JP4923172B2 - Method for modifying polylactic acid fiber structural material - Google Patents

Description

  The present invention relates to a modification treatment method for improving the heat resistance of a polylactic acid fiber structural material.

  In recent years, petroleum-based polymer materials have been devised and improved in various ways to meet the diverse needs of the world. However, fossil fuels are finite and will eventually be depleted, and there is an urgent need to develop alternative polymer materials that use plants (biomass) to be regenerated as a result of the sun. For example, polylactic acid materials made from corn, sugarcane, etc. are attracting attention. Polylactic acid has a relatively high melting point among biomass plastics, and exhibits suitable characteristics as a molding material such as high transparency, high rigidity, and excellent processability. Therefore, textiles, film / sheets and molding resins, clothing, planting pots / herbicidal sheets, food packaging / containers / tableware, stationery, and mobile phones / computers, etc. Widely deployed.

  However, the temperature at which polylactic acid dissolves (melting point) is at most 170 ° C to 180 ° C compared to other biomass plastics, and the temperature at which thermal deformation occurs (load deflection temperature) is as low as 60 ° C. . For this reason, products that are not exposed to high temperatures, such as stationery, may be deformed during transportation, as well as being developed in food containers that require heat resistance (tray) and clothing that requires ironing. , The development of daily products is hindered.

For this reason, many attempts have been made to increase the heat resistance of polylactic acid. Specifically, an agent that promotes crystallization of polylactic acid is added (see Patent Document 1), and a network of networks is formed between polylactic acid molecules to crosslink and restrain the movement of the molecules (Patent Document 2 and 3) and improving the heat resistance by combining with other plastic materials.
JP 2006-212897 A JP 2005-336325 A JP 2006-276235 A

  In the conventional method for improving the heat resistance of polylactic acid described above, the melting point and the deflection temperature under load can be improved to 200 ° C. and 130 ° C., respectively. Is a pre-processing method in which a drug is kneaded into polylactic acid.

  However, if processing is carried out to improve the heat resistance of polylactic acid in such a previous stage, it may be necessary to review the processing and production processes according to the polylactic acid material that has undergone such processing. There is a problem that it is not possible to respond flexibly when, for example, it is desired to impart heat resistance to a product manufactured at

  Then, this invention aims at providing the modification | reformation processing method of the polylactic acid fiber structure material which can improve heat resistance by post-processing the polylactic acid fiber structure material comprised by a polylactic acid fiber. It is.

  The method for modifying a polylactic acid fiber structure material according to the present invention includes an impregnation process step of heating while immersing the polylactic acid fiber structure material in a dispersion in which a cross-linking agent is dispersed, and an impregnation step with the cross-linking agent. And a cross-linking formation step of irradiating the polylactic acid fiber structure material with high energy rays. Furthermore, in the impregnation treatment step, a vinyl monomer having two or more vinyl groups in the molecule is used as a crosslinking agent. Furthermore, in the impregnation treatment step, after the polylactic acid fiber structural material is immersed at 2 ° C. to 80 ° C., the temperature is increased from 90 ° C. to 120 ° C. at a temperature rising rate of 0.1 ° C./min to 10 ° C./min. It is characterized by heating at a constant temperature for a period of minutes to 120 minutes. Furthermore, in the cross-linking formation step, the electron beam is irradiated at a dose of 20 kGy to 400 kGy.

  The present invention has the above-described configuration, so that the polylactic acid fiber structure material is impregnated with a crosslinking agent in the impregnation treatment step, and the polylactic acid fiber structure material is irradiated with high energy rays in the crosslinking formation step to be heat resistant. The polylactic acid fiber structure material with improved properties can be obtained, and post-processing can be performed to improve the heat resistance of the polylactic acid fiber structure material. In the present specification, the polylactic acid fiber structural material means a structural material composed of polylactic acid fibers, and the structural material includes yarns, woven fabrics, knitted fabrics, non-woven fabrics, and clothing manufactured using these. Of textile products.

  In the present invention, polylactic acid which is a polyester having L-lactic acid and / or D-lactic acid as a main repeating unit is used from the viewpoint of high melting point and high heat resistance. The method for producing polylactic acid includes L-lactic acid and / or D-lactic acid as a raw material, a lactide which is a cyclic dimer once generated, and then ring-opening polymerization, L-lactic acid and / or Alternatively, a one-step direct polymerization method in which dehydration condensation is directly performed in a solvent using D-lactic acid as a raw material is known. The polylactic acid used in the present invention may be obtained by any method.

