Thermo-mechanical properties of composite filaments for 3D printing of fabrics

Abstract

The objective of this study is to manufacture composite filaments to be used in three-dimensional (3D) printing of fabrics using fused deposition modeling (FDM) method. The primary properties of a fabric are flexibility and strength which are lacking in the available 3D printed materials. Polylactic acid (PLA), thermoplastic polyurethane (TPU) and poly(ethylene) glycol (PEG) were blended in different proportions and extruded using twin-screw extruder to obtain composite filaments. The properties of the filaments were studied using various material characterization methods such as uniaxial tensile test, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscope (SEM). With the addition of PEG in the PLA/TPU composition, it was found that the yield stress and Young’s modulus of the composite filaments have significantly decreased compared to that of pure PLA filament. It was also noted that there was no significant difference in ultimate tensile strength whereas the elongation at break was increased by more than 500%. Using the composite filament, a plain weave fabric structure was 3D printed to investigate the printing ability of a complex structure. It is concluded that the composite filaments developed are suitable for 3D printing but non-uniformity in diameter affects the print quality and hence the overall properties of fabrics.

Keywords

Polylactic acid thermoplastic polyurethane poly(ethylene) glycol composite filament 3D printing fabrics

Introduction

Additive manufacturing (AM) (also known as three-dimensional (3D) printing) is the process of producing 3D objects by adding materials layer upon layer using the computer-aided design (CAD) models. The first patent related to additive manufacturing was granted in the United States in 1984,¹ entitled “Apparatus for production of three-dimensional objects by Stereolithography”. However, this technology was commercialized after the expiration of the patent.² Hence this technology is quite new, but its use has been skyrocketed in recent years. AM has been used in almost all industries including automotives, aerospace, defense, military, fashion, textiles, medicine, and many others.

The application of polymer-based additive manufacturing in fabric industry has fascinated the researchers.^3–10 Different polymers such as polylactic acid (PLA), acrylonitrile butadiene styrene (ABS) and thermoplastic polyurethane (TPU) have been tried.¹¹ Among all the 3D printing methods, fused deposition modeling (FDM) is probably the most popular method. With the FDM method, various products can be manufactured within a limited time frame with minimal waste. It holds a potential to produce complex geometries.^12–14 Although it has several advantages, the parts produced with FDM do not have high mechanical strength, high flexibility, and tear resistance.¹⁵ The reasons might be the presence of voids, weaker bonding between the layers, process parameters and the polymer used itself.^16,17 Most of the polymers which are readily available for FDM 3D printing do not have high mechanical strength, and high flexibility together. To compensate for these drawbacks of the materials, a material having high mechanical strength (e.g., PLA) and another polymer having high flexibility (e.g., TPU) are blended together in different weight percentages (wt %) in this work. Poly(ethylene) glycol (PEG) is used as a plasticizer in varying proportions to enhance the properties of the composite filaments.

Composite materials affect the mechanical and chemical properties of the final product.^17–19 They either improve or degrade the mechanical properties depending on the compatibility between the polymers.^20,21 Depending on the quantity and type of materials used, the mechanical properties of the final product can be customized.^22–25 PLA is a biodegradable and renewable thermoplastic polymer that is obtained from renewable resources such as corn, wheat, rice, and sugarcane.^26,27 The properties such as low thermal stability, high degradation rate during processing, brittleness in nature with less than 10% elongation at break,^28,29 low toughness and moisture sensitivity limit its applications.^30,31 TPU is a biocompatible, and linear segmented block copolymer,^32,33 and is synthesized by the reaction of polyols with aliphatic or aromatic diisocyanates. The polyols may be ether-, ester-, and carbonate-based diols. These polyols form the soft segments of the material whereas the diisocyanates such as diphenylmethane-4.4-diisocyanate (MDI) make the hard segments of TPU by addition of a chain extender such as butanediol. The soft segment (SS) interconnects two hard segments (HS) and the hard segments are bonded together with the presence of hydrogen bonds to form physical crosslinks.^34–36 PEG is biocompatible, yields non-immunogenicity, non-antigenicity, protein rejection, and non-toxic.^37,38 PEGs having molecular weight (MW) less than 1000 are viscous, colorless, water soluble and hygroscopic liquids at room temperature. The higher MW PEGs are waxy white solids. Its melting point is dependent on its MW and exhibits a plateau at about 67°C.^39,40

