GB2138016A - Adhesive bonding - Google Patents

Abstract

Improvement of green strength of adhesive bonds formed to plasticised polyvinyl chloride using moisture curable isocyanate bearing polyurethane prepolymer adhesives is achieved by use of selected primer compositions. The primer compositions comprise an organic tertiary amino compound having in its molecule at least one saturated heterocyclic ring containing not less than two tertiary nitrogen atoms. Preferred examples are triethylene diamine and hexamethylene tetramine. Primers according to the invention are of particular interest in the field of bonding PVC soles to shoes.

Description

SPECIFICATION Adhesive bonding Adhesive compositions based on isocyanate terminated prepolymers have been proposed and their use in various bonding operations has also been proposed including attaching soles to shoes. In our GB Patent Specification No. 1540634, there is described and claimed a method of adhesively bonding a shoe sole to a shoe upper comprising the steps of providing on attaching surfaces of one or both of the sole and upper a layer of a fluid composition comprising a urethane prepolymer from reaction of a polyol having a crystalline melting point from 40"C to 90"C with a polyisocyanate in amounts to provide a ratio of reactive isocyanate groups to reactive hydroxyl groups from 1.25:1 to 3.0::1 and the urethane prepolymer being such that it may be chain extended to provide a polyurethane which is tacky and capable of adhesive union when heated, bringing a compound having active hydrogen atoms into contact with the urethane prepolymer for reaction with NCO groups of the composition to bring about chain extension of the urethane prepolymer to provide said polyurethane, and heating the composition to bring it to tacky adhesive condition, pressing the attaching surfaces of the sole and upper together with the composition between them and bringing about cooling of the composition to form an adhesive bond between the sole and the upper.

In a method according to said specification, the fluid composition comprising urethane prepolymer is in a fluid form for application to the surface with the fluidity needed for wetting adhesive engagement with the surface. This composition may have the necessary fluid form as a result of heating to a relatively low temperature which is nevertheless somewhat above the crystalline melting point of the prepolymer, or the composition may comprise a solution of the prepolymer dissolved in an inert organic solvent which is removed for example by evaporation prior to the chain extension reaction. The term "crystalline melting point" where used herein indicates the temperature at which crystalline segments of the polymer melt, and is determined as the temperature of the major endotherm peak in a differential thermal analysis.When the polymer is brought above its crystalline melting point in application or activation, the crystalline segments melt and cause softening of the polymer. After application, the urethane prepolymer is preferably brought below its crystalline melting point and is allowed to crystallise to develop a resistance to flow and distortion of the layer at temperatures used in the succeeding step of chain extension. In those cases where the fluid composition comprises a solution of the prepolymer in an inert volatile solvent, it may be applied at room temperature or at slightly elevated temperatures. Solvents which may be used are volatile organic liquids which do not contain active hydrogen. Among those which may be employed are xylene, toluene, dimethyl formamide, acetone, methyl ethyl ketone, ethyl acetate, cellulose acetate, methylene chloride and mixtures of these.Particularly useful solvents include mixtures of toluene with up to about 85% by weight based on the weight of the solvent mixture of methylene chloride or methyl ethyl ketone. Because of the nature of the urethane prepolymers employed in the method, relatively high solids content solutions, for example 60% by weight and higher, have viscosities low enough for application in wetting engagement with surfaces. Also, small percentages of solvents may be useful where it is desired to apply the urethane prepolymer at moderately elevated temperatures below temperatures needed to melt the urethane prepolymer in the absence of solvents.

