Introduction
It is well known that in certain circumstances permeation through fluorpolymers can cause issues with lined piping systems. What follows is an explanation of the important factors that influence permeation. In particular, consideration will be given to (i) the mechanisms of permeation (ii) factors that influence the rate of permeation, and (iii) a discussion of the relative merits of different fabrication methods used in manufacturing fluoropolymer lined piping systems with regard to permeation. Nb. It should be noted that what follows will only give consideration to PTFE and PFA.
(i) Mechanisms of Permeation
Polymer Structure
To understand how molecules can permeate through fluoropolymers, it is necessary to understand the basic chemical structure of these materials. Both PTFE and PFA are made up of long chains of carbon atoms, surrounded by fluorine atoms Each carbon atom in the chain (which may be a thousand or more atoms long), has two fluorine atoms bonded to it. Given that: (1) the carbon carbon bond is strong and the carbon fluorine bond is one of the strongest chemical bonds known,
(2) the resulting molecule is very simple in structure (only carbon and fluorine atoms) and (3) the shape of the molecule is such that the exterior of the molecule is made up of a closely packed helical sheath of fluorine atoms protecting the carbon atoms that make up its backbone, the result is an extremely strong molecule, that is almost entirely impervious to chemical attack.
When PTFE and PFA molecules are in their bulk form, they comprise a mixture of crytalline and amorphous (non-crystalline) components. When these two structures are examined in detail, it is found that the crystalline components are denser in comparison to the amorphous ones.