Polyamide-imide and mica pulp particles and paper-like sheets made therefrom

Pulp particles consisting essentially of 50 to 90% by weight of mica particles and 50 to 10% by weight of polyamide-imide, said polyamide-imide forming a continuous phase in which said mica particles are dispersed discontinuously. A paper-like sheet comprising an integrated intimate mixture of 20 to 95% by weight of the pulp particles and 80 to 5% by weight of staple fibers of a thermally stable polymer.

This invention relates to pulp particles and a sheet made from a mixture of 
them with staple fibers by a paper-making process. 
Natural pulp particles have previously been best known as pulp particles 
for use in the production of paper. In recent years, pulp particles 
obtained from synthetic polymer have attracted attention as materials for 
electrical insulating paper because of their superior thermal stability 
and electrical insulation. 
For example, U.S. Pat. No. 2,999,788 discloses paper pulp particles 
composed of synthetic polymers. These paper pulp particles, however, have 
the defect that when they are processed and used as electrical insulating 
sheet, its impregnating ability, thermal resistance and flame resistance 
of the sheet are not sufficient. In rendering electrical machinery such as 
an electrically driven power generator small-sized and light in weight, an 
electrical insulating sheet having superior thermal resistance and 
insulating oil impregnating ability is required. The above paper pulp 
particles do not meet these requirements fully. 
On the other hand, British Patent Specification No. 1,129,097 discloses a 
high-temperature-resistant sheet-like structure suitable for use in 
electrical insulation comprising an intertwined mixture of particulate 
mica and substantially unfused aromatic polyamide fibrids. However, this 
sheet-like structure has insufficient thermal resistance and impregnating 
ability as an electrical insulating material for electric machinery and 
appliances which are to be rendered small-sized and light in weight. 
Furthermore, this prior art technique suffers from the defect that the 
paper-making operation meets with extreme difficulty because of the 
separation of the mica from the fibrids at the time of producing such a 
sheet-like structure. This sheet-like structure also has the defect that 
the mica is easily removed by rubbing. 
Further U.S. Pat. No. 3,080,272 discloses a sheet-like structure comprising 
a fused homogeneous waterleaf of synthetic organic polymer fibrids and 
inorganic flake-like material in which the fabrids form a continuous phase 
and mica is dispersed therein as the flake-like material. Likewise U.S. 
Pat. No. 3,523,061 discloses a porous sheet-like structure comprising 
rod-like staple fibers of heat fusible polymer such as polyethylene 
terephthalate and mica flakes. 
The sheet-like structures disclosed in these prior art references are 
however a similar type of the sheet-like structure disclosed in the 
aforementioned British Patent Specification No. 1,129,097, and are, 
therefore, not free similarly from various defects involved in said prior 
art technique. 
We have now found that pulp particles comprising specific polyamide-imide 
polymer and mica having a specific range of particle size are free from 
these defects of the prior methods, and instead have superior 
sheet-formability, impregnating ability, electric insulation, thermal 
resistance, flame resistance and resistance to removal. We have also found 
that a paper-like sheet produced on a paper-making machine from the pulp 
particles and staple fibers of a thermally stable polymer has excellent 
properties. 
According to the present invention, there are provided pulp particles 
consisting essentially of 50 to 90% by weight of mica particles having a 
particle size in a range of 60 - 3000 Tyler mesh and 50 to 10% by weight 
of a film-forming polyamide-imide at least 70 mol % of which recurring 
units consist of at least one or more of recurring units of the following 
formula 
##STR1## 
wherein X is at least one member selected from the group consisting of 
alkylene, alkylidene, cycloalkylene or cycloalkylidene having respectively 
1 - 6 carbon atoms, 
##STR2## 
in which R is an organic radical having 1 - 10 carbon atoms; Y and Y' may 
be same or different and each is at least one member selected from the 
group consisting of a hydrocarbon group having 1 - 6 carbon atoms, a 
halogen atom, an alkoxy group having 1 - 3 carbon atoms, an aryloxy group 
having 6 - 10 carbon atoms, a carbalkoxy group having 2 - 10 carbon atoms 
and an alkoxycarbonyl group having 1 - 5 carbon atoms; m and n may be same 
or different and each is a number from 0 to 3; in which said 
polyamide-imide forms a continuous phase and said mica particles are 
dispersed discontinuously in said continuous phase. 
According to the present invention, there is also provided a sheet 
comprising a hot-pressed, integrated intimate mixture of 20 to 95% by 
weight of the pulp particles described above and 80 to 5% by weight of 
staple fibers of a thermally stable polymer. 
Now the present invention will be explained further in detail. 
The feature of the pulp particles of the present invention resides in 
selection of specific polyamide-imide as a polymer component and of mica 
an inorganic filler. 
The mica used in this invention may be those available at low cost and 
having superior electric insulation, thermal stability and flame 
resistance, examples of which include muscovite, sericite, or phlogopite, 
biotite, etc. 
The particle size of the mica to be used in the present invention should be 
in a range of 60 - 3000 Tyler mesh. 
In the pulp particles of this invention, at least 70 mol %, preferably 85 
mol % or more of polyamide-imide as the polymer component should be 
composed of recurring units which are expressed by the following formula 
##STR3## 
in which X, Y, Y', m and n have the same meanings as defined already. 
Of those, especially preferred are --CH.sub.2 --, --O-- and --SO.sub.2 -- 
as X; a methyl group, a halogen atom and a methoxy group as Y and Y'; and 
0 or 1 as m and n. 
