CA2088003A1

CA2088003A1 - Prepolymeric imidazole compounds containing urea and/or urethane groups, and their use as curing agents for epoxy resins

Info

Publication number: CA2088003A1
Application number: CA 2088003
Authority: CA
Inventors: Christian Burba; Werner Mrotzek; Joerg Volle
Original assignee: Schering AG
Current assignee: Lanxess Organometallics GmbH
Priority date: 1992-02-19
Filing date: 1993-01-25
Publication date: 1993-08-20
Also published as: DE4204921A1; EP0558929A1

Abstract

ABSTRACT

The invention relates to prepolymeric imidazole derivatives containing urea and/or urethane groups and to their use as curing agents in fiber-reinforced plastics based on epoxy resins.

Description

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The invention relates to new prepolymeric imidazolyl derivatives, their use as curing agents in epoxy-resin com-positions, curable epoxy-resin compositions containing them and consisting of an epox~ resin and of ~ompounds containing urea and/or urethane group~ and based on addition products of imidazole compounds and prepolymeric isocyanates, and option-ally commonly us~d curing agents and solvents, for the pro-duction of molded articles.
Essentially two methods are used today to manu~ac-ture composite materials.
With the wet lay-up technique, the reinforcing materials are impregnated with the curable mixture and heat-cured in one step to the thermoset final state.
In the two-step method, so-called prepregs are first produced from the reinforcing materials and the curable mix-ture, and these prepregs are then processed further in a second step to finished parts. A distinction is made between solvent-cQntaining and solvPntless systems.
The prepregs are normally ~ormed in a continuous process in which the reinforcing ~aterials are conducted through an impregnating bath of the resin/curing agent mixture, or then the impregnating mix~ure is prepar~d only just before it is applied to the base material by means of a pecial knife coater. The quantity of impr~gnating mixture deposi~ed on a given base material is regulated not only .
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PATENT
51~3910-2555 through the viscosity of the impregnating mixture but also through squeeze rolls located downst:ream.
With solvenk-containing systems, the solvent con~
tained in the impregnating solution is evaporated through khe input of heat after khe impregnating operation, and the resin system is converked at the same time from the A skage to the B
stage. Depending on the process conditions and khe resln syskem used, the rein~orcing materials impregnated with a liquid to highly viscid impregnating mixture are thus turned into a slightly tacky to almost dry prepreg. It is important that in this process step the ~olve~t be completely eliminaked from the impregnating mixture and that kha latent curing agent needed to cure the prepreg in the second step not be activated just yet as this would cause the reaction of the impr~gnated reinforcing makerials to go prematurely to completion.
Depending on the chemical composition of the resin system, with solventless sys*ems the impregnated material also undergoes a shork heat treatment, or then the rein~orcing materials are ~aced on both sides with release ~heets imme-diately after impregnation, withsut any heak kreatment, and then placed into intermediate storage appropriate to the sys-tem. Duri~g khis storage, either a gradual ~ransition of the resin system to the B stage kakes place or the impregnating mixture is flxed on the base materials through physical effects alone and largely without undergoing chemical modi-~ication.

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51g910-~555 The prepregs so obtained can be ~tored and shipped as rolls before thay are cut to size for a given end use and 6tacked to the thickness o~ a finished part. Under ~he ~imul-taneous action o~ pressure and heat, the prepreg stack is com-pletely cured into a high-strength ~lolded part, with the still fluid l~w-molecular-weight resin passing into the high-molecular-weight C stage of a thermoset.
While the one-step method requires long open times and short cure times a~ low curing temperatures, prolonged storage stability of the prepregs is an additional re~uirement in the two-step method. ~torage temperatures below room tem-perature are increasingly considered unacceptable.
Regardless of how the prepregs are made, they should cure thoroughly at the lowest possible temperature and within a reasonably short time, the maximum temperature of the exo-thermic reaction should remain at a low level even with moder-ately thick layers, and the profile of physical properties of the inished products should meet actual requirements.
While curing systems have been described in the past which permit curing to occur at well below 100 C, there is still a demand ~or very-low-temperature curing agents charac-terized by good storage stabllity of the reactive mixture.
This demand is enc~untered particularly in ~ields where siz-able molded ~tructures (prepregs) have to be fabricated and their complete cure at elevated temperatures would necessitate substantial outlays for equipment, ~;

