GB2101522A - Producing high tenacity monofilaments - Google Patents

Abstract

A monofilament of a thermoplastic resin having a high tenacity is produced by a process in which a monofilament melt spun from a thermoplastic resin is subjected to a single or multi-stage stretching under the conditions satisfying the following equations: <IMAGE> wherein i is a number of stretching stages, e is a base of natural logarithm (i.e. 2.71828), V1 is the first take-off linear velocity (m/min), Vi+1 is the final take-off linear velocity (m/min) at the i-stretching stage, DRTi is the total stretching ratio at the i-stretching stage, DRTiw is the DRTi from which the monofilament begins to become whitened at the i-stretching stage, Tm is the melting point of the thermoplastic resin and theta i is the temperature of the filament at the i-stretching stage.

Description

SPECIFICATION Process for producing a monofilament having high tenacity The present invention relates to a process for producing a monofilament having a high tenacity from a thermoplastic resin, such as polyethylene, polypropylene, polyamide, polyester and the like, by a melt spinning and stretching technique.

Heretofore, monofilaments obtained from a melt spinning and stretching of therrribplastic resins have been generally produced as follows. For instance, the thermoplastic resin is extruded through nozzles each having a round cross-sectional area, and usually passes through a cooling bath, or is optionally solidified by using a treatment bath to form fibrous materials. The fibrous materials are then stretched or drawn art a low stretching ratio of, for example, 3 through 10 and at an optimum temperature depending upon the type of resin used. Thus, monofilaments having a straight strength of 2 g/d through 7 g/d are produced. In order to increase the tenacity of the monofilaments, the use of a higher stretching ratio of, for example, 11 through 20 is required.However, in this case, although the straight strength is increased, the knot strength is remarkably decreased with the increase in the stretching ratio. Furthermore, in order to increase the stretching ratio, unstretched filaments having a higher denier should be used and, as a result, vacuum bubbles are generated in the filaments due to the deviation of the heat shrinkage at the cooling step in the inner portions of the filaments. The bubbles cause frequent stretching failure. In addition, in the case where filaments are stretched at a higher stretching ratio, other problems including the whitening of the filaments, the generation of fluff and powdering on the surface of the filaments and the like, occur.

Especially, monofilaments made of polyethylene are widely used as fibrous materials for marine industries, since the density of the polyethylene is less than 1. However, the strength of polyethylene is remarkably inferior to those of other synthetic fibrous materials such as polyesters, polyamides and the like. For instance, in the case of ropes, the strength of the ropes made of high-density polyethylene is at most approximately 70% of that of polyester ropes having the same diameter and is at most approximately 50% of that of nylon ropes having the same diameter. For this reason, use of the polyethylene is limited in products, such as towing ropes for large oil tankers, in which high strength is required.

Accordingly, an object of the present invention is to obviate the above-mentioned problems of the prior arts and to provide a process for producing a monofilament having a high tenacity from thermoplastic resins, in which the problems of the decrease in the knot strength and the stretching failure at a high stretching ratio are effective solved.

Another object of the present invention is to provide a process for producing a monofilament of thermoplastic resins having a high tenacity of approximately 1.5 through 2.0 times of that of the conventional monofilaments without causing the whitening of the filaments and having a good operating efficiency.

Other objects and advantages of the present invention will be apparent from the following descriptions.

In accordance with the present invention, there is provided a process for producing a monofilament having a high tenacity from a thermoplastic resin, wherein a monofilament melt spun from a thermoplastic resin is subjected to multi-stage stretching under the conditions satisfying the following equations.

0. < T, - 37 (i = 1) Tm - 27 < s3i~Tm17 (i > = 2) wherein i is a number of stretching stages, e is a base of natural logarithm (i.e. 2.71828), V, is the first take-off linear velocity (m/min), V+1 is the final take-off linear velocity (m/min) at the i-stretching stage, DR TI is a total I stretching ratio at the i-stretching state, DRTjW is the DRTj from which the monofilament begins to become whitened at the i-stretching stage, Tm is the melting point of the thermoplastic resin and Oj is the temperature of the filament at the i-stretching stage.

The present invention will now be better understood from the following descriptions presented in connection with the accompanying drawings in which: Fig. 1 is a schematic drawing illustrating a desirable embodiment of an apparatus in which a monofilament having a high tenacity is produced; Fig. 2 (a), (b) and (c) and 3 (a), (b) and (c) are schematic drawings illustrating cross sections of examples of monofilaments of high-density polyethylene obtained from the process of the present invention; and Figs. 4 and 5 are schematic drawings illustrating cross sections of examples of monofilaments high-density polyethylene having a thick denier obtained from the process of the present invention.

According to the presen, invention, monofilaments melt spun from a thermoplastic resin, such as polyethylene, polypropylene, nylon, polyester or the like, are stretched in a multi-stage stretching, at a high stretching ratio, without causing the whitening of the filaments and the stretching failure. The optimum stretching ratio at each stretching stage is determined based on the stretching ratio from which the whitening begins and the number of the stretching stages. The optimum filament temperature at each stage is determined based on the melting point of the filaments and the number of the stretching stages.

