Kaiserslautern - Fachbereich Maschinenbau und Verfahrenstechnik
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- finite element method (9)
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Fabric reinforced thermoplastic composites, suitable for the production of thin-walled, high
strength structural parts, are available on the market today with various fibre/matrix combinations.
However, further market penetration and series production are inhibited as long as forming
technologies are not well understood. In this thesis, the potential for series production of different
forming technologies is evaluated. Stamp forming is an efficient way to produce parts in
very short cycle times. A limiting factor to part complexibilty is undesired wrinkle formation as
a consequence of insufficient fabric shear. Fabric shear and other important deformations of impregnated
fabrics were examined by means of new test devices. Evidence was found that membrane
tension is the crucial factor to avoid wrinkle formation. New tool concepts and processing
Windows were developed to produce fabric reinforced thermoplastic parts free of wrinkles and
distortions.
Prozeßanalyse und Modellbildung bei der Herstellung gewebeverstarkter, thermoplastischer Halbzeuge
(1999)
Werkstoff- und verarbeitungsspezifische Vorteile gewebeverstärkter, thermoplastischer Faser-Kunststoff-
Verbunde bieten ein erhebliches Potential für die Umsetzung konsequenter Leichtbauweisen in den verschiedenen Anwendungsbereichen der Industrie. In der Regel erfolgt die
Herstellung von Bauteilen in einer Prozeßkette, die sich aus den beiden Kernprozesse Verbundbildung (Imprägnierung und Konsolidierung) und Formgebung zusammensetzt. Die Verbundbildung erfolgt mit Hilfe diskontinuierlicher (Autoklav, Etagenpresse) oder kontinuierliche (Kalander, Doppelbandpresse) Verfahren, die z.B. umformbare Halbzeuge wie ORGANOBLECHEN, zur weiteren Verarbeitung bereitstellen. Diese Entkopplung des Imprägnierungsprozesses der Verstärkungstextilien mit thermoplastischer Matrix von der Bauteilherstellung ermöglicht - analog der Blechumformung - eine großserientaugliche Prozeßtechnik mit kurzen Taktzeiten. Alle wesentlichen Eigenschaften des Verbundwerkstoffes werden so während der Halbzeugherstellung determiniert, während die Bauteilherstellung dann nur der Umformung dient. Auf dem Verbundwerkstoffmarkt ist ein Reihe von Halbzeugen mit verschiedenen Eigenschaftsprofilen verfügbar. Infolge des ständigen Wandels der werkstofflichen Anforderungsprofile
ist jedoch eine permanente Weiterentwicklung der Halbzeuge notwendig. Hieraus resultiert für die Werkstoffhersteller ein großes technisches und wirtschaftliches Risiko, da die
aufwendige Werkstoffentwicklung - aufgrund mangelnder wissenschaftlicher Grundlagen - mit erheblichen Unsicherheiten behaftet ist. Derzeit wird die Neuentwicklung von anforderungsspezifischen Halbzeugen mit Hilfe von Trial- and-Error Versuchen hinsichtlich der universellen Prozeßparameter (Temperatur, Zeit und Druck) betrieben. Derartige Versuchsreihen bedingen einen beträchtlichen finanziellen und zeitlichen Umfang und stellen somit ein wesentliches Entwicklungsrisiko dar. Darüber hinaus sind die Ergebnisse nur für Anlagen gleichen Typs und Bauart uneingeschränkt anwendbar. Eine Übertragbarkeit der Prozeßparameter auf andere Anlagenkonzepte ist nicht gegeben, so daß die Prozeßfenster jeweils immer individuell ermittelt werden müssen. Aufgrund
dieser aufwendigen Vorgehensweise werden deshalb intensive Anstrengungen unternommen, die während der Materialherstellung auftretenden Vorgänge mathematisch zu
modellieren, um dann durch rechnerische Simulation der Verarbeitungsprozesse eine Abschätzung des energetischen und zeitlichen Aufwandes zu ermöglichen. Daraus ergab sich die Aufgabe, das Imprägnierungs- und Konsolidierungsverhalten gewebeverstärkter, flächiger thermoplastischer Halbzeuge durch umfangreiche Untersuchungen an verschiedensten Materialkombinationen zu analysieren. Ziel war es, die bei der Halbzeugherstellung
auftretenden Effekte zu charakterisieren, Ansätze zu deren Modellierung zu entwickeln und Erkenntnisse zur Optimierung der Prozeß- und Materialparameter zu gewinnen. Wesentliche Ergebnisse der Prozeßanalyse sind das Auftreten von transversalen Makro- und Mikroströmen sowie das Entstehen von festigkeits- und steifigkeitsreduzierenden Bindenähten
und Lufteinschlüssen. Ferner laufen innerhalb der Faserbündel, temperatur- und druckabhängige Lösungs- und Diffusionsvorgänge bei der Eliminierung der Lufteinschlüsse sowie Wechselwirkungen zwischen elastischen Geweben und strömender Matrix ab. Zentrales Ergebnis der Untersuchung stellt ein abgeleitetes System von funktionalen Abhängigkeiten dar, welches einen ersten, grundlegenden Schritt zur Erstellung eines umfassenden Simulationsmodells des komplexen Prozesses bildet. Hiermit schließt die vorliegende Arbeit erstmals eine Lücke zwischen anlagenspezifischen Versuchsreihen und der allgemeinen numerischen Prozeßsimulation, die es ermöglicht, das
Imprägnierungs- und Konsolidierungsverhalten gewebeverstärkter thermoplastischer Halbzeuge auf experimenteller Basis anlagenunabhängig zu beschreiben. Dazu wurden die regelbaren Prozeßgrößen Verarbeitungstemperatur T (°K), Verarbeitungszeit t (s) und
Konsolidierungsdruck P (Pa) zu einer dimensionslosen, matrixspezifischen Prozeßkonstanten - B-Faktor (tP/n(T))- zusammengefaßt, die den Verarbeitungszyklus energetisch und zeitlich bilanziert. Diese Vorgehensweise erlaubt die Untersuchung des funktionalen Zusammenhangs zwischen Prozeßregelgrößen und Imprägnierungsqualität innerhalb bestimmter Grenzen sowie den Vergleich verschiedenartiger diskontinuierlicher und kontinuierlicher Anlagentypen und
