By Jan H. Schut
One of the most competitive areas in automotive technologies these days is weight loss. American carmakers need to lighten cars to improve fuel efficiency and offset weight added by more advanced gadgets. European carmakers are under even more pressure to lose weight because they have to reduce CO2 emissions (to 130 g/km) for 65% of new cars starting next year and for all cars by 2015. So chemists, compounders, and engineers are finding creative new ways to replace heavy materials with light.
Here are some of the latest developments in automotive lightweighting, introduced at two current conferences from the Society of Plastics Engineers’ Automotive and Composites Division and Detroit Section in Troy, Mich.–the Automotive Composites Conference & Exhibition Sept. 13-15 (speautomotive.com/comp), and the 2011 Automotive TPO Conference Oct. 3-5 (auto-tpo.com), both held in Troy.
The ACCE conference introduced a new super lightweight continuous-glass thermoplastic molding technology for the first time. The TPO conference features several new technologies to reduce weight by reducing talc, including the first beta nucleation of talc-filled PP and the first part validation of a new magnesium-based fiber. The percentage of weight saving is less dramatic than from glass reduction, but still significant. The ACCE papers will be available free at http://speautomotive.com/aca at the end of the year.
REDUCING WEIGHT BY REDUCING GLASS
At the ACCE conference, a paper from Timo Huber, an engineer at the Fraunhofer Institut in Pfinztal, Germany (www.ict.fraunhofer.de), presented “Local Continuous Fiber Reinforcement” technology for the first time anywhere. (The paper was published in the proceedings for a conference in Marakesh earlier this year, but wasn’t actually given.) The technology places heated continuous glass fiber around metal inserts, only where needed along stress axes in the part and can save up to 80 wt% of glass vs glass-filled PP.
Continuous-fiber-reinforced structures have been molded before using woven fiber glass shapes and unidirectional fiberglass tape, like the auto seat system developed by BASF Corp. (www.basf.com) and the French auto parts maker Faurecia (www.faurecia.com), which was originally introduced as a concept at auto shows three years ago. (It was also presented at the ACCE.) Both BASF/Faurecia and Fraunhofer’s continuous glass thermoplastic parts create a separate glass-reinforced structure, place it in the mold, and mold over it. Both achieve dramatic weight savings replacing metal. But Fraunhofer’s cats-cradle approach to glass roving is about as minimal and strategic as glass use gets.
Fraunhofer starts by making unidirectional roving out of commingled PP and glass fiber, first heated, then pulled over three consolidating rollers for even distribution and wetting, and then pulled through a die for shaping. Temperatures in the heating zone, consolidating rollers and die are important to make roving without voids, Huber says. The roving is hand placed over the pins while hot, but in production could be placed by robot. Non-metal pins could also be used, and automotive parts that already have metal thread inserts for attachment points can use these to hold the glass fiber roving. Roving can be designed into structural ribs for further weight saving.
Sample parts with 6.5 wt% unidirectional glass showed higher tensile strength than glass-filled composites with 30% of either long or short glass fiber. Samples with only 2.3 wt% unidirectional glass achieved nearly the same tensile strength as 15% short or long glass composites—approximately 47 MPa for continuous glass vs. 49 MPa for short glass and 52 MPa for long glass. All parts were molded with impact copolymer PP (C711-70RNA from Dow Chemical), which has a MFR of 70 g/10 min.
Two papers present successful cross-linking of PP, which is unusual in itself. At the ACCE conference, Scott Miller, application engineer at Dow Corning Corp., Midland, Mich. (www.dowcorning.com), reported on “Closing the Gap Between PP and PA Composites with New Silane Grafting Technology” (initially presented at the SPE’s ANTEC conference in May). Dow Corning calls it the first successful free-radical silane grafting of PP while minimizing beta scission. Chemical grafting and cross-linking of PP typically break molecular chains and reduce properties. PP can be commercially grafted with chain extenders like maleic anhydride, but these only allow coupling to fibers, not cross-linking.
The patent-applied-for technology (U.S. Pat. Applic. # 20110178198) grafts a silane molecule onto the backbone of the PP chain. Silane-grafted PP can then either couple either with reinforcing fibers, cross-link neat PP, or do both to improve performance. Dow Corning’s silane-grafting technology shows improvements over maleated PP for heat stability, long-term aging, moisture resistance and mechanical strength with fibers, Dow Corning says. It has been successfully tested with glass, flax, hemp, ligno cellulosic, and wood fibers and reportedly improves bio-fiber composites enough to replace glass in some applications.
