Title:
LAPPING TAPE FOR INSULATING ELECTRICAL MACHINERY
United States Patent 3808086
Abstract:
The present invention relates to a lapping tape for insulating electrical machinery by impregnation comprising a porous substrate material, mica paper, and an adhesive, characterized in that the combined adhesive and curing accelerator for the subsequently added impregnating resin is an oxyamino resin having the formula: ##SPC1## Is used in which R1 and R2 each denotes a straight-chain alkyl group having up to four carbon atoms or together denote a lower alkylene group which can be interrupted by a heteroatom, the oxyamino resin being prepared by quantitative reaction of an epoxy resin having a melting point above 50° C (to ASTM E 28) and having at least two ethylene oxide groups per molecule with a secondary amine having the formula: ##SPC2## In which R1 and R2 have the meaning just given.
US Patent References:
/3556925.html
Mertens - January 1971 - 3556925
/3592711.html
Senarclens - July 1971 - 3592711
/3695984.html
Rogers - October 1972 - 3695984
/3556925.html
Mertens - January 1971 - 3556925
/3592711.html
Senarclens - July 1971 - 3592711
/3695984.html
Rogers - October 1972 - 3695984
Inventors:
Mosimann, Hans (Breitenbach, CH)
Heim, Peter (Basel, CH)
Lutz, Walter (Laufen, CH)
Heim, Peter (Basel, CH)
Lutz, Walter (Laufen, CH)
Application Number:
05/242229
Publication Date:
04/30/1974
Filing Date:
04/07/1972
View Patent Images:
Export Citation:
Assignee:
Schweizerische, Isola-werke (Breitenbach, CH)
Primary Class:
Other Classes:
156/331.100, 442/326, 156/307.300, 442/324, 174/110R
International Classes:
H01B3/04; H01B3/40; H01B3/50; H01B3/02; H01B3/18; B32B19/06; B32B19/02
Field of Search:
161/93,163,88 174/25,110,120,121,121.4 156/331 260/47EP,47R,47CZ,89.7R,89.7N,2EC,571,573,47CP
Primary Examiner:
Mccamish, Marion E.
Attorney, Agent or Firm:
Wenderoth, Lind & Ponack
Claims:
1. A lapping tape for use in insulating electrical machinery after impregnation, comprising a porous substrate material, mica paper and an adhesive to bind the mica to the porous substrate, characterized in the use of a material which serves as a combined adhesive and a curing accelerator for a subsequently added impregnating resin, said material being an oxyamino resin having the formula: ##SPC6##
2. A tape according to claim 1, characterised in that the porous substrate
3. A tape according to claim 1, characterised in that the oxyamino resin used as a combined adhesive and accelerator is derived from an epoxy resin of the novolak kind or from an epoxy resin on a bisphenol or heterocyclene
4. A tape according to claim 1, characterised in that R1 and R2
5. A process for preparing a lapping tape for use in insulating electrical machinery after impregnation comprising a porous substrate material, mica paper and an adhesive to bind the mica to the porous substrate; characterized in the use of a material which serves as a combined adhesive and a curing accelerator for a subsequently added impregnating resin, said material being an oxyamino resin having the formula: ##SPC8##
6. A process according to claim 5 wherein the organic solvent is a ketone aromatic hydrocarbon.
2. A tape according to claim 1, characterised in that the porous substrate
3. A tape according to claim 1, characterised in that the oxyamino resin used as a combined adhesive and accelerator is derived from an epoxy resin of the novolak kind or from an epoxy resin on a bisphenol or heterocyclene
4. A tape according to claim 1, characterised in that R1 and R2
5. A process for preparing a lapping tape for use in insulating electrical machinery after impregnation comprising a porous substrate material, mica paper and an adhesive to bind the mica to the porous substrate; characterized in the use of a material which serves as a combined adhesive and a curing accelerator for a subsequently added impregnating resin, said material being an oxyamino resin having the formula: ##SPC8##
6. A process according to claim 5 wherein the organic solvent is a ketone aromatic hydrocarbon.
Description:
The conductors of medium and high voltage and power electric motors and generators are insulated by mica products. In high-voltage windings, the conductor is insulated before being placed in its slot, the insulating being in the form of a mica product either in foil form or in the form of a lapping. The total impregnation process is used at present for windings required to operate at up to about 6 kV -- i.e., the complete winding is insulated with porous mica strips, and after it has been placed in the slot the finished winding is impregnated with a solvent-free impregnating resin.
