Silicones stress cracking

Bronzes are somewhat similar to brasses in mechanical properties and to high-zinc brasses in corrosion resistance (except that bronzes are not affected by stress cracking). Aluminum and silicon bronzes are very popiilar in the process industries because they combine good strength with corrosion resistance. 

In the case of crystalline polymers it may be that solvents can cause cracking by activity in the amorphous zone. Examples of this are benzene and toluene with polyethylene. In polyethylene, however, the greater problem is that known as environmental stress cracking , which occurs with materials such as soap, alcohols, surfactants and silicone oils. Many of these are highly polar materials which cause no swelling but are simply absorbed either into or on to the polymer. This appears to weaken the surface and allows cracks to propagate from minute flaws. 

PMMA is prone to stress cracking and contact with plasticized PVC, rubber seals and silicone sealants must be avoided or carefully studied. 

A plausible explanation of the detrimental effects of ammonia comes from the work of Hichalske and Bunker on stress cracking in silicates [19] They observed that small molecules such as ammonia and water, in which a lone electron pair is within about 1 6 A of a labile hydrogen, are uniquely effective in promoting stress cracking in silicates The postulated mechanism comprises weakening of si-0 bonds by formation of complexes in which there are sorbate hydrogen bonds to silicate oxygens coupled with interactions between the sorbate electron pairs and silicons. Complementary molecular orbital calculations... 

Dilute solutions or emulsions containing 0.5-1% of a silicone fluid are extensively used as a release agent for rubber molding. However, their use has been restricted with thermoplastics because of the tendency of the fluids to cause stress cracking in polymers. 

Polyurethane is a stiffer and firmer material than silicone introduced in 1977 for its better tear strength and low coefficient of friction. Due to documented cases of stress cracking discovered a few years after implantation, some processing changes were made, increasing performance satisfaction. Compared with silicone, it allows for a thinner lead body and when moist is slippery. This represents an advantage in cases in which two... 

The adhesives will bond almost all materials (though a primer may be needed with some), except polyolefin plastics (eg Polythene) and other low surface-energy types such as fluoropolymers (eg Teflon) and silicone-based rubbers. Alkaline glass may cause premature bond failure and all glasses should be silane primed if at all possible, as this considerably improves the joint s humidity resistance. May stress crack stressed mouldings or susceptible plastics - polycarbonate, for example. 

For a polymeric silica gel prepared by the hydrolysis of silicon tetraethoxide, =0.25 J/m k — 10 nm and po =0.50, so that tensile stresses, cracking will result. For a colloidal silica gel, the corresponding values are y v 0.25 J/ m, Iq 100 nm, po = 0.25, and CTmax 3 MPa that is, the maximum stress is considerably smaller. 

Environmental stress cracking (ESQ is the name given to a phenomenon by which a plastic product under high stresses may crack when in contact with certain active environments such as, detergents, fots and silicone fluids. 

Figure 5.313 shows the stress-crack initiating effect of non-additivated, chemically differently structured lubricant oils. Poly-a-olefins, silicone oils, and perfluoro-ether oils have a strong stress-crack initiating effect. Ester oils and polyglycols (soluble and insoluble in water) counteract stress-cracking [965]. 

The American Petroleum Institute and the National Association of Corrosion Engineers publish documents that indicate whether weldments made with certain wire/flux combinations have experienced failure in HjS environments under conditions in which failures were unexpected. Weldments with high manganese and silicon contents have been shown to be very susceptible to sulfide stress cracking at levels of hardness at or below 22 HRC, even after thermal treatments. 

Siiicon Dow Corning DC 230 silicon fluid 23 7 2 Exposure stress is the critical stress (level at which part stress cracks) Strained in bending fixture Cycolac GE Plastics... 

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