Question: What is a type III glass and what is this usually used for?
Answer: A Type III glass usually is a soda-lime glass with a higher content of alkali and alkaline earth metals like sodium (Na), potassium (K), barium (Ba), calcium (Ca) and magnesium (Mg). The glass is therefore softer and easier to convert. They have a CTE of higher than 9.0 10-6/K. This makes them more prone susceptible to the thermic shock breakage and they are not recommended to be used for lyophilisation processes, as also written in the Ph. Eur. Due to this composition it also has a much lower hydrolytic resistance. This is why in the USP and the Ph. Eur. it is described as ‘Type III are not used for parenteral products or for powders for parenteral use.’ Type III containers are very well suitable for orals.
Question: Why is the CTE of a glass important to know?
Answer: The Coefficient of Thermal Expansion (CTE) is a measure for how much the glass expands and shrinks in volume when it is heated and cooled down again. The smaller the CTE the less is the movement. Pure quartz glass e.g. has a CTE of below 1 10-6/K. The CTE is a consequence of the chemical composition of the glass. The higher the amount of Alkali (Na, K) and Alkaline earth (Ca, Mg, Ba) metals in the glass is, the higher is also the CTE. Among the Borosilicate glasses there are basically three groups: The group of 3.3 CTE glasses. That is where the Boro8330TM, Duran(R) and Pyrex glasses are located. Then we have the group of around 5.0 CTE glasses, where FIOLAX(R) belongs to. And at last there is the group of 7.0 CTE glasses, to which most of the cheaper glasses belong. Apart from indicating the quality of the glass the CTE is also quite important for temperature applications. Especially for freeze-drying applications glasses with CTE of 3.3 or 5.0 are highly recommended. During freezing and heating the temperature difference can be quite high so that breakage due to temperature shock can occur. And this comes easier for 7.0 expansion or soda-lime glasses than for 3.3 and 5.0 CTE glasses.
(To feature in this column, email your queries to Dr Bettine Boltres, SCHOTT AG: [email protected])
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