CA2003221A1

CA2003221A1 - Flame-retardant high-temperature resistant paperlike materials based on polyimide fibers

Info

Publication number: CA2003221A1
Application number: CA002003221A
Authority: CA
Inventors: Walter Loy; Robert Vodiunig; Klaus Weinrotter; Manfred Schobesberger; Claus Schobesberger
Original assignee: Lenzing AG
Current assignee: Lenzing AG
Priority date: 1988-11-29
Filing date: 1989-11-17
Publication date: 1990-05-29
Also published as: BR8906237A; AU613540B2; DK585089D0; IL92371A; NO894719L; DK585089A; IL92371A0; JPH02259199A; NO894719D0; FI89526C; FI895405A0; PT92426A; TR24477A; FI89526B; NZ231505A; EP0371957A2; EP0371957A3; AU4535389A; KR900007909A

Abstract

ABSTRACT OF THE DISCLOSURE:

There is disclosed a process for producing high-temperature resistant and flame-retardant paperlike materials based on polyimide polymers of the general formula which material has a weight per unit area of between 20 and 1,100 g/m2, a LOI-value of at least 32 % O2 and a TG
point of at least 300°C. A pulp consisting of an aqueous suspension of polyimide fibers and, if desired, polyimide fibrids having structural units of the general formula (I) is processed on a paper machine to a nonwoven, which is wet-pressed and dried. The polyimide fibers are used in a disintegrated state, i.e., having fiber lengths of from 0.01 to 120 mm. Paperlike materials according to the invention having a weight per unit area of between 60 and 290 9/m2 are producible by contacting a manipulatable sheet-like structure of polyimide fibers with a polyimide solution, drying and, if desired, compacting the same.

Description

~QO;~Zl The invention relates to flame-retardant, high-temperature resistant paperlike material based on thermostable polymers as well as to processes for producing the same.
Synthetic papers of thermostable polymers are known and are mainly used for electric insulations. A further applicability is the manufacture of core materials (honeycombs) for composites.
The known processes use conventional paper-oaking techniques. To this end, it is, however, necessary to prepare pulps as starting materials for such papers, that contain not only fibers, but even fibrils and/or fibrids.
The latter type of fiber has a surface structure as is possessed by cellulosic fibers naturally. This structure is imperative for the preparation of synthetic papers from pulps.
Such preparation is described, e.g., in US-A -3,756,908. Starting materials are fibers and fibrids of aromatic polyamides (m-aramides), the fibers being produced according to a known spinning process and the fibrids being prepared by precipitation of a polymer solution. The aqueous slurry of a fiber-fibrid mixture is processed to paper by means of a paper machine, which paper may still be calendered afterwards.
Also EP-B-0 019 113 deals with a paperlike sheet.
The fibrous starting material and amorphous particles are commonly suspended. From this pulp, paperlike materials ~003~Z~

are obtained according to a known paper-making procedure, whose strengths are increased by the addition of cross-linking agents and radiation.
US-A - 2,999,788 relates to the production of pulp from fibrids of various polymers and to the structures produced therefrom.
The preparation of fibrids from m-aramides, which may subsequently be utilized for the production of synthetic papers, are described by several Japanese patents (JP 59-47695, JP 60-126400, JP 61-157532, JP 62-85014, JP 62-85015, JP 62-85018).
The production of fibrids from synthetic polymers also is described in US-A - 3,018,091.
The synthetic papers known so far, in particular those based on aromatic polyamides, no longer have proved satisfactory in many fields of applications with regard to high temperature resistance, aging stability and stability at elevated temperatures.
It is the ob~ect of the invention to avoid these disadvantages. It consists in providing high-temperature resistant and flame-retardant paperlike materials based on polyimide polymers of the general formula O O

~ - N A N-R - ~ ,~l) ~ / \ /
_ C C _ n O O

~00~21.

wherein n is an integer greater than l and A represents a four-valent aromatic group selected from ~ /~

~\ ~ ~

wherein X is CO, CH2, O, S, CF2 and R represents at least one of the following divalent aromatic groups selected from 20~ -CH~

~ or \~/

2003;~Zl.

which material exhibits the following properties in combination: a weight per unit area of between 20 and 1,100 g/m2, in particular of between 60 and 290 g/m2, a LOI (limited oxygen index) value of at least 32 ~ 2 and a TG point of at least 300C.
Further properties of the material according to the invention may be:
- a tearing strength in the longitudinal direction of between 30 and 120 N/mm2, and - a dielectricbreakdown strength of 10 to 65 kV/mm at direct voltage and of 15 to 50 kV/mm at alternating voltage.
The polyimide fibers partially may be replaced by other high-temperature resistant organic or inorganic fibers without substantially altering the spectrum of properties.
Paperlike materials according to the invention having a weight per unit area of between 60 and 290 9/m2 are capable of being produced in that a manipulatable sheet-like structure based on polyimide fibers of the general formula O O

~ N A N-R~ , (I) l \ / \ / J n 2(~0~

wherein n, A and X have the meanings indicated above and R stands for a divalent aromatic group, is contacted with a polyimide solution, dried and, if desired, compacted.
The polyimide fibers mentioned are known and are producible, for instance, according to the process described in AT-B - 377,016.
Advantageously, wovens, knits, nonwovens or needle felts, preferably in the heat-shrunk state, are used as manipulatable sheet-like structures.
Needle felts having a weight per unit area of from 40 to 150 g/m or pre-shrunk needle felts having a weight per unit area of from 60 to 200 9/m2 are particularly suited.
The sheet-like structures are impregnated with a solution of the polyimide of the general formula I, wherein n, A, X and R have the meanings indicated above Solvents, preferably, are DMF, N-methyl pyrrolidone (NMP), dimethyl acetamide (DMAc), DMSO or other strongly polar solvents and mixtures of these solvents. As mixing components, even less polar or non-polar solvents, such as dioxan, chlorated hydrocarbons and the like, may be used. The polymer portion of the solution preferably varies between 3 and 40 ~ by weight.
Impregnation may be carried out according to one of the conventional impregnation procedures, the temperature of the solution ranging between 10 and 100C.
In the upper temperature range, the reduced 200~21 viscosity of the solution allows for more rapid impregnation and, thus, higher production rates.
Suitably, the material sheet is passed through a solution of polyimide and subsequently is freed from solvent. In an advantageous mode of procedure, the sheet is guided through a washing tank, the solvent being extracted with hot water, preferably at 60 to 90C.
Subsequent drying may be effected by radiation, contact heat or convection.
The impregnated and dried sheet-like structures still may be compacted on a twin-roll machine, a multi-roll calender or in a plate press, preferably at a temperature of between 50 and 350C, to equalize their thickness. Most suitably, it is operated at ~line pressures" (roll separation force per unit length~ of lO
to l,000 kN/m.
It turned out that the paperlike materials according to the invention having a weight per unit area of between 60 and 290 g/m2 are capable of being produced also by using paper-making techniques known per se, in that a pulp consisting of an aqueous suspension of polyimide fibers and polyimide fibrils and/or polyimide fibrids having structural units of the general formula _ O O

_-- N A N-R ~ . (I) _ C C _ n O O

2003~21 wherein n, A, X and R have the meanings indicated above, in a manner known per se, is processed on a paper machine to a nonwoven, which is then wet-pressed and dried.
Paperlike materials according to the invention having a weight per unit area of between 20 and 1,100 g/m2 are capable of being produced in that a pulp consisting of an aqueous suspension of polyimide fibers and, if desired, polyimide fibrids, having structural units of the general formula N A N-R ~
_ C C _ n O O
15 wherein n, A, X and R have the meanings indicated above, in a manner known per se, is processed on a paper machine to a nonwoven, which is then dried, the polyimide fibers required for the formation of the nonwoven being used in a disintegrated state, i.e., having fiber lengths of from 0,01 to 120 mm.
Preferably, a mixture of fibers of different lengths is used, thus attaining a particularly high uniformity and strength of the paper. The titers of the fibers used, preferably range between 0.7 and 20 dtex.
In addition to polyimide fibers and polyimide fibrids, fibers of polyvinyl alcohol may also be contained in the pulp to be processed. Furthermore, 200~

