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Home > Infosheets > PTFE Coating Thursday, 24 April 2014 13:03

Benefits of PTFE on stud bolts

Once tightened is supposed to be tight up to the moment of repair or maintenance. Corrosion is not desired. The remedy is to burn, grind, tap and oversize tap. An expensive and time costly operation. Coating is a perfect solution to stop corrosion!

PTFE (PolyTraFluoroEthyleen) was discovered by accident in 1938 by Dr. Roy Plunkett of Dupont USA. They were looking for a non-poison coolant based on fluor polymer. After several experiments with compounds Dr. Plunkett discovered that the compound was polymerized during the night. After several weeks of testing they found out that the new material was resistant to all:

  • Acids
  • Solvents
  • Temperature up to +260 ºC
  • Temperature up to -204 ºC

Furthermore they learned that the coefficient of friction was the lowest of all known solid materials. The newly discovered polymer offered many new appliances for various industries. One of the most important applications was found in defense, air force and nuclear industry. That is the reason why Teflon (de original name of Dupont) was not released before 1946.  

PTFE is applicable as solid material or as extruding substance, divided in powder and dispersion. Fluor polymer dispersions consist of:

  1. Resin             for binding
  2. Humidifier       for fluidity
  3. Pigment         for color
  4. Bearer            water or solvent
  5. Additives        for hardening, etc

Then there is the environmental aspect. PTFE is a perfect substitute for cadmium. It equalizes cadmium in corrosion resistance and exceeds it in coefficient of friction. The maximum temperature is 260 ºC and PTFE can be produced in different colors.

Before producing it is important to know which criteria to meet in the design. For example, some resins have a lower maximum temperature. The following parameters determine the composition of the coating:

  1. Corrosion resistance                           7   Consistency after coating
  2. Chemical resistance                            8   Coefficient of friction
  3. Lubrication                                          9   Hardness
  4. Contact corrosion                              10  Layer thickness
  5. Hydrogen embrittlement                    11  Color
  6. Temperature                                      12  Non Stick

PTFE on fasteners
The quality of coated stud is often of poor quality. Frequently specifications only mention the supplier's name and layer thickness. This will lead to great confusion.

To get a good result an proper preparation is a necessity. Especially for appliances with strict conditions like offshore and (petro)chemical. When you apply PTFE coating on a blasted substrate, the advantage of ease of use will remain, but the material still isn't corrosion resistant. Therefore the material first needs to be treated with a corrosion resistant primer before covering it with PTFE. This is not necessary at Stainless Steel.

To apply the coating correctly to the bolted joint the conditions during heating are of great importance. This needs to be done in special furnaces, in which the layers melt into each other. This results in a hard, corrosion resistant coating (ASTM B-117), which protects even after many times dismantling and assembling.

The guaranteed layer thickness is between 25 en 30 mµ. When applied to oversized tapped nuts please be aware of an extra layer thickness.  The average coefficient of friction is between 0,12 and 0,16. This can be adapted by raising or lowering the amount of PTFE as solid material in the hardened coating.

Thanks to a quicker dismantling and assembling you can save a considerable amount of money considerably by coating of stud bolts. Do you want to know how much you can save? We will be happy to calculate your profit.

Considering the great amount of Fluor polymer a good advice is essential. Often a test coating is enough to retrieve all the necessary information to design the right polymer and applicator.