Determination of the Gelation Time by DLTMA - METTLER TOLEDO

Determination of the Gelation Time by DLTMA

Purpose

To determine the gelation time (gel point) of an epoxy powder. 

 

Sample

Uncured KU600 epoxy resin powder. 

 

Conditions

Measuring cell: TMA/SDTA

Probe: Ball-point probe (quartz glass); diameter 3 mm

Sample preparation: After the temperature program had been started and the temperature of 160 C reached, the furnace was opened and several milligrams of coating powder spread directly on the sample support so that the molten powder coalesced and formed a drop under the probe. The furnace was then immediately closed. 
After the measurement, the sample was burned off with a small flame and the residue brushed away after cooling. 
Note: The residue must not become glassy and stick to the probe or sample holder.

TMA measurement: Isothermal measurement at 160 °C for 40 min

Load: every 6 s alternating between +0.05 N and -0.05 N (period 12s), i.e. the probe is raised from the sample support for 6 s, and rests on the sample or sinks into the sample for 6 s. 

Atmosphere: Static air


Interpretation

The upper part of Figure 1 shows the DLTMA curves of the coating in bending mode for the 1st (black) and 2nd (blue) heating runs. The lower part of the figure shows the two bending modulus curves on a logarithmic ordinate scale calculated from the two DLTMA curves. 
At T1, the uncured coating softens. The glass transition is apparent as an increase in the DLTMA amplitude and the corresponding decrease of the modulus. Since the viscosity of the resin decreases, the probe penetrates further into the coating until it rests directly on the metal blade (at T2). The elastic modulus between T2 and T3 corresponds roughly to that of steel. 
At T3, the modulus increases again, the sample becomes harder, a consequence of the crosslinking reaction. The fact that the material is fully cured is shown by the behavior of the sample in 2nd heating run: the DLTMA amplitude below T4 is very small and the bending modulus correspondingly large. The glass transition temperature is significantly higher than T1. 

Conclusion

The bending measurements with changing probe force (DLTMA mode) not only show the charges of the bending modulus at the glass transition, but also the mechanical changes that occur during the curing reaction, that is, the increases in stiffness on cross-linking. Simultaneously measured DLTMA and SDTA curves allow the mechanical and enthalpy changes to be compared. 

 

Investigation of the Curing Reaction using Bending Measurements | Thermal Analysis Application No. HB 22 | Application published in METTLER TOLEDO TA Application Handbook Thermosets Volume 1