Figure 2 . Mechanical and material characterization of the engineered two-stage resin DIW ink. (a) tanδ and E’ quantified over a range of temperatures via DMA. (b) Features of tensile properties for solely photo cure resin (P), several ratios of photopolymer to epoxy constituents (PE30, PE50, PE70), and solely epoxy resin (E). (c) DoC quantified via FTIR versus time superimposed over Young’s modulus of PE70 resin at 1, 15, 30, and 60 seconds of UV exposure. (d) DoC quantified via FTIR versus time superimposed over Young’s modulus of PE70 resin at 0 and 72 hours after photo cure.
Furthermore, we performed FTIR experiments to understand the curing kinetics of both the photo cure and epoxy resin networks, DoC with respect to time are presented in Figure 2c, d for photo cure and photo epoxy resin networks, respectively. DoC for both resin constituents was determined through analysis of the absorbance intensity of selected wavenumbers corresponding to bonds which drop in frequency as the respective polymer networks develop (Figure S1a, b). Because our inverted and horizontally printed structures are significantly impacted by gravitational forces, a rapidly solidifying material is essential for parts to maintain their structure. As previously discussed, the high presence of bonding sites of the triacrylate TMPTA monomer allows for rapid polymerization, and therefore the development of a suitably stable/solidified network for the purpose of self-support. When exposed to UV irradiation with light intensity of 66 mW/cm2 for one second, the photopolymer network constituent began rapidly developing (42.5% DoC) – and therefore solidifying – with a Young’s modulus of 0.915 ± 0.224 MPa, thereby laying the groundwork for layers to retain their deposited shape (Figure 2c). After five seconds of exposure, the network reaches 82% DoC, and is nearly fully developed after 15 seconds of UV irradiation. As a result, the solidified layers have a Young’s modulus of 4.90 ± 0.24 MPa, a 430% increase within 14 seconds. Based on these results, we elected to expose printed layers to 15 seconds of UV radiation to assure the printed structures would remain stable during fabrication. It should be noted Young’s modulus continues to increase to 7.10 ± 0.80 MPa and 9.86 ± 1.76 MPa for 30 and 60 second exposure, respectively due to the cumulative UV exposure from curing subsequent layers in fact strengthens prior layers, thus enabling creation of larger structures. With the modulus of ~5MPa-10MPa, the previously printed layer can sustain a large structure without exhibit visible deformation. For the epoxy resin, FTIR analysis (Figure S1 in Supplementary Information) revealed that the DoC of the epoxy resin constituent rose to 50% after 24 hours, increasing further to 73.2% after 48 hours, and finally achieving 86.3% DoC after 72 hours in ambient, room-temperature conditions; thereby indicating autonomous network formation behavior under room-temperature conditions (Figure 2d). This curing mechanism proved highly effective in strengthening the material, after 72 hours, PE70 resin underwent a three order of magnitude increase in Young’s modulus (9.86 MPa to 1.9 GPa) and a two order of magnitude increase in tensile strength (0.79 to 81 MPa) (Figure 2c, d, Figure S2a, b).
To confirm the suitability of the two-stage resin ink for DIW 3D printing, we examined viscosity, G’, G”, and tanδ. Viscoelastic behavior of the hybrid ink was determined via oscillatory stress sweep as shown in Figure S3a. The ink exhibited a stable plateau of storage modulus G’ over loss modulus G”, with a value of 2800 Pa and possesses a critical stress (τc) of 186 Pa, indicating a high stiffness of the formulated two-stage DIW ink. This is crucial for inverted and horizontal DIW AM to maintain the printed shape versus gravitational sagging post-extrusion. Additionally, we observed that the two-stage resin combined with 7 wt% fumed silica exhibited desirable shear-thinning behavior with respect to viscosity (Figure S3b). Finally, we conducted an analysis of viscosity versus time in order to determine an approximate a working life wherein the two-stage ink was reasonably printable (Figure S3c). Based on the dramatic increase in viscosity at t = 10,000 seconds, corroborated by qualitative experimental experience, we determined the working life of the two-stage ink to be approximately three hours. This working time allowed for fabrication of multiple structures, for example, pillar samples used in the following section took approximately 25 minutes to print.
3.2    Adhesion
To demonstrate the robustness of the two-stage resin bond, and therefore end-use application viability we printed 5 x 10 x 20 mm pillars on several materials (Figure 3a). The substrates selected were acrylic, wood, glass, aluminum, and concrete due to their ubiquity in construction and consumer applications. Adhesion strength is quantified by the peak stress (MPa) required to separate the as-printed (with 15 second UV cure) and the fully cured (72 hours) printed pillars from the substrates (the test apparatus is shown in Figure 3b). The results of the adhesion experiments are presented in Figure 3c. Printed pillars tested directly after printing exhibited similar adhesion strengths ranging from 0.17 to 0.33 MPa with acrylic substrates exhibiting the lowest adhesion strengths and glass the highest. This range suggests that the photopolymer network can rapidly create a substrate-agnostic bond, indicating that structures of comparable size can be printed in a variety of conditions. Importantly, considering the density of the two-stage resin is 1.2 g/cm3, it is possible to create self-supporting structures up to 28.15 A cm3 in volume where A is the area of the base of the printed structure (Equation S1). For example, for the pillar with 5 x 10 mm base, the length can be up to 2 m on a wooden substrate. After 72 hours, the formation of the epoxy resin matrix doubled and even quintupled in the case of acrylic the adhesion strengths of printed pillars. Indeed, 72-hour cure samples, acrylic and wood substrates exhibited the highest adhesion strengths (1.04 ± 0.7 MPa and 0.99 ± 0.7 MPa, respectively). This is potentially due to greater hydrogen bond prevalence caused by hydroxide and ester group interactions as well as the surface roughness of wood. Whereas glass, aluminum, and concrete provided similar, lower adhesion strengths (0.64 ± 0.1, 0.66 ± 0.1, and 0.6 ± 0.02 MPa, respectively). The smoothness of the glass and aluminum substrates is a likely contributor to the lower adhesion strength, while failure on the concrete substrate was due to loosened mineral particles at the pillar-substrate interface. Despite these differences, the fully cured printed two-stage resin structures exhibited impressive adhesion strength on a breadth of substrates. As a result, considering the same 5 x 10 mm base pillar on a wooden substrate, the pillar can grow up to 8 m after four cycles of 72 hour curing. These findings further reinforce the importance of the autonomous ambient temperature epoxy curing mechanism for temperature-sensitive substrates such as wood and acrylic.