Hydroxypropyl methylcellulose HPMC is a non-ionic cellulose mixed ether among various mixed ethers with ionic methylcarboxymethylcellulose. It does not react with heavy metals. The differences in the content of hydroxypropyl methylcellulose, hydroxypropyl methylcellulose, and hydroxypropyl content ratio and viscosity of the oxygen free gene have completely different varieties in terms of performance. For example, varieties with high methoxyl content and low hydroxypropyl content have different performance. Close to methylcellulose and low methoxy content varieties. Compared with varieties with higher hydroxypropyl content, its performance is close to that of the hydroxypropyl methylcellulose produced. However, although each variety contains only a small amount of hydroxypropyl group or a small amount of methoxy group, there are great differences in solubility in organic solvents or flocculation temperature in aqueous solutions.
1. Solubility of hydroxypropyl methylcellulose
Solubility of hydroxypropyl methylcellulose in water Hydroxypropyl methylcellulose is actually a propylene oxide (methylhydroxypropyl ring) modified methylcellulose, so it still has the same properties as methylcellulose It has similar properties of being soluble in cold water but insoluble in hot water. However, the gelling temperature of modified hydroxypropyl in hot water is much higher than that of methylcellulose. For example, a 2% hydroxypropyl methylcellulose aqueous solution with methoxy group content DS=0.73 and hydroxypropyl group content MS=0.46 has a viscosity of 500mpa at 20°C. The gel temperature of S product is close to 100°C, while that of methylcellulose at the same temperature is only about 55°C. As for its solubility in water, it has also been greatly improved. For example, after hydroxypropyl methylcellulose is crushed (the particle shape is 0.2~0.5mm, the viscosity of 4% water at 20°C is 2pA·S, it can be used at room temperature without cooling. Easily dissolves in water).
(2) The solubility of hydroxypropyl methylcellulose in organic solvents The solubility of hydroxypropyl methylcellulose in organic solvents is also better than that of methylcellulose. Methylcellulose requires a methoxy substitution degree of 2.1 The above products contain high-viscosity hydroxypropyl methylcellulose with hydroxypropyl MS=1.5~1.8 and methoxy DS=0.2~1.0, with a total substitution degree of more than 1.8, and are easily soluble in anhydrous methanol and ethanol solutions. Thermoplastic and water-soluble. It is also soluble in chlorinated hydrocarbons such as methylene chloride and chloroform, and organic solvents such as acetone, isopropyl alcohol and diacetone alcohol. Its solubility in organic solvents is better than water solubility.
2. Factors affecting the viscosity of hydroxypropyl methylcellulose
Hydroxypropyl methylcellulose viscosity factor The standard viscosity measurement of hydroxypropyl methylcellulose is the same as other cellulose ethers, and is measured at 20°C with a 2% aqueous solution as the standard. The viscosity of the same product increases as the concentration increases. For products with the same concentration and different molecular weights, the product with a larger molecular weight has a higher viscosity. Its relationship with temperature is similar to that of methylcellulose. When the temperature rises, the viscosity begins to decrease, but when it reaches a certain temperature, the viscosity suddenly rises and gelation occurs. Low viscosity products have a higher gelling temperature than high viscosity products. The gel point is not only related to the viscosity of the ether, but also related to the composition ratio of methoxy and hydroxypropyl groups in the ether and the total degree of substitution. It must be noted that hydroxypropyl methylcellulose is also pseudoplastic; its solutions are stable when stored at room temperature and do not exhibit any viscosity decrease, except for possible enzymatic degradation.
3. Hydroxypropyl methylcellulose is acid and alkali resistant
Hydroxypropyl methylcellulose is acid and alkali resistant. Hydroxypropyl methyl cellulose is acid and alkali resistant. It is generally stable and will not be affected by the pH value in the range of PH2~12. It can withstand a certain amount of weak acids, such as formic acid, acetic acid, and lemon. Acid, succinic acid, phosphoric acid but concentrated acid has the effect of reducing viscosity. Alkali such as caustic soda, caustic potassium, and lime water have no effect on it, but the effect of slightly increasing the viscosity of the solution will gradually decrease later.
4. Hydroxypropyl methylcellulose can be mixed
Hydroxypropyl methylcellulose solution can be mixed with water-soluble polymer compounds to form a uniform and transparent solution with higher viscosity. These polymer compounds include polyethylene glycol, polyvinyl acetate, polysiloxane, polymethyl vinyl siloxane, hydroxyethyl cellulose and methyl cellulose. Natural polymer compounds such as acacia gum, locust bean gum, locust gum, etc. also have good mixability. its solution. Hydroxypropyl methylcellulose can also be mixed with stearic acid or mannitol palmitate or sorbitol, and can also be mixed with glycerin, sorbitol and mannitol. These compounds can be used as hydroxypropyl methylcellulose plasticizers agent.
