AU613540B2

AU613540B2 - Flame-retardant high-temperature resistant paperlike materials based on polyimide fibers

Info

Publication number: AU613540B2
Application number: AU45353/89A
Authority: AU
Inventors: Walter Loy; Claus Dr. Schobesberger; Manfred Schobesberger; Robert Vodiunig; Klaus Dr. Weinrotter
Original assignee: Lenzing AG; Chemiefaser Lenzing AG
Current assignee: Lenzing AG
Priority date: 1988-11-29
Filing date: 1989-11-21
Publication date: 1991-08-01
Anticipated expiration: 2009-11-21
Also published as: CA2003221A1; FI89526B; NZ231505A; NO894719D0; IL92371A0; NO894719L; IL92371A; JPH02259199A; FI89526C; EP0371957A2; DK585089A; BR8906237A; EP0371957A3; FI895405A0; PT92426A; AU4535389A; TR24477A; DK585089D0; KR900007909A

Description

AUSTRALIA D PATENTS ACT 1952 Form COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE Short Title: Int. Cl: Application Number: Lodged: Complete Specification-Lodged: Accepted: Lapsed: Published: S Priority: Related Art: i TO BE COMPLETED BY APPLICANT Name of Applicant: LENZING AKTIENGESELLSCHAFT Address of Applicant: A 4860 LENZING

AUSTRIA

Actual Inventor: Address for Service: GRIFFITH HACK CO., 601 St. Kilda Road, Melbourne, Victoria 3004, Australia.

Complete Specification for the invention entitled: FLAME-RETARDANT HIGH-TEMPERATURE RESISTANT PAPERLIKE MATERIALS BASED ON POLYIMIDE FIBERS.

The following statement is a full description of this invention including the best method of performing it known to me:-

I

i The invention relates to flame-retardant, hightemperature 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-making t techniques. To this end, it is, however, necessary to at a S prepare pulps as starting materials for such papers, that S* contain not only fibers, but even fibrils and/or fibrids.

a 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.

a ut Such preparation is described, in US-A S3,756,908. 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 2 ii are obtained according to a known paper-making procedure, whose strengths are increased by the addition of crosslinking 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- CC 85014, JP 62-85015, JP 62-85018).

The production of fibrids from synthetic polymers t" 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 object 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 C C n 0 0 -3 2 wherein n is an integer greater than 1 and A represents a four-valent aromatic group selected from or o 0 a o o *o 00 0 o o wherein X is CO, CH 2 0, S, CF 2 and R represents at least one of the following divalent aromatic groups selected from

CH

3

CH

2 0O

CHM

3 CH3 or -4which material exhibits the following properties in combination: a weight per unit area of between 20 and 1,100 g/m 2 in particular of between 60 and 290 g/m2, a LOI (limited oxygen index) value of at least 32 02 and a TG point of at least 300 C.

Further properties of the material according to the invention may be: -a tearing strength in the longitudinal direction of 2 between 30 and 120 N/mm 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 i-norganic 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 g/m 2 are capable of being produced in that a manipulatable sheet-like structure based on polyimide fibers of the general formula 0 0 o o SN A N-R (I) C C n O

O

5 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 2 40 to 150 g/m or pre-shrunk needle felts having a weight per unit area of from 60 to 200 g/m2 are particularly i o t 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 100 0

C.

In the upper temperature range, the reduced 6- *o S a o0 oo o o o o oo o oO e o i 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 90 0

C.

Subsequent drying may be effected by radiation, contact heat or convection.

10 The impregnated and dried sheet-like structures still may be compacted on a twin-roll machine, a multiroll calender or in a plate press, preferably at a temperature of between 50 and 350 0 C, to equalize their thickness. Most suitably, it is operated at "line pressures" (roll separation force per unit length) of to 1,000 kN/m.

It turned out that the paperlike materials according to the invention having a weight per unit area of between 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 C C SN A N-R

(I)

C C n 0 0 7 0 o o 0 0 0 o o o o o o o o o o o 0 0 0 0 0 0 o o 0 0 0 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/m 2 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 O O C C SA N-R (I) C C n 0 0 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, having fiber lengths of from 20 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 polyimi f'bers and polyimide fibrids, fibers of polyvin alcohol may also be contained in the pulp to b- ocessed. Furthermore, 8 fibers, fibrids and fibrils of other polymers or materials, 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, 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 of nonwovens, because they decompose at temperatures above 200°C, 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.

