Additive Manufactured Carbon Nanotube/Epoxy Nanocomposites for Heavy-Duty Applications.

A solid epoxy resin formulation containing 2.5 wt % carbon nanotubes is 3D printed into self-standing parts, which after thermal curing result in CNTs/epoxy nanocomposites with mechanical properties attractive for heavy-duty applications.

Additive manufacturing, also known as 3D printing, is a process where materials are joined layer-by-layer, guided by 3D model data, to produce complex objects.[1] Additive manufacturing is advantageous over conventional casting and subtractive methods because of low material wastage and high flexibility allowing for prototyping of complex shaped parts. Since its first commercialization in 1990,[2] fused deposition modeling (FDM) has become the most used 3D printing process due to its simple machine design and low cost.[3] FDM showed great industrial potential to produce parts for practical applications in medical devices,[4] heavy-duty parts,[5] and vehicles.[6] A wide range of thermoplastic polymers can be used in FDM, ranging from polylactic acid (PLA), to poly(acrylonitrile-co-butadiene-co-styrene) (ABS), to high performance polymers, such as polyether ether ketone (PEEK).[7] Furthermore, polymers containing metals,[8] ceramics,[9] and carbon nanotubes (CNTs)[10] as fillers and/or reinforcements have been used in FDM to produce composite parts with enhanced stiffness. However, FDM parts exhibit anisotropic mechanical properties, characterized by different properties of the printed parts in a direction perpendicular and parallel to the layer deposition.[11] This is caused by the presence of interlayers, which possess mechanical properties different from those of each layer of material.[12] The most used solutions to reduce anisotropy in FDM parts included post-treatments, such as post-layer-fusion,[13] radiation induced cross-linking of layers,[14] and polymer infiltration to lock the layers.[15] On the other hand, since the majority of raw materials for FDM are thermoplastic polymers, the chemical and thermal resistance of the resulting parts is poor (with exception of high performance polymers). These properties limit the applicability of FDM parts for heavy-duty applications, which require stiffness and inertness to be useful in harsh conditions.

We propose to address the above challenges by printing thermosetting resins, which can be cured to produce duromer (i.e., cross-linked) parts. A covalently cross-linked polymer network is anticipated to form throughout the parts (existing in both inner- and interlayers), thus reducing the materials’ anisotropy. Furthermore, the cross-linked polymer network provides enhanced solvent and thermal resistance. Compton et al.[16] developed short fiber-in-epoxy inks; 3D printing and subsequent curing of the inks resulted in bioinspired wood mimics. Pierson et al.[17] printed (by direct writing) epoxy with chopped carbon fibers. The resulting cured composites had a low void content as well as highly orientated fibers, resulting in a tensile modulus close to the theoretical values. Indeed, the liquid nature of epoxy resin inks promoted fusion between liquid layers as they merged together. However, liquid inks must have a sufficiently high zero-shear viscosity[18] (solid-like behavior) to allow printing complex structures, as otherwise, the ink would flow apart. Nevertheless, using viscous liquid epoxy or filler-in-epoxy inks still restricts the height of the FDM parts, as the increasing part weight will cause the non-cross-linked inks to flow. Using solid epoxy resin to address the problem of insufficient zero-shear viscosity of as-printed parts has rarely been reported. Ming et al.[19] utilized a solid epoxy resin, which melted at an elevated temperature to impregnate continuous carbon fibers. After being printed, the epoxy-resin-impregnated carbon fibers solidified and supported parts for subsequent curing.[19] Nevertheless, the authors printed and cured only carbon fiber reinforced epoxy parts to a height of 10 mm.

