CS233068B1 - Latent epoxy system - Google Patents

Abstract

Latent epoxy systems contain latent hardeners that do not react with the epoxy resin used at normal or slightly elevated temperatures up to 50 - 60 °C, or only to a very limited extent. Curing occurs only when the temperature is increased and pressure is applied. According to the invention, a mixed hardener consisting of 0.5-15 parts by weight of substituted bisurea and 15-0.25 parts by weight of dicyandiamide is added to 100 parts by weight of organic polyepoxides containing functional 1,2-epoxy groups, the dosage of the mixed hardener being based on R of a low-molecular unmodified diane-type epoxy resin with an epoxy equivalent of 0.50 mol/100 g. The system is cured at a temperature of 100-190 °C for 200-3 min., optionally under a pressure of 0.05-30 MPa, followed by post-curing at temperatures of 140-190 °C without pressure.

Description

The invention relates to a latent epoxy system composed of organic polyepoxides containing functional 1,2 epoxy groups and a latent hardener based on dicyandiamide or bisurea.

The problem being solved is to reduce the curing temperature of the latent epoxy system and shorten the curing time while maintaining or extending the service life of this system.

Latent epoxy systems contain latent hardeners that do not react with the epoxy resin used at normal or slightly elevated temperatures of up to approximately 50 - 60°C, or react only to a very limited extent. Curing occurs only at higher temperatures in the range of 120 - 190°C under the possible effect of increased pressure. This property predetermines latent epoxy systems for use in a wide range of different applications. These include, for example, one-component epoxy adhesives suitable for bonding highly stressed metal structural components, prepregs based on carbon, glass and other reinforcing fibers, epoxy binders used in drawing and precision winding technologies, powder or solvent-based coatings, impregnation baths, and the like. Before curing, latent epoxy systems can be stored for up to one year in an unchanged state and without affecting the subsequent curing reaction, depending on the purpose of use and the type of their composition. The lifespan of these systems can be further extended by storing them at temperatures below freezing.

In the above latent epoxy systems, the most commonly used latent hardeners are dicyandiamide alone, aromatic diamines - 4,4*diaminodiphenylsulfone, 4,4*diaminodiphenylmethane, boron trifluoride complexes, some dicarboxylic acid anhydrides, and substituted mono- and bisureas. Hardeners based on a combination of substituted monoureas with dicyandiamide are also known.

The most widely used of these latent hardeners, for example for the production of one-component epoxy adhesives and prepregs, is dicyandiamide. Its disadvantage is its poor solubility in epoxy resins and organic solvents and the need to cure latent epoxy systems.

233 088 at temperatures of 160 - 180°C, even in the case of acceleration by tertiary or quaternary amines, if the curing time is to be within a technologically acceptable range, because reducing the curing temperature requires a disproportionately long curing time. For example, an impregnation bath made from a mixture of 100 parts by weight of low-molecular epoxy resin of the diane type with an epoxy equivalent of 0.48 - 0.53 mol/100 g, trade name CHS Epoxy 15, manufactured by the National Chemical and Metallurgical Enterprise Association (SpoiLchemie), and 7 parts by weight of dicyandiamide, has a gelation time at a temperature of 115°C longer than 1000 min according to standard ČSN 64 0341.

Curing at temperatures above 140°C is necessary for most latent epoxy systems containing the known latent hardeners mentioned. Lowering the curing temperature could only be achieved at the expense of the life of the latent epoxy system. The exceptions are the use of substituted monourea, such as N,N-dimethyl-N'-p'-chlorophenylurea, or its combination with dicyandiamide, and the use of bisurea of the general formula ch ₃ ^X N-C0-NH-R

CH ₃ ch ₃

NH - CO - N ^{X x} ch ₃ where R is a divalent aromatic radical such as unsubstituted phenylene, methylphenylene, dimethylphenylene or methylenediphenylene. Latent epoxy systems containing substituted ureas or combinations of substituted monoureas with dicyandiamide can be cured at temperatures of 125 - 130°C for about 60 min.

