Microstructure Evaluation and Thermal–Mechanical Properties of ABS Matrix Composite Filament Reinforced with Multi-Walled Carbon Nanotubes by a Single Screw Extruder for FDM 3D Printing

: This paper reports the synthesis of a new printable ABS–MWCNT composite ﬁlament, for use in fused deposition modeling (FDM), using an extrusion technique. Acrylonitrile butadiene styrene (ABS) and multi-walled carbon nanotubes (MWCNTs) were the initial materials used for fabricating the ﬁlaments. The MWCNTs were dispersed in ABS resin, then extruded through a single-shaft extruder in ﬁlament form, with MWCNT contents of 0.5%, 1%, 1.5%, 2%, 3% or 4% by weight. After extrusion, the diameter of the ﬁlaments was about 1.75 mm, making them appropriate for FDM. The as-synthesized ﬁlaments were then used in FDM to print out samples, on which tensile tests and other analyses were carried out. The results demonstrate that the sample with 2% MWCNTs had the highest strength value, 44.57 MPa, comprising a 42% increase over that of the pure ABS sample. The morphology and dispersion of MWCNTs in the composite were observed by ﬁeld emission scanning electron microscopy (FESEM), demonstrating the uniform distribution of MWCNTs in the ABS matrix. The thermal behavior results indicated no signiﬁcant change in the ABS structure; however, the melt ﬂow index of the ﬁlaments decreased with an increase in the MWCNT content.


Introduction
Additive manufacturing (or 3D printing) technology has recently become a trending technology for manufacturing parts from 3D digital models, due to its remarkable benefits, such as the potential to form complex geometric parts, reduced manufacturing time, low cost, flexible design, print-on-demand, reduced waste, and high-quality products. Various popular 3D printing technologies have been developed, including fused deposition modeling (FDM), stereolithography (SLA), powder bed fusion (PBF), multiphoton lithography (MPL), sintering laser melting (SLM), digital light processing, and so on [1][2][3][4][5][6][7]. Due to its technology-and cost-related advantages, the FDM process has become a practical 3D printing technology for various applications. Moreover, the process operations are straightforward and tenable, in which thermoplastic-based materials, such as acrylonitrile butadiene styrene (ABS), polyamide, polycarbonate, polyethylene, and polypropylene, are typically used. However, their mechanical properties are usually lower in comparison with those of conventional products, and they usually present anisotropic behavior [8,9]. Thus, the demand for new high-quality printable materials for the FDM process has significantly grown recently, due to its various applications in the rapid tooling and manufacturing areas [10][11][12]. Motivated by this fact, composite materials have emerged as a promising alternative candidate to expand the 3D printable materials spectrum, in terms of physicochemical properties. Polymer matrix composites reinforced by carbonaceous materials, such as graphene and carbon nanotubes (CNT), have gained attention in previous reports [13][14][15][16]. These studies have demonstrated that FDM 3D printing products fabricated using carbon fiber or carbon nanotube fillers in the polymer matrix had remarkably enhanced electrical/heat conduction, mechanical strength, modulus of elasticity, toughness, and/or durability, revealing the great potential of such materials in various practical applications. By introducing fibers or particles into a polymer matrix, 3D-printed objects in the biomedical, mechanical, and electrical areas, among others, have been widely studied and applied [17,18]. Many researchers have recently made efforts towards investigating the effects of process and infill parameters on the mechanical properties of 3D-printed specimens fabricated by FDM [19][20][21], and have developed predictive models using analytical, numerical, and experimental techniques [22,23].
In the present study, a MWCNT filler is used as the reinforcement material, which is added to the ABS matrix-a strong, durable production-grade thermoplastic-in order to create a unique ABS-MWCNT composite, which can be used in the FDM process to improve the desired properties of the printed parts. It is worth noting that the conventional approaches to ABS-CNT composites have utilized a relatively high weight percentage of filler, ranging from 5-10 wt% [24][25][26][27], while few studies using low loading CNT content (<5 wt%) in ABS matrix have been carried out. In addition, standard solution mixing or thermal blending techniques used in the fabrication of ABS-CNT composite require complicated and high melting temperature treatments, limiting the quality of the obtained products due to a low homogeneous distribution between ingredients. Herein, we propose the possibility of directly synthesizing ABS-MWCNT composite filaments with MWCNT content up to 4 wt% for commercial FDM 3D printing with a standard filament diameter of about 1.75 mm. An extrusion FDM machine is then used to produce various small-scale objects using the ABS-MWCNT filaments. Moreover, the thermal-mechanical characteristics of the composite filaments and the 3D printed parts are further analyzed.
Multi-walled carbon nanotubes (MWCNTs), manufactured and supplied by the Vietnam Academy of Science and Technology (2-20 nm diameter, 10 µm length, 130-160 m 2 /g surface area, 0.04-0.08 g/cm 3 density, and~90% purity) were utilized as the reinforcement material. The MWCNTs were chemically functionalized with a carboxylic acid (-COOH) functional group at a temperature of 70 • C for 5 h. After functionalization, the MWCNT-COOH was dispersed into ethanol by a sonication process for 1 h, in order to prepare the MWCNT suspension. This step was applied to deagglomerate and enhance the uniform dispersion stability of the purified MWCNTs (see Figure 1).

