PTFE Absorption Properties

In contrast to metals, plastic and elastomers absorb varying quantities of the materials they contact, especially organic liquids. Absorptivities in PTFE are unusually low, and a chemical reaction between the plastic and the other substances is a rarity (with the few exceptions noted previously). However, when absorption is combined with other effects, this property can influence the serviceability of these resins in a particular chemical environment. For example, if rapid fluctuations in temperature or pressure occur, circumstances may be created that are physically damaging. The wider service temperature range for PTFE resins exposes them to this type of physical damage more frequently that other plastics.

By way of explanation, let us consider the “steam cycle” test described in ATSM standards* for lined pipe. Samples of lined pipe are subjected to 0.8MPa (125 psi) steam, alternating with low pressure cold water, causing very severe thermal and pressure fluctuations indeed. This is repeated for 100 cycles. Steam created a pressure and temperature gradient through the liner causing absorption of a small quantity of steam which condenses to water within the liner wall. On pressure release, or on reintroduction of steam, the entrapped water can expand to vapor causing an original micro pore. The repeated pressure and thermal cycling enlarges the micro pores, ultimately causing visible water-filled blisters within the liner. The ASTM standards note that the blisters do not adversely affect pipe liner performance – the chemical barrier thickness is still intact.

Similarly, in an actual process, the plastic component may absorb process fluids and repeated temperature or pressure cycling can then cause blisters. Such an occurrence may be surprising to one familiar with the extreme chemical inertness of PTFE. This effect is not seen in metals because they fail mainly by corrosion. It is rarely seen in most other plastics and elastomers because temperatures above the boiling point of liquid chemicals are normally beyond their capability. Hence this effect is new to most corrosion engineers and chemical processors, and requires new understanding for proper selection, design, testing, and use of these fluoro-polymers. Further clarification is contained in the section on testing.

There are corrosive measures which reduce the severity of blistering. Thermal insulation of a lined pipe or vessel reduces the temperature gradient in the liner, thereby often preventing condensation and subsequent expansion of absorbed fluids. It also reduced the speed and magnitude of temperature changes, thereby minimizing blistering. Thus, by reducing the resin, insulation can provide a protective measure in many cases. Additional protection can be provided by using operating procedures or devices which limit the rate of process pressure reductions or temperature increases.

Related effects can occur with process materials which may be absorbed and subsequently react, decompose or solidify within the structure of the existing plastic. Prolonged retention of absorbed chemicals can lead to their decomposition within the fluorocarbon component. Absorbed monomers can polymerize similarly. Although unusual, these events can happen, emphasizing the value of in-process testing.