Mixed blocked isocyanate prepolymers, a process for their production and their use for the production of flexible epoxy resin systems

Isocyanate prepolymers based on an aromatic diisocyanate and a polyether polyol in which at least 95% of the isocyanate groups are blocked by blocking agents selected from PA1 A) about 50 to about 97 equivalent % of a phenol or substituted phenol, PA1 B) about 30 to about 50 equivalent % of a pyrazole or a substituted pyrazole and PA1 C) 0 to about 10 equivalent % of a blocking agent different from A) and B). These blocked isocyanate prepolymers are particularly useful for the production of flexible epoxy resin systems.

BACKGROUND OF THE INVENTION 
The present invention relates to new isocyanate prepolymers having 
predominantly or exclusively blocked isocyanate groups in which at least 
two different blocking agents are present, a process for their production 
and their use for the production of flexible epoxy resin systems. 
Synthetic resins based on epoxy resins have a number of advantageous 
properties. Such desirable properties include, for example, good adhesion 
to organic and inorganic substrates, good solvent resistance and high 
resistance to chemicals. However, due to their high cross-linking density, 
amine-cured epoxy resins (particularly those based on diphenylolpropane 
(bisphenol A) and epichlorohydrin) are often brittle, having glass 
transition regions above 20.degree. C. Consequently, these synthetic 
resins do not generally meet the impact strength, shock resistance and 
high flexibility requirements for many applications. This is particularly 
true with respect to construction applications where durable bridging of 
shrinkage cracks (e.g., in concrete) is necessary. 
The elasticity of these resins can be increased internally to a certain 
extent by reducing the cross-linking density and externally by adding 
softeners. 
External elasticizers such as tar, phthalate esters, high-boiling alcohols 
and vinyl polymers are unreactive and are not incorporated into the 
duromer network. They bring about expansion solely by occupying space. 
Internal elasticizing through reduction of the cross-linking density can 
be achieved by reducing the functionality of the curing agent. Longchain, 
low-functional aminoamides based on dimerized fatty acids have been 
successfully used to reduce crosslinking density. However, these 
aminoamides cannot be employed in all application areas due to 
insufficient elasticizing. 
A good and lasting elasticizing of epoxy resins can be achieved by 
combination of the resin with a polyurethane. Elasticized plastics 
composed of epoxy resins, polyfunctional carbamate aryl esters and 
polyamines are described, for example, in DE-AS 2,152,606, but the 
plastics thus prepared do not satisfy all of the practical requirements 
for such materials. More specifically, the carbamate aryl esters described 
in DE-AS 2,152,606 have a very high viscosity. Therefore, in many cases 
they have to be processed with added softeners. Further, the mechanical 
properties of these prior art plastics do not meet the technical 
requirements for many applications. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide novel blocked 
isocyanate prepolymers. 
It is also an object of the present invention to provide blocked isocyanate 
prepolymers which are useful for the production of flexible epoxy resin 
systems which have mechanical properties that satisfy the requirements for 
many practical applications and which have lower viscosities than prior 
art materials. 
It is another object of the present invention to provide a process for the 
production of blocked isocyanate prepolymers which are useful in the 
production of flexible epoxy resins having improved properties. 
It is a further object of the present invention to provide a process for 
the production of flexible epoxy resin systems. 
These and other objects which will be apparent to those skilled in the art 
are accomplished by reacting (a) an aromatic diisocyanate with (b) a 
polyether polyol having an average hydroxyl functionality of from about 
1.5 to about 4 and (c) a combination of at least two blocking agents. The 
blocking agents are selected from (A) phenols and substituted phenols, (B) 
pyrazoles and substituted pyrazoles and optionally (C) other known 
blocking agents which are outside the scope of (A) and (B). The phenol or 
substituted phenol blocking agent may be used in an amount of from about 
50 to about 97 equivalent percent, based on the total amount of blocking 
agents employed. The pyrazole or substituted pyrazole blocking agent may 
be used in an amount of from about 3 to about 50 equivalent percent, based 
on the total amount of blocking agents employed. The optional blocking 
agent (C) may be used in an amount of from 0 to about 10 equivalent 
percent, based on the total amount of blocking agents employed. The 
prepolymer formed by this reaction has at least 95% of its isocyanate 
groups blocked with blocking agent. From about 1.3 to about 7% of the 
total weight of this prepolymer is the sum of the weight of any free 
isocyanate groups plus the weight of the blocked isocyanate groups. The 
blocked isocyanate prepolymers of the present invention may be combined 
with an epoxy resin and an organic polyamine and then cured to form an 
epoxy resin composition. 
