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The development, design, and modeling of a rapid continuous processing scheme is developed to economically manufacture conductive polymer composite bipolar plates for fuel cells. Bipolar plates are required to possess several important properties for fuel cell operation, with the most sought after being electrical conductivity and mechanical strength. The polymer composite material generated at Virginia Tech is based on material generated by a wet-lay process and uses polyethylene terepthalate (PET) or polyphenylene sulfide (PPS) as the binder, although PPS is mainly used. In order to reach sufficient conductivity for use in generating bipolar plates, the polymer is doped with high levels of conductive graphite particles in the range of 70-80 wt%. The polymer system is reinforced with 6-9 wt% glass or carbon fibers. When compression molded into a solid, flat preform, the wet-lay material exhibits excellent bulk (in-plane) conductivity (> 250 S/cm). The material also exhibits tensile and flexural strengths of 57.5 and 95.8 MPa, respectively, higher than other polymer composite material being considered for bipolar plate production. However, formability and through-plane conductivity needs improvement. The laminate bipolar plates developed at Virginia Tech are made using wet-lay material in the core and a thermoplastic/graphite mixture on the surfaces. The wet-lay material provides mechanical integrity, while a powder form of PVDF or PPS and graphite mixture added to the surfaces to improve through-plane conductivity and formability. The manufacturing scheme for the production of laminate bipolar plates is based on the pre-consolidation of the wet-lay material, which establishes a solid, flat surface for the continuous addition of laminate powder. Because the laminate powder only requires heating, radiation heating is used in the process design to pre-heat the preform prior to compression molding. The heated preform passes underneath a press, where forming of channels takes place along with cooling of the bipolar plate. It is estimated that the entire process can take one minute to produce a bipolar plate. The cost of manufacturing a bipolar plate is estimated to be $8/kW, below the goal of $10/kW. The annual production is determined to be 250,000, with over 500,000 possible depending on certain design factors. / Ph. D.

Wheat gluten, wheat gliadin, and corn zein agricultural proteins were evaluated as multifunctional additives that: (1) provided reinforcement, (2) improved thermal stability, and (3) lowered the cost of polymer composites. Wheat proteins were utilized in two polymer matrices: poly(vinyl alcohol) (PVA) and synthetic cis-1,4-polyisoprene rubber (IR). The proteins were hydrolyzed and dispersed in the polymer matrix, where they cooperatively self-assembled into nanostructures called amyloids. Amyloids have the potential for high rigidity and stability due to high β-sheet content. In Chapter II, trypsin hydrolyzed wheat gluten (THWG) proteins were incubated in aqueous PVA solutions, then the composite solutions were air dried and compression molded into films. Anisotropic protein aggregates formed through a typical mechanism of β-sheet self-assembly, where a greater molding time and pressure and/or a lower PVA molecular weight allowed for more protein aggregation. The larger protein structures provided less reinforcement. In Chapters III and IV, THWG and trypsin hydrolyzed gliadin (THGd), a component protein in wheat gluten, were compounded in synthetic polyisoprene rubber to form nanocomposites. The reinforcement correlated to the protein β-sheet content and varied with protein concentration, protein batch preparation, processing temperature, and compounding time. The isotropic β-sheet containing structures were very thermally stable, even under harsh rubber compounding conditions. By optimizing the processing parameters uniform protein dispersion and optimal IR reinforcement were achieved, although the protein and IR phases had poor compatibility. In Chapter V, the THGd-IR composites were cured using a typical cure package and molding process. Protein aggregation into nanostructured β-sheets was observed during the curing process. Rubber reinforcement increased as a function of protein concentration and curing time. In Chapter VI, a hydrophobic protein (zein) was substituted for the hydrophilic protein (gliadin) used previously to improve protein-IR compatibility. The zein protein was better at reinforcing IR, while gliadin improved mechanical stability. Both zein and gliadin improved the thermal stability of IR. The results from Chapters II-VI showed an interesting concept: in situ filler formation in polymer matrices where the choice of protein, polymer, and processing conditions influenced the final morphology and composite properties. / Doctor of Philosophy / We use plastics every day for a wide range of applications, from food packaging to automobile tires. Many of these plastics are composite materials, called “polymer composites,” meaning they