| Citation: | June-Ho Choi, Myeong Rok Ahn, Chae-Hwi Yoon, Yeon-Su Lim, Jong Ryeol Kim, Hyolin Seong, Chan-Duck Jung, Sang-Mook You, Jonghwa Kim, Younghoon Kim, Hyun Gil Cha, Jae-Won Lee, Hoyong Kim. Enhancing compatibility and biodegradability of polylactic acid/biomass composites through torrefaction of forest residue[J]. Journal of Bioresources and Bioproducts, 2025, 10(1): 51-61. doi: 10.1016/j.jobab.2024.10.003
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Plastics are widely used in daily life due to their lightweight and convenience. Global plastic production, which reached 450 million tons annually in 2022, is expected to exceed 900 million tons annually by 2045 (Bergmann et al., 2022). However, plastic disposal poses significant environmental challenges, including the emission of toxic substances and greenhouse gases during incineration, contamination of soil and groundwater through landfills, and the formation of plastic islands from indiscriminate dumping (Shen et al., 2020). One of the most critical issues with plastics is their extended biodegradation period, during which they persist in oceans and on land, releasing harmful substances and contributing to long-term pollution (Stubbins et al., 2021).
Despite active regulatory efforts worldwide, the immediate restriction of plastic use is challenging due to the global dependence on these materials (Xanthos & Walker, 2017). A promising solution lies in biodegradable plastics, a subset of bioplastics. By 2025, the biodegradable plastic market is projected to reach 6.73 billion yuan (US dollar), representing 44% of the broader bioplastic market, with polylactic acid (PLA) accounting for 13.9% of the biodegradable plastics segment (Abraham et al., 2021; Taib et al., 2023). Polylactic acid, a polymer derived from the polymerization of lactic acid obtained from renewable sources such as corn or sugarcane, is used in a variety of applications, including disposable products, 3D printing materials, and medical supplies (Taib et al., 2023). However, the degradation of PLA in nature depends significantly on environmental conditions such as temperature, humidity, and the presence of microorganisms (Zaaba and Jaafar, 2020). Under optimal conditions, i.e., sufficient geothermal heat, pressure, humidity from groundwater, and microbes in sediment layers, PLA decomposes naturally (Qi et al., 2017). Yet, outside of these conditions, the biodegradation process may take years or even decades (Xiao et al., 2012).
The PLA biodegradation occurs through several stages: deterioration, biofragmentation, assimilation, and mineralization (Jaiswal et al., 2020). In biofragmentation, the process starts with hydrolysis of the carbonyl group by penetrating water. However, the inherent surface hydrophobicity of PLA delays biodegradation by limiting water infiltration (Li et al., 2023). Additionally, PLA derived from crop starch, such as corn or sugarcane, experiences significant fluctuations in supply chains and raw material costs depending on market conditions, leading to instability in supply and low price competitiveness (Ma et al., 2012). One potential solution to these disadvantages is the fabrication of composite materials incorporating biomass.
Forest residue (FR), an underutilized byproduct of logging and forest management, is often discarded or left on-site. However, its use offers an opportunity to maximize biomass resource efficiency, reducing carbon debt (Tripathi et al., 2019). The FR is also more cost-effective than other lignocellulosic biomass, as it does not require separate cultivation and can be harvested at relatively low costs. Thus, utilizing FR efficiently processes surplus resources generated from forest management, promoting sustainable forest use. Systematic forest management and converting these byproducts into energy or chemical resources not only help maintain forest health but also enhance environmental value (Braghiroli and Passarini, 2020). Using large quantities of unused lignocellulosic biomass in biodegradable plastic composites can reduce resin demand, increase consumer awareness of eco-friendly products, and shorten biodegradation times by promoting moisture penetration and creating more initiation points for biodegradation (Raj et al., 2022). However, PLA/biomass composites often suffer from poor interfacial compatibility due to differences in their surface properties, particularly the hydrophilicity of biomass and the hydrophobicity of PLA, which causes reduced tensile strength (Wang et al., 2021).
To address this issue, the surface properties of biomass can be improved through torrefaction, a process that selectively decomposes the amorphous hemicellulose regions, thereby reducing the energy required for size reduction and facilitating even dispersion in the composite (Niu et al., 2019). Unlike traditional biomass carbonization, torrefaction occurs at lower temperatures, ranging from 200 to 300 ℃ in an oxygen free environment (Javanmard et al., 2023). This technique has been widely used to improve solid fuels for thermal power generation by removing moisture and volatile organic compounds (Abdulyekeen et al., 2021). The growing focus on carbon reduction and renewable energy has spurred recent interest in this field. The changes induced by torrefaction in biomass surface properties and grindability suggest its potential applications in industries beyond fuel, including plastic production (Javanmard et al., 2023). Additionally, utilizing biomass byproducts from forest activities supports resource recycling and contributes to a circular economy, offering environmentally friendly and sustainable solutions for energy and material production.
