Corrosion of Polymer (Plastic) Materials
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between solvent and resin is important. Slightly polar resins, such as polyesters and vinyl esters, are attacked by mildly polar solvents.
Generally, saturated long-chain organic molecules, such as the straightchain hydrocarbons, are handled well by the polyesters.
Orthophthalic, isophthalic, bisphenol, and chlorinated or brominated polyesters exhibit poor resistance to such solvents as acetone, carbon disulfide,
toluene, trichloroethylene, trichloroethane, and methyl ethyl ketone. The
vinyl esters show improved solvent resistance. Heat-cured epoxies exhibit
better solvent resistance. However, the furan resins offer the best all-around
solvent resistance.
Stress corrosion is another factor to consider. The failure rate of glass-reinforced composites can be significient. This is particuarly true of composites
exposed to the combination of acid and stress.
Under stress, an initial fiber fracture occurs, which is a tensile type of
failure. If the resin matrix surrounding the failed fiber fractures, the acid is
allowed to attack the next available fiber, which subsequently fractures. The
process continues until total failure occurs.
5.6 Environmental Cracking
Stress cracks develop when a tough polymer is stressed for an extended
period of time under loads that are small relative to the polymer’s yield
point. Cracking will occur with little elongation of the material. The higher
the molecular weight of the polymer, the less likelihood of environmental
stress cracking, other things being equal. Molecular weight is a function
of the length of individual chains that make up the polymer. Longer chain
polymers tend to crystallize less than polymers of lower molecular weight or
shorter chains, and they also have a greater load-bearing capacity.
Crystallinity is an important factor affecting stress corrosion cracking.
The less crystallization that takes place, the less likelihood of stress cracking. Unfortunately, the lower the crystallinity, the greater the likelihood
of permeation.
Resistance to stress cracking can be reduced by the absorption of substances that chemically resemble the polymer and will plasticize it. In addition, the mechanical strength will also be reduced. Halogenated chemicals,
particuarly those consisting of small molecules containing fluorine or chlorine, are especially likely to be similar to the fluoropolymers and should be
tested for their effect.
The presence of contaminents in the fluid may act as an accelerator. For
example, polypropylene can safely handle sulfuric or hydrochloric acids, but
iron or copper contamination in concentrated sulfuric or hydrochloric acid
can result in the stress cracking of polypropylene.
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between solvent and resin is important. Slightly polar resins, such as polyesters and vinyl esters, are attacked by mildly polar solvents.
Generally, saturated long-chain organic molecules, such as the straightchain hydrocarbons, are handled well by the polyesters.
Orthophthalic, isophthalic, bisphenol, and chlorinated or brominated polyesters exhibit poor resistance to such solvents as acetone, carbon disulfide,
toluene, trichloroethylene, trichloroethane, and methyl ethyl ketone. The
vinyl esters show improved solvent resistance. Heat-cured epoxies exhibit
better solvent resistance. However, the furan resins offer the best all-around
solvent resistance.
Stress corrosion is another factor to consider. The failure rate of glass-reinforced composites can be significient. This is particuarly true of composites
exposed to the combination of acid and stress.
Under stress, an initial fiber fracture occurs, which is a tensile type of
failure. If the resin matrix surrounding the failed fiber fractures, the acid is
allowed to attack the next available fiber, which subsequently fractures. The
process continues until total failure occurs.
5.6 Environmental Cracking
Stress cracks develop when a tough polymer is stressed for an extended
period of time under loads that are small relative to the polymer’s yield
point. Cracking will occur with little elongation of the material. The higher
the molecular weight of the polymer, the less likelihood of environmental
stress cracking, other things being equal. Molecular weight is a function
of the length of individual chains that make up the polymer. Longer chain
polymers tend to crystallize less than polymers of lower molecular weight or
shorter chains, and they also have a greater load-bearing capacity.
Crystallinity is an important factor affecting stress corrosion cracking.
The less crystallization that takes place, the less likelihood of stress cracking. Unfortunately, the lower the crystallinity, the greater the likelihood
of permeation.
Resistance to stress cracking can be reduced by the absorption of substances that chemically resemble the polymer and will plasticize it. In addition, the mechanical strength will also be reduced. Halogenated chemicals,
particuarly those consisting of small molecules containing fluorine or chlorine, are especially likely to be similar to the fluoropolymers and should be
tested for their effect.
The presence of contaminents in the fluid may act as an accelerator. For
example, polypropylene can safely handle sulfuric or hydrochloric acids, but
iron or copper contamination in concentrated sulfuric or hydrochloric acid
can result in the stress cracking of polypropylene.