DETAILED DESCRIPTION OF THE PRESENT INVENTION 
The present invention is directed to isocyanate prepolymers based on 
aromatic diisocyanates and polyether alcohols having an (average) hydroxyl 
functionality of from about 1.5 to about 4. At least 95% of the isocyanate 
groups of this prepolymer are blocked by blocking agents. The total 
content of free and blocked isocyanate groups (calculated as NCO) is from 
about 1.3 to about 7% by weight. The blocking agent used to produce the 
blocked isocyanate prepolymers of the present invention is made up of 
(A) from about 50 to about 97 equivalent percent of at least one phenol or 
substituted phenol, 
(B) from about 3 to about 50 equivalent percent of at least one pyrazole or 
substituted pyrazole, and 
(C) from 0 to about 10 equivalent percent of one or more blocking agents 
different from (A) and (B), 
with the total percentage of (A), (B) and (C) being 100. 
The present invention also relates to a process for the production of 
isocyanate prepolymers containing isocyanate groups having an NCO content 
of from about 1.4 to about 8.5% by weight. These isocyanate prepolymers 
are based on polyether polyols having an (average) hydroxyl functionality 
of from about 1.5 to about 4. At least 95 equivalent percent of the 
isocyanate groups of these prepolymers are blocked with blocking agents 
which include: 
(A) from about 50 to about 97 equivalent percent of a phenol or a 
substituted phenol, 
(B) from about 3 to about 50 equivalent percent of a pyrazole or a 
substituted pyrazole, and 
(C) from 0 to about 10 equivalent percent of other blocking agents which 
are different from (A) and (B), 
with the total amount of blocking agents being 100 equivalent percent. 
The present invention also relates to a proceeds for the production of 
flexible epoxy resin systems from the isocyanate prepolymers of the 
present invention. 
The use of substituted pyrazoles as blocking agents has already been 
disclosed in DE-OS 3,403,436 and EP-A 0,159,117. Each of these 
publications describes one-component systems which cure to form polymeric 
coatings only when exposed to elevated temperatures. Neither publication 
suggests that certain pyrazole-blocked polyisocyanates would be curable by 
polyamines at room temperature. Moreover, there is no indication in either 
of these disclosures that the disclosed pyrazoles should be combined with 
a phenolic blocking agent. 
The isocyanate prepolymers on which the blocked polyisocyanates of the 
present invention are based are reaction products of aromatic 
polyisocyanates with polyether polyols containing urethane groups and 
having an NCO content of from about 1.4 to about 8.5% by weight, 
preferably from about 1.6 to about 5% by weight. 
Any of the known aromatic polyisocyanates may be used to prepare the 
prepolymers and blocked polyisocyanates of the present invention. 
Diisocyanates having molecular weights in the range of from about 174 to 
about 300 are preferred. Specific examples of preferred diisocyanates 
include: 2,4-diisocyanatotoluene and technical mixtures thereof in which 
preferably up to 35% by weight, based on the mixture is 
2,6-diisocyanatotoluene; 4,4'-diisocyanatodiphenylmethane and technical 
mixtures thereof with 2,4'- and optionally 
2,2'-diisocyanatodiphenylmethane and/or its higher homologs. 
Diisocyanatotoluene is particularly preferred. 
The polyether polyols useful for the production of the isocyanate 
prepolymers employed in the practice of the present invention have an 
(average) molecular weight, calculated from the hydroxyl functionality and 
hydroxyl group content, of from about 800 to about 10,000, preferably from 
about 1,000 to about 6,000, and an (average) hydroxyl functionality of 
from about 1.5 to about 4, preferably from about 2 to about 4. They may be 
prepared in known manner by alkoxylation of the appropriate starter 
molecules, preferably using ethylene oxide and/or propylene oxide in any 
order or combination in the course of the alkoxylation reaction. 
