Effect of the direct introduction of disulfide groups with 2-iminothiolane hydrochloride on wrinkle recovery of natural protein fibers

Abstract

To develop environmentally friendly natural protein fibers with good wrinkle recovery without changing their excellent physical properties, thiol groups were directly introduced into silk and wool fibers by chemical modification using 2-iminothiolane hydrochloride. Thereafter, disulfide (–SS–) groups were introduced by oxidative cross-linking. The wrinkle recovery of the wool fabrics improved with the increase in 2-iminothiolane hydrochloride concentration. In particular, introducing the reduction process between the 2-iminothiolane hydrochloride treatment and the oxidation process improved the wrinkle recovery of wool fabrics. In contrast, the silk fabrics treated with 0.04 wt% 2-iminothiolane hydrochloride exhibited good wrinkle recovery. In addition, combustion-ion chromatography revealed that sulfur was introduced into the silk fabrics by oxidation after 2-iminothiolane hydrochloride treatment. According to these experiments, directly introducing –SS– groups using 2-iminothiolane hydrochloride into silk and wool fibers effectively improves wrinkle recovery without changing the physical properties of natural protein fabrics.

Keywords

Natural protein (silk and wool) fibers chemical modification wrinkle recovery disulfide (–SS–) groups thiol (–SH) groups

Silk is known as the ‘queen of fibers’, owing to its excellent lustre, hand feel, texture, moisture absorption, air permeability, and dyeability. However, it is a difficult fiber to handle because of its susceptibility to sweat and water wrinkles. Moreover, it is also susceptible to fibrillation due to friction, and yellowing due to ultraviolet rays and heat.^1–3 Crepe silk fabrics with a high-twist yarn are used in Kyoyuzen and Nishijin textile products; however, they have poor wrinkle recovery when worn.¹ In contrast, wool is known as the ‘king of fibers’. The wrinkles and deformation during wear can be recovered by resting it on a hanger.⁴ This is due to its high elastic recovery rate. However, some lightweight wool fabrics do not have the high level of wrinkle recovery expected from high-quality products.

Previous studies present improvements in wrinkle recovery of natural protein fibers and their products, focusing on wool and silk. Various improvement methods for wool fabrics have been proposed, including incorporating some bulky molecules,^5,6 cross-linking,⁵ resin treatment,⁵ high flexibility polymer treatment,⁶ natural polymers,⁷ and mercury (II) acetate.⁵ For silk fabrics, various improvement methods have been proposed, for example, cross-linking,^1,8 resin treatment,^1,2,9 and graft-copolymerization.^1,3 However, these methods cannot improve wrinkle recovery without changing the physical properties of natural fabrics, such as hand feel, weight, and color tone.

Previous research has demonstrated that wrinkle formation in wool is caused by the rearrangement (disconnection and reconnection) of hydrogen bonds, while disulfide (–SS–) groups act as an inhibitor of wrinkle formation.¹⁰ 2-Iminothiolane hydrochloride (2-IT) is used as a protein cross-linking reagent, and can introduce –SS– groups into proteins.¹¹ A new wrinkle resistant finish for wool^4,10 was found using 2-IT, by introducing new –SS– groups, thus providing good wrinkle recovery without changing the physical properties of wool fabrics. In addition, it was shown that chemical modification of hair keratin fibers using 2-IT was effective for a permanent waving treatment.¹² Moreover, it was confirmed that 2-IT reacts with the amino group of L-phenylalanine used as the model compound of fibrous protein, and that the thiol (–SH) group replaces the amino group by ¹H-nuclear magnetic resonance and secondary ion mass spectrometry.¹³ The reaction mechanism of 2-IT is shown in Figure 1.

However, the relationship between the 2-IT reaction concentration and wrinkle recovery for wool keratin fibers and comparison of the wrinkle recovery evaluation (difference of the open angle) and wear test have not been reported. In addition, past studies have not investigated the direct introduction of –SS– groups, an excellent property (such as structural stability) of keratin fibers, into silk fibers (between fibril molecules) by chemical modification using 2-IT.

