The Role of Residual Lignin in Microfibrillated Cellulose in Properties of Polylactic Acid Biocomposites

TL;DR

This study shows that residual lignin in microfibrillated cellulose improves its performance when combined with polylactic acid, enhancing mechanical and thermal properties of biocomposites.

Contribution

The study demonstrates that residual lignin in lignin-containing MFC improves dispersion and interfacial interactions in PLA composites.

Findings

Higher lignin content in LMFC increased dispersion in PLA from 24.2% to 35.1%.

LMFC improved tensile strength and elastic modulus while reducing elongation at break.

Residual lignin enhanced interfacial interactions, as shown by dynamic mechanical analysis.

Abstract

Microfibrillated cellulose (MFC) derived from wood sources is a biodegradable and eco-friendly reinforcing material for polymer composites. However, the high polarity of MFC is a challenge in homogeneous distribution into the hydrophobic PLA matrix, which limits its reinforcing efficiency. In this study, lignin-containing MFC (LMFC) with different residual lignin contents was prepared to investigate its dispersion behavior and reinforcing effect in polylactic acid (PLA). The aspect ratio and neutral sugar composition of LMFC remained similar regardless of lignin content, whereas the dispersion degree in PLA, quantified using a log-normal distribution model, increased from 24.2% to 35.1% with increasing lignin content. Mechanical testing showed that LMFC incorporation enhanced tensile strength and elastic modulus while reducing elongation at break. Higher residual lignin content in LMFC positively affected the tensile strength of the LMFC–PLA composites. Dynamic mechanical analysis revealed an increase in storage modulus and a decrease in loss factor with higher lignin content and LMFC loading (1–10 wt%), indicating enhanced interfacial interactions. Differential scanning calorimetry showed reductions in glass transition temperature (5–8 °C) and cold crystallization temperature (8–16 °C) compared to neat PLA. These findings indicate that residual lignin in LMFC enhances dispersion and interfacial interactions in PLA, leading to improved mechanical and thermal performance and highlighting its potential as an effective reinforcing component in sustainable biocomposites.

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1. Introduction

Composite materials enhance the mechanical performance of a polymer matrix through the incorporation of reinforcing phases, which enable the material to withstand external loads [1,2,3,4,5]. Typical reinforcements include carbon fibers, glass fibers, and aramid fibers [6,7], where morphological properties such as fiber diameter, aspect ratio (length-to-diameter), isotropic and anisotropic arrangements, and the number of fibers, such as single fibers or fiber bundles, strongly influence stress transfer, crack initiation/propagation, and the overall failure response of the composite. Owing to these characteristics, fibrous materials are used as reinforcements to achieve high strength at comparatively low loadings and are employed in various industries. However, these fibers are non-renewable, not environmentally friendly, consume significant amounts of energy during the manufacturing process, and have harmful effects on the human body [8,9,10]. These constraints motivate the exploration of sustainable, biomass-derived reinforcement solutions.

In response, various studies on utilizing natural fibers as reinforcing materials are in progress, and among them are fibers derived from wood, which exhibit excellent performance, processability, and cost advantages. Moreover, as a reinforcement material, wood fibers can improve the mechanical properties of polyolefins with low thermal properties. The reinforcement properties of the fibers have been reported to be affected by the shape of the particles, which is determined during processing, or the compatibility of hydroxyl groups present on the surface [11,12,13,14].

However, as interest in environmental issues increases, studies on using reinforcement materials to improve physical and thermal properties, while adding biodegradability, have recently been conducted not only on polyolefins but also on biodegradable matrices such as PLA [15,16,17,18]. Tingaut et al. [19] used MFC, which has a high aspect ratio and good mechanical properties, as a reinforcing material for PLA. MFC was acetylated to improve its compatibility with PLA, resulting in improved dispersion and thermal properties of the final composite. Furthermore, Winter et al. suggested that microfibrillated cellulose (MFC), microfibrillated wood (MFW), and microfibrillated lignocellulose (MFLC) can be used as reinforcement materials. In this case, composites were prepared by extrusion using a double screw extruder, and the experimental results showed that both MFW and MFLC exhibited improved dispersion in PLA, but there was no significant difference in tensile strength [20]. Notably, despite the prevalence of lignin in lignocellulosic fibers and its potential to affect interfacial behavior, systematic investigations into the role of residual lignin content on both mechanical reinforcement and dispersion of lignin-containing wood fibers in PLA composites remain limited.

