Impregnated insulating tape for fabricating an insulating sleeve for electric conductors

An insulating tape for manufacturing an insulating sleeve for electric conductors, impregnated with a heat-hardening epoxy resin-acid anhydride hardener mixture, contains a self-hardening bonding agent-accelerator composition of an epoxy resin as the bonding agent and a quarternary onium salt as the accelerator. To prevent migration of the charge carriers present in the accelerator, the accelerator further has a group which can react with the bonding agent-impregnating resin mixture. This group causes a fixation of the accelerator in the lattice produced during the hardening.

BACKGROUND OF THE INVENTION 
The invention relates to an insulating tape for fabricating an insulating 
sleeve for electric conductors which is impregnated with a heat-hardening 
epoxy resin-acid anhydride hardener mixture. It is especially useful for 
the winding bars or coils of electric machines. 
Insulating tapes which incorporate hardened epoxy resins are known, see for 
example U.S. Pat. No. 4,336,302. Also known are tapes with epoxy resins 
which incorporate bonding agent-accelerator systems. When the systems 
employ quaternary onium salts as accelerators, effective binding for most 
epoxy resin is provided. Furthermore, use of quaternary onium salts as 
accelerators will cause practically no ionic polymerization of epoxy 
compounds at room temperature. As a result, an insulating tape impregnated 
with an epoxy compound binder-quaternary onium accelerator can be stored 
at room temperature for up to more than three months without changing its 
properties. The quaternary onium salts also can cause polymerization of an 
epoxy resin-acid anhydride mixture at temperatures near 60.degree. C. and 
the polymerization time is substantially accelerated, so that it is 
possible to harden the portion of impregnating resin penetrating the 
insulating tape in an economically short time. 
The bonding agent-accelerator system provided in this manner remains fully 
soluble in the impregnating resin under the conditions of drying and 
preheating as would be applied to insulating sleeves wound from insulating 
tapes prior to the impregnation of the sleeves with resin-hardener. There 
is no decrease in the hardening and accelerator reactivity relative to the 
reactivity of a combination of an impregnating resin and bonding agent 
accelerator system which is added to an insulating sleeve wound from tape 
unimpregnated with resin or the system. 
According to present understanding, the known accelerators based upon 
quaternary onium salts are preserved in the system unchanged during the 
hardening of the impregnating resin and do not join the hardened bonding 
agent-impregnating resin body. Since they contain electrically charged 
particles, the possibility therefore exists that a field-oriented 
migration or alignment of these particles could occur due to the voltages 
applied to the insulating sleeves in operation. As a result, the 
insulation resistance would drop. 
Although no indication has been observed so far in the insulating sleeves 
made with known bonding agent-accelerator mixtures and investigated as to 
such migration in a formed, hardened bonding agent-impregnating resin 
material, it is an object of the invention to prevent such a process from 
the start in an insulating tape of the type described above. 
SUMMARY OF THE INVENTION 
These and other objects are achieved by the invention which is directed to 
an insulating tape containing an epoxy resin, inorganic material, a 
flexible substrate, and a bonding agent-accelerator composition. More 
specifically the tape includes an epoxy resin with an organic acid 
anhydride hardener and planar, breakdown-proof, inorganic material such as 
mica scales or fine mica layers, which is applied to a flexible support 
and cemented thereto, cemented to itself and, if desired, to a cementing 
terminating cover layer by means of a composition comprising a bonding 
agent/accelerator mixture which is self-hardening at the setting 
temperature of the impregnating resin, the bonding agent being an epoxy 
resin and the accelerator being a quaternary onium salt of the following 
general structure formula 
EQU M[R.sub.1 R.sub.2 R.sub.3 R.sub.4 ].sup.+ X.sup.- 
wherein M is an atom of the 5th main group of the periodic system, 
especially N and P, R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are identical or 
different aliphatic, aromatic, heterocyclic or aryl-aliphatic radicals 
which may contain still further quaternary atoms, and X is an anion such 
as Cl, Br, I, F, NO.sub.3, ClO.sub.4, an organic acid radical such as 
acetate, or a complex anion such as BF.sub.4.sup.-, PF.sub.6.sup.- and the 
like. 
The R radicals (R.sub.1, R.sub.2, R.sub.3, R.sub.4) or two thereof can be 
bound, to the central atom, or in addition can be bound to each other. 
Also, the quaternary central atom M can be incorporated into a ring system 
which may contain multiple bonds. 
According to the invention, at least one of the substituents of the central 
atom M or of the heterocyclic system formed with the central atom has a 
group which is capable of reacting with the bonding agent-impregnating 
resin mixture. In the hardening of the impregnating resin, fixation of the 
accelerator compound in the lattice produced is possible with this group. 
Thus, migration of the accelerator is not possible. 
It is particularly advantageous for the foregoing purpose if the 
accelerator contains an epoxy group or a hydroxyl group because these make 
particularly highly storage-stable insulating tapes. This is true also if 
the bonding agent contains hydroxyl as well as epoxy groups. 
