Did you know that the carbon fibers we see today, which are increasingly being utilized as a reinforcing material due to their high strength and high modulus, have seen little increase in tensile and compressive strength since their introduction nearly 30 years ago?

It's a surprising fact, considering the wide range of applications these fibers have in various industries, from aerospace to automotive and beyond.

This unexpected plateau in performance is a result of the defect-limiting nature of carbon fiber strength. It's a challenge that has been persistently present in the industry, but it's also a challenge that has sparked a wave of innovation and research.

In this blog, we'll take you on a journey through the world of carbon fibers. We'll explore their manufacturing process, the impressive properties they possess, the challenges the industry faces, and the innovative solutions being developed to overcome these challenges.

We'll delve into the next generation of carbon fibers, alternative precursors like lignin, textile-grade PAN, and polyethylene, and advanced carbon fibers and their composites. We'll also look at how these innovations are pushing the boundaries of what's possible with carbon fibers, breaking through the mechanical property ceiling of 7 GPa of commercially available carbon fibers.

So, buckle up and get ready for a deep dive into the fascinating world of carbon fibers. It's a journey filled with unexpected twists and turns, challenges and solutions, and a glimpse into the future of a material that's set to revolutionize various industries.

TL;DR

• Carbon fibers are high-strength, high-modulus materials increasingly used as reinforcing materials in various industries.
• The manufacturing process of carbon fibers significantly affects their structure and properties.
• Carbon fibers possess impressive tensile properties, compressive properties, fracture toughness, and thermal and electrical properties.
• The carbon fiber industry faces several challenges, including limited increase in strength, complexity of the manufacturing process, high cost of precursors, and the mechanical property ceiling of commercially available carbon fibers.
• The industry is addressing these challenges through defect minimization, optimization of the manufacturing process, use of alternative precursors, and development of advanced carbon fibers.
• The next generation of carbon fibers is being developed from alternative precursors such as lignin, textile-grade PAN, and polyethylene.
• Advanced carbon fibers and their composites are being developed, including carbon fibers utilizing carbon nanofillers, low-density and hollow carbon fibers, and carbon nanotube-based hierarchical carbon fiber composites.
• The future of carbon fibers is bright, with continued innovation expected in the industry.

Section 1: The Carbon Fiber Journey

Understanding Carbon Fibers

Carbon fibers are increasingly being utilized as a reinforcing material due to their high strength and high modulus, which are imparted into the properties of the final composite. But how are these fibers made? Let's take a journey through the carbon fiber manufacturing process.

From Polymerization to Carbonization

The journey of a carbon fiber starts with the polymerization of Polyacrylonitrile (PAN). This is the first step in creating the base material for the carbon fiber.

• Polymerization: The process begins with the formation of a polymer from monomers. This polymer, usually Polyacrylonitrile (PAN), forms the base material for the carbon fiber.
• Fiber Spinning: The polymer is then spun into fibers. This process aligns the polymer chains in the direction of the fiber, which is crucial for the strength and stiffness of the final carbon fiber.
• Stabilization: The fibers are then stabilized by heating them in air at temperatures between 200-300°C. This step involves oxidation and cyclization reactions that stabilize the fiber structure and prepare it for the next step.
• Carbonization: The stabilized fibers are then carbonized in an inert environment at temperatures between 1000-1700°C. During this process, non-carbon elements are removed, and the fiber is converted into pure carbon. An additional high-temperature carbonization step can be performed, heating the fiber up to 3000°C, to further increase the carbon content and the modulus of the fiber.

The Impact of the Manufacturing Process

The manufacturing process significantly affects the structure and properties of the carbon fiber. The alignment of the polymer chains during fiber spinning, the stabilization reactions, and the carbonization process all contribute to the high strength, high modulus, and high carbon content of the fiber.

For example, as the heat treatment temperature is increased during carbonization, the carbon fiber modulus increases, and the carbon content continues to increase.

It's also important to note that the entire manufacturing process needs to be optimized to achieve the desired properties in the final carbon fiber. This includes the initial polymerization stage and continues throughout the entire process. The manufacturing process also influences the microstructure of the carbon fiber, which in turn affects its mechanical properties.

Section 2: The Strength of Carbon Fibers

Unleashing the Power: Tensile Properties

Carbon fibers are renowned for their tensile strength, which is primarily limited by the presence of defect structures. These defects can reside on the fiber surface or within the internal fiber structure. The tensile strength of carbon fibers is a defect-limiting property, with ultimate strength being determined by the size of the largest defect. This is why carbon fibers are often used in applications where high strength-to-weight ratios are crucial.