ABS-MWCNT Composite Filament Preparation and Printing Process
The ABS pellets and MWCNTs were mixed and extruded using a single screw extruder equipment. Six types of filaments were prepared and matched, with different MWCNT contents of 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 3 wt%, or 4 wt%. Figure 2 illustrates the schematic of preparing the ABS-MWCNT composite filament. The mixture of polymer granules and MWCNT was loaded into the extruder. They were caught by the rotating screws and pushed forward inside the extruder, then were melted in the heated chamber (operated at 220 °C), due to the provided heat and shearing of the materials between the screw and barrel. The melting-phase containing MWCNTs occurred due to the combination of shearing and kneading behavior, with the mixture becoming homogenous after some time. Under the effect of the press, the ABS incorporated into MWCNT mixture was extruded through nozzles to form semisolid filaments, which were cooled down by an air-fan and water system, reaching the required filament diameter of 1.75 ± 0.1 mm. In this work, a three-time extrusion process was applied for each type of proposed recipe, in order to increase the homogeneity level. The as-synthesized composite filaments were then fed into a FDM machine, in order to evaluate their printing ability, and print samples were retained for further analyses.

ABS-MWCNT Composite Filament Preparation and Printing Process
The ABS pellets and MWCNTs were mixed and extruded using a single screw extruder equipment. Six types of filaments were prepared and matched, with different MWCNT contents of 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 3 wt%, or 4 wt%. Figure 2 illustrates the schematic of preparing the ABS-MWCNT composite filament. The mixture of polymer granules and MWCNT was loaded into the extruder. They were caught by the rotating screws and pushed forward inside the extruder, then were melted in the heated chamber (operated at 220 • C), due to the provided heat and shearing of the materials between the screw and barrel. The melting-phase containing MWCNTs occurred due to the combination of shearing and kneading behavior, with the mixture becoming homogenous after some time. Under the effect of the press, the ABS incorporated into MWCNT mixture was extruded through nozzles to form semisolid filaments, which were cooled down by an air-fan and water system, reaching the required filament diameter of 1.75 ± 0.1 mm. In this work, a three-time extrusion process was applied for each type of proposed recipe, in order to increase the homogeneity level. The as-synthesized composite filaments were then fed into a FDM machine, in order to evaluate their printing ability, and print samples were retained for further analyses.

Characterizations
Tensile tests: SDLs MX2D printing was used to print the tensile test samples, according to ASTM D638 standard. All samples were printed horizontally in raster angle [45, +45] at 100% infill. Pure ABS filament was also included, in order to print reference samples. The tensile test was carried out on a DEVOTRANS machine with a constant tensile rate of 20 mm/min at room temperature.
Melt flow index (MFI): the melt flow index of the samples was measured using Tinus Olsen equipment, according to ASTM D1238 under experimental conditions at a high temperature of 230 • C and 2.16 kg loading. DSC analysis: differential scanning calorimetry (DSC) experiments were conducted using a DSC EXSTAR 7020, in order to study the thermal behaviors of the samples. The

Characterizations
Tensile tests: SDLs MX2D printing was used to print the tensile test samples, according to ASTM D638 standard. All samples were printed horizontally in raster angle [45, +45] at 100% infill. Pure ABS filament was also included, in order to print reference samples. The tensile test was carried out on a DEVOTRANS machine with a constant tensile rate of 20 mm/min at room temperature.
Melt flow index (MFI): the melt flow index of the samples was measured using Tinus Olsen equipment, according to ASTM D1238 under experimental conditions at a high temperature of 230 °C and 2.16 kg loading. DSC analysis: differential scanning calorimetry (DSC) experiments were conducted using a DSC EXSTAR 7020, in order to study the thermal behaviors of the samples. The samples were heated in the temperature range of 25-300 °C with an increase rate of 10 °C/min. An inert atmosphere was maintained by purging nitrogen into the equipment during the test.

