Reactive Extrusion as an Environmentally Friendly Technology for the Production of Bio(Nano)Composites: Implementation and Characterization

Abstract

The influences of reactive extrusion of poly(lactic acid) (PLA)-based bio(nano)composites on their properties are described. Reactive compatibilizers were used to enable good dispersion of natural (nano)fibers in the thermoplastic matrix consisting of PLA/poly(butylene adipate-co-terephthalate) (PBAT) and PLA/polycarbonate (PC) blends. At the same time, chain extenders were used for the modification of immiscible thermoplastics, PLA and PBAT, in order to achieve good miscibility of the PLA/PBAT blend. In the experimental part, the main obstacle of PLA, its brittleness, was improved in three different series of bio(nano)composites. Reactive extrusion with PLA/PBAT blends and the addition of hops as a chain extender and compatibilizer increased the elongation at break of the bio(nano)composite by more than 240% and the impact strength by 200% compared to neat PLA. Reactive extrusion of PLA/PBAT blends and addition of 1% nanocrystalline cellulose (NCC) with additives increased the elongation at break by more than 730% compared to pure PLA, and the sample did not break during the impact testing. Reactive extrusion with PLA/PC blends and the addition of 1 wt% NCC with additives increased the elongation at break by more than 90% and the impact strength by more than 160% compared to pure PLA.

1. Introduction

Biopolymers, biopolymer blends, and biocomposites are becoming more and more interesting for research and industry because they have less impact on the environment. Researchers are making great efforts to avoid the disadvantages of biopolymers. Environmentally friendly materials, especially biodegradable ones, such as PLA, poly(3-hydroxybutyrate) (PHB), poly(ɛ-caprolactone) (PCL), poly(butylene succinate) (PBA), and PBAT, are attracting great interest from researchers and industry [1]. The properties of PLA together with its processability on conventional equipment make it possible to replace conventional petroleum-based thermoplastics [2, 3]. There are numerous research efforts in the field of reactive modification of biopolymers using various reactive agents, such as organic peroxides and multifunctional coagents, for modification with crosslinking [4, 5, 6]. Researchers reported the degradation of PLA in combination with crosslinking by modifying PLA with peroxides [7, 8, 9, 10, 11, 12]. Compatibilization can be used for the modification of biopolymers. Compatibilization of immiscible polymers can be performed by adding nonreactive agents, reactive agents, crosslinking, and double-functionalized polymers or with mechanochemistry where temperature and shear create macroradicals during compounding. Reactive compatibilization is a very cost-effective processing technology, an environmentally friendly process because it is solvent free, requires no special equipment, and can be easily up-scaled to industrial production [13]. A chain extender was used for the PLA/PBAT blend, which improved elongation at break, tensile strength, and impact resistance [14]. For the PLA/poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/PBAT ternary blend, an epoxy-based styrene-acrylic oligomer with low functionality was used, which improved tensile strength and elongation at break [15]. For the PLA/PBAT blend, a bio-based chain extender (epoxidized cardanol prepolymer) was used, which improved tensile strength, elongation at break, and toughness [16]. Epoxy-functionalized oligomer as a chain extender was used for PLA/PBAT/flax fiber composites, where stiffness and strength were improved [17]. For microcellulose and nanocellulose, the cellulose surface is chemically modified to improve the surface interaction of cellulose with the polymer matrix, usually by esterification and silanization or by plasma and corona surface treatment, which is also required in the case of PLA matrix [18]. Unmodified bacterial cellulose nanowhiskers were incorporated into the PLA matrix by electrospinning, followed by the incorporation of nanostructured fiber into the PLA matrix by melt blending. The stiffness and strength were increased, while the ductility remained at the level of pure PLA [19]. Major research efforts have been devoted to improve the reactive compatibility of PLA, including with petroleum-based polymers. Chain extenders were used for the PLA/PC blend to increase toughness [20]. The tougher PLA base blend was mixed with PC, hydrogenated styrene-butadiene-styrene block copolymer and using a reactive compatibilizer and poly(ethylene-co-glycidyl methacrylate). Thermal stability and excellent toughness were achieved [21].

