3128 with and without BN431 obtained by using the capillary rheometer at 163°C by using second technique.  Figure 5-7 and 5-8 show a transient capillary experiment in the oscillating regime for the virgin resin and B N 431 filled resin at the apparent shear rate of 100 s". 1  CHAPTER 5- RESULT AND DISCUSSION  73  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABfLITY OF MOLTEN POLYMER  m-LLDPE Exact, L7D=40 ,D=0.05", T=163°C _  600 -,  CD  o 400 o v. O  20020  25  30  35  40  barrel distance (in)  Figure 5-7: Transient capillary experiments for m-LLDPE Exact® 3128 at the apparent shear rate of 100 s" Pressure drop oscillations are obtained at this shear rate.  1  m-LLDPE containing 0.1% BN431, D=0.05", L/D=40, T=163°C — 600 CO  :* 400 o <£ 2 0 0 £  o  38  33  28  length of barrel (cm)  Figure 5-8: Transient capillary experiments for 0.1% B N filled m-LLDPE Exact® 3128 at various shear rates. Note that the force at 430 kg representing the shear rate of 100 s" . It is surprising the absence of oscillations at the shear rate of 100s" , mainly due to the presence of BN. 1  1  CHAPTER 5- RESULT AND DISCUSSION  74  THE EFFECT OF BORON NITRTDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  It can be clearly seen from figure 5-7 that the force (pressure drop) fluctuates between 530 kg and 550 kg for the pure resin, at the shear rate of 100 s". However, from 1  figure 5-8 it can be seen that there is no oscillation in the presence of BN431 into the resin at the same shear rate of 100 s . Other types of B N were also studied by using the crosshead die at various concentrations and temperatures. The results for maximum shear rates yielding smooth extrudates in extrusion of m-LLDPE Exact® 3128 at 163°C for two concentrations of B N 427-431 (200ppm and lOOOppm) are summarized in figure 5-9.  mPE Exact T=163°C Crosshead D/d =3.1 mm/1.524mm  virgin 431,0.02% 431,0.1% 430, 0.02% 430, 0.1% 429, 0.02% 429, 0.1% 428, 0.02% 428, 0.1% 427, 0.02% 427, 0.1% ctuf, 0.02% ctuf, 0.1% ctf5, 0.02% ctf5, 0.1% 500 =melt fracture  Apparent shear rate, s-1  1000 = smooth  Figure 5-9: The effect of various boron nitride types on the maximum shear rate yielding a smooth extrudate in extrusion of m-LLDPE Exact  3128 at 163°C for two concentrations of BN (200 ppm and 1000 ppm)  CHAPTER 5- RESULT AND DISCUSSION  75  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER From figure 5-9, it can be seen that BN431 and CTF5 exhibit the best performance in terms of maximizing the critical shear rate for the onset of melt fracture. There are some common characteristics for these two boron nitrides. The particle sizes are small (5-10 um) as well as they have no agglomerated particles. CTUF, BN427 and BN429 also have small particle sizes but some of their particles are agglomerated. Therefore, we can conclude that small average particle size of the B N and free of any agglomerated particles into the B N powder are the main requirements for maximizing the critical shear rate for the onset of melt fracture. Figure 5-10 shows the melt fracture results for virgin P E resin, B N 428, 430 and CTF5 at two different temperatures 163 °C and 204°C. It can be seen that the critical shear rates for the onset of melt fracture increase with temperature in most cases.  m-LLDPE, T=163 4 204"C Crosshead Q/d =3.1mm/1.$24rom  428, 0.02% ,204C 428, 0.1% ,204C 428, 0.02% ,163C 428, 0.1% ,163C virgin, 204C virgin, 163C 430, 0.02% ,204C 430, 0.1% ,204C 430, 0.02% ,163C 430, 0.1% ,163C ctf5,0.1% ,204C ctf5,0.02% ,204C ctf5,0.02% ,163C ctf5, 0.1% ,163C 500 =melt fracture  Apparent shear rate, s"  1000 • smooth  Figure 5-10: The effect of temperature on the melt fracture performance of BN 428, 430 and CTF5 filled mLLDPE resin at two different temperatures of 163°C and 204°C.  CHAPTER 5- RESULT AND DISCUSSION  76  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  n  1 0  1  -i  1—i—i—i i i  1  1  1—i—i i i  o [_m-LLDPE,T=163°C Crosshead D/d=3.1 mm/1.524mm  .0  o 0  to  D_  0  to to CD  0  "to  0  1_  CO CD  SZ to  1  0  0  0  c  CD i CO  Q.  5 " 10"  1  Q  0  So  O  pure resin  • A V O  BN427 BN428 BN429 BN430  0  BN431  0 _1  10  I  1  '  '  _i  '  10  1  i  i  2  i  i i i 11  10  3  Apparent shear rate, s"  Figure 5-11: The flow curve of pure resin Exact® 3128 and of Exact 3128 resin containing BN427 to BN431 (0.1 weight % in all cases) using the crosshead die at 163 °C.  Figure 5-11 shows the flow curve for the pure m-LLDPE resin and BN427 to BN431 filled resins (0.1 weight % in all cases) by using the crosshead die at 163 °C. The flow curve for all the filled resins are about the same except for that of BN431 filled one.  CHAPTER 5- RESULT AND DISCUSSION  77  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  The flow curve for BN431 is lower than all the others. Moreover, from figure 5-9, it can be seen that 0.1% BN431 filled resin has the best performance over others. From these data, one may suggest that BN431 not only reduces the driving pressure of extrusion but also postpones the onset of melt fracture to much higher shear rate. As will be shown below, this reduction in pressure is due to the occurrence of polymer slip. Summarizing this part of the study, one may conclude that for optimum performance of B N the following requirements apply: (1) the B N additive should consist of fine particles (average size of about 5-10 u.m), (2) it should be thoroughly dispersed in the resin, and (3) it must be used at its optimal concentration depending on the type of the polymer and the extrusion temperature, and (4) it must not contain B2O3 which possibly increase its surface energy and as a result polymer adhesion on the surface of B N might become a factor. The reproducibility of the data for this apparatus is high. Experimental error as reproducibility is checked to be within 5%.  