Study on the effect of an inert atmosphere for the heat treatment coloration of wool fabric

Abstract

This article is focused on a novel method to color wool fabric via heat treatment under an inert atmosphere. It can not only give new color to the fabric but also minimize pollution, because it is water and dye-free. The effects of temperature, time and different inert atmospheres (nitrogen, argon) used in the heat treatment on wool fabric color were studied. The bending stiffness, the crease recovery angle and tensile testing were used to analyze the mechanical properties of wool fabric before and after heat treatment. The color fastness to soaping and light of wool fabric after inert atmosphere heat treatment were compared with that of traditional basic yellow dyed wool fabric. The results showed that the K/S value of wool fabric treated with a nitrogen and argon atmosphere increased with the increasing temperature and time. Under the same heat treatment conditions, the maximum K/S value of fabric heat treated under nitrogen was higher than that under argon. The bending stiffness and crease recovery angle performance were improved and positively correlated with the heat treatment temperature and time. The samples treated under the same conditions under nitrogen showed higher bending stiffness and a lower crease recovery angle than under argon. The contact angle of the wool fabric after the treatment would decrease first and then increase with the increasing temperature. The tensile strength of the wool fabric would decrease with increasing temperature and time of the heat treatment in both nitrogen and argon, and the tensile strength of the wool fabric after treatment was higher than 80% of the original tensile strength, although the breaking elongation decreased. The color fastness to soaping and light of wool fabric after inert atmosphere heat treatment were better than for the traditional basic yellow dyed wool fabric. Therefore, the use of inert atmosphere heat treatment to endow wool fabric color is a potential research direction.

Keywords

coloration wool fabric heat treatment inert atmosphere

Wool fiber is one of the oldest natural fibers and is widely used in the textile industry because of its good elasticity, hygroscopicity and thermal insulation, soft handle and reliable mechanical property.^1–4 However, due to the special hydrophobic lipid layer on the surface of the wool fiber, it causes much difficulty for wool dyeing, which requires a great deal of fresh water and energy and is a negative influence in the textile industry.^5–7 Some physical and chemical methods try to modify the surface of the wool to solve this problem, but this brings ecological and economic problems.^3,8–11 After dyeing, the color fastness is an important parameter to evaluate the quality of wool fabric. To improve the color fastness is another big problem in the wool fabric dyeing process.

Heat treatment is a novel environment friendly method for dyeing due to it being water- and dye-free.¹² Pioneers have tried to use heat treatment for dyeing cellulose materials, such as wood, to increase the color homogeneity and avoid chemical dye.^13–15 The mechanism of dyeing is attributed to the degradation and oxidation of the hemicelluloses and celluloses.^13,16,17 In the chemical fiber field, the carbon fiber precursor polyacrylonitrile (PAN) is a typical example of color change after heat treatment; the color of PAN fiber changes from white to brown after the oxidation process, and then to black after heat treatment under high temperature for carbonization.¹⁸ Some researchers also used wool for heat treatment. In Michlik's¹⁹ research, wool yarns were heated to 120–180℃ in the air; the color changed to yellow, but the mechanical properties were reduced significantly. In a previous study on wool fiber heat treated under nitrogen, the color changed to yellow successfully and, due to the protection of nitrogen, the mechanical properties did not decrease much compared to fiber heated under air.²⁰

In this article, heat treatment was used to color the wool fabric under two inert atmospheres (nitrogen and argon). This novel method will not use any chemical or synthetic dyes and does not need any water during the treatment process. The color strength (K/S) of wool fabric could be controlled by heating time, heat temperature and heat atmosphere. The bending stiffness, crease recovery angle, mechanical properties and color fastness were investigated. The results show that the fabric could be colored yellow successfully through the heat treatment method. By controlling the temperature and time, the color of the treated wool fabric can be adjusted from light yellow to brownish yellow. The fabric has reliable mechanical properties and good color fastness, and therefore shows a great potential for use in the textile industry in water-deficient areas.

Methodology

Raw materials

Industrially scoured white wool serge (2/2 twill, 230 g/m²) was supplied by Huatong Co., Ltd, China. The linear density of both warp and weft yarns was 16 × 2 tex with the thickness of 0.32 mm. The warp and weft yarns densities of the fabric were 318/10 cm and 302/10 cm, respectively.

Heat treatment procedure

In order to study the effect of different inert atmospheres on the heat treatment of wool fabrics, a quartz tube furnace (TL-1200, Nanjing Boyuntong Instrument Technology Co., Ltd, Nanjing, China) was used for the heat treatment of wool fabric in nitrogen (purity, 99.999%) and argon (purity, 99.999%) atmospheres. The gas flowed through the tube furnace at the rate of 150 ml/min during the heat treatment process. The determined temperatures were 180℃, 190℃ and 200℃ at a heating rate of 15℃/min. The heat treatment times were 3, 5 and 8 h at a certain temperature. The experimental conditions of the heat treatment and sample designation are shown in Table 1.

Table 1.

Experimental conditions for the heat treatment of wool fabric

Atmosphere	Treatment time	Heat treatment temperature
Atmosphere	Treatment time	180℃	190℃	200℃
Nitrogen	3 h	N-180-3 h	N-190-3 h	N-200-3 h
	5 h	N-180-5 h	N-190-5 h	N-200-5 h
	8 h	N-180-8 h	N-190-8 h	N-200-8 h
	3 h	A-180-3 h	A-190-3 h	A-200-3 h
Argon	5 h	A-180-5 h	A-190-5 h	A-200-5 h
	8 h	A-180-8 h	A-190-8 h	A-200-8 h

Characterization of materials

The color of wool fabric was observed by optical imaging. The color strength (K/S) of wool yarn was characterized by spectrophotometry (Color Eye 7000A, Gretag Macbeth, USA). The ultraviolet-visible (UV-vis) absorption spectra of wool fabric were recorded by a Varian Cary 5000 UV-VISNIR spectrophotometer with a diffuse reflectance accessory (DRA-2500). The bending stiffness of the fabric was characterized by a bending stiffness tester (YG207, Ningbo Textile Electronic Technology Co., Ltd, China), the sample size was 25 mm × 25 mm, the angle was set to 41.5° and five specimens were tested under each condition. The crease recovery angle of the fabric was measured by an automatic fabric crease recovery tester according to GB/T3819-1997 (YG541E, Ningbo Textile Electronic Technology Co., Ltd, China) and five specimens were tested under each condition. The contact angle was characterized by a contact angle tester (JY-PHb, Chengde Jinhe Instrument Manufacture Co., Ltd, China), the liquid was water and 10 specimens were tested under each condition. Tensile testing was used to measure the breaking strength of the fabric according to ASTM D434-1995 and five specimens were tested under each condition in both warp and weft directions with specimens of 250 mm ± 2 mm length and 50 mm ± 2 mm width. The tensile strength retention (TSR(%)) was calculated via the following equation²¹

TSR (%) = \frac{TS}{T S_{0}} \times 100 %

(1)

where TS is the breaking strength of treated wool fabric and TS₀ is the breaking strength of untreated wool fabric.

The color fastness to soaping was measured according to GB/T3921-2008. The CIE lab coordinate values (L*, a* and b*) for the fabric before and after washing were measured. L* is the lightness/darkness, the a* value represents the chromaticity coordinate for red or green and b* represents yellow/blue chroma.

The color difference before and after washing ΔE was calculated via the following equation²²

Δ E = [(Δ L^{*}) 2 + (Δ a^{*}) 2 + (Δ b^{*}) 2]^{1 / 2}

(2)

The color fastness to light was measured by color difference (ΔE) before and after exposure (XENOTEST ALPHA + test instrument, Atlas, American) according to GB/T 8427-2008. The irradiation intensity was 42 W/m², the wavelength was 300–400 nm, the temperature was less than 45℃ and the exposure time was 60 h.

Results and discussion

Coloration based on heat treatment

Two inert gases, nitrogen and argon, were used to provide a shielding atmosphere for the heat treatment of wool. The colors of wool fabrics changed to yellow from white (Figure 1) during heat treatment under both inert gases, which is consistent with our previous report.²⁰ The color of wool fabrics changed from white to light yellow, yellow and brownish yellow as the heating time was prolonged at the same temperature (e.g. 180^oC). The corresponding change trends of colors were similar for nitrogen and argon protective gases. When the treatment time was kept constant (3 h), the color of the fabric was deepened with an increase in heating temperature (Figure 1). Comparing the colors of wool fabric heat treated under different inert gases, the color of samples treated under a nitrogen atmosphere were deeper than those treated under an argon atmosphere. This is possibly caused by the reaction between nitrogen gas and wool fibers.

Figure 1.

Optical photographs of the wool fabric treated under different conditions.

The color strength (K/S) curves of wool fabric treated under different conditions are shown in Figure 2. The K/S value of the wool fabric increased gradually with increasing heat temperature or heating time under an inert atmosphere. The wool fabric showed the maximum K/S values between 365 and 381 nm under a nitrogen atmosphere. The peaks for K/S curves were located in the range of 362–379 nm for the argon atmosphere. Wool fabrics treated under both inert atmospheres showed a similar color range, which may be due to the degeneration of the protein molecules in the wool fibers.

Figure 2.

