Current status and development of hemp fiber degumming process

Source: Zhang Fang; Qu Lijun
Keywords: hemp fiber degumming process characteristics analysis comparison: development direction
Foreword With the intensification of global environmental pollution, people's lifestyles and life concepts are changing quietly. People are more advocating nature and advocating returning to nature. The ancient textile material of hemp fiber has been favored as a clothing fabric with its unique excellent performance. However, there are many problems in the textile processing technology of cannabis. The degumming process has always been the bottleneck affecting the processing quality of cannabis products. The degumming effect is not ideal, which will directly affect the subsequent process and even affect the performance of hemp textiles. Degumming is a key link in the production of hemp textiles. At present, the main methods of degumming used in domestic and foreign countries are the following: natural water ramie method, chemical degumming method, microbial degumming method, biological enzyme degumming method, biochemical combined degumming method and ultrasonic treatment method, and flash explosion treatment method.
1 The characteristics of hemp fiber and its relationship with degumming are directly removed from the cane stem. The original hemp contains a certain amount of non-cellulosic components, including lignin and hemicellulose. , wax lipids, pectin and some water solubles and ash. If the raw hemp is made of spinnability, these non-cellulosic components must be removed to obtain fine dry hemp, that is, degumming. Due to its structure and chemical composition, the degumming of marijuana is much more difficult than delining and ramie. The degumming effect directly affects the subsequent process. The effects of different areas, different varieties and even maturity of marijuana degumming are all different. Table l shows the structure of hemp fiber and the chemical composition of cannabis from different places.

1.1 Structural characteristics of hemp fiber Hemp fiber is initially arranged by a chain of macromolecules in which glucose-based oxygen bridges are arranged in parallel, polymerizing the sub-group system, and then forming a fibrous skeleton of the excellent void-fibril system. Hemp fiber contains colloids in three levels: the colloidal system of fiber and fiber, the colloidal system between filament systems, and the colloidal system of the chain molecular cluster system. Therefore, the cannabis monofilament cells are interlaced into a network under the bonding of intercellular substances, while the ramie fiber cells are arranged in parallel, close to each other, and the hemp fiber cells are chemically bonded to hemicellulose. This also increases the difficulty of degumming. More importantly, the cannabis single fiber is too short, generally 7 to 50 mm, and the fiber uniformity is poor. If the intercellular layer material is completely removed (ie, full degumming), it will inevitably cause short pile and lose spinnability, so it must be The single fiber is adhered by the residual glue to form a fiber bundle process fiber for spinning, which is a so-called "moderate degumming" process. Due to the above characteristics, the degumming of hemp is difficult.
1.2 Chemical composition of hemp fiber The content of lignin and hemicellulose in hemp fiber directly affects the difficulty of degumming and the fiber quality after degumming. As can be seen from Table 2, the content of lignin and hemicellulose in cannabis is much higher than that of flax and ramie, so the difficulty of degumming is also increased.

2 Cannabis fiber degumming method 2.1 Traditional natural water ramie degumming method Natural water ramie is to bundle the hemp into small bundles, or bundle the hemp peeled from the cane stems, soak in ponds, ditches, lakes In the natural waters such as rivers, microbial anaerobic fermentation degumming is carried out, and the combination of various microorganisms in the water is used to decompose the hemp of the polymer compound into a small molecule compound, thereby extracting the cellulose.
This method requires a large amount of water, which is limited in areas where water resources are not abundant; and is affected by seasons and climate, and the quality of degumming is unstable; the process of ramie is caused by the water quality of the water source and the different types of microorganisms in the water. It is difficult to control; the degumming time is longer, about 10 days in summer, and 3 to 4 months in winter: this method is serious for water pollution. This method is the most traditional method adopted by Chinese farmers, and it can not adapt to modern society. The mode of production is currently used in very few areas.

