Characterisation of poly(acrylamide‐co‐acrylic acid) mixed with anthocyanin pigment from hibiscus sabdariffa l.

Abstract

PurposeThe purpose of this paper is to develop a new water based coating system composed of poly(acrylamide‐co‐acrylic acid) and anthocyanin colourant extracted from Hibiscus sabdariffa L. and characterise the system.Design/methodology/approachAnthocyanin from Hibiscus sabdariffa L. (roselle) calyxes was extracted using optimised water extraction method with ratio of calyxes to water being 1:2. UV‐Vis absorption spectroscopy was conducted on the anthocyanin extract to monitor its degradation. Poly(acrylamide‐co‐acrylic acid) was mixed with the extracted anthocyanin solution in two different weight ratios. The mixtures were coated on glass substrate and let to cure for approximately one day. FTIR spectroscopy was conducted on the samples to determine their functional groups and identify the compounds in the samples. Cross hatch test was performed on the samples to determine the adhesion properties. Thermal degradation of the samples was determined through thermogravimetric analysis. Surface roughness of the samples was studied by atomic force microscopy. Colour stability was determined before and after UV irradiation.FindingsAnthocyanin pigment from roselle extracted with water was found to be stable with 6.0 per cent drop in absorbance value over the 15 day period. The reaction rate was found to be 0.000181 h−1, and the half life was calculated to be 3,850 hours. Better adhesion of the samples to the glass substrate was due to the higher concentration of poly(acrylamide‐co‐acrylic acid). Thermogravimetric analysis revealed similar thermal stability of the samples. Surface roughness study revealed that sample with higher anthocyanin content has higher surface roughness. Colour stability of the 20PBR was found to be better than 10PBR.Research limitations/implicationsUsage of poly(acrylamide‐co‐acrylic acid) as coating is limited by its weak water resistance property. Improvement could be made in this direction for future applications.Practical implicationsMixture of water‐based polymer and anthocyanin colourant from roselle has been developed into coating for the first time. Commercialisation is possible if more research is conducted towards water resistance property of poly(acrylamide‐co‐acrylic acid), storage condition for the pigment and coating, and mass production of the pigment.Originality/valueIncorporation of anthocyanin pigment into water based coating is the first of its kind.