Introduction

Environmental-friendly polymers such as poly(lactic acid) (PLA) have been drawn increasingly attention, because they can be totally decomposed by some microorganisms in the environment. PLA can be produced from plant like corn, cassava with starch compounds, which is more recyclable and renewable than petroleum-based polymers [1, 2]. It has good biodegradability, biocompatibility and relatively excellent mechanical properties due to its main functional groups of the ester group, hydroxyl group and carboxyl group, and it can be applied in food packaging, medical treatment and other aspects [2,3,4,5]. However, due to the poor toughness of PLA, the elongation at break is less than 10%, PLA cannot meet the requirement of plastic deformation under high stress.

The properties of fillers have a significant influence on the properties of PLA martix. As a biomass filler, culms of A. donax filler (ADF) was used as reinforcement for PLA biocomposites. The tensile modulus increased by 39% than that of neat PLA. Results showed that the ADF stiffness can reinforce the neat PLA [6, 7]. Caitlyn et al. used cellulose-nanofibrils (CNFs) and cellulose nanocrystals (CNCs) to reinforce PLA. Results showed that the addition of CNC and CNF can affect the crystal microstructure of PLA [8]. Some researchers used biomass residues and carbon materials such as graphene oxide, carbon nanotubes, and carbon fiber nanoparticles as fillers to improve the mechanical properties of polymer matrix [9]. Multi-walled carbon nanotube was used to reinforce PLA/PEG biocomposites. Results showed that the tensile strength and tensile modulus were increased by 400% and 50%, respectively [10]. With a large aspect ratio, high specific strength and specific stiffness, bamboo fiber has unique advantages as polymer reinforced fillers. Our previous research studied ultrafine bamboo-char (UFBC) and bamboo particle (BP) to strengthen and toughen the PLA biocomposites. Results showed that the maximum tensile strength, modulus, and elongation at break of 45.20 MPa, 540.50 MPa, and 7.53% were achieved, respectively with the addition of UFBC and BP. Interestingly, a concrete-like microstructure system was confirmed in the PLA/BP/UFBC biocomposites [11]. However, the flow ability and toughness of PLA still have potential for improvement.

It is crucial to improving the toughness of PLA biocomposites reinforced by rigid fillers. PLA biocomposites could been reinforced by rigid fillers such as carbon nanotubes, metal hydroxide, layered and hydroxyapatite. Although strength is important for polymers, toughness is also indispensable [12]. PEG as an effective plasticizer can be used to improve the toughness and reducing the brittleness of PLA materials [13]. PEG has been used in many fields including drug-delivery, electroplating, pesticides, metal processing and food processing due to its nontoxicity, biocompatibility and good solubility in organic solvents [14]. Compatibility of PEG with other polymers allows the processing efficiency improvement of polymer matrix, resulting in good thermal stability, mechanical properties, and permeability of the biocomposites [15, 16].

The plasticization performance of PEG is related to the molecular weight. The low molecular weight of PEG (376–460 g/mol) can improve the compatibility and plasticity of the PLA matrix significantly [17]. Raisipour-Shirazi et al. [18] prepared the nano-fibrillated cellulose whisker (NFC) and used PEG (350 g/mol) as a plasticizer for PLA/NFC by solution casting. The crystallinity of PLA/NFC-PEG increased by 18% in comparison with PLA/NFC. Deng et al. found that PEG-grafted cellulose nanofibers dispersed more uniformly in PLA matrix. The high strength (343 MPa) of the PLA composite film with PEG-grafted cellulose nanofibers was achieved. PEG can increase the ductility and tough of PLA matrix composites [19]. However, few researches have investigated the effect of PEG on the crystallinity and mechanical properties of PLA ternary biocomposites.

