EP0313068A2

EP0313068A2 - Polyvinyl alcohol-based synthetic fibers having a slender cross-sectional configuration and their use for reinforcing shaped articles

Info

Publication number: EP0313068A2
Application number: EP88117568A
Authority: EP
Inventors: Akio Mizobe; Tomoo Saeki; Masakazu Nishiyama; Isao Sakuragi; Akitsugu Akiyama; Hideki Hoshiro; Hiroshi Sugishima
Original assignee: Kuraray Co Ltd
Current assignee: Kuraray Co Ltd
Priority date: 1987-10-22
Filing date: 1988-10-21
Publication date: 1989-04-26
Anticipated expiration: 2008-10-21
Also published as: ES2077560T3; EP0313068A3; DE3854253T2; DE3854253D1; EP0313068B1

Abstract

Described is the spinning of a boric acid-containing aqueous polyvinyl alcohol solution in an alkaline coagulation bath. The coagulation bath is maintained at a temperature of 55 to 95°C. The resulting tow of monofilaments is drawn in a draw ratio of not less than 17. This process provides polyvinyl alcohol-based synthetic fibers having a slender cross-sectional configuration and superior mechanical properties. By using these fibers as a reinforcement material for synthetic resins, rubbers, hydraulic inorganic materials, etc., there can be obtained shaped articles possessing improved mechanical properties.

