Why this is a family decision first
Before choosing a particular grade for a tank, it helps to decide the family. Orthophthalic, isophthalic and vinyl ester resins are processed in similar ways — hand lay-up, spray-up, winding, room-temperature MEKP cure — but they are positioned for very different service, and the family sets the boundaries of what a laminate can be expected to withstand.
The three families
Orthophthalic polyester, built on phthalic anhydride, is the general-purpose family: economical and widely used for water tanks, sheets and moulded parts in mild service. Isophthalic polyester, built on isophthalic acid, is the family the catalogue specifies where a laminate faces water immersion, chemical service or sustained outdoor conditions. Vinyl ester, made by reacting an epoxy resin with methacrylic acid, is chosen for more corrosive service.
Within isophthalic, the catalogue has service-specific lining grades: SPR 301 OT for oil storage and SPR 301 OS for solvents, sewage and effluent.
Published typical values
| Property | GP Clear Resin (SPR 201 GPC) | ISO Polyester Resin (SPR 301) | Oil Storage Tank Grade Resin (SPR 301 OT) | Vinyl Ester Resin (SPR 501 VE) |
|---|---|---|---|---|
| Chemistry | Orthophthalic, Unsaturated Polyester | Isophthalic Unsaturated Polyester Resin | Isophthalic unsaturated polyester | Bisphenol-A Epoxy Vinyl Ester |
| Viscosity (typical) | 350–450 cPs | 500 cPs (mPa·s) at 25 °C | 300–600 cPs at 25 °C | 350 cPs (at 25°C) |
| Gel time / pot life (typical) | < 8 minutes | 15 minutes at 25 °C | 15–35 minutes at 25 °C | 15–20 minutes at 25°C (see TDS test condition) |
| Styrene content (typical) | 33–40% | 40% by weight | 33–45% by weight | 40% |
| Source of values | GP Clear Resin TDS | ISO Polyester Resin TDS | 2026 catalogue; grade TDS | Vinyl Ester Resin TDS |
Typical values, reproduced from each product’s own published data. They describe the liquid resin at its reference condition, not a finished part, and are not guaranteed limits.
The corrosion barrier
In a chemical-service tank, the inner corrosion barrier — resin-rich, usually veil-reinforced, backed by chopped strand — is the part that meets the medium. It is common to use a more resistant resin in the barrier than in the structural laminate behind it. So the family decision can be made separately for the barrier and the structure, and often is.
What decides the answer
The stored medium by name, its concentration, the operating and upset temperatures, and whether there is a governing design standard. None of the families is universally best, and none has a published compatibility chart on this site; the choice is confirmed against the actual service.