Thickening and thixotropy of cellulose ether
The second effect of cellulose ether – thickening depends on: cellulose ether polymerization degree, solution concentration, shear rate, temperature and other conditions. The gelation property of solution is unique to alkyl cellulose and its modified derivatives. Gelation characteristics are related to degree of substitution, solution concentration and additives.
For hydroxyl alkyl modified derivatives, gel properties are also related to the degree of hydroxyl alkyl modification. For MC and HPMC with low viscosity, 10%-15% concentration solution can be prepared. Medium viscosity MC and HPMC can prepare 5%-10% solution while high viscosity MC and HPMC can only prepare 2%-3% solution. Usually cellulose ether viscosity grading is also to 1%-2% solution grading.
High molecular weight cellulose ether has high thickening efficiency. The same concentration solution, different molecular weight polymer has different viscosity, viscosity and molecular weight can be expressed as follows, [η]=2.92 ×10-2 (DPn) 0.905, DPn is the average polymerization degree is high.
Low molecular weight cellulose ether to add more to achieve the target viscosity. Its viscosity is less dependent on shear rate, high viscosity to achieve the target viscosity, the amount needed to add less, viscosity depends on the thickening efficiency. Therefore, to achieve a certain consistency, a certain amount of cellulose ether (concentration of solution) and solution viscosity must be guaranteed.
The gelation temperature of the solution decreased linearly with the increase of the concentration of the solution, and gelation occurred at room temperature after reaching a certain concentration. HPMC has a high gelation concentration at room temperature.
The consistency can also be adjusted by selecting particle size and cellulose ethers with different degrees of modification. The so-called modification is the introduction of hydroxyl alkyl group in 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 methoxy and hydroxyl groups. Various properties of cellulose ether are required by changing the relative substitution values of two kinds of substituents.
FIG. 4 shows the relationship between consistency and modification. In Figure 5, the addition of cellulose ether affects the water consumption of mortar and changes the water-binder ratio of water and cement, which is the thickening effect. The higher the dosage, the more water consumption.
Cellulose ethers used in powdery building materials must dissolve quickly in cold water and provide the right consistency to the system. If a given shear rate is still flocculent and colloidal it is a substandard or poor quality product.
The consistency of cement slurry also has a good linear relationship with the dosage of cellulose ether. Cellulose ether can greatly increase the viscosity of mortar. The larger the dosage is, the more obvious the effect is, as shown in Figure 6
Cellulose ether aqueous solution with high viscosity has high thixotropy, which is one of the characteristics of cellulose ether. Aqueous solutions of Mc type polymers usually have pseudoplastic, non-thixotropic fluidity below their gel temperature, but Newtonian flow properties at low shear rates.
Pseudoplasticity increases with the increase of molecular weight or concentration of cellulose ether. And it is independent of substituent type and degree. Therefore, cellulose ethers of the same viscosity grade. Whether MC, HPMC or HEMC, always show the same rheological properties as long as the concentration and temperature remain constant.
When the temperature increases, structural gel is formed and high thixotropic flow occurs. Cellulose ethers with high concentration and low viscosity exhibit thixotropy even below the gel temperature. This property is of great benefit to the construction of building mortar to adjust its flow and flow hanging property. It should be noted that the higher the viscosity of cellulose ether, the better the water retention. However, the higher the viscosity, the higher the relative molecular weight of cellulose ether, its solubility decreases. This has a negative impact on mortar concentration and construction performance.
The higher the viscosity, the more obvious the thickening effect of mortar, but it is not a complete proportional relationship. Some low viscosity but modified cellulose ether in improving the structural strength of wet mortar has a more excellent performance. With the increase of viscosity, cellulose ether water retention improved.
That’s all content about thickening and thixotropy of cellulose ether.
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