Glass fiber reinforced PLA composite with enhanced mechanical properties, thermal behavior, and foaming ability

Polymer 181 (2019) 121803

Contents lists available at ScienceDirect

Polymer journal homepage: www.elsevier.com/locate/polymer

Glass fiber reinforced PLA composite with enhanced mechanical properties, thermal behavior, and foaming ability

T

Guilong Wanga,∗, Dongmei Zhanga, Gengping Wanb, Bo Lia, Guoqun Zhaoa a b

Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan, Shandong, 250061, China Key Laboratory of Chinese Education Ministry for Tropical Biological Resources, Hainan University, Haikou, Hainan, 570228, China

H I GH L IG H T S

glass fibers (GF) were incorporated to enhance polylactic acid (PLA). • Silane-modified led to simultaneously enhanced strength, rigidness and toughness of PLA. • GF increased the heat deflection temperature of PLA. • GF can greatly improve the foaming ability of PLA. • GF • Low-density microcellular PLA/GF foams were prepared.

A R T I C LE I N FO

A B S T R A C T

Keywords: Polylactic acid Glass fiber Composite Microcellular foam

Polylatic acid (PLA) and PLA foams show a promising prospect for replacing the traditional petroleum-based polymers and foams. However, PLA shows poor ductility, thermal stability and foaming ability, and its application is significantly limited. Herein, silane-modified glass fibers (m-GF) were adopted to improve the mechanical properties, thermal stability, and foaming ability of PLA. PLA/m-GF composites with different GF contents were firstly prepared by twin-screw compounding. Microscopic morphology analysis showed that silane-modified GF has a good bonding with PLA matrix, and increasing GF content led to slight decreased of GF length. Mechanical testing showed that GF led to simultaneously enhanced strength, rigidness, and toughness. The higher the GF content is, the more obvious the reinforcement effect is. With 20 wt% GF, the PLA/m-GF composite shows almost 2-fold enhanced strength and rigidness, and more than 3-fold enhanced impact toughness than the pure PLA. The outstanding mechanical properties arises from the strengthening effect of the GF network skeleton that shows good bonding with PLA matrix. Thermal analysis showed that GF led to increased heat deflection temperature but reduced melt flow index of PLA. Foaming experiments showed that GF can dramatically improve the foaming ability by increasing expansion ratio and refining cellular morphology. Microcellular PLA/m-GF foam with an expansion ratio of up to 20-fold and cell sizes less than 10 μm was achieved. Thus, the strong PLA/m-GF composites and their foams show a promising future in preparing lightweight structural components used in many applications such as automotive and aircraft industries.

1. Introduction The extensive use of petroleum-based plastics has caused growing environmental concerns not only because the production of petroleumbased plastics consumes a lot of unsustainable petrochemical resources and generates a lot of green-house gas emissions but also the ubiquitous plastic residue poses a serious potential ecological threat [1–3]. The development of high-performance biopolymers to replace the petroleum-based polymers becomes increasingly important for achieving

∗

sustainable development of our society [4]. Polylactic acid (PLA) is a linear aliphatic thermoplastic polyester derived from fully renewable sources such as corn and wheat [5], which has high level of biocompatibility, good biodegradability, high stiffness, and UV stability [6,7]. Therefore, PLA is today the most used biopolymers, and has been widely used in biomedical, textile and food packaging industries [8–10]. With the advance of production technology and the increase of production scale [11,12], the cost and price of PLA reduces gradually in recent years [13], which in turn promotes the more widely application

Corresponding author. E-mail address: [email protected] (G. Wang).

https://doi.org/10.1016/j.polymer.2019.121803 Received 20 July 2019; Received in revised form 8 September 2019; Accepted 11 September 2019 Available online 13 September 2019 0032-3861/ © 2019 Elsevier Ltd. All rights reserved.

Polymer 181 (2019) 121803

G. Wang, et al.

Due to its good mechanical properties and low cost, GF is a very popular reinforcing filler in the industry. However, due to its brittle nature, the incorporation of GF usually led to reduced ductility and toughness of composite, especially when the bonding between fibers and polymer matrix is not good [34,42,43]. Compared with GF, carbon nanotubes and carbon nanofibers show greater potential in enhancing the mechanical properties of PLA [44,45]. However, it is still challenging to uniformly disperse these nanomaterials in polymer matrix, which is the key factor limiting the mass application of them in industry. For polymer composites, the bonding interface between fillers and polymer matrix plays a crucial role in affecting their mechanical properties [33]. Generally, a high bonding interface is a guarantee of outstanding mechanical performance of composite. Thus, significant research efforts have been devoted to improve the bonding interface. Monticelli et al. found that the ductility of the PLA/PCL immiscible blend can be dramatically increased with the incorporation of the polyhedral oligomeric silsesquioxane (POSS) based compatibilizing agent [26]. It was reported that the impact strength of PLA/cellulose composite can be remarkably enhanced by modifying the cellulose interface with maleinated additive [34]. After modification, the fiber pull-outs became the dominant mode of failure, and hence the impact energy increased extraordinarily. Dong and coworkers reported that the PLA/halloysite nanotube (HNT) composite mats showed significantly increased tensile strength and modulus by introducing coupling agents [46]. Lee et al. found that even a small amount of silane coupling dramatically increased the mechanical properties of the PLA/kenaf fiber composite [47]. Crikós et al. prepared the maleic anhydride grafted PLA coupling agents, which was further found to be very efficient in increasing the strength of PLA/wood composites [48]. In summary, although many efforts have been made to improve the mechanical performance of PLA, it is still challenging to coincidentally enhance strength and toughness, especially considering the requirements for industrialization. So far, there is little literature regarding reinforcing PLA with silane coupling agent-modified GF. In this study, we took an experimental study to investigate the effect of GF on the mechanical, thermal and processing properties of PLA, towards fabricating high-performance PLA/m-GF composites with obviously enhanced strength, rigidness and toughness. The PLA/m-GF composites with different fiber fractions were prepared to optimize the material formulation. In order to improve the interfacial bonding of GF and the PLA matrix, the surface of GF was modified with silane coupling agents [49]. To characterize the strength, rigidness and toughness of the PLA/m-GF composites at room temperature, tensile, flexible and Izod impact tests were conducted, respectively. Scanning electron microscope (SEM) was employed to analyze the morphology of PLA/m-GF composites and also the composite fracture surfaces obtained in Izod impact testing. Furthermore, the dynamic mechanical thermal analysis (DMTA) was conducted to evaluate the response of PLA/m-GF composites under varied temperatures. The heat deflection temperatures were also measured to evaluate the effect of GF on the thermal resistance of PLA. Moreover, the melt flow index (MFI) was measured to evaluate the effect of GF on the flowability of the PLA at molten state. Finally, foaming experiments with carbon dioxide as blowing agents were conducted to investigate the effect of GF on the foaming behavior of PLA.

