GB2197326A

GB2197326A - Adhesive compositions

Info

Publication number: GB2197326A
Application number: GB08724865A
Authority: GB
Inventors: David Francis O'sullivan
Original assignee: HB Fuller Adhesives Ltd
Current assignee: HB Fuller Adhesives Ltd
Priority date: 1986-10-24
Filing date: 1987-10-23
Publication date: 1988-05-18
Anticipated expiration: 2007-10-23
Also published as: GB8625528D0; GB2197326B; GB8724865D0

Abstract

An adhesive composition applicable in melt form to surfaces to be bonded comprises (a) a silane-grafted ethylene-vinyl acetate copolymer, the ethylene-vinyl acetate copolymer precursor having a vinyl acetate content of from 18 to 40% and a melt flow index of 20 to 150, preferably 35 to 55, (b) a catalyst for the condensation of said silane-grafted ethylene-vinyl acetate copolymer, and, optionally, (c) a plasticizer. The adhesive composition is particularly suitable for use in product assembly. Components (a) and (b) may be packed in separate containers.

Description

SPECIFICATION Adhesive compositions The invention relates to adhesive compositions, in particular to hot melt adhesive compositions comprising cross-linkable silane grafted ethylenevinyl acetate copolymers.

Hot melt adhesive systems which utilise polyurethanes which cross-link under the action of moisture to achieve a curing of the adhesive after application are known. One such system is that described by Bostik Limited in GB-A-1540634.

The present invention relates to a further cross-linkable hot melt adhesive system which by the use of silane grafted ethylene-vinyl acetate copolymers may yield adhesive bonds having particularly advantageous properties in terms of resistance to organic solvents, high cohesive strength, resistance to creep failure, high yield temperature and high adhesion to certain substrates.

Silane grafted ethylene homo- and co-polymers are widely used for the preparation of cable jacketing, wire insulation and piping and have been suggested for use as spacers in laminate materials. US-A-3646155 for example describes the production of such material. Polyethylene is reacted with an unsaturated silane, e.g. vinyl triethoxysilane, in the presence of a peroxide initiator to produce the silane grafted polymer. A catalyst, for example dibutyl tin dilaurate is incorporated into the grafted polymer which is then formed into the desired shape. Catalysed hydrolysis and cross-linking of the silane groups occurs to give the hardened end product.The reaction procedure may thus be represented as follows:- (A) Grafting -(CH2CH2CH2CH2CH2)a + CH2=CH,Si(OC2H )

peroxide - (CH2CH2CHCH2CH2) a C2CH2Si(OC2H5)3 silane graft polymer (B) Hydrolysis

(C) Cross-linking

- (CH2CH2CHCH2CH2) a I - (CH2CH2CHCH2CH2) a CH2CH 2Si(OH)3 CH2CH2S\O + catalyst 0 /0 CH2CH2Si(OH)3 CH2 CH2Si-O - (CH2CH2C5CH2CH2) a - (CH2CH2CHCH2CH2 a We have now surprisingly found that certain silane grafted medium range molecular weight ethylene-vinyl acetate copolymers may be used as the base for a cross-linkable hot melt adhesive system particularly suitable for use in product assembly in the furniture and motor industries for example.

According to one aspect of the present invention therefore we provide an adhesive composition appliable in melt form to surfaces to be bonded, said composition comprising (a) a silane-grafted ethylene-vinyl acetate copolymer, (hereinafter SGEVA) the ethylene-vinyl

catalyst ~ (CH2CH2C1HCH2CH2)a + 3 C2H50H H2CH2Si(OH)3 acetate copolymer (hereinafter EVA) precursor where for having a vinyl acetate content of from 18 to 40%, preferably 20-35%, and most preferably about 28%, and a melt flow index (MFIexpressed in g/l0min.) of 20 to 150, preferably 20 to 100, especially preferably 35 to 55 and particularly preferably about 40, (b) a catalyst for the condensation of said silane-grafted ethylene-vinyl acetate copolymer, and preferably also (c) a plasticizer, preferably a polybutene plasticizer.

