On the asymmetry of soda lime silicate float glass structure and chemistry

Soda lime silicate glass produced by the float process is among the materials with the largest use worldwide. The manufacturing process introduces an asymmetry in the glass sheets that causes well-known modifications in terms of properties between the so-called “air side” and “tin side”. Nevertheless, not much is known about the structural changes induced by the diffusing tin ions in the network. In the present work, we show that surprisingly, the short range of the tin side is akin to that of the bulk, while the network connectivity in the air side vicinity is enhanced with a larger concentration of bridging oxygens. This correlates with a partial depletion of modifiers from the air side as detected by secondary ion mass spectrometry. On the other hand, the incorporation of tin markedly changes the medium range. In fact, the tin side is characterized by: (i) longer correlation distances, (ii) a more homogeneous structure depicted by a more reduced splitting of the medium frequency Raman features into R and Rc bands, (iii) a substantially lower amount of free volume, and (iv) a slight reduction in the average free volume size. Furthermore, the spontaneous hydration layer on the two surfaces is studied, and its effect on the free volume is discussed. These results provide new insight into the soda lime silicate glass structure and have various technological implications for processes like chemical tempering or phenomena like surface defects nucleation.