Synthesis principle of hydroxyethyl cellulose
When the molar substitution degree (MS) of hydroxyethylcellulose is 0.05-0.5. It is an alkali-soluble product. HEC with an MS above 1.3 is soluble in water. The MS range of common industrial HEC products currently on the market is 1.3 to 2.5. And the DS range of most water-soluble HEC products is 0.8 to 1.2.
Analysis of heavily hydroxyethylated products. The results show that although MS is greater than 3.0. The DS value does not increase significantly. This shows that the newly added ether group is mainly connected to the side chain of the hydroxyethoxy group or oligomer ether. If the chain length of the side chain oligomer ether is defined as MS/DS, the commercial HEC side chain length value is in the range of 1.5-2.5.
Increasing the molar substitution degree can enhance the water solubility of the product, but the growth of side chains will increase the difficulty of application. Ordinary low MS HEC cannot obtain transparent solutions in organic solvents such as acetone, ethanol and lower ether. High MS HEC is partially soluble in methanol and some mixed solvents composed of water and water-soluble organic solvents.
The raw materials used in industrial HEC production include high-purity wood pulp, hydroxypropyl methylcellulose cotton cellulose, sodium hydroxide and ethylene oxide. It was difficult to prepare HEC in the early days. But later it was produced by a slurry method using an inert solvent as a medium. The quality of the product is stable and its performance has been greatly improved.
There are many kinds of inert solvents (reaction media) used in slurry production. Such as acetone, fatty alcohols, tetrahydrofuran, alkoxyalkanols, aromatic hydrocarbons and alkanes, etc. The key to the production process is strict control of water content, inert solvent type, reaction time, reaction temperature and alkali concentration.
The purification process also requires the use of inert solvents. Whose water content should ensure that the product is insoluble but can dissolve impurities such as salts and ethylene glycol.
Cross-linked HEC can be obtained by treating it with glyoxal or chloroformate under certain acid conditions.
Synthesis principle of hydroxyethyl cellulose
There are many patent reports on the production of HEC by slurry method. Such as adding surfactants, using mixed solvent systems, adding boric acid or borate, mechanical pretreatment technology of cellulose raw materials, etc. It was first used to use glyoxal cross-linked HEC which is insoluble in water to improve the washing effect.
Since the entire reaction process does not consume alkali, the amount of acid used for neutralization can be estimated by the amount of alkali added. However, the amount of acid used during the neutralization process should be slightly higher than the calculated value. So that the pH value of the system after neutralization is between 5 and 7. In order to effectively carry out glyoxal treatment and achieve cross-linking. This results in fast-dispersing, fast-dispersing crystals.
The rate of hydroxyethylation is proportional to the concentration of ethylene oxide at a certain temperature (49°C). Controlled by the diffusion rate of ethylene oxide in alkali cellulose. Usually the amount of ethylene oxide added is excessive, and its utilization rate is between 50% and 70%. The occurrence of side reactions increases the consumption of ethylene oxide and also increases the difficulty of post-storage treatment. The sodium acetate and by-products ethylene glycol and polyethylene glycol produced during the production process not only increase costs. It also increases the COD and BOD indicators of discharged water, causing serious pollution to the water system.
The reaction rate of ethylene oxide increases as the concentration of NaOH increases in the range of 0.4 to 1.5 mol/L, and when the alkali concentration is greater than 2. 5 mol/L. The etherification reaction proceeds rapidly in cellulose, and X-ray diffraction boron studies indicate that the reaction occurs intramolecularly or within unit cell sheets. In the reaction between gaseous ethylene oxide and solid alkali cellulose, the reaction temperature plays an important role in obtaining products with good water solubility. The temperature is generally controlled between 33 and 75°C.
Some other etherifying agents can also be selected to prepare HEC. For example, 2-chloroethanol is used to prepare low-MS cellulose ethers. ethylene glycol carbonate is used to hydroxyethylate the glucose ring in the presence of DMSO and NaH than ethylene oxide. Alkane conversion rate is high.