5. Hydroxypropyl methylcellulose is insoluble in water
Hydroxypropyl methylcellulose is insoluble in water-soluble cellulose ethers and can be surface-cross-linked with aldehydes, causing these water-soluble ethers to precipitate in the solution and become insoluble in water. And make hydroxypropyl methylcellulose insoluble in aldehydes, formaldehyde, glyoxal, succinic acid, dialdehyde, etc. When using formaldehyde, special attention should be paid to the pH value of the solution. Among them, glyoxal reacts quickly, so it is used in industry Glyoxal is commonly used as a cross-linking agent in production. -Crosslinking agent. The dosage of this type of cross-linking agent in the solution is 0.2% to 10% of the mass of ether, and the best is 7% to 10%. If glyoxal is used, 3.3% to 6% is most suitable. The general treatment temperature is 0~30℃ and the time is 1~120min. The cross-linking reaction needs to be carried out under acidic conditions. Generally, inorganic strong acid or organic carboxylic acid is added to the solution to adjust the pH of the solution to about 2 to 6, preferably between 4 and 6, and then aldehydes are added to perform the cross-linking reaction. The acids used include hydrochloric acid, sulfuric acid, phosphoric acid, formic acid, acetic acid, glycolic acid, succinic acid or citric acid, among which formic acid or acetic acid is the best, and formic acid is the best. Acids and aldehydes can also be added simultaneously to cross-link the solution within the desired pH range. This reaction is often used in the final step of the cellulose ether preparation process to make the cellulose ether insoluble and facilitate washing and purification with 20~25°C water. When using the product, you can add alkaline substances to the product solution to adjust the pH of the solution to be alkaline so that the product can quickly dissolve in the solution. This method can also be used when a film is prepared using a cellulose ether solution and then the film is processed into an insoluble film.
6. Hydroxypropyl methylcellulose anti-enzyme
The cellulose derivatives of hydroxypropyl methylcellulose are theoretically resistant to enzymes. For example, each anhydroglucose group is firmly bonded with a substituent group and is not susceptible to microbial erosion and infection. However, in fact, the substitution value of the finished product exceeds 1. It can also be degraded by enzymes, which shows that the degree of substitution of each group in the cellulose chain is uneven, and microorganisms can erode nearby unsubstituted anhydroglucose groups to form sugar, which can be absorbed by microorganisms as food. Therefore, if the degree of ether substitution of cellulose increases, the resistance of cellulose ethers to enzymatic attack will increase. It is reported that under controlled conditions, hydroxypropyl methylcellulose (DS=1.9), methylcellulose (DS=1.83), methylcellulose (DS=1.66), hydroxyethylcellulose (1.7%) The residual viscosities are 13.2%, 7.3%, 3.8% and 1.7% respectively. Hydroxypropyl methylcellulose has strong anti-enzyme capabilities. It can be seen that hydroxypropyl methylcellulose has excellent enzyme resistance, coupled with its good dispersion, thickening and film-forming properties, it can be used in emulsion coatings, etc., and generally does not require the addition of preservatives. However, in order to prevent the solution from long-term storage or possible external contamination, preservatives can be added, and the selection of preservatives can be determined based on the final requirements of the solution. Phenylmercuric acetate and manganese fluorosilicate are effective preservatives, but they are toxic and must be handled with care. Generally, 1 to 5 mg of phenylmercuric acetate can be added to each liter of solution.
7. Performance of hydroxypropyl methylcellulose membrane
Hydroxypropyl methylcellulose film-forming properties Hydroxypropyl methylcellulose has excellent film-forming properties. When its aqueous solution or organic solvent solution is coated on a glass plate, it becomes a colorless, transparent and tough film after drying. . It has good moisture resistance and remains solid at high temperatures. If hygroscopic plasticizers are added, the elongation and flexibility can be enhanced, and the flexibility can be improved. Plasticizers such as glycerol and sorbitol are the most suitable. The general solution concentration is 2%~3%, and the plasticizer dosage is 10%~20% of the cellulose ether. If the plasticizer content is required to be high, colloid syneresis will occur under high humidity. The tensile strength of the film with plasticizer added is much greater than that of the film without plasticizer, and it increases with the amount of plasticizer added. The hygroscopicity of the film also increases with the amount of plasticizer.