New system coextrudes foam PP barrier sheet.

* It's believed to be a world first: commercially available machinery for coextruding six-layer packaging sheet of polypropylene foam with barrier, tie, and seal layers. The new system from Reifenhauser GmbH in Germany (U.S. office is in Lawrence, Mass.) uses butane or pentane gas to foam PP to a density of only 0.2 g/cc. Overall sheet density is only 0.3 g/cc. According to Reifenhauser sources, conventional approaches use chemical blowing agents, which get the PP density down only to 0.5 g/cc. Then a multi-layer film is laminated on top. Overall density is 0.6-0.7 g/cc.

Reifenhauser is applying for patents on the coex foam process and product. The new technology first went into production last June on a relatively small line at Silver Plastics, a large producer of foamed polystyrene packaging in Troisdorf, Germany. The line is making test-market quantities of barrier packaging.

Twin-screw foam extrusion

The basic structure is foamed PP/solid PP/tie layer/EVOH (or nylon) barrier/tie/LLDPE (or LDPE) seal layer. PP foam is extruded on an 85-mm Reitruder corotating twin screw with gas injected a third of the way down the barrel. This is itself an unusual (and patented) approach, since foam sheet is typically made with two single-screw cascade extruders. The line also has two 65-mm single screws for barrier and seal layers and two 35-mm satellite extruders for tie layers. The six-layer Reicell HDS die can be built in up to 280 mm diam.

The line forms a horizontal bubble, similar to PS foam sheet. It has a layflat width of 1.5 meters and is slit into two 750-mm webs. Output is between 550 and 770 lb/hr, depending on thickness, which ranges from 0.8 to 3.5 mm. Footprint of the system, complete with winders, is 14 meters long by 2.5 meters wide.

Reifenhauser says use of a physical rather than chemical blowing agent requires a PP homopolymer with higher melt strength. Reifenhauser solved the problem by blending standard homopolymer with 4050% of Pro-fax PF-814, a partially cross-linked homopolymer designed for foaming and made in Canada by Montell Polyolefins, Wilmington, Del. PP of similar melt strength is reportedly made by PCD-Borealis in Germany and by two Japanese suppliers, but none of these resins are sold in the U.S.

Reifenhauser's next step will be direct foaming with C[O.sub.2] rather than hydrocarbons. "We expect to be foaming PP (and also polystyrene) with C[O.sub.2] early next year," says Chris Nicholls, area sales manager for North America. "We've already run successful lab trials on PP with 100% C[O.sub.2]."

Other PP foam activity

Reifenhauser worked on its coex gas-foamed process for over five years. Two years ago, Reifenhauser supplied to Silver Plastics a two-layer coex sheet line for direct-gassed PP foam with an EVA moisture barrier. It makes sheet for institutional food-service plates [ILLUSTRATION FOR PHOTO OMITTED].

Almost all other multi-layer PP foam sheet is laminated, not coextruded. However, two other PP-foam coex packaging lines have been running for over two years one for up to six layers at TiroPak Convenience Foods in Romont, Switzerland; and a five-layer line at Polyraz Ltd. in Israel. Polyraz's sister company, Advanced Packaging Technologies (APT) LLC in Edgerton, Wis., is just starting up a similar line in Paragold, Ark., believed to be the first coex PP foam in the U.S.