Wool is a type of high-grade natural textile material. It has many features such as soft to the hand, elasticity, etc. However, wool fibers have cuticles, which make wool fabrics unstable and easy to shrink and deform in the washing process. Therefore, in order to improve the product’s dimensional stability, the scales on the surface of the wool fiber need to be processed to reduce its directional friction effect (D.F.E.) and to be machine-washable [1
The Chlorine-Hercosett treatment that combines mid-chlorination pretreatment with cationic polymer deposition on wool fiber surface is most widely used in conventional wool shrinkproofing finishing. The chlorination process causes fiber yellowing, and the use of resin results in the fiber being stiffer to handle. What is more, during the mid-chlorination pretreatment, a large amount of chlorine gas is released into the air, which threatens the health of the operator and is seriously detrimental to the atmosphere. Adsorbable organic halogens (AOX) are generated by reaction of chlorine with wool amino acid residues in the process of chlorination [2
]. There is a high concentration of AOX in the waste liquid. AOX not only pollutes the environment but also causes serious damage to humans [3
]. Chlorine-Hercosett technology is unsustainable and does not meet the requirements of eco-textiles processing. However, there is still no effective treatment method that can replace chlorine treatment in wool fibers shrinkproofing finishing. Therefore, when the eco-label (2002/371/EC) was established by the European Community for textile products, they were forced to make the following decision: “Halogenated shrink-resistant substances or preparations shall only be applied to wool slivers
”. It is urgent to study a cleaner, efficient, and industrialized wool shrinkproofing technology.
At present, many sustainable technologies for eco-friendly processes of wool shrinkproofing methods have been brought forward under the concept of environmental protection, including enzyme treatment [4
], plasma processing [5
], ozone gas processing [6
], and processing with nano materials and so on [7
]. The enzyme method is not only eco-friendly but also has mild conditions [8
]. From the point of view of cleaner industrial application, enzyme treatment is the most likely to replace the chlorination treatment of these research methods. More and more researchers have focused on the shrinkproofing treatment with enzymes [9
]. Part of the scales of wool can be removed by enzyme treatment through the hydrolysis reaction of enzymes on the peptide bonds of wool fibers [10
]. Shrinkproofing finishing with enzyme is a kind of environmentally friendly method which has many extra advantages such as easy operation, saving energy, and so on [11
However, there are two main restricting factors for the cleaner industrial development of shrinkproofing finishing with enzyme. On the one hand, the reaction of enzyme with wool takes a long time to process and it is difficult to achieve the requirement of machine washable of the treated wool only through enzyme treatment [12
] because there are a lot of hard keratins that have a high content of disulfide bonds in the cuticles of the wool fiber [13
]. Disulfide bonds inhibit the reaction of the enzyme macromolecules with the wool scales. Therefore, a combination of pretreatment and enzyme exhaustion treatment is widely used in wool shrinkproofing finishing [14
], such as KMnO4
pretreatment, etc. [15
], which is time-consuming and uncontrollable [16
]. On the other hand, the mechanical properties of wool fibers treated by enzyme are usually significantly affected by damage [17
]. Enzyme molecules diffuse to the inside of the wool fibers during the extensive time of enzyme treatment and destroy the wool fiber cortex [18
], causing irreversible damage to the wool fibers. Methods of transglutaminase treatment and graft modification of enzyme and polymer deposition have been used to improve the wool tensile strength after treatment [19
]. Transglutaminase can catalyze the cross-linking reaction between protein molecules and improve the mechanical properties of the wool fiber. Transglutaminase treatment was reported to be used in improving the tensile strength of damaged wool fibers [20
] but the increase of fiber tensile strength is limited through cross-linking of proteins. In order to inhibit the enzyme molecular diffusion to the cortex and reduce the mechanical damage of wool in extended time finishing with enzyme, the molecular weight of the enzyme can be increased by graft modification [21
]. Jinsong Shen [22
] proposed modifications to the proteases so that the reaction between enzyme molecules and wool was restricted to the cuticle scales of the fibers. However, the enzyme activity and the homogeneity of the treatment are affected by the graft modification [23
]. In addition, the method of polymer deposition with organics or resin has also been suggested to improve the tensile strength of the treated wool fiber [7
]. However, the feeling of the wool becomes rough by the polymer deposition process. Consequently, the enzymatic treatment of wool shrinkproofing can hardly be industrialized [24
In order to realize the industrialization of cleaner and environment-friendly shrinkproofing finishing with enzyme, it is extremely necessary to improve the efficiency of action and reduce the time of enzyme treatment. The enzyme treatment must be completed before the enzyme diffuses into the cortex and the cell membrane complex of the wool fiber. In this way, the reaction between the enzyme and the wool fiber is controlled only on the surface of the fiber. To achieve rapid enzyme shrinkproofing finishing, the rate of migration of the enzyme on the surface of the wool must be accelerated, so that enzyme acts evenly on the surface of the wool fiber in a short time to avoid its irreversible damage to the wool at the adsorption position. Therefore, multiple padding treatments can be used to achieve rapid enzyme shrinkproofing finishing by improving the migration efficiency of the enzyme on the wool surface during the process.
In this work, the wool top was treated with Savinase 16L and activator in one bath in shrinkproofing finishing by using multiple padding techniques. The activator does not only maintain enzyme activity, but also can break down disulfide bonds in the wool scales [25
]. The adsorption and reaction properties of enzymes on wool were investigated. The mechanism of rapid and highly efficient shrinkproofing finishing by multiple padding was explored. All these investigations were expected to guide future industrial application of eco-friendly and high efficient enzyme-based wool shrinkproofing technology.
To summarize, the multiple padding process can improve the efficiency of enzyme on wool fiber and reduce the enzyme treatment time. The area shrinkage of treated wool was 2.4% (lab) and 4.3% (plant) by the multiple padding process for 2.5 min. The tenacity loss of the treated wool was less in the plant (5.6%) than in the lab (7.4%). The adsorption performance of enzyme revealed that the multiple padding treatment can control the adsorbance of the enzyme onto the wool fiber and promote enzyme desorption and migration, so that the reaction between enzyme and wool fiber is controlled on the surface of the wool and the treatment is more uniform avoiding excessive damage to the wool fibers. Amino acid analysis and XPS analysis suggested that the multiple padding process obviously accelerated the damage of disulfide bonds. The removal efficiency of the disulfide bond was about 15 times as much as in the exhaustion treatment in 2.5 min. The reduction of disulfide bonds obviously promoted the hydrolysis of enzyme to the wool. The average catalytic rate of the padding process was 14 times faster than in the exhaustion process, and the process time (2.5min) decreased by 32.5 min compared with the exhaustion process (35 min). Due to the high efficiency of the enzyme and the short processing time, excessive damage caused by the extensive time enzyme exhaustion treatment to wool fibers can be avoided.
The multiple padding wool shrinkproofing process with the Savinase 16L and activator is continuous, easy to operate and has no AOX in the effluent. This technology was preliminarily industrialized at the Changshu Xinguang Wool Top Treatment Co. Ltd. (Changshu, China). It can be widely applied in industry and used in place of the environmental polluting chlorine treatment. AOX zero emission can be realized in wool shrinkproofing treatment in the near future. In addition, the results of this study are instructive in the research field of enzyme application.