GB2317177A - Organic phosphonates and metal complexes thereof for use as coating agents and especially for pretreating steel - Google Patents

Abstract

The application discloses a method of pre-treating steel articles, in particular galvanised steel articles, prior to application of one or more paint layers. A pre-treatment compound is disclosed which comprises the complex formable in solution by immersion of a metallic species in a solution of the compound R-PO(OH) 2 , where R includes an organic group. The metal can be in native form or as a salt. R is preferably entirely organic. A solution of this complex can be used as a pre-treatment bath for steel items, resulting in superior adhesion of the paint layer. The application refers to the complex, a solution of the complex, the use of the complex as a coating, and to a method of coating a steel article involving the complex. The R group preferably includes at least one of an epoxy, hydroxy, unsaturated hydrocarbon, amide or carboxylic group, or a combination thereof. Suitable unsaturated hydrocarbon groups include alkenes, vinyl or acrylate groups. Other preferred forms are those in which the R group includes an aliphatic chain to which the paint layer can form an interlocking network. Such a chain is preferably at least C 4 long and may be substituted. A solution of the invention preferably includes an accelerator compound, such as peroxide, which may be present in a catalytic amount.

Description

Pre-treatment of Steel The present invention relates to the pre-treatment of steel articles, and zinc and zinc alloy surfaces such as galvanised steel, prior to application of one or more paint layers.

Galvanised steels with organic coatings, such as paint, are of major commercial importance in a number of commercial applications. A steel substrate provides the strength required of the product, and is generally coated with a metallic layer of zinc or zinc alloy. The metallic coating is then covered by pre-treatment layers, an organic primer layer container corrosion inhibiting pigments, and a topcoat which can be either second organic paint layer or a laminate film. This is shown schematically in fig 1.

Organic paints do not adhere to a metallic substrate sufficiently well to meet the demands of the various applications, which require a product that can be formed into a variety of shapes, with no loss of adhesion to the coatings, or reduction in the corrosion resistance. Therefore, before the organic coatings are applied, the metal substrate is generally treated with adhesion promoters to improve the bonding of subsequent coatings, and the corrosion resistance of the finished product. The pre-treatment systems presently in use are effective at providing adhesion and corrosion resistance, and typically involve the use of phosphates, chromates, or mixed metal oxides to bond to the metal substrate. These coatings are then usually exposed to an acidic chromate solution, which forms a second pre-treatment layer.

This system is able to produce a technically satisfactory product, but has the serious disadvantage that some of the chemical agents involved are the subject of significant environmental concern. This applies in particular to the Chromium (VI) compounds. There is a definite prospect that such compounds will be the subject of environmental control legislation.

The present invention is the result of the inventors' efforts to find an alternative pre-treatment process which does not rely on the above environmentally sensitive compounds. The phosphonate group of chemicals have been identified as having potential for use as pre-treatments in this context. This group has the advantage that the molecules can be designed so that they have the ability to form chemical bonds with both the metal, and with the organic coating. This is shown schematically in fig. 2, where a single layer has been shown for simplicity. The actual coating may be several molecular layers in thickness.

Phosphonates have been used previously within the water treatment industry, such as in the oil and gas fields, where a large amount of brine is produced and hence control of scale and corrosion is required. Research on phosphonates and their complexes in aqueous media using electrochemical and gravimetric methods has been carried out to show that these inhibitors slowed down both the cathodic and anodic reactions on low carbon steel.

A number of patents have been published concerning the use of phosphonates on metallic substrates, including aluminium, steel and galvanised steel. These disclosures discuss adhesion and corrosion resistance. In addition, there are several papers describing the use of aminophosphonic acids such as nitrilotris(methylene)triphosphonic acid to prevent the conversion of aluminium oxide surfaces to aluminium hydroxide, without reducing the mechanical linkage to the oxide by organic materials such as adhesives and paints.

One example is US-A-4,777,091 which proposes the treatment of steel or galvanised steel with aminophosphonic acid compounds prior to coating with adhesive compositions.

Another example is US-A-4,308,079 which discloses a phosphonatebased pre-treatment for aluminium surfaces. It states that the phosphonate acts as a hydration inhibitor for the aluminium oxide layer, ie preventing conversion to aluminium hydroxide, and therefore allows a good mechanical key to form between the irregular anodised oxide surface and the paint layer.

The present invention primarily relates to the complex formable in solution by immersion of a metallic species in a solution of the compound R PO(OH)2 where R includes an organic group, and also to the use of that complex as a coating agent. The metal can be in the native form or as a salt.

