Closing the loop on bottle caps – Greenpath and Tomra pioneer mechanical sorting solution for separating polyolefins by polymer type and colour

The full-service recycler, processor and manufacturer Greenpath has partnered with Tomra Recycling Sorting to install and optimise a plastic flake sorting production line designed specifically to identify and separate polyolefin caps from carbonated and non-carbonated beverage bottles.

Greenpath’s vision is to create the same type of effective recycling system for bottle closures that exists for bottle recycling. Operating for more than 25 years with locations in California, Nevada and Texas, the vertically integrated Greenpath accepts a wide range of materials, and the flow of inbound materials can be somewhat inconsistent. However, this is where Joe Castro, President Greenpath Enterprises sees the company’s advantage in offering value-added services. Greenpath creates value being a one-stop solution for suppliers and customers alike by taking mixed trailer loads and variable supply streams and producing consistent, quality products.

Materials received include paper, metals, and plastics. Considering only the polyolefin materials – low-density polyethylene (LDPE), high-density polyethylene (HDPE) and polypropylene (PP) – Greenpath accepts bottles, film, rigid plastics, and, of course, bottle caps. PE and PP have very close density characteristics and are challenging to separate into their different polymer types. That is why mixed polyolefins are primarily downcycled into more forgiving applications that allow for more contamination or a wider range of material specifications. “Alternatively, other companies use a ‘solution by dilution’ approach to include mixed polyolefins, using primarily virgin material and only a small fraction of recycled plastic,” says Joe Castro.

With Greenpath’s focus on delivering value-added services to its customers, the polyolefin cap conundrum has kept Castro up at night for the last 15 years when they first started washing and processing recycled bottle caps. He had a vision for a sorting process solution to deliver high quality, consistent separation of HDPE from PP to give brand owners and manufacturing companies the flexibility to confidently include PCR in their injection moulding applications. “We are ultimately targeting 98% or greater purity levels,” offers Castro.

The quest for cap to cap

A beverage container commonly consists of four parts: the bottle, cap or closure, ring and label. States with recycled content laws for bottles have seen packaging companies and recyclers take the first step, focusing on the PET bottle, which comprises the bulk of the container’s weight. A more mature PET recycling infrastructure has led to brand owners incorporating up to 100% post-consumer recycled (PCR) material in their bottles. As regulations stiffen to increase the percentage of recycled content by weight, the next logical container component to address is the cap. “Following Greenpath’s approach, the industry can move toward a 100% PCR by weight package using only mechanical sortation,” says Eric Olsson, Area Segment Manager, Plastics, Tomra Recycling Sorting.

For the cap to get to a recycler like Greenpath, the bottle is the carrier through the Material Recovery Facility (MRF). “Otherwise, the cap will get screened out in the front end of the circuit and more likely be landfilled,” mentions Castro. The bottles, labels, caps and rings are shredded, and a sink-float process is used to separate the PET bottles from the PE and PP caps. The heavier PET sinks and the lighter polyolefins float and are skimmed off as a secondary recycling by product.

Logic may dictate using PET for both bottle and cap to solve the issue, but Olsson explains it’s not that simple. “Mono-material packaging is a dream that a lot of recyclers, brand owners and chemical companies are striving for, but we are not there yet. Different polymers have different strengths and weaknesses,” he says. “Building in the threading on a cap that’s required to tighten and keep the container sealed is a property that polyolefins, PE and PP, lend themselves especially well too.”

While some bottle recyclers see mixed polyolefins as a waste material, Castro sees opportunity. Because of their experience with a variety of materials and philosophy of one-stop solutions for his suppliers, Greenpath has extensive processing capabilities with different lines to sort diverse feedstock. The company is adept at mechanical, multi-material, and batch recycling processes. To realise the opportunity to close the loop on caps and offer a more sustainable market for polyolefins, Greenpath investigated various technologies to sort the mixed polyolefins. “PE and PP densities are very close to one another, so using float/sink tanks is not effective,” explains Castro. Alternative recycling technologies were also considered but didn’t offer the yield or recovery for both PE and PP resins that was desired. According to Castro, the cost-benefit analysis just didn’t justify the space and capital required. “The yields are low, and you have to consider the expenditure, space and waste generated from the separation process,” he says.

Greenpath began to zero in on a mechanical solution using flake sorters. Company officials initiated conversations with Tomra, who had introduced a new type of flake sorting technology. One of the technology’s design parameters was to extract value from multicomponent plastic waste streams. Olsson mentions that the industry hasn’t begun to reach the ceiling limit for mechanical polyolefin sorting. “It’s best to pursue a path that introduces the least amount of change to the material and sorts in the most efficient way,” he says. “Mechanical and chemical sorting technology OEMs are just getting the first processes off the ground to produce materials resulting from the 2025 and 2030 circularity commitments made in the twenty-teens. Producing 95% or greater purity-by-polymer streams is critical for many types of downstream recycling processes, and flake sorting can get us there.”

Flexibility in mechanical sorting

Discussions related to Greenpath’s goals led down a path to selecting Tomra’s Innosort Flake sorter. As the sorter does not require a large footprint, it allowed an easy integration at the Colton, Calif. facility. After discussing the purity, yield and throughput necessary to meet Greenpath’s objectives, flake sorters were installed.

Designed for flexibility, Innosort Flake incorporates a combination of sensor technologies, including colour cameras capable of identifying 16.8 million colour variations. With two-sided cameras, “one is positioned on each side of the chute where the materials fall through, so it can discern the difference between one side of the flake versus the other,” explains Olsson. It helps to identify in-mould labelling that can be a contaminant. “Features like this cater the machine especially well to the polyolefin recycling supply chain, which is in dire need of quality gap closure,” he adds. Its Flying Beam lighting system is claimed to offer fast and reliable material detection by the sorter’s near-infrared (NIR) sensors. Providing a homogeneous light distribution across the entire 2 m machine width, it is claimed to ensure classification of PE and PP materials, as well as other polymers and materials. Integrated into the scanner box for protection against damage, the illumination technology offers up to 80% energy savings over conventional light sensors.

Configuration flexibility allows for single or multiple step sorting with the same unit. “We can split the machine into multiple sorting tasks by chute and even split a chute into two different process streams or more,” explains Olsson. “We can do multiple passes through a machine or link many machines in series to achieve the results we need.” Castro admits the inconsistent feedstock Greenpath receives does present sorting challenges, but the company has been able to overcome these variations through years of experience to provide a good high-end recycled product. Still, the company ended up with a mixed polyolefin material.

Optimising for consistency

What makes Greenpath an asset for its suppliers also makes it a hindrance to polyolefin flake sorting. The inconsistent feed – plus the fact that this is the first operation in North America to attempt this type of mixed polyolefin sorting – has demanded special attention to detail with sorting optimisation. “These machines are themselves optimal in a steady state stream,” says Olsson, “and, thus, have to work together as a holistic process system, along with other types of processing equipment.” Getting it right to meet Greenpath’s high purity goals of mono-polymer PE and PP from inconsistent mixed polyolefin feedstock has both parties analysing the entire sorting process. While there is still some work to be done at the Colton facility, “mechanical viability is there,” confirms Olsson.

The system is now to the point where the circuit can first sort by polymer with the Innosort Flake units and then by colour. “If a customer wants a natural or specific sorted colour such as red, white, blue orange, or green PE or PP product, we can purify and provide them a sorted colour that meets their specific application using PCR,” says Castro.