Additivation of PLA/PBAT Blends to Expand Applications in the Biodegradable Packaging Sector

Abstract

The present study investigates the influence of different organic additives on PLA/PBAT blends, focusing on enhancing their thermal, mechanical, rheological, and microstructural properties for biodegradable packaging applications. Five additives were tested: citric acid, lauric acid, gum rosin, cellulose nanocrystals, and poly(vinyl acetate) (PVAc). The blends were prepared in an internal mixing chamber and injection molded for mechanical testing. Characterizations included mechanical tests (tensile, flexural, Izod impact), thermal analyses (DSC, TGA), rheological assessments (torque, melt flow index), and SEM. Among the additives, PVAc showed the most promising results, improving phase dispersion through a core-shell structure and enhancing tensile strength (11.9%), toughness (32%), and flexural stress (26.5%). These results highlight PVAc as the most promising additive for optimizing the performance of PLA/PBAT blends. This study provides valuable insights into developing sustainable materials with improved properties for biodegradable packaging.

Keywords:
PLA/PBAT blends; biodegradable packaging; natural additives; polymer additive; sustainable materials

1. Introduction

The global production of plastics has more than doubled in recent decades, increasing from 234 million tonnes in 2000 to 460 million tonnes in 2019, with projections reaching 736 million tonnes by 2040 in the absence of more ambitious policies¹. In Brazil alone, 4,548 million tonnes of plastic waste were generated in 2023, with low-density polyethylene (LDPE/LLDPE) accounting for the largest share, followed by significant contributions from products made of polypropylene (PP), polyethylene terephthalate (PET), and high-density polyethylene (HDPE)². While efforts have led to a 23.9% increase in the Brazilian production of post-consumer recycled resins since 2018, the volume of plastic waste remains a critical environmental concern¹. Non-biodegradable plastic waste, particularly from multilayer and chemically complex packaging, is difficult to recycle, contributing to environmental pollution and climate change³,⁴. By 2040, mismanaged plastic waste is expected to rise by 47%, with environmental leakage increasing by 50% compared to 2020 levels¹. Conventional plastics, regardless of origin, can persist for centuries, fragmenting into micro- and nanoplastics that infiltrate ecosystems and bioaccumulate through food chains⁵.

To address the detrimental impact of traditional plastic packaging, the development of biobased and biodegradable alternatives has gained increasing attention. Poly(lactic acid) (PLA) is a promising bio-based polyester that has been extensively researched for use in packaging applications due to its renewability, biodegradability, and good mechanical properties. Nevertheless, PLA is brittle and exhibits limited flexibility, restricting its application in flexible and durable packaging products⁶. Blending PLA with other biodegradable polymers, such as poly(butylene adipate-co-terephthalate) (PBAT), is an effective strategy to enhance its ductility, toughness, and processability while maintaining biodegradability⁷,⁸. PBAT’s flexibility complements PLA’s rigidity, resulting in blends with improved mechanical properties suitable for various packaging applications⁹. However, the immiscibility of PLA and PBAT often leads to phase separation, which can compromise the overall performance of the blend⁹.

To overcome the limitations associated with PLA/PBAT immiscibility, the incorporation of compatibilizers or additives is necessary to enhance the interfacial adhesion and overall properties of the blend. Conventional compatibilizers, such as peroxides, isocyanates, and epoxy-functionalized compounds, have demonstrated efficacy in improving PLA/PBAT blends but may introduce toxic residues or inhibit biodegradability⁸,¹⁰. Consequently, the exploration of natural and biodegradable additives is a growing area of interest, as they offer the dual benefits of enhancing blend performance while preserving environmental compatibility.

This study investigates the use of bio-based or low-toxicity additives — including anhydrous citric acid (CA), lauric acid (LA), gum rosin (GR), cellulose nanocrystals (CNC), and poly(vinyl acetate) homopolymer (PVAc) — as modifiers for PLA/PBAT blends aimed at biodegradable packaging applications. The objective of this work is to evaluate the influence of these additives on the mechanical, thermal, rheological, and microstructural properties of PLA/PBAT blends, providing insights into the development of environmentally sustainable packaging materials. By enhancing blend performance through the use of natural or low-toxicity additives, this research contributes to the advancement of biodegradable packaging solutions, addressing the pressing need to mitigate the environmental impact of plastic waste.

2. Experimental

2.1. Materials

Poly(lactic acid), known as Ingeo™ Biopolymer 4043D (NatureWorks, USA), is designed explicitly for biaxially oriented film applications. It has a melt flow rate of 6 g/10 minutes at 210 °C and 2.16 kg, a relative viscosity of 4.0, a glass transition temperature ranging from 55 to 60 °C, and a peak melt temperature between 145 and 160 °C.

Poly(butylene adipate-co-terephthalate), known as Ecoflex^® F Blend C1200 (BASF, Germany), is designed for the production of flexible films. It can be processed using either a blown film or a cast film method. This material has a melt flow rate ranging from 2.7 to 4.9 g/min at 190 °C under a load of 2.16 kg. It features a mass density between 1.25 and 1.27 g/cm³, and its melting point is within the range of 110 to 120 °C.