Description

This invention relates to polyvinyl alcohol (hereinafter referred to sometimes as PVA)-based synthetic fibers having a slender cross-sectional configuration and a large crystal length-to-breadth ratio. In another aspect, the invention relates to shaped articles reinforced with the same fibers.
PVA-based fibers are characterized by the highest strength and tenacity of all conventional synthetic fibers and, because of these properties, have heretofore been used for the reinforcement of various molding materials inclusive of organic molding materials such as plastics and rubbers and hydraulic inorganic materials such as cement and gypsum.
The most important requirements that any reinforcing PVA fiber must meet are high strength and a high modulus of elasticity. Unless the reinforcing fiber is adequate in strength and modulus, the finished reinforced article will necessarily lack toughness.
The second important requirement of such a reinforcing fiber is a high adhesive affinity for matrices (e.g. said plastics, rubbers, cement and gypsum). If the adhesion between the reinforcing fiber and the matrix is insufficient, the strength and modulus of the fiber will not be fully exploited when the article responds to an external stress, with the result that the article may undergo cracking or breakdown. In order to establish an improved bond between the reinforcing fiber and the matrix, it is an effective procedure to increase the surface area of the reinforcing fiber by, for example, flattening the cross-sectional configuration of the fiber, deforming the fiber otherwise, or reducing the fineness or denier number of the fiber. For this purpose of increasing the strength and modulus of PVA fibers, a variety of methods have been proposed and some of them actually used.
The most representative method for manufacturing such a PVA fiber comprises wet-extruding an aqueous PVA dope in a dehydrating salt-containing coagulation bath at ambient temperature and subjecting the resulting tow to drawing and heat treatment and, if necessary, to acetalization. As is well known, the PVA fiber manufactured by the above method has a double-layer construction consisting of a skin layer and a core, resembling that of a cocoon, and as such has a comparatively large surface area but can only be drawn or stretched in a total draw ratio of about 8. Accordingly, the strength of the fiber is only about 7 g/d which is far less than the requirement for a reinforcing fiber.
In this category of fiber production technology, the processes which appear to be the most effective and promising are the process described in Japanese Patent Publication No. 14901/1969 and the process described in Japanese Patent Publication No. 3513/1969. The characteristic aspect of these processes is that an aqueous PVA solution of comparatively high concentration is extruded in a dehydrating salt-containing coagulation bath maintained at a high temperature of not less than 65°C and, then, quenched immediately on withdrawal from the bath. The underlying principle of this technology is to provide a homogenous fiber having no discrete skin and core structure to thereby implement a high strength characteristic. By these processes, the total draw ratio can be increased to about 14 and the strength of the fiber improved over that of the earlier fiber. However, the strength value is only just above 10 g/d and the fiber cross-sectional configuration is close to circular. Thus, the fiber is not fully satisfactory.
Processes for manufacturing PVA fibers having still higher strength values are described in Japanese Patent Publications No. 32623/1973 and No. 1368/1978. These processes are characterized in that an aqueous PVA spinning solution containing boric acid or a salt thereof is extruded in a dehydrating salt-containing alkaline coagulation bath at 20-50°C. The fibers obtainable by these processes are greater in homogeniety than the fibers manufactured by the methods of the aforementioned Patent Publications No. 3513/1969 and 14901/1969 and the cross-sectional configuration of these fibers is approximately circular, thus assuring a smaller surface area. Furthermore, by the former process, the total draw ratio is increased to about 22 and the mechanical properties are improved, providing a strength value of about 17 g/d, but the fiber has only a small surface area and is not yet satisfactory for reinforcing purposes. The latter patent literature refers to the length of fiber crystals but does not refer to the ratio of length (L) to width (W) (hereinafter referred to as L/W ratio) of the crystals. At any rate, the total draw ratio of 16 and the strength value of 12 g/d as shown in the examples of this patent are considerably lower than the values obtainable according to this invention. As will be explained hereinafter, the reinforcing effect is improved only when the L/W ratio, rather than the value of L, is increased. Moreover, since the fiber of this prior art is circular in cross section and has a low surface area, it is not satisfactory for use as a reinforcing material.
A further technology is disclosed in Japanese Unexamined Patent Application KOKAI No. 126312/1985 (corresponding to U.S.P. 4,603,083 and European Patent Laid-open Specification No. 0146084) and Japanese Unexamined Patent Application KOKAI No. 108712/1986. This technology generally comprises dissolving PVA in a solvent such as dimethyl sulfoxide or glycerol, subjecting the solution to dry-jet-wet spinning (the spinning method in which the spinning dope extruded in the air from a spinneret nozzle is immediately dipped in a coagulation bath) or quench-gelation spinning, and, after removal of the solvent, drawing the resulting tow in a high draw ratio. The fiber obtainable by this technology has been remarkably improved in mechanical properties,but because of its exceptionally high homogeneity and circular cross section with a small surface area, it is not satisfactory for use as a reinforcing fiber.
Thus, the attempts so far made to improve the strength and modulus of PVA fiber are all intended to provide an increased fiber homogeneity,and although the fibers so manufactured have been improved in mechanical properties, they are substantially circular in cross section and inadequate in surface area, thus failing to simultaneously meet the aforementioned two requirements for reinforcing materials.
On the other hand, as a positive approach to increasing the surface area of a fiber, there is known a technology comprising extruding a spinning dope from a "deformed" nozzle to provide a fiber having an irregular cross-sectional configuration. As far as PVA fibers are concerned, this technology requires an increased spinning draw and the drawability is drastically reduced as compared with the case in which a nozzle having circular orifices is employed. Consequently, the strength of the fiber is low and the spinnability and productivity are also adversely affected.
Another approach for obtaining an increased surface area per unit weight of fiber is to reduce the denier number of the fiber. Reducing the denier number, however, tends to reduce both spinnability and productivity and, in addition, adversely affects the dispersibility of the fibers in the matrix.
Thus, by any of the technologies proposed so far, it has been impossible to obtain a PVA fiber having high strength and modulus and a sufficiently large surface area.
Thus, the technical problem underlying the present invention is to provide PVA fibers of sufficiently high strength and modulus, with a slender cross-sectional configuration and a large surface area, all of which are the essential requirements for reinforcing fibers.
