Researchers study the case of the disappearing glass

Scientists know how iron rusts and why copper turns green but now researchers at Penn State University are investigating how glass corrodes.

‘We know that a silica rich layer forms on the outside of some silicate glasses when it is exposed to aqueous media,’ said Nathan P. Mellott, Ph.D. candidate in materials science. ‘We do not yet know exactly the mechanism behind the formation of this layer, what compositional/structural aspects of the original glass promote or inhibit layer formation, or how the physical properties of the glasses are affected.’

The silica layer that forms by aqueous corrosion on most glass can affect optical properties, transport properties and mechanical strength. For normal windowpane glass, the layer forms slowly and goes unnoticed during the lifetime of most windows. However, with more reactive glasses, aqueous corrosion can occur more rapidly.

Mellott and Dr. Carlo Pantano, director of Penn State’s Materials Research Institute and distinguished professor of materials science and engineering, are looking at modified e-glass, the type of glass used for fibreglass reinforcement and insulation.

This class is made of silicon dioxide, calcium oxide, sodium oxide and aluminium oxide.

The researchers looked at three different compositions of glass in which the calcium and sodium oxides varied and the proportion of aluminium and silica oxides remained the same.

They found that the sodium, calcium and aluminium ions were all leaving the surface, replaced by either water or hydrogen ions. The varying glass compositions did not affect the layer composition or structure, but did alter the thickness of the corrosion layer.

‘We want to determine if the glass structure at the surface remained the same during corrosion, or the glass network breaks up and repolymerizes,’ said Mellott. ‘Evidence points to the glass structure breaking up and repolymerizing.’

One reason to look at corrosion of fibreglass is that US health regulations require that glass fibres dissolve fairly quickly if inhaled into the lungs.

‘Regulations require that fibre can only stay in the lung for a specific length of time before it dissolves,’ said Mellott. ‘If we have a better idea of how fibre breaks down, we can modify the glass compositions that make the fibres dissolve more quickly.’

Understanding of the silica layer could also help to make the fibres stronger.

When glass fibres are pulled, they are typically reactive. To protect the fibres, they are often coated with a polymer that can be deposited on the fibre in a strongly acidic or basic media.

‘The formation of the silica layer is dependent on the acidity of the liquid the glass fibre is in and the temperature,’ said the Penn State researcher. ‘Our current experiments use a very acidic solution to create the silica layer we are characterising.’

After the researchers characterise the silica layer, they will look at the optical and mechanical properties of these glasses.