Influence of Ultraviolet Irradiation and Protease on Scale Structure of Alpaca Wool Fibers

2.1 Materials

The alpaca wool used in this paper was 90.8 mm in length and 21.6 μm in fineness provided by Jiangsu Hong textile Co., Ltd., China. Savinase 8.0 T protease was sourced from Novoxin Biomedical Co., Ltd., China. The analytical grades of hydrogen peroxide, anhydrous sodium carbonate, sodium bisulfite, and glacial acetic acid were purchased from Chemical Reagent Co., Ltd., China. The dispersing agent was obtained from Lusen Chemical Co., Ltd. Lanasu red dye was procured from Shanghai Xiawang Garment Washing and Dyeing Co., Ltd., China. Low-temperature dyeing assistant (LTA) was sourced from Laide Textile Co., Ltd., Shanghai, China. The analytical grade sodium pyrophosphate was purchased from Chemical Reagent Co., Ltd., China. The leveling agent was purchased from Jiafeng Chemical Co., Ltd., Shanghai, China.

2.2 Methods

2.2.1 UV irradiation treatment

Considering that the alpaca wool fibers were fully exposed to UV radiation, 3 g of alpaca wool fibers was aligned at one end and suspended in the UV irradiation device in each irradiation experiment. The fibers were kept in the middle of the two lamp tubes maintaining 10 cm distance from each lamp tube, and the irradiation environment was preserved as much regularly and uniformly as possible. The width of the hung fibers was only 2 cm only ensuring the fibers on the sides and at the center are exposed to the same level of UV irradiation. The UV intensity at 15 cm from 36 W at the wavelength of 254 nm was approximately 1780 mW/cm. The irradiation time was 0.5, 1, 2, and 4 hours, respectively. The untreated samples were set as the control group. The laboratory UV irradiation device setup is shown in Figure 1.

Figure 1

UV irradiation device for alpaca wool.

UV, ultraviolet.

2.2.2 UV/protease anti-shrinkage treatment

The bio-protease treatment process was performed according to the actual experimental and shrinkage-proof process conditions, referring to the procedure recommended by Novozymes Savinase as presented in Figure 2.

Figure 2

Process chart of protease treatment for alpaca wool.

2.2.3 Dyeing process

The low-temperature dyeing process prescription of reactive dyes is summarized in Table 1. The specific dyeing process was carried out following the procedure shown in Figure 3.

Table 1

Procedure of dyeing the alpaca fibers.

Chemical name	Amount
Red lanolin dye	3% (o.w.f.)
LTA	1% (o.w.f.)
Leveling agent	2% (o.w.f.)
Sodium bicarbonate	10 g/L
Acetic acid	0.6%
Dyeing pH	5–6
Anhydrous sodium carbonate	10 g/L
Fixing pH	8–8.5
Bath ratio	1:30

1. LTA, low-temperature dyeing assistant.

Figure 3

Dyeing curve of alpaca wool fiber.

2.3 Alpaca hair-related performance test

3.1 Effect of UV/protease on surface structure

SEM was used to view the scales on the surface of alpaca wool fibers treated by UV radiation and combined with UV radiation and protease at the magnification of 1000 times (100 μm). From the photograph as shown in Figures 4 and 5, it was evident that the scale layer structure on the surface of alpaca wool directly affected the shrinkage and dyeability of alpaca wool.

Figure 4

Effect of UV irradiation on scale structure for (A) pristine (B) 0.5 hours (C) 1 hour (D) 2 hours (E) 4 hours.

UV, ultraviolet.

Figure 5

Effect of protease treatment on scale structure for (A) pristine (B) 0.5 hours (C) 1 hour (D) 2 hours (E) 4 hours.

It is unblemished from Figure 4B–D that no heavyweight damage occurred on alpaca wool fiber's scale layer after 0.5, 1, and 2 hours of UV irradiation. However, with the extension of time up to 4 hours, only local scales and grooves were found on the fibrous scales. This indicates that the UV irradiation alone could not cause significant damage to the scale structure of the fibers.

From Figure 5, it is evident that, compared to the untreated fibers, the scales of the protease-treated fibers began to be damaged, the aggregation of scales had been decreased, and the discontinuity of scales had been increased and accompanied by local scales. With the prolongation of UV irradiation time, the damage of scales became intensified. Notably, in the case of Figure 5D and E, the structure of fibrous scales was damaged noticeably, and many local scales were disappeared, which made the fibers smoother. The results showed that under long-term UV irradiation, the scales were no longer compact and were degraded and passivized. The phenomena behind this type of change may be the impact of proteinase on keratin degradation of alpaca wool fibers, leading to the digestion and destruction of some scales by proteinase.

3.2 Effect of UV irradiation on the whiteness index

Alpaca wool fibers belong to the natural protein fibers in which yellowing and fiber brittleness have occurred under sunlight. UV irradiation at 254 nm causes the decomposition of amino acids in the fibers, which results in yellowing of fibers observed by the naked eye. The specific test results are summarized in Table 2.

The abovementioned results explain that the yellowing degree of alpaca wool fiber was increased with the increment of UV irradiation time.

3.3 Effect of UV/protease on fiber strength

Alpaca wool fiber has good strength and elasticity. After UV irradiation for some time, not only the fibers were degraded, brittle, and turned into yellow seriously but also the strength was decreased. After protease shrink-proof treatment, the scales of the fibers were peeled off apparently, and the strength reduced again, and the degree of deterioration was higher than that of fibers undergone with UV treatment. The specific strength test results are shown in Figure 6.

Figure 6

Strength of alpaca fibers before and after UV/protease treatment.

UV, ultraviolet.