The polyamide-imide to be used in the present invention may also include 
not more than 30 mol %, preferably not more than 15 mol % of recurring 
units which are expressed by the following formula 
##STR4## 
in which R is an alkylene having 2 - 15 carbon atoms, 
##STR5## 
(Y has the same meaning as defined above), 
##STR6## 
Not more than 30 mol%, preferably not more than 15 mol% of recurring units 
of the polyamide-imide that is used in the present invention may contain 
nitrogen-containing polyheterocyclic compounds such as 
polyamidebenzimidazole, aromatic polyimide, etc. or aromatic polyamide to 
be described in later-appearing paragraphs. 
The polyamide-imide to be used in the present invention has film-forming 
ability and has a logarithmic viscosity (measured in a solvent of 
N-methyl-2-pyrrolidone at a concentration of 0.5 g/100 ml at 30.degree. 
C.) in a range of 0.35 - 2.0, preferably 0.6 - 0.9. In the pulp particles 
of this invention, the amount of the mica particles to be used should be 
50 to 90% by weight of the total amount of the pulp particles. If the 
amount is less than 50% by weight, the effect of the mica is not produced, 
and a paper-like sheet obtained from such pulp particles does not possess 
good thermal resistance, flame resistance, and impregnating ability. If, 
on the other hand, the amount exceeds 90% by weight, the mechanical 
properties of the resultant paper-like sheet, such as strength and 
elongation, are reduced. Preferably, the amount of the mica is 60 to 80% 
by weight, especially 60 to 70% by weight. 
The pulp particles of the present invention are produced by dissolving the 
polyamide-imide in a suitable solvent, adding mica particles to the 
solution, and introducing the resulting solution into a suitable 
precipitating agent to precipitate it as fine particles. At this time the 
precipitating agent is stirred at high speed; and the process is operated 
so that upon the elimination of the solvent from the solution, a shearing 
or beating action will be generated. The solvent used in this process is 
suitably a water-soluble solvent which dissolves the polymer component of 
the pulp particles but is inert to the mica. Examples of such solvents are 
inorganic solvents such as sulfuric acid, hydrofluoric acid, fuming 
sulfuric acid, chlorosulfuric acid, fluorosulfuric acid or polyphosphoric 
acid, and organic solvents such as N-methyl-2-pyrrolidone, 
N,N-dimethylformamide, N-N-dimethyl acetamide, dimethyl sulfoxide, 
hexamethyl phosphorylamide, or tetramethyl urea. These inorganic or 
organic solvents may be used in admixture. The solvent may also contain an 
inorganic salt such as lithium chloride or calcium chloride in order to 
increase its polymer solubilizing power. 
The concentration of the polymer in the solution varies according to the 
type and the degree of polymerization of the polymer, but is preferably 
about 2 to 15% by weight. 
The precipitating agent used in the above described method is desirably a 
liquid or solution which is miscible with the solvent of the polymer 
solution but is a non-solvent for the polymer. The type of the 
precipitating agent is selected according to the type of the solvent used. 
Where the organic solvent is used, the precipitating agent that can be 
used may be water, glycerine, ethylene glycol, a mixture of water with 
glycerine or ether, or may also be an aqueous solution containing at least 
one salt expressed by the formula MX.sub.n wherein M is Li, Na, K, Mg, Ca, 
Sr, Ba, Sn, Zn, Al or Ni, X is Cl, NO.sub.3, Br, CH.sub.3 COO or SCN, and 
n is an integer of 1 to 4. Examples of such salts are calcium chloride, 
lithium chloride, sodium chloride, magnesium chloride, zinc chloride, 
strontium chloride, aluminum chloride, stannic chloride, nickel chloride, 
calcium bromide, calcium nitrate, zinc nitrate, aluminum nitrate, sodium 
acetate, potassium thiocyanate, and calcium thiocyanate. Calcium chloride, 
lithium chloride, aluminum chloride, calcium thiocyanate and sodium 
acetate are especially suitable. Of these, an aqueous solution of calcium 
chloride is especially preferred because of its ease of handling. Of these 
various precipitating agents, aqueous systems are especially suitable. 
Where the inorganic solvent is used, that precipitating agent may be water 
alone or the inorganic solvent diluted with water. 
The pulp particles of this invention produced by the method described above 
have such a structure that a number of fine particles of mica are embedded 
in the finely dispersed state as a discontinuous phase in a continuous 
phase of the polymer. Accordingly, the mica particles have very good 
resistance to removal. Furthermore, as a number of sharp-edged concavities 
and convexities are present on the surface of the pulp particles, the 
intertwining of the pulp particles among one another is very good. By 
subjecting a mixture of these particles with staple fibers to a 
paper-making process, a paperlike sheet of good quality can be prepared. 
Furthermore, since the pulp particles of this invention comprises mica 
which is inexpensive and has superior electrical insulation, thermal 
resistance and flame resistance, the paper-like sheet obtained from such 
pulp particles and staple fibers has markedly improved electrical 
insulation, thermal resistance and flame resistance and is available at 
low cost. 
The formation of a sheet from a mixture of the pulp particles of this 
invention with staple fibers is preferably carried out by the wet process 
using a Fourdrinier machine or cylinder paper machine just as in the case 
of the conventional paper-making using natural pulp. 
