A clean and efficient processing system to improve the fluffiness of down fibers based on multi-enzymes

Abstract

Down fiber – a natural and environmentally recyclable insulation material – is applied mainly in the area of natural-filled products. However, owing to its natural deficiencies of low fluffiness and thermal stability, large-scale application is limited. In our work, an enzyme preparation (transglutaminase (TGase)) was used as a fluffy agent to improve the fluffy degree of down. With fluffy as the main index, the single factor condition of only using TGase enzyme was optimized, and fluffiness was increased by 22%. On the basis of this optimum condition, the compound dosage of papain and TGase was further optimized. The fluffy degree of down was improved by hydrolysis of papain and crosslinking of TGase. Amino group content, thermogravimetry (TG), differential scanning calorimetry (DSC), moisture absorption, tensile strength, and elongation at break of down treated with different enzyme reagents were tested. The results showed that the thermal stability of down fiber treated with the multi enzyme increased significantly, the denaturation temperature was increased from 53.7°C to 77.4°C, and the moisture absorption was also improved. The elastic property also exhibited a great enhancement; elongation at break increased from 12.4% to 37.7%, nearly three times higher than the original property, and the tensile strength increased from 180 MPa to 370 MPa, almost 2.1 times as much as before, increased by 105.6%.

Keywords

down fibers TGase cross-linking fluffiness

As a natural sustainable material, down fiber is used widely in thermal insulation textiles, especially in winter clothing, due to its superior thermal insulation performance and soft touch.¹,² At the same time, fluffiness is one of the most important indexes reflecting the quality of down fibers, which is of great significance to the extensive application of natural down fibers.³ Fluffiness refers to the volume of down fibers of a certain quality in the same container under the same constant pressure. The higher the fluffiness of down, the larger the volume of down of the same quality. Filling the fabric with high-fluffiness down will make the fabric’s insulation layer thicker, and warmth retention will be better. However, the thermal stability, mechanical strength, and structural stability of down fibers obtained directly from animals are often low, and there is the problem that the fluffiness of down fibers does not reach the standard required for use, which restricts the application of natural down fibers.¹ Therefore, it is necessary to process natural down fiber to improve its fluffiness.

In previous studies, polysiloxane compounds have been used widely in the modification of down fibers as leavening agents.⁴ However, as the polysiloxane compound is physically adsorbed only on the surface of the down fiber, the shedding of polyoxyl radical compounds after washing will lead to the decreased performance of down. In addition, some researchers found that the fluffiness of down fibers could be increased greatly by adding oxidants and surfactants.^5–7 Nevertheless, these methods are still limited, with damage to down fibers and unsatisfactory effects on improving fluffiness. Glutaraldehyde is a leather crosslinking agent also used as a fluffing agent for down.⁸ However, down fibers treated with glutaraldehyde become yellow easily, which seriously affects the quality of down products. Importantly, one of the main disadvantages of these aldehyde crosslinkers is that they are unsafe for both humans and the environment. Some researchers have used alkaline zirconium sulfate as a crosslinking agent for down fibers to improve fluffiness, and achieved good results.³ Although zirconium tanning agents and their products are less harmful to the environment, with the emergence of increasingly strict international trade rules, it is still possible that the content of metal ions in down products will be limited in the future. Therefore, researching and designing an ideal down fluffing agent suitable for down processing that does not affect the environment or damage the physical and chemical properties of the down itself will make a very important contribution to the development of the down industry.

Transglutaminase (TGase) is a clean, efficient, and ideal biological crosslinking agent. Currently, it is used widely in textiles, foods, and other fields.^9–11 TGase can effectively catalyze nucleophilic reactions and form covalent cross-links within or between protein molecules. This covalent bond is very strong and can maintain the stability and improve the elasticity of the protein.¹⁰,¹² Therefore, TGase is used commonly to repair dyed wool fibers in the textile field.^13–15 Wool fiber needs water washing and enzyme treatment before dyeing, which destroys the breaking strength and elasticity of the wool fiber to a certain extent. The catalytic cross-linking properties of TGase enable the treated wool fibers to form strong covalent bonds within or between molecules. Wool fiber damaged by chemical or enzymatic action showed a consistent and significant improvement in elasticity, shrink-proof performance, breaking strength, and hydrophilicity when treated with TGase. In the same way, the catalytic crosslinking properties of TGase can be used to improve the fluffiness of down fibers.

