Papermaking! Vol12 Nr1 2026

PAPER making! The e-magazine for the Fibrous Forest Products Sector

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Volume 12 / Number 1 / 2026

PAPER making! FROM THE PUBLISHERS OF PAPER TECHNOLOGY INTERNATIONAL ® FROM THE PUBLISHERS OF PAPER TEC Volume 12, Number 1, 2026 CONTENTS: FEATURE ARTICLES: 1. Coatings : Lignin nanoparticles for barrier coatings.

2. Recycling : Removal of fillers and chemical reagents from waste paper. 3. Hygiene : Impact of different hand drying methods on viral aerosol formation. 4. Nano-Additives : Review of current nano-additives in Pulp & Paper. 5. Enzymes : Cellulase-xylanase formulations for enhanced dewatering & bonding. 6. Drives : Leveraging VFD data for NDT and predictive maintenance. 7. Refining : Comparing ultra-fine bar refining and Valley Beating on Kraft Pulps. 8. Sustainability : Plant-level carbon accounting on China’s Pulp & Paper Industry. 9. Testing : Discriminating paper origin using Multivariate Analysis. 10. Social Media : Using X for business. 11. Artificial Intelligence : Writing effective prompts for Microsoft Copilot. 12. Attention to Detail : How to Improve your Attention to Detail.

SUPPLIERS NEWS SECTION: News / Products / Services : x

Section 1 – PITA CORPORATE MEMBERS ABB / BIM KEMI / PILZ / VALMET

Section 2 – NON-PITA MEMBERS IBS / MARE Advertisers: ABB / BLACKBURN CHEMICALS / SPRAYING SYSTEMS / VALMET / ZURICH INSURANCE

DATA COMPILATION: Events : PITA Courses & International Conferences / Exhibitions Installations : Overview of equipment orders and installations between Nov. ’23 & Apr. ‘24. Research Articles : Recent peer-reviewed articles from the technical paper press. Technical Abstracts : Recent peer-reviewed articles from the general scientific press.

The Paper Industry Technical Association (PITA) is an independent organisation which operates for the general benefit of its members – both individual and corporate – dedicated to promoting and improving the technical and scientific knowledge of those working in the UK pulp and paper industry. Formed in 1960, it serves the Industry, both manufacturers and suppliers, by providing a forum for members to meet and network; it organises visits, conferences and training seminars that cover all aspects of papermaking science. It also publishes the prestigious journal Paper Technology International ® and the PITA Annual Review , both sent free to members, and a range of other technical publications which include conference proceedings and the acclaimed Essential Guide to Aqueous Coating .

Contents

Page 1 of 1

PAPER making! FROM THE PUBLISHERS OF PAPER TECHNOLOGY INTERNATIONAL ® FROM THE PUBLISHERS OF PAPER TEC Volume 12, Number 1, 2026

Lignin nanoparticle stabilized Pickering emulsion coating for fabricating water- and oil-proof, biodegradable, and recyclable paper ZHENKE WEI, JIAYU LIU, YONGGUI WANG, ZEFANG XIAO & YANJUN XIE Paper derived from plant fibers is considered a highly promising alternative to non-degradable plastic packaging. However, its widespread use in packaging is limited by its poor water- and oil- proof properties. To address this challenge, this study developed a polyvinyl alcohol (PVA)/lignin nanoparticles (LNPs)/stearic acid (SA) (PLS) Pickering emulsion composite coating to fabricate high-performance biodegradable paper. In the emulsion system, the PVA acted as an oil-repellent in the aqueous phase, the SA served as a water-repellent in the oil phase, and the LNPs functioned as emulsifiers that effectively stabilized the emulsion. Owing to the synergistic effect among PVA, SA, and LNPs, the coated paper exhibited good water- and oil-proof properties (with a water con-tact angle of 111.2°, Cobb 60 value of 17.73g/m 2 , and Kit rating exceeding 9/12), along with high mechanical strength (including a dry tensile strength of 7.12kN/m and a wet tensile strength of 0.97kN/m). While significantly enhancing performance, the PLS emulsion coating retained the environmental benefits of paper, could be easily removed to facilitate fiber recycling, and the coated paper was fully degradable in soil within 120d. Overall, the PLS emulsion coating effectively enhanced the properties of paper without compromising its eco-friendly characteristics, demonstrating significant potential for promoting the substitution of plastic with paper. Contact information: Key Laboratory of Bio-based Material Science and Technology (Ministry of Education), College of Materials Science and Engineering, Northeast Forestry University, Harbin 150040, China. Journal of Bioresources and Bioproducts, Available online 27 January 2026, 100235, corrected proof

https://doi.org/10.1016/j.jobab.2026.100235 Creative Commons Attribution 4.0 License

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Article 1 – Barrier Coatings

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Contents lists available at ScienceDirect

Journal of Bioresources and Bioproducts

journal homepage: https://www.keaipublishing.com/en/journals/journal-of- bioresources-and-bioproducts/

Research Article Lignin nanoparticle stabilized Pickering emulsion coating for fabricating water- and oil-proof, biodegradable, and recyclable paper Zhenke Wei, Jiayu Liu, Yonggui Wang ∗ , Zefang Xiao, Yanjun Xie ∗