The following additives were investigated as potential compatibilizers for the PLA/PBAT blend:

• Citric acid anhydrous (CA; Neon Comercial Reagentes Analíticos LTDA, Brazil), purity ≥99.5%

• Lauric acid (LA; Dinamica Química Contemporânea LTDA, Brazil), purity ≥98%

• Gum rosin (GR; Quibras Química Brasileira LTDA, Brazil) batch no. KE21.221-Q

• Cellulose nanocrystals (CNC; CelluForce NCC® NCV100, CelluForce, Canada)

• Poly(vinyl acetate) homopolymer (PVAc; VINNEX® 2522, Wacker Chemie AG, Germany), Softening point in 119 °C, Melting temperature in 153.39 °C

The molecular structures of these compatibilizers are shown in Figure 1, to assist the reader in understanding their chemical nature and potential interactions with the polymer matrix.

Figure 1
Molecular structures of the compatibilizers used in the PLA/PBAT blends: (a) PVac, (b) CA, (c) LA, (d) GR, and (e) CNC.

2.2. PLA/PBAT blend preparation

PLA and PBAT granules were dried in a circulating air oven at 50 °C for 3 hours prior to processing. The polymer blends were prepared using a fixed PLA/PBAT weight ratio of 70:30, selected based on preliminary studies that indicated this composition provided a balance between mechanical performance and processability. To evaluate the effect of different additives on the compatibility and properties of the blend, a concentration of 3 parts per hundred of resin (3 phr) was used for each additive.

Each sample was formulated by incorporating only one compatibilizer at a time into the PLA/PBAT matrix, generating six distinct formulation families. These included a control sample composed solely of the PLA/PBAT blend without additives, and five modified blends with the addition of one of the investigated additives. The formulations are identified by the following abbreviations, which are used consistently throughout the figures and tables:

• PLA₇₀PBAT₃₀– control blend without additives

• CA-Blend – PLA/PBAT with 3 phr of citric acid

• LA-Blend – PLA/PBAT with 3 phr of lauric acid

• GR-Blend – PLA/PBAT with 3 phr of gum rosin

• CNC-Blend – PLA/PBAT with 3 phr of cellulose nanocrystals

• PVAc-Blend – PLA/PBAT with 3 phr of polyvinyl acetate

All blends were processed in an internal mixing chamber (HAAKE™ Rheomix OS, Thermo Fisher Scientific, USA) equipped with Roller Rotors. The mixing was performed at 170 °C and 60 rpm for 6 minutes, with a batch size of 65 g. This processing condition was maintained for all samples to ensure consistency and comparability across the different formulations.

Using the torque versus time curves recorded during the mixing process, it is possible to calculate the specific mechanical energy (EME) required to process each sample. The total torque for mixing the blend can be understood as the integral of the area in the torque vs time graph after 6 minutes of mixing. To perform the EME calculation, curves from at least three batches were considered for each sample; the values were expressed as the mean and standard deviation. The EME, expressed in kJ/kg, can be calculated according to Equation 1.

$EME=\frac{2\pi N}{m}\underset{t_0}{\overset{t_{mixture}}{{\displaystyle \int }}}M\left(t\right)dt$ (1)

where N is the rotation speed (rpm), m is the total mass of the sample (g), t is the processing time (min) and M(t) is the total torque produced during the processing time (Nm). The necessary data was extracted from the torque versus mixing time curves.

2.3. Injection molding

Mechanical test specimens for tensile, flexural, and impact resistance tests were prepared using a pure PLA/PBAT blend and modified blends with individual additives, through injection molding. The material collected from the internal mixing chamber was immediately cut into small pieces using scissors while still softened, facilitating further processing. These pieces were then used directly as granules for injection molding. The granules were stored in a desiccator prior to being processed in an injection molding machine, specifically the HAAKE MiniJet II (Thermo Fisher Scientific, USA). The injection processing conditions included a temperature of 170 °C, an injection pressure ranging from 410 to 510 bar, and a holding pressure between 250 and 400 bar. The mold temperature was maintained between 30 and 60 °C.

2.4. Melt flow index (MFI) measurements

The tests were performed according to Method A of ASTM D1238-13¹¹, using a Ceast Modular Melt Flow Tester (model 7026.000) under a load of 2.16 kg and at a temperature of 170 °C, with a 2-minute residence time. The cutting time was adjusted to ensure consistent sample length, and the results are expressed in g/10 min.

2.5. Thermal characterization

The thermal properties of the additive-loaded blends were assessed using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). This comprehensive thermal analysis provided crucial data on the phase transitions, crystallinity, and thermal stability of the PLA/PBAT blends, elucidating the influence of additives on their thermal behavior and structural properties.

TGA was performed with a TA Instruments TGA Q50, where approximately 15 mg of each sample was heated from room temperature to 750 °C at a heating rate of 20 °C/min under a nitrogen atmosphere with a flow rate of 10 mL/min. This analysis was conducted to evaluate the thermal stability and degradation profile of the blends.

DSC analyses were performed using a Q20 differential scanning calorimeter (TA Instruments, USA) under a nitrogen atmosphere (50 mL/min). The samples, in the form of granules obtained during the mixing process, were subjected to a heating ramp from 25 °C to 200 °C at a rate of 10 °C/min. Pure PLA and PBAT samples were also analyzed after undergoing a single processing cycle on the internal mixer to assist in identifying the thermal events associated with each polymer phase.

