Composite materials and laminates
by Chris Woodford. Last updated: June 19, 2016.
Unlike many natural and artificial materials, which find applications by chance only after they have been discovered or invented, composites are often carefully designed with a particular application in mind. Originally developed as light and strong materials for the aerospace industry in the mid-20th century, they have now found their way into a wide range of products, from stealth bombers to Smart cars and from bridges to oil rigs. Laminates are composites in which layers of different materials are bonded together with adhesive, to give added strength, durability, or some other benefit.
Photo: Testing composite materials onboard Space Shuttle Mission STS-32, 1990. Picture courtesy NASA JSC Digital Image Collection.
What is a composite?
Composites are made by combining two or more natural or artificial materials to maximize their useful properties and minimize their weaknesses. One of the oldest and best-known composites, glass-fiber reinforced plastic (GRP), combines glass fibers (which are strong but brittle) with plastic (which is flexible) to make a composite material that is tough but not brittle. Composites are typically used in place of metals because they are equally strong but much lighter.
Most composites consist of fibers of one material tightly bound into another material called a matrix. The matrix binds the fibers together somewhat like an adhesive and makes them more resistant to external damage, whereas the fibers make the matrix stronger and stiffer and help it resist cracks and fractures. Fibers and matrix are usually (but not always) made from different types of materials. The fibers are typically glass, carbon, silicon carbide, or asbestos, while the matrix is usually plastic, metal, or a ceramic material (though materials such as concrete may also be used).
Photo: A simple model of a composite. I've used layers of sticky plastic fastener (Blu-Tack) as the matrix and matchsticks as the fibers, so this is (loosely speaking) a kind of polymer matrix composite. It would be easy to turn this into a science fair experiment: build yourself a large sample of composite like this and then compare its properties to those of the materials from which you've made it.
These three types of matrix produce three common types of composites:
- Polymer matrix composites (PMCs), of which GRP is the best-known example, use ceramic fibers in a plastic matrix.
- Metal-matrix composites (MMCs) typically use silicon carbide fibers embedded in a matrix made from an alloy of aluminum and magnesium, but other matrix materials such as titanium, copper, and iron are increasingly being used. Typical applications of MMCs include bicycles, golf clubs, and missile guidance systems; an MMC made from silicon-carbide fibers in a titanium matrix is currently being developed for use as the skin (fuselage material) of the US National Aerospace Plane.
- Ceramic-matrix composites (CMCs) are the third major type and examples include silicon carbide fibers fixed in a matrix made from a borosilicate glass. The ceramic matrix makes them particularly suitable for use in lightweight, high-temperature components, such as parts for airplane jet engines.
Photo: A typical PMC (polymer matrix composite). This brown powder, N-CAS (nanocomposite absorbent solvent), can remove poisonous arsenic from drinking water. It's made by embedding nanoparticles of metal oxide, which absorb the arsenic, in a polymer matrix. Picture courtesy of Idaho National Laboratory and US Department of Energy.
Objects made from glass-reinforced plastic, such as row boats, are frequently made by hand by cutting layers of composite from a continuous reel and sticking them into a mold with resin. The reel of composite used for this process is typically manufactured by drawing continuous lengths of the fiber material through a tank containing the resin that will act as the matrix. The coated fibers are then pressed onto a backing tape and formed into long sheets or continuous rolls called laminates. The tape is a composite whose fibers all run in the same direction. This makes it anisotropic, which means its physical properties vary in different directions. For example, it is stronger and stiffer in one direction than in others.
That may be a good thing if the material needs to be particularly strong along one axis but, for something like a boat hull or the fuselage or an airplane, the composite needs to be equally strong in every direction. The problem can be solved in one of two ways. First, when the hull or fuselage is being manufactured, the laminates can be pasted in so that each successive layer has its fibers pointing in a different direction. Alternatively, another kind of laminate can be used in which the fibers have been chopped up before they are stuck to the backing tape. Although this type of laminate is weaker in any one direction, it has the advantage of being equally strong in all directions.
