The treatment of natural starch by chemical, physical, biological and other methods changes the chemical structure of some D-glucopyranose units in the starch molecule, and at the same time changes the physical and chemical properties of natural starch to varying degrees. The modified starch It is called modified starch or starch derivative. The development of modified starch is to overcome the shortcomings of natural starch, thereby expanding the starch.
Application of powder in industry. Modified starch mainly refers to the starch modified by technical means such as physical methods, chemical methods, biotechnology and genetic engineering technology. The scope of application of modified starches continues to expand with the development of other related industries. It can be seen that in the 21st century and beyond, modified starches have very broad development prospects.
(1) Hydroxyalkyl starch Hydroxypropyl starch was invented in the late 1930s and early 1940s. It not only possesses many excellent characteristics of hydroxyethyl starch, but also the boiling point of propylene oxide than ethylene oxide. High, making the manufacturing process safer. However, the high price of propylene oxide and the low efficiency of the derivatization reaction limit its wide application. It was not until the 1950s that the U.S. Food and Drug Administration approved hydroxypropyl starch as a direct additive to food, and industrial production began. Due to the high stability and non-ionic properties of hydroxyalkyl starch paste, it has great potential for industrial application. The annual output in the United States reaches more than 90,000 tons.
① Structure of hydroxyalkyl starch. Hydroxyalkyl starch is a kind of etherified starch. The most practical ones in industry are hydroxyethyl and hydroxypropyl starch. They are produced by the reaction of starch with ethylene oxide or propylene oxide under alkali catalysis. Made. When preparing hydroxyalkyl starch, ethylene oxide or propylene oxide can not only react with any one of the three active hydroxyl groups of the starch anhydroglucose monomer, but also react with the substituted hydroxyalkyl group to form a Oxyalkyl branched chain. As a result of this chain reaction, there are more than three molecules of ethylene oxide or propylene oxide reacting with an anhydroglucose monomer, resulting in greater than theoretical
The apparent degree of substitution of the upper maximum degree of substitution (DS=3). Therefore, hydroxyalkyl starch is different from many starch derivatives such as phosphate starch and cationic starch. The degree of reaction is expressed by the degree of molecular substitution (MS).
②Hydroxyalkyl starch properties. Hydroxyalkyl starch is a non-ionic starch ether, which has high stability when replacing ether bonds. In the process of chemical reactions such as hydrolysis, oxidation, dextrinization, and cross-linking, the ether bond will not break, the substituted group will not fall off, and it is less affected by the electrolyte and pH value, and can be used under a wide pH value. . Hydroxyalkyl starch is hydrophilic, which weakens the internal hydrogen bond strength of the starch granule structure. As the degree of substitution increases, the gelatinization temperature drops, and it can eventually expand in cold water. Products with higher degrees of substitution can be dissolved in methanol or ethanol. Hydroxyalkyl starch gelatinization is easy, the paste liquid has high transparency, good fluidity, and weak retrogradation.
High stability. It can be stored at low temperature or frozen and then melted, repeated many times, and still maintain the original colloidal structure. In addition, the paste has good film-forming properties, the film is transparent, flexible and smooth, has good folding resistance, and because there are no micropores, the grease resistance is improved.
③ Preparation process of hydroxyalkyl starch. Starch is a polyhydroxy compound with the chemical activity of polyhydric alcohols. Under certain conditions, these hydroxyl groups can easily react with substances containing hydroxyl groups to form ether compounds. Hydroxyalkyl starch is the product of the reaction of propylene oxide with starch under alkaline conditions. When other nucleophilic ions are present in the reaction medium, by-products may be generated. There are three preparation processes for hydroxyalkyl starch: water phase method to prepare hydroxyalkyl starch with low degree of etherification. The product is mainly used in papermaking and textile industry. The main process is to disperse starch in water system under the action of catalyst. React with propylene oxide, the method is simple and easy to implement,
Low cost; solvent method to prepare high etherification hydroxyalkyl starch, the product is mainly used in medicine, industry, such as artificial substitutes
Plasma is hydroxyalkyl starch with high degree of etherification and high purity. The main process is to disperse the starch in an organic solvent (such as isopropanol, etc.), and react under the action of a catalyst. The cost of this method is that the organic solvent is used. High; F method, carried out in a sealed autoclave, is a gas-solid reaction of starch and alkylene oxide. The advantage is that the degree of etherification is high, but the disadvantage is that the finished product is difficult to purify. In addition, alkylene oxide is catalyzed by alkali It is easy to produce polymerization, there is a danger of explosion, and it is difficult to realize industrialization.