Several researchers have worked on PLA/TPU composition for various applications, but the work related to FDM 3D printing of fabrics is not found in the literature. Zhou et al.,⁴¹ studied PC/PLA/TPU blend and found improvement in toughness, increase in flexibility, but decrease in tensile strength and fracture response of the composite. Jaso et al.,⁴² investigated the PLA/TPU composition having varying weight percentages (wt%) of poly(lactic) acid (PLA) in TPU matrix and observed improvement in tensile strength, and elongation but decrease in elastic recovery. Mi et al., produced scaffolds for medical and tissue engineering applications using the PLA/TPU composition. They demonstrated increase in tensile and compressive strength, flexibility and larger pores with the addition of TPU content in the blends.³³ Feng and Ye fabricated tensile and impact specimens using hot-press-molding process with PLA/TPU polymers. They observed yield and neck formation which signifies the improvement of ductile behavior of the composite. They argued the presence of hydrogen bonding between PLA and TPU molecules which caused their partial miscibility.⁴³ Xu et al.,⁴⁴ studied PLA/TPU composites and found the increase in crystallinity and melt strength and confirmed the immiscibility of these polymers. Kaynak et al.,⁴⁵ studied the mechanical performance of neat PLA, glass fiber (GF)-reinforced PLA, and TPU-blended PLA composites using injection molding and 3D printing methods. They used twin-screw extruders to obtain filaments for 3D printing using 10 wt% of TPU/15 wt% of GF, 3D printed dog-bone specimens and compared their performance with the dog-bone samples obtained from injection molding.

It should be noted that no information related to manufacturing of PLA/TPU/PEG composite filaments mixed in different proportions has been found in the literature. The mechanical, thermo-mechanical and morphological characterization of these filaments are missing in the literature as well. In the present work, PLA is blended with TPU and PEG in varying weights to obtain composite filaments with optimal strength and flexibility which can be used for 3D printing of flexible structures. The microstructure, mechanical and thermo-mechanical properties of the composite filaments obtained are investigated and used for 3D printing of plain weave fabric structures in this work.

Materials and methods

Materials

PLA pellets were purchased from Filabot (Barre VT, USA) having a melt flow index (MFI) of 3 g/10 min at 190°C, density of 1.24 g/cm³, and the glass transition temperature ( $T_{g}$ ) of 55–60°C.⁴⁶ TPU pellets were supplied by LNS Technologies (Scotts Valley, CA, USA) with the properties such as shore hardness of 90A, density of 1.14 g/cm³, $T_{g}$ of – 33°C, and MFI of 32 g/10 min at 224°C.⁴⁷ PEG powder was purchased from Acros Organics (New Jersey, USA) having an average MW of 6000 Dalton.

Sample preparation and filament manufacturing

The pellets were dried in oven at 60°C for 12 h and then blended in a defined ratio as mentioned in Table 1. The polymer blend was fed into twin-screw extruder (Leistritz Mic 18/GL 40D, Nuremberg, Germany) and composite filaments were obtained as shown in Figure 1. Seven different temperature zones namely one to seven were allocated on the twin-screw extruder and the temperatures were fixed in the respective order: 155°C, 165°C, 165°C, 175°C, 180°C, 180°C, 170°C. The screw speed was set at 30 r/min. The composite filaments produced had varying diameters as listed in Table 1.

Table 1.

Composition and diameters of the filaments manufactured.

Samples	PLA (wt %)	TPU (wt %)	PEG (wt%)	Diameter (mm) (mean ± SD)
PLA	100	0	0	1.73 ± 0.073
PLA_TPU30_PEG0.5	69.5	30	0.5	1.73 ± 0.083
PLA_TPU30_PEG1	69	30	1	1.81 ± 0.070
PLA_TPU30_PEG2	68	30	2	1.80 ± 0.068
PLA_TPU30_PEG3	67	30	3	1.76 ± 0.057
PLA_TPU30_PEG4	66	30	4	1.75 ± 0.093
PLA_TPU30_PEG5	65	30	5	1.80 ± 0.082
PLA_TPU40_PEG0.5	59.5	40	0.5	1.79 ± 0.083
PLA_TPU40_PEG1	59	40	1	1.80 ± 0.066
PLA_TPU40_PEG2	58	40	2	1.78 ± 0.127
PLA_TPU40_PEG3	57	40	3	1.70 ± 0.082
PLA_TPU40_PEG4	56	40	4	1.79 ± 0.156

Note: 1. PLA: poly(lactic) acid, TPU: thermoplastic polyurethane, PEG: poly(ethylene) glycol, and SD: standard deviation.

Figure 1.

Representative composite filaments manufactured: (a) PLA_T40_P3, (b) PLA_T40_P0.5.

Material characterization methods

Uniaxial tensile tests of composite filaments

Proper selection of filaments is necessary to precisely predict the mechanical properties of the produced composite filaments. For this purpose, the filaments having diameter as shown in Table 1 were considered for the uniaxial tensile tests. Instron 5565 (Norwood, MA, USA) was employed to test the mechanical properties with a load cell of 1 kN, gage length of 50 mm, and crosshead speed of 25 mm/min (Figure 2). Five specimens were tested for each composition and the average values were reported.

Figure 2.

Uniaxial tensile testing of composite filaments manufactured. (a) Before extension on the Instron 5565 machine, and (b) After the extension.

