ASTM D7028 Glass Transition Temperature Testing for Epoxy by DMA
The glass transition temperature (Tg) is determined using the standard testing method ASTM D7028-07. Dynamic Mechanical Analysis (DMA) technique is used for testing. The values are reported in the SI unit.
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- Overview
- Scope, Applications, and Benefits
- Test Process
- Specifications
- Instrumentation
- Results and Deliverables
Overview
ASTM D7028-07 describes a test method for determining the glass transition temperature (Tg) of polymer matrix composites using dynamic mechanical analysis (DMA). The glass transition is the temperature range over which the polymer matrix transitions from a rigid, glassy state to a rubbery, viscoelastic state, thereby significantly reducing the composite’s stiffness and load-bearing capacity.
This test is critical for aerospace, automotive, and structural composite applications where maximum service temperature must remain well below Tg to preserve structural performance.
Scope, Applications, and Benefits
Scope
ASTM D7028-07 applies to continuous fiber-reinforced polymer matrix composites and measures Tg via:
- Storage modulus (E’) onset temperature
- Loss modulus (E”) peak temperature
- Tan delta (loss factor), peak temperature. The method covers the single-cantilever, dual-cantilever, and three-point-bending DMA modes.
Applications
- Aerospace composite structural qualification
- Automotive composite part temperature rating
- Cure verification and post-cure optimization
- Thermal degradation and hot-wet conditioning assessment
- Material comparison for high-temperature composite selection
Benefits
- Non-destructive, small specimen requirement
- Measures multiple Tg indicators simultaneously
- Detects incomplete cure or moisture-induced plasticization
- Provides data for maximum service temperature determination
- Essential for aerospace composite airworthiness documentation
Test Process
Specimen Preparation
Rectangular composite specimens are cut to defined dimensions and conditioned per test requirements (dry or wet).
1DMA Setup
Specimens are clamped in the appropriate fixture (single-cantilever, dual-cantilever, or 3-point bend) and loaded into the DMA furnace.
2Temperature Ramp
A dynamic oscillatory force is applied while temperature is increased at a controlled rate (typically 3–5°C/min); E', E'', and tan δ are continuously recorded.
3Tg Determination
Tg is identified from the onset of E' reduction, E'' peak, and tan δ peak; all three values are reported.
4Technical Specifications
| Parameter | Details |
|---|---|
| Test Principle | Dynamic mechanical analysis (DMA) |
| Applicable Materials | Continuous fiber-reinforced polymer matrix composites |
| Heating Rate | 1–5°C/min |
| Frequency | 1 Hz (default) |
| Measured Outputs | E' onset Tg, E'' peak Tg, tan δ peak Tg |
Instrumentation Used for Testing
- Dynamic mechanical analyzer (DMA) with environmental furnace
- Single-cantilever, dual-cantilever, or 3-point bending fixture
- Liquid nitrogen cooling option for subambient testing
- Calipers for specimen dimension measurement
- Data analysis software with modulus and phase angle calculation
Results and Deliverables
- Storage modulus (E’) vs. temperature curve
- Loss modulus (E”) vs. temperature curve
- Tan delta (loss factor) vs. temperature curve
- Tg values from all three methods
- Full DMA test report with specimen and test conditions
Frequently Asked Questions
Aerospace specifications typically use the E' onset or E'' peak, which are more conservative than tan δ peak; the appropriate value depends on the governing specification or OEM requirement.
Absorbed moisture plasticizes the polymer matrix, reducing Tg — this is why aerospace composites are often tested in both dry and conditioned (hot-wet) states.
The method applies to thermoset (epoxy, bismaleimide, PEEK) and thermoplastic matrix composites with continuous fiber reinforcement.
Typical specimens are 50–60 mm long, 10–15 mm wide, and 2–4 mm thick; exact dimensions depend on the DMA fixture selected.
Yes — the measured Tg can be compared to the theoretical fully cured Tg to estimate degree of cure; under-cured laminates show a lower Tg than fully cured material.
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