An ecofriendly bleaching method for cashmere fiber with hydrogen peroxide and laccase in SCF-CO₂ by avoiding heavy effluent discharge

3.1.1 pH effect

We investigated the impact of the working solution pH on the bleaching efficiency for cashmere sample by using the one-step enzymatic-oxygen bleaching method in the SCF-CO₂ system. We varied the pH in the working solution from 5 to 10, which contained 0.3 g LA, 0.6 g PEG1000, 20 mL H₂O₂, and 20 mL buffered solution, under a pressure at 21.0 MPa and a bleaching temperature at 55 °C for a duration of 60.0 min. Fig. 2 shows the obtained whiteness and alkali solubility of the bleached samples.

Close

Fig. 2

Impact of the working solution pH value on the alkali solubility ( ) and whiteness ( ) of cashmere fiber bleaching according to the one-step supercritical enzymatic-oxygen method for 60 min at a pressure of 21.0 MPa and a temperature of 55 °C, using a solution with 0.6 g PEG1000, 20.0 mL buffer solution, 0.3 g LA, and 20.0 mL H₂O₂ in SCF-CO₂.

Export to PPT

As the pH value increased from 5 to 10, Fig. 2 shows that the pH of the working solution notably affected the bleaching efficiency of the cashmere sample in the system. Particularly, Fig. 2 shows that when the working solution pH value increased from 5 to 8, the sample whiteness gradually improved; however, as the pH value increased over 8 to 10, no further improvement was observed. Therefore, we determined that by appropriately increasing the working solution pH value in the supercritical bleaching system, the removal and decomposition of natural colorants and/or other impurities had a positive effect on the cashmere fiber.

In aqueous solution, H₂O₂ is a weak acid to generate different active spices by changing the pH value of the bleaching system, which effectively alters the decomposition rate and stability of H₂O₂, and then varies its ability to bleach the cashmere fiber efficiently. Whereas, H₂O₂ is usually relative stable in acidic and room temperature conditions. Therefore, its removal efficiency in the supercritical bleaching treatment was poor, since its decomposition was slow to generate spices in an acidic environment to oxidize natural colorants. Consequently, the whiteness of the cashmere sample was poor when the working solution with a pH value at 5–6. After the solution pH increased to a neutral environment at about 7, in an aqueous environment, the H₂O₂ could easily be ionized to H⁺, HO₂^–, HO^• and $O_2^{2-}$ , as shown in Eqs. (3), (4) and (5) (Liu et al., 2019; Abdel-Halim and Al-Deyab, 2013; Zheng et al., 2021). Thus, the generated active species of HO₂^–, HO^•, etc., could oxidize colorants and other impurities on the cashmere fiber surfaces to improve the fiber whiteness. Moreover, the H₂O₂ in the bleaching system quickly decomposed and generated more HO₂^–, HO^• and $O_2^{2-}$ spices as the pH value increased to 8, namely at an alkaline condition, as shown in Eqs. (5), (6), and (7) (Liu et al., 2019; Abdel-Halim and Al-Deyab, 2013; Zheng et al., 2021). These active spices of HO₂^–, HO^•, etc., demonstrated good ability to oxidize impurities and colorants on the cashmere fiber surfaces (Gonçalves et al., 2014). Thus; as more active spices generated and transferred onto the fiber surfaces, the more impurities and colorants could be oxidized and catalyzed for degradation. Thus, we achieved the enhanced bleaching efficiency and notably improved whiteness when the pH value increased to 8, as shown in Fig. 2. As the pH value continued to increase over 8, however, we did not observe a continuous improvement in whiteness. Probably, the improved strong alkaline environment caused the H₂O₂ to decompose too quickly, thus accelerating the ineffectiveness of bleaching system’s rate of decomposition of H₂O₂, as shown in Eq. (8).

