Colloidal properties of epoxidized natural rubber latex prepared via membrane separation technology

Abstract

To date, epoxidized natural rubber (ENR) latex is still far from being commercially available for the production of dipped latex goods although the process was reported in 1922. Therefore, the objective of this work is to establish an understanding on the colloidal properties of ENR latex and relate them to coagulation and film formation. To do so, ENR latex was first prepared from low ammonia preserved natural rubber (LATZ) latex and then concentrated (C-ENR) using a patented process via membrane separation technology. NMR was used to determine the epoxidation level and ring opening in the ENR latex. The LATZ latex and both epoxidized latexes (ENR and C-ENR) were characterized for their particle size distribution, morphology, zeta potential and rheological properties. NMR analysis revealed an epoxidation level of 26% with < 2% ring opening. Particle size analysis showed C-ENR latex to have a narrow size distribution with no flocculation upon sonication. Zeta potential analysis as a function of pH showed a lower isoelectric point for C-ENR latex compared to LATZ latex. Zeta potential was also low for C-ENR latex compared to LATZ latex above pH 5 indicating an outward shift in the shear plane of the latex particle due to adsorption of the nonionic surfactant. Rheology of the latexes was studied using steady-state and oscillatory measurements. Elastic overshoot was observed for C-ENR latex at volume fraction (ϕ) of 0.490–0.584 indicating strong particle–particle interaction. The calculated maximum packing fraction (ϕp) of C-ENR latex was 0.721 compared to the ϕp of 0.815 for LATZ latex. The lower ϕp was attributed to the adsorbed nonionic surfactant layer on C-ENR latex particles. Oscillatory measurements showed that C-ENR latex behaved as dominantly elastic material from ϕ of 0.4975–0.584. At ϕ of 0.497 (= 48.5 wt%), the adsorbed polymer layers start to interact with each other. At this ϕ, the calculated adsorbed nonionic surfactant layer thickness (Δ) was 35 nm. As the latex concentration was increased, the latex became more elastic due to the interpenetration and compression of the surfactant layers. At the highest latex ϕ of 0.584 (= 57.6 wt%), tan δ was 0.3 resulting in an Δ of 20 nm. This thick layer of adsorbed surfactant hinders the coagulation of particles by salt during dipping thereby prohibiting the film formation process required to make the latex dipped products.