BIODEGRADABLE FIBER

Abstract

 The present invention provides a biodegradable fiber which does not become brittle in a moist heat environment, and which has excellent alkali resistance. Disclosed is a biodegradable fiber, containing a blend polymer containing a polyethylene terephthalate, a polybutylene adipate terephthalate and a polylactic acid, wherein the content of the polyethylene terephthalate in the fiber is 80 mass% or more. It is preferable that the content of the polyethylene terephthalate in the fiber is at least 93 mass% or more, the content of the polybutylene adipate terephthalate in the fiber is 0.4 mass% to 2.4 mass%, the content of the polylactic acid in the fiber is 0.4 mass% to 2.4 mass%, and the ratio of the content of the polybutylene adipate terephthalate to the content of the polylactic acid is 40/60 to 60/40 in terms of the mass ratio.

Description

Technical Field

[0001] The present invention relates to a biodegradable fiber containing a polyethylene terephthalate as a main component.

Background Art

[0002] Polyethylene terephthalate fibers are favorably used for a variety of applications due to their excellent mechanical and chemical properties.

[0003] On the other hand, in recent years, biodegradable fibers have been attracting attention in view of environmental issues. Typical examples of such fibers are polylactic acid fibers.

[0004] Polylactic acid fibers have poor alkali resistance and the texture thereof cannot be improved through alkali weight reduction like polyethylene terephthalate, making them unsuitable for clothing applications. In addition, they undergo hydrolysis and become brittle in a moist heat environment, making them unsuitable for use as vehicle interior materials, which are used in high-temperature environments, and so their applications have been limited.

[0005] As a biodegradable fiber having excellent alkali resistance, a core-sheath type composite fiber with a polyethylene terephthalate sheath and a polylactic acid core has been proposed (Patent Literature 1).

Citation List

Patent Literature

[0006] Patent Literature 1: Japanese Patent Laid-Open No. 2008-208482

Summary of Invention

Technical Problem

[0007] However, although the surface of the fiber of Patent Literature 1 is covered with polyethylene terephthalate, the core becomes brittle in a moist heat environment, and the fiber cannot prevent a decrease in its strength of the fiber. Further, upon alkali weight reduction, the polylactic acid core dissolves. Furthermore, while the polylactic acid used in the core is biodegradable, the polyethylene terephthalate used in the sheath is not biodegradable, so the fiber is insufficient in terms of environmental load.

[0008] As described above, there have been no biodegradable fibers that can be used in a moist heat environment and exhibit alkali resistance.

[0009] Therefore, an object is to provide a fiber having excellent biodegradability while containing a polyethylene terephthalate as a main component.

[0010] Further, an object is to provide a biodegradable fiber, which does not become brittle even in a moist heat environment and has excellent alkali resistance. Solution to Problem

[0011] As a result of extensive research, the present inventors have found that a fiber comprising a blend polymer containing a polyethylene terephthalate as a main component and specific amounts of a polybutylene adipate terephthalate and a polylactic acid can be effectively biodegraded although polyethylene terephthalate fibers are inherently almost non-biodegradable, and thus have completed the present invention.

[0012] Specifically, an object of the present invention is achieved by a biodegradable fiber containing a blend polymer containing a polyethylene terephthalate, a polybutylene adipate terephthalate and a polylactic acid, wherein the content of the polyethylene terephthalate in the fiber is 80 mass% or more.

[0013] Further, the present inventors have found that it is possible to obtain a fiber comprising a blend polymer containing a polyethylene terephthalate as a main component and specific amounts of a polybutylene adipate terephthalate and a polylactic acid in a specific mixing ratio, which does not become brittle even in a moist heat environment, has good alkali resistance to an extent that it can be reduced in weight by alkali, and further, is biodegradable as a whole fiber to an extent that equals to or greater than the content of the polybutylene adipate terephthalate and the polylactic acid.

