CA1067244A - Process for producing pulp-forming particles and synthetic paper-like sheets made therefrom - Google Patents

Abstract

Abstract of the Disclosure A process for preparing pulp-forming particles comprises adding a solution in an aqueous organic solvent of a thermally stable aromatic nitro-gen-containing polymer selected from aromatic polyamides and nitrogen-containing polyheterocyclic compounds to a precipitant to form a dispersion containing pulp-forming particles of the polymer. The precipitant consists of water and an amide-type solvent for the polymer present in selected ranges depending on the polymer used. The pulp-forming particles are heated to 30 to 90°C in the presence of at least 2 parts by weight ofa mixture of aqueous organic solvent and precipitant, per part by weight of the pulp-forming particles.
The resulting pulp-forming particles can be formed into synthetic paper by combination with thermally stable staple fibers. The synthetic paper has superior thermal stability, impregnating ability, texture uniformity and electrical insulation.

Description

1067Z4~

FIELD OF THE INVEN~ION
This invention relates to a new process for producing pulp-forming particles of a thermally stable ~ynthetic polymer having very good sheet-formability, and a synthetic paper-like sheet prepared therefrom and - having superior thermal stability, impregnating ability, texture uniformity and electrical insulation.
BAC~GROUND OF THE INVENTIO~
Wood pulp has been best known as pulp for paper-making, and a great maJority of electrically insulating - sheets heretofore in use are papers from wood pu~p. ~he wood pulp papers, however, have a serious defect of hav-ing poor thermal stability, and their thermal stability is far from that required for reducing the size and weight of electric machinery such as motors or trans-formers.
Recently, pulp-forming particles of synthetic pol~mers have attracted much attention as materials for electrically insulating sheets because of their superior

2~ thermal stability and electrical insulation. For example, United States Patent 2,999,788 discloses pulp-forming particles composed of a synthetic polymer. A sheet formed : from the pulp-forming particles disclosed in this patent, however, is unsuitable as an electrical insulating sheet 2~ because of its insufficient impregnating abilit~. In G/~ ~o~ye J addition, since water drainin6 from a sheet-forming screen . . .
is poor at the time of sheet forming, the resulting sheet has a no~-uniform texture. It is difficult therefore to 29 obtain a sheet having a uniform thic~ness, and the resulting sheet has insufficient electriGal insulation. Especially when it is desired to reduce the size and weight of electric machinery such as motors and transforemers, the electricahy insulating sheet used should have not only superior thermal stability and impregnating ability, but also a uniform texture. These requirements cannot be sufficiently met when the pulp-forming particles disclosed in the above patent are used.
British Patent Specification No. 1,129,097 discloses a high tempera~ure-resistant structure suit-able for electrical insulation which is composed of an intertwined mixture of mica particles and substantially unfused aromatic polyamide fibrids. However, the aromatic polyamide fibrids used in this patent cause poor water drainage from the sheet-forming screen, and cannot afford a sheet of uniform texture. - -It is an obaect of this invention to provide pulp-forming particles which have good sheet-formability and permit good water drainage from a sheet-forming screen, and can afford synthetic paper-like sheets having a good uniformity of texture and superior thermal stability, im-pregnating ability and electrical insulation.
SUMMARY 0~ THE INVENTION
~he present invention provides a process for preparing pulp-forming particles, which comprises adding a solution of a thermally stable aromatic nitrogen-containing polymer selected from the group consisting of aromatic polyamides and nitrogen-containing polyhetero-cyclic compounds in an aqueous organic solvent consisting ~067Z44 of an organic solvent and water to a precipitant consisting of an amide-type solvent for the polymer and water with stirring, there-by to form a dispersion containing pulp-forming particles of said polymer, the water content of the polymer solution being 1 to 10%
by weight based on the total amount of the aqueous organic sol-vent, and the polymer and the concentration of said amide-type solvent in the precipitant being 15 to 48% by weight when the poly-mer is the aromatic polyamide, and 40 to 75% by weight when the polymer is the nitrogen-containing polyheterocyclic compound; and heating the resulting pulp particles to 30 - 90C in the presence of at least 2 parts by weight, per part by weight of the pulp particles, of said aqueous solvent and said precipitant.
The invention further provides a synthetic paper-like sheet composed of the pulp-forming particles prepared by the above process and thermally stable staple fibers.
DETAILED DESCRIPTION OF THE INVENTION
A method has already been known to provide pulp-forming particles by adding a solution of a synthetic polymer in an organic solvent to a precipitant with stirring. However, the pulp-forming particles produced by the conventional methods have the defect that water drainage from the sheet-forming screen is poor, and a sheet of uniorm texture is difficult to obtain (in the present applica-tion, this defect is expressed as "poor sheet-formability"). We have unexpectedly found that the presence of a specified amount ~1 to 10% by weight~ of water ~n the polymer solution markedly 106729~4 improves the sheet-formability of the pulp-forming particles and the electrically insulating properties of synthetic paper-like sheets prepared from the pulp-forming particles. It has generally been thought that the presence of water in a solution of a polymer in an organic solvent should desirably be avoided since it causes a reduc-tion in the solubility of the polymer and results in an unstable solution. In view of this fact, it is surprising that the superior advantage mentioned above can be obtained by using an aqueous organic solvent containing a fairly large amount of water.
When the water content of the solution is less then 1% by weight there is insufficient effect of the water addition, and when it exceeds 10% by weight, the solution becomes exceedingly unstable.
The preferred water content of the solution is 3 to 9% by weight.
We have further found that a proper choice of the preci-pitant markedly improves the sheet-formability of the pulp-forming particles and the electrical insulating properties of synthetic paper-like sheets made therefrom. Based on this finding, we knew that when the polymer used is the aromatic polyamide, a precipi-tant composed of water and 15 to 48% by weight, preferably 30 to 45%
by weight, of the amide-type solvent should be used, and when the polymer used is the nitrogen-containing polyheterocyclic compound, a precipitant composed of water and 40 to 75% by weightJ preferably 60 to 70% by weight, of the amide-type solvent should be used.