In both cases -- i.e., high-voltage conductors which are fully insulated first, then placed in the slot, and high-voltage conductors treated by being fully impregnated once the winding is in the slot -- the mica insulation must be completely impregnated with a solvent-free synthetic resin.
If the resin is introduced into a winding by impregnation after the tape has been wound on a conductor bar built up from insulated separate conductors and stuck together, the mica insulation must, if the process is to be satisfactory, be completely impregnated with the resin in a vacuum pressure process, and so the lapping tape used must be porous if it is to be able to take up the resin, particularly in the case of layers having a thickness of several millimeters. Lapping tapes comprise a substrate, e.g. a glass silk fabric having a density of approximately 25 g/m 2 or a matt of glass or synthetic fibres, and a layer of mica. The purpose of the substrate is to provide the composite material with the necessary mechanical strength. To enable the material to be handled, however, the substrate and the mica must be stuck together by an adhesive. For this procedure to be satisfactory, the starting materials must meet the following requirements.
The tape must be mechanically strong and able to withstand mechanical lapping, yet it must be substantially adhesive free so that the resin can impregnate the lapping thoroughly. Clearly, these two requirements are contradictory, for the mechanical strength of a lapping tape is greatr in proportion as the bond provided between the substrate and the mica by a flexible adhesive is greater, yet the adhesive is a hindrance to subsequent impregnation. Endeavors are therefore made for the sticking of the substrate to the mica to be as far as possible on a spot basis.
Of course, the adhesive used for lapping tapes which, because of the poor tensile strength of the mica paper, are a composite product consisting of mica paper and a high tensile heat-resistant fabric (glass fabric or synthetic fibres) must meet the following requirements:
1. a very small quantity of binder must stick the mica paper to the substrate satisfactorily enough for mechanical lapping;
2. although sticking must be satisfactory, the adhesive must penetrate the mica paper to a very reduced extent and must serve substantially or entirely to stick the mica paper to the substrate (for the sake of proper impregnation subsequently); and
3. the adhesive must be compatible with the impregnating resin which will be used subsequently.
The impregnating resin, besides having all the required insulating properties, must have a low enough viscosity to enter the porous insulation and stick it to the group of conductors after curing to form a compact inclusion-free insulation.
The impregnating resin must meet a number of requirements:
1. the resin must wet the mica insulation satisfactorily:
2. the viscosity must be low and, if at all possible, be less than 300 cP at the impregnating temperature;
3. if the resin is to be kept in a tank with very little alteration, its viscosity must remain constant in time despite frequent heating in operations to from 50° to 60° C;
4. viscosity must rise very rapidly in curing, to reduce resin droppings, and curing must be a rapid operation to ensure a short dwell in the furnace; and
5. the resin must be sufficiently heat-resistant for the lapping to be used at the usual operating temperatures (class F, 155° C). The resin must therefore not soften appreciably at such temperature and it must lose little, if any, weight at this temperature in continuous operation.
High voltage insulation is required to have low dielectric losses -- i.e., its tg δ must rise very little with voltage and temperature. The insulation must not soften below peak operating temperatures. These two requirements make it impossible to use resins diluted with large quantities of a reactive diluent. Reactive diluents are monofunctional low-viscosity epoxy compounds which, because of their monofunctional feature, act as chain stoppers and thus inhibit the formation of long polymer chains. After curing, therefore, the Marten point (to DIN 53 462) or the dimensional stability (to ISO R 75) shifts to lower temperatures as compared with a diluent-free mixture. If the diluted resins already discussed are excluded, few appropriate impregnated-resin systems having viscosities below 1,000 cP at 20° C are left.
Since a low-viscosity resin penetrates the winding more readily than a high-viscosity resin, endeavors are made to increase the temperature. The impregnating resin therefore starts to react so that viscosity increases; unless the throughput is large so that fresh resin can always be added, the mixture soon becomes unserviceable for the proposed purpose. It would therefore be desirable to have a resin system which reacts very little at the temperature of impregnation; and example of such a system would be an epoxy resin and a liquid anhydride. Unfortunately, such systems drip for a long time, since they react slowly even at elevated temperature.
