Reinforcement of Polylactic Acid for Fused Deposition Modeling Process with Nano Particles Treated Bamboo Powder

The focus of this report was to understand the tensile properties and dynamic mechanical properties of bamboo powder (BP) reinforced polylactic acid (PLA) composite filaments which were treated with nano calcium carbonate (CaCO3), cellulose nanofibers (CNF), and micro-crystalline cellulose (MCC) using impregnation modification technology. The storage modulus (E’) of nano CaCO3-BP/PLA, MCC-BP/PLA, and CNF-BP/PLA composite filaments increased compared with BP/PLA composite filaments before the glass transition temperature Tg. When the temperature was above Tg, the reinforcement effect of nano CaCO3, MCC, and CNF gradually became less apparent. The loss modulus (E’’) and loss factor (tan δmax) of the nano CaCO3-BP/PLA, MCC-BP/PLA, and CNF-BP/PLA composite filaments was higher than that of BP/PLA composite filaments produced by the “one-step” method. The tensile strength (TS) results showed a similar trend. Compared with the control samples, the TS of nano CaCO3-BP/PLA, MCC-BP/PLA, and CNF-BP/PLA composite filaments produced by the “one-step” method (and the “two-step” method) increased by 40.33% (and 10.10%), 32.35% (and −8.61%), and 12.32% (and −12.85%), respectively. The TS of nano CaCO3-BP/PLA, MCC-BP/PLA, and CNF-BP/PLA composite filaments produced by the “one-step” method was slightly higher than those produced by the “two-step” method. The elongation at break (EAB) of BP/PLA composite filaments was higher than that of BP/PLA samples treated with nano CaCO3, MCC, or CNF. The PLA and modified BP were readily accessible through a simple mixing process. The rheological investigation of such mixtures showed that nano CaCO3, CNF, and MCC have different effects on the processability and rheological properties of composites.


Introduction
Natural fibers are renewable materials that are derived abundant sustainable resources (e.g., bamboo, cotton, flax, hemp, jute, kenaf, sisal, ramie, pineapple, coir, etc.), which can be isolated, treated, and functionalized for a multitude of applications, for instance, polymeric matrix composites [1], etc. In polymeric matrix composites, natural fibers as a reinforcing phase, provide positive environmental benefits with respect to ultimate disposability and raw material use [2]. And natural fiber has many advantages such as low density, low cost, low energy consumption, high specific strength, and modulus, as well as a relatively reactive surface [3]. Natural fiber reinforced polymer composites have seen

Preparation of Modified Bamboo Powder
Four specimen groups of BP were prepared for the experiment. The first specimen group was unmodified BP which was used as a control group. The other three specimen groups of BP were subjected to the impregnation modification process and were treated by nano CaCO 3 , MCC, or CNF, respectively. At 25 • C, 100 g of BP were dissolved in 2000 mL of deionized water for 30 min at 60 rpm, 1.7 g of EDTA-2Na, and 20 g of nano CaCO 3 , MCC, or CNF were then added to the mixture, which was then mixed for an additional 25 min. The suspensions were then rinsed on a 200-nylon mesh net with deionized water and then air dried. The modified bamboo powder (MBP), which includes nano CaCO 3 -BP, MCC-BP, and CNF-BP were obtained and preserved in a constant temperature and climate box at 23 • C and 50% relative humidity (RH).