Preferably, the R group is entirely organic, i.e. with no inorganic substituents.

The present invention further relates to a solution of the abovedefined complex, preferably in the substantial absence of solid metal. This solution can be used as a pre-treatment bath for steel items without causing an initial weight loss that the present invention shows to be associated with phosphonic acid treatments.

The present invention further relates to a method of coating a steel article, comprising the steps of; (i) providing a solution of the compound R-PO(OH)2 where R includes an organic group (ii) contacting the solution with a metal species or a salt thereof other than the steel article thereby to form a solution of a metal-phosphonate complex; (iii) optionally, removing the metal, if any remains; (iv) applying the thus formed complex solution to the surface of the steel article; and (v) coating the steel article with an organic coating.

The present invention also proposes the use of the compound R PO(OH)2 where R includes an organic group but not an amino group, or a metallic complex thereof, as a surface treatment for a steel article prior to application of a paint or other organic layer thereto.

The present invention envisages the production of an article comprising a steel substrate on which is formed, successively, an optional zinc or zinc alloy galvanising layer, an intermediate layer, and one or more paint layers, wherein the intermediate layer is formed by immersion of the steel substrate with optional zinc layer in a solution of the compound R PO(OH)2, or a metallic complex thereof, where R includes an organic group to which the paint layer(s) can bind.

Preferably, the bonding between the R group and the paint layer is by way of a chemical bond.

Finally, the present invention relates to a method of finishing a steel item comprising the steps of; (i) optionally, galvanising the steel item; (ii) applying in aqueous form the compound R-PO(OH)2 or a metallic complex thereof, where R includes an organic group to which a paint layer can bind; and (iii) applying at least one paint layer.

Preferably, the compound is present in a solution. This solution can be applied to the said steel item by at least substantial immersion, by spraying, or by roller coating, or any combination thereof.

Preferred forms of the invention according to all the above aspects are ones in which the R group includes at least one epoxy group, or at least one hydroxy group, or at least one unsaturated hydrocarbon group, or at least one amide group, or at least one carboxylic group, or a combination thereof.

Such groups bind well with most paints. Suitable unsaturated hydrocarbon groups include alkenes, vinyl groups, or acrylate groups. Other preferred forms are those in which the R group includes an aliphatic chain with which the paint layer can form an interlocking network. In this case, the R group becomes strongly associated with the paint layer but not directly chemically bonded. Such chains should be at least C4 long preferably larger than C8 and more preferably larger than C12. Such chains may be substituted, for example with epoxy, vinyl, hydroxy or other groups, or may contain one or more unsaturated regions. Another preferred from for the R group is one in which amino groups are not present.

Further such preferred forms are those in which the solution of the said compound or complex includes an accelerator compound. Preferably, the latter is present in a catalytic amount. A suitable such compound is a peroxide.

The present invention will now be described in more detail with reference to the accompanying figures, in which; Figure 1 is a schematic view showing the structure of a typical organic coated steel product; Figure 2 is a schematic illustration of the phosphonic acid molecule to which the present invention relates; Figures 3 to 1 7 are graphs and SEM micrographs showing the result of investigations into suitable phosphonic acid-based compounds.

The present invention results from work carried out to investigate the effect of phosphonic acids on a zinc substrate, and to establish their potential as pre-treatments for organic coated steels. The phosphonic acids conform to two groups of chemicals, which are similar, but with one carbon difference in the backbone of the molecule. The first pair are 2carboxyethylphosphonic acid (2-CEPA) and phosphonoacetic acid (PAA), which are capable of forming bonds with zinc at both ends of the molecule, either by the carboxylic acid group, or by the phosphonic acid group. The second pair are 1,3-propylbisphosphonic acid (1,3-PBPA), and 1,2ethylbisphosphonic acid (1,2-EBPA), which are also capable of bonding at either end of the molecule to zinc, by the phosphonic acid groups. The structures are: HOOC-CH2-CH2-PO(OH)z (HO)2OP-CH2-CH2-CH2-PO(OH)2 2-CEPA 1 ,3-PBPA HOOC-CH2-PO(OH)2 (HO)2OP-CH2-CH2-PO(OH)2 PAA 1 ,2-EBPA EXPERIMENTAL DETAILS The zinc foil used was purchased from Goodfellows; thickness, 0.5mm, purity, 99.95 + %, with a typical analysis of (ppm): Ca 1, Cd 20, Cu 15, FelO, In 10, Mg < 1, Ni 1, Pb 100, Si 2, Sn 8.