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These requirements concerning curing behavior and profile of properties apply also to epoxy~resin sys~ems to be processed by the wet lay-up method.
For some applications, a partial cure proYiding dimensional stability of the molded iarticles is all that is required or even desired, with compl~ete curing, possibly after inter~ediate storage, taking place in a subsequent tempering operation at the requisite temperatures. However, it is important that the heat resi~tance of the material exceed the curing temperature even during the initial cure as otherwise demolding becomes possible only a~ter the temperature of the molded articles has dropped.
Dicyandiamide, long used as a latent curing agent in curable mixtures based on epoxy resins, is usually combined with co-curing agents and/or accelerators to achieve the desired properties. A great many proposals are known from the literature in this field.
~ he present invention has as its object to overcome the drawbacks of the prior art and to provide curable mix-tures, based on epoxy compounds and latent curing agents soluble or homogeneously disper~ible in epoxy resins, wh~ch partially cure to the dimensionally stable state and com-pletely cure to the thermoset final state at low temperatures within a practical period of time without high peak exothermic ~emperatures, posse~s~a heat re~1stance that meets practical :: :

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requirements, and in prepregs exhibit adequate storage stabil-ity at room temperature.
~ his object i6 accc3mplished by the use of new com pounds containing urea and/or urethane qroups and based on addition products of imidazole compounds and prepolymeric i~o-cyanates.
The invention thus relates to compounds of the gen-eral formula A-R-(xR1x-R)e-xR1x-R A (I) where ~2 C~ = C
A is [Hll-(CH;~)3 ] -nac IC~3 R is -OC-~-C~ ~-C~ E-CO-C 1~ 3/ C ~ 3 R~ is ll~ C~;~(Po)l,-(Bo~c(po)d-c~a-c~
:~ Clr, C~3 l s - o - o r - ~ E o I s - C II"- C l1 2 - O -: a hd PO: i5 -0-C8-C~
C~

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~ being 0 or 1, b, ~ and d being between 0 and 50, the sum o~ b, cand d being :between 0 and 50, and preferably betwe~n 10 and 40, and ~ore p~rticularly between 15 and 35, and e heing between 0 and 10, and preferably between 0 and :;; and R2 and R3 = hydrogen, CH3, C2H5 or the phenyl group, indepen dently of one another.

The invention further relates to compounds of the general formula (I), prepared by adding in a first ~tep diols and/or diamines containing ether groups to isophorone diiso-cyanate (IPDI) at temperatures of from 80 to 140~ C, option-ally uslng solvents and aatalystc, in a mol ratio o~ diol and~or dia~ine to isophorone dii~ocyanate of from 1:1 8 to 1:2, and reacting in a~econd step these adducts cont;ining f~ee isocyanate groups with imidazole ompounds of the general : : formula ~ ~ .

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5199~0-2555 where R2 and R3 may be, independently o~ each other, hydrogen, CH3, C2H5 or the phenyl group and ~ .is 0 or 1, optionally usiny ~iolvents, at temperatures of from 100 to 120 C in equivalent amounts.
In one embodiment of the invention, the compounds are used as curing agents in amounts of from 10 to 50 g, and more particularly from 20 to 30 g~ per 100 g of epoxy resin.
In another embodiment of the invention, glycidyl ethers based on bisphenol A, bisphenol ~ and novolacs with epoxy values of from 0.18 to 0.6, and more particularly from O.39 to 0.55, are used as epoxy resins. Commercial halogenated, and particularly bromindted, epoxy resins with approximately 18 percent by weight of bromine, based on the aforeæaid staring materials, may also be used.
The inventive compounds of the general formula (I) can be prepared by adding in a first step icoph~rone diiso~
cyanate to diamin~s or diols containing ether groups at tem-:peratures of ~rom 20 to 90 C~ and;pre~erably from 60 to 700 C, optionally using catalysts such ae dibutyltin dilaurate (DBTL):, and optionally using solvents, in a mol ratio of diol or ~iamine to isophorone diisocyanate of~approximately 1:~.8 to 1:2, and rsacting in a second step the addition product containing ~ree is~ocyanate groups with imida201e compounds and/or N-ami~oalkyli.m1dazole compounds at tempexaturee~of from 80 to 140:3~ C, and preferably from 100 to 120~;C, op~ionally using solvents,; in a mol ratio of prepol~meric isocyanates to : - ~ .: . . ~ ~ , PATENT

