Hydroxypropyl methyl cellulose HPMC is a kind of non-ionic cellulose mixed ether, which is different from ionic methyl carboxymethyl cellulose mixed ether, and it does not react with heavy metals. Due to the different ratios of methoxyl content and hydroxypropyl content in hydroxypropyl methylcellulose and different viscosities, it has become a variety of varieties with different properties, for example, high methoxyl content and low hydroxypropyl content Its performance is close to that of methyl cellulose, and the varieties with low methoxyl content and high hydroxypropyl content, its performance is close to that of hydroxypropyl methylcellulose. However, in various varieties, although only a small amount of hydroxypropyl group or a small amount of methoxy group is contained, the solubility in organic solvents or the flocculation temperature in aqueous solution are very different.
1. The solubility of hydroxypropyl methylcellulose
①Solubility of hydroxypropyl methyl cellulose in water Hydroxypropyl methyl cellulose is actually a methyl cellulose modified by propylene oxide (methyl oxypropylene), so it still has the same properties as methyl cellulose. Cellulose is similar in cold water solubility and hot water insolubility. However, due to the modified hydroxypropyl group, its gelation temperature in hot water is much higher than that of methyl cellulose. For example, the viscosity of the aqueous solution of hydroxypropyl methylcellulose with 2% methoxyl content DS=0.73 and hydroxypropyl content MS=0.46 is 500 mpa?s at 20°C. Its gel temperature It can reach close to 100°C, while methylcellulose at the same temperature is only about 55°C. As for its solubility in water, it has also been greatly improved. For example, the pulverized hydroxypropyl methylcellulose (granular shape of 0.2~0.5mm at 20°C with a viscosity of 4% aqueous solution up to 2pa?s can be used in At room temperature, it is easily soluble in water without cooling.
②The solubility of hydroxypropyl methyl cellulose in organic solvents The solubility of hydroxypropyl methyl cellulose in organic solvents is also better than that of methyl cellulose, and methyl cellulose needs to have a degree of methoxyl substitution at Products above 2.1, and high-viscosity hydroxypropyl methylcellulose containing hydroxypropyl MS=1.5~1.8 and methoxy DS=0.2~1.0, and total substitution degree above 1.8 are soluble in anhydrous methanol and ethanol solutions medium, thermoplastic and water-soluble. It is also soluble in chlorinated hydrocarbons such as dichloromethane and chloroform, and in organic solvents such as acetone, isopropanol and diacetone alcohol. Its solubility in organic solvents is better than water-soluble.
2. Factors affecting the viscosity of hydroxypropyl methylcellulose
Influencing factors of hydroxypropyl methyl cellulose viscosity The standard viscosity determination of hydroxypropyl methyl cellulose, like other cellulose ethers, is based on a 2% aqueous solution at 20°C. The viscosity of the same product increases with the increase of concentration. For products with different molecular weights at the same concentration, the products with larger molecular weights have higher viscosity. Its relationship with temperature is similar to that of methyl cellulose. When the temperature increases, the viscosity begins to decrease, but when it reaches a certain temperature, the viscosity suddenly increases and gelation occurs. The gel temperature of low-viscosity products is higher. is high. Its gel point is not only related to the viscosity of the ether, but also to the composition ratio of methoxy and hydroxypropyl in the ether and the total degree of substitution. It must be noted that hydroxypropyl methylcellulose is also pseudoplastic and its solution is stable at room temperature without any degradation of viscosity except the possibility of enzymatic degradation.
3. Hydroxypropyl methylcellulose acid and alkali resistance
Hydroxypropyl methylcellulose acid and alkali resistance Hydroxypropyl methylcellulose is generally stable to acid and alkali, and is not affected in the range of pH 2~12. It can withstand a certain amount of light acid. Such as formic acid, acetic acid, citric acid, succinic acid, phosphoric acid, boric acid, etc. However, concentrated acid has the effect of reducing viscosity. Alkali such as caustic soda, caustic potash and lime water have no effect on it, but can slightly increase the viscosity of the solution, and then there will be a phenomenon of slow decline in the future.
4. Mixability of hydroxypropyl methylcellulose
Miscibility of Hydroxypropyl Methyl Cellulose Hydroxypropyl methyl cellulose solution can be mixed with water-soluble polymer compound to become 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 gum arabic, locust bean gum, karaya gum, etc. also have good mixability with its solution. Hydroxypropyl methylcellulose can also be mixed with mannitol or sorbitol esters of stearic acid or palmitic acid, and can also be mixed with glycerin, sorbitol and mannitol. These compounds can be used as hydroxypropyl methylcellulose. Cellulose-based plasticizer.
