How are composite materials made?

In an advanced society like ours we all depend on composite materials in some aspect of our lives. Fibreglass was developed in the late s and was the first modern composite. It's still the most common, making up about 65 per cent of all the composites produced today. It's used for boat hulls, surfboards, sporting goods, swimming pool linings, building panels and car bodies. You may well be using something made of fibreglass without knowing it.

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Composites made from metal oxides can also have specific electrical properties and are used to manufacture silicon chips that can be smaller and packed more densely into a computer. This improves the computer&#;s memory capacity and speed. Oxide composites are also used to create high temperature superconducting properties that are now used in electrical cables.

Nowadays many composites are made for functions other than simply improved strength or other mechanical properties. Many composites are tailored to be good conductors or insulators of heat or to have certain magnetic properties; properties that are very specific and specialised but also very important and useful. These composites are used in a huge range of electrical devices, including transistors, solar cells, sensors, detectors, diodes and lasers as well as to make anti-corrosive and anti-static surface coatings.

As for fibreglass, it&#;s made from plastic that has been reinforced by filaments or fibres of glass. These filaments can either be bundled together, and woven into a mat, or they are sometimes cut up into short lengths which are randomly oriented in the plastic matrix.

Composites have been made from a form of carbon called graphene combined with the metal copper, producing a material 500 times stronger than copper on its own. Similarly, a composite of graphene and nickel has a strength greater than 180 times of nickel.

Another well-known composite is concrete. Here aggregate (small stones or gravel) is bound together by cement. Concrete has good strength under compression, and it can be made stronger under tension by adding metal rods, wires, mesh or cables to the composite (so creating reinforced concrete).

Humans have been using composite materials for thousands of years. Take mud bricks for example. If you try to bend a cake of dried mud, it will break easily but it is strong if you try to squash, or compress it. A piece of straw, on the other hand, has a lot of strength when you try to stretch it but almost none when you crumple it up. When you combine mud and straw in a block, the properties of the two materials are also combined and you get a brick that is strong against both squeezing and tearing or bending. Put more technically, it has both good compressive strength and good tensile strength .

Composites exist in nature. A piece of wood is a composite, with long fibres of cellulose (a very complex form of starch) held together by a much weaker substance called lignin. Cellulose is also found in cotton and linen, but it is the binding power of the lignin that makes a piece of timber much stronger than a bundle of cotton fibres.

Composite materials are formed by combining two or more materials that have quite different properties. The different materials work together to give the composite unique properties, but within the composite you can easily tell the different materials apart &#; they do not dissolve or blend into each other.

Making a composite

Most composites are made up of just two materials. One material (the matrix or binder) surrounds and binds together a cluster of fibres or fragments of a much stronger material (the reinforcement). In the case of mud bricks, the two roles are taken by the mud and the straw; in concrete, by the cement and the aggregate; in a piece of wood, by the cellulose and the lignin. In fibreglass, the reinforcement is provided by fine threads or fibres of glass, often woven into a sort of cloth, and the matrix is a plastic. 

Examples of various forms of glass reinforcements to be used in the creation of fibreglass. Image source: Cjp24 / Wikimedia Commons.

The threads of glass in fibreglass are very strong under tension but they are also brittle and will snap if bent sharply. The matrix not only holds the fibres together, it also protects them from damage by sharing any stress among them. The matrix is soft enough to be shaped with tools, and can be softened by suitable solvents to allow repairs to be made. Any deformation of a sheet of fibreglass necessarily stretches some of the glass fibres, and they are able to resist this, so even a thin sheet is very strong. It is also quite light, which is an advantage in many applications. 

Over recent decades many new composites have been developed, some with very valuable properties. By carefully choosing the reinforcement, the matrix, and the manufacturing process that brings them together, engineers can tailor the properties to meet specific requirements. They can, for example, make the composite sheet very strong in one direction by aligning the fibres that way, but weaker in another direction where strength is not so important. They can also select properties such as resistance to heat, chemicals, and weathering by choosing an appropriate matrix material. 

Choosing materials for the matrix

For the matrix, many modern composites use thermosetting or thermosoftening plastics (also called resins). (The use of plastics in the matrix explains the name 'reinforced plastics' commonly given to composites). The plastics are polymers that hold the reinforcement together and help to determine the physical properties of the end product. 

