Epoxy resin composition

A novel epoxy resin composition is comprised of a mixture of a cyclic epoxide type epoxy resin having an epoxy equivalent of 200 or below and a phenoxy resin having a molecular weight of approximately 30,000, a curing agent, and an inorganic filler. This resin composition is useful in the fabrication of cast insulators to be employed in electric machines.

BACKGROUND OF THE INVENTION 
The present invention relates to an epoxy resin composition useful in the 
making of cast insulators employed in electric machines. 
An expoxy resin combined with an acid anhydride cures to provide a product 
that has superior electrical, mechanical and chemical properties and which 
is extensively used as an epoxy resin cast insulator in electric machines 
including those employed in power transmission and distribution. If 
particularly good electrical and mechanical properties (e.g. high heat 
resistance) are required, cycloaliphatic epoxy resins having two or more 
epoxy groups in the molecule are used either alone or in combination with 
bisphenol A type epoxy resins. In order to improve the productivity of 
epoxy resin cast insulators using a smaller number of molds, a method 
commonly referred to as the superatmospheric gelling process which is 
capable of reducing the release time is currently employed. In this 
method, an epoxy resin blend of interest held within a cold pressurized 
tank is injected into a heated mold through a pipeline and a die head, 
while the mold is pressurized to compensate for any contraction of the 
resin being cured, thereby producing the desired casting within a 
shortened period of curing. The epoxy resin blend employed in this method 
must have a low viscosity and a long pot life within the cold pressurized 
tank, and must be capable of curing rapidly within the heated mold. 
Those epoxy resins which exhibit low viscosities at low temperatures have 
low molecular weights and, hence, they exhibit on extremely high degree of 
shrinkage during curing and are highly likely to give cured products with 
sink marks and cracks. This problem is particularly serious with 
cycloaliphatic epoxy resins which are commonly employed for the purpose of 
providing improved heat resistance. In addition, epoxy resins that are 
highly reactive at elevated temperatures will also exhibit comparatively 
high reactivity at low temperatures and suffer from a shorter pot life. 
Common practice for dealing with these problems is to employ the 
superatmospheric gelling method with a view to preventing the occurrence 
of sink marks and cracks during the curing process and to use a latent 
accelerator for the purpose of extending the pot life of the resin blend. 
A problem arises, however, from the fact that epoxy resins of low 
molecular weights, such as cycloaliphatic epoxy resins, that will exhibit 
low viscosities at low temperatures are less resistant to thermal shock 
than the solid epoxy resins which are commonly employed in ordinary 
casting methods other than the superatmospheric gelling process. 
One conventional method employed for improving the resistance of 
low-viscosity epoxy resins to thermal shock is to use them in combination 
with bisphenol A type epoxy resins, but the intended resistance to thermal 
shock cannot be attained without adding a large amount of bisphenol A type 
epoxy resin. Furthermore, the resulting epoxy resin composition has an 
excessively high viscosity at low temperatures and presents considerable 
difficulty in working operations at low temperatures. An alternative to 
the use of bisphenol A type epoxy resins is to add plasticity providing 
agents, such as high-molecular weight oligomers that has molecular weights 
within the range of from about 500 to 5,000 and which are comprised of 
polyester, polyether, polybutadiene or the like in the backbone chain. 
However, as the addition of these oligomers in increased, the viscosity of 
the epoxy resin is increased significantly while its heat resistance is 
considerably reduced. On the other hand, if the addition of such oligomers 
is insufficient, there is little possibility of improvement in the 
resistance of the produce against thermal shock. Plasticity providing 
agents such as those having polyamide in the backbone chain have the 
advantage that they will not substantially increase the viscosity of the 
resin blend but then, the resin blend incorporating such plasticity 
providing agent is highly reactive and has a shorter pot life. 
In the superatmospheric gelling method, an epoxy resin blend having a low 
viscosity at low temperature is injected into a mold that is heated to a 
temperature higher than that of the resin blend. Within the mold, the 
viscosity of the resin blend is reduced temporarily to cause precipitation 
of the filler, giving rise to such problems as surface defects (e.g. flow 
marks) on the cured product and an appreciably high degree of unevenness 
in the distribution of filler's level in the cured product. This latter 
problem causes nonuniformity in the dielectric constant of the cured 
product and an insulator made of that product will have an unequal 
potential distribution in its cross section. An electric machine using 
this insulator will not only suffer from such electrical problems as 
reduced a.c. breakdown voltage but also has degraded mechanical properties 
such as low crack resistance. 
In ordinary casting methods other than the super-atmospheric gelling 
process, the above-mentioned problems have hitherto been coped with by 
using an increased amount of filler or reducing its particle size so that 
the overall viscosity of the epoxy resin composition is sufficiently 
increased to prevent precipitation of the filler. However, this method is 
unable to produce a resin blend having a low viscosity at low temperature, 
and most of the problems associated with the drop in the viscosity of the 
resin blend that results from its injection into a mold having a higher 
temperature remain unsolved. 
