The water solubility of cellulose ether depends on the type of etherification group and its degree of substitution (DS), and may also be affected by the production process.
1.Types of etherification groups:
Cellulose ethers can be monoethers or mixed ethers, and their properties vary to some extent. Low substituted hydrophilic groups, such as hydroxyethyl groups, on cellulose macromolecules can impart a certain degree of water solubility to the product, while hydrophobic groups, such as methyl, ethyl, etc., can only impart a certain degree of water solubility to the product with a moderate degree of substitution. Low substituted products only swell in water or can dissolve in dilute alkaline solutions. Cellulose ethers can be classified based on the types of substituents, ionization properties, and solubility differences.
2.The size of degree of substitution (DS):
Many industrially produced cellulose ethers are soluble in water or organic solvents. For anionic type, to obtain water solubility, the DS value should be above 0.4, while for non-ionic type, the DS value should be above 1. If hydrophobic etherification groups have an advantage, water solubility disappears when the DS value is higher than 2. In all cases, cellulose ethers with lower molecular weight have stronger solubility. The solubility of hydrophobic ether in water will be affected at high temperature. The dissolved product will undergo gel or agglomeration when heated, and will dissolve again when cooled. This is the unique thermal gel performance and phenomenon of hydrophobic cellulose ether, which has an important impact on production and application.
3.The impact of production process:
Most applications require cellulose ether solutions to be clear or even transparent, but some cellulose ether products can only form turbid solutions, which may contain insoluble particles or free fibers. The main reasons are insufficient and uneven stirring and mixing of reactants during the reaction process, or uneven substitution caused by irregular cellulose molecular chains (wide molecular weight distribution, large differences in raw material sources) and uneven aggregation structure (difficult to replace in high crystalline regions). Impurities such as lignin and ash in cellulose raw materials, or the presence of crosslinking agents in etherification reactants, can all lead to the formation of insoluble residues.
