Composites on the road to the big time?

Fifty years ago the new Chevrolet Corvette, with its fibreglass body, seemed to set the motor industry on the path of automotive composites. But did it actually do so?

A first glance might suggest not. Half a century later the composite car body, tending now to be of carbon as much as glass, is still limited to supermodels and niche vehicles while the vast mass production market remain a nut waiting to be cracked. If there are few composite bodies, there are even fewer composite chassis or space frames, and fewer still integral composite full vehicle structures minus only the mechanicals and trim. The fact is that the 50 intervening years have not fitted composites for a full direct assault on steel in its mass-production bastion, basically because they are still too expensive and take too long to fabricate.

A more considered look, though, reveals a less discouraging story. By constantly nibbling away round the edges of established materials usage, reinforced plastics have made inroads and, because even a small share of a massive market is substantial, are now shipped to car makers in millions of tonnes per annum. Inside the car, under the bonnet (hood), at the front and rear ends, in the trim and increasingly in the structure too, composites are doing what they generally do — saving weight, reducing part count, adding ‘style’, and lasting longer. In addition they are contributing to safety, cutting fuel usage and emissions and, in low production runs, avoiding high tooling and setting costs.

History

Back-track to before the 1953 Corvette. The first plastic of all to win its way into cars was a brown thermoset resin called Bakelite, used from the 1940s in items like distributor caps and steering wheels. While they did not look great by today's standards, these gave designers new latitude and freedom. In a logical evolutionary line from Bakelite was bulk moulding compound (BMC), a resin mixed with catalyst and short mineral or glass fibres to form a mouldable ‘dough’ that could similarly be made into bulk plastic parts. Glass fibres were used because they were strong and affordable. Manufacture then was by compression moulding. BMCs have since evolved into more advanced compounds having fibre volume fractions of up to 50% with commensurate properties, and are today more often injection moulded.

Conceptually, rolling a BMC out flat results in a sheet moulding compound (SMC), something that has always appealed to the motor industry because, like steel, it comes in sheets and can be press moulded, albeit with heating. However, the short fibre and mineral reinforcement used to achieve a doughy consistency may not give adequate strength for load bearing applications. For that a stiffer resin and longer fortifying fibres are needed. Back in the 1950s glass fibre reinforced plastic (GRP) enthusiasts experimented by distributing fibres of glass randomly into polyester resin spread into an open mould. Cured results were encouraging but lacked consistency. This problem was eased when mats of chopped fibres held within a partially cured resin were introduced. Chopped strand mat (CSM), which could be laid up by hand in layers, provided some consistency in the final part and a higher fibre content. CSM became a mainstay of the emerging GRP industry. Thriving alongside it, the original pre-mat methods have been progressively refined into the spray-up technology we have today.

These materials (glass, polyester, CSM, BMC, SMC) and techniques (hand lay-up, spray-up and press forming of sheet), were the foundation for the automotive composites sector and are still important. Subsequently, more advanced composites using vinyl ester, epoxy, phenolic and other thermosetting resins along with aramid (eg Kevlar), carbon and more recently natural fibres, plus recyclable reinforced thermoplastics, have widened the materials choice while fabricators have tackled the cycle time issue with a range of closed mould systems, pre-impregnated material solutions adopted from aerospace, and recent promising ‘semi-preg’ approaches.

With these weapons, the plastics industry has been able to encroach onto metals' territory, meeting the automotive sector's chief requirements for mechanical properties, freedom from corrosion, speed of production, cost and a top-quality Class A finish. The future requirements list will have to include recyclability, especially in the European Community where the End of Life Vehicle Directive requires that cars made from 2015 will have to be 95% recyclable.

Today’s technology

Some of the earliest technology is still in use. Basic open moulding of glass laminate each year yields thousands of GRP parts ranging from motor cycle fairings to composite roofs for truck sleeper cabs. But it takes hours to prepare a mould, spread in the gel-coat that will constitute a smooth outer surface, lay-up the required number of CSM plies — and then cure the resulting laminate, with or without heat. Slightly faster can be spray-up, especially when automated. Refinements have reduced problems of overspray and styrene emissions, and improved control of the process. New designs of nozzle, for instance, reduce atomisation and work well with unfilled resins while innovations like fluid impingement technology (FIT®) from Magnum Venus Products have proved successful with filled resins. FIT limits emissions by utilising low-pressure impingement of large droplets.

Both hand lay-up and spray-up are suitable for production runs into dozens and will continue to be of interest for prototypes, concept cars, ‘limited edition’ vehicles and a few, like motor caravans, that will never make it into the big numbers but require attractive interior fittings and body parts. But notching production levels up a gear necessitates a change of process. This shift, which faces many producers of niche vehicles when they start to become successful, is well exemplified by US company Commuter Cars Corp, which has great hopes of a nippy traffic-beating tandem two-seat electric vehicle called Tango. Prototypes have been built with the simplest of GRP technologies, lay-up by hand of CSM plies into a coated urethane mould. But it takes nearly five hours to lay up the main body and another 12 hours to cure it at room temperature without vacuum, so for hoped-for production volumes in the hundreds to low thousands, Commuter Cars and its Canadian moulder SLP plan to convert from open to closed moulding. According to SLP production manager Joe Taverna, the choice is likely to be between resin transfer moulding (RTM) and low-pressure compression moulding.

