IL92371A - Flame-retardant high-temperature resistant paperlike materials based on polyimide fibers - Google Patents

Description

FLAME- RETARDANT HIGH-TEMPERATURE RESISTANT PAPERLIKE MATERIALS BASED ON POLYIMIDE FIBERS ninpi>nn jnion * Β»Ν>^Α »a>o by ooiann M*I >IOT imn ABSTRACT OF THE DISCLOSURE: There is d isclosed a process for producing high-temperature resistant and f 1 a m e - r e t a r d a n t 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 % 02 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 invent ion having a weight per unit area of between 60 and 290 g/m are producible by contacting a m a n i u 1 a t a b 1 e sheet-like structure of polyimide fibers with a polyimide solution, drying and, if desired, compacting the same.

The invention relates to f 1 a m e - r e t a r d a n t , high-temperature resistant paperlike mater ial 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 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 artd fibrids of aromatic polyamides ( m - a r a m i d e s ) , 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 ate obtained according to a k own paper-making procedure, whose strengths are increased by he addition of cross- linking agents and radiation.

US-Λ - 2,999,788 relates to the production of pulp from fi r ids of var ious polymers and to the structures roduced therefrom. Ε Ρ - Λ 0 1 1 9 1 8 5 relates to polyimlde fibers and their production/. These may serve as starting material for the production of the products of the present invention. e reparation of fibrids from m-aramides, which may subsequen ly be utilized for he production of synthetic apers, are described by several Japanese patents (JP 59-47695, JP 60-126400, JP 61-157532, JP 62- 85014, JP 62-85015, JP 62-85018). he roduct ion of f ibrids from synthetic polymers also is described in US-Λ - 3,018,091.

The syn hetic papers known so far, in particular those based on aromatic polyamides, no longer have proved satisfactory in many fields of applicatio s witli regard to hi h tem erature resistance, aging stability and stability at elevated emperatures.

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 olymers of the ge eral formula wherein n is an integer greater than 1 and A represents a four-valent aromatic group selected from which material exhibits the following properties in combination: a weight per unit area of between 20 and 2 2 1,100 g/m , in particular of between 60 and 290 g/m , 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/ram , and - a d i e 1 e c t r i cb r ea k d o w n 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 propert ies .

Paperlike materials according to the invention 2 having a weight per unit area of between 60 and 290 g/m are capable of being produced in that a manipulatable sheet-like structure based on polyimide fibers of the general formula 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 man i pu 1 a t a b 1 e 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 2 per unit area of from 60 to 200 g/m 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 ( D Ac ) , 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°C.

In the upper temperature range, the reduced viscos i ty of the solut ion 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°C. 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 350°C, to equalize their thickness. Most su itably, it is operated at "l ine pressures" (roll separation force per unit length) of 10 to 1,000 kN/m.

It turned out that the paperlike materials according to the invention having a weight per unit area of between 2 60 and 290 g/m 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 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 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 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, fibers, f ibrids and fibrils 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 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.

This may be effected at line pressures of between 0.1 and 1,000 kN/m and at temperatures of preferably 70°C to 450°C. It is also possible to compress multi-layer nonwovens, the coherence of the individual layers being ensured by the t he r mopl as t i c i t y 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 requ ire, 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 2 unit area of between 60 and 290 g/m and of between 20 and 1,100 g/m being described in examples 1 to 6 and 10 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 the following assay methods: LOI (limiting oxygen index) ASTM D-2863 TGA ( t he r tnog r a v i me t r i c analysis) device: Perkin Elmer TGA/2 heating rate: 20°C/min 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 5345 Example l i_ Starting materials: Polyimide needle felt, produced from benzophenone- 3 , 3 ' , 4 , 4 ' - 1 e t r aca r boxy 1 i c acid dianhydride and 4 , 4 ' - m e t hy 1 eneb i s ( pheny 1 i socy ana t e ) and 2,4- and 2 , 6 - t o 1 u e n e d i i s o c a n a t e (trade name P 84, producer: Lenzing Ak t i enge s e 11 scha f t ) , 2 weight per unit area: 85 g/m width: about 150 mm.

