|Title of Invention||
A PROCESS FOR PRODUCING THERMOSETTING ADHESIVES FROM ACRYLIC COPOLYMERS HAVING PENDANT PHENOLIC GROUPS
|Abstract||This invention relates to a thermosetting adhesive from acrylic copolymers having pendant phenolic groups. An alkenyl cyanlde morlomer, an acrylic acid ester monomer and a monomer containing both an unsaturation and a phenol group are copolymerised in the presence of a free radical initiator. The copolymer is isolated and converted to thermosetting adhesive by adding stoichiometric quantity of a polyelectrophile to form a thermosetting adhesive.|
This invention relates to a process for producing a thermosetting adhesive from acrylic copolymers having pendant phenolic groups.
Film adhesives have great application potentiality in high-tech areas like aerospace. The bonding requirements in these areas are generally met by use of glue like and solvent-based resinous polymeric adhesives. Use of film adhesive facilities the bonding manoeuvre and helps regulate the glue line thickness and minimise wastage of adhesive and accumulation of dead weight which is very critical when weight saving becomes a matter of concern. A few film adhesives useful in structural and non structural applications have been reported. Some of these film adhesives and their applications have been cited by E. Lavins and J.A.Snelgrove in the Hand Book of Adhesives (Editor, Skiest; Publisher, Van Nostrand and Reinold, 1977) and by L.T.Eby and H.P.Brown in Treatise on Adhesion and Adhesives (Editor, R.L.Patrick; Publisher, Harcel Dekker, 1988). Majority of the reported film adhesives are based on high molecular weight epoxy resin and a few are based on acrylic copolymers. Two component vinyl acetalphenollcs constitute another less-preferred film adhesive. The decreased preference is due to the condensation nature of the cure reaction which generates voids In the bond, thereby weakening it, Epoxy film adhesive dominate the industry and this system invokes need for blending with polymers suoh as nylon, rubber etc. to confer film-forming characteristics. The
physical and adhesive properties are hence dictated by the nature of the additives. Those based on acrylic films uses vinyl polymerisation for orosslinklng which is unpredictable in aerobic conditions due to interference of atmospheric oxygen acting as polymerisation inhibitor. Whereas the present invention discloses thermoplastic and thermoset film adhesives formed from phenol-functional acrylic polymers with built-in film forming characteristics wherein, the thermoset adhesive undergoes facile cure by addition reaction without evolution of volatile byproducts and the thermoplastics, simply by melting and consolidation. Both systems have very good shelf-life at ambient conditions.
The object of this invention is in producing a thermosetting adhesive suitable for fabrication of dry, thin films and methods for their application as thermoplastic and thermoset adhesive for bonding a variety of substrates including metals, rubbers, plastics, wood, composites etc.
Our co-pending applciatlon /HAS/89 discloses and claims a process for producing acrylic copolymers with pendant phenol function.
formamide can be chosen as the solvent. The polymerisation is performed at temperatures between 60-90°C for 3 to 10 hours and the formed polymer is isolated by pouring the resinous solution to any nonsolvent. The nonsolvent can be chosen among hydrocarbons, such as hexane, heptane, octane, cyclohexane or their mixtures in any combination and proportion. The nonsolvents can also be alcohols such as ethanol, methanol, propanol, isopropanol, or a mixtures of these in any combination and proportion. Typically methanol can be chosen. In such a copolymer formulation, the alkyl aorylate part can vary from 55-65% by weight, the nitrile monomer can vary between 20-35% by weight and the phenol functional monomer is varied from 3 to 15% by weight. The free radical initiator is taken at a concentration ranging from 0.05 to 0.2 percent by weight of the total monomer.
The phenol group in the polymer offers amenability for structural modifications and orosslinking through its reactions with other functional electrophiles. Thus, the polymer can be crosslinked through reaction of the phenol groups with any polyelectrophile such as polyepoxide typically a diepoxide.
Accordingly the present invention provides a process for producing a thermosetting adhesive from acrylic copolymers having pendant phenolic groups which comprises the steps of copolymerising an alkenyl cyanide monomer, an acrylic acid ester monomer containing both an unsaturation and a phenol group in the
presence of a free radical initiator. Isolating the copolymer by known methods, converting said copolymer by adding stoichiometric quantity of polyelectrophile thereto to form a thermosetting adhesive.
The following examples illustrate typical details of synthesis of the copolymer and the film adhesive from it.
PROCESS FOR PHEMOL-FUMCTIORAL ACRYLIC POLYMERS
59.6 parts of butyl acrylate and 31 parts of acrylonitrile are mixed with 9.5 parts of 4 maleimido phenol in 100 ml freshly distilled dimethyl formamide containing 0,10 part of azobisisobutyronitrile all taken in a 500 ml round bottomed flask fitted with a vaccum adaptor. The solution is cooled to -50°C or preferably -70°C and evacuated using a vacuum pump and then closed under vacuum. The solution is then kept in a water bath heated to 60 - 70°C and maintained at this temperature for 2 to 8 hours. The viscous solution is then poured drop-wise to 2 litres of methyl alcohol under agitation. The resinous polymer formed is separated by decanting the supernatant liquid. The entire precipitate is then dissolved in 150 parts of tetra hydro furan (THF) and then reprecipitated as described. The polymer is then dried at 40 to 50°C under vacuum for 4 to 7 hours. The polymer is formed in 80-90% yield. The polymer is characterised by Gel Permeation Chromatography to determine the molecular weight (Mn)
in the range 50,000-60,000 with a poly dispersity index of 1.7 to 2.0. The infra red (IR) spectrum of the polymer shows an absorption at 1780 cm-1 characteristic of the amide and at 1740 cm-1 due to ester carbonyl groups and a strong absorption at 2150 cm-1 due to the nitrile group. The broad peak at 3300-3500 cm-1 is indicative of the hydroxyl groups.
PROCESS FOR THERMOSET ADHESIVE (TSA) : EXA MPLE - 2
Acrylic copolymers with pendant phenolic group produced by the above steps is transformed to a thermosetting film as follows. The polymer is dissolved in methyl ethyl ketone and is premixed with a bisphenol-A-based diepoxide for an epoxy/phenol equivalent ratio of unity along with 1% by weight (of the resin) of triphenyl phosphine catalyst. The phenol equivalent of the resin is calculated to the first approximation from the relative weight of phenol monomer in the monomer mixture assuming that the copolymer has the same composition as the monomer feed which is correct since the conversion is near quantitative. Typically a diepoxide like Lapox B-1: (supplied by Hindustan Ciba Geigy, Mumbai) with epoxy-content of 5.5 equivalents/kg can be used for blending prior to casting the film. This film is used as thermoset adhesive (TSA). The adhesive formed is cast into film by known means and the formed film is tested for its mechanical properties after curing it following a step-wise cure schedule of 30 minutes each at 80°c, 100°C and 150°C. Mechanical properties are included in table 2.
APPLICATION OF ADHESIVE IN BONDING : EXAMPLE - 3
The adhesive properties, Lap Shear Strength (LSS) and T-Peel Strength (TPS) of the film adhesives of example 2 are determined usinj chromic acid-etched B-51-SWP aluminium alloy as per ASTH D-1002 and ASTH-D 872 methods respectively. The film of required area is cut and interposed between the joints with contact pressure. The step-wise cure schedule of 30 minutes each at 80°C, 100°C and 150°C is adopted both for thermoplastic adhesive and thermoset adhesive (TSA, example 2). The aluminium alloy can be replaced by nitrile rubber in which case no etching is given.
Table 1 gives the relative concentrations of the monomers employed for realising the phenol-functional acrylic polymers (PFAP) described in example 1.
Table 2 gives typical mechanical properties of both thermoplastic adhesive and cured thermoset adhesive (TSA, as in example 2).
