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Impact modifiers (IMs) are a class of toughening functional additives that increase the impact strength of the plastic articles. Many commodity thermoplastics, such as PVC and PS, are brittle at ambient conditions (i.e., poor impact strength) and easily undergo cracking and crazing. In order to meet the physical requirements for certain applications, an IM additive is used. IMs are elastomeric and rubbery and have a lower modulus than the host polymer system. When effectively dispersed into the polymer matrix, the rubbery phase of the IM acts to absorb or dissipate the energy from impact in order to stop craze or crack propagation. IMs can be grafted to the polymer during polymerization or physically blended during compounding. Styrenic oligomers/copolymers, such as ABS and methyl methacrylate‐butadiene‐styrene, make up the largest category of IMs, accounting for about 45% of the market (Markarian 2004). These along with acrylics that command 30% of the market and are used mostly in PVC. Elastomers, including ethylene‐propylene‐diene terpolymer (EPDM) and thermoplastic elastomers, make up about 10% used with polyolefins. The remainder is made up of chlorinated polyethylenes (CPE) and other types. IMs, such as ABS, EPDM, and CPE, are also “stand‐alone” plastic products. These materials are used as IMs in their oligomeric forms (i.e. MWs of 5000–20 000 g/mol; Ambrogi et al. 2017). Similar to plasticizers and FRs, IMs are often used at relatively high concentrations in the plastic formulation. However, since most IMs are large molecules, leaching from the plastic into the environment has not been a major concern.

Table 2.3 Examples of common antioxidant additives used in plastics.