Suitable starter molecules are low-molecular weight compounds containing 
primary or secondary amino groups and/or hydroxyl groups which when 
alkoxylated will produce a polyol having an average hydroxyl functionality 
of from about 1.5 to about 4. Specific examples of suitable starter 
molecules include: water, ethanol, ethylene glycol, propylene glycol, 
trimethylo-propane, glycerol, pentaerythritol, sorbitol, ethylenediamine 
or any mixtures of such starter molecules which will produce a polyol 
having an average hydroxyl functionality of from about 1.5 to about 4 when 
alkoxylated. The polyether polyols may also be mixtures of polyether 
polyols prepared separately and then mixed, provided that such mixtures 
have an average molecular weight of from about 800 to about 10,000 and an 
average hydroxyl functionality of from about 1.5 to about 4. 
To prepare isocyanate prepolymers useful in the practice of the present 
invention, polyisocyanates such as those identified above are reacted with 
polyether polyols such as those identified above to form. urethane groups 
while maintaining an NCO/OH equivalent ratio of from about 1.8:1 to about 
10:1. This reaction is generally carried out within the temperature range 
of from about 40 to about 100.degree. C. 
The nature and proportions of the starting components and the NCO/OH 
equivalent ratio are selected from the above-described materials and 
equivalent ratios so that isocyanate prepolymers are formed having an NCO 
content of from about 1.4 to about 8.5%. A lower NCO/OH ratio of, for 
example, from about 1.5:1 to about 2.5:1 is used if a chain extending 
reaction is desired. Otherwise an isocyanate excess, that is, an NCO/OH 
equivalent ratio of from about 5:l to about 10:1, is often used. When an 
isocyanate excess is used, it is preferred that the excess starting 
isocyanate be removed upon completion of the prepolymer formation by 
distillation if the isocyanate is distillable (e.g., by thin film 
distillation). Where non-distillable starting isocyanates remain in the 
prepolymer, the excess starting isocyanate may also be removed by 
extraction using suitable solvents such as, for example, cyclohexane or 
isooctane. 
The blocking agents used in the practice of the present invention are 
preferably employed in a total quantity such that at least 95, most 
preferably from about 97 to about 100 equivalent percent of the total 
equivalents of NCO groups present in the isocyanate prepolymer are 
blocked. The blocking agent composition of the present invention is 
generally made up of (A) from about 50 to 97 equivalent %, preferably from 
about 60 to about 95 equivalent %, of a phenol or substituted phenol, (B) 
from about 3 to about 50 equivalent %, preferably from about 5 to about 40 
equivalent %, of a pyrazole or substituted pyrazole and (C) from 0 to 
about 10 equivalent %, preferably 0 equivalent % of blocking agent which 
is different from (A) and (B), with the total equivalent % of (A) plus (B) 
plus (C) being 100 equivalent %. 
In principle, any compound having at least one phenolic OH group and which, 
except for this OH group, is inert to isocyanate groups, may be used as 
blocking agent (A). Particularly useful phenols include monohydric, 
C.sub.1 -C.sub.18 alkyl-substituted phenols, preferably C.sub.6 -C.sub.12 
alkyl-substituted phenols, phenols having preferably an alkyl substituent 
or ester groups. Specific examples of suitable phenols include: phenol; 
the isomeric cresols; the isomeric xylenols; 2-sec-butylphenol; 
4-tertbutylphenol; the isomeric nonylphenols; dodecylphenols; 
octadecylphenols; the isomeric alkyl hydroxybenzoates having from 1 to 18, 
preferably 1 to 4 carbon atoms in the alkyl radical; and mixtures of these 
phenols. Technical mixtures of nonylphenol isomers (hereinafter referred 
to as "isononylphenol") are particularly preferred. 
Examples of the pyrazoles and substituted pyrazoles which are useful as 
blocking agent (B) include: pyrazole; 3-(5)methylpyrazole; and 
3,5-dimethylpyrazole. Pyrazole and 3,5-dimethylpyrazole are particularly 
preferred. 
Examples of the optional blocking agents (C) include: butanone oxime; 
.epsilon.-caprolactam; and secondary monoamines such as di-n-butylamine. 
In the process of the present invention, it is preferred that blocking 
agent (A) be reacted first with the isocyanate prepolymer to be blocked. 
This reaction of the phenolic blocking agent and isocyanate prepolymer is 
carried out in known manner, preferably at a temperature of from about 50 
to about 120.degree. C, optionally in the presence of a catalyst until the 
calculated NCO content is attained. Any of the catalysts known to be 
useful in such blocking reactions may be used. The pyrazole blocking agent 
(B) may then be added in portions. The temperature is maintained within 
the range of from about 20 to about 100.degree. C, preferably from about 
20 to about 60.degree. C during the addition of this blocking agent. 
It is also possible, but less preferred, to react the isocyanate prepolymer 
with the pyrazole blocking agent (B) before the isocyanate prepolymer is 
reacted with the phenolic blocking agent (A). 
Where an optional blocking agent (C) is used, blocking agent (C) may be 
reacted with the isocyanate prepolymer at any point, optionally mixed with 
blocking agent (A) or (B). 
The advantage to reacting the isocyanate prepolymer with the phenolic 
blocking agent (A) first and then with blocking agent (B) is that the less 
reactive blocking agent is reacted first and the NCO groups still 
remaining react with the more reactive blocking agent. This makes it 
possible to achieve virtually complete blocking of the isocyanate groups 
without the need to use an excess of blocking agents. 
The reaction of isocyanate prepolymer with blocking agent composition is 
carried out preferably in the solid state. But the use of organic solvents 
or softeners is, in principle, also possible. Examples of suitable 
solvents include: n-butyl acetate; methoxypropyl acetate; toluene; xylene; 
and higher aromatic solvent mixtures which are commercially available 
(e.g., Solvesso solvent which is marketed by Exxon Chemie). 
The predominantly or completely blocked isocyanate prepolymers of the 
present invention are characterized by a total content of unblocked and 
blocked aromatic isocyanate groups (calculated as NCO) of from about 1.3 
to about 7% by weight, preferably from about 1.5 to about 4% by weight. 
The blocked polyisocyanate prepolymers of the present invention 
surprisingly have distinctly lower viscosities than do the corresponding 
exclusively phenol-blocked products of prior art. 
In combination with organic polyamines, the blocked prepolymers of the 
present invention cure within a day to form elastic non-adhesive plastics. 
Any organic polyamine having a molecular weight within the range of from 
about 60 to about 1000, preferably from about 60 to about 500, which has a 
total of at least two primary and/or secondary amino groups per molecule 
(preferably, two primary amino groups per molecule) may be used in this 
reaction. Specific examples of suitable polyamines include: 
ethylenediamine, 1,2-diaminopropane, 1,3-diaminopropane, 
1,4-diaminobutane, 1,6-diaminohexane, and 2,2,4- and/or 
2,4,4-trimethylhexamethylenediamine. Polyamines which in addition to two 
or more primary amino groups also have secondary amino groups such as 
diethylenetriamine or triethylenetetramine may also be used. 
Plastics which are prepared by reacting the blocked isocyanate prepolymers 
of the present invention with diamines having one or more cycloaliphatic 
rings and a molecular weight within the range of from about 60 to about 
1000 are particularly preferred. Examples of suitable diamines having at 
least one cycloaliphatic ring include: 1,4-diaminocyclohexane; 
4,4'-diaminodicyclohexylmethane; 1,3-diaminocyclopentane; 
4,4'-diaminodicyclohexylpropane-2,2; 4-isopropyl- 1,2-diaminocyclohexane; 
3,3'-diaminodicyclohexylpropane-2,2; 
3,3'-dimethyl-4,4'-diaminodicyclohexylmethane; 
3-aminomethyl-3,3,5-trimethylcyclohexylamine (isophorone diamine); and 
technical bis-aminomethyltricyciodecane (which is commercially available 
under the name "TCD-Diamin" from Hoechst AG). Aminopolyethers having a 
molecular weight within the above-given ranges and having on statistical 
average from 2 to 3 terminal primary amino groups per molecule such as 
those which are commercially available under the trademark Jeffamine 
(Texaco) may also be used to produce plastics from the blocked isocyanate 
prepolymers of the present invention. 
The plastics obtained by cross-linking of the blocked poly-isocyanate 
prepolymers of the present invention have better mechanical properties 
than do plastics produced from comparable exclusively phenol-blocked 
systems of the prior art. 
In practice, the blocked polyisocyanate prepolymers of the present 
invention are primarily used in three-component systems made up of epoxy 
resins, polyamines and the blocked polyisocyanate prepolymers. Mixtures of 
this type may be cured under either hot or cold conditions. 