In this study, PLA was blended with torrefied forest residue (TFR), in which the amorphous regions were selectively decomposed. Torrefaction increased the crystallinity and hydrophobicity of the biomass surface, preventing a decrease in the PLA/biomass composite's physical properties while enhancing its biodegradability by facilitating water penetration.
The FR was obtained from the National Institute of Forest Science, Republic of Korea, and supplied as wood chips. These wood chips were ground using a cutting mill (PULVERISETTE 19, FRITSCH GmbH, Idar-Oberstein, Germany) equipped with a 0.5 mm sieve. The ground material was used for all analysis and experiments except for chemical composition analysis. The resulting wood powder was dried with a moisture content lower than 10% and stored at 4 ℃ until further use. The PLA, processed by mixing NatureWorks Ingeo Biopolymer 4032D and 4060D in a 9꞉1 ratio, was used to blend with the biomass to prepare composites.
The FR was fully dried at 105 ℃ for 24 h before torrefaction. Torrefaction was conducted at 220, 250, and 280 ℃ for 50 min in a carbonization furnace (Drying Engineering Inc., Gwangju, Republic of Korea) under a nitrogen gas flow rate of 50 mL/min and a temperature ramping rate of 3 ℃/min. The torrefied samples were pulverized for 5–15 min at 350 r/min in a planetary ball mill (PM 100, Retsch GmbH, Haan, Germany) using zirconium balls. The milled samples were then passed through a 100 mesh sieve using an electric sieve shaker to obtain fine particles. The torrefied biomass samples were designated as TFR220, TFR250, and TFR280, corresponding to the torrefaction temperatures of 220, 250, and 280 ℃, respectively.
The chemical compositions of FR and TFR were analyzed using National Renewable Energy Laboratory procedures. The samples were ground using a cutting mill (Pulverisette 19, FRITSCH GmbH, Idar-Oberstein, Germany) with a 0.2 mm sieve. High-performance liquid chromatography (Agilent 1200 Infinity, Agilent Technologies Korea Inc., Seoul, Republic of Korea), equipped with an Aminex 87H column (300 mm × 7.8 mm; Bio-Rad Laboratories, Hercules, California, USA), was used to identify monomeric sugars such as glucose, xylose, mannose, galactose, and arabinose, along with sugar derivatives.
The elemental composition of FR and TFR, including carbon, hydrogen, nitrogen, sulfur, and oxygen, was determined using an elemental analyzer (Flash 2000, Thermo Electron Corporation, Waltham, Massachusetts, USA). Oxygen content was calculated by subtracting the weight percentages of C, H, N, and S from the total.
Thermal decomposition characteristics of FR and TFR were investigated using a thermogravimetric analyzer (TGA-Q500, TA Instruments, New Castle, Delaware, USA). Approximately 10 mg of the sample was placed on a platinum plate and heated to 105 ℃ for 15 min to eliminate moisture. Subsequently, the temperature was increased from 105 to 800 ℃ at a rate of 20 ℃/min. After reaching the final temperature, the furnace was maintained at 800 ℃ for 15 min. The samples were then oxidized in air for 10 min. Thermogravimetric data were used to calculate the fixed carbon, volatile matter, and residual ash content based on mass changes during heating.
The surface morphologies of the FR and TFR were examined using field-emission scanning electron microscopy (FE-SEM, SUPRA 55VP, Carl Zeiss, Oberkochen, Germany). For the analysis, fully dried samples were fixed onto aluminium stubs using carbon tape. The samples were then sputter-coated with a Pt-Pd alloy to a thickness of ~10 nm. Surface images were obtained using a 2 kV electron beam to capture detailed structural features.
The water absorption capacities of PLA, FR, and TFR were evaluated following a drying process in an oven at 105 ℃ for 24 h. After drying, the samples were placed in a desiccator containing a 35% NaCl solution to maintain a relative humidity of 70%–80%. The sample weights were measured over 32 h to assess water absorption at specified intervals, allowing for a detailed comparison of the absorption properties.