[0014] Specifically, an object of the present invention is achieved by a biodegradable fiber containing a blend polymer containing a polyethylene terephthalate, a polybutylene adipate terephthalate and a polylactic acid, wherein the content of the polyethylene terephthalate in the fiber is at least 93 mass% or more, the content of the polybutylene adipate terephthalate in the fiber is 0.4 mass% to 2.4 mass%, the content of the polylactic acid in the fiber is 0.4 mass% to 2.4 mass%, and the ratio of the content of the polybutylene adipate terephthalate to the content of the polylactic acid is 40/60 to 60/40 in terms of mass ratio.

[0015]  Further, the biodegradable fiber is preferably characterized in that the content of the polyethylene terephthalate in the fiber is 98 mass% or more, and the ratio of the content of the polybutylene adipate terephthalate to the content of the polylactic acid (polybutylene adipate terephthalate/polylactic acid) is 50/50 to 60/40 in terms of mass ratio. This makes it possible to prepare the biodegradable fiber having higher resistance to a moist heat environment, biodegradability, and alkali resistance.

[0016] It is also preferable that the biodegradable fiber has a biodegradation rate of 15% or more after 135 days in the ASTM D5511 test.

[0017] It is also preferable that the biodegradable fiber has a reduction rate of strength at break of 25% or less after the following moist heat environment test.

(Moist heat environment test)

[0018] A tubular knitted fabric is produced, and a 120 mm × 150 mm test piece is taken from the produced tubular knitted fabric, and after heat setting, measurement is performed using an AG-IS autograph (registered trademark) tensile tester manufactured by Shimadzu Corporation under the conditions of a sample width of 50 mm, a test length of 50 mm, and a constant tensile speed of 100 mm/min. The maximum load in the load-elongation curve is taken as the strength at break (cN). The test piece is measured twice in each of the vertical direction and the horizontal direction, with the average value taken as the strength at break before the moist heat environment test. Using a still air temperature and humidity chamber PR-3KP manufactured by ESPEC Corporation, the test piece is left to stand in a moist heat environment at a temperature of 80°C and a relative humidity of 95%. After 400 hours, the strength at break is measured twice by the above tensile test after the moist heat environment test in the same manner as in that before the moist heat environment test, and then the result is taken as the strength at break after the moist heat environment test. The reduction rate of strength at break in a moist heat environment is calculated using the average value according to the following formula.

[0019] Reduction rate (%) of strength at break under moist heat environment = {(strength at break before moist heat environment test - strength at break after moist heat environment test)/(strength at break before moist heat environment test)}×100

[0020] In addition, the mass reduction rate of the biodegradable fiber after the alkali resistance test described below is preferably 15% or less.

(Alkali resistance test)

[0021] A tubular knitted fabric is produced, a 100 mm × 100 mm test piece is taken from the produced tubular knitted fabric, the mass (W1) in a moisture equilibrium state is measured, and then the test piece is immersed in a 4 mass% concentration of an aqueous sodium hydroxide solution kept at 98 ± 2°C. After 30 minutes, the test piece is taken out, washed with water, dried, and then brought back to a moisture equilibrium state, and thus the mass (W2) at that time is measured. This measurement is performed twice, and the mass reduction rate is calculated using the average value according to the following formula.

Advantageous Effects of Invention

[0022] According to the present invention, a biodegradable fiber can be obtained, wherein even a polyethylene terephthalate, which is inherently non-biodegradable, exhibits biodegradability, and the fiber does not dissolve immediately upon alkali weight reduction because of its good alkali resistance, and does not become brittle easily even in a moist heat environment. Furthermore, the strength at break and the elongation at break of the obtained biodegradable fiber are equivalent to those of polyethylene terephthalate fibers.

Description of Embodiments

[0023]  The biodegradable fiber of the present invention must contain a blend polymer containing a polyethylene terephthalate (hereinafter referred to as PET) as a main component, a polybutylene adipate terephthalate (hereinafter referred to as PBAT) and a polylactic acid. By blending PET with PBAT and the polylactic acid, biodegradability is imparted to the PET fiber that is inherently non-biodegradable.

