Polyimide precursor solution process for the production thereof coating or film obtained therefrom and process for producing the film

This invention provides a polyimide precursor solution having high concentration and low viscosity and a production process thereof, and polyimide coatings and films having excellent physical properties obtained therefrom and a production process thereof. Particularly, it relates to a polyimide precursor solution which contains a salt of a specific diamine with a specific tetracarboxylic acid, as its solute; to a process for the production of the polyimide precursor solution, which comprises allowing 1 mole of a specified diamine to react with 0.3 to 0.9 mole of a specific tetracarboxylic acid dianhydride, thereby obtaining a diamine, and subsequently adding 0.95 to 1.05 moles of a specified tetracarboxylic acid to 1 mole of the thus obtained diamine; to a polyimide coating or film obtained from the aforementioned polyimide precursor solution; and to a process for the production of the polyimide coating or film, which comprises coating the aforementioned polyimide precursor solution on a substrate and heating the coat to effect imidization.

FIELD OF THE INVENTION 
This invention relates to a polyimide precursor solution and a process for 
the production of the solution and to a polyimide coating or film obtained 
from the polyimide precursor solution and a process for the production of 
the film. 
BACKGROUND OF THE INVENTION 
Polyimide compounds are useful for their applications to the field of 
electronics and have been used as insulating films and protective coatings 
on semiconductor devices. Particularly, all aromatic polyimide compounds 
have been contributing greatly to the increase in integration and multiple 
function of flexible circuit substrates, integrated circuits and the like, 
because of their excellent heat resistance, mechanical characteristics and 
electric characteristics. Polyimide precursor solutions have been used 
conventionally when layer-insulating films or protective films are formed 
on LSI chips. Solutions comprising poly(amic acid)s represented by the 
following general formula are known as such polyimide precursor solutions. 
##STR1## 
These poly(amic acid) solutions are produced by allowing an aromatic 
diamine compound to react with an aromatic tetracarboxylic acid 
dianhydride in a solvent, and various solutions have been proposed in 
which aprotic polar solvents are used as described, for example, in 
JP-B-36-10999 and GB 898651A (the term "JP-B" as used herein means an 
"examined Japanese patent publication"), JP-A-62-275165 (the term "JP-A" 
as used herein means an "unexamined published Japanese patent 
application), JP-A-64-5057 (corresponding to U.S. Pat. No. 5,073,828 and 
EP-A-297413), JP-B-2-38149 (corresponding to U.S. Pat. No. 4,533,574), 
JP-B-2-38150 (corresponding to U.S. Pat. No. 4,562,100), JP-A-1-299871, 
JP-A-58-122920 (corresponding to U.S. Pat. No. 4,454,276), JP-B-1-34454, 
JP-A-58-185624 (corresponding to U.S. Pat. No. 4,438,256), Journal of 
Polymer Science, Macromolecular Reviews Vol. 11, p. 199 (1976), U.S. Pat. 
No. 4,238,528, JP-B-3-4588 (corresponding to U.S. Pat. No. 4,525,507 and 
EP-B-151801), JP-B-7-30247, JP-A-7-41556, JP-A-7-62095 (corresponding to 
U.S. Pat. No. 5,478,914), JP-A-7-133349, JP-A-7-149896, JP-A-6-207014, 
JP-B-7-17870 and JP-B-7-17871 (corresponding to U.S. Pat. No. 5,254,361), 
and IBM Technical Disclosure Bulletin Vol. 20, No. 6, p. 2041 (1977) or 
solutions in which a mixed solvent selected from water-soluble ether 
compounds, water-soluble alcohol compounds, water-soluble ketone compounds 
and water is used as described in JP-A-6-1915 (U.S. Pat. Nos. 5,463,016 
and 5,466,732). 
With regard to the polyimide precursor as the solute of the polyimide 
precursor solution, various polymers are known in addition to poly(amic 
acid)s. For example, a poly(amic acid) ester represented by the following 
general formula 
##STR2## 
is disclosed in Macromolecules, Vol. 22, p. 4477 (1989) and Polyimides and 
Other High Temperature Polymers, p. 45 (1991); a poly(amic acid) 
trimethylsilyl ester represented by the following general formula 
##STR3## 
is disclosed in Macromolecules, Vol. 24, p. 3475 (1991); and a poly(amic 
acid) bis(diethylamide) represented by the following formula 
##STR4## 
is disclosed in Journal of Polymer Science Part B, Vol. 8, p. 29 (1970), 
Journal of Polymer Science Part B, Vol. 8, p. 559 (1970), Nippon Kagaku 
Kaishi, Vol. 1972, p. 1992 (published by the Chemical Society of Japan) 
and Journal of Polymer Science Polymer Chemistry Edition, Vol. 13, p. 365 
(1975). 
