Fibre Reinforced Facade Encyclopedia: Composition
Epoxy resin is a thermoset formed by reacting an epoxide prepolymer, most often a bisphenol A diglycidyl ether, with a stoichiometric hardener such as an amine or an anhydride. It gives the highest mechanical performance, the best adhesion and the lowest cure shrinkage of the common composite matrices, contains no styrene, and costs three to five times as much as polyester resin. It is also the least ultraviolet stable, so it is almost never left exposed.
Epoxy shrinks 1 to 3 percent on cure against 5 to 9 percent for polyester, which is why epoxy is used for tooling and for structural bond lines where dimensional accuracy decides the result.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Cast resin tensile strength | 60 to 85 MPa | Unfilled neat resin, not the laminate |
| Tensile modulus | 2.8 to 4.0 GPa | Fibre sets laminate stiffness |
| Elongation at break | 3 to 6% | Tougher matrix than polyester |
| Cure shrinkage | 1 to 3% by volume | Lowest of the four architectural thermosets |
| Glass transition temperature | 60 to 200 °C | Set by the hardener and the post cure |
| Lap shear adhesion to steel | 15 to 30 MPa | Best adhesion of the four |
| Mix ratio tolerance | Within about 2% by weight | Not a catalysed system; ratio is chemistry |
| Relative resin cost | 3 to 5x polyester | Vinyl ester is 1.5 to 2x polyester |
| Fire | Combustible in every grade | No epoxy resin is non-combustible |
How epoxy differs from a catalysed resin
Polyester resin and vinyl ester resin are catalysed: a small percentage of peroxide starts a chain reaction and the exact dose shifts the speed, not the chemistry. Epoxy is not catalysed. The hardener is a reactant consumed in the network, so the mix ratio is fixed by stoichiometry and getting it wrong leaves permanently unreacted material, a lower glass transition temperature and a soft, weak laminate. Ratios are specified by weight or volume, commonly between 100:20 and 100:50, and must be held to a few percent.
Because there is no styrene, there is nothing to evaporate and nothing to cross-link through a volatile monomer. That is the source of the low cure shrinkage, the low volatile organic compound emission and the excellent adhesion. It is also the source of the high viscosity, which makes epoxy harder to wet out through a heavy woven roving without vacuum assistance or heat.
Cure behaviour
Pot life is the working time of the mixed batch in the pot, and it is shorter than the open time on the laminate because the exotherm is concentrated in the mass. A large batch in a deep container can reach boiling and smoke; the same resin spread thin on a mould cures quietly. Mix small batches or use a shallow tray.
Room temperature cure gives a usable laminate but rarely the full glass transition temperature. A post cure, typically 8 to 16 hours at 60 to 80 °C, and higher for high temperature hardeners, is what converts the laminate to its published properties. Amine blush, a waxy carbamate film formed when amine hardeners react with atmospheric moisture and carbon dioxide, must be washed off with water before any subsequent bonding or coating.
Where epoxy earns its cost
- Structural bonding: bonded joints, anchor grouting and bonded stiffeners, where adhesion governs
- Carbon fibre laminates: the fibre is too expensive to under-use with a lower performing matrix
- Tooling and pattern work: low shrinkage means the mould holds the shape it was made to
- Prepreg and autoclave work: heat cured epoxy films give the highest fibre volume fraction and lowest void content
- Repair and secondary bonding: epoxy bonds well to cured polyester, but not the reverse
For a sprayed or hand laid architectural facade laminate in glass fibre, the cost premium usually cannot be justified by the wind load. The decision is driven by the bond lines, the fibre type and the tolerance, not by the panel stress.
Sensitisation and workplace exposure
Epoxy removes the styrene problem and replaces it with a sensitisation problem. Uncured epoxy resin and, more strongly, amine hardeners are skin sensitisers: repeated contact can produce allergic contact dermatitis, and once a worker is sensitised the reaction recurs at very low doses and can end their ability to work with the material. Some amine hardeners are also respiratory sensitisers. Safe Work Australia classifies the common hardeners accordingly, and the control is nitrile gloves changed frequently, no bare skin contact, no solvent washing of skin, and mechanical mixing rather than hand contact.
Vapour exposure is generally low at ambient cure, which is why epoxy shops need less extraction than open mould polyester shops. Sanding cured epoxy generates respirable dust and needs extraction and respiratory protection of its own.
Weathering and fire behaviour
Epoxy has the worst ultraviolet resistance of the four resins. The aromatic bisphenol backbone absorbs ultraviolet light, and an exposed epoxy surface yellows, chalks and loses gloss quickly. Epoxy laminates intended for exterior use are always coated, normally with a polyurethane or fluoropolymer topcoat, and the coating rather than the resin becomes the weathering system. This is a real difference from a gel coated polyester panel, where the pigmented gel coat is formed in the mould.
Standards and references
- ISO 3673 parts 1 and 2 (epoxy resins: designation and determination of properties)
- ASTM D1652 (epoxy content) and ASTM D2471 (gel time and peak exotherm)
- ASTM D3171 (constituent content of composite materials) and ASTM D2584 (ignition loss)
- ASTM D7028 (glass transition temperature by dynamic mechanical analysis)
- Safe Work Australia Model Code of Practice: Managing Risks of Hazardous Chemicals in the Workplace
- AS 5637.1 (group number) and NCC 2022 Volume One Specification 7 (check the edition adopted in your jurisdiction)
- ASTM E84 and EN 13501-1 (reaction to fire classification of the finished laminate)
Related terms
Questions specifiers ask
Is epoxy stronger than polyester?
As a neat resin, yes, and the difference shows up in adhesion, interlaminar shear, fatigue and impact tolerance. In a laminate loaded along the fibres, strength is dominated by the glass and the fibre volume fraction, so the tensile difference is smaller than the price difference suggests.
Why is epoxy not used for architectural glass fibre panels?
Cost, viscosity and ultraviolet stability. A wind loaded glass fibre panel does not need epoxy mechanical performance, epoxy wets out heavy reinforcement less easily, and an exposed epoxy surface degrades under sunlight where a pigmented gel coat does not.
Does epoxy contain styrene?
No. Epoxy systems are styrene free, which is a genuine advantage for workplace air quality. The hazard shifts to skin and respiratory sensitisation from the resin and the amine hardener.
Can epoxy be bonded to a cured polyester laminate?
Yes, epoxy bonds well to a properly abraded and cleaned polyester laminate. The reverse does not hold: polyester bonds poorly to cured epoxy, which matters when planning repairs.
Is epoxy fire resistant?
No. Unmodified epoxy generally performs worse than polyester in reaction to fire tests and produces heavy smoke. Fire retarded grades improve classification, but no epoxy resin is non-combustible.
Specifying a composite facade against a fire and durability brief? Our fibre reinforced polymer system and its test evidence are set out in full. See ShapeShell-RT specifications
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.