2. Materials and methods

2.1 Samples

Commercially available PLA with the trade name Ingeo 4043D was provided by Plastika Trček, Slovenia. A commercially available PC with the trade name Lexan 243 R was purchased from the company Sabic, Austria. NCC was donated by the company Navitas, Slovenia. Commercially available SEBS-g-MA with the trade name FG 1901 GT was purchased from Kraton, Germany. Commercially available TPU copolymer with trade name Kuramiron U TU-S5265 was purchased from Kuraray, Germany. Commercially available CaCO₃ with the trade name Calplex Extra was donated by Calcit, Slovenia. A commercially available chain extender with the trade name Joncryl ADR 4468 was purchased from the company BASF, Netherlands. Commercially available PBAT with the trade name Ecoflex F Blend C1200 was purchased from the company BASF, Netherlands. Commercially available hops with the trade name Styrian Aurora were donated by the Slovenian Institute of Hop Research and Brewing, Slovenia.

Three different series of samples were produced. The first series was a toughness modification of PLA by blending with PBAT, adding a Joncryl chain extender to improve the miscibility of PLA and PBAT, and adding a modified TPU compatibilizer to improve the interactions between the hops and the thermoplastic matrix, since hops without surface treatment were used. The composition of the samples of the first series is shown in Table 1.

Sample	PLA (wt%)	PBAT (wt%)	TPU (wt%)	Joncryl (wt%)	NCC (wt%)
PLA	100	0	0	0	0
PLA15PBAT10H	70	15	4.5	0.5	10
PLA15PBAT5H	75	15	4.5	0.5	5
PLA20PBAT5H	70	20	4.5	0.5	5

Table 1.

Composition of the samples of the first series.

For the second series of samples, the toughest version from the first batch of samples was used, to which 1 wt% NCC was added. The composition of the second series of samples is shown in Table 2.

Sample	PLA (wt%)	PBAT (wt%)	TPU (wt%)	Joncryl (wt%)	NCC (wt%)
PLA20PBAT	74.5	20	5	0.5	0
PLA20PBAT 1NCC-1	73.5	20	5	0.5	1

Table 2.

Composition of the samples of the second series.

For the third series of samples, PLA was blended with PC to increase toughness and maintain stiffness and strength at a high level. NCC was added to the thermoplastic blend, to which two compatibilizers were added, at three different concentrations. In addition to the modified TPU, modified SEBS was also used to maximize the toughness of the bio(nano)composite because it has a high content of PC and the toughness is limited. The reactive compounding was performed twice. The composition of the third series of samples is shown in Table 3.

Sample	PLA (wt%)	PC (wt%)	SEBS (wt%)	TPU (wt%)	CaCO3 (wt%)	NCC (wt%)	Compounding cycles
PLAPC	42	40	10	5	3	0	1
PLAPC 1NCC-1	41	40	10	5	3	1	1
PLAPC 1NCC-2	41	40	10	5	3	1	2
PLAPC 2NCC-1	40	40	10	5	3	2	1
PLAPC 2NCC-2	40	40	10	5	3	2	2
PLAPC 5NCC-1	37	40	10	5	3	5	1
PLAPC 5NCC-2	37	40	10	5	3	5	2

Table 3.

Composition of the samples of the third series and the number of compounding cycles.

2.2 Reactive compounding

Reactive compounding was used to improve the surface interaction of NCC and hops with the thermoplastic matrix. The NCC and hops used were not surface-treated. The role of the compatibilizer was to ensure good surface interaction of the NCC and hops with the thermoplastic matrix and to ensure good dispersion of the NCC and hops in the thermoplastic matrix by qualitatively wetting the surface of the NCC and hops to prevent its agglomeration. To ensure good wettability of the NCC surface and its dispersion, high shear was used in reactive compounding and, in the case of the PLAPC samples, multiple compounding cycles were used. High shear was achieved by high screw speeds during reactive compounding and the lowest possible processing temperature for the bio(nano)composites. In parallel, multiple reactive compounding cycles can be used to mimic the recycling of bio(nano)composites. The behavioral changes during multiple processing of bio(nano)composites can be studied. Reactive compounding is an existing technology for modifying PLA. Thus, the main drawback of PLA, namely its brittleness, can be improved by reactive compounding by preparing a blend of PLA and a tough thermoplastic with the addition of natural fibers and a reactive additive.

