The constituent materials, along with the processes by which they are combined, determine the properties of a finished composite part. The most commonly used raw materials are described here.
E-glass is the most widely used reinforcing fibre. Silica sand, limestone and other minerals are melted in a furnace and allowed to fall through tiny holes in a platinum plate to create fibres around 5-24μm diameter. As they emerge they are ‘sized’ (coated to enable handling and bonding to the matrix), bundled together to form a ‘roving’, and wound directly onto a bobbin. Other types of reinforcing glass exist for higher strength or chemical resistance.
Carbon fibre is made from a precursor fibre, usually polyacrylonitrile (PAN). This is pyrolysed and converted at high temperature into a graphite (carbon) fibre about 6-7μm diameter. Carbon fibres make the stiffest and strongest composites.
Aramid (aromatic polyamide) fibres such as Kevlar® and Twaron® are noted for their excellent impact resistance, as well as other properties, and are widely used in armour applications. There are other high performance polymer fibres such as UHMWPE (ultra high molecular weight polyethylene) and HMPP (high modulus polypropylene).
Natural fibres such as flax and hemp are often used in automotive interiors and have good damping properties, which can work well hybridised with carbon. Basalt fibres are increasingly used, especially where alkali resistance is important.
The resins used in composites are polymers – made of long, chain-like molecules. These may be thermoplastic or thermoset.
Thermoplastics melt when heated and solidify when cooled. Commonly used thermoplastics include PP, PET, nylon, or, for high performance applications, PEI or PEEK.
Thermosets are cured by a chemical reaction when resin and hardener or catalyst are mixed, causing cross-links between the polymer chains. They will not melt, though will eventually break down when heated. Thermosets have better mechanical properties and chemical resistance than all but the most expensive thermoplastics.
The most commonly used resin in glass fibre composites (GFRP) is unsaturated polyester. Vinyl esters are tougher and more water resistant than polyesters. Epoxy resins outperform most other resins and are usually used with carbon fibre. Phenolic resins have lower mechanical properties but very good fire resistance. Several other resin systems also exist.
Fillers and Additives
Mineral fillers such as calcium carbonate are often included to reduce cost, as less resin is needed, while improving other properties. Other additives can be included, e.g. fire retardants, UV absorbers or toughening agents.
Fibres can be incorporated directly in some processes, but more often are converted into fabrics which may be uni-/ bi-/ multi-axial, woven, knitted, braided, needle-punched or simply chopped and bound into a random-oriented fabric. Textile engineering is an important aspect of optimising composite manufacture, and 3D preforms are increasingly being used, to align fibres exactly where they will provide the best properties.
Some fabrics, especially carbon fibre, are pre-impregnated with resin to form a ‘prepreg’, which usually needs to be kept in a freezer to stop the resin from going off until ready for use.
Bulk moulding compound (BMC) is a mixture of short chopped fibres, resin and filler, used in injection and compression moulding. It is also called dough moulding compound (DMC). Sheet moulding compound (SMC) is made by chopping longer (25mm+) fibres onto a layer of resin.
Reinforcing and thermoplastic fibres can be commingled to make a fabric which can be referred to as a thermoplastic prepreg. When heated, the thermoplastic fibres melt to form a matrix. Also, short fibres can be ‘compounded’ with thermoplastic resin for injection moulding.
Using a sandwich of strong skins with a lighter core material is a great way to increase stiffness and reduce weight. Commonly used core materials may be polymer foams such as polyurethane, PVC or acrylic, or honeycomb structures made from aluminium, paper, Nomex® – (a Kevlar®-based paper) or other polymers. Balsa wood is an excellent natural core material with good fire resistance.