CHAPTER 5- RESULT AND DISCUSSION  78  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  5.3  The Effect of Boron Nitride on the rheological behavior of Teflon FEP 4100 and m-LLDPE: parallel-plate rheometer and sliding- plate rheometer studies  In order to explain the effect of B N on the processability on P E Exact® 3128 during the extrusion in a wire coating process, the following possible mechanisms should be considered.  >  Change of the rheological behavior: parallel plate rheometer  >  Effect of die geometry: various contraction angles of orifice die  >  Wall slip: capillary and sliding plate rheometer  5.3.1  Change of the rheological behavior  To study possible effects of the B N addition to the resin on its rheology, linear oscillatory shear experiments were carried out for the metallocene P E (Exact® 3128) and Teflon® FEP 4100 with and without B N using a Rheometrics System IV parallel-plate rheometer.  5.3.1.1 Metallocene L L D P E Exact® 3128  Frequency sweep experiments for m-LLDPE Exact® 3128 were carried out at 163°C for different levels of B N type. Only CTF5 and BN431 were considered in this study since they gave the best performance in terms of melt fracture performance (see section 5.2). Figure 5-12 shows dynamic mechanical data for virgin m-LLDPE and mL L D P E filled with three different levels of B N (type CTF5). Surprisingly, no difference was found in the linear viscoelastic behavior of the virgin and CTF5 filled resins. Similar  CHAPTER 5- RESULT AND DISCUSSION  79  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  results were obtained with the C T U F and CTL40 type of B N . This set of data agrees well with those in figure 5-4. The rheological behavior does not change by the addition of CTF5 up to concentrations of 0.5 wt%.  Figure 5-12. Linear viscoelastic data of m-LLDPE Exact® 3128 at 163°C with and without BN (type CTF5).  CHAPTER 5- RESULT AND DISCUSSION  80  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  Since B N 431 performs best in eliminating the melt fracture (see figure 5-9) and the driving pressure in extrusion is lower than that of all other B N (see figure 5-11), its effect on the rheological behavior of m-LLDPE was investigated at 163 °C and plotted in figure 5-13.  10 6  pi  1—i  i i l  1—i  1111  i i i i ill  1—I  I I I 11II  1—I  I I I I ill  1—I  I I I I UJ  m-LLDPE, T=163°C  o  10  5  10  4  CD 1  o E o |  10  10  3  2  Q 10  1 A  ^  O  O  G', virgin m-LLDPE  •  G", virgin m-LLDPE  A  G \ w/1000 ppm of BN431 -  V  G", w/1000 ppm of BN431  A  G', w/500 ppm of BN431  T  G", w/500 ppm of BN431 '  •  G', w/200 ppm of BN431  • 10° 10-  2  t i i i 11  11  10"  10°  1  G", w/200 ppm of BN431 t  i t i i i 111  10  1  i  nl  10  2  i  i i iiii  10  3  Frequency, a> (rad/s)  Figure 5-13: Linear viscoelastic data ( G \ G") of m-LLDPE Exact® 3128 at 163°C with and without BN (type 431).  CHAPTER 5- RESULT AND DISCUSSION  81  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILrTY OF MOLTEN POLYMER  It can be clearly noticed that both the storage and loss moduli, G ' and G " , of BN431 filled resin are lower than that of the virgin one. This observation agrees well with that shown in figure 5-11 where BN431 had a similar effect on the flow curve of the mL L D P E Exact® 3128. It is also noted that higher B N concentrations (500ppm and lOOOppm filled resin) resulted into lower moduli values. The same effect can also be observed for the flow curve of virgin and BN431 filled resin plotted in figure 5-14. The question that has to be answered now is whether or not this reduction in the measured linear viscoelastic properties is a reduction in the rheological properties of the bulk of the material or a result of slippage of the resin at the wall. Sliding plate rheometer studies were carried out to answer this question. The results are presented in a later section.  i  ,  10"  2  i  ,  10"  1  i  10°  ,  i  10  . 1  10  2  10  3  F r e q u e n c y , rad/s  Figure 5-14: The wall shear stress vs frequency for the virgin m-LLDPE and m-LLDPE with BN431 with different levels concentration by using the parallel plate rheometer at 163 °C.  CHAPTER 5- RESULT AND DISCUSSION  82  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  5.3.1.2 Teflon FEP " 4100 18  Teflon FEP® 4100 was also studied using parallel plate rheometer in order to determine the effect of B N on the rheology of fluoropolymer. Three different concentrations of B N (CTF5), namely 0.025%, 0.05%, and 0.17% along with the pure resin were used. Frequency sweeps were carried out at various concentrations and temperatures. Figure 5-15 depicts the dynamic moduli and complex viscosity of Teflon FEP® 4100 at the temperature of 300°C.  <J Q0 I 1 0  -2  1  '  i i i t  nl  I  I  10-1  I I I I  1 0  III  I  I  I I I I  o  Frequency,  1 0  III  I  1  I I I  1  11  ll  10  2  1  1  I I I III  10  3  a *co (rad/s) T  Figure 5-15: Linear viscoelastic data for Teflon FEP 4100 at the reference temperature of 300°C with and without BN (type CTF5).  CHAPTER 5- RESULT AND DISCUSSION  83  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  The incorporation of B N (CTF5) increases all the material functions to a certain degree particularly at small frequencies. This can be attributed to the presence of B N particles that seem to have an effect on the rheology of Teflon FEP®. All three material functions, G ' , G", and 77*, increase with the increase in the B N concentration. Therefore the effect of B N (CTF5) on the rheological behavior of Teflon FEP® 4100 was different to that of m-LLDPE Exact® 3128. As discussed before B N had no effect on the rheological behavior of m-LLDPE Exact® 3128. Figures 5-16 to 5-18 plot the master curves of dynamic moduli of Teflon FEP® 4100 with various levels of B N at 300 °C. In all cases, good time-temperature superposition is obtained.  CHAPTER 5- RESULT AND DISCUSSION  84  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  Figure 5-17: The dynamic moduli, G'&G", of 0.05% CTF5 added Teflon FEP 4100 at reference temperature 300°C.  