GB2135903A

GB2135903A - Adhesive foams

Info

Publication number: GB2135903A
Application number: GB08305751A
Authority: GB
Inventors: Thomas Wickel; Heinz Gerhard Gilch
Original assignee: Bostik Ltd
Current assignee: Bostik Ltd
Priority date: 1983-03-02
Filing date: 1983-03-02
Publication date: 1984-09-12
Also published as: GB8305751D0

Abstract

A foamed adhesive layer having pressure sensitive adhesive properties and a stable foam structure is prepared by mixing a gas comprising oxygen with an aqueous dispersion comprising an unsaturated polyester, a pressure sensitive adhesive component and an accelerator comprising an organic compound effective for oxidative curing of the polyester. Preferred formulations contain an alkyd resin, a carboxylated acrylic polymer and a naphthenate or octoate, and are frothed with air.

Description

SPECIFICATION Adhesive foams This invention is concerned with adhesive foams.

It is known to bond a sheet of surface covering onto a substrate surface by use of a foamed adhesive. US patent specification 4036673 for example discloses depositing a quantity of a foamed adhesive on the substrate surface, laying the sheet on the adhesive, applying pressure to the sheet so as to spread the adhesive and cause the adhesive to conform to any irregularities of the substrate surface, and allowing the adhesive to dry, thereby bonding the sheet to the substrate surface. The specification discloses the use of a filled vinyl acrylic copolymer latex emulsion as a component of the foamed adhesive.

We have observed that for satisfactory performance in various applications, it is desirable for the foamed adhesive to retain its structure in the final bond, or to retain its structural form on drying, together with its adhesive properties.

Among objects of the present invention are to provide an improved method of providing a foamed adhesive layer and a foamable adhesive composition for use in providing a foamed ahesive layer.

We have now found that adhesive foams which retain their structural and adhesive properties may be produced by incorporation of air bubbles in a foamable adhesive composition comprising a polymeric material having pressure sensitive adhesive properties, an ethylenically unsaturated polyester material and an accelerator comprising an organic compound effective for oxidative curing of the polyester material.

The present invention provides in one of its aspects a method of providing a foamed adhesive layer having a stable foam structure and pressure sensitive adhesive characteristics comprising mixing a gas comprising oxygen with an aqueous dispersion comprising an unsaturated polyester, a pressure sensitive adhesive component, and an accelerator comprising an organic compound effective for oxidative curing of the polyester, the aqueous dispersion having a viscosity of about 8.000 to about 30,000 mpas (Brookfield RVT Spindle No 3, after 3 minutes constant stirring at 2.5 rpm) at 21"C plus or minus 2"C, and applying the mixture as a froth to a substrate to provide a stable foam structure having a tacky pressure sensitive adhesive surface.

A method according to the invention may be used to provide a foamed adhesive layer on a first substrate which is then bonded to a second substrate by pressing the two substrates together with the foamed adhesive layer between them. Preferably water is removed from the foam before the two substrates are brought together, although this is less necessary when one or both of the substrates is pervious to water.

Examples of materials which may be bonded in this way include uneven substates for example hammered aluminium, polyurethane and polystyrene foams and gypsum board as well as smooth substrates for example foils of pvc. The bond may be formed between the two substrates at any time after the foam has dried i.e. within a few hours of application of the froth, or after storage for several months. In the latter case it is convenient to protect the adhesive foam with a release paper during storage. If desired, the froth may be applied to both sides of the first substrate and each foamed layer may be protected by a release paper thus to provide a double sided adhesive tape or foil.

In a method according to the invention, water may be removed from the applied adhesive layer by any convenient means, for example by allowing the adhesive layer to remain exposed to air at room temperature, or by heating with radiant heaters. Setting of the froth commences upon mixing of air with the dispersion and continues until a self-supporting foam structure is produced, and ultimately a stable foam is achieved. The process is accelerated by heating the froth. It is important therefore to ensure that removal of water from the froth (i.e. drying) is conducted under conditions which do not adversely influence setting of the material to provide the desired stable foam.

