Method for preparing polyamide-imide resin

A polyamide-imide resin which can give a varnish having a high resin content, excellent in storage stability and heat resistant final products can be produced by reacting an aromatic diisocyanate with a tricarboxylic acid anhydride in the presence of a basic solvent while adjusting the resin content at 40% by weight or more, and adding a specific amount of lactam and a specific amount of an alcohol and/or an oxime separately before, during or after the above-mentioned reaction so as to adjust the reduced viscosity of the resin to 0.10 to 0.27.

BACKGROUND OF THE INVENTION 
This invention relates to a process for producing a polyamide-imide resin 
which can give a varnish having a high resin content, excellent in storage 
stability and is useful particularly as a varnish for heat-resistant 
electric wire. 
In the prior art, as polyamide-imide resins used in varnishes for 
heat-resistant electric wire, there have heretofore been used those which 
are obtained by using N-methylpyrrolidone (NMP) as a solvent for 
synthesis, have a reduced viscosity (concentration: 0.5 g/dl, solvent: 
dimethylformamide, measurement temperature: 30.degree. C.) of more than 
0.4 and have a sufficiently increased molecular weight. On the other hand, 
since the solution viscosities of varnishes for electric wire are set at 
about 30 poises (30.degree. C.) in the case of die coating because of 
restriction as to coating workability, the resin content of the 
above-mentioned polyamide-imide resins with a high molecular weight 
satisfying this requirement has an upper limit of about 30% by weight even 
if a good solvent NMP is used. Therefore, when such polyamide-imide resins 
with a high molecular weight are used in varnishes for electric wire, a 
large amount of expensive NMP must be used, and this poses a problem from 
the viewpoint of the cost. 
One method for reducing the cost by decreasing the used amount of NMP and 
increasing the resin content is to lower the molecular weight of the 
resin. However, when the molecular weight of a polyamide-imide resin 
obtained from a diisocyanate and a tricarboxylic acid anhydride is lowered 
so that the reduced viscosity of the resin may be 0.4 or lower, the 
terminal functional group concentration of the resin increases, so that 
the viscosity of the resulting varnish increases gradually with the lapse 
of time, which results in causing a problem of marked lowering of the 
storage stability. In the case where the viscosity has increased as days 
go by, when the resin is used, for example, as a varnish for electric 
wire, there are caused inconveniences such as the alteration of initially 
set coating conditions and the adjustment of the viscosity by diluting the 
varnish having an increased viscosity with a solvent, and the 
characteristics of a protective coating film formed by volatilizing the 
solvent sometimes vary. 
There is also a proposal aiming at removing these disadvantages, on a 
process for producing a stabilized polyamide-imide resin capable of having 
a high resin content in which terminal functional groups are masked with a 
specific active-hydrogen-containing compound. This process is greatly 
improved in the storage stability of a polyamide-imide resin having a 
lowered molecular weight, but is required to employ more strict 
stabilizing technique for polyamide-imide resins which have a lowered 
molecular weight for making the reduced viscosity 0.3 or lower and have a 
greatly increased resin content. That is to say, it is necessary to devise 
a stabilizing method so that polyamide-imide resins stabilized in such a 
low molecular weight region may show a sufficient cure reactivity at the 
time of baking and curing. Particularly when there is used such an 
active-hydrogen-containing compound wherein the terminal functional groups 
are masked by thermally irreversible bonding groups in a usual baking 
temperature range, the resulting resin is greatly lowered in cure 
reactivity, though it is excellent in storage stability. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide a process for producing a 
stabilized polyamide-imide resin which is free from such problems, can 
have a greatly increased resin content, is excellent in storage stability 
and cure reactivity, and is useful particularly as a varnish for 
heat-resistant electric wire. 
The present inventors effected resinification reactions under various 
synthesis conditions, varying the kind of agents for masking the terminal 
functional groups of the resulting polyamide-imide resin (masking agents), 
their used amounts, the molecular weight of the resin and the like, and 
investigated in detail relationships between the resin composition of the 
resulting resin and its practical performance characteristics to 
accomplish this invention. 
This invention provides a process for producing a polyamide-imide resin by 
reacting an aromatic diisocyanate (I) and a tricarboxylic acid anhydride 
(II) in approximately equimolar amounts in the presence of a basic solvent 
while adjusting the resin content at 40% by weight or more, the 
improvement wherein a lactam (III) in an amount of 0.1 to 1.0 mole per 
mole of the aromatic diisocyanate (I), and if necessary, an alcohol (IV) 
and/or an oxime (V) in an amount of 0.01 to 0.5 mole per mole of the 
aromatic diisocyanate (I) is added simultaneously or separately before, 
during or after the above-mentioned reaction so as to make the reduced 
viscosity of the resin 0.10 to 0.27. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
The most preferable method for adding masking agents for obtaining 
excellent practical performance characteristics without damaging the cure 
reactivity is to add first a lactam before, during or after the reaction 
to mask terminal functional groups (mainly isocyanate groups) and then to 
add an alcohol and/or an oxime after the reaction to mask terminal 
functional groups (mainly acid anhydride groups). From the viewpoint of 
the cure reactivity of the resulting resin, the isocyanate group is 
preferably masked by the lactam. But it is possible to react an alcohol 
and/or an oxime with a tricarboxylic acid anhydride, and then reacting 
therewith an aromatic diisocyanate, followed by the reaction with a 
lactam. It is also possible to react an alcohol and/or an oxime with the 
tricarboxylic acid anhydride, and subsequently add thereto an aromatic 
diisocyante and a lactam to react at the same time. 
According to this invention, there can be obtained a polyamide-imide resin 
which can give a varnish having a high resin content of about 40-55% by 
weight, excellent in long-term storage stability, and can be used 
particularly in varnishes for heat-resistant electric wire. 
The aromatic diisocyanate used in this invention includes, for example, 
tolylene diisocyanate, xylylene diisocyanate, 4,4'-diphenylether 
diisocyanate, naphthalene-1,5-diisocyanate, 4,4'-diphenylmethane 
diisocyanate, etc. When the heat resistance and the like are taken into 
consideration, it is preferable to use 4,4'-diphenylmethane diisocyanate 
or tolylene diisocyanate. If necessary, there may be co-used aliphatic 
diisocyanates such as 1,6-hexamethylene diisocyanate, isophorone 
diisocyanate and the like, alicyclic diisocyanates, trimers thereof, 
isocyanurate-ring-containing polyisocyanates obtained by trimerization 
reaction of the aforesaid aromatic diisocyanates, polyphenylmethyl 
polyisocyanates, e.g., a phosgenated condensate of aniline and 
formaldehyde, etc. In particular, isocyanurate-ring-containing 
polyisocyanates obtained by trimerization reaction of tolylene 
diisocyanate or 4,4'-diphenylmethane diisocyanate which are effective for 
improving the heat resistance, are preferred, and their used amount is 
preferably 0.03 to 0.20 equivalent per equivalent of the aromatic 
diisocyanate when the resulting varnish is used for heat-resistant 
electric wire. 
As the tricarboxylic acid anhydride, there can be used, for example, 
compounds represented by the general formulas (i) and (ii): 
##STR1## 
wherein X is --CH.sub.2 --, --CO--, --SO.sub.2 --, --O-- or the like. When 
the heat resistance, the cost and the like are taken into consideration, 
trimellitic acid anhydride is preferred. 
If necessary, polycarboxylic acids or acid anhydrides thereof other than 
the tricarboxylic acid anhydrides described above may also be co-used. As 
such polycarboxylic acids, there can be used, for example, trimellitic 
acid, trimesic acid, tris(2-carboxyethyl)isocyanurate, terephthalic acid, 
isophthalic acid, succinic acid, adipic acid, sebacic acid, 
dodecanedicarboxylic acid and the like. 
