Polymeric coatings designed for corrosion protection are usually tougher and are applied in heavier films than are appearance coatings. Requirements of such coatings are much more stringent: They must adhere well to the substrate and must not chip easily or degrade from heat, moisture, salt, or chemicals.
Environmental factors also drive the technology behind polymer coatings replacing chrome and cadmium coatings. This is partly due to increasing concern about heavy metals. Also, automakers must now contend with acid rain in addition to salt spray, and polymers surpass chrome and cadmium in acid rain resistance.
Acrylics and alkyds are widely used for farm equipment and industrial products requiring good corrosion protection at a moderate cost. Alkyd resins, particularly, play a major role in maintenance painting because of their good weathering characteristics and ease of application with low-cost, low-toxicity solvents. Alkyd paints are also relatively high in solids, permitting good buildup of a paint film with a minimum number of coats.
Silicone modification of organic resins improves overall weatherability and durability. Because of the general acceptance of alkyds in maintenance painting, these resins were singled out by Dow Corning in its investigation of silicone-modified organic coatings. Compared to organic coatings in general, silicones have greater heat stability, longer life, better resistance to deterioration from sunlight and moisture, and greater biological and chemical inertness.
The investigations have resulted in the commercial availability of water-base silicone-modified alkyds. These materials have the weatherability, gloss, application ease, and other performance characteristics of solvent-base coatings, but they contain only small amounts of volatile organic compounds. The solvent-base formulations have proved their durability and fade resistance on chemical and natural-gas storage tanks, above-deck structures of oil tankers, and other outdoor structures subjected to industrial and marine environments.
For optimum weatherability, silicone content should be 25 to 30%. Performance of the water-borne formulations is proving to be almost identical to that of the solvent-base coatings.
For coatings requiring higher heat resistance, silicone resins can be used alone for the paint vehicles, or they can be blended with various organic resins. These finishes are used on space heaters, clothes dryers, and barbecue grills. Similar formulations are used on smokestacks, incinerators, boilers, and jet engines. Performance of formulations containing ceramic frits approaches that of ceramic materials.
Polyurethane enamels are characterized by excellent toughness, durability, and corrosion resistance. These thermosetting materials, available in both one and two-part formulations, cost more than the alkyds and acrylics.
Urethane chemistry is versatile enough to provide a hard, durable, environmentally resistant film, a tough, elastomeric coating, or a surface somewhere between. Urethanes have traditionally been available as solvent-based coatings containing 25 to 45% solids, but environmental concerns have prompted manufacturers to also supply them in high solids, 100% solids, and water-borne formulations.
Coating thickness of polyurethanes ranges from about 2 mil for average requirements to as much as 30 mil for applications requiring impact and/or abrasion resistance as well as corrosion resistance. Typical uses are on conveyor equipment, aircraft radomes, tugboats, road-building machinery, and motorcycle parts. Abrasion-resistant coatings of urethanes are applied on railroad hopper cars, and linings are used in sandblasting cabinets and slurry pipes.
Epoxy finishes have better adhesion to metal substrates than do most other organic materials. Epoxies are attractive economically because they are effective against corrosion in thinner films than are most other finishing materials. They are often used as primers under other materials that have good barrier properties but marginal adhesive characteristics.
Coating thickness can vary from 1 mil for light-duty protection to as much as 20 mil for service involving the handling of corrosive chemicals or abrasive materials. Performance of epoxies is limited in the heavier thicknesses, however, because they are more brittle than other organic materials.
Nylon 11 coatings provide attractive appearance as well as protection from chemicals, abrasion, and impact. Applied by electrostatic spray in thicknesses from 2.5 to 8 mil, nylon coatings are used on office and outdoor furniture, hospital beds, vending-machine parts, and building railings. Heavier coatings -- to 50 mil -- are applied by the fluidized-bed method and are used to protect dishwasher baskets, food-processing machinery, farm and material-handling equipment, and industrial equipment such as pipe, fittings, and valves.
Fluorocarbons are more nearly inert to chemicals and solvents than all other polymers. The most effective barriers among the fluorocarbons for a variety of corrosive conditions are PFA, PTFE, ECTFE, FEP, and PVDF.
For impact service, PVDF and ECTFE coatings are recommended, in that order. PTFE, FEP, and PFA are also suitable, but they have a greater tendency to creep under load. For abrasive conditions, PVDF is outstanding among the fluorocarbons. Recommended for high service temperatures -- drying ovens and steam-handling equipment, for example -- are PFA and PVDF. These materials are also used on engine components and welders. PVDF also has the highest compressive strength of the fluorocarbons. PTFE has the highest allowable service temperature (600°F) of the fluorocarbons.
