According to the current situation of HPMC production in my country, the factors that affect the quality of hydroxypropyl methylcellulose are analyzed, and on this basis, how to improve the quality level of hydroxypropyl methylcellulose is discussed and studied, so as to into production.
Key words: hydroxypropyl methylcellulose; quality; control; research
Hydroxypropyl methylcellulose (HPMC) is a non-ionic water-soluble cellulose mixed ether made from cotton, wood, and etherified with propylene oxide and methyl chloride after alkali swelling. Cellulose mixed ether is The modified derivative of single substituent ether has better unique properties than the original monoether, and can play the performance of cellulose ether more comprehensively and perfectly. Among the many mixed ethers, hydroxypropyl methylcellulose is the most important. The preparation method is to add propylene oxide to alkaline cellulose. Industrial HPMC can be described as a universal product. The degree of substitution of the methyl group (DS value ) is 1.3 to 2.2, and the molar substitution degree of hydroxypropyl is 0.1 to 0.8. It can be seen from the above data that the content and properties of methyl and hydroxypropyl in HPMC are different, resulting in the final product viscosity and The difference in uniformity leads to fluctuations in the quality of finished products of various production enterprises.
Hydroxypropyl methylcellulose produces ether derivatives through chemical reactions, which have profound changes in its composition, structure and properties, especially the solubility of cellulose, which can be varied according to the type and amount of alkyl groups introduced. Obtain ether derivatives soluble in water, dilute alkali solution, polar solvents (such as ethanol, propanol) and non-polar organic solvents (such as benzene, ether), which greatly expands the varieties and application fields of cellulose derivatives .
1. Effect of hydroxypropyl methylcellulose alkalization process on quality
The alkalization process is the first step in the reaction stage of HPMC production, and it is also one of the most critical steps. The inherent quality of HPMC products is largely determined by the alkalization process, not the etherification process, because the alkalization effect directly affect the effect of etherification.
Hydroxypropyl methylcellulose interacts with alkaline solution to form alkali cellulose, which is highly reactive. In the etherification reaction, the main reaction of the etherification agent to the swelling, penetration, and etherification of cellulose and The rate of side reactions, the uniformity of the reaction and the properties of the final product are all related to the formation and composition of alkali cellulose, so the structure and chemical properties of alkali cellulose are important research objects in the production of cellulose ether.
2. Effect of temperature on the quality of hydroxypropyl methylcellulose
In a certain concentration of KOH aqueous solution, the adsorption amount and swelling degree of hydroxypropyl methylcellulose to alkali increase with the decrease of reaction temperature. For example, the output of alkali cellulose varies with the concentration of KOH: 15 %, 8% at 10°C, and 4.2% at 5°C. The mechanism of this trend is that the formation of alkali cellulose is an exothermic reaction process. As the temperature rises, the adsorption of hydroxypropyl methylcellulose on alkali The amount is reduced, but the hydrolysis reaction of alkali cellulose is greatly increased, which is not conducive to the formation of alkali cellulose. It can be seen from the above that lowering the alkalization temperature is conducive to the generation of alkali cellulose and inhibits the hydrolysis reaction.
3. The effect of additives on the quality of hydroxypropyl methylcellulose
In the cellulose-KOH-water system, the additive—salt has a great influence on the formation of alkali cellulose. When the concentration of KOH solution is lower than 13%, the adsorption of cellulose to alkali is not affected by the addition of potassium chloride salt. When the concentration of lye solution is higher than 13%, after adding potassium chloride, the apparent adsorption of cellulose to alkali The adsorption increases with the concentration of potassium chloride, but the total adsorption capacity decreases, and the water adsorption increases greatly, so the addition of salt is generally unfavorable to the alkalization and swelling of cellulose, but salt can inhibit hydrolysis and regulate the system The free water content thus improves the effect of alkalization and etherification.
4. The influence of production process on the quality of hydroxypropyl methylcellulose
At present, hydroxypropyl methylcellulose production enterprises in my country mostly adopt the production process of solvent method. The preparation and etherification process of alkali cellulose are all carried out in an inert organic solvent, so the raw material refined cotton needs to be pulverized to obtain a larger Surface area and reactivity to ensure the quality of the finished product.
Add the pulverized cellulose, organic solvent and alkali solution into the reactor, and use powerful mechanical stirring at a certain temperature and time to obtain alkali cellulose with uniform alkalization and less degradation. Organic dilution solvents (isopropanol, toluene, etc.) have a certain inertness, which makes hydroxypropyl methylcellulose emit uniform heat during the formation process, showing a stepwise release progress, while reducing the hydrolysis reaction of alkali cellulose in the opposite direction To obtain high-quality alkali cellulose, usually the concentration of lye used in this link is as high as 50%.
After cellulose is soaked in lye, fully swollen and evenly alkalized alkali cellulose is obtained. The lye osmotically swells cellulose better, laying a good foundation for the subsequent etherification reaction. Typical diluents mainly include isopropanol, acetone, toluene, etc. The solubility of lye, the type of diluent and stirring conditions are the main factors affecting the composition of alkali cellulose. The upper and lower layers are formed when mixing. The upper layer is composed of isopropanol and water, and the lower layer is composed of alkali and a small amount of isopropanol. The cellulose dispersed in the system is fully in contact with the upper and lower liquid layers under mechanical stirring. The alkali in the system The water balance shifts until cellulose is formed.
As a typical cellulose non-ionic mixed ether, the content of hydroxypropyl methylcellulose groups is on different macromolecular chains, that is, the distribution ratio of methyl and hydroxypropyl groups is different on the C of each glucose ring position. It has greater dispersion and randomness, making it difficult to guarantee the quality stability of the product.