The degree of crystallinity in percentage (X_C) of the PLA/PBAT blend films was evaluated according to Equation 2:

$X_c=\frac{\Delta H_m}{f\times \Delta H_m^0}\times 100\%$ (2)

where ∆Hm is the melting enthalpy obtained from the DSC curve (J/g); ∆H⁰_m is the theoretical enthalpy of melting for a 100% crystalline polymer, taken as 93 J/g for PLA and 114 J/g for PBAT; and f is the weight fraction of PLA in the blends¹²,¹³.

2.6. Mechanical characterization

Tensile properties of injected blends were evaluated using a universal testing machine model, Instron Emic 23 SD (Instron, USA), in accordance with ASTM D638¹⁴. The tests were conducted at a crosshead speed of 10 mm/min. For each formulation, a minimum of five and up to seven specimens were considered in the calculation of the mean values and standard deviations.

The flexural properties of the blends were determined through three-point bending tests performed according to ASTM D790-86¹⁵, utilizing an Instron 5967 universal testing machine. The tests were conducted at room temperature with a crosshead speed of 2 mm/min. The span between supports was set to 40 mm for PLA/PBAT blends and 25 mm for additive-loaded blends. Injected samples with dimensions of 80 × 12.7 × 3 mm were used. Per the standard, testing was terminated either when the sample fractured or upon reaching 5% strain, whichever occurred first. Five to seven specimens were used to calculate the mean and standard deviation of each group.

The impact resistance (Izod) was conducted using the pendulum impact tester model Ceast Impactor II (Instron, USA) with an 11 J pendulum, following the ASTM D256-24 standard¹⁶. Each test evaluated at least five specimens, and the results were presented as representative curves in the graphs. The values reported in the table of mechanical properties represent the average of five individual samples.

2.7. Scanning electron microscopy (SEM)

The morphology of the blends was evaluated using a scanning electron microscope (SEM), model Zeiss EVO MA10 (ZEISS Microscopy, USA), with an acceleration voltage of 10 kV after cryogenic fracture of flexural resistance test specimens. Prior to observation, all samples (obtained by cryogenic fracture) were mounted on metal stubs using conductive carbon tape and sputter-coated with gold to minimize charging and prevent sample damage.

2.8. Statistical evaluation

Quantitative data were presented as mean ± standard deviation. Statistical comparisons were performed using one-way ANOVA followed by Tukey’s post-hoc test. A significance level of 0.05 was applied to determine the statistical relevance of the results. In tables, Means followed by the same letter in the same column do not differ statistically among themselves by Tukey test (p < 0.05). All analyses were conducted using OriginPro software (2021 version).

3. Results and Discussion

3.1. Rheological study

The rheological analysis of the PLA₇₀PBAT₃₀ blend revealed that the incorporation of different additives significantly altered its processing behavior, as shown in Table 1. These modifications are directly associated with the chemical nature of each additive and the specific interactions they promote within the matrix.

Table 1
Rheological properties of pure blend and additive-modified samples, extracted from mixing curves and melt flow index tests.

Sample	Torque at 5 min (Nm)	Total Torque (Nm)	SME (kJ/kg)	MFI (g/10min)
PLA70PBAT30	4.1±1.2ab	38.0±0.7ab	217.2±25.0ab	8.7±1.0a
CA-Blend	0.5±0.1d	22.1±1.4c	125.9±8.2b	Flows freely
LA-Blend	1.8±0.2cd	25.4±0.5c	145.3±3.1a	83.8±14.9b
GR-Blend	2.9±0.0bc	33.5±0.7b	191.7±3.8c	51.8±8.8c
CNC-Blend	3.9±0.0ab	37.6±1.6ab	214.9±9.1c	24.0±1.4d
PVAc-Blend	4.3±0.2a	41.3±0.9a	236.3±5.1ab	16.2±0.7e

CA-Blend exhibited the most pronounced plasticizing effect, drastically reducing both the torque at 5 minutes (0.5 ± 0.1 Nm) and the total torque (22.1 ± 1.4 Nm), as well as lowering the specific mechanical energy (SME) to 125.9 ± 8.2 kJ/kg. Its behavior during the MFI test confirmed extremely high fluidity at 170 °C, with material flowing freely under testing conditions. This behavior may be attributed not only to the plasticizing action of citric acid but also to catalyzed hydrolytic degradation. The first step in the degradation mechanism of PLA occurs through the hydrolysis of its ester groups. Citric acid, which contains multiple carboxylic groups, hydrolyzes segments of the polymer, breaking it down into smaller fragments¹⁷. This process results in a significant reduction in molecular weight, which, in turn, increases the flowability of the material. These results suggest that although citric acid improves flow, it may compromise the mechanical properties, which will be discussed later.

LA-Blend also demonstrated plasticizing behavior, albeit less intensively. It reduced both torque and SME (145.3 ± 3.1 kJ/kg), while displaying the highest MFI (83.8 ± 14.9 g/10 min), indicating a strong reduction in melt viscosity. LA acts as a plasticizer by reducing intermolecular forces within the polymer, increasing free volume, and lowering the glass transition temperature¹⁸. This suggests improved processability for applications requiring high flow rates, though possibly at the expense of mechanical strength.

