Cellulose derivatives are produced by esterification or etherification of hydroxyl groups in cellulose polymers with chemical reagents. According to the structural characteristics of the reaction products, cellulose derivatives can be divided into three categories: cellulose ethers, cellulose esters, and cellulose ether esters. The cellulose esters that are actually commercially used are: cellulose nitrate, cellulose acetate, cellulose acetate butyrate and cellulose xanthate. Cellulose ethers include: methyl cellulose, carboxymethyl cellulose, ethyl cellulose, hydroxyethyl cellulose, cyanoethyl cellulose, hydroxypropyl cellulose and hydroxypropyl methyl cellulose. In addition, there are ester ether mixed derivatives.
Properties and uses Through the selection of substituting reagents and process design, the product can be dissolved in water, dilute alkali solution or organic solvent, or have thermoplastic properties, and can be used to manufacture chemical fibers, films, film bases, plastics, insulating materials, coatings, slurry, polymeric dispersant, food additives and daily chemical products. The properties of cellulose derivatives are related to the nature of the substituents, the degree DS of the three hydroxyl groups on the glucose group being substituted, and the distribution of the substituents along the macromolecular chain. Due to the randomness of the reaction, except for the uniformly substituted product when all three hydroxyl groups are substituted (DS is 3), in other cases (homogeneous reaction or heterogeneous reaction), the following three different substitution positions are obtained: Mixed products with unsubstituted glucosyl groups: ① monosubstituted (DS is 1, C, C or C position is substituted, structural formula see cellulose); ② disubstituted (DS is 2, C, C, C, C Or C, C positions are substituted); ③ full substitution (DS is 3). Therefore, the properties of the same cellulose derivative with the same substitution value may also be quite different. For example, cellulose diacetate directly esterified to a DS of 2 is insoluble in acetone, but cellulose diacetate obtained by saponification of fully esterified cellulose triacetate can be completely dissolved in acetone. This heterogeneity of substitution is related to the basic laws of cellulose ester and etherification reactions.
The basic law of cellulose esterification and etherification reaction in the cellulose molecule, the positions of the three hydroxyl groups in the glucose group are different, and the influence of adjacent substituents and steric hindrance are also different. The relative acidity and degree of dissociation of the three hydroxyl groups are: C>C>C. When the etherification reaction is carried out in an alkaline medium, the C hydroxyl group reacts first, then the C hydroxyl group, and finally the C primary hydroxyl group. When the esterification reaction is carried out in an acidic medium, the difficulty of the reaction of each hydroxyl group is opposite to the order of the etherification reaction. When reacting with a bulky substitution reagent, the steric hindrance effect has an important influence, and the C hydroxyl group with a smaller steric hindrance effect is easier to react than the C and C hydroxyl groups.
Cellulose is a crystalline natural polymer. Most of the esterification and etherification reactions are heterogeneous reactions when the cellulose remains solid. The diffusion state of the reaction reagents into the cellulose fiber is called the reachability. The intermolecular arrangement of the crystalline region is tightly arranged, and the reagent can only diffuse to the crystalline surface. The intermolecular arrangement in the amorphous region is loose, and there are more free hydroxyl groups that are easy to contact with reagents, with high accessibility and easy reaction. Generally, raw materials with high crystallinity and large crystal size are not as easy to react as raw materials with low crystallinity and small crystal size. But this is not entirely true, for example, the acetylation rate of dry viscose fibers with lower crystallinity and smaller crystallinity is significantly lower than that of cotton fiber with higher crystallinity and larger crystallinity. This is because some hydrogen bonding points are generated between adjacent polymers during the drying process, which hinders the diffusion of reagents. If the moisture in the wet cellulose raw material is replaced by a larger organic solvent (such as acetic acid, benzene, pyridine) and then dried, its reactivity will be greatly improved, because drying cannot completely drive out the solvent, and some The larger molecules are trapped in the “holes” of the cellulose raw material, forming so-called contained cellulose. The distance that has been enlarged by swelling is not easy to recover, which is conducive to the diffusion of reagents, and promotes the reaction rate and uniformity of the reaction. For this reason, in the production process of various cellulose derivatives, there must be corresponding swelling treatment. Usually water, acid or a certain concentration of alkali solution is used as swelling agent. In addition, the difficulty of the chemical reaction of the dissolving pulp with the same physical and chemical indicators is often very different, which is caused by the morphological factors of various types of plants or cells with different biochemical and structural functions in the same plant. of. The primary wall of the outer layer of plant fiber hinders the penetration of reagents and retards chemical reactions, so it is usually necessary to use corresponding conditions in the pulping process to destroy the primary wall in order to obtain dissolving pulp with better reactivity. For example, bagasse pulp is a raw material with poor reactivity in the production of viscose pulp. When preparing viscose (cellulose xanthate alkali solution), more carbon disulfide is consumed than cotton linter pulp and wood pulp. The filtration rate is lower than that of viscose prepared with other pulps. This is because the primary wall of sugarcane fiber cells has not been properly damaged during pulping and the preparation of alkali cellulose by conventional methods, resulting in difficulty in the yellowing reaction.
Pre-hydrolyzed alkaline bagasse pulp fibers] and Figure 2 [bagasse pulp fibers after alkali impregnation] are electron microscope scanning images of the surface of bagasse pulp fibers after pre-hydrolyzed alkaline process and conventional alkaline impregnation respectively, the former can still be seen to clear pits; in the latter, although the pits disappear due to the swelling of the alkali solution, the primary wall still covers the entire fiber. If the “second impregnation” (ordinary impregnation followed by a second impregnation with a dilute alkali solution with a large swelling effect) or dip-grinding (common impregnation combined with mechanical grinding) process, the yellowing reaction can proceed smoothly, the viscose filtration rate is significantly improved. This is because both of the above two methods can peel off the primary wall, exposing the inner layer of the relatively easy reaction, which is conducive to the penetration of reagents and improves the reaction performance (Fig. 3 [secondary impregnation of bagasse pulp fiber], Fig. Grinding Bagasse Pulp Fibers]).
In recent years, non-aqueous solvent systems that can directly dissolve cellulose have emerged. Such as dimethylformamide and NO, dimethyl sulfoxide and paraformaldehyde, and other mixed solvents, etc., enable cellulose to undergo a homogeneous reaction. However, some of the above-mentioned laws of out-of-phase reactions no longer apply. For example, when preparing cellulose diacetate soluble in acetone, it is not necessary to undergo hydrolysis of cellulose triacetate, but can be directly esterified until the DS is 2.