As it is found that the incorporation of a plasticizer into the adhesive system improves substrate wetting by the hot melt as well as providing improved flexibility in the cured adhesive it is particularly preferred that the adhesive system should comprise at least one plasticizer, such as for example an ester based plasticizer (e.g. benzoate plasticizers such as glycol dibenzoate and dipropylene glycol dibenzoate (available as Benzoflex 9-88)). Polybutene plasticizers however are especially preferred. The plasticizer conveniently constitues 5 to 40%, preferably 10-35%, of the adhesive composition.

In the SGEVAS used according to the invention, the silane graft is conveniently of general formula (I) ~A~Si(R)3 m(X)m (I) wherein A represents an optionally substituted, straight or branched C26 alkylene chain, preferably an ethylene, n-propylene or n-butylene chain; X represents a leaving group displaceable by hydrolysis, conveniently a halogen atom (for example chlorine) or a group of formula -OR' or R'COO- where R' is a C14 alkyl or alkoxyalkyl group; m is 1, 2 or 3, preferably 2 or 3; and R represents a blocking group not displaceable by hydrolysis, for example a C14 alkyl group.

The silane graft constitutes a minor part by weight of the SGEVAs, generally up to 5%, preferably 0.5-2% and especially preferably about 1%.

Thus the SGEVA component for inclusion in the adhesive composition of the invention may be prepared by reacting the EVA precursor with up to 5% of an unsaturated silane of formula II A'Si(R)3 m(X)m (II) (wherein A' represents an optionally substituted straight or branched chain C26 monoalkenyl group, preferably a group CH2=CH-(CH2)n- where n is 0, 1 or 2, and R, X and m are as defined above) in the presence of a catalyst, conveniently a peroxide catalyst.

The unsaturated silanes of formula Il can be prepared by conventional methods.

The catalyst used in the preparation of the SGEVAs is preferably a free-radicai generating compound such as for example benzoyl peroxide, dicumyl peroxide or other such catalysts referred to by LIS-A-3646155.

The SGEVA component preferably forms up to 40%, e.g. 10-40%, and especially preferably about 30%, of the adhesive composition.

The MFI ranges for the EVA precursor specified above are important as by operating within the specified ranges an adhesive may be produced which has sufficient initial strength when substrates are first bonded in product assembly applications.

In the adhesive composition of the invention, SGEVAs having a 3-5 membered chain linking the silicon atoms with the EVA backbone will result in a cured adhesive having a generally more open structure than that achieved with only a 2-membered linking chain. The open structure will advantageously permit both a high degree of curing and the inclusion of relatively high concentrations of tackifying resin and other components in the uncured adhesive composition. In this way, the viscosity, tack and melting characteristics of the uncured adhesive may be tailored to particular desired ievels, for example to permit the use with the new adhesive of conventional hot melt adhesive melting, mixing and application apparatus.

As the catalyst for the SGEVA hydrolysis and cross-linking, a silanol condensation catalyst may be used. Many such materials are known to the art. Thus suitable catalysts include, for example, metal carboxylates, e.g. dibutyl tin dilaurate, organometallics, e.g. tetrabutyl titanate, organic bases, e.g. ethylamine, and mineral and fatty acids. Several such compounds are identified for example in US-A-3646155. Among suitable condensation catalysts, organotin compounds, such as dibutyl tin dilaurate are preferred. The catalyst, conveniently makes up from 0.05-0.2%, preferably about 0.1%, of the adhesive composition as a whole.

The adhesive composition of the invention conveniently also contains further components selected from: antioxidants; tackifying resins; further polymers; and diluents and modifiers.

The tackifying resin component of the hot melt adhesive composition of the present invention may comprise any suitable resin or resin mixture, for example those conventional for hot melt adhesives. The resin or resin mixture should however be selected to achieve the desired balance between compatibility with the SGEVA and the other components of the adhesive, the melt flow properties of the adhesive as a whole and the specific adhesion to the substrates intended to be bonded with the adhesive. In this respect, suitable tackifying resins may include: aromtic modified resins such as < x-methyl styrene homopolymers or copolymers, e.g.Krystalex F100 (a- methyl styrene polymer), Krystalex FR75 (a modified a-methyl styrene copolymer) or Piccotex (a vinyl toluene-a-methyl styrene copolymer), all three available from Hercules Chemical Co.; aliphatic petroleum hydrocarbon resins; styrene-modifed hydrocarbon resins; and, particularly preferably, alicyclic hydrocarbon resins, e.g. Escorez 5300 (available from Exxon).

