Solventless resin composition having minimal reactivity at room temperature

A resin composition is provided that is suitable for impregnating sheet material, including fabrics, films, paper and tapes of the type employed to form prepregs, such as tapes used to form electrical insulation layers on electrical components. The resin composition includes a solid or semi-solid epoxy resin having an epoxide functionality of at least 2.5, a metal acetylacetonate for catalyzing the epoxy resin, and an accelerator of bisphenol A-formaldehyde novolac catalyzed by an acidic catalyst and having a hydroxyl equivalent weight of 120. The resin composition is essentially unreactive at room temperature and at elevated temperatures sufficient to enable permeation of the sheet material by the resin in its manufacture, is essentially unreactive at temperatures required to remove moisture and volatiles during processing of articles wrapped or taped with the sheet material, and cures at a higher temperature without adversely affecting the cure characteristics of the resin composition. The resin composition of this invention achieves the desirable reactivity characteristics noted above, while exhibiting enhanced mechanical, thermal and electrical properties necessary for its use within an electrical insulation composite.

This invention generally relates to epoxy resin technology. More 
particularly, this invention is directed to a solventless epoxy resin 
composition characterized by high viscosity, solid or semi-solid 
consistency, long shelf life, and minimal reactivity at heat processing 
temperatures, and further characterized by reactivity at higher curing 
temperatures to form a solid having excellent mechanical and electrical 
insulating properties, such that the epoxy resin composition is highly 
suitable for use as a binder material for sheet materials used as 
electrical insulation. 
BACKGROUND OF THE INVENTION 
Polymer-impregnated sheet material, such as fabrics, films, paper and 
tapes, have been widely employed to form electrical insulation for various 
electrical equipment and components, including high voltage stator bars of 
a generator. Formation of such insulation generally involves the use of a 
pre-impregnated sheet material, often referred to as a prepreg, that can 
be applied directly to a member to be insulated. Various materials can be 
employed as the sheet material and the impregnation material, depending on 
the requirements of the applications. 
As taught by U.S. Pat. Nos. 3,812,214, 4,603,182 and 4,656,090 to 
Markovitz, each assigned to the assignee of this invention, a prepreg of 
mica paper backed with a woven fabric backer, such as woven fiberglass, is 
often used in the manufacturing of high voltage stator bars. The mica 
paper can be employed with a single backer or in combination with two 
backers, in which one backer can be a woven fabric such as fiberglass 
while the second can be another woven fabric, a non-woven fabric such as a 
polyester mat, or a polyester or polyimide film. In each case, an epoxy 
resin binder is used to permeate through the mica paper and the backers, 
and to bond each backer to the mica paper so as to form a prepreg sheet. 
Prepregs of the type taught by Markovitz are typically slit into tapes that 
can be more readily wrapped around a conductor, such as a stator bar of a 
generator. Typically, multiple layers of tape are tightly wrapped around 
the conductor, usually overlapping by one-half the width of the tape. 
After being wrapped with a sacrificial release film to protect the tape 
and prevent contamination, the conductor and its tape wrapping are then 
placed in an autoclave for vacuum heat treatment and subsequent curing. 
Vacuum heat treatment is carried out to remove air, moisture and any 
solvent or volatile compound present in the resin binder, so as to prevent 
formation of voids in the cured insulation that would otherwise adversely 
affect the quality of the insulation and induce premature insulation 
failure due to breakdown under electrical stress. Thereafter, the taped 
conductor undergoes a cure under pressure to consolidate the tape 
insulation, such that the resin binder bonds the mica paper and each of 
its backers together to form a void-free solid insulation. 
In order to reliably form a high quality insulation, several requirements 
must be met in the manufacture and processing of resin-impregnated sheet 
materials, such as the mica tape and the taped conductor noted above. 
Preferably, the resin binder is a semi-solid or solid at room temperature, 
yet sufficiently flexible to make the sheet material pliable. Furthermore, 
the binder must have a sufficiently high molecular weight to act as an 
adhesive for bonding the prepreg components together, and must be 
substantially tack-free to prevent the prepreg from sticking together 
(i.e., blocking). 
In addition, during the manufacture of the prepreg, the resin binder must 
be able to permeate through the sheet materials, as well as act as an 
adhesive to the one or two backers used in the construction of the 
prepreg. One known approach is to add a solvent, such as methyl ethyl 
ketone, acetone or toluene, to a semi-solid or solid resin so as to reduce 
its viscosity. While this approach is effective, a shortcoming is the 
relatively large amount of solvent that must be removed during the 
subsequent vacuum heat treatment of the taped conductor, an amount that is 
typically much greater than the moisture content of the tape. As noted 
previously, if the solvent is not completely removed, the retained solvent 
will adversely affect the cured properties of the binder and can promote 
the formation of voids in the cured insulation. An additional shortcoming 
of the use of solvents is the environmental and safety concerns associated 
with their use. 