[0024] The biodegradable fiber of the present invention is biodegraded by the synergistic effect of two components: because of a hydrolyzable material, polylactic acid, the fiber is hydrolyzed in a high temperature and high humidity environment in soil, promoting biodegradation by the action of microorganisms; and because of an enzymatically degradable material, PBAT, the fiber is biodegraded directly by the action of microorganisms.

[0025] The biodegradable fiber of the present invention preferably contains at least 93 mass% or more of PET. It is also preferable that the content of PET in the fiber is 95 mass% or more. If the content of PET in the fiber is 93 mass% or more, the high strength at break, alkali resistance, and other properties of a fiber made only of PET are not lost, and the strength is less likely to decrease in a moist heat environment, making it possible to use the fiber in the same applications as PET fibers.

[0026] PET in the present invention may be not only homo-PET, but also copolymerized PET copolymerized with an alkali metal salt of sulfoisophthalic acid, etc.

[0027] To PET in the present invention, modifiers such as light resistance agents, heat resistance agents, and matting agents may be added to improve various physical properties.

[0028] The content of PBAT in the present invention is preferably 0.4 mass% to 2.4 mass%, also preferably 0.6 mass% to 2.2 mass%, and more preferably 0.8 mass% to 2.0 mass%. If the content is 0.4 mass% or more, biodegradability can be imparted to PET. If the content is 2.4 mass% or less, the alkali resistance is good and the strength at break and the elongation at break are less likely to decrease.

[0029] The content of the polylactic acid in the present invention is preferably 0.4 mass% to 2.4 mass%. Further, the content is also preferably 0.6 mass% to 2.2 mass%, and more preferably 0.8 mass% to 2.0 mass%. If the content is 0.4 mass% or more, the resultant is easily hydrolyzed in soil, and biodegradability can be imparted to PET. If the content is 2.4 mass% or less, the resultant is less likely to become brittle even in a moist and heat environment, has good alkali resistance, and is less likely to decrease in strength at break and elongation at break.

[0030] The ratio of the content of PBAT to the content of the polylactic acid in the present invention is preferably 40/60 to 60/40 in terms of mass ratio. The ratio is preferably 45/55 to 55/45, and more preferably 48/52 to 52/48. When the content ratio is in the range of 40/60 to 60/40, the synergistic effect of PBAT and the polylactic acid can impart biodegradability to PET. In addition, the strength is less likely to decrease even in a moist heat environment.

[0031] The biodegradable fiber of the present invention has a biodegradation rate of preferably 15% or more, more preferably 20% or more, and particularly preferably 25% or more after 135 days in the ASTM D5511 test. If the biodegradation rate is 15% or more after 135 days, the fiber exhibits sufficient biodegradability.

[0032] The biodegradable fiber of the present invention has a biodegradation rate of preferably 35% or more, more preferably 40% or more, and particularly preferably 45% or more after 360 days in the ASTM D5511 test.

[0033] The biodegradable fiber of the present invention has a biodegradation rate of preferably 57% or more, more preferably 60% or more, and particularly preferably 65% or more after 675 days in the ASTM D5511 test.

[0034] The biodegradable fiber of the present invention has a reduction rate of strength at break of preferably 25% or less, more preferably 15% or less, and particularly preferably 10% or less after the moist heat environment test described below. The biodegradable fiber with a reduction rate of strength at break of 25% or less can also be used in applications where there is a possibility of a moist heat environment, similar to PET fibers.

[0035] The biodegradable fiber of the present invention preferably has a mass reduction rate of 15% or less after the alkali resistance test described below. If the mass reduction rate is 15% or less, the texture can be appropriately improved by alkali weight reduction, similar to PET fibers.