Urethane prepolymers used in the process of UK Patent Specification No 1 540634 include--NCO terminated prepolymers from reaction of a polyol having a crystalline melting point in the range of from 40"C to 90"C, with an excess of diisocyanate. Suitable polyols include the hydroxyl terminated polycaprolactones and polyesters of 6 to 1 2 carbon atom aliphatic dicarboxylic acids such as sebacic, adipic, azelaic, suberic and dodecanedioic acids with, preferably even numbered, glycols having from 2 to 6 carbon atom chains such as 1,4butanediol.The acid component of the polester polyol may include from 5% to 25% on a molar basis of cycloaliphatic acid such at 1 ,4-cyclohexane dicarboxylic acid or 1,2-cyclohexane dicarboxylic acid to promote adhesion particularly to resin-rubber surfaces and from 5% to 25% on a molar basis of aromatic dicarboxylic acid such as terephthalic acid or isophthalic acid to improve resistance to plasticisers. The glycol component may include from 5% to 15% on a molar basis of diethylene glycol or cycloaliphatic glycols such as 1 ,4-cyclohexane dimethanol to improve tack and wetting properties and give better reactivity with water vapour.These polyols should have a molecular weight in the range from 1 ,000 to 10,000 and preferably from 2,000 to 4,000, and preferably have a crystalline melting point in the range from 40"C to 65"C. The urethane prepolymer for application to the surface to be bonded is made by reaction of one or more of the above polyols with a diisocyanate in proportions to give an -NCO to -OH ratio in a range from 1.25:1 to 3.0:1, preferably from 1.5:1 to 2.5:1. Any of the available diisocyanates may be used including tolylene diisocyanates, diphenylmethane diisocyanates, and dicyclohexyl methane diisocyanates.

It has been found that urethane prepolymers formed using diphenylmethane diisocyanate, react with water much more rapidly than do urethane prepolymers from reaction using tolylene diisocyanate, but that the latter urethane prepolymers are more stable on storage. Improved storage stability coupled with rapid reaction rate may be obtained by use of a combination of both diisocyanates. Because of the differences in rate of reaction, it is preferred to add the tolylene diisocyanate first and allow it to react for a limited time which enables the -NCO group in the para position to react with the polyol, after which diphenylmethane diisocyanate may be added. Useful ratios of thlese diisocyanates may be from equal parts on a molar basis to 80 moles of diphenylmethane diisocyanate with 20 moles of tolylene diisocyanate.

From the foregoing, it will be apparent that the urethane prepolymer is of comparatively low molecular weight and has crystalline segments in the polymer molecule having a relatively low crystalline melting point, and can be applied on a surface at a relatively low temperature at which it is freely fluid for entering wetting adhesive engagement with the surface. Reaction of the diisocyanate with the polyol has been found to yield a urethane prepolymer with a crystalline melting point only a few degrees (i.e. of the order of 5"C) above the crystalline melting point of the polyol itself.For example, a butanediol sebacate polyol may have a crystalline melting point of 49"C, and the polyurethane from reaction of this polyol with diphenylmethane diisocyanate in a ratio of 1.5 -NCO to 1 -OH may have a crystalline melting point of only 54"C.

In a method according to said patent specification, the step of chain extension of the urethane prepolymer may preferably be carried out at warm or moderately elevated temperatures with a chain extending agent to bring the adhesive to a tough flow resistant, but heat softenable condition. The chain extension treatment involves exposing the surface of the adhesive coating to a compound having two active hydrogens for reaction with the -NCO groups of the urethane prepolymer. A preferred chain extending agent is water, preferably in the form of vapour or steam, for example, in a high humidity room or in proximity to a steam or vapour source. Other chain extending agents such as diamines or glycols may also be used.

The chain extension reaction should be carried to an extent at which the film becomes elastoplastic on heat activation, but not markedly cross-linked, i.e. the film is still soluble or attacked by active polyurethane solvents such as dimethyl formamide or tetrahydrofuran. This chain extended polymeric polyurethane has a crystalline melting point which is substantially unchanged from that of the original urethane prepolymer, so that when heated above the crystalline melting point of the polymeric polyurethane the chain extended compound becomes viscoelastic, namely, somewhat rubbery, but deformable and flowable under pressure and is tacky and capable of adhesive union with a compatible surface.