Each of the aforementioned polyimide precursors are in the form of a 
solution of a high molecular weight polymer. 
When a polyimide coating is obtained from any of these polymer solutions, 
the film is obtained generally by coating the polymer solution on a 
substrate such as a copper, glass or the like and heating the solution to 
effect removal of solvent and imidization. 
However, when such a high molecular weight polymer solution is coated, the 
high molecular weight causes a problem in that concentration of the solute 
must be reduced in order to obtain suitable viscosity of the solution for 
its coating. Also, when concentration of the solute is increased in order 
to improve productivity, it causes another problem in that its coating 
cannot be made due to high viscosity of the solution, or, even if the 
coating can be made, a coating or film having excellent mechanical and 
thermal characteristics cannot be obtained. In addition, since the polymer 
solution cannot withstand long-term preservation, it is extremely 
difficult to preserve the solution for a prolonged period of time while 
keeping its molecular weight. 
SUMMARY OF THE INVENTION 
In view of the above, it is an object of the present invention to provide a 
polyimide precursor solution having high solid contents and low viscosity 
and a process for the production thereof, as well as a polyimide coating 
obtained from the precursor solution, which has excellent physical 
properties, and a process for the production thereof. 
With the aim of overcoming the aforementioned problems, the inventors of 
the present invention have conducted extensive studies and found that, 
when specific monomers are used in combination, a polyimide coating having 
excellent physical properties can be obtained even from a solution 
containing monomers per se, not in a polymer form. In other wards, it was 
found that a polyimide precursor solution which contains a salt of 
monomers of a diamine compound represented by the following general 
formula (1) and a tetracarboxylic acid and/or a tetracarboxylic ester 
represented by the following general formula (2) shows a low viscosity 
even if the monomer salt is dissolved in the solution in a high 
concentration of and, moreover, that a high strength polyimide coating can 
be obtained from the solution. The present invention has been accomplished 
on the basis of these findings. Such findings are extremely surprising 
when taken into consideration the fact that only high molecular weight 
polymers have been known as polyimide precursors for constituting the 
polyimide precursor solutions. 
Accordingly, the first embodiment of the present invention is a polyimide 
precursor solution which comprises, as its solute, a salt of (i) a diamine 
represented by the following general formula (1) 
##STR5## 
(wherein R represents a tetravalent aromatic residue containing at least 
one 6-membered carbon ring, wherein the four carbonyl groups are directly 
connected to different carbon atoms of R and each of two pairs of the four 
carbonyl groups is connected to adjacent carbon atoms in the 6-membered 
carbon ring, and R' represents a divalent aromatic residue containing at 
least one 6-membered carbon ring) and (ii) a tetracarboxylic acid and/or a 
tetracarboxylic ester represented by the following general formula (2) 
##STR6## 
(wherein R" represents a tetravalent aromatic residue containing at least 
one 6-membered carbon ring, wherein the four carbonyl groups are directly 
connected to different carbon atoms in the residue and each of the two 
pairs of the four carbonyl groups is connected to adjacent carbon atoms in 
the 6-membered carbon ring, and R'" represents a hydrogen atom or an alkyl 
group having 1 to 5 carbon atoms). 
The second embodiment of the present invention is a process for the 
production of a polyimide precursor solution, which comprises reacting 1 
mole of a diamine represented by the following general formula (3) 
EQU H.sub.2 N--R'--NH.sub.2 ( 3) 
(wherein R' represents a divalent aromatic residue containing at least one 
6-membered carbon ring) to react with 0.3 to 0.9 mole of a tetracarboxylic 
acid dianhydride represented by the following general formula (4) 
##STR7## 
(wherein R represents a tetravalent aromatic residue containing at least 
one 6-membered carbon ring, wherein the four carbonyl groups are directly 
connected to different carbon atoms of R and each of two pairs of the four 
carbonyl groups is connected to adjacent carbon atoms in the 6-membered 
carbon ring) in a solvent to give the diamine represented by the 
aforementioned general formula (1), and 
adding 0.95 to 1.05 moles of the tetracarboxylic acid and/or 
tetracarboxylic ester represented by the aforementioned general formula 
(2) to 1 mole of the thus obtained diamine. 