In this study, we used a green and efficient method to deal with down fibers that did not meet the required standard. TGase was used as a bulking agent that could be cross-linked with natural down fibers to achieve the purpose of increasing fluffiness. At the same time, papain was selected as the softening agent. Other properties of the treated down fibers were also tested. With the aim of ensuring the other properties were not affected, a set of optimal solutions were developed to improve the fluffiness of down fibers.

Materials and methods

Materials and equipment

Untreated down fibers were supplied by Anhui Wuwei DongLong Co., Ltd., China. TGase was obtained from Shanghai Aladdin Biochemical Technol. Co., Ltd., China. Papain was obtained from Suzhou Fulu Biological Technol. Co., Ltd., China. The enzyme units used in the experiment were all international units (IU; 1 enzyme activity unit refers to the amount of enzyme that can cause 1 micromole substrate transformation in 1 min under optimal conditions at 25°C). Nonionic degreaser (LQ-5) was purchased from Sichuan Decision Chemical Co., LTD., China.

The energy dispersive spectrometer (S4800) was provided by the Hitachi (Japan); X-ray diffractometer (Advance-D8) was produced by Bruker (Germany); Thermogravimetric (STA409PC) and differential scanning calorimetry (Q2000) were made by TA Instruments (New Castle, DE, USA). Inductively coupled plasma atomic emission spectroscopy (2100DV) was supplied by Perkin Elmer (USA). Single fiber strength meter (XQ-2) was obtained from Hangzhou Liangyu (China).

Methods

Down productive process

The fluffiness, cleanliness, and residual fat rate of untreated down fibers was tested before the experiment. Down processing technology is divided into the following steps.

Washing

Down fibers (100 g) were added to water (4000 ml), washed at 35°C for 20 min, and washing was repeated three times.

Degreasing

The above down fibers (100 g) were placed into 3000 ml of water with 2 g of nonionic degreasing agent. Degrease proceeded at 40°C for 1 h. After degreasing, washing was carried out three times to remove the degreasing agent from the feather fibers.

Oxidation

The defatted down fibers were added to 2000 ml of an aqueous solution containing 1.5 ml of hydrogen peroxide, and then subjected to oxidation treatment at 30°C for 20 min. Three consecutive oxidation treatments were carried out, with water washing required after each oxidation treatment.

Soften

After the oxidation treatment, the down fibers were added to 3000 ml of an aqueous solution containing a certain amount of papain. The pH of the solution was adjusted to 6 with 0.1 M HCl and 0.1 M NaOH solutions, and the softening treatment was performed at 40°C for 30 min.

Cross-linking

After the softening treatment, the down fibers were added to 2000 ml of an aqueous solution containing a certain amount of TGase, the pH was adjusted to 6, and the cross-linking treatment was carried out at 40°C for 2 h.

Drying

After the treatment was completed, the fibers were washed again, then spin-dried with a centrifugal dryer machine, and finally dried at 40°C.

After all the steps were processed, the samples were placed at 20 ± 2°C/65 ± 4% relative humidity in the testing room.

X-ray diffractometry

X-ray diffraction (XRD) measurements in the 2θ range of 5∼50° were performed with Cu Kα1 radiation (λ = 145 0.1541 nm) at 40 kV and 40 m. The crystallization performance of all the samples was measured. Samples were evaluated at least three times under each experimental condition.

Analysis of down fiber properties

Thermogravimetry

Thermogravimetry (TG) was used to analyze the stability of the sample. The samples were tested under a temperature range from 20 to 600°C at a constant heating rate of 10°C/min under a nitrogen flow. Samples were evaluated at least three times under each experimental condition.

