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Characteristics Of Cellulose Derivatives
Jan 24, 2018

Cellulose is a crystalline natural polymer, most of the ester, etherification reaction is a heterogeneous reaction in the case of cellulose to maintain a solid state, the reaction reagent to the proliferation of cellulose fiber, said the reachability and degree. Crystalline molecules arranged in close, the reagent can only spread to the crystalline surface. Intergranular amorphous molecules arranged loosely, more free hydroxyl easily contact with the reagent, reachable and higher, easier to react. In general, a raw material that has a high degree of crystallinity and a large crystal size is less reactive than a material with a low degree of crystallinity and a small crystal size. However, this is not entirely true. For example, the rate of acetylation of less crystalline and less crystalline dry viscose fibers is significantly lower than that of higher crystallinity and more crystalline cotton fibers. This is because some hydrogen bonds occur between adjacent polymers during drying, hindering reagent diffusion. If the moisture in the wet cellulosic feedstock is replaced with a larger volume of organic solvent (such as acetic acid, benzene, pyridine) and then dried, its reactivity is greatly improved because all of the solvent can not be driven out by drying and some Larger molecules are entrapped in the "holes" of the cellulosic feedstock to form so-called containment cellulose. It has been difficult to recover the swelling distance, which is conducive to the proliferation of reagents to promote the reaction rate and the uniformity of the reaction. To this end, in a variety of cellulose derivatives in the production process, there must be a corresponding swelling treatment. Usually water, acid or a concentration of alkali solution as a swelling agent. In addition, the ease and chemical reaction of dissolving slurries with the same physical and chemical indices often differ greatly due to the morphological factors of various cells with different biochemical and structural functions in various plants or in the same plant of. The primary wall of the outer layer of plant fiber hinders reagent permeation and delayed chemical reaction, so the primary wall is usually damaged by the corresponding conditions in the pulping process to obtain the dissolving pulp with better reaction performance. For example, bagasse pulp is a poorly reactive raw material in the manufacture of viscose fibers and consumes more carbon disulfide than cotton fluff pulp and wood pulp in the preparation of viscose (cellulose xanthate solution) The filtration rate is lower than the viscose prepared with other slurries. This is because the primary wall of sugarcane fiber cells in the process of pulp and pulp preparation by conventional methods were not properly destroyed, resulting in yellow reaction difficult.

Pre-hydrolyzed bagasse pulp fiber] and Figure 2 [alkali-impregnated bagasse pulp fiber] were pre-hydrolyzed alkaline bagasse and alkaline bagasse alkaline pulp after immersion electron microscope scanning electron microscopy images, the former can still see To the clear pits; the latter due to swelling of the alkali solution pits disappear, but the primary wall is still coated with the entire fiber. If the "second impregnation" (after the common method and then the swelling of the greater swelling of the dilute alkali solution for the second impregnation) or immersion - grinding (common law impregnation combined with mechanical grinding) process, the yellowing reaction can be smoothly , Viscose filtration rate increased significantly. This is because the above two methods can spall off the primary wall to expose the inner secondary wall that is easier to react, which facilitates the reagent permeation and improves the reaction performance (FIG. 3 [secondary impregnated bagasse pulp fiber], FIG. 4 [ Grinding bagasse pulp fiber]).

In recent years, a non-aqueous solvent system that can directly dissolve cellulose has emerged. Such as dimethylformamide and NMMO, dimethyl sulfoxide and paraformaldehyde and other mixed solvents, so that cellulose can be homogeneous reaction. However, some of the above-mentioned heterogeneous reactions no longer apply. For example, when making cellulose diacetate soluble in acetone, it is not necessary to be hydrolyzed with cellulose triacetate, and it can be directly esterified until DS is 2.





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