Key Laboratory of Bio-based Material Science and Technology (Ministry of Education), College of Materials Science and Engineering, Northeast Forestry University, Harbin 150040, China

a r t i c l e

i n f o

a b s t r a c t

Paper derived from plant fibers is considered a highly promising alternative to non-degradable plastic packaging. However, its widespread use in packaging is limited by its poor water- and oil- proof properties. To address this challenge, this study developed a polyvinyl alcohol (PVA)/lignin nanoparticles (LNPs)/stearic acid (SA) (PLS) Pickering emulsion composite coating to fabricate high-performance biodegradable paper. In the emulsion system, the PVA acted as an oil-repellent in the aqueous phase, the SA served as a water-repellent in the oil phase, and the LNPs functioned as emulsifiers that effectively stabilized the emulsion. Owing to the synergistic effect among PVA, SA, and LNPs, the coated paper exhibited good water- and oil-proof properties (with a water con- tact angle of 111.2°, Cobb 60 value of 17.73 g/m 2 , and Kit rating exceeding 9/12), along with high mechanical strength (including a dry tensile strength of 7.12 kN/m and a wet tensile strength of 0.97 kN/m). While significantly enhancing performance, the PLS emulsion coating retained the environmental benefits of paper, could be easily removed to facilitate fiber recycling, and the coated paper was fully degradable in soil within 120 d. Overall, the PLS emulsion coating ef- fectively enhanced the properties of paper without compromising its eco-friendly characteristics, demonstrating significant potential for promoting the substitution of plastic with paper.

Keywords: Lignin nanoparticle Pickering emulsion Paper coating Water- and oil- proof Biodegradable

1. Introduction

Traditional petroleum-based plastic packaging, such as low-density polyethylene (LDPE) and polypropylene (PP), faces challenges due to their non-renewable characteristics, poor degradability, and pollution issues arising during the recycling processes, making it difficult to meet the demands of global green and low-carbon development ( Xia et al., 2021 ; Zhang et al., 2022 ; Peydayesh et al., 2025 ). With the widespread promotion of sustainable development concepts, the design and development of modern packaging materials must not only meet increasingly diverse functional requirements but also adhere to environmentally friendly principles, thereby minimizing negative impacts on the ecological environment. Paper made from plant fibers offers advantages such as wide availability, renewability, biodegradability, and ease of recycling, and is now widely regarded as a highly promising alternative to traditional plastic packaging ( Zhai et al., 2024 ; Chen et al., 2025 ; Yang et al., 2025 ). Despite the significant environmental benefits and immense market potential of paper-based packaging materials, they still possess inherent limitations that severely restrict their effective replacement of plastic packaging. Notably, the poor water- and oil-proof properties of paper significantly affect its packaging

Peer review under the responsibility of Editorial Office of Journal of Bioresources and Bioproducts. ∗ Corresponding authors. E-mail addresses: wangyg@nefu.edu.cn (Y. Wang), yxie@nefu.edu.cn (Y. Xie) .

https://doi.org/10.1016/j.jobab.2026.100235 Received 21 October 2025; Received in revised form 17 December 2025; Accepted 24 December 2025 Available online xxx 2369-9698/© 2026 The Authors. Publishing Services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ) Please cite this article as: Z. Wei, J. Liu, Y. Wang et al., Lignin nanoparticle stabilized Pickering emulsion coating for fabricating water- and oil-proof, biodegradable, and recyclable paper, Journal of Bioresources and Bioproducts, https://doi.org/10.1016/j. jobab.2026.100235

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functionality and durability ( Jing et al., 2021 ). Therefore, enhancing the water- and oil-proof properties of paper is a crucial step toward achieving the goal of substitution of plastic with paper. Coating technology is an important means to improve the water- and oil-proof properties and functionalization of paper ( Basak et al., 2024 ). Per- and polyfluoroalkyl substances (PFAS) coatings can significantly reduce the surface energy of paper, thereby producing paper-based materials with excellent water- and oil-proof properties. However, due to the bioaccumulation and toxicity of PFAS, they pose potential threats to the ecological environment and human health and have been gradually phased out ( Zabaleta et al., 2020 ; Neng et al., 2025 ). Currently, thermoplastic materials such as LDPE, which are non-renewable and difficult to biodegrade, are commonly used as coatings to impart water- and oil-proof properties to paper. These non-degradable coatings have significant draw- backs. First, they are difficult to separate effectively from the paper substrate, which severely limits the recycling and reuse of paper. Second, when waste paper-based materials coated with these substances are disposed of in landfills, the coatings do not naturally degrade, inevitably causing environmental pollution and ecological burdens ( Kansal et al., 2020 ). Therefore, developing environ- mentally friendly, non-toxic, easily separable, and scalable water- and oil-proof coatings has become a critical challenge. In recent years, paper coatings based on biodegradable materials such as polysaccharides, proteins, and polyvinyl alcohol (PVA) have garnered widespread attention due to their environmental friendliness and sustainability. These materials have been extensively applied to enhance the mechanical strength of paper, impart oil-proof properties, or introduce additional functionalities such as antibacterial properties ( Chi et al., 2020 ; Huang et al., 2023 ; Cheng et al., 2024 ; Li et al., 2024 ; de Amorim dos Santos et al., 2025 ). However, their high hydrophilicity often results in poor water-proof performance after coating, which limits their application in high-humidity environments or in fields requiring high barrier properties. To address the shortcomings in water- and oil-proof properties of pa- per treated with biodegradable coatings, current strategies mainly include hydrophobic modification of hydrophilic biodegradable polymers ( Hamdani et al., 2020 ; Tan et al., 2023 ; Fang et al., 2025 ), multilayer composite coating ( Kansal et al., 2020 ; Zhu et al., 2023 ), and emulsion coating ( Zhang et al., 2023 ; Peng et al., 2024 ). Emulsion coating technology presents a promising solution with significant advantages. This strategy cleverly combines oil-proof and water-proof components through emulsification, creating a stable aqueous dispersion system that simultaneously enhances both the water- and oil-proof properties of paper in a single coating process. This method not only avoids complex chemical modifications but also simplifies the coating procedure, aligning with the principles of green manufacturing. However, conventional emulsion coatings typically depend on synthetic surfactants to disperse and stabilize the oil phase. Although these systems avoid the use of organic solvents, the surfactants may introduce potential ecotoxicity, and the resulting emulsions often exhibit insufficient thermodynamic stability ( Cui et al., 2021 ). For example, cetyltrimethylammo- nium bromide (CTAB) is frequently used as a surfactant for alkyl ketene dimer (AKD) emulsions in industrial applications. While such emulsions can impart excellent hydrophobicity to paper, CTAB itself exhibits high biological toxicity ( Timmer et al., 2019 ). Therefore, developing alternative stabilizers that are efficient, non-toxic, and derived from renewable sources is essential for advanc- ing emulsion coating technologies. Pickering emulsion coating technology, stabilized by solid particles, particularly those derived from biomass-based nanomaterials, has emerged as a pivotal breakthrough in recent years, offering a novel pathway for creating high-performance and environmentally friendly coatings. The stabilization mechanism relies on the irreversible adsorption of solid particles at the oil-water interface, forming a dense barrier with high mechanical strength, which results in an emulsion system with exceptional stability. A Pickering emulsion coating was developed using cellulose nanocrystals (CNC) as the stabilizer, with chitosan and glutinous rice starch dissolved in the aqueous phase, while polylactic acid (PLA) was dissolved in dichloromethane to form the oil phase. This coating significantly enhanced the paper’s hydrophobicity and oil-proof properties. The PVA is a water-soluble synthetic polymer that is widely used due to its excellent film-forming ability, biocompatibility, and biodegradability. The molecular chains of PVA are rich in hydroxyl groups, which can form strong hydrogen bonds with the cellulose paper substrates, thereby providing good coating adhesion and a continuous, defect-free film structure. This makes PVA an ideal paper coating material that can significantly enhance the mechanical properties and oil-proof performance of paper ( Liu et al., 2022 ; Choe et al., 2024 ). However, paper coated with PVA exhibits poor water-proof performance, necessitating further modification or compounding with hydrophobic components to overcome this limitation ( Kwon et al., 2024 ). Stearic acid (SA), a long-chain saturated fatty acid, is regarded as an environmentally friendly hydrophobic material due to its biocompatibility, biodegradability, and renewa- bility ( Zhou et al., 2024 ). However, the hydrophobicity of stearic acid makes it difficult to achieve stable dispersion in water, and its compatibility with hydrophilic PVA is poor, thereby limiting the synergistic combination and practical application of both in aqueous systems. Lignin is the second most abundant natural polymer, primarily sourced from the black liquor of the paper industry, and is currently mostly utilized in low-value applications or directly incinerated. In recent years, with the development of nanotechnology, the high-value utilization of lignin has shown new prospects. Lignin nanoparticles (LNPs), prepared via nanonization, possess the characteristic advantages of nanomaterials, including small size and high specific surface area ( Yao et al., 2025 ). Furthermore, since lignin molecules contain both hydrophobic structures (such as phenyl rings and methoxy groups) and hydrophilic groups (such as hydroxyl and carboxyl groups), they can self-assemble into amphiphilic nanoparticles with high surface activity ( Gan et al., 2025 ). These characteristics make LNPs ideal emulsifiers for Pickering emulsions. To address the limitations of paper regarding water- and oil-proof properties, this study developed a ternary hybrid oil-in-water (O/W) PLS Pickering emulsion coating, composed of biodegradable PVA, biomass-derived SA and LNPs. Unlike the conventional LNPs-stabilized Pickering emulsions reported in previous studies, the PLS emulsion coating achieved a synergistic composition of hydrophilic PVA and hydrophobic SA. In this system, LNPs acted as emulsifiers, effectively reducing the interfacial tension and stabilizing the oil-water interface. The PVA, present in the continuous aqueous phase, imparted film-forming ability and oil-proof properties to the coating while also contributing to emulsion stability. Meanwhile, SA, as the hydrophobic oil phase, was uniformly dispersed as micrometer-sized droplets, providing essential hydrophobicity to the coating. When applied to the paper surface, the PLS emulsion coating formed a protective “armor ” layer that significantly enhanced the paper’s proof properties to water and oil, as well

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Fig. 1. Schematic diagram illustrating the application of a PVA/LNPs/SA (PLS) emulsion coating to fabricate high-performance, biodegradable paper LNPs: lignin nanoparticles; PVA: polyvinyl alcohol; SA: stearic acid; PLS: PVA/LNPs/SA.

as its water vapor barrier properties and tensile strength ( Fig. 1 ). Moreover, all components of the coating are biodegradable, enabling the coated paper to retain its environmentally friendly characteristics and degrade completely in soil within 120 d. These promising results indicate that the PLS emulsion coating effectively addresses key challenges related to barrier performance and mechanical strength in paper-based materials, offering a potential solution for developing high-performance, sustainable green packaging.

2. Material and methods

2.1. Materials

Industrial alkali lignin was supplied by Shandong Duoyuan Co., Ltd. (Liaocheng, China). Sodium hydroxide (NaOH, ≥ 96%) was purchased from Tianli Chemical Reagent Co., Ltd. (Tianjin, China). Hydrochloric acid (HCl, 37%) was obtained from Shanghai Hushi Chemical Co., Ltd. (Shanghai, China). Dimethyl sulfoxide (DMSO, ≥ 99.75%) was supplied by Tianjin Fuyu Fine Chemical Co., Ltd. (Tianjin, China). Stearic acid (SA, ≥ 99.75%) was obtained from Tianjin Guangfu Technology Development Co., Ltd. (Tianjin, China). Toluene ( ≥ 99.5%) was obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The n -heptane ( ≥ 98%), castor oil (chemically pure), and PVA1799 (degree of alcoholysis: 99.17%, acetyl group content: 0.79% ( w )) were acquired from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Fluorescein sodium (85%) was purchased from Macklin Biochemical Technology Co., Ltd. (Shanghai, China). Lumogen F Red 305 was purchased from BASF Aktiengesellschaft (Ludwigshafen, Germany). Uncoated bleached paper was provided by Yueyang Forest & Paper Co., Ltd. (Yueyang, China). Uncoated corrugating medium paper was purchased from Xiaoying Paper Co., Ltd. (Suzhou, China). All experiments were conducted by using deionized water.

2.2. Preparation of LNPs

The LNPs were fabricated through a solvent exchange method. Initially, industrial alkali lignin was subjected to a purification process. Specifically, the lignin was dissolved in a NaOH solution with a pH between 12 and 13. After complete dissolution, the solution was filtered to remove insoluble impurities. Under continuous stirring, HCl solution was slowly added to the filtrate to adjust the pH to 2–3, inducing the precipitation of the alkali lignin. The precipitate was collected by vacuum filtration and washed three times with deionized water to remove residual impurities. The purified alkali lignin was subsequently dissolved in DMSO to obtain a 10 mg/mL solution. This solution was then injected dropwise into a 10-fold volume of deionized water under continuous magnetic stirring. The obtained LNPs suspension was transferred to a dialysis bag with a molecular weight cut-off of 14 000 and dialyzed against deionized water for 96 h with water changes every 12 h to ensure the complete removal of DMSO.

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Table 1 The coding and formulation of emulsion coatings.

Sample

LNPs (2%) (g)

PVA (12%) (g)

SA (g)

PLS 0

5 5 5 5 5 5

PLS 0.25 PLS 0.5

0.15

0.3 0.6 0.9 1.2

PLS 1

PLS 1.5

PLS 2

LNPs, lignin nanoparticles; PVA, polyvinyl alcohol; SA, stearic acid; PLS, PVA/LNPs/SA.

2.3. Preparation of Pickering emulsion coatings

Firstly, PVA was dissolved in deionized water under constant stirring at 90°C to prepare a 12% ( w/w ) aqueous solution. Subse- quently, 5 g of this PVA solution was mixed with 5 g of a 2% ( w/w ) LNPs suspension and a specified amount of SA. The mixture was heated to 80°C in a water bath to completely melt the SA, and then homogenized using a high-shear emulsifier (D-500, Wiggens, Germany) at 15 000 r/min for 5 min to form a stable emulsion. After homogenization, the emulsion was rapidly cooled in an ice-water bath to induce SA recrystallization. The resulting emulsion coatings were labeled as PLS x emulsion coatings, where x represents the mass ratio of SA to PVA ( Table 1 ).

2.4. Preparation of PLS emulsion coated paper

The PLS emulsion was uniformly applied on both sides of the bleached paper using a coating machine (ZY-TB-X3, Zhongyi, China) equipped with a 100 μm coating bar, operating at a constant speed of 5 mm/s. The coated paper was initially dried at room temperature, followed by complete drying in a vacuum oven at 50°C until a constant weight was achieved. The obtained paper was named P-PLS x , while the paper treated with the coating without SA was designated as P-PLS 0 . The paper coated with non-emulsified PLS 2 coating was named P-PLS 2 − M.

2.5. Characterization

The particle size and zeta potential of LNPs were determined using a particle nano sizer and zeta-potential tester (Zetasizer Nano ZS90, Malvern Instruments, UK). Morphological analysis of the LNPs was performed by transmission electron microscopy (TEM, JEM 2100, JEOL, Japan) at an accelerating voltage of 200 kV. The average droplet diameter and size distribution of PLS emulsions were measured with a laser particle size analyzer (Mastersizer 3000, Malvern Instruments, UK). The morphology of emulsions was characterized by confocal laser scanning microscopy (CLSM, LSM 900, ZEISS, Germany). The PVA and LNPs were stained with fluorescein sodium, while the SA was stained with Lumogen F Red 305. The excitation/emission spectra were 488/515 nm for fluorescein sodium and 561/616 nm for Lumogen F Red 305. The morphology of dried emulsions and coated paper was examined by using scanning electron microscopy (SEM, Adpreo S HiVac, Thermo Scientific, USA). The dynamic rheological properties of PLS emulsions were evaluated with a rotational rheometer (AR2000ex, TA Instruments, USA) at 25°C. The chemical properties of the coated paper surface were measured by using an X-ray photoelectron spectrometer (XPS, SCALAB 250Xi, Thermo Scientific, USA). According to the Chinese standard GB/T 453 —1989 (Paper and board —Determination of tensile strength —Constant rate of elongation method ‌ ), the tensile strength of the coated paper was assessed using an electronic universal testing machine (UTM 2503, Suns, China). To determine the wet tensile strength, the paper was first immersed in deionized water for 20 min. Surface wettability was characterized using a contact angle meter (Attension Theta, Biolin Scientific, Sweden). Paper absorbency was quantified with a Cobb sizing instrument (ZZ-100, Changchun Paper Machinery Co., Ltd., China) according to the Chinese standard GB/T 1540 —2002 (Paper and Board-Determination of Water Absorption-Cobb Method, China). Oil resistance was evaluated following TAPPI T 559 cm- 12 (Grease Resistance Test for Paper and Paperboard), with test solvents prepared by blending castor oil, n -heptane, and toluene in specified ratios. The water vapor transmission rate (WVTR) of paper was determined in accordance with the Chinese standard GB/T 1037 —2021 (Test Method for Water Vapor Transmission of Plastic Film and Sheet-Desiccant Method and Water Method, China) using a water vapor permeability tester (C360M, Languang, China) under conditions of (23 ± 2)°C and (50 ± 2)% relative humidity. The oxygen transmission rate (OTR) of the paper was determined using a gas permeability tester (VAC-V2, Languang, China) in accordance with Chinese standard GB/T1038.1 —2022 (Plastics-Film and Sheeting-Determination of Gas-Transmission Rate-Part 1: Differential-Pressure Methods, China) under conditions of (23 ± 2)°C and (50 ± 2)% relative humidity.

2.6. Fruit preservation test

To evaluate the effect of paper packaging on fruit preservation, bayberries, grapes, and cherry tomatoes were wrapped in bags made of uncoated paper and P-PLS 2 , with the openings sealed using adhesive tape. These packaged fruits were then compared with unpackaged control samples exposed to ambient conditions. The experiment was conducted under natural environmental conditions,

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with temperatures ranging from 13 to 34°C and relative humidity levels varying between 60% and 90%. Fruit weight was monitored at regular intervals to assess moisture loss and preservation efficacy.

2.7. Recyclability of PLS emulsion coated paper

The recyclability of PLS emulsion-coated paper was evaluated through a repulping process. Specifically, the P-PLS 2 was cut into approximately 2 cm × 2 cm pieces and subjected to hot water treatment at 100°C with continuous stirring to facilitate coating detachment. The pulp fibers were then filtered and collected, followed by the use of a fiber disintegrator to dissociate the fibers before paper formation. The recycled paper prepared from uncoated paper and from P-PLS 2 were designated as R-Uncoated and R-P-PLS 2 , respectively.

2.8. Assessment of biodegradability

The degradation experiment was conducted on the campus of Northeast Forestry University from April 20 to August 18, 2025. Polyethylene (PE) film, uncoated paper, and P-PLS 2 were cut into 5 cm × 5 cm samples and buried in soil. The samples were periodically taken out for photographic recording and weight measurement to assess their biodegradability.

3. Results and discussion

3.1. Characterization of LNPs

Alkali lignin, a natural polyphenolic polymer, exhibits extremely low solubility in aqueous media due to its highly cross-linked aromatic structure and hydrophobic characteristics. To enhance its applicability in water systems, the assembly of lignin into nanopar- ticles has emerged as an effective strategy. A solvent exchange-induced self-assembly approach was employed to fabricate LNPs. Initially, alkali lignin was dissolved in the DMSO to form a homogeneous solution. Subsequently, this solution was added dropwise into water, where the DMSO in the system was rapidly diluted with water, leading to a significant decrease in the solubility of lignin. During this process, the hydrophobic aromatic ring regions of the lignin molecules folded and aggregated to minimize contact with the aqueous phase, forming a hydrophobic core; meanwhile, the hydrophilic groups (e.g., hydroxyl and carboxyl groups) in the molecular structure oriented toward the aqueous phase, resulting in the formation of stable amphiphilic nanoparticles ( Fig. 2 a). This self-assembly process was driven by multiple intermolecular interactions, including van der Waals forces, hydrogen bonding, 𝜋 - 𝜋 stacking, and hydrophobic effects ( Moreno and Sipponen, 2024 ; Tao et al., 2024 ). The TEM images revealed that the obtained LNPs exhibited a spherical-like morphology, with diameters ranging from 8 to 35 nm and an average particle size of (20.8 ± 5.0) nm ( Figs. 2 b and 2 c). Notably, dynamic light scattering (DLS) measurements indicated a larger hydrodynamic size (67.37 nm) compared to TEM observations ( Fig. 2 d), which can be attributed to the swelling of LNPs in aqueous environments. Zeta potential analysis confirmed a strong negative surface charge ( − 37.6 mV) ( Fig. 2 e), primarily resulting from the ionization of phenolic hydroxyl and carboxyl groups in lignin. Colloidal systems with an absolute zeta potential greater than 30 mV are highly stable. When the absolute value of the zeta potential on the particle surface exceeds 30 mV, the electrostatic repulsion between particles is sufficient to over- come van der Waals attraction, thereby ensuring the kinetic stability of the colloidal system ( Vanderfleet and Cranston, 2021 ). The high negative zeta potential further demonstrates the excellent dispersion stability of LNPs in water.