imidazole/N-aminoalkylimidazole compounds of approximately equivalent amounts.
The diols and diamines containing ether groups which in accordance with the invention are also used are preferably based on polyethylene oxides or polypropylene oxides and their copolymers or block polymers. They are commercial products, marketed under the trademaxks Desmophen~ (trademark of Bayer AG), Lupranol~ (trademark of BASF) and Jeffamine~ (trademark of Texaco Chemical Co.), for example.
In accordan~e with the invention, diols and amines with molecular weights of approximately 400 to 3,00~, and preferably 600 to 2,500, and more particularly 1,000 to 2,000, are used.
The isophorone diisocyanates used are commarcial-grade products.
The imidazole compounds used are also commercial products. Imidazole, 2-methylimidazole, 2-ethyl-4 methyl-imidazole, 2-phenylimidazole:and N-(3-aminopropyl)imidazole, used alone or in mixtures, are pre~erred. ~.
~ The compounds in accordance with the invention are industrlal products which, with;a mol ratio o~ diol and/or diamine to isophorone diiso~yanate of 1:~ overall, ar~ ~ormed predominantly as co:mpounds of the general formul~ (I) with e = -0,~that is, largely:without higher-molecular-weight ~

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PATENT
519910-~555 chain-propagation reaction products. In addition, minor amount~ of propagation-reaction proclucts where e>O are present. With a mol ratio of diol alnd/or diamine to IPDI o~
from 1:1.8 to 1:<2, these propagation-reaction products in-crease, that is, in the reaction mixture the molecular-weight di6tribution of *he products increases, with e assuming val~es ranging from approximately 0.1 to 10, and usually ~rom 0.5 to 5.
In accordance with the invention, compounds are pre-ferred i.n which polyether polyols or polyether amines based on propylene oxides are used. Optionally, a certain proportion of ethylene oxide may be incorporated in the molecule, dis-tributed randomly or in block fashion, provided that this proportion does not exceed that of propyl~ne oxide. The ~olecular weights of these polyether compounds preferably range from approximately 600 to 2,500, and more particularly from about 1,000 to 2,000.
The compounds in accordance with the invention act bo$h as curing agents and as accelerators so that the concurrent use o~ commonly used accelerators and curing agents ~ay be dispensed with. on the other hand, the inventive compounds can be used uccessfully as accelerators in commercial curing agents such as dicyandiamide, for example.
The quantity of curing agents or accelerators can be varied over a wide range. It is determined by the intended nd;use and the curing conditions which it may impose.~ The , ~ . . . .

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PATF,NT

compounds of the invention are used in amounts ranging from 10 to 50, and preferably 15 to 40, part;s by weight per 100 parts by weight of epoxy compound. In the case of halogenated (and fipecifically brominated) epoxy resins, amounts between 5 and 10 g /100 g of epoxy resin ha~e proved advantageous.
The epoxy resins used in ~he preparation of the cur-able mixture are glycidyl e~ters and ethers having two or more epoxy groups per molecule, and preferably glycidyl ethers based on mono- or polyhydric phenols. Pre~erred are, in accor-dance with the invention, the glycidyl ethers of 2,2-bist4-hydroxyphenyl)propane (bisphenol A) with epoxy values of from 0~2 to 0.6, and particularly the compounds with epoxy values of about 0.39 to 0.55 which at room temperature are semisolid, ranging from highly to moderately viscous. Glycidyl ethers based on bisphenol F and the novolacs have also proved advantageous.

.
Commercial halogenated, and preferably brominated, ~poxy resins with the usual bromine contents, preferably about : 18 weight percent, based on the aforesaid resins, may also be ;~; used.