When the concentration of the amide-type solvent in the precipi-tant is smaller than the lower limit of the above concentration range, the resulting pulp-forming particles become coarser and larger rod-like particles, and their sheet-formability is very much deteriorated. Consequently, synthetic paper-like sheets made from these particles have poor electrical insulation. On the other hand, when the concentration of the amide-type solvent exceeds the upper limit of the above-specified range, the pulp-forming particles flocculate into a mass, and their sheet-formability is exceedingly reduced. Thus, synthetic paper-like sheets prepared from these pulp-forming particles have po0r electrical insulation.
We have further ~ound for the first time that when pulp-forming particles obtained by adding the polymer solution to the stirred precipitant to precipitate the polymer are heated to a temperature of 30 to 90C, preferably to 35 to 70C, in the presence of at least 2 parts by weight, preferably at least 5 parts by weight, more preferably at least 30 parts by weight, per part by weight of the pulp-forming particles ~solid), of the aqueous organic solvent and precipitant, the sheet-formability of the pulp-forming part-icles can be markedly enhanced. The upper limit of the total amount of the aqueous organic solvent and the precipitant to be present is not critical. However, the lower limit of this amount is critical, and when the amount of the aqueous organic solvent and the precipitant is less then 2 parts by weight per part by weight of the pulp-forming particles, the heat-treatment renders the pulp-forming particles leather-like or massy, and makes subsequent operations such as washing, beating and sheet-forming troublesome. The heat-treatment tempera-ture is also important. When the temperature is lower than 30 C, sheet-formability is poor. With increasing treatment temperature the required heat-treatment time becomes correspondingly reduced, but when the temperature exceeds 90C, the appropriate treatment time becomes so short that a uniform quality of the pulp-forming particles cannot be maintained.
The present invention is based on the above three new discoveries, and one of the most important advzntages of the present invention is to provide pulp-forming particles having superior sheet-formability. The mean specific filtration resistance value measured by the method to be described is a very good measure for evaluating the sheet-formability of the pulp-forming particles. When the pulp-forming particles have a mean specific filtration resistance of 5 x 108 to 100 x 108 cm/g, they have acceptable sheet-formability. When the mean specific filtration resistance exceeds 100 x 10 cm/g, water drainage from the sheet-forming screen becomes poor, and it is difficult to obtain sheets having a very uniform texture. On the other hand, when it is smaller than 5 x 108 cm/g, water draining is too fast and the paper texture is deteriorated.
The invention is described in greater detail below.
Thermally stable aromatic nitrogen-containing polymer The aromatic polymer used as a starting polymer in this invention means a polymer in which a considerable portion of the main chain is composed ~067Z44 of an aromatic ring. The thermally stable polymer used as a starting polymer means a polymer which has a softening point of at least 155 C, preferably at least 250C, and in respect of which properties, such as tensile strength, elongation, dielectric constant, degree of polymerization and color, will not undergo great change on exposure to temperatures of at least 155C, preferably at least 180C, in air for long periods of time. The thermally stable aromatic nitrogen-containing polymer used in this invention is either an aromatic polyamide or a nitrogen-containing polyheterocyclic compound.
Preferably, the polymer used in the present invention is a polymer which has a solubility of at least 3% by weight, preferably at least 5% by weight, in organic solvents at room temperature and forms a stable solution. The aromatic polyamides and nitrogen-containing polyheterocyclic compounds are known, and examples of the thermally stable aromatic nitrogen-containing polymers used in the present invention are shown below.
1. Aromatic polyamides (1) Polyamides formed by condensation between dicarboxylic acids con-taining an aromatic ring (preferably, highly active derivatives such as acid halides) and diamines containing an aromatic ring. For example, they are homopolymers derived from one kind of dicarboxylic acid such as terephthalic acid or isophthalic acid and one kind of diamine such as m-phenylenediamine, 4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl methane or xylylene diamine, or copolymers derived from the dicarbo-xylic acid component and the diamine component either one or both of which consist of at least two kinds of compounds.
Typical examples include poly(m-phenylene iso-phthalamide), poly(m-xylylenediamine terephthalamide)? poly (N-methyl p-phenylene terephthalamide), and a copolyamide of m-phen~lene diamine, isophthalic acid and terephthalic acid.
1~ (2) Polyamides obtained by condensing aminocarboxylic acids containing an aromatic ring preferably after they have been activated. ~hey may be homopolymers derived from one kind of aminocarboxylic acid such as p- or m- ~
aminobenzoic acid, or p--aminomethylbenzoic acid, or copoly-mers derived from at least two kinds of the aminocarboxylic acid. A condensation product of p-aminobenzoic acid is a typical example~ - -

(3) Copolyamides obtained by copolymerizing (1) and (2) above. A polyamide obtained by condensing m-phenylene-diamine, isophthaloyl chloride and p-aminobenzoyl chloride hydrochloride.
2. ~itrogen-containing polyheterocyclic compounds (1) Aromatic polyamideimides Polyamideimides containing a recurring unit of the following formula -HN-C- ~ -C~N
--C' O

9 ~

1067Z4~

wherein R is ~ or - ~ - X -O
- in which X is -0-, -S02-, -C--, or a lower alkylene groupO
The polyamideimides may contain an inert substituent such as a methyl group, an alkoxy group or a halogen atom.
(2) Aromatic polyamide hydrazides containing a Fecurring unit of the following formula O O
" "
- C - ~ - C - NE~H_ These ~olymers may contain an inert substituent such as a methyl group, an alkoxy group or a halogen atom (3) Aromatic polyamide imidazoles containing a recurring unit of the following formula .

H~- C - ~ ~C - R -wherein R is the same as defined in (l) above.
These polymers may contain an inert substituent such as a methyl group, an alkoxy group or a halogen atom.

(4) Aromatic polyimides containing a recurring unit of the following formula O O
Il 11 ~C ~ , C\

`C '~/~C~
Il 11 .
'O O

~06724~ .

wherein R is the same as defined in (1) above.
These polymers may contain an inert substitu~nt such as - a methyl group, an alXoxy group or a halogen atom.