Composite Filaments Processing: Preparation of PLA-Based Composite Filaments by the "One-Step" Method and "Two-Step" Method
In order to remove all absorbed moisture and prevent void formation, the BPs (particle size <74 µm), nano CaCO 3 -BPs, MCC-BPs, and CNF-BPs were dried at 103 • C in an oven until the moisture content (MC) was less than 2 wt%. The PLA was first dried in an oven at 80 • C for 4 h, then at 100 • C for 4 h, and lastly at 110 • C for 20 h before processing. The composite filaments were prepared by the following steps: First, using a high-speed mixer, 20 wt% of BP or MBP (e.g., nano CaCO 3 -BP, MCC-BP, and CNF-BP) and 80 wt% of PLA were added and mixed fully.
Second, in this study, two methods were used to manufacture the composite filaments, that is, the "one-step" method and "two-step" method. (1) "One-step" method ( Figure 1): the composite filament was manufactured by placing the mixture directly in an intermeshing counter-rotating conical twin-screw extruder (PolyLab QC, HAAKE, Karlsruhe, Germany); (2) "Two-step" method ( Figure 2): Polymers 2019, 11, 1146 4 of 12 the mixture was granulated using twin-screw extruder. The mixture underwent melt mixing in the twin-screw extruder and then was passed through the die of the extruder forming the composites. The composites were then pelletized in a pelletizer machine to produce the granules. The granules obtained from the pelletizer machine were then placed in the twin-screw extruder with a die diameter of 3 mm; where they were subjected to melt mixing and then water cooling to produce the composite filaments. In addition, the process was carried out at a temperature difference range of 175 • C, 175 • C, 170 • C between the feeding zone to die zone.
At last, the composite filaments with diameters of 1.75 ± 0.05 mm were obtained by using a laser diameter measuring (LDM) gauge (Mercury-Tech) and the wiredrawing-winding machine (WWM-001, independent research and development).
Polymers 2019, 11, x FOR PEER REVIEW 4 of 12 filament was manufactured by placing the mixture directly in an intermeshing counter-rotating conical twin-screw extruder (PolyLab QC, HAAKE, Karlsruhe, Germany); (2) "Two-step" method ( Figure 2): the mixture was granulated using twin-screw extruder. The mixture underwent melt mixing in the twin-screw extruder and then was passed through the die of the extruder forming the composites. The composites were then pelletized in a pelletizer machine to produce the granules. The granules obtained from the pelletizer machine were then placed in the twin-screw extruder with a die diameter of 3 mm; where they were subjected to melt mixing and then water cooling to produce the composite filaments. In addition, the process was carried out at a temperature difference range of 175 °C, 175 °C, 170 °C between the feeding zone to die zone. At last, the composite filaments with diameters of 1.75 ± 0.05 mm were obtained by using a laser diameter measuring (LDM) gauge (Mercury-Tech) and the wiredrawing-winding machine (WWM-001, independent research and development).

Rheological Studies
The melt viscosity of the composites was studied using a HAAKE Polylab torque rheometer (HAAKE, Karlsruhe, Germany) equipped with a Rheomix 600 QC counter-rotating roller rotors mixing chamber. The mixing chamber was loaded at 70% volume capacity and all measurements were performed over a constant revolution speed 40 rpm at a temperature of 170 °C.

Tensile Properties
Tensile tests were carried out according to ASTM D 638-2010 using an Instron universal testing machine (Instron, Norwood, MA, USA). Tests were performed with a load cell of 500 N and a cross-head speed of 2 mm/min on composite filament at room temperature (25 °C). The reported data were tensile strength (TS) and elongation at break (EAB). A minimum of five samples were tested for each formulation to get an average and standard deviation.

Dynamic Mechanical Analysis (DMA)
The storage modulus (E'), loss modulus (E''), and loss factor (tan δ) of composite filaments were carried out according to ASTM D 7028-07 ε1 using a dynamic mechanical analyzer (DMA Q800, TA Instruments, New Castle, PA, USA). DMA samples were vibrated with a tensile fixture at a Figure 1. The composite filaments prepared by the "one-step" method.
Polymers 2019, 11, x FOR PEER REVIEW 4 of 12 filament was manufactured by placing the mixture directly in an intermeshing counter-rotating conical twin-screw extruder (PolyLab QC, HAAKE, Karlsruhe, Germany); (2) "Two-step" method ( Figure 2): the mixture was granulated using twin-screw extruder. The mixture underwent melt mixing in the twin-screw extruder and then was passed through the die of the extruder forming the composites. The composites were then pelletized in a pelletizer machine to produce the granules. The granules obtained from the pelletizer machine were then placed in the twin-screw extruder with a die diameter of 3 mm; where they were subjected to melt mixing and then water cooling to produce the composite filaments. In addition, the process was carried out at a temperature difference range of 175 °C, 175 °C, 170 °C between the feeding zone to die zone. At last, the composite filaments with diameters of 1.75 ± 0.05 mm were obtained by using a laser diameter measuring (LDM) gauge (Mercury-Tech) and the wiredrawing-winding machine (WWM-001, independent research and development).

