Research on quality of cellulose ethers used in dry mortar

Research on quality of cellulose ethers used in dry mortar

In recent years, with the gradual implementation of the relevant policies of adhering to the scientific development concept and building a resource-conserving society, China’s construction mortar is facing a revolution from traditional mortar to dry-mix mortar. The construction of dry-mix mortar industry has entered rapid development. trail of. As the most important admixture in building dry-mix mortar products, cellulose ether plays a key role in the performance and cost of dry-mixed mortar. There are two types of cellulose ethers: one is ionic, such as sodium carboxymethyl cellulose (CMC), and the other is nonionic, such as methyl cellulose (MC), hydroxyethyl cellulose (HEC), Hydroxypropyl methylcellulose (HPMC) and the like. Currently, most of the global cellulose ether products are used in building materials.

With the rapid development of the dry-mix mortar industry, China’s cellulose ether has basically achieved localization, and the situation of foreign product control market has been broken. With the increasing popularity of dry-mixed mortar products, China will become the world’s largest producer of dry-mixed mortar. The application of cellulose ether will further increase, and its manufacturers and products will be more and more. How the performance of cellulose ether in dry mortar is the focus of the producer and the user.

The most important property of cellulose ethers is their water retention in building materials. If no cellulose ether is added, the thin layer of fresh mortar will dry out quickly, making the cement unable to hydrate in the normal way, resulting in the mortar not hardening and good adhesion. At the same time, the addition of cellulose ether makes the mortar have good plasticity and flexibility, and improves the bonding strength of the mortar. Let’s talk about the impact on the application of dry mortar from the product properties of cellulose ether.
۱, the fineness of cellulose ether
The fineness of cellulose ether affects its solubility. For example, the lower the fineness of cellulose ether, the faster its dissolution rate in water and the better water retention performance. Therefore, cellulose ether fineness should be included in one of its investigation properties. In general, the sieve opening of cellulose ether fineness exceeding 0.212 mm should be no more than 8.0%.
۲, weight loss rate
The weight loss rate of drying refers to the drying of cellulose ether at a certain temperature, and the mass of the lost material accounts for the percentage of the mass of the original sample. For a certain quality of cellulose ether, the weight loss rate is too high, which will reduce the content of active ingredients in cellulose ether, affect the application of downstream enterprises, and increase the purchase cost. Generally, the weight loss rate of cellulose ether drying is not more than 6.0%.

  1. Sulfate ash of cellulose ether
    For a certain amount of cellulose ether, the ash content is too high, which will reduce the content of active ingredients in cellulose ether, affecting the application effect of downstream enterprises. The sulfated ash of cellulose ether is an important measure of its own performance. In combination with the current production status of cellulose ether production enterprises in China, the ash content of MC, HPMC and HEMC should not exceed 2.5%, and the ash content of HEC cellulose ether should be no more than 10.0%.
    ۴, the viscosity of cellulose ether
    The water retention and thickening of the cellulose ether is mainly determined by the viscosity and the amount of the cellulose ether itself added to the cement slurry.
  2. PH value of cellulose ether
    After the cellulose ether product is stored at a higher temperature or for a longer period of time, the viscosity gradually decreases, and the performance of the high viscosity product is particularly remarkable, so the pH is required to be limited. It is generally preferred that the pH range of the cellulose ether be controlled to be 5-9.
    ۶, the transmittance of cellulose ether
    The light transmittance of cellulose ether directly affects its application effect in building materials. The main factors affecting the transmittance of cellulose ether are: (1) the quality of raw materials; (2) the effect of alkalization; (3) the process Proportion; (4) solvent ratio; (5) neutralization effect.

According to the effect of use, the transmittance of the cellulose ether should be not less than 80%.

