Impregnation of pinewood with softwood Kraft lignin

Abstract Scots pine wood was impregnated under vacuum with an aqueous 60% acetone solution containing about 10% softwood Kraft lignin. The impregnation treatment incorporated 6–8% of lignin (on wood dry mass) to the wood samples and caused a noticeable change in color from pale yellow to dark brown, but microscopy images showed that the lignin remained mostly within 200 µm from the wood surface. Pinewood samples impregnated with lignin and subjected to water leaching trials had a mass loss of only about 1%, in accordance with the insolubility of Kraft lignin in water. The impregnated wood exhibited lower hygroscopicity and considerably lower surface wettability, as indicated by an increase of more than 100% in water contact angle. Lignin impregnation also improved the fungal decay resistance of the wood against the brown rot fungus Rhodonia placenta, but had no effect against Coniophora puteana. On the other hand, lignin impregnation worsened the flammability properties of the pinewood as determined by cone calorimetry, especially in relation to smoke formation. Enhancing the penetration of lignin into the wood structure may help improving further the resistance to fungal decay, while the combination of flame retardant additives with the lignin may be required in order to improve the flammability properties of pinewood.