[0036] The total fineness of the biodegradable fiber of the present invention is not particularly limited and may be the same as the total fineness used for ordinary PET fibers, and is preferably 1 dtex to 300 dtex in terms of spinning operability and mechanical strength. If the total fineness is 1 dtex to 100 dtex, the fiber will maintain good texture when used mainly for clothing applications. Also, if the total fineness is 30 dtex to 300 dtex, the fiber will maintain good strength when used for vehicle applications.

[0037] The biodegradable fiber of the present invention preferably has a single filament fineness of 0.8 dtex to 25 dtex. If the single filament fineness is 0.8 dtex or more, the fiber will maintain good strength, when used mainly for clothing applications. If the single filament fineness is 25 dtex or less, the specific surface area of the fiber will be large and the fiber will be easily biodegraded.

[0038] The biodegradable fiber of the present invention has a strength at break of preferably 2.0 cN/dtex or more, more preferably 2.5 cN/dtex or more, and particularly preferably 3.0 cN/dtex or more. If the strength at break is 2.0 cN/dtex or more, the spinning operability and the processability in the knitting and weaving process are good, and the fiber can be used in the same applications as PET fibers.

[0039] The biodegradable fiber of the present invention preferably has an elongation at break of 20% or more. If the elongation at break is 20% or more, the spinning operability and the processability in the knitting and weaving process are good, and the fiber can be used in the same applications as PET fibers.

[0040] The biodegradable fiber of the present invention may be circular or an irregular cross section. Examples of the irregular cross section include multi-lobed, triangular, flat, and elliptical shapes.

[0041] The biodegradable fiber of the present invention can also be used as long fiber, but can be used as woven or knitted fabrics. They can also be processed from the long fibers to produce short fibers, and can also be used as cotton batting. They can also be used as nonwoven fabrics.

Examples

[0042] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. In addition, the physical properties in the examples were each measured and evaluated as follows.

(Tensile test)

[0043] Measurement was performed in accordance with JIS L 1013 using an AGS-1kNG autograph (registered trademark) tensile tester manufactured by Shimadzu Corporation under conditions of a sample yarn length of 200 mm and a constant tensile speed of 200 mm/min. The maximum load in the load-elongation curve was divided by the fineness to determine the strength at break (cN/dtex), and the elongation rate at that time was determined as the elongation at break (%). The measurement was performed three times, and the average value was found.

(Biodegradability evaluation)

[0044] An anaerobic biodegradability test was conducted at 52 ± 2°C according to the ASTM D5511 standard.

(Moist heat environment test)

[0045] A circular knitting machine (NCR-EW) (manufactured by Eiko Industrial Co., Ltd.) was used to perform circular knitting using two combined yarns, producing a tubular knitted fabric with 30 wales/inch and 37 courses/inch. A 120 mm × 150 mm test piece was taken from the produced tubular knitted fabric, and after heat setting at 190°C for 1 minute, measurement was performed using an AG-IS autograph (registered trademark) tensile tester manufactured by Shimadzu Corporation under the conditions of a sample width of 50 mm, a test length of 50 mm, and a constant tensile speed of 100 mm/min. The maximum load in the load-elongation curve was taken as the strength at break (cN), and the test piece was measured twice in each of the longitudinal direction and the transverse direction, with the average value taken as the strength at break before the moist heat environment test. Using a still air temperature and humidity chamber PR-3KP manufactured by ESPEC Corporation, the test piece was left to stand in a moist heat environment at 80°C and 95% relative humidity. After 400 hours, the strength at break of the test piece was measured twice in each of the longitudinal direction and the transverse direction, with the average value taken as the strength at break after the moist heat environment test. The reduction rate of strength at break in a moist heat environment was calculated using the following formula.

