TECHNOLOGY AREA(S): MaterialsOBJECTIVE: Develop an additive manufacturing technique that allows for processing of thermally-cured thermoset polymers.DESCRIPTION: Thermally cured thermosets, such as polyurethanes and polydimethylsiloxes (PDMS), are widely used in a myriad of industrial and military-relevant applications, such as machine parts, protective coatings, and medical devices, as they possess high thermal and mechanical stability. Additionally, as polymers, these materials possess the attractive features of being lightweight, ease of manufacturing relative to other high strength materials (e.g., metals/alloys), and inexpensive. Because of these attributes, thermally-cured thermosets currently dominate the traditional manufacturing space for thermoset materials. Highly desirable, however, is an additive manufacturing (AM) methodology amenable to processing these materials, as this would enable an on demand , energy-efficient means of their production. AM has also been demonstrated as a platform to rapidly fabricate customizable parts. This would be particularly impactful for DOD applications where manufacturing at the point of use may provide critical capabilities while decreasing and/or eliminating supply chain and logistical challenges.To date, the difficulty in 3D printing thermally-cured thermosets largely stems from a need for extremely rapid heating/cooling cycles (sub microsecond) that span large temperature changes a requirement that cannot be easily met using conventional bulk heating. Recently, researchers have demonstrated novel methods of internal heating using nanoscale heat sources, such as photothermal curing (ref 1), or pulsed microwave irradiation (ref 2) that could support the rapid cure cycles required with additive manufacturing. Additionally, the aforementioned photothermal curing method has demonstrated tunable mechanical and physical properties based on the intensity of light irradiation, thus offering potential access to 3D printed parts with tailored properties (ref 3).There is an essential need to develop an additive manufacturing technique that enables the processing of purely thermally-cured thermoset polymers. The technique should also be generalizable include different types of thermally-cured thermosets. Additionally, the proposed method should not require an oven to fix the final print, and the final part should demonstrate mechanical and thermal stability akin to cast parts made from the same polymer formulation.PHASE I: Develop a methodology that enables only thermally-activated curing of 1 of the following thermosets: epoxy-amines, PDMS, or polyurethanes, using only commercially available components. Please note that resins that are easily polymerized via photoinitiation, such as cationic epoxies and (meth)acrylates, will not be considered. The 3D printing technique should be capable of curing at the point of extrusion, and preferably not require and oven to fix the print. If an oven is used to post-cure, the printed part should be stable for 1hr prior to oven curing.The printer should have a minimal average speed of 10mm/s throughout a print and be able to continuously print for a minimum of 20 minutes. The technique should also demonstrate the ability to stop/restart after 10 minutes with no need to clean the printer. The final print part should demonstrate a resolution (layer thickness and length) less than 1mm. 3DBenchy and other common 3D printing stress tests should be performed to ensure (i) the Young's modulus, tensile strength, and glass transition temperature are similar to cast parts from the same polymer formulation, (ii) good adherence between layers, and (iii) solvent and light resistance similar to cast parts from the same polymer formulation. The performers should demonstrate the ability to systematically and controllably vary the thermal and mechanical properties to render parts that range from elastomeric to glassy.PHASE II: Demonstrate the method developed in Phase I can be extend to use a different thermally-cured thermoset than the one the team selects in Phase I and should also extend Phase I capabilities to enable print speeds to a minimum of 50 mm/s continuously for 4 hours. Additionally, the printer should be able to change the resolution of the print (1mm to 0.1mm) and the print speed (10mm/s to 50mm/s), and also demonstrate the ability to print without user intervention. The final print parts for both classes of thermosets should demonstrate a resolution down to 0.1mm, enable printing of complex shapes, and demonstrate inclusion of specified hollow features.To validate the ability to cure and lock in the part, key structures should be printed.One such example include scaffolds and a mathematical geometric comparison of the printed geometry vs the expected geometry should be determined.Another example includes a tall structure, such as a cylinder, should be prepared to assess for slumping of the part as pressure from above layers could cause not fully cured towards the bottom of the part to flow and cause distortions and slumping of the part.Additionally, the final printed parts should demonstrate mechanical, thermal, and performance properties that exceed that of common AM resins. Solutions that also demonstrate the ability to monitor stress-development during cure, as well as the ability to co-print two different thermally-activated thermosets are highly desired.PHASE III: The proposed technology has a broad range of civilian and military applications as thermoset polymers are widely used as machine parts for automotive and aerospace applications, as wound dressings for biomedical applications, as well as protective coatings. This technology could have transformative implications for DoD as it will enable the ability to print thermally-cured thermosets on-demand greatly simplifying the supply chain. In the civilian sector, in addition to health care implications, this technology may also enable mass customization of consumer products comprised of thermoset materials.KEYWORDS: thermoset polymers; additive manufacturing; 3D printing; polymer curing; mechanical stabilityReferences:Fortenbaugh, R.J.; Lear, B.J., On-demand Curing of Polydimethylsilozane (PDMS) using the photothermal effect of gold nanoparticles. Nanoscale 2017, 9, 8555.; Choi, W.; Choi, K.; You, C., Ultrafast Nanoscale Polymer Coating on Porous 3D Structures Using Microwave Irradiation. Advanced Functional Materials 2018, 28, 1704877.; Fortenbaugh, R.J.; Carrozzi, S.A.; Lear, B.J., Photothermal Control over the Mechanical and Physical Properties of Polydimethylsiloxane. Macromolecules 2019, 52, 3839.
