The word "polyester" is used in two different worlds, and the confusion is constant. In textiles, polyester is a PET-based thermoplastic fibre. In the composite industry it means unsaturated polyester resin (UPR): a thermosetting binder that cannot be re-melted once it has cured.
This article is about the second — the resin inside every GRP part, from a boat hull to a train panel.
What Unsaturated Polyester Is
Unsaturated polyester is a polymer produced by the polycondensation of a dibasic acid (phthalic anhydride, isophthalic acid or maleic anhydride) with a glycol (propylene glycol, ethylene glycol). The adjective "unsaturated" comes from the double bonds left along the polymer chain, ready to react — curing proceeds through exactly those bonds.
As produced, the resin is close to solid at room temperature. To become usable it is dissolved in a reactive solvent — almost always styrene. Commercial polyester resin typically contains 30–45% styrene. Styrene is not merely a thinner: during cure it bridges the polymer chains and becomes part of the structure.
How Curing Works
Polyester cures by free-radical polymerisation. Three components are required:
- Resin + styrene — the base system.
- Accelerator — usually cobalt octoate, most often pre-added at the factory ("pre-accelerated" resin).
- Catalyst / initiator — MEKP (methyl ethyl ketone peroxide), added at roughly 1–2% by weight.
The moment MEKP is added, the cobalt decomposes it into free radicals; the radicals attack the double bonds in styrene and in the polyester chains, and a three-dimensional cross-linked network forms. The process is exothermic. In thick laminates, uncontrolled exotherm causes cracking and discolouration.
Safety note: never mix cobalt accelerator directly with MEKP. In neat form the two react violently. The accelerator always goes into the resin first, and the catalyst into the mixture afterwards.
Gel time, cure and post-cure
- Gel time: the period until the mixture begins to lose its flow, typically 15–40 minutes. It is tuned by temperature, catalyst level and resin grade.
- Cure: within a few hours the part is firm enough to demould.
- Post-cure: final mechanical and chemical resistance takes weeks at ambient temperature. A few hours at 40–80 °C shortens this and improves properties.
Polyester Resin Grades
| Grade | Character | Typical application |
|---|---|---|
| Orthophthalic | General purpose, economical, moderate chemical and water resistance | Interior panels, general GRP parts, bodywork |
| Isophthalic | Higher water, hydrolysis and chemical resistance; better toughness | Boat hulls, water and chemical tanks, exterior |
| Isophthalic-NPG | Outstanding water and UV resistance, low osmosis risk | Marine gelcoat, yacht hull barrier coat |
| Bisphenolic | High temperature and aggressive chemical resistance | Chemical plant tanks, stacks, acid lines |
| DCPD | Low shrinkage, low styrene emission, good surface | Automotive panels, visible surfaces |
| Flame retardant | ATH-filled or halogenated/phosphorus formulation | Rail, marine and public transport interiors |
Vinylester: between polyester and epoxy
Vinylester resin has an epoxy backbone but is dissolved in styrene and cured with MEKP like polyester. It offers markedly better water and chemical resistance and better impact toughness than polyester, while remaining cheaper and easier to process than epoxy. In yacht hulls, the first few plies behind the gelcoat are frequently vinylester as an osmosis barrier.
Strengths and Limits
Strengths: low cost, ambient-temperature cure, easy processing, wide formulation latitude, excellent compatibility with glass fibre, good surface quality.
Limits: 5–8% volumetric shrinkage on cure, lower adhesion and mechanical performance than epoxy, styrene emission requiring ventilation and occupational controls, and osmosis risk under prolonged water contact.
That last point is the most debated one, and it is directly tied to gelcoat selection.
Styrene, Safety and Environment
Styrene is a volatile organic compound; inhalation and skin contact must be controlled. Closed-mould processes — vacuum infusion and RTM — reduce styrene emission dramatically compared with open hand lay-up. This is one of the strongest arguments for moving from open to closed moulding, for worker health and for ISO 45001 and ISO 14001 compliance alike.
Polyester at Zenginler İmalat
Zenginler İmalat has been manufacturing with polyester since 1977. The laminate used in rail applications is produced with flame-retardant Büfa Firestop 8175-W-1 resin and Şişecam glass fibre, with Büfa Firestop GC S285-SV fire gelcoat on the surface. This recipe was tested by TSE under TS EN 45545-2:2023 and documented in classification report no. 304218 as meeting requirement set R1 at hazard levels HL1, HL2 and HL3.