CHAPTER 5- RESULT AND DISCUSSION  85  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  10-  10-  2  1  10°  10  10  1  2  10  3  F r e q u e n c y , co*aT (rad/s)  Figure 5-18: The dynamic moduli, G'&G", of 0.17% CTF5 added Teflon FEP 4100 at reference temperature 300°C.  The shift factors, a , to superpose the linear viscoelastic data were also determined T  at each temperature together with the zero shear viscosity, ri . 0  CHAPTER 5- RESULT AND DISCUSSION  86  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER Figures 5-19 to 5-21 summarize the results for the zero shear viscosity at various temperatures.  0.00  0.02  0.04  0.06  0.08  0.10  0.12  0.14  0.16  0.18  % of BN added  Figure 5-19: The effect of BN concentration on the zero shear viscosity of Teflon FEP  CHAPTER 5- RESULT AND DISCUSSION  4100 at 300°C.  87  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABrLITY OF MOLTEN POLYMER  6500  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  i  I  i  i  i  i I  Teflon F E P 4100 T= 320°C  6000 * CO  0_  5500  8  5000  > co CD w  o  4500  CD  N  4000  3500  ' ' ' ' ' ' ' '' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' *' ' ' *' ' ' ' *' ' ' ' 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 1  1  1  1  1  1  1  % of BN added  Figure 5-20: The effect of BN concentration on the zero shear viscosity of Teflon FEP  CHAPTER 5- RESULT AND DISCUSSION  4100 at 320°C.  88  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  3800 rr-r  3600 h  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 i i i i  i  i i i  Teflon F E P 4100 T=340°C  tn  * co  3400  D_ o  8 tn  3200  co CD to  3000  CD  N  2800  2600  ' ' ' * ' ' ' ' * ' ' ' ' * ' ' ' ' * '' ' ' * ' ' ' ' * ' ' ' ' * ' ' ' *''''^ ' ' ' 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 1  % of BN added  Figure 5-21: The effect of BN concentration on the zero shear viscosity of Teflon FEP  4100 at 340°C.  It can be seen from figure 5-19, 5-20 and 5-21 that the zero shear viscosity increases with the increase of B N concentration. On the other hand increase of temperature results into decrease of the zero shear viscosity.  Based on the determined shift factor, the activation energies for flow, E , were a  calculated for the various B N concentrations based on an Arrhenius type equation (Eq. 225):  CHAPTER 5- RESULT AND DISCUSSION  89  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER ( a = expl T  R  j_  1  T  T,ref  (Eq 2-25)  where R is the gas constant and T f is the reference temperature. Figure 5-22 depicts the re  relationship between the activation energy for flow and the B N concentration. It clearly shows that the activation energy decreases with increase of the B N concentration.  Figure 5-22: The relationship between the activation energy for flow, Ea, and the B N (CTF5) concentration in Teflon FEP 4100.  This finding means that the viscosity becomes less temperature dependent with increase of B N (CTF5) concentration in Teflon FEP® 4100. The reproducibility of the data  CHAPTER 5- RESULT AND DISCUSSION  90  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  for this apparatus is high. Experimental error as reproducibility was checked to be within 2%. 5.3.2  Effect of the die geometry on the B N performance It has be seen that the effect of B N is more pronounced in the crosshead die rather  than the regular capillary die. This implies that the die geometry must play a very important role in this phenomenon. We can see from figure 2-7 that the crosshead provides a much smoother streamlined polymer flow. The polymer entering the die through port 11 flows along the conically converging annular channel defined by surfaces 12 and 22 to enter the die inlet smoothly. It is known that gross melt fracture is originated from the die entrance. This is why the streamlined flow at the entrance part of the die is so important to the melt fracture. In order to model the effect of the streamlined flow on melt fracture, various entrance angle of orifice dies (L/D=0) were used in the capillary rheometer. The entrance angles for this study were 8 °, 15 °, 30 °, 60°, 90° and 150°. Figures 5-23 to 5-24 depict the end pressure as a function of apparent shear rate of m-LLDPE Exact 3128 with and without CTF5 for six orifice dies having various entrance angles. As can be seen, in spite of the significant differences of the flow curve for each entrance angle, the entrance angle has almost no effect on the extrudate appearance. For all dies, the surface melt fracture starts at around 70 s' . This means that the streamlined inlet region is a necessary 1  but not a sufficient condition for the effect of BN. In fact, this effect is a combination of annular geometry of the crosshead die and the slowly converging entrance flow that provide the best condition for delaying the onset of melt fracture. Finally it is noted that B N has no effect on the pressure in orifice dies. In other words, the data plotted in figures 5-23 and 5-24 are identical.  CHAPTER 5- RESULT AND DISCUSSION  91  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  ~i  i  i  -i—i—i—i i i i  i i i ii  m-LLDPE Exact @ 163°C 10  A  q 9  to  Q_  o nioro D o o n ooo  03 i_ (/) <D  Q-  M  A  1 Q  0  ol  LU  O  ^  o° o<> o  M  8 o oo  O  o  150° 90° 60° 30° 15° 8°  O  • A  melt fracture  V O  10-1 10  _i 1  i  i  i  i  i  1 1 1  10  2  o  0  1  i  i  i  i  i  _i  11  10  i  i  i  i  1 1 1  10  3  4  -1  Apparent shear rate, s  Figure 5-23: The end pressure as a function of apparent shear rate of m-LLDPE Exact® for six orifice dies having various entrance angles.  CHAPTER 5- RESULT AND DISCUSSION  92  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  1 — i — i — i  ~i  ~i  i i i  1—i—i—i  i i i  n — i — i — i — i  m-LLDPE Exact C T F 5 @ 1 6 3 ° C 10  1  A A  A  $  o  co D_  0  C i_D Z5  </) tn  o. •o  "  Q  o  1 °  o  10°  oo o  O  a  °  oo  10  1  i  i  i  _i  111  10  2  0  V  90° 60° 30° 15°  O  8°  •  melt fracture  i  0  150°  O O  j  o  o  cz LU  10-  0 8  0  0°  o o  V  O  § 55 o<  111—  6 • o  0 A  A A  | A  A  i  i  i  i  i  A  j  111  10  i  i  i i  1 1 1  10  3  4  -1  Apparent shear rate, s  Figure 5-24: The end pressure as a function of apparent shear rate of 0.05% CTF5 filled m-LLDPE Exact for six orifice dies having various entrance angles.  