Role of PVOH and kaolin on colloidal stability of liquid and powder EVA and SB latexes in cement pore solution

Abstract

The influence of polyvinylalcohol (PVOH) and kaolin on the stability of an ethylene–vinylacetate (EVA) and a carboxylated styrene–butadiene (SB) latex copolymer in synthetic cement pore solution (SCPS) was investigated by photometric turbidity measurements. The dispersions were prepared from liquid EVA/SB latexes or re-dispersible powders (RDPs) obtained by spray drying of the mother liquor with PVOH and kaolin. Colloidal properties of the EVA and SB latex particles were captured by dynamic light scattering (DLS), environmental scanning electron microscopy (ESEM), zeta and streaming potential measurements. The amount of PVOH sorbed onto SB particles was quantified via total organic carbon (TOC) method. It was found that in water, EVA particles generally coagulate and settle with time as a consequence of their nonionic character. In contrast, dispersions of the anionic styrene–butadiene latex show high stability due to repulsion from the pressure of their counter ions clouds. In synthetic cement pore solution, however, the liquid SB latex becomes unstable and shows strong coagulation as a consequence of calcium interaction. Surprisingly, the corresponding SB re-dispersible powder is much more stable in SCPS and exhibits only slight sedimentation. The enhanced stability is attributed to a surface coating of the SB powder particles with a film of PVOH during spray drying. The PVOH coating embeds some of the carboxylate groups located on the surface of the SB powder, as evidenced by a reduced anionic charge density. This way, interaction with calcium is weakened and precipitation via latex–calcium complexation is much reduced. Consequently, addition of PVOH/kaolin during the spray drying of latex polymers not only prevents coalescence and caking of the powder particles, but also enhances their colloidal stability in cementitious systems.