The suggestion was therefore made to add an accelerator to the lapping tape. As conventional kinds of accelerator for systems of this kind there can be considered metal naphth-enates and octoates, e.g. cobalt or zinc naphthenate or octoate, tertiary amines, such as e.g. benzyldimethylamine, dimethylaminomethyl-phenol 2, 4, 6-tri (dimethylaminoethyl) phenol or tri (methoxycarbonylethyl) amine, and boron fluoride-amine complexes, e.g. boron fluoride ethylamine or piperidine or pyridine. For instance, German Auslegeschrifts Pat. Nos. 1,162,898 and 1,219,554 disclose a process for dipping the rolls of tape before lapping into accelerator solution and drying them. After lapping, the impregnating resin in the tape contacts the accelerator and it is considered that reaction acceleration occurs substantially locally in the lapping. Since the substances concerned are low-molecular substances, some of them are dissolved out of the impregnating resin and therefore reach the resin supply. Consequently, the acceleration acts in the supply too, more particularly because the resin is at a temperature of from about 50° to 60° C during impregnation.
Surprisingly, an adhesive for sticking mica paper to substrate has been found which meets all requirements very satisfactorily and also acts catalytically on the stable resin curing agent systems mentioned. The novel adhesive comprises an oxyamino resin which has the formula and which is prepared by quantitively reacting an epoxy resin having the formula II, a melting point above 50° C (to ASTM E 28) and at least two ethylene oxide groups per molecule, with a secondary amine having the formula III: ##SPC3##
in which R 1 and R 2 each denote a straight-chain alkyl group having up to four carbon atoms or together denote a lower alkylene group which can be interrupted by a heteroatom.
This reaction leads to a new class of tertiary amine, viz. oxyamino resins having the partial formula I, which are very suitable as adhesives between the substrate and the mica and also as accelerators in the curing of the epoxy resin systems added subsequently for impregnation.
The reaction must be quantitative; if it is not, the tertiary amine groups involved initiate the reaction of the ethylene oxide groups remaining in the molecule and the adhesive is unstable.
The novolak kinds of epoxy resin are very suitable, as are the kinds on a base of bisphenol or heterocyclene; cycloaliphatic resins, however are less satisfactory.
Epoxy novolaks have the following basic structure: ##SPC4##
These are therefore chain-like phenol-formaldehyde-novolaks with the phenolic hydroxyl groups substituted by glycidyl groups. Since the resulting oxyamines usually have a lower melting point than the starting resins, the starting resins used are those having a relatively high melting point. Examples of appropriate amines having straight-chain alkyl groups and of appropriate cyclic amines are dimethylamine, diethylamine, di-n-proplyamine, pyrrolidine, piperidine, morpholine, thiomorpholine and so on. In the case of branched chains, of course, steric effects preclude or limit the accelerator effect. For practical reasons the reaction with diethylamine is preferred.
Advantageously, the reaction with the amine proceeds in a solvent, the amine being introduced in a quantity of from 1 to 3 equivalents referred to the epoxy resin. So that the resulting oxyamine can be freed readily and without damage from excess amine and from solvent, it is preferred to use as solvent ketones or aromatic hydrocarbons or their mixtures whose boiling point is preferably below 150° C.
A glass-silk fabric or a felt of glass fibers or synthetic fibers makes a particularly good porous substrate material.
The adhesive and accelerator can be prepared e.g. as follows:
1. 260 g of an epoxy novolak resin having an epoxy equivalent of approximately 200 and a melting point of 80° C are dissolved in the same quantity of toluene at 100° C. After cooling to about 50° C, 280 g of diethylamine are added with vigorous agitation. After 5 hours of reflux reaction at from 60° to 70° C the temperature is increased in steps and the surplus diethylamine and the toluene are removed by distillation. The resulting product is a resin having a melting point of 75° C.
It is not necessary to use an epoxy novolak resin, and equally satisfactory results are obtained by using a bisphenol resin, as will now be described.