As shown in Fig. 2, when the pH value increased from 5 to 8, alkali solubility slowly increased. Then as the pH value increased even more, we observed an additional enhancement, which indicated that less damage was caused to the integrated fiber under relatively mild conditions with a bleaching pH value higher than 5 to 8. Otherwise, the fibers would seriously deteriorate. In addition, these results also indicate that with the bleaching oxidation of colorants on the cashmere fiber surfaces, some components, macro-chains and/or bonds on the cashmere fiber itself could also be affected, which increase its alkali solubility; particularly, an excessive pH value in the bleaching system caused a significant damage to cashmere fibers. As s result, a pH value at about 8 was suitable to achieve relatively high whiteness while limiting deterioration in the fiber in the one-step supercritical bleaching system.

3.1.2 Effect of PEG1000

We investigated the effect of PEG1000 on the bleaching efficiency of the one-step enzymatic-oxygen system for cashmere fiber in SCF-CO₂ system. We set the bleaching temperature at 55 °C for 60.0 min under a pressure at 21.0 MPa. Different PEG1000 dosages were utilized for investigation in the working solution which contained 0.3 g LA, 20 mL H₂O₂, and 20 mL buffered solution along with a pH value at 8. Fig. 3 shows the whiteness and the alkali solubility of the bleached samples.

Close

Fig. 3

Impact of the PEG1000 dosage on the alkali solubility ( ) and whiteness ( ) of cashmere fiber bleaching according to the one-step supercritical enzymatic-oxygen method at a temperature for 60 min at a pressure of 21.0 MPa and a temperature of 55 °C, by using a solution with an adjusted pH value to 8 with 20.0 mL buffer solution (pH=8), 0.3 g LA, and 20.0 mL H₂O₂ in SCF-CO₂.

Export to PPT

When we used PEG1000 in the one-step enzymatic-oxygen bleaching system in SCF-CO₂ system, we observed the improved whiteness of the bleached sample by increasing the dosage from 0.0 to 0.8 g. As we increased the PEG1000 dosage from 0.8 g to 1.0 g, however, whiteness remained constant. In general, PEG1000 is not only a good solvent as well as a surfactant for extensive utilizations in industries, but also it is a nonionic phase transfer reagent possessing a certain solubility in SCF-CO₂, especially with the presence of some cosolvents (Heldebrant and Jessop, 2003; Weidner et al., 1997; Mishima et al., 1999). Therefore, PEG1000 was instrumental to the formation of (H₂O₂ + LA+PEG1000 + H₂O)/SCF-CO₂ system in the supercritical fluid step by step, as the supposed supercritical bleaching mechanism in Scheme 1 (a → c). This system can be mutually soluble with a variety of organic or inorganic substances with evenly and stably distributed in the whole fluid system (Heldebrant and Jessop, 2003; Weidner et al., 1997; Mishima et al., 1999). Particularly, some interactions such as weak Coulomb forces, hydrogen bonds and van de Waals forces, etc., between the PEG1000 chains and the active species (such as the HO₂^–, $O_2^{2-}$ , HO^•, etc.) might be occurred, so as to realize and improve the transfer of those effective bleaching ingredients from the bleaching working solution to the cashmere sample, as the supposed mechanism in the Scheme 1 (b, c and d). Therefore, with the presence of phase transfer reagent of PEG1000, H₂O₂ and LA in the supercritical bleaching system, those colorants and other impurities on cashmere fiber were readily removed and oxidized, as the depicted mechanism in the Scheme 1 (d and e). As the PEG1000 dosage increased to 0.8 g, the increased distribution of the PEG1000 molecules in the SCF-CO₂ system were able to transfer more active bleaching ingredients (HO₂^–, HO^•, or H₂O₂, etc.) to the fiber surfaces, oxidizing and decomposing more colorants. As the PEG1000 dosage increased over 0.8 g, the whiteness did not continue to improve probably because the transfer of the H₂O₂ and LA spices reached equilibrium in SCF-CO₂ system. As a result, additional PEG1000 did not transfer more active substances for further improvement in the fiber bleaching efficiency in the system.

Close

Scheme 1

Supercritical bleaching mechanism for cashmere according to the procedures of the proposed method: (a) PEG1000 with CO₂ fluid to form the PEG1000/SCF-CO₂ system; (b) the PEG1000/SCF-CO₂ system with the bleaching ingredients; (c) LA, H₂O₂, and active spices (e.g. HO₂^–, $O_2^{2-}$ , HO^•, etc.); (d) active species and bleaching ingredients after being transferred onto the cashmere fibers; and (e) enzymatic-oxygen bleaching with SCF-CO₂.

