Rubber microcracks

Cyclization is generally very effective in improving the adhesion of TR and SBR to polyurethane adhesives. Rubbers treated with concentrated sulfuric acid yield a cyclized layer on the surface. This layer is quite brittle, and when flexed develops microcracks, which are believed to help in subsequent bonding by favoring the mechanical interlocking of the adhesive with the mbber. 

Hirai (1970) found that the rubber particles in ABS materials harden on oxidation and develop microcracks. These structures propagate in the continuous plastic phase parallel to the irradiated surface (Figure 3.32). 

Merz et.al. 16 Energy Absorption by Rubber Particles and Microcrack Formation... 

One problem with cured SMC and BMC products, as with unreinforced UP products, is their brittleness and susceptibility to microcracking. Parts are subject to failure or visible damage under low impact conditions. In autobody-panel applications surface appearance is key. Matrix micro-cracking which can affect surface appearance is a problem in these materials. Additions of small amounts (e.g. lOpph resin) of liquid rubber have been reported (McGarry, Rowe and Riew, 1978) to improve the microcrack resistance of SMC type compounds. 

The study revealed that water vapour diflfiised rapidly in silicone rubber and that in a PDMS-salt compound surrounded by water, the water that entered the rubber was absorbed by the salt particles, creating an osmotic pressure in the rubber. This pressure gave rise to internal cracks in the rubber, which eventually percolated and resulted in a salt exchange between the rubber-salt compound and the surrounding pure water. Equations describing the onset and propagation of the microcracks in the silicone were derived as a function of mbber fracture toughness, tensile modulus and osmotic pressure of the salt. 22 refs. 

Although crazes are typical of amorphous glassy polymers, crazes or crazelike features have also been observed in many other amorphous polymers (see below), semicrystalline polymers (eg, in PE, PP, PA), and in rubber-modified high impact polymers (eg, HIPS, ABS). In the literature they were often designated as microcracks, deformation zones, or jdeld zones (28). 

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