Hydroxypropylmethylcellulose (HPMC) is a kind of cellulose variety whose output and consumption have increased rapidly in recent years. It is made of refined cotton after alkalization treatment.
Using propylene oxide and methyl chloride as etherifying agent, it is a non-ionic cellulose mixed ether prepared through a series of reactions. The degree of substitution is generally 1.2~2.0. Its properties vary depending on the ratio of methoxyl content to hydroxypropyl content.
(1) Hydroxypropyl methylcellulose is easily soluble in cold water, but hardly soluble in hot water. But its gel temperature in hot water is significantly higher than that of methylcellulose. Compared with methylcellulose, the solubility in cold water is also greatly improved than methylcellulose.
(2) The viscosity of hydroxypropyl methylcellulose is related to its molecular weight, the larger the molecular weight, the higher the viscosity. Temperature also affects its viscosity, which decreases with increasing temperature. However, its viscosity is high, and the influence of temperature is lower than that of methyl cellulose. Its solution is stable on storage at room temperature.
(3) The water retention performance of HPMC depends on its addition amount, viscosity, etc., and its water retention rate is higher than that of methyl cellulose at the same addition amount.
(4) Hydroxypropyl methylcellulose is stable to acid and alkali, and its aqueous solution is very stable in the range of pH=2~12. Sodium hydroxide and lime water have little effect on its performance, but alkali can speed up its dissolution and slightly increase its viscosity.
Hydroxypropyl methylcellulose is stable to common salts, but when the concentration of salt solution is high, the viscosity of hydroxypropyl methylcellulose solution tends to increase.
(5) Hydroxypropyl methylcellulose can be mixed with water-soluble polymers to form a uniform solution with high viscosity. Such as polyvinyl alcohol, starch ether, vegetable gum, etc.
(6) Hydroxypropyl methylcellulose has better enzyme resistance than methylcellulose, and its solution is less likely to be enzymatically degraded than methylcellulose
. The adhesion of hydroxypropyl methylcellulose to mortar construction is higher than that of methylcellulose.
Ionic cellulose ether is made from natural fibers (cotton, etc.) through alkali treatment, using monochloroacetic acid as etherification agent, and a series of reaction treatments. The degree of substitution is generally 0.4~1.4, and its performance is greatly affected by the degree of substitution. Its aqueous solution has the functions of thickening, film formation, bonding, water retention, colloid protection, emulsification, suspension, etc., and is widely used in petroleum, food, medicine, textile and papermaking industries. It is one of the most important cellulose ethers.
(1) Carboxymethyl cellulose has strong hygroscopicity and will contain a lot of water when stored under normal conditions.
(2) Carboxymethyl cellulose aqueous solution does not produce gel, and the viscosity decreases with the increase of temperature. When the temperature exceeds 50°C, the viscosity is irreversible.
(3) Its stability is greatly affected by pH. Generally, it can be used for gypsum-based mortar, but not for cement-based mortar. When highly alkaline, it loses viscosity.
(4) Its water retention is much lower than that of methyl cellulose. It has a retarding effect on gypsum-based mortar and reduces its strength. However, the price of carboxymethyl alkaline cellulose is significantly lower than that of methyl cellulose.
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