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Thermoplastics – particularly reinforced thermoplastics – are on a roll, steadily making progress in all markets. Converting processes have been improved, too, and the use of long fibres has developed. Here are the latest trends in that area.
(Published on June-July 2005 – JEC Magazine #18)
In the quest for more affordable materials with the lowest possible specific weight, automotive and aeronautical engineers in particular have discovered a new way to lighten the load. Thermoplastics reinforced with long fibres boast not only savings in weight, but also a string of other advantages. Components made from long-fibre thermoplastics, or LFTs, are endowed with greater strength and reliability as well as increased functionality and a longer service life. What’s more, direct LFT (D-LFT) and other such novel processes add efficient and inexpensive processing to the list of pros.
Long gone are the days when a narrow rectangle carved in the car roof – a socalled sunroof – attempted unconvincingly to create the illusion of driving in a roadster with the wind whipping through your hair. The latest generation of cars are fitted out with expansive new-style panoramic roofs and are advertised as offering a new sense of space and heavenly views. In the case of the Citroën C3 Pluriel, for example, the roof retracts completely. The BMW X3 and 5 Series Touring, as well as the latest VW Golf and Opel Astra, also have sport roofs that open up wide.
In order to open up the coveted view of the sky without sacrificing the stability of the roof unit, key components need to be both strong and lightweight – a veritable juggling act for designers and engineers, which is why long-fibre thermoplastics seem such a saving grace.
Head in the clouds
For drivers of French automaker Citroën’s Pluriel, all it takes is the touch of a button and a couple of flicks of the wrist to have their head in the clouds. The Pluriel’s retractable roof provides the key to transforming the conventional saloon into a convertible, spider or pick-up with magical ease. This multifunctionality hinges primarily on a special frame, which has the roof/rear window assembly stowed away in the boot in a matter of seconds. Coloured to match the special Citroën shade of grey, the frame is manufactured from a polyamide (PA 6.6) reinforced with long glass fibres, supplied by Ticona GmbH.
Ticona is also the supplier of a glassfibre- reinforced polypropylene (PP) used to produce a part that accommodates the rear-window section of the Pluriel’s roof unit. The part is run off using outsert moulding, a special variation on injection moulding where all the functional elements can be moulded directly onto a metal base. The polymer reinforced with long glass fibres combines the required strength with the necessary functionality. Both of these partially-visible LFT components are not only integral to the overall design of the roof construction, but also provide an aesthetically pleasing solution.
It all started with GMT
So these components are not there just to make the car attractive – they also help to trim the weight and assembly costs and to improve mechanical properties. Notably, the strength, stiffness and heat deflection temperature of parts are altered distinctly for the better with LFT materials. The degree to which physical properties are bumped up depends on the length, quantity and orientation of the fibres. Not least among the influences on the physical properties is how carefully the mixture of fibres and matrix is processed. In this regard, there is still plenty of scope for development. The search is still on to find the ultimate method of embedding the longest possible undamaged fibres into polymers (and thus into end-products) under the most favourable conditions and at the lowest cost.
The reinforcing method with the longest history is glass-mat thermoplastics (GMT) technology as it has long been practised in the manufacture of glass-fibre reinforced plastics (GRP). Here, semifinished glass-mat thermoplastics are generally formed in presses. Depending on the requirements to be met, the choice expands to include other technologies. Topping the list of alternatives is the conventional injection moulding of long-fibre thermoplastics, while transfer moulding is the technique employed when long-fibre pellets are processed. In recent times, D-LFT technology has not only cut itself a niche in the market for processing long fibres but is strengthening its position.
Skipping the semi-finished product stage
One of the pioneers of intelligent D-LFT technology is the press manufacturer Dieffenbacher. As early as the start of the long-fibre trend, Dieffenbacher came up with the ingenious idea of using in-line compounding (ILC) to bypass the semifinished- product stage in the conventional and decades-old GMT process. Here, the necessary compound material, for example consisting of a polypropylene matrix with the relevant additives and 20% to 40% glass-fibre content, is mixed in a compounder and transferred directly to the compression press, where the melt is formed into a component. This eliminates both the cost of manufacturing semifinished GMT products tailored to size for processing, and supply-chain costs. On top of that, energy consumption shrinks because the glass mats no longer need to be heated.
