WO2002097159A2

WO2002097159A2 - Coating

Info

Publication number: WO2002097159A2
Application number: PCT/GB2002/002215
Authority: WO
Inventors: Bruno Frederic Stengel; John Ridland
Original assignee: J0Hnson Matthey Plc
Priority date: 2001-05-29
Filing date: 2002-05-16
Publication date: 2002-12-05
Also published as: WO2002097159A3; AU2002313105A1; GB0112899D0

Abstract

A metal coating composition comprises an organometallic compound which is the reaction product of an orthoester, condensed orthoester or oxo-alkoxide of at least one metal selected from titanium, zirconium, aluminium, zinc, molybdenum and tungsten, an alcohol containing at least two hydroxyl groups, an organophosphorus compound and a base.

Description

Coating

The present invention relates to coatings for metals and to additives useful for the manufacture of metal coatings.

Coatings for metals are commonly used in industry in order to protect the metal from corrosion or oxidation or to provide a suitable base on which to apply further coatings such as paints. The coating must be easy to apply, form a uniform coating over the metal and be resistant to removal. Metal coatings based on titanium compounds are known in the art. These coatings typically comprise a titanium orthoester such as tetra (isopropyl) titanium and may also contain a solvent, resins and other ingredients such as acrylates, polyesters, epoxy resins, polyurethanes, silicones and silicates to impart certain desirable properties such as corrosion resistance. These products are normally cured at temperatures in excess of 200 °C.

It is an object of the present invention to provide an improved metal coating composition and / or an additive therefor.

According to the invention a metal coating composition comprises an organometallic compound which is the reaction product of an orthoester, condensed orthoester or oxo- alkoxide of at least one metal selected from titanium, zirconium, aluminium, zinc, molybdenum and tungsten, an alcohol containing at least two hydroxyl groups, an organophosphorus compound and a base.

The organometallic compound has been found to be effective when used on its own as a metal coating composition or as an additive to a metal coating composition containing other ingredients. One benefit of using the metal coating composition of the invention has been found to be that curing may be effected at lower temperatures compared with commercially available coating compositions. For example, it is possible using certain compositions of the invention to cure the coating at about 150 °C compared with over 200 °C for some prior art coating compositions.

The organometallic compound comprises the reaction product of an orthoester, condensed orthoester or oxo-alkoxide of at least one metal selected from titanium, zirconium, aluminium, zinc, molybdenum and tungsten. In this specification, when we refer to an oxo- alkoxide we mean a compound which contains one or more metal-oxygen multiple bonds, which are of the general formula MO_x(OR)_y where M is Mo or W or other metal. Normally an orthoester, condensed orthoester or oxo-alkoxide of one of the selected metals is used but it is within the scope of the invention to use an orthoester, condensed orthoester or oxo-alkoxide of more than one of the selected metals.

Preferred metals are titanium, zirconium and aluminium, especially titanium or zirconium. For clarity we refer hereinafter to a titanium, zirconium, aluminium, zinc, molybdenum or tungsten orthoester or condensed orthoester, and all such references should be taken to include orthoester, condensed orthoester or oxo-alkoxide of more than one metal, e.g. to a mixture of titanium and zirconium orthoesters.

The organometallic compound is the reaction product of a titanium, zirconium, aluminium, zinc, molybdenum or tungsten orthoester, condensed orthoester or oxo-alkoxide, an alcohol containing at least two hydroxyl groups, and an organophosphorus compound containing at least one P-OH group. Preferably, the orthoester has the formula M(OR) , AI(OR)₃ or Zn(OR)₂ where M is titanium, zirconium, molybdenum or tungsten and R is an alkyl group. More preferably R contains 1 to 6 carbon atoms and particularly suitable orthoesters include tetraisopropoxy titanium, tetra-n-butoxy titanium, tetra-n-propoxy zirconium, tetra-n- butoxy zirconium and tri-iso-butoxy aluminium. Oxo-alkoxides of the type MO_x(OR)_y , in which x = 0 - 2 and y = 1 - 4, may also be used when M is Mo or W. Such compounds may be made, for example, by the reaction of the corresponding metal oxy-chloride with alcohol in the presence of a base.

