GB2113235A

GB2113235A - Coating of materials

Info

Publication number: GB2113235A
Application number: GB08301370A
Authority: GB
Inventors: John Powell; D P Willis; L G Barratt
Original assignee: Whitford Plastics Ltd
Current assignee: Whitford Plastics Ltd
Priority date: 1982-01-19
Filing date: 1983-01-19
Publication date: 1983-08-03
Also published as: GB2113235B; GB8301370D0

Abstract

Heat-resistant coating layers for a substrate e.g. steel strip or an article e.g. cooking, tools, household goods comprise (a) an aromatic polysulphone with repeating units of formula -Ar-SO2 in which Ar is an aromatic radial e.g. <IMAGE> where Y is -O-, -S-, or an aromatic diol residue, and preferably of formula <IMAGE> (b) an aromatic sulphide polymer with repeating units of formula -A-S- where -A- is a mono- or bi-nuclear, optionally alkyl-substituted organic radical e.g. <IMAGE> and (c) optionally, a fluorocarbon polymer such as polytetrafluoroethylene.

Description

SPECIFICATION Coating of materials This invention relates to coating compositions for metal surfaces which comprise aromatic polysulphones, and more particularly to such coating compositions further comprising fluorocarbon polymers. It further relates to coatings formed from such compositions, to coated articles for substrates, and coating methods.

It is known to coat metal surfaces with aromatic polysulphones as a heat-resistant coating, for example on the non-cooking surfaces of metal cookware as described in Patent No.

1 417 239. This earlier patent is concerned with cookware with an aromatic polysulphone coating on a non-cooking surface, optionally pigmented and patterned. The aromatic polysulphones comprise repeating units of the formula tAr-SO 2 where Ar is itself preferably of the formula

in which Y is preferably oxygen but may also be sulphur or an aromatic diol residue. Such coatings may be formed by applying a thermoplastic aromatic polysulphone as a solution. Usually it is advisable to increase the molecular weight of the polymer, in order to improve the resistance of the coating to washing-up procedures.This can be done by irradiation (beta-ray, ultra-violet or electron beam) at temperatures up to 4000C, optionally followed by 300o400cC heattreatment, or by such heat-treatment alone if the initial coating contains added sulphur, dimephides or bisthiols. If the polymer has reactive endgroups such as -OH, heat-treatment in the presence of oxygen also increases the molecular weight.

Patent No. 1 426 342 sets out to improve such aromatic polysulphone coatings in a different wav, by the incorporation of additional polymer material. It is concerned with a coating composition based on a solution or dispersion of the thermoplastic aromatic polysulphone having further dispersed therein a fluorocarbon polymer such as lubricant grade ptfe (polytetrafluoroethylene), the weight of polysulphone being from 1090%, preferably 1 5-85% and most preferably less than 60% of the total weight of polysulphone and polyfluorocarbon). Where the thermoplastic aromatic polysulphone is a dispersion, it can be aqueous, made by milling the polymer with water, thereby forming a vehicle to which the fluorocarbon polymer can be added.

Such coatings can be applied to the article (e.g.

by spraying, dipping or brushing) dried, and sintered to given an adherent non-stick coating for cooking, handling or working surfaces of equipment.

The above patents further describe variant and optional features, and examples of specific polymers and methods, which are incorporated herein by way of reference.

The present invention is based on the realisation that a further polymeric component can be added to coating compositions as described above to give coated articles of good coating thickness and resistance to normal use.

The invention provides (a) coating layers for articles or substrates; (b) substrates or articles so coated (c) coating compositions suitable for application to provide such coating layers (d) methods of application of such coating compositions and (e) methods of production of such coating compositions.

In the first aspect the invention provides a coating layer for a substrate or article comprising both an aromatic polysulphone and an aromatic sulphide polymer.

The aromatic polysulphone can comprise the repeating units Ar-SO2- in which Ar is a divalent aromatic radical and at least some of the Ar units have the structure

in which Y is oxygen, sulphur or the residue of an aromatic diol such as 4,4'-bisphenol. Other units of formula Ar can comprise those described in the above-mentioned British Patent Specification.

Preferably the group Ar has the formula:

The aromatic sulphide polymer can comprise the repeating unit -A-S- where -A- is a mononuclear or binuclear divalent aromatic radical optionally substituted with alkyl groups, and is preferably a phenylene radical i.e. the radical:

Thus, a preferred coating comprises those, two polymers with the respective repeating groups

referred to below for convenience as polyethersulphone (PES) and polyphenylene (PPS) respectively.

The relative proportions of aromatic polysulphone and aromatic sulphide polymer can vary over a wide range. If desired the aromatic sulphide polymer can predominate. It can constitute from 10 to 90% of the combined weights. More usually, however, it ranges from 20 to 50% e.g. about 30 40%.

