Can Polyhydroxyurethane-Derived Covalent Adaptable Networks Provide Environmental Benefits in Composite Applications?

Area of Science:

• Materials Science and Engineering
• Sustainable Chemistry
• Life Cycle Assessment

Background:

• Covalent adaptable networks (CANs) and CO2-derived polyhydroxyurethanes (PHUs) are explored as sustainable alternatives to conventional thermosets like epoxy (EP).
• The environmental impact and recyclability of CAN-based composites and thermoset PHUs remain largely unassessed.
• Composite materials often utilize epoxy matrices reinforced with carbon fibers or natural fibers (NFs).

Purpose of the Study:

• To conduct a comprehensive life cycle assessment (LCA) of PHUs, synergetic hybrid EP-PHU CANs, and EP composites.
• To evaluate the environmental implications of synthesis, processing, and recycling strategies for these materials.
• To compare the sustainability of composites using carbon fibers versus natural fibers.

Main Methods:

• Life cycle assessment (LCA) methodology applied to polyhydroxyurethanes (PHUs), epoxy (EP), and hybrid EP-PHU covalent adaptable networks (CANs).
• Evaluation of composite systems incorporating either carbon fibers or natural fibers (NFs).
• Analysis of synthesis routes, including cyclic carbonate monomer production from EP and supercritical CO2, and assessment of curing energy requirements.
• Investigation of recycling pathways: chemical recovery of carbon fibers and mechanical recycling of CAN matrices.

Main Results:

• Producing cyclic carbonate monomers from EP and supercritical CO2 presents potential environmental advantages.
• PHUs offer environmental benefits to EP but are limited by high energy inputs for curing.
• Synergetic EP-PHU CAN composites demonstrate significant environmental gains compared to EP and PHU composites, with favorable recycling pathways.
• Chemical recovery of carbon fibers via oxidative depolymerization is environmentally beneficial compared to virgin material production.
• For NF composites, mechanical recycling of CAN matrices is more suitable than chemical recycling due to environmental impacts.
• The optimal sustainability strategy is highly dependent on the specific raw materials used and cannot be universally generalized.

Conclusions:

• Synergetic EP-PHU CANs represent a promising sustainable composite material with enhanced recyclability, particularly when combined with carbon fibers.
• The choice between chemical and mechanical recycling strategies for CAN-based composites is critically dependent on the type of reinforcing fiber (carbon vs. natural).
• Further strategies for improving composite sustainability, considering both material choice and end-of-life management, are essential and discussed.