Polymer melts friction

These additives reduce the friction between the polymer aggregates or crystallites and also lower the adhesion between the polymer melt and metal surfaces (which occurs in rollers, extruders). This results in ... 

With a finite value of necessarily some intramolecular hydrodynamic interaction or shielding must occur. The importance of eq. (3.53) lies at the present time, in the fact that it can be adapted for concentrated, solvent free systems like polymer melts. As Bueche (13) pointed out, in these systems every chain molecule is surrounded by chain molecules of the same sort. As all these molecules are necessarily equivalent, one cannot speak of a hydrodynamic shielding effect. This would imply that certain chains are permanently immobilized within the coils of other chains. The contrary is expected, viz. that the centre of gravity of each chain wiH independently foHow, in the average, the affine deformation of the medium as a continuum. From this reasoning Bueche deduces that the free-draining case should be applicable to polymer melts. Eq. (3.53) is then used (after omission of rj0) for the evaluation of an apparent friction factor . After introduction of this apparent friction factor into eq. (3.50), the set of relaxation times reads ... 

The extruder is used since it mixes, melts, and degases the polymer. However, as said in the previous paragraph, from an exergetic point of view, it is not an efficient apparatus, since it dissipates mechanical work into heat by frictional forces. A possible alternative to the extruder could therefore be a separate degasser, a static mixer, or a gear pump to push the polymer melt through the mixer and perforated plate (Figure 11.6). 

Many of the comments in the previous chapter about the selection of grade, additives and mixing before moulding apply equally in preparation for extrusion. It is important of course that the material should be appropriate for the purpose, uniform, dry, and free from contamination. It should be tested for flow and while many tests have been devised for this it is convenient to classify them as either for low or high rates of shear. The main terms used in such testing ( viscosity , shear rate , shear strain , etc.) are defined in words and expressed as formulae in ISO 472, and it is not necessary to repeat them here. Viscosity may be regarded as the resistance to flow or the internal friction in a polymer melt and often will be measured by means of a capillary rheometer, in which shear flow occurs with flow of this type—one of the most important with polymer melts—when shearing force is applied one layer of melt flows over another in a sense that could be described as the relationship between two variables—shear rate and shear stress.1 In the capillary rheometer the relationship between the measurements is true only if certain assumptions are made, the most important of which are ... 

Melting of the solid polymer is the result of heat from two sources conduction from the heated barrel, and friction between the barrel and the polymer. Most of the heat comes from friction between the barrel, the viscous melt film, and the remaining solid polymer. This frictional heating is most efficient when the melt viscosity is high, and when the melt film is thin. Thinness of the melt film depends on designing and maintaining a very small clearance between the screw flights and the barrel surface. 

The present paper is organized as follows. In Sect. 2 we present an experimental system for which both chemically end-grafted layers with high grafting densities and irreversibly adsorbed surface anchored layers can be formed (Sect. 2.1). We then discuss how the internal structure of these two kinds of surface layers can be analyzed and compared to the different available models (Sect. 2.2). In Sect. 3, we review what is expected for the interdigitation between such surface-anchored layers and a polymer melt, and compare these expectations with neutrons reflectivity results. In Sects. 4 and 5, we analyze the experimental data and the different models which allow to understand under which conditions such surface anchored layers can be used to promote adhesion or to reduce friction. 

Figure 8. Results obtained for a polymer melt of molecular weight 9.6 10 flowing on a silica surface covered with a pseudo brush made from a melt of molecular weight 1.93 10. a) Vj as a function of Vt b) slip length b as a function of Vj. The friction in the low shear regime is comparable to that obtained on a low density surface layer, but the critical velocity V is much...

Slip and friction of polymer melt flows N. El Kissi, J-M. Rau... 

The existence of a static friction stress, corresponding in fact to the flow rate q on flow curves. Thus in the conditions used in this study, the polymer melts only slip above a critical stress threshold. They adhere when at rest. 

An attempt was made to determine the effect of wall material on friction with polymer melt slip, in terms of variations in the extrapolation length "b". 

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