An additional requirement is that, during processing of a conductor wrapped 
or taped with the prepreg, the vacuum heat treatment must sufficiently 
lower the viscosity of the resin binder within the sheet material and 
increase the vapor pressure of its volatile compounds, so as to enable the 
removal of the volatile components. Such a requirement is particularly 
important in the use of multiple, tightly-wrapped layers of mica tape. For 
example, vacuum cycles of at least about five hours and often up to about 
twelve hours, at temperatures of up to about 120.degree. C, are typically 
required to remove the volatile compounds from the multiple tape layers 
around a stator bar. However, a significant problem with this step is the 
tendency for the resin binder to be reactive at the vacuum heat treatment 
temperatures necessary to adequately reduce the viscosity of the binder so 
as to remove the volatiles. As a result, the binder will begin to gel, 
particularly if the temperature is too high, the duration of the cycle is 
excessive, or if the prepreg is aged such that the resin binder has 
already begun to react. 
Finally, the resin binder must be able to flow under pressure during the 
curing stage in order to fill all voids between the prepreg layers and 
between the prepreg and the conductor. However, if gelation has occurred 
during the vacuum heat treatment cycle, there will be insufficient resin 
flow during cure, such that voids will likely remain and degrade the 
effectiveness and the life of the cured insulation. While the reactivity 
of the binder could be reduced in order to prevent gelation during vacuum 
heat treatment, the result has been a reactivity which is inadequate to 
achieve sufficient cure during the cure cycle within a practical process 
cycle of about twelve hours at about 165.degree. C. 
While the above-noted U.S. Pat. Nos. 3,812,214, 4,603,182 and 4,656,090 to 
Markovitz advanced the art of resin binders that are suitable for forming 
prepreg mica tapes, the disclosed resins do not exhibit an optimized 
reactivity. Specifically, these resins exhibit some degree of reactivity 
at temperatures necessary to completely remove their volatile components. 
As a result, gelation tends to occur during the vacuum heat treatment 
cycle, preventing the elimination of voids and thereby degrading the 
effectiveness and life of the insulation. In the use of these resins, 
gelation is avoided only by carefully monitoring the hot vacuum cycle, 
thereby complicating processing. Furthermore, these resins have tended to 
react over extended periods at room temperature, such as periods in excess 
of one month, necessitating that they be refrigerated in order to promote 
their shelf life. 
Finally, the resin compositions taught by U.S. Pat. Nos. 4,603,182 and 
4,656,090 included styrene or vinyl toluene as diluents, which are 
reactive and volatile compounds, that can be removed during vacuum heat 
treatment, and thereby result in a variable product depending on how much 
was removed. Because these resin compositions have a tendency to gel 
during the hot vacuum cycle, such gelation hinders the ability to achieve 
adequate compaction in order to obtain a void-free insulation. 
Accordingly, the mechanical and electrical properties of a resulting cured 
insulation can be diminished. 
In view of the above, it would be desirable if a resin binder were 
available that exhibited more optimal reactivity properties, specifically 
in terms of being: essentially unreactive at room temperature for storage 
stability; essentially unreactive at about 50.degree. C. to about 
120.degree. C. in order to enable manufacture of the prepreg by hot melt 
soaking; essentially unreactive and having a low viscosity at a suitable 
vacuum heat treatment temperature so as to remove air, moisture and 
volatiles during processing of a conductor wrapped or taped with the sheet 
material; and highly reactive at practical curing temperatures. If such a 
resin binder were to exist, a substantial improvement could be achieved in 
the shelf life of mica tapes, the removal of volatile components during 
processing of the taped conductor, the avoidance of void formation during 
curing, and the effectiveness and life of the resulting insulation. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a resin composition that 
enables the manufacture of a resin-rich prepreg by heat treating at 
temperatures on the order of about 50.degree. C. to about 120.degree. C., 
wherein the resin is able to penetrate through the prepreg components 
without the presence of any solvents in the resin. 
It is a further object of this invention that such a resin composition 
exhibit little or no reactivity at room temperature and temperatures 
sufficient to reduce the viscosity of the resin and remove any volatiles 
within the resin. 
It is another object of this invention that such a resin composition enable 
the manufacture of a resin-rich prepreg that can be vacuum heat treated 
without the risk of pre-gelation. 
It is still a further object of this invention that such a resin 
composition be reactive at an elevated temperature and therefore curable 
within a practical time period so as to form a solid that is characterized 
by desirable mechanical and electrical insulating properties. 
It is yet another object of this invention that such a resin composition 
have an extended shelf life of at least one year without requiring 
refrigeration. 
