By Youngjae Lee, Jiyong Park, Woogeun Kim and Namjoo Kim, Miwon Specialty Chemical
3D printing (3DP) technology has found applications in diverse fields, including jewelry, dentistry, sports equipment, construction, fashion and medicine. This study aims to conduct an experimental investigation of acrylate and methacrylate monomers for use in Digital Light Processing (DLP) 3DP technology. These monomers, with their varied structures, show significant potential as key components in DLP 3D printing formulations.
Through a series of systematic experiments, the authors assess the suitability of various acrylate and methacrylate monomers in terms of their mechanical and thermal properties for DLP 3D printing applications. Specifically, the focus is on monofunctional monomers with diverse cyclic and aromatic structures. This study encompasses a comprehensive analysis of their essential characteristics relevant to the DLP process.
Experimental Details
To analyze the properties of monomers, we evaluated several characteristics for each monomer. The measured and evaluated monomer properties include viscosity, tensile properties, Shore hardness, glass transition temperature (Tg), flexural properties, impact resistance (IZOD) and resolution of the model (Eiffel tower). Typically, monomers cannot be used alone for printing due to their low viscosity and relatively poor mechanical properties. Therefore, an aliphatic urethane difunctional acrylate oligomer was added to the formulation. The oligomer was selected based on two criteria: moderate viscosity and relative flexibility.
The viscosity of neat monomers was measured by a Brookfield viscometer, while that of formulations was measured by TA Instrument’s rheometer. Other than viscosity measurement, the instruments used for assessing each property and the test specimens employed will be described in their corresponding sections.
Oligomer
As mentioned before, an aliphatic urethane difunctional acrylate oligomer (UO) was used in the formulation, and its 3DP properties are shown in Table 1.
Monomers
A total of nine monomers were tested, and all the monomers examined in this study are monofunctional and have a cyclic structure. Among these monomers, three are acrylate monomers, and the remaining six are methacrylate monomers. The information about these monomers is shown in Table 2.
Formulation
For all experiments, the same formulation was used for each monomer. 50 parts of the specific oligomer were blended with 50 parts of each neat monomer and 1 part of photo-initiator, TPO. All formulations were heated in a convection oven for a sufficient time so that the TPO dissolved in the formulation, and then the formulation was mixed well manually. The mixture was then cooled down to room temperature.
Test Specimens
For the tensile properties and Shore hardness measurement, ASTM D638 Type V test specimen was used. After the test specimens were printed with the Carima IMC DLP printer (all test specimens were printed using this printer in XY [flat] direction), the specimens were soaked in isopropyl alcohol (IPA) and sonicated three times: two minutes, two minutes and one minute. Rinsing with IPA removed uncured resin from the surface. After the process of sonication, the specimens were dried for 10 minutes in a fume hood to remove any residual IPA. Then the specimens were placed on the tray for an additional curing using the Carima CL300Pro curing machine twice, for two minutes each time. The specimens were flipped before their second cure with the curing machine. Lastly, the test specimens were conditioned at the temperature of 23±2° C and the humidity of 50±5% for 24 hours.
Two different types of specimens were used for the measurement of oligomer and monomer Tg. The specimen for the measurement of oligomer Tg has dimensions of 3 cm 0.5 cm x 0.025 cm (length x width x wet thickness). The oligomer with a photo-initiator was applied on a glass and cured with the Heraeus UV-curing machine with a Halogen bulb (Total UV: 2300 mJ/cm2). Then, it was conditioned at the temperature of 23±2° C and the humidity of 50±5% for 30 minutes before measurement.
The specimen for the measurement of monomer Tg has dimensions of 3.5 cm × 0.5 cm × 0.02 cm (length × width × thickness). This specimen was printed with the same process and the post-treatments for the Type V test specimen.
Tensile Properties
The tensile properties evaluated are tensile strength, elongation and Young’s modulus. To assess these properties, ASTM D638 Type V test specimens were examined using a Universal Testing Machine (UTM). Seven test specimens were printed, and at least five of them were tested based on their resolution. An Instron Model 3366 Dual Column Table Frame was used with mechanical wedge action tensile grips. The test specimens were tested by pulling at a rate of 10 mm/min. Tensile strength is the maximum stress applied to the specimen during the test. Elongation is the ratio of the change in length to the initial length of the specimen. Young’s modulus is the ratio of the stress applied to the specimen to the strain caused by the stress.
In addition to the tensile properties, toughness was determined from the stress-strain curve. The area under the stress-strain curve represents toughness, defined as energy absorbed per unit volume before fracture.
Flexural Properties
The flexural mechanical properties are flexural modulus and flexure strength. Both the measurement procedures and the preparation of test specimens, including their conditioning, were conducted in accordance with the ASTM D790 test method.
Shore Hardness
For the measurement of Shore hardness, TECLOCK durometers were used. The Shore hardness was measured on the D-shaped parts of Type V test specimens. For each specimen, Shore D hardness was measured.
Izod Impact Resistance
Izod impact resistance was measured by using Instron CEAST 9050 manual model pendulum tester. The Izod impact resistance test was performed on ASTM D256 specimen samples. The specimens were printed with the 3D printer and then notched using an Instron manual notching machine.
Tg
A Perkin Elmer DMA 8000 with tension mode clamp was used to measure the glass transition (Tg) of formulations. As mentioned in the test specimen section, two different types of test specimens were used for oligomers and monomers.