As the polycarboxylic acid anhydrides, there can be used dianhydrides of 
tetrabasic acids, for example, aliphatic and alicyclic tetrabasic acids 
such as 1,2,3,4-butanetetracarboxylic acid, cyclopentanetetracarboxylic 
acid, ethylenetetracarboxylic acid, bicyclo-[2,2,2]-octo-(7)-ene-2:3, 
5:6-tetracarboxylic acid and the like; aromatic tetrabasic acids such as 
pyromellitic acid, 3,3',4,4'-benzophenonetetracarboxylic acid, 
bis(3,4-dicarboxyphenyl) ether, 2,3,6,7-naphthalenetetracarboxylic acid, 
1,2,5,6-naphthalenetetracarboxylic acid, ethylene glycol bistrimellitate, 
2,2'-bis(3,4-biscarboxyphenyl)-propane, 2,2',3,3'-diphenyltetracarboxylic 
acid, perylene-3,4,9,10-tetracarboxylic acid, 3,4-dicarboxyphenylsulfonic 
acid and the like; heterocyclic tetrabasic acids such as 
thiophene-2,3,4,5-tetracarboxylic acid, pyrazinetetracarboxylic acid and 
the like; etc. 
These polycarboxylic acids or acid anhydrides thereof may be used for 
improving resin characteristics such as flexibility, solubility in 
solvents, melt-flow characteristics (processability) in molding and 
processing, cure reactivity, and the like. In particular, 
3,3',4,4'-benzophenonetetracarboxylic acid dianhydride effective for 
improving the cure reactivity is preferred. The using amount of 
3,3',4,4'-benzophenonetetracarboxylic acid dianhydride is preferably in 
the range of 0.03 to 0.2 mole per mole of the tricarboxylic acid 
anhydride. 
The aromatic diisocyanate and the tricarboxylic acid anhydride are reacted 
in approximately equimolar amounts. When they are reacted in approximately 
equimolar amounts, a polyamide-imide resin having a sufficiently high 
molecular weight is obtained at the time of baking and curing, and shows 
the best heat resistance and flexibility. Although the diisocyanate 
compound may be added in slightly excessive amount of moles in 
consideration of the fact that a small amount of water contained as an 
impurity in the reaction solvent reacts with isocyanate groups, the amount 
of the aromatic diisocyanate compound must not be more than 1.1 moles per 
mole of the tricarboxylic acid anhydride. 
As the basic solvent, there can be used those which are substantially inert 
to the aromatic diisocyanates. For example, N-methylpyrrolidone, 
dimethylformamide, dimethylacetamide and the like can be used. As a 
synthesis solvent for the aromatic diisocyanate and the tricarboxylic acid 
anhydride, N-methylpyrrolidone is preferred. As a dilution solvent used 
after the reaction, dimethylformamide is preferred. Dimethylformamide has 
an effect of lowering the solution viscosity of the resulting varnish, and 
contributes to the increase of the resin content. 
As to the resin concentration during the reaction, when the resin content 
is less than 40% by weight, the excess solvent should be removed after the 
synthesis by complicated procedures such as condensation or the like, so 
that an economical disadvantage is brought about. When the cost, the 
performance characteristics and the like are taken into consideration, the 
resin content is preferably 40 to 80% by weight. Here, the term "resin 
content" means the concentration of the sum of the aromatic diisocyanate 
and the tricarboxylic acid anhydride in the reaction system. However, the 
amount of the lactam, alcohol and oxime to be used is not included in this 
calculation. 
As the lactam, alcohol and oxime used in this invention as agents for 
masking the terminal functional groups of the produced polyamide-imide 
resin (masking agents), there may be used, for example, lactams such as 
2-pyrrolidone, .epsilon.-caprolactam, lauryllactam and the like; alcohols 
having 1 to 10 carbon atoms, such as methanol, ethanol, n-butanol, 
t-butanol, methyl Cellosolve, ethyl Cellosolve, methylcarbitol, benzyl 
alcohol, cyclohexanol, .omega.-hydro.perfluoroalcohol and the like; oximes 
such as 2-butanone oxime, formaldoxime, acetaldoxime, cyclohexanone oxime 
and the like; etc. These masking agents preferably contain one active 
hydrogen in the molecule. When masking agents contain two or more active 
hydrogens, they become chain-elongating agents for the resin, so that the 
controlling of the molecular weight and the solution viscosity become 
difficult, and that moreover the heat resistance is sometimes lowered. 
