Effect of Cold Plasma Treatment on Contact Angle and Surface Color Properties of Untreated and Thermally Modified Scots Pine Wood

Abstract

Scots pine wood (Pinus sylvestris) specimens were thermally modified using the rectification method and subsequently exposed to air-plasma under atmospheric pressure for 5 and 7 min. The water contact angle and CIELAB color parameters, including lightness (L*), red–green (a*), and yellow–blue (b*) coordinates, were measured. The results showed that plasma treatment decreased L*, particularly in thermally modified wood, while inducing a slight decrease in untreated wood. The b* value gradually increased in untreated samples, whereas thermally modified specimens exhibited pronounced increases in both a* and b*. Analysis of variance revealed that wood type, plasma exposure time, and their interaction had statistically significant effects on all three-color parameters. Plasma treatment markedly reduced the water contact angle in both solid pine and thermo-wood samples, demonstrating enhanced surface wettability. The strongest effect was observed in thermo wood exposed to plasma for 7 min, which exhibited the lowest contact angle and the highest level of hydrophilicity among all treatments. FTIR spectroscopy indicated a gradual reduction of hydroxyl groups and ester carbonyls in hemicellulose and an accumulation of polymerized lignin caused by thermal modification, alongside partial degradation of polar groups, surface activation, and the formation of new carbonyl functionalities.