The tackifying resin will conveniently make up from 5 to 40%, preferably 10-40%, and especially preferably about 30%-35%, of the adhesive.

The hot melt adhesive of the invention preferably contains at least one antioxidant. In this respect, conventional EVA antioxidants, such as butylated hydroxytoluene (BHT) may be used. A preferred antioxidant is pentaerythritol-tetrakis-3-(3,5-di-tert. butyl-4-hydroxyphenyl)-propionate, which is available under the trade name Irganox 1010 from Ciba-Geigy UK Ltd. of Manchester.

The antioxidant will generally be present in the adhesive at about 0.1-2.5%, preferably 0.5-1%.

The adhesive composition of the invention preferably also comprises a further polymer or polymer mixture. The further polymer or polymer mixture, which should not be cross-linkable under the action of the catalyst, will be selected to achieve, inter alia, a balance between compatability with the catalyst and the other components of the adhesive and the viscosity and tack characteristics of the adhesive. Since the SGEVA is based on an EVA backbone, the further polymers are preferably selected from EVAs, conveniently EVAs having VA contents of 12-40%, preferably 18-35% and especially preferably about 28% and MFls of 2 to 2500, conveniently 5-800, preferably 20-500 and most preferably 150-400.The further polymer conveniently constitutes up to 15%, preferably 1 to 10%, and especially preferably about 5%, of the adhesive of the invention and generally will be present at about half the concentration of the SGEVA.

To avoid premature cross-linking, it may be preferable not to contact the condensation catalyst with the SGEVA until the adhesive composition is melted for application. Thus according to a further aspect of the invention we provide a hot melt adhesive system comprising a first composition comprising an SGEVA as defined above and a second composition comprising a catalyst for the condensation of said SGEVA, said first and second compositions being packed in separated containers.

The adhesive composition or system may also contain further components such as diluents or modifiers, conveniently as about 5-40%, preferably about 30%, of the total adhesive. These components may serve to regulate the viscosity and setting speed of the adhesive and may be included to enhance the wicking of the adhesive into the substrates to be bonded. In this respect, polyolefins (for example low crystallinity range molecular weight homo- or copolymers, such a that available under the trade name Vestoplast 608 from Hulls (UK) Limited of Manchester) and conventional diluents and modifiers for hot melt adhesives, such as waxes (e.g.

petroleum waxes such as paraffin waxes or microcrystalline waxes such as Okerin 8981 from Astor Chemicals of West Drayton, Middlesex), low molecular weight polyethylene, atactic polypropylene, hydrogenated animal or vegetable fats (e.g. hydrogenated castor oil or hydrogenated tallow), and synthetic waxes, such as Fischer Tropsch waxes may be used. Where waxes or low molecular weight polyethylenes are used as diluents, they should conveniently have softening temperatures in the range of 50-120"C. Microcrystalline waxes, Fisher Tropsch waxes and paraffin waxes having softening temperatures in the ranges 65 to 94"C (e.g. 79"C), 110 to 120"C and 54 to 72"C respectively are particularly suitable.A mixture of low molecular weight polyethylene, such as polyethylene AC6 or AC8 available from Allied Chemical Corporation International NV SA of Birmingham, and a microcrystalline wax having a softening temperature of about 90"C, such as Micro 549 available from Holmes Chemical Company of Uxbridge, may also be particularly suitable, especially where the polyethylene and the wax constitute about 5 and about 15% by weight respectively of the adhesive.

Where the adhesive is formulated as a two part system, the SGEVA and the catalyst being separately packaged, the optional components can appear in one or both of the separate compositions, the SGEVA-containing composition will however preferably contain the antioxidant and the tackifier and the catalyst containing composition will preferably contain a further polymer; the plasticizers, diluents and modifiers may be in either or both compositions. In a two composition system, the catalyst containing composition particularly conveniently comprises 99.8% of an EVA (e.g. 28-400 EVA, i.e. an EVA with a VA content of 28% and an MFI of 400) and 0.1% of a catalyst (e.g. dibutyl tin dilaurate).