2-Carboxyethylphosphonic cid (2-CE PA), and phosphonoacetic acid (PAA), were purchased from Aldrich. 1 ,2-ethylbisphosphonic acid (2-EBPA) and 1 ,3-propylbisphosphonic acid 1,3-PBPA were synthesised by the well known Arbusov reaction (see Kosalapott, J. Am. Chem.Soc. 1944,66, 1 511) of triethylphosphite with the appropriate dibromoalkane, followed by hydrolysis of the phosphonate using the method of GB-A-1497992, which involves reaction with formic acid, and distillation of ethyl formate.

Anal. Calcd for (C2H8O6P2): C.12.6%: H.4.2% found: C.12.5%: H.4.5% 1H NMR (D2O): +4.8 (s,OH):+1.8(d.CH2) 31P(1H)NMR(D2O):+28.6 Anal. Calcd for (C3HtoO6P2) C.17.6%:H.5.0% found: C.17.7%:H.5.4% 'H NMR (D2O): +4.8 (s,OH):+1.8(m.CH2) 31P(1H)NMR(CDC13):+30.0 A series of experiments wee carried out in which pieces of zinc foil were immersed in aqueous solutions of phosphonic acids under various conditions, and then rinsed in distilled water, before drying at room temperature to constant weight. For the weight loss/gain studies, unless otherwise stated, the phosphonic acid solutions were prepared at a concentration of 0.5%w/w using distilled water, and kept in a water bath at 600 C, and the pieces of zinc foil had a total surface area of 12cm2. For the results in fig 2 and fig 3, only one zinc piece was used for each experiment, which was returned to the solution after each weighing. For the remainder of the work, however, a fresh solution and a fresh piece of zinc foil was used for each result.

The solutions were analysed for zinc content using a Perkin Elmer Plasma 400 Inductively Coupled Plasma Spectrometer. The zinc surfaces were analysed using a Jeol 35C SEM with an Oxford Instruments Link ISIS EDX system and Autobeam attached. The samples were analysed at 20kV, x 550 magnification, and the results quoted are an average of five analyses of different areas of the sample. 1H NMR were recorded on a Bruker WM360 operating at 360 MHz, and 31P(1H) NMR were recorded on a Jeol FX90Q operating at 36 MHz. All NMR spectroscopy data is quoted in ppm.

31P (1H) NMR spectra were referenced to 85% H3PO4 (Oppm), and 'H NMR spectra were referenced to TMS (Oppm). An insert containing triphenylphosphone in deuterated acetone, which gave an observed peak at 2.81 ppm was used as an external reference to study the position of the phosphonic acid peaks, and to enable semi-quantitive analysis of the solutions. For the semi-quantitive analysis, a pulse delay of 5 seconds was used for 325 scans. This pulse delay was determined to be adequate for comparison of the levels of phosphorous present in the solutions of these compounds, as further increase in pulse delay did not significantly influence the results.

EXPERIMENTAL RESULTS 2-CEPA The change in the weight of the pieces of zinc foil after immersion in aqueous solutions of 2-CEPA at temperatures from 20C to 6O0C are shown in fig. 3. All of the pieces of zinc foil lost weight, until a certain level of zinc in solution was reached, which appeared to be independent of temperature.

After this weight loss had occurred, the pieces of zinc foil then gained weight. Both the rate of weight loss and the rate of weight gain increase as the temperature of the solution is raised.

Figure 4 shows the changes in weight that took place when pieces of zinc foil were added to three solutions of 2-CEPA in a water bath at 500C.

The control shows the previously described weight changes. After 7 hours, when the piece of zinc foil in a second solution had started to gain weight, a further portion of 2-CEPA was added. The zinc foil lost weight rapidly, before once again starting to gain weight, at a slightly faster rate than the control. The pieces of zinc foil in the third solution were replaced with fresh samples after 7 and 16 hours. both of the fresh pieces of zinc foil increased in weight after they had been in solution, although the rate of weight gain was slightly slower than that of the control sample.

Other experiments included changing the solutions every hour, but using the same piece of zinc foil, which resulted in continued loss in weight of the sample, giving a straight line graph. The concentration of the solutions was also varied between 0.25 and 0.75%w/w. This concentration range did not influence the time taken to reach the maximum zinc loss of the samples, although it did effect the amount of weight loss, which increased as the concentration of phosphonic acid in solution was increased.

