GB1564501A - Coatings - Google Patents

Description

(54) COATINGS (71) We, GRANT & WEST LIMITED, a Company registered under the laws of England, of Stoke Mill, Guildford, Surrey GUI IQB, and JONAS WOODHEAD LIMITED, a Company registered under the laws of England, of 177 Kirkstall Road, Leeds LS4 2AQ, Yorkshire do hereby declare the inventiton, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to compositions for forming protective coatings for metal parts especially ferrous parts, to methods for forming such coatings and to the coatings when formed.It is particularly useful where such materials are to be protected from both fretting and corrosion, and it finds its primary application in the coating of stressed components prone to failure by metal fatigue where fretting can be an over-riding cause of the failure. Examples of such components are leaf springs and coil springs.

The invention is concerned particularly with the problem of fretting of leaf springs.

This occurs between adjacent faces of each leaf in a pack of spring leaves and also between the spring leaves and the clamping plates which hold the spring together in the region of the centre bolt. The exact mechanism involved is not clearly understood but the resultant detrimental pitting and cracking of the tension surface of the spring leaves is apparently caused by some form of surface welding.

Ordinary lubricating with oils or greases does not significantly reduce the incidence of fretting which often results in the spring breaking at the localized region where fretting has occured. The problem occurs with multi leafspringse.g. of 6 to 10 or more leaves but is particularly critical with minimum leaf springs having 5 or less leaves e.g. 2, 3 or 4 leaves.

It is an object of the present invention to provide protective coatings for use in such' situations which gives improved protection against fretting and corrosion, and in addition which are "air-drying" and less expensive than conventional coatings.

We have found that such results can be obtained with a novel two component and two stage treatment, neither of which in themselves provide the desired degree of protection but which together give excellent results.

Thus according to one aspect of the present invention a method of forming a protective coating on a metal part comprises applying to the clean grease free part one or more coats of a primer composition comprising a resin and a chromate pigment, drying the coating to form a layer at least 0.01 mils thick e.g. 0.1 to 1 mil thick or 0.2 to 0.4 mils thick, which is of low permeability to water vapour and liquid moisture, and applying one or more coats of a surface coat composition comprising a resin, an alkali metal or alkaline earth metal alkyl benzene sulphonate and a solvent, the surface coat composition adhering to the primer coating and drying the surface coating to form a layer at least 0.1 mils thick e.g. 0.5 to 6 mils thick or 0.8 to 1.0 mils thick, which provides a corrosion resistant coating.

It is not known how this coating achieves its antifretting properties and the surface is.

tacky and polymery in feel. Thus whilst it appears that some form of lubrication is occurring the surface has not been described as a lubricating surface since its appearance does not conform to that of conventional lubricants.

The resin in the primer composition is preferably a polyvinyl butyral resin e.g. one giving a solution of relatively low viscosity e.g. in the range 20 to 30 centipoise (as a 5% solution in n-butanol at 20"C as measured by the Hoppler method (DIN 53015)).

Alternative resins such as phenolic resins or higher molecular weight polyvinyl butyral resins (e.g. having solution viscosity of 60 to 90 centipoise) may be used but we have found that these are less satisfactory and tend to form thinner films which cannot contain as much chromate pigment and which have lower physical strength.

Up to 500it e.g. 10 to 40% by weight of the low solution viscosity polyvinyl butyral may however be replaced by phenolic resins or by the higher solution viscosity polyvinyl butyrals.

The resin used in the primer formulation is desirably alcohol or ketone soluble, film forming from such solutions, fully cured and thermally stable, i.e. not heat hardening to any such degree as to accelerate rupturing of the film in use, up to at least 1000C or 150"C and preferably up to 2000C.

The chromate pigment is used to confer corrosion protection. A number of metal chromates could be used e.g. Barium or zinc chromate, or zinc tetroxy chromate but these, we have found, tend to be unstable on storage depositing out from the composition.

We thus prefer to use strontium chromate as the chromate pigment.

The primer composition preferably contains 5 to 15% of resin e.g. 8 to 12% by weight, 1 to 15% e.g. 3 to 10% of chromate pigment, and desirably 0.1 to 5% e.g. 0.5 to 2.5 of phosphoric acid e.g. of 1.65 specific gravity. The composition may also contain a small amount of carbon black e.g. 1 to 2% and the balance of the composition is a volatile solvent or solvent blend e.g. a mixture of methyl-iso-butyl-ketone and methanol or any other appropriate solvent for the resin.

Thus a preferred class of primer compositions for use in accordance with the invention comprises 8 to 12% of polyvinyl butyral resin of solution viscosity 20 to 30 centipoise, 36 to 52% of methyl-iso-butylketone, 28 to 42% of methanol, 1 to 2% of carbon black, 3 to 10% of strontium chrdmate and 0.5 to 2.5% of phosphoric acid of specific gravity 1.65.

The primer layer has excellent adhesion to ferrous materials and affords good corrosion protection.

The resin of the surface coat composition serves as a binder for the alkyl benzene sulphonate serving to adhere it securely to the primer coat. Any appropriate resin for this function can be used but a petroleum resin sold under the name "Inkovar AB 165" has been found very satisfactory. This material according to the manufacturers has a coal tar colour of 3, resin colour of F, a softening point of 165"C, an acid number of less than 1, a saponification number of less than 1, a specific gravity of 1.075, is soluble in ink solvent KB No=43, and in ink solvent KB No=27, and in aromatic solvents but is insoluble in alcohols and glycols. It has a Stokes viscosity in ink solvent KB=43 (55% n.v. at 25"C) of 0.85, a Gardner viscosity of C and the result of the dilution test is pass at 1000%.

The resin binder could be replaced wholely or in part by a coumarone resin or a vinyl toluene resin but these are not preferred because they have certain problems e.g. they are liable to embrittlement by ultra violet radiation in use, although they have good resistance to water and salt.

Other resins include rosin esters, indene resins, modified alkyd resins, oxidized petrolatum or fatty acid ester resins but the petroleum resins are preferred especially those have softening points in the range 1650C to 1800C.

The composition may contain 1 to 10% e.g. 4 to 8 ó of resin.

The alkyl benzene sulphonate is preferably blended with a compatible salt e.g. a carbonate e.g. calcium alkyl benzene sulphonate (e.g. C22H4sC6H4SO3Ca) with calcium carbonate. The alkyl group may be a Cl0 to C30 e.g a C20 to C25 chain. The blend is thought to be a complex but this is not certain.

The mode in which this material imparts the anti-fretting properties to the coating is not known for certain and the invention is not dependent thereon but it is believed that a flexible coating of polarised microscopic platelets that overlap each other in a dense "fish scale" pattern is produced. It is also believed that the coating has the property of reforming if ruptured by scratching or abrasion.

The composition may contain 15 to 40% e.g. 2W32% of this blend or complex.

The balance of the surface coating composition is preferably a volatile solvent e.g. an aliphatic petroleum fraction solvent, desirably one sold as SBP 2 which has a boiling rnge of 70 to 980C, xylene or toluene or white spirit.

A preferred class of surface coat composition comprises 4 to 8% of "Inkovar AB 165" petroleum resin (having a melting point of 165 C), 20 to 30% of a calcium alkyl benzene sulphonate ((C22H4sC6H4SO3) Ca)/calcium carbonate blend or complex sold as SACI 700 by the Witco Chemical Corporation and 60 to 75 /" of solvent SBP 2.

The manufacturers of SACI 700, state that it contains 22 to 26% by weight of calcium alkyl benzene sulphonate.

The invention also extends to a ferrous metal article carrying a primer layer 0.01 to 1 mil thick of polyvinyl butyral resin containing 1.25:1 to 0.25:1 parts of strontium chromate per part of resin and an adherent surface layer 0.1 to 2 mils thick of a petroleum resin containing 8:1 to 2.5:1 parts of an alkali metal or alkaline earth metal alkyl benzene sulphonate per part of petroleum resin.

In a preferred structure the surface layer also contains a water insoluble alkali metal or alkaline earth metal salt e.g. a carbonate.