die Ableitung von Substitutionsbeziehungen zwischen den Regelgrößen. Dabei konnte gezeigt werden, daß die imprägnierungsbedingte Abnahme der Halbzeugdicke in Abhängigkeit des B-Faktors unabhängig von der verwendeten Anlagentechnik einem exponentiellen Gesetz folgt, dessen halbzeugspezifische Regressionskoeflizienten die während der Imprägnierung und Konsolidierung auftretenden Effekte zusammenfassen und die Ableitung optimaler Prozeßeinstellungen erlauben. Damit ist es möglich, für bestimmte Materialpaarungen mit Hilfe des B-Faktors einen Verarbeitungsbereich zu identifizieren, der aufgrund der
anlagenunabhängigen Betrachtungsweise der Parameter Temperatur, Imprägnierzeit und Konsolidierungsdruck
unter Berücksichtigung verschiedener Randbedingungen prinzipiell auf jede beliebige Anlagenkonfiguration zur Imprägnierung und Konsolidierung übertragen werden kann. Dies schließt deshalb nicht nur diskontinuierliche und kontinuierliche Prozesse zur Herstellung flächiger Halbzeuge, sondern darüber hinaus auch Verfahren zur Weiterverarbeitung dieser Materialien ein. Für jede potentielle Materialpaarung kann, ähnlich wie bei rheologischen Kennwerten für Kunststoffe, mit standardisierten Methoden eine Datenbasis geschaffen werden, die die Abhängigkeit des Imprägnierungsgrades von den Verarbeitungsbedingungen beschreibt. Damit kann der Versuchsaufwand bei der Einführung eines neuen Werkstoffsystems auf einer Anlage reduziert, die Verarbeitung wirtschaftlich optimiert, die Beanspruchung der Werkstoffkomponenten bei der Verarbeitung minimiert und die Einsatzstoffe verarbeitungsgerecht modifiziert werden.
Um eine weitere Zunahme der Umweltbelastung infolge des stetig anwachsenden Leergewichtes heutiger Kraftfahrzeuge zu vermeiden, wird dem vermehrten Einsatz von Faser-Kunststoff-Verbundwerkstoffen (FKV) auch für hochbelastete Sekundärstrukturen zukünftig eine zentrale Rolle beim intelligenten Fahrzeug-Leichtbau zukommen. Die hierzu erforderliche Substitution metallischer Bauweisen durch FKV-Lösungen bietet das Potential, aufgrund des herausragenden gewichtsspezifischen Eigenschaftsprofils der Werkstoffgruppe und der Möglichkeit einer weitreichenden Teile- und Funktionsintegration eine deutliche Gebrauchswerterhöhung und auch Kostenreduktion zu erzielen.
Die im Rahmen dieser Arbeit durchgefüh1ie Analyse des heutigen Entwicklungsstandes von
Fahrzeug-Sitzstrukturen und Fahrwerkselc1nenten in FKV-Bauweise zeigte, daß bei vorderen
Pkw-Sitzen die Grenzen des mit FKV derzeit Umsetzbaren erreicht sind. Bei Pkw-Fahrwerken konnte festgehalten werden, daß bislang keine längsorientierte, nicht angetriebene Hinterachse in FKV-Bauweise ohne zusätzliches Querelement bekannt ist, obwohl diese Achsbauart zunehmend eingesetzt wird. Zielsetzung der Arbeit war es daher, innovative Ansätze für die integrierte Gestaltung vorderer Sitzstruktur-Komponenten zu entwickeln, die Leichtbau und hohe Funktionalität mit Großserienfertigung verbinden und weiter Vorschläge zur Gestaltung längsorientierter, nicht angetriebener Pkw-Hinterachsen in FKV-Bauweise zu erarbeiten. Die Arbeit geht zunächst detailliert auf die Besonderheiten der FKV-Bauweisenentwicklung im Vergleich zum klassischen Konstruktionsablauf bei metallischen Strukturen ein. Darauf aufbauend wird die werkstoff- und fertigungsgerechte Spritzgieß-Konstruktion einer oberen Lehnen-Quertraverse aus diskontinuierlich langglasfaserverstärktem Thermoplasten (DLFRP oder LFT) vorgestellt, die zwei Kopfstützen-Führungselemente und die eigentliche metallische Quertraverse integriert. Zudem ermöglicht die Funktionserweiterung der axialen Drehbarkeit des gesamten Bauteils ein verbessertes Anstellen der Kopfstütze an den Insassen-Hinterkopf. Quasi-statische und auch dynamische Kopfaufprall-Prüfungen an Prototypen zeigten eine gute
Übereinstimmung mit den numerischen Simulationsergebnissen und bestätigten das geforderte „gutmütige" Versagensverhalten des Bauteils durch den Einsatz von Langfasern.
Im weiteren stellt die Arbeit erstmals ein schlüssiges werkstoff- und fertigungsgerechtes Konzept für eine längsorientierte, nicht angetriebene Hinterachse in FKV-Bauweise auf der Basis eines funktionsintegrierten CFK-Doppel-Blattfeder-Elementes mit in Reihe geschalteter FKV-Drehrohrfeder-Anordnung vor. Die im Vergleich zu herkömmlichen Metallbauweisen um etwa 40 % leichtere Konstruktion verzichtet auf ein mitfederndes Querelement zur Aufnahme der Seitenkräfte und ermöglicht ein elastokinematisch basiertes In-Vorspur-Gehen des kurvenäußeren Hinterrades. Die grundsätzliche Funktionstüchtigkeit der Konstruktion konnte in ausführlichen strukturmechanischen Simulationsrechnungen nachgewiesen werden.
The demand for material, energy and weight saving in many industrial fields promotes the use of
novel lightweight construction materials like fibre reinforced plastics (FRP). FRP with
thermoplastic matrices provide a high potential for lightweight construction together with the
possibility of process automation, a good medium resistance, a favourable impact behaviour and
good recyclability. However, the employment of these materials raises joining problems since
usual joining technologies can scarcely be used. Preliminary studies showed that welding
technologies are superior to the conventional joining technologies riveting and adhesive bonding
with regard to the mechanical seam properties.