Our previous study used the silane-compatibilized bamboo cellulose nanowhiskers (BCNW) toughen the PLA composites. Results showed that the tough strength and modulus of the PLA biocomposites decreased [20]. On the other hand, the bamboo cellulose BCNW was prepared by complex hydrolysis process. The bamboo particles (BP) and ultrafine bamboo-char (UFBC) can improve the tensile strength and modulus of PLA and PP biocomposites significantly [21]. The bamboo-char was produced by pyrolysis of bamboo. Bamboo-char is ground by high-energy ball mill to produce the ultrafine bamboo-char. The ultrafine bamboo-char was added into the biocomposites to increase the tough strength of the composites. The toughness of PLA/BP/UFBC biocomposites was decreased after adding the rigid additives. To improve the toughness of PLA/BP/UFBC biocomposites, PEG was used to improve the interface of the PLA matrix and the additives in this study. This work aims to improve the toughness of PLA/BP/UFBC biocomposites and investigate the combination between the PLA matrix and the rigid fillers including BP and UFBC with PEG (400 g/mol). The mechanical and thermal properties of the PLA/BP/UFBC-PEG biocomposites were analyzed. The surface functional groups of the biocomposites were analyzed by FTIR. The fracture surface morphology of the biocomposites were observed by SEM. The mechanism of PEG improving the toughness of PLA/BP/UFBC-PEG biocomposites were analyzed. These results will provide a new idea to promote the performances of PLA ternary biocomposites in engineering fields.

Materials and Methods

Materials

PEG 400 (400 g/mol, pH 4–7) was purchased from Dongnan Industrial Co, Shanghai, China. Moso-bamboo particles (BP), supplied by China National Bamboo Research Center in Hangzhou, was harvested in Anji, Zhejiang, China. Poly(lactic acid) PLA, (melt flow index: 16.2 g/min at 190 °C and 2.16 kg), ES701, Tongjieliang Biomaterial Co., Ltd. Shanghai, China. Ultrafine bamboo-char (UFBC) was supplied by Tonfu Agri. Biomass Sci. Co. Hangzhou, China. The particle size distributions of BP and UFBC were analyzed by a laser particle size analyzer (Mastersizer 3000, England).

Preparation of PLA/BP/UFBC-PEG Ternary Biocomposites

BP and UFBC were dried in a blast drying oven for 16 h at 105 °C. PLA was placed in a vacuum drying oven and dried under vacuum for 16 h at 55 °C and cooled to room temperature. The weight of PLA/BP/UFBC was set to 50 g. The ratio of the PLA, BP and UFBC were set as our previous study [11]. The composites formulations are shown in Table 1.

Table 1 Biocomposites formulations
Full size table

The biocomposites including the PLA, BP and UFBC were mixed with different ratios of PEG (0, 2, 4, 6, 8 and 10 wt% to the mixtures, respectively). The mixtures were fed into a double roll lab-scale compounder (HL-200, Sci. & Edu. Instru. Co., Jilin Univer., China) at 180 °C for 20 min. After melting and mixing, the mixtures were placed in a grinder (QE-2000, Yili Co., Ltd., Zhejiang, China) and grounded into small particles of 1.5 ± 0.05 mm diameter. The small particles were placed in a micro-injection molding machine (WZS10D, Xinshuo Co., Ltd., Shanghai, China) for pre-heating at 180 °C for 5 min. Then, the small particles were injected into an injection molding cylinder at 5 MPa and held for 10 s, extruded injection to form a biocomposites sample. All samples were kept in a desiccator for mechanical and thermal tests. The pure PLA specimens were prepared in the same conditions.

Mechanical Properties

Tensile and flexural tests of PLA/BP/UFBC-PEG ternary biocomposites and PLA specimens were carried out with a universal mechanical testing machine (CMT4503, MTS Inc) according to ASTM D638 and ASTM D790. The gauge length and the crosshead speed for tensile tests were set to 50 mm and 20 mm/min, support span for flexural tests were set to 60 mm and 5 mm/min, respectively. The data provided for the analysis was the average value of 3 specimens.

Differential Scanning Calorimetry

The thermal property was investigated by differential scanning calorimetry (DSC, 200F3, Netzsch, Germany). Approximately 10 mg of dry sample was weighed and hermetically sealed in an aluminum crucible. The sample was heated from room temperature to 200 °C at a rate of 10 °C /min and held isothermally for 5 min, followed by cooling down to room temperature at a rate of 10 °C/min. The reheated up to 200 °C at a rate of 10 °C/min. The nitrogen purge rate was 50 mL/min. Parameters include glass-transition temperature (Tg), melt crystallization temperature (Tmc), melt crystallization enthalpy (△Hmc), melting crystallinity (Xmc), cooling crystallization temperature (Tcc), cooling crystallization enthalpy (△Hcc), melting temperature (Tm), melting enthalpy (△Hm) and crystallinity (Xc). The second heating process was recorded.