5. Insolubility and water solubility of hydroxypropyl methylcellulose
The insolubilized water-soluble cellulose ethers of hydroxypropyl methylcellulose can be surface-crosslinked with aldehydes, and these water-soluble ethers are precipitated in the solution and become insoluble in water. The aldehydes that make hydroxypropyl methylcellulose insoluble include formaldehyde, glyoxal, succinaldehyde, adipaldehyde, etc. When using formaldehyde, special attention should be paid to the pH value of the solution, among which glyoxal reacts faster. Therefore, glyoxal is commonly used as a crosslinking agent in industrial production. The dosage of this type of crosslinking agent in the solution is 0.2%~10% of the ether mass, preferably 7%~10%, and 3.3%~6% is the most suitable for glyoxal. 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 2~6, preferably between 4~6, and then aldehydes are added to carry out the cross-linking reaction. . The acids used include hydrochloric acid, sulfuric acid, phosphoric acid, formic acid, acetic acid, hydroxyacetic acid, succinic acid or citric acid, among which formic acid or acetic acid is suitable, and formic acid is the most optimal. The acid and aldehyde can also be added simultaneously to allow the solution to crosslink in the desired pH range. This reaction is often used in the final treatment process in the preparation process of cellulose ethers. After the cellulose ether is insolubilized, it is convenient to wash and purify with water at 20~25°C. When the product is in use, an alkaline substance can be added to the solution of the product to adjust the pH of the solution to be alkaline, and the product will dissolve in the solution quickly. This method is also applicable to the cellulose ether solution made into a film and then the film is processed to make it an insoluble film.
6. Hydroxypropyl methylcellulose is resistant to enzymes
Hydroxypropyl methylcellulose is resistant to enzymes. In theory, cellulose derivatives, such as each anhydroglucose group, have a firmly bonded substituent group, which is not easy to be infected by microorganisms, but in fact the finished product When the substitution value exceeds 1, it will also be degraded by enzymes, which means that the degree of substitution of each group on the cellulose chain is not uniform enough, and microorganisms can erode near unsubstituted anhydroglucose groups to form sugars. , which are absorbed as nutrients for microorganisms. Therefore, if the etherification substitution degree of cellulose increases, the resistance to enzymatic erosion of cellulose ether is also enhanced. It is reported that the residual viscosity of hydroxypropyl methylcellulose (DS=1.9) is 13.2%, methylcellulose (DS=1.83) is 7.3%, and methylcellulose (DS=1.66) is 3.8%, and hydroxyethyl cellulose is 1.7%. It can be seen that the anti-enzyme ability of hydroxypropyl methylcellulose is strong. Therefore, the excellent enzyme resistance of hydroxypropyl methylcellulose, combined with its good dispersibility, thickening and film-forming properties, is generally used in water-emulsion coatings, etc., and generally does not require the addition of preservatives. However, for the long-term storage of the solution or the possible contamination of the outside world, preservatives can be added as a precaution, and the choice can be determined according to the final requirements of the solution. Phenylmercuric acetate and manganese fluorosilicate are effective preservatives, but they have both Toxicity, attention must be paid to the operation, and the dosage is generally 1~5mg of phenylmercuric acetate per liter of solution.
7. Properties of hydroxypropyl methylcellulose film
The performance of hydroxypropyl methylcellulose film Hydroxypropyl methylcellulose has excellent film-forming properties, and its aqueous solution or organic solvent solution is coated on a glass plate, and it becomes colorless and transparent after drying. And tough film. It has good moisture resistance and remains solid at high temperatures. For example, adding a hygroscopic plasticizer can enhance its elongation and flexibility. To improve flexibility, plasticizers such as glycerin and sorbitol are the most suitable. The general solution concentration is 2%~3%, and the amount of plasticizer is 10%~20% of cellulose ether. If the content of plasticizer is too high, the shrinkage phenomenon of colloid dehydration will occur under high humidity. The tensile strength of the film with plasticizer added is much larger than that without plasticizer, and it increases with the increase of the added amount. As for the hygroscopicity of the film, it also increases with the increase of the plasticizer amount.
Post time: Oct-13-2022