We aim to produce rigid, solvent resistant thermoset CNT/epoxy nanocomposite parts for applications at harsh conditions. To allow printing of large objects, a solid epoxy resin formulation was used. Solid epoxy resin (with an uncured glass transition temperature of 35 °C), latent hardener, accelerator, and 2.5 wt % CNTs were blended at 62 °C in a Brabender mixer. After mixing and cooling to room temperature, the formulation solidified and was ground into powder. In a preliminary experiment, the resin powder was transferred into a syringe fitted with a needle of an inner diameter of 1.6 mm. The syringe was attached to a 3D printing platform (Printrbot Simple Bro base model) (Figure a). The syringe and needle were heated to 77 °C, at which point the resin containing 2.5 wt % CNTs melted with a complex viscosity of 3000 Pa s measured at a shear frequency of 2 Hz (as compared to 2300 Pa s for the resin formulation without CNTs). A motor drove the plunger of the syringe to press the molten resin through the nozzle to print cubes (15 mm × 15 mm × 15 mm) and rectangles (80 mm × 10 mm × 3.8 mm) (Supporting Information, Section S1). We found that the CNT loading can be up to 2.5 wt % with respect to the epoxy resin, while a higher CNT loading (5 wt %) led to a very viscous (14 500 Pa s) and uneven flow at 77 °C. Once extruded through the needle, the resin solidified rapidly in air, resulting after printing in uncured yet self-standing objects. To cure the printed objects, they were covered in wet clay, which was dried to form a stiff shell to support the printed parts (a method similar to that reported by Ming et al.[20]). The printed parts were then placed into an oven at 50, 60, and 70 °C, respectively, for 24 h. The curing protocol was designed to allow for curing, while avoiding the (wrapped) resin to flow at these temperatures (Supporting Information, Section S1). After curing at 70 °C, the epoxy resin was postcured at 120 and 150 °C for 1 h. The absence of a reaction exotherm during differential scanning calorimetry (DSC) (Supporting Information, Figure S4a) confirmed that the objects were fully cured. The glass transition temperature of the nanocomposites was 118 °C (Supporting Information, Figure S4b).

Figure 1

Design of the FDM printer using syringe and plunger (a) and single screw extruder (b). Design a was used to print cubic and rectangular CNT/epoxy composites for compression and flexural tests, while that in b was used to print dog-bone-shaped composites for tensile tests.

The bulk and skeletal densities of printed and cured objects were measured using pycnometry on sample pieces and powders, respectively. The printed parts had a bulk and skeletal density of 1.16 ± 0.01 and 1.22 ± 0.01 g/cm3, respectively, resulting in a void content of 6%. The mechanical properties of the printed and cured objects were measured in compression in the layer-stacking direction (Z direction) and perpendicular to it (XY direction). The elastic modulus and compression strength in the Z direction were 1.3 ± 0.2 GPa and 81 ± 3 MPa, and in the XY direction, 1.4 ± 0.1 GPa and 79 ± 6 MPa, respectively. An unpaired two-sample t test confirmed that the properties were identical within error. The rectangular specimens were subjected to flexural tests; they possessed a flexural modulus of 2.0 ± 0.1 GPa and a strength of 58 ± 4 MPa.

The syringe and plunger printer allowed only batch-by-batch printing, limiting the size of the objects to the volume of the raw material in the syringe. Furthermore, the back-pressure increased during extrusion, increasing the power required to drive the plunger. Once the power exceeded the maximum output of the motor, the process ceased, causing materials wastage of up to 50%. Therefore, the 3D printer was fitted with a single screw extruder (Figure b), which was used already for direct extruding/printing polymer pellets[21] and powder.[22] A nozzle with an inner diameter of 1 mm was connected to the barrel heated to 80 °C. The molten resin was extruded through the nozzle and was printed into dog bones with a thickness of 3 mm, according to standard EN ISO 527-2 1BA. In each layer of the specimen, a 1-perimeter shell was printed, followed by infilling. The rectangular infills had raster angles of 0°, 45°, and 90° with respect to the length of the objects. The layer thickness was 0.6 mm (Supporting Information, Section S1). Printed parts were covered with clay and cured as described above.

