3D printing polyurethane acrylate(PUA) based elastomer and its mechanical behavior

Liquid-crystal display(LCD) 3D printing, also known as light during 3D printing or photopolymer 3D printing, is a type of additive manufacturing technology that uses light-sensitive resin to create three-dimensional objects. This technology has gained popularity in recent years owing to its ability to create high-resolution, detailed objects with a wide range of materials, including shape-memory polymers, toughness resins, and elastomers. Elastomers are a type of polymer material that has the ability to stretch and deform under an applied force, but return to their original shape when the force is removed. The superior deformation recovery rate contributes to elastomer use in various industries, including automotive, aerospace, medical, and consumer goods. In this study, a UV-curable polyurethane acrylate(PUA) elastomer with an elongation of 100%–200% was developed. Using LCD 3D printing, we were able to fabricate Triply periodic minimal surface(TPMS) lattice structures with this elastomer investigated the compressive behavior of TPMS structures with different compressive ratios of 20%–50%. Our results demonstrate that this approach enables the creation of flexible energy-absorbing structures under cyclic loading. This study highlights the potential of LCD 3D printing technology for the production of elastomeric materials with tunable mechanical properties.


Introduction
The rapid development of 3D printing has enabled the application of new structures or materials [1][2][3]. The process involves laying down successive layers of material to build the object. Elastomers are a type of polymer material that have the ability to stretch and deform under an applied force, but return to their original shape when the force is removed. This makes them ideal for applications, such as flexible hinges, damping, and energy absorption [4]. LCD 3D printing is a specific type of 3D printing technology that uses a digital light projector to cure resins and create three-dimensional objects. This technology is well suited for printing elastomer materials because of its ability to create high-resolution, detailed objects with elastic properties.
Polyurethane acrylate (PUA) elastomers are a class of materials that are highly elastic and resistant to chemicals, making them suitable for use in various applications such as seals, gaskets, and shock-absorbing components [4][5][6]. The development of 3D printing techniques enables the application of 3D PU structures in damping or energy absobtion [7][8][9]. Additive manufacturing technologies, such as photocuring 3D printing and LCD 3D printing, offer a flexible and efficient way to produce structures with high resolution and good surface finish [3]. LCD 3D printing exhibits high resolution, which is dependent on the LCD and high efficiency, and can be used to fabricate parts of a wide range of materials, including shape memory polymers [10,11], biomaterials [12], and elastomers [13,14]. However, the mechanical behavior of LCD 3D printed PUA elastomers has not yet been fully investigated, and there is a need to understand how the printing process and other factors may affect Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. the properties of these materials. In addition, commercial photocuring elastomer inks used for UV 3D printing often exhibit poor mechanical strength, inferior resilience, and lower elongation at break. This limits the potential applications of these materials and structures, which can be produced using UV 3D printing.
The compressive behavior of 3D printing parts is not only related to the material properties, but also to the structural features. TPMS are mathematical structures that are known for their unique geometry and mechanical properties [15][16][17]. In recent years, 3D printing technology has been utilized to fabricate TPMS lattice structures using elastomeric materials such as TPU, silicone, and rubber [18,19]. One advantage of using 3D printing to fabricate TPMS lattice structures is its ability to create complex and highly customizable geometries. Lattice structures can be designed to have specific mechanical properties, such as high stiffness or high energy absorption, by controlling the size and shape of the unit cells. Additionally, 3D printing allows the creation of graded material properties within the structure, which can improve the overall performance of the structure.
Several studies have demonstrated the potential of 3D printed TPMS lattice structures for various applications. For example, these structures have been used as lightweight and highly efficient energy absorbers for automotive and aerospace applications [20,21]. They have also been used as scaffolds in tissue engineering [22,23]. Structural elements in heat transfer [18,24]. One challenge in the use of 3D printed TPMS lattice structures is the limited selection of materials suitable for 3D printing. Although elastomeric materials such as silicone and rubber are frequently used because of their capacity to deform under load and revert to their original shape, they tend to have greater hardness than silicone rubber.
Hence, in this study, we aimed to address the issue of low extension by developing PUA photocuring inks that can be used for LCD 3D printing and exhibit excellent mechanical properties. The TPMS structure may then be used to refer to a compressive lattice structure created using LCD printing and a PUA-based elastomer. This study highlights the potential of LCD 3D printing technology for the production of elastomeric materials with tunable mechanical properties.

