Cellulose and Polylactic Acid: Biomimetic Multifunctional Packaging Film Rapidly Degrades in Soil Environment

Figure 3. Mechanical performance tests of LEAFF in dry and wet states, demonstrating its superiority over traditional bioplastics and petrochemical film materials.

In the dry state, the tensile strength of LEAFF reaches 118.1 ± 8.6 MPa, and the elastic modulus reaches 10.6 ± 1.2 GPa, far exceeding pure CNF (83.7 ± 6.7 MPa, 7.6 ± 0.9 GPa), pure PLA (67.4 ± 8.3 MPa, 2.9 ± 0.2 GPa), and non-crosslinked CNF/PLA composite films (92.2 ± 1.6 MPa, 7.3 ± 0.4 GPa). After 36 hours of water immersion, LEAFF retains 77% of its tensile strength (91.1 ± 6.9 MPa) and 63% of its elastic modulus (6.7 ± 0.7 GPa), significantly higher than pure CNF (51%) and CNF/PLA (68%). Swelling tests show that the degree of swelling of LEAFF is close to that of pure PLA, indicating excellent water stability. This is closely related to its surface PLA coating, which is continuous and pore-free, effectively blocking water penetration.

Figure 4. Multifunctional evaluation of LEAFF packaging, including oxygen/water vapor barrier properties, transparency, and printability.

In terms of gas barrier properties, LEAFF exhibits a water vapor permeability (WVP) of 0.794 g·mm·m⁻²·d⁻¹, significantly lower than pure CNF (1.146), pure PLA (1.162), and non-crosslinked CNF/PLA (1.382); its oxygen transmission rate (OTR) is 0.772 cm³·m⁻²·day⁻¹·atm⁻¹, only 0.7% of pure PLA (108.5), demonstrating superior gas barrier performance compared to conventional plastics such as polyethylene and polypropylene. Regarding transparency, LEAFF has a light transmittance of approximately 49% at 600 nm, higher than CNF (33%) and CNF/PLA (29%), allowing clear visibility of text through it. Printability tests show that ink diffusion on its surface is less than that on traditional A4 paper, meeting packaging label requirements.

Figure 5. Degradation images of LEAFF in ambient temperature soil and analysis of microbiome changes reveal its rapid degradation mechanism.

Soil degradation experiments showed that LEAFF completely degraded within 5 weeks under conditions of 25°C and 40-60% humidity, while pure PLA exhibited no significant degradation. At 2 weeks, the remaining weight of LEAFF was about 40%, and SEM images revealed extensive microbial colonization on its surface, a phenomenon not observed on pure PLA. Microbial community analysis indicated that the Shannon diversity index in the LEAFF degradation environment increased from 6.5 (in soil) to 7.1, reflecting enhanced microbial diversity. Notably, the abundance of Planctoellipticum variicoloris—a Planctomycete bacterium capable of macromolecule uptake—rose significantly, suggesting that it may accelerate the degradation process by promoting PLA hydrolysis.

Our hypothesis is that the challenges of PLA biodegradation under environmental conditions are twofold: firstly, the chemical limitation of energy balance, and secondly, the physical limitation caused by the crystalline structure of PLA. The degradation of PLA involves an initial energy input for ester bond hydrolysis, followed by the release of low-energy lactide monomers that are converted into pyruvic acid and then funneled into central metabolism. Therefore, we hypothesize that the co-localization of high-energy cellulose provides energy for this energy-deficient microbial metabolic process, which is supported by the observation that soil microbial communities proliferate on LEAFF films but not on pure PLA films. Additionally, we engineered the films to enhance the contact between cells and secreted hydrolytic enzymes with the PLA polymer chains. Furthermore, the internal structure of CNF fibers allows soil microbes to grow and aggregate near the PLA material, further enhancing its biodegradability compared to pure PLA.

This work proposes a biomimetic hierarchical structure design to overcome the current bottleneck of “mutual exclusivity between high strength and biodegradability” in bioplastics. Inspired by the structure of plant leaves, CNF cores and PLA coatings are crosslinked via HMDI to achieve synergistic interfacial effects: on the one hand, the tensile strength and elastic modulus surpass those of conventional plastics (such as polyethylene and polypropylene); on the other hand, PLA is endowed with ambient soil biodegradability, addressing the limitation of PLA’s requirement for industrial composting. Meanwhile, by regulating the crystalline structure and densification, the material simultaneously achieves high transparency, water stability, and excellent gas barrier properties, integrating the multifunctionality required for packaging materials. In addition, it is discovered for the first time that specific microorganisms (such as Planctoellipticum variicoloris) can promote degradation, revealing a mechanism by which enhanced microbial diversity accelerates degradation. This provides a new structure–property–degradability correlation approach for bioplastic design.