(5) Aromatic polyazoles ~xamples are polybenzimidazole, polybenzoxazole and polybenzothiazole. These polymers may contain an inert substituent such as a methyl group, an alkoxy group or a halogen atom.
R ~70/y~e~ o ~ c~ Z.~ o ~7 ~LJ (6) Polyguinazolinedione, pol~bcn~o~adi~one, pGlyquinazolone, and polyquinoxaline.
(7) Polythiazole, polyoxazole, polyoxadiazole, polyhydantoin, and polyparabanic acid.
(8) Polymers containing a recurring unlt of - the following formula -- O O
H~-- C - ~ C - NH - R -C - OH
O
wherein R is the same as defined i~ (1) aboveO
These polymers are precursors of polyamideimidesO
(9) Aromatic polyhydrazides and aromatic polyurea. The compounds (8) and (9) are considered as precursors, but are also included within the definition of the nitrogen-containing polyheterocyclic compounds used in the present inventionO It is also possible to use polymers obtained by copolymerizing the precursor (8) or (9) with an aromatic dicarboxylic acid such as iSophthalic acld or terephthalic acid, benzophenonetetra-- 106724~

carboxylic anhydride, or pyromellitic anhydride.
Preferred aromatic polyamides used in the present invention are poly(m phenylene isophthalamide) and poly(m-phenylene isophthalamide terephthalamide) 9 and preferred nitrogen-containing polyheterocyclic compounds are aromatic polyamideimides~ Polyamide-imides containing a recurring unit of the following formNla O O
~ C -,~,_ C ~
~ C ,N - R
- _ O

wherein R is -- ~ - X -- ~ - in which X is a lower alky]ene group, preferably a methylene grou~, are especially preferredO
PolYmer solutions In the process of this invention, the above thermally stable aromatic nitrogen-containing polymer is dissolved in an agueous organic solvent consisting of an organic solvent and water to form a polymer solu-tion. The water content of the polymer solution should be 1 to 10% by weight, preferably 3 to ~/0 by weight, based on the total amount of th-e polymer, organic solvent and waterO Suit~ble organic solvents include N-methyl-2-pyrrolidone~ N,N-dimethyl formamide, N,N-dimethyl acetamide, dimethyl sulfoxi~e, hexamethyl phosphorylamide-, and tetramethylurea. These solvents can-be used either singly or in admixture of two or moreO

If desired, an inorganic salt such as calcium chloride or lithium chloride may be added to the above organic solvent to increase its polymer solubilizing powerO
~he polymer conc~ntration in the solution differs according to the type and the degree of polymer-ization of the polymer, but generally, ranges from 2 to - 15% by weightO
It is not altogether necessary to add to the solution a finely divided solid inorganic substance which does not substantially react with the solution and is insoluble in the solutionO This is however preferred in order to further improve the electrical insulation and thermal stability of sheets made from the resulting polymer solutionO Micas having good electrical insula-- tion and thermal stability are preferred as the solid inorganic substanceO Spherical flaky substances such as asbestos, glass flakes, quartz powder, talc, kaolin, alumina, and calcium sulfate are also preferred. The amou~t of the solid inorganic substance that may be added is 50 to 900/0 by weight, preferably 100 to 400% by weight, based on the weight of the polymerO
When the solid inorganic substance is to be added to the polymer solution, it is preferred to dis-perse it therein as uniformly as possibleO ~xamples of preferred devices used for this purpose are an Attritor (a product of Mitsui Miike Seisakusho COD~ Ltdo) and a T.K. Homomixer (a product of Tokushu Eika Kogyo CoO, Ltdo)o Various methods c~n be used in the present in-vention-for including water in the polymer solutionO

For example, it is preferred to employ a method which comprises mixing the solvent with a required amount of water, adding the polymer to the mixture, and stirring the entire mixture to form a solution. The polymer to be added to the mixture may be in the form of a solid such as a powder or granule, or a highly concentrated solution. When the polymer is a solid, it is possible to cause a part or the whole of the required amount of water to be absorbed by or adhere to the polymer, and then to mix the polymer with the solvent.
When the polymer is obtained by solution polymerization, a part or the whole of the required amount of water may be added during or before the polymerization so long as it does not adver-sely affect the polymerization.
The required amount of water may also be added after dissolving the polymer in the solvent to a certain polymer concen-tration. When the addition of water causes a local precipitation of the polymer, it is necessary to dissolve the polymer completely in the solvent by a proper measure such as the prolongation of the mixing-stirring time or the heating of the solution.
As previously stated, the presence of a predetermined water in the polymer solution offers an advantage of improving the sheet-formability of the resulting pulp-forming particles and the electrical insulation of the resulting sheet. Since the polymer solution used in the process of this invention contains a fairly large amount of water, the water content of the polymer solution does not appreciably change by mois~re absorption in subsequent handling of the polymer solution. This offers another advantage that the ` 1067Z44 degree of undersirable fluctuations in the quality of pulp-forming particles caused by such fluctuations in the water content of the polymer solution is far smaller than in a conventional process using a substantially anhydrous polymer solution, and therefore, the process steps can be very easily controlled.
Preparation of the pulp-forming-~articles When the polymer solution prepared as mentioned above is added to a stirred precipitant composed of an amide-type solvent for the polymer and water, the polymer precipitates to yield pulp-forming particles. The concentration of the amide-type solvent in the precipitant is 15 to 48% by weight, preferably 30 to 45% by weight, when the polymer is the aromatic polyamide, and 40 to 75%
by weight, preferably 60 to 70% by weight, when the polymer is the nitrogen-containing polyheterocyclic compound. Preferably, the polymer solution is added while the precipitant is stirred at high speed to bring about a shearing action or beating action on the polymer solution and simultaneously to extract the solvent from it.
The amide-type solvent used in the present invention includes, for example, N,N-dimethyl formamide, N,N-dimethyl acetamide, N-methyl-2-pyrrolidone, N-acetyl-pyrrolidone, N-methyl-caprolactam, N-acetyl-caprolactam, hexamethyl-phosphorylamide, and tetramethylurea.
Of these, a solvent composed mainly of N-methyl-2-pyrrolidone is preferred.
In the process of this invention, it is preferred to use the same amide-type solvent to dissolve the polymer and for forming the precipitant.

