EP0407901B1 - Process for the fabrication of polyethylene fibres by the high speed spinning of ultra-high molecular weight polyethylene - Google Patents

Description

The invention relates to a process for the production of polyethylene threads by rapid spinning of solutions of ultra-high molecular weight polyethylene, which due to their good strength and their high modulus, e.g. for use as technical yarns, for plastic reinforcement in general, etc. are suitable.

It is known to use threads and industrial yarns from a whole range of polymers such as regenerated cellulose, polyester, polyamides and the like. to manufacture. In all of these processes, efforts are made to obtain threads with high strength, high modulus, in particular high initial modulus and the lowest possible elongation at break; in addition, efforts are made to work with the highest possible production speeds and with the simplest possible procedures.

There has also been no lack of attempts to produce such yarns from polyethylene. Polyethylene has a number of advantages due to its chemical structure, e.g. against polymers as obtained by polycondensation. For example, not the risk of hydrolysis, which is frequently observed with the ester bonds or amide bonds of polyesters and polyamides.

Polyethylene, as a synthetic material that can be produced in practically any quantity, is also less susceptible to the fluctuations in supply and demand, as is the case with pulp, quite apart from the fact that the depletion of forests means that the raw material base for pulp is increasingly at risk.

It is easiest to manufacture polyethylene threads after the melt spinning process. There are limits to melt spinning polyethylene, however, because with higher molecular weights, which are important for high strengths and moduli, the viscosity of the melt increases so much that it becomes difficult to spin. The spinning temperature cannot be increased arbitrarily, since decomposition of the polyethylene is to be feared at temperatures above approximately 240 ° C. With higher molecular weights, the elasticity of the polymer melts also increases, which leads to problems particularly at higher extrusion speeds.

Attempts have also been made to circumvent these difficulties by spinning solutions of polyethylene into threads. Similar difficulties also arise with these processes, since the viscosity and elasticity of solutions also increase significantly with increasing molecular weight of the dissolved polymer.

The Dutch patent application 79/04990 describes a process for the production of polyethylene threads with high strength and a high modulus, in which, as can be seen in particular from the examples, relatively low concentration solutions are used. In order to obtain satisfactory mechanical properties, it is necessary to heat-draw the filaments after spinning, winding and extracting, which reduces the productivity of the process.

Pennings and co-workers describe in "Polymer Bulletin" 16, 167-174 (1986) how one can spin ultra high molecular weight polyethylene under different conditions. In order for the polyethylene threads to have useful mechanical properties, the threads must be drawn, as in the process described in NL-OS 79/04990, the threads also being extracted before drawing.

Although a number of processes for the production of polyethylene threads by spinning ultra high molecular weight polyethylene are already known, there is still a need for improved processes which in particular ensure increased productivity and in which it is not necessary to carry out a drawing after spinning and winding to obtain useful mechanical properties.

The object of the invention is therefore to provide a method for high-speed spinning of ultra-high molecular weight polyethylene which allows high productivity, which works without stretching the spun threads and which in a simple manner provides polyethylene threads which have good mechanical properties, in particular high strengths and have a high modulus and are suitable for use as technical yarns, as reinforcing material for plastics, etc.

This object is achieved by a process for the production of polyethylene threads by rapid spinning of solutions of ultra-high molecular weight polyethylene, which is characterized in that polyethylene having a molecular weight is used M _w ≧ 1 x 10⁶ and a solvent produces an approximately 1 to 6% by weight solution and the solution at an extrusion temperature T _E = 180-250 ° C and an extrusion speed V _E = 5 to 150 m / min through spinnerets with nozzle openings, whose cross-section becomes smaller towards the nozzle exit area, extruded into a spinning shaft, which is kept below the nozzle exit area by means of a heating device at a temperature of 100 to 250 ° C, one blows the threads below the heating zone with a gas, the threads at a speed V _w ≧ subtracts 500 m / min and freed from the solvent without further stretching.

Preferably the molecular weight is M _w ≧ 3.5 · 10⁶.

vorzugsweise ≦ 3.In a particularly advantageous embodiment of the method according to the invention, the molecular non-uniformity of the polymer is expressed as $$\text{U =}\frac{{\overline{\text{M}}}_{\text{w}}}{{\overline{\text{M}}}_{\text{n}}}\text{≦ 5}$$

preferably ≦ 3.

The temperature below the nozzle exit surface is preferably set to 150-190 ° C. It is advantageous to work with a take-off speed of at least 1000 m / min. Take-off speeds of 1500 to 4000 m / min are very advantageous.

