GB2113689A - Process for imparting flexibility to epoxide resins - Google Patents

Abstract

Flexibility is imparted to epoxide resins by means of polymers containing carboxyl groups as modifying agents. These polymers are obtainable by reacting alkali metal salts of polyoxyalkylene monools or polyols with halogenoalkylcarboxylic acid salts followed by acidification. The modifying agents have a low viscosity, so that the modified expoxide 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, before curing, in such quantities that 1 to 60 mol % of the epoxide groups react with the carboxyl groups of the polymer.

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 fur 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 1 600C 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 polymerising together, in the presence of a regulator which contains mercapto groups and which has at least one carboxyl group:: a) 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 glycidyl 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 the modifying agent, a high degree of elasticisation is made possible, without impairing the adhesion 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, on the one hand, 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 reduced, and, on the other hand, are as far as possible non-specific in respect of curing agents, so that those skilled in the art have a greater choice in respect of the curing agents which seem to them suitable and are not, as hitherto, dependent on specific combinations of curing agents. This applied especially 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 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 an alkali metal salt of a polyoxyalkylene monool or polyol having a functionality of 1 to 3, and whose polyoxyalkylene blocks together have a molecular weight of 500 to 3,500, with at least an equivalent quantity of a halogenoalkylcarboxylic acid salt at 50 to 140"C, if appropriate in the presence of an inert solvent, and subsequently liberating the carboxylic acid by acidification and removal of salts formed.

Preferably, the polyoxyalkylene blocks together have a molecular weight of 800 to 2,000.

Alkali metal salts of polyoxyalkylene monools or polyols can be obtained, in particular, by two different routes: thus, it is possible to effect an addition reaction between an alkylene oxide, specifically, in particular, ethylene oxide, propylene oxide, tetrahydrofuran or mixtures thereof, and compounds containing acid hydrogen, such as compounds containing carboxyl groups, hydroxyl groups or CH groups which are activated by adjacent carbonyl groups. Monohydric to trihydric alcohols are particularly preferred as starting compounds. In this procedure, the alkylene oxide is caused to undergo an addition reaction in a manner which is in itself known, for example using catalytic amounts of an alkali metal methoxide at temperatures of 80 to 1400C and under elevated pressure.Suitable starting alcohols are monohydric lower alcohols having 1 to 5 carbon atoms, for example methanol, ethanol, propanol, butanol or isobutanol, dihydric alcohols, such as ethylene glycol, propylene glycol or butylene glycol, ortrihydric alcohols, such as glycerol.

In a preferred procedure alkylene oxides are caused to undergo an addition reaction with aryl monools or polyols or alkyl monools or polyols. Examples of suitable aryl polyols are resorcinol, phloroglucinol, hydroquinone, 2,7-dihydroxynaphthalene or the 2,6- or 1,8-isomer or 2,6-dihydroxyanthracene. 2,2-Bis-(4- hydroxyphenyl )-propane, bis-(4-hydroxyphenyl)-methane, 1,1,2-tris-(4-hydroxyphenyl)-ethane, 1,1,3-tris-(4- hydroxyphenyl)-propane, bis-(2-hydroxyphenyl)-ethane or 2,2-bis-(4-hydroxymethylphenyl)-propane are also particularly suitable for use as starting alcohols.

The polyoxyalkylene monools or polyols thus obtained are then suitably converted into alkali metal salts by means of an alkali metal methoxide, in particular sodium methoxide, at 150 to 2000C, the methanol being removed by distillation.

Another route for the preparation of alkali metal salts of polyoxyalkylene monools or polyols consists in first preparing the alkali metal salts of the compounds used as starters and then attaching the alkylene oxides by an addition reaction under the conditions of stoichiometric polymerisation. In this mode of polymerisation, each growing chain contains a terminal alkali metal group.

The alkali metal salts of polyoxyalkylene monools or polyols which are desirably obtained by one of the two processes mentioned above preferably are sodium salts and/or having at least 50% by weight of oxypropylene groups. They are then suitably reacted with at least equivalent quantities of halogenoalkylcarboxylic acid salts. Preferred halogenoalkylcarboxylic acid salts are the alkali metal salts, in particular the sodium salts. The preferred halogen radical is the chlorine radical. The lower halogenoalkylcarboxylic acid salts, in particularthose having 1 to 3 carbon atoms in the alkyl radical, are particularly preferred. The sodium salt of chloroacetic acid is particularly preferred.

These halogenoalkylcarboxylic acid salts are reacted with alkali metal salts of polyoxyalkylene monools or polyols at 50 to 1400C. The use of inert solvents, such as toluene or cyclic ethers, such as dioxane, is advisable. The alkali metal salt formed in the reaction can then be precipitated in a crystalline form.

Alkali metal salts formed in the reaction are then converted into the free acids by acidification, in particular using a mineral acid, such as hydrochloric acid. They are advantageously freed from the precipitated salts by filtration and the solvent is removed by distillation, together with excess mineral acid. The product which has been purified in this way can, if appropriate, be filtered again in order to remove residues of salts.

The resulting products are clear, oily compounds which are liquid at room temperature and are soluble in organic solvents.

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 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, am ides and heterocyclic nitrogen bases, such as N,N-diglycidylaniline, N,N-diglycidyltoluidine, N,N,N',N'-tetraglycidyl- bis-(p-aminophenyl)-methane, 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 Enzyklopadie dertechnishen Chemie" ("Ullmann's Encyclopaedia of Industrial Chemistry"), volume 10, page 563 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 resins. 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 as low as room temperature, it is still preferable to warm the mixture to temperatures of 100 to 150"C. The reaction then 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 resin.It is only necessary to take care that this modified fractional 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 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 have a composition differing from that df 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 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 or tetrabutylphosphonium 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 130into about 220 C: 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 particularly 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 industryforthe 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 Example serves to illustrate the present invention.

