GB2123010A - A process for flexibilising epoxy resins - Google Patents

Abstract

An epoxy resin is flexibilised by addition, before hardening of the epoxy resin, of 3 to 45% of an acrylate polymer which has been obtained by copolymerisation of A1) 35 to 83% by weight of one or more alkyl esters of acrylic and/or methacrylic acid having 1 to 8 carbon atoms in the alkyl radical, A2) 10 to 40% by weight of vinyl acetate and/or acrylonitrile, A3) 7 to 35% by weight of glycidyl acrylate and/or glycidyl methacrylate and A4) 0 to 35% by weight of one or more acrylic and/or vinyl monomers different from the monomers A1) to A3) but not acrylic, methacrylic or itaconic acid and which has a mean vapour pressure osmometry molecular weight of 1,000 to 3,000 and on average at least one glycidyl group per molecule. The thus-modified epoxy resin can be used for bonding surfaces together.

Description

SPECIFICATION A process for flexibilising epoxy resins The invention relates to a process for flexibilising epoxy resins by adding acrylate polymers.

There are various prior art ways of flexibilising epoxy resins. For example, flexibilisation can be achieved by using special hardening agents, such as, for example, polyaminoamides. In many cases, however, there is no choice in respect of the hardening agent, because, for example, the hardening temperature, the rate of hardening or the glass transition temperature of the hardened epoxy resin are prescribed. In these cases one is forced to use such hardening agents as dicyanodiamide, polycarboxylic anhydrides or short-chain aliphatic polyamines, which, however, lead to brittle hardening products. It is then necessary to flexibilise the epoxy resins by adding modifiers.However, even in the case of flexible hardening agents, for example polyaminoamides, additional flexibilisation of the epoxy resin is frequently desirable, and the modifiers can then be distributed in the epoxy resin in the form of a physical mixture or react with the epoxy resin. Most of the known modifiers belong to the group of nonreactive additives. For this state of the art, reference is made to a book by H. Jahn, namely "Epoxidharze [Epoxy Resins]", published by VEB Deutscher Verlag für Grundstoffindustrie, Leipzig, 1969. The products chosen for use as reactive modifiers have groups which are capable of reacting with the epoxy groups of the epoxy resin, for example carboxyl groups.Those skilled in the art will understand that, although the reaction with the modifier should involve only some of the epoxy groups (to ensure that the epoxy resins can still harden), the incorporated modifier content should be sufficiently sizeable to ensure, the desired degree of flexibilisation.

U.S. Patent 3,948,849 discloses a reactive modifier which is a butadiene-acrylonitrile copolymer which has a molecular weight of 3,000 and terminal carboxyl groups. The epoxy resins are modified, before they become hard, by heating them together with the modifier at 1 6000 for about 30 minutes.

Used as adhesives, epoxy resins modified in this way give adhesive bonds which have resilient joints.

However, these compounds have a disadvantage in that some olefinic double bonds remain in the structure of the hardened modified epoxy resin and, because of their oxidisability, impair the chemical and physical properties of the hardened resins. European Patent Application 81/107,629.8 discloses a process for flexibilising epoxy resins by adding carboxyl-containing polymers, which is characterised in that copolymers obtained by CO2 polymerisation of a1) 40 to 87% by weight of one or more alkyl esters of acrylic and/or methacrylic acid which has 1 to 8 carbon atoms in the alkyl radical, a2) 10 to 40% by weight of vinyl acetate and/or acrylonitrile, a3) 1 to 20% by weight of acrylic, methacrylic and/or itaconic acid, a4) 1 to 5% by weight of glycidyl acrylate and/or glycidyl methacrylate, and aS) O to 35% by weight of acrylic and/or vinyl monomers which differ from the monomers a,) to a4) in the presence of a mercapto-containing regulator which has at least one carboxyl group, and having a mean vapour pressure osmometry molecular weight of 1 ,000 to 3,000, are added in such amounts to the epoxy resins before they are hardened that 1 to 60 mol% of the epoxy groups react with the carboxyl groups of the copolymer.

