Under the hottest starch oxidation modified adhesi

2022-08-08
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In order to maintain the agglomeration of starch (insoluble in water) and achieve the purpose of partial oxidation, Xu Lai and Luo Guoping chose Ganoderma lucidum for oxidation reaction under weak alkaline conditions at room temperature (21 ~ 38 ℃). At this reaction temperature, the starch is suspended in water as swelling pellets, and the oxidant mainly acts on the low crystalline and non crystalline regions of the pellets. This part is mainly amylopectin molecules and a small amount of amylose molecular chain segments. Under the test conditions, the hydroxyl group on part of carbon 6 (C6) is oxidized to aldehyde group (- CHO) or carboxyl group (- COOH), A small amount of hydroxyl on position 2 and 3 (c2c3) is oxidized to ketyl Ling (>c = 0) or c2c3 bond breaks into two carboxyl groups. The position and degree of oxidation are related to reaction temperature, pH value, oxidant concentration and other factors

the method of preparing oxidized corn starch by Yang Mingfeng and others is: starch milk with a concentration of 40%, stirring constantly, and the speed of the mixer is 60R/min. Add 2% NaOH to adjust the pH value to 8 ~ 10, slowly add sodium hypochlorite solution (effective chlorine 11 ~ 13%), the amount of sodium hypochlorite uses effective chlorine to account for 5% of the mass of absolutely dry starch, and add dilute hydrochloric acid (2%) to maintain the pH. In the process of oxidation, the formation of carboxyl group affects the decline of pH value, so dilute alkali solution should be added to keep the pH value constant. After adding sodium hypochlorite solution to reach the required oxidation degree, neutralize it to pH 6.0 ~ 6.5, reduce the remaining sodium hypochlorite with sodium sulfite, filter it with vacuum filter, and clean it with water. After washing, it is dried at 60 ℃ to 10% - 12% moisture, which is the product of oxidized starch

2 modification progress of oxidized starch

there are still two problems in the practical application of oxidized starch adhesive in corrugated boxes and other fields. The first is the gel phenomenon. The adhesive often appears gel phenomenon after the storage period (generally 5. Hanging the pendulum on the release device for months); This is because the molecules associate with each other to form a cross-linked structure. When water combines with dispersed starch molecules to re associate, gel phenomenon occurs. Secondly, the adhesive has weak initial viscosity and slow natural drying speed. Since the solid content of the adhesive itself is only 18%-25%, and the moisture content is as high as 75%-82%, the moisture in the adhesive is easy to penetrate into the paper. The initial water content of the carton produced with this adhesive is high. Because there is a moisture gradient in the corrugated paper, if it is not dried in time, the paper blank will become soft, causing edge running and corrugation. In view of the shortcomings of oxidized starch adhesive, such as weak initial adhesion, slow natural drying speed and short storage period, as well as the contradiction between the fluidity and adhesion of the adhesive itself, there are more and more reports on the modification of oxidized starch adhesive by adding inorganic or organic substances

2.1 adding inorganic filler drier

a significant weakness of corn starch adhesive is the natural drying speed of steamed bread. An important measure to improve its performance is to add some inorganic fillers such as sodium modified bentonite, kaolin and other driers to accelerate the film-forming speed of the adhesive and shorten the drying time. This is because the ingredients of this kind of inorganic filler are mainly A1 and Si (bentonite is Al2O3 · 4si02 · 3h20; kaolin is Al2O3 · 2SiO2 · 4h20), which is colloidal after dispersion in water, has huge specific surface area and strong adsorption, and can absorb electrolytes with positive and negative charges. The experimental machine data of Jinan new era Gold Testing Instrument Co., Ltd. is very comprehensive, forming a bridge with negatively charged paper fibers, which not only improves the solid content of the glue, Reducing moisture can also block and fill the gap of paper fibers (especially straw pulp paper), reduce moisture permeability, and improve the initial adhesion and drying speed of the adhesive. Experiments show that bentonite is an ideal quick drying agent with good effect and low price. The appropriate addition amount is 1.% of starch. When the addition amount is too much, the drying speed is accelerated, but the viscosity and fluidity become worse. In addition, active light calcium carbonate is added (the dosage is generally about 1.5%), and Chen Xianhua adds about 1% to the glue solution with 19% solid content. 7% active light calcium carbonate and a small amount of dispersant reduced the drying time from 50rain to 28min

2.2 add high molecular substances

in the oxidized starch adhesive, add organic high molecular substances such as urea formaldehyde glue, modified urea formaldehyde resin, styrene acrylic lotion, epichlorohydrin, etc., so that the polar groups in it can cross-linked with the hydrophilic carboxyl, aldehyde, hydroxyl and other groups in the oxidized starch molecule to form polymers, and the hydrophobic ability is enhanced. However, most of the added macromolecules have fast film-forming speed. In addition, they often form a hydrophobic structure after blending with oxidized starch adhesive. Once painted on the corrugated board, a film will be formed, so as to prevent water from penetrating into the board and improve the drying speed and initial adhesion of the adhesive

