Title of Invention

A PROCESS FOR PRODUCING TACK-FREE NON-MELTING ACRYLIC FILM ADHESIVES

Abstract

A process for producing tack-free non-melting acrylic adhesive comprising the steps of blending an acrylic copolymer having pendant epoxy groups obtained by copolymerising a mixture of an alkyl acrylate, an alkenyl cyanide and a glycidyl ester of an unsaturated organic acid with an aliphatic diamine such as piperazine to partially cure the epoxy group at ambient temperature and aromatic diamine (at a temperature of 180°C) to cure the remaining epoxy group to obtain an adhesive composition and then adding fillers such as silica powder in a solvent medium and casting said composition as a film on a support to obtain said adhesive

Full Text

This invention relates to a process for producing tack-free non-melting acrylic film adhesives, Non-melting thin films are useful as thermosetting film adhesives for bonding a variety of substrates including polyimide films. Film adhesives have great application potential in high-tech areas like aerospace. Bonding is the technique employed in the manufacture of several critical structure components for aerospace applications. Notable components include, honeycomb structures, antennas, solar sail and solar panel for satellites. Solar sail is made by bonding polyimide film segments. A polyimide suitable for such applications is marketed by DuPont in the trade name KAPTON. The bonding requirements in majority of these areas ar e generally met by use of glue-1ike and solvent-based resinous polymeric adhesives. There is an increased preference for dry film adhesives for such applications. This is because use of film adhesive can faci1itate bonding manoeuvre, avoid use of solvents and help regulate the flue line thickness and minimise wastage of adhesive and accumulation of dead weight which are very critical when weight saving becomes a matter of concern. A few film adhesives for structural and general purpose applications are known and the majority of them are based on high molecular weight epoxy resin and some on acrylic copolymers and vinyl acetal-phenolic systems. Thermosetting epoxy film adhesives require blending with other polymeric additives such as rubber, nylon etc. for deriving film-farming characteristics which makes the process complex and at times lead to problems of phase separation of the various components of the film. Those based on polyvinyl acetai-phenolic also have the short-comings of a two-component system and are associated with condensation cure reaction which is accompanied by evolution of volatiles during cure. This necessitates application of pressure during bonding to get defect—free bonds. Thus, E Lavins and J A Snelgrove describe in the Hand Book of Adhesives (Editor, Skiest; Publisher, Van Nostrand and Reinold, 1977) and L T Eby and H P Brown describe in Treatise on Adhesion And Adhesives (Editor R L Patrick, publisher, Marcel Dekker, 1966) vinyl acetal-phenolic structural adhesives in which polyvinyl acetal serves as co-reactant, flexibi1ising agent and flow control component. In this heterogeneous system, adhesive properties depend on the particle size of resole dispersed in polyvinyl acetal. Conventional acrylic film adhesives make use of vinyl polymerisation for crosslinking• Such cure reactions proceed with great difficulty and are incomplete under aerobic environments due to interference of atmospheric oxygen acting as polymerisation inhibitor- The poor shelf-life of the resultant films is also of concern - In many applications, the film adhesives is required to be non-tacky under ambient conditions, non-melting and non-flowing while heating and curing as otherwise melt-flow of the ahesive can cause many problems arising from the spreading of adhesives to unwanted areas of the adherends. Majority of the above mentioned systems do not possess these qualities, whereas the present invention possesses them. The main objective of the present invention relates to a process for production of a non-tacky, non-melting film adhesive suitable for the above referred applications using a combination of acrylic polymerisation and epoxy curing. The synthesised acrylic copolymer is blended with the fillers and curatives and cast in to tack-free, thin film adhesive for qse in bonding. The adhesives are selected so as to induce partial cure of the film at ambient conditions (or B-staging) and to render it non-tacky. The film does not show any melt-flow characteristics like many reported film adhesives. The film adhesive of the present invention can be used as a thermoset adhesive with good lap shear strength and peel strength for a variety of substrates including polyimide film. In the first step, a pendant epoxy-functional polymer is made by free radical copolymerization of a mixture of three vinyl monomers in a solvent. One such monomer is an alkeny1 cyanide monomer like acrylonitrile or methacrylonitrile. The second monomer of the mixture is an alkyl ester of acrylic or methacrylie acid where the alkyl part can be constituted by one among methyl, ethyl, propyl, butyl, pentyl, isopentyl, hexyl, or ethyl hexyl group an^ preferably butyl group. The third component of the mixture is constituted by glyeidyl ester of an unsaturated acid. Glyeidyl acrylate or glycidyl methacrylate are examples. The free radical initiator used can be any azo initiator, example azobis isobutyronitrile or azobiscyanocycloheaxane or it can be any peroxide initiator such as benzoyl peroxide or ditertiary butyl peroxide. The polymerisation is done in an organic