By reason of the viscoelastic condition and the somewhat rubbery physical state of the chain extended composition at temperatures above the crystalline melting point of the polyurethane, bonds established by contact of the adhesive surface with another surface have a very high initial bond strength so that in assembly of a shoe sole and a shoe upper, tendency of the sole to separate from the upper because of springiness of the sole is minimised.

One example of adhesive which may be used in a manner as disclosed in UK Patent Specification No 1 540634 is available from Bostik GmbH under the name Bostik 7801 and comprises a reaction product of a poly(hexane diol adipate) with toluene diisocyanate and 4,4' diphenylmethane diisocyanate in presence of an organo tin catalyst to an NCO?OH ratio of 1.8.

It has been found that this proposed adhesive has properties which render it capable of use in the attachment of soles to shoes by a process in which adhesive is applied to the sole and the shoe upper, and the adhesive on the sole is allowed to undergo chain extension under room temperature conditions and then heated to bond forming condition whereas the adhesive applied to the shoe upper is subjected to chain extension by use of a heated humidity chamber. Good bond strengths to many materials have been achieved in this way. However, "green strength" of bonds formed between leather upper materials and soling materials of plasticised polyvinyl chloride has been shown in laboratory tests to be less perfect than is desirable for normal shoemaking conditions. By green strength is meant the peel bond strength measured 1 5 seconds after pressing to form the bond.

It is one object of the present invention to provide for improved green strength of bonds formed to plasticised polyvinyl chloride using adhesive compositions comprising selected urethane polymers having isocyanate groups available for reaction.

The invention provides in one of its aspects a method of forming adhesive bonds between surfaces one at least of which comprises plasticised polyvinyl chloride using an adhesive composition comprising a urethane polymer having isocyanate groups available for reaction, and capable of polymerisation by atmospheric moisture to provide a polyurethane which is tacky and capable of adhesive union when heated, the method comprising applying to the PVC surface a primer composition comprising an organic tertiary amino compound having in its molecule at least one saturated heterocyclic ring containing not less than two tertiary nitrogen atoms to provide a primer coat thereon, applying the adhesive composition onto the primer coat, subjecting the adhesive composition to a moisture treatment to bring it to a condition in which when heated it is tacky and capable of adhesive union, heating the adhesive coating and bringing the surfaces together with the adhesive between them in heated condition.

In a method according to the present invention, the adhesive composition comprises a urethane polymer having isocyanate groups available for reaction, and capable of polymerisation by atmospheric moisture to provide a polyurethane which is tacky and capable of adhesive union when heated. Examples of suitable adhesives include those disclosed in GB 1 540634 and Bostik 7801.

For sole attaching purposes, the adhesive composition preferably comprises a polyurethane which is a prepolymer formed from reactants comprising aromatic diisocyanate and substantially linear hydroxy polyester having a molecular weight in the range 1,500 to 4,500. Preferably the prepolymer is further reacted with a mono functional reactant in an amount sufficient to react with up to 40 mol % of the NCO groups of the prepolymer. Preferably, the adhesive composition is applied as a solvent free material, in molten form.

In a method according to the invention, the primer composition comprises an organic tertiary amino compound having in its molecule at least one saturated heterocyclic ring containing not less than than two tertiary nitrogen atoms. Preferred organic tertiary amino compounds have molecules in which each nitrogen atom has three valencies satisfied by different carbon atoms.

Examples of such materials are 1,4-dimethyl piperazine, triethylene diamine and hexamethylene tetramine. The latter two compounds are particularly favoured; in these two compounds each nitrogen atom has three valencies satisfied by different carbon atoms which are also members of a saturated heterocyclic ring system. These compounds also do not have groups available for reaction with the free isocyanate groups of the prepolymer.

By use of the selected primer composition in a method according to the invention, we have found that improved green strength of adhesive bonds to plasticised PVC may be achieved.

