Solvent-Free Dyeing of Solid Wood in Water-Saturated Supercritical Carbon Dioxide

Coloring of massive wood is still a major challenge in high-end wood processing, which aims to provide solid wood at a broad range of colors to furniture manufacturers. Here we report that water-saturation of supercritical carbon dioxide (scCO2)—a promising green dyeing medium—greatly improves solid wood dyeing. This has been demonstrated for disperse blue 134 and three European hardwood species, namely, birch (Betula pendula), beech (Fagus sylvatica), and oak (Quercus robur). Variation of dyeing temperature (45–100 °C) and pressure (10.0–30.0 MPa) revealed that 20.0 MPa and 100 °C pose an optimal parameter combination. Coloration of the interior of small wood cubes (2 cm3), expressed as ΔE color change within the CIELAB color space, was here 40.6 (±1.2) for birch, 28.7 (±3.1) for beech, and 16.3 (±4.4) for oak. Since these values are significantly higher compared to water-free scCO2, it can be concluded that water—even if poorly soluble in scCO2—was effectively transported throughout the semiporous wood matrices. This causes the latter to swell, which efficiently improves the accessibility of individual cell wall constituents by dyeing reagents, for example. Three-point bending tests indicate that water-saturated scCO2 dyeing effectuates some morphological or chemical alterations to the wood structure, since a moderate loss of both flexural strength and stiffness (15–20%) was found. Nevertheless, the proposed approach which can be adapted to a broad variety of biobased materials represents a green, sustainable, and highly versatile process, as it is free of organic solvents and waste products.