"Manufacturing Europe 1993", Sterling Publications Ltd.

Non-stick, water repellentprotective coatings

Keith Kent, Kiss-Cote Incorporated

A novel family of non-stick inert coatings, under the trade nameKiss-Cote,can beapplied to any material to protect the surface from moisture corrosionand chemical or biological fouling and prevent water, ice, mildew, paint and

biological wastefrom adhering to treated surfaces. Thegas permeablecoatings are also tolerant to extreme temperatures and pressures. Their

potential is outlined in this article.

T he physical and chemicalproperties of a surface determinehow it interacts with the environ­

ment. The more stable it is, the less itinteracts with the environment. Thenon-aesthetic purpose of a coating is tostop a surface from interacting withwhat comes in contact with it.

The very characteristics that makeinert materials desirable also makethem difficult to handle and appl y.How can a non-reactive material bebonded to a surface?

Slightly reactive materials, such asP"IFE, can be bonded using surfaceetching, heating, primers and pressure- all of which are costly - some withlimited success. These materials havepoor wear resistance and are proneto migrate, causing unwanted contami­nation of surrounding materials .Silicones, especially polytdi-methylsiloxane), are the most inert and chemi­cally stable materials known to man.They are so non-reactive that they can­not be bonded to a surface.

A new catalytic process can makethese highly inert polymers partiallyreactive. The resulting Kiss-Cote mate­rial has a reactive side of the polymerwhich forms a very secure bond withthe substrate while its inert outer sur­face 15 both passive and resistant tochemical and environmental attack.

Surface protectionTraditionally, two distinct approach­

es have been taken in the developmentof surface protection:

I. Ablative surfaces sacrifice externalperformance to improve appearance.These self-polishing surfaces are

removed by weathering or wear andcan vary in texture and colour.

2. Non -reactive or inert surfaces haveadvantageous non-stick fouling-releasecharacteristics and are not prone toenvironmental attack. But the inertmaterials are difficult to apply as acoating that bonds to a substrate.

Silicone developmentDuring the Second World War sili­

cones were developed as lubricantsand insulators for submarines andhigh-altitude aircraft due to their abili­ty to withstand extremes in tempera­ture and pressure in a variety ofenvironments. Silicone rubber is stillthe most inert and chemically stablematerial.

Attempts at developing a non-stickcoating have had to address the para­dox that the quality that makes such acoating desirable - its lack of reactivity- also makes it hard to apply. Mostnon-stick coatings are susceptible tocracking and de-lamination or becomeworn down with exposure.

In the past, researchers haveattempted to incorporate inert materi­als like silicone or Teflon into othercoatings as a matrix or binder, hopingthat low levels of the stable materialwould aggregate along the surface ofthe cured coating. Results were disap­pointing. Adding silicone to a paintusually turns the material into a non­paint, and the material quickly peelsaway from the substrate.

Mixing with other materials such asbinders and fillers also diminishes theinert material's beneficial characteris­tics, and the performance of the coating

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may only be enhanced for a short time .The binders introduce other problems.For example, they are usually rigidwith a differen t coefficient of expan­sion than the surface of the substrate .With fluctuations in temperature, thecoating expands and contracts at a dif­ferent rate than the underlying materi­al, causing the coating to crack andde-laminate.

The primary objective of Kiss-Coletechnology was to develop a coat ingthat presents a completely inert faceto the environment yet is securelybonded to the substrate it protects . Thechosen polymer, polytdimethvl silox­ane), is one of the most non-reactivesilicones known.

The critical step was to develop acatalytic process to make part of thepolymer chain reactive, so that a securebond could form between the coatingand the substrate. The chosen polymeris resistant to most chemicals, providesa non-stick surface and can onlv beremoved by taking away the surfacelayer of the substrate to which it is •bonded .

Silicone robber manufactureThe manufacture of silicone rubber

involves extensive cross-linkingbetween chains of a polymer. As thereaction proceeds, the sites on eachchain react with each other, formin g ahighly cross-linked network of poly­mer. Silicone rubber is made by lettingthis cross-linking reaction proceeduntil alI the reactive sites are linked .

Kiss-Cote materials are made by apatented process that modifies thepolymerisation process . Inhibitors are

added to the polymer to halt the cross­linking process prematurely at a pre­selected point. This leaves many highlyreactive sites on the polymer chainavailable for bonding to the substrate.The non-reactive side of the cross­linked chain forms the inert face withthe uri-r eacted sites reacting with thesubstrate to bond the inert layer to it.

This family of materials is a uniquelyformulated type of surface treatmentwith most of the properties of the sili­cone-base polymer: temperature, pres­sure and chemical resistance andwater-repellent capabilities; yet itsticks to surfaces and will not migrate.

The key attributes of this treatmentat each int erface depend on surfacephenomena. The intermediate material- between the reactive bonding com­ponent. and the inert surface, whichprovides protection - has no value andshould be minimised . Correctlyapplied, th is surface treatment is amono-molecular layer approximately120 Angstroms <O.0l21l) thick.

The coating adapts to the surface ofthe substrate and causes no majorchanges in the coated product.

Surface chemistryThe chemical reactivity of a material

is related to its surface energy.Materials with a low surface energytend to be non-reactive. Materials witha high surface energy tend to be veryreactive. Kiss-Cote has a very low sur­face energy - 20-22 dynes /ern'.

The contact angle measurement isthe height or angle of the bead of thefluid as it contacts the surface. The con­tact angle of a material to water andother fluids relates to its reactivity .

Reactive surfaces readily wet orsheet with water as they ha ve a lowcontact angle to it. Non-reactive sur­faces do not react well with water, andhave a high contact angle. Reacti ve sur­faces are hydrophilic (wa ter-loving)and non-reactive materials arehydrophobic (water-hating).

Kiss-Cote modified poly(dimethylsilo xanes) are non-reactive and inert.They exh ibit a well-organised methy­lated su rface layer. Their surface ener­gy is low and coated surfaces arehydrophobic. Unlike PTFE, they do notwet as well with most other liquids aswith water, so treated surfaces are non­stick with a fouling-release character.

Drag reduction and lubricityWhen a solid is at rest in a fluid its

boundary layer is stationary, for thesame reason that water along the bankof a stream does not mo ve. When rela­tive movement occurs between thesolid and the flu id there is resistance,which is related to the visco-elasticcharacteristics of the fluid and themolecular forces at the solid/fluidinterface, which bind the boundary

layer to the surface. This resistance, ordrag, results in a shear force which istransmitted through the boundarylayer to the surface of the solid.

The stability of the boundary layer,at a molecular level, depends upon therelationship between the force thatbinds the flu id molecule to the surfaceof the solid and the kinetic energy ofthe free molecules in the flu id stream.A proportion of the d rag arising fromthe relative movement of the solid andthe fluid is dependent on surfacechemistry interactions occurring at theinterface between the solid object andthe fluids moving against its surface.

The inert Kiss-Cote treatment is usedto lubricate coated surfaces, reducingsurface friction and drag with a non­stick finish. Laboratory and field testsof the lubrication capabilities of thecoatings have only just begun. Earlyevidence demonstrates that the yincrease the wear cycle and lifetime oftreated wear parts, particularly thosein sliding contact with one an other.

For example, Lightfield Ammu­nitions Incorporated coated the frontend of project iles and reported a 10 percent increase in exit muzzle velocity.These coatings have also been used toimprove performance of many record ­setting and world championshippower and sa iling boats since 1986.

Most conventional lubricants andmould release agents are slippery andthey separate because of interfacialshear between the molecules of thelubricant or release agent. They do notaffect the solid surfaces thev lubricate.Kiss-Cote based lubrication ~nd releaseagents operate on a different concept:The coating changes the properties ­including th e friction coefficient - ofthe solid material itself. Coated sur­faces require smaller amounts of lubri­cants and less frequent lubricantchanges than non-coated surfaces.

A Kiss-Cote lining protects any typeof mould from most acids, bases , sol­vents, and detergents. The non-stickfinish permits multiple releases with­out re-application . When the mould isseparated the release agent does nottransfer to the finished parts, so con­tamination and migration do not occur.

The quality of protection providedby the Kiss-Cote treatment dependsupon:

1. A stable substrate which can with­stand the mechanical stresses of theenvironment in which the productfunctions

2. As smooth a surface as possible toprevent mechanical retention of foul­ing materials, and

3. A substrate surface which is non­reactive to gas vapour. The surfacetreatment is resistant to most acid s,alkalis, solvents and detergents. It ishydrophobic (non-wetting), but is

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vapour-permeable. Therefore, suscep­tible substrate surfaces should be pre­treated to prevent corrosion caused bygaseous agents.

ImpactThe family of coating products is

expected to have a broad impact ondozens of industries. The coatings willbe applied to a range of materials fordifferent reasons, at varying stages ofthe manufacturing process.

This innovative surface treatmenthas marine, automotive, aviation, con ­struction, power generation , arnrnuni­tion and weaponry, computer andelectronic, consumer goods, medicaland dental applications . lt serves as ahigh-performance mould release, alubricant and lubricant additive.

These chemical- and corrosion-resis­tant, water-repellent, gas-permeableinert coatings can be applied to virtual­ly any material to serve any or all of thefollowing purposes:

Protecting the coated material frommoisture, corrosion and chemical orbiological fouling

Lubricating the coated surface byreducing surface friction and drag and ,when used as an addit ive , improvingthe slipperiness and temperaturestability of other lubricants or surfacetreatments

Preventing unwanted substances ­water, ice, mildew, pa int , natural orman-made detritus - from adhering totreated surfaces

Polishing and preserving metal, wood,resin and ceramic surfaces, serving as areplacement for high-gloss wax

Sealing equipment and structuressubjected to repeated heat and freezecycles and keeping treated surfacesdry.

Keeping water off on e side of a treat­ed surface wh ile allowing vapours(includ ing water vapour) from the dryside to pass out.

These new coatings share most ofTeflon's de sirable performance charac­teristics, but few of its liab ilities . Thevare easier to apply and requ ire no pre­application treatments or post-applica­tion curing. Polymers are non-toxic ,non-volatile and environmentallyfriendly . In addition, if the coating isdamaged in use, it can easily berepaired. C

Dr Keith Kent is chairmanof Kiss-CoteIncorporated, of Tampa, Florida , USA , asiliconecoatings manufacturer;and ofKent IntegratedScientific SvstemsIncorporated, a research and developmentcompany specialising in scientific andbiomedical materials and devices,andindustrial problem solt'ing. He alsohasUS and foreign patents on new familiesof silicones and other polymers.