GB2222592A - Treatment/coating of metal surfaces - Google Patents

Abstract

A metal surface is coated with a specified polyhydroxybenzene or a derivative thereof and a hot-cure epoxy composition (e.g. an epoxy adhesive). The polyhydroxybenzene or derivative may be pyrogallol, gallamide or a gallate. The metal surface may be coated with the polyhydroxybenzene and thereafter with the epoxy composition or may be treated with a mixture of the polyhydroxybenzene and at least one component of the epoxy composition. The presence of the polyhydroxybenzene compound gives a bond with improved durability when exposed to water or humid conditions.

Description

METAL COATING The present invention relates to the coating of a metal surface and in particular to the coating of metal surfaces with an adhesive formulation. More specifically the invention is directed to coating of a metal surface with an adhesive and a material which modifies the characteristics of the adhesive bond.

It is desirable for many purposes, for example the manufacture of composite articles, of which printed circuit boards are an example, to first coat a metal surface, for example the surface of a steel or copper article, with an adhesive composition, typically an adhesive organic compound and particularly with polymeric materials such as acrylic or epoxy adhesives.

Improved adhesion of a metal to a substrate, which may be a polymeric material, a metal or other material, is usually obtained by increasing the surface area of the metal by chemical or physical methods. Alternatively, the metal surface may be treated with a proprietary chemical prior to application of an adhesive, to obtain improved adhesive bonding. Such proprietary chemicals are typically called adhesion promoters. A common class of adhesion promoter is the organosilanes, particularly those containing at least one functional group, which compounds are used to promote adhesion to metal oxide or glass surfaces.Examples of such organosilanes include gamma-glycidoxypropyltrimethoxysilane; N-beta-aminoethyl-gamma-aminopropyltrimethoxysilane; gamma-mercaptopropyltrimethoxysilane; gamma-methacryloxypropyltrimethoxysilane; and ganma-aminopropyltrimethoxys ilane, all of which are available commercially. An effective class of adhesion promoter for bonding of copper substrates is the range of substituted triazoles, imidazoles, indazoles, thiazoles, oxazoles, carbamates, xanthates and phthalazines disclosed in United States Patent 3837964. In United States Patent 4448847 is disclosed the use of beta-diketones or mercaptoacetic acid esters of tri- or tetra-hydroxy alcohols to improve the characteristics of the bonding of epoxy adhesives to metals.United States Patent 4139693 discloses that adhesive compositions having an enhanced bond strength can be obtained by mixing a monomeric ester of 2-cyanoacrylic acid with an anionic polymerisation inhibitor and an adhesion promoter which is gallic acid or a derivative thereof such as methyl, propyl or hexyl gallate. A commercially available procedure involves pretreatment of a metal surface with a chromate solution prior to the application of stoving paints or prior to adhesive bonding.

However, the use of chromates is becoming environmentally undesirable because of toxicity (carcinogenicity) problems in manufacture and use, and difficulties in disposing of effluents containing chromium.

One problem which has been observed is a deterioration in the bond strength over a period of time. An adhesion promoter may increase the initial bond strength but over a period of time the strength of the bond decreases to an unsatisfactorv level, this decrease being particularly noticeable in the presence of water or at high humidity. Other adhesion promoters may give little improvement in the initial bond strength but the deterioration in the bond strength may occur more slowly thus prolonging the satisfactory life of the bond. However, further improvements are desirable, in particular it is desirable to prolong still further the retention of an adequate bond strength.

According to the present invention, there is provided a process which comprises coating a metal surface with a polyhydroxyaryl compound of the formula

and also with a hot-curing epoxy composition and heating to a temperature of at least 1000C to effect curing of the epoxy composition, wherein each X is independently an -OH group or a group -ROH and at least one of the groups X is an -OH group; Y is a hydrogen atom, a group R , a group COR or a group 34 NR R4; R is a divalent hydrocarbon or substituted hydrocarbon group; R1 is a hydrocarbyl or substituted hydrocarbyl group; R is a group R OR R4; R3 is a hydrogen atom, a hydrocarbyl or a substituted hydrocarbyl group; R4 is a hydrogen atom, a hydrocarbyl or a substituted hydrocarbyl group; and 4 R and R can be the same or different.

The group -ROH, if present, may be an oxime group, for example of the type CR3=NOH or may be an hydroxyalkyl group of the type -(CH2)nOH, where n is an integer and is preferably one. We have obtained useful results when all of the groups X are -OH groups.

The group Y may be a hydrogen atom but it is preferred that Y is either a group R1, particularly a substituted hydrocarbyl group, 2 2 or especially a group COR . In the group COR it is preferred that 2 is either 3 group OR 34 3 R2 is either a group OR or a group NR R . The group OR can be a hydroxy or a substituted or unsubstituted hydrocarbonoxy group. The group NR3R4 can be a substituted or unsubstituted amino group, wherein the groups attached to the carbon atom may be substituted, for example as in an ethanolamino group. The group Y may include at least one hydrocarbyl group and this may contain up to 18 carbon atoms and may be saturated or unsaturated. If the hydrocarbyl group contains a substituent, this may be a halogen atom, an amino-group, a hydroxy group or an epoxy group.

Thus, as a preferred aspect of the present invention, the polyhydroxyaryl compound is of the formula:

where R2 is either a group OR3 or a group NR3R4; and R3 and R4 are as defined.

The polyhydroxyaryl compound may be pyrogallol, gallic acid, methyl gallate, propyl gallate, dodecyl gallate, gallamide, a gallamide derivative such as N-(2-hydroxy-ethyl)3,4,5-trihydroxybenzoic acid amide or a substituted alkyl pyrogallol such as 5-(2-aminoethyl)-1 , 2, 3-trihydroxybenzene.

