White Paper · Resin Selection
An FRP grating installed in the wrong resin is a corrosion problem wearing a non-corrosive material. The chemistry lives in the polymer matrix; the glass fibers only carry load. This paper explains how chemicals attack thermosets, compares the five principal resin families, and provides a 20-chemical × 5-resin compatibility matrix to make the selection defensible.
- Match the resin to the worst-case chemical across all concentrations and temperatures seen in service.
- Temperature accelerates attack: reaction rate roughly doubles per 10 °C rise (Arrhenius).
- A C-veil / surfacing veil keeps glass fibers below the surface and prevents "bloom" under chemical attack.
Three Mechanisms — and Temperature
1. Hydrolysis (ester-bond cleavage)
Polyesters contain ester linkages that acids, alkalis and hot water can hydrolyze, gradually weakening the resin and exposing glass fibers. Orthophthalic resin is most susceptible; isophthalic improves it; vinyl ester uses fewer ester linkages and resists hydrolysis far better. Strong alkali (caustic) is especially hard on polyesters.
2. Oxidation
Strong oxidizers — chlorine bleach (NaOCl), concentrated peroxides, chlorine dioxide, nitric acid — oxidize the polymer backbone and can embrittle or discolor a general-purpose resin. Bisphenol-A and especially novolac vinyl esters are formulated to survive oxidizing service.
3. Solvent swelling
Organic solvents (ketones, aromatics, some alcohols) penetrate the resin network and swell it, reducing strength and bonding. Cross-linked vinyl esters and phenolics resist better than orthophthalic; ketones such as acetone are hostile to nearly all room-temperature thermosets.
Temperature effect (Arrhenius)
Chemical degradation is a chemical reaction; its rate approximately doubles for every 10 °C rise. A resin rated "excellent" at 25 °C may only be "fair" at 60 °C. Always select on the maximum continuous temperature, and de-rate compatibility ratings as temperature rises.
Five Families and Where Each Belongs
| Resin System | General Profile | Strongest Suit | Typical Service Limit |
|---|---|---|---|
| Orthophthalic (OP) | General-purpose, lowest cost | Water, mild environments | ~60 °C |
| Isophthalic (ISO) | Mid-grade, better moisture/acid resistance | Dilute acids, wastewater, brine | ~70–80 °C |
| Vinyl Ester (VE, bisphenol-A) | Strong acid / oxidizer / solvent resistance | Acids, bleach, moderate solvents | ~90–110 °C |
| Novolac Vinyl Ester (NVE) | Highest chemical/heat resistance | Strong acids, oxidizers, hot service | ~120–150 °C |
| Phenolic | Fire-resistant, low smoke, moderate chemical | Fire/low-smoke areas, dilute acids | ~120 °C (fire-limited) |
Service temperatures indicative; exact values per resin data sheet and ASTM D648 heat-deflection test. Novolac VE is the premium choice for concentrated acids/oxidizers.
20 Chemicals × 5 Resin Families (room temperature)
Indicative A–D ratings from industry chemical-resistance charts. Verify at your actual temperature and concentration with coupon testing (ASTM C581) if critical.
| Chemical (concentration) | Orthophth. | Isophthalic | Vinyl Ester | Novolac VE | Phenolic |
|---|---|---|---|---|---|
| Hydrochloric acid (HCl) 10% | B A A A B | ||||
| Hydrochloric acid (HCl) 37% | D C B A C | ||||
| Sulfuric acid (H₂SO₄) 10% | B A A A A | ||||
| Sulfuric acid (H₂SO₄) 50% | C B A A B | ||||
| Sulfuric acid (H₂SO₄) 98% | D D C B D | ||||
| Sodium hydroxide (NaOH) 10% | D C B B B | ||||
| Sodium hydroxide (NaOH) 50% | D D C C B | ||||
| Sodium chloride brine | A A A A A | ||||
| Chlorine bleach (NaOCl) 12% | D D B A C | ||||
| Hydrogen peroxide 30% | D C B A D | ||||
| Methanol | D C B A B | ||||
| Acetone | D D D C D | ||||
| Toluene | D C B A B | ||||
| Acetic acid 50% | D C B A C | ||||
| Phosphoric acid 85% | C B A A A | ||||
| Ammonium hydroxide | D C B B B | ||||
| Ferric chloride solution | B A A A A | ||||
| Seawater | A A A A A | ||||
| Diesel / diesel oil | C B A A B | ||||
| Crude oil | C B A A B |
Legend: A Excellent (25+ yr expected) B Good (long service, monitor) C Fair (limited life, confirm by test) D Not recommended at this concentration/temperature.
Matrix is indicative for selection; manufacturer chemical-resistance guides and ASTM C581 coupon immersion govern final material approval. Ratings degrade as temperature rises.
Ratings Degrade as Temperature Rises
Select on maximum continuous temperature; the matrix above is room-temperature.
| Resin System | Typical Continuous Service Limit | Representative Duty |
|---|---|---|
| Orthophthalic | 60 °C (140 °F) | General water/washdown |
| Isophthalic | 70–80 °C (158–176 °F) | Dilute acid, wastewater |
| Vinyl ester (bisphenol-A) | 90–110 °C (194–230 °F) | Acids, bleach |
| Novolac vinyl ester | 120–150 °C (248–302 °F) | Strong oxidizers/hot acids |
| Phenolic | ~120 °C (fire-limited) | Fire/low-smoke zones |
Per ASTM D648 heat-deflection temperature and resin data sheets; oxidative service temperatures are often lower than mechanical HDT would suggest.