Rheological Studies
The melt viscosity of the composites was studied using a HAAKE Polylab torque rheometer (HAAKE, Karlsruhe, Germany) equipped with a Rheomix 600 QC counter-rotating roller rotors mixing chamber. The mixing chamber was loaded at 70% volume capacity and all measurements were performed over a constant revolution speed 40 rpm at a temperature of 170 °C.

Tensile Properties
Tensile tests were carried out according to ASTM D 638-2010 using an Instron universal testing machine (Instron, Norwood, MA, USA). Tests were performed with a load cell of 500 N and a cross-head speed of 2 mm/min on composite filament at room temperature (25 °C). The reported data were tensile strength (TS) and elongation at break (EAB). A minimum of five samples were tested for each formulation to get an average and standard deviation.

Dynamic Mechanical Analysis (DMA)
The storage modulus (E'), loss modulus (E''), and loss factor (tan δ) of composite filaments were carried out according to ASTM D 7028-07 ε1 using a dynamic mechanical analyzer (DMA Q800, TA Instruments, New Castle, PA, USA). DMA samples were vibrated with a tensile fixture at a

Rheological Studies
The melt viscosity of the composites was studied using a HAAKE Polylab torque rheometer (HAAKE, Karlsruhe, Germany) equipped with a Rheomix 600 QC counter-rotating roller rotors mixing chamber. The mixing chamber was loaded at 70% volume capacity and all measurements were performed over a constant revolution speed 40 rpm at a temperature of 170 • C.

Tensile Properties
Tensile tests were carried out according to ASTM D 638-2010 using an Instron universal testing machine (Instron, Norwood, MA, USA). Tests were performed with a load cell of 500 N and a cross-head speed of 2 mm/min on composite filament at room temperature (25 • C). The reported data were tensile strength (TS) and elongation at break (EAB). A minimum of five samples were tested for each formulation to get an average and standard deviation.

Dynamic Mechanical Analysis (DMA)
The storage modulus (E'), loss modulus (E"), and loss factor (tan δ) of composite filaments were carried out according to ASTM D 7028-07 ε1 using a dynamic mechanical analyzer (DMA Q800, TA Instruments, New Castle, PA, USA). DMA samples were vibrated with a tensile fixture at a frequency of 1 Hz. The samples were subjected to an amplitude of 15 µm in a temperature range of −20 • C to 120 • C at a heating rate of 2 • C/min.

Morphology Observation
The instrument used to analyze the surface morphology of the composite filaments and the interfacial quality between phases was a field emission environmental scanning electron microscope (ESEM; XL30 ESEM-FEG; FEI Co., Philips, The Netherlands). The samples were sputter coated with a thin layer of gold in a vacuum chamber for conductivity before the examination and were analyzed in a vacuum chamber that was less than 5 × 10 −5 Pa at a voltage of 7 kV.

Rheological Properties
Interfacial bonding between the reinforcement and matrix plays a vital role in determining the mechanical properties of the composites and the interfacial interactions of the composites can be reflected by rheology tests. Rheology investigates the flow of matter via recording the change in the parameters such as torque (T), melt temperature (TM), and total energy (E) at a constant rotational speed. In general, the polymer melts exhibit non-Newtonian viscosity. Thus, aiming to investigate the effect of CNF, MCC, and nano CaCO 3 on the rheological properties of composites in this study. Figure 3 showed a typical T-t, TM-t, and E-t plots of the blends test (e.g., BP and PLA, CNF-BP and PLA, MCC-BP and PLA, nano CaCO 3 -BP and PLA). There was a high initial T value during the fusion of all the polymers followed by a sharp decrease during the 20 min test. The TM of all the polymers rose sharply and then was followed by a general decrease, which is because the energy input reached a maximum value and the degradation of polymer reduced the viscosity during polymer melting and mixing process. As can be seen from Figure  respectively. BP treated by CNF impregnation modification was the easiest to mix with PLA, which is due to a lower T max and E max . It can be inferred that the nano CaCO 3 , CNF, and MCC each has a different impact on the rheological properties, suggesting that the incorporation of nano-particles into BP reinforced PLA composite filaments can result in disparate dispersion in the reinforcing phase due to the nanoscale dispersion and unique characteristics of the nano-particles. The good dispersion nano-particles can decrease the void numbers and provide strong interfacial hydrogen bond, thus developing better interfacial adhesion and improving the tensile properties of the composite filaments. It also can be inferred from these results that adopting nano CaCO 3 to treat BP was better than using MCC and CNF to treat BP on the basis of the impregnation modification method.

