Mechanism of Action of Redispersible Latex Powder in Dry-Mix Mortar
Release Time:
2022-03-04
Source:
Mechanism of Action of Redispersible Latex Powder in Dry-Mix Mortar
Redispersible latex powder can be physically blended with other inorganic binders—such as cement, hydrated lime, gypsum, and clay—as well as various aggregates, fillers, and other additives—including hydroxypropyl methylcellulose, polysaccharide ethers (starch ethers), and cellulose fibers—to produce dry-mix mortars. When water is added to the dry-mix mortar and the mixture is stirred, the hydrophilic protective colloids and mechanical shear forces facilitate the rapid dispersion of latex-powder particles into the water, enabling the redispersible latex powder to form a continuous film. The specific composition of the latex powder influences the mortar’s rheological behavior and various application properties: its affinity for water upon redispersion, the resulting viscosity after dispersion, its impact on air content and bubble distribution, and its interactions with other admixtures—all of which determine whether a particular latex powder enhances flowability, thixotropy, or viscosity. It is generally accepted that the mechanism by which redispersible latex powder improves the workability of fresh mortar is as follows: the latex powder, particularly its protective colloids, exhibits strong affinity for water upon dispersion, increasing the slurry’s consistency and thereby enhancing the internal cohesion of the applied mortar.
After fresh mortar containing a latex-powder dispersion is placed and allowed to set, water gradually diminishes as the substrate absorbs it, hydration reactions consume it, and it evaporates into the air. Consequently, the resin particles draw closer together, the interfacial boundaries become increasingly blurred, and the resins progressively coalesce until they ultimately form a continuous film. The polymer film-forming process unfolds in three stages. In the first stage, polymer particles in the initial emulsion move freely via Brownian motion. As water evaporates, particle mobility becomes increasingly restricted, and the interfacial tension between water and air drives the particles to align and aggregate. In the second stage, once the particles begin to make contact, network-like water is removed by capillary evaporation, and the high capillary tension acting on the particle surfaces induces deformation of the latex spheres, promoting their fusion. The remaining moisture fills the pores, and the film begins to take shape. In the third and final stage, polymer molecular diffusion—sometimes referred to as self-adhesion—results in the formation of a truly continuous film. During film formation, isolated, mobile latex particles consolidate into a new thin-film phase that exhibits high tensile stress. Clearly, for redispersible latex powder to successfully form a film within hardened mortar, the minimum film-forming temperature (MFT) must be lower than the mortar’s curing temperature.