This invention is, therefore, concerned with a PVA fiber having a degree of cross-sectional roundness not greater than 65% and a L/W ratio, as defined herein, of at least 2.1. The term cross-sectional roundness is defined below (see page 19).
In the study of the reinforcing effects of various types of PVA fibers, it was found that, for reasons not yet understood, the reinforcing effect is more closely associated with the L/W ratio of the fiber than with the strength and modulus of the fiber. In order to realize a sufficient reinforcing effect, the L/W ratio should not be less than 2.1 and preferably not less than 2.3 and the degree of cross-sectional roundness be not greater than 65 percent and preferably not greater than 60 percent. The length L of the fiber crystal, taken independently, is not much correlated with the reinforcing effect and mechanical properties of the fiber but rather shows high correlation with them only in the context of L/W.
As an effective process for producing such fibers,this invention provides a process which comprises wet-spinning an aqueous PVA solution containing boric acid or a salt thereof in a dehydrating salt-containing alkaline coagulation bath at 55 to 95°C and drawing the resulting tow in a draw ratio of at least 17.
The most outstanding feature of this method is that whereas the conventional methods employ a coagulation bath temperature in the range of 20 to 50°C, the method of the invention uses a coagulation bath at a high temperature of 55 to 95°C. Thus, although the resulting fiber is opacified and presents a slender cross-sectional configuration as a result of the coagulation at such a high temperature, the drawability is markedly improved and the L/W ratio of the fiber is increased to realize a very large increase in the reinforcing effect and a marked improvement in mechanical properties. Furthermore, even in the case of PVA with a high degree of polymerization which exhibits only very low drawability with the consequent failure to achieve an improvement in mechanical properties, this invention permits a high draw ratio to fully exploit the effect of the degree of polymerization of vinyl alcohol, thereby yielding a fiber with a large L/W ratio and having a slender cross-sectional configuration. The ratio involved is not sufficiently clear but it is suspected that the mechanism of coagulation is completely different from that involved in conventional spinning technology.
In another aspect, this invention is concerned with shaped articles reinforced with the above-described PVA-based fibers,particularly fiber-reinforced plastic products (hereinafter referred to briefly as FRP), fiber-reinforced hydraulic products (hereinafter referred to briefly as FRC) and fiber-reinforced rubber articles.
An example of the production method according to this invention is described below in detail.
The degree of polymerization of the PVA employed is at least 1,500, preferably at least 2,000, and more desirably at least 3,000. When the degree of polymerization of PVA is less than 1,500, crystal growth in the axial direction of the fiber is too small to assure an L/W ratio of not less than 2.1 and an improvement in mechanical properties.
The spinning dope used in accordance with this invention is an aqueous solution containing 5 to 30 weight percent of said PVA and additionally 0.5 to 5 weight percent, based on said PVA, of boric acid or a salt thereof. The concentration of PVA is preferably 6 to 25 weight % and, for even better results, 7 to 18 weight %. The concentration of PVA may vary to some extent according to its degree of polymerization, but when the concentration is too low, the growth of crystals along the fiber axis will not be sufficient to assure a large L/W ratio, although the drawability will be satisfactory, and when conversely the concentration of PVA is too high, the drawability is greatly impaired.
The temperature of the spinning dope is 85 to 125°C and preferably in the range of 95 to 120°C. When the dope temperature is too low, the drawability is greatly impaired, while an excessively high dope temperature results in boiling of the dope. For the purpose of stabilizing spinnability, an organic acid such as acetic acid or an inorganic acid such as nitric acid may be added to the spinning dope in an appropriate amount.
The temperature of the coagulation bath is 55 to 95°C and preferably in the range of 60 to 80°C. When the bath temperature is below 55°C, the fiber is not so slender in its cross-sectional configuration and the drawability is too low to assure a sufficiently large L/W ratio. Conversely when the coagulation bath temperature is over 95°C, boiling of the bath and adhesion of monofilaments tend to occur.
To establish the alkalinity of the coagulation bath, an alkali hydroxide such as sodium hydroxide, potassium hydroxide, etc. is employed and its concentration is 2 to 200 g/ℓ and preferably 5 to 50 g/ℓ. The salt to be incorporated in the coagulation bath is a salt having dehydrating properties, such as sodium sulfate, sodium carbonate, etc. and its concentration is 100 g/ℓ to saturation and preferably close to saturation. As spinneret nozzle, an ordinary circular-orifice nozzle or a nozzle similar thereto is employed.
The fiber obtained can be post-treated by the conventional procedures of neutralization, wet heat drawing, rinsing, drying, heat drawing and heat treating (tempering). The draw ratio in wet condition is at least 3 and preferably not less than 5. The total draw ratio, inclusive of that in wet condition and that in dry condition, is not less than 17 and preferably not less than 20. The greater the total draw ratio, the larger is the L/W ratio. When the total draw ratio is less than 17, the L/W ratio does not reach 2.1.
The shaped resin articles reinforced with the above fibers are described below.
The size of PVA fibers may be similar to that used in the ordinary fiber-reinforced plastic (FRP) products, viz. in the range of 0.1 to 100 deniers, for instance.
There is no limitation on compatible matrix plastics. Thus, any of the thermosetting resins such as unsaturated polyester resins, phenolic resins, epoxy resins and melamine resins, etc. and the thermoplastic resins such as polyester resins, polyamide resins, polyethylene resins, polypropylene resins, polyvinyl chloride resins and so on can be employed.
In accordance with this invention, the PVA fibers can be used e.g. in the form of short staples or chopped strands or, depending on use, may be used in the form of e.g.rovings, woven fabrics or nonwoven fabrics.
The proportion of the PVA fibers in a shaped plastic article is preferably 1 to 50 weight percent. Such a shaped plastic article may further contain, in addition to PVA fibers and matrix resin, other fibers, fillers, curing or hardening agents, thixotropic agents, modifiers of flow properties, pigments and so on.
In one method of production of such a shaped article, the PVA fibers and a thermosetting resin are blended using a kneader and the resulting bulk molding compound is molded by compression molding or injection molding. Another method comprises laminating a sheet molding compound of a thermosetting resin with chopped strands of the PVA fibers, covering the laminate with a polyethylene film or the like on either side and molding the assemblage. Shaped articles may also be obtained by the usual hand lay-up method. In any process, the PVA fibers of this invention can be used in the same manner as glass fibers in the manufacture of shaped articles.
The FRP obtainable by the method of this invention has the following characteristics.

