Blowing PP opens new markets at low cost for film processors

Until now, polypropylene has not had much of a career being processed on traditional lines designed to blow polyethylene (PE) because of PP''s poor melt strength, long cooling period, and poor optics. Those times are coming to a end.

This allows PE processors to differentiate their businesses and break into niche film applications without substantial new equipment investments.

The change occurs as new grades come onto the market allowing all-PP coextruded structures and equipment tweaking to improve output and film clarity. "Until recently it hasn''t been possible to produce PP on conventional PE blown film lines. Blown PP allows serious down-gauging so that end products (IV bags, retortable pouches, stretch or non-fusion shrink hoods, and heavy-duty sacks) are lighter, much more transparent, and in certain cases, similarly priced [to PE]," says market analyst Patrick Ellis, who is based in Mercurey, France.

Coextruding PP between layers of LLDPE to give a film high gloss, low haze, and sealability, is not new, says Bernd Schroeter, project manager film extrusion lines, Reifenhauser Maschinenfa- brik (Troisdorf, Germany; Ipswich, MA). Nor is blending 20% styrene-ethylene/butylenes-styrene (SEBS) into PP to get improved transparency, melt strength, and bubble stability—but generally this has a downside of increased costs, says Ellis.

The key quality of multilayer blown PP is stiffness, Schroeter reports. Blowing biorients the web, producing clear film because it breaks the large polymer crystals into smaller ones with lower light wavelength interference, says German Laverde, marketing director for Battenfeld Gloucester Engineering, based in Gloucester, MA.

Resin producer Borealis (Kongens Lyngby, Denmark) has just come out with a random/terpolymer, Borclear RB707CF, specifically for conventional blown film lines with air cooling. Anton Wolfsberger, skills center manager-film at Borealis says a 40-µm RB707CF blown film has an MFI of 1.8 g/10 min with gloss of 70% and haze of 4%, better than most standard LDPE/LLDPE blend films of the same thickness (30% gloss and 10% haze). The material''s grease and heat resistance and stiffness reportedly allow it to fulfill hot-fill stand-up pouch and retortable applications as well as protective films for technical parts such as acrylic sheets.

Dow (Midland, MI; Horgen, Switzerland) has come out with its high melt strength grade, Inspire 114, specifically for blown film. Although transparency was not initially the key to this development, Mark Murphy, development manager PP, says the company is working on a polyolefin elastomer for blowing to go commercial in 2005, which will have high clarity and can substitute for polyethylene or EVA sealing layers in coextruded PP films.

"Processors like the idea of being able to expand their market by producing a new product on their existing line," Murphy says. "For example, packagers often have to buy-in cast PP to laminate on reverse printed PET/PE packaging films. Now the same processor can produce the PP on his own [PE] line and cut costs." Murphy and others warn that the learning curve can be steep for PE processors. "The key is to run the material hotter and hotter sooner," he says.

Dow offers processors a bevy of Internet tips for processing blown PP. With its 114 grade Dow recommends screw diameters of 90 mm or 3.5 inches with a 30:1 L/D. Die gap should be 1.8 mm with a blow up ratio 2.25:1. The higher heat (suggested is 215C for the feed zone, 220C for the remaining zones, and 230 to 250C at the die) should eliminate unmelted material, the major problem, he says. With low shear decompression screws designed for LLDPE, grooved barrel extruders could require even higher barrel and feed zone temperatures. He recommends a die gap of 1 to 1.75 mm. "We saw a reduction on output when running PP on conventional blown film line[s]. There is a sacrifice in how many pounds per hour can be pumped. So screws must be optimized," says Battenfeld''s Laverde.

Andreas Goeldel, application development manager PP film Europe for Basell (Hoofddorp, The Netherlands) says high-clarity Catalloy thermoplastics polyolefins grades with Adsyl sealing layers produce good-clarity blown film. Stiffness, grease resistance, and dart impact make them ideal for pet food packaging, he says. PP alone can be too splitty for most applications and therefore blending often gives the properties required. Basell has an experimental grade, Exp11698531 to be marketed next year for skin layers, combined with a random copolymer in the core which provides toughness for cutlery wrap, take-away food bags, and perforated bakery wrap.

Ganesh Nagarajn, application development group Basell North America, says the better orientation of blown PP, compared to cast, with PP''s heat resistance, allow hot-fill bagging applications such as salt or polycarbonate pellets. "If you want to bag these in PE, you have to wait until the contents cool down—that takes time and results in extra costs," Nagarajn, says.

Not all the improvements are coming from the resin side. The new resins generally process well on equipment with internal bubble cooling (IBC), high air volume, and chillers, says Mark Miller, R&D technical director, Atofina. (La Porte, TX). Although machinery makers prefer selling processors their latest lines, they admit that retrofitting is possible, and in tough times like these they are not adverse to selling individual parts such as dies or air rings, says Frank Alexy, area sales manager, Kuhne (Sankt Augustin, Germany; Norwich, CT).

Jochen Hennes, technical director at blown film equipment maker Kiefel Extrusion (Worms, Germany; Wrentham, MA) points to improvements of its latest Kirion C line he says make a big difference in blowing PP. "We offer a spiral mandrel with 48 outlets for good outflow that translates into better bubble stability than dies with fewer," Hennes says. "The melt needs to exit the die at the lowest possible temperature to facilitate cooling, which means a low- temperature screw giving good mixing yet very low shear."

PP film output on standard blown lines can be up to 30% less than processing PE, says Dow''s Murphy. Hennes says that by redesigning the Kirion''s air flow channels and air ring geometry to allow a larger air contact surface to the melt as close to the die lips as possible, the line provides 30% improved cooling and better output. Besides an IBC, chilled air, (8 to 15C) gives best results, he says. Borealis'' Wolfsberger sees Kiefel''s A-frame with nylon brushes as providing an optimal means of collapsing the bubble with little contact and pressure to eliminate micro scratches on the film surface. Non-contact turning bars such as Kiefel''s Zero Touch, Reifenhauser''s REIcofly, or Nostic from Windmoller & Holscher (Lengerich, Germany; Lincoln, RI) are also recommended.

Kiefel, which displayed a high-gloss, high-transparency, three-layer 35-µm PP film at NPE 2003 of Borclear RB707CF and skins of RB307MO (a nucleated random copolymer), says winding is equally important in producing highclarity PP web.

"PP has a lot of post shrinkage when cooling since the crystallization changes. Either you wind blown PP very soft to compensate for shrinkage so the PP can change once the roll is stored, or you wind as stiff as possible to prevent any change during the post-shrinkage process," says Hennes. So far he favors the second method which has shown better results.

Robert Colvin [email protected]