of PLA. However, PLA suffers from some obvious shortcomings including its brittleness and poor temperature resistance [14,15]. These shortcomings greatly limit its usage in many engineering areas where high toughness and high temperature resistance are commonly essential properties. In turn, this makes it hard for PLA to widely replace the conventional petroleum-based plastics such as polyethylene (PE), polypropylene (PP), and polystyrene (PS). In this context, to enhance the mechanical and thermal resistance properties of PLA is of great significance to further extend its applications [15,16]. In the past decade, to enhance the mechanical properties of PLA has been a hot topic in both academic and industrial areas [17,18]. Stereocomplex crystallization has been demonstrated to be an effective method to enhance the properties of polylactides (PLA), including mechanical performance and thermal stability [19–22]. However, high cost limits its wide applications in industry. Blending modification of PLA is a cost-effective approach to enhance its mechanical performance. The commonly used fillers for modification include some tough polymers and fibers [23]. It was reported that adding thermoplastic elastomers (TPE) into PLA led to significantly improved ductility, but the mechanical properties including strength and stiffness were reduced gradually with the increase in TPE content [24]. Blending PLA with poly(hydroxybutyrate) (PHB) also led to enhanced ductility but decreased strength, stiffness and thermal resistance [25]. In the presence of compatibilizing agents, poly(ε-caprolactone) (PCL) can be used to improve the ductility of PLA, yet the tensile strength and modulus are decreased significantly [26]. For organic additives, it is general that the toughness is improved while sacrificing the strength. Compared with the second-phase tough polymers used as fillers, the fibers used as fillers are generally more effective in improving the mechanical properties of the PLA matrix, because the latter is much stronger than the former. The fibers used to reinforce PLA include natural fibers and synthetic fibers [27]. Generally, the natural fibers are more environmentally friendly than the synthetic fibers, but their strength is relatively poorer. Moreover, since natural fibers are generally highly polar than synthetic fibers, they are more difficult to disperse in polymer and result in a higher water absorption of the prepared composite. The commonly used natural fibers include lignin [28,29], jute [28], tannin [29], cellulose [30–33], abaca [34,35], coir [36], pulp [37], ramie [38], and other plant fillers [39–41]. Anwer and coworkers found that both lignin and tannin lead to the reduced tensile strength of PLA, and they slightly accelerate the thermal degradation of PLA [29]. Delgado-Aguilar and coworkers reported that mechanical properties of PLA can be significantly improved with the incorporation of 30 wt% jute fibers, and it is inferred that the proper dispersion of the fibers within PLA matrix and the good chemical bonding between PLA and fibers are responsible for the enhanced tensile mechanical properties [28]. For the filler material of cellulose nanofibrils, it was found that the proper addition leads to enhanced tensile strength, modulus and toughness [30]. In the presence of epoxidized soybean oil, the PLA/nanocellulose composite shows dramatically enhanced ductility [30]. Graupner and coworkers found that the reduced fiber aspect ratio resulted from reprocessing led to remarkably reduced strength of the PLA/cellulose fiber composite [32]. Compared with GF and cellulose, the positive effect of fiber abaca on improving the strength of PLA is much poorer, and it significantly increases the brittleness of PLA [34]. Dong et al. reported that untreated coir fibers led to significantly reduced tensile strength, modulus and toughness of PLA due to the poor bonding between fibers and PLA matrix, and moreover coir fibers reduced the onset temperature of thermal degradation of PLA [36]. In addition, the incorporation of pulp fibers or ramie fibers can significantly increase both strength and rigidness of PLA, however this is usually achieved with sacrificing the ductility and toughness [37,38]. Aside with natural fibers, the synthesized fibers including GF, carbon fibers and carbon nanotubes are also used as filler materials for reinforcing PLA. Compared with the natural fibers, the synthesized fibers are generally much stronger, and hence they are more effective in enhancing the mechanical properties of PLA.

2. Experimental section 2.1. Materials An injection molding grade PLA, Ingeo™ biobopolymer 3052D, was purchased from NatureWorks LLC, Minnetonka, MN, USA. It is an amorphous PLA designed for the applications where the requirements are clarity with heat deflection temperatures lower than 49 °C. The PLA has a density of 1.24 g/cm3 and 4% of D-lactide content. Its melt flow index is 14 g/10 min, measured at 210 °C with a load of 2.16 kg. The 2

Polymer 181 (2019) 121803

G. Wang, et al.

glass fibers (GF), ECS303A, were purchased from Chongqing Polycomp International Corporation, Chongqing, China. The GF has an average diameter of about 13 μm and a pre-compounded length of about 4 mm. To improve the interfacial bond between GF and PLA, the surface of GF was functionalized with organic silane coupling agent Hexadecyltrimethoxysilane CAS 16415-12-6, manufactured by Hangzhou Jessica Chemicals Co.,Ltd. Carbon dioxide with a purity of 99.99% was used as the physical blowing agent in foaming experiments, and it was purchased from Linde Gas LLC.