One surprising effect that can be observed from figures 5-23 and 5-24 is that the end pressure does not change monotonically with entrance angle. This can be seen clearly in figure 5-25 where the end pressure is plotted as a function of entrance angle for several  CHAPTER 5- RESULT AND DISCUSSION  93  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF M O L T E N POLYMER  values of the apparent shear rate. It can be seen that at about 30°, we obtain the maximum end pressure. This observation can be very helpful in assessing the relative important of shear and extensional components of the various contraction angles, as well as in assessing the usefulness of contraction flows in determining the elongational viscosity of polymer melts. (Cogswell, 1977).  12  -|—I—I—I—|—I—I—I—I—I—I—I—I—I—I—I—I—I—1—1—I—I—T-  -|—I—I—I—I—I—I—I—l~  m-LLDPE, T = 1 6 3 ° C  - o - 47 s* •Q- 70 s  .a.  10 h  _1  •A 94 s"  CO Q_  •'/<>.  •v- 117 s O 164 s  d/. '/v //A  (D  yb.  W  (0 (i_ D Q.  1  /p.  •-.<>•.  _1  _1  •a- 234 s" •o- 351 s"  1  •D- 469 s"  1  1  '•.'v..  '•..A.. \ 'Ul:'.  •A- .•  ' C  (6/  LU  O:  ' i:;:::?S'8 ;;;;  'o-  r O  I I , , I II  20  40  . . . .  60  80  I . . . . I . . . •  100  120  140 160  Contraction angle, 2 a  Figure 5-25: The effect of contraction angle on the wall shear stress for m-LLDPE at 163°C.  CHAPTER 5- RESULT AND DISCUSSION  94  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  5.3.3  Wall slip  From the experimental data presented in section 5.2, it was seen that the use of B N 431 into m-LLDPE resulted into reduction of the wall shear stress compared to that of the other powders. This particular B N powder exhibited the best performance in not only eliminating the surface and stick-slip melt fracture, but also postponing the gross melt fracture to a higher apparent shear rate. As discussed before, one may argue that the reduction of the wall shear stress is due to the presence of wall slip. To answer this, capillary rheometer experiments were carried out using various die diameters. The results are summarized in figures 5-26 and 5-27. These figures plot the flow curve of pure mL L D P E Exact® 3128 with and without addition of BN431 by using the capillary rheometer with dies having various diameters namely 0.254, 0.508 and 1.27mm at 163°C. The length to diameter ratio was 40 for all cases to keep the effect of pressure constant.  10°  Trrp-  m - L L D P E , L / D = 40 T=163°C  A  sharkskin  CL  smooth r f k  in in  oscilla tion  ID  D ra <D  10-'  a a  D A O •  •  CO  5  D = 0.01" D = 0.02" D = 0.05"  D  • io-  :  10°  10  1  10  2  Apparent shear  10  10*  s  rate, s  10  5  •1  Figure 5-26: The flow curve of pure m-LLDPE Exact® 3128 using capillary dies having various diameters dies (0.254, 0.508 and 1.27mm).  CHAPTER 5 - RESULT AND DISCUSSION  95  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  10°  1  1—r 1 T i l l ]  1  1—TTTTTT]  1  1—TTTTTT]  1  1—F I 1 1 1 1 ]  m-LLDPE containing 0.1%of BN431 L/D=40, T=163°C  A  1  sharkskjrv  <0 Q_  gross  smooth  in in  O co d)  10-  D = 0.01" D = 0.02" D = 0.05"  A  in  •  •  O  •  •  •  ••j Q-2  I  10°  i  i  * i i i 111  i  i  10  I  i  10  1  2  i  • ' i i i il  10  i  i  A p p a r e n t shear rate, s '  i i i 11 i l  i  10  3  4  1—i i i i 111  10  5  1  Figure 5-27: The flow curve of m-LLDPE Exact® 3128 with addition of BN431 for capillary dies having various diameters (0.254, 0.508 and 1.27mm).  It can been seen from figure 5-26 that all three flow curves coincide. It means that there is no slip in the case of pure Exact® 3128. However, the flow curves diverge when the wall shear stress exceeds the critical value with addition of BN431 in figure 5-27. This indicates that slip occurs at the die wall. Thus, we can conclude that BN431 does affect the flow profile by causing the polymer to slip along the metal wall. This is shown below more convincingly by carrying out the sliding plate rheometer studies. Figure 5-28 shows the flow curve of m-LLDPE with the addition of 0.1% BN431 determined by means of the sliding plate rheometer having various gaps spacing. It can be seen that the flow curve becomes gap dependent. Smaller gap spacing shifts the flow curve towards smaller wall shear stress values. This type of gap dependence is consistent  CHAPTER 5 - RESULT AND DISCUSSION  96  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER with the occurrence of wall slip. The reproducibility of the data for this apparatus is high. Experimental error as reproducibility is checked to be within 5%.  10  3  —I—  II  1—I . 11 ...I  1—I I . . 11.1  1—  I  m-LLDPE with addition of 0.1% BN431 T=163°C V 10  2  V o  to in <a CO  v o o  CD  <a  x: CO  10  o o  \  0  o o  1  V O 0  o 0  10° 10"  1  10°  10  1  Gap = 0.46 mm Gap =0.19 mm Gap = 0.10 mm  10  2  10  3  Apparent shear rate, s'  Figure 5-28: Theflowcurve of pure m-LLDPE Exact 3128 with addition 0.1% of BN431 determined by the sliding plate rheometer using various gap spacing.  CHAPTER 5 - RESULT AND DISCUSSION  97  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESS ABILITY OF MOLTEN POLYMER  5.4  Flow Visualization on Polypropylene To identify the mechanism of gross melt fracture elimination by the use of B N as  a processing aid, visualization experiments were performed with the experimental set-up presented in section 2.3.4. It is hoped to see that the flow pattern developed in the entrance region to the die is different in the presence of BN. This is demonstrated in this section by using polypropylene as the base resin. Figure 5-29 plots the flow curve of polypropylene with and without B N (CTF5) by using the transparent quartz capillary die. It can be seen that the flow curves corresponding to 0.1% B N added resin result in a small reduction of the wall shear stress at all apparent shear rates used. .  -T  1  1  1  1  1—T~  ~I  1  1  f~  pp <a 200°c ro ioCL  •  1  H  A  s  in in  Onset of melt fracture  H  B  CD  • •  CO CD JZ  • A  A  •  P P  A  P P + 0.1%  BN(CTF5)  10-  ;  10  10  1  2  10  3  Apparent shear rate, s"  Figure 5-29: The flow curve of polypropylene with and without BN (CTF5) by using the transparent quartz capillary die.  CHAPTER 5 - RESULT AND DISCUSSION  98  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  Photographs of the capillary entry region were taken by using the microscope (Nikon SMZ-2T) and Nikon FM-2 35 mm photographic camera. The effect of shear rate and additive concentration on the flow pattern development at the entry of the capillary are investigated. Gross melt fracture is known to originate from the die entrance [Bergem, 1976]. The observation of flow pattern may provide additional insight on the effect of additives on the meltfracturephenomena of molten polymers. Figure 5-30a shows three pictures of the flow of polypropylene at various apparent shear rates (32.4, 324 and 650 s") at 200°C. It can clearly be seen that the flow 1  entry angle is higher at the low shear rate, it also has a bigger corner vortex. As the shear rate increases, the flow entry angle become more bend and the corner vortex becomes smaller. Moreover, one can see that there is discontinuous particle motion on the picture at 650 s". Gross meltfractureis observed and the streamlines are no longer smooth. It is 1  caused by a discontinue motion of the polymer flow. In fact, the flow in the entry region break into layers, which they seem to move with different velocities. The closer the layer is to the center of the stream, the larger step and more frequent is this discontinuous motion. A schematic diagram to explain this flow pattern development is shown in figure 5-30b. The flow in the entry region appears to be broken into several layers, and each layer moves with its own velocity. At regular time intervals, different in each layer, the motion stops for a brief period. The closer the layer is to the center of the stream, the larger and morefrequentare the jumps inside it.  CHAPTER 5 - RESULT AND DISCUSSION  99  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  Figure 5-30a: Pictures of the flow of polypropylene at various apparent shear rates (left 32.4 s", middle 320 s' and right 650 s~) at 200"C. 1  !  Entry region  motion stops  Capillary Figure 5-30b:  A schematic diagram to explain the flow pattern development at 650s'  CHAPTER 5 - RESULT AND DISCUSSION  1  100  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  Figure 5-31 shows the flow of polypropylene with and without the addition of 0.1% B N (CTF5) at the shear rates of 650 s" at 200°C. It can be seen that the addition of 1  0.1% B N eliminates the discontinuous streamlines even at the highest shear rate. The streamlines are now smooth and the flow seems to be more organized. This suggests that B N is a good processing aid for eliminating not only the surface melt fracture but also postponing the onset of gross melt fracture to higher shear rates. The mechanism by which the bulk flow is affected by the addition of B N remains to be explained. At least in this section, we have observed the change of the flow pattern development at the entrance to the capillary.  (a)  (b)  Figure 5-31: Pictures of the flow of polypropylene with (b) and without (a) the addition of 0.1% BN (CTF5) at the shear rate of 650s" at 200°C. 1  CHAPTER 5 - RESULT AND DISCUSSION  101  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  Figure 5-32 shows the extrudate of PP at the shear rate of 450s" both with and 1  without B N (CTF5) obtained with the quartz transparent die. The extrudate of PP with 0.1 %BN is smooth (left), while the extrudate of pure PP exhibits gross melt fracture (right). This again proves that B N works in eliminating the gross melt fracture of polymers, in this case of polypropylene.  (a)  (b)  Figure 5-32: PP extrudate at the shear rate of 450s' with (a) and without boron nitride (b) obtained with the quartz transparent die. The extrudate of PP with 0.1%BN is smooth (a), while the extrudate of pure PP exhibits gross melt fracture (b). 1  CHAPTER 5 - RESULT AND DISCUSSION  102  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  5.5  The Effect of Boron Nitride on the blow moulding process of H D P E It has been demonstrated that B N is a suitable P A for the extrusion of polyolefms.  In this section, we will examine whether or not B N works as a processing aid for the blow moulding process of high-density polyethylene. It is more convincing if a P A is demonstrated that it works in real processes and not just in extrusion using lab-scale equipment. As discussed two HDPE's were tested (see section 4.1). The technnique used to prepare them was the one that makes use of the twin screw extruder. A masterbatch of 5% B N concentrate instead of 10% B N used this time. The chemical and physical properties of two H D P E are listed in section 4-1. The type of B N powder used in these blow moulding studies was CTF5. A twin screw extruder was used to blend two types of HDPE, namely Resin A and Resin B. These are produced by Petromont company, Montreal, Quebec. Resin A is a high molecular weight high density polyethylene resin with a broad molecular weight distribution produced using Union Carbide's UNTPOL process. Resin B is a high density polyethylene resin with an intermediate molecular weight distribution and also produced using Union Carbide's UNTPOL® process. This resin is intended primarily for use in intermittent high shear rate blow moulding equipment designed for high speed production of blow moulded containers. As discussed before, a Battenfeld/Fisher 50mm blow moulding machine was empolyed in our blow moulding experiments. The processing details are discussed in section 2.3.5. 