Prior to processing, both PLA and the biomass were dried separately in a vacuum oven at 60 and 120 ℃, respectively. The PLA and biomass were mechanically mixed and melted to produce a composite containing 20% biomass (w/w, based on composite) using a twin-screw micro-compounder (Haake Minilab II, Thermo Fisher Scientific, Waltham, USA) equipped with an injection molding machine (Haake MiniJet, Thermo Fisher Scientific, Waltham, USA). The composite mixture was thoroughly mixed via internal circulation at 190 ℃ and 50 r/min for 3 min. The resulting mixture was injected into a mold heated to 100 ℃ at 50 MPa for 30 s. Dumbbell-shaped samples were formed according to to ASTM D638-2022 (the Definitive Guide to Plastic Tensile Testing, USA) and conditioned at a constant temperature (≤ 25 ℃) and humidity (≤ 30%) for 24 h. The PLA/biomass composites blended with FR and the respective TFR samples (TFR220, TFR250, and TFR280) were labelled as FR-C, TFR220-C, TFR250-C, and TFR280-C, respectively.
To analyze the fracture surfaces of PLA and the composites, the samples were first frozen in liquid nitrogen and then fractured. The fracture surface morphologies were observed using a field-emission scanning electron microscope (FE-SEM, MIRA3, Tescan, Brno, Czech Republic) with an accelerating voltage of 1 kV and a working distance of 16 mm. Samples were sputter-coated with a Pt-Pd alloy to a thickness of ~10 nm prior to analysis.
The water contact angle of the PLA/biomass composites was measured using a drop shape analyzer (DSA25, KRÜSS, Hamburg, Germany) with the sessile drop method. Water droplets of 10 μL were placed on the surface of the composites, and images were captured for 3 s after deposition to assess the water contact angle.
The tensile strength and elongation of the PLA/biomass composites were determined according to the ASTM D638-2022 using a 100 kN load cell on a universal testing machine (5984, Instron, Norwood, USA). The test was conducted with a 26 mm gauge length between grips and a constant strain rate of 10 mm/min. The tensile strength and elongation at break were calculated as the average of the test results from 10 specimens, excluding the highest and lowest values for accuracy.
The crystallization behavior of neat PLA and PLA/biomass composites was examined using differential scanning calorimetry (DSC, Discovery DSC, TA Instruments, New Castle, USA). All DSC analysis were performed under an N2 flow of 50 mL/min. The first heating scan was conducted over a temperature range below the thermal decomposition threshold at a rate of 10 ℃/min, and the temperature was held for 2 min to eliminate the thermal history. The furnace was then cooled to 20 ℃. In the second heating scan, temperatures ranged from 20 to 200 ℃ at the same heating rate. Thermal properties such as glass transition temperature (Tg), crystallization temperature (Tc), cold crystallization enthalpy (∆Hc), melting temperature (Tm), and melting enthalpy (∆Hm) were derived from the second heating scan curves. X-ray diffraction (XRD) analysis was performed to quantify the crystallinity of neat PLA and PLA/biomass composites. XRD patterns were acquired using an X-ray diffractometer (D8 ADVANCE with DAVINCI, Bruker, Massachusetts, USA) with CuKα radiation (λ = 0.154 06 nm), covering a 2θ range of 0°–75° at a scan speed of 1°/min. The crystallinity index was calculated using the Ruland-Vonk method (Ruland, 1961; Thygesen et al., 2005).