are made of two or more chemically distinct materials where one material is a polymer. For reference, a polymer is a long chain molecule made of many (“poly-”) units (“- mer”). Polymer composites often contain additives which modify the properties of the polymer. For example, many soft polymers, such as tire rubber, need to be made stiffer and so a “reinforcing additive” is used to improve the stiffness of the rubber. Many composite materials are made stiffer so less material can be used. This process is called “lightweighting.” The automotive industry and food packaging industry use this process to reduce weight and fuel costs. In this research, plant proteins are tested as reinforcing additives in polymer composites. Plant proteins, such as wheat gluten, are abundant, non-toxic, sustainable, and can self-assemble into extremely small, stiff structures. For these reasons, plant proteins offer an environmentally friendly alternative to typical reinforcing additives. This dissertation shows that plant proteins can reinforce two polymers with very different properties. The first polymer is poly(vinyl alcohol) (PVA), which is biodegradable, hydrophilic (i.e., “water loving”), and is commonly used in flexible food packaging. The second polymer is synthetic cis-1,4-polyisoprene rubber (IR), which is non-biodegradable, hydrophobic (i.e., “water fearing”), and is commonly used in automotive tires. In Chapters II-V, the wheat gluten protein is hydrolyzed, i.e., chemically “chopped” into short chain peptides, to encourage the self-assembly of the plant protein into small, stiff structures. The self-assembled protein structures survive typical industrial processing techniques, such harsh rubber compounding conditions which involve high heat, pressure, and shear forces (i.e., the material is pushed in opposing directions). In Chapter VI, full corn and wheat proteins are incorporated into IR using standard industrial mixing and curing processes. The corn and wheat proteins reinforce the synthetic rubber and inhibit the degradation of the chemical structure of cured rubber under high heat. At certain protein concentrations, the proteins improve the elasticity and lessen the permanent deformation in the polymer composite. Together, Chapters II-VI show that proteins from diverse plant sources can be used to improve the performance of polymers with dissimilar properties.

In der vorliegenden Dissertationsarbeit wird auf die Thematik der Entwicklung von Polymerwerkstoffen, basierend auf vollständig natürlichen Resourcen, eingegangen. Die vorliegende Lösung beruht auf der Compoundierung von Polylactid mit unterschiedlich modifizierten Vanillin. Ziel war es, flammschutzwirkende Komponenten einzubringen und die Abhängigkeiten zwischen Zusammensetzung und Eigenschaften aufzuklären. Dem liegt die Absicht zugrunde, optimale Werkstoffe zur Verfügung zu stellen, die sich durch deutlich verbesserte flammhemmende und mechanische bzw. thermo-mechanische Eigenschaften auszeichnen. Die erzeugten modifizierten Vanillin-Derivate sowie deren Composite wurden hinsichtlich der physikalischen und chemischen Struktur mittels REM, EDX, FTIR, NMR, DSC, TGA, SEC und Zugversuch charakterisiert. Zur Bestimmung der flammwidrigen Eigenschaften wurden UL-94 V, LOI und CCT durchgeführt. Es hat sich gezeigt, dass System aus PLA und einem Vanillin-Phosphorsäure-Ester in Bezug auf werkstofflichen Eigenschaften insgesamt die optimale Leistung aufwies. Die Materialen ergaben eine verbesserte Zähigkeit und erheblich erhöht flammwidrige Eigenschaften. In einem weiteren Schritt wurden MMT und APP, zwei kommerzielle Flammschutzmittel, mit dem PLA/VP System kombiniert. Die daraus abgeleiteten Resultate bewiesen eine synergistische Wirkung zwischen VP und MMT bzw. APP und führten zu besseren Brandklassen bei LOI und UL-94 Brandtests. / The present work demonstrates the development of fully bio-based polymeric composites. It was realized by the compounding of poly(lactic acid) and differently modified vanillin. The aim of this work was to introduce flame retardant components into PLA and to study the flame retardant mechanism. The intention of this approach is the preparation of optimized PLA composites with significantly improved flame retardant, mechanical as well as thermo-mechanical properties. The modified vanillin and the PLA composites based on those vanillin derivatives were characterized by means of SEM, EDX, FTIR, NMR, DSC, TGA, SEC and tensile test for their physical and chemical structures. UL-94 V, LOI and CCT were carried out to determine the corresponding flame retardant properties. The results showed that, the PLA/VP system represented the best overall performance. The PLA/VP composite exhibited increased toughness and significantly improved flame retardancy. In addition, two commercialized flame retardants, MMT and APP, were introduced into the PLA/VP system, respectively. It was suggested that there were synergic effects between VP and MMT as well as APP. The combined used flame retardants resulted in an improved classification in UL-94 and LOI tests.

