Resin decides chemical resistance

Chemical resistance in FRP grating is not a single number. It is set by the resin, the glass fibre and the interface between them. The medium meets the resin first; only after the resin is attacked does the fibre become exposed, and once it is exposed the attack accelerates and load capacity falls with it.

The common resins run roughly from orthophthalic polyester, through isophthalic polyester, to vinyl ester, with phenolic alongside. Orthophthalic is cheapest and least resistant; isophthalic handles general acid and alkali; vinyl ester resists alkali, chlorine and some solvents; phenolic's strength is fire performance and low smoke, with moderate alkali resistance.

For the product range and resin options, see the chemical-resistant FRP grating

Glass fibre is attacked too

Many people assume glass fibre is glass, and therefore immune. In fact E-glass is attacked by strong alkali and by hydrofluoric acid. Acid is gentler, but long immersion still reduces fibre strength, and this point is often missed when the material is chosen.

Alkali attacks glass continuously: it dissolves the silica skeleton at the fibre surface, the fibre thins and embrittles, and the grating loses capacity. That is why alkali duty usually calls for vinyl ester resin with a surface veil rather than ordinary polyester.

Hydrofluoric acid is a special case. It attacks almost every silicate material, glass fibre included. Standard FRP grating is not suitable where hydrofluoric acid is present; the manufacturer has to confirm the duty, and another material is often needed.

The fibre-resin interface is another weak point. Water or the medium creeps along the interface and debonds fibre from resin, showing as white marks, blistering and delamination. A surface veil works by adding a dense resin layer that slows that ingress along the interface.

Acids, alkalis, salts and solvents

Acids: most mineral acids have limited effect on isophthalic and vinyl ester resin, but strong oxidising acids such as nitric or concentrated sulphuric degrade the resin. Attack rises with concentration and temperature; dilute acid at ambient temperature is usually mild.

Alkalis: alkali attacks the glass more than the resin, especially sodium and potassium hydroxide. Alkali duty has to consider resin and fibre together; vinyl ester is the usual choice and ordinary polyester is normally not enough.

Salts: neutral salt solutions such as sodium chloride or sodium sulphate usually have little effect, but salt spray leaves crystals and repeated wet-dry cycles age the resin surface. Coastal projects also have to add UV to the salt exposure.

Solvents: solvents soften or swell the resin, and aromatic, ketone, ester and chlorinated solvents are the most aggressive. Solvent attack often shows no colour change at all, only weight gain, a soft surface and lost strength, which makes it harder to spot than acid or alkali. For more on matching media to products, see chemical-resistant grating selection

Concentration and temperature

The same chart can pass at ambient temperature and fail once heated. Reaction rate roughly doubles for every ten degrees, so both resin degradation and medium penetration speed up; the temperature column is the first thing to confirm when reading a chart.

Concentration is not linear. Dilute acid can attack some resins harder than concentrated acid, because the concentrated acid passivates the surface while the dilute acid keeps penetrating; with alkali, concentrated is usually the more dangerous, so the strength cannot be ranked by concentration alone.

The contact mode matters as much as the chemistry: continuous immersion, intermittent spray, splash, vapour and washdown are different duties. Charts are usually written for continuous immersion; splash duty may be relaxed, but only with a stated basis.

How to read a chemical resistance chart

A usable chart states the medium, its concentration, the temperature, the resin type, what the rating means and the test standard behind it. If any of these is missing, the conclusion is not reliable and cannot be used as an acceptance basis.

Ratings are often written as recommended, conditionally recommended or not recommended, or as A, B and C, but the definitions vary between makers. Read what conditional actually means: a temperature cap, a concentration cap, or short-term contact only.

Separate the resin rating from the finished product rating. A resin that resists a medium does not guarantee that a laminate with glass, filler and surface veil will resist it too, and cut edges, drilled holes and joints are places the resin does not fully cover.

Mixtures are often overlooked. Two media that pass separately can attack the resin together more than either alone, and cleaning agents, disinfectants and process additives belong on the list as well, not just the main medium.

When vinyl ester is required

Vinyl ester is not universal, but several duties leave little choice: strong alkali, sodium hypochlorite and chlorine environments, some solvents, and acid or alkali at elevated temperature. Isophthalic polyester in those duties often falls short of the design life.

Compared with isophthalic polyester, vinyl ester has fewer ester linkages, which gives better hydrolysis and alkali resistance. Compared with epoxy, it cures quickly and processes easily, which suits volume products such as grating.

Phenolic resin is the other route. Its strengths are fire performance, low smoke and resistance to some solvents, but it is brittle, only moderately alkali resistant and dark in colour; chemical platforms needing a fire certificate may justify it.

Moving one resin grade up costs more, but it is usually cheaper than replacing a whole installation a few years later. The point is to select from the medium, not from a blanket rule that chemical plants always use vinyl ester.

Surface veil and structural layer

The surface veil is the part most often dropped from a chemical-resistant grating. It forms a dense resin layer at the surface that slows ingress along fibres and interfaces; without it, the medium finds a path into the fibre quickly and corrosion starts from inside.

Veil material matters too. C-glass veil resists acid well, synthetic veil resists alkali and solvents better, and polyester veil is cheap but only moderately resistant. Specify it together with the resin rather than writing surface veil and stopping there.

The structural layer matters as well. Bearing bars, nodes and cut edges all need enough resin cover, and cut or drilled faces must be sealed with edge resin. On chemical projects that sealing step is mandatory, not optional.

Ordering and verification

State the medium, concentration, temperature, contact mode (immersion, spray or splash), cleaning and disinfection method, resin type and surface veil on the inquiry. A request that says only acid and alkali resistant leaves the supplier unable to choose a resin, and the quotation will default to a standard product.

Structural parameters and edge sealing follow the FRP grating specification guide

• List each medium with its concentration and temperature.
• Say whether the duty is continuous immersion or intermittent spray.
• Specify resin and surface veil together.
• Require sealed cuts and drilled faces.