Therefore, the aim of the present work was the development of plant configurations and process
windows for welding thermoplastic FRP with which a material and component spectrum as big
as possible can be joined economically. The investigated materials were fabric reinforced
thermoplastics (polypropylene, polyamide 12, polyamide 6.6 and polyphenylene sulphide) with
glass fibre and/or carbon fibre reinforcement and fibre volume fractions above 35%.
The evaluation of the existing welding technologies with regard to technological, economical
and ecological aspects showed that vibration welding and induction welding are most suitable to
welding of thermoplastic FRP. Therefore, these two welding technologies were investigated in
detail in the present work.
For vibration welding the parameter influences determined in different works on unreinforced
thermoplastics were confinned qualitatively. However, for the exa1nined fabric reinforced
thermoplastics the process parameters differed quantitatively compared to those for unreinforced
thermoplastics. The optimum welding pressure as well as the necessary welding time were three
times that for unreinforced thermoplastics. Despite the abrasion of the reinforcing fibres due to
the friction forces, a very good tensile shear strength was achieved. For a glass-fibre fabric
reinforced polyamide 12, for example, a weld factor of l was achieved. A process-controlled
welding pressure reduction during the vibration phase 3, which was proposed for unreinforced
thermoplastics, was integrated into a developed system controller programme. In this the melt
displacement course is analysed online and the pressure is reduced automatically. For T-profiles
with welded braces of glass-fibre reinforced polypropylene this procedure led to an essential
strength and rigidity increase. However, for single lap joints of FRP no strength increase could
be observed. As technological and economical alternative to the vibration welding technology, a continuous
induction welding process was developed, the necessary plant was built and the process was
analysed and modelled. Current flow in the laminate was identified as the dominant mechanism
of induction heating of carbon-fibre fabric reinforced plastics, due to the contact of the crossing
fibre bundles. The essential quality relevant feature of the developed process is the course of the
laminate temperature during the four process phases. This was analysed and the influencing
process parameters were determined and quantified.
A simple model based on fibre contact in the laminate was developed, with which the necessary
induction heating time for different lmninate structures was estimated. The differing fibre contact
areas in the different fabrics were considered by the introduction of a fabric factor. In order to
obtain a more exact determination of the temperature distribution in the laminate a finite element
model was developed. With this model the temperature distribution and the absolute temperature
in carbon-fibre reinforced laminates during induction heating were predicted. It was sufficient to
model the inhomogeneous laminate in a simplified manner as monolithic material with anisotropic
properties. The three cooling phases were modelled with Fourier's law of thermal conduction
in its three-dimensional form, which was solved with the Binder-Schmidt explicit method.
The difference between measured and calculated values was less than IO %. With the developed
models it is possible to determine optimum process parameters with the aid of a few easy
preliminary experiments.
Like for vibration welding optimum process windows for carbon-fibre and glass-fibre reinforced
thermoplastics were developed for induction welding, too. The achieved tensile shear strength of
induction welded single lap joints was only slightly lower than that of vibration welded
specimens concerning equivalent laminates.
Finally, the developed welding technologies were compared with each other regarding technological
and economical aspects. It was found that vibration and induction welding complement
each other very well. Vibration welding should be used for mass production and simple shaped
parts with small to medium sizes, while induction welding is more suitable for small series of
parts with almost any shape and size.
Aufgrund des spezifischen Eigenschaftsprofils weisen Faser-Kunststoff-Verbunde (FKV) wesentliche
komparative technische Vorteile gegenüber den Konkurrenzwerkstoffen in einer Reihe
von industriellen Anwendungsfeldern auf. Trotz des daraus resultierenden Marktpotenzials
konnten sich die FKV noch nicht in dem erwarteten Maße durchsetzten. Ursache hierfür ist die
im Allgemeinen zwischen FKV und traditionellen Materialien bestehende signifikante "Wirtschaftlichkeitslücke".
Um eine Lösung dieses Problems voranzutreiben, wurden neue Ansätze bei der Entwicklung
innovativer FKV-Anwendungen erforderlich. Optimierungspotenziale sind möglichst früh im
Entwicklungsprozess zu identifizieren und konsequent zu erschließen. Hierzu wird ein geeignetes
Analyse- und Planungsinstrument benötigt. Die bisher zur Verfügung stehenden Methoden
der Wirtschaftlichkeitsanalyse auf Basis von Herstellkosten erweisen sich als unzureichend, da
innovative FKV-Anwendungen, die hohe Herstellkosten bedingen, aber gleichzeitig im Betrieb
Kostenvorteile gegenüber herkömmlichen Alternativprodukten aufweisen, benachteiligt werden.
Deshalb wurde ein neuartiges, werkstoffgerechtes Instrument der Wirtschaftlichkeitsanalyse
für FKV entwickelt, welches sich auf die Methode der Lebenszykluskostenrechnung stützt.
Das neuartige Instrument wurde auf verschiedene Fallstudien aus den Bereichen Verkehrstechnik,
Anlagenbau, Bauwesen und Offshore-Industrie angewendet. Dies diente der Modellvalidierung,
dem Aufzeigen von Anwendungsoptionen der Methodik bzw. des Modells im Rahmen
von konkreten FKV-Entwicklungsaufgaben sowie der Bereitstellung einer Grundlage für die
abschließende Ableitung einer wirtschaftlichkeits- und damit zukunftsorientierten Entwicklungsstrategie
für FKV.
Die vorliegende Arbeit beschäftigt sich mit der Herstellung von thermoplastischen Elastomeren
(TPE) mit co-kontinuierlicher Phasenstruktur auf Basis von Polyester/Elastomer Blends. Die
eingesetzten Elastomere wurden dazu gezielt chemisch funktionalisiert, um die Verträglichkeit mit
den Polyestern zu verbessern.
Die Funktionalisierung der Elastomere wurde durch radikalisch initiierte Pfropfung von
Glycidylmethacrylat (GMA) in der Schmelze erreicht. Anhand von Ergebnissen umfangreicher
Untersuchungen an Ethylen/Propylen Copolymeren wurden die wesentlichen Einflussfaktoren auf
die Produkteigenschaften, wie dem GMA-Pfropfungsgrad, dem Vernetzungsgrad und dem Anteil
Nebenprodukte evaluiert und optimiert.
