PET bottle life cycle and NIR heating technology

More than 500 billion PET bottles are used globally to pack different products but unlike other types of plastic, PET is 100% recyclable.

NIR radiation is applied to various stages of the PET bottle lifecycle: from pallets and preform injection to bottle blow moulding and its recycling. The global trend to switch to rPET contributes to it — let’s see why.

IR radiation and thermoplastic polymers

PET is a thermoplastic polymer belonging to the polyester group. It is fully recyclable and widely used to produce bottles for beverages and jars for food, containers for vegetable oil and vinegar, household chemicals and cosmetics.

Polyethylene terephthalate is often recycled into fibres, sheets, fabrics and definitely in new PET packaging. In its natural state, the resin is colourless, semi-crystalline and highly flexible. Once processed, it can be semi-rigid and rigid.

Being thermoplastic polymer, heat impact leads to its softening and deformation. In bottle manufacturing, a PET preform once heated and passed to the mould takes the shape of the latter. Thus, it is difficult to overestimate the importance of the heating system of a reheat PET stretch blow moulding machine and lamps in particular.

Infrared lamps emit a wide range of frequencies. There are three types of infrared radiation:

• Wave number near infrared with frequencies of 2500–714 nm (4000–14,000 cm^-1).
• Wave number mid infrared with frequencies of 25,000–2500 nm (400–4000 cm^-1).
• Wave number far infrared with frequencies of 2,500,000–25000 nm (4–400 cm^-1).

NIR lamps and bottle blow moulding

Different wavelengths interact with different compounds in different ways. Each type is defined either by the source of light to trigger an interaction between constituent wavelengths and compounds present in the sample or the electromagnetic spectrum field analysed. IR and NIR spectroscopy both fall into the latter category.

Both IR and NIR spectroscopy use spectral data to characterise matter. The main difference is the field of the electromagnetic spectrum studied. NIR spectroscopy focuses on the 714 nm to 2500 nm range. This is just outside the range normally visible to the naked eye.

Found in the 19th century, the near infrared energy expanded into industrial applications in the mid-20th century as an add-on unit for other optical devices. In the 21st century its application expanded to PET stretch blow moulding.

Near infrared radiation penetrates PET and reheats PET preforms. In fact, the radiation in the NIR range can penetrate the PET samples completely to a depth. Beer-Lambert Law is the main principle and the basis for the different methods of spectrophotometry, including NIR spectroscopy. According to it, the amount of light absorbed by a sample (in our case, it is PET preform) is directly related to the volume of sample the light passes through and the concentration of the sample.

The importance of NIR technology is difficult to overestimate for manufacturing of such PET bottles as:

• asymmetric bottles
• refillable/returnable bottles
• hot fill bottles
• cylindrical bottles at high speed
• colour PET bottles.

The heating process is claimed to be faster in comparison with the conventional infrared (IR) system. It reduces energy consumption and permits to design a smaller (shorter) heater of the PET stretch blow-moulding machine, and the equipment has a smaller footprint and is designed to be easier to transport.

PET Technologies with 24 years of experience in the industry was among the first companies that adapted NIR heating to the linear PET stretch blow moulding machines. The APF-Max series of blow moulders are among the first SBM machines of linear type where it was implemented.

It is compact, has a user-friendly concept and has high output while its dimensions are more compact.

The advantages of NIR heating system for PET bottles production include:

• uniform temperature distribution between the inner and outer parts of preform wall
• reduced heating time
• compact heater size
• reduced energy consumption of blow moulder.

NIR radiation and recycling

There are controversies surrounding the use of coloured (ie, any colour other than transparent and transparent blue) PET bottles. They have been banned by law in India and South Korea, and there are similar discussions in other countries.

By contrast, the European PET Bottle Platform Technical Committee states that transparent coloured PET bottles should not be banned as they can increase the total value of PET bottle recycling.

Near infrared (NIR), in combination with optical sorting, is an efficient and quick way to sort PET bottles automatically by colour. Once sorted they are 100% recyclable and reusable for different applications. Additional sorting by individual colour (eg, green or amber) is also possible. It additionally increases the rPET value.

NIR and rPET

Finally, this recycled PET bottle is used again to produce a new PET bottle. This can repeat unlimited number of times. They are both eco-friendly and transparent, resistible and flexible.

Recycled PET uses 79% fewer greenhouse emissions and less energy than the virgin one. Some other benefits include:

• up 50% less energy is consumed for its production
• eliminates plastics wastes
• less water is used for production.

Still, not all blow moulding machines are capable to process preform from rPET due to the technical limitations. They come from the properties and characteristics of the material. Some of the reasons why not all machines are capable of processing it:

• Material properties (melt flow, crystallinity, molecular weight).
• Wear and tear (rPET contains impurities that cause additional wear of blow moulding machine components).
• Melt temperature and thermal stability (rPET requires a higher processing temperature due to its molecular changes during recycling).

The entire range of the reheat stretch blow moulding machines developed by PET Technologies is designed to be compatible with rPET. The role of NIR heating system in bottles blow moulding from rPET preform can increase with the percentage of recycled content in the sample.

Image credit: iStock.com/Calin Hanga