3D Printing Chemistry 101: Polyurethanes and Carbon’s RPU

If you’re anything like this author, you got a C- in high school chemistry and never looked back. With a newfound interest in the topic, I’m hoping to revisit the molecular science of some of the most popular materials in 3D printing to understand them—not just in terms of applications and physical properties, but chemical makeup.

In our first installment in this series, we looked at PAEK plastics, given their popularity in demanding, high-performance applications. Here we look at a specific form of polyurethane from 3D printing unicorn Carbon. What’s worth considering about Carbon’s materials and why they deserve special consideration is that, through the use of a post-print heating step, further physical properties are activated, rendering these photopolymers with characteristics akin to thermoplastics.

Polyurethanes are rubber-like materials, with about 75 percent of the world’s use of the going toward rigid and flexible foams. Applications include upholstery, foam seals and gaskets, elastomeric wheels and tires, automotive suspension bushings, and more. While there are thermoplastic polyurethanes, most polyurethanes are thermosets, meaning they don’t melt when heated. In 3D printing, thermosets are the domain of vat photopolymerization technologies, which harden photopolymer resins using ultraviolet light.

Polyurethanes are created via reactions between isocyanates, a class of chemicals made from derivatives of ammonia called amines, and polyols, organic materials with multiple groups of oxygen bonded to hydrogen. The polylols contribute to the flexibility of the polymer and the amount of crosslinking determines how tough or rigid it is. While long chains with low crosslinking result in a stretchy material, short chains with a great deal of crosslinks result in a hard one. Long chains with intermediate crosslinking create a polymer that can be useful for foam materials.

Polyurethane synthesis, in which the urethane groups NH−(C=O)−O− connect molecular units.

In the production of polyurethanes, the isocyanates reacts with water to create urethane and urea bonds. In the case of Carbon’s RPU 130, a rigid polyurethane, these bonds are necessary for the resulting physical properties.

Jason Rolland, senior vice president of Materials at Carbon, explained to 3DPrint.com, “RPU 130 is a rigid dual-cure polyurethane-urea material. The presence of urethane and urea bonds in the material allow for significant hydrogen bonding to occur. This gives the material a high degree of toughness as well as a high softening temperature.”

Carbon’s Digital Light Synthesis (DLS) technology is a company-specific form of digital light projection (DLP), in which ultraviolet light is cast onto a vat of photopolymer resin and, through the use of an oxygen-permeable window, cures the material continuously at a rapid rate. This process enables isotropic physical properties, that is properties that are the same in all directions of the part.

Whereas most photopolymers used in 3D printing result in weak, brittle parts more suitable for prototyping, components made with DLS feature engineering-grade mechanical characteristics due to a heating step that is applied once the printing process is complete. Carbon explained how this works:

“Dual-cure materials developed by Carbon, including RPU 130, are a blend of photocurable and thermally curable chemistries,” Rolland said. “The photocurable groups activate and polymerize during printing when the resin is exposed repeatedly to patterned UV light. This allows us to precisely define the shape of the part during printing. The thermally active groups are triggered during the post bake, forming a separate polymer network. The final properties of the material are determined by both the UV and thermal polymer networks.”

Because of the aforementioned issues with photopolymers, they have, historically, not been suitable for end part production via 3D printing. Advances from companies like Carbon, however, are changing that. Carbon told 3DPrint.com that the properties of photopolymers results in an attempt to balance impact strength and thermal characteristics, which is less of an obstacle for thermoplastics.

“A key challenge for UV curable materials is the tradeoff between impact strength and thermal properties. Generally, you need to pick one. Thermoplastics like nylon do a better job of balancing these properties. RPU 130 is unique in that it has very high impact strength (>30 J in Gardner impact test) and a high heat deflection temperature (120 oC),” Rolland explained. “The combination of properties realized with RPU 130 makes it highly differentiated in the space of additive manufacturing; it is a great substitute for common thermoplastics like ABS, nylon, or polypropylene for a variety of applications. Low-volume automotive part production involving notable cost savings has been demonstrated using RPU 130 and Carbon’s technology. Other examples of potential industrial and consumer product applications for RPU 130 include air ducts and brake caliper covers for vehicles, sunglasses, tool housings, and device enclosures.”

As we have discussed elsewhere, there is an urgent need to shift our plastics production from fossil fuel derived sources to renewable sources. There are numerous research endeavors underway that seek to develop plant-based photopolymers for 3D printing, many of which show great promise.

A pipe holder fastener 3D printed by assembly technology manufacturer ARaymond for automotive production. Image courtesy of Carbon.

Until those materials can be scaled to commercial levels, companies like Carbon are attempting to use more renewable stock for their materials. Rolland said of Carbon’s rigid polyurethane:

“Nearly 30% of RPU 130 is composed of a plant-derived raw material called Susterra propanediol, a raw material produced by DuPont Tate & Lyle Bio Products that is derived from corn. This is directly in line with Carbon’s commitment to principles of sustainability in the development of new materials. New polymer material innovations can go hand-in-hand with environmentally conscious principles. RPU 130 shows that enhanced material performance and improved material sustainability do not have to be opposing goals. Plant-derived raw materials generally have a lower carbon footprint than petroleum-derived materials, and Susterra propanediol—which is a USDA certified 100% Bio-based product—produces 48% less greenhouse gas emissions and uses 46% less nonrenewable energy from cradle-to-gate compared to conventional petroleum-based alternatives.”