Effects of plasma treatment on biodegradation of natural and synthetic fibers

The lightweight materials industry increasingly demands textiles that meet stringent hygiene, durability, and performance standards, while addressing environmental concerns associated with synthetic polymers that are slow to decompose. Approximately 30% of polymer waste is incinerated, 30% recycled, and 30% remains undisposed, motivating the development of biodegradable polymers and treatments. Plasma treatment has emerged as a promising approach to modify polymer surfaces to enhance biodegradation by altering surface structure and chemistry without affecting bulk properties. Despite advances in biodegradable polymers and plasma processing, gaps remain regarding biodegradation pathways across polymer types, factors influencing biodegradation, resultant products, and methods to enhance degradation. Some studies suggest that grafting natural polymers onto synthetic ones can facilitate microbial degradation initiation on the natural segment that propagates into the synthetic chain. This study aims to measure the biodegradability of plasma-treated wood fiber-reinforced polypropylene composites to determine whether plasma modification alters the rate of biodegradation. Secondary objectives are to examine low molecular weight components generated during biodegradation and to explore the composition and properties of substances isolated from plasma-treated samples. The work also attempts to experimentally substantiate the mechanism behind plasma-induced decomposition of cellulose.

Prior studies have demonstrated improved mechanical performance in natural-fiber-reinforced polymer composites and the impact of chemical treatments on fiber properties. For instance, epoxy composites reinforced with alkali-treated Zanthoxylum acanthopodium bark fibers (up to 25 wt.%) achieved tensile strengths of 47.3 MPa, though with increased water uptake. Vachellia farnesiana fibers subjected to HCl and NaOH treatments showed significant changes in cellulose content and properties, with HCl causing degradation due to its acidity. Plasma treatment is recognized as a dry, clean, environmentally friendly, and cost-effective method for surface modification of natural fibers, improving adhesion by etching and introducing functional groups and free radicals while reducing chemical use and process time. Literature notes that plasma treatment can reduce energy consumption for synthetic fiber production by about 25% compared to other methods, and modifying natural fibers requires 14–50% less energy than producing synthetic fibers, depending on plant growth factors and plasma parameters. Studies in biodegradable composites have shown that including more biodegradable constituents accelerates degradation (e.g., triple compositions degrade faster than double ones). Plasma treatment is reported to preserve textile properties while enhancing mechanical attributes and adhesion in biodegradable composites, often outperforming chemical treatments in efficacy and sustainability. Research on various fibers (e.g., flax, cotton, wool, ramie) and matrices indicates improved fiber–matrix adhesion and modified surface energy after plasma exposure, supporting the use of cold plasma as an eco-friendly alternative that fosters sustainable composite applications.

Experimental design comprised: selection of research objects (natural softwood fibers and cellulose-containing materials), production of polypropylene/cellulose composites, low-temperature plasma treatment, and assessment of biodegradability and mechanical strength. Materials and characterization: Natural fibers included unbleached softwood sulfate pulp and softwood sulfate lignin. SEM analysis involved sample preparation, loading into a vacuum chamber, electron beam interaction and detection to image surface topography and morphology. Wood fibers had an average length of 190 µm (range 100–400 µm; SD 63 µm) and width of 50 µm (range 20–100 µm; SD 19 µm). Approximate composition (%): Natural fiber—cellulose 46–48, hexosans 10–12, pentosanes 8–10, lignin 28–30, organic acids 4–5; Cellulose composite—cellulose 18–20, hexosans 12–14, pentosanes 8–9, lignin 21–23, organic acids 3–4, polypropylene 35–37. Composite preparation: Solid-state shear pulverization reduced cellulose particle size. Cellulose fibers were mixed with polypropylene (Haake Rheocord 9000) at 60 rpm, 185 °C for 8 min; blends were compression molded at 185 °C, 10 MPa for 15 min, then conditioned 5 days at room temperature. Plasma reactor and treatment: A radio-frequency induction coil around a borosilicate glass tube formed the reactor, with gas supply, vacuum pump and an internal rotating sample tube driven by a stepper motor. Low-pressure, low-power plasma minimized heating. Plasma parameters: current 4.8–5.2 mA; power/voltage U = 25–28 kW (as reported); gas medium sulfur hexafluoride (SF6) with additional H2O; exposure 10–30 min; pressure P ≈ 10^2 Pa. Procedure: (1) Clean glass tube and sample holder; use nitrile gloves. (2) Weigh 5 g fiber sample into internal glass tube holder; insert into reactor and attach to stepper motor. (3) Evacuate air with vacuum pump via diaphragm valve. (4) Once pressure < 10 Pa, start rotation to mix fibers; brief pressure rise occurs from outgassing. (5) Introduce selected process gas via needle valve or mass flow controller; during pumping, pressure increases to ~10^2 Pa for ~2 min; stop gas; repeat after 2–5 min; continue pumping to remove moisture and ensure sufficient gas. (6) Final pumpdown to ~10^-3 Pa. X-ray sensitive fluorocarbon bonds consideration: initial phase 250–350 eV; final 0–1,150 eV; high-resolution oxygen spectrum binding energy 526–540 eV. Equipment: ChroZen UHPLC, GC-MS (GCMSQP2010 Plus), TD-20 thermal desorber, ATR-8200HA (Pike Tech), SEM (Sigma VP ZEISS), probe microscope (MultiMode 8). Chemical analysis per GOST; GC with Agilent 7820. Mechanical testing: Tensile tests (ISO 527) on a UTM, gauge length 6 cm, crosshead speed 5 mm/min; results averaged over 10 samples. Biodegradability testing: Samples were buried in soil at 10–15 cm depth for 30 days; soil humidity 25–75% and temperature 15–25 °C maintained for microbial activity. After 30 days, samples were exhumed; residue amount and mass loss used to quantify degradation. Statistics: One-way ANOVA with significance at p ≤ 0.05; results reported as means ± SD of at least five experiments.

• Mechanical strength: Reinforcing with cellulose increased tensile strength from 18 to 21 MPa (+16.7%). Low-temperature plasma treatment of natural fibers increased strength from 18 to 25 MPa (+33.3%). Plasma treatment of reinforced fibers increased strength from 18 to 29 MPa (+50%). Table 1: natural fiber 18 MPa; reinforced fiber 21 MPa; plasma-treated natural fiber 25 MPa; plasma-treated reinforced fiber 29 MPa.
• Microscopy and morphology: Non-treated cellulose fibers showed chips and voids; plasma-treated fibers exhibited structural changes resembling wood charring and hollowed segments, with diminished reinforcing microfibril elements leading to enhanced flexibility and disrupted structural integrity. In cellulose-reinforced polypropylene composites, plasma-treated fibers had fewer defects, were better embedded in the matrix, and showed polymer adhesion at multiple points with no slip lines or voids on fracture surfaces.
• Interfacial interactions and biodegradation: High interfacial shear strength can decelerate biodegradation, yet plasma modifies cellulose structure and generates low molecular weight components during depolymerization. Approximately 50 products were identified from plasma-chemical decomposition. Notable yields included: levoglucosan (C6H10O5) 7.92%; 3-dioxyglucosenone (MW ~144 g/mol) 10.1%; methyl maltol ~6.12%; pyrogallol ~4.56%; anhydrosucrose (MW 126 g/mol) ~20.1%; furfural (C5H4O2) 5.11%. Molecules below 90 Da were attributed to O2 and CO2 decay products. Plasma treatment altered molecular masses and yields compared with untreated fibers, indicating modified degradation pathways.
• Lignin content and degradation rates: Natural fibers contained 28–30% lignin; composites 21–23% (also reported: plasma-treated natural fibers 21% lignin and composites 18%). Dehydration and decomposition velocities of non-composites were almost 2 times lower than those of composites, indicating faster degradation of composites under natural conditions (influenced by filamentous fungi acting on lignin).
• Biodegradation timeline: Plasma-treated specimens initiated biodegradation on day 12, whereas untreated specimens initiated around day 19—an acceleration of approximately 7 days. After 30 days of soil burial, plasma-treated samples became brittle and thinner with prominent changes; plasma-treated cellulose-reinforced polypropylene composites exhibited clear signs of decomposition.

The study demonstrates that low-temperature plasma treatment substantially modifies the surface and microfibrillar structure of cellulose fibers, improving fiber–matrix adhesion in polypropylene composites and enhancing mechanical performance. These surface and structural alterations also influence chemical decomposition pathways, as evidenced by the distinct profiles and yields of low molecular weight products generated during biodegradation. The shift in degradation products and earlier onset of biodegradation (day 12 vs day 19) indicate that plasma treatment accelerates biodegradation by about one week. While stronger interfacial bonding generally can hinder biodegradation, the plasma-induced changes in cellulose (including increased formation of low molecular weight species and altered lignin context) promote biodegradation in both fibers and composites. The faster degradation observed in composites compared to non-composites aligns with fungal activity on lignin and the presence of more biodegradable constituents. These findings support the research hypothesis that plasma modification alters biodegradation rates and mechanisms, providing a pathway to engineer composites that balance mechanical performance with eco-friendly end-of-life behavior. The results corroborate prior reports that plasma processing is an effective, cleaner alternative to chemical treatments for tailoring surface properties, adhesion, and durability while facilitating microbial interactions in biodegradable systems.

This work shows that low-temperature plasma treatment of cellulose fibers and cellulose-reinforced polypropylene composites simultaneously enhances mechanical strength (up to 50% increase in tensile strength for reinforced fibers) and accelerates biodegradation (onset advanced by about 7 days). SEM revealed reduced defects, improved fiber–matrix adhesion, and microfibril restructuring in plasma-treated samples. Chemical analyses identified altered distributions of low molecular weight degradation products following plasma treatment, evidencing modified degradation pathways. The study underscores plasma treatment as an eco-friendly, efficient method to tune composite surface chemistry and morphology for improved performance and controlled biodegradability, offering insights for assessing the biodegradability of other cellulose-based and synthetic materials.