Increasing the size of the zinc pieces however, did result in a reduction in the time taken for the maximum loss in weight to occur.

In Figure 5, the changes to the weight of the pieces of zinc foil in solutions of 2-CEPA are compared with the zinc content in the corresponding solution. The initial loss in weight of the pieces of zinc foil is mirrored by the increasing zinc content in solution. After two hours there is a reduction in the amount of zinc present in solution, and, the pieces of zinc foil start to gain weight.

The phosphorus content in the solutions of 2-CEPA, measured by NMR, fig. 6 decreases after 3 hours, but what is more interest, is the change in the position of the phosphorus peak with time, when compared to the position of the peak due to reference triphenylphosphine (external). The peak moves upfield for the first hour, but there is less movement in the second hour, and after four hours there is a slight shift back downfield.

Analysis of the surface of the pieces of zinc foil by SEM, figs 7 and 8, show that over the first hour there was a reduction in the level of carbon present on the surface, with little change over the following hour, and an increase in the level detected after two hours. The amount of zinc detected increased over the first half and hour, and then stayed at a similar level until after the second hour, when there was a rapid decrease in the amount detected. The levels of phosphorus and oxygen both increase after two hours.

The surfaces showed increased pitting with time spent in solution for the first hour, and flat rectangular platelet type crystals, growing away from the surface, were observed after one hour. The surface coverage by these crystals increased with time, but even after 8 hours, there appeared to be some small areas of the zinc surface visible.

PAA The weight loss of the pieces of zinc foil in solutions of PAA, fig. 9 is accounted for by a corresponding increase in zinc content of the PAA solutions. There was no obvious increase in the weight of the samples, as there was for those in solutions of 2-CEPA, however, there was a slight increase in weight after 6 hours, although this was of less significance.

There was also no detectable reduction in the zinc content of the solutions.

The amount of phosphorous in solution measured by 31P NMR, remains relatively constant, although the position of the peak does change, fig. 11.

The phosphorous peak moved upfield for the first hour, then it remains in the same position for the next hour, before moving back downfield, to a position further downfield than the initial position of the peak.

Analysis of the samples by SEM, fig 11, showed similar results to those for 2-CEPA, with increased pitting of the surface seen as the time in solution was increased. No coating was observed on the surface until the pieces of zinc had been in solution for 6 hours, after which time there was a large increase in the amount of phosphorus detected. The amount of zinc detected after the coating had formed was higher than the amount detected on the samples that had been in solutions of 2-CEPA for the same length of time. The coating completely covered the surface in a flat layer, with small spherical crystal growths randomly dotted on top of the coating. It is possible that the 'crazy paving' appearance of the surface is caused by the electron beam.

1,3-PBPA The weight changes of the pieces of zinc foil in solutions of 1 ,3-PBPA, fig. 12, are similar to those for PPA, with weight loss occurring over several hours, but then little or no weight gane. Phosphorus was detected on the surface after 2 hours, however, a visible surface coating fig. 13 was only seen after 4 hours. The crystal growth was the same as that for PAA, a flat coating with a 'crazy paving' appearance possibly due to damage caused by the electron beam, with spherical growths randomly positioned.

1 2-EBPA The pieces of zinc foil in solutions 1 ,2-EBPA, fig. 14 lost weight over the first hour, and then showed little change in weight for the next two hours. After three hours, they started to gain weight, with a corresponding decrease observed in the zinc content of the solutions. Surface analysis, fig.

15 showed an increase in the amount of phosphorus detected on the zinc foil after one hour, and small round crystal growths were also observed after this time. The number and size of these growths increased as the piece of zinc foil was in the solution for a longer time. After 8 hours they had merged to form an almost continuous coating, although there were still some small areas of the zinc substrate visible.

The weight changes with time for the pieces of zinc foil in solutions of the four phosphonic acids are shown for comparison in fig. 16. All show an initially similar rate of weight loss which reaches a maximum and then levels off. The molar ratios to the phosphonic acid in solution and the maximum zinc content in solution are shown in Table 1. The pieces of zinc foil in solutions of 2-CEPA and 1 ,2-EBPA, after two hours, then start to increase in weight. This weight gain also reaches a maximum and then levels off, with 1 ,2-EBPA showing the biggest increase in weight. For the zinc pieces in solutions of 1,3-PBPA and PAA it is not so obvious if any weight increase has taken place. Possibly a slight weight increase for the zinc pieces in solutions of 1 ,3-PBPA occurs after 3 hours, and for PAA after 7 hours. The pH changes for the solutions of 2-CEPA and PA, fig. 17 were very similar, as they both increased to around pH 4 after three hours.