Tensile Properties of Composite Filaments
The tensile strength (TS) results of BP/PLA, nano CaCO3-BP/PLA, MCC-BP/PLA, and CNF-BP/PLA composite filaments are shown in Figure 4. When the composite filaments were prepared by "one-step" method, the TS of nano CaCO3-BP/PLA, MCC-BP/PLA, and CNF-BP/PLA composite filaments increased by 40.33%, 32.35%, and 12.32%, respectively, compared with BP/PLA composite filaments. The TS enhancement observed in these composite filaments can be explained by the formation of a percolating network in the polymer matrix, in which stress transfer is facilitated by hydrogen bonding. The hydrogen bonding was best exemplified in the paper where these secondary interactions provide the basis of its mechanical strength. Furthermore, well-dispersed nanometer-sized elements in the polymer matrix may also serve as nucleating agents in the foaming process [31]. Moreover, crystalline cellulose is much stronger and stiffer than the amorphous cellulose and cellulose itself, which means that MCC can be a better reinforcing agent than cellulose [32]. Khalia et al. [33] also reported that MCC with high crystallinity can deliver a strong reinforcing ability because of the high modulus, which is capable of improving the mechanical properties of biocomposites. However, when the composite filaments were manufactured by the "two-step" method, the TS of nano CaCO3-BP/PLA increased by 10.10%, while MCC-BP/PLA and CNF-BP/PLA composite filaments both decreased by 8.61% and 12.85%, respectively. This is due to the fact that MCC and CNF existed in a net structure, which will reunite easily and sturdily because of the strong hydrogen bonding, which can affect the reinforcing effect. From Figure 4, it can be observed that except BP/PLA composite filaments, the TS result values of nano CaCO3-BP/PLA, MCC-BP/PLA, and CNF-BP/PLA composite filaments manufactured by the "one-step" method were slightly higher than those produced by "two-step" method. These higher TS values are due to the preparation process of the composite filaments, which causes PLA to degrade. The molecular chain of MCC and CNF contains a large number of hydroxyl groups, which can make PLA degrade enormously. It should be noted that CNF has more surface hydroxyl groups than MCC [34], which resulted in CNF-BP/PLA composite filaments having lower TS values than MCC-BP/PLA composite filaments. It also can be inferred that the effect of nano CaCO3 impregnation modification on TS of composite filaments was better than MCC and CNF impregnation modification.