Differential scanning calorimetry

Down samples (3.0–5.0 mg), were added into aluminum pans and sealed. Differential scanning calorimetry (DSC) was carried out in the range from 20 to 200°C with a scanning rate of 10°C/min under a nitrogen atmosphere. Samples were evaluated at least three times under each experimental condition.

Quality standard test of down fibers

The fluffiness, cleanliness, and residual fat rate of the down and feathers were tested according to the requirements of the International Down and Feather Bureau (IDFB). A total of five measurements were performed, and the averages are presented along with the standard deviation.

Detection of fluffiness

Samples (∼30.0 g) were extracted from the fluffy treated samples and dried in a 70°C oven for 45 min. The samples were then placed in the pre-treatment oven and stored for 24 h at (20±2)°C and (65±2)% relative humidity. Samples (28.5 g) were weighed, and a national standard fluffiness meter was used to measure the fluffiness.

Detection of cleanliness

Samples (10.0 g) were weighed and placed in a 2000 ml plastic jar. Then, 1000 ml distilled water was added into the plastic bottle to fully wet the down fibers. The plastic jar was placed horizontally on the oscillator for about 30 min. After the oscillation ended, the filtrate was collected and poured slowly into the transparency meter. The filtrate was discharged gradually from the hose below the transparency meter, and the red double cross line at the bottom was observed from the top of the transparency meter until it had just stopped discharging. The corresponding scale on the surface of the liquid, that is, the cleanliness of the sample, was recorded.

Detection of residual fat rate

Dried samples (2.0–3.0 g) were weighed accurately using an analytical balance, and then placed in a cleaned Soxhlet extractor. Ethyl ether (120 ml) was poured into the bottom flask, which had been weighed and cleaned. The bottom flask was placed in a 50°C water bath and connected to a Soxhlet extractor, which made the ethyl ether contra-flow 6 times per hour, for a total of 20 times. The round bottom flask was placed in an oven at 105°C until a constant weight was recorded. Residual fat rate (F_r) was calculated using the following expression:

F_{r} = \frac{A - B}{C} \times 100 %

(1)

where A is the constant weight of the round flask with residual fat; B is the weight of the clean round bottom flask; and C is the weight of the sample.

All three readings were in grams and corrected to 0.0001 g.

Breaking strength

Both ends of the down fiber were pasted on the plastic sample board; 40 single down fibers were fixed on each plastic sample board, and a small amount of epoxy resin mixture was dropped on both ends of the down fiber. The resin mixture was made by mixing epoxy resin A and epoxy resin B in a ratio of 2:1. After the above steps were completed, the sample board was placed in a 70°C oven to dry for 48 h. After the epoxy resin was completely cured, the short fiber mechanical property tester JSF08 was used to test it. All results were measured five times to obtain an averaged result. The calculation methods of elongation at break and tensile strength of down fiber are shown in equations (2) and (3) respectively:

f = \frac{l_{1} - l_{2}}{l_{1}} \times 100 %

(2)

T = \frac{F}{S} \times 1 0^{- 3}

(3)

where f is the elongation at break; l₁ is the original length (mm); l₂ the tensile length (mm); T is the tensile strength; F is the breaking load (mN); and S the area of fracture (mm²).

The degree of cross-linking of down fibers

The content of amino groups of down fibers before and after treatment can directly reflect the degree of cross-linking between down fibers and TGase. The content of amino groups is usually determined by assay.¹⁶,¹⁷ Typically, leucine is used as the standard solution to draw a standard curve. A small amount of shredded down fibers is added to the leucine solution, the absorbance was measured at 570 nm, and the amino content calculated using the drawn standard curve. The degree of cross-linking (C_m) was calculated using the following expression:

C_{m} = \frac{M_{0} - M_{1}}{M_{0}} \times 100 %

(4)

where M₀ is the content of amino group before TGase catalyzed cross-linking, and M₁ the content of amino group after TGase catalyzed cross-linking.