3.2. Characterization of PLS emulsions

Stable O/W Pickering emulsions stabilized by LNPs were successfully developed, effectively achieving the synergistic composite and stable dispersion of water-soluble PVA and hydrophobic SA. In the binary PVA/SA system without LNPs, the absence of effective interfacial stabilizers prevented the formation of a stable emulsion, resulting in overall solidification during cooling due to the crystallization of SA. In contrast, after incorporating LNPs as emulsifiers, both the LNPs/SA and PVA/LNPs/SA systems formed uniformly stable emulsions, demonstrating the effective emulsifying capability of LNPs in complex multiphase systems (Fig. S2). As shown in Fig. 3 a, initially, PVA and LNPs were dissolved and dispersed, respectively, in the aqueous phase, while hydrophobic SA existed as a separate phase due to its immiscibility with water. Following high-speed homogenization, SA was dispersed into micrometer-sized droplets within the aqueous phase. At this stage, LNPs, owing to their significant amphiphilicity and high specific surface area, adsorbed onto the newly formed oil-water interface, partially embedding in the oil phase and partially remaining in the aqueous phase. Through irreversible interfacial adsorption and steric hindrance effects, LNPs effectively reduced the interfacial tension between oil and water, thereby stabilizing SA droplets ( Xu et al., 2024 ; Li et al., 2025 ). Meanwhile, PVA acted as a protective colloid. Driven by interfacial energy, the hydrophobic segments (residual acetoxy groups) of its molecular chains further adsorbed onto the surfaces of LNPs and extended into the uncovered regions of the oil-water interface, thereby enhancing the continuity and stability of the interfacial layer. In this composite interfacial structure, LNPs served as “bricks ” constructing the basic framework of the interfacial barrier, while PVA acted as the “cement, ” bridging, filling, and reinforcing the interface ( Nakamae et al., 1999 ; Kaewsaneha et al., 2022 ). The prepared PLS emulsions exhibited excellent long-term storage stability. After 30 d of static storage, no significant phase separation or stratification was observed in any of the samples, indicating superior physical stability ( Fig. 3 b). The SEM and CLSM images revealed that the particles in the emulsion were dispersed and exhibited a spherical-like morphology ( Figs. 3 c, 3 d, S3 and

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Fig. 2. Preparation and characterization of lignin nanoparticles (LNPs). (a) Schematic diagram illustrating the preparation and self-assembly mech- anism of LNPs. (b) Transmission electron microscope (TEM) image of the LNPs. (c) Particle size distribution of LNPs based on statistical analysis of TEM images. (d) Hydrodynamic size distribution of LNPs measured by dynamic light scattering (DLS). (e) Zeta potential of LNPs dispersion. DMSO, dimethyl sulfoxide.

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Fig. 3. Preparation and characterization of PLS emulsions. (a) Schematic diagram of the formation mechanism of PLS emulsions. (b) Digital image of the appearance of PLS emulsions. (c) Confocal laser scanning microscopy (CLSM) image of PLS 2 emulsion. (d) Scanning electron microscope (SEM) image of PLS 2 emulsion droplets. (e) Initial size distribution of PLS emulsions. (f) The surface mean diameter (D 3,2 ) of PLS emulsions. (g) Shear viscosity of PLS emulsions. D 3,2 , surface mean diameter.

S4). Laser particle size analysis revealed that all PLS emulsions exhibited a distribution peak within the range of 0.3–3 μm. As the amount of SA increased, the particle size distribution gradually shifted from a unimodal to a bimodal distribution, evidenced by the emergence of a second peak in the 3–20 μm range for PLS 0.5 , PLS 1 , PLS 1.5 , and PLS 2 emulsion ( Fig. 3 e). This indicates that higher SA content promotes the formation of some larger droplets. Meanwhile, the surface mean diameter (D 3,2 ) also gradually increased with the increase of SA content ( Fig. 3 f). Notably, after 30 d of storage, the particle size distribution and D 3,2 values of all emulsions showed no significant changes, further confirming their long-term stability ( Figs. 3 f and S5). Rheological tests showed that PLS emulsions exhibited pronounced shear-thinning behavior, primarily due to the disruption and reorganization of the three-dimensional network structure within the emulsion under shear ( Fig. 3 g). This rheological characteristic is advantageous for casting and forming processes during coating operations. With increasing SA content, the apparent viscosity of the PLS emulsions significantly increased, which is mainly attributed to the higher overall volume fraction of the dispersed phase. This leads to stronger interactions among SA particles and between SA particles and the continuous phase molecules, resulting in increased viscosity ( Fig. 3 g). Moreover, the elevated viscosity effectively suppresses the Brownian motion and aggregation tendencies of SA particles, reducing the likelihood of collisions

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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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Fig. 4. Structure and mechanical properties of PLS emulsion coated paper (P-PLS). The SEM images of (a) uncoated paper, (b) PLS 0 emulsion coated paper (P-PLS 0 ), and (c) PLS 2 emulsion coated paper (P-PLS 2 ). (d) Schematic diagram illustrating structure of P-PLS 0 , and PLS 0.25, 0.5, 1, 1.5, 2 emulsion coated paper (P-PLS 0.25, 0.5, 1, 1.5, 2 ). (e) Tensile strength of uncoated paper, and P-PLS. (f) Wet tensile strength of uncoated paper, and P-PLS. (g) Photographs showing that a 2 cm-wide P-PLS 2 can easily lift a 4 kg kettlebell after being soaked in water for 20 min.

