Bubble growth in a confined heated polymer: the example of safety glass

TL;DR

This paper investigates the mechanisms of bubble growth in laminated safety glass, combining experiments and a physical model to understand how gases and temperature affect bubble formation and stability.

Contribution

It introduces a comprehensive physical model for bubble growth in polymer composites considering multiple gases, thermal effects, and rheology, validated by experimental data.

Findings

Two gases, air and water vapor, drive bubble growth.

Bubble growth depends on initial bubble size and polymer relaxation.

Anomalous air oversaturation can cause catastrophic bubble expansion.

Abstract

Laminated safety glass (LSG) is a composite assembly of glass and polyvinyl butyral (PVB), a viscoelastic polymer. LSG can be found in building facades, important landmarks around the world, and every major form of transportation. Yet, the assembly suffers from unwanted bubbles which are anathema to one of the most important features of glass: optical transparency. In here, we present an in-depth study of the reasons behind these bubbles, either during high-temperature quality control tests or normal glass operating conditions. We provide a physical model for bubble growth that deals with two gases, thermal effects on gas solubility and diffusivity, and a time-temperature dependent rheology. The model can be extended to n-component bubbles or other materials beyond PVB. By combining experiments and theory, we show that two gases are at play: air trapped in interfacial bubbles in the assembly during lamination and water initially dissolved in the polymer bulk. Both gases work in tandem to induce bubble growth in finished assemblies of LSG provided that (i) the original bubble nucleus has a critical size and (ii) the polymer relaxes (softens) sufficiently enough, especially at elevated temperatures. The latter constraints are relaxed in a condition we termed anomalous air oversaturation that may even trigger a catastrophic, yet beautiful ice flower instability.