Differential Scanning Calorimetry (DSC): Tg, Tm, Hf & Crystallinity Kinetics

Differential Scanning Calorimetry (DSC) is the most widely used thermal analysis technique in polymer science and engineering. It characterizes key thermal transitions ($T_g, T_c, T_m$), heat capacity changes, and phase kinetics. In industrial production (e.g., compounding, blow moulding, film blowing), DSC is essential to check if a semi-crystalline polymer has achieved the target degree of crystallinity. Indian testing labs (e.g., CIPET, SGS India) utilize DSC to identify unknown polymers, evaluate polymer blends, and qualify raw materials.

• Explain the working principle of heat-flux and power-compensated DSC instruments.
• Interpret a DSC thermogram, identifying glass transition ($T_g$), cold crystallization ($T_{cc}$), melting ($T_m$), and crystallization ($T_c$).
• Calculate the degree of crystallinity ($X_c$) from melting peak enthalpy ($\Delta H_m$) data.
• Solve the kinetics of crystallization using the Avrami equation.
• Identify ASTM D3418 (transition temperatures) and ISO 11357 standards.

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PET (Polyethylene Terephthalate)

3.1 Instrument Principles

• Heat-Flux DSC: Sample and reference sit on a common thermoelectric disk in a single furnace. Heat flow is calculated from temperature difference $\Delta T = T_S - T_R$.
• Power-Compensated DSC: Sample and reference sit in separate furnaces with independent heaters. The power required to keep $T_S = T_R$ is measured directly.

3.2 Thermal Transitions on a DSC Curve

A standard heating run plots heat flow (mW/W or W/g) vs. temperature. Transitions include:

1. Glass Transition ($T_g$): Baseline step change (endothermic). Represents a second-order thermodynamic transition (change in heat capacity $\Delta C_p$).
2. Cold Crystallization ($T_{cc}$): Exothermic peak. Occurs when a quenched amorphous phase gains mobility above $T_g$ and packs into crystals.
3. Melting ($T_m$): Endothermic peak. First-order transition (breaking of crystalline structures).

3.3 Calculating Degree of Crystallinity ($X_c$)

$$X_c (\%) = \frac{\Delta H_m - \Delta H_{cc}}{\Delta H_m^\circ} \times 100\%$$

Where:

• $\Delta H_m$: Enthalpy of melting (area under the melting peak, J/g)
• $\Delta H_{cc}$: Enthalpy of cold crystallization (if present, J/g)
• $\Delta H_m^\circ$: Enthalpy of melting for a $100\%$ crystalline reference of the same polymer.
  • For PP: $\Delta H_m^\circ = 207$ J/g
  • For PE: $\Delta H_m^\circ = 290$ J/g
  • For PET: $\Delta H_m^\circ = 140$ J/g

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  "subtitle": "ISO 10628 Unit Operations & Thermodynamic Control Sequence",
  "steps": [
    { "step": 1, "title": "Raw Material Feed & Metering", "description": "Drying to < 0.02% moisture and gravimetric dosing", "parameters": "80°C - 100°C", "output": "Conditioned Feed" },
    { "step": 2, "title": "Reaction & Polymer Melt Processing", "description": "Controlled thermal shear dissipation and phase transition", "parameters": "220°C - 260°C", "output": "Homogenous Melt" },
    { "step": 3, "title": "Shaping & Solidification", "description": "Die extrusion or mold cavity pressure packing", "parameters": "60 - 120 bar", "output": "Formed Component" },
    { "step": 4, "title": "Quality Inspection & Testing", "description": "ASTM D638 / ISO 527 tensile & MFI compliance check", "parameters": "QA Passed", "output": "Finished Grade" }
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  "recycleLoop": "Closed-loop sprues and edge-trim regrind re-fed into main hopper at maximum 15% blend ratio.",
  "notes": "Standard Indian Industrial Practice (CIPET / Reliance / Supreme Petrochem Protocol)"
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<div className="problem-statement">

Problem: A semi-crystalline Polypropylene (PP) sample is analyzed via DSC. The melting peak shows an enthalpy of melting $\Delta H_m = 92.5$ J/g. No cold crystallization exotherm is observed. If the reference melting enthalpy for 100% crystalline PP is $\Delta H_m^\circ = 207$ J/g, calculate the degree of crystallinity ($X_c$) of this PP sample.

</div> <div className="solution-step">

Solution: Since $\Delta H_{cc} = 0$:

$$X_c = \frac{\Delta H_m}{\Delta H_m^\circ} \times 100\%$$ $$X_c = \frac{92.5 \text{ J/g}}{207 \text{ J/g}} \times 100\% = 0.44686 \times 100\% = \mathbf{44.69\%}$$

Interpretation: The PP sample has a crystalline fraction of 44.69%, which is typical for injection-moulded homopolymer PP. This degree of crystallinity will provide a balance of tensile strength and stiffness.

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  "drawingNumber": "DWG-POLY-2026-676",
  "scale": "1:1 (FULL)",
  "material": "P20 Tool Steel / Hardened Alloy (HRC 48-52)",
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    { "label": "Thermal Expansion Allowance", "value": "1.5%", "tolerance": "Volumetric Shrinkage" },
    { "label": "Draft Angle Minimum", "value": "1.0° - 1.5°", "tolerance": "Core/Cavity Ejection" }
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    "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."
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CIPET (Central Institute of Petrochemicals Engineering & Technology) centres across India use DSC for polymer blend analysis. For example, verifying recycling contamination levels in PP-PE PCR (Post-Consumer Resin) blends by evaluating the distinct melting peaks of PP (165°C) and HDPE (135°C) per ASTM D3418.

• $T_g$ (Glass Transition): Step change in baseline; polymer transitions from glassy to rubbery state.
• $\Delta H_m$: Heat absorbed to melt the crystalline region; area under the endothermic peak.
• $X_c$ (Crystallinity): Percentage of crystalline phase in the semi-crystalline polymer.
• Exothermic: Upward peak in standard DSC convention (releasing heat).
• Endothermic: Downward peak (absorbing heat).

1. ASTM D3418 — Standard Test Method for Transition Temperatures and Enthalpies of Polymers by Thermal Analysis
2. ISO 11357-1 — Plastics — Differential scanning calorimetry (DSC) — Part 1: General principles
3. ISO 11357-3 — Determination of temperature and enthalpy of melting and crystallization

1. Draw a typical DSC heating thermogram for a quenched semi-crystalline PET sample. Clearly label $T_g$, $T_{cc}$, and $T_m$.
2. Why is $T_g$ classified as a second-order transition while $T_m$ is first-order? Reference the behavior of entropy and volume.
3. A PET sample displays $\Delta H_m = 38.0$ J/g and $\Delta H_{cc} = 12.0$ J/g. Calculate $X_c$ if $\Delta H_m^\circ = 140$ J/g.

Q1. On a standard DSC heating thermogram, the glass transition ($T_g$) appears as a:

• A) Step change in the baseline (endothermic shift)

Q2. Which transition represents an exothermic reaction during heating of a quenched polymer?

• C) Cold crystallization ($T_{cc}$)

Q3. What is the reference melting enthalpy ($\Delta H_m^\circ$) for 100% crystalline Polypropylene?

• C) 207 J/g

Q4. The degree of crystallinity of PE is calculated using a reference enthalpy ($\Delta H_m^\circ$) of:

• B) 290 J/g

Q5. Which international standard is commonly used to assign polymer glass transition temperature via DSC?

• A) ASTM D3418