Reduction rate (%) of strength at break under moist heat environment ={ (strength at break before moist heat environment test - strength at break after moist heat environment test)/(strength at break before moist heat environment test)}×100

(Alkali resistance test)

[0046] A circular knitting machine (NCR-EW) (manufactured by Eiko Industrial Co., Ltd.) was used to perform circular knitting with two combined yarns, producing a tubular knitted fabric with 30 wales/inch and 37 courses/inch. A 100 mm × 100 mm test piece was taken from the produced tubular knitted fabric, and the mass (W1) in a moisture equilibrium state was measured. The test piece was then immersed in a 4 mass% concentration of an aqueous sodium hydroxide solution kept at 98 ± 2°C. After 30 minutes, the test piece was taken out, washed with water, dried, and then brought back to a moisture equilibrium state, and thus the mass (W2) at that time was measured. This measurement was performed twice, and the mass reduction rate was calculated using the average value according to the following formula. The mass reduction rate was used as an index of alkali resistance. Mass reduction rate (%) = {(W1-W2)/W1}×100

(Example 1)

[0047] A blend polymer was obtained by mixing so that the PET content was 98 mass%, the PBAT content was 1.0 mass% and the polylactic acid content was 1.0 mass%, with the PBAT/polylactic acid content ratio being 50/50 in terms of mass ratio, and then melt extruded at 294°C. The resultant was stretched 3.1 times with GR1 (peripheral speed of 1350 m/min and a temperature of 90°C) and GR2 (peripheral speed of 4200 m/min and a temperature of 140°C) to produce a biodegradable fiber with 84 dtex/36f. Using the obtained biodegradable fiber, a tensile test, biodegradability evaluation, a moist heat environment test, and an alkali resistance test were conducted.

(Example 2)

[0048] A biodegradable fiber was produced in the same manner as in Example 1, except that the PBAT content was 1.1 mass%, the polylactic acid content was 0.9 mass%, and the PBAT and polylactic acid were mixed to give a content ratio of 55/45 in terms of mass ratio. Using the obtained biodegradable fiber, a tensile test, biodegradability evaluation, moist heat environment test, and alkali resistance test were conducted in the same manner as in Example 1.

(Example 3)

[0049] A biodegradable fiber was produced in the same manner as in Example 1, except that the PBAT content was 0.9 mass%, the polylactic acid content was 1.1 mass%, and PBAT and polylactic acid were mixed to give a content ratio of 45/55 in terms of mass ratio. Using the obtained biodegradable fiber, a tensile test, biodegradability evaluation, moist heat environment test, and alkali resistance test were conducted in the same manner as in Example 1.

(Example 4)

[0050] A biodegradable fiber was produced in the same manner as in Example 1, except that the PET content was 96 mass%, the PBAT content was 2.0 mass%, and the polylactic acid content was 2.0 mass%. Using the obtained biodegradable fiber, a tensile test, a biodegradability evaluation, a moist heat environment test, and an alkali resistance test were conducted in the same manner as in Example 1.

(Example 5)

[0051] A biodegradable fiber was produced in the same manner as in Example 1, except that the PET content was 96 mass%, the PBAT content was 2.2 mass%, the polylactic acid content was 1.8 mass%, and PBAT and polylactic acid were mixed to give a content ratio of 55/45 in terms of mass ratio. Using the obtained biodegradable fiber, a tensile test, biodegradability evaluation, moist heat environment test, and alkali resistance test were conducted in the same manner as in Example 1.

(Comparative example 1)

[0052] Except that melt spinning was performed using only PET, a multifilament was produced in the same manner as in Example 1. Using the obtained multifilament, a tensile test, a biodegradability evaluation, a moist heat environment test, and an alkali resistance test were conducted in the same manner as in Example 1.

(Comparative example 2)

[0053] Polylactic acid alone was melt extruded at 230°C and melt spun in a conventional manner to produce a multifilament with 84 dtex/36f. The multifilament obtained was subjected to a tensile test, biodegradability evaluation, and an alkali resistance test in the same manner as in Example 1. In addition, a moist heat environment test was conducted in the same manner as in Example 1, except that the heat setting temperature was set to 120°C.

(Comparative example 3)

[0054] A multifilament was produced in the same manner as in Example 1, except that the PBAT content was 2.0 mass% and no polylactic acid was contained. Using the obtained biodegradable fiber, a tensile test, biodegradability evaluation, a moist heat environment test, and an alkali resistance test were conducted in the same manner as in Example 1.