RTM, which relies on highly rigid matched metal tools inside which both heat and pressure are applied, is favoured for its ability to turn out in minutes parts that would take hours or days to produce by traditional open moulding. Resin injected into the closed mould under pressure permeates a dry fibre preform rapidly and can be cured quickly under heat. As well as being fast, the process is repeatable and can be automated to run with minimal intervention. One of the leaders in ‘industrialising’ the process, UK-based company Plastech TT, has developed automated injection systems which mix and inject the resin at optimum pressure for the moulding cycle, then flush out the remnants afterwards, all with high consistency and minimal waste.

Plastech proprietor and managing director Alan Harper points out that a Class A standard of finish can be achieved, on both sides if necessary, if the mould working surfaces are well prepared. While modern vinyl ester surfaces will last for 18 000 parts or more, he says, replaceable tool faces such as Plastech's multiple insert tooling (MIT) system can reduce cost and extend mould tool life almost indefinitely. Another advantage is that the removable faces permit pre-loading of release agent, gel-coat etc, off-line before insertion into the tool. RTM enables parts to be moulded complete with inclusions such as threaded inserts for panel attachments, metal reinforcements and core inserts.

A downside of RTM is the costs of tooling which, with integral injection points, sprues and heaters, can be substantial. However, vehicle manufacturers attuned to the costs of tooling up for steel will find them modest in comparison.

Twins

An alternative to RTM is compression moulding, generally of SMC. According to the Composites Fabricators Association (now the American Composites Manufacturers Association), much of the 90% of automotive composites that are made up of glass reinforced polyester rely on the twin workhorses SMC and BMC. The Ford Motor Company, a century old this year (2003), used SMC extensively in its 2002 Thunderbird, 60% of the car's body panels being of this GRP thermoset material hot press moulded. The company points to the freedom composites give designers to execute aerodynamic and aesthetically pleasing shapes, and to tooling costs around half those needed for steel stamping. The modern Chevrolet Corvette, too, is a largely SMC car, in keeping with its illustrious GRP forebear a half century ago.

French auto maker Renault confirms these advantages of SMC composite, adding that greater parts consolidation within one-piece mouldings reduces production costs overall, while lower tooling costs facilitate periodic style changes and modifications. The company, which adopted SMC large-scale for its composite-bodied Espace ‘people mover’ and has continued its use in its latest Avantime model, says that moulded parts are not only lighter and corrosion free, but resist minor impacts better than steel. Latest SMCs are able to tolerate the high temperatures of traditional auto painting facilities. Most moulded Avantime parts, for instance, are painted on-line at 90°C.

Finish has indeed been a nagging issue with SMC because it has proved difficult to eliminate ‘paint pops’, pinprick imperfections that can arise in a painted surface due to slight outgassing of the composite during the high temperature spray operation. Motor manufacturers would rather have plastics able to tolerate standard finish processes than have to paint them separately at lower temperatures. Various ‘fixes’ have been tried, including sealers like the two-stage curing DynaSeal™ from BASF Coatings.

An alternative approach is to avoid the need for painting altogether by moulding colour in. Some success has been achieved, notably by Plasticolors Inc working with Ashland Specialty Chemicals. Tests showed that a ultraviolet (UV) light-stable SMC introduced by them with pigment already incorporated, maintained its colour even after a 10 000 hour weathering test. Though more expensive than standard SMC, by eliminating the painting step it could prove economical overall.

Another issue with SMC is limited shelf life, aggravated by the difficulty of ensuring that batches have not thickened beyond acceptable limits during storage. BMC/SMC shelf lives have progressively been extended, to around 30–50 days currently while a number of suppliers, DSM Resins for one, is working to develop analytical methods that deliver consistency test results rather faster than the 3–7 days required by current tests.

Some of the SMCs available today far surpass in properties those of the earliest compounds. Manufacturers can, for instance, use S-glass to hike modulus and other mechanical properties. Several groups are working on low-density formulations that more reliably provide a Class A finish. Another example of continuing improvement is the SMCAplus roving recently introduced for automotive exterior Class A-finished body panels. These rovings have been optimised for the higher strength requirements of tailgates, spoilers, deck lids and similar items. First commercial application is the tailgate for the latest Renault Laguna station wagon. Nevertheless, experts like Thomas Schuh, manager body polymer components for DaimlerChrysler, believe that there is further to go. Schuh proposes that there should be greater co-operation between all parties to reach the goal of a premium SMC that is truly suitable for quality exterior panels.

An interesting insight into the potential for moulding compounds was afforded by the JEC Composites Show last April in Paris. The automotive village there was graced by a SMC/BMC concept car exhibited by the

European Alliance for SMC

and its members. An all-composite space frame drastically reduced the weight of this notional car and, clothed with SMC body panels plus major items such as integrated front and rear modules, spoilers, fenders and a rear boot lid, it provided a model of automotive plastics application for the composites community to aspire to. Even parts of the frame, particularly the pillars, were moulded in SMC, in two halves subsequently bonded together.