Standing Firm: Compressive Properties

The compressive strength of carbon fibers is equally impressive. It's the performance of the carbon fiber composite that determines its applicability. A higher composite compressive strength is achieved as the anisotropy parameter is reduced. Maximizing the longitudinal shear modulus of the fiber, while maintaining a high fiber axial modulus, is required to improve carbon fiber composite compressive strength.

Toughness Personified: Fracture Toughness

Fracture toughness is a critical material property used in the design of structures and components. It measures the amount of stress required to propagate a preexisting flaw. A direct correlation exists between the reinforcing carbon fiber fracture toughness and the fracture resistance of the carbon fiber composite.

More Than Just Strength: Thermal and Electrical Properties

Carbon fibers are not just about strength and toughness. They also exhibit excellent thermal and electrical properties. The thermal conductivity of carbon fibers ranges from about 5 W/m/K to 800 W/m/K, depending on the type of carbon fiber. Similarly, the electrical conductivity of carbon fibers typically falls in the range of 104 - 105 S/m, generally scaling with tensile modulus. The improved orientation enables greater electron mobility along the fiber axis, resulting in improved electrical conductivity.

Section 3: Challenges in the Carbon Fiber Industry

Despite the impressive properties and wide-ranging applications of carbon fibers, the industry faces several challenges. Let's delve into these issues and understand their real-world implications.

The Strength Ceiling

High-performance aerospace-grade carbon fibers such as IM7 and T800H have been in production for nearly 30 years. However, there has been little increase in the carbon fiber tensile and compressive strength since their introduction. This stagnation is due to the defect-limiting nature of the carbon fiber strength. In other words, the strength of carbon fibers is limited by the size of the largest defect in the fiber structure.

The Complexity of Manufacturing

The manufacturing process of carbon fibers is complex and requires careful control at each stage. The optimization of the carbon fiber microstructure begins during the polymerization stage and persists throughout the entire manufacturing process. Any misstep can affect the properties of the final product.

The High Cost of Precursors

The carbon fiber market is currently dominated by PAN-based carbon fibers, which comprise about 96% of the total market. However, PAN and pitch-based carbon fibers are expensive to produce. This high cost is a significant barrier to the wider adoption of carbon fiber technologies.

Breaking Through the Mechanical Property Ceiling

The industry is also striving to break through the mechanical property ceiling of 7 GPa of commercially available carbon fibers. This involves efforts to minimize defects, tailor the carbon fiber microstructure, and increase the carbon fiber interfacial shear strength.

Real-World Implications

These challenges have real-world implications. The limited increase in strength means that the performance of carbon fiber composites in high-stress applications has plateaued. The complexity of the manufacturing process increases the cost and reduces the scalability of carbon fiber production. The high cost of precursors makes carbon fibers an expensive material, limiting its use in cost-sensitive applications. Finally, the mechanical property ceiling restricts the performance of carbon fiber composites in extreme applications.

Section 4: Innovations in Carbon Fiber Production

Introduction to the Next Generation of Carbon Fibers and Alternative Precursors

The next generation of carbon fibers is being developed from alternative precursors such as lignin, textile-grade PAN, and polyethylene. These materials are being explored with the aim of achieving modest carbon fiber mechanical properties at a reduced price compared to commercially available PAN and pitch-based carbon fibers. The research suggests that PAN-based carbon fiber prepared from a textile-grade PAN will soon be available in the commercial marketplace

Advanced Carbon Fibers and Their Composites

Carbon Fibers Utilizing Carbon Nanofillers

Carbon fibers utilizing carbon nanofillers are part of the next generation of carbon fibers. These fibers are being developed to break through the mechanical property ceiling of 7 GPa of commercially available carbon fibers. Efforts are being made to minimize defects, tailor the carbon fiber microstructure in PAN/CNT fibers, and increase the carbon fiber interfacial shear strength through CNT grafting techniques

Low-Density and Hollow Carbon Fibers

Low-density and hollow carbon fibers are being processed from PAN, pitch, and other precursor fibers. These fibers are advantageous in reducing the final density of the carbon fiber composite. For example, if the carbon fiber density is reduced to 0.80 g/cm3, the final composite density would be equal to 0.96 g/cm3 for the same volume fraction of reinforcing fiber. Hollow carbon fibers have been shown to possess a tensile strength of 1.6 GPa, tensile modulus of 244 GPa, at a density of 1.20 g/cm3

Carbon Nanotube-Based Hierarchical Carbon Fiber Composites

Carbon nanotube-based hierarchical carbon fiber composites are being developed to provide carbon fibers with additional functionality that is otherwise non-existent in traditional pitch and PAN-based carbon fibers. These composites are capable of self-sensing damage, acting as resistance heating elements, and consolidating parts within the aircraft, automobile, or other structures. Graphene fibers, which were first developed in 2011, are commonly manufactured through a wet-spinning technique that takes advantage of the lyotropic liquid crystalline behavior of graphene and graphene oxide.

