The Role of Redispersible Latex Powder in Putty
Release Time:
2021-02-27
Source:
Redispersible latex powder is a thermoplastic resin powder obtained by spray-drying a polymer emulsion to remove moisture.
The Role of Redispersible Latex Powder in Putty
Redispersible latex powder is a thermoplastic resin powder produced by spray-drying polymer emulsions to remove moisture; upon rehydration, it can be redispersed into an emulsion. Owing to this property, it finds extensive application in areas that require modification of inorganic cementitious materials, such as single-component JS waterproof coatings, adhesive mortars for polystyrene insulation boards used in building thermal insulation, flexible surface protection mortars, polystyrene-bead insulation coatings, tile adhesives, self-leveling mortars, dry-mix mortars, and putties. The addition of redispersible latex powder enhances product strength; whether incorporated into putty powders or mortars, it significantly improves their mechanical performance and markedly boosts hardness. It exhibits outstanding water resistance and excellent bond strength, while also increasing mortar elasticity and extending its open time. Moreover, it imparts superior alkali resistance, enhancing mortar adhesion, flexural strength, wear resistance, and workability. In particular, when used in flexible crack-resistant mortars, it provides exceptional flexibility.
The role of redispersible latex powder in putty powders: it provides strong adhesion and excellent mechanical properties, outstanding water resistance and permeability, as well as superior alkali resistance and abrasion resistance. It also enhances water retention, extends open time, and improves durability.
1. On the role of fresh mortar
1) Improve constructability.
2) Enhances water retention and improves cement hydration.
3) Enhance workability.
4) Avoid early cracking.
2. On the role of hardened mortar
1) Reduce the elastic modulus of the mortar to enhance its compatibility with the substrate.
2) Enhance flexibility and crack resistance.
3) Enhance resistance to powder shedding.
4) Hydrophobicity or reduced water absorption.
5) Enhance adhesion to the substrate.
Re-dispersible latex powder forms a polymer emulsion upon contact with water; during the mixing and drying process, the emulsion undergoes secondary dehydration. The performance of the latex powder in putty is governed by a composite system involving cement hydration and the film-forming process of the latex powder, which unfolds in four distinct steps:
① When the redispersible latex powder is thoroughly mixed with water in the putty powder, it disperses into fine polymer particles.
② During the early stages of cement hydration, cement gel gradually forms, and the liquid phase becomes saturated with Ca(OH)2 produced during hydration; meanwhile, latex powder forms polymer particles that deposit on the surface of the cement gel–unhydrated cement particle mixture.
③ As cement hydration progresses, the moisture in the capillary pores decreases, gradually confining the polymer particles within these pores and leading to the formation of a densely packed layer on the surfaces of the cement gel–unhydrated cement particle mixture and the filler.
④ Under the combined effects of hydration reactions, substrate absorption, and surface evaporation, the moisture content continues to decrease, causing the already formed aggregate layer to coalesce into a thin film that binds the hydration reaction products together, thereby forming a continuous network structure. The synergistic action of cement hydration and the film-forming properties of latex powder results in an enhanced dynamic crack resistance of the putty.
From the perspective of practical application, putty used as a transition layer between exterior wall external thermal insulation and coating should not have a strength higher than that of finishing mortar; otherwise, cracking is likely to occur. Within the entire thermal insulation system, the flexibility of the putty should exceed that of the substrate. Only in this way can the putty better accommodate substrate deformation and cushion its own deformation induced by external environmental factors, thereby alleviating stress concentration and reducing the likelihood of coating cracking and spalling.