BMC, despite the material's original bulk connotations, has found a large specialist market in vehicle headlights for which, thanks to the exceptional temperature performance of the latest formulations, some 50 million BMC reflectors a year are being manufactured. High thermal tolerance is needed to withstand temperatures approaching 200°C produced by tungsten halogen lamps. Additives to achieve this, as well as low shrink, good UV resistance and other properties, help optimize resin formulations, while high fibre volumes, with addition of carbon is some cases, have contributed the necessary rigidity and strength. Rapid-cycle injection moulding is used, with cycle times as low as 30 seconds being achieved for this thermoset plastic. A smooth reflecting surface is gained by metalising the moulded surface via a urethane-acrylate base coat, applied as a sealer against minor porosity. BMC suppliers (Bulk Molding Compounds Inc {BMCI} of Chicago and Menzolit Fibron in Europe are just two examples) are seeking to improve formulations and processes still further so that cost can be reduced by dispensing with the base coat.

It is easier to mould plastic than form metal to the exact shapes required for optimum beam focusing. Furthermore, plastic lends itself to part consolidation and designers take pleasure in integrating the latest reflectors with those for sidelights and turn indicator lights, along with overall clear lenses in stylish modules which enhance a vehicle?s aesthetic appearance.

BMC, as well as contributing the characterful ‘eyes’ of a vehicle, is used for under-bonnet applications where certain formulations have proved equal to the hot, corrosive, vibrating environment for which engine compartments are noted. Although distributor caps have become a rarity in these days of electronic ignition, BMC has found a new role as a replacement for stamped steel and die-cast aluminium in engine valve (rocker box) covers. High-modulus formulations enable such covers to be made as direct replacements, with the same bolt holes as those used for metal. Oil filler necks, wiring ducts and other elements are no longer additions as they can be moulded integrally. According to BMCI, while some 10 million plastic covers have been fitted since they were introduced at the beginning of the 1990s, no field failures have been reported. Up to a third lighter than metal, BMC covers are also said to be cheaper. It may be possible to reduce costs still further if present efforts to engineer a high-Tg polyester able to withstand oil at temperatures of up to 150°C prove successful. It would then be possible to discontinue the current use of vinyl ester resin, which is thermally tolerant but more expensive.

BMC has also been enlisted for brush holders and housings in electric motors, which continue to proliferate in modern cars. The new roles the material has found should help maintain market volumes, and hence economies of scale, enabling this enduring automotive plastic to hold its own against lighter metals like magnesium, and thermoplastics.

Bringing BMC and SMC technologies together results in a hybrid called thick moulding compound (TMC). Injection moulded TMC valve covers now on the market equip, for example, six-cylinder engines on Ford Explorer and Ranger models as well as vehicles by General Motors, Jaguar, DaimlerChrysler and Saturn.

Closed moulding

RTM, compression and injection moulding are just part of the closed moulding armoury. For manufacture of large, planar surfaces, typically on large vehicles, manufacturers are developing various resin/vacuum infusion processes such as vacuum assisted RTM (VARTM), vacuum resin infusion moulding (VARIM) and Seeman Composites resin infusion moulding process (SCRIMP). Infusion can be a useful route for producing demonstrator vehicles — one example, a diminutive three-wheel ‘personal transit module’ built by Corbin Motors of California, has a body moulded by vacuum infusing an Owens Corning woven fabric — and is a candidate for small electric and hybrid vehicles.

These processes, as with RTM, rely on infusion of dry fibre stacks or preforms by a highly liquid resin optimized for the purpose. Attempts to produce thicker preforms, to save laying down multiple layers, have been led by textile-based companies with various weaving solutions. However, wovens have fibres that are crimped, and hence weakened, in at least one axis. An interesting variation in which straight fibres are used in all three orthogonal axes has come from US company 3TEX Inc. Perpendicular z fibres are pulled vertically between the fibres in the horizontal x and y layers by the weaving machine needles. 3TEX proprietor Dr Mansour Mohamed says that preform structures up to 1.8 m (72 in) wide and 2.5 cm (1 in) thick can be produced in most reinforcement/matrix combinations. Under a current contract with the US Navy, the company is producing preforms some 15.2 cm (6 in) thick. Fibreglass preforms have been developed for vehicle bumpers, suspension items and structural parts. Multiaxial weaves integrating ±45° angles from braids are seen as a next step for this innovative straight fibre technology.

Attempts are being made to reduce cost by automating the production of preforms. Systems such as Ford's programmable preform process (F3P) used in producing parts for, among others, the Aston Martin Vanquish, can effect substantial savings in volume production.

Another solution altogether, at least for parts having a constant cross-section, is pultrusion. Normally continuous and automated, the process draws rovings, aligned down the major axis, through a bath of thermoset resin then through a heated die which both shapes the section and brings about cure. Emerging drawn section is cut to the lengths required. Despite the initial investment in dies, puller, forming guides and so on, the process can be most cost effective for high volumes. Various continuous strand mats and fabrics (woven braided and knitted) can be used as well as rovings, and the process is today used to ‘pull’ components as large as panels for trucks.

