CompositesWorld

OCT 2017

CompositesWorld

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OCTOBER 2017 32 CompositesWorld WORK IN PROGRESS positive response from that conference, the company says it is devoting a significant portion of its 2017 R&D; budget to mature the technology, with an emphasis on identifying projects and partners to test soluble and structural cores. To date, Persico's structural composites research has focused on a variety of cores combined with autoclave-cured carbon fiber/ epoxy prepreg skins or braided carbon fabrics infused with epoxy via high-pressure resin transfer molding (HP-RTM). at work might soon pay off: automakers in Italy and the UK are said to be looking for applications for the technology on two platforms. Versatile technology To date, the company says it has successfully molded cores from conventional rotomolding resins: polypropylene (PP), polyamide 6 (PA 6), polyvinyl chloride (PVC), liquid and powdered crosslinked polyethylene (XLPE), linear polyethylene (PE), polyethylene terephthalate (PET), polycarbonate (PC), polystyrene (PS), acry- lonitrile butadiene styrene (ABS) and the weatherable polymer, acrylic styrene acrylonitrile (ASA). For a greener core, they have rotomolded post-industrial recycled PE, bio-based polyolefins and several water-soluble polymers. Persico's most recent and surprising material is polyetheretherketone (PEEK). Although this very stiff, high-perfor- mance aromatic ther- moplastic's melt temperature (343°C) is significantly higher than that of most conventional rotomolding resins, Carrara says Persi- co's ability to rotomold it is a function of the efficiency of the elec- trical heating systems embedded in its SMART tool system, which helps them quickly reach PEEK's higher temperature. He also credits unique vacuum circuits, which are said to greatly enhance rotomolding of PEEK in powder and micro-pellet forms. Work is underway to optimize processing conditions for PEEK in granular form, and future work might evaluate other high-temperature thermoplastics. Depending on selection of skin and core materials, the cores can be designed to permanently bond to skins or detach after demolding. Not surprisingly, non-polar olefins offer poor adhesion without surface preparation. On the other hand, powdered PET provides very good adhesion to carbon composites. Similarly, high- gloss cores offer poorer adhesion than cores with textured surfaces. Interestingly, researchers say they've seen no correlation between the selected process — at least in terms of autoclave cure vs. HP-RTM — and core adhesion. More study is underway in this area. So versatile is the technology that there even is the option, when water-soluble polymers are used, to remove cores from skins by dissolving them in hot water. Early work with ethylene vinyl alcohol (EVOH) was found to take up to 48 hours to fully dissolve the cores. However, more recent work, focused on polyvinyl alcohol-polyvinyl acetate (PVOH-PVA), a food/pharmaceutical-grade packaging polymer (Gohsenol, from Nippon Gohsei, Osaka, Japan) has accel- erated dissolution times. "More and more we are convinced that an effective combination of soluble plastics and an agile production technique represents a competitive advantage over other methods of producing removable cores in complex shapes," explains Gaetano Donizetti, sales manager, Persico Industrial. He contends that rotomolded cores can surpass silicone bladders when part shape reaches a certain complexity. To advance the technology to a pre-commercial phase in 2016, Persico produced a special test tool called a "cuboid" and fitted it to a SMART rotomolding machine. e tool produces rotomolded parts that are 290 mm long by 290 mm wide by 150 mm tall. "We use the cuboid to define the practical limits for our cores, such as maximum and minimum wall thickness, survivable forming pres- sures and how fast we can dissolve them in water," adds Donizetti. He notes that when 0.6-kg cuboid cores rotomolded in PVOH-PVA with 2-mm nominal walls were soaked in 40°C water, the cores completely dissolved in 50 minutes. When soaked in 80°C water, Rotomolded soluable cores more easily removed Persico has explored the use of water-soluble polymers, which can be dissolved away in hot water after the skins are cured. In the two images at right, carbon fiber/epoxy prepreg was wrapped around an EVOH core (near right), the sandwich was autoclave cured, and then the core was soaked away leaving behind a hollow ball (far right). Read this article online | short.compositesworld.com/RotoCores

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