EP0136659A2

EP0136659A2 - Wet-degradable fibers

Info

Publication number: EP0136659A2
Application number: EP84111518A
Authority: EP
Inventors: Kunio Nishimura; Mamoru Tsumoto
Original assignee: Teijin Ltd
Current assignee: Teijin Ltd
Priority date: 1983-09-30
Filing date: 1984-09-27
Publication date: 1985-04-10
Also published as: EP0136659A3

Abstract

A wet-degradable fiber having a covering resin layer is described. The moisture permeability (R) of a resin forming the covering layer is at least 2.5 g/m²·24 hours as determined at a relative humidity difference of 0% and 90% with respect to a thickness of 0.1 mm. The strength wet reduction ratio (AS) of the fiber is within a range satisfying requirements represented by the following formulae:

and

wherein ΔS₁ represents the strength reduction ratio after two hours' immersion in water at 20°C and AS₂ represents the strength reduction ratio after fifty hours' immersion in water at 20°C.

Description

in water to the point of initiation of the dissolution is very short, and since the dissolution rate is very high, the fiber is dissolved out in a very short time after dipping in water or the strength of the fiber is lost in a very short time. This time is ordinarily two hours or shorter. There has been proposed a method in which the hydrophilic nature of such a water-soluble fiber is modified so that the fiber is prevented from being completely dissolved.
In this proposal, the fiber is not dissolved out but the reduction of the strength is ordinarily controlled to a certain reduction ratio in the range of from 30 to 60%. Furthermore, this certain reduction ratio is ordinarily reached within 2 hours after immersion in water. This dissolution time or reduction ratio depends on the temperature, and the higher the temperature, the shorter the dissolution time and the higher the strength reduction ratio.
A water-swelling fiber such as rayon, cellulose or cellulose acetate is not dissolved out but degradation is stopped at a certain strength reduction ratio, e.g., 20 to 50%, and this certain strength reduction ratio is ordinarily reached within 2 hours.

SUMMARY OF THE INVENTION

It is a primary object of the present invention to provide a fiber, the strength of which is gradually reduced with the lapse of time when it is immersed in water or is wet with water. By the expression "gradually reduced with the lapse of time" used herein, it is meant that the time required for initiation of degradation after immersion in water is longer than in a conventional water-soluble or water-swelling fiber, the dissolution rate is low, and the time required for the dissolution is long.
In accordance with the present invention, there is provided a wet-degradable fiber having a covering resin layer, wherein the moisture permeability (R) of a resin

WET-DEGRADABLE FIBERS

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wet-degradable fiber. More particularly,'it relates to a fiber, the strength of which is gradually reduced with the lapse of time when the fiber is in the wet state or immersed in water.

2. Description of the Prior Art

As the fiber of which the strength is reduced in water or in the state wet with water, there are known fibers of water-soluble polymers such as polyvinyl alcohol and polyethylene glycol and fibers of water-swelling polymers having hydrophilic groups, such as rayon, cellulose, and cellulose acetate. Reduction of the strength in water or dissolution in water in these fibers is industrially utilized in various fields.
For example, because of an excellent water solubility, a polyvinyl alcohol fiber is used as an adhesive binder, a cracking-preventing agent in a molded article, a fiber for a crepe fabric, a fiber for a chemical lace fabric, a marking fiber for a knitted fabric, a selvage curl-preventing fiber for a woven or knitted fabric, a fiber for preventing extension of an elastic yarn, and a fiber for pressing a strongly twisted yarn.
These known fibers, however, are commonly characterized in that degradation of the strength is manifested in a very short time after immersion in water, or the strength is reduced to a certain level in a short time after immersion in water and then further reduction of the strength is halted. The time required for initiation of the reduction of the strength after immersion in water is very short, and the degradation or dissolution is completed in a very short time. For example, in the case of a water-soluble fiber composed of polyvinyl alcohol or polyethylene glycol, the time from the point of immersion forming the covering layer is at least 2.5 g/m².24 hours as determined at a relative humidity difference of 0% - 90% with respect to a thickness of 0.1 mm, and the strength wet reduction ratio (AS) of the fiber is within a range satisfying requirements represented by the following formulae:

and
wherein AS₁ represents the strength reduction ratio after two hours' immersion in water at 20°C and AS₂ represents the strength reduction ratio after fifty hours' immersion in water at 20°C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The wet-degradable fiber of the present invention has a moisture-permeable covering resin layer. As the skeleton fiber on which this moisture-permeable covering resin layer is formed, there can be mentioned a fiber in which reduction of the strength is advanced in the wet state, such as a water-soluble fiber or a water-swellable fiber, and a fiber which is hydrolyzed in the wet state in the presence of a hydrolysis promoter (such as an amine) to advance deterioration or degradation of the fiber, such as a polyester fiber.
By the term "water-soluble fiber" used herein is meant a fiber which is dissolved when it is immersed in water at a temperature higher than 0°C, with the result that the strength of the fiber is reduced as compared with the strength in air. This fiber may be composed of a natural polymer, a semi-synthetic polymer, or a synthetic polymer. For example, there can be mentioned polyvinyl alcohol, polyethylene oxide, sodium polyacrylate, and collagen.
By the term "water-wetting fiber" used herein is meant a fiber which is not dissolved but swollen when it is immersed in water at a temperature higher than 0°C, with the result that the strength of the fiber is reduced as compared with the strength in air. This fiber also may be composed of a natural polymer, a semi-synthetic polymer, or a synthetic polymer. For example, there can be mentioned rayon, cellulose acetate, Vinylon, wool, polyvinyl alcohol, polyethylene oxide, sodium polyacrylate, and collagen.
As the fiber which is hydrolyzed in the wet state in the presence of a hydrolysis promoter such as an amine, there can be used a polyester fiber, that is, a fiber composed of a polymer having ester bonds in the main chain. For example, there can be mentioned a polyester comprising terephthalic acid as the main acid component and at least one glycol, preferably at least one alkylene glycol selected from ethylene glycol, trimethylene glycol, tetramethylene glycol, pentamethylene glycol, and hexamethylene glycol, as the main glycol component.
A polyester in which a part of the terephthalic acid component is replaced by another bifunctional carboxylic acid component also can be used, and a polyester in which a part of the glycol component is substituted by a diol component other than the above-mentioned glycol also can be used.
As the bifunctional carboxylic acid other than terephthalic acid, there can be mentioned aromatic, aliphatic and alicyclic bifunctional carboxylic acids such as isophthalic acid, napthalene-dicarboxylic acid, diphenyl-dicarboxylic acid, diphenoxyethane-dicarboxylic acid, 0-hydroxyethoxybenzoic acid, p-hydroxybenzoic acid, 5-sodium-sulfoisophthalic acid, adipic acid, sebacic acid, and 1,4-cyclohexane-dicarboxylic acid. As the diol compound other than the above-mentioned glycol, there can be mentioned aliphatic, alicyclic and aromatic diol compounds such as cyclohexane-l,4-dimethanol, neopentyl glycol, bisphenol A, and bisphenol S, and polyoxyalkylene glycols.
A polycarboxylic acid such as trimellitic acid or pyromellitic acid or a polyol such as glycerol, trimethylolpropane or pentaerythritol may be copolymerized with the polyester, so far as the polyester is substantially linear (ordinarily, the amount of the comonomer is up to 1 mole %). ^-
When a fiber composed of a copolycondensate derived from terephthalic acid, 5-sodium-sulfoisophthalic acid, and ethylene glycol or a fiber composed of a polyester in which 0.1 to 5.0 mole % of sodium alkylsulfonate or trimethyl phosphate is incorporated is used as the skeleton fiber, the effect of gradually advancing reduction of the strength in the wet state with the lapse of time is enhanced. Sodium alkylsulfonate and trimethyl phosphate may be used alone or in the form of a mixture of both compounds.
The configuration of the skeleton fiber used in the present invention is not particularly critical. For example, an unprocessed yarn having a circular or non- circular section may be used. However, it is preferred that a fiber having a configuration modified by false twisting, fluid processing, rubbing processing, stuffing processing, asymmetric structure-forming processing, shrinkage difference-utilizing processing, or raising processing be used as the skeleton fiber. Moreover, a spun yarn formed by tow spinning, false twist spinning, or composite spinning can be used. These yarns are effective for fixing a covering resin tightly to the surfaces of fibers, and the durability of the applied resin can be increased by the anchoring effect.
A resin having a moisture permeability (R) of at least 2.5 g/m 2 '24 hours is used as the resin to be covered on the skeleton fiber in the present invention. The moisture permeability (R) referred to herein is a value determined according to the method of JIS Z-0208. Namely, the value of the moisture permeability (R) is expressed by the amount (g) of water vapor which passes through a film having a thickness of 0.1 mm and an area of 1 m² for a period of 24 hours under a relative humidity difference of 90% - 0% (which means that the atmosphere on one side of the film is maintained at a relative humidity of 0% and the atmosphere on the other side of the film is maintained at a relative humidity of 90%).
When moisture permeability is lower than 2.5 g/m^2.24 hours, the amount of water vapor passing through the covering resin layer is limited and no substantial wet degradation is attained. If the moisture permeability is very high, for example, 500 to 1000 g/m^2.24 hours, the degradation rate is too high and the thickness of the covering layer has to be increased, and the resulting fiber has a thick covering layer such that it is of almost no practical use.
Among resins having a moisture permeability (R) of at least 2.5 g/m^2·24 hours, a resin having a flexibility but being not water-soluble or water-swellable is used. For example, there may be used vinyl and vinylidene resins such as polyacrylic acid, polymethyl methacrylate, polyacrylonitrile, polyvinyl chloride, polyvinyl acetate, polyvinylidene chloride, polyethylene, polypropylene, and polybutene, acetal resins such as a formal resin and polyvinyl butyral, polyamide resins such as nylon 6 and nylon 66, unsaturated polyester resins, polyurethane resins such as other type polyurethane and ester type polyurethane, diene type rubbers such as polybutadiene, styrene-butadiene rubber, acrylonitrile-butadiene rubber, polyisobutylene, and butyl rubber, olefin rubbers such as ethylene-propylene rubber and polyisobutylether, polysulfide rubbers such as Thiokol rubber, silicone rubbers, fluorine resins such as polytetrafluoroethylene and polytrifluoroethylene, and cellulose derivatives such as methyl cellulose and othyl cellulose.
The thickness of the covering resin layer is appropriately determined according to the desired strength wet reduction ratio of the wet-degradable fiber. Ordinarily, the thickness is varied according to the kind of resin, but it is preferred that the thickness of the covering resin layer be 0.05 to 5 mm, more preferably 0.1 to 2 mm. If the thickness of the covering resin layer is smaller than 0.05 mm; a uniform covering is hardly formed and the permeability of the covering layer becomes uneven. If the thickness exceeds 5 mm, the contribution of the covering resin layer to the strength becomes excessive, and control of the strength wet reduction ratio becomes difficult.
When the skeleton fiber is a polyester fiber, an amine is incorporated in the covering resin. In this case, the resin not only shows a moisture permeability but also exerts a function of fixing the amine to the surface of the fiber, preventing falling of the amine by rubbing or shock and preventing scattering of the amine into air, dissolution of the amine in water, or adhesion of the amine to the finger or skin.
By the term "amine" used herein is meant an aliphatic, aromatic or alicyclic hydrocarbon compound in which an organic group (inclusive of hydrogen) is substituted with ammonia, a primary amine (RNH₂) or a secondary amine (R₂NH). The number of nitrogen atoms contained in one molecule is not particularly critical, and one or more of nitrogen atoms may be contained. It is preferred that an amine which provides a hydrogen ion concentration (pH) of at least 9.0 when dissolved at a concentration of 5% by weight in water or a solvent having a largest solubility to the amine be used.
As specific examples of the amine used in the present invention, there can be mentioned o-tolylbiguanide, piperazine anhydride, N-methylethanolamine, guanidine carbonate, hexamethylenediamine carbamate, sodium diethyldithiocarbamate, N-methyl-D-glucamine, L-alginine, 2-amino-2-methyl-1,3-propanediol, 2-amino-2-ethyl-l,3-propanediol, 2-amino-2-methyl-l-propanol, _N-methyldiethanolamine, 2,4,6-tris(dimethylaminomethyl) phenol, tetraethylammonium hydroxide, polyethylene-imine, 2-methylimidazole, 2-ethyl-4-methylimidazole, modified aliphatic polyamines, modified aliphatic polyamine adducts, and ketimine.
It is preferred that the-content of the amine in the covering resin be 2 to 100% by weight, more preferably 10 to 70% by weight, based on the resin. If the content of the amine is lower than 2% by weight, the hydrolysis-promoting action is insufficient. If the content of the amine exceeds 100% by weight, the hydrolysis-promoting action is too violent and desirable strength wet reduction cannot be obtained.
Any known methods may be adopted for covering the moisture-permeable resin on the skeleton fiber. For example, there may be adopted a hot melt extrusion coating method, a method in which the fiber is dipped in a dope of the resin and is squeezed by a mangle, a method in which the fiber is dipped in a dope of the resin and passed through a fine hole, and a method in which the fiber is sprayed with a solution of the resin and is then heated and dried. When an amine is incorporated in the resin, use of the resin in the form of an aqueous liquid such as an aqueous emulsion or an aqueous suspension should be avoided.
In order to gradually reduce the strength in the wet state with the lapse of time, the strength wet reduction ratio (AS) of the wet-degradable fiber of the present invention should satisfy requirements represented by the following formulae:

and
Supposing that the tensile strength of the wet-degradable fiber in the normal state is Sa and the tensile strength of the fiber, measured in the wet state when the fiber is dipped in water at 20°C and is taken out after the lapse of a certain time, is S B , the strength wet reduction ratio (AS) is represented as follows:
Incidentally, in the above formulae, ΔS₁ represents the strength wet reduction ratio after 2 hours' immersion in water at 20°C, and àS₂ represents the strength wet reduction ratio after 50 hours' immersion in water at 20°C.
The wet-degradable fiber of the present invention can be used for engineering work and ocean engineering work. Especially, the wet-degradable fiber of the present invention is preferably used for a diameter-expansion type sand bag (pack drain) for solidifying soft ground, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 57-143010. Namely, the wet-degradable fiber of the present invention is suitable as a fiber manifesting the expansion of diameter required for this diameter-expansion type pack drain.
The present invention will now be described in detail with reference to the following examples. These examples by no means limit the scope of the invention.
In the examples, the tensile strength was measured according to the tensile test method for filamentary yarns stipulated in JIS L-1070. A constant-speed stretching type tensile tester was used for the measurement. The measurement conditions are as follows.

Grip distance: 20 cm
Drawing speed: 100%/min

Example 1

Three filament yarns of a polyvinyl alcohol fiber (300 denier/50 filaments) (Solubron® SX supplied by Nichibi K.K.) were doubled and twisted in the Z direction at 200 T/m. The twists were set by dry heat-setting at 170°C for 2 minutes under tension. The thus obtained yarn was coated with a resin by using an extrusion coating machine. The resin used was an ethylene-vinyl acetate copolymer resin having a moisture permeability R of 140 g/m^2.24 hours (Ultrathene® UE 722 supplied by Toyo Soda Manufacturing Co.). The thickness of the covering layer was 0.6 mm and the amount of the coated resin was 1850% by weight based on the fiber.
When this wet-degradable fiber was immersed in water at 20°C and the strength wet reduction ratio was determined, it was found that ΔS₁ was 3.8, AS₂ was 60.1, and AS₂/AS₁ was 15.82, and it was confirmed that the fiber had an excellent wet degradability (see Table 1).