A Repeatable Six-Step Procedure
- List every chemical, its concentration and its maximum continuous temperature seen over the year — including washdown chemicals and seasonal upsets.
- Identify the worst case: the chemical × temperature combination with the lowest rating in the matrix.
- Match the resin to that worst case (A or B rating required; C only with coupon confirmation).
- Apply secondary factors: fire rating (E84/IMO), UV exposure, food/pharma contact requirements, color.
- De-rate for temperature: if operating temperature is elevated, move ratings down one band and re-check.
- Validate with coupons when uncertain: ASTM C581 immersion at service temperature, then check weight change, flexural retention and visual appearance.
Why a C-Veil Protects the Matrix
The top and bottom faces of a good FRP panel are covered by a rich-resin surfacing veil (often a "C-glass" veil) that places 100% resin over the first layer of glass. Without it, the outermost glass fibers are directly exposed; chemical attack penetrates along the fiber/matrix interface (fiber "bloom"), weakening the panel long before the bulk resin fails. A C-veil is the cheapest chemical-resistance upgrade available and should be specified for any corrosive service. For secondary containment or immersion service, increase resin-rich face thickness and consider a chopped-strand-mat-rich barrier.
ASTM C581 and Field Evidence
The definitive method for qualifying a resin against a specific fluid is ASTM C581 — Standard Practice for Determining Chemical Resistance of Thermosetting Resins Used in Glass-Fiber-Reinforced Structures Intended for Liquid Service. Coupons are immersed at service temperature for an extended period (typically 30, 90 and 180 days), then measured for flexural strength retention, weight change, thickness change and visual appearance. Retention above 80–85% with negligible weight gain is generally acceptable. For non-critical, room-temperature, benign fluids, the compatibility matrix plus manufacturer guidance is sufficient; for primary containment or oxidizer service, require C581 data on the exact resin batch.
Conclusions
Chemical resistance is a resin-selection problem, not a material-form problem. Build the chemical inventory at worst-case concentration and temperature, pick the resin that rates A or B against that combination, specify a C-veil face, and require ASTM C581 coupon validation for oxidizer or immersion service. A general-purpose orthophthalic panel will not survive a bleach sump no matter how many kilograms of glass it contains.
References
- Industry molded grating technical catalog (manufacturer data) — resin systems and fire/chemical grades. Manufacturer technical data available upon request.
- Industry pultruded grating technical brochure (manufacturer data) — resin grades and 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 Thermosetting Resins in Glass-Fiber-Reinforced Structures; ASTM D648 (heat deflection), D570 (water absorption).
- Industry-standard resin chemical-resistance compatibility charts (leading manufacturers) for A–D ratings.
How Chemicals Attack Thermoset Resins
Three mechanisms dominate. Hydrolysis — water and caustic attack the ester linkages in polyester and vinyl ester backbones; vinyl ester's fewer ester groups are why it resists hot alkaline service better. Oxidation — strong oxidizers (bleach, hydrogen peroxide, chlorine dioxide) attack the resin surface and, in concentrated form, the polymer network itself; this is the chemistry that separates vinyl ester from isophthalic in pulp and chlor-alkali plants. Solvent attack / stress cracking — ketones and aromatics swell and craze the matrix, often accelerated by applied load; the chemical list must be checked for solvents, not just acids and bases. Temperature multiplies every mechanism — reaction rate roughly doubles per 10 °C rise — so the design chemical is the worst case at the highest service temperature, not the average.
The Five Resin Families in One View
| Resin | Chemical Role | Temperature | Typical Cost |
|---|---|---|---|
| Orthophthalic polyester | General-purpose baseline; low acid resistance | ~60–70 °C | Lowest |
| Isophthalic polyester | Weak acids, brine, wastewater workhorse | 70–80 °C | Baseline 1.0× |
| Vinyl ester | Strong acids, oxidizers, solvents, heat | 90–110 °C | 1.4–1.8× |
| Phenolic | Fire/low-smoke; modest chemical range | ~120 °C (fire-limited) | 2.5–3.5× |
| Modified (novolac epoxy vinyl ester) | Highest chemical/heat envelope | 110–120+ °C | Highest |
Making the Selection Defensible
Assemble the full chemical list with concentrations and temperatures, check each against the resin chart, and select the resin that survives the worst case — not the one that survives the average. Record the judgement in the specification so the choice is auditable. Where the list spans families (weak acid plus bleach, for example), select for the aggressive member; where temperature pushes a borderline resin past its limit, move up a family rather than relying on margin. This is the process that turns resin selection from an opinion into an engineering decision.
Assemble the full chemical list with concentrations and temperatures, check each against the resin chart, and select the resin that survives the worst case — not the one that survives the average. Record the judgement in the specification so the choice is auditable. Where the list spans families (weak acid plus bleach, for example), select for the aggressive member; where temperature pushes a borderline resin past its limit, move up a family rather than relying on margin. This is the process that turns resin selection from an opinion into an engineering decision.Get a Resin Recommendation for Your Chemicals
List your chemicals, concentrations and temperatures — ZeAllgrate returns a resin match with C581-style data and a grit/C-veil specification.