Hydroxypropyl methyl cellulose ether (HPMC) for dry powder mortar

The Chinese name of HPMC is hydroxypropyl methylcellulose. It is non-ionic and is often used as a water-retaining agent in dry-mixed mortar. It is the most commonly used water-retaining material in mortar.

The production process of HPMC is mainly a polysaccharide-based ether product produced by alkalization and etherification of cotton fiber (domestic). It has no charge itself, does not react with the charged ions in the gelling material, and has stable performance. The price is also lower than other types of cellulose ethers, so it is widely used in dry-mixed mortar.

Function of hydroxypropyl methylcellulose: It can thicken the freshly mixed mortar to have a certain wet viscosity and prevent segregation. (Thickening) Water retention is also the most important characteristic, which helps to maintain the amount of free water in the mortar, so that after the mortar is constructed, the cementitious material has more time to hydrate. (Water retention) It has air-entraining properties, which can introduce uniform and fine air bubbles to improve the construction of mortar.

The higher the viscosity of hydroxypropyl methylcellulose ether, the better the water retention performance. Viscosity is an important parameter of HPMC performance. At present, different HPMC manufacturers use different methods and instruments to measure the viscosity of HPMC. The main methods are HaakeRotovisko, Hoppler, Ubbelohde and Brookfield.

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. Regarding particle size, the finer the particle, the better the water retention. After the large particles of cellulose ether come into contact with water, the surface immediately dissolves and forms a gel to wrap the material to prevent water molecules from continuing to infiltrate. Sometimes it cannot be uniformly dispersed and dissolved even after long-term stirring, forming a cloudy flocculent solution or agglomeration . It greatly affects the water retention of cellulose ether, and solubility is one of the factors for choosing cellulose ether.

Fineness is also an important performance index of methyl cellulose ether. The MC used for dry powder mortar is required to be powder, with low water content, and the fineness also requires 20%~60% of the particle size to be less than 63um. The fineness affects the solubility of hydroxypropyl methylcellulose ether. Coarse MC is usually granular, and it is easy to dissolve in water without agglomeration, but the dissolution rate is very slow, so it is not suitable for use in dry powder mortar.

In dry powder mortar, MC is dispersed among cementing materials such as aggregate, fine filler and cement, and only fine enough powder can avoid methyl cellulose ether agglomeration when mixing with water. When MC is added with water to dissolve the agglomerates, it is very difficult to disperse and dissolve. Coarse fineness of MC is not only wasteful, but also reduces the local strength of the mortar. When such a dry powder mortar is applied in a large area, the curing speed of the local dry powder mortar will be significantly reduced, and cracks will appear due to different curing times. For the sprayed mortar with mechanical construction, the requirement for fineness is higher due to the shorter mixing time. Generally speaking, the higher the viscosity, the better the water retention effect. However, the higher the viscosity and the higher the molecular weight of MC, 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, that is, 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. That is, 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.

The water retention of HPMC is also related to the temperature used, and the water retention of methyl cellulose ether decreases with the increase of temperature. However, in actual material applications, dry powder mortar is often applied to hot substrates at high temperatures (higher than 40 degrees) in many environments, such as exterior wall putty plastering under the sun in summer, which often accelerates Curing of cement and hardening of dry powder mortar. The decline of water retention rate leads to the obvious feeling that both workability and crack resistance are affected, and it is particularly critical to reduce the influence of temperature factors under this condition.

In this regard, methyl hydroxyethyl cellulose ether additives are currently considered to be at the forefront of technological development. Although the amount of methyl hydroxyethyl cellulose is increased (summer formula), the workability and crack resistance still cannot meet the needs of use. Through some special treatment on MC, such as increasing the degree of etherification, etc., the water retention effect can be maintained at a higher temperature, so that it can provide better performance under harsh conditions.

The dosage of HPMC should not be too high, otherwise it will increase the water demand of the mortar, it will stick to the trowel, and the setting time will be too long, which will affect the constructability. Different mortar products use HPMC with different viscosities, and do not use high-viscosity HPMC casually. Therefore, although hydroxypropyl methylcellulose products are good, they are applauded when they are used well. Choosing the right HPMC is the primary responsibility of enterprise laboratory personnel. At present, many unscrupulous dealers are compounding HPMC, and the quality is quite poor. When selecting a certain cellulose, the laboratory should do a good job in the experiment to ensure the stability of the mortar product, and don’t be greedy for cheap and cause unnecessary losses.