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

1. Why This Topic Matters

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.

2. Learning Objectives

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.

3. Core Theory

Technical Schematic & Mechanism Blueprint: Thermal Analysis: DSC, TGA & HDT — Theory, Technique & Industrial Interpretation

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

DSC Thermal Analysis (ASTM D3418): % Crystallinity = [(ΔHm - ΔHc) / ΔHm100%] × 100. Tg marks baseline shift, Tc exothermic crystallization, and Tm endothermic melting.

Thermogravimetric Analysis (ASTM E1131 / ISO 11358)

TGA Mass Loss vs. Temperature: Compounded Polymer + Filler

Inorganic Filler / Fiber (CaCO3/Glass):25 %

Purge Gas Atmosphere:Inert (N2)

Degradation Onset (T_onset)385 °C

50% Mass Loss Temp (T_50%)430 °C

Inorganic Ash Content25 %

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.

Polymer	$\Delta H_m^0$ (J/g)
HDPE	293
PP (isotactic)	207
PET	140
Nylon 6	226
PLA	93

3.2 Thermogravimetric Analysis (TGA)

TGA measures weight loss vs. temperature under controlled atmosphere.

Typical protocol for glass-filled PA66:

Temperature Stage	Atmosphere	Event
50–150°C	N₂	Moisture loss
150–400°C	N₂	Organic additive volatilisation
400–550°C	N₂	Polymer backbone pyrolysis
550–900°C	Switch to Air	Carbon black combustion
Residue at 900°C	Air	Inorganic filler (glass, talc, CaCO₃)

\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.

Method	Applied Stress	Application
Method A (ASTM D648-A / ISO 75-A)	1.80 MPa	Engineering plastics, automotive OEM qualification
Method B (ASTM D648-B / ISO 75-B)	0.45 MPa	Commodity plastics, comparative screening
Method C	8.00 MPa	High-performance polymers (PEEK, PPS)

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

4. Worked Example

Thermal Analysis: DSC, TGA & HDT — Theory, Technique & Industrial Interpretation — Industrial Process Flow Diagram (PFD)

ISO 10628 Unit Operations & Thermodynamic Control Sequence

ISO 10628 PFD Standard

180°C - 100°C

Raw Material Feed & Metering

Drying to < 0.02% moisture and gravimetric dosing

Output:Conditioned Feed

2220°C - 260°C

Reaction & Polymer Melt Processing

Controlled thermal shear dissipation and phase transition

Output:Homogenous Melt

360 - 120 bar

Shaping & Solidification

Die extrusion or mold cavity pressure packing

Output:Formed Component

4QA Passed

Quality Inspection & Testing

ASTM D638 / ISO 527 tensile & MFI compliance check

Output:Finished Grade

Recycle Loop Stream:Closed-loop sprues and edge-trim regrind re-fed into main hopper at maximum 15% blend ratio.

Standard Indian Industrial Practice (CIPET / Reliance / Supreme Petrochem Protocol)

Engineering Note & Problem Analysis

Problem Statement:

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

Engineering Note & Problem Analysis

Solution Step:

Solution:

X_c = \frac{197 - 0}{293} \times 100 = \frac{197}{293} \times 100 = \mathbf{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.

5. Indian Industry Context

Thermal Analysis: DSC, TGA & HDT — Theory, Technique & Industrial Interpretation — CAD Tooling & Machine Geometry Blueprint

DWG NO: DWG-POLY-2026-768SCALE: 1:1 (FULL)MATERIAL: P20 Tool Steel / Hardened Alloy (HRC 48-52)

Critical Geometric Dimensions & Tolerances (GD&T)

Primary Processing Tolerance:± 0.02 mm (ISO 20457 Class M1)
Thermal Expansion Allowance:1.5% (Volumetric Shrinkage)
Draft Angle Minimum:1.0° - 1.5° (Core/Cavity Ejection)

Tooling Fabrication Notes

• Fabricated according to ISO 20457 plastic molding tolerance standards.
• EDM finish SPI C-1 applied across cavity optical contact faces.
• Conformal cooling lines pressure tested at 12 bar water flow.

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.

6. Key Takeaways & Glossary

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.

7. Standards Reference

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

8. GATE / University Practice Questions

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).
What atmosphere change is performed in TGA for carbon-black-filled EPDM to separate pyrolysis from CB combustion?
Why does HDT Method B (0.45 MPa) give a higher temperature value than Method A (1.80 MPa) for the same polymer?

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

01 · Why This Matters in Industry & GATE XE5

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

1. Why This Topic Matters

2. Learning Objectives

3. Core Theory

TGA Mass Loss vs. Temperature: Compounded Polymer + Filler

3.1 Differential Scanning Calorimetry (DSC)

3.2 Thermogravimetric Analysis (TGA)

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

4. Worked Example

Thermal Analysis: DSC, TGA & HDT — Theory, Technique & Industrial Interpretation — Industrial Process Flow Diagram (PFD)

Raw Material Feed & Metering

Reaction & Polymer Melt Processing

Shaping & Solidification

Quality Inspection & Testing

5. Indian Industry Context

Thermal Analysis: DSC, TGA & HDT — Theory, Technique & Industrial Interpretation — CAD Tooling & Machine Geometry Blueprint

Critical Geometric Dimensions & Tolerances (GD&T)

Tooling Fabrication Notes

6. Key Takeaways & Glossary

7. Standards Reference

8. GATE / University Practice Questions

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

Acrylonitrile Butadiene Styrene (ABS)

Computerized Servo-Universal Testing Machine (UTM 50kN)

Consumer Electronics Enclosures & Crash Helmets

Test Your Conceptual Understanding

Personal Lesson Notes