The primer layer is preferably 0.2 to 0.4 mils thick and the surface layer is preferably 0.8 to I mil thick. The ratio of the thickness of the surface layer to the thickness of the primer layer is preferably in the range 5:1 to 2:1 e.g. 4:1 to 2.5:1.

The ratio by weight of chromate to resin in the primer layer is preferably in the range 0.375:1 to 0.8:1.

The ratio of alkyl benzene sulphonate to resin in the surface layer is preferably in the range 5:1 to 4:1.

The invention extends particularly to minimum leaf springs coated with the novel two layer composition.

The surface coating composition is desirably resistant to thermal degradation up to a temperature of at least 1000C, 150"C or 200"C.

The invention can be put into practice in various ways and a number of examples will be described to illustrate the invention.

Example 1 The procedure is preferably carried out as follows.

The spring blade is either freshly shot peened or solvent cleaned to ensure a clean grease free surface. A layer of the preferred class of primer having the following composition: polyvinyl butyral resin Mowital (Registered Trade Mark) B30T 12.0% (sold by Hoechst) methyl-iso-butyl-ketone 48.0% methanol 27.0 /" carbon black 1.3% strontium chromate 9.3% phosphoric acid (sp. gr. 1.65) 2.4 is applied by spraying, brushing, dipping or centrifugal coating and dried at 600F for 10 minutes to give a coating 0.2-0.4 mils thick.Mowital B30T is quoted by the manufacturers as having the following properties: 69-71% acetal groups in terms of polyvinyl butyral, 1% acetyl groups in terms of polyvinyl acetate, 24--27% hydroxyl groups in terms of polyvinyl alcohol; solids content above 98%, viscosity (Hoppler method DIN 53015) at 200C, as a 6% solution in methanol 4--6 centipoise as, as a 5% solutiton in n-butanol 20-30 centipoise, as a 10% solution in n-butanol 90 to 150 centipoise; water absorption of a 0.1 mm thick film after 24 hours immersion in water at 20"C; density about 1.1.

It is insoluble in white spirit, petrol and mineral oils.

A layer of the surface coat material having the following composition Inkovar AB 165 6.0",/, (C221145C,114SO3)Ca CaCo3 complex 24.0 Solvent SBP 2 70.0% was then formed on the surface of the primer coating in the same manner and dried at 600F for 25 minutes to give the second coating.

Example 2 The formulation of Example 1 was tested by applying the coating, as in Example 1 to two flat straight plates of spring metal which had been heat treated and shot peened. The plates were of identical size and shape and were clamped together at one end so that they were in superposed contacting relationship in the horizontal plane.

The lower plate was supported on two rollers and the load was applied downwardly to the top surface of the upper plate by two rollers positioned between each of the lower rollers and the ends of the plate. This arrangement is known as 4 point bending loading on rollers and evens out the stress preventing localization of the stress.

The plates were then loaded with stresses through the two upper rollers, the stress varying between 20 and 60 tons p.s.i. each cycle, there being 575 cycles per minute.

The test was carried out until the plates or one of them fractured.

The surface coating was produced by dipping and the total coating thickness was 0.003 to 0.006" average value 0.0055 inches.

This withstood 3,959,700 cycles to failure.

An identical plate to tfiat described above was tested for corrosion resistance by the ASTM B117--73 salt corrosion test in which the article is sprayed with a 5% aqueous sodium chloride solution at 95+2-3"F and the degree of corrosion checked periodically. It only showed trace corrosion after 520 hours.

Example 3 This is a comparison example An identical pair of metal plates to those used in Example 2 were given a conventional hot zinc phosphate coating followed bya zinc rich primer coating by dipping. The coating was 0.0004 to 0.008 inches thick average 0.00045 to 0.0006 inches thick. The sample failed after 851,950 cycles.

On salt spray corrosion testing the material showed trace corrosion after 48 hours and continuing increases in corrosion at 96 hours, 168 and 400 hours when the test was stopped.