Thermosetting plastics are liquid when prepared but harden and become rigid (ie, they cure) when they are heated. The setting process is irreversible, so that these materials do not become soft under high temperatures. These plastics also resist wear and attack by chemicals making them very durable, even when exposed to extreme environments. 

Thermosoftening plastics, as the name implies, are hard at low temperatures but soften when they are heated. Although they are less commonly used than thermosetting plastics they do have some advantages, such as greater fracture toughness, long shelf life of the raw material, capacity for recycling and a cleaner, safer workplace because organic solvents are not needed for the hardening process. 

Ceramics, carbon and metals are used as the matrix for some highly specialised purposes. For example, ceramics are used when the material is going to be exposed to high temperatures (such as heat exchangers) and carbon is used for products that are exposed to friction and wear (such as bearings and gears). 

An electron microscope image, in false colour, of a magnesium matrix composite reinforced with titanium aluminium carbide. Image source: ZEISS Microscopy / Flickr.

Choosing materials for the reinforcement

Although glass fibres are by far the most common reinforcement, many advanced composites now use fine fibres of pure carbon. There are two main types of carbon that can be used &#; graphite and carbon nanotubes. These are both pure carbon, but the carbon atoms are arranged in different crystal configurations. Graphite is a very soft substance (used in &#;lead&#;pencils) and is made of sheets of carbon atoms arranged in hexagons. The bonds holding the hexagons together are very strong, but the bonds holding the sheets of hexagons together are quite weak, which is what makes graphite soft. Carbon nanotubes are made by taking a single sheet of graphite (known as graphene) and rolling it into a tube. This produces an extremely strong structure. It&#;s also possible to have tubes made of multiple cylinders &#; tubes within tubes. 

Carbon fibre composites are light and much stronger than glass fibres, but are also more expensive. Of the two, graphite fibres are cheaper and easier to produce than carbon nanotubes. They are used in aircraft structures and in high performance sporting equipment like golf clubs, tennis rackets and rowing boats, and are increasingly being used instead of metals to repair or replace damaged bones. 

Even stronger (and more costly) than carbon fibres are threads of boron. Nanotubes of boron nitride have the additional advantage of being much more resistant to heat than carbon fibres. They also possess piezoelectric qualities, which means they can generate electricity when physical pressure is applied to them, such as twisting or stretching.

Polymers can also be used as the reinforcement material in composites. For example, Kevlar, originally developed to replace steel in radial tyres but best known for its use in bullet-proof vests and helmets, is a polymer fibre that is immensely strong and adds toughness to a composite. It is used as the reinforcement in composite products that require lightweight and reliable construction (eg, structural body parts of an aircraft). Even stronger than Kevlar is a substance made from a combination of graphene and carbon nanotubes.

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Source: NASA Goddard / YouTube. View

Source: NASA Goddard / YouTube. View video details and transcript

Choosing the manufacturing process

Making an object from a composite material usually involves some form of mould. The reinforcing material is first placed in the mould and then semi-liquid matrix material is sprayed or pumped in to form the object. Pressure may be applied to force out any air bubbles, and the mould is then heated to make the matrix set solid. 

The moulding process is often done by hand, but automatic processing by machines is becoming more common. One of these methods is called pultrusion (a term derived from the words 'pull' and 'extrusion'). This process is ideal for manufacturing products that are straight and have a constant cross section, such as bridge beams. 

In many thin structures with complex shapes, such as curved panels, the composite structure is built up by applying sheets of woven fibre reinforcement, saturated with the plastic matrix material, over an appropriately shaped base mould. When the panel has been built to an appropriate thickness, the matrix material is then cured. 

Sandwich composites

Many new types of composites are not made by the matrix and reinforcement method but by laying down multiple layers of material. The structure of many composites (such as those used in the wing and body panels of aircraft), consists of a honeycomb of plastic sandwiched between two skins of carbon-fibre reinforced composite material.

A honeycomb composite sandwich structure from NASA. Image source: NASA / Wikimedia Commons.

These sandwich composites combine high strength, and particularly bending stiffness, with low weight. Other methods involve simply laying down several alternating layers of different substances (for example, graphene and metal) to make the composite. 

