EP0645479A1 - High strength and high modulus polyacrylonitrile fibers, process for their production and their use - Google Patents

Abstract

Described are fibres containing polyacrylonitrile homopolymer or copolymer having a weight average molecular weight of less than 210,000 as fibre-forming substance, said fibres having tenacities of more than 80 cN/tex, initial moduli, based on 100 % extension, of more than 1,800 cN/tex and breaking extensions of less than 10 %. Also described is a process for producing high strength polyacrylonitrile fibres, comprising the following measures: a) preparing a spinning solution containing an organic aprotic solvent or a mixture of such solvents and at least 15 % by weight, based on the spinning solution, of a polyacrylonitrile homopolymer or copolymer having a weight average molecular weight of less than 500,000 or a mixture of such polymers, b) spinning this spinning solution by a wet spinning process or by a dry jet wet spinning process into a coagulation bath at an extrusion speed of at least 5 m/min, c) coagulating the spun filaments in the coagulation bath and withdrawing these filaments from the coagulation bath, and d) aftertreating the spun filaments by performing one or more drawing steps, the draw ratio between the device for withdrawing the spun filaments from the coagulation bath and the point of exit from the aftertreatment zone being at least 1 : 12.

Description

The present invention relates to high-strength polyacrylonitrile fibers (hereinafter referred to as PAN fibers) of high modulus, and to a particularly adapted process for their production and their use, in particular as reinforcing materials or for the production of filters or friction linings.

PAN fibers of high strength are known per se. For example, Dobrecov et al. in soviet. Contributions to fiber research and textile technology, 9 , pp. 407-411 (1972) described PAN fibers of high strength and high modulus, which are derived from PAN types of high molecular weight; Examples of this are molecular weights of 3 * 10⁶.

From EP-A-165,372 and -255,109 fibers of strengths with more than 8.83 cN / dtex and processes for their production are known, in which PAN types of high molecular weight are also used. According to EP-A-255,109 PAN types with a molecular weight of more than 500,000 (weight average) are used, while according to EP-A-165,372 PAN types with an intrinsic viscosity of more than 2.5 are used, which has a molecular weight of more than 210,000 (weight average) corresponds.

In the abovementioned publications, PAN types of unusually high molecular weight are used without exception. Usual molecular weight values for PAN fibers range approximately from 80,000 to 180,000 (cf. the statements by Falkai et al. In "Synthesefaser", p. 200, Verlag Chemie (1981) or by Masson et al. In " Fiber Producer ", June 1984, pp. 34-37).

The use of the high molecular weight PAN types specified in these documents also causes problems in the production of these fibers. Because of the lower solubility of the high molecular weight PAN types, the spinning mass concentration for the production of a spinning mass must be reduced. For example, when processing PAN types of lower molecular weight, it is possible to work with spinning mass concentrations of 19-21%. In the above documents, on the other hand, reduced spinning mass concentrations of at most 10-15% are used. This means a significant loss in productivity of 25 - 70% for a production plant. For example, according to EP-A-165,372, a spinning mass concentration of between 6 and 12% is used (Examples 5-7). That means a loss of productivity between 45 - 70%.

Furthermore, the residence time for dissolving the PAN in the solvent increases considerably with increasing molecular weight of the polymer. Here new systems for solving have to be purchased in order to maintain the effectiveness of the solving process. Additional measures usually have to be taken to work with a higher spinning mass concentration. EP-A-255,109 attempts to reduce the viscosity by adding 1-10% water to the spinning mass in order to be able to work with a higher spinning mass concentration. However, this is associated with a risk of corrosion for the systems; in addition, this measure only allows limited viscosity reductions.

PAN fibers of high strength have also become known which have been produced with PAN types of the usual molecular weight. For example, GB-A-1,193,170 describes PAN fibers which have strengths of up to 17.5 g / denier. The elongation at break of the fibers described is, however, at more than 15% too high for many applications.

From EP-A-44,534 high-strength PAN fibers of high modulus are known, which have also been produced with PAN types of the usual molecular weight are. Fibers with strengths of up to 81 cN / tex or with initial moduli of up to 1989 cN / tex are described. PAN fibers which have strengths of more than 80 cN / tex and at the same time initial moduli of more than 1800 cN / tex are not described in this document.

