2. Basic Properties of Chitosan and Its Derivatives Chitosan (abbreviated as cs) is rich in sources and affordable, and is synthesized by chitin (extremely present in shells and fungi such as shrimp, crab, insects, etc.) every year. About 10 billion tons, it is the second largest renewable resource on earth after cellulose, and it is also quite abundant in China's coastal areas.) A natural homogeneous straight amino group obtained after deacetylation under concentrated alkali. Streptan is the only basic polysaccharide in nature and contains free amino groups. Its chemical name is (1,4) -2, amino-2, β-D-glucose, and its molecular formula is (C6H13 · N05) n. Its structural formula is shown in Figure 1:
The appearance of chitosan is a white or light yellow translucent flake or powder solid with a slight pearl luster; the relative molecular weight ranges from hundreds of thousands to millions; due to the presence of amino groups, its chemical properties are greatly activated, although insoluble In water and alkaline solutions, but under acidic conditions, NH2 is protonated to NH2 +, so it can be dissolved in most dilute acids, such as dilute hydrochloric acid, acetic acid, lactic acid, formic acid and other aqueous solutions. The polymer chain can be slowly hydrolyzed to form a solution, which is also one of its main and most useful properties. Deacetylation degree and viscosity (average relative molecular weight) are the two main performance indicators of chitosan.
Due to the existence of active groups such as amino, hydroxyl, acetylamino, oxygen bridge, and electron-rich pyran ring in the molecular structure, chitosan can perform a variety of chemical modification reactions, and its derivatives have unique physical, Chemical properties and physiological functions, such as good film-forming properties, good biocompatibility, antibacterial properties, regulating immune function, and degradability. Therefore, it has shown broad application prospects in the fields of medicine, food, functional materials, textile printing and dyeing, environmental protection, daily chemical industry, fruit and vegetable preservation, and biological tissue technology.
3. Packaging suitability analysis of chitosan material Since the composition and structure of chitosan are polysaccharides like natural cellulose, it can be made into a thin sheet material with the following requirements for packaging suitability:
(1) Protective properties Protective properties mainly refer to the mechanical strength, air permeability, stability and other aspects of packaging materials .
①Mechanical strength Due to the strong internal force of the polysaccharide substance chitosan, the interlacing force of chains, the van der Waals force between molecules, and the force between hydroxyl and water molecules, the sheet made of it has a certain Tensile strength, tensile strength, breaking strength, flexural strength, abrasion resistance, etc.
② Gas permeability The permeability of packaging materials to water vapor, oxygen, and organic volatile gases is a more important characteristic, which directly affects the quality of packaged products, especially food. Chitosan macromolecules have many amino groups bound to water molecules, which have excellent water absorption and water retention properties, and they can be made into materials with a porous structure by various methods (such as polyethylene glycol as a microporous agent). It has good gas permeability, which is very important for many packaging products.
③ Stability Stability refers to the light resistance, chemical resistance, organic solvent resistance, heat and cold resistance of packaging materials. Chitosan does not undergo chemical changes under the action of light, heat, cold, and most organic solvents. It is non-mildew, non-rot, and sterile. Its stability is better than paper.
(2) Workability The workability of packaging materials refers to work strength, sealability, non-adhesion, applicability to adhesives, non-curling, etc. According to the structure and characteristics of chitosan, it can be known that its strength is moderate, it has good stiffness, can be adhered, and can meet the requirements of adaptability and mechanical properties in packaging operations.
(3) Commerciality Mainly refers to the gloss, smoothness, and dyeability of packaging materials. Chitosan has strong film-forming ability, its sheet is smooth, and it has good absorption performance. It can fully absorb the dye solution and make it dyed into various colors.
(4) Ease of use and hygiene Chitosan material has moderate strength, and its ease of use, such as unsealing performance, is superior to polymeric materials. In addition, the chitosan material is used for food packaging and pharmaceutical packaging . It is non-toxic, odorless, odorless, has excellent thermal stability, and has obvious properties of inhibiting mold, bacteria, and yeast, which meets the requirements of food and drug management regulations.