2. 175 g of an epoxy resin on a bisphenol base, with a melting point of 80° C and an epoxy equivalent of 555 are dissolved in 120 g of toluene at 80° C. After cooling to 50° C, 70 g of diethylamine are added and heating with reflux is given at 60° C for 4 hours, whereafter the temperature is increased slowly to distill the surplus diethylamine and the toluene; the final residues thereof are removed in vacuo at 150° C. The resultig product is a resin which has a melting point of 78° C, an epoxy equivalent which cannot be identified and a hydroxyl number of 230.
This product can be dissolved in acetone or better in methylethylketone and dissolves warm in xylene but reprecipitates cold. Alcohols and aqueous solvents do not dissolve it.
3. 560 g of a mixture comprising two epoxy resins (a bisphenol type and a novolak type) and having a melting point of 70° C and an epoxy equivalent of 580 are dissolved in 500 g of toluene at 100° C. After cooling to 60° C, 102 g of diethylamine are added and the solution is reacted for 3 hours with reflux and without heating, whereafter a distillation top is connected and the temperature is increased slowly to 150° C, surplus diethylamine and toluene being removed by distillation, whereafter the mixture is maintained in vacuo at the same temperature until distillation ceases. No epoxy products can be detected in the resulting product, whose melting point is 70° C.
4. 575 g of an epoxy resin mixture on a bisphenol base and having an epoxy equivalent of 560 are dissolved in 500 g of toluene at 100° C. After cooling to approximately 60° C, 140 g of dipropylamine are added and the solution is agitated at 60° C for 3 hours, whereafter the temperature is increased slowly to 150° C and the surplus amine and the toluene are removed by distillation. The reaction mixture is maintained in vacuo at the same temperature until distillation ceases. The resulting product softens at 70° C and is soluble in methylethylketone.
In the thinly liquid epoxy curing agent mixtures such as are used to impregnate finished lappings, the adhesive does not dissolve cold but becomes increasingly soluble above 60° C.
The adhesive thus prepared can be used to stick mica paper to substrate. To this end, the resin is dissolved in an appropriate solvent, e.g. a ketone or an aromatic hydrocarbon or mixtures thereof, and applied to the substrate material, e.g. a thin glass cloth. The concentration of the solution is so adjusted in dependence upon the particular applicator device used that approximately from 2 to 20 g of resin per square metre of surface are applied. After this treatment the glass cloth is stuck to the mica paper in machine width. Strip cutting machines can cut the lapping tapes from the material, which is coiled into rolls or reels. The amount of resin used should be the minimum necessary to provide a firm connection between the two layers, so that the tapes can be cut and given mechanical lapping readily. The smaller this quantity, the more readily subsequent impregnation of the finished lapping can be carried out.
Since the tape, and the lapping produced thereby also contain the accelerator for the impregnating resin, the same can be stored in the impregnating tank without accelerator and therefore has a much longer life than previously, yet the impregnated lapping cures very rapidly in the oven due to the action of the accelerator present in the adhesive in the tape. Since the accelerator too is resin-like, it is not washed out during impregnation even at temperatures as high as from 50° to 60° C, whereas metal salts enter the impregnating resin in similar conditions and shorten its durability.
The following figures will make these points clear.
A liquid epoxy resin having an epoxy equivalent of approximately 180 is mixed with an equivalent quantity of hexahydrophthalic acid anhydride. As freshly prepared, the viscosity of the mixture at 20° C is from 900 to 100 cps.
The following experiments were made using this mixture:
a. the mixture was kept in a glass vessel at a thermostatically controlled temperature of 50° C and over a period of time samples were taken and its viscosity at 20° C checked.
b. the mixture was placed in contact with a tape according to the invention and left in contact with the tape at 50° C for 2 hours, whereafter the tape was removed and the mixture kept at 50° C. The viscosity of the mixture was again checked at 20° C.
c. as in (b) a tape was used which has previously been impregnated with a 1 percent cobalt naphthenate solution (accelerator). After removal of the tape the viscosity of the mixture was determined at 20° C.
The results are shown in table I.