In the case of DRTi > DRTjW, the problems such as the whitening of the filament, the generation of fluff and powdering on the surface of the filaments occurs, whereby the commercial value is lost. On the other hand, in the case of DRTjw > DRTi > DRTjW x (1.0 - 0.0970 e-0.312 x x 1), frequent stretching failure undesirably occurs although no whitening of the filaments is caused.

Therefore, according to the present invention, the multi-stage stretching should be carried out under the conditions satisfying the following equations: DO TO < DRTjw x (1.0 - 0.0970-0.312 X I) Furthermore, the temperature 8i of the filament should be: i7ij ~ Tm37 (i = 1) Tm-27##iTm-17 (i#2) In the case where the temperature 3j of the filament is not within the above-mentioned range, the whitening phenomenon occurs or the strength is not improved, even if the stretching can be carried out.

In the case where the above-mentioned stretching ratio at each stage and the above-mentioned temperature of the filaments are maintained, filaments having a high tenacity, i.e. more than, 1.5 through 2.0 times that of the conventional filaments, can be effectively produced, without causing the whitening of the filaments.

The monofilaments to be stretched are generally extruded through a screw type extruder.

Although any conventional type screw type extruder can be used in the process of the present invention, a screw type extruder having a metering portion of a groove depth Hm of 0.1 57D0.719 through 0.269D 7'9 (wherein D is a bore diameter (mm) of the extruder) can be desirably used in the present invention. In the case where the groove depth is less than 0.157D0.719, the production capacity tends to be decreased and, further, the heat generation of the resin tends to occur, whereby various problems, such as the occurrence of the swing of the filament and smoking during the extrusion and the generation of fluff and powdering, are likely to be caused.Contrary to this, in the case where the groove depth is more than 0.269D0.719, the discoloration of the filaments and the stretching failure are likey to occur due to the decrease in the mixing of the resin.

The nozzles through which the monifilaments are extruded at a melt spinning step can be those having any known cross-sectional shape, such as a perfect round shape, an oval shape, a capsule shape, or a dumb-bell shape. Especially, nozzles having a cross-sectional area S (mm2) which satisfies the following equation: 0.503 mm2 ~ S < 3.14 mm2 0.09 < - # 0.30 52 can be preferably used at the melt spinning step. In the above equation, I represents a maximum crosssectional secondary moment, max (Ix, 1y). that is, the maximum secondary moment in the cross-sectional secondary moments with respect to the main x axis and y axis passing through the center of the gravity of the cross-section.

The desirable cross-sectional shapes of the nozzles used in the present invention are those having an oval shape, a capsule shape (or elongated circle shape), a dumb-bell shape and the like and having a cross-sectional area S of 0.503 through 3.14 mm2 and a maximum cross-sectional secondary moment of 0.09 S2 through 0.30 s2 mm4. Especially, the use of the oval shaped nozzle having a ratio of the long axis a to the short axis b (i.e., a/b) of 1.2 through 1.6 is desirable. This is because the manufacture of the nozzles becomes difficult and expensive as the cross-sectional shapes of the nozzles become complicated. The desirable L/De [wherein L: land length (mm), De: perfect circule corresponding

diameter (mm) = 2] 7r of the nozzles is 10 through 1 5.Although there is no limitation in the structure of the land, the straight type land is desirable in view of the manufacturing cost and the precision of the manufacture (or cutting). The desirable arrangement of the nozzles in the die is such that x or y axis passing through the center of gravity of the cross-section of the nozzles and having a smaller cross-sectional secondary moment is tangential to the pitch circle diameter (P.C.D.). If the nozzles are reversely arranged, the deviation of the heat shrinkage generated in the unstretched filaments cannot be remarkably obviated.

By the use of the above-mentioned nozzles, it is difficult for vacuum bubbles to be formed in the unstretched filaments and, even in the case where the filaments are stretched at a high stretching ratio, the undesirable stretching failure does not occur and filaments having a high knot strength can be obtained.

As mentioned hereinabove, the nozzles used in the melt spinning step desirably have a crosssectional area S of 0.503 through 3.14 mm2 and a maximum cross-sectional secondary moment of 0.09 S2 through 0.30 s2 mm4. In the case where the cross-sectional area S is less than 0.503, the manufacture of the nozzles becomes difficult and, since melt fracture tends to be generated during the melt spinning step, the stretching at a high stretching ratio cannot be effected. On the other hand, in the case where the cross-sectional area is more than 3.14 mm2, the spinning pressure becomes low, so that the discharge becomes uneven and the filaments tend to be cut directly under the nozzle whereby the yield of the filament becomes less.

