Urushi is the oldest and most precious lacquer used since antiquity in East Asia.
For artistic purposes, in order to obtain suitable rheological properties, the lacquer is usually
mixed with a vegetable oil. In this work we investigated the curing process of urushi/tung oil
mixtures in order to highlight the chemical interactions at the molecular level between the two
materials.
A multi-analytical approach was adopted, based on thermogravimetry (TG), differential scanning
calorimetry (DSC), gas chromatography-mass spectrometry (GC-MS), evolved gas analysis
mass spectrometry (EGA-MS), analytical pyrolysis coupled with gas chromatography and mass
spectrometry (Py-GC-MS) and high performance liquid chromatography-mass spectrometry
(HPLC-MS). Fresh and aged mixtures were analysed and the results were compared with those
obtained from the analysis of the individual materials.
The data highlighted that different polymerization and oxidation mechanisms take place in
oil/urushi mixtures compared to the pure materials. Py-GC-MS and GC-MS showed that the
profile of aliphatic mono- and di-carboxylic acids was drastically different for the aged film of
pure tung oil compared to the mixtures. The ratio between the relative content of azelaic and
palmitic acids was much lower in the mixtures than in the pure oil, highlighting a lower level of
oxidation. On the other hand, the relative content of short chain carboxylic acids, which are
produced by pyrolysis of the cross-linked oil network, increased as the concentration of urushi in
the mixtures increased, thus indicating an increasing level of reticulation. HPLC-MS showed a
relatively higher amount of triglycerides with hydroxylated fatty acids – the intermediate
oxidation product of polyunsaturated fatty acids - in the mixtures with respect to pure tung oil
