CompositesWorld

FEB 2018

CompositesWorld

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FEBRUARY 2018 44 CompositesWorld FEATURE / Thermoplastic Composites high-performance thermoplastics: polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyaryletherketone (PAEK), poly- etherimide (PEI) and polyphenylene sulfide (PPS). Some manu- facturers offer tapes prepregged with commodity thermoplastic resins, such as polyamide (PA), polypropylene (PP) and others, but these are generally considered unsuitable for large aerostructures. For the aerospace industry, the materials that hold the most promise are PEEK and PEKK. "In general, both polymers provide excellent high-temperature performance, good toughness and chemical solvent resistance, along with low moisture absorp- tion," says Mike Buck, product manager, thermoplastics, at prepreg supplier Barrday Corp. (Millbury, MA, US). "PEKK also offers a higher T g for improved temperature resistance and a lower melt temperature for processing." Arnt Offringa, director, R&D;, at aerospace manufacturer GKN Aerospace Fokker (Hoogeveen, e Netherlands), says PEKK's ability to deliver performance on a par with PEEK, but process at a lower melt temperature, makes it the prime candidate for future growth in commercial airframe applications. ermoplastic resins typically are applied to the fiber via solvent-based or water-based powder application. e prepreg method used affects the interface properties with the fiber, and the water-based process creates a smoother surface than the solvent- based process. e fiber volume fraction of most thermoplastic tapes ranges from 40-60%, with aerospace-grade materials in the 50-60% range. "Sixty percent fiber volume is important if mechanical prop- erties are the primary focus and you have a production process with a longer cycle time and plenty of flexibility for pressure and temperature," says Buck. "Higher speed or lower pressure processes, such as AFP/ATL, out-of-autoclave/oven processing, etc., would benefit from higher resin contents." Tapes can be slit to create narrower tapes, or tows. Tapes also can be cut, like thermoset prepregs, to prescribed shapes to create blanks. ermoplastics tapes do not have backing paper like ther- moset tapes do. e use of thermoplastics changes considerably the manufac- turing steps to make finished parts. e most important difference is that a thermoplastic, by nature, is solid at room temperature and must be heated to melt temperature for forming. As noted, PEKK has a higher T g than PEEK (160°C vs. 140°C); PEEK has a higher melt temperature than PEKK (390°C vs. 340°C). Jim Pratte, technical fellow, composite materials research and innovation group at Solvay Composite Materials (Alpharetta, GA, US), says material use depends on the application, noting that PEKK is a co-polymer that can be tailored for different tempera- tures and crystallinity rates, while PEEK has an extensive database and crystallizes faster. Such temperature requirements immediately vault these mate- rials beyond the temperatures required to cure epoxy or any other thermoset material. e most commonly used process to manu- facture parts from thermoplastic tapes today is stampforming, where tapes, cut to a prescribed shape and then stacked, are inserted into a preheating oven to be softened and preconsoli- dated. is stackup is then transferred to a forming press, which usually consists of a matched metal tool that fully consolidates and cools the tapes under high pressure (250-500 psi). "Generally, I have found that processing of thermoplastics often requires a 'backwards' type of thought process vs. processing thermosets," says Buck. "For example, with thermoplastics, you FIG. 2 Viable processes: Stampforming/compression molding Most parts made with thermoplastic tapes are manufactured using stampforming or compression molding, which offers relatively quick cycle times. Further, ther- moplastics can be welded, which eases and speeds assembly. The most commonly used thermoplastics for aerospace applications are PEEK and PEKK. Source | Tri-Mack Plastics FIG. 3 Viable processes: In-situ consolidation One promising process for the use of thermoplastic tapes is in-situ consolidation via automated tape laying (ATL) or automated fiber placement (AFP). In this process, prepregged thermoplastic tapes or tows are heated at the tape head to their melting point (>300°C), placed on the tool, and then immediately consoli- dated by the end-effector. Although an additional high-pressure consolidation step is required at this time to achieve porosity targets, ultimately, this process is expected to provide 100% consolidation in a single step. Source | Automated Dynamics

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