Composite Material
Composite materials are composed of two or more distinct substances combined to create a new material with enhanced properties. Each component retains its individual characteristics, contributing to the composite's overall performance, which can differ significantly from the properties of the individual materials. Composites can be natural, like wood and bone, or man-made, such as concrete and fiberglass. Historically, humans have utilized composites for various applications, including adobe for construction, which combines mud and straw, and reinforced concrete for building structures.
In contemporary contexts, fiber-reinforced plastics (FRPs) are prevalent, incorporating materials like carbon or fiberglass with plastic resins to achieve lightweight and strong structures. These materials are commonly used in industries such as aerospace, automotive, and construction. Despite their advantages, such as low maintenance and design flexibility, the high cost of raw materials and environmental concerns related to disposal have become significant challenges. Efforts are being made to enhance the sustainability of composites through recycling and the use of biodegradable materials, representing an important area of ongoing research and development.
Composite Material
Composite materials are combinations of two or more substances. These materials are combined, but not blended or dissolved—each substance is clearly observable in the composite, although not necessarily visible to the naked eye. Together, these materials have characteristics that are different from those they have individually. Composite materials, often called simply composites, may be natural or man-made.
Natural Composites
Parts of many organisms are composites. For example, wood is composed of cellulose fibers bound together by lignin. Separately, lignin and cellulose are weak, but together they form a strong substance. Bones are made of hydroxyapatite, which is mostly calcium phosphate and is hard but easily broken, and collagen, a soft and elastic protein. Collagen alone is not very strong—it is a major component of hair and fingernails, for example—but combined with hydroxyapatite, it helps bones perform some necessary functions, such as supporting the body and absorbing impact.
Man-made Composites
Man-made composite materials have been created and used for centuries. Most man-made composites contain two materials: a binder, or matrix, and a reinforcement material, such as a fiber.
In ancient times, people learned to make and use composite materials to build shelters. One composite material, adobe, is a mixture of mud and straw, and has been widely used in construction by many peoples, including ancient Egyptian, Arabic, African, North and South American, and English builders. Adobe, which is known as cob in England, may contain a variety of materials, such as sticks or animal dung, depending on the location and materials available. The mud—often clay and sand—binds together the organic material (structural fiber), which provides the strength and structure. Adobe is commonly used in regions where lumber is unavailable and is generally a low-cost building option.
Many adobe structures have been in continuous use for thousands of years. These composite materials are still used around the world for building homes and other structures, especially in hot, dry regions.
Concrete is another composite developed in ancient times and still widely used today. It contains cement, sand, and aggregate such as gravel. Modern builders use reinforced concrete, which contains metal rods or wires to improve tensile, or bending, strength. Another common modern building material is plywood. This composite material is made of sheets of wood veneer and glue.
Modern Composites
Modern composites usually combine a structural fiber and a plastic. These composites are often called fiber-reinforced plastics, or FRPs. The fibers may include aramid fiber, basalt fiber, boron fiber, carbon fiber, fiberglass, or natural fibers such as flax, hemp, or wood. Plastic resins include epoxy, polyester, polypropylene, polyurethane, and vinyl ester. FRPs are commonly used in aircraft, automotive components, body armor, building materials, sporting equipment, and watercraft.
Fiberglass, the first modern composite, was developed during the late 1930s. Fiberglass contains threads of glass that are often woven together into a cloth and then bonded with a plastic or resin. It is commonly used in automobile bodies and surfboards.
Many advanced composites developed since then rely on carbon fibers instead of glass. They are more expensive to create than fiberglass, but the materials are lighter and stronger. Carbon composites are often found in golf clubs and other sporting equipment. Disc brake pads on cars contain a matrix of carbon fibers and reinforcement of silica, resulting in a composite that can take high heat without wearing out quickly. Researchers are also experimenting with using carbon nanotubes. These are extremely expensive, but result in lighter and stronger composites.
FRP development is flexible and can be adapted to many specific uses. Designers and engineers may alter the proportions of resins and fiber reinforcement; change the orientation of the fiber—for example, by weaving it, add fillers, or change the fabrication process, such as the method of molding. FRPs are often used in construction for bathtubs, countertops, hot tubs, sinks, and swimming pools. Power is generated using FRP wind turbine blades, transmission towers, and circuit boards. Many pipes, tanks, and vessels are made using these composites.
In 2015 researchers from Deep Springs Technology and the New York University Polytechnic School of Engineering announced the development of a new metal matrix composite. The magnesium alloy matrix composite—which is light enough to float on water—is a syntactic foam. Syntactic foam composites are made of ceramic, metal, or polymer matrixes that contain hollow particles. The magnesium alloy matrix is reinforced with hollow pieces of silicone carbide. Researchers believe it could be used to manufacture boat hulls. This would allow a vessel to stay afloat even if it was damaged. Magnesium-based metal matrix composites also hold potential for biomedical applications because magnesium is a common element in the body and therefore a composite containing the metal is likely to be highly compatible.
Some researchers are exploring ways of using biodegradable materials to make composites. Many waste products from crops, for example, are sustainable resources that could be used as reinforcement materials. Many of these composites would eventually break down, limiting their use. However, developing composites using biodegradable material would reduce manufacturing's reliance on products made using petroleum-based plastics. Several products, including a surfboard and a utensil, have been developed. The surfboard uses harakeke fiber instead of fiberglass and has proven to hold up under wet conditions. The biodegradable utensil, which is designed to scoop the flesh out of the skin of a kiwifruit, is made of a bioplastic using kiwifruit waste fiber.
Benefits and Drawbacks of Composites
Modern composites such as FRPs have many advantages over traditional building materials, such as metal, stone, and wood. For example, FRPs are strong but very light. Vehicles such as cars and boats are more fuel efficient when they are made with lightweight materials. Sporting equipment such as tennis rackets and golf clubs are more effective when they are lighter because less force is required to use them.
Composites are also nonconductive and noncorrosive. They are flexible, which allows them to withstand more force and strain. Composites are low maintenance and have a long life. They are also easy to mold and form, allowing for more design options.
Although composites have many benefits, the main drawback is the cost. The raw materials used to make them are often very expensive. Furthermore, disposal of goods that reach their end of life (EoL) is becoming increasingly difficult. Germany banned composite disposal in landfills in 2009, and in 2019 many of the early composite wind turbines began reaching EoL and decommissioning. Recycling and a focus on sustainable materials in composites has thus become more important to many industries.
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