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John Bambara, president and CEO of Sentinel Products Corp. in Hyannis, Mass., and holder of over a hundred plastics patents, is launching yet another proprietary technology. Bambara, whose previous patents include silane-crosslinked polyolefin foams, is now commercializing what is believed to be the first coextruded PP foam sheet with a low-density core and solid skins.
The sheet is made with an annular die and physical blowing agents. That combination, he says, creates a very low-density core but the overall sheet is “so stiff it’s like steel.” A 5-mm sheet with 5-pcf foam constituting 30% of its thickness has a flexural modulus of more than 8000 psi. Sentinel has made three-layer ABA and ABC structures and five-layer ABCDE structures. Samples were displayed for the first time at two shows this spring, one being NPE in Chicago last month.
The PP foam layer is made with a modified twin-screw extruder and is coextruded with unfoamed layers from up to four single-screw satellite extruders. Bambara has achieved foam-layer densities as low as 4 pcf with the twin-screw alone. Now, he’s trying to get that down to 2 or 3 pcf by using a tandem-extruder arrangement with a water-cooled, 34:1 L/D single-screw extruder downstream of the twin-screw. The foam is very lightly crosslinked to provide extra melt strength while retaining recyclability.
Sentinel’s foams are blown with a cocktail of physical blowing agents—mostly CO2 plus a blend of butane, nitrogen, and one proprietary gas. As far as Bambara knows, the only other PP foams made in the U.S., Europe, or Japan are monolayer and use chemical blowing agents. The difference, he says, is that “chemically blown foams are softer and the cells tend to collapse when thermoformed. Ours don’t collapse.”
Bambara is offering to license his new PP foam technology for non-automotive applications such as packaging. Sentinel is collaborating with Davis-Standard Corp. of Pawcatuck, Conn., to supply a package of equipment and know-how. Possible applications include thermoformed microwavable food trays that would stay rigid even in contact with boiling water. Another possibility is trays for modified-atmosphere packaging (MAP), in which a barrier film could be laminated to the foam.
For thermoforming, coex foam has several advantages over unfoamed PP. It doesn’t sag in the oven; cycle time is up to 50% faster; and trays trim more easily, Sentinel says.
For automotive applications, Sentinel is making products and marketing the technology itself. Discussions are under way for nine automotive programs, and Sentinel is sampling 1.5- to 6-mm-thick coex PP foam sheet to Tier 1 and 2 automotive suppliers for applications like headliners, door shields, and air ducts. Monolayer crosslinked-PP foams are already used for headliners, but they’re so soft that they must be laminated on both sides with a fiberglass mat for adequate stiffness. Sentinel’s 5-mm, 6-pcf coex foam sheet is said to be stiffer on its own than monolayer crosslinked foams with two fiberglass laminated layers.
Bambara’s creative cycle runs a typical course: First, he gets an idea, then he files for a patent, develops the process, commercializes it, licenses it, sells it, and then moves on to the next new idea. Over the years, his firm has manufactured products as diverse as blister packaging and floppy disks, and has spun off many licenses and eight joint ventures worldwide.
Now, at 71, Bambara doesn’t look or feel his age, but he has made one change: “Now I’m focused,” he says. “I’m staying with PP foam.”