fibers, fibrids and f ibrils of other polymers or materials, e.g., asbestos, carbon, may be admixed to the pulp. Any kind of filler that may be used in conventional paper-making is suitable.
The starting materials even may be spun-dyed in order to produce colored papers. Spun-in additives, such as carbon black, impart an inherent electric conductivity to the paper.
The formation of a nonwoven may be realized both on a paper machine and on a wet fleece molder or a sheet former.
To improve the strength of the nonwoven, additives and binders, e.g., based on polyvinyl alcohol or silicon, may be added to the pulps. It is, however, also possible to apply the same on the wet-pressed nonwoven by spraying,injecting, sprinkling or dipping, whereupon the nonwoven is dried.
Binders based on silicon are well suited for the compaction o~ nonwovens, because they decompose at temperatures above 200C, merely leaving most finely divided silicon dioxide in the paper, which practically does not affect the properties of the final product.
A preferred embodiment of the process according to the invention is characterized in that the dried nonwoven is compressed in a plate press or in a multi-roll calender. It is also possible to compress together several superimposed nonwovens.

Z003~

This may be effected at line pressures of between 0.1 and 1,000 kN/m and at temperatures of preferably 70C
to 950C. It is also possible to compress multi-layer nonwovens, the coherence of the individual layers being ensured by the thermoplasticity of the polyimide, which may even be increased by the addition of binders.
The paperlike material produced according to the invention has a uniform and smooth surface and is capable of being coated by known techniques, e.g., in order to obtain colored, conductive or high-gloss surfaces.
The paperlike material according to the invention also is well suited for application in fields that require, i.a., a high thermostability, e.g., for insulating materials in the electric industry, for engines, generators and transformers, furthermore, as sheet sealing materials in the engineering industry, for instance, as seals in combustion engines, such as, e.g., cylinder head gaskets and the like.
The invention will be explained in more detail in the following examples, the production of paperlike materials according to the invention having weights per unit area of between 60 and 290 9/m2 and of between 20 and l,100 9/m2 being described in examples 1 to 6 and 10 to 24, respectively. Examples 7 and 9 relate to the preparation of polyimide fibrids and fibrils known per se. The reported properties of the paperlike materials produced according to the invention were determined by 2~)0~2~1.

the following assay methods:
LOI (limiting oxygen index~ ASTM D-2863 TGA (thermogravimetric analysis) device: Perkin Elmer TGA/2 heating rate: 20C/min determination of weight losses by onset DSC (differential scanning calorimetry) device: Perkin Elmer DSC/4 heating rate: 20C/min Dielectric breakdown strength according to DIN 53481 Tensile strength and elongation according to DIN 53455 Exa_ple l_ Starting materials:
Polyimide needle felt, produced from benzophenone-3,3',4,4'-tetracarboxylic acid dianhydride and 4,4'-methylenebis(phenylisocyanate) and 2,4- and 2,6-toluenediisocyanate ttrade name P 84, producer: Lenzing Aktiengesellschaft), weight per unit area: 85 g/m2 width: about 150 mm.
Polyimide solution, 20 % by weight of P 84 in DMF.
The needle felt was impregnated at 22C, dried and compacted by means of a twin-roll calender by maintaining the following parameters:
Roll temperature: 250 to 245C
Roll gap: 0.04 mm ~oo;~

Pressure of the rolls: 10 kN
The paperlike material obtained according to the invention had a weight per unit area of 167 9/m2 and a mean thickness of 0.20 mm.
Thermal properties:
LOI: 36 to 37 ~ 2 TG~: maximum weight loss at 530C
TG point: 312C
Mechanical properties:
Tearing strength - longitudinal: 30 N/mm2 - transverse: 27 N/mm2 Elongation - longitudinal: 8 ~
- transverse: 4 %
Dielectric breakdown strength at direct voltage: 40 kV/mm alternating voltage: 12 kV/mm Exam~le 2_ Starting materials:
Polyimide needle felt according to Example 1, but with weight per unit area of 120 g/m2 width about 150 mm.
Polyimide solution, 10 % by weight of P 84 in DMF.
The needle felt was impregnated at 15C, dried and compacted by means of a twin-roll calender by ~aintaining the following parameters:

"~,QQ.~21.

