Determination of the Glass Transition from Bending Measurements

Purpose

To determine the glass transition temperature using a method based on the change of elastic behavior. 

Sample

Cured KU600 epoxy resin powder as an adhesive layer between two metal plates.

Conditions

Measuring cell: TMA

Probe: Ball-point probe (quartz glass); diameter 3 mm, 3-point bending accessory. 

Sample preparation: The KU epoxy powder coating was cured between two 0.1-mm thick steel plates. These were pieces of razor blades, 166 mm long and 5 mm wide. The thickness of the layer was 0.06 mm. This produced a sandwich structure such as could occur in adhesive bonding. The cured sample was mounted on the bending accessory so that the full length of 14 mm could be used between the supports:

TMA measurement: Heating from 50 to 240 °C at 5 K/min
Load: alternating between 0.5 N and 1.0 N every 6 s (period 12 s)

Atmosphere: Static Air

Interpretation

The sandwich structure bends slightly due to the force applied to the probe. The change in force of 0.5 N produces a change in bending of about 40 um as long as the adhesive layer (i.e the cured epoxy powder) is below the glass transition temperature. Above the glass transition, sample bending is more pronounced, about 200 um at 1 N. The change in bending with the changing force becomes larger, about 130 um. This is recorded in the DLTMA curve and allows the softening or glass transition temperature to be determined. The temperature can be evaluated as the onset or midpoint of the mean curve (red). 

The upper and lower envelopes (dashed lines) show the bending profiles for probe forces of 0.5 N and 1.0 N. The difference between these envelopes is a measure for the elasticity respectively for the modulus of bending of the sample. It can be assumed that the elastic modulus of the steel blades hardly changes so that the changes are only due to the thermoset. The curve of the bending modulus of the sandwich structure can also be used to determine the T_g of the "adhesive" as shown by the onset in the lower diagram. 

Conclusions

Bending the measurements with static and changing probe force are sensitive methods for determining the softening or glass transition temperature of coatings, adhesive bonding layers and filled thermosets. Since only the temperature region of the change of bending behavior of the modulus is of interest, no time-consuming multiple measurements are necessary, and sample preparation is usually simply a matter of adapting the geometry of the sample to the bending measurement accessory. Since the maximum force that can be applied in the TMA is limited, the bending samples should be as thin as possible so that minimal bending and change in bending can be measured.

As previously mentioned with other methods, when reporting the results of bending measurements, it is important to state how the measurements were performed and how the results were evaluated. 

Apart from DMA, a DLTMA measurement is often the only possibility to determine the softening point or glass transition of highly filled or highly reinforced thermosets. With such materials, the changes in heat capacity and expansion coefficient at the glass transition are often very small. 

^{Determination of the Glass Transition from Bending Measurements | Thermal Analysis Handbook No.HB20 | Application published in METTLER TOLEDO TA Application Handbook Thermosets, Volume 1}