incorporation of SA into the coating, the WCA of the coated paper gradually increased, demonstrating hydrophobic properties, with all WCAs exceeding 90°, and the maximum WCA of P-PLS 2 reaching (111.2 ± 4.5)° ( Fig. 5 a). The improvement in WCA can be attributed to the incorporation of hydrophobic SA particles into the hydrophilic PVA matrix within the PLS emulsion coating. Therefore, the partially exposed SA particles on the surface provide the coating with enhanced hydrophobic properties. Due to the large number of SA particles adhering to its surface, P-PLS 2 − M also showed a high WCA ( Fig. 5 a). Furthermore, compared to the rapid absorption of water droplets on uncoated paper, the absorption rate on PLS emulsion-coated paper was slower due to the dense coating structure and the hydrophobicity of SA. Specifically, P-PLS 1.5 and P-PLS 2 , which contain higher SA content, showed only a slight decrease in WCA over 180 s, exhibiting resistance to water absorption (Fig. S12). The water-proof performance of the coated paper was further confirmed by the Cobb 60 test, which represents the amount of water absorbed per unit area of paper within 60 s. Due to the poor water-proof performance of uncoated paper, its Cobb 60 value reached as high as (89.59 ± 5.63) g/m 2 . Due to the dense structure of the coating and the hydrophobic properties of the SA particles, the Cobb 60 value of the coated paper was significantly reduced. As the SA content increased, the Cobb 60 value gradually decreased, reaching a minimum of (17.73 ± 0.09) g/m 2 (P-PLS 2 ), representing an 80.2% reduction compared to the uncoated paper ( Fig. 5 b). This further demonstrates the good water-proof properties imparted to

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Fig. 5. Water- and oil-proof properties of P-PLS. (a) The water contact angle (WCA) of uncoated paper, P-PLS, and paper coated with non-emulsified PLS 2 coating (P-PLS 2 − M). (b) Cobb 60 value of uncoated paper, P-PLS, and P-PLS 2 − M. (c) Contact angle of common beverages on the surface of P-PLS 2 . (d) Kit rating of uncoated paper, P-PLS, and P-PLS 2 − M. (e) Photographs showing the barrier properties of uncoated paper and P-PLS 2 against different liquids after 24 h of direct contact. (f) Photographs showing uncoated paper and P-PLS 2 in direct contact with jelly, fried chicken, and soybean oil (dyed with oil red) at various temperatures for 10 min. the paper by the PLS emulsion coating. P-PLS 2 − M demonstrated a lower Cobb 60 value compared to the uncoated paper, which can be attributed to the abundant SA particles on its surface ( Fig. 5 b). However, its Cobb 60 value remained higher than that of P-PLS 2 , likely due to the uneven dispersion of SA particles that exposed the hydrophilic PVA. Despite the improvement in water-proof properties achieved by the PLS emulsion coated paper, a significant gap remains compared to conventional LDPE coated paper commonly used in daily applications (Table S2). Additionally, the PLS 2 emulsion-coated paper exhibited good barrier performance against common beverages such as coffee, cola, tea, and milk, with contact angles exceeding 105° ( Fig. 5 c). The barrier properties against oil are equally important for paper-based packaging materials, especially for food packaging papers. The Kit rating of coated paper was measured to evaluate its oil-proof performance, with a higher Kit rating indicating better oil-proof performance. Uncoated paper, due to its porous structure, allows easy penetration of oil, resulting in a Kit rating of 0/12. In contrast, the oil-proof performance of coated paper was significantly improved by the PLS emulsion coating. The P-PLS 0 exhibited the best oil-proof properties, with a Kit rating of (11.4 ± 0.55)/12, primarily due to the strong oil-proof performance of PVA. However, the introduction of SA in the coating slightly decreased the oil-proof performance because of the lipophilic nature of SA and the reduced proportion of PVA. Nonetheless, all coated papers maintained a Kit rating exceeding 9/12, which is sufficient for daily applications and comparable to the oil-proof properties of LDPE coated paper ( Figs. 5 d and S13, Table S2).

