Today's KNOWLEDGE Share : Pressing laminated glass below the glass transition temperature (Tg)

Today's KNOWLEDGE Share

❄️Pressing laminated glass below the glass transition temperature (Tg) for the interlayer can produce glass that looks sealed, clean, and acceptable, yet carries hidden defects that only reveal themselves during autoclaving or months later in service.

the proper transition temperature (Tg)

• PVB > ~40 °C

• SGP > ~55 °C

🔥Above Tg, polymer chains become soft and mobile, allowing the interlayer to flow and wet the glass surface fully.

This mobility enables molecular bonding and hydrogen bonding to form between the interlayer and glass, creating a strong, durable adhesion essential for laminate integrity.

❄️Below Tg, polymer chains are rigid and stuck, so adhesion is weak or incomplete.

⚠️ When glass is pressed too cold, several hidden quality problems already exist, even if the glass looks acceptable.

• Weak temporary adhesion, polymer chains can’t rearrange, so bonding is fragile.

• Loss of transparent edge zone, normally a clear 15–20 mm edge indicates good flow and air evacuation, when pressed too cold, micro-air pockets remain, causing hazy or opaque edges (an early warning sign).

❗Even with a sealed edge, pressing too cold is a problem if the center doesn’t reach the required temperature. The seal prevents moisture ingress, but can’t fix poor polymer flow or trapped air inside, leading to hidden defects.

🔥 What Happens After Autoclaving?

High heat (~130–145 °C) and pressure (~10–14 bar) cause trapped air to expand (Ideal Gas Law), creating bubbles or “blow-in.” The softened interlayer allows air to migrate, exposing defects formed during pressing.

❗ Autoclaving doesn’t create defects; it reveals and worsens them.

⏳ Even if no visible bubbles appear immediately after autoclaving, pressed-too-cold glass may suffer from long-term failures:

• Edge delamination

• Poor humidity resistance

• Reduced adhesion durability

• Increased sensitivity to thermal cycling

❗ Autoclaving consolidates adhesion; it does not create it

❌ So Autoclaving Can't Fix Pressed-Too-Cold Glass

It cannot remove trapped air

It cannot rebuild missing molecular adhesion

Even if the glass seemed okay after autoclaving, glass may suffer from long-term failures as we mentioned earlier

✅But, how can we fix this issue?

🔁 Re-Pressing can Fix It, but only BEFORE autoclaving, and only under strict conditions:

✔ No contamination or moisture ingress

✔ Correct pressing temperature (safely above Tg)

✔ Uniform heat and pressure

🔬 Why it works:

Reheating above Tg restores polymer mobility, allowing:

• Better wetting

• Air evacuation

• Formation of stronger temporary adhesion

❌ Once autoclaved, re-pressing is ineffective.

source : Mohamed Elderaa