How does the PFA work in our valves?

How does the PFA work in our valves?

SIESA PFA valves are corrosion resistant and guarantee zero leaks. They are highly recommended in the semiconductor, optoelectronics, pharmaceutical, food and beverage, petrochemical, refining, biochemical, cosmetics and high purity industries.

What is PFA?

Perfluoroalkoxy or PFA copolymers are a class of fluoropolymers with properties similar to polytetrafluoroethylene (PTFE). It differs from PTFE resins in that it is processable in the melt state using conventional injection molding and screw extrusion methods. 

PFA was invented by DuPont, introduced to the market in 1972, and is sold under the trade name Teflon PFA (Teflon is the better-known trade name for PTFE). Other trade names are Daikin's Neoflon PFA or Solvay's Hyflon PFA. 

PFA, like PTFE, has a low coefficient of friction and low reactivity, but is more easily formable. PFA copolymers are softer than PTFE and melt at 305ºC.

Properties and characteristics of PFA

PFA polymers are completely fluorinated (except for the CO bonds) and melt processable. They have chemical resistance and thermal stability comparable to PTFE. The specific gravity of perfluoroalkoxy resins is in the range of 2.12 to 2.17. PFA, like PTFE, has a continuous use temperature limit of 250°C in chemicals at any concentration.

The crystallinity and specific gravity of PFA parts decrease as the cooling rate of the molten polymer increases. The lowest crystallinity obtained by quenching the PFA melt on ice was 48% (specific gravity 2,123). 

PFA has excellent electrical properties such as high insulation resistance, low dielectric constant (2,1), and low dissipation factor. The dielectric constant and dissipation factor remain practically unchanged in the range of -40 to -250°C and 100 Hz at 2,4×1010 Hz. Dielectric strength (short term) is 80 kV/mm for a 0,25 mm thick film. 

The chemical properties of PFA are similar to those of PTFE. PFA is attacked by radiation, and begins to degrade in air at a slightly higher dose than PTFE, which begins to degrade at 0,02 Mrad.

Features of PFA coating

For several years, fluoropolymers have played an important role in the chemical and similar industries, protecting plants and equipment against a wide range of corrosive media. This is because PFA has substantially better chemical resistance and thermal stability than other engineering plastics. PFA, a perfluoro-alkoxy polymer that has been used successfully for 20 years as a lining material, is now a thermoplastic successor to PTFE. Compared with PTFE, PFA has the equivalent thermal and chemical resistance, while it is superior in processability, translucency, penetration resistance and mechanical strength.

In the chemical industry, PFA is more applied to linings than PTFE because using PTFE is more difficult for lining metal parts of complicated shape, such as valves and pumps. PFA material, a thermoplastic material with a well-defined melting point, can be processed by transfer molding to achieve better coating quality.

The granular material is melted in a melt pot and then fed into the hot tool. This transfer molding method makes very precise wall thicknesses, even on tight radii and undercuts. Additionally, no mechanical finishing is needed except removal of the esprada and smoothing the mating faces of the flanges. Finally, the uniformity of the wall thickness can vary, especially in the case of a more complicated shape such as valve housings.

Prevention against permeation

Unlike metals, thermoplastics absorb variable amounts of the media in which they come into contact. Absorption can be followed by permeation through the coating. Such problems can be solved by increasing the wall thickness of the liner or by installing devices to eliminate the gap between the fluoropolymer liner and the metal inner wall. It is clear that in terms of resistance against permeation and absorption, PFA, or melt-processed fluoropolymers, is better than PTFE.

Learn more about permeation here

Use of SIESA valves

  1. Highly corrosive non-acidifying acids, such as chlorides, sulfuric acid, humid gases, phosphoric acid, acetic acid and hydrofluoric acid.
  2. Fluid transport systems where there is interaction between acids and alkalis.
  3. Semiconductor and pharmaceutical manufacturing process where the precipitation of organic compounds and the leaching of metal ions occur.
https://www.youtube.com/watch?v=4JeOw9N7pWQ
SIESA PTFE AND PFA valves

 

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Reference

http://www.buenoeco.com/index.php?option=product&lang=sp&task=pageinfo&id=1126&belongid=971&index=1

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