Permeation can be defined as passage of gases and liquids through a second material such as solid. It is significant consideration in the selection of the plastic material for the construction of the chemical processing equipment because process fluids may travel across the thickness of the polymer by permeation. Permeated species in sufficient quantities could cause corrosion, contamination.
Permeation is molecular migration through microvoids either in the polymer (if the polymer is more or less porous) or between polymer molecules. In neither case is there an attack on the polymer. This action is strictly a physical phenomenon. However, permeation can be detrimental when a polymer is used to line piping or equipment. In lined equipment, permeation can result in:
- Failure of the substrate from corrosive attack.
- Bond failure and blistering, resulting from the accumulation of fluids at the bond when the substrate is less permeable than the liner or from corrosion/reaction products if the substrate is Attacked by the permeant loss of contents through substrate and liner as a result of the eventual failure of the substrate All polymers do not have the same rate of permeation. In fact, some polymers are not affected by permeation. The fluoro-polymers are particularly affected.
Some control can be exercised over permeation that is affected by :-
- Temperature and pressure
- The permeant concentration
- The thickness of the polymer
Increasing the temperature will increase the permeation rate because the solubility of the permeant in the polymer will increase, and as the temperature rises, polymer chain movement is stimulated, permitting more permeant to diffuse among the chains more easily. The permeation rates of many gases increase linearly with the partial pressure gradient, and the same effect is experienced with the concentration of gradients of liquids. If the permeant is highly soluble in the polymer, the permeability increase may be nonlinear. The thickness will generally decrease permeation by the square of the thickness.
The density of the polymer as well as the thickness will have an effect on the permeation rate. The greater the density of the polymer, the fewer voids through which permeation can take place. A comparison of the density of sheets produced from different polymers does not provide an indication of the relative permeation rates. However, a comparison of the sheets' density produced from the same polymer will provide an indication of the relative permeation rates. The denser the sheet, the lower the permeation rate.
The thickness of the liner is a factor affecting permeation. For general corrosion resistance, thicknesses of 0.010–0.020 in. are usually satisfactory, depending on the combination of lining material and the specific corrodent. When mechanical factors such as thinning to cold flow, mechanical abuse, and permeation rates are a consideration, thicker linings may be required. Increasing a lining thickness will normally decrease permeation by the square of the thickness. Although this would appear to be the approach to follow to control permeation, there are some disadvantages. First, as thickness increases, the thermal stresses on the boundary increase that can result in bond failure. Temperature changes and large differences in coefficients of thermal expansion are the most common causes of bond failure. The plastic's thickness and modulus of elasticity are two of the factors that influence these stresses. Second, as the thickness of the lining increases, installation becomes more difficult with a resulting increase in labor costs. The rate of permeation is also affected by the temperature and the 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 :-
- Ease of condensation of the permeant. Chemicals that readily condense will permeate at higher rates.
- The higher the intermolecular chain forces (e.g., van der Waals hydrogen bonding) of the polymer, the lower the permeation rate.
- The higher the level of crystallinity in the polymer, the lower the permeation rate.
- The greater the degree of cross-linking within the polymer, the lower the permeation rate.
- Chemical similarity between the polymer and permeant when the polymer and permeant both have similar functional groups, the permeation rate will increase.
- The smaller the molecule of the permeant, the greater the permeation rate.