1) Unlike the conventional PVA fibers, the fibers used in this invention have a larger L/W ratio, superior mechanical properties, a degree of cross-sectional roundness of not greater than 65 percent, which means a slender cross-sectional configuration, and a large surface area. Therefore, the FRP manufactured using such fibers is superior in strength, modulus of elasticity and impact strength.
2) The mechanical properties of the FRP according to this invention are by far superior to the usual glass fiber-reinforced plastics. Moreover, the FRP of the invention is superior in surface smoothness and lighter in weight.

By virtue of the above excellent properties, the fiber-reinforced resin composition according to this invention can be used in a wide range of purposes, such as architectural materials, building materials, industrial parts, transportation devices, automobiles, boats, and so on.
The shaped hydraulic products reinforced with the PVA fibers according to this invention are described below.
For the fabrication of a fiber-reinforced hydraulic product, typically a fiber-reinforced cement (FRC) product, the PVA fibers can be used in any form appropriate to the molding process or procedure. For example, they can be used as short monofilaments or chopped strands or in the form of continuous filament yarns or bundled filament yarns. They may also be used in the shape of fiber rods. The PVA fibers can also be used in such varied forms as nonwoven fabrics,mats,mesh sheets,knitted fabrics,or other two-dimensional or three-dimensional constructions. Furthermore, the PVA fibers can be used in combination with reinforcing steel or further used in combination with e.g. glass fibers, steel fibers, acrylic fibers.
On the other hand, there is no limitation to the molding method that can be used; the ordinary molding processes for FRC can be employed with equal success. For example, a wet-casting method can be used for the manufacture of sheets. The Hatschek process (see British patent 20 40 331) is typical of the method.
In the case of mortar or concrete, casting, sprayup, and pouring can be mentioned as examples of field molding technology. As examples of factory molding technology, vibration molding, centrifugal casting, and extrusion molding can be mentioned.
The level of addition of the PVA fibers to a hydraulic molding composition is 0.2 to 20 weight percent and preferably 1 to 5 weight percent. The aspect ratio (the length of fiber divided by the diameter of a circle equivalent to the cross-section of the fiber) in the case of short staples is 50 to 2,000 and preferably 150 to 600.
Cement is a representative hydraulic material and Portland cement is a typical example. Blast furnace cement, flyash cement, alumina cement, etc. may also be used. These hydraulic materials can be used singly or in combination. Furthermore, these cements may be used in admixtures with sand or gravel to provide a mortar or a concrete. As other examples of the hydraulic material, gypsum, gypsum slag, magnesia, etc. can be mentioned. After all, any hydraulic material suited to the intended application can be used.
As auxiliary additives, mica, sepiolite, attapulgite, etc. can be used.
Compared with the conventional FRC composition, the FRC compositions of this invention contain reinforcing PVA fibers having a larger L/W ratio and a slender cross-sectional configuration with a degree of cross-sectional roundness not greater than 65% and, as such, assure excellent mechanical properties. Therefore, the FRC composition of this invention can be used in all varieties of cement and concrete applications such as plates, pipes, blocks, wall panels, roof tiles, room dividers, road pavements, tunnel lining, surface protection and so on.
Rubber articles reinforced with the PVA fibers of this invention are described below.
The rubber to be used may be of any type.
Thus, natural rubber (NR) and various synthetic rubbers such as styrene-butadiene rubber SBR), chloroprene rubber (CR), nitrile rubber (NBR), ethylene-propylene diene rubber (EPDM), etc. can be employed. These rubbers can be used singly or in combination, and may be used in the form of a latex.
The PVA fibers of this invention are used in the form of a fabric or in the form of twisted yarn. Depending on uses, however, nonwoven fabrics, short staples, chopped strands, etc. may prove useful forms of application.
For the manufacture of shaped rubber articles using the PVA fibers of this invention, the per se conventional technology can be utilized.
For example, the PVA fibers are twisted to form a cord and, if necessary, after treatment with an adhesive, the cord is woven or knitted into a fabric and superposed on a rubber matrix. Or the PVA fibers are woven or knitted and, if necessary, after treatment with an adhesive, they are molded together with rubber.
The shaped rubber articles of this invention have the following advantages.

