The significance of a particle-flow process

A. R. Berens and V. L. Folt, B. F . Goodrich Research Center, Brecksville, Ohio 44141

The Significance of a Particle-Flow Process in PVC Melts

Introduction

previous paper from this laboratory (1)presented evidence that the melt flow of polyvinyl chloride (PVC), under certain conditions, involves the slippage of resin particles past one another, rather than a homogeneous deformation ofthe melt. Fracture-surface electron micrographs provided clear evidence that resin particles, as formed during polymerization, can maintain their identity and shape during such melt flow processes as milling or extruding. Flow by a particle-slippage mechanism was shown to be characterized by relatively low apparent melt viscosity and elasticity. Particle identity could be obliterated by sufficientlysevere thermalmechanical treatments, resulting in a more homogeneous flow process and an increase in melt viscosity and elasticity. Major differences in melt flow properties were thus related to resin particle structure and degree of fusion, i. e. to supermolecular structural features quite distinct from molecular structure parameters. This paper examines in greater detail some of theinterrelations among PVC preparation and history, supermolecular structure, and melt flow properties. The contribution of a particle-slippage melt flow process will be shown to have a significant bearing on some of the problems encountered in practical processing of PVC. Experimental-Our principal experimental evidence has been the behavior of straight, uncompounded PVC samples in a constant load capillaryrheometer at 160210°C. The rheometer used has a 0.5-inch diameter barrel and plunger and is driven by a pneumatic system capable of rapidly applying a constant force of from 500 to 2000 pounds. Flat-entry dies of 0.04 and 0.06inch diameter and 0.3-inch length were used. Powder samples were cold-pressed into pellets, and sheet samples were diced for loading into the rheometer. Flow rates weremeasured by a timed, cut-and-weigh procedure or estimated from the total flow time of a two-gram sample. Melt elasticity was compared through measurements of extrudate diameters. Rates of shear ranged from 10 to 5000 sec-l. Fracture surfaces were examined by electron microscopy of shadowed, gelatin-carbon replicas of samples broken at dry ice temperature.
POLYMER ENGINEERING AND SCIENCE, JANUARY, 7968A

Effects of Polymerization Process and Particle Structure In view of the evidence that resin particles can act as the flow units in melt deformation processes, one might expect that the size and shape of the particles would affect melt flow behavior. The particle structure, in turn, is controlled by the type of polymerization process employed. Emulsion and suspension processes, for example,yield quite different types of particles, and corresponding differences in melt flow behavior are observed. As an example, Figrire 1 shows the flow rates, under fixed conditions in the constant load rheometer, for a series of emulsion PVC’s (ca. 7000 A particle size) and suspension PVC’s of varied molecular weight (intrinsic viscosity). Figure 2 shows the post-extrusion swelling ratio (extrudatediameter/die diameter) for the same samples; values of the swell ratio less than 1.00 reflect the measurement of extrudate diameter at room temperature, for a thermal shrinkage of 3 to 4 % is to be expected upon cooling from the extrusion temperature. It is seen that the emulsion PVC’s have significantly

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INTRINSIC VISCOSITY

Figure I . Relations between melt pow rate and intrinsic uiscosity for emulsion and suspension PVC’s; extruded at 180°C, 1000-lb. load, 0.042 x 0.311 inch die.
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Figure 2. Relations between extrudate swell ratio, Dezt/ D d...