(Comparative example 4)

[0055] A multifilament was produced in the same manner as in Example 1, except that no PBAT was contained and the polylactic acid content was 2.0 mass%. Using the obtained biodegradable fiber, a tensile test, biodegradability evaluation, a moist heat environment test, and an alkali resistance test were conducted in the same manner as in Example 1. These results are also shown in Table 1.

[Table 1]

			Example 1	Example 2	Example 3	Example 4	Example 5	Comparative example 1	Comparative example 2	Comparative example 3	Comparative example 4
Content	PET	mass%	98	98	98	96	96	100	0	98	98
	PBAT	mass%	1.0	1.1	0.9	2.0	2.2	0	0	2.0	0
	Polylactic acid	mass%	1.0	0.9	1.1	2.0	1.8	0	100	0	2.0
	Content ratio of PBAT to polylactic acid (PBAT/polylactic acid)		50/50	55/45	45/55	50/50	55/45	-	0/100	100/0	0/100
Fiber physical properties, Evaluation results	Strength at break	cN/dtex	4.30	4.26	4.32	4.12	4.24	4.25	3.42	4.20	4.25
	Elongation at break	%	35.8	36.7	35.5	36.5	36.6	36.2	35.2	36.0	35.2
	Biodegradation rate on day 135 in ASTM D5511 test	%	26.3	24.0	23.5	39.5	35.2	2.4	89.8	12.0	10.0
	Biodegradation rate on day 360 in ASTM D5511 test	%	49.0	45.8	42.9	56.2	54.3	2.8	98.8	34.5	31.8
	Biodegradation rate on day 675 in ASTM D5511 test	%	67.8	65.5	62.7	73.2	72.5	4.6	99.5	56.3	52.5
	Reduction rate of strength at break in moist heat environment test	%	2.2	2.0	5.5	7.0	6.0	0.5	99.7	11.2	15.0
	Mass reduction rate in alkali resistance test	%	8.1	7.9	8.3	9.1	8.5	7.5	100	7.6	8.5

[0056] As a result of evaluating the physical properties of the fibers, it was found that PET fibers, which are inherently non-biodegradable, exhibit excellent biodegradability over a long period of time when both polylactic acid and PBAT are contained (Examples and Comparative Examples 1, 3, and 4).
Furthermore, the results of each Example and Comparative Examples 3 and 4 show that the fibers of the Examples were able to suppress a decrease in strength even when exposed to a high-temperature, high-humidity environment.

[0057] Furthermore, the results of Comparative Example 2 confirmed that the fibers of the Examples contain PET as a main component and therefore have excellent alkali resistance, and that by setting the PET content to 97 mass% or more, particularly excellent alkali resistance can be obtained (Examples 1, 2, and 3). It was also confirmed that particularly excellent alkali resistance is obtained by setting the PBAT/polylactic acid content ratio to 1 or more (Examples 2 and 3, 4 and 5).

[0058] Furthermore, in the case of Examples 1 and 2, it was found that the fiber can be made to have excellent biodegradability while maintaining a particularly low strength at break of 4% or less after a moist heat environment test.

[0059] As shown in Table 1, Examples 1 to 5 exhibited sufficient biodegradability. Furthermore, the strength was sufficiently maintained even in a moist heat environment, and the alkali resistance was also excellent. The strength at break and the elongation at break were equivalent to those of PET without any decrease. The obtained biodegradable fiber was suitable as a fiber for clothing and vehicle interior materials.

[0060] Comparative Example 1 exhibited no reduction in strength and high alkali resistance in a moist heat environment, but did not exhibit biodegradability. Comparative Example 2 exhibited excellent biodegradability, but the strength was significantly reduced in a moist heat environment. Further, the resultant had also poor alkali resistance, dissolving entirely after 10 minutes of immersion in an aqueous sodium hydroxide solution. Comparative Examples 3 and 4 exhibited poor biodegradability, with a biodegradation rate of less than 15% after 135 days in the ASTM D5511 test.

