Performance relationships in PVC–plasticizer dry blending

Abstract

The phenomenon of plasticizer acceptance by poly(vinyl chloride) (PVC) in hotprocess dry blending is examined via scanning electron microscopy, mercury intrusion porosimetry, and torque rheometer measurements. The effects of granule porosity, resin molecular weight, and synthesis recipe in PVC manufacture by the suspension process are related to the rate of plasticizer acceptance. For a PVC resin to dry blend, i.e., to become a free‐flowing powder when mixed with plasticizer under hot‐processing conditions, the resin granules must be porous. Porosity arises from interstices between primary PVC particles. At a given granule porosity, an increase in primary particle agglomeration adversely affects dry blend performance. At constant molecular weight and for resins manufactured by a given recipe, dry‐blend performance is quantitatively described by granule porosity. With an increase in resin molecular weight, a greater granule porosity is required to maintain an equivalent dry‐blend time (DBT). Accordingly, for most suspending agent recipes, DBT is dependent directly upon granule porosity and inversely upon molecular weight. However, if the suspending agent used in resin manufacture is an excessively rapid film former, dry‐blend performance with molecular weight variation is dependent upon the suspending agent's concentration, not upon granule porosity, which must be adequate, nor upon the resin's molecular weight. An interfacial film‐forming suspending agent enhances fusion of primary PVC particles at the suspension granule—water interface, increasing the granule's “pericellular membrane” thickness. This membrane, a PVC skin, does not significantly influence dry‐blend performance with low‐ or intermediate‐viscosity plasticizers. The particle skin does impede dry‐blend rates with high‐viscosity, poorly solvating plasticizers, but this effect can be negated in part by increasing the diameter of pore openings in the topographical skin. Dry blending occurs below the glass transition temperature (T_g) of PVC with low‐viscosity plasticizers and above the T_g with high‐viscosity, poorly solvating modifiers. The influence of resin and plasticizer variables indicates the dry‐blend phenomenon to be a diffusion‐controlled process. The rate of dry blending is dependent upon two mechanisms: (1) the rate of pore penetration—which exposes the plasticizer to a much greater surface area than if it remained exterior, encapsulating the granule—and (2) the rate of plasticizer diffusion into the PVC matrix.