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Starch adhesives have many advantages, such as abundant raw materials, low price, degradable, etc. They can be widely used in corrugated cardboard packaging boxes, fiberboard, construction and other fields. However, the shortcomings of unmodified starch adhesives, such as poor fluidity, difficult sizing, poor water resistance and easy moisture absorption and opening in wet environment, limit the further application of starch adhesives. Therefore, modification of starch adhesive can expand its application field.
Starch is a natural macromolecule compound and polysaccharide compound. It is also a kind of degradable and renewable substance which can not pollute the environment. Natural starch is chemically treated and some chemical groups are introduced to change its molecular structure and physicochemical properties to form starch derivatives. There are usually two kinds of unmodified starch structures: amylose and amylopectin, which are polysaccharides. Usually because of the poor water solubility, modified starch, namely water-soluble starch, is often used. Soluble starch is a kind of modified starch derivative obtained by different methods. It is insoluble in cold water, ethanol and ether, soluble or dispersed in boiling water to form colloidal solution or emulsion liquid. Modified starch is made from natural starch by specific chemical, physical and enzymatic methods. Starch with improved original properties has been widely used in food, medicine, leather, foundry, paper making, textile, water treatment and other industries.
Modified Starch Production Line:
Starch is a natural polysaccharide polymer with a large number of hydrophilic hydroxyl groups on the molecular chain. Under the action of hydrophilicity and hydrogen bond of starch molecular chain, starch adhesive has high viscosity and poor water resistance. In recent years, it has been reported that chemical crosslinking can improve the water resistance of starch. However, crosslinking modification can improve the water resistance of starch adhesives, at the same time, the system viscosity increases correspondingly, which makes it difficult to apply in high-speed corrugated board production line. Ammonium persulfate (APS) was used to partially oxidize and degrade corn starch. By reducing the length of starch molecular chain, the problems of high viscosity and poor fluidity of adhesives were solved. On the basis of oxidative degradation of starch, the oxidative cross-linking modified starch adhesive with good water resistance and fluidity and reticular molecular structure was prepared by the reaction of high functionality melamine formaldehyde (MF) with hydroxyl group of starch molecular chain.
The varieties and specifications of modified starch are more than 2,000. The classification of modified starch is generally based on the treatment methods. In processing refined white starch, sweet potato varieties with high starch content must be selected. Although the processed starch is made from natural raw materials, the modified starch can not be regarded as natural by artificial processing. Modified starch for food is not harmful to the body.
At present, there are more than 2000 varieties and specifications of modified starch. The classification of modified starch is generally based on treatment methods, such as physical modification, chemical modification, biological modification and composite modification.
1) Physical Modification:
The gelatinization method first destroys the structure of starch agglomeration, causes the agglomeration to swell, and makes the starch molecules hydrate and dissolve. There are several pasting methods: indirect heating method, electrified heating method and high-pressure pasting method. Among them, indirect heating is the most basic way of starch gelatinization, which often needs to add a large amount of water, and through traditional heating treatment such as cooking and baking to achieve gelatinization. Electric heating method is characterized by fast heating rate, uniform heating, no heat transfer surface, no pollution of heat transfer surface, high thermal efficiency (more than 90%), easy continuous operation, and complete gelatinization of starch in a relatively short time. However, the technology of electrified heating is still in the exploratory stage in China. The advantage of high pressure gelatinization is to save energy. It refers to the gelatinization of starch-water suspension under high pressure. However, because of its large size, it is not suitable for starch gelatinization in laboratory.
2) Chemical modification:
Modified starch prepared by various chemical reagents. There are two kinds of starch: one is to decrease the molecular weight of starch, such as acidified starch, oxidized starch, baked dextrin, and the other is to increase the molecular weight of starch, such as cross-linked starch, esterified starch, etherified starch, and grafted starch.
3) Biological modification (enzymatic modification):Starch was treated with various enzymes. Such as alpha, beta, gamma-cyclodextrin, maltodextrin, amylose and so on.
4) Composite Modification:
Modified starch was prepared by two or more treatment methods. Such as oxidized cross-linking starch, cross-linking esterified starch, etc. The modified starch obtained by composite modification has the respective advantages of two kinds of modified starch. In addition, modified starch can also be classified according to the production process routes, such as dry method (such as phosphate starch, acidolysis starch, cationic starch, carboxymethyl starch, etc.), wet method, organic solvent method (such as ethanol as solvent for carboxyl starch preparation), extrusion method and drum drying method (such as natural starch or modified starch as raw materials to produce pre-gelatinized starch).
Because of its good biodegradability and biocompatibility, starch and its modifiers have potential applications in medical and health fields. The swelling, dissolution, gelation, rheological properties, mechanical properties and enzyme digestion characteristics of starch are the main factors that affect the application of starch in the medical field. Marques et al. Prepared a new starch polymer by mixing starch with cellulose acetate and ethylene-vinyl alcohol copolymers and reacting with hydroxyapatite. It has the advantages of high cell biocompatibility and low cytotoxicity. It is used as bone support or substitute material in medicine.
Modified starch, as a promising new water treatment agent, has attracted more and more attention. Although the direct application of flocculant in natural raw water is not effective, the flocculant with polyaluminium chloride shows good flocculant aid performance in the treatment of low temperature and turbidity water.A series of cationic starches were synthesized, which had good flocculation effect on silica suspension.A high density cationic polymer flocculant was synthesized. When the relative molecular weight of the flocculant is 660,000, the clarification effect of the oil wastewater is better than that of the polyacrylamide flocculant with the relative molecular weight of 8 million.
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