Export to PPT

As shown in Fig. 3, when the PEG1000 dosage increased from 0.0 g to 1.0 g, the effect on the sample alkali solubility was reduced. These phenomena indicate that the PEG1000 application in the one-step enzymatic-oxygen bleaching system exhibited a protection effect on the substrate, accompanying with a mild and slight damaging on cashmere fiber. Therefore, to improve whiteness and protect the fiber from damage, a PEG1000 dosage of 0.8 is recommended in the one-step supercritical enzymatic-oxygen system.

3.1.3 Effect of buffer solution

We examined the impact of buffer solution dosage on the bleaching efficiency for cashmere fiber by using the one-step supercritical enzymatic-oxygen bleaching method. We set the bleaching temperature at 55 °C for 60.0 min under a system pressure at 21.0 MPa. We changed the dosage of the buffer solution (adjusted pH value to 8), which included 0.3 g LA, 0.8 g PEG1000, and 20 mL H₂O₂. The results are shown in Fig. 4.

Close

Fig. 4

Impact of the buffer solution dosage on the alkali solubility ( ) and whiteness ( ) of cashmere fiber bleaching according to the one-step supercritical enzymatic-oxygen method for 60 min at a pressure of 21.0 MPa and a temperature of 55 °C, by using a solution with an adjusted pH value to 8 with 0.8 g PEG1000, 0.3 g LA, and 20.0 mL H₂O₂ in SCF-CO₂.

Export to PPT

Fig. 4 shows that the whiteness of the cashmere samples was enhanced when the buffer solution dosage increased to 20 mL in the supercritical bleaching system. We then observed a tendence to decrease as the buffer solution dosage continued to 35 mL. Theoretically, buffer solution can stabilize the pH of the bleaching working solution and supply enough concentrations of bleaching substances and/or active species in the system. In the one-step supercritical enzyme-oxygen bleaching system, the buffer solution should be able to stabilize the pH of the working solution to stabilize the bleaching substances. This is particularly true for generated reaction species in the working solution, as well as in the supercritical fluid media. Additionally, LA expression requires a small amount of water to trigger biological oxidation. Therefore, by increasing the buffer solution dosage from 10 mL to 20 mL, the bleaching working solution not only efficiently generated stable active bleaching spices by promoting the decomposition of H₂O₂ but also improved the activity of LA. As a result, the whiteness was significantly increased and improved the bleaching efficiency, as shown in Fig. 4. Sample whiteness did gradually reduce, however, as the buffer solution dosage increased to more than 20 mL. Excess water likely caused the concentrations of the bleaching ingredients to decrease gradually in the working solution and in the inner cores of the supercritical microemulsion. Therefore, their bleaching activities, as well as LA activity, were hindered on the sample surfaces.

Fig. 4 also shows a slow uptick in alkali solubility when the buffer solution dosage increased to 25 mL. We observed an even more significant enhancement when the buffer solution was greater than 25 mL. When the application of the buffer solution was appropriate, it not only removed various colorants on the fibers but also reduced the damages to the cashmere fibers. It is likely that the LA activity and the rate of H₂O₂ decomposition remained relatively stable, which enabled the bleaching reactions to remove colorants and other impurities from the cashmere fiber. However, the obvious enhancement in the sample alkali solubility with the buffer solution dosage further increased was probably due to the more excess alkali buffer solution, particularly some excess active species, transferred on the fiber sample which might cause some more decompositions on the macrochains of the fiber. To sum up, an application and 20 mL dosage of buffer solution could be implemented according to the sample whiteness and the alkali solubility during the supercritical bleaching for cashmere fibers with the one-step enzymatic-oxygen method.

3.1.4 H₂O₂ effect

We investigated the impact of H₂O₂ on the bleaching efficiency for cashmere sample in the one-step enzymatic-oxygen bleaching system. We set the bleaching temperature at 55 °C for 60.0 min under a bleaching pressure of 21.0 MPa. We used different H₂O₂ (30 %) dosages in the working solution (adjusted pH value to 8), which included 0.3 g LA, 0.8 g PEG1000, and 20 mL buffered solution. The achieved results are as shown in Fig. 5.

