Crazing in Plastics

If a plastic article has been machined then it is likely that crazes will form at the surface. In moulded components, internal nucleation is common due to the presence of localised residual stresses. 

---

Another interesting aspect is that shower enclosures are usually either ceramic (grouted tiles) or plastic (methacrylate, or Plexiglas ). One patent emphasizes the choice of surfactant and its effect on plastic. Nonionic surfactants have been said to cause crazing in plastic surfaces [300], and one shower rinse formulation claims the use of amphoteric surfactants for superior soil removal and greater safety to plastic surfaces [301],... 

The rapid cooling of certain plastic products can result in frozen in stresses and strains (particularly with injection molding). The stresses may decay with time in a viscoelastic manner. However, they will act like any other sustained stress to aggravate cracking or crazing in the presence of aggressive media and hostile environments like UV radiation. 

Water molecules combine the tendency to cluster, craze and plasticize the epoxy matrices with the characteristic of easily diffusion in the polymer1 10). The morphology of the thermoset may be adversaly influenced by the presence of the sorbed moisture. The diffusion of the water in glassy polymers able to link the penetrant molecules is, therefore, characterized by various mechanisms of sorption which may be isolated giving useful information on the polymer fine structure. 

As a consequence, the overall penetrant uptake cannot be used to get direct informations on the degree of plasticization, due to the multiplicity of the polymer-diluent interactions. The same amount of sorbed water may differently depress the glass transition temperature of systems having different thermal expansion coefficients, hydrogen bond capacity or characterized by a nodular structure that can be easily crazed in presence of sorbed water. The sorption modes, the models used to describe them and the mechanisms of plasticization are presented in the following discussion. 

Fig. 8.1. Toughening mechanisms in rubber-modified polymers (1) shear band formation near rubber particles (2) fracture of rubber particles after cavitation (3) stretching, (4) debonding and (5) tearing of rubber particles (6) transparticle fracture (7) debonding of hard particles (8) crack deflection by hard particles (9) voided/cavitated rubber particles (10) crazing (II) plastic zone at craze tip (12) diffuse shear yielding (13) shear band/craze interaction. After Garg and Mai (1988a).

In environmental crazing the specimen fails by the development of a multitude of very fine cracks in the presence of an organic liquid or its vapour. This phenomenon may manifest itself even without the presence of mechanical stress the internal stresses, always present in plastic specimens, can be sufficient. 

Formation in Polystyrene, paper presented at British Plastics Institute, Research Meeting on the Effect of Structure on the Fracture of Plastics— The Role of Craze in Fracture, Univ. of Liverpool, Liverpool, England (April 14, 1972). 

Crack tip blunting is attributed to localized yielding at the crack tip. Localized yielding may result from shear deformation, or normal stress deformation. Unlike shear deformation, which occurs at constant colume, normal stress deformation involves a volume dilatation and is considered to be responsible for the formation of crazes in thermoplastics. Since crazes are not observed in highly crosslinked epoxies, it is generally assumed that plastic deformation at the crack tip takes place via a shear yielding process. 

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... 

At the same stress amplitude, rubber modified polymers fail sooner in fatigue than do the unmodified polymers even though they have superior resistance to fatigue crack propagation. This is a result of much earlier initiation of crazing, localized plastic deformation, and subsequent crack development due to the stress concentrating effect of the dispersed second phase particles. 

---