Attempts to achieve comparable improvements by use of primers based on other amino compounds in conjunction with Bostik 7801 have been unsuccessful. Examples of materials which have been found not to be suitable for use as primers are triethylamine, imidazole, 1methyl imidazole, 4-methyl morpholine, 2, 4, 6 di(methoxy methyl) amino 1, 3, 5 triazine, pyrazine and hexamethylene dinitrile. Whilst not wishing to be bound by any particular theory, it is believed that the favourable properties of 1,4-dimethyl piperazine, and especially triethylene diamine and hexamethylene tetramine are due to outwardly directed lone pairs of electrons associated with the nitrogen atoms.It is noted that with the latter two compounds, tertiary nitrogen atoms are located in the molecules in such a way as to preclude oscillation of the nitrogen atoms, and thus their associated lone pairs of electrons remain outwardly directed with respect to the molecule.

The primer composition for use in a method according to the invention is preferably in the form of a solution of dispersion. Suitable solvents include for example acetone, methyl ethyl ketone, and ethyl acetate each of which may be diluted with substantial amounts of water if desired. Extremely dilute solutions are effective, and may contain as little as about 0.5 to 5% by weight of the preferred organic nitrogen compound.

The invention provides in another of its aspects a primer composition for use in conjunction with an adhesive composition having reactive isocyanate groups for bonding plasticised PVC, comprising an organic tertiary amino compound having in its molecule at least one saturated heterocyclic ring containing not less than two tertiary nitrogen atoms and a solvent therefore.

A method according to the invention is particularly intended to be employed in bonding shoe soles of plasticised polyvinyl chloride to shoe uppers of leather or other materials. However, other materials may be bonded and other bonding operations may be carried out using a method according to the invention.

When bonding soles of plasticised polyvinyl chloride to leather shoe uppers by a method according to the invention, the primer composition is applied to the sole e.g. by brush or sponge applicator or more preferably by wiping with a cloth, to provide a thin primer coat thereon.

Much of the solvent is removed by the wiping or brushing action. After evaporation of the remaining solvent, the adhesive is applied onto the primer coat, e.g. as a hot melt. A similar coating of the same adhesive is preferably applied to the shoe upper. The adhesive layers are subjected to moisture treatment to effect chain extension, and one or both of the layers is brought to a condition in which it is tacky and capable of adhesive union. The sole and upper are then spotted together and pressed to complete the bond.

In order that the invention may become more clear there now follows a description of seven illustrative primer compositions and an illustrative method of forming adhesive bonds, according to the invention. It is to be understood that these illustrative primer compositions and method are selected for description to illustrate the invention by way of example only.

In order to indicate beneficial effects achieved by use of the illustrative primers, sample bonds were formed using various primers and adhesives. In the following description, the following identifications for adhesives are used. Adhesives 1 and 2 were employed for comparative purposes to show the order of green strength values achieved with commercially used adhesives.

Adhesive 1 was a one part polyurethane solvent based adhesive available to the shoe manufacturing trade in Germany as Bostik 2828, and used for attaching soles to shoes. This adhesive is an isocyanate free polyurethane which yields strong bonds of good green strength.

Adhesive 2 was a single part polyurethane solvent based adhesive available to the shoe manufacturing trade in Great Britain as Bostik 11 50. This adhesive is also an isocyanate free polyurethane which gives strong bonds of good green strength.

Adhesive 3 was a single part, moisture curable polyurethane adhesive available in Germany as Bostik 7801.

Adhesive 4 was a single part, moisture curable polyurethane adhesive made from poly(butane diol adipate) (molecular weight about 3,000) and 4,4' diphenylmethane diisocyanate using dibutyl tin dilaurate catalyst, to produce a prepolymer having about 1.6% NCO groups available for reaction (i.e. NCO:OH ratio 1.8:1). This prepolymer was reacted with sufficient 2 ethyl hexanol to react with 10% of the available NCO groups. The prepolymer, in admixture with about 4% coumarone indene resin was stored under moisture free conditions until required for use.

Adhesive 5 was similar to Adhesive 4, but was made using an organo tin mercaptide catalyst.