Close

Fig. 5

Impact of the H₂O₂ dosage on the alkali solubility ( ) and whiteness ( ) of cashmere fiber bleaching according to the one-step supercritical enzymatic-oxygen method for 60 min at a pressure of 21.0 MPa and a temperature of 55 °C, by using a solution with an adjusted pH value to 8 with 0.8 g PEG1000, 20.0 mL buffer solution, and 0.3 g LA in SCF-CO₂.

Export to PPT

Fig. 5 illustrates the remarkable improvement in whiteness when the H₂O₂ dosage increased to 20.0 mL. When the H₂O₂ dosage increased from 20.0 mL to 50.0 mL, however, whiteness decreased. H₂O₂ was present in the aqueous solution primarily in an alkaline environment as HO₂^– through the ionization shown in Eq. (3), which effectively oxidized natural colorants thanks to its strong oxidizing power. The formation of more HO₂^–, HO^• and $O_2^{2-}$ , was achieved by increasing the H₂O₂ dosage following the reactions given in Eq. (3), (5) and (6). The phase transfer reagent, PEG1000, was able to transfer more active bleaching ingredients when the H₂O₂ dosage increased to 20 mL in the supercritical system. Then it was possible to degrade and remove additional colorants and impurities from the cashmere substrate, thus improving whiteness. When the H₂O₂ dosage exceeded 20 mL in the working solution, however, whiteness decreased because the self-decomposition of H₂O₂ was effective, as shown in Eq. (8). Excessive H₂O₂ also caused oxidative degradation of the substrate fiber during one-step supercritical enzymatic-oxygen bleaching. As a result, whiteness deteriorated, which resulted in a yellowing of the cashmere fiber sample.

Fig. 5 also demonstrates the sample alkali solubility notably improved when the H₂O₂ dosage increased to 50.0 mL, in particular, when the H₂O₂ dosage exceeded 20.0 mL. The H₂O₂ dosage likely increased the alkali solubility because of an adverse oxidation effect on the fiber, which might have been caused by an excessive amount of H₂O₂, thus damaging the cashmere fiber. Therefore, an appropriate dosage of H₂O₂ is preferred to protect the cashmere in the supercritical enzymatic-oxygen bleaching process. To achieve whiteness and limit fiber damage, as an oxidizing agent, an H₂O₂ dosage of 20.0 mL is recommended to achieve high bleaching efficiency for cashmere fiber by using the one-step supercritical enzymatic-oxygen bleaching method.

3.1.5 LA effect

We examined the impact of LA application on the bleaching efficiency for cashmere sample by using the one-step enzymatic-oxygen bleaching method in the system. We set the bleaching temperature to 55 °C for 60.0 min under a pressure of 21.0 MPa. We changed the LA dosage in the working solution (adjusted pH value to 8) which included 0.8 g PEG1000, 20 mL H₂O₂, and 20 mL buffered solution. The results are as shown in Fig. 6.

Close

Fig. 6

Impact of the LA dosage on the alkali solubility ( ) and whiteness ( ) of cashmere fiber bleaching according to the one-step supercritical enzymatic-oxygen method for 60 min at a pressure of 21.0 MPa and a temperature of 55 °C, by using a solution with an adjusted pH value to 8 with 0.8 g PEG1000, 20.0 mL buffer solution, and 20.0 mL H₂O₂ in SCF-CO₂.

Export to PPT

Fig. 6 shows that sample whiteness was enhanced significantly with the application of LA. We increased its dosage to 0.5 g by achieving a constant value until we utilized a dosage of 0.9 g. In general, some of the colorants on the cashmere fibers were covered by the scale layers, which can’t be oxidized and destroyed efficiently and completely by those transferred active species. Additionally, the chemical compositions and structures for most of the colorants are relative to phospholipid-sterol bilayer envelopes and carrier membranes. Thus, as a redox enzyme, LA can readily change the structures of the colorants as well as those envelopes or membranes to a certain degree at an appropriate condition (Wang et al., 2018; Lan et al., 2010), which help those colorants can be easily oxidized and removed by the various bleaching ingredients in the system. When the LA dosage was increased to 0.5 g, efficient removal of those colorants was achieved through oxidative degradation, by reaching a maximum whiteness value at 82.26 for the cashmere sample in the one-step supercritical enzymatic-oxygen bleaching system.