9 :t 1 This may be effected at line pressures of between 0.1 and 1,000 kN/m and at temperatures of preferably 70 0

C

to 450 C. 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, 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, a high thermostability, 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 g/m 2 and of between and 1,100 g/m 2 being described in examples 1 to 6 and to 24, respectively. Examples 7 and 8 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 10 the following assay methods: LOI (limiting oxygen index) ASTM D-2863 TGA (thermogravimetric analysis) device: Perkin Elmer TGA/2 heating rate: determination of weight losses by onset DSC (differential scanning calorimetry) device: Perkin Elmer DSC/4 heating rate: 20 C/min Dielectric breakdown strength according to DIN 53481 Tensile strength and elongation according to DIN 53455 ExamEle 1: Starting materials: Polyimide needle felt, j produced from benzophenone-3,3',4,4'-tetracarboxylic acid dianhydride and 4,4'-methylenebis(phenylisocyanate) and 2,4- and 2,6-toluenediisocyanate (trade name P 84, producer: Lenzing Aktiengesellschaft), weight per unit area: 85 g/m 2 width: about 150 mm.

Polyimide solution, 20 by weight of P 84 in DMF.

The needle felt was impregnated at 220C, dried and compacted by means of a twin-roll calender by maintaining the following parameters: Roll temperature: 250 to 245 0

C

Roll gap: 0.04 mm 11 Pressure of the rolls: 10 kN SThe paperlike material obtained according to the invention had a 2 weight per unit area of 167 g/m and a mean thickness of 0.20 mm.

Thermal properties: LOI: 36 to 37 02 TGA: maximum weight loss at 530 0

C

TG point: 312 C i 10 Mechanical properties: Tearing strength longitudinal: 30 N/mm 2 transverse: 27 N/mm 2 Elongation longitudinal: 8 transverse: 4 Dielectric breakdown strength at direct voltage: 40 kV/mm alternating voltage: 12 kV/mm Example 2: Starting materials: Polyimide needle felt according to Example 1, but with a weight per unit area of 120 g/m width about 150 mm.

Polyimide solution, 10 by weight of P 84 in DMF.

The needle felt was impregnated at 15 0 C, dried and compacted by means of a twin-roll calender by maintaining the following parameters: 12

I

r I I I

'I

Roll temperature: 250°C Roll gap: 0.04 mm Pressure of the rolls: 43 kN and 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 g/m2 and a mean thickness of 0.24 mm.

Thermal properties: LOI: 36 to 37 02 TGA: maximum weight loss at 530°C TG point: 312 0

C

Mechanical properties: Tearing strength longitudinal: 50 N/mm 2 transverse: 30 N/mm 2 Elongation longitudinal: 6.5 transverse: 3 Dielectric breakdown strength at direct voltage: 30 kV/mm alternating voltage: 17 kV/mm Example 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 20 0 C, dried and 13r compacted by means of a twin-roll calender.

The paperlike material obtained according to the invention had a weight per unit area of 222 g/m and a mean thickness of 0.28 mm.

Thermal properties: LOI: 36 to 37 02 TGA: maximum weight loss at 5300C TG point: 312°C Mechanical properties: Tearing strength longitudinal: 33 N/mm 2 Elongation longitudinal: 7.5 Dielectric breakdown strength at direct voltage: 25 kV/mm I 15 alternating voltage: 9 kV/mm Example 4: Starting materials: Polyimide needle felt according to Example 1, but with a 2 weight per unit area of 60 g/m 2 width about 150 mm.

Polyimide solution, 30 by weight of P 84 in DMF.

The needle felt was impregnated at 55°C, dried and compacted by means of a twin-roll calender by maintaining the following parameters: Roll temperature: 250 0

C

Roll gap: 0.04 mm 14 Pressure of the rolls: 10 kN The paperlike material obtained according to the invention had a 2 weight per unit area of 90 g/m and a mean thickness of 0.15 mm.

Thermal properties: LOI: 36 to 37 02 2 TGA: maximum weight loss at 530 0

C

TG point: 312°C Mechanical properties: 2 Tearing strength longitudinal: 50 N/mm I transverse: 30 N/mm Elongation longitudinal: 4 transverse: 3 15 Dielectric breakdown strength at direct voltage: 55 kV/mm i alternating voltage: 40 kV/mm Example Starting materials: Polyimide needle felt according to Example 1, but with a weight per unit area of 240 g/m 2 width about 150 mm.

Polyimide solution, 10 by weight of P 84 in DMF.

The needle felt was impregnated at 20 0 C, dried and compacted by means of a twin-roll calender by maintaining the following parameters: 15 'I i- Roll temperature: 250 0

C

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 g/m 2 and a mean thickness of 0.26 mm.

Thermal properties: LOI: 36 to 37 02 TGA: maximum weight loss at 530 0

C

TG point: 312 0

C

Mechanical properties: Tearing strength longitudinal: 60 N/mm 2 transverse: 40 N/mm 2 Elongation longitudinal: 9 transverse: 6 Dielectric breakdown strength at I direct voltage: 25 kV/mm alternating voltage: 10 kV/mm Ii Example 6: Starting materials: Polyimide needle felt according to Example 1, but with a weight per unit area of 260 g/m 2 width about 150 mm.

