GB2113690A - Process for imparting flexibility to epoxide resins - Google Patents

Abstract

Flexibility is imparted to expoxide resins by adding reaction products of polyoxyalkylene monools or polyols with cyclic carboxylic acid anhydrides in a molar ratio of 1 mol of hydroxyl group to 0.3 to 1 mol of acid anhydride group, subject to the proviso that, on average, at least 1 carboxyl group is present in the molecule in the reaction product. The modifying agents have a low viscosity, so that the modified epoxide resins may hardly deviate from their initial viscosity and the modified epoxide resins are non-specific in respect of curing agents and can, for example, be cured under cold conditions by means of any desired polyaminoamides.

Description

SPECIFICATION Process for imparting flexibility to epoxide resins The invention relates to a process for imparting flexibility to epoxide resins by adding polymers containing carboxyl groups, in such quantities that 1 to 60 mol % of the epoxide groups react with the carboxyl groups of the polymer before curing.

Various possible means of imparting flexibility to epoxide resins are known from the state of the art. Thus, for example, it is possible to impart flexibility by using special curing agents, such as polyaminoamides. In many cases, however, there is no possibility of choice with regard to the curing agents, for example if the curing temperature, the rate of curing or the glass temperature of the cured epoxide resin is prescribed. In these cases it is compulsory to use curing agents, such as dicyandiamide, polycarboxylic acid anhydrides or short-chain aliphatic polyamines, which, however, result in brittle cured products. It is then necessary to impart flexibility to the epoxide resins by adding modifying agents. Even in the case of flexible curing agents, for example the polyaminoamides, however, it is frequently desirable to impart additional flexibility to the epoxide resin.

In this connection, the modifying agents can be distributed in the epoxide resin in the form of a physical mixture or can react with the epoxide resin.

Most of the known modifying agents belong to the group of unreactive additives. In relation to this state of the art, reference is made to the book by H. Jahn "Epoxidharze" ("Epoxide resins"), VEB Deutscher Verlag für Grundstoffindustrie, Leipzig, 1969.

Products which are selected as reactive modifying agents are those containing groups which are capable of reacting with the epoxide groups of the epoxide resin, for example carboxyl groups. It will be understood by those skilled in the art that, in the reaction with the modifying agent, in order to make it still possible for the epoxide resins to cure, only a fraction of the epoxide groups can be allowed to react, but the proportion of modifying agent incorporated must be sufficiently large for the desired flexibility to be achieved.

Butadiene-acrylonitrile copolymers having a molecular weight of 3,000 and terminal carboxyl groups are disclosed as reactive modifying agents in U.S. Patent Specification 3,948,849. Modification of the epoxide resins is effected before curing by heating the epoxide resins containing the modifying agent at 160"C for about 30 minutes. If epoxide resins which have been modified in this manner are used as adhesives, adhesive bonds having elastic joints are obtained.

A particular disadvantage of these compounds consists in the fact that they have a high viscosity and are therefore, on the one hand, difficult to disperse in the epoxide resin and, on the other hand, increase the viscosity of the epoxide resin very greatly. The result of this, however, is to restrict considerably the possibility of adding fillers to the epoxide resin in order to reduce its cost and/or to affect its properties.

Low-viscosity epoxide resins are often desired for reasons of technical performance in use too.

European Patent Application 81,107,629.8 describes a process for imparting flexibility to epoxide resins by adding polymers containing carboxyl groups, which is characterised in that copolymers which have been obtained by polymerizing together, in the presence of a chain regulator which contains mercapto groups and which has at least one carboxyl group:: a1) 40 to 87% 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) 1 to 20% by weight of acrylic, methacrylic and/or itaconic acid, a4) 1 to 5% by weight of giycidyl acrylate and/or glycidyl methacrylate and a5) 0 to 35% by weight of acrylic and/or vinyl monomers different from the monomers a1 to a4, the copolymers having an average molecular weight, determined in a vapour pressure osmometer, of 1,000 to 3,000, are added to the epoxide resins before curing in quantities such that 1 to 60 mol % of the epoxide groups react with the carboxyl groups of the copolymer.