In the case where the maximum cross-sectional secondary moment i is less than 0.09 s2 mm4 (c.f.

in the case of the perfect circles, s2 1=-=0.0796 S2 mm4), 4# vacuum bubbles tend to be generated in the unstretched filament and, therefore, the desired high stretching ability cannot be achieved. If the high stretching is carried out, frequent stretching failure is caused and only filaments having a low knot strength can be obtained at a low yield. On the other hand, in the case where the maximum cross-sectional secondary moment I is more than 0.30 s2 mm4, although the above-mentioned problems can be solved, thinner portions are generated in the monofilaments, so that monofilaments tend to be torn off from the thinner portions during the stretching step and, also, the manufacture of, for example, ropes becomes difficult.

In addition, the nozzles used,at the melt spinning step in the present invention are desirably such that thermoplastic resins can be melt extruded at a nozzle shear rate of 150 through 900 sec-l. In the case where the nozzle shear rate is less than 1 50 sec-', the spinning pressure is lowered and, therefore, the extrusion rate is varied, whereby products having an uneven denier are produced. Contrary to this, in the case where the nozzle shear rate is more than 900 sec-', the melt fracture tends to be formed at a spinneret during a long period of operation, whereby the filaments tend to be cut under the nozzle.

Furthermore, the stretching ratio [i.e. f= V,/V0 wherein V0 is an extrusion linear velocity at a nozzle discharge (m/min.), V, is a take-off linear velocity (m/sec)] is desirably within the range of 1.00 through 3.50 (usually, 0.5 through 1.5 in the case of a perfect circle) in the present invention. In the case of a perfect circle) in the present invention. In the case where the stretching ratio is less than 1.00, the desirable increase in the strength of filaments is not obtained due to the insufficient molecule orientation. Contrary to this, in the case where the stretching ratio is more than 3.5, problems, including the stretching failure, the whitening of the stretching filament and the like, tend to occur.

The typical embodiment of the process of the present invention will now be illustrated with reference to the accompanying drawing.

As shown in Fig. 1, a thermoplastic resin is melt extruded from a screw-type extruder 1 and, then, passes through a cooling bath, whereby unstretched filaments 11 are produced. The filaments can optionally be solidified by using a treatment bath (not shown in Fig. 1).

The unstretched monofilaments 11 are stretched at a high stretching ratio at an optimum temperature depending upon the type of the thermoplastic resin. For instance, as shown in Fig. 1, the starting monofilaments 11 are first subjected to a first-stage stretching in a heated water bath 4 via first take-off rolls 3. Then, the filaments, pass through second take-off rolls 5 and preheating rolls 6, wherein the filaments are preheated to an optimum temperature depending upon the thermoplastic resin used.

The filaments thus preheated are subjected to a second stage stretching as they pass through the heat rolls 7. The stretched monofilaments are wound through final take-off rolls by using a winder 10, after, optionally, being annealed by means of the annealing heat rolls 9. Although the shapes, arrangements and surface finishing of the rolls used in the process of the present invention are not specifically limited, the use of nip type rolls is desirable, so that the filaments will not slip.

The high stretching ratio can be effected by any known technique, for example, wet type stretching, heat roll type stretching, heat plate type stretching, heated bath type stretching and the like.

These stretching methods can be used alone or in any combination thereof.

As is known in the art, the straight strength of stretched fibrous materials is largely affected by tie stretching ratio. Since an extremely high stretching ratio can be effected according to the present invention, filaments having a high tenacity can be produced. In addition, the knot strength of the filaments produced by the present process is higher, by 30 through 50%, than that of the conventional filaments at the same stretching ratio. In addition, according to the present invention, filaments having a high elongation are also produced.

According to another embodiment of the present invention, neck stretching by which necking deformation occurs is desirably effected by wet stretching and ultra-stretching after the necking deformation is completed by means of heat rolls. Multi-stage stretching usually means that filaments are stretched in two or more stages. The physical properties, especially the strength, of the filaments are improved with the increase in the number of the stretching stages. However, the installation cost is raised with the increase in the number of the stretching stages. For these reasons, a three or four stage stretching is suitably used from a practical point of view.

As mentioned hereinabove, according to the present invention, neck stretching by which necking deformation occurs is desirably effected by wet stretching. Especially, in the case where the stretching by which the necking formation occurs is carried out at a deformation velocity of 50 min-' or less and where the subsequent multi-stage stretching after the completion of the necking deformation is carried out at a deformation velocity of 20 min-' or less, desirable results can be obtained. The deformation velocity at the stretching is defined by V,+1 -- V, Li wherein Lj is an effective stretching distance (m) at the i-stage stretching, Vi is a delivery linear velocity (m/min) of the filament at the i-stretching stage and Vj+, is a take-off linear velocity (m/min) of the filament at the i-stretching stage.

If the deformation velocity during the neck stretching is more than 50 min-', problems, including the formation of voids in the filaments, the whitening of the surface of the filaments and the occurrence of the stretching failure, tend to be caused. Contrary to this, if the deformation velocity during the multistage stretching after the completion of the necking deformation is more than 20 min-1, frequent stretching failure tends to occur and, therefore, sufficient high ratio stretching cannot be effected.

