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. 2026 Mar 12;11(11):17329–17338. doi: 10.1021/acsomega.5c09503

Optimizing Sustainable Poly(lactic acid) (PLA) Composites: A Comprehensive Study on the Impact of Thermoplastic Starch (TPS) and α‑Cellulose Ratios

Pisit Dhamvithee †, Nawadon Petchwattana ‡, Weeraya Thong-on §, Wasinee Channuan §,*
PMCID: PMC13019243  PMID: 41908402

Abstract

Poly­(lactic acid) (PLA) composites were developed by a melting extrusion technique with different thermoplastic starch (TPS) and α-cellulose ratios (30:0–15:15), while keeping constant PLA content and PLA-grafted glycidyl methacrylate (PLA-g-GMA) compatibilizer content of 60 and 10 wt %, respectively. PLA-g-GMA, which had a titrated epoxy content equivalent to 12.72%, improved interfacial adhesion by causing a nucleophilic ring-opening reaction between its epoxy functional groups and the hydroxyl functionality of hydrophilic fillers (TPS or α-cellulose) to form covalent ether linkages. Although homogeneous dispersion was achieved by this mechanism at low levels of α-cellulose loading, concentrations ≥10 wt % resulted in the reaggregation of the filler and worse morphological defects. Performance analysis showed considerable “trade-offs”: α-cellulose-free composite (PLA/T30/C0) maintained tensile strength between composites and neat PLA, while much higher increases in elongation at break value were observed (3.29 ± 0.21%); the reaggregation at higher α-cellulose loadings led inversely to a significant drop in both tensile strength and brittle behavior of biocomposites. Thermally, the hydrophilic fillers played the roles of heterogeneous nucleating agents, greatly enhancing crystallinity degree (X c up to 22.7%) but also causing residual moisture, inducing hydrolytic degradation and lowering slightly thermal stability (T max) as well as giving rise to rheological instability due to a high MFI increase up to 58% upon moderate loadings. The important features are that the addition of hydrophilic materials such as cellulose significantly reduced the barrier property to water vapor, which is 7.1 times higher than that of neat PLA. In addition, the migration investigation conducted on the best PLA/T30/C0 formula revealed that it exceeded the maximum regulatory limit (30 mg/dm3) for total dissolved substances in distilled water (30.2 mg/dm3) and 20% ethanol simulants (38.2 mg/dm3). These poor barrier and migration properties of the composite are such that it cannot be commercialized as a high-moisture or alcohol-containing food, despite having been tested as compliant with fatty foods. Thus, although the increased ductility of the TPS-only composite and its susceptibility to hydrolytic degradation resulted from high water absorption offer advantages for applications in which fast deterioration under natural conditions is required, contributing to sustainable waste management (e.g., horticultural pots), it is not well suited for food packaging where protection against moisture is required.

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1. Introduction

Conventional plastics have been applied on a large scale, and recycling rates are quite low; therefore, inefficient disposal poses heavy pressure on environmental sustainability and hinders progress on the global Sustainable Development Goals (SDGs), especially SDG12 (Responsible Consumption and Production) as well as challenges the progress of SDG13­(Climate action). Thus, biodegradable plastics have been developed as environmentally friendly solutions for waste reduction and promoting the circular bioeconomy. , Poly­(lactic acid) (PLA), one of the most promising biodegradable thermoplastics, is endowed with both a biocompatible nature and high-strength mechanical properties. Its universal utilization in the packaging field, however, is restricted due to its shortcomings: the production cost is too high, and it has poor impact strength and a low crystallization rate. − For cost-effectiveness and an increase in functionality, the blending of PLA with its rich counterparts in polysaccharides (e.g., thermoplastic starch (TPS), cellulose, etc.) is being reported. −

However, these blends typically have poor interfacial adhesion because of the polarity difference between the hydrophobic PLA matrix and hydrophilic fillers, despite their economic merits. This incompatibility usually results in phase separation and poor mechanical and barrier properties. , Reactive compatibilization via glycidyl methacrylate (GMA) is widely adopted to bridge these phases. − As a bifunctional monomer, GMA contains epoxy moieties that react with the hydroxyl and carboxyl groups of the biopolymers, while its acrylic group enables free-radical grafting onto polymer chains. Although GMA-grafted PLA (PLA-g-GMA) has been reported to enhance interfacial adhesion and thermal stability in binary blends and biocomposites, work on its efficiency as a compatibilizer in ternary PLA/TPS/cellulose systems is limited.

