Dispersible polymer powder and other inorganic adhesives (such as cement, slaked lime, gypsum, clay, etc.) and various aggregates, fillers and other additives [such as hydroxypropyl methylcellulose, polysaccharide (starch ether), fiber Fiber, etc.] are physically mixed to make dry-mixed mortar. When the dry powder mortar is added to water and stirred, under the action of hydrophilic protective colloid and mechanical shearing force, the latex powder particles can be quickly dispersed into the water, which is enough to make the redispersible latex powder fully film. The composition of rubber powder is different, which has an impact on the rheology of the mortar and various construction properties: the affinity of the latex powder for water when it is redispersed, the different viscosity of the latex powder after dispersion, the effect on the air content of the mortar and the distribution of bubbles, The interaction between rubber powder and other additives makes different latex powders have the functions of increasing fluidity, increasing thixotropy, and increasing viscosity.
It is generally believed that the mechanism by which redispersible latex powder improves the workability of fresh mortar is that the latex powder, especially the protective colloid, has an affinity for water when dispersed, which increases the viscosity of the slurry and improves the cohesion of the construction mortar.
After the fresh mortar containing the latex powder dispersion is formed, with the absorption of water by the base surface, the consumption of hydration reaction, and the volatilization to the air, the water gradually decreases, the resin particles gradually approach, the interface gradually blurs, and the resin gradually fuses with each other. finally polymerized into a film. The process of polymer film formation is divided into three stages. In the first stage, the polymer particles move freely in the form of Brownian motion in the initial emulsion. As the water evaporates, the movement of the particles is naturally more and more restricted, and the interfacial tension between the water and the air causes them to gradually align together. In the second stage, when the particles start to contact each other, the water in the network evaporates through the capillary, and the high capillary tension applied to the surface of the particles causes the deformation of the latex spheres to make them fuse together, and the remaining water fills the pores, and the film is roughly formed. The third and final stage enables the diffusion (sometimes called self-adhesion) of the polymer molecules to form a truly continuous film. During film formation, the isolated mobile latex particles consolidate into a new thin film phase with high tensile stress. Obviously, in order for the dispersible polymer powder to be able to form a film in the rehardened mortar, the minimum film forming temperature (MFT) must be guaranteed to be lower than the curing temperature of the mortar.
Colloids – polyvinyl alcohol must be separated from the polymer membrane system. This is not a problem in the alkaline cement mortar system, because the polyvinyl alcohol will be saponified by the alkali generated by the cement hydration, and the adsorption of the quartz material will gradually separate the polyvinyl alcohol from the system, without the hydrophilic protective colloid. , The film formed by dispersing the redispersible latex powder, which is insoluble in water, can not only work in dry conditions, but also in long-term water immersion conditions. Of course, in non-alkaline systems, such as gypsum or systems with only fillers, since polyvinyl alcohol still partially exists in the final polymer film, which affects the water resistance of the film, when these systems are not used for long-term water immersion , and the polymer still has its characteristic mechanical properties, dispersible polymer powder can still be used in these systems.
With the final formation of the polymer film, a system composed of inorganic and organic binders is formed in the cured mortar, that is, a brittle and hard skeleton composed of hydraulic materials, and redispersible polymer powder is formed in the gap and solid surface. flexible network. The tensile strength and cohesion of the polymer resin film formed by the latex powder are enhanced. Due to the flexibility of the polymer, the deformation capacity is much higher than the rigid structure of the cement stone, the deformation performance of the mortar is improved, and the effect of dispersing stress is greatly improved, thereby improving the crack resistance of the mortar.
With the increase of the content of dispersible polymer powder, the whole system develops towards plastic. In the case of high content of latex powder, the polymer phase in the cured mortar gradually exceeds the inorganic hydration product phase, the mortar will undergo qualitative changes and become an elastomer, and the hydration product of cement will become a “filler” “. The tensile strength, elasticity, flexibility and sealing properties of the mortar modified with dispersible polymer powder were improved. Incorporation of dispersible polymer powders allows a polymer film (latex film) to form and form part of the pore walls, thereby sealing the highly porous structure of the mortar. The latex membrane has a self-stretching mechanism that applies tension to its anchorage with the mortar. Through these internal forces, the mortar is held as a whole, thereby increasing the cohesive strength of the mortar. The presence of highly flexible and highly elastic polymers improves the flexibility and elasticity of the mortar. The mechanism for the increase in yield stress and failure strength is as follows: when a force is applied, microcracks are delayed due to the improvement in flexibility and elasticity, and do not form until higher stresses are reached. In addition, the interwoven polymer domains also hinder the merging of microcracks into through-cracks. Therefore, the dispersible polymer powder increases the failure stress and failure strain of the material.
The polymer film in the polymer-modified mortar has a very important effect on the hardening of the mortar. The redispersible polymer powder distributed on the interface plays another key role after being dispersed and formed into a film, which is to increase the adhesion to the materials in contact. In the microstructure of the interface area between the powder polymer-modified ceramic tile bonding mortar and the ceramic tile, the film formed by the polymer forms a bridge between the vitrified ceramic tile with extremely low water absorption and the cement mortar matrix. The contact area between two dissimilar materials is a special high-risk area where shrinkage cracks form and lead to loss of adhesion. Therefore, the ability of latex films to heal shrinkage cracks plays an important role in tile adhesives.
At the same time, the redispersible polymer powder containing ethylene has more prominent adhesion to organic substrates, especially similar materials, such as polyvinyl chloride and polystyrene. A good example of
Post time: Oct-31-2022