Determination of moisture absorption of down fiber

The moisture absorption of down fibers is used to determine the percentage of water absorbed by down fiber in the original quality of down fiber in a constant temperature and humidity environment for a certain time, which could reflect the hydrophilic groups on the surface of down fibers, and also reflect the sanitary properties to a certain extent. The moisture absorption is determined according to QB/T 1811-1993. The specific operation method is as follows: the down sample is placed in an electric blast drying oven at a temperature of 100°C and dried for 120 min, and then weighed. This weight is recorded as W₁ (g). After placing the sample in a constant temperature and humidity environment, adjusting the temperature to 25°C and humidity to 65% for a certain time, the weight was recorded again as W₂ (g). The moisture absorption is calculated using formula (5).

Moisture absorption = \frac{W_{1} - W_{2}}{W_{1}} \times 100 %

(5)

where W₁ is the quality of down fiber before placing it in a constant temperature and humidity environment; and W₂ is the quality of down fiber after putting it in a constant temperature and humidity environment.

Results and discussion

Effect of TGase enzyme dosage on down fluffiness

The dosage of TGase is one of the most important factors affecting the fluffy degree of down fiber. The crosslinking diagram is shown in Figure 1.

Figure 1

Schematic diagram of cross-linking down fibers.

If the dosage of TGase is too low, the effect of improving the fluffy of down fiber is not obvious. At the same time, excessive usage of the cross-linking enzyme TGase may cause over cross-linking of down fiber, resulting in a decrease in the fluffy score, which is also a waste of raw materials. Therefore, the relationship between the amount of TGase enzyme and down fluffiness was investigated in this experiment. The results are shown in Figure 2.

Figure 2

Effects of the dosage of glutamine transaminase on fluffiness of down fibers.

It can be seen from Figure 2 that, compared with the untreated down samples, the fluffiness of down fibers after TGase enzyme treatment has been improved to varying degrees. In a certain range, the fluffy degree of down fiber increased with the increase of cross-linking enzyme dosage. When the amount of cross-linked enzyme was 40 U/g, the fluffiness of down fiber was increased by 22% compared with untreated down. This is because TGase, as a highly effective biocatalyst, can catalyze the amido group transfer reaction between primary amine and glutamine in proteins, and TGase can form ɛ-(ɤ-glutamine) lysine bonds in down fibers through intramolecular or intermolecular cross-linking.⁹,¹⁷ The chemical bond of ɛ-(ɤ-glutamine) lysine bond has a strong force, which can be seen in the macroscopic view as a main factor in increasing the fluffiness of down fiber. However, as the dosage of cross-linking enzyme continued to increase, the fluffy degree of down fiber decreased slightly and became stable. This is because the surface of down fiber will be excessively cross-linked with increasing amounts of cross-linking enzyme, resulting in a slight decrease in the fluffy degree of down fiber. Meanwhile, the number of active groups of down fiber of a certain quality is also defined, so any improvement is not obvious as the amount of enzyme continues to increasing. Therefore, according to the actual fluffy effect seen after treatment, the dosage of TGase enzyme was selected as 40 U/g.

Effect of multi-enzyme treatment on down fluffiness

With the aim of determining the optimal dosage of TGase alone to increase the fluffiness of down, and taking fluffiness as the main index, with residual fat rate, cleanliness, and whiteness as secondary indexes, the multi-enzyme synergy method was used to optimize the amount of papain and TGase enzyme to maximize the performance of down fiber. The results are shown in Table 1.