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

The needle felt was impregnated at 22°C, dried and compacted by means of a twin-roll calender by maintaining the following parameters: Roll temperature: 250 to 245°C Roll gap : 0.04 mm Pressure of the rolls: 10 kN The paperl ike material obtained according to the invention had a weight per unit area of 167 g/m and a mean thickness of 0.20 mm.

Thermal properties: LOI: 36 to 37 » Oj TGA: maximum weight loss at 530°C TG point: 312°C Mechanical properties: 2 Tearing strength - longitudinal: 30 N/mm 2 - transverse: 27 N/mm 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 a 2 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°C, dried and compacted by means of a twin-roll calender by maintaining the following parameters: Roll temperature: 250°C Roll gap: 0.0 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 5 a weight per unit area of 238 g/m 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°C Mechanical properties: 2 Tearing strength - longitudinal: 50 N/mm 2 - transverse: 30 N/mm Elongation - longitudinal: 6.5 % - transverse: 3 % Dielectric breakdown strength at direct voltage: 30 kV/mm alternating voltage: 17 kV/mm E ample 3j^ 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°C, dried and compacted by means of a twin-roll calender.

The paperlike material obtained according to the invention had a 2 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 530°C TG point: 312°C Mechanical properties: 2 Tearing strength - longitudinal: 33 N/mm Elongation - longitudinal: 7.5 % Dielectric breakdown strength at direct voltage: 25 kV/mm alternating voltage: 9 kV/mm ExampJLe 4^ Starting materials: Polyimide needle felt according to Example 1 but with 2 weight per unit area of 60 g/m 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°C Rol 1 gap : 0.04 mm Pressure of the rolls: 10 kN The paperlike material obtained according to the invent ion had a weight per unit area of 90 g/m and a mean thickness of 0.15 mm.

Thermal properties: LOI : 36 to 37 I Oj TGA: maximum weight loss at 530°C TG point: 312°C Mechanical properties: Tearing strength - longitudinal: 50 N/mm 2 - transverse: 30 N/mm Elongation - longitudinal: 4 % - transverse : 3 % Dielectric breakdown strength at direct voltage: 55 kV/mm alternating voltage: 40 kV/mm Examp_l.e 5j_ Starting materials: Polyimide needle felt according to Example 1, but with a 2 weight per unit area of 240 g/m width about 150 mm.

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

The needle felt was impregnated at 20 °C , dried and compacted by means of a twin-roll calender by maintaining the following parameters: Roll temperature: 250°C Roll gap: 0.04 mm Pressure of the rolls: 40 to 52 kN The paperlike material obtained according to the invention had 2 a weight per unit area of 260 g/m and a mean thickness of 0.26 mm.

Thermal properties: LOI: 36 to 37 % 02 TGA: maximum weight loss at 530°C TG point: 312°C Mechanical properties: Tearing strength - longitudinal: 60 N/mm 2 - transverse: 40 N/mm Elongation - longitudinal: 9 % - transverse: 6 % Dielectric breakdown strength at direct voltage: 25 kV/mm alternating voltage: 10 kV/mm Example 6j^ Starting materials: Polyimide needle felt according to Example 1» but with a weight per unit area of 260 g/m width about 150 mm.

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

The needle felt as impregnated at 20°C, dried and compacted by means of a twin-roll calender by maintaining the following parameters: Roll temperature: 230 to 240°C Roll gap: 0.04 mm Pressure of the rolls: 35 kN The paperlike material obtained according to the invention had a 2 weight per unit area of 290 g/m and a mean thickness of 0.35 mm.

Thermal properties: LOI : 36 to 37 % 02 TGA: maximum weight loss at 530°C TG point: 312°C Mechanical properties: 2 Tearing strength - longitudinal: 110 N/mm 2 - transverse: 90 N/mm 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, 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 to a binary spinneret by means of a gear pump and was atom ized into the aqueous precipitat ion bath by compressed air.

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

Ex amgl. e B i_ 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 Example 9χ 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 %.