In a method according to the invention, the froth may be formed using conventional frothing apparatus for example a whisk. However, we prefer to employ a machine of the static mixer type namely a continuous foam beater for whipped cream making type "Mini Bako" or a hand mixer. Preferably the dispersion and air or other gas comprising oxygen are fed into the static mixer at rates commensurate with deposition of froth at a density between 0.18 and 0.60 g/ml more preferably about 0.5 i 0.06 g/ml. In this condition (which may exist for between 15 minutes and 24 hours) the froth may be shaped into desired physical configuration. The resulting dried foam is shape retaining, resilient and compressible to some extent, but does not change density or collapse under light pressure.Preferably the fully dried foam has a density of between about 0.4 and 0.9 g/ml, more preferably about 0.45 to 0.65 g/ml. The froth may be applied in any desired thickness normally used in adhesive applications commensurate with achieving a final bond line of required thickness.

Of course, the amount of froth applied is selected partly in view of the magnitude of any substrate irregularities which are to be accommodated by the froth or foam, and the expected density of the foam in the bond line. For many purposes, particularly with respect to production of foam tapes and foils for subsequent bonding applications, it is convenient to employ sufficient froth to provide a foamed adhesive layer of about 50 mil thickness, i.e. to apply about 100 mil thick layer of froth to the chosen substrate.

An aqueous dispersion for use in a method according to the invention may comprise from about 40 to about 70 wt % solids, and preferably comprises from about 50 to about 60 wt % solids. In order to ensure optimum froth application characteristics it is desirable to include in the dispersion a foam stabilizer and a thixotropic or structural viscosity regulating agent hereinafter referred to as the thixotropic agent. The thixotropic agent is preferably a natural or synthetic inorganic or organic material or combination of such materials and is employed to ensure that material emerging from the mixer has a measure of "body".

Aqueous dispersions having viscosity outside the specified range are difficult to dispense in the form of a froth of uniform structure. We prefer to arrange that the aqueous dispersion has a viscosity at 21"C ~ 2"C (Brookfield RVT, spindle No 3, after 3 minutes constant stirring at 2.5 rpm) of about 10,000 mpas.

An important aspect of the present invention resides in the conversion of the frothed dispersion to a firmly resilient foam of sufficient storage stability to be regarded as permanent. This conversion is a result of the presence of the unsaturated polyester and the accelerator which enhances drying or oxidative curing of the unsaturated polyester when frothed with air to form a cross-linked matrix in which bubbles are trapped. The unsaturated polyester is selected with a view to providing not only the sought after curing effect but also a required degree of elasticity or flexibility in the resulting foam. Suitable unsaturated polyesters are those alkyd resins having a somewhat branched structure for example, and having sufficient unsaturation to permit required cross-linking between polyester molecules to form the matrix.Suitably, the ethylenic unsaturation arises from use of unsaturated oil fatty acids in the manufacture of the alkyd resin, for example, safflower, tall, soya bean, cottonseed, dehydrated castor or linseed oil fatty acids. Preferred materials have an oil content (i.e. fatty acid content) of about 60 to about 70% derived from safflower oil or tall oil, a phthalic acid content of about 15 to about 30%, and are derived from e.g. pentahydric alcohol. Preferred materials have an acid value (mg KOH/g) of about 0 to about 20. Preferred materials also have a viscosity at 20"C of about 10,000 to 30,000 mpas. Examples of such materials are commercially available under the trade names Soalkyd 9023 and Soalkyd 9015. The preferred materials also provide an advantage when preparing the aqueous emulsion.They are so-called self emulsifying alkyd resins, which may be added directly to aqueous components of the composition to become emulsified therein without the need to first dissolve the resin in organic solvent. We prefer to employ sufficient alkyd resin to provide from about 5 to about 20 wt % of the dispersion, more preferably from about 10 to about 15 wt % of the dispersion.