From the viewpoint of an effect of stabilizing the viscosity of the 
produced varnish, the easiness of thermal dissociation, the cost and the 
like, there can preferably be used .epsilon.-caprolactam as the lactam, 
methanol as the alcohol, and 2-butanone oxime as the oxime. 
Although lactams are preferred because they hardly damage cure reactivity, 
they are sometimes insufficient in an effect of imparting storate 
stability. This problem tends to be caused particularly when the resin has 
a low molecular weight and a high terminal functional group concentration. 
When the storage stability is insufficient as described above, the problem 
can be solved by using the above-mentioned methanol and/or oxime according 
to the need. 
The using amount of the lactam is 0.1 to 1.0 mole per mole of the aromatic 
diisocyanate. When it is less than 0.1 mole, the storage stability becomes 
insufficient. When it exceeds 1.0 mole, there is brought about a 
relatively slight inhibitory effect on cure reactivity, however a large 
amount of free lactam remains and hence the resulting varnish has a 
decreased resin content. When a lactam alone is used as a masking agent, 
its used amount is preferably in the range of 0.3 to 1.0 mole per mole of 
the aromatic diisocyanate. When a mixed system of a lactam and an alcohol 
and/or an oxime is used as masking agent, the using amount of the lactam 
is preferably in the range of 0.2 to 0.8 mole per mole of the aromatic 
diisocyanate. 
The using amount of the alcohol and/or the oxime is 0.01 to 0.5 mole per 
mole of the aromatic diisocyanate. When it is less than 0.01 mole, there 
is brought about only an insufficient effect on the storage stability. 
When it exceeds 0.5 mole, the cure reactivity is greatly lowered, so that 
a baked coating film formed by using the resulting varnish is deteriorated 
in practical performance characteristics. It is particularly preferably in 
the range of 0.01 to 0.3 mole. 
The addition of the lactam and of the alcohol and/or the oxime to be used 
if necessary is conducted before, during or after the reaction described 
above. They may be added after the polyamide-imide resin is produced and 
then diluted with another solvent. They may be added either in full at a 
time or stepwise. It is preferable to add a part of or the whole of the 
lactam before or during the reaction from the viewpoint of controlling the 
polymerization and suppressing the formation of bubbles by rapid 
decarboxylation. However, the alcohol and/or the oxime are preferably 
added after the reaction because they sometimes damage the polymerization. 
When the alcohol and/or the oxime are added and reacted before or during 
the above-mentioned reaction, it is preferable to use the alcohol/or the 
oxime in a proportion of 0.25 mole or less per mole of the aromatic 
diisocyanate. When they are used in a proportion of 0.25 mole or more, 
they tend to damage polymerization or cure reaction. 
When at least one masking agent is added before or during the reaction, it 
is preferable to conduct the reaction at a temperature of 80.degree. to 
200.degree. C. The reaction temperature of 160.degree. C. or lower is 
preferable in order to suppress side reactions such as network formation. 
It is most suitable to conduct the reaction at about 130.degree. C. When 
at least one masking agent is added after the reaction, it is preferable 
to conduct the reaction at a temperature of 80.degree. to 160.degree. C. 
The reaction temperature can be made lower and lower when the reaction is 
effected at higher and higher resin content. For example, when the resin 
content is 60% by weight, the reaction temperature is most suitably about 
110.degree. C. 
In a method by which at least one masking agent is added after the 
reaction, it is necessary to completely mask the terminal functional 
groups by further effecting the reaction at 0.degree. to 130.degree. C. up 
to 7-8 hours after the addition. The reaction temperature in this case is 
most suitably about 90.degree. C. 
The polyamide-imide resin in this invention should have a reduced viscosity 
of 0.10 to 0.27. When the reduced viscosity is lower than 0.10, the 
storage stability or practical performance characteristics such as heat 
resistance, flexibility and the like become insufficient. When the reduced 
viscosity exceeds 0.27, the resin content is lowered so as to make it 
impossible to achieve the object of this invention. The reduced viscosity 
can be adjusted by measuring the solution viscosity during the reaction. 