As explained above, the conventional epoxy resins are unable to have high 
resistance to both heat and thermal shock. In addition, when an epoxy 
resin blend having a low viscosity at low temperature is injected into a 
mold heated to a temperature higher than that of the blend, the filler 
will precipitate to cause not only surface flaws on the cured product but 
also nonuniformity or degradation in the properties of the final product. 
SUMMARY OF THE INVENTION 
The primary object, therefore, of the present invention is to eliminate the 
aforementioned problems by providing a novel epoxy resin composition that 
has a low viscosity and long pot life at low temperatures while retaining 
rapidly curing properties at high temperatures and exhibiting high 
resistance to thermal shock without sacrificing its heat resistance and 
which will not cause precipitation of any portion of the filler in the 
resin composition.

DETAILED DESCRIPTION OF THE INVENTION 
The epoxy resin composition of the present invention comprising an epoxy 
resin (A) prepared by mixing a cyclic epoxide type epoxy resin having an 
epoxy equivalent of 200 or below with a phenoxy resin having a molecular 
weight of approximately 30,000; a curing agent made of a mixture of a 
polybasic carboxylic acid anhydride and an aromatic ester; and a filler 
made of an inorganic powder. 
The cyclic epoxide type epoxy resin as one component of the mixture (A) may 
be 3,4-epoxycyclohexylmethyl-(3,4-epoxy)cyclohexanecarboxylate, and this 
may be mixed with 10-50% of its own weight of a phenoxy resin. 
The aromatic ester as one component of the curing agent may be a diester of 
Formula (I) 
##STR1## 
wherein R.sub.1 and R.sub.2 are each a saturated or unsaturated cyclic 
hydrocarbon group having 6-8 carbon atoms. 
The diester of Formula (I) may be added in an amount of 0.05-0.3 mole per 
mole of the polybasic carboxylic acid anhydride which is the other 
component of the curing agent. 
Almost all of the commercial cyclic epoxide type epoxy resins having an 
epoxy equivalent of 200 or below may be used in the present invention, but 
epoxy dicyclopentenylphenylglycidyl ether and dicycloaliphatic diether 
diepoxy are unsuitable since their viscosities are very high and mixtures 
with a phenoxy resin have such high viscosities that working at low 
temperatures (20.degree.-80.degree. C.) will become either very difficult 
or entirely impossible. If the cyclic epoxide type epoxy resin has a very 
low molecular weight, an increased amount of curing agent has to be used 
in order to cause the desired degree of curing in the resin blend. 
Therefore, cyclic epoxide type epoxy resins that are advantageous for use 
in the present invention are those having an epoxy equivalent of 200 or 
below and a viscosity of no higher than 1,000 cp at 25.degree. C. Among 
the compounds that satisfy these requirements, those having a molecular 
structure of the same type as that of 
3,4-epoxycyclohexylmethyl-(3,4-epoxy)cyclohexanecarboxylate are 
particularly preferable. 
The phenoxy resin is preferably added in an amount of 10-50% by weight of 
the cyclic epoxide type epoxy resin. If less that 10 wt% of the phenoxy 
resin is used, the resulting resin blend is such that it cannot be 
injected into a mold having a higher temperature without causing 
precipitation of the filler since the decrease in the viscosity of the 
resin blend will be as great as in the case where only the cyclic epoxide 
epoxy resin is used as the resin component. If, on the other hand, more 
than 50 wt% of the phenoxy resin is used, the resin blend will have such a 
high viscosity that considerable difficulty is involved in working the 
blend at low temperatures. 
Any polybasic carboxylic acid anhydride that is liquid at low temperatures 
(20.degree.-80.degree. C.) may be employed as one component of the curing 
agent, and suitable examples are hexahydrophthalic acid anhydride, 
methylhexahydrophthalic acid anhydride, tetrahydrophthalic acid anhydride, 
and methyltetrahydrophthalic acid anhydride, and these may be used either 
independently or in combination. 
The aromatic ester used as the other component of the curing agent may be 
any compound of Formula (I) where R.sub.1 and R.sub.2 are each a saturated 
or unsaturated cyclic hydrocarbon group having 6-8 carbon atoms, and 
illustrative aromatic rings include cyclohexane, methyl-substituted 
cyclohexane, benzene, cyclohexene and methyl-substituted cyclohexene 
rings. 
