Fibre Reinforced Facade Encyclopedia: Composition
Polyester resin is an unsaturated polyester dissolved in styrene, cured at room temperature by a peroxide initiator to form a rigid thermoset. It is the default matrix for glass fibre laminates because it is the cheapest of the four architectural thermosets, wets glass out readily and cures without heat or pressure. It is also the weakest of the four, the most brittle, the most styrene-heavy and the least resistant to alkali and ultraviolet light.
A hand laid polyester laminate at 25 percent fibre by volume costs roughly a third of the same laminate in epoxy, and reaches roughly two thirds of its interlaminar shear strength.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Cast resin tensile strength | 40 to 75 MPa | Unfilled neat resin, not the laminate |
| Tensile modulus | 3.0 to 4.0 GPa | Laminate stiffness is set by the fibre |
| Elongation at break | 1.5 to 3% | Lowest strain to failure of the four resins |
| Cure shrinkage | 5 to 9% by volume | Highest of the four; drives print-through |
| Styrene content | 35 to 45% by weight | Reactive diluent and the main exposure hazard |
| Heat distortion temperature | 60 to 90 °C | Isophthalic grades at the upper end |
| Gel time at 20 °C | 15 to 40 minutes | 1 to 2% MEKP with a cobalt accelerator |
| Relative resin cost | 1.0x, the industry baseline | Vinyl ester 1.5 to 2x, epoxy 3 to 5x |
| Fire | Combustible in every grade | No polyester resin is non-combustible |
What the resin actually is
An unsaturated polyester is made by condensing a glycol with a mixture of a saturated and an unsaturated dibasic acid. The unsaturated sites are the reaction points. The polymer is then dissolved in styrene monomer, which acts as both the solvent that makes the resin pourable and the co-reactant that bridges the chains when cure begins. Cured polyester is therefore a styrene cross-linked network, and the styrene fraction is not a solvent that evaporates away; most of it becomes part of the solid.
Three families matter in practice. Orthophthalic resins are the general purpose, lowest cost grade. Isophthalic resins cost more and give better hydrolysis resistance, higher heat distortion temperature and better weathering, and are the sensible minimum for any laminate exposed to weather.
Cure behaviour
Cure is initiated by a peroxide, normally methyl ethyl ketone peroxide at 1 to 2 percent, with a cobalt salt accelerator already present in the supplied resin. The reaction is free radical and strongly exothermic. A laminator sees an open working time, a gel point where the resin stops flowing, then an exothermic peak. Thick sections must be built in stages or the exotherm will scorch the resin and distort the moulding.
Full property development needs a post cure, typically 16 to 24 hours at 40 to 60 °C, or several weeks at ambient. Subsequent layers bond chemically if applied before the previous layer has fully cured; after that the bond is mechanical only and the surface must be abraded. High cure shrinkage is the other practical consequence. It reads through a thin laminate as print-through of the reinforcement pattern, which is why chopped strand mat is used as a barrier layer under woven roving and why gel coat thickness is controlled.
Mechanical performance in context
In a fibre dominated direction the laminate properties come from the glass, so polyester, vinyl ester and epoxy laminates of the same fibre volume fraction have similar tensile strength along the fibres. The resin decides everything else: interlaminar shear strength, compressive behaviour, fatigue life, impact damage tolerance and the strain at which the matrix starts to micro-crack. Polyester has the lowest strain to failure of the four, so it micro-cracks first, and once the matrix has cracked the laminate takes up water and loses stiffness.
For architectural panels carrying wind load rather than cyclic structural load this is usually acceptable. For anything fatigue loaded, immersed or chemically exposed, vinyl ester resin or epoxy resin earns its cost.
Weathering and chemical resistance
Unprotected polyester yellows and chalks under ultraviolet light. The industry answer is not a better resin but a sacrificial surface: a pigmented gel coat containing ultraviolet absorbers, applied against the mould face, or a coating applied after demoulding. Expect surface maintenance on an exposed laminate rather than permanence.
Polyester is an ester, so it is hydrolysed by alkali. Prolonged contact with fresh cement, alkaline cleaning agents or damp masonry attacks the matrix. This is a separate issue from the alkali attack on the fibre described on the alkali resistant glass fibre page, and it is one reason polymer laminates and a cement matrix are kept as distinct systems rather than combined.
Styrene exposure and ventilation
Styrene is the dominant occupational hazard in open mould polyester work. The Safe Work Australia workplace exposure standard is 50 ppm as an eight hour time weighted average with a 100 ppm short term exposure limit. The odour threshold is far below both figures, so the fact that a workshop smells of styrene proves nothing about compliance; only monitoring does.
Three controls reduce exposure. Low styrene emission grades carry a wax that blooms to the laminate surface and suppresses evaporation. Closed moulding, meaning vacuum infusion or resin transfer moulding, cuts emissions by roughly an order of magnitude compared with hand lay-up because the resin is never open to the air. Local exhaust ventilation at the mould, rather than general dilution, does the rest.
Fire behaviour
Polyester resin is an organic polymer. It ignites, it burns, it contributes heat, and it produces dense black smoke. Its limiting oxygen index is around 19 to 21 percent, which is below the oxygen content of air. Filling the resin with aluminium trihydrate at 40 to 60 percent by weight, or using a brominated or phosphorus modified grade, improves reaction to fire results significantly, at the cost of higher viscosity, harder wet-out and reduced mechanical properties.
Standards and references
- AS 5637.1 (determination of group number) and NCC 2022 Volume One Specification 7 (check the edition adopted in your jurisdiction)
- ISO 3672 and ISO 1675 (unsaturated polyester resins: designation, density and viscosity)
- ASTM D2584 Standard Test Method for Ignition Loss of Cured Reinforced Resins
- ASTM D2583 (Barcol hardness, used as a cure check) and ASTM D5117 (post cure verification)
- Safe Work Australia Workplace Exposure Standards for Airborne Contaminants: styrene 50 ppm TWA, 100 ppm STEL
- ASTM E84 and EN 13501-1 (reaction to fire classification of the finished laminate)
- BS 8414 parts 1 and 2, AS 5113 (full scale facade fire testing)
Related terms
Questions specifiers ask
Is polyester resin strong enough for a facade panel?
Yes, for wind and self weight. Laminate strength along the fibres comes from the glass, and a well made polyester laminate is used routinely for architectural panels. The resin choice matters more for fatigue, chemical exposure and long term micro-cracking than for peak strength.
Why does polyester cost less than epoxy?
Raw material cost and process. Polyester is made from commodity glycols and acids and is thinned with styrene, which is inexpensive. Epoxy uses more costly precursors, needs a matched hardener and often a heated post cure.
Is polyester resin fire retardant?
Standard grades are not. Filled and halogenated grades achieve much better reaction to fire results and are how good group numbers and Euroclasses are reached in glass fibre reinforced polymer panels. None of them is non-combustible.
Does polyester resin need a gel coat?
For external use, effectively yes. A pigmented gel coat with ultraviolet absorbers is the sacrificial weathering layer and the source of the finished colour and gloss. Without it the laminate yellows and chalks.
Can polyester resin be used with alkali resistant glass in concrete?
No. Polyester and cement are different systems. Polyester laminates use E glass fibre; cement composites use alkali resistant glass fibre. The resin itself is hydrolysed by alkali, so it is not a route to a cement compatible composite.
Comparing resin systems for a facade package? ShapeShell-RT is our fibre reinforced polymer system, with its fire classifications set out in full. See ShapeShell-RT specifications
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.