(1) Solubility
The solubility of cellulose ether in alkaline aqueous solution, water, or organic solvent depends on the nature of the etherification group and the degree of substitution (DS). Substances with a DS value below 0.1 are generally insoluble and differ from cellulose only in some physical and technical parameters, such as tensile strength, surface potential energy, water absorption capacity, or dyeability. Modification of cellulose, to this extent, is mainly used for the reprocessing of cellulose in the textile and paper industries and is not marketed as cellulose ether products.
When the DS range of the product reaches 0.2~0.5, it begins to dissolve in an alkali aqueous solution, such as 5%~8% NaOH. The dissolving performance depends on the etherification group. As the degree of substitution increases, cellulose ether gradually dissolves in water. Good solubility is maintained at very high DS levels for anionic and highly hydrophilic nonionic types, but if hydrophobic substituents dominate, the solubility disappears at higher DS levels.
Many industrially produced cellulose ethers are soluble in water and organic solvents. For the anionic type to obtain water solubility, the DS value must be above 0.4; for the nonionic type, the DS value must be above 1. If hydrophobic etherifying groups dominate, water solubility disappears at DS values above 2, and it is soluble in protic or polar aprotic solvents, such as low-aliphatic alcohols, ketones, or ethers. Hydrophobic cellulose ethers are also soluble in chlorinated hydrocarbons but less in pure hydrocarbons. Cellulose ethers containing only anionic groups are virtually insoluble in organic solvents in all DS ranges except in polar aprotic solvents such as dimethyl sulfoxide. In all cases, lower molecular weight cellulose ethers are more soluble. The solubility of hydrophobic ethers in water will be affected at high temperatures. The dissolved products will gel or agglomerate when heated and dissolve again when cold. This is the unique thermogelling property of hydrophobic cellulose ethers, which significantly impact production and application.
Most applications require cellulose ether solutions to be precise or even transparent, but some cellulose ether products only form turbid solutions, which may contain insoluble particles or free fiber strands. The main reason is that the reactants are not sufficiently stirred and mixed during the reaction, or the cellulose molecular chain is very irregular (the molecular weight distribution is too broad, and the sources of raw materials vary greatly), and the aggregate structure is uneven (highly crystalline areas are difficult to replace) ) caused by uneven substitution. Impurities in cellulose raw materials, such as lignin, ash, etc., or the presence of cross-linking agents in etherification reactants may produce insoluble residues.
(2) The viscosity of the solution
The viscosity range of cellulose ether solution is extensive. At room temperature, the viscosity range of 2% neutral cellulose ether aqueous solution can reach 5-10⁵ mPa·s or even more comprehensive. Its size depends on concentration, temperature, the average chain length of macromolecules (or degree of polymerization), and salt or other additives. The chain length of raw cellulose macromolecules can be shortened through chemical treatment during the cellulose ether production process to obtain the required final product with lower viscosity.
Under defined concentration and temperature conditions, the solution’s rheological properties may be Newtonian, pseudoplastic, thixotropic, or even gelatinous, depending on the chain length, substituent distribution, and the nature of the etherifying group.
(3)Physical properties
Cellulose ethers are white or yellowish solids, usually in granular form or powder (moisture content up to 10%). The apparent density range of powder is 0.3~0.5g/cm³. The apparent density of some (uncrushed) fibrous products is lower than 0.2g/cm³. According to different uses, manufacturers can adjust different purity levels. Products with high purity have no odor or taste. Untreated products may contain up to 40% (mass fraction) of sodium salts such as NaCl, sodium acetate, etc. The product can be mixed with additives to ensure stability, dissolution controllability, and ease of processing.
In addition, most cellulose ether industrial products can be mixed with other water-soluble polymers, such as starch products, natural resins, natural colloids, polyacrylamide, etc., to obtain compound products with the required rheological and other physical properties.
(4)Stability
Cellulose ethers are easily affected by cellulases and microorganisms. The enzyme preferentially attacks unsubstituted anhydroglucose units, which will cause hydrolysis and scission of macromolecular chains, reducing product viscosity. The ether substituent can protect the cellulose main chain. Therefore, the stability of cellulose ether increases with the increase of DS or the improvement of substitution uniformity, and only a few unsubstituted anhydroglucose units are attacked by hydrolase.
Cellulose ether is relatively stable and not easily affected by air, moisture, sunlight, moderate heating, and general pollutants. Potent oxidizing agents can generate peroxide and carbonyl groups, leading to further degradation of cellulose ethers under alkaline conditions. When the cellulose alkaline solution is heated, the viscosity decreases significantly. Strong acids can also degrade molecular chains by directly hydrolyzing cellulose acetal bonds. Like other organic polymers, under the action of high-energy radiation, the chain structure of cellulose ether will also be damaged and degraded. Industrial cellulose ether products can be added with biocides, buffers, or reducing agents where permitted according to the application to achieve long-term storage stability and constant viscosity under appropriate storage conditions.
Solid cellulose ethers are stable at temperatures as high as 80 to 100°C. Higher temperatures or prolonged heating may, in some cases, cause cross-linking and form insoluble networks. The solid product degrades slightly in the range of 130-150°C. When heated to 160-200°C, it will undergo muscular degradation and turn brown. This is related to both the type of ether and the heating conditions. Neutral aqueous solutions will not cause a decrease in viscosity when heated for a long time and then cooled to room temperature. Moderate heating, gelation, or agglomeration will also not affect the viscosity.