CNC-Blend and PVAc-Blend exhibited increased resistance compared to the other additized blends, as indicated by their higher torque and specific mechanical energy (SME) values (214.9 ± 9.1 and 236.3 ± 5.1 kJ/kg, respectively), along with lower MFI values (24.0 ± 0.1 g/10 min for CNC and 16.2 ± 0.7 g/10 min for PVAc). The behavior of PVAc-Blend suggests enhanced polymer chain interactions and possibly improved phase adhesion or compatibilization, resulting in greater melt strength—an important characteristic for applications that demand dimensional stability and mechanical integrity. In the case of CNC-Blend, the observed results may indicate the role of CNC as a filler or potential reinforcement agent, which will be further explored in the discussion of mechanical properties.

GR-Blend presented intermediate behavior, with torque and SME values between those of the plasticized and reinforced systems. It reached a notably high MFI of 51.8 ± 8.8 g/10 min, suggesting a balanced effect: increased flowability without a drastic reduction in melt resistance. This could be advantageous for injection molding or extrusion processes where moderate viscosity and mechanical properties are required.

Overall, these findings reinforce the need to tailor additive selection according to processing and end-use requirements. Plasticizers such as CA and LA favor flow and ease of processing, while CNC and PVAc enhance melt strength and resistance, potentially improving final material performance.

3.2. Thermal properties

Thermogravimetric analysis (TGA) is a valuable method for assessing the thermal stability of materials. In this study, we measured two parameters to evaluate the thermal behavior of both neat and additized blends. The first parameter, known as T onset, is obtained from the TG curves (Figure 2a) and indicates the lowest temperature at which the material begins to experience mass loss. The second parameter, T max, is derived from the peak of the derivative thermogravimetry (DTG) curve (Figure 2b) and corresponds to the temperature at which the rate of degradation is at its highest.

Figure 2
Thermogravimetric analysis (TGA) of neat and additized PLA/PBAT blends: (a) weight loss curves and (b) derivative weight loss (DTG) curves.

When comparing the thermal stability profiles of the samples, it was observed that all exhibited a double-step thermal degradation, which is linked to the PLA and PBAT phases. Previous research has shown that PBAT is more thermally stable than PLA, demonstrating a higher onset temperature (T_onset)⁷,¹⁹. The TG curves reveal that all additives reduced the thermal stability of the PLA/PBAT blend, as evidenced by the decrease in T onset (Table 2) compared to the sample without additives.

Table 2
TGA data for PLA/PBAT neat blend and additive-containing formulations.

Sample	Tonset (°C)	Tmax PLA (°C)	Tmax PBAT (°C)
PLA70PBAT30	348.6	368.9	405.3
CA-Blend	303.4	337.8	409.8
LA-Blend	305.6	327.3	404.5
GR-Blend	307.0	331.2	403.1
CNC-Blend	312.6	339.9	410.5
PVAc-Blend	324.6	349.7	404.3

The PLA₇₀PBAT₃₀ sample exhibited the highest thermal resistance, followed by PVAc and CNC, while the samples containing GR, CA, and LA demonstrated the lowest thermal stability. According to the literature, LA decomposes in a single step and is thermally stable up to 140 °C, whereas CA undergoes decomposition primarily in one step within the 150–220 °C range, where 95% of the transformation occurs, and the DTG peak appears at around 210 °C¹⁸,²⁰. This analysis suggests that additives such as PVAc and CNC, which exhibit greater compatibility with the polymer matrix, exert a less negative impact on the thermal stability of the blend.

Analyzing the DTG curve reveals a shift in the maximum degradation kinetics of the PLA phase to lower temperatures. This indicates that the presence of additives makes the PLA phase more susceptible to thermal degradation. This effect is likely due to interactions between the additives and the PLA chains, which promote bond breakage at lower temperatures. In contrast, the maximum decomposition kinetics of PBAT remained stable, occurring within the same temperature range as the pure sample. This suggests that the PBAT phase is less affected by additives in terms of thermal stability.

To better identify the thermal transitions observed in the blends, differential scanning calorimetry analyses were performed on the pure polymers after processing in the internal mixer, using the same mixing parameters applied to the PLA/PBAT blends. Figure 3a shows the cooling curves at a controlled rate of 10 °C/min following a first heating step that erased the thermal history. Figure 3b presents the second heating scan, after inducing controlled crystallization during the cooling step.

Figure 3
DSC thermograms of processed neat PLA and PBAT: (a) cooling scan at 10 °C/min and (b) second heating scan. Key thermal events such as glass transition (T_g), cold crystallization (T_CC), and melting (T_m) are indicated.

For PLA, a T_g of approximately 59.2 °C was observed in both scans. A cold crystallization peak appears at 112.0 °C (ΔH_CC 22.72 J/g) during the second heating, followed by a bimodal melting behavior with endothermic peaks at 148.1 °C and 154.6 °C (ΔH_m 22.87 J/g). According to other researchers, the presence of two melting peaks is common in semi-crystalline polymers and polymer blends²¹. This phenomenon arises from a melt–recrystallization–remelt process, in which the lower-temperature peak is associated with the melting of less stable, imperfect crystals, while the higher-temperature peak corresponds to the melting of more stable, well-formed crystals. The calculated crystallinity degree of PLA was 24.6%. The PLA cooling curve did not show an exothermic crystallization peak, which may indicate that, under the applied cooling rate, only nucleation occurred without sufficient time for crystal growth to develop — a behavior also reported in the literature²².