The epoxy composition can be any such material which is cured by heating to an elevated temperature, such as at least 1000C.

It is preferred that the epoxy composition is one which requires heating to a temperature of at least 1200C, and especially at least 1500C, in order to effect curing of the epoxy composition. As a particularly preferred aspect of the present invention, curing is effected at a temperature of 180 + 10 C.

The epoxy- composition may be any such composition which is used for coating onto metal surfaces and may be an epoxy paint or particularly an epoxy adhesive composition.

The epoxy composition is only partially polymerised and curing of the partially polymerised material is effected after it has been coated onto the metal surface. A curing agent is incorporated into the epoxy composition either before, during or after coating of the composition onto the metal surface. Preferably the epoxy composition which is coated onto the metal surface includes a curing agent. Curing of the epoxy composition is effected under the appropriate conditions for the particular composition. Thus, curing is effected at an elevated temperature of at least 1000C..

If the epoxy composition is an adhesive composition curing may be effected under an applied pressure. The pressure may be that achieved by holding two pieces of metal together with clips around the edges thereof. Alternatively, the applied pressure may be at least one kg/cm2 and especially at least ten kg/cm2. Typically, two metal surfaces are coated with the polyhydroxyaryl compound and with an epoxy adhesive composition and are then bonded together by pressing the surfaces together under conditions effective to cause curing of the epoxy adhesive.

A wide range of hot cure epoxy adhesive compositions are available commercially and these proprietary materials can be used in the process of the present invention using the manufacturer's recommended curing conditions of temperature, pressure and time.

The curing time for many hot cure epoxy adhesive compositions is at least one minute but generally does not exceed two hours. Epoxy resins and their use as adhesives are discussed in "Encyclopedia of Polymer Science and Engineering", Second Edition, Volume 6 (1986) at pages 322 to 382 and in "Synthetic Adhesives and Sealants" edited by W.C.Wake, in the section at pages 73 to 88 entitled "Toughened epoxy resin adhesives".

The process of the present invention is particularly suitable for coating steel surfaces. The metal surface which is to be coated is preferably brightly polished and/or freshly cleaned.

However, we have found that the process can be applied using a metal in the "as received" condition when it may be lightly rusted.

The coating of the metal surface with the polyhydroxyaryl compound may be effected by applying the polyhydroxyaryl compound alone 0 the surface of the metal. However, it is generally preferred that coating is effected by applying the polyhydroxyaryl compound in a suitable medium to the metal surface. More specifically, the polyhydroxyaryl compound can be applied to the metal surface in the form of a solution in a suitable organic solvent, in water, or in an organic solvent/water mixture. The coating of the metal surface with the polyhydroxyaryl compound may be effected before the application of the epoxy adhesive composition.

Alternatively, the polyhydroxyaryl compound may be incorporated into the epoxy adhesive composition and the mixture thereof applied to the metal surface.

Conventional organic solvents may be used for the polyhydroxyaryl compound and include for example alcohols, ethers, ketones and aliphatic and aromatic hydrocarbons. Especially preferred solvents are those having good wetting and drying properties and include for example toluene, xylene, chloroform, 1,1,1-trichloroethane, ethanol, isopropanol, octanol, acetone, methylethyl ketone, water and glycol ethers.

The polyhydroxyaryl compound, or a composition containing the compound, may include a dyestuff so that the treated area can be clearly seen from the colouration resulting from the dyestuff.

If the polyhydroxyaryl compound is coated onto the metal surface before applying the epoxy composition, the process of coating with the polyhydroxyaryl compound may be repeated, if desired several times, before applying the epoxy composition.

The polyhydroxyaryl compound, or the solution or emulsion thereof, may be applied to the metal in conventional manner, for example by dipping, spraying, brushing or roller coating. The temperature of the application may be any suitable temperature such as from 0 to 500C, for example ambient temperature. Typically, solutions-of the polyhydroxyaryl compound may contain from 0.1 to 20% by weight of polyhydroxyaryl compound. The presence of from 0.1 to 2% by weight of the polyhydroxyaryl compound in a surface coating formulation is generally sufficient to provide improved adhesion of the epoxy adhesive composition.

According to an alternative procedure, the polyhydroxyaryl compound may be formulated into the epoxy composition.

The polyhydroxyaryl compound may be formulated into the epoxy composition using any suitable blending technique. It is desirable that the polyhydroxyaryl compound is distributed as uniformly as is possible in the epoxy composition. We have found that the technique used to achieve the desired uniform incorporation is dependent on the particular polyhydroxyaryl compound, particularly the solubility thereof in the epoxy composition. The polyhydroxyaryl compound and the epoxy composition, or a component thereof, may be mixed as solutions in a suitable solvent.Thus, the polyhydroxyaryl compound and the epoxy composition or a component thereof can be separately dissolved in a suitable solvent such as acetone, the solutions mixed and the solvent evaporated off to give a solution of the polyhydroxyaryl compound in the epoxy composition or component thereof and, if necessary, other components may be added to give a hot-cure epoxy composition containing the polyhydroxyaryl compound. However, we have found that whilst gallic acid and the esters thereof can be incorporated into an epoxy composition using this technique, other polyhydroxyaryl compounds such as gallamide are less soluble and are reprecipitated as the solvent is evaporated off.

Using epoxy adhesive compositions, we have also found that essentially complete removal of the solvent from the mixture is very desirable in order to achieve joints having a satisfactory shear strength. If the solvent is not adequately removed, the shear strength of the resulting joint is reduced compared to the shear strength attainable by applying the polyhydroxyaryl compound first and then the epoxy adhesive composition to the metal surface.

The polyhydroxyaryl compound can be mixed into the epoxy composition, or component thereof, using a simple blending technique such as simply stirring the two materials together. However, this technique does not readily achieve the desired uniformity of dispersion of the polyhydroxyaryl compound in the epoxy composition or component thereof. More effective dispersion can be achieved by the use of a high shear mixing device such as, for example an Ultra Turrax or a Silverson high shear mixer.

