Use of surfactants to remove water-based inks from plastic film: effect of calcium ion concentration and length of surfactant hydrophobe

Abstract

Effective plastic film deinking could permit the reuse of recycled polymer to produce clear film, reduce solid waste for landfills, reduce raw material demand for polymer production, and aid process economics. In this study, the deinking of a commercial polyethylene film printed with water-based ink was studied using surfactants in the presence of hardness ions (calcium ions) at various pH levels. The electrostatic properties of ink particles in a washing bath were also investigated. Synthetic anionic surfactant or fatty acid soap in the presence of calcium ions at alkaline pH levels was found to be nearly as effective at deinking as cationic, nonionic, or amphoteric surfactants alone. However, adding calcium ions decreases the deinking effectiveness of cationic, nonionic, and amphoteric surfactants. Increasing the length of the ionic surfactant hydrophobe enhances deinking. Zeta potential measurements showed that water-based ink particles in water reach the point of zero charge (PZC) at a pH of about 3.6, above which ink particles are negatively charged, so cationic surfactant tends to adsorb better on the ink than anionic surfactant above the PZC in the absence of calcium. As the cationic surfactant concentration is varied between 0.005 and 25 mM, the zeta potential of the ink particles reverses from negative to positive owing to adsorption of cationic surfactant. For anionic surfactants, added calcium probably forms a bridge between the negatively charged ink and the negatively charged surfactant head groups, which synergizes adsorption of the surfactant and aids deinking. In contrast, calcium competes for adsorption sites with cationic and nonionic surfactants, which inhibits deinking. All the surfactants studied here disperse ink particles effectively in the washing bath above pH 3 except for the ethoxylated amine surfactant.