~067Z44 Heat-treatment of pulp-orming partlcles The pulp-forming particles so prepared are then heat-treated at a temperature of 30 to 90C, preferably 35 to 70C, in the presence of at least 2 parts by weight, preferably at least 5 parts by weight, more preferably at least 30 parts by weight, per part by weight of the pulp-forming particles (solid content), of the aqueous organic solvent and precipitant used. The pulp-forming particles may be heat-treated either as dispersed in liquid or as fabricated into a web form. For example, when the polymer solution is added to a stirred precipitant in a preciptator, the polymer precipitates to yield pulp-forming particles. Since these pulp-forming particles are dispersed in~a liquid composed of the aqueous organic solvent and the precipitant and the dispersion has sufficient flowability, it can be directly transferred to a heater such as a double-walled heater and continuously heat-treated there. AlternativelyJ
the dispersion can be transferred to a tank equipped with a heating device, and heat-treated there batchwise. When, on the other hand, the amount of the aqueous solvent and precipitant is relatively small and a dispersion having flowability cannot be obtained (for example, when a greater portion of the liquid is separated by filtration of the above dispersion), the pulp-forming particles, to which the remainder of the liquid has adhered, are processed into a web form, and the resulting web-like pulp-forming particles having adhered there-to the aqueous organic solvent and the precipitant are passed ~067Z44 ..

between heated rolls to heat-treat them. Suitable heat-treatment times differ according to the type of the polymer? the type of the precipitant, and particularly the heating temperature~ But the heat-treatment time is usually several seconds to several hours. Generally, the heat treatment time is preferably shorter with higher - heat-treating temperaturesO The optimum heat-treatment time should be selected so that a preferred mean specific filtration resistance valùe can be obtained. This can be easily determined experimentally by those skilled in the art.
Production_of ~y~nthetic ~a~er-like sheets from ~ul~-formin~
particles Paper-like sheets of excellent quality can be obtained by subjecting a mixture of the pulp-forming par-ticles prepared by the process of this invention with thermally stable staple fibers to a sheet-forming processO
- Conveniently, the sheet formation from this mixture is - carried out by the wet method using a paper machine of the ~ourdrinier or cylinder type as in conventional-papermaking from wood pulpo The staple fibers may be of any thermally stable staple fibers, and include, for example, the follo~lingO
(1) Staple fibers of the same aromatic poly-amides as hereinbefore described~
(2) ~taple fibers of the same nitrogen-containing polyheterocyclic compounds.
(3) Staple fibers of the same precursors of nitrogen-containing polyheterocyclic-compounds as described .

hereinabove (the polymers ~8) and (9) mentioned above under paragraph 2).
(4) Staple fibers of polyphenylene oxide or polyarylene oxides.
~ 5) Staple fibers of aromatic polyesters.
Examples of the aromatic polyesters are as follows:
(a) Polyethylene-2,6-naphthalate and/or polyethylene-2,7-naphthalate.
(b) Copolyesters containing at least 85 mole% of an ethylene-2,6-naphthalate unit and/or an ethylene-217-naphthalate unit, preferably copolyesters using an aromatic dicarboxylic acid as an acid component.
(c) (i) Mixed polyesters containing polyethylene-2,6-naphthalate and/or polyethylene-2,7-naphthalate.
(ii) Mixed polyesters containing a copolyester containing at least 85 mole% of an ethylene-2,6-naphthalate and/or an ethylene-2,7-naphthalate unit.
(d) Polyethylene terephthalate.
(e) Copolyesters containing at least 85 mole% of an ethylene terephthalate unit, preferably copolyesters using an aromatic dicar-boxylic acid as an acid component.
(f) (i) Mixed polyesters containing polyethylene terephthalate.
(ii) Mixed polyesters containing a copolyester containing at least 85 mole% of an ethylene terephthalate unit.

(6) Staple fibers of inorganic compounds, such as glass fibers, asbestos fibers, rock wool, slag wool, ~067Z~14 . .
fused silica fibers, bauxite fibers, boron fibers, potassium titanate fibers, magnesia fibers, and whiskers of alumina - or silicon nitride.

(7) ~atural fibers such as cellulose fibers, regenerated cellulose fibers, and cellulose acetate fibers.
0f the above staple fibers, fibers from polymers may be made of the same or different polymer as or from that which constitutes the polymer solutionO
The denier size of each of the individual staple fibers is 0. 5 to 10 denier, preferably lo 5 to 300 denierO
The length of the staple fibers differs according to the single fiber denier, but is generally 1 to 10 mm, prefer-ably 3 to 8 mmO
The sheet thus obtained from the pulp forming 15 particles of this invention contains the pulp--forming particles in an amount of 20 to 95% by weight, preferably 40 to 60% by weight, based on the weight of the sheetO
When the amount of the pulp-forming particles is less than 20% by weight, the properties of the sheet, such as dielectric strength, tensile strength and elongation become poorO When the amount of the pulp-forming particles is larger than 95/0 by weight, the impregnating ability, tensile strength and elongation of the sheet become poorO
The wet sheet prepared is dried, and heated 25 under pressure by means of a-hot press or heated rolls, etc. to afford a synthetic paper-like sheet cf excellent quality. ~he heating temperature somewhat differs accord-ing, for example, to the crystallinity and the degree of polymerization of the pulp--forming particles and staple fibers, but suitable heating temperatures are llO to 320C.
At temperatures below 110C, the pressing is insufficient and a tough sheet cannot be obtained. On the other hand, at temperatures higher than ~20C, the polymer portions of the pulp-forming particles completely melt-adhere to one another and become film~like, and the sheet obtained lacks flexibility. ~hus, temperatures outside the above-specified range are not preferredO ~he pressure also differs somewhat according to the crystallinity and the degree of polymerization of the pulp-forming particles, and is preferably not more than 200 Kg/cm~O When the pressure is above 200 Kg~cm2, the polymer portions of the pulp-forming particles completely melt adhere to one ano--ther especially at elevated temperatures and become film-likeO ~hus, the resulting sheet lacks flexibility~
- The following ~xamples and Comparative ~xamples - illustrate the present inventionO ~he various properties described in these examples were measured by the following methods.
Logarithmic viscosity (~ lh) .. , 1 ~
Measured at 30C for a soiution of the sample polymer dissolved in N-methyl-2-pyrrolidone in a concentra-tion of 0.5 g/100 mol~
Dielectric stren~th Measured in accordance with JIS C 2111 using an alternate current voltage.
Impre~natabilit~ (permeabilit~) The sample cut in a circular shape with a diameter of 2 cm is made afloat on the surface of an .
.

- --r~

` 1067Z44 insulating oil (JIS No. 1), and the time required until the insulating oil permeates onto the surface of the sample is measured.
Texture uniformit~
~he surface condition of the sample sheet is visually observed. Also, the condition of the sheet is visually observed through light rays of the visible region. Thus, the texture uniformity of the sheet is.
evaluated and rated on a scale of good and poor.
Mean specific filtration resistance Into a glass tube having an inside diameter of 34 mm and a length of 130 cm and equipped with a stopper and a 200-mesh wire screen at its bottom, a 0.5% aqueous suspension of pulp-forming particles is placed up to a - 15 height of 120 cm from the surface of the wire screen.
The stopper is removed, and the liquid surface level descreasing with time is measuredO ~he mean specific - - filtration resistance is calculated from the following - equationO
Mean specific ,p x g x b filtration resistance ~ --~
~ x c x Ho wherein ~: the density of water ( ~ cm3) g: the gravity, the acceleration 980 (cm~sec2) b: the discharge water resistance (sec) ~: the viscosity of water ( ~cm,secO) c: the concentration of the pulp suspension ( ~cc) -Ho: the initial liquid surface level 120 (cm) - ~ .
~he discharge water resistance (b) is determined - in the following manner. Let the liquid surface level after the ~apse Oftseconds from the beginning of the liquid level decrease by the removing of the stopper be H, and the value of H/ko be x. A graph is prepared by plotting t and (x - ~n x - 1) (where ~n represents natural logarithm)O
Except for ve,ry small values of t, a straight line is ob-tained. ~he inclination of this line is b, and with regard B to this straight line ortion, the relation t=b(x -~nx i0 is establishedO
~xample 1 Preparation of a ~ol~mer solution ~rimellitic anhydride and 4,4'-diaminodiphenyl ~ methane were condensed in a molar ratio of 2:1 in N-methyl-2-pyrrolidone to form a bis-imide compoundO ~he bis-imide compound was reacted with 2 moles, per mole of the 4,4'-diaminodiphenylmethane, of trimellitic anhydride and 3 moles, per mole of the 4,4'-diaminodiphenylmethane, of 4,4'-diphenylmethane diisocyanate to from a solutlon containing 2,~/o of polyamideimide (having a logarithmic viscosity in N-methyl-2-pyrrolidone of 0.73) in the N-methyl-2-pyrrolidone (solution A)o Separately~ 190 parts of deionized water was added to 2,~50 parts of N-methyl-2-pyrrolidone~ and 418 parts of a mica powder having a particle size, measured by the Andreasen pipette method, of 400 to 1,000 mesh was addedO
The mixture was then stirred for 40 minutes by means of a T.K. Homomixer (a product of Tokushu Kika Kogyo Co., Ltd.) to form a mixture consisting of N~methyl-2-pyrrolidone, - ~2 -~067244 water and mica (mixed solution B).
gOO parts of the solution A was added to 2958 - parts of the mixed solution B, and they were stirred until a homogeneous solution consisting of the polyamide imide, N-methyl-2-pyrrolidone, water and mica was obtained (solution C). ~he concentration of the polyamideimide in the solution C was 60 ~/o by weight, and its water content was 5.~/o by weight (the above weight percentages were calculated with the omission of the amount of the mica).
PreParation of a nreci~itant 30 Parts of water was added to 70 parts of N-methyl-2-pyrrolidone, and the mixture was simply stirred to form a precipitantO
- Pre~aration of ~ul~-forming ~a~ticle~ -A continuous precipitator of the in-line mixer type consisting of a combination of a stator equipped with a buffle and a turbine vane-type rotor and equipped with a feed inlet for the precipitant and the polymer solution and .
an opening for discharging a pulp-forming particle slurry after precipitation was charged simultaneously with 005 - E ~min. of solution C and 5 Kg/min. of the precipitant, and the resulting slurry of pulp-forming particles was discharged out of the discharge openingO At this time, the temperature of the precipitant was adjusted to ~5C, and the temperature of the solution C, to 35C The speed of the rotor was adjusted to 5,000 rpm~
Heat-treatmert ~ ive liters of the resulting-slurry of pulp-forming particles was placed in a 7-liter tank, and stirred '1067Z44 ,~o sse c~
at a speed of about 60 rpm. Warm water was flow~
into a jacket attached to the tank to heat-treat the slurry at a temperature of 60C for 5 minutes.
After the heat-treatment, the pulp--forming particle slurry was placed in a centrifugal separator and a - greater portion of the precipitant was separated as a filtrate. ~hen, deionized water was fed to wash the pulp--forming particles sufficientlyO The result-ing pulp-forming particles had a mean spec~fic filtra-tion resistance of 33 x 108 cm/gO
Sheet formin~
An aqueous dispersion containing 2016 g (solids content) of the resulting pulp-formirg particles and poly(m-phenylene isophthalamide) fibers with a single fiber denier of 2 denier and a fiber length of 7 mm (CONEX, a registered trademark for a product of Teijin Limited) was processed on a TAPPI
~ 0~7 ~ ~e standard sheet machine. Water drninin'~ from the wire screen was good, and the pulp-forming particles ex--~o hibited good sheet-formability. The resulting sheet had good uniformity of texture. The staple fiber content of the sheet was 20% by weightO
The wet sheet was hot-pressed at 230C and 200 K ~cm2 to form a sheet having a thickness of 113 microns. The sheet had a dielectric strength of 29 KV/mm, an impregnatability of 800 sec/mm, a tensile strength of 3.7 Kg/cm2 and an elongation of 6.0%.
The sheet was immersed in a silicone oil at 240C for lOOO hours. ~ut its above properties ..
scarcely changed, and its termal stability was good.
Comparative Exam~le 1 Pulp-forming particles and sheets were prepared in the same way as in Example 1 except that the amount of water in solution B was varied as shown in ~able 1. ~he properties of the sheets - -obtained were measured and the results are shown in Table 1.

.

