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In a first for stretch-blow molding, Chicago-based Pechiney Plastic Packaging, Inc. (PPPI) is launching a family of polypropylene barrier food containers that are claimed to be as clear as multi-layer PET bottles. The first of these Gamma Clear PP containers is a wide-mouth, three-layer design in a 26-oz size intended for oxygen-sensitive foods. PPPI is testing these containers with customers and plans to design custom versions in the future.
Until now, high-clarity PP food bottles have been made almost exclusively by extrusion blow molding, which typically produces up to 30% trim scrap and requires tie layers for multi-layer barrier structures. Stretch-blowing, however, eliminates trim scrap and produces a more rigid and precise neck finish. It is also possible—as in this case—to do without tie layers. The only PP bottles previously known to have been processed by stretch-blow molding are monolayer containers like baby bottles and those for water, tea, and soy sauce.
PPPI’s patent-pending technology is a cost-effective alternative to multi-layer PET bottles, says Martin Matushek, global director of marketing and strategy. The three-layer (PP/EVOH/PP) barrier containers are made using modified reheat stretch-blow machines from Sidel, while preforms are molded on coinjection equipment from Husky.
Pros and cons
Industry sources say PPPI’s achievement has been to match PET barrier bottles in clarity, oxygen barrier, and mechanical properties while using a PP structure that could offer improved cost-performance. PET-like clarity is achieved primarily due to orientation of the PP skin layers, Matushek reports. He declines to identify the particular PP random copolymer and clarifier used.
The oxygen transmission rate (OTR) of the containers is 0.003 cc/package/day, Matushek says, making them viable for packaging tomato-based sauces, salsa, pickles, preserved fruit, jams, and jellies. In addition, the bottles withstand hot filling at up to 205 F (96 C), which exceeds the performance of heat-set multi-layer PET bottles. Despite the absence of tie layers between the PP skins and EVOH core, the containers reportedly exhibit outstanding delamination resistance and cold drop-impact performance.
At first glance, PP barrier containers appear to offer major cost advantages over PET. One industry analyst estimates the price of clarified random copolymer to be 41¢/lb vs. 55¢/lb for bottle-grade PET. Furthermore, PP has a 44% density advantage over PET.
On the other hand, industry sources note that PET containers are typically 15% lighter than PP and cycle times for both preforms and bottles are significantly longer for PP. Also, PP could encounter resistance on recyclability because PP bottles would be hard to distinguish and separate from PET versions.
PPPI expects to explore Gamma Clear’s potential in non-carbonated beverages like water and juices, as well as meat and dairy products and personal-care applications. The company also anticipates working with development partners to apply this technology in additional market areas.