GB2100276A - Curable adhesive - Google Patents

Abstract

A curable adhesive, to be prepared before use by mixing the components, which contains a polyisocyanate and/or a prepolymer thereof and a polyol which is a reaction product of an aryl or aralkyl polyol having a molecular weight of 100 to 400 with an oxirane of formula <IMAGE> wherein R<3> is hydrogen or an optionally halogenated hydrocarbon group of 1 to 4 carbon atoms, or a group -CH2OR<4> in which R<4> is an alkyl, alkenyl or aryl group, and wherein the groups R<3> within the polymeric molecule are identical or different and on average 1 to 5 mols of the oxirane are attached per hydroxyl group of the polyol, the proportions of polyol and polyisocyanate being such that 0.95 to 1.25 isocyanate groups of the polyisocyanate are present per one hydroxyl group of the polyol.

Description

SPECIFICATION Curable adhesive The invention relates to a curable adhesive, to be prepared before use from the components by mixing the latter, which contains, as the effective, reactive constituents, at least one polyol and a polyisocyanate and/or a prepolymer thereof. In addition, the adhesive can contain customary additives, such as accelerators, adhesion auxiliaries, pigments and/or fillers.

Curable adhesives based on reaction products of polyether-ols or polyester-ols with polyisocyanates or polyisocyanate prepolymers are known and are described, for example, in Kunststoff-Handbuch ("Plastics Handbook"), Volume Viol, "Polyurethanes", Vieweg-Höchtlen, Carl Hanser Verlag, Munich, 1966, pages 719 et seq.

If polyether-ols are employed as the polyol component, the poor adhesion of these adhesives to plastic surfaces is a disadvantage.

Although reaction products formed from polyester-ols and polyisocyanates or prepolymers thereof adhere better to plazstic surfaces, their adhesive properties deteriorate under the influence of moisture or water.

There has therefore been a requirement to improve curable, two-component adhesives based on a polyol and a polyisocyanate or prepolymers thereof in such a way that the good adhesive properties are retained even under the influence of moisture or water, but, at the same time, good adhesion to plastic surfaces is also achieved or retained. The curing temperature and the curing rate of the polyolisocyanate adhesive should, as far as possible, not be affected thereby.

The required advantageous technical properties in use can be achieved in accordance with the adhesive of the present invention which contains, as the polyol, a reaction product of an aryl or aralkyl polyol having a molecular weight of 1 00 to 400 with an oxirane of the general.formula

wherein R3 is a hydrogen radical or an optionally halogenated hydrocarbon group having 1 to 4 carbon atoms, or the group -CH20R4 in which R4 denotes an alkyl, alkenyl or aryl group, and wherein the groups R3 within the polymeric molecule are identical or different, on average 1 to 5 mols of the oxirane having been attached per hydroxyl group of the polyol, and the polyol and the polyisocyanate being present in a ration such that 0.95 to 1.25 isocyanate groups of the polyisocyanate correspond to one hydroxyl group of the polyol.

A preferred adhesive according to the invention contains, as the polyol, the reaction product of a compound of the general formula R'(Cn2OH)mx wherein R1 is a radical derived from benzene, naphthalene or anthracene, R2 is a diva lent aliphatic hydrocarbon radical having 1 to 4 carbon atoms, n is O or 1 and m is 2 or 3, with an oxirane. The divalent aliphatic hydrocarbon radical R2 can be a methylene, ethylene, propylene or butylene radical, and the propylene and butylene radical can also be in a branched form.

If the index n is 0, these products are aryl polyols; if the index n is 1, they are aralkyl polyols. The following are examples of compounds of the general formula I R1(R2n0H)m resorcinol, pyrogallol, phloroglucinol, 1 ,4-bis-(hydroxymethyl)-benzene, hydroquinone, 2,7 dihydroxynaphthalene, 2,6-dihydroxynaphthalene, 1 ,8-dihydroxynaphthalene and 2,6 dihydroxyanthracene.

The adhesives according to the invention can also contain, as the polyols, the reaction product of a compound of the general formula

wherein Z is q-valent aliphatic hydrocarbon radical having 1 to 6 carbon atoms, p is 1 or 2, q is 2 or 3 and R2 and n are as defined above, with an oxirane.

