A. Detailed Gel Time & Exotherm Protocol (Probe + Thermocouple Method)

Scope & Applicability

• Designed for thermosetting resins (e.g. UPR, vinyl ester) with gel times > ~5 minutes.

• Useful in QC labs and pilot sizing to ensure cure consistency.

• Based on “withdrawn” ASTM D2471 but widely referenced as a practical QC method.

Materials & Equipment

• Resin sample, well stirred

• Catalyst / accelerator (as per formulation)

• Mixing cup, stirrer

• Gel-time reaction cup / block (metal) of fixed volume (e.g. 15 mL or 45 mL)

• Type K or T thermocouple, fine-wire

• Data logger with time stamping (1 s resolution or better)

• Stopwatch or timer

• Glass rod or probe (e.g. uncoated glass stirring rod, 3 mm diameter)

• Safety PPE (gloves, goggles, fume hood)

Procedure

1. Equilibrate resin to lab temperature (e.g. 23 °C). Record initial temperature T0T_0T0​.

2. Weigh a fixed mass or volume of resin (e.g. 20 g or specific mL) into mixing cup.

3. Add catalyst (e.g. 1–2 % MEKP or as per formulation). Start timer immediately.

4. Mix rapidly and uniformly (e.g. stir for 5–10 s) and transfer the mixture to the reaction cup.

5. Insert thermocouple centrally (ensuring good thermal contact with resin, not touching walls). Start logging.

6. Every 10–15 s, probe the resin surface with the glass rod: withdraw and inspect for resin film adherence. When no film adheres (or very weak), record that time as “gel time.”

7. Continue logging until the temperature peaks and begins to decline. From data, determine the time to exotherm peak and peak temperature.

8. Stop the experiment when temperature falls and the reaction is complete (or after a safe timeout).

Data Reporting

• Gel time (mm:ss)

• Time to peak (mm:ss)

• Peak temperature (°C)

• Heating rate (°C/min) in linear region

• Plot of temperature vs time curve

• If repeated runs, report mean ± standard deviation

Cautions & Tips

• Use identical volume / vessel geometry for each test to ensure consistency. Differences in cross-section or insulation will shift times and temperature.

• Calibrate thermocouples and data logger periodically.

• Keep mixing / transfer times minimal and constant to reduce timing error.

• Be aware of heat loss to vessel walls; use insulating supports if necessary.

• For very reactive resins (very short gel times), this method may become imprecise; consider rheometer or automated gel timers.