In Situ Filler Addition for Homogeneous Dispersion of Carbon Nanotubes in Multi Jet Fusion–Printed Elastomer Composites

Abstract The dispersibility of fillers determines their effect on the mechanical properties and anisotropy of the 3D‐printed polymeric composites. Nanoscale fillers have the tendency to aggregate, resulting in the deterioration of part performance. An in situ filler addition method using the newly developed dual‐functional toughness agents (TAs) is proposed in this work for the homogeneous dispersion of carbon nanotubes (CNTs) in elastomer composites printed via multi jet fusion. The CNTs added in the TAs serve as an infrared absorbing colorant for selective powder fusion, as well as the strengthening and toughening fillers. The printability of the TA is theoretically deduced based on the measured physical properties, which are subsequently verified experimentally. The printing parameters and agent formulation are optimized to maximize the mechanical performance of the printed parts. The printed elastomer parts show significant improvement in strength and toughness for all printing orientations and alleviation of the mechanical anisotropy originating from the layer‐wise fabrication manner. This in situ filler addition method using tailorable TAs is applicable for fabricating parts with site‐specific mechanical properties and is promising in assisting the scalable manufacturing of 3D‐printed elastomers.


Fusion-printed elastomer composites
Jiayao Chen, a,b, † Ran An, a,b, † Wei Shian Tey, a,b Qingyun Zeng, a Lihua Zhao, a,c and Kun Zhou a,b,* The carbon nanotube (CNT) loading fractions of the premixed CNT/thermoplastic polyurethane (TPU) powder and the CNT/TPU part fabricated using one-time dispensing of the toughness agent (TA) containing 3 wt% CNTs and 13 wt% 2-pyrrolidinone were investigated by thermogravimetric analysis (TGA).As shown in Figure S1, the weight loss percentage was almost identical between the two samples, indicating same loading fraction of CNTs between the premixed and in-situ addition methods.Figure S3 shows the optical microscope images of the fused CNT/TPU layers obtained from the two CNT addition methods.The thin-layer samples were fabricated as follows.First, the neat TPU powder and premixed CNT/TPU powder were respectively loaded onto the glass substrates individually, with heat-resistant double layer tapes to fix a thin layer of powder.An air gun was used to remove redundant powder on the substrates.The TA was programed to dispense on the substrate loaded with a thin layer of the neat TPU powder (as the same volume of that dispensed during the printing process).Finally, the samples were then heat in an oven at 150 °C to fuse.Compared with the sample with TA dispensing, a few dark aeras with various size can be seen in the premixed CNT/TPU layer, confirming the higher tendency of CNT aggregation.Table S2.Various effects on the tensile properties of the CNT/TPU parts using TAs and the TPU parts using FA.Table S3.Average values of the mechanical properties in the x, y, z directions for the CNT/TPU and TPU parts.

Specimen UTS (MPa) Young's modulus (MPa) Elongation at break (%)
Toughness (MJ/m 3 ) CNT/TPU 11.5 75.4 502.3 43.9 TPU 7.9 78.0 251.0 17.2 To investigate the variation of hydrogen-bonding ratio after adding CNTs, Fourier-transform infrared spectroscopy (FTIR) was analyzed.Figure S7 shows the FTIR spectra of the CNT/TPU and TPU parts.The positions of the characteristic bands of the functional groups remained the same between the CNT/TPU and TPU parts, while the intensity ratios of some groups were different.The hydrogen bonds can be formed between N-H (3500-3200 cm -1 ) and C=O groups (1800-1600 cm -1 ), of which band intensities were increased in the CNT/TPU part as compared with those of the TPU part.The hydrogen-bonding index (HBI) that reflects the hydrogen-bonding ratio can be calculated by where Abonded, Afree, and Atotal are the integrated absorbance peaks of the hydrogen bonded groups, free groups, and total groups, respectively.Figure S8 shows the peak fitting results of the N-H and C=O groups of the TPU and CNT/TPU parts, and the HBI values of the groups are listed in Table S4.The CNT/TPU part possessed higher HBI values of the N-H (0.760) and C=O groups (0.548) than the TPU part, indicating the enhanced hydrogen-bonding interaction between the carboxylated CNTs and TPU.Based on the estimated consumable cost in Table S6, the benefit of manufacturing toughened CNT/TPU part manufacturing can be further calculated.The cost of the CNT/TPU parts fabricated by the proposed process, cCNT/TPU, is evaluated to 0.26 USD per cm 3 model.We take the toughness enhancement as an example to calculate the benefit of the CNT/TPU parts comparing with that of the normal TPU parts provided by HP. [6] The average increment of the toughness of the CNT/TPU parts is 155%, compared with those of the neat TPU parts.

Figure S1 .
Figure S1.TGA curves of the premixed CNT/TPU powder and the CNT/TPU part fabricated using TA dispensing.The latter was calibrated by removing a proportion of solvent, as the solvent with high boiling point in the TA remains in the part.

Figure S2 .
Figure S2.Optical microscope images of the TPU powder with TA dispensing (A) into two lines and (B) on the designated region.

Figure S5 .
Figure S5.Density of the CNT/TPU parts fabricated using TAs with CNT content from 1 to 5 wt% and the neat TPU parts using FA.

Figure S6 .
Figure S6.Rheological properties of the TPU part and CNT/TPU part at melting temperature.(A) Variations of the shear rate against viscosity.Variations of the frequency against (B) elastic modulus and viscous modulus (G′ and G′′, respectively) and (C) Tanδ.

Figure S7 .
Figure S7.FTIR spectra of the CNT/TPU and TPU parts.Two characteristic bands were zoomed-in, corresponding to N-H and C=O groups.

Figure S8 .
Figure S8.Peak fitting results of the N-H and C=O groups of the CNT/TPU and TPU parts.

Table S4 .
HBI values of the N-H and C=O groups of the CNT/TPU and TPU parts.

Table S5 .
Mechanical properties of the recently reported CNT-filled TPU composites fabricated via FDM and casting.

Table S6 .
To fulfill the same requirement of toughness, the consumption of the CNT/TPU part, xCNT/TPU, is 1/(1+155%) times of the neat TPU parts (xTPU=1).Hence, the benefit of CNT/TPU part manufacturing can be estimated as Consumable cost of the CNT/TPU parts applying 3 wt% TA and the neat TPU applying estimated by using HP MJF 4200 commercial printer.