Section 5: Addressing the Challenges

The carbon fiber industry is actively addressing the challenges it faces, with a focus on defect minimization, optimization of the manufacturing process, use of alternative precursors, and development of advanced carbon fibers.

Defect Minimization

The defect-limiting nature of carbon fiber strength is a significant challenge. However, efforts are being made to minimize defects, particularly in small diameter fibers and those with higher PAN molecular weight. Advanced spinning techniques are also being explored to reduce defects and increase the performance of the final carbon fiber composite.

Optimization of the Manufacturing Process

The manufacturing process of carbon fibers is complex and requires careful control at each stage. The industry is working on optimizing the carbon fiber microstructure, a process that begins during the polymerization stage and persists throughout the entire fiber manufacturing process. This optimization is crucial for the continued improvement in carbon fiber mechanical properties.

Use of Alternative Precursors

The high cost of PAN and pitch-based carbon fibers is a significant barrier to the wider adoption of carbon fiber technologies. To address this, the industry is exploring alternative precursor materials such as lignin, lignin/polymer blend, textile-grade PAN, and polyethylene. These materials aim to achieve modest carbon fiber mechanical properties at a reduced price compared to commercially available PAN and pitch-based carbon fibers.

Development of Advanced Carbon Fibers

The industry is also developing advanced carbon fibers and their composites to break through the mechanical property ceiling of 7 GPa of commercially available carbon fibers. This includes carbon fibers utilizing carbon nanofillers, low-density and hollow carbon fibers, and carbon nanotube-based hierarchical carbon fiber composites. These advanced fibers are expected to increase the performance of the final carbon fiber composite.

Real-World Examples

In the real world, these solutions are being implemented by major manufacturers such as Toray, Toho Tenax (Teijin), Mitsubishi Rayon Company, Hexcel, Cytec, Zoltek, and SGL. These companies are investing in research and development to overcome the challenges faced by the carbon fiber industry and push the boundaries of what's possible with carbon fibers.

Conclusion

• The carbon fiber industry has come a long way since the investigation of carbon fibers and filaments began in 1958. Today, carbon fibers are a critical component in various industries due to their high strength, high modulus, and the unique properties they impart to the final composite.
• However, the industry faces several challenges, including the defect-limiting nature of carbon fiber strength, the complexity of the manufacturing process, the high cost of precursors, and the mechanical property ceiling of commercially available carbon fibers. These challenges have led to a plateau in the performance of carbon fiber composites in high-stress applications and have limited the wider adoption of carbon fiber technologies.
• Despite these challenges, the industry is making significant strides in addressing these issues. Efforts are being made to minimize defects, optimize the manufacturing process, use alternative precursors, and develop advanced carbon fibers. These efforts are paving the way for the next generation of carbon fibers, which promise to push the boundaries of what's possible with carbon fibers.
• Looking toward the future, we can expect to see continued innovation in the carbon fiber industry. The development of carbon fibers from alternative precursors, the use of advanced spinning techniques, and the creation of hierarchical carbon fiber composites are just a few of the exciting developments on the horizon. These innovations have the potential to revolutionize various industries, from aerospace to automotive, and beyond.

As we continue to push the boundaries of what's possible with carbon fibers, one thing is clear: the future of carbon fibers is bright, and the best is yet to come.

Call to Action

As we've seen, the carbon fiber industry is a dynamic and rapidly evolving field. The innovations and advancements we've discussed today are just the tip of the iceberg. There's so much more to explore and understand.

• We encourage you to stay informed about the latest developments in the carbon fiber industry. Whether it's the introduction of new precursors, the development of advanced carbon fibers, or breakthroughs in manufacturing processes, each new development brings us one step closer to realizing the full potential of carbon fibers.
• We also invite you to share your thoughts or experiences related to carbon fibers. Whether you're an industry professional, a researcher, or simply someone with a keen interest in the field, your insights and experiences can add value to the conversation.
• Remember, the future of carbon fibers is not just in the hands of researchers and industry professionals. It's also in the hands of informed individuals like you, who can drive demand for more sustainable, efficient, and high-performing materials.

So let's continue the conversation. Share this blog with your colleagues, leave a comment with your thoughts, or get in touch with us if you have any questions or insights you'd like to share. Together, we can shape the future of the carbon fiber industry.

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