The reduced viscosity is measured in the following manner. To 1 liter of 
water is added 15 g of a solution prepared by adding N-methylpyrrolidone 
to a part of a resin solution obtained by the above-mentioned reaction so 
as to adjust the concentration to 10% by weight, whereby the resin is 
precipitated. Subsequently, the precipitate is dried under a vacuum of 0.3 
mmHg at 60.degree. C. for 10 hours to obtain a solid resin. The solid 
resin is made into a dimethylformamide solution having a concentration of 
0.5 g/dl, and the viscosity of the thus obtained solution is measured at 
30.degree. C. by using a Cannon-Feske viscometer (viscometer number 50). 
When the polyamide-imide resin obtained in this invention is made into a 
varnish, there may be used, as co-solvents, xylene, NISSEKI HISOL-100, 150 
(trade names, mfd. by Nippon Petrochemicals Co., Ltd., aromatic 
hydrocarbons obtained from petroleum, b.p. 80.degree.-300.degree. C.), 
methyl Cellosolve acetate, ethyl Cellosolve acetate, .gamma.-butyrolactone 
and the like in combination with the basic organic solvents described 
above. 
If necessary, catalysts for promoting cure or catalysts for dissociating 
urethane may be co-used in the polyamide-imide resin obtained in this 
invention. There are used, for example, tertiary amines such as 
triethylamine, triethylenediamine, dimethylaniline, dimethylethanolamine, 
1,8-diaza-bicyclo(5,4,0)-undecene-7 (or its organic acid salts), and the 
like; organotin compounds such as dibutyltin dilaurate, dibutyltin 
dioctoate and the like; organic titanium compounds such as 
tetrabutoxytitanate, tetraisopropoxytitanate, chelate or acylate compounds 
thereof, and the like; trialkylphosphine; etc. In particular, the tertiary 
amines are preferred. If necessary, various additives such as curing 
agents, surfactants and the like may be added to the polyamide-imide 
resin. 
As the curing agents, there are used epoxy resins, amino resins, 
phenol-formaldehyde resins, polyester resins having one or more hydroxyl 
groups and/or carboxyl groups, adducts of an aromatic polyisocyanate with 
any of the previously described compounds containing one active hydrogen 
in the molecule, etc. There are preferably used adducts of any of the 
previously described aromatic diisocyanates or trimers thereof with a 
compound having one active hydrogen in the molecule, particularly 
preferably an .epsilon.-caprolactam adduct of 4,4'-diphenylmethane 
diisocyanate. 
As another additive, benzoin is preferably used. Benzoin can improves the 
smoothness of the resulting coated film. 
When used, for example, as a varnish for heat-resistant electric wire for 
die coating, the thus obtained varnish can have a high resin content of 
about 40 to 55% by weight when the solution viscosity is set at 25 to 35 
poises (30.degree. C.). When used as a varnish for heat-resistant electric 
wire for felt coating, the obtained varnish can have a high resin content 
of about 20 to 35% by weight when the solution viscosity is set at 0.6 to 
0.8 poise (30.degree. C.). These varnishes are excellent in logn-term 
storage stability, and the resulting baked coated films fromed by using 
them are good in heat resistance and flexibility and moreover excellent in 
Freon resistance and crazing resistance. 
Although the polyamide-imide resin obtained in this invention is used 
mainly as a varnish for heat-resistant electric wire, it is useful for 
other purposes, for example, heat-resistance sheets, heat-resistant 
laminate materials, heat-resistant molded articles, heat-resistant 
adhesives, heat-resistant composite materials with glass fiber or carbon 
fiber, impregnation for electric insulation, casting varnish, etc.

This invention is explained below referring to Examples and Comparative 
Examples. 