The third embodiment of the present invention is a polyimide coating or 
film obtained from the aforementioned polyimide precursor solution. 
The fourth embodiment of the present invention is a process for the 
production of a polyimide coating or film, which comprises coating the 
aforementioned polyimide precursor solution on a substrate and heating the 
coat to effect imidization. 
Other objects and advantages of the present invention will be made apparent 
as the description progresses. 
DETAILED DESCRIPTION OF THE INVENTION 
First, technical terms used in the present invention are described. 
(1) Salt 
A complex which is obtained by mixing a diamine component with a 
tetracarboxylic acid component in a solvent. The amino group and carboxyl 
group may be linked to each other under any condition (ionic bond or 
non-ionic bond). 
(2) Polyimide 
An organic polymer in which 80 mol % or more of the repeating units of the 
polymer chain has the imide structure. This polymer generally shows no 
melting point or softing point at less than 400.degree. C. 
(3) Polyimide precursor 
An organic compound which becomes a polyimide by ring closure by heating or 
a chemical action. In this instance, the term "ring closure" means 
formation of an imide ring structure. 
(4) Polyimide solution 
A solution in which a polyimide precursor is dissolved in a solvent. The 
solvent in this case is a compound which is liquid at 25.degree. C. 
(5) Viscosity 
Viscosity measured at 20.degree. C. by a rotational DVL-BII digital 
viscometer (Brookfield viscometer) manufactured by Tokimec Co. 
(6) Solute concentration 
Weight ratio of the polyimide precursor in the solution, expressed by 
percentage. 
(7) Polyimide coating 
A polyimide formed on a substrate such as copper, aluminum, glass or the 
like. Of these polyimides, those which are used in the adhered state to 
substrates are called polyimide coatings. 
(8) Polyimide film 
A polyimide formed on a substrate such as copper, aluminum, glass or the 
like. Of these polyimides, those which are used after peeling from the 
substrates are called polyimide films. 
The present invention is described further. 
In the polyimide precursor solution of the present invention, a salt of a 
diamine represented by the general formula (1) and a tetracarboxylic acid 
and/or a tetracarboxylic ester represented by the general formula (2) is 
dissolved in a solvent as the solute. 
In this case, R represents a tetravalent aromatic residue containing at 
least one 6-membered carbon ring, wherein the four carbonyl groups are 
directly connected to different carbon atoms in the residue and each of 
the two pairs of the four carbonyl groups is connected to adjacent carbon 
atoms in the 6-membered carbon ring. Illustrative examples of R include 
the following groups. 
##STR8## 
The following groups are particularly preferred as R. 
##STR9## 
R' is a divalent aromatic residue which has at least one 6-membered carbon 
ring. Illustrative examples of R' include the following groups. 
##STR10## 
The following groups are particularly preferred as R'. 
##STR11## 
In the tetracarboxylic acid and/or tetracarboxylic ester represented by the 
general formula (2) of the present invention, R" represents a tetravalent 
aromatic residue containing at least one 6-membered carbon ring, wherein 
the four carbonyl groups are directly connected to different carbon atoms 
in the residue and each of the two pairs of the four carbonyl groups is 
connected to adjacent carbon atoms in the 6-membered carbon ring. R'" 
represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms. 
Illustrative examples of R" include those which are shown as the examples 
of R, and, in the salt of the diamine of general formula (1) and 
tetracarboxylic acid of general formula (2), the same group or different 
groups may be used as R and R". 
As R", the following groups are particularly preferred. 
##STR12## 
Illustrative examples of R'" include the following groups. 
##STR13## 
As R'", the following groups are particularly preferred. 
EQU --H,--CH.sub.3 
Any solvent can be as the solvent of the solution of the present invention, 
provided that it can dissolve the salt of a diamine represented by the 
general formula (1) and a tetracarboxylic acid and/or a tetracarboxylic 
ester represented by the general formula (2). 