Zu diesem Zweck wurden entsprechende Analysewerkzeuge entwickelt und an das spezifische
System angepasst. Durch Kombination von FTIR- und 1H-NMR-Analysemethoden konnte eine
normalisierte und allgemein auf Polymere mit Ethylenblocksequenzen anwendbare
Kalibrierfunktion zur Bestimmung des GMA-Pfropfungsgrades entwickelt werden.
Weiterhin konnte das optimierte Funktionalisierungsverfahren erfolgreich auf andere Elastomere,
wie Ethylen/Propylen/Dien Terpolymere (EPDM) und Nitrilkautschuke (NBR) übertragen werden.
Die funktionaliserten Elastomere wurden mit und ohne dynamische Vulkanisation mit
Polyethylenterephthalat (PET) bzw. Polybutylenterephthalat (PBT) compoundiert. Neben PET
Neuware wurde auch PET Recyclat aus gebrauchtem Getränkeflaschenmaterial in die
Untersuchungen mit einbezogen. Dabei konnten die mechanischen Eigenschaften der TPE nicht
durch die dynamische Vulkanisation verbessert werden.
Die Blends wurden diskontinuierlich im Innenmischer und kontinuierlich im
Doppelschneckenextruder reaktiv compoundiert und anschließend mittels mechanischer,
thermomechanischer, thermischer und morphologischer Untersuchungsmethoden charakterisiert.
Es zeigte sich, dass die GMA-funtionalisierten Elastomere deutlich verträglicher sind mit den
Polyestern als nicht unfunktionalisierte Elastomere. Dies dokumentieren die feineren selbstdurchdringenden
Phasenstrukturen, einhergehend mit höheren mechanischen Kennwerten. Insbesondere GMA-gepfropfter Nitrilkautschuk mit hohem Acrylnitrilgehalt zeigte, auch verglichen
mit kommerziellen Verträglichkeitsmachern, ein großes Potential in den hergestellten TPE.
Bei Verwendung von PET Recyclat konnten sehr gute mechanische Kennwerte erzielt werden.
Damit stellen solche TPE eine interessante, wertschöpfende Recyclingoption für gebrauchtes
PET Getränkeflaschenmaterial dar.
This thesis aimed at developing thermoplastic elastomers (TPE) with co-continuous phase
structures based on polyester/elastomer blends. The employed elastomers were chemically
functionalized in order to improve the compatibility with the polyesters.
The elastomers were melt functionalized by free-radical initiated grafting of glycidyl methacrylate
(GMA). Major parameters of the grafting reaction affecting the grafting degree, the degree of
crosslinking and the amount of undesired by-products were studied and optimized for an
ethylene/propylene rubber system.
Suitable analytical tools were developed and adapted to characterize the GMA grafting degree.
By combining FTIR and 1H-NMR techniques a normalized and universally applicable calibration
function for the determination of the GMA grafting degree was established for polymers
containing ethylene block sequences.
1H-NMR measurements evidenced that the epoxide rings of the grafted glycidyl methacrylate
were not affected (i.e. ring opened) by the free-radical grafting reaction.
Increasing inititor concentration did not affect the total amount of polymerized GMA but shifted
the ratio from grafted to homopolymerized GMA while increasing the crosslinking degree of the
elastomer.
In order to achieve a high grafting degree the reaction temperature has to be adjusted as low as
possible. Moreover the GMA loss due to evaporation is reduced, too. Even though GMA has a
high melting point of 189°C it is highly volatile at lower temperatures.
The type of initiator proved to be a key parameter of the grafting reaction. All the investigated
peroxides can be utilized for the grafting, but the grafted products differed significantly. For a
certain type of peroxide no difference between liquid and solid types could be found.
The best grafting performance was reached by using 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane.
This was the only initiator that combined high grafting degrees with an extremely
low gel content. Grafting yields of ~80% were reached without using further coagents or
comonomers. Surprisingly, this initiator is not customary used for grafting reactions.
Furthermore the optimized grafting method was successfully transfered to other elastomers, e.g.
ethylene/propylene/diene terpolymers (EPDM) and nitrile rubbers (NBR).
NMR-analysis of the NBR-g-GMA revealed that the opoxide rings may react with the nitrile
functions forming reactive oxazolines. A stereochemically controlled reaction pathway following
Anti-Markoffnikoff rule was supposed for their formation.
The functionalized elastomers with and without dynamic curing were melt blended with
poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT). Beside typical virgin
bottle grade PET material, discarded post-consumer softdrink bottle material was employed.
The reactive compounding of the blend was conducted both discontinuously using a batch mixer
and continuously in an twin-screw extruder. The blends were characterized according to their
mechanical, thermomechanical, thermal and morphological properties.
It was found that GMA functionalized rubber exhibits a much better compatibility towards
polyesters than non-functionalized elastomers. This was evidenced by scanning electron
microscopy (SEM) and the transmission electron microscopy (TEM) analysis. Improved
compatibility was reflected in the formation of a finer dispersed co-continuous phase structure
yielding a better mechanical performance.
By varying the blend composition ratio the region of co-continuous phase structures (IPN) was
determined. Later, the blend composition was fixed at 50:50 wt.% polyester : elastomer. This
composition yielded IPN structures in all examined blends and dynamic vulcanizates.
The technique of dynamic curing could not be adopted to the polyester/elastomer blends. The high melt temperatures for polyester processing were inappropriate for peroxidic curing systems.
As a consequence a 2-step dynamic curing sequence was applied. In the first step a dynamic
vulcanizate masterbatch using the functionalized elastomer along with a further plastomer was
prepared. In the second step this masterbatch was blended with the polyesters. It turned out, that
the mechanical performance of the TPE could not be improved by dynamic vulcanization.
All extruded and injection moulded TPE using the prior funtionalized elastomers exhibited good
mechanical performance. In particular GMA grafted nitrile rubbers with high acrylonitrile content
performed very well and showed the capacity to compete with typical commercial ethylene/GMA
copolymer grades.
Blends with recycled PET material showed outstanding mechanical performance. As a
consequence the production of such TPE materials using discarded PET may be a value-added
recycling option for post-consumer PET waste.