PAN fibers are in demand as reinforcement materials due to their high resistance in aggressive environments, for example in strongly alkaline environments, or against radiation. High strengths and high initial moduli with low elongation at break are particularly in demand for technical applications. There is a need for PAN fibers with such a property profile, in particular for PAN fibers that can be obtained by high productivity processes.

The present invention relates to fibers containing polyacrylonitrile homopolymer or copolymer with a weight average molecular weight of less than 210,000 as a fiber-forming substance, the fibers having strengths of more than 80 cN / tex, initial moduli, based on 100% elongation, of more than 1800 cN / tex and have elongations at break of less than 10%.

The precipitation or solution polymers prepared by the customary processes can be used as polymer raw materials. Depending on the requirements for the areas of application, both homo- and copolymers of acrylonitrile can be used. The purity of the monmer used should be as high as possible. Suitable comonomers are all unsaturated compounds copolymerizable with acrylonitrile, preferably unsaturated carboxylic acids, such as acrylic acid, methacrylic acid or itaconic acid; unsaturated sulfonic acids, such as allyl, methallyl or styrene sulfonic acid; unsaturated carboxamides, such as acrylamide or methacrylamide; Esters of unsaturated carboxylic acids, such as the methyl, ethyl, propyl or butyl esters of acrylic or methacrylic acid or polyfunctional hydroxyethyl or aminoethyl esters or their derivatives of acrylic or methacrylic acid; Esters of carboxylic acids with unsaturated alcohols or ethers based on unsaturated alcohols, such as vinyl esters and ethers, for example vinyl acetate, vinyl stearate, vinyl butyrate, vinyl bromoacetate, vinyl dichloroacetate or vinyl trichloroacetate; unsaturated aldehydes or ketones, such as acrolein or crotonaldehyde; Acid halides of unsaturated carboxylic acids, such as acrylic or methacrylic acid chlorides; or further monomers copolymerizable with acrylonitrile, such as styrene, butadiene, propylene or vinyl halides, for example vinyl chloride, vinylidene chloride or vinyl bromide.

The polymers used preferably have a content of at least 90% by weight, in particular at least 99% by weight, of acrylonitrile units.

Polyacrylonitrile homopolymers or copolymers whose molecular weight (weight average) is 175,000 to 210,000 are used in particular.

The strengths of the fibers according to the invention are preferably 89 to 100 cN / tex.

The initial moduli, based on 100% elongation, of the fibers according to the invention are preferably 1850 to 2150 cN / tex, very particularly preferably 1900 to 2150 cN / tex.

The elongations at break of the fibers according to the invention are preferably 7 to 9%.

Also preferred are fibers, as defined above, which have knot strengths of more than 15 cN / tex, in particular from 17 to 20 cN / tex.

The titers of the fibers according to the invention are usually in the range of textile titers, for example in the range of less than or equal to 3.5 dtex. Fibers with a titre of 1.5 to 2.5 dtex are preferred.

It has been found in the course of development that high-strength PAN fibers can be produced with high productivity if certain process conditions are observed.

• a) Herstellung einer Spinnlösung enthaltend ein organisches aprotisches Lösungsmittel oder eine Mischung derartiger Lösungsmittel und mindestens 15 Gew.%, bezogen auf die Spinnlösung, eines Polyacrylnitrilhomopolymeren oder -copolymeren mit einem Gewichtsmittel des Molekulargewichtes von weniger als 500,000 oder einer Mischung aus solchen Polymeren,
• b) Verspinnen dieser Spinnlösung nach einem Nassspinnverfahren oder einem Trockendüsen-Nassspinnverfahren in ein Koagulationsbad, wobei eine Spritzgeschwindigkeit der Spinnlösung von mindestens 5 m/min gewählt wird,
• c) Koagulation der ersponnenen Fäden im Koagulationsbad und Abziehen dieser Fäden aus dem Koagulationsbad, und
• d) Nachbehandeln der ersponnenen Fäden unter Durchführung einer oder mehrerer Verstreckungen, wobei der Verstreckgrad zwischen Abzugsvorrichtungen der ersponnenen Fäden aus dem Koagulationsbad und Ausgang der Nachbehandlungsstrecke mindestens 1 : 12 beträgt.