Before processing, both PLA in pellets form and GF in bundle form were dried at 80 °C for 8 h to remove any moisture. Afterwards, PLA pellets and GF were compounded using a co-rotating twin-screw extruder (SHJ-30, KY Solution Group, China) with a screw diameter of 21 mm and length to diameter ratio of 36 to prepare PLA/m-GF composites. In order to reduce the breakage of GF resulted from strong shear at the first several sections of barrel, the GF was directly fed to the middle range of barrel by a side feeding system. The temperature profile of extruder from the hopper to the die was set to be 160 °C, 185 °C, 190 °C, 190 °C, 190 °C, 185 °C, and 165 °C. The screw speed was set at 100 rpm, and the feed rate was kept at about 3.5 kg/h. The extruded composite strand was first cooled in a water-cooling bath and then chopped using a pelletizer. After compounding, all the prepared PLA/m-GF composite pellets were dried once again at 80 °C for 8 h to remove any moisture. Afterwards, a 68 tons injection molding machine, XL-680, Ningbo Surely Meh. Co., Ltd, China, was employed to mold these composite pellets into standard samples for tests. In injection molding procedure, the main processing parameters including melt temperature, mold temperature, injection speed, packing pressure, and packing time were set to be 190 °C, 45 °C, 20 cm3/s, 5.0 MPa, and 3.0 s, respectively. Solid-state batch foaming process was used to foam pure PLA and PLA/m-GF composites. Before foaming, all the materials were dried using a vacuum oven at 75 °C for 6 h to remove any moisture. In a representative foaming procedure, an extruded cylindrical sample with a dimension of Φ3 mm × 15 mm was firstly placed in a temperatureregulated cylindrical chamber at room temperature (~23 °C). Afterwards, the chamber was charged with CO2 at a pressure of 20 MPa by using a syringe pump. After the CO2 pressure stabilizes, the chamber was heated from room temperature to the designated temperature, during which CO2 pressure was maintained at 20 MPa. At the designated temperature, a dwelling duration of 40 min was used to completely impregnate the sample with CO2. Afterwards, CO2 pressure was released at a depressurization rate of about 900 MPa/s to trigger foaming. Finally, the chamber was transferred to an ice water bath for rapid cooling and hence stabilizing foam structure. Before formal foaming experiments, trials were conducted to find the optimum foaming temperature. It was found that pure PLA showed the best cellular structure at 120 °C, and hence the temperature was determined as the designated temperature for comparing the foaming ability of pure PLA and PLA/m-GF composites.

microscope was employed to investigate GF length. To prepare specimens for observation, PLA/m-GF composite were sandwiched between two glass slides by using hot compression. With the obtained optical microscope photographs, the ImageJ software was used to calculate GF length. For each specimen, more than 50 glass fibers were measured to calculate the average GF length. To evaluate the mechanical performance of the injection molded PLA/m-GF samples, tensile, flexural and Izod impact tests were conducted according to the standards of ASTM D638, D790, and D256, respectively. A universal electronic testing machine, SANS CMT5205, was used to conduct the tensile and flexural tests, while a XC-5.5D impact tester was used to conduct the Izod impact test. In tensile and flexural tests, the head speed rates were kept at 5.0 mm/min and 2.0 mm/min, respectively. In Izod impact test, the impact energy and impact speed were 2.75 J and 3.50 m/s, respectively. For each condition, at least five samples were measured, and the average value was used for discussion. A dynamic mechanical analyzer, NETZSCH DMA 242, was used to analyze the dynamic mechanical thermal properties of the injection molded PLA/m-GF composite samples. The testing sample has a dimension of 32 mm × 12.7 mm × 3.2 mm. The bending test with a single cantilever was conducted. In testing, the temperature was raised from 20 °C to 120 °C at a heating rate of 3.0 °C/min, and the frequency was kept at 1 Hz. The storage modulus (E′) and loss factor (tan δ) were recorded for discussion. The thermal stability of the PLA/m-GF composites was characterized by using a thermogravimetric analyzer (TGA Q50, TA Instruments, USA) under nitrogen atmosphere from 20 °C to 600 °C at a heating rate of 10 °C/min. To characterize the thermal resistance of the composites, the heat deflection temperature tests were conducted according to ASTM D648. In measurement, the bars with a dimension of 127 mm × 12.7 mm × 3.2 mm were placed under the deflection measuring device. A load of 1.8 MPa was placed on each sample, and temperature was raised at 2.0 °C/min using an oil bath. For each case, the tests were repeated by five times, and the average value was recorded. A melt flow indexer (MFI-1211, JJ-TEST, China) with an automated cut-off mechanism was used to characterize the flow ability of the PLA/ m-GF composites. The timer was programmable to change the cut-off time for a time interval of 10 s. The test was conducted at a constant temperature of 190 °C with a load mass of 2.16 kg. The cut-off time was changed in order to obtain at least five samples for each condition. The cellular structure of foams was investigated by using the same SEM machine as stated above. To prepare SEM specimens, all the foams were dipped into liquid nitrogen for 30 min, and then cryofractured using pliers. Afterwards, the fracture surface was coated a thin gold layer (10–20 nm) using an automatic sputter coater right before the SEM observations. The obtained cellular structure was analyzed by using the ImageJ software to evaluate the cell size of foam. At least 200 cells were measured for each foamed sample to calculate average cell size. According to the cell number (n) in a certain area (A), the cell density with respect to the unfoamed volume was calculated with the following equation:

2.3. Measurement and characterization methods

N0 = (n/ A)3/2 × ϕ

A scanning electron microscope (SEM), Zeiss EVO MA10, was used to analyze the morphology of the PLA/m-GF composites and also the impact fracture surface of the PLA/m-GF composite samples. To prepare the specimen for SEM observation, the extruded PLA/m-GF composite strand was cryofractured in liquid nitrogen. The fracture surface was coated with a thin gold layer (10–20 nm) using an automatic sputter coater right before the SEM observations. GF length plays an important role in determining the mechanical properties of GF-reinforced polymer composites. Thus, an optical

where ϕ is the expansion ratio of foam. Foam density was measured using the water displacement method according to ASTM D792-00. The expansion ratio of foam was then calculated with the following equation:

2.2. Preparation of PLA/m-GF composites and their foams

ϕ = ρs / ρf

(1)

(2)

where ρs is the bulk density of the pure PLA or PLA/m-GF composites, and ρf is the measured density of the foamed sample. 3

Polymer 181 (2019) 121803

G. Wang, et al.

Fig. 1. Cryogenic breakage morphology of PLA/m-GF composites: (a) 5 wt%, (b) 10 wt%, (c) 15 wt%, and (d) 20 wt%.