taking polyacrylamide (PAM) as an example, the compatibility between PAM and starch in the film reduces the force between starch molecules, thereby increasing the flexibility of starch molecules. On the other hand, the addition of PAM suggested that POM joints should not be used, which was similar to the reinforcement skeleton in reinforced cement, further inhibited the damage of the adhesive film, and played the role of toughening, tackifying and thickening. The appropriate dosage of polyacrylamide is 0.6% - 0.7% of starch. Qiu Qinghua added 2% - 10% modified urea formaldehyde glue to the glue solution, which shortened the drying time of the glue solution on the paperboard by 46%. Guo Chengyuan added 8% ~ 10% urea formaldehyde resin to the glue solution and painted it on the surface of the carton. It was surface dry at room temperature for 1H and had good moisture-proof performance. Zhang Xianhong introduced that adding about 5% of the plasticized urea formaldehyde resin can not only shorten the drying time and improve the initial adhesion, but also make the carton have better gloss after drying. Ma Wenwei introduced that adding 5% epichlorohydrin modified urea formaldehyde resin to oxidized starch glue, the drying time on paperboard is only one third of that of ordinary oxidized starch glue. The paperboard can be formed in 0.5 ~ 1H after adding PVA and urea formaldehyde glue at the same time. The adhesive prepared by adding epichlorohydrin, polyvinyl alcohol, carboxymethyl cellulose and styrene acrylate lotion to the preparation of oxidized starch can also shorten the drying time

Another practical problem encountered in the application of oxidized starch adhesive is the condensation of glue liquid. The ways to solve this problem are: first, strictly control the oxidation degree of oxidation reaction; Second, change the production process and directly make oxidized starch adhesive into solid, which not only solves the problem of poor stability, but also facilitates long-distance transportation. Of course, the production cost also increases. The new way is to add water-soluble polymer dispersants, such as polyvinyl alcohol, polyvinyl formal, etc. to the glue solution, which can prolong the storage life of the glue solution. The polyvinyl formal dispersion system is made of polyvinyl alcohol. The concentration of the dispersion system must be in proportion to the amount of starch. In the whole dispersion system, the pure solid content of polyvinyl formal should not be less than 4%, and the pure solid content of starch should not exceed 10%. Using this, without oxidative cracking, the glue liquid directly gelatinized by starch is placed for one year without mildew, poor fluidity and adhesion. Commonly used methods include adding vinyl acetate and polyvinyl formal glue to the glue solution, and mixing starch acetate with oxidized starch

2.3 the influence of gelatinizer

the scope of sodium hydroxide as a gelatinizer industry has doubled. It combines with the non oxidized hydroxyl group in oxidized starch, destroys the hydrogen bond, so that the force between starch molecules is weakened, and dissolves and gelatinizes. It reacts with the functional groups of oxidized starch to form sodium salt, which increases the hydrophilicity and solubility of starch. It also converts starch, pectin, protein and sugar in corn starch into colloid respectively, It increases the viscosity and hardness of the adhesive. At the same time, it gives the starch glue anti freezing property, increases the fluidity of the starch glue, and makes it easy to store. If the amount is too much, although the adhesive has good transparency, good fluidity and long storage time, the viscosity of the adhesive will decrease, and it is easy to produce foam, resulting in degumming; And the alkalinity is too strong, which has a certain corrosive effect on paper fibers; If the dosage is too small, it will cause insufficient gelatinization of starch, low cohesion and poor fluidity. The experiment shows that the amount of alkali should be controlled at 8% - 10% of the amount of starch

2.4 effect of crosslinking agent

in order to improve the crosslinking degree and initial viscosity of the adhesive, borax is added as crosslinking agent. Borax can form polynuclear complexes with hydroxyl and aldehyde groups of gelatinized oxidized starch in water, which has the function of crosslinking and thickening, increases the viscosity and surface tension of starch adhesive, improves the cohesion and stability, and thus improves the adhesion and water resistance. However, if the amount of glue is too large, the glue will be easy to be drawn after it is put on the machine, and the paper will not absorb the glue, and even lose its fluidity and viscosity; If the amount is too small, the bonding is not enough, making the glue too thin and poor adhesion. The experiment determined that the amount of borax should be controlled at 2% - 3% of the amount of starch

3 prospect of oxidized modified starch products

oxidized starch adhesive after physical and chemical modification research, the initial adhesion, drying speed, stability and other aspects of the adhesive have been significantly improved compared with ordinary oxidized starch adhesive, which has been popularized and applied in practice. But people have also done a lot of work in starch modification. Wen Weihua is concerned with the preparation of adhesives by replacing starch with corn flour, converting corn gluten into sodium alkyl sulfonate to improve fiber affinity, saponifying the fat in corn under alkaline conditions, and improving the anti-aging ability of adhesives. In the paper industry, anionic oxidized cyanohexyl starch used in coated base paper, phosphorus ammonia double modified starch used in the production of high-grade paper such as coated paper and food industry have expanded the application range of modified products of starch. Liu Yanli et al. Systematically tested the size and size film properties of oxidized esterified composite modified starch, oxidized crosslinked composite modified starch and original starch, and analyzed the results. There are many kinds of modified starch, mainly including acidified starch, oxidized starch, crosslinked starch, esterified starch powder, grafted starch, etc. Due to the degradation of oxidized starch to a certain extent and the introduction of carboxyl groups, the degree of polymerization is reduced and the molecular fluidity is improved. Other natural polymer materials, such as konjak, have better properties than starch and are competitive in adhesives. There are still many problems to be solved for modified starch adhesive

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