Tips & Tricks
Factors Influencing the DSC and TGA Measurement Results
In order to establish the origin of a failure or shortcoming, the DSC and TGA measurement results of raw material manufacturers and processors are carefully compared – not only in round-robin tests conducted by various laboratories, but also in failure analysis, especially for such areas as plastic parts.
The operators both on the supplier and customer side of course discuss their respective measurement parameters with each other, but are often surprised to find that there are still differences in the measurement plots – not to mention different interpretations of the measurement curves.
The following table shows an overview of the great variety of criteria influencing DSC and TGA measurement results, with a description of each.
| Influential factor | Criterion | Recommendations/Examples |
|---|---|---|
| Sample preparation | Sampling | sampling point on the polymer mold, near/away from the gate |
| Sample preparation | cutting with a scalpel, punching out | |
| Sample pre-treatment | tempering at defined storage temperatures, moisture | |
| Sample mass | sample weight of 10 +/-0.1 mg | |
| Sample density | especially important for powders (bulk density) | |
| Sample shape, surface | flat disk for a large area of contact on the DSC sensor | |
| DSC/TGA instrument | Sensor type | type of thermocouple and sample carrier |
| Temperature calibration | dependent on the heating rate | |
| Sensitivity calibration | dependent on the atmosphere, crucible and sensor type (thermocouple) | |
| Type of purge gas (atmosphere surrounding sample) | inert gas (e.g. nitrogen) or reaction gas (e.g. oxygen) | |
| Purge gas flow | 50 ml/min | |
| Protective gas flow | 20 ml/min nitrogen in order to avoid condensation effects in the low-temperature range | |
| Cooling type | intracooler, liquid nitrogen, air compressor for DSC | |
| Vacuum | lowering of the boiling point of solvents, plasticizers for TGA | |
| Drift behavior of the baselines | for TGA/STA and DSC | |
| Buoyancy behavior | for TGA/STA | |
| Measuring parameters | Temperature range | final temperature max. 40 K over the last expected thermal effect for DSC |
| Heating/cooling rate | 10 K/min | |
| Reheating | for DSC measurements on polymers, a 2nd heating is required since the 1st heating also includes the thermomechanical history | |
| Temperature/time program | TM-DSC, isothermal steps instead of linear heating rate | |
| Crucible type (shape, material, volume) | crucibles with pierced lid, pressure crucibles for polycondensation, thermal conductivity of the crucible material, compatibility between sample and crucible material | |
| Reference crucible for DSC/STA | empty or filled with inert materials | |
| Gas change | Oxidative Induction Time, Oxidations-Induktionszeit (OIT) und oxidations-onset temperatur (OOT)Oxidations-Induktionszeit (isothermische OIT): Relatives Maß der Beständigkeit eines (stabilisierten) Werkstoffs gegen oxidative Zersetzung, das durch die kalorimetrische Messung des Zeitintervalls bis zum Beginn der exothermen Oxidation des Werkstoffs bei einer festgelegten Temperatur in einer Sauerstoff- oder Luftatmosphäre bei Atmosphärendruck bestimmt wird.OIT, in an oxygen atmosphere | |
| Correction measurement | taking a correction measurement into consideration (e.g. buoyancy for TGA) | |
| Curve evaluation | Smoothing of the measurement curves | avoid too high of a smoothing factor |
| Correction of the baseline | BeFlat® for DSC | |
| Correction of the time constant and thermal resistance | Tau-R® Mode for DSC | |
| Evaluation standards | ISO 11357 for midpoint temperature of the glass transition or linear baseline for enthalpy of melting for DSC | |
| Advanced calculations | degree of crystallinity, Solid Fat Content (SFC), kinetic analysis |