If acrylic polymers are used as modifying agent, a high degree of elasticisation is made possible, without impairing the adhesing of the modified epoxide resins to interfaces. The viscosity of the epoxide resins is increased moderately by the modifying agents.

It has therefore been a requirement of the present Applicants to find modifying agents, containing carboxyl groups, which have a very low viscosity, so that the viscosity of the modified epoxide resin is not significantly greater than the initial viscosity of the epoxide resin, or the viscosity of the modified epoxide resin is decreased, and which, additionally, are as far as possible non-specific in respect of curing agents, so that those skilled in the art have available to them a fairly large selection of curing agents which appear to them to be suitable, and are not, as hitherto, dependent on a few combinations of curing agents. This applies particularly to the possibility of using polyaminoamides in cold-curing epoxide resin systems.

The present invention provides a process for imparting flexibility to an epoxide resin by adding thereto before curing, at least one polymer containing carboxyl groups, in such a quantity that 1 to 60 mol % of the epoxide groups in the epoxide resin react with the carboxyl groups of the polymer, in which process the polymer containing carboxyl groups is one obtainable by reacting a polyoxyalkylene monool or polyol, having an OH functionality of 1 to 5 and whose polyoxyalkylene blocks together have a molecular weight of 500 to 3,500, with a cyclic carboxylic acid anhydride in a molar ratio of 1 mol of hydroxyl groups to 0.3 to 1 mol of acid anhydride groups, subject to the proviso that, on average, at least one carboxyl group is present per molecule in the polymer produced.

Polyoxyalkylene monools or polyols used for the reaction with the carboxylic acid anhydrides are suitably obtained by an addition reaction between ethylene oxide, propylene oxide, tetrahydrofuran or mixtures thereof and compounds containing acid hydrogen, such as compounds containing carboxyl or hydroxyl groups or CH groups which are activated by adjacent carbonyl groups. Monohydric to pentahydric alcohols are preferred as starting compounds. The addition reaction with the alkylene oxide is effected in a manner which is in itself known, for example using an alkali metal methoxide as catalyst and at temperatures of 80 to 140"C and a corresponding pressure.Suitable starting alcohols are monohydric lower aliphatic alcohols with 1 to 5 carbon atoms, such as methanol, ethanol, propanol, butanol or isobutanol, dihydric alcohols, such as ethylene glycol, propylene or butylene glycol, trihydric alcohols, such as glycerol, tetrahydric alcohols, such as pentaerythritol, or pentahydric alcohols, such as arabitol.

In a preferred procedure an addition reaction is carried out between alkylene oxides and aryl monools or polyols or alkaryl monools or polyols. Examples of suitable aryl polyols are resorcinol, phloroglucinol, hydroquinone, 2,7-dihyd roxynaphthalene or the 2,6- or 1,8-isomers, or 2,6-dihydroxyanthracene. 2,2-Bis-(4- hydroxyphenyl)-propane, bis-(4-hydroxyphenyl)-methane, 1,1 ,2-tris-(4-hydroxyphenyl)-ethane, 1,1 ,34ris-(4- hydroxyphenyl)-propane, bis-(2-hydroxyphenyl)-ethane or 2,2-bis-(4-hydroxymethyl phenyl)-propane are also particularly suitable for use as starting alcohols.

Polyoxyalkylene polyols which can be obtained by an addition reaction between alkylene oxides and novolaks having a molecular weight of 300 to 600 are, however, also suitable. In this context novolaks are, as is known, to be understood as meaning the reaction products of phenol with formaldehyde in a molar ratio of 1:0.5 to 0.9.

Polyoxyalkylene monools or polyols which are particularly preferred are those in which at least 50% by weight of the oxyalkylene groups are oxypropylene groups.

