CS235001B2 - Method of polyolefin fibres production with high tensile strength and with modulus of elasticity in tension - Google Patents

Abstract

A process for making polymer filaments which have a high tensile strength and a high modulus by stretching a polymer filament which contains a substantial amount, eg at least 25 wt of polymer solvent at a temperature between the swelling point and the melting point of the polymer. A solution of the polymer may be spun to a filament through a spinning aperture and the spun filament cooled to below the dissolution temperature of the polymer without substantial evaporation of solvent from the filament and then brought to a temperature between the swelling point and the melting point of the polymer, and stretched. By the process polyethylene filaments may be obtained having a tensile strength of at least 1.2 GPa.

Description

The present invention relates to a process for the production of high tensile strength and high tensile modulus polyolefin fibers.

The fibers are produced by spinning linear polymers. In the process, the polymer is brought into a liquid shape (melt, solution) and spun. Randomly oriented strands of molecules in the thus obtained fiber must then be longitudinally oriented in the fiber by elongation. Although other substances may also be fiberizable, macromolecules are The side branches have an adverse effect on fiber formation and mechanical properties, and therefore fiber production is based on the use of polymers which are as linear as possible, although a limited degree of branching will in most cases be inevitable and thus permissible.

By stretching the fibers, the macromolecules of the chain are oriented longitudinally and the strength of the fibers increases. In many cases, however, the strength remains far below what one would theoretically expect. Many attempts have already been made to produce fibers whose tensile strength and modulus of strength are closer to the values that are theoretically possible. These experiments are reviewed in Juyn in Plastica 31 (1978) 262-270 and Bigga in Polymer Eng. Sci. 16. (1976) 725-734, failed to give satisfactory results. In many cases, it has been possible to sufficiently improve the tensile modulus but not the tensile strength, while, in addition, the formation of fibers is so slow that economical production is impossible.

It has now been found that polyolefin fibers of high tensile strength and high tensile modulus can be made by drawing polyolefin fibers; fibers which contain a significant amount of polymer solvent, at a temperature between the temperature at which the swollen polymer contains 10% solvent ("swelling temperature") and between the melting point.

It is therefore an object of the present invention to provide a polyolefin fiber having a high tensile strength and a high modulus of elasticity by spinning a polyolefin solution followed by elongation of the fiber obtained, characterized in that the polyolefin solution at a concentration of 1 to 5% by weight is spun and the resulting fiber is cooled below the minimum temperature of dissolving the polyolefin in the solvent used without promoting evaporation of the solvent to form a gel fiber that elongates at least partially: evaporation of the solvent.

In a process known as dry spinning, which is generally known on a commercial scale, a solution of a spinable polymer is spun in a shaft that blows air, usually heated, to evaporate all or most of the solvent from the fiber. The shaft temperature is below the melting point of the polymer, so that the polymer precipitates when the solvent is evaporated; this will increase the mechanical strength of the fiber, which is still very low at the exit of the spinning hole. The strength is further increased in the following drawing process at temperatures below the melting point of the polymer.

According to the invention, evaporation of the solvent from the fiber just after spinning is not supported during the cooling period. · The fiber may be cooled: below the dissolution temperature and below the particular temperature * swelling the polymer in a solvent ^л, · and · in any suitable manner, such as passing the fibers · water bath or a shaft, wherein the shaft is blown no or almost no air. Some evaporation of the solvent from the fiber often occurs spontaneously and cannot be prevented. However, this does not matter at all if evaporation is not actively promoted and the amount of solvent in the fiber is not reduced to a low value, for example less than 25 weight percent solvent relative to the polymer, preferably not less than equal weight amounts of solvent and polymer. If necessary, evaporation of the solvent can be reduced or suppressed by spinning in an atmosphere containing the solvent vapor.

