In certain cases, conventional lubricants can be replaced by a silicone-based inert coating that forms a non-stick surface. These silicones are suitable for use on conveyors, as a high-performance mold release and a lubricant on other manufacturing equipment. They have an applied cost of pennies per square foot.
Self-bonding polymers* (SBP) were developed originally to keep dental plaque from sticking to teeth and to keep blood cells from sticking to the inside of artificial hearts. When it was discovered that they kept barnacles from sticking to boat bottoms (see Figure 1), industrial applications of thin film (mono-molecular layer) protective materials were explored and proven.
Figure 1. The polymers can lubricate boat hulls to improve race results.
Conventional non-stick coatings are toxic and difficult to apply. They're the epitome of paradox. The very qualities and characteristics that make them desirable,their lack of reactivity,also makes them difficult to handle and apply.
It's the reactivity of a material that permits chemical bonding to take place. The trick is to get an inert material to react with a substrate surface. Most non-stick coatings bind to the substrate poorly, which makes them susceptible to cracking and delamination. They also erode with exposure and use. For example, consider the appearance of the surface finish on a well-used non-stick frying pan.
Conventional non-stick polymers, such as PTFE, can't be repaired or replaced on site. Their formulations are toxic and require special handling. They are difficult to apply, usually requiring an acid-etch pretreatment, high temperature and pressure to bond. It's a costly proposition. They have poor wear resistance and are prone to migrate, which contaminates surrounding materials.
Silicones are the most inert and chemically stable materials known to man. They are so inert that, in their standard state, they can't be bonded to any surface. Many researchers have attempted to use other coatings as a matrix or binder for silicone or Teflon. Results have been disappointing. For example, adding a non-stick material to a paint produced a "non-paint" that either doesn't bond or peels away quickly. Once a surface is contaminated with non-stick material, repairing or repainting it is nearly impossible. Marketing the so-called benefits of minute amounts of non-stick polymers incorporated into other materials, however, continues to be popular, despite the inherent problems.
A primary objective has been to develop a completely inert non-stick coating that bonds securely to the substrate it protects. This required making at least one part of the polymer chain reactive, so it attaches to the substrate. The chosen base polymer, poly (dimethyl siloxane), is resistant to most chemicals, provides a non-stick non-wetting surface, and can only be removed by removing the surface layer of the substrate to which it is bonded.
The reactive sites on the molecule (see Figure 2) results in a family of non-stick, self-bonding polymers that can be applied in a few seconds with a simple wipe or by brushing. This means, for example, that presser bars no longer need to be sent for re-coating. When rubbed into a monomolecular layer and polished, polymers produce a slippery, protective, easy-to-clean surface on metals, plastics, wood, fiberglass and fabrics.
Resistant to most acids, bases and detergents, SBPs produce a finish that reduces surface friction. Its lubricity can benefit many manufacturing processes, including reducing the power needed to drive conveyors and providing a non-stick finish for packaging and processing equipment.
Figure 2. Monolayer polymers require some number of reactive sites to make them adhere to solid surfaces.
SBP materials have many of the same properties of silicone base polymers: resistance to temperature and pressure changes, while providing chemical resistance and water-repellency. Because these polymers are elastic, they withstand thermal cycling quite well.
These chemical- and corrosion-resistant, water-repellent, gas-permeable inert coatings can serve any or all of the following purposes:
Lubricating surfaces and, when used as a additive, improving slipperiness and temperature stability of other lubricants and surface treatments.
Preventing unwanted material from adhering to surfaces.
Keeping fluids off a treated surface while allowing gases to pass.
Protecting against moisture, corrosion, chemical or biologic fouling, and some forms of oxidation.
The difference from other non-sticks
They share most of Teflon's desirable performance characteristics, but have few of its liabilities. Unlike Teflon, the coatings are easy to apply and require no pretreatment or post-application curing. SBPs are non-toxic, non-volatile and environmentally friendly. If damaged in use, they can be repaired easily and readily, which stands in contrast to Teflon, whose repair is difficult and costly.
Unlike any other inert material, SBPs readily bond to a substrate without pretreatment. They are applied as thin films, whereas other inert materials must be applied in bulk. SBP films have advantages over bulk coatings. They are lighter and require no polymerization. They are spread on a surface and the excess is removed by wiping with an absorbent cloth. SBPs leave no voids in the polymer network.
SBPs may be applied to virtually any substrate, including other silicones and Teflon, to enhance their performance. SBP may even be applied underwater, achieving a coverage of 150 to 500 square feet per gram. The coating offers long-term protection for submerged parts and does not wash off the substrate. This easy application also permits quick re-coating of damaged surfaces without having to relocate them to another environment.
Thinner is better
SBP surface treatment is a mono-molecular layer (see Figure 3) approximately 120 Angstroms (0.012 micron) thick. When applied in such a thin layer, it is transparent and adapts to the substrate surface intimately, with no significant change in dimensions or surface topography.
Figure 3. The molecular structure of one family of self-bonding polymers.
How it works
Most conventional lubricants and mold release agents are slippery because of interfacial shear between lubricant molecules. They don't usually affect the surfaces they lubricate. SBP lubricants operate on a different concept: the coating changes the properties,including coefficient of friction,of the solid material itself. Coated surfaces require smaller amounts of lubricants and less frequent lubricant changes than uncoated surfaces. Coated surfaces show less wear than uncoated parts.
SBPs function as dry lubricants for door track assemblies, conveyor assemblies, bearings and turbine fan blades. They enhance fluid flow, making them ideal for coating heat exchanger tubes, which enhances heat transfer.
SBP coatings restore or replace other non-stick finishes on products ranging from cookware to gas turbines. Coating fan blades can reduce drag and HVAC maintenance requirements, while reducing bacterial and fungal growth. Most important, coated fans blades use four percent less power and operate with less noise.
Because of their ease of application, SBP coatings can be introduced at any point in a product's manufacturing or use cycle. In many industrial applications, they add value in a particular setting, and are easily applied where they are needed.
First-time users usually apply the material as spot treatments. As confidence in their performance grows, users begin to experiment with various formulations for coating entire structures and equipment, as well as adding them as ingredients to enhance other materials in the workplace.
SBP performance depends on having a stable substrate, which is able to withstand the mechanical stresses of the environment in which the product functions. For best results, apply it to the smoothest surface possible to prevent mechanical retention of contamination. Of course, one shouldn't put them on walkways or where a label, decal or paint will be applied.
Dr. Keith Kent is chairman of KISS-COTE, Inc., He can be reached at (813) 962-2703 or [email protected]