By increasing the number of crosslinking molecules within existing elastomers, the team found they could give the materials customized levels of strength. The polymers’ covalent permanent networks also demonstrated the ability to detach or reattach their chemical links once exposed to high temperatures. Consequently, any broken bonds within the newly-designed polymers could be ‘healed’ by simply heating them up. According to the research team, the potential future applications of the technology range from artificial limbs to flexible aerospace components.
“We have made an exciting group of materials whose properties can be fine-tuned to get either the softness of rubber or the strength of load-bearing plastics,” said Dr. Svetlana Sukhishvili, Professor in the Department of Materials Science and Engineering at Texas A&M. “Their printability and the ability to self-heal within seconds, make them suited for not just more realistic prosthetics and soft robotics, but also ideal for broad military applications such as agile platforms for air vehicles and futuristic self-healing aircraft wings.”
3D printing with elastomer materials
Biological tissues such as skin and tendons feature a number of characteristics that are also desirable within 3D printing. Fibrous tissues are able to seamlessly integrate with different tissues and to heal once broken. Integrating similar features into flexible polymer printing materials could potentially open new areas to 3D printing within soft robotics and consumer electronics, but this has proved difficult thus far.
Existing techniques such as Suspended Layer 3D printing, utilize complex liquid ink formulations to achieve good interlayer adhesion, but the cost has prevented their widespread adoption. Commonly-used printing methods such as Fused Deposition Modelling (FDM) are more cost-effective but don’t offer the same level of adhesion or mechanical strength.
Permanently crosslinked elastomer resins are inefficient too, providing enhanced strength compared to ordinary photopolymers, but at the expense of being non-recyclable. Previous research into reversing the crosslinking process has also experimented with thermal post-processing, but this can affect the material’s strength and ability to be reprocessed.
Dr. Sukhishvilli explained the importance of recyclable crosslinking, comparing the process to stitching in cloth making. “Crosslinks are like stitches in a piece of cloth, the more stitches you have, the stiffer the material gets and vice versa,” said Sukhishvili. “But instead of having these ‘stitches’ be permanent, we wanted to achieve dynamic and reversible crosslinking so that we can create materials that are recyclable.”
Dynamic covalent polymer networks offer a unique alternative in that they provide both enhanced layer adhesion and the reusability not yet provided by existing photopolymers. Elastomer networks are also compatible with the Diels-Alder (DA) reaction, meaning that its links can be ‘clicked together’ and ‘unclicked’ without any by-products. DA reactions have the added benefit of thermal reversibility too, giving affected materials the ability to break apart at temperatures over 120oC, then reattach once cooled.
DA-based polymers are essentially able to self-heal via the heating-induced dissociation of covalent bonds, releasing furan and maleimide moieties that can repair its damaged networks. Leveraging this reversible covalent DA reaction, the researchers produced a family of reprintable covalently crosslinked polymer networks.
The research team’s new family of polymers
The resins produced by the researchers consisted of a mixture of an oligomeric linear prepolymer and abismaleimide (BMI) crosslinker. Different amounts of BMI were ‘studded’ onto the polymer, and attached via a thermally reversible DA reaction with two cross-linking molecules, furan and maleimide. Changing the amount of crosslinking molecules was found to adjust the material’s stiffness, potentially increasing it by up to 1,000 times the level of a standard photopolymer.
During testing, the researchers were able to produce solid objects using their new elastomer and FDM 3D printing, but not without drawbacks. Once cooled down to below 120oC, the material’s viscosity sharply increased, indicating its reformation into a solid dynamic network.
If exposed to temperatures above 140oC, the DA-printed (DAP) materials suffered irregularities which rendered them unrecyclable. In order to better understand the cooling process, the researchers reduced the temperature of their elastomer rapidly from 120oC down to room temperature and monitored the results.
Evaluating their experiment, the team discovered that shifts at 80ppm and 176ppm indicated that when the crosslinker ran out, the whole process slowed down. Compared to molded samples, the 3D printed DAPs also exhibited 0.5-0.7 MPa greater tensile strength, indicating the materials’ ability to fill gaps found within conventional printed parts.
Further testing their DAP networks, the team matched different crosslink densities with varied mechanical properties to create a part with mechanically mismatched interfaces. The object was 3D printed with three different materials using multiple syringe extruders, each with an individual DAP. Summary pressure testing showed that the component exhibited higher strength than parts produced using conventional FDM polymer materials.
Successive testing had therefore exhibited the potential of the materials for creating parts with gradients in both strength and elastic modulus. Adjusting the amount of BMI used to create the parts also allowed them to be created with on-demand characteristics for specific applications. As a result, the research team concluded that their novel family of materials needed further tuning, but in the future they could be utilized to create a range of mechanically diverse objects.
“Right now, we can easily achieve around 80 percent self-healing at room temperature, but we would like to reach 100 percent. Also, we want to make our materials responsive to other stimuli other than temperature, like light,” said Dr. Frank Gardea, Research Engineer in the United States Army Research Laboratory. “Further down the road, we’d like to explore introducing some low-level intelligence so that these materials know to autonomously adapt without needing a user to initiate the process.”
Elastomer materials in 3D printing
The flexibility and strength provided by elastomer 3D printing materials have often led to their application in consumer products such as insoles. In recent years, a number of researchers have developed enhanced elastomer-based resins, in an attempt to expand their usage within different industries.
A team of scientists led by Virginia Tech University (VTU) collaborated with Michelin North America to create a novel 3D printed elastomer rubber material. Adding photoinitiators and other compounds to a latex liquid mixture allowed the team to integrate a scaffold into the 3D printing process, providing additional strength.
Researchers from the University of Colorado Denver and the Southern University of Science and Technology developed a honey-like Liquid Crystal Elastomer (LCE) resin. Once 3D printed into lattice structures, the resin begins to mimic cartilage, creating new medical applications for the material.
3D printing companies such as 3DSystems have also launched their own elastomer materials, with the firm releasing its Figure 4 RUBBER-65A BLK material earlier this year. The material displays high elongation at break, providing a high level of flexibility and durability.
The joint project was carried out by Texas A&M University and the U.S. Army Combat Capabilities Development Command Army Research Laboratory. The researchers’ findings are detailed in their paper titled “A Tailorable Family of Elastomeric‐to‐Rigid, 3D Printable, Interbonding Polymer Networks,” which was published in the Advanced Functional Materials journal. The report was co-authored by Qing Zhou, Frank Gardea, Zhen Sang, Seunghyun Lee, Matt Pharr, and Svetlana A. Sukhishvili.
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Featured image shows the research team’s newly-developed elastomer 3D printing material. Photo via Texas A&M University.