Suitable accelerators are the so called siccative materials familiar in the chemistry of unsaturated alkyd resins. We prefer to use naphthenates or octoates of cobalt, barium or zinc or mixtures thereof.

An important component of an aqueous dispersion for use in a method according to the invention is the pressure sensitive adhesive component. By pressure sensitive adhesive component where used herein is meant a component which in the finished product provides pressure sensitive adhesive properties i.e.

renders the product capable of adhering at room temperature to a surface by briefly applied pressure alone.

Pressure sensitive adhesives known in the art usually comprise as a major component a linear high polymer and a lower molecularweighttackifying additive. Aqueous dispersions of many pressure sensitive adhesive types are suitable as the pressure sensitive adhesive component in the present invention for example those based on polyacrylates, polyvinyl acetates, or styrene-butadiene copolymers which form films with high permanent tack depending on the polymer and/ortackifying resins used. We prefer to employ a carboxylated aqueous acrylic dispersion to confer tacky pressure sensitive properties on the froth and the ultimate foam.

We prefer to employ sufficient of this carboxylated acrylic polymer to provide from about 38 to about 52 wt % of the dispersion, more preferably about 42 wt % of the dispersion.

Conveniently the ingredients of the aqueous dispersion are so chosen that the dispersion remains stable in storage for several weeks and preferably for several months. The dispersion should have a pH value in excess of about 5.5, more preferably of about 8. We prefer to include an emulsifying agent comprising an anionic and ionic tensides dissolved in plasticiser, and a foam stabiliser e.g. a sodium salt of a carboxylic acid e.g. sodium stearate or sodium oleate. We prefer to employ the emulsifying agent to provide about 0.1 to 5 wt %, more preferably about 1.5 wt % of the weight of the dispersion. We prefer to use the sodium salt material to provide about 0.1 to about 1 wt %, more preferably about 0.5 of the weight of the dispersion.

The dispersion may be prepared in any convenient manner. For example an aqueous dispersion of the pressure sensitive adhesive may be formed together with emulsifying and thixotropic agents. The unsaturated polyester may be dispersed in this dispersion. The accelerator may be subsequently added together with foam stabiliser, plasticiser, bacteriocide, additional water and other desired additives. Acidity of the dispersion may be adjusted with ammonia if desired prior to addition of the unsaturated polyester.

Alternatively one may pre-emulsify the unsaturated polyester with water and add the emulsion to the aqueous dispersion of pressure sensitive adhesive, or one may dilute the aqueous dispersion of pressure sensitive adhesive with water, and add the unsaturated polyester to this dispersion.

The present invention provides in another of its aspects an adhesive composition comprising a dispersion of an ethylenically unsaturated polyester material, a pressure sensitive adhesive component, frothing adjuvants and an accelerator comprising an organic compound effective for oxidative curing of the polyester material, the composition being such that when air is intimately incorporated therein to provide a froth, a froth is formed of a density in the range of about 0.18 to 0.6 g/ml which when dried provides a resilient storage stable foam structure having a pressure sensitive tacky surface.

By use of a method and composition according to the invention one may form adhesive bonds between substrates which have gaps or unevenly mating surfaces, and one may do so in order to achieve good wetting of rough surfaces without excessive penetration into for example foam surfaces, or to achieve thick flexible bond lines with minimum adhesive, good thermal and sound insulation resulting from the foam structure of the bond line together with good tack retention of the adhesive and good retention of the shape ofthe cured dry foam.

In order that the invention may become more clear there now follows a description of example adhesive compositions provided by the invention and illustrative thereof, and of their use in providing a foamed adhesive layer. It is to be understood that the illustrative compositions have been selected for description to facilitate understanding of the invention and not by way of limitation thereof.

Each of the illustrative compositions comprises an aqueous dispersion of an ethylenically unsaturated polyester, a pressure sensitive adhesive component and an organic compound effective for oxidative drying of the polyester. The illustrative compositions each also comprise selected emulsifying and thixotropic agents, foam stabilisers, plasticisers and bacteriocides.

