WERNER HUFENBACH, MAIK GUDE, SIRKO GELLER�), ANDRZEJ CZULAKTechnische Universität DresdenInstitute of Lightweight Engineering and Polymer TechnologyHolbeinstraße 3, 01307 Dresden

Manufacture of natural fiber-reinforced polyurethane composites

using the long fiber injection process

Summary — This paper gives an overview of recent activities in the production of natural fiber--reinforced polyurethane (PUR) composites. Extensive research is being conducted to developmarketable solutions for the production of bio-based plastics. In composite applications, naturalfibers can be used as a substitute for synthetic fibers and help to make a significant contributiontowards ecological and efficient lightweight components. In addition, an outlook on current deve-lopments in Long Fiber Injection (LFI) processing for the application of natural fiber-reinforcedpolyurethane composites, specifically in electric vehicles is given.Keywords: natural fibers, fiber reinforced polymers, long fiber injection, polyurethane.

WYTWARZANIE KOMPOZYTÓW POLIURETANOWYCH WZMOCNIONYCH W£ÓKNAMINATURALNYMI W PROCESIE WTRYSKIWANIA Z U¯YCIEM D£UGICH W£ÓKIENStreszczenie — Niniejszy artyku³ zawiera informacje na temat aktualnych trendów zwi¹zanychz produkcj¹ kompozytów poliuretanowych (PUR) wzmocnionych w³óknami naturalnymi. Zwró-cono w nim uwagê na szeroko zakrojone badania nad opracowaniem rozwi¹zañ w zakresie pro-dukcji tworzyw polimerowych na bazie materia³ów biologicznych, nadaj¹cych siê do wprowadze-nia na rynek. W³ókna naturalne mog¹ byæ stosowane jako substytut w³ókien syntetycznych poma-gaj¹c w otrzymaniu lekkich materia³ów i s³u¿¹c jednoczeœnie ekologii. Omówiono tak¿e perspek-tywy rozwoju zastosowañ procesu wtryskiwania z u¿yciem d³ugich w³ókien (LFI z ang. long fiberinjection) w szczególnoœci do wytwarzania czêœci do pojazdów elektrycznych.S³owa kluczowe: w³ókna naturalne, polimery wzmocnione w³óknami, wtrysk z u¿yciem d³ugichw³ókien, poliuretan.

INTRODUCTION

Natural fiber-reinforced composites have tradition-ally found applications mainly in the automotive indus-try, replacing primarily glass fiber-reinforced plastics[1—3]. In addition to the obvious environmental issues,another motivation for natural fiber application is thepotential for weight saving, since natural fibers havelower density than glass fibers.

The application of natural fibers gives rise to severalchallenges, especially with regard to reproducibility ofmaterial properties. Further, no limitations in compari-son to the currently used synthetic fibers concerningsmell, aging or fire behavior are likely to result. To ensurea reproducible quality of a natural product seems to bethe ultimate challenge. Here, the fiber preparation andfurther processing of the natural fibers play a decisiverole. For good composite quality, the structure of the fiberbundles and the resulting fiber-matrix adhesion are of

special importance. This requires specific solutions,depending on the types of fiber and matrix material used.

Natural fibers are traditionally integrated into fibercomposites in the form of mats or other non-woven fab-rics [4]. In addition to several other thermoset or thermo-plastic matrix systems, especially polyurethanes (PUR)have been reinforced with natural fibers. A classicmethod for processing non-woven reinforcements isstructural reaction injection molding (SRIM). In thismethod, fiber mats are cut or punched to net shape andfurther processed into a preform. They are optionallythermoformed and then inlayed into the mold. Compar-able to resin transfer molding, the preform is placed in aclosed mold with a thermoset polyurethane system. Afurther possibility for producing fiber-reinforced PURcomposites is to spray the reinforcement with the matrixsystem followed by compression molding. Therefore,various PUR spray coat methods are available.

However, cutting of the semi-finished products aswell as preparing and handling the preform representtime and cost intensive process steps. Further, the re-

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�) Corresponding author; e-mail: s.geller@ilk.mw.tu-dresden.de

maining pieces of mats produce waste, which is harmfulfrom the ecological point of view. For these reasons thereis an increasing demand for a cost-effective alternativefor the production of reinforced PUR parts. Since pro-ducts containing long fibers can have properties compa-rable to a non-woven reinforcement, the long fiber injec-tion (LFI) process was developed by KraussMaffei Tech-nologies GmbH. This highly productive technology ischaracterized by the direct processing of rovings in aone-step, fully automated process.

Long fiber injection and natural fiber injectionprocess

The LFI process was developed as an economical al-ternative to SRIM and is characterized by the processingof reactive polyurethane systems in conjunction withlong fiber reinforcement [5—7]. It is particularly suitedfor the production of large lightweight components, es-pecially for carriers or cover panels in interior and exte-rior of passenger and commercial vehicles [8, 9]. Typicalinterior applications for example are dashboard carriers,door trim panels, parcel shelves or trunk floors. Exteriorcomponents like roof modules are produced in the LFIprocess using foam-backed thermoplastic films, whicheliminates subsequent painting [10].

