4. Homogeneous method (dissolution method) production process of HPMC
In the alkali leaching, slurry and gas-solid processes, the reaction system is in a multiphase state, which aggravates the uncertainty of the product structure. The structure determines the properties, and the properties determine the application. Therefore, the randomness and inhomogeneity of the type, position and number of substituent groups have a potential impact on the structure and application performance of the product. Dissolve cellulose. Reacting with reagents in a homogeneous state helps to solve this problem.
Solvents for cellulose are divided into derivatized solvents and non-derivatized solvents. The former is a soluble cellulose intermediate such as ester, ether or acetal formed with cellulose molecules through covalent bonds. Further modification under homogeneous reaction conditions, such as CF3COOH/CF3(C0)2O, DMF/N2O4, HCOOH, DMSO/paraformaldehyde, NaOH/CS2, H3PO4 aqueous solution, Me3SiCl/pyridine, etc.; instead of derivatized solvents The structure of cellulose is changed only by intermolecular forces, resulting in the dissolution of cellulose. Such as DMSO/methylamine, NMMO, DMAc/LiCl, DMAc/LiBr, DMF/LiCI, DMP/LiBr, DMSO/LiCI, HMOT/LiCI, DMSO/TBAF • 3H2O, liquid ammonia/NH4SCN system and ionic liquids of various structures Wait.
For many years, people have carried out chemical modification attempts such as etherification and esterification of cellulose in cellulose solvent systems. Heinze has published a large number of research results on acylation and esterification of cellulose in ionic liquids. The chemical structure of the corresponding cellulose derivatives can be controlled by the reaction process, and the reaction process is mild and the cycle is short, and the solvent can also be recycled and reused. On this basis, Heinze also applies click chemistry to cellulose. Chemical modification, introducing groups such as carboxylate, thiophene, azide and aniline. The reaction process has few side reactions, strong practicability and high selectivity, and has potential application prospects.
In the research of homogeneous preparation of cellulose esters, Zhang Lina et al. prepared hydroxypropyl cellulose, methyl cellulose and cyanoethyl cellulose with NaOH/CS2 and other systems, and made some progress. The process is to directly add the pulverized cellulose into a horizontal reactor with a strong stirring device, disperse it in a 5-8 times NaOH/thiourea or urea system to dissolve the cellulose, and then add a quantitative amount of lye. And the acylating agent to react, after the reaction. The reacted cellulose ether is precipitated with a large amount of acetone, and then washed with hot water, dried, pulverized and sieved to obtain a finished product. This process is the frontier exploration of the current production process. It can replace a very uniform high-quality product with a small amount of etherifying agent. Due to the large amount of solvent used to dissolve cellulose, high cost, and other factors, the current process It is still in the research stage and has not yet been industrialized.
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