In each of the illustrative compositions, the ethylenically unsaturated polyester is provided by an air drying alkyd resin, and the pressure sensitive adhesive component is provided by an acrylic ester polymer used in the form of an aqueous dispersion.

Each illustrative composition comprises ingredients in amounts by weight as shown in Table 1.

TABLE 1 Composition of illustrative composition Ingredient Illustrative composition 1 2 3 4 5 6 Acrylic Dispersion 82.32 74.77 75.84 75.84 75.84 75.84 Alkyd Resin 1 7.2 - 11.04 - 11.02 Alkyd Resin 2 - 11.2 - 10.73 - 11.03 Emulsifier 1.5 1.5 1.48 1.46 1.48 1.48 Foam Stabiliser/ Plasticiser 0.5 0.5 0.49 0.49 0.49 0.49 Bacteriocide 0.2 0.2 0.2 0.2 0.2 0.2 Thickening Agent 3.3 4.1 3.43 4.0 3.42 3.38 Accelerator 1 0.16 0.2 0.11 0.10 - Accelerator - - - - 0.18 0.18 Ammonia (25%) 0.02 0.03 0.05 0.04 0.03 0.06 Water 4.8 7.5 7.36 7.14 7.34 7.34 In the illustrative compositions: The acrylic dispersion used is a carboxylated aqueous acrylic dispersion supplied by Synthomer Chemie under the trade name VL 1247, which is believed to comprise polybutyl acrylate, carboxylated to an extent of about 1 to 2% by weight.It is understood to comprise about 57 % solids. At 25"C it has a Brookfield viscosity of 150 - 300 mpas and a pH value of 7 to 9. It has a specific gravity of 1.1 g/ml.

The alkyd resins used comprise members of the Soalkyd series supplied by Brenntag.

"Alkyd Resin 1" is an air drying alkyd resin derived from phthalic acid, tall oil and a penta-hydric alcohol.

The fatty acid content of the alkyd resin is 66%, the phthalic acid content is 20% and it has a viscosity of approximately 25,000 mpas at 20"C and an acid value of 7 to 13 mg KOH/g.

"Alkyd Resin 2" is a self emulsifying, air drying alkyd resin derived from phthalic acid, safflower oil and a penta-hydric alcohol. It has an oil content of 65% and a phthalic acid content of 22%. The viscosity at 20"C is approximately 20,000 mpas, and it has an acid value of 8 to 13 mg KOH/g.

The emulsifier is provided by Atmer 152 which comprises a 50% solution of organic anionic and non ionic surface active material in a phthalate plasticiser.

The foam stabiliser comprises sodium stearate which is added as a powder.

The thickening agent used is Dis-Thix liquid supplied by Schwegmann and is a modified non-ionic polyether and serves to confer thixotropic and or structural viscosity properties.

The bacteriocide used is Preventol D3 which comprises a mixture of alkyl methanol and halogenalkylacyla minomethanol.

Accelerator 1 used is Octa-soligen which is a cobalt salt of a carboxylic acid and is believed to comprise cobalt octoate or naphthenate. Accelerator 2 is a mixture of carboxylates of cobalt, barium and zinc.

The illustrative compositions are formed by blending together at room temperature the various ingredients to provide a stable aqueous dispersion having thixotropic properties. Properties of the illustrative compositions are shown in Table 2. The viscosity of the blend was measured before foaming using the test of Brookfield RVT spindle 3 after 3 minutes constant stirring at 2.5 rpm.

Each of the illustrative compositions was employed to provide a foamed adhesive layer having a stable foam structure and pressure sensitive adhesive properties in the following way.

