The affinity of latex powder to water when it is redispersed, the different viscosities of latex powder after dispersion, the influence on the air content of mortar and the distribution of air bubbles, the interaction between rubber powder and other additives, etc., make different latex powders have increased fluidity. , Increase thixotropy, increase viscosity and so on.
Improve workability
It is generally believed that the redispersible latex powder improves the workability of fresh mortar: the latex powder, especially the protective colloid, has an affinity for water and increases the viscosity of the slurry and improves the cohesion of the construction mortar. After the freshly mixed mortar containing 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 will gradually decrease, the particles will gradually approach, the interface will gradually become blurred, and they will gradually merge with each other and finally aggregate. film forming. 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 water and air forces 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 fuse them together, and the remaining water fills the pores, and the film is roughly formed. The third, final stage allows the diffusion (sometimes called self-adhesion) of the polymer molecules to form a true continuous film. During film formation, isolated mobile latex particles consolidate into a new film phase with high tensile stress. Obviously, in order to enable the redispersible latex powder to form a film in the hardened mortar, it is necessary to ensure that the minimum film forming temperature (MFT) is lower than the curing temperature of the mortar.
Increase material stress
With the final formation of the polymer film, a system consisting of inorganic and organic binder structures, that is, a brittle and hard skeleton composed of hydraulic materials, and a film formed by redispersible latex powder in the gaps and solid surfaces are formed in the cured mortar. 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 ability is much higher than that of the cement stone rigid structure, 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 redispersible latex powder, the whole system develops towards plastic. In the case of high latex powder content, the polymer phase in the cured mortar gradually exceeds the inorganic hydration product phase, and the mortar will undergo a qualitative change and become an elastomer, while the hydration product of cement becomes a “filler”. “. The tensile strength, elasticity, flexibility and sealability of the mortar modified by redispersible latex powder are all improved. The blending of redispersible latex powder allows the polymer film (latex film) to form and form part of the pore wall, thereby sealing the high porosity structure of the mortar. The latex membrane has a self-stretching mechanism that exerts tension where it is anchored to the mortar. Through these internal forces, the mortar is maintained 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 until higher stresses are reached due to improved flexibility and elasticity. In addition, the interwoven polymer domains also hinder the coalescence of microcracks into penetrating cracks. Therefore, the redispersible polymer powder improves the failure stress and failure strain of the material.
Post time: Mar-09-2023