Properties of Glass: Chemical
The properties of glass can be varied and regulated over an extensive range by modifying the composition, production techniques, or both. In any glass, the mechanical, chemical, optical, and thermal properties cannot occur separately. Instead, any glass represents a combination of properties. And in selecting an individual glass for a product, it is this combination that is important. Usually one property cannot be changed without causing a change in the other properties. It is the art of the glass scientist to produce the most favorable combination possible.
Glass is much more resistant to corrosion than most materials, so much so that it is easy to think of it as corrosion-proof. Glass windows after several years exposure to the elements remain clear and apparently unaffected. Glass bottles hold a wide range of liquids that would dissolve other materials. In the laboratory, reactions are carried out in glass beakers and flasks without damage to the beakers or contamination of the solutions reacting.
But, in spite of these indications that glass is indestructible by chemical attack, under certain conditions it will corrode, even dissolve. In these cases, it is important to choose the right type of glass, since some are more corrosion resistant than others. Only a few chemicals aggressively attack glass -- hydrofluoric acid, concentrated phosphoric acid (when hot, or when it contains fluorides), hot concentrated alkali solutions and superheated water. Hydrofluoric acid is the most powerful of this group; it attacks any type of silicate glass. Other acids attack only slightly; the degree of attack can be measured in laboratory tests but such corrosion is rarely significant in service for acids other than hydrofluoric and phosphoric.
Acids and alkali solutions attack glass in different ways. Alkalis attack the silica directly while acids attack the alkali in the glass.
When an alkali solution attacks a glass surface, the surface simply dissolves. This process continuously exposes a fresh surface which in turn is dissolved. As long as the supply of alkali is sufficient, this type of corrosion proceeds at a uniform rate.
Acid corrosion behaves quite differently. By dissolving the alkali in the glass composition, a porous surface is left that consists of the silica network with holes where the alkali has been removed by the acid. This porous surface slows the rate of attack since the acid must penetrate this surface layer to find alkali to dissolve.
Corrosion by water is similar to acid corrosion in that alkali is removed from the glass surface. Water corrosion acts at a much slower rate. At high temperatures, however, water corrosion can become significant. Gauge glasses for steam boilers are a case in point. These products must be protected from the superheated water by a sheet of mica or replaced on a schedule that insures that they will not be seriously weakened.
Many laboratory tests have been devised for testing corrosion resistance. Some of them aim at accelerating rate of corrosion by employing high temperatures or by grinding the glass to a specified grain size to expose more surface area to the corroding solution. After treatment for the specified time and at the specified temperature, the weight loss of the glass can be measured or the amount of alkali extracted can be determined.
Many factors influence the rate of corrosion and no laboratory test to date is capable of predicting service behavior under all conditions. Concentration and rate of agitation of the corroding solution are important factors. As corrosion progresses, the test solution becomes contaminated with components extracted from the glass; this contamination may speed or slow corrosion rate. Some glass products have a silica-rich skin, so the surface will show a different corrosion rate from the interior. A powder test on the glass from such a product will miss this surface effect completely. Comparisons between glasses from accelerated tests will be reversed sometimes at normal service temperatures.
Comparative values of resistance to acids, alkalis and water may be found in the literature, but these should be employed as guides only. Results must be checked under actual service conditions.
Electrical Properties (See: PROPERTIES OF GLASS: Electrical)
Mechanical Properties (See: PROPERTIES
OF GLASS: Mechanical)
Optical Properties (See: PROPERTIES
OF GLASS: Optical)
Thermal Properties (See: PROPERTIES OF GLASS: Thermal)