In this process, the modifier can be added to the total amount of epoxy resin. However, it is also possible to add the total amount of modifier only to a proportion of the total amount of epoxy resin. In doing so, care should however be taken to ensure that this modified, fractional amount of epoxy resin still has sufficient epoxy groups to ensure, on mixing of this amount with the remaining epoxy resin and subsequent hardening, reactive incorporation of this modified, fractional amount. The epoxy resins thus modified combine high resilience with firm adhesion to bounding surfaces. The viscosity of epoxy resins is only slightly increased by the modifiers. In the hardened state, the modified epoxy resins are chemically and physically stable.

This process for modifying epoxy resins has a certain disadvantage in that, before the epoxy resin is hardened, either the total amount of epoxy resin or a fraction thereof has to be reacted with the acrylate polymer acting as modifier. This additional reaction stage increases the cost of preparing the finished epoxy resin.

The present invention provides a process for flexibilising an epoxy resin, which process comprises adding to the epoxy resin before hardening a copolymer which has been obtained by copolymerisation of A1) 35 to 83% by weight of one or more alkyl esters of acrylic and/or methacrylic acid having 1 to 8 carbon atoms in the alkyl radical, A2) 10 to 40% by weight of vinyl acetate and/or acrylonitrile, A3) 7 to 35% by weight of glycidyl acrylate and/or glycidyl methacrylate, and A4) 0 to 35% by weight of one or more acrylicand/or vinyl monomers which differ from the monomers A1) to A3) and which are free of groups capable of reacting with the glycidyi groups of component A3) under the copolymerisation conditions, and which has a mean vapour pressure osmometry molecular weight of 1 ,000 to 3,000 and on average at least one glycidyl group per molecule, in an amount of 3 to 45% by weight relative to the total weight of the copolymer and epoxy resin.

The copolymer acts as a reactive modifier for epoxy resins which retains the abovementioned combination of advantages of high flexibilisation of epoxy resins with firm adhesion to bounding surfaces while, however, avoiding the necessity of the additional step of reaction between modifiers and epoxy resin before hardening.

The copolymers added to the epoxy resin particularly preferably have a mean molecular weight of 1,500 to 2,500.

Those copolymers are particularly preferred for use as modifiers which have on average 1.5 to 2.2 glycidyl groups per molecule.

The copolymers are obtained in a manner known in itself, namely by free-radical polymerisation of the monomers. This polymerisation is started in a known manner with an initiator such as azodiisobutyrodinitrile. The desired moelcular weight is obtained by, for example, using mercaptocontaining regulators or by telomerisation in free-radical-forming solvents, such as carbon tetrachloride.

Those compounds are preferably used as mercapto-containing regulators which also have at least one hydroxyl group. Examples of such mercapto-containing regulators are 2-mercaptoethanol and thioglycerol.

It is advisable to carry out the polymerisation in the presence of solvents, such as, for example, toluene. The polymerisation is effected at temperatures of 100 to 1200C, advantageously at the temperature of the boiling solvent, for example at the boiling point of toluene. Freed from solvent and from residual monomers, the polymerisation products are moderately to highly viscous at room temperature.

The monomers A,), which constitute the main content of the copolymer, are alkyl esters of acrylic and/or methacrylic acid having 1 to 8 carbon atoms in the alkyl radical, for example methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate, 2-ethylhexyl acrylate or 2-ethylhexyl methacrylate. Alkyl acrylates are particularly preferred.

The monomers A4), which can but need not be present in the copolymer, can be, for example, acrylamide, methacrylamide, styrene, isopropylidene acetate or vinyl propionate. The monomers should be free of groups which are capable under the polymerisation conditions of reacting with the glycidyl groups.

The copolymers are added to the epoxy resins in such amounts that the copolymers constitute 3 to 45% of the total weight of copolymer and epoxy resin. The copolymer content in the mixture is preferably 10 to 25% by weight.

Any epoxy resin known to those skilled in the art can, in principle, be used as epoxy resins. It is particularly preferable for epoxy resins to be based on bisphenol A or F and epichlorohydrin. Further examples of epoxy resins are the diglycidyl ether of hydrogenated bisphenol A, p-glycidyloxy-N,Ndiglycidylaniline and cycloaliphatic polyepoxy compounds as described in, for example, H. Jahn's "Epoxidharze [Epoxy Resins]", published by VEB Deutscher Verlag für Grundstoffindustrie, Leipzig, 1969, or H. Batzer and F. Lohse's article in "Ullmanns Enzyklöpadie der technischen Chemie [Ullmann's Encyclopaedia of Industrial Chemistry]", Volume 10, pages 563 et seq., 4th edition, Verlag Chemie, Weinheim, 1975.