Roll temperature: 250C
Roll gap: 0.04 mm Pressure of the rolls: 43 kN and 35 kN after a further passage with the remaining settings unchanged.
The paperlike material obtained according to the invention had a weight per unit area of 238 9/m2 and a mean thickness of 0.24 mm.
Thermal properties:
LOI: 36 to 37 % 2 TGA: maximum weight loss at 530C
TG point: 312C
Mechanical properties:
Tearing strength - longitudinal: 50 N/mm2 - transverse: 30 N/mm2 Elongation - longitudinal: 6.5 %
- transverse: 3 Dielectric breakdo~n strength at direct voltage: 30 kV/mm alternating voltage: 17 kV/mm Exa_ele 3-Starting materials:
Polyimide needle felt according to Example 2 Polyimide solution, 15 % by weight of P 84 in DMF.
The needle felt was impregnated at 20C, dried and ~003~

compacted by means of a twin-roll calender.
The paperlike material obtained according to the invention had a weight per unit area of 222 9/m2 and a mean thickness of 0.28 mm.
Thermal properties:
LOI: 36 to 37 ~ 2 TGA: ~aximum weight loss at 530C
TG point: 312C
Mechanical properties:
Tearing strength - longitudinal: 33 N/mm2 Elongation - longitudinal: 7.5 Dielectric breakdown strength at direct voltage: 25 kV/mm alternating voltage: 9 kV/mm Exa_~le 4_ Starting materials:
Polyimide needle felt according to Example 1, but with a weight per unit area of 60 9/m2 width about 150 mm.
Polyimide solution, 30 ~ by weight of P 84 in DMF.
The needle felt was impregnated at 55C, dried and compacted by means of a twin-roll calender by maintaining the following parameters:
Roll temperature: 250C
Roll gap: 0.04 mm Z0~ 2Z~

Pressure of the rolls: 10 kN
The paperlike material obtained according to the invention had a weight per unit area of 90 9/m2 and a mean thickness of 0.15 mm.
Thermal properties:
LOI: 36 to 37 % 2 TGA: maximum weight loss at 530C
TG point: 312C
Mechanical properties:
Tearing strength - longitudinal: 50 N/mm2 - transverse: 30 N/mm2 Elongation - longitudinal: 4 ~
- transverse: 3 %
Dielectric breakdown strength at direct voltage: 55 kV/mm alternating voltage: 40 kV/mm _xam~le 5:__ __ __ Starting materials:
Polyimide needle felt according to Example 1, but with a weight per unit area of 240 9/m2 width about 150 mm.
Polyimide solution, 10 % by weight of P 84 in DMF.
The needle felt was impregnated at 20C, dried and compacted by means of a twin-roll calender by maintaining the following parameters:

Z0~ 2~.

Roll temperature: 250C
Roll gap: 0.04 mm Pressure of the rolls: 40 to 52 kN
The paperlike material obtained according to the invention had a weight per unit area of 260 9/m2 and a mean thickness of 0.26 mm.
Thermal properties:
LOI: 36 to 37 % 2 TGA: maximum weight loss at 530C
TG point: 312C
Mechanical properties:
Tearing strength - longitudinal: 60 N/mm2 - transverse: 40 N/mm2 Elongation - longitudinal: 9 %
- transverse: 6 Dielectric breakdown strength at direct voltage: 25 kV/mm alternating voltage: 10 kV/mm Exa_~le 6_ Starting materials:
Polyimide needle felt according to Example l, but with a weight per unit area of 260 g/m2 width about lS0 mm.
Polyimide solution, 15 ~ by weight of P 84 in DMF.
The needle felt was impregnated at 20C, dried and ~003~

compacted by means of a twin-roll calender by maintaining the following parameters:
Roll temperature: 230 to 240C
Roll gap: 0.04 mm Pressure of the rolls: 35 kN
The paperlike material obtained according to the invention had a weight per unit area of 290 9/m2 and a mean thickness of 0.3S mm.
Thermal properties:
LOI: 36 to 37 % 2 TGA: maximum weight loss at 530C
TG point: 312C
Mechanical properties:
Tearing strength - longitudinal: 110 N/mm2 - transverse: 90 N/mm2 Elongation - longitudinal: 15 %
- transverse: 12 %
Dielectric breakdown strength at direct voltage: 12 kV/mm alternating voltage: 9 kV/mm Exa_ple 7_ The preparation of fibrids may be realized in a known manner, e.g., by spraying the polymer solution into an aqueous precipitation bath.
To this end, a 5 % solution of P 84 in DMF was fed ~o~

to a binary spinneret by means of a qear pump and was atomized into the aqueous precipitation bath by compressed air.
Spinneret diameter: 2.1 mm Feed rate (polymer solution): 100 cm3Jmin Compressed air pressure: 6 bar The diameter of the fibrids obtained, on an average, amounted to 2-3 mm.

