The molecular design of photo-curable and high-strength benzoxazine for 3D printing

Article abstract of DOI:10.1039/d0cc07801h

Low viscosity photo-curable benzoxazines (BZs) are designed and synthesized for use in stereolithography 3D printing. An initial investigation shows that the thermally polymerized polybenzoxazines (PBZs) have remarkably highT_g(264 °C) and flexural modulus (4.91 GPa) values. Subsequently, the formulated photoprintable resins are employed for use in high-resolution projection micro-stereolithography (PμSL) printing. Complex PBZ 3D structures can be achieved from the as-printed objects after they are thermally treated. These findings advance the design of BZ monomers for photopolymerization-based 3D printing and offer a method for the efficient fabrication of high-performance thermosets for various demanding engineering applications.

Full text of DOI:10.1039/d0cc07801h

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The molecular design of photo-curable and
high-strength benzoxazine for 3D printing†
Cite this: Chem. Commun., 2021,
57, 3375
Yong Lu, ‡a Kok Wei Joseph Ng,‡b Hui Chen,a Xuelong Chen,b
abc
Song Kiat Jacob Lim,a Weili Yanb and Xiao Hu
*
Received 30th November 2020,
Accepted 23rd February 2021
DOI: 10.1039/d0cc07801h
rsc.li/chemcomm
Low viscosity photo-curable benzoxazines (BZs) are designed and
Additive manufacturing (AM), commonly known as 3D printing,
synthesized for use in stereolithography 3D printing. An initial is a rapidly developing technology that has advanced product
investigation shows that the thermally polymerized polybenzox- fabrication in prototyping and tooling, and has provided a revolu-
azines (PBZs) have remarkably high Tg (264 8C) and flexural modulus tionary alternative for material processing away from traditional
(4.91 GPa) values. Subsequently, the formulated photoprintable manufacturing methods, with the major advantage of accurately
resins are employed for use in high-resolution projection micro- producing complex structures/shapes.17,18 Amongst various AM
stereolithography (PlSL) printing. Complex PBZ 3D structures can technologies, photopolymerization-based 3D printing using tech-
be achieved from the as-printed objects after they are thermally nologies such as stereolithography (SLA), digital light processing
treated. These findings advance the design of BZ monomers for (DLP), and continuous liquid interface production (CLIP) have
photopolymerization-based 3D printing and offer a method for the attracted special attention from material scientists and engineers
efficient fabrication of high-performance thermosets for various due to the feasibility of obtaining high resolution and low feature
demanding engineering applications.
sized products.19 A large number of polymer materials which are
difficult to process and manipulate with traditional manufacturing
Polybenzoxazines (PBZs) are a class of high-performance methods have been successfully fabricated, normally via two-stage
thermosetting phenolics which have demonstrated a range processing, into complex structures by photopolymerization-based
of desirable features to overcome some drawbacks of using 3D printing.20–22 Two new low viscosity photo-curable BZs, BZ-C2
conventional novolac and resole type phenolics.1–3 Normally, and BZ-C5, for high-resolution PmSL processing are reported here.
they have high thermal stability and mechanical strength, Studies of the corresponding PBZs show excellent thermal and
high char yield, excellent flame resistance, low water absorp- mechanical properties, indicating their high-performance char-
tion and near-zero volumetric shrinkage.4–7 Therefore, a variety of acteristics. The PBZ 3D structured objects are demonstrated, which
applications have been explored for using PBZs in important fields are produced by thermally curing the PmSL printed parts. These
such as corrosion protection coatings, electronics, aerospace com- findings establish the BZ monomer design for favourable photo
posites, blends and alloys.8–13 Although PBZs offer a variety of resins, offer a way for PBZ fabrication using high-resolution PmSL
advantages, there are some inherent shortcomings that they face, printing, and also benefit the approach of processing high-
such as their brittle natures and undesirable processability due to performance thermosets with AM technology for demanding func-
the high curing temperatures (generally 180–250 1C) required. tional applications.
Therefore, use of conventional manufacturing methods such as
The synthetic approach to the monoacrylate functionalized
extrusion and melting are difficult to process PBZs into complicated BZ-C2 and BZ-C5, as well as a diacrylate functionalized BZ-BA
structures and this limits their wide implementation.14–16
is shown in Fig. 1a. A typical Mannich condensation was
used first, to synthesize the hydroxyl-terminated BZs. Phenol/
bisphenol A (BPA), 2-aminoethanol/5-amino-1-pentanol and
paraformaldehyde were reacted in chloroform (CHCl3) solution
at 70 1C overnight. Afterwards, the reaction mixture was washed
with an alkaline solution to remove unreacted acidic impuri-
ties, followed by a generalized aqueous workup to yield the
desired products. The as-prepared hydroxyl BZs were continuously
reacted with acryloyl chloride to form the acrylate functionalized
a Temasek Laboratories@NTU, Nanyang Technological University, Research Techno
Plaza, 50 Nanyang Drive, 637553, Singapore
b School of Materials Science and Engineering, Nanyang Technological University,
639798, Singapore. E-mail: ASXHU@ntu.edu.sg
c Nanyang Environment and Water Research Institute, Nanyang Technological
University, 637141, Singapore
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0cc07801h
1
‡ These authors contributed equally.
