Crazing agents

Much attention has been focused on the microstructure of crazes in PC 102,105 -112) in order to understand basic craze mechanisms such as craze initiation, growth and break down. Crazes I in PC, which are frequently produced in the presence of crazing agents, consist of approximately 50% voids and 50% fibrils, with fibril diameters generally in the range of 20-50 nm. Since the plastic deformation of virtually undeformed matrix material into the fibrillar craze structure occurs at approximately constant volume, the extension ratio of craze I fibrils, Xf , is given by... 

Brown et al have shown that, in IMMA, both environmental and non-en-vironmental ( intrinsic ) crazing can occiu, and have shown how the incidence of the former depends on the partial pressure of the crazing agent. It is not certain whether intrinsic crazing occurs in the low temperature deformation of cross4inked glasses or whether craze formation is wholly enviroiunental in these cases. 

An inventor took his new PMMA kitchen bowl from the dishwasher, which had just completed its duties. The bowl shattered when he poured a bottle of gin into it. Subsequent investigation showed that the PMMA absorbed about 2 % of water under hot, wet conditions in the dishwasher, and that only the surface layers dried out during the dry heating stage. The surface was therefore under tensile stress due to differential swelling when the ethanol exhibited its powers as a solvent crazing agent. 

Brown has pointed out that gases at sufficiently low temperatures make almost all linear polymers craze [59-61,63]. Parameters such as the density of the crazes and the craze velocity increase with the pressure of the gas and decrease with increasing temperature. It was concluded that the surface concentration of the absorbed gas was a key factor in determining its effectiveness as a crazing agent. 

Kitagawa, M. (1976) Craze initiation of glassy polymers under action of crazing agent. 

The crazing agent thus acts through its presence within the polymer matrix. In increasing the chain mobility (lowering Tg) it facilitates the primary and secondary steps of craze initiation nucleation and stabilization of a craze. This leads to the lowering of 0 and ej in brittle polymers such as PS. Easier nucleation and stabilization even cause the appearance of crazes in otherwise ductile materials such as PPO, PSU, PVC, or PC. 

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