Closing the loop on bottle caps

Greenpath and TOMRA pioneer mechanical sorting solution for separating polyolefins

“Developing a mechanical sorting process to close the loop for food-grade cap-to-cap recycling is groundbreaking. It’s a game changer to create a system to consistently separate mixed polyolefins by polymer and color with high purity,” comments Joe Castro, president of Greenpath Enterprises, Colton, California/USA.

The full-service recycler, processor and manufacturer has partnered with TOMRA Recycling Sorting to install and optimize 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. “We are at the genesis of creating a sorting system for caps similar to what is more common today for the bottle,” adds Eric Olsson, area segment manager, plastics for TOMRA Recycling Sorting.

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 Castro 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. “Polypropylene and polyethylene (PE) are together the most abundant plastic family on earth, but the market is limited for valuable applications for mixed polyolefins,” comments Olsson.

Unfortunately, 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,” informs 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 molding 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 Olsson.

For the cap to get to a recycler like Greenpath, the bottle is the carrier through the 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 realize 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,” comments 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 Discussions related to Greenpath’s goals led down a path to selecting TOMRA’s INNOSORT™ FLAKE sorter. One appealing feature for the company, the sorter does not require a large footprint, allowing for easier 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 unique combination of sensor technologies, including color cameras capable of identifying 16.8 million color 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-mold labeling 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 offers 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 ensures 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. “TOMRA’s technology is allowing us to take the material to another level, to purify at a higher level that opens up the whole circular opportunity with going cap to cap,” he says.


Optimizing 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 optimization. “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.”

Both companies have been committed to getting the process right. “We shopped around for a company who would support us on this journey before selecting TOMRA. A manufacturer could just say ‘you know the equipment and what it does, so go figure it out yourself,” says Castro. “However, TOMRA has been very supportive in this project, and it has been more like a partnership and understanding that extra support would be needed to get the process right.”   

Getting it right to meet Greenpath’s high purity goals of mono-polymer PE and PP from inconsistent mixed polyolefin feedstock has both parties analyzing 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 color. “If a customer wants a natural or specific sorted color such as red, white, blue orange, or green PE or PP product, we can purify and provide them a sorted color that meets their specific application using PCR,” says Castro.

TOMRA’s Olsson shows optimism for what has been accomplished with Greenpath, its impact on the mixed polyolefin market and what it means for packaging companies trying to meet recycled content targets. “We are just now starting to act on this opportunity,” he offers. “We’re at the beginning for mixed waste streams of polyolefins going closed loop. It won’t just be a pipe dream for polyolefins anymore.”


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