Solving engineering challenges

Teflon, or polytetrafluoroethylene (PTFE), is one of the most versatile industrial coatings available. Best known for its non-stick properties, Teflon offers an extremely low coefficient of friction, chemical inertness, and excellent dielectric stability.

Far more than a product for cookware, its non-stick properties can 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 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.

Precision application

The application of Teflon and other functional coatings has advanced to a precise, highly automated operation. Applicators of Teflon coatings – who have invested in production equipment and meet environmental regulations – are partnering with companies that previously performed this work in their own facilities or subcontracted it to local applicators.

However, within this group there can be a broad range of capabilities and quality levels.

Osterhout says the difference in quality between applicators is usually defined by two primary factors:

• Proper, thorough pretreatment of parts prior to coating
• Ability to apply coatings as thin as 10µm, at a tolerance of ±3µm. The industrial norm is 10µm to 20µm per side of coating

To assure that level of precision, applicators must design and build their own production lines from the ground up. These systems must be configured to be flexible enough to accommodate a wide range of parts in different sizes and geometries.

Orion uses a flexible cellular manufacturing approach to increase production speed for a wide variety of parts, while controlling material cost and energy.

The stationary equipment, or robotic cell, is moved into position, allowing 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. Parts are coated in these cells with minimal overspray and coating waste.

“In our process, we meter the coating material out of the gun so it is the same every time,” Osterhout says. “There is 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 ovens, parts can often be cured 3x faster than with traditional convection ovens.

Non-stick

Teflon’s non-stick qualities can be important for preventing the accumulation of foreign particles, contaminants, and wear properties.

In many applications, the build-up of foreign material can prevent the proper function of machine or engine components. Thin Teflon coatings can minimize surface contamination.

Teflon’s non-stick properties make it an excellent corrosion barrier. Coated parts can be used to prevent galvanic corrosion and shed corrosive fluids such as salt water, process chemicals, fuels, and lubricants.

Formulations can also include anti- microbial agents, benefiting a range of applications.

Conformable coatings

For applications where very tight clearances between parts are required, such as rotary screw compressors, Teflon-based conformable coatings can narrow the gap without allowing metal-to-metal contact during operation. Benefits to decreasing the gap include reducing noise, stopping air or fluid leakage, or creating a tighter hydrodynamic seal. In rotary screw compressors, a conformable coating can increase efficiency 10% or more without changing machining tolerances.

Ultra-thin conformable coatings, such as DB L-908, 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. 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 and 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 – a better 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. Orion’s FluoroBond LSR was designed to reduce the coefficient of friction of molded silicone rubber from 20% to 60%. 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 contacts – even other silicone surfaces – 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 uses chemical vapor deposition to form a physical bond. However, Parylene is rigid, can crack, has no anti-microbial properties, only processed parts that fit in the vapor chamber can be coated, and interior dimensions are difficult to coat.

FluoroBond LSR provides a chemical bond to silicone rubber to reduce the coefficient of friction. With its good elongation properties, the coating does not crack and anti-microbial properties can be added.

Dry film lubricants

Typically, engineers are educated 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 longer. Dry film coatings also 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, including bearing-type applications where one part rolls or slides over another part. Graphite coatings are generally used in wet service or at elevated temperatures.

As a dry film lubricant, Teflon is typically used for light-to-moderate applications. It is often incorporated into unique formulations with moly and graphite.

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 a product with no coating, or a coating that isn’t performing as expected, and they need a resource to explore the options.

“Engineers need to establish how the Teflon coating is going to function and they need to know the thickness that will be applied to compensate for it to fit within their existing dimensions,” Osterhout says.

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, and research and develop their own for specific customer applications.

Some labs incorporate the latest quality control (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.

“Having the in-house lab, 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,” Osterhout says.