The present invention provides a resin composition that is suitable for 
impregnating sheet materials, including fabrics, films, paper and tapes, 
to form a resin-rich prepreg, such as the type employed to form electrical 
insulation for electrical components. More particularly, the resin 
composition of this invention has been determined to be essentially 
unreactive at room temperature and at elevated temperatures that enable 
permeation of the sheet material by the resin, and essentially unreactive 
during a subsequent vacuum heat treatment of an article wrapped or taped 
with the prepreg for the purpose of reducing resin viscosity and removing 
volatiles, without adversely affecting the cure characteristics of the 
resin composition. 
Accordingly, the resin composition is particularly suitable for forming an 
electrical insulation layer on a conductor, such as a high voltage stator 
bar of a generator. The resin composition of this invention achieves the 
desirable reactivity characteristics noted above, while exhibiting 
enhanced mechanical and electrical properties necessary for its use within 
an electrical insulation system. 
The above advantages are achieved by a solventless thermosetting resin 
composition composed of a solid or semi-solid epoxy resin having an 
epoxide functionality of at least 2.5, a metal acetylacetonate for 
catalyzing the solid or semi-solid epoxy resin, and an accelerator of 
bisphenol A-formaldehyde novolac catalyzed by an acidic catalyst and 
having a hydroxyl equivalent weight of 120. 
In accordance with the teachings of this invention, the resin composition 
is substantially unreactive at a temperature of up to at least about 
120.degree. C., and is reactive at a temperature of about 165.degree. C. 
to form a solid material having mechanical and electrical properties that 
are suitable to enable the solid material to serve as an electrical 
insulating material. The resin composition enables the manufacture of 
resin-rich prepregs by heat treating at temperatures on the order of about 
5.degree. 0C. to about 120.degree. C., during which the viscosity of the 
resin composition is sufficiently reduced to enable the resin composition 
to penetrate through a sheet material without the presence of any solvents 
in the composition. 
In accordance with this invention, the solid or semi-solid epoxy resin 
constitutes the primary epoxy component for obtaining the desirable 
adhesive properties for a prepreg impregnated with the resin composition, 
and for achieving the desirable semi-solid or solid consistency of the 
resin composition. A liquid epoxy resin having an epoxide functionality of 
2 can replace a portion of the solid or semi-solid epoxy resin, if a 
softer and more pliable sheet material is desired. It has been determined 
that such a substitution can be carried out for up to about fifty weight 
percent of the solid or semi-solid epoxy resin without significantly 
affecting the reactivity of the resin composition or its cured properties. 
A significant advantage of this invention is that the resin composition 
exhibits more optimal reactivity properties than the resin compositions of 
the prior art, particularly in terms of being essentially unreactive at 
temperatures up to and including those necessary for sufficiently reducing 
the viscosity of the resin composition for vacuum heat treatment. 
Accordingly, the resin composition of this invention provides for 
substantial improvements in the shelf life of impregnated sheet materials, 
such as mica tapes used to wrap electrical components, to the extent that 
a shelf life of more than a year can be achieved without refrigeration. 
In addition, the resin composition is also unreactive at temperatures 
necessary to enable the removal of volatile components from a sheet 
material impregnated with the resin composition. As such, the resin 
composition enables the manufacture of resin-rich prepregs, such as mica 
tapes, that can be vacuum and heat treated without the risk of 
pre-gelation, while also promoting the effectiveness and life of an 
insulation system formed by the prepreg. 
Another significant advantage of this invention is that the above is 
achieved without adversely affecting the cure characteristics of the 
prepreg and the mechanical and electrical properties desired for materials 
of the type used to form insulation layers for electrical components. 
Specifically, the resin composition of this invention is highly reactive 
at a suitable curing temperature above the vacuum heat treatment 
temperature, so as to be curable within a practical time period to form a 
solid that is characterized by enhanced mechanical and electrical 
insulating properties. 
Other objects and advantages of this invention will be better appreciated 
from the following detailed description

DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides a resin composition whose processing 
characteristics and mechanical and electrical properties make the 
composition particularly well suited for use as a binder for a prepreg 
electrical insulating material for electrical equipment and components, 
such as a high voltage stator bar for a generator. The resin composition 
of this invention is characterized by being stable and unreactive at 
temperatures of up to about 120.degree. C. and reactive and curable via an 
epoxy-epoxy reaction to form a solid or semi-solid thermoset material at 
temperatures of about 165.degree. C. and above. At temperatures of about 
50.degree. C. to about 120.degree. C., the resin composition is further 
characterized as having a viscosity that is sufficiently low so as to 
enable the resin composition to readily permeate a sheet material, such as 
a fabric film, paper or tape, and thereby form a prepreg. In view of the 
resin composition being essentially unreactive at about 50.degree. C. to 
about 120.degree. C., the resin composition does not begin to gel and 
cause the prepreg to stiffen during the period in which the resin 
composition is required to permeate the sheet material. In addition, the 
prepreg can be subsequently vacuum and heat treated at about 120.degree. 
C. for periods as long as about twelve hours to remove air, moisture and 
other volatiles, without risk of pregelation. 
Referring to FIG. 1, a stator bar 10 for a generator is represented that 
illustrates some of the general concepts of this invention, as well as a 
suitable application for the resin composition of this invention. As 
shown, the stator bar 10 is composed of a number of conducting copper 
strands 12 that are insulated from each other by strand insulation 13, as 
is conventional in the art. In addition, the conductor strands 12 are 
arranged to form two arrays that are separated by a strand separator 14. 
Surrounding both arrays is a groundwall insulation 15 formed by multiple 
wrappings of a mica paper tape 16 manufactured in accordance with the 
teachings of this invention. 
As illustrated in FIGS. 2 and 3, the mica paper tape 16 is a prepreg 
composed of a mica paper 17 backed by a single woven backing 18 (FIG. 2) 
or a pair of backings 18aand 18b(FIG. 3), and impregnated with the resin 
composition of this invention. In the latter configuration, one of the 
backings 18a, or 18b can be a woven fabric such as fiberglass while the 
second can be another woven fabric, a non-woven fabric such as a polyester 
mat, or a polyester or polyimide film. In each case, the resin composition 
of this invention is used to permeate through the mica paper 17 and to 
bond each backing 18, 18a and 18b to the mica paper 17, thereby forming 
the prepreg tape 16. As such, the resin composition affects the properties 
of the mica paper tape 16 both while in the prepreg state, and after 
vacuum heat treatment and curing steps are performed by which the 
mechanical and electrical properties of the mica paper tape 16 and the 
groundwall insulation 15 are acquired. 
The above-described stator bar 10 is merely intended to represent generally 
conventional conductors over which it is desirable to provide electrical 
insulation layers formed by a resin-impregnated sheet material. Therefore, 
the teachings of this invention are not limited to the specific 
configuration shown in the FIG., and are equally applicable to various 
other electrical components and assemblies that benefit from the presence 
of electrical insulation layers. It is also foreseeable that the resin 
composition of this invention could be employed for various applications 
other than those involving a prepreg. Accordingly, those skilled in the 
art will recognize that numerous applications for the resin composition of 
this invention are possible, all of which are within the scope of this 
invention. 
As noted above, a key feature of this invention is that the resin 
composition is formulated to have reaction characteristics that facilitate 
the manufacture of a resin-impregnated sheet material, such as the mica 
paper tape 16 shown in FIG. 1. In accordance with this invention, it has 
been unexpectedly determined that the desired properties and 
characteristics described above are achieved through the combination of a 
solid or semi-solid epoxy resin having an epoxide functionality of at 
least 2.5, a metal acetylacetonate as a catalyst for the epoxy resin, and 
a bisphenol A-formaldehyde novolac as an accelerator. A preferred 
formulation for the resin composition is a metal acetylacetonate content 
of about 0.1 to about 1 percent by weight of the epoxy resin, and an 
accelerator content of about 5 to about 15 percent by weight of the epoxy 
resin. The above formulation can be modified by substituting a liquid 
epoxy resin having an epoxide functionality of 2 for up to about fifty 
weight percent of the solid or semi-solid epoxy resin. Solventless resin 
compositions formulated in accordance with the above are very viscous, 
solid or semi-solid at a temperature of about 25.degree. C., but become 
fluid liquids with a viscosity of less than about 10,000 centipoise (cps) 
at about 120.degree. C. 
A preferred accelerator for the resin composition is a bisphenol 
A-formaldehyde novolac available from Georgia-Pacific Resins, Inc., under 
the name BRWE 5300. The preferred accelerator is made with an acidic 
catalyst, usually oxalic acid, and is characterized by a melt viscosity of 
about 800 to about 1600 cps at about 125.degree. C, a hydroxyl equivalent 
weight of 120, and a Mettler softening point of about 80.degree. C. to 
about 105.degree. C. Other bisphenol A-formaldehyde novolacs having lower 
or higher melt viscosities or softening points, indicative of a lower or 
higher molecular weight, can be used as the accelerator, with the 
limitation that their hydroxyl equivalent weight is 120, since bisphenol A 
must be used in the acid-catalyzed reaction with formaldehyde in order to 
yield an accelerator having the desired reactivity properties. As a 
nonreactive accelerator, the bisphenol A-formaldehyde novolac must be 
present in an amount less than the stoichiometric level in terms of the 
epoxy:novolac and epoxy equivalent:phenolic hydroxyl equivalent ratios. 