It is necessary that the amount of the pulp particles be 20 to 95% by 
weight, preferably 20 to 80% by weight, more preferably 40 to 70% by 
weight, based on the total amount of the sheet. If the amount of the pulp 
particles is less than 20% by weight, the properties of the resulting 
sheet, such as dielectric breakdown voltage, strength or elongation, 
become poor. On the other hand, if the amount of the pulp particles 
exceeds 95% by weight, the resulting sheet has poor strength and 
elongation and impregnating ability with insulating oils. 
The staple fibers used in the preparation of the sheet of this invention 
need to be thermally resistant. Usually, it is desirable that such fibers 
have a denier of 0.5 to 10 and a length of 1 to 20 mm. Various polymers 
can be used to form such heat-resistant fibers. Examples of such fibers 
include: 
1. Staple fibers composed of the nitrogen-containing polyheterocyclic 
compounds. 
Examples of the nitrogen-containing polyhetrocyclic compounds are as 
follows: 
a. Aromatic polyamide-imides as cited above for use in producing the pulp 
particles. 
b. Aromatic polyamide-benzimidazoles containing units of the formula 
##STR7## 
wherein R is the same as defined above, and R' is hydrogen or a monovalent 
organic group. These polyamide-imidazoles may have an inert substituent 
such as a methyl group, alkoxy group or halogen atom. 
c. Aromatic polyimides containing units of the formula 
##STR8## 
wherein R is the same as defined above and Z represents at least one of 
divalent groups selected from 
##STR9## 
--SO.sub.2 --, and --O--. These polyimides may have an inert substituents 
such as a methyl group, alkoxy group or halogen atom. 
d. Polyazoles; such as polyoxazole, polyoxadiazole, polythiazole, 
polythiadiazole 
e. Polybenzazole, such as polybenzimidazole, polybenzthiazole, 
polybenzoxazole 
f. Polyhydantoin, polyparabaniacid, polyquinazolinedione, polyquinazolone 
g. Polyquinoxaline, polyoxadinone 
2. Staple fibers composed of aromatic polyamides. Examples of the aromatic 
polyamides are (a) condensed polyamides formed between dicarboxylic acids 
having aromatic rings, preferably high active derivatives thereof such as 
acid halides, and diamines having aromatic rings, such as a homopolymer of 
one dicarboxylic acid selected from terephthalic acid and isophthalic acid 
and one diamine selected from m-phenylene diamine, 4,4'-diaminodiphenyl 
ether, 4,4'-diaminodiphenylmethane, xylylene diamine and 
N-methyl-p-phenylene diamine, or a copolymer of at least two of said 
dicarboxylic acids and said diamines or of one dicarboxylic acid or 
diamine with at least two of the other component. Typical examples are 
polymethaphenylene isophthalamide, polymetaphenylene terephthalamide, and 
a copolymer of metaphenylene diamine, isophthalic acid and terephthalic 
acid; (b) polyamides obtained by polycondensation of aminocarboxylic acids 
having aromatic rings, preferably by activation prior to polycondensation 
such as homopolymers of para- or meta-aminobenzoic acid or 
para-aminomethylbenzoic acid, or copolymers of at least two of such 
aminocarboxylic acids. A typical example is a polycondensate of 
paraaminobenzoic acid; and (c) copolyamides obtained by copolymerization 
of (a) and (b). A typical example is a polyamide composed of metaphenylene 
diamine, isophthalic acid and paraaminobenzoyl chloride hydrochloride. 
3. Staple fibers composed of polyphenylene oxide or polyarylene oxides. 
4. Staple fibers composed of aromatic polyesters, examples of which are (a) 
polyethylene-2,6-naphthalate and polyethylene-2,7-naphthalate; (b) 
copolyesters containing at least 85 mol% of ethylene-2,6-naphthalate or 
ethylene-2,7-naphthalate units, those containing an aromatic dicarboxylic 
acid as an acid component being preferred; (c) mixed polyesters containing 
(i) polyethylene-2,6-naphthalate and/or polyethylene-2,7-naphthalate, 
and/or (ii) a copolyester containing at least 85 mol% of 
ethylene-2,6-naphthalate and/or ethylene-2,7-naphthalate; (d) polyethylene 
terephthalate; (e) copolyesters containing at least 85 mol% of ethylene 
terephthalate units, those containing an aromatic dicarboxylic acid as an 
acid component being preferred; and (f) mixed polyesters containing (i) 
polyethylene terephthalate and/or (ii) copolyesters containing at least 85 
mol% of ethylene terephthalate units. 
5. Staple fibers composed of inorganic compounds, such as glass fibers, 
asbestos fibers, rock wool, slug wool, fused silica fibers, glassy silica 
fibers, porcelain fibers, kaoline fibers, bauxite fibers, boron fibers, 
potassium titanate fibers, magnesia fibers, alumina whiskers, and silicon 
nitride whiskers. 
6. Natural fibers such as cellulose fibers, regenerated cellulose fibers, 
or cellulose acetate fibers. One or more kinds of the staple fibers may be 
used. Since the difference in specific gravity between the pulp particles 
and the staple fibers is small, they can be dispersed uniformly within the 
dispersion in the sheet-forming process using the pulp particles of this 
invention. This makes the sheet-forming operation easy, and gives a very 
uniform sheet. 
The resulting sheet is dried, and hot pressed using such a device as a hot 
press or hot roll to give a paper-like sheet of good quality. The 
temperature for hot pressing differs somewhat according to the 
crystallinity, the degree of polymerization, etc. of the pulp particles 
and the staple fibers, but is suitably between 140.degree. and 320.degree. 
