Latex, colloidal and film properties of concentrated skim latex prepared using membrane filtration process

Abstract

For a long time, skim latex has been regarded as waste from the latex centrifugation process. To recover the rubber, skim latex is coagulated using strong acid due to the high presence of non-rubbers, resulting in products of low quality and unpleasant odour. Additionally, the acid discharged into effluent ponds causes malodour and adversely affects the health of nearby residents. Considering this, a novel method employing a membrane filtration system was utilised, successfully recovering and concentrating skim latex while removing water-soluble non-rubbers. The resulting latex concentrate was then compared to raw skim latex in terms of latex, colloidal, and film properties. Latex properties, namely total solids content (TSC), dry rubber content (DRC), alkalinity, nitrogen content, and gel content were determined. For the colloidal properties, particle size and distribution, zeta potential, and rheological properties (flow and viscoelastic behaviours), were characterised. The film properties assessed included the swelling index, appearance, and tensile properties. Significant improvements in latex properties after concentrating were observed, with both TSC and DRC increasing to 42% and 36%, respectively, while alkalinity, nitrogen, and gel contents decreased to 0.61%, 2.23%, and 54%, respectively. Particle size distribution in raw skim latex (RSL) exhibited a tri-modal distribution, but after membrane filtration, concentrated skim latex (CSL) showed a bi-modal distribution with particle sizes ranging from 0.03 to 0.3 µm and 0.3 to 5 µm and average sizes of 0.07 and 1 µm, respectively. Changes were also noted in zeta potential and rheological behaviour after membrane filtration. The isoelectric point of zeta potential shifted to a higher pH, from 4.2 for RSL to 4.9 for CSL, and the absolute zeta potential values decreased with increasing pH values. CSL also demonstrated a different flow behaviour, fitting well to the Herschel-Bulkley model, unlike RSL, which conformed to the Bingham model. Membrane concentration resulted in significant increases in CSL yield stress, consistency index, and shear thinning behaviour, leading to higher moduli values and indicating increased interaction between CSL particles. Conversely, CSL films prepared via casting swelled more in toluene, approximately two times more than RSL films. Nevertheless, RSL films were more opaque and only became transparent after leaching, indicating high non-rubber contents. Regarding tensile properties, both unleached and leached CSL films exhibited a similar tensile versus elongation profile, with ultimate tensile strength and elongation at break values ranging between 1.4 and 1.6 MPa and 650 and 700%, respectively.