To assess biodegradability, the PLA/biomass composite was cryogenically ground to a particle size of smaller than 50 μm using a freezer mill (SPEX 6875D, SPEX, Metuchen, USA). The ground samples were tested for aerobic biodegradability under composting conditions, in collaboration with the Korea Textile Inspection and Testing Institute, a nationally accredited organization. An official report was issued based on the analysis. Biodegradability tests followed the ISO 14855-1-2012 (Determination of the Ultimate Aerobic Biodegradability of Plastic Materials Under Controlled Composting Conditions-Method by Analysis of Evolved Carbon Dioxide, Switzerland), comparing the degradation of neat PLA and the composite with cellulose (as a standard material) over a 90 d period. The theoretical CO2 output from the test materials was calculated using the following equation (Luo et al., 2019):
| $$ \text { Theoretical } \mathrm{CO}_2 \text { output }=\text { Dry mass of the test material } \times \text { Total organic carbon content } \times \text { Molecular weight ratio of } \mathrm{CO}_2 \text { to } \mathrm{C} $$ | (1) |
The biodegradation rate was calculated using the accumulated CO2 emissions as shown below (Luo et al., 2019):
| $$ \begin{aligned} & \text { Biodegradation rate }=\left(\text { Cumulative } \mathrm{CO}_2 \text { generated from the test material in the composting container }- \text { Average } \mathrm{CO}_2\right. \text { emitted } \\ & \text { from the control sample }) / \text { Theoretical } \mathrm{CO}_2 \text { output } \times 100 \% \end{aligned} $$ | (2) |
Fig. 1 shows photographs of the FR and TFR at different torrefaction severities. As the torrefaction severity increased, TFR exhibited a darker color, indicative of heat-induced carbonization, along with a noticeable reduction in particle size due to the decomposition of carbohydrates, particularly hemicellulose. The chemical composition and crystallinity of the biomass under varying torrefaction conditions were summarized in Table 1. The degradation of biomass accelerated with increasing torrefaction severity, primarily due to the breakdown of hemicellulose and structural sugars, such as xylan, galactan, and mannan. The FR, predominantly consisting of hardwood species, displayed a higher xylan content compared to its relatively low mannan content. During torrefaction, the xylan content in TFR significantly decreased due to the decomposition of heterogeneous polysaccharides such as glucuronoxylan, highlighting hemicellulose's high susceptibility to thermal degradation (Ozonoh et al., 2020).
| Sample | Structural sugar (%) | Total lignin (%) | Ash (%) | Recovery yield (%) | Crystalline region (%) | Amorphous region (%) | |||
| Glucan | Xylan | Galactan | Mannan | ||||||
| FR | 29.9 ± 1.0 | 10.5 ± 0.4 | 3.6 ± 0.2 | 1.6 ± 0.1 | 27.6 | 1.4 ± 0.1 | – | 24.9 | 75.1 |
| TFR220 | 32.2 ± 0.5 | 10.3 ± 0.4 | 3.5 ± 0.1 | 1.5 ± 0.2 | 37.6 | 0.6 ± 0.0 | 94.7 ± 0.9 | 40.1 | 59.9 |
| TFR250 | 33.3 ± 0.2 | 8.4 ± 0.3 | 3.2 ± 0.2 | 1.4 ± 0.1 | 41.6 | 1.5 ± 0.2 | 88.0 ± 1.0 | 42.5 | 57.5 |
| TFR280 | 32.1 ± 0.6 | 3.1 ± 0.1 | 3.0 ± 0.1 | 0.9 ± 0.1 | 53.1 | 2.0 ± 0.3 | 74.5 ± 1.1 | 35.4 | 64.6 |
| Notes: FR, forest residue; TFR220, TFR250, and TFR280, corresponding to the torrefied biomass samples with torrefaction temperatures of 220, 250, and 280 ℃, respectively. | |||||||||
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Interestingly, the glucan content showed a different trend compared to xylan, with the highest glucan concentration observed in TFR250. This suggested that as hemicellulose degraded, the relative glucose content increased, but further increased torrefaction severity eventually led to cellulose breakdown (Ong et al., 2021). The degradation of hemicellulose contributes to the rise in relative glucan content, while excessive torrefaction ultimately results in cellulose decomposition (Chen et al., 2019).
Torrefaction-induced changes in chemical composition also affected the morphological structure of the biomass. Fig. 1 illustrates the surface morphology of FR and TFR. During torrefaction, the dense outer surface layer covering the fiber bundles was removed, leading to roughened cell wall surfaces and the formation of porous structures (Ong et al., 2021). As the torrefaction severity increased, the destruction of fiber bundles and the deformation of cell walls became more obvious as seen in Fig. 1. These changes in cell wall structure were largely attributable to alterations in chemical composition during the torrefaction process (Lateef & Ogunsuyi, 2021). The collapse of cell wall structures and the exposure of rough surfaces with fibers and pores during torrefaction can be explained by hemicellulose degradation, a key component of lignocellulosic biomass (Adeleke et al., 2021). Hemicellulose plays an essential role in linking cellulose and lignin through lignin-carbohydrate complexes (Xie et al., 2024). The preferential degradation of hemicellulose, as shown by the chemical composition analysis, occurred before the breakdown of cellulose and lignin during torrefaction, leading to cell wall disruption (Ong et al., 2021). These structural changes are expected to facilitate better resin infiltration during composite formation, improving the physical adhesion between the biomass and the resin.
