Sustainable packaging solutions may be the buzz in the industry, but the fact that environmentally-conscious consumers have started making it a condition of purchase means this is a shift that’s here to stay. The pandemic has hastened this trend as consumers have focused more on what’s truly important to them, and environmental issues fall at or near the top of the list.

Recyclability, as well as minimizing the use of plastics altogether, is at the center. The value of recycling is that it keeps components in play rather than allowing them to wind up in landfills or the ocean. However, while recycling efforts have been in place for some time, confusion and misconceptions remain – what it means to be recyclable, compostable, biodegradable, and more specifically, what needs to be true for the circular economy to come to fruition.

As this trend progresses, print service providers (PSPs) will have no choice but to become fluent in the specifics about recyclability and the factors that impact it so that they can guide their clientele toward appropriate sustainable solutions. While the label may represent only a small portion of the package, the simple fact that it is adhered to the container makes it critical to container recyclability. The science around plastics recycling is complex, and the technology is different depending on the polymer.

This article will provide a general overview about plastics recycling as it relates to the labeling of rigid and semi-squeeze polyester (PET) and high-density polypropylene (HDPE) containers – two of the most common container types on the consumer goods market today.

Plastics recyclability
Reprocessing plastic is more complicated than reprocessing glass or metal due to the variety of chemistries involved. There is no universal solution. To get clean recycled resin, steps must be taken to (a) sort it appropriately, and (b) remove contaminants that would result in poor resin quality. Furthermore, the demand for recycled plastic is growing. Reclaimers harvesting the plastics need to get a high yield, making it vital that contamination in the recycle stream be minimized. Consequently, the Association of Plastic Recyclers (APR) has developed very specific critical guidance protocols for labeling to increase the likelihood that containers will, in fact, be recycled.

Containers deposited into recycle bins by consumers get sorted based on the material – types of plastic, glass, aluminum, etc. There are multiple steps to the sorting process that utilize different technologies. For plastic containers, near infrared (NIR) systems are used to sort the different polymer types. This technology relies on the varied reflectivity of polymers and distinguishes between their individual wavelength signatures to separate them. Optical detection systems may be used to further sort based on color and, specifically, to ensure that clear containers end up in the clear recycle stream.

There are certain factors around labeling that impact successful sorting of plastic containers such as film type, label coverage and metalization. Proper sorting is essential to achieving higher purity resins that can be more readily reused. Contamination can mean changes in color, as well as physical and mechanical properties that can limit their reuse. These factors vary by container type.

Following sortation, the reclaim process varies among the different types of plastics, as well. The choice of label material can either facilitate or hinder sortation, reclamation, or both.

(a) Label coverage. Size matters. Color sorting equipment must be able to detect container color beneath the printed label, and high surface coverage on a container can impede this process, resulting in containers ending up in the wrong color stream. Too great a percentage of label coverage can also cause false NIR readings resulting in a mingling of polymers. The APR has set out specific recommendations regarding acceptable coverage percentages for PET containers to minimize incorrect color sortation. For containers that are 550 ml or less, the label coverage area should be 55% or less; whereas containers above 550 ml can have up to 70% label coverage. To date, no guidelines have been established as to label coverage on HDPE containers or NIR functionality.

(b) Metalization. Sorting equipment is designed to detect and eliminate metal from the recycle stream. Even a very thin metalized label can be detected and cause the container to be kicked out, resulting in yield loss.

(c) Film type. Label materials must be compatible with the reclamation process specific to the container to which they will be adhered. Using the wrong film type can either cause containers to be kicked out of the system or contaminate the stream.

PET containers
Recycled polyester (rPET) can be turned into new PET containers such as water or soda bottles. In fact, some major brands, such as Coke, Pepsi, Evian and Hellmann’s, are promoting the use of rPET containers in certain markets. rPET can also be used to make shoes, handbags, industrial strapping, rope, automotive parts, construction materials and fiberfill for jackets and sleeping bags, as well as fibers that are then made into fabric for clothing and carpet. Plastic manufacturers are investigating additives that might be utilized to ensure that rPET performs with the right thermal properties, flow and melt to ensure its usability.

