Unsaturated polyester resin cures by a free-radical chain-growth mechanism: a peroxide initiator (typically MEKP at 1–2 wt%) decomposes to radicals that open the C=C bonds in the polyester backbone and in the styrene co-monomer, which usually makes up 30–50% of the mix [S2].
Epoxy resin cures by a step-growth polyaddition between epoxide rings and a hardener, almost always an amine or anhydride, with no volatile by-products and far less shrinkage than the polyester/styrene route [S2][S3]. The two routes deliver different gel times, different post-cure mechanical behaviour, and very different cost-per-kilogram, so the selection must follow the chemistry, not the brand label.
Reaction Mechanism: Radical Chain-Growth vs Step-Growth Addition
Polyester relies on a free-radical chain reaction. The peroxide catalyst (MEKP is the workhorse) generates radicals that attack the C=C double bonds on the fumarate or maleate units along the polyester chain and on the diluent styrene, building a crosslinked network through copolymerisation of the two unsaturated species [S2]. No hardener is mixed in stoichiometric ratio; instead, the resin is single-component, and the catalyst dose (often 1–2% by mass) controls gel and peak exotherm.
Epoxy is a two-part system. The epoxide (oxirane) ring on each end of the diglycidyl ether of bisphenol-A (DGEBA) molecule opens and reacts with the active hydrogen of an amine (e.g. TETA, DETA, or polyamidoamine) or anhydride hardener, and the network grows step by step until the stoichiometric ratio is consumed [S3]. Mix ratio is tight, typically 5:1 to 3:1 by mass depending on the hardener, and the user is far more sensitive to off-ratio errors than with polyester [S6].
The practical upshot: polyester tolerates ambient workshop dust and mild off-ratio better because the chain-growth keeps propagating as long as radicals survive, while epoxy stoichiometry is unforgiving, and an off-ratio batch will stay tacky or under-cured even after a week [S6].
Cure Profile, Gel Time, and Exotherm
Polyester gel time is short and tunable. With 1% MEKP at 25°C, an orthophthalic laminating resin will gel in 6–15 minutes and reach a usable Barcol hardness within 2–4 hours, depending on cobalt accelerator dose and ambient temperature [S3]. Full cure is still slow: most fabricators de-mould after 24 hours and post-cure before service.
Epoxy gel time is hardener-driven. A standard DGEBA + TETA system at 25°C gels in 30–60 minutes and reaches handling strength in 4–8 hours, while slow polyamidoamine hardeners extend working life past 90 minutes for large layups [S3]. One Resin Library reference places polyester cure at "approximately 12 hours" and notes that epoxy can be accelerated with mild heat up to about 38°C (100°F) without yellowing or distortion [S3][S5].
Exotherm differs sharply. Polyester releases the heat of polymerisation rapidly because every chain extension is a radical event in a thick, styrene-rich mass; thick castings (>12 mm) can smoke, crack, or warp from runaway exotherm. Epoxy step-growth releases heat more gently, which is why deep-pour river tables and electrical potting use epoxy almost exclusively [S3].
Mechanical Properties After Cure
Epoxy delivers higher bond strength, roughly 2000 psi typical lap-shear per published resin-supplier data, and noticeably better flexural and tensile performance on carbon-fibre laminates than polyester on glass-fibre laminates of equivalent weight [S3][S5]. Cured polyester is described as "brittle and less impact-resistant" than epoxy in the same fabric format, which is why polyester in structural work is always paired with glass-fibre reinforcement [S1][S5].
Recycled-carbon-fibre test data show that epoxy matrices give higher fracture strain than unsaturated polyester matrices on identical fibre layups, attributable to the higher ductility of the cured epoxy network [S5]. For a structural repair, that ductility margin is the reason the boat-design and composite-trades literature unanimously recommends epoxy for any load-bearing patch [S4][S5].
Shrinkage on cure also differs. Polyester contracts around 5–8% volumetrically as styrene copolymerises and the network densifies; epoxy contracts under 2% because addition polymerisation does not release small-molecule by-products [S3]. The polyester shrinkage is the reason moulds must be over-sized and why gelcoat surfaces pull away from inserts on thick castings.
Chemical, Water, and UV Resistance
Epoxy shows "superior resistance to environmental factors" and does not degrade as fast as polyester when immersed in water or exposed to many solvents, per direct comparison data published in 2024 [S4]. Cured epoxy is also more moisture-resistant after the gel-to-cure window closes, because the polar hydroxyl and ether groups in the network hydrogen-bond to water rather than letting it diffuse freely [S3].
