In dry mortar, cellulose ether plays the role of water retention, thickening and thixotropy, air-entraining and retarding properties. Good water retention capacity makes cement hydration more complete, can improve the wet viscosity of wet mortar, increase the bonding strength of mortar, and in ceramic tile bonding mortar, it can increase the opening time and adjust the time. Adding cellulose ether to mechanical spraying mortar can improve the structural strength of the mortar. Self-leveling can prevent settlement, segregation and stratification, etc. Therefore, cellulose ether is being widely used in dry powder mortar as an important additive. In order to give full play to the application of cellulose ether in dry-mixed mortar, it is also crucial to choose the type of cellulose ether and determine its application range.
1. Water retention of cellulose ether
①The higher the viscosity of the cellulose ether, the better the water retention performance, and the viscosity of the polymer solution. Depending on the molecular weight (polymerization degree) of the polymer is also determined by the chain length of the molecular structure and the shape of the chain, and the distribution of the types and quantities of the substituents also directly affects its viscosity range.
②The greater the amount of cellulose ether added in the mortar, the better the water retention performance, and the higher the viscosity, the better the water retention performance.
③ 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 infiltrating. 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.
2. Thickening and thixotropy of cellulose ether
The second function of cellulose ether – thickening depends on: the degree of polymerization of cellulose ether, solution concentration, shear rate, temperature and other conditions. The gelling property of the solution is unique to alkyl cellulose and its modified derivatives. The gelation properties are related to the degree of substitution, solution concentration and additives. For hydroxyalkyl modified derivatives, the gel properties are also related to the modification degree of hydroxyalkyl. For mc and HPMC with low viscosity, 10%-15% concentration solution can be prepared, 5%-10% solution can be prepared for medium viscosity mc and HPMC, and 2%-3% solution can be prepared for high viscosity mc and HPMC, and usually The viscosity classification of cellulose ether is also graded with 1%-2% solution. High molecular weight cellulose ether has high thickening efficiency. In the same concentration solution, polymers with different molecular weights have different viscosities. High degree. The target viscosity can only be achieved by adding a large amount of low molecular weight cellulose ether. Its viscosity has little dependence on the shear rate, and the high viscosity reaches the target viscosity, and the required addition amount is small, and the viscosity depends on the thickening efficiency. Therefore, to achieve a certain consistency, a certain amount of cellulose ether (concentration of the solution) and solution viscosity must be guaranteed. The gel temperature of the solution also decreases linearly with the increase of the concentration of the solution, and gels at room temperature after reaching a certain concentration. The gelling concentration of HPMC is higher at room temperature.
Consistency can also be adjusted by selecting particle size and selecting cellulose ethers with different degrees of modification. The so-called modification is to introduce a hydroxyalkyl group with a certain degree of substitution on the skeleton structure of mc. By changing the relative substitution values of the two substituents, that is, the DS and ms relative substitution values of the methoxy and hydroxyalkyl groups that we often say. Various performance requirements of cellulose ether can be obtained by changing the relative substitution values of the two substituents.
The addition of cellulose ether affects the water consumption of the mortar and changes the water-to-cement ratio, which is the thickening effect. The higher the dosage, the greater the water consumption.
Cellulose ethers used in powdered building materials must dissolve quickly in cold water and provide a suitable consistency for the system. If given a certain shear rate, it still becomes flocculent and colloidal block, which is a substandard or poor quality product.
There is also a good linear relationship between the consistency of cement paste and the dosage of cellulose ether. Cellulose ether can greatly increase the viscosity of mortar. The larger the dosage, the more obvious the effect.
High-viscosity cellulose ether aqueous solution has high thixotropy, which is also a major characteristic of cellulose ether. Aqueous solutions of Mc-type polymers usually have pseudoplastic and non-thixotropic fluidity below their gel temperature, but Newtonian flow properties at low shear rates. Pseudoplasticity increases with the molecular weight or concentration of cellulose ether, regardless of the type of substituent and the degree of substitution. Therefore, cellulose ethers of the same viscosity grade, no matter mc, HPMC, HEMC, will always show the same rheological properties as long as the concentration and temperature are kept constant. Structural gels are formed when the temperature is raised, and highly thixotropic flows occur. High concentration and low viscosity cellulose ethers show thixotropy even below the gel temperature. This property is of great benefit to the adjustment of leveling and sagging in the construction of building mortar. It needs to be explained here that the higher the viscosity of cellulose ether, the better the water retention, but the higher the viscosity, the higher the relative molecular weight of cellulose ether, and the corresponding decrease in its solubility, which has a negative impact on the mortar concentration and construction performance. The higher the viscosity, the more obvious the thickening effect on the mortar, but it is not completely proportional. Some medium and low viscosity, but the modified cellulose ether has better performance in improving the structural strength of wet mortar. With the increase of viscosity, the water retention of cellulose ether improves.
Post time: Mar-14-2023