Characterisation of environmentally friendly fibres for composites

  • Fiona Gentles

Student thesis: Doctoral Thesis


Throughout the past decade, Polypropylene (PP) has been used as a favoured matrix material with different fibres being used as reinforcement in automotive composites. One of the most popular fibres for reinforcing PP matrix is Glass fibre (GF) due to its strength and impact resistance. More recently there has been interest expressed in further reducing the weight of the vehicle and the environmental impact of the vehicle's lifecycle. One plausible way to reduce the 'environmental impact' of the vehicle is by replacing GF with lighter and more 'environmentally friendly fibres'. In order to develop environmentally friendly composites to be used in the automotive industry, it is vital to have an extensive understanding of the fibre reinforcement properties and how it contributes to the overall composite performance. In this thesis, full characterisation of three environmentally friendly fibres, Polyethylene Terephthalate (PET), Flax and Sisal, were carried out and an insight into the interaction between the fibre under investigation and the PP matrix is given. Further to this, an investigation into the accuracy of using the fibre's diameter to calculate Natural fibre (NF) properties was carried out. due to NF being non-circular in cross section. Characterisation of the fibre properties was carried out using various techniques. Single fibre tensile tests were used to investigate strength and modulus of each fibre. It was found that using the actual cross-sectional area (CSA) of the Natural fibres gave more accurate results than assuming circularity of NF. The thermoelastic properties of the investigated fibres were determined through a combination of experimental measurements and micromechanical modelling. Dynamic mechanical thermal analysis and thermal mechanical analysis techniques were employed to characterise unidirectional fibre-polyester composites over a range of off-axis loading angles. The results were input into a number of micromechanical and semi-empirical models. It was found that the investigated fibres were highly anisotropic with the fibres longitudinal modulus being greater than the fibres transverse modulus over a range of temperatures. Single fibre pull out was used to investigate the interfacial shear strength (IFSS) between the fibre under consideration and PP matrix containing various percentages of maleic anhydride Polypropylene (MAPP). It was discovered that the IFSS increased when the MAPP content increased. The IFSS was found to be low even with 10% MAPP and this was revealed to be caused by the anisotropic nature of the fibre. Furthermore, it was found that assuming circularity of NF in determining IFSS gave less accuracy than using the actual perimeter of the NF. Therefore it is highly recommended that when possible the actual perimeter and area should be used to calculate NF properties. In order to use the fibres under consideration in composites, a method to obtain the composite fibre weight fraction has to be investigated. The traditional method cannot be used as the matrix is burnt off leaving the reinforcement fibres behind however the matrix and fibres being considered have similar thermal characteristics. The DSC was found to give reasonably accurate results for obtaining the weight fraction of composite. Along with the investigation into fibre weight fraction, the fibre morphology between the PP matrix and reinforcement fibres was examined. It was found that no transcrystallisation occurred in PP at a melt flow index (MFI) of 47.
Date of Award27 May 2015
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SupervisorJames Thomason (Supervisor) & James Wood (Supervisor)

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