K/S curves of the wool fabric treated under different conditions: (a)–(c) nitrogen; (d)–(f) argon.

Figure 3 shows the plots of the maximum K/S value of wool fabric as a function of heating time under different temperatures and inert atmospheres. The maximum K/S value increased as the temperature was increased or the heating time prolonged. The maximum K/S values of wool fabric treated under nitrogen were higher than for those treated under argon under the same heat treatment conditions (temperature and time). For example, the maximum K/S values of wool fabric treated under nitrogen and argon (at 180℃ for 3 h) were 3.19 and 3.13, respectively. As the heat treatment time was increased to 5 and 8 h, the maximum K/S value increased by 36%, 92% for nitrogen and 31%, 87% for argon, respectively. When the treatment time was kept at 3 h and the temperature increased to 190℃ and 200℃, the maximum K/S value of wool fabric under nitrogen and argon increased by 77%, 303% and 67%, 274%, respectively. The K/S value in the range of 400–700 nm (visible light) presented a declining trend for both samples. The K/S value at around 555 nm (the most sensitive to light for greenish-yellow) showed a similar trend as the maximum K/S value. The samples treated under higher temperature or longer time had higher K/S values, which indicated that there was a deeper yellow color for the samples treated under longer time and higher temperature. It can be inferred that heating temperature and time are dominant influencing factors on the heat-induced colorations of wool fabrics.

Figure 3.

Maximum K/S values of wool fabric treated under different conditions.

The UV-vis absorbance spectra of wool fabric treated under different conditions are shown in Figure 4. As the time and temperature increased, the shoulder band broadened and red-shifted. Under the same heat treatment conditions, the absorption peak intensity of wool fabric treated under an argon atmosphere is lower than for that treated under a nitrogen atmosphere. The red-shift of the R band shows the same characteristics as the wool fiber. When the heat treatment time was 3 h and the temperatures were 180℃, 190℃ and 200℃, the absorption peaks of the R band for fabric treated under nitrogen and argon were 1.06, 1.12, 1.16 and 1.03, 1.06, 1.08, respectively. This is mainly because the protection of the argon atmosphere is more effective than nitrogen and the oxidation reaction of the carbonyl group in the tryptophan was suppressed during the heating period, which leads to the chromophore C-O group being formed by oxidation decrease.²⁰ The results of UV-vis absorption spectroscopy were consistent with the K/S results and optical photographs.

Figure 4.

The ultraviolet-visible absorption spectra of wool fibers treated under different conditions: (a)–(c) nitrogen; (d)–(f) argon.

Mechanical properties of wool fabric from heat treatment

Figure 5 shows that the bending stiffness of wool fabric increased with the increasing of heat treatment temperature and time under both atmospheres. The bending stiffness increased gradually with increasing temperature from 180℃ to 200℃ under same heat treatment time, while the bending stiffness increased significantly with the time increasing from 3 to 8 h under the same heat treatment temperature. The bending stiffness of wool fabric under an argon atmosphere shows the same trend with the temperature and time increased. Under the same conditions, the bending stiffness for fabric treated under nitrogen is higher than for that treated under argon. It is presumed that some proteins in the wool fabric were oxidized and resulted in the denaturation of protein, which makes the surface of the fabric stiff and increases the modulus. Compared to the nitrogen atmosphere, the protection of argon is stronger, which leads to the degree of oxidation being lower, leading to the fabric treated under nitrogen being stiffer.

Figure 5.

Bending stiffness of wool fabric treated under different conditions.

Figure 6 shows the crease recovery angle of the fabric treated under different conditions. As Figure 6 shows, in general, the crease recovery angles increased with increasing temperature and time. The influence of argon was stronger than nitrogen. The crease recovery angle of wool fabric under argon increased 15°and 28.7° under 180℃ and 200℃ for 3 h, respectively. The crease recovery angle increased 38.4° under 180℃ for 8 h. One of the possible reasons is that wool fabrics were oxidized, damaging the protein to make the fabric stiff and reduce elasticity.

Figure 6.

Crease recovery angles of wool fabric treated under different conditions.

The contact angle of wool fabric decreased first and then increased as the heat time and temperature increased, as Figure 7 shows. Compared with the untreated wool fabric, the contact angle of wool fabric treated under 180℃ for different times (3, 5, 8 h) did not change significantly under either nitrogen or argon atmosphere. With the temperature increasing to 190℃, the contact angle reduced slightly. When the temperature continued increasing to 200℃, the contact angle increased compared to that of the untreated sample. The scale layers on the surface of the wool fibers were slightly damaged and the lipids in the F layer were degraded at 190℃, which led to the hydrophilic critical micelle concentration (CMC) layer being exposed and reduced the contact angle. With the temperature continuing to increase to 200℃, the hydrophilic bonds on the surface of the wool fibers were gradually destroyed and the contact angles were increased.