For the first reactive extrusion cycle, the materials were mixed separately and extruded on the Labtech LTE 20–44 twin-screw extruder. The screw diameter was 20 mm, the L/D ratio was 44:1, and the screw speed was 600 rpm. The temperature profile for the PLAPC and PLAPBAT samples increased from the hopper (165°C and 145°C, respectively) to the die (200°C and 180°C, respectively). Vacuum extraction was performed during reactive extrusion to remove the volatile gaseous products of reactive extrusion. The vacuum was set at 50 mbar. After compounding, the two produced filaments with a diameter of 3 mm were cooled in a water bath and formed into pellets with a length of about 5 mm and a diameter of 3 mm.

In the case of the second reactive extrusion cycle in the samples PLAPC, the produced pellets of bio(nano)composites were extruded on the same extruder with identical extruder settings.

2.3 Injection molding

Injection molding was performed on Krauss Maffei 50–180 CX injection molding machine with a screw diameter of 30 mm and a clamping force of 500 kN. The cold runner mold was used to produce the samples. The mold had two cavities, one with a dumbbell-shaped mold of type 1BA (ISO 527-1), and the second with a cuboid shape (ISO 178/ISO 179). The temperature profile for the PLAPC and PLAPBAT samples was increasing from the hopper (185°C and 165°C, respectively) to the mold (200°C and 185°C, respectively), the injection speed was set to 60 mm/min, and the mold temperature was set to 30°C and 20°C, respectively, and the cooling time was set to 10 s and 15 s, respectively. During plastification, the backpressure for the PLAPC and PLAPBAT samples was set to 150 bar and 250 bar, respectively, and the screw speed was set to 50 rpm and 200 rpm, respectively. The high backpressure was used to remove air pockets in the melt and to achieve the best possible homogeneity of the melt. For the PLAPC samples, a low screw speed was used to prevent thermal degradation of the bio(nano)composite melt and to minimize shear during processing due to the higher processing temperature of the PLAPC samples.

2.4 Methods for characterization of the bio(nano)composites

Flexural and tensile tests were performed on the Shimadzu AG-X plus according to ISO 178 and ISO 527-1, respectively. Five measurements were taken for each specimen. In tensile tests, tensile stiffness (E_t), tensile strength (σ_m), tensile yield strain (ɛ_m), and elongation at break (ɛ_tb) were determined. In bending, the flexural stiffness (E_f), flexural strength (σ_fM), and yield strain (ɛ_fM) were evaluated.

Thermomechanical properties were investigated using a Perkin Elmer DMA 8000 dynamic mechanical analyzer. TT_DMA software, version 14,310, was used to evaluate the results. The viscoelastic properties of the samples were analyzed by recording the storage modulus (E’), loss modulus (E”), and loss factor (tan δ) as a function of temperature. The viscoelastic analyses were performed on specimens with dimensions of approximately 42 x 5 x 2 mm. The samples were heated at 2°C/min from room temperature (23°C) to 180°C under an air atmosphere. A frequency of 1 Hz and an amplitude of 20 μm were used in dual-cantilever mode.

Thermal measurements were performed using a differential scanning calorimeter (DSC 2, Mettler Toledo) under a nitrogen atmosphere (20 mL/min). The temperature of the samples was raised from 0 to 200°C at a heating rate of 10°C/min and held in the molten state for 5 min to erase their thermal history. After cooling at 10°C/min, the samples were reheated at 200°C at 10°C/min. The crystallization temperature (T_c), crystallization enthalpy (ΔH_c), glass transition temperature (T_g), cold crystallization temperature (T_cc), cold crystallization enthalpy (ΔH_cc), melting temperature (T_m), and melting enthalpy (ΔH_m) were determined using the cooling and the second heating scan.

Crystallization behavior on samples PLAPC was determined on Mettler Toledo Flash DSC 1 with Huber intercooler TC45 and nitrogen purge gas (50 mL/min). Samples were cooled from melt (200°C) to the desired temperature, rapidly heated to the aging temperature (90°C for 100 s), rapidly cooled to 15°C, reheated at 120°C, and then cold crystallized at 120°C at various times (from 0.1 s to 2400 s). All cooling and heating segments were rapidly cooled and heated (500°C/s) to prevent crystallization during cooling and heating. The first heating run was performed from 15–200°C. For the evaluation of the heating section, segment No. 12 was taken and the melting temperature and melting enthalpy were characterized. The mass of the samples was determined using the normalized change in specific heat capacity at the glass transition based on the evaluation of DSC 2 measurements.

Impact tests were performed on Pendel Dongguan Liyi test equipment, type LY -XJJD5 impact testing machine according to ISO 179. The impact test specimens were injection molded according to ISO 179 and had dimensions of 80 x 10 x 4 mm. The pendulum with 5 J was used for the evaluation of the impact test.

3. Results and discussion

3.1 Mechanical properties

The tensile and flexural results are shown in Table 4 and Figure 1. The results for the PLA sample are used as reference values for all other modifications by reactive compounding and for evaluating the usability of reactive compounding.

Sample	Tensile test results			Flexural test results
	Et (GPa)	σm (MPa)	ɛtb (%)	Ef (GPa)	σfM (MPa)	ɛfM (%)
PLA	3.17 ± 0.21	71.8 ± 1.0	4.8 ± 0.3	3.38 ± 0.07	105.2 ± 0.8	4.49 ± 0.04
PLA15PBAT 10H	2.47 ± 0.23	46.9 ± 0.7	3.9 ± 0.2	2.87 ± 0.02	71.5 ± 0.2	3.38 ± 0.01
PLA15PBAT 5H	2.59 ± 0.17	50.4 ± 0.5	8.9 ± 1.2	2.84 ± 0.01	74.1 ± 0.3	3.50 ± 0.01
PLA20PBAT 5H	2.45 ± 0.33	42.9 ± 0.8	16.5 ± 2.0	2.48 ± 0.01	64.3 ± 0.2	3.85 ± 0.04
PLA20PBAT	2.65 ± 0.24	44.7 ± 0.5	61.2 ± 10.6	2.16 ± 0.02	55.2 ± 0.4	3.74 ± 0.05
PLA20PBAT 1NCC	2.15 ± 0.15	44.1 ± 0.4	39.9 ± 5.3	2.13 ± 0.01	53.2 ± 0.3	3.84 ± 0.03
PLAPC	2.17 ± 0.16	31.6 ± 0.3	4.8 ± 0.6	2.03 ± 0.02	48.2 ± 1.1	3.13 ± 0.28
PLAPC 1NCC-1	2.37 ± 0.31	40.5 ± 0.2	9.6 ± 0.4	2.08 ± 0.01	57.6 ± 0.3	4.68 ± 0.08
PLAPC 1NCC-2	2.44 ± 0.24	40.7 ± 0.4	9.5 ± 0.8	2.02 ± 0.01	55.6 ± 0.3	4.71 ± 0.06
PLAPC 2NCC-1	2.38 ± 0.27	37.4 ± 0.2	8.7 ± 0.9	1.95 ± 0.01	52.6 ± 0.3	4.84 ± 0.11
PLAPC 2NCC-2	2.33 ± 0.11	37.3 ± 0.4	8.8 ± 1.0	1.88 ± 0.01	51.1 ± 0.2	4.86 ± 0.14
PLAPC 5NCC-1	2.23 ± 0.07	36.6 ± 0.5	8.6 ± 0.6	1.94 ± 0.01	51.6 ± 0.5	4.77 ± 0.13
PLAPC 5NCC-2	2.01 ± 0.13	31.5 ± 0.6	4.6 ± 0.3	1.81 ± 0.01	40.4 ± 2.4	3.00 ± 0.49

Table 4.

Summarized results from the tensile and flexural tests.

Figure 1.

Summarized results of the tensile strength (bars) and strain at break (line).

3.1.1 Results of the first-reactive compounding series

When hops were added to the blend of PLA and PBAT, increasing hops decreased tensile stiffness, strength and elongation at break, slightly increased flexural stiffness, decreased flexural strength, and flexural elongation. Simultaneously decreasing the hops content and increasing the PBAT content had no effect on tensile stiffness and decreased strength, but dramatically increased elongation at break, decreased flexural stiffness and strength, and increased elongation at flexural strength. It can be concluded that the addition of hops to the biocomposites lowered the strength, tensile stiffness, flexural stiffness, and elongation. The addition of PBAT lowered the stiffness and strength, but dramatically increased the elongation at break. Adding PBAT to PLA can improve PLA’s biggest drawback, its brittleness. The second conclusion is that reactive compounding for the combination of the thermoplastic matrix of PLA and PBAT modified with the chain extender and the compatibilizer with the addition of hops is the right technological approach for the production of biocomposites. The miscibility of the thermoplastics was achieved by the correct choice of the chain extender at the appropriate concentration, as the elongation at break at 20 wt% addition of PBAT to PLA and simultaneous addition of 5 wt% hops increased dramatically compared to the other biocomposites. Compared to the reference values for pure PLA, the stiffness and strength decreased significantly, but at the same time the elongation at break and an indicator of toughness was drastically increased.

3.1.2 Results of the second-reactive compounding series

The addition of NCC was compared with the mixture PLA/PBAT as a reference for the second series of samples. The addition of 1 wt% NCC decreased the stiffness, strength, and elongation at break, but increased the flexural strain. Compared with the sample from the first series PLA20PBAT 5H, the stiffness of the sample PLA20PBAT 1NCC was lower, the strength was higher, and the elongation at break was much higher. The flexural properties were all lower. It can be concluded that the addition of 1 wt% NCC decreased the strength, stiffness, and elongation due to the poorer wettability of NCC. The processing conditions of bio(nano)composites are not optimal for the incorporation of NCC into the thermoplastic matrix. Nevertheless, the elongation at break of bio(nano)composites with NCC is significantly higher compared to the hops composite, indicating that NCC is a suitable additive to increase the toughness of PLA-based bio(nano)composites. Despite the nonoptimal reaction conditions, a drastically higher toughness was achieved compared to PLA/PBAT/hops biocomposites and also to the PLA reference.

3.1.3 Results of the third-reactive compounding series

The third series of samples was used to test NCC and the effect of multiple compounding cycles on the properties of bio(nano)composites. For this series, the PLAPC blend was used as a reference. Compared with the pure PLA, the blend exhibited lower stiffness and strength and the same elongation at break. Further addition of NCC increased tensile stiffness and flexural elongation at 2 wt% addition, but decreased strength and elongation at break. Adding 5 wt% NCC decreased stiffness, strength, and elongation. An additional compounding cycle with 1 wt% NCC addition increased tensile stiffness and strength and maintained elongation at break, increased flexural strength, and decreased flexural stiffness and strength. An additional compounding cycle with 2 wt% NCC additive lowered tensile stiffness while maintaining tensile strength and elongation at break, lowered flexural stiffness and strength, and increased flexural strength. An additional compounding cycle with 5 wt% NCC addition reduced stiffness, strength, and elongation. Elongation was lower than for the PLA reference. It can be concluded that the reaction compounding conditions ensure good interfacial interactions between the thermoplastic matrix and the NCC and ensure good dispersion of the NCC in the thermoplastic matrix at NCC concentrations below 5 wt%. Comparing the first and the second reactive compounding cycles, it can be concluded that with one additional cycle of the reactive compounding with 2 wt% and 5 wt% NCC addition, degradation of PLA and possibly also of NCC already occurs, as evidenced by a decrease in tensile strength and elongation, which in the case of the PLA matrix is a good indicator of the onset of degradation of the PLA matrix, while the lower stiffness is an indicator of the onset of degradation of NCC. The degradation is most likely due to the high temperatures during reactive compounding and injection molding. It is more pronounced at higher NCC content, indicating simultaneous partial degradation of both the thermoplastic matrix and NCC. Reactive compounding of PLA and PC in the presence of a combination of two compatibilizers and a filler provides good miscibility of PLA and PC while ensuring good interfacial interactions and dispersion of NCC in the thermoplastic matrix at NCC concentrations below 2 wt%.

The highest tensile stiffness and strength were obtained for PLA15PBAT 5H with 2.59 GPa and 50.4 MPa, lower than the PLA reference (3.17 GPa and 71.8 MPa). The highest elongation at break was obtained for PLA20PBAT 1NCC with 39.9%, much higher than the pure PLA reference (4.8%). If good thermal stability of the bio(nano)composite is also required, then PLAPC 1NCC with a tensile stiffness of 2.37 GPa, strength of 40.5 MPa, and elongation at break of 9.6% would be the best choice.

3.2 Thermomechanical properties

The results of the dynamic mechanical evaluation are shown in Figures 2–9. The results for the PLA sample are used as reference values for all other modifications by reactive compounding and for the evaluation of the usability of the reactive compounding.

Figure 2.

Summarized results of storage modulus for the first series of the samples.

Figure 3.

Summarized results of loss factor for the first series of the samples.

