Thermal Analysis: DSC, TGA & HDT — Theory, Technique & Industrial Interpretation

Thermal characterisation is essential for polymer quality control, formulation development, and failure investigation. A DSC thermogram tells you whether a resin is truly crystalline, if it contains the right additives, and whether a rubber compound is fully cured. TGA quantifies filler loading, detects moisture, and confirms thermal stability. HDT is mandated by OEMs (Tata, Maruti) to qualify engineering plastics for under-hood automotive components.

• Interpret DSC curves: identify $T_g$, $T_m$, $T_c$, crystallinity percentage, and cure enthalpy.
• Use TGA weight-loss curves to determine filler content, moisture, and onset of decomposition.
• Distinguish HDT Method A (1.80 MPa) and Method B (0.45 MPa) and interpret results.
• Calculate degree of crystallinity ($X_c$) from DSC melting enthalpy.
• Identify ASTM E1356 (DSC), ASTM E1641 (TGA), and ASTM D648 (HDT) standards.

Nylon-6,6 (30% Glass Fiber Reinforced)

3.1 Differential Scanning Calorimetry (DSC)

DSC measures heat flow into or out of a polymer sample vs. temperature (typically 10–20°C/min scan rate).

Key thermal events:

• $T_g$ (Glass Transition): Endothermic baseline step — increase in heat capacity with no peak.
• $T_c$ (Crystallisation): Exothermic peak — observed on cooling or cold crystallisation on slow heating.
• $T_m$ (Melting): Endothermic peak — peak area gives melting enthalpy $\Delta H_m$.
• Cure exotherm (thermosets): Exothermic peak on first heat — measures degree of cure and $\Delta H_{cure}$.

Degree of Crystallinity from DSC:

$$X_c = \frac{\Delta H_m - \Delta H_c}{\Delta H_m^0} \times 100 \%$$

Where: $\Delta H_m$ = measured melting enthalpy (J/g), $\Delta H_c$ = cold crystallisation enthalpy (J/g), $\Delta H_m^0$ = 100% crystalline reference enthalpy.

3.2 Thermogravimetric Analysis (TGA)

TGA measures weight loss vs. temperature under controlled atmosphere.

Typical protocol for glass-filled PA66:

$$\text{Filler Content (\%)} = \frac{\text{Residue at 900°C}}{\text{Initial Sample Mass}} \times 100$$

3.3 Heat Deflection Temperature (HDT) — ASTM D648 / ISO 75

HDT measures the temperature at which a polymer bar deflects by 0.25 mm under a 3-point bending load.

Important: HDT is NOT the same as continuous service temperature — it is a relative stiffness index under a specific transient load.

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Problem: A DSC scan of an HDPE bottle shows melting enthalpy $\Delta H_m = 197$ J/g. No cold crystallisation peak observed. Calculate $X_c$.

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Solution:

$$X_c = \frac{197 - 0}{293} \times 100 = \frac{197}{293} \times 100 = \textbf{67.2\%}$$

Interpretation: 67% crystallinity is consistent with blow-moulded HDPE bottles. Higher crystallinity improves barrier properties and stiffness. Rapid quench cooling reduces crystallinity to ~50–55%, lowering barrier performance.

Tata AutoComp (Pune) specifies HDT ≥ 120°C at 1.80 MPa for under-hood plastic parts — ruling out standard PP (HDT ~90–100°C at 0.45 MPa) and specifying 30% glass-filled PA66 or PBT.

SRF Ltd (Technical Textiles division, Bhiwadi) uses DSC to monitor PET and Nylon 6 crystallinity in tyre cord fabric — crystallinity directly controls dimensional stability and tenacity-to-weight ratio in automotive tyres.

• DSC: Measures heat flow events ($T_g$, $T_m$, $T_c$, cure exotherm) to characterise transitions and crystallinity.
• TGA: Measures weight loss vs. temperature to quantify moisture, fillers, and thermal stability.
• $X_c$: Degree of crystallinity — ratio of measured melting enthalpy to 100%-crystalline reference.
• HDT Method A (1.80 MPa): Standard for engineering plastics in automotive OEM qualification.
• HDT Method B (0.45 MPa): Lower stress; for commodity plastics comparative screening.
• $\Delta H_m^0$: 100% crystalline melting enthalpy — reference for crystallinity calculation.

1. ASTM E1356 — DSC measurement of glass transition temperatures
2. ISO 11357-3 — DSC determination of melting and crystallisation enthalpy
3. ASTM E1641 — Decomposition kinetics by TGA
4. ISO 11358 — General principles of TGA for polymers
5. ASTM D648 / ISO 75 — Heat deflection temperature under flexural load

1. A PET DSC scan shows $\Delta H_m = 42$ J/g with $\Delta H_c = 18$ J/g. Calculate $X_c$ ($\Delta H_m^0 = 140$ J/g).
2. What atmosphere change is performed in TGA for carbon-black-filled EPDM to separate pyrolysis from CB combustion?
3. Why does HDT Method B (0.45 MPa) give a higher temperature value than Method A (1.80 MPa) for the same polymer?

Q1. In DSC, $T_g$ appears as:

• C) A baseline shift (step change in heat capacity)

Q2. $\Delta H_m^0$ for HDPE (100% crystallinity) is:

• C) 293 J/g

Q3. TGA residue at 900°C in air represents:

• D) Inorganic filler (glass, talc, CaCO₃)

Q4. HDT Method A fibre stress:

• B) 1.80 MPa

Q5. Standard governing DSC measurement of glass transition temperature:

• A) ASTM E1356