Why is it necessary to add redispersible latex powder in dry-mixed mortar?

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New Earth Group


Why is it necessary to add redispersible latex powder in dry-mixed mortar?

Dry-mixed mortar is maderedispersible latex powderWith other inorganic adhesives (such as cement, hydrated lime, gypsum, clay, etc.) and various aggregates, fillers and other additives [such as hydroxypropyl methyl cellulose, poly (starch ether), cellulose fiber, etc.] made by physical mixing. When the dry mortar is added to water and stirred, under the action of hydrophilic protective colloid and mechanical shear force, the latex powder particles can be quickly dispersed into water, and sufficient to make the redispersible latex powder fully film. The different composition of rubber powder will affect the rheological properties of mortar and various construction properties, such as: the affinity of latex powder to water when it is redispersed, the different viscosity of latex powder after dispersion, the influence on the air content of mortar and the distribution of bubbles, the interaction between rubber powder and other additives, etc., so that different latex powders have the effects of increasing fluidity, increasing thixotropy, increasing viscosity, etc.
It is generally believed that the redispersible latex powder improves the workability of the fresh mortar: the affinity of the latex powder, especially the protective colloid, to water when dispersed, increases the viscosity of the slurry and improves the cohesion of the construction mortar. After the fresh mortar containing latex powder dispersion is formed, with the absorption of water on 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 becomes blurred, the resin gradually fuses with each other, and finally polymerizes into a film. The process of polymer film formation is divided into three stages. In the first stage, the polymer particles are free to move in the form of Brownian motion in the initial emulsion. With the evaporation of water, the movement of particles is naturally more and more restricted, and the interfacial tension between water and air causes them to gradually arrange together. In the second stage, when the particles begin to contact each other, the network-like moisture 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, the remaining moisture is filled in the pores, and the film is roughly formed. The third, final stage allows diffusion (sometimes referred to as self-adhesive) of the polymer molecules into a truly continuous film. During film formation, isolated mobile latex particles consolidate into a new film phase with high tensile stress. Obviously, in order to make the re-dispersible latex powder can re-harden mortar film, it is necessary to ensure that the minimum film-forming temperature (MFT) is lower than the curing temperature of the mortar.
With the final formation of the polymer film, a system of inorganic and organic binder structure is formed in the cured mortar, that is, a brittle skeleton composed of a hydraulic material, and a flexible network composed of a film formed by a redispersible latex powder in the gap and a solid surface. The tensile strength of the polymer resin film formed by the emulsion powder is enhanced and the cohesive force is improved. Due to the flexibility of the polymer, the deformation capacity is much higher than that of the rigid structure of the cement stone, the deformation performance of the mortar is improved, and the effect of the dispersed stress is greatly improved, thereby improving the crack resistance of the mortar. With the increase of the dosage of redispersible latex powder, the whole system develops towards plastics. In the case of high latex powder content, the polymer phase in the solidified 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 sealing properties of the mortar modified by redispersible latex powder are improved. The blending of the redispersible emulsion powder allows a polymer film (latex film) to form and form part of the pore wall, thereby closing the highly porous structure of the mortar. The latex film has a self-stretching mechanism to apply tension to its anchorage with 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 of the increase in yield stress and failure strength is as follows: When a force is applied, the microcracks are delayed due to the improvement in flexibility and elasticity until a higher stress is reached. In addition, the interwoven polymer regions also act as an impediment to the coalescing of the microfractures into through-fractures. Therefore, the redispersible emulsion powder increases the failure stress and failure strain of the material.
The polymer film in polymer modified mortar has a very important effect on hardening mortar. The redispersible emulsion powder distributed on the interface plays another key role in the dispersion and film formation, that is, it increases the adhesion to the materials in contact. In the microstructure of the interface zone between the powder polymer modified ceramic tile-bonded mortar and the ceramic tile, the film formed by the polymer formed a bridge between the vitrified ceramic tile with very low water absorption and the cement mortar matrix. Areas of contact between two dissimilar materials are particularly at high risk of shrinkage crack formation and resulting in loss of bonding force. Therefore, the ability of the latex film to heal shrinkage cracks has an important role in tile adhesives.

Key words:

Redispersible latex powder