Polyester relies on a wax or surface veil plus the residual styrene-rich skin to block water uptake. Over time, hydrolysis of the ester linkages in the backbone is the dominant aging mechanism, especially in warm, wet service; isophthalic grades resist this better than orthophthalic, which is why marine and chemical-tank specifiers insist on isophthalic or vinyl ester instead of general-purpose orthophthalic [S2].
Neither resin is intrinsically UV-stable. Both need a topcoat, gelcoat, or UV-absorbing additive to prevent chalking and yellowing; the same source notes that "the ideal way to make sure that the epoxies don't yellow with UV rays is to use UV resistant transparent urethane as the top coating" [S3]. For styrene-free systems, the wider category of synthetic resin chemistries now includes many low-odour, low-VOC alternatives driven by styrene's carcinogen concerns in the US and EU.
Cost, Shelf Life, and Mix Tolerance
Polyester is the cost-leader. Resin Library states that polyester "holds around 75% of the resin market in the UK" specifically because of its price advantage and the simplicity of single-component MEKP cure [S5]. Thomasnet's 2025 overview confirms that polyester remains "more affordable than alternatives such as epoxy" for the same fabricated part [S1].
Shelf life is the trade. Polyester inhibited with hydroquinone or similar is typically rated 6–12 months at 25°C and degrades faster once the drum is opened and exposed to moisture or contamination; epoxy resins, when stored sealed, routinely hit 24 months on the shelf [S3][S6]. For low-volume maintenance shops that buy drums twice a year, polyester's shelf life is rarely a problem; for project-based buyers who keep inventory across seasonal work, epoxy is the lower-risk choice [S6].
Mix tolerance is the daily shop issue. Polyester "has to be mixed in extremely narrow proportion" of catalyst, and the user chases gel time by feel rather than by stoichiometry, while epoxy is "a bit more forgiving" on the hardener side but absolutely will not cure if the ratio is off by more than a few percent on either side [S6]. Operators who care more about pot-life control than absolute peak mechanicals tend to pick epoxy; operators who want fast turn-around on a mould and accept the brittleness penalty tend to pick polyester.
Selection Matrix by Use Case
For cost-driven non-structural FRP (boat hulls, flat-roofing, pond linings, bund walls, general mouldings), pick orthophthalic or isophthalic polyester with MEKP and 1.5–2% cobalt-accelerated cure; glass-fibre reinforcement is mandatory because cured polyester alone is too brittle [S1][S5]. For structural repair, aerospace, racing, marine-keel, or carbon-fibre laminates, pick epoxy with an amine hardener matched to the layup temperature; budget for the 30–80% material cost premium and accept the longer pot life [S3][S5]. For thick castings, deep-pour river tables, or electrical potting where exotherm control is critical, epoxy is the only safe default [S3]. For road and bridge polyester-concrete overlays, agencies in the US spec isophthalic polyester cut with up to 50% styrene for fast return-to-traffic, accepting the styrene-emission controls that come with it [S2]. For drinking-water pipe and tank linings, specify styrene-free or vinyl ester systems to keep residual styrene below contact-water limits; the same article pool on synthetic resin chemistry tracks the regulatory drivers behind this shift.
Decision Criteria in One Frame
Compare the two systems on criteria supported by the research: (1) cost and market share, polyester is more affordable than epoxy [S1][S5] and holds around 75% of the UK resin market [S5]; (2) reinforcement compatibility, polyester is totally compatible with glass fibre, enabling the production of strong, lightweight and durable fibreglass composites [S5]; (3) shelf life, polyester has a short shelf life whereas epoxy does not, and epoxy is more forgiving to mix than the narrow proportions required for polyester [S6]; and (4) cured mechanical behaviour, cured polyester tends to be more brittle and less impact-resistant than epoxy [S1]. By those four criteria, the rule of thumb is unambiguous: polyester wins on throughput and unit cost in non-structural FRP; epoxy wins on mechanicals, chemistry, and mix forgiveness in anything load-bearing or precision-cast.
The next spec to watch is styrene-free unsaturated polyester and vinyl-ester systems, which are now specified for US drinking-water and food-contact applications and are gradually displacing high-styrene (>40%) orthophthalic formulations in California and EU plant builds. For process engineers weighing mould-vs-cast decisions against tolerances tighter than ±0.3% of nominal dimension, the higher cure shrinkage of polyester rules it out, and the higher exotherm of thick polyester castings rules it out for pours over 12 mm; both boundaries shrink the applicable envelope of polyester in precision work every year [S2][S3].
For the relevant spec sheets and selection criteria, see peek, and pom.
See also our earlier report, Magnetic Tape vs QR Code AGV Guidance: A Spec-Level Decision Guide.