Fibre Reinforced Facade Encyclopedia: Composition
Phenolic resin is a phenol formaldehyde thermoset that cures by condensation and chars rather than burns cleanly away, giving the lowest flame spread and the lowest smoke of the common composite matrices. It is the honest reason phenolic exists in facade and transport work. It is also harder to process than polyester resin or vinyl ester resin, gives a dark and limited colour range, and yields mechanical properties around 10 to 20 percent below an equivalent polyester laminate.
Phenolic resin chars to 50 to 60 percent of its original mass in a fire. Polyester and epoxy leave almost no char, so almost all of their mass is available as fuel.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Limiting oxygen index | 35 to 45% | Polyester and epoxy are about 19 to 22% |
| Char yield | 50 to 60% by mass | The mechanism behind the low heat release |
| Cast resin tensile strength | 35 to 55 MPa | Below polyester; laminates follow |
| Cure shrinkage | 6 to 10% by volume | Condensation cure releases water |
| Cure by-product | Water, about 10% by mass of resin | Must be removed or it causes voids |
| Colour range | Amber through dark red brown | Cannot be pigmented to light or clean colours |
| Free formaldehyde exposure standard | 1 ppm TWA, 2 ppm STEL | Safe Work Australia workplace exposure standard |
| Relative resin cost | 1.5 to 2.5x polyester | Processing cost adds further |
| Fire | Combustible, but the best classified resin | No phenolic resin is non-combustible |
Why it is specified
Every other common composite matrix decomposes into volatile fuel when heated. Phenolic instead cross-links further and carbonises, forming a rigid char layer that insulates the material below it and cuts the supply of volatiles to the flame. The measurable consequences are a low peak heat release rate in the cone calorimeter, a low flame spread index in ASTM E84, very low smoke development, and low toxic gas yield. Phenolic laminates routinely reach the best classification band available to an organic composite in rail, marine and aerospace interior specifications, and they are the standard route to a good group number in a fibre reinforced polymer lining.
Cure by condensation, and what that costs
Polyester, vinyl ester and epoxy cure by addition: nothing is expelled. Phenolic resole cures by condensation, expelling roughly 10 percent of its own mass as water. That water has to go somewhere. If it is trapped it forms voids and blisters, drops interlaminar strength and leaves a laminate that looks sound and tests poorly. The practical answers are heat and pressure, meaning press moulding, vacuum bagging or a controlled oven schedule, and thin sections built up in stages rather than thick single laminates.
Cure is acid catalysed, usually with a sulfonic acid. The catalyst is corrosive, so mixing gear, pumps and moulds need to be corrosion resistant, and mould life is shorter than in polyester work. Phenolic also wets glass out less readily than polyester, and the sizing on the reinforcement has to be matched to phenolic rather than to a styrene system. None of this is exotic, but it is a different production line, which is why phenolic laminating is a specialist capability rather than a resin substitution.
Colour and surface
Cured phenolic is amber to dark red brown, and it darkens further with age and ultraviolet exposure. It cannot be pigmented to a light, clean or pastel colour; the resin colour always shows through. Where a specific facade colour is required the surface has to be a gel coat or an applied coating, and that coating is itself an organic layer that has to be accounted for in the fire assessment of the finished panel. A phenolic laminate behind a thick conventional gel coat can perform worse in a surface flammability test than the laminate alone, so the coating and the laminate must be tested together as the product that will be installed.
Surface quality off the mould is also harder to control than in polyester work, because of the condensation water and the higher cure shrinkage. Expect more finishing labour.
Mechanical performance
A phenolic glass laminate is typically 10 to 20 percent below the equivalent polyester laminate in flexural and tensile strength, with lower strain to failure and lower impact tolerance. The matrix is more brittle and the fibre to matrix bond is weaker unless the sizing is correctly matched. Fibre volume fraction is generally lower in hand laid phenolic than in an equivalent polyester laminate because of the wet-out difficulty.
For a wind loaded facade panel this is usually a design adjustment rather than a barrier: add thickness, add a stiffening rib, or change the panel geometry. It becomes significant where the panel is also impact rated or where deflection governs.
Workplace exposure
Phenolic systems contain no styrene, which removes the dominant hazard of polyester work. In its place are free phenol and free formaldehyde in the uncured resin, plus the acid catalyst. The Safe Work Australia workplace exposure standard for formaldehyde is 1 ppm as an eight hour time weighted average with a 2 ppm short term limit, and formaldehyde is classified as a carcinogen, so exposure is controlled to as low as reasonably practicable rather than simply to the limit. Phenol has its own exposure standard and is absorbed through the skin. Low free formaldehyde resole grades, closed mixing and local exhaust ventilation at the mould are the normal controls.
Standards and references
- AS 5637.1 (determination of group number) and NCC 2022 Volume One Specification 7 (check the edition adopted in your jurisdiction)
- AS 1530.3 (ignitability, flame propagation, heat release and smoke release)
- ASTM E84 (surface burning characteristics: flame spread index and smoke developed index)
- EN 13501-1 and EN 45545-2 (reaction to fire for construction products and for rail vehicles)
- ISO 5660-1 cone calorimeter (heat release rate), the basis of the AS 5637.1 group number
- Safe Work Australia Workplace Exposure Standards for Airborne Contaminants: formaldehyde 1 ppm TWA, 2 ppm STEL
- AS 5113 and BS 8414 parts 1 and 2 (full scale facade fire testing)
Related terms
Questions specifiers ask
Is phenolic resin non-combustible?
No. Phenolic is an organic polymer that ignites and contributes heat. It chars instead of burning freely, which gives very low flame spread and very low smoke, but non-combustibility is a separate classification tested to AS 1530.1 that no resin achieves.
Why is phenolic not used for every composite facade panel?
Processing and appearance. Condensation cure releases water that must be driven out under heat or pressure, the acid catalyst is corrosive to plant, the resin is dark and cannot be pigmented to light colours, and the laminate is 10 to 20 percent weaker than polyester.
What colour can a phenolic panel be?
The resin itself is amber to dark red brown and darkens with age. Any other colour has to come from a gel coat or an applied coating, and that coating must be included in the fire test of the finished product.
Does phenolic contain styrene?
No. It contains free phenol and free formaldehyde instead, and is acid catalysed. That is a different exposure profile, not an absence of one; formaldehyde is a carcinogen with a 1 ppm eight hour exposure standard in Australia.
How does phenolic compare with a cement composite for fire?
They are different categories. Glass fibre reinforced concrete has a cement matrix with no polymer to burn and can be tested as non-combustible to AS 1530.1. Phenolic is the best classified organic resin but is still assessed by reaction to fire and full scale testing.
Working to a fire brief on a composite facade? Our published fire classifications, testing bodies and test standards are set out in one place. See the fire performance evidence
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.