C. The pressure also differs according to the crystallinity, the degree of 
polymerization, etc. of the pulp particles and the staple fibers, but is 
preferably not more than 200 Kg/cm.sup.2. 
In the paper-like sheet of this invention, fine pores are embraced in the 
fine texture of the paper-like sheet by means of a number of sharp-edged 
concavities and convexities present on the surfaces of the pulp particles. 
Accordingly, this sheet has a better impregnating ability with insulation 
oils than a paper-like sheet obtained from pulp particles composed only of 
synthetic polymers. 
Utilizing superior thermal resistance and electrical insulation, the sheet 
obtained from the pulp particles of this invention can be used effectively 
as an electrical insulating sheet, and also finds applications as building 
and structural materials utilizing its good flame resistance and 
mechanical properties. 
The methods of measuring the various properties are shown below. 
Logarithmic Viscosity (.eta. inh) 
Measured in N-methyl-2-pyrrolidone in a concentration of 0.5 g/100 ml. at 
30.degree. C. 
Dielectric Strength 
Measured in accordance with JIS C 2111 using an alternate current voltage. 
Flame Resistance 
The sample with a width of 12.5 mm is contacted with 2.5 cm flame of a 
Bunsen burner for 2 seconds. Then, the flame is brought away from it. When 
the sample is burned with flame, the flame resistance is evaluated as 
poor, and when it is not at all burnt, the flame resistance is evaluated 
as good. 
Impregnating Ability (Permeability) 
The sample cut in a circular shape with a diameter of 10 mm is made afloat 
on the surface of an insulating oil (JIS No. 1), and the time required 
until the insulating oil permeates into all over the surface of the sample 
is measured. 
Surface Strength of the Sheet 
Measured in accordance with the method of JIS P 8129.

The following Example will illustrate the present invention. 
EXAMPLE 1 
Production of Pulp Particles 
Trimellitic anhydride and 4,4'-diaminodiphenylmethane were subjected to a 
condensation reaction in a mol ratio of 2:1 to prepare a bis-imide, after 
which trimellitic anhydride and 4,4'-diphenylmethane diisocyanate were 
further added in a mol ratio of 2:3 to one mol of the 4,4'-diamino 
diphenylmethane to prepare a polyamide-imide. The resulting 
polyamide-imide has a logarithmic viscosity of 0.5 in 
N-methyl-2-pyrrolidone. 
Ten grams of the resulting polyamide-imide was dissolved in 90 g of 
N-methyl-2-pyrrolidone, and thereafter the resulting solution was mixed 
respectively with 2.5 g, 10 g, 23.3 g and 190 g of fine particles of 
sericite having a particle size within a range of 325 - 3000 Tyler mesh. 
Each of the resultant mixtures was fed into 1 liter of water placed in a 
homomixer agitated at high speed to form precipitates. The resulting pulp 
particles are designated Nos. 1, 2, 3 and 4. Pulp particles Nos. 2 and 3 
are within the scope of the present invention, whereas pulp particles Nos. 
1 and 4 are outside the scope of the present invention. 
For comparison, 10 g of the same polyamide-imide as above was dissolved in 
90 g of N-methyl-2-pyrrolidone. Without addition of mica, the solution was 
fed into one liter of water placed in a homomixer stirred at high speed to 
induce precipitation, and to produce pulp particles consisting only of the 
polyamide-imide. The resulting pulp particles were designated as No. 5. 
In the production of the pulp particles described above, the solution 
containing the precipitate was centrifuged. The pulp particles Nos. 3 and 
4 consisting predominantly of the sericite were found easier to separate 
than the pulp particle No. 5 which consisted only of the polyamide-imide, 
and could be separated in 6 and 2 minutes respectively at 5,200 rpm. In 
contrast, the pulp particle No. 5 did not separate fully even after 60 
minutes. 
Production of Sheet 
Each of the pulp particles Nos. 1 to 5 were fully washed, and dispersed in 
water to form an aqueous dispersion containing 1.4 g of the pulp 
particles. Separately, the same polyamide-imide as used to produce the 
pulp particles was dissolved in N-methyl-2-pyrrolidone to form a spinning 
solution. The spinning solution was wet spun into a coagulating bath at 
60.degree. C. comprising an 42 weight % aqueous solution of calcium 
chloride, drawn to 2.4 times in dry heat at 320.degree. C., and cut to a 
length of 5 mm to form staple fibers. Each of the above aqueous 
dispersions of the pulp particles was well mixed with an aqueous 
dispersion containing 0.6 g of the staple fibers, and the mixture was 
poured onto a stainless steel wire screen and subjected to a paper-making 
process to form a sheet. The resulting sheets were designated Nos. 1 to 5. 
Since the pulp particles and the staple fibers were very uniformly 
dispersed in the dispersions, the sheet-forming operation was easy. The 
sheets were dried at 100.degree. C., and hot pressed at 270.degree. C. 
and 200 Kg/cm.sup.2 to form paper-like sheets each having a thickness of 
about 100 microns. 
Various tests were performed on these paper-like sheets, and the results 
obtained are shown in Table 1. 
Table 1 
__________________________________________________________________________ 
Amount of 
Amount of Paper 
sericite pulp par- 
Weight ratio Tensile 
Dielec- Impreg- 
Color- 
in pulp ticles in 
in the sheet Sheet 
Tensile 
elonga- 
tric Flame 
nating 
ation 
Run 
particles 
the sheet Seri- form- 
strength 
tion strength 
resis- 
ability 
after 
Nos. 
