First glass

A common perception is that glass is heavy, brittle and has had its day. John Henderson, business development manager at Glass Technology Services, looks at the case for using glass in the pharma sector

Glass is renowned for its inert nature and its general stability as a packaging material, and is therefore widely trusted as a medium for sensitive products. It has a small but significant share of the pharmaceutical packaging market and its use is growing as glass manufacturers adapt to the increasing demands placed on their product.

As pharma technology becomes more advanced and sensitive formulations demand ever greater stability from their packaging, the question arises 'how inert is inert?' When standard soda lime glass is used for bottles that hold products such as beer, wine or tablets, it is considered totally inert. However, this is not necessarily true in all cases. A container chosen solely to meet the European and US pharmacopoeia Type 1 standard may not be entirely appropriate for its intended use if all the factors relating to the interaction between pack and contents has not been taken into account.

Aqueous solutions can attack glass in two distinct ways. In neutral or acidic conditions (pH<8) the predominant action is the leaching of metal ions from the surface of the glass. These ions are usually alkali ions. In alkaline conditions (pH>9) the solution actually mounts an 'assault' on the silicate network itself, such that in extreme cases the surface can become pitted and 'fogged'.

unintentional confusion

The glass industry has not set out to confuse the pharmaceutical industry but by default it appears to have done so. As demands on glass have changed over the years, individual glass manufacturers have adapted their basic compositions and introduced new ones to meet the needs of the pharmaceutical market while retaining previous variants. This has resulted in a larger than necessary number of glass compositions being used for pharmaceutical applications.

The least expensive and most widely made composition is based on the soda-lime-silica system, which generally falls in the Type III category. This is essentially the same glass that is used for bottles and jars for foodstuff packaging. It is sometimes coloured amber to give protection from UV light. A typical composition of soda-lime-silica glass is given in table 1.

high cost

Vitreous silica is an extremely inert material, but its cost is too high to allow it to be used widely in the pharmaceutical industry. This glass is very difficult to melt and form, requiring temperatures exceeding 1900oC. Its chemical durability is second to none but its price is such that it can be justified only in the most demanding applications and where a price premium is available.

Full or hard borosilicate glass is highly chemically resistant has the added advantage of excellent thermal shock resistance. For this reason it is generally encountered as laboratory ware. From the typical composition (see table 1) it can be seen that there is very little alkali in this glass. It is the boron in the composition that allows its commercial production. The lack of alkali gives these glasses excellent pharmacopoeia performance, which generally puts them into the Type I category.

But developments in the pharmaceutical field quickly led to the realisation that neither soda-lime-silica nor borosilicate was the full answer to the industry's requirements. In classic tradition a compromise was arrived at and the neutral borosilicate was born. This can give Type I performance for little more than soda-lime cost.

Makers of pharmaceutical containers like to manufacture in large numbers for volume markets and pharmaceutical companies looking for smaller runs of specialist product tend not to be encouraged. However, the significant economic advantage of soda-lime-silica glasses has driven the need to find ways of improving their durability without unduly affecting primary cost. There are a number of ways this has been achieved:

Thanks to its very stable and chemically durable nature, glass has a well deserved reputation as an inert packaging material.

But it is important when looking at materials for pharmaceutical packaging to investigate and understand the product/packaging interactions. This is as true for glass as any other material.