Results and Discussions
The tensile test was conducted for all the samples; that is, ABS and 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 3 wt%, and 4 wt% MWCNT samples. Figure 3a presents images of the samples, showing fractures after the tensile tests. Additionally, the stress-strain curves are illustrated in Figure 3b. Generally, the tensile strength of the samples improved along with the addition of MWCNTs as filler (0.5 wt% to 2.0 wt%), compared to the pure ABS sample. Specifically, the 0.5 wt% samples showed a tensile strength value of 38.85 MPa, higher than the value of 31.43 MPa for the reference sample. This improvement revealed the reinforcement effect of MWCNTs, in terms of enhancing the mechanical properties of the ABS matrix, even with a small amount such as 0.5 wt%. The positive pattern was the same with the loading content increasing up to 2.0 wt%, for which the tensile strength was 41.81% and achieved a value of 44.57 MPa. However, there was no change in tensile strength as the MWCNT concentration reached 3.0 wt%, but was slightly reduced in the 4.0 wt% MWCNT sample, compared to the pure ABS condition. (2) cooling fan; (3) water cooling system; (4) motor of cooling box; (5) motor of filament extruder; (6) pulling system; (7) guiding system; (8) coiling system.

Results and Discussions
The tensile test was conducted for all the samples; that is, ABS and 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 3 wt%, and 4 wt% MWCNT samples. Figure 3a presents images of the samples, showing fractures after the tensile tests. Additionally, the stress-strain curves are illustrated in Figure 3b. Generally, the tensile strength of the samples improved along with the addition of MWCNTs as filler (0.5 wt% to 2.0 wt%), compared to the pure ABS sample. Specifically, the 0.5 wt% samples showed a tensile strength value of 38.85 MPa, higher than the value of 31.43 MPa for the reference sample. This improvement revealed the reinforcement effect of MWCNTs, in terms of enhancing the mechanical properties of the ABS matrix, even with a small amount such as 0.5 wt%. The positive pattern was the same with the loading content increasing up to 2.0 wt%, for which the tensile strength was 41.81% and achieved a value of 44.57 MPa. However, there was no change in tensile strength as the MWCNT concentration reached 3.0 wt%, but was slightly reduced in the 4.0 wt% MWCNT sample, compared to the pure ABS condition.
Thus, the strengthening effect of MWCNTs depends on the arrangement and distribution of the filler within the matrix. Interestingly, low content MWCNT loading delivered the best homogeneous and alignment mixing between the reinforcer and ABS substrate; therefore, all components could be adhesively bonded, giving an optimum recipe for the highest strength and stiffness properties. In addition, the MWCNT particles tended to locally agglomerate under higher content, resulting in their poor dispersion, non-uniform alignment, and a weak interaction between the MWCNTs and ABS matrix.
The elongation investigations of samples are further included in Figure 3b. The percentage of tensile elongation at the breakpoint was more significant in cases of ≥1.5 wt% MWCNT samples, while the pieces with 0.5 wt% and 1 wt% exhibited a slightly higher extension, compared to pure ABS. This phenomenon was observed at the non-uniform plastic deformation region of the stress-strain curves (Figure 3b Thus, the strengthening effect of MWCNTs depends on the arrangement and distribution of the filler within the matrix. Interestingly, low content MWCNT loading delivered the best homogeneous and alignment mixing between the reinforcer and ABS substrate; therefore, all components could be adhesively bonded, giving an optimum recipe for the highest strength and stiffness properties. In addition, the MWCNT particles tended to locally agglomerate under higher content, resulting in their poor dispersion, non-uniform alignment, and a weak interaction between the MWCNTs and ABS matrix. The elongation investigations of samples are further included in Figure 3b. The percentage of tensile elongation at the breakpoint was more significant in cases of ≥1.5 wt% MWCNT samples, while the pieces with 0.5 wt% and 1 wt% exhibited a slightly higher extension, compared to pure ABS. This phenomenon was observed at the non-uniform plastic deformation region of the stress-strain curves (Figure 3b), illustrating the greater brittleness with a high concentration of MWCNTs (1.5, 2, 3 and 4 wt%) incorporated into the ABS. Notably, at 2.0 wt%, the sample elongation and tensile strength properties were inversely proportional.
The DSC and MFI measurements were carried out to confirm the effect of MWCNTs on the thermal behavior of the as-synthesized nanocomposites. Figure 4 shows the thermogram profiles of the samples during the heating process, in which the temperature increased from 50 to 350 °C. The glass transition temperature (Tg) and melting point (Tm) for the different ABS-MWCNT composites were determined at around 90 °C and 240 °C, respectively. This observation indicated a minor effect of MWCNTs, as a filler, on the thermal properties of the ABS matrix. The peaks at about 230-240 °C in the DSC thermogram ( Figure 3) correspond to the melting temperature Tm of the composites, which was the same as that of pure ABS matrix, even with the presence of the MWCNT filler. The DSC and MFI measurements were carried out to confirm the effect of MWCNTs on the thermal behavior of the as-synthesized nanocomposites. Figure 4 shows the thermogram profiles of the samples during the heating process, in which the temperature increased from 50 to 350 • C. The glass transition temperature (Tg) and melting point (Tm) for the different ABS-MWCNT composites were determined at around 90 • C and 240 • C, respectively. This observation indicated a minor effect of MWCNTs, as a filler, on the thermal properties of the ABS matrix. The peaks at about 230-240 • C in the DSC thermogram ( Figure 3) correspond to the melting temperature T m of the composites, which was the same as that of pure ABS matrix, even with the presence of the MWCNT filler.       