The illustrative primers each comprised a solution of tertiary amino compound having in its molecule at least 1 saturated heterocyclic ring containing not less than two tertiary nitrogen atoms in a solvent.

The illustrative primer compositions comprised ingredients in amounts by weight as shown in Table 1.

The primer compositions were prepared by dissoiving the tertiary amino compound in the solvent.

Peel tests and green strength tests were carried out on sample adhesive bonds using the adhesives as described above both with and without use of the illustrative primer compositions.

The results are shown in Tables 2 to 4. Each of the samples comprised a 30mm x 100mm strip of roughed upper leather and a strip (30mm X 100mm) of plasticised polyvinyl chloride soling material (PVC). The test materials and test methods used were according to FEICA recommendations.

Example 1 A sample unprimed adhesive bond was formed by applying adhesive 3 at 100"C to the PVC surface to be bonded. The adhesive was moisture-cured for 24 hours at room temperature and humidity (22"C + 2"C and greater than 10% RH). After this exposure of the upper leather surface and after cooling subjected to airborne moisture in a humidity chamber (90"C 26% RH).

After a residence time of 10 to 1 5 minutes, the sample strips were removed from the humidity chamber and allowed to cool at room conditions (20"C + 2"C and not less than 10% RH) for about 1 5 minutes. The adhesive layer on the soling material was exposed to an infra-red heater (USM Model DVHG F) set at 85"C to 90"C with the layers and heater elements spaced approximately 45 mm apart for 4 to 5 seconds. The soling strip was then pressed against the upper leather strip under normal sole pressing conditions of 0.5 N/mm2 fo 12 seconds. The primed samples were formed in a similar way. However, before applying the prepolymer to the PVC surface, it was wiped with a cloth impregnated with a solution of the primer.

The peel tests were carried out using an Instron machine (100mm/min). The green strengths were determined by measuring the peel bond strength 1 5 seconds after pressing.

Test samples using Adhesives 1 and 2 were made in accordance with normal shoe factory procedures. The PVC soling was solvent wiped, and the adhesive applied to the soling and roughed upper leather strips. After the adhesive films had dried, they were heat activated and pressed together.

In the tables, peel test 1 was carried out after ageing bonded samples for 7 days at 60"C and 90% relative humidity. Peel test 2 was carried out after storage of samples for 7 days under normal room temperature and humidity conditions.

Table 2 indicates the green strength for sample bonds of PVC substrates bonded to leather upper material with Adhesive 3 as above described. Table 3 compares peel strengths of bonds formed using Adhesives 1 and 2 with bonds formed using Adhesive 3 and illustrative primer 6.

TABLE 1 Ingredient Illustrative Primer Composition 1 2 3 4 5 6 7 Hexamethylene tetramine 0.5 1 1 Triethylene diamine 0.5 1 0.5 1 ,4-dimethyl piperazine 1 Water 90 Acetone 99 99.5 99 99.5 99 8.5 Methyl ethyl ketone 99.5 Tenside 0.5 TABLE 2 Illustrative Primer Green Strength (N/cm) No primer 0 1 4 2 17-20 3 20-30 4 17-20 5 30-40 TABLE 3 Adhesive Illustrative Green Peel Test 1 Peel Test 2 Primer Strength (N/cm) (N/cm) Composition (N/cm) 1 - 3-6 7+1 65+6 2 - 6-10 20+7 88+6 3 - 0 4+1 136+21 3 Illustrative 10-15 74 + 28 62 i 22 Primer 6 In all examples it was found that there was no substantial difference in the creep tests between primed and unprimed samples.

Example 2 Adhesive compositions 4 and 5 were used to form bonds between further sample strips of roughed upper leather (1), and PVC soling material. Prior to bonding the PVC soling material was primed by wiping with a cloth soaked with illustrative primer 7. A thin film of the melted adhesive composition was applied to the soling samples at 1 00 C. These samples were exposed to airborne moisture under ambient conditions i.e. 20"C + 2"C and not less than 10% RH.