The solid or semi-solid epoxy resin serves as the primary epoxy component 
for obtaining the desired adhesive properties and the desired pliability 
of the mica paper tape 16 in its prepreg state. Preferred solid or 
semi-solid epoxy resins for the resin composition include epoxy novolacs 
such as DEN 439 and DEN 438, available from Dow Chemical Co., though it is 
foreseeable that other epoxy resins having an epoxide functionality of at 
least 2.5 could be used. The DEN 439 resin is particularly preferred, and 
is characterized by an epoxide functionality of 3.8, an epoxide equivalent 
weight of 191 to 210, and a Mettler softening point of about 48.degree. C. 
to about 58.degree. C. DEN 438 is characterized as having an epoxide 
functionality of 3.6, an epoxide equivalent weight of 176 to 181, and a 
viscosity of about 20,00 to about 50,000 cps at about 52.degree. C. 
Another epoxy novolac resin that can be used is DEN 485, also manufactured 
by Dow Chemical Co. DEN 485 has an epoxide functionality of 5.5, an 
epoxide equivalent weight of 165 to 195, and a softening point of about 
66.degree. C. to about 80.degree. C. 
Other solid or semi-solid epoxy resins with an epoxide functionality of at 
least 2.5 include epoxy cresol novolacs made by the Ciba Chemical Co., 
such as: ECN 1235 with an epoxide functionality of 2.7, an epoxide 
equivalent weight of 200 to 227, and a melting point of about 34.degree. 
C. to about 42.degree. C. ; ECN 1273 with an epoxide functionality of 4.8, 
an epoxide equivalent weight of 217 to 233, and a melting point of about 
68.degree. C. to about 78.degree. C.; ECN 1280 with an epoxide 
functionality of 5.0, an epoxide equivalent weight of 213 to 233, and a 
melting point of about 78.degree. C. to about 85.degree. C.; and ECN 1299 
with an epoxide functionality of 5.4, an epoxide equivalent weight of 217 
to 244, and a melting point of about 85.degree. C. to about 100.degree. C. 
Suitable solid or semi-solid epoxy resins with an epoxide functionality of 
at least 2.5 also include tetra functional phenol such as MT0163, 
available from Ciba Chemical Co. and having an epoxide functionality of 4, 
an epoxide equivalent weight of 179 to 200, and a melting point of about 
55.degree. C. to about 95.degree. C., and Epon 1031, available from Shell 
Chemical Co. and having an epoxide functionality of 3.5, and an epoxide 
equivalent weight of 200 to 240, which is a solid resin having a kinematic 
viscosity of about Z2 to about Z7 at about 25.degree. C. as an 80 percent 
weight solution in methyl ethyl ketone. 
Other suitable solid epoxy resins with an epoxide functionality of at least 
2.5 include modified epoxy novolacs such as the EPI-REZ SU resins made by 
Shell Chemical Co., such as EPI-REZ SU-2.5 with an epoxide functionality 
of 2.5, an epoxide equivalent weight of 180 to 200, and a melt viscosity 
of about 2500 to about 4500 centistokes at about 52.degree. C., EPI-REZ 
SU-3.0 with an epoxide functionality of 3.0, an epoxide equivalent weight 
of 187 to 211, and a melt viscosity of about 20,000 to about 50,000 
centistokes at about 52.degree. C. and EPI-REZ SU-8 with an epoxide 
functionality of 8.0, an epoxide equivalent weight of 195 to 230, and a 
melting point of about 77.degree. C. to about 82.degree. C. 
As noted above, up to about fifty weight percent of the solid or semi-solid 
epoxy resin can be replaced in the resin composition with a liquid epoxy 
resin having an epoxide functionality of 2. Substitution of the liquid 
epoxy resin for the solid or semi-solid epoxy resin can be employed to 
produce a softer and more pliable tape 16 without introducing volatile 
components or significantly affecting the reactivity and cure properties 
of the resin composition of this invention. A preferred difunctional epoxy 
resin is the liquid bisphenol A-diglycidyl ether epoxy resin Epon 826, 
available from Shell Chemical Co. and characterized by an epoxide 
functionality of 2, an epoxide equivalent weight of 178 to 186, and a 
viscosity of about 6500 to about 9500 cps at about 25.degree. C. Offsets 
of Epon 826 include Araldite GY 6008 available from Ciba Chemical Co., DER 
330 available from Dow Chemical Co., and EPOTUF 37-139 available from 
Reichhold Co. 
Other suitable liquid bisphenol A-diglycidyl ether epoxy resins include the 
following made by Shell Chemical Co.: Epon 828 with an epoxide equivalent 
weight of 185 to 192 and a viscosity of about 11,000 to about 15,000 cps 
at about 25.degree. C.; Epon 830 with an epoxide equivalent weight of 190 
to 198 and a viscosity of about 17,700 to about 22,500 cps at about 
25.degree. C.; and Epon 834 with an epoxide equivalent weight of 230 to 
280 and a Gardner-Holdt viscosity O-V at about 25.degree. C. when measured 
as a 70 percent weight solution in diethylene glycol monobutyl ether. Many 
other similar liquid bisphenol A-diglycidyl ether epoxy resins made by 
different manufacturers could also be foreseeably used. 
