GB2152946A

GB2152946A - Non-stick coatings

Info

Publication number: GB2152946A
Application number: GB08401707A
Authority: GB
Inventors: Dennis Joseph Harman; John Gascoyne
Original assignee: Postans Ltd
Current assignee: Postans Ltd
Priority date: 1984-01-23
Filing date: 1984-01-23
Publication date: 1985-08-14
Also published as: GB8401707D0; GB2152946B

Abstract

Non-stick coatings are produced from a composition comprising 100 parts by weight of a cross-linkable silicone polyester resin, from 20 to 200 parts by weight of a laminar solid, preferably mica, and a liquid carrier for the resin.

Description

SPECIFICATION Non-stick coating The present invention relates to a non-stick coating particularly, but not exclusively, for use on the internal surfaces of items of cookware such as frying pans, milk saucepans, baking trays and casseroles.

A non-stick coating for an item of cookware should retain its non-stick properties over a wide range of service conditions, for instance after contact with hot oil or boiling salt solutions. The coating should adhere strongly to the item of cookware, so that it does not peel off after repeated use, and should also be abrasion resistant, so that it can resist the effects of cooking implements, washing-up cloths and scouring pads.

Preferably the coating has a high hardness value, so that it can resist rupture, and is formable so that it can be used, for instance, on metal substrates which are formed by bending or drawing into kitchen utensils.

At present most non-stick coatings are based on polytetrafluoroethylene (PTFE) resins. These may be provided on an item of cookware as a single or multiple layer coating.

A single layer PTFE coating has relatively good non-stick performance, but, due to its inherent properties, has poor adhesion and low abrasion resistance. It thus cannot meet the more severe requirements for a non-stick coating. For instance, such a coating cannot be used effectively on a frying pan.

A multiple, for instance triple, layer PTFE coating has improved abrasion resistance. However its penetration and abrasion resistances are still relatively poor. Such a coating therefore does not solve all the problems of single layer PTFE coatings.

There are also manufacturing problems associated with the production of items of cookware having PTFE coatings. In order to achieve as good adhesion as possible, it is necessary to prepare the substrate surface to a very high standard, which is much higher than is normally used for surface preparation in general industry.

Generally, the substrate surface is shot-blasted before the coating is applied. The PTFE resins are relatively difficult to handle, which makes their application difficult. Temperatures in excess of 400"C are needed to cure the resins on the substrate. The PTFE resins themselves are relatively expensive and difficult to manufacture. It can thus be seen that the cost of producing a PTFE coating on an item of cookware is high.

The processing costs are increased still further if a multiple layer PTFE coating is to be produced as the energy and labour costs are increased and productivity is decreased when compared to the production of a single layer coating.

There is a large amount of literature available describing coatings based on fluorocarbon polymers, such as PTFE, and methods of applying them, for instance, to cookware. The literature includes the following patent specifications: GB-A-1 400 458; - 1 482 652; - 1 496 554; 1 500 022; 1 557 230; 1 560 781; 2 090 772; US-A-3 931 084; - 4351 882; EP-A-O 022 256; - 0 022 257; - 0 043 862; - 0 056 280; and - 0 075908.

The use of silicone resins as the basis for non-stick coatings is known. However, these materials are brittle and have only fair adhesion, and therefore have only a short service life. Their use is restricted mainly to commercial bakeware which needs to be recoated after as few as 500 repeat uses, even though the bakeware is not man-handled or scraped during use. Clearly such resins could not economically be used on domestic cookware.

Formulations based on silicone polyester resins are known and are used to produce heat resistant coatings for exterior surfaces of hollow-ware, such as cookware, but these formulations cannot be used to provide non-stick coatings for the internal surfaces of such articles.

The use of silicone of polyorganosiloxane resins in providing surface coatings is described, for instance, in the following patent specifications: GB-A-1 448985; - 1 476314; - 1 512505; US-A-4 066817; EP-A-O 003 760; - 0 017 958; - 0 081 444; DE-A-28 51 740; SU-A-722 927; and - 805970.

It is an object of the present invention to provide a non-stick coating for use of the internal surfaces of cookware which overcomes at least in part the disadvantages of presently known non-stick coatings.

According to a first aspect of the present invention, there is provided a non-stick coating comprising: 100 parts by weight of a cross-linked silicone polyester resin; and from 20 to 200 parts by weight of a laminar solid.

According to a second aspect of the present invention, there is provided a composition, for use in producing a non-stick coating, comprising: 100 parts by weight of a cross-linkable silicone polyester resin; from 20 to 200 parts by weight of a laminar solid; and a liquid carrier for the resin.