The crystallinity of biomass also fluctuated in response to torrefaction conditions (Table 1). Following torrefaction, the crystallinity of the biomass increased due to the preferential decomposition of the amorphous regions, which contained both volatile matter and hemicellulose (Cao et al., 2023). However, when torrefaction was performed under more severe conditions, cellulose degradation occurred in both amorphous and crystalline regions, leading to a decrease in the overall crystallinity (Cao et al., 2021).
The effects of torrefaction on the volatile matter, fixed carbon, and ash content were analyzed and shown in Table 2 and Fig. S1. The TFR exhibited lower volatile matter and higher fixed carbon content compared to FR, with these trends becoming more pronounced at higher torrefaction temperatures. The reduction in the volatile matter is largely due to the loss of O-containing functional groups during torrefaction, which decomposes at elevated temperatures (Oyebode & Ogunsuyi, 2021). Additionally, the increase in fixed carbon content is attributed to the higher lignin content in TFR. Lignin, compared to hemicellulose and cellulose, has a higher fixed carbon content due to its dense network of phenolic moieties formed by frequent hydrogen bonding interactions at high temperatures (Zakaria et al., 2023). These findings from the proximate analysis were consistent with the results of the ultimate analysis, which showed that TFR had a higher C content and lower O content than FR, particularly under harsher torrefaction conditions, leading to a reduced O/C ratio (Alvarez-Chavez et al., 2019). During torrefaction, hemicellulose and cellulose, rich in hydroxyl groups, underwent decomposition, particularly O-derived hydroxyl groups and functional groups with low thermal stability. This decomposition results in a lower O/C ratio (Li et al., 2021). Such chemical modifications, including an increase in fixed carbon and a reduction in the O/C ratio, alter the surface properties of biomass, contributing to increased hydrophobicity (Wang et al., 2022).
| Content (%) | FR (%) | TFR (%) | ||
| 220 ℃ | 250 ℃ | 280 ℃ | ||
| Volatile matter | 81.6 | 81 | 78.2 | 71.8 |
| Fixed carbon | 16.5 | 17.2 | 19.9 | 26.3 |
| Ash | 1.9 | 1.8 | 1.9 | 1.9 |
| C | 46.1 | 47.6 | 50.2 | 52.8 |
| H | 6.7 | 6.5 | 6.2 | 6.1 |
| N | 0.8 | 1.2 | 1.3 | 1.5 |
| S | 0.0 | 0.0 | 0.0 | 0.0 |
| O | 46.5 | 44.7 | 42.3 | 39.6 |
| O/C | 1.01 | 0.94 | 0.84 | 0.75 |
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Fig. 2 illustrates the moisture absorption properties of TFR in comparison to PLA and FR. Biomass is generally hygroscopic and readily absorbs water in environments with high humidity (Yu et al., 2022). The water absorption capacity of biomass is influenced by its components, with cellulose, a polymer of glucose molecules, being insoluble and immiscible in water, yet capable of forming strong hydrogen bonds with water molecules, leading to high water absorption (Mishra et al., 2024). Biomass also contains lignin and hemicellulose, key components of plant cell walls that differ in their water absorption capabilities. Hemicellulose, with numerous O-containing functional groups, has a higher water absorption capacity, while lignin has a comparatively lower capacity (Xu et al., 2023). The overall composition of biomass, including its structural sugars and lignin, plays a significant role in its moisture absorption behavior, and chemical alterations during torrefaction can significantly affect this property.
The PLA showed a gradual and minimal increase in moisture uptake over time. In contrast, FR rapidly absorbed moisture, reaching approximately 11.2% saturation after 16 h, indicating a substantially higher moisture uptake rate and capacity than PLA. The TFR, however, demonstrated a reduction in moisture uptake rate and capacity, with 9.4% saturation after 16 h. This decrease in water absorption was attributed to the increased hydrophobicity of biomass following torrefaction. The decomposition of carbohydrates and the loss of O-containing functional groups during torrefaction weaken the formation of hydrogen bonds between the biomass and water molecules, reducing its water absorption capacity (Yang et al., 2019). Despite this, torrefied biomass still has greater water absorption properties compared to PLA, which may influence the composite's physical properties and biodegradability.