Table 3 gives the LSS and TPS of both types of adhesive both for aluminium-aluminium and rubber-aluminium systems as described in example
Scheme 1 depicts the synthesis of the polymer as in example 1 and curing of the thermoset adhesive resulting from it as in example ,
Table 1: Typical composition and molecular characteristics of phenol-functional acrylic polymers (PEAP)
Table 2. Mechanical properties of thermoplastic adhesive (TPA) and cured thermoset adhesive ( TSA). The number refer to adhesives derived from corresponding PFAP of table 1.( Example: TPA-1 and TSA-1 are derived from PFAP-1)
Table 3 : LSS (in kg/cm2) and T-peel strength (in kg/cm) of TPA and TSA polymers under ambient conditions
The advantages of the present invention are:
l.The film adhesives disclosed in the present invention is easily synthesised and their composition and consequently the mechanical properties can be conveniently tailored.
2.The polymer readily forms film which is transparent and is stable at ambient conditions indefinitely.
3.The dry film can be rendered thennoplastic or thermoset through easy formulation while casting film from it.
4.The absence of condensation cure reaction avoids probability for formation of microvoids between bonded substrates.
5.It can be easily used as adhesive for different adherends for moderately load bearing applications.
1. A process for producing a thermosetting adhesive from
acrylic copolymers having pendant phenolic groups comprising the
steps of copolymerising an alkenyl cyanide monomer an acrylic
acid ester monomer and a monomer containing both an unsaturation
and a phenol group in the presence of a free radical initiator,
isolating the copolymer by known methods, converting said
copolymer by adding stoichiometric quantity of polyelectrophile
thereto to form a thermosetting adhesive.
2. The process as claimed in claim 1, wherein said acyrlic
copolymer with pendant phenolic groups and the polyelectrophile
are separately dissolved in organic solvents and then admixed.
3. The process as claimed in claims 1 and 2 wherein said polyelectrophile is a diepoxide resin.
4. The process as claimed in claim 3, wherein said resin is a biphenol A based diepoxide resin.
5. The process as claimed in claims 1 to 4, wherein polyelectrophile is added to said copolymer in the presence of triphenyl phosphine catalyst.
6. The process as claimed in claim 1 wherein the alkenyl cyanide monomers is acrylonitrile or methaorylonitrile.
7. The process as claimed in claim 1 wherein the acrylic acid ester is an ester of methanol, ethanol, propanoic isopropanol, butanol, isobutanol, pentanol or ethylhexyl alcohol.
8. The process as claimed in claim 1, wherein said monomer containing an unsaturated and a phenol group is selected from hydroxy phenyl maleimide, hydroxy phenyl citraconimide, hydroxy phenyl itaconimide and hydroxy styrene.
9. The process as claimed in claim 4, wherein said phenol group is located on ortho, meta or para positions with respect to the unsaturated group.
10. The process as claimed in claim 1, wherein the free radical initiator is an azo compound preferably azo bis isobutyronitrile or azobis cyanocyolohexane.
11. The process as claimed in claims 1 and 2 wherein the oopolymerisation is carried in the presence of a peroxide catalyst.
12. The process as claimed in claim 3, wherein the peroxide catalyst is selected fron benzoyl peroxide, ditertiary butyl peroxide, dicunyl peroxide and cyclohexanone peroxide.
13. The process as olained in claims 1-4 wherein the polymerisation is carried at 60 to 90°C for 10 hours.
14. The process as claimed in claims 1-5, wherein said polymerisation is carried out in the presence of an organic solvent selected from tetra hydrofuran; dioxane, methyl ethyl ketone, methyl isobutyl ketone, dimethylformamide, dimethylacetamide, either alone or in combination.
15. The process as claimed in claims 1-8, wherein 55 to 65% by wt of alkyl acrylate monomer, 20-35% by weight of the nltrile monomer and 3 to 15% by weight of phenol functional polymer are copolymerised.
16. The process as claimed in claims 1-6 wherein said
thermosetting adhesive is cast into films.
17, A process for producing a thermosetting adhesive
substantially as herein described.
|Indian Patent Application Number||454/MAS/1999|
|PG Journal Number||50/2007|
|Date of Filing||21-Apr-1999|
|Name of Patentee||M/S. INDIAN SPACE RESEARCH ORGANISATION|
|Applicant Address||ANTARIKSH BHAVAN, NEW BEL ROAD, BANGALORE 560 094.|
|PCT International Classification Number||C 09 J 04/02|
|PCT International Application Number||N/A|
|PCT International Filing date|