Non-stick coatings

Abstract: A non-stick coating composition comprising fluorocarbon resins and from 5 to 25% by weight of refractory fibres. The composition preferably additionally includes a liquid medium in which the fibres are dispersed and the fluorocarbon is dispersed. The composition is then applied, the liquid medium driven off and the fluorocarbon resin cured.

Description: The present invention relates to non-stick coatings, particularly for cookware, though this invention is also applicable to other coated articles such as domestic and industrial tools and appliances. Conventionally, non-stick coatings have included a fluorocarbon layer such as PTFE (polytetrafluoroethylene) applied directly to a metal substrate. This has been achieved for example by roll bonding the coating either as a sheet or powder, or by a liquid bonding process. In the latter, the coating material is applied as a solution or dispersion, the liquid medium is driven off and the coating is then cured. Such coatings however tend to degrade with use and in some instances, the coating material flakes away from the substrate. Higher quality coatings have been obtained by initially applying a basecoat to a roughened substrate and then applying a fluorocarbon rich topcoat to the basecoat. The liquid mediums used in these systems are evaporated at relatively low temperatures prior to curing at about 400°C. While such coatings have improved adherence, the surfaces are still rather soft and therefore prone to damage by abrasion. It is an object of the present invention to provide a non-stick coating which has improved resistance to wear and abrasion. According to the present invention, a coating composition for providing a non-stick coating comprises one or more fluorocarbon resins and refractory fibres, the fluorocarbon representing 40 to 90% by weight and the fibres representing 1 to 25% by weight of the final dry coating, the fibres have diameters in the range of 1 to 20 µm, and the lengths of the fibres vary between 70 and 150 µm, characterised in that the composition additionally including a binder resin comprising one or more of polyamideimide, polyphenylene sulphide, polyether sulphone and silicone resins. The fibres and the fluorocarbon are preferably dispersed in a liquid medium. The composition may also include pigments. The fibres may represent 5 to 25% by weight of the dry final coating, preferably 7.5 to 20%, for example 10%. (This is approximately equivalent respectively to 2 to 10%, 3 to 8% and 4% by weight of the wet coating composition). The fluorocarbon may represent 60 to 70% by weight of the dry final coating, preferably 65%. Where a liquid medium is employed, this may be present as 40g to 80g, based on 100g of the wet coating formulation as applied, preferably 50g to 70g, for example, 54.5g. Where other components are included, they may represent 10 to 30% by weight of the final coating, preferably 15 to 25% for example 22%. The fibres are preferably alumina fibres, however, carbon fibres, glass fibres, and similar materials may be used. The fibres as supplied may have diameters in the range 1 to 5 µm. Their lengths vary between 70 and 150 µm, however, fibres having a length to diameter ratio between 5:1 and 20:1 can also be used. Suitable fluorocarbons may include PTFE (Polytetrafluoroethylene), FEP (Fluorinated Ethylene Propylene), PFA (Perfluoroalkoxy) though the preferred materials are PTFE and PFA, particularly PTFE. The coating is preferably applied using a liquid medium. Suitable liquid mediums include blends of Water, aromatic hydrocarbons NMP (N-Methyl Pyrrolidone), DMF (Di-methyl formamide) Glycols, and Alcohols, though the preferred materials are blends of Water, Alcohols, NMP (N-MethylPyrrolidone) and Glycols The coating may be applied by spray, roller or coil coating techniques or alternatively, the coating may be applied using a powder coating technique. The thickness of the coating system may vary between 20 and 50 µm, but is preferably 30-35 µm. The substrate material is preferably steel (including stainless steel) or aluminium, though other materials such as copper, glass or ceramics may be used, particularly when articles other than cookware are to be coated. In some cases, when liquid medium coating methods are used, the substrate is coated while flat and is subsequently shaped, although the coating of performed articles can be undertaken. The invention is particularly suitable in the case of multi-layer coatings, especially two-layer coatings. One or more of the coating layers may be