  1. Gel temperature of cellulose ether
    Cellulose ethers are primarily used as tackifiers, plasticizers, and water retention agents in cement products, so viscosity and gel temperature are important metrics for characterizing the quality of cellulose ethers. The gel temperature is used to determine the cellulose ether type, which is related to the degree of substitution of the cellulose ether. In addition, salts and impurities can also affect the gel temperature. When the temperature of the solution rises, the cellulose polymer gradually loses water, and the viscosity of the solution decreases. When the gel point is reached, the polymer is completely dehydrated to form a gel. Therefore, the temperature of the cement product is usually controlled below the initial gel temperature. Under this condition, the lower the temperature, the higher the viscosity, and the more obvious the viscosity-increasing water retention effect

What is the difference between HPMC and MC


MC is methyl cellulose. After the refined cotton is treated with alkali, chloromethane is used as an etherifying agent, and a series of reactions are made to prepare cellulose ether. The degree of substitution is generally 1.6 to 2.0, and the degree of substitution differs depending on the solubility. It belongs to nonionic cellulose ether.
(۱) The water retention of methyl cellulose depends on the amount of addition, viscosity, particle fineness and dissolution rate. Generally, the amount of addition is large, the fineness is small, and the viscosity is large, and the water retention rate is high. The addition amount has the greatest influence on the water retention rate, and the viscosity level is not proportional to the water retention rate. The rate of dissolution depends mainly on the degree of surface modification of the cellulose particles and the fineness of the particles. Among the above cellulose ethers, methyl cellulose and hydroxypropyl methylcellulose have a high water retention rate.
(۲) mc is soluble in cold water, and it is difficult to dissolve in hot water. The aqueous solution is very stable in the range of pH=3~12. It has good compatibility with starch, guar gum and many other surfactants. When the temperature reaches the gelation temperature, a gelation phenomenon occurs.
(۳) 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 temperature of the mortar exceeds 40 ° C, the water retention of methyl cellulose will be significantly deteriorated, which seriously affects the workability of the mortar.
(۴) Methyl cellulose has a significant effect on the workability and adhesion of the mortar. “Adhesiveness” herein refers to the adhesive force felt between the worker’s application tool and the wall substrate, that is, the shear resistance of the mortar. The adhesion is large, the shear resistance of the mortar is large, the force required by the worker during use is also large, and the workability of the mortar is poor.
Methylcellulose adhesion is intermediate in cellulose ether products.

HPMC is hydroxypropylmethylcellulose, which is a nonionic cellulose mixed ether prepared by a series of reactions using acetal oxide and methyl chloride as an etherifying agent after alkalization of refined cotton. The degree of substitution is generally 1.2 to 2.0. Its nature is different depending on the ratio of methoxyl content and hydroxypropyl content.
(۱) Hydroxypropyl methylcellulose is easily soluble in cold water, and it may be difficult to dissolve in hot water. However, its gelation temperature in hot water is significantly higher than that of methyl cellulose. The dissolution in cold water is also much better than methyl cellulose. –
(۲) The viscosity of hydroxypropylmethylcellulose is related to the molecular weight thereof, and the viscosity is high when the molecular weight is large. The temperature also affects its viscosity, the temperature rises, and the viscosity decreases. However, its viscosity has a lower temperature than methyl cellulose. Its solution is stable at room temperature.
(۳) hpmc is stable to acids and bases, and its aqueous solution is very stable in the range of pH=2~12. Caustic soda and lime water do not have much effect on their properties, but alkali accelerates their dissolution rate and increases viscosity. Hydroxypropyl methylcellulose has stability to general salts, but when the concentration of the salt solution is high, the viscosity of the hydroxypropyl methylcellulose solution tends to increase.
(۴) The water retention of hydroxypropyl methylcellulose depends on the amount of addition, viscosity, etc., and the water retention rate under the same amount is higher than that of methylcellulose.
(۵) Hydroxypropyl methylcellulose can be mixed with a water-soluble polymer compound to form a uniform, higher viscosity solution. Such as polyvinyl alcohol, starch ether, vegetable gum and the like.
(۶) Hydroxypropyl methylcellulose has higher adhesion to mortar than methyl cellulose.
(۷) Hydroxypropyl methylcellulose has better enzymatic resistance than methylcellulose, and its solution is less likely to be enzymatically degraded than methylcellulose

sanaat mahdi