④ Application of hydroxyalkyl starch. Hydroxyalkyl starch with low degree of etherification is an excellent papermaking aid. According to relevant applications, the gelatinized etherified starch is added to paper and can become paper enhancer. The hydroxyalkyl starch is used in the surface treatment of printing paper to give the paper better strength and folding resistance. It can form a smooth film, inhibit ink penetration, make the printing effect good, and improve colorability, fluidity, Water retention. In the food industry, hydroxyalkyl starch can be used as a food paste agent. In the textile industry, it is used as a textile water slurry. In the petroleum industry, as a well fluid additive for petroleum drilling. Pure Hydroxyethyl Lake
The powder can be used as artificial plasma substitute in medicine. Hydroxyalkyl starch is a non-ionic compound, which has the characteristics of good dispersibility in water and stable physical and chemical properties, so it has a strong commercial appeal. It is an ideal surface sizing agent and coating adhesive, which can effectively improve the physical properties of paper, such as abrasion resistance, hand feel and paper smoothness; it can solve the shortcomings of paper fuzzing and powder falling; inhibit printing ink The penetration of the printing paper makes the printing paper ink clear and uniform, and the film reduces ink consumption. Due to the non-ionic nature of hydroxyalkyl starch, it is rarely used as an additive in slurry. In addition, hydroxyalkyl starch has good water retention and film-forming properties. It is widely used in paper bags, cartons, labels, envelopes and other adhesives. In the textile industry, hydroxyalkyl starch with low substitution degree can be used for fiber yarn sizing. Or it can be mixed with polyvinyl alcohol. Generally, it can reduce the cost of sizing by about 35%. It is mostly used for permanent anti-wrinkle finishing.
(2) Cross-linked starch Cross-linked starch was used in commercial production in the 1940s. So far, cross-linked starch is not only a major variety of modified starch, but also combined with other starch modification methods, and is widely used in the production of composite modified starch.
① Cross-linked starch structure. Cross-linked starch is a derivative of starch that reacts with a chemical reagent with two or more functional groups to form etherified or esterified bonds between the hydroxyl groups of starch molecules to form a derivative of cross-linking. Any chemical reagent with two or more functional groups that can react with two or more hydroxyl groups in the starch molecule can be used as a cross-linking agent. There are many cross-linking agents reported in the literature, but they are not widely used in industrial production. , There are mainly sodium trimetaphosphate and phosphorus oxychloride. The former has 2 functional groups and the latter has 3 functional groups. These two crosslinking agents are non-toxic, and the made crosslinked starch can be used in the food industry.
② Classification of cross-linked starch. There are multiple hydroxyl groups in starch, and each glucose group contains two secondary hydroxyl groups and one primary hydroxyl group. These hydroxyl groups have the chemical reaction properties of alcoholic hydroxyl groups and can react with many compounds. When a compound has two or more groups that can react with a hydroxyl group, there is a possibility of a reaction, forming a cross-linked bond between the hydroxyl groups of the same molecule or different molecules. A considerable number of cross-linking agents have been reported in patents or literature. Generally, cross-linked starches are classified according to different cross-linking agents, which can be divided into the following five categories: dibasic or tribasic compounds, such as tripolyphosphoric acid Salt, trimetaphosphate, adipate, citrate; halide, such as epichlorohydrin, phosphorus trichloride, phosgene, aliphatic dihalide
Etc.; Aldehydes, such as formaldehyde, adipaldehyde, acrolein, etc.; mixed acid anhydrides, such as adipic acid and acetic acid mixed anhydrides, carbonic acid and organic acid mixed anhydrides, etc.; amino and imino compounds, such as urea, urea formaldehyde resin , Dimethylolurea, etc.