~able 1 Amount of Mean ~ .
water specific .
. filtration Dielectric Impregnat .
Run . resistance Texture strength ability No. Parts 0/~ (x 108 cm/g) uniformity (KV/mm) (sec/mm) _ _ _ . ~ __ 1 0 0 . 4 Poor 4O1 800 . __ . _ ~ __ 2 15 0.46 4 . Poor 9 7 830 3 381 _ 10.5 4 . Poor _ _9-3 . 7 _ _ ~he polymer precipitated upon addition of 4 485.6 13.0 the solution A to the mixed so'ution B, . ~ _ and solution C could not be formed.
__ In Run ~oO 1 in which no water was added to the polymer solution and in Run NoO 2 in which the amount of water added was too small, the pulp-forming particles in - the slurry flocculated to form a massO Thus, they had a ~er~ low mean specific filtration resistanceO me result-ing sheet had an exceedingly uneven surface, and poor pro-. perties, especially poor dielectric strengthO
On the other hand, in Run No. 3 where the amount of water was too large, the precipitation rate was so fast that the pulp-forming particles formed had a large size, and had a very low mean specific filtration resistance.
A sheet from these pulp-forming particles had a rough surface and lnsufficient dielectric s.rengthO In Rhn ~o. 4 in which the amount of water was further increased, it was impossible to form a solution corresponding to solution C in Example 1, and subsequent operations failedO
It can be appreciated that in Example 1 in accord-ance with this invention, the pulp-forming particles had 106724~

a far higher mean specific filtration resistance, and the resulting sheet had f~r superior uniformity of texture - and electrical insulation, than in these comparative runs.
ExamPle 2 5 g of a powder of poly(m-phenylene isophthal-amide terephthalamide) obtained by interfacial polymeri--zation and having a logarithmic viscosity of 109 (iso-phthaloyl chloride/terephthaloyl chloride = 97/3 (molar ratio)~ was dissolved in a solvent consisting of 95 g of N-methyl-2-pyrrolidone and 6 g of water (the water content of the solution was 5066% by weight), and then 9.8 g of - the same mica as used in Example 1 was addedO ~his mixed solution was introduced into a precipitant composed of 550 g of water and 450 g of N-methyl-2-pyrrolidone stirred at high speed in a home blender (National MX-130~ Type, a registered trademark) to form a slurry of pulp-forming particles. ~he resulting slurry was transferred into a flask, and heat-treated at 60Co for 5 minutesO ~he slurry was then subjected to a centrifugal separator to separate it into the precipi-tant and the pulp-forming particles. Ihe precipitant remaining adhered to the pulp-forming particles was removed by washing with a large quantity of waterO The resulting pulp-forming particles had a mean specific filtration re-sistance of 35 x 108 cm/g.
An aqueous dispersion containing 204~ g (solids content) of the resulting pulp-forming particles and 0~27 g of the same aromatic polyamide fibers as used in Example 1 having a single fiber denier of 2.0 denier and a length of 5 mm was processed on a TAPPI standard sheet machine.

1~67Z44 ~he wet sheet obtained was dried, and hot-pressed at 200 Eg~cm2 by means of a press whose surface temperature was held at 250C. to form a sheet havin~ a thickness of about 105 microns. ~he sheet had a staple fiber content e 5 Of 10% by weight. In the sheet formation, water drnining from the wire screen was fast, and the sheet-formability of the puip-forming particles was good. The resulting sheet had excellent uniformity of texture.
The sheet obtained had a dielectric strength of 49.7 EV/mm, an impregnatability of 5800 sec/mm, a tensile strength of 400 Kg/mm2 and an elongation of 5.~/00 When the sheet was allowed to stand in air at 270C. for 7 days, the above properties scarcely changed. ~he sheet was sub-- stantially free from coloration, and proved to have good thermal stabilityO
~ or comparison, the above procedure was repeated except that water was not added in forming the polymer solution. ~he resulting sheet had a dielectric strength of 32 EV/mm which was insufficient for a sheet containing only 20 10% by weight of staple fibers~ -- Example ~
Pre~aration of pol~mer solution A polymer solution C having a polyamideimide concentration of 6.1% by weight and a water content of 5.6% by weight was prepared in the same way as in ~xample 1 except that the polyamideimide used to form solution A had a logarithmic viscosity of 0~75; in the preparation of mixed solution B, 1423 parts of ~-methyl-2-pyrrolidone, 115 parts of deionized water and 232 parts of a mica powder were mixed; and that the solution.C was prepared by mix-ing 1770 parts of the mixed solution B and 500 parts of the solution Ao Preparation of pre~ tant N-Methyl-2-pyrrolidone was mixed with water to form three precipitants having an N-methyl-pyrrolidone .
concentration of 45, 60, and 70/0 by weight respectivelyO
Using the three precipitants, pulp-forming - particles were formed and heat-treated in the.same way as in Example 1 except that the speed of the rotor in the precipitator was changed to 7100 rpm, and the temperature of the precipitant was adjusted to 34Co ~heets were pre-pared from the pulp-forming particles and poly(m-phenylene isophthalamide) fibers in the same way as in Example 2 ex-cept that the pressing of the sheet was effected at 230Co and 200 Kg/cm2 and the thickness of the resulting sheets was-.
100 micronsO
~he properties of the pulp-forming particles and the sheets were measured, and the results are shown in Table 2 . able 2 _.. .
- Concent- ~ean specific .
ration of filtration Uniformity Dielectric.
Run the precipitant resistance of strength NoO (wt.%) (x 108 cm/g) texture(KV/mm) 1 45 _ Good 47.5 2 60 - 53 Good 52O0 3 70 42 Good 51 o O

- 1067Z4~

~ hese three kinds of sheets were allowed to stand in air at 210C. for 1000 hours, but their pro-perties scarcely changed~ When they were dipped in a silicone oil at 240Co for 1000 hours, they were scarcely colored, and their tensile strength and elongation were not deteriorated appreciably. ~heir thermal stability were also good.
Compara i e_Example 2 ~xample 3 was repeated except that the concent-ration of N-methyl-2-pyrrolidone in the precipitant was - changed to 0, 10, 30, 77, and 90% by weight respectivelyO
- ~he properties of the resulting pulp-forming particles and sheets were measured, and the results are shown in ~able ~O
Table ~_ ~

Concent- Mean specific ration of the filtration Uniformity Dielectric Run precipitant resist~nce of stren~th NoO (wt.%) (x 10~ cm/g) texture(KV7mm) _. ___. __ ._ .
1 _ 3 Poor 80 8 2 10 - - Poor lloO
. . . _ _ 3 30 _ Poor 18 0 3 4 77 _ _ _ _ Poor 5O9 Pbulpiforming particles could not be ~ . ._. _ In Runs Nos. 1 to 3 in which the concentration of N-methyl-2-pyrrolidone in the precipitant was too low, the pulp-forming particles were of a rod-like shape with a large size, and thus had a very low mean specific filtration re-sistance. ~he resulting sheets had a low dielectric strength.