The invention therefore also relates to a method for producing high-strength polyacrylonitrile fibers comprising the following measures:

• a) preparation of a spinning solution containing an organic aprotic solvent or a mixture of such solvents and at least 15% by weight, based on the spinning solution, of a polyacrylonitrile homopolymer or copolymer with a weight average molecular weight of less than 500,000 or a mixture of such polymers,
• b) spinning this spinning solution using a wet spinning process or a dry-jet wet spinning process in a coagulation bath, a spray speed of the spinning solution of at least 5 m / min being selected,
• c) coagulation of the spun threads in the coagulation bath and removal of these threads from the coagulation bath, and
• d) post-treatment of the spun threads by carrying out one or more stretching, the degree of stretching between draw-off devices of the spun threads from the coagulation bath and exit of the post-treatment section being at least 1:12.

Any organic aprotic solvent or a mixture of such solvents can be used as the spinning solvent. Examples of such solvents are dimethyl sulfoxide (DMSO), dimethylacetamide (DMAC) and in particular dimethylformamide (DMF).

In the process according to the invention, polymers are used which have a weight average molecular weight of less than or equal to 500,000, preferably 74,000 to 500,000. Polymers are particularly preferably used whose weight average molecular weight is 75,000 to 400,000, in particular 175,000 to 250,000.

In order to ensure the highest possible productivity of the process, it is necessary to use a PAN polymer with the molecular weights defined above. With these PAN polymers of relatively low molecular weight, it is possible to prepare spinning solutions with a sufficiently high concentration of the polymer without having to use auxiliaries which may interfere with the process.

If PAN polymers with higher molecular weights are used, productivity losses can be expected directly.

Classic processes and machines can be used to produce the spinning solution. In the method according to the invention, the spinning mass concentration is at least 15%, preferably 17 to 22%, in particular 18 to 22%. When using dope concentrations of less than 15%, problems with the nozzle run usually occur; i.e. irregularities occur at the nozzles during spinning and, as a result, sticking can occur. Furthermore, the productivity of the aftertreatment line decreases directly with the decrease in the spinning mass concentration.

The spinning mass is usually filtered before spinning. This removes gel particles and any impurities that may be present. Filtration is of great importance in the method according to the invention, since this measure can considerably reduce the error rate during spinning and aftertreatment. Spinning defects can subsequently lead to windings on the drawing rollers during contact and wet drawing of the fiber.

The filtration can be carried out with the known devices, for example with filter presses, in which the spinning mass is pressed through several compact fabric layers. A measure of the filter effect is the so-called continuity; this represents an upper limit for those particle diameters that still pass through the filter.

In the case of spinning solutions with DMF as solvent, filters with a continuity of 5 to 15 µm are preferably used. This means that particles with a diameter of less than 5 to 15 microns can still pass through the filter. If the filtration of the spinning mass is not correct, i.e. in the case of spinning solutions with DMF as solvent with a coarser filter than 15 µm, later production disruptions are to be expected.

In the case of DMF spinning solutions, the filtration temperature is preferably between 80 and 90 ° C.

The speed at which the threads emerge from the spinneret must be selected so that the fibers practically do not bend when immersed in the liquid and maintain their previous direction of movement. This is ensured if the spinning solution has a spray speed of at least 5 m / min, preferably 5 to 7 m / min.