The emulsion was fed through a static mixer namely a continuous foam beater type Mini Bako, where the air (or other oxygen containing gas) is incorporated into the blend and the resulting froth continuously supplied from the nozzle. The froth is applied onto a substrate e.g. a pvc foil and allowed to dry for a sufficient period (e.g. 15 minutes to 24 hours) to provide a stable foam structure which has a tacky pressure sensitive adhesive surface and is of low compressibility.

The initial density of the froth is as shown in Table 3. However, on drying, this increases to a value as also shown in Table 3.

Removal of water from the dispersion may be allowed to occur by merely allowing the froth to be exposed to atmosphere at room temperature. If desired, acceleration of the water removal phase may be achieved by heating. Following production of the foam structure a second substrate may be pressed onto the foam structure whereby to adhesively bond the two substrates together. Alternatively the foam structure may be stored for later use, e.g. protected by a release paper.

In order to examine properties of foams formed by use of the illustrative compositions, sample strips of foamed material were formed, using the method just above described.

Each of the illustrative compositions was frothed with air and extruded onto a substrate consisting of cotton fabric or a foil of plasticised polyvinyl chloride. In each case, the froth was allowed to dry overnight in air at room temperature (21"C + 2"C). The density of the froth in g/ml is shown in Table 3.

The illustrative compositions were applied to a pvc foil with a doctor blade in a quantity such that the wet foam layer as laid down was 50 mil thick. After drying in air at room temperature (21"C + 2"C) the rolling ball tack was measured. The results are given in Table 3. The dry foam density of the samples made using illustrative compositions 3 to 6 was determined after extrusion of the froth, and after the samples had dried.

The results are given in Table 3.

Peel strength of adhesive bonds between layers of cotton fabric were examined using illustrative compositions 3 to 6. Each composition was extruded onto a layer of cotton fabric. After elapse of a few minutes, a second layer of cotton fabric was pressed lightly onto the froth to form a composite. After drying the samples for seven days at room temperature, the bonds were peeled apart on an Instron tester. Results are shown in Table 3 as "Peel Strength A".

Peel strength of adhesive bonds between layers of pvc foil was examined using illustrative compositions 3 to 6. Each composition was extruded onto a pvc foil at a froth density as shown in Table 3 and a film thickness of 100 mil. The froth was allowed to dry overnight, and then a layer of silicone release paper was applied to the foam. The samples were aged for 9 months, and then the release paper was stripped off. A second pvc foil was pressed by hand against the exposed surface of the foam. The peel strength of these samples was then measured on an Instron tester. The results are recorded in Table 3 as "Peel Strength B".

Stability of the foams was examined. It was found that using the first illustrative composition, a froth of density 0.4 g/ml was formed which was stable in air at room temperature for 24 hours. The samples used for the rolling ball tests were resilient and exhibited low compressibility on pressing. The samples made using the second illustrative composition exhibited resilience and a volume decrease of 25% when pressed by hand. Optical evaluation of the response of foamed films on pvc foil made using the third to sixth illustrative compositions and covered with release paper to hand pressure after nine months ageing showed the samples were resilient. Their foam stability was assessed as shown in Table 3.

TABLE 2 Physical properties of illustrative compositions Illustrative foamed compositions 1 2 3 4 5 6 1. Viscosity (mpas) 7,680 10,200 10,800 10,440 10,000 9,720 2. pHValue 7.9 8.3 8.0 7.9 7.9 8.0 3. Total solids content of(%) 54 55 60 56 TABLE 3 Properties of foam samples made using illustrative composition Illustrative foamed compositions 1 2 3 4 5 6 1. Froth density (g/ml) 0.46 0.44 0.47 0.51 0.5 0.54 2. Rolling ball tack (cm) 1.3 0.6 1.9 2.4 1.5 1.1 3. Dry foam density (g/ml) 0.469 0.634 0.493 0.518 4. Peel StrengthA(N/cm) 9.4 11.6 12.0 9.8 5. Peel Strength B (N/cm) 2.4 1.1 3.4 2.7 6. Dry foam stability good poor excellent excellent