TABLE I
Experiment Initial Viscosity measured at 20°C viscosity after after after of 2h. 24 h. 3 days mixture at 50°C at 50°C at 50°C Control (a) 930 cps 1112 cps 1573 cps 2004 cps With tape according to invention (b) 930 cps 1022 cps 1731 cps 1960 cps With tape previously dipped in cobalt naphthenate solution (c) 930 cps 1068 cps 1806 cps 4065 cps
It may therefore be concluded that the tape according to the invention does not affect the resin/curing agent system of the experimental mixture, i.e., accelerator is not dissolved out of the tape. On the other hand, as the appreciable increase in viscosity shows, the low-molecular accelerator dissolves out of the tape pretreated with cobalt naphthenate. It can also be shown that the adhesive contained in the tape according to the invention greatly reduces the gelling time of the same resin/curing agent mixture as was used in the previous experiments at elevated temperature, the gelling time being determined at 130° C and 160° C in a thermostat. The term "gelling time" is to be understood as denoting the period between the instant of time at which the resin curing agent accelerator mixture comes to the reaction temperature in the thermostat, and the instant at which the mixture ceases to flow freely and becomes a gel. This can be checked by dipping a thin glass rod into the mixture every few seconds and pulling it out. As long as drops form, there is no gelling. As viscosity increases, filaments form, and when gelling occurs the filaments break off. ##SPC5##
As Table II shows, the accleration provided by the new adhesive is even more striking than an accelerator frequently recommended in practical use for resin/curing agent systems of the kind specified.
As tables I and II show, the resins having tertiary amine groups such as are used as adhesives of the tape according to the invention not only are not dissolved out of the tape by the solvent-free impregnating resin but also considerably accelerate curing by action on the impregnating resin remaining in the lapping at the temperature at which lappings are pressed in practice. On the other hand, a correspondingly devised combined adhesive accelerator based on dialkylamines having branched alkyl radicals has substantially no accelerating effect.
The invention therefore provides a considerable technical advance:
1. The new lapping tape enables the impregnating process to be carried out without the tape having to be given further treatment for the provision of an accelerator, for the adhesive and the accelerator are the same product in the invention.
2. The adhesive used for the tape greatly accelerates the curing of the solvent-free resin used for impregnation.
3. Since the adhesive of the tape is virtually insoluble in the solvent-free impregnating resin during the impregnating time, the impregnating resins used can be slow-curing resin systems which can be stored cold substantially without change and whose viscosity rises little even at the impregnating temperature.
In both cases -- i.e., high-voltage conductors which are fully insulated first, then placed in the slot, and high-voltage conductors treated by being fully impregnated once the winding is in the slot -- the mica insulation must be completely impregnated with a solvent-free synthetic resin.
If the resin is introduced into a winding by impregnation after the tape has been wound on a conductor bar built up from insulated separate conductors and stuck together, the mica insulation must, if the process is to be satisfactory, be completely impregnated with the resin in a vacuum pressure process, and so the lapping tape used must be porous if it is to be able to take up the resin, particularly in the case of layers having a thickness of several millimeters. Lapping tapes comprise a substrate, e.g. a glass silk fabric having a density of approximately 25 g/m 2 or a matt of glass or synthetic fibres, and a layer of mica. The purpose of the substrate is to provide the composite material with the necessary mechanical strength. To enable the material to be handled, however, the substrate and the mica must be stuck together by an adhesive. For this procedure to be satisfactory, the starting materials must meet the following requirements.
The tape must be mechanically strong and able to withstand mechanical lapping, yet it must be substantially adhesive free so that the resin can impregnate the lapping thoroughly. Clearly, these two requirements are contradictory, for the mechanical strength of a lapping tape is greatr in proportion as the bond provided between the substrate and the mica by a flexible adhesive is greater, yet the adhesive is a hindrance to subsequent impregnation. Endeavors are therefore made for the sticking of the substrate to the mica to be as far as possible on a spot basis.
Of course, the adhesive used for lapping tapes which, because of the poor tensile strength of the mica paper, are a composite product consisting of mica paper and a high tensile heat-resistant fabric (glass fabric or synthetic fibres) must meet the following requirements:
1. a very small quantity of binder must stick the mica paper to the substrate satisfactorily enough for mechanical lapping;
2. although sticking must be satisfactory, the adhesive must penetrate the mica paper to a very reduced extent and must serve substantially or entirely to stick the mica paper to the substrate (for the sake of proper impregnation subsequently); and
3. the adhesive must be compatible with the impregnating resin which will be used subsequently.
