The objective of this project was to use renewable resources derived from plants and other sources to prepare high-performance carbon fiber and thermosetting matrix resins with high-strength and high-thermal resistance. The scientific objectives of this work were to:

• Develop methods for breaking down, modifying, and processing renewable resources to make epoxy resins, vinyl resins, and carbon fibers; and,
• Determine structure-property relationships for these novel materials.

Several natural resources are currently used to produce biobased chemicals, including lignin, carbohydrates, cellulose, hemicellulose, chitin, and plant oils. This project focused on developing high-transition temperature (T_g) biobased resins by selecting or preparing cyclic and aromatic derivatives of lignin, carbohydrates, cellulose, hemicellulose, chitin, and hybrid monomers based on multiple renewable sources. Biobased fibers were prepared from lignin based oligomers. The biobased resins and fibers were combined to make high performance biobased composites for DOD applications.

The researchers developed numerous scientific and engineering advancements in this project. Bacteria can successfully decompose lignin into useable structures for the formation of small filaments that might be able to be converted into carbon fibers. Approximately 300 strains of bacteria that decompose lignin were identified, and some have the potential to make lignin into fiber-forming oligomers, including newly identified species of Serratia. However, scale-up of this process proved problematic and unfeasible for completion in this project.

The researchers chemically fractionated lignin to alter its molecular weight distribution and alter its usefulness for separating chemically modified lignin. Various chemical modifications of lignin have been used successfully in carbon fiber development. These methods include acetylation and methacrylation, and the researchers developed separation strategies to produce carbon fiber precursors. Both melt- and solution spinnable lignin-based fibers were produced. Thermo-oxidation and UV curing were successful stabilization methods for these fibers. Carbon fibers were produced from a few types of lignin and chemically modified lignin. The resulting mechanical properties were relatively poor, but there are obvious steps that need to be taken to improve these properties. Electrical conductivity of these fibers ranged from moderately conductive, similar to that of polyacrylonitrile (PAN)-based fibers, to highly conductive, indicating a significant graphitic content. The highest mechanical properties were achieved for lignin-based carbon fibers by stretching the fibers during processing resulting in modulus of 35 gigapascals (GPa), strength of 1 GPa, and elongation to failure of 3%, significantly exceeding the state of the art in ligninbased fibers. However, life cycle analysis of the lignin-based carbon fibers is not favorable due to the recent reduction in cost of PAN-based carbon fibers.

Numerous biobased resins were developed, including epoxies, vinyl esters (VEs), unsaturated polyesters (UPEs), and polyurethanes, many with excellent properties that can be used in high-performance polymers, composites, and coatings applications. Lignin-based cross-linkers have been prepared that have performance similar to that of bisphenol A cross-linkers while having significantly reduced toxicity. The researchers developed isosorbide methacrylate with the highest ever transition temperature of greater than 250 degrees celcius (°C) for a VE system. This project produced numerous reactive diluents and viscosity reducers for VE and UPE technology based on fatty acids, lignin, and isosorbide that will maintain or increase polymer performance while reducing hazardous emissions. The isosorbide-based viscosity reducer and lignin-based reactive diluents are promising for scale-up and commercialization. The researchers identified biobased lignin-derived resins with high bioatomic efficiency that are good for the lower end of high-performance composites and most coatings applications. Furan epoxies are very promising with good thermomechanical properties and very high toughness, making them excellent candidates for composites and coatings applications. The researchers developed higher performing UPE resins using isosorbide as an additive or component, but the feasibility for scale-up is low due to the long reaction times required. Life cycle analysis of these resins shows that many of these renewable technologies have lower or similar cost relative to commercial resin technologies and thus have high potential for commercial transition.

This work benefited the Department of Defense by developing fibers and resins to reduce reliance on petroleum-derived materials, developing materials with properties unachievable using petroleum, offering solutions to reducing hazardous emissions, and reducing the toxicity of high-performance polymers and fibers.