Upon cooling below the dissolution temperature, particularly below the swelling temperature of the polymer in the solvent, the polymer precipitates out of solution and forms a gel. The fiber consisting of this polymer gel has sufficient mechanical strength for further processing, for example by means of guides, rollers, etc., as commonly used in spinning techniques. A fiber of this kind is heated to a temperature between the swelling point of the fiber in the solvent and between the melting point of the polymer and is elongated at this temperature. This can be done by guiding the fiber into a zone containing it. gaseous or liquid medium maintained at the desired temperature. A tube dryer with air as a gaseous medium is very suitable, but it is also possible to use liquid baths or any other suitable apparatus. The gaseous environment is easier to handle and therefore should be given priority. When the fiber is elongated, the solvent evaporates or, if a liquid medium is used, the solvent dissolves in the liquid. Preferably, evaporation is promoted by suitable measures, for example by removing the solvent vapor, for example by guiding a gas or air stream around the fiber in the stretching zone. At least a portion of the solvent should be evaporated, but preferably at least a major portion of the solvent is evaporated, so that at the end of the attenuation zone the fiber contains at most a small amount - e.g.

little ¹ percent, calculated on a solid-solvent basis. The fiber optionally obtained must be solvent-free and it is preferable to use such conditions that the fiber is free or practically free of solvent already in the stretching zone.

Contrary to expectations, fibers which are considerably stronger than fibers of the same material produced by any of the conventional known dry spinning processes, i.e. fibers of significantly higher tensile strength and modulus, can be produced by the method of the invention. Using the procedures described in the above-mentioned treaties by Juyn and Biggo, perhaps fibers with a higher modulus have been obtained, but the tensile strength is still insufficient. In addition, the productivity of these processes is low.

The process of the present invention differs from dry spinning processes in that a fiber containing a significant amount of solvent of the spinning material is drawn at a temperature at which the spinning material in the solvent at least swells while removing the solvent, while in the commonly used spinning processes the drawing is subjected to fibers. solvent-free.

The requirement for dry spinning is that the linear polymer is soluble in a suitable solvent. A variety of solvents are known for any soluble polymer. One skilled in the art - can select from them without difficulty a suitable solvent whose boiling point is not so high that the evaporation of the solvent from the fiber is sufficient and not so low that it becomes too volatile to prevent the formation of the fiber due to rapid evaporation or - under pressure to prevent this.

Dissolution of the polymer in a suitable solvent involves swelling. While the solvent is absorbed and the volume increases, a strongly swollen gel is formed, but due to its consistency and shape stability, it can still be considered as a kind of solid. It is generally believed that the polymer is composed of ordered (crystalline) and less ordered (amorphous) regions. It is believed that the ordered regions act as anchoring points and thus impart shape stability to the gel. Gel formation and dissolution are time dependent. A given polymer can be dissolved in a given solvent only above a given temperature. Below that dissolution temperature - only swelling occurs and the lower the temperature, the less swelling is at a certain point is negligible. The swelling point or swelling temperature is considered to be the temperature at which there is a noticeable increase in volume and a noticeable absorption of solvent - 5 to 10% by weight of the polymer. For simplicity, the swelling temperature above which the drawing is to be carried out is the temperature at which 10% of the solvent is absorbed in the swelling polymer.

For dry-spinning processes, as is generally the case, solutions with a concentration of 5 to 30% by weight are generally used for technical and economic reasons. Such solutions are also suitable for the process of the invention, although lower concentration solutions will usually be used.

Preferably solutions of up to 5% by weight are used. Sometimes even lower concentrations can be used, although these concentrations have no advantages and are wasteful.

Suitable draw ratios can be readily determined by experiment. The tensile strength and fiber modulus are within certain limits approximately proportional to the draw ratio. Depending on whether the fibers are to be stronger, a higher draw ratio will have to be chosen.

The draw ratio is at least 5, preferably at least 10, and in particular 20. High draw ratios of 30 to 40 and even higher can be used without objection, yielding fibers whose tensile strength and modulus are considerably higher than those produced by conventional processes .

In dry spinning processes, as commonly used, the diameters of the spinning holes in the spinnerets are usually small. In general, these diameters are of the order of 0.02-1.0 mm. Especially when using small spinning holes (/ 0.2 mm), the spinning process has a high sensitivity to the presence of impurities in the spinning solution, so that this solution must be carefully cleaned of impurities and kept free of solid impurities. In most cases, a filter is placed on the spinnerets. In spite of this, it appears that the spinnerets have to be cleaned after a short period of time and that, more often, clogging occurs. Larger spinning apertures, more than 0.2 millimeters, for example 0.5 to 2.0 mm or more, can be used in the process of the invention since considerably higher draw ratios can be used and, moreover, generally lower polymer concentrations in the spinning are used. solution.