Tensile Properties of Composite Filaments
The tensile strength (TS) results of BP/PLA, nano CaCO 3 -BP/PLA, MCC-BP/PLA, and CNF-BP/PLA composite filaments are shown in Figure 4. When the composite filaments were prepared by "one-step" method, the TS of nano CaCO 3 -BP/PLA, MCC-BP/PLA, and CNF-BP/PLA composite filaments increased by 40.33%, 32.35%, and 12.32%, respectively, compared with BP/PLA composite filaments. The TS enhancement observed in these composite filaments can be explained by the formation of a percolating network in the polymer matrix, in which stress transfer is facilitated by hydrogen bonding. The hydrogen bonding was best exemplified in the paper where these secondary interactions provide the basis of its mechanical strength. Furthermore, well-dispersed nanometer-sized elements in the polymer matrix may also serve as nucleating agents in the foaming process [31]. Moreover, crystalline cellulose is much stronger and stiffer than the amorphous cellulose and cellulose itself, which means that MCC can be a better reinforcing agent than cellulose [32]. Khalia et al. [33] also reported that MCC with high crystallinity can deliver a strong reinforcing ability because of the high modulus, which is capable of improving the mechanical properties of biocomposites. However, when the composite filaments were manufactured by the "two-step" method, the TS of nano CaCO 3 -BP/PLA increased by 10.10%, while MCC-BP/PLA and CNF-BP/PLA composite filaments both decreased by 8.61% and 12.85%, respectively. This is due to the fact that MCC and CNF existed in a net structure, which will reunite easily and sturdily because of the strong hydrogen bonding, which can affect the reinforcing effect. From Figure 4, it can be observed that except BP/PLA composite filaments, the TS result values of nano CaCO 3 -BP/PLA, MCC-BP/PLA, and CNF-BP/PLA composite filaments manufactured by the "one-step" method were slightly higher than those produced by "two-step" method. These higher TS values are due to the preparation process of the composite filaments, which causes PLA to degrade. The molecular chain of MCC and CNF contains a large number of hydroxyl groups, which can make PLA degrade enormously. It should be noted that CNF has more surface hydroxyl groups than MCC [34], which resulted in CNF-BP/PLA composite filaments having lower TS values than MCC-BP/PLA composite filaments. It also can be inferred that the effect of nano CaCO 3 impregnation modification on TS of composite filaments was better than MCC and CNF impregnation modification.  Having used any of the fore-mentioned preparation methods, the EAB of BP/PLA composite filaments was higher than that of BP/PLA treated by nano CaCO3 or MCC or CNF. This is due to the formation of nano CaCO3, MCC, and CNF, which act as stress concentrators that lead to an increase of brittleness, which in turn reduces the EAB. Moreover, nano CaCO3 or MCC or CNF probabilistically come into contact with BP directly, leading to stress cracks that propagated much more easily through the material [35]. Compared to nano CaCO3-BP/PLA composite filaments manufactured by the "one-step" method, the EAB of MCC-BP/PLA and CNF-BP/PLA composite filaments were increased by 10.04% and 6.99%, respectively, this is because the melt plasticization process of MCC-BP/PLA and CNF-BP/PLA composite filaments during the "one-step" method process was better than that of nano CaCO3-BP/PLA composite filaments. It also can be seen that the EAB of MCC-BP/PLA and CNF-BP/PLA composite filaments manufactured by the "two-step" method, decreased by 11.93% and 23.05%, respectively, in comparison to nano CaCO3-BP/PLA composite filaments, indicating that MCC and CNF made the PLA in the composite filaments manufactured by the "two-step" degrade much more than in CaCO3.

Dynamic Mechanical Properties
The storage modulus E', loss modulus E'' and loss factor tan δ of the composite filaments as a function of temperature were determined by DMA and are shown in Figure 6. The DMA plots  Having used any of the fore-mentioned preparation methods, the EAB of BP/PLA composite filaments was higher than that of BP/PLA treated by nano CaCO 3 or MCC or CNF. This is due to the formation of nano CaCO 3 , MCC, and CNF, which act as stress concentrators that lead to an increase of brittleness, which in turn reduces the EAB. Moreover, nano CaCO 3 or MCC or CNF probabilistically come into contact with BP directly, leading to stress cracks that propagated much more easily through the material [35]. Compared to nano CaCO 3 -BP/PLA composite filaments manufactured by the "one-step" method, the EAB of MCC-BP/PLA and CNF-BP/PLA composite filaments were increased by 10.04% and 6.99%, respectively, this is because the melt plasticization process of MCC-BP/PLA and CNF-BP/PLA composite filaments during the "one-step" method process was better than that of nano CaCO 3 -BP/PLA composite filaments. It also can be seen that the EAB of MCC-BP/PLA and CNF-BP/PLA composite filaments manufactured by the "two-step" method, decreased by 11.93% and 23.05%, respectively, in comparison to nano CaCO 3 -BP/PLA composite filaments, indicating that MCC and CNF made the PLA in the composite filaments manufactured by the "two-step" degrade much more than in CaCO 3 .  Having used any of the fore-mentioned preparation methods, the EAB of BP/PLA composite filaments was higher than that of BP/PLA treated by nano CaCO3 or MCC or CNF. This is due to the formation of nano CaCO3, MCC, and CNF, which act as stress concentrators that lead to an increase of brittleness, which in turn reduces the EAB. Moreover, nano CaCO3 or MCC or CNF probabilistically come into contact with BP directly, leading to stress cracks that propagated much more easily through the material [35]. Compared to nano CaCO3-BP/PLA composite filaments manufactured by the "one-step" method, the EAB of MCC-BP/PLA and CNF-BP/PLA composite filaments were increased by 10.04% and 6.99%, respectively, this is because the melt plasticization process of MCC-BP/PLA and CNF-BP/PLA composite filaments during the "one-step" method process was better than that of nano CaCO3-BP/PLA composite filaments. It also can be seen that the EAB of MCC-BP/PLA and CNF-BP/PLA composite filaments manufactured by the "two-step" method, decreased by 11.93% and 23.05%, respectively, in comparison to nano CaCO3-BP/PLA composite filaments, indicating that MCC and CNF made the PLA in the composite filaments manufactured by the "two-step" degrade much more than in CaCO3.