Bonds were made by use of soling samples aged for 14 days using adhesive 4 or for 8 weeks using adhesive 5 in this way. Upper leather samples were prepared by application of the melted adhesive at 1 00 C. After cooling, the upper material samples were exposed to airborne moisture in a humidity chamber set at 90"C and 26% RH. After a residence time in the chamber of 10 to 1 5 minutes, the sample strips were removed from the chamber, and allowed to cool at room conditions (20"C + 2"C and not less than 10% RH) for about 1 5 minutes.The aged soling samples were then heat activated by exposure to an infra-red heater set at 85 to 90"C for a few seconds with the layers and heater elements spaced approximately 45mm apart and pressed together. Green strength of the bond, and peel strength of the bonds after ageing 5 days at room conditions were determined. Results are shown in Table 4.

TABLE 4 Adhesive Green Strength Peel Strength N/cm N/cm 4(14 days 31 + 5 72 + 5 ageing of soles) 5 (8 weeks 30 + 8 79 + 19 ageing of soles) Illustrative Method In the illustrative method an adhesive bond was formed between substrates comprising a PVC outsole and a lasted shoe upper of leather. The PVC surface was primed by wiping with a cloth impregnated with illustrative primer 3. Adhesive 3 was then melted and brought to a temperature of 100"C and applied as a coating 0.03mm thick both to the primed PVC outsole and to the previously roughened sole attaching surface of the shoe upper.

The coating applied to the PVC sole was moisture cured at room temperature for 24 hours for reaction of the moisture with -NCO groups of the prepolymer to effect chain extension. The coating applied to the shoe upper was cured at 90"C for approximately 10 minutes at 27% relative humidity, and aged at room temperature.

The material of the coatings became tough, strongly adherent to the attaching surface and capable of being softened to tacky condition adhesive to like adhesive surfaces at temperatures of about 95"C. The coating on the sole surface was subjected to infrared heat activation for 5 seconds in a conventional heater set at 100"C.

The PVC sole was assembled against the attaching surfaces of the bottom of the shoe upper, and the assembly pressed under normal sole pressing conditions. A strong initial bond was formed with no separation or "grinning" and the bond strength was considered satisfactory for use of the shoe.

CLAI MS 1. A method of forming adhesive bonds between surfaces one at least of which comprises plasticised polyvinyl chloride usinmg an adhesive composition comprising a urethane polymer having isocyanate groups available for reaction, and capable of polymerisation by atmospheric moisture to provide a polyurethane which is tacky and capable of adhesive union when heated, the method comprising applying to the PVC surface a primer composition comprising an organic tertiary amino compound having in its molecule at least one saturated heterocyclic ring containing not less than two tertiary nitrogen atoms to provide a primer coat thereon, applying the adhesive composition onto the primer coat, subjecting the adhesive composition to a moisture treatment to bring it to a condition in which when it is tacky and capable of adhesive union, heating the adhesive coating and bringing the surfaces together with the adhesive between them in heated condition.

2. A method according to claim 1 wherein each nitrogen atom of the tertiary amino compound has three valencies satisfied by different carbon atoms.

3. A method according to clain 2 wherein said carbon atoms are all members of a saturated heterocyclic ring system.

4. A method according to any one of the preceding claims wherein the primer composition is applied to the surface in the form of an aqueous solution.

5. A method according to any one of the preceding claims wherein one of the surfaces to be bonded comprises a shoe sole prepared from plasticised PVC.

6. A method according to claim 5 wherein the adhesive is applied as a hot melt.

7. A method according to either one of claims 4 and 5 wherein the adhesive is applied to both surfaces and chain extended by moisture treatment.

8. A method according to claim 1 substantially as the illustrative method hereinbefore described.

9. A primer composition for use in conjunction with an adhesive composition having reactive isocyanate groups for bonding plasticised PVC, comprising an organic tertiary amino compound having in its molecule at least one saturated heterocyclic ring containing not less than two tertiary nitrogen atoms and a solvent therefore.