(% wt.) 
(% wt.) 
Polymer 
cite 
Fibers 
ability 
(Kg/mm.sup.2) 
(%) (KV/mm) 
tance 
(min.) 
heating 
__________________________________________________________________________ 
1 20 70 56 14 30 Good 3.1 3.5 13 Poor 
30 Heavy 
2 50 70 35 35 30 Good 3.0 3.1 12 Good 
20 Slight 
3 70 70 21 49 30 Good 3.2 3.4 10 Good 
20 Slight 
4 95 70 3.5 66.5 
30 Good 1.5 2.0 8 Good 
10 Slight 
5 0 70 70 0 30 Good 2.7 2.0 15 Poor 
40 Heavy 
6 0 35 35 35 30 Poor 2.0 2.2 12 Good 
30 Slight 
__________________________________________________________________________ 
Nos. 2 and 3 were the sheets in accordance with the present invention, and 
exhibited good strength, elongation and dielectric strength. They also 
showed good flame resistance. In an impregnation test, an insulating oil 
permeated into all over the surfaces of these sheets in 20 minutes, and 
these sheets showed good impregnating ability. Furthermore, when these 
sheets were allowed to stand for 5 hours at 290.degree. C., their 
coloration was very slight. These sheets Nos. 2 and 3 had a surface 
strength of 18A, showing very good resistance to the removal of mica. 
In sheet No. 1, the amount of the gericite in the pulp particles was small. 
It exhibited good strength, elongation and dielectric strength, but poor 
flame resistance and impregnating ability. When left to stand for 5 hours 
at 290.degree. C., it was heavily colored owing to the heat. 
Sheet No. 4 has poor strength, elongation and dielectric strength because 
the amount of the polymer component in the pulp particles was too small. 
Sheet No. 5 which did not contain the sericite showed poor strength, 
elongation, flame resistance, impregnating ability and coloration (after 
heating at 290.degree. C. for 5 hours) as compared with sheets Nos. 2 and 
3. 
For comparison, sheet No. 6 was prepared, and tested. Sheet No. 6 was 
produced by preparing an aqueous dispersion containing the pulp particles 
composed only of the polymer component (used in the preparation of sheet 
No. 5), sericite and polyamide-imide staple fibers in a weight ratio of 
35:35:30, mixing the three components thoroughly in the aqueous 
dispersion, and subjecting it to a paper-making process using a 100 mesh 
stainless steel wire screen, followed by drying and hot pressing to form a 
paper-like sheet having a thickness of about 100 microns. In the 
preparation of sheet No. 6, the sericite precipitated rapidly because of 
its high specific gravity, and one surface of the sheet consisted almost 
entirely of the sericite. When the sheet was removed from the paper-making 
wire screen, part of the sericite remained spottingly on the wire screen. 
This undesirably resulted in poor sheet-formability. The resulting 
paper-like sheet No. 6 had good flame resistance, and dielectric strength, 
but poor strength, elongation and impregnating ability. This sheet had a 
surface strength of less than 2A, exhibiting a far poorer resistance to 
the removal of mica than the sheets Nos. 2 and 3 of the present invention. 
EXAMPLE 2 
A paper-like sheet (designated sheet No. 7) was produced in the same way as 
in Example 1 except that the pulp particles of No. 2 in Example 1 (the 
weight ratio of the polymer to the mica 1:1) were used, and the weight 
ratio of the pulp particles to the polyamide-imide staple fibers was 
changed to 1:9. The paper-like sheet obtained after hot pressing was 
tested, and the results are shown in Table 2. (Sheet No. 7 is outside the 
scope of the present invention.) 
Table 2 
__________________________________________________________________________ 
Amount of Amount of Paper 
sericite pulp par- 
Weight ratio Tensile 
Dielec- Impreg- 
Color- 
in pulp ticles in 
in the sheet Tensile 
elonga- 
tric Flame 
nating 
ation 
Run particles 
the sheet Seri- strength 
tion strength 
resis- 
ability 
after 
No. (% wt.) 
(% wt.) 
Polymer 
cite Fibers 
(kg/mm.sup.2) 
(%) (KV/mm) 
tance 
(min.) 
heating 
__________________________________________________________________________ 
7 50 10 5 5 90 1.0 2.0 5 Poor 15 Heavy 
__________________________________________________________________________ 
The material for sheet No. 7 had good sheet-formability, but the resulting 
sheet had poor dielectric strength, strength, elongation and flame 
resistance because of its small content of the pulp particles. 
EXAMPLE 3 
Ten grams of the same polyamide-imide as used in Example 1 was dissolved in 
90 g of N-methyl-2-pyrrolidone. The resulting solution was mixed with 15 g 
of sericite having a particle size of 325 - 3000 Tyler mesh. With other 
conditions being the same as in Example 1, pulp particles were produced. A 
mixture of the pulp particles and the same polyamideimide staple fibers 
having a length of 5 mm as used in Example 1 was formed into a sheet, 
which was then hot pressed to produce a paper-like sheet. The amount of 
the pulp particles in the sheet was changed to 85, 90 and 98% by weight, 
and three paper-like sheets designated Nos. 8, 9 and 10 were produced. 
These sheets were tested in the same way, and the results are shown in 
Table 3. Sheets Nos. 8 and 9 are within the scope of the present 
invention, and sheet No. 10 is outside the scope of the present invention. 
