Reinforced Thermoplastics: LFRT/GMT Roundup

In the world of composites, thermoplastics with long (6.35 mm/0.25 inch or greater) fiber reinforcement make up one of the fastest growing categories. Leading this expansion is one of the oldest forms, glass mat thermoplastic (GMT) and two of the segment’s newest: precompounded (pelletized) LFRTs (long-fiber reinforced thermoplastics), also known as LFTs, and inline compounded (ILC) or direct LFTs (D-LFTs). (These are defined and their competitive positions are compared on p. 26.)

The automotive industry, still the core market for all reinforced thermoplastics (RTPs), provides significant market pressure for innovation and evolution. The need to improve profits — particularly in slow North American and European markets — and reduce fuel consumption has made cost and weight reduction high priorities. Although RTPs usually have a higher raw material cost, they typically yield parts of lower weight and can be produced at lower system cost than parts made of either metals or thermoset composites. Moreover, the long-fiber reinforcement in GMT and both forms of LFT offer those charged with redesigning previously metallic components unprecedented opportunities to reduce part count and tooling costs and simplify assembly/finishing operations, yet maintain rapid production cycles. Similar concerns are driving long-fiber RTP adoption in heavy truck and mass transit applications, and recreational vehicle (RV) OEMs reportedly are exploring thermoplastic composites as a way to take significant weight out of their “houses on wheels.”

Arguably, GMT has changed more in the last decade than it did in the first 30 years of its existence. Whether this has been stimulated by the need to hold onto market share in the face of tough LFRT/D-LFT competition, or simply the culmination of three decades of processing and compounding experience is probably irrelevant. In the world of materials science, the edict is “evolve or face extinction.” Producers of all three types are making sure product evolution isn’t left to chance.

EXPANDING WITH NEW MATRICES

Although polypropylene (PP) has been the dominant matrix for GMT composites for almost four decades, expansion into engineering thermoplastics has helped create a family of high-performance GMTs. AZDEL Inc. (Forest, Va.) now markets polycarbonate (PC) and polyetherimide (PEI) versions of its SUPERLITE low-density GMT products into mass-transit and aviation applications because of the materials’ inherent flame retardant properties and higher thermal performance in a very low-density yet reasonably stiff panel. Quadrant Plastic Composites AG (Lenzburg, Switzerland) has introduced a new nylon (polyamide or PA) grade in its chopped mat format that is currently being trialed for automotive applications in Europe.

On the precompounded LFRT side, a broader offering of matrices has been available for some time, although PP still dominates sales globally. Ticona Engineering Polymers (Florence, Ky.) is introducing new matrices into its Celstran LFRT product line, featuring conductive and nonconductive acetal (polyoxymethylene [POM]) and linear-polyphenylene sulfide (PPS). This family also includes PP, PA and thermoplastic polyurethane (TPU) grades reinforced with one or more types of 10-mm/0.4-inch long fibers, such as glass, carbon, aramid or stainless steel, for injection molding. A second family, called Compel, features even longer (25 mm/1.0 inch) fibers for processing via compression molding.

Evolutionary expansion has progressed to the point that there is reportedly a program underway to apply D-LFT technology to thermosets as a means to make preproduction of semifinished SMC obsolete, reducing material costs. According to Dr. Frank Henning, director of the Department of Polymer Engineering, Fraunhofer ICT (Pfinztal, Germany) and also a consultant to Dieffenbacher GmbH & Co. KG (Eppingen, Germany), “In Germany, there is a big development project with thermoset materials called ‘DuroVision.’ This program is now in the prototype stage and results are expected to be published by year’s end.” The process is said to provide for compounding of the material immediately before placement in the press, giving the molder the same flexibility in terms of fiber loading and additive content that is now available to thermoplastics molders via inline compounding. “Direct thermosets also overcome a lot of the shelf-life and storage restrictions common to conventional thermosets.”

Meeting Environmental Mandates

Suppliers of long-fiber RTPs are at the forefront of efforts to develop moldable materials that conserve dwindling resources and reduce environmental impact. Among the largest and fastest growing are those reinforced with natural fibers. Although they offer lower stiffness and strength than composites reinforced with glass, they are usually lighter in weight and cost less. As such, they make good replacements for fiberboard and wood-fiber composites in interior trim panels, where structural strength is of secondary importance, on passenger vehicles and heavy trucks. A wide variety of natural fibers are used and are generally classified by the part of the plant from which the fibers derive, such as bast (skin) fibers from flax, hemp, jute and kenaf; leaf fibers from sisal, henequen, pineapple and banana; and fruit/seed fibers from cotton, kapok and coir (coconut).