Methods and materials
2.1. LCD based 3d printing LCD (Liquid crystal display (LCD) 3D printing is a type of 3D printing technology that uses a liquid crystal display (LCD) screen to cure photosensitive resin into a solid object. This process is known as stereolithography and allows for the creation of highly detailed and accurate 3D prints.In LCD 3D printing, in which a layer of photosensitive resin is placed on a platform within the printer. An LCD screen is located below the platform and is used to project an image of the cross section of the 3D model being printed. Light from the LCD screen cures the resin, solidifying it into the desired shape. The printer then lowers the platform slightly and repeats the process, building the object layer by layer until it is complete. One of the main advantages of LCD 3D printing is that it produces high-quality prints with fine detail and smooth surfaces. It is also relatively fast compared to other 3D printing technologies(such as SLA, SLS, or FDM), and it allows for the use of a wider range of custom materials with different mechanical behaviors.
The resolution of an LCD 3d printer is highly dependent on that of the LCD screen. Owing to the development of high-resolution LED screen, 8 K based LCD 3D printers have also been developed. Here, we used a Phrozen Sonic Mini 8 K 3D Printer to fabricate the PUA elastomer and 3D printer structures.The mini 8 K 3D printer utilizes a 7.1' Mono LCD screen with a 22 μm resolution, can print 165 × 72 × 180 mm build volume parts; the minimal printed layer can be set to 25 μm. Here, we print structures with 50 μm layers fabricated using a patterned 405 nm. The bottom layer was exposed for 30 layers to ensure that the printed structure could adhere to the printed platform, and the other layers were exposed for 15s per layer. After printing, the parts were cleaned with 99.9% ethanol to remove the residual uncured resin. The parts were then soaked in a solution of liquid detergent and water and scrubbed by hand until they appeared clean.

Materials
In this study, we developed photocuring inks for LCD 3D printing using monofunctional-aliphatic urethane acrylate RJ425, hydroxyethyl Acrylate (HEA), and Isobornyl Acrylate (IBOA) in different ratios to achieve the desired elasticity and strength. RJ425 exhibits high stretch properties [25]; however, RJ 425 has a low adhesive strength, is prone to falling off the platform, and fails to form 3D parts. HEA could efficiently improve the adhesive when mixed with RJ425. A thinner IBOA can be used to modify mechanical performance, such as the stretchable strain and tensile strength. Hence, we used three different mixture ratios, RJHI-1, RJHI-2, and RJHI-3, to investigate the mechanical performance of 3D printing PUA with a mixture of RJ425, HEA, and IBOA (table 1). The mechanical properties of the resulting structures were characterized by tensile testing. The behavior of the TPMS structures under compressive loading was evaluated. Diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide-819 was used as the photoinitiator and the amount of photoinitiator was varied to control the curing rate and mechanical properties of the inks.Here, we control the content photoninitiator-819 as a 2% volume fraction of the PUA liquids. The PUA was then used to fabricate tensile sample and compressive TPMS lattice structures using the mini 8K LCD 3D printer.