Other radicals capable of reaction are mercaptan-groups (SH), carboxyl 
(COOH) groups, amino or imino (NH.sub.2, NH--) groups, which may be 
present individually or mixed.

DETAILED DESCRIPTION OF THE INVENTION 
The accelerator according to the invention solves the prospective problem 
of charged group migration in an insulation material impregnated with 
binder which is to be used for electrical applications. This is 
accomplished by the provision that at least one of the substituents on the 
quaternary onium accelerator compound is capable of reacting with the 
impregnating epoxy resin and/or bonding agent. 
Examples for such quaternary onium salts are, for instance, the reaction 
products of corresponding tertiary amines or phosphines such as 
tributylamine, dimethylbenzylamine, 1,2-dimethylimidazol, pyridine, 
tributylphosphine, triphenylphosphine with compounds such as 
epichlorohydrin, epibromohydrin, 2,3-dihydroxypropylchloride or bromide or 
hydrogen halogenide addition products of epoxy or polyepoxy compounds. 
In the insulating tape designed in accordance with the invention, the 
following epoxy resins can be used as the bonding agent: 
Epoxy compounds such as bisphenol A-diglycidyl ether, resorcine diglycidyl 
ether, epoxidized phenol or cresol novolakes as well as N-glycidyl 
compounds such as heterocyclic epoxy compounds with a hydantoin base. 
Further epoxies are listed in "Handbook of Epoxy Resins" by H. Lee and K. 
Neville (McGraw-Hill Book Co., 1967). Particularly advantageous is the use 
of higher-functional epoxidized phenol or cresol novolakes and hydantoin 
epoxy resins, which are excellent bonding agents for the insulating tapes 
provided because of their high viscosity and adhesion power. 
The bonding agent concentration is chosen so that it is not too high a 
value in manufacturing the insulating tape. As a result, the impregnating 
resin can penetrate in a facile fashion into the voids in the tape during 
the impregnation. The weight content of the compounds used as the bonding 
agent for cementing the insulating tape is therefore between 3 and 10%, 
relative to the total weight of the insulating tape. With such a bonding 
agent content it is advisable to set the amount of accelerator 
approximately in the order of 0.05 to 5%, relative to the total weight of 
the insulating tape. The amount of accelerator added to the insulating 
tape in its manufacture depends on how much inorganic material, for 
instance, mica, is contained in the tape and on how large the content of 
bonding agent used for the cementing is in the insulating tape. 
A further improvement of the insulating constructed from insulating tapes 
according to the invention, which have great stiffness, is addition of 
further hardeners to the bonding agent-accelerator mixture which become 
effective only at the hardening temperature of the heat-hardening epoxy 
resin-acid anhydride mixture. Thereby, the dimensional heat stability of 
the self-hardening bonding agent-accelerator mixture as well as of the 
mixture between the bonding agent and the penetrated impregnating resin is 
increased further. These additions, however, have no adverse effect on the 
storage ability of the insulating tape. Also, this binding 
agent-accelerator mixture with additional hardener remains fully soluble 
under the conditions of drying and preheating prior to the impregnation of 
the insulation. 
Particularly well suited are additives acting as hardeners which are 
insoluble in the bonding agent up to approximately 80.degree. C. and have 
a sufficiently high melting point relative to the setting temperature, 
such as 2-phenyl-4,5 dihydroxymethylimidazol. Such additives show their 
activity only at the temperatures of hardening, while polymerizing of the 
impregnating resin penetrating into the insulating sleeve is produced in 
the temperature range about 60.degree. C. by the quaternary onium 
compounds which are likewise present in the bonding agent. 
As additives are further suited complex compounds or adducts which are 
decomposed into hardening-effective components only at the hardening 
temperatures. These are compounds as described in U.S. Pat. Nos. 4,189,577 
or 4,205,156, or salts of trimellitic acid with imidazols such as 
1-cyanoethyl-2-methylimidazol trimellitate. 
It is further advantageous to use as additives encapsulated hardener 
systems or hardener systems which are absorbed by substances with a large 
active surface. These are released and become active only at the hardening 
temperature or through exchange with other compounds. Thus, tertiary 
amines absorbed, for instance, in molecular sieves can be used. 
The amount of these hardeners admixed to the bonding agent depends on their 
specific effectiveness and comprises the range between 0.05 and 10% by 
weight referred to the bonding agent. 
In the following description, the behavior of insulating tapes constructed 
in accordance with the invention will be explained, by reference to Tables 
1 to 3. 
TABLE 1 
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Wt. Parts 
Storage 
Accel. Stability 
per of the 
1000 Wt. Mixtures 
Parts (Days) Rxn 
Epoxy Accel- Epoxy at Time 130.degree. C. 
Agent erator Resin R.T. 70.degree. C. 