Suitable liquid bisphenol F-diglycidyl ether epoxy resins include Epon 
DPL-862, made by Shell Chemical Co. and having an epoxide equivalent 
weight of 166 to 177 and a viscosity of about 3000 to about 4500 cps at 
about 25.degree. C., and bisphenol F-diglycidyl ether epoxy resins made by 
Ciba Chemical Co., such as Araldite GY 281 with an epoxide equivalent 
weight of 158 to 175 and a viscosity of about 5000 to about 7000 cps at 
about 25.degree. C., and Araldite GY 308 with an epoxide equivalent weight 
of 173 to 182 and a viscosity of about 6500 to about 8000 cps at about 
25.degree. C. 
Other liquid epoxy resins with an epoxide functionality of 2.0 that could 
be used include cycloaliphatic epoxy resins such as: 
3,4-epoxycyclohexylmethyl -3,4-epoxycyclohexane carboxylate (ERL 4221 from 
Union Carbide), with an epoxide equivalent weight of 131 to 143 and a 
viscosity of about 350 to about 450 cps at about 25.degree. C.; 
2-(3,4-epoxycyclohexyl -5,5-spiro-3,4-epoxy) cyclohexane metadioxane (ERL 
4234 from Union Carbide), with an epoxide equivalent weight of 133 to 154 
and a viscosity of about 7000 to about 17,000 cps at about 38.degree. C.; 
and 3,4-epoxy-6-methylcyclohexylmethyl adipate (ERL 4289 from Union 
Carbide), with an epoxide equivalent weight of 205 to 216 and a viscosity 
of about 500 to about 1000 cps at about 25.degree. C. Offsets of any of 
the epoxy resins made by other manufactures or mixtures of epoxy resins 
can also be used. 
The metal acetylacetonate of the resin composition is preferably aluminum 
acetylacetonate (Al(C.sub.5 H.sub.7 O.sub.22).sub.3). 
Notably, a solvent is completely omitted from the resin composition 
formulated in accordance with the above. Consequently, the complications 
and defects attributable to the requirement to completely remove a solvent 
from the tape 16 during thermal processing is altogether avoided. In 
addition, the resin composition surprisingly is essentially unreactive 
over a broad temperature range of up to at least about 120.degree. C., yet 
is fully reactive at a temperature of about 165.degree. C. The proximity 
of these temperatures is highly advantageous from a processing standpoint, 
since the relatively high maximum temperature at which the resin 
composition is unreactive permits a vacuum and heat treatment cycle to be 
conducted for as long as about fifteen hours without gelation of the resin 
composition. As such, curing can be successfully carried out at about 
165.degree. C. for a duration of as little as about six hours to yield a 
consolidated tape 16 characterized by a fully compacted mica paper 
insulation and a fully cured resin in which voids are essentially absent. 
Those skilled in the art will gain a better understanding of the present 
invention and its advantageous properties and characteristics from the 
following illustrative examples, which have been carried out 
experimentally using resin compositions formulated in accordance with this 
invention. 
EXAMPLE 1 
(Prior Art) 
The processing characteristics and properties of a resin composition 
formulated in accordance with the prior art, and widely used to 
manufacture resin-rich tapes, is illustrated in this Example. Generally, 
the resin composition evaluated under Example 1 employed a combination of 
epoxy resins and a phenol-formaldehyde novolac curing agent. Mica paper 
tapes of the type shown in the Figures are typically manufactured by 
diluting the resin composition with a solvent, since the pot life 
stability of the resin is too short for hot melt processing. However, the 
resin composition employed in this Example was processed by hot melting 
the resin in order to eliminate possible complications associated with the 
use of a solvent when comparing the results of Example 1 with subsequent 
Examples employing resin compositions formulated in accordance with this 
invention. Accordingly, all of the test samples used solventless resins. 
The resin composition of Example 1 was composed of 50 parts-by-weight (pbw) 
of the epoxy novolac DEN 438, 50.0 pbw of the bisphenol A-diglycidyl ether 
epoxy resin Epon 828, and 60.0 pbw of a phenol-formaldehyde novolac resin 
having a weight average molecular weight of about 1500 to about 1800. The 
shelf life stability of the resin was about three months at room 
temperature (about 18.degree. C. to about 32.degree. C.). During heating 
at about 120.degree. C., samples of the resin composition were found to 
gel within about six hours. A cure cycle was then performed in which the 
resin was cured for about twelve hours at about 165.degree. C. to produce 
solid specimens. The Shore D hardnesses of the specimens were about 70 to 
about 90 at room temperature (about 25.degree. C.) and about 30 to about 
40 at a temperature of about 160.degree. C. The percent dissipation 
factors for the specimens at 60 hertz and 10 volts per mil (Vpm) were 
0.16, 3.46 and 36.04 at room temperature, 155.degree. C. and 200.degree. 