Dynamic Mechanical Properties
The storage modulus E', loss modulus E'' and loss factor tan δ of the composite filaments as a function of temperature were determined by DMA and are shown in Figure 6. The DMA plots

Dynamic Mechanical Properties
The storage modulus E', loss modulus E" and loss factor tan δ of the composite filaments as a function of temperature were determined by DMA and are shown in Figure 6. The DMA plots clearly revealed that the storage modulus E' of the nano CaCO 3 -BP/PLA, MCC-BP/PLA, and CNF-BP/PLA composite filaments increased compared with the BP/PLA composite filaments at the temperature ranging from −20 • C to the glass transition temperature (T g ). The reinforcement effect of nano CaCO 3 , MCC, and CNF gradually became less apparent when the temperature was above T g. This is because the rigid nanoparticles (e.g., nano CaCO 3 , CNF, and MCC) further limited the movement of the matrix molecular segments, thus causing the improvement in rigidity of the composite filaments. The data showed a remarkable modulus enhancement in the glassy state, where at −20 • C, E' increased from 3.345 GPa (BP/PLA-0) by 56.17% (to 5.224 GPa), 58.03% (to 5.286 GPa), 38.48% (to 4.632 GPa) for nano CaCO 3 -BP/PLA-0, MCC-BP/PLA-0, and CNF-BP/PLA-0 composite filaments, respectively; while E' increased from 4.322 GPa (BP/PLA-1) by 16.06% (to 5.016 GPa) and 20.34% (to 5.201 GPa) for nano CaCO 3 -BP/PLA-1 and MCC-BP/PLA-1 composite filaments, respectively. It also can be seen that the E' −20 • C of nano CaCO 3 -BP/PLA-0, and MCC-BP/PLA-0 composite filaments were a slight proportion higher than that of nano CaCO 3 -BP/PLA-1 and MCC-BP/PLA-1 composite filaments, respectively, which is consistent with the TS results. As can be seen from Figure 6b that the E" of the nano CaCO 3 -BP/PLA, MCC-BP/PLA, and CNF-BP/PLA composite filaments were higher than that of BP/PLA-0, which means that the large friction occurred due to mutual movement. It can be inferred that the energy of the thermally activated molecular movement was different for all the composite filaments. The tan δ max value of BP/PLA-0 composite filaments were lower than nano CaCO 3 -BP/PLA, MCC-BP/PLA, and CNF-BP/PLA composite filaments (Figure 6c), which indicated that the CaCO 3 , MCC, and CNF had an effect on the movement of the PLA chain segments. Moreover, the crystallinity (CrI) of material can affect the dynamic mechanical properties, as it can be seen in the DMA curves [36], an increase in both storage and loss modulus in the temperature range of 90-120 • C was observed, this is due to the re-crystallization exotherm of PLA, leading to the crystals formed during the cold crystallization of PLA around 95 • C [37]. Additionally, it can be seen that the data of CNF-BP/PLA-1 (two-step method) composite filaments were not shown in Figure 6. This is because the EAB of CNF-BP/PLA-1 composite filaments was lower than the other materials, causing the higher brittleness, thus leading to the breakage which happened during the test. And according to the literature [38], the existence of CNF can improve the degradation rate from the beginning of degradation rather than in the process of degradation, which obviously improved the degradation of PLA. clearly revealed that the storage modulus E' of the nano CaCO3-BP/PLA, MCC-BP/PLA, and CNF-BP/PLA composite filaments increased compared with the BP/PLA composite filaments at the temperature ranging from −20 °C to the glass transition temperature (Tg). The reinforcement effect of nano CaCO3, MCC, and CNF gradually became less apparent when the temperature was above Tg. This is because the rigid nanoparticles (e.g., nano CaCO3, CNF, and MCC) further limited the movement of the matrix molecular segments, thus causing the improvement in rigidity of the composite filaments. The data showed a remarkable modulus enhancement in the glassy state, where at −20 °C, E' increased from 3.345 GPa (BP/PLA-0) by 56.17% (to 5.224 GPa), 58.03% (to 5.286 GPa), 38.48% (to 4.632 GPa) for nano CaCO3-BP/PLA-0, MCC-BP/PLA-0, and CNF-BP/PLA-0 composite filaments, respectively; while E' increased from 4.322 GPa (BP/PLA-1) by 16.06% (to 5.016 GPa) and 20.34% (to 5.201 GPa) for nano CaCO3-BP/PLA-1 and MCC-BP/PLA-1 composite filaments, respectively. It also can be seen that the E'−20°C of nano CaCO3-BP/PLA-0, and MCC-BP/PLA-0 composite filaments were a slight proportion higher than that of nano CaCO3-BP/PLA-1 and MCC-BP/PLA-1 composite filaments, respectively, which is consistent with the TS results. As can be seen from Figure 6b that the E'' of the nano CaCO3-BP/PLA, MCC-BP/PLA, and CNF-BP/PLA composite filaments were higher than that of BP/PLA-0, which means that the large friction occurred due to mutual movement. It can be inferred that the energy of the thermally activated molecular movement was different for all the composite filaments. The tan δmax value of BP/PLA-0 composite filaments were lower than nano CaCO3-BP/PLA, MCC-BP/PLA, and CNF-BP/PLA composite filaments (Figure 6c), which indicated that the CaCO3, MCC, and CNF had an effect on the movement of the PLA chain segments. Moreover, the crystallinity (CrI) of material can affect the dynamic mechanical properties, as it can be seen in the DMA curves [36], an increase in both storage and loss modulus in the temperature range of 90-120 °C was observed, this is due to the re-crystallization exotherm of PLA, leading to the crystals formed during the cold crystallization of PLA around 95 °C [37]. Additionally, it can be seen that the data of CNF-BP/PLA-1 (two-step method) composite filaments were not shown in Figure 6. This is because the EAB of CNF-BP/PLA-1 composite filaments was lower than the other materials, causing the higher brittleness, thus leading to the breakage which happened during the test. And according to the literature [38], the existence of CNF can improve the degradation rate from the beginning of degradation rather than in the process of degradation, which obviously improved the degradation of PLA.