The following components may additionally be present in both the coating and the composition.

A cross-linking agent for the resin. If used, this will be incorporated in the cross-linked resin in the coating.

However, it is preferred that the resin is of the self cross-linking type and therefore a cross-linking agent will not be required. The cross-linking agent may be used where it is desired to reduce the stoving temperature of the composition. Generally a composition according to the invention will comprise at most 5%, by weight of the composition, of a cross-linking agent.

Processing aids. These may be used to aid in manufacture, for instance to improve the dispersion of filler in the composition or to improve the stability of the composition, andlor to aid in the application of the composition, for instance by promoting substrate wetting by the composition. They do not necessarily improve the properties of the coating. However, the preferred processing aids, such as fluorocarbon and acrylate auxiliary polymers provide an additional benefit of enhancing the non-stick properties of the coating. Preferably from 10 ppm to 1.0% by weight (based on the weight of the final coating) of these processing aids are used.

Non-stick promoters. These are used to enhance the non-stick properties of the final coating and preferably from 0.01 to 5.0% by weight (based on the weight of the final coating) of these promoters are present. Suitable promoters include fluorocarbon and silicone oils and certain waxes of the polyethylenel polytetrafluoroethylene (PE/PTFE) type.

The exact amounts of the processing aids and non-stick promoters to be used will depend on the particular resin, solid and solvent used, but can readily be determined by trial and error by a person skilled in the art.

Preferably, the silicone polyester resin is capable of self cross-linking at elevated temperatures and provides not only non-stick properties but also excellent heat resistance, hardness and flexibility in the coating. A preferred such resin comprises from 20 to 80%, most preferably from 45 to 65%, (by weight of the resin) of a phenyl alkyl silicone resin. A suitable resin is sold under the trade designation Plastokyd SC 55 by Croda Resins Ltd., of Belvedere, Kent. If a lower level of heat resistance is required the level of silicone modification may be reduced, but the reduction should not be so great as to reduce the non-stick properties of the coating.

Any laminar solid may be used in the coating or composition. By 'laminar' is meant a solid whose particles have both length and breadth significantly greater than their depth, and therefore having a plate-like form.

The preferred laminar solid is mica, although other laminar minerals such as vermiculite or glass flakes may also be used. Alternatively the laminar solid may be a flake metal such as aluminium or stainless steel flake.

If desired a mixture of laminar solids may be used.

Where the laminar solid is a mineral, there should be at least 40 parts, and preferably at least 90 parts, thereof present. Where the laminar solid is a metallic material, there may be present only 20 parts thereof.

The composition or coating may additionally contain a pigment or a mixture of pigments. A preferred pigment is carbon black. However, other black pigments, such as cobalt oxide, may be used instead of, or in addition to, the carbon black. Alternatively, coloured pigments, such as titanium dioxide, red iron oxide, chromium oxide, titanates, coated micas, and certain cobalt compounds may be used to provide a desired colourforthe coating. Using the coating or composition of the present invention, it is possible to obtain pastel colours by appropriate mixtures of pigments. This is not possible with PTFE based coatings.

It will be appreciated that, since the coating is intended principally for use on cookware, any pigment used should be not only heat and chemical resistant, but also non-toxic.

It is preferred that the amount of pigment used should be kept as low as possible commensurate with its hiding power and colour. The exact amount of pigment to be used in any particular composition can readily be determined by a person skilled in the art by trial and error.

The liquid carrier used in the composition according to the invention is preferably an organic solvent capable of dissolving the polyester resin. The solvent may be pure, but it is envisaged that in practice the solvent will be a blend of components. Suitable solvent components include xylol, cyclohexanone, aromatic hydrocarbons, such as toluene, and glycol ether esters.

Alternatively, the liquid carrier may merely suspend or may form an emulsion with the polyester resin.

The amount of liquid carrier used will be determined in accordance with the method by which the composition is to be applied to a substrate. However, typically it will be between 30 and 50% by weight (based on the total weight of all the components). Suitably the composition will have a viscosity of about 100 seconds (as measured using the British Standard BSB4flow cup method) at 20"C.

The amount and type of liquid carrier used will be selected so as to control the properties of the composition after application in respect of flow, levelling, liquid carrier boil and wrinkling.

The composition may be applied to a substrate by any method used in industry to apply paint, for instance spraying or coil coating. The substrate need only be prepared to normal standards. There is no need to prepare itto the high standards used in producing PTFE coatings.