  The average molecular weight of polylactic acid is usually at least 50,000, preferably at least 100,000, preferably 100,000 to 300,000. When the average molecular weight is lower than 50,000, the strength properties of the fiber are lowered, which is not preferable. If it exceeds 300,000, the melt viscosity becomes too high, and melt spinning may be difficult.

  The polylactic acid in the present invention may be a copolymerized polylactic acid obtained by copolymerizing other components having ester forming ability in addition to L-lactic acid and D-lactic acid. Examples of other copolymer component units include polyvalent carboxylic acids, polyhydric alcohols, hydroxycarboxylic acids, lactones, and the like. Specifically, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid , Azelaic acid, sebacic acid, dodecanedioic acid, fumaric acid, cyclohexanedicarboxylic acid, terephthalic acid, isophthalic acid, phthalic acid, 2,6-naphthalenedicarboxylic acid, 5-sodium sulfoisophthalic acid, 5-tetrabutylphosphonium sulfoisophthalic acid Polycarboxylic acids such as ethylene glycol, propylene glycol, butanediol, heptanediol, hexanediol, octanediol, nonanediol, decanediol, 1,4-cyclohexanedimethanol, neopentyl glycol, glycerin, trimethylo Propane, pentaerythritol, bisphenol A, polyhydric alcohols such as dihydric glycol, triethylene glycol, polyethylene glycol, polypropylene glycol, polytetramethylene glycol, etc., glycolic acid, Hydroxycarboxylic acids such as hydroxybutyric acid, 4-hydroxybutyric acid, 4-hydroxyvaleric acid, 6-hydroxycaproic acid hydroxybenzoic acid, glycolide, ε-caprolactone glycolide, ε-caprolactone, β-propiolactone, δ-butyrolactone, β A lactone such as-or γ-butyrolactone, pivalolactone, or δ-valerolactone can be used. These copolymer components can be used alone or in combination of two or more.

  Furthermore, an internal plasticizer and an external plasticizer can be contained, and a matting agent, a deodorant, a flame retardant, a yarn friction reducing agent, an antioxidant, a color pigment, and the like can be added as necessary.

  Using polylactic acid fibers obtained by spinning polylactic acid as described above by a known production method, yarns, woven fabrics, knitted fabrics, and nonwoven fabrics are produced as polylactic acid fiber structural materials by a conventionally known method. Fabricate fiber products. In addition to these structural materials, any material having a structure capable of forming a crosslink by irradiation with high energy rays can be subjected to a modification treatment.

  Examples of the crosslinking agent impregnated into the polylactic acid fiber structural material thus prepared include polyfunctional acrylic monomers, polyfunctional allyl monomers, and mixed monomers thereof, among which polyfunctional allyl monomers are preferable.

  Specific examples include, for example, ethylene oxide-modified bisphenol A di (meth) acrylate, 1,4-butanediol di (meth) acrylate, diethylene glycol di (meth) acrylate, and dipentaerythritol hexaacrylate. , Dipentaerythritol monohydroxypentaacrylate, caprolactone modified dipentaerythritol hexaacrylate, pentaerythritol tri (meth) acrylate, pentaerythritol tetra (meth) acrylate, polyethylene glycol di (meth) acrylate, trimethylolpropane triacrylate, EO modified tri Methylolpropane tri (meth) acrylate, PO-modified trimethylolpropane tri (meth) acrylate, Tris Acryloxyethyl) isocyanurate, tris (methacryloxyethyl) isocyanurate, and mixtures thereof are common. Examples of the polyfunctional allylic monomer include triallyl cyanate, triallyl isocyanate, diallyl phthalate, diallyl benzene phosphonate, and mixtures thereof.

  In particular, it has two or more vinyl groups such as triallyl isocyanate (TAIC), triallyl cyanate (TAC), trimethylolpropane triacrylate (TMPTA), trimethylolpropane trimethacrylate (TMPTMA), divinylbenzene (DVB) in the molecule. Vinyl monomers are preferably used. Such vinyl monomers are excellent in that a crosslinking material having a high gel fraction can be obtained at a low monomer concentration and a low dose.