Table 3 
__________________________________________________________________________ 
Amount of Amount of Paper 
sericite pulp par- 
Weight ratio Tensile 
Dielec- Impreg- 
Color- 
in pulp ticles in 
in the sheet 
Sheet 
Tensile 
elonga- 
tric Flame 
nating 
ation 
Run particles 
the sheet Seri- form- 
strength 
tion strength 
resis- 
ability 
after 
Nos. 
(% wt.) 
(% wt.) 
Polymer 
cite 
Fibers 
ability 
(Kg/mm.sup.2) 
(%) (KV/mm) 
tance 
(min.) 
heating 
__________________________________________________________________________ 
8 60 85 34 51 15 Good 2.2 3 27 Good 20 Slight 
9 60 90 36 54 10 Good 2.0 3 32 Good 25 Slight 
10 60 98 39.2 
58.8 
2 Good 1.1 1 45 Good 40 Slight 
__________________________________________________________________________ 
Sheets Nos. 8 and 9 had good dielectric strength, flame resistance and 
impregnating ability, and their coloration after heating for 5 hours at 
290.degree. C. was slight. These sheets had a surface strength of 18A and 
the small fragments of the sericite did not separate, showing good 
resistance to the removal of mica. Sheet No. 10 (comparison) had good 
dielectric strength and flame resistance and slight coloration after 
heating for 5 hours at 290.degree. C., but because of its high content of 
the pulp particles, had poor strength, elongation and impregnating 
ability. 
EXAMPLE 4 
This Example shows that the use of mica as an inorganic filler to be 
included in pulp particles is a very important requirement, and if 
asbestos is used instead of the mica, the operability is remarkably 
reduced, and the resulting sheet has poor properties. 
Production of Pulp Particles 
Trimellitic anhydride and 4,4'-diaminodiphenylmethane were subjected to a 
condensation reaction in a mol ratio of 2:1 to prepare a bis-imide, after 
which trimellitic anhydride and 4,4'-diphenylmethane diisocyanate were 
further added in a mol ratio of 2:3 to one mol of the 4,4'-diamino 
diphenylmethane to prepare a polyamide-imide. The resulting 
polyamide-imide has a logarithmic viscosity of 0.8 in 
N-methyl-2-pyrrolidone. Five parts of the resulting polyamide-imide was 
dissolved in 95 parts of N-methyl-2-pyrrolidone to form a solution. The 
solution was mixed with each of mica and asbestos as a filler having a 
particle size of 100 - 1000 Tyler mesh in the ratios indicated in Table 4. 
Each of the mixtures obtained was put into a 42% wt. aqueous solution of 
calcium chloride placed in a homomixer agitated at high speed, to form 
pulp particles. The mixture containing the asbestos had very poor 
flowability as compared with the mixture containing the mica particles. 
Because of this the operability of producing pulp particles was reduced 
greatly. The results are shown in Table 4. 
Table 4 
______________________________________ 
Amount of 
Composition 
the filler 
of the in the pulp 
mixture particles Operability 
(parts) (wt. %) Mica Asbestos 
______________________________________ 
Polymer 5 The mixture had poor 
Filler 2.1 
30 Good flowability, leading to 
Solvent 95 difficult operation 
Polymer 5 The mixture had very 
Filler 3.8 
45 Good poor flowability, lead- 
Solvent 95 ing to very difficult 
operation 
Polymer 5 The mixture scarcely had 
Filler 7.5 
60 Good flowability, leading to 
Solvent 95 failure of operation 
______________________________________ 
The pulp particles were thoroughly washed, and dispersed in water. 
Furthermore, in the same way as in Example 1, the same polyamide-imide as 
used in the preparation of the pulp particles in the present Example was 
wet spun, drawn, and cut to a length of 4 mm to form staple fibers which 
were then dispersed in water to form an aqueous dispersion. This 
dispersion was mixed with the pulp particle aqueous dispersion, and 
subjected to a sheet-forming process in the same way as in Example 1. The 
resulting sheet was dried, and pressed at 230.degree. C. and 200 
Kg/cm.sup.2 to form a paper-like sheet. In the preparation of the sheet, 
the weight ratio of the pulp particles to the staple fibers was adjusted 
to 7:3. The resulting paper-like sheets were tested, and the results are 
shown in Table 5 below. 
Table 5 
______________________________________ 
Amount of 
filler in 
pulp particles Filler 
(wt. %) Test items Mica Asbestos 
______________________________________ 
Tensile strength (Kg/mm.sup.2) 
4.4 4.7 
30 Tensile elongation (%) 
9.4 6.6 
Dielectric strength (KV/mm) 
43 30 
Tensile strength (Kg/mm.sup.2) 
3.5 3.0 
43 Tensile elongation (%) 
7.9 4.8 
Dielectric strength (KV/mm) 
44 14 
Tensile strength (Kg/mm.sup.2) 
2.2 * 
60 Tensile elongation (%) 
5.0 
Dielectric strength (KV/mm) 
27 
______________________________________ 
*Sheet formation was not performed because it was impossible to produce 
pulp particles. 
The results of Table 5 clearly show that when asbestos is used as the 
filler, not only is the production of pulp particles difficult, but also 
sheets produced by using these pulp particles had poor physical properties 
such as tensile elongation and dielectric strength. 
EXAMPLE 5 
This Example intends to exemplify that particle size of the mica to be used 
as a filler plays an important role in the pulp particles of the present 
invention. 
