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1. Cellulose ether
Cellulose ether is the general name of a series of products produced by the reaction of alkali cellulose and etherifying agent under certain conditions. Alkali cellulose is replaced by different etherifying agents to obtain different cellulose ethers. According to the ionization properties of substituents, cellulose ethers can be divided into ionic type (such as carboxymethyl cellulose) and non-ionic type (such as methyl cellulose). According to the types of substituents, cellulose ethers can be divided into monoethers (such as methyl cellulose) and mixed ethers (such as hydroxypropyl methyl cellulose). According to different dissolubility, it can be divided into water solubility (such as hydroxyethyl cellulose) and organic solvent solubility (such as ethyl cellulose). Dry mixed mortar is mainly water soluble cellulose, and water soluble cellulose can be divided into instant type and delayed dissolution type after surface treatment.
The action mechanism of cellulose ether in mortar is as follows:
(1) After the cellulose ether in the mortar is dissolved in water, the effective and uniform distribution of the cementitious material in the system is ensured due to the surface activity. As a protective colloid, the cellulose ether “wraps” the solid particles and forms a layer of lubricating film on its outer surface, which makes the mortar system more stable, and also improves the fluidity of the mortar in the mixing process and the smoothness of the construction.
(2) Due to the molecular structure characteristics of cellulose ether solution, the water in the mortar is not easy to lose, and gradually released in a long period of time, giving the mortar good water retention and workability.
1.1.1 Methylcellulose (MC)
The refined cotton is treated with alkali, and the cellulose ether is prepared through a series of reactions with methyl chloride as etherifying agent. Generally, the degree of substitution is 1.6~2.0, and the solubility varies with the degree of substitution. It belongs to non-ionic cellulose ether.
(1) Methylcellulose is soluble in cold water, but it is difficult to dissolve in hot water, and its aqueous solution is stable in the range of pH=3~12. It has good compatibility with starch, guar gum and many surfactants. When the temperature reaches the gel temperature, gel phenomenon will occur.
(2) The water retention of methyl cellulose depends on its addition amount, viscosity, particle fineness and dissolution rate. Generally, the water retention rate is high with large addition amount, small fineness and large viscosity. The addition amount has a great influence on the water retention rate, and the viscosity is not in direct proportion to the water retention rate. The dissolution rate mainly depends on the surface modification degree and particle fineness of cellulose particles. Among the above cellulose ethers, methyl cellulose and hydroxypropyl methyl cellulose have higher water retention.
(3) The change of temperature will seriously affect the water retention rate of methyl cellulose. Generally, the higher the temperature, the worse the water retention. If the mortar temperature exceeds 40 ℃, the water retention property of methyl cellulose will be significantly worse, which will seriously affect the workability of mortar.
(4) Methylcellulose has obvious influence on the workability and adhesion of mortar. The “adhesiveness” here refers to the adhesive force felt between the workers’ painting tools and the wall substrate, that is, the shear resistance of the mortar. The adhesiveness is large, the shear resistance of mortar is large, and the strength required by workers in the use process is also large, so the construction of mortar is poor. In cellulose ether products, the adhesion of methyl cellulose is at a medium level.
1.1.2 Hydroxypropyl methyl cellulose (HPMC)
Hydroxypropyl methyl cellulose (HPMC) is a kind of cellulose whose yield and dosage are increasing rapidly in recent years. It is a non-ionic cellulose mixed ether made from refined cotton after alkalization treatment, using epoxy propane and methyl chloride as etherifying agent through a series of reactions. The degree of substitution is generally 1.2~2.0. Its properties are different depending on the proportion of methoxy content and hydroxypropyl content.
(1) Hydroxypropyl methyl cellulose is easily soluble in cold water, but it is difficult to dissolve in hot water. However, its gel temperature in hot water is significantly higher than that of methyl cellulose. Compared with methyl cellulose, the solubility in cold water is also greatly improved.
(2) The viscosity of hydroxypropyl methyl cellulose is related to its molecular weight, and the higher the molecular weight, the higher the viscosity. The temperature will also affect its viscosity, and the viscosity will decrease when the temperature rises. However, the effect of high viscosity and temperature is lower than that of methyl cellulose. The solution is stable at room temperature.
(3) The water retention of HPMC depends on its addition amount and viscosity, and its water retention rate is higher than that of methyl cellulose at the same addition amount.
(4) Hydroxypropyl methylcellulose is stable to acids and bases, and its aqueous solution is stable in the range of pH=2~12. Caustic soda and lime water have little effect on its performance, but alkali can accelerate its dissolution rate and improve the viscosity pin. 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 methyl cellulose can be mixed with water-soluble macromolecular compounds to form a uniform solution with higher viscosity. Such as polyvinyl alcohol, starch ether, vegetable gum, etc.
(6) Hydroxypropyl methyl cellulose has better enzyme resistance than methyl cellulose, and the possibility of enzymatic degradation of its solution is lower than that of methyl cellulose.
(7) The adhesion of hydroxypropyl methyl cellulose to mortar construction is higher than that of methyl cellulose.
1.1.3 Hydroxyethyl cellulose (HEC)
The refined cotton is prepared by reacting with ethylene oxide as etherifying agent in the presence of acetone after alkali treatment. The degree of substitution is generally 1.5~2.0. It has strong hydrophilicity and is easy to absorb moisture.
(1) Hydroxyethyl cellulose is soluble in cold water, but difficult to dissolve in hot water. The solution is stable at high temperature and does not have gel property. It can be used for a long time under the medium and high temperature of mortar, but its water retention is lower than that of methyl cellulose.
(2) Hydroxyethyl cellulose is stable to common acids and bases. Alkali can accelerate its dissolution and slightly improve its viscosity. Its dispersibility in water is slightly worse than that of methyl cellulose and hydroxypropyl methyl cellulose.
(3) Hydroxyethyl cellulose has good performance in anti sagging of mortar, but it has a long retarding time for cement.
(4) The performance of hydroxyethyl cellulose produced by some domestic enterprises is significantly lower than that of methyl cellulose due to its high water content and ash content.
1.1.4 Carboxymethyl cellulose
The ionic cellulose ether is prepared from natural fibers (cotton, etc.) after alkali treatment, using sodium monochloroacetate as etherifying agent, and through a series of reactions. Its degree of substitution is generally 0.4~1.4, and its performance is greatly affected by the degree of substitution.
(1) Carboxymethyl cellulose is highly hygroscopic, and it will contain a large amount of water when stored under general conditions.
(2) The carboxymethyl cellulose aqueous solution will not produce gel, and the viscosity will decrease with the increase of temperature. When the temperature exceeds 50 ℃, the viscosity is irreversible.
(3) Its stability is greatly affected by pH. Generally, it can be used in gypsum based mortar, but not in cement based mortar. In high alkalinity, viscosity will be lost.
(4) Its water retention is much lower than that of methyl cellulose. It has retarding effect on gypsum based mortar and reduces its strength. However, the price of carboxymethyl cellulose is significantly lower than that of methyl cellulose.