RRIM/SRIM

Infusion processes may find a place in public transport and niche vehicles, but mainstream car makers need technologies better suited to mass production. Here likely candidates include structural reaction injection moulding (SRIM) and its associate reinforced reaction injection moulding (RRIM). These can be regarded as faster cousins of RTM; in either case taking just minutes for a typical moulding cycle. Like RTM, SRIM aims to infuse pre-formed fibres with liquid thermoset resin but, unlike RTM, the resin components (highly reactive isocyanate and polyol) have to be mixed in an impingement head just outside the mould immediately before use. Once mixing has occurred the preform has to be infused fast, before the resin can polymerise. SRIM was employed initially for modestly sized parts but the process has since been re-engineered by members of the Automotive Composites Consortium to suit production of larger parts. This is amply demonstrated by the 1.9 m (6.5 ft) long composite cargo box produced for GM/Chevrolet's Silverado pick-up, the world's largest SRIM produced part when it went into production in 2001. The box weighs 50% less than a conventional welded steel equivalent, is corrosion-free, and can withstand an enormous amount of rough use without suffering scratches and dents.

GM supplier Meridian Automotive Systems developed a preform production cell and equipped it with Cannon Compotec chopped fibre preform technology. Chopper guns fed with a special glass roving developed by Owens Corning deposit 7.6 cm (3 in) long fibres onto a preform screen, under robotic control to ensure even distribution. A glass content of 50% by weight is achieved in a Baydur resin optimized for the process by the Bayer Corp. Resin injection takes a mere 5 seconds. Once this is complete, the two halves of a 2500 ton moulding press are closed together and a two-minute 94°C cure cycle takes place. Meridian has shown it can produce some 320 boxes a day with present facilities. Similar technology is used to manufacture equally rugged mid-gate and inner tailgate modules for other GM pick-ups.

RRIM is another high speed enabler for economic production. An illustration of RRIM benefits is afforded by The Budd Company's use of the material for a Chevrolet pick-up. A previous two-piece steel and SMC fender was replaced by a single-piece RRIM manufactured unit. The replacement fits better and is quicker to produce, with fewer secondary operations. The 12.2 kg (27 lb) fender is produced from 20% mineral-filled Bayflex 190 polyurethane from Bayer's Polyurethane Division, which works closely with Owens Corning Automotive Solutions Business Group.

Though the process is normally regarded as suitable for build runs typically around 10 000, Australian company UP Tooling has developed a variation suitable for shorter runs. Aiming to avoid some of Australia's imports of aftermarket body enhancements such as fenders (bumpers), grilles and side skirts, the company asked Huntsman Chemicals to formulate a special PU resin for the mineral fibre reinforcement it had selected for stiffness, strength and low coefficient of thermal expansion. The result is moulded components that can be primed and clear-coat painted to yield the required Class A surface finish. Recently the company has been supplying thousands of body kits to Ford Australia for its Falcon range and to Ford Malaysia for use in its 4 by 4 Rover model.

According to the 2002 Automotive Plastics Report from Market Search of Ohio, USA, SRIM/RRIM parts are gaining on thermoset SMCs for major components in the light truck market.

Thermoplastics

Thermoplastics are a rising star of automotive composites. One telling advantage is that, unlike thermosets, they are more like traditional plastics in being serially recyclable. They can be melted down and re-moulded a number of times before their properties become too degraded for further use. This feature can survive addition of reinforcement and justifies claims of environmental acceptability. Automotive polyurethane (PU), polypropylene (PP) and polyamide (PA) thermoplastics, suitably reinforced, can be tough, durable, and highly resistant to chemicals and impacts.

Any of the standard fibre reinforcements can be used in short, long or continuous form. Growing in popularity, particularly for applications requiring high mechanical strength and dimensional stability over time, are long fibre reinforced thermo-plastics (LFTs). Long fibres, as well as conferring the usual reinforcement strength and stiffness, prevent creep. Long fibre-reinforced glass mat thermoplastics (LFGMTs), based on alternate layers of thermoplastic resin film and glass fibre mat compressed into a sheet product, are also winning converts among moulders who like to work with sheet.

Used since the 1970s for a range of items such as bumper beams, instrument panels, seat pans, and rocker panels, GMT has recently extended its application scope, becoming what is believed to be the first structural GMT automotive part to be associated with a car's frame. Volvo has used a glass/PP GMT support for the rear differential on the all-wheel drive V70 model it introduced in 2000. Replacing the cast aluminium plate previously used to support the 40 kg (88 lb) differential, the new support also transfers rotational forces from the differential to the chassis and to the rear axle, to which it is bolted. In addition it helps protect the differential from road debris and, thanks to a reflective aluminium heat shield, from heat generated by the exhaust pipe. The 480 mm (19 in) long by 250 mm (10 in) wide part weighs almost 40% less than an aluminium equivalent, which would have been more expensive. Moulder Polytec Composites Sweden AB uses blanks of a special mat produced by Quadrant Plastic Composites to meet the rigorous specification. This is based on a woven form for part thickness and performance, and utilises a standard GMT reinforced with Twintex fabric from European supplier Saint Gobain Vetrotex. Twintex combines glass reinforcement with PP, the co-mingled result being noted for its toughness and impact resistance. Quadrant terms its GMT/Twintex hybrid GMTex.

