Glass Transition Temperature (Tg) in Laminated Glass Interlayers
What Is the Glass Transition?
The glass transition is a reversible change in an amorphous polymer from a hard, rigid, glassy state to a soft, flexible, rubbery state. Unlike melting (a sharp first-order phase transition), the glass transition occurs gradually over a temperature range of 10–30°C. The midpoint of this range is reported as the glass transition temperature Tg.
The structural consequence is dramatic: the modulus drops by 2–3 orders of magnitude (from ~103 MPa to ~100 MPa) across the transition. For a laminated glass interlayer, this means the difference between near-monolithic composite action and essentially no shear coupling.
The Free Volume Explanation
The physical basis for the glass transition lies in free volume — the unoccupied space between polymer chains that allows molecular rearrangement (Ferry, 1980, Ch. 11, Sec. C).
Above Tg, free volume is sufficient for configurational rearrangements: chains can slide past each other, and the material flows in response to sustained stress. Below Tg, free volume has collapsed to a critical minimum and chains are effectively frozen — only small-amplitude vibrational motions remain.
fg ≈ 0.025 ± 0.005
The fractional free volume at Tg is approximately 2.5% for the great majority of amorphous polymers, regardless of chemical structure (Ferry, 1980, p. 288). Different polymers reach this critical threshold at different temperatures because of their different molecular structures and interaction strengths.
Tg is rate-dependent. Because the glass transition is kinetic (not a true thermodynamic phase transition), the measured Tg depends on the observation time scale. Faster measurements (higher DMA frequency, faster cooling rate) give higher apparent Tg. A change in time scale by a factor of 10 shifts the apparent Tg by approximately 3°C (Ferry, 1980, Eq. 28).
Measuring Tg from DMA Data
From a DMA temperature sweep, four methods can determine Tg (Anton Paar, Basics of DMA, p. 6–7):
| Method | Signal used | Tg value | Notes |
|---|---|---|---|
| Peak of tan(δ) | Loss factor | Highest | Most commonly reported in literature |
| Peak of G″ (or E″) | Loss modulus | Intermediate | Clearest physical meaning: maximum energy dissipation |
| Step method on G′ | Storage modulus | Lower | Intersection of tangent lines in glassy and transition regions |
| Inflection point of G′ | Storage modulus | Lower | Point of steepest decline |
Always state the method. The difference between methods can be 5–15°C. Comparing Tg values obtained by different methods is a common source of confusion in practice.
Tg Values for Commercial Interlayers
| Interlayer | Type | Tg (°C) | Source |
|---|---|---|---|
| EVALAM, EVASAFE | EVA | < −10 | Centelles et al. (2021): “The Tg of TPU and EVA is below −10°C” |
| TPU | Thermoplastic polyurethane | < −10 | Centelles et al. (2021) |
| PVB BG-R20 | Standard PVB | ≈ +8 | Galic et al. (2022): “For PVB foil, [Tg] is approximately +8°C” |
| PVB ES, Saflex DG-41 | Structural PVB | > +8 (higher than standard) | Centelles et al. (2021): less plasticiser → higher Tg |
| SentryGlas | Ionomer | ≈ +55 | Centelles et al. (2021): “highest Tg of all tested materials” |
The Plasticiser Effect in PVB
Different PVB products differ primarily in plasticiser content. Centelles et al. (2021, p. 6) explain:
“PVB becomes stiffer by reducing the amount of plasticiser. By increasing the amount of plasticiser, the glass transition temperature decreases. Therefore, PVB ES is stiffer and has a higher glass transition temperature than PVB BG-R20.”
The mechanism is explained by free volume theory (Ferry, 1980, Sec. D4, p. 301): plasticiser molecules are small relative to the polymer chains. They insert between chains, increasing the total free volume. More free volume means chains can rearrange at lower temperatures — so Tg decreases and the material softens.
| Product type | Plasticiser | Tg | G (long-term) | Application |
|---|---|---|---|---|
| Acoustic PVB | High | Very low | Very soft | Sound insulation |
| Standard PVB (BG-R20) | Medium | ≈ 8°C | ~0.5 MPa | Safety glazing |
| Stiff PVB (DG-41, ES) | Low | > 8°C | ~1–2 MPa | Structural glazing |
| Extra Stiff PVB (ES PRO) | Very low | Highest PVB | Highest PVB | Post-breakage performance |
Tg and Structural Glass Design
Tg is the single most important material parameter for choosing an interlayer. The relationship between Tg and design temperature determines the interlayer’s structural contribution:
| Design condition | PVB (Tg ≈ 8°C) | SentryGlas (Tg ≈ 55°C) |
|---|---|---|
| Winter (−10°C) | Below Tg → stiff | Far below Tg → very stiff |
| Spring/autumn (20°C) | Above Tg → softening | Below Tg → still stiff |
| Summer (35°C) | Well above Tg → rubbery | Below Tg → still stiff |
| Hot facade (50°C) | G ≈ 0 → no coupling | Near Tg → transitioning |
| Extreme heat (70°C) | No shear coupling | Above Tg → softening |
This is why SentryGlas is specified for structural applications (balustrades, canopies, glass floors, overhead glazing): it maintains meaningful shear coupling up to ~50°C, while standard PVB loses coupling above ~20°C.
Ferry (1980, p. 290) notes that “the temperature dependence of relaxation processes is indeed independent of chemical structure except as reflected by Tg itself.” This means if you know only one thing about an interlayer, know its Tg — it determines the entire temperature range of structural usefulness.
Compare Interlayer Performance
See how G varies with temperature and load duration for all interlayer types. Our EN 16613 Reference shows the data at all 11 standard conditions.
Launch EN 16613 Reference