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10/24/2014 | 6 MINUTE READ

Long-Fiber Thermoplastics Grab Attention At Automotive Composites Conference

Close Up on Technology: Thermoplastic Composites
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LFT looks promising in automotive, with lightweighting, balanced stiffness and toughness, and processing advances as key attractions.

Long-fiber thermoplastics (LFT) and continuous-fiber thermoplastics were prominent at the 2014 SPE Automotive Composites Conference & Exhibition (ACCE), held in Novi, Mich, Sept. 9-11. Notable presentations included new nylons for LFT and continuous-glass composites, novel glass reinforcements for automotive composites, a study of direct long-fiber thermoplastic (DLFT) with carbon fiber in nylon 6, and another that investigated injection molded, long-glass PP and nylon 6 integral foams using “breathing mold” technology.

Meanwhile, LFT thermoplastics compounder PlastiComp, Winona, Minn., showcased a semi-truck fender liner made of long-glass PP. A week after ACCE, the same company revealed it has developed hybrid long-glass and carbon-fiber composites.

Invista, Wichita, Kan., unveiled its Torzon modified nylon 66 with low viscosity and high flow to help bind the resin to continuous- or long-glass fibers. Trials with narrow unidirectional tapes made with the new modified nylon revealed significant improvements in load transfer efficiency from continuous fibers to the polymer matrix, resulting in improved properties versus typical nylon 6 or 66 continuous-fiber composites. LFT with the modified nylon showed better fiber wetting, threefold improvement in tensile strength, and doubling of flexural modulus relative to standard long-glass nylon. Also, the low moisture gain of LFT with the new resin resulted in 90% retention of properties.

DuPont, Wilmington, Del., reported on its development of high-glass-transition-temperature (Tg ) nylon overmolding resins for use in high-glass-content continuous-fiber laminates. Vizilon TPC overmolding composites are available in pellets with either a nylon 66 or a high-Tg PPA (partially aromatic nylon), and as continuous glass-reinforced sheet made with these resins. These have been formulated for a robust process window, according to researcher Paul Kane. A key challenge for thermoplastic composites is meeting stiffness requirements for automotive applications at elevated temperature. Typically, 90 C (194 F) testing is used for automotive components such as cross-car beams, lift gates, and seating.

Trials of beams were made with Vizilon high-Tg PPA and nylon 66 resins combined with 50% short glass for overmolding onto stamped continuous-glass nylon inserts. The beam overmolded with PPA showed nearly twice the bending stiffness at 90 C and 49% higher torsional stiffness compared with the beam overmolded with nylon 66. At 90 C, the behavior of the PPA-overmolded beams was nearly the same as those overmolded with nylon 66 at 23 C. When the beams were compared at 90 C, the PPA-overmolded beams absorbed around 30% more energy than their nylon 66 counterparts. Kane says the higher stiffness offered by the high-Tg PPA will allow designs with less weight or bulk.

Both Owens Corning, Toledo, Ohio, and PPG Industries, Pittsburgh, presented new glass reinforcements for LFT or continuous-fiber composites. Corey Melvin, Owens Corning’s global market manager, said its new “premier” glass roving, Performax SE4849, increases compounding line speed for LFT PP by 30% while providing a new level of end-use performance through higher glass loadings and improved glass-PP matrix bonding.

Dispersion of SE4849 is said to be 20% better than the company’s standard product and 40% better than competing products. Similarly, its tensile strength of air-spliced roving strands is 38-46% better than existing products. According to Melvin, it has been shown to deliver up to 40% improved processing and wetout, up to 20% higher modulus, and 50-83% less fuzz generation than competitors’ products.

PPG has developed a novel glass composed of different oxides that combines both high elongation and high modulus. It will be launched commercially in late 2015. PPG worked with materials engineer Brandon Strohminger of A&P Technology, Cincinnati, a specialist in braided reinforcements, to test composite automotive crush tubes. The researchers developed braided crush tubes using an epoxy matrix with the new glass and with carbon fiber or a hybrid of glass/carbon fibers in order to simulate crash loads at high strain rates. PPG showed evidence that the best specific energy absorption (SEA) was achieved by the hybrid of glass and carbon composite. Ryan Emerson, applications development group leader, said this result should apply to thermoplastic composites, as well.

Emerson sees significant potential for the new glass in continuous-glass formats with thermoplastics such as PP and nylons. He said PPG is now exploring use of long glass in preforms made by in-situ polymerization of nylon 6 from caprolactam through reactive casting. Automotive applications to target would be large structural parts.
Zoltec Corp., St. Louis and the Fraunhofer Project Center for Composites Research at the Univ. of Western Ontario have been investigating DLFT with carbon fiber and nylon 6 in compression molded automotive parts such as large exterior panels.

The researchers are working toward using carbon as a drop-in replacement for glass fibers to increase performance and weight reduction while utilizing currently available equipment with minor modifications.

The researchers initial processing study showed that mechanical performance was better in the flow direction than the cross-flow direction, due to fiber orientation. This directionality became stronger the farther the material flowed during pressing, but the fiber weight percentage was relatively consistent across the panel. Shorter fibers appeared to influence performance more than the longer fibers, possibly due to bundling of the longer fibers. The maximum processable limit of carbon was found to be 45% by wt. with this matrix and fiber configuration. Also, carbon-specific fiber guiding would reduce fuzz generation and part homogeneity, according to the researchers. They concluded that DLFT technology can be further enhanced by application of local continuous reinforcements in forms of fabrics, profiles, and preforms to produce tailor-made parts.

The Canadian researchers, together with the Dept. of Polymer Engineering of the Fraunhofer-Institute for Chemical Technology (ICT) in Germany, discussed their investigations into injection molded, long-glass PP and nylon 6 foams using “breathing mold” technology, better known as the core-back technique. During injection, the cavity is initially filled, and then the core side of the mold retracts, allowing foam expansion between solid skins.

The researchers found that combining foam injection molding with LFT allowed production of large sandwich components with a high lightweighting potential in one shot. At a constant surface weight, the bending stiffness in these structures could be increased by up to 600%. An instrumented impact penetration test of nylon 6 with 50% long glass showed higher energy absorption with increasing density reduction. Charpy impact bending test results for PP with 30% long glass showed nearly constant behavior with increasing density reduction. Delay time before mold opening was shown to influence the thickness of the solid skin. Thicker skins tended to increase bending and impact strengths, while the tensile properties slightly decreased.

PlastiComp has developed an innovative set of hybrid thermoplastic composites—dubbed LFT Complet, that combine long glass fiber and long carbon fiber reinforcement together in a single, ready-to-mold composite pellet (see photo). A nylon 66 hybrid pellet with 20% long glass + 20% long carbon fibers has a tensile strength of 41,200 psi (284 MPa), which is 96% as high as 40% long carbon fiber alone and 24% higher than 40% long glass alone. It has a flexural modules of 2.6 million psi, which is 87% that of carbon alone and 86% higher than glass alone. The inclusion of long glass allows the material to retain 78% of the unnotched impact strength of glass at 18.8 ft-lb/in. (1004 J/m), which is a 25% improvement over carbon fiber alone. 

“With these new hybrid long glass + carbon fiber composites, PlastiComp has a stepping-stone material in the middle of the glass-fiber to carbon-fiber gap that can be 20-50% less expensive with hardly any performance hit,” says business-development manager Eric Wollan. PlastiComp will offer custom hybrid and other LFT composites in polymers from PP to PEEK.