For PBAT, a crystallization peak was detected at 67.3 °C (ΔH_C 14.91 J/g) in the cooling scan, and the glass transition of its rubbery phase was observed at −32.8 °C. The melting of PBAT occurred at 125.8 °C (ΔH_m 9.19 J/g). These thermal events align with previously reported data¹². PBAT exhibited a low crystallinity degree (8.1%), as expected for a flexible copolymer. It must be noted that the PBAT melting peak and the PLA cold crystallization peak occur within a similar temperature range. Therefore, the overlap of these energetically opposing transitions complicates the accurate determination of PLA crystallinity in blend samples²³.

Regarding the blends, the cooling and second heating curves of differential scanning calorimetry are presented in Figure 4a and 4b, respectively. The characteristic thermal events are summarized in Table 3 to provide context for the effect of each additive.

Figure 4
DSC curves of PLA/PBAT blends: (a) cooling step and (b) second heating step. The graph includes both the neat blend and the blends modified with different additives.

Table 3
Thermal properties of PLA/PBAT neat and additive blends determined from the second heating cycle in the DSC analysis.

	Cooling step		Second heating step
Sample	* Tc (°C)	ΔHC (J/g)	**Tg (°C)	**TCC (°C)	ΔHCC (J/g)	**Tm (°C)	ΔHm (J/g)	**XC (%)
PLA70PBAT30	51.95	1.84	54.32	106.25	15.91	145.00; 153.59	18.10	27.80
CA-Blend	Unidentified	-	43.38	93.53	18.49	134.99; 146.14	21.27	33.65
LA-Blend	59.85	2.95	46.42	92.31	17.64	139.30; 150.62	20.74	32.82
GR-Blend	66.59	8.78	50.45	103.45	18.40	142.27; 151.59	20.20	31.96
CNC-Blend	Unidentified	-	59.23	116.21	2.24	147.71; 155.27	2.49	3.93
PVAc-Blend	62.61	6.354	53.22	112.20	15.70	145.98; 153.78	17.70	28.05

• ^*
  Relating to the PBAT phase. **Relating to the PLA phase.

The sample without additives exhibited a T_g of 54.32°C, a small decrease compared to neat PLA (59.2 °C), which suggests that PLA and PBAT are not thermodynamically compatible. If they were compatible, both T_g of PLA and PBAT would move to combine with each other, and T_g peaks should be weaker and wider after they were blended²⁴. In the additive-free blend, it was observed that the cold crystallization temperature of PLA is slightly reduced compared to neat PLA. This implies that the crystallization of PLA is facilitated by blending with 30 wt% of PBAT, which explains the slight increase in PLA's crystallinity for the PLA70PBAT30 sample. It is well-documented by other authors that the presence of PBAT influences the crystallization behavior of PLA, acting as a nucleating agent and increasing its crystallization rate²⁴-²⁶.

In the cooling curves of the DSC analysis shown in Figure 4a, the crystallization temperature associated with the PBAT phase could not be identified in the samples containing LA and CNC. This result suggests that these additives interfere with the nucleation and growth processes of the PBAT phase, preventing its crystallization. In contrast, the PVAc-Blend sample exhibited a higher Tc than the neat PLA/PBAT blend without additives. This result indicates that PVAc is capable of interacting with both polymer phases, promoting a better dispersion of the rubbery PBAT phase within the PLA matrix, suggesting that it acts as a compatibilizing agent for this system.

Among all formulations, the CA-Blend exhibited the most pronounced shift in thermal behavior, with the greatest reduction in T_CC, the highest cold crystallization enthalpy (ΔH_CC), and the highest crystallinity degree (X_C) for the PLA phase. This can be attributed to hydrolytic degradation during melt processing, which likely shortened the polymer chains, reducing entanglement and promoting more efficient molecular alignment and crystallization.

In contrast, the LA-Blend also showed increased crystallinity, which may be related to enhanced dispersion of the PBAT phase within the PLA matrix. This improved phase distribution likely facilitated crystallization during heating, in line with the plasticizing behavior previously observed in the rheological analysis.

CNC resulted in the lowest crystallinity degree (X_C = 3.93%), despite increasing the T_CC, which suggests limited crystallite formation or poor filler dispersion. This additive also affected the PBAT phase, as evidenced by the absence of an exothermic peak associated with PBAT crystallization in the cooling curve (Figure 4a). This observation indicates that the addition of CNC hinders the crystal growth process. Poor dispersion of nanoparticles within polymer matrices is a well-known issue widely reported in the literature, as the main challenge in using cellulose nanoparticles lies in achieving stable and uniform dispersion across various matrices. This difficulty arises from the strong tendency of cellulose nanoparticles to aggregate due to their high surface area and the abundance of hydroxyl groups, which promote hydrogen bonding between particles²⁷,²⁸. These findings suggest that the low crystallinity observed in this case may be attributed to the inadequate distribution of the nanofiller, which hindered the crystallization of both polymer phases in the blend.

