Inorganic salts as additives for natural oriental lacquer

Oriental lacquer, a milk-white sap collected from lacquer trees, can be cured automatically to bright and dense films in the presence of air, thus is worthy of being regarded as a type of natural green coating, as it does not emit any volatile organic compounds. It has been used in Asia for more than 8000 years attributing to its excellent characteristics. However, during harvesting and storage process, the lacquer sap surface can automatically polymerise into lacquer blocks under the catalysis of laccase present in the sap, wasting a significant amount of resources. It is an arduous task how to slow down the curing of oriental lacquer in storage without affecting the normal curing process in operation.

Increased dryness time

Now the curing of oriental lacquer was adjusted using Na₂CO₃ and NaCl as additives. The results indicated that the addition of Na₂CO₃ quenched the semiquinone radical during the polymerisation of urushiol, the main component of oriental lacquer, without affecting the laccase activity. Under the appropriate condition (25 °C and 80% relative humidity), the touch-free dryness time of the oriental lacquer mixed with Na₂CO₃3 increased from 3 h to 15 days. Moreover, the addition of same amount of Na₂CO₃ and NaCl to oriental lacquer improved the curing time to 6 h, which was similar to that of pristine lacquer without the addition of any salts. The polymerisation rate of the lacquer sap returned to the normal again, that may be due to the blocking of the reaction between urushiol and CO₃^2‐ after the introduction of NaCl.

The study has been published in Progress in Organic Coatings, Volume 161, December 2021.