  As a dispersion for dispersing the cross-linking agent, it is dissolved in a proportion of 1 to 30% of the cross-linking agent with respect to the polylactic acid fiber structure material before the treatment, and the emulsion dispersion is 0.1 to 10 times the amount of the added cross-linking agent. A solution prepared by dissolving an agent is used. If the crosslinking agent is less than 1%, the degree of crosslinking when impregnated is insufficient, and if it exceeds 30%, the polylactic acid fiber structural material deteriorates and becomes brittle. Further, when the emulsifying dispersant is less than 0.1 times, the dispersing action is not exhibited, and when it exceeds 10 times, the impregnation rate is lowered.

  Examples of the emulsifying dispersant include tamol dispersants such as naphthalenesulfonic acid / formalin condensate soda, and surfactants having an HLB range of 20 to 7 such as polyoxyethylene polyoxypropylene alkyl ether. The total weight of the dispersion liquid may be set to 5 to 200 times the weight of the polylactic acid fiber structure material to be immersed. When the weight of the dispersion is less than 5 times, impregnation unevenness is likely to occur, and when it exceeds 200 times, the impregnation rate is lowered.

  When the impregnation treatment is performed by immersing the polylactic acid fiber structure material in the dispersion liquid, the liquid temperature of the dispersion liquid is set to 2 ° C. to 80 ° C. in advance, and the polylactic acid fiber structure material is charged. After the addition, the temperature of the dispersion is increased at a rate of 0.1 ° C./min to 10 ° C./min, and set to 90 ° C. to 120 ° C. After raising the temperature, the mixture is heated for 1 minute to 120 minutes at that temperature. By performing such a heating treatment, a sufficient amount of the crosslinking agent can be impregnated uniformly. If the initial liquid temperature is lower than 2 ° C or higher than 80 ° C, impregnation unevenness is likely to occur. Further, if the rate of temperature rise is slower than 0.1 ° C./min, the processing time is increased and the production efficiency is poor. Further, if the heating time is shorter than 1 minute, impregnation unevenness is likely to occur, and if it is longer than 120 minutes, there is a problem in terms of production efficiency.

  Next, the polylactic acid fiber structure material that has been sufficiently dried is irradiated with a high energy ray in an inert gas atmosphere such as nitrogen or in the air, and a crosslinking reaction of the impregnated crosslinking agent is performed. Examples of the high energy rays to be irradiated include electromagnetic waves (γ rays, X rays), electron beams (EB), particle rays (α rays, neutron rays) and the like. In particular, it is preferable to use an electron beam (EB). In that case, the acceleration voltage is preferably in the range of 100 to 5,000 KV. In the electron beam irradiation, it is necessary to adjust the acceleration voltage and the intensity of the irradiation dose according to the thickness of the polylactic acid fiber structure material. If the strength is too strong, the strength of the polylactic acid fiber is reduced. It is necessary to balance the improvement in heat resistance and the reduction in strength due to irradiation. Therefore, the irradiation dose is preferably in the range of 20 kGy to 400 kGy. When the irradiation dose is lower than 20 kGy, crosslinking is difficult, and when the irradiation dose is higher than 400 kGy, the deterioration of the polylactic acid fiber structure progresses and becomes weak.

<Polylactic acid fiber structural material>
As the polylactic acid fiber structural material, a polylactic acid fabric (manufactured by Toray Industries, Inc., 56T / 56T Ecodia Taffeta) was used.

<Impregnation treatment process>
A sample was prepared by cutting the polylactic acid structural material into a predetermined size, and the weight was measured (W1). Weighing a cross-linking agent (TAIC; manufactured by Kanto Chemical Co., Inc. or Nippon Kasei Co., Ltd.) and an emulsifying dispersant (Tamol type dispersant (manufactured by Kao Corporation) or surfactant (RS-929; manufactured by Takamatsu Yushi Co., Ltd.)) After kneading, it was dissolved in water, and the total amount of the aqueous solution was 25 times the weight of the dough (bath ratio 1:25). The prepared dispersion was put together with the prepared sample into a dyeing pot, and this was put into a dyeing tester (manufactured by Sakurai dyeing machine industry). The dyeing pot was charged after heating to 50 ° C. Thereafter, the temperature was raised to 110 ° C. in 50 minutes, and heated at 110 ° C. for 30 minutes. After the impregnation treatment, the sample was taken out and weighed again (W2). The impregnation rate of the polylactic acid sample with the crosslinking agent was calculated from the following equation.
Impregnation rate (%) = 100 * (W2-W1) / W1

<Crosslinking formation process>
The impregnated sample was sufficiently dried and irradiated with an electron beam. The electron beam irradiation was performed in a nitrogen atmosphere using an electron beam irradiation apparatus (manufactured by NHV Corporation).

