PROCESS FOR THE PRODUCTION OF POLYMER FILAMENTS HAVING HIGH TENSILE STRENGTH AND MODULUS

Abstract: The invention relates to a process for the production of polymer filaments by spinning a solution of a polymer, having a weight-average molecular weight M w higher than 4.10⁵ kg/kmole with at least 80 % by weight of solvent at a temperature above the gel point of that solution, cooling the spun product to below the gel point and stretching the obtained filament to a filament having a tensile strength of more than 1,5 GPa at room temperature. The polymer has preferably a weight/number-average molecular weight ratio Mw/Mn lower than 5. During stretching the filament can be twisted around its axis.

Description: The invention relates to a process for the production of polymer filaments having high tensile strength by spinning a solution of high-molecular polymer and stretching the filaments. Such processes are described in applicant's British patent specifications 2.042.414 and 2.051.667. In these known processes polyalkene polymers of very high molecular weights are used and/or high degrees of stretching are applied. It has now been found that comparable tensile strengths and moduli can be reached while using lower molecular weights and/or lower stretch ratios, or that substantially higher tensile strengths and moduli can be reached while using the same molecular weights and stretch ratios, if one specifically uses solutions of polymers having a weight/number-average molecular weight ratio Mw/Mn which is lower than those applied in the known processes. In the mentioned known processes there are used polyalkene polymers, in particular polyethylenes, having a Mw/Mn ratio in the range of 6.5 to 7.5 and above. In the process according to the invention a solution of an ethylene polymer or copolymer containing at most 5% by wt of one or more alkenes with 3 to 8 carbon atoms and having a weight-average molecular weight Mw higher than 4.10⁵ kg/kmole and a weight/number-average molecular weight ratio Mw/Mn lower than 5 with at least 80% by wt of solvent (in respect of the solution) is spun at a temperature above the gel point of that solution, the spun product is cooled to below the gel point and the filament obtained is stretched, in the form of a gel containing or not containing a solvent, to form a filament having a tensile strength higher than 1.5 gigapascal (GPa). Linear high molecular weight ethylene polymers having specific Mw/Mn ratios as required for the invention can be prepared by fractionating polymers having a broader molecular weight distribution (reference is made in this respect to Fractionation of Synthetic Polymers by L.H. Tung), or by using polymers that have been obtianed with specific catalyst systems and/or under specific reaction conditions (reference is made in this respect to L.L. Böhm, Die Angewandte Makromolekulare Chemie 89 (1980), 1-32 (nr. 1910)). The method according to the invention involves an improved stretching efficiency of the polymers in that for the same E modulus a substantially higher tensile strength is obtained than in the known processes. It has further been found that the tensile strength and moduli of stretched high molecular polymer filaments can be improved by twisting the filaments around their stretching axis during the stretching. Thus a twisted filament is obtained which has a reduced tendency to fibrillation and which has a substantially improved knot strength compared to the knot strength of straight-stretched filaments. The polymers to be applied according to the process of the invention must be highly linear and must comprise more in particular fewer than 1 side chain per 100 carbon atoms, preferably fewer than 1 side chain per 300 carbon atoms. Specifically the ethylene polymers to be used in accordance with the invention can contain up to at most 5% by wt. of one or more other alkenes copolymerized therewith, such as propylene, butylene, pentene, hexene, 4-methylpentene, octene, etc. The polyethylene materials used may also contain minor quantities, preferably 25% by wt at most, of one or more other polymers, particularly an alkene-1 polymer, such as polypropylene, polybutylene or a copolymer of propylene with a minor quantity of ethylene. The advantages of the process according to the invention manifest themselves very strongly in its preferred embodiment, in which ethylene polymers having a Mw/Mn ratio lower than 4 are used. The solutions to be spun must contain at least 80% by weight of solvent in respect of the solution. Very low polymer concentrations in the solution, such as in particular lower than 2% by wt polymer, may be important when applying polymer materials of an ultra-high molecular weight. When using polymer materials within the preferred Mw and Mw/Mn ranges for the process according to the invention, viz. a Mw between 5.10⁵ and 1.5 10⁶ kg/kmole and a Mw/Mn lower than 4, preference is given to the use of solutions