Composites are manufactured using one of the following techniques:

More detailed descriptions than those given below are available in 'Composites - a guide to best practice: Section 4. Manufacturing composites'.

Wet lay-up

The component is built up in a mould by applying several layers of reinforcement and wet resin, which is distributed by a roller, until the desired thickness is achieved. The resin is then cured, using applied heat if necessary, to produce the finished component.

This process is widely used in the marine industry to prepare glass-fibre reinforced polyester resins. Material costs are relatively low and the process is very flexible. However, it is labour intensive and suffers the drawback of high styrene emissions.

A spray gun is used to apply chopped fibre reinforcement and wet resin to a mould until the desired thickness of material is built up. The resin is then cured. The process is faster and cheaper than wet lay-up, but mechanical properties are lower.

The process is typically used for large, relatively simple structures such as bathtubs, boat hulls and storage tanks.

Compression moulding

A charge of fibre and resin, either sheet moulding compound or bulk moulding compound, is placed in a preheated mould, which is then closed, and held under pressure until the resin is cured. The process can generate a Class A surface finish, and the similarity to the stamping process used for sheet metals has led to applications in the automotive industry.

The high investment in heated tooling means that the process is only suitable for medium to high volume production.

Injection moulding

Bulk moulding compound is heated and injected into a heated mould, where it is held under pressure until the resin has cured. The process is used for relatively small components for which a short cycle time can be achieved. The high cost of tooling means that the process is suitable only for medium to high volume production.

Resin transfer moulding (RTM)

A fibre preform or fabric is placed in a heated mould. Reactive resin is mixed and injected into the mould under pressure. Pressure is maintained until the resin has cured and the part is removed. The process is suitable for complex, highly-loaded parts, and is used in a wide range of industries. Vacuum assisted resin transfer moulding (VARTM) is a variant of the process in which vacuum is applied to the closed mould, allowing resin to be injected under low pressure. The loads on the tooling are therefore lower, allowing cheaper, larger tools to be used to fabricate large structures such as boat hulls or wind turbine blades. A further variant is structural reaction injection moulding(SRIM). This uses highly reactive resins such that the mould does not need to be heated to cure the resin, although it often is heated in order to reduce cycle time.

Vacuum infusion

Vacuum is used to cause a low viscosity resin to impregnate a fibrous preform. Most commonly, the resin is caused to flow over the surface of the preform, and then to impregnate through the thickness, minimising the distance the resin must travel through the preform. The process is well-suited to large components such as boat hulls or wind turbine blades. The tooling does not have to carry substantial loads during the process.

Filament winding

A fibre tow is passed through a resin bath and applied, under tension, to a convex mandrel. The mandrel is rotated and the fibre release is moved to lay down fibres in the desired geometry until the required thickness is achieved. The composite is then allowed to cure, using elevated temperature if necessary. The process can be automated for high volume production, and is used for tubular structures such as pipes and driveshafts, as well as more specialised structures such as pressure vessels or monocoque bicycle frames. In the last two applications, the mandrel will remain inside the component. In a variant of the process, pre-impregnated fibre tows or slit prepreg can be used, removing the need for a resin bath, but requiring a high-temperature curing stage.

Pultrusion

Fibre tows are taken from a creel and fed through a resin bath, before being pulled through a heated extrusion die, which cures the resin to produce an extruded part with axial reinforcement and constant cross-section. Applications include gratings, ladder sections, bridge parts and handrails. The process can be automated and uses low-cost raw materials, making it suitable for high volume applications.

Prepreg

Prepreg (pre-impregnated fibre) consists of fibres, fabrics or mats impregnated with resin. Thermosetting prepregs include hardeners in the resin, and therefore have a limited shelf life and are usually stored under refrigeration. Before use, the prepreg is brought to room temperature to avoid condensation, and plies are cut to the required shape and orientation. These are stacked in a mould to the required thickness, using a roller to avoid entrapment of air. The laminate is sealed in a vacuum bag, which is evacuated, and the part is then cured, either in a conventional oven, or in an autoclave, under additional pressure. Autoclave processing gives less porosity and superior mechanical properties, but requires a slow cycle time, and expensive equipment, particularly for large components. Prepreg is used where high mechanical properties are required and the high cost of processing can be justified. Examples include aerospace structures, sporting goods and wind turbine blades.

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