Fig. 3 shows images of neat PLA and PLA/biomass composites along with FE-SEM images of their fracture surfaces. In composites blended with untreated FR (FR-C), the FR particles were clearly distinguishable. However, in composites blended with TFR, the color of the composite darkened as torrefaction severity increased, and the distinction between the TFR particles and PLA became less clear. This can be attributed to the weakening of the biomass's cell wall structure due to torrefaction, making the particles smaller and more easily dispersed during the blending process (Yu et al., 2019). In contrast, the fracture surface of neat PLA, as seen in the FE-SEM image, was very smooth. In the composites, the fracture surfaces exhibit rougher and more irregular textures. The FR composite (FR-C) showed smaller, well-defined pits and a relatively clean surface compared to the TFR composite (TFR220-C and TFR280-C). This was due to the lower compatibility between the surface properties of untreated FR and PLA. The poor interface between FR and PLA led to easier separation upon fracturing, resulting in clean, distinct pits on the fracture surface (Asadollahzadeh et al., 2022). Conversely, the surface properties of TFR, which were modified by the torrefaction process, enhanced compatibility with PLA, making it more difficult for the materials to separate during fracture. As a result, biomass detaches in cohesive masses, leading to larger pits and a more uneven fracture surface (Yang et al., 2024).
Blending materials with different surface properties typically reduces the compatibility and mechanical performance of the resulting composites (Mamunya et al., 2016; Awad et al., 2021). The contact angle of PLA/biomass composites offers insights into surface properties, such as surface energy, which is an indirect indicator of composite compatibility. As surface energy increases, the contact angle decreases, indicating a more hydrophilic surface. Biomass typically has a higher surface energy and is more hydrophilic than PLA (Zouari et al., 2022). Fig. 4a shows that the contact angle of FR-C decreased from 76.4° (neat PLA) to 74.0°, reflecting the influence of hydrophilic functional groups on the biomass surface, which increased surface energy (Silva et al., 2019).
In the case of TFR composites, as torrefaction intensity increased, the contact angle increased, indicating a decrease in surface energy. For example, the TFR220-C composite exhibited a contact angle of 76.1°, similar to that of neat PLA, suggesting improved compatibility due to more similar surface properties (Zuo et al., 2020). However, composites made from biomass torrefied under harsher conditions showed higher contact angles than neat PLA. This could be attributed to the loss of hydrophilic functional groups caused by excessive carbonization during torrefaction (Wei et al., 2020). Consequently, the reduced hydrophilicity of the biomass weakened its compatibility with PLA under more severe torrefaction conditions.
The compatibility between PLA and biomass plays a critical role in determining the mechanical properties of PLA/biomass composites. Improved compatibility enhances the mechanical performance of the composite, while poor compatibility often leads to a reduction in mechanical strength (Han et al., 2020). Fig. 4b shows the tensile strength and elongation at break for PLA/biomass composites containing 20% biomass under different torrefaction conditions. The FR-C composite exhibited a tensile strength of 58.7 MPa and an elongation at break of 1.6%, which were substantially lower than the tensile strength (69.7 MPa) and elongation at break (5.8%) of neat PLA. This reduced performance was attributed to the inadequate compatibility and dispersion of the untreated biomass within the composite, which impeded the uniform distribution of stress under external forces, leading to a decline in strength (Arteaga-Pérez et al., 2015).
However, this mechanical deterioration can be mitigated by modifying the surface characteristics of the biomass through torrefaction. Although most composites displayed low elongation due to the rigid and brittle nature of PLA, the TFR220-C, exhibiting a contact angle similar to neat PLA, demonstrated an improved tensile strength of 62.3 MPa, surpassing that of the FR-C. Torrefaction enhances the dispersion and compatibility of the composite by increasing the hydrophobicity of the biomass surface, thereby promoting better adhesion between the PLA matrix and the biomass (Ke et al., 2022). However, excessive torrefaction can degrade cellulose chains, resulting in weakened mechanical properties of the composite. Severe torrefaction conditions can exacerbate the damage to cellulose, further diminishing the mechanical strength of the composite (Wang et al., 2017). Therefore, torrefaction under optimal conditions improves the dispersion and compatibility of PLA and biomass in the composite, while maintaining mechanical integrity. This approach offers a practical method for utilizing biomass as a bulk-loading filler in PLA composites.
Fig. 5 presents the DSC heat flow curves and XRD patterns of neat PLA and PLA/biomass composites. The DSC heat flow curves revealed that the composites containing heterogeneous materials exhibited slightly lower Tm and slightly higher Tg compared to neat PLA. This shift in thermal properties is likely due to the interference of biomass particles with the arrangement of PLA chains and the intermolecular forces between them (Ruz-Cruz et al., 2022). From a thermodynamic perspective, crystallinity is closely related to crystallization energy, which represents the amount of energy required for the crystallization process (da Cunha et al., 2023). Generally, crystallinity and crystallization energy are proportional, meaning that materials with higher crystallization energy tend to exhibit greater crystallinity.