Figure 7.

Contact angles of wool fabric treated under different conditions.

The TSR and breaking elongation are two important factors to evaluate the quality of wool fabric. The TSR and breaking elongation of untreated fabric, treated fabric and dyed fabric (Auramine O (basic yellow 2)) are shown in Figure 8. As Figure 8(a) shows, most of the TSRs reduced with increasing heat treatment temperature and prolonging heat treatment time. The TSR under argon was higher than under nitrogen under the same conditions (time and temperature). The TSR was 87.6% after 3 h at 180℃ under nitrogen. With the time increasing to 8 h, it only reduced to 82.4%. By maintaining the heat treatment time at 3 h and increasing the temperature to 200℃, the TSR was reduced to 78.3%. The TSRs of wool fabric after 3 h under 180℃ and 200℃ were 89.6% and 80.5%, respectively. The TSR of dyed fabric was 94.8%, which was higher than for the treated fabric. However, most of the TSR after heat treatment in different conditions was higher than 80%, which was still strong enough for textile industry use. It can be seen from Figure 8(b) that the breaking elongation after heat treatment reduced significantly with increasing temperature and time. The breaking elongations of untreated fabric and dyed fabric were 35% and 31.2%, respectively. After 8 h heat treatment at 200℃, it reduced to 10% and 11.3% under nitrogen and argon, respectively. It can be seen that the heat treatment has a great influence on the breaking elongation.

Figure 8.

(a) Tensile strength retention and (b) breaking elongation of wool fabric treated under different conditions.

Color fastness of wool fabric from heat treatment

The color fastness to soaping and to light are two important parameters to evaluate the quality of fabric. In order to further research it, Auramine O (basic yellow 2) was used to dye the wool fabric and make a comparison with the fabric treated under nitrogen and argon for 3 h at 180℃ and 200℃ (Fb-180-3 and Fb-200-3, respectively). The results of color fastness to soaping are shown in Figure 9(a), which shows that the color of the untreated wool fabric was almost unchanged. In addition, the color difference (ΔE) of the treated fabric under nitrogen and argon has no obvious difference. The color of Fb-180-3, Fb-200-3 and dyed wool fabric changed obviously when compared to the untreated sample. The color difference (ΔE) of fabric treated under both atmospheres is nearly the same and very close to 1.5. The color difference (ΔE) of dyed wool fabric is higher and is close to 2.25. It is indicated that the wool fabric using heat treatment under an inert atmosphere has better color fastness to soaping than the dyed fabric (Auramine O (basic yellow 2)).

Figure 9.

(a) Color fastness to soaping and (b) color fastness to light of wool fabric treated under different conditions.

Figure 9(b) shows the color fastness to light for the six samples. All of the six wool fabrics have a color change to different degrees after 60 h light treatment. The color of untreated fabric changed to light yellow and the ΔE was 2.9, which may be caused by the oxidation of the fabric. The ΔE of Fb-180-3 and Fb-200-3 were very close; both of them are smaller than for the dyed wool fabric. It can be concluded that the color fastness to light treatment under both nitrogen and argon was similar, both of which were better than for the dyed wool fabric.

Conclusion

This work has demonstrated that heat treatment under an inert atmosphere could color the wool fabric to yellow successfully. By controlling the temperature and time, the color of the treated wool fabric can be adjusted from light yellow to brownish yellow. By comparing the heat treatment under nitrogen and argon in the same conditions, the fabric obtained under nitrogen showed a deeper color. The bending stiffness and crease recovery angle performance were improved and positively correlated with the heat treatment temperature and time. The fabric treated under argon was softer and more elastic compared to the fabric treated under nitrogen. The contact angle of the wool fabric after the treatment would decrease first and then increase with the increasing temperature. In addition, the tensile strength of the wool fabric decreased with increasing heat treatment temperature and time under both nitrogen and argon atmospheres. The TSR of the treated wool fabric was generally higher than 80% that of the untreated fabric, which is a reliable strength for use in textile field. The breaking elongation also decreased. Finally, the fabric also showed a good color fastness to soaping. The heat treatment method for coloring wool fabric is an environment friendly dyeing method that does not need any chemical and synthetic dyes. The mechanical properties and color fastness of the fabric are reliable and could be used in the textile industry. The method does not need water during any part of the process, which could make it possible to color wool fabric in a water-deficient area. In conclusion, the heat treatment method for color wool fabric has a great potential for future textile fields.

Footnotes

Declaration of conflicting interests

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

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Key Laboratory of Textile Fiber & Product (Wuhan Textile University), Ministry of Education (FZXW2017002).

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