Approach of new coating formulations based on cationic starch for kraftliner paper

Abstract

Traditional packaging materials are based mainly on polyethylene and polypropylene, which are not biodegradable and compromise their disposal if left in the environment. As an alternative, paper is already widely used as a packaging material and a low-cost option with adequate manufacturing know-how. However, it has limitations in terms of permeability and hydrophilicity. The coating technique on the paper surface is an excellent option to provide better barrier properties and reduce its hygroscopicity. Additionally, making coatings with natural or biodegradable materials, such as starches, is preferable to reduce environmental pressure. Using natural components mixed with starch in its cationic form with high viscosity at low solid content for coating kraftliner paper for packaging purposes is underexplored in literature. Therefore, this work aimed to evaluate cationic starch-based coatings on kraftliner paper to create high-quality multilayer papers with biodegradation and recycling potential with barrier properties. In the first scientific article, cationic starch suspensions were mixed with carnauba wax in proportions of 1:0, 3:1, 1:1, 1:3, and 0:1 (m:m). The second scientific article studied the addition of bentonite in 5%, 7%, and 10% (m/m) to cationic starch suspensions. Then, the third scientific article evaluated the cationic starch suspensions mixed with poly(vinyl alcohol)(PVOH) in proportions of 1:0, 3:1, 1:1, 1:3, and 0:1 (m:m). For each scientific article, the mixtures coated the kraftliner paper in two layers of 15.0±0.5 g/m² and compared to uncoated kraftliner paper (86±1 g/m²) double wet-and-dry kraftliner paper. The suspensions were evaluated for solids content, pH, and viscosity. Also, we analyzed the multilayer papers for chemical, physical, morphological, and mechanical characteristics, in addition to barriers to water, water vapor, oil, and oxygen. All suspensions showed pH >4.0 with good interaction without phase separation. The scanning electron microscopy images showed a thin film for each formulation on the paper, except for the treatment formed by 100% carnauba wax; adding cationic starch to the wax corrected this non-film formation. With coating application, papers showed a decrease of up to 36% in tensile mechanical strength caused by water in the suspensions, which reduced the interaction between paper fibers. Kraftliner paper coated with cationic starch showed high hydrophilicity with a Cobb test of 189±2 g/m² but decreased by 28% and 66% when adding carnauba wax and bentonite, respectively. Furthermore, adding PVOH to cationic starch decreased the water vapor permeability of the papers by 57% and increased the oil barrier from 10 to 12 in the kit oil. Compared to pure cationic starch, the oil barrier also increased when adding 7% bentonite (m/m) to cationic starch. Finally, an oxygen barrier of up to 10 cc/(mm².day) was achieved by adding 75% (m/m) of cationic starch to the PVOH polymer. Incorporating carnauba wax, bentonite, and PVOH into the cationic starch improved the barrier properties to water vapor and oxygen, serving as low-moisture packaging for foods, such as bakery elements. These coatings meet market demands for primary and secondary packages, focusing on an excellent human-environment relationship.