in accordance with the invention. In a particularly preferred form, a basecoat would include a fibre content of 1 to 20% by weight of its final dry composition, preferably 7.7%, while a topcoat would include a fibre content of up to 10% by weight of its final composition, preferably about 1% to 10%, particularly about 1%. The thickness of the basecoat may vary between 5 and 30 µm, and is preferably between 15 and 25µm, for example 25 µm. The thickness of the topcoat in may vary between 5 and 20 µm, and is preferably between 10 and 15 µm, for example 15 µm. Optionally, a midcoat can be applied to assist in the application of the coating system. The invention therefore also extends to an article coated using a composition as defined above, or in its broadest sense an article having a non-stick coating comprising a fluorocarbon and refractory fibres. According to another aspect of the invention, a method of applying a non-stick coating to an article comprises dispersing refractory fibres and dispersing a fluorocarbon in a liquid medium to form a "wet" coating composition, applying the wet composition to the article, driving off the liquid medium to leave a "dry" coating, and curing the coating at an elevated temperature to provide a cured coating. In the case of a multi layer coating each layer is preferably dried before the next layer is applied though the layers may be applied wet-on-wet. The curing of the various layers may be conducted separately or simultaneously. The drying step (when employed) may be carried out at a temperature in the range of 50°C to 100°C, preferably at about 80°C. The time taken may vary between 2 and 20 mins, though typically may take about 5 mins, though this is dependent upon the power of the oven. The pressure may be between 700 and 800mm Hg (93 and 107 kPa), but is preferably 760mm of Hg (101 kPa). The curing step may be carried out at a temperature in the range of 370°C to 450°C, preferably 390°C to 420°C, for example 400°C. The time taken at peak metal temperature may vary between 5 and 20 minutes, preferably 5 to 10 minutes, for example 5 mins. The pressure may be between 700 and 800 (93 and 107 kPa), but is preferably about 760mm Hg (101 kPa). The curing step may be conducted under an atmosphere of air. Preferably, prior to coating, the substrate to be coated is prepared in order to remove all contaminants such as oil, grease, detergent, blast media etc. Oils and greases may be removed by vapour degreasing, for example, using a chlorinated or fluorinated cleaning solvent. Alternatively, oils and greases may be removed by an alkali wash or by high temperature "burn-out" in which the substrates may be exposed to a pre-bake at 420°C to 450°C for 15 minutes. Preferably, the surfaces are also abraded, for example, by grit blasting. Suitable grits, e.g. for aluminium substrates, include iron-free aluminium oxide grit of 40 to 60 mesh (250 to 375 µm), using an air pressure of 100 psi (689 kPa) to an average blast profile (Ra) of 4 to 8 µm, as measured using surface profile measuring equipment such as a Perthometer. All blast media should then be cleaned from the substrate prior to coating. In the case of cast aluminium, it is possible that the metal may retain some contaminants from the casting process which might not be removed by grit blasting or degreasing. In such a case, high temperature "burn-out" is recommended to be employed prior to degreasing and grit blasting. It has been found that coatings in accordance with the invention have improved resistance to abrasion at high temperatures and also improved resistance to staining, while no degradation in adherence has been experienced. The invention may be carried into practice in various ways and will now be illustrated in the following examples. Example 1 The following basecoat composition was assembled. Component % by weight (dry) % by weight (wet) Polyamide-imide 17.00 7.50 Organic solvent 14.00 Deionised water 21.50 Alumina fibres 7.50 3.30 Black pigment 10.00 4.70 Wetting agents 1.50 PTFE dispersion 62.50 45.00 PFA dispersion 3.00 2.50 The first six components were thoroughly mixed in a ball mill then the remaining components were added at room temperature using a slow speed 100 rpm paddle mixer. The mixture was passed through a 100 µm sieve. A pressed aluminium substrate was vapour degreased using a conventional chlorinated cleaning solvent. The substrate was abraded by gritblasting using a 40 to 60 mesh aluminium oxide grit at an air pressure of 100 psi (689 kPa) to an average blast profile (Ra) of 6 µm. The