An epoxy adhesive composition which includes a polyhydroxyaryl compound can be applied to the metal surface and cured using similar, or identical, conditions as can be used for the same epoxy adhesive composition which is free from any polyhydroxyaryl compound.

The process of the present invention may be used to bond together a metal and a plastics material or to bond together two sheets of metal. We have found that the initial adhesive strength of the bond is generally the same as, or slightly greater than, the initial adhesive strength of a bond obtained when using the epoxy adhesive alone. However, in tests of the durability of the bond, we have found that the use of the polyhydroxyaryl compound results in a slower deterioration of the strength of the bond.

Durability of the bond can be determined by immersion of the bond in water maintained at 500C and testing the strength of the bond after various periods of immersion. We have found that the strength of the adhesive bond is reduced to half of its initial value after immersion in water for 500 hours when using the epoxy adhesive alone.

However, using a polyhydroxyaryl compound together with a hot cure epoxy adhesive, the strength of the adhesive bond can still be greater than half of its initial value even after immersion in water for a considerably greater period of time, for example up to 5000 hours. Failure of the bond is mainly cohesive rather than adhesive, hence failure is occurring within the adhesive rather than at the interface between the metal surface and the adhesive.

Preferred polyhydroxyaryl compounds are those which result in an adhesive bond which retains at least 50% of its initial strength after immersion in water at 500C for a period of 1500 hours, preferably for 3000 hours and especially for 5000 hours. This effect is not obtained if the tolyhydroxyaryl compound is used with an acrylic adhesive or with a cold cure epoxy adhesive.

The present invention also provides a metal at least part of one surface of which has been coated with a polyhydroxyaryl compound of the formula

and also with a hot cure epoxy composition, wherein X and Y are both as hereinbefore defined.

The polyhydroxyaryl compound can be as previously described.

The hot cure epoxy composition can be any such composition as previously described and is particularly a hot cure epoxy adhesive composition.

The present invention also includes an article having a metal to metal bond wherein the metal surfaces bonded together have been treated with the polyhydroxyaryl compound and with a hot cure epoxy adhesive composition. Preferred bonds retain at least half of their initial adhesive strength after being immersed in water at 500C for at least 3000 hours.

The present invention may be applied to the production of a composite article having a metal to polymer bond or may be used for the production of articles wherein a metal is bonded to the same or a different metal using a hot cure epoxy adhesive composition.

Various embodiments of the present invention are set out in more detail in the following, non-limiting, examples.

Test Procedure (A) Test strip preparation Mild steel strips (102 x 25.4 x 1.6mm) purchased from Q-Panel Co., 102 Taylorson Street South, Salford M5 3EL, were used unless stated to the contrary. The steel was identified as being a low carbon, cold rolled steel (Type CR1 as ISO 3574). One surface of the strips had been abrasively ground, by Q-Panel Co., to remove the mill surface and provide 0.4-0.65 micrometres roughness, according to ASTER Test Method D609.2B. Before use the strips were cleaned and degreased by immersion in boiling 1,1, l-trichloroethane for 15-30 minutes, allowed to cool to ambient temperature and subjected to a further degreasing stage, suspended in the vapour from refluxing l,1,1-trichloroethane for two minutes.The strips were allowed to cool and dry in a clean, dry environment to limit recontamination of the surfaces. All subsequent work up to curing of the adhesive was carried out in a clean, dry area and the strips were handled at the end remote from the surface to be bonded in order to prevent transfer of contamination from fingers to the surfaces to be bonded.

(B) Formation of bond Pairs of strips were formed into single lap joints using a commercial adhesive and following the methods described in ASTM Test Method D1002-64. (Standard Method of test for strength properties of adhesives in shear by tension loading - metal to metal). A paste adhesive (ESP 106, purchased from Permabond Adhesives Limited, Woodside Road, Eastleigh, Hampshire S05 4EX), which is described as a toughened, single part, epoxy adhesive, was spread over the surfaces to be joined, and the strips arranged in a template fitted with pegs and spacing pieces designed to provide an overlap of 12.5mm, and prevent movement of the strips before curing of the adhesive.Ten or twelve single-lap joints were prepared in the template at one time. The adhesive- as cured,-according to the manufacturer's recommendations, -by heating ehe assembledXjoints in the template to 1800C between the plates of a heated hydraulic press, raised to a gauge pressure of approximately 20 kg/cm2 and maintaining this pressure at 180 + 2"C for one hour before cooling to ambient temperature. Each joint was inspected visually and measured with a ruler. Any joint which was out of alignment, or had an incorrect overlap, was discarded. From each set of joints, one or two were measured by micrometer to determine the thickness of the glue-line (normally 0.1-0.2mm) and any joints which were substantially different were also rejected.

(C) Testing of durability of bonds Twenty joints of acceptable quality were selected, the shear strength of 5 joints was measured as described hereafter. The remaining 15 joints were subjected to a durability test according to ASTM Test Method D1151-84. The joints were placed in a glass jar containing sufficient distilled water, at 50"C, to almost totally cover the metal strips, inclined close to the vertical. The jar was sealed to prevent water evaporation and was stored in an oven at a controlled temperature of 50 + 2"C. After 500 hours (+ 12 hours), 3 or 4 joints were removed, allowed to dry at ambient temperature and humidity for four hours and shear strength measured as described hereafter.Further sets of 3 or 4 joints were also removed after 1500 hours, 3000 hours and 5000 hours (+ 24 hours), allowed to dry (recover) for four hours at ambient temperature then shear strength measured as described hereafter.