To carry out the method according to the invention, spinnerets with nozzle openings are used, the cross-section of which becomes smaller in the direction of extrusion. For example, spinnerets with nozzle openings can be used, the cross-sectional profile of which can be called trumpet-shaped or funnel-shaped or pseudo-hyperbolic. Such a favorable pseudo-hyperbolic cross-sectional shape is shown in the figure.

A pseudo-hyperbolic cross-sectional shape is to be understood in the course that approximates a hyperbolic course, but can have more or less large deviations both at the beginning and at the end.

Such a solvent is preferably used to prepare the solutions so that the solution has a viscosity of 1 to 100 Pa.s at the extrusion temperature. Paraffin oil is particularly suitable here. The viscosity is measured at a speed gradient D = 1s⁻¹.

Polyethylene which is as linear as possible is used in the preparation of the solutions, which does not preclude the possibility of branching to a small extent. Preferably the polymer used is a polyethylene obtained by low pressure polymerization. It is commercially available and is often referred to as HDPE (high density polyethylene).

It is particularly advantageous to use a polyethylene as the polymer which is wholly or largely present as a homopolymer. However, in certain cases it is also possible to use a copolymer, e.g. a copolymer composed of up to about 5% by weight of monomers other than ethylene such as propylene or butylene. Of course, copolymers can also be used which contain more or less of the other monomer (s).

The polyethylene used for the production of the polyethylene threads according to the invention belongs to the types of polyethylene which are generally referred to as ultra-high molecular weight polyethylene. This is understood to mean polyethylene, which is a molecular weight M _w of at least 1 million, under M _w The weight average is to be understood, which can be determined, for example, by the GPC method. M _n is the number average, which can be determined, for example, using osmotic methods.

soll vorzugsweise ≦ 5 insbesondere ≦ 3 sein.Although it is also possible to use polyethylenes with a customary molecular weight distribution which can be more or less wide and, for example, have a non-uniformity of 20, for example, it is nevertheless advantageous to use a polyethylene which has the narrowest possible molecular weight distribution has, whose values for the inconsistency are as low as possible. The non-uniformity, which is defined by the ratio of the weight average molecular weight to the number average molecular weight $$\text{U =}\frac{{\overline{\text{M}}}_{\text{w}}}{{\overline{\text{M}}}_{\text{n}}}$$

should preferably be ≦ 5, in particular ≦ 3.

The non-uniformity of the polymer used can be controlled by the way in which it is produced; it is of course also possible to go from a polyethylene with a very broad molecular weight distribution by fractionation to a polymer with a narrow molecular weight distribution.

Compounds are used as solvents which are still sufficiently viscous at the extrusion temperature, which is between 180 and 250 ° C, possibly between 180 and 230 ° C, i.e. have a viscosity of preferably at least 3-10 Pa.s, measured at D = 1s⁻¹.

The polyethylene solvent system should be chosen so that the solution forms a gel by cooling to temperatures below the extrusion temperature.

The gel formation temperature should preferably be 130 ° C. or lower. It can also be below 70 ° C.

The spinning solutions mentioned are elastic. The dissolution of the polyethylene in the solvent preferably takes place at temperatures which correspond to the extrusion temperature. It is advantageous if the dissolving under an inert atmosphere, e.g. takes place under nitrogen.

A stabilizing agent can be added to the solution.

Paraffin oils are particularly suitable as solvents. Hydrocarbons such as cyclooctane, paraxylene, decalin or petroleum ether can also be used.

In the context of the invention, solutions with concentrations of approximately 1 to 6% by weight can be used, preferably those with concentrations of 1-3% by weight.

However, the most advantageous are concentrations of about 1 to 2% by weight.

Extrusion speed is to be understood as the amount of spinning liquid which leaves the nozzle in the time unit per unit area of the nozzle outlet openings. It is given in m³ / m² x min or m / min.

The linear speed, in m / min, at which the threads are drawn off at the lower end of the spinning shaft is indicated under take-off speed. Since the threads are no longer fed after being drawn further, this take-off speed generally corresponds to the take-up speed.

The withdrawal speeds that can be achieved depend on the concentration selected. In general it can be said that the maximum withdrawal speed decreases with increasing concentration of the polyethylene. However, spinning may be difficult in the lower concentration range; these can be remedied by reducing the extrusion rate. The most suitable combinations of extrusion speed, draw-off speed and concentration of the solution can be determined by a few experiments.

In general, it can be said that the maximum achievable extrusion rate decreases with increasing polymer concentration.

As a device with which the spinning shaft is brought to the required temperature below the spinneret, e.g. simple annular heaters can be used. Depending on the size of the spinning equipment used, the length of the heating zone can be between a few centimeters, e.g. 4 cm up to 200 cm.