Commercial products are available primarily in the GMT and D-LFT formats. Quadrant Plastic Composites, through its subsidiary Quadrant Natural Fiber Composites (Lambrecht, Germany), is one of the world’s largest natural-fiber composites manufacturers, producing a broad line of GMT products with a PP matrix and a blend of kenaf, hemp and flax in areal weights ranging from 300 g/m² to 3,000 g/m². Because natural fibers are inherently variable from plant to plant, season to season and region to region, blending several types ensures relatively consistent performance (see “Learn More”).

Still other work is underway using bio-based polymer matrices derived from annually renewable plant materials, such as soy, corn, potatoes and various grasses. These matrices are often coupled with natural-fiber reinforcements to form so-called “green composites,” which are generally defined as produced entirely from renewable agricultural resources. Green composites are less structural than conventional composites but provide sufficient mechanicals, along with good sound and heat insulation, making them suitable for interior trim panels in vehicles.

When natural fibers are combined with a bio-based matrix, they are said to be carbon-neutral. That is, the plants from which fiber and matrix materials are derived absorb as much carbon during their lifecycle as is expended converting the materials to a finished composite. Additionally, some bio-composites are reportedly biodegradable at end of life, making them compostable.

In Europe, where legislation mandates vehicle recyclability, auto OEMs are pushing for composites that can be reclaimed for reuse. Thermoplastics’ melt-reprocessability gives them an inherent advantage over thermoset composites, provided the reinforcement can be reclaimed. The greatest success, thus far, has been achieved with so-called self-reinforcing polymers (SRPs), which comprise a matrix and reinforcement from the same polymer family — e.g., Curv from Propex Fabrics Inc. (Chattanooga, Tenn.). Suppliers of other RTPs face the difficulty of finding a matrix/reinforcement combination that can meet end-of-life constraints without sacrificing the mechanical properties contributed by glass.

A potential solution, on the GMT side, is the use of basalt fiber. Basalt-reinforced composites are attractive in geographies where parts are energy-cycled (incineration with energy capture) at the end of their useful life. In an incinerator, glass fibers melt, fall to the bottom and build up on the incinerator’s interior walls, forcing costly work stoppage to remove the accumulated melt. But basalt, with a higher melt temperature because of its volcanic origin, reportedly pulverizes to a dust and is easily removed. Basalt also provides stiffness between that of glass and carbon but at lower cost than carbon or S-glass, offering the possibility of achieving glass-like mechanicals at lower fiber loadings, thus mitigating basalt’s higher cost compared to conventional E-glass. Recently, AZDEL introduced a basalt-based GMT called Volcalite, which was developed especially for headliners and trim panels in the Japanese automotive market, where energy-cycling is now mandatory. In fact, a basalt-based low-density headliner for Honda Motors was judged Most Innovative Use of Plastics in the Environmental category at the 2006 SPE Automotive Innovation Awards Gala.

TRENDING TOWARD “TOTAL SOLUTIONS”

Recently, long-fiber thermoplastics have demonstrated their great potential for design and decorative versatility. They can be overmolded on, comolded with or bonded to metals, glass, “paint” films, carpeting, fabric and other decorative materials, enabling cost-effective manufacture of complex finished components out of the mold that are lightweight, durable and aesthetically pleasing. These facing materials also can be colaminated with GMT composites during initial production.

GE Plastics (Pittsfield, Mass.) and AZDEL, for example, have teamed to use thermoplastic tapes and paint films as skins for AZDEL’s SUPERLITE low-density, aerated mat GMT to create a very stiff, lightweight sandwich composite they call HPPC (High-Performance thermoPlastic Composite). When thermostamped on tools equipped with RocTool’s (Le Bourget du Lac, France) CAGE inductive mold heating system, the HPPC sandwich composites are said to offer a Class A surface out of the tool. The group recently achieved a four-minute cycle time for a thermoplastic vehicle hood (see “Related Content,” at left). Such technology may finally make horizontal body panels a reality for thermoplastics. In fact, the GM Volt and Hyundai QarmaQ, concept cars introduced this year, both feature such body panels molded with GE/AZDEL HPPC.

