The combination of lightweighting, recyclability and post-consumer resins (PCR) will be three key areas of focus as industry stakeholders continue to work on making caps and cap resins more sustainable.(Cosmetic packaging has also gradually begun to innovate, paying more attention to sustainability: PP Airless Pump Bottles, Plastic Pump, Plastic Bottles, etc.)
In the past two decades, the weight of beverage packaging has shown a trend of getting lower and lower. Advances in part design, processing and materials have enabled the industry to develop bottle and closure solutions that have a lower environmental impact because fewer raw materials need to be produced, processed and transported.
An update to the design criteria for the finish of the neck, i.e. the area of application of the cap, supports the trend towards lightweighting. For example, transitioning from the PCO 1810 finish used for carbonated soft drinks to the now standard PCO 1881 design resulted in a 25-35% reduction in closure weight, with a similar reduction in the finish 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 the opportunity to reduce weight has stalled. Therefore, increasing production capacity has greater significance. As performance and dimensional tolerances for these lightweight closures tighten, 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 closures.
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 add stress-crack resistance needed for thin-walled parts, improve creep and barrier properties needed for carbonated or dosed 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 and lighter closures, using modern high performance PE resins to ensure performance.
Design For Recyclability
As circularity continues to gain momentum, design for recyclability has become an area of greater concern than ever across the value chain. Closures that once consisted of two or more materials for requirements such as tamper resistance or sealing performance now have fully recyclable single-material solutions.
While cost and production efficiency remain important factors, new recycling guidelines and regulations will shape modern closures and increase interest in elements such as binding and recyclable barrier resins.
In the past, two-piece closures consisting of a polypropylene (PP) outer shell with a low-density PE (LDPE) or alternative liner to ensure adequate sealing performance were standard. Two-piece closures are still common in some parts 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 ) caps. These one-piece closures not only simplify the supply chain, reduce neck distortion and improve seals, but also facilitate the adoption of a single-material closure flow.
Another area where recyclable closure designs are considered is the flexible food packaging market. The all-PE stand-up pouch (SUP) is designed to replace traditional SUPs, which use a blend of materials to help meet performance requirements.
With the development of advanced PE resins with significantly improved properties such as barrier and sealability, 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 spouts and fittings made of PP or PE, especially for products such as baby food, dressings and sauces. To make the entire package recyclable, it is expected that the use of all 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 on all containers under 3 liters to remain attached/tethered. This legislation caused major disruption to the beverage industry and required new and innovative designs.
Following the same reasoning as in the EU, similar legislation could be introduced in North America and elsewhere. Tethering also presents an opportunity to differentiate products and may become popular in North America as multinational companies seek to integrate global closure 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.
Combined With PCR
For years, processors have successfully incorporated post-industrial resins (PIR), also known as scrap/regrind materials, into molded parts in certain industries. Take PCR 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 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 a challenge during the processing of the line, which is designed to use much lower viscosity resins, which are commonly used in caps and closures. Other supply considerations include quality, consistency and reliability.
At a high level, the main technical considerations for developing caps with PCR content are materials, processing, application requirements, and part design. The PCR properties differ from those of the original resin. Material properties, such as mechanical properties, rheology, color and optics, and sensory, 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 if additional steps related to storage, handling and compounding are required.
In most cases, the material specification for PCR is very wide. 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 closure production, further challenges may arise downstream, during bottling and end use.
To address this challenge, the design of the tip should be evaluated in conjunction with the application requirements. From a sustainability perspective, it is important to weigh the benefits of lightweighting against the benefits of PCR to see which approach is more viable and which brings greater benefits relative to environmental impact.
“PCR-ready” virgin resins tailored for PCR are likely to 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 facilitate 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 closure resins are expected to continue over the next 10 years as lawmakers and consumer demands drive new commercial solutions. Collaboration between supply chain players (fabricators, toolmakers, brand owners and material suppliers) will advance this effort year by year.
It's impossible to say exactly how different closures will be by the end of the century, but they'll certainly incorporate today's nascent sustainability innovations.
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