The condensed orthoesters suitable for preparing the organometallic compounds used in this invention are typically prepared by careful hydrolysis of titanium, zirconium, aluminium, zinc, molybdenum or tungsten orthoesters. Titanium or zirconium condensed orthoesters are frequently represented by the formula: R¹0[M(OR¹)₂0]nR¹ in which R¹ represents an alkyl group and M represents titanium or zirconium. Preferably, n is less than 20 and more preferably is less than 10. Preferably, R¹ contains 1 to 12 carbon atoms, more preferably, R¹ contains 1 to 6 carbon atoms and useful condensed orthoesters include the compounds known as polybutyl titanate, polyisopropyl titanate and polybutyl zirconate.

Preferably, the alcohol containing at least two hydroxyl groups is a dihydric alcohol and can be a 1 ,2-diol such as 1 ,2-ethanediol or 1,2-propanediol, a 1 ,3-diol such as 1 ,3-propanediol, a 1 ,4-dioI such as 1 ,4-butanediol, a diol containing non-terminal hydroxyl groups such as 2- methyl-2,4-pentanediol or a dihydric alcohol containing a longer chain such as diethylene glycol or a polyethylene glycol. The preferred dihydric alcohol is 1 ,2-ethanediol. The organometallic compound can also be prepared from a polyhydric alcohol such as glycerol, trimethylolpropane or pentaerythritol. Preferably, the organometallic compound is prepared by reacting a dihydric alcohol with an orthoester, condensed orthoester or oxo-alkoxide in a ratio of from 1 to 32 moles of dihydric alcohol to each mole of metal titanium, zirconium, aluminium, zinc, molybdenum and tungsten. More preferably, the reaction product contains 2 to 25 moles of dihydric alcohol per mole of metal (total) and most preferably 4 to 25 moles dihydric alcohol per mole of metal (total).

The organophosphorus compound which contains at least one P-OH group can be selected from a number of organophosphorus compounds including phosphates, phosphate salts, pyrophosphates, esters or salts of phosphonic acid or of alkyl or aryl phosphonic acid, esters or salts of phosphinic acid or of alkyl or aryl phosphinic acid, phosphites, esters or salts of thiophosphonic acid or of alkyl or aryl thiophosphonic acid and phosphorous derivatives of hydroxy carboxylic acids, eg. citric acid.

Preferably, the organophosphorus compound is a salt of an alkyl or aryl phosphonic acid, a substituted or unsubstituted alkyl phosphate, a substituted or unsubstituted aryl phosphate or a phosphate of an alkylaryl glycol ether or an alkyl glycol ether or a substituted or unsubstituted mixed alkyl or aryl glycol phosphate. Useful compounds include tetrabutyl ammonium phenyl phosphonate, monoalkyl acid phosphates and dialkyl acid phosphates, zinc or copper salts of alkyl or aryl phosphonic acid and mixtures of these. Convenient organophosphorus compounds are the compounds commercially available as alkyl acid phosphates and containing, principally, a mixture of mono- and di-alkyl phosphate esters. When an alkyl phosphate is used as the organophosphorus compound, the organic group preferably contains up to 20 carbon atoms, more preferably up to 8 carbon atoms and, most preferably, up to 6 carbon atoms. When alkyl-aryl or alkyl glycol ether phosphates are used the carbon chain length is preferably up to 18 carbon atoms and, more preferably, 6 to 12 carbon atoms.

Alternative organophosphorus compounds suitable for use in preparing the coating compositions of the invention are the reaction products obtainable by reacting phosphorus pentoxide and a polyhydric alcohol, particularly a glycol. Such products can be prepared, for example, by heating a mixture of phosphorus pentoxide and a polyhydric alcohol until a uniform liquid is formed. Conveniently, the amount of polyhydric alcohol used to prepare such a product is in excess of the stoichiometric amount required to fully react with the phosphorus pentoxide. The excess polyhydric alcohol acts as a solvent for the organophosphorus reaction product. Moreover, when a product containing excess polyhydric alcohol is used, this excess polyhydric alcohol comprises at least a portion of the alcohol containing at least two hydroxyl groups used to prepare the organometallic compound. Suitable products contain up to 16 moles of polyhydric alcohol per mole of phosphorus (P). Preferably the products contain from 3 to 10 moles of polyhydric alcohol per mole of phosphorus.

Particularly preferred organophosphorus compounds include butyl acid phosphate, mixed butyl-ethylene glycol phosphates, polyethylene glycol phosphate, aryl polyethylene glycol 5 phosphates and a product of reaction of ethylene glycol and phosphorus pentoxide and the reaction product of an alkyl phosphonate and a hydroxy-functionalised carboxylic acid such as citric acid.

