Process for rendering epoxy resins flexible.

The invention relates to a process for making epoxy resins more flexible by adding carboxyl-containing polymers.

Various possibilities are known from the prior art for making epoxy resins more flexible. For example, flexibility can be achieved by using special hardeners, such as polyaminoamides. In many cases, however, there is no choice regarding the hardener, for example if the hardening temperature, the hardening speed or the glass transition temperature of the hardened epoxy resin are prescribed. In these cases one is forced to use hardeners such as dicyandiamide, polycarboxylic anhydrides or short chain aliphatic polyamines, which however lead to brittle curing products. It is then necessary to make the epoxy resins more flexible by adding modifying agents. But also in the case of flexible hardeners, for example the polyaminoamides, additional flexibility of the epoxy resin is often desirable. The modifiers can be distributed in the form of a physical mixture in the epoxy resin or can react with the epoxy resin. Most of the known modifiers belong to the group of non-reactive additives. For this state of the art, reference is made to the book by H. Jahn "Epoxidharze", VEB Deutscher Verlag für Grundstoffindustrie, Leipzig, 1969. Products which have groups which are able to react with the epoxy groups of the epoxy resin, for example carboxyl groups, are selected as reactive modifiers. It is understandable to the person skilled in the art that only part of the epoxy groups react during the reaction with the modifying agent. Ren may, in order to make the curing of the epoxy resins still possible, but the proportion of the modifier incorporated must be so large that the desired flexibility is achieved.

Butadiene acrylonitrile copolymers having a molecular weight of 3,000 and having terminal carboxyl groups are known as reactive modifiers from US Pat. No. 3,948,849. The epoxy resins are modified before curing by heating the epoxy resins containing the modifier to 160 ° C. for about 30 minutes. When using epoxy resins modified in this way as adhesives, adhesive bonds with elastic joints are obtained. A disadvantage of these compounds, however, is that olefinic double bonds remain in the skeleton of the hardened modified epoxy resin, which, owing to their oxidizability, impair the chemical and physical properties of the hardened resins.

The invention has for its object to find reactive modifiers for epoxy resins that allow the highest possible elasticity without the _Hthe modified epoxy resins at the interfaces is impaired, the viscosity of the epoxy resins for the modifying agents should be increased as little as possible and the modified epoxy resins being chemically and physically stable in the hardened state.

This object according to the invention is achieved in that the epoxy resins are cured before curing by copolymers which have been polymerized together<ul id="ul0001" list-style="none"><li>a₁40 to 87% by weight of one or more alkyl esters of acrylic and / or methacrylic acid with 1 to 8 carbon atoms in the alkyl radical,</li><li>a₂) 10 to 40% by weight of vinyl acetate and / or acrylonitrile,</li><li>_a3) ₁ up to 20% by weight of acrylic, methacrylic and / or itaconic acid,</li><li>a₄) 1 to 5% by weight of glycidyl acrylate and / or glycidyl methacrylate,</li><li>a₅) O to 35 wt .-% acrylic and / or vinyl monomers, which of the monomers a₁) to a₄) are different,</li></ul>in the presence of a mercapto group-containing regulator which has at least one carboxyl group, have been obtained, and the copolymers having an average molecular weight, measured in the vapor pressure osmometer, of 1000 to 3000, is added in amounts such that 1 to 60 mol% of the epoxy groups react with the carboxyl groups of the copolymer.

Copolymers with an average molecular weight of 1500 to 2500 are particularly preferred as modifiers.

A particularly good modification is achieved when the copolymers have an average of 1.5 to 2.5 carboxyl groups per molecule.

The copolymers are obtained in a manner known per se by radical polymerization of the monomers.

The monomers a1), which form the main part of the copolymer, are alkyl esters of acrylic and / or methacrylic acid with 1 to 8 carbon atoms in the alkyl radical, for example methyl acrylate or methacrylate, ethyl acrylate or methacrylate, butyl acrylate or methacrylate, 2-ethylhexyl acrylate or - methacrylate.

The monomers a₅), which may or may not be present in the copolymer, can be, for example, acrylamide or methacrylamide, styrene, isopropylidene acetate or vinyl propionate.

In particular, thioglycolic acid, 3-mercaptopropionic acid or thiolactic acid can be used as regulators containing mercapto groups which have at least one carboxyl group in the molecule.

In principle, the epoxy resins known to the person skilled in the art can be used as epoxy resins. Epoxy resins based on bisphenol-A or F and epichlorohydrin are particularly preferred. ; Further examples of epoxy resins are the diglycidyl ether of hydrogenated bisphenol-A, p-glycidyloxy-N, N-diglycidylaniline or cycloaliphatic polyepoxide compounds, as described, for example, in H. Jahn "Epoxidharze", VEB Deutscher Verlag für Grundstoffindustrie, Leipzig, 1969, or in H Batzer and F. Lohse in "Ullmann's Encyclopedia of Technical Chemistry", Volume 10, pp. 563 ff., 4th edition, Verlag Chemie, Weinheim 1975.