In addition to mechanical reinforcement and improved dispersion, cellulose-based fillers have also been reported to affect the thermal and thermo-mechanical behavior of PLA composites [21]. Gregor-Svetec et al. [22] showed that PLA filaments reinforced with nanofibrillated cellulose and lignin-modified nanofibers exhibited changes in storage modulus and crystallization behavior, indicating modified thermo-mechanical stability compared to neat PLA. Similarly, other studies [23,24] on PLA/lignin-containing nanocomposites have demonstrated that lignin incorporation can enhance thermal degradation resistance and modify decomposition mechanisms, depending on the type and amount of lignin used.

Prior work on the dispersion method for MFC in PLA has shown that improved dispersion can be achieved through processing methods such as injection molding, thereby enhancing tensile strength [25]. Additionally, the applicability of well-dispersed MFC–PLA composites to additive manufacturing has been confirmed by preparing specimens via 3D printing and evaluating their mechanical and thermal properties, demonstrating their feasibility as a filament [26]. Although dispersion methods and surface modification have been explored for cellulose-based reinforcements, the specific influence of residual lignin content in wood-derived microfibrils on the performance of PLA composites has not been comprehensively elucidated. In particular, earlier reports primarily focused on processing feasibility or general mechanical enhancement, without quantitatively correlating residual lignin content with dispersion uniformity and thermo-mechanical performance while maintaining comparable fibril morphology. Therefore, it remains unclear whether residual lignin acts merely as a polarity modifier or as an active interfacial component that governs stress transfer and viscoelastic behavior in PLA composites. Based on these considerations, it was hypothesized that increasing the residual lignin content in lignin-containing microfibrillated cellulose (LMFC) would improve its dispersion within the PLA matrix by modifying interfacial interactions, thereby enhancing the mechanical and thermal properties of the resulting composites. To address this hypothesis, the present study investigates the effect of residual lignin content in the LMFC by evaluating the mechanical and thermal properties of PLA composites with controlled lignin levels. Furthermore, dispersion was quantitatively evaluated by statistical analysis of the LMFC free-path spacing using a log-normal distribution model (D_0.2_).

2. Materials and Methods

2.1. Materials

PLA, in pellet form (2002D, NatureWorks LLC, Minneapolis, MN, USA), was used after drying for 2–3 days in a vacuum drying oven. The specific gravity, according to ASTM D792 [27], is 1.24, and the melting point is 210 °C. Radiata pine wood chips were used to manufacture MFC. The Klason lignin content was measured according to TAPPI 222 om-02 and was found to be 28.1 ± 0.9%. All measurements were conducted in triplicate.

2.2. Fabrication of Lignin-Containing Microfibrillated Cellulose (LMFC)

Organosolv pulping was used to prepare LMFC, which contains lignin. The reaction was carried out at 120 °C for 120 min in an autoclave with a liquid ratio of 1:2 (v/v) using a wood chip and glycol ether sulfuric acid mixed reagent (glycol ether: H_2_SO_4_ = 97:3 (v/v)). After the reaction, the mixture was filtered using a solution of 0.5 N NaOH, acetone, and distilled water. The yield of the prepared pulp was 60.2 ± 0.7% and the Klason lignin content was 25.1 ± 0.4% (L20). As a pretreatment step for LMFC production, a laboratory Valley beater (DM-822, Daeil Machinery Co., Ltd., Daejeon, Republic of Korea) was used to beat until a Canadian Standard Freeness (CSF) of 100 mL was achieved. The pulp slurry concentration was 2%, prepared by passing the pulp through a Super Masscolloider (MKCA6-2, Masuko Sangyo Co., Ltd., Kawaguchi, Japan) three times. The gap size between the grinder’s stones (MKGA 6-46, Masuko Sangyo, Kawaguchi, Japan) was adjusted to −90 ± 5 µm. Figure 1a presents SEM images illustrating the morphology of the LMFC.