Table 1

Effects of multi-enzyme co-treatment on the properties of down fibers

Sample	Fluffiness (cm³)	Residual fat rate (%)	Whiteness (%)	Cleanliness (mm)
Untreated	456 ± 6	1.22 ± 0.035	60.51 ± 2.68	462 ± 8
1:20	565 ± 4	0.83 ± 0.043	61.34 ± 4.38	610 ± 10
1:10	575 ± 5	0.78 ± 0.027	59.82 ± 3.25	595 ± 5
1:8	560 ± 6	0.82 ± 0.035	61.56 ± 1.95	590 ± 5
1:5	540 ± 3	0.81 ± 0.056	58.66 ± 4.21	590 ± 5
1:4	510 ± 8	0.76 ± 0.064	59.68 ± 3.52	580 ± 5
1:2	420 ± 4	0.79 ± 0.072	57.45 ± 4.37	560 ± 5

Note: The sample ratio is the ratio between papain and TGase, and the specific dosage is based on 40 U/g TGase.

It can be seen from the table that, with the increase in the ratio of papain and TGase enzyme, the fluffiness of down fiber first increased and then decreased. The main function of papain is to hydrolyze the peptide bonds in the down fibers and expose the active groups to make the down fibers reach a loose state.¹⁹ The down fiber will increase its contact with TGase after exhibiting this loose state, thereby improving the intermolecular cross-linking efficiency of the down fiber, which also improves the fluffiness of the down fiber. Because the effect of papain and TGase on down fiber can be regarded as the combined effect of first hydrolysis and then cross-linking, the binding force between down fiber molecules can be better improved. Therefore, the effect of the multi-enzyme treatment on improving the fluffiness of the down fiber is better than that of single enzyme treatment.²⁰ When the dosage ratio of papain and TGase enzyme was 1:10, the effect of improving down fiber fluffiness was best. Compared with the untreated down, fluffiness increased by 26%, and, compared with down treated only with TGase enzyme, the improvement effect was also obvious. With continued increase in the dosage ratio between the two enzymes, the fluffiness of the down fiber appeared to decrease. When the dosage was 1:2, the fluffiness of down fiber dropped from the highest 575 cm³ to 420 cm³, which was a large decrease. The main reason was that, as the amount of papain increases, the degree of hydrolysis of down fiber also increases, the polypeptide skeleton in the down fiber molecule is degraded, and a large number of peptide bonds are broken. The structure of the down fiber has been severely damaged, greatly reducing the final down fiber fluffiness. According to the actual effect of down fiber fluffiness, the dosage ratio of papain and TGase enzyme is 1:10, that is, the dosage of papain is 4 U/g and that of TGase enzyme is 40 U/g.

On the other hand, the ratio of papain to TGase had little effect on the residual fat rate and whiteness of down fiber, and the changes were not obvious. However, with the increase of the dosage ratio between the two enzymes, the cleanliness of down fiber decreased, which may be due to the use of a large number of enzyme preparations attached to the surface of the down fiber, resulting in a decline in cleanliness of the down fiber.

Amino group content analysis

TGase can catalyze the acylamino transfer between primary amine and glutamine in down fiber molecules.⁹,¹⁷ Therefore, the content of amino groups in down fibers before and after treatment can directly reflect the degree of crosslinking between down fibers and TGase. Figure 3 shows the effect of different treatment methods on the amino content of down fibers.

Figure 3

The curve of free amino of down fibers treated with softening enzyme (A), untreated down fibers (B), down fibers treated with TGase enzyme (C), and down fibers treated with multiple enzymes (D).

It can be seen from the figure that the amino content of untreated down fiber was 2.76 μg/ml. The content of amino groups in down fibers treated with papain increased to 3.31 μg/ml. The reason is that papain can hydrolyze down fibers. After hydrolysis, the molecular chains of down fibers are opened, exposing a large number of active groups at both ends, which increases the amino content. After treating with TGase alone, the content of amino groups in down fibers decreased to 2.36 μg/ml. Because TGase catalyzes the cross-linking of fibers and consumes a certain amount of active amino groups, the amino group content is reduced. Similarly, the effect of papain and TGase also greatly reduced the amino content of papain alone, which was 2.48 μg/ml. Finally, as calculated according to formula (3), the cross-linking degree of down fiber with TGase alone was 14.5%, and the cross-linking degree of down fiber with TGase after softening with papain was 25.1%. Therefore, the effect of papain and TGase can increase the cross-linking degree of down fibers, which can also better improve the fluffiness of down.