Example 4 This is a comparison example An identical pair of metal plates to those used in Example 2 were given a conventional dipped zinc rich primer coating. The coating was 0.002 to 0.0045 inches thick. The sample failed after 628,850 cycles. On salt spray corrosion testing the material showed trace corrosion at 100 hours and corrosion on the edges of the sample at 500 hours.

Example 5 This is a comparison example An identical pair of metal plates to those used in Example 2 were dipped in the surface coating composition of Example 1 only to given a single coating 0.003 to 0.006 inches thick. This sample failed after 1,462,950 cycles. Thus it can be seen that in the absence of the primer coating the material has inferior properties.

On salt spray testing the sample showed trace corrosion after 520 hours and also peeling away of the coating at the edges of the sample.

Example 6 Example 2 was repeated except that the coatings were applied by spraying to give a total thickness coating in the range 0.0005 to 0.0015 inches which permits rapid drying in production (the thicker coat in Example 2 took longer to dry than would be feasible in production).

This sample survived 1,800,000 cycles of bending without failure and 250 hours on the salt spray test with no more than trace corrosion.

Example 7 This is a comparison example Example 5 was repeated except that the coating was applied by spraying to a thickness of 0.00075 inches. This sample failed completely after 114 hours on the salt spray corrosion test.

Examples 8, 9 and 10 Identical..4 plate automobile front springs were coated by spraying to the same thicknesses with the formulation of Example, 2 (Example 8), Example 3 (Example 9) and Example 5 (Example 10).

Examples 9 and 10 are comparison examples.

The coated springs were then fatigue tested under identical typical loading conditions to failure. The spring of Example 8 survived 31 /7n more cycles than that of Example 9 and 36% more cycles than that of Example 10.

WHAT WE CLAIM IS: 1. A method of forming a protective coating on a metal part which comprises applying to the clean grease free part one or more coats of a primer composition comprising a resin and a chromate pigment, drying the coating to form a layer at least 0.01 mils thick, which is of low permeability to water vapour and liquid moisture, and applying one or more coats of a surface coat composition comprising a resin, an alkali metal or alkaline earth metal alkyl benzene sulphonate and a solvent, the surface coat composition being selected to adhere to the primer coating and drying the surface coating to form a layer at least 0.1 mils thick, which provides a corrosion resistant coating.

2. A method as claimed in Claim 1 in which the primer coating is 0.2 to 0.4 mils thick.

3. A method as claimed in Claim 1 or Claim 2 in which the surface coating is 0.5 to 6 mils thick.

4. A method as claimed in any one of Claims 1, 2 or 3 in which the resin used in the primer formulation is alcohol or ketone soluble, film forming from such solution, fully cured and thermally stable.

5. A method as claimed in any one of Claims 1, 2, 3 or 4 in which the resin component in the primer composition is a polyvinyl butyral resin, which has a solution of viscosity in the range 20 to 30 centipoise (as a 5% solution in n-butanol at 20"C as measured by the Hoppler method (DIN 53015), or a mixture of such a low viscosity resin and up to 50% of phenolic resins or higher solution viscosity polyvinyl butyrals, having a solution viscosity of 60 to 90 centipoise as measured by the same method.

6. A method as claimed in any one of claims 1 to 5 in which the chromate pigment is strontium chromate.

7. A method as claimed in any one of Claims 1 to 6 in which the primer composition contains 5 to 15% of resin and 1 to 15% of chromate pigment and the balance of the composition is a volatile solvent or solvent blend.

8. A method as claimed in Claim 7 in which the primer composition contains 0.1% to 5% of phosphoric acid of 1.65 specific gravity.

