Hardener for epoxide resin masses

The present invention provides a hardner for epoxide resin masses based on dicyandiamide, wherein it contains PA1 a) dicyandiamide with a particle size of .gtoreq.90% .ltoreq.10 .mu.m. and PA1 b) 0.1 to 30% by weight of silicon dioxide and/or of an oxide of a metal of Group IIA or IIB of the Periodic Table with a large specific surface area.

The present invention is concerned with a hardner for epoxide resin masses 
based on dicyandiamide. 
It is known to use dicyandiamide for the hardening of epoxide resins (cf. 
U.S. Pat. Nos. 2,637,715 and 3,391,113). The advantages of the 
dicyandiamide are, in particular, its toxicological harmlessness and the 
absence of smell, as well as its chemically inert behaviour, sb that the 
resin-hardner mixture has a good storage stability. 
The disadvantages of dicyandiamide are its poor solubility and the 
non-optimal distributability of the hardner in the epoxide resin which, 
for example, can lead to an increased consumption of hardner. However, 
this increased amount used gives rise, in turn, to the effect that the 
hardened regions with a high content of non-consumed hardner and thus can 
have defects in the end product. 
Attempts have not been lacking to improve this problem of the poor 
distributability in liquid epoxide resins or in epoxide resins melting at 
a higher temperature. Depending upon the intended use, the following ways 
have been employed: 
the use of an appropriate solvent or solvent mixture for the production of 
impregnation solutions from epoxide resin and hardner (cf. Federal 
Republic of Germany Patent Specification No. 30 26 706 and German 
Democratic Republic Patent Specification No. 133 955); 
the use of substituted dicyandiamides with improved distributability in 
epoxide resin masses (cf., for example, French Patent Specification No. 22 
07 911); and 
the use of dicyandiamide dispersions in liquid epoxide resin mixtures. 
Whereas, according to the first way, only a few solvent mixtures could be 
found which display the necessary criteria, such as good solubility or 
good compatibility with the epoxide resin mixtures, in the case of the 
second way, the provision of substituted dicyandiamides is uneconomical 
because the production of these products gives rise to high operational 
and investment costs. In the case of the last-mentioned way, the 
difficulty arises that dicyandiamide must be used which is as finely 
divided as possible in order to achieve a good distribution in powdered 
epoxide resin-hardner mixtures or to obtain dispersions in liquid epoxide 
resin mixtures and, on the other hand, finely-divided dicyandiamide has a 
marked tendency to cake so that finely-divided dicyandiamide forms 
agglomerates within a very short space of time which also can scarcely be 
broken up again by grinding. In the case of dicyandiamide suspensions, 
this agglomeration of finely-ground dicyandiamide leads to the formation 
of comparatively large flocks, which give rise to considerable problems in 
the epoxide resin mixtures. 
Therefore, it is an object of the present invention to provide a hardner 
for epoxide resin masses based on dicyandiamide which does not display the 
above-mentioned known disadvantages, which is characterised by good 
application-technical properties and which can be produced economically. 
Thus, according to the present invention, there is provided a hardner for 
epoxide resin masses based on dicyandiamide, wherein it contains 
a) dicyandiamide with a particle size of .gtoreq.90% .ltoreq.10 .mu.m. and 
b) 0.1 to 30% by weight of silicon dioxide and/or of an oxide of a metal of 
Group IIA or IIB of the Periodic Table with a large specific surface area. 
Surprisingly, we have found that the hardner according to the present 
invention for epoxide resin masses has an improved distributability in the 
epoxide resin, as well as an increased storage stability in the epoxide 
resin-hardner dispersions. Furthermore, the additions of silicon dioxide 
and/or of oxides of metals of Group IIA or IIB of the Periodic Table bring 
about a sometimes considerable shortening of the gel times in comparison 
with normal dicyandiamide, which was also not foreseeable. 
The epoxide resin hardner according to the present invention based on 
dicyandiamide consists of a dicyandiamide with a particle size of 
.gtoreq.90% .ltoreq.10 .mu.m., as well as silicon dioxide and/or an oxide 
of a metal of Group IIA or IIB of the Periodic Table with a large specific 
surface area, the specific surface area preferably being at least 50 
m.sup.2 /g. according to BET, in order to achieve the action according to 
the present invention. Types of silicon dioxide which are commercially 
available under the trade name Aerosil (Degussa), as well as HDK (Wacker), 
have proved to be especially advantageous. 
Especially preferred oxides of metals of Group IIA and IIB of the Periodic 
Table include magnesium oxide, as well as mixtures thereof but, in 
principle, other oxides, for example those of barium or of strontium, can 
also be used. 
The amount of the silicon dioxide or of the oxide of a metal of Group IIA 
or IIB of the Periodic Table is 0.1 to 30% by weight and preferably from 
0.2 to 20% by weight, referred to the weight of the dicyandiamide. 
The production of the epoxide resin hardner according to the present 
invention preferably takes place by continuously dosing the silicon 
dioxide and/or the oxide of a metal of Group IIA or IIB of the Periodic 
Table to the dicyandiamide before grinding to the desired grain size so 
that the metal oxides have the same particle size distribution as the 
dicyandiamide. 
This prevents the finely-ground dicyandiamide from clumping together 
immediately after the grinding procedure, which would result in the 
formation of undesired agglomerates. Thus, the mixing of the components 
can be completed in one step. The grinding of the dicyandiamide and/or of 
the metal oxides can be carried out without problems in conventional 
technical devices, such as grader mills or the like, i.e. without 
separation of dicyandiamide and of the metal oxides during the grinding 
process. 
The dicyandiamide-containing epoxide hardner according to the present 
invention, which has a bulk density of 300 to 500 g./liter, can be 
optimally distributed in solid or liquid epoxide resins, it thereby being 
possible to reduce the amount of hardner in comparison with conventional 
hardners based on dicyandiamide. We have found that amounts of 3 to 6% by 
weight, referred to the epoxide equivalent weight, are sufficient. 
The hardening reaction with the hardner according to the present invention 
can be additionally activated with conventional hardening accelerators, 
for example 2-methylimidazole, substituted ureas (monouron, diuron), 
dimethylbenzylamine or substituted methylenedianilines. The hardened 
epoxide resin has a clearly improved chemical resistance and adhesion in 
comparison with products produced with conventional 
dicyandiamide-containing hardners.