Illustrative examples of the solvent include aprotic polar solvents such as 
N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, 
hexamethylphosphoramide, N-methylpyrrolidone and the like; ether compounds 
such as 2-methoxyethanol, 2-ethoxyethanol, 
2-(methoxymethoxy)ethoxyethanol, 2-isopropoxyethanol, 2-butoxyethanol, 
tetrahydrofurfuryl alcohol, diethylene glycol, diethylene glycol 
monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol 
monobutyl ether, triethylene glycol, triethylene glycol monoethyl ether, 
tetraethylene glycol, 1-methoxy-2-propanol, 1-ethoxy-2-propanol, 
dipropylene glycol, dipropylene glycol monomethyl ether, dipropylene 
glycol monoethyl ether, tripropylene glycol monomethyl ether, polyethylene 
glycol, polypropylene glycol, tetrahydrofuran, dioxane, 
1,2-dimethoxyethane, diethylene glycol dimethyl ether, diethylene glycol 
diethyl ether and the like; and water-soluble alcohol compounds such as 
methanol, ethanol, 1-propanol, 2-propanol, tert-butyl alcohol, ethylene 
glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 
2,3-butanediol, 1,5-pentanediol, 2-butene-1,4-diol, 
2-methyl-2,4-pentanediol, 1,2,6-hexanetriol, diacetone alcohol and the 
like, and these compounds may be used alone or as a mixture of two or 
more. Of these compounds, particularly preferred are 
N,N-dimethylformamide, N,N-dimethylacetamide, 2-methoxyethanol, diethylene 
glycol monomethyl ether or 1-methoxy-2-propanol as single solvent and a 
combination of N-methylpyrrolidone with diethylene glycol monomethyl 
ether, N-methylpyrrolidone with methanol or N-methylpyrrolidone with 
2-methoxyethanol as a mixed solvent. 
Concentration of the polyimide precursor in the polyimide precursor 
solution of the present invention may be preferably 30% by weight or more, 
more preferably 35% by weight or more, most preferably 40% by weight or 
more, based on the total weight of the solution. The concentration less 
than 30% by weight may not provide sufficient effects in improving 
producibility of coating procedure. Preferable upper limit of the 
concentration is about 80% by weight. The concentration exceeding 80% by 
weight may cause insufficient dissolution of the polyimide precursor. 
Also, the polyimide precursor solution may have a viscosity of preferably 
100 poise or less, more preferably 85 poise or less, most preferably 60 
poise or less. The viscosity exceeding 100 poise is not preferable since 
the coating procedure becomes difficult. 
The polyimide precursor solution of the present invention can be produced 
by adding a tetracarboxylic acid and/or a tetracarboxylic ester 
represented by the general formula (2) to a solution of a diamine 
represented by the general formula (1). 
As a preferred example, the following describes a method for the production 
of a polyimide precursor solution, in which a tetracarboxylic acid 
dianhydride is allowed to react with a diamine in a mixture of an aprotic 
polar compound with an ether compound, thereby obtaining a solution of the 
diamine of general formula (1), and then the tetracarboxylic acid and/or 
tetracarboxylic ester of general formula (2) is added to the thus obtained 
diamine solution. 
First, an aromatic tetracarboxylic acid dianhydride having R as its 
skeleton is allowed to react with the aforementioned aromatic diamine 
having R' as its skeleton in a mixture of an aprotic polar compound with 
an ether compound. Next, the aforementioned aromatic tetracarboxylic acid 
and/or tetracarboxylic ester having R" as its skeleton is added to the 
resulting reaction solution. 
The temperature for the first reaction to obtain the compound of the 
general formula (1) is preferably from -30.degree. to 60.degree. C., more 
preferably from -20.degree. to 40.degree. C. At the temperature deviating 
from the range of -30.degree. to 60.degree. C., it is difficult to control 
of the temperature. The reaction is carried out preferably from 30 minutes 
to 24 hours, more preferably from 1 to 12 hours. If the reaction is 
carried out only less than 30 minutes, the diamine compound produced may 
sometimes show insufficient dissolution. In general, the reaction 
terminates within 24 hours. 
The temperature for the second reaction between the compound of the general 
formula-(1) and the compound of the general formula (2) is preferably from 
-30.degree. to 60.degree. C., more preferably from -20.degree. to 
40.degree. C. At the temperature deviating from the range of -30.degree. 
to 60.degree. C., it is difficult to control of the temperature. The 
reaction is carried out preferably from 30 minutes to 24 hours, more 
preferably from 1 to 12 hours. If the reaction is carried out only less 
than 30 minutes, the salt produced may sometimes show insufficient 
dissolution. In general, the reaction terminates within 24 hours. 