9. A method as claimed in Claim 7 or Claim 8 in which the primer composition contains 1 to 20% carbon black.

10. A method as claimed in anyone of Claims 1 to 9 in which the primer composition comprises 8 to 12% of polyvinyl butyral resin of solution viscosity

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (29)

**WARNING** start of CLMS field may overlap end of DESC **. material showed trace corrosion after 48 hours and continuing increases in corrosion at 96 hours, 168 and 400 hours when the test was stopped. Example 4 This is a comparison example An identical pair of metal plates to those used in Example 2 were given a conventional dipped zinc rich primer coating. The coating was 0.002 to 0.0045 inches thick. The sample failed after 628,850 cycles. On salt spray corrosion testing the material showed trace corrosion at 100 hours and corrosion on the edges of the sample at 500 hours. Example 5 This is a comparison example An identical pair of metal plates to those used in Example 2 were dipped in the surface coating composition of Example 1 only to given a single coating 0.003 to 0.006 inches thick. This sample failed after 1,462,950 cycles. Thus it can be seen that in the absence of the primer coating the material has inferior properties. On salt spray testing the sample showed trace corrosion after 520 hours and also peeling away of the coating at the edges of the sample. Example 6 Example 2 was repeated except that the coatings were applied by spraying to give a total thickness coating in the range 0.0005 to 0.0015 inches which permits rapid drying in production (the thicker coat in Example 2 took longer to dry than would be feasible in production). This sample survived 1,800,000 cycles of bending without failure and 250 hours on the salt spray test with no more than trace corrosion. Example 7 This is a comparison example Example 5 was repeated except that the coating was applied by spraying to a thickness of 0.00075 inches. This sample failed completely after 114 hours on the salt spray corrosion test. Examples 8, 9 and 10 Identical..4 plate automobile front springs were coated by spraying to the same thicknesses with the formulation of Example, 2 (Example 8), Example 3 (Example 9) and Example 5 (Example 10). Examples 9 and 10 are comparison examples. The coated springs were then fatigue tested under identical typical loading conditions to failure. The spring of Example 8 survived 31 /7n more cycles than that of Example 9 and 36% more cycles than that of Example 10. WHAT WE CLAIM IS:

1. A method of forming a protective coating on a metal part which comprises applying to the clean grease free part one or more coats of a primer composition comprising a resin and a chromate pigment, drying the coating to form a layer at least 0.01 mils thick, which is of low permeability to water vapour and liquid moisture, and applying one or more coats of a surface coat composition comprising a resin, an alkali metal or alkaline earth metal alkyl benzene sulphonate and a solvent, the surface coat composition being selected to adhere to the primer coating and drying the surface coating to form a layer at least 0.1 mils thick, which provides a corrosion resistant coating.

2. A method as claimed in Claim 1 in which the primer coating is 0.2 to 0.4 mils thick.

3. A method as claimed in Claim 1 or Claim 2 in which the surface coating is 0.5 to 6 mils thick.

4. A method as claimed in any one of Claims 1, 2 or 3 in which the resin used in the primer formulation is alcohol or ketone soluble, film forming from such solution, fully cured and thermally stable.

5. A method as claimed in any one of Claims 1, 2, 3 or 4 in which the resin component in the primer composition is a polyvinyl butyral resin, which has a solution of viscosity in the range 20 to 30 centipoise (as a 5% solution in n-butanol at 20"C as measured by the Hoppler method (DIN 53015), or a mixture of such a low viscosity resin and up to 50% of phenolic resins or higher solution viscosity polyvinyl butyrals, having a solution viscosity of 60 to 90 centipoise as measured by the same method.

6. A method as claimed in any one of claims 1 to 5 in which the chromate pigment is strontium chromate.

7. A method as claimed in any one of Claims 1 to 6 in which the primer composition contains 5 to 15% of resin and 1 to 15% of chromate pigment and the balance of the composition is a volatile solvent or solvent blend.

8. A method as claimed in Claim 7 in which the primer composition contains 0.1% to 5% of phosphoric acid of 1.65 specific gravity.

9. A method as claimed in Claim 7 or Claim 8 in which the primer composition contains 1 to 20% carbon black.

10. A method as claimed in anyone of Claims 1 to 9 in which the primer composition comprises 8 to 12% of polyvinyl butyral resin of solution viscosity

20 to 30 centipoise, 36 to 52 Sn of methyl-isobutyl-ketone, 28 to 420/;: of methanol, 1 to 2% of carbon black, 3 to 10% of strontium chromate and 0.5 to 2.5% of phosphoric acid of specific gravity 1.65.

