Crystallinity stress cracking

Flexural modulus increases by a factor of five as crystallinity increases from 50 to 90% with a void content of 0.2% however, recovery decreases with increasing crystallinity. Therefore, the balance between stiffness and recovery depends on the appHcation requirements. Crystallinity is reduced by rapid cooling but increased by slow cooling. The stress—crack resistance of various PTFE insulations is correlated with the crystallinity and change in density due to thermal mechanical stress (118). 

Materials of these types have T s of some 290-300°C and some grades are claimed to be stable to about 400°C. Whilst resistant to hydrocarbons, halogenated hydrocarbons, ethers and acids the polymers are soluble in such materials as dimethylformamide, N-methylpyrrolidone and pyridine. Bases can cause stress cracking. These non-crystalline polymers are tough at temperatures as low as -46°C whilst at 260°C they have the strength shown by PTFE at room temperature. The polymers also exhibit excellent electrical insulation properties. 

Many engineering thermoplastics (e.g., polysulfone, polycarbonate, etc.) have limited utility in applications that require exposure to chemical environments. Environmental stress cracking [13] occurs when a stressed polymer is exposed to solvents. Poly(aryl ether phenylquin-oxalines) [27] and poly(aryl ether benzoxazoles) [60] show poor resistance to environmental stress cracking in the presence of acetone, chloroform, etc. This is expected because these structures are amorphous, and there is no crystallinity or liquid crystalline type structure to give solvent resistance. Thus, these materials may have limited utility in processes or applications that require multiple solvent coatings or exposures, whereas acetylene terminated polyaryl ethers [13] exhibit excellent processability, high adhesive properties, and good resistance to hydraulic fluid. 

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. 

Commercial polymer with an overall CTFE-to-ethylene ratio of 1 1 contains ethylene blocks and CTFE blocks of less than 10 mol% each. The modified copolymers also produced commercially exhibit improved high-temperature stress cracking. Typically, the modified copolymers are less crystalline and have lower melting points.88 Modifying monomers are hexafluoroisobutylene (HFIB), perfluorohexylethylene, and perfluoropropylvinyl ether (PFPVE).89... 

Stress and strain Nature of environment Temperature Molecular weight Molecular architecture Crystallinity Orientation > < Dissolution Softening Stress-cracking Embrittlement Chemical degradation Photochemical degradation k Biological degradation... 

Some of these plastics often compete for the same applications. Strength, modulus of elasticity, impact strength, and other properties vary greatly with type, degree of crystallinity, and their preparations that result in different densities. Their stress-crack resistance and useful service temperature ranges may also vary with type of polyolefin, their crystalline structure, etc. 

Besides the stress of state in the polymer, environmental stress cracking in polymers involves both solubility and absorption rate phenomena. Sensitizing media that cause ESC can be divided into two categories those that swell or wet the polymer and those that chemically react with the polymer. The medium may be gaseous or liquid. The former mechanism has been the subject of numerous studies and is commonly recognized as the primary cause of the majority of chemically induced failures of polymers. Although both amorphous and semicrystalline polymers are susceptible to ESC, it is well known that amorphous polymers tend to be more at risk. The close packing of chains in the crystalline domains of semicrystalline polymers acts as a barrier to fluid. 

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