Determination of the Softening Temperature of Thin Coatings

Purpose

To determine the softening temperature and thus obtain information about the glass transition temperature range.

Sample

Aluminum plate, 1.5 mm thick with a coating of cured KU600 epoxy resin powder about 27 μm thick.

Conditions

Measuring cell: TMA

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

Sample preparation: A piece of about 5x5 mm was sawn out of the plate. The probe was placed directly on the coating so that it exerted a large compressive stress on the sample. 

TMA measurement: Heating from 40 to 190 °C at 5 K/min

Atmosphere: Static air

Interpretation 

The softening of the thin coating is clearly apparent from the penetration of the ball-point probe (about 1.5 μm). The shape of the probe, the applied force, and the fillers prevent the probe from penetrating very far into the coating. The rate of expansion before and after the effect corresponds to that of the aluminum plate 25⋅10^-6K^-1 because the thin coating hardly contributes to the expansion (about 0.3 μm over 100 K). An evaluation similar to that in the previous example is therefore not possible.

Conclusion

The TMA is an excellent method for determining the softening temperature of very thin paint or varnish coatings and thereby indirectly obtaining information about the glass transition. Usually, special sample preparation is not necessary, that is, the coating does not have to be physically separated from the support material. The softening temperature is directly determined from the first heating curve. 
The measurement and evaluation method used must always be stated when quoting results because the probe force and shape of the probe tip influence the extent and location of the penetration. For this reason, the softening temperatures of penetration measurements are not exactly identical to the glass transition temperature of the expansion measurement. 

_{Determination of the Softening Temperature of Thin Coatings | Thermal Analysis Handbook No.HB20 | Application published in METTLER TOLEDO TA Application Handbook Thermosets, Volume 1}