The disadvantage of substituted monoureas and their combination with dicyandiamide is their poorer solubility in epoxy resins and organic solvents. Another disadvantage of substituted monoureas in their production is the more difficult availability of monoisocyanates, which constitute one of the starting raw materials.

For example, an impregnation bath made from a mixture of unmodified low molecular weight epoxy resin of the dian type with an epoxy equivalent of 0.48 - 0.53 mol/100 g in an amount of 100 parts by weight in a ratio of 3 parts by weight of bisurea

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Formulas

CH

9 ^H 3 ^CH 3 ^X N - CO - NH -ZÍx- NH - CO - N ^{X CH} 3^ V ^X CH ₃ has a gelation time according to the standard ČSN 64 0341 of 55 min. at a curing temperature of 115°C. Even though in this case the curing temperatures are in the lower range of values achieved by other known latent hardeners, it is desirable to further strive to reduce these values in order to further reduce the energy requirements of curing and shorten production times.

The problem is solved according to the invention by a latent epoxy system based on organic polyepoxides containing functional 1,2-epoxy groups, on the one hand, on the other

The essence of the invention lies in the fact that the latent epoxy system consists of 100 parts by weight of organic polyepoxides containing functional 1, 2 - epoxy groups, for example 100 parts by weight of dianene, chromium-plated or chlorinated dianene, cycloaliphatic, epoxyvinyl, multifunctional - nitrogen-containing epoxy resin, or mutual combinations of these epoxy resins, and of a mixed hardener, consisting of 15 - 0.25 parts by weight of dicyandiamide, and of 0.5 - 15 parts by weight of substituted bisurea of the general formula ^V N - 00 - NH - R - NH - CO - N ^{X CH} Z ^X ° ^H 3 , where R is a divalent aromatic radical, for example unsubstituted phenylene, methylphenylene, dimethylphenylene or methylenediphenylene, the dosage of the individual components of the mixed hardener233 068 la being based on a low-molecular unmodified a dian type epoxy resin with an epoxy equivalent of 0.50 mol/100 g.

The advantage of the latent epoxy system with a mixed hardener according to the invention is a significant reduction in the curing time at a relatively low temperature. For example, in an impregnation bath formed from a mixture of 100 parts by weight of unmodified low molecular weight epoxy resin of the palm type with an epoxy equivalent of 0.48 - 0.53 mol/100 g, 7 parts by weight of dicyandiamide and 3 parts by weight of bisurea of the said type, the gelation time is reduced to only 8 minutes at 115°C. The latent epoxy systems according to the invention are significantly more reactive compared to systems containing dicyandiamide or substituted bisurea separately. The mechanical values of the cured epoxy systems are better than in the case of the separate use of substituted ureas as latent hardeners, especially at elevated temperatures.

The advantage of using a combination of substituted bisureas with dicyandiamide over combinations of substituted monoureas with dicyandiamide is their better solubility in epoxy resins and organic solvents, such as n-butanol and ethylene glycol monomethyl ether. Another advantage is the good availability of diisocyanates, which are one of the starting materials in the production of substituted bisureas.

Latent epoxy systems according to the invention can be used in particular as single-component epoxy adhesives, impregnation baths for the production of prepregs with carbon, glass and other reinforcement, single-component powder and solvent-based coatings, impregnation baths when using drawing and precision winding technologies, requiring long service life of epoxy systems, and casting and molding compounds.

In the production of these latent epoxy systems, it is possible to use a mixed hardener of the selected composition either in the form of a solution, for example in ethylene glycol monomethyl ether, in connection with solvent-based coatings or impregnation solutions, or in the form of a paste, prepared by dispersing the mixed hardener with a smaller proportion of the low-molecular epoxy resin of the type used, for example on a three-roll friction roller. The paste is suitable for use in the production of one-component adhesives or solvent-free impregnation baths. In the case of epoxy powder adhesives and coatings, the mixed hardener can be dosed in powder form into a mixture of a suitable epoxy resin, fillers, pigments, flexibilizers and the like, and homogenized together with these components, for example in a ball mill or on a calender.

