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Fiber-reinforced Polymer composites, a new material for a new era

20 Jun 2022 10:00 AM | Anonymous member (Administrator)

Author: This article is part of an expert piece written by Hicham Ghoussein, CEO and Founder at Endeavor Composites, Inc. Ghossein earned his Ph.D. in 2018 from The University of Tennessee, Knoxville. He served as an entrepreneurial fellow at the Innovation Crossroads Program at Oak Ridge National Laboratory, where he worked on scaling up and commercializing a carbon fiber nonwoven technology that allowed the production of turnkey preform solutions for the advanced composites manufacturing. He was awarded The Heart of Smoke and Fire challenge coin by Scot Forge Space Program in 2016, The Eisenhower School of Defense - Advanced Manufacturing challenge coin and The Secretary of The Army - Civilian Aide challenge coin in 2018. 

The advancement of artificial intelligence, computational capacity, and improved sources of power for transportation have created a paradigm similar to the discovery of the steam engine that led to the first industrial revolution. In the automotive industry, this evolution can be witnessed in the electrification of fleets and the push to expand vehicle range. Similar efforts can be seen in other transportation industries. Even the aerospace community is buzzing with efforts to develop eVTOL (Electric Vertical Take-off and Landing) as a potential future model of urban transportation. 

Many will attribute these advancements to the enhancement of output for electrical batteries and electric engines. However, an evolution is also happening behind the scenes with the building blocks of these vehicles. Original Equipment Manufacturer (OEMs) realized that reducing the weight of their products would allow them to extend the range of their vehicles given the same amount of energy. To do so, many are experimenting and investigating the usage of fiber-reinforced polymer (FRP) composite materials in place of aluminum and metal alloys. With its high strength to weight ratio, FRP has been the material of choice for high-end vehicles and jet fighters. Today, innovation efforts are underway to manufacture it with cost efficiency that meets the needs of everyday vehicles. A lighter vehicle yields lower emissions, hence making the vehicle more environmentally friendly.

A composite, as the name suggests, is the combination of two or more materials that carries the properties and benefits of its components. These components are defined as reinforcement and matrix. The reinforcement is the element that provides much of the strength to the material. The matrix encapsulates the reinforcement and provides the first contact with the surrounding elements. A perfect example, even though it is not a polymeric matrix, is reinforced concrete, where the concrete acts as matrix and the metallic rebars act as reinforcement. The interest in FRP stems from the lightweight nature of the materials that rivals that of aluminum and metal Alloys. Additionally, most FRP are recyclable or at least have reinforcing fibers that can be reclaimed. The flow chart in Figure 1 details the categories of FRP based on the type of reinforcement. These categories are defined by the ratio of fiber length to its diameter, known as the Aspect ratio. 

Figure 1: reinforcement categories flow chart

The Reinforcements: 

Continuous reinforcements are first to come to mind when talking about composites. They have the iconic weave pattern seen in Figure 2; they add elegance and reflect order in the design on top of their high performance. Continuous reinforcements are used in areas of structural load and as encapsulating shells. They are limited in manufacturing methods, as they have proven challenging when processing complex shape parts with deep draw or angles. The 

Figure 2: Typical weave pattern for carbon fiber reinforcement. 

Next comes the discontinuous reinforcement, mainly present in injection molding, extrusion compression molding and 3D printing techniques. A discontinuous reinforcement is when the fiber length is shorter than the part length. Recently efforts to produce non-woven fabrics with discontinuous reinforcements have gained traction in the market. Discontinuous RFPs are desired for complex shape parts, due to their high formability and ease of production. The fibers can be man-made such as carbon, glass, aramid, or basalt. But they can also be natural, such as bamboo, flax, hemp, jute, coir, or banana. Natural fibers are gaining traction recently due to their inherent sustainability and affordability. These different materials all have their niche. Man-made fibers remain dominant in high performance applications. Carbon fibers add stiffness and rigidity to the composite, while glass improves impact and insulation. Aramids are great in ballistic protection in their woven format, and in skid resistance in discontinuous format. 