Manifold benefits

A major success for thermoplastics has been their widespread adoption for air inlet manifolds which, as an essential engine item, have to survive an aggressive operating environment. Dr Michael Fischer, who markets Ultramid® glass/PA thermoplastic for BASF believes that up to 35% of US models now have plastic manifolds, in many cases pigmented so that they resemble the stamped steel and die-cast aluminium manifolds they have replaced! Last year, says Fischer, some 18 million PA manifolds were produced globally, the majority moulded as two halves for subsequent vibration welding together, but 40% being moulded complete, with internal details formed by the lost core process. He estimates that penetration will reach 25% in Europe by 2005, and 65% or more in Japan. Improved resins have proved both reliable and durable, while permitting full integration of other parts like air filters, fuel injection rails and cylinder head covers. But higher engine running temperatures resulting from lean burn and emission reduction technologies have increased thermal stress, prompting BASF to introduce a new 30-35% glass-reinforced Ultramid offering superior performance. Thermal and hydrolysis resistance for the new grade exceed those of PA 66 (nylon) whilst mechanical properties, including suitability for multiple re-processings, are also said to be more than adequate.

Other manufacturers have likewise optimized thermoplastics for this use. For instance, manifolds made from a 30% glass reinforced, heat stabilized polyamide 6 from Bayer's Durathan® range equip the 2.2 litre Ecotec engine used in General Motors' Chevrolet Cavalier, Pontiac Sunfire and Saturn sports utility vehicle (SUV) models.

A logical addition to a GRP intake manifold would be a plastic throttle, thereby making most of the air pathway plastic. This has been brought a step closer by the introduction of glass/PA throttle bodies, notably on several Renault, Peugeot and Citroen models. These bodies, produced in 30 and 35% glass content Ultramid grades, turn out to be some 50% lighter than metal predecessors, as well as less expensive to produce. Because they can be fabricated in optimum shapes, engine performance is enhanced. They have superior vibrational characteristics, and resist attack from fuel and lubricants. Thermoplastic throttle pedals, too, are on the way. An example now in production for a major car producer is that from UK company Birkby's Plastics Ltd. Moulded pedals offer OEMs, as the company points out, choice of design, colour and branding. The variant currently in production is for an electronic ‘drive by wire’ system and incorporates a position sensor.

Another growing under-bonnet application is parts for vehicle radiators and cooling/heating systems. One of the companies with a product able to withstand the hot water/glycol environment is Rhodia Engineering Plastics. Its 25% and 30% glass fibre/PA Technyl resins are enhanced PA 66 grades designed for use at continuous temperatures of up to 130°C and peak temperatures 60°C higher still. They therefore overcome a tendency to deteriorate noted in standard grades. A 60% glass reinforced PA branded Technyl Star has provided stiff and durable parts for air distribution grilles, to which they have also brought enhanced appearance. Rhodia has concentrated on developing resins which flow easily so that processing time and costs are minimized.

Up-front

Outside the car, thermoplastics have an important role in replacing front end modules made from multiple metal pieces. Single-piece moulded carriers are cheaper to produce and can have lights, fan, cooling circuit pipes and other items attached simply and quickly. It is also possible to incorporate conical, columnar or other energy absorption devices to mitigate the effects of frontal collisions. This approach was famously taken by Renault for its Clio front end, though in that case the cone system used was of BMC.

One interesting front end example is that from the ‘new’ Mini produced by BMW, which marked the debut of a new material, StaMax P. Produced by StaMax Inc, a collaboration of DSM Engineering Plastics with the Owens Corning Automotive Solutions business, the long fibre-reinforced PP is described by StaMax general manager Leon Jacobs as ‘bridging the gap between short fibre compounds and GMT’. Because the composite's properties and moulding process were tailored to the carrier application, says Jacobs, system cost is reduced. StaMax P can, he asserts, replace GMT in certain applications and is able to be processed on normal injection moulding, injection-compression moulding and extrusion-compression moulding equipment, with minimal further investment being needed. Andrew Hopkins, general manager Owens Corning Automotive, says that StaMax is an example of how composites offer material alternatives that are flexible and easy to process. “This is especially important today as there are more and more niche vehicles,” says Hopkins. “The costs of retooling to build these in metal, in limited production volumes, would be high.”

StaMax P and similar LFTs could be just as applicable to the carriers that exist inside car doors to support window winding mechanisms, handles, locks, speakers, cable trays, clips etc. In fact such a carrier, now being supplied for the Ford Fiesta in Europe, is the second major application for StaMax, in this case a 30% glass loaded PP grade.

Thermoplastics are also big in fenders, closures such as tail gates, covers, door trim and some interior components such as instrument panel mouldings. Atofina's Pyltrex®, for instance, is a long glass fibre reinforced PP used for several of these items, including robust bumper beams that can absorb the energy from minor ‘bumps’ without damage. Creep resistance conveyed by the long fibres was critical in the selection of this material for the under-engine shield of the Citroen C5, while it is also used for the engine cover on the Renault Laguna 2. Grades with fibre lengths of 12–25 mm are suitable for traditional forming technologies including injection and compression moulding. TowFlex®, a continuous glass fibre reinforced PA 6 from Hexcel Composites, was selected for the re-cyclable bumpers fitted to the BMW M3 sports coupe. Replacing with an integrated unit a previous metal bumper beam and separately produced shock absorbing components, the new bumper has beams and crush columns made with a fast matched-mould thermoforming process using flat sheet stock and tubular profiles. High frequency welding is used to join the parts together. The new unit is some 60% lighter than the old.