The impregnating resin, besides having all the required insulating properties, must have a low enough viscosity to enter the porous insulation and stick it to the group of conductors after curing to form a compact inclusion-free insulation.
The impregnating resin must meet a number of requirements:
1. the resin must wet the mica insulation satisfactorily:
2. the viscosity must be low and, if at all possible, be less than 300 cP at the impregnating temperature;
3. if the resin is to be kept in a tank with very little alteration, its viscosity must remain constant in time despite frequent heating in operations to from 50° to 60° C;
4. viscosity must rise very rapidly in curing, to reduce resin droppings, and curing must be a rapid operation to ensure a short dwell in the furnace; and
5. the resin must be sufficiently heat-resistant for the lapping to be used at the usual operating temperatures (class F, 155° C). The resin must therefore not soften appreciably at such temperature and it must lose little, if any, weight at this temperature in continuous operation.
High voltage insulation is required to have low dielectric losses -- i.e., its tg δ must rise very little with voltage and temperature. The insulation must not soften below peak operating temperatures. These two requirements make it impossible to use resins diluted with large quantities of a reactive diluent. Reactive diluents are monofunctional low-viscosity epoxy compounds which, because of their monofunctional feature, act as chain stoppers and thus inhibit the formation of long polymer chains. After curing, therefore, the Marten point (to DIN 53 462) or the dimensional stability (to ISO R 75) shifts to lower temperatures as compared with a diluent-free mixture. If the diluted resins already discussed are excluded, few appropriate impregnated-resin systems having viscosities below 1,000 cP at 20° C are left.
Since a low-viscosity resin penetrates the winding more readily than a high-viscosity resin, endeavors are made to increase the temperature. The impregnating resin therefore starts to react so that viscosity increases; unless the throughput is large so that fresh resin can always be added, the mixture soon becomes unserviceable for the proposed purpose. It would therefore be desirable to have a resin system which reacts very little at the temperature of impregnation; and example of such a system would be an epoxy resin and a liquid anhydride. Unfortunately, such systems drip for a long time, since they react slowly even at elevated temperature.
The suggestion was therefore made to add an accelerator to the lapping tape. As conventional kinds of accelerator for systems of this kind there can be considered metal naphth-enates and octoates, e.g. cobalt or zinc naphthenate or octoate, tertiary amines, such as e.g. benzyldimethylamine, dimethylaminomethyl-phenol 2, 4, 6-tri (dimethylaminoethyl) phenol or tri (methoxycarbonylethyl) amine, and boron fluoride-amine complexes, e.g. boron fluoride ethylamine or piperidine or pyridine. For instance, German Auslegeschrifts Pat. Nos. 1,162,898 and 1,219,554 disclose a process for dipping the rolls of tape before lapping into accelerator solution and drying them. After lapping, the impregnating resin in the tape contacts the accelerator and it is considered that reaction acceleration occurs substantially locally in the lapping. Since the substances concerned are low-molecular substances, some of them are dissolved out of the impregnating resin and therefore reach the resin supply. Consequently, the acceleration acts in the supply too, more particularly because the resin is at a temperature of from about 50° to 60° C during impregnation.
Surprisingly, an adhesive for sticking mica paper to substrate has been found which meets all requirements very satisfactorily and also acts catalytically on the stable resin curing agent systems mentioned. The novel adhesive comprises an oxyamino resin which has the formula and which is prepared by quantitively reacting an epoxy resin having the formula II, a melting point above 50° C (to ASTM E 28) and at least two ethylene oxide groups per molecule, with a secondary amine having the formula III: ##SPC3##
in which R 1 and R 2 each denote a straight-chain alkyl group having up to four carbon atoms or together denote a lower alkylene group which can be interrupted by a heteroatom.
This reaction leads to a new class of tertiary amine, viz. oxyamino resins having the partial formula I, which are very suitable as adhesives between the substrate and the mica and also as accelerators in the curing of the epoxy resin systems added subsequently for impregnation.
The reaction must be quantitative; if it is not, the tertiary amine groups involved initiate the reaction of the ethylene oxide groups remaining in the molecule and the adhesive is unstable.