Tensile testing and compressive stength testing
For the tensile testing, specimens were prepared with a size of 10 × 4 × 0.5 mm, and the gauge length was 4 mm. The tensile properties of the specimens were measured using a universal testing machine (Zhiqu machine, China) at a strain rate of 5 mm min −1 . The mechanical properties of the specimens were determined by measuring the tensile strength, elongation at break, and toughness. The TPMS structures were fabricated using an LCD 3D printer with a size of 20 × 20 × 20 mm, and the porous fraction varied from 25% to 40%. The compressive behavior of the different TPMS structures was tested using a universal testing machine at a strain rate of 5 mm min −1 . The deformation within the structure was analyzed at various stages of the compressive ratio from 20% to 50%. Compressive deformation of the TPMS structure was investigated using a 1080p camera (Huiboshi Camera W1-1080p, China). Figure 1(a) shows the mini 8 K LCD 3D printing scheme and the material molecular formula Chitu slicer is used to slice the printed parts for the mini 8 K 3D printer, which has a 22 μm resolution. A 405 nm UV light source was used to expose the slice image layer by layer to form 3D parts. For printing materials, RJ425, a PUA monomer, is the main photocuring ink, and was used to improve the strength of the UV resin, BOA was used to improve the flow of resin and 819, as a photoinitiator, enabling the liquid resin to form a solid layer under UV light irradiation. Figure 1(b) shows the TPMS compressive lattice structure fabricated using photocuring inks. There are four different TPMS lattice structures: I-WP, Diamond, Gyroid and Primitive. The TPMS lattice structures were composed of 2 × 2 unit cells with a size of 10 × 10 × 10 mm −1 and a wall thickness of 500 μm−800 μm with different volume fractions. The structure was printed with a layer thickness of 50 μm and a printing resolution of 22 μm. The structure could withstand compressive loading without collapse, indicating its potential as a flexible energy-absorbing structure. Figure 2 shows the tensile stress-strain curves of the PUA UV resin developed in this study. The PUA UV resin shows similar level tensile strength to hydrogel [26], approximately 250-300 kPa, and the PUA UV resin also shows a high elongation at break, which is larger than 100%. As shown in figure 2(a), the tensile stress-strain curve of RJHI-1 exhibited a nonlinear relationship. However, RJHI-2 and RJHI-3 showed a low nonlinear extension compared with RJHI-1. Hence, with an increase in HEA, the PUA UV resin tends to exhibit elastic behavior. Figure 3 shows the mechanical properties of the three PUA UV resins. As we can see, with the increase of the content of HEA, the tensile strength increased from 0.28 ± 0.06,0.29 ± 0.02 to 0.30 ± 0.03 MPa. The elongation and toughness decreased with an increase in the HEA content. The extension of RJHI-1 up to 2.22 ± 0.14 is approximately 1.34 times more than that of RJHI-2 (1.66 ± 0.09) and 1.49 times more than that of RJHI-3 (1.49 ± 0.32). The toughness of RJHI-1, RJHI-2 and RJHI-3 are 0.64 ± 0.09, 0.46 ± 0.02, and 0.42 ± 0.12.The reason is that RJ425 has good elongation, attributed to better extension, while HEA improves the tensile strength but results in a low elongation of the sample. Hence, we selected RJHI-1 to print the TPMS structures in sections 3.3 and 3.4.       changes after five cycles of loading and unloading, which suggests that the Diamond TPMS lattice structure exhibits good deformation recovery. The deformation of the structure can be seen in figure 4(g). Similar to the Diamond TPMS lattice structure, the compressive strength increased with an increase in the compressive ratio or volume (as shown in figure 5). The compressive strength of the pristine TPMS lattice structure was larger than that of the Diamond TPMS lattice structure at the same compressive ratio/volume (as shown in figure 6). However, the pristine TPMS structure exhibited a larger hysteresis effect after unloading. This may be due to the primitive TPMS structure exhibiting local buckling during the compressive loading. Hence, the pristine TPMS structure exhibited better energy absorption.

Mechnical properties of 3D printing PUA elastomer
The energy absorption of the TPMS structure was investigated further. We conducted an egg with 52.42g fall off onto a cylindrical TPMS structure (40% volume) from 1 m high. As shown in figures 7(a), (b), and (d), a onelayer cylinder TPMS structure (with a radius of 20 mm and height of 20 mm) and a two-layer cylinder TPMS structure (with a radius of 20 mm and height of 40 mm) were used to investigate the energy abortion efficiency. The results showed that the egg fractured when the one-layer cylindrical TPMS structure was used (as shown in figure 7(c)). The egg rebounced (see the Supporting Information SI video) and then fell off the structure without damage when the two-layer cylinder TPMS structure was used (as shown in figure 7(e). Hence, the two-layer cylindrical TPMS structure can withstand impact resistance and protect the egg from failure, indicating its potential as a flexible energy-absorbing structure.
Overall, these results demonstrate the potential of LCD 3D printing for the production of elastomeric materials with tunable mechanical properties. The photocuring inks developed in this study exhibited excellent mechanical properties, and the compressive lattice structures produced using these inks exhibited promising behavior under loading. This study has potential applications in the production of flexible energy-absorbing structures.

Conclusion
In conclusion, LCD 3D printing technology was used to fabricate PUA elastomers, and the compressive behavior and energy absorption of PUA TPMS structures were inverted. The three tensile strengths of different elastomer-RJHI-1s, RJHI-2, and RJHI-3 were investigated, and RJHI-1 exhibited high elongation and excellent toughness. The compressive behavior of the TPMS structure-diamond and primitive types was examined, and our results showed that the TPMS structure showed good recovery through load and unload cycles. The fall in testing shows that the PUA-based TPMS structure exhibits excellent energy absorption. This study highlights the potential of LCD printing technology for the production of elastomeric materials with tunable mechanical properties and energy absorption applications.

Data availability statement
The data cannot be made publicly available upon publication because they are not available in a format that is sufficiently accessible or reusable by other researchers. The data that support the findings of this study are available upon reasonable request from the authors.