Industrial Applicability

[0061] The biodegradable fiber of the present invention can be suitable for use in vehicle interior materials used in high temperature environments, for example, because the fiber exhibits little reduction in strength even when used in a moist heat environment, despite the fact that it exhibits biodegradability in soil via hydrolysis. Furthermore, since the fiber has excellent alkali resistance, the texture of the fiber can be improved by alkali weight reduction in the same manner as PET fibers, and therefore the fiber can be suitably used in clothing applications, for example. In addition, the fiber can be suitably used in the same applications as ordinary PET fibers, such as industrial materials and daily necessities.

Claims

1. A biodegradable fiber, containing a blend polymer containing a polyethylene terephthalate, a polybutylene adipate terephthalate and a polylactic acid, wherein the content of the polyethylene terephthalate in the fiber is 80 mass% or more.

2. A biodegradable fiber, containing a blend polymer containing a polyethylene terephthalate, a polybutylene adipate terephthalate and a polylactic acid, wherein the content of the polyethylene terephthalate in the fiber is at least 93 mass% or more, the content of the polybutylene adipate terephthalate in the fiber is 0.4 mass% to 2.4 mass%, the content of the polylactic acid in the fiber is 0.4 mass% to 2.4 mass%, and the ratio of the content of the polybutylene adipate terephthalate to the content of the polylactic acid is 40/60 to 60/40 in terms of mass ratio.

3. The biodegradable fiber according to claim 1 or 2, wherein the content of the polyethylene terephthalate in the fiber is 98 mass% or more, and the ratio of the content of the polybutylene adipate terephthalate to the content of the polylactic acid is 50/50 to 60/40 in terms of mass ratio.

4. The biodegradable fiber according to any one of claims 1 to 3, having a biodegradation rate of 15% or more after 135 days in the ASTM D5511 test.

5. The biodegradable fiber according to any one of claims 1 to 4, having a reduction rate of strength at break of 25% or less after the following moist heat environment test.

(Moist heat environment test)

A tubular knitted fabric is produced, a 120 mm × 150 mm test piece is taken from the produced tubular knitted fabric, and after heat setting, measurement is performed using an AG-IS autograph (registered trademark) tensile tester manufactured by Shimadzu Corporation under the conditions of a sample width of 50 mm, a test length of 50 mm, and a constant tensile speed of 100 mm/min. The maximum load in the load-elongation curve is taken as the strength at break (cN), and the test piece is measured twice in each of the longitudinal direction and the transverse direction, with the average value taken as the strength at break before the moist heat environment test.

Using a still air temperature and humidity chamber PR-3KP manufactured by ESPEC Corporation, the test piece is left to stand in a moist heat environment at 80°C and 95% relative humidity. After 400 hours, the strength at break of the test piece is measured twice in each of the longitudinal direction and the transverse direction, with the average value taken as the strength at break after the moist heat environment test. The reduction rate of strength at break in a moist heat environment is calculated using the average value, according to the following formula.

Reduction rate (%) of strength at break in moist heat environment = {(strength at break before moist heat environment test - strength at break after moist heat environment test)/(strength at break before moist heat environment test)}×100

6. The biodegradable fiber according to any one of claims 1 to 5, having a mass reduction rate of 15% or less after the following alkali resistance test.
(Alkali resistance test)
A tubular knitted fabric is produced, a 100 mm × 100 mm test piece is taken from the produced tubular knitted fabric, and the mass (W1) in a moisture equilibrium state is measured. The test piece is then immersed in a 4 mass% concentration of an aqueous sodium hydroxide solution kept at 98 ± 2°C. After 30 minutes, the test piece is taken out, washed with water, dried, and then brought back to a moisture equilibrium state, and thus the mass (W2) at that time is measured. This measurement is performed twice, and the mass reduction rate is calculated using the average value according to the following formula.