Comparative Characterisation of Seven Interlayer Materials

The Seven Interlayers

Centelles et al. (2021) tested seven commercial interlayer materials under identical conditions, enabling direct comparison. All specimens were standalone interlayer films tested in an RSA3 DMA (TA Instruments) in tension mode, at temperatures from −10°C to 50°C (extended to 80°C for SentryGlas).

Three Distinct Behavioural Groups

The isothermal relaxation curves reveal three fundamentally different types of viscoelastic behaviour:

Group 1: PVB materials (BG-R20, DG-41, ES)

Extreme temperature sensitivity in the 0–40°C range. E(t) at 1 s ranges from ~1000 MPa at −10°C to ~0.5 MPa at 50°C — a 2000× drop across 60°C. Strong time dependence (significant relaxation at each temperature). T_g near +8°C places the glass transition right in the structural service temperature range.

Group 2: SentryGlas (ionomer)

Moderate temperature sensitivity up to 50°C. E(t) remains ~100–700 MPa across the design range because T_g ≈ 55°C is above most service temperatures. Only at 60–80°C does significant softening occur. Retains E_∞ = 80.5 MPa — meaningful long-term structural stiffness due to ionic cross-links.

Group 3: EVA and TPU

Low temperature sensitivity across the entire range. E(t) ≈ 2–14 MPa at all service temperatures (T_g < −10°C — already in rubbery plateau). Very little time dependence — nearly elastic. TPU is approximately 2× stiffer than EVASAFE at all temperatures. EVA shows a secondary crystallisation anomaly near 44°C.

Master Curve Comparison

The TTS-shifted master curves at T₀ = 20°C reveal the full relaxation behaviour:

• PVB group: classic S-shaped curve from glassy plateau (~1000 MPa) to rubbery plateau (~0.5–2 MPa) over 6–8 decades of log(t). PVB ES and Saflex DG-41 are nearly indistinguishable.
• SentryGlas: S-shape shifted to much longer reduced times. Maintains high modulus to timescales where PVB has already reached its rubbery plateau.
• EVA/TPU: nearly flat master curves — E₀ and E_∞ differ by less than 10×.

The stiffness crossover

At short times (wind gust, 3 s): PVB ES ≈ Saflex DG-41 > PVB BG-R20 ≈ SentryGlas >> TPU > EVA.

At long times (permanent load, 50 years): SentryGlas >> PVB ES > Saflex DG-41 >> PVB BG-R20 >> TPU > EVA.

The key insight: SentryGlas ranks below PVB for short loads but far above PVB for sustained loads.

Damping: tan(δ) Peak Values

Prony Coefficients Compared

The E₀/E_∞ ratio is the clearest single indicator of material character. PVB BG-R20 drops 2054×: from rigid glass-like to near-fluid under sustained load at 20°C. SentryGlas drops only 8.5×, retaining 80 MPa — a structurally meaningful modulus.

Key Finding: PVB ES ≈ Saflex DG-41

One of the most significant practical findings: PVB ES (Trosifol/Kuraray) and Saflex DG-41 (Eastman) — stiff PVB products from different manufacturers — have very similar viscoelastic properties. Their master curves nearly overlap, and their Prony coefficients give comparable results (E_∞ = 1.79 vs 1.16 MPa; C₁ = 20.2 vs 17.5).

This means engineers can treat these products as structurally interchangeable for design purposes, simplifying specification and sourcing.

Choosing the Right Interlayer