Solving Engineering Challenges With Teflon
Teflon, or polytetrafluoroethylene (PTFE), is one of the most versatile industrial coatings available today. Best known for its nonstick properties, Teflon offers an extremely low coefficient of friction, chemical inertness and excellent dielectric stability.
While almost everyone has heard of Teflon, due to ongoing advancements and more precise application techniques, the expanding range of potential uses for PTFE and other functional coatings can often be overlooked.
Far more than a product for cookware, Teflon’s nonstick properties can be used to solve a variety of design challenges including preventing build-up of contaminants, corrosion and bacteria; reducing friction or sticking between parts that come into contact; and even as an ingredient in heavy-wear, high-load applications.
“Teflon is the ultimate problem solver for design issues that can include sticking, release, wear, noise and abrasion,” says George Osterhout of Orion Industries, one of the largest Chemours-licensed applicators for Teflon coatings in North America. “Engineers often seek assistance when they are experiencing problems, whether it’s a part that has no coating or a coating that isn’t performing as expected.”
According to Osterhout there is no limit to the type of substrates that can be coated with Teflon. This includes carbon steel, aluminum, stainless steel, steel alloys, brass, titanium and magnesium as well as glass, fiberglass, silicone rubber, rubber and plastics.
There is also no limit to the kinds of products that are coated. A short list includes cookware, supercharger rotors, automotive throttle shafts, solenoids, airplane toilet bowls, the exterior of light bulbs, rubber O-rings and the rollers used to cook hot dogs in convenience stores, to name a few.
Today, the application of Teflon and other functional coatings has advanced into a precise, highly automated operation. Due to the investment in production equipment and the rigors of meeting environmental regulations, applicators of Teflon coatings are partnering with companies that previously performed this work in their own facility or that used to job it out to a local applicator.
However, within this group there can be a broad range of capabilities and levels of quality. According to Osterhout, the difference in quality between applicators is usually defined by two primary factors: the proper and thorough pretreatment of parts prior to coating, and the ability to apply precise thickness of coatings as low as 10 microns with little to no variance between parts and at a tolerance of +/- 3 microns. The industrial norm is 10-20 microns per side of coating.
To assure that level of precision, applicators must design and build their own production lines from the ground up. Furthermore, these systems must be configured to be flexible enough to accommodate a wide range of parts in different sizes and geometries.
As an example, Orion uses what they call a flexible cellular manufacturing approach to speed production for a wide variety of parts, while controlling the material cost and energy.
The stationary equipment or robotic cell is moved into position and allows the coatings to be applied exactly to specification while using conveyers and ovens to do the flash and curing process or to completely cure the part. The parts are coated in these cells with very little overspray and coating waste.
“In our process we meter out the coating material out of the gun so it is the same every single time,” says Osterhout. “There is very little overspray because we apply the coating where it needs to be, not all over the coating booth.” By using infrared ovens mixed with convection air oven, parts can often be cured three times faster than with traditional convection ovens.
Nonstick, More Than Just Food
Although Teflon is very popular as a highly durable and reliable coating for cookware, its nonstick qualities can be just as important for preventing the accumulation of foreign particles, contaminants and wear properties.
In many applications, the build-up of dirt, ice, soot, scale, food and other foreign material can prevent the proper function of machine or engine components. If contamination of a surface is anticipated, it can be minimized with thin Teflon coatings.
Teflon’s nonstick properties make it an excellent corrosion barrier, as well. Coated parts can be used to prevent galvanic corrosion and shed corrosive fluids like salt water, process chemicals, fuels and lubricants.
Today’s formulations can also include antimicrobial agents that can benefit a wide range of applications.
For applications where very tight clearances between parts are required, such as rotary screw compressors, Teflon-based conformable coatings can be used to narrow the gap without allowing metal-to-metal contact during operation.
There are many benefits to decreasing the gap, including reducing noise, stopping air or fluid leakage, or creating a tighter hydrodynamic seal. In the case of rotary screw compressors, a conformable coating can increase efficiency over 10% or more without changing machining tolerances.
Ultrathin conformable coatings, like the DB L-908 from Orion for instance, contain a mixture of polyimide and other resins. The formulation contains nanometer-sized wear-resisting particles, as well as PTFE.
The coating is applied in thicknesses specified during initial design and testing, such that the two coated surfaces initially contact each other while allowing the tips clearance to pass in a rotary screw application.
Small air pockets in the polymer allow the coating to compress under mechanical pressure. Once compressed, the air pockets remain crushed so the coating holds the new profile. The PTFE in the formulation prevents the surfaces from galling or sticking to each other during this initial contact.
This is a superior alternative to surface hardening, which only delays galling, or more precise machining of parts with extremely tight tolerances.
Given the extremely tight clearance, these conformable coatings must hold ±0.001” tolerance on all surfaces at a thickness of 0.002” to 0.006”.
Low-Friction Coatings for Silicone Rubber
In addition to being a licensed applicator of Teflon coatings, some manufacturers have developed their own line of proprietary coatings. Case in point, Orion’s FluoroBond LSR was designed to reduce the coefficient of friction of molded silicone rubber from 20-60%. In addition, it also aides in reducing the collection of dust or dirt on the finished coated surface.
Silicone rubber, though widely used for tubes, seals and rollers, is tacky and can cling to any object it comes in contact with – even other silicone surfaces. This is due to the material’s high friction coefficient, which can exceed 1.0.
To lower the coefficient of friction on the surface of silicone parts, many applications use a Parylene coating that utilizes a chemical vapor deposition process to form a physical bond. However, Parylene is rigid, can crack and also has no antimicrobial properties. Furthermore, only process parts that fit in the vapor chamber can be coated, and interior dimensions are very difficult to coat.
FluoroBond LSR instead provides a chemical bond to silicone rubber to reduce the coefficient of friction. Due to its excellent elongation properties, the coating does not crack, and antimicrobial properties can be added.
Dry Film Lubricants
Typically engineers are schooled in oil- or grease-based lubrication techniques, but not nearly as much time or attention is spent on dry film lubricants.
Unlike oil and grease that can migrate away, dry film lubricants continue to transfer back and forth on the mating surfaces and stay in place for a much longer time. Also, dry film coatings serve as a thin cushion, spreading high point loads in bearings and reducing element fatigue.
For this, graphite and molybdenum disulfide (Moly) are often used. Moly- disulfide coatings are recommended for conditions of heavy wear, particularly high-load situations. This includes bearing-type applications where one part rolls or slides over another part. Graphite coatings are generally used in wet service or at elevated temperatures.
Teflon can also serve a dry film lubricant, though typically for light to moderate applications. Teflon is often incorporated with Moly and graphite into unique formulations.
R&D Laboratory Access
Osterhout admits the variety of coating formulations as well as the variety of products that could benefit is daunting. In many cases engineers have product with no coating, or a coating that isn’t performing as expected, and need a resource to explore the options.
“Engineers need to establish how the Teflon coating is going to function, and they also need to know the thickness that will be applied to compensate for it to fit within their existing dimensions,” says Osterhout.
To assist in developing specialized materials and application methods, a select number of applicators now maintain an in-house R&D laboratory to test new coatings available on the market, and research and develop their own for specific customer applications.
Some labs are even equipped with the latest QC equipment for testing wear, lubrication, coefficients of friction and other key application parameters. In addition, customers and prospects can consult with in-house engineering staff when custom coatings are required.
Among the benefits of having the in-house lab, says Osterhout, is that “We have access to baseline performance information from past research and experimentation so we can compare one coating to another that we have developed. This allows us to know that we made substantial improvements in new coatings performance.”