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New Material: Vitrimers Promise to Impact Composites Reinforced Plastics Volume 00, Number 00 June 2017 www.reinforcedplastics.com New material: vitrimers promise to impact composites FEATURE Denis A. Kissounko, Philip Taynton and Chris Kaffer Strong and durable like thermosets, yet moldable and recyclable like thermoplastics, vitrimers are ‘malleable thermosets’ which are challenging the status quo in the composites industry. Mallinda, a startup company founded by some of the pioneering inventors of the technology, is developing malleable CFRPs for rapid (30 s) production cycle times. Background malleable state, their total crosslink density remains constant, but For over 50 years, synthetic polymers have been divided into two the rate of bond exchange increases with temperature (see Fig. 2), general categories: thermosets which have excellent mechanical since all chemical reactions run faster at higher temperatures. This properties, but must be irreversibly cured; and thermoplastics, leads to a gradual decrease in viscosity with temperature, which which can be melted down and reprocessed, but have inferior differs from the relatively abrupt drop in viscosity associated with thermal and mechanical properties (Fig. 1) [1]. This characteristic the melting transition of thermoplastic materials. The reversible allows thermoplastics to be molded relatively quickly for high nature of vitrimers enables welding, molding, reshaping, and volume manufacturing, using techniques such as injection mold- recycling of fully cured materials. ing. Futhermore, these materials can be recycled by melt-proces- Mallinda co-founders invented the first known vitrimeric mate- sing. On the down side however, the majority of thermoplastics rials which do not require a catalyst, and behave like traditional are susceptible to mechanical deformations and creep at elevated thermosets under ambient conditions. Mallinda’s catalyst-free temperatures; therefore, they do not have a sufficiently robust vitrimers exhibit malleable behavior only when heated above dimensional stability at high temperature. In addition, commercial the glass transition temperature (Tg) of a given formulation. This thermoplastics are typically of higher cost in comparison to analo- means that below the glass transition, the polymer network is gous thermoset resins. Thermosets are the resins of choice for frozen, and is thus indistinguishable from a traditional thermoset. structural composites applications, such as aerospace composite Mallinda’s vitrimeric polyimine platform is also quite tractable. Tg, part fabrication where stiffness and durability are critical. While for example, can be tuned with formulation from below room thermoset resins offer significant performance benefits compared temperature to upwards of 2008C. to their thermoplastic counterparts, significant cure times for ther- moset resins make them unsuitable for high volume applications What does this mean for composites? such as automotive. Furthermore, due to the inability to reprocess The Mallinda team has shown that when structural composite thermoset resins after they have been cured, these materials are materials, like carbon fiber reinforced plastics (CFRPs), are made refractory to repair and remolding. Recently, this paradigm has using vitrimeric resins, several distinct advantages arise. First, been challenged by the arrival of malleable thermoset materials, unlike traditional thermosets, vitrimeric composite materials also known as vitrimers. can be reprocessed after being cured. Fully cured materials can Vitrimers are covalently-bonded crosslinked network polymers, be heated, remolded, and reshaped without losing their original like thermosets, but with the distinguishing feature that the strength. Second, these composite materials can be heat-welded chemical bonds in the network are exchangeable. Typically, a together to form monolithic cured multilayer laminates. Finally, catalyst is included in the resin formulation, which enables bond CFRPs made with vitrimeric resins can be easily recycled in a exchange reactions when the material is heated above the vitri- solution-based process in which the fiber and resin can be sepa- meric transition temperature [2]. When vitrimers are heated to the rated, recovered and reused to their original strength [3]. 