With an increasing number of companies using or considering recycled PET (rPET) as part of the package composition, it’s important to understand the properties that are most likely to impact your molding processes and bottle qualities.
The first point to understand about rPET is that it remains PET. Even though it might have some altered aesthetics such as changes in color or clarity, rPET still retains the main, desirable physical properties of PET including tensile strength, barrier, impact resistance, thermal properties, and more.
And while the bulk of the properties are retained, the specifications are wider in recycled materials. For example, color shows the most variation, but there is also a wider specification in the solution viscosity that will affect how the material behaves during injection and blowmolding.
Most manufacturing facilities are used to dealing with slight variation when processing PET. However, with rPET, the variations/material specifications are broader. Therefore, the package may need to be designed to accommodate a wider processing window.
When we’re considering rPET content for a packaging application, it should be qualified like any other resin change. That typically means looking for changes in how the material processes in injection and blowing, and dimensional or performance changes in the resulting parts.
Be wary of those wider spec ranges.
Wider material specifications must be accounted for when qualifying rPET. This could be variation between different suppliers, for example, food-grade rPET resin from supplier A will be different from supplier B or from lot to lot. This will require additional effort during the qualification process; for example, larger sample sizes than are typically used for virgin resin will help to capture the true variation in appearance or performance due to the wider material specifications. The goal is to make sure the material’s impact on the container is properly understood during qualification.
Similar expectations will hold for incoming material inspection. The containers that are collected for recycling vary over time and with location, so wider specifications should be expected for the resin that is created from them. Monitoring of the material to ensure adherence to acceptable values is more important as rPET percentages increase.
At the end of the day, each brand owner also needs to determine what the acceptable shelf appearance is for their product. For example, if your container is highly colored or covered with a full-body shrink sleeve, changes in color, clarity or gloss can be tolerated. However, in situations where the container clarity is critical to marketing the contents, then appearance variations become even more critical.
The key takeaway is that while you shouldn’t expect tremendous differences in how the rPET behaves during processing, there may be differences in how the material looks (e.g., color, haze). Working with your engineers to compensate for the impact of wider specifications on material processing will allow production of acceptable containers with recycled materials.
Martin Geithmann is senior project engineer of PTI. He has 20-plus years of experience in preform and container design and optimal material selection for package performance. PTI is a plastic packaging resource for preform and package design, package development, rapid prototyping, preproduction prototyping, and material evaluation engineering.