Thermogravimetric analysis

After different enzyme treatments, the internal structure of down fiber will change, and the binding forces will change accordingly, which will affect the thermal stability of down fiber. Therefore, the samples of down before and after treatment were analyzed by thermogravimetry. It can be clearly seen from Figure 4 that down fiber basically presented a three-step thermal degradation mode. In the first stage of degradation, with the heating temperature range of 25–100 °C, the weight loss of all samples was not obvious. The main reason for the weight loss is the evaporation of free water molecules and bound water molecules in the down fibers. In the second step, when the temperature increased to 250–450°C, the weight loss of all down fiber samples was very obvious. In this stage, weight loss could be assigned to hydrogen bond cracking, inclusion compound degradation, disulfide bond breaking, amino acid decomposition, and the release of some volatile gases such as sulfur dioxide, carbon dioxide, carbon monoxide, hydrogen sulfide, etc. At this stage, the greatest weight loss is seen in down fibers treated with papain only, followed by untreated down fibers, and then down fibers treated with TGase only . It is worth noting that the weight loss rate of down fiber treated with multiple enzymes is lower than that of the other samples. This phenomenon shows that the down fibers have the strongest intermolecular binding force after multi-enzyme treatment, followed by down fibers treated with TGase alone. Both of these treatment methods can increase the thermal stability of down fibers, and the use of papain alone to treat down fibers will reduce the thermal stability of down fibers. The most direct reason for this phenomenon is that TGase can cross-link with down fibers, thereby enhancing the binding force between down fiber molecules. When the down fiber is first treated with papain and then cross-linked with TGase, the down fiber is first slightly hydrolyzed to form a loose state. At the same time, the slight hydrolysis provides the required active groups for the reaction, making TGase interact better with the down fiber. More intramolecular and intermolecular crosslinks occur, so the binding force between molecules of down fibers is better enhanced, and the down fibers have the highest thermal stability after multi-enzyme treatment. Using papain alone will cause down fiber to hydrolyze, destroying the structure of down fiber, weakening the binding force between molecules, and reducing thermal stability. The third stage is when the temperature is higher than 450°C. This stage is mainly the carbonization process of down fiber, and the weight loss of the fiber gradually became flat. From the above observation, it can be seen that different enzyme reagents change the protein structure of down fiber and change the thermal stability of down fiber.

Figure 4

Thermogravimetric (TG) thermograms of down fibers treated by enzyme reagents.

Differential scanning calorimetry analysis

The denaturation temperature (T_d) was used to investigate the stability of down fibers, including untreated down, softened down, TGase treated down and down fibers treated with multi enzymes. When the down fiber is heated, the three helices in keratin will separate or disperse from each other, and the molecular chain will tend to relax gradually. According to the heat capacity of down samples during thermal activation and denaturation, the peak value on the DSC curve represents the denaturation temperature (T_d) of down fiber.²¹ When the temperature reaches the denaturation temperature (T_d), the triple helix structure of down fiber will begin to decompose. Therefore, the wider peak in the DSC curve indicates that down fiber with different thermal stability has a stronger binding force and more stable molecular structure. From the DSC curves of down fiber with different treatment methods shown in Figure 5, it can be seen that the DSC curves of all samples were a single peak, with denaturation temperatures (T_d) of 53.72°C, 62.33°C, 68.54°C and 77.36°C, respectively. The results showed that the thermal stability of down fiber treated with papain was lower than that of untreated, TGase treated, and multi enzyme treated down fiber. The direct reason is that papain hydrolyzes the peptide bonds of the down fibers, which greatly reduces the binding force between molecules, which leads directly to a decrease in thermal stability of the down fibers. At the same time, compared with down treated with TGase, the thermal stability of down treated with multi enzyme was stronger. A possible reason is that the pretreatment with protease increases the contact between TGase enzyme and down fiber, and improves the catalytic efficiency of the enzyme. At the same time, slight hydrolysis provides reaction groups for cross-linking. Therefore, the binding force between down fiber molecules treated with multi enzyme synergy is the strongest of all. The results of T_d analysis and thermal stability analysis are consistent.

