Surface micro-crazing

Often the most obvious effect of exposure to sunlight is colour fading this is essentially related to the pigments or dyestuffs, and not to the polymer. The other effects are the results of surface oxidation of the polymer, in a similar way to heat ageing. The first visible effect is discoloration, in the direction of yellow and then brown. Micro-crazing develops concurrently, but may not become evident until a general loss of gloss is apparent. How... 

The EPDM particles were formd either to initiate crazes or to terminate them, depending on the interfacial bonding, the particle size, the concentration, and the interparticle distances. The variation of notched Izod impact strength of PPBC blends with different EPDM concentrations is shown in Fig. 10.16. The fracture surface of a blend with 10 wt% EPDM, as examined under SEM, is shown in Fig. 10.20. The hemispherical embeddings and hoUows (representing the removed EPDM particles) are clearly visible on the fracture surface. The ribbonlike structures visible on the fractured surfaces are probably the micro-shear bands in the blends. 

Both principal fracture mechanisms, shear yielding and crazing, are influenced by the particle size. In PPBC matrix, where spherical elastomeric particles are chemically bonded, the energy absorption takes place mainly by deformation of the matrix. In such systems, a large amount of shear yielding is to be expected. The shear yielding becomes more prominent upon increasing the concentration of EPDM as well as reduction of their particle size. The micro-shear bands in the fracture surface (Pig. 10.23e) clearly support these expectations. 

In both glassy and semi-ciystalline polymers the cohesive zone often takes the physical form of a craze a colinear crack extension whose surfaces are bonded by micro-fibrillar or film material. Following the work of Williams [1] we represent such a zone most simply as a Dugdale-Barenblatt zone bearing a uniform stress. 

---