Investigation of the Curing Reaction Using Bending Measurements
Investigation of the curing reaction using bending measurements
Purpose
To determine the curing reaction and the glass transition temperatures based on the change in elastic behavior.
Sample
Uncured KU600 epoxy resin powder as a coating on a metal blade.
Conditions
Measuring cell: TMA/SDTA
Probe: Ball-point probe (quartz glass) ; diameter 3 mm, 3-point bending accessory
Sample preparation: A small amount of KU600 epoxy resin powder was spread in a thin layer over a steel blade 0.1 mm thick, 16 mm long and 5 mm wide (a piece of a razor blade) and heated to 100 °C so that the powder melted and coalesced. This produced a film thickness of 0.5 mm. This sample was mounted on the bending accessory so that there was a length of 14 mm between the supports. The probe was placed directly onto the coating.
TMA measurement: 1st and 2nd heating curves of the same sample, measured from 30 to 260 °C at 5 K/min
Force: alternating between 0.5 and 1.0 N every 6 s (period 12 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 (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 the 2nd heating run: the DLTMA amplitude before T4 is very small and the bending modulus correspondingly large. The glass transition temperature is significantly higher than T1.
Conclusions
The bending measurements with charging prob force (DLTMA mode) not only show the changes of the bending modulus at the glass transition. but also the mechanical changes that occur during the curing reaction, that is, the increase in stiffness on cross-linking. Simultaneously measured by 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