Morphology Observation
Typical environmental scanning electron micrographs (ESEM) of the tensile fractured surfaces were presented in Figure 7. These figures were used to investigate the interface quality between nano-particles (e.g., nano CaCO 3 , MCC, and CNF), BP, and PLA. Figure 7 showed the corresponding ESEM images of BP/PLA, CNF-BP/PLA, MCC-BP/PLA, and nano CaCO 3 -BP/PLA composite filaments. Obviously, the level of interface adhesion between the reinforcement and matrix was reflected qualitatively by the ESEM micrographs. It can be seen that strong interface adhesion between modified BP (nano CaCO 3 -BP, MCC-BP, and CNF-BP) and matrix PLA was obtained since less space or voids between both phases can be observed. During failure at the interface, the BP may not be able to support a load and could be easy to pull out from matrix PLA. It also can be noticed that the reinforcing phases broke under tensile tests except for the reinforcement pullout, which indicated that the load was transferred from the PLA matrix to the reinforcements effectively. It can be inferred that the development of interface bonding was a sign of the tensile properties' improvement in the composite filaments. The better interfacial interaction increased the tensile properties of the composite filaments manufactured by the "one-step" method, which was confirmed by the tensile testing results. In addition, according to the research [39,40], the tensile properties were related not only to the interfacial compatibility but also to the raw materials themselves. For nano-particles reinforced composites manufactured by the "two-step" method, the effect of raw materials on tensile strength was more than the interfacial compatibility between reinforcement and matrix. It also can be observed that the "two-step" method made the matrix PLA of nano-particles reinforced composite filaments decrease much more than the "one-step" method, thus, decreasing the molecular weight of PLA. However, this phenomenon did not happen for the BP/PLA composite filaments. Typical environmental scanning electron micrographs (ESEM) of the tensile fractured surfaces were presented in Figure 7. These figures were used to investigate the interface quality between nano-particles (e.g., nano CaCO3, MCC, and CNF), BP, and PLA. Figure 7 showed the corresponding ESEM images of BP/PLA, CNF-BP/PLA, MCC-BP/PLA, and nano CaCO3-BP/PLA composite filaments. Obviously, the level of interface adhesion between the reinforcement and matrix was reflected qualitatively by the ESEM micrographs. It can be seen that strong interface adhesion between modified BP (nano CaCO3-BP, MCC-BP, and CNF-BP) and matrix PLA was obtained since less space or voids between both phases can be observed. During failure at the interface, the BP may not be able to support a load and could be easy to pull out from matrix PLA. It also can be noticed that the reinforcing phases broke under tensile tests except for the reinforcement pullout, which indicated that the load was transferred from the PLA matrix to the reinforcements effectively. It can be inferred that the development of interface bonding was a sign of the tensile properties' improvement in the composite filaments. The better interfacial interaction increased the tensile properties of the composite filaments manufactured by the "one-step" method, which was confirmed by the tensile testing results. In addition, according to the research [39,40], the tensile properties were related not only to the interfacial compatibility but also to the raw materials themselves. For nano-particles reinforced composites manufactured by the "two-step" method, the effect of raw materials on tensile strength was more than the interfacial compatibility between reinforcement and matrix. It also can be observed that the "two-step" method made the matrix PLA of nano-particles reinforced composite filaments decrease much more than the "one-step" method, thus, decreasing the molecular weight of PLA. However, this phenomenon did not happen for the BP/PLA composite filaments. CNF-BP/PLA-1 composite filaments were manufactured by the "two-step" method ( Figure 2).