3.3. Mechanical properties

The incorporation of additives influenced the tensile response in various ways, reflecting the specific interaction between each additive and the PLA/PBAT blend, as observed in Figure 5a. The mechanical properties are summarized in Table 4. The influence of the additives was primarily observed in tensile strength, strain at break, and toughness (calculated as the area under the stress-strain curve). The mechanical response of the samples containing citric acid is not presented, as these samples exhibited brittle behavior and fractured under the pressure of the clamping device.

Figure 5
Representative stress–strain curves of PLA/PBAT and additive-modified blends: (a) tensile test and (b) three-point bending test.

Table 4
Summary of mechanical properties extracted from the mechanical tensile test.

Sample	Young Modulus (MPa)	Max strength (MPa)	Yield Strain (%)	Mechanical toughness (MJ/m3)
PLA70PBAT30	1133.4±47.9a	40.54±1.18b	4.68±0.12ab	7.66±0.68b
CA-Blend	-	-	-	-
LA-Blend	1173.6±30.9a	28.05±1.40c	2.83±0.17d	3.60±1.17c
GR-Blend	1161.0±59.4a	40.83±2.79b	4.04±0.8c	0.95±0.14d
CNC-Blend	1172.1±47.4a	39.94±1.24b	4.45±0.21b	3.97±1.17c
PVAc-Blend	1182.9±55.3a	45.38±2.44a	4.85±0.16a	10.11±1.93a

The Young’s modulus values presented in Table 4 indicate that there are no statistically significant differences between the samples, with values ranging from 1133.4 to 1182.9 MPa. This suggests that the overall stiffness of the blend was only minimally affected by the additives. However, properties related to strength and deformation showed notable variations. The highest tensile strength was observed for the PVAc-containing sample (45.4 ± 2.4 MPa), which demonstrated significantly greater strength compared to most other formulations, suggesting that PVAc contributes to improved matrix cohesion. The PVAc-blend also exhibited the highest mechanical toughness, approximately 32% higher than the blend without additives. This supports the previously mentioned hypothesis that PVAc enhances the interaction between the PLA and PBAT phases, which will be further confirmed through morphological analysis.

As shown in Figure 5a, the GR-Blend fractured within the elastic regime, indicating brittle failure, although its maximum strength was statistically similar to that of the PLA70PBAT30 and CNC-Blend samples. This brittle behavior, evidenced by the lowest toughness value among all blends, can be correlated with its higher crystallinity degree, which leads to increased stiffness and reduced flexibility.

Among all the additives, the LA-Blend sample exhibited the lowest maximum strength (28.0 ± 1.4 MPa) and strain at yield (2.8 ± 0.2%). However, as shown in the representative stress–strain curves in Figure 5a, these samples fractured in the plastic region, confirming the plasticizing effect that increased polymer chain mobility. This also explains the higher MFI and the reduced structural strength, as evidenced by the decrease in T_g.

Regarding strain at break, the PVAc-Blend demonstrated the highest elongation capacity among the additive-containing formulations, indicating better preservation of the PLA/PBAT blend's ductility. Combined with the greater toughness provided by this additive, this formulation may be particularly advantageous for applications requiring both flexibility and impact resistance. Conversely, the GR-Blend and CNC-Blend exhibited low elongation at break, suggesting that these additives promote a more rigid structure that is more susceptible to fracture. Overall, the incorporation of PVAc achieved a favorable balance between strength and ductility, while gum rosin led to reduced ductility and a higher tendency toward brittle failure—highlighting the importance of selecting appropriate additives to tailor the mechanical behavior of PLA/PBAT blends for specific end-use applications.

The representative curves in Figure 5b from the three-point bending test and the flexural properties summarized in Table 5 reveal significant variations in flexural modulus, stress, and strain at break among the different samples, reflecting the specific impact of each additive on the flexural strength and flexibility of the PLA/PBAT blend. The PLA₇₀PBAT₃₀ sample exhibited the lowest flexural modulus (2018.3 ± 66.8 MPa) and did not fracture up to 5% deflection, indicating good flexibility and, consequently, lower stiffness.

Table 5
Summary of mechanical properties extracted from the mechanical three-point bending test.

Sample	Flexural Modulus (MPa)	Stress at break (MPa)	Strain at break (%)	Type of failure
PLA70PBAT30	2018.3±66.8a	61.9±2.0a*	5.0±0.0a*	Not fractured
CA-Blend	4372.6±167.7b	21.1±2.1b	0.6±0.0b	Brittle
LA-Blend	4624.2±241.2b	42.6±1.5c	2.8±0.6c	Ductile
GR-Blend	5344.5±106.5c	73.3±5.1d	1.9±0.2d	Brittle
CNC-Blend	3747.0±133.7d	67.2±3.1a*	4.9±0.1a*	Ductile or not fractured
PVAc-Blend	4585.9±281.6b	78.3±1.2e*	5.0±0.0a*	Not fractured

• ^*
  For samples that did not fracture during the test, this value refers to the stress and strain values recorded upon reaching the limit of 5% established by the standard.

Citric acid and lauric acid significantly increased the flexural modulus, indicating increased stiffness. However, their failure characteristics differed: CA-Blend showed brittle fracture, with low strain at break (0.6 ± 0.0%), while LA-Blend exhibited ductile failure, with more significant deformation (2.8 ± 0.6%), suggesting enhanced stiffness and some capacity to absorb deformation before fracture.