The operation principle of the LFI process is based onthe processing of highly reactive two component poly-urethane systems. These are mixed together in a mixinghead within a very short time using a high pressure coun-ter-flow injection procedure. The mixing head is pre-sented schematically in Figure 1.

This allows for very low cycle times and thus the pro-duction of parts for automobile applications. The rein-

forcing glass fiber rovings are fed directly from a spoolinto a cutting unit on the mixing head and then cut to de-fined lengths. Fiber lengths of 12.5 to 200 mm are theore-tically possible, but usually fiber lengths up to maximum100 mm are used. The chopped fibers are conveyed bymeans of blowing air through the mixing head to its out-put, where they are wetted with the output componentstream. The spray cone can be influenced by oscillatingair, for example to create a wider spray angle or even toalign the fibers [11]. During the discharge, the fiberlength and fiber mass content can be varied locally andadjusted to the loads of the component.

The fiber-PUR mixture is discharged into an openmold, whose cavity is covered with the material. Afterthe mold is closed, the matrix material cures, whereby itexpands to varying degrees, depending on the PUR sys-tem used. Due to the spray-up like discharge, the highpressures usually associated with filling the cavity withlong fibers are not necessary. Changes in the cross sectionof the part such as stiffening fins occur during the expan-sion process of the foaming material. Thereby, expansionpressures usually do not exceed 1 MPa. The moderateprocess parameters, especially in comparison to com-pression or injection molding, are advantageous for theintegration of functional elements [12, 13].

In comparison to other composite technologies likeSRIM, in the LFI procedure processing steps for prepar-ing, thermoforming and handling of textile preforms areeliminated. Consequently, the process time and invest-ment costs of the required installations can be saved. Lastbut not least, due to the direct processing of rovings,waste of glass can be avoided. Thus, load-adapted light-weight components with high strength and stiffness canbe fabricated much more economically and ecologicallysustainable then before.

Cover panels manufactured in the LFI process aretraditional applications of natural fibers. Early on,especially door trim panels have been reinforced withnatural fiber mats [14, 15]. Consequently there is astrong motivation for adapting the LFI technology tothe usage of natural fibers. However, since naturalfibers cannot be broken such as glass fibers, the cuttingtechnology is a special challenge. The high output rateresulting from the requirements of a serial productionwith short cycle times is particularly difficult toachieve with natural fibers.

In 2003, KraussMaffei Technologies GmbH presentedthe natural fiber injection (NFI) process as an evolution ofthe LFI technology [16]. The cutting system was adaptedto the requirements of natural fiber processing, retainingthe specific technological advantages of the LFI process.Sisal or sisal hemp-blend fibers were used as reinforce-ment. Although the method could have proved its func-tionality, the breakthrough failed to appear. Today, due toan increasing demand for natural fiber applications, thedevelopment of appropriate cutting methods for naturalfibers is required.

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polyol isocyanate

glass fibres

cutting unit

venturi nozzle

mixingchamber

Fig. 1. Scheme of the LFI mixing head

Application of natural fiber-reinforced compositesin electric vehicles

As a part of the efforts to reduce the emissions of car-bon dioxide, the trend to electrification in the automobilesector is evident. For electric vehicles, lightweight con-struction is a crucial factor and even more important thanin conventionally powered vehicles, due to the currentlylimited ranges and the heavy weight of batteries. In thispurpose and in the context of conservation of resources,the use of natural fiber composites is of particular inter-est. For this reason, within the framework of the EU-pro-ject MATLEV together with the Warsaw University ofTechnology and SZTK TAPS company, at the TechnischeUniversität Dresden (Institute of Lightweight Engineer-ing and Polymer Technology) novel solutions are in de-velopment for the production of high load-bearing light-weight components with improved sustainability for usein electric vehicles. One part of these research activities isthe development of the LFI process for the production ofnatural fiber-reinforced polyurethane composites. Thisincludes extensive manufacturing studies on the applica-tion of natural fibers in several forms of reinforcement,such as long fibers or continuous fibers and several speci-fications concerning linear density, fiber length or fiberpreparation. With regard to applications in electric vehi-cles, adapted material and process parameters are in de-velopment to meet the requirements in reproducibility ofmechanical properties. For the integration of long fiberreinforcements, suitable cutting methods are being deve-loped.

The variability in terms of fiber mass content and fiberlength as well as the resulting specific values in the LFIprocess are very high, but however, due to long fiber rein-forcement, the level of mechanical properties is limited.For highest mechanical loadings, continuous fiber rein-forcement is necessary. Therefore, corresponding solu-tions are developed which enable automated processingof natural fiber textile semi-finished products. This in-cludes appropriate handling systems as well as adaptedmold designs for reproducible positioning of the rein-forcing layers.

CONCLUSIONS

The motivation for the usage of natural fibers is stron-ger than ever before. Beyond weight reduction, especially

in the automobile industry but also in other industrialfields, there is a high demand on processing of ecologi-cally efficient materials. To further increase natural fiberapplications, research is necessary on reproducible quali-ties in terms of processability and resulting mechanicalproperties, which both are essential in industrial applica-tions. Beyond improvements in natural fiber processing,corresponding manufacturing technologies have to bedeveloped which enable automated production of fibercomposites. Here, the LFI process provides a great poten-tial for the fabrication of long fiber-reinforced compo-sites.

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