Characterisation of cellular composites using synchrotron X-ray microtomography

Background:
Cellular materials are common. In nature they can be found in form of wood, bone and cork. Manmade cellular materials are usually honeycombs and foams. A great demand for bio-based, renewable and lightweight products has surfaced, in the wake of climate change and increasing oil prices. For example, the European project SustainComp (http://www.sustaincomp.eu) aims to develop wood based sustainable materials for packaging, transport and construction applications. One of the objectives of SustainComp is to make bio-based reinforced foams primarily based on polylactic acid (PLA) and wood fibres and/or cellulose nanofibrils. Fibre reinforced foams have the potential of being lighter, stiffer and stronger than conventional foams.

Project description:
The main goal of this thesis is to characterise the morphology of wood fