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Mechanism of Redispersible Emulsion Powder in Dry Mix Mortar

Mechanism of Redispersible Emulsion Powder in Dry Mix Mortar

Redispersible latex 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 made into dry-mixed mortar by physical mixing. When the dry powder mortar is added to the water and stirred, under the action of the hydrophilic protective colloid and mechanical shear force, the latex powder particles can be quickly dispersed into the water, which is enough to fully form the redispersible latex powder into a film. The composition of the rubber powder has different effects on the rheological properties of the mortar and various construction properties: the affinity of the latex powder to water when it is redispersed, the different viscosities of the latex powder after dispersion, the impact on the air content of the mortar and the distribution of air bubbles, The interaction between rubber powder and other additives makes different latex powders have the effects of increasing fluidity, increasing thixotropy, and increasing viscosity.

It is generally believed that the mechanism of redispersible latex powder to improve the workability of fresh mortar is: the affinity of latex powder, especially the protective colloid, to water when it is dispersed, 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 resin particles will gradually approach, the interface will gradually blur, and the resins will gradually fuse with each other. eventually 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 water and air forces them to gradually align together. In the second stage, when the particles come into contact with each other, the water in the network evaporates through capillary tubes, 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 polymer 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.

Colloids – polyvinyl alcohol must be separated from the polymer film 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 one-time dispersion of redispersible latex powder, which itself is insoluble in water, can function not only in dry conditions, but also in long-term water immersion conditions. Of course, in non-alkaline systems, such as gypsum or filler-only systems, because 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 unique mechanical properties, and the redispersible polymer powder can still be used in these systems.

With the final formation of the polymer film, a system composed of inorganic and organic binder structures is formed in the cured mortar, that is, a brittle and hard skeleton composed of hydraulic materials, and redispersible latex powder forms a film between the gap and the 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 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 walls, thereby sealing the highly porous 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.

The polymer film in polymer modified mortar has a very important effect on hardening mortar. The redispersible latex powder distributed on the interface plays another key role after being dispersed and film-forming, which is to increase the adhesion to the contacted materials. In the microstructure of the powder polymer modified tile bonding mortar and the tile interface, the film formed by the polymer forms a bridge between the vitrified tiles with extremely low water absorption and the cement mortar matrix. The contact zone between two dissimilar materials is a particularly high risk area for shrinkage cracks to form and lead to loss of cohesion. Therefore, the ability of latex films to heal shrinkage cracks is of great importance for tile adhesives.

At the same time, the redispersible latex powder containing ethylene has more outstanding adhesion to organic substrates, especially similar materials, such as polyvinyl chloride and polystyrene. A good example is when it comes to masks.

Post time: May-04-2023
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