Polymers containing carboxyl groups and which can be used with advantage are obtained by using polyoxylakylene monools or polyols in which the polyoxyalkylene blocks together have a molecular weight of 800 to 2,000, and it is particularly preferable if the reaction products contain, on average, 1 to 2.5 carboxyl groups per molecule.

Cyclic polycarboxylic acid anhydrides which are employed preferentially are succinic anhydride, maleic anhydride or phthalic anhydride. Further suitable cyclic polycarboxylic acid anhydrides are dichloromaleic anhydride, dodecenylsuccinic anhydride, glutaric anhydride, tetrahydrophthalic anhydride, 3,6- dimethyltetrahydrophthalic anhydride, cis-hexahydrophthalic anhydride, methylhexahydrophthalic anhydride, endomethylenetetrahydrophthalic anhydride, methylendomethylenetetrahydrophthalic anhydride, hexachloroendomethylenetetrahydrophthalic anhydride, pyromellitic dianhydride, ciscyclopentanetetracarboxylic acid dianhydride, hemimellitic anhydride, trimellitic anhydride and naphthalene-1,8-dicarboxylic acid anhydride.

The reaction of the polyoxyalkylene monools or polyols with the cyclic polycarboxylic acid anhydrides is suitably effected in a manner which is in itself known by heating the reactants at 100 to 150"C for a period of about 0.5 to 10 hours. Depending on its content of carboxyl groups, the reaction product is of low viscosity to very mobile, the viscosity of the reaction product increasing with the content of carboxyl groups.

It is not absolutely necessary to add a solvent in the reaction. The addition of a catalyst is also superfluous, since the free carboxyl groups formed in the reaction themselves catalyse the further reaction.

In principle, the epoxide resins known to those skilled in the art can be used as the epoxide resins. Epoxide resins based on the reaction products of bisphenol A or F and epichlorohydrin are particularly preferred.

Further examples of epoxide resins are the diglycidyl or polyglycidyl ethers of polyhydric aliphatic alcohols, such as 1,4-butanediol, or of polyalkylene glycols, such as propylene glycols; diglycidyl or polyglycidyl ethers of cycloaliphatic polyols, such as 2,2-bis-(p-hydroxycyclohexyl)-propane; diglycidyl or polyglycidyl ethers of polyhydric phenols, such as resorcinol or 2,2-bis-(4'-hydroxy-3',5'-dibromophenyl)-propane, or of condensation products, obtained under acid conditions, of phenols with formaldehyde, such as phenol novolaks and cresol novolaks; polyglycidyl esters of polybasic carboxylic acids, such as phthalic acid, terephthalic acid,-tetrahydrophthalic acid and hexahydrophthalic acid;N-glycidyl derivatives of amines, amides and heterocyclic nitrogen bases, such as N,N-diglycidylaniline, N,N-diglycidyltoluidine, N,N,N',N' tetraglycidyl-bis-(p-aminophenyl)methan4, triglycidyl isocyanurate, N,N'-diglycidylethyleneurea, N,N' diglycidyl-5,5-dimethylhydantoin, N,N'-diglycidyl,5,5-dimethyl-6-isopropyl-5,6-dihydrouracil, and further epoxide resins, such as are described, for example, in H. Jahn "Epoxidharze" ("Epoxide resins"), VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig, 1969, or in H. Batzer and F. Lohse "Ullmanns Enzyklop die der technischen Chemie" ("Ullmann's Encyclopaedia of Industrial Chemistry"), volume 10, page 653 et seq., 4th edition, Verlag Chemie, Weinheim, 1975.