The Tc of neat PLA was 101.2 ℃, with a crystallization enthalpy (∆Hc) of 21.5 J/g (Fig. 5a). However, all PLA/biomass composites exhibited lower crystallization temperatures and enthalpies compared to neat PLA, regardless of the torrefaction conditions. In materials with lower crystallinity, the alignment of molecular chains is more hindered, necessitating weaker crystallization energy. Conversely, in highly crystalline materials, the increase in intermolecular interactions increases the overall energy required for crystallization. This phenomenon was further illustrated by the ∆Hm values. Melting enthalpy is often used as a predictor of material crystallinity. Neat PLA exhibited a ∆Hm of 37.0 J/g, higher than that of FR-C (30.3 J/g) and all TFR-C, which were below 30 J/g. Typically, materials with higher crystallinity have a denser molecular structure and stronger intermolecular bonds, requiring more energy to melt. These results suggested that torrefaction reduced the crystallinity of the biomass, which in turn affected the overall thermal and biodegradation properties of the PLA/biomass composites.
This phenomenon was further validated by the XRD patterns of the PLA/biomass composites (Fig. 5b). The most characteristic peak indicating the crystalline structure of PLA appeared at 2θ of 16.7°, corresponding to a crystallinity of 12.8%. This peak is associated with the α-form crystal structure, one of the stable crystalline forms of PLA (Liu et al., 2023). In contrast, the XRD patterns of the PLA/biomass composites show a blunted peak at 2θ of 16.7°, indicating a reduction in crystallinity. Notably, FR-C exhibited the lowest crystallinity (2.9%), while the TFR-C composites showed a slight improvement in crystallinity. This suggested that the addition of biomass disrupted the crystallization of PLA, increasing the crystallization energy of the composite and ultimately leading to a decrease in its overall crystallinity.
Fig. 6 illustrates the biodegradability of neat PLA and TFR220-C. The standard material (cellulose) reached 100% biodegradability after 90 d, confirming the validity of the biodegradability measurement method. Neat PLA demonstrated a biodegradability of 33.3% at 45 d and 88.8% at 90 d. In contrast, TFR220-C showed a faster initial decomposition rate, reaching 54.0% after 45 d and 94.9% after 90 d. Biomass, being biodegradable in natural environments, facilitates the activities of microorganisms and enzymes, which is further enhanced by the water absorption properties of biomass. This increased water permeability helps initiate hydrolysis in the composite, accelerating the biodegradation process. Furthermore, the reduction in crystallinity observed in the composite contributed to the enhanced biodegradation. Typically, crystalline regions have denser molecular structures that slow down biodegradation by hindering access to microorganisms and enzymes. As shown in Fig. 5, the addition of TFR220 disrupted crystallinity formation, which increased the initial degradation rate and improved overall biodegradability.
This study investigated the effects of torrefaction on FR and its application as a bulk-loading filler in PLA/biomass composites. The following key findings emerged: (1) Torrefaction at various temperatures (220, 250, and 280 ℃) altered the chemical composition of FR by increasing fixed carbon and reducing volatile matter, while also improving crystallinity; (2) The process enhanced the hydrophobicity of FR, which improved compatibility in PLA/biomass composites; (3) The TFR improved the tensile strength of the composites, although excessive torrefaction negatively impacted mechanical properties; (4) The addition of TFR accelerated the biodegradability of PLA/biomass composites by reducing crystallinity and increasing water permeability, leading to faster decomposition compared to neat PLA. These results demonstrated the potential of TFR in creating eco-friendly and biodegradable materials for sustainable applications.
Acknowledgments: This work was supported by the Korea Research Institute of Chemical Technology (KRICT) Project (No. KS2442-10) and the R & D Program (No. 20017973) of the Ministry of Trade, Industry, and Energy (MOTIE/KEIT).|
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Figures(6) / Tables(2)
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June-Ho Choi, Myeong Rok Ahn, Chae-Hwi Yoon, Yeon-Su Lim, Jong Ryeol Kim, Hyolin Seong, Chan-Duck Jung, Sang-Mook You, Jonghwa Kim, Younghoon Kim, Hyun Gil Cha, Jae-Won Lee, Hoyong Kim. Enhancing compatibility and biodegradability of polylactic acid/biomass composites through torrefaction of forest residue[J]. Journal of Bioresources and Bioproducts, 2025, 10(1): 51-61. doi: 10.1016/j.jobab.2024.10.003
| Sample | Structural sugar (%) | Total lignin (%) | Ash (%) | Recovery yield (%) | Crystalline region (%) | Amorphous region (%) | |||
| Glucan | Xylan | Galactan | Mannan | ||||||
| FR | 29.9 ± 1.0 | 10.5 ± 0.4 | 3.6 ± 0.2 | 1.6 ± 0.1 | 27.6 | 1.4 ± 0.1 | – | 24.9 | 75.1 |
| TFR220 | 32.2 ± 0.5 | 10.3 ± 0.4 | 3.5 ± 0.1 | 1.5 ± 0.2 | 37.6 | 0.6 ± 0.0 | 94.7 ± 0.9 | 40.1 | 59.9 |
| TFR250 | 33.3 ± 0.2 | 8.4 ± 0.3 | 3.2 ± 0.2 | 1.4 ± 0.1 | 41.6 | 1.5 ± 0.2 | 88.0 ± 1.0 | 42.5 | 57.5 |
| TFR280 | 32.1 ± 0.6 | 3.1 ± 0.1 | 3.0 ± 0.1 | 0.9 ± 0.1 | 53.1 | 2.0 ± 0.3 | 74.5 ± 1.1 | 35.4 | 64.6 |
| Notes: FR, forest residue; TFR220, TFR250, and TFR280, corresponding to the torrefied biomass samples with torrefaction temperatures of 220, 250, and 280 ℃, respectively. | |||||||||
DownLoad:
CSV
| Content (%) | FR (%) | TFR (%) | ||
| 220 ℃ | 250 ℃ | 280 ℃ | ||
| Volatile matter | 81.6 | 81 | 78.2 | 71.8 |
| Fixed carbon | 16.5 | 17.2 | 19.9 | 26.3 |
| Ash | 1.9 | 1.8 | 1.9 | 1.9 |
| C | 46.1 | 47.6 | 50.2 | 52.8 |
| H | 6.7 | 6.5 | 6.2 | 6.1 |
| N | 0.8 | 1.2 | 1.3 | 1.5 |
| S | 0.0 | 0.0 | 0.0 | 0.0 |
| O | 46.5 | 44.7 | 42.3 | 39.6 |
| O/C | 1.01 | 0.94 | 0.84 | 0.75 |
DownLoad:
CSV
| Sample | Structural sugar (%) | Total lignin (%) | Ash (%) | Recovery yield (%) | Crystalline region (%) | Amorphous region (%) | |||
| Glucan | Xylan | Galactan | Mannan | ||||||
| FR | 29.9 ± 1.0 | 10.5 ± 0.4 | 3.6 ± 0.2 | 1.6 ± 0.1 | 27.6 | 1.4 ± 0.1 | – | 24.9 | 75.1 |
| TFR220 | 32.2 ± 0.5 | 10.3 ± 0.4 | 3.5 ± 0.1 | 1.5 ± 0.2 | 37.6 | 0.6 ± 0.0 | 94.7 ± 0.9 | 40.1 | 59.9 |
| TFR250 | 33.3 ± 0.2 | 8.4 ± 0.3 | 3.2 ± 0.2 | 1.4 ± 0.1 | 41.6 | 1.5 ± 0.2 | 88.0 ± 1.0 | 42.5 | 57.5 |
| TFR280 | 32.1 ± 0.6 | 3.1 ± 0.1 | 3.0 ± 0.1 | 0.9 ± 0.1 | 53.1 | 2.0 ± 0.3 | 74.5 ± 1.1 | 35.4 | 64.6 |
| Notes: FR, forest residue; TFR220, TFR250, and TFR280, corresponding to the torrefied biomass samples with torrefaction temperatures of 220, 250, and 280 ℃, respectively. | |||||||||
| Content (%) | FR (%) | TFR (%) | ||
| 220 ℃ | 250 ℃ | 280 ℃ | ||
| Volatile matter | 81.6 | 81 | 78.2 | 71.8 |
| Fixed carbon | 16.5 | 17.2 | 19.9 | 26.3 |
| Ash | 1.9 | 1.8 | 1.9 | 1.9 |
| C | 46.1 | 47.6 | 50.2 | 52.8 |
| H | 6.7 | 6.5 | 6.2 | 6.1 |
| N | 0.8 | 1.2 | 1.3 | 1.5 |
| S | 0.0 | 0.0 | 0.0 | 0.0 |
| O | 46.5 | 44.7 | 42.3 | 39.6 |
| O/C | 1.01 | 0.94 | 0.84 | 0.75 |
DownLoad:
