Reactions and Mechanisms of Redispersible Polymer Powder
The unique ability of RPP to redisperse in water and form stable polymer dispersions is crucial for its effectiveness in various applications. This chapter will explore the reactions and mechanisms involved in the redispersion process and the interactions between RPP and other components in cementitious systems.
a.Redispersion Mechanism
The redispersion of RPP in water involves the breakdown of the powder particles and the formation of a stable polymer dispersion. This process is influenced by several factors, including the composition and morphology of the RPP particles, the presence of additives, and the conditions of the aqueous medium.

b.Particle Swelling and Dissolution
When RPP is introduced into water, the first step is the swelling of the polymer particles. Water penetrates the particle structure, hydrating the protective colloids and water-soluble agents present in the powder[1, 2]. This swelling process is crucial for the subsequent dissolution of the polymer particles.
The rate and extent of particle swelling are influenced by the composition of the RPP, particularly the type and amount of hydrophilic monomers used in the shell layer. Monomers such as methacrylic acid (MAA) and acrylic acid increase the hydrophilicity of the shell, promoting water uptake and swelling[3].

c.Electrostatic Repulsion and Steric Stabilization
As the RPP particles swell and dissolve, the protective colloids and surfactants adsorbed on the particle surface play a crucial role in stabilizing the resulting polymer dispersion. These additives provide electrostatic repulsion and steric stabilization, preventing the aggregation of polymer particles[1, 4].
Electrostatic repulsion occurs when the particles carry a similar electrical charge, causing them to repel each other. This is achieved through the dissociation of carboxyl groups in the shell layer, which form negatively charged carboxylate ions in the presence of a base[5]. The zeta potential, a measure of the electrical potential difference between the dispersion medium and the stationary layer of fluid attached to the dispersed particle, is an indicator of the stability of the redispersion[3].
Steric stabilization is provided by the adsorbed protective colloids, such as polyvinyl alcohol (PVA), which form a physical barrier around the polymer particles[1]. The protective colloids extend into the aqueous medium, creating a "hairy" structure that prevents the particles from coming into close contact and aggregating[5].

d.Interactions with Cementitious Systems
When RPP is incorporated into cementitious systems, such as mortars and concrete, it undergoes various interactions with the cement hydration products and other components. These interactions are essential for the performance and functionality of RPP in construction materials. e.Film Formation and Polymer-Cement Co-Matrix As the cementitious system hydrates and sets, the redispersed polymer particles come into close contact with the cement hydration products. The polymer particles coalesce and form a continuous film within the cement matrix, creating a polymer-cement co-matrix[4, 6]. The formation of this co-matrix is influenced by the glass transition temperature (Tg) of the polymer and the curing conditions. Polymers with a low Tg, such as those based on ethylene-vinyl acetate (EVA) or styrene-acrylic copolymers, are able to form flexible films at ambient temperatures, enhancing the flexibility and crack-bridging properties of the cementitious system[4]. f.Interfacial Bonding and Adhesion The polymer film formed by RPP also plays a crucial role in the adhesion between the cementitious matrix and various substrates, such as tiles or reinforcing fibers. The polymer film penetrates the pores and irregularities of the substrate surface, creating a strong mechanical interlock[6].
In addition to mechanical bonding, the functional groups present in the RPP, such as carboxyl and ester groups, can form chemical bonds with the substrate and the cement hydration products[7]. This chemical interaction further enhances the adhesion and durability of the polymer-modified cementitious system.

g.Hydrophobicity and Water Resistance
RPPs containing hydrophobic monomers, such as vinyl esters of versatic acid (VeoVa), can impart water resistance and hydrophobicity to the cementitious system[7]. The branched, carbon-rich structure of these monomers reduces the water uptake and permeability of the polymer film, protecting the cementitious matrix from moisture-related deterioration.


References
1. Huan-qin, C. Advances of water redispersibility mechanism for redispersible polymer powder. 2011.
2. Zhong, Z., Influence of Superplacticister and Redispersible Polymer Powder on the Strength of Crumb Rubber Mortar. Journal of Beijing University of Technology, 2009.
3. Li, Z., Influencing Mechanism of Methacrylic Acid on the Redispersion Properties of Redispersible Polymer Powders. Acta Physico-chimica Sinica, 2010.
4. Caimi, S., et al., Core-Shell Morphology of Redispersible Powders in Polymer-Cement Waterproof Mortars. Polymers (Basel), 2018. 10(10).
5. Huan-qin, C., Mechanism of the Influence of Hydrophobic Interaction on Water-Dispersibility of Redispersible Polymer Powder. Journal of Chemical Engineering of Chinese Universities, 2011.
6. Herold, D.H. FIXING OF STONEWARE "PORCELANIC" TILES BY MEANS OF CERAMIC TILE ADHESIVES MODIFIED WITH REDISPERSIBLE POLYMER POWDER. 2007.
7. Ding, J.J., et al., Composition Analysis of an Unknown Redispersible Powder. Key Engineering Materials, 2017. 726: p. 55 - 59.