The following Examples are given for the purpose of illustrating the 
present invention. For comparison purposes, epoxide resin hardners based 
on dicyandiamide are produced by grinding dicyandiamide with the following 
additives and compared with one another: 
hardner a) dicyandiamide with 1.3% by weight silicon dioxide HDK N 20 with 
a specific surface area of 200 m.sup.2 /g. (Wacker) 
hardner b): dicyandiamide with 10.0% by weight calcium oxide 
hardner c): dicyandiamide with 0.8% by weight magnesium oxide 
hardner d): dicyandiamide with 5.0% by weight zinc oxide 
hardner e): dicyandiamide without addition 
EXAMPLE 1 
Determination of the Average Grain Diameter by Means of X-ray Electron 
Microscopy 
hardner a): 4-5 .mu.m. (little agglomeration) 
hardner b): 4-5 .mu.m. (scarcely any agglomeration) 
hardner c): 4-5 .mu.m. (scarcely any agglomeration) 
hardner d): 4-5 .mu.m. (scarcely any agglomeration) 
hardner e): 15 .mu.m. (agglomerates up to 40 .mu.m.). 
EXAMPLE 2 
Determination of the Particle Size by Wet Sieving with a Sieve Mesh Size of 
10 .mu.m 
For the wet sieving, there is produced a 15% suspension of the qualities 
obtained in ethyl acetate saturated with dicyandiamide. For homogeneous 
mixing, the suspension is treated for 5 minutes in an ultrasonic bath and 
then filtered through a vibrating, previously weighed out filter with a 
mesh size of 10 .mu.m. 
After the filter has been rewashed with a further 50 g. ethyl acetate, it 
is dried and reweighed. From this is given the percentage content of 
dicyandiamide with a grain diameter of less than 10 .mu.m. 
hardner a): 99% &lt;10 .mu.m. 
hardner b): 98% &lt;10 .mu.m. 
hardner c): 98% &lt;10 .mu.m. 
hardner d): 98% &lt;10 .mu.m. 
hardner e): 45% &lt;10 .mu.m. 
EXAMPLE 2 
Determination of the Gel Times 
The gel times of various epoxide hardners according to the present 
invention are determined with Epikote 1007 and compared with conventional 
dicyandiamide with an average particle size of 80 .mu.m. 
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hardner (5% by wt. dicyandiamide 
gel time at 180.degree. C. 
ref to Epikote 1007) in min. 
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dicyandiamide (80 .mu.m.) 
7.0 
epoxide resin hardner a) 
6.0 
epoxide resin hardner b) 
5.0 
epoxide resin hardner c) 
6.0 
epoxide resin hardner d) 
5.0 
dicyandiamide (80 m.) + 
3.0 
0.25% 2-methylimidazole 
epoxide resin hardner a) + 
2.5 
0.25 2-methylimidazole 
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EXAMPLE 4 
Determination of the Storage Stability 
Dispersions are produced each of 4% of the hardner according to the present 
invention in Epikote 828 and compared with a dispersion of 4% 
dicyandiamide (80 .mu.m.) within a period of time of 4 weeks: 
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dispersion after 4 weeks 
hardner storage 
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dicyandiamide (80 .mu.m) 
almost all the material 
deposited 
hardner a) stable dispersion 
hardner b) stable dispersion 
hardner c) stable dispersion 
hardner d) stable dispersion 
hardner e) about 50% deposited as flocks 
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EXAMPLE 5 
The dispersions described in Example 4 are applied in the form of a lacquer 
layer to a cleaned steel sheet and hardened at 180.degree. C. for one 
hour, the following observations thereby being made: 
dicyandiamide (80 m.): punctate coating with unused hardner, not completely 
hardened 
hardner a): smooth, transparent coating 
hardner b): smooth, white-pigmented coating 
hardner c): smooth, almost transparent coating 
hardner d): smooth, slightly pigmented coating 
hardner e): uneven, slightly punctate coating with unused hardner