(hours) 
______________________________________ 
A 1 10 100 30 120 
B 1 10 100 30 100 
2 25 100 30 80 
C 1 25 100 30 150 
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The compositions described in Table 1 are the bonding agent accelerator 
mixtures. The compilation presented illustrates the storage stability of 
these mixtures, which were stored in thin layers in an open vessel. The 
mixtures are formulated from three characteristic epoxy agent types A, B, 
C with different compounds 1 and 2 serving as accelerators, of the 
mixtures. The accelerators are as follows: 
1. Reaction product of 1-methylimidazol (1 mol) and epibromohydrin (1 mol). 
2. Reaction product of 1-methylimidazol (1 mol) with the addition product 
of 1 mol bisphenol A-diglycidyl ether and 1 mol hydrogen bromide. 
The compounds were dissolved while heating (at 70.degree. C.) or by means 
of methylene chloride as a solution-promoting agent in the respective 
epoxy agents A, B and C, where A means a glycidyl ether of bisphenol A 
with an epoxy equivalent of 174.+-.2 and a viscosity of 5000+500 mPas at 
25.degree. C., B, a semisolid epoxidized phenol novolak with an epoxy 
equivalent of 178.+-.5 and a viscosity of 1600.+-.300 mPas at 80.degree. 
C., and C, a semisolid trifunctional hydantoin epoxy resin with an epoxy 
equivalent of 166 and a viscosity of about 6000 mPas at 80.degree. C. 
From Table 1, the small effectiveness of these compositions with respect to 
ionic polymerization in the temperature range of about 70.degree. to 
80.degree. C. can clearly be seen. 
TABLE 2 
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Bonding 
Wt. Parts Bonding 
Polymerizing Time (min) 
Agent- Agent-Accelerator 
of the Epoxy 
Accel- Mixture per H.-Acid Anhydride Bonding 
erator 100 Wt. Parts Agent-Accelerator Mixture 
Mixture 
Epoxy H.-Acid Anh. 
70.degree. C. 
90.degree. C. 
______________________________________ 
A1 22 124 31 
B2 25 114 28 
C1 22 205 58 
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Table 2 shows the excellent accelerator effect of these types of 
compositions on bonding agent-accelerator mixture on epoxy resin acid 
anhydride hardener polymers. The tests were carried out with an epoxy 
resin acid anhydride of a glycidyl ether of bisphenol A (epoxy resin A) 
and methylhexahydro-phthalic acid anhydride. 
The examples shown in the tables clearly show the effects of the quaternary 
onium compounds as the accelerator portion of a bonding agent-accelerator 
mixture composition of an insulating tape designed in accordance with the 
invention. The insulating tapes made with the bonding agent-accelerator 
compositions detailed here can be stored for more than 3 months at room 
temperature and, after the insulating sleeve is impregnated with an 
epoxy-acid anhydride resin, it is assured in the hardening through the 
special choice of the bonding agent-accelerator mixture that all points 
are set, i.e., also points, at which the bonding agent was not resorbed by 
the impregnating resin, since the bonding agent-accelerator mixture is 
self-hardening. Thus, an insulation with very good electrical properties 
and high dimensional heat stability is obtained. 
TABLE 3 
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Storage 
Stability 
Wt. Parts 
of the Rxn 
Hardener 
Mixtures Time 
per 100 
(Tgen) at at 
Epoxy Accel- Wt. Parts 
Room 130.degree. C. 
Resin erator Hardener Epoxyh. 
Temp. 70.degree. C. 
(hours) 
______________________________________ 
B (a) 1 5 100 5 10 
D 2 5 100 3 8 
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(a) Reaction Product of 1 mol (1methylimidazol) and 1 mol 
(epibromohydrin). 
Table 3 contains data on the storage stability of bonding agent mixtures 
with a quaternary onium compound as accelerator and some specific 
hardeners as additives for this bonding agent-accelerator mixture. Also, 
from this is clearly seen a low reactivity of the additives on the bonding 
agent of the insulating tape up to a temperature range of 70.degree. C. A 
comparison with Table 1 shows for the temperature range about 70.degree. 
C. a somewhat higher reactivity overall, but at room temperature, the 
excellent stability of the mixtures prevails also here. As bonding agent 
were used, for instance, epoxy resin B and epoxy resin 
D=tetraglycidyl-p,p'-methylenedianiline with an epoxy equivalent of 
130.+-.10 and a viscosity of about 1800 mPas at 80.degree. C. As additives 
served the hardeners 1 and 2, where 1 is 
2,4-diamino-6-(2'-methylimidazolyl)-(1'-ethyl)-s-triazine; and 2 is 
2-phenyl-4,5-dihydroxymethylimidazole. 
Through the increase of the vitrification point of the hardened bonding 
agent, achievable by means of the additives, by about 30.degree. to 
40.degree. C., the stiffness of the insulating sleeve is increased further 
and its mechanical strength at high temperatures is raised further.