Differential scanning calorimetry analysis

Differential scanning calorimetry (DSC) analysis was conducted using a Q2000 TA instrument. All the experiments were operated under a nitrogen atmosphere at the flow rate of 50 mL/min. The samples, weighing from 5 to 10 mg, were first heated from −70°C to 220°C and then were cooled from 220°C to −70°C and again heated to 220°C at a heating and cooling rate of 10°C/min. The parameters such as glass transition temperature ( $T_{g}$ ), melting temperature ( $T_{m}$ ), crystallization point, and enthalpies were measured. The first heating cycle was performed to remove the thermal histories of the composite filaments and all data are collected from the second heating and cooling cycles.

Thermogravimetric analysis

Thermogravimetric analysis (TGA) was conducted using Q500 TA instrument under a nitrogen atmosphere at a flow rate of 60 mL/min. The samples, weighing from 6 mg to 10 mg, were kept in the platinum pan and were heated from room temperature to 700°C at a heating rate of 10°C/min.

Dynamic mechanical analysis

The thermomechanical properties of the samples were evaluated by performing the dynamic mechanical analysis (DMA) using an RSA3 TA instruments. The dynamic temperature ramp test was conducted from −50°C to 80°C using the three-point bending fixture (flexural mode) at a ramp rate of 3°C/min, frequency of 1 Hz, and strain of 0.5%. Liquid nitrogen was used during the tests to regulate the heating and cooling process. Before conducting the tests, specimens with dimensions 40 mm × 10 mm × 1 mm were manufactured for each composition using the hot press with the temperature set at 200°C.

Fourier transform infrared spectroscopy

Fourier transform infrared spectroscopy (FTIR) analysis of the composite filaments was performed using a Nicolet 6700 FTIR (ThermoFisher Scientific, Madison, WI, USA). It was conducted for a total of 64 scans with a resolution of 4 cm⁻¹ and having the wavenumbers in the range of 400–4000 cm⁻¹.

Scanning electron microscope

Scanning electron microscope (SEM) was used to study the cross-section and surfaces of the composite filaments. The filaments were cryogenically fractured by immersing them in liquid nitrogen to prevent any alteration in the surface morphology. The specimens were gold-sputtered using EMS Q150R sputter coating device since they were non-conductive. The study was performed at different magnifications at an accelerating voltage of 20 kV and a working distance of approximately 10 mm–15 mm.

3D printing of fabric using composite filament

A 3D model of plain weave fabric structure was constructed using computer aided design (CAD) software, Solidworks® as shown in Figure 3. The details of design and construction can be found in ref [12]. The print settings used were infill density of 100%, printing temperature of 225°C, and print speed of 10 mm/s.

Figure 3.

CAD model of plain weave fabric for 3D printing.

Statistical analysis

Analysis of variance (ANOVA) test was done for tensile test results by using OriginPro Statistical Software to evaluate the effect of fillers and plasticizers on the composite filaments. All analyses were carried out at 0.05 significance level (i.e., α = 0.05). To determine exactly which group means are significantly different, a Tukey test was performed.

Results and discussion

Tensile test results

The tensile tests on pure PLA filaments and composite filaments were conducted, and the results are presented in Figure 4, Tables 2 and 3. This data show that the yield stress ( $σ_{y}$ ) of the composite filaments decreased due to the addition of TPU and PEG. The yield stress of pure PLA filament was found to be 47.52 ± 2.37 MPa, whereas that of other composite filaments were in the range of 26–30 MPa. The reason behind the lower yield stress might be the presence of SS of TPU polymers which are elastomeric in nature. The ultimate tensile stress (UTS) of pure PLA filament was determined to be 49.83 ± 1.26 MPa. With the addition of 30 wt% TPU and the varying wt% of PEG, the UTS of composite filaments were in the range of 42–58 MPa. Similarly, with the addition of 40 wt% TPU and the varying wt% of PEG, the values of UTS were in the range of 51–56 MPa. However, the statistical analysis showed that there is no statistically significant difference in the values of composite filament UTS compared to the pure PLA filament (p > .05). However, a significant drop in the Young’s modulus (E) can be seen for all the composite filaments. For composite filaments having 30 wt% TPU and 0.5 wt% PEG, the modulus is decreased by 38.6% compared to that of pure PLA (p < .05). On applying Tukey test, it was found that there is no significant difference between the modulus of PLA_TPU30_PEG0.5 and PLA_TPU30_PEG1. With the addition of 2 and 3 wt % of PEG, the modulus has been increased by 15% and 28.5%, respectively compared to that of PLA_TPU30_PEG0.5, although all the modulus values are still lower than that of pure PLA. The modulus of PLA_TPU30_PEG4 is not significantly different than that of PLA_TPU30_PEG3. The modulus of PLA_TPU30_PEG5 is decreased by 7.8% compared to PLA_TPU30_PEG4. The modulus of PLA_TPU40_PEG0.5 decreased by 39.5% compared to that of pure PLA. The statistical analysis showed that there is no significant difference between the modulus of PLA_TPU40_PEG0.5 and PLA_TPU40_PEG1. The modulus of PLA_TPU40_PEG3 and PLA_TPU40_PEG4 decreased by 28.5% and 34.5% respectively, compared to the modulus of pure PLA filament. On the other hand, the % elongation at break is increasing for all the composite filaments. The elongation at break of PLA filament is 4.64% whereas it is 386.75% for PLA_TPU30_PEG0.5 and 486.32% for PLA_TPU40_PEG0.5 filaments. There is no statistically significant difference between the % elongation at break for PLA_TPU30_PEG4 and PLA_TPU30_PEG5 samples (p > .05). The elongation at break of PLA_TPU40_PEG4 filament is 512%. This analysis showed that the addition of 4 wt% PEG in both compositions (30 wt% TPU and 40 wt% TPU) exhibited the optimal mechanical properties.