Figure 4.

Summarized results of storage modulus for the second series of the samples.

Figure 5.

Summarized results of loss factor for the second series of the samples.

Figure 6.

Summarized results of storage modulus for the first compounding cycle.

Figure 7.

Summarized results of loss factor for the first compounding cycle.

Figure 8.

Summarized results of storage modulus for the second compounding cycle.

Figure 9.

Summarized results of loss factor for the second compounding cycle.

3.2.1 Results of the first-reactive compounding series

When hops were added to the blend of PLA and PBAT, the increasing amount of hops lowered the storage modulus in the glass transition region and allowed cold crystallization at lower temperatures. Simultaneously reducing the amount of hops and increasing the PBAT content further lowered the storage modulus from room temperature to the glass transition region. The onset of cold crystallization was comparable, and the height of the storage modulus was lower than that of the PLA15PBAT sample. The height of the peak of the loss factor at the glass transition of PLA in biocomposites decreased with increasing PBAT content. The position of the peak decreased with increasing hop content, and it also decreased with increasing PBAT content. Compared with the pure PLA reference material, all PLAPBAT biocomposites with hops had a lower storage modulus and also a lower glass transition temperature. The onset of cold crystallization in the biocomposites indicated good interfacial adhesion between the thermoplastic matrix and hops. PLA and PBAT, as well as homogenized hops, were successfully blended into biocomposites by reactive compounding.

3.2.2 Results of the second reactive compounding series

The addition of NCC was compared with the mixture of PLA/PBAT as a reference for the second series of samples. The addition of 1 wt% NCC reduced the storage modulus at the beginning of the glass transition and in other regions was comparable with the reference PLA20PBAT. Compared with the sample from the first series PLA20PBAT 5H, the storage modulus of the sample PLA20PBAT 1NCC was lower due to the higher TPU content. The same results are shown in the dissipation factor, where the dissipation factor of the sample PLA20PBAT 1 NCC at the beginning of the glass transition was slightly higher than that of the reference PLA20PBAT. In addition, the onset of cold crystallization is seen slightly earlier and the peak is higher. The reactive compounding allowed good surface interactions between the thermoplastic matrix and the NCC, homogeneous dispersion of the NCC in the matrix, and good mixing of PLA and PBAT.

3.2.3 Results of the third-reactive compounding series

Storage modulus curves showed the first drop in glass transition temperature for PLA. The drop is more significant for PLA-based blends compared to PLA-based blends with NCC. The storage modulus was higher in this range (75–100°C) with higher NCC content. Above 100°C, the storage modulus increased due to cold crystallization of the material. The lowest peak for the cold crystallization temperature was for sample PLAPC (114°C), and the highest peak was for sample PLAPC 1NCC-2 (119°C). For all samples with NCC addition, the peak for cold crystallization temperature in the second compounding cycle was at a higher temperature than in the first compounding cycle. The addition of NCC to PLA-based compounds inhibited the cold crystallization of PLA. At low NCC loading (1 wt%), the NCC acted as a reinforcement for the PLA-based blend; at higher loadings (2 wt% and 5 wt%), the stiffness of the nanocomposites decreased below the stiffness of the neat matrix. Despite the inhibitory effect of NCC on cold crystallization, the NCC prevented softening of the PLA-based matrix after the glass transition temperature of PLA. The dissipation factor curve shows two sharp peaks at 69°C and 160°C (Figures 4 and 5) for PLA and PC matrices, respectively. The height of the first peak is the lowest for sample PLAPC 1NCC and the highest for sample PLAPC. The height of the second peak is the lowest for sample PLAPC and the highest for sample PLAPC 1NCC. In the second compounding cycle, the heights of the peaks were higher, indicating the beginning of the degradation of the matrix. The most elastic behavior is exhibited by sample PLAPC 1NCC after the first compounding cycle. The good surface interaction of NCC with the matrix due to the compatibilizer in sample PLAPC 1NCC increased the storage modulus maintained the storage modulus at a high level and decreased the height of the peak of the loss factor for PLA. The higher peak of dissipation factor for sample PC is due to the highest cold crystallization temperature for sample PLAPC 1NCC, and thus the overlap of the glass transition temperature of PC and the onset of melting of PLA.

