High performance fluoropolymer coatings.

High performance fluoropolymer coatings offer engineers and designers a diverse range of low friction, dry-lubricant materials suitable for a variety of applications. These coatings combine the unique capabilities of two 'engineering plastics'; fluoropolymers, which have the lowest coefficient

By combining these materials, it is possible to create tough, lubricating coatings capable of operating at temperatures which, at the low end (ie, under cryogenic temperatures), would render ordinary fluid lubricants as brittle as glass and, at the high end, would char them to ash.

Whereas in the past the application of fluoropolymer coatings has been limited, they now represent the most sensible, safe and economical method of lubricating and protecting many mechanical parts.

Enter the matrix

In the early stages of their development, fluoropolymer coatings were formulated as soft films (the kind found on frying pans) and used in limited industrial applications. In 1969, a team of polymer chemists and engineers devised a matrix concept by which coatings were designed to protect the soft fluoropolymer from wear, while taking advantage of its low-frictional property. This matrix construction not only revolutionised the fluoropolymer coating industry, but also formed the base from which Xylan, the world's largest and most complete range of fluoropolymer coatings, was developed.

Coating components

A bonded fluoropolymer coating matrix comprises three basic elements;

* a solid lubricant.

* a pigment or filler.

* a polymer binder.

Solid lubricants

Small particles of low-frictional materials such as polytetrafluoroethylene (PTFE), molybdenum disulphide and graphite are included in the matrix to reduce friction at the surface of the coating. These materials tend to be softer than the matrix as a whole, and smear across the surface of the coating when parts rub together.

PTFE is most the commonly used solid lubricant, because it has the lowest coefficient of friction, is stable and effective at high and low temperatures, and is inert to chemical attack. Its low surface tension also makes it an excellent release agent. Other widely used fluoropolymer lubricants include FEP and PFA, both of which have excellent non-stick and low-frictional properties and which offer the specific benefits of superb release and abrasion resistance respectively.

Pigments and fillers

Coatings are modified by pigments and fillers to provide properties not inherent in their primary formulation. These modifications may be used, for example, to make coatings harder, more corrosion resistant, or to add colour. A good example of modification through pigmentation is found in Whitford's electrically conductive coatings, where carbon or metal particles are introduced to the matrix.

Polymer binder

Polymer binders hold the lubricating particles and fillers in place and enable them to adhere to a wide range of substrates. Coatings derive most of their corrosion and thermal resistant characteristics from these materials.

The key properties of a 'binder' are its toughness and stability at extreme temperatures. Whitford uses various classes of binder, each suited to different areas of application. One class of binder is a thermosetting alloy, which combines superior toughness and temperature stability (up to 290 [degrees] C) with a flexible cure schedule. A second example is a high-temperature-stable thermoplastic which is used for its resistance to abrasion and chemicals, and its ability to operate effectively in the presence of strong acids, bases and solvents. Another typical example is a lower-temperature thermoset which, while not as tough as some polymers, is economical, and provides good corrosion protection and an exceptional array of colours.

Matrix coatings are infinitely adjustable in as much as the properties of a coating depend entirely upon the relative proportions of the materials used in its formulation. Coatings can thus be formulated for specific applications where, for example, superior release, wear resistant or chemical resistant properties are required.

Coatings at work

High performance fluoropolymer coatings can be formulated to meet a variety of performance criteria and to suit a selection of applications. A number of their key characteristics are considered below.

Friction

The coefficient of friction between two surfaces depends upon the pressure, speed and temperature under which components are operating. Friction commonly causes heat, wear and loss of energy in dynamic applications. In severe circumstances, friction can also cause overheating and seizure (eg, in bearings).

Excessive friction is also detrimental in static situations such as bolted joints where much of the tightening torque can be expended overcoming thread-to-thread and bearing face friction. In such instances, the bolt is not properly tensioned (preloaded) and the joint can be unexpectedly weak in service. Improperly fastened parts may also be subject to 'backout' when vibration occurs.

Frictional coefficients typically vary from approx. 0.06 for PTFE materials to approx. 0.15 for molybdenum coatings. Whitford is able to formulate coatings with frictional values as low as 0.02.

Wear

Every metal component, no matter how smooth, has small peaks, or asperities, on its surface. Initial contact between metal components causes a momentary welding of these asperities which, as the parts continue to move, are ripped off the metal surfaces leaving behind minute pits. Wear is common in bearing-type applications such as rolling element bearings, slide assemblies, rocker arms, ball joints and other applications where one component rolls or slides over another.

