Every few years, there have been excited predictions that oriented polypropylene (OPP) bottles would make a breakthrough in replacing PET and glass in a broad range of food and non-food containers. Today, OPP is re-emerging as a contender due to its high-heat capability, light weight, improved clarity, and cost competitiveness. The primary focus this time is hot-fill applications, especially barrier containers with multilayer structures or coatings. Higher machinery outputs, optimized preform design, and improved PP resins and clarifier additives give OPP its best shot ever at replacing glass in hot-fill barrier packaging for jams, jellies, salsas, pickles, and tomato-based products.
Meanwhile, PET heat-setting technology has not stood still. New “panel-less” bottle designs reduce weight and broaden design freedom. New tooling developments and machine enhancements have raised outputs to unprecedented levels.
These developments come at a time when the booming hot-fill packaging sector appears to be cooling somewhat. The hot-fill juice market is said to be flat, while sports drinks and isotonics show moderate growth. The new hot markets are ready-to-drink teas and enhanced waters.
New momentum for OPP
Despite the renewed attention to OPP, heat-set PET is firmly entrenched as the dominant factor in plastic hot-fill packaging. Multilayer PET containers using MSD6 nylon barrier and oxygen scavengers are used in bottles for hot-filled tea and natural fruit juices. PET boasts glass-like clarity and better inherent barrier properties than OPP. New technology has been developed to eliminate vacuum panels that were previously required to prevent PET bottle collapse when hot-filled beyond 180 F. However, PET’s limitations in heat resistance and production cost make it vulnerable to OPP.
OPP containers made on reheat stretch-blow machines can withstand hot filling at up to 205 F. PET bottles can be hot-filled only to 130 F to 150 F without heat setting, a process which heats the preform to just under its crystallization temperature, thus enhancing the bottle’s thermal stability and barrier properties. OPP bottles don’t require heat-setting and can be used in many hot-filled applications.
OPP’s future will be dictated largely by cost. Historically, a 10¢ to 15¢/lb advantage made PP an attractive option to PET, especially given PP’s 40% lower density. However, the price differential has now narrowed to where both materials are equal in cost/lb basis, which still leaves PP with a density advantage. Observers see the price pendulum shifting farther in favor of PP over the next year as PET market demand catches up with recent increases in supply capacity.
Industry experts point to shifting consumer demand from carbonated soft drinks to “healthier” drinks like juices, teas, and enhanced waters that are hot-filled. “With growing demand for fresher, less processed, and more natural products, OPP is attractive relative to PET and glass because it can be hot-filled and offers lighter weight,” explains Surendra Agarawal, senior technology consultant at Kraft Foods, Glenview, Ill.
Barrier bottles emerge
Perhaps the key limiting factor for OPP is gas barrier. With 30 times the oxygen and CO2 permeation of PET, OPP is precluded from use in some containers unless coated or coinjected with a barrier resin. “If a cost-effective barrier can be identified, it’s a no-brainer for OPP in hot-fill,” asserts Shawn Sheppard, product development manager for Milliken Chemical, which makes additives for PP and PET. Indeed, the temperature limitation for heat-set PET is about 185 F.
Thus it’s no surprise that much of the current development work centers on multilayer OPP containers. A leader in this area is Ball Corp., Broomfield, Colo., which acquired innovative multilayer OPP technology from Alcan Packaging in 2006. Gamma-Clear wide-mouth OPP containers can be hot-filled at up to 200 F and are aimed at oxygen-sensitive foods.
The three-layer (PP/EVOH/PP) barrier containers are made on modified SBO reheat stretch-blow machines from Sidel from preforms coinjected on equipment from Kortec and Husky Injection Molding Systems. A key ingredient of these containers is a maleic anhydride additive for the PP. This special compatibilizer is claimed to be a first in eliminating PP delamination, which was previously a major obstacle to such multilayer structures. Ball has introduced Gamma-Clear OPP bottles as a glass replacement in two commercial soup and tomato-sauce containers in North America. The 26-oz bottles have a 63-mm neck finish and weigh 52 g. Preform design is critical in attaining optimum clarity, according to Mike Vaughn, Ball’s v.p. of packaging innovation. Ball is working on other bottle sizes and higher production rates.