Ziel der Untersuchung war es, eine Charakterisierung des Dauerlaufverhaltens dynamisch und tribologisch belasteter Maschinenteile aus Wood Polymer Composite (WPC) in tragenden Anwendungen durchzuführen. Zu diesem Zweck wurde ein Hybridprofil aus Aluminium und WPC (70% PP, 30 % Weichholz) zu dem bestehenden Profil aus Aluminium in ein Hängefördersystem eingebaut und die dynamische Belastung (Vertikalbeschleunigung) auf das Fördergut während des Anlagenbetriebes gemessen. Im Testbetrieb wurde das System mit Ersatzlasten beladen und die Amplituden der Vertikalbeschleunigung zu Versuchsbeginn und deren Veränderung nach 1000h Laufzeit hinsichtlich beider Profilarten bewertet und das Hybridprofil auf sichtbare Schäden überprüft. Zu Beginn und nach 1000h Versuchszeit waren keine relevanten Veränderungen des charakteristischen Beschleunigungsbildes bezüglich beider Profilaufbauten feststellbar. Trotz Verschleiß am Antrieb des Fördersystems blieb das Hybridprofil voll funktionsfähig (kein sichtbarer Verschleiß, keine merkliche Schädigung). Darauf aufbauend wird der Versuch mit höherer Belastung fortgesetzt und die Entwicklung eines Profils, welches nur aus WPC besteht, vorangetrieben. / Aim of the study was to analyze the characteristics of dynamic and tribological stressed wood polymer machine elements. For this purpose an endurance test with an overhead conveyor was executed. A sectional beam in hybrid design (Aluminum & WPC [70 % PP / 30 % softwood]) was implemented in this aluminum beam overhead conveyor. During the conveying process the vertical acceleration of the transported material (dummy loads) was measured. At the beginning of the endurance test the acceleration patterns of the aluminum and the hybrid beams did not differ. Despite of mechanical wear of the drive system after 1000 operating hours, alterations of the acceleration patterns still could not be detected in the different beams. The hybrid beam remained fully functional with no visible wear or damage after 1000 operating hours. Based on these encouraging results the endurance test is continued with enhanced dummy loads and the development of a beam completely composed of WPC is aspired.

Laminados compósitos com matrizes poliméricas, respectivamente termorrígida e termoplástica, reforçadas com fibras contínuas de carbono foram submetidos a impacto único transversal com diferentes níveis de energia. Os danos imprimidos aos materiais estruturais foram avaliados por termografia ativa infravermelha na modalidade transmissão. Em geral, os termogramas do laminado termoplástico apresentaram indicações mais claras e bem definidas dos danos causados por impacto, se comparados aos do compósito termorrígido. O aquecimento convectivo das amostras por fluxo controlado de ar se mostrou mais eficaz que o realizado por irradiação, empregando-se lâmpadas incadecentes. Observou-se também que tempos mais longos de aquecimento favoreceram a visualização dos danos. O posicionamento da face impactada do espécime, relativamente à câmera infravermelha e à fonte de calor, não afetou a qualidade dos termogramas no caso do laminado termorrígido, enquanto que influenciou significativamente os termogramas do compósito termoplástico. Os resultados permitiram concluir que a termografia infravermelha é um método de ensaio não-destrutivo simples, robusto e confiável para a detecção de danos por impacto inferior à 5 Joules em laminados compósitos poliméricos reforçados com fibras de carbono. / Continuous carbon fiber-reinforced thermosetting and thermoplastic composite laminates were exposed to single transversal impact with different energy levels. The damages marked to the structural materials were evaluated by active infrared thermography in transmission mode. In general, the thermoplastic laminate thermograms showed more clear and delineated damage indications when compared to the ones from thermosetting composite. The convective heating of the samples by controlled hot air flow was more efficient than via irradiation using lamp. It was also observed that longer heating times improved the damage visualization. The positioning of the specimen´s impacted face regarding the infrared camera and the heating source did not affect the thermo-imaging of thermosetting specimens, whereas it substantially influenced the thermograms of thermoplastic laminates. The results allow concluding that infrared thermography is a simple, robust and trustworthy methodology for detecting impact damages as light as 5 Joules in carbon fiber composite laminates.