Printed and cured dog bone specimens consisting of five layers were tensile tested (Supporting Information, Section S1). The tensile loading direction was perpendicular to the printing direction. Specimens with infill raster angles of 0°, 45°, and 90° were tested (Supporting Information, Section S1). All specimens fractured perpendicular to the loading direction rather than followed the interface between printed layers. The spherical voids seen in the fracture surfaces (Figure ) randomly distributed throughout the nanocomposites and along the interlayer formed by entrapment of air during melting and compaction of the resin powder in the small single screw extruder (Figure b) and during printing, i.e., air entrapment between a solidified layer and the next layer being printed. The densities of the printed specimens with infill raster angles of 0°, 45° and 90°, which did include different amounts of interlayers of the infillings, were 1.13 ± 0.02, 1.12 ± 0.01, and 1.10 ± 0.02 g/cm3, resulting in a void content of 7%, 8%, and 10%, respectively. On the other hand, the CNT/epoxy composites printed using the FDM with a plunger had a void content of 6%. The larger nozzle diameter allowed for the printing of thicker layers, which resulted in composites with fewer interlayers, and thus voids, per part volume. Without vacuum assisted degassing prior to curing, which is typically used to produce void-free polymer composites using conventional composite manufacturing methods,[23] the voids in 3D printed CNT/epoxy nanocomposites were difficult to remove. Other than voids, no clear interlayer could be seen (Figure b), indicating fused layers in the printed CNT/epoxy nanocomposites. The stress–strain curves were characteristic for stiff but brittle polymers (Figure a). From linear regions, the Young’s moduli were determined; specimens with raster angles of 0°, 45°, and 90° had moduli of 2.0 ± 0.2, 1.9 ± 0.2, and 1.8 ± 0.1 GPa. The 0.2% offset yield of these specimens occurred at strains of 2.2 ± 0.4%, 2.4 ± 0.4%, and 2.0 ± 0.4%, resulting in yield strengths of 43 ± 7, 43 ± 4, and 38 ± 5 MPa, respectively. The specimens with raster angles of 0°, 45°, and 90° had failure strains of 4.1 ± 1.2%, 4.2 ± 0.3%, and 4.9 ± 1.0% with ultimate strengths of 58 ± 9, 56 ± 3, and 52 ± 6 MPa, respectively (Figure b). The CNT/epoxy nanocomposites had rough fracture surfaces (Figure ). Figure d and f show the fracture propagation (red arrows) indicating that the fracture initiated at void clusters in the nanocomposites (red circles). The fracture propagated through the specimens following a radiating pattern but was not governed by interlayer inhomogeneity as reported for 3D printed PLA[12] and ABS parts.[24] This indicated the formation of homogeneous CNT/epoxy nanocomposites. As such, the cross-linked epoxy network and the incorporation of CNTs rather than the hypothetical anisotropy dominated the mechanical behavior of the 3D printed nanocomposites. The mechanical properties of our 3D printed nanocomposites are comparable to literature values of other 3D printed epoxy composites (Table S1 in Supporting Information Section S3).

Figure 2

Tensile fracture surfaces of 3D printed CNT/epoxy nanocomposites with infills with raster angles of 0° (a, b), 45° (c, d), and 90° (e, f). Red circles show initial cracks formed at void clusters; red arrows indicate crack propagation fronts.

Figure 3

Characteristic tensile stress–strain curves of printed dog bone specimens with raster angles of 0°, 45°, and 90°. Young’s moduli (■) and ultimate strengths (○) of nanocomposites as a function of bulk density.

The chemical resistance of the nanocomposites was investigated by immersing them in acetone, isopropanol, dimethylformamide (DMF), tetrahydrofuran (THF), toluene, benzene, (Shell) diesel, dimethyl sulfoxide (DMSO), and 37% HCl (experimental details and results are summarized in Supporting Information Section S4). The nanocomposites swelled massively in DMF, THF, and DMSO, leading to sample disintegration within 24, 48, and 48 h, respectively, while acetone caused a volume increase by 30% in 10 days. The nanocomposites possessed excellent resistance to all other solvents, in which they did not experience any volume change in 10 days.

In previous work on FDM epoxy and epoxy composites using liquid formulations, the printing height was limited. This was caused by the weight of the stacked layers, which deformed previously printed but uncured layers once the load exerted by the part weight exceeded the zero-shear viscosity of the resin.[25] As a result, the height of printed parts was typically limited to 10 mm.[26,27] Only some articles reported high printed epoxy parts with dual cured epoxy resin[28] or fast gelling epoxy formulations.[29] Solid epoxy resins do allow for overcoming the height limitation; to demonstrate this, we printed and cured a nanocomposite duck (the icon of our group) with a height of 48 mm (seen in the abstract graphic). The heights of the model and the overhangs show the advantage of solid epoxy formulations for printing large, complex parts. The usefulness of our material for heavy-duty applications was demonstrated using a nanocomposite chain ring connected by two carabineers. The printed ring withstood the load exerted by the weight (90 kg) of a person (Supporting Information Video 1). We also printed a 2.5 cm tall wrench (spanner) and used it to tighten/loosen a 1/4 in. tube connector at 120 °C as well as in acetone, respectively (Supporting Information Video 2). These trials demonstrated the usefulness of CNT/epoxy nanocomposites at harsh conditions.

In summary, solid epoxy, hardener, accelerator, and CNTs were melt-blended and subsequently ground; this powder was used as ink for additive manufacturing of nanocomposites. The compression moduli and strengths of the printed nanocomposites were 1.4 GPa and 81 MPa, and tensile moduli and strengths, 2.0 GPa and 55 MPa, respectively. Even when printed with different raster angles, the nanocomposites had mechanical properties that were identical within error. Fracture surfaces showed that the crack propagation was not governed by the interlayers. This indicated the isotropic mechanical properties of printed nanocomposites in contrast to those of typical FDM polymer materials. These nanocomposites had a glass transition temperature of 118 °C and exhibited good chemical and thermal resistance, indicating their potential to be used in heavy-duty applications.