The amount of organophosphorus compound present in the reaction product which comprises the organometallic compound of the coating composition of the invention is 10 usually in the range 0.1 to 4.0 mole of phosphorus to 1 mole of metal (titanium, zirconium, aluminium, molybdenum or tungsten), preferably in the range 0.1 to 2.0 mole phosphorus to 1 mole metal and most preferably in the range 0.1 to 1.0 mole phosphorus to 1 mole metal. When the metal is zinc, the amount of organophosphorus compound is usually in the range 0.1 to 2.0 mole of phosphorus to 1 mole of metal.

15 The organometallic compound additionally comprises a base, however when the organophosphorous compound comprises the reaction product of a base and a phosphate or alkyl or aryl phosphonic acid, it is not always essential to add a base to the components of the organometallic compound. For example, an alkali-metal salt or a quaternary ammonium salt of a phosphate or alkyl or aryl phosphonic acid derivative may be used as

20 the organophosphorus compound.

Suitable inorganic bases include inorganic oxides and hydroxides, e.g. alkali metal and alkaline earth hydroxides for example sodium hydroxide, potassium hydroxide, calcium hydroxide, lithium hydroxide and ammonium hydroxide. Oxides or hydroxides of zinc, molybdenum or tungsten are also bases for this application. Preferred organic bases

25 include quaternary ammonium compounds such as tetrabutyl ammonium hydroxide, choline hydroxide (trimethyl(2-hydroxyethyl)ammonium hydroxide) or benzyltrimethyl ammonium hydroxide, or alkanolamines such as monoethanolamine, diethanolamine, triethanolamine and triisopropanolamine. Usually, the amount of base used is in the range 0.1 to 4.0 mole base per mole of metal (titanium, zirconium or aluminium). The preferred amount is in the

30 range 0.1 to 2.0 mole base per mole of metal and frequently the amount of base present is in the range 0.1 to 1.0 mole base per mole of metal.

The organometallic compound can be prepared by mixing the components (orthoester or condensed orthoester, alcohol containing at least two hydroxyl groups, organophosphorus compound and base, if present) with removal, if desired, of any by-product, (e.g. isopropyl

J 5 alcohol when the orthoester is tetraisopropoxytitanium), at any appropriate stage. In one preferred method the orthoester, condensed orthoester or oxo-alkoxide and a dihydric alcohol are mixed and, subsequently, a base is added, followed by the organophosphorus compound.

When the coating composition comprises the organometallic compound in addition to other ingredients, the organometallic compound may simply be added to the other ingredients by mixing. For example, the organometallic compound may be added to a conventional paint or other coating composition, including solvent-based and water-based paints such as emulsions, by adding it to the paint or coating with stirring. The organometallic compound is preferably added in an amount of 1 - 20% by weight of the total coating weight, more preferably between 3% and 12%, e.g. about 5 - 10% by weight.

The metal coating composition may be applied to the metal substrate by dipping, pouring, spraying, brushing or by any other convenient method. It may be applied to a bare metal surface or to another material which covers the surface, e.g. to a protective coating of e.g. iron phosphate. The coating is generally cured at elevated temperature after application. Typically the coatings of the invention are cured at temperatures between 100 and 250 °C for a period of 5 - 30 minutes.

Specific embodiments of the invention will be further described in the following Example.

Preparation of the coating according to the invention

Ethylene glycol (100 g, 1.6 moles) was added from a dropping funnel to stirred titanium n- butoxide (34 g, 0.1 mole) in a 250ml conical flask fitted with stirrer. An aqueous solution of sodium hydroxide, containing 32% NaOH by weight (12.5g, 0.1 mole) was added drop-wise to the reaction flask with mixing to yield a clear pale yellow liquid. To this liquid a combined reaction product of P₂0₅ (7.1 g, 0.05 mole) and ethylene glycol (55 g, 0.9 moles) was slowly added and the resulting mixture was stirred for several minutes. The P₂0₅ reaction product was prepared by dissolving P₂0₅ in ethylene glycol, with a combination of mixing and carefully controlled heating; this was subsequently allowed to cool. After removing n- butanol at 70 °C under vacuum to constant weight the product was a pale yellow liquid with a Ti content of 2.96% by weight.

Coating of metal plate Stainless steel plates, pre-treated with iron phosphate, were coated with the organometallic compound described above by holding the plate at approximately 45 ° and pouring the composition evenly over the plate, the top edge being left uncoated to allow for handling. The plates were then allowed to drain in a vertical position for 4 minutes and any excess coating collected at the bottom edge was removed. The coated samples were then placed in an air oven at a predetermined temperature for a period to allow the coating to cure. Different cure times and temperatures were used for comparison. The coatings were examined visually and then tested for adhesion and solvent resistance by the MEK rub test. All coatings had the appearance of an even, silver-grey film.