Dabei bedeuten:

S =

Spritzgeschwindigkeit (m/min)

F =

Fördermenge (cm³/min)

Z =

Anzahl der Düsenlöcher

d =

Düsenlochdurchmesser (mm)

Infolge der hohen Spritzgeschwindigkeit treten die ersponnen Fäden ohne merkliche Änderung der Richtung in das Koagulationsbad bzw. durch die Oberfläche des Koagulationsbades ein. Ändert sich die Richtung der Fäden beim Eintauchen in das Koagulationsbad wesentlich, so ist mit Verklebungen der Fasern miteinander und an der Oberfläche der Düse zu rechnen. Im Koagulationsbad kann sich die Bewegungsrichtung der Fäden ändern.The spraying speed S is calculated according to the following equation:

Here mean:

S =

Spraying speed (m / min)

F =

Delivery rate (cm³ / min)

Z =

Number of nozzle holes

d =

Nozzle hole diameter (mm)

As a result of the high spraying speed, the spun threads enter the coagulation bath or through the surface of the coagulation bath without any noticeable change in direction. If the direction of the threads changes significantly when immersed in the coagulation bath, the fibers must stick together and on the surface of the nozzle. The direction of movement of the threads can change in the coagulation bath.

As a result of the high spraying speed and the relatively high viscosity of the highly concentrated spinning solution, a high pressure can build up in the spinneret. This pressure can lead to problems during spinning and to a spillage of the spinneret, i.e. sticking of drops of the spinning mass on the nozzles. It is therefore advisable to increase the temperature of the spinning solution when such problems occur before it passes through the spinneret in order to lower the viscosity of the spinning solution. In the case of DMF as a spinning solvent, it is advisable to heat the spinning mass to at least 100 ° C shortly before it enters the nozzle.

Spinning mass temperatures of these spinning solutions below 100 ° C. generally cause the problems described above. At dope temperatures above 130 ° C, vaporization of the DMF and yellowing of the dope are to be expected. In the case of DMF, temperatures of the spinning solution in front of the spinneret of 100 to 130 ° C. are preferred.

The correct choice of the nozzle hole diameter significantly influences the clean and perfect entry of the threads into the coagulation bath. The required high spray speeds of the method according to the invention are difficult to achieve, in particular when choosing large nozzle hole diameters. In these cases, problems with spinning and spilling of the spinneret can be expected. If such problems occur, it is advisable in individual cases to reduce the nozzle diameter.

In addition, the behavior of the thread when pressed into the liquid of the coagulation bath can be influenced by the choice of the thickness of the thread. As already mentioned, the threads must be pressed into the coagulation bath under such conditions that the fibers do not bend when immersed in the liquid and lose their previous direction of speed. This can also be influenced by the choice of the diameter of the nozzle holes.

Typically, the nozzle hole diameters are less than 120 µm; nozzle hole diameters of 60 to 120 μm are preferred.

The spinning can be carried out according to the wet spinning method or dry nozzle wet spinning method known per se. For this purpose, the spinneret can be immersed in the coagulation bath or the spinneret is mounted at a predetermined distance from the surface of the coagulation bath, which causes the spinning to take place through an air gap. The distance between the spinneret and the coagulation bath surface can be varied over a wide range, preferably the distance is less than 10 millimeters, in particular 1 to 10 mm.

The coagulation bath is usually an aqueous mixture containing an organic aprotic solvent - for example a solution, dispersion or suspension of this organic aprotic solvent in water. The organic aprotic solvent in the coagulation bath is preferably the spinning solvent selected in each case.

The concentration of the organic aprotic solvent should be chosen in such a way that there is sufficiently rapid and complete coagulation. When working with relatively highly concentrated spinning solutions, it must be ensured that the concentration of the organic aprotic solvent in the coagulation bath is not or will not be too high. Will the concentration of the organic aprotic solvent in the coagulation bath chosen high, so the fibers can be glued to the take-off godet, since complete coagulation of the fibers is not guaranteed.

Typically, concentrations of the organic aprotic solvent of less than 75% by weight, based on the solution in the coagulation bath, are used. 60 to 75% by weight are preferred.

After the coagulation, the fiber is post-treated; this can be done on a known aftertreatment system.

Surprisingly, it was found that PAN fibers can be obtained by the spinning according to the invention, which can be drawn very high. In order to produce high-strength PAN fibers, the spun threads are post-treated by performing one or more stretching operations, the degree of stretching between devices for drawing off the spun threads from the coagulation bath and the exit of the post-treatment section being at least 1:12, preferably 1:14 to 1:18.

