With the exception of most PVC pressure pipe and a few profile applications where high modulus is the major consideration, most rigid and semirigid PVC compounds have impact performance requirements that can be met only through the use of impact modifiers. We present an overview of the mechanisms of impact modification and the effects of impact modifier type and concentration, interaction with other formulation ingredients, and PVC morphology as they are influenced by processing conditions. Typical end use formulations are given to illustrate the selection and use of impact modifiers for various applications.
HISTORY OF IMPACT MODIFICATIONarly attempts to extend the utility of PVC were E directed to softening or plasticization, both to enhance processability and to produce tougher (and flexible) articles (1). As the search continued for nonmigrating plasticizers and as the need for more rigid compounds became evident, internal modification or grafting/copolymerization of PVC with other monomers and polymers gained momentum (2). The major rubber companies (which also became the major PVC producers) began to develop and supply various modified PVC compounds that were intended for use "as-supplied" (3).In the early 1960s, powder-blending became more feasible and economically attractive to the end-user fabricators. The ability to inventory large quantities of PVC and a relatively small number of additives that could be used to tailor-make a wide variety of compounds improved the economics as well as performance and utility of rigid PVC compounds and products. The success of powder blending was largely due to the development of polymeric processing aids and impact modifiers.The "evolution" of materials useful as impact modifiers might be expressed chronologically as elastomers (41, elastomer/PVC grafts (5), ABS terpolymers (6), core-shell multipolymers (7). and "superfine" inorganic fillers.
MODES AND MECHANISMS OF IMPACT MODIFICATIONUnlike "glassy" polymers that have sharp melt transitions, PVC goes through a virtual metamorphe sis involving the breakdown of its particulate structure (8). At each stage of the breakdown, PVC has different physical characteristics (9). Faulkner ( 10) and Summers, et al. (1 11, have shown that complete fusion of PVC is necessary to give the ultimate in toughness, primarily as reflected in optimum tensile strength. Other investigators suggest that unmodified rigid PVC compounds develop optimum toughness (as measured by impact resistance) when 60 to 70% is in the heterogeneous state where some of the primary particles still exist in the crystalline melt matrix (12). It is this dependence on melt morphology -and the interdependence of modifier and PVC morphology-that makes ' a e modification of PVC different.Merz and his co-workers concluded that impact resistance developed from absorbing energy in the fracture of rubbery particles as cracks were propagated through them (13). Newman and Strella found this explanation unsatisfactory because the increase in energy absorption far exceeded tha...
