What each resin is for

Resin is the only continuous phase in FRP grating: it encases the fibre and meets the corrosive medium first. The resin system therefore decides what medium, what temperature and what fire class the grating can be used in.

The four common resins are not a simple price ladder but four property curves: orthophthalic is cheapest and least resistant, isophthalic is the general-purpose workhorse, vinyl ester resists alkali and chlorine, and phenolic wins on fire and low smoke.

The first step is not choosing a resin but completing the medium list: main medium, cleaners, disinfectants and process additives, each with its concentration and temperature. For a point-by-point approach see the FRP grating chemical resistance guide

Orthophthalic: cost first

Orthophthalic polyester is the cheapest general-purpose resin, suited to ordinary atmosphere, weak acid and alkali, and clean water at ambient temperature. Its ester linkage density is high, hydrolysis stability is limited, and long contact with alkali or hot water leads to blistering and whitening.

In grating it is common where the corrosion load is light and the budget is tight, such as general plant walkways, dry platforms and decorative screening. Used around a tank that is continuously wet with acid or alkali, it usually falls short of the design life.

That does not make it unusable. Short splash, occasional washdown and dry indoor service are well within its range. The problem is that many projects read the word chemical as severe corrosion, so they either over-specify or, when it matters, under-specify.

Isophthalic: the general acid and alkali resin

Isophthalic polyester has a more regular molecular structure and a lower ester linkage density than orthophthalic, giving better hydrolysis and general acid and alkali resistance. It is one of the most common resins in chemical plants, wastewater works and coastal projects.

Its range covers dilute sulphuric and hydrochloric acid at ambient temperature, some organic acids, neutral salt solutions, and intermittent washdown or splash duty. Strong alkali, sodium hypochlorite, concentrated nitric acid and some solvents still exceed it.

Isophthalic performance depends heavily on cure. Under-cured panels have a tacky surface and poor resistance, and they are usually the first to fail an immersion test. Ask for a cure or Barcol hardness report, not just the resin grade name.

The same grade can be formulated differently by different makers, so request the chemical resistance table, cure system and typical immersion data for that grade.

Vinyl ester: alkali, chlorine and solvents

Vinyl ester keeps the epoxy backbone at each end of the molecule with only a few ester linkages in between, so hydrolysis, alkali and sodium hypochlorite resistance are clearly better than polyester, with better toughness and lower shrinkage as well.

Typical uses are around strong alkali tanks, sodium hypochlorite preparation rooms, chlorine environments, flue gas desulphurisation areas and some solvent duty. Isophthalic polyester in those places often blisters and exposes fibre within a year or two.

Vinyl ester is not universal either. Concentrated nitric acid, strong oxidisers, some chlorinated solvents and hot concentrated alkali can still exceed it, and ketones and esters attack it readily, so each medium still has to be checked against a table.

Battery plants combine sulphuric acid mist, electrolyte splash and cleaning agents, which is typical vinyl ester duty; see FRP grating in battery plants

Phenolic: fire and low smoke first

Phenolic resin's greatest strength is fire behaviour: it is hard to ignite, gives low smoke density and few toxic gases, and is often specified for chemical platforms, offshore facilities and tunnels that need a fire certificate.

Its weaknesses are toughness and alkali resistance. Phenolic has high crosslink density and is brittle, so impact resistance is below polyester and vinyl ester, strong alkali is not a good fit, and the colour is dark, which matters where appearance is specified.

Processing differs as well. Cutting and drilling produce finer dust and edges chip more easily, so sharp tooling and dust extraction are needed on site, and edge sealing must use a phenolic-compatible resin.

Temperature ceiling and derating

The resin heat deflection temperature sets the long-term service ceiling. General orthophthalic and isophthalic polyesters usually sit between 60 and 100 degrees Celsius, vinyl ester a little higher, phenolic higher still, depending on formulation and cure.

Temperature does not reduce capacity linearly. Near the heat deflection temperature the resin modulus falls quickly, the grating creeps under sustained load, and deflection grows with time, so an ambient load table cannot be applied to hot duty.

Heat also accelerates chemical attack: reaction rate roughly doubles for every ten degrees, speeding both medium penetration and resin degradation. Selection has to combine the highest temperature with the highest concentration.

For hot projects, ask the manufacturer for the heat deflection temperature report and the derating curve for that grade rather than estimating from the resin family.

Fire performance and cost

Fire rating and chemical resistance are independent properties. A product can be fire retardant but only moderately resistant, or highly resistant with ordinary fire performance; chemical projects often need both and each has to be confirmed separately.

Fire data should cover flame spread, smoke density, burning drips and toxic gases. ASTM E84, EN 13501 and GB 8624 use different methods and classes that do not convert directly, so the report must name its basis.

On the same panel type, orthophthalic is cheapest, isophthalic slightly higher, vinyl ester typically 30 to 50 percent more, and phenolic higher again because of process and volume. The gap moves with the market and should not be fixed in a budget.

Cost has to be read over the full life. Isophthalic in strong alkali may need replacing in three to five years, while the extra spent on vinyl ester is usually recovered within two or three replacement cycles.

Matching the medium and asking the right questions

Do not ask a supplier only whether the product is acid and alkali resistant. Ask how long that grade can be continuously immersed, at what concentration and temperature, whether there is third-party immersion data, and what the surface veil is made of.

Ask about cure and batch consistency as well. With the same grade, different catalyst ratios, ambient temperatures and moulding times give very different resistance, so batch projects should require cure records.

Requirements for site cutting, openings and edge sealing are set out under FRP grating fabrication and edge sealing

Panel depth, bearing bar spacing and resin grade are listed together in the FRP grating specification

• List every medium with its concentration, temperature and contact mode.
• Specify resin and surface veil together.
• Request third-party immersion data for the grade.
• Put cure and edge sealing into the acceptance clause.