A study of the mechanical properties of vapour grown carbon fibres and carbon fibre-thermoplastic composites

Abstract

As fibras de carbono produzidas na fase de vapor (VGCFs) combinam custos de produção potencialmente baixos com propriedades mecânicas, térmicas e eléctricas favoráveis. Isto torna-as de especial interesse para as aplicações onde as fibras de carbono, baseadas no pitch ou no poliacrilonitrilo (PAN) (designadas por fibras 'convencionais') são demasiado caras e as fibras de vidro não apresentam as propriedades necessárias. O presente projecto de investigação visou três objectivos. Em primeiro lugar, estudar sistematicamente as diferentes morfologias em que as VGCFs podem ser produzidas e avaliar o seu efeito nas propriedades mecânicas. Em segundo lugar, obter conhecimentos sobre a produção de compósitos de VGCF e matriz termoplástica. A determinação das propriedades mecânicas dos compósitos permite avaliar o desempenho das VGCFs como reforço de termoplásticos. Finalmente, pretende-se desenvolver modelos micromecânicos para prever as propriedades mecânicas mais relevantes dos materiais produzidos. Usando estes modelos inversamente, é possí­vel derivar as propriedades das fibras. No caso de VGCFs com diâmetros menores que 1 mm (VGCFs sub-micrométricas), é esta a unica maneira para determinar estas propriedades. Estudaram-se sistemáticamente as diferentes morfologias em que as VGCFs podem ser produzidas e avaliou-se o efeito da forma sobre as propriedades mecânicas das fibras. Concluiu-se que a forma não influencia significativamente o valor do módulo à tracção. No entanto, as fibras com forma diferente de cilindros perfeitos têm uma resistência de ruptura à tracção mais baixa. Globalmente, o módulo e a resistência à tracção são significativamente mais baixos do que os das fibras de carbono, ex-pitch ou ex-PAN, comercialmente disponí­veis. Mostrou-se também que o método da fragmentação não pode ser usado para avaliar a qualidade da interface destas fibras em compósitos de matriz polimérica, qualquer que seja a morfologia. Isto deve-se ao tipo de rotura, que é inerente à estrutura interna das VGCFs. Produziam-se e processaram-se compósitos termoplásticos reforçados com VGCFs submicrométricas usando tecnologias commerciais, sem problemas significativos, sempre que se utilizou o equipamento apropriado. Para avaliar o desempenho das VGCFs, as propriedades dos compósitos foram determinadas e comparadas com as dos reforçados com fibras convencionais. Verificou-se que os compósitos de VGCFs podem ser produzidos com resistência à ruptura e coeficiente de expansão térmica (CTE) comparáveis, embora com rigidez mais baixa, do que as daqueles compósitos. Usaram-se modelos micromecânicos disponí­veis na literatura e um novo modelo para prever a rigidez, o CTE e a resistência à ruptura de compósitos reforçados com fibras curtas, a partir das propriedades da fibra e da matriz. Os modelos foram verificados experimentalmente e aplicados inversamente para calcular as propriedades das VGCFs sub-micrométricas. Concluiu-se que as VGCFs têm um CTE aparente mais alto do que o das fibras de carbono ex-PAN e rigidez mais baixa. Embora a resistência à ruptura das fibras não possa ser calculada, dado que o comprimento da maioria das fibras é inferior ao comprimento crí­tico, a metodologia de modelação inversa permite determinar a resistência ao corte interfacial. Mostra-se que a adesão interfacial entre as VGCFs e a matriz termoplástica é comparável à das fibras de carbono convencionais. As diferenças de propriedades entre os compósitos de VGCF e os reforçados com fibras de carbono ex-PAN, podem ser atribuí­das à diferença de propriedades das fibras. Além disso, concluiu-se que a rigidez e o CTE aparentes das VGCFs sub-micrométricas são, pelo menos, tão boas como as das fibras de vidro.

Vapour Grown Carbon Fibres (VGCFs) combine potentially low production costs with encouraging mechanical, thermal and electrical properties. This makes them of specific interest for applications where ex-pitch- and ex-polyacrylonitrile (PAN) carbon fibres (designated by 'conventional' fibres) are too expensive, and glass fibres cannot provide the required properties. A research was carried out with three goals. First, to study systematically the different morphologies in which VGCFs can be produced and to evaluate their effect on the mechanical properties. Second, to develop know-how on the production of thermoplastic-VGCF composites. The determination of the mechanical properties of the composites allows the assessment of VGCFs as reinforcements of thermoplastics. Finally, to develop micromechanical models to predict the more relevant mechanical properties of the materials produced. By using these models inversely, it is possible to derive the properties of the fibres. In the case of VGCFs with diameters below 1 mm (submicron VGCFs) this is the only way to determine these properties. The different morphologies in which VGCFs can be grown were studied systematically and the effect of the shape on the mechanical properties of the fibres evaluated. It was concluded that the shape of the VGCFs has a small influence on the value of the tensile modulus. However, fibres with shapes different from perfect cylinders, have a lower tensile strength. Overall, both the tensile modulus and strength were significantly lower than those of commercially available ex-pitch- or ex-PAN carbon fibres. Furthermore, it was shown that the fragmentation method cannot be used to assess the quality of the interface of these fibres in polymeric matrix composites, irrespective of the morphology. This is due to the failure mode, which is inherent to the inner structure of the VGCFs. The production and processing of submicron VGCF-reinforced thermoplastic composites was done with commercial technologies, without major difficulties, provided the appropriate equipment was used. To evaluate the performance of the fibres, the properties of the composites were determined and compared to those reinforced with conventional ones. It was found that VGCF-composites can be produced with comparable strength and coefficient of thermal expansion (CTE) but with lower stiffness. Micromechanical models available in the literature and a newly developed model were used to predict stiffness, CTE and strength of short fibre reinforced composites from the fibre and matrix properties. The models were validated experimentally and then applied inversely to calculate the submicron VGCFs properties. It was concluded that VGCFs have an apparent CTE that is higher than that of ex-PAN carbon fibres and a lower stiffness. Although the fibre strength could not be calculated, as most of the fibres are well below the critical length, the inverse modelling methodology allows the determination of the interfacial shear strength. It was shown that the interfacial adhesion between VGCFs and the thermoplastic matrix is comparable to that of conventional carbon fibres. The differences in properties between VGCF- and ex-PAN carbon fibre composites, can be attributed to the differences in fibre properties. Furthermore, it was concluded that the apparent stiffness and CTE of submicron VGCFs are, at least, as good as those of glass fibres.