1) Compared with the conventional PVA fibers, the fibers of this invention have a larger L/W ratio, superior mechanical properties, and a larger surface area because of their slender cross-sectional configuration with a degree of cross-sectional roundness not over 65%. Therefore, rubber articles reinforced with the fibers of this invention assure an improved bond between the fibers and the rubber matrix, a higher residual strength after flexural fatigue, higher tenacity, higher modulus and higher durability.
2) Compared with the conventional PVA fibers,the PVA fibers of this invention can reinforce shaped articles to the same degree with a smaller fiber weight, with the result that the shaped articles are lighter in weight and have a smoother surface.

Thanks to the above superior properties, the fiber-reinforced rubber composition according to this invention can be used in various applications such as tire parts, tarpaulin, sheet, hoses, diaphragms, and so on.
While, as described in detail hereinbefore, the PVA fibers of this invention have a large L/W ratio, superior mechanical properties and a large surface area due to a slender cross-sectional configuration with a degree of cross-sectional roundness not greater than 65% and therefore they are applied as reinforcing fibers for cement, plastics and rubber.Their excellent mechanical properties can be best exploited in such industrial products as ropes and cables.
This invention will hereinbelow be described in further detail by way of examples. In these examples, the degree of cross-sectional roundness (degree of slenderness; the smaller the value, the flatter the fiber), the crystal length-to-width ratio (L/W) and mechanical properties (dry strength and elongation at break and initial modulus of elasticity) were determined as follows.

o Degree of cross-sectional roundness

A photograph of the cross-section of a sample fiber is enlarged to the size of about 100 mm² and the cross-sectional area F is measured.
Then, the largest width B of the cross-section is measured. The degree of cross-sectional roundness is calculated by means of the following formula.
The degree of cross-sectional roundness was calculated for 20 randomly sampled monofilaments from a multifilament yarn and the mean of the values was defined as the degree of cross-sectional roundness of the fiber constituting the multifilament yarn.

o Crystal length-to-width ratio

The conventional method of wide-angle x-ray diffraction analysis was applied under the following conditions.

(1) Rotary cathode x-ray diffraction analyzer (Type RAD-rA, Rigaku Denki Co. Ltd.; 40 KV, 100 mA, CuKα (graphite monochromator) scintillation counter
(2) Goniometer
Slit system: DS1/2°, SS1/2°, RS 0.15 mm
Scanning speed: 2ϑ = 1/2°/min
(3) Specimen (a fiber specimen weighing 125 mg is set in parallel lengths of 2.5 cm in a width of 1.5 cm on a specimen mount)

The diffraction curves for planes (020) and (100) were determined by the transmission method and the half width B (hkℓ) was found for each curve.

