CompositesWorld
Issue link: https://cw.epubxp.com/i/930524
FEBRUARY 2018 46 CompositesWorld FEATURE / Thermoplastic Composites performance characteristics similar to epoxy, but they are gener- ally tougher than epoxy. Further, they also do not exotherm, and they resist corrosion, wear and fire very well. ermoplastics also avoid the storage and out-time limits of thermoset prepreg — not a trivial matter considering the expense of investment in freezers, and the task of managing prepreg expi- ration dates to determine which rolls of prepreg to use when. Indeed, the cost of disposing of/recycling expired material can add considerably to the cost of manufacturing parts with ther- moset prepreg. Further, because they do not cure and crosslink, "ermoplastic composites can be remelted/reprocessed," notes Buck. at makes them relatively easy to recycle. "Recycling is a big driver in auto- motive because volumes are so much larger and the economics are so much more challenging," notes Solvay's Pratte. "If you can't recycle materials, it's going to be that much more of a barrier to adoption." Although it's particularly important to automakers, recycling is becoming an increasingly vital consideration in aerocomposites manufac- turing as well, as aircraft OEMs, too, contem- plate lifecycle management (LCM) and product end-of-life issues. ermoplastics also enable part bonding in a way that is not possible with thermosets. ey offer the poten- tial for welding/fusing parts together, which could negate the need for adhesives in some applications." GKN Aerospace Fokker is famous for its use of thermoplastics welding, and Offringa says there is a need to develop a variety of welding techniques, including resistance welding, induction welding and conduction welding (see Learn More). Porcher's Quéfélec points out that AFP/ATL is a form of continuous welding and reports that many of his customers are looking to use welding to join smaller thermoplastic parts into larger structures. As composites use increases in aerospace and automotive, the demand for automation will increase. is represents a real oppor- tunity for thermoplastic tapes. "In my opinion, this is an area where thermoplastics will shine," says Buck. Disadvantages (aka, opportunities) For all their advantages, thermoplastic tapes lack the maturity of thermoset tapes and, therefore, present some challenges. Web Industries (Marlborough, MA, US), which slits and formats ther- moset and thermoplastic tapes, sees these challenges. Grand Hou, director of research and technology at Web, says a thermoplastic resin, because of its toughness, is more difficult to slit and meet tolerance requirements. e material, he says, is also springy, so it requires a different winding pattern with different winding control than those used with thermosets. Jim Powers, business development manager at Web, notes that thermoset tapes are available in widths of up to 60 inches/1,524 mm and can go thousands of feet without a defect, while ther- moplastic tapes typically top out at 12 inches and show as many as 30 defects in just 700 ft. "ermosets went through the same development curve," says Hou. "We are probably closer than we realize [to major quality improvement]. And as soon as there is a large program using thermoplastic composites, then you will see tremendous improvement in quality." e other challenge posed by the resin is its application. ermoset tapes are typically prepregged using resin in film form, which allows prepreggers to apply resin precisely and uniformly, with minimal thickness variation. ermoplas- tics, by contrast, rely on a powder-based application process that is more difficult to control and can create resin-rich and dry areas. Such non-unifor- mity can lead to problematic interply porosity. Web's Powers says solvent-based resins tend to have a rougher surface and generate more gaps, while water-based systems tend to be flatter, with minimal gauge variation and a smoother surface. "Rougher solvent-based materials offer more surface area," he notes, "but from a spooling standpoint, water-based systems run quicker and provide better rolls." Another variable is the fact that at room temperature, ther- moplastic tapes are characterized by an unusual boardiness, which produces a stiff, occasionally uneven tape that is prone to producing gaps and splits. e boardiness also can cause material waviness, which may lead to some tape width inconsistency. is causes subsequent problems during tape slitting, which relies on consistent tape widths to stay within specifications. Further, unlike a thermoset prepreg, which has tack at room temperature that facilitates ply-to-ply adhesion during layup, a thermoplastic is dry and tackless. Additional means are required FIG. 4 Viable processes: Continuous compression molding Not widely used, but considered a highly viable process for aerostructures manufacture, is continuous compression molding (CCM), in which thermoplastic tapes are passed continuously through a series of forming tools to create composite profiles, like this one. CCM is viewed as a good candidate for the manufacture of stringers and frames for an aircraft fuselage. Source | xperion Thermoplastics' remeltability and reprocessability makes them much easier to recycle than thermosets.