(D) Determination of joint shear strength Measurement of the shear strength of each single lap joint was carried out according to ASTM Test Method D1002-64 referred to above. The joint, which was gripped at each end in the jaws of an INSTRON-1195 testing machine, was subjected to increasing tension in shear at ambient temperature and a cross head speed of lmm/minute, until fracture occurred. The shear strength of the joint was calculated from the load at failure. Both faces of the fractured joint were inspected visually and the apparent locus, or locii, of failure was noted. For each set of determinations, the average shear strength, and the loss in average shear strength with exposure were calculated.

Compaztive Example A Bonds were formed as hereinbefore described in Stages A and B using the adhesive only, that is without the prior use or inclusion of an adhesion promoter. In the initial tests, failure of the joint occurred by a mixture of cohesive and adhesive failure.

After exposure to water, failure of the joint occurred at the steel-adhesive interface.

Comparative Example B After cleaning the steel strips as described in Stage A, the surfaces of both strips were pre-treated before bonding as described in Stage B. Pre-treatment was effected using an aqueous solution containing hexavalent chromium (available as ACCOMET C from Albright and Wilson, PO Box 80, Oldbury, Warley, West Midlands B69 4LN) which had been obtained by diluting ACCOMET C with water in the proportions, by volume, of one part of ACCOMET C to four parts of water. The diluted solution was applied by brush to the metal surfaces, which were then allowed to dry in air at ambient temperature. Bonding was then effected as described in Stage B.

The joints were subjected to the durability test as described in Stage C and the joint shear strength was determined as described in Stage D.

Initially failure was adhesive at the ACCOMET C/adhesive interface but subsequent failures were by a mixture of adhesive and cohesive failure.

Comparative Example C A 1% w/v solution of gamma-glycidoxypropyltrimethoxysilane (purchased from Aldrich Chemical Co.Ltd., The Old Brickyard, New Road, Gillingham, Dorset SP8 4JL) in water was prepared. Steel strips, cleaned as described in Stage A, were immersed in the freshly prepared solution at pH 6-7 for 30 minutes at ambient temperature.

The treated strips were dried in a stream of nitrogen. Joints were prepared and tested as described in Stages B, C and D.

Initially failure was cohesive but subsequent failures were by a mixture of adhesive and cohesive failure.

Comparative Examples D, E and F Steel strips were pretreated using a procedure essentially as disclosed in United States Patent Specification 4448847. The steel strips were cleaned as described in Stage A. Ammonium hydroxide solution was added to a solution of 3g of citric acid in 100cm3 of distilled water to give a pH of about 7. The resulting solution was heated to a temperature of 800C and the clean steel strips were immersed in the solution at 800C for ten minutes. The treated strips were rinsed, in turn, with distilled water and methanol. The washed strips were immediately immersed, for 10 to 15 minutes, in a solution of a reagent which solution was being maintained at reflux. The strips were then washed with methanol, dried and then bonded as described in Stage B. The joints obtained were tested as described in Stages C and D.

The solutions of reagent used were: Comparative Example D - 0.0025M solution of dibenzoyl methane in methanol.

Comparative Example E - 0.0025M solution of 1,2,6-hexanetriol trimercaptoacetate in methanol.

Comparative Example F - 0.0025M solution of pentaerythritol tetramercaptoacetate in methanol.

The initial failure in Comparative Example D was a mixture of adhesive and cohesive whereas that in Comparative Examples E and F was cohesive. Subsequent failures were generally a mixture of adhesive and cohesive failure.

Example 1 The procedure used was generally as described for Comparative Example B with the exception that a different pre-treatment reagent and process was used.

A 0.1% w/v solution of gallamide (3,4,5-trihydroxybenzoic acid amide) in a mixed solvent of commercial grade ethanol (two parts by volume) and acetone (three parts by volume) was prepared. This solution was coated, by brush, onto steel strips cleaned as described in Stage A and the solvents were allowed to evaporate at ambient temperature. A further coating of this solution was applied and allowed to dry. The strips were then bonded together as described in Stage B. The joints formed were tested as described in Stages C and D. Even after 5000 hours, failure of the bond was essentially by cohesive failure with little, if any, failure at the steel-adhesive interface.

Example 2 The procedure of Example 1 was repeated using a 0.1% w/v solution of 2,6-bis(oximinomethyl)-4-methylphenol in commercial grade ethanol.

Failure occurred by a mixture of cohesive and adhesive failure, with the proportion of adhesive failure increasing as the duration of the test was increased.

Example 3 The procedure of Example 1 was repeated using a 0.1% w/v solution of 2,6-bis(hydroxymethyl)-4-methylphenol in commercial grade ethanol.

Initial failure was predominantly cohesive but subsequent failure was increasingly adhesive failure.

Example 4 The procedure of Example 1 was repeated using a 0.1% w/v solution of N-beta-hydroxyethylgallamide in a mixed solvent of commercial grade ethanol (two parts by volume) and acetone (three parts by volume).

In durability tests up to 3000 hours, failure of the bond was predominantly cohesive failure.

Example 5 The procedure of Example 1 was repeated using a 0.1: w/v solution of gallic acid (3,4,5-trihydroxybenzoic acid) in the mixed solvent.

In durability tests up to 3000 hours, failure of the bond was predominantly cohesive failure.

Examples 6 and 7 These were repeats of Example 1. In durability tests up to 5000 hours, failure of the bond was predominantly cohesive failure.

Comparative Example G The procedure of Example 1 was repeated using only the mixed solvent of commercial grade ethanol and acetone. After 5000 hours, failure of the bond was predominantly adhesive failure.

Example 8 The procedure of Example 1 was repeated using a 0.1 w/v solution of pyrogallol in the mixed solvent.

Failure in most cases was cohesive with only a slight amount of adhesive failure.