Dieffenbacher and Fraunhofer ICT have codeveloped a process for selectively reinforcing D-LFT composites with continuous fibers in the form of profiles or fabrics like Twintex, produced by Saint Gobain Vetrotex (Chambery, France). Strategic placement of uni tapes or fabric, as determined by such means as finite element analysis, makes it possible to add stiffness or other properties only where needed. This tactic reduces the need to over-engineer parts to satisfy safety margins, and it limits the use of more expensive reinforcements, thus reducing part weight and cost.

Quadrant’s GMTex products — sandwiches created at the mold by stacking layers of chopped mat with layers of various types of woven and/or nonwoven continuously reinforced textile mats, all with a PP matrix — are used for applications subject to severe loading conditions, such as rear-axle support brackets and bumper beams.

There is a strong trend toward hybrid structures of thermoplastic composite and steel. For example, the front-end module (FEM) for the Volkswagen Polo features an injection molded subcomponent of DLGF9311 long glass-reinforced PP from Dow Automotive (Midland, Mich.) that is structurally bonded to a steel member. The hybrid LFRT/steel FEM is reportedly thinner, lighter and stronger than an RTP-only system, and the PP/steel bond is said to be stronger than that achieved with overmolded systems. The resin is part of Dow’s recently launched family of precompounded LFRT-PP products whose standard fiber length is 12.7 mm/0.5 inch (10 to 60 percent glass content) but can be compounded with much longer fibers. The adhesive, dubbed LESA, was formulated by Dow specifically to optimize PP/steel adhesion.

Suppliers of precompounded and inline compounded LFRTs also are making it easier for molders to achieve targeted performance and maintain quality control by supplying thermoplastic composites enhanced with specialty additive packages. On the precompounded side, GE Plastics’ LNP Specialty Compounds (Exton, Pa.) recently introduced a new line of its Verton LFRT products under the Xtreme sub-brand that can incorporate a much broader offering of properties — including flame retardance, molded-in color and weatherability — in a single-pellet solution. These prepackaged systems relieve molders of the responsibility and potential for quality variance inherent in metering and dry-blending colorants, additives, flame retardants, UV stabilizers and other additives with the glass-reinforced resins at the press, which can lead to separation, settling and other consistency/repeatability issues. GE reports that Steelcase (Grand Rapids, Mich.), a global manufacturer of office furniture, switched to the new compounds to improve color saturation on the seat-back frame of its award-winning Leap office chair to eliminate issues with color separation, poor finish and an unacceptably high rate of rejects on these appearance-critical parts. The new compound not only improved color consistency dramatically but also improved notched Izod impact while maintaining strength and flexibility. Future work is said to be focused on enhancing lubricity/wear resistance for parts, such as gears and washers, and enhancing conductivity for powder-coated LFRT parts.

On the D-LFT side, there have also been breakthroughs in additive packages for inline compounding that incorporate multiple functionality into a single system. Typically, the formulation includes colorant (usually black), stabilizers and coupling agents to deliver improved mechanicals and longer service life. Addcomp Holland BV’s (Nijverdal, Holland) new ADD-VANCE combination masterbatch formulations are said to have been optimized to provide good mechanical properties as well as low odor, low VOCs and low emissions — all important properties for components used on vehicle interiors. For more demanding applications, halogen- and antimony-free flame retardant systems also are available. Paul Stassen, Addcomp’s marketing/sales director, says, “The addition of our ADD-VANCE combi-masterbatch to a D-LFT formulation is typically made at around the 2 percent level, but it contributes to more than 80 percent of the quality of the final product.” Addcomp guarantees both the mechanical properties and service life of its systems in D-LFT products if inline compounding and molding conditions are handled correctly.

MOLDING A NEW FUTURE

Both GMT and LFRT/D-LFT composites got their start in the automotive industry, and the developments noted above have enabled designers and OEMs to aggressively incorporate long-fiber RTPs into many applications that previously were in metal, such as engine and, now, full underbody noise shields, front-end modules, seat structures, instrument-panel carriers, roof racks, bumper beams, knee bolsters, wheel wells, battery trays, trunks/rear storage tubs and door hardware modules. One integrated door hardware module, produced by the Ranger Group (Carate Brianza, Italy) for Italian automaker Lancia’s Ypsilon subcompact, meets all European side-impact and hip-protection requirements with no foam pad, injection-molded inlay or other carrier reinforcement. The design was evaluated in compression-molded D-LFT, injection-molded LFT pellets, a sandwich of chopped-mat GMT with selective use of textile-reinforced GMT, and a 40 percent chopped-glass mat GMT. After hip- and side-impact testing, both LFT parts failed but the GMT grades passed. The chopped-mat GMT was selected because it provided the best performance/cost balance for the part.