The curing temperature of all latent epoxy systems according to the invention can be selected in the range of 100 - 190°C depending on the dosage of the individual components and the required length of the curing time. If increased heat resistance is required, the manufactured part can be cured at lower temperatures and, if necessary, under pressure, further cured at elevated temperatures in the range of 140 - 190°C without pressure.

Typical uses of the latent epoxy system of the invention are apparent from the following examples.

Example 1. - Prepreg

a) an isomeric mixture of substituted bisurea is used in the weight ratio A : B = 80 : 20, where A) is 2,4 tolylene bis (N*, N*- dimethylurea) of the formula

CH. i A

ch ₃

N

CH.

^X 0H ₃

B) is 2,6 tolylene bis (N*, N*-dimethylurea) of the formula

b)

CH ₃ CH ₃ ch ₃ ^X N - CO - ΒΉ - rÁs - ΝΗ - CO - N (9 ^X ch ₃ prepare an impregnation solution in ethylene glycol monomethyl ether, consisting of

100 parts by weight of unmodified low molecular weight palm type epoxy resin with an epoxy equivalent of 0.48 - 0.53 mol/100 g, trade name

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CHS - Epoxy 15, manufacturer Spolchemie, parts by weight of substituted bisurea according to point a), and parts by weight of dicyandiamide, so that the resulting viscosity of the impregnation solution is approximately 20 sec/4 mm Ford at 25°C,

c) impregnate a glass fabric with a weight of 215 g/m, for example with the trade name type

145i manufacturer interglas GmbH, the solvent is evaporated from the thus formed prepreg blank, which is further dried at a temperature of 120°C for 3 min,,

d) after drying, the produced prepreg has a shelf life of at least one month at a temperature of 20°C,

e) prepreg is processed by pressing under a pressure of 0.45 MPa at a curing temperature of 120°C for 30 minutes,

f) the manufactured laminate with a cured epoxy system and a glass content of 75% by weight has a bending strength of 1250 MPa and a bending modulus of elasticity of 41 Gřa.

Example 2. - Solvent-free impregnation bath for the production of hollow laminates

a) an isomeric mixture of substituted bisurea is used in accordance with point a) of Example 1·,

b) a paste is prepared on a three-roll friction roller, consisting of parts by weight of substituted bisurea according to point a), 2 parts by weight of dicyandiamide, and parts by weight of a low-molecular epoxy resin of the diane type with an epoxy equivalent of 0.50 - 0.58 mol/100 g, modified with 15% by weight of a reactive solvent, for example 1,4-butanediol bisglycidyl ether,

c) 10 parts by weight of the resulting paste are added to

100 parts by weight of epoxy resin of the type and properties identical to the type used for the preparation of the paste in point b), for example known under the trade name CHS - Epoxy 110 BG - 15, manufactured by Spolchemie, after which homogenization is carried out with a high-speed mixer,

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d) the initial viscosity of the resulting impregnation bath reaches 60 s /6 mm Ford at 25°0, after 7 days of storage it increases to 70 and after a month of storage to 120, while the applicable limit for processing the impregnation bath is 160 s /6 mm Ford at 25°C,

e) a glass roving with an elementary fiber thickness of 10 /t/m, with a number of 60 associated strands, E glass, for example roving type 701, manufactured by Vertex National Enterprise, is mechanically impregnated with an impregnation bath, which is wound onto a separated steel mandrel, onto which the same glass roving is further applied by drawing technology in the direction of the mandrel axis. After being wrapped with polypropylene tape, the resulting hollow laminate semi-finished product is cured in an oven at a temperature of 120°C for 60-120 min.

f) After curing, solid parts of the laminate containing % by weight of glass exhibit the following mechanical properties: curing time (min) 60 120 flexural strength (MPa) 1310 1483 flexural modulus of elasticity (GPa) 51.9 53.6