solvent selected among tetrahydrofuran (THF), dioxane, methylethy1 ketone (MEK), methyl isobutyl ketone (MIBK), dimethyl formamide (DMF), dimethyl acetamide (DMAc) , N-methy 1 pyrrol idone (MNP) or dimethyl sulfoxide (DliSO) or a mixture of two or more of any of these solvents in any proportion. The preferred solvent is DMF. The polymerisation is performed at temperatures between 6O-80°C. The reaction is carried out for 5-10 hours and the resultant polymer is isolated by pouring the resinous solution to any non solvent. The nonsolvent can be chosen from among hydrocarbons, such as hexane, heptane, octane, cyclohexane or their mixtures in any combination and proportion. The nonsolvents can also be an alcohols such as ethanol, methanol, propanol, ispropanol, or a mixtures of these in any combination and proportion in the presence or absence of added water. The preferred one is methanol. In such systems, the alkyl acrylate or methacrylate imparts the required flexibility whereas the polar nitrile component contributes to enhanced wetting, adhesion and cohesive strength of the polymers. The epoxy groups incorporated through suitable monomer serves as the functional group for curing of the film adhesive. The polymer is blended with a filler, preferably silica powder of particle size below 10 micron, an aliphatic diamine for curing part of the epoxy groups at room temperature and an aromatic diamine to induce curing of the remaining epoxy groups at higher temperature. The part curing at room temperature by the aliphatic amine prevents the melt-flow of the resin on heating« The aliphatic diamine can be chosen from among piperazine or substituted piperazine or N,N' dialkyl alkane diamine where the alkane moiety can range from ethane to dodecane and the alkyl part from methyl to hexyl• The preferred diamine compound is 'piperAzine' The aromatic diamine is chosen among 3,3'- diamino diphenyl sulfone, 4,4-'diamino diphenyl sulfone, 3,3 -diamino diphenyl ketone, 4,4'-diamino diphenyl ketone or a mixture of these in any proportion, pr&f&renc& being given to 4,4'- diamino diphenyl sulfone. In such a copolymer formulation, the alkyl aerylate part can vary from 55-65X by weight, the nitrile monomer can vary between 20-35X by weight and the epoxy monomer-content is varied such that the polymer has an epoxy functionality in the range 0.15-0.3 equivalents/kg. The free radical initiator is taken at a concentration ranging from 0.01 to 0.02 percent by weight of the total monomer. The si 1ica fi1ler in the polymer formulation is varied in the range 10-20% by weight of the polymer. The aliphatic and aromatic diamines are added such that each accounts for 50X of the epoxy groups on an equivalent basis. In this calculation, the aliphatic diamine and aromatic diamine are both considered to possess two functional groups each for reaction with epoxy groups of the polymer on a 1:1 equivalent basis. Accordingly the present invention provides a process for producing tack-free non-melting acrylic adhesives comprising the steps of blending an acrylic copolymer with pendant epoxy groups obtained by copolymerising a mixture of an alky 1 aerylate, an alkenyl cyanide and a glyeidyl ester of an unsaturated organic acid with a mixture of an aliphatic diamine and an aromatic diamine to produce said adhesive and if desired adding known fillers such as silica powder in a solvent medium and casting said composition as a film on a support- The following examples illustrate the details of synthesis of the copolymer and the fomulated film adhesive derived from it. Example 1, Synthesis of epoxy-functional acrylic polymer (EFAP) A mixture containing 39 parts of butyl aerylate, 21 parts of acrylonitrile, 1 part of glyeidyl methacrylate and 39 parts of freshly disti1 led dimethyl formamide containing 0.1%.(with respect to the three monomers) of azo-bis-iso-butyronitrile, all taken in a 500 ml round bottomed flask fitted with a vacuum adaptor. The solution is cooled to -50°C or preferably -70°C and evacuated using a vacuum pump and then closed under vaccum. The solution is then kept in a water bath heated to 70°C and maintained at this temperature for 6 hours-The viscous solution is then poured drop-wise to 10 times its volume of methyl alcohol under agitation. The resinous polymer formed is separated by decanting the supernatant liquid. The precipitate is then dissolved in tetrahydrofuran (THF) to make approximately 10X (weight/weight) solution and then reprecipitated into methanol as described above. The polymer is then dried at 40 to 60°C under vacuum for 3 to 7 hours. The polymer is formed in 80-90% yield• It is characterised by Gel Permeation Chromatography (GPC) to determine the number average molecular weight (Mn) which is in the range 45000-55000 with a poly dispersity of 1.7 to 2.0. The IR spectrum of the polymer shows an absorption at 1740 cm" characteristic of the ester carbonyl groups and a strong absorption at 2150 cm" due to the nitrile group derived from acrylonitrile. The weak peak at 910 cm" is indicative of the epoxy groups. Example 2, Preparation of film adhesive 100 parts of the above dried polymer (EFAP) is dissolved in 280 parts of methyl ethyl ketone and is then mixed with 10-20 parts of fine silica filler. The mixture is transferred to a jar mill and mixed for 10-20 hours. The viscous fine dispersion is diluted with methyl ethyl ketone (MEK) containing equimolar mixture of piperazine and 4,4'-diamine diphenyl sulfone together equivalent to the epoxy-content of the polymer, so as to form a 10% (weight/weigh