Corrosion of Linings
189
permeation rates increase linearly with the partial pressure gradient, and
the same effect is experienced with concentration gradients of liquids. If the
permeant is highly soluble in the polymer, the permeability increase may
not be linear.
The thickness of the polymer affects the permeation. An increase in
thickness will generally decrease the permeation by the square of the
thickness. However, there are disadvantages to this approach. First, as the
lining thickness is increased, thermal stresses on the bond are increased,
resulting in bond failure. Temperature changes and large differences in
coefficients of thermal expansion are the most common causes of bond
failure. The thickness and modulus of elasticity of the lining material are
two of the factors that influence these stresses. In addition, as the thickness of the sheet lining material increases, it becomes more difficult to
form, and heat may have to be supplied. Also, the thicker sheets are also
more difficult to weld. A third factor is cost. As the thickness of the material increases, not only does the material cost more, but the labor cost also
increases because of the greater difficulty of working with the material.
If polymers such as fluorinated ethylene propylene (FEP), polyvinylidene
fluoride (PVDF), or polytetrafluorethylene (PTFE) are being used, the cost
may become prohibitive.
The density of the polymer in addition to its thickness will also have an
effect on the permeation rate. The higher the specific gravity of the sheet,
the fewer the voids that will be present through which permeation can take
place. A comparision of the specific gravity between two different polymers
will not give an indication of the relative permeation rates. However, a comparision of two liners of the same polymer will provide the difference in the
relative permeation rates. The liner having the greater density will have the
lower permeation rate.
Other chemical and physiochemical properties affecting permeation are:
1. Ease of condensation of the permeant. Chemicals that readily condense 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 permenant. When
the polymer and the permeant both have similar functional groups,
the permeant rate will increase.
6. The smaller the molecule of the permeant, the greater the permeation
rate.
189
permeation rates increase linearly with the partial pressure gradient, and
the same effect is experienced with concentration gradients of liquids. If the
permeant is highly soluble in the polymer, the permeability increase may
not be linear.
The thickness of the polymer affects the permeation. An increase in
thickness will generally decrease the permeation by the square of the
thickness. However, there are disadvantages to this approach. First, as the
lining thickness is increased, thermal stresses on the bond are increased,
resulting in bond failure. Temperature changes and large differences in
coefficients of thermal expansion are the most common causes of bond
failure. The thickness and modulus of elasticity of the lining material are
two of the factors that influence these stresses. In addition, as the thickness of the sheet lining material increases, it becomes more difficult to
form, and heat may have to be supplied. Also, the thicker sheets are also
more difficult to weld. A third factor is cost. As the thickness of the material increases, not only does the material cost more, but the labor cost also
increases because of the greater difficulty of working with the material.
If polymers such as fluorinated ethylene propylene (FEP), polyvinylidene
fluoride (PVDF), or polytetrafluorethylene (PTFE) are being used, the cost
may become prohibitive.
The density of the polymer in addition to its thickness will also have an
effect on the permeation rate. The higher the specific gravity of the sheet,
the fewer the voids that will be present through which permeation can take
place. A comparision of the specific gravity between two different polymers
will not give an indication of the relative permeation rates. However, a comparision of two liners of the same polymer will provide the difference in the
relative permeation rates. The liner having the greater density will have the
lower permeation rate.
Other chemical and physiochemical properties affecting permeation are:
1. Ease of condensation of the permeant. Chemicals that readily condense 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 permenant. When
the polymer and the permeant both have similar functional groups,
the permeant rate will increase.
6. The smaller the molecule of the permeant, the greater the permeation
rate.