3. Results and discussions

modulus was increased by more than one-fold, for the composite with 20 wt% GF. It clearly demonstrates that the addition of GF is very powerful in enhancing both the strength and the rigidness of PLA. Compared with the PLA/m-GF composites reported in Refs. [43,55], the PLA/m-GF composites in this study shows improved tensile properties, the reason for which should be correlated with the improved compatibility between GF and the PLA matrix by using the organic silane [56]. Generally, the reinforcement mechanism is that the stress is transferred from the matrix to the fiber by a shear transfer, while a good bond between the GF and the PLA matrix is the guarantee of the stress transfer. Compared with common natural fibers [37], the positive effect of GF on reinforcing PLA is much more pronounced. However, it can be seen from Fig. 2c that the elongation at break reduces gradually with the increase of GF. This phenomenon is normal for the rigid fiber reinforced polymer composites because rigid fibers show poorer ductility than polymer matrix, and they restrict the deformation of polymer matrix before breakage [54,55]. The flexural tests were also conducted to estimate the bending strength and rigidness of the prepared PLA/m-GF composites. Fig. 3 plots the flexural strength and modulus of the PLA/m-GF composites. It can be found that both flexural strength and flexural modulus increase linearly with increasing in the GF content, and the variation tendency is the same as the variation of the tensile strength and tensile modulus. With 20 wt% GF, the tensile strength and the tensile modulus are increased by 42.6% and 99.6%, respectively. As expected, GF shows remarkably positive effect in enhancing the rigidness because of its strong mechanical anchoring effect [57,58]. It is expected that the load is transferred from the PLA matrix to the strong fibers. It is inferred that the good adhesion between GF and PLA matrix contributes to the continuous increase of strength and modulus with increasing in the GF content. To characterize the toughness of the composites at high deformation rates, the Izod impact testing was conducted. Fig. 4 plots the measured impact strength of the pristine PLA and the PLA/m-GF composites. It

3.1. Composite morphology The dispersion of fillers in the polymer matrix has a significant effect on the properties of composite. Fig. 1 shows the dispersion of GF in the PLA matrix. Overall, the fibers disperse relatively uniformly in the PLA matrix without any obvious agglomerations. Even for the composite with a high GF content of 20 wt%, it can be clearly seen that each GF disperse separately and distribute uniformly in the PLA matrix. It indicates that the organic silane modified GF shows a good compatibility with the PLA matrix, which could benefit the mechanical properties of the composites. In addition to the dispersion and distribution of GF, the length to diameter ratio of GF also has a significant effect of the properties of the PLA/m-GF composites [50]. Thus, the length distribution of GF in PLA matrix was measured. Fig. S1 shows the frequency distribution of GF. It can be seen that the GF has a diameter of several hundred micrometers at relatively low GF content (5 wt%, 10 wt%), and it reduces gradually with the increase of fiber content. It indicates that the GF length is reduced significantly by twin-screw compounding. With the increase of fiber content, the chance of fiber collisions increases, which exacerbates fiber breakage during compounding. This phenomenon is normal and has been extensively reported in preparing other GF reinforced polymer composites through twin-screw compounding [51–53]. 3.2. Mechanical properties Fig. 2 shows the measured tensile properties of the PLA/m-GF composites. It can be observed from Fig. 2a & b that both the tensile strength and the tensile modulus increase nearly linearly with the increase of GF. This linear relationship is consistent with theoretical expectation [54]. Compared with the properties of the pristine PLA, the tensile strength was increased by nearly one-fold while the tensile 4

Polymer 181 (2019) 121803

G. Wang, et al.

Fig. 2. Tensile properties of the PLA/m-GF composites: (a) strength, (b) modulus, and (c) elongation at break.

can be seen that the GF increases the impact strength significantly. The higher the GF content is, the higher the impact strength is. Notably, as the GF content increases from 0 wt% to 20 wt%, the impact strength is increased from 30.9 J/m to 102.8 J/m, with an increment of more than 2-fold. Compared with the reported impact strength of the PLA/m-GF composite in Ref. [42], the PLA/m-GF composite prepared in this study exhibits superior impact strength. The more pronounced enhancement of impact strength can be owing to the following two factors. First, the fibers disperse separately and uniformly in PLA matrix (Fig. 1), which reduces the chance of crack initiation and propagation due to the agglomeration of fibers. Second, due to the functionalization of fiber surfaces, the GF and the PLA matrix have a good adhesion, which not only reduces the stress concentration around GF and hence hinders crack initiation, but also increases energy consumption as the GF was pulled out of PLA matrix. Fig. 5 shows the impact fracture surface of the PLA/m-GF composites. It can be seen that the fibers disperse separately and uniformly in the PLA matrix. Because the strong shear flow during injection molding [59], the fibers tend to orient along melt flow direction. Since the impact fracture surface is vertical to melt flow direction, the fibers are perpendicular to the impact fracture surface, as shown in Fig. 5. It is worth noting that the fibers at the central layer of the sample orient parallel to the impact fracture surface, and this phenomenon is much more obvious for the PLA/m-GF composites with relatively high GF content (15 wt%, 20 wt%). At the central layer, the flow rate gradient is very small, which leads to weak shear flow [59,60]. Thus, the fiber orientation is mainly determined by the fountain flow at the melt flow front during injection molding, and thus the fibers are vertical to the flow direction and consequently parallel to the impact fracture surface [61]. In addition, it can be seen from Fig. 5 that the fracture surface becomes rougher with increasing in the GF content, which also indicates the improvement of impact strength by adding more GF. Generally, fiber pull-out is one of the failure mechanisms in fiber-reinforced composite materials. It consumes a lot of energy and thus contributes to impact strength. Fig. S2 shows a representative impact fracture surface in a large magnification. It can be seen that nearly all of the fibers have been pulled out of the PLA matrix, which demonstrates the fiber pullout is the main failure mechanism. Notably, the GF surface is attached

Fig. 4. Impact strength of the PLA/m-GF composites.

with a thin layer of the PLA matrix, which indicates the good adhesion between the GF and the PLA matrix. The good bonding interface will definitely increase the energy consumption during pulling out the fiber, and hence leads to the high impact strength of the PLA/m-GF composites.