BZs and their chemical structures were verified by H-NMR and
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large molecule structure. It has been reported that the viscosity of
a photopolymerization printable formulation should be in the
order of 5 Pa sÀ1, otherwise, the printing resin is not able to
flow sufficiently and recover the building platform evenly if the
viscosity is higher than this value.23 Due to the highly viscous
nature, BZ-BA is not suitable for formulating a photo-curable resin
without adequate addition of monomer diluents. Moreover,
BZ-BA has poor stability in air and can gelatinize within several
hours. Conversely, BZ-C2 and BZ-C5 are fairly stable and possess a
low viscosity, which is favourable for photo-curable resins. In
Fig. 1c, the UV-vis absorption of BZ-C2 and BZ-C5 in dilute CHCl3
is shown and both of them exhibit similar absorption bands with
a maximum absorption at 277 nm. Because they do not show
UV-vis absorption beyond 400 nm, the resins based on them will
not affect the light penetration of the UV light source with a
wavelength longer than 400 nm.
In order to obtain PBZ structures of the two BZ monomers,
thermal polymerization of BZ-C2 was examined using DSC
analysis. The sample was prepared with a dual-curing process,
in which the BZ-C2 monomer and the photo initiator BAPO
were mixed vigorously with a vortex mixer to form a homo-
geneous resin. The resin was deposited in a silicone mould and
solidified within B10 s under UV irradiation. The photo curing
process was continued until full cure of the samples occurred,
as shown by the significantly decreased FT-IR absorption of the
vinyl structure (Fig. S2, ESI†). Afterwards, a progressive thermal
treatment was applied to the photocured sample and a speci-
men was collected at each curing stage for DSC scanning. As
presented in Fig. 1d, the photocured BZ-C2 exhibited a broad
exothermic peak at 235 1C (onset at 162 1C), corresponding to
the oxazine ring-opening polymerization. Being thermally
treated from 160 to 220 1C, the exothermic peak decreases
gradually at each stage, indicating the consecutive ring-opening
of the BZs and the incomplete PBZ network within the sample.
When the curing temperature reaches 240 1C, the exotherm
disappears completely, suggesting that entire conversion to the
PBZ network has occurred. Therefore, the maximum treatment
temperature of 240 1C was applied to all thermal curing
processes in this work. The thermal stability of the resultant
PBZs was examined using TGA analysis in a N2 atmosphere.
Fig. 1e shows that the 5% and 10% weight loss temperatures
(T5 and T10) of photocured BZ-C2 are 252 and 314 1C, which are
significantly improved to 326 and 358 1C, respectively, for the
resultant PBZ-C2. Similar findings are observed for photocured
BZ-C5 as well, with the T5 and T10 enhanced from 249 and
292 1C to 327 and 360 1C, respectively, after thermal curing. The
greatly increased thermal stability of the PBZs is attributed to the
highly crosslinked network in their structures. Interestingly, both
PBZ-C2 and PBZ-C5 exhibit a similar initial degradation tempera-
ture because the network degradation is always initiated from the
cleavage of their Mannich base formed by ring-opening. After the
initial degradation, all the tested samples display a second
thermal degradation attributed to the thermal decomposition of
the polymer main chains. It is worth noting that the char yield at
800 1C for PBZ-C2 is B30%, which is much higher than that of
the commercial resin (B7%) (Fig. S3, ESI†), suggesting the
Fig. 1 (a) Synthetic routes, and (b) plots of viscosity vs. shear rate for
BZ-C2, BZ-C5, and BZ-BA. (c) UV-vis absorption spectra of BZ-C2 and
BZ-C5 in dilute chloroform solution. (d) DSC thermograms for PBZ-C2
cured at different temperatures. (e) The TGA curves of photocured BZ-C2/
C5 and PBZ-C2/C5 under a N2 atmosphere.
FT-IR resonances. As shown in Fig. S1a and b (ESI†), the char-
acteristic proton resonances (Ar–CH2–N– and –O–CH2–N–) of the
oxazine ring appear at 4.85 and 4.05 ppm for BZ-C2 and at 4.83 and
3.96 ppm for BZ-C5. The multiplets in the range of 7.13 – 6.77 ppm
and 7.09 – 6.75 ppm are assigned to their aromatic protons. The
peaks at 6.42, 6.14, 5.85 ppm in Fig. S1a (ESI†) are assigned to the
vinyl protons of BZ-C2. In Fig. S1b (ESI†), the chemical shifts at
6.38, 6.10, 5.79 ppm belonged to the vinyl protons of BZ-C5. The
structure of BZ-BA is verified with 1H-NMR as well (Fig. S1c, ESI†).
The FT-IR absorption results further confirm the chemical struc-
tures of BZ-C2 and BZ-C5. As can be seen from Fig. S2a (ESI†), the
characteristic absorption band of C–O–C and N–C–O of the oxazine
ring are clearly observed at B1224 and B935 cmÀ1, respectively.