Ten grams of the polyamide-imide used in Example 1 was dissolved in 90 g of 
N-methyl-2-pyrrolidone, and the resulting solution was mixed with 15 g 
each of the following muscovites having different particle sizes; 
muscovite (A): passing through 3000 Tyler mesh 
muscovite (B): passing through 60 Tyler mesh but not through 3000 Tyler 
mesh 
muscovite (C): not passing through 60 Tyler mesh 
Each of the resulting mixtures was fed into a 42% by weight aqueous 
solution of calcium chloride placed in a homomixer agitated at high speed 
to form pulp particles by precipitation. In this case, the mixed solutions 
using the muscovites (A) and (B) had excellent fluidity, but the mixed 
solution using the muscovite (C) had an inferior fluidity. 
Using three types of pulp particles so prepared and staple fibers of the 
polyamide-imide used in Example 1, paper-like sheets of a thickness of 
about 100 microns were prepared in the same way as in Example 1. 
Impregnating ability and dielectric strength measured on these paper-like 
sheets are shown in Table 6. 
Table 6 
__________________________________________________________________________ 
Amount of pulp 
Amount of 
particles in 
staple fibers 
Impregnating 
Dielectric 
Run Type of 
the sheet 
in the sheet 
ability 
strength 
No. muscovite 
(% wt) (%. wt) (min.) (KV/mm) 
__________________________________________________________________________ 
11 A 50 50 35 13 
12 B 50 50 10 15 
13 C 50 50 5 10 
14 A 70 30 35 19 
15 B 70 30 15 22 
16 C 70 30 3 15 
17 A 90 10 40 29 
18 B 90 10 17 40 
19 C 90 10 10 20 
20 A 98 2 1300 35 
21 B 98 2 60 50 
22 C 98 2 40 30 
__________________________________________________________________________ 
The resulting paper-like sheet using the muscovite (C) had a coase, uneven 
surface and poor dielectric strength, while the paper-like sheet using the 
muscovite (A) had an inferior impregnating ability. 
EXAMPLE 6 
This Example intends to exemplify that the use of the polyamide-imide as 
the polymer component plays an important role in the preparation of the 
pulp particles of the present invention. 
Run No. 23: The paper-like sheet (designated sheet No. 3 in Example 1) was 
left standing for 24 hours in an atmosphere kept at a temperature of 
25.degree. C. and relative humidity of 60%, and thereafter its moisture 
absorption was measured. The result was about 1.0%. The value of 
dielectric strength at a relative humidity of 60% was 11 (KV/mm). 
Run No. 24: Pulp particles were prepared in the same way as in No. 3 of 
Example 1 except that polymetaphenylene isophthalamide was used instead of 
the polyamide-imide. 
Separately, polymetaphenylene isophthalamide was wet spun, drawn to 2.5 
times in a boiling bath, drawn further to 1.4 times on a heat panel at 
300.degree. C. and then cut into a length of 5 mm of form staple fibers. 
Using the abovementioned pulp particles and the staple fibers, a paper-like 
sheet was prepared in the same way as in Example 1, and the resulting 
paper-like sheet was subjected to measurement of moisture absorption and 
dielectric strength in the same way as Run No. 23 mentioned above. As the 
results, the moisture absorption was about 2.6%, and the dielectric 
strength was 7 (KV/mm). 
Run No. 25: Pulp particles were prepared in the same way as in No. 3 of 
Example 1 except that a copolyamide consisting of 80% by weight of 
caprolactam unit and 20% by weight of adipamide unit was used instead of 
the polyamide-imide, and also a calcium chloride-methanol solution was 
used instead of N-methyl-2-pyrrolidone as the solvent. Using the resulting 
pulp particles and staple fibers (2 denier, 5 mm length) of the 
above-mentioned copolymer, a paper-like sheet was prepared in the same way 
as in Example 1, and the resulting paper-like sheet was subjected to 
measurement of moisture absorption and dielectric strength in the same way 
as Run No. 23 mentioned above. As the result, the moisture absorption was 
about 2.0%, and the dielectric strength was 3 (KV/mm). 
From the results of the abovementioned Experiments Run Nos. 23-25, it can 
be seen that pulp particles prepared from polyamide-imide provide a 
paper-like sheet which has a smaller moisture absorption ratio and 
superior dielectric strength when compared to pulp particles prepared from 
polyamide. 
EXAMPLE 7 
Five grams of a copolyamide consisting of 80% by weight of caprolactam unit 
and 20% by weight of adipamide unit was dissolved in a solvent consisting 
of 6 g of calcium chloride and 89 g of methanol, and the resulting 
solution was added by 7.5 g of miscovite having a particle size in a range 
of 60 - 1000 Tyler mesh and mixed uniformly, after which the solution was 
fed into a 42% by weight calcium chloride aqueous solution placed in a 
homomixer stirred at a high speed to prepared pulp particles. 
Separately, staple fibers (2 denier, 5 mm length) were prepared from the 
abovementioned copolymer polyamide. Using the staple fibers thus prepared 
and the pulp particles prepared as above in the proportion of 30:70 (Run 
No. 26) and also 10:90 (Run No. 27), sheet were prepared on a stainless 
steel wire screen in a paper-making process in the same way as in Example 
1. The sheets were dried at 100.degree. C., and hot pressed at 200.degree. 
C. and 200 Kg/cm.sup.2 to form paper-like sheets. 
Separately pulp particles of this invention were prepared in the same way 
as in the above experiment except that the solution of the copolyamide in 
the calcium chloride-methanol solvent was replaced by a solution which was 
obtained by dissolving 5 g of the polyamide used in Example 1 in 95 g of 
N-methyl-2-pyrrolidone. Using the pulp particles so obtained and staple 
fibers (2 denier, 5 mm length) of the polyamide-imide in the proportion of 
70:30 (Run No. 28) and also of 90:10 (Run No. 29), sheets were prepared in 
the same way as above, followed by heat-drying at 100.degree. C. and 
hot-pressing at 200.degree. C. and 200 Kg/cm.sup.2. 