The highest tensile stiffness and strength were obtained for PLA15PBAT 5H with 2.59 GPa and 50.4 MPa, which is lower than the PLA reference (3.17 GPa and 71.8 MPa).

The highest storage modulus up to the glass transition zone was achieved by the PLAPBAT samples with the addition of hops. The highest temperature stability was achieved with sample PLAPC 1NCC. All DMA results show that reactive extrusion is a suitable processing technology for bio(nano)composites even without surface modification of natural fibers.

3.3 Thermal properties

The results of the DSC evaluation are shown in Table 5 and Figures 10 and 11. The results for the PLA sample were used as reference values for all other modifications by reactive compounding and for the evaluation of the usability of the reactive compounding.

Sample	Tg (°C)	ΔCp (J/gK)	Tcc (°C)	ΔHcc (J/g)	Tm (°C)	ΔHm (J/g)	Diff. ΔHm (J/g)
PLA	59.7	0.24	118.5	13.32	152.5	13.47	0.15
PLA15PBAT 10H	58.5	0.14	126.4	7.48	153.7	8.07	0.59
PLA15PBAT 5H	59.5	0.19	129.4	4.75	154.0	5.05	0.30
PLA20PBAT 5H	59.9	0.16	131.6	1.82	154.3	2.74	0.92
PLA20PBAT	60.5	0.33	125.8	4.01	150.8	5.82	1.81
PLA20PBAT 1NCC	60.3	0.36	139.1	0.29	150.9	0.60	0.31
PLAPC	59.8	0.11	123.6	5.35	152.0	5.37	0.02
PLAPC 1NCC-1	60.8	0.09	133.3	0.32	152.2	0.37	0.05
PLAPC 1NCC-2	60.6	0.09	128.3	0.04	152.2	0.33	0.29
PLAPC 2NCC-1	61.1	0.08	133.4	0.13	152.4	0.31	0.18
PLAPC 2NCC-2	60.7	0.09	131.6	0.18	152.8	0.52	0.34
PLAPC 5NCC-1	61.1	0.06	130.4	0.24	153.2	0.48	0.24
PLAPC 5NCC-2	60.1	0.08	128.4	1.28	152.9	1.67	0.39

Table 5.

Summarized results from the second heating from DSC tests.

Figure 10.

Summarized results of crystallization enthalpy of the samples after aging at 90°C for 100 s and various cold crystallization times at 120°C.

Figure 11.

Summarized results of melting temperature of the samples after aging at 90°C for 100 s and various cold crystallization times at 120°C.

3.3.1 Results of the first-reactive compounding series

When hops were added to the blend of PLA and PBAT, the increasing amount of hops lowered the cold crystallization temperature and melting temperature and improved the crystallinity. The glass transition temperature was not affected. Simultaneously reducing the hops content and increasing the PBAT content increased the cold crystallization temperature, melting temperature, and crystallinity. Increasing the PBAT content increased the cold crystallization temperature and crystallinity. Compared with the pure PLA reference, the cold crystallization temperature, melting temperature, and crystallinity were increased for biocomposites.

3.3.2 Results of the second-reactive compounding series

The addition of NCC was compared with the mixture of PLA/PBAT as a reference for the second series of samples. With the addition of 1 wt% NCC, the cold crystallization temperature increased and the crystallinity was reduced.

3.3.3 Results of the third-reactive compounding series

At the first compounding cycle, the cold crystallization temperature, melting temperature, and crystallinity were increased for all bio(nano)composite samples compared with the PLAPC reference. During the second compounding cycle, the cold crystallization temperatures were decreased and the crystallinity was increased.

Higher crystallinity indicates good homogeneity of the bio(nano)composites.

The crystallization kinetics were characterized with flash DSC for the third series of samples (Figures 10 and 11). The onset of formation of the crystal units was after aging at 90°C for 100 s and crystallization at 120°C after 100 s for all samples. The shorter time is not sufficient for the formation of the crystal moieties formation. Higher NCC loading promoted the crystal moieties formation as well as the second compounding cycle. The fastest and largest increase in crystal moieties was for sample PLAPC 5NCC-2 up to the cold crystallization time 600 s, and then for sample PLAPC 1NCC-2. Therefore, the conclusion can be made that NCC inhibited cold crystallization at shorter times and enhanced cold crystallization at longer times at elevated temperatures. The mobility of PLA chains at elevated temperatures reached a threshold for the formation of crystal units after 600 s at 1 wt% and 2 wt% NCC loading. At 5 wt% NCC loading, 100 s is sufficient due to the higher amount of NCC particles in the matrix. From the results, we can also conclude that agglomeration of NCC occurs to a smaller extent and increases with increasing NCC content.