The presence of highly hydrophilic TPS has brought a dual functionproviding nutrition for microorganisms to proliferate and promoting the ductility/elongation at break of the matrix, thus accelerating the breakdown of PLA. , In compatibilized PLA/TPS blends, an amount of 20–30 wt % TPS has been stated to be optimal in terms of compromise between toughness, strength, and film-formability, but without excessive phase separation. − Meanwhile, α-cellulosemicrocrystalline cellulose (MCC) or cellulose nanocrystals (CNCs)provides a significant strength effect owing to its high tensile strength and modulus. Cellulose nanomaterials (CNMs) are also subject to low loadings (<10 wt %) to avoid aggregation. MCC was successfully encapsulated at 5 wt % to enhance modulus, without affecting the other mechanical properties. , Moreover, the addition of 10–30 wt % lignocellulosic fibers (e.g., bamboo flour or banana fibers) together with 10–15 phr PLA-g-GMA was found to give better phase morphology and mechanical properties than noncompatibilized systems, without losing processability for normal melt compounding. ,

In this work, the cooperation mechanisms of TPS and α-cellulose in ternary composites are systematically studied. GMA was grafted-copolymerized with PLA as a compatibilizer by a free-radical reaction initiated with benzoyl peroxide (BPO). The TPS/α-cellulose ratio was modified in order to adjust the microstructure of the material, keeping the PLA content fixed at 60 wt %. Biocomposites were produced and characterized in order to understand how phase morphology influenced the mechanical performance and barrier characteristics. This methodology addresses a gap in the literature regarding practical and affordable packaging solutions.

2. Materials and Methods

2.1. Materials

Poly­(lactic acid) (PLA) (Ingeo Biopolymer 4043D) was obtained from NatureWorks LLC. Thermoplastic starch (TPS) (TAPIOPLAST TPS) was purchased from SMS, Thailand. Glycidyl methacrylate (GMA) and benzoyl peroxide (BPO) were obtained from Tokyo Chemical Industry, Japan. α-Cellulose was obtained from Sigma-Aldrich. All chemicals such as potassium hydroxide (KOH), hydrochloric acid (HCl), dichloromethane (CH2Cl2), and methanol (CH3OH) were of analytical research grade and supplied by RCI Labscan Limited, Thailand.

2.2. Material Preparation

2.2.1. Preparation of PLA Grafted with GMA (PLA-g-GMA)

Poly­(lactic acid)-graft-glycidyl methacrylate (PLA-g-GMA) was synthesized by the melt grafting method via a twin-screw extruder (LUH-206; Chareon TUT Co., Thailand) using BPO as the initiator. The PLA pellets were dried at 85 °C for 4 h for the removal of moisture prior to grafting. According to the literature, ,, the contents of GMA and BPO were 10 wt % of PLA and 0.5 wt % of PLA, respectively. The GMA liquid and BPO were added to the dried PLA pellets, mixed, and then processed in the extruder. The operation was performed at a screw speed of 115 rpm. The temperature of the extruder barrel was gradient, from 100 to 170 °C with equal intervals (100 °C, 155 °C, 160 °C, 165 °C, and 170 °C), where the die head was maintained at 175 °C. The extrudate was cooled in a water bath and pelletized to obtain PLA-g-GMA pellets. The success that required GMA grafting was further confirmed through Fourier transform infrared (FTIR) analysis and chemical titration.

2.2.2. Preparation and Film Fabrication of PLA Composites

The procedure for composite preparation included two steps: compounding and film formation. For the compounding process, TPS, PLA, and PLA-g-GMA were first predried in a hot-air oven at 85 °C for 4 h. They were dry mixed, with mixing by weight at the ratios in Table . The compound was then processed in a twin-screw extruder with a temperature profile ranging from 70 to 190 °C and a screw speed of 117 to 121 rpm. The resulting extrudate was cooled in a water bath, pelletized, and again dried for 2 h at 80 °C. The second step, in making the composite film, involved feeding dried pellets into a single screw extruder with a cast film line (CF-W350; Chareon TUT Co., Thailand). This was carried out at temperatures ranging from 60 to 150 °C and a screw speed of 65–81 rpm. The PLA composite sheet films obtained had thicknesses between 0.20 and 0.40 mm.

1. PLA Composites Consisting of PLA-g-GMA, PLA, TPS, and α-Cellulose in Different Weight Ratios.
code PLA-g-GMA (wt %) PLA (wt %) TPS (wt %) α-cellulose (wt %)
PLA/T30/C0 10 60 30 0
PLA/T25/C5 10 60 25 5
PLA/T20/C10 10 60 20 10
PLA/T15/C15 10 60 15 15
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2.3. Evaluation and Characterization

2.3.1. Grafting Degree of PLA-g-GMA

The degree of grafting of PLA-g-GMA was evaluated by chemical titration, which was used to calculate the number of open epoxide rings. A 1.5 g sample of the PLA-g-GMA film was refluxed in 100 mL of dichloromethane at 30 °C for 30 min. Then, 20 mL of the resulting solution was pipetted into an Erlenmeyer flask. 10 mL of 0.1 M hydrochloric acid in methanol solution (HCl/CH3OH) was immediately added to open the epoxide rings on the GMA chains. The resulting solution was then titrated with 0.1 M potassium hydroxide in methanol (KOH/CH3OH). The degree of grafting of PLA-g-GMA was calculated using the following eq .

grafting degree(%)=M×(V−V0)×Cm×1000×100 1

where V and V 0 stand for the consumed volumes (mL) of KOH/CH3OH solution in the case of grafted and blank, respectively; C is the molar concentration (mol/L) of titrant; M is the molecular weight of GMA 142.15 (g/mol); and m represents mass (g) used from the PLA-g-GMA sample aliquot.