Figure 4.

Engineering stress – strain curves of pure PLA filaments and composite filaments. (a) Stress-strain curves of PLA and composite filaments up to 10% of their elongation, and (b) Stress-strain curves of PLA and composite filaments up to their elongation at break.

Table 2.

Mechanical properties of the composite filaments manufactured with 30 wt% TPU and varying wt% PEG.

Samples	Yield stress ( $σ_{y}$ ) (MPa)	Ultimate tensile stress (UTS) (MPa)	Young’s modulus, E (MPa)	Elongation at break (%)
Pure PLA	47.52 ± 2.37	49.83 ± 1.26	2511.77 ± 115.50	4.64 ± 0.16
PLA_TPU30_PEG0.5	27.78 ± 1.45	45.20 ± 2.16	1541.71 ± 46.83	386.75 ± 8.55
PLA_TPU30_PEG1	27.74 ± 3.12	42.22 ± 2.80	1470.16 ± 129.68	350.61 ± 14.73
PLA_TPU30_PEG2	28.04 ± 2.15	47.16 ± 2.32	1773.04 ± 66.80	477.79 ± 13.00
PLA_TPU30_PEG3	29.10 ± 3.69	50.27 ± 3.47	1982.63 ± 73.71	477.74 ± 22.91
PLA_TPU30_PEG4	27.33 ± 1.25	55.36 ± 0.77	1849.22 ± 76.99	530.41 ± 6.40
PLA_TPU30_PEG5	26.10 ± 2.65	58.78 ± 2.73	1705.34 ± 41.89	562.07 ± 22.45

Table 3.

Mechanical properties of the composite filaments manufactured with 40 wt% TPU and varying wt% PEG.

Samples	Yield stress ( $σ_{y}$ ) (MPa)	Ultimate tensile stress (UTS) (MPa)	Young’s modulus, E (MPa)	Elongation at break (%)
Pure PLA	47.52 ± 2.37	49.83 ± 1.26	2511.77 ± 115.50	4.64 ± 0.16
PLA_TPU40_PEG0.5	23.00 ± 4.36	51.63 ± 3.50	1518.85 ± 73.20	486.32 ± 10.94
PLA_TPU40_PEG1	30.44 ± 5.33	54.94 ± 2.01	1548.20 ± 80.48	495.82 ± 6.26
PLA_TPU40_PEG3	30.70 ± 3.75	55.90 ± 2.32	1796.84 ± 60.60	460.85 ± 7.71
PLA_TPU40_PEG4	26.05 ± 2.60	55.34 ± 1.28	1643.45 ± 128.80	512.79 ± 6.02

DSC results

The study of thermal properties of materials becomes crucial for applications in thermal environments and for this purpose, DSC was used in this research. The DSC curves showing the heat/cool/heat cycles of neat PLA is shown in Figure 5 and the properties obtained from the cooling and second heating cycle are presented in Table 4. The important phases such as glass transition temperature ( $T_{g}$ ), cold crystallization temperature ( $T_{c c}$ ), cold crystallization enthalpy (∆ $H_{c c}$ ), melting temperature ( $T_{m}$ ), and enthalpy of fusion (∆ $H_{m}$ ) are evaluated using the heating cycle whereas the crystallization temperature ( $T_{c}$ ) and enthalpy of crystallization (∆ $H_{c}$ ) are obtained from the cooling cycle.

Figure 5.

Heat/cool/heat cycle of PLA.

Table 4.

Summary of DSC results of the composite filaments.