Polyimide solution, 15 by weight of P 84 in DMF.

The needle felt was impregnated at 20 C, dried and 16 I A T- compacted by means of a twin-roll calender by maintaining the following parameters: Roll temperature: 230 to 240 0

C

Roll gap: 0.04 mm Pressure of the rolls: 3, kN The paperlike material obtained according to the invention had a weight per unit area of 290 g/m2 and a mean thickness of 0.35 mm.

Thermal properties: LOI: 36 to 37 02 l TGA: maximum weight loss at 530°C TG point: 312°C Mechanical properties: Tearing strength longitudinal: 110 N/mm 2 transverse: 90 N/mm 2 Elongation longitudinal: 15 transverse: 12 Dielectric breakdown strength at direct voltage: 12 kV/mm alternating voltage: 9 kV/mm Example 7: The preparation of fibrids may be realized in a known manner, by spraying the polymer solution into an aqueous precipitation bath.

To this end, a 5 solution of P 84 in DMF was fed 17

I

to a binary spinneret by means of a gear pump and was atomized into the aqueous precipitation bath by compressed air.

Spinneret diameter: 2.1 mm Feed rate (polymer solution): 100 cm 3 /min Compressed air pressure: 6 bar The diameter of the fibrids obtained, on an average, amounted to 2-3 mm.

Example 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 1,500 rpm Intake pressure: 0.5 bar Run-out pressure: 3.5 bar Duration time: 40 min Example 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 18 ii it .1g.

I

I Example 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/m 2 and was pressed on a plate press at 280 C and 290 bar.

Vt If

V

lit I

I,,

Thermal properties: LOI: 37 to 38 02 TGA: onset at 5640C TG point: 312°C Mechanical properties: Tensile strength longitudinal: transverse: Elongation longitudinal: transverse: Dielectric breakdown strength at direct voltage: 12 kV/mm alternating voltage: 6 kV/mm.

I V V Vi 31 N/mm 2 26 N/mm 2 12 10 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 grou-iJ polyimide fibers having lengths of from 0.01 to mm and polyvinyl alcohol fibers at a quantitative 19 T 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 g/m2 and was pressed on a plate press at 70 C and 490 bar.

Thermal properties: LOI: 37 to 38 02 TGA: onset at 564 0

C

TG point: 312 0

C

Mechanical properties: Tensile strength longitudinal: 27 N/mm 2 transverse: 22 N/mm 2 Elongation longitudinal: 19 transverse: 15 Dielectric breakdown strength at direct voltage: 47 kV/mm alternating voltage: 26 kV/mm.

Example 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 molder, wet-pressed and dried.

The nonwoven thus produced had a weight per unit area of 102 g/m 2 and was pressed on a plate press at 450 0 C and bar.

20 Thermal properties: LOI: 39 to 40 02 TGA: onset at 564 C TG point: 334°C Mechanical properties: Tensile strength longitudinal: 67 N/mm 2 -transverse: 56 N/mm 2 Elongation longitudinal: 11 transverse: 9 Dielectric breakdown strength at direct voltage: 11 kV/mm alternating voltage: 6 kV/mm.

Example 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 g/m and was pressed on a plate press at 3500C and 250 bar.

Thermal properties: LOI: 38 to 39 02 TGA: onset at 564 0

C

TG point: 328 0

C

Mechanical properties: 21 Tensile strength longitudinal: 81 N/mm 2 transverse: 68 N/mm 2 Elongation longitudinal: 7 transverse: 5 Dielectric breakdown strength at direct voltage: 14 kV/mm alternating voltage: 7 kV/mm.

Example 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 g/m2 and was pressed on a tworoll calender at 350 0 C and 500 N/m.

Thermal properties: LOI: 38 to 39 02 TGA: onset at 564°C TG point: 328 0

C

Mechanical properties: Tensile strength longitudinal: 30 N/mm 2 transverse: 25 N/mm 2 Elongation longitudinal: 12 transverse: 9 Dielectric breakdown strength at direct voltage: 13 kV/mm 22 il~i i i mm~ alternating voltage: 7 kV/mm.

Example 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 350 C and 1,000 kN/m.

Thermal properties: LOI: 38 to 39 02 TGA: onset at 564°C TG point: 328 C Mechanical properties: Tensile strength longitudinal: 117 N/mm 2 transverse: 98 N/mm 2 Elongation longitudinal: 8 transverse: 5 Dielectric breakdown strength at direct voltage: 75 kV/mm alternating voltage: 42 kV/mm.

Example 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 g/m 2 and 23 T- i was pressed on a plate press at 330°C and 340 bar.

