ARTICLE IN PRESS
JID: JOBAB
[m3GeSsc;February 6, 2026;11:5]
Z. Wei, J. Liu, Y. Wang et al.
Journal of Bioresources and Bioproducts xxx (xxxx) xxx
and aggregation among dispersed SA particles, thereby lowering the risk of phase separation ( Alavi and Chen, 2022 ; Kuang et al., 2023 ). This mechanism likely contributes to the excellent long-term stability observed in the PLS emulsions. Temperature-dependent shear viscosity measurements revealed that the viscosity of the PLS emulsions decreased as the temperature increased (Fig. S6). This behavior can be attributed to the enhanced thermal energy, which weakens intermolecular interactions within the emulsion and increases the mobility of polymer chains and dispersed particles, thereby reducing internal frictional resistance. This behavior not only enhances the processing fluidity of the PLS emulsion coating at elevated temperatures but also promotes more uniform film formation during application. In addition, to enhance the scalability of the PLS emulsion, a PLS 2 emulsion with a solid content of 30.65% ( w ) was prepared (Fig. S7). The higher solid content contributes to improved coating efficiency and drying efficiency.
3.3. Structure and mechanical properties of PLS emulsion coated paper
The two-side coating process was employed to uniformly apply the PLS emulsion to both sides of the paper, followed by drying to produce coated paper. After the coating treatment, the basis weight of the paper increased from the original (66.44 ± 0.29) g/m 2 to between 86 and 92 g/m 2 (Fig. S8). The surface morphology of the coated paper was characterized by SEM, revealing significant differences in microstructure between the uncoated and coated paper. The surface of the uncoated paper exhibited a disordered interweaving of fibers, forming a rough and porous structure. This porous structure facilitates the permeation of liquids (such as water and oil) and gases, severely compromising its barrier performance. Additionally, this loose and porous structure results in poor mechanical strength. After PLS emulsion coating treatment, the fibers and pores on the paper surface were completely covered by a continuous and dense coating. Notably, differences were observed in the surface morphology of coated paper treated with different components of PLS emulsion. The surface of P-PLS 0 was smooth and uniform, while with the incorporation of SA, dispersed SA particles were observed in the coating. These SA particles were captured and immobilized by PVA in the composite coating, leading to increased surface roughness. Furthermore, as the amount of SA increased, the surface roughness of the coated paper gradually increased ( Figs. 4a–4d and S9). Furthermore, the surface chemistry of the coated paper was analyzed by XPS. The high-resolution C 1s spectra revealed an enhancement in the C–C peak for P-PLS 2 , which is likely attributable to the presence of SA. This observation suggests that some SA particles may be exposed on the surface of the paper coated with the PLS emulsion (Fig. S10). In contrast to PLS emulsion-coated paper, a large number of SA particles were observed adhering to the surface of P-PLS 2 − M. However, these particles were not embedded within the coating matrix but were merely attached unevenly to the surface (Fig. S11A). This loose configuration made the SA particles susceptible to easy removal by hand (Fig. S11B), which proved detrimental to practical applications. Mechanical properties are critical evaluation indicators for assessing the practical applications of paper packaging materials, as they directly determine the reliability of the materials under actual usage conditions. After treatment with the PLS emulsion coating, the coated paper exhibited a significant enhancement in tensile strength. Notably, the P-PLS 0 demonstrated the highest dry tensile strength ((7.26 ± 0.61) kN/m), primarily attributed to the high strength of PVA, the hydrogen bond network formed between the coating and the paper fibers, and the effective repair of surface defects on the paper by the coating ( Xie et al., 2021 ; Zhao et al., 2023 ; Wei et al., 2025 ). However, the introduction of SA resulted in a slight reduction in tensile strength compared to the P-PLS 0 . This phenomenon may be ascribed to the phase-separated microstructure formed by SA particles within the coating, which disrupts the continuity of the PVA matrix, thereby causing a stress concentration effect ( Fig. 4 e). Furthermore, wet tensile strength is another crucial parameter for paper applications, particularly in high-humidity or liquid-exposure environments. Sufficient wet strength ensures structural integrity and durability under moist conditions. After immersing the paper in water for 20 min, the wet tensile strength was measured. Due to the high hydrophilicity of untreated paper, water molecules readily penetrate the fiber network, disrupting intermolecular hydrogen bonds and leading to a drastic decline in strength, resulting in a wet tensile strength of only (0.14 ± 0.02) kN/m for uncoated paper. In contrast, the P-PLS 0 exhibited improved wet strength and water resistance ((0.53 ± 0.05) kN/m), owing to the inherent robustness and dense structure of the coating. More importantly, the incorporation of SA further enhanced the paper’s ability to inhibit water penetration and diffusion, thereby mitigating the detrimental effects of moisture on both the fibers and the coating structure ( Wennman et al., 2023 ). Consequently, the wet strength increased to a maximum of (0.97 ± 0.07) kN/m (P-PLS 2 ), which was 6.93 × that of the uncoated paper and 1.83 × that of P-PLS 0 ( Fig. 4 f). The exceptional wet tensile strength demonstrates that the PLS emulsion coating can effectively maintain the mechanical strength and structural integrity of paper under high-humidity conditions, suggesting its promising potential for applications in wet environment. These tensile strengths outperform those of commercial LDPE coated paper, demonstrating that the PLS emulsion coated paper can meet daily application requirements (Table S2). To visually highlight the superior wet strength of PLS emulsion coated paper, uncoated paper and P-PLS 2 were cut into 2 cm-wide strips and immersed in water for 20 min before undergoing a weight-bearing test using a 4 kg kettlebell. Due to the superior wet strength of P-PLS 2 , it was able to easily withstand the weight of the kettlebell ( Fig. 4 g and Video S1). In contrast, the uncoated paper fractured immediately under the same load (Video S2).
3.4. Water- and oil-proof properties of PLS emulsion coated paper
Uncoated paper exhibits poor liquid barrier properties due to its inherently porous fibrous structure, which significantly limits its application in the packaging industry. The wettability of both uncoated and coated paper was evaluated by measuring the water contact angle (WCA). Uncoated paper showed a lower WCA of (63.2 ± 2.6)° due to the high hydrophilicity of the fibers and the extensive pore network ( Fig. 5 a). More importantly, water droplets were rapidly absorbed upon contact with the paper surface through capillary action, causing the WCA to drop to 0° within 8 s (Fig. S12), indicating poor water-proof performance ( Lu et al., 2025 ). In contrast, the WCA of P-PLS 0 increased to (87.0 ± 1.6)°, attributed to the dense structure of the coating. Notably, with the
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