By Jan H. Schut
For over 10 years R&D has gone on among European car makers, machine builders, material suppliers and research institutes to adapt conventional resin transfer molding machines from thermosets to thermoplastics. Instead of pumping two-component epoxy or PU into a mold to cure, they pump thermoplastic monomer with catalyst and activators into the mold to polymerize in situ. The goal for thermoplastic RTM is to mass produce continuous-glass-fiber-reinforced thermoplastic parts for cars, which would have advantages over thermosets of being tougher, stronger, weldable, and recyclable. T-RTM technologies are mostly focused on in situ molding of PA6 from epsilon-caprolactam monomer.
Because caprolactam has very low viscosity, it wets fiber structures rapidly in about 30 seconds without disturbing their position, achieving high directional fiber content of up to 65 volume %. Caprolactam, which melts at 69 C, comes either in liquid form in heated containers or in flake form. Usually two tanks are used in a modified RTM dosing machine. One tank is for caprolactam with catalyst, the other for caprolactam with activators and additives. The two caprolactam streams are combined typically 1:1 in a specially designed mixing head heated to around 100 C, then pumped into an RTM mold heated to around 150 C. The mix polymerizes in 2-5 minutes, depending on part characteristics and volume.
Caprolactam, however, isn’t easy to work with. It has very low, watery viscosity of 5-10 mPas vs 200-300 mPas for liquid PU, so leaks are an issue with conventional RTM machinery and molds designed for PU. Caprolactam also has to be protected from oxygen and moisture (<0.01%) throughout the in situ process since moisture slows or stops polymerization. So continuous fiber structures have to be predried before they’re put into the mold. PA 6 polymerizes to a solid using an anionic “ring-opening” polymerization reaction at over the melt temperature of the monomer, but below the melt temperature of the polymer.
LOTS OF EUROPEAN R&D
Among others, Porsche AG in Stuttgart, Germany (www.porsche.com), worked with the Fraunhofer Institute for Chemical Technology in Pfinztal, Germany (www.ict.fraunhofer.de), to develop “Cast Polyamide” thermoplastic RTM, shown at the JEC Composites show in Paris in 2006. Porsche and the Fraunhofer showed the process again in 2010 at a composites conference in Germany with a demo trunk liner for a Porsche Carrera 4, which weighed 50% less than an aluminum trunk liner.
Volkswagen AG in Wolfsburg, Germany (www.volkswagenag.com), which bought Porsche in 2012, recently successfully tested high-pressure thermoplastic RTM molding of continuous-glass-filled PA6 “B pillar” reinforcements that could be glued into steel B pillar frames and weigh 36% less than high-strength steel B pillars in production for the North American market. The tests were done in VW’s fiber-reinforced plastics test plant in Wolfsburg, Germany, and reported in Kunststoff magazine in March this year.
First an asymmetrical woven fiber structure with a sizing compatible with anionic polymerization of PA6 was preshaped in a separate mold with a binder, also compatible with caprolactam. Preforms were kept dry in a drying oven from HK-Praezionstechnik GmbH, Oberndorf am Neckar, Germany (www.hk-pt.de), then put into an RTM mold heated to 150 C. Molding was done on an existing 1000-ton injection molding machine, using a two-sided mold, modified to prevent leakage of caprolactam. Caprolactam injection must be moisture-free, so the mold was rinsed with nitrogen each time before filling.
KraussMaffei Technologies GmbH in Munich, Germany (www.kraussmaffeigroup.com), which worked with VW, developed a new high-pressure caprolactam mixing head, electrically heated to about 100 C and pumping with nearly 100 bars of pressure. KraussMaffei also modified its RTM machines for caprolactam with heated hoses to transfer melted caprolactam from dosing tanks to the mixing head. Even coupling pieces in the hoses needed heater cartridges to keep temperature constant. KraussMaffei already built caprolactam mixing heads and dosing machines for NYRIM, a reactive in situ casting process for PA6 copolymers, developed in the 1980s by Monsanto Co., St. Louis, MO (www.monsanto.com), then sold to DSM NV in the Netherlands (www.dsm.com). NYRIM puts caprolactam with activated elastomeric polymer in one tank and caprolactam with catalyst in the other, then mixes them.
Hennecke GmbH, Sankt Augustin, Germany (www.hennecke.com), developed an even higher pressure T-RTM system. Hennecke optimized its counter flow RTM mixing head for thermoset PU to mix lower viscosity caprolactam. The high pressure caprolactam mixing head uses more than twice the pressure of Hennecke’s PU mixing head, which is roughly 200 bars. Hennecke’s caprolactam mixing head is self-cleaning only through counter flow.
Engel Austria GmbH, Schwertberg, Austria (www.engelglobal.com), worked with the Fraunhofer-ICT from 2009 to 2011 to develop a high pressure, servo-motor-powered thermoplastic RTM machine based on melting caprolactam flake in a modified injection molding unit, not on dosing liquid caprolactam from tanks. Engel uses an Engel e-victory injection molding machine with two injection units, modified for low viscosity caprolactam with special valves and seals. This was shown for the first time at an Engel open house in June, 2012, along with Engel’s in situ thermoplastic RIM process (see previous blog June 11, 2014), which is also based on injection molding of caprolactam.
Mahr Metering Systems GmbH, Goettingen, Germany (www.mahr.com), adapted a metering machine to process reactive PA for low-pressure T-RTM casting. Mahr also recently developed a new mixing head for caprolactam, catalyst and activator, which is self-cleaning using nitrogen. It was shown for the first time at the JEC Composites show in Paris in March. Mahr’s dynamic mixing head for T-RTM uses high-precision gear pumps for process pressure from 20 up to nearly 50 bars. It is also designed for three components instead of two, allowing caprolactam to be combined with catalyst, activators and colorants separately.
Resin suppliers are working on reactive caprolactam formulations. BASF SE, Ludwigshafen, Germany (www.basf.com) worked with VW and Krauss Maffei to develop reactive PA6 systems with caprolactam, catalyst, activators, and additives,. BASF has also done development work for low-pressure T-RTM with Mahr. Lanxess AG, Koeln, Germany (www.lanxess.com), has done development work with Engel on thermoplastic RIM PA6 (see blog June 11, 2014). Brueggemann Chemical, Heilbronn, Germany (www.brueggemann.com), which acquired DSM’s NYRIM business over a decade ago, has done development work with Hennecke.
No auto maker so far has announced plans to commercialize thermoplastic RTM. Cycle time is apparently still an issue for large production quantities. “Two to three minute cycle time is OK for 100,000 parts a year,” notes a researcher in in situ RTM at a major auto maker, “but not for 200,000 or 300,000 parts a year.” But a spokesperson from BASF thinks the first car parts could be in series production by 2018 or 2019.