Example 9 The procedure of Example 1 was repeated using a 0.1X w/v solution of n-dodecyl gallate.

Initial failure was bv a mixture of cohesive and adhesive failure and after 5000 hours failure was also by a mixture of cohesive and adhesive failure.

Example 10 In this example the adhesion promoter was incorporated into the adhesive formulation.

Gallamide was mixed with the adhesive, as used in Stage B, in the proportion of 0.5% w/w of gallamide relative to the total composition. The steel strips, which had been cleaned as described in Stage A, were coated with the gallamide/adhesive mixture as described in Stage B. The strips were bonded together and the joints tested as described in Stages B, C and D.

Failure after 3000 hours was predominantly cohesive failure.

Comparative Example H The procedure of Comparative Example A was repeated using a different adhesive formulation. The adhesive was a toughened epoxy adhesive containing aluminium powder (obtained from Permabond Adhesives Limited as grade ESP 110).

Example 11 The procedure of Example 10 was repeated with the exception that the adhesive used was ESP 110.

The results obtained in Comparative Examples A to H and Examples 1 to 11 are set out in Table One.

Table One

x.or Initial 500 hours 11500 hours 3000 hours 5000 hours Comp. Strength Str. % Str. % Str. % Str. l% Ex. (Mpa) (Mpa) loss (Mpa) loss (Mpa) loss (Mpa) loss (a) (a) (a) (a) (a)(b) A 38.9 17.3 55.5 16.8 57 11.8 69.6 ND ND B 29.7 29.7 0 22.1 25.6 20.6 30.6 ND ND C 39.4 33.1 16 29.1 26.1 23.4 40.6 ND ND D 38.8 28.5 26.5 22.1 43.2 21.3 45 ND ND E 38.8 29.7 23.4 28.7 26 25.1 35.2 ND ND F 45.9 28.5 37.9 26* 43.2 25.2* 45.1 ND ND 1 43.3 31.9 26.3 30.2 30.2 29.2 32.5 23.6 45.5 2 38.8 22.1 43.0 18.4 t 52.5 19.5 49.7 ND ND 3 42.7 22.5 47.2 21.6 i 49.4 21.8 48.9 17.8 58.3 4 46.6 35.6 23.6 30.4 1- 35 27.8 40.5 24.2 48.2 5 45.2 32.7 27.6 31.9 29.5 28.7 36.5 25.3 44.0 6 46.8 34.5 26.3 32.1 31.4 29.8 36.3 26.1 44.3 7 48.2 32.7 32.3 30.5 36.7 30.1 37.5 26.1 45.9 G 42.0 20.5 51.2 21.1 49.7 18.8 55.1 15.3 63.5 8 42.8 31.6 26.11 24.2 43.3 28.5 33.4 27.1 36.6 9 37.6 25.6 : 32.0 23.7 37.0 21.5 42.8 19.7+ 47.6 10 45.3 33.5 26.2 27.2 40.0 23.1 49.0 ND ND H 42.7 26.6 37.8 22.9 46.4 19.2 55.1 19.3+ 54.8 11 38.8 24.2 i 23.2 23. 40.2 13.7 64.6 ND ND t Notes to Table One (a) Shear strength determined as described in Stage D.

* These results exclude one or two results in which the bonds were defective.

(b) In Examples 1 and 4 and Comparative Examples G and H, three determinations were effected.

In Example 3, only one determination was effected.

In Examples 5 to 9, four determinations were effected.

ND means not determined.

(c) in these experiments, the joint strength was determined as described in (D), but was measured using a Monsanto Tensometer T-2O.

Example 12 and Comparative Example I The procedures of Example 1 and Comparative Example A respectively were repeated with the exception that determinations of the bond strengths were effected after shorter periods of time.

The results obtained are set out in Table Two.

Table Two

Ex.or ) Initial -1 50 hours 100 hours 1 200 hours 500 hours Comp. Strength Str. % Str. % Str. % Str. Z Ex. (Mpa) (Mpa) I loss (Mpa) I loss ( (Mpa) I loss (Mpa) loss (a) (a) ~ (a) (a) (a) 12 43.9 41.8 4.8 36.5 16.9 35.5 19.2 31.3 28.6 I 41.9 37.7 10 30.4 27.4 28.3 32.5 19.5 53.4 Notes to Table Two (a) is as defined in Notes to Table One.

Example 13 and Comparative Example J The procedures of Example 1 and Comparative Example A respectively were repeated using different steel strips which had been treated in a different manner.

The steel strips were of the same size as described in Stage A and were formed from cold rolled steel sheet produced by British Steel Corporation. The strips had a milled surface and were at least six months old. The strips had been oiled to protect the surfaces during storage. No attempt was made to clean the steel or to remove corrosion products. The strips were treated with 1,1,1-trichloroethane to remove grease as described in Stage A. The degreased strips were then used as described in Example 1 and Comparative Example A respectively.

The results obtained are set out in Table Three.

Table Three

s Ex.or Initial 500 hours 1 1500 hours 1 3000 hours Comp. Strength Strength % Strength % Strength % Ex. (Mpa) (a) (Mpa) (a) loss (Mpa)(a) loss (Mpa) (a) loss 13 15.9 11.5 27. 9.7 1 39.2 9.6 39.8 K 16.2 6.8 57.7 8.1 46.8 6.2 61.8 Notes to Table Three (a) is as defined in Notes to Table One.

Example 14 and Comparative Example K The procedure described for Example 13 and Comparative Example J was repeated using the steel strips obtained from Q-Panel Co., as described in Test Procedure A, with the exception that the treatment was applied to the surface of the strips which had not been ground and which had a milled surface resulting from the rollingprocess.

The results obtained are set out in Table Four.