3.3. Dynamic mechanical thermal analysis (DMTA) The dynamic mechanical properties of the PLA/m-GF composites were measured by DMTA. The temperature dependences of the storage modulus (E′) and the loss factor (tan δ) at 1 Hz are given in Fig. 6a and b, respectively. As plotted in Fig. 6a, the E′ values of the pristine PLA and the PLA/m-GF composites firstly maintain at a relatively high level with the increase of temperature and then reduce sharply at a certain temperature, eventually reaching a low level plateau. When the temperature is below 50 °C, the PLA/m-GF composites have higher E′ values than the pristine PLA, particularly at high GF content, which indicates GF could act as an effective reinforcing filler at relatively low temperatures. The sudden drop of E′ values when the temperature rises higher than 50 °C is because materials begin to transfer from the glassy state to the high-elastic state. As the polymer composite becomes molten gradually with further increasing the temperature, the reinforcing effect of fibers becomes inconspicuous, and thus the pristine PLA and the PLA/m-GF composites finally shows a similar level of E′. The

Fig. 3. Flexural properties of the PLA/m-GF composites: (a) strength, and (b) modulus. 5

Polymer 181 (2019) 121803

G. Wang, et al.

Fig. 5. The Impact fracture surface of the PLA/m-GF composites: (a) schematic of SEM sampling, (b) 5 wt%, (c) 10 wt%, (e) 15 wt%, and (e) 20 wt%.

shows the dependence of weight loss ratio on the temperature. It can be found that the pristine PLA and its composites have a similar degradation peak at about 360 °C, and the thermal degradations of all the materials nearly finish at the same temperature of about 385 °C. All these facts further confirm that the GF shows little effect on the thermal stability of the PLA matrix. Although the thermal stability is not improved, the performance of GF is still superior to many natural fillers because the addition of natural fibers usually accelerates the thermal degradation of PLA [36,42,64]. In addition, based on the residual mass percentage after complete thermal degradation, the actual GF contents in the PLA/m-GF composites are determined to be 6.2 wt%, 11.1 wt%, 17.43 wt% and 24.6 wt%, respectively, which are very close to the expected composition ratio.

similar phenomenon has also been reported by other researchers [38,62]. Moreover, it can be seen from Fig. 6a that E′ has a jump when GF increased from 5 wt% to 10 wt% at low temperatures, which could be attributed to the formation of GF networks within PLA matrix as GF concentration reaches the percolation threshold [63]. As Fig. 6b shows, tan δ remains zero at relatively low temperatures (< 50 °C), which indicates that the internal damping is zero. As the temperature rises, tan δ increases sharply due to the dramatically increase of viscous dissipation when materials transfer from the glassy state to the high-elastic state. Generally, the temperature at which the loss tangent is manifested is interpreted as the glass transition temperature. By comparing the temperature corresponding to the peak value of tan δ, it can be found that the glass transition temperature of the PLA/m-GF composites shifts slightly to lower temperatures. Compared with the molecular segments, the size of GF is too macro to affect their mobility. Thus, the micro fillers usually show very limited effect on the glass transition temperature of the polymer matrix, although in some cases the glass transition temperature could rise or decrease slightly [29,31,42]. It is inferred that the slightly decreased glass transition temperature of the PLA/m-GF composites might be owing to the addition of organic silane which improves the molecular mobility of the PLA matrix.

3.5. Heat deflection temperature (HDT) The measured heat deflection temperature (HDT) of the pristine PLA and the PLA/m-GF composites are shown in Fig. 8. As seen in Fig. 8, the HDT value increases almost linearly with the GF content. In the presence of 20 wt% GF, the HDT of the PLA/m-GF composite is increased by about 3 °C compared to the pristine PLA. The strong rigid fibers can prevent the deformation of the PLA matrix, and hence lead to the increased HDT. Moreover, it is inferred that the surface treatment of GF using silane also contributes to the increased HDT [38], which is derived from the increase in modulus as well as the interaction between GF and the PLA matrix [38].

3.4. Thermogravimetric analysis (TGA) The thermal stability of the pristine PLA and the PLA/m-GF composites was investigated with TGA, and the results are plotted in Fig. 7. From the weight loss curve in Fig. 7a, it can be found that the thermal degradations of the pristine PLA and the PLA/m-GF composites nearly all occurs at about 330 °C. For the composites with high GF content (10 wt%, 15 wt%), it seems that the degradation is slightly shifted to higher temperatures, but it is not obvious. It indicates that the addition of GF shows very limit effect on improving thermal stability. Fig. 7b

3.6. Melt flow index (MFI) To evaluate the effect of GF on the flowability, the melt flow index of PLA and PLA/m-GF composites were measured. As shown in Fig. 9, the presence of reinforcing fibers deteriorates the flowability of PLA melt. The higher the GF content is, the smaller the MFI value is. The

Fig. 6. DMTA curves of PLA and PLA/m-GF composites: (a) storage modulus, and (b) loss factor. 6

Polymer 181 (2019) 121803

G. Wang, et al.

Fig. 7. Thermal stability of PLA and PLA/m-GF composites: (a) thermogravimetric analysis (TGA) curves, and (b) derivative thermogravimetric analysis (DTGA) curves.

achieved with 10 wt% GF. Too much GF led to reduced absorbed blowing agent and increased foaming resistance, thereby reducing foam's expansion ratio. From Fig. 11c, it is observed that cell density increases gradually with increasing in GF content. With 20 wt% GF, cell density is increased by about 3 orders of magnitude, indicating the outstanding effect of GF in promoting cell nucleation. In addition, it is worth noting in Fig. 10f that a large number of small cells attach on GF surface, indicating a good bonding between PLA foam matrix and GF. All the above phenomena about pure PLA and PLA/m-GF foams clearly indicate that glass fibers are very effective fillers for improving the foaming ability of PLA. The underlying mechanism of improved foaming ability is analyzed as following. Firstly, the incorporation of GF provides a large number of heterogeneous interfaces, which reduces the energy barrier of cell nucleation and hence promotes cell nucleation [65]. Secondly, GF can accelerate the crystallization of PLA [56], thereby promoting cell nucleation and stabilizing foam structure [66,67]. Moreover, GF can enhance the viscoelasticity of PLA, which can not only promote cell nucleation by increasing local energy fluctuation [68], but also stabilize cell structure by reducing cell collapse and coalescence [69].

Fig. 8. Heat deflection temperature of the pristine PLA and the PLA/m-GF composites.

4. Conclusions In summary, PLA/m-GF composites with enhanced mechanical properties, thermal behavior, and foaming ability were prepared. The incorporation of GF led to simultaneously enhanced strength, rigidness, and toughness of PLA. In the presence of 20 wt% GF, the composite showed almost 2-fold increase in strength and rigidness, and more than 3-fold enhancement in impact toughness. This remarkably improved mechanical properties was derived from the anchoring effect of the strong GF and the good chemical bonding between GF and PLA matrix. Moreover, GF contributed significantly to the thermal resistance of PLA, and the heat deflection temperature increased linearly with increasing GF content. GF showed little influence on the glass transition temperature and thermal stability of PLA, but it led to reduced flowability of PLA. Furthermore, GF can greatly improve the foaming ability of PLA by dramatically increasing expansion ratio, reducing cell size, and enhancing cell nucleation. With 15 wt% GF, microcellular PLA/m-GF foam with an expansion ratio larger than 20 and cell sizes less than 10 μm was achieved. The PLA/m-GF composite and its foam show promising future in providing strong and lightweight product that can be used in many areas.

