Corrosion of Polymer (Plastic) Materials
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follow to control permeation, there are disadvantages. First, as thickness
increases, the thermal stresses on the boundary increase, which can result
in bond failure. Temperature changes and large differences in coefficients of
thermal expansion are the most common causes of bond failure. Thickness
and modulus of elasticity are two of the factors that influence these stresses.
Second, as the thickness of a lining increases, installation becomes difficult
with a resulting increase in labor costs.
The rate of permeation is also affected by the temperature and temperature
gradient in the lining. Lowering these will reduce the rate of permeation.
Lined vessels, such as storage tanks, that are used under ambient conditions
provide the best service.
Other factors affecting permeation consist of these chemical and physiochemical properties:
1. Ease of condensation of the permeant: chemicals that condense readily will permeate at higher rates.
2. The higher the intermolecular chain forces (e.g., van der Waals
hydrogen bonding) of the polymer, the lower the permeation rate.
3. The higher the level of crystallinity in the polymer, the lower the
permeation rate.
4. The greater the degree of crosslinking within the polymer, the lower
the permeation rate.
5. Chemical similarity between the polymer and the permeant: when
the polymer and the permeant have similar functional groups, the
permeation rate will increase.
6. The smaller the molecule of the permeant, the greater the permeation rate.
5.3 Absorption
Polymers have the potential to absorb varying amounts of corrodents with
which they come into contact, particularly organic liquids. This can result in
swelling, cracking, and penetration to the substrate of a lined component.
Swelling can cause softening of the polymer, introduce high stresses, and
cause failure of the bond on lined components. If the polymer has a high
absorption rate, permeation will probably take place. An approximation of
the expected permeation and/or absorption of a polymer can be based on the
absorption of water. These data are usually available. Table 5.5 provides the
water absorption rates for the more common polymers, Table 5.6 gives the
absorption rates of various liquids by FEP, and Table 5.7 provides the absorption rates of representative liquids by PFA.
163
follow to control permeation, there are disadvantages. First, as thickness
increases, the thermal stresses on the boundary increase, which can result
in bond failure. Temperature changes and large differences in coefficients of
thermal expansion are the most common causes of bond failure. Thickness
and modulus of elasticity are two of the factors that influence these stresses.
Second, as the thickness of a lining increases, installation becomes difficult
with a resulting increase in labor costs.
The rate of permeation is also affected by the temperature and temperature
gradient in the lining. Lowering these will reduce the rate of permeation.
Lined vessels, such as storage tanks, that are used under ambient conditions
provide the best service.
Other factors affecting permeation consist of these chemical and physiochemical properties:
1. Ease of condensation of the permeant: chemicals that condense readily will permeate at higher rates.
2. The higher the intermolecular chain forces (e.g., van der Waals
hydrogen bonding) of the polymer, the lower the permeation rate.
3. The higher the level of crystallinity in the polymer, the lower the
permeation rate.
4. The greater the degree of crosslinking within the polymer, the lower
the permeation rate.
5. Chemical similarity between the polymer and the permeant: when
the polymer and the permeant have similar functional groups, the
permeation rate will increase.
6. The smaller the molecule of the permeant, the greater the permeation rate.
5.3 Absorption
Polymers have the potential to absorb varying amounts of corrodents with
which they come into contact, particularly organic liquids. This can result in
swelling, cracking, and penetration to the substrate of a lined component.
Swelling can cause softening of the polymer, introduce high stresses, and
cause failure of the bond on lined components. If the polymer has a high
absorption rate, permeation will probably take place. An approximation of
the expected permeation and/or absorption of a polymer can be based on the
absorption of water. These data are usually available. Table 5.5 provides the
water absorption rates for the more common polymers, Table 5.6 gives the
absorption rates of various liquids by FEP, and Table 5.7 provides the absorption rates of representative liquids by PFA.