Cellulose ethers’Water-retaining agent is the key additive to improve the water-retaining performance of dry-mixed mortar, and it is also one of the key additives to determine the cost of dry-mixed mortar materials. The main source of water-retaining agent is cellulose ether.
1.1Hydroxypropyl Methyl Cellulose Ether-HPMC
Hydroxypropyl methyl cellulose (HPMC) is a general term for a series of products produced by alkali cellulose reacting with etherifying agent under certain conditions.Alkaline 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 two categories: ionic sub-type (such as HEC) and non-ionic type (such as methyl cellulose).According to cellulose ethers solubility, they can be divided into water solubility (e.g. hydroxyethyl cellulose) and organic solvent solubility (e.g. ethyl cellulose).
Water-soluble cellulose is mainly used in dry-mixed mortar . Water-soluble cellulose ether can be divided into instant-soluble type and delayed-soluble cellulose ether after surface treatment.
The working mechanism of cellulose ether in mortar is as follows:
(1) When cellulose ether in mortar dissolves in water, its surface activity ensures the effective and uniform distribution of cementitious materials in the system. As a protective colloid, cellulose ether “wraps” solid particles and forms a layer on its outer surface.
The lubrication film makes the mortar system more stable, and improves the fluidity of the mortar in the mixing process and the smoothness in the construction process.
(2)Cellulose ether solution, due to its molecular structure, makes the moisture in mortar not easy to lose, and gradually releases over a long period of time, giving the mortar good water retention and workability.
1.1.1Molecular Formula of Methylcellulose (MC)[C6H7O2(OH)3-h(OCH3)n]x
The refined cotton was treated with alkali and methyl chloride was used as etherifying agent to produce cellulose ether through a series of reactions. Generally, the degree of substitution is 1.6-2.0, and the solubility varies with the degree of substitution. It belongs to nonionic cellulose ether.
(1)Methylcellulose is soluble in cold water and difficult to dissolve in hot water. Its aqueous solution is very stable in the range of pH=3~12. It has good compatibility with starch, guar gum and many surfactants. When the temperature reaches the gelatinization temperature, it will appear gelatin phenomenon.
(2)The water retention of methylcellulose depends on its dosage, viscosity, particle size and dissolution rate. Generally, the water retention rate is high when the addition amount is large, the fineness is small and the viscosity is large. Among them, the addition amount has the greatest influence on the water retention rate, and the viscosity is not proportional to the water retention rate. The dissolution rate mainly depends on the degree of surface modification and particle fineness of cellulose particles. Among the above cellulose ethers, methyl cellulose and hydroxypropyl methyl cellulose have higher water retention rates.
(3)The change of temperature will seriously affect the water retention of methylcellulose. The higher the general temperature, the worse the water retention. If the temperature of mortar exceeds 40 C, the water retention of methyl cellulose will be significantly worse, which seriously affects the workability of mortar.
(4)Methyl cellulose has obvious influence on the construction and adhesion of mortar. The “adhesion” here refers to the shear resistance of mortar, which is the adhesive force felt between the worker’s tool and the wall base material. Great adhesiveness, high shear resistance of mortar.
(5)In the process of using, people need a lot of strength, and the construction ability of mortar is poor. In cellulose ether products, the adhesion of methyl cellulose is in the middle level.
1.1.2 Hydroxypropyl methylcellulose (HPMC)
Hydroxypropyl methyl cellulose (HPMC) is a kind of cellulose which has been increasing rapidly in yield and dosage in recent years. It is a non-ionic cellulose mixture-ether made from refined cotton after alkali treatment, using propylene oxide and chloromethane as etherifying agents through a series of reactions. The degree of substitution is generally 1.2-2.0. Its properties are different depending on the ratio of methoxy content to hydroxypropyl content.
(1)Hydroxypropyl methyl cellulose is soluble in cold water, but difficult to dissolve in hot water. But its gelation temperature in hot water is much higher than that in methyl cellulose. The solubility of methyl cellulose in cold water is also improved.
(2)The viscosity of hydroxypropyl methyl cellulose is related to its molecular weight, while the viscosity is high when its molecular weight is large. Temperature also affects its viscosity, which increases and decreases. However, the effect of viscosity and high temperature is lower than that of methyl cellulose. The solution is stable at room temperature.
(3)Hydroxypropyl methyl cellulose (HPMC) has higher water retention than methyl cellulose (MC) at the same amount of HPMC.
(4)Hydroxypropyl methyl cellulose is stable to acids and bases, and its aqueous solution is very stable in the range of pH=2~12. Caustic sodium and lime water have little effect on its performance, but alkali can accelerate its dissolution speed and improve its viscosity pin. Hydroxypropyl methyl cellulose is stable to common salts, but the viscosity of hydroxypropyl methyl cellulose solution tends to increase when the concentration of salt solution is high.
(5)Hydroxypropyl methyl cellulose (HPMC) can be mixed with water-soluble macromolecule compounds to form a homogeneous solution with higher viscosity. Such as polyvinyl alcohol, starch ether, vegetable gum, etc.
(6)Hydroxypropyl methyl cellulose (HPMC) has better resistance to enzymes than methyl cellulose (MC), and the possibility of enzymatic degradation in solution is lower than that of methyl cellulose.
1.1.2Hydroxyethyl Cellulose (HEC)
The refined cotton was prepared by alkali treatment and ethylene oxide as etherifying agent in the presence of acetone. 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 and difficult to dissolve in hot water. The solution is stable at high temperature without gelation. The mortar can be used for a long time at high temperature, but its water retention is lower than that of methyl cellulose.
(2)Hydroxyethyl cellulose is stable to general acids and bases. Alkali can accelerate its dissolution and slightly increase its viscosity. Its dispersion in water is slightly worse than that of methyl cellulose and hydroxypropyl methyl cellulose.
(3)Hydroxyethyl cellulose (HEC) has a good anti-hanging property for mortar, but has a long retarding time for cement.
1.1.3 Carboxymethyl Cellulose (CMC) [C6H7O2（OH）2OCH2COONA\n
CMC cellulose ether was prepared from natural fibers (cotton, etc.) after alkali treatment and sodium chloroacetate as etherifying agent through a series of reactions. Its degree of substitution is generally 0.4-1.4, and its performance is greatly affected by degree of substitution.
(1)Carboxymethyl cellulose (CMC) has a high moisture absorption and can contain a large amount of water under general storage conditions.
(2)Carboxymethyl cellulose aqueous solution does not produce gels, and viscosity decreases with increasing temperature. When the temperature exceeds 50 degrees, the viscosity is not reversible.
(3)Its stability is greatly affected by pH. Generally, it can be used in gypsum-based mortar, but not in cement-based mortar. In the case of high alkalinity, the viscosity will be lost.