White Paper · Selection
Chemical resistance is decided in the polymer matrix, not in the glass. Selecting the wrong resin turns a non-corrosive material into a corrosion problem. This paper presents a structured selection methodology: how the principal resin families compare, how concentration and temperature together set the rating, which test methods validate the choice, and the failure modes to watch for in service. It complements the earlier engineering paper by focusing on the decision procedure rather than the attack chemistry.
- Resin family selection is driven by the worst-case chemical × temperature combination, not the average one.
- Concentration and temperature are a matrix: a resin rated "excellent" at 25 °C / 10% may be "poor" at 60 °C / 37%.
- ASTM C581 coupon immersion is the definitive validation for critical or uncertain service.
Five Families and Their Duty
Indicative service limits per ASTM D648 heat-deflection and manufacturer resin data sheets; exact values depend on formulation.
| Resin System | Profile | Strongest Suit | Typ. Service Limit | Best-Fit Service |
|---|---|---|---|---|
| Orthophthalic (OP) | General-purpose, lowest cost | Water, mild washdown, indoor | ~60 °C | Not for acids/alkalis/oxidizers |
| Isophthalic (ISO) | Mid-grade, better moisture/acid | Dilute acids, wastewater, brine | ~70–80 °C | Workhorse for chemical plants |
| Bisphenol-A vinyl ester | Strong acid / oxidizer / solvent | Bleach, acids, moderate solvents | ~90–110 °C | Standard aggressive service |
| Novolac vinyl ester | Highest chemical / heat resistance | Concentrated acids, oxidizers, hot | ~120–150 °C | Premium; highest cost |
| Phenolic | Fire / low-smoke, moderate chemistry | Fire zones, dilute acids | ~120 °C (fire-limited) | Selected for fire, not corrosion |
Resin families compared for selection; the matrix below applies the A–D rating method at room temperature.
14 Conditions × 5 Resin Families
Indicative A–D ratings from industry chemical-resistance charts. Temperature and concentration both move the rating; always select on the worst-case combination.
| Chemical / Concentration / Temperature | OP | ISO | VE | NVE | PH |
|---|---|---|---|---|---|
| HCl 10% / 25 °C | B A A A B | ||||
| HCl 37% / 25 °C | D C B A C | ||||
| HCl 37% / 60 °C | D D C B D | ||||
| H₂SO₄ 10% / 25 °C | B A A A A | ||||
| H₂SO₄ 50% / 60 °C | D C B A C | ||||
| H₂SO₄ 98% / 25 °C | D D C B D | ||||
| NaOH 10% / 25 °C | D C B B B | ||||
| NaOH 50% / 25 °C | D D C C B | ||||
| NaOCl bleach 12% / 25 °C | D D B A C | ||||
| H₂O₂ 30% / 25 °C | D C B A D | ||||
| Acetic acid 50% / 25 °C | D C B A C | ||||
| Seawater / brine / 25 °C | A A A A A | ||||
| Methanol / 25 °C | D C B A B | ||||
| Acetone / 25 °C | D D D C D |
Legend: A Excellent B Good C Fair (confirm by test) D Not recommended.
Indicative for selection; manufacturer chemical-resistance guides and ASTM C581 immersion govern final approval. Ratings degrade as temperature rises (roughly one band per 10–15 °C).
How a Selection Is Validated
| Method | What It Measures | When to Require It |
|---|---|---|
| ASTM C581 | Chemical resistance of thermoset resins in liquid service (immersion, 30/90/180 day) | Primary containment, oxidizer service, uncertain chemistry |
| ASTM D570 | Water absorption over 24 h | Moisture-exposed baseline |
| ASTM D648 | Heat-deflection temperature (HDT) | Continuous service limit |
| ASTM D790 (post-immersion) | Flexural strength retention after immersion | C581 companion measurement |
| Manufacturer chemical charts | A–D ratings at stated concentration/temperature | Preliminary selection; room temperature, benign fluids |
For non-critical, room-temperature, benign fluids the manufacturer chart is sufficient; for critical service require C581 data on the exact resin batch.
The Atmosphere Above the Sump
Specifiers often list only the liquid underfoot and forget the vapour above it. In a chlorine bleach plant, an acid sump or a solvent area, the vapour phase can be more aggressive than the liquid because it reaches every surface — including the top of the grating, cut edges and clip hardware — and condenses on cold faces. A resin that resists 10% HCl liquid may still chalk under concentrated HCl vapour at 60 °C. Inventory the vapour-phase chemicals too, and apply the same concentration × temperature logic to them. For vapour service, specify the resin one grade higher than the liquid rating suggests.