The stretching ratio in each step can be desirably set in such a manner that the stretching ratio is lower, by 0.2 through 0.5 times, than that in which the whitening occurs. Furthermore, it is recommended that the neck stretching is effected at a temperature of 1 000C or less and that the subsequent multi-stage stretching after the completion of the necking deformation is effected at a temperature of 1 000C or more.

In the case where the monofilament is produced from polyethylene according to the present invention, polyethylene having a melt index of 0.1 through 0.9 g/1 0 min can be desirably used. In the case where the melt index of the polyethylene is less than 0.1 g/1 0 min, problems, including the generation of melt fracture at spinning, a poor stretching property, a decrease in the stretching ratio in which the whitening occurs and a high ratio stretching is impossible, tend to occur and monofilaments having a high tenacity cannot be obtained. On the other hand, in the case where the melt index of the polyethylene is more than 0.9 g/1 0 min, it is difficult to obtain monofilaments having a high tenacity, although high ratio stretching can be effected.

Furthermore, it is desirable that a ratio of a high-load melt index to a melt index (i.e. high-load melt index/melt index) of polyethylene is 40 or less. In the case where the ratio of a high-load index is more than 40, not only the desired straight strength and knot strength of the monofilament cannot be obtained, but also the spinnability is decreased, whereby, unless the nozzles having a diameter corresponding to the desired denier are used at the time when the denier of the monofilament is changed, the monofilaments are cut under the nozzles.

Among the polyethylene resins having the above-mentioned melt index and ratio of the high-load melt index to the melt index, medium and high density polyethylene resins can be desirably used in view of the moldability and strength thereof. These resins can be a homopolymer of ethylene and copolymer thereof with other monomer(s). These resins can optionally contain a heat stabilizer, a weathering agent, a lubricant, a matting agent, a pigment, a flame retarder, a foaming agent and the like.

In addition to polyethylene, other thermoplastic resins capable of melt spinning, such as, for example, polyamides, polyesters, polypropylene and the like, can be also used in the production of monofilaments according to the present process.

In the case where a high-density polyethylene having a melt index of 0.1 through 2.0 g/1 0 min, a density of 0.950 through 0.960 g/cm3 and a HLMI/MI ratio of 20 through 35 is used in the present invention, high-density polyethylene high tenacity monofilaments having the following characteristics can be continuously produced.

Tensile Strength (gid) : 11.0-1 5.0 Elongation at Break (%) : 4.0--10.0 Young's Modulus (kg/mm2) : 1600--3200 Melting Point(OC) ; 136-145 The denier of the high-density polyethylene filaments produced by the present invention is desirably as thick so 600 denier or more in view of the simplicity of the fabrication. The shapes of the cross-sectional area can be in any shapes. Examples of such shapes are shown in Figs. 2 through 5.

Among these shapes, the filaments having cross-sectional areas of Figs. 4 and 5, especially Fig. 5 are desirable, since these shapes simplify the subsequent winding and twisting steps of the manufacture of ropes and produce ropes having a high tenacity and a high flexibility.

These high-density polyethylene filaments having a high tenacity can be advantageously used, in lieu of nylon ropes, in the fields of, for example, ropes for large ships (e.g. mooring ropes, tag ropes), since the tensile strength is substantially identical to that of nylon, the density is lower than that of water, the snap back is small and the production cost is less than a half of that of nylon.

As mentioned hereinabove, according to the present invention, the monofilaments having a high tenacity, which is larger, by 50 through 100%, than that of conventional monofilaments can be obtained. Furthermore, in the case where a wet type stretching, the heat transfer coefficient of which is highest, is utilized in the first stretching stage, the necking point can be fixed and uniform filaments can be obtained. In addition, in the case where heat rolls are utilized in the second and the subsequent stretching steps, the freedom of the selection of numbers of the stretching stages becomes large and, as compared with other technique including hot plate type, heated air type and the like, the installation cost of the apparatus is decreased and the workability is improved.

The present invention now will be further illustrated by, but by no means limited to, the following Examples together with the Comparative Examples.

EXAMPLES 1-4 and COMPARATIVE EXAMPLES 1 High-density polyethylene having a melt index of 0.45 g/1 0 min and a density of 0.955 g/cm3 was melt extruded and was subjected to a multi-stage stretching, after cooling, under the conditions as shown in Table 1 below. Thus, monofilaments were produced. The results are shown in Table 1.

The common production conditions, other than those shown in Table 1, are as follows.

