Structural study of oriental lacquer films during the hardening process

Area of Science:

• Materials Science
• Polymer Chemistry
• Surface Chemistry

Background:

• Oriental lacquer, a natural resin from lac trees, forms a hard, insoluble film essential for industrial coatings.
• Traditional processing methods like Nayashi and Kurome are known to influence lacquer properties.

Purpose of the Study:

• To investigate the functional expression process during the hardening of two oriental lacquer types: Kiurushi (raw urushi) and Kuromeurushi.
• To elucidate the chemical differences and polymerization mechanisms contributing to lacquer film properties.

Main Methods:

• Fourier transform infrared spectroscopy (FT-IR) for functional group analysis.
• Thermogravimetry/differential thermal analysis-mass spectrometry (TG/DTA-MS) for thermal degradation studies.
• Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) to differentiate lacquer compositions and polymerization.

Main Results:

• FT-IR and TG/DTA-MS showed no significant differences between Kiurushi and Kuromeurushi films.
• Py-GC/MS revealed distinct ratios of alkenylcatechols to pentadecylcatechol, with Kiurushi having higher ratios.
• These ratios decreased during hardening due to polymerization, indicating faster cross-linkage formation in Kuromeurushi.
• A strong correlation was observed between Py-GC/MS peak area ratios and pencil hardness testing results.

Conclusions:

• Oriental lacquer achieves extreme hardness and insolubility through accelerated nucleus-side chain and side chain-side chain cross-linkages.
• Traditional treatments (Nayashi and Kurome) effectively enhance hardening rates by activating these cross-linkages.
• Py-GC/MS is a key technique for differentiating lacquer types and understanding their hardening mechanisms.