Cement-based tile adhesive is the largest application of the current special dry-mixed mortar. It is a kind of organic or inorganic admixture with cement as the main cementing material and supplemented with grading aggregate, water retention agent, early strength agent and latex powder. mixture. Generally, it only needs to be mixed with water. Compared with ordinary cement mortar, it can greatly improve the bond strength between the facing material and the substrate, has good anti-slip property and has excellent water resistance and heat resistance. It is also used for the decoration of interior and exterior wall tiles, floor tiles and other decorative materials. It is widely used in the decoration of interior and exterior walls, floors, bathrooms, kitchens, etc. It is the most widely used tile. Bonding material.
Usually, when we judge the performance of a tile adhesive, we should pay attention to its mechanical strength and opening time in addition to its operational performance and anti-slipping ability. In addition to affecting the rheological properties of porcelain rubber, such as the smoothness of operation, the condition of the sticking knife, etc., the cellulose ether has a strong influence on the mechanical properties of the tile adhesive.
1. Open time
When Redispersible polymer powder and cellulose ether coexist in the wet mortar, some data models show that the rubber powder has stronger kinetic energy attached to the cement hydration product, and the cellulose ether is more present in the interstitial fluid, which affects more. The viscosity and setting time of the mortar. The surface tension of the cellulose ether is larger than that of the rubber powder, and enrichment of more cellulose ether at the mortar interface is beneficial to form a hydrogen bond between the base surface and the cellulose ether.
In the wet mortar, the water in the mortar evaporates, the cellulose ether is enriched on the surface, and a film is formed on the surface of the mortar within 5 minutes, which reduces the subsequent evaporation rate, as more water is thicker from the mortar. Part of the migration to the thinner layer of the mortar layer, the initial opening of the membrane is partially dissolved, and the migration of water will bring more cellulose ether to the surface of the mortar.
The film formation of cellulose ether on the surface of the mortar has a great influence on the performance of the mortar:
First, the formed film is too thin, it will be dissolved twice, can not limit the evaporation of water, reduce the strength.
Second, the formed film is too thick, the concentration of cellulose ether in the mortar interstitial fluid is high, and the viscosity is large. When the tile is pasted, it is not easy to break the surface film.
From this, it is understood that the film forming properties of the cellulose ether have a large influence on the opening time. The type of cellulose ether (HPMC, HEMC, MC, etc.) and the degree of etherification (degree of substitution) directly affect the film-forming properties of cellulose ether, and the hardness and toughness of the film.
In addition to imparting the various beneficial properties described above to the mortar, the cellulose ether retards the hydration kinetics of the cement. This retardation is mainly due to the adsorption of cellulose ether molecules on various mineral phases in the hydrated cement system, but generally speaking, the cellulose ether molecules are mainly adsorbed on water such as CSH and calcium hydroxide. On the chemical product, it is rarely adsorbed on the original mineral phase of the clinker. In addition, due to the increase in viscosity of the pore solution, the cellulose ether reduces the mobility of ions (Ca2+, SO42-, …) in the pore solution, thereby further delaying the hydration process.
Viscosity is another important parameter that represents the chemical properties of cellulose ethers. As mentioned above, the viscosity mainly affects the water retention capacity and also has a significant effect on the workability of the fresh mortar. However, experimental studies have found that the viscosity of cellulose ether has almost no effect on the hydration kinetics of cement. The molecular weight has little effect on hydration, and the biggest difference between different molecular weights is only 10 min. Therefore, molecular weight is not a key parameter for controlling cement hydration.
“Application of Cellulose Ether in Cement-Based Dry-mixed Mortar Products” clearly states that the retardation of cellulose ether depends on its chemical structure. The general trend summarized is that for MHEC, the higher the degree of methylation, the smaller the retardation of cellulose ether. In addition, hydrophilic substitutions (such as substitutions to HEC) are more repressive than hydrophobic substitutions (such as substitutions to MH, MHEC, MHPC). The retarding effect of cellulose ether is mainly affected by two parameters of the type and amount of the substituent group.
Our system experiments also found that the content of the substituents plays an important role in the mechanical strength of the tile adhesive. We evaluated the performance of HPMC with different degrees of substitution in the tile adhesive, and tested the cellulose ether pairs with different groups under different curing conditions. The influence of the mechanical properties of the tile adhesive, Figure 2 and Figure 3 are the effects of changes in methoxy (DS) content and hydroxypropoxy (MS) content on the pull-down strength of the tile adhesive at room temperature.
In the test, we consider Hydroxypropyl methyl cellulose( HPMC), which is a complex ether. Therefore, we should put the two figures together. For HPMC, we need a supply to ensure the water solubility and light transmittance. We know the content of the substituents. It also determines the gel temperature of HPMC, which determines the environment in which HPMC is used. Therefore, the content of the commonly used HPMC is also framed in a range. How to combine methoxy and hydroxypropoxy groups in this range To achieve the best results is what we study. Figure 2 shows that within a certain range, the increase of methoxyl content will bring about a downward trend of the pull strength, while the hydroxypropoxyl content will increase and the pull strength will increase. For open time, there are similar effects.
Post time: Dec-18-2018