Below the heating zone, the threads are blown with a gas to reduce the temperature. It is advantageous to set a gradient-like or graduated temperature profile by blowing on the threads, so that after the heating zone, in which e.g. there is a temperature of 160 ° C, initially there is a zone in which the temperature is only around e.g. Falls 10 ° C, e.g. to about 150 ° C, which is then followed by a next zone within which the temperature drops to, for example, 110 ° C, which then connects to a zone in which, using gas at room temperature, cooling to temperatures below 50 ° C takes place so that the threads have cooled sufficiently when they reach the take-off device. Temperature gradations can initially also be carried out with the help of one or more heating devices with which temperature gradations or temperature gradients can be set.

The cross-sectional shape of the spinning orifices is of great importance for the method according to the invention. It is imperative that the spinning orifices on the side where the spinning mass enters the nozzle orifices have an enlarged opening, ie that the cross section of the nozzle orifices on the exit side becomes smaller. Nozzle openings with a pseudo-hyperbolic course are very suitable. There is a course under pseudo-hyperbolic to understand, which is approximated to a hyperbolic course and can deviate from an exactly hyperbolic course both in the more curved and in the more linear region. The figure schematically shows such a design.

However, it is also possible to use nozzles with nozzle openings which initially have a funnel-shaped opening part, which can be trumpet-shaped or else conical, which then either changes abruptly or after a transition to a conical shape in which the cone has a more acute opening angle than that Cone or the parabola of the inlet part. It is possible to design the last part of the nozzle opening with a constant cross section.

It was particularly surprising that it is possible to use the process according to the invention to process ultra-high molecular weight polyethylene into threads with good mechanical properties such as high modulus and high breaking strength. The process according to the invention is particularly advantageous in comparison with known processes in that it is a so-called one-step process, i.e. in that it works without the previously required post-stretching. This makes the process particularly economical and allows high production speeds.

It was also particularly surprising that the process according to the invention permits spinning of high molecular weight polyethylene without the resulting fear of spinning breaks, which can be observed especially when spinning high molecular weight polyethylene in the form of elastic melts or solutions in the previously known processes . So the number of melt breaks, which in the known processes are often attributed to processes that already take place inside the spinneret, is considerably reduced or avoided entirely.

The process according to the invention allows take-off speeds of up to the order of 4000 m / min and above.

The threads obtained have such good mechanical properties that post-stretching is no longer necessary and is sometimes no longer possible.

Due to their properties, the threads, which can also be cut into staple fibers, are particularly suitable for use as technical yarns. They can be used very well with protective clothing e.g. bulletproof vests and the like, processing rope, parachutes etc.

The threads are very suitable, in particular as staple fibers in the reinforcement of plastic.

Although the processes that take place within the nozzle and in the spinning shaft in the process according to the invention are not explained in detail, it is assumed that the process according to the invention results in a particularly advantageous molecular structure, ie a particularly favorable molecular structure in the thread. It can be assumed that the process according to the invention produces sufficient longitudinal chains of molecules which also act as binding chains and that the longitudinal molecules and the lamellar regions are in a favorable relationship to one another and that errors occur due to chain fold defects only to a minor extent.

The invention is illustrated by the following examples:

Comparative Example 1

A 1.5% by weight solution of an ultra high molecular weight polyethylene is prepared in the following way: 48.7 g of a polymer with an intrinsic viscosity of 33.38 dl / g, measured at 135 ° C. in decalin, one M w = 5.5. 10⁶ kg / kmol and M _n = 2.5. 10⁶ kg / kmol are added to 3,200 g of paraffin oil and 16.2 g of the antioxidant 2,6-di-t-butyl-4-methyl-cresol and stirred at a temperature of 120 ° C in a 5 liter kettle. The mixture is homogenized by stirring, while being heated to 150 ° C. The stirrer is switched off as soon as the polyethylene is completely dissolved and the so-called Weißenberg effect occurs. The temperature is then kept at 150 ° C. for 48 hours. The solution is cooled to room temperature, at about 130 ° C a gel forms. The gel is fed to a spinning device with spinning orifices which have a trumpet-shaped cross-sectional shape, as shown in the figure. The outlet openings of the nozzle openings have a diameter of 0.5 mm. The solution is extruded at 220 ° C at a speed of 1 m / min, the threads are quenched in air and wound up at the same speed. After the paraffin oil has been extracted, the fiber thus obtained can be stretched to a ratio of 200 at a temperature of 148 ° C., producing fibers with a strength of 7.0 GPa.

Comparative Example 2

The solution described in Example 1 is processed in the same way, only an extrusion speed of 100 m / min and a winding speed of 500 m / min are used. The fiber thus obtained can no longer be hot drawn; the strength after extraction of the paraffin oil with n-hexane was 0.3 GPa.