5.5.1  Flow Curves of Resins Figure 5-33 shows the flow curve of Resin A and B by using the crosshead die. It  can be seen that addition of B N (CTF5) result in reduction of the wall shear stress by about 5% in both cases.  CHAPTER 5- RESULT AND DISCUSSION  103  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILrTY OF MOLTEN POLYMERS  10°  -i  1  -i  1—i—i—i—r~  1  H D P E (Resin A and Resin B) C r o s s h e a d D/d = 3.1 mm/1.524mm  G  e  O •  CO CD  O  CO  5  O •  a  A V  A V  O •  A V  1  A V  A  A V —1  10  I  1  A  e  a A V  Resin A Resin A containing 1 OOOppm BN Resin B Resin B containing 1000ppm BN  1-  10  1  1—i—i—r-  A V  •  10-  1  e  CO Q_  1  10  2  Apparent shear rate, s"  3  1  Figure 5-33 : The flow curve of Resin A and B by using the crosshead die. The reduction on wall shear stress by addition of BN is cleanly seen.  Figure 5-34 shows the effect of die gap on the head pressure at the screw speed of 30 rpm for the pure HDPE Resin A and Resin B. The processing temperature were 190 °C and 185°C for Resin A and Resin B respectively. Processing below these temperatures may cause damage of the extrusion screw inside the extruder. It can be seen from figure 5-34 that as the die gap increase, there is less resistant to polymer flow and therefore the head pressure decreases. This is due to the fact that the apparent shear rate decreases with increase of the die gap.  CHAPTER 5- RESULT AND DISCUSSION  104  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABrLITY OF MOLTEN POLYMERS  Figure 5-34 : The head pressure varied with the die gap of the pure HDPE Resin A and Resin B at the screw speed of 30 rpm.  CHAPTER 5- RESULT AND DISCUSSION  105  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  Figure 5-35 and 5-36 show the effect of shear rate at the die exit on the head pressure of Resin A and Resin B at the die gap of 0.5mm. It can clearly be seen that B N shifts the flow curve to lower head pressure. In other words, for a given head pressure, higher shear rate (flow rate) can be obtained with the addition of BN. Furthermore, this means that the head pressure can be reduced at certain shear rate with addition of B N for both resins. It can be seen that the reduction in pressure drop with the addition of lOOOppm B N is of the order of 3 to 10 %. The data were collected and plotted as the average of two runs. This reduction is not due to the experimental error as reproducibility was checked to be within 1% (±20psia).  2200  T—1—1—I—I—I—I—r-  -]—i—i—i—i—|—i—i—i—I—|—i—i—i—i—|—i—i—i—i—|—i—i—I—r-  Shear rate at the die exit vs head pressure  2000  <3> 1 9 0 ° C for Resin A  o> 1800 TJ <U  "5 1600 CO co i  2 JZ  1400 1200  1000 32 00  3300  3400  3500  Head  3600  pressure,  3700  3800  39 00  psia  Figure 5-35 : The effect of shear rate at the die exit on the head pressure of Resin A at the die gap of 0.5mm. The screw speed was 30 rpm.  CHAPTER 5- RESULT AND DISCUSSION  106  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  3200  -i—i—i—i—I—i—i—i—r-  3000 t 2800  - i — i — i — I — i — i — r  ~i—i—i—r  Shear rate at the die exit vs head pressure @ 185°C for Resin B  2600 X  CD  g> CD CO CD CO CO CD  sz CO  2400 2200 2000 1800 1600  virgin Resin B 0.025% BN added  1400  0.1% BN added  1200 1000 2800  •  3000  i  •  i  3200  i  i  i  i  i  3400  i  i_  . L  3600  3800  Head pressure, psia  Figure 5-36 : The effect of shear rate at the die exit on the head pressure of Resin B at the die gap of 0.5mm. The screw speed was 30rpm.  5.5.2  Rheological Measurement of Resins A and B Figure 5-37 and 5-38 plot the storage and loss moduli of Resins A and B at the  processing temperatures. It can be seen that the addition of B N has only a negligible  CHAPTER 5- RESULT AND DISCUSSION  107  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS effect on the complex viscosity, n*, for both resins A and B. This is exactly what has been observed for most other B N powders.  i—i—i—i—i  10  i 111  — i — i — i — i — i 1111—  1—i—i—i i M I  Resin A(DMDA -6147) 5  O  ^ O u  <> n  o °  u  D  •  Q  6  e  9 10  D  4  6  O  G  •  G " , virgin Resin A  A  G , 0.025% added  V  virgin Resin A  0.025% BN added  O  .0.1% BN added  0  \ 0 . 1 % B N added , virgin Resin A , 0.025% BN added ,0.1% BN added  10  3  _LXL  10-  10°  _l  I  I '  t  i  l  10  ' I  1  l l  I  10  2  Frequency, co (rad/s)  Figure 5-37: The dynamic moduli and complex viscosity, n*, for resin A @190°C.  CHAPTER 5- RESULT AND DISCUSSION  108  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  T  10  1  1  1—I  I  I I  I  1  1  1  -i—i—i—i—i 1 1  I I  1—I  5  Resin B (DMDA -6200)  a  CO £L  CO CO  CD  ID  i  o  "5  V)  •a o  10  E  o o to  4  O  o  E  Q  CO  c >» Q  0  a  O  G  10  G", virgin Resin B  A  G', 0.025% added  V  G", 0.025% BN added  X 0) CL  |  E  o O  <£> G', 0.1 % BN added  I  10,-1  •>  £  •  O  G", 0.1 % BN added  9  1*. virgin Resin B  •  if, 0.025% BN added  A 3  ' , virgin Resin B  I I I  n*. 0.1 % BN added  I  10°  I  I  I  I  I  t  I I I  10  i  1  i  i  i  i  i  10  2  Frequency, co (rad/s)  Figure 5-38: The dynamic moduli and complex viscosity, r\*, for resin B @185°C.  CHAPTER 5- RESULT AND DISCUSSION  109  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  5.3.3  Transient Extrusion Experiments Figure 5-39 plots the transient extrusion experiments for resin A with and without  BN. The head pressure is plotted as a function of time for pure Resin A and Resin A containing 250 ppm and 1000 ppm of BN. It can be seen that the final steady-state pressure is different for various levels of B N filled resin. The effect of B N on the head pressure is significant and noticeable here. Higher B N concentration reduces the steadystate pressure. What is, however, significant to note is the induction time to attain steady state. While for the case of pure resin A, steady-state is attained fast, it takes about 3 minutes for resin A containing 250 ppm B N and about 8 minutes for resin A containing 1000 ppm of BN. It is apparent that as polymer flows through the die, B N diffuses to the wall and provides some kind of conditioning to the die that allows slippage at the wall. As a result a gradual reduction in the head pressure is observed.  