The radical Z is q-valent aliphatic hydrocarbon radical having 1 to 6 carbon atoms, for example the radical

The following are examples of polyols of the general formula II which, having been reacted with an oxirane, are present in the adhesive according to the invention: 2,2-bis-(4-hydroxyphenyl)-propane, bis (4-hydroxyphenyl)-methane, 1,1 ,2-tris-(4-hydroxyphenyl)-ethane, 1,1 ,3-tris-(4-hydroxyphenyl)propane, bis-(2-hydroxyphenyl)-ethane or 2,2-bis-(4-hydroxymethylphenyl)-propane.

Suitable polyols which, having been reacted with an oxirane, are present in the curable adhesives according to the invention, are also compounds of the biphenyl type of the general formula

In this formula, the radical R2 denotes a divalent aliphatic hydrocarbon radical having 1 to 4 carbon atoms, as already described in the case of formula I. The index n is O or 1 and the index p is 1 or 2. The following are examples of suitable compounds: 2,2'-dihydroxybiphenyl, 4,4'-dihydroxybiphenyl, 2,2',4'trihydroxybiphenyl and 2,2'-bis-(hydroxymethyl)-biphenyl.

The aromatic rings present in the formulae I to III can, in addition, be substituted by alkyl groups containing 1 to 4 carbon atoms. The following are examples of such compounds: methylphloroglucinol, 2,5,6-trimethylresorcinol, 4-ethyl-5,6-dimethylresorcinol, 4-propyl-2,7-dihydroxynaphthalene or 4,6-ditert.-butylpyrocatechol.

Particularly advantageous results in respect of adhesion of the adhesives to plastic surfaces and in respect of behaviour towards the action of moisture and water are exhibited by adhesives of the type according to the invention if they contain, as the polyol, the reaction product of an aryl or aralkyl polyol with 1.5 to 3 mols of an oxirane.

Compounds of general formula

can be used as the oxiranes.

The group R3 can be a hydrogen radical. The oxirane is then ethylene oxide. The group R3 can be a hydrocarbon, e.g. alkyl, group having 1 to 4 carbon atoms. R3 is preferably an alkyl group having 1 to 4, particularly 1 or 2, carbon atoms. If R3 is methyl group, the oxirane is propylene oxide. The alkyl group can optionally be halogenated and can contain, for example, chlorine radicals. An example of a halogenated group R3 of this type is the CH2CI group and the CH2-CCl3 group. The hydrocarbon group can also be unsaturated. An example of an unsaturated group of this type is the vinyl group or the allyl group.

The group R3 can also denote the group -CH2-0-R4, wherein R4 is an alkyl, alkenyl or aryl group. An example of a suitable alkyl group is the butyl group; examples of suitable alkenyl groups are the vinyl and allyl groups; examples of suitable aryl groups are phenyl, chlorophenyi and nitrophenyl.

In addition to ethylene oxide and propylene oxide which have aiready been mentioned, the following compounds, for example, can be used as the oxiranes: butylene oxide; isobutylene oxide; epichlorohydrin; epibromohydrin; trifluoromethylethylene oxide; 1 ,2-dichloro-3,4-epoxybutene; 1,1 dichloro-2,3-epoxybutane; 1,1,1 -trichloro-3,4-epoxybutane; methyl glycidyl ether; ethyl glycidyl ether; isopropyl glycidyl ether; tert.-butyl glycidyl ether; n-hexyl glycidyl ether; ethylhexyl glycidyl ether; phenyl glycidyl ether; chlorophenyl glycidyl ether; nitrophenyl glycidyl ether; vinyl glycidyl ether; allyl glycidyl ether and butadient monoxide.

Within the polymeric molecule, the groups R3 can have identical or different meanings.

Propylene oxide is preferably used as the oxirane.

The addition reaction catalysts used are the catalysts known from the state of the art, in particular basic catalysts, such as, sodium methylate, sodium hydroxide, potassium hydroxide or calcium hydroxide, and also acid catalysts, such as, boron trifluoride-etherate.

Up to 85% by weight of the polyol obtained by reacting an aryl or aralkyl polyol with an oxirane can be replaced by other polyhydroxy compounds. Polyhydroxy compounds of this type are difunctional or polyfunctional aliphatic polyalkylene oxide derivatives, such as are obtained, for example, by an addition reaction of oxiranes with aliphatic alcohols having at least two hydroxyl groups. It is also possible to use the polyester-ols which are in themselves known, and also castor oil. It is also possible to use low-molecular polyacrylate polymers or copolymers having at least two hydroxyl groups. The addition of these compounds makes it possible to modify the properties of the adhesives in the manner desired and to make them, for example, more flexible or to reduce their viscosity.

