1. The main function of cellulose ether

In ready-mixed mortar, cellulose ether is a main additive that is added in a very low amount but can significantly improve the performance of wet mortar and affect the construction performance of mortar.

2. Types of cellulose ethers

The production of cellulose ether is mainly made of natural fibers through alkali dissolution, grafting reaction (etherification), washing, drying, grinding and other processes.

According to the main raw materials, natural fibers can be divided into: cotton fiber, cedar fiber, beech fiber, etc. Their degrees of polymerization vary, which affects the final viscosity of their products. Currently, major cellulose manufacturers use cotton fiber (by-product of nitrocellulose) as the main raw material.

Cellulose ethers can be divided into ionic and nonionic. The ionic type mainly includes carboxymethyl cellulose salt, and the non-ionic type mainly includes methyl cellulose, methyl hydroxyethyl (propyl) cellulose, hydroxyethyl cellulose, etc.

At present, the cellulose ethers used in ready-mixed mortar are mainly methyl cellulose ether (MC), methyl hydroxyethyl cellulose ether (MHEC), methyl hydroxypropyl cellulose ether (MHPG), hydroxypropyl Methyl cellulose ether (HPMC). In ready-mixed mortar, because ionic cellulose (carboxymethyl cellulose salt) is unstable in the presence of calcium ions, it is rarely used in ready-mixed products that use cement, slaked lime, etc. as cementing materials. In some places in China, carboxymethyl cellulose salt is used as a thickener for some indoor products processed with modified starch as the main cementing material and Shuangfei powder as the filler. This product is prone to mildew and is not resistant to water, and is now being phased out. Hydroxyethyl cellulose is also used in some ready-mix products, but has a very small market share.

3. Main performance indicators of cellulose ether

(1) Solubility

Cellulose is a polyhydroxy polymer compound that neither dissolves nor melts. After etherification, cellulose is soluble in water, dilute alkali solution and organic solvent, and has thermoplasticity. Solubility mainly depends on four factors: first, solubility varies with viscosity, the lower the viscosity, the greater the solubility. Second, the characteristics of the groups introduced in the etherification process, the larger the group introduced, the lower the solubility; the more polar the group introduced, the easier the cellulose ether is to dissolve in water. Third, the degree of substitution and the distribution of etherified groups in macromolecules. Most cellulose ethers can only be dissolved in water under a certain degree of substitution. Fourth, the degree of polymerization of cellulose ether, the higher the degree of polymerization, the less soluble; the lower the degree of polymerization, the wider the range of degree of substitution that can be dissolved in water.

(2) Water retention

Water retention is an important performance of cellulose ether, and it is also a performance that many domestic dry powder manufacturers, especially those in southern regions with high temperatures, pay attention to. Factors affecting the water retention effect of mortar include the amount of cellulose ether added, viscosity, particle fineness and the temperature of the use environment. The higher the amount of cellulose ether added, the better the water retention effect; the greater the viscosity, the better the water retention effect; the finer the particles, the better the water retention effect.

(3) Viscosity

Viscosity is an important parameter of cellulose ether products. At present, different cellulose ether manufacturers use different methods and instruments to measure the viscosity. For the same product, the viscosity results measured by different methods are very different, and some even have doubled differences. Therefore, when comparing viscosity, it must be carried out between the same test methods, including temperature, rotor, etc.

Generally speaking, the higher the viscosity, the better the water retention effect. However, the higher the viscosity, the higher the molecular weight of cellulose ether, and the corresponding decrease in its solubility will have a negative impact on the strength and construction performance of the mortar. The higher the viscosity, the more obvious the thickening effect on the mortar, but it is not directly proportional. The higher the viscosity, the more viscous the wet mortar will be. During construction, it is manifested as sticking to the scraper and high adhesion to the substrate. But it is not helpful to increase the structural strength of the wet mortar itself. During construction, the anti-sag performance is not obvious. On the contrary, some medium and low viscosity but modified methyl cellulose ethers have excellent performance in improving the structural strength of wet mortar.

(4) The fineness of the particles:

The cellulose ether used for ready-mixed mortar is required to be powder, with low water content, and the fineness also requires 20% to 60% of the particle size to be less than 63 μm. The fineness affects the solubility of cellulose ether. Coarse cellulose ethers are usually in the form of granules, which are easy to disperse and dissolve in water without agglomeration, but the dissolution rate is very slow, so they are not suitable for use in ready-mixed mortar (some domestic products are flocculent, not easy to disperse and dissolve in water, and prone to caking). In ready-mixed mortar, cellulose ether is dispersed between aggregates, fine fillers and cement and other cementing materials. Only fine enough powder can avoid cellulose ether agglomeration when mixing with water. When cellulose ether is added with water to dissolve the agglomeration, it is very difficult to disperse and dissolve.

(5) Modification of cellulose ether

The modification of cellulose ether is the extension of its performance, and it is the most important part. The properties of cellulose ether can be improved to optimize its wettability, dispersibility, adhesion, thickening, emulsification, water retention and film-forming properties, as well as its impermeability to oil.

4. Effect of ambient temperature on water retention of mortar

The water retention of cellulose ether decreases with the increase of temperature. In practical material applications, mortar is often applied to hot substrates at high temperatures (higher than 40°C) in many environments. The drop in water retention resulted in a noticeable impact on workability and crack resistance. Its dependence on temperature will still lead to the weakening of mortar properties, and it is particularly critical to reduce the influence of temperature factors under this condition. Mortar recipes were adjusted appropriately, and many important changes were made in seasonal recipes. Although increasing the dosage (summer formula), the workability and crack resistance still cannot meet the needs of use, which requires some special treatment of cellulose ether, such as increasing the degree of etherification, etc., so that the water retention effect can be achieved at a relatively high temperature. It maintains a better effect when it is high, so that it provides better performance in harsh conditions.

5. Application in ready-mixed mortar

In ready-mixed mortar, cellulose ether plays the role of water retention, thickening and improving construction performance. Good water retention performance ensures that the mortar will not cause sanding, powdering and strength reduction due to water shortage and incomplete hydration. The thickening effect greatly enhances the structural strength of the wet mortar. The addition of cellulose ether can significantly improve the wet viscosity of wet mortar, and has good viscosity to various substrates, thereby improving the wall performance of wet mortar and reducing waste. In addition, the role of cellulose ether in different products is also different. For example, in tile adhesives, cellulose ether can increase the opening time and adjust the time; in mechanical spraying mortar, it can improve the structural strength of wet mortar; in self-leveling, it can prevent settlement, Segregation and stratification. Therefore, as an important additive, cellulose ether is widely used in dry powder mortar.