(5) Economics At present, chitosan is mainly extracted from shrimp shells or crab shells. This method has two disadvantages: firstly, the raw materials are limited, the collection is limited by the season, and the production cost is high; secondly, due to the type of raw materials, origin and production process Different, the quality of chitosan is unstable. At home and abroad, research on the method of producing chitosan by fermentation has been started since the 1990s. For example, Asahi Glass Co., Ltd. in Japan has developed a process of producing chitosan by fermentation in 1993. Beijing University of Chemical Technology also successfully developed a fermentation process for the production of chitosan in 1998. This method can greatly reduce the production cost of chitosan, making the production of chitosan by fermentation instead of using shrimp shells or crab shells as raw materials. may. Therefore, although the cost of chitosan may be temporarily higher than that of polymeric materials and paper, its economic feasibility is worth looking forward to.
4. Analysis of printing suitability of chitosan materials In addition to packaging, most packaging materials also need to be printed. In printing, the packaging material is used as a substrate, so it must meet the following requirements for printability:
(1) Porous materials made of elastic chitosan with good mechanical compression will reduce the volume under the action of external force. When the external force is removed, due to the separation characteristics of pores and pores, the porous structure and the original Volume, thus ensuring a good elastic recovery rate in printing, and its compression deformation is small.
(2) Good printing smoothness Compared with vegetable fiber, chitosan has a smaller degree of polymerization, a uniform distribution state, and a better film-forming property. Therefore, its sheet has good smoothness, and it can definitely obtain finer graphic accuracy with printing, and it can reproduce color and multi-level well.
(3) Hygroscopicity and dimensional stability The amino group on the chitosan macromolecule makes it absorb and absorb water well. It does not swell after absorbing water, so its size is stable after absorbing water during printing. This overcomes the disadvantages of water absorption and swelling of paper and other materials, eliminating the need for humidity conditioning before printing, and is not affected by environmental temperature and humidity during printing, ensuring color overprint accuracy.
(4) Ink absorption Chitosan has strong adsorptivity and is suitable for the adsorption of water-based inks and resin inks.
5. The specific application of chitosan material in packaging For chitosan packaging material, there have been many market applications abroad, some of which have achieved very good packaging effects, and its various properties in some packaging applications It is more excellent than traditional packaging materials and is a green packaging material that can replace traditional packaging materials.
5.1 Application of Chitosan in Fresh-keeping Packaging of Fruits and Vegetables Once the fruits and vegetables are picked, the supply of water and nutrients from the root system is interrupted, but their physiological functions such as respiration and transpiration still exist, and they will continue to consume nutrients and lose water. The moisture content of most fruits and vegetables is between 65% and 96%. As long as the water content of 5% is lost, the gloss will be lost, wrinkles will appear on the surface, the product quality will decrease, and the value will decrease. Fruits and vegetables are not easy to store, so reducing water evaporation and inhibiting respiration are necessary requirements for fruit and vegetable fresh-keeping packaging. In the past, the most used packaging films for fruits and vegetables were plastic products. In addition to the pollution caused by plastic packaging, the biggest problem is the inability to keep the fruits fresh. Fresh-keeping packaging film made with chitosan as the main raw material, the raw material is natural biopolysaccharide, safe, non-toxic, can be biodegraded, there is no problem of residual toxicity; it has the inner packaging film of fruits and vegetables. Certain gas and water vapor barrier properties and good tensile strength can regulate the physiological metabolism of fruits after harvest; at the same time, chitosan and its degradation products also have certain antibacterial properties. Some scholars have studied the application of chitosan packaging film to tomatoes, kiwi and other vegetables and fruits. A large number of practices have proven that the shelf life of vegetables and fruits using chitosan as an inner packaging film can be greatly extended, which effectively guarantees the value of vegetable and fruit products during transportation and sales. Therefore, chitosan has a good application prospect in fruit and vegetable preservation packaging.
5.2 Application of Chitosan to Meat and Fat Food Packaging According to domestic reports, chitosan can be processed into a composite packaging film (paper) through a special process, which has high tensile strength, elongation and good water resistance. And oil resistance, and chitosan has a significant effect on keeping pork fresh. In recent years, some large and medium-sized meat processing companies in China are planning to adjust the product structure, actively develop meat fresh-keeping packaging products, standardize the packaging of meat products, adopt natural packaging fresh-keeping technology, and extend the shelf life of meat products to ensure the quality of products. And value. In the past, the sterilization temperature of meat products was low, the sterilization was not thorough, and the storage temperature was slightly higher, which would easily cause the growth of microorganisms and cause deterioration. Even under refrigerated conditions, unqualified hygienic indicators often appeared, which seriously affected product quality. After the edible chitosan fresh-keeping packaging film is applied to meat food packaging, its oil blocking and antibacterial properties help improve the quality of meat food and extend its shelf life. The chitosan packaging film not only forms the modified atmosphere of the packaging of meat products, but also inhibits the physiological activities of microorganisms such as molds, bacteria, and yeasts, and plays a double sterilization effect to achieve the purpose of keeping fresh.
