This month, polymer expert and technical blog author, Dr Charlie Geddes, ‘clears’ the myths about ABS and looks into how changing the chemical competition can affect its properties
August Technical Blog
Mentioning transparency in the context of ABS moulding materials can raise a few eyebrows because ABS is normally taken to be opaque and indeed the vast majority of grades of ABS are opaque.
ABS is a two-phase system, which can be considered as small rubbery particles dispersed in a rigid thermoplastic matrix of styrene-acrylonitrile copolymer (SAN). The SAN provides stiffness and the rubber particles provide impact resistance. One feature of a two-phase system is that light gets scattered as it passes from phase to phase, resulting in very low light transmission levels. It is possible to minimise scattering of light and maximise transmission if the two phases have similar refractive index values.
Manufacturers of ABS soon discovered that incorporating methyl methacrylate (MMA) monomer into the SAN rigid phase and tinkering with the composition of the rubbery phase resulted in matched refractive index values and hence high transparency could be achieved.
Of course tampering with the chemical composition has a knock-on effect on other properties. Clear ABS (strictly speaking MABS) cannot compete with the impact resistance of impact grades of ABS nor the heat resistance of high heat grades of ABS. However clear ABS can outperform polystyrene, SAN and PMMA for impact resistance and has similar heat distortion temperatures. Polycarbonate can outperform clear ABS for both impact resistance and high temperature performance but clear ABS scores well for stress crack resistance and processability, with good melt flow characteristics.
Typical applications include domestic goods, cosmetic packaging, medical components and sports and leisure goods.