What is CO2 3D Printing and Why Does It Matter?

Additive manufacturing, or 3D printing, is being reimagined through the integration of carbon dioxide (CO2). This approach uses the greenhouse gas as a building block for materials, treating a waste gas as a resource. This is an area of active research exploring how to transform captured carbon into solid, functional products.

How CO2 is Incorporated into 3D Printing

Carbon dioxide is integrated into 3D printing by serving as a chemical feedstock for new materials. One pathway involves the chemical conversion of CO2 into polymers. Through catalytic reactions, CO2 can be combined with other chemicals, such as epoxides, to form aliphatic polycarbonates like polypropylene carbonate (PPC). These polymers can incorporate a substantial amount of CO2 by weight, locking the gas into a stable plastic.

Another method transforms CO2 into durable carbon nanomaterials. In a specialized two-step process, CO2 is first converted into carbon monoxide (CO) through electrolysis. This CO is then fed into a reactor where, in the presence of a catalyst like steel wool, it decomposes to form carbon nanotubes (CNTs). These high-strength nanotubes are then compounded with polymers like polylactic acid (PLA) to create advanced composite materials.

CO2 is also used directly in the fabrication process, particularly in construction. In a method for 3D printing concrete, gaseous CO2 is injected along with steam into the fresh concrete mix as it is being printed. The carbon dioxide reacts with calcium hydroxide in the cement, forming solid calcium carbonate (limestone) and water. This reaction, a form of carbon mineralization, accelerates the curing process.

Printing Techniques for CO2-Derived and CO2-Processed Materials

For polymer-based applications, CO2-derived material often takes the form of a filament for Fused Deposition Modeling (FDM) printers. For instance, carbon nanotubes synthesized from CO2 are mixed with a binder like PLA to create a composite filament. This filament can be printed on standard FDM machines to create a preliminary shape, or preform, from which the binder can later be removed by heat to leave a pure, porous CNT structure.

CO2-based polymers are also formulated for other printing methods, including paste extrusion and powder-based systems. Polyalkylene carbonates are used as binders in pastes for printing ceramics or in powders for Selective Laser Sintering (SLS). In these applications, the binder provides green strength to the printed part. A property of these binders is their clean decomposition at low temperatures, burning off into CO2 and water vapor without leaving residue, which is beneficial for creating pure final parts.

Polypropylene carbonate (PPC) on its own has limitations like low mechanical strength and poor heat resistance, affecting the durability of printed objects. To overcome this, PPC is often blended with other materials, such as inorganic particles or chain extenders, to create composites with improved stability. For the direct injection of CO2 in concrete printing, the machinery is modified with specialized pumps and jets to introduce the gas and steam precisely at the moment of deposition.

Applications and Developments in CO2 3D Printing

Carbon nanotube composites enable the printing of lightweight yet strong objects with complex geometries, such as honeycomb structures. After printing, the resulting pure CNT scaffold can be infused with an epoxy resin. This process yields high-performance nanocomposites with enhanced strength and electrical conductivity, suitable for applications in aerospace or electronics.

In the construction sector, the direct injection of CO2 into 3D-printed concrete is a notable development. This technique produces concrete blocks that are stronger than those made with conventional methods, with some studies showing increases in weight-bearing capacity of over 35%. This method allows for the on-site fabrication of building components that actively store carbon, turning buildings into carbon sinks. The improved properties also mean less material may be needed.

CO2-derived polymers are also used as binders for creating intricate ceramic or metal parts. These binders are being used in paste extrusion and SLS printing to manufacture components for the energy sector, including fuel cells and solid-state batteries. The ability of these binders to decompose cleanly allows for the production of highly pure and dense final parts, a requirement for many advanced applications.

Environmental Significance of CO2 in Additive Manufacturing

The use of carbon dioxide in 3D printing is a form of carbon capture and utilization (CCU), a strategy for mitigating climate change by converting captured CO2 into products. By transforming a greenhouse gas into polymers, composites, or mineralized carbonates, this technology creates a value chain for captured carbon. This process helps reduce the dependency on virgin fossil fuels as the primary feedstock for plastic production.

From a lifecycle perspective, the environmental benefits depend on the specific pathway and energy sources used. Life cycle assessments have shown that producing polymers from CO2 can lead to a lower global warming impact compared to their petroleum-based counterparts. For example, some CO2-based rubbers have been shown to reduce global warming impacts by up to 34%. However, these processes can be energy-intensive, and the overall carbon footprint is influenced by the energy source used for chemical conversions.

This technology also contributes to the principles of a circular economy. It provides a method for upcycling industrial emissions into higher-value materials, creating a more circular flow of carbon. The resulting products can also be designed for sustainability; for instance, many CO2-derived aliphatic polycarbonates are biodegradable, offering an alternative to persistent plastics.