Epoxy resin
Two liquids that become a solid by reacting with each other — the structural adhesive, and the matrix holding most composites together.
Epoxy is a two-part thermoset: a resin bearing epoxide groups and a hardener that opens them, reacting together into a rigid crosslinked network. Nothing evaporates and nothing is driven off, so it cures with very little shrinkage — which is why it holds tolerance in a bonded joint where a solvent-based adhesive would pull the parts.
It adheres to almost everything, resists most chemicals, and is a good electrical insulator. Its structural role is larger than its adhesive one: most high-performance composites are epoxy holding fibres in place, and the fibres carry the load while the matrix transfers it between them and keeps them from buckling.
Being a thermoset, it cannot be melted or reshaped after cure. That is the source of both its temperature resistance and its recycling problem.
Processing
Mixed in a specified ratio and cured, at room temperature for general-purpose systems or in an oven for structural ones — a higher cure temperature gives a higher service temperature, and an under-cured epoxy is permanently weaker rather than merely slower.
The ratio is not a guideline. Unlike a polyester resin, where catalyst quantity mainly changes the speed, an epoxy's hardener is a reactant consumed stoichiometrically, and getting it wrong leaves unreacted material in the network and a joint that never reaches strength.
Composite processes: hand lay-up, vacuum infusion, filament winding, and pre-impregnated fabric cured under pressure in an autoclave, which is how aerospace structure is made.
Uses
The matrix of carbon and glass fibre composites — aircraft structure, wind turbine blades, boat hulls, sporting goods. Structural adhesives in aerospace, automotive and construction. Protective coatings and floor systems in industry. Potting and encapsulation of electronics. The laminate of every printed circuit board, which is glass fabric in epoxy.
In construction, as the grout that anchors reinforcing bar into hardened concrete, and as the injection resin that repairs cracked structures.
History
Developed independently in Switzerland and the United States in the late 1930s and commercialised after the Second World War. Its rise tracks the rise of composites exactly: epoxy was the matrix that made glass fibre structural and later made carbon fibre worth having, because it bonds to the fibre well enough to transfer load into it.
Environmental impact
Not recyclable in any ordinary sense. A cured epoxy composite can be ground into filler, burned for energy with the fibres lost, or pyrolysed to recover shortened fibres at some cost in their properties. None of those returns the material to what it was.
This is the wind industry's visible problem: turbine blades are large epoxy composite structures with a twenty-five year life and no established end-of-life route, and the first large cohort is now retiring. Recyclable and cleavable epoxy chemistries are in development and none is yet standard.
Uncured resin and hardener are the health issue rather than the cured solid. Epoxy is a well-documented occupational skin sensitiser: once someone is sensitised the reaction is permanent and can end a trade. Cured epoxy is inert.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
contains
- Carbon — element · the aromatic backbone of the common bisphenol-based resins
- Oxygen — element · in the epoxide rings that give the resin its name, and that the hardener opens to build the network
- Hydrogen — element · the balance
- Nitrogen — element · in the amine hardeners that cure most systems — and the component responsible for the sensitisation risk
is produced by
- Polymerisation — process · the resin is made first and the network second — cure is a polymerisation carried out by the user rather than the manufacturer
is used as
- Adhesion and bonding — application · the structural adhesive: it fills gaps, cures without shrinking, and bonds metals and composites that cannot be welded to each other
- Structural engineering — application · as the matrix of the composites that carry load in aircraft, wind turbines and hulls
is used in
- Construction — industry · anchoring reinforcement into hardened concrete, and injection repair of cracked structure
- Electronics manufacture — industry · the laminate of every printed circuit board, which is glass fabric in epoxy
- Shipbuilding — industry · hull composites and protective coatings
- Wind turbine blade — object · the matrix, and the reason a retired blade cannot be melted down
- Printed circuit board — object · the matrix of the laminate, with bromine bound into it for the flame retardancy the grade is named for
is an alternative to
- Portland cement — material · as a repair and anchoring material in concrete construction, where epoxy bonds to the existing structure and cementitious grout mostly does not
is a component of
- Glass fibre composite — material · or, more often and more cheaply, an unsaturated polyester resin
- Carbon fibre composite — material · the matrix, which holds the fibres in place and transfers load between them
belongs to the group
- Plastic — material · a thermoset: it cures once and cannot be melted again, which is where the recycling problem starts
Sources
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- Material WorldOur own writing