Glass

Considerations when using glass packaging

Advantages

• Glass is an inert material. Glass has excellent barrier properties and chemical resistance, making it an ideal material for products with a long shelf life.
• Used glass is easy to clean. That is why it has been used for reusable packaging materials, such as beverage bottles, for a long time.
• The high degree of transparency of glass improves product visibility and makes this material highly appealing for marketing purposes. The material is relatively form free (it can easily be turned into different shapes and sizes).
• Many countries have an efficient glass collection system in place and the material is relatively easy to recycle.

Drawbacks

• Glass breaks relatively easily.
• Glass is relatively heavy and therefore less efficient during the logistical process.
• The production of glass and the packaging process are two separate processes. This means that glass packaging materials must be transported to the packager while empty.
   

Raw materials

Glass is a hard, inert material with a mineral origin. Glass can be either transparent (white) or coloured, depending on the raw materials used. There are different types of glass; in this article, we will focus on the type of glass that is used as a packaging material, i.e. soda-lime-silica glass.

The main component of glass is silicon dioxide (SiO2), better known as sand. The metal oxides sodium oxide (Na2O) and potassium oxide (K2O) are added to lower the melting temperature, which also increases the material's water solubility. Calcium oxide (CaO) and barium oxide (BaO) are then added to reduce the water solubility. Furthermore, aluminium oxide (Al2O3) can be added to limit the glass’ expansion when exposed to heat. In the past, lead was added to achieve more luster and lower the melting point.

For coloured glass, certain ingredients are added to the raw materials during the production process. For example, adding iron oxides will result in a green, yellow or brown colour, while adding cobalt oxide (Co2O3) creates a blue colour. Colour is used to protect the packaged product against the influence of light. The degree of protection differs per colour. Furthermore, colour can be used for marketing purposes. The use of colourants affects the recyclability of glass.
 

Production

The production of glass is a melting process. The dry raw materials are mixed and transported to a furnace. In the furnace, used glass cullets which are being recycled - are added to the mixture. Melting and producing glass is a continuous process; the glass furnace is generally always on. Eventually, the molten glass is transported to a dosing mechanism. With scissors, the hot and syrupy stream of glass is cut into droplets with the weight of a single packaging. These droplets are also known as "gobs.” The gobs can then be used to produce various types of packaging materials.

Blow and blow production process for bottles
To produce bottles, the so-called blow and blow process is utilised. It consists of two steps. The first step is making a pre-form. The first mould is used for this. The gob drops into the mould and is pressed into the mould from above using compressed air. The aperture forms at the bottom of the mould. Next, air is blown in from underneath. At this stage, the distinct shape of a bottle begins to form. The pre-form of the bottle is turned over and placed in the second mould. More air is blown in to ensure the glass is evenly distributed on the inside of the bottle as well. The bottle is still warm when it comes out of the second mould. By allowing the glass to cool in a controlled manner (a process known as “annealing"), any residual internal stresses are eliminated. This reduces the risk of breakage.

Press and blow production process for bottles and jars
This process can be used for the production of both bottles and jars. Once again, the packaging is made via a two-step process. The first step is making a pre-form in the first mould. The gob drops into the mould and is pressed into the pre-form with a plunger. The pre-form is turned over. In the second mould, the packaging is shaped like a bottle or jar by blowing air through the packaging's aperture. Lastly, the packaging is left to cool in a controlled manner.

The Narrow Neck Press and Blow (NNPB) process is used to produce thinner, lightweight bottles. By manufacturing the plunger out of material that can withstand higher temperatures, it is possible to make it narrower. As a result, the plunger can fit through the narrow neck of the pre-form, resulting in more precise control over the wall thickness of the bottle. An added benefit is that the pre-form cools faster during pressing, thereby increasing the production speed. (Source: Vetropack)
 

Recycling

In order to properly recycle glass and maintain the high quality of the recycled material even after multiple recycling processes, the quality of the collected stream of packaging glass has to meet certain requirements. For example, porcelain (tableware), earthenware (some alcoholic beverages), mirrors, drinking glasses and oven dishes should not be disposed of in the packaging glass stream. These materials negatively affect the recycling process and lower the quality of the recyclate because they do not melt or have a different melting temperature than regular packaging glass.
 

Situation in various countries

Netherlands
In 2018, 502 kiloton of soda-lime-silica-glass was used for packaging materials in the Netherlands, of which 86% was eventually recycled to make new glass. (Source: Afvalfonds Verpakkingen)