PTFE The Mechanics
PTFE coatings are extraordinarily resistant to chemical attack and the surface energy is very low. This means that liquids do not readily wet the solid and other solids do not adhere strongly to the PTFE coating. These properties render PTFE very valuable for forming protective surface coatings, in a wide range of applications from non-stick cookware to surgical sutures. However, the very properties that make PTFE coatings so useful in such applications also make it very difficult to form PTFE coatings which are sufficiently adherent to their substrates. In practice, PTFE coatings do rely on a mechanical key for their adherence to their substrates and by sintering at an elevated temperature, this does depend upon the type of PTFE coating selected. However, some coating systems, particularly single coat systems can be applied without the requirements of mechanical surface impact preparation, in which case, please discuss your particular specification with us.
The most common method of application is by means of spray application and provides for a very accurate method of applying PTFE coatings. The spray method is applicable to PTFE coatings on a wide range of substrates, this method is believed to be the best solution for geometric components.
The bond between the substrate and the PTFE coating is created by means of blasting the substrate with a dry abrasive. This method has more than one positive effect on the substrate. Firstly and most important, it provides for a key for the coating to adhere to. Secondly, it increases the surface area of the substrate, which takes advantage of the applied coating to search every opportunity to attach to the substrate. Thirdly, the blasting process aids to the cleaning process of the substrate prior to the application of the PTFE coating removing any surface contamination.
Typical dry film thickness (DFT), range from 10 to 100 microns. However, the thickness of the applied coating will be subject to the coating system and the requirements of the customer. The part can be made of any metallic substrate. The surface of the part has to be specially prepared to receive the PTFE. Firstly, the part must be free from all surface contaminants, oils or surfactants, and therefore it is most important to degrease the part fully. This can be achieved by temperature or by means of a solvent. In specific cases there may be a need for a chemical cleaning operation, subject to the specification required.
Now the part is ready for the PTFE coating. As described previously, the coating is applied by spray method, by the use of an air aspirated spray gun. The coating may be of a single coat system or a multi coat system, applying one or more coats which can include a prescribed primer. The exact formulation of the coating systems are retained by each manufacturer. Once the coating has been applied, the coating will be allowed to “flash off”, this is to ensure the “carriers” (solvents or water) have had sufficient time to evaporate, this part of the process takes place at a low temperature. Should the PTFE coating system be of more than one coat, sufficient time is allowed between each applied coating. Some coating will require a “stepped” cure schedule before the full cure temperature and duration is achieved. Once the component has been fully cured in accordance with the prescribed technical data sheet, the components are cooled to an ambient temperature.
Quality control measures take places at various stages throughout the application process. Such measures can include viscosity checks on the coating material and mechanical test of the cured coating to determine the adherence and hardness of the fully cured coated system. In process controls includes recording of both time and temperature of the cure schedule, this is achieved by digital recording equipment. Such information is retained for further scrutiny by our internal Quality Management System and by our external certifying authority, British Standards. In addition, material manufacturers visit our premises to ensure adherence to the specifications, guidelines and values of the products used. Discreet documentation can be secured from material manufacturers to support customer’s requirements.