C., respectively. The percent weight losses for ten gram discs having a 
diameter of about 5.7 centimeters and aged twenty-eight days at about 
180.degree. C. and about 200.degree. C. were 0.75 and 1.32, respectively. 
EXAMPLE 2 
A clear, semi-solid resin was made in accordance with this invention by 
combining about 90 pbw DEN 439, about 10 pbw Epon 826 (i.e., Epon 826 was 
substituted for about 10 weight percent of DEN 439), about 7.5 pbw BRWE 
5300, and about 0.25 pbw aluminum acetylacetonate, followed by heating and 
stirring the mixture at about 100.degree. C.. The stability of the resin 
composition was in excess of about twenty-four months. The resin 
composition had not gelled after about fifteen hours at about 120.degree. 
C., and was thereafter fully cured after ten hours at about 165.degree. C. 
The resulting specimens had Shore D hardnesses of about 85 to about 90 at 
room temperature (about 25.degree. C.) and about 75 to about 85 at a 
temperature of about 160.degree. C. The percent dissipation factors for 
the samples (60 Hz, 10 Vpm) were 0.116, 2.508, 2,970 and 2.730 at room 
temperature, 155.degree. C., 180.degree. C. and 200.degree. C., 
respectively. The percent weight losses for ten gram discs having a 
diameter of about 5.7 centimeters and aged twenty-eight days at about 
180.degree. C. and about 200.degree. C. were 0.15 and 0.80, respectively. 
EXAMPLE 3 
A clear, semi-solid resin was made in accordance with this invention by 
combining about 80 pbw DEN 439, about 20 pbw Epon 826, about 7.5 pbw BRWE 
5300, and about 0.25 pbw aluminum acetylacetonate, followed by heating and 
stirring the mixture at about 100.degree. C. As with the resin composition 
of Example 2, the resin composition of this Example had a stability in 
excess of about twenty-four months, did not gel after about fifteen hours 
at about 120.degree. C., and was fully cured after ten hours at about 
165.degree. C. The resulting samples had Shore D hardnesses of about 80 to 
about 90 at room temperature and about 60 to about 75 at about 160.degree. 
C. The percent dissipation factors for the samples (60 Hz, 10 Vpm) were 
0,103, 2,854, 3.120 and 2,690 at room temperature, 155.degree. C., 
180.degree. C. and 200.degree. C., respectively. The percent weight losses 
for ten gram discs having a diameter of about 5.7 centimeters and aged 
twenty-eight days at about 180.degree. C. and about 200.degree. C. were 
0.17 and 0.84, respectively. 
EXAMPLE 4 
A clear, semi-solid resin was made in accordance with this invention by 
combining about 70 pbw DEN 439, about 30 pbw Epon 826, about 7.5 pbw BRWE 
5300, and about 0.25 pbw aluminum acetylacetonate, followed by heating and 
stirring the mixture at about 100.degree. C. As with the resin 
compositions of Examples 2 and 3, the resin composition of this Example 
had a stability in excess of about twenty-four months, did not gel after 
about fifteen hours at about 120.degree. C., and was fully cured after ten 
hours at about 165.degree. C. The resulting specimens had Shore D 
hardnesses of about 85 to about 90 at room temperature and about 65 to 
about 85 at about 160.degree. C. The percent dissipation factors for the 
specimens (60 Hz, 10 Vpm) were 0.105, 3,040, 3,250 and 2,847 at room 
temperature, 155.degree. C., 180.degree. C. and 200.degree. C., 
respectively. The percent weight losses for ten gram discs having a 
diameter of about 5.7 centimeters and aged twenty-eight days at about 
180.degree. C. and about 200.degree. C. were 0.21 and 0.94, respectively. 
EXAMPLE 5 
A clear, semi-solid resin was made in accordance with this invention by 
combining about 70 pbw DEN 439, about 30 pbw Epon 826, about 10 pbw BRWE 
5300, and about 0.25 pbw aluminum acetylacetonate, followed by heating and 
stirring the mixture at about 100.degree. C. As with the resin 
compositions of Examples 2 through 4, the resin composition of this 
Example had a stability in excess of about twenty-four months, and did not 
gel after about fifteen hours at about 120.degree. C. After curing at 
about 165.degree. C. for about eight hours, the resulting specimens had 
Shore D hardnesses of about 80 to about 90 at room temperature and about 
75 to about 85 at about 160.degree. C. The percent dissipation factors for 
the specimens (60 Hz, 10 Vpm) were 0.104, 2.02, 3.980 and 1.540 at room 
temperature, 155.degree. C., 180.degree. C. and 200.degree. C., 
respectively. The percent weight losses for ten gram discs having a 
diameter of about 5.7 centimeters and aged twenty-eight days at about 
180.degree. C. and about 200.degree. C. were 0.17 and 0.87, respectively. 