As shown in Fig. 6, alkali solubility of the sample followed an upward trend as the LA dosage increased to 0.5 g. With the application of LA in the supercritical enzymatic-oxygen bleaching system, natural colorants were efficiently oxidized and removed from the surfaces of the cashmere fiber sample and the cashmere fibers also had less damage. Obviously, an appropriate application of LA in this developed supercritical bleaching system is also necessary and important. Thus, to achieve satisfactory whiteness and limit damage to the cashmere fiber, an LA dosage of 0.5 g is recommended.

3.2.1 System pressure

We explored the effect of system pressure on the bleaching efficiency of cashmere sample by employing the one-step enzymatic-oxygen bleaching method. We set a system temperature of 55 °C for 60.0 min and applied different pressures with the working solution (adjusted pH value to 8) which included 0.5 g LA, 0.8 g PEG1000, 20 mL H₂O₂ (30 %), and 20 mL buffer solution. The results are shown as in Fig. 7.

Close

Fig. 7

Impact of the bleached pressure on the alkali solubility ( ) and whiteness ( ) of cashmere fiber bleaching according to the one-step supercritical enzymatic-oxygen method for 60 min at a temperature of 55 °C, by using a solution with an adjusted pH value to 8 with 0.8 g PEG1000, 20.0 mL buffer solution (pH=8), 0.5 g LA, and 20.0 mL H₂O₂ in SCF-CO₂.

Export to PPT

Fig. 7 illustrates the significant improvement in bleaching whiteness of the cashmere sample when the system pressure increased from 9.0 MPa to 18.0 MPa. We observed a constant value and then a small decrease in whiteness when the system pressure increased to 23.0 MPa. System pressure has a significant effect on solute solubility, which forms the transfer system easily because of enhanced density (Heldebrant and Jessop, 2003; Weidner et al., 1997; Mishima et al., 1999). Therefore; a higher system pressure increased the solubility of PEG1000 in SCF-CO₂, and resulted in the improving transfer efficiency of the active bleaching ingredients onto the fiber substrate. Moreover, the PEG1000 could be distributed more evenly in the treatment unit under higher system pressure, which resulted in more active spices accessed uniformly onto the surfaces of cashmere fibers for the oxidation of colorants. However, no continuous enhancement or even a slight decrease in the whiteness as a further increase in the system pressure over 18 MPa, was probably due to a fact that the excessive pressure was not contributed to further improve the solubility of the PEG1000, and/or showed some affects on the generation of the bleaching ingredients as well as on the LA activity in some degree.

Fig. 7 also shows that the alkali solubility increased by indicating some more damages to the cashmere fiber as pressure increased to 23.0 MPa. Particularly, if system pressure was higher than 18 MPa, which notably increased the sample alkali solubility. At an excessive system pressure, it is likely that undesirable interactions occurred between the fiber macrochains and the active oxidative spices as well as the system media. In brief, the system pressure had a remarkable influence on bleaching efficiency of the cashmere fiber. If the bleaching pressure was appropriate, it was able to remove the colorants and protect the substrate fiber simultaneously by employing the one-step enzymatic-oxygen bleaching method. Therefore, a bleaching pressure at 18.0 MPa is recommended according to sample whiteness and alkali solubility.

3.2.2 System temperature

We examined the impact of system temperature on the bleaching efficiency for cashmere fiber by using the one-step enzymatic-oxygen bleaching method in the SCF-CO₂ system. We applied different system temperatures under a pressure of 18.0 MPa for 60.0 min, by using a working solution (adjusted pH value to 8) that included 0.5 g LA, 0.8 g PEG1000, 20 mL H₂O₂ (30 %), and 20 mL buffered solution. The results are depicted in Fig. 8.