The polyisocyanates used can, quite generally, be those which are known from the state of the art It is preferable to use polyisocyanates of the following formula

wherein r ~ 3 or

wherein R5 denotes the radical

These isocyanates are commercially available.However, other polyisocyanates are also suitable, for example: 1 ,6-hexamethylene diisocyanate; 2,4,4-trimethyl-1 ,6-hexamethylene diisocyanate; 3 isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate; 1 ,4-tetra methylene diisocyanate; 2,4hexahydrotoluylene dissocyanate and 2,6-hexahydrotoluylene diisocyanate; hexahydro-1 ,3-phenylene diisocyanate and hexahydro-1,4-phenylene diisocyanate; 1,3-phenylene diisocyanate and 1,4phenylene diisocyanate; 2,4-toluylene diisocyanate and 2,6-toluylene diisocyanate; 4,4'diisocyanatodiphenylmethane; 1 ,5-naphthylene diisocyanate; m-xylylene diisocyanate; tris-(4isocyanatophenyl) thiophosphate; 4,4' ,4"-triisocyanatotriphenylmethane; 2,4,6-triisocyanatotoluene or 2,4,4'-triisocyanatodiphenyl ether.The isocyanates must, however, fulfil the condition that they contain on average at least two isocyanate groups per molecule.

The prepolymers of the isocyanates which are used can be the partial reaction products of the latter with polyether-polyols or polyester-polyols having a molecular weight < 2,000, the polyether polyols or polyester-polyols containing at least two hydroxyl groups in the average molecule, and the polyisocyanates being reacted with these polyols in a ratio such that 1.5 to 3 isocyanate groups correspond to one hydroxyl group.

Examples of suitable polyether-ols are linear or branched polyether-ols based on ethylene oxide, propylene oxide and butylene oxide, polythioethers and products from the addition reaction of ethylene oxides with polyamines and alkoxylated phosphoric acids.

Examples of suitable polyester-ols are linear or branched polyester-ols such as are obtained in a customary manner from polybasic, preferably dibasic, carboxylic acids, such as adipic acid, sebacic acid, phthalic acid, halogenated phthalic acids, maleic acid and benzene-1 ,2,4-tricarboxylic acid, monomeric, dim erin or trimeric fatty acids and polyhydric alcohols, such as ethylene glycol, polyethylene glycol, propylene glycol, polypropylene glycols, 1,3-butanediol, 1,4-butanediol, 2,2-dimethyl-1 ,3-propanediol, 1 ,6-hexanediol, 1,1,1 -trimethylolpropane, hexanetriols or glycerol.

The reaction of the polyisocyanates with the polyether-polyols or polyester-polyols is therefore effected in a ratio such that on average at least two isocyanate groups are present per molecule in the reaction product.

The curing time of the adhesive can be substantially shortened, and/or its curing temperature can be substantially lowered, in a manner which is in itself known, by adding an accelerator. A particularly preferred adhesive is, therefore, an adhesive which contains, as the accelerator, up to 2% by weight, relative to the total of polyols and polyisocyanates, of a tertiary amine and/or an organo-tin compound.

Tertiary amines which are particularly suitable are dimethylbenzylamine, dicyclohexylmethylamine, dimethylpiperazine, dimethylaminoethanol, 1 2-dimethylimidazole, N-methylmorpholine, Nethylmorpholine, dimethylcyclohexylamine, 1 -azo-bicyclo(3,3,0)-octane or 1 ,4-diaza-bicyclo-(2,2,2)octane. Examples of organo-tin compounds which can be used with advantage are dibutyl-tin dilaurate ortin(ii) octoate.

It is also possible to add pigments and/or fillers to the adhesive as auxiliaries. Both inorganic and organic products are suitable for this purpose, for example, titanium dioxide, iron oxide, chromium oxide, barium sulphate, quartz powder, talc, calcium carbonate, Neuburg silica or bentonite.

The adhesives according to the invention cure at temperatures as low as room temperature or a moderately elevated temperature. Since the adhesives are liquid, the use of a solvent is not necessary.