③The properties of cross-linked starch. After starch is cross-linked, when the particles are heated or gelatinized by the compound, it shows the effect of cross-linking on the particles. Cross-linking mainly strengthens the hydrogen bonds in the particles. The chemical bonds act like bridges between molecules and cross-link starch. The hydrogen bonds may be weakened when heated, but the chemical bonds keep the particles with varying degrees of integrity. The characteristics of cross-linked starch depend on the degree of cross-linking. From the Brabender viscosity curve in Figure 2-1, the difference between native starch and cross-linked starch with different degrees of cross-linking can be clearly seen. A is native starch, which is consistent with the description of the gelatinization phenomenon. B is a starch with low degree of cross-linking (one cross-linked bond per 1000 glucose residues), and the viscosity change trend is consistent with that of native starch. The B curve is located above the A curve, indicating that cross-linking has no effect on the expansion and gelatinization of starch granules. However, the cross-linked bond formed by cross-linking increases the molecular weight of starch molecules, which shows that the paste viscosity increases on the curve. Curve C shows moderately cross-linked starch (one cross-linked bond for every 440 glucose residues), which can prevent the expansion of starch molecules. The D curve is a highly cross-linked starch (each 100 glucose residues has a cross-linking bond), it can not swell and gelatinize at all, and the viscosity is very low, so it cannot be measured. The E curve is the heating temperature change curve. After the raw starch is cross-linked, its hot paste viscosity is relatively stable, and it is resistant to acid, alkali, and shear. The improvement of these properties makes cross-linked starch widely used in food, paper, textile, petroleum drilling, battery and other industries. . Many properties of cross-linked starch are superior to native starch, so the application range is much wider. First, cross-linked starch increases the gelatinization temperature and viscosity, and the shear resistance is much higher than that of native starch paste. The viscosity of native starch paste Affected by shearing force, it is greatly reduced, and the stability of cross-linked starch against acid and alkali is much better than that of native starch.
④ Preparation of cross-linked starch. Phosphorus oxychloride is used as the cross-linking agent. This reagent is non-toxic and can be used to prepare mulch film. The reaction process of starch and epichlorohydrin is: the first step is the reaction of starch and sodium hydroxide to produce alkaline powder and water; the second step is the reaction of alkaline starch and epichlorohydrin to produce cross-linked starch and sodium chloride; The third step is that additional starch and sodium hydroxide react similarly to step two to produce a cross-linked product. Prepare the original corn starch as a 6% aqueous solution, fully dissolve and partially gelatinize at 80°C, apply quantitatively on a glass plate immediately, and dry at 70~80°C. Store the viscosity (Bu) after conjunctiva in a ventilated and dry place .
A starch suspension of a certain concentration is prepared, 100 g of raw corn starch and 150 mL of alkaline sodium sulfate solution (containing 0.66 g of sodium hydroxide and 16.66 g of anhydrous sodium sulfate) are mixed to form a suspension under stirring. The reaction system is placed in a heating jacket, and a pressure regulator is used to maintain the temperature required for the reaction; the function of sodium sulfate is to inhibit the expansion of starch granules. Dissolve the required amount of epichlorohydrin in 50ml. alkaline sodium sulfate solution. After the whole system is equilibrated for a period of time, drip the starch milk in 3~5min to seal the reaction system; after a period of constant temperature, use a certain amount of hydrochloric acid Adjust the pH of the system to about 6, and end the reaction. Coat immediately and remove the film.
⑤ Application of cross-linked starch. Cross-linked starch can be used in food, medicine, textile, papermaking, etc. In the food industry, the use of cross-linked starch has higher freezing stability and freeze-thaw stability, and is especially suitable for frozen food. If freezing or freezing and thawing are repeated for a long time at low temperature, the food can still maintain the original structure. , No change, cross-linked starch can be used as a thickener for salad dressings. It has the characteristics of not reducing viscosity at low pH and in the case of homogeneous and high-speed stirring. At the same time, it has good stability when stored at low pH. Sex. Cross-linked starch can show low initial viscosity and high heat during high temperature and rapid disinfection of cans
Transfer and rapid heating effect, and then increase the consistency to provide the necessary suspension state and organizational form. Similarly, it is widely used in the production of soup cans, gravy, sauce seasoning, baby food and fruit filling, pudding and fried food In medicine, foreign medical surgical gloves, latex gloves and other latex products use this modified starch as a lubricant. Highly cross-linked starch does not gelatinize when heated, has compact granular structure and good fluidity. It is suitable for anti-sticking agent and lubricant of rubber products. Because it has a good smooth feeling, it is harmless to human body and is non-irritating. It will gradually replace the previous use. Talcum powder. Highly cross-linked starch can be used as a carrier for insecticides and herbicides, and it is convenient to spray; cross-linked starch can be used as a perspiration agent, and cross-linked starch ethers containing carboxymethyl or hydroxyalkyl groups are suitable for human health absorbents and moisture absorption It has a capacity of 20 times and is widely used in toilet paper, surgical cotton swabs, and absorbents for patient body fluids.
