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PET packaging demystified
Polyethylene terephthalate (PET) packaging is widely used for packaging in the drink industry, representing about 75% of UK bottled water packaging, according to Zenith International. However, the bottles have to answer a wide and expanding range of expectations.
The reason why amorphous extrudable PET grades weren’t easily recyclable lies in the drying processJasmine Lejeune
To fulfil these expectations completely, different manufacturing processes have to be used, as well as chemical alteration of the material to make it fit for purpose.
There are two main families of manufacturing processes: extrusion and injection molding. Extrusion is used mainly for five-gallon (19-litre) bottles with handles, which are going to be used for cold fill, mainly for dairy and juices but sometimes packaged water. This process also allows a wide range of different-shaped designs for the pack.
A plasticating screw heats the plastic granulates. The melted polymer goes through a die to form a hollow tube. The hollow tube, still warm, is cut and enclosed in a mold where air is injected and the hollow tube takes shape.
Injection molding is used for regular bottle shapes with less mechanical resistance expectations. It’s the most common process used on bottling lines around the world, as it’s the most cost-efficient for producing the majority of the smaller format packs (33cl to five litres).
Two main types of processes exist:
- One-step injection molding – the melted polymer is injected into the final shape of the container in a cold mold. It’s then released.
- Two-step injection molding – the first step is the injection of melted plastic in a mold, resulting in the creation of a preform. The cold preform is then taken onto another machine, enclosed in a mold, heated, stretched with a pin to reach the desired length and then blown in the shape of the chosen mold.
PET takes different states as a material during the above processes: amorphous and crystalline.
- Crystallisation is a structured organisation of atoms within the material. Crystallisation can be partial or complete.
- Amorphous is a term used to define a material lacking a crystal structure. For example, when a solid melts, it changes from the crystalline to the amorphous state, in which it remains until recrystallisation takes place.
PET project
A new PET blend, changing the whole efficiency and process of the amorphous extrudable PET grades recycling, was patented mid-2008 by PepsiCo Inc (EP1918327).
By blending a slow crystallising PET and a faster crystallisation PET, the company has managed to solve some recurrent industry problems, such as obtaining a PET blend resin useful for extrusion blow molding that can be easily recycled.
The new blend combines the following characteristics:
- A sufficient melt strength, which makes it suitable for extrusion.
- A sufficient crystallinity to reduce sticking and agglomeration, enabling the PET blend to be recycled.
The reason why amorphous extrudable PET grades weren’t easily recyclable lies in the drying process. Recycled PET containers are ground, washed and dried to be recycled. The drying process (typically done at 160˚C for four hours) allows for an adequate crystallinity to be developed in the ground flake to prevent sticking or clumping at levels exceeding 20% in recycled PET.
With this new PET blend, this problem will not occur, and will allow more material to be recycled. Avoiding clumping also simplifies the recycling process.
What is PET?
PET is a term misleadingly used to simply communicate PET-based thermoplastic polymers. Actually, PET is blended with other compounds for use in different applications. For example, some compounds are added for aesthetic reasons. A classic use of cobalt and pigments controls the colour and transparency of the packaging.
On the other hand, the sensitivity of some beverages to UV (less UV means a longer shelf life), oxygen (less oxygen allows a better resistance to thermal degradation) and sometimes their carbonation introduces the need to use a PET blend, creating an adequate barrier. This can be achieved through multi-layered or surface-coated bottles, associating different material, each of them having one or more of the required properties.
However, the ‘ideal’ route is to use a single-layer PET blend material for reasons of simplicity of the process and lower capital investments. To achieve this mono-layer, PET-based blend, oxygen-scavenging agents, barrier-improving agents, toners and nucleating agents are traditionally used. The proportion and the choice of the additives is an area of expertise, and is still developing, making more options available to the industry.
Jasmine Lejeune is operations consultant at Zenith International. She is part of the technical team, advising clients on all aspects of beverage operations, from bottling plant design, build and commissioning to problem solving and regulations. Contact Jasmine here.
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