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Polymer Coatings

Polymer coatings are applied to engineering parts and consumer products to confer non-stick, low friction, corrosion protection, wear resistance or combinations of those attributes. The polymers are sprayed, either with a wet carrier or as a dry, electrostatic powder. The range of materials is vast, extending to paints and elastomeric coatings, but those of interest tribologically are mostly fluorocarbons, that is PTFE, FEP and PFA.

Some polymers are 'thermoplastics', which melt and flow under heat-treatment, forming a glassy surface, usually ideal for non-stick. Others are 'thermosets' which cross-link their molecular chains during heat-treatment so that the structure is permanently changed, usually to provide coatings with good wear resistance.

Important polymer types and how they work

PTFE

The most well known is PTFE (polytetrafluoroethylene), used primarily for its low friction and non-stick. It is a thermoplastic, with a melting point around 325°>C, but it begins to degrade at 260°C. It is noteable for one of the lowest friction coefficients of any material (measured when sliding against stainless steel), with a value of 0.05 at higher loads. In sliding situations, it functions by transfering a layer of PTFE to the rubbing counterface, as a very thin film of highly aligned polymer molecules, so that the PTFE slides across itself. This process is more effcient at higher pressure, so that the friction coefficient decreases as the load increases; an unusual property. Because it relies on the transfered film, fluids like water tend to disrupt the process, so that wear and friction can then be higher.

FEP

FEP (fluorinated ethylene propylene) is softer than PTFE, and easily melt flows (melting at 260°C), creating a very low energy surface with exceptional non-stick properties, superior to those of PTFE. It gives low friction, but because it is softer, loads must be light. Its non-stick properties are augmented by good resistance to clean-down fluids, like strong detergents, making it ideal for parts needing regular cleaning to remove tenacious product deposits (glues, melted plastics, sticky food products, burnt food products, etc.)

PFA

PFA (perfluoroalkoxy) is also a melt-flow fluorocarbon, melting at 305°C, creating a good non-stick surface. It is superior to PTFE and FEP in several respects; it is tougher, more resistant to chemicals (with better substrate protection, e.g. under salt spray) and has a high dielectric strength.

All three of the above are used in bulk form, but the interest here is in their performance as a thin coating (typically 25 microns), sprayed on to a part and then heat-cured. By definition, the polymers are non-stick, so considerable skill is required through surface preparation and the use of primers to ensure that the polymer coating does not simply delaminate from the component.

The Apticote 200 Range

Poeton use other polymers for their Apticote 200 range, the composition being proprietry to Poeton. There are six Apticote 200 products, each aimed at different application areas.

Uses

Apticote 200 polymer coatings are used in a wide variety of industries, including:

By understanding the problem and specifying the correct Apticote 200 grade, Poeton can boast a long list of successes:

Substrates

Apticote 200 coatings can be applied to a range of metallic substrates. In practise, the curing temperature is critical, so that the low temperature cure with A200B or A200L is recommended for temper-sensitive tool steels. A200C is best suited for stainless steel substrates.

Thickness

Apticote 200 coatings are typically 25µ thick, with a variation around 5µ, and it is unusual for an application to require any tight tolerances. The clear A200C is thinner, 5µ, and A200N can be applied thicker (50µ) for corrosion protection.

Uniformity and coverage

Coverage on flat surfaces is even, but the spraying process is line-of-site, so that bores and re-entrant features are difficult to cover. For such complex parts, electrostatic spraying is best. The coverage of sharp corners depends on the spray technique; electrostatic spraying providing good replication. In contrast, wet spraying can sometimes lead to only thin coverage on corners.

Surface finish

The melt-flow polymers tend to give the smoothest finish, whereas the sprayed and cured products sometimes show an ‘orange peel’ surface effect. Typically, a thick polymer coating will have a surface roughness up to 2µ Ra.

Colour

The colour varies with the type of polymer, some products being clear (A200C). Whilst the colour of any one polymer will be consistent, there will be subtle variations, so that colour matching cannot be guaranteed.

Post-finishing

Polymer coatings are used as-sprayed. They should not be polished or finished in any way.

Cleaning

Poeton will advise on the appropriate chemicals to be used for in-service clean-down, depending on the coating/product combination.

Curing temperature

After deposition, the parts are baked to melt flow or cross-link the polymer, depending on the type. This obviously has a bearing on the substrate. Poeton can advise on what coating is best for your substrate material and application.

In-service temperature

The limiting operating temperatures of the Apticote 200 coatings are:

The lower operating temperature limit is around -70°C

Sliding wear/non-stick

As a general rule, the best non-stick coatings tend to be softer, and therefore not as wear resistant. So it’s important to understand the application.

A sticky product simply pressing against a surface needs pure non-stick. But if the product is also abrasive (such as a filled plastic) and is flowing into a mould before curing, the coating needs an emphasis on more hardness and wear resistance.

We have evaluated all our Apticote 200 coatings for non-stick using pull-off tests, against a range of products. Also, we have evaluated their wear resistance in pin-on-disc sliding. The Poeton technical team will guide you as to the best coating for your application.

Friction

In our laboratory friction tests, using a flat-ended 6mm diameter stainless steel pin vs a rotating disc, with a 10N load, the friction coefficients were as follows:

Abrasive wear resistance

One relevant property for a polymer coating might be its ability to resist abrasion by aggressive products, for instance a filled polymer or a gritty food product.

Using a modification to a Taber Abrasion Test, the Apticote 200 series is ranked in the following order (where 1 is best, 5 is worst).

The abrasion resistance of A200B can be improved by increasing its cure temperature.

Chemical resistance

Aggressive chemicals like solvents and alkaline cleaning fluids can soften or degrade some polymers. Overall, the chemical resistance of the Apticote 200 coatings are ranked as follows (where 1 is best, 5 is worst).

If the coating is also required to give corrosion protection to the substrate (perhaps a non-stainless, hardened tool steel), the coating needs to be thick and with 100% coverage. A200N is the best, giving up to 500 hrs salt mist endurance.

Mechanical properties

The curing temperature could be critical to the properties of the substrate. Many tool steels used for moulds are temper-sensitive, particularly low alloy steels with 1% carbon and small amounts of Cr. It is important to know the maximum temperature to which the substrate can be subjected, since this may constrain the grade of polymer coating that can be used.

Physical properties

Poeton Apticote 200 polymer coatings have the following properties.

USDA/FDA Compliance

Under the Code of Federal Regulations (CFR21), the USDA/FDA allows materials listed in several regulations to be used in combination to achieve a final coating product. The resultant coating is compliant with the regulations, but the USDA/FDA no longer formally approve a coating.


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