What is Hydroxypropyl methyl cellulose HPMC？
1, solubility
However, the gelation temperature of modified hydroxy propyl is much higher than that of methyl cellulose in hot water. For example, the viscosity of hydroxypropyl methylcellulose aqueous solution with 2% methoxy content DS=0.73 and hydroxypropyl content MS=0.46 is 500 mpa at 20℃. The gel temperature of S’s product is close to 100℃, while that of methyl cellulose of the same temperature is only about 55℃. As for its solubility in water, also greatly improved, for example, after the crush of hydroxypropyl methylcellulose (grain shape 0.2~0.5mm at 20℃ 4% aqueous viscosity of 2pA? S products can be easily dissolved in water without cooling at room temperature.
(2) Hydroxypropyl methyl cellulose in organic solvents solubility of hydroxypropyl methyl cellulose in organic solvents solubility, is also better than methyl cellulose, methyl cellulose needs in the methoxy substitution degree of 2.1 or more products, and contains hydroxypropyl MS=1.5~1.8 and methoxy DS=0.2~1.0, High viscosity hydroxypropyl methyl cellulose with total substitution degree above 1.8 is soluble in anhydrous methanol and ethanol solutions, and has thermoplastic and water solubility. It is also soluble in chlorinated hydrocarbons such as dichloromethane and trichloromethane, and organic solvents such as acetone, isopropyl alcohol, and diacetone alcohol. Its solubility in organic solvents is superior to water solubility.
2, viscosity of the influencing factors
Hydroxypropyl methylcellulose viscosity factors hydroxypropyl methylcellulose standard viscosity determination, and other cellulose ether is the same, are at 20℃ with 2% aqueous solution as the standard determination. The viscosity of the same product, with the increase of concentration and increase, the same concentration of different molecular weight products, molecular weight of the product is high viscosity. Its relationship with temperature is similar to that of methyl cellulose. When the temperature rises, the viscosity begins to decline, but when it reaches a certain temperature, the viscosity suddenly rises and gelation occurs. The gelation temperature of products with low viscosity is higher than that of products with high viscosity. The level of its gel point, in addition to the high and low viscosity of ether, but also with ether methoxy and hydroxypropyl group composition ratio and the total degree of substitution are related. It must be noted that hydroxypropyl methylcellulose is also pseudoplastic; its solution is stable when stored at room temperature and does not show any degradation of viscosity except for the possibility of enzymatic degradation.
3,acid and alkaline resistance
Hydroxypropyl methylcellulose acid alkali hydroxypropyl methylcellulose acid and alkali, generally is stable, in the ph PH2~12 range is not affected, it can withstand a certain amount of light acid, such as formic acid, acetic acid, citric acid, succinic acid, phosphoric acid, boric acid, etc. But the concentrated acid has the effect of reducing the 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 decrease slowly in the future.
4, can be mixed
Hydroxypropyl methyl cellulose solution can be mixed with water-soluble polymer compounds, and become a uniform transparent solution with higher viscosity. These high molecular compounds are polyethylene glycol, polyvinyl acetate, polysilicone, polymethyl vinyl siloxane and hydroxyethyl cellulose and methyl cellulose, etc. Natural polymer compounds such as gum Arabic, locust bean gum, thorn tree gum and so on also have a good mixture with its solution. Hydroxypropyl methyl cellulose can also be mixed with stearic acid or palmitic acid mannitol ester or sorbitol ester, but also with glycerol, sorbitol and mannitol, these compounds can be used as hydroxypropyl methyl cellulose plasticizer.
Hydroxypropyl methyl cellulose insoluble water-soluble cellulose ether, can be surface cross-linking with aldehydes, and make these water-soluble ether precipitated in solution, become insoluble in water. And make hydroxypropyl methylcellulose insoluble aldehyde, formaldehyde, glyoxal, succinaldehyde, dialdehyde, etc., the use of formaldehyde should pay special attention to the PH value of the solution, in which glyoxal reaction faster, so in industrial production commonly used glyoxal as a cross-linking agent. The dosage of this kind of crosslinking agent in solution is 0.2%~10% of ether mass, the best is 7%~10%, such as the use of glyoxal with 3.3%~6% is the most appropriate. The general treatment temperature is 0~30℃, 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 for cross-linking reaction. The acids used are hydrochloric acid, sulfuric acid, phosphoric acid, formic acid, acetic acid, hydroxy acetic acid, succinic acid or citric acid, among which formic acid or acetic acid is the best, while formic acid is the best. Acids and aldehydes may also be added at the same time to allow the solution to be cross-linked in the desired PH range. This reaction is often used in the final process of cellulose ether preparation process, so that cellulose ether does not dissolve, easy to use 20~25℃ water to wash and purify. When the product is used, alkaline substances can be added to the solution of the product to adjust the PH of the solution to be alkaline, and the product is quickly dissolved in the solution. This method can also be used when cellulose ether solution is used to make a film and then the film is treated to make an insoluble film.
6, anti-enzyme
Hydroxypropyl methyl cellulose enzyme resistance of cellulose derivatives in theory, such as each anhydroglucose groups such as there is a solid combination of replacing groups, erosion of microorganisms is less susceptible to infection, but in fact the finished product to replace the value more than 1, also by enzyme degradation, this is the description of each group in the cellulose chain substitution degree is not uniform, Microorganisms can erode near unsubstituted dehydrated glucose groups to form sugars, which can be absorbed by microorganisms as food. Therefore, if the etherification substitution degree of cellulose increases, the resistance of cellulose ether to enzymatic erosion will be enhanced. It is reported that under controlled conditions, the residual viscosity of hydroxypropyl methyl cellulose (DS=1.9), methyl cellulose (DS=1.83), methyl cellulose (DS=1.66), and hydroxyethyl cellulose (1.7%) was 13.2%, 7.3%, 3.8%, and 1.7%, respectively. Hydroxypropyl methyl cellulose has strong anti-enzyme ability. Thus hydroxypropyl methylcellulose excellent anti-enzyme, combined with its good dispersion, thickening and film formation, applied in emulsion coatings, etc., generally do not need to add preservatives. However, in order to prevent long-term storage of the solution or possible contamination from the outside world, preservatives can be added, whose selection can be determined according to the final requirements of the solution. Phenylmercuric acetate and manganese fluosilicate are effective preservatives, but they are toxic and must be carefully operated. Generally, 1~5mg phenylmercuric acetate can be added to each liter of solution.
7, membrane performance
Hydroxypropyl methyl cellulose film performance of hydroxypropyl methyl cellulose has excellent film, its aqueous solution or organic solvent solution, coated on the glass plate, after drying becomes colorless, transparent and tough film. It has good moisture resistance and remains solid at high temperatures. Such as the addition of hygroscopic plasticizer, can enhance its elongation and flexibility, to improve flexure, glycerol and sorbitol and other plasticizer is the most appropriate. The general solution concentration is 2%~3%, the plasticizer dosage is 10%~20% of cellulose ether. If the content of plasticizer is exigent, the shrinkage phenomenon of colloid dehydration can occur in high humidity. The tensile strength of the film added plasticizer is much larger than that not added, and increases with the increase of the amount of added, as for the hygroscopicity of the film is also increased with the increase of the amount of plasticizer.