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Rheology and processing of molten poly(methyl methacrylate) resins Stamboulides, Christos

Abstract

The rate of production in many commercial polymer processing operations is limited by the appearance of flow instabilities. Flow instabilities might manifest themselves as fine and periodic distortions on the extrudate surface (sharkskin or surface melt fracture) or in the form of helix and/or chaotic distortions (gross melt fracture). To postpone such phenomena to higher shear rates processing aids are used. External processing aids, such as fluoropolymers and waxes, have been found to be effective in eliminating sharkskin. Internal processing aids, such as boron nitride and clays, have been found effective in postponing the onset of gross melt fracture. In addition, different polymers have also been used as processing aids in the processing of other polymers with both polymers to be immiscible. In many of these cases melt fracture phenomena have been eliminated. The first objective of this present work was to study the rheological behavior of poly(methyl methacrylate) resins under shear and extension. Several frequency sweep experiments were carried out using the concentric parallel plate rheometer at various temperatures. By applying time temperature superposition, master curves for the linear viscoelastic moduli were obtained and the activation energy of flow was found to be independent of molecular weight. Following, extensional measurements using the Sentmanat Extensional RJieometer (SER) were attempted. It was found that poly(methyl methacrylate) resins exhibit shear thickening effects only at high shear rates. The main objective of this work was to identify suitable processing aids for the extrusion of poly(methyl methacrylate). Three different molecular weight poly(methyl methacrylate)s were tested together with various processing aids through capillary rheometer studies. First, it was found that poly(methyl methacrylate) polymers exhibit spiral/helical type of distortions at a critical shear stress value (0.35 ± 0.03 MPa) independent of temperature and molecular weight. "Traditional" processing aids used mainly in the extrusion of polyolefins and some other commercial polymers were found ineffective in eliminating instabilities in the case of poly(methyl methacrylate) processing. On the other hand, mixing of poly(methyl methacrylate) with a proprietary blend of synthetic resins and fatty glycerides with modified organic fatty acids, MoldWiz® INT-35UDH, was able to reduce the extrusion pressure and postpone the onset of gross melt fracture to higher shear rates. Also, the addition of different polyethylenes (LLDPE, LDPE and HDPE) resulted into a significant pressure reduction along with significant postponement of gross melt fracture to higher shear rates.

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