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In addition, further demonstrations illustrated the good water- and oil-proof properties of P-PLS 2 . First, the long-term barrier performance of P-PLS 2 against water, milk, and cola was evaluated. When 100 mL of these liquids were poured into a filtration device containing uncoated paper and P-PLS 2 , the uncoated paper allowed liquid penetration after 24 h. In contrast, P-PLS 2 effectively prevented liquid passage due to its enhanced water-proof properties ( Figs. 5 e and S14). Furthermore, jelly, fried chicken, and soybean oil at different temperatures were placed on the paper surface. Due to the poor water- and oil-proof performance of uncoated paper, after 10 min, water from the jelly, oil from the fried chicken, and soybean oil had penetrated and spread into the paper. In comparison, P-PLS 2 exhibited an effective barrier that inhibited the penetration and diffusion of both water and oil ( Fig. 5 f). These demonstrations fully confirm the good water- oil-proof properties of P-PLS 2 . To verify the universality of the PLS emulsion coating, it was applied to the surface of corrugating medium paper, producing PLS emulsion-coated corrugating medium paper (CP-PLS). The uncoated corrugating medium paper exhibited hydrophobicity due to the high lignin content in its fibers. After treatment with the PLS emulsion coating, CP-PLS with lower SA content showed a reduced WCA and an increased Cobb 60 value, primarily due to the dominant surface hydrophilicity imparted by the PVA in the coating. As the SA content in the PLS emulsion coating increased, the water-proof properties of CP-PLS gradually improved, as indicated by an increase in WCA and a decrease in Cobb 60 value. Notably, the water-proof performance of CP-PLS 2 (WCA of (120.1 ± 4.5)°, Cobb 60 value of (19.65 ± 1.50) g/m 2 ) surpassed that of the uncoated corrugating medium paper (Figs. S15A and S15B). Regarding oil-proof performance, CP-PLS exhibited a trend similar to P-PLS, with both demonstrating significantly better oil-proof performance than uncoated paper. Although the Kit rating slightly decreased with increasing SA content, all CP-PLS maintained a Kit rating exceeding 8/12 (Figs. S15C and S15D). These results demonstrate the broad applicability of the PLS emulsion coating. The PLS emulsion coating endowed the coated paper with good water- and oil-proof properties, which can be attributed to the synergistic interaction among PVA, SA, and LNPs, collectively forming a dense and hydrophobic functional surface. In terms of water- proofing, the dense structure of the coating effectively compensates for the deficiencies of the inherently porous fiber network of the paper, significantly suppressing water penetration caused by capillary action. Additionally, the incorporation of hydrophobic SA microparticles, effectively emulsified by LNPs, further enhances the hydrophobic performance of the coating, thereby significantly improving the overall water-proof properties of the coated paper ( Zhang et al., 2023 ). Regarding oil-proof properties, the good oil- proof performance primarily arises from the physical barrier formed by the tight interconnection of PVA macromolecular chains. Although the SA component exhibits oleophilicity, PVA, as the continuous phase, plays a dominant role in oil-proofing. Its molecular chains construct a dense structure that effectively obstructs the penetration and migration of oily substances ( Li et al., 2024 ; Peng et al., 2024 ). Consequently, even with the incorporation of SA into the system, the composite coating can still rely on the continuous protective barrier provided by PVA to maintain a high Kit rating.

3.5. Gas barrier performance of PLS emulsion coated paper

The water vapor barrier performance plays a crucial role in many packaging applications as high water vapor barrier performance ensures the extended shelf life of moisture-sensitive or water-losing foods. Due to the extensive porous structure of uncoated paper, it exhibited a high WVTR. In contrast, the WVTR of P-PLS 0 and P-PLS 2 was reduced to 599.72 and 557.30 g/(m 2 ·d), respectively, demonstrating an enhancement in water vapor barrier performance ( Fig. 6 a). This enhancement can primarily be attributed to two factors. First, the dense coating uniformly covers the paper surface, effectively sealing its porous structure, thereby inhibiting the rapid transmission of water vapor ( Chen et al., 2024 ). Second, the hydrophobic SA particles embedded in the PLS emulsion coating obstruct the passage of water vapor molecules and increase their diffusion path, resulting in a lower WVTR for P-PLS 2 compared to P-PLS 0 ( Zhong et al., 2015 ; Zhang et al., 2024 ). Similar to the WVTR, the OTR of P-PLS 2 was significantly lower than that of both the uncoated paper and P-PLS 0 ( Fig. 6 b). Although the gas barrier properties of P-PLS 2 were improved, they remain inferior to those of LDPE coated paper and PLA coated paper, indicating a need for further optimization in future work (Table S3). The improved water vapor barrier performance of PLS emulsion coated paper offers significant potential for food preservation. To demonstrate its effectiveness in fruit preservation, an assessment was conducted using P-PLS 2 on three types of fruits (bayberry, grape, and cherry tomato). The preservation effectiveness of these fruits was compared with those exposed to the natural environment and those wrapped in uncoated paper. Compared to bare fruits and those wrapped in uncoated paper, fruits wrapped in P-PLS 2 effectively inhibited moisture loss due to its enhanced water vapor barrier properties. Across all fruits, quality gradually declined during the testing period, and their appearance underwent varying degrees of shrinkage ( Figs. 6c–6f and S16). However, the weight loss of all fruits wrapped in P-PLS 2 was lower than that of bare fruits and those wrapped in uncoated paper ( Figs. 6c–6e ). Notably, the effect was most pronounced in bayberries, where the weight loss of bare and uncoated paper-wrapped fruits exceeded 70% by the 9th day, resulting in surface cracking due to excessive dehydration. In contrast, the weight loss of bayberries wrapped in P-PLS 2 was < 40% on the 9th day, with no significant surface cracking observed ( Fig. 6 f). These results underscore the good fruit preservation capability of P-PLS 2 , attributed to its PLS 2 emulsion coating, which effectively retains moisture.

3.6. Recyclability and biodegradability of PLS emulsion coated paper

The recyclability of packaging paper is essential to the circular economy, as recyclable paper can significantly reduce costs and improve environmental sustainability. Currently, common coated papers (such as LDPE coated paper) face considerable challenges during recycling ( Aayanifard et al., 2024 ). These plastic coatings are difficult to separate from paper fibers, which adversely affects the mechanical properties and uniformity of recycled paper products. Therefore, the recyclability of P-PLS 2 was evaluated. When the coated paper was immersed in water at 100°C, the PVA in the PLS emulsion coating completely dissolved due to the disruption of its

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