Permeation of PTFE
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, as shown in the diagrams below.
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.
As a result of its unique structure, there are 3 distinct ways in which permeation can occur:
Permeation Type 1
Physically very small molecules, such as helium, water or carbon dioxide can permeate through PTFE and PFA. This happens because the molecules are sufficiently small to allow them to pass through the structure of the polymer in the gaps between the individual polymer molecules. It has been found that this type of permeation is largely absent in the crystalline components of PTFE and PFA because within crystals the individual molecules form an orderly structure, leaving little space for other molecules to pass through. In the amorphous components, the molecules are arranged in a random fashion, resulting in a structure that is about 15% less dense than the crystalline one, thus leaving more space between individual polymer molecules, making the structure more permeable. It should be noted that while these very small molecules can permeate through the structure of PTFE and PFA, the permeation process does not cause any damage to its structure, its corrosion resisting and its non-stick properties.
Permeation Type 2
Atoms that are chemically similar to fluorine, such as chlorine and bromine, can permeate through the structure of PTFE and PFA. Here the permeation mechanism is one of substitution of atoms in the polymer chains. A chlorine atom, say, takes the place of a fluorine atom on a PTFE polymer chain on the surface of the PTFE. It can then jump from there to a PTFE molecule further into the structure, and so on through the entire thickness of the material.
It should be noted that at a molecular level, this transfer of individual atoms between molecules is quite normal, in this case fluorine atoms jumping from one PTFE molecule to another. Therefore, the transfer of other atoms through the thickness of the PTFE does not cause any damage to the overall structure of the polymer.
(ii) Factors Influencing the Rate of Permeation
The rate at which materials can permeate through the thickness of a piece PTFE or PFA is governed by a wide variety of factors, which result in greatly varying rates of permeation. In many circumstances, a piece of lined equipment may be in service for 20 or more years without any evidence of permeation. However, there can be circumstances where permeation will become evident in a matter of weeks or months after a piece of equipment is put into service. Over the course of many years of research, the following factors have been found to have a major influence on the rate of permeation:
1. The polymer layer thickness
2. The temperature
3. The pressure differential across the polymer layer
4. The concentration of permeant in the contained fluid
Dealing with each of these in turn:
1. Polymer Layer Thickness
If two polymer layers, made of identical material, fabricated in the same manner were tested for rate of permeation, it would be found that the rate of permeation through the thicker layer would be lower than that through the thinner layer. In most circumstances the fall off in permeation rate is non-linear with thickness, often decaying in roughly logarithmic manner. However, as the thickness continues to increase it has been found that the permeation rate tends to plateau, rather than continue to fall.
2. Temperature
As the temperature increases, the rate of permeation through PTFE and PFA increases, in a non-linear fashion. This is driven by several factors, namely, as the temperature increases (a) the permeant will become more soluable in the polymer, (b) there is an increase in the amount of swapping of individual atoms between the polymer chains, and (c) the polymer increases in volume, leading to more space between the individual polymer chains giving more space for atoms to pass between them. It should be noted that not all of these mechanisms may occur in any one particular set of circumstances.
3. Pressure Differential Across the Polymer Layer
As the pressure differential across the polymer layer increases, the rate of permeation through PTFE and PFA increases in a roughly linear fashion in most circumstances.
4. Permeant Concentration
As the permeant concentration in a liquid, or partial pressure of a gas increases, the rate of permeation increases in a linear fashion.
Important Note
For all of the above factors, it should be noted that most research work that has been carried out into permeation rates has been on polymer layers ranging from a few tens of microns thick up to a few tenths of a millimetre thick. The results obtained are, on the whole, extremely variable, and owe much to the method of fabrication of the test membrane, and the exact test methodology used. Attempts to translate this thin film data into meaningful data for liners used in piping systems (with liners typically ranging from 3 – 10mm thick) have proved to be singularly unsuccessful.