Crystal size ratio (L/W)

From the half widths B (hkℓ) for (020) and (100) planes as found by the transmission method, the respective crystal sizes were calculated by the Scherrer equation.
D(hkℓ) = K λ/Bo(hkℓ)cos ϑ(hkl)
K = 0.9
λ = 1.5418 (Å)
Bo = the dispersion of the diffraction curve after slit correction by the method of Jones (radian)
ϑ(hkℓ) = Bragg's angle (deg.)
It was assumed that L/W = D(020)/D(100).

o Dry strength and elongation at break and initial modulus of elasticity

(1) Specimen: Multifilament yarn
(2) Dry strength and elongation at break and initial modulus of elasticity: In an atmosphere of 20°C and 65% R.H., determinations were made according to JIS-1017 using an Instron tester at a sample length of 20 cm and a pulling speed of 10 cm/min. The initial modulus of elasticity was determined from the elongation-load curve.
(3) Number of experiments: Ten determinations were made and the mean result was used.

Examples 1 through 3 and Comparative Examples 1 through 3

A completely saponified PVA (degree of polymerization = 3,500) was dissolved in water to a final concentration of 9 wt. %, followed by addition of 3.5 wt. % of boric acid based on the PVA to give a spinning dope. This spinning dope was heated at 105°C and extruded into coagulation baths containing 15 g/ℓ of sodium hydroxide and 350 g/ℓ of sodium sulfate at 60°C (Example 1), 70°C (Example 2), 90°C (Example 3), 40°C (Comparative Example 1), 100°C (Comparative Example 2) and 30°C (Comparative Example 3) from nozzles having 1,000 circular orifices. The resulting tows were taken out from the baths at a rate of 6 m/min. Thereafter, the respective tows were subjected to the routine treatment of drawing between rolls, neutralization, 1.5 times wet heat drawing, rinsing and drying in a wet draw ratio of 6. The tows were then subjected to dry heat drawing at 230°C and taken up on bobbins.

The non-heat drawn monofilaments obtained using a coagulation bath at 30°C or 40°C were comparatively transparent and had a high degree of cross-sectional roundness but as the temperature of the coagulation bath was increased, the monofilaments rapidly gained opacity and the degree of cross-sectional roundness decreased. The properties of the yarns obtained are shown in Table 1. The yarn construction was invariably 1800d/1000f.

Table 1

	Example 1	Example 2	Example 3	Comparative Example 1	Comparative Example 2	Comparative Example 3
Coagulation bath temperature (°C)	60	70	90	40	100	30
Draw ratio (times)	22.5	26.8	25.4	14.5	Not spinnable	14.6
Cross-sectional roundness (%)	48	36	34	68		68
L/W	2.5	2.8	2.7	1.9		1.7
Strength at break (g/d)	19.7	22.5	20.3	13.5		13.5
Elongation at break (%)	4.9	4.3	4.7	7.2		7.4
Initial modulus of elasticity (g/d)	470	550	513	310		310

Thus, as the coagulation bath temperature was increased to the levels of Examples 1, 2 and 3, the degree of cross-sectional roundness decreased, so that even in the case of PVA with a comparatively high degree of polymerization, the drawability showed marked improvements, giving rise to fibers having large L/W ratios and excellent mechanical properties.

Example 4

A completely saponified PVA (degree of polymerization = 1,700) was dissolved in water to a final concentration of 14 wt. %, followed by the addition of 1.5 wt. % of boric acid based on the PVA. The solution was heated at 100°C and extruded from a 1,000-circular orifice nozzle into a coagulation bath containing 30 g/ℓ of sodium hydroxide and 340 g/ℓ of sodium sulfate at 80°C. The resulting tow was taken from the bath at a rate of 8 m/min. The tow was then subjected to the routine treatment series of drawing between rolls, neutralization, 2 times wet heat drawing, rinsing and drying in a wet draw ratio of 7. The tow was further subjected to dry heat drawing at 225°C and taken up on a bobbin.
The resulting yarn showed marked opacity and a cross-sectional roundness of 50%. The total draw ratio was 25 and the L/W ratio was as large as 2.4. This fiber was very suitable for the reinforcement of cement.