However, if it is desirable for the preparation to have a very low viscosity, it is possible to use inert solvents, such as, lower ketones, ethers or esters, or aromatic solvents.

The adhesives according to the invention can be processed easily; since the polyol component present in accordance with the invention is liquid. The reaction with the polyisocyanates takes place at temperatures as low as room temperature apd can be accelerated by warming. The adhesives have a good adhesion to a very wide variety of substrates, such as plastic surfaces, metals, wood or silicate surfaces. The adhesive properties are also not influenced, or only to an insignificant extent, by the action of water or moisture.

The adhesives according to the invention can be used with particular advantage in cases where the glued joints are intended to have a particularly good peeling resistance which is only reduced to a slight extent, or not at all, by the action of water.

In the Example which follows, the composition and properties of the adhesives according to the invention are described in greater detail. All the quantity data in the Example relate to parts by weight.

EXAMPLE Preparation of the polyols to be used in accordance with the invention.

50 g of pyrogallol were dissolved in 140 g of propylene oxide, 0.42 g of potassium methylate were added and the mixture heated at a temperature of 1 250C for 1.5 hours in an autoclave. The temperature was maintained until virtually the whole of the propylene oxide has undergone an addition reaction and the pressure in the reaction vessel had decreased again. This was the case after about 3 hours. After cooling, small quantities of unreacted propylene oxide were removed in vacuo at 800 C, the residue was neutralised with hydrochloric acid and the salt formed was removed.The dried polyol, which was obtained in a yield of virtually 100%, had a hydroxyl number of 365 mg of KOH/g and an average molecular weight of 465, determined by measurement in a vapour pressure osmometer. The polyol thus contained, on average, 2 mols of propylene oxide attached per mol of hydroxyl group of the pyrogallol. The viscosity of the polyol was 7,000 mPas at 250C.

Further polyols, which are described in Table 1, were prepared in a simiiar manner from other polyhydroxy compounds (= starting polyols) and propylene oxide or other oxiranes. (In this connection, the reaction with the oxiranes could also be carried out in suitable inert solvents, such as, ethylene glycol dimethyl ether or dioxane.

Preparation of the adhesives according to the invention and testing their adhesive properties a) Preparation of the adhesives from polyols and polyisocyanates.

The adhesives according to the invention were prepared by mixing the polyols with various polyisocyanates in the mixing ratio indicated in Table 2. (As a rule, the polyols are used in a solvent-free form. If the adhesives are required to have low processing viscosities, it is possible to employ inert solvents, such as esters, ketones or aromatic compounds, as diluents.) Four commercially available products were used as the polyisocyanates: b1) A polyphenyl-polymethylene polyisocyanate (crude MDI) of the general formula

b2) A triisocyanate of the formula

both of these polyisocyantes b1 and b2 were used in a solvent-free form.) b3) A polyisocyanate of the formula

wherein R5 denotes the radical

employed in the form of a 75% strength by weight solution in ethyl acetate.

Furthermore, a 60% strength by weight solution in methyl ethyl ketone of a reaction product formed from 3 mols of 2,4-toluylene diisocyanate and 1 mol of a trifunctional polyester-ol having a molecular weight of 600 and obtained from adipic acid, ethylene glycol, 2,2-dimethyl-1 ,3-propanediol and trimethylolpropane, was employed under the designation b4.

The content of free isocyanate groups, relative to the solid, was as follows: Polyisocyanate b, 31.2% by weight Polyisocyanate b2 22.7% by weight Polyisocyanate b3 13.0% by weight Polyisocyanate b4 1 1.2% by weight Further additives, such as accelerators, adhesion auxiliaries, pigments and fillers, can be dissolved or dispersed, it being advantageous to add these additives to the polyol before the latter is mixed with the polyisocyanate.

The exact formulations of the adhesives can be seen in Table 2. The formulations of Nos. 1 to 11 describe the adhesives according to the invention. Comparison adhesives then follow under formulations Nos. 12 to 13.

Comparison adhesive No. 1 2 contains a difunctional polyether-ol based on propylene oxide and ethylene oxide and having a hydroxyl number of 90 mg of KOH/g, instead of the polyols according to the invention.