Thermal properties: I LOI: 37 to 38 02 TGA: onset at 564°C TG point: 319°C Mechanical properties: Tensile strength: 79 N/mm 2 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 i 15 5.0 mm were slurried in water and molded to a nonwoven I on a sheet former, wet-pressed, sprayed with binder and i dried. The nonwoven thus produced had a weight per area unit of 105 g/m2 and was pressed in three layers on a plate press at 350 C and 480 bar. The weight of the paper K 20 per area unit was 315 g/m 2 Subsequent separation of the 1 three layers was no longer possible.

Thermal properties: LOI: 38 to 39 02 TGA: onset at 564 0

C

TG point: 328 0

C

Mechanical properties: Tensile strength: 75 N/mm 2 Elongation: 13 24 Dielectric breakdown strength at direct voltage: 68 kV/mm alternating voltage: 39 kV/mm.

Example 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 wetfleece molder, wet-pressed and dried. The nonwoven thus produced had a weight per nit area of 183 g/m 2 and, immediately upon the drying process, was pressed on a twin-roll calender at 22oC and 5' N/m.

Thermal properties: LOI: 37 to 38 02 TGA: onset at 564°C TG point: 312°C Mechanical properties: Tensile strength longitudinal: 33 N/mm 2 transverse: 28 N/mm 2 Elongation longitudinal: 17 transverse: 14 Dielectric breakdown strength at direct voltage: 66 kV/mm alternating voltage: 37 kV/mm.

25 i -g i

I

U

12 ij/ Examje 19: Ground polyimide fibers having lengths of 0.01 to mm were slurried in water and molded to a nonwoven on a sheet former, wet-pressed and dried at 3500C. The nonwoven thus produced had a weight per area unit of 100 g/m and, immediately upon the drying process, was pressed on a plate press at room temperature 210C and 480 bar.

Thermal properties: LOI: 37 to 38 02 TGA: onset at 564°C TG point: 312°C Mechanical properties: Tensile strength: 15 N/mm 2 Elongation: 18 Dielectric breakdown strength at direct voltage: 44 kV/mm alternating voltage: 25 kV/mm.

J'-ii 20 Exam2le Ground polyimide fibers having lengths of 0.01 to mm were slurried in water and molded to a nonwoven on a wet fleece molder, wet-pressed, sprayed with silicon finish and dried at 150°C. The nonwoven thus produced had a weight per area unit of 205 g/m 2 and was pressed on a plate press at 320 0 C and 350 bar.

Thermal properties: 26 -27 LOI: 37 to 38 02 TGA: onset at 564°C TG point: 312 0

C

Mechanical properties: Tensile strength longitudinal: transverse: Elongation longitudinal: transverse: Dielectric breakdown strength at direct voltage: 24 kV/mm alternating voltage: 13 kV/mm.

75 N/mm 2 63 N/mm 2 10 8 Example 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 1050C. The nonwoven thus produced had a weight per area unit of 1,090 g/m 2 and was pressed on a plate press at 3500C and 380 bar.

Thermal properties: LOI: 38 to 39 02 TGA: onset at 564°C TG point: 3280C Mechanical properties: Tensile strength: 57 N/mm 2 Elongation: 14 27 Dielectric breakdown strength at direct voltage: 31 kV/mm alternating voltage: 17 kV/mm.

Example 22: Polyimide fibrids were impacted into water and molded to a nonwoven on a sheet former, wet-pressed and dried at 105 C. The nonwoven thus produced had a weight per unit area of 1,090 g/m and was pressed on a plate press at 350°C and 380 bar.

Thermal properties: LOI: 38 to 39 02 TGA: onset at 564 0

C

TG point: 328 0

C

Mechanical properties: Tensile strength: 57 N/mm 2 Elongation: 14 Dielectric breakdown strength at direct voltage: 31 kV/mm alternating voltage: 17 kV/mm.

Example 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 150 0 C. The nonwoven thus produced had a weight per unit area of 210 g/m 2 and was 28

I

'1 pressed on a plate press at 320 0 C and 350 bar.

Thermal properties: LOI: 37 to 38 02 TGA: onset at 564°C TG point: 3120C Mechanical properties: Tensile strength: 88 N/mm 2 Elongation: 12 Dielectric breakdown strength at direct voltage: 21 kV/mm alternating voltage: 10 kV/mm.

Example 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 150 0 C. The nonwoven thus 2 produced had a weight per unit area of 503 g/m 2 and was pressed on a plate press at 320 0 C and 350 bar.

Thermal properties: LOI: 37 to 38 02 TGA: onset at 564°C TG point: 312°C Mechanical properties: Tensile strength: 81 N/mm 2 Elongation: 20 The electric puncture strength was 9 Direct voltage: 19 ky/mm Alternating voltage: 7 ky/mm.

It 14

K

I I S I 30