The novolak kinds of epoxy resin are very suitable, as are the kinds on a base of bisphenol or heterocyclene; cycloaliphatic resins, however are less satisfactory.
Epoxy novolaks have the following basic structure: ##SPC4##
These are therefore chain-like phenol-formaldehyde-novolaks with the phenolic hydroxyl groups substituted by glycidyl groups. Since the resulting oxyamines usually have a lower melting point than the starting resins, the starting resins used are those having a relatively high melting point. Examples of appropriate amines having straight-chain alkyl groups and of appropriate cyclic amines are dimethylamine, diethylamine, di-n-proplyamine, pyrrolidine, piperidine, morpholine, thiomorpholine and so on. In the case of branched chains, of course, steric effects preclude or limit the accelerator effect. For practical reasons the reaction with diethylamine is preferred.
Advantageously, the reaction with the amine proceeds in a solvent, the amine being introduced in a quantity of from 1 to 3 equivalents referred to the epoxy resin. So that the resulting oxyamine can be freed readily and without damage from excess amine and from solvent, it is preferred to use as solvent ketones or aromatic hydrocarbons or their mixtures whose boiling point is preferably below 150° C.
A glass-silk fabric or a felt of glass fibers or synthetic fibers makes a particularly good porous substrate material.
The adhesive and accelerator can be prepared e.g. as follows:
1. 260 g of an epoxy novolak resin having an epoxy equivalent of approximately 200 and a melting point of 80° C are dissolved in the same quantity of toluene at 100° C. After cooling to about 50° C, 280 g of diethylamine are added with vigorous agitation. After 5 hours of reflux reaction at from 60° to 70° C the temperature is increased in steps and the surplus diethylamine and the toluene are removed by distillation. The resulting product is a resin having a melting point of 75° C.
It is not necessary to use an epoxy novolak resin, and equally satisfactory results are obtained by using a bisphenol resin, as will now be described.
2. 175 g of an epoxy resin on a bisphenol base, with a melting point of 80° C and an epoxy equivalent of 555 are dissolved in 120 g of toluene at 80° C. After cooling to 50° C, 70 g of diethylamine are added and heating with reflux is given at 60° C for 4 hours, whereafter the temperature is increased slowly to distill the surplus diethylamine and the toluene; the final residues thereof are removed in vacuo at 150° C. The resultig product is a resin which has a melting point of 78° C, an epoxy equivalent which cannot be identified and a hydroxyl number of 230.
This product can be dissolved in acetone or better in methylethylketone and dissolves warm in xylene but reprecipitates cold. Alcohols and aqueous solvents do not dissolve it.
3. 560 g of a mixture comprising two epoxy resins (a bisphenol type and a novolak type) and having a melting point of 70° C and an epoxy equivalent of 580 are dissolved in 500 g of toluene at 100° C. After cooling to 60° C, 102 g of diethylamine are added and the solution is reacted for 3 hours with reflux and without heating, whereafter a distillation top is connected and the temperature is increased slowly to 150° C, surplus diethylamine and toluene being removed by distillation, whereafter the mixture is maintained in vacuo at the same temperature until distillation ceases. No epoxy products can be detected in the resulting product, whose melting point is 70° C.
4. 575 g of an epoxy resin mixture on a bisphenol base and having an epoxy equivalent of 560 are dissolved in 500 g of toluene at 100° C. After cooling to approximately 60° C, 140 g of dipropylamine are added and the solution is agitated at 60° C for 3 hours, whereafter the temperature is increased slowly to 150° C and the surplus amine and the toluene are removed by distillation. The reaction mixture is maintained in vacuo at the same temperature until distillation ceases. The resulting product softens at 70° C and is soluble in methylethylketone.
In the thinly liquid epoxy curing agent mixtures such as are used to impregnate finished lappings, the adhesive does not dissolve cold but becomes increasingly soluble above 60° C.