As is conventional in this art, the coating can contain pigments or fillers, and be constituted as continuous or a patterned application.

It is particularly preferred, however, for the coating to further comprise at least one fluorocarbon polymer, preferably the homopolymer of tetrafluoroethylene i.e. pffe but possibly a copolymer thereof with a compatible comonomer such as ethylene, vinyl chloride or hexafluoropropylene. Thus, the preferred coating of the invention comprises PES, PPS and ptfe.

The weight proportion of fluorocarbon polymer to the total weight of the three polymeric components varies widely but is preferably from 10% to 70%. Usually, it is less than 50%. If a filler or pigment is also present the total weight proportion of fluorocarbon plus filler (and/or pigment) to the total mixture is usually less than 60%; the maximum content of filler itself is usually 50%.

A second aspect of the invention is constituted by substrates or articles having a coating as defined above. These substrates can include glass, metal especially ferrous metal or aluminium, and ceramics; or substrates with composite surfaces e.g. sprayed ceramic or metal powder. An especially valuable substrate is steel strip, provided as a coil for further fabrication.

The polysulphone/sulphide polymer coating per se is heat-resistant and can be used for suitable decorative and protective purposes on articles exposed to heat, e.g. the non-cooking surfaces of cooking utensils. When fluorocarbon is also present, non-stick or lubricant properties are imparted, rendering the coating useful for the cooking surface or cooking utensils (frying pans, saucepans, bakeware) industrial processing equipment (moulds, rollers, stirrers, mixers, chutes, hoppers, heat-sealing jaws) domestic equipment (iron sole plates, food mixers, ice separators) tools (saw blades, secateur blades) or electrical components (insulators).

A third aspect of the present invention is constituted by coating compositions for application to substrate surfaces, and consists in a coating composition comprising a dispersion of aromatic sulphide polymer e.g. as defined above in a solution (or possibly a dispersion) of at least one aromatic polysulphone e.g. as defined above in a liquid dispersing medium. The sulphide polymer can constitute from 10 to 90%, preferably 2050% e.g. 3040% by weight of the total of both polymer.

It is much preferred for the polyaromatic sulphone to be completely dissolved in a suitable organic solvent such as dimethyl formamide, Nmethyl pyrrolidone or cyclohexanone. Further miscible organic diluents, which may or may not be solvent for the polyaromatic sulphone, can be present. It is preferred if the amount of such diluents is not so great as to cause the polyaromatic sulphone to precipitate.

The coating composition can also contain fillers and pigments. In particular, it can comprise a dispersion, throughout the dispersing medium, of fluorocarbon polymer e.g. as defined above.

The preferred relative proportions of such fluorocarbon polymer are also preferably those given above.

The total proportion of dispersing medium and any diluent to be used is variable over a wide range, and depends upon the final viscosity desired and the evaporation rate, bearing in mind the intended mode and site of application.

Various application techniques are known in the art and described, for example in the Patents above referred to.

The fourth aspect of the invention is concerned with methods of application of such compositions. One method consists in a method of coating an article or substrate in which the coating composition as described above is applied to the surface and the molecular weight of at least the aromatic polysulphone is increased. This can be effected by irradiation, with optional subsequent heat-treatment, or by heat-treatment after incorporation of sulphur, sulphide or bisthiols or by heat-treatment when the polysulphone containsOH groups, all as described in Patent 1 417 239.

Alternatively, or additionally, the coating can be dried and heat-treated to sinter the fluorocarbon particles when such are present.

The fifth aspect of the invention consists in a method of preparing the coating composition, in which the aromatic sulphide polymer is dispersed within a solution (or possibly dispersion) of aromatic polysulphone. Optionally diluents are subsequently added. It is preferred to mill the sulphide polymer into a proportion of an aromatic polysulphone solution, and thereafter to add the remaining desired proportion of such solution.

However, it can also be milled into solvent, or diluent, or any combination. Where fluorocarbon polymer is to be added it can be milled or stirred into a previously formed suspension of the sulphide polymer.

Preferably, a number of premixes are formed and thereafter mixed as described in more detail in the Example of the invention Example In this example, all parts are by weight.

Premix I A mixture of the following composition was milled in a ball mill until homogeneous: Titanium dioxide 46.52 Polyphenylene sulphide 53.52 N-methyl pyrrolidone 99.96 Premix II A mixture of the following composition was milled together in a bead mill until homogeneousL Titanium dioxide 24.72 Polyether sulphone 14.28 N-methyl pyrrolidone 60.99 solvent Premix Ill A mixture of the following composition was milled together in a bead mill until homogeneoust Polytetrafluoroethylene 14.00 Polyether sulphone 11.50 N-methyl pyrrolidone 74.49 solvent Final mixture The following mixture was stirred together until homogeneous: Premix 1 20 Premix II 40 Premix lil 40 Its composition was as follows: :- Polyphenyl sulphide (PPS) 5.35 Polyether sulpbone (PES) 10.30 Polytetrafluoroethylene (PTFE) 5.60 Filler (+pigment) 14.54 Solvent 14.54 Diluents 49.65 Ratios PPS -34.18 PPS+PES PTFE =26.35 PPS+PES+PTFE PTFE+FILLER =56.27 PPS+PES+PTFE+FILLER FILLER =39.79 PPS+PES+PTFE+FILLER This final mixture was applied by roller to one side of a mild steel strip which was then cured at 3800--4000C. At this temperature the polyether sulphone and the dispersed polyphenylene sulphide melt together to form a layer securely adhering the sintered polytetrafluoroethylene particles to the steel.