Another material featuring in bumpers (fenders) is the Saint Gobain Vetrotex Twintex hybrid, a dry prepreg which has polypropylene in both the reinforcement, where it is co-mingled with glass, and as the matrix. Up to 70% fibre volume fraction can be achieved. Noted for its combination of high impact strength plus flexural and tensile strength, this injection-mouldable thermoplastic is used for, among other items, the bumper beam on Chevrolet's Venture minivan, and a range of automotive externals including tailgates and other closures. In lower grades, Twintex is used for parts of passenger interiors and instrument panels.

Twintex, with a similar aligned glass/PP material, Plytron, figure in a programme currently being undertaken in UK by the Warwick Manufacturing Group at the University of Warwick, BI Composites and SCL (Security Composites Ltd). The APPLE (Advanced Polymeric Panels with Low Environmental Impact) programme, aimed at extending the use of thermoplastics in the automotive sector, is examining the potential for vacuum forming exterior panels.

US market researcher BRG Townsend Inc confirms that long-fibre thermoplastics are one of the fastest growing plastic materials, and estimates that consumption in 2001 was some 59 million kg (130 million lb). Growth would be further accelerated by any substantial migration into car bodies, so it is interesting that DaimlerChrysler is considering it for side panels in an enlarged four-passenger version of its Smart car. Dow Automotive appears to have solved one of the few difficulties associated with LFT, having developed a new adhesive which can successfully bond to the low-energy surface of PP, enabling thermoplastic parts to be bonded to each other and to metal.

A further filip to LFT prospects is likely from technology designed to allow OEMs to compound the materials for themselves, thereby eliminating the need to store and use semi-finished product. Self-compounding at time and point of use fits well with just-in-time manufacturing, reducing both warehousing/logistics costs and energy consumption. Self compounding lines pioneered by Fraunhoffer Institutes and commercial companies in Germany are already in service. Manfred Bruemmer of Dieffenbacher GmbH told delegates at the Society of Plastics Engineers (SPE) automotive composites conference in September, held at Michigan State University, that the LFT-D on-line compounding process for glass/PP, now well established in Europe, has recently reached the US too. Potential to automate in-line compounding may result in further cost savings in the future.

Advanced

Race cars, a miracle of low weight, strength, stiffness and crashworthiness, rely on advanced composites, mainly carbon/epoxy. But penetration of these materials into the production car sector has been negligible, until recently.

A number of carbon concept cars — most notably GM's Ultralite in 1992 and BMW's Z22 a couple of years ago — have shown what can be done. The Ultalite's monocoque body weighed only 191 kg (420 lb) and was stiffer than steel. Twenty CFRP parts for the Z22 yielded a 50% weight saving compared with steel. But carbon is expensive to buy and difficult to process. To illustrate, the material in the ‘92 Ultralite cost 18 times what the appropriate amount of steel would have cost. And, although this prototype was hand-laid, processing of carbon has often been by the established, but pricey, aerospace route of moulding from pre-impregnated materials and curing at high pressure and temperature in an autoclave.

Today ultra-high modulus carbon remains expensive, but efforts to bring prices down for material having lower but still highly competitive modulus have been succeeding. The visionary work of Zolt Rummy and his Zoltek company in targeting $5/lb and building production capacity ahead of demand in order to break the ‘low demand, therefore high price, therefore low demand’ circle, is particularly impressive. Unfortunately, it looks as if Zoltek may have over-extended itself but, even if it does not get to enjoy the fruits of its efforts, industry insiders believe that other manufacturers will soon be able to deliver prices of $6–7 by avoiding aerospace overheads and producing the material on substantial scale from inexpensive PAN precursor. This level of affordability will, many believe, transform the prospects for carbon/graphite solutions.

Initially, carbon has been making its mark in components and parts, an example being hard tops now being moulded for Morgan sports cars by Autocarbon.com Inc of California. Owner James Miller has set his company, which also supplies carbon spoilers, bumpers, hoods wings and other components, the goal of becoming a leading provider of carbon composite items to the automotive industry. Another application is composite drive shafts that are lighter and last longer than metal predecessors. A race car driveshaft has, for example, been produced from Hexcel Composites' HexPly unidirectional carbon/epoxy prepreg and other shafts have been produced by filament winding.

Substantial use in vehicle bodies will come first to the performance and ‘supercar’ sector. Ferrari has produced an extensively carbon model and Ford is expected to have CFRP body panels on its new GT40 model, due for launch this year as something of a centenary celebration. The limited edition vehicle will recreate in modern form the classic that gave Porsche and Ferrari a run for their money in the 1960s.