For the measurement of oligomer Tg, five parts of hydroxy dimethyl acetophenone were blended with 100 parts of neat oligomer. For the measurement of monomer Tg, specimens of monomers formulated with difunctional urethane oligomer were printed using a 3D printer.
The initial measuring temperature range for Tg was between -80° C and 150° C. If the Tg was not shown in that range or appeared on the edge of the graph, the temperature range was adjusted to be at least between 50° C below and 50° C above the Tg, and then it was re-measured with a new specimen. The temperature ramp rate was set to 5° C/min, the oscillation frequency to 1 Hz and the strain distance to 0.020 mm. With these settings, the tan δ max point was reported as Tg for this study.
Resolution
To distinguish the resolution, three different models (Eiffel tower, dental mold and jewelry, Figure 1) were printed. Among those models, the Eiffel Tower was chosen because of its complex structure, which can have several points of distinction. The Eiffel Tower model has a height of 6 cm and its base dimensions are 6.7 cm × 6.7 cm. For resolution assessment, the Eiffel Tower model was divided (not physically) into four parts: top, second floor, four legs and entire body. The top part of the model has four half-circle holes (0.2 cm × 0.1 cm each). The second floor has 64 square holes (0.05 cm × 0.05 cm each). Each leg has 24 triangular holes (0.2 cm × 0.1 cm each). Formulations with high resolution have clear holes, while those with mediocre or low resolution have holes blocked with cured resin.
Results and Discussion
Mainly, the tensile properties data were used to categorize each monomer into three different types: rigid, tough and flexible. For this study, the type of each product is relative to that of other products. In other words, a monomer categorized as rigid in this study may be sufficiently rigid for some applications but not rigid enough for others. The measured properties of each monomer are shown in Table 3.
The monomers IBOMA, TMCHMA and TBCHMA are categorized as rigid types, exhibiting properties such as high Tg, high tensile strength, high Young’s modulus and high Shore D hardness (see Table 6). They also demonstrate relatively high resolution compared to other monomer types. Among these, IBOMA has the highest Tg (91° C), Young’s modulus (810 MPa), flexural modulus (16,800 MPa) and Shore D hardness (83). In contrast, the tough monomers IBOA and CHDCIEA possess relatively high tensile properties and elongation. Notably, CHDCIEA exhibits the highest toughness at 21.28 MJ/m³ along with high resolution. Lastly, the flexible monomers CTFA, PHEMA, THFMA and IPGMA exhibit high elongation exceeding 120%, with CTFA achieving the highest elongation at 154%. Although PHEMA and THFMA are elastic during testing, they lack sufficient tensile strength to be classified as tough types. Compared to other categories, flexible monomers generally show relatively high IZOD impact resistance, ranging from 38 J/m to 50 J/m.
Conclusion
In this study, the nine monomers with various cyclic structures were assessed to investigate the properties of each monomer, intended for use in Digital Light Processing (DLP) 3DP technology.
Based on the properties of the monomers studied, we can suggest various potential applications. Rigid monomers such as IBOMA, TMCHMA and TBCHMA, characterized by high Tg, high tensile strength, highYoung’s modulus and high Shore D hardness, are well-suited for use in automotive components that require impact resistance and lightweight properties. These monomers could also be utilized in the production of durable electronic device casings or high-performance industrial equipment parts where strength and rigidity are crucial.
Tough monomers like IBOA and CHDCIEA, with their relatively high tensile properties and elongation, are appropriate for applications in pipe installation systems for construction and infrastructure projects. They also could be employed in the manufacturing of resilient household appliances or robust outdoor furniture, where a combination of strength and flexibility is necessary.
Flexible monomers such as CTFA, PHEMA, THFMA and IPGMA, exhibiting high elongation and high IZOD impact resistance, can find applications in the development of flexible printed circuit boards for electronics or in the production of vibration-damping materials for industrial machinery. Their elastic properties make them suitable for creating flexible seals and gaskets in automotive and aerospace industries, where direct skin contact is not a concern.
It’s important to note that while these monomers offer valuable properties for various applications, their use should be limited to products and components that do not come into direct contact with skin or mucous membranes. For any application where human contact is possible, further safety testing and regulatory approval would be necessary.
In conclusion, this study offers new insights into the design and application of polymer materials, contributing to increased industrial applicability.
Acknowledgement
The authors sincerely thank Gaeun Lee for her invaluable contribution. All experiments were conducted by Gaeun, and her dedication was essential to the successful completion of this work.
Jiyong Park is the director of the R&D Institute at Miwon Specialty Chemical. He earned a master’s degree in thermodynamics from Korea University, South Korea. Park can be reached at email: jiyong@miwonsc.com, www.miwonsc.com/eng/.
Woogeun Kim is the head of the Basic Materials Lab at Miwon Specialty Chemical. He earned a master’s degree in polymer chemistry from Hanyang University, South Korea. Kim can be reached at email: woogeun@miwonsc.com.
Namjoo Kim is the leader of the R&D team at Miwon Specialty Chemical He earned a bachelor’s degree in polymer engineering from Sungkyunkwan University, South Korea. Kim can be reached at email: njkim@miwonsc.com.
Youngjae Lee is on the staff of the R&D team at Miwon Specialty Chemical. He earned a bachelor’s degree in chemical and biomolecular engineering from the University of Illinois at Urbana-Champaign, Illinois. Lee can be reached at email: youngjae.lee@miwon.sc.com.