Close

Fig. 8

Impact of the bleached temperature on the alkali solubility ( ) and whiteness ( ) of cashmere fiber bleaching according to the one-step supercritical enzymatic-oxygen method for 60 min at a pressure of 18.0 MPa, by using a solution with an adjusted pH value to 8 with 0.8 g PEG1000, 20.0 mL buffer solution, 0.5 g LA, and 20.0 mL H₂O₂ in SCF-CO₂.

Export to PPT

Fig. 8 illustrates that the whiteness of the bleached cashmere sample increased from 72.51 to 82.26 as system temperature increased from 35 °C to 55 °C. When the temperature exceeded 55 °C, however, whiteness decreased. When the bleaching temperature was appropriate, it had a positive effect on the decomposition and removal of colorants on the cashmere sample. The generation rate of active spices from LA and H₂O₂ in the supercritical system was improved by increasing the temperature appropriately. Then it was possible to transfer powerful oxidative spices onto the cashmere fiber surfaces to oxidize and remove colorants and other impurities, which enhanced the supercritical bleaching efficiency, as shown in Fig. 8. Whereas, the sample whiteness likely decreased when the system temperature was higher than 55 °C because the excess temperature inhibited LA activities and enhanced fiber damage as well as made it yellow via some serious oxidative attacks from the active spices. Obviously, applying an appropriate temperature plays a crucial role in the cashmere bleaching by employing the one-step enzymatic-oxygen bleaching method in SCF-CO₂.

Fig. 8 also illustrates the increase in alkali solubility of the bleached cashmere samples when the system temperature was between 35 °C and 55 °C. Especially, when the system temperature rose from 55 °C to 85 °C, alkali solubility increased rapidly. The rate of H₂O₂ decomposition and its active spices generation increased with system temperature. Then various oxidants were produced in the bleaching system quite quickly. Therefore, increasingly intense oxidation reactions and attacks damaged the fiber substrate other than the colorants by the transferred active spices at the overhigh system temperatures. As a result, alkali solubility tended to increase as bleaching temperature increased, which was particularly notable when the bleaching temperature exceeded 55 °C. Additionally, system temperature could show remarkable influences not only on the removing various colorants of the cashmere sample but also on the fiber itself damages simultaneously. Thus, an appropriate temperature at 55 °C is recommended for the cashmere by utilizing the one-step enzymatic-oxygen bleaching method in SCF-CO₂ system.

3.2.3 Bleaching duration

We examined the impact of bleaching duration on the bleaching efficiency for cashmere fiber by using the one-step enzymatic-oxygen bleaching method. We set the temperature to 55 °C under a pressure of 18.0 MPa with different bleaching durations in a working solution (adjusted pH value to 8) including 0.5 g LA, 0.8 g PEG1000, 20 mL H₂O₂ (30 %), and 20 mL buffered solution. All results are shown in Fig. 9.

Close

Fig. 9

Impact of the bleached duration on the alkali solubility ( ) and whiteness ( ) of cashmere fiber bleaching according to the one-step supercritical enzymatic-oxygen method at a pressure of 18.0 MPa and at a temperature of 55 °C, by using a solution with an adjusted pH value to 8 with 0.8 g PEG1000, 20.0 mL buffer solution, 0.5 g LA, and 20.0 mL H₂O₂ in SCF-CO₂.

Export to PPT

Fig. 9 illustrates that a prolonged bleaching duration from 30.0 min to 180.0 min caused a significant influence on bleaching whiteness. When the bleaching duration increased to 60.0 min, we achieved a remarkable enhancement in the whiteness. Further improvement was also observed and then a constant value was maintained as the bleaching duration reached and then exceeded 90.0 min. Generally, with a longer bleaching duration, it was possible to generate more active spices and additional cycles of supercritical fluid circulation. Thus, it was also possible to transfer more LA and H₂O₂ active spices for bleaching to the supercritical system and then onto the surfaces of the samples. As a result, additional oxidative reactions occurred between the colorants (as well as other impurities) and the active spices for degradation and removal, which enhanced bleaching whiteness significantly when duration increased from 30.0 min to 60.0 min and then from 60.0 min to 90.0 min. After 90.0 min, however, the bleaching efficiency did not improve, probably because the bleaching reaction reached equilibrium between the colorants (as well as other impurities) and the active ingredients. Fig. 9 also shows that alkali solubility of the sample was gradually enhanced as the bleaching extended from 30.0 min to 180.0 min. By ensuring that the bleaching duration was suitable, a relatively low degree of fiber damage could be considered. In conclusion, bleaching duration had a significant effect on the whiteness of the cashmere fiber in the supercritical enzymatic-oxygen bleaching system, and then caused a mild degree of fiber deterioration. A bleaching duration of 60.0 min or 90.0 min not only improved the sample whiteness but also reduced fiber damage for the cashmere substrate by employing the one-step supercritical enzymatic-oxygen method.