2.3.2. Fourier Transform Infrared Spectrometry (FTIR) Characterization

The FTIR spectra of PLA, PLA-g-GMA, and its composites were recorded with an attenuated total reflectance (ATR) attachment for identifying the chemical structure. The spectra were collected with a Nicolet iS5 FTIR spectrometer (Thermo Fisher Scientific, Thailand) in the range of wavenumbers from 4000 to 400 cm–1.

2.3.3. Scanning Electron Microscopy (SEM)

The surface morphology of PLA and PLA composite films was investigated by using SEM (JSM-IT300LV, JEOL Ltd., Japan). The samples were coated with gold before analysis to acquire conductive images. The micrographs were taken at a 15 kV accelerating voltage.

2.3.4. Differential Scanning Calorimetry (DSC)

DSC (DSC25) from TA Instruments was employed for thermal analysis under a nitrogen atmosphere. The heating–cooling–heating cycle was carried out in a normal procedure. At first, the samples were heated from room temperature to 200 °C at a heating rate of 10 °C/min and maintained isothermally at this temperature for 5 min in order to erase the thermal history from previous processing. Then, the samples were cooled to −50 °C at a speed of 10 °C/min. The last scan again heated the samples from −50 °C up to 200 °C at 10 °C/min. The data for this second heating were employed to investigate the thermal behavior and chain segment motion of the material.

The degree of crystallinity (X c) was determined by the second heating scan and calculated according to eq .

Xc(%)=(ΔHmΔH°m)×100 2

where ΔH m is the enthalpy of melting from the DSC second heating scan (J/g) and ΔH°m is the theoretical enthalpy of melting for a 100% crystalline PLA sample, which was known to be 93.0 J/g.

2.3.5. Thermogravimetric Analysis (TGA)

The thermal stability of the samples was evaluated by a thermogravimetric analyzer (TGA 550, TA Instruments). About 10 mg of the sample was analyzed in a temperature range between 50 and 600 °C. The TGA measurement was carried out under nitrogen flushing at 50 mL/min with a heating rate of 10 °C/min.

2.3.6. Melt Flow Index (MFI)

MFI was determined using a melt flow index testing machine (MFR1 from Chareon TUT, Thailand), in accordance with the ASTM D1238. The test conditions applied were 190 °C and a load of 2.16 kg. The extrudate was collected and weighed, and the MFI values for grams per 10 min (g/10 min) were determined. Each sample was measured three times.

2.3.7. Mechanical Properties

The tensile properties of the films were measured using a Hounsfield H50KS universal testing machine (Calserve, Thailand) according to ASTM D882–10. The machine was equipped with a load cell of 1000 kN, a gauge length of 80 mm, and a crosshead speed of 50 mm/min. Tests were performed on ten replicates of each sample in the form of 15 mm × 120 mm specimens.

2.3.8. Water Absorption

The water uptake characteristics of the films were tested according to Pulgarin et al. The film samples were cut into 2 cm × 2 cm squares and first dried at 50 °C for 24 h. After obtaining the dry weight (W 0), the films were immersed in distilled water for 6 h at room temperature. The surface moisture was removed by gently draining, and the final weight (W t) of the wet films was determined. The experiment was performed in triplicate. The water absorption by weight (%) was determined using eq .

water absorption(%)=Wt−W0W0×100 3

where W t is the weight of films after the specified immersion time interval (g) and W 0 is the initial dry weight of the film sample (g).

2.3.9. Water Vapor Permeability (WVP)

WVP of the films was calculated based on the water vapor transmission rate (WVTR) measured with a WVTR tester (C360M, Kinetics Corporation Ltd., Thailand), according to the ASTM E96–00 Desiccant method. Circular film samples (with a diameter of 7 cm and an average thickness of about 0.4–0.6 mm) were first air-dried at room temperature for 24 h. Each test cup was then sealed with a specimen adhered to the mouth of the cup, and a molecular sieve (the desiccant) was placed inside the cup. The sealed cup was subsequently transferred to a conditioning chamber at 23 °C and 85% relative humidity (RH). WVP values were determined in triplicate for each film.

2.3.10. Migration Testing for Food Contact Materials

Only the PLA composite with the greatest mechanical strength was submitted to migration tests because this material would require more challenging conditions of use. The test was conducted in accordance with the JETRO 2008 procedures and testing methods for implements, containers, and packaging. This procedure characterized the amount of nonvolatile residue remaining after the evaporation of the simulant and indicates overall migration. Specific migration limits were determined based on the toxicological hazard of single or related substances. To simulate actual conditions for products used at temperatures lower than 100 °C, the measurement was performed at 60 °C for 30 min. Migration of the PLA composite’s chemical migrants into four food simulants was performed (as described in Table ). Four film samples, each measuring 6 cm × 10 cm, were tested with each simulant.