Extruder 40 mum0, L/D = 24 Screw : Compression Ratio of 3.2 Breaker Plate : 2.0 mm0 x 86H Screen Pack : Five (80, 100, 120, 150 and 100 meshes) No. of Nozzle Holes: 40 ExtruderTemperature (OC): C = 160, C2=250, C3 = 290, D, = 290, D2=290 Air Gap: 5 cm Temperature of Cooling bath: 1 70C Stretching Temperature: First Stage; 1 000C (wet type) Second Stage; 115 C (Heat Roll type) Third Stage; 11 50C (Heat Roll type) Fourth Stage; 1200C (Heat Roll type) Test Methods of Physical Properties of Monofilament:: JIS (Japanese Industrial Standards)-L-1 070 Chuck Distance = 30 cm, Take-off Speed = 30 cm/min, Temperature = 200C, Relative Humidity = 60% TABLE 1 EXAMPLE COMPARATIVE EXAMPLE MOLDING CONDITIONS 1 2 3 4 1 2 3 4 Draft Ratio F 2.16 2.58 2.59 2.75 1.26 3.49 2.80 2.21 Crose Sectional Area (mmw) 2.01 1.89 2.01 2.19 0.785 2.01 2.01 1.99 l/S2 0.1011 0.1036 0.2577 0.1272 0.07958 0.07958 0.07958 0.08649 Shape Elonated Oval Dumb-bell Oval perfect Perfect Perfect Elongated Circle Circle Circle Circle Circle Flatness Ratio a/b 1.3 1.3 2.3 1.6 1.0 1.0 1.0 1.11 First-Stage Stretching 13.0 13.0 13.0 13.0 10.0 10.5 10.0 13.0 Second-Stage Stretching 1.13 1.13 1.13 1.13 - - 1.25 1.13 Third-Stage Stretching 1.04 1.04 1.04 1.02 - - 1.15 1.04 Ratio &alpha;3 Fourth-Stage Stretching - - - 1.13 - - 1.05 Ratio &alpha;;4 TOTAL Stretching Ratio DRT 15.3 15.3 15.3 16.9 10.0 10.5 15.5 15.5 TABLE 1 (Continued) EXAMPLE COMPARATIVE EXAMPLE PHYSICAL PROPERTIES 1 2 3 4 1 2 3 4 Denier of Unstretches 6120 5100 5100 5700 4000 4200 4950 6120 Filament [De] Denier of Stretched 399 337 340 335 398 395 319 396 Filament [De] Stretchability No Failure No Failure No Failure No Failure One/6 Hr Two/3 Hr Fifteen/2 Hr Five/3 Hr (No,of stretching for 6 Hr for 6 Hr for 6 Hr for 6 Hr Failure/Times) or more or more or more or more Straight Strength 12.8 13.3 12.0 14.0 8.0 8.8 12.2 12.5 [g/d] Knot Strength 3.6 3.9 4.0 3.7 5.7 5.3 2.7 2.8 [g/d] Straight Elongation 8.8 8.9 8.3 8.0 18.0 15.0 7.8 8.4 [%] Knot Elongation 2.2 2.6 2.8 2.2 9.0 7.6 1.9 2.1 [%] As is clear from the results shown in Table 1 above, according to the present invention, the stretchability could be improved and no substantial stretching failure occurs. Furthermore, as to the strength of the monofilaments thus obtained, monofilaments having a high straight strength and high knot strength could be obtained in Examples 1-4. Contrary to this, the straight strength was low in Comparative Example 1 probably due to low draft ratio f and low stretching ratio. In Comparative Example 2, the straight strength was also low probably due to the low stretching ratio. In Comparative Example 3, the stretchability was very poor probably due to the low maximum cross-sectional secondary moment, although the draft ratio and the stretching ratios were increased. In Comparative Example 4, the stretchability was also poor due to the low maximum cross-sectional secondary moment.

EXAMPLES 5-9 and COMPARATIVE EXAMPLES 5-7 High-density polyethylene having a melt index of 0.45 g/1 0 min and a density of 0.955 g/cm3 was melt extruded and was subjected to a multi-stage stretching, after cooling, under the conditions as shown in Table 2 below. Thus, monofilaments were produced. The results are shown in Table 2.

The common production conditions, other than those shown in Table 2, are as follows.