Table Four

Ex.or Initial 500 hours J 1500 hours 3000 hours 1 5000 hours Comp. Strength Str. % Str. L Str. % Str. 2 Ex. (Mpa) (Mpa) loss (Mpa) loss (Mpa) loss (Mpa) loss (a) (a) ~ (a) (c) (a) (c) (a) (c) 14 41.4 31.7 23.4 28.8+ 30.3 27.6+ 33.3 22.8+ 44.9 K 34.6 16.9 51.2 17.5+ 49.4 11.2+ 67.6 8 3 8.3+ 76.0 Notes to Table Four (a) and (c) are both as defined in Notes to Table One.

Example 15 The procedure of Example 1 was repeated using a 0.1% w/v solution of methyl gallate in commercial grade ethanol.

In durability tests up to 5000 hours, failure of the bond was predominantly cohesive failure.

Example 16 The procedure of Example 1 was repeated using a 0.1% w/v solution of n-propyl gallate in commercial grade ethanol.

In durability tests up to 5000 hours, failure of the bond was predominantly cohesive failure.

Example 17 The procedure of Example 1 was repeated using a 0.1% w/v solution of the hydrogen chloride salt of 5-(2-aminoethyl)1,2,3-trihydroxybenzene in commercial grade ethanol.

In durability tests up to 5000 hours, failure of the bond was predominantly cohesive failure.

The results obtained in Examples 15 to 17 are set out in Table Five.

Table Five

Ex.or Initial 500 hours 1 1500 hours 1 3000 hours 1 5000 hours Comp. Strength Str. % Str. % Str. % Str. % Ex. (Mpa) (Mpa) loss (Mpa) loss (Mpa) loss (Mpa) loss (a) (a) (c) (a) (c) (a) (c) (a)(c) 15 39.5 31.1 21.3 29.7+ 24.8 26.7+ 32.4 24.1+ 39.0 16 41.1 32.5 20.9 30.0+ 27.1 27.6+ 32.9 23.7+ 42.3 17 43.1 32.5+ 24.6 31.6+ 26.7 1 28.1+ 34.7 25.0+ 41.9 f Notes to Table Five (a) and (c) are both as defined in Notes to Table One.

Example 18 The procedure of Example 1 was repeated using a 0.1% w/v solution of gallamide in the mixed solvent with the exception that the adhesive was ESP 110.

In durability tests up to 5000 hours, failure of the bond was predominantly cohesive failure.

Example 19 16mm3 of a 1% w/v solution of gallic acid in commercial grade ethanol were applied from a micropipette onto an area of approximately 2.5cm x 2.5cm on the surface of a strip as used in Example 1. The solution was spread over the area and the solvent allowed to evaporate. A further 16mm3 of the solution was then applied and the solvent allowed to evaporate. The procedure thereafter was as described in Example 1 with the exception that the adhesive was ESP 110.

In durability tests up to 5000 hours, failure of the bond was predominantly cohesive failure.

Example 20 The procedure of Example 19 was repeated using a 1% w/v solution of gallic acid in commercial grade ethanol with the exception that the adhesive was ESP 106.

In durability tests up to 5000 hours, failure of the bond was predominantly, or solely, cohesive failure.

The results obtained in Examples 18 to 20 are set out in Table Six.

Table Six

Ex.or Initial 500 hours 1500 hours 3000 hours 5000 hours Comp. Strength Str. % Str. % Str. % Str. Z Ex. (Mpa) (Mpa) loss (Mpa) loss (Mpa) loss (Mpa) loss (a)(c) (a)(c) I I (a)(c) I I (a)(c) I (a)(c) 18 38.9 32.6 16.0 28.4 27.0 25.2+ 35.2 24.2+ 37.8 19 46.7+ 36.8+ 21.3 29.1+ 37.7 26.6+ 43.0 24.7+ 47.1 20 52.3+ 35.4+ 32.3 - 30.3+ 42.0 26.7+ 48.9 24.2+ 53.7 Notes to Table Six (a) and (c) are both as defined in Notes to Table One.

In all of the following Examples and Comparative Examples the shear strength of the joints were determined as described in (D) but were measured using a Monsanto Tensometer T-20.

Example 21 Gallic acid was added to the paste adhesive ESP 106 in the proportion of 1% w/v of gallic acid to the adhesive formulation, acetone was added to the mixture which was stirred to give a freely mobile liquid. The mixture was warmed to 500C and the acetone was removed under water pump pressure.

Bonds were formed as described for Stages A and B of the Test Procedure using the mixture of gallic acid and ESP 106 prepared from the acetone mixture with the exception that after the adhesive was applied, glass beads (0.12 - 0.2mm diameter) were sprinkled onto the adhesive to control the glue-line thickness, the joint was assembled and secured with two bulldog type clips. Curing was effected for one hour at 180 C.

In durability tests up to 3000 hours, failure of the bond was predominantly adhesive failure and the initial strength of the bond was much lower than was attained in Example 5.

Example 22 The procedure of Example 21 was repeated using n-propyl gallate rather than gallic acid. Similar results were obtained.

Comparative Example L The procedure of Example 21 was repeated with the exception that no gallic acid, or other polyhydroxyaryl compound, was added to the mixture.

The results obtained in Examples 21 and 22 and Comparative Example L are set out in Table Seven.

Table Seven

Ex.or Initial 1 500 hours 1 3000 hours Comp. Strength Strength % Strength % Ex. (Mpa) (a) (Mpa) (a)~J loss ~ (Mpa) (a) loss~ ~ 21 21.0 11.2 46.6 9.3 55.7 22 28.5 15.9 44.1 1 13.0 54.4 L 21.8 11.9 45.3 8.8 59.5 Notes to Table Seven (a) is as defined in Notes to Table One.

