Improving single-step scCO2 dyeing of wood by DMSO-induced micro-swelling

Abstract

Supercritical dyeing of wood is a smart green technology. Beyond being of interest to wood industry, it could help to preserve tropical forests and decrease the environmental pollution from overseas shipping. However, scCO2 dyeing is still in its infancies. This paper demonstrates at the example of three major European hardwood species and the dye Naphthazarin that the uptake of colorants can be improved if scCO2 is complemented with small amounts of DMSO. The latter triggers micro-swelling of wood, thereby improving dye penetration. Systematic variation of process parameters revealed that optimal colouration is reached at 20.0 MPa and 85 °C. Colour gradients as observed in the interior of halved specimen were evaluated by two methods. While the Euclidean distance ΔE (CIELAB) represents relative colour change only, a recently proposed scanning densitometric approach translates colour depth into absolute dye surface concentration. Morphological investigations conceal this study investigating the impact of selected process conditions.