Application of Redispersible Latex Powder in Mortar Systems
Release Time:
2021-01-04
Source:
Redispersible latex powder can be physically blended with other inorganic binders—such as cement, hydrated lime, and gypsum—as well as various aggregates, fillers, and other additives—including methyl hydroxypropyl cellulose, starch ethers, lignocellulosic fibers, and water-repellent agents—to produce dry-mix mortars. When such dry-mix mortars are mixed with water, the hydrophilic protective colloids and mechanical shear forces cause the latex-powder particles to disperse uniformly in the aqueous phase. The dispersion of conventional redispersible latex powder typically requires only a very short time; for example, in dry-sprayed concrete repair mortars, the dry mortar containing redispersible latex powder is mixed with water just at the nozzle exit for about 0.1 seconds before being sprayed onto the substrate, which is sufficient for the latex powder to achieve complete dispersion and film formation. Even during the early mixing stage, the latex powder begins to influence the mortar’s rheological properties and workability. Depending on the inherent characteristics of the latex powder and the specific modifications applied, these effects vary: some enhance flow, while others increase thixotropy. The underlying mechanisms are multifaceted, involving the latex powder’s affinity for water during dispersion, the resulting differences in viscosity after dispersion, the role of the protective colloids, interactions with cement and water, the impact on air content and bubble distribution, and the synergistic effects arising from the latex powder itself and its interactions with other admixtures. A widely held view is that redispersible latex powder generally increases the air content of the mortar, thereby providing a lubricating effect that enhances workability; moreover, the latex powder—particularly the protective colloids—exhibits strong water affinity and increased viscosity upon dispersion, which helps to improve the internal cohesion of the fresh mortar and thus its ease of placement. Subsequently, the wet mortar containing the latex-powder dispersion is applied to the substrate, where, as moisture is progressively removed through three pathways—the substrate’s absorption, consumption by cement hydration, and evaporation from the surface—the resin particles draw closer together, their interfaces gradually coalesce, and ultimately form a continuous polymeric film. This process primarily occurs within the mortar’s pores and on the surfaces of solid particles.
To render this process irreversible—such that the polymer film does not redisperse upon subsequent exposure to water—the protective colloid of the redispersible latex powder, namely polyvinyl alcohol, must be removed from the polymer film system. In alkaline cement mortar systems, this is not a significant challenge, because the alkali generated by cement hydration saponifies the polyvinyl alcohol, and the adsorption of polyvinyl alcohol onto quartz-based materials further facilitates its gradual removal from the system. With the hydrophilic protective colloid eliminated, the film formed by the initial redispersion of the redispersible latex powder—being inherently insoluble in water—can perform effectively not only under dry conditions but also under prolonged immersion in water. In non-alkaline systems, such as gypsum-based systems or systems consisting solely of fillers, polyvinyl alcohol may, for various reasons, remain partially embedded in the final polymer film, thereby compromising its water resistance. However, since these systems are generally not intended for long-term immersion in water and the polymer still retains its characteristic mechanical properties, the presence of residual polyvinyl alcohol does not adversely affect the application of redispersible latex powder in such systems.