substrate was then cleaned of all blast media using conventional techniques. The coating material was applied to the substrate as a base coat using a Binks model 630 spray gun equipped with a 1.8 mm fluid nozzle and an air nozzle adapted to use 13 cu. ft. (0.37m 3 ) of air per minute at 30 psi (206 kPa). The base coat was applied uniformly over the substrate by spraying with a steady even motion. After the base coat had been applied to the required wet film thickness (70 to 90 µm), the solvents, including water, were flashed off at a temperature of 80 to 100°C and the coated substrate was allowed to cool. Example 2 A top coat formulation was assembled using the following components: Component % by weight (dry) % by weight (wet) Silicone resin 7.40 3.30 Organic solvent 6.60 Deionised water 16.91 Alumina fibres 2.20 0.99 Glycol 1.98 Wetting agents 2.29 PTFE dispersion 83.20 61.49 PFA dispersion 7.20 6.44 The top coat composition was prepared in the same way as in Example 1 and was applied to the cooled base coat to a wet film thickness of 50 µm. The solvents including water were flashed off at a temperature of 80 to 100°C, for about 10 minutes. The substrate with its two layer coating was then cured for 5 minutes at 400°C (though in fact, a maximum cure of 5 minutes at 420°C or a minimum cure of 4 minutes at 400°C would have been sufficient). These temperatures represent the peak metal surface temperatures. The resultant product included a base coat of 21 µm in thickness and a top coat of 11 µm. The coating had a smooth, semi-gloss finish. Testing showed a practical operating temperature range of -20°C to 290°C.

Claims: A coating composition providing a non-stick coating which comprises one or more fluorocarbon resins and refractory fibres, the fluorocarbon representing 40 to 90% by weight and the fibres representing 1 to 25% by weight of the final dry coating, the fibres have diameters in the range of 1 to 20 µm, and the lengths of the fibres vary between 70 and 150 µm, characterised in that the composition additionally including a binder resin comprising one or more of polyamide imide, polyphenylene sulphide, polyether sulphone and silicone resins. A composition as claimed in Claim 1, characterised in that it additionally includes a liquid medium, the fibres and fluorocarbon being dispersed in the medium. A composition as claimed in any preceding Claim, characterised in that the fluorocarbon represents 60 to 70% of the final coating. A composition as claimed in Claim 2 or Claim 3, characterised in that the liquid medium is present as 40 to 60g based on 100g of the coating formulation, preferably 50 to 55g based on 100g of the coating formulation. A composition as claimed in any preceding Claim, characterised in that the fibres are alumina fibres, carbon fibres or glass fibres. A composition as claimed in any preceding Claim, characterised in that the fluorocarbons comprise PTFE (polytetrafluoroethylene) and/or FEP (fluorinated ethylene propylene) and/or PFA (Perfluoroalkoxy). A composition as claimed in any of Claims 2 to 6, characterised in that the liquid medium is a blend of some or all of water, xylene, Toluene, MNP (N-methyl pyrrolidone), DMF (Di-methyl formamide), Glycol and Alcohol. A method of applying a non-stick coating to a substrate which comprises dispersing a coating composition as claimed in any of Claims 1 to 7 in a liquid medium to form a wet coating composition, applying the wet coating composition to the substrate, driving off the liquid medium to leave a dry coating, and curing the coating at an elevated temperature to provide a cured coating. A method as claimed in Claim 8, characterised in that the drying step is carried out at a temperature in the range of 20 to 100°C for a time between 2 and 20 minutes and/or the curing step is carried out at a temperature in the range of 370 to 450°C for a time between 5 and 20 minutes. A method as claimed in Claim 8 or Claim 9, characterised in that, prior to coating, the substrate is treated to remove contaminants, preferably by subjecting the substrate to a temperature of 420 to 450°C. A method as claimed in any of Claims 8 to 10, characterised in that the substrate is steel, aluminium, copper, glass or ceramic. A method as claimed in any of Claims 8 to 11, characterised in that a plurality of layers of coating are applied to the substrate.

Inventors: BATE T J, WILSON H C, BATE, T. J., WILSON, H. C.
Applicants: WHITFORD PLASTICS, WHITFORD PLASTICS LIMITED
Application year: 1993
Main classification: C09D127
IPC codes: C08K7, C09D127, C09D179