Comparative Example 4

The same procedure as Example 2 was followed up to the drying stage and the dry heat drawing ratio was decreased to give a total draw ratio of 16.
The resulting yarn had the same degree of cross-sectional roundness of 36 %, as that of the yarn according to Example 2 but had a smaller L/W ratio of 2.0, thus being inferior in mechanical properties. The strength at break was 12.8 g/d and the modulus of elasticity was 280 g/d. Compared with the yarn according to Example 2, this fiber was by far inferior in its reinforcing effect.

Comparative Example 5

The same PVA as used in Examples 1 through 3 was dissolved in dimethyl sulfoxide to a concentration of 10 wt. %. This spinning solution was heated at 70°C and extruded from a 50-circular orifice nozzle by the dry-jet-wet spinning method into a methanol coagulation bath at 10°C. The distance between the discharge surface of the nozzle and the liquid surface in the bath was 15 mm. The resulting tow was subjected to wet drawing with removal of the solvent and drying in a draw ratio of 6. Then, the yarn was further subjected to dry heat drawing at 240°C to give a total draw ratio of 24.
The resulting filament was approximately circular with a degree of cross-sectional roundness of as high as 92%. With an L/W ratio of as large as 2.6, the fiber showed excellent mechanical properties. The strength at break was 21.0 g/d and the modulus of elasticity was 530 g/d.
As shown in Examples 11 and 12 and Comparative Examples 11 to 13 which appear hereinafter, the reinforcing property of this fiber was appreciably inferior to that of the fibers according to Examples 1 to 3. Electron microscopic examination of the broken surfaces after bending test of hardened cement samples revealed that whereas substantially no slip-out had occurred in the case where the fibers of Examples 1 through 3 having slender cross-sectional configurations with low cross-sectional roundness values were used, many slipouts had occurred in the samples prepared with fibers having a circular cross-section.

Example 5

A completely saponified PVA (degree of polymerization = 4,500) was dissolved in water to a concentration of 8.5 wt. %, followed by addition of 4.0 wt. % of boric acid based on the PVA to give a spinning dope. This spinning dope was heated at 110°C and extruded from a 1,000-circular orifice nozzle into a coagulation bath containing 8 g/ℓ of sodium hydroxide and 360 g/ℓ of sodium sulfate. The resulting tow was taken from the bath at a rate of 4 m/min. The tow was subjected to the routine series of drawing between rolls, neutralization, wet heat drawing, rinsing and drying and, then, to dry heat drawing and finally taken up on a bobbin. The draw ratio under wet conditions was 4 and the total draw ratio was 27.
The monofilaments were very flat in cross-section with a cross-sectional roundness of 32%. With an L/W ratio of 2.9, the yarn had excellent mechanical properties, i.e. strength at break was 22.9 g/d, elongation at break was 4.3%, and initial modulus of elasticity was 560 g/d. As anticipated, the yarn was excellent in its reinforcing effect on cement.

Examples 6 through 8 and Comparative Examples 6 through 8

The fibers prepared in Examples 1-3 and Comparative Examples 3 and 5 were used for reinforcing an unsaturated polyester resin. Thus, a kneader was charged with 150 parts of an unsaturated polyester resin (Polymar 6709, Takeda Chemical Industries, Ltd.), 2 parts of MgO (Kyowamag 40 F, Kyowa Chemical Co., Ltd.), 8 parts of a mold release agent (zinc stearate, Nippon Oils and Fats Co., Ltd.), 0.5 part of curing agent (Perbutyl Z, Nippon Oils and Fats Co., Ltd.) and 275 parts of CaCO₃ (S light No. 1200, Nitto Powder Industries, Ltd.) and after 15 minutes of mixing, 3 mm staples of the above-mentioned PVA fibers or commercial glass fibers(strength at break = 6.1 g/d, initial modulus = 302 g/d,cross-sectional roundness = 100%) were added in a proportion of 100 parts. The contents were further stirred at low speed for 5 minutes to give a bulk molding compound (BMC). This BMC was compression-molded at 160°C and 100 kg/cm² for 10 minutes to give an FRP with a thickness of 5 mm. The results are shown in Table 2.
It is apparent from Table 2 that the shaped articles obtained by using PVA fibers with L/W ≧ 2.1 and cross-sectional roundness ≦ 65% are superior in bending strength and impact resistance.