Samples	$T_{g}$ (°C), PLA	$T_{c c}$ (°C)	∆ $H_{c c}$ (J/g)	$T_{m}$ (°C)	∆ $H_{m}$ (J/g)	$T_{c}$ (°C)	∆ $H_{c}$ (J/g)
PLA	59.52	134.11	6.803	154.36	8.76	—	—
PLA_TPU30_PEG0.5	59.53	122.67	13.05	153.13	15.48	96.29	2.11
PLA_TPU30_PEG1	59.28	124.75	13.69	151.32	16.80	97.11	2.18
PLA_TPU30_PEG2	59.60	123.96	11.71	150.98	17.31	93.35	1.95
PLA_TPU30_PEG3	58.25	125.74	12.23	152.34	17.48	93.21	1.55
PLA_TPU30_PEG4	58.69	124.71	12.29	150.50	17.28	88.80	1.54
PLA_TPU30_PEG5	59.47	124.00	12.24	151.48	13.78	86.32	1.25
PLA_TPU40_PEG0.5	59.51	122.15	11.12	150.71	14.35	97.78	2.37
PLA_TPU40_PEG1	59.83	124.76	11.03	150.99	13.17	97.05	2.97
PLA_TPU40_PEG2	58.84	119.22	17.56	150.55	17.93	96.81	1.77
PLA_TPU40_PEG3	59.22	120.52	12.28	150.50	15.54	92.99	1.98
PLA_TPU40_PEG4	58.52	120.32	13.12	150.81	14.01	91.27	1.98

Note: 2. $T_{g}$ : glass transition temperature; $T_{c c}$ : cold crystallization temperature; ∆ $H_{c c}$ : cold crystallization enthalpy; $T_{m}$ : melting temperature; ∆ $H_{m}$ : enthalpy of fusion; $T_{c}$ : crystallization temperature; ∆ $H_{c}$ enthalpy of crystallization.

The $T_{g}$ of neat PLA is 59°C, which remains almost the same for all the samples. This result implies that adding 30 wt% or 40 wt% TPU and varying wt% of PEG is not showing significant change in the $T_{g}$ of the whole composition. This might have happened because the $T_{g}$ of neat TPU is around −30°C, and that of PEG is around −15°C.⁴⁸ Therefore, TPU and PEG are already in rubbery state and hence do not either assist or hinder the molecular chain mobility of PLA.

The cold crystallization peaks are visible for all the samples, and these are exothermic in nature. The $T_{c c}$ and associated enthalpy for neat PLA are 134°C and 6.8 J/g, respectively. With the addition of TPU and PEG, the $T_{c c}$ has been decreased for all the samples whereas ∆ $H_{c c}$ found to be increased. For example, PLA_TPU30_PEG5 has a $T_{c c}$ of 124°C and ∆ $H_{c c}$ of 12.24 J/g. Similarly, PLA_TPU40_PEG4 has a $T_{c c}$ of 120°C and ∆ $H_{c c}$ of 13.12 J/g. Cold crystallization peaks are visible when the polymer chains do not get enough time to fully get ordered. It appears after the glass transition temperature when the chains get a chance to move and get ordered. Crystallization peak is visible during the cooling cycle which happens due to the polymer chain mobility and ordering in the material.^49,50 The enthalpy of crystallization shows the energy used during crystallization by the polymer. This can be interpreted as a direct proportional relation with the number of crystalline structures. The higher the enthalpy, the higher the number of crystalline structures. From Table 4, it is evident that ${∆ H}_{c}$ is quite low for all the composite filaments. It clearly indicates that the chains of the polymers in the composites are not getting enough time to get fully ordered and therefore experience low crystallization. That’s why the exothermic cold crystallization peaks were visible. To solve this issue, lower cooling and heating rates may be applied.

The endothermic melting peaks appear after the exothermic cold crystallization peaks. The melting temperature of neat PLA is 154°C and the enthalpy of fusion is 8.76 J/g. The melting temperatures found to be decreased slightly whereas the enthalpy of fusion (∆ $H_{m}$ ) increased for all the composites. For example, PLA_TPU30_PEG5 has $T_{m}$ of 151.48°C and ∆ $H_{m}$ of 13.78 J/g. Likewise, PLA_TPU40_PEG4 has $T_{m}$ of 150.81°C and ∆ $H_{m}$ of 14.01 J/g. The increase in enthalpy might have happened due to the alignment of chains during cold crystallization phase. An ordered polymeric chain will require higher energy to break down and melt than the unordered chains.

DMA results

The thermo-mechanical behavior of PLA and the composite filaments were studied using DMA test method. The storage modulus ( $E^{'})$ , loss modulus ( $E^{″})$ , and tanδ curves as a function of temperature are shown in Figure 6. Storage modulus is related to the Young’s modulus and indicates the stiffness of a material. It represents the tendency of a material to store energy applied on it.⁵¹ The $E^{'}$ of PLA was higher than all other polymer compositions. It implies that PLA has a higher stiffness. With the addition of TPU and PEG in varying weight percentages, it was found that the storage modulus decreased for all the polymeric composite filaments. The decrease in rigidity and stiffness of composite filaments might have happened due to the presence of SS in TPU which caused increased flexibility of molecular chains. In addition, the plasticizing effect of PEG in the blend might have also contributed to decrease in modulus.