TABLE 1 Molar ratios of the phosphonic acids in solution, and the maximum zinc content of the solutions.

Molar ratio of Phosphonic acid Phosphonic acid maximum zinc in solution 2-CEPA 1.7:1 PAA 1 .1:1 1 ,3-PBPA 1.2:1 1 ,2-EBPA 1 .3:1 Over the concentration range used in these experiments, the first process that occurs with all of the phosphonic acids, involves a reaction with the zinc to form a soluble complex. The process continues until a certain level of this complex in solution is achieved, figs. 7, 9, 12 and 14 and Table 1. The formation of a coating on the surface of the zinc substrate can occur concurrently with the removal of zinc from the substrate as was seen for the zinc pieces in solutions of 2-CEPA. A few crystals were observed on the surface after one hour, fig. 8, whilst weight loss continues for another hour. The bulk of the coating formation, however, only occurs after a certain level of zinc in solution has been reached. This was shown by replacing the phosphonic solution every hour, which produced a continual weight loss in the zinc piece, by adding a further portion of phosphonic acid after weight gain had started, which caused a further loss in weight in the sample, and by changing the zinc piece after weight gain had started, which showed an increase in weight with no previous weight loss, fig 4.

Therefore, a solution can be produced which starts for form a coating as soon as the substrate is immersed.

Changing the temperature of the solution, fig. 3, results in a change to both the rate of reaction of the phosphonic acid with the zinc, and the rate of coating formation. The time taken to reach maximum weight loss of the zinc piece was not affected by changing the concentration of the phosphonic acid between 0.25 and 0.75%w/w, however, increasing the surface area of the zinc piece did reduce the time taken for this to occur.

There are considerable differences in the time taken for these events to occur, and the type of surface coatings formed, depending on the phosphonic acid used. The phosphonic acids 2-CEPA and PAA, both have a phosphonic acid group at one end of the molecule, and a carboxylic acid group at the other, but 2-CEPA has three carbons in the backbone of the molecule, and PAA has only two. The phosphonic acids 1 ,2-EBPA and 1,3 PBPA, have two phosphonic acid groups at either end of the molecule, and there is also a difference of one carbon in the backbone. All four phosphonic acids therefore have groups capable of forming chemical bonds to zinc at either end of the molecule. If the same structure of salt was formed for both pairs of phosphonic acids, and if just the solubility of the salt determined the coating weight, it would be expected that 2-CEPA and 1 ,3-PBPA would produce the highest coating weights, as they are the largest molecules, but it is 1 ,2-EBPA which shows the biggest increase in coating weight, suggest either that solubility is not the biggest factor in coating formation, or that different types of sale are formed. Another possibility is that the types of coatings formed by 2-CEPA and 1 ,2-EBPA encourage film growth by having readily accessible functions for attachment to either molecules in solution, whereas those formed by PAA and 1 ,3-PBPA do not.

The zinc salt of 2-CEPA, ZN[Zn(O3PCH2CH2CO2)2.3H20 was prepared from zinc chloride, and was structurally characterised. This study found that the structure contains one set of zinc atoms four co-ordinated by oxygen atoms of the phosphonate groups and another set five co-ordinated by oxygen atoms of the carboxyl groups and lattice water molecules. If the movement of the phosphorus peaks in solutions of PAA was due to chelate formation then this would be likely to produce a different type of coating, and may account for the differences.

It is then a simple matter to adopt a chemical structure based on one or more of the above but which includes a group to which an organic layer such as paint can adhere. Such groups include epoxy groups and/or vinyl groups with which paints can react, or short to medium chain compounds with which paint layers can form an interconnecting network. These latter compounds can be substituted, for example with one or more of the former, and/or contain unsaturated areas.

The present invention also envisages the inclusion of accelerator or catalyst compounds. Such compounds may be able to ensure completion of the reaction quickly enough to allow the invention to be used as part of a continuous process. A suitable compound is a peroxide, which need only be present in catalytic amounts, An important development allowed by the invention is to form a solution of the relevant phosphonic acid compound and a metallic species, and to use this complex solution for the pre-treatment step. This would mean that the weight loss experienced by the experimental samples above would be eliminated and the reaction would proceed in place in a correspondingly shorter time.

