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additives feature | Polymer foams

Chris R&P Compounding is targeting Vistamaxx PBE at a variety of applications including footwear

foam structure that maintains the barrier, chemical, and temperature resistance of PVDF resins with the light-weighting and cost-saving advantages of foam. Foamed wire and cable jacketing, for example, is not only lighter weight and lower cost than solid jacketing, but has an improved, lower dielectric constant, is more flexible, and is easier to strip, claims Seiler. Other foamed applications, such as foam-core PVDF piping, are in development.

Improving foam properties

ExxonMobil Chemical’s metallocene-catalyzed Vistamaxx propylene- based elastomer (PBE) is finding use in foaming applications. Chris R&P Compounding of Guangdong Province in China recently developed a special formula for foaming with Vistamaxx PBE that uses existing equipment but creates foams with improved properties. Vistamaxx PBE has a semi-crystalline structure of propylene and

ethylene that contributes to making foams softer, with better elongation, more flexibility, and better slip resistance than ethylene vinyl acetate (EVA) and polyolefinic elastomer (POE) alternatives, says Chris R&P. The Vistamaxx PBE-based foams are also more weather resistant than EPDM rubber and lower in cost than polyurethane and rubber-based alterna- tives, adds the company. Chris R&P has used Vistamaxx PBE successfully in a range of

products, including footwear, toys, and sporting goods. ❙

liquid foaming agents, including what it says is the world’s first liquid foaming agent for PVC sheet. The Excelite product creates a finer, more consistent cell structure within the extruded polymer to create a smoother surface finish. This aesthetic quality is important in applications such as digital printing, says Bjoern Klaas, director of new product and technology development at the company. Liquid foaming agents, like liquid colorants, bring improved dispersion, highly accurate dosing, and reduced waste, notes Klaas. Arkema recently introduced a patent-pending CFA in

a masterbatch designed for foaming its polyvinylidene fluoride (PVDF) resins during extrusion (eg wire and cable jacketing, profiles, pipes, film, sheet) or injection moulding. “For years, no foaming agent was available that could make a consistent structure with fully-closed cells in PVDF, and PVDF applications were primarily served with solid products rather than foamed prod- ucts,” notes Dave Seiler, Americas business manager and global advisor for fluoropolymers at Arkema. The new technology, however, yields a closed-cell

40 COMPOUNDING WORLD | January 2013

Nucleating and coupling agents Particles such as calcium carbonate and talc can be used to nucleate bubble formation in chemical foaming applications. Direct gas extrusion systems can use mineral particles as passive nucleators or endothermic CFAs as active nucleating agents. In general, nucleating agents act to reduce cell size and increase cell density, which helps maintain physical properties and thus allow further weight reduction. Smaller cell sizes typically lead to improved surface finish and thermal insulation. In polypropylene (PP) foam, talc acts as both a

reinforcing filler and a cell nucleator. Studies investigat- ing talc as a nucleator of cell formation found that a microcrystalline talc morphology performs better than the more conventional macrocrystalline, lamellar morphology, explains Dr Gilles Meli of Imerys Talc. Meli and his team presented this work at the Society

of Plastics Engineers Foams 2012 conference held in September in Barcelona, Spain. Meli explained that the energy barrier required to initiate heterogeneous nucleation depends on the surface geometry of the nucleating site. Microcrystalline talc nucleates cells more effectively because it is has a higher surface area, which enables it to entrap more gas, thereby generating a higher number of nuclei. PP foam containing micro- crystalline talc displayed cell density more than two times higher than that of PP foam containing lamellar talc, reported Meli. Applied Minerals’ Dragonite halloysite, a natural aluminosilicate clay with a hollow tubular morphology, provides effective reinforcement and nucleation because of its high aspect ratio and surface area. Dragonite’s surface area is 65 m2 to about 10-20 m2 m2

/g for typical talc and approximately 5 /g for calcium carbonate, notes Dr Chris DeArmitt,

the company’s CTO. In a presentation at AMI’s Polymer Foam 2012

conference, DeArmitt showed that using halloysite to nucleate cell formation improved production speeds by 30-40% in a foamed extruded HDPE sheet. In addition, the formulation required 30% less endothermic blowing agent, and it improved mechanical properties and surface finish compared to an existing commercial

/g or higher, compared

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