Crystallinity (Xc) was estimated according to the following equation.

$$X_{{\text{c}}} \left( \% \right) = \frac{{\Delta H_{{\text{m}}} { - }\Delta H_{{{\text{cc}}}} }}{{\Delta H_{{0}} \times X_{{{\text{PLA}}}} }} \times 100\% ,$$
(1)

where ∆Hm refers to the melting enthalpy of PLA/BP/UFBC-PEG ternary biocomposites; ∆Hcc refers to the cold crystallization enthalpy of PLA/BP/UFBC-PEG biocomposites, XPLA refers to the weight ratio of PLA in PLA/BP/UFBC-PEG ternary biocomposites. ∆H0 refers to the melting enthalpy of pure PLA, which is 93.6 J/g [22, 23].

Fracture Surface Morphology

Surface morphology of UFBC, tensile fracture surfaces of PLA and PLA/BP/UFBC-PEG ternary biocomposites were observed using a scanning electron microscopy (SEM) (SU8010, Hitachi, Japan). The samples were coated with gold before observation. The launching voltage of the electron microscope was 10.0 kV.

Fourier Transform Infrared Spectroscopy (FTIR)

A Fourier transform infrared spectroscopy (FTIR) (Nicolet 6700, Thermo Fisher Nicolet Corporation, USA) was used to analyze the functional groups on the surface of samples. The data were obtained in the wavenumber range from 4000 to 500 cm−1 [24]. The KBr pellet technique was used to obtain the spectra. The details are as follows: 200 mg KBr and 1–2 mg sample were put into an agate mortar and grind them in one direction for 1–2 min. Put the grinded sample evenly into the mold with the medicine spoon, then put the mold on the tablet press, tightened the screw and closed the vent valve, pressurize to 20 MPa, and stay for 1–2 min. PLA/BP (70/30) biocomposites and PLA/BP/UFBC (70/27/03) ternary biocomposites were also characterized by FTIR according to the same method as above.

Results and Discussion

Particle Size Distribution (PSD) Analysis

Table 2 showed the particle size distribution of BP and UFBC. The particle size of BP is much larger than UFBC. The role of BP is to increase the toughness of the PLA matrix. Most of UFBC is usually smaller than 45 μm, which has strong toughening and reinforcing effects on the PLA. On the other hand, UFBC with smaller particle size can disperse better in the PLA matrix during the melt and mix process. The particle size distribution of BP and UFBC have a significant influence on the mechanical properties of the biocomposites [25].

Table 2 The particle size distribution of BP and UFBC
Full size table

Mechanical Properties

The tensile strength, tensile modulus, and elongation at break of PLA/BP/UFBC-PEG biocomposites with different contents of PEG are shown in Fig. 1a. The tensile strength of the ternary biocomposites decreased with the addition of PEG. It may be due to PEG increased the movement of PLA molecular chain in the PLA/BP/UFBC biocomposites, weakened the intermolecular interaction force of the biocomposites [26]. The tensile strength of 31 MPa and tensile modulus of 870 MPa of PLA/BP/UFBC-PEG were achieved with the addition of 6 wt% PEG, which is much higher than that of PLA/BP/UFBC. It indicates that the molecular chain movement of PLA reached a relatively balanced state with the appropriate PEG addition, and the biocomposites have higher stiffness and less deformation. The addition of 10 wt% PEG decreased the tensile strength of biocomposites. The tensile strength decreased from 38 to 18 MPa. The tensile modulus decreased from 530 to 440 MPa. When the addition amount was 10 wt%, the elongation at break of biocomposites increased to 6.1% in comparison to PLA/BP/UFBC. As shown in the DSC analysis, the 70/27/03-PEG6 biocomposites have higher melt crystallization temperature (Tmc) and melt crystallinity (Xmc). It indicates that the addition of PEG can effectively improve the mobility of PLA molecular chain and the plasticity of the PLA biocomposites. It may attribute the increase of tensile modulus. On the other hand, the molecular weight of PEG has influence on the tensile flexural. The molecular weight of PEG is only 400 mol/g. When the addition of PEG is more than 6%, it may hinder the availability of the PEG molecules to the PLA macromolecules, resulting in decreasing the flexural modulus [27].