COMATIVE EXAMPLE 1 
In a 2-liter four-necked flask equipped with a thermometer, a stirrer and 
an Allihn condenser, 452.5 g of 4,4'-diphenylmethane diisocyanate, 347.5 g 
of trimellitic acid anhydride and 1485.7 g of N-methylpyrrolidone were 
placed and reacted with stirring in a nitrogen stream at 100.degree. C. 
for 1 hour, at 115.degree. C. for 2 hours, and then at 120.degree. C. for 
2 hours, and subsequently heated to 135.degree. C. to promote the reaction 
(the resin content was 35% by weight). The reaction solution was diluted 
by adding 381 g of xylene. The resin content (calculated value) of the 
thus obtained varnish of a polyamide-imide resin was 30% by weight, and 
the initial viscosity (B-type viscometer, 30.degree. C.) was 31 poises. 
The reduced viscosity (0.5 g/dl, dimethylformamide, 30.degree. C.) of the 
polyamide-imide resin was 0.42. The varnish underwent no change in 
viscosity at all even when allowed to stand at 40.degree. C. for 1 month. 
COMATIVE EXAMPLE 2 
In a 2-liter four-necked flask equipped with a thermometer, a stirrer and 
an Allihn condenser, 452.5 g of 4,4'-diphenylmethane diisocyanate, 347.5 g 
of trimellitic acid anhydride and 533.3 g of N-methylpyrrolidone were 
placed and reacted with stirring in a nitrogen stream at 100.degree. C. 
for 1 hour and then at 115.degree. C. for 2 hours (the resin content was 
60% by weight). The reaction solution was diluted by adding 267 g of 
N-methylpyrrolidone. The resin content (calculated value) of the thus 
obtained varnish of a polyamide-imide resin was 50% by weight, and the 
initial viscosity (B-type viscometer, 30.degree. C.) of the varnish was 32 
poises. The reduced viscosity (0.5 g/dl, dimethylformamide, 30.degree. C.) 
of the polyamide-imide resin was 0.15. The varnish had a viscosity of 
1,000 poises or higher after being allowed to stand at 23.degree. C. for 
10 days, and was thus very low in storage stability. 
COMATIVE EXAMPLE 3 
In a 2-liter four-necked flask equipped with a thermometer, a stirrer and 
an Allihn condenser, 45.25 g of 4,4'-diphenylmethane diisocyanate, 347.5 g 
of trimellitic acid anhydride, 145.0 g of .epsilon.-caprolactam and 533.3 
g of N-methylpyrrolidone were placed and reacted with stirring in a 
nitrogen stream at 90.degree. C. for 1 hour and then at 115.degree. C. for 
1 hour, after which the reaction was further proceeded at 135.degree. C. 
The reaction solution was cooled to 70.degree. C. when the Gardner 
viscosity at 30.degree. C. became 30 seconds. Thereto was added 20.5 g of 
methanol, and the resulting solution was allowed to react at said 
temperature for 1 hour and then at 90.degree. C. for 2 hours. The reduced 
viscosity of the resulting resin was 0.09 (0.05 g/dl, dimethylformamide, 
30.degree. C.). The resin content (calculated value) of the thus obtained 
varnish of the polyamide-imide resin was 60% by weight. The varnish had an 
initial viscosity of 42 poises (30.degree. C.), and had a viscosity of 56 
poises after being allowed to stand at 40.degree. C. for 1 month. 
EXAMPLE 1 
Immediately after the synthesis of the polyamide-imide resin varnish (resin 
content: 50% by weight) obtained in Comparative Example 2, 96.7 g of 
.epsilon.-caprolactam (0.473 mole per mole of the aromatic diisocyanate) 
was added, and the resulting solution was allowed to react at 110.degree. 
C. for 3 hours. The solution was then cooled to 60.degree. C., and 37.4 g 
of 2-butanone oxime (0.237 mole per mole of the aromatic diisocyanate) was 
added thereto, after which the thus obtained solution was allowed to react 
at said temperature for 1 hour and then at 90.degree. C. for 3 hours. The 
resulting resin had a reduced viscosity of 0.15 (0.5 g/dl, 
dimethylformamide, 30.degree. C.). The thus obtained varnish had an 
initial viscosity of 33 poises, and had a viscosity of 34.5 poises after 
being allowed to stand at 40.degree. C. for 1 month: it thus showed 
excellent storage stability. Both of films obtained by applying this 
varnish to each glass plate and baking it at 200.degree. C. for 30 minutes 
or at 250.degree. C. for 30 minutes, respectively, had such excellent 
flexibility that they were not broken even when wrinkled several times. 
