Hydroxypropyl methyl cellulose ether is a kind of nonpolar cellulose ether soluble in cold water obtained from natural cellulose through alkalization and etherification modification.
Keywords:hydroxypropyl methylcellulose ether; alkalization reaction; etherification reaction
1. Technology
Natural cellulose is insoluble in water and organic solvents, stable to light, heat, acid, salt and other chemical media, and can be moistened in dilute alkali solution to change the surface of cellulose.
Hydroxypropyl methyl cellulose ether is a kind of non-polar, cold water-soluble cellulose ether obtained from natural cellulose through alkalization and etherification modification.
2. The main chemical reaction formula
2.1 Alkalization reaction
There are two possibilities for the reaction of cellulose and sodium hydroxide, that is, according to different conditions to generate molecular compounds, R – OH – NaOH; or to generate metal alcohol compounds, R – ONa.
Most scholars believe that cellulose reacts with concentrated alkali to form a fixed substance, and thinks that each or two glucose groups are combined with one NaOH molecule (one glucose group is combined with three NaOH molecules when the reaction is complete).
C6H10O5 + NaOH→C6H10O5 NaOH or C6H10O5 + NaOH→C6H10O4 ONa + H2O
C6H10O5 + NaOH → (C6H10O5 ) 2 NaOH or C6H10O5 + NaOH → C6H10O5 C6H10O4 ONa + H2O
Recently, some scholars believe that the interaction between cellulose and concentrated alkali will have two effects at the same time.
Regardless of the structure, the chemical activity of cellulose can be changed after the action of cellulose and alkali, and it can react with various chemical media to obtain meaningful species.
2.2 Etherification reaction
After alkalization, the active alkali cellulose reacts with etherification agent to form cellulose ether. The etherifying agents used are methyl chloride and propylene oxide.
Sodium hydroxide acts like a catalyst.
n and m represent the degree of substitution of hydroxypropyl and methyl on the cellulose unit, respectively. The maximum sum of m + n is 3.
In addition to the above-mentioned main reaction, there are also side reactions:
CH2CH2OCH3 + H2O→HOCH2CH2OHCH3
CH3Cl + NaOH → CH3OH + NaCl
3. Process description of hydroxypropyl methylcellulose ether
The process of hydroxypropyl methyl cellulose ether (“cellulose ether” for short) is roughly composed of 6 processes, namely: raw material crushing, (alkalinization) etherification, solvent removal, filtration and drying, crushing and mixing, and finished product packaging.
3.1 Raw material preparation
The natural short-lint cellulose purchased in the market is crushed into powder by a pulverizer to facilitate subsequent processing; the solid alkali (or liquid alkali) is melted and prepared, and heated to about 90 °C to make 50 % caustic soda solution for use. Prepare reaction methyl chloride, propylene oxide etherification agent, isopropanol and toluene reaction solvent at the same time.
In addition, the reaction process requires auxiliary materials such as hot water and pure water; steam, low-temperature cooling water, and circulating cooling water are required to assist power.
Short linters, methyl chloride, and propylene oxide etherification agents are the main materials for producing etherified cellulose, and short linters are used in a large amount. Methyl chloride and propylene oxide participate in the reaction as etherification agents to modify natural cellulose , the amount of usage is not large.
Solvents (or diluents) mainly include toluene and isopropanol, which are basically not consumed, but in view of entrained and volatilized losses, there is a slight loss in production, and the amount used is very small.
The raw material preparation process has a raw material tank area and an attached raw material warehouse. Etherifying agents and solvents, such as toluene, isopropanol, and acetic acid (used to adjust the pH value of the reactants), are stored in the raw material tank area. The supply of short lint is sufficient , can be provided by the market at any time.
The crushed short lint is sent to the workshop with a cart for use.
3.2 (Alkalinization) etherification
(Alkaline) etherification is an important process in the process of etherification of cellulose. In the earlier production method, the two-step reactions were carried out separately. Now the process is improved, and the two-step reactions are combined in one stage and carried out simultaneously.
First, vacuumize the etherification tank to remove the air, and then replace it with nitrogen to make the tank free of air. Add the prepared sodium hydroxide solution, add a certain amount of isopropanol and toluene solvent, start stirring, then add short cotton wool, turn on the circulating water to cool down, and after the temperature drops to a certain level, turn on the low-temperature water to lower the system material temperature Drop to about 20 ℃, and maintain the reaction for a certain period of time to complete alkalization.
After alkalization, add the etherifying agent methyl chloride and propylene oxide measured by the high-level metering tank, continue to start stirring, use steam to raise the system temperature to nearly 70 ℃ ~ 80 ℃, and then use hot water to continue heating and maintaining The reaction temperature is controlled, and then the reaction temperature and reaction time are controlled, and the operation can be completed by stirring and mixing for a certain period of time.
The reaction is carried out at about 90 °C and 0.3 MPa.
3.3 Desolvation
The above-mentioned reacted process materials are sent to the desolventizer, and the materials are stripped and heated with steam, and the toluene and isopropanol solvents are evaporated and recovered for recycling.
The evaporated solvent is first cooled and partially condensed with circulating water, and then condensed with low-temperature water, and the condensate mixture enters the liquid layer and separator to separate the water and solvent. The mixed solvent of toluene and isopropanol in the upper layer is adjusted in proportion. Use it directly, and return the water and isopropanol solution in the lower layer to the desolventizer for use.
Add acetic acid to the reactant after desolvation to neutralize excess sodium hydroxide, then use hot water to wash the material, make full use of the coagulation characteristic of cellulose ether to hot water to wash the cellulose ether, and refine the reactant. The refined materials are sent to the next process for separation and drying.
3.4 Filter and dry
The refined material is sent to the horizontal screw separator by high-pressure screw pump to separate free water, and the remaining solid material enters the air dryer through the screw feeder, and is dried in contact with hot air, and then passes through the cyclone separator and air Separation, the solid material enters the subsequent crushing.
The water separated by the horizontal spiral separator enters the water treatment tank after sedimentation in the sedimentation tank to separate the entrained cellulose.
3.5 Crushing and mixing
After drying, the etherified cellulose will have uneven particle size, which needs to be crushed and mixed so that the particle size distribution and overall appearance of the material meet the product standard requirements.
3. 6 Finished product packaging
The material obtained after crushing and mixing operations is the finished etherified cellulose, which can be packaged and put into storage.
4. Summary
The separated wastewater contains a certain amount of salt, mainly sodium chloride. The waste water is evaporated to separate the salt, and the evaporated secondary steam can be condensed to recover condensed water, or directly discharged. The main component of the separated salt is sodium chloride, which also contains a certain amount of sodium acetate because of the neutralization with acetic acid. This salt has industrial utilization value only after recrystallization, separation and purification.