<Evaluation test>
In order to investigate the degree of cross-linking of the polylactic acid fiber structural material modified as described above, the gelation rate and the volume swelling degree were measured. As a measuring method, a modified polylactic acid fiber structural material (2 cm × 20 cm) is weighed (WG1), immersed in 40 ml of a chloroform solution, and stirred at room temperature for 24 hours. Subsequently, the solution is filtered through glass filter paper, and the gel remaining on the filter paper is taken out and weighed (WG2). Then, it is dried at room temperature for 27 hours, further dried at 60 ° C. for 3 hours, and weighed again (WG3). The gelation rate and the volume swelling degree of the polylactic acid fiber structural material are obtained by the following equations.
Gelation rate (%) = 100 × (WG3-WG1) / WG1
Volume swelling degree = 0.838 (WG2-WG1) / WG1

  Moreover, in order to investigate the heat resistance of the modified polylactic acid fiber structural material, the following four evaluation tests were performed.

(1) Iron test A polylactic acid fiber structural material was cut into 2 cm × 2 cm to prepare a sample, and an iron heated to a predetermined temperature was pressed against the prepared sample for 1 minute. The temperature of the heating surface of the iron was set in three stages: low (90 ° C. to 120 ° C.), medium (150 ° C. to 170 ° C.), and high (180 ° C. to 200 ° C.). After the heat pressure welding, the change in the appearance of the sample and the dimensional change rate were evaluated.

(2) Dry heat test A sample was prepared by cutting a polylactic acid fiber structural material into 6 cm × 6 cm, and left in a thermostat set at a predetermined temperature for 3 minutes. The temperature was set at 120 ° C, 140 ° C and 150 ° C. After the dry heat treatment, the length of the four sides of the sample was measured, and the dimensional change rate due to heat was calculated by the following equation.
Dimensional change rate (%) = (B ² / A ² −1) * 100
Here, A is the average value of the four sides before the heat treatment, and B is the average value of the four sides after the heat treatment.

(3) Tensile test A tensile tester (AGS-J manufactured by Shimadzu Corporation) was used. In the measurement, a sample was prepared by cutting a polylactic acid fiber structural material into 5 cm × 40 cm, and the test speed was set to 30 cm / min with a sample holding interval of 20 cm. As the measurement results, the strength (N) and elongation (%) at break were obtained.

(4) Burst strength test A sample was prepared by cutting a polylactic acid fiber structural material into 6 cm x 6 cm, and left in a thermostat set at a predetermined temperature for 3 minutes. The temperature was set at 120 ° C, 140 ° C and 150 ° C. After the dry heat treatment, the fracture strength (kPa) was measured based on the JIS L-1018 A method (Murren method).

<Example 1>
Using the polylactic acid woven fabric, the above-described impregnation treatment was performed with the concentration of the crosslinking agent (TAIC) set to 5% owf, 15% owf, and 30% owf. The emulsifying dispersant (Tamol dispersant) was used in an amount three times the amount of the crosslinking agent. The impregnation rates were 1.8% (5% owf), 6.69% (15% owf), and 11.19% (30% owf), confirming that a sufficient amount of the crosslinking agent was impregnated. .

<Example 2>
Using the polylactic acid woven fabric, the concentration of the crosslinking agent (TAIC) was set to 15% owf, and the above impregnation treatment was performed. The emulsifying dispersant (Tamol dispersant) was used in an amount three times the amount of the crosslinking agent. And in order to confirm the effectiveness by temperature rising, the dyeing pot was heated to 110 degreeC, and it maintained as it was for 80 minutes, and the impregnation process was performed. When the temperature was raised and heated, the impregnation rate was 7.22%, but when the temperature was not raised, the impregnation rate was 6.57%, and the case where the temperature was raised and heated was higher. It was confirmed that more crosslinking agent could be impregnated.

<Example 3>
Using the polylactic acid woven fabric, the concentration of the crosslinking agent (TAIC) was set to 15% owf, and the above impregnation treatment was performed. The emulsifying dispersant (Tamol dispersant) was used in an amount three times the amount of the crosslinking agent. The polylactic acid fabric thus impregnated (impregnation rate 6.5%) was irradiated with an electron beam. The results are shown in Table 1.

