Applications of Rheology to Rubber Processing

Abstract

Abstract Recent years have seen great advances in the general field of polymer processing; these have been to a large extent connected with the very rapid growth of the thermoplastics industry, particularly polyolefins, nylon, and dacron (terylene). Because these latter materials have been simple (pure) polymers, usually used without plastizers or other additives, and relatively stable at and just above their ‘melt’ points, they could be treated theoretically as uniform materials, with well defined physical properties. However, variation of molecular weight, molecular weight distribution, and chain branching within a given type of polymer causes physical properties to vary with the method of synthesis, even from batch to batch, and so tabulation of the numerical values of these properties has been a laborious and expensive business. Nevertheless the very fact that such properties were largely unaffected by melt processing operations (particularly for the earliest of the polyolefins, low-density polyethylene) encouraged engineers, physicists and technologists to analyze and predict the flow behavior of these materials during processing. On the one hand there has been a growing interest in the results of modern theoretical rheology, and in the extent to which they can be used to investigate the rheological properties of polymer melts by experimental means; this work has been based both on concepts of material structure, the molecular approach, and on strictly mathematical ideas, the continuum approach; it has led in most cases to idealized mechanical (or mathematical) models of rheological behavior, using as few material parameters (or parametric functions) as possible, consistent with a model behavior approximating that of commonly used materials. On the other hand, efforts have been made to apply the methods of traditional Newtonian fluid dynamics to analyse material flow patterns in relatively complicated situations, such as occur in actual processing operations. In general, the models used for thermoplastic melts in this latter context have been a good deal simpler than those known to be needed from general rheological experiments, while quite simple, but generally realistic, models prove intractable in complex situations. Attention has therefore been directed to approximate methods, which use whatever information is available ab initio about a particular situation to simplify the equations used to describe the situation. As an example, in the manufacture of three mil sheet by calendering, one realizes at once that the thickness of the sheet is small compared with its other dimensions, and also compared with the roll diameters. This allows lubrication theory to be employed. Such techniques, though not easy to describe fully, are highly important because of their power. They have always been used in physics and engineering, often unconsciously when little was known about the behavior of materials, and usually in isolation. They depend on our being able, when studying a particular flow pattern, to reject much of the complex rheological information available about a material because much of it proves irrelevant; in consequence, we are led to make just those rheological measurements that are necessary to understand the flow in question.