2. Food Simulant Used for Migration Testing.
food simulant type of food
distilled water food pH > 5
acetic acid 4% (v/v) food pH < 5
ethanol 20% (v/v) alcoholic food
n-heptane fatty food
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2.4. Statistical Analysis

All of the experiments were conducted in triplicate across three batches. All values were expressed as the mean ± standard deviation (SD). Statistics were computed by using the SPSS program (version 27). The significance of differences between all of the means was determined by one-way analysis of variance (ANOVA) and followed by least significant difference (LSD) for post hoc analysis. A p-value ≤ 0.05 was considered statistically significant.

3. Results and Discussion

3.1. PLA Grafting with GMA (PLA-g-GMA)

FTIR spectroscopy was applied to monitor chemical changes in the GMA grafting. Spectra of neat GMA, neat PLA, and the synthesized product (PLA-g-GMA) were depicted in Figure . Neat GMA exhibited a characteristic absorption peak at around 1637 cm–1 attributed to CC stretching of the methacrylate vinyl group. This peak is very characteristic and provides a reference point for the unsaturation. This unique peak was largely consumed when GMA was reacted with PLA under thermal and free-radical (BPO) initiation during twin-screw extrusion in the resulting spectra of PLA-g-GMA. A decrease in this peak shows that vinyl groups are involved in free-radical reactions associated with melt processing. ,−

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FTIR spectra of GMA, PLA, and PLA-g-GMA.

In addition to the loss of the vinyl signal, significant modifications in other parts of the spectrum were seen. In particular, the synthesized PLA-g-GMA still had a unique absorption peak at about 905 cm–1 attributed to the C–O–C asymmetric stretching vibration of the epoxy ring. , This is important because it means that the three-membered epoxy rings were not fully consumed during the initial melt processing. Instead, they were still chemically intact and maintained reactivity for subsequent reactions in the composite formation. The existence and intensity of this epoxy peak are essential to the mechanisms of compatibilization, as also discussed below.

3.2. Chemical Titration of GMA Content

The epoxy functionality of the final product was measured by chemical titration, yielding a grafting degree of 12.72% for the sample containing 10 wt % GMA. This number indicates that a large proportion of epoxy groups was retained after the melt processing, thus showing that GMA-derived functionalities were successfully introduced into the polymer matrix. However, the measure indicates only unreacted epoxy groups and does not give a picture of their distribution or bonding state.

Depending on the system employed, the titrated epoxy groups may be (i) covalently and directly grafted to the PLA chains (the wanted grafting structure), (ii) self-polymerization of GMA segments physically dispersed through the polymer, or a combination of both, as common in free-radical systems. Since titration only measures the reactive groups, it is not possible to distinguish between theman inherent limitation of chemical determination rather than an experimental flaw.

The spectroscopic data indicate that grafting has been achieved. The absence of the CC band and the presence of the epoxy peak at 905 cm–1 support that the GMA monomer is reacted without significantly affecting its reactive group. These FTIR properties are in accordance with the suggested grafting mechanism, but FTIR alone cannot determine the molecular position of grafted units.

3.3. Behavior of PLA-g-GMA in Composite Mixtures

The epoxy signal at 905 cm–1 was progressively attenuated or disappeared when PLA-g-GMA was added into the composite formulations of TPS and α-cellulose (Figure ). This observation suggests that during the melt compounding, some of the residual epoxy groups in PLA-g-GMA underwent reaction. The reaction has been suggested to proceed by nucleophilic ring-opening of the strained epoxide by hydroxyl groups in TPS and α-cellulose, resulting in interfacial ethers (C–O–C) and the generation of new secondary hydroxyls. These interactions can act as chemical bridges between the PLA matrix and the polar fillers, resulting in reactive compatibilization. ,

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FTIR spectra of PLA-g-GMA and PLA composites.

Despite the inherent analytical limitations in distinguishing grafted moieties from homopolymers, the combined results of titration and FTIR provide strong indirect evidence of a reactive functionality. Furthermore, as will be demonstrated in the morphological analysis (Section and Figure ), the effective dispersion of the filler phase at low loadings serves as functional evidence supporting the successful interfacial modification by the synthesized agent.

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SEM micrographs at 15 kV and 1000× magnification of (a) PLA, (b) PLA/T30/C0, (c) PLA/T25/C5, (d) PLA/T20/C10, and (e) PLA/T15/C15.