E x a mp^e 1 Poly i in ide 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 280°C and 290 bar.

Thermal properties: LOI: 37 to 38 % 02 TGA: onset at 564°C TG point: 312°C Mechanical properties: 2 Tensile strength - longitudinal: 31 N/mm 2 - transverse: 26 N/mm Elongation - longitudinal: 12 % - transverse : 10 % Dielectric breakdown strength at direct voltage: 12 kV/mm alternating voltage: 6 kV/mm.

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 oiolder, 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°C TG point: 312°C Mechanical properties: 2 Tensile strength - longitudinal: 27 N/mm 2 - transverse: 22 N/mm 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/m2 and was pressed on a plate press at 450°C and 50 bar .

Thermal properties: LOI: 39 to 40 % O2 TGA: onset at 564°C TG point: 334°C Mechanical properties: 2 Tensile strength - longitudinal: 67 N/ram - transverse: 56 N/mm Elongation - longitudinal: 11 % - transverse: 9 % Dielectric breakdown strength at direct voltage: 11 kV/mm alternating voltage: 6 kV/n.

Example 1 ± 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 350°C and 250 bar .

Thermal properties: LOI: 38 to 39 % 02 TGA: onset at 564°C TG point: 328°C Mechanical properties: 2 Tensile strength - longitudinal: 81 N/mm 2 - transverse: 68 N/mm Elongation - longitudinal: 7 % - t ransver se : 5 % Dielectric breakdown strength at direct voltage: 14 kV/mm alternating voltage: 7 kV/mm.

Ex a triple 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 2 weight per unit area of 40 g/m and was pressed on a two-roll calender at 350°C and 500 N/m.

Thermal properties: LOI: 38 to 39 % 02 TGA: onset at 564°C TG point: 328°C Mechanical properties: Tensile strength - longitudinal: 30 N/mm 2 - transverse: 25 N/mm Elongation - longitudinal: 12 % - transverse: 9 % Dielectric breakdown strength at direct voltage: 13 kV/mm alternating voltage: 7 kV/mm.

Ex ample 1.5^ 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 2 unit of 55 g/m 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: 2 Tensile strength - longitudinal: 117 N/mm 2 - transverse: 98 N/mm Elongation - longitudinal: 8 % - transverse: 5 % Dielectric breakdown strength at direct voltage: 75 kV/mm alternating voltage: 42 kV/mm.

Example 16j.

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 2 thus produced had a weight per unit area of 252 g/m and was pressed on a plate press at 330°C and 340 bar.

Thermal properties: LOI: 37 to 38 % 02 TGA: onset at 564°C TG point: 319°C Mechanical properties: Tensile strength: 79 N/mm Elongat ion : 10 % Dielectric breakdown strength at direct voltage: 21 kV/mm alternating voltage: 11 kV/mm.

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 2 unit of 105 g/m and was pressed an three layers on a plate press at 350°C and 480 bar. The weight of the paper 2 per area unit was 315 g/m . Subsequent separation of the three layers was no longer possible.

Thermal properties: LOI: 38 to 39 % 02 TGA: onset at 564°C TG point: 328°C Mechanical properties: Tensile strength: 75 N/mm Elongation: 13 % Dielectric breakdown strength at direct voltage: 68 kV/mm alternating voltage: 39 kV/mm.

Ex ample 18j_ 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 had a weight per unit area of 183 g/m and, immediately upon the drying process, was pressed on a twin-roll calender at 22°C and 500 kN/m.

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

Example ^9j_ 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 350°C. The nonwoven thus produced had a weight per area unit of 100 2 g/m and, immediately upon the drying process, was pressed on a plate press at room temperature 21°C and 480 bar .

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

E ample 20^ Ground pblyimide 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 150°C. The nonwoven thus produced had a weight per area unit of 205 g/m and was pressed on a plate press at 320°C and 350 bar.