' '''~F

In Runs Nos. 4 and 5 the N-methyl-2-pyrrolidone concentration of the precipitant was too higho In Run No. 4, the pulp-forming particles flocculated and had a very low mean specific filtration resistance, and the resulting sheet had a low dielectric strength. In Run No. 5, the concentration of the solvent was higher than in Run No. 4,, and the precipitation of polymer hardly occurred, and it was impossible to obtain pulp-forming particles.
Example 4 60 Parts of a powder of poly(m-phenylene iso-phthalamide) obtained by interfacial pol~merization a~d having a logarithm;c viscosity of 1~83 was dispersed in a solvent consisting of 940 parts of N-methyl-2-pyrrolidone and 65 parts of water and cooled to 5C. (the solution - had a water content of 6.1% by weight), and 111 parts of a mica powder having a particle diameter, as measured by the Andreasen pipette method, of 400 to 1000 mesh was addedO
~he mixture was then heated to about 40C. to dissolve the polymer completelyO
On the other hand, N-methyl-2-pyrrolidone was - mixed with water to make three kinds of precipitants having an N-methyl-2-pyrrolidone concentration of 20, 30 and 40%
by weight respectivelyO
A continuous precipitator of the inline mixer type consisting of a cylindrical stator having a diameter of 40 mm and a rotor with two turbine vane-like blades and including openings for the polymer-solution and the pre-cipitant and an opening for discharging the resulting 106724~

slurry of pulp-forming particles was charged with 0.5 Kg/
min. of the polymer solution and 5 Kg/min. of the pre-cipitant at the same time, and the resulting slurry of pulp-forming particles was taken out from the dischaxge opening. ~t this time, both the precipitant and the poly-mer solution were maintained at a temperature of 20C.
~he speed of the rotor was adjusted to 9,000 rpmO
The resulting pulp-forming particles were heat-treated in the same way as in E~ample lo An aqueous dispersion containing 2016 g (solids content) of the resulting pulp-forming particles and 0O54 g of poly(m-phenylene isophthalamide) fibers (CONEX, a re-gistered trademark, a product of ~eijin ~imited) having a single fiber denier of 2 denier and cut to a length of 5 mm was processed on a ~APPI standard sheet machine, dried, and pressed at 300Co and 200 Eg/cm2 to form a sheet having a thickness of about 120 microns. ~he sheet had a staple fiber content of 20% by weightO The properties of the pulp-form-ing particles and the sheets were measured, and the results are shown in ~able 4O
_able 4 Concentra- _ _ tion of the filtration Dielectric Tensile Run precipitant resis~ance Uniformity strength streng~h No. (wto%) (x 10 cm/g) of texture (KV/mm) (Kg/cm ) 1 20 _ Good 34 _.___ 2 30 37 Good 40 5-3 3 40 2 Good 36 5~4 _ _ _ __ ______ 106~244 - ~he three sheets were allowed to stand in air at 270C. for 7 days~ But the above properties of the sheets hardly changedO ~he degree of coloration of the sheets was low, and their thcrmal stability was good.
_mParati e Example_~
Example 4 was repeated except that the N-methyl-2-pyrrolidone concentration of the precipitant was changed to 0, 10 and ~/o by wei~ktO The properties of the result-ing pulp-forming particles and sheets were measured, and the results are shown in ~able 5 ~able_5 _, __ _ ~_ Concentra- Mean tion of the filtration Dielectric Tensile Run precipitant resis~ance Uniformity strength streng~h No. (wto%) (x 10 cm/g) of texture (KV/mm) (Kg/cm ) _____ _ . __. .__ _ , 1 0 4 Poor lOoO 4O0 _ _ . _ . _~ , 2 10 4 Poor 22O5 3.8 _._ _ ____._ , ____ __ ~ __ 3 50 2 Poor 14.9 4O3 -_ _ _ _ In Runs Nos. 1 and 2 in which the concentration of N-methyl-2-pyrrolidone of the precipitant was too low, the resulting pulp-forming particles were of a rod-like shape with a large size and had a very low mean specific filtration resistance. Ihe resulting sheet had a low dielectric resistanceO
In Run No~ 3 in which the concentration of N-methyl-2-pyrrolidone was too high, the pulp-forming par-ticles flocculated and had a very low mean specific , ~067Z4~

filtration resistanceO ~he sheet had a low dielectric resistance.
Exam~le ~
By the same method as in Example l, a 2~/~ by weight solution of polyamideimide with a logarithmic viscosity of 0.78 in N-methyl-2-pyrrolidone was prepared.
- ~he resulting solution was diluted with N-methyl-2-pyr-rolidone and water to form a solution having a polymer concentration of 6% by weight and a water content of 6% by weightO A slurry of pulp-forming particles was prepared by the same method as in Example 1 using a mixture of 60 parts of N-methyl-2-pyrrolidone and 40 parts of water as a pre-cipitant, except that the temperatures of the precipitant and the polymer solution were adjusted to 1~Co ~ and the speed of the rotor was adjusted to 7100 rpmO
~he resulting slurry of pulp-forming particles was heat-treated by the same method as in Example l at vary-~ ing heating temperatures for varying periods as shown in Table 6. In Run No. 7 in ~able 6 in which the heat-t~ atmen~
r~ ~D~ec~ 0,~
--20 temperature was adjusted to 95C., steam was flowcd in a tank jacket for heating.
~heets were prepared from the resulting pulp-forming particles and aromatic polyamide staple fibers in the same manner as in Example 2 except that the pressing f the sheets was performed at 1700C. and 200 Kg/cm2 to form sheets with a thickness of about 170 microns.
I~he properties of the pulp-forming particles and sheets were me~sured, and the results are shown in ~ble 6.