Example 3

Festigkeit

2,3 GPa

Young Modulus

36 GPa

Bruchdehnung

8%

A solution according to Example 1 is spun at an extrusion speed of 100 m / min, but a distance of 20.5 cm below the exit surface of the spinneret is kept at a temperature of 160 ° C. by means of a cylindrical furnace. The threads are drawn off at a speed of 4,000 m / min. These threads can no longer be hot drawn, but have the following properties after extraction of the paraffin oil:

strength

2.3 GPa

Young modulus

36 GPa

Elongation at break

8th%

Example 4

A spinning solution is processed as indicated in Example 3, but the process is carried out at an extrusion temperature of 190 ° C. and a winding speed of 2,000 m / min. The strength of the extracted fibers is 1.7 GPa.

Example 5

A spinning solution is processed as in Example 3, but with an extrusion speed of 10 m / min and a winding speed of 2,000 m / min. The strength of the extracted fiber is 1.9 GPa.

Example 6

The spinning solution is processed according to Example 3, but at an extrusion speed of 5 m / min using a spinneret with spinning orifices which have a diameter at the exit point of 1 mm. In deviation from Examples 1 to 4, in which a spinning shaft of 0.5 m length was used, a spinning shaft of 4 m length is used here. This length was necessary to allow the extruded filaments to cool sufficiently before they were wound up. The winding speed is 2,000 m / min. After extraction, the threads have a strength of 1.4 GPa. on.

Example 7

In the same way as described in Example 1, a 3% spinning solution is prepared from a polyethylene which has an Mw = 4. 10⁶ and an Mn = 2. 10⁵ has. The extrusion temperature is 190 ° C. and the take-off speed is 3,000 m / min. The strength of the extracted fiber is 0.8 GPa.

Example 8

A spinning solution according to Example 7 is used at an extrusion temperature of 220 ° C. and at a winding speed of 4,000 m / min. The strength of the extracted threads is 0.8 GPa.

Example 9

A spinning solution corresponding to Example 7, but with a concentration of 5% by weight is extruded at a temperature of 220 ° C., the take-off speed is 3 500 m / min. The strength of the extracted fiber is 0.6 GPa.

Example 10

A spinning solution is prepared analogously to Example 1, but using decalin as the solvent. The spinning mass is extruded at an extrusion temperature of 180 ° C. at a spinning speed of 100 m / min and wound up at 1,000 m / min. The strength of the extracted fiber is 0.9 GPa.

The examples show that when working without the use of a heating device according to the invention below the spinneret, useful strengths can only be achieved by post-stretching in the heat. However, very low extrusion speeds must be used. If higher extrusion speeds are used, subsequent stretching is no longer possible and that Strengths are so low that the threads are unusable for most purposes.

Examples 3 to 10 according to the invention, on the other hand, show that it is possible to work in a one-step process without the need for post-stretching, and in this way one obtains strengths which are twice or more than the strength compared to the procedure of Example 2nd

Claims (9)

1. A process for producing polyethylene filaments by high-speed spinning of solutions of ultra-high-molecular polyethylene, characterized in that an approximately 1 to 6 wt% solution is produced from polyethylene of molecular weight M _w ≧ 1 x 10⁶ and a solvent, the solution is extruded at an extrusion temperature T_E = 180-250 °C and an extrusion rate V_E = 5 to 150 m/min through spinnerets, with spinneret holes whose cross-section diminishes towards the orifice plane, into a spinning shaft that below the orifice plane is maintained at a temperature of 100-250 °C by means of a heating device, the filaments are blown on below the heating zone with a gas, and the filaments are drawn off at a rate V_w ≧ 500 m/min and freed from the solvent without further stretching.
2. A process according to claim 1, characterized in that a polyethylene of molecular weight M _w ≧ 3.5 x 10⁶ is used.
3. A process according to one of claims 1 or 2, characterized in that a polyethylene with a molecular non-uniformity $$\text{U =}\frac{{\overline{\text{M}}}_{\text{w}}}{{\overline{\text{M}}}_{\text{n}}}\text{≦ 5}$$

   

   is used.
4. A process according to claim 3, characterized in that U is ≦ 3.
5. A process according to one of claims 1 to 4, characterized in that a temperature of 150-190 °C is established below the orifice plane by means of the heating device.
6. A process according to one of claims 1 to 5, characterized in that the drawing-off rate is V_w ≧ 1000 m/min.
7. A process according to claim 6 characterized, in that the drawing-off rate is V_w = 1500-4000 m/min.
8. A process according to one of claims 1 to 7, characterized in that solvents are used such that the solution has a viscosity at the extrusion temperature of 1 to 100 Pa.s, measured at a shear rate of D = 1 s⁻¹.
9. A process according to claim 8, characterized in that paraffin oil is used as solvent.

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