The amount of packaging made entirely of plastic is growing constantly. More and more, packaging made of other materials also contains plastic to provide airtightness, weldability, etc.
Plastic is not just one material. Plastic is made from polymers and additives. A polymer consists of many small molecules joined together. Depending on the chemistry of the molecules and the way in which they are joined together, the polymers can have entirely different properties. It is often necessary to add fillers, stabilisers, softeners etc. to make the polymer suitable for practical use. The finished material is called plastic.
Although plastics are now starting to be developed from the new "bio-fuels", these have not yet reached an acceptable price/performance ratio for the industry in general and the raw material for most plastics is still oil or natural gas that is refined in several steps. Between 30-40% of the production of the plastic industry goes to packaging of different kinds. But still approximately only 1.5% of the total global oil consumption is used for the production of plastic for packaging. Plastic is mainly used to save resources during production, distribution and use.
The most commonly used packaging plastics are polyethylene (PE for short), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), polyamide (PA) thermo-plastic polyester (PET) and polycarbonate (PC). For more information see "An Introduction to Plastic Resins and their Applications"
Practically all packaging plastics belong to the large group of thermoplastics. This means that they soften and melt when heated and are easy to work and shape. When cooled, the shape is maintained. Thermoplastics can be re-shaped many times through re-heating and cooling. This method is used e.g. when weld-sealing packaging whereby the welding surface is heated to melting point, squeezed up and then cooled.
The properties of thermoplastics also make them easy to re-cycle through remelting. This method is frequently utilised by plastic processing companies, not least to prevent the production spill from becoming waste.
Normally, only a small amount of plastic material is necessary to contain a large amount of product, e.g. in films, bags and thin bottles. Therefore, plastic packages have a low energy and resource consumption calculated per amount of packed product. Plastics are mainly used where some kind of barrier is needed to protect the product from the environment or the environment from the product. Most packaging plastics protect the product from humidity, many from smell and flavour (aromas) and some from gases.
Even extremely thin plastic packages can be made air and watertight and have good barrier properties to give best possible product protection.
By combining plastics with other materials, e.g. with cardboard, the tightness and weldability of the plastic and the rigidity and stability of the cardboard are utilised. This is what we see in milk and juice packaging and it is also a very good example of how combinations of different materials contribute to minimising the total material consumption in a package. The alternative – packaging made of homogenous, uniform material such as rigid plastic or glass – means higher weight and larger volumes to handle.
Plastics are used within practically all areas of packaging, with the possible exclusion of packaging for certain aggressive products that dissolve plastics. However, even this problem can often be eliminated by choosing the right plastic and coating.
Plastics are also used where it is desirable to see through the package. There are plastics which are almost as clear as glass but are neither fragile nor heavy like glass. The PET-bottle for carbonated drinks is a good example.
Plastics are also used for incorporating loads. Stretch film around packages and entire pallet loads or shrink film encasing bottles, cans or cardboard capsules are examples of this. At the same time, the product is protected against rain, dirt, pilferage, etc.
Plastics also serve as glue or as thin surface layers in the form of lacquers or laminated surface layers on other materials, e.g. for moisture protection. One major area of application for packaging plastics is shock protection, such as fittings of cellular plastic in transport packages such as CD-players, computers or other fragile products.
source - Packaging and the Environment (Feb 2001) - Packforsk - www.packforsk.se