In this study, to develop environmentally friendly natural protein fibers with a good wrinkle recovery without changing their excellent physical properties, –SH groups were directly introduced into wool and silk protein fibers by chemical modification using 2-IT of varying concentrations, and then –SS– groups were introduced by oxidative cross-linking. The effect of the direct introduction of –SS– groups with 2-IT on the wrinkle recovery of silk and wool fabrics was investigated. In addition, the sulfur content introduced into wool and silk fabrics was calculated by combustion-ion chromatography.

Experimental section

Materials

An undyed 100% merino wool (plain weave, 2/60 ×2/60, 175 g/m²) (UDW) and chrome dyed 100% merino wool (plain weave, 2/60 × 2/60, 167 g/m²) (DW) were used as the wool fabric samples. Moreover, scoured silk (crepe silk, tango-chirimen,154 g/m²) (CS) was used for the silk fabric samples.

2-IT was the protein cross-linking reagent, l-cysteine was the reducing reagent, and polyoxyethylene (10) octylphenyl ether (OPE) was purchased from Fujifilm Wako Pure Chemical Co.

Preparation of wool fabrics treated with 2-IT

The natural protein fibers were processed by 2-IT treatment followed by oxidation or by 2-IT treatment followed by reduction and then oxidation. The preparation condition of the wool samples (UDW and DW) is shown in Table 1.

Table 1.

Preparation condition of the samples

Sample no.	Fabric	Introduction of –SH groups					Reduction
		2-IT	Ratio of wool to solution	pH	Temp.	Time
		(wt%)	Ratio of wool to solution	pH	(^oC)	(h)
1	UDW	0.00	1:26	8.03	50	2	U
2	UDW	0.01	1:26	7.98	50	2	U
3	UDW	0.10	1:26	7.69	50	2	U
4	UDW	0.30	1:26	7.43	50	2	U
5	UDW	1.00	1:26	7.03	50	2	U
6	UDW	0.00	1:26	8.03	50	2	T
7	UDW	0.01	1:26	7.98	50	2	T
8	UDW	0.10	1:26	7.69	50	2	T
9	UDW	0.30	1:26	7.43	50	2	T
10	UDW	1.00	1:26	7.03	50	2	T
11	DW	0.00	1:28	8.00	70	4	U
12	DW	0.01	1:28	7.74	70	4	T
13	CS	0.00	1:25	8.00	50	2	U
14	CS	0.04	1:25	7.86	50	2	U
15	CS	0.40	1:25	7.50	50	2	U

2-IT: 2-iminothiolane hydrochloride; UDW: undyed wool; DW: dyed wool; CS: crepe silk; U: untreated; T: treated.

The production method of sample 7 as the representative sample was as follows: UDW (5 × 24 cm²) was immersed in a solution of 0.01 wt% 2-IT/0.05-M phosphate buffer (pH 7.98) and 0.10 wt% OPE at a ratio of wool to solution of 1:26. The wool sample was soaked for 2 h at 50°C, and continuously shaken, which introduced thiol (–SH) groups (2-IT treatment). The sample was then washed in distilled water to remove unreacted 2-IT. Afterwards, it was immersed in a solution of 0.40 wt% l-cysteine and 0.10 wt% OPE at a ratio of wool to solution of 1:52. The wool sample was soaked at 25°C for 30 min, and continuously shaken, which disconnected the –SS– groups (reduction process). Then, it was squeezed three times with a paper towel. Finally, the wool was set with a steam iron (15 s/side × 2, 150 ± 5°C, constant steam pressure, weight 2.0 kg), and then was dried at room temperature. A steam iron was placed twice on both sides (right and wrong sides) for 15 s/side. Sample 1 was prepared as the control fabric of samples 2, 3, 4, 5, 6, 7, 8, 9 and 10. Sample 6 with the post-treatment reduction was prepared as the control fabric of samples 7, 8, 9 and 10.