3.3.1 Surface morphology of the bleached cashmere fiber

We used SEM (S-4800, Hitachi) to further validate the efficiency of the one-step supercritical enzymatic-oxygen bleaching method. We examined the morphologies of the cashmere fibers before and after bleaching, as shown in Fig. 10(a1–a3) and Fig. 10(b1–b3).

Close

Fig. 10

Control cashmere fiber samples (a-1) before bleaching and SEM images at a magnification of (a-2) × 3000 and (a-3) × 5000; (b) Supercritically bleached fibers at a temperature of 55 °C and a pressure at 18.0 MPa for 90 min with the optimized working solution in the (b-1) SCF-CO₂ system and the SEM images at a magnification of (b-2) × 3000 and (b-3) × 5000.

Export to PPT

Fig. 10(a-1), (a-2), and (a-3) show that the control cashmere fibers before bleaching exhibited a yellowish color and their surfaces were relative clean by involving less other impurities such as some sediments, natural oils and animal dander, after a previous supercritical pretreatment in our previous work (Wang et al., 2024). Natural surface stripes and special scale layers of the control cashmere fiber were evident. Fig. 10(b-1), (b-2), and (b-3) show that the cashmere fiber surfaces featured thin circular scales and neat, clean scale surfaces along with natural stripes after being bleached in SCF-CO₂. Particularly, the heavy yellowish colors on the control fiber sample were disappeared, and the cashmere sample became pure white, which indicate that this developed supercritical one-step enzymatic-oxygen bleaching method played a fantastic role in the degradation and removal of those colorants and/or other impurities as well as imparted less damages on the fiber surfaces.

Because PEG1000 is an amphiphilic phase transfer reagent, it can form a special type of PEG1000/SCF-CO₂ dispersion system in SCF-CO₂. Accordingly, this system can dissolve and disperse organic and inorganic substances with a stable distribution across the supercritical system to achieve the efficient transfer of active spices from the working solution into the supercritical hydrophobic media. Along with PEG1000 and under appropriate conditions, LA and H₂O₂ as well as their generated active spices transferred onto the cashmere fiber surfaces and triggered oxidative bleaching reactions to degrade and remove colorants and/or other impurities during supercritical fluid circulation, as shown in Scheme 1. Briefly, we further validate that pure whiteness of bleached cashmere and intact fiber surface morphologies are possible as cashmere fibers are bleached by following the one-step enzymatic-oxygen bleaching method in SCF-CO₂.

3.3.2 Surface elemental compositions of the bleached cashmere fiber

To explore the efficiency of the one-step supercritical enzyme-oxygen bleaching method further, we also used EDS to investigate the surface chemical components of cashmere fiber before and after bleaching, as shown in Fig. S1(A) and Fig. S1(B) and Table 1.

Fig. S1(A) and Fig. S1(B) show the different element distributions on the control and on the bleached cashmere fiber surfaces, respectively. According to Fig. S1(A, a → g) the elements C, N, O, Si, S, and Al were distributed on the cashmere fiber surface, which indicated that these relative clean and neat fiber surfaces had very fewer impurities still. According to Fig. S1 (B, a → f), the cashmere surface was increasingly pure after bleaching with the one-step supercritical enzymatic-oxygen method. Then we observed a significant reduction in the remaining elemental species distributions and only detected C, N, O, and S on the bleached fiber surface. Table 1 shows that other than the basic fiber elements C, H, O, N, and S, only Al, the extraneous element, was identified on the control cashmere fiber surface, which indicating that some inorganics, etc., still remained on the control fiber. However, compared with the control sample in Table 1, we did not detect the extraneous element or its relative compounds on the cashmere sample after supercritical bleaching. In addition, we also found that N, and S were reduced significantly, which likely was due to the removal and decomposition of natural colorants and/or other impurities from the cashmere fibers. Therefore, the developed bleaching method provided excellent efficiency in the degradation and removal of colorants and other impurities from the cashmere sample.