Polyolefins such as polyethylene, polypropylene, copolymers of ethylene and propylene and higher polyolefins can be dissolved without difficulty in hydrocarbons such as saturated, aliphatic and cyclic as well as aromatic hydrocarbons or mixtures thereof, for example the mineral oil fraction. Aliphatic or cyclic hydrocarbons such as nonane, decane, undecane, dodecane, tetralin, decalin etc., or mineral oil fractions having an appropriate boiling range are very suitable. The polyethylene or polypropylene is preferably dissolved in decalin or dodecalin. The process according to the invention is particularly suitable for preparing fibers from polyolefins, preferably polyethylenes.

The fibers of the invention can be used for many purposes. They can be used to reinforce a variety of materials known to be fibrillated or fiber reinforced, such as tire cords, and for all possible applications where low weight combined with high strength is desirable. However, the scope of possible applications is not limited to those mentioned above.

The invention will now be illustrated by the following examples, but is not limited thereto.

Example 1

High molecular weight polyethylene, with an M _w of 1.5 χ molar, was dissolved in decall at 145 ° C to form a 2 weight percent solution. This solution was spun through a spinneret with a spinneret hole of 0.5 mm diameter at 130 ^° C. The fiber was passed into a water bath where it was cooled. · The cooled fiber with a thickness of 0.7 mm, which had a gel-like appearance and · still contains about 98 percent of the solvent was conducted · · tube furnace, heated at 12o ^C · C and stretching using various draw ratios. This procedure is shown schematically in Fig. 1.

Figure 1 shows the polymer solution A, the cooling bath B, the wet fiber C, the feed roller D, the dryer E and the drawing roller F.

Giant. 2 and 3 show the tensile strength and the modulus applied against the draw ratio. Both figures show the values in the next table. In Fig. 2, a tensile strength expressed in GPa, and b denotes the draw ratio. In Fig. 3, a module expressed in GPa and b denotes the extension ratio.

Thus, Fig. 2 shows the tensile strength and Fig. 3 shows the modulus applied against the draw ratio.

A modulus of greater than 60 GPa and a tensile strength of nearly 3 GPa can be achieved, while the module of polyethylene fibers produced in the conventional manner is 2 to 3 GPa and a tensile strength of about 0.1 GPa. The modulus and tensile values of the fibers formed with different draw ratios as applied in FIG. 2 and 3 are compiled in Table 1.

Polyethylene fibers having a tensile strength greater than 1.2 GPa can be easily produced by the process of the invention.

TABLE 1

Experiment Extension ratio GPa modulus Tensile strength GPa

1 1 2.4 0.09 2 3 5.4 0.27 3 7 17.0 0.73 4 8 17.6 0.81 5 11 23.9 1.32 6 12 37.5 1.65 7 13 40.9 1.72 8 15 Dec 41.0 1.72 9 17 43.1 2.11 10 25 69.0 2.90 11 32 90.2 3.02 Example 2 Example 3

As described in Example 1, a 2 weight percent solution of a mixture of equal parts of high molecular weight polyethylene _Mw · 1.5 χ 10 ⁶ and high molecular weight polypropylene _Mw ss 3.0 · x 106 was spun at 140 ° C and drawn at 140 ° C. 130 ° C using a draw ratio of 20. The fibers had a tensile strength of 1.5 GPa.

Prepared as in Example 1, a solution of 25 weight percent isotactic polypropylene having a _Mw of 3.0 10 ⁶ χ spun at 140 c and stretched at 130 ° C using a draw ratio of 20. Tensile strength · resultant filaments was 1 GPa.

Claims (4)

A process for producing polyolefin fibers having a high tensile strength and a high modulus of elasticity in tensile by spinning a polyolefin solution followed by drawing the obtained fiber, characterized in that the polyolefin solution having a concentration of 1 to 5% by weight is spun and the formed fiber is cooled below the minimum temperature of dissolution of the polyolefin in the solvent used without promoting evaporation of the solvent to form a gel fiber which elongates with at least partial evaporation of the solvent. 2. The method of claim 1, wherein the spinning fiber is cooled below a temperature at which the swollen polymer contains 10% solvent and is then drawn at a temperature between said temperature and between the melting point. 3. A method according to claim 1, characterized in that the fiber comprises at least 100% by weight of solvent, based on the polymer. 4. The method of claim 1, wherein the draw ratio is at least 10.