Die Polyurethane stellen eine extrem vielgestaltige Kunststoffklasse dar, außerdem zählen
sie, von den Produktionskosten her, zu den höherwertigen Kunststoffen. Ersteres erschwert die
Entwicklung von Recyclingverfahren, letzteres ist der Grund, weshalb trotzdem seit längerem
an Wiederverwertungsmethoden für Polyurethane gearbeitet wird. Eine ganze Reihe von
Verfahren existieren bereits und werden mit Erfolg angewendet. Da es aber immer noch PURTypen
gibt, die bisher nicht erfolgreich wiederverwertet werden können, besteht weiterhin
Bedarf an zusätzlichen Verfahren.
Bei der Hydrolyse von Polyurethan-Abfällen wird das Material unter Zugabe von Wasser
weitgehend in seine Ausgangsbestandteile zerlegt. Aufgrund einiger schwierig zu bewältigender
Verfahrensschritte wird die Hydrolyse bisher nur im Labor- und Technikumsmaßstab
angewendet. In dieser Arbeit wurde nun ein Hydrolyseverfahren entwickelt, bei dem die
Auftrennung in die Bestandteile nur bis zu einem bestimmten Grad durchgeführt wird, also ein
partieller Abbau stattfindet.
Der partielle hydrolytische Abbau wurde in einem Doppelschneckenextruder ausgeführt. Die
Produkte („hygrothermisch a bgebautes P olyur ethan“; HA-PUR) wurden durch Bestimmung
des unlöslichen Rückstands und der Viskosität, mittels Infrarotspektroskopie sowie mit Hilfe
der Thermogravimetrie mit angeschlossener Massenspektrometrie charakterisiert.
Ausgehend von den Eigenschaften des Zwischenproduktes HA-PUR wurde nach
Anwendungsmöglichkeiten gesucht.
HA-PUR lässt sich hervorragend mit Duromeren mischen. Diese Tatsache wurde genutzt,
um die heute als Zähmodifikator für Duromere gebräuchlichen, aber teuren funktionalisierten
Flüssigkautschuke durch ein preisgünstiges Recyclingprodukt zu ersetzen. Tatsächlich wirkte
sich ein Zusatz von HA-PUR zu Duromeren günstig auf deren mechanische Eigenschaften, wie
Bruchzähigkeit, Bruchenergie und Schlagzähigkeit aus. Weiterhin konnte HA-PUR auch als
Härter für Epoxidharze eingesetzt werden.
Die kautschukähnlichen Eigenschaften von HA-PUR legten dessen Verwendung als Füllstoff in Kautschukrezepturen nahe. In Anteilen von 10-20 Gew.-% bewirkte HA-PUR bei einigen
Kautschuksorten eine beschleunigte Vulkanisation sowie eine Verbesserung der mechanischen
Eigenschaften. Im Falle einer Verschlechterung des Eigenschaftsprofils war es möglich, diese
durch geringfügige Variationen der Rezeptur auszugleichen.
HA-PUR besitzt gewisse thermoplastische Eigenschaften. Daher wurde auch die Möglichkeit
erprobt, es als Zähmodifikator für Polyoxymethylen (POM) einzusetzen. (Die Verwendung von
thermoplastischem Polyurethan für diese Zwecke ist heute bereits Stand der Technik.) Bei
Zusatz von 5 Gew.-% HA-PUR wurde eine leichte Erhöhung der Schlagzähigkeit festgestellt.
This work was aimed at studying the hygrothermal decomposition of polyester urethanes and
the usability of the products in thermosets, rubbers and thermoplastics.
Polyurethanes (PUR) are one of the most versatile groups of plastic materials. The variety of
PUR types reaches from flexible foams and rigid foams over thermoplastic elastomers to
adhesives, paints and varnishes. This variety is one of the reasons, why the development of
cost-efficient reycling methods is very difficult. On the other hand, the production of PUR is
rather expensive - compared to the mass-produced plastic materials like the polyolefins. This
fact was the reason for the development of recycling methods for PUR since the 60s. The
recycling routes for PUR can be devided in mechanical and chemical methods. Mechanical
methods cover e.g. grinding of PUR waste, compression moulding, adhesive pressing,
bonding. Chemical methods (also called feedstock recycling) change the chemistry of the
material. A third group of recycling methods is the recovery of energy. This can mean simple
incineration of the PUR waste or the decomposition by pyrolysis or hydrogenation and the
combustion of the products.
Chemical methods are e.g. glycolysis and hydrolysis. Glycolysis, which is already used on a
commercial scale, means the decomposition of PUR by diols (e.g. glycol) at elevated
temperatures through a transesterification reaction. The reaction products are polyols which are
similar to the virgin components and can be directly used for the manufacture of new PUR.
Amines can be products of side reactions of the glycolysis.
Hydrolysis of polyurethane waste means decomposition of the material to its virgin
components by treatment with water at elevated temperatures. The products are polyols and
amines which are related to the virgin isocyanates. After purification, the polyols can be used
for the production of new PUR, as well as the amines - after conversion into isocyanates by
phosgenation. Since there are still some problems with the processing (e.g. the separation of the amines), the hydrolysis of PUR waste has not yet been used on a commercial scale.
In this work, a process of hydrolysis has been worked out which does not lead to the virgin
components. The formation of these virgin components can be avoided by stopping the process
before reaching the state of complete decomposition.
This partial hygrothermally decomposition was carried out in a twin-screw extruder at
temperatures between 150 and 250 °C and addition of 10 wt.-% of water. The material used
for this process was polyester-PUR waste from the footwear industry and was ground into particles of 1-3 mm size. The products („hygrothermally decomposed polyurethane“; HDPUR)
were characterized by determination of the insoluble residue and melt viscosity. The
hygrothermal decomposition was traced by infrared spectroscopy and by thermogravimetry
combined with mass spectrometry. These examinations allowed a monitoring of the
decomposition degree. Further, some information about the chemical processes during
decomposition could be obtained.
Based on the specific properties (consistency upon decomposition stage, compound
containing primary and secondary amines) of HD-PUR attempts were made to check its use in
selected thermoset, rubber and thermoplastic combinations.
