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.