2.2.2.3 Starch blends
The starch used in the starch blend can be native starch, physically modified starch or chemically modified starch, or a copolymer formed by reaction with monomers. Synthetic resins that can be blended with starch include polyethylene, polyvinyl alcohol, polyvinyl chloride, polystyrene, polyester, etc. Among them, polyethylene or polyvinyl alcohol as the base material and adding starch are the main research objects for degradable plastics. Blends of starch and other natural and synthetic biodegradable plastics are another important product. Among them, natural polymers include pectin, cellulose, galactose, chitin, etc., which can be used to prepare packaging materials or food containers.
(1) The untreated starch of starch and traditional synthetic resin blending plastic has the following shortcomings: poor compatibility with polyethylene and other polymers; poor dispersion; hydrophilicity affects the dimensional stability of the finished product; thermal stability Poor performance, processing temperature can not be higher than 230 ℃, or even lower. There are many treatment methods for starch, including simple surface treatment, gelatinization treatment, various denaturation treatments, and graft modification of starch, etc.: strong drying to make the moisture content less than 1%; coupling agent treatment, silane, epoxy modification Dimethicone (add corn oil) to make it hydrophobic; compatibilizer, EAA, EVA, EVOH; graft modification, graft vinyl acetate, PMMA, PS, MAH, PAA, SBS, acrylic acid ester. The treated starch is mixed with other additives such as self-oxidizing agent, and mixed with polyethylene and other carriers in a vented co-rotating twin-screw extruder to make a masterbatch. The masterbatch is added to general-purpose plastics in the required proportions and processed into products on the usual molding equipment. Starch-filled plastics are currently the most fully studied starch-based plastics at home and abroad. Generally, natural starch or its derivatives are used as fillers and added to the polymer in the form of particles.
In the past, it was generally added to general-purpose plastics (such as polyethylene, polystyrene, etc.). When starch is blended into non-degradable traditional plastics, the prepared plastics are often not biodegradable; and the plastics prepared when blended with other biodegradable polymers can also be biodegradable.
(2) Degradable plastics of starch and biodegradable materials Starch can be compounded with natural macromolecules such as gum, cellulose, galactose, chitin, etc. to form completely biodegradable materials, which are used to prepare packaging materials or food containers. The starch blend biodegradable plastic is the blending of starch and other biodegradable polymers, such as polycaprolactone, polylactic acid, etc., and then molding. Starch and other natural polymer materials such as cellulose, hemicellulose, lignin, pectin, chitin, protein, etc. are compounded to produce fully biodegradable plastics, which is a kind of all-natural biomaterials developed in recent years. The blend of starch and synthetic biodegradable polymer has biodegradable properties. The biodegradable materials blended with starch are mainly polyvinyl alcohol, polyhydroxyalkanoates (such as polyhydroxybutyrate, polyhydroxyvalerate, polyhydroxybutyrate valerate, etc.), polycaprolactone, polyhydroxyalkanoate, etc. Lactic acid, polybutenoic acid succinate, etc. The conditions for preparing the starch of biodegradable starch-based plastics with excellent mechanical properties: the content of amylose in starch is high; the compatibility of starch and polymer is good, and the ideal state of biodegradable starch-based plastics should be close to the molecular level of starch A form compatible with the polymer; it is best to have a continuous starch phase to ensure the degradation of microorganisms by enzymes. Degradable materials made from starch and natural polymer materials have many advantages. The raw materials it needs are renewable resources. Its unit price is far lower than that of traditional plastics. The average cost decreases with the increase in output, and it can be completely biodegraded and degraded. The product is not harmful to the environment,
It does not produce harmful gases when burned; at the same time, it also has a certain degree of thermoplasticity, which can be heat-sealed and further deep-drawn. It is an ideal biodegradable material. Its film sheet can be used as a packaging material or It can be used as a raw material to be processed into various shaped products with a wide range of uses.