Conclusions
Based on our main objective to produce natural fiber reinforced composite materials with strong interfacial adhesion for 3D printing, especially for fused deposition modeling, this work introduced nano-particles impregnation modification. In summary, the BP/PLA composite filaments were treated with nano CaCO3, CNF and MCC using impregnation modification and were successfully produced using both a "one-step" and "two-step" method. The TS of the composite filaments manufactured by the "one-step" method were remarkably improved by adding nano CaCO3, CNF, and MCC. The TS of MCC-BP/PLA and CNF-BP/PLA composite filaments manufactured by the "two-step" method decreased due to the serious degradation of PLA. The TS of nano CaCO3-BP/PLA, MCC-BP/PLA, and CNF-BP/PLA composite filaments manufactured by the "one-step" method was slightly higher than those produced by the "two-step" method. The EAB of nano CaCO3-BP/PLA, MCC-BP/PLA, and CNF-BP/PLA was lower than that of the BP/PLA composite filaments, which is due to the increase in brittleness caused by impregnation modification. The E' of the composite filaments increased through the addition of nano CaCO3, CNF, and MCC at the temperature ranging from −20 °C to the Tg, while the reinforcement effect of nano CaCO3, MCC, and CNF gradually became less apparent when the temperature was above Tg. The nano CaCO3-BP, MCC-BP, or CNF-BP and PLA were readily accessible by the simple mixing process. A detailed rheological investigation of such mixtures showed that the nano CaCO3, CNF, and MCC had a different impact on the processability and rheological properties of the composites. The effect of nano CaCO3 on the properties of composite filaments was better than MCC and CNF impregnation modification.
Author Contributions: C.W. and H.C. conceived and designed the experiments; W.Z., M.L., and C.W. performed the experiments; C.W. and W.Z. analyzed the data; H.C., S.Z., and G.W. provided materials/equipment/analysis tools; C.W. and W.Z. wrote the paper; L.M.S. and S.Q. Shi modified the paper.