Where the adhesive of the invention is formulated as a single composition it will advantageously be packaged in water-tight containers, for example aluminium cartridges, which containers advantageously will also include a desiccant, for example a sachet of silica gel at the end of a cartridge for a hot melt adhesive applicator. Thus, according to a further aspect of the invention we provide a hot-melt adhesive applicator cartridge comprising a water-tight container containing therein a desiccant and a hot-melt adhesive composition, said composition comprising an SGEVA as hereinbefore defined and a catalyst for the condensation thereof.

Besides the components discussed above, it may be desirable to incorporate into the hot melt adhesive a colouring agent, for example a whitener such as titanium dioxide. Such colouring agents are particularly readily dispersed within the hot melt adhesive if introduced as solid dispersions in EVA. Thus for example small quantities, e.g. 0.1-4%, of a 1:1 dispersion of TiO2 in EVA (available under the trade name E/Va M/B from ICI Plastics Division of Welwyn Garden City) may be included in the hot melt adhesive.

In use, the adhesive of the present invention will be heated to melting, generally to 150-200 C preferably about 160-170"C, mixed (e.g. in a cartridge loaded applicator or by the mixing in a mixer head of the two compositions of a two composition system) and applied to the substrates to be bonded, generally in a film thickness of up to about 3 mm, preferably up to about 1 mm, although this can be achieved by applying a larger amount and scraping off the excess. The adhesive characteristics of the system should be sufficient to maintain a bond between the substrates while the curing of the SGEVA component by hydrolysis and crosslinking occurs.

For the hydrolysis of the SGEVA, water is required. In general, the necessary water can be supplied by ambient moisture. However, if desired, the adhesive may contain further components which gradually release moisture into the system, for example fillers with surface-bound moisture.

The adhesive of the invention may be used in most applications where hot melt adhesives have been used and where full bonding strength is not required immediately after the application of the adhesive. The adhesive of the invention is thus particularly suited to use in product assembly (for example in the furniture and automobile industries), packaging and labelling.

Thus according to a further aspect of the invention we provide the use of the adhesive composition and system of the invention in product assembly.

Because of the silane cross-linking reaction, the adhesive of the invention is capable of chemically bonding to the surfaces of certain substrates to produce an enhanced adhesive effect.

Thus the adhesive of the present invention may be particularly suitable for the bonding of cellulosic and silicaceous substrates, for example paper and glass.

The SGEVA and catalyst components in the adhesive of the present invention may be selected to achieve particular desired characteristics, for example rate or controlability of curing, and by varying the VA content and the MFI of the EVA precursor for the SGEVA, the formulation characteristics (such as viscosity, softening point etc.), for the adhesive may be controlled to give a readily processable material.

The preferred SGEVAs used according to the present invention have particularly beneficial properties as hot melt adhesive components and thus the invention also provides a silanegrafted ethylene-vinyl acetate copolymer being a reaction product of an ethylene-vinyl acetate copolymer having a vinyl acetate content of 18 to 40% and a melt flow index of 35 to 55 with an unsaturated silane of formula ll A'~Si(R)3 m(X)m (II) (wherein A' represents an optionally substituted straight or branched chain C26 monoalkenyl group, X represents a hydrolyticaily displaceable leaving group, m is 1, 2 or 3 and R represents a non-hydrolytically displaceable blocking group). The silane content of the grafted copolymer is preferably 0.5 to 5%. The ethylene-vinyl acetate copolymer precursor preferably has a melt flow index of about 40 and a vinyl acetate content of 20-35%, especially about 28%.