In the aftertreatment, the fiber can, for example, be washed one or more times after leaving the coagulation bath, an additional coagulation being able to take place in these steps. The fiber is usually wet-drawn and / or softened during at least one washing step. After washing, drying is usually carried out. The fibers are then post-drawn in a further drawing step; this can be done by stretching in a hot air bath and / or by contact stretching, for example using heated godets. The fibers are then drawn off, preferably under tension. Furthermore, it is possible and preferred to fix the drawn fibers after the post-drawing. The fibers can then be fed to a cutting device or the fibers are further processed as filaments, for example wound up.

Such post-treatment processes for the PAN fibers are known per se and are described, for example, in EP-A-44,534, -165,372 and -255,109.

The PAN fibers according to the invention can be used for a wide variety of applications. These fibers are typically used for technical purposes. Examples of this are the use as reinforcing material for the production of composite materials, for example for the production of fiber-reinforced thermoplastic or thermosetting plastics or in particular for the production of fiber-reinforced hydraulically setting materials, for example in concrete.

Furthermore, the PAN fibers according to the invention can be used for the production of nonwovens which e.g. can be used as a filter or as geotextiles.

Another preferred area of application of the PAN fibers according to the invention is the production of friction linings, in particular brake linings.

The following examples illustrate the invention without limiting it

example 1

A PAN polymer with a molecular weight of 200,000 (Example 1a) and a PAN polymer with a molecular weight of 550,000 (Example 1b) are dispersed in cold DMF and dissolved at 80-90 ° C. in a dissolver.

The concentration of the solution is chosen so that a ball fall time of 700 ± 50 sec results. The ball fall time is measured by the method of K. Jost (Rheologica Acta, Vol. 1, page 303) at 60 ° C. The following table shows the production conditions and the properties of the spinning solution: Dwell time (min) Bullet fall time Spinning mass conc. (%) Example 1a 30th 690 19th Example 1b 60 685 12th

Example 2

The dope according to example 1a was filtered once through a 10 μ filter (example 2a) and once through a 25 μm filter (example 2b). The spinning masses were then fed to a 100 hole / 120 μm nozzle. The following table shows the dependence of the nozzle pressure on the dwell time of the dope after filtration: Time (min) Example 2a Nozzle pressure (bar) Example 2b Nozzle pressure (bar) 1 7.0 7.0 10th 7.0 8.2 60 7.0 10.1 120 7.1 12.2 180 7.0 spun

Example 3

A PAN polymer with a molecular weight of 200,000 was dissolved as described in Example 1a with a concentration of 16, 18 and 22% in DMF, filtered through a 10 μm filter and a 100 hole / 120 μ nozzle fed. The precipitation bath concentration was 70% DMF in water, the precipitation bath temperature 0 ° C. The results can be found in the following table: example Spinning mass concentration (%) Nozzle run 3a 16 unsatisfactory because of fiber tear at the nozzle 3b 18th very good 3c 22 unsatisfactory because of a sharp increase in nozzle pressure

Example 4

A spinning mass was produced as described in Example 3b. The spinning mass temperature was varied between 85 - 120 ° C. The results are shown in the following table: example Spinning mass temp. ° C Nozzle run 4a 85 bad - run not possible 4b 90 bad - run not possible 4c 95 bad - run with interruptions 4d 100 bad - runs about 30 min. 4e 105 Nozzle is running - individual interruptions 4f 110 good nozzle run 4g 120 very good - nozzle run very good

Example 5

The spinning mass from Example 4g was taken and pressed through nozzles of different diameters. The results are shown in the following table: example jet Nozzle run 5a 100/60 µm satisfactory 5b 100/80 µm Good 5c 100/120 µm Good 5d 100/150 µm bad

Example 6

The dope was prepared as in Example 5c and the spray speed varied. The results are shown in the following table: example Spraying speed (m / min) Nozzle run 6a 3rd not possible 6b 5 good nozzle run 6c 7 good nozzle run

Example 7

A spinning mass and spinning arrangement was prepared as described in Example 6b and the precipitation bath concentration was varied. The results are shown in the following table: example Precipitation bath concentration (%) Gluing 7a 90 very much 7b 80 much 7c 70 non-sticky 7d 60 non-sticky