The production method of sample 12 as the representative sample was as follows: DW (2.81 kg, 11 m) was immersed in a solution of 0.01 wt% 2-IT/0.05-M phosphate buffer (pH 7.74) and 0.1 wt% OPE at a ratio of wool to solution of 1:28. The wool sample was soaked for 4 h at 70°C, and continuously shaken. Thereafter, the sample was washed in groundwater. Afterwards, it was immersed in a solution of 0.4 wt% l-cysteine and 0.10 wt% OPE at a ratio of wool to solution of 1:28. The wool sample was soaked for 30 min at 40°C, and was continuously shaken (reduction process). Thereafter, it was washed in groundwater, underwent centrifugal dehydration, and was dried at room temperature. Finally, the wool sample was set with a high-voltage decatizing device. Sample 11 was prepared as the control fabric of sample 12.

Preparation of silk fabrics treated with 2-IT

The preparation condition of the silk samples (CS) is shown in Table 1. The production method of sample 13 as the representative sample was as follows: CS (5.5 × 10.5 cm²) was immersed in a solution of 0.04 wt% 2-IT/0.1-M phosphate buffer (pH 7.86) and 0.10-wt% OPE at a ratio of wool to solution of 1:25. The silk sample was soaked for 2 h at 50°C, and continuously shaken, which introduced –SH groups (2-IT treatment). Then, it was washed in distilled water, and squeezed three times using a paper towel to remove unreacted 2-IT. Finally, it was set with an iron (15 s/side ×2, 120 ± 5°C, weight 947 g) and then was dried at room temperature. An iron was placed twice on both sides (right and wrong sides) for 15 s/side. Sample 13 was prepared as the control fabric of samples 14 and 15.

Evaluation of the wrinkle recovery of wool

The aging history is essential for assessing the effectiveness of a chemical modification of wool fabrics.^14,15 In this regard, the wrinkle recovery of wool fabrics was evaluated the next day after sample preparation. The test method¹⁶ is presented below.

The specimens, cut to 40 mm × 15 mm along the warp, were preconditioned at a relative humidity (RH) of 56–58% at 20°C for 24 h. The specimens were then conditioned at RH of 90% at 30°C for 24 h. Finally, each specimen was placed on a metal plate holder. Each specimen was wrinkled along a weft, by adding 800 g of load at RH of 56–58% at 20°C for 90 min. After removing the load, the open angle after 30 min was measured at RH of 56–58% at 20°C using a Monsanto-type wrinkle tester. Because the standard deviation of the open angle after 30 min (between 120° and 155°) was between 1.3 and 2.6 (n = 5), and the coefficient of variation (CV) was 0.86–1.81% (see Table 1 in Kuzuhara and Hori),¹⁰ the wrinkle recovery was evaluated by the difference of the open angles of the untreated and treated wool fabrics (n = 1).

Wearing test using wool pants

Dress pants were sewn using samples 11 and 12. Half of the dress pants was used for sample 11 (right side), while the other half of dress pants was used for sample 12 (left side).

The dress pants were conditioned at RH of 62% at 22°C for 24 h, then wrinkled at RH of 62% at 22°C for 1 min by sitting and bending the knees. Afterwards, the wrinkled area was observed at RH of 62% at 22°C for 1 h.

Evaluation of the wrinkle recovery of silk

The wrinkle recovery of silk fabrics was evaluated on the day following sample preparation. The specimens, cut to 40 mm × 15 mm along the warp and weft, were preconditioned at RH of 60% at 20°C for 24 h. Each specimen was placed on a metal plate holder. Each specimen, cut to 40 mm × 15 mm along the warp, was wrinkled along a weft by adding 500 g of load at RH of 60% at 20°C for 60 min. Moreover, each specimen, cut to 15 mm × 40 mm along the weft, was wrinkled along a warp by adding 500 g of load at RH of 60% at 20°C for 60 min. After removal of the load, the open angles between 0 and 40 min were measured at RH of 60% at 20°C using the Monsanto-type wrinkle tester. The wrinkle recovery was evaluated by the difference of the open angles of the untreated and treated silk fabrics (n = 2), because the standard deviation of the open angle after 0–30 min (135–141°) along the weft (right and wrong sides) was 1.5–2.5 (n = 3), and the CV was 1.88–1.06%.