The adhesive thus prepared can be used to stick mica paper to substrate. To this end, the resin is dissolved in an appropriate solvent, e.g. a ketone or an aromatic hydrocarbon or mixtures thereof, and applied to the substrate material, e.g. a thin glass cloth. The concentration of the solution is so adjusted in dependence upon the particular applicator device used that approximately from 2 to 20 g of resin per square metre of surface are applied. After this treatment the glass cloth is stuck to the mica paper in machine width. Strip cutting machines can cut the lapping tapes from the material, which is coiled into rolls or reels. The amount of resin used should be the minimum necessary to provide a firm connection between the two layers, so that the tapes can be cut and given mechanical lapping readily. The smaller this quantity, the more readily subsequent impregnation of the finished lapping can be carried out.
Since the tape, and the lapping produced thereby also contain the accelerator for the impregnating resin, the same can be stored in the impregnating tank without accelerator and therefore has a much longer life than previously, yet the impregnated lapping cures very rapidly in the oven due to the action of the accelerator present in the adhesive in the tape. Since the accelerator too is resin-like, it is not washed out during impregnation even at temperatures as high as from 50° to 60° C, whereas metal salts enter the impregnating resin in similar conditions and shorten its durability.
The following figures will make these points clear.
A liquid epoxy resin having an epoxy equivalent of approximately 180 is mixed with an equivalent quantity of hexahydrophthalic acid anhydride. As freshly prepared, the viscosity of the mixture at 20° C is from 900 to 100 cps.
The following experiments were made using this mixture:
a. the mixture was kept in a glass vessel at a thermostatically controlled temperature of 50° C and over a period of time samples were taken and its viscosity at 20° C checked.
b. the mixture was placed in contact with a tape according to the invention and left in contact with the tape at 50° C for 2 hours, whereafter the tape was removed and the mixture kept at 50° C. The viscosity of the mixture was again checked at 20° C.
c. as in (b) a tape was used which has previously been impregnated with a 1 percent cobalt naphthenate solution (accelerator). After removal of the tape the viscosity of the mixture was determined at 20° C.
The results are shown in table I.
TABLE I
Experiment Initial Viscosity measured at 20°C viscosity after after after of 2h. 24 h. 3 days mixture at 50°C at 50°C at 50°C Control (a) 930 cps 1112 cps 1573 cps 2004 cps With tape according to invention (b) 930 cps 1022 cps 1731 cps 1960 cps With tape previously dipped in cobalt naphthenate solution (c) 930 cps 1068 cps 1806 cps 4065 cps
It may therefore be concluded that the tape according to the invention does not affect the resin/curing agent system of the experimental mixture, i.e., accelerator is not dissolved out of the tape. On the other hand, as the appreciable increase in viscosity shows, the low-molecular accelerator dissolves out of the tape pretreated with cobalt naphthenate. It can also be shown that the adhesive contained in the tape according to the invention greatly reduces the gelling time of the same resin/curing agent mixture as was used in the previous experiments at elevated temperature, the gelling time being determined at 130° C and 160° C in a thermostat. The term "gelling time" is to be understood as denoting the period between the instant of time at which the resin curing agent accelerator mixture comes to the reaction temperature in the thermostat, and the instant at which the mixture ceases to flow freely and becomes a gel. This can be checked by dipping a thin glass rod into the mixture every few seconds and pulling it out. As long as drops form, there is no gelling. As viscosity increases, filaments form, and when gelling occurs the filaments break off. ##SPC5##
As Table II shows, the accleration provided by the new adhesive is even more striking than an accelerator frequently recommended in practical use for resin/curing agent systems of the kind specified.
As tables I and II show, the resins having tertiary amine groups such as are used as adhesives of the tape according to the invention not only are not dissolved out of the tape by the solvent-free impregnating resin but also considerably accelerate curing by action on the impregnating resin remaining in the lapping at the temperature at which lappings are pressed in practice. On the other hand, a correspondingly devised combined adhesive accelerator based on dialkylamines having branched alkyl radicals has substantially no accelerating effect.
The invention therefore provides a considerable technical advance:
1. The new lapping tape enables the impregnating process to be carried out without the tape having to be given further treatment for the provision of an accelerator, for the adhesive and the accelerator are the same product in the invention.
2. The adhesive used for the tape greatly accelerates the curing of the solvent-free resin used for impregnation.
3. Since the adhesive of the tape is virtually insoluble in the solvent-free impregnating resin during the impregnating time, the impregnating resins used can be slow-curing resin systems which can be stored cold substantially without change and whose viscosity rises little even at the impregnating temperature.