Ageing behaviour and analytical pyrolysis characterisation of diterpenic resins used as art materials: Manila copal and sandarac

摘要

Artificial indoor and outdoor exposure conditions have been applied in order to investigate the photo-ageing behaviour of two natural resins once used as art materials. Copals and sandarac consist of free labdane diterpenoids and of a highly cross-linked fraction of polycommunic acid. To determine the nature and the composition of the cross-linked fraction, pyrolysis is required. Thermally assisted hydrolysis and methylation, coupled with gas chromatography-mass spectrometry (THM-GC/MS), has been used to identify the acidic compounds. Many secondary pyrolysis products have been recognised and distinguished from the original resin components. Further details on the composition and ageing behaviour of Manila copal and sandarac have been obtained from Fourier transform infrared spectroscopy (FTIR), size exclusion chromatography (SEC) and direct temperature-resolved mass spectrometry (DTMS). During ageing, cross-linking and cleavage reactions were found to affect largely the chemical structure of the two resins, together with minor degradation processes such as isomerisation, defunctionalisation and oxidation.