0034-3617/ß 2017 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.repl.2017.06.084 1 Please cite this article in press as: D.A. Kissounko, et al., Reinf. Plast. (2017), http://dx.doi.org/10.1016/j.repl.2017.06.084 REPL-1297; No of Pages 5 FEATURE Reinforced Plastics Volume 00, Number 00 June 2017 FEATURE FIGURE 1 Thermoplastics vs. thermosets. Mallinda is using vitrimer technology to develop a new class of o97q-2oDPfQ). Within 1-min the formed part can be demolded ‘pre-cured pre-impregnated laminates (prepreg)’ which will im- and handled immediately, with full rigidity occurring upon cool- pact current prepreg users as well as high throughput producers ing. Scrap material can be further repurposed to form other parts, who currently rely on Resin Transfer Molding (RTM) processes or can be recycled in the solution-based process wherein the fiber with quick-cure resins. The envisioned pre-cured prepreg mallea- and resin materials are recovered and reused (see demonstration ble composite materials can be cured upstream in the manufactur- video at https://youtu.be/FNd2bBt8CMU) (Fig. 3). ing process as individual laminates in a roll-to-roll process, and At present, prepreg materials offer several advantages over stored indefinitely at room temperature. To manufacture compos- other CFRP manufacturing approaches such as Resin Transfer ite parts, any number of plies can be heated to the material’s Molding (RTM) processes. Prepreg materials offer guaranteed specifically-formulated Tg, laid up in a mold to the desired thick- (rather than estimated) fiber/resin ratios, and consistent mechan- ness, and both consolidated and formed into a final part through ical properties. They are more reliable than RTM materials, as void compression forming [3] (in a video demonstration, a 20 s space, and trapped bubbles are common problems for RTM mate- compression forming step is used: https://youtu.be/ rials. They offer a simpler, cleaner, and safer lay-up process, since FIGURE 2 Temperature-dependent stress relaxation behavior of vitrimers shows that malleable flow of vitrimers correlates with reaction-rate of bond exchange. 2 Please cite this article in press as: D.A. Kissounko, et al., Reinf. Plast. (2017), http://dx.doi.org/10.1016/j.repl.2017.06.084 REPL-1297; No of Pages 5 Reinforced Plastics Volume 00, Number 00 June 2017 FEATURE FIGURE 3 High speed consolidation and molding for Mallinda’s malleable materials. there is no need to handle wet chemicals on site, as opposed to FIGURE 4 RTM. The drawbacks of traditional thermoset prepreg materials New recycling paradigm of the polyimine-based composites. are related to shelf-life, refrigeration logistics, cost, and a higher scrap rate. FEATURE Due to the irreversible nature of traditional thermoset How is it made? curing, prepreg materials, are partially cured in the roll-to-roll While there are many features which are unique to malleable fiber impregnation process (called B-staging), and immediately thermosets, one feature which is shared with traditional thermoset frozen to prevent completion of the curing process until the materials is the initial curing step. Similar to traditional thermoset material is used to manufacture a composite part. The materials, malleable thermosets must be formed in a polymeriza- manufacturing step typically involves vacuum bagging the lay- tion reaction in which a liquid precursor solution is cured into a up over a mold, followed by autoclave treatment for up to several polymeric solid. As with traditional materials, this initial curing hours to cure the resin. Compared with current prepreg materials, step is the optimal step for combining the polymer with reinfor- pre-cured prepregs offer the advantages of extended shelf-life cing fibers, since the low viscosity of the uncured resin enables stability, ambient shipping and storage (no refrigeration), faster consistent and even flow into, and impregnation of, the fine cycle time (no waiting for cure during layup/consolidation), and reinforcing fibers. This means that vitrimeric resins can be infused recyclability of cured scrap and end of life material. If the pre- into reinforcing fibers using existing prepregging equipment. A cured prepreg materials are made at sufficient scales, these mate- key differentiation in processing vitrimeric prepreg materials is rials will become available at a fraction of the price of present that the resins are cured in-line immediately after resin infusion. prepreg materials. Mallinda has partnered with an existing prepreg supplier to pilot TABLE 1 Lightweighting potential of materials. 3 Please cite this article in press as: D.A. Kissounko, et al., Reinf. Plast. (2017), http://dx.doi.org/10.1016/j.repl.2017.06.084 REPL-1297; No of Pages 5 FEATURE Reinforced Plastics Volume 00, Number 00 June 2017 FEATURE FIGURE 5 CFRP Mfg costs. the infusion process with initial success, and the company is competitive with epoxy resins when developed to similar scales. moving to scale-up its industrial processing for its introductory Due to other efficiencies, Mallinda expects its prepregs to be less product offering. expensive than competitive materials at launch. Why are vitrimer composites easier to recycle? What are Mallinda’s near-term and long-term plans? Due to the same
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