Figure 6.

Thermo-mechanical analysis of composite filaments. (a) $E^{'}$ versus temperature, (b) $E^{″}$ versus temperature, and (c) tanδ versus temperature.

Loss modulus represents the viscous behavior of a material and its tendency to dissipate energy applied to it.⁵¹ The initial loss modulus is lower for PLA filaments than other composites. This result represented that there was a change in polymer motion⁵² and an increase in TPU and PEG content increased the molecular friction which might be the reason for higher loss modulus of the composite filaments. It should be noted that $E^{″}$ depends on different molecular motions, transition, morphology and structural heterogeneities.⁵¹

Tanδ denotes the damping factor of the materials and is defined as the ratio of loss modulus to storage modulus (tanδ = $\frac{E^{″}}{E^{'}}$ ). Generally, the maximum value of the curve occurs at the glass transition temperature of the materials. It can be observed that PLA has only one peak near 60°C which represents its $T_{g}$ . The composites have two peaks indicating the presence of two glass transition temperatures. The lower temperature close to −40°C indicates the $T_{g}$ of TPU and the higher temperature at 60°C indicates the $T_{g}$ of PLA. The $T_{g}$ of PEG is not observed clearly. The appearance of different peaks is an indication of phase separation between the polymers.⁵³ It can be said that all the materials showed a broad tanδ peak due to several relaxation mechanisms occurring in the system. This fact showed that these materials contain damping properties and the composition can also be used in shock absorbers and isolators.⁵⁴

TGA results

TGA is a useful method to understand the thermal degradation behavior of the substances.⁵⁵ The loss of mass and rate of loss at different temperatures help to determine the application of substances in thermal environments. The thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of pure PLA filaments and composite filaments are shown in Figure 7. Effects of fillers and plasticizers in PLA matrix on the thermal degradation was studied. Neat PLA is observed to be more stable than all the composite materials. The onset thermal degradation temperature ( $T_{o n s e t}$ ) of neat PLA is 312°C. Similarly, the maximum thermal degradation temperature ( $T_{\max}$ ) is 363°C at which about 96% of the weight is lost. The final thermal degradation temperature (FDT) is 380°C and the char residues are only 3.13% of the initial weight (Table 5). With the addition of TPU and PEG, the onset temperature and maximum temperature decreased compared to pure PLA. This shows their rapid degradation behavior and lower thermal stability. The char residues of all the composites at 700°C are almost negligible. This happened due to the lower decomposition temperature of TPU.⁴¹ However, the final degradation temperatures of all the composites are higher than pure PLA. The DTG curves show multiple peaks which demonstrates that the TPU has different functional groups and structural elements having different decomposition temperatures. All the composites have similar thermal stability and degradation behavior. The composites followed similar residual weight plateau after 400°C. Hence, it can be said that these composites can be used in applications where the temperature does not rise over 285°C. In case of pure TPU filaments, it is suitable for applications below 270°C around which its onset thermal degradation temperature lies.

Figure 7.

TGA thermograms of PLA filament, and the composite filaments manufactured. (a) TG curves showing the percentage weight loss of the composition at different temperatures, and (b) DTG curves showing the rate of weight loss.

Table 5.

Characteristic temperatures of the composite filaments obtained from TGA.

Samples	$T_{o n s e t}$ (°C)	$T_{\max}$ (°C)	FDT (°C)	Char % at 700°C
PLA	312	363	380	3.13
PLA_TPU30_PEG0.5	290	323	610	0
PLA_TPU30_PEG1	290	327	525	0
PLA_TPU30_PEG2	290	327	580	0
PLA_TPU30_PEG3	293	328	390	0
PLA_TPU30_PEG4	290	325	560	0
PLA_TPU30_PEG5	285	325	400	0
PLA_TPU40_PEG0.5	290	327	538	7.91
PLA_TPU40_PEG1	293	323	580	0
PLA_TPU40_PEG2	300	330	600	0
PLA_TPU40_PEG3	295	323	620	0
PLA_TPU40_PEG4	300	326	600	1.45
TPU	273	280, 335, 420	450	4.42

Note: 3. $T_{o n s e t}$ is the onset thermal degradation temperature, $T_{\max}$ is the maximum thermal degradation temperature, and FDT is the final thermal degradation temperature.

FTIR results

To determine the functional groups of the polymers in the composites, FTIR spectroscopy was performed, and the results are shown in Figure 8. The spectra of the composite filaments having 30 wt% TPU, and varying wt% of PLA and PEG is shown in Figure 8(a) whereas the spectra of the composite filaments having 40 wt% TPU and varying wt% of PLA and PEG is shown in Figure 8(b). The functional groups associated with their wavenumber and intensity of PLA and TPU are listed in Table 6.