3.3.3 Tensile strength property of the bleached cashmere fiber

We used an Instron5967 universal material testing machine, to evaluate tensile strength of the cashmere fiber before and after bleaching with the developed method in SCF-CO₂. Then we further validated the methodology according to properties of the bleached cashmere fiber, as shown in Fig. S2 and Table 2.

Fig. S2 shows the tensile fracture of cashmere fibers before and after bleaching in the SCF-CO₂ system, which satisfied Hooke's law. Tensile fracture of cashmere fibers occurred in three stages. First, the large molecular chain of cashmere fiber was deformed in a relatively low stress range, which produced a small strain and caused the tensile curve of the cashmere fiber to from a straight line, as the A district depicted in Fig. S2. Second, when stress increased, a continuous and prolonged strain occurred, which broke the secondary valence bonds of the cashmere fiber in the amorphous regions and caused evident macrochain slips, as the B district described in Fig. S2. Third, the cashmere fiber underwent a tensile breaking as stress increased because the macromolecular backbone and secondary bonds completely broke, as the C district shown in Fig. S2. The stress–strain curve of the bleached fiber moved to a lower position along the strain axes than the control cashmere fiber. Except for a slight lower breaking stress recorded in Fig. S2, this shift was accompanied by nearly closed stress–strain tendency with the control fiber. These phenomena were probably due to some slightly integral damages occurred to the cashmere fiber, particularly for its internal macrochain compactness, which led to the slightly reduced strength and the shift of the fracture curve.

According to the results in Table 2, the tensile breaking strength of the bleached fiber decreased from 6.82 N to 6.01 N compared with the control. This decreased in breaking strength was accompanied by an improvement in elongation from 46.76 % to 47.39 %. Accordingly, we verify that the one-step supercritical enzymatic-oxygen method for cashmere bleaching only slightly affected the mechanic property of the fiber. As a result, under suitable conditions, this developed supercritical bleaching method could achieve efficient bleaching efficiency and satisfactory mechanic property for cashmere fibers in SCF-CO₂.

3.3.4 Thermal property of the bleached cashmere fiber

We also used TG-DTG (SDT2960) to investigate the thermal properties of cashmere samples before and after bleaching according to the developed method, in order to further validate the applicability of this methodology in practice, as shown in Fig. S3 and Table 3.

According to Fig. S3(a) and Fig. S3(b), the TG-DTG analysis shows that the thermal property of the bleached cashmere fiber was similar to the control. Additionally, the thermal decomposition curves and stages were closely aligned without any obvious changes. Additionally, Fig. S3(a) and Fig. S3(b) also show that the mass fractions of the cashmere samples slightly decreased in the temperature range of 25 °C to 100 °C, and we also observed that each sample had a weak peak in the DTG curve which was caused primarily by water evaporation. We also observed that weight loss in each sample was significantly accelerated in the temperature range of 200 °C to 400 °C. Because of the large number of internal chemical bonds that were broken, and then the fiber macrochains were seriously degraded, which caused the low molecular substances to become volatile and finally led to the significantly improved weight loss. Moreover, between 250 °C and 350 °C, we observed that each sample had a similar thermal decomposition rate and shape. Additionally, the fastest rate of weight loss was also indicated by a strong decomposition peak with the temperature over 200 °C to 400 °C in the corresponding DTG curves.

Table 3 shows the detailed results for the TG-DTG analysis. We obtained similar results for the bleached cashmere sample and the control one with a weight loss at 550 °C (%), a residual weight at 600 °C, and the fastest decomposition point temperature (T_p). These results clearly demonstrate that the generated and transferred active bleaching spices in the supercritical system showed some good selection to those natural colorants and/or other impurities on the cashmere substrate, which imparted very less effect on the fiber thermal properties on the whole. Consequently, the results of this study also verified the possible application of this developed method for cashmere bleaching in SCF-CO_2.