表１をみると、電子線の線量が１００ｋＧｙ以上でゲル化されていることが示されており、架橋形成には１００ｋＧｙ以上の線量が必要であることがわかる。

Table 1 shows that the electron beam dose is gelled at 100 kGy or more, and it can be seen that a dose of 100 kGy or more is necessary for the formation of a cross-link.

<Example 4>
Using the polylactic acid woven fabric, the concentration of the crosslinking agent (TAIC) was set to 15% owf, and the above impregnation treatment was performed. The emulsifying dispersant (Tamol dispersant) was used in an amount three times the amount of the crosslinking agent. The polylactic acid fabric thus impregnated (impregnation rate: 7.22%) was irradiated with an electron beam for crosslinking formation treatment, and an iron test was performed on the treated polylactic acid fabric. The results are shown in Table 2.

表２をみると、１００ｋＧｙ以上の線量を照射した場合には融解しないものの、２００ｋＧｙの線量を照射した場合に１５０℃程度で縮小するようになった。これに対して、１００ｋＧｙの線量を照射した場合には１７０℃程度まで変化が見られなかった。こうした結果は、２００ｋＧｙの線量を照射することでポリ乳酸織物の劣化が進んだものと考えられる。したがって、１００ｋＧｙの線量を照射することで架橋形成による耐熱性の向上と電子線照射による劣化のバランスが良好に行われているものと考えられる。

Table 2 shows that although it does not melt when irradiated with a dose of 100 kGy or more, it shrinks at about 150 ° C. when irradiated with a dose of 200 kGy. In contrast, no change was observed up to about 170 ° C. when a dose of 100 kGy was applied. From these results, it is considered that the deterioration of the polylactic acid woven fabric has progressed by irradiating a dose of 200 kGy. Therefore, it is considered that the balance between the improvement in heat resistance due to cross-linking and the deterioration due to electron beam irradiation is satisfactorily performed by irradiating a dose of 100 kGy.

<Example 5>
Using the polylactic acid woven fabric, the concentration of the crosslinking agent (TAIC) was set to 5% owf, and the above impregnation treatment was performed. The emulsifying dispersant (RS-929) was used in the same amount as the crosslinking agent. The polylactic acid woven fabric thus impregnated (impregnation rate 1.8%) was irradiated with an electron beam to perform a crosslinking formation treatment, and a dry heat test was performed on the treated polylactic acid woven fabric. The results are shown in Table 3.

表３をみると、線量が１００ｋＧｙに設定した場合に温度変化に対して寸法変化率が小さいことがわかる。線量が２００ｋＧｙでは、高温になると寸法変化率が大きくなるのは、電子線照射により劣化が進んだものと考えられる。

Table 3 shows that the dimensional change rate is small with respect to the temperature change when the dose is set to 100 kGy. When the dose is 200 kGy, the dimensional change rate increases as the temperature rises. It is considered that the deterioration has progressed due to electron beam irradiation.

<Example 6>
Using the polylactic acid woven fabric, the concentration of the crosslinking agent (TAIC) was set to 15% owf, and the above impregnation treatment was performed. The emulsifying dispersant (RS-929) was used in the same amount as the crosslinking agent. The polylactic acid fabric impregnated in this way (impregnation rate 6.5%) was irradiated with an electron beam to undergo a crosslinking formation treatment, and the treated polylactic acid fabric was subjected to a dry heat test. The results are shown in Table 4.

表４をみると、表３と同様に線量が１００ｋＧｙに設定した場合に温度変化に対して寸法変化率が小さいことがわかる。架橋剤の濃度が大きくなったことによる影響は見られなかった。

When Table 4 is seen, it turns out that a dimensional change rate is small with respect to a temperature change when a dose is set to 100 kGy similarly to Table 3. FIG. There was no effect of increasing the concentration of the cross-linking agent.

<Example 7>
Using the polylactic acid woven fabric, the concentration of the crosslinking agent (TAIC) was set to 15% ow.f, and the above impregnation treatment was performed. The emulsifying dispersant (Tamol dispersant) was used in an amount three times the amount of the crosslinking agent. The polylactic acid woven fabric thus impregnated (impregnation rate: 7.22%) was irradiated with an electron beam to perform a crosslinking formation treatment, and the treated polylactic acid woven fabric was subjected to a tensile test. The results are shown in Table 5.