Claims (36)

1. The complex formable in solution by immersion of a metallic species in a solution of the compound R-PO(OH)2 where R includes an organic group.

2. The complex of claim 1 wherein the metal is in one of the native form or a salt thereof.

3. The complex of any preceding claim wherein the R group is wholly organic.

4. The complex of any preceding claim wherein the R group includes at least one of the following groups; epoxy; hydroxy; unsaturated hydrocarbon; amide; carboxylic; or a combination thereof.

5. The complex of claim 4 including an unsaturated hydrocarbon groups being one of an alkene, vinyl or acrylate group.

6. The complex of any preceding claim in which the R group includes an aliphatic chain with which the paint layer can form an interlocking network.

7. The complex of claim 6 in which the aliphatic chain is at least C4 long.

8. The complex of claim 6 in which the aliphatic chain is larger than Cg.

9. The complex of claim 6 in which the aliphatic chain is larger than C12.

10. The complex of any one of claim 6 to 9 in which the aliphatic chain is substituted.

11. The complex of claim 10 wherein substitution is by one of epoxy, vinyl, hydroxy or other groups, or the chain contains one or more unsaturated regions.

12. The complex of any one of claims 6 to 11 wherein the R group contains no amino groups.

13. A solution of the complex of any preceding claim.

14. A solution according to claim 13 in the substantial absence of solid metal.

15. A solution according to claim 13 or claim 14 including an accelerator compound.

16. A solution according to claim 15 wherein the accelerator compound is present in a catalytic amount.

17. A solution according to claim 15 or claim t6 wherein the accelerator compound is a peroxide.

t8. The use of the complex or solution of any preceding claim as a coating agent.

19. The use of the complex or solution of any preceding claim as a coating agents for steel.

20. A method of coating a steel article, comprising the steps of; (i) providing a solution of the compound R-PO(OH)2 where R includes an organic group (ii) contacting the solution with a metal species or a salt thereof other than the steel article thereby to form a solution of a metal-phosphonate complex; (iii) optionally, removing the metal, if any remains; (iv) applying the thus formed complex solution to the surface of the steel article; and (v) coating the steel article with an organic coating.

21. Use of the compound R-PO(OH)2 where R includes an organic group but not an amino group, or a metallic complex thereof, as a surface treatment for a steel article prior to application of a paint or other organic layer thereto.

22. A method for the production of an article comprising a steel substrate on which is formed successively, an optional zinc or zinc alloy galvanising layer, an intermediate layer, and one or more paint layers, wherein the intermediate layer is formed by immersion of the steel substrate with optional zinc layer in a solution of the compound R-PO(OH)2, or a metallic complEx thereof, where R includes an organic group to which the paint layer(s) can bind.

23. A method according to claim 22 wherein the bonding between the R group and the paint layer is by way of a chemical bond.

24. A method of finishing a steel item comprising the steps of; (i) optionally, galvanising the steel item; (ii) applying in aqueous form the compound R-PO(OH)2 or a metallic complex thereof, where R includes an organic group to which a paint layer can bind; and (iii) applying at least one paint layer.

25. A method according to claim 24 wherein the compound is present in a solution.

26. A method according to claim 25 wherein the solution is applied to the said steel item by at least substantial immersion, by spraying, or by roller coating, or any combination thereof.

27. A method according to any one of claims 22 to 26 wherein the R group includes at least one epoxy group, or at least one hydroxy group, or at least one unsaturated hydrocarbon group, or at least one amide group, or at least one carboxylic group, or a combination thereof.

28. A method according to claim 27 wherein the R group includes an unsaturated hydrocarbon groups being one of an alkene, vinyl or acrylate group.

29. A method according to any one of claims 22 to 28 in which the R group includes an aliphatic chain with which the paint layer can form an interlocking network.

30. A method according to claim 29 wherein the chain is at least C4 long, preferably larger than Cg and more preferably larger than Cl2.

31. A method according to claim 30 wherein the chain is substituted, for example with epoxy, vinyl, hydroxy or other groups, or contains one or more unsaturated regions.

32. A method according to any one of claims 22 to 31 in which the R group contains no amino groups.

33. A method according to any one of claims 22 to 32 using a solution including an accelerator compound.

34. A method according to claim 33 wherein the accelerator compound is present in a catalytic amount.

35. A method according to claim 33 or claim 34 wherein the accelerator compound is a peroxide.

36. A steel pretreatment compound, solution or method substantially as herein described with reference to the accompanying figures.