Evaluation of stiffness, weight, and reflectance of silk

The specimens, cut to 20 mm × 145 mm along the weft, were preconditioned at a RH of 60% at 20°C for 24 h. According to the JIS A method (45° cantilever method), the tip of each specimen was placed on a smooth horizontal table with a 45° slope at one end, aligned with the 0 positions on the scale. The specimen was then pushed towards the slope while being held down with a holding plate to prevent it from lifting on the horizontal table. The stiffness length (mm) that the specimen moved when the center point of one end of the specimen contacted the slope was measured (right and wrong sides, n = 3).

The weight of the crepe silk fabric (5.5 × 10.5 cm²) was measured before and after processing. The reflectance of the crepe silk fabric (n = 4) was measured at a wavelength of 530 nm using a reflectometer (model TR-1000D; Tokyo Densyoku Co. Ltd., Tokyo, Japan).

Ion chromatography

The S content in the wool and silk fabrics was quantified using combustion-ion chromatography. The absorption solution was implemented by the combustion method. A 0.02 g sample was dried for 2 h at 110°C, was placed in a magnetic ball and burned in a combustion chamber at 1350°C with a flowing mixture of oxygen and argon gas using an automatic processing combustion unit AQF-2100S (Mitsubishi Chemical Analytic Corp., Yamato, Japan). The gas was then absorbed in an absorption solution (10 mM hydrogen peroxide solution). After the volume of the absorption solution was fixed at 30 mL, ${SO}_{4}^{2 -}$ in the solution was measured using an ion chromatograph ICS-1500 (Dionex, Inc., Sunnyvale, USA). Finally, the ${SO}_{4}^{2 -}$ was converted into S.

Results and discussion

Wrinkle recovery of wool

The natural protein fibers were processed by 2-IT treatment followed by oxidation or by 2-IT treatment followed by reduction and then oxidation. The process of directly introducing disulfide (–SS–) groups into natural protein fibers such as wool and hair is shown in Figure 2.

First, 2-IT reacts with the amino groups in keratin fibers, and the –SH groups replace the amino groups (introduction of –SH groups: 2-IT treatment).¹³ Next, –SS– groups in keratin fibers disconnect due to reducing agents. We demonstrated that thioglycolic acid as a reducing agent diffused gradually beyond the cuticle region, and towards the insides of the cortex region along with the disconnection of –SS– groups using Raman spectroscopy and microspectrophotometery.¹⁷ Furthermore, we confirmed that l-cysteine disconnects –SS– groups in hair keratin fiber.¹⁸

Finally, the newly introduced –SH groups were oxidized to create new –SS– groups (oxidative cross-linking). Raman spectroscopy revealed increasing –SS– content in wool¹⁰ and hair keratin fibers¹² after 2-IT treatment and the oxidation. In addition, we used Raman spectroscopy to verify the considerable increase in the S–S bond intensity from the cuticle region to the center of the cortex region of virgin white human hair after 2-IT treatment and oxidation.¹⁹

The effect of introducing a reduction process between 2-IT treatment and oxidation on wrinkle recovery in wool fabrics was investigated. The wrinkle recovery of samples 1–10 is summarized in Tables 2 and 3. For oxidation after the 2-IT treatment, the wrinkle recovery of samples 2 and 3 improved compared with sample 1 (control). This indicates that new –SS– groups were introduced into the wool fabrics by a thiol/disulfide interchange.²⁰ Moreover, for oxidation after the 2-IT treatment and then reduction, the wrinkle recovery of sample 7 (difference of the open angle 15°) significantly improved compared with sample 2 (difference of the open angle 4°). For oxidation after reduction, the wrinkle recovery of sample 6 (difference of the open angle 9°) significantly improved compared with sample 1 (control). This indicates that the rearrangement of –SS– groups into the wool fabrics accelerated due to a thiol/disulfide interchange by the oxidation process after the 2-IT treatment followed by the reduction process. Thus, introducing the reduction process between the 2-IT treatment and the oxidation process effectively improves the wrinkle recovery of wool fabrics.

Table 2.

