The process and product performance of carboxymethyl cellulose cross-linked by N, N-methylenebisacrylamide to prepare superabsorbent resin were studied, and the concentration of alkali, the amount of cross-linking agent, alkali etherification, and solvent were discussed. The effect of dosage on the water absorption performance of the product. The adsorption mechanism of the water-absorbent resin to water is explained. Studies have shown that the water retention value (WRV) of this product reaches 114ml/g.
Key words: cellulose ether; methylenebisacrylamide; preparation
1、Introduction
Superabsorbent resin is a polymer material with strong hydrophilic groups and a certain degree of crosslinking. Common water-absorbing materials such as paper, cotton, and hemp have low water absorption rate and poor water retention capacity, while super-absorbent resins can absorb water dozens of times their own weight, and the gel formed after absorbing water will not dehydrate even with slight pressure. Excellent water retention capacity. It is neither soluble in water nor in organic solvents.
There are a large number of hydroxyl groups, carboxyl groups and sodium hydrate ions on the molecular chain of the super absorbent material made of cellulose. After absorbing water, the water is surrounded by a hydrophilic macromolecular network and can be retained under external pressure. When water moistens the adsorption resin, a layer of semi-permeable membrane is formed between the resin and water. Due to the high concentration of mobile ions (Na+) in the water-absorbent resin, according to Donnan’s equilibrium principle, this ion concentration difference can cause osmotic pressure. Poor, forming a moistening and swelling weak power, water passes through this layer of semi-permeable membrane and combines with hydrophilic groups and ions on the macromolecules of the superabsorbent resin, reducing the concentration of mobile ions, thereby showing high water absorption and swelling. This adsorption process continues until the osmotic pressure difference caused by the difference in the concentration of mobile ions is equal to the resistance to further expansion caused by the cohesive force of the molecular network of the polymer resin. The advantages of superabsorbent resin prepared from cellulose are: moderate water absorption rate, fast water absorption speed, good salt water resistance, non-toxic, easy to adjust pH value, can be degraded in nature, and low cost, so it has a wide range of uses. It can be used as water blocking agent, soil conditioner, and water retaining agent in industry and agriculture. In addition, it has good development and application prospects in health, food, microbiology, and medicine.
2. Experimental part
2.1 Experimental principle
The preparation of cotton fiber superabsorbent resin is mainly to form a cross-linked structure with a low degree of substitution on the fiber skin. Cross-linking to compounds that generally have two or more reactive functional groups. The functional groups capable of cross-linking include vinyl, hydroxyl, carboxyl, amide, acid chloride, oxirane, nitrile, etc. The water absorption ratio of superabsorbent resins prepared with different cross-linking agents is different. In this experiment, N, N-methylenebisacrylamide is used as a cross-linking agent, including the following steps:
(1) Cellulose (Rcell) reacts with alkaline solution to generate alkali cellulose, and the alkalization reaction of cellulose is a rapid exothermic reaction. Lowering the temperature is conducive to the formation of alkali fibers and can inhibit their hydrolysis. Adding alcohols can increase the disorder of cellulose, which is beneficial to alkalization and subsequent etherification.
RcellOH+NaOH→RcellONa+H2O
(2) Alkali cellulose and monochloroacetic acid generate sodium carboxymethyl cellulose, and the etherification reaction belongs to the nucleophilic substitution reaction:
RcellONa+ClCH2COONa→RcellOCH2COONa+NaCl
(3) N, N-methylenebisacrylamide cross-linked to obtain a super absorbent resin. Because there are still a large number of hydroxyl groups on the molecular chain of carboxymethyl fiber, the ionization of the hydroxyl group of cellulose and the ionization of the acryloyl double bond on the molecular chain of N, N-methylenebisacrylamide can be triggered under the action of alkali catalysis, and then Cross-linking between cellulose molecular chains occurs through Michael condensation, and immediately undergoes proton exchange with water to become a water-insoluble cellulose superabsorbent resin.
2.2 Raw materials and instruments
Raw materials: absorbent cotton (cut into linters), sodium hydroxide, monochloroacetic acid, N, N-methylenebisacrylamide, absolute ethanol, acetone.
Instruments: three-necked flask, electric stirring, reflux condenser, suction filter flask, Buchner funnel, vacuum drying oven, circulating water vacuum pump.
2.3 Preparation method
2.3.1 Alkalinization
Add 1 g of absorbent cotton to the three-necked bottle, then add a certain amount of sodium hydroxide solution and absolute ethanol, keep the temperature below room temperature, and stir for a while.
2.3.2 Etherification
Add a certain amount of chloroacetic acid and stir for 1h.
