Two thixotropic grades, one question
Both grades exist for the same lay-up problem: large, steep cooling-tower and wind-energy mouldings where resin must not drain before it gels. They differ in chemistry and working window, and that is where the choice lies.
The chemistry difference
SPR 201 CTG is orthophthalic, the general-purpose family, sharing its typical liquid values with GP Clear Resin. SPR 301 CT is isophthalic, the family the catalogue specifies for heat, chemical and corrosion resistance. A cooling tower’s circulating water is often warm and dosed with treatment chemicals, which is the service CT is positioned for.
Published typical values
The catalogue publishes a thixotropic index and two-speed viscosity for CT, and only the thixotropic role for CTG; the numbers are not a like-for-like sag comparison.
| Property | Orthophthalic Cooling Tower Grade Resin (SPR 201 CTG) | Isophthalic Cooling Tower Grade Resin (SPR 301 CT) |
|---|---|---|
| Chemistry | Orthophthalic unsaturated polyester | Isophthalic unsaturated polyester |
| Appearance | Clear liquid | Cloudy to amber liquid |
| Viscosity (typical) | 350–450 cPs | 400–650 cPs (60 rpm); 1,500–2,000 cPs (6–12 rpm) |
| Thixotropy | Not published | 2.0–3.5 |
| Gel time / pot life (typical) | Under 8 minutes | 15–35 minutes at 25 °C |
| Styrene content (typical) | 33–40% | 37–45% by weight |
| Source of values | 2026 catalogue; grade TDS | 2026 catalogue; grade 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.
Working window
At their reference cures, CTG gels in under 8 minutes typical and CT in 15–35 minutes at 25 °C. A short window suits small stages and fast turnaround; a long one suits large panels built in stages. Either way, the window on the real mould depends on shop temperature and must be confirmed.