Comparison adhesive No. 13 contains a polyester-ol having a functionality of approximately 2.5 and a hydroxyl number of 109 mg of KOH/g, instead of the poly-ols to be used in accordance with the invention. The polyester was obtained by subjecting adipic acid to a condensation reaction with diethylene glycol, ethylene glycol and trimethylolpropane.

b) Testing the adhesive properties The drum peel-strength test, as specified in DIN 53,295, is used to test the technical properties in use of the adhesives. The drum peel-strength is determined on two different bonds: An aluminium/polyethylene (PE) bond An aluminium/epoxide resin glass-fibre laminate (GRP) bond The aluminium sheets are of grade Al Cu Mg 2pl and are degreased and subjected to a chromate/sulphuric acid pickling process before being glued. The polyethylene is roughened and pretreated, and the epoxide glass-fibre laminate (GRP) is ground.

The adhesives are applied to the surface which are to be made to adhere, at a rate of 80 g/m2 of solid, and the solvents, if present, are evaporated off at room temperature or at an elevated temperature, for example 700C. The curing of the adhesives is effected at 90 C for 20 minutes under an applied pressure of 0.5 N/mm2. Peeling is carried out at 2O0C, after cooling.

In addition, storage in hot water at 950C for three days is carried out with the aluminium/GRP test specimens. Before the peeling test, the test specimens are cooled in water at 200 C, and are peeled while still moist. This storage in hot water is very suitable as a short-duration test for testing the resistance of the adhesives to water and moisture.

The strength values obtained are listed in Table 3. It can be seen from the values that comparison adhesive No. 12, which contains a polyether-ol, gives low strength values, because its adhesion is poor.

Although comparison adhesive No. 1 3, based on a polyester-ol, adheres better to polyethylene and glass-fibre laminate, its strength value declines in the hot water test.

The Figure of the accompanying Drawing shows the relationship between the drum peel- strength of the aluminium/polyethylene bond and the quantity of propylene oxide which has undergone an addition reaction with 2,2-bis-(4-hydroxyphenyl)-propane. The polyisocyanate component employed was polyisocyanate b1. It can be seen that high strengths are obtained only within a narrow range of attached propylene oxide.

TABLE 1 Polyols

Polyol Molar amount of oxirane attached per mole of OH number Viscosity at Formulation OH of the starting of the of the polyol No. Starting polyol polyol polyol at 250C (mPas) Adhesives according to the invention 1 Pyrogallol 2.0 Propylene oxide 365 7,000 2 Phloroglucinol 1.7 Propylene oxide 415 84,000 3 1 ,4-bis-(Hydroxymethyl)- benzene 1.5 Propylene oxide 357 220 4 2,2-bis-(4-Hydroxyphenyl)- propane 1.5 Propylene oxide 284 80,000 5 bis-(4-Hydroxyphenyl) methane 1.5 Propylene oxide 295 50,000 6 2,7-Dihydroxynaphthalene 1.7 Propylene oxide 316 14,000 7 2,2'-Dihydroxybiphenyl 1.6 Propylene oxide 304 17,000 8 1 ,4-bis-(Hydroxymethyl)- benzene 2.0 Ethylene oxide 300 200 9 2,2-bis-(4-Hydroxy- 0.8 Ethylene oxide + phenyl)propane 1.0 Propylene oxide 283 60,000 10 2,2'-Dihydroxybisphenyl 1.9 Propylene oxide 280 11,000 11 2,2-bis-(4-Hydroxyphenyl) propane 1.5 Propylene oxide 284 80,000 Adhesives not according to the invention 12 | Polyether-ol 90 3.000 13 - Polyester-ol 109 4,000 TABLE 2 Adhesive formulations

Polyol, Formulation parts by Polyisocyanate, Molar ratio, Other additives, No. weight parts by weight OH:NCO parts by weight 1 100 96 b1 1:1.1 2 100 119 b1 1:1.2 - 3 100 102 b1 1:1.2 20 Titanium dioxide 4 100 78 b, 1:1.15 - 5 100 102 b2 1:1.05 0.02 Dibutyl-tin dilaurate 6 100 94 b1 1:1.1 15 Propylene glycol MW 400 7 100 88 b1 1:1.2 - 8 100 86 b1 1::1.2 - 9 | 100 | 163 b3 | 1:1.0 | 10 100 187 b4 1:1.0 11 100 128b1 1:1.2 100 Castor oil + 200 Calcium carbonate 12 100 26 b1 1:1.2 13 100 31 b1 1:1.2 TABLE 3 Testing the technical properties in use of the adhesives

