Two-stage stretch blowmolding of polypropylene (PP) has long been touted as the next big thing for packaging blowmolders. PP recently has been coextrusion blowmolded with barrier materials for a number of applications where it replaced PET, glass, or metal (June 2004 MP/MPI). But for the very high-volume, two-stage stretch blowmolding (SBM) applications now almost exclusively in PET, the promise of PP''s lower cost has been offset by much slower cycle times and the need to modify existing PET equipment to process it. Now, say some material suppliers, that problem may be solved.
Michael Rousset, marketing manager at additives supplier Milliken (Spartanburg, SC; Gent, Belgium), says slow processing is a result of PP''s higher thermal capacity compared to PET, necessitating more time to get heat into the material. But he says advances in the firm''s Hyperform nucleating agents have made PP a viable competitor.
Dialing it in
Tests run on a Sidel SBM machine-the most common in the industry-attained outputs of 1500 bottles/hr per cavity, comparable to PET rates. Bottle clarity rivals that of PET bottles except in the injection molded neck, which is still whitish in color. Preforms can be molded at rates similar to PET ones, he says, and he anticipates no problems filling large-cavity preform molds should such high-volume applications appear.
Limitations still exist, and until now only simple bottle designs have been proven. "We''ve not been able to achieve [these outputs] with flat oval designs yet," admits Rousset. Milliken expects to have samples of the first commercial stretch blowmolded PP bottles made using its nucleator for this month''s K show.
Commercial since early this year, but first publicized during a press event in mid-June, is a stretch blowmolded bottle for denatured alcohol and other solvents made using the new RF926MO PP grade from polyolefins supplier Borealis (Lyngby, Denmark). Solvents supplier GSG (Gussago, Italy) captively processes the bottles using a preform designed and made by Otto Hofstetter AG (Uznach, Switzerland) and an SBM unit from Smiform (San Giovanni Bianco, Italy). The bottles can be 30% lighter than the HDPE ones they replaced, without sacrificing mechanical properties.
Other potential end users and their bottle suppliers are now testing the material, says André van Uffelt, Borealis VP for molding. At the firm''s press event he showed sample bottles with necks that were nearly transparent. He notes, "It''s still slightly translucent, but we''ve realized a big step change." Uffelt says the Borealis material contains a nucleation package but didn''t name the supplier.
Because of PET''s better barrier properties, it is unlikely PP will replace it in applications such as carbonated water or soft drink packaging. But Rousset and Uffelt believe PP can capture some of the dish detergent or household cleanser packaging markets, ones that have been transitioning from extrusion-blow high-density polyethylene to PET. PP also makes sense for hot-filled applications as its heat stability far exceeds that of PET.
Still, not all are convinced that PP''s day has come. At Husky Injection Molding Systems, the world''s largest manufacturer of PET preform molds, Dave Cook, president of the firm''s European operations, says, "In spite of all the interest in PP, it has not really made any inroads into the market. The molding of the preforms is not difficult but the stretch blowmolding is."
It''s just possible, though, that developments such as those from Milliken and Borealis are about to force a new reckoning of PP in the market.
Matthew Defosse email@example.com