Example 2

Nine filament yarns of a cellulose acetate fiber (100 denier/25 filaments) (Rosel® supplied by Teijin Ltd.) were doubled and twisted in the Z direction at 200 T/m. The twists were set by wet heat-setting at 70°C under tension for 30 minutes. The fiber was then dried at 100°C for 30 minutes. The thus obtained fiber was covered with a resin by using an extrusion coating machine. The resin used was the same as the resin used in Example 1. The thickness of the covering layer was 0.5 mm and the amount of the coated resin was 1290% by weight based on the fiber.
When this wet-degradable fiber was immersed in water at 20°C and the strength wet reduction ratio was determined, it was found that ΔS₁ was 2.5, Δ5₂ was 37.5, and ΔS₂/ΔS₁ was 15.00, and it was confirmed that the fiber had an excellent wet degradability (see table 1).

Example 3

A twisted filament yarn having a twist number of 200 T/m (900 denier/150 filaments) was prepared from the same polyvinyl alcohol fiber as used in Example 1. The obtained yarn was immersed in a cyclohexane solution containing 10% by weight of 1,2-polybutadiene having a moisture permeability R of 70 g/m²·24 hours (JSR-_RB830 supplied by the Nippon Synthetic Chemical Industry Co.) and then the immersed yarn was passed through a small hole of a polytetrafluoroethylene plate along the center line of the hole, whereby a uniform covering resin layer was formed on the surface of each fiber. The coated fiber was then dried at 100°C for 80 seconds. This resin processing was repeated 8 times. In the obtained fiber, the thickness of the covering-layer was 0.30 mm and the amount of the coated resin was 541% by weight based on the fiber.
When this wet-degradable fiber was immersed in water at 20°C and the strength wet reduction ratio was measured, it was found that ΔS₁ was 9.8, ΔS₂ was 64.7, and ΔS₂/ΔS₁ was 6.60, and it was confirmed that the fiber had an excellent wet degradability (see Table 1).

Example 4

A twisted filament yarn having a twist number of 200 T/m (900 denier/150 filaments) was prepared from the same polyvinyl alcohol fiber as used in Example 1. Low density polyethylene having a moisture permeability R of 2.5 g/m²·24 hours (F028 supplied by Ube Industries Ltd.) was dissolved in xylene by heating. The solution temperature was 95°C and the polyethylene concentration was 8% by weight. The twisted filament yarn was immersed in the solution while maintaining the solution temperature at 95°C. The yarn was passed through a polytetrafluoroethylene slit to remove the excessive solution, and then the yarn was dried at 150°C for 85 seconds. This resin processing was repeated 9 times. In the obtained yarn, the thickness of the covering resin layer was 0.4 mm and the amount of the coated resin was 612% by weight based on the fiber.
When this wet-degradable fiber was immersed in water at 20°C and the strength wet reduction ratio was determined, it was found that ΔS₁ was 0.4, AS₂ was 21.8, and ΔS₂/ΔS₁ was 54.5, and it was confirmed that the fiber had an excellent wet degradability (see table 1).

Comparative Example 1

A twisted filament yarn having a twist number of 200 T/m (900 denier/150 filaments) was prepared from the same polyvinyl alcohol fiber as used in Example 1. When this yarn was directly immersed in water at 20°C, the yarn was dissolved out completely in about 5 minutes and the strength could not be measured (see Table 1).

Comparative Example 2 ^-

A twisted filament yarn having a twist number of 200 T/m (900 denier/225 filaments) was prepared from the same cellulose acetate fibers as used in Example 2. When the yarn was directly immersed in water at 20°C and the strength wet reduction ratio was determined, it was found that AS₁ was 37.4, AS₂ was 37.5, and ΔS₂/ΔS₁ was 1.00, and it was confirmed that wet degradation was caused but it was substantially completed within 2 hours from the point of immersion in water (see Table 1).

