Many rigid PVC molded and extruded products – including some pipe, siding, window profiles, and sheet – require impact strengths beyond that attainable with PVC alone, which is a rather brittle polymer. The inclusion of a small amount of another polymer with a rubber “backbone” will, with proper dispersion into the vinyl melt, provide the shock absorbing mechanism required to enhance impact strength of the PVC product by absorbing the energy of impact (measured as ft./lbs. in the crack propagating notched Izod test per ASTM D-256-72, or as a falling weight per ASTM D-2444).

Several types of specialty polymers will furnish the necessary marginal compatibility with PVC to function as impact modifiers. Chlorinated polyethylene (CPE), acrylonitrile-butadiene-styrene (ABS), methacrylate-butadiene-styrene (MBS), and certain acrylate polymers are the generic classes which are most commonly used. More recently, ethylene-vinyl acetate (EVA) has shown promise as an impact modifier. Each type offers its unique advantages – and disadvantages – which relate to weathering, clarity, effect on melt viscosity during processing, and heat stability. For example:
Weathering Resistance – favors acrylates, CPE, EVA
Clarity & Minimal Stress Whitening – favors acrylates, MBS, certain ABS types
Impact Efficiency – favors MBS, ABS
Chemical Resistance – favors no modifier – all will detract

Since impact modifiers are more costly than PVC, naturally no more should be used than necessary to provide required impact strength for the intended application. The relationship between use level and impact strength is not linear, but follows as “S” curve, characteristic for each type of modifier.

Sufficient work input, or shear, must be achieved during extrusion to arrive at the optimum dispersion of modifier particles in order to obtain the maximum benefit from the quantity used. 2-5 phr, 7-10 phr, and 12-15 phr are very typical use levels to achieve desired impact strengths for pipe, profile, and sheet applications respectively.