t) solution. For calculating the composition of the polymer and the epoxy equivalent, the formed polymer is supposed to have the same composition as the feed of three monomers which is correct since the conversion is near quantitative. The solution is then poured in to a rectangular metallic mould containing polypropylene IPP) sheet of around 200 micron thickness fixed on its bottom. The solvent is allowed to evaporate at room temperature for 24 hours. The quantity of solution poured in to the mould is regulated such that the final film has a thickness of 50-100 micron. The film with the PP sheet support is taken out and can be served directly as adhesive after releasing the back-up PP sheet. It is non-tacky and it doesn't melt on heating but only softens and consolidates. Example 3, Application of film adhesive for bonding The following procedure is typical for evaluation of adhesive properties of the film adhesive of example 2 using polyimide film. Thus, adhesive properties. Lap Shear Strength (LSS) and T-peel strength (TPS ) are determined with aluminised KAPTON film (procured from M/s. Dupont, USA) as per ASTM- D 1002 and ASTM-D 872 methods respectively. The film adhesive is cut and interposed between the polyimide adherends which are then bonded by heating them in an air oven to preferably 180°C and optionally 150°C for half an hour. A contact pressure of SO-100 psi and optionally 50 psi is applied while curing. The bonded specimens are tested in an IBSTRON UTM as per ASTM procedures for lap shear strength (LSS)_ and T-peel strength (TPS). Table 1 gives the typical characteristics of the polymer obtained from the above exmaple 1. Table 2 gives typical characteristics of the formulated resin described in example 2 for use in the production of film adhesive as described in the same example. Table 3 gives typical lap shear strength and T-peel strength of the film adhesive using aluminised KAPTON polyimide film as adherend as described in example 3 under different test conditions. Scheme 1 depicts the synthesis and curing of the polymer adhesives as described in examples 1 and 3 respectively. » The film adhesive disclosed here is advantageous over the conventional ones in the following respects. 1. It is easily synthesised; 2. The formulated resin can be easily cast into film; 3. The film is partly B-staged, non tacky at ambient conditions and it does not flow while curing at high temperature; 4. No volatile by-products are generated on curing, thus eliminating the possibility for generation of microvoids between adherends; 5. The film adhesive can be potentially served for other substrates as well. This includes the following adherends such as metals, rubbers, plastics and wood in any combination. Table 1: Typical composition and molecular characteristiics of epoxy functional acrylic polymer (EFAP) as in example 1 Table 2. Typical composition and characterisitcs of formulated resin as in example 2 Table 3s Lap shear strength (in kg/cm2 ) and T-peel strength (in kg/cm) of dry film polymers under different conditions using aluminised KAPTON film (Al—Al and Al—KAPTON surface) as in example 3 It is to be understood that the forgoing description and the appended claims do not exclude obvious equivalents known to persons skilled in the art. Our copending application No. /MAS/99 described and claims a process for synthesising high molecular weight acrylic copolymers with pendant epoxy functions. These copolymers are produced by copolymerising in a known manner a mixture of an alkyl acrylate, an alkenyl cyanide, and a glycidyl ester of an unsaturated organic acid in the presence of a known free radical initiator. WE CLAIM: 1. A process for producing tack-free non-melting acrylic adhesive comprising the steps of blending an acrylic copolymer with pendant epoxy groups obtained by copolymerising a mixture of an alkyl acrylate, an alkenyl cyanide and a glycidyl ester of an unsaturated organic acid with a mixture of an aliphatic diamine and an aromatic diamine to produce said adhesive and if desired adding known fi1lers such as si 1ica powder in a solvent medium and casting said composition as a film on a support. 2. The process as claimed in claim 1, wherein the aliphatic diamine is selected from substituted or unsubstituted piperazine, N, N'- dialkyl alkane diamine wherein the alkane group is selected from ethane to dodecane either alone or in combination - 3. The process as claimed in claims 1-2, wherein the aromatic diamine is selected from 3,3-'diamine diphenyl sulphone, 4,4- diamino diphenyl sulphone, 3,3-'diamino diphenyl ketone, 4,4'- diamine diphenyl ketone, either alone or in combination. 4- The process as claimed in claim 1, wherein 10-20% by wt of silica is optionally added to said adhesive prior to film forming. 5. The process as claimed in claims 1-4, wherein the film is formed by casting the solution on a support and drying at room temperature. 6- The process as claimed in claim 5, wherein the support is made of polypropylene or polyethylene sheet. 7. A process for producing tack-free non melting acrylic film adhesive substantially as herein described.

Documents:

452-mas-1999-abstract.pdf

452-mas-1999-claims duplicate.pdf

452-mas-1999-claims original.pdf

452-mas-1999-correspondance others.pdf

452-mas-1999-correspondance po.pdf

452-mas-1999-description complete duplicate.pdf

452-mas-1999-description complete original.pdf

452-mas-1999-drawings.pdf

452-mas-1999-form 1.pdf

452-mas-1999-form 19.pdf

452-mas-1999-form 26.pdf

452-mas-1999-form 3.pdf

452-mas-1999-form 5.pdf