Figure 5

Differential scanning calorimetry (DSC) profiles of down fibers treated by enzyme reagents.

Analysis of moisture absorption of down fiber before and after treatment

Moisture absorption is a criterion for evaluating the comfort of fabric. The moisture absorption of down fiber before and after treatment with different methods was tested. It can be seen from Figure 6 that, with the increase of time, the moisture absorption of all samples was gradually improved. This is because there are a lot of hydrophilic groups, such as hydroxyl groups, carboxyl groups, and amino groups, on the surface of down fiber. After absorbing water, the weight of down fiber is gradually increased, so the absorbability is improved. It is worth noting that the moisture absorption of down fiber treated with papain was better than that of other samples. The direct reason is that the hydrolysis of papain destroys the surface structure of down fiber, breaks part of the peptide bond, and opens the molecular chain between fibers, which increases the hydrophilic group in the molecule, which leads to the improvement of the moisture absorption of down fiber. With addition of the cross-linking enzyme TGase, the intramolecular or intermolecular cross-linking of down fiber occurs again, and a certain number of active groups are consumed. Therefore, compared with the down fiber treated with papain, the moisture absorption of down fiber treated with TGase and multi enzyme treatment was lower.

Figure 6

Hygroscopic properties of down fibers treated by enzyme reagents.

Changes in breaking strength and elongation at break of down before and after treatment

After softening and crosslinking treatment, the molecular binding force of down fiber will change, and the physical properties of down fiber will be affected. Therefore, the tensile strength and elongation at break of down fiber treated with different enzyme reagents were investigated. It can be seen from Figure 7 that the tensile strength and elongation at break of the down fiber treated by softening enzyme were smaller than those of other samples. The direct reason is that the softening enzyme acts on the peptide bonds of down fibers, which hydrolyzes the down fiber, destroys the stable structure, and greatly weakens the force between the fiber molecules. The tensile strength and elongation at break of down fiber treated with multi enzyme were higher than those treated with TGase alone, which indicates that the binding force and elasticity of the down fiber are stronger after multi-enzyme treatment, which directly explains the higher fluffiness after multi-enzyme treatment. It is worth noting that, compared with the untreated down fiber, the tensile strength of down treated with multi enzyme increased by 105.6%, from 180 MPa to 370 MPa, and the elongation at break increased from 12.4% to 37.7%, nearly 3.0 times more than the original property.

Figure 7

Tensile strength (left panel) and elongation at break (right panel) of down fibers treated with softening enzyme (A), untreated down fibers (B), down fibers treated with TGase enzyme (C) and down fibers treated with multiple enzyme (D).

Conclusion

In this paper, utilising mild reaction conditions, clean and efficient enzyme reagents were selected as fiber bulking agents to improve the fluffiness of down fibers. The single action of TGase, and the dosage and proportion of papain in the synergistic action of multiple enzymes with TGase were analyzed in detail. The results indicate that TGase has a very obvious effect on the improvement of down fluffiness. At the same time, papain, as a softener of down fiber, can expose the active groups of down fiber, promoting contact between down fiber and TGase, and making cross-links occur within and between molecules, so as to improve the fluffiness of down fiber. In summary, the use of enzyme reagents as a mild, efficient and green down-leavening agent can greatly improve the performance of natural down fibers and has good development prospects.

Footnotes

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 funded by the Key Scientific Research Group of Shaanxi Province (2020TD-009), Key Scientific Research Program of Shaanxi Provincial Education Department (Collaborative Innovation Center project) (20JY003), Science and Technology Plan Project of Xi’an Weiyang District (No. 201907) and the Youth Innovation Team of Shaanxi Universities.

ORCID iD

Taotao Qiang

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