Example 23 The coating procedure of Example 19 was repeated with the exceptions that the adhesive was a toughened epoxy adhesive capable of being cured at 1200C and above (obtained from Permabond Adhesives Limited as grade ESP 109), the joint was formed as described in Example 21 and was cured by being placed in an oven maintained at 1200C for 75 minutes.

In durability tests up to 3000 hours, failure of the bond was predominantly cohesive failure becoming progressively more adhesive failure and being predominantly adhesive failure after 3000 hours and 5000 hours.

Comparative Example M The procedure of Example 23 was repeated with the exception that the coating with gallic acid solution was omitted.

Example 24 The procedure of Example 23 was repeated with the exception that curing was effected at 1500C for 40 minutes.

Comparative Example N The procedure of Example 24 was repeated with the exception that the coating with gallic acid was omitted.

Example 25 The procedure of Example 23 was repeated with the exception that curing was effected at 1800C for 10 minutes.

Comparative Example O The procedure of Example 25 was repeated with the exception that the coating with gallic acid solution was omitted.

The results obtained in Examples 23 to 25 and Comparative Examples M to 0 are set out in Table Eight.

Table Eight

Ex.or I Initial 1 500 hours 1 3000 hours 1 5000 hours Comp. Strength Strength % Strength % Strength % Ex. (N a) (a) (N a) (a) loss (Mpa)(a) loss (N a) (a) loss 23 29.5 18.9 36.0 16.7 43.4 14.2 51.8 M 27.9 16.6 40.5 12.0 57.0 10.0 64.2 24 36.6 22.4 39.0 22.2 39.3 17.7 1 51.7 N 35.5 19.3 45.7 17.8 49.8 14.7 58.4 25 ! 34.4 25.5 25.2 18.1 46.9 18.8 44.9 0 t 36.7 20.1 1 45.2 17.8 51.6 14.9 59.4 Notes to Table Eight (a) is as defined in Notes to Table One.

Example 26 Gallic acid was added to the adhesive ESP 106 in an amount of 1% w/v of gallic acid to ESP 106. The materials were mixed for approximately five minutes using a Silverson high shear mill. The resulting mixture was used to form a bond as described under Test Procedure, Stages A and B using the resultant milled mixture with the exception that the bonds were formed as described in Example 21.

In durability tests up to 3000 hours, failure of the bond was predominantly cohesive.

Example 27 The procedure of Example 26 was repeated with the exception that n-propyl gallate was used rather than gallic acid.

Comparative Example P The procedure of Example 26 was repeated with the exception that no gallic acid, or other polyhydroxyaryl compound, was added to the adhesive.

Example 28 The procedure of Example 26 was repeated with the exceptions that 2.5% w/w of gallic acid was incorporated into the adhesive, the adhesive was ESP 109 and curing was effected at 1800C for 30 minutes.

Example 29 The procedure of Example 28 was repeated with the exception that curing was effected at 1200C for 90 minutes.

Example 30 The procedure of Example 1 was repeated using a 0.1% w/v aqueous solution of gallic acid.

Failure of the bond was adhesive initially but after 3000 hours was a mixture of adhesive and cohesive failure. The bond strength was essentially unchanged after 3000 hours.

Comparative Example Q The procedure of Comparative Example B was repeated with the exception that ACCOMET C was diluted with water in the proportions, by volume, of one part of ACCOMET C to 24 parts of water and the joint was formed in the manner described in Example 21.

The results obtained in Examples 26 to 30 and Comparative Examples P and Q are set out in Table Wine.

Table Eight

Ex.or Initial 500 hours 3000 hours Comp. Strength Strength % Strength % Ex. (Mpa) (a) (Mpa) (a) loss (Mpa)(a) loss 26 34.7 30.6 11.8 20.9 39.7 27 32.9 30.8 6.3 22.9 30.3 P 34.8 24.4 30.1 16.2 53.3 28 26.4 1 24.6 6.6 ND ND 29 23.2 20.2 13.0 ND ND 30 22.4 25.5 - 22.8 Q 42.8 36.2 15.5 28.6 33.4 Notes to Table Nine (a) is as defined in Notes to Table One.

Comparative Examples R to Z Experiments were carried out using cold cure epoxy adhesives. The procedures used were essentially as described for Example 1 with the exception that the epoxy adhesives were two part systems which were premixed, the mixture applied to the strips, joints were formed as described in Example 21 and the adhesive allowed to cure at room temperature. Further details of the procedures used are as follows.

Comparative Example R The metal strips were coated using ARALDITE adhesive (obtained from Ciba-Geigy as a product for use by the domestic handyman). The two parts of the adhesive, resin and hardener, were mixed in equal parts and applied to the metal surface. Curing was allowed to occur at ambient temperature (15-20 C) over a period of 24 hours.

Comparative Example S The procedure of Comparative Example R was repeated with the exception that the metal strips were first coated with a 0.1% w/v solution of gallamide in commercial grade ethanol using the procedure of Example 1.

Comparative Example T The procedure of Comparative Example R was repeated with the exception that the adhesive was a different two part epoxy adhesive (E32, obtained from Permabond Adhesives Limited) and curing was effected for 24 hours at ambient temperature followed by one hour at 50"C.

Comparative Example U The procedure of Comparative Example T was repeated with the exception that the metal strips were first coated with two 16mm3 portions of a 0.1% w/v solution of gallic acid in ethanol using the procedure of Example 19.

Comparative Example V The procedure of Comparative Example T was repeated with the exception that 1.6t w/v allyl gallate were mixed with the resin component of the adhesive prior to mixing the resin and the hardener.

Comparative Example W The procedure of Comparative Example U was repeated using a 0.1% w/v solution of allyl gallate in commercial grade ethanol.

Comparative Example X The procedure of Comparative Example V was repeated with the exception that 1.6 w/v of the hydrogen chloride salt of 5-(2-amino ethyl)-1,2,3-trihydroxybenzene was mixed with the resin component of the adhesive.