The modified epoxy resins can be hardened with state of the art hardening agents. The following hardening agents can be used, in particular in hot hardening, that is to say in hardening at temperatures of greater than 1300C to about 2200 C: dicyanodiamide and its derivatives, polycarboxylic anhydrides, such as phthalic anhydride, methylhexahydrophthalic anhydride, and pyromellitic anhydride. Warm hardening, at temperatures of about 1000C, can be carried out with aromatic polyamines, such as mphenylenediamine, or cycloaliphatic polyamines. Room temperature hardening can be carried out with polyaminoamides, polyaminoimidazolines, modified aliphatic polyamines or polyetherpolyamines.Room temperature hardening is particularly advantageously carried out with mixtures of polyaminoamides or polyaminoimidazolines on the one hand the aliphatic polyamines or polyetherpolyamines on the other.

The use of such a hardening agent mixture with the modified epoxy resins gives particularly high strength values on bonding. A particular hardening temperature and/or hardening time can be lowered or shortened respectively by using l nown accelerators. Examples of such accelerators are tertiary amines.

Epoxy resins modified in accordance with the invention have the increased flexibility required.

They adhere firmly to bounding surfaces, and form a resilient adhesive joint. After modification but before hardening, the epoxy resins can also be used for impregnating webs, such as, for example, glass fibre webs or fabrics, which are then hardened to give laminates. These laminates can be used, for example, in the electrical industry for preparing circuit boards. These prepregs, as they are called, can also be used for moulded parts, such as, for example, in boatbuilding, or for repairs, for example in work on car-bodies. The modified epoxy resins can advantageously also be used as paint bases or as casting resins.

It is true that in European Patent Application 81/107,629.8, already mentioned above, the monomer mixtures for preparing the modifying copolymers contain 1 to 5% by weight of glycidyl acrylate and/or glycidyl methacrylate. However, at least some of this relatively low glycidyl compound content reacts with the carboxyl groups of the acrylic, methacrylic and/or itaconic acid also present in the monomer mixture and with the carboxyl group of the mercapto-containing regulator, and any residual glycidyl group content is too low to ensure that the modifier is incorporated into the epoxy resin when the resin is being hardened via these groups.

The Examples below contain a description of how the modifiers are prepared and of the properties of the hardened modified epoxy resins.

EXAMPLE 1 Preparation of low molecular weight acrylic polymers A solution of 450 g of monomer mixture, 1.8 g of azodiisobutyrodinitrile and 13.5 to 18 g of 2mercaptoethanol is added dropwise, with stirring and while nitrogen is passed in, to 85 g of boiling toluene in the course of 1.5 hours, during which refluxing conditions should be upheld. When all of the solution has been added, the mixture is refluxed for 2 hours. On addition of a further 1.3 g of azodiisobutyrodinitrile, the mixture is refluxed for a further 1.5 hours. The solvent and small amounts of unconverted monomers are distilled out in vacuo at a temperature from 90 to 1200C. The resulting 100% pure acrylic polymers are moderately to highly viscous at room temperature.

The composition of the mixture of monomers, the regulator content and some data measured on the acrylic polymers are given in Table 1.

The mean molecular weights contained in Table 1 were measured by vapour pressure osmometry.

TABLE 1 Composition and properties of the acrylic polymers

Mean Viscosity Serial Composition of monomer mixture Regulator1) molecular at 25 C Epoxide No. % by weight % by weight weight mPas equivalent 1 75.0 of n-butyl acrylate a1) 15.0 of vinyl acetate a2) 3.0 2000 40,000 1300 10.0 of glycidyl acrylate a2) 2 75.0 of n-butyl acrylate al) 12.5 of vinyl acetate a2) 4.0 1550 22,000 1050 12.5 of glycidyl acrylate a3) 3 74.0 of butyl acrylate al) 12.0 of acrylonitrile a2) 4.0 1600 85,000 960 14.0 of glycidyl acrylate a2) 4 51.0 of butyl acrylate a) 14.0 of but acrylate a,) 1800 2ethylhexyl acrylate a1) 3.5 1800 28,000 1030 12.0 of vinyl acetate a2) 13.0 of glycidyl acrylate a2) 5 67.0 of n-butyl acrylate al) 11.0 of vinyl acetate a2) 3.0 2050 55,000 1330 110.0 of glycidyl acrylate a3) 12.0 of vinyl propionate a4) 1) 2-mercaptoethanol, relative to 100 parts by weight of monomer mixture.