The following Examples are provided to illustrate the present invention further without limiting the scope of protection sought therefor (all percentages and parts referred to herein are by weight unless otherwise specified): Example 1 One part adhesive composition The composition is the admixture of the following components:: SGEVA+ 31.3% Catalyst/EVA blend (1% dibutyl tin dilaurate) 1.7% 175"C Micro wax (microcrystalline wax) 20.0% Sasol Wax (Fischer Tropsch wax) 10.0% Inganox 1010 (antioxidant) 1.0% Escorez 5300 (hydrogenated alicyclic hydrocarbon-tackifying resin) 26.0% Piccotex 100 (a-methyl styrene-vinyl) toluene copolymer-tackifying resin) 10.0% + THE SGEVA (Batch 8704-AEl Compounds, Gravesend, Kent) is the reaction product of vinyltrimethoxysilane and 28-40 EVA, prepared using the procedure of US-A-3646155.

The composition is homogenized, preferably after preblending of the catalyst and the SGEVA, and filled into aluminium cartridges for a hot melt adhesive applicator. A silica gel sachet is placed in the filled cartridge which is then sealed.

Example 2 One part adhesive composition The composition is the admixture of the following components: SGEVA 31.3% Catalyst/EVA blend (1% dibutyl tin dilaurate) 1.7% Irganox 1010 (antioxidant) 1.0% Escorez 5300 (hydrogenated alicyclic hydrocarbon-tackifying resin) 26.0% Piccotex 100 (a-methyl styrene-vinyl toluene copolymer-tackifying resin) 10.0% Hyvis 30 (polybutene-plasticizer) 20.0% Benzoflex 9-88 (dipropylene glycol dibenzoate-plasticizer) 10% + The SGEVA (Batch 8704-AEI Compounds, Gravesend, Kent) is the reaction product of vinyltrimethoxysilane and 28-40 EVA, prepared using the procedure of US-A-3646155.

Example 3 adhesive system Two The first composition comprises the following components: SGEVA" 34% Wingtack 95 (polyterpene resin) 33% Okerin 8981 (microcrystalline wax) 32.4% Irqanox 1010 (antioxidant) 0.6% The SGEVA is the reaction product of 100 parts of 28-40 EVA copolymer with 1.5 parts of an unsaturated silane of formula CH2=CHCH2Si(OCH3)3 (the grafting reaction may be performed using the peroxide catalysed procedure of US-A-3646155).

The second composition comprises the following components: 28-400 EVA 99.8% Dibutyl tin dilaurate 0.2% In use, the first and second compositions are melted, mixed in a weight ratio of 95:5 and the mixture is applied to the substrates to be bonded.

If desired, a plasticizer may be incorporated in the two-composition system of Example 3, for example in place of the microcrystalline wax in the first composition. In this regard a 2:1 mixture of plasticizers such as Hyvis 30 and Benzoflex 9-88 may be used.

Claims

1. An adhesive composition appliable in melt form to surfaces to be bonded, said composition comprising (a) a silane-grafted ethylene-vinyl acetate copolymer the ethylene-vinyl acetate copolymer precursor wherefor having a vinyl acetate content of from 18 to 40% and a melt flow index of 20 to 150, (b) a catalyst for the condensation of said silane-grafted ethylene-vinyl acetate copolymer, and, optionally (c) a plasticizer.

2. A composition as claimed in claim 1 further comprising a plasticizer.

3. A composition as claimed in claim 2 comprising a polybutene plasticizer.

4. A composition as claimed in either one of claims 2 and 3 containing a plasticizer as from 5 to 40% of the total composition weight.

5. A composition as claimed in any one of claims 1 to 4 for which said ethylene-vinyl acetate copolymer precursor has a melt flow index of from 20 to 100.

6. A composition as claimed in claim 5 for which said ethylene-vinyl acetate copolymer precursor has a melt flow index of from 35 to 55.

7. A composition as claimed in claim 5 for which said ethylene-vinyl acetate copolymer precursor has a melt flow index of about 40.

8. A composition as claimed in any one of claims 1 to 7 for which said ethylene-vinyl acetate copolymer precursor has a vinyl acetate content of about 28% by weight.

9. A composition as claimed in any one of claims 1 to 8 wherein said silane-grafted ethylene-vinyl acetate copolymer is a reaction product of a said ethylene-vinyl acetate copolymer precursor and an unsaturated silane of formula II A'Si(R)3 m(X)m (II) wherein A' represents a group CH2=CH-(CH2)n- (where n is 0, 1 or 2), X represents a hydrolytically displaceable leaving group, m is 1, 2 or 3, and R represents a non-hydrolytically displaceable blocking group.