The aromatic ester is advantageously added in an amount of 0.05-0.3 mole 
per mole of the polybasic carboxylic acid anhydride. If less than 0.05 
mole of the aromatic ester is used, the curing agent is no more effective 
than one composed of only the polycarboxylic acid anhydride and the cured 
resin blend will have an undesirably low resistance to thermal shock. If, 
on the other hand, more than 0.3 mole of the aromatic ester is used, a 
cured product having improved resistance to thermal shock is obtained but, 
then, it will not have a high value of HDT (deflection temperature under 
flexural load). 
An inorganic powder material that will not deteriorate the electrical or 
mechanical properties of the blend may be used as a filler in the present 
invention. Suitable, but by no means limiting, examples are alumina, 
hydrated alumina, quartz and fused quartz powders. 
The epoxy resin composition of the present invention may be prepared and 
cast by the following procedures: the cyclic epoxide type epoxy resin 
having an epoxy equivalent of 200 or below is mixed with the phenoxy resin 
having a molecular weight of approximately 30,000 to form a mixture (A); 
this mixture is blended with the curing agent made of the polybasic 
carboxylic acid anhydride and the aromatic ester, the inorganic filler 
powder, and a suitable accelerator (if desired) at 20.degree.-80.degree. 
C., preferably under subatmospheric pressure, thereby making the epoxy 
resin composition; the composition then is injected directly into a 
preheated mold (90.degree.-160.degree. C.) through a pipeline; the mold is 
subsequently pressurized at 0.5-5.0 kg/cm.sup.2 G for 1-30 minutes until 
curing of the composition is completed to produce a casting. 
The accelerator that may be incorporated in the epoxy resin composition is 
illustrated by, but by no means limited to, metal salts of organic 
carboxylic acids (e.g. zinc octylate), tertiary amines, boron 
trifluoride-amine complex, and imidazole. The amount of the accelerator 
added is to be adjusted to such a value that curing of the blend will be 
completed in 1-30 minutes at the mold temperature of 
90.degree.-160.degree. C. 
The epoxy resin composition offered by the present invention may also 
contain a colorant, a coupling agent or an internal release agent on the 
condition that they will not deteriorate any of the desirable 
characteristics such as the viscosity, long pot life, and fast curing 
property of the resin blend, as well as the resistance to precipitation of 
the filler, absence of color unevenness, high HDT and thermal shock 
resistance of the cured product of the resin blend. 
The following Examples and Comparative Examples are provided for the 
purpose of further illustrating the composition of the present invention. 
In the examples and comparatives, all parts were based on weight. 
EXAMPLE 1 
A hundred parts of a mixture of a cyclic epoxide type epoxy resin (CY-179 
of Ciba Geigy) and 15 wt% of a phenoxy resin (PKHH of UCC), 74 parts of 
methyl-THPA (acid anhydride), 12 parts of a methyl tetrahydrophthalic acid 
diester of bisphenol A, 1 part of zinc octylate and 440 parts of an 
alumina powder were agitated at 60.degree. C. under vacuum so as to 
prepare an epoxy resin composition. The initial viscosity of the resin 
composition, its pot life, gelling time and time vs. viscosity profile 
were determined by the following methods. The results are summarized in 
the following Table 1 and the accompanying FIGURE (----). 
Three test pieces were prepared from the resin composition by first gelling 
it at 150.degree. C. and by then curing at 130.degree. C..times.24 hours. 
These test pieces were used in evaluation of crack resistance, HDT and 
filler precipitation, respectively, by the methods shown below. The 
results of evaluation are also summarized in Table 1. 
Initial viscosity 
The epoxy resin composition was agitated under vacuum at 60.degree. C. for 
30 minutes and its viscosity was measured. 
Pot life 
The viscosity of the epoxy resin composition was measured at 60.degree. C. 
and at intervals of 30 minutes. The time required for the viscosity to 
increase to 10.sup.5 cp was measured. 
Gelling time 
The epoxy resin composition was heated in a hot-air drying oven held at 
150.degree. C. and the time required for the resin composition to gel was 
measured. 
Time vs. viscosity profile 
The epoxy resin composition was put in a vessel held at 60.degree. C. and 
the vessel was placed in an oil bath also held at 60.degree. C. Viscosity 
measurements were done at 30-minute intervals for plotting the 
time-dependent variations in viscosity. 
Crack index 
A test piece of the epoxy resin composition was examined for its crack 
resistance by the method recommended by the IEC in IEC Publication 455-2. 
HDT 
A test piece was prepared from the epoxy resin composition and evaluated in 
accordance with ASTM-D 648. 
Filler precipitation 
A test piece having an outside diameter of 20 mm and a height of 200 mm was 
cured from the epoxy resin composition and the concentration of the filler 
in the top 5-mm portion of the sample was measured. The amount of filler 
precipitation was determined by substracting the measured value of filling 
from the initial value. 