What Goes Wrong When the Resin Is Wrong
Fiber bloom
Without a C-glass surfacing veil, chemical attack penetrates along the fiber/matrix interface and glass fibers emerge on the surface as a white "bloom." The panel still looks whole but has lost surface strength. Fix: specify a resin-rich C-veil on both faces.
Swelling & softening
Solvents penetrate the cross-linked network, swelling the resin and dropping flexural strength. Acetone and ketones are hostile to nearly all room-temperature thermosets; vinyl ester resists better but is not immune.
Crazing & cracking
Thermal cycling under chemical exposure creates surface micro-cracks that accelerate further attack. Often seen at cut edges where the resin barrier was not sealed.
Blistering
Osmotic blistering in immersion service when fluid wicks along micro-voids between glass and resin. Indicates poor wet-out or insufficient barrier; require C581 and resin-rich faces.
Where Chemical Selection Actually Fails
Most in-service chemical failures are not in the panel face — they are at field-cut edges and at clips. A factory panel has a C-veil barrier; a cut edge exposes glass fibers to wicking. Chemical (or condensate) migrates along the fiber/matrix interface, producing bloom, blistering and eventual delamination — starting at the edge and working inward. Stainless clips in chloride service can also pit at the contact. The specification response is cheap and deterministic:
- Seal every cut edge with matching resin or FRP banding, not just paint.
- Use A4/316 (not A2/304) clips in chloride, bleach and wastewater service.
- Avoid leaving bare panel edges exposed to condensate; frame them where possible.
Six Steps, Repeatable on Any Project
- Inventory every chemical, concentration and maximum continuous temperature — including washdown, seasonal upsets and vapour phase.
- Identify the worst-case combination (lowest A–D rating) across the matrix.
- Match the resin to that worst case; require A or B, or C only with coupon confirmation.
- Overlay secondary requirements: fire (E84/IMO), UV, food/pharma contact, color.
- De-rate for temperature if service is elevated; move ratings down one band per 10–15 °C above the chart baseline.
- Validate with C581 coupons when uncertain or critical; check weight change, flexural retention and visual appearance.
When the Chart Is Enough
Manufacturer A–D compatibility charts are derived from decades of field and coupon data and are adequate for room-temperature, benign, non-critical service. They are not a substitute for ASTM C581 immersion when any of the following is true: the fluid is an oxidizer; concentration is above ~50%; temperature is above ~50 °C; the panel is primary containment or immersion; or the chemistry is unusual (new solvents, mixed process fluids). Require 30/90/180-day coupon data on the exact resin batch, with flexural retention above ~80–85% and negligible weight change. The chart gets you to 90% of decisions; the coupon closes the last 10% that matter.
Five Lines to Take Away
- Resin, not glass, decides corrosion. The strongest glass in the wrong matrix still fails.
- Match the worst case, not the average: highest concentration, highest temperature, and the vapour above the sump.
- Always specify a C-veil and seal cut edges; that is where chemical selection actually fails in the field.
- De-rate one rating band per 10–15 °C above room temperature; the matrix accelerates by Arrhenius.
- Coupon-test oxidizer, immersion or unusual service with ASTM C581; the chart is for selection, not approval.
Put the Chemical List in the Spec
A defensible selection is also a documented one. Attach the chemical inventory — name, concentration, maximum continuous temperature, vapour phase and duration — to the specification, state the resin chosen and the rating it relies on, and note any C581 coupon data that backed the choice. This does three things: it prevents a later substitute from swapping in a cheaper resin, it gives the next engineer the basis for the decision, and it turns a field failure investigation into a straightforward comparison of expected versus observed performance rather than a guess about what was specified.
Conclusions
Chemical selection is a procedure, not a guess. Inventory every chemical at its worst-case concentration and temperature, look it up in the resin matrix, require A or B, de-rate for temperature, and validate critical service with ASTM C581 coupons. Specify a C-veil face to prevent fiber bloom, and recognize the four failure modes — bloom, swelling, crazing, blistering — as the evidence that the resin was mismatched.
Sources Cited in This Paper
- Industry molded grating engineering guide (manufacturer PDF) — resin grades and chemical ratings. Manufacturer technical data available upon request.
- Industry pultruded grating metric design manual (manufacturer PDF) — resin service temperatures. Manufacturer technical data available upon request.
- ZeAllgrate — Guidelines for the Engineer/Designer (ACMA) — material selection for corrosive service.
- ASTM C581 (chemical resistance of thermosets), D570 (water absorption), D648 (heat deflection), D790 (flexural).
- Industry-standard resin chemical-resistance compatibility charts (leading manufacturers) for A–D ratings.
Get a Resin Match for Your Chemicals
List chemicals, concentrations and temperatures — ZeAllgrate returns a resin recommendation with C581-style data and a C-veil specification.