Fig. 1
figure 1

The mechanical behavior of PLA/BP/UFBC-PEG biocomposites. a Tensile properties, b Flexural properties

Full size image

The flexural strength, flexural modulus, and flexural strain at break of PLA/BP/UFBC-PEG biocomposites with different addition content of PEG are shown in Fig. 1b. The addition of PEG reduced flexural strength, flexural modulus, and increased the flexural strain at break compared to PLA/BP/UFBC. When the addition of PEG content was in the range of 2–8 wt%, the flexural modulus of PLA/BP/UFBC-PEG was maintained within the range of 413–352 MPa. The flexural modulus decreased significantly with the addition of 10 wt% PEG (163 MPa). The flexural strain at break increased sharply and reached to 6.79% with the addition of 10 wt% PEG. The flexural strain at break of PLA/BP/UFBC-PEG10 increased by 565.69% in comparison with PLA/BP/UFBC. Figure 2 shows the deformation of PLA/BP/UFBC-PEG8 and PLA/BP/UFBC-PEG10 biocomposites during the flexural performance test. The flexural resist ability of the 70/27/03-PEG10 was much higher than that of the 70/27/03-PEG8, which is consistent with the higher flexural fracture strain of the biocomposites.

Fig. 2
figure 2

Flexural properties measurements of PLA/BP/UFBC-PEG biocomposites. a PLA/BP/UFBC-PEG8% biocomposites; b PLA/BP/UFBC-PEG10% biocomposites

Full size image

Thermal Properties

Figure 3 showed the DSC thermograms of the PLA/BP/UFBC-PEG biocomposites. Exothermic melt crystallization peaks of the PLA/BP/UFBC2 and PLA/BP/UFBC-PEG4 biocomposites were not detected. The crystallization rate of PLA was slow and the PLA molecular chain did not complete rearrangement and was difficult to melt and crystallize [28]. However, when the biocomposites were reheated for the second time, exothermic cold crystallization peaks were observed. It may attribute that nucleation from the glass state is much easier than that from the molten state [29]. The PLA/BP/UFBC, PLA/BP/UFBC-PEG2, and PLA/BP/UFBC-PEG4 biocomposites with slow crystallization rate were prone to cold crystallization rather than melting crystallization. Results showed that when the addition of PEG was less than 4 wt%, the crystallization rate of PLA did not change significantly. Table 3 is thermal property parameters of the biocomposites, including melt temperature (Tmc), melt crystallization enthalpy (△Hmc), melting crystallinity (Xmc), glass transition temperature (Tg), cold crystallization temperature (Tcc), cold crystallization enthalpy (△Hcc), melting temperature (Tm), melting enthalpy (△Hm) and crystallinity (Xc). As shown in Table 3, comparing with the PLA/BP/UFBC biocomposites, the glass transition temperature (Tg) and cold crystallization temperature (Tcc) of the PLA/BP/UFBC-PEG biocomposites decreased as the amount of PEG increased from 2 to 4 wt%, indicating that the addition of the PEG increased the mobility of the PLA molecular chain and improved the crystallization rate of the biocomposites. The PEG had a plasticizing effect on PLA. It might attribute to that the small molecular PEG could go into PLA molecular chain, resulting in improving the lubrication of the PLA molecular chain [30]. It is equivalent to shielding the groups of PLA and reducing the physical crosslinking points [25].

Fig. 3
figure 3

The DSC thermograms of PLA/BP/UFBC-PEG ternary biocomposites: a cooling processing, b second heating processing

Full size image
Table 3 Thermal behaviors of PLA/BP/UFBC-PEG composites
Full size table

As shown in Fig. 3b, the melting crystallization temperature (Tmc) of the PLA/BP/UFBC-PEG6 was 76.8 °C. And as the amount of PEG increased from 6 to 10 wt%, there was no obvious difference in the melting crystallization peak of the biocomposites. The PLA/BP/UFBC-PEG6 biocomposites had a higher melting crystallization temperature (Tmc). It indicates that when the amount of addition reached 6–10 wt%, the addition of PEG could significantly increase the PLA crystallization rate and the biocomposites were more easily to melt and crystallize [31]. Table 3 shows that the PLA/BP/UFBC-PEG6 biocomposites had a higher melting crystallization temperature (Tmc) and melt crystallinity (Xmc), which confirmed the mechanical properties change of PLA/BP/UFBC-PEG6 biocomposites.

It can also be seen from Fig. 3b and Table 3 that the PLA/BP/UFBC-PEG biocomposites with PEG addition of 6 wt%-10 wt% did not undergo glass transition and cold crystallization in the second heating process, which confirmed the melting crystallization behavior of PLA/BP/UFBC-PEG biocomposites [32, 33]. The cold crystallization behavior of biocomposites disappeared with the addition of 7 wt% PEG. PEG can be used as a high-efficiency plasticizer for PLA and can increase the PLA crystallization rate [34]. The melting temperature of PLA/BP/UFBC-PEG biocomposites gradually decreased when PEG was increased from 6 to 10 wt% and the mobility and plasticity of PLA molecular chain were improved. The melting crystallinity and cold crystallinity of PLA/BP/UFBC-PEG biocomposites decreased with the increase of PEG addition.