Fig. 9. Melt flow rates of PLA and PLA/m-GF composites.

reduced flowability is derived from the anchoring effect of the strong rigid GF which prevents the flow of PLA melt. However, since the rigid GF tends to orient along the flow direction due to shearing, its negative effect on the flowability of polymer melt is not as pronounced as other ductile or soft fibers [35], which is a significant advantage for the molding processes, such as injection molding and extrusion, that need high melt flowability. 3.7. Foaming behavior To investigate the effect of GF on the foaming behavior of PLA, solid-state batch foaming was conducted to foam pure PLA and PLA/mGF composites under 20 MPa at 120 °C. Fig. 10 shows the representative cellular morphology of pure PLA and PLA/m-GF composite foams, and the quantitative information of foam structure including expansion ratio, cell size, and cell density are plotted in Fig. 11. It can be clearly seen from Figs. 10 and 11b that the incorporation of GF reduces cell size significantly. Real microcellular structure with cell size less than 10 μm can be achieved at high GF contents (15 wt% and 20 wt%). Moreover, as Fig. 11a shows, the incorporation of GF leads to an obvious increase in foam's expansion ratio. The maximum expansion ratio of 23.8-fold is

Acknowledgements The authors are grateful to the National Natural Science Foundation of China (NSFC, Grant No. 51875318, 21706046), and the Qilu Outstanding Scholar Program of Shandong University for the funding support. 7

Polymer 181 (2019) 121803

G. Wang, et al.

Fig. 10. SEM images of (a) pure PLA foam, and PLA/m-GF composite foams with (b) 5 wt%, (c) 10 wt%, (d) 15 wt%, and (e) 20 wt% GF. (f) is the local enlarged image of (e).

Fig. 11. (a) Expansion ratio, (b) cell size, and (c) cell density of pure PLA and PLA/m-GF composite foams.

Appendix A. Supplementary data

Sci. 32 (2007) 455–482. [9] K. Hamad, M. Kaseem, H.W. Yang, F. Deri, Y.G. Ko, Properties and medical applications of polylactic acid: a review, Express Polym. Lett. 9 (2015) 435–455. [10] P. Wen, D. Zhu, K. Feng, F. Liu, W. Lou, N. Li, M. Zong, H. Wu, Fabrication of electrospun polylactic acid nanofilm incorporating cinnamon essential oil/betacyclodextrin inclusion complex for antimicrobial packaging, Food Chem. 196 (2016) 996–1004. [11] Y. Hu, W.A. Daoud, K.K. L Cheuk, C.S.K. Lin, Newly developed techniques on polycondensation, ring-opening polymerization and polymer modification: focus on poly(lactic acid), Materials 9 (2016) 133. [12] E. Castro-Aguirre, F. Iniguez-Franco, H. Samsudin, X. Fang, R. Auras, Poly(lactic acid)-Mass production, processing, industrial applications, and end of life, Adv. Drug Deliv. Rev. 107 (2016) 333–366. [13] M. Nofar, D. Sacligil, P.J. Carreau, M.R. Kamal, M.C. Heuzey, Poly (lactic acid) blends: processing, properties and applications, Int. J. Biol. Macromol. 125 (2019) 307–360. [14] S. Saeidlou, M.A. Huneault, H. Li, C.B. Park, Poly(lactic acid) crystallization, Prog. Polym. Sci. 37 (2012) 1657–1677. [15] R.M. Rasal, A.V. Janorkar, D.E. Hirt, Poly(lactic acid) modifications, Prog. Polym. Sci. 35 (2010) 338–356. [16] K. Hamad, M. Kaseem, M. Ayyoob, J. Joo, F. Deri, Polylactic acid blends: the future of green, light and tough, Prog. Polym. Sci. 85 (2018) 83–127. [17] B. Imre, G. Keledi, K. Renner, J. Móczó, M. Murariu, P. Dubois, B. Pukánszky, Adhesion and micromechanical deformation processes in PLA/CaSO4 composites, Carbohydr. Polym. 89 (2012) 759–767. [18] Y. Ding, W. Feng, D. Huang, B. Lu, P. Wang, G. Wang, J. Ji, Compatibilization of

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.polymer.2019.121803. References [1] R. Geyer, J.R. Jambeck, K.L. Law, Production, use, and fate of all plastics ever made, Sci Adv 3 (2017) e1700782. [2] L.C.M. Lebreton, J. van der Zwet, J.W. Damsteeg, B. Slat, A. Andrady, J. Reisser, River plastic emissions to the world's oceans, Nat. Commun. 8 (2017) 15611. [3] D.K. Schneiderman, M.A. Hillmyer, 50th anniversary perspective: there is a great future in sustainable polymers, Macromolecules 50 (2017) 3733–3749. [4] S.K. Sansaniwal, M.A. Rosen, S.K. Tyagi, Global challenges in the sustainable development of biomass gasification: an overview, Renew. Sustain. Energy Rev. 80 (2017) 23–43. [5] Y. Zhu, C. Romain, C.K. Williams, Sustainable polymers from renewable resources, Nature 540 (2016) 354–362. [6] L.T. Lim, R. Auras, M. Rubino, Processing technologies for poly(lactic acid), Prog. Polym. Sci. 33 (2018) 820–852. [7] V. Nagarajan, A.K. Mohanty, M. Misra, Perspective on polylactic acid (PLA) based sustainable materials for durable applications: focus on toughness and heat resistance, ACS Sustain. Chem. Eng. 4 (2016) 2899–2916. [8] B. Gupta, N. Revagade, J. Hilborn, Poly(lactic acid) fiber: an overview, Prog. Polym.

