Bulletin No. 37 - Venting of PTFE Lined Pipe and Equipment

Venting of PTFE Lined Pipe and Equipment

Permeation Through Plastics

What is Permeation?

Permeation is the molecular transport of a chemical component through a polymer. In any polymer/fluid (either gas or liquid), a degree of permeation is inevitable. The driving force behind permeation is chemical potential, normally quantified as concentration or pressure gradient. Combining Henry’s law governing solubility and Fick’s laws of diffusion, the permeation flow Q through a flat membrane can be described by:

Q = P . A . Δp/d                      [1]

with P the permeation coefficient, a material, chemical and temperature dependent parameter, A the membrane area, Δp the vapor pressure gradient over the membrane and d the membrane thickness.

In other words, permeation is the product of solubility and diffusion of a chemical component in a polymer.

Figure 1. Permeation

What are the effects?

Permeation can have several effects, often detrimental to the operating life of polymer lined systems in chemical environments.
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