The preparation of MC is to react cellulose with an alkali solution to generate alkali cellulose and then respond with methyl chloride under a certain pressure and temperature to obtain a crude product, which is separated, washed, dried, pulverized, and packaged to get the product. The preparation methods include the gas phase method, liquid phase method, and homogeneous phase method.
1. Gas-phase method
After pulverization, alkalization, and aging, alkali cellulose reacts with circulating methyl chloride gas. The reaction pressure is about 4.9×10⁵Pa. Since it is a gas-solid phase reaction, the reaction is uneven, and the degree of substitution and ether efficiency is low.
2. Liquid phase method
Using etherifying agent methyl chloride as the dispersion medium, alkali cellulose is suspended in liquid methyl chloride for reaction. Since methyl chloride has a low boiling point (23.6°C under normal pressure), to keep it fluid during the reaction, the reaction pressure is 70°C. It is about 1.7×10⁶Pa, which makes the etherification process in a liquid-solid phase reaction state. The reaction is more uniform, and the degree of substitution and ether efficiency are higher.
Another liquid phase method alkalinizes and etherifies cellulose in the presence of an inert organic solvent. The organic solvent plays a crucial role in reducing the pressure of the reaction system [usually (4~6)×10⁵ Pa], which facilitates transfer. This, in turn, significantly reduces the occurrence of side reactions, making this method a highly effective alternative.
3. Homogeneous method
There are many homogeneous preparation methods of methylcellulose, as follows.
(1) Dissolve cellulose in a DMSO/PF solvent system, using NaOH and iodoform (CH₃I) as etherifying agents.
(2) Cellulose is dissolved in the DMAc/LiCl system, and butyllithium-iodomethane is used to methylate the cellulose.
(3) Due to the difficulty in solvent recovery, the homogeneous production of MC has not yet been industrialized.
The formula and process conditions of methylcellulose production vary according to the required product performance.
The methylation reaction is generally carried out in an autoclave with a jacket, stirring paddle, and nickel lining. The autoclave’s working pressure can reach a gauge pressure of 1.2-1.5MPa, while the pressure of most preparation processes is 0.4-0.5MPa.
The end-point NaOH concentration of the methylation reaction depends on the desired degree of substitution of methylcellulose. A portion of the etherifying agent reacts with NaOH, and the final base concentration depends on the starting concentration, the amount of water produced, and the NaOH consumption. The effective reaction almost stops when the NaOH concentration drops to 30%, and the substitution degree increases above DS=2.0.
As a solvent and carrier for NaOH, water is indispensable as a diluent for the etherification agent, which is beneficial to the penetration of alkali and promotes the etherification reaction. In other words, etherification is almost impossible in the absence of water. However, in the presence of water, the stirring method, stirring degree, and reaction temperature all affect the degree of substitution, and the etherification efficiency decreases as the concentration of water increases. This is because part of the etherifying agent will be hydrolyzed into by-products due to the presence of water. As a swelling agent, water can effectively promote the etherification of the entire cellulose when the alkali concentration is high. The increase in water leads to a decrease in alkali concentration, which weakens the driving force of the etherification reaction and reduces the etherification efficiency.
The efficiency of the methylation reaction refers to the utilization rate of the etherifying agent methyl chloride, which is about 70% to 80% for alkali-soluble methylcellulose and 40% to 50% for water-soluble methylcellulose.
(1) Production technology of alkali-soluble methylcellulose
MC with a low degree of substitution (DS=0.1~0.9, that is, the methoxy group content is 2%~16%) is soluble in 2%~10% NaOH aqueous solution. The preparation process is as follows: First, change the ball drop time (representing the degree of polymerization, which refers to The corresponding falling ball times measured when using a falling ball viscometer to measure refined cotton linters with different degrees of polymerization) are 6s, 15s, and 600s. The cotton liners are treated with 27.5%~45.0% NaOH aqueous solution at 15~35℃. Prepare alkali cellulose containing NaOH with a mass ratio of cellulose of 0.35 to 0.60, and then react alkali cellulose with methyl chloride (mass ratio of cellulose of 0.15 to 0.5) at 35 to 75°C for 4 to 10 hours until the pressure drops to 0, then neutralize with acidic hot water, wash, and dry to obtain the finished product. The product contains 0.9% to 13.5% methoxy groups and can completely dissolve in 4% alkali solution. It can be used for fabric sizing, printing, and dyeing sizing, and it can also be used for the permanent gluing of clothing or processing into a celluloid-like film.
