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 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, 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, 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-RMode 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