The GR-Blend had the highest flexural modulus (5344.5 ± 106.5 MPa) and fracture stress (73.3 ± 5.1 MPa) but fractured in a brittle manner with limited deformation (1.9 ± 0.2%), suggesting that resin imparts high stiffness but drastically reduces flexibility. This brittleness aligns with the observations from the tensile test. In contrast, CNC-Blend and PVAc-Blend displayed a mixed behavior of high stiffness (3747.0 ± 133.7 MPa and 4585.9 ± 281.6 MPa, respectively) while avoiding fracture up to 5% deflection. Fracture stress was exceptionally high for PVAc-Blend (78.3 ± 1.2 MPa), suggesting an effective combination of stiffness and fracture resistance without visible failure up to the test limit.

The results of the unnotched Izod impact test, illustrated in Figure 6, show significant variation in the impact resistance of PLA/PBAT blends with different additives. All samples exhibited complete fracture, meaning they were broken into two or more parts during the impact. This fracture type indicates that, regardless of the additive, the blend structures were sufficiently compromised to fracture completely under the 11 J hammer impact.

Figure 6
IZOD impact strength for PLA/PBAT neat blend and with additives.

The PLA₇₀PBAT₃₀ sample exhibited the highest impact resistance (49.90±11.68 kJ/m²), standing out for its natural combination of toughness and energy absorption capacity provided by the PLA and PBAT phases. Among the additive-containing samples, PVAc-Blend maintained relatively high impact resistance (42.62 ± 7.44 kJ/m²), consistent with its strong performance in tensile and flexural tests, where it displayed a combination of stiffness and ductility. This behavior indicates that PVAc improves cohesion in the PLA/PBAT matrix and enhances toughness, making it a favorable choice for applications that require impact resistance.

In contrast, CA-Blend drastically reduced impact resistance (3.18±0.45 kJ/m²), aligning with the brittle behavior observed in both tensile and flexural tests. Despite increasing blend stiffness, the addition of citric acid made the structure significantly more prone to sudden failure under impact. LA-Blend and GR-Blend also exhibited lower impact resistance (30.27±8.46 kJ/m² and 26.02±11.47 kJ/m², respectively), consistent with the tensile and flexural tests, in which they revealed greater stiffness but a higher tendency for brittle fracture, particularly for the addition of GR.

CNC-Blend displayed intermediate impact resistance (33.24 ± 5.19 kJ/m²), suggesting a balance between stiffness and impact absorption capacity. For this sample, the mechanical results were below expectations for a nanocomposite, most likely due to poor dispersion of the nanofiller, as observed in the morphological images.

These results indicate that additives increasing stiffness, such as citric acid and gum rosin, tend to compromise impact resistance, while poly(vinyl acetate) and nanocellulose improve impact absorption, ensuring more balanced performance for applications demanding mechanical strength and toughness.

3.4. Morphology (SEM)

The Scanning Electron Microscopy (SEM) images of the cross-section obtained from the cryogenic fracture of the injected specimens (Figure 7) show morphological changes influenced by the addition of various additives to the PLA/PBAT blend. These observations are crucial for understanding how the choice of additive impacts the morphology and, in turn, the final performance of the blend in specific applications.

Figure 7
SEM micrographs of the cryo-fractured cross-sections of the injected samples: (a) PLA70PBAT30, (b) CA-Blend, (c) LA-Blend, (d) GR-Blend, (e) CNC-Blend, and (f) PVAc-Blend. Images were captured at magnifications of 10,000× and 20,000×.

The micrograph of the PLA₇₀PBAT₃₀ sample, visible in Figure 7a, exhibits a rough matrix, with few incrustations. As described by other authors, PLA/PBAT blends typically display a characteristic morphology of immiscible systems marked by clear phase separation. These authors reported that cryo-fractured surface micrographs of a film with 70% PLA and 30% PBAT, PBAT particles dispersed in the PLA matrix, were observed, forming larger clusters in samples with higher PBAT content. This clustering phenomenon reduces the interfacial area and weakens phase interaction, which in turn negatively impacts Young’s modulus and maximum stress, decreasing the mechanical strength of the blend.

The CA sample, in Figure 7b, exhibited an irregular surface, characterized by numerous clusters that were more numerous than those observed in the neat blend. The agglomerates appear to adhere well to the matrix, which has notable porosity. This effect may be attributed to the size and polarity of the citric acid molecule, which increases polymer chain mobility due to the possible hydrolytic degradation of the blend components. As a result, the presence of shorter chain segments promotes a more dispersed phase distribution.

The LA sample demonstrated an immiscible blend morphology, which is characterized by elongated fibrils assigned to rubbery PBAT in a continuous matrix. In Figure 7c, the pointed, somewhat cylindrical morphology is attributed to the phase (PBAT), indicating that PBAT still occupies some positions within the pores. The voids represent the PBAT that was removed during the cryogenic fracture.

In the SEM images of the GR sample, in Figure 7d, phase separation was not visualized. The sample was characterized by a single phase marked by a uniform roughness. What was observed differs from what has already been reported by other authors, where the incorporation of 5 phr of Gum Rosin in a blend with 20% PBAT significantly altered the cryo-fractured surface morphology, resulting in larger PBAT domains (0.5–1.5 µm) with a near-spherical shape. This increase in size and spherical shape was attributed to the loss of affinity and miscibility between PLA and PBAT after rosin addition²⁹.