The epoxy resins can be modified in various ways. It is thus possible to add the amount of modifier to the total amount of the epoxy resin. Even if the reaction of the carboxyl groups of the modifier with the epoxy groups of the epoxy resin starts at room temperature, it is preferred to heat the mixture to temperatures of 100-150 ° C. The reaction then takes from 30 minutes to 4 hours. However, the total amount of the modifying agent can also be added to only part of the total amount of the epoxy resin. It is only necessary to note that this modified part of the epoxy resin still has enough epoxy groups that when this amount is mixed with the rest of the epoxy resin and the subsequent curing, a reactive incorporation of this modified part is guaranteed. It is sufficient if the modified portion still has about 40 mol% epoxy groups. The advantage of this procedure is that the modification can take place at the manufacturer. It is also possible to carry out the partial modification with an epoxy resin, the composition of which differs from that of the remaining amount of epoxy resin. In particular, the partial modification can be carried out with a low-viscosity epoxy resin. The diglycidyl ethers of aliphatic diols, for example 1,4-butanediol; Hexanediol-1.6 or neopentyl glycol.

It can be advantageous to add a catalyst in effective amounts to the mixture of epoxy resin and copolymer containing carboxyl groups, which catalyst accelerates the modification reaction. Quaternary ammonium or phosphonium compounds, such as, for example, tetramethylammonium chloride or iodide, benzyltrimethylammonium chloride, tetrabutylphosphonium chloride or acetate, are particularly preferred as catalysts.

The hardeners known from the prior art can be used to harden the modified epoxy resins. The following hardeners are particularly useful for hot curing, ie curing at temperatures above 130 ° C to about 220 ° C: dicyandiamide and its derivatives; Polycarboxylic anhydrides such as phthalic anhydride; Methyl hexahydrophthalic anhydride; Pyromellitic dianhydride. Aromatic polyamines such as m-phenylenediamine are suitable for heat curing at temperatures around 100 ° C .; cycloaliphatic polyamines. Curing at room temperature can be done with polyaminoamides; Polyaminoimidazolines; modified aliphatic polyamines or polyether polyamines can be carried out. Mixtures of polyaminoamides or polyaminoimidazolines on the one hand and aliphatic polyamines or polyether polyamines on the other hand are particularly advantageous for curing at room temperature. When using such a hardener mixture, particularly high strength values ​​are obtained with the modified epoxy resins during bonding. The respective curing temperature and / or the curing time can be reduced or shortened by using known accelerators. Such accelerators are, for example, tertiary amines.

Although it was already known from the magazine "Farbe und Lack", 82, (1976), page 1105 ff., In turn it was possible to cure acrylate resins containing low molecular weight carboxyl groups, which are used as stoving lacquers, with relatively small amounts of an epoxy resin. However, it could not be concluded from the compatibility of these products that an epoxy resin as the predominant main product can be modified with relatively small amounts of a copolymer containing low molecular weight carboxyl groups in such a way that a high degree of elasticity is achieved when the cured epoxy resins retain their adhesion. ,

The epoxy resins modified according to the invention are particularly suitable as adhesives, since they adhere well to the interfaces to be connected and form an elastic adhesive joint. But you can also impregnate with the modified, but not yet cured epoxy resin carrier sheets, such as glass fiber nonwovens or fabric, and cure to form laminates. They can be used, for example, in the electronics industry to harden printed circuits. Another possible use of these so-called prepregs is in the production of molded parts, such as in boat building, and for repair purposes, for example in body construction. It is also advantageous to use the modified epoxy resins as coating raw materials or as casting resins.

The following examples describe the preparation of the modifying agents which is not claimed, the modification of epoxy resins according to the invention and the properties of the not yet cured and of the cured modified epoxy resins.

Production of the low molecular weight acrylic polymers

A solution of 400 g of monomer mixture, 1.6 g of azodiisobutyronitrile and 12 to 14 g of thioglycolic acid is added dropwise to 120 g of boiling toluene over the course of 1.5 hours, while stirring and introducing nitrogen, the reflux being maintained. The mixture is then refluxed for a further 2 hours. After adding 1.2 g of azodiisobutyronitrile, the mixture is heated under reflux again for 1.5 hours. The solvent and small amounts of unreacted monomers are removed in vacuo at a temperature of 80 to 110 ° C. The resulting 100% acrylic polymers are medium to high viscosity at room temperature.