EXAMPLE 2 Demonstration of the flexibilising action of the acrylic polymers in epoxy resins The flexibilising action of the acrylic polymers in epoxy resins is demonstrated in a strength test, namely the DIN 53,289 peel test by means of reels. In this test, 85 g of an epoxy resin which is based on bisphenol A and has an epoxide equivalent of 185 is mixed with 15 g of acrylic polymer No. 1 to 5, and the mixture is admixed with the equivalent amount of various hardening agents. The mixtures are applied at a rate of 50 to 100 g/m2 to Al Cu Mg 2pl strips of aluminium (the strips are 0.5 or 1.6 mm thick, 250 mm long and 30 mm wide) which have been etched with chromate/sulphuric acid. The strips of aluminium are joined together, and the mixture is hardened under a slight contact pressure.

The following hardening agents and hardening conditions are used: a) dicyanodiamide, hardening at 1 850C for 1 hour, b) commercially available modified cycloaliphatic polyamine having an H equivalent of 111, hardening at 1000C for 30 minutes, and c) a commercially available modified polyamine having an H equivalent of 92, hardening at room temperature in the course of 2 days, followed by heating at 800C for 30 minutes.

The hardened test specimens are then subjected at 200C to the DIN 53,289 peel test by means of reels where the 0.5 mm thick strip of aluminium is peeled off.

The peel strength values found are shown in Table 2. As can be seen, the epoxy resin systems modified in accordance with the invention have considerably higher peel strength values and are accordingly much more flexible than unmodified epoxy resin systems.

TABLE 2 Peel strength by means of reels of various hardened epoxy resin systems

Peel strength in N/mm Hardening agent Modified with acrylic polymer a) dicyano- b) cycloaliphatic No. diamide polyamine c) polyamine 1 4.1 2.3 5.6 0 2 3.6 2.5 5.2 =0 3 > D 3 3.8 2.7 4.9 ot > .E 4 4.0 2.4 5.1 5 3.3 2.6 5.0 o c U a) o unmodified epoxy resin 0.9 0.4 0.3 0 =

Claims (7)

1. A process for flexibilising an epoxy resin, which process comprises adding to the epoxy resin before hardening a copolymer which has been obtained by copolymerisation of A,) 35 to 83% by weight of one or more alkyl esters of acrylic and/or methacrylic acid having 1 to 8 carbon atoms in the alkyl radical, A2) 10 to 40% by weight of vinyl acetate and/or acrylonitrile, A3) 7 to 35% by weight of glycidyl acrylate and/or glycidyl methacrylate, and A4) 0 to 35% by weight of one or more acrylic and/or vinyl monomers which differ from the monomers A,) to A3) and which are free of groups capable of reacting with the glycidyl groups of component As) under the copolymerisation conditions, and which has a mean vapour pressure osmometry molecular weight of 1 ,000 to 3,000 and on average at least one glycidyl group per molecule, in an amount of 3 to 45% by weight relative to the total weight of the copolymer and epoxy resin.

2. A process according to claim 1, wherein the copolymer added has a mean molecular weight of 1,500 to 2,500.

3. A process according to claim 1 or 2, wherein the copolymer added has on average 1.5 to 2.2 glycidyl groups per molecule.

4. A process according to any one of the preceding claims, wherein the copolymer added has been obtained in the presence of a mercapto-containing regulator.

5. A process according to claim 4, wherein the mercapto-containing regulator also contains at least one hydroxyl group.

6. A process according to claim 1 wherein the copolymer added is any one of acrylic polymers 1 to 5 hereinbefore specified.

7. A process for flexibilising an epoxy resin substantially as hereinbefore described.