Claims

1. A method of providing a foamed adhesive layer having a stable foam structure and pressure sensitive adhesive characteristics comprising mixing a gas comprising oxygen with an aqueous dispersion comprising an unsaturated polyester, a pressure sensitive adhesive component and an accelerator comprising an organic compound effective for oxidative curing of the polyester, the aqueous dispersion having a viscosity of about 8,000 to about 30,000 mpas (Brookfield RVT spindle No 3, after 3 minutes constant stirring at 2.5 rpm) at 21"C t 2"C, and applying the mixture as a froth to a substrate to provide a stable foam structure having a tacky pressure sensitive adhesive surface.

2. A method according to claim 1 wherein the gas comprises air and the froth is applied at a density between about 0.18 and 0.60 g/ml.

3. A method according to claim 2 wherein the dried foam has a density between about 0.4 and 0.9 g/ml.

4. A method according to any one of the preceding claims wherein the dried foam has a uniform thickness of about 50mil.

5. An adhesive composition comprising a dispersion of an ethylenically unsaturated polyester material, a pressure sensitive adhesive component, frothing adjuvents and an accelerator comprising an organic compound effective for oxidative curing of the polyester material, the composition being such that when air is intimately incorporated therein to provide a froth, a froth is formed of a density in the range of about 0.18 to 0.6 g/ml which when dried provides a resilient storage stable foam structure having a pressure sensitive tacky surface.

6. A composition according to claim 5 wherein the dispersion comprises from 40 to 70 wt % total solids.

7. A composition according to claim 6 wherein the dispersion comprises from about 50% to about 60% by weight total solids.

8. A composition according to any one of claims 5,6 and 7 wherein the unsaturated polyester provides from about 5 to about 20 wt % of the dispersion.

9. A composition according to claim 8 wherein the unsaturated polyester provides from about 10 to about 15 wt % of the dispersion.

10. A composition according to any one of claims 5 to 9 wherein the ethylenically unsaturated polyester comprises an air drying alkyd resin.

11. A composition according to claim 10 wherein the alkyd resin has an acid value (mg KOH/g) of less than 20.

12. A composition according to claim 11 wherein the alkyd resin has a viscosity at 20"C of about 10,000 to about 30,000 mpas.

13. A composition according to any one of claims 10, 11 and 12 wherein the alkyd resin is a self emulsifying resin.

14. A composition according to any one of claims 10 to 13 wherein the alkyd resin has an oil content from about 60 to about 70% and a phthalic acid content of about 15 to about 30%.

15. A composition according to any one of claims 10 to 14 wherein the alkyd resin is derived from phthalic acid, safflower oil, tall oil, soyabean oil, cottonseed oil, dehydrated castor or linseed oil fatty acids and penta-hydric alcohol.

16. A composition according to any one of claims 5 to 15 wherein the pressure sensitive adhesive component comprises a carboxylated acrylic polymer.

17. A composition according to claim 16 wherein the carboxylated acrylic polymer provides from about 38 to about 52 wt % of the dispersion.

18. A composition according to claim 17 wherein the carboxylated acrylic polymer provides about 42 wt % of the dispersion.

19. A composition according to any one of claims 5 to 18 wherein the accelerator comprises a naphthenate or octoate of cobalt, barium or zinc or a mixture of any two or more thereof.

20. A composition according to any one of claims 5 to 19 wherein the dispersion has a pH value of about 8.

21. A composition according to any one of claims 5 to 20 wherein the composition comprises a thixotropic and or viscosity regulating agent.

22. A composition according to claim 5 substantially as hereinbefore described with reference to any one of the illustrative compositions.

23. A method according to any one of claims 1 to 4 wherein the aqueous dispersion used is an adhesive composition according to any one of claims 5 to 22.

24. A foamed structure in the form of an adhesive tape having a pressure sensitive adhesive surface formed by a method according to any one of claims 1,2,3,4 and 23 protected by a layer of release paper.