Table 6 Heat-treatment conditions Mean _ _ specific ~empera- filtration Dielectric Run tu~e ~ime resis~ance Uniformity strength No. ( CO) (min) (x 10~ cm/g) of texture (KV/mm) 1 Untreated 211 Poor 5800 2 40 60 98 Good 6503 _ . _ ____ ____ . . ___ __._ 3 50 20 _ ~ 9 Good 64 9 4 _ _ __ _ 5 53 Good 69.8 _ 7 5 12 Good 5708 6 ~ 271440 205 Poor 58.3 7 95 00~ 4 Poor 2708 ~ . _ In Table 6, Runs Nos. 1, 6 and 7 are comparative runs. In Run No. 6, the pulp-forming particles had an ex-cessively high mean specific filtration resistance becauseof the low heating temperature, although the treatment was B carried out for as long as 24 hoursO Consequently, water Jro~ ~ ~g e dr~irirg from the wire screen was poor at the time of sheet forming, and the resulting sheet had poor uniformity of texture. In Run NoO 1, no heating was performed, and the results were the same as in Rhn No. 6. On the other hand, in Rhn No. 7, the heat-treatment temperature was high, and the pulp-forning particles flocculated and melt-adhered and had a very low mean specific filtration resistanceO ~he resulting sheet had a very uneven surface and low di-electric strength.

Runs Nos. 2 to 5 were in accordance with the process of this invention. ~he resulting pulp-forming particl.es had a suitable mean specific filtration re-sistance, and the resulting sheets had good uniformity of texture and dielectric strengthO
ExamPle 6 - . 6 Parts of poly(m-phenylene isophthalamide) obtained by interfacial polymerization and having a loga-rithmic viscosity of 1.8 was dissol~ed in a solvent con-sisting of 94 parts of N-methyl-2-pyrrolidone and 6 parts of water (the solution had a water content of 5066% by weight), and then 11.1 parts of a mica powder having a particle size, as determined by the Andreasen pipette method, of 400 to lOO meshO
On the other hand, a precipitant consisting of 40 parts of N-methyl-2-pyrrolidone and 60 parts of water was preparedO In the same way as in ~xample 5, a slurry of - pulp-forming particles ~Jas prepared, heat-treated and washedO
~he mean specific filtration resistances of the resulting pulp-forming particles are shown in Table 70 In the same way as in ~xample 5, sheets were formed from the resulting pulp-forming particles, dried, and hot-pressed at 250 Eg/cm20 ~he sheets had a thickness of about llO micronsO ~he texture uniformity and dielectric strength of the sheets were measured, and the results are shown in ~able 7.

Table 7 Treatment _ _ _ _ conditio ns ¦ Mean T filtration Dielectric empera- Tim resistanceUniformity resistance NRuOn tu~OrC.) ~min ) (x 108 cm/g) of texture ~KV/mm) _ 1 Untreated 159 Poor 51.4 _--50 _ ~_ ~_ 3 70 2 50 Good 50.0 4 27 1440 _ 151 Poor ¦ 52.1 5 95 0.3 3 Poor I 18.3 Runs Nos. 1, 4 and 5 were comparison runs. In Run No. 4, the mean specific filtration resistance of the pulp-forming particles was too high in spite of the 24-hour treatment, and since water drainage from the wire screen was poor at the time of sheet forming, the resulting sheet had poor uniformity of texture. The sheet had a good dielectric strength on an average, but individual values varied widely. In Run No. 1 in which no heating was performed, the results were substantially the same.
In Run No. 5 in which the heat-treatment temperature was too high, the pulp-formlng particles flocculated and adhered and had very low mean specific filtration resistance. The resulting paper had a rough surface and a markedly low dielectric strength.
In Runs Nos. 2 and 3 which were in accordance with the pro-cess of this invention, the pulp-forming particles had a suitable mean specific filtration resistance, and the resulting sheets had good uniformity of texture and dielectric stren~thO

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Claims (14)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for preparing pulp-forming particles, which comprises adding a solution of a thermally stable aromatic nitrogen-containing polymer selected from the group consisting of aromatic polyamides and nitrogen-containing polyheterocyclic compounds in an aqueous organic solvent consisting of an organic solvent and water to a precipitant consisting of an amide-type solvent for the polymer and water with stirring, thereby to form a dispersion containing pulp-forming particles of said polymer, the water content of the polymer solution being 1 to 10% by weight based on the total amount of said aqueous organic solvent and said polymer, and the concentration of said amide-type solvent in the precipitant being 15 to 48% by weight when the polymer is an aromatic polyamide, and 40 to 75% by weight when the polymer is a nitrogen-containing polyheterocyclic compound; and heating the precipi-tated pulp-forming particles to 30 to 90°C. in the presence of at least 2 parts by weight in total of said aqueous organic solvent and said precipitant, per part by weight of the pulp-forming particles.

2. The process of claim 1 wherein the heat-treatment is carried out by heating said dispersion containing the pulp-forming particles.

3. The process of claim 1 wherein the heat-treatment is carried out by forming said pulp-forming particles having adhered to thereto the aqueous organic solvent and the precipitant into a web and heating the web.

4. The process of any one of claims 1 to 3 wherein said aromatic polyamide is poly(m-phenylene isophthalamide) or poly(m-phenylene isophthal-amide terephthalamide).

5. The process of claim 1 wherein said nitrogen-containing polyhetero-cyclic compound is an aromatic polyamideimide.

6. The process of claim 5 wherein said aromatic polyamideimide is a polymer containing a recurring unit of the following formula wherein R is in which X is a lower alkylene group.

7. The process of claim 1 wherein said amide-type solvent as a component of the precipitant is selected from the group consisting of N,N-dimethyl formamide, N,N-dimethyl acetamide, N-methyl-2-pyrrolidone, N-acetyl-pyrrolidone, N-methyl-caprolactam, N-acetyl-caprolactam, hexamethylphosphoryl-amide and tetramethylurea.

8. The process of claim 1 or 7 wherein the organic solvent as a com-ponent of the aqueous organic solvent is the same as the amide-type solvent as a component of the precipitant.

9. The process of claim 1 wherein the water content of the polymer solution is 3 to 9% by weight.

10. The process of claim 1 wherein the concentration of the amide-type solvent in the precipitant is 30 to 45% by weight when the polymer is an aromatic polyamide, and 60 to 70% by weight when the polymer is a nitrogen-containing polyheterocyclic compound.

11. The process of claim 1 wherein the heat-treatment temperature is 35 to 70°C.

12. The process of claim 1 wherein the concentration of the polymer in the polymer solution is 2 to 15% by weight.

13. A synthetic paper-like sheet composed of the pulp-forming particles prepared by the process of claim 1 and thermally stable staple fibers.

14. The sheet of claim 13 composed of 20 to 95% by weight of the pulp-forming particles and the remainder being the staple fibers.