Behaviour of lignin during thermal treatments of wood

Different thermal treatments or processes (combustion, welding, thermowood) of several wood species were examined and compared. Each procedure resulted in different temperature loads on wood. The changes of lignin after thermal treatments were determined by different suitable methods, both wet chemical and instrumental analyses. The characterisation of the wood meals by means of FTIR spectroscopy has shown that changes of the aromatic skeletal vibrations occur, which indicate condensation reactions of lignin. The variations within the FTIR spectra of hardwood were more evident than those of softwood. The decomposition of lignin has also been proven in the acetone extracts of "thermowood", which contained a lignan, identified by GC/MS. Moreover, the thioacidolysis showed the decrease of monomeric degradation products in all thermal treated samples. This is due to the cleavage of structural units bound by arylgycerol-β-ether linkages. However, it can be assumed that in addition to the degradation of specified types of bonds also condensation reactions have occurred and these products were not detectable by means of GC/MS. This conclusion would also support the above-mentioned results of FTIR spectroscopy.