Process Aids Give Pe A High-performance Punch

Polymer processing aids and stabilizer systems for polyethylene resins are enabling this most widely-used polymer to expand from simple commodity applications into higher-performance sectors not previously considered, says Claudia Beyer, director of marketing and applications at Great Lakes Chemicals, Fraunfeld, Switzerland.

The polymer processing aids (PPA) are a boon for production of linear low-density polyethylene (LLDPE) films, particularly from metallocene resins, which can be hard to process, notes Chris Fisher, global business manager for Viton FreeFlow at DuPont Dow Elastomers, Wilmington, DE. Benoît Barrière, development engineer at Atofina, Paris, adds that while PPAs help solve operating problems, one of their main benefits is increasing output to save processors money.

The PPAs migrate to the extrusion die and other metal surfaces to provide a fine coating that serves as a lubricant. It cuts down on die buildup, reduces melt fracture that can degrade the appearance and strength of films, and typically speeds up extrusion lines by two to three times, explains Dave Seiler, business manager of Kynar for Atofina in Philadelphia, PA.

Joris Briers, application and product development engineer for Dyneon, Zwijndrecht, Belgium, says processors, despite pressures to cut costs, would be hard-pressed to try to scrimp on fluorinated ppas if they want to increase output and eliminate problems with gels, black spots, surface defects, and shark-skin in LLDPE blown and cast films. “Although they cost money, ppas can actually save money by allowing a higher blend ratio of cheaper LLDPE with less-easy-to-process LDPE in most film applications,” Briers notes. “Our customers tell us the cost of PPA is less than the savings gained.” Atofina’s Barrière says PPAs generally add _0.01/kg to processing costs.

Formulations extend PPA capabilities

While fluoroelastomer and fluoropolymer PPAs have been around for years, new products promise significantly improved performance.

For instance, last September, DuPont Dow launched Viton FreeFlow Z100 and Z200, which feature so-called rheology-modified fluoroelastomers and are used at lower concentrations than conventional PPAs. The products are made via proprietary technology that provides larger and rounder fluoroelastomer particles. These spread easily to more effectively coat the die surface and stand up to abrasive and aggressive pigments and additives, says Fisher. Initial uses center on blown films, but Fisher notes there is also interest for cast films.

Both products carry the same price. Z100 uses polyethylene glycol as the interfacial agent, but at a lower level than other PPAs. Z200 employs a patent-pending, polycaprolactone-based agent that boasts greater oxidative and thermal stability, gives no olefactory effects in products, and virtually eliminates screw slippage, notes Fisher. Processors can expect over 10% savings in PPA costs and better performance, he adds.

Meanwhile, Dyneon introduced in late February the first two products in a more-robust family of fluoroelastomer PPAs, says Bob Moore, global business manager in Oakdale, MN. The PPAs, whose designations were not available at presstime, are used at lower loadings and promise a 20 to 30% improvement in efficiency and lower usage costs than current materials, he adds.

The improvements stem from the molecular composition of the elastomer, not particle size, explains Steve Amos, group leader for new product development. The fluoroelastomers also are teamed with a synergist of slightly different composition.

Dyneon additionally is working on a synergist that would reduce interactions, such as absorption of elastomer onto inorganic particles, and abrasion of the fluoroelastomer, says Amos. It could minimize the need to change loadings for different resins. Introduction might be later in 2003 or in 2004.

Processors seeking less-expensive PPAs have long turned to fluoropolymers, notes Seiler. Such materials cost as little as half that of fluoroelastomers, but don’t set up as fast, he admits. Atofina developed Kynar Flex 2500, a fluoropolymer that has a melting point about 25°C lower than other Kynars, enabling easier dispersing, says Seiler, who adds that it costs approximately 10 to 15% more. It is being trialed in Europe for LLDPE and HDPE films.

The company also hopes to commercialize, perhaps as soon as 2004, a PPA aimed at processors rather than masterbatchers, the usual buyers of neat material. A modified Kynar Flex formulation, it will tackle problems posed by titanium dioxide and silicas, which scrub the barrel and screw surfaces and thus can negate the effects of PPAs.

Dyneon intends to introduce globally a lower-cost fluoropolymer PPA during the first half of this year, notes Moore. A pure, fully-fluorinated polymer, it is based on a different polymer structure, add Amos.

Alternative PPAs remain in development

Meanwhile, Saint-Gobain Advanced Ceramics, Amherst, NY, is working on a PPA based on boron nitride. Called CarboGlide, the material is specifically targeted at metallocene LLDPE blown film, explains Stephen Lyle, product and market manager. It promises better extrusion rates and gage control and allows friction control. However, commercialization prospects have been complicated, he says, by the launch of new fluoroelastomer PPAs, because they provide nearly as good performance. Research continues on optimizing the CarboGlide formulation.

A PPA being developed for medium-molecular-weight HDPE (mmw-hdpe) blown films, including metallocenes, aims not to improve initial film production, but to make films suitable for machine direction orientation (MDO), which can improve film gloss, clarity, stiffness, and moisture barrier. Equistar Chemical, Houston, TX, is now providing samples of three grades of MMW-HDPE containing a proprietary PPA that overcomes the problems of uneven stretching that have hindered MDO (Oct 02 MP, 42; MPI, 58), says Kelly Williams, market development manager for flexible packaging. It poses no property tradeoffs or compatibility issues with other additives, he claims.

A decision on which PPA-containing MMW-HDPE formulations to market should be made by year-end, notes Williams. The grades will command an as-yet-undecided premium in price. Equistar is talking to suppliers of MDO equipment.

Stabilizer packages also advance

Great Lakes expects to introduce, in the third quarter, a stabilization package, Anox ProcessPlus, designed to prevent color degradation that can occur in chromium catalyst-produced HDPE, such as material manufactured by ChevronPhillips and its licensees. Great Lakes’ Beyer says the “chromium acts like a color chameleon to distort tones of highly color-sensitive applications such as food or cosmetics packaging.”

Also to be commercialized this year is another package to prevent moisture-absorbing additives from clumping in high-humidity regions. This should give additives a longer shelf life and reduce handling problems, such as the need to break up lumps before dosing.

Ciba Specialty Chemicals, Basel, Switzerland, in February launched a new stabilizer package, Irgastab Film NPO, for LLDPE blown film. The solid, non-tris-nonylphenol phosphite (TNPP), which replaces TNPP formulations, Irgafos 168, and high-performance phosphites, not only suits markets that demand environmentally-friendly stabilizers (TNPP is suspected of mimicking estrogen) — such as Scandinavia, Japan, and Germany — but produces less blooming and reduces gels. It is also free of nonylphenol.

Meanwhile, Clariant, Huningue, France, is offering a stabilizer and additives program that analyzes the combined effect that additives will have in a PE application, says Mohamed Sidqi, manager, business line plastics additives. “On average, we are able to help a customer save 15 to 20%, and in one case up to 40%, in costs with this simulation,” he says.

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