For modification of the properti2s of the end ~ .:

: product, modifying or auxiliary substances such as phenolic ; ~ resins,:melamine resins or silicone resins may be used, in :
~: addition to other epoxy resins.
~, To~obtain:~the desired viscosity, resins of di~ferent ~.
viscosities or diluents may be employed, as may commonly used ~: :

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solvents such as dimethylformamide, acetone, methyl ethyl ketone, methyl glycol and propylene glycol monomethyl ether, or mixtures thareof.
In prepregging, organic and inorganic fibers, non-woven and woven fabrics and particularly glass are u~ed~
The curing agents of the invention can be used successfully in both solventless and solvent-containing prepregs based on epoxy resins and, optionally, commonly used curing agents. To prepare solventless prepregs, the base materials are impregnated at elevated temperature, if indi-cated, with the ~inder systems by methods which are known per se and then appropriately stored before they are processed further like solvent-containing systems.
Solvent-containing prepregs are formed by the con-ventional method, the base material~ being impregnated in an impregnating bath with the reactive resin mixture and, after the excess resin has been squeegead s~f; continuously con-verted from the A stage to the B stage with input of energy ~mostly heat) and simultaneous elimination of the solvent.
Depending on the desired prepreg consistency ~viscid:to solid), the prepregs are then provided on both sides with a release ~heet and:rolled up for storage and shipping~ The ~urther proceasing~consists of~cutting the individual~prspreg layers to size and placing:~hem one on top of the other to form a stack,~ ~rom which a highly crosslinked part is produced by~moldlng with simultaneous heat input.

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Further examples of solventless systems are heat-curing one-component adhesivesr for example, ~or the adhesive-bonding of body sections in the automotive industry, and epoxy-resin coatings applied either wet or as a powder.

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PATENT

PREP~RATION OF INVENTIVE CURING AGENTS
ExamPle ;~
(a) 44.4 g of isophorone diisocyanate is introduced under a nitrogen atmosphere into a reaction vessel and/ after being heated to about 500 C, miLxed with 200 g o~ Jeff-amine~ D 2000 (polyoxypropylene diamine; trademark o~
Texaco Chemical Co.). The mixture is allowed to react for another hour at 100 C. ~he product has the follow-ing characteristics:
I~ocyanate numbex: Approx. 45 Amine number: <1 Viscosity: 10 Pa-s/60 C

~b) To the a~oresaid product, 16.4 g o~ methylimidazole is added in portions at about 100 C. The mixture is then allowed to react ~or another hour at 100 c. The product has the following characteristics:
Amine number: Approx. 45 ; Viscosity: 85 Pa-s/60 c .

Example 2 ~a) 200 g of Desmophen l900 U (polyoxypropylene glycol;
~ ~ : trademark of Bayer AG) is introduced under a nitrogen :: atmosphere intv a reaction vess~l and dewatered for about : 2 hrs~at 100 C and~a vacuum of 40 mbarO After cooling to~about 35 C and additlon of 0.~ g of Netatin~ 712 : (dibutyltin dilaurate; ~rademark o~ Acima AG~, ~4.4 g of :

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PATENT
519910 ~555 isophorone diisocyanate is adde~d over a period of about 60 minutes. The mixture is allowed to react for about another 2 hrs at ?5 C. The relaction product has the foll~wing characteristics:
Isocyanate number: Approx. 45 Viscosity: 6 Pa-s/25 C

(b) To the aforesaid product, there is added in portions, after heating to about 100 C, 13.6 g o~` imidazole.
Stirring is continued ~or 1 hr at 120 C. The reaction product has the following characteri~tics:
Amine number: Approx. 45 Viscosity: 8.5 Pa-s/60 C

Exa~m~e 3 A~ter being heated to 100 C, 244.6 g of the reac-tion product described in Example 2 ~a) is mixed with 25 g of N-(3-aminopropyl)imidazole. Stirring is continued for 1 hr at 120 C. The reaction product has the followi~g character-ietics:
Amine number: Approx. a o Viscosity: 2903 Pa-s/60 C

Example 4 ;~ ~ After being heated to 100 C, 244.6 g of the reaction ~ product described in Example 2 (a) is mixed with 16.4 g of :
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PATENT
519glO-2555 2-methylimidazole. Stirring is continued for 1 hr at 120 C.
The reaction product has the ~ollowing characteristics:
Amine number: Approx. 45 Viscosity: 3~0 Pa-s/25 C

Example 5 (a~ 310 g of polyoxypropylene glyco:l 620 is introduced under a nitrogen atmosphere into a reaction vessel and de-watered for about 2 hrs at 100 C and a vacuum o~ 40 mbar.