To control the lignin content of the prepared LMFC, lignin was removed using the chlorite method. First, 1 g of sodium chlorite (extra pure grade, Daejung Chemicals & Metals Co., Ltd., Siheung, Republic of Korea) and 0.2 mL of extra acetic acid (Pure grade, Duksan Pure Chemical Co., Jecheon, Republic of Korea) were added as a slurry at a concentration of 3% to 10 g of LMFC. After 60 min of stirring in a constant-temperature water bath at 70 °C, an additional 1 g of sodium chlorite and 0.2 mL of glacial acetic acid were added to adjust the lignin content. To control lignin content, chlorite delignification was performed for 60 and 90 min to obtain partially delignified samples with different residual lignin levels. These conditions yielded Klason lignin contents of 8.4 ± 0.3% (L8) and 3.9 ± 0.2% (L4), respectively. The reaction was filtered under reduced pressure with distilled water, and the filtrate was washed until neutral (Figure 1b).

2.3. Fabrication of LMFC–PLA Filament

The solvent exchange method proposed by Beaumont et al. [28] was used to prepare LMFC powder via solvent substitution. tert-Butyl alcohol (TBA, extra pure, Daejung, Korea) was selected as a substitution solvent. tert-Butyl alcohol was added at a weight ratio of 50 times the total LMFC weight, with a solid content of approximately 10%, and the mixture was stirred for 12 h. After stirring, the supernatant was removed by centrifugation and freeze-dried (TFD5505A, ilShinBioBase, Gyeonggi, Republic of Korea). The freeze-dried LMFC was dispersed at concentrations ranging from 1–10 wt% of the total filament weight. After homogenizing in dichloromethane using an experimental homogenizer (Homogenizer, HG-15A, DAIHAN Scientific Co., Ltd., Wonju, Republic of Korea), PLA was added and stirred occasionally until all the PLA had dissolved. Dichloromethane was removed by solvent casting. The filament extruder (Filabot EX2 filament extruder, Triex^®^ LLC, Barre, VT, USA) equipped with a single screw was employed. The screw had a diameter of 2.85 mm and a length/diameter ratio of 12. The extrusion nozzle temperature was 154 °C, with an extrusion speed of 17.5 rpm, resulting in a filament diameter of 1.7 mm (Figure 2).

Tensile strength and dynamic mechanical analysis (DMA) specimens were prepared from 3D printed LMFC–PLA filaments. A 3D printer (Moment 1, Moment Co., Ltd., Guri, Republic of Korea) was used with a nozzle temperature of 180 °C using a prepared 1.7 mm filament. The specimens for tensile testing, in accordance with ASTM D638 [29], were prepared by 3D printing in five layers to a thickness of 1 mm without any void space between the layers. For the DMA test, 3 layers were deposited to a thickness of 0.4 mm. The 3D printing was layered perpendicularly to the printing direction. Tensile strength and DMA specimens are illustrated in Figure 3.

Dynamic mechanical analysis (DMA) was performed using a DMA 8000 (PerkinElmer, Inc., Waltham, MA, USA). The measurements were carried out at 1 Hz over a temperature range from −10 °C to 120 °C, with a heating rate of 2 °C/min.

The thermal properties of the 3D printed LMFC–PLA specimens were evaluated using differential scanning calorimetry (DSC, Discovery 25, TA Instruments, New Castle, USA). Approximately 5 mg of the sample was loaded into the Tzero pan and lid. The initial temperature was set to 20 °C, and the samples were heated to 200 °C at 5 °C/min to determine the phase transition temperatures, including the glass transition temperature (T_g_), cold crystallization temperature (T_cc_), and melting temperature (T_m_). The degree of crystallinity was determined as follows: X_c_ (%) = (ΔH_m_ − ΔH_c_)/ΔH_f_. Here, ΔH_f_ is the theoretical heat of fusion of 100% crystalline PLA, which is 93 J/g [30]. ΔH_m_ is the enthalpy of melting, and ΔH_c_ is the enthalpy of cold crystallization. The observed variations were below 3%.

2.4. Morphological and Chemical Properties of LMFC

To determine the morphological characteristics of the LMFC, the aspect ratio of the LMFC was calculated as the length (l) divided by the average width (d). The LMFC was dispersed in deionized water and observed by an optical microscope (BX 50, Olympus Optical Co. Ltd., Tokyo, Japan). The number of LMFC samples (n) was 300 for each LMFC (L4, L8, and L20). The length was measured once, and the diameter was averaged after three measurements. The chemical composition of the sugar was analyzed by neutral sugar analysis (alditol-acetate method), in which the pretreated samples were dissolved in 2 mL of acetone and analyzed by gas chromatography (HP-6890, Agilent, Santa Clara, CA, USA).

2.5. Dispersion Quantity (D0.2)

The dispersion degree of LMFC was measured by the following method. First, 10 wt% LMFC-reinforced PLA filaments were cut at 10 cm intervals, and the cross-sections were observed with FE-SEM (SU8220, Hitachi Ltd., Tokyo, Japan) at a magnification of 500×. Prior to SEM observation, the samples were sputter-coated with platinum to prevent charging, and the images were acquired at an accelerating voltage of 5 kV. The distance between LMFCs was measured manually as a horizontal linear distance using the method of Luo et al. [31] and expressed as a log-normal distribution. The dispersion degree was quantified using a dispersion quantity, denoted D_0.2_, calculated from the log-normal distribution using Equations (1)–(4).

$$[eqn]$$

and

$$[eqn]$$

Here, μ is the mean particle free-path spacing and σ is the standard deviation. The log mean (m) and log standard deviation (n) are calculated.

$$[eqn]$$

In the above expression (Equation (3)), x indicates the size of the free-path spacing. Lastly, dispersion quantity (D) includes all spacing between a certain percentage of µ:

$$[eqn]$$

3. Results and Discussion

3.1. Morphology and Chemical Properties of LMFC

The aspect ratio of the reinforcement has been reported to enhance the mechanical properties of the composite material by dispersing it within the range of 10 to 80 [32]. According to Cheng et al. [33], for high tensile strength, the critical fiber length is the maximum strength per unit length of the fiber. Moreover, this approach remains effective until the fibers reach an aspect ratio of 50, which is sufficient to allow cutting while still maintaining stress in the composite material [34]. Using chlorine bleaching to remove residual lignin minimized the decrease in cellulose’s degree of polymerization [35]. Also, bleaching to control the residual lignin content in the LMFC may remove lignin from the fiber surface, forming pits [36]. Despite differences in lignin content, the LMFC aspect ratio remained in a similar range, approximately 19.9–26.6 (Figure 4).

Neutral sugar analysis was performed to determine the change in sugar composition (Table 1). The results indicated that glucose loss was reduced by approximately 1% when the total equivalent weight was 100%. Therefore, there was no significant difference in the form or chemical composition of the LMFC except for the residual lignin content and, therefore, it was determined that it could be used as a reinforcing material for PLA-based materials.

3.2. Dispersion Quantity

Upon evaluating the 3D printed specimens, no defects were visually observed; however, the color darkened as the LMFC content increased. This color change is attributed to the residual lignin in the LMFC. As shown in Figure 5, the lignin incorporated in the composite material ranged from 0.05 wt% to 2.0 wt% in the LMFC–PLA composite material and from 1 wt% LMFC (L4) to 10 wt% LMFC (L20) in PLA composite material. A uniform dispersion is important for LMFCs to exhibit reinforcing effects. Therefore, the dispersion quantity was calculated by using a log-normal distribution to quantify the dispersion degree. Figure 6 shows the distance between the LMFCs in an LMFC–PLA composite as a probability on a smooth curve based on a frequency function. It was found that, as the residual lignin content increased from L4 to L20, the mean value and standard deviation decreased (Table 2). A decreasing standard deviation indicates that the data are close to the mean, and thus the LMFC maintains a uniform distance. However, since these results are estimates from the population, it is necessary to estimate the variance (D_0.2_) using probability [31]. This behavior may be attributed to changes in PLA melt fluidity induced by residual lignin on the LMFC surface.

3.3. Tensile Strength

The mechanical properties of the 3D printed LMFC–PLA composites are summarized in Table 3 and Figure 7. While Figure 7 highlights variations in ultimate tensile strength, Table 3 presents the modulus of elasticity and elongation at break, which are key indicators of stiffness and ductility, respectively. As shown in Table 3, incorporation of LMFC into the PLA matrix resulted in a pronounced increase in the modulus of elasticity compared to neat PLA (1.56 ± 0.04 GPa). Depending on LMFC type and loading level, the modulus increased to 2.17 ± 0.09 GPa. This enhancement reflects the reinforcing contribution of cellulose fibrils and is consistent with previous reports showing that the mechanical properties of PLA-based composites are strongly influenced by the relative contributions of cellulose, lignin, and hemicellulose in lignocellulosic reinforcements [37]. In contrast, elongation at break decreased progressively with LMFC incorporation, indicating a transition toward more brittle behavior. Whereas neat PLA exhibited an elongation at break of 7.2 ± 0.9%, LMFC-reinforced composites showed substantially reduced values in the range of approximately 3–5%. Such a trade-off between stiffness and ductility is commonly observed in fiber-reinforced polymer systems and has been attributed to the restricted mobility of polymer chains and interfacial stress concentration during deformation [38]. Previous studies [39,40] have reported that residual lignin reduces polarity mismatch with PLA, thereby improving dispersion and enhancing load transfer within the composite matrix, improving tensile performance. Although dispersion was not directly correlated with tensile strength on a per-sample basis, the non-linear dependence of tensile strength on LMFC content and its correlation with residual lignin content suggest that the dispersion state of LMFC likely plays an important role in governing mechanical performance. The ultimate tensile strength of PLA composites incorporating LMFC at loadings of 1–10 wt% with different residual lignin contents is presented in Figure 7. Tensile strength increased with additional LMFC loading from 3–5 wt%. In addition, the higher ultimate tensile strength was obtained with an increase in Klason lignin content of LMFC from 4% to 10%. The results indicate that the higher residual lignin promotes the mechanical properties of the LMFC–PLA composite, along with the reinforcing effect of the microfibrillated structure of LMFC. In the previous study, the mechanical performance of LMFC–PLA composites is influenced not simply by filler content or fibril geometry but also by the dispersion state of the reinforcing phase [41]. Effective dispersion facilitated by residual lignin, therefore, appears to be an important factor enabling efficient stress transfer and enhanced mechanical performance in the LMFC–PLA system.

3.4. Dynamic Mechanical Analysis

The storage modulus (log E’) and loss factor (tan δ) were measured to investigate the viscoelastic properties of LMFC–PLA composites during heating (Figure 8). In the rubbery stage at 110 °C, a higher storage modulus was observed in the LMFC–PLA composites rather than in neat PLA. In addition, increasing the LMFC loading from 1 to 10 wt% resulted in an increased storage modulus. This suggests that LMFC plays a reinforcing role, maintaining the modulus at higher temperatures. Moreover, the stabilized storage modulus at increased temperature was attributed to the cold crystallization by incorporating LMFC, compared to the neat PLA. At 10 wt% LMFC loading, the storage modulus was 3631.3 MPa for L4-10, 3549.5 MPa for L8-10, and 4626.9 MPa for L20-10. The L20-10 sample has a higher log-normal distribution (35.1%) than L4-10 (24.2%), resulting in better high-temperature resistance, aided by improved LMFC dispersion.

For the loss factor (tan δ), defined as the ratio of loss modulus to storage modulus, a higher tan δ indicates greater viscous energy dissipation compared to the elastic energy during deformation [42]. All LMFC–PLA composites show a lower peak intensity of the loss factor compared to neat PLA. This result is attributed to the incorporation of LMFC influenced by the restricted segmental mobility of PLA. In a previous study, nanocellulose fibrils in PLA decreased the intensity of the loss factor due to the strong interfacial adhesion between reinforcement and matrix material [40]. In addition, the loss factor also decreased when the LMFC loading content was increased from 1 wt% to 10 wt%. The loss factor is influenced by the number of mobile units, related to the relaxation phenomenon [43]. Overall, LMFC–PLA composites were more resistant than neat PLA near the glass transition temperature.

3.5. Differential Scanning Calorimetry

The phase transition temperatures of the LMFC–PLA composite are presented in Figure 9 and Table 4, for the relative comparison of LMFC loading effect. The T_g_ decreased by approximately 5–8 °C, while the T_cc_ decreased by approximately 8–16 °C with the addition of LMFC, compared to the neat PLA. These results indicate that the low-molecular-weight lignin and hemicellulose components of LMFC, prepared by organosolv pulping, could impact crystallization kinetics and nucleating activity. As the LMFC content increased, T_g_ increased by 1–2 °C while T_cc_ decreased by 4–7 °C. In addition, the substrate’s phase changes due to the hydrogen bonding between the phenolic hydroxyl group of lignin and the carbonyl group of PLA are consistent with the literature [44,45]. Compared to neat PLA, double melting peaks were observed in LMFC–PLA composite. Even though the hydrogen bonding occurs between the LMFC and PLA, the heterogeneity of the LMFC remains in the composite materials, leading to the additional melting peak in the LMFC–PLA composite. These results confirm that the constituents of the LMFC have an effect on the thermal properties of PLA. However, this needs to be improved through further investigation since the PLA has enthalpic relaxation and crystal reorganization behavior during heating.

Overall, the sample with higher residual lignin content (L20) showed better dispersion within the PLA matrix and improved mechanical properties. In terms of filler loading, 3–5 wt% LMFC offered a more balanced improvement.

4. Conclusions

This study investigated the mechanical and thermal behavior of PLA composites reinforced with LMFCs (L4, L8, and L20) possessing similar aspect ratios, with particular emphasis on the influence of residual lignin content. The degree of LMFC dispersion within the PLA matrix, quantified using a log-normal distribution model, increased from 24.2% to 35.1% with increasing lignin, demonstrating that lignin contributes to improved fiber distribution within the PLA matrix. Enhanced dispersion and higher LMFC loading led to increased tensile strength and elastic modulus, accompanied by reduced elongation at break. Additionally, PLA composites incorporated with a higher residual lignin content of LMFC showed increased tensile strength. Dynamic mechanical analysis showed increases in the storage modulus and decreases in the loss factor with higher lignin and LMFC content, indicating enhanced viscoelastic stability. In addition, DSC analysis revealed reductions in the T_g_ and T_cc_ of PLA, with more pronounced effects at higher lignin levels. These results indicate that residual lignin in LMFC acts not merely as a passive filler but as an active component that enhances dispersion and strengthens interfacial interactions, thereby simultaneously improving the mechanical and thermal properties of PLA composites. These findings highlight the potential of LMFC as a sustainable and effective reinforcing component for high-performance biocomposites. Future work will further investigate LMFC surface chemistry and its influence on interfacial interactions, as well as the effect of lignin type on composite performance.