The modification of the epoxide resins can be effected in various ways. Thus, it is possible to add the intended quantity of modifying agent to the total quantity of epoxide resin. Even if the reaction of the carboxyl groups of the modifying agent with the epoxide groups of the epoxide resin starts at a temperature as low as room temperature, it is still preferable to warm the mixture to temperatures of 100 to 1 500C. The reaction takes place in the course of 30 minutes to 4 hours. However, the intended quantity of modifying agent can be added to only a fraction of the total quantity of the epoxide resins.It is only necessary to take care that this modified fraction quantity of the epoxide resin still has sufficient epoxide groups to ensure that, when this quantity is mixed with the remainder of the epoxide resin and subsequently cured, this modified fractional quantity is incorporated by means of a reaction. It is sufficient for the modified fractional quantity still to contain about 40 mol % of the epoxide groups originally present. The advantage of this procedure consists in the fact that the modification can be effected at an early stage, even by the manufacturer. it is also possible to carry out the partial modification using an epoxide resin having a composition differing from that of the remaining quantity of epoxide resin. In particular, the partial modification can be carried out using a low-viscosity epoxide resin.The diglycidyl ethers of aliphatic diols, for example of 1,4-butanediol, 1,6-hexanediol, or neopentyl glycol, are particularly suitable for this purpose.

It can be advantageous if a catalyst which accelerates the modifying reaction is added in effective quantities to the mixture of epoxide resin and modifying agent. Catalysts which are particularly preferred are quaternary ammonium or phosphonium compounds, such as, for example, tetramethylammonium chloride or iodide, benzyltrimethylammonium chloride ortetrabutylphosphonium chloride or acetate.

The curing agents which are known from the state of the art can be used for curing the modified epoxide resins. The following curing agents are particularly suitable for curing under hot conditions, that is to say curing at temperatures from above 1 300C to about 2200C: dicyandiamide and derivatives thereof; polycarboxylic acid anhydrides, such as phthalic anhydride; methylhexahydrophthalic anhydride; or pyromellitic dianhydride. Aromatic polyamines, such as m-phenylenediamine or cycloaliphatic polyamines, are suitable for curing under warm conditions at temperatures of about 100 C. Curing at room temperature can be carried out using polyaminoamides; polyaminoimidazolines; or modified aliphatic polyamines or polyether-polyamines.

Polyaminoamides or polyaminoimidazolines are suitable for curing at room temperature. Particularly high increases in strength values are obtained by means of them when using the modifying agents according to the invention.

The particular curing temperature and/or curing time can be reduced or shortened by using known accelerators. Examples of such accelerators are tertiary amines.

Epoxide resins which have been modified in accordance with the invention are particularly suitable as adhesives, since they adhere well to interfaces to be joined and form an elastic adhesive joint. However, it is also possible to impregnate carrier webs, such as glass fibre non-wovens or fabrics, with the modified, but not yet completely cured epoxide resins, and to cure these webs to give laminates. They can be used, for example, in the electrical industry for the manufacture of printed circuits. A further possible use of these so-called prepregs consists in the production of shaped articles, such as in boat-building, and also for repair purposes, for example in the construction of car bodies. Use of the modified epoxide resins as paint raw materials or as casting resins is also advantageous.

The following Example serves to illustrate the invention.

PREPARATION EXAMPLE Modifying agents A mixture of 175 g of a product from the addition reaction of propylene oxide with glycerol, having a hydroxyl number of 160 and the polyoxypropylene blocks having together a molecular weight of 960 described below as polyether I - and 26.0 g of succinic anhydride (0.52 mol of anhydride/mol of OH group) is heated to a temperature of 1200C in the course of 20 minutes, while stirring and passing nitrogen over the mixture, whereby the succinic anhydride dissolves in the reaction mixture. The reaction mixture is kept at this temperature and the acid number is determined regularly. When an acid number of 72 to 74 (theoretical acid number 72.6) has been reached, a vacuum is applied for approx. 15 minutes in order to remove small quantities of unreacted anhydride, and the mixture is then cooled.A pale yellow liquid which has an acid number of 73 and a viscosity of 1,680 mPas at 25"C is obtained.