A~ter cooling to about 35 C and addition of 0.3 g of Metatin~ 712 (dibutyltin dilaurate; trademark of Acima AG), 222 g of isophorone diisocyanate is added over a period of about 1 hr. Stirring is continued for about 2 hrs at 75O C.

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: ~ The reaction product has:the ~ollowing ch~racteristics:

;Isocyanate number: Approx. 100 ~ ~ Yiscosity: ~ ~ Approx. 50 Pa~s/25 C

;~ ~b) After being h ated~to~about 100 ~, 68.1 g of imidaæole ~; is added in portions~to ~he product described above.

tirring:is continued :for l hr:at 120 C.

: The product has the followi~g charaoteristic:

Am1ne nu=ber~ : 92 -~
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Q~3 PA'rENrr Example_6 111 g of isophorone diisocyanate is introduced under a nitrogen atmosphere into a reaction vessel and, after being heated to about 140 c, mixed with :IOo g of Jef~amine~ D 400 (polyoxypropylene diamine; trademarlc of Texaco Chemical Co.).
The mixture is allowed to react for another hour at 100 C.
Then 41 g of 2-methylimidazole is added in portionæ at 150-160 C. The mixture is then allowed to react ~or another hour at 170 C. The product has the ~ollowing character-istics:
Melting range: 80-90 C
~mine number: Approx. 150 ~ , ~ o 244.4 g of the reaction product described in Example 1 (a), there is added, after heating to about 100~ C

22.0 g of 2-ethyl-4-methylimi~azole. The mixture is allowed to react for another hour at 120 C. The product has the following~characteristics: ~

Amine number: ~ ~ Approx. 50 Viscosity: ~ 36.4 Pa-s/75 C

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51991~-2555 Determination of effect of curina agent in the cmase of resins based_on bisp~ A

100 g of an epoxy resin (epoxide equivalent weight about lso) is mixed with 15 g of reaction product 1 of the invention and used to make prepregs.
The prepregs are made on the laboratory scale by spreading the reaction mixture onto a satin weave glass filament fabric measuring about 0.1 m2 which after impregna-tion is lined on both sides with a release 6heet and then stored at room te~perature. Very tacky prepregs are so obtained. Their consistency should not appreciably change during storage. For further processing, several layers of these prepregs are superposed on one a~other and laminated at a temperature of 60~ C for 16 hrs,:or at 120 C for 2 hrs, under light pressure (approximately 0.1 to 0.5 bar).
For determ~nation of the period vf invariant prepreg consistency given in Table 1, the prepregs were ~tored under standard climatic conditions and a prepreg lamina~ion was carried out at 24-hr intervals under the~aforesaid conditions The first day on which a change in resin flow is observed ~arks the ~nd of the period o~ invariant prepreg coneIstencyO
; The heat:resistance, also given in Table l, o~ the ~ateriaI undergolng testing was determined by the torsion pen~
duIum test in~conformity~with DIN~53,455 on test speci~ens,~
measuring~IOO;x I0 LL, ~ taken fron~the fInIsh~d laLinates.

PATEN~
51g91~-2555 The glass-transition temperatures and periods of invariant prepreg consistency given in Table 1 for the other examples were determilled as in the c:ase of Example 1;

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5 P. ~ 111 N N N N ~'1 ~ 3 ~ N

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51991~-2~55 Determination of e~f~
in the case of brQminated resins ~xamp~e 1 100 g o~ a brominated epoxy resin ~epoxy value about 0.18, bromine content about 18~) dissolved in 25 g of methyl ethyl ketone (MEK) is mixed with 10 g of a 50% curing-agent solution o~ the inventiYe reaction product and methyl ethyl ketone and used in prepregging. The curing-agent solution is prepared at room temperature. Its concentration may generally be chosen at will.
For prepregging on the laboratory scale, a woven glass-filament fabric about o.l m2 in size is *irst impreg-nated with the resin/curing-agent solution. This is followed by a two-minute heat treatment at 120 c in a forced-air oven of the rei~forcing material so impregnated. During pre-pregging, the sslvent is removed ~rom the impregnating solu-tion and the reactants are converted from the A stage to the B
stage. Almost dry, ~lexible prepregs are so obtained which after cooling can be stored at room temperature for several weeks before they are processed further.
The ~urther processing of the prepregs to laminates about 1 mm thick is carried out in one hour at 150 G with a compacting pressure of about 1 bar. The thoroughly c~red end product so obtained exhib1ts no ~laws so far as adhesion of the individual prepreg layers is con~ernad.