(2) Production process of water-soluble methylcellulose
Conventional equipment is used to prepare MC; methyl chloride amounts are small, and the mass is no more than 1.5 to 3 times the dry base cellulose. For example, 8 parts of refined cotton are immersed in 210 parts of 33% NaOH aqueous solution at 22 to 24°C, then pressed to about 20 parts, and then put into an autoclave. After vacuuming, 248 parts of liquid methyl chloride are added, and the temperature is continued to 50°C. Maintain this temperature for 1 hour, then slowly raise the temperature to 70°C for further etherification, distill to remove excess methyl chloride, wash with hot water to remove NaCl or the generated methanol, and dry to obtain a product with a methoxy group content of 33%.
Using a two-stage process, water-soluble dimethylcellulose with a moderate degree of substitution can also be used to produce high-viscosity products. First, alkali cellulose is conventionally reacted with excess methyl chloride to obtain a product with a methoxy group content of 14% to 21%. Then, alkali liquid and a certain amount of methyl chloride are added for secondary etherification, and finally, a product containing methoxy group is obtained based on 29% to 34% methylcellulose.
(3) Production technology of organic solvent-soluble methylcellulose
Organic solvent-soluble MC with a degree of substitution DS= 2.1 or above is prepared by two-stage etherification. In the alkali cellulose used, the NaOH content is 0.8 to 1.1 times that of cellulose, and the water content is 0.28 to 0.38 times that of cellulose. The methyl chloride used in the etherification process is 1 to 4 times that of cellulose. In the first reaction stage, methylcellulose with a methyl substitution degree of 1.5 to 1.9 is obtained. In the second stage of the etherification reaction, a sufficient amount of solid NaOH is added to increase the content (including the original content) to 1.1 to 1.1 of the mass of the cellulose 3 times; continue etherification until the NaOH concentration drops to at least about 55% (but not less than 30%), the reaction is completed, the obtained uniformly substituted methylcellulose is separated, and post-processed to get the product.
The liquid phase two-step method used by Kalle Company in Germany is: alkalize 100 parts of crushed cellulose in 135.5 parts of 25% NaOH solution for 1 hour, then add 600 parts of liquid methyl chloride and etherify at 85°C until the methoxy group When the content is 14% to 21%, recover unreacted methyl chloride to obtain the methylcellulose intermediate product, then use 133.5 parts of 42% NaOH solution for 15 min, add another 600 parts of methyl chloride to the autoclave, and add 133.5 parts of 42% NaOH solution to the autoclave. Etherify again for 1 hour at ℃. After recovering methyl chloride, the etherified product is placed in hot water of 80 to 90 ℃ for flocculation and purification. Neutralize with 2 parts of glacial acetic acid, centrifuge, and dry at 105 ℃ to obtain a 30.5% methoxyl group of methylcellulose. The process formula reported by Dow Chemical Company in the United States is: put 568 parts of wood pulp and 1136 parts of 50% NaOH solution in a kneader, alkalize at 60°C for 20 minutes, knead, and pulverize; the alkali cellulose composition is NaOH/fiber Element =0.9～1.5, water/cellulose=0.9～1.5. Then, put the crushed alkali cellulose (about 1.6kg) into the autoclave, add liquid methyl chloride, and etherify at 5.7×10⁵ Pa and 75℃ for 12.5h. The product, after recovering the methyl chloride, is heated at 80~90℃. The water was flocculated and purified, washed and purified with hot water twice, centrifuged, and dried (140°C) to obtain MC with a methoxy group content of 29% (DS of 1.8).