Uma maneira de quantificar a resistência da interface de materiais compósitos é medindo a tensão necessária para separar o filme do substrato. Esta tensão é frequentemente usada como um parâmetro de projeto para desenvolver estes materiais. Entretanto, muitos métodos de medição da adesão não são capazes de eliminar as falhas de coesão das falhas de adesão filme/substrato. Neste trabalho um novo método de medição de adesão é proposto com o objetivo de eliminar a interferência da falha de coesão por cisalhamento, inerente ao filme delaminado, da medida da tensão normal de adesão feita por método padrão. O novo método provou ser eficiente e seus resultados mostraram serem mais precisos que os do método normalizado. Foram testados os seguintes compostos obtidos por evaporação de cobre usando canhão de elétrons através do processo de deposição física de vapor (PVD), formando um filme metálico sobre quatro substratos poliméricos: polipropileno (PP), poliamida 6 (PA 6), poliestireno de alto impacto (HIPS) e poli(tereftalato) de etileno (PET). A análise de espectroscopia de retroespalhamento Rutherford foi utilizada para caracterizar os filmes e o ângulo de contato para caracterizar a interface. As superfícies dos polímeros foram modificadas através de flambagem e lixamento para validar os resultados do novo método. Finalmente os polímeros delaminados através dos métodos padrão e proposto foram observados por microscopia óptica e eletrônica de varredura (MEV), comprovando assim, que somente no método novo ocorre a separação da interface metal-polímero livre da influência da falha coesiva do filme de cobre. / One of the methods to quantify the interface strength of composites is to measure the tensile stress necessary to separate a film from the substrate surface. Such value is often used as a project parameter to develop the composite. However, most of methods cannot avoid the interference of the cohesion bulk failures from the film/substrate adhesion measures. In this work a new method is proposed in order to eliminate the influence of the cohesion shear failure inherent to the delaminated film of the normal adhesion stress measured by the standard method. The new method has proved to work and their results have become more accurate than the standard pull-off method. The experiment consisted in delaminate a copper film deposited by physical vapor evaporation (PVD) through electron gun on four polymers: polypropylene (PP), polyamide 6 (PA 6), high impact polystyrene (HIPS) and polyethylene terephthalate (PET). The Rutherford backscattering spectrometry was used to characterize the films and the contact angle analysis to characterize the interfaces. The polymer surfaces have also been modified to verify the adhesion strengths of the copper film through sanding and flaming processes to validate the new method. Finally the substrates delaminated were analyzed for both methods, standard and proposed, through optical and scanning electron microscopies, proving that only the new method is effective in pulling-off the metal/polymer interface without the cohesive failure influence of the copper film.

