Resin for FRP Cooling Tower Manufacturing

Cooling tower fabricators in Nepal work on two kinds of tower. Package (bottle) towers arrive as moulded FRP shells — the round blue units seen beside plant rooms, hotels and hospitals — and field-erected induced-draft towers are assembled on site from panels and structural sections.

The FRP scope usually covers the casing and shell panels, the fan stack or fan cylinder, air-inlet louvres, the cold-water basin or basin lining, internal partitions and baffle walls, fan blades and motor supports where the design calls for them, and access items such as ladders, handrails and walkway gratings. Not every part of a tower is FRP: the fill pack and drift eliminators are normally moulded thermoplastic (usually PVC or PP) supplied as a separate item, and the motor, gearbox and pipework are bought in.

Every grade below cures at room temperature with the same two-part workshop system — MEKP hardener with a cobalt octoate accelerator, each added to the resin separately — so a fabricator can move between them without re-equipping. What separates them is body, gel time and chemistry.

GP Clear Resin is the general-purpose starting point: 350–450 cPs, a gel time under 8 minutes and 33–40% styrene, which wets chopped strand mat out quickly on flat and gently curved panels. GP Yellow Resin is the same orthophthalic chemistry in a thicker, slower grade — 450–600 cPs and a 10–15 minute gel time — which buys working time when several large panels are laminated in one shift. GP White Resin is the white-pigmented grade on that same GP base specification, and the supplier lists cooling towers among its uses: worth considering where a white laminate is wanted without a separate pigment addition.

The two catalogue cooling-tower grades are thixotropic: formulated to thicken at rest and thin again under a brush or roller, so the resin stays where it is put on a vertical shell instead of draining to the bottom of the panel. Orthophthalic Cooling Tower Grade Resin (SPR 201 CTG) is the orthophthalic one, listed at 350–450 cPs with a gel time under 8 minutes. Isophthalic Cooling Tower Grade Resin (SPR 301 CT) is the isophthalic one, and the catalogue publishes its rheology in full: 400–650 cPs at 60 rpm against 1,500–2,000 cPs at 6–12 rpm, a thixotropic index of 2.0–3.5, a gel time of 15–35 minutes and 37–45% styrene, with heat, chemical and corrosion resistance and high strength listed against it.

Four things act on a cooling tower laminate at once, and each one points at a different part of the decision.

• Permanent wetting. Internal surfaces are wet whenever the tower runs, and damp even when it does not. This is the reason isophthalic chemistry is offered for the job at all: Isophthalic Cooling Tower Grade Resin (SPR 301 CT) and ISO Polyester Resin are the isophthalic grades in the range.
• Warm water. The circulating water leaves the process warm. No service-temperature limit is published for these grades on this site, so where a plant runs hotter than ordinary comfort-cooling duty, ask for the operating temperature to be checked against the grade before ordering.
• Water treatment. Biocides, chlorine, scale and corrosion inhibitors are dosed into the circuit. The chemical, its concentration and the temperature decide whether a polyester laminate is appropriate or whether Vinyl Ester Resin is the right route. Neither this page nor the product pages publish a chemical-resistance chart.
• Sun and weather. The outside of a tower sits in full sun. A pigmented gelcoat is what takes that exposure, not the structural laminate underneath.

The mould-face surface of a tower shell is usually a gelcoat, laid into the mould before the structural laminate. GP Gelcoat Resin is the general white, high-gloss, thixotropic gelcoat in the range. ISO Gelcoat is the isophthalic one; its TDS summary publishes a pre-accelerated system, 40,000 cPs at low shear against 1,500 cPs at high shear, a thixotropic index of 26, an 8-minute gel time and a 700 µm target wet film within a 600–800 µm range, and the 2026 catalogue positions the isophthalic gelcoat where the surface faces weathering or water contact.

Note the difference in cure: the ISO Gelcoat is supplied pre-accelerated, so only MEKP hardener is added at the bench, while the laminating grades above are non-accelerated and take both catalyst and accelerator. Colour, film thickness and the finish a customer will accept are worth settling in writing before the first shell is pulled.

Casing panels, shells and basin sections are laminated by hand lay-up, and larger shops chop and spray with a spray-up gun. chopped strand mat is laid into the mould and wetted out with resin catalysed by MEKP hardener and accelerated with cobalt octoate; the two are added to the resin separately and never mixed with each other first. Layers are built up to the thickness the design calls for, with extra plies at ribs, edges, bolt lines and lifting points.

Where the mould face is vertical or steeply curved — a package tower shell, a fan cylinder — a thixotropic grade holds the wet laminate in place while it gels. Panels cure at room temperature, then are trimmed, drilled and assembled with the tower’s steelwork or bolted shell-to-shell. Confirm the catalyst and accelerator dosage for the shop temperature on the day, and run a trial laminate before committing a production run to a grade that is new to the shop.

• Which component is being laminated — an external casing panel, an internal partition, a fan stack or the basin?
• Is the lay-up flat, or vertical and steeply curved enough to need a thixotropic grade?
• What treatment is dosed into the circulating water, at what concentration, and how hot does the water return?
• How big is a single lay-up, and how long must the resin stay workable to finish it in one pass?
• What is the workshop temperature during the production run, and has the catalyst and accelerator dosage been set for it?
• Does the end customer’s specification name a resin type, a gelcoat colour or a laminate thickness?
• Is there a fire requirement for the plant room or enclosure the tower sits in?
• What quantity is needed, and by when, so supply into Nepal can be planned?

• Harsh water chemistry. Where the dosing is aggressive or the water is hot, Vinyl Ester Resin is the chemical-service route; the decision belongs to the named chemical, its concentration and the temperature, not to the resin’s name.
• General wet or chemical service without a vertical lay-up. ISO Polyester Resin is the plain isophthalic laminating grade — 500 cPs, a 15-minute gel time, 40% styrene and a 90 °C heat deflection temperature in its TDS summary, which is resin data rather than a service limit.
• A fire requirement. If the enclosure or plant room specifies fire-retardant laminates, that requirement belongs to the fire-retardant grades, and it is decided by the finished, tested part rather than by the resin alone.
• Structural sections rather than mouldings. Walkway and support sections pulled as continuous profiles are a pultrusion job — see Pultrusion Grade Resin (SPR 401 PT) and the pultrusion guide.
• Translucent roof panels over the tower. That is sheet work, covered by the roof-light sheet guide, not by a cooling-tower grade.