Effect of calcium nitrate and a hydrophobically modified polymeric surfactant on the stability of natural rubber latex

The control of the colloidal stability of latex particles is essential to obtain homogenous films. The manufacture of latex gloves is required to control the flocculation of latex particles, induced by the addition of calcium salts. This flocculation can, however, be retarded by the addition of very small amounts of surfactants. The present publication describes the effect of Ca(NO 3 ) 2 on the stability and rheological properties of natural rubber latex in the presence of small concentrations of a hydrophobically modified polyfructose (Inutec NRA). This surfactant strongly adsorbs on the surface of the latex particles, forming a repulsion layer. However, addition of Ca(NO 3 ) 2 causes a shrinkage of this layer. The colloidal stability, as a function of surfactant and Ca(NO 3 ) 2 concentrations was studied by determining surfactant adsorption, zeta potential and rheological properties. The addition of Ca(NO 3 ) 2 increased surfactant adsorption, due to the poorer solvency of the medium for the polyfructose chains, which became less hydrated in the presence of Ca 2+ . The addition of calcium also reduced the absolute value of zeta potential, due to compression of the repulsion layer. However, as the surfactant was adsorbed onto the surface of the particles, the latex particles remained stable even after charge reversal at high Ca 2+ concentration, due to the higher steric stabilisation conferred by the surfactant. Flow behaviour studies showed a reduction in the maximum packing fraction ( ϕ_max ) of latex with surfactant concentration, due to formation of the repulsion layer that increased the distance between particles. However, the presence of Ca(NO 3 ) 2 increased ϕ_max , indicating a reduction in the interparticle distance caused by compression of the repulsion layer between particles. Moreover, the addition of Ca(NO 3 ) 2 also decreased the cohesive energy density of the latex, resulting in a more viscous behaviour. These results demonstrate the ability of calcium ions to dehydrate surfactant molecules, which is crucial for explaining the macroscopic properties of latex suspensions. Graphical abstract