New Green Paper Additives - Chitin and Chitosan

Shrimp shells and crab shells are solid waste produced by the aquaculture industry. With the development of marine and artificial shrimp and crab farming in China, the amount of these wastes has grown and caused serious environmental pollution.

On the other hand, they are also a valuable biological resource. Since the French discovered chitin in 1811, chitin extracted from such wastes, its deacetylated chitosan, and various modified chitosans have been widely used as preservatives and sedimentation agents in the food industry. , Dye deepeners for textile printing and dyeing industry, flocculants for water treatment, precious metal ion recovery aids for metallurgical industry, additives for daily chemical industry, and applications in agriculture, papermaking and chemical industries.

1 Chitin

Chitin is a natural macromolecule compound that is second only to cellulose on the earth and it is widely found in the hard shells of cell-like walls, insects, and crustaceans. It is estimated that there are nearly 10 billion tons of chitin biosynthesized in nature every year. In addition, chitin is the largest number of nitrogen-containing organic compounds on Earth.

The chemical structure of chitin and cellulose is very similar. The molecular chain is a linear straight chain. The difference is that there is an acetamide group (ch3conh-) on the chitin c2. Deacetylation of chitin results in chitosan. The three structures are as follows.

Chitin is white or gray amorphous, translucent solid, insoluble in water, dilute acid, dilute alkali and common organic solvents, soluble in concentrated inorganic acids, but at the same time the main chain will degrade to generate α-glucosamine. Not all structural units in chitin have n-acetyl groups. Elemental analysis shows that about 12.5 amino groups of natural chitin molecules are not n-acetylated, and the insolubility of chitin limits its application. Therefore, research and development in recent years has mainly focused on chitosan and its modification.

Chitosan

Chitin is generally converted to chitosan by removing the acetyl group heterogenously in 100-180 degrees Celsius, 40%-60% sodium hydroxide solution. In the industry, chitosan with a degree of deacetylation of more than 70% and a nitrogen content of 7.0% to 8.7% is regarded as a qualified product. It is a white, amorphous, transparent, slightly pearly solid, insoluble in water and alkaline solutions. Its chemical structure is shown in the above structural formula. Chitosan is the only natural sugar polymer found in nature. Its most important property is that it is soluble in most dilute acids and forms salts. However, it also hydrolyzes in dilute acid, making it less viscous. Chitosan has good adsorption, film formation, permeability, fiber formation, hygroscopicity and moisture retention.

2 Chitosan modification

In order to improve the various properties of chitosan, chemical or physical modification methods can be used to change its structure to meet certain special needs. At present, a large amount of research has been done in this field at home and abroad, and a considerable amount of papers have been published. . Chitosan modification is now mainly concentrated on the following aspects: hydroxyethylation, carboxymethylation, cyanoethylation, quaternary ammonium cationization, graft copolymerization with vinyl monomers, and aldehydes and anhydrides Self-crosslinking and re-acetylation of chitin, etc.

Studies have shown that chitin and chitosan can do anything that cellulose does and can do better, and the acid-soluble nature of chitosan is similar to the paper-making acidic media in papermaking and is a subject that needs to be studied. There are promising new paper additives.

In theory, chitosan is a linear polymer with multiple hydroxyl and amino groups in its molecular structure. These groups can fully contact with the cellulose surface, have the ability to form hydrogen bonds, and have a large molecular weight, have sufficient interfiber bridging capacity, and at the same time, they act as a natural cationic polymer under acidic conditions, have many positive charge centers and The hydrogen bond center can be combined with the negative charge of cellulose to form an ionic bond. Therefore, chitosan is a good reinforcing agent. It can increase the dry strength of the paper sheet and can also be used as a wet strength agent for the paper sheet.

Experimental studies have found that chitin and chitosan can simultaneously increase the dry strength and wet strength of newsprint and other types of paper when mixed with slurries. They can also increase the wet strength of cartons, paper bags, diapers, and improve the efficiency of pulping.

Because chitosan significantly increases paper strength, papermakers can replace some good fibers with lower quality, less expensive fibers, while at the same time ensuring that the quality of the paper is not reduced, thereby reducing costs. Experiments have found that chitosan can reduce the beating process by up to $%, and adding #% chitosan to the slightly beaten kraft pulp can make the pulp strength reach the level of high beating.Chitosan due to the relative molecular weight Large and more positive charge density, can play agglomerate and agglomeration at the same time, and the retention effect is good.Chitosan strengthens the connection between the fibers, reduces the "pulling" phenomenon in the printing process, and makes the paper sheet have a smooth The surface and improve its printability.

In addition, chitosan can also be obtained through special processing porous chitosan body. It has a lower density and a larger specific surface area, and can be widely used as an insulator, an adsorbent, an ion exchange resin, a carrier, etc. in industry.