Vapour Grown Carbon Fibres (VGCFs) combineren een potentieel lage kostprijs met veelbelovende mechanische, thermische en electrische eigenschappen. Dit maakt hen bijzonder geschikt voor toepassingen waar ex-pitch en ex-polyacrylonitriel (PAN) koolstofvezels (hier ‘conventionele’ vezels genoemd) te duur voor zijn en glasvezels de vereiste eigenschappen niet kunnen bieden. Een onderzoek is uitgevoerd, gericht op drie doelen. Ten eerste het systematisch bestuderen van de verschillende morphologieën waarin VGCFs geproduceerd kunnen worden en hun invloed op de mechanische eigenschappen. Ten tweede het ontwikkelen van kennis op het gebied van de vervaardiging van VGCF-thermoplastische composieten. Door de mechanische eigenschappen van de composieten te bepalen, kan de de rol van VGCFs als versterking voor thermoplasten vastgesteld worden. Tenslotte het ontwikkelen van micromechanische modellen die de relevantere eigenschappen van de geproduceerde materialen kunnen voorspellen. Door deze modellen omgekeerd te gebruiken, kunnen de eigenschappen van de vezels afgeleid worden. Dit is de enige manier om deze eigenschappen te bepalen voor VGCFs met diameters kleiner dan 1 mm (submicron VGCFs). De verschillende morphologieën waarin VGCFs geproduceerd kunnen worden, zijn systematisch bestudeerd en het effect van de vorm van de vezel op de mechanische eigenschappen is geëvalueerd. De vorm van de VGCFs blijkt weinig invloed te hebben op de hoogte van de trekstijfheid. Vezels met een andere dan een perfecte cylinder-vorm, hebben echter een lagere treksterkte. In het algemeen waren zowel de trekstijfheid als de treksterkte van de VGCFs significant lager dan die van commercieel beschikbare ex-pitch of ex-PAN koolstofvezels. Daarnaast is aangetoond dat de fragmentatie-test niet gebruikt kan worden om de kwaliteit van de interface van deze vezel in composieten met een polymeer-matrix te bepalen, ongeacht hun morphologie. Dit komt door hun bezwijkgedrag, dat inherent is aan de interne structuur van de VGCFs. Submicron VGCF-versterkte thermoplastiche composieten zijn zonder noemenswaardige problemen geproduceerd en verwerkt met behulp van commerciele technologieën, onder voorwaarde dat de geschikte apparatuur gebruikt werd. Om de prestaties van de vezels te evalueren, zijn de eigenschappen van de composieten bestudeerd en vergeleken met die van composieten versterkt met conventionele vezels. Het bleek dat VGCF-composieten geproduceerd kunnen worden met een vergelijkbare sterkte en thermische uitzettingscoefficient (CTE) maar met een lagere stijfheid. Micromechanische modellen beschikbaar uit de literatuur en een nieuw ontwikkeld model zijn gebruikt om de stijfheid, CTE en sterkte van korte-vezel versterkte composieten te voorspellen vanuit de vezel- en matrixeigenschappen. De modellen zijn experimenteel gevalideerd en vervolgens omgekeerd toegepast om de submicron VGCF-eigenschappen te berekenen. Geconcludeerd kan worden dat submicron VGCFs een schijnbare CTE hebben die hoger is dan die van ex-PAN koolstofvezels en een lagere stijfheid. Hoewel de sterkte van de vezels niet direct berekend kon worden, omdat de meeste vezels ruim beneden de kritische lengte zijn, maakt invers modelleren wel de afleiding mogelijk van de afschuifsterkte van de interface tussen matrix en vezel. De hechting tussen VGCFs en de thermoplastische matrix blijkt vergelijkbaar met die van conventionele koolstofvezels. De verschillen in eigenschappen tussen VGCF- en ex-PAN koolstofvezel versterkte composieten kunnen worden toegeschreven aan de verschillen in vezeleigenschappen. Daarnaast is geconcludeerd dat de schijnbare stijfheid en CTE van submicron VGCFs zeker zo goed zijn als die van glasvezels.