Effects of the die wall conditions on the extrusion of linear polyolefins

Linear polyolefins such as high density polyethylenes, linear low-density polyethylene and polypropylenes are the most commonly produced plastic resins due to their versatility, low cost and good mechanical properties. However, due to the poor flow properties of the high molecular weight linear polyolefins, the transformation of these resins into commercial products is usually limited by the onset of surface distortions such as sharkskin and melt fracture. These distortions are believed to be accompanied by the presence of polymer slippage at the die surface; however, the mechanisms of wall slip and their effect on the occurrence of flow instabilities are still not fully known.; The objective of this investigation is to determine the effects of the chemical and morphological conditions of the die wall on the extrudability of linear polyolefins. The relationship between the critical shear stress for the onset of flow instabilities and the thermodynamic work of adhesion between the polymer and the die wall will be evaluated to identify the interfacial conditions that favor a particular mechanism for wall slip. Since limited information is available about the effect of the chemical composition of steels on the extrusion of polyolefins, particular attention will be given to this aspect.; The critical shear stress for the onset of flow instabilities or wall slip was considerably affected by the chemical nature of the die. Low surface energy materials such as TeflonRTM, boron nitride, chromium and nickel coatings induced slippage at the wall while tool and stainless steels, and brasses promoted surface defects. The chemical composition of the steels affected the conditions for the onset of surface distortions somewhat. Tool steels presented improved adhesion compared to stainless steels due to the strong interactions between the polymer and the iron oxides available at the surface. The morphological condition of the die also played an important role on the occurrence of flow instabilities. An average 20% increase in critical shear stress for the onset of flow instabilities was achieved when the surface roughness of the die increased from 0.1 mum to 15 mum. This increase is however, accompanied by the presence of lines and deep grooves in the flow direction on the surface of the extrudates. (Abstract shortened by UMI.)...