Further modifying agents, the characteristic data of which can be seen in Table 1, are prepared by this method from polyether I and further polycarboxylic acid anhydrides and also from polyethers II and III and polycarboxylic acid anhydrides. Polyethers II and III have the following composition: Polyether II is a bifunctional product from the addition reaction of propylene oxide with bisphenol A. Its hydroxyl number is 110 and the molecular weight of the sum of the polyoxypropylene blocks is 800.

Polyether Ill is a bifunctional product from the addition reaction of a mixture of 70% by weight of propylene oxide and 30% by weight of ethylene oxide with 1 ,4-butanediol, the reaction being carried out in such a way that the oxyethylene groups are preferentially located at the end of the chain. The hydroxyl number is 56 and the molecular weight of the sum of polyoxyalkylene blocks is 1,910.

EXAMPLE Modification of epoxide resins The modification is carried out by heating mixtures of an epoxide resin, formed from bisphenol A/epichlorohydrin and having an epoxide equivalent of 185 g/mol, and varying quantities of the modifying agents, after adding 0.03% by weight of tetramethylammonium chloride, for 2 hours at 120"C while stirring and passing nitrogen over the mixture. The ratios of epoxide and modifying agent can be seen from Table 1.

After cooling, epoxide resins which are liquid at room temperature, the viscosity of which is slightly reduced or only slightly increased compared with that of the unmodified epoxide resin (approx. 10,000 mPas at 25"C), are obtained. The viscosity and the epoxide equivalent of the modified epoxide resins can be seen from Table 1.

TABLE 1 Composition and properties of modified epoxide resins Serial Modifying agents no. Poly- Polycarboxy- Mols of Viscosity ether liy acid anhydride/ Acid at 25DC, no. anhydride mols of no. mPas OH group 1 I Succinic 0.52 73 1680 anhydride 2 I Succinic 0.67 92 2500 an hydride 3 I Maleic 0.52 70 1840 anhydride 4 I Phthalic 0.52 67 3440 anhydride 5 11 Succinic 0.75 71 8400 anhydride 6 11 Maleic 0.75 38 620 anhydride Modified epoxide resin Serial no.Epoxide resin, Modifying agent, Epoxide Viscosity parts by weight parts by weight equiva- at 25"C lent mPas 1 80 20 250 11000 2 85 15 230 15000 3 85 15 228 9500 4 80 20 245 12500 5 85 15 230 16500 6 80 20 240 8900 Two adducts are prepared using the modifying agent listed in Table 1 under no. 1, formed from polyether I and 0.52 mol of succinic anhydride per mol of hydroxyl group: A) 67 g of modifying agent no. 1 and 33 g of epoxide resin formed from bisphenol A/epichlorohydrin, epoxide equivalent 185 B) 70 9 of modifying agent no. and 30 g of neopentyl glycol diglycidyl ether, technical purity, epoxide equivalent 150.

The adducts, obtained by heating the mixtures at 120"C for 2 hours with the addition of 0.03% by weight of tetramethylammonium chloride have epoxide equivalents of A) 1,100 B) 950.

Two further modified epoxide resins are prepared from the adducts and the above epoxide resin (epoxide equivalent 185) by mixing at room temperature in the following ratio: Modified epoxide resin Serial no. Composition Epoxide Viscosity at equivalent 25"C, mPas 7 25 9 of adduct A 235 9500 75 g of epoxide resin 8 25 9 of adduct B 230 8600 75 g of epoxide resin Properties of the cured, modified epoxide resins Two different polyamine curing agents were employed for curing: a) a commercial polyaminoamide, H equivalents 165, viscosity at 75"C 800 mPas; b) a commercial polyaminoimidazoline, H equivalent 95, viscosity at 25"C 2,500 mPas.

The curing agents are added to the modified epoxide resins in equivalent quantities.

The technological properties in use of the epoxide resin/curing agent mixtures are tested by determining the bonding strength (tensile shear strength) as specified in DIN 53,283.