Coatings based on PTFE are being used to reduce wear in the U.S. automotive industry. Fluoropolymer coatings prevent binding and galling in disc brake systems at temperatures over 100°C. PTFE is also used as a dry lubricant. In addition, PTFE can be used as a coating on automotive fasteners, and a new process uses PTFE to prevent seizure in valve springs. In FluoroPlate impingement, a process developed by Orion Industries, a mixture of inorganic and organic particles bombard the spring surface, relieving internal stresses and reducing surface flaws. The coating also helps the springs to repel oil.
A new class of coating -- an alloy of fluoropolymer and other resins -- has a different viscosity behavior than that of the earlier organics. Viscosity of "fastener-class" coating resins decreases sharply as film shear increases (as in application by the dip/spin process). Then, when the spinning basket stops, viscosity returns almost instantaneously to its original value. Thus, when applied to the dip/spin process, the coating clings to sharp edges, threads, and points.
Film thickness typically ranges from 0.5 to 0.7 mil, but formulations can be adjusted to provide films of 0.3 to 0.4 mil for parts with fine threads or other intricate features. Not only do these extremely tough coatings provide a more uniform barrier to corrosives, but they are also based on polymers that are inherently stable in the presence of a wide spectrum of acids, bases, and aqueous solutions.
Combination coatings blend the advantages of anodizing or hard-coat platings with the controlled infusion of low-friction polymers and/or dry lubricants. The coatings become an integral part of the top layers of metal substrates, providing increased hardness and other surface properties.
These coatings, typified by a series of proprietary coatings developed by General Magnaplate Corp., are different for each class of metals. For example, the company's Tufram coating for aluminum combines the hardness of aluminum oxide and the protection of a fluorocarbon topcoat to impart increased hardness, wear and corrosion resistance, and permanent lubricity.
In the multistep process, the surface is first converted to aluminum oxide. Submicron particles of PTFE are then fused into the porous anodized surface, forming a continuous plastic/ceramic surface that does not chip, peel, or delaminate. The coating is claimed to have greater abrasion resistance than case-hardened steel or hard chrome plate.
Another proprietary coating which penetrates PTFE into precision hardcoat anodizing is Nimet Industries' Nituff. The coating achieves a self-curing, self-lubricating surface with low friction, high corrosion resistance, and dielectric properties superior to ordinary hardcoat anodizing. It is used extensively in aerospace, textile, food processing, packaging, and other industries, where it allows manufacturers to benefit from the light weight and easy machinability of aluminum enhanced by the durability, cleanliness, and dry lubrication of the Nituff surface.
Other proprietary combination coatings have been developed for steel, stainless steel, copper, magnesium, and titanium that provide similar surface improvement. Coatings are also available that enhance specific properties such as lubricity, corrosion resistance, or wear resistance.
Another type of coating, powder coating, combines properties of both plastics and paints. The coatings are manufactured using typical plastics-industry equipment. They are first sent through a melt-mix extruder and then ground. When applied as a coating, however, the powder becomes a coating film that is exactly like paint.
These coatings have been developed in response to pressures to reduce volatile organic compound (VOC) emissions, which have increased over the past few years. Overspray from liquid paints contains solvents that are released into the atmosphere. Even with recovery systems, some volatile components escape. Powder coatings, on the other hand, are completely recyclable. Overspray can be collected easily and reused. If a small amount becomes too contaminated for recycling, safe disposal techniques are available.
Powder coatings also show promise as a substitute for clear coats in the automotive industry. Present solvent-based paints could be replaced by a clear powder coating that cures at roughly the same temperature as conventional paints. Powder coatings may also replace the baked-on porcelain enamel used for appliance parts. Washer and dryer lids are now powder coated by The Glidden Co.
Applications are not all that is new, however. Materials have changed. The majority of powder coatings have relied on either an epoxy or polyester resin base. Acrylics, however, are becoming more important, and other possible bases include nylon, vinyl, and various fluoropolymers.
Two processes for applying coatings have undergone refinements. With electrostatic spraying, the most popular method, powder is given a charge and sprayed onto electrically grounded parts. Baking them completes the cure. Nonconductive parts must be primed or heated to provide them with more electrostatic attraction.
In the fluid-bed process, air passes through a porous membrane at the bottom of a tank and aerates the powder so that it swirls around in the tank. A part is then heated and dipped into the tank, so that the powder melts on the surface. This process is used for thick-film protection coatings, and is suitable only for metal parts that can retain heat long enough to be coated.
Work continues at Glidden and elsewhere on reducing the cure temperature for powder coatings. While these coatings are presently comparable to liquid paints that cure at 300 to 400°F, researchers hope to develop coatings that can be cured at lower temperatures for use on plastic parts.