TABLE OF CONTENTS Unmodified PVC-U has relatively poor impact strength at, and below, ambient temperatures and the inclusion of an impact modifier significantly improves its performance, in particular the notch sensitivity (sensitive to stress concentrations at the notch).
Impact modification is achieved by incorporating rubber domains into the polymer matrix, which, of course, must fit well within the matrix. Good dispersion and adhesion to the PVC matrix is essential.
The mechanism for impact improvement depends on the rubber component being able to absorb impact energy without the propagation of crazes and cracks.
Examples of the different generic types of impact modifier that can be added to PVC formulations are:
Methacrylate butadiene styrene (MBS) terpolymer
Acrylate polymethacrylate copolymer (acrylic)
Chlorinated polyethylene (CPE)
Ethylene vinyl acetate copolymer (EVA)
Acrylonitrile butadiene styrene terpolymer (ABS)
Addition levels are typically 5 12 phr and the incorporation of impact modifiers can influence gelation and melt viscosity. Having limited compatibility with PVC, it is believed that the impact improvement is achieved via the interruption of the homogeneous morphology into a heterogeneous structure.
Acrylic modifiers have good processing characteristics linked to reduced die swell and good outdoor weathering stability. Their use in window profile and siding applications is well documented. Based on a crosslinked butylacrylate elastomer core and polymethacrylate shell technology, produced by emulsion polymerisation, the elastic core of the modifier has a hard shell to keep the product free flowing and couple the modifier to the PVC matrix. Much work has been carried out covering various aspects of their...
