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Many thermoplastics such as polystyrene and PMMA are brittle when stressed, a property that limits applications. A good way of strengthening such polymers is to copolymerise elastomeric chains during manufacture. The elastomer chains form separate phases in the solid, typically 10-20 micrometres in diameter, so that when the material is strained, crazes form at their surfaces, increasing the energy needed to break the material. In other words, the material is toughened. The technique is known as rubber toughening, and has been applied to a wide range of polymers.
While incorporation of 10-25% rubber into rigid thermoplastics increases the toughness, the much lower tensile modulus of the rubber component lowers the stiffness of the product. The copolymer will be sensitive to all the chemicals which may attack the rigid chains as well as the elastomeric chains, so care is needed when specifying such rubber-toughened grades in place of a rigid polymer.
One of the best known examples is Acrylonitrile butadiene styrene or ABS plastic, where polybutadiene is grafted onto the styrene-acrylonitrile chains, so converting a brittle polymer into a much tougher material. The method has also been used with PMMA, high-impact polystyrene or HIPS, epoxy resin and nylon.