EXAMPLE 2 
In a 2-liter four-necked flask quipped with a thermometer, a stirrer and an 
Allihn condenser, 452.5 g of 4,4'-diphenylmethane diisocyanate, 347.5 g of 
trimellitic acid anhydride, 96.7 g of .epsilon.-caprolactam and 533.3 g of 
N-methylpyrrolidone were placed and reacted with stirring in a nitrogen 
stream at 100.degree. C. for 1 hour, at 115.degree. C. for 2 hours and 
then at 125.degree. C. for 1 hour, after which the reaction was further 
proceeded at 135.degree. C. The reaction solution was cooled to 
100.degree. C. when the Gardner viscosity at 30.degree. C. of the solution 
prepared, as a sample for judging the end point, by diluting a part of the 
reaction solution so as to adjust the resin content to 40% by weight, 
became 25 seconds. Thereafter, the reaction solution was diluted by adding 
272.4 g of N-methylpyrrolidone, 345.3 g of dimethylformamide and 2.7 g of 
methanol. Subsequently, the resulting solution was allowed to react at 
90.degree. C. for 3 hours. The reduced viscosity of the resulting resin 
was 0.24 (0.5 g/dl, dimethylformamide, 30.degree. C.). The resin content 
(calculated value) of the thus obtained varnish of the polyamide-imide 
resin was 41% by weight. The varnish had an initial viscosity of 28 poises 
(30.degree. C.), and had a viscosity of 29 poises (30.degree. C.) after 
being allowed to stand at 40.degree. C. for 1 month; it thus showed 
excellent storage stability. 
EXAMPLE 3 
In a 2-liter four-necked flask equipped with a thermometer, a stirrer and 
an Allihn condenser, 456.4 g of 4,4'-diphenylmethane diisocyanate, 343.6 g 
of trimellitic acid anhydride, 533.3 g of N-methylpyrrolidone and 48.4 g 
of .epsilon.-caprolactam were placed and reacted with stirring in a 
nitrogen stream at 100.degree. C. for 1 hour, at 115.degree. C. for 2 
hours, and then at 125.degree. C. for 1 hour, after which the reaction was 
further proceeded at 135.degree. C. The reaction solution was cooled to 
100.degree. C. when the Gardner viscosity at 30.degree. C. of the solution 
prepared, as a sample for judging the end point, by diluting a part of the 
reaction solution so as to adjust the resin content to 44% by weight, 
became 29 seconds. Thereafter, the reaction solution was diluted by adding 
484.9 g of dimethylformamide and 18.6 g of 2-butanone oxime. Subsequently, 
the resulting solution was allowed to react at 90.degree. C. for 3 hours. 
The reduced viscosity of the resulting resin was 0.21 (0.5 g/dl, 
dimethylformamide, 30.degree. C.). The resin content (calculated value) of 
the thus obtained varnish of the polyamide-imide resin was 44% by weight. 
The varnish had an initial viscosity of 34 poises (30.degree. C.), and had 
a viscosity of 36 poises (30.degree. C.) after being allowed to stand at 
40.degree. C. for 1 month; it thus showed excellent storage stability. 
EXAMPLE 4 
To 1,300 g of the varnish (resin content: 44% by weight) obtained in 
Example 3 were gradually added 177.7 g of N-methylpyrrolidone and 905.7 g 
of xylene at 70.degree. C. with stirring to obtain a varnish. The resin 
content (calculated value) of the obtained varnish was 24% by weight. The 
varnish had an initial viscosity of 0.68 poise (30.degree. C.), and had a 
viscosity of 0.72 poise (30.degree. C.) after being allowed to stand at 
40.degree. C. for 1 month; it thus showed excellent storage stability. 
