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Why Rigid PVC uses Impact Modifiers

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

Several types of specialty polymers will furnish the necessary marginalcompatibility with PVC to function as impact modifiers. Chlorinatedpolyethylene (CPE), acrylonitrile-butadiene-styrene (ABS),methacrylate-butadiene-styrene (MBS), and certain acrylate polymers are thegeneric classes which are most commonly used. More recently, ethylene-vinylacetate (EVA) has shown promise as an impact modifier. Each type offers itsunique advantages - and disadvantages - which relate to weathering, clarity,effect on melt viscosity during processing, and heat stability. Forexample:
Weathering Resistance - favors acrylates, CPE, EVA
Clarity & Minimal Stress Whitening - favors acrylates, MBS, certain ABStypes
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 beused than necessary to provide required impact strength for the intendedapplication. The relationship between use level and impact strength is notlinear, but follows as "S" curve.

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