It is envisaged that the principal use for coatings according to the present invention will be for providing non-stick internal surfaces in domestic cookware. For such uses it is preferred that all the components of the coating meet the requirements of various regulating authorities, such as the U.S. FDA or the German BGA, for materials allowable for use in contact with food. The coating exemplified below, for instance, meets FDA requirements.

However, the application of the coating is in no way limited to cookware. The coating may be used on for instance bolts, stud fasteners, valves, hoppers, sifters and cylinders to provide low friction dry lubrication.

Alternatively the coatings may be used in the confectionery and preserve manufacturing industries to provide good release surfaces in cooking pots and on production lines. Further, the coating may be applied to the insides of moulds to provide good mould release.

The coatings of the present invention will be of particular use where abrasion resistance is required, since this cannot readily be provided in PTFE coatings.

The present invention also includes products, such as cookware and kitchen utensils, have applied thereto a coating according to the invention.

The present invention is illustrated by the following example, in which all parts and percentages are by weight unless otherwise stated.

The following components were placed in a ball mill.

1234 parts of a silicone polyester resin solution containing 60% resin solids dissolved in Oxitol acetate.

1123 parts of mica.

101 parts of carbon black.

525 parts of a solvent comprising a mixture of aromatic hydrocarbons and glycol ether esters.

The silicone polyester resin was Plastokyd SC 55 referred to above. The mica was Micro Mica ATI supplied by Norwegian Talc of Liverpool. The carbon black was Special Black 4, supplied by Degussa of Frankfurt, W.

Germany. The solvent comprised a 1:1 by volume blend of Solvesso 200, supplied by Esso Chemicals and Oxitol Acetate, supplied by Shell Chemicals.

The components were ground in the ball mill until a good dispersion was obtained.

To the dispersion was added a further 844 parts of the silicone polyester resin solution and 750 parts of the solvent. 4.5 parts of a fluorocarbon resin (Fluorad F.C. 430, supplied by 3M Commercial Chemicals of Bracknell) was also added to the dispersion. The fluorocarbon resin acts both as a processing aid and as a non-stick promoter. These components were thoroughly mixed to provide a composition for producing a non-stick coating.

The composition had a viscosity of about 100 seconds (determined as set out above) at 20"C and could be stored up to six months in a normal airtight container.

The composition comprised: Silicone polyester resin 100 parts Filler (comprising 92% mica) 98.2 parts Fluorocarbon resin (acting as 0.36 parts processing aid and non-stick promoter) Solvent blend 46%* * based on total weight of all the components.

The composition was then coated onto the internal surface of an item of cookware, by spraying or coil coating. The surface had only been prepared to normal industrial standards and not to the high standards required for preparing PTFE coatings. The coated item was then placed in a convection oven at 250 C for 30 minutes to cure the resin and form a non-stick coating according to the invention on the item.

This represents a significant cost saving when compared to the production of PTFE-based coatings wherein each coating step, of which there may be up to three, involves the use of an oven temperature typically of 425"C.

The coating had a black gloss finish after curing. It was tested for desirable properties as follows.

Non-stick performance - The coating fully met the requirements of British Standard BS 4861 - Non-stick Unreinforced Plastics Coating on Domestic Cookware - and BS 5206 - Non-stick Hard Base Plastics Coating on Domestic Cookware. These include tests for non-stick properties (and also adhesion) after contact with hot oil and boiling salt solution.

Single layer PTFE coatings will also pass the first specification, but are unlikely to pass the second specification. Multiple layer PTFE coating will pass both these specifications.

Adhesion - The coating passed British Standard BS 3900, Part E6, at 1 mm spacings with no loss of adhesion and thus showed excellent adhesion properties.

A triple layer PTFE coating will also pass this test, but it is considered unlikely that a single layer PTFE coating would.

Abrasion Resistance - The coating remained intact after 300 double rubs with a wet Brillo pad, thus showing excellent abrasion resistance.

Although a triple layer PTFE coating also shows this level of abrasion resistance, a single layer PTFE coating will only remain intact for less than 50 double rubs.

Hardness - The coating is not ruptured by a 3H pencil in British Standard test BS Au 148, part 6, clause 3 for film hardness. This hardness is retained during service at elevated temperatures.

In contrast, both single and multiple layer PTFE coatings can only resist rupture by an F pencil, thus showing they are significantly less hard than the present coating.

Moreover, it has been shown that the present coating is sufficiently flexible for it to be formed by bending or drawing of a metal substrate to which it is adhered for instance to form kitchen utensils.

It can thus be seen that the coating of the present invention is much easier and less expensive to apply than a PTFE coating and has better properties when formed as a single layer coating than a triple layer PTFE coating.