Developed in the final years of the last century, the new D-LFT technology was showcased by Dieffenbacher at Düsseldorf’s K 2001, where it was already being displayed in combination with in-line compounding. In the same year, the company collected the “Innovation Award” presented by the Frankfurt-based Arbeitsgemeinschaft Verstärkte Kunststoffe Technische Vereinigung (working group for reinforced plastics – technical association or AVK-TV). Plant design for the technology centres around two separate units for producing the fibre-reinforced melt. A conventional twin-screw extruder (the compounder) prepares the polymer, usually polypropylene (PP). Downstream, the fibres are mixed wet with the now viscous and additive-enriched melt in another twin-screw device. Finally, a standard hydraulic press forms the LFT parts. At the end of March 2004, Dieffenbacher unveiled an expanded version of the D-LFT/ILC concept. The newly developed process is the fruit of collaboration with the Fraunhofer Institut für Chemische Technologie (Fraunhofer Institute for Chemical Technology – ICT). It opens the way to using high-quality engineering plastics, including polyamide (PA 6 or PA 6.6), polybutylene terephthalate (PBT), and ABS. This optimised approach to the direct processing of LFT is also said to offer potentially dramatic savings. According to the project’s partners, the prototype plant turned out parts at a considerably slashed price – up to 35% in components containing varying proportions of glass fibres – compared to that of the traditional semi-finished products method.
PP and PA as polymers matrix
As a supplier of engineering thermoplastics, Ticona also ultimately benefits from the success of this development. In Kelsterbach (near Frankfurt Airport), glass, carbon and aramid fibres and, more rarely, stainless-steel filaments are employed as reinforcing materials. Ticona’s portfolio already boasts two ready-formulated long-fibre-reinforced thermoplastics with its Celstran and Compel products. These are produced with a specially patented pultrusion process, using PP or PA as matrix polymers. Celstran features fibres 12 millimetres long, and Compel, 25 millimetres. Fibre proportions range from 30% to 60%. The specific application is what determines which plastic is best suited as the matrix and what fibre content to use. Ticona’s LFT products can be used in nearly all of the current methods, including injection, transfer and blow moulding, as well as extrusion and thermoforming. The processing of such thermoplastic varieties is purely physical, of course, while the curing of thermoset-moulded components and the resulting cross-linking are chemical processes.
Fibres from nature
The choice of long fibres responsible for the long life and high strength of plastic components is not necessarily limited to glass, carbon or aramid. Natural fibres, such as flax in a PP matrix, also have the potential to break into the circle of conventional reinforcing fibres. Research carried out at the Thüringisches Institut für Textil- und Kunststoff-Forschung e.V. (Thuringian Institute for Textile and Plastics Research – TITK) in Rudolstadt has shown that composites reinforced with natural fibre mats, with fibre lengths of over 30 millimetres, have already made themselves at home on the production line. One such application that is spreading the word about natural fibres is for automotive interior trim panels made from flaxreinforced PP.
Activities in Rudolstadt have proven that such composite materials can be successfully processed via extrusion. Not only does this unlock a greater degree of structural design freedom but production waste can be eliminated – or at least minimised and recycled inline. However, the approach has yet to set a precedent with natural fibres in the industry. For this reason, the TITK’s investigations switched over to the manufacturing of composites reinforced with long natural fibres through a plastification and compression-moulding process. Working with Dieffenbacher, TITK made use of D-LFT, this time modified to meet the demands of natural fibres and rounded out with industry-friendly long-fibre metering.
Hard evidence that the process works comes from the plant in the Dieffenbacher technical centre in Eppingen, where automobile underbody panels were made from PP reinforced with sisal and flax. With an adapted layout strategy, it only took a few attempts to achieve fully-filled moulds and good surface quality. The Thuringian Institute, however, also established that not all parameters of the manufactured components’ inherent physical values are yet on a par with those of compressionmoulded glass-fibre products.
But the researchers in Rudolstadt still feel confident that the current properties displayed can be upped by further optimising processing conditions, fibre-matrix bonds and by the inclusion of an appropriate impact modification additive.