Furthermore, Figure shows a significant reduction in the intensity of the carbonyl (CO) stretching vibration peak at 1747 cm–1 upon incorporation of α-cellulose. This change points to the presence of favorable intermolecular hydrogen bonding between the PLA matrix and the α-cellulose reinforcing phase. Although the CO shift is indicative of beneficial physical interactions (hydrogen bonding), the covalent linkages formed by the epoxy groups establish a stronger chemical bonding mechanism (as implied by the disappearance of 905 cm–1). This dual-interaction system, driven by strong covalent grafting, is the key factor behind the significant enhancement in the overall performance of composites.

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FTIR spectra of PLA and PLA composites.

3.4. Morphology of the PLA Composites Surface by SEM

Figure displays the SEM images of neat PLA and PLA composites, which were primarily evaluated to assess the dispersibility and distribution of α-cellulose within the polymer matrix. The neat PLA exhibits a typically smooth fracture surface, whereas the surface roughness of the PLA composites increases proportionally with the α-cellulose content. SEM images at low α-cellulose loadings (0–5 wt %) reveal excellent encapsulation and homogeneous dispersion of the filler. This optimal morphology is consistent with the proposed mechanism, where the epoxy groups of the PLA-g-GMA compatibilizer chemically react with the abundant hydroxyl (−OH) groups on the α-cellulose surface, leading to the formation of strong covalent linkages and robust interfacial adhesion. Conversely, SEM images captured at higher α-cellulose loadings (≥10 wt %), which show significant agglomerated sites and a proportional increase in surface roughness, confirm the detrimental effects of excessive filler concentration. These morphological features verify two critical issues: (1) The large agglomerates act as stress concentration points within the composite matrix, which can prematurely initiate crack propagation and reduce the material’s overall mechanical integrity; , and (2) the presence of gaps and voids at the interface between the large agglomerates and the polymer matrix is clear evidence of deterioration in interfacial compatibility and adhesion. This poor bonding severely compromises the load transfer efficiency, leading to inferior mechanical performance in the high-loading composites, ,, as quantitatively confirmed by the tensile results in Section (Figure ). The shift from homogeneous dispersion (where chemical bonding is dominant) at low loadings to large agglomerates (where physical interactions dominate) at high loadings underscores the limitation of the available compatibilizer (PLA-g-GMA) relative to the total surface area of the filler particles. ,,,

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(a) Tensile strength; (b) elastic modulus; and (c) elongation at break of PLA and PLA composites.

3.5. Thermal Property

3.5.1. DSC Analysis

Analysis of the thermal properties (Table ) revealed a complex interplay between the components: a simultaneous decrease in the glass transition temperature (T g) from neat PLA (58.8 °C) to the composites (56.0–56.5 °C) and crystallization temperature (T c from 125.2 to 116.3 °C), alongside a significant increase in the degree of crystallinity (X c) (from 12.3% to 22.7%). This counterintuitive relationship (increased chain mobility (T g reduction) coupled with increased crystalline ordering (X c increase)) is resolved by recognizing two independent mechanisms operating on distinct polymer phases. First, the plasticization effect dominates the amorphous phase: the significant T g depression (56.0 −56.5 °C) is attributed to the migration of small molecules (such as glycerol from TPS) into the PLA matrix. These molecules increase the free volume and segmental mobility of the PLA chains by reducing intermolecular forces. ,− Second, the decrease in T c and the significant acceleration of X c are governed by heterogeneous nucleation. Both the rigid TPS and α-cellulose particles act as nucleating agents, providing energetically favorable surfaces for crystallization. ,, This effect drastically accelerates the crystallization kinetics, allowing crystallization to occur more rapidly and at a lower temperature (T c is reduced), resulting in a substantially higher final overall crystallinity (X c) than neat PLA. Thus, thermal behavior confirms that plasticization controls the mobility of the amorphous fraction, while heterogeneous nucleation controls the kinetics and extent of crystallization.

3. Thermal Transition Temperatures of PLA and PLA Composites .
polymer T g (°C) T c (°C) ΔH c (J/g) T m (°C) ΔH m (J/g) X c (%) T onset (°C) T max (°C) T end (°C)
PLA 58.8 125.2 10.1 149.6 11.4 12.3 297.9 320.1 329.4
PLA/T30/C0 56.4 117.3 17.0 147.7 19.6 21.1 297.2 312.8 327.6
PLA/T25/C5 56.5 119.9 16.7 148.9 18.5 19.8 297.8 312.2 326.2
PLA/T20/C10 56.0 119.0 18.4 148.1 20.0 21.5 297.4 316.1 331.7
PLA/T15/C15 56.2 116.3 19.8 147.9 20.1 22.7 295.6 313.5 329.9
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X c: Degree of crystallinity.

