THE VISCOELASTIC BEHAVIOR OF JAPANESE LAC QUER FILM I - MECHANICAL RELAXATION PROCESSES OF LACQUER FILMS

The storage modulus (E') and loss tangent (tan delta) of three kinds o f Japanese lacquer films were measured in the temperature range of -15 0 degrees C to 400 degrees C, and the mechanical relaxation processes detected were assigned in relation to the structure of urushiol, the p rincipal constituent of the lacquer. The El of the films decreased rem arkably in the temperature range of -50 degrees C to 100 degrees C, an d then turned to increase above 150 degrees C, and remained almost unc hanged above 200 degrees C. With respect to tan delta, three relaxatio n precesses labeled alpha, beta, and gamma were observed at 11 Hz and 30 degrees C to 100 degrees C, -60 degrees C, and -140 degrees C, and the apparent activation energies of these processes were 65 to 75 kcal /mol, 13 kcal/mol and 8 to 9 kcal/mol, respectively. It was suggested that the alpha process with the remarkable drop in E' was due to the m icro-Brownian motion of polymerized urushiol molecules, and the increa se of E' in a higher temperature range was caused by the oxidative pol ymerization of urushiol. The beta and gamma processes were attributed to the molecular motion related to the absorbed water and that of meth ylene groups in the side chains of urushiol molecules, respectively. T he polymerization of urushiol progressed during storing at a room temp erature for 2 months or at 200 degrees C for 2 hours,which caused a sh ift of the alpha peak to a higher temperature range.