The improvement of physical properties, particularly impact strength, is the role of an important group of additives, both for thermoplastics and thermosets. The aim is to compensate for inherent brittleness, or embrittlement occurring at sub-zero temperatures, notch-sensitivity and crack propagation. The mechanism is normally to introduce a component which can absorb the energy of an impact, or dissipate it. One of the main methods is to introduce microscopic particles of rubber, but there is also considerable interest in the surface treatment of fillers and other additives, such as pigments, to give them an impact modification function also and so add to their value. John Murphy discusses the options for the compounder.
An important aspect of an impact modifier is its ability to bond, either mechanically or, more recently, chemically, with the matrix polymer. It is important, however, to differentiate between impact modification and reinforcement. In some polymer matrices, reinforcement such as glass fibre actually makes the matrix more brittle.