Figure 8.

FTIR spectra of PLA, TPU and composite filaments. (a) The composite filaments have 30 wt% TPU and varying wt% of PLA and PEG, (b) The composite filaments have 40 wt% TPU and varying wt% of PLA and PEG.

Table 6.

FTIR spectra of PLA, TPU and PEG polymers used in this study.^55,56

Material	Wavenumber (cm⁻¹)	Assignment
TPU	3298	Stretching vibration of NH in urethane group.
	2848 and 2921	Presence of Poly(tetramethylene glycol) (PTMG), known as soft segment (SS).
	1700 and 1733	C = O stretching vibration (amide I band).
	1637	C = C stretching vibration in the aromatic ring.
	1530 and 1560	N–H bending vibration (amide II band).
	1311	C–N (amide III band) stretching vibration.
	1218	C−O−C stretching vibration in esters.
	1106	C−O−C stretching vibration in ethers.
	815	C–H out-of-plane bending in aromatic ring.
PLA	2852 and 2925	Asymmetric and symmetric stretching vibration of ${C H}_{3}$ .
	1750	C = O stretching vibration.
	1452	${C H}_{3}$ antisymmetric bending vibration.
	1359 and 1382	Deformation and symmetric mode of CH group.
	1043, 1080, 1128, and 1180	C−O−C stretching vibration.

For pure TPU, a medium peak at 3298 cm⁻¹ corresponds to the stretching vibration of NH in urethane group. The spectrum also exhibits characteristic bands at 2848 cm⁻¹ and 2921 cm⁻¹, attributed to the presence of Poly(tetramethylene glycol) (PTMG).⁵⁶ PTMG forms the SS of the material. Soft segments provide the flexibility and elastomeric behavior. They may be either polyester or polyether and interconnect two HS of the material. They are bonded together with the presence of hydrogen bonds and form physical crosslinks.^34,35,57 The bands at 1700 cm⁻¹ and 1733 cm⁻¹ are due to the stretching vibration (amide I band) of C = O carbonyl compound. The peak at 1637 cm⁻¹ resembles the presence of stretching vibration of C = C bonds in the aromatic ring, whereas the peak at 815 cm⁻¹ shows the presence of C–H out-of-plane bending in aromatic ring. The peaks at 1530 cm⁻¹ and 1560 cm⁻¹ show the N–H bending vibration (amide II band). The presence of peak at 1311 cm⁻¹ was due to C–N (amide III band) stretching vibration. The bands at 1218 cm⁻¹ and 1106 cm⁻¹ were due to C−O−C stretching vibration in esters and ethers, respectively.

For neat PLA, two peaks of medium intensity at 2852 cm⁻¹ and 2925 cm⁻¹ can be seen which is due to the presence of asymmetric and symmetric stretching vibration of ${C H}_{3}$ whereas the peak at 1452 cm⁻¹ was due to ${C H}_{3}$ antisymmetric bending vibration. A signal of strong intensity at 1750 cm⁻¹ is attributed to the C = O stretching vibration. The peaks at 1359 cm⁻¹ and 1382 cm⁻¹ shows the deformation and symmetric mode of CH group. The peaks at 1043, 1080, 1128, and 1180 cm⁻¹ are due to C−O−C stretching vibration.

In the composite filaments, the intensity of peaks in the range of 2850–2950 cm⁻¹ has decreased. This might be because of the lower weight percentages of TPU in the composition. This is the range for PTMG which provides the flexibility and elastomeric behavior to the materials. Due to lower weight percentages, the flexibility will be lower than that of pure TPU. There are not any notable changes observed at around 3300 cm⁻¹. Almost similar intensity of peaks is seen at around 1750 cm⁻¹, which might be due to the presence of C = O stretching vibration in both TPU and PLA. Since the amount of PEG added is very low compared to the other two polymers, its effect has not been noted significantly on the spectra. From these spectra, it can be said that the composite samples contain all polymer properties.

Scanning electron microscopy

The SEM images of the cross-section of cryogenically fractured PLA and composite filaments are shown in Figure 9. The cross-section of PLA filaments appears flat with little roughness and showed brittle failure (Figure 9(a)). The addition of 30 wt% TPU and 2 wt% PEG caused an increase in spherical aggregates and voids in the cross-section (Figure 9(b)). Mi et al.,³³ also observed similar surface morphology in the PLA/TPU blend. The aggregates are shown by arrows and voids by dotted circles. The aggregates are uniformly distributed on the cross-section, which have been appeared due to the coalescence of TPU particles. The increase in TPU particles increases the flexibility of the filaments, which is seen in tensile tests. However, the formation of TPU aggregates is the indication of phase separation between PLA matrix and TPU additives, which is also seen in tanδ curves during DMA tests. The voids are present on the fractured surface, which might have occurred due to poor adhesion between the PLA matrix, TPU additive and PEG plasticizer. This further provides evidence of phase separation and immiscibility of these polymers. On further increasing the PEG content to 5 wt%, the TPU aggregates and void formation was reduced (Figure 9(c)). This might have happened due to the bonding between the polymers. As a result, the elongation has increased during tensile tests. With the addition of 40 wt% TPU, the flexibility of the composite filaments increased. This is due to the hydrogen bond formation between PLA and TPU.⁴³ The uniform dispersion of TPU aggregates and voids are seen in Figure 9(d), which are reduced with the addition of 4 wt % PEG.