10. A composition as claimed in any one of claims 1 to 9 wherein said silane-grafted ethylene-vinyl acetate copolymer is a vinyltrimethoxysilane-grafted ethylene-vinyl acetate copolymer.

11. A composition as claimed in any one of claims 1 to 10 wherein the silane graft constitutes from 0.5 to 2% by weight of the said silane-grafted ethylene-vinyl acetate copolymer.

12. A composition as claimed in any one of claims 1 to 11 containing said silane-grafted ethylene-vinyl acetate copolymer as from 10 to 40% of the total composition weight.

13. A composition as claimed in any one of claims 1 to 12 containing said catalyst as from 0.05 to 2% of the total composition weight.

14. A composition as claimed in any one of claims 1 to 13 wherein said catalyst comprises dibutyl tin dilaurate.

15. A composition as claimed in any one of claims 1 to 14 comprising at least one further component selected from: antioxidants; tackifying resins; non-silane grafted ethylene-vinyl acetate copolymers; and modifiers and diluents.

16. A composition as claimed in claim 15 containing an antioxidant as from 0.1 to 2.5% of the total composition weight.

17. A composition as claimed in either one of claims 15 and 16 containing a tackifying resin as from 5 to 50% of the total composition weight.

18. A composition as claimed in any one of claims 15 to 17 containing a non silane grafted ethylene-vinyl acetate copolymer as up to 1 5% of the total composition weight.

19. A composition as claimed in any one of claims 15 to 18 containing at least one modifier or diluent selected from microcrystalline waxes, paraffin waxes and Fischer Tropsch waxes as from 5 to 40% of the total composition weight.

20. A hot melt adhesive system comprising a first composition comprising a silane-grafted ethylene-vinyl acetate copolymer as defined in claim 1 and a second composition comprising a catalyst for the condensation of said silane-grafted ethylene-vinyl acetate copolymer, said first and second compositions being packed in separate containers.

21. An adhesive system as claimed in claim 20 wherein said first composition further contains a plasticizer.

22. An adhesive system as claimed in either one of claims 20 and 21 wherein said silanegrafted ethylene-vinyl acetate copolymer is a reaction product of a said ethylene-vinyl acetate copolymer precursor and an unsaturated silane of formula II At~Si(R)3 m(X)m (II) wherein A' represents a group CH2=CH-(CH2)n- (where n is 0, 1 or 2), X represents a hydrolytically displaceable leaving group, m is 1, 2 or 3, and R represents a non-hydrolytically displaceable blocking group.

23. An adhesive system as claimed in any one of claims 20 to 22 for which said ethylenevinyl acetate copolymer precursor has a melt flow index of from 20 to 100.

24. An adhesive system as claimed in any one of claims 20 to 23 for which said ethylenevinyl acetate copolymer precursor has a melt flow index of from 35 to 55.

25. An adhesive system as claimed in any one of claims 20 to 24 wherein said second composition further contains a non silane grafted ethylene-vinyl acetate copolymer.

26. A hot melt adhesive applicator cartridge comprising a water-tight container containing therein a desiccant and a hot melt adhesive composition as claimed in any one of claims 1 to 19.

27. Adhesive compositions substantially as herein disclosed in any one of the Examples.

28. The use of an adhesive composition as claimed in any one of claims 1 to 19 or an adhesive system as claimed in any one of claims 20 to 25 in product assembly.

29. A silane-grafted ethylene-vinyl acetate copolymer being a reaction product of an ethyienevinyl acetate copolymer having a vinyl acetate content of 18 to 40% and a melt flow index of 35 to 55 with an unsaturated silane of formula Il A'Si(R)3 m(X)m (II) (wherein A' represents an optionally substituted straight or branched chain C26 monoalkenyl group, X represents a hydrolytically displaceable leaving group, m is 1, 2 or 3, and R represents a non-hydrolytically displaceable blocking group).