Exa__le 8.
Polyimide fibers having a staple length of 5 mm and a titer of 2.2 dtex were impacted into water. The suspension was fed to a cone refiner and maintained there until the degree of fibrillation (portion of fibrils~ was about 40 %.
Consistency 4 ~
Conical rotor (8):1,500 rpm Intake pressure:0.5 bar Run-out pressure:3.5 bar Duration time: 40 min Exam_le 9_ Polyimide fibers having a staple length of 5 mm and a titer of 2.2 dtex were circulated through an impact mill (plate impact mechanism, screen ring 0.5 mm, corrugated trapezoid, 13,900 rpm) until the degree of fibrillation was about 90 %.

2003;~

Exa_ple 10.
Polyimide fibers having a fiber titer of 2.2 dtex and staple lengths of 2.5 mm, 5.0 mm and 10.0 mm as well as ground polyimide fibers having lengths of from 0.01 to 5.0 mm and polyvinyl alcohol fibers at a quantitative ratio of 16:16:15:50:3 were slurried in water, molded to a nonwoven on a wet fleece molder, wet-pressed and dried. The dry nonwoven had a weight per unit area of 183 g/m2 and was pressed on a plate press at 280C and 290 bar.
Thermal properties:
LOI: 37 to 38 ~ 2 TGA: onset at 564C
TG point: 312C
Mechanical properties:
Tensile strength - longitudinal: 31 N/mm2 - transverse: 26 N/mm2 Elongation - longitudinal: 12 - transverse: 10 Dielectric breakdown strength at direct voltage: 12 kV/mm alternating voltage: 6 kV/mm.

Example 11_ Polyimide fibers having a fiber titer of 2.2 dtex and staple lengths of 2.5 mm, 5.0 mm and 10.0 mm as well as ground polyimide fibers having lengths of from 0.01 to 5.0 mm and polyvinyl alcohol fibers at a quantitative 200~

ratio of 16:16:15:50:3 were slurried in water, molded to a nonwoven on a wet fleece molder, wet-pressed and dried. The nonwoven thus produced had a weight per unit area of 183 9/m2 and was pressed on a plate press at 70C
and 490 bar.
Thermal properties:
LOI: 37 to 38 % 2 TGA: onset at 564C
I'G point: 312C
Mechanical properties:
Tensile strength - longitudinal: 27 N/mm2 - transverse: 22 N/mm2 Elongation - longitudinal: 19 %
- transverse: 15 %
Dielectric breakdown strength at direct voltage: 47 kV/mm alternating voltage: 26 kV/mm.

Exam~le 12_ Polyimide fibers having a fiber titer of 0.7 dtex and a staple length of 2.5 mm as well as ground polyimide fibers having lengths of from 0.01 to 5.0 mm and polyvinyl alcohol fibers at a quantitative ratio of 48.5:48.5:3 were impacted into water, molded to a nonwoven on a wet fleece ~older, wet-pressed and dried.
The nonwoven thus produced had a weight per unit area of 102 9/m2 and was pressed on a plate press at 450C and 50 bar.

;~003~1 Thermal properties:
LOI: 39 to 40 ~ 2 TGA: onset at 564C
TG point: 334C
Mechanical properties:
Tensile strength - longitudinal: 67 N/mm2 - transverse: 56 N/mm2 Elongation - longitudinal: 11 %
- transverse: 9 Dielectric breakdown strength at direct voltage: 11 kV/mm alternating voltage: 6 kV/mm.

Exam~le 13_ Polyimide fibers having a fiber titer of 1.7 dtex and staple lengths of 2.5 mm and 5.0 mm as well as polyvinyl alcohol fibers at a quantitative ratio of 60:37:3 were slurried in water and molded to a nonwoven on a wet fleece molder, wet-pressed and dried. The nonwoven thus produced had a weight per unit area of 70 g/m2 and was pressed on a plate press at 350C and 250 bar.
Thermal properties:
LOI: 38 to 39 % 2 TGA: onset at 564C
TG point: 328C
Mechanical properties:

~Q03~2~

Tensile strength longitudinal: 81 N/mm2 - transverse: 68 N/mm2 Elongation - longitudinal: 7 %
- transverse: 5 %
Dielectric breakdown strength at direct voltage: 14 kV/mm alternating voltage: 7 kV/mm.

