Controlling colorant staining during the de-inking of flexible plastic packaging

Abstract Several technologies for de-inking plastics have been developed, yet, most face a similar problem, namely colorant staining. This means the ink may be removed, but some colorant re-adsorbs to the plastic resulting in recyclates with a slightly colored shine after extrusion. This re-adsorption or staining of colorants occurs as they become available in the medium during de-inking by dissolution or solubilization, followed by dynamic adsorption–desorption of the dissolved and solubilized pigments between the medium and the plastic surfaces. In this paper, staining was analyzed, and methods for reducing staining are proposed. First, it was confirmed that staining is a reversible adsorption-desorption process. Next, kinetic models were established to describe the de-inking, dissolution-solubilization, and adsorption-desorption for a yellow diarylide pigment (P Y13) and red azo lake pigment (PR 57:1) in caustic soda with detergent. First-order models were able to describe these processes, and were implemented in Aspen Plus to examine the effect of residence time, ink load, wastewater treatment efficiency, and flow pattern. It was shown that colorant adsorption increases with residence time, and that an increase in ink load or decrease in wastewater treatment efficiency also results in an increased adsorption. Next, it was shown that crossflow results in negligible adsorption of colorants (< 0.5 ppm), and counter-current also results in reduced adsorption (up to 85 %) compared to single reactor de-inking (around 200 ppm and higher). Washing with fresh media enables colorant desorption when its driving force exceeds that of adsorption. Regardless, it can be argued that it is industrially unfeasible to completely counteract staining by merely altering the process flow, and more advanced techniques are needed to prevent staining. • Staining is a reversible adsorption-desorption process. • First-order kinetic models can describe the staining of plastics during de-inking. • Wastewater treatment should be optimized to reduce the driving force of adsorption. • High affinity of colorants for plastics limits desorption in fresh de-inking medium. • Counter-current and crossflow configurations reduce staining up to 85 % and 99.9 %.