PREPARATION EXAMPLE Modifying agents Polyether I is a product formed by an 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 controlled in such a way that the oxyethylene groups are preferentially attached at the end of the chain. The polyether has a hydroxyl number of 56, and the polyoxyalkylene blocks together have a molecular weight of 1,910. 21.6 g of sodium methoxide and 40 g of methanol are added to 400 g of polyether land the mixture is heated to 1200C. Avacuum is applied. The temperature is raised to 190"C. All the methanol distills off in the course of 1.5 to 2 hours.The mixture is then cooled to 60"C, 46.6 g of sodium chloroacetate are added and the mixture is allowed to react at 100C,while stirring, until an alkali number of 2 has been reached. After adding 50 g of water and 80 g of concentrated hydrochloric acid, the mixture is stirred at room temperature for 1 hour and the excess of hydrochloric acid and the water are then removed in vacuo at 90 C and the precipitated sodium chloride is removed by filtration. A yellow-brown liquid having an acid number of 51 (theoretical acid number = 53) and a viscosity of 970 mPas is obtained.

Further modifying agents, the characteristic data of which can be seen in Table 1, are prepared by this method from polyethers II and Ill and sodium chloroacetate. Polyethers II and Ill have the following composition: Polyether II is a bifunctional product from the addition reaction of a mixture of 55% by weight of propylene oxide and 45% by weight of ethylene oxide with ethylene glycol. The hydroxyl number is 70 and the molecular weight of the sum of the polyoxyalkylene blocks is 1,550. In preparing the polyether, the reaction was controlled in such a way that the oxyethylene groups are preferentially attached at the end of the chain.

Polyether Ill is a bifunctional product from the addition reaction between propylene oxide and bisphenol A.

The hydroxyl number is 110 and the molecular weight of the sum of the polyoxypropylene blocks is 800.

EXAMPLE Modification ofepoxide resins The modification is carried out by heating mixtures of an epoxide resin, formed from bisphenol Nepichlorohydrin 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 the quantities of epoxide resin and modifying agent can be seen in Table 1.

After cooling, epoxide resins are obtained, which are liquid at room temperature and 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). The viscosity and the epoxide equivalent ofthe modified epoxide resins can be seen in Table 1.

TABLE 1 Composition and properties of modified epoxide resins Serial Modifying agent Epoxide resins no. parts by weight Poly- Acid Viscosity ether number at 25"C, no. mPas 1 I 51 970 80 2 11 62 650 80 3 lil 91 6500 80 4 lil 91 6500 85 Serial Modifying agent Epoxide Viscosity at no. parts by weight equivalent 25"C, mPas 1 20 240 8000 2 20 245 7300 3 20 250 17000 4 15 230 12000 Two adducts are prepared using the modifying agent formed from polyether I and sodium chloroacetate, which is listed in Table 1 under no. 1:: A) 75 g of modifying agent no. 1 25 g of epoxide resin formed from bisphenol A/epichlorohydrin, epoxide equivalent 185 B) 75 g of modifying agent no. 1 25 g of neopentylglycol diglycidyl ether, technical purity, epoxide equivalent 150.

The adducts obtained by heating the mixtures at 1 200C for 2 hours with the addition of 0.03% by weight of tetramethylammonium chloride have epoxide equivalents of A) 1,500 B) 1,050.

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: Serial Modified epoxide resin no. Composition Epoxide Viscosity at equivalent 25"C, mPas 5 25 9 of adduct A 238 7500 75 g of epoxide resin 6 259 of adduct B 235 6000 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 equivalent 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 pickling 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 OO"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,lt can be seen that, even when using as the curing agent a polyaminoamide which in itself already imparts a relatively high 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

Modified Bonding strength Roll peel strength, epoxide DIN 53,283 N/mm2 DIN 53,289 N/mm resin Polyamino- Polyamino- Polyaminoamide a) no. amide a) imidazoline b) 1 26.8 29.4 3.5 2 24.1 26.0 2.8 3 3 27.8 28.9 3.2 w c r Hc 4 26.1 27.2 2.7 5 5 24.8 28.7 4.2 6 25.4 27.0 3.8 a, unmodified O epoxide 19.8 23.1 0.9 resin w C resin oO >

Claims (12)

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 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 an alkali metal salt of a polyoxyalkylene monool or polyol having a functionality of 1 to 3, and whose polyoxyalkylene blocks together have a molecular weight of 500 to 3,500, with at least an equivalent quantity of a halogenoalkylcarboxylic acid salt at 50 to 1 400C, if appropriate in the presence of an inert solvent, and subsequently liberating the carboxylic acid by acidification and removal of salts formed.

2. A process according to claim 1, in which the alkali metal salt of a 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 3 hydroxyl groups.

3. A process according to claim 1 or 2, in which the alkali metal salt of a 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 alkali metal salt of a 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 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 2000.

6. A process according to any of the preceding claims, in which a sodium salt of a polyoxyalkylene monool or polyol is used.

7. A process according to any one of the preceding claims, in which the halogenoalkylcarboxylic acid salt is a chloroalkylcarboxylic acid salt.

8. A process according to any one of the preceding claims, in which the halogenoalkylcarboxylic acid salt is a sodium salt of a halogenoalkylcarboxylic acid.

9. A process according to any one of the preceding claims, in which the halogenoalkylcarboxylic acid salt has 1 to 3 carbon atoms in its alkyl group.

10. A process according to any one of the preceding claims, in which the halogenoalkylcarboxylic acid salt is the sodium salt of chloroacetic acid.

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

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