Ex_mple 14.
Ground polyimide fibers having lengths of about 0.01 mm to 5.0 mm were slurried in water and molded to a wet nonwoven on a paper machine, wet-pressed, sprayed with binder and dried. The nonwoven thus produced had a weight per unit area of 40 9/m2 and was pressed on a two-roll calender at 350C and 500 N/m.
Thermal properties:
LOI: 38 to 39 % 2 TGA: onset at 564C
TG point: 328C
Mechanical properties:
Tensile strength - longitudinal: 30 N/mm2 - transverse: 25 N/mm2 Elongation - longitudinal: 12 %
~5 - transverse: 9 Dielectric breakdown strength at direct voltage: 13 kV/mm Z003XX~

alternating voltage: 7 kV/mm.

Exa_ple 15_ Ground polyimide fibers having lengths of 0.01 to 5.0 mm were slurried in water, molded to a wet nonwoven on a paper machine, wet-pressed, sprayed with binder and dried. The nonwoven thus produced had a weight per area unit of 55 g/m2 and was pressed on a twin-roll calender at 350C and 1,000 kN/m.
Thermal properties:
LOI: 38 to 39 ~ 2 TGA: onset at 564C
TG point: 328C
Mechanical properties:
Tensile st}ength - longitudinal: 117 N/mm2 - transverse: 98 N/mm2 Elongation - longitudinal: 8 - transverse: 5 Dielectric breakdown strength at direct voltage: 75 kV/mm alte}nating voltage: 42 kV/mm.

Exa_~le 16.
Ground polyimide fibers having lengths of 0.01 to 5.0 mm were slurried in water and molded to a nonwoven on a sheet former, wet-pressed and dried. The nonwoven thus produced had a weight per unit area of 252 9/m2 and 20e)32~

- was pressed on a plate press at 330C and 390 bar.
Thermal properties:
LOI: 37 to 38 % 2 TGA: onset at 564C
TG point: 319C
Mechanical properties:
Tensile strength: 79 N/mm2 Elongation: 10 %
Dielectric breakdown strength at direct voltage: 21 kV/mm alternating voltage: 11 kV/mm.

Examele 17_ Ground polyimide fibers having lengths of 0.01 to 5.0 mm were slurried in water and molded to a nonwoven on a sheet former, wet-pressed, sprayed with binder and dried. The nonwoven thus produced had a weight per area unit of 105 9/m2 and was pressed in three layers on a plate press at 350C and 480 bar. The weight of the paper per area unit was 315 g/m2. Subsequent separation of the three layers was no longer possible.
Thermal properties:
LOI: 38 to 39 % 2 TGA: onset at 564C
TG point: 328C
Mechanical properties:
Tensile strength: 75 N/mm2 Elongation: 13 %

200~

Dielectric breakdown strength at direct voltage: 68 kV/mm alternating voltage: 39 kV/mm.

Exa_ple 18.
Polyimide fibers having a fiber titer of 2.2 dtex and staple lengths of 2.5 mm, 5.0 mm and 10.0 mm as well as ground polyimide fibers having fiber lengths of from 0.01 to 5.0 mm at a quantitative ratio of 19:16:15:50 were slurried in water, molded to a nonwoven on a wet-fleece molder, wet-pressed and dried. The nonwoven thus produced h3d a weight per unit area of 183 9/m2 and, immediately upon the drying process, was pressed on a twin-roll calender at 22C and 500 kN/m.
Thermal properties:
LOI: 37 to 38 % 2 TGA: onset at 564C
TG point: 312C
Mechanical properties:
Tensile strength - longitudinal: 33 N/mm2 - transverse: 28 N/mm2 Elongation - longitudinal: 17 ~6 - transverse: 14 9 Dielectric breakdown strength at direct voltage: 66 kV/mm alternating voltage: 37 kV/mm.

Z0~32~

Exa_~le 19.
Ground polyimide fibers having lengths of 0.01 to 5.0 mm were slurried in water and molded to a nonwoven on a sheet former, wet-pressed and dried at 350C. The nonwoven thus produced had a weight per area unit of 100 g/m2 and, immediately upon the drying process, was pressed on a plate press at room temperature 21C and 480 bar.
Thermal properties:
LOI: 37 to 38 % 2 TGA: onset at 564C
TG point: 312C
Mechanical properties:
Tensile strength: 15 N/mm2 Elongation: 18 %
Dielectric breakdown strength at direct voltage: 44 kV/mm alternating voltage: 25 kV/mm.