At the recent K 2007 show in Dusseldorf, Kortec Inc., a supplier of turnkey coinjection systems, announced a licensing agreement to develop and sell the Gamma-Clear technology outside North America. Kortec will be the machinery, material, and technology provider for Gamma-Clear multilayer OPP systems. Husky is expected to be a supplier of the coinjection equipment.
Kortec expects to produce high-output Gamma-Clear systems with up to 48 cavities for 26-oz containers. Wide-neck finishes range from 63 to 85 mm for tomato sauce. Systems with up to 144 cavities can be produced for narrow-neck (28- to 40-mm) containers.
Ball has also developed Gamma PP bottles that withstand retort sterilization temperatures up to 250 F. These six-layer extrusion blow molded containers have an EVOH barrier between tie layers. A patented sealing surface is said to remain uniform and smooth at high temperatures and to permit no barrier distortion. The first commercial application is an 8-oz nutraceutical drink.
Coatings show promise
Coatings are another way to improve the barrier performance of OPP containers. Container Corporation of Canada, Richmond Hill, Ont., partnered with the Council for Scientific Innovation Research in South Africa to develop a dip/flow/spray-coating process that raises oxygen barrier 45-fold compared with untreated PET. The FDA-compliant coating, which is said to be recyclable, is applied on the outside of the bottle and also requires a topcoat in some instances, says Norman Gottlieb, CCC president. The company developed a machine that can treat up to 1000 bottles/hr and is working to reach a rate of 15,000/hr. Several application development projects are under way. CCC also says its coating process improves the oxygen barrier of heat-set PET bottles 45-fold.
CCC claims to be the only North American manufacturer of two-stage reheat OPP bottles, both barrier and non-barrier. Its coating has been used in test quantities to make 500-cc hot-fill containers for juices and tomato-based products. They are made on CCC’s EnviroClear stretch-blow machines, built by a partner in China and available to other molders. These bottles boast high clarity, 30% lighter weight, and lower cost than heat-set PET.
Meanwhile, Dow Plastics is investigating the use of plasma barrier coatings like the Plasmax silicon oxide coating system from SIG Plasmax GmbH in Germany. Dow has two experimental PP stretch-blow molding grades based on its Inspire line, which are characterized by high toughness and clarity. Initial tests show that plasma treatment can reduce the oxygen transmission rate (OTR) for 500-cc hot-fill bottles from 0.8 cc/pkg/day to 0.02 cc/pkg/day. A 50-angstrom (5-nanometer) thick coating is applied inside the bottle using conventional silicon oxide plasma coating technology, according to Terry Glass, application technology leader. Dow believes barrier coatings offer better recycling benefits and less flavor/odor scalping than multilayer structures using EVOH.
OPP bottles are also getting a leg up in hot-fill applications from additive advances. At the K 2007 show, Milliken Chemical launched Millad NX8000, its fourth-generation sorbitol acetal clarifier. It reportedly cuts haze by 50% versus its previous product, Millad 3988i, which was considered the industry standard. NX8000 addresses haze issues in the neck and base, reduces cycle time, and enhances process stability, according to Milliken.
The introduction of new clarified PP grades has bolstered OPP’s position in hot-fill bottles. Basell introduced Profax RP448S clarified random copolymer, a 40-MFR reactor grade that contains no peroxide and is based on new catalyst technology. In addition to being optimized for barrier coating, the new grade allows faster preform molding at a lower melt temperature, according to David McKeeman, new-business development manager. It is being used by CCC in barrier OPP hot-fill applications. The material is also being evaluated in Latin America for single-serve juice containers in both one-step and two-step stretch-blow molding.
In Europe, Basell introduced Stretchene PP resins for stretch-blow molding, including hot-fill uses. They are claimed to offer better stiffness, toughness, clarity, and productivity than previous PPs. In particular, Stretchene RP1685 has an HDT of 109 C and very high topload performance.
Total Petrochemicals USA plans to commercialize in six months a new 15-MFR clarified random copolymer. In stretch-blown hot-fill containers, it will offer improved processability, better drop impact, and greater production efficiencies than other random copolymers. A 23-g preform has been molded for a 500-cc bottle with output up to 1500 bottles/hr/cavity. Total is working with several suppliers on barrier coatings.
