CompositesWorld
Issue link: https://cw.epubxp.com/i/532917
JULY 2015 6 CompositesWorld PERSPECTIVES & PROVOCATIONS » Shortly after I moved from Dow Chemical to Fiberite (now part of Cytec Industries) in 1987, I attended a sales meeting where a colleague from Fiberite's parent company, Imperial Chemical Industries (ICI), avowed that, within a decade, thermo- plastic composite prepregs would displace thermoset prepregs throughout the aerospace industry. ICI had developed carbon fber prepregs based on high-temperature PEEK thermoplastic, and the advocate cited advantages, including infnite shelf life, lack of lengthy cure times, easier recyclability and high impact strength. Almost 30 years later, his prediction is unfulflled: Ter- moplastics have made some inroads, on leading edges of some Airbus planes and in various beams, fuselage clips and other structures. And most modern thermoset prepreg systems incorporate thermo- plastic polymers at some level to improve damage tolerance. But more than 95% of aerospace prepregs are still thermoset. Full disclosure: I have spent most of my career either promoting or working with thermoset materials. Tey have attributes that make them attractive for highly stressed parts, including lower processing viscosities for RTM, infusion or prepreg manufacture (which permit high fber volumes), superior adhesion to fbers (especially true of epoxies), paint and non-composite materials, and high thermal resistance, especially to compressive creep. Tis enables polyimides to survive the high-temperature zones in jet engines, and phenolic compounds to resist the compressive forces placed on molded brake pistons on a long downhill descent. Composites for wind turbine blades are almost exclusively ther- moset, as are the major structures in the Boeing 787 and Airbus A350. To date, carbon fber-intensive automotive structures, from Italian supercars to the BMW i3, feature thermoset matrices. Decades back, the knock on thermosets was that they cured too slowly, but I have seen electrical circuit breakers produced from polyester BMC in multi-cavity injection molds in cycle times of less than 15 seconds. A number of suppliers now ofer resins for RTM with the potential for less than 90-second cure. On the downside, many thermosets have limited shelf lives and there is the risk of spoilage. Whether or not fabricators recognize it, every thermoset mold is a chemical reactor — each part cycle is a polymerization that has to be carefully controlled to avoid exotherm and to achieve the desired state of cure. And if it goes wrong, you cannot grind it up and remold it — at least not easily. Tat said, a fair portion of my time was spent competing against thermoplastics for certain applications, especially in the indus- trial sector, so I became very familiar with thermoplastics and how they are processed. For many years, thermoplastics have enjoyed a reputation as being "easy to process" — pourable, easy to automate, able to melt, solidify and remelt — with properties essentially defned by the material supplier, who had full control of the chemistry. Such materials were recyclable, as long as they were unreinforced or contained only short fbers or mineral fllers. Te development of glass mat thermoplastics, which can be compres- sion molded like SMC, and long fber (typically 6 to 25 mm) pellets, which can be compression or injection molded, opened up new applications, especially in auto components. More recent introductions of continuous unidirectional and fabric-based ther- moplastic materials, including carbon fber versions, based on more economical and easier-to-process resins than PEEK, have positioned thermoplastics squarely against thermoset prepregs. Recently, I have embraced a more neutral stance, having found numerous applications where thermoplastics might make more sense than thermosets, especially in composite structures. Having a broader palette is very exciting. Two emerging technologies for thermoplastics seem especially appealing. One, structural injection overmolding, uses a continuous-fber structural "backbone," either a fat sheet or a formed/consolidated insert, which is preheated and placed into a mold, behind which reinforced discontinuous mate- rials are injected to form ribs, attachment points and other features (see CW's coverage, p. 19). Te second is thermoplastic RTM, where the native high viscosity of thermoplastic resins is circumvented by injecting low-viscosity monomers into a heated mold containing a fber preform (similar to thermoset RTM). Tis is followed by what some call in-situ polymerization, resulting in a conventional ther- moplastic polymer composite. So far, caprolactam (which forms polyamides) and acrylics show great promise using this technique. (Read more at short.compositesworld.com/InSituPoly). For decades, thermosets and thermoplastics competed partly by pointing out each other's weaknesses. I don't see as much of that today. Instead, I see healthy competition to develop material systems and processes that make composites overall more competitive with legacy materials, providing added value and new avenues to high performance. Tis is the right battlefront, and the one on which our industry — united — needs to prevail. Thermosets vs. thermoplastics: Is the battle over? I see healthy competition ... that makes composites overall more competitive with legacy materials. Dale Brosius is the chief commercialization ofcer for the Institute for Advanced Composites Manufacturing Innovation (IACMI), a DoE-sponsored public-private partnership targeting high-volume applications of composites in energy-related industries including vehicles and wind. He is also head of his own consulting company, which serves clients in the global composites industry. His career has included positions at US-based frms Dow Chemical Co. (Midland, MI), Fiberite (Tempe, AZ) and successor Cytec Industries Inc. (Woodland Park, NJ), and Bankstown Airport, NSW, Australia-based Quickstep Holdings. He served as chair of the Society of Plastics Engineers Composites and Thermoset Divisions. Brosius has a BS in chemical engineering from Texas A&M; University and an MBA.