Figure 5-39: The transient extrusion experiment for resin A with and without BN. The screw speed was 30 rpm.  CHAPTER 5- RESULT AND DISCUSSION  110  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABELITY OF MOLTEN POLYMERS  Figure 5-40 plots similar results to figure 5-39 for resin B. The head pressure for 0.025% and 0.1% B N added to resin B are lower than that of the pure resin. The observation at the induction time is also seen here and confirms with the result obtained for Resin A. Visual observation also conducted on the filled resins in order to check the quality of the B N dispersion in the two resin. The result of dispersion is more uniform in Resin B i.e. the parison was uniform in color with Resin B containing 250 and 1000 ppm of B N . However, Resin A containing B N had a white-colored streamlines running along the parison. This indicated that the B N exhibit only medium dispersion in Resin A , mainly due to the high viscosity. This dispersion quality observation related directly to the performance of BN.  h  '  1  Head pressure Paribus With time  Extrusion time, minutes  Figure 5-40: The transient experiment for resin B with and without BN. The screw speed was 30rpm.  CHAPTER 5- RESULT AND DISCUSSION  in  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  5.5.4  Visual Observation of the extrudate  In order to examine the appearance of the extrudate surface visually i.e., bottle, pictures are taken and these are shown in figures 5-41 to 5-45. Figure 5-41 shows the surface appearance of a piece of bottle made by using resin A with and without BN. The processing temperature was 190°C, the die gap was 1.6mm and the screw speed was set at 30 rpm. The samples were collected in the regime where pressure is in steady state. The black lines/dots in the picture represent distortions (melt fracture). It can clearly be seen that the surface appearance improves as the concentration of B N increases. Although the performance of B N cannot completely eliminate the melt fracture in this case, it can clearly improve the surface appearance to a certain degree.  A  B  C  Figure 5-41: The surface appearance of extrudate (bottle) made by Resin A at the shear rate of2800s" . A) Pure resin A, B) Resin A containing 0.025% BN and C) Resin A containing 0.1% BN. 1  CHAPTER 5- RESULT AND DISCUSSION  112  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  Figure 5-42: SEM pictures showing the surface appearance of the bottle made by using resin A with and without the addition of BN. A) Pure resin A at the top and B) Resin A containing 0.1% BN at the bottom. It can be seen that the amplitude periodic distortions decrease by the addition of BN into the resin.  S E M pictures are also taken on part of the bottle surface made by using Resin A. Figure 5-42 shows the surface appearance of the bottle made by resin A with and without addition of 0.1% BN. Although B N cannot completely eliminate the melt fracture in this case, it can be seen that the amplitude of the periodic distortions dramatically decreases by the addition of B N into this resin. Figure 5-43 shows the effect of B N on bottle surface made by resin B. The processing temperature was 185°C at screw speed of 30 rpm and die gap of 1.6mm. The melt index of resin B is almost five times higher than that of resin A. It can be seen that the surface exhibits melt fracture for the pure resin case. However, the surface for the cases of resin containing 0.025% and 0.1% B N remains smooth. One possible explanation for the differences in the performance of B N for the two HDPE is different in melt index and molecular weight of the resins. Resin A has a low melt index and a much higher molecular weight than Resin B. This might have affected  CHAPTER 5- RESULT AND DISCUSSION  113  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  the dispersion of B N in the more viscous fluid and thus the full effect of B N is not seen in Resin A. More experiments using different screw configuration to obtain a better B N dispersion are required.  Figure 5-43: Surface appearance of extrudate (part of bottle) made by using Resin B at 2800s" A) Pure resin B on the left, B) Resin B containing 0.025% B N on the middle C) Resin B containing 0.1% BN of the right. 1  The effect of temperature on the extrudate appearance was also examined for Resin A. Experiments were carried out at 190°C and 205 °C. Figure 5-44 shows the temperature effect on the appearance of the bottle surface. It can be seen that a bottle having smooth surface was made at 205 °C, as opposed to the one having melt fractured surface made at 190°C.  CHAPTER 5- RESULT AND DISCUSSION  114  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  A  B  Figure 5-44: The effect of temperature on the bottle surface appearance. The bottle was collected at 190°C (A) exhibits melt fracture while that made at 205°C (B) isfreeof any defects.  In figure 5-40, it was seen that the pressure decreased with time during a transient extrusion experiment and it became stable only after around 8 minutes for resin containing BN. In order to investigate the effect of the induction time on the surface appearance of the bottle. Samples were collected at two different time instants; i.e., after 1 and 10 minutes from the start-up of the extrusion process. Figure 5-45 shows the surface appearance of the bottle at 1 and 10 minutes respectively. It can clearly be seen that the bottle surface was smooth at the time instant of 10 minutes compared to the sample exhibits melt fracture at 1 minute. This observation implies that B N requires a finite period of time (induction time) in order to migrate to the die wall, promote slip and as a result to eliminate melt fracture.  