Manufacture of natural fiber-reinforced polyurethane composites using the long fiber injection process

Manufacture of natural fiber-reinforced polyurethane composites using the long fiber injection process
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  WERNER HUFENBACH, MAIK GUDE, SIRKO GELLER  )  , ANDRZEJ CZULAK Technische Universität DresdenInstitute of Lightweight Engineering and Polymer TechnologyHolbeinstraße 3, 01307 Dresden Manufacture of natural fiber-reinforced polyurethane compositesusing 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 WTRYSKIWANIAZ U¯YCIEM D£UGICH W£ÓKIEN Streszczenie  — 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-dukcjitworzywpolimerowychnabaziemateria³ówbiologicznych,nadaj¹cychsiêdowprowadze-nianarynek.W³óknanaturalnemog¹byæstosowanejakosubstytutw³ókiensyntetycznychpoma-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,agingorfirebehaviorarelikelytoresult.Toensurea reproducible quality of a natural product seems to bethe ultimate challenge. Here, the fiber preparation andfurther processing of the natural fibers play a decisiverole.Forgoodcompositequality,thestructureofthefiber bundles and the resulting fiber-matrix adhesion are ofspecial importance. This requires specific solutions,dependingonthetypesoffiberandmatrixmaterialused.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- POLIMERY 2013,  58  , nr 6 473  ) Corresponding author; e-mail:  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 mixingheadwithinaveryshorttimeusingahighpressurecoun-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, butusually fiber lengths up tomaximum100 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 withlongfibersarenotnecessary.Changesinthecrosssectionof 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-mentcostsoftherequiredinstallations canbesaved.Last but 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 can be 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 presentedthenaturalfiberinjection(NFI)processasanevolutionofthe 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,thebreakthroughfailedtoappear.Today,duetoan increasing demand for natural fiber applications, thedevelopment of appropriate cutting methods for naturalfibers is required. 474 POLIMERY 2013,  58  , nr 6  polyol isocyanateglass fibrescutting unitventuri nozzlemixingchamber  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-structionisacrucialfactorandevenmoreimportantthanin 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 extensivemanufacturing studies ontheapplica-tion of natural fibers in several forms of reinforcement,suchaslongfibersorcontinuousfibersandseveralspeci-fications concerning linear density, fiber length or fiberpreparation. With regard to applications in electric vehi-cles, adapted material and process parameters are in de-velopmenttomeettherequirements in reproducibility ofmechanical properties. For the integration of long fiberreinforcements, suitable cutting methods are being deve-loped.Thevariabilityintermsoffibermasscontentandfiberlength as well as the resulting specific values in the LFIprocessareveryhigh,buthowever,duetolongfiberrein-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 Themotivation for theusage of natural fibers is stron-gerthaneverbefore.Beyondweightreduction,especiallyin 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,researchisnecessaryonreproduciblequali-ties in terms of processability and resulting mechanicalproperties,whichbothare essential in industrial applica-tions. Beyond improvements in natural fiber processing,corresponding manufacturing technologies have to bedeveloped which enable automated production of fibercomposites.Here,theLFIprocessprovidesagreatpoten-tial for the fabrication of long fiber-reinforced compo-sites.REFERENCES 1. Müssig J., Schönfeld U., von Buttlar H.-B., Schmehl M.: Kunststoffe international  2007,  3  , 50.2. Bledzki A. K., Faruk O., Sperber V. E.:  Macromol. Mater. Eng. 2006,  291  , 449.3. Schlott S.:  Kunststoffe international  2006,  3  , 83.4. Medina L., Schledjewski R., Lahm M., Jungmann H.:  Kunst-stoffe international  2004,  3  , 92.5. Starke J., Renkl J., Söchtig W.: „Internationale AVK-TVTagung für verstärkte Kunststoffe und duroplastischeFormmassen”, 1999,  2  , pp. A9.1—A9.9.6. FrehsdorfW.,SöchtigW.: KunststoffePlastEurope 2001, 3  ,23.7. Partusch G.: „Fachtagung Polymerschäume”, SKZ, Würz- burg 2003, pp. D.1—D.11.8. RauH.,BachsteffelJ.:„KunststoffeimAutomobilbau”,VDI,Düsseldorf 2004, pp. 229—243.9. Hack T., Schmidt R., Nonnbroich K., Burmeister J.: „Kunst-stoffeimAutomobilbau”,VDI,Düsseldorf2008,pp.67—76.10. Rosenberger J.: „PUR 2009 , VDI, Düsseldorf 2009, pp.21—32.11. Hufenbach W., Gude M., Geller S., Zichner M.:  Kunststoffeinternational  2011,  11  , 36.12. Hufenbach W.  et al. : „3rd Scientific Symposium of theCRC/Transregio 39”, TU Chemnitz, Chemnitz 2011, pp.65—69.13. Hufenbach W.  et al. : „Verbundwerkstoffe und Werkstoffver- bunde”, TU Chemnitz, Chemnitz 2011, pp. 283—288.14. Gassan J.:  Kunststoffe international  2003,  8  , 31.15. Pöltl J., Schäfer H.: „DVM-Tag 2000 , DVM, Berlin 2000,pp. 187—204.16. Frehsdorf W., Schmidhuber S.:  Kunststoffe Plast Europe  2004, 10  , 1. Received 17 VIII 2012. POLIMERY 2013,  58  , nr 6 475
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