having polymer concentrations ranging from 2% by wt to 15% by wt for Mw values ranging from 1.5.10⁶ to 5.10⁵. The choice of the solvent is not critical. Thus, in the case of polyethylene any suitable solvent can be used, such as halogenated or non-halogenated hydrocarbons. In most solvents polyethylene is soluble only at temperatures of at least 90 °C. In customary spinning processes the space in which the filaments are spun is under atmospheric pressure. Low-boiling solvents are, therefore, less desirable, because they may evaporate from the filaments so rapidly that they will come to function more or less as foaming agents and will disturb the structure of the filaments. In the said concentration range solutions of polymer materials, when cooled rapidly, will pass into a gel below a critical temperature (gel point). This gel point is defined as the temperature of apparent solidification when cooling the polymer solution. During the spinning a liquid solution must be used, and the temperature must therefore be above this gel point. During the spinning process the temperature of the polyethylene solution is preferably at least 100 °C and more in particular at least 120 °C, and the boiling point of the solvent is preferably at least 100 °C and particularly at least equal to the spinning temperature. The boiling point of the solvent must not be so high that is is difficult for the solvent to be evaporated from the filaments spun. Suitable solvents are aliphatic, cyclo-aliphatic and aromatic hydrocarbons having boiling points of at least 100 °C, such as octane, nonane, decane or isomers thereof and higher straight or branched hydrocarbons, petroleum fractions having boiling ranges in excess of 100 °C, toluenes or xylenes, naphtalene, hydrogenated derivatives thereof, such a tetralin, decalin, but also halogenated hydrocarbons and other solvents known in the art. Owing to the low cost, preference will be given mostly to non-substituted hydrocarbons, including also hydrogenated derivatives of aromatic hydrocarbons. The spinning temperature and the dissolution temperature must not be so high as to result in substantial thermal decomposition of the polymer. The chosen temperature will therefore generally not be above 240 °C. Although for reasons of simplicity the spining of filaments is spoken of in this specification, it will at once be clear to the expert that, in applying the present process, spinning heads with slit dies can be used as well. The term filaments as used herein therefore not only comprises filaments having more or less round cross sections, but also covers small ribbons produced in a similar manner. The essence of the invention is the manner in which stretched structures are made. In that process the shape of the cross section is of minor importance. The spun product is cooled down to below the gel point of the solution. This may be done in any suitable manner, for instance by passing the spun product into a liquid bath, or through a chamber. In the cooling process to below the gel point of the polymer solution the polymer will form a gel. A filament consisting of this polymer gel has enough mechanical strength to be processed further, for instance via the guides, rolls, etc. customary in the spinning technique. The gelfilament thus obtained is subsequently stretched. During the stretching the gel may still contain substantial quantities of solvent, up to quantities hardly lower than those present in the polymer solution spun. This will happen when the solution is spun and cooled under such conditions as not to promote the evaporation of the solvent, for instance by passing the filament into a liquid bath. Part or even essentially all of the solvent can be removed from the gel filament also before the stretching, for instance by evaporation or by washing-out with an extractant. The stretching of gel filaments still containing substantial quantities of more than 25% by wt and preferably more than 50% by wt of solvent is preferred, because thus a higher final degree of stretching and consequently a higher tensile strength and modulus of the final filament can be obtained; in certain technical emdobiments it may be more advantageous, however, to recover most of the solvent before the stretching. The filaments spun are preferably stretched at a temperature of at least 75 °C. On the other hand, the stretching will preferably be performed below the melting point or solution point of the polymer, because above that temperature the mobility of the macromolecules will soon be so high that the desired orientation cannot or not sufficiently be effected. The intramolecular heat development resulting from the stretching energy expended on the filaments must be taken into account. At high stretching speeds the temperature in the filaments may thus rise considerably, and care should be taken that it does not come near or even above the melting point. The filaments can be brought to