Morphology Analysis

Figure 4 showed the SEM of the tensile fracture surface of PLA/BP/UFBC-PEG biocomposites. As shown in Fig. 4a, fault phenomenon on the tensile fracture surface of PLA/BP/UFBC biocomposites were observed. It indicates that the interaction between PLA and fillers in PLA/BP/UFBC biocomposites were weaker than that of the PLA/BP/UFBC-PEG biocomposites. As shown Fig. 4b, PLA/BP/UFBC-PEG biocomposites have been elongated and deformed in the direction of molecular tension. The wire-drawing phenomenon can be observed in the tensile fracture surface of PLA/BP/UFBC-PEG [35]. The addition of PEG improved the dispersion and adhesion of fillers including UFBC and BP in PLA/BP/UFBC-PEG ternary biocomposites. The interface compatibility was improved by adding PEG. The unfractured bamboo particles can be observed. PEG increased the interface adhesion between PLA matrix and fillers, which contributed to transfer the flexural stress when the biocomposites bending [36].

Fig. 4
figure 4

SEM images: the tensile fracture surface a PLA/BP/UFBC, b PLA/BP/UFBC-PEG, c and d the flexural fracture surface of PLA/BP/UFBC-PEG

Full size image

FTIR Analysis

Fig. 5 showed the infrared spectrum of PLA/BP, PLA/BP/UFBC, and PLA/BP/UFBC-PEG biocomposites. The broad peak in the wavelength range of 3300–3500 cm-1 is mainly caused by the stretching vibration of the −OH bond and the characteristic peak exists in the spectrum of each sample. In addition, In comparison with the characteristic absorption band of PLA/BP biocomposites, two characteristic peaks were found on the infrared spectrum of PLA/BP/UFBC ternary biocomposites, which is −CH3 stretching vibration peak at 2840 cm−1 and C=C bond stretching vibration peak in the range of 1390–1650 cm−1. This indicates UFBC with a large amount of aromatic functional groups on the surface, which changed the surface group of the biocomposites material. This could explain the improvement of flowability and surface hydrophilicity of the PLA/BP biocomposites after adding UFBC. As shown in FTIR analysis, plasticizer PEG has an influence on the surface functional groups of the PLA biocomposites and the interaction between PLA, BP and UFBC. The characteristic absorption peak of PLA/BP/UFBC-PEG8 biocomposites are similar to that of PLA/BP/UFBC. The characteristic absorption bands of PEG at 3400 cm−1, 2896 cm−1 and 1456 cm−1 ascribed to the stretch vibration of OH, stretch vibration −CH3, and shear vibration of −CH, respectively [37]. The characteristic spectrums of PLA/BP/UFBC-PEG10 biocomposites changed significantly. The intensity of the −CH3 bond at 2840–2910 cm−1 decreased as well as the stretching vibration of aromatic C=C bond at 1390–1650 cm−1 [38]. As shown in Fig. 5, the intensity of the −CH3 bond at 2840–2910 cm−1 decreased significantly after adding PEG in the PLA/BP/UFBC biocomposites, especially PLA/BP/UFBC biocomposites with the 10 wt% PEG. It indicates that PEG improved the interaction between PLA molecule chains, resulting in improvement of the toughness of PLA biocomposites.

Fig. 5
figure 5

FTIR spectra of PLA-based biocomposites

Full size image

Conclusions

In this study, the addition of PEG improved the interaction between PLA molecular chains and fillers, and enhanced the toughness of the PLA/BP/UFBC ternary biocomposite as well. With the increase of PEG loading, the flexural strain at break increased to 6.79%, and the flexural toughness was significantly improved. PLA/BP/UFBC-PEG6 biocomposites had the higher melting crystallization temperature, compared to other PEG-plasticized PLA/BP/UFBC biocomposites. The crystallization rate and the plasticity of PLA/BP/UFBC ternary biocomposites increased and the processability was improved when PEG addition was 6–10 wt%. This study provides an approach for toughing PLA/BP/UFBC ternary biocomposites and balancing the rigidity and toughness properties of PLA biocomposites, which will help to expand its application.