~xam~le 20-.
Ground polyimide fibers having lengths of 0.01 to 5.0 mm were slurried in water and molded to a nonwoven on a wet fleece molder, wet-pressed, sprayed with silicon finish and dried at 150C. The nonwoven thus produced had a weight per area unit of 205 9/m2 and was pressed on a plate press at 320C and 350 bar.
Thermal properties:

200~

LOI: 37 to 38 ~ 2 TGA: onset at 569C
TG point: 312C
Mechanical properties:
Tensile strength - longitudinal: 75 N/mm2 - transverse: 63 N/mm2 Elongation - longitudinal: 10 ~
- transverse- 8 a Dielectric breakdown strength at direct voltage: 24 kV/mm alternating voltage: 13 kV/mm.

Exa_ple 21.
Polyimide fibrids and ground polyimide fibers having lengths of 0.01 to 5.0 mm at a quantitative ratio of 50:50 were slurried in water and molded to a nonwoven on a sheet former, wet-pressed and dried at 105C. The nonwoven thus produced had a weight per area unit of 1,090 9/m2 and was pressed on a plate press at 350C and 380 bar.
Thermal properties:
LOI: 38 to 39 ~ 2 TGA: onset at 564C
TG point: 328C
Mechanical properties:
Tensile strength: 57 N/mm2 Elongation: 14 %

2003~

Dielectric breakdown strength at direct voltage: 31 kV/mm alternating voltage: 17 kV/mm.

Exa_ple 22-Polyimide fibrids were impacted into water and molded to a nonwoven on a sheet former, wet-pressed and dried at 105C. The nonwoven thus produced had a weight per unit area of 1,090 9/m2 and was pressed on a plate press at 350C and 380 bar.
Thermal properties:
LOI: 38 to 39 ~ 2 TGA: onset at 564C
TG point: 328C
Mechanical properties:
Tensile strength: 57 N/mm2 Elongation: 14 ~
Dielectric: breakdown strength at direct voltage: 31 kV/mm alternating voltage: 17 kV/mm.

Exam~le 23_ Polyimide fibers having a length of 2.5 mm and a titer of 2.2 dtex were slurried in water and molded to a nonwoven on a sheet former, wet-pressed, sprayed with silicon finish and dried at 150C. The nonwoven thus produced had a weight per unit area of 210 9/m2 and was 2()0;~2~

pressed on a plate press at 320C and 350 bar.
Thermal properties:
LOI: 37 to 38 % 2 TGA: onset at 564C
TG point: 312C
Mechanical properties:
Tensile strength: 88 N~mm2 Elongation: 12 %
Dielectric breakdown strength at direct voltage: 21 kV/mm alternating voltage: 10 kV/mm.

Exa_~le 24.
Polyimide fibers having a length of 120 mm and a titer of 20 dtex were slurried in water and molded to a nonwoven on a sheet former, wet-pressed, sprayed with silicon finish and dried at 150C. The nonwoven thus produced had a weight per unit area of 503 g/m2 and was pressed on a plate press at 320C and 350 bar.
Thermal properties:
LOI: 37 to 38 ~ 2 TGA: onset at 564C
TG point: 312C
Mechanical properties:
Tensile strength: 81 N/mm2 Elongation: 20 ~
The electric puncture strength was 200~

Direct voltage: 19 kV/mm Alternating voltage: 7 kV/mm.

Claims

1. A flame-retardant high-temperature resistant paperlike material based on polyimide fibers of the general formula (I) wherein n is an integer greater than 1 and A represents a four-valent aromatic group selected from or wherein X is CO, CH2, O, S, CF2 and R represents at least one of the following divalent aromatic groups selected from or which paperlike material comprises the following properties in combination: a weight per unit area of between 20 and 1,100 g/m2, a LOI (limited oxygen index) value of at least 32 % O2 and a TG point of at least 300°C.

2. A paperlike material as set forth in claim 1, wherein said weight per unit area ranges between 60 and 290 g/m .

3. A paperlike material as set forth in claim 1, further comprising - a tearing strength in the longitudinal direction of between 30 and 120 N/mm2, and - a dielectric breakdown strength of 10 to 65 kV/mm at direct voltage and of 15 to 50 kV/mm at alternating voltage.