Thermal properties: LOI : 37 to 38 » Oj TGA: onset at 564°C TG point: 312°C Mechanical properties: 2 Tensile strength - longitudinal: 75 N/mm 2 - transverse: 63 N/mm Elongation - longitudinal: 10 % - transverse : 8 % Dielectric breakdown strength at direct voltage: 24 kV/mm alternating voltage: 13 kV/mm.

Exam le 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 105°C. The nonwoven thus produced had a weight per area unit of 1,090 g/m2 and was pressed on a plate press at 350°C and 380 bar.

Thermal properties: LOI : 38 to 39 t O.

TGA: onset at 564°C TG point: 328°C Mechanical properties: 2 Tensile strength: 57 N/mm Elongat ion : 14 % Dielectric breakdown strength at direct voltage: 31 kV/mm alternating voltage: 17 kV/mm.

Exam le 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°C TG point: 328°C Mechanical properties: 2 Tensile strength: 57 N/mm Elongat ion : 14 % Dielectric breakdown strength at direct voltage: 31 kV/mm alternating voltage: 17 kV/mm.

Example 23j.

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°C. The nonwoven thus 2 produced had a weight per unit area of 210 g/m and was pressed on a plate press at 320 °C and 350 bar, Thermal properties: LOI: 37 to 38 % 02 TGA: onset at 564°C TG point: 312°C Mechanical properties: 2 Tensile strength: 88 N/mm Elongation: 12 % Dielectric breakdown strength at direct voltage: 21 kV/mm alternating voltage: 10 kV/mm.

E x ampl. e 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°C. The nonwoven thus produced had a weight per unit area of 503 g/m and was pressed on a plate press at 320°C and 350 bar.

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

Claims (26)

The claims defining the invention are as follows:

1. A f 1 a m e - r e t a r d a n t high-temperature resistant paperl ike material based on polyimide fibers of the general formula wherein n is an integer greater than 1 and A represents a four-valent aromatic group selected from wherein X is CO, CH2, 0, S, CF2 and R represents at least one of the following divalent aromatic groups selected which paperlike material comprises the following properties in combination: a weight per unit area of 2 between 20 and 1,100 g/m , a LOI (limited oxygen index) value of at least 32 % 02 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/m2.

3. A paperlike material as set forth in claim 1, further comprising - a tearing strength in the longitudinal direction of 2 between 30 and 120 N/mm , and - a dielectric breakdown strength of 10 to 65 kV/mm at direct voltage and of 15 to 50 kV/mm at alternating vol t age .

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 f 1 a m e - r e t a r d a n t high-temperature resistant paperlike material having a weight 2 per unit area of between 60 and 290 g/m , a LOI (limited oxygen index) value of at least 32 % O- and a TG point of least 300 C, which process comprises providing a m an i pu 1 a t ab 1 e sheet-like structure based polyimide fibers of the general formula wherein n is an integer greater than 1 and A represents a four-valent aromatic group selected from contacting said sheet-like structure with a polyimide so 1 u t on, 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 poly im ides 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 ( D Ac ) 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 f irth 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. 1 14. A process for producing a f 1 a m e - r e t a r d an t high- 2 temperature resistant papedike material having a weight 3 per unit area of between 60 and 290 g/m , a LOI (limited 4 oxygen index) value of at least 32 % Oj and a TG point of 5 at least 300°C, which process comprises g providing a pulp essentially consisting of an aqueous 7 suspension comprising polyimide fibers, and at least one 8 of polyimide fibrils and polyimide fibrids having 9 structural units of the general formula

14. Q wherein n is an integer greater than 1 and A represents a 1 four-valent aromatic group selected from wherein X is CO, CH-, O, S, CF- and R represents at least 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 f lam e- r e t a r dan t high-temperature resistant paperlike material having a weight per unit area of between 20 and 1,100 g/m , a LOI (limited oxygen index) value of at least 32 % 02 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 wherein η is an integer greater than 1 and A represents a four-valent aromatic group selected from wherein X is CO, CHj, 0, S, CF2 and R represents at least one of the following divalent aromatic groups selected processing said pulp , in a manner known per s e , to a non w oven , 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. COHEH¾DE * RAPAPO T