Comparison of Stabilization Systems for Soybean Wax Emulsions to Produce Sustainable Water-Resistant Paper Based Packaging: Surfactant vs. Pickering

Soybean wax is a sustainable alternative to synthetic polymeric coatings in packaging due to its renewable, environmentally benign, and hydrophobic properties. In order to be effectively applied, however, soybean wax must be emulsified in water. The present work compares two stabilization approaches for soybean wax emulsions: a conventional surfactant-based emulsion (SE) using a mixture of nonionic surfactants (Span-80 and Tween-80), and a Pickering emulsion (PE) using cellulose nanocrystals combined with sodium alginate (CNC-SA) as an anionic stabilizer. The SE produced stable emulsions at 6 wt% Span-80/Tween-80 (at a HLBmix value of 10) with a mean droplet size of 449 nm but limited storage stability (approximately 7 days under ambient conditions), while the PE achieved superior stability (approximately 1 month) at 1 wt% CNC-SA with a mean droplet size of 740 nm. The stabilized SE and PE were subsequently applied as coatings on three different types of paper substrates: northern bleached kraft (NBK) paper, copy paper, and cellulose nanofiber (CNF)-coated NBK paper. When applied to northern bleached kraft (NBK) paper, the SE coatings provided minimal improvements in barrier performance. The Cobb 60 value decreased slightly from 125 g/m2 (control-no coating) to 86 g/m2, indicating a negligible water barrier with immediate water absorption upon contact. In contrast, the Cobb 60 value of the PE-coated NBK paper decreased markedly from 125 g/m2 to 39 g/m2, confirming that the PE coating substantially enhances water resistance. The SE coating displayed a significant loss of water contact angle (WCA) from 85° to 0° within 20 s, showing limited water holdout capacity, whereas PE-coated NBK paper demonstrated strong water holdout, with the WCA decreasing only from 94° to 85° over 5 min. The SE coating achieved only a 14% reduction in water vapor transmission rate (WVTR), while the PE coating provided a greater reduction of 30%. In terms of oil resistance, both emulsion systems significantly enhanced the kit rating of the papers tested, e.g., from kit number 0 to 6–9 (paper dependent). The SE coating, however, experienced a substantial reduction in barrier integrity after folding, while the PE coating largely retained its oil barrier properties. Furthermore, the SE coating reduced the tensile strength of NBK paper by 41%, whereas the PE coating reduced it by only 7%. Overall, the comparative findings indicate that although the SE generated a smaller mean particle size, it offered minimal improvement in the water and oil barrier performance of paper and had a limited storage life. In contrast, the PE generated a larger mean particle size, but provided substantially greater water and oil resistance, and enhanced mechanical strength retention. In addition, the PE displayed an effective storage life of at least one month. The Pickering emulsion, formulated with all biologically derived components, therefore represents a viable, sustainable, bio-based alternative to synthetic polymeric coatings for packaging applications.