4. A paperlike material as set forth in claim 1, wherein said polyimide fibers partially are replaced by other high-temperature resistant organic or inorganic fibers.

5. A process for producing flame-retardant high-temperature resistant paperlike material having a weight per unit area of between 60 and 290 g/m2, a LOI (limited oxygen index) value of at least 32 % O2 and a TG point of at least 300°C, which process comprises providing a manipulatable sheet-like structure based on polyimide fibers of the general formula (I) wherein n is an integer greater than 1 and A represents a four-valent aromatic group selected from or wherein X is CO, CH2, O, S, CF2 and R represents at least one of the following divalent aromatic groups selected from or contacting said sheet-like structure with a polyimide solution, and drying said sheet-like structure.

6. A process as set forth in claim 5, further comprising compacting the dried sheet-like structure.

7. A process as set forth in claim 5, wherein said manipulatable sheet-like structures comprise wovens, knits, nonwovens and needle felts.

8. A process as set forth in claim 7, wherein said needle felts have been heat-shrunk.

9. A process as set forth in claim 5, wherein said polyimide solution is comprised of a solution of said polyimides of the general formula I, wherein n, A, X and R
have the meanings indicated in claim 5, in a polar solvent.

10. A process as set forth in claim 9, wherein said polar solvent is selected from the group consisting of dimethyl formamide, dimethyl sulfoxide, N-methyl pyrrolidone (NMP), dimethyl acetamide (DMAc) and mixtures thereof.

11. A process as set forth in claim 9, wherein said polyimide solution has a polymer portion amounting to between 3 and 40 % by weight.

12. A process as set forth in claim 5, wherein the dried sheet-like structure is compacted on a twin-roll, on a multi-roll calender or in a plate press.

13. A process as set forth in claim 12, wherein compaction is effected at a temperature of between 50 and 350°C

14. A process for producing a flame-retardant high-temperature resistant paperlike material having a weight per unit area of between 60 and 290 g/m2, a LOI (limited oxygen index) value of at least 32 % O2 and a TG point of at least 300°C, which process comprises providing a pulp essentially consisting of an aqueous suspension comprising polyimide fibers and at least one of polyimide fibrils and polyimide fibrids having structural units of the general formula (I) wherein n is an integer greater than 1 and A represents a four-valent aromatic group selected from or wherein X is CO, CH2, O, S, CF2 and R represents at least one of the following divalent aromatic groups selected from or processing said pulp , in a manner known per se, to a nonwoven on a paper machine, and wet-pressing and drying said nonwoven.

15. A process for producing a flame-retardant high-temperature resistant paperlike material having a weight per unit area of between 20 and 1,100 g/m2, a LOI
(limited oxygen index) value of at least 32 % O2 and a TG
point of at least 300°C, which process comprises providing a pulp essentially consisting of an aqueous suspension comprising polyimide fibers having structural units of the general formula (I) wherein n is an integer greater than 1 and A represents a four-valent aromatic group selected from or wherein X i9 CO, CH2, O, S, CF2 and R represents at least one of the following divalent aromatic groups selected from or processing said pulp , in a manner known per se, to a nonwoven,and wet-pressing and drying said nonwoven, wherein said polyimide fibers required for the formation of the nonwoven are used in a disintegrated state having fiber lengths of from 0,01 to 120 mm.

16. A process as set forth in claim 15, wherein said aqueous suspension further comprises polyimide fibrids.

17. A process as set forth in claim 15, wherein said aqueous suspension further comprises fibers of polyvinyl alcohol.

18. A process as set forth in claim 15, wherein processing to a nonwoven is carried out on a paper machine.

19. A process as set forth in claim 15, wherein processing to a nonwoven is carried out on a wet fleece molder.

20. A process as set forth in claim 15, wherein processing to a nonwoven is carried out on a sheet former.

21. A process as set forth in claim 15, further comprising adding binders and additives to said pulp.

22. A process as set forth in claim 15, further comprising applying binders and additives to said nonwoven after wet-pressing.

23. A process as set forth in claim 22, wherein a silicon compound is sprayed on said nonwoven after wet-pressing and before drying.

24. A process as set forth in claim 15, further comprising compressing the dried nonwoven in a plate press.

25. A process as set forth in claim 15, further comprising compressing the dried nonwoven in a multi-roll calender.

26. A process as set forth in claim 24 or 25, wherein several superimposed nonwovens are compressed together.