2.2 Chemical degumming At present, China's hemp textile enterprises mainly adopt chemical degumming process. The principle is to remove the colloid in the original hemp by using the difference in the stability of the colloid and cellulose in the hemp fiber to preserve the cellulose content. In the process, the alkali agent is mainly used, supplemented by oxidizing agent, auxiliary agent and certain mechanical action to achieve the purpose of degumming. The basic process route is: original hemp - cage - pickling - washing - scouring - knocking hemp - bleaching - washing - pickling - washing - dehydration - opening - cage - oil - deoiling water - shaking - Drying - fine dry hemp.
However, there are still some problems in the actual production of the above process: the lignin content in the cannabis gum is relatively high, up to 4 to 7%, and the removal of lignin in the above process is not ideal, but the presence of lignin has a quality for textiles. Great impact. The fiber with less lignin content has good gloss, whiteness, softness, elasticity, and good spinnability and dyeability. On the contrary, the fiber quality is poor. The bleaching of lignin in an acidic environment is mainly chlorination, and then treated with a dilute alkali solution, which can be oxidized and dissolved by hypochlorite. The re-treatment of the lignin can achieve a dry numbness content of O.8%. The following, color white, soft, can meet the requirements of post-processing of textile printing and dyeing, but the process requires strict control of the bleaching and lye processing parameters, otherwise it is difficult to obtain the fine dry hemp that meets the requirements of the subsequent process. Although the degumming effect of this method has been improved, it is not very satisfactory.
According to research reports, the combination of chemical degumming and mechanical separation can obtain fine dry hemp fiber with better degumming effect. The main process flow is: scouring waste alkali soaking - head road hemp - scouring - pickling - bleaching - washing - oiling - drying - carding - process fiber. The entire process uses a relatively mild chemical action, supplemented by the mechanical action of two alternate hemps, so as to achieve moderate degumming while maintaining proper strength and length, creating conditions for combing. The hemp fiber produced by this method can be spun 36-40 tex pure hemp yarn, and ll, l4, 16, 2 ltex hemp/cotton, hemp/polyester blended yarn.
In general, chemical degumming has a long processing time. In recent years, there have been efforts to shorten the processing time, such as pre-oxygen treatment and pre-urethane treatment combined with the scouring process, which greatly shortens the degumming time. . It is worth mentioning here that the alkaline oxygen one bath method degumming. The method reasonably combines the traditional two steps of scouring and bleaching, and the two steps of boiling and rinsing are completed in one step. The alkali and hydrogen peroxide interact with each other, and the alkali acts to remove the colloid, hemicellulose and wood in the marijuana. The action of the element and other impurities provides an alkaline environment for the decomposition of hydrogen peroxide, which makes the alkali and hydrogen peroxide exert the maximum effect: hydrogen peroxide is stable in acidic medium, the decomposition rate is very low, and it can be activated by alkali in alkaline medium. The hydrogen peroxide molecules dissociate and can bleach the hemp fiber. It is especially important to oxidize the lignin. The lignin can be dissolved in the alkaline oxygen bath solution after oxidation, thereby helping to better remove lignin and other impurities. A new degumming aid magnesium sulfate and a strong alkali bath hydrogen peroxide stabilizer are used, so that the alkaline oxygen one bath degumming reaction is very stable and protects the cellulose. Alkaline oxygen degumming process: original hemp unpacking - pre-soaking acid - water washing - alkali oxygen - bath - knocking hemp - washing - pickling - washing - dehydration - shaking hemp - oil - dehydration - drying / drying alkali The new method and process of oxygen degumming are short, the cost is low, and the refined and dry residue ratio and lignin content are low. It is suitable for spinning medium and high count yarns and making high-grade clothing fabrics. After the new method is degummed, the finished hemp fabric can be finished without finishing the bleaching process, which shortens the finishing process and improves the stability of the product quality.
Although the above-mentioned chemical degumming process can meet the requirements of comb spinning, there are still many problems. After the chemical treatment of marijuana, some chemicals are inevitably adhered to the fibers. These chemicals may enter the fibers and damage the internal structure of the fibers. It destroys the medicinal ingredients of hemp fiber itself, especially the reputation of cannabis "green fiber". In addition, these chemicals have brought a series of problems to the export industry of some cannabis textiles in our country.
2.3 microbial degumming

The aforementioned “natural water ramie” is also a microbial degumming, but it only uses the natural action of microorganisms. Introduced here is the artificially selected microbial degumming method. The microbial degumming method uses natural or artificially cultured bacteria to produce enzymes, and the enzyme decomposes the colloids to obtain a degumming method of the separated fiber bundles - the essence of which is the result of the cycle of "gum culture, seedling production, enzyme degumming".
According to the latest research on polysaccharide chemistry, colloidal components should be classified according to their structure. Divided into polypentaose, polyhexose and its heteropolysaccharides and lignin. Most isoxic microorganisms can use polypentose and polyhexose as their sugar source and energy source. The enzyme secreted by the microorganism decomposes various gums in the bast into small molecular compounds to separate the gelatin. Complete degradation of some heteropolysaccharides is likely to require synergistic effects of several bacteria. Lignin is not easily used by microorganisms and is the most difficult part of decomposition in plants. Therefore, the main object of microbial degumming is to remove polypentose, polyhexose and its heteropolysaccharides other than cellulose and essence. The lignin can only be removed by certain post-treatment measures.