In the TPO conference, Jesse Baldwin, technical director of Toray Plastics (America) Inc.’s foam division in Front Royal, Va., (www.torayfoam.com), will present “New Cross-Linked Olefin Foams for Soft Touch Automotive Interior Trim Applications.” The patent-applied-for (U.S. Pat. Applic. # 20110014835) ToraSoft foam is a closed-cell PP foam with a combination of extreme softness–70% softer than Toray’s previous PP foams–with good haptics, or sense of touch and feel, good thermoformability, and good rebound characteristics. According to the patent application, it combines an olefin block copolymer with PP, a chemical blowing agent, and chemical cross-linking agent. The extruded sheet is cross-linked with an electron beam and heated for foaming in a molten bed of salt, a process Toray has used since the 1960s for even heat transfer and free foam expansion. ToraSoft, introduced commercially in January 2010, is under qualification and has been specified for upcoming automotive programs for interior trim parts.
WEIGHT-WATCHER RECIPES FOR TPO
Several recent developments introduced at the TPO conference take weight out of TPO parts by reducing EPDM rubber or talc. Philip Jacoby, v.p. of technology at Mayzo Corp., a specialty chemical company in Suwanee, Ga., (www.mayzo.com) will present “New PP Beta Nucleation Technology for Improving the Impact-Stiffness Balance of Neat and Talc-Filled PP and TPO Parts.” Beta nucleation is unusual in the first place. (A few Asian companies also make beta nucleants, but Mayzo believes it is the only maker of beta nucleant masterbatches.) Mayzo’s new third-generation beta nucleant masterbatch, MPM 2000, however, has the highest beta nucleating activity anywhere. It can reportedly produce PP with up to 70% of crystals in beta form. The crystal phase of homopolymer PP is typically over 95% alpha crystalline, which melts at 165 degrees C, and only 5% beta crystals, which melt at 14-15 degrees C lower temperature.
Beta nucleation produces hexagonal crystals in PP, whereas alpha nucleation produces clusters of monoclinic crystals (a rectangular prism with a parallelogram at its base). Hexagonal beta crystals improve impact strength and impart opacity, but lower tensile strength and flex modulus. By improving impact strength of talc-filled TPOs, beta nucleation allows APDM reduction. Mayzo introduced beta nucleant masterbatches commercially six years ago, but the first two versions had the limitation that they couldn’t be used with alpha nucleants like talc, certain pigments, or alpha-nucleated PP, which many resin producers make by adding sodium benzoate, NA-11, NA-21 or the clarifier Millad 3988. Mayzo’s second-generation beta nucleant solved the problem by adding an alpha nucleator “killer,” which prevented interference from sodium benzoate, but did not stop other alpha nucleants.
The new third-generation beta nucleant overcomes alpha nucleation by very high activity of the beta nucleator. Beta-nucleated PP can also make breathable biaxially oriented, waterproof film with densities as low as 0.28 g/cc. During biaxial stretching or thermoforming, beta crystals convert to alpha crystals and form micro voids, making the plastic white without pigment. When thermoforming web scrap is reprocessed, however, the beta nucleant is still present and retains its initial nucleation activity, so new beta nucleant only has to be added for the 50% that is virgin material.
At the TPO conference Wouter Reyntjens, business development manager at Milliken & Company, headquartered in Spartanburg, S.C. (www.millikenchemical.com) is presenting an ongoing OEM validation of a recently introduced magnesium-based synthetic mineral fiber reinforcement. The fiber, called Hyperform HPR803i, was introduced two years ago at the 2009 National Plastics Exposition in Chicago, but this is the first data on a part. HPR803i is 99% magnesium oxysulfate according to the MSDS with high aspect ratio and is sold in powder form. Loaded at 3%-10% with or without talc, it reportedly achieves better scratch resistance than PP with talc alone and weight savings from reduced talc of 7%-20%, depending on part requirements.
Milliken will present data on a developmental B-pillar trim part, made by Mecaplast Group, a tier-one automotive molder in Monaco (www.mecaplast.com), using a compound developed by Inno-Comp Ltd., a compounder in Hungary (www.inno-comp.hu). The compound has 13% mineral loading of talc and HPR-803i with a density of 0.98 g/cc and performs like 20% talc-filled PP for a density of 1.04 g/cc. But it weighs 7% less and has better scratch resistance.