Understanding the transparent to cloudy transition of silicate interlayers in fire-resistant glass

Alkali silicate systems have been widely used to improve the fire resistance of glass, wood, steel and polymers. However, a key durability challenge with these materials as interlayers in fire-resistant glass is their transparent to cloudy transition. Though several empirical measures were reported for mitigation, the lack of clarity of the underlying mechanism has limited their effectiveness. In this work, the structural transformations of alkali silicate systems of varying molar ratios at different length and time scales are investigated with the aid of multiple characterization techniques, including ²⁹Si magic angle spinning nuclear magnetic resonance (MAS NMR), small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). It is found that the distribution state of particles and the interparticle distance significantly contribute to the transition. Specifically, the formation of interparticle networks and aggregates can lead to light scattering, resulting in irreversible loss of transparency. Additionally, heat treatment can induce the coalescence of particles, resulting in a matrix-dominated structure devoid of scattering features. This transformation can lead to an opaque to transparent reversal. The fundamental insights gained from this research are crucial for developing durable silicate systems suitable for fire-resistant glass and other potential applications. The novel research protocol presented here can be further applied to future research on similar material systems and their associated behaviors.