Mechanisms and applications in thermoplastics

It is desirable for plastics articles to be able to withstand cracking when subject to minor impact. Failure to resist cracking is a common feature of plastics and this entry discusses the way in which polymers can be used as toughening additives to overcome the problem. 

Several thermoplastics, both of the commodities kind [polystyrene (PS), polyacrylonitrile (PAN), polymethylmethacrylate (PMMA), polypropylene (PP), polyvinylchloride (PVC) etc.] and engineering pol)uners [polyamides (PA), polyesters (PE), polycarbonates (PC), polyimides (PI), polysulfones (PSF), polyoxymethylene (POM), polyphenylene oxide (PPO) etc.] exhibit glass transition temperatures (Tg) higher than or close to room temperature (R.T.). As a consequence they show, at R.T. or below it, the shortcoming of brittle impact behaviour, which limits their commercial end-uses. 

Three main classes of pol)aner matrices can be identified with respect to their failure characteristics  

brittle amorphous pol)maers, such as PS and SAN, with low unnotched and notched impact strengths, for which crack initiation and propagation stresses are lower than the yield stress  

pseudo-ductile engineering pol5nners, such as PC, PA, PI, PE and PSF, with high imnotched and low notched impact strengths they exhibit high crack initiation and low crack propagation energies and a brittle-to-ductile transition temperatures as well  

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