Comparison study on the effect of acid and nitrate salt on the properties of poly(vinyl alcohol) coating doped with dye consist of anthocyanin

Abstract

Purpose – The purpose of this paper is to observe the thermal and colour stabilities of coating films consisting of natural dye anthocyanin colourant with acid and nitrate salt as additives. Design/methodology/approach – The natural dye was extracted from Hibiscus sabdariffa L. (Roselle) by using distilled water as solvent. The extracted dye was mixed with 1 weight per cent hydrochloric acid and 5 weight per cent calcium nitrate to conduct the comparison study. The dye samples were than mixed with poly (vinyl alcohol) (PVA). The mixtures were applied on glass panels to form coating films. The coating films were left to dry for 24 hours. The thermal stability of coating films was investigated by using thermogravimetric analysis (TGA). The amorphousness coating films molecular structures were observed by using x-ray diffraction (XRD). CIE L*a*b* colour coordinate space technique was used to investigate the coating films colour stability against ultraviolet (UV). Findings – The results show that addition of hydrochloric acid (HCl acid) enhanced thermal stability of PVA-anthocyanin coating and shifted the degradation temperature to higher temperature at 300°C. HCl acid also improved UV stability of the coating system significantly with the reduced ΔE from 26.67 to 16.89. Addition of calcium nitrate (Ca(NO3)2) salt promotes good interaction with PVA-anthocyanin system that caused structural changes, improved thermal stability and increased Tg from 61.5 to 83.1°C. Research limitations/implications – The potential of using natural dye extracted from plant as renewable material. Practical implications – The coating films developed in this works are suitable for glass substrate application. Originality/value – Development of water-based coating from PVA binder with anthocyanin colourant is introduced in this study.