EXAMPLE 6 
A clear, semi-solid resin was made in accordance with this invention by 
combining about 70pbw DEN 439, about 30 pbw Epon 826, about 12.5 pbw BRWE 
5300, and about 0.25 pbw aluminum acetylacetonate, followed by heating and 
stirring the mixture at about 100.degree. C. As with the resin 
compositions of Examples 2 through 5, the resin composition of this 
Example had a stability in excess of about twenty-four months. The resin 
composition had not gelled after about twelve hours at about 120.degree. 
C., and was cured after about eight hours at about 165.degree. C. The 
Shore hardness was about 80 to about 90 at room temperature and about 80 
to about 85 at about 160.degree. C. The percent dissipation factors for 
the specimens (60 Hz, 10 Vpm) were 0.123, 1.850, 3.418 and 1.420 at room 
temperature, 155.degree. C., 180.degree. C. and 200.degree. C., 
respectively. The percent weight losses for ten gram discs having a 
diameter of about 5.7 centimeters and aged twenty-eight days at about 
180.degree. C. and about 200.degree. C. were 0.17 and 0.89, respectively. 
EXAMPLE 7 
A clear, very viscous resin was made in accordance with this invention by 
combining about 50 pbw DEN 438, about 50 pbw Epon 826, about 12.5 pbw BRWE 
5300, and about 0.25 pbw aluminum acetylacetonate, followed by heating and 
stirring the mixture at about 100.degree. C. As with the resin 
compositions of Examples 2 through 6, the resin composition of this 
Example had a stability in excess of about twenty-four months. The resin 
composition had not gelled after about twelve hours at about 120.degree. 
C. After a cure of about twelve hours at about 165.degree. C., the Shore 
hardness was about 80 to about 90 at room temperature and about 70 to 
about 80 at about 160.degree. C. The percent dissipation factors for the 
specimens (60 Hz, 10 Vpm) were 0.284, 1.466 and 1.650 at room temperature, 
155.degree. C. and 180.degree. C., respectively. The percent weight losses 
for ten gram discs having a diameter of about 5.7 centimeters and aged 
twenty-eight days at about 180.degree. C. and about 200.degree. C. were 
0.47 and 1.24, respectively. 
From the above examples, it can be seen that the resin compositions 
formulated in accordance with this invention exhibited superior stability, 
mechanical and electrical properties, and shelf life and aging 
characteristics as compared to the prior art composition of Example 1. 
Most notably, the shelf life of the formulations of Examples 2 through 7 
exceeded that of the prior art by a factor of eight, and were unreactive 
for exposures of up to about fifteen hours at 120.degree. C. Thereafter, 
curing of the formulations resulted in specimens that demonstrated the 
physical, thermal and electrical superiority of the resin composition of 
this invention as compared to the prior art resin composition. 
Accordingly, it can be seen that a significant advantage of resin 
compositions formulated in accordance with this invention is that they are 
essentially unreactive at storage temperatures. Accordingly, resin 
compositions of this invention provide for substantial improvements in the 
shelf life of a mica paper tape or other resin-rich prepreg, to the extent 
that a shelf life of at least two years can be achieved without 
refrigeration. 
In addition, the resin compositions of this invention are also unreactive 
at temperatures necessary to reduce the viscosity of the resin composition 
during vacuum heat treatment of a prepreg, so as to enable the removal of 
volatile components from the prepreg. As such, the resin composition 
enables the manufacture of resin-rich prepregs that can be vacuum and heat 
treated without the risk of pre-gelation, while also promoting the 
effectiveness and life of an insulation composite formed from the prepreg. 
Another significant advantage of this invention is that the above 
processing characteristics are achieved without adversely affecting the 
cure properties desired for tapes 16 of the type shown in the FIGS. 
Specifically, the resin composition of this invention is highly reactive 
at a suitable curing temperature above the vacuum heat treatment 
temperature, so as to be curable within a practical time period to form a 
solid that is characterized by enhanced mechanical and electrical 
insulating properties. 
While our invention has been described in terms of a preferred embodiment, 
it is apparent that other forms could be adopted by one skilled in the 
art. For example, while the disclosure is directed to resin-rich prepregs 
of the type that are cured in an autoclave, various resin-impregnated 
sheet materials can be manufactured with the resin composition of this 
invention. Because of the high reactivity of these prepregs at 165.degree. 
C. and higher temperatures, the various resin-impregnated sheet and tape 
materials could be used for insulating conductors by curing in a hot press 
instead of an autoclave. Furthermore, the resin composition of this 
invention can be used in the manufacture of precured sheet materials, 
fabrics and films. Accordingly, the scope of our invention is to be 
limited only by the following claims.