The combination of lightweighting, recyclability and post-consumer resin (PCR) will be three key areas of focus as industry stakeholders continue to work on making caps and cap resins more sustainable.
01. See Lightweight
Over the past two decades, the weight of beverage packaging has been decreasing. Advances in part design, processing and materials have allowed the industry to develop bottle and closure solutions that have a lower environmental impact, as fewer raw materials need to be produced, processed and shipped.
An update to the design standard for neck finishes, namely the application area of the cap, supports the trend towards lightweighting. For example, transitioning from a PCO 1810 finish for carbonated soft drinks to the current standard PCO 1881 design resulted in a 25-35% reduction in cap weight, with a similar reduction in the finish itself. The adoption of the latest neck finish standard (26/22) is expected to mark another major change in the next few years.
In applications such as lightweight water bottles, some believe that the opportunity to reduce weight has stalled. Therefore, increasing production capacity has greater significance. As the performance and dimensional tolerances of these lightweight closures get tighter, the specific grades of polyethylene (PE) resins that were common only a decade ago are often no longer able to meet the processing and functional requirements in new designs.
To address these limitations of commodity resins, material suppliers have developed high-performance PE resins with enhanced mechanical properties that can further reduce the weight of caps and caps.
These properties stem from advances in resin design, such as bimodal molecular weight distribution and the use of octene comonomers, which offer unique advantages over traditional PE resins.
High-performance resins can increase stress crack resistance required for thin-walled parts, improve creep resistance and barrier properties required for carbonated or metered beverages, and provide higher flow for larger, high-cavity molds that make more parts at once . The industry will continue to move forward with thinner, lighter caps, using modern high-performance PE resins to ensure performance.
02. Design For Recyclability
As circularity continues to gain momentum, design for recyclability has become an area of greater focus across the value chain than ever before. Caps that were once composed of two or more materials to meet requirements such as tamper resistance or sealing properties now have fully recyclable single-material solutions.
While cost and production efficiency remain important factors, new recycling guidelines and regulations will shape modern caps and increase interest in elements such as binding and recyclable barrier resins.
In the past, a two-piece cover consisting of a polypropylene (PP) shell with a low density PE (LDPE) or alternative liner to ensure adequate sealing performance was standard. Two-piece closures are still common in some regions of North America and in some beverage applications, but in most regions, two-piece closures are being phased out in favor of sensory, unlined, high-density PE (HPDE ) bottle cap. These one-piece caps not only simplify the supply chain, reduce neck deformation and improve sealing, but also facilitate the adoption of single-material cap flows.
Another area of consideration for closure designs for recyclability is the flexible food packaging market. All PE stand-up pouches (SUPs) are designed to replace traditional SUPs that use hybrid materials to help meet performance requirements.
With the development of advanced PE resins with significantly improved properties such as barrier and sealing properties, more and more applications can be replaced with PE-based film structures to create fully recyclable stand-up pouches. Some of these bags also contain nozzles and accessories made of PP or PE, especially for products such as baby food, condiments and sauces. In order to make the entire package recyclable, it is expected that the use of full PE bags, fittings and closures will become more common.
Tether is an emerging option in North America that helps ensure caps are collected with the container while helping to reduce litter. In 2024, as part of the Single-Use Plastics Directive, the European Union (EU) will require plastic closures for all containers under 3 liters to remain attached/tethered. This legislation has caused significant disruption to the beverage industry, requiring new and innovative designs.
Following the same reasoning as the EU, similar legislation could be introduced in North America and elsewhere. Tethering also offers an opportunity to differentiate products and may become popular in North America as multinational companies seek to integrate global closed designs. Co-development between resin suppliers and closure designers will facilitate these new designs and unlock the potential of materials with higher tensile strength and flow properties for tape and hinged tether closures.
03. Binding PCR
Processors have successfully incorporated post-industrial resins (PIR), also known as scrap/regrind materials, into molded parts in certain industries for many years. Incorporating PCR goes a step further by using content previously used by consumers or in industrial end-use applications.
Many factors must be considered to effectively incorporate PCR into seals for food contact and non-food contact applications. Going forward, these factors will play a greater role in determining how much PCR is feasible to mix with virgin resin, as organizations aim to maximize the amount of recycled content in caps and caps.
Arguably the most difficult aspect of incorporating PCR into caps today is the limited availability of suitable PCR supply streams, especially for food and beverage packaging. For PE, recycled HDPE (rHDPE) from blown milk and jugs is currently the only mainstream material approved for food contact applications.
One of the challenges with using resins originally designed for blow molding is that their properties are not optimized for injection molding (and compression molding, to a lesser extent). These production methods are quite different, and the high viscosity of blow molding resins presents challenges during processing on a production line designed to use much lower viscosity resins commonly used in caps and closures. Other supply considerations include quality, consistency and reliability.
At a high level, the main technical factors to consider in developing caps with PCR content are materials, processing, application requirements and part design. PCR properties differ from those of the original resin. Material properties, such as mechanical properties, rheology, color and optics, and sensory properties, determine whether a material is suitable for a given application.
At the same time, molders must determine whether parts can be produced without major changes to existing molding equipment and throughput, or whether additional steps related to storage, handling and mixing are required.
In most cases, the material specifications for PCR are very broad. When combined with the inherent property differences between the PCRs available today and the original cap and closure resins, what emerges is an altered process window with additional variability in size and part performance. If molders do not fully understand and address these differences during cap production, further challenges can arise downstream, bottling and end-use.
To address this challenge, the design of the tip should be evaluated in relation to the application requirements. From a sustainability perspective, it is important to weigh the benefits of lightweighting combined with the benefits of PCR to understand which approach is more feasible and which offers greater benefits relative to environmental impact.
"PCR-ready" virgin resins tailored for PCR may become more common as more efforts are made to introduce new recycled PE supply streams. In many cases, the same developments in cap resins that promote lightweighting also offer improved mechanical and rheological properties, enabling processors and brand owners to use more recycled content in caps and food packaging.
These sustainability-driven innovations in closures and cap resins are expected to continue over the next 10 years as demand from lawmakers and consumers drives new business solutions. Collaboration between supply chain actors (fabricators, toolmakers, brand owners and material suppliers) will advance this work year by year.
It's impossible to say exactly how different the closures will be by the end of the century, but they will certainly incorporate today's nascent sustainability innovations.
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