TABLE 1 
______________________________________ 
Resin composition 
initial Cured product 
vis- pot gelling filler pre- 
cosity life time HDT crack cipitation 
Run No. (cp) (hr) (min) (.degree.C.) 
index (wt %) 
______________________________________ 
Ex. 1 19500 &gt;360 19 141 6.8 2.0 
Ex. 2 29000 &gt;360 20 140 7.0 1.1 
Ex. 3 37000 355 22 140 7.3 0.6 
Comp. Ex. 
1 27000 330 18 145 4.0 21.5 
2 69000 165 30 132 6.9 0.6 
3 22000 360 25 140 3.5 1.7 
4 45000 310 28 130 6.6 1.2 
______________________________________ 
EXAMPLE 2 
A hundred parts of a mixture of a cyclic epoxide type epoxy resin (CY-179) 
and 30 wt% of a phenoxy resin, 55 parts of methyl-THPA, 28 parts of a 
methyl tetrahydrophthalic acid diester of bisphenol A, 1 part of zinc 
octylate and 430 parts of an alumina powder were agitated under vacuum to 
prepare an epoxy resin composition. The characteristics of this 
composition and the cured product thereof were evaluated as in Example 1. 
The results are summarized in Table 1 and the accompanying figure (----). 
EXAMPLE 3 
A hundred parts of a cyclic epoxide type epoxy resin (CY-179) and 45 wt% of 
a phenoxy resin, 40 parts of methyl-THPA, 41 parts of a methyl 
tetrahydrophthalic acid diester of bisphenol A, 1 part of zinc octylate 
and 425 parts of an alumina powder were agitated under vacuum to prepare 
an epoxy resin composition. The characteristics of this composition and 
the cured product thereof were evaluated as in Example 1. The results are 
summarized in Table 1 and the accompanying FIGURE (--.quadrature.--). 
COMATIVE EXAMPLE 1 
A hundred parts of a mixture of a cyclic epoxide type epoxy resin (CY-179) 
and 6 wt% of a phenoxy resin, 66 parts of methyl-THPA, 34 parts of a 
methyl tetrahydrophthalic acid diester of bisphenol A, 1 part of zinc 
octylate and 470 parts of an alumina powder were agitated at 60.degree. C. 
under vacuum so as to prepare an epoxy resin composition. The 
characteristics of this composition and the cured product thereof were 
evaluated as in Example 1. The results are summarized in Table 1 and the 
accompanying FIGURE (----). 
COMATIVE EXAMPLE 2 
A hundred parts of a mixture of a cyclic epoxide type epoxy resin (CY-179) 
and 60 wt% of a phenoxy resin, 46 parts of methyl-THPA, 23 parts of a 
methyl tetrahydrophthalic acid diester of bisphenol A, 1 part of zinc 
octylate and 400 parts of an alumina powder were agitated at 60.degree. C. 
under vacuum so as to prepare an epoxy resin composition. The 
characteristics of this composition and the cured product thereof were 
evaluated as in Example 1. The results are summarized in Table 1 and the 
accompanying FIGURE (--.DELTA.--). 
COMATIVE EXAMPLE 3 
A hundred parts of a mixture of a cyclic epoxide type epoxy resin (CY-179) 
and 30 wt% of a phenoxy resin, 70 parts of methyl-THPA, 2 parts of a 
methyl tetrahydrophthalic acid diester of bisphenol A, 1 part of zinc 
octylate and 400 parts of an alumina powder were agitated at 60.degree. C. 
under vacuum so as to prepare an epoxy resin composition. The 
characteristics of this composition and the cured product thereof were 
evaluated as in Example 1. The results are summarized in Table 1 and the 
accompanying FIGURE (----). 
COMATIVE EXAMPLE 4 
A hundred parts of a mixture of a cyclic epoxide type epoxy resin (CY-179) 
and 30 wt% of a phenoxy resin, 40 parts of methyl-THPA, 54 parts of a 
methyl tetrahydrophthalic acid diester of bisphenol A, 1 part of zinc 
octylate and 455 parts of an alumina powder were agitated under vacuum at 
60.degree. C. so as to prepare an epoxy resin composition. The 
characteristics of this composition and the cured product thereof were 
evaluated as in Example 1. The results are summarized in Table 1 and the 
accompanying FIGURE (--X--). 
In Examples 1 to 3 and Comparative Examples 1 to 4, the filler occupied 
about 70% by weight of the epoxy resin composition, and the amount of the 
curing agent used per equivalent of the epoxy resin was 0.8. 
As will be understood from the data shown above, the epoxy resin 
composition of the present invention will provide a cast insulator that 
has high resistance to both heat (as evidenced by high HDT values) and 
thermal shock and which also exhibits a uniform dispersion of filler. In 
addition, the composition will ensure efficient production of cast 
insulators with consistent quality. Minimum loss of the resin during 
casting operations will also provide for substantial saving of the 
resources.