8

Polymer 181 (2019) 121803

G. Wang, et al.

[19]

[20] [21]

[22]

[23]

[24]

[25]

[26]

[27] [28]

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39] [40]

[41]

[42]

[43]

[44] Y. Wang, Y. Mei, Q. Wang, W. Wei, F. Huang, Y. Li, J. Li, Z. Zhou, Improved fracture toughness and ductility of PLA composites by incorporating a small amount of surface-modified helical carbon nanotubes, Compos. B Eng. 162 (2019) 54–61. [45] M.A.S. Anwer, H.E. Naguib, Study on the morphological, dynamic mechanical and thermal properties of PLA carbon nanofibre composites, Compos. B Eng. 91 (2016) 631–639. [46] Y. Dong, J. Marshall, H.J. Haroosh, S.M. Moghadam, D. Liu, X. Qi, K. Lau, Polylactic acid (PLA)/halloysite nanotube (HNT) composite mats: influence of HNT content and modification, Compos Part A-Appl S 76 (2015) 28–36. [47] B. Lee, H. Kim, S. Lee, H. Kim, J.R. Dorgan, Bio-composites of kenaf fibers in polylactide: role of improved interfacial adhesion in the carding process, Compos. Sci. Technol. 69 (2009) 2573–2579. [48] Á. Csikós, G. Faludi, A. Domján, K. Renner, J. Móczó, B. Pukánszky, Modification of interfacial adhesion with a functionalized polymer in PLA/wood composites, Eur. Polym. J. 68 (2015) 592–600. [49] S. Park, J. Jin, Effect of silane coupling agent on mechanical interfacial properties of glass fiber‐reinforced unsaturated polyester composites, J. Polym. Sci., Polym. Phys. Ed. 41 (2002) 55–62. [50] C. Geng, J. Su, C. Zhou, H. Bai, G. Yang, Q. Fu, Largely improved toughness of polypropylene/long glass fiber composites by b-modification and annealing, Compos. Sci. Technol. 96 (2014) 56–62. [51] W. Wang, G. Zhao, Y. Guan, J. Wang, C. Wang, Study on effects of short glass fiber reinforcement on the mechanical and thermal properties of PC/ABS composites, J. Appl. Polym. Sci. 131 (2014) 40697. [52] A. Ameli, M. Nofar, S. Wang, C. Park, Lightweight polypropylene/stainless-steel fiber composite foams with low percolation for efficient electromagnetic interference shielding, ACS Appl. Mater. Interfaces 6 (2014) 11091–11100. [53] S. Gu, L. Zhu, C. Mercier, Y. Li, Glass-fiber networks as an orbit for Ions: fabrication of excellent antistatic PP/GF composites with extremely low organic salt loadings, ACS Appl. Mater. Interfaces 9 (2017) 18305–18313. [54] M. Delgado-Aguilar, F. Julián, Q. Tarrés, J.A. Méndez, P. Mutjé, F.X. Espinach, Bio composite from bleached pine fibers reinforced polylactic acid as a replacement of glass fiber reinforced polypropylene, macro and micro-mechanics of the Young's modulus, Compos. B Eng. 125 (2017) 203–210. [55] X. Yin, J. Bao, Glass fiber coated with graphene constructed through electrostatic self-assembly and its application in poly(lactic acid) composite, J. Appl. Polym. Sci. 133 (2016) 43296. [56] G. Wang, D. Zhang, B. Li, G. Wan, G. Zhao, A. Zhang, Strong and thermal-resistance glass fiber-reinforced polylactic acid (PLA) composites enabled by heat treatment, Int. J. Biol. Macromol. 129 (2019) 448–459. [57] J. Sliseris, L. Yan, B. Kasal, Numerical modelling of flax short fibre reinforced and flax fibre fabric reinforced polymer composites, Compos. B Eng. 89 (2016) 143–154. [58] J. Karger-Kocsis, H. Mahmood, A. Pegoretti, Recent advances in fiber/matrix interphase engineering for polymer composites, Prog. Mater. Sci. 73 (2015) 1–43. [59] G. Wang, G. Zhao, X. Wang, Effects of cavity surface temperature on mechanical properties of specimens with and without a weld line in rapid heat cycle molding, Mater. Des. 46 (2013) 457–472. [60] F. Ansari, L.A. Granda, R. Joffe, L.A. Berglund, F. Vilaseca, Experimental evaluation of anisotropy in injection molded polypropylene/wood fiber biocomposites, Compos Part A Appl S 96 (2017) 147–154. [61] T.B.N. Thi, M. Morioka, A. Yokoyama, S. Hamanaka, K. Yamashita, C. Nonomura, Measurement of fiber orientation distribution in injection-molded short-glass-fiber composites using X-ray computed tomography, J. Mater. Process. Technol. 219 (2015) 1–9. [62] Y. Zhou, L. Lei, B. Yang, J. Li, J. Ren, Preparation of PLA-based nanocomposites modified by nanoattapulgite with good toughness-strength balance, Polym. Test. 60 (2017) 78–83. [63] A. Bunde, W. Dieterich, Percolation in composites, J. Electroceram. 5 (2000) 81–92. [64] M. Ho, K. Lau, H. Wang, D. Hui, Improvement on the properties of polylactic acid (PLA) using bamboo charcoal particles, Compos. B Eng. 81 (2015) 14–25. [65] G. Wang, G. Zhao, G. Dong, Y. Mu, C. Park, Lightweight and strong microcellular injection molded PP/talc nanocomposite, Compos. Sci. Technol. 168 (2018) 38–46. [66] G. Wang, G. Zhao, L. Zhang, Y. Mu, C. Park, Lightweight and tough nanocellular PP/PTFE nanocomposite foams with defect-free surfaces obtained using in situ nanofibrillation and nanocellular injection molding, Chem. Eng. J. 350 (2018) 1–11. [67] N. Hossieny, M. Barzegari, M. Nofar, S. Mahmood, C. Park, Crystallization of hard segment domains with the presence of butane for microcellular thermoplastic polyurethane foams, Polymer 55 (2014) 651–662. [68] G. Wang, J. Zhao, K. Yu, L. Mark, G. Wang, P. Gong, C. Park, G. Zhao, Role of elastic strain energy in cell nucleation of polymer foaming and its application for fabricating sub-microcellular TPU microfilms, Polymer 119 (2017) 28–39. [69] G. Wang, G. Zhao, S. Wang, L. Zhang, C. Park, Injection-molded microcellular PLA/ graphite nanocomposites with dramatically enhanced mechanical and electrical properties for ultra-efficient EMI shielding applications, J. Mater. Chem. C 6 (2018) 6847–6859.

