Of all the properties of PTFE/Teflon, the one that people tend to know best is that it is non-stick. While this characteristic is usually attributed to the now increasingly discontinued application of PTFE in non-stick cookware, it does have wider applications in surface protection, sliding elements, and self-lubricating bearing materials.
However, there exist many applications where PTFE is required to be bonded to other surfaces. Most notable among these would be in structural, sliding bearings, where a PTFE sheet must be bonded on one side to a metal surface, while the other side is exposed as a sliding element. In such an arrangement, the PTFE sheet is bonded to a metal plate and a stainless-steel plate is placed on top of the PTFE sheet. The low coefficient of friction between PTFE and polished stainless-steel means that the stainless-steel sheet can slide freely along the PTFE sheet’s surface. The stainless-steel plate is itself welded to another metal surface and a vertical load is applied to it. These PTFE sliding bearings are used in infrastructure to accommodate both loads and movement. However, the high vertical loads also mean that shear loads exist, which act directly on the bond between the PTFE and the metal, making it essential that the bonding process is done with the utmost care and technical understanding.
Here we look at some of the factors that affect the bonding and throw light on the precautions and preparations needed to ensure a strong, reliable bond.
Appearance: Virgin PTFE is white in color and does not bond to surfaces unless it is chemically treated (etched) using a special process. The formulation of the chemical etchant is proprietary, with each processor using a method that suits them best. Once etched, the surface of the PTFE changes color to brown. This brown surface can be bonded easily using standard industrial grade adhesives.
Surface preparation: The metal surface to be mounted with PTFE can be prepared by the normal machining methods such as grinding, milling, shaping, and planning. The surface roughness of all forms of preparation should be preferably between Ra = 1.6 µm and Ra = 3µm and not more than Ra = 6µm. Once roughened the surfaces can be cleaned with trichloroethylene, perchloroethylene or acetone. As with any bonding process, we would need to ensure the surface of the metal is free from grit and debris.
Bonding PTFE: For bonding of PTFE the following resin adhesive can be used: Araldite - Hardener - HV 953U; Araldite AW106. The Araldite should be applied both to metal and PTFE sheet and be spread as uniformly as possible by means of a serrated spatula. To obtain the best dispersion of the adhesive, when spreading on the surface brush in the longitudinal direction; when spreading on the metal, brush in the transverse direction. The total quantity of bonding should be approximately 200gm per sq. mt.
Other bonding agents can also be looked into, but usually a good industrial grade agent would be recommended.
Hardening: After mounting the PTFE a clamping pressure of between 10-15 Kg/cm2 is recommended. It is important to keep the pressure constant during the hardening process. Due to the differences in the thermal expansion coefficient of the materials, maximum curing temperature should not exceed 40°C. The hardening times for various temperatures are 20°C min 15 hours; 25°C min 12 hours; 40°C min 5 hours.
Finishing: After curing of the adhesive, the PTFE can be machined by conventional means – if required. The choice depends on the machinery available viz.: grinding; grindstone.
Grinding: For grinding of PTFE use the same speed as grinding cast iron, taking care that sufficient cooling is used with an ‘open stone’. The grindstone should be preferably silicon carbide based with rubber or polyurethane binding; grain size 80-30. Alternatively, aluminum oxide with rubber bonding may also be used for soft, fine grinding action, pre-polishing and pre-mating treatment.
Oil Grooves: PTFE pads can be machined with oil grooves using the same methods and cutting data as used for cast iron. The form and depth of the oil grooves are optional. However, the oil grooves should never pierce through the PTFE. Oil grooves should be away from the edges by 6mm.
Maintenance: Bonded PTFE must be maintained, as the strength of the bond can be impacted by adverse environmental factors such as excessive sunlight, corrosion, and heat. Temperatures around the bonded areas should not exceed 120°C, while any presence of corrosive elements – such as sea-air and/or chemical fumes can – affect the metal surface and eat into the bond. Usually, when exposed to adverse elements, the bond strength can get affected along the edges and any corrosion along the metal can slowly eat its way into the middle.
In such a case, the bond can be reapplied along the edge of the sheet after cleaning any debris/rust from the affected area. However, care must be taken to apply a protective coating around the bonded area to ensure long-term functionality.
It should be noted that even with etching, PTFE remains a material resistant to bonding. While the etching process allows for a reasonably strong bond to metals, there is a limit to the strength of this bond. Even well bonded surfaces offer a bond strength of only 4-5 Mpa (40-50Kg/cm2). As such, in areas where the shear load is expected to be higher, the PTFE sheet may need to be supported by clamping or bolting.