Applied stress, environmental resistance

Resistance to Chemical Environments and Solubility. As a rule, amorphous plastics are susceptible, to various degrees, to cracking by certain chemical environments when the plastic material is placed under stress. The phenomenon is referred to as environmental stress cracking (ESC) and the resistance of the polymer to failure by this mode is known as environmental stress cracking resistance (ESCR). The tendency of a polymer to undeigo ESC depends on several factors, the most important of which are applied stress, temperature, and the concentration of the aggressive species. 

The environmental stress crack resistance (ESCR) of glass fiber-reinforced SPS/nylon blends was characterized by exposing the surface of test bars with 0% and 1 % strains applied to drops of solvent and then observing the damage to the surface. In addition, 7-day and 30-day chemical inunersion resistance were evaluated. The results of this testing for glass fiber-reinforced SPS/nylon blends are shown in Table 16.7. 

Cebrian, G., Sagarzazu, N., Pagan, R., Condon, S., and Manas, P. 2008. Resistance of Escherichia coli grown at different temperatures to various environmental stresses. Journal of Applied Microbiology 105 271-278. 

As we saw in the preceding discussion, several mechanical parameters can be derived from stress-strain tests. Two of these parameters are of particular significance from a design viewpoint. These are strength and stiffness. For some applications, the ultimate tensile strength is the useful parameter, but most polymer products are loaded well below their breaking points. Indeed, some polymers deform excessively before rupture and this makes them unsuitable for use. Therefore, for most polymer applications, stiffness (resistance to deformation under applied load) is the parameter of prime importance. Modulus is a measure of stiffness. We will now consider how various structural and environmental factors affect modulus in particular and other mechanical properties in general. 

A plastic material that is resistant to a given chemical in an unstressed state may crack when exposed under stress to the same chemical. This stress may arise not only from externally applied forces but also from internal stresses due to molding operations. Most plastics suffer from this phenomenon of environmental stress cracking, and it is the cause of many failures in service. Several test methods have been developed to quantify this effect. 

As a concluding comment it must be pointed out that, in considering styrene-based plastics for particular applications, the poor solvent resistance might be a limitation. Simultaneous application of stress, in the presence of a solvent, may give premature failure due to environmental stress cracking. These comments apply equally to PS, HIPS, ABS and the PPO-PS blends. 

Thermal effects are the most cormnon of the environmental effects, since all components will have a temperature coefficient of expansion and matty will have a temperature coefficient of resistance. Thns, besides possible resistance changes with temperatnre, there will be dimension changes, with the resulting possibility of stress forces being applied to mounted components. Since semiconductor devices are pressine sensitive, ambient temperature changes may have a marked effect on the performance of equipment and circuits incorporating any snch... 

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