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To determine the heat resistance of the matrix material, test specimens measuring .100 x 10 mm are then taken from the laminates and Rubjected to a torsion pendulum test in conformity with DIN 53,455. The results of this test are presented in Table ~ for various mixing ratios.
The glass-transition temperatures also listed in Table 2 for the othex examples were determined as in Example 1 with the epoxy resin there specified. ~or comparison, the glass-transition temperature of a matrix material much used at present, with dicyandiamide as curing agent and N-methyl-imidazole as accelerator, is included in the table. (Mixing ratio of brominated epoxy resin/dicyandiamide/N-methyl-imidazole: 100:5:0.8.) . .
, , PATENT

Table 2 ~eat resistance o~ ~rominated-resin lamina~s ,- - -- .. __ _ ~ _ -~
i Curing Mixing Gla~P -t~an~ition t0mp~rature ( C) agen'c t:urlng a E~cample agent 1 day 4 weeka ¦ 12 weeka , _ _==~ ~ ~
I 1 100: 10 82 I
100:20 125 130 12 _ _ . ~ _ _ .

3 ' ioo:20 lOl ~ ~ . .._ _~ - . 1 4 100:20 105 . v___ ~ _~. _ _ - . ._ ---_11 100: 10 104 100:20 117 , .. _ ~ ~ . ___ . _ _ _ . _ _11 i Compar- .
ative 100:5:0.R 118 .~ ... ---~= ~- -~----Determination of ef~ect of curinq agent in the case o~ bisphenol A-based resins - ~or determination of the propertie~ of the material when salected curing agents are used, mixtures c~nsisting solely of epoxy resin and curing agent were fully cured and tested so as to exclude the distorting influence of rPinforce-ments and additives~
In the examples listed in Table 3, a glycidyl ether based on bisphenol A with an epoxy value o~ 0.53 was used as epoxy resin.

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To make the test specimens, 100 g of the epoxy resin was mixed in each case at room temperature with the quantity of curing agent indicated in Table 3 and thorou~hly cured in a steel mold to give flat molded pieces 4 mm thick. From these ~olded pieces, test specimens were tiaken by sawing or milling, and on these the values of the prope;rties listed in Table 3, determined in conformity with the test standards specified below, are based.
Test-specimen dimension~
.. . _ . . .. _ Flexural ~trength DIN 53,482 80 x 10 x 4 mm De~lection DIN 53,452 80 x 10 x 4 ~m Impact strength DIN.53,453 50 x 6 x 5 mm Tensile strength DIN 53,455 Dumbbell No. 3 Elongation DIN 53,455 Dumbbell No. 3 Modulus o~ elasticity DIN 53,457 Dumbbell No. 3 Marten's temperature DIN 53,458 60 x 15 x 4 ~m Heat-distortion temperature nIN 53,461 120 x 10 x 4 Glass-transition temperature DIN 53,445 80 x 10 x 1 mm , ~ ~ ~3 (~

PATENT

Ta~le 3 Properties o~ ~E~a~or~ed molded~

Curing agent ¦ Exampl~ 1¦ Example 2 ¦
~_ ~ ~
Curing-agent quantity (g) 25 25 ICuring conditions 16 hrs/60C 16 hrs/60C
I _ __ .. ~ _~
IFlexural strength (N/mm2) 70 53 j ._ ..~ __ ~ ~1 De~lection (mm) 12 10 I ~ ~ ~ I
Impact strength (kJ/m2) 25 20 ~ .= _ 11 l Tensile ~trength (N/mm2) 35 47 ~ . - - . .11 Elongation (%) .3.9 3.4 ~ __ . ___ . _ ~ ~
Modulus o~ elasticity (N/mm2) 1,500 2,000 .. ~ .___ ~
~arten~s temperature (C) 53 40 .. . ~ .. .___ Heat-distortion temperature (C) 75 67 .._ ._ . ~ ~ . . ~
Glass-transition temperature (C~ ~4 85 ~ -- ' ' .!1 ' ~:

Claims

1. A compound of the general formula A-R-(xR1x-R)e-xR1x-R-A (I) where ;
;
;

a is 0 or 1; b, c and d are between 0 and 50;
the sum of b, c and d is between 0 and 50; and R2 and R3 are, independently of one another, hydrogen, CH3, C2H5 or the phenyl group.

2. A compound as defined in claim 1, wherein the sum of b, c and d is between 10 and 40.

3. A compound as defined in claim 2, wherein the sum of b, c and d is between 15 and 35.

4. A compound as defined in claim 1, wherein e is between 0 and 5.

5. A method for preparing a compound as defined in claim 1, which comprises adding in a first step at least one diol, at least one diamine or a combination of at least one diol and at least one diamine containing ether groups to isophorone diisocyanate (IPDI) at temperatures of from 80 to 140° C, optionally using solvents and catalysts, in a mol ratio of diol, diamine or both to isophorone diisocyanate of from 1:1.8 to 1:2, and reacting in a second step at least one adduct containing free isocyanate groups with at least one imidazole compound of the general formula (II) where R2 and R3 are, independently of one another, hydrogen, CH3, C2H5 or the phenyl group and a is 0 or 1, optionally using solvents, at temperatures of from 100 to 120° C in equivalent amounts.

6. A method as defined in claim 5, wherein at least one diol, at least one diamine or a combination of at least one diol and at least one diamine based on polypropylene oxide and having molecular weights of from 600 to 2,500 are used as the at least one diol, diamine, or combination containing polyether groups.

7. A method as defined in claim 5, wherein imidazole, 2-methylimidazole, 2-ethyl-4 methylimidazole, 2-phenylimidazole, N-(3-aminopropyl)imidazole or combinations of these are used as the at least one imidazole compound.

8. A curing agent for epoxy resins comprising at least one compound as defined in claim 1 in an amount of from 10 to 50 g per 100 g of epoxy resin.

9. A curable epoxy-resin composition comprising (a) an epoxy resin having on average more than one epoxy group per molecule, and (b) at least one compound as defined in claim 1.

10. A curable epoxy resin composition in which reinforcements and embedments are impregnated at room tem-perature with a binder, comprising (a) an epoxy resin having on average more than one epoxy group per molecule;
(b) at least one compound as defined in claim 1; and optionally (c) at least one solvent, filler, pigment, auxiliary agent or a combination of these, converted to the semisolid but still fusible state (the B stage), optionally at elevated temperature.

11. A curable epoxy-resin composition as defined in claim 9, wherein at least one glycidyl ether based on bisphenol A, bisphenol F or novolacs with epoxy values of from 0.18 to 0.6 is used as epoxy resins.

12. A curable epoxy-resin composition as defined in claim 10, wherein at least one glycidyl ether based on bisphenol A, bisphenol F or novolacs with epoxy values of from 0.18 to 0.6 is used as epoxy resins.

13. A method for preparing a molded epoxy-resin article, comprising in a first step impregnating reinforcements and embedments at room temperature with a binder, comprising (a) an epoxy resin having on average more than one epoxy group per molecule;
(b) at least one compound as defined in claim 1; and optionally (c) at least one solvent, filler, pigment, auxiliary agent or a combination of these, and converting to the B stage, and in a second step, molding or placing between substrates to be bonded the laminates or prepregs and thoroughly curing at elevated temperature and under pressure.

14. A process for manufacturing fiber-reinforced base materials for the electrical industry comprising converting in a first step a reinforcing material impregnated with a binder based on at least one epoxy resin and amine curing agent to the B stage by the use of heat and optionally pressure and thoroughly curing in a second step at elevated temperature, wherein (a) at least one brominated epoxy resin having on average more than one epoxy group per molecule is used as epoxy resin and (b) at least one of the compounds defined in claim 1 is used as amine curing agent.