By applying a fluoropolymer coating to the surfaces of the metal components, it is possible to prevent the asperities on mating surfaces from making physical contact with each other, and thereby eliminate objectionable wear. The coating can also serve as a thin cushion, spreading high point loads (where applicable) and reducing element fatigue.

Release

Release is the property of a surface which results in the inability of substances to adhere to it. Release is commonly associated with cookware (Coated to release food materials). However, it is equally relevant to the industrial sector in applications such as rubber moulding, thermoforming and adhesive assemblies where a build-up of foreign materials can cause serious problems.

If contamination of a surface is anticipated, a thin coating of a suitable high release fluoropolymer coating is usually sufficient to eliminate the problem; foreign matter is unable to cling to the waxy surface of the coating and falls off.

Corrosion

The electrochemical process of corrosion is complex and can result from single or multiple sources. Oxidising fluids such as salt water, electrolytes, acid fumes, fuels and process chemicals can all attack metal. Dissimilar metal unions (galvanic corrosion) and vibration between tightly joined components (fretting) can also cause corrosion.

Fluoropolymer coatings can form an excellent corrosion barrier, protecting the substrate from attack, even when applied as a thin film. Fasteners used in highly corrosive environments such as offshore oil drilling rigs, for example, can be protected for thousands of hours and remain functional if coated prior to service.

Vibration

Vibration generates noise. Vortices trailing impeller blades, impacting gear teeth, bearings spinning in races and slapping piston skirts are just a selection of common sources of vibration that can be effectively dampened using fluoropolymer coatings. Under impact, the coating absorbs the energy and transmits reduced energy to resonant surfaces.

Temperature extremes

Few fluid lubricants are recommended for use at cryogenic temperatures or above 205 [degrees] C as they become solid or oxidise respectively. Dry lubricant coatings, however, can operate comfortably at both temperature extremes.

Excalibur and Quantum

The most recent advances in the development of Whitford's high performance fluoropolymer coatings are the introduction of Excalibur and Quantum. Excalibur is a stainless steel reinforced coating system which offers the toughest, longest lasting, most durable non-stick in the world. Originally developed for use on cookware, independent tests have proved Excalibur to last up to 100 times longer than other leading brands.

The key to Excalibur's durability lies in its external reinforcement. This involves blasting the substrate and then spraying the surface with particles of molten stainless steel which cool and harden to form 'peaks' and 'valleys' [ILLUSTRATION FOR FIGURES 1-4 OMITTED]. A stainless steel mesh is thus welded to the substrate so as to provide a permanent base for the non-stick coating in the form of a permeated matrix.

Quantum is an internally reinforced three-coat non-stick fluoropolymer coating system which incorporates a novel approach to coating formulation. Quantum utilises both chemical and mechanical bonding technology to provide exceptional durability, non-stick and abrasion resistance.

By combining chemical and mechanical bonding techniques, Whitford has been able to overcome a key difficulty facing all fluoropolymer scientists - how to make non-stick coatings stick to the surface to which they are applied. In conventional coating systems, this problem is partially solved by incorporating fluoropolymer elements in the primer coat which fuse chemically with fluoropolymer elements in the midcoat. However, this is a compromise, as fluoropolymer is a release agent and so limits the primer's adhesion to the substrate.

Quantum's primer coat has no fluoropolymer content; it is 100% 'glue', dedicated to adhesion to the substrate. It has a coral-like construction in addition to metallic filaments, both of which lock the midcoat in place to form a remarkably strong mechanical bond.

Applications

The need for effective coating protection is demonstrated nowhere better than in the harsh marine environment encountered on the off-shore platforms of the North Sea. Repairing and recovering equipment on drilling installations can become both time consuming and hazardous once the installations have been exposed to the elements for any period of time. These difficulties multiply when service operations have to be conducted underwater. The petrochemical industry has benefitted from the use of Xylan coatings which provide the anti-corrosive, wear resistant and lubricant properties to ensure smooth operation in hostile conditions.

Another typical application is the use of flexible coatings on the weather seals and rubber channels on car doors and windows. Flexible coatings act as a protective barrier against extreme temperatures, and provide low coefficients of friction, good release properties and excellent wear resistance. A vital characteristic of the coating system is its ability to match or exceed the substrate's flexibility. As a result, they are now used in the production of components for Rover, Peugeot, Renault, Toyota and Saab cars.

Julie Knowles is with Whitford Plastics Ltd, Runcorn, in the UK.