Influence of 2-IT concentration on wrinkle recovery of wool fabrics (50°C, 2 h)

Sample no.	2-IT (wt%)	Reduction	Wrinkle recovery^a
1	0.00	U	106
2	0.01	U	110 (4)
3	0.10	U	119 (13)
4	0.30	U	128 (22)
5	1.00	U	128 (22)

^aOpen angle (difference of the open angle) (n = 1).

2-IT: 2-iminothiolane hydrochloride; U: untreated.

Table 3.

Influence of 2-IT concentration on wrinkle recovery of wool fabrics (50°C, 2 h)

Sample no.	2-IT (wt%)	Reduction	Wrinkle recovery^a
1	0.00	U	111
6	0.00	T	122 (9)
7	0.01	T	126 (15)
8	0.10	T	130 (19)
9	0.30	T	130 (19)
10	1.00	T	131 (20)

^aOpen angle (difference of the open angle) (n = 1).

2-IT: 2-iminothiolane hydrochloride; T: treated; U: untreated.

The relationship between the difference in the open angle and 2-IT concentration is shown in Figure 3. For oxidation after the 2-IT treatment, the difference in the open angle increased with the 2-IT concentration. The difference in the open angle was constant for 2-IT concentrations above 0.30 wt%. Whereas for oxidation after 2-IT treatment followed by reduction, the difference in the open angle was constant for 2-IT concentrations above 0.10 wt%. Therefore, a good wrinkle recovery (difference of the open angle 15°) could be obtained even at a 2-IT concentration of 0.01 wt% by combining the 2-IT treatment process with the reduction process.

The influence of the dye adsorbed in wool fabrics on the wrinkle recovery was investigated. The wrinkle recovery of dyed wool fabrics (DW) treated with 2-IT and then subjected to reduction is summarized in Table 4. The wrinkle recovery of sample 12 (DW: difference of the open angle 11°) did not improve compared with sample 7 (UDW: difference of the open angle 15°), it improved compared with sample 11 (control). This indicates that the –SS– content introduced into the dyed wool fabrics (sample 11) decreased compared with that of undyed wool fabrics (sample 6) due to a decrease in free amino groups with the adsorption of dye molecules.

Table 4.

The wrinkle recovery of dyed wool fabrics (DW) treated with 2-IT and then subjected to reduction

Sample no.	2-IT (wt%)	Reduction	Wrinkle recovery^a
11 (control)	0.00	U	116
12	0.01	T	127 (11)

^aOpen angle (difference of the open angle) (n = 1).

2-IT: 2-iminothiolane hydrochloride; T: treated; U: untreated.

The wear test and wrinkle recovery evaluation of wool fabrics were compared. The wrinkle recovery of dress pants (100% dyed merino wool fabric, plain weave, 2/60 × 2/60, 167 g/m²) treated with 2-IT (left side: sample 12) and untreated (right side: sample 11) are shown in Figure 4. The dress pants were conditioned at RH of 62% at 22°C for 24 h, and then wrinkled at RH of 62% at 22°C for 1 min. Immediately after wrinkling (a), the number of wrinkles in sample 11 was more than that of sample 12. In addition, the wrinkle depth of sample 10 was greater than sample 11. After 1 h (b), the wrinkles of sample 11 disappeared, whereas those of sample 10 did not disappear. Moreover, the difference in the number of wrinkles and wrinkle disappearance time between samples 11 and 12 increased by increasing the wrinkling time. Thus, the wrinkle recovery of the wool pants caused by wearing can be improved, when the difference of the open angles of the treated and untreated (control) wool fabrics is over 10° (Table 4).