In Figure 7e, the micrographs of the CNC-Blend show a consistent morphology with a uniform roughness across the fracture surface. The SEM image of the outer surface of the sample (Figure 8a) reveals that the blend containing nanocellulose exhibits a surface marked by "stretch marks." At magnifications of 10,000x and 20,000x (Figures 8b and 8c), numerous dark spots, attributed to the presence of nanocellulose, can be observed within the yellow circle. This aggregation of nanoparticles may explain why nanocellulose did not act as a nucleating agent, justifying the low crystallinity degree calculated for this sample. According to observations published by other authors, increasing the amount of nanocellulose from 3 to 5% in a PLA/PBAT blend reduces the size of dispersed PBAT regions³⁰. This effect occurs because nanocellulose can reduce the viscosity ratio between PLA and PBAT, facilitating PBAT droplet fragmentation. This finding supports the notion that nanocellulose acts as a reinforcing agent, promoting a more cohesive and compact distribution of particles, which is reflected in a denser and more regular morphology.

Figure 8
SEM images of the outer surface of the injected CNC-Blend specimen at different magnifications: (a) 2,000×, (b) 10,000×, and (c) 20,000×. The yellow circle highlights a structure attributed to CNC agglomeration.

In their work on PLA/PBAT/clay nanocomposites, Barbosa et al. (2019) also reported that the morphologies observed by SEM and TEM indicated that the addition of clay altered the crystallization behavior of both PLA and PBAT³¹. Additionally, studies on PLA crystallization kinetics have shown that PLA chains are susceptible to the presence of a second phase and processing conditions³². Therefore, it can be concluded that the incorporation of nanoparticles into PLA/PBAT blends may interfere with the growth of spherulitic crystals and hinder the proper organization of PLA chains. This could explain why CNC, despite being a crystalline filler, failed to induce crystallinity in this system effectively.

The MEV analysis of the PVAc sample revealed a microstructure characteristic of a core-shell structure typically observed in toughened polymer blends²¹,³³. The addition of PVAc promoted a more uniform distribution of the rubbery PBAT phase within the PLA matrix. This morphological improvement aligns with the highest mixing torque recorded for this sample, as well as the highest toughness observed in the tensile test, suggesting a stronger interaction between the blend components. This confirms the role of PVAc as a compatibilizing agent, as evidenced by the significant number of finely dispersed PBAT spheres observed within the PLA matrix, further supporting the enhanced compatibility.

This improved microstructure is a key factor contributing to the superior mechanical performance of the PVAc sample compared to blends with other additives. These findings underscore the role of poly(vinyl acetate) not only in modifying the morphology but also in enhancing the phase interactions within the PLA/PBAT blend³⁴, corroborating the improved overall material properties observed in other characterizations.

4. Conclusions

Among the five additives tested—citric acid (CA), lauric acid (LA), gum rosin (GR), cellulose nanocrystals (CNC), and poly(vinyl acetate) (PVAc)—PVAc demonstrated the most promising results. The incorporation of PVAc led to improved phase dispersion through a core-shell structure, which translated into superior mechanical performance, including an increase in tensile strength (11.9%), toughness (32%), and flexural stress (26.5%). Additionally, the PVAc-Blend exhibited the highest thermal stability among the formulations. The choice of non-toxic additives, such as those investigated in this study, is crucial for disposable packaging, as these materials will eventually decompose in controlled landfills.

In contrast, the CA-Blend and LA-Blend exhibited the lowest viscosity during both mixing and the melt flow index test, making them unsuitable for standard injection molding and extrusion techniques. Citric acid, in particular, caused a pronounced plasticizing effect, drastically reducing torque and specific mechanical energy, and displaying extremely high fluidity, suggesting catalyzed hydrolytic degradation that may compromise mechanical properties. Gum rosin (GR) promoted high stiffness but resulted in brittle failure of the blend. The CNC-Blend, despite increasing resistance and specific mechanical energy, did not achieve the expected performance for a nanocomposite, likely due to poor nanofiller dispersion, which also hindered the crystallization of the polymer phases.

This work contributes to the expansion of PLA/PBAT blend applications in the packaging sector. By demonstrating how the selection of the appropriate additive can modulate the mechanical (stiffness, strength, ductility, and toughness) and thermal properties of the blends, this study offers insights for developing sustainable materials with optimized characteristics. This is fundamental for driving the next generation of biodegradable packaging solutions and mitigating the growing environmental impact of plastic waste.

5. Acknowledgments

This work has been supported by the following Brazilian research agencies: CNPq, PIBIC CNPq, and CAPES-PROEX. The authors would also like to express their gratitude to the Núcleo de Sustentabilidade Group at the Federal University of Rio Grande do Sul for providing essential resources, infrastructure, and technical support throughout the development of this study.

• Data Availability
  Research data is available in the body of the article.

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Edited by

• Associate Editor:
  Leonardo Gondim de Andrade e Silva.

• Editor-in-Chief:
  Luiz Antonio Pessan.

Data availability

Research data is available in the body of the article.

Publication Dates

• Publication in this collection
  01 Sept 2025
• Date of issue
  2025

History

• Received
  10 Jan 2025
• Reviewed
  19 June 2025
• Accepted
  16 July 2025