A Competition Among Plastics
By Aaron Brody
All plastics are not alike. Thousands have been developed but, because of functionality, safety and cost, only about two dozen are used for packaging. A select few of these have entered into vigorous competition in bottle and jar making: high density polyethylene (HDPE) and a form of polyester called PET currently dominate because of their semi-rigid properties and relative ease of fabrication; and a third, polypropylene, is increasingly being considered.
High Density Polyethylene
HDPE has long been the plastic of choice for household chemicals, personal care liquids and fluid milk packaging, though PET, because of its stiffness and transparency, has recently penetrated some of these markets. HDPE is translucent and relatively easy to fabricate and recycle. And while it does not offer as much of an oxygen barrier as PET, it does offer excellent water and water vapor barriers. HDPE bottles can be fabricated in-line and with high output, considerably reducing costs. For years, they have had labels applied on the bottle surface in the mold; though, more recently, HDPE, like all plastic bottles, has benefited from the arrival of full-body shrink sleeves.
A modestly expensive material often referred to as “plastic glass,” PET was originally developed for carbonated beverages. But it has since penetrated other markets because it is a tough material with oxygen and water vapor barrier.
PET is not easy to fabricate, but modern technology has overcome this issue in what has become a two-stage manufacturing process: injection molding plus blowing. But as bottle makers have climbed the PET bottle-fabrication learning curve, in-line and on-site PET bottle making has emerged. The unit volumes and the cost of shipping large quantities of empty bottles have also shifted PET bottle making from the converter to the packager. Today, injection molders largely make the monolayer preforms and then ship them to packagers who blow mold them into bottles and convey them to adjacent packaging lines.
Polypropylene is a heat-resistant plastic that is not yet in widespread use – it is more difficult to fabricate than HDPE. The material is typically employed for hot-fill products like syrup and for products that require high oxygen barrier, and it can be applied to high-barrier plastics to form bottles that meet the special shelf life needs of products such as the new Wolfgang Puck and Hillside self-heating coffee. Polypropylene sits between PET and HDPE it terms of cost. And while it offers excellent water vapor and water barrier, it exhibits poor oxygen barrier. Polypropylene also suffers from transparency issues, which are being resolved with polymer additives.
None of these materials is without its challenges but, fortunately, the issues they present can, in part, be resolved by enhancing structure—by co-extruding, coating, blending or heat setting—and also by improving fabrication technologies. For example, marrying two or more plastics into a multilayer structure like a ketchup bottle can improve oxygen barrier. Coating PET with glass can add to the gas barrier. And, a more recent innovation allows for embedding PET with active components such as oxygen scavengers, which enhances the material for such applications as beer. BP
The author, Aaron L. Brody, Ph.D., is President/CEO of Packaging/Brody Inc., a consultancy in food, packaging technology and marketing. Contact Dr. Brody at 770.613.0991 or email@example.com