Comparative Example Y The procedure of Comparative Example U was repeated using a 1.0% w/v solution of the hydrogen chloride salt of 5-(2-aminoethyl)1,2,3-trihydroxybenzene in commercial grade ethanol.

Comparative Example Z The procedure of Comparative Example Y was repeated using a 0.1% w/v solution of the salt in commercial grade ethanol.

In all cases, failure of the bond was solely or predominantly adhesive failure.

The results obtained in Comparative Examples R to Z are set out in Table Ten.

Table Ten

Ex.or Initial 500 hours ( 1500 hours 3000 hours r 5000 hours Comp. Strength Str. % Str. Z Str. % Str. X Ex. (Mpa) (Mpa) loss (Mpa) loss (Mpa) loss (Mpa) loss (a) (a) (d) (a) (e) (a) (a) R 12.7 13.3 - 9.6 24.7 7.3 42.5 5.0 60.0 S 12.6 9.6 24 10.0 20.8 7.4 41.3 4.3 65.9 T 23.9 9.0 62.5 0++ 100 ND ND ND ND U 26.1 7.3 72.3 O 100 ND ND ND ND V 20.8 15.7** 24.5 O 100 ND ND ND ND W 25.0 17.2** 31.1 O 100 ND ND ND ND X 22.2 4.6 79.0 Oft 100 ND ND ND ND Y 12.9 0 100 Oft 100 ND ND ND ND Z 1 : 24.3 3.0 88.0 Oft 1100 ND ND ND ND ND Notes to Table Ten (a) is as defined in Notes to Table One.

(d) ** measured after 300 hours.

(e) measured after 1000 hours.

Tests were also carried out using a toughened acrylic adhesive supplied in two parts as resin and initiator (F245, obtained from Permabond Adhesives Limited). The polyhydroxyaryl compounds were applied first to the metal strips, followed in turn by the initiator and then the resin. In some experiments the polyhydroxyaryl compound was omitted or was pre-mixed with the resin.

Curing was effected at ambient temperature for 16 hours and then at 500C for four days. The durability tests were effected at 4O0C.

In all cases failure of the bond was adhesive failure. The strength of the bonds decreased rapidly, to less than half the original value after 1000 hours. No improvement compared to the adhesive alone could be seen.

Claims (17)

1. A process which comprises coating a metal surface with a polyhydroxyaryl compound of the formula

and also with a hot-curing epoxy composition and heating to a temperature of at least 1000C to effect curing of the epoxy composition, wherein each X is independently an -OH group or a group -ROH and at least one of the groups X is an -OH group; Y is a hydrogen atom, a group R , a group COR or a group NR3R4 R is a divalent hydrocarbon or substituted hydrocarbon group; R1 is a hydrocarbyl or substituted hydrocarbyl group; R is a group R1 OR3 34 3 R3 is a hydrogen atom, a hydrocarbyl or a substituted hydrocarbyl group; R4 is a hydrogen atom, a hydrocarbyl or a substituted hydrocarbyl group; and 3 4 R3 and R4 can be the same or different.

2. A process as claimed in claim 1 wherein the epoxy composition is-an epoxy -adhesive composition.

3. A process as claimed in either claim 1 or claim 2 wherein all of the groups X are -OH groups.

4. A process as claimed in any one of claims 1 to 3 wherein Y 2 2 3 34 is a group COR in which R is either OR3 or NR3R4.

5. A process as claimed in any one of claims 1 to 4 wherein the polyhydroxyaryl compound is pyrogallol, gallic acid, methyl gallate, propyl gallate, dodecyl gallate, gallamide, a gallamide derivative or an aminoalkyl pyrogallol derivative.

6. A process as claimed in any one of claims 1 to 5 wherein the epoxy composition is cured at a temperature of at least 1200C.

7. A process as claimed in claim 6 wherein the epoxy composition is cured at a temperature of at least 1500C.

8. A process as claimed in claim 7 wherein the epoxy composition is cured at a temperature of 180 + 100C.

9. A process as claimed in any one of claims 1 to 8 wherein a steel surface is coated.

10. A process as claimed in claim 2 wherein the surfaces of two pieces of metal are coated and the pieces of metal are bonded together by contacting the coated surfaces under conditions to form an adhesive bond.

11. A process as claimed in any one of claims 2 to 10 wherein the metal surface is coated at least once with the polyhydroxyaryl compound and thereafter is coated with an epoxy adhesive composition.

12. A process as claimed in any one of claims 2 to 10 wherein the metal surface is coated with a mixture of the polyhydroxyaryl compound and an epoxy adhesive composition.

13. A process as claimed in claim 12 wherein the mixture is obtained by a process including a high shear mixing step.

14. A metal having on at least part of one surface thereof a coating of a polyhydroxyaryl compound of the formula

and also of a hot cure epoxy composition wherein each X is independently an -OH group or a group -ROH and at least one of the groups X is an -OH group; Y is a hydrogen atom, a group R , a group COR or a group NR3R4 R is a divalent hydrocarbon or substituted hydrocarbon group; R1 is a hydrocarbyl or substituted hydrocarbyl group; R is a group R1 OR3 34 or NR R R3 is a hydrogen atom, a hydrocarbyl or a substituted hydrocarbyl group; R is a hydrogen atom, a hydrocarbyl or a substituted hydrocarbyl group; and R3 and R4 can be the same or different.

15. A metal as claimed in claim 14 wherein the epoxy composition is an epoxy adhesive composition.

16. A metal as claimed in claim 15 wherein the metal is bonded to a further material which is a plastics material, or a metal which may be the same or different metal.

17. A metal bonded to a metal wherein the bond between the metals retains at least half of its initial adhesive strength after being immersed in water at 500C for at least 3000 hours.