It is thought that the improved properties of the coating are at least to some extent provided by the laminar solid, which appears to provide a continuous overlapping film. This film provides most of the toughness and abrasion resistance and contributes to the non-stick properties of the coating. However, the applicants do not wish to be limited in any way by this explanation.

It can therefore be seen that the present invention provides an improved and less expensive non-stick coating.

Claims

1. A non-stick coating comprising: 100 parts by weight of a cross-linked silicone polyester resin; and from 20 to 200 parts by weight of a laminar solid.

2. The coating of claim 1, further comprising a processing aid.

3. The coating of claim 2, wherein the processing aid comprises a fluorocarbon or acrylate auxiliary polymer.

4. The coating of claim 2 or claim 3, comprising from 100 ppm to 1.0% of the processing aid.

5. The coating of any one of claims 1 to 4, further comprising a non-stick promoter.

6. The coating of claim 5, comprising from 0.01 to 5% of the non-stick promoter.

7. The coating of claim 5 or claim 6, wherein the non-stick promoter is a fluorocarbon or silicone oil, or a polyethylenelpolytetraf luoroethylene wax.

8. The coating of any one of claims 1 to 7, wherein the resin comprises from 45 to 65% by weight of the resin of a phenyl alkyl silicone resin.

9. The coating of any one of claims 1 to 7, further comprising a cross-linking agent for the resin.

10. The coating of claim 9, comprising up to 5% of the cross-linking agent.

11. The coating of any one of claims 1 to 10, wherein the laminar solid is mica, vermiculite, glass flake or aluminium or stainless steel flake.

12. The coating of claim 11, wherein the laminar solid is a mineral, the coating comprising at least 40 parts thereof.

13. The coating of claim 12, wherein the coating comprises at least 90 parts of the mineral laminar solid.

14. The coating of any one of claims 11 to 13, wherein the laminar solid is mica.

15. The coating of claim 11, wherein the laminar solid is metallic and the coating comprises at least 20 parts thereof.

16. The coating of any one of claims 1 to 15, further comprising a pigment.

17. A composition, for use in producing a non-stick coating, comprising: 100 parts by weight of a cross-linkable silicone polyester resin; from 20 to 200 parts by weight of a laminar solid; and a liquid carrierforthe resin.

18. The composition of claim 17, further comprising a processing aid.

19. The composition of claim 18, wherein the processing aid comprises a fluorocarbon or acrylate auxiliary polymer.

20. The composition of claim 17 or claim 18, comprising from 100 ppm to 1.0% of the processing aid.

21. The composition of any one of claims 17 to 20, further comprising a non-stick promoter.

22. The composition of claim 21, comprising from 0.01 to 5% of the non-stick promoter.

23. The composition of claim 21 or claim 22, wherein the non-stick promoter is a fluorocarbon or silicone oil, or a polyethylene! polytetrafluoroethylene wax.

24. The composition of any one of claims 17 to 23, wherein the resin is self-cross linking.

25. The composition of any one of claims 17 to 24, wherein the resin comprises from 45 to 65% by weight of the resin of a phenyl alkyl silicone resin.

26. The composition of any one of claims 17 to 23, further comprising a cross-linking agent for the resin.

27. The composition of claim 26, comprising up to 5% of the cross-linking agent.

28. The composition of any one of claims 17 to 27, wherein the laminar solid is mica, vermiculite, glass flake or aluminium or stainless steel flake.

29. The composition of claim 28, wherein the laminar solid is a mineral, the coating comprising at least 40 parts thereof.

30. The composition of claim 29, wherein the coating comprises at least 90 parts of the mineral laminar solid.

31. The composition of any one of claims 28 to 30, wherein the laminar solid is mica.

32. The composition of claim 28, wherein the laminar solid is metallic and the coating comprises at least 20 parts thereof.

33. The composition of any one of claims 17 to 32, further comprising a pigment.

34. The composition of any one of claims 17 to 33, wherein the liquid carrier is an organic solvent or solvent blend capable of dissolving the polyester resin.

35. The composition of any one of claims 17 to 34, comprising from 30 to 50% by weight (based on the total weight of all the components) of the liquid carrier.

36. The composition of any one of claims 17 to 35 having a viscosity of about 100 seconds.

37. A non-stick coating substantially as hereinbefore described with reference to the Examples.

38. A composition, for use in producing a non-stick coating, substantially as hereinbefore described with reference to the Examples.

39. A product having thereon a non-stick coating according to any one of claims 1 to 16 and 37.

40. A product having thereon a non-stick coating prepared by applying to the product a composition according to any one of claims 17 to 36 and 38.