Example 8

The fiber from Example 7c was drawn off with a take-off godet and wet-drawn, washed, finished, dried, contact-drawn and removed under tension on a classic aftertreatment line. The total draw was 1:12 (example 8a) or 1:10 (example 8b). The results are shown in the following table:
The fiber values are as follows: Measurement 8a 8b Titer (dtex) 2.0 2.0 Strength (cN / tex) 90 80 Knot strength (cN / tex) 20th 14 Strain (%) 8th 10th Module (cN / tex) 2000 1400

The determination of the fiber strength, the fiber elongation and the knot strength was carried out according to DIN 53816.

Claims (23)

Fibers containing polyacrylonitrile homopolymer or copolymer with a weight average molecular weight of less than 210,000 as a fiber-forming substance, the fibers having strengths of more than 80 cN / tex, initial moduli, based on 100% elongation, of more than 1800 cN / tex and elongation at break of less than 10%. Fibers according to claim 1, characterized in that the polyacrylonitrile homopolymer or copolymer has a weight average molecular weight of 175,000 to 210,000. Fibers according to claim 1, characterized in that they have strengths of 89 to 100 cN / tex. Fibers according to Claim 1, characterized in that these initial moduli, based on 100% elongation, have from 1850 to 2150 cN / tex. Fibers according to Claim 4, characterized in that these initial moduli, based on 100% elongation, are from 1900 to 2150 cN / tex. Fibers according to claim 1, characterized in that they have elongations at break of 7 to 9%. Fibers according to claim 1, characterized in that they have knot strengths of more than 15 cN / tex. Fibers according to claim 7, characterized in that they have knot strengths of 17 to 20 cN / tex. Fibers according to claim 1, characterized in that these titers have from 1.5 to 2.5 dtex. Process for producing high-strength polyacrylonitrile fibers comprising the following measures: a) preparation of a spinning solution containing an organic aprotic solvent or a mixture of such solvents and at least 15% by weight, based on the spinning solution, of a polyacrylonitrile homopolymer or copolymer with a weight average molecular weight of less than 500,000 or a mixture of such polymers, b) spinning this spinning solution according to a wet spinning process or dry-nozzle wet spinning process into a coagulation bath, a spray speed of the spinning solution of at least 5 m / min being selected, c) coagulation of the spun threads in the coagulation bath and removal of these threads from the coagulation bath, and d) post-treatment of the spun threads by performing one or more stretching, the degree of stretching between the withdrawal device of the spun threads from the coagulation bath and the exit of the post-treatment section being at least 1:12. A method according to claim 10, characterized in that dimethylformamide is used as the spinning solvent. A method according to claim 10, characterized in that a polyacrylonitrile homopolymer or copolymer with a weight average molecular weight of 175,000 to 250,000 is used. A method according to claim 11, characterized in that the spinning mass containing polyacrylonitrile homopolymer or copolymer and dimethylformamide is filtered through a filter of 5-15 µm patency before spinning. Process according to Claim 11, characterized in that 18 to 22% by weight, based on the spinning solution, of the polyacrylonitrile homopolymer or copolymer is present in the spinning solution. A method according to claim 11, characterized in that the temperature of the spinning solution before the spinneret is 100 to 130 ° C. A method according to claim 11, characterized in that the nozzle hole diameter is 60 to 120 µm. A method according to claim 10, characterized in that the spraying speed is 5 to 7 m / min. A method according to claim 10, characterized in that the spinning is carried out by a dry-jet wet spinning process, the width of the air gap between the spinneret and the surface of the coagulation bath being 1 to 10 mm. A method according to claim 10, characterized in that the coagulation bath is an aqueous mixture of the spinning solvent, the concentration of the spinning solvent being less than 70% by weight, based on the coagulation bath. Use of the fibers according to claim 1 as reinforcing materials for the production of composite materials. Use according to claim 20, characterized in that the composite materials are fiber-reinforced hydraulically setting materials. Use of the fibers according to claim 1 for the production of nonwovens, in particular for the production of filters or geotextiles. Use of the fibers according to claim 1 for the production of friction linings.