Epoxy resins useful in such three-component systems generally contain, on 
average, more than one epoxide group per molecule. Suitable epoxy resins 
include: glycidyl ethers of polyhydric alcohols such as butanediol, 
hexanediol, glycerol, hydrogenated diphenylolpropane; and polyhydric 
phenols such as resorcinol, diphenylolpropane and phenolaidehyde 
condensates. The glycidyl ethers of polybasic carboxylic acids such as 
hexahydrophthalic acid or dimerized fatty acid may also be used. 
Liquid epoxy resins based on epichlorohydrin and 2,2-diphenylol-propane 
(bisphenol A) and having a molecular weight of from about 340 to about 450 
are particularly preferred. If desired, the viscosity of the mixtures can 
be lowered by including monofunctional epoxide compounds. The addition of 
such monofunctional epoxide compounds will also improve the pot life of 
the system. Examples of suitable monofunctional epoxide compounds include 
aliphatic and aromatic glycidyl ethers such as butyl glycidyl ether and 
phenyl glycidyl ether; glycidyl esters such as glycidyl acrylate; and 
epoxides such as styrene oxide and 1,2-epoxydodecane. 
"Epoxy resin systems" which contain a blocked isocyanate prepolymer of the 
present invention, an epoxy resin and a polyamine (particularly a 
cycloaliphatic polyamine) cure to form high quality plastics when from 
about 0.4 to about 0.9 (preferably from about 0.5 to about 0.8) primary 
amino groups and from about 0.02 to about 0.5 (preferably from about 0.03 
to about 0.4) blocked isocyanate groups per epoxy group are present. 
Mixtures which are ready for use may be made by including any of the common 
auxiliary substances and additives such as fillers, pigments, reaction 
accelerators or viscosity regulators in epoxy resin systems made up of an 
epoxy resin, a polyamine and a blocked polyisocyanate of the present 
invention. Examples of such auxiliary substances and additives include: 
reaction accelerators such as salicylic acid, bis(dimethylamino- 
methyl)-phenol and tris(dimethyl-aminomethyl)-phenol; fillers such as 
sand, any of the powdered minerals, silica, powdered asbestos, kaolin, 
talc, powdered metal, tar, tar pitch, asphalts, cork granules, and 
polyamides; softeners such as phthalate esters; and viscosity regulators 
such as benzyl alcohol. Epoxy resin-curing agent combinations in which the 
blocked isocyanate prepolymers of the present invention are used or 
applied concomitantly as curing agents are suitable for the production of 
coatings, adhesives, sealants, and structural pads. These resins are 
characterized by good bonding, resistance to chemicals, high impact 
strength and shock resistance, improved flexibility and elasticity.

The following Examples serve to explain the invention further. All 
percentage data and the PO/EO ratio refer to percentage by weight. 
EXAMPLES 
Example 1 
1000 g of a polyether triol having the OH number 28 (prepared by 
propoxylation of trimethylolpropane and subsequent ethoxylation of the 
propoxylation product (PO/EO ratio =83:17)) and 500 g of a polyether diol 
having the OH number 56 (prepared by propoxylation of propylene glycol), 
were reacted with 174 g of 2,4-diisocyanatotoluene for 5 hours at 
80.degree. C. to form a prepolymer in which the theoretical NCO content of 
2.5% was attained. 