Figure 5 presents the relationship between the MWCNT content and the melt flow index of the samples. The MFI value reached 6.3 g/10 min and 5.6 g/10 min in the blank ABS and 0.5 wt% MWCNT samples, respectively. These values were significantly reduced under higher loading of MWCNT contents, approaching 0.9 g/10 min for the 2.0 wt% MWCNT sample. Meanwhile, the 3.0 wt% and 4.0 wt% MWCNT samples presented low MFI values. The decrease of MFI values could be attributed to an increase of composite viscosity, induced by the formation of a nanofiller network. In practice, the low MFI value filament materials lead to many difficulties during the FDM feeding and extrusion processes; therefore, high MWCNT content composites are not favorable for commercial applications. FESEM measurement was conducted to analyze the surface fractures of the broken printed parts and the morphology of MWCNTs distributed within the samples. Figure 6 presents an FESEM image of the destruction surface of an FDM composite sample after FESEM measurement was conducted to analyze the surface fractures of the broken printed parts and the morphology of MWCNTs distributed within the samples. Figure 6 presents an FESEM image of the destruction surface of an FDM composite sample after the tensile test. A layered structure is revealed, along with the presence of voids between layers and segments.
Appl. Sci. 2021, 11, 8798 7 of 11 the tensile test. A layered structure is revealed, along with the presence of voids between layers and segments. The fracture morphologies under 1.0 wt%, 2.0 wt%, and 3.0 wt% content of ABS-MWCNT composites, as depicted by FESEM measurements, are presented in Figure 7a, Figure 7b and Figure 7c, respectively. The 1.0 wt% MWCNT sample exhibited a typical fracture surface with shear lips and few dimples, which means the sample retained high ductility during the tensile test, compared to the ABS polymer. In Figure 7b,c, with higher MWCNT content, a more considerable degree of shear lips can be observed. Although the The fracture morphologies under 1.0 wt%, 2.0 wt%, and 3.0 wt% content of ABS-MWCNT composites, as depicted by FESEM measurements, are presented in Figures 7a, 7b and 7c, respectively. The 1.0 wt% MWCNT sample exhibited a typical fracture surface with shear lips and few dimples, which means the sample retained high ductility during the tensile test, compared to the ABS polymer. In Figure 7b,c, with higher MWCNT content, a more considerable degree of shear lips can be observed. Although the addition of the MWCNT filler could enhance the tensile strength of the ABS polymer by over 40%, as mentioned in the previous part, as the increase was not significant for concentrations above 2.0 wt%, adding an appropriate amount of nanofillers to the matrix is essential. Importantly, voids and crack tracks were generated more significantly in the surface of higher MWCNT content samples, revealing the optimum loading content of MWCNTs in the ABS matrix as 2.0 wt%. The pull-out of the MWCNTs from the fracture surface, shown in Figure 6 at the selected regions, is presented more clearly in Figure 8a,b, having a higher magnification. This observation confirmed that the dispersion and distribution of MWCNTs in the matrix were better with lower MWCNT content, resulting in an improvement of the tensile strength. However, it was observed that the dispersion of MWCNTs was not perfect with higher MWCNT content loading into the ABS matrix: they were aggregated and pulled out by the tensile stress, as shown in Figure 8c,d. The agglomeration in 2.0 wt% and 3.0 wt% samples was more severe than in the case of 1.0 wt% MWCNTs, and the tensile strength no longer increased for the 3.0 wt% sample. The presence of MWCNT agglomerates in the matrix demonstrated the poor separation effect during the production and printing of the ABS-MWCNT composite filaments. In addition, the agglomeration of MWCNTs reduced the adhesion level between MWCNTs and the ABS matrix, due to the lower degree of interfacial contact between them. Furthermore, the aggregation of MWCNTs could result in cracking when tensile strength is applied to the sample. There- The pull-out of the MWCNTs from the fracture surface, shown in Figure 6 at the selected regions, is presented more clearly in Figure 8a,b, having a higher magnification. This observation confirmed that the dispersion and distribution of MWCNTs in the matrix were better with lower MWCNT content, resulting in an improvement of the tensile strength. However, it was observed that the dispersion of MWCNTs was not perfect with higher MWCNT content loading into the ABS matrix: they were aggregated and pulled out by the tensile stress, as shown in Figure 8c,d. The agglomeration in 2.0 wt% and 3.0 wt% samples was more severe than in the case of 1.0 wt% MWCNTs, and the tensile strength no longer increased for the 3.0 wt% sample. The presence of MWCNT agglomerates in the matrix demonstrated the poor separation effect during the production and printing of the ABS-MWCNT composite filaments. In addition, the agglomeration of MWCNTs reduced the adhesion level between MWCNTs and the ABS matrix, due to the lower degree of interfacial contact between them. Furthermore, the aggregation of MWCNTs could result in cracking when tensile strength is applied to the sample. Therefore, in this study, we suggest maintaining the content of MWCNTs at 2 wt% or less in the composite filaments.
Appl. Sci. 2021, 11, 8798 9 of 11 Figure 8. (a,b) Morphology of the MWCNT pull-out from the fracture surface of tensile sample with 2% wt at the magnification of ×200,000 and ×300,000, respectively; (c,d) Images of MWCNTs at the agglomeration area in samples with 2% wt and 3% wt at the magnification of ×50,000 and ×300,000, respectively.