Extruder 40 mm, I/D = 24 Screw Compression Ratio of 3.2 Breaker Plate 2.0 mmX x 86H Screen Pack Five (80, 100, 120, 1 50 and 100 meshes) No. of Nozzle Holes: 40 ExtruderTemperature (OC): C, = 160, C2=250, C3 = 290, D, = 290, D2=290 Air Gap: 5 cm Temperature of Cooling Bath: 170C Test Methods of Physical Properties of Monofilament: JIS (Japanese Industrial Standards)-L-1070 Chuck Distance = 30 cm, Take-off Speed = 30 cm/min, Temperature = 20"C, Relative Humidity = 60% TABLE 2 EXAMPLE COMPARATIVE EXAMPLE 5 6 7 8 9 4 5 6 Nozzle Cross Sectional Area (mm2) 2.01 2.01 1.89 2.01 2.01 2.01 2.01 1.99 1 0.07958 0.1011 0.1036 0.07958 0.1272 0.07958 0.07958 0.08649 S2 Shape Perfect Elongated Oval Perfect Oval Perfect Perfect Elongated Circle Circle Circle circle Circle Circle Flatness Ratio a/b 1.0 1.3 1.3 1.0 1.6 1.0 1.0 1.1 Stretching Ratio DRT1 13.0 13.0 13.0 7.0 13.0 10.0 13.0 13.0 DRT2 14.7 14.7 14.7 10.0 14.7 13.0 14.7 14.7 DRT3 15.3 15.3 15.3 13.0 15.8 15.6 15.3 15.3 Whitening Beginning Stretching Ratio DRT1W 14.0 14.5 14.5 14.0 14.3 14.0 14.0 14.0 DRT2W 15.5 15.7 15.7 11.5 15.5 13.5 14.7 15.5 DRT3W 16.4 16.8 16.8 14.0 16.4 14.2 14.7 16.4 Upper Limit Stretching Ratio DRT1M 13.0 13.5 13.5 13.0 13.3 13.0 13.0 13.0 DRT2M 14.7 14.9 14.9 10.9 14.7 12.8 13.9 14.7 DRT3M 15.8 17.2 16.2 13.5 15.8 13.7 14.1 15.8 Filament Temperature #1 ( C) 100 100 100 95 100 100 100 100 #2 ( C) 115 115 115 110 110 115 105 115 #3 ( C) 115 115 115 110 120 115 130 130 *DRTiM = DRTiW x (1.0 - 0.0970e-0.312 x i) TABLE 2 (Continued) EXAMPLE COMPARATIVE EXAMPLE 5 6 7 8 9 4 5 6 Stretching Method Frist Stage Second wet wet wet wet wet wet wet wet and Further Stage Heat Roll Heat Roll Heat Roll Heat Roll Heat Roll Heat Roll Heat Roll Heat Roll Denier of Unstretched Filament 5120 6120 5100 4550 6320 5000 5120 6120 [De] Denier of Stretched Filament 334 399 337 352 403 320 333 396 [De] Condition of Filament Tansparent Transparent Transparent Transparent Transparant Partially Whitening Transparent Glossy Glossy Glossy Glossy Glossy Whitening Glossy Glossy Little Glossy Stretchability One/6 Hr No Failure No Failure One/6 Hr No Failure Ten/2Hr Two/3 Hr Five/3 Hr No. of Failure/Time 6 Hr or more 6 Hr or more 6 Hr or more Straight Strenght (g/d) 12.7 12.8 13.3 11.6 13.5 12.2 11.7 12.5 Knot Strength (g/d) 2.9 3.6 3.9 54.3 3.5 2.7 2.9 2.8 Straight Elongation (%) 8.7 8.8 8.9 9.8 7.1 7.8 7.0 8.4 Knot Elongation (%) 2.0 2.2 2.6 3.8 2.0 1.9 2.7 2.1 As is clear from the results shown in Table 2 below, according to the present invention, the monofilaments having a high tenacity could be effectively produced without causing whitening of the monofilaments. Contrary to this, in Comparative Example 4, the stretchability is poor and whitening partially occurred due to DRTi > DRTiw x (1.0-0.0970e-0.312x1).

Similarly, in Comparative Example 5, whitening occurred in the products and the strength was somewhat decreased due to the fact that #1 did not satisfy the correlation Tm-27##i#Tm-17.

EXAMPLES 10-13 Production of High Tenacity Filament High-density polyethylene having a melt index of 0.51 g/10 min according to a JlS-K-6760 method and a density of 0.053 g/cm3 was melt extruded under the conditions as shown in Table 3 below and was subjected to a multi-stage stretching after quench. Thus, monofilaments were produced.

The results are shown in Table 3 below.

The common production conditions, other than those shown in Table 3, are as follows.

Extruder 50 mm, L/D = 24 Screw : Compression Ratio of 4.0 Breaker Plate 2.0 mm > c x 130H Screen Pack : Five (80, 100, 1 20, 1 50 and 100 meshes) No. of Nozzle Hole: 60 ExtruderTemperature ( C): C1 = 160, C2=250, C3 = 290, D, = 290, D2 = 290 Air Gap: 5 cm Temperature of Cooling Bath ( C): 1 50C Stretching Temperature: First Stage: 1 000C (Wet type) Second Stage; 115 C (Heat Roll) Third Stage; 115 C (Heat Roll) Fourth Stage; 140 C (Heat Roll) Production Rate: 16 Kg/Hr Manufacture of Rope Rope having a thickness of 12 mm was prepared, according to a JlS-L-2705 method, by using the high tenacity polyethylene monofilaments produced above.

The results are shown in Table 4 below.

Test Method of Samples The physical properties of the monofilaments were determined according to JlS-L-1 070 and 1073 methods, wherein a chuck distance of 30 cm, a take-off speed of 30 cm/min, a temperature of 200C and a relative humidity of 60% were used.

The physical properties of the ropes were determined according to JlS-L-2704, 2705 and 2706 methods, wherein a temperature of 20 + 2 C and a relative humidity of 65 + 2% were used.