In addition, the edible chitosan fresh-keeping packaging film can also be effectively applied to the inner packaging of processed foods with relatively high oil content such as fried foods and cakes. In the past, the inner packaging of this type of food was mostly paper packaging. However, paper packaging has low mechanical properties, poor moisture resistance, and oil absorption. These characteristics have limited the development of paper packaging. Although some plastic packaging has relatively Good mechanical properties, but not the best choice for green packaging, will bring pollution burden to the environment. After effective processing of chitosan, whether it is used for inner packaging or outer packaging of oily foods, it has very good advantages. Chitosan is expected to become a new type of alternative to traditional packaging materials.
5.3 Chitosan is used to make packaging paper As early as the middle of the last century, researchers in the Soviet Union and Japan added chitosan to papermaking raw materials. This bold attempt greatly improved the water resistance and water resistance of paper. Later research showed that paper treated with chitosan acetic acid solution can improve printing quality, improve mechanical properties, water resistance and electrical insulation properties. If chitosan is added to the paper, the paper absorbs water, reduces strength, and also has certain antibacterial properties. This is used in the manufacture of special papers such as plain paper, label paper, cigarette packaging paper, and in waterproof packaging paper and transparent packaging. Paper and other aspects have unique applications. These characteristics have greatly improved the application performance of paper packaging materials.
6. Application of chitosan materials in printing At present, more researches are focused on the application of chitosan as a surface sizing agent in offset printing paper. Offset printing is a more common printing method. Due to the type of ink used and the paper wetting constantly during printing, offset printing paper needs to have high surface and internal strength, as well as high surface energy and a uniform microporous structure. Porosity (such as the number of pores, pore size, and shape) affects the printing quality. In order to allow the ink media to penetrate into the inside of the paper page and the ink particles remain on the surface of the paper page, the offset printing paper should have a uniform pore size and pore distribution. If the pores are too large, the ink completely penetrates into the paper sheet, resulting in a decrease in gloss and print-through; while the porosity is lower than a certain value, it is difficult to discharge water during the thermosetting process, and foaming occurs; if the pore distribution or pore size Non-uniform, spots or missing prints will appear. Paper with a high porosity structure has high water absorption, which makes the paper surface soft, and the evacuation of fibers or fillers causes the surface of the paper to shed.
In order to improve the surface properties of offset printing paper, it can be surface-sized. The use of natural or synthetic polymers as surface sizing agents, such as oxidized starch, is prone to spots, and the carboxyl groups in the oxidized starch are sensitive to polar water-alcohol fountain solution, which can cause hair loss on the paper surface. Oxidized starch may also be produced during production Enter the paper mill wastewater, resulting in an increase in BOD5 value. Synthetic polymers are generally less prone to speckles, but can have a negative impact on workshop workers and the environment. Therefore, cationic biochemical polymers such as chitosan or polyglucosamine, as they have no side effects on the environment and human health, may be a substitute to meet these conditions. Chitosan sizing has the following characteristics, making it the first choice for offset printing paper sizing paper: The quality of paper sizing with chitosan is better than that of oxidized starch or polyvinyl alcohol; chitosan can reduce paper This property is very important in offset printing; Chitosan has cationic properties and is adsorbed on fibers during papermaking to reduce the burden of BOD5 in paper mill wastewater; Chitosan has biodegradability and is good for the environment ; Chitosan can increase paper strength and can be used in low basis weight paper.
In addition, research on chitosan as an emulsifier, thickener, release agent, filler, etc. in printing inks is also ongoing.
7. Conclusion Green packaging and printing, which the industry has been advocating in recent years, is not just a slogan. It should also be an industry's development direction and innovation force. As a new type of green packaging and printing material, chitosan needs to be recognized by the packaging and printing industry, and this industry needs continuous research and improvement in this regard. I believe that chitosan packaging materials will appear in more and more green packaging and printing. I also hope that chitosan, the largest nitrogen-containing organic compound on the planet, will make our planet less polluted and more natural.