HD-PUR is quite well miscible with thermosets such as epoxy resins (EP) phenolic resins
(PF), and unsaturated polyester resins (UP). This fact was utilized for replacing the expensive
functionalized liquid rubbers, which are used for toughening of thermosets, by this costefficient
recycling product. The mixing of HD-PUR, especially with EP, leads to a clear
improvement of the mechanical properties like fracture toughness, fracture energy, and impact
toughness. Due to this promising results, the emphasis for further investigations was placed on
experiments with HD-PUR in EP. Two EPs (one trifunctional and one tetrafunctional) of Ciba
were used. Examinations of fracture surfaces by scanning electron microscopy gave some
information about the phase structure and the toughening mechanism. Dynamic-mechanical
thermoanalysis made it possible - apart from the investigation of other mechanical properties - to
determine the crosslink density which was then correlated with the fracture mechanical data. The
addition of HD-PUR in small amounts (up to 20 wt.-%) led to improved toughness along with
only slightly reduced stiffness. It should be noted, that even mixtures with 80 wt.-% HD-PUR
gave a curable resin yet with reduced stiffness and temperature resistance. HD-PUR alone could
act as hardener for epoxy resins. Further, one phenolic resin, one unsaturated polyester resin,and one vinylester-urethane hybrid resin were examined. The results were, compared to the
experiments with EP, less promising.
Due to its rubber-like properties, especially when extruded at lower temperatures, HD-PUR
seemed to be qualified for using as polymeric filler and extender in rubber recipes. Five sorts of
rubbers (natural rubber, nitrile-butadiene-rubber, styrene-butadiene-rubber, epoxidized rubber
and fluoro rubber) were mixed with HD-PUR in ratios of 10-20 wt.-%. If possible, standard
recipes without further additives were used. The changes of the rheological properties and the
vulcanization behaviour were checked. The results showed, that HD-PUR not only could be
regarded as neutral filler, but also as a kind of reactive plasticizer which could influence the vulcanization behaviour and the mechanical properties. Indeed, the vulcanization rate and the
tear strength of natural rubber was increased. If there was any deterioration of the performance,
this could be compensated by small variations of the related recipes. Some experiments were
conducted with regard to the comparison of two different vulcanization systems and two
different grades of carbon black.
The applicability of HD-PUR as modifier for thermoplastics has been checked by adding
HD-PUR to poly(oxymethylene) (POM). The modification of POM with thermoplastic PUR is
already the state of the art. Due to its thermoplastic properties, HD-PUR should be suitable for
this application. Mixing of HD-PUR with POM was possible in amounts from 5 - 40 wt.-%. If
5 wt.-% of HD-PUR was added, the impact toughness of POM was slightly increased. Higher
amounts of HD-PUR led to a decrease of impact toughness, tensile strength and Young’s
modulus.
Future works could provide the complete clarification of the chemical reactions during the
hygrothermal decomposition.The related information could serve for improved process control
and for extending the decomposition on PURs of polyether type. Further, the applicability of
HD-PUR as toughening agent for other (brittle) materials should be checked. The modification
of thermoplastics still offers a wide field of applications. Also the use of HD-PUR as reactive
filler in rubber recipes could be worked out. Finally, some other applications for HD-PUR, e.g.
as pressure sensitve adhesive, as sealing material or for sound and vibration damping could be
tested.
Die mechanischen Eigenschaften von Verbundwerkstoffen und Werkstoffverbunden werden
in erheblichem Maß durch die Eigenschaften der Grenzfläche bestimmt. Oftmals ist die
Grenzfläche sogar das schwächste Element. Eine zuverlässige Beschreibung der mechanischen
Grenzflächenqualität ist von großer Bedeutung für die Wahl optimaler Werkstoffkombinationen
und Kontaktbildungsverfahren. Bei mechanisch-technologischen Charakterisierungsmethoden
unterliegen die Zielgrößen, wie etwa die Grenzflächenscherfestigkeit, oftmals
einer starken Streuung. In der vorliegenden Arbeit wird deshalb das Konzept der linearelastischen
Bruchmechanik zur Grenzflächencharakterisierung herangezogen. Für die dazu
notwendige Spannungsanalyse des Prüfkörpers mit einem öffnungsdominierten Grenzflächenriß
werden FE-Modelle erstellt. Im Nachgang zu Experiment und Datenreduktion werden die
Voraussetzungen für die Anwendbarkeit des linear-elastischen Konzeptes verifiziert.
Da die Grenzflächenzähigkeit c G empfindlich von der Zweiachsigkeit ψ des örtlichen Beanspruchungszustandes
abhängt, wird eine Belastungseinrichtung konzipiert, mit der ψ im gesamten,
der linear-elastischen Bruchmechanik zugänglichen Mixed-Mode-Intervall stufenlos
variiert werden kann. Ergänzend zur Bestimmung der (ψ ) c G -Grenzflächenbruchkurve
wurde das Rißwachstum lichtmikroskopisch verfolgt und der Einfluß thermischer Eigenspannungen
abgeschätzt.
An nicht-linearen FE-Modellen wird der Einfluß des Rißuferkontaktes sowie des plastischen
Fließens als Kleinbereichstörung auf die Modenabhängigkeit der Grenzflächenbruchenergie
untersucht. In beiden Beispielen wird durch Annahme von Verzerrungskriterien im Inneren
der jeweiligen Nichtlinearitätszone eine Verbindung zwischen Festigkeitslehre und Bruchmechanik
hergestellt. Für den Fall der Kleinbereichplastizität werden außerdem die Ligamentnormalspannungen
im Rahmen eines weakest-link-Modells für rißbehaftete Körper bewertet.Es zeigt sich, daß die U-Gestalt der (ψ ) c G -Grenzflächenbruchkurve qualitativ nachvollzogen
werden kann, wenn man die Ligamentnormalspannungen als rißtreibende Kraft bewertet.
Ein Beitrag zur methodischen Integration von neuen Werkstoffen in die Fahrzeugcrashberechnung
(2000)
In the automotive industry, the market share of innovative and individual product variants has increased dramatically in recent years. The vehicle-independent requirements relating to quality and equipment are growing. Summarizing, the expenditure on product development is rising, forcing car producers to reduce development times and costs. The need for reengineering of the product development process is increasingly being met by virtual product development. A key factor is the reduction of time- and cost-consuming prototype tests by pre-designing automotive components virtually. The prerequisite for this are numerical methods which meet a high quality standard and allow predictive statements to realize concept decisions on the basis of calculation results. Questions of product liability are becoming increasingly important and may also concern test results based on numerical results in future.