Wrinkle recovery of silk

The relationship between the open angle and recovery time of crepe silk fabrics (wrinkled along a warp, n = 2) treated with 0.04 wt% 2-IT and untreated is shown in Figure 5. The mean and standard deviation (CV, %) of the open angle after 0–30 min of crepe silk fabrics (wrinkled along a warp, n = 2) treated with 0.04 wt% 2-IT and untreated are shown in Table 5. Because the CV of the open angle after 0–30 min was 0.0–2.77%, it can be judged that this test method provides an accurate estimate of wrinkle recovery. The open angle of the crepe silk fabric increased by increasing recovery time, and was constant at approximately 30 min. For all recovery times, the open angle of the silk fabrics treated with 2-IT (sample 13) (right side: difference of the open angle after 30 min 20.5°, wrong side: difference of the open angle after 30 min 14.5°) was significantly larger than that of the untreated silk fabrics (sample 12). The relationship between the open angle and recovery time of the crepe silk fabrics (wrinkled along a weft, n = 2) treated with 0.04 wt% 2-IT and untreated is shown in Figure 6. The mean and standard deviation (CV, %) of the open angle after 0–30 min of crepe silk fabrics (wrinkled along a weft, n = 2) treated with 0.04 wt% 2-IT and untreated are shown in Table 6. Because the CV of the open angle was 0.00–8.32% except for 0 min of the untreated silk fabrics, it can be judged that this test method provides a relatively accurate estimate of wrinkle recovery. The open angle of the wool fabrics treated with 2-IT (sample 13) (right side: difference of the open angle after 30 min 14°, wrong side: difference of the open angle after 30 min 17°) was considerably larger than that of the untreated silk fabrics (sample 12). This indicates that new –SS– groups were introduced into the crepe silk fabrics by the oxidation process after the 2-IT treatment process. Immediately after wrinkling (0 min), the open angle (84–89°) of untreated fabric (Figure 6, wrinkled along the weft) was smaller than that (102–108°) of the untreated fabric (Figure 5, wrinkled along the warp). This suggests that the wrinkle formation of the warp that uses untwisted raw silk is stronger than the weft that uses raw silk twisted approximately 3000 turns per meter.

Table 5.

Mean and standard deviation (CV, %) of the open angle after 0–30 min of crepe silk fabrics (wrinkled along a warp, n = 2) treated with 0.04 wt% 2-IT and untreated

Recovery time (min)	Untreated, R	Untreated, W	0.04 wt% 2-IT, R	0.04 wt% 2-IT, W
0	102.0 ± 2.00 (1.96)	108.5 ± 0.50 (0.46)	114.0 ± 1.00 (0.88)	111.0 ± 0.50 (0.45)
5	118.5 ± 1.50 (1.27)	120.0 ± 0.00 (0.00)	135.5 ± 0.50 (0.37)	131.0 ± 2.00 (1.53)
10	122.0 ± 1.00 (0.82)	120.0 ± 2.00 (0.00)	141.0 ± 1.00 (0.71)	132.0 ± 2.00 (1.52)
15	126.5 ± 0.50 (0.40)	125.0 ± 1.00 (1.80)	144.0 ± 2.00 (1.39)	134.0 ± 2.00 (1.49)
20	129.0 ± 1.00 (0.78)	127.0 ± 1.00 (0.79)	148.0 ± 0.50 (0.37)	138.0 ± 2.00 (1.45)
25	131.0 ± 1.00 (0.76)	129.5 ± 0.50 (0.39)	151.5 ± 0.50 (0.33)	143.5 ± 1.50 (1.05)
30	132.0 ± 0.00 (0.00)	131.0 ± 1.00 (0.76)	152.5 ± 1.50 (0.98)	145.5 ± 2.50 (1.72)

CV: coefficient of variation; 2-IT: 2-iminothiolane hydrochloride; R: right side; W: wrong side.

Table 6.

Mean and standard deviation (CV, %) of the open angle after 0–30 min of crepe silk fabrics (wrinkled along a weft, n = 2) treated with 0.04 wt% 2-IT and untreated