Figure 9.

SEM images showing cross-sections of (a) PLA, (b) PLA_PU30_PEG2, (c) PLA_TPU30_PEG5, (d) PLA_PU40_PEG1, (e) PLA_TPU40_PEG4. The aggregates are shown by arrows and voids by dotted circles.

Figure 10 a shows a smooth surface with little roughness of PLA filament. The roughness might have occurred by stretching during the extrusion process. On the other hand, the composite filaments (Figure 10(b)–(e)) exhibit rough surfaces and fine chiplets can be seen clearly. This happened due to the presence of PEG and TPU in the PLA matrix, which are immiscible. This also proves that they have poor compatibility and possess phase separation.

Figure 10.

SEM images showing the surfaces of (a) PLA, (b) PLA_TPU30_PEG2, (c) PLA_TPU30_PEG5, (d) PLA_TPU40_PEG1, and (e) PLA_TPU40_PEG4.

3D printed fabric structure

A plain weave fabric structure (Figure 11) having eight warp yarns and five weft yarns was 3D printed in horizontal orientation using the PLA_TPU_30_PEG5 composite filament. The spacing between the yarns is 10 mm. The width of the 3D printed fabric is 70 mm along weft direction whereas the length is 40 mm along the warp direction. Several fibrils can be seen in the printed part. A weft yarn as indicated by an ellipse is broken while 3D printing. The inconsistency in warp yarns is clearly visible indicated by the arrows. The reason for this is the non-uniform extrusion of filament during the printing process, which happened due to the inconsistent diameter of filament that was produced using the extruder. This defect in the filament affected the print quality, dimensional accuracy, and the overall properties of the printed parts. The work related to improving the print quality and fabric properties will be considered in our future research.

Figure 11.

3D printed plain weave fabric structure using PLA_TPU30_PEG5 composite filament.

Conclusions

Different polymers such as PLA, TPU and PEG are blended in different proportions and extruded to obtain composite filaments. The amount of TPU content in the composition is kept constant either being 30% or 40%. PLA and PEG are added in varying weight percentages. The filaments produced had varying diameters, but the optimal range was considered for this study. The following conclusions can be drawn from this work:

1. The tensile test results showed that the yield stress of the composite filaments decreased significantly but the ultimate tensile stress was in the same range compared to pure PLA filament. Moreover, the Young’s modulus of composite filaments also decreased compared to pure PLA filament whereas the elongation at break increased by more than 550%. This reveals the fact that the flexibility and ductility of composites increased which might be due to the presence of soft segments in TPU.

2. The thermal analysis showed that the glass transition temperature of PLA filament is almost the same in all the composites, which was observed using DSC. TGA results revealed that the onset thermal degradation temperature of composite filaments was around 285°C which is slightly lower than that of pure PLA filament. It can be said that these composites can be used in applications where the temperature does not rise over 285°C.

3. The thermo-mechanical tests demonstrated that pure PLA filaments hold higher storage modulus than the composite filaments. This implies that pure PLA filament is stiffer than the composite filaments. The increase in TPU and PEG content make the filaments flexible and results in lower storage modulus. On the other hand, the initial loss modulus of the pure PLA filament is the lowest. The increase in TPU and PEG content enhanced the molecular friction which might have caused higher loss modulus of the composite filaments.

4. The FTIR spectroscopy revealed that there is not any additional chemical bonding between the polymers which can affect their properties.

5. The 3D printed plain weave fabric structure using the FDM method with PLA_TPU30_PEG5 composite filament demonstrated the feasibility of these filaments in the 3D printing of complex structures.

Footnotes

Acknowledgements

The authors would like to thank Dr Ramsis Farag and John Osho for their help during this project. SEM imaging were done at the AU Research Instrumentation Facility in Biological Sciences Department at Auburn University.

Declaration of conflicting interests

The authors declare no potential conflict of interest with respect to the research, authorship and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was partially supported by the Department of Mechanical Engineering, Auburn University and partially by the Interdisciplinary Center for Advanced Manufacturing Systems (ICAMS) with funding from the Industrial Base Analysis & Sustainment Program of the Industrial Base Policy Office of the Office of the Secretary of Defense, awarded by US Army Contract W52PLJ-20–9-3045.

ORCID iDs

Ajay Jayswal

Jia Liu

Gregory Harris

Sabit Adanur

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