154 g of a technical mixture of isomeric nonylphenols (isononylphenol) was 
then added to the prepolymer and reacted in the presence of 0.4 g of 
tin(II) octoate while stirring for 6 hours at 60.degree. C. 20 g of solid 
pyrazole were then added and the mixture was stirred for a further 30 
minutes at 60.degree. C. A total of 99 equivalent %, based on the 
isocyanate groups of the prepolymer, of blocking agents were used. The 
blocked isocyanate prepolymer thus obtained had the following 
characteristic data: 
Blocked NCO content: 2.25% (calculated) 
Viscosity (22.degree. C): 49,000 mPa.s 
187 g of the prepolymer were homogeneously mixed with 11.9 g of 
3,3'-dimethyl-4,4'-diaminodicyclohexylmethane. The mixture cured overnight 
at ambient temperature to form a clear, elastic plastic having the 
following mechanical properties: 
Shore A hardness: 65 
Elongation at tear: 1050% 
Breaking strength: 4.7 N/mm.sup.2 
Tear strength: 17N/mm 
Example 2 
1000 g of the same polyether triol which was used in Example 1 and 1000 g 
of a polyether diol having the OH number 28 (prepared by propoxylation of 
propylene glycol and subsequent ethoxylation of the propoxylation product 
(PO/EO ratio =80:20)) were reacted with 174 g of 2,4-diisocyanatotoluene 
for 5 hours at 80.degree. C. to form a prepolymer having the theoretical 
NCO content of 1.9%. 176 g of isononylphenol were then added, the mixture 
was catalyzed using 0.5 g of tin(II) octoate and stirred for 6 hours at 
60.degree. C. 19 g of solid 3,5-dimethylpyrazole were then added and the 
mixture was stirred for an additional 30 minutes at 60.degree. C. A total 
of 100 equivalent %, based on the isocyanate groups of the prepolymer, of 
blocking agents were used. The blocked isocyanate prepolymer thus obtained 
had the following characteristic data: 
Blocked NCO content: 1.7% (calculated) 
Viscosity (22.degree. C): 39,000 mPa.s 
247 g of this prepolymer were homogeneously mixed with 11.9 g of 
3,3'-dimethyl-4,4'-diaminodicyclohexylmethane. The mixture cured overnight 
at ambient temperature to form a clear, elastic plastic having the 
following mechanical properties: 
Shore A hardness: 61 
Elongation at tear: 1500% 
Breaking strength: 3.9 N/mm.sup.2 
Tear strength: 20 N/mm 
Example 3 
1000 g of the same polyether diol which was used in Example 2 and 800 g of 
a polyether triol having the OH number-35 (prepared by propoxylation of 
trimethyloipropane and subsequent ethoxylation of the propoxylation 
product (PO/EO ratio 87:13)) were reacted with 174 g of 
2,4-diisocyanatotoluene for 5 hours at 80.degree. C. to form a prepolymer 
having the theoretical NCO content of 2.1%. 176 g of isononylphenol were 
then added, the mixture was catalyzed using 0.4 g of tin(11) octoate and 
stirred for 6 hours at 60.degree. C. 13 g of solid pyrazole were then 
added and the mixture was stirred for an additional 30 minutes at 
60.degree. C. A total of 99 equivalent %, based on the isocyanate groups 
of the prepolymer, of blocking agents were used. The blocked isocyanate 
prepolymer thus obtained had the following characteristic data: 
Blocked NCO content: 1.9% 
Viscosity (22.degree. C): 61,000 mPa.s 
221 g of this prepolymer were then homogeneously mixed with 1.9 g of 
3,3'-dimethyl-4,4'-diaminodicyclohexylmethane. The mixture cured overnight 
at ambient temperature to form a clear, elastic plastic having the 
following mechanical properties: 
Shore A hardness: 54 
Elongation at tear: 1050% 
Breaking strength: 2.8 N/mm.sup.2 
Tear strength: 18 N/mm 
Example 4 (Comparative) 
The NCO prepolymer corresponded to that produced in Example 1. This 
prepolymer was blocked exclusively with isononylphenol as follows: 1674 g 
of the NCO prepolymer together with 248 g of isononylphenol and 0.4 g of 
tin(II) octoate were stirred for 7 hours at 60.degree. C. 113 equivalent 
%, based on isocyanate groups of the prepolymer, of blocking agent were 
used. 
The blocked isocyanate prepolymer thus obtained had the following 
characteristic data: 
Blocked NCO content: 2.2% 
Viscosity (22.degree. C): 78,000 mPa.s 
193 g of this prepolymer were then homogeneously mixed with 11.9 g of 
3,3'-dimethyl-4,4'-diaminodicyclohexylmethane. The mixture cured overnight 
at ambient temperature to form a clear, elastic plastic having the 
following mechanical properties: 
Shore A hardness: 63 
Elongation at tear: 190% 
Breaking strength: 3.4 N/mm.sup.2 
Tear strength: 14 N/mm 
Example 5 
(Production of an elasticized epoxy resin) 
100 g of the blocked prepolymer from Example 1 and 100 g of a commercially 
available epoxy resin made up of bisphenol A and epichlorohydrin and 
having an average molecular weight of 380 were homogeneously mixed with 
37.5 g of 3,3'-dimethyl-4,4'-diaminodicyclohexylmethane. The mixture cured 
overnight to form an impact-resistant plastic having a Shore D hardness of 
70. 
Although the invention has been described in detail in the foregoing for 
the purpose of illustration, it is to be understood that such detail is 
solely for that purpose and that variations can be made therein by those 
skilled in the art without departing from the spirit and scope of the 
invention except as it may be limited by the claims.