In heat-set PET, most of the major bottle manufacturers like Amcor PET Packaging, Graham Packaging Co., Ball, and Constar International have developed different versions of panel-less technology. Previously, the conventional way to design a hot-fill PET bottle was to build in vacuum panels to absorb distortion as a hot-filled beverage cools to room temperature. Panel-less bottles eliminate “label crinkling” and provide more aesthetic and design options for brand owners.
Amcor in Ann Arbor, Mich., is working to use its Powerflex panel-less technology in a hybrid combination with paneled designs, according to Len Zabinko, v.p. of custom beverage marketing. The goal is to control material distribution and incorporate subtle shapes and vacuum panels into the bottle design using specially developed control systems. These proprietary designs have been presented to several brand owners, according to Zabinko.
Amcor also recently launched the first non-beverage application for its Powerflex bottle. Claude’s Sauces Inc., El Paso, Texas, introduced eight varieties of sauces and marinades in Amcor’s 16-oz long-neck Powerflex bottle. Amcor also recently signed up its first national brand and introduced its first custom Powerflex bottle, a 14-oz juice container for Welch’s.
Meanwhile, Graham Packaging in York, Pa., said it will launch the second generation of its Active Transverse Panel (ATP) panel-less bottle early next year. It will be lighter, more flexible, and have broader appeal to fillers, according to Mark Leiden, v.p. of global marketing. A key feature is an inward base instead of the former outward base, which eliminates the need for a cup, or “puck,” system that transfers bottles through the filling line.
Graham also says it is finding success with a proprietary stretch-blow technology that cuts the weight of hot-fill PET containers without design changes or loss of performance. Few details have been released but the patent-pending “super lightweight” process features a special mold and preform design and precise parison distribution to cut weight by 7% to 12% for bottles from 8 oz to 1 gal.
Another panel-less approach, called vertical compensation technology, has been developed by Philadelphia-based Constar. The design for a 10-oz bottle incorporates horizontal rings that allow the bottle to shrink about 0.090-in. to accommodate the vacuum as it cools. Lightweighting and excellent topload strength are claimed. Constar will introduce a 10-oz stock bottle for hot-fill juices in the first quarter of 2008. A range of sizes from 6.75 to 12 oz is planned.
The first aluminum mold for heat-set PET bottles was developed jointly by Sidel and Graham as an alternative to stainless steel. With one-third the weight of stainless steel, the new F300 mold boosts output and reduces changeover time. A new aluminum alloy containing silver is more heat resistant than standard aluminum and withstands up to 180 C (356 F).
Suppliers of stretch-blow machines have increased speeds and boosted output for hot-fill bottles to a level that nearly matches those for cold-fill PET. For example, Sidel has introduced a hot-fill version of its Universal SBO rotary reheat unit that produces 1800 bottles/cavity/hr, a 20% improvement over previous models. Key features include a specially adapted preform feeder for larger neck finishes, heated molds, and 20% more oven banks. Bottle cost is about 30% lower than the previous model, according to Dominique Martin, marketing v.p. and product manager.
SIPA North America has adapted its SFR rotary reheat line for hot-fill production. It boasts an output of 1500 bottles/cavity/hr and features an electric-heated mold and an oven configuration situated closer to the blowing wheel. SIPA also introduced a heat-set version of its linear reheat blow molder. The six-cavity SFL unit makes 8000 bottles/hr.
SIG Corpoplast reports significant lightweighting of hot-fill PET bottles with two-stage reheat machines that stretch with a unique, fully mechanical device instead of pneumatic cylinders. It uses a double-guided cam to reduce wall-thickness variability and control shrinkage. As a result, thinner walls and higher speeds reportedly can be achieved. For example, one customer reduced the weight of a 0.5L hot-fill PET container from 36 g to 29 g.
Machine makers are also seeking to make the PET heat-setting process more energy efficient. Krones Inc. has introduced the Air Wizard H, an air recycling system that reportedly cuts air consumption 52% to 60%. Exhaust air is captured and is converted into low-pressure air for the plant or routed back to the compressor. Sidel also has an air-recovery system that provides 25% savings vs. previous heat-set machines. (See also Learn More.)
To boost heat-setting output, Nissei ASB Co. has streamlined its linear reheat machine by eliminating a cold blow mold. The HSB double-blow machine reportedly produces bottles with better hot-fill performance at up to 205 F than comparable heat-set machines. The 10-cavity unit makes 7000 bottles/hr with neck finishes up to 38-mm for juices and teas.