Comparative Example 3

Three filament yarns of the same polyvinyl alcohol fiber (Solubron® SX, 300 denier/50 filaments) as used in Example 1 were doubled and twisted in the Z direction at 200 T/m. The twists were set by dry heat-setting at 170°C under tension for 2 minutes. The obtained yarn was immersed in a tetrahydrofuran solution containing 10% by weight of a polyvinylidene chloride resin having a moisture permeability R of 0.3 g/m²·24 hours (Saran® F-216 supplied by Asahi Kasei Kogyo K.K.) and was then passed through a fine hole made on a polytetrafluoroethylene plate to uniformly coat the resin on each fiber. The coated fiber was heated at 90°C for 60 seconds. This resin processing was repeated 8 times. In the obtained fiber, the thickness of the covering layer was 0.3 mm and the amount of the coated resin was 583% by weight based on the fiber.
When this fiber was immersed in water at 20°C and the strength wet reduction ratio was determined, it was found that ΔS₁ was 0.1, àS₂ was 0.1, and AS₂/^AS₁ was 1.00 (see Table 1).

Example 5

A polyethylene terephthalate filament yarn (75 denier/24 filaments) was subjected to spindle false twisting at a heater temperature of 220°C, a speed of 120 m/min, and a twist number of 3305 T/m to prepare a woolly finished yarn having a total crimp degree (TC) of 37%.
The twelve woolly filament yarns thus prepared were doubled and twisted in the Z direction at a twist number of 100 T/m to obtain a woolly yarn (900 denier/288 filaments).
25 parts by weight of piperazine anhydride (supplied by Kawaken Fine Chemicals Co.), 25 parts by weight of polyvinyl alcohol having a moisture permeability R of 366 g/m²·24 hours (Gosefimer® L-7514 supplied by The Nippon Synthetic Chemical Industry Co.), and 50 parts by weight of methanol (first class chemical) were weighed. At first, methyl alcohol was stirred and heated on a warm water bath maintained at 60°C, and polyvinyl alcohol was gradually added thereto with stirring to dissolve the polyvinyl alcohol in methanol. Then piperazine anhydride was similarly added to the solution and dissolved therein with stirring. The woolly yarn was immersed in the thus prepared solution maintained at normal temperature and then the woolly yarn was passed through the center of a small hole having a diameter of 1.2 mm, which was made through a polytetrafluoroethylene plate, to uniformly coat the woolly yarn with the resin solution. The yarn was dried at 100°C for 1 hour. In the dried yarn, the amount of the coated amine-containing resin was 96% by weight based on the woolly yarn.
When the strength wet reduction ratio of this wet-degradable fiber was determined, it was found that ΔS₁ was 25.2, àS₂was 64.8, and ΔS₂/ΔS₁ ratio was 2.57, and it was confirmed that the fiber had an excellent wet degradability (see Table 2).

Example 6

A filament yarn (75 denier/24 filaments) of a polyester obtained by copolycondensing terephthalic acid containing 2.6 mole % of 5-sodium-sulfoisophthalic acid with ethylene glycol was subjected to spindle false twisting at a heater temperature of 180°C, a speed of 120 m/min, and a twist number of 3305 T/m to obtain a woolly yarn having a TC of 35%. Twelve woolly yarns thus prepared were doubled and twisted in the Z direction at a twist number of 100 T/m to obtain a woolly yarn (900 denier/288 filaments).
The woolly yarn was processed with the same resin solution as used in Example 5 in the same manner as in Example 5, and then the yarn was dried at 100°C for 1 minute. In the obtained covered yarn, the amount of the coated resin was 90% by weight based on the fiber.
When the resulting wet-degradable fiber was immersed in water at 20°C and the strength wet reduction ratio was determined, it was found that ΔS₁ was 28.4, AS₂ was 73.5, and AS₂/AS₁ was 2.59, and it was confirmed that the fiber had an excellent wet degradability (see Table 2).