An example of these methods of calculation is the crash calculation using the finite elements method, which is being used increasingly to improve passive safety within the general context of reengineering. In view of the growing requirements relating to occupant safety, this method of calculation, which is established highly efficient in the design of the body-in-white, will have to be improved in a way, that realizes a virtual design of interior safety components in the future. This will include especially simulating the complex material behaviour of the materials used in the vehicle interior. Describing these materials with sufficient material models affords a high quality of reproduction, which often can only be achieved at considerable cost and effort. This paper introduces a general process for the methodical integration of these new materials in the crash calculation. The objectives of the integration process are to improve and assure detail reproduction quality, as well as to increase efficiency using standardizable material modelling procedures.
The integration process will be developed on the basis of the so-called Quality Improvement Paradigm and subdivided into four phases: the planning phase, the performance phase, the evaluation phase and the know-how acquisition phase. During the planning phase, the necessary fundamental data on the material, the areas of application of the material within the vehicle and the associated application boundary conditions will be formulated. Targets will be defined for the integration process on the basis of these fundamentals. They will include a clear definition of assumptions regarding material behaviour and requirements relating to the planned material model. Under consideration of the validity of these assumptions and requirements, further action will be described in a project plan and assessed on the basis of a risk appraisal. Material characterization, as well as material modelling and validation will take place in the performance phase of the integration process. Material characterization involves formulating all the fundamental data required for development of the model so that material behaviour can be described. The first step - material modelling - involves examining the models existing in commercial calculation programs and literature, and validating their suitability for detail reproduction of material behaviour. The objective is to select a model that can be used for crash calculation in the productive development process, or for the necessary further developments. Further development of the model will be subdivided into formulation of the materials equation, implementation of the materials equation in the finite element tool and verification of development steps. The implemented material model has to be validated. During the evaluation phase of the process, the process steps will be analyzed in detail and validated. The quality of the developed material model will be proved. This will include a definition of quality specifications for the model and for components to be designed by numerical simulation. The final step in the integration process is the know-how acquisition phase, which serves to improve the quality of material models and leads to a long-term improvement of the process efficiency. In addition to empirical process data acquisition, this phase will include saving all experimental and numeric data. In addition, know-how will be derived by using the material model in the productive development process.
The integration process will be shown for the first time within this paper by using a polymer brittle foam with high energy absorption capacity as an example. The fundamental data required for this purpose will be formulated in the process planning phase. The foams are synthetically manufactured materials with a low specific weight and a cellular. The material behaviour of the foams is affected by the properties of the basic materials and the characteristics of the cellular structure. Mechanical behaviour is characterized by the compressive plateau- or crush-stress over a large compression strain and by a low elastic recovery rate after pressure relief. The usual areas of application with regard to occupant protection are the headliner trim, the instrument panel and the side door trim. These foams are also used in bumpers for vehicle protection in lower impact collisions and for pedestrian safety. Regardless of the area of application, polymer brittle foams are primarily subjected to compressive stresses. As a result of the formulated fundamental data, it is necessary to account for the changes in mechanical properties in dependence on varying conditions, such as strain rate, temperature and the application of force - characterized by varying sample and impactor geometries - in order to characterize the material. Classifying polymer brittle foams, two representative materials are determined which are used for material characterization. The thermoplastic and closed-cell particle foam system Noryl® EF will be considered in addition to the thermoset and nearly open-cell polyurethane-based Bayfill® EA foam system. Based on the procedure defined by the integration process, the material characterization will be realized by material tests such as the uniaxial compression test and the shear test, as well as application-oriented basic tests in accordance with the actual area of application of the foams. These tests will be conducted under varying application boundary conditions. Evaluation and analysis of the test results shows that the material behaviour of the two foam systems can be described by an elastic-plastic approach. The main characteristic of
this approach is the dependence of plasticity on the first and second stress invariants which is represented by a closed yield surface the multiaxial stress space. The material behaviour observed, is heavily dependent upon the application boundary conditions strain rate and temperature, as well as the density of the foam system. Failure of the Bayfill® EA also occurred when it was subjected to tensile and shear stresses, which have a strong influence on the behaviour of components under application-oriented loads.
The examination of existing material models which have the potential to reproduce the polymer brittle foam, leads to the selection of a model developed for aluminium foams by Professor N. Fleck at Cambridge University. The material model will be improved based on the results of the material characterization. This further development necessitates adapting the flow rule, introducing a law for the strain rate dependency of the material behaviour and formulating a damage model for modelling the failure mechanism of brittle polymer foams. The modified material model will be implemented as a user material in the ABAQUS/Explicit finite elements program and validated on the basis of the material tests and application-oriented tests. The potential of the integration process will be established during the evaluation phase. Though being of a pilot nature, this process supports the quality-controlled modelling of polymer brittle foams to a high degree. This is clearly reflected in the detailed process descriptions which allow a target-oriented distribution of labour among the development partners involved. Thus, the material characterization will be performed in close cooperation with IVW GmbH and material modelling will be carried out in cooperation with HKS Inc. The transparency and reproducibility of the process are also essential for long-term quality improvement and assurance. Analysis of the model validation in the evaluation phase leads to an acceptance of the developed material model for numerical simulation of the material behaviour of polymer brittle foams. A prerequisite for use is compliance with various quality specifications for the material model and for components to be designed based on numerical simulation. The components are required to exhibit a homogeneous density distribution and little scatter within defined bounds of production. The main requirement for use of the material model is quality-controlled calibration of the material parameters. The necessary material tests must be conducted on the foam system used for the component. The parameters also have to be validated by applicationoriented basic tests. Calibration and the validation should be supervised by the component manufacturer and carried out by a test institute with the necessary equipment and experience. Following the implementation of the integration process, two different calculation models which are representative of the area of application of the foams - the bumper test according to NHTSA, Part 581 and the head impact test according to FMVSS 201 – demonstrate the productive use.
The goal of this thesis was to improve the surface quality of highly reinforced polymer
composites in order to make these materials applicable to the painted exterior of passenger
cars.
