Engineering Self-Healing into 3D Printing to Enhance Sustainability

Sales of photopolymers for additive manufacturing grew steadily until plateauing around 2021 at approximately $650 million [1]. However, the growing interest in functional materials—such as self-healing polymers—may reinvigorate the market and drive a renewed upward trend in photopolymer use for 3D printing. Additive manufacturing enables the production of complex geometries with minimal start-up and tooling costs. The use of materials with inherent functionality to create highly engineered parts for specialized applications further expands the market potential for additive manufacturing.

Light-driven 3D printing technologies offer high-resolution manufacturing while allowing for customization of the material system, enabling tailored material properties in the final product. Self-healing polymers are able to recover a portion of their original mechanical integrity following self-repair. This ability would extend the life of a product which may have significant economic and environmental impact. This is especially important for aerospace applications [2-4], where repairing a component is more economical than carrying an inventory of replacement parts or delivering them on an as-needed basis. Between 1993 and 2009, five space missions were conducted to service the Hubble telescope [5], involving the transport of needed replacement components. In a 2004 report to the U.S. Senate estimating the cost of these missions, it was projected to range between $1.7 – $2.4 billion, with $43 million allocated for just the payload processing [6]. The implementation of high performance, self-healing materials would, at the very least, reduce the weight and payload requirements. SHP are not limited to just aerospace, but may find applications in various domains including biomedical [7-9], soft robotics [10-12], coatings [4, 13, 14], and military [15]. 3D printing (3DP) of these materials opens the door to manufacture complex geometries with self-healing capabilities, enhancing sustainability and enabling its application into these domains.

My research aims to develop a straightforward approach for 3D printing self-healing photopolymer systems [16]. Self-healing is achieved by combining a low-melting temperature thermoplastic polymer with a conventional thermoset polymer for 3D printing. The thermoset/thermoplastic resin must be transparent to allow for light to pass through it for the thermoset resin to be cured in the presence of the thermoplastic polymer; as such, the two-material system must form a transparent, homogeneous resin. Upon curing, the materials separate and create two distinct yet intertwined solid phases. Healing occurs when the thermoplastic phase is melted, allowing it to flow into the damaged area while the thermoset phase, which cannot be remelted, provides geometric stability. Upon cooling, the thermoplastic phase solidifies, effectively acting as an adhesive to repair the damaged material. We have found several formulations that exhibited near-complete recovery of mechanical properties. The addition of this thermoplastic polymer was found to also improve mechanical properties as well, creating both a tougher and stronger material than the conventional 3D printing thermoset resin. While the addition of the thermoplastic led to promising enhancements of photopolymers for additive manufacturing, its incorporation increases the difficulty of 3D printing these blends. My research provides insight into various factors that play a role in balancing the 3D printability, mechanical properties, and self-healing capabilities of this unique material system.

References                                                            

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16.       V. Mei, K. Schimmelpfennig, E. Caravaca, S. Colvin and C. L. Lewis, "Investigating the Structure–Property−Processing Relationship of Polycaprolactone-Based 3D Printed Self-Healing Polymer Blends," ACS Applied Polymer Materials, vol. 6, pp. 2177-2187, 2024.