But greater penetration will require simpler process technologies than those used for carbon hitherto. One likely candidate is compression moulding of sheet product. Two years ago Zoltek introduced a commercial grade large-tow carbon fibre intended for the SMC market, and US auto makers are known to be working on development of carbon SMCs. Indeed, a carbon-based SMC used on the 2003 Dodge Viper is believed to be the first volume production use of CFRP in the motor industry. In Europe Hexcel Composites has launched a carbon/epoxy SMC with 60% fibre content. With an average five-minute cure time, this material is intended for compression moulding of small to medium-sized components.

A company that says it has a mission, to ‘take the “advanced” out of advanced composites’ is SP Systems in the UK. First developing its PRIME low-temperature curing epoxy resins for moulding under vacuum bag pressure in ovens rather than in expensive autoclaves, it then went on to introduce its SP resin infusion technology, SPRINT™. This ‘semi-preg’ comprises resin film interleaved with layers of carbon (or glass) fabric. Applying heat and vacuum causes the pre-catalysed resin to migrate outwards and, because the resin only has to infuse across the thickness of the material, wet-out is rapid and thorough. The dry layers provide escape paths for air being sucked out by vacuum. Fabricators using SPRINT have been able to achieve high-quality void-free laminates. There are even greater hopes for a form of the product that incorporates syntactic material enabling a laminate thick enough to mimic 1 mm steel (for example), to be laid up as a single ply. It can also avoid the need to use sandwich construction to achieve panel stiffness. SPRINT CSB infuses and cures rapidly, in sharp contrast to the laborious processing of conventional prepregs. It can yield ‘ready to go’ surfaces with minimal rework needed to achieve a Class A finish. On-going panel tests suggest that print-through is not a problem, though use of a surface veil will obviate this risk altogether. Epoxy resins with their low volatiles and high stability when cured, can be painted without the precautions needed for SMCs.

According to Joe Summers of the company's automotive group, nearly every recent supercar has utilized SPRINT or SPRINT CSB. Ronart was the first and TVR, currently in pre-production with its Tuscan R model, is the latest. The latter, with its CFRP body on a steel tubular chassis, is a connoisseur's car and production is expected to be in the dozens rather than the hundreds. A recent agreement to supply a leading European manufacturer (identity undisclosed) with body panels for a new sports model should take production levels to several hundred a year, possibly rising to over 1000 eventually. Summers believes that, given progress being made in a body panel prototype shop at the company's Newport, Isle of Wight, headquarters, it will be possible by 2004 to process up to 2000 panels from a single mould. He says the material will also come into its own for sales differentiating enhancements, such as carbon bonnets (hoods) on cars where retooling for steel, given limited production runs, would be prohibitively expensive. A Porsche Boxter, for example, has appeared at various composites and motor shows sporting a SPRINT CSB composite bonnet, which weighs 75% less than the metal equivalent.

Similar ‘semipreg’ technology has also been developed or is under development by Hexcel Composites (HexFIT™), Advanced Composites Group (Z-Preg), Cytec Fiberite and others. However, full prepreg technology is likely, experts believe, to remain in use for the primary vehicle structure (chassis, space frame) where stiffness and strength are vital. More structural emphasis has been noted in Europe than in the US where the greater focus has been on body panels.

Carbon thermoplastics, for some time evident in aerospace, are now coming into the automotive business. Preformed blanks of 60% carbon fibre loaded PA were used by Colorado based Hypercar for its light, fuel-efficient, sports utility concept vehicle, the Revolution. Hypercar says that the novel high-rate compression stamping process it used to produce the prototype, whose 62-part body structure weighs only half as much as a comparable metal body, could deliver a production run of 50 000 plus at competitive prices.

‘Greening’

Composites may soon face their greatest challenge, the recycling issue — and their biggest opportunity, the ‘greening’ of road transport.

Experts accept there could be a problem. A researcher at Ford's Dunton research centre, UK, told Reinforced Plastics that a certain amount of ‘rethink’ was taking place in view of the European Community's strict end of vehicle life requirements. He thought it would be ironic if we were pushed back towards metals for their inherent degradability. Alan Foster, global product leader reinforcements for Owens Corning agrees that recycling is a challenge but thinks that the current major research focus around the world will deliver answers. Moreover, he says, recyclability should not be over-emphasised as safety and energy efficiency may be just as important.

David Cripps of SP Systems points out that much depends on what is meant by recycling. If another use for reclaimed material outside the automotive sector is acceptable, then it is easier to manage the situation than if ‘a piece of car has to be recycled to another piece of car’. Separation of phases (fibres from matrix) using fluidised beds, pyrolysis or other means may not be feasible or economically worthwhile, leaving grinding into small fragments the only viable option. Hence solutions like IDI's use of post- processed coarse ground sprues, runners and cured parts mixed back into BMC are a possible way forward.

Thermoplastics are inherently more recyclable than thermosets but even these reach the end of the road, due to progressive degradation of properties. Nevertheless, with their amenability to repeated thermoforming, they do go some way to addressing the issue. Thermoplastic recyclate can be added to new material in proportions that do not excessively reduce properties. A one-step long fibre transfer moulding process devised by German company Ramaplan Anlagenbau supports this possibility by accommodating recyclate of a number of thermoplastic types including GMT.

