Less is more: design flair giving PET a clear lead

PET is increasingly used for packaging, in particular for bottles, where it has now become ubiquitous, used for everything from mineral water to wine. Strictly speaking, polyethylene terephthalate, better known as PET, doesn't contain any polyethylene, being a thermoplastic polymer resin of the polyester family. It has numerous advantages as a packaging material, being both lightweight and relatively cheap to produce. It's easily formed into different shapes, making it unlikely to shatter and easily resealable. It also ticks the green box, being easy to recycle.

The plastic chemicals do not leach so it's extremely safe as a packaging solution for drinking water and other foodstuffs. Consequently, up to 70% of soft drinks are sold in PET plastic bottles, according to the British Plastics Federation.

PET is formed from the reaction between two chemicals - purified terephthalic acid (PAT) and ethylene glycol (EG). PET that is to be used for bottles is polymerised to create long molecular chains. The bottles are made in two steps. First, injection moulding is used to create a PET preform, essentially a long thin cylinder. This is then blow moulded to stretch the preform to create the bottle shape.
In some cases, the preform is injection moulded and then blow moulded into its final shape in a single step, but both methods are equally efficient with no difference in quality. In either case the final container is a single piece with no joint, giving greater strength and reducing the risk of breakages.

The fundamental approach has been much improved in recent years, leading to lighter weight packaging without compromising the protection of the products. Jonathan Bloom, senior executive for industrial issues with the British Plastics Federation, says that today's plastics packaging is up to 80% lighter than it was 20 years ago.

"Technical advances and design flair have reduced the quantity of plastics packaging needed to pack a given quantity of product over time, without sacrificing the pack's strength or durability," Bloom says. "For example a 1-litre plastic detergent bottle that weighed 120g in 1970 now weighs just 43g: a 64% reduction.

"Society's consumption levels rose by 20% between 1999 and 2004. However, the use of plastics packaging only rose by 4%, which has to be credited to our members' continuous innovation in product design," he adds.

This helps to reduce both the costs of manufacturing the PET packaging and its carbon footprint through the supply chain. A good example of this is the new projects developed by P.E.T. Engineering. The company has teamed up with Husky Injection Molding Systems, and with resins specialist Novapet, to create a series of lighter weight bottles.

This includes a 0.5-litre EvoLight design, with a weight of 7.5g, which P.E.T. Engineering claims is 31.5% lighter than the average weight of bottles currently on the market. This is achieved by implementing a new Husky 26/22 neck finish that weighs 1.91g, with P.E.T. Engineering's Swerve Neck solution and patented Sunbase bottom, which gives more resistance and rigidity to this area, even with the presence of nitrogen. A 1.5-litre EvoLight version can be used for both still and carbonated water applications on the same filling line without the need of a neck finish conversion. The still water version weighs in at 18.9g, while the sparkling version is 22.5g.

Behind the barrier
However, there are some performance issues that PET producers have to overcome. One limitation for PET has been that over time oxygen can be released from the container sidewall or can pass through the sidewall. Countering this has required the addition of various types of gas barrier, depending on the product involved. For example products such as fruit and vegetable juices are sensitive to oxygen, while carbonated drinks risk not only oxygen getting into the bottle, but carbon dioxide leaking out.

Some products, ranging from dairy drinks to cosmetics are sensitive to light, which can affect taste, texture or simply shorten shelf life.

There's also a danger that acetaldehyde, or AA, can form during the manufacturing process and lead to an unpleasant taste, which is particularly noticeable in products such as mineral water.

To counter these issues several companies have developed barrier additives. Bjoern Klaas, director of new products and operations for plastics additives specialist ColorMatrix, says: "The barrier market is attractive because customers are looking for lightweight packaging and for other characteristics."