Aluminium sheets 1.6 mm thick, of the quality Al Cu Mg 2pl, are used for the bonding strength test. Before adhesion, the sheets are degreased and subjected to a chromate/sulphuric acid picking process.

The adhesive is applied to the test sheets in a quantity of 50 g/m2 and is cured at room temperature for 3 days. The test specimens are then stored at 1 00'C for a further hour in order to complete curing. The bonding strengths obtained at room temperature are listed in Table 2. They show a considerable increase in the case of the epoxide resins which have been modified in accordance with the invention.

The roll peel strength as specified in DIN 53,289 is also determined using the polyaminoamide curing agent a) (H equivalent 165) and the epoxide resins, aluminium sheets 0.5 mm thick being peeled from aluminium sheets 1.6 mm thick.

The quality of the aluminium sheets, their pretreatment and the curing conditions for the adhesives are unchanged. The peel strength values obtained are listed in the last column of Table 2. It can be seen that, even when using as the curing agent a polyaminoamide which in itself already imparts a higher flexibility to the epoxide resins, a further increase in the peel strength values is obtained if the epoxide resins which have been modified in accordance with the invention are used.

TABLE 2 Strength values of adhesives formed from modified epoxide resins andpolyamine curing agents Bonding strength, Roll peel strength, DIN 53,283 N/mm2 DIN 53,289 N/mm Modified epoxide Polyamino resin Polyamino- imidazoline Polyaminoamide no. amide a) b) a) 1 26.7 27.8 3.9 2 27.6 28.4 3.8 3 25.8 28.6 4.7 4 27.1 27.3 3.0 5 29.0 29.5 4.1 6 24.5 26.8 3.2 7 25.5 27.8 3.7 8 26.7 28.4 3.5 Comparison, unmodified epoxide resin 19.8 23.1 0.9 according to the invention

Claims (10)

1. A process for imparting flexibility to an epoxide resin by adding thereto before curing, at least one polymer containing carboxyl groups, in such a quantity that 1 to 60 mol % of the epoxide groups in the epoxide resin react with the carboxyl groups of the polymer, in which process the polymer containing carboxyl groups is one obtained by reacting a polyoxyalkylene monool or polyol, having an OH functionality of 1 to 5 and whose polyoxyalkylene blocks together with a molecular weight of 500 to 3,500, with a cyclic carboxylic acid anhydride in a molar ratio of 1 mol of hydroxyl groups to 0.3 to 1 mol of acid anhydride groups, subject to the proviso that, on average, at least one carboxyl group is present per molecule in the polymer produced.

2. A process according to claim 1, in which the polyoxyalkylene monool or polyol has been obtained by an addition reaction between ethylene oxide, propylene oxide and/or tetrahydrofuran and a compound having 1 to 5 hydroxyl groups.

3. A process according to claim 1 or 2, in which the polyoxyalkylene monool or polyol contains, as oxyalkylene groups, at least 50% by weight of oxypropylene groups.

4. A process according to claim 1, in which the polyoxyalkylene monool or polyol has been obtained by an addition reaction between an alkylene oxide and an aryl monool or polyol or alkaryl monool or polyol.

5. A process according to claim 1, in which the polyoxyalkylene monool or polyol has been obtained by an addition reaction between an alkylene oxide and a novolak having molecular weight of 300 to 600.

6. A process according to any one of the preceding claims, in which the polyoxyalkylene blocks of the polyoxyalkylene monool or polyol together have a molecular weight of 800 to 2,000.

7. A process according to any one of the preceding claims, in which the polymer added to the epoxide resin contains, on average, 1 to 2.5 carboxyl groups per molecule.

8. A process according to any one of the preceding claims, in which the carboxylic acid anhydride is succinic anhydride, maleic anhydride or phthalic anhyride.

9. A process according to claim 1 substantially as hereinbefore described with reference to the foregoing Example.

10. A cured epoxide resin in which, prior to the curing, the epoxide resin has been rendered flexible by a process as claimed on any one of the preceding claims.