Examples 9 through 10 and Comparative Examples 9 through 10

The PVA fibers having a monofilament fineness of 2 deniers, indicated in Table 3, were treated with 1%/monofilament of styrene-soluble polyvinyl acetate (Dainippon Ink and Chemicals, Inc.) and cut into 25,4 mm lengths, which were then constructed into chopped strand mats. Each mat was impregnated with a homogenous mixture of 100 parts of said unsaturated polyester resin, 2 parts of MgO, 8 parts of a mold release agent, 3 parts of a curing agent (Perbutyl Z) and 300 parts of CaCO₃ to give a sheet molding compound (SMC). This SMC was compression-molded at 160°C and 100 kg/cm² for 10 minutes to give an FRP with a thickness of 5 mm. It will be apparent from Table 3 that the FRPs manufactured from the SMCs reinforced with the PVA fibers according to this invention are superior in bending strength and impact resistance.
The results of evaluating the quality of FRP using filaments of various cross-sectional roundness and L/W were similar to the results described in Table 2.

Examples 11 through 12 and Comparative Examples 11 through 13

The fibers prepared in Examples 1 and 2 and Comparative Examples 3-5 were cut into 6 mm lengths. Using a Hatschek machine, a composition of 2 parts of the fiber, 4 parts of pulp and 94 parts of Portland cement was wet-cast and subjected to spontaneous cure for 15 days to give cement plates with a thickness of 5 mm. The physical properties of such plates are shown in Table 4. The bending strength of each plate was measured in accordance with JIS K6911.
Using the FRC's obtained in Examples 11 and 12 and Comparative Examples 11 through 13, the broken surfaces of the respective specimens after the bending test were examined with an electron microscope. Whereas the specimens corresponding to Comparative Examples 11 and 13 showed the reinforcing fiber not cut but pulled out, this phenomenon was more pronounced in the case of the specimen of Comparative Example 13.

Examples 13 through 15 and Comparative Examples 14 through 16

The PVA fibers obtained in Examples 1 through 3, the PVA fibers.(cross-sectional roundness = 43%, L/W = 2.0, strength at break = 14.4 g/d, initial modulus = 345 g/d) manufactured in the same manner as Example 2 except that the total draw ratio was 16.2, and the PVA fibers obtained in Comparative Examples 3 and 5 were used as reinforcing fibers for the manufacture of fiber-reinforced rubber sheets. Thus, each of the fibers was twisted at 30 T/10 cm for both first and final twists to provide 1,800 D/1 x 2 cords, which were then treated with RFL to give dipped cords. RFL means rubber latex containing a resorcinol-formaldehyde resin.
The dipped cords were arranged in the form of tire cord fabrics and rubber was layed up and vulcanized to give a fiber-reinforced rubber sheet. This sheet was bent on a flexural fatigue tester and the cords were then taken out and measured for residual strength. The results are set forth in Table 5.
It is apparent from Table 5 that the rubber sheets manufactured using the PVA fibers of this invention are superior in adhesion and in residual strength after flexural fatigue.

Claims

1. Polyvinyl alcohol-based synthetic fibers having a degree of cross-sectional roundness not greater than 65 percent and a crystal length-to-width ratio of at least 2.1.

2. The polyvinyl alcohol-based synthetic fibers according to Claim 1 wherein the degree of polymerization of polyvinyl alcohol is at least 1,500.

3. The polyvinyl alcohol-based synthetic fibers according to Claim 1 wherein the degree of polymerization of polyvinyl alcohol is at least 3,000.

4. A method for manufacturing polyvinyl alcohol-based synthetic fibers which comprises extruding a spinning dope comprising an aqueous polyvinyl alcohol solution containing boric acid or a salt thereof into a salt-containing alkaline coagulation bath at a bath temperature of 55 to 95°C and drawing the resulting tow of monofilaments at a total draw ratio of at least 17.

5. The method according to Claim 4 wherein the temperature of said coagulation bath is between 60°C and 80°C.

6. The method according to Claim 4 wherein the total draw ratio is not less than 20.

7. A shaped article reinforced with polyvinyl alcohol-based synthetic fibers having a degree of cross-sectional roundness not greater than 65 percent and a crystal length-to-width ratio of at least 2.1.

8. The shaped article according to Claim 7 wherein the article reinforced is made of a member selected from the group consisting of synthetic resins, hydraulic inorganic materials and rubbery polymers.