The breaking strength of alpaca wool fibers was reduced when irradiated by UV light alone for a prolonged time, but not in an extreme manner. When treated with protease, the breaking strength of alpaca wool fibers was dropped significantly. The hydrolysis of protease on alpaca hair fibro keratin was also verified, which condensed the coverage of scales on the surface of fibers and weakened the strength of fibers. The experimental data have suggested that UV/protease treatment lessen the strength of fibers, and the impact of this reduction was accompanied by UV light. Therefore, prolonged exposure time affects the fiber strength which means that the fiber strength was decreased with the increment of exposure time.

3.4 Friction coefficient of UV/protease-treated fibers

Due to the scale structure on the surface of wool fibers, some frictional effect was prominent in the fibers, which can directly disturb the production and processing of wool fibers and the shrinkage of fabrics. UV light and protease damage to alpaca hair scale layer can also be studied by friction coefficient. The friction coefficient of alpaca wool before and after treatment is summarized in Tables 3 and 4.

From the abovementioned data, the two phenomena are more than evident. First, when the alpaca wool fibers were irradiated by UV light alone, the friction coefficient of the fibers forwardly and reversely was not changed firmly, and the friction effect was always about 15.5%. With the prolongation of irradiation time, the friction coefficient of the fibers showed a downward trend as a whole. Second, when treated with UV light/protease, the static and dynamic friction coefficients of the fibrous scales were close to each other. Although the friction coefficient also showed a deterioration tendency, this time the interval was more than previous. Besides, the friction effect was lower. Because of having severe damage in the fiber scale layer, the friction coefficient of the advancing and inverse scales was close to each other, and with the growth of UV irradiation time, the value of friction coefficient of the fibers was declined significantly. The results showed that UV light stimulated the protease to destroy the scale layer and at the presence of UV light, this destruction of the scale layer was led to the passivation of the scale layer, which was beneficial for the protease to degrade the protein in the inner layer of the scale.

3.5 Experiments on fleece shrinkage

The empirical data of the diameter of chronic balls at different periods after both actions are summarized in Table 5.

It was observed that with the raising of UV irradiation time, the diameter of the chorionic ball was enlarged slowly. However, after protease treatment, the width of the chorionic ball was amplified significantly. The reduction in cross-winding between fibers has indicated that UV irradiation promoted the digestion and decomposition of proteinase in alpaca wool scale layer by enhancing the anti-shrinkage influence of protease.

3.6 Dyeing experiment for K/S value

The K/S value is the ratio of the dye uptake (absorption coefficient) to the total dye amount (scattering coefficient). The higher the K/S value, the more the concentration of colored substances and the darker the color of the solid surface is. After the dyeing time is set to 60 minutes as the full fixing time of fibers, the K/S values measured by different dyeing samples are treated as shown in Figure 7.

Figure 7

Dyeing K/S value of alpaca wool after protease treatment on UV-treated samples

UV, ultraviolet.

3.7 Dyeing experiment for dyeing uptake

Previous test results showed that UV light destroys the scales of alpaca wool fibers. Therefore, in the study of dyeability, the dyeing changes were analyzed by comparing among the three groups only. In the experiment, the critical time points were selected as, i.e., dyeing rate at 5, 35, and 60 minutes. The change of dyeing rate was analyzed according to the curve as shown in Figure 8.

Figure 8

Dyeing rate curve of alpaca wool fibers after different treatments.

The data maps of controlled groups (single UV irradiation for 4 hours, only protease shrink-proof treatment and UV irradiation after 4 hours of protease treatment) were compared and analyzed. The different dyeing performance of alpaca wool fibers was measured by studying the changes of initial dyeing rate, dyeing rate (slope characterization), and substantial dyeing amount after various treatment.

From Figure 8, it is proved that the initial value of dyeing and dyeing rate of alpaca wool fibers treated with UV light/protease are significantly higher than those untreated with protease, and the equilibrium dyeing rate remains about 92%. The initial dyeing rate and dyeing rate of alpaca wool fibers treated with UV light were also significantly higher than those of alpaca wool fibers untreated with UV light. The results showed that the dyeing performance of the fibers was improved by UV irradiation, whereas this dyeing performance was better when protease treatment was combined with the UV irradiation system. This may be because the cell membrane of alpaca wool treated by UV light had been destroyed, which enhanced the affinity of protease to the surface of fibers and the degradation effect of protease on keratin at a time. Therefore, dye molecules were easily diffused into the fibers and colored the fibers.

3.8 Infrared spectrum analysis of alpaca hair before and after dyeing

The infrared spectra of alpaca wool fibers before and after dyeing were studied by Fourier transform infrared spectroscopy to investigate the changes in the structure of alpaca wool fibers after dyeing. The results are shown in Figure 9.

Figure 9

Infrared spectra of alpaca wool (1) before and (2) after dyeing.

The principle of long-time UV irradiation is to degrade some amino acids in the scale layer near a specific wavelength, and the purpose of protease treatment is to damage keratinase in the scale layer on the surface of the fiber by protease leading to the destruction of the structure of the scale layer. However, these treatments did not destroy the lamellar keratin of scales to produce new substances. Nevertheless, it is necessary to check whether the dyed fibers formed new products.

The infrared spectra of the untreated original fibers and the dyed alpaca wool fibers treated with UV light/protease have little difference. Infrared spectra showed only a sharp absorption peak at 1022 cm⁻¹. It was the characteristic absorption peak of cystamine sulfonate (SO₃⁻), which was caused by S–O stretching vibration [21]. The appearance of this new absorption peak has indicated that the cysteine disulfide bond in fibers was broken, indicating that dyeing forced the disulfide bond in fibro keratin to shatter and form new products.