Drum peel-strength, DIN 53,295 Nmm/mm Al/GRP Adhesive after 3 days' storage formulation No. Al/PE Al/GRP in hot water 1 302 70 62 2 295 68 60 3 285 65 61 4 288 67 62 5 265 - 59 55 6 291 68 61 7 287 63 57 8 275 60 55 9 250 51 45 10 240 49 42 11 305 57 54 Comparison adhesives 12 5 | 3 2 13 145 39 15

Claims (14)

1. A curable adhesive which contains at least one polyol and a polyisocyanate and/or a prepolymer thereof, wherein the polyol is a reaction product of an aryl or aralkyl polyol having a molecular weight of 100 to 400 with an oxirane of formula

wherein R3 is hydrogen, an optionally halogenated hydrocarbon group of 1 to 4 carbon atoms or a group of formula -CH2OR4 in which R4 is an alkyl, alkenyl or acryl group, and wherein the groups R3 within the polymeric molecule are identical or different, and on average 1 to 5 mols of the oxirane are attached per hydroxyl group of the polyol, the proportions of polyol and polyisocyanate being such that 0.95 to 1.25 isocyanate groups of the polyisocyanate are present per one hydroxyl group of the polyol.

2. An adhesive according to claim 1, wherein the polyol is the reaction product of a compound of formula R1(Rn2OH)rn, wherein R' is a radical derived from benzene, naphthalene or anthracene, R2 is a divalent aliphatic hydrocarbon radical of 1 to 4 carbon atoms, n is O or 1 and m is 2 or 3, with an oxirane.

3. An adhesive according to claim 1 wherein the polyol is the reaction product of a compound of formula

wherein Z is a q-valent aliphatic hydrocarbon radical of 1 to 6 carbon atoms, p is 1 or 2, q is 2 or 3 and R2 and n are as defined in claim 2, with an oxirane.

4. An adhesive according to claim 1 wherein the polyol is the reaction product of a compound of formula

wherein R2 and n are as defined in claim 2, and p is 1 or 2, with an oxirane.

5. An adhesive according to claim 1 wherein the polyol is as defined in claim 2, 3 or 4 with the modification that the aromatic ring(s) in the polyol is/are additionally substituted by one or more alkyl groups of 1 to 4 carbon atoms.

6. An adhesive according to any one of the preceding claims, wherein on average 1.5 to 3 mols of the oxirane are attached per hydroxyl group of the polyol.

7. An adhesive according to any one of the preceding claims, wherein the polyol is a reaction product of an aryl or aralkyl polyol with propylene oxide.

8. An adhesive according to any one of the preceding claims in which up to 85% by weight of the poiyol has been replaced by one or more difunctional or polyfunctional aliphatic polyalkylene oxide derivatives, polyester-ols, caster oil or low-molecular polyacrylates containing at least two hydroxyl groups.

9. An adhesive according to any one of the preceding claims, wherein the polyisocyanate is of formula

wherein r < 3 or of formula

or of formula

wherein R5 is

10. An adhesive according to any one of the preceding claims wherein the polyisocyanate prepolymer is a partial reaction product of one or more polyisocyanates with a polyether-polyol or polyester-polyol having a molecular weight < 2,000, the polyetherpolyol or polyester-polyol containing an average of at least two hydroxyl groups per molecule, and the polyisocyanate being reacted with the polyol in a ratio such that 1.5 to 3 isocyanate groups are present per one hydroxyl group.

11. An adhesive according to any one of the preceding claims which also contains one or more additives selected from accelerators adhesion auxiliaries, pigments and fillers.

12. An adhesive according to claim 11 which contains, as an accelerator, up to 2% by weight, based on the total weight of the polyols and polyisocyanates, of a teriary amine and/or an organo-tin compound.

13. An adhesive according to claim 1 substantially as hereinbefore described with reference to the Example.

14. A two-component pack comprising in a first container, at least one polyol as defined in any one of claims 1 to 7 and, in a second container, a polyisocyanate and/or a prepolymer thereof as defined in claim 1, 9 or 10, the contents of the first and second containers being capable, when mixed, of forming a curable adhesive as claimed in any one of the preceding claims.