Recovery time (min)	Untreated, R	Untreated, W	0.04 wt% 2-IT, R	0.04 wt% 2-IT, W
0	88.5 ± 9.50 (10.7)	84.0 ± 16.0 (19.1)	100.0 ± 2.00 (2.00)	98.5 ± 1.50 (1.52)
5	104.5 ± 4.50 (4.31)	104.5 ± 7.50 (7.18)	119.0 ± 7.00 (5.88)	116.0 ± 2.00 (1.72)
10	108.0 ± 4.00 (3.70)	106.5 ± 7.50 (6.10)	124.0 ± 4.00 (3.23)	124.5 ± 1.50 (1.20)
15	113.0 ± 1.00 (0.88)	108.5 ± 6.50 (5.99)	126.0 ± 4.00 (4.49)	129.5 ± 0.50 (0.39)
20	116.0 ± 1.00 (0.86)	112.5 ± 6.50 (5.78)	130.0 ± 2.00 (3.17)	130.5 ± 0.50 (0.38)
25	117.0 ± 1.00 (0.85)	115.0 ± 7.00 (6.09)	131.0 ± 1.00 (0.76)	132.0 ± 1.00 (0.76)
30	118.0 ± 2.00 (1.69)	117.0 ± 9.00 (7.69)	132.0 ± 0.00 (0.00)	134.0 ± 0.00 (0.00)

CV: coefficient of variation; 2-IT: 2-iminothiolane hydrochloride; R: right side; W: wrong side.

The relationship between the open angle and recovery time of the crepe silk fabrics (wrinkled along the warp, n = 2) treated with 0.40 wt% 2-IT and untreated is shown in Figure 7. The mean and standard deviation (CV, %) of the open angle after 0–30 min of crepe silk fabrics (wrinkled along a warp, n = 2) treated with 0.40 wt% 2-IT and untreated is shown in Table 7. For all recovery times, the open angle of the silk fabrics treated with 2-IT (sample 14) was slightly smaller than that of the untreated silk fabrics (sample 12), which indicates that the wrinkle recovery of silk fabrics did not improve with the increased 2-IT concentration, unlike for the wool fabrics. The unimproved wrinkle recovery of the crepe silk fabrics may be attributed to the increasing stiffness of the crepe silk fabrics treated with 0.40 wt% 2-IT, which, in turn, could be attributed to the fact that a thiol/disulfide interchange is less likely to occur due to the low –SS– content originally existing in silk fabrics, unlike wool fabrics.

Table 7.

Mean and standard deviation (CV, %) of the open angle after 0–30 min of crepe silk fabrics (wrinkled along a warp, n = 2) treated with 0.40 wt% 2-IT and untreated

Recovery time (min)	Untreated, R	Untreated, W	0.40 wt％2-IT, R	0.40 wt％2-IT, W
0	102.0 ± 2.00 (1.96)	108.5 ± 0.50 (0.46)	100.5 ± 0.50 (0.88)	101.0 ± 0.00 (0.00)
5	118.5 ± 1.50 (1.27)	120.0 ± 0.00 (0.00)	118.5 ± 1.50 (0.37)	118.0 ± 0.00 (0.00)
10	122.0 ± 1.00 (0.82)	120.0 ± 2.00 (0.00)	118.5 ± 1.50 (0.37)	121.0 ± 0.00 (0.00)
15	126.5 ± 0.50 (0.40)	125.0 ± 1.00 (1.80)	120.0 ± 1.00 (1.39)	122.5 ± 0.50 (0.41)
20	129.0 ± 1.00 (0.78)	127.0 ± 1.00 (0.79)	118.5 ± 1.50 (0.37)	124.5 ± 0.50 (0.40)
25	131.0 ± 1.00 (0.76)	129.5 ± 0.50 (0.39)	118.5 ± 1.50 (0.37)	125.5 ± 0.50 (0.40)
30	132.0 ± 0.00 (0.00)	131.0 ± 1.00 (0.76)	128.5 ± 0.50 (0.98)	125.5 ± 0.50 (0.40)

CV: coefficient of variation; 2-IT: 2-iminothiolane hydrochloride; R: right side; W: wrong side.

Stiffness, weight, and reflectance of silk fabrics

The stiffness, weight, and reflectance of crepe silk fabrics are shown in Table 8. The stiffness of the crepe silk fabrics treated with 0.04 wt% 2-IT (sample 14) was slightly higher than the untreated sample (sample 13). However, this processing method did not change the weight and reflectance of the crepe silk fabrics.

Table 8.