3.5.2. TGA

The thermal stability analysis, quantified by the maximum degradation temperature (T max), demonstrated a consistent, albeit slight, reduction across all composite formulations compared to neat PLA (decreasing from 320.1 °C to a range of 312.2–316.1 °C) (see Table ). This overall decrease in thermal robustness is mechanistically attributed to accelerated hydrolytic degradation of the PLA matrix during high-temperature melt processing. The incorporation of highly hydrophilic components, specifically TPS and α-cellulose, introduces residual moisture, which acts as a potent catalyst for the chain scission of PLA ester bonds. , This molecular weight reduction compromises the inherent thermal stability of the matrix. While the reactive compatibilization via PLA-g-GMA is intended to enhance interfacial adhesion (which can potentially delay decomposition), the dominant factor dictating T max remains the severe degradation induced by the hydrophilic nature of the additives. Furthermore, the marginally lower degradation onset temperature (T onset) observed in the high α-cellulose content sample (PLA/T15/C15, 295.6 °C) may also reflect the initiation of thermal decomposition due to structural defects or residual traces of the radical initiator (BPO) used during compounding.

3.6. Melt Flow Index (MFI)

MFI data (presented in Table and Figure ) display complex rheological behavior governed by competing mechanisms. Neat PLA (5.19 ± 0.80 g/10 min) and the TPS-only composite (PLA/T30/C0, 5.02 ± 0.24 g/10 min) showed no significant difference in terms of flow properties (p > 0.05). However, at moderate α-cellulose loadings (5 to 10 wt %), the MFI values increased significantly (p ≤ 0.05), peaking at 8.20 ± 0.58 g/10 min for PLA/T20/C10a 58% increase relative to neat PLA. This remarkable improvement of flowability is ascribed to chain scission taking place on a great scale during melt processing. The hydrophilic nature of TPS and α-cellulose brings the remaining water in the matrix to act as a strong catalyst for PLA ester bond hydrolysis, according to the high extruder head temperature. , This decrease in the molecular weight is the major factor for the initial lowering of melt viscosity. Conversely, at the highest α-cellulose loading (15 wt %), the MFI sharply decreased to 6.04 ± 0.31 g/10 min. This drop is characterized by a critical sharp change in rheology and is also observed directly through SEM evidence (Figure ) as massive reagglomeration of the α-cellulose particles. Given the high volume fraction of hard agglomerates, a percolating immobile particulate network forms in the matrix, creating a strong rheological hindrance that slows PLA chain mobility, leading to an increase in melt viscosity. The poor effect of the PLA-g-GMA compatibilizer at 15 wt % α-cellulose is notable here, due to the overwhelming filling surface area at this loading, which obscures any compatibility effect and leads to severe agglomeration that will dominate the rheological response over that of reacted polymer degradation. This dramatic MFI variability (from 5.19 ± 0.80 to 8.20 ± 0.58 and back to 6.04 ± 0.31 g/10 min) also reflects the rheological instability of the system, which poses a significant consideration for commercial processing and demands strict control over degradation and filler dispersion.

4. MFI Values of PLA and PLA Composites .

polymer MFI (g/10 mim)
PLA 5.19 ± 0.80b
PLA/T30/C0 5.02 ± 0.24b
PLA/T25/C5 7.37 ± 1.03a
PLA/T20/C10 8.20 ± 0.58a
PLA/T15/C15 6.04 ± 0.31b
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a

a and b denote the mean of the vertical data with significant differences (p ≤ 0.05).

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MFI values of PLA and PLA composites.

3.7. Mechanical Properties

From Table , the mechanical properties of the composites further demonstrate the interrelationship between the PLA matrix and the added components. The tensile strength (Figure a) of the TPS-only composite (PLA/T30/C0, 42.54 ± 3.29 MPa) showed no statistically significant change from that of neat PLA (43.34 ± 5.88 MPa). However, the addition of α-cellulose led to a significant reduction in strength, with the lowest values (17.78 ± 1.71 MPa to 20.72 ± 2.53 MPa) recorded at high loadings (≥10 wt %). This deficiency is consistent with the serious reagglomeration of α-cellulose in similar SEM images (Figure ). This is due to the excessively large agglomerates formed, which act as stress concentration points and have weak interfacial adhesion, resulting in premature failure. This supports our conclusion that α-cellulose is unable to act as an effective reinforcing filler at high-loading concentrations due to poor load transfer.

5. Mechanical Properties of PLA and PLA Composite Films .

polymer tensile strength (MPa) elongation at break (%) elastic modulus (MPa)
PLA 43.34 ± 5.88a 2.31 ± 0.15b 2560 ± 400.31a
PLA/T30/C0 42.54 ± 3.29a 3.29 ± 0.21a 2070 ± 176.78b
PLA/T25/C5 25.44 ± 2.76b 2.19 ± 0.23b 1480 ± 185.07c
PLA/T20/C10 17.78 ± 1.71c 1.53 ± 0.08c 1452 ± 66.11c
PLA/T15/C15 20.72 ± 2.53c 1.62 ± 0.11c 1544 ± 285.18c
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a

a–c denote the mean of the data in the vertical direction with significant differences (p ≤ 0.05).