PET containers are strong and lightweight and don’t react with water or food, making them ideal for food and beverage applications. rPET must be very pure to be usable. For the PET recycling process to produce high-quality rPET, it’s necessary for the label materials to be separated from the container because the polymers (typically a PP film with an acrylic adhesive) will lower the quality of the recycled resin.

After sortation, the containers are shredded and chopped into small pieces, which then go into a warm caustic water bath – typically a sodium hydroxide solution. This solution breaks the bond of the label to the container, effectively washing the label off. The polypropylene film and adhesive are lighter than water and, therefore, float to the top of the bath. The PET is heavier than water, so the flake sinks to the bottom. To be clear, the adhesive does not dissolve. Through filtration, the label material is separated from the PET flake, which is then collected for reuse.

What does this mean in terms of label production? Using label materials that are recognized by the APR for PET container recyclability is an excellent start. Specifically, the film must have a density of <1 so that it will float, and the adhesive must be able to “wash off.” Label design, including size and artwork, should also be taken into consideration as they impact sortation, as noted above.

HDPE containers
Recycled polyethylene (rHDPE) can be used to make waste and recycling receptacles, decking, bike racks, benches, picnic tables, fencing and furniture.

rHDPE does not have to be as pure as rPET to be usable, so a certain amount of label material, typically PE or PP, is allowable in their recycling. Consequently, the recycling process for HDPE is simpler than that of PET because it is not necessary to separate the label from the container. The containers, along with their labels, are simply cleaned and shredded and incorporated into the rHDPE that is used to form new items. The choice of label materials, then, depends more on desired label performance and converting preference.

Polyethylene (PE) is a like polymer to HDPE, resulting in the least contamination (the adhesive only) in the recycle stream. Its conformability makes it less prone to darting, making it a good choice for semi-squeeze containers. It does require a certain amount of finesse in dispensing, however, because it tends to stretch.

A certain amount of PP resin is allowable in the HDPE recycling process. PP films can be made very thin, which means more labels per roll, lower shipping costs and less inventory space. It’s also easier to print and is less expensive than PE. A very thin PP will require some finesse during both converting and dispensing.

Why paper is bad
In all cases, paper labels are detrimental to the recycling of plastic containers, regardless of the polymer, because the paper contaminates the wash water and can stick to the flake, resulting in reduced quality. The hot caustic bath used for PET recycling, in particular, renders paper labels into pulp that can’t be filtered out. Small fibers will remain and carbonize when the material is extruded, resulting in unacceptable quality. Non-pulping paper can resist the bath, but it’s heavier than water and sinks with the PET flake, again contaminating the rPET.

Labels are part of a bigger picture
The label is one of the smallest components of a consumer package, and brands that are strong in sustainability are looking at the total package – the container type, any coatings on the container, label materials, closures, and pumps, all of which impact recyclability. They are dissecting package design and trying to redesign different aspects to improve sustainability. For recycled content to be incorporated into package design, steps must be taken to keep the recycle stream as clean as possible.

Each time a plastic is recycled, a little more discoloration occurs, and the recycle stream will always have a mixture of virgin plastic and recycled plastic that’s been through any number of cycles. One water bottle may have recycled content in its third cycle while another may have content in only its first cycle, but both of those containers will be put into the recycle stream together. That makes the effect of contamination cumulative. Additives are used to mitigate this when new recycled resins are blended, but the cleaner the recycle stream can be kept in the first place, the greater chance there is of recycled polymers being more widely utilized in the long-term. To that end, steps must be taken toward producing the purest recycled resin possible, and new ways of improving its functional properties will be critical to the continued production of items from recycled plastic.

In the end, proper sortation is critical to achieving high quality, usable recycled resins, and there are numerous factors that go into achieving it. Any steps that can be taken toward that end related to the labeling of the containers will become more important as time goes on. PSPs that are knowledgeable about the factors that enable recyclability can serve as a resource to brands in ensuring their packages have the greatest chance of being recycled without contaminating the recycle stream.