Conclusions
Optimal ABS-MWCNT composite filaments for FDM 3D printing were fabricated successfully using a single screw extruder. The fabricated 3D printing filament had a diameter of 1.75 ± 0.1 mm and met the requirements of FDM 3D printing. First, the printability of the composite filaments was evaluated through printing tensile test samples from a digital file. Moreover, the thermal-mechanical properties of the ABS-MWCNT composites were also determined. The 3D-printed samples exhibited varying mechanical properties, which were influenced by the loading content of the MWCNTs in the composite filament. DSC analysis determined that the presence of MWCNTs had no adverse effects on the thermal behavior of the ABS matrix; however, the MFI data indicated a decreasing trend of MFI values with increasing MWCNT loading content. According to the obtained results discussed above, the optimum MWCNT fraction for the fused deposition modeling process is 2.0 wt%, with the resultant samples offering feasible mechanical and thermal properties for practical applications.  Morphology of the MWCNT pull-out from the fracture surface of tensile sample with 2% wt at the magnification of ×200,000 and ×300,000, respectively; (c,d) Images of MWCNTs at the agglomeration area in samples with 2% wt and 3% wt at the magnification of ×50,000 and ×300,000, respectively.

Conclusions
Optimal ABS-MWCNT composite filaments for FDM 3D printing were fabricated successfully using a single screw extruder. The fabricated 3D printing filament had a diameter of 1.75 ± 0.1 mm and met the requirements of FDM 3D printing. First, the printability of the composite filaments was evaluated through printing tensile test samples from a digital file. Moreover, the thermal-mechanical properties of the ABS-MWCNT composites were also determined. The 3D-printed samples exhibited varying mechanical properties, which were influenced by the loading content of the MWCNTs in the composite filament. DSC analysis determined that the presence of MWCNTs had no adverse effects on the thermal behavior of the ABS matrix; however, the MFI data indicated a decreasing trend of MFI values with increasing MWCNT loading content. According to the obtained results discussed above, the optimum MWCNT fraction for the fused deposition modeling process is 2.0 wt%, with the resultant samples offering feasible mechanical and thermal properties for practical applications. Data Availability Statement: All data generated from this study is available within the text of this paper.