The monomer composition, the regulator content and the average molecular weight, which was measured in the vapor pressure osmometer, are shown in Table 1.

Production of the modified epoxy resins

To produce the modified epoxy resins, mixtures of an epoxy resin made from bisphenol-A / epichlorohydrin with an epoxy equivalent of 185 g / mol with varying amounts of the acrylic copolymers after addition of 0.03% tetramethylammonium chloride are heated at 120 ° C. with stirring for 2 hours, the reaction is carried out under nitrogen. The proportions of epoxy resin and acrylic copolymer are shown in Table 1.

After cooling, liquid epoxy resins are obtained at room temperature, the viscosity of which is only slightly increased compared to that of the unmodified epoxy resin (approx. 9000 mPas at 25 ° C). The viscosity and the epoxy equivalent of the modified epoxy resins can be found in Table 1.

Two adducts are produced from the acrylic copolymer listed in Table 1 under No. 5 from 82% by weight of n-butyl acrylate, 15% by weight of vinyl acetate, 2% by weight of acrylic acid and 1.0% by weight of glycidyl methacrylate:<ul id="ul0002" list-style="none"><li>a) 70 g acrylic copolymer No. 5 30 g epoxy resin from bisphenol-A / epichlorohydrin, epoxy equivalent 185</li><li>b) 80 g of acrylic polymer No. 5 20 g of neopentyl glycol diglycidyl ether of technical purity, epoxy equivalent 150</li></ul>

The by heating for 2 hours at 120 ° C with the addition of ₀,₀₃% Of adducts obtained from tetramethylammonium chloride have epoxy equivalents of<ul id="ul0003" list-style="none"><li>a) 900</li><li>b) 1800. Two additional modified epoxy resins are prepared from the adducts and the bisphenol A / epichlorohydrin epoxy resin (epoxy equivalent 185) by mixing at room temperature in the following ratio:<img file="EP0056427A2_D0001.tif" /><img file="EP0056427A2_D0002.tif" /></li></ul>

Properties of the cured modified epoxy resins

a) Thermosetting epoxy resin systems

Two different polyamine hardeners are used for curing:<ul id="ul0004" list-style="none"><li>a) Triethylenetetramine with an H equivalent of 25 g / val</li><li>b) a modified cycloaliphatic polyamine with an H equivalent of 111 g / val.</li></ul>

The hardeners are added to the modified epoxy resins in equivalent amounts.

Two strength tests are used for the application test of the epoxy resin / hardener mixtures.<ul id="ul0005" list-style="none"><li>1) Bond strength (tensile shear test) according to DIN 53 283</li><li>2) Angular peel strength according to DIN 53 282</li></ul>

Aluminum sheets 1.6 mm thick are used for the bond strength, 0.5 mm aluminum sheets of the quality Al Cu Mg 2pl are used for the angular peel strength. The sheets are degreased before gluing and subjected to a chromate-sulfuric acid pickling process (pickling pickling).

The epoxy resin / hardener mixtures forming the adhesive are in an amount of approx. 50 g / m² applied to the test panels and cured at 100 ° C for 20 minutes under light pressure.

After cooling, the bond and angle peel strength are determined at 20 ° C. The values ​​are shown in Table 2. They show that both the angular peel strength and the bond strength compared to the unmodified epoxy resin are increased considerably when the modification of the epoxy resins according to the invention is used.<tables id="tabl0001" num="0001"><img file="EP0056427A2_D0003.tif" /></tables>

b) thermosetting epoxy resin systems

Heat-curing, one-component epoxy resin systems are formulated from the modified epoxy resins and dicyandiamide as hardener. The adhesives are applied to ground steel specimens and cured at 180 ° C for 60 minutes under light pressure. The bond strength is then measured in accordance with DIN 53 283. The results are shown in Table 3.<tables id="tabl0002" num="0002"><img file="EP0056427A2_D0004.tif" /></tables>

c) Cold curing epoxy resin systems

From the modified epoxy resins and a commercially available polyamino amide (H equivalent 95, viscosity 2 500 mPas at 25⁰C) and its mixture with an aliphatic ether diamine (H equivalent 50, viscosity 15 mPas at 25 ° C) or a modified aliphatic polyamine (H equivalent 75, viscosity 400 mPas at 25 ° C), two-component adhesives are formulated on the Aluminum sheets (pretreatment as under a)) are applied and hardened for 3 days at room temperature.

The bond strength is then measured in accordance with DIN 53 283. The results are shown in Table 4. It also shows here that improved strength values ​​are obtained with the epoxy resins modified according to the invention.<tables id="tabl0003" num="0003"><img file="EP0056427A2_D0005.tif" /></tables>