The reaction of a tetracarboxylic acid dianhydride with a diamine for the 
formation of the diamine of general formula (1) may be carried out using 
preferably 0.3 to 0.9 mole, more preferably 0.4 to 0.6 mole, most 
preferably 0.45 to 0.55 mole, of the tetracarboxylic acid dianhydride 
based on 1 mole of the diamine. When amount of the tetracarboxylic acid 
dianhydride based on 1 mole of the diamine is outside the range of 0.3 to 
0.9 mole, there is a tendency that formation of the diamine of general 
formula (1) becomes difficult. Also, the aromatic tetracarboxylic acid 
and/or tetracarboxylic ester having R" as its skeleton may be added at a 
ratio of preferably 0.95 to 1.05 moles, more preferably 0.97 to 1.03 moles 
based on the diamine. When the ratio of the aromatic tetracarboxylic acid 
and/or tetracarboxylic ester having R" as its skeleton is outside the 
range of 0.95 to 1.05 moles, there is a tendency that formation of the 
salt becomes difficult. 
When a solution of the diamine of the general formula (1) is produced, the 
monomers and solvent may be mixed in optional order. When a mixture of 
solvents is used as the solvent, a solution of the diamine of the general 
formula (1) can be obtained by dissolving or suspending each monomer in 
each solvent, mixing the resulting solutions or suspensions and then 
stirring the mixture at a predetermined temperature for a predetermined 
period of time to effect reaction. With regard to the method for the 
addition of the tetracarboxylic acid and/or tetracarboxylic ester of 
general formula (2), one or both of them may be added directly as solid(s) 
or in the form of solution(s) to the aforementioned diamine solution under 
stirring. 
In addition, when required, conventional additive agents such as organic 
silanes, pigments, fillers including conductive carbon black and metal 
particles, abrasive agents, dielectric substances, lubricants and the like 
may be added to the polyimide precursor solution of the present invention 
in such amounts that they do not spoil the effect of the present 
invention. Other polymers and water-insoluble solvents such as ethers, 
alcohols, ketones, esters, halogenated hydrocarbons, hydrocarbons and the 
like may also be added in such amounts that they do not spoil the effect 
of the present invention. 
When a polyimide film is molded from a polyimide precursor solution, the 
precursor solution is applied to the surface of a substrate by extruding 
the solution from a slit-type nozzle or using a bar coater or the like, 
dried to remove the solvent and then subjected to imidization, and the 
film formed is separated from the substrate. 
The imidization is carried out preferably at 200.degree. to 400.degree. C., 
more preferably 250.degree. to 350.degree. C. At the temperature deviating 
this range, imidization may become insufficient or deformation or 
deterioration of coating may occur by heat. 
When a polyimide coating is produced, a polyimide precursor solution is 
applied to the surface of a substrate by a conventional means such as spin 
coating, spray coating, dipping or the like, dried to remove the solvent 
and then subjected to imidization. 
Thus, the polyimide precursor solution of the present invention and films 
and coatings obtained from the solution can be used in the production, for 
example, of a heat resistant insulating tape, a heat resistant adhesive 
tape, a high density magnetic recording base, a condenser, a film for FPC 
use and the like. Such materials are also useful for the production of 
molding materials and moldings such as a sliding means in which a fluoride 
resin, graphite or the like is packed, a structural element reinforced 
with glass fiber, carbon fiber or the like, a bobbin for miniature coil, a 
sleeve, a terminal-insulating tube and the like. They can be used also for 
the production of laminated materials such as an insulating spacer for 
power transistor, a magnetic head spacer, a power relay spacer, a 
transformer spacer and the like. They are also useful for the production 
of enamel coating materials for use in insulation coating of electric 
wires and cables, a solar battery, a low temperature storage tank, a space 
heat insulator, an integrated circuit, a slot liner and the like. They are 
also useful for the production of an ultrafiltration membrane, a reverse 
permeation membrane, a gas separation membrane and the like. They can also 
be used for the production of thread, woven fabric, non-woven fabric and 
the like having heat resistance.