Effects of coating weight and nanoclay content on functional and physical properties of bionanocomposite-coated paper

Sodium caseinate (NaCas)-based formulations reinforced with halloysite nanotubes (HNTs) were applied as coating materials on paper sheets. NaCas/HNT nanocomposite-based films were preliminary characterized by FTIR spectroscopy, DSC, and XRD analyses. The effects of coating weight and HNT content on mechanical, optical, and barrier properties of coated papers were investigated using a two-factor, five-level central composite design. Increasing coating weight and HNT content reduced water vapor permeability (WVP) of coated papers within the range of 47–79% and improved their grease resistance. Depending on coating weight and HNT content, NaCas/HNT nanocomposite-based coatings were able to increase the tensile strength (TS) and elongation at break (%E) of coated papers, respectively, in the ranges of 19–50% and 41–58%. Specular gloss of the treated paper sheets was positively influenced by coating weight but not affected by HNT content. Such improvement in the functional and physical properties of the developed coated papers can be attributed not only to the increase of their surface smoothness and homogeneity, but also to the strong interfacial interactions occurring between the incorporated nanotubes and the NaCas matrix, along with their good dispersion in the reinforced material, as confirmed by FTIR and SEM analysis. Based on the multi-response optimization procedure, a coating weight level of 15.99 g/m² and an HNT content of 1.45 wt% were found to be the optimum parameters for maximum TS, %E, and grease resistance and minimum WVP with an overall desirability of 0.98.