Meanwhile, the stretching absorption of the carbonyl and vinyl
groups in the acrylate structure are found at about 1718 and
1638 cmÀ1, respectively. These results prove the successful synthe-
sis of the acrylate BZs.
The viscosity data of the BZ monomers were collected using
a rotational rheometer with parallel plate geometry at ambient
temperature. The shear rate was varied in the range from 0.5 to
5 sÀ1. Fig. 1b shows that the viscosity of BZ-BA, BZ-C2 and
BZ-C5 is 300, 0.9 and 0.1 Pa sÀ1, respectively. Obviously, the
diacrylate BZ-BA monomer has a much higher viscosity than the
monoacrylate-based BZ-C2 and BZ-C5 due to its comparatively
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Fig. 2 (a) Storage moduli (insets: Specimen bars used for testing) and (b) tan d values plotted as functions of temperature for photocured BZ-C2/C5 and
PBZ-C2/C5. (c) Flexure stress–strain curves of PBZ-C2 and PBZ-C5 thermally cured at different temperatures. (d) A general ring opening reaction
mechanism and the corresponding network structures of UV cured BZ and PBZ.
potential applications of flame retardants and carbon precursors the significantly enhanced storage modulus and Tg. In parti-
for PBZ-C2.
cular, the broad tan d peaks of the PBZ-C2 and PBZ-C5 curves
The thermomechanical behaviours of photocured BZ-C2/C5 suggest that there is a heterogenous network in their structures,
and PBZ-C2/C5 were studied using DMA. Fig. 2a and b show the consisting of both highly and loosely crosslinked regions.
temperature dependence of the storage modulus and tan d, Encouraged by the DMA results, the mechanical performances
whereas the maximum value of the tan d curve is used to of PBZ-C2 and PBZ-C5 were further studied with a 3-point
determine the Tg. For the photocured BZ-C2 and BZ-C5, the bending test and representative flexure stress–strain curves
initial storage moduli are 478 and 7 MPa, respectively, and are shown in Fig. 2c. As shown, the modulus of the PBZs is
these decrease sharply as the temperature rises, indicating the greatly improved with the progressive thermal treatment due to
elastomer behaviour of the samples. However, PBZ-C2 and PBZ- the increased crosslinking of the samples. Remarkable moduli
C5 exhibit remarkably enhanced initial storage moduli of 4.3 of 4.91 and 3.00 GPa are achieved for the fully cured PBZ-C2
and 1.9 GPa, respectively, which are maintained at tempera- and PBZ-C5, respectively. As shown in Fig. S5 (ESI†), compared
tures up to nearly 150 1C. Additionally, a Tg of 54 1C and less to commonly used plastic resins, both PBZ-C2 and PBZ-C5
than 30 1C is found for the photocured BZ-C2 and BZ-C5, demonstrate high Tg and moduli which are associated with
respectively, whereas PBZ-C2 and PBZ-C5 showed highly photo printability, indicating that the two PBZs are high-
improved Tg of 264 and 208 1C, respectively. Considering the performance thermosets with excellent and promising
dual curing process of photo and thermal photopolymerization properties.
as well as the intermolecular hydrogen bonding formed
Due to their low viscosity and interesting features of the
(Fig. 2d), a triple network (Fig. S4, ESI†) is eventually estab- resultant PBZs, BZ-C2 and BZ-C5 monomers were formulated
lished within the PBZ structures and this contributes mostly to into photo resins to demonstrate their use in manufacturing
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Table 1 The diverse 3D printed structures and height changes before and
after thermal treatment
as 264 1C and a remarkable flexural modulus of 4.91 GPa due to
the highly crosslinked triple network within its structure. PBZ
objects with complex 3D structures are demonstrated using
PmSL printing and follow-up post-curing. The benefits of this
work are that it expands the existing library with a new class of
PBZs for popular additive manufacturing and it explores an
approach for processing high-performance thermosets for various
advanced applications.
The authors acknowledge support from the Nanyang Techno-
logical University (NTU) with grant number: 04IDS000677N040
and support from the School of Materials Science and Engineering
NTU for the present work. The authors also acknowledge Lim Zhi
Han Chester for his help with designing and generating the
CAD files.
Nanyang Technological University, Singapore, is filing a
patent based on this work.
a
b
*Height change percentage in brackets. BZ-C5 based resin. BZ-C2
based resin.
Conflicts of interest
There are no conflicts to declare.
high-performance PBZ thermosets with a two-stage fabrication
process, consisting of PmSL 3D printing and post thermal
curing. For the PmSL printing, a UV light source with a wave-
length of 405 nm and an intensity of 17.5 mW cmÀ2 were used
for all the printing. The formulated resin was exposed to UV
irradiation for 6 s per layer and generated a designed pattern
with roughly 100 mm thicknesses. As the printing process
progressed, the structural geometry was built up in a layer-by-
layer manner. Once the printing was completed, the structure
was removed, photocured further and transferred for a second
stage thermal curing. Table 1 demonstrates the printed objects
with various 3D structures before and after thermal treatment.
Measured using a micrometre calliper, only small height shrink-
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