Application of Redispersible Latex Powder in Mortar Systems

Colloidal polyvinyl alcohol must be separated from the polymer film system. In alkaline cement mortar systems, this is not a significant challenge, as the polyvinyl alcohol is saponified by the alkali generated during cement hydration, and the adsorptive action of quartz materials gradually drives the polyvinyl alcohol out of the system. Without the hydrophilic protective colloid, the film formed by the redispersible latex powder—though itself insoluble in water—can perform effectively not only under dry conditions but also under prolonged immersion in water. Of course, in non-alkaline systems, such as gypsum-based formulations or systems consisting solely of fillers, some polyvinyl alcohol remains in the final polymer film, which can compromise its water resistance. However, when these systems are not intended for long-term immersion in water and the polymer still retains its characteristic mechanical properties, redispersible latex powder can still be used in such applications.
With the final formation of the polymer film, a composite system comprising inorganic–organic binder structures is established within the cured mortar: a brittle, rigid skeleton formed by hydraulic materials, and a flexible network created by redispersible latex powder that forms films in the voids and on solid surfaces. The polymer resin film formed by the latex powder enhances tensile strength and increases cohesive forces. Owing to the flexibility of the polymer, the deformation capacity of the mortar is significantly higher than that of the rigid cementitious matrix, thereby improving the mortar’s deformation performance and substantially enhancing its ability to distribute stresses, which in turn improves its crack resistance.

As the dosage of redispersible latex powder increases, the entire system shifts toward a more plastic behavior. At high latex-powder contents, the polymer phase in the hardened mortar gradually surpasses the inorganic hydration-product phase, leading to a fundamental transformation: the mortar becomes an elastomer, while the cement hydration products assume the role of a “filler.” Modification with redispersible latex powder enhances the mortar’s tensile strength, elasticity, flexibility, and impermeability. The incorporation of redispersible latex powder facilitates the formation of a polymer film (latex film) that forms part of the pore walls, thereby sealing the highly porous structure of the mortar. The latex film exhibits a self-stretching mechanism, which can exert tensile forces at its bond interface with the mortar matrix. Through these internal forces, the mortar is held together as a cohesive whole, thus increasing its cohesive strength. The presence of highly flexible and elastic polymers further improves the mortar’s flexibility and elasticity. The mechanisms underlying the increases in yield stress and ultimate strength are as follows: when external loads are applied, the enhanced flexibility and elasticity delay the initiation of microcracking, requiring higher stresses before such cracks develop. Moreover, the interwoven polymer network impedes the coalescence of microcracks into through-cracks. Consequently, redispersible latex powder elevates both the material’s ultimate stress and ultimate strain.

The polymer film in polymer-modified mortar plays a critically important role in the hardened mortar. The redispersible latex powder dispersed at the interface not only forms a film but also exerts another key function: it enhances adhesion to the materials it contacts. In the microstructure of the interfacial zone between powder-polymer-modified tile adhesive mortar and ceramic tiles, the polymer film creates bridging bonds between the vitrified tiles—with their extremely low water absorption—and the cement-based mortar substrate. The contact zone between these two dissimilar materials is a particularly high-risk area where shrinkage cracks tend to form, leading to a loss of bond strength. Therefore, the ability of the latex film to heal such shrinkage cracks is of great significance for tile adhesives.
Meanwhile, redispersible latex powders containing ethylene exhibit particularly strong adhesion to organic substrates, especially to similar materials such as polyvinyl chloride and polystyrene; this is well illustrated by their use in polymer-modified dry-mix mortars for bonding and finishing over polystyrene boards.