CHAPTER 5- RESULT AND DISCUSSION  115  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  Figure 5-45: The effect of the induction time on the surface appearance of the bottle. The sample (A) collected at time after 1 minute from the start-up of extrusion is fractured compared the smooth one (B), which was collected at 10 minutes after the start-up. A) Sample collected at time = 1 minute and B) Time =10 minutes  CHAPTER 5- RESULT AND DISCUSSION  116  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  6  Conclusions Experiments were carried out in parallel plate and capillary rheometers with a  Nokia Maillefer crosshead for m-LLDPE Exact® 3128, Teflon® FEP 4100, H D P E (Resin A and Resin B) and polypropylene polymers. Eight different boron nitride powders were evaluated in terms of their effect on the rheology of polymer, ability to eliminate the sharkskin and postpone the gross melt fracture to higher shear rates. Boron nitride was found to act as an effective processing aid in the extrusion of both fluoropolymers and polyolefins. For the first time, a processing aid was shown not only to eliminate sharkskin and stick-slip (oscillating) melt fracture, but also to postpone gross melt fracture to significantly higher shear rates. The critical conditions and the influence of operating parameters such as the temperature, B N type and concentration were determined. It is found that B N type CTF5 and BN431 has the best peformance in order to postpone the gross melt fracture. They reduce not only the driving pressure of extrusion but also postpones the onset of melt fracture to much higher shear rate. For optimum performance of B N the following requirements apply: (1) the B N additive should consist of fine particles (average size of about 5-10 u.m), (2) it should be thoroughly dispersed in the resin, and (3) it must be used at its optimal concentration depending on the type of the polymer and the extrusion temperature, and (4) it must not contain B2O3 which possibly increase its surface energy and as a result polymer adhesion on the surface of B N might become a factor. Capillary and sliding plate experiments have shown that slip does occur in the presence of BN431 filled resins (section 5.3.3). The change of the entrance flow  CHAPTER 6- CONCLUSIONS  117  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  structures (section 5.4) may also be one of the possible mechanisms of B N action, postponing the melt fracture to higher shear rate. Finally, it has been demonstrated that B N works as a processing aid not only in continuous extrusion using lab-scale equipment but also in industrial scale blow moulding process of high-density polyethylene (section 5.5). The performance of B N in blow moulding machine highly depends on the resin type, B N concentration, induction time and processing temperature.  CHAPTER 6- CONCLUSIONS  118  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMER  7 Recommendations For Future Work  •  Additional experiments should be carried out with other types of boron nitride. Some other processing aid should also be used with boron nitride such as talc, calcium tetraborate, molybdenum sulfide and black carbon. These should be included in the extrusion of polymers at various concentrations to determine its optimal concentration as a function of the processing temperature and type of polymer. Experiments with dies of various geometry are also desirable to see how the die geometry affects the resin processability.  •  The flow visualization study should be continued in much more detail, in order to identify the mechanism by which B N and other processing aids affect the extrusion of molten polymers. The visualization technique could also be used independently to measure the slip velocity directly.  CHAPTER 7 - RECOMMENDATIONS FOR FUTURE WORK  119  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  References Atwood, B.T., Schowalter, W.R., Measurement ofslip at the wall duringflowof high density polyethylene through a rectangular conduit. Rheol. Acta, 28, 134-146 (1989). T.F.Ballenger, I.J. Chen,J.W. Crowder, G.E.Hagler., Polymer meltflow instabilities in extrusion: investigation of the mechanism and material and geometric variables, Trans.Soc. Rheol. 15 (1971) 195-215. Bagley, E.B., End corrections in the capillaryflowofpolyethylene. J. 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References  124  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABILITY OF MOLTEN POLYMERS  Notation a  shift factor  b  Rabinowitsch correction  D  capillary diameter, m  D  tip diameter, m  T  E  activation energy for flow, J  e  Bagley end correction or energy  G  shear modulus, Pa  G'  storage modulus, Pa  G"  loss modulus, Pa  G*  complex modulus, Pa  G  amplitude ratio in oscillatory shear  h  gap between plates, m  I  melt polydispersity  K  power-law consistency index, MPa • s"  L  capillary length or length of sample, m  n  power-law exponent  P  absolute pressure, Pa  a  d  P  ambient pressure, Pa  P'  driving pressure, Pa  P .  Bagley correction, Pa  APcx  exit pressure drop, Pa  AP ,  entrance pressure drop, Pa  Q  volumetric flow rate, m /s  R  capillary radius, m or universal gas constant  T  absolute temperature, K  a  end  en  NOTATION  3  125  THE EFFECT OF BORON NITRIDE ON THE RHEOLOGY AND PROCESSABrLITY OF MOLTEN POLYMERS  time, s or wall thickness, m  t  glass transition temperature, K Tref  reference temperature, K  u  melt velocity, m/s  u  slip velocity, m/s  Ax  plate displacement, m  s  Greek Letters a  pressure coefficient of viscosity, Pa"  8  mechanical loss angle  r(t)  shear strain  Y  shear rate, s'  YA  apparent shear rate, s"  Y A,s  apparent shear rate, corrected for slip, s"  K  wall shear rate, s"  Yo  strain amplitude in oscillatory shear  n  viscosity, Pa • s  1  1  1  1  zero-shear viscosity, Pa • s  n*  complex viscosity, Pa • s  p  density, kg/m  3  critical shear stress for the onset of melt fracture, Pa wall shear stress, Pa Co  stress amplitude in oscillatory shear, Pa  CO  frequency, rad/s or specific volume, cm /g  NOTATION  3  126  

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