Earlier this year ColorMatrix launched Amosorb Plus, an oxygen scavenging solution for PET packaging that builds on the success of the original Amosorb, launched back in 2001. It is a crystallised PET-based concentrate with a built-in catalyst to promote oxidation, offering product protection for non-carbonated foods & beverages for up to six months or longer, depending on the container design, allowing customers to tailor shelf life.

Clear thinking
Its main advantage is improved clarity. Klaas explains: "Amosorb Plus uses the same chemistry but the processes have changed to reduce the haze." This also means that since it is based on a previously approved technology it can be launched straightaway with no further regulatory approval procedures.

It is also possible to combine several different barriers, for example for those products that are sensitive to both oxygen and UV light - but this can be more complex to manufacture and can affect the clarity of the PET.

To get around this ColorMatrix has developed Amosorb SolO2, which is a combination of scavenging technologies with both active and passive barriers with a transparent appearance. It's also possible to combine colours, but as Klaas notes: "If you are working with scavenging technology then you are usually looking for clear glass-like containers."

But there are exceptions such as brown PET bottles that are used to protect beer from UV light. As Klaas points out, "Beer is a complicated drink" needing also an active oxygen barrier and a passive barrier to prevent CO2 egress.

Last year ColorMatrix introduced an interesting product called HyGuard, which Klaas says is still in its infancy. HyGuard gives a completely clear, glass-like appearance, with a much lighter weight bottle. Instead of using a thicker bottle to keep the oxygen out, HyGuard allows oxygen to seep into the bottle but then reacts the oxygen away, mixing it with hydrogen and turning it to water. The cap is a vital element containing a hydrogen activator, while there are tiny amounts of a catalyst within the container walls. The hydrogen is released once the bottle is sealed. As oxygen enters through the container wall, it binds with the hydrogen on the surface of the catalyst to form nano-particles of odourless and colourless water. The amount of water produced is at a parts-per-million level and does not affect the contents in any way.

The result is a much thinner bottle, which is lighter in weight and much more transparent. It's also easier to recycle because there's less plastic content and no contaminants in the PET wall.

However, the bottle must be designed together with the cap since that's where the hydrogen is stored. HyGuard can be combined with other barriers, such as for UV protection, without compromising its performance.

Wrexham-based rigid plastic packaging provider APPE has the exclusive European license for HyGuard, which it markets under the brand name ActiveSeal. APPE says that it works best with smaller containers and is currently used for juice bottles, but APPE says it is working on containers for wine and for food.

Hot stuff
APPE has also been working on PET containers suitable for high temperature applications - hitherto a significant limitation of PET, since many products are filled at high temperature. To this end APPE has worked with Japan-based plastics machinery manufacturer Nissei ASB to develop the Thermalite range of jars, which are suitable for hot fill and pasteurisation up to 95°C. APPE claims this is up to 85% lighter than glass equivalents, and has a smaller footprint.

Thermalite is suitable for use with products such as cooking sauces, tomato-based products, pickled vegetables as well as fruit purées, jams and spreads. It requires less energy to be used during the filling process, with reduced noise levels and is less prone to breakages on the line. It is currently being used by G's Fresh to package the M&S Simply Pickles range, which includes beetroot
and pickled onions.

Kinza Sutton, APPE's marketing manager, says that there are also a number of shelf-life trials under way with many other producers and fillers across Europe. There's a choice of six sizes from 370ml to 720ml in round and octagonal designs, with further shapes and sizes in development.

In conclusion, PET is increasingly replacing glass as the material of choice for most bottling requirements. One major exception has been wine bottles, partly because wine is stored in bottles for many years and glass has the advantage of not allowing oxygen in, which would otherwise lead to a short shelf life. But even this is changing with supermarket chain Sainsbury's having recently launched a number of wines packaged in standard sized 75cl PET bottles.

The key factor is that the PET bottles must mimic the look of the glass bottles, both in their shape and in the clarity of the glass. And of course, PET has the advantage of being fairly robust, even as the containers become thinner, leading to fewer breakages than with glass - surely something we can all drink to.