3.4 Impact properties

The results of the toughness evaluation are shown in Figure 12. The results for the PLA sample are used as reference values for all other modifications by reactive compounding and for the evaluation of the usability of the reactive compounding.

Figure 12.

Summarized results of impact strength.

3.4.1 Results of the first-reactive compounding series

Higher hops content and lower PBAT content in biocomposites PLAPBAT/hops lowered impact strength. On the contrary, a lower hops loading and a higher PBAT loading led to a larger scatter of the measurement results.

3.4.2 Results of the second-reactive compounding series

The addition of NCC was compared with the mixture of PLA/PBAT as a reference for the second series of samples. Both the reference and the PLA20PBAT 1NCC sample showed excellent impact resistance, as they did not break during the impact test.

3.4.3 Results of the third-reactive compounding series

The addition of NCC to the PLAPC mixture improved the toughness of the bio(nano)composites. The exception is the sample PLAPC 5NCC-2 due to the degradation of PLA and NCC. The best toughness was obtained for sample PLAPC 1NCC-1. Higher NCC loading decreased the toughness as well as the second compounding cycle.

Sample PLA20PBAT 1NCC showed the best toughness, followed by sample PLAPC 1NCC and sample PLA20PBAT 5H. It is obvious that NCC has successfully improved the toughness of bio(nano)composites through the appropriate processing technology—reactive compounding.

4. Conclusion

Reactive compounding was used for bio(nano)composites with PBAT and PC in addition to the main PLA matrix together with appropriate compatibilizers and (processing) additives. The adequacy of the reactive compounding was evaluated by characterizing the mechanical, thermomechanical, and thermal properties, as well as toughness.

The evaluation of mechanical properties showed that novel properties were achieved by the addition of NCC. The blend was able to achieve either high toughness with the addition of PBAT or high-temperature stability with the addition of PC. The prepared bio(nano)composites showed good miscibility of PLA and PBAT or PLA and PC and good surface interaction between the thermoplastic matrix and the natural fibers, although the surface of the natural fibers was not modified. Furthermore, the flash DSC results showed an altered morphology behavior of the PLAPC 1NCC-2 bio(nano)composite. Longer residence time at elevated temperature accelerates crystallization as a result of the degradation of PLA and NCC due to shorter PLA chains and smaller NCC particles, which acts as nuclei for the initiation of heterogeneous crystallization of PLA. At the same time, we can observe that the NCC is well distributed in the thermoplastic matrix due to the increasing crystallinity. For the PLA/PBAT blends, good miscibility was achieved with the proper processing parameters and by using appropriate chain extenders. Good surface interaction between the thermoplastic matrix and the natural fibers was achieved with the proper compatibilizers and loading. The adequacy of the reactive compounding was evaluated by the simultaneous increase in stiffness and elongation at break in the tensile test, the change in storage modulus and loss factor in DMA, the change in cold crystallization temperature and crystallinity in DSC, and the increase in impact strength. The prepared bio(nano)composites showed tougher behavior while maintaining high stiffness and strength. The addition of NCC also affected the morphology of the bio(nano)composites, which can be controlled by the processing parameters. The second reactive compounding cycle at 1 wt% NCC loading showed that recycling of the novel bio(nano)composites can also be performed without much influence on the properties of the recycled products. The present work shows that the existing polymer processing equipment is suitable for the production of bio(nano)composites and their recycling. Sustainable design was the guiding principle for conducting the research with surface-unmodified natural fibers to avoid the use of chemicals and thus minimize the impact of bio(nano)composites on the environment.

Reactive compounding is a suitable processing technology for bio(nano)composites, even if the surface of natural fibers is not modified, to achieve novel properties of PLA-based blends with natural fibers (preferably NCC). The desired properties can be developed by suitable compatibilizers and processing additives during reactive compounding. To describe the dependence on the amount of added NCC in bio(nano)composites, the addition of less than 1 wt% NCC in PLA-based blends bio(nano)composites should be investigated in further research.

Acknowledgments

We would like to thank Navitas for providing nanocrystalline cellulose.