The world of composites
Photo: The B2 stealth jet planes use clever design and composite materials to evade radar detection. Picture by Lance Cheung courtesy of US Air Force.
The strength and lightness of composites has made them particularly attractive for transportation. From the $42 million B-2 "Stealth" Bomber to low-cost, home-build glider kits, composites have made airplanes lighter, more economical, and more affordable and solved problems such as cracking and metal fatigue. In 2005, Boeing's 787 Dreamliner became the first commercial jet with wings and fuselage made from carbon-fiber composites (representing half the plane's total weight). Composites have also made possible new craft called tiltrotors—airplanes with a swiveling propeller at the end of each wing that can hover or take off vertically like a helicopter. Made from traditional materials such as aluminum, craft of this sort would have been simply too heavy to carry their cargo. Space rockets and satellites are also benefiting from composites, and in some unusual ways. Instead of having fuel tanks that must be jettisoned part way through a mission, the next generation of spacecraft may have tanks made from composites that can themselves be burned up as fuel.
Composites are not just useful in making things fly. Cars of the future must be safer, more economical, and more environmentally friendly, and composites could help achieve all three. Although composites such as GRP have been used in the manufacture of automobile parts since the 1950s, most cars are still made from steel. Engineers believe carefully designed composites could cut the weight of a typical steel car by as much as 40 percent, increasing fuel economy by as much as a quarter, yet maintaining body strength and crash-resistance. High-temperature ceramic-matrix composites are also making possible cleaner-burning, more fuel-efficient engines for both cars and trucks.
Photo: Smart cars are lightweight, composite cars. A steel safety shell holds together a variety of different parts and panels mostly made of plastics, including polypropylene (PP), polyvinyl butyral (PVB), polycarbonate (PC), and polyethylene terephthalate (PET). As on most cars, the "rubber" tires are actually composites made from rubber and numerous other materials, such as silica.
Composites are increasingly used in place of metals in machine tools. Apart from being lighter and stronger, they can offer better performance than metals at high temperatures and do not develop potentially dangerous weaknesses such as fractures and fatigue.
The strength and lightness of composites has made them equally popular in the design of sports equipment. Tennis rackets were once made from wood, though they were heavy, had a tendency to break, and often warped in extreme heat. Sometimes they were made of lightweight aluminum, though that lacks strength. Now they're typically made from graphite or graphite-based composites, and improved composites based on fiberglass, Kevlar (aramid fibers), titanium, and ceramics are now being developed. The composite fibers used in tennis rackets are angled specifically to reduce bending and twisting and to improve stability. Similar materials are used in other types of sports equipment. The latest composite hockey sticks made from aerospace-grade carbon fibers in a nylon polymer matrix are twice as tough and six times stronger than sticks made from ordinary composites. The same material is used to make wheels for mountain bicycles, but more advanced Kevlar composites are used to make the light, super-strong, solid wheels used in Olympic-style cycles.
Composites are so versatile that they are now being used even to build large-scale structures. Bridges made from composites are about a fifth as heavy as equivalent metal bridges and don't suffer problems such as corrosion and metal fatigue. (For more on the advantages of composite bridges, see this article on building more durable bridges from the Federal Highway Administration.) Back in 2003, Composites World estimated that there were approaching 350 bridges worldwide built from composite materials (compared to about a dozen in 1993). And bridges aren't the only large-scale application of composites. NASA scientists and industry engineers are currently developing composites that could be used in place of metals to construct offshore oil platforms and the pipelines that carry their oil to shore. Once developed, the technology is expected to yield extra-durable pipes that could be used for everyday applications such as sewage disposal.
Photo: Laminating a paper poster in a heat-treating machine. Photo by Michael Winter courtesy of US Navy.
