PTFE Coating Process
The term "PTFE" as used is intended to embrace both PTFE homopolymers and polymers formed by copolymerising tetrafluoroethylene with other monomers. Polymers of fluoroethylene containing other halogens are also included, as are mixtures of polymers of different composition. Polymers may be of different chain lengths (molecular weights), molecular weight. If desired, other particles, such as mineral particles of a pigmented character, may be included with PTFE particles.
The bonding of the PTFE coating and the material of the substrate is as a result of mechanical keying between the PTFE and the surface of the substrate.
The PTFE particles are preferably less than 100 microns in diameter, and more preferably less than 50 microns in diameter. Particularly preferred are PTFE particles having a maximum diameter less than 30 microns.
In conventional or air atomized spraying, the coating is supplied to a spray gun by siphon, gravity, or pressure feed. When the gun trigger is pulled, the coating flows through the nozzle as a fluid stream. Compressed air from the centre of the nozzle surrounds the fluid with a hollow cone as it leaves the nozzle, breaking the coating into small droplets and transferring velocity to it. Additional jets of compressed air from the nozzle break up the droplets further and form an elliptical pattern.
In electrostatic coating, the fluid is atomized, then negatively charged. The part to be coated is electrically neutral, making the part positive with respect to the negative coating droplets. The coating particles are attracted to the surface and held there by the charge differential until cured.
With an electrostatic spray gun, the droplets pick up the charge from an electrically charged electrode at the tip of the gun. The charged particles are given their initial momentum from the fluid pressure/air pressure combination. Electrostatic spraying offers high transfer efficiency (65 % to 95 %) and excellent edge coverage. The attraction between paint droplets and the part is strong enough to cause paint overspray that misses the part to curve back, which contributes to the high transfer effiencies.
Electrostatic application does not coat recessed areas (Faraday cages) as well as nonelectrostatic application. The charged droplets tend to be attracted to the sides of the recess and sharp edges instead of penetrating to the bottom.
All electrically conductive materials near the spray area such as the material supply, containers, and spray equipment must be grounded to prevent static buildup. All hangers, conveyors, etc. must be kept clean to ensure conductivity to ground. Charges build up on ungrounded surfaces. Operators grounding out these surfaces may receive a severe electrostatic shock.
Conventional air spray is the oldest spray process. It offers the best control of spray patterns and degree of atomization. This system produces the finest atomization and, therefore, the finest finishes. Conventional spray will also spray the widest range of coating materials of the four techniques.
The PTFE particles may be delivered to the surface of the applicator in the dry state, but it has been found to be more convenient to "deliver the PTFE particles to the surface of the applicator in the form of a liquid dispersion. Preferably, the dispersing solvent is sufficiently volatile to evaporate almost instantly, leaving the particles in a substantially dry state, A suitable dispersing solvent is trichlorotrifluoroethane, though other low-boiling halogenated hydrocarbons can also be used.
PTFE is extraordinarily resistant to chemical attack, and the surface free energy of solid PTFE is very low. This means that liquids do not readily wet the solid, and other solids do not adhere strongly. These properties render PTFE very valuable for forming protective surface coatings, in a wide range of applications from non-stick cookware to engineered products. Methods employed for coating with PTFE is to spray onto the substrate a dispersion of low molecular weight solids of PTFE suspended in a suitable liquid such as solvents or water, known as the carrier and then to evaporate the carrier. Such coatings are sometimes sintered or even buffed in order to increase the uniformity of the coating. However, coatings formed by this method can be used as a single coat application such as in mould release applications or as release layer. The method of application can be applied to a wide range of substrates, and produces a type of PTFE coating which is both tough and durable. Some PTFE coating systems can be of a multi coat type, which includes both primers and mid coats being applied before the final coat. Typical film thicknesses are from 10 to 100 microns thick, for example from 20. to 50 nm thick. . A usual characteristic of the process is that the aqueous PTFE coatings are effectively self-limiting in thickness. However, PFA (powder) coating can be deposited with additional thickness.