the stretching temperature by passing them into a zone containing a gaseous or liquid medium, which is kept at the desired temperature. A tubular furnace with air as a gaseous medium is very suitable, but a liquid bath or any other device appropriate for that purpose can also be used. During the stretching (any) solvent present will be separated off from the filament. This is preferably promoted by measures appropriate for that purpose, such as the discharge of the solvent vapour by passing a hot gas or air stream along the filament in the stretching zone, or by stretching in a liquid bath comprising an extractant for the solvent, which extractant may optionally be the same as the solvent. The final filament must be free of solvent, and to good advantage the chosen conditions will be such that this condition is reached, or at any rate virtually reached, already in the stretching zone. The moduli (E) and tensile strengths (σ) are calculated by means of force/elongation curves as determined at room temperature by means of an Instron Tensile Tester, at a testing speed of 100% stretching/min (ε· = 1 min⁻¹), and reduced to the original diameter of the filament sample. In applying the present process high stretch ratios can be used. It has been found, however, that by using polymer materials having low molecular weight ratios Mw/Mn, according to the invention, filaments having a considerable tensile strength can be obtained already if the stretch ratio at least equals where the value Mw is expressed in kg/kmole (or g/mole). The filaments according to the invention are suitable for many uses. They can be used as reinforcement in many materials of which the reinforcement with fibres or filaments is known and for all uses in which a small weight combined with great strength is desirable, such as, for instance, rope, nets, filter cloths, etc. If so desired, minor quantities of usual additives, stabilizers, fibre treating agents and the like, particularly quantities of 0. 001-10% by weight in respect of the polymer, can be incorporated in or on the filaments. The invention will further be elucidated by the following examples without being limited by them. Example 1 A high-molecular linear polyethylene having a Mw of about 1.1 x 10⁶ kg/kmole and a Mw/Mn of 3.5 was dissolved at 160 °C to form a 2% by wt solution in decalin. This solution was spun in a water bath at 130 °C through a spinneret with a spinneret hole having a diameter of 0.5 mm. The filament was cooled in the bath so that a gel-like filament was obtained still containing more than 90% solvent. This filament was stretched in a 3. 5-metre-long stretch oven, which was kept at 120 °C. The stretching speed was about 1 sec⁻¹. The stretch ratio was varied between about 20 and 50. Of the filaments stretched with different stretch ratios the moduli (E) and the tensile strengths (σ) were determined. The value of the stretch ratios, moduli and tensile strengths are shown in table 1 and are compared with the values obtained for a polyethylene sample having the same Mw of 1.1 10⁶ kg/mmole and a Mw/Mn of 7.5, which sample was treated under comparable conditions. Example 2 Under essentially the same processing conditions as described in example 1, except that 8% wt solutions were used, a polyethylene sample having a Mw of about 500,000 kg/kmole and a Mw/Mn of 2.9 and a polyethylene sample having a Mw of about 500,000 kg/kmole and a Mw/Mn of 9 were processed to form filaments and compared.

Claims: Process for the production of polyethylene filaments having high tensile strength, wherein a solution of an ethylene polymer or copolymer containing at most 5 % by wt of one or more alkenes with 3 to 8 carbon atoms and having a weight-average molecular weight Mw higher than 4.10⁵ kg/kmole with at least 80 % by wt of solvent is spun at a temperature above the gel point of that solution, the spun product is cooled to below the gel point and the filament obtained is stretched, in the form of a gel contianing or not containing a solvent, to form a filament having a tensile strength of more than 1.5 GPa, measured at room temperature, characterized in that a polyethylene is used having a weight/number-average molecular weight ratio Mw/Mn lower than 5. Process according to any one of claims 1-2, characterized in that polymer or copolymer is used having a weight/number-average molecular weight ratio Mw/Mn lower than 4. Solution spun high molecular weight polymer filament, characterized in that the filament consists of an ethylene polymer or copolymer containing at most 5 % by wt of one or more alkenes with 3 to 8 carbon atoms, having a weight-average molecular weight Mw higher than 4.10⁵ kg/kmole and a weight/number-average molecular weight ratio Mw/Mn lower than 5.

Inventors: KIRSCHBAUM, ROBERT, LEMSTRA, PIETER JAN, PIJPERS, JACQUES PETER LAURENTIUS, SMITH, PAUL
Applicants: DSM N.V.
Application year: 1982
Main classification: D01F6
IPC codes: D01F6, D01D5