At present, the most research and application is biological enzyme-chemical combined degumming, and some manufacturers have formed quite mature processes. The process flow is: strain-culture-fermentation-production bacteria-bacteria degumming-refining-knocking-rinsing-rinsing acid-oil-water-shake-drying. The utility model has the advantages that the environmental pollution can be greatly reduced, the energy and chemical consumption are reduced, the fiber damage is small, the obtained fine and dry hemp quality is excellent, and the hand feels fluffy and soft.
Microbial degumming has strong specificity and mild conditions. It can improve the processing quality of hemp, reduce production cost, improve economic efficiency, reduce environmental pollution, and is simple and easy to process. It is a technology with great development potential. However, the technology still needs further improvement. For example, the existing degumming strains are modified, and new techniques are applied to further strengthen the research on the modification of existing degumming strains, and to improve the enzyme-producing ability and enzyme activity of degumming bacteria, and the stability of enzyme activity. And further strengthen the screening and cultivation of new strains.
2.4 Enzymatic degumming At present, the degumming research of hemp crops has been deepened to the molecular level, that is, enzymatic degumming. In 1972, Fogarty WM. and Ward. OP proposed the possibility of utilizing a pectinase-producing microorganism or pectinase preparation in a degumming process. Since the 1980s, many researchers in China have isolated and screened degumming strains and used a single strain to carry out the degumming test of raw hemp. The main purpose is to ramie, flax, kenaf, jute, etc. Only research is done in the year.
Enzymatic degumming means that the degumming bacteria are cultured until the senescence phase of the growth of the bacteria, followed by filtration or centrifugation, and the crude enzyme solution is used to impregnate the raw hemp, or the crude enzyme solution is purified and concentrated into a liquid agent, or the concentration may be concentrated. The liquid is dried to become a powder. When used, the liquid agent is diluted or the powder is dissolved in water, and the original hemp is immersed in the enzyme diluent for enzymatic degumming.
Regarding the enzymatic degumming, from the research on other hemp crops, the advantages are obvious, the process is simple and easy, no special equipment is needed, fast and efficient, no pollution, and the quality of the produced fine dry hemp is good. However, from the current point of view, a single enzymatic degumming can not be applied to industrial production, mainly because the enzyme activity of the enzyme-producing strain or enzyme preparation is too low, and the original hemp after the enzyme degumming also contains more colloid, which must pass chemistry. The refining process can make up the quality requirements of degumming.
2.5 Ultrasonic degumming technology Ultrasonic degumming is a new process that has recently emerged with the characteristics of “explosive” peeling. It firstly produces a large number of cracks in the outer colloidal layer, and then, under the continuous action of the cavitation bubbles, a colloidal mass is formed, which is detached into the water and then expanded into the water by ultrasonic cavitation. Huge pressure and tensile force to comminute and impact the exfoliated colloidal mass, which is ultrasonically pulverized into very small gelatinous plasmids and even decomposed. These colloidal particles are stably dispersed in the liquid, thereby completing the pretreatment of the cannabis degumming quickly and well. Ultrasonic degumming is mainly based on strong ultrasonic "cavitation effects". Ultrasound can be used to improve and accelerate the degumming of hemp, and the production of ultrasonic itself and the role played in the pre-degumming treatment does not involve any chemical agents and is a physical process. Today, when the "protection of the environment" is very loud, it is of great significance. This unique processing mode and mechanism of action of ultrasonic in the degumming pretreatment of hemp makes it have great potential in the degumming of hemp.

2.6 Flash Explosion Degumming The flash explosion technology for hemp fiber processing is also a degumming method that has developed rapidly in recent years, is effective in fiber separation and degumming, and is low-cost and non-polluting. Flash degumming is divided into two steps: first, thermal action under high temperature and high pressure conditions. The moisture in the pretreated raw hemp and the absorption of high heat energy by each component degrade the hemicellulose into soluble sugar, while the lignin softens and partially degrades, and the strength of the fiber is reduced, providing selective mechanical separation for the flash explosion process. . Secondly, the flash explosion process uses high-temperature and high-pressure hot steam and high-temperature liquid water to work together on the original hemp polymer, and the adiabatic expansion process is completed instantaneously, and the work is performed externally. During the flash explosion process, the expanded vapor acts on the raw hemp polymer in the form of a shock wave, which causes the cellulose molecular chain in the raw hemp polymer (also has a hemicellulose molecular chain, a lignin molecular chain, and a pectin mass). In the chain), the shearing deformation movement occurs under softening conditions, since the deformation speed of the raw hemp polymer is much smaller than that of the shock wave, so that shearing is caused multiple times to separate the fibers.
The ratio of hemp fiber cellulose after degumming is significantly increased, the non-cellulosic component such as lignin is obviously reduced, and the degumming effect is ideal, and the practical industrialization of the treatment method has certain potential.
3 Conclusion Chemical degumming has been a commonly used method in cannabis production. With the increasing awareness of environmental protection, this method will be gradually eliminated; biological degumming is difficult, usually with other methods; ultrasonic waves appearing in recent years The method and the flash degumming method are both environmentally friendly and the degumming effect is ideal. Therefore, mechanical, physical or new chemical methods and even new biological treatment methods can make the gel removal more thorough, the fineness is higher, the lignin content in the fiber is lower, and the fiber is not damaged and the environment is not polluted. The method of advantage of cannabis "green" textile materials will be the development direction of cannabis degumming.