Stiffness, weight, and reflectance of crepe silk fabrics

	Stiffness length (mm) (n = 3)		Weight (g)		Reflectance (n = 4)
Crepe silk	Right side	Wrong side	Before	After	530 nm
Untreated	53.3 ± 0.47 (0.88)	50.6 ± 0.94 (1.86)	0.800	0.800	77.9 ± 0.40 (0.53)
0.04 wt% 2-IT	55.5 ± 0.41 (0.74)	54.7 ± 0.85 (1.55)	0.811	0.811	77.6 ± 1.33 (1.71)

2-IT: 2-iminothiolane hydrochloride.

Sulfur content in natural protein fibers

The S contents in the wool and silk fabrics are summarized in Table 9. The S content (0.80%) of the untreated crepe silk fabric (sample 13) was lower than that (2.97%) of the untreated wool fabric (sample 1). This is due to the 1/2 cystine content (amino acid composition as residues per 100 residues: 0.2)²¹ of silk fibroin, which is very low compared to the 1/2 cystine content (amino acid composition as residues per 100 residues: 13.1)²² of wool keratin.

Table 9.

S content in wool and silk fabrics

Sample no.	Treatment condition	S content (%)	Introduced S content (%)
1	Undyed wool (untreated)	2.97	0.00
3	0.10 wt% 2-IT	3.30	0.33
8	0.10 wt% 2-IT + reduction	3.30	0.33
13	Crepe silk (untreated)	0.08	0.00
14	0.04 wt% 2-IT	0.20	0.12
15	0.40 wt% 2-IT	0.23	0.15

2-IT: 2-iminothiolane hydrochloride.

The introduced S content (0.33%) of the wool fabrics treated with 0.10 wt% 2-IT (sample 3) was higher than that (0.12%) of the silk fabrics treated with 0.04 wt% 2-IT (sample 14). This suggests that the amino group content in the wool fabrics is higher than that of the silk fabrics. In addition, the S content of sample 8 was equal to that of sample 3, indicating that the 1/2 cystine content of the wool fabrics was unchanged by introducing the reduction process between the 2-IT treatment and the oxidation process. Therefore, we assumed that the rearrangement of –SS– groups into the wool fabrics was more accelerated due to the thiol/disulfide interchange, thereby improving the wrinkle recovery of the wool fabrics.

Moreover, the S content (0.20%) of silk fabrics (sample 14) treated with 0.04 wt% 2-IT increased compared with the S content (0.08%) of the untreated silk fabrics (sample 13), which indicates that –SH groups were introduced into the silk fabrics. According to this experiment, the wrinkle recovery of the crepe silk fabrics could be improved by introducing a small amount of –SH groups.

Conclusions

The effect of chemical modification using 2-IT on the wrinkle recovery of wool and silk fabrics was investigated by changing the 2-IT reaction concentration. The wrinkle recovery of the wool fabrics improved with the increase in the 2-IT concentration, which indicates that new –SS– groups were introduced into the wool fabrics by a thiol/disulfide interchange. The improved wrinkle recovery of the wool fabrics could have been caused by: (a) more disulfide (–SS–) groups; (b) rearrangement of the –SS– groups, which is aided by the reducing agent (c) both (a) and (b). This can be attributed to the introduction on the reduction process between the 2-IT treatment and the oxidation process. When the difference in the open angles of the treated and untreated (control) wool fabrics was over 10°, the wrinkle recovery of the wool pants caused by wearing was improved.

The silk fabrics treated with 0.04 wt% 2-IT exhibited a decent wrinkle recovery. However, the wrinkle recovery of the silk fabrics did not improve with the increase in 2-IT concentration. In addition, the S content (0.20%) of the silk fabrics treated with 0.04 wt% 2-IT increased compared with the S content (0.08%) of untreated silk fabrics. This indicates that –SH groups were introduced into the silk fabrics by the oxidation after 2-IT treatment.

According to these experiments, the direct introduction –SS– groups using 2-IT into silk and wool fibers is an effective means of improving wrinkle recovery without changing the physical properties of natural protein fabrics.

Footnotes

Acknowledgements

The authors thank Ms. K. Okuyama, and Ms. Y. Okano for their experimental assistance.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by grant-in-aid for Scientific Research (C) (19K02322) from the Japan Society for the Promotion of Science (JSPS).

ORCID iD

Akio Kuzuhara

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