Claims

1. A coating layer for a substrate or article comprising both an aromatic polysulphone and an aromatic sulphide polymer.

2. A coating layer as claimed in Claim 1 in which the aromatic polysulphone comprises the repeating units -Ar-SO2- in which Ar is a divalent aromatic radical and at least some of the Ar units have the structure.

in which Y is oxygen, sulphur or the residue of an aromatic diol.

3. A coating layer as claimed in Claim 2 in which at least some of the Ar units have the formula

4. A coating layer as claimed in any one preceding claim in which the aromatic sulphide polymer comprises the repeating unit-A-S- where-A-is a mononuclear or binuclear divalent aromatic radical either unsubstituted or substituted with one or more alkyl groups.

5. A coating layer as claimed in Claim 4 in which -A- is a phenylene radical of formula

6. A coating layer as claimed in Claim 5 wherein the aromatic polysulphone comprises at least some units of formula

and the aromatic sulphide polymer comprises at least some units of formula

7. A coating layer as claimed in any one preceding claim in which from 20 to 50% by weight, of the total polysulphone and aromatic sulphide polymer, is constituted by the aromatic sulphide polymer.

8. A coating layer as claimed in Claim 7 in which the said weight ratio is from 30 to 40%.

9. A coating layer as claimed in any one preceding claim which further comprises at least one fluorocarbon homopolymer or copolymer.

10. A coating layer as claimed in Claim 9 in which the fluorocarbon homopolymer is polytetrafluoroethylene.

11. A coating layer as claimed in any one of Claims 9 or 10 in which the weight proportion of fluorocarbon homopolymer or copolymer to total weight of the three polymeric constituents is from 10 to 70%.

12. A coating layer as claimed in Claim 11 in which the said weight ratio is less than 50%.

13. A coating layer as claimed in any one preceding claim comprising polyether suiphone, polyphenylene sulphide and polytetrafluoroethylene.

14. A coating layer as claimed in any one preceding claim further containing at least one pigment and/or at least one filler.

1 5. A coating layer as claimed in Claim 14 in which the weight ratio of filler and/or pigment, in relation to total mixture, is less than 50%.

1 6. A coating layer as claimed in Claim 14 or 15, in which fluorocarbon homopolymer or copolymer is present, and in which the weight ratio of such fluorocarbon polymer plus filler and/or pigment to the total weight is less than 60%.

1 7. A substrate or an article at least partially coated with a layer as claimed in any one preceding claim.

18. A substrate as claimed in Claim 17 in the form of steel strip.

1 9. An article as claimed in Claim 1 7 in the form of a cooking utensil, industrial processing equipment, household equipment, garden tool or electrical components.

20. A coating composition for application to the surface of an article or substrate, comprising a dispersion of the aromatic sulphide polymer of the type and in amounts as set forth in any one of Claims 1 to 8 in a solution or dispersion of the aromatic polysulphone of the type and in amounts as set forth in any one of Claims 1 to 8 in a liquid solvent or dispersing medium.

21. A coating composition as claimed in Claim 20 in which the aromatic polysulphone is dissolved in dimethyl formamide, N-methyl pyrrolidone or cyclohexanone.

22. A coating composition as claimed in Claim 21 further containing an organic diluent in an amount whereby the composition is still solvent for the aromatic polysulphone.

23. A coating composition as claimed in any of Claims 20, 21 or 22 further comprising a fluorocarbon homopolymer or copolymer of the type, and to the amounts, defined in any of Claims 9,10,11 or 12.

24. A method of formation of a coating layer from the coating composition as claimed in any one of Claims 20 to 23 in which the composition is applied to the surface and the molecular weight of at least the aromatic polysulphone is increased.

25. A method of formation of a coating layer from the coating composition as claimed in Claim 23 in which the coating is dried and heat-treated to sinter the fluorocarbon polymer particles.

26. A method of preparing a coating composition as claimed in any one of Claims 20 to 23 in which the aromatic polysuiphone, aromatic polysulphide polymer and any fluorocarbon polymer are formed into two or more separate premix suspensions or solutions each containing at least one of the said components, and the premixes themselves are mixed together to form the coating composition.