Example 7

20 parts by weight of o-tolylbiguanide (Nocselar® BG supplied by Ouchi Shinko Kagaku Kogyo K.K.), 15 parts by weight of ethyl cellulose having a moisture permeability R of 262 g/m^2.24 hours (N-7 supplied by Hercules K.K.) and 65 parts by weight of ethyl alcohol (first class chemical) were weighed. At first, ethyl alcohol was heated and stirred at 60°C on a warm water bath, and then ethyl cellulose was gradually added to ethanol and dissolved therein with stirring. Then, o-tolylbiguanide was similarly added to the solution and dissolved therein with stirring. The same woolly yarn as used in Example 6 was processed with the thus-formed solution in the same manner as described in Example 6. The amount of the coated resin containing the amine was 88% by weight based on the fiber.
When the strength wet reduction of this wet-degradable fiber was determined, it was found that AS₁ was 26.1, AS₂ was 66.3, and ΔS₂/ΔS₁ was 2.54, and it was confirmed that the fiber had an excellent wet degrad-. ability (see Table 2).

Comparative Example 4

The procedures of Example 5 were repeated in the same manner except that drying was carried out at 40°C for 3 minutes instead of drying at 100°C for 1 minute. The amount of the coated resin was 97% by weight based ' on the fiber. The obtained fiber was characterized by AS₁ of 2.8, AS₂ of 3.0, and ΔS₂/ΔS₁ of 1.07, and it was confirmed that the degradation was low and the fiber had no practical utility (see Table 2).

Comparative Example 5

The procedures of Example 6 were repeated in the same manner except that drying was carried out at 180°C for 1 minute instead of drying at 100°C for 1 minute. The amount of the coated resin was 54% by weight based on the fiber. When this fiber was immersed in water at 20°C, the strength was drastically reduced, and it was confirmed that the fiber had no practical utility (see Table 2).

Comparative Example 6

A polyvinyl alcohol filament yarn (100 denier/30 filaments) (Solubron® SL supplied by Nichibi) was dipped in water at 20°C and the strength wet reduction was determined. It was found that ΔS₁ was 52.7, ΔS₂ was 52.8, and ΔS₂/ΔS₁ was 1.00. The degradation was very fast and the fiber was different from the fiber of the present invention in which the strength was gradually reduced with the lapse of time (see Table 2).

Claims

1. A wet-degradable fiber having a covering resin layer, wherein the moisture permeability (R) of a resin forming the covering layer is at least 2.5 g/m²·24 hours as determined at a relative humidity difference of 0% and 90% with respect to a thickness of 0.1 mm, and the strength wet reduction ratio (ΔS) of the fiber is within a range satisfying requirements represented by the following formulae:

and

wherein AS₁ represents the strength reduction ratio after two hours' immersion in water at 20°C and AS₂ represents the strength reduction ratio after fifty hours' immersion in water at.20°C.

2. A wet-degradable fiber as set forth in claim 1, wherein said covering resin layer is formed on a water soluble fiber or a water-swellable fiber.

3. A wet-degradable fiber as set forth in claim 1, wherein the strength wet reduction ratio (AS) is within a range satisfying requirements represented by the following formulae:

and

wherein ΔS₁ and ΔS₂ are as defined in claim 1.

4. A wet-degradable fiber as set forth in claim 1, wherein said covering resin layer is formed on a polyester fiber and contains an amine.

5. A wet-degradable fiber as set forth in claim 4, wherein the strength wet reduction ratio (AS) is within a range satisfying requirements represented by the following formulae:

and

wherein ΔS₁ and AS₂ are as defined in claim 1.

6. A wet-degradable fiber as set forth in claim 4, wherein the polyester is a copolycondensate of terephthalic acid and 5-sodium-sulfoisophthalic acid with ethylene glycol.

7. A wet-degradable fiber as set forth in claim 4, wherein the polyester contains 0.1 to 5.0 mole % of sodium alkylsulfonate.

8. A wet-degradable fiber as set forth in claim 4, wherein the polyester contains 0.1 to 5.0 mole % of trimethyl phosphate.