10. A primer composition according to claim 9 wherein the solvent includes water.

11. A primer composition according to either one of claims 9 and 10 wherein the tertiary amino compound is as specified in either one of claims 2 and 3.

1 2. A primer composition according to any one of claims 9 to 11 wherein the tertiary amino

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

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. TABLE 4 Adhesive Green Strength Peel Strength N/cm N/cm 4(14 days 31 + 5 72 + 5 ageing of soles) 5 (8 weeks 30 + 8 79 + 19 ageing of soles) Illustrative Method In the illustrative method an adhesive bond was formed between substrates comprising a PVC outsole and a lasted shoe upper of leather. The PVC surface was primed by wiping with a cloth impregnated with illustrative primer 3. Adhesive 3 was then melted and brought to a temperature of 100"C and applied as a coating 0.03mm thick both to the primed PVC outsole and to the previously roughened sole attaching surface of the shoe upper. The coating applied to the PVC sole was moisture cured at room temperature for 24 hours for reaction of the moisture with -NCO groups of the prepolymer to effect chain extension. The coating applied to the shoe upper was cured at 90"C for approximately 10 minutes at 27% relative humidity, and aged at room temperature. The material of the coatings became tough, strongly adherent to the attaching surface and capable of being softened to tacky condition adhesive to like adhesive surfaces at temperatures of about 95"C. The coating on the sole surface was subjected to infrared heat activation for 5 seconds in a conventional heater set at 100"C. The PVC sole was assembled against the attaching surfaces of the bottom of the shoe upper, and the assembly pressed under normal sole pressing conditions. A strong initial bond was formed with no separation or "grinning" and the bond strength was considered satisfactory for use of the shoe. CLAI MS

1. A method of forming adhesive bonds between surfaces one at least of which comprises plasticised polyvinyl chloride usinmg an adhesive composition comprising a urethane polymer having isocyanate groups available for reaction, and capable of polymerisation by atmospheric moisture to provide a polyurethane which is tacky and capable of adhesive union when heated, the method comprising applying to the PVC surface a primer composition comprising an organic tertiary amino compound having in its molecule at least one saturated heterocyclic ring containing not less than two tertiary nitrogen atoms to provide a primer coat thereon, applying the adhesive composition onto the primer coat, subjecting the adhesive composition to a moisture treatment to bring it to a condition in which when it is tacky and capable of adhesive union, heating the adhesive coating and bringing the surfaces together with the adhesive between them in heated condition.

2. A method according to claim 1 wherein each nitrogen atom of the tertiary amino compound has three valencies satisfied by different carbon atoms.

3. A method according to clain 2 wherein said carbon atoms are all members of a saturated heterocyclic ring system.

4. A method according to any one of the preceding claims wherein the primer composition is applied to the surface in the form of an aqueous solution.

5. A method according to any one of the preceding claims wherein one of the surfaces to be bonded comprises a shoe sole prepared from plasticised PVC.

6. A method according to claim 5 wherein the adhesive is applied as a hot melt.

7. A method according to either one of claims 4 and 5 wherein the adhesive is applied to both surfaces and chain extended by moisture treatment.

8. A method according to claim 1 substantially as the illustrative method hereinbefore described.

9. A primer composition for use in conjunction with an adhesive composition having reactive isocyanate groups for bonding plasticised PVC, comprising an organic tertiary amino compound having in its molecule at least one saturated heterocyclic ring containing not less than two tertiary nitrogen atoms and a solvent therefore.

10. A primer composition according to claim 9 wherein the solvent includes water.

11. A primer composition according to either one of claims 9 and 10 wherein the tertiary amino compound is as specified in either one of claims 2 and 3.

1 2. A primer composition according to any one of claims 9 to 11 wherein the tertiary amino

compound comprises one or more of 1 4-dimethyl piperazine, triethylene diamine and hexamecompound comprises one or mo thylene tetramine.