Thermoformers of large technical parts seeking to use high-melt-strength poly-propylene (HMS-PP) as a less costly material alternative to acrylonitrile-butadiene-styrene and polyethylene will need to be patient.

Processors avoid using conventional polypropylene for large-part thermoforming because of its low melt strength and extensibility in the melt phase. These problems also have challenged deep-draw thermoforming of small products, such as drinking cups, as PP’s shrinkage is difficult to control between forming and trimming. But they have been largely overcome in packaging in the last three years, through developments in tooling, web cooling, and trimming.

A breakthrough seemed to have occurred last year when Basell, Wilmington, de, began marketing its Entegrity HMS-PP grade for thermoforming of large parts, such as those for niche vehicles and marine applications. The supplier engaged Spartech, St. Louis, mo, as the exclusive sheet processor of Entegrity materials in North America, and sought extruders in other regional markets.

Entegrity was used along with a high-gloss PP capstock for coextruded sheet, and Basell predicted the material would be a game-changer, since the capstock would yield thermoformed parts that did not require painting, giving thermoformers a decided advantage over injection molded and composites parts. However, while thermoforming of disparate substrate and capstock materials — such as acrylic with abs or polyvinylfluoride with tpo — is possible and can realize surface finishes, large-part making generates edge-trim volumes up to 50%. When disparate materials are used, the edge trim is not easily reprocessed, thus pushing material costs to excessive levels.

The Entegrity program appears to have stalled. Spartech officials did not return calls, but the processor is understood to be having some success using the PP capstock with a non-PP substrate.

Meanwhile, other suppliers are unlikely to develop materials for the technical-part market. Vic DiNardo, market manager at Sunoco Chemicals, Pitts-burgh, pa, says it has done investigative work for HMS-PP for heavy-gage parts processing. But thus far, “we’re sitting back and watching...We continue to look at it, but it is low priority. In post-extrusion, what we’ve seen is not economical,” since, DiNardo points out, scrap levels are high.

Scrap levels skyrocket because, even for HMS-PP, the processing window is so narrow, machine operators often have only seconds to decide when the sheet is sufficiently heated for forming. If overheated, the sheet sags and falls onto the forming machine’s heaters; if sheet is underheated, it won’t stretch.

BP Chemicals, Houston, tx, say it has no offerings and does not intend to enter the market. Borealis, Lyngby, Denmark, offers a thermoformable grade of its Daploy HMS-PP, but says it is used only for foam trays.

One supplier considering the market is Dow Plastics. Mark Murphy, European technical manager for polyolefins, in Horgen, Switzerland, says it experienced some success in Europe with its HMS-PP grades, Inspire 112 and the still-developmental D 114.00, in thick-sheet applications.

Dow brought Inspire 112 to market in 2000 for film extruders. Murphy says Dow’s global marketing force is identifying potential thermoforming applications and expects feedback by summer. However, Murphy says most processors are switching to D 114.00. “It’s an improvement on [grade] 112,” he notes, “with lower [gelling], up to 30% increase in extrusion rates compared to standard fractional-mfr (melt-flow-rate) copolymers, and a higher modulus.” Murphy expects broad commercialization of D 114.00 by year end.