A hybrid chopped mat GMT/steel lower instrument panel carrier designed for Ford, Volvo and Mazda vehicles (see photo at right) is the first IP carrier to meet or exceed all world safety standards for full frontal and offset crashes. It reduces cost by 12 percent and weight by 2 kg to 3 kg (4.4 to 6.6 lb) and provides better noise/vibration/harshness (NVH) values than previous versions. It also consolidates parts and provides 90 percent of the vehicle’s cockpit fixing points, streamlining assembly. The cross-car beam has been reduced to a steel tube, which is fully encapsulated at junction points during compression molding without the knitlines that are seen in injection molding.

Newer programs are working on Class A exterior body panels and complete tailgates. For example, a tailored compression-molded D-LFT tailgate (see photo, p. 33) for the convertible model smartfortwo micromini car provides stowage for the vehicle’s soft top and A pillars when the canopy is down. The component is selectively stiffened with unidirectional continuous-glass profiles across the part to fulfill rear crash requirements, and is comprised of an inner and outer shell, the former with a grained surface and the latter with a good B surface that requires no painting. The tailgate is mounted as-is, without the need for secondary finishing operations.

While automotive still represents their largest market, long-fiber RTPs are claiming territory elsewhere. GMT products, in fact, have long been used in the industrial, materials handling and agricultural/lawn and garden markets and are now making inroads into the building trade. LFRTs are replacing metals in housings and chasses for portable electronics — personal data assistants (PDAs), mobile phones, portable MP3 players, notebook computers, appliances, shower head components, marine swim platforms and boat hatches. Injection-molded LFRTs also are expanding into the handheld power tool market where they are replacing painted die-cast metal. Senco Tools uses Ticona’s Celstran TPU-GF50-01 50 percent glass-reinforced thermoplastic polyurethane for the handle of its nail guns because of the material’s stiffness, strength, creep resistance and aesthetics at temperatures ranging from -40° to 150°F/-40°C to 66°C (see top photo, this page). Additionally, Alessandro Belli, founder/CEO of Italian design firm Tecnologie Urbane, has developed the first all-reinforced-plastic folding bicycle. It measures 48 cm by 36 cm by 12 cm (18.9 inches by14.2 inches by 4.7 inches) and weighs only 4.0 kg/8.8 lb. Its frame is Ticona’s 40 percent glass-reinforced Celstran LFRT (see opening photo, p. 1).

Despite the myriad applications already captured by long-fiber RTPs, those who develop them claim there’s plenty of room for new ideas and broader applications. Industry veteran Gerry Battino, who began his career at PPG Industries (Pittsburgh, Pa.) where GMT was developed, spent several decades at AZDEL and later became president of Quadrant Plastic Composites Inc. (Quadrant’s North American arm), says, “GMT was originally developed to replace sheet metal and die-cast parts.” Given the incursion of LFRTs into part categories once dominated by GMTs, Battino predicts that “the fastest growth area for GMT in the future will be as a flat-sheet product to replace other materials, such as plywood, luan and thermoset composites. It might be cut or shaped a bit, but it will not be flow molded.” Applications could include concrete forms, scaffolding, large containers, the sides and floors for railway cars or heavy truck trailers, and load floors for buses. “When you think about how easily you can vary glass content and construction, sheet thickness, resins, colorants and other additives, and how you can combine GMT sheet with other materials to make sandwich panels,” he points out, “you’ve got a whole new way to use GMT materials.”

Precompounded LFRT, says Maria Ciliberti, Ticona’s automotive regional sales manager, Ticona Engineering Polymers, “is an engineering materials solution that is in its infancy. We can offer many derivations of LFRT products by varying the resin matrix, type of reinforcement, length of the reinforcement and even the product form beyond cylindrical pellets.” Noting that LFRT suppliers are already incorporating carbon and basalt fibers and even thermoset matrices, she claims, “We have really only just begun.”

Fraunhofer’s Dr. Henning is equally upbeat about D-LFTs. “The direct process offers the opportunities for a lot of different material and reinforcement combinations,” he says. “Glass, carbon, natural, and synthetic fibers and different types of polymers — olefins, engineering plastics, bio-based resins — are already being introduced and will be especially interesting for tailored LFT.”