COMPARATIVE EXAMPLES 7-9 The physical properties of commercially available polyethylene filaments, polypropylene multifilaments and nylon multi-filaments and ropes having a thickness of 12 mm comprised by each filament were determined in a manner as described in Examples 10-13. The results are shown in Tables 3 and 4.

TABLE 3 EXAMPLE COMPARATIVE EXAMPLE 10 11 12 13 7 8 9 Commercially Commercially Commercially Available Available Available polyethylene Polypropylene Nylon Samples High Density Polyethylene Having High Tenacity Fitament Filament Filament Cross-Sectional Area 2.01 2.01 2.19 2.01 - - S (mm2) of Nozzle First Stretching Ratio 13.0 13.0 12.3 12.3 - - Second Stretching Ratio 1.13 1.13 1.08 1.08 - - Molding Third Stretching Ratio 1.04 1.04 1.04 1.04 - - Condition Fourth Stretching Ratio - 1.15 1.04 1.16 - - Total Stretching Ratio 15.3 17.6 14.3 16.0 - - z Cross-Sectional Approximately Flat Circle Flat Circle Five Parallel Approximately Multi- Multi Shape of Filament Perfect Circle Filament of Perfect Circle Filament Filament Flat Circle Denier of Filament [De] 400 300 400 2000 400 680 1260 Physical Properties Tensile Strength (g/d) 12.4 15.0 12.8 12.2 8.0 7.5 8.0 of Filament Knot Strength (g/d) 3.59 3.26 3.69 2.53 4.45 5.00 5.53 Elongation at Break (%) 7.4 7.0 10.0 4.2 12.6 20.0 18.2 Young's Modulus (kg/mm2) 1950 2800 1600 1900 780 580 360 Melting Point ( C) 140 141 139 138 134 170 218 TABLE 4 EXAMPLE COMPARATIVE EXAMPLE Rope Having 12 mm Diameter 10 11 12 13 7 8 9 Weight kg/200m 13.5 15.6 14.8 14.5 14.5 14.3 18.2 Breaking Power (t) 2.53 3.12 2.66 2.80 1.43 1.70 2.83 Elongation at Break (%) 26.0 17.0 21.0 15.0 32.0 38.7 53.0 Physical Strength per Unit 37.4 40.0 36.0 38.5 19.7 23.8 31.1 Properties Weight (kg m/g) of Rope Gloss and Color Good Good Good Good Good Good Flexibility Fairly Good Very Good Very good Fairly good Fairly Good Good Very Good Snap Back Very Small Very Small Very Small Very Small Small Fairly Large Very Large in Water Float Float Float Float Float Float Sink Relative filament Cost 110 105 114 100 167 173 264 per Unit Strength EXAMPLES 14-16 and COMPARATIVE EXAMPLES 10-16 High density polyethylene containing 0.5% of zinc stearate, 0.1% of 2,6-di-tert.butyl-4methylphenol, 0.1% of calcium stearate, 0.05% of dimyristylthiodipropionate was melt extruded and stretched, after water cooling, in the conditions as shown in Table 5 below. Thus, monofilaments were produced. The results are shown in Table 5 below.

The common production conditions, other than those shown in Table 5, are as follows.

Extruder 40 mmm LID = 24 Screw Compression Ratio of 3.2 Breaker Plate 2.0 mm x 86H Screen Pack Five (80, 100, 120 150 and 100 meshes) No. of Nozzle Holes: 40 Extruder Temperature (OC): C, = 160, C2=250, C3=290, D = 290, D2=290 Air Gap: 5 cm Spinning Speed (High Speed Side): 110 m/min.

Temperature of Cooling Bath: 1 70C Stretching Temperature: First Stage; 1 000C (Wet type) Second Stage; 115 C (Heat Roll type) Third Stage; 115 C (Heat Roll type) Fourth Stage; 1 200C (Heat Roll type) Test Methods of Physical Properties of Monofilament: JIS (Japanese Industrial Standards)-L-1 070 and 1073 Chuck Distance = 30 cm, Take-off Speed = 30 cm/min.