Various tests were performed on these four types of paper-like sheets, and 
the results obtained are shown in Table 7. 
Table 7 
__________________________________________________________________________ 
Amount of pulp 
Amount of 
Type particles in 
staple fibers 
Tensile 
Tensile 
Impregnating 
Dielectric 
Run 
of the sheet 
in the sheet 
strength 
elongation 
ability 
strength 
No. 
polymer 
(wt. %) (wt. %) (Kg/cm.sup.2) 
(%) (min.) (KV/mm) 
__________________________________________________________________________ 
26 copolymer 
polyamide 
70 30 1.5 13.3 18 5 
27 " 90 10 1.5 3.2 45 7 
28 polyamide- 
70 30 3.0 7.0 10 20 
imide 
29 " 90 10 1.9 3.0 20 54 
__________________________________________________________________________ 
The abovementioned four types of the paper-like sheets were left standing 
for one hour at 250.degree. C. and thereafter subjected to various tests. 
The results are shown in Table 8. 
Table 8 
______________________________________ 
Color- 
Tensile Tensile Dielectric 
Bending 
ation 
Run strength elongation 
strength 
Time* after 
No. (Kg/mm.sup.2) 
(%) (KV/mm) (times) 
heating 
______________________________________ 
26 0.4 1.5 6 3 heavy 
27 0.1 2.4 3 3 heavy 
28 2.0 5.0 19 &gt;100 slight 
29 2.4 2.4 50 &gt; 100 slight 
______________________________________ 
*Bending time: bending times required till the sample was cut off by 
bending it to 180.degree.. 
From Table 8, it can be seen clearly that the paper-like sheets prepared 
from the pulp particles of the present invention have extremely superior 
dielectric strength and thermal resistance in comparison with the 
paper-like sheets prepared from the pulp particles which are prepared from 
caprolactamhexamethyleneadipamide copolyamide and muscovite. 
EXAMPLE 8 
Using an aqueous dispersion containing 1.4 g of the pulp particles of the 
present invention used in Example 7 comprising polyamide-imide and 
muscovite, three types of sheets were produced in a paper-making process 
from three type of aqueous dispersions which contain the following staple 
fibers prepared by ordinary methods; 
i. an aqueous dispersion containing 0.6 g of polyethylene terephthalate 
staple fibers (2 denier, 5 mm length); 
ii. an aqueous dispersion containing 0.6 g of polyethylene-2,6-naphthalate 
staple fibers (2 denier, 5 mm length); and 
iii. an aqueous dispersion containing 0.6 g of polymetaphenylene 
isophthalamide staple fibers (2 denier, 5 mm length). 
The sheets so produced were dried and then not pressed at 230.degree. C. 
and 200 kg/cm.sup.2 to form paper-like sheets. The resulting paper-like 
sheets were tested, and the results are shown in Table 9 below. 
Table 9 
__________________________________________________________________________ 
Staple Tensile 
Tensile 
Impregnating 
Dielectric 
Run 
fiber strength 
elongation 
ability 
strength 
No. 
used (Kg/mm.sup.2) 
(%) (min.) (IV/mm) 
__________________________________________________________________________ 
30 polyethylene 
terephthalate 
2.5 3.0 4 20 
31 polyethylene- 
2,6-naphthalate 
3.1 4.3 2 20 
32 polymetaphenylene 
isophthalamide 
2.9 4.3 20 20 
__________________________________________________________________________ 
These paper-like sheets were left standing for 1 week at 250.degree. C., 
and thereafter subjected to various tests. The results of the tests are 
shown in Table 10. 
Table 10 
______________________________________ 
Tensile Tensile Dielectric 
strength elongation strength 
Run No. (Kg/mm.sup.2) 
(%) (KV/mm) 
______________________________________ 
30 1.0 1.0 18 
31 2.8 3.0 18 
32 2.5 3.0 18 
______________________________________ 
EXAMPLE 9 
Using an aqueous dispersion containing 1.4 g of the pulp particles of the 
present invention used in Example 7 comprising polyamide-imide and 
muscovite, a sheet was prepared in a paper-making process from an aqueous 
dispersion containing 0.6 g of an ordinary glass fiber (diameter 5 micron, 
5 mm length). The sheet so produced was dried and then hot pressed at 
230.degree. C. and 200 kg/cm.sup.2 to form a paper-like sheet. The 
resulting paper-like sheet was tested, and the results are shown in Table 
11. 
Table 11 
______________________________________ 
Tensile Tensile Impregnating 
Dielectric 
Run strength elongation 
ability strength 
No. (Kg/mm.sup.2) 
(%) (min.) (KV/mm) 
______________________________________ 
33 3.0 3.0 2 20 
______________________________________ 
This paper-like sheet was left standing for 1 week at 250.degree. C. and 
subjected to various tests are shown in Table 12. 
Table 12 
______________________________________ 
Tensile Tensile Dielectric 
strength elongation strength 
Run No. (Kg/mm.sup.2) 
(%) (KV/mm) 
______________________________________ 
30 3.0 2.0 20 
______________________________________