表５をみると、電子線照射により劣化が進み強度が低下したことがわかる。また、照射する線量が大きくなるほど強度が低下しており、劣化の度合いが大きいことがわかる。

When Table 5 is seen, it turns out that deterioration progressed by electron beam irradiation and the intensity | strength fell. Moreover, it turns out that intensity | strength is falling, and the degree of deterioration is so large that the irradiated dose becomes large.

<Example 8>
Using the polylactic acid woven fabric, the concentration of the crosslinking agent (TAIC) was set to 5% owf, and the above impregnation treatment was performed. The emulsifying dispersant (RS-929) was used in the same amount as the crosslinking agent. The polylactic acid fabric thus impregnated (impregnation rate: 1.8%) was irradiated with an electron beam for crosslinking formation treatment, and a burst test was performed on the treated polylactic acid fabric. The results are shown in Table 6.

<Example 9>
Using the polylactic acid woven fabric, the concentration of the crosslinking agent (TAIC) was set to 15% owf, and the above impregnation treatment was performed. The emulsifying dispersant (RS-929) was used in the same amount as the crosslinking agent. The polylactic acid fabric impregnated in this way (impregnation rate 6.5%) was irradiated with an electron beam to undergo a crosslinking formation treatment, and the treated polylactic acid fabric was subjected to a dry heat test. The results are shown in Table 7.

  When Table 6 and Table 7 were seen, the tendency for the burst strength to fall by the polylactic acid fabric by which the modification process was carried out by crosslinking agent impregnation or electron beam irradiation was seen. Further, when considering the correlation between the dry heat treatment temperature and the burst strength, in the case of only electron beam irradiation, the strength decrease due to the temperature increase became larger as the dose increased. Even in the case of only the crosslinking agent impregnation, a decrease in strength due to an increase in temperature was observed as the impregnation amount increased. In “TAIC impregnation + electron beam irradiation”, a decrease in burst strength due to this temperature increase was suppressed. It should be noted that when the concentration of TAIC was increased in “TAIC + 200 kGy irradiation”, a significant increase in strength was observed. From these facts, the formation of clear crosslinks can be seen.

  When comparing “electron beam irradiation only” and “TAIC impregnation + electron beam irradiation”, the burst strengths of “electron beam irradiation only” and “TAIC + 20 kGy” at 20 kGy showed similar values, but at 50 kGy, TAIC The value of the burst strength was greater for the material impregnated with. The tendency was confirmed even at 100 kGy, and the tendency became more remarkable at 200 kGy. From this result, it is speculated that the formation of crosslinks started from irradiation with 50 kGy, and that the crosslinks were clearly formed at 100 kGy or more.

  In addition, looking at “TAIC impregnation + electron beam irradiation”, (1) 20 kGy → 50 kGy tends to decrease the burst strength, (2) 50 kGy → 100 kGy, the burst strength increases, and (3) When 100 kGy → 200 kGy, the burst strength tended to decrease. In the stage (1), it is presumed that the influence of the deterioration of electron beam irradiation was greater than that of the cross-linking formation. In the stage (2), it is presumed that the formation of cross-linking is promoted, and damage due to deterioration of electron beam irradiation is compensated. In the stage of (3), it is presumed that damage due to deterioration of electron beam irradiation prevailed over the formation of crosslinks. From these results, it can be seen that irradiation with 100 kGy is optimal. Based on the above results, it can be said that irradiation at 100 kGy is most preferable.

Claims (4)

  An impregnation treatment step of heating while immersing the polylactic acid fiber structure material in the dispersion in which the crosslinking agent is dispersed, and irradiating the polylactic acid fiber structure material impregnated with the crosslinking agent in the impregnation treatment step with high energy rays A method for modifying a polylactic acid fiber structure material, comprising a crosslinking formation step.   The reforming treatment method according to claim 1, wherein in the impregnation treatment step, a vinyl monomer having two or more vinyl groups in the molecule is used as a crosslinking agent.   In the impregnation treatment step, after the polylactic acid fiber structure material is immersed at 2 ° C. to 80 ° C., the temperature is increased from 90 ° C. to 120 ° C. at a temperature rising rate of 0.1 ° C./min to 10 ° C./min for 1 minute The reforming method according to claim 1 or 2, wherein heating is performed at a constant temperature for 120 minutes.   The modification treatment method according to any one of claims 1 to 3, wherein, in the cross-linking formation step, an electron beam is irradiated at a dose of 20 kGy to 400 kGy.