Chemical Properties of the Containers

Mario Moretti

Description

The containers are mainly used for packaging of foodstuffs and pharmaceutical products; consequently, the chemical resistance of the glass which is used is very important. The chemical interaction between the surface of the glass and the liquids in contact with it is normally minimal, but it cannot be ignored; the factor which is the main determiner of this is the chemical composition of this glass.
The majority of containers are made of silico-sodium-calcium glass. In terms of chemical resistance, what is fundamental is the introduction of small percentages of alumina, an increase in the level in calcium and magnesium oxides and at the same time the reduction of the content in sodium oxide.
This glass is used to produce all types of objects for foodstuffs, like bottles, vases for conserved food, drinking glasses; perfume bottles, but also bottles for pharmaceutical solutions, whose inner surface is first sulphurised.
Sulphur dioxide, SO2, is used in this treatment, in the gas state: part of the alkali are removed from the glass and a surface with a lower chemical transfer compared to the original non-treated one, is obtained.
A second type of glass used for containers is boron-silicate, with which bottles, vials etc. for pharmaceutical use are produced.
This type of glass is called 'neutral glass' because when in contact with the liquid it contains, gives rise to unimportant chemical reactions.
Boron-silicate glass, along with sodium-calcium glass treated by sulphurisation, are types of glass which are chemically resistant to sterilisation, in autoclave, of the contained solution (30 minutes at a temperature of 120°C).
The colour of the glass containers at times has a precise objective, at other times it has only an aesthetic effect. For example, the dark green colour of spumante bottles or the yellow-amber of beer bottles is to protect the contents from ultraviolet rays which alter the taste of the product.