Fibres and resins of polymer-matrix composites

Four fibres do most of the work

The fibres most widely used in fibre-reinforced polymers are glass, carbon, aramid and boron, and of these glass and carbon are the most common.^[1]

Each fibre names a class of composite. Glass and carbon fibre go into both thermoset and thermoplastic matrices — epoxy is the usual matrix of aerospace carbon-fibre structure — while the matrices the sources read here name for aramid fibre are thermoplastics, and natural fibre composites carry plant fibres in thermoplastics such as polypropylene or thermosets such as epoxy.^[1,2,3,4]

Glass fibre comes in an electrical grade, E-glass, drawn from a borosilicate glass, and stronger structural grades, S-glass and S2-glass, made from a magnesia-alumina-silicate. It does not conduct electricity, resists corrosion and costs less than the other fibres, which is why it is used for wind blades, boat hulls, consumer goods and the secondary structure of aircraft.^[1,2]

How carbon fibre is made

Most carbon fibre starts as a polyacrylonitrile (PAN) fibre. It is first stabilised by oxidation while stretched at 200–300 °C, which turns a thermoplastic into a ladder compound that will no longer melt, then carbonised at about 1000 °C in nitrogen so that everything but carbon leaves as gas — roughly half the precursor’s mass survives — and may finally be heat-treated far hotter to graphitise it.^[1,2,3]

Carbon and graphite fibres are often named interchangeably, but they are not the same: both are built from graphene layers, and only when the layers are stacked with three-dimensional order is the material graphite, which takes longer and hotter processing and costs more.^[1]

Carbon fibre is three to ten times stiffer than glass and carries primary aircraft structure. It is also costly, conducts less well than aluminium so that exposed parts need lightning protection, and causes galvanic corrosion against metal fasteners.^[1]

Aramid and plant fibres

Aramid fibre is light, strong and tough and resists impact, so it goes where impacts happen and into armour. It is weak in compression and absorbs water — service reports describe parts taking up as much as eight per cent of their weight — so aramid parts have to be sealed against the environment.^[1,2]

Plant fibres such as flax, hemp and ramie are cellulose-based and the natural fibres best suited to structural composites. Most of them begin to degrade above about 200 °C, and that one fact restricts the matrices and processes that can be used with them.^[1,2]

Thermosets and thermoplastics

A thermosetting resin — epoxy, polyester, vinyl ester, phenolic, polyimide, bismaleimide — cures by irreversible cross-linking into an insoluble solid. A thermoplastic softens on heating and hardens on cooling, again and again, with no chemical cure at all.^[1,2]

The difference decides how a part is made. Thermosets are liquid enough before cure to soak quickly into a dry fibre preform, which is why most carbon- and glass-fibre composites used them when the U.S. Department of Energy surveyed the industry in 2015. A thermoplastic melt is far more viscous and does not readily impregnate fibre tows, and heating it further to thin it risks degrading it.^[1,2]

The same difference decides what happens at the end of a part’s life: a thermoplastic can be re-melted and separated from its fibres, a cured thermoset cannot.^[1,2]

Commonly misunderstood