Temperature=20 C, Relative humidity=60% TABLE 5 EXAMPLE COMPARATIVE EXAMPLE 14 15 16 10 11 12 13 14 15 16 Polyethylene Density (g/cm2) 0.954 0.948 0.953 0.964 0.945 0.954 0.958 0.953 0.953 0.953 Resin M.l. (g/10 min) 0.83 0.2 0.60 0.35 0.2 1.5 0.51 0.35 0.35 0.35 H.L.M.l./M.l. 36 35 24 57 43 38 32 45 45 45 Groove Depth of Metering Zone (mm) 3.0 3.6 3.0 2.4 3.6 2.0 2.4 1.7 2.4 2.4 Nozzle Shear Rate (sec1) *1 400 3200 590 590 400 320 1250 590 590 590 Extrusion Surface Roughing Degree *2 1 1 1 1 3 1 4 1 1 1 No. of Filaments Cut under Nozzle *3 0 0 0 2 4 0 10 12 0 0 Stretching First Stage 40.0 28.1 40.0 30.0 40.0 28.1 40.0 40.0 72.0 40.0 Deformation Second Stage 12.0 6.9 12.0 - 12.0 6.9 12.0 12.0 - 12.0 Velocity Third Stage 4.2 3.5 4.2 - 4.2 3.5 4.2 4.2 - (min 1) Fourth Stage - 15.6 - - - 15.6 - - - First Stage 100 100 100 100 100 100 100 100 100 90 Stretching Stretching Second Stage 115 115 115 - 115 115 115 115 - 98 Temp. ( C) Third Stage 115 115 115 - 115 115 115 115 - 98 Fourth Stage - 140 - - - 140 - - - Stretching Ratio 15.3 16.0 15.3 13.0 15.3 16.0 15.3 15.3 13.0 15.3 Stretchability *4 0 0 0 1 15 2 5 20 5 7 Straight Strength (g/d) 12.0 15.0 14.5 9.9 10.5 12.2 9.9 10.4 Physical Knot Strength (g/d) 4.5 3.4 4.0 3.2 Stretching 3.7 2.7 Stretching 3.0 1.7 Properties Straight Elonation (%) 9.2 7.0 8.0 10.9 impossible 7.0 7.8 Impossible 11.2 6.5 of Filament Knot Elongtion (%) 2.9 2.3 2.5 3.9 1.8 1.9 3.5 2.0 *1 Nozzle Shear Rate 40 # = #R3 O: Extrusion Volume (cm3/sec) R: Nozzle Relative Radius (cm) *2 Surface Roughening Degree was visually observed according to the following standards.

1 : Very Good 2 Good 3 : Stretching Possible Limit 4 : Surface Roughening 5 : Extremely Surface Roughening *3 No. of filaments cut under the nozzles during 1.5 hours' spinning operation was counted.

*4 No. of filaments cut during 1.5 hours' stretching operation was counted.

Claims (8)

1. A process for producing a monofilament having a high tenacity from a thermoplastic resin, wherein a monofilament melt spun from a thermoplastic resin is subjected to multi-stage stretching under the conditions satisfying the following equations.

#i # Tm - 37 (i = 1) Tm - 27 # #i # Tm - 17 (i # 2) wherein i is a number of stretching stages, e is a base of natural logarithm (i.e. 2.71828), V1 is the first take-off linear velocity (m/min), Vj+, is the final take-off linear velocity (m/min) at the i-stretching stage, DO TO is a total stretching ratio at the i-stretching state, DRTiw is the DO TO from which the monofilament begins to become whitened at the i-stretching stage, Tm is the melting point of the thermoplastic resin and #i is the temperature of the filament at the i-stretching stage.

2. A process as claimed in claim 1, wherein the monofilament is extruded from the thermoplastic resin, at a melt spinning step, through a nozzle having a cross-sectionai area S (mm2) satisfying the following equations.

0.503 mm2#S#3.14 mm2 0.09#-#0.30 s2 wherein I is a maximum cross-sectional secondary moment max (Ix, ly) (i.e. the maximum secondary moment in the cross-sectional secondary moments with respect to the main x axis and y axis passing through the center of the gravity of the cross-section).

3. A process as claimed in claim 1 or 2, wherein a neck stretching by which necking deformation occurs is effected during wet stretching and multi-stage stretching is effected by means of heated rolls after the completion of the necking deformation.

4. A process as claimed in claim 1 or 2, wherein polyethylene having a melt index of 0.1 through 0.9 g/10 min. and a ratio of a high-load melt index to a melt index of 40 or less is used.

5. A process as claimed in claim 1 or 2, wherein the extrusion of the monofilament is effected through a screw type extruder having a metering portion having a groove depth Hm of 0.1 57D0.719 through 0.259D C.719 mm, wherein D is a bore diameter (mm) of the extruder.

6. A process as claimed in claim 5, wherein polyethylene having a melt index of 0.1 through 0.9 g/l 0 min and a ratio of a high-load melt index to a melt index of 40 or less is melt extruded at a nozzle shear rate of 150 through 900 sec-' and the extruded monofilament is stretched.

7. A process as claimed in claim 6, wherein neck stretching by which necking deformation occurs is effected at a deformation velocity of 50 min-1 or less and multi-stage stretching is effected at a deformation velocity of 20 min- or less Vi+1 - Vi L1 wherein Li is an effective stretching distance (m) at the i-stage stretching, Vi is a delivery linear velocity (m/min) of the filament at the i-stretching stage and V+, is the final take-off linear velocity (m/min) of the filament at the i-stretching stage.

8. A process as claimed in claim 7, wherein neck stretching is effected at a temperature of 1000C or less and the multi-stage stretching after the completion of the neck stretching is effected at a temperature of 1 00 C or more.