Treatment has been conducted on samples in a macroscopic scale. A strong interaction between wood and the polymerized oligomers can be attested.
The composite material obtained by mean of the short treatment without catalyst consists in impregnated wood, partially dry. No grafting or strong polymerization seems to have occurred. The long treatment induces a complete drying.
Obtained by mean of catalyzed short treatment, the material obtained is strongly softened, partially dry and highly darkened. Completely dry after the long treatment, the composite appears hard and brittle.
A strong swelling is noticed after heating step: up to 30%. As oligomers impregnation induces only 2% swelling, this indicates that oligomers inside wood lumens penetrate the cell walls during heating step. Figure 5 shows a transverse section of long treated pine where lumens appear empty.
Chemical catalysis causes another physical reaction: a strong darkening of wood during heating step. According to Formula (2), figure 6 shows the AE* of pine sapwood and beech samples due to heating.
Fig. 5. SEM micrographs of treated pine sapwood sample
Colour change of samples treated without catalyst is in the range of classical colour changes due to heat treatment on wood. With catalyst, darkening is strong enough to modified wood appearance.
PLA darkening while processing is often reported in the literature. PLA purity, related to the L-isomers content is considered as one possible reason of coloration. But interactions between oligomers and lignocellulosic compounds could also lead to such a darkening. Every constituent of the composite has been subjected to short and long heating: oligomers, wood, cellulose, extracted wood, extractives, and lignin sulfonate powder. Darkening is observed on wood subjected to short and long catalysed treatment, on low purity oligomers subjected to sulphuric acid catalysed long treatment, on cellulose subjected to catalysed long treatment, on extractives subjected to sulphuric acid catalysed treatment. Lignin sulfonate powder, already brown, is strongly darkened in every case.
Wood softening is translated by the wood structure disintegration into single fibres. Middle lamella, mostly composed of lignin, seems to be destroyed by the chemical treatment and do not fix cells anymore. Figure 7 attests this observation.
Fig. 7. Treated pine sapwood whose tracheids are easily manually disjoined
Softening and darkening are strongly related and both phenomenons imply a lignin modification.