Colour of polyurethane foams filled with wood and wood derivatives exposed to two xylophagous fungi

This study addresses changes in aesthetical features of bio-based polyurethane (PU) foams filled with four forest-based particles (namely wood flour, wood bark, kraft lignin, and paper sludge), which were decayed by two wood-rot fungi. White-rot and brown-rot fungi were inoculated on these foams and further characterization results were obtained using colourimetric parameters measured according to the CIELab method. The fungi penetrated into both intra and intercellular spaces into the PU foams, although it neither changed the foam weight nor chemically interacted with the polymer cell wall. These fungi changed overall the appearance of the foams, which indicates that a protective strategy against these microorganisms may be of interest, especially for foam parts used in contact with wood products. The neat PU presented smaller colour stability if compared to the filled foams. Among the fillers, powdered kraft lignin stood out and can be indicated to produce valuable darkened PU parts with stable colour patterns.