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This paper presents and reviews findings in relation to three key areas where polymer processing instabilities occur. The paper also describes methods that can be utilised to reduce, or eliminate, the particular instability. Using previously published results in each of the three areas and work presented in the paper, physical insight into the three mechanisms is reviewed and compared. Extrusion instabilities develop with increasing extrusion rate and the onset of extrusion instability is often a key limitation to the maximum output of an extrusion line. The sharkskin instability is an exit effect instability that can be modified by changing exit geometries and eliminated using certain additives. The stick-spurt instability is intimately related to wall boundary conditions which can be influenced by certain wall and polymer formulations. Finally volume instabilities occur in the entry region of a die and result in ahighly distorted product. The instabilities are related to viscoelastic effects within the die and can be minimised by appropriate die and polymer modification. The paper provides sufficient experimental background to identify the key physical aspects associated with each of the instabilities and this in turn provides insight into the different way each instability occurs and how they can be minimised.

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Changes in the local conformations of poly­(ethylene 2,5-furandicarboxylate) (PEF) and poly­(ethylene terephthalate) (PET) after crystallization from the amorphous state were studied and compared by infrared analysis. Both strain-induced crystallization (SIC) under uniaxial loading (D-PEF and D-PET) and crystallization under static conditions (TC-PEF and TC-PET) were used to determine the effect of mechanical stretching. It appears that, although the crystalline systems are the same, the inner organization of the amorphous phases is influenced by stretching. In addition to the increased incidence of the more extended aliphatic chain conformation (trans), stretching appears to reduce the number of activated hydrogen bonds between segments, which are more constrained in PEF than in PET. The effect of stretching on the β-relaxation, which is related to local motions of carbonyl groups, showed that PEF carbonyl groups of amorphous regions are freed by the stretching. We conclude that the different chain architectures of the polyesters induce different sensitivities of the amorphous phase to the drawing process.

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