Polymer electrolyte membrane fuel cells (PEMFCs) have the potential to play a major role as energy generators for transportation and portable applications. One of the current barriers to their commercialization is the cost of the components and manufacturing, specifically the bipolar plates. One approach to preparing PEMFCs for commercialization is to develop new bipolar plate materials, related to mass production of fuel cells. Thermoplastic/carbon filler composites with low filler loading have a major advantage in that they can be produced by a conventional low-cost injection molding technique. In addition, the materials used are inexpensive, easy to shape, and lightweight. An optimal bipolar plate must possess high surface and bulk electronic conductivity, sufficient mechanical integrity, low permeability, and corrosion resistance. However, it is difficult to achieve high electrical conductivity from a low-cost thermoplastic composite with low conductive filler loading. Concerns over electrical conductivity improvement and the injection processability of composites have brought forth the idea of producing a polypropylene/three-carbon-filler composite for bipolar plate application. The thesis addresses the development of synergistic effects of filler combinations, investigating composite conductive materials and using composite bipolar plate testing in PEMFCs. One significant effect of conductive network formation is the synergetic effects of different carbon filler sizes, shapes, and multiple filler ratios on the electrical conductivity of bipolar plate materials. A polypropylene resin combined with low-cost conductive fillers (graphite, conductive carbon black, and carbon fibers with 55 wt% of filler loading) compose the main composite for all investigations in this research. Numerous composite formulations, based on single-, two-, and three-filler systems, have been created to investigate the characteristics and synergistic effects of multiple fillers on composite conductivity. Electrical conductivity measurements corresponding to PEMFC performance and processing characteristics were investigated. Experimental work also involved other ex-situ testing for the physical requirements of commercial bipolar plates. All combinations of fillers were found to have a significant synergistic effect that increased the composite electrical conductivity. Carbon black was found to have the highest influence on the increase of electrical conductivity compared to the other fillers. The use of conjugated conducting polymers such as polypyrrole (PPy) to help the composite blends gain desirable conductivities was also studied. Electrical conductivity was significantly improved conductivity by enriching the conducting paths on the interfaces between fillers and the PP matrix with PPy. The conductive network was found to have a linkage of carbon fibers following the respective size distributions of fibers. The combination of Fortafil and Asbury carbon fiber mixture ameliorated the structure of conductive paths, especially in the through-plane direction. However, using small fibers such as carbon nanofibers did not significantly improve in electrical conductivity. The useful characteristics of an individual filler and filler supportive functions were combined to create a novel formula that significantly improved electrical conductivity. Other properties, such as mechanical and rheological ones, demonstrate the potential to use the composites in bipolar plate applications. This research contributes a direction for further improvement of marketable thermoplastic bipolar plate composite materials.

Research in nanoparticle-reinforced composites is predicated by the promise for exceptional properties. However, to date the performance of nanocomposites has not reached its potential due to processing challenges such as inadequate dispersion and patterning of nanoparticles, and poor bonding and weak interfaces. The main objective of this dissertation is to improve the physical properties of polymer nanocomposites at low nanoparticle loading. The first step towards improving the physical properties is to achieve a good homogenous dispersion of carbon nanofibers (CNFs) and single wall carbon nanotubes (SWNTs) in the polymer matrix; the second step is to manipulate the well-dispersed CNFs and SWNTs in polymers by using an AC electric field. Different techniques are explored to achieve homogenous dispersion of CNFs and SWNTs in three polymer matrices (epoxy, polyimide and acrylate) without detrimentally affecting the nanoparticle morphology. The three main factors that influence CNF and SWNT dispersion are: use of solvent, sonication time, and type of mixing. Once a dispersion procedure is optimized for each polymer system, the study moves to the next step. Low concentrations of well dispersed CNFs and SWNTs are successfully manipulated by means of an AC electric field in acrylate and epoxy polymer solutions. To monitor the change in microstructure, alignment is observed under an optical microscope, which identifies a two-step process: rotation of CNFs and SWNTs in the direction of electric field and chaining of CNFs and SWNTs. In the final step, the aligned microstructure is preserved by curing the polymer medium, either thermally (epoxy) or chemically (acrylate). The conductivity and dielectric constant in the parallel and perpendicular direction increased with increase in alignment frequency. The values in the parallel direction are greater than the values in the perpendicular direction and anisotropy in conductivity increased with increase in AC electric field frequency. There is an 11 orders magnitude increase in electrical conductivity of 0.1 wt% CNF-epoxy nanocomposite that is aligned at 100 V/mm and 1 kHz frequency for 90 minutes. Electric field magnitude, frequency and time are tuned to improve and achieve desired physical properties at very low nanoparticle loadings.