Adhesion test

The adhesion of the coatings to the substrate was tested by ASTM method D3359-95

(method A). An X-cut was made in the coating of the steel test panel with a sharp steel scribing tool. A strip of 25 mm wide 'Scotch' transparent cellophane adhesive tape, approximately 10 cm long, was applied over the X-cut and pressed down over its full length. After 30-90 seconds the tape was slowly peeled off the steel panel and the tape and panel were assessed for removal of the coating from the panel. The results are shown in the table.

MEK rub test

A folded strip of cloth, approximately 2-3 cm wide, was dipped into about 100 ml of 2- butanone (methyl ethyl ketone) in a beaker, so as to totally wet the cloth. The test panel was laid flat inside a fume cupboard and the soaked cloth was rubbed with moderately heavy finger pressure along the length of the test panel in a regular motion at an even rate such that around 100 double rubs (up and back) were carried out in one minute along the middle 00 mm or so of the panel. The effect of the rubbing was observed and the time was noted when removal of coating first started, when bare metal was first revealed and when the whole of the coating had been removed along the length of the panel. The results are shown in the table.

The results show that the coating compositions of the invention may be cured at relatively low temperatures and produce a coating with good adhesion to the substrate and excellent solvent resistance.

Claims

1. A metal coating composition comprising an organometallic compound which is the reaction product of an orthoester, condensed orthoester or oxo-alkoxide of at least one metal selected from titanium, zirconium, aluminium, zinc, molybdenum and tungsten, an alcohol containing at least two hydroxyl groups, an organophosphorus compound and a base.

2. A coating composition as claimed in claim 1 , wherein said orthoester has the formula M(OR)₄ , AI(OR)₃ or Zn(OR)₂ where M is titanium, zirconium, molybdenum or tungsten and R is an alkyl group.

3. A coating composition as claimed in claim 2, wherein R contains 1 to 6 carbon atoms.

4. A coating composition as claimed in claim 1 , wherein said oxo-alkoxide has the formula MO_x(OR)_y , in which x = 0 - 2 and y = 1 - 4, when M is Mo or W and R is an alkyl group.

5. A coating composition as claimed in claim 1 , wherein said condensed orthoeser is represented by the formula R¹0[M(OR¹)₂0]_nR¹ in which R¹ represents an alkyl group, M represents titanium or zirconium and n is less than 20.

6. A coating composition as claimed in any of the preceding claims, wherein the alcohol containing at least two hydroxyl groups is a dihydric alcohol.

7. A coating composition as claimed in claim 6, wherein the diol is selected from 1 ,2- ethanediol, 1 ,2-propanediol, 1 ,3-propanediol, 1 ,4-butanediol, 2-methyl-2,4-pentanediol, diethylene glycol, or a polyethylene glycol.

8. A coating composition as claimed in any of the preceding claims, wherein the organophosphorus compound which contains at least one P-OH group is selected from the group consisting of phosphates, phosphate salts, pyrophosphates, esters or salts of phosphonic acid or of alkyl or aryl phosphonic acid, esters or salts of phosphinic acid or of alkyl or aryl phosphinic acid, phosphites, esters or salts of thiophosphonic acid or of alkyl or aryl thiophosphonic acid and phosphorous derivatives of hydroxy carboxylic acids.

9. A coating composition as claimed in claim 8, wherein the organophosphorus compound is selected from butyl acid phosphate, mixed butyl-ethylene glycol phosphates, polyethylene glycol phosphate, aryl polyethylene glycol phosphates, a product of the reaction of ethylene glycol and phosphorus pentoxide and the reaction product of an alkyl phosphonate and a hydroxy-functionalised carboxylic acid.

10. A coating composition as claimed in any of the preceding claims, wherein the amount of organophosphorus compound in the reaction product which comprises the organometallic compound is such as to give a ratio of P:M in the range 0.1 - 4.0 moles of phosphorus to 1 mole of metal.

11. Use of an organometallic compound which is the reaction product of an orthoester, condensed orthoester or oxo-alkoxide of at least one metal selected from titanium, zirconium or aluminium, zinc, molybdenum and tungsten, an alcohol containing at least two hydroxyl groups, an organophosphorus compound and a base for the manufacture of a metal coating composition.