Having read all about composites, you might have come to the conclusion that they're not the kind of materials ordinary people are likely to come across very often—but you'd be wrong! Have you ever fastened sticky-backed plastic onto a book to protect the cover? Or glued cardboard to paper to make it stronger? Perhaps you've coated a poster you've printed on your computer with plastic to make it weatherproof? If you've done any of these things, you've made yourself a laminate: a particular kind of composite material formed by bonding together layers of two or more other materials with adhesives.
What are laminates?
You'll find your dictionary defines a lamina as a thin sheet or plate of material: a layer, in other words. Fix two or more sheets of material together and you get a laminate, which is essentially just a material made up of layers. Since the layers are usually different materials, laminates are examples of composites, though the materials aren't integrated together in the same way as with other (matrix) composites. It's also important to remember that a laminate isn't simply several layers of materials: the materials have to be permanently bonded together with something like adhesive, so they behave as one material, not several. You can think of the adhesive (or adhesives—because there might be more than one) as an additional material in a laminate.
Why would you want to make a laminate? Generally, because a material you'd normally use by itself (say paper, wood, or glass) isn't strong or durable enough to survive by itself. Paper isn't waterproof, for example, while plastic is relatively hard to print on. But what if you print on the paper then coat it with plastic? The laminated composite material you've made gives you the best of both worlds.
What are laminates used for?
Laminates tend to be based on four main materials: wood, glass, fabric, and paper.
Laminated floors are very popular because they're really hard wearing. Unlike a traditional hard wood floor, a laminate floor is typically made of four layers. The top might be something like a thin layer of transparent plastic designed to resist stains and scratches. Underneath that, there's a thin layer of patterned wood (or even paper printed with a wood pattern) that gives the floor its attractive appearance. The next layer is the core: the bulk of the material, made from low-grade fiberboard. Finally, on the base, there's a thin layer of hard, moisture-proof board. Many low-cost furniture products that resemble solid wood are actually laminates made of lower-grade wood products (known as chipboard or particle board) with a thin coating of veneer, plastic, or even paper. The main drawback of laminated floors is that they can split apart and warp if they get wet.
Car windshields and bulletproof glass are actually very heavy laminates made from several layers of glass and plastic. The outer layers of glass are weatherproof and scratchproof, while the inner plastic layers provide strength and a small amount of flexibility to stop the glass from shattering. You can read more in our main article about bulletproof glass. As we've already seen, glass is also laminated with plastic to make composites such as GRP (Glass Reinforced Plastic).
Photo: Bulletproof glass is an energy-absorbing sandwich of glass and plastic. You can think of it as a composite (because it's a combination of materials) or a laminate (because it involves sheets of material bonded together). Picture courtesy of US Air Force.
Most shoes and many outdoor clothes are made from laminated materials. A typical raincoat usually has a waterproof membrane between a hard-wearing outer layer and a soft, comfortable inner layer. Sometimes the membrane is directly bonded to the inner and outer layers to make a very tough and durable piece of clothing; this is known as a 3-layer laminate. If the membrane is bonded to the outer fabric with no inner lining, that's called a 2.5 layer laminate. Waterproof clothes made this way tend to be more "breathable" than 3-layer laminates since moisture can escape more easily.
Photo: Looking inside a laminated 2.5-layer waterproof nylon jacket. It looks like a single layer of nylon, but it's actually two layers laminated together. You can tell that because the inner and outer surfaces look totally different. The ultra-waterproof black outer layer is made of rip-stop nylon. The inner white surface is an extra coating that improves air circulation and breathability.
Many people own small laminating machines that coat pieces of paper, card, or photographs in a thin but tough layer of durable plastic. You simply buy a packet of plastic "pouches", insert your paper item inside, and run this "sandwich" through the machine. It heats or glues the plastic and presses it firmly together to make a weatherproof and durable coating. Identification (ID) cards and credit cards are also laminated with clear plastic so they can survive several years of use.