Example 3. - Prepreg adhesive film

a) a substituted bis-Urea with the chemical name methylene-di-p-phenylene bis (N*, N*- dimethylurea) of the formula is used

CH _q 0 0 CH ₇

N - C - NH GH ₂ - - NH - C - N

CH ₃ ch ₃

b) prepare a solution of mixed hardener consisting of 7 parts by weight of dicyandiamide, parts by weight of bisurea according to point a), and 70 parts by weight of ethylene glycol monomethyl ether. Dissolution can be accelerated by heating the ethylene glycol monomethyl ether to 50 - 60°C,

c) by dissolving at a temperature of 50°C in methyl ethyl ketone, an 80% solution is prepared, unmodified high molecular weight palm-type epoxy resins with an epoxy equivalent

0.19 - 0.22 mol/100 g,

d) an impregnation solution is prepared, containing

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100 parts by weight of epoxy resin according to point c), for example, resin with the trade name CHS - Epoxy 1/33» manufactured by Spolchemie, parts by weight of low molecular weight unmodified epoxy resin of the diane type with an epoxy equivalent of 0.48 -0.53 mol/100 g, for example, trade name CHS - Epoxy 15» manufactured by Spolchemie, parts by weight of liquid butadiene acrylonitrile rubber, terminated with carboxyl groups with a content of 0.057 mol of these groups per 100 g of this rubber, and with a content of 26% by weight of acrylonitrile groups, and parts by weight of a mixed hardener solution according to point b).

The above components are homogenized by stirring and diluted with ethylene glycol monomethyl ether so that the resulting impregnation solution reaches a viscosity of 20 - 25 s /4 mm Ford at 25°C.

e) The impregnation solution is mechanically impregnated onto a glass fabric, for example a plain weave, with the trade name

E 57-80, manufactured by Vertex National Enterprise, the solvents are evaporated, and the resulting prepreg adhesive film semi-finished product is dried at a temperature of 90°C for 10 - 15 minutes. Then the resulting prepreg adhesive film with the separation film is wound together on a roll.

f) The finished prepreg adhesive film has a shelf life of at least one month at a temperature of 20°C and at least six months at a temperature of -18°C.

g) The bonded duralumin-duralumin joint after curing at a temperature of 120 C and under a pressure of 0.7 MPa for 30 minutes exhibits the following mechanical properties:

tensile shear strength 30 MPa, peel strength according to

Wintera 4.5 H/mm, width of glued joint.

Claims (1)

Latent epoxy system 7Í _t organic ^{7 4} polyepoxides, containing functional 1, 2 - epoxy groups, ρΡ,'ρ&άνέ others. components, such as organic ^solvents including reactive solvents, inorganic and/or organic reinforcements, reactive liquid rubbers or other flexibilizers, fillers, blowing agents, dyes, flame retardants, and other auxiliaries, and a hardener consisting of dicyandiamide or substituted bisurea, which system exhibits the ability to cure at a temperature of 110-130°C for a period of 200-20 min., characterized in that it consists of 100 parts by weight of organic polyepoxides containing functional 1, 2-epoxy groups, for example 100 parts by weight of dian, chromized or chlorinated dian, cycloaliphatic, epoxy-lacquer, multifunctional - nitrogen-containing epoxy resin, or mutual combinations of these epoxy resins, and of a mixed hardener, consisting of 15-0.25 parts by weight of dicyandiamide, and from 0.5 - 15 parts by weight of a substituted bisurea of the general formula TARGET TARGET N-CO.-NH-R-NH-CO-N ch ₃ gh ₃ where R is a divalent aromatic radical, for example unsubstituted phenylene, methylphenylene, dimethylphenylene or methylenediphenylene, the dosage of the individual components of the mixed hardener being based on a low molecular weight unmodified dian type epoxy resin with an epoxy equivalent of 0.50 mol/100 g.