Surface-initiated crazes

Fig. 2, Distribution of surface-initiated crazes in a thin sheet sample...

ESC is mostly a surface-initiated failure of multiaxially stressed polymers in contact with surface-active substances. These surface-active substances do not cause chemical degradation of the polymer, but rather accelerate the process of macroscopic brittle-crack failure. Crazing and cracking may occur when a polymer under multiaxial stresses is in contact with a medium. A combination of external and/or internal stresses in a component may be involved. 

The EPDM particles were found to either initiate crazes or to terminate them, depending on the interfacial bonding, the particle size, concentration and the interparticle distances. The variation of notched Izod impact strength of PPBC blends with different EPDM concentrations is shown in Figure 12.16. The fracture surface of a blend with 10 wt% EPDM, as examined under SEM, is shown in Figure 12.20. The hemispherical embeddings and hollows (representing the removed... 

Ultraviolet light initiates free radical oxidation at the exposed surface of an elastomeric product to generate a layer of oxidized rubber. Heat, moisture, or high humidity can then initiate crazing of the surface, which subsequently can be abraded off. Such degradation of the surface is more severe with nonblack stocks than with black compounds. Nonblack compounds such as white tire sidewalls thus require higher levels of nonstaining antioxidants than carbon black-loaded formulations. 

The applied global set of stresses is concentrated at surface grooves, where the pair of stresses and produce different but complementary effects. To initiate crazing nascent nano-cavities need to be produced by local plastic flow under... 

In homo-polymers, when crazes are surface-initiated, generally imder a substantial stress, premature fracture follows from craze breakdown initiated from entrapped dust particles of low adhesion to craze matter. To counteract this, various types of compliant particles having a variety of morphologies that are effective in craze initiation under lower stresses are incorporated into the homo-polymers through synthesis or blending. We discuss this practice in Chapter 13 on toughening of brittle polymers. 

To achieve craze plasticity with substantial potential to alleviate brittleness, well under the stress levels given above, a definite strategy needs to be followed, which is not possible through surface-initiated growth of crazes. 

Poly(ethylene terephthalate), (PET), is a thermoplastic polymer widely used in the production of fibers and films on exposure to near ultraviolet light, PET fibers tend to lose their elasticity and break easily PET films become discolored, brittle and develop crazed surfaces. Such deterioration in properties has been attributed to photochemical reactions initiated by the... 

Figure 16. SEM images of the fracture surface of an RH-cycled (80-120% RH) sample, showing crack initiation and growth from a craze site near the anode and membrane interface. A crazing site found on Hi-RH cycled samples. The half-penny shaped craze closely resembles crazes formed on free surfaces.

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