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 DensityThe mass density is defined as the ratio between mass and volume. density | especially important for powders (bulk DensityThe mass density is defined as the ratio between mass and volume. 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 | 20 ml/min | |
| Protective gas flow | 50 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, IsothermalTests at controlled and constant temperature are called isothermal.isothermal steps instead of linear heating rate | |
| Crucible type (shape, material, volume) | crucibles with pierced lid, pressure crucibles for polycondensation, Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.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, Oxidative-Induction Time (OIT) and Oxidative-Onset Temperature (OOT)Oxidative Induction Time (isothermal OIT) is a relative measure of the resistance of a (stabilized) material to oxidative decomposition. Oxidative-Induction Temperature (dynamic OIT) or Oxidative-Onset Temperature (OOT) is a relative measure of the resistance of a (stabilized) material to oxidative decomposition.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 TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition or linear baseline for enthalpy of melting for DSC | |
| Advanced calculations | degree of Crystallinity / Degree of CrystallinityCrystallinity refers to the degree of structural order of a solid. In a crystal, the arrangement of atoms or molecules is consistent and repetitive. Many materials such as glass ceramics and some polymers can be prepared in such a way as to produce a mixture of crystalline and amorphous regions.crystallinity, Solid Fat Content (SFC), kinetic analysis |