2.3.2 Crosslinking
In the later stage of etherification, N,N-methylenebisacrylamide was added in proportion to carry out cross-linking, and stirred at room temperature for 2 hours.
2.3.4 Post-processing
Use glacial acetic acid to adjust the pH value to 7, wash away the salt with ethanol, wash away the water with acetone, filter with suction, and vacuum dry for 4 hours (at about 60°C, vacuum degree 8.8kPa) to obtain a white cotton filament product.
2.4 Analytical testing
The water absorption rate (WRV) is determined by sieving, that is, 1g of the product (G) is added to a beaker containing 100ml of distilled water (V1), soaked for 24 hours, filtered through a 200-mesh stainless steel screen, and the water at the bottom of the screen is collected (V2). The calculation formula is as follows: WRV=(V1-V2)/G.
3. Results and discussion
3.1 Selection of alkalization reaction conditions
In the process of producing alkali cellulose by the action of cotton fiber and alkaline solution, the process conditions have a significant impact on the performance of the product. There are many factors in the alkalization reaction. For the convenience of observation, the orthogonal experiment design method is adopted.
Other conditions: The solvent is 20ml of absolute ethanol, the ratio of alkali to etherifying agent (mol/md) is 3:1, and the crosslinking agent is 0.05g.
The experimental results show that: primary and secondary relationship: C>A>B, the best ratio: A3B3C3. The concentration of lye is the most important factor in the alkalization reaction. The high concentration of lye is conducive to the formation of alkali cellulose. However, it should be noted that the higher the concentration of lye, the greater the salt content of the prepared superabsorbent resin. Therefore, when washing the salt with ethanol, wash it several times to ensure that the salt in the product is removed, so as not to affect the water absorption capacity of the product.
3.2 Effect of crosslinking agent dosage on product WRV
The experimental conditions are: 20ml of absolute ethanol, 2.3:1 ratio of alkali to etherification agent, 20ml of lye, and 90min of alkalization.
The results showed that the amount of cross-linking agent affected the cross-linking degree of CMC-Na. Excessive cross-linking leads to a tight network structure in the product space, which is characterized by low water absorption rate and poor elasticity after water absorption; when the amount of cross-linking agent is small, the cross-linking is incomplete, and there are water-soluble products, which also affect the water absorption rate. When the amount of cross-linking agent is less than 0.06g, the water absorption rate increases with the increase of the amount of cross-linking agent, when the amount of cross-linking agent is more than 0.06g, the water absorption rate decreases with the amount of cross-linking agent. Therefore, the dosage of crosslinking agent is about 6% of the cotton fiber mass.
3.3 Effect of etherification conditions on product WRV
The experimental conditions are: alkali concentration 40%; alkali volume 20ml; absolute ethanol 20ml; cross-linking agent dosage 0.06g; alkalization 90min.
From the chemical reaction formula, the alkali-ether ratio (NaOH:CICH2-COOH) should be 2:1, but the actual amount of alkali used is greater than this ratio, because a certain free alkali concentration must be guaranteed in the reaction system, because: certain A higher concentration of free base is conducive to the completion of the alkalization reaction; the cross-linking reaction must be carried out under alkaline conditions; some side reactions consume alkali. However, if the amount of alkali is added too much, the alkali fiber will be seriously degraded, and at the same time, the efficiency of the etherification agent will be reduced. Experiments show that the ratio of alkali to ether is about 2.5:1.
3.4 Influence of solvent amount
The experimental conditions are: alkali concentration 40%; alkali dosage 20ml; alkali-ether ratio 2.5:1; cross-linking agent dosage 0.06g, alkalization 90min.
The solvent anhydrous ethanol plays the role of dispersing, homogenizing and maintaining the slurry state of the system, which is beneficial to disperse and transfer the heat released during the formation of alkali cellulose, and can reduce the hydrolysis reaction of alkali cellulose, thereby obtaining uniform cellulose . However, if the amount of alcohol added is too much, the alkali and sodium monochloroacetate will dissolve in it, the concentration of reactants will decrease, the reaction rate will decrease, and it will also have an adverse effect on the subsequent crosslinking. When the amount of absolute ethanol is 20ml, the WRV value is large.
In summary, the most suitable conditions for preparing superabsorbent resin from absorbent cotton alkalized and etherified carboxymethyl cellulose cross-linked by N, N-methylenebisacrylamide are: alkali concentration 40%, solvent-free 20ml of water and ethanol, the ratio of alkali to ether is 2.5:1, and the dosage of crosslinking agent is 0.06g (6% of the amount of cotton linters).