Types of glass
In the various pharmacopoeias a distinction is made between different types of glass, e.g. in the European Pharmacopoeia, paragraph 3.2.1 "Glass Containers for Pharmaceutical Use".
Neutral glass is borosilicate glass with a significant proportion of boric oxide, alumina lye and/or alkaline earth oxide. Due to its composition, neutral glass has a higher hydrolytic resistance and a high resistance to thermal shock.
Sodium oxide glass is made from silicon oxide mixed with alkali metal oxides (especially narium oxide) and alkaline earth oxides (especially calcium oxide). However, due to its composition, sodium oxide glass has a more limited hydrolytic resistance than borosilicate glass.
Pharmaceutical glass is classified on the basis of the hydrolytic stability of the glass. Hydrolytic stability refers to the resistance to the release of soluble minerals in water that occurs under certain conditions when glass comes into contact with water. Hydrolytic stability is determined by titrating the alkalis released.
Glass can therefore be classified on the basis of hydrolytic resistance as follows:
Type I glass: neutral (borosilicate) glass with a high hydrolytic resistance due to the chemical composition of the glass itself.
Type II glass: Normally sodium oxide glass with a high hydrolytic resistance due to a suitable treatment of the inner surface.
Type III glass: Normally made of sodium oxide glass with only limited hydrolytic resistance.
Although the suitability of the selected, validated and/or registered glass packaging is the responsibility of the respective pharmaceutical manufacturer, we generally recommend the following to our customers:
Type I glass is suitable for most parenteral nutrition (human) preparations and is the most commonly requested glass.
Type II glass is suitable for most acidic, neutral and aqueous preparations for (veterinary) parenteral administration.
Type III glass is generally suitable for non-aqueous preparations for parenteral administration, powders for parenteral administration (except for freeze-dried preparations) and non-parenteral preparations.
In addition, glass packaging can generally be used with hydrolytic resistance above that recommended for a particular type of preparation.
The outer surface can be treated, e.g. to reduce resistance and increase durability, by the so-called hot treatment (i.e. the application of metal oxide) and cold treatment (i.e. the application of a smooth layer).
To increase the hydrolytic stability of glass vials, their internal surface area can be increased by the interdiffusion/ion exchange from the glass and the subsequent reaction of the sulfates with the sodium available on the surface, resulting in sodium sulfate. The sodium sulphate remains on the glass surface as a water-soluble crystalline precipitate which must be rinsed off before filling.