High fractions of dense carbon or other fibres can reduce recyclability. Researchers at Queen Mary, University of London, UK, suggest that this can be avoided by using thermoplastic fibre to reinforce thermoplastic matrix. Thus a PP/PP, dubbed PURE, developed by the University in collaboration with the Advanced Materials Centre at Ford, Dunton, UK, is a material with similar properties to GRP which can be melted and remoulded as a single entity, without inhibition from fibre content. The PP fibres do not seriously degrade during thermal recycling and Ford's Alan Harrison believes it should prove possible to substitute them for glass in a range of items including body panels, under trays and bumper beams.

“We want this with no loss of quality, no increase in cost and no reduction in function,” Harrison told us. “PURE offers hope that we can achieve that aim.”

Most recyclable of all would be naturally derived fibres and resins, which are combustible, compostable, renewable and carbon dioxide neutral. These substances have the further advantages of being widely available, and very low in weight. This all explains why manufacturers around the world are experimenting with fibres from jute, hemp, kenaf, soy, cotton, hemp, flax, coir, kapok, coconut and bast; and resins based on soybean and corn. Mercedes Benz introduced jute-based door panels into its A-Class vehicles as long as eight years ago. The North American market alone for natural fibre composites is growing at some 50% annually, from a base of $150 million in 2000. A landmark agreement between Kafus Biocomposites and Ford supplier Visteon Automotive Systems should see natural fibre composites used for items such as in interior panels, linings and fittings. Although a natural fibre/thermoset composite might only be half as strong as a glass composite, and even less stiff, this can sometimes suffice and is appropriate technology. Other companies active in natural fibres include UK hemp producer Hemcore Ltd, and Cargill Ltd of Canada who market short and long fibre items made from flax and other agricultural products. Kafus Biocomposites is probably furthest ahead since sizeable kenaf composite manufacturing facilities exist.

In resins, a notable initiative is that by Ashland Specialty Chemicals' Polymer Composites Division to combine grain-derived organic systems with polyester. The company reports, for instance, performance equal to or better than current polyesters for a hybrid in which a quarter of the total resin was renewable.

Mega opportunity

Balancing the recycling challenge is a major opportunity for composites within an impending automotive revolution. The technology of fuel cells, in which the controlled combination of hydrogen fuel with oxygen from the air produces electricity, heat and water, is widely seen as the natural successor to internal combustion of oil-derived fuels, which one day will no longer be available. Composites can be used for the cases containing the battery-like cells, and for the conductive bipolar plates which encase the active electrode and membrane assemblies. Work to incorporate them is already surprisingly advanced.

Several BMC suppliers are focussing on the bipolar plates, where vinyl ester based systems highly filled with con- ductive graphite can serve instead of metals previously used, with substantial weight benefit. This could be another instance in the BMC tradition of capturing an application which is small in terms of an individual vehicle, but vast in total potential. Each one percent of the automotive market could, it has been estimated, consume between 50–100 million lbs of the material.

Carbon, as a component of BMC or in some other form of composite, could have a definite roll in bipolar and end plates. For instance, GRAFCELL® flexible graphite from Ucar is used in bipolar plates supplied to fuel cell leader Ballard Power Systems. Porvair Fuel Cell Technology relies on a slurry moulding process developed by the US Department of Energy's Oakridge National Laboratory to mould carbon fibre/phenolic bipolar plates complete with necessary flow channels.

An associated requirement is that to contain hydrogen fuel. In the future every road vehicle may have, instead of today's fuel tank, a strong cylinder in which hydrogen is contained under pressure. The task of achieving this safely is a ‘natural’ for composites since metals, apart from being heavy, are apt to fatigue and can be embrittled by hydrogen. Because carbon and other composites are so strong in tension, the walls of cylinders made to withstand internal pressures of between 5000 and 10 000 psi do not have to be unduly thick. A typical structure comprises a plastic (sometimes metal) liner over-wrapped with resin-impregnated continuous filament carbon. During manufacture, the liner acts as a mandrel for filament winding of wet or prepreg overwrap fibres. Cylinders must be certificated to rigorous standards laid down by the International Standards Organisation and national authorities.

Canada's Dynetek is using specially modified European filament winding machines to achieve rapid production since efficient manufacture will be key to competing successfully in this potentially huge market. IMPCO spent $100 million engineering gaseous fuel system technology for future hydrogen powered vehicles. Each of its Quantum Type IV TriShield cylinders has a one-piece ultra-high molecular weight polyethylene liner overwrapped with multiple layers of carbon/epoxy laminate, including a proprietary external protective layer to confer impact resistance. TriShield cylinders are part of the hydrogen fuel system on Hyundai's Santa Fe fuel cell sports utility vehicle (SUV).

This is an application in which composites clearly surpass all alternative materials. Another is likely, judging from models that already exist, to be the production of electric and hybrid electric vehicles that will be the fuel efficient cars of tomorrow. Plastics already figure strongly in several fuel cell-powered concept cars. Future application could extend to full integral body/chassis structures for all these necessarily low-weight vehicles. If the progress composites are now making in concept and niche vehicles is maintained, this will open up immense prospects for them in the radical new automobiles of 10 to 20 years time. Because these materials and the future hydrogen economy belong together like the horse and carriage from a previous transport era, composites could make it into the mass production automotive mainstream ‘big time’.

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