For the evaluation of the application sector of automotive exterior components, a catalogue of
requirements was drawn up from technical specifications, internal standards, and legal
requirements. Components in the horizontal decorative section of the outer skin, like front hood,
boot lid, and roof, have to fulfil the highest optical and structural requirements. A survey of the
automobile market concerning applications of fibre reinforced plastics in the exterior of cars
showed the state of the art and certain tendencies. So far, only non-reinforced, short-fiber- or
random-fiber-reinforced plastics have been able to fulfil the high suriace requirements. Up to
now, high material prices, the lack of mass production concepts, and insufficient suriace quality
have prohibited serial applications of CFRP in the outer skin of passenger cars. Therefore,
different manufacturing technologies for exterior components in composites were examined and
compared in an overview of processes. The process of resin transfer moulding (RTM) was
identified to have great potential for serial production:because of its achievable suriace quality
together with high specific mechanical properties of 1he composites.
The goal of the current research was to find optimized combinations of materials, processes,
and coatings, in order to realize a Class-A suriace quality for CFRP parts in the RTM process.
The main problem with the suriace quality is the print-through of the reinforcement caused by
the inhomogeneous distribution of the reinforcing fibres and the chemical and thermal shrinkage
of the matrix material during processing. In order to periom a systematic investigation of the
composite materials, the process parameters, and surtace treatments, an experimental RTM
tool with a plate cavity was designed and produced in the suriace quality standard of a serial
tool.
Within the material optimization the comparison of five epoxy resins showed that the system 82
was the most promising for further investigations with regard to surface quality and cycle time.
Within the comparison of the fibre reinforcements, the woven fabrics displayed a minor surface
quality compared to the non-woven and non-crimp fabrics. lt was found out that multiaxial
stitched fabrics with optimized placement technique, texturized, multifilament stitching yarns,
and trikot-franse stitching pattern currently provide the best combination of surface quality and
processability, Even better surface results were achieved with non-crimp fabrics that are fixed
by an adhesive to a polyester mesh. However, the difficult processing and infiltration with matrix
material still provide a hurdle to a possible serial application. As a result of the investigation, one
type of randomly oriented cut glass mat with minimal fibre diameter and even fibre distribution
was preferred as a core material to the commonly used continuous strand mats. Within the
great variety of different surface veils, a few types could be identified to offer an effective
reduction of long term waviness (from LW>20 to LW<20) and short term waviness (from SW>35 to SW<15). These selected surface veil types are mechanically or binder fixed and made of
glass or PAN fibres with an areal weight of 50 to so g/m2
.
Statistical methods for the design of experiments and the analysis of the results were used in
the process optimization with the epoxy system 82. After the identification of the main predictors
and responses a D-optimal experimental plan was designed and perfomed. The method of
multiple regression was used to create a process modell which describes the observed system
behaviour and deviation to a very high degree.
It was discovered that high pressures on the liquid matrix system right after injection contribute
to a high surface quality by compensating a great part of the reaction shrinkage. fn order to
achieve high pressures in the cavity exceeding 100 bar, the processing af)d tooling equipment
was modified beyond conventional RTM process capabilites. Optimal settings for vacuum and
temperature difference depend on tool temperature and post pressure levels. The simultaneous
analysis of curing temperature and demoulding time showed that the best surface quality can be
achieved if the part is demoulded from the tool as soon as the saturation T9, depending on the
current tool temperature, is reached. Longer curing times neither increase the T9 of the part nor
do they improve surface quality. From these results a first strategy for high suriace quality can
be derived with a high tool temperature and a short demoulding time. The second strategy with
a !ow tool temperature and a long demoulding time, however, is easier and safer to periom in
terms of process stability.
In order to compare highly reactive thermoset matrix materials and to measure the volume
shrinkage throughout the whole reaction, a novel shrinkage measurement cell, or dilatometer,
was designed. This created the new opportunity to determine the processing shrinkage in its
chemically and thermally induced proportions depending on matrix material, curing temperature,
and time. Because of the good correlation of the laboratory results with the previous RTM
experiments, a high experimental effort for hardware investigations to characterize new epoxy
systems can be saved in the future. Matrix system 82 displayed the lowest shrinkage values in
combination with a high reactivity. It could also be observed that a great proportion of the
reaction shrinkage takes place very quickly after the start of reaction. Therefore, the post
pressure on the matrix system must be applied as early as possible in order to compensate this
shrinkage. Curing at lower temperatures always leads to lower chemical and thermal shrinkage.
In comparison to literature the newly developed method presented in this thesis provides
plausible results with high accuracy, and for the first time also for highly reactive thermoset
systems.
Suriace coatings offer the opportunity to reduce or cover surface structures and defects in order
to achieve a high quality of the painted part surface. The exploration of in-process coatings lead
to thermoplastic films and gel-coats as technologies with a high potential for the improvement of
surface quality. In comparison, epoxy surfacing films and inmould-powder-coatings result in
more effort to adapt the materials and application methods0to the current RTM process. It was
shown that the post-process coating with a plastic paint system contributes to an improvement of the surface quality. In this study different priming coat materials and thicknesses were
identified that cover part of the surface texture with an acceptable structure of the coat itself. In
addition, two surface finishing methods with manual sanding were found to raise the surface
quality of the painted part up to the required standard if required.
The results of the different subsystems materials, RTM-process, and surface coatings can be
combined in different combinations of various emphasis to the overall system of the painted
ATM-part, complying with the requirements of the specific outer skin region.
Short-term solutions for outer skin parts with vertical surfaces {as A-, B-, C-pillars, sills, or rear
side wings) were found and proven with sample plates for the first time. In order to achieve the
high quality required for horizontal exterior components (as front hood, roof, and trunk lid) at the
current state of development, a higher performance of the subsystems is necessary. But even
for this Class-A suriace quality, sample parts could be produced for the first time with high effort
in the ATM-process. At the beginning of this investigation, sample plates produced in RTM
displayed surtace waviness values of LW>35 and strong fibre marking over the whole surtace.
With the combination of optimization results, sample plates with LW<5 could be produced. A
visual evaluation could not determine any regular, oriented surface texture.
The presented work showed solutions in material-process-coating-combinations and
development potential to reach the required Class-A surface quality of automobile exterior parts
with advanced composites. This provides the necessary foundation for further developments
with the aim of a serial application.