EXAMPLE 5 
In a 2-liter four-necked flask equipped with a thermometer, a stirrer and 
an Allihn condenser, 452.5 g of trimellitic acid anhydride, 96.7 g of 
.epsilon.-caprolactam and 533.3 g of N-methylpyrrolidone were placed and 
reacted with stirring in a nitrogen stream at 100.degree. C. for 1 hour, 
at 115.degree. C. for 2 hours, and then at 125.degree. C. for 1 hour, 
after which the reaction was further proceeded at 135.degree. C. The 
reaction solution was cooled to 100.degree. C. when the Gardner viscosity 
at 30.degree. C. of the solution prepared, as a sample for judging the end 
point, by diluting a part of the reaction solution so as to adjust the 
resin content to 45% by weight, became 28 seconds. Thereafter, the 
reaction solution was diluted by adding 444.4 g of dimethylformamide. 
Thereto was added 13.7 g of methanol, and the resulting solution was 
allowed to react at 90.degree. C. for 3 hours. The reduced viscosity of 
the resulting resin was 0.21 (0.5 g/dl, dimethylformamide, 30.degree. C.). 
The resin content (calculated value) of the thus obtained varnish of the 
polyamide-imide resin was 45% by weight. The varnish had an initial 
viscosity of 30 poises (30.degree. C.). The varnish had a viscosity of 30 
poises (30.degree. C.) after being allowed to stand at 40.degree. C. for 1 
month and thus showed excellent storage stability. 
Each of the varnishes obtained in Example 2, 3 and 5 and Comparative 
Examples 1 and 3 was coated on a copper wire and baked by a conventional 
method to obtain an enamel wire, and characteristics of the thus obtained 
enamel wires were evaluated. The result are shown in Table 1. 
TABLE 1 
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Comparative 
Comparative 
Example No. Example 1 
Example 3 
Example 2 
Example 3 
Example 5 
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Flexibility* (10% elonga- 
1X OK 4X OK 1X OK 1X OK 1X OK 
tion) 
Cut through temperature* 
&gt;400 390 &gt;400 &gt;400 &gt;400 
(2kg) (.degree.C.) 
Abrasion resistance* 
130 95 130 120 120 
(600 g) (times) 
Heat shock* 1X OK 4X OK 1X OK 1X OK 1X OK 
(240.degree. C. - 1 hr) 
BDP Retention** (260.degree. C.) (%) 
&gt;80 &gt;75 &gt;80 &gt;80 &gt;80 
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Note to Table 1: - 
Baking conditions: 
Diameter of wire: 
1 mm, 
Coating conditions: 
8 times by using a die 
Furnace length: 
4.5 m 
Furnace temperature: 
inlet 260.degree. C. 
middle 360.degree. C. 
outlet 400.degree. C. 
Linear speed of wire: 
10 m/min 
*According to JIS C 3003 
**After deteriorated with heating for 168 hours, retention rate of 
breakdown voltage was compared with the initial value. 
It is shown that Examples 2, 3 and 5 having individual resin contents of 41 
to 45% by weight and individual reduced viscosity of 0.20 to 0.24 
stabilized by suitable amounts of the combination of a lactam and an 
alcohol or of a lactam and an oxime possess greatly improved storage 
stability, as compared with Comparative Example 2 which was not stabilized 
and with Comparative Example 3 having a reduced viscosity of 0.09 although 
stabilized under the same conditions as with Example 5. Further the 
storage stabilities in Examples 2, 3 and 5 are equal to Comparative 
Example 1 of the prior art. Moreover, it is shown that the enamel wire 
performance characteristics of Examples 2, 3 and 5 are excellent and equal 
to those of Comparative Example 1. 
As is clear from the above results, the polyamide-imide resins obtained by 
the production process of this invention are not only good in storage 
stability but also excellent in heat resistance, flexibility and abrasion 
resistance, can be used in various heat-resistant materials including 
varnishes for heat-resistant electric wire, and hence are industrially 
very effective.