immiscible PLA-based biodegradable polymer blends using amphiphilic di-block copolymers, Eur. Polym. J. 118 (2019) 45–52. R. Bao, W. Yang, W. Jiang, Z. Liu, B. Xie, M. Yang, Q. Fu, Stereocomplex formation of high-molecular-weight polylactide: a low temperature approach, Polymer 53 (2012) 5449–5454. Y. Sun, C. He, Synthesis and stereocomplex crystallization of poly(lactide)–graphene oxide nanocomposites, ACS Macro Lett. 1 (2016) 79–713. X. Wei, R. Bao, Z. Cao, W. Yang, B. Xie, M. Yang, Stereocomplex crystallite network in asymmetric PLLA/PDLA blends: formation, structure, and confining effect on the crystallization rate of homocrystallites, Macromolecules 47 (2014) 1439–1448. R. Bao, W. Yang, X. Wei, B. Xie, M. Yang, Enhanced formation of stereocomplex crystallites of high molecular weight poly(l-lactide)/poly(d-lactide) blends from melt by using poly(ethylene glycol), ACS Sustain. Chem. Eng. 2 (2014) 2301–2309. I. Burzic, C. Pretschuh, D. Kaineder, G. Eder, J. Smilek, J. Másilko, W. Kateryna, Impact modification of PLA using biobased biodegradable PHA biopolymers, Eur. Polym. J. 114 (2019) 32–38. S. Jia, Z. Wang, Y. Zhu, L. Chen, L. Fu, Composites of poly(lactic) acid/thermoplastic polyurethane/mica with compatibilizer: morphology, miscibility and interphase, RSC Adv. 5 (2015) 98915–98924. I. Armentano, E. Fortunati, N. Burgos, F. Dominici, F. Luzi, S. Fiori, A. Jiménez, K. Yoon, J. Ahn, S. Kang, J.M. Kenny, Processing and characterization of plasticized PLA/PHB blends for biodegradable multiphase systems, Express Polym. Lett. 9 (2015) 583–596. O. Monticelli, M. Calabrese, L. Gardella, A. Fina, E. Gioffredi, Silsesquioxanes: novel compatibilizing agents for tuning the microstructure and properties of PLA/PCL immiscible blends, Eur. Polym. J. 58 (2014) 69–78. K. Lau, P. Hung, M. Zhu, D. Hui, Properties of natural fibre composites for structural engineering applications, Compos. B Eng. 136 (2018) 222–233. M. Delgado-Aguilar, H. Oliver-Ortega, J. Méndez, J. Camps, F. Espinach, P. Mutjé, The role of lignin on the mechanical performance of polylactic acid and jute composites, Int. J. Biol. Macromol. 116 (2018) 299–304. M.A.S. Anwer, H.E. Naguib, A. Celzard, V. Fierro, Comparison of the thermal, dynamic mechanical and morphological properties of PLA-Lignin & PLA-Tannin particulate green composites, Compos. B Eng. 82 (2015) 92–99. Z. Yang, J. Si, Z. Cui, J. Ye, X. Wang, Q. Wang, K. Peng, W. Chen, S. Chen, Biomimetic composite scaffolds based on surface modification of polydopamine on electrospun poly(lactic acid)/cellulose nanofibrils, Carbohydr. Polym. 174 (2017) 750–759. A.N. Frone, S. Berlioz, J.F. Chailan, D.M. Panaitescu, Morphology and thermal properties of PLA-cellulose nanofibers composites, Carbohydr. Polym. 91 (2013) 377–384. J. Shojaeiarani, D. Bajwa, N. Stark, Spin-coating: a new approach for improving dispersion of cellulose nanocrystals and mechanical properties of poly (lactic acid) composites, Carbohydr. Polym. 190 (2018) 139–147. H. Kyutoku, N. Maeda, H. Sakamoto, H. Nishimura, K. Yamada, Effect of surface treatment of cellulose fiber (CF) on durability of PLA/CF bio-composites, Carbohydr. Polym. 203 (2019) 95–102. A. Jaszkiewicz, A.K. Bledzki, P. Franciszczak, Improving the mechanical performance of PLA composites with natural, man-made cellulose and glass fibers – a comparison to PP counterparts, Polimery 58 (2013) 435–442. A. Jaszkiewicz, A. Meljon, A.K. Bledzki, M. Radwanski, Gaining knowledge on the processability of PLA-based short-fibre compounds – a comprehensive comparison with their PP counterparts, Compos Part A Appl S 83 (2016) 140–151. Y. Dong, A. Ghataura, H. Takagi, H.J. Haroosh, A.N. Nakagaito, K. Lau, Polylactic acid (PLA) biocomposites reinforced with coir fibres: evaluation of mechanical performance and multifunctional properties, Compos Part A Appl S 63 (2014) 76–84. Y. Du, T. Wu, N. Yan, M.T. Kortschot, R. Farnood, Fabrication and characterization of fully biodegradable natural fiber-reinforced poly(lactic acid) composites, Compos. B Eng. 56 (2014) 717–723. T. Yu, J. Ren, S. Li, H. Yuan, Y. Li, Effect of fiber surface-treatments on the properties of poly(lactic acid)/ramie composites, Compos Part A Appl S 41 (2010) 499–505. R. Scaffaro, A. Maio, E.F. Gulino, B. Megna, Structure-property relationship of PLAOpuntia Ficus Indica biocomposites, Compos. B Eng. 167 (2019) 199–206. L. Quiles-Carrilo, N. Montanes, D. Garcia-Garcia, A. Carbonell-Verdu, R. Balart, S. Torres-Giner, Effect of different compatibilizers on injection-molded green composite pieces based on polylactide filled with almond shell flour, Compos. B Eng. 147 (2018) 76–85. X. Niu, Y. Liu, Y. Song, J. Han, H. Pan, Rosin modified cellulose nanofiber as a reinforcing and co-antimicrobial agents in polylactic acid/chitosan composite film for food packaging, Carbohydr. Polym. 183 (2018) 102–109. M. Huda, L. Drzal, A. Mohanty, M. Misra, Chopped glass and recycled newspaper as reinforcement fibers in injection molded poly(lactic acid) (PLA) composites: a comparative study, Compos. Sci. Technol. 66 (2006) 1813–1824. S.D. Varsavas, C. Kaynak, Effects of glass fiber reinforcement and thermoplastic elastomer blending on the mechanical performance of polylactide, Compos Commun 8 (2018) 24–30.

9