Permeation: Key Factor When Looking for Coated Pipe Systems

Permeation: Key Factor When Looking for Coated Pipe Systems

PERMEATION IN PTFE AND PFA COATED PIPING SYSTEMS

We seek to establish the main points to consider when selecting fluoropolymer-coated piping systems, due to the phenomenon known as permeation.

PTFE (Polytetrafluoroethylene) and PFA (Perfluoroalkoxy) coated pipes are an ideal solution for processes that handle highly corrosive fluids. They contain unique properties such as:

  • Inertia
  • Low thermal and electrical conductivity,
  • Resistance to high service temperatures
  • Mechanical resistance given to the system by the metal casing.

Since ancient times, the unique properties of PTFE and PFA were found to be directly attributable to the strength of the chemical bonding of the Carbon-Fluorine chain, this same chemical bonding introduced a complexity in processing, since the material is so viscous at temperatures melting, that standard manufacturing methods for plastics, such as extrusion or injection molding, are simply not feasible, and instead require specific manufacturing techniques that are performed on specialized processing equipment.

The modification of the basic Carbon-Fluorine structure, mainly in the form of side chains, introduced new properties that improved the processing capacity and, in most cases, modified costs and yields, these improvements caused the level to increase. of competition between fluoropolymer suppliers, giving rise to the search for technical improvements, among them, the study of the phenomenon of permeation stands out in a special way.

Consequences of permeation


Permeation through fluoropolymers is known to cause problems with coated piping systems,

In particular, the phenomenon of permeation in fluoropolymer-coated pipes is a simple 3-stage physical process that involves the absorption of a fluid on the pipe wall.
container, diffusion through that wall and exhaust, or evaporation on the outside of the container wall, the container being the metal casing itself, although there are interfacial spaces.

How can permeation occur?

As a result of the unique structure that PTFE and PFA have, there are 3 different ways in which permeation can occur:

  1. Physically very small molecules (such as Helium for example) can penetrate through PTFE and PFA, this is due to the intermolecular voids that exist in the structure of said fluoropolymers. It should be noted that this form of permeation does not damage the structure of the polymer, nor does it alter its properties, such as corrosion resistance and anti-adherence.
  2. Atoms that are similar, from a chemical point of view (like fluorine for example), can penetrate through the structure of PTFE and PFA, substituting atoms in the polymer chains. This intermolecular atom transfer is normal, and therefore does not harm the overall structure of the polymer.
  3. In a strict sense, it is not a matter of “permeation” as such, although the end result is the same, that is, there is movement of material through the thickness of the fluoropolymer layer, which is the result of defects in its manufacturing process. .

Damage due to permeation can occur in different ways, among which are:

  1. Damage to the outer cover. That is, damage to the metal casing, significantly reducing mechanical resistance and creating potentially dangerous conditions.
  2. Emissions or inadvertent release of media that are potentially hazardous.
  3. Deformation of the coating. Due to the accumulation of corrosion by-products at the interface of the metal casing. This being the most extreme case, since it reduces the vacuum resistance of the coating, seriously affecting the flow of the fluid in the piping system.

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Key Factors Affecting Permeation

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