Manufacturing of Carbon Fiber Precursors Containing High Loadings of Plant-Based Lignin

Background

In 2019, Forbes reported the European Union’s (EUs) proposal to regulate a 37.5% drop in carbon fuel emissions by passenger vehicles in 2030! 92 miles per US gallon of fuel was set as a 2030 guideline for automotive manufactures. This technology addresses the sustainable production of carbon fiber that will serve as frames for cars and passenger vans. The use of carbon fiber is expected to allow for vehicle compliance to the aggressive, fuel emission standards of EU legislative bodies and the United State Environmental Protection Agency (EPA). Carbon fiber is 4.6 times lighter than steel- thus making carbon fiber an ideal construction material for lighter, more fuel-efficient passenger vehicles. The CF industry will continue to grow in terms of revenues (at 3.1% until 2024 from $2.2 billion), market entry, and labor.

This technology addresses the sustainable and cost-effective manufacturing of carbon fiber through the use of biorenewable lignin. The technology describes (1) the incorporation of lignin at more than 30% of the acrylic-based fiber and (2) ‘green’ liquid formulations for the environmental sound manufacturing of precursor fiber Lignin is a sustainable biofuel and byproduct of the pulping and paper industries. It is the world’s most naturally abundant source of aromatic polymer, and has a high char yield for carbonization. Researchers have sought to use lignin as an inexpensive alternative to synthetic polymers for the manufacture of carbon fiber. To reduce the cost of carbon fiber from $10 to $5/lb for the automotive industry, lignin- from low-cost wood and non-wood extraction technologies- should compose 30-50% of the acrylic fiber. To date, 51% of the cost for carbon fiber manufacturing comes from the precursor fiber.

The team at NC State University utilizes fiber spinning approaches (like the process for high strength, high modulus yarns of ultra-high molecular weight polyethylene (UHMWPE) for soft armor protection) towards the manufacture of lignin-based fibers. NC State researchers have mapped formulations that enable the retention of lignin throughout the steps for fiber processing. This mapping strategy allows the team to formulate industrially responsible coagulation baths for fibers processing.

Biobased technologies are typically characterized has lacking the mechanical performance that is associated with synthetics. Nevertheless, NC State have formulated lignin/acrylic polymer spinning solutions to yield notable mechanical performance upon the addition of a novel sugar derivative at less than 5% of the overall fiber weight!

Technology Overview

Currently, 50 million tons of lignin waste is released by the pulp and paper industry each year (Bruijinicnx et al. Green Chemistry, 2015). As the cement within plants, lignin is naturally rigid and makes woody plants inedible for human consumption. The fundamental goal of this technology was to reconstruct the plant-cell wall synthetically. Herein, the flexible acrylic polymer templates lignin. In nature, this type of templating is among plant structures of fibrillar cellulose surrounded by lignin. Researchers have attempted to convert lignin into low-cost precursors for carbon fiber by melt extrusion and solution spinning; however, these attempts were unsuccessful. Lignin biopolymers are more irregular in their structure than synthetic polymers (Figure 1), which makes it difficult to align them during fiber spinning. Further, the lignin-based fibers were often weak and brittle even when they were spun with synthetic resin.

This technology employs fiber spinning technologies that are known to result in highly aligned and crystalline fibers. Moreover, the (patented) technologies for keeping high loadings of lignin within the spun fibers are crucial to the development of flexible fibers having desirable mechanical properties. At 2:1 lignin to acrylic the modulus of fibers increased by 66% from that of neat acrylic fiber; both fibers had linear density values between 12-13 decitex. From those technologies (see Figure 2), the researchers synthetically produced plant microstructures within spun fibers of lignin at 33% acrylic (i.e. polyacrylonitrile). The fracture tip of lignin/PAN fibers revealed submicron fibrils of PAN surrounded by lignin cement- thus providing evidence that this is a promising approach for further exploration at TRL 4. So far, the technology was demonstrated at TRL 3, wherein single and multifilament fibers of 100 grams or less have been made.

The technology readiness level is a ~4, where product development at pilot scales of production (in other words several pounds of fibers) are required. Prototypes of single-filament fibers were demonstrated to exhibit noteworthy values of mechanical strength and stiffness at high loadings of lignin relative to acrylic polymer.

Prototypes of multi-filament work have been fabricated onsite and an outside laboratory for the pilot-scale spinning of solution spun fibers. Nevertheless, multi-filament yarn development is needed, where fibers consistently hit target values of mechanical performance at targeted values of fiber diameter. Performance at fine diameters is needed for follow-up trials for stabilization and carbonization by a commercial partner or national laboratory. Further, stabilization/carbonization trials typically require 500+ filaments to determine its feasibility for commercial processing.

Benefits

Lignin is a low-cost and biorenewable resource from woody, nonwoody plants, and agricultural crop residues. It is often removed from the cellulose pulp that is found in consumer and industrial products.

This technology maintains high lignin content among blended fibers (between 30-50%) of the acrylic polymer, unlike previous technologies where lignin was shown to leach from fibers during processing. This weight percentage of lignin is need to ensure the resulting precursor to carbon fiber can meet the targeted goals of $1 to $3/lb.

This process for lignin-acrylic fiber spinning utilizes ‘green’ liquid formulations for lignin retention during processing. This enables the tuning of process to meeting environmental and occupational standards for health and safety.

Notable performance properties are achieved among the lignin-acrylic fibers, especially in terms of fiber modulus, due to the ability to form an internal structure of lignin-mortar surrounding fibrils of highly organized acrylic polymer.

Applications

The technology describes lignin-acrylic fibers that can serve as precursors to low-cost carbon fiber; for which the carbon fiber can be used to build passenger vehicles, as a building material, and/or in recreational equipment.

Lignin-acrylic fibers also have use in outdoors furniture and geotextiles, due to the ability of lignin to block UV-radiation and the durability of acrylics for furniture use.

Opportunity

NC State University would like to work with industry 1) through investment, 2) as partner(s) for technology development, and 3) through licensing.

Investment would support work at the pilot scale within labs at NC State. The goal is to produce up to 500 grams of fiber using NC State’s own personnel, who will focus on the development of multi-filament lignin-acrylic yarns of 50+ filaments in the tow. The lignin-acrylic tows show possess fine-diameters and mechanical properties becoming those of technical fibers.

The technology focuses on the development of carbon fiber precursors. A developmental partner would aid in the conversion of lignin-acrylic precursors into actual carbon fiber at developmental scales of tow (at 100+ filaments and several kilograms of fiber).

The goal is to work with industry to validate the technology and to enable its transfer for production on increasing larger scales. Through developmental partnership and licensing, the technology is expected to undergo commercialization.