Softer Materials Like Polyethylene Terephthalate Can be Harder to Mold

By Bob Hatch

Related links: Polypropylene (PP) | Polyethylene Terephthalate (PET)

The rules change with TPE & PET, but you still have to leave room for the material to flow.

Long considered a secondary material option for injection molded containers, polypropylene (PP) is coming back into favor with designers and engineers. Its rise stems largely from the ever-changing packaging market. Today’s developments result from the drive toward smaller, stronger containers, an increased focus on sustainability, and the need for better performance on hot-fill lines. Now, the same attributes that made PP an option 10 years ago are the ones that all rigid packages need.

Former Landscape

Materials selection for rigid packaging changed dramatically over the past decade. New consumer and retailer expectations about what a container should do (together with enhanced performance features of many packaging materials) have transformed the way rigid packaging is engineered. Since its introduction, PP could be found predominantly in automotive applications and as a low-cost alternative to Polyethylene Terephthalate (PET). Most designers didn’t consider the material’s unique and valuable properties.

Recent developments have expanded PP’s potential for use. Improvements in such factors as clarity, impact resistance, and processability have allowed PP to penetrate typical HDPE (high-density polyethylene) market segments with a key advantage: lower container weights. In addition, PP is a lower-cost option than polystyrene, PET, and HDPE when density and modulus properties are optimized for container applications.

Expectations Evolve

Today, brand owners face a whole new set of consumer and retailer expectations for rigid containers. They must meet industry standards for smaller containers, adhere to retailer sustainability guidelines, and create more effective branded packaging. Finally, this must all be done within a tight budget.

While attention-grabbing containers that visually leap off the shelf are ideal, the basic functions of a package must be met as well. Brand owners need a high-quality container that performs similar to or better than all other containers in the competitive space. Rigid packages should be convenient to handle, transport, and use. At the same time, packagers have an evolving set of needs when it comes to the performance of specific materials. Ideally, they look for a balance of features like clarity, stiffness, temperature performance, and sustainability.

Increased consumer and retailer expectations have created more opportunities for the use of PP. Material suppliers have enhanced the performance of this material, putting the resin on a level plane with other packaging materials. In addition, characteristics that were not valued in the past are now considered advantages.

A Balance of Clarity and Toughness

Consumers demand clean, lightweight, and visually appealing packaging. For packagers, these qualities equate to two important material functions: clarity and toughness. The importance of clarity in packaging is due, in part, to the subconscious assumption that “clean” is associated with clear. In the past, packagers may have believed that PP couldn’t produce clear packaging solutions, but this is no longer the case. PP can be used in a variety of demanding applications that require high clarity, including rigid containers, thanks to recent advances in clarifiers and material formulations. In packaging, performance goes hand-in-hand with visual appeal. The industry and the consumer may judge a product by, among other things, its looks and its ability to withstand wear and tear. Material suppliers have responded by improving stiffness and toughness while maintaining high levels of clarity.

At Dow, the Inspire Performance Polymers line is an example of new-generation PP with combined stiffness, toughness, and clarity. These qualities are important in food packaging applications such as syrup, ketchup, and fruit juice, as they allow for stronger handles and “cleaner-looking” bottles. The toughness features of these materials also offer processors an opportunity to meet club stores’ need for large, bulk-size packaging.

Improved Temperature Performance

For packagers, high temperature performance is another key characteristic of today’s effective packaging. Hot-fill packaging lines continue to be used to control sterilization of many fruit juices, teas, and isotonic (sports performance) drinks. Today’s high-performance PP grades offer high heat deflection temperatures, making them excellent options for hot-fill applications because they do not distort when filled at 210°F, compared to PET, which can soften at approximately 180°F.

Sustainable Benefits

The recent Wal-Mart initiative that requires its suppliers’ packaging to meet targets on an environmental scorecard gives all designers a new goal: Today’s packages must be engineered with sustainability in mind. Besides Wal-Mart, other major retailers have set standards for products sold in their stores, evaluating environmentally friendly qualities and other properties such as product-to-package ratio.

For designers, balancing form and function is a constant battle. Packagers consistently aim to improve sustainable attributes, but remain concerned that final performance may suffer as a result. Many have found that the enhanced properties of current PP grades create opportunities for reduced wall thickness through down-gauging. As a result, brand owners are able to improve their product-to-package ratio and conserve resources.

After a decade of changes, PP is now a more competitive option with a long list of attributes that make it excellent for packaging applications. Enhanced products will continue to provide the industry with a much-needed option for combining stiffness, toughness, and clarity in one material. As expectations for packaging evolve, PP is no longer viewed as a secondary rigid packaging option, but prized for its ability to diversify packaging offerings.

December, 2007 - Reprinted with permission from Injection Molding Magazine. Copyright © Canon Communications LLC.