The Elastic modulus (Figure b) exhibited a statistically significant and consistent reduction across all composite compositions compared to neat PLA (2560 ± 400.31 MPa), dropping to a minimum of 1452 ± 66.11 MPa. This widespread matrix softening contradicts the expected stiffening effect from rigid α-cellulose particles (which have a modulus up to 140 GPa). This compelling evidence indicates that substantial degradation of PLA chains (thermal scission) occurs during melt compounding, promoted by the BPO activator and heat, completing a hiding process that overlays any possible stiffening effect from the fillers.

In terms of ductility, the inclusion of TPS alone (PLA/T30/C0) resulted in a considerable toughening impact, with an elongation at break (EAB) increased to 3.29 ± 0.21% from 2.31 ± 0.15% of neat PLA (Figure c). On the other hand, the addition of α-cellulose suppressed this reinforcement, and EAB decreased markedly (1.53 ± 0.08% to 1.62 ± 0.11%), thus corroborating that the high-loading composite was more brittle. This reduction is reflected in the solid-like nature of α-cellulose, more specifically the cellulose agglomerates, which limit polymer mobility and accelerate crack propagation due to retarded large-scale plastic deformation.

The mechanical property evaluation indicated that PLA/T30/C0 has the same tensile strength as neat PLA, but with enhanced flexibility. Thus, in this investigation, PLA/T30/C0 was chosen to evaluate the food contact material and assess its potential for future packaging development.

3.8. Water Sorption and Permeability Results

Table displays the water uptake (WU) and water vapor permeability (WVP) of PLA and the PLA composites. The water barrier characteristics, comprising both WU and WVP, were strongly correlated and verified the very hydrophilic nature of the additives. The maximum WU was observed for the α-cellulose-free composite, PLA/T30/C0 (9.44 ± 0.97%), representing a significant 6.5-fold increase compared to neat PLA (1.45 ± 0.40%). While α-cellulose-containing compositions (e.g., PLA/T15/C15) showed lower WU values (3.91 ± 0.41%) than the PLA/T30/C0, their values remained 2.6–2.7-fold higher than neat PLA, confirming the inherent hydrophilicity of the filler. This chemical behavior, caused by the presence of highly polar hydroxyl (−OH) groups on TPS and on α-cellulose, provides direct moisture sorption through strong hydrogen bonding. ,, Moreover, this high water content can promote the hydrolysis of PLA ester linkages, putting at risk the long-term mechanical performance of the composite. ,

6. Water Uptake and Water Vapor Permeability of PLA and PLA Composites .

polymer water uptake (%) water vapor permeability (ng/m·s·Pa)
PLA 1.45 ± 0.40d 0.013 ± 0.00c
PLA/T30/C0 9.44 ± 0.97a 0.086 ± 0.01a
PLA/T25/C5 3.81 ± 0.40c 0.092 ± 0.01a
PLA/T20/C10 6.79 ± 0.82b 0.074 ± 0.01ab
PLA/T15/C15 3.91 ± 0.41c 0.067 ± 0.01b
Open in a new tab
a

a–d denote the mean of the vertical data with significant differences (p ≤ 0.05).

Similarly, WVP was directly correlated with WU. The highest WVP (0.092 ± 0.01 ng/m·s·Pa) was observed for the PLA/T25/C5 composite, resulting in 7.1 times less vapor barrier properties compared to neat PLA. Although the WVP slightly decreased under high α-cellulose content (PLA/T15/C15), this indicates that there might be some tortuous path effect due to rigid particles. The WVP was still significantly increased (5.2-fold compared with neat PLA). Importantly, the structural data from SEM (reagglomeration of incompatible α-cellulose at high concentration) could explain this poor barrier performance. The interfacial voids, gaps, and porosity induced by such defects, in turn, serve as fast transport channels for water vapor, which leads to a large flow of water vapor molecules through the material and results in severe destruction of the polymer’s nonspecific moisture barrier properties.

The significant changes in WVP and WU suggest a key compromise for potential packaging applications. The severe 7-fold increase in WVP compared to neat PLA suggests that the composite’s natural moisture barrier effectiveness is drastically degraded. Therefore, such material would not be suitable for the packaging of products requiring high moisture protection (e.g., dry food, electronics), as it would result in poor product shelf life. Nevertheless, this loss of barrier performance is correlated with the addition of hydrophilic components (TPS and α-cellulose) that work in synergy to compensate for their disadvantages by improving poor sustainability and inciting rapid material degradation. This combination of properties indicates that the composite is most suitable for short-term preservation packaging or products with medium moisture sensitivity. Additionally, the material is suited for applications where rapid breakdown in the presence of moisture is desired, such as horticultural articles (e.g., seedling pots) or certain disposable single-use products designed for environmental exposure.