The reinforcement role, like the name indicates, is to carry the load applied to the material and resist deformation. In a well fabricated part with perfect bonding between matrix and reinforcement, the latter will be first to fail, this is why the choice of reinforcement type and quantity is highly dependent on the application itself. Figure 3 shows the connection between the mechanical properties and the length of the reinforcing fibers. 

Figure 3: Relative property of a composite in relation to the fiber length in the reinforcement. The graph shows an increase in mechanical properties of the composite in relation to the fiber length, while processability decreases in relation to fiber length. The modulus of a material indicates its stiffness and ability to maintain its shape under constant load. Strength indicates the highest load a material can withstand before deformation and Impact is the maximum resistance to a blunt force damage that the material can take. Processability indicates the ease of use of the material in different manufacturing techniques. 

Photo credit: Nguyen B.N. et al., long fiber thermoplastic injection molded composites: From process modeling to property prediction, SPE Automotive Composites Conference and Exposition, Troy, MI, CD-ROM Proceedings, 2005.

The Matrices:

Polymeric matrices fall under two categories, Thermoset and Thermoplastic. The thermosets are crosslinked polymers, and often require a catalyst to initiate the crosslinking reaction. The chemical reaction is directly dependent on temperature, and once crosslinking occurs the matrix is set and non-reversible. Its most famous products are the epoxy groups. High performance parts are made from a prepreg preform, where the thermoset is already introduced to the fabric in a tacky state and as heat is applied the reaction accelerates to finalize the crosslinking process in a mold. This technique is mainly used in the aerospace industry and yields parts with up to 50% fibers by volume. Other methods of fabrication include resin transfer molding (RTM) and Vacuum Assisted Resin Transfer Molding (VARTM). These techniques are mostly used in the automotive and marine industries thanks to their low cost and ease of production. Finally, sheet molding compound (SMC) is one of the most processed materials in transportation due to its ability to form a part in less than a minute. SMC is made from a putty of discontinuous fibers and resin, that gets squeezed in a heated metallic mold. 

Thermoplastics become pliable with the application of heat. Hence, they can be softened and formed into a desired shape. Their polymeric chains do not form a crosslinking bond, allowing for fluidity once brought to a temperature above their melting point. Thermoplastics can be made from commodity plastics, like polypropylene (PP), polyethylene (PE), or polyvinyl chloride (PVC). They are produced in high volumes for consumer applications where high mechanical performance is not required. Another category of thermoplastics is engineering plastics that are designed to withstand different mechanical and environmental conditions. These engineered plastics can be found in high end applications such the eVTOL airplanes. 

The Market:

The composites market is expected to grow at a compound annual growth rate of 6.6% from 2019 to 2028, going from $89.04 billion to $144.5 billion. This is due to the increase in demand for performance materials in various industries such as automotive and transportation; wind energy; and aerospace and defense. The market is made up of a few key players holding most of the market share, yet the industry is seeing a rise in the number of new startups and companies that are bringing major manufacturing innovation and addressing supply chain and environmental challenges. 

Chart, bar chart Description automatically generated
Figure 3: photo source (Statista) 

For years, the supply chain lacked visibility and its organization entropy built up to cultivate a disaster spurred by the pandemic. Shipment delays and reliance on international supplies proved challenging. Today, the industry is evaluating new suppliers, digitizing its supply chain and relying more on recycled materials when possible to reduce and reuse, limiting its dependencies on foreign supplies. An assessment of the manufacturing ecosystems will lead to producing raw materials closer to the source in every continent, triggering the establishment of new refineries as well as growth of new crops for natural fibers and bio resins supplies. The shipping industry will have to pivot and prepare for a change in demand as a consequence of the operational changes large corporations are undertaking as they adopt these new materials. 

As a closing note, the world is pivoting towards a new era: The era of advanced composites. This relatively new class of material is booming now, and will reshape the industry. The cementing fact of this growth is the speed of adoption following the pandemic. Corporations’ demand for lighter, faster, and stronger materials redefined their supply chain structure and set them back to work towards a brighter future.

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