11. A method as claimed in any one of Claims I to 10 in which the resin of the surface coat composition comprises a petroleum resin and serves as a binder for the alkyl benzene sulphonate securely adhering it to the primer coat.

12. A method as claimed in Claim 11 in which the resin of the surface coating composition comprises a petroleum resin having a softening point in the range 1650C to 1850C or a mixture thereof with a minor proportion of a coumarone resin or a vinyl toluene resin or rosin esters, indene resins, modified alkyd resins, oxidized petrolatum or fatty acid esters.

13. A method as claimed in Claim 11 or 12 in which the petroleum resin is that sold under the name "Inkovar AB 165", which according to the manufacturers has a coat tar colour of 3, resin colour of F, a softening point of 165"C, an acid number of less than 1, a saponification number of less than 1, a specific gravity of 1.075, is soluble in ink solvent KB No=43, and in ink solvent KB No=27, and in aromatic solvents but is insoluble in alcohols and glycols, has a Stokes viscosity in ink solvent KB=43 (55% n.v. at 250C) of 0.85, a Gardner viscosity of C and the result of the dilution test is pass at iooo.

14. A method as claimed in claim 11, 12 or 13 in which the surface coating composition contains I to 10% of resin.

15. A method as claimed in any one of Claims 1 to 14 in which the alkyl benzene sulphonate is blended with a compatible salt.

16. A method as claimed in Claim 15 in which the alkyl benzene sulphonate is calcium alkyl benzene sulphonate (e.g.

C22H45C6H4SO3Ca) and is blended with calcium carbonate.

17. A method as claimed in any one of Claims 1 to 16 in which the alkyl group of the alkyl benzene sulphonate is a C,O to C30 chain.

18. A method as claimed in any one Claims 1 to 17 in which the surface coating composition contains 15 to 40% of alkyl benzene sulphonate or a blend thereof with a compatible salt.

19. A method as claimed in any one of Claims 1 to 18 in which the balance of the surface coating composition is a volatile aliphatic petroleum fraction solvent, desirably one sold as SBP 2 which has a boiling range of 70 to 980C, Xylene or toluene or white spirit.

20. A method as claimed in any one of Claims 1 to 19 in which the surface coat composition comprises 4 to 8% of "Inkovar AB165" petroleum resin, 20 to 30 ,/ of a calcium alkyl benzene sulphonate ((C22H4sC6H4SO3)Ca)/calcium carbonate blend or complex sold as SACI 700 by the Witco Chemical Corporation and 60 to 75 /n of solvent SBP 2.

21. A method as claimed in Claim 1 and substantially as specifically described herein with reference to Example 1, or Example 2 or Example 6, or Example 8.

22. A metal article whenever coated by a method as claimed in any one of Claims 1 to 22.

23. A ferrous metal article carrying a primer layer 0.01 to 1 mil thick of polyvinyl butyral resin containing 1.25:1 to 0.25:1 parts of strontium chromate per part of resin and an adherent surface layer 0.1 to 2 mils thick of a petroleum resin containing 8:1 to 2.5:1 parts of an alkali metal or alkaline earth metal alkyl benzene sulphonate per part of petroleum resin..

24. An article as claimed in Claim 23 in which the surface layer also contains a water insoluble alkali metal or alkaline earth metal salt.

25. An article as claimed in Claim 23 or Claim 24 in which the primer layer is 0.2 to 0.4 mils thick and the surface layer is 0.8 to 1 mil thick.

26. An article as claimed in Claim 23, 24 or 25 in which the ratio of the thickness of the surface layer to the thickness of the primer layer is in the range 5:1 to 2:1.

27. An article as claimed in Claim 23, 24, 25 or 26 in which the ratio by weight of chromate to resin in the primer layer is in the range 0.375:1 to 0.8:1.

28. An article as claimed in Claim 23, 24, 25, 26 or 27 in which the ratio of alkyl benzene sulphonate to resin in the surface layer is in the range 5:1 to 4:1.

29. An article as claimed in any one of Claims 23 to 28 in which the article is a minimum leaf spring coated with the novel two layer composition.