3.9. Migration Tests

The PLA/T30/C0 formulation was chosen for the measurement of overall migration (60 °C for 30 min) to assess its appropriateness as a food contact material, where residues were extracted in the four standard simulants listed in Table . The migration values into distilled water (30.2 mg/dm3) and 20% ethanol (38.2 mg/dm3) exceeded the TIS (TIS 655 Part 1–2010; No. 435/2022) permissible limits of migration for Thai regulatory laws, which are set at 30 mg/dm3. Degradation is mainly due to the extensive migration of hydrophilic components of TPS (e.g., glycerol plasticizer) , and low molecular weight PLA oligomers. On the other hand, the PLA composite showed partial compliance in n-heptane (0 mg/dm3) and successfully passed the 4% acetic acid test (27.2 mg/dm3), ensuring safe contact only with high-fat or certain weakly acidic food substances. Accordingly, the material cannot be used for packaging high-moisture, high-sugar, or alcoholic foods and does not merge with dry food contact or non-food-contacting articles.

7. Migration Tests into Food Simulants.

food simulant residue content (mg/dm3) color release (mg/dm3) maximum permitted concentrations (mg/dm3)
distilled water 30.2 <30
4% acetic acid 27.2 <30
20% ethanol 38.2 <30
n-heptane at 25 °C for 60 min 0.0 <30
Open in a new tab
a

According to the maximum permitted concentration of PE, PP, PS, and PET.

4. Conclusion

This work systematically explores the synergistic effect of TPS and α-cellulose on the properties of PLA biocomposites with PLA-g-GMA acting as a reactive compatibilizer. The presence of reactive epoxy groups on the PLA backbone was indicated by FTIR and titration analyses, and the grafting degree was 12.72%.

Morphological observation indicated that PLA-g-GMA significantly improved the interfacial adhesion by enhancing the nucleophilic ring-opening reaction between its epoxy groups and the hydroxyl-functionalized fillers, TPS or α-cellulose, to form covalent bonds. This led to a uniform distribution of α-cellulose filler at low concentrations (0–5 wt % %), but at 10 wt %, the amount of incorporated filler surpassed the effectiveness of having a fixed concentration level commensurate with compatibilizer concentration, hence maximum reagglomeration. Accordingly, the α-cellulose-free composition (PLA/T30/C0) maintained the tensile strength of neat PLA and displayed considerably increased flexibility (3.29 ± 0.21% elongation at break), whereas increasing levels of α-cellulose rendered the films brittle while decreasing their ability to resist tension as a consequence of points susceptible to stress concentration induced by agglomerate formation.

Concerning thermal and rheological properties, the addition of hydrophilic fillers resulted in higher residual moisture that promoted faster hydrolytic degradation of the PLA matrix. This chain scission caused a uniform decrease of the T max and the MFI ranging up to 58%, indicating an urgent process control requirement. In contrast, α-cellulose proved to be an effective heterogeneous nucleating agent, producing drastic increases in matrix crystallinity (X c) up to 22.7% and reductions of the crystallization temperature (T c).

In spite of the structural improvements, the incorporation of TPS and α-cellulose reduced the water vapor barrier performance by 7.1 times compared to neat PLA. Moreover, the migratory test of the best PLA/T30/C0 formulation showed that the amount of total dissolved substances in both distilled water (30.2 mg/dm3) and 20% ethanol solutions (38.2 mg/dm3) exceeded the legal migration limit (30 mg/dm3). Although the composite had partial compliance in n-heptane (fatty food contact) and mildly acidic foods, these significant migration and barrier deficiencies make it effectively unsuitable for commercial packaging of high-moisture, high-sugar, or alcoholic food. Accordingly, although the good ductility and disintegration tendency induced by moisture sensitivity of the material may be more preferred in quick-deteriorating products such as short-life packaging products or non-food-contacting articles (e.g., horticultural containers) over longer-term physical barrier and stability requirements, more in-depth studies to ascertain the long-term stability of retention and barrier, including under realistic high-humidity and refrigerated storage conditions, are required before full commercial qualification for any long-duration application.

Acknowledgments

This research project was funded by the Srinakharinwirot University Fundamental Fund for fiscal year 2023, through a grant from Thailand Science Research and Innovation (TSRI) under grant number 001/2023.

Glossary

Abbreviations

PLA

poly­(lactic acid); biodegradable polymer used as the main composition in biocomposites.

TPS

thermoplastic starch is a modified tapioca starch that is biodegradable.

C

α-cellulose is derived from natural sources and is biodegradable. It has potential for sustainability and enhanced properties.

GMA

glycidyl methacrylate, which is a bifunctional monomer that consists of acrylic and epoxy groups, is employed for grafting onto PLA.

PLA-g-GMA

GMA was grafted onto PLA to act as a coupling agent in biocomposites

PLA/T30/C0

PLA composites with 30 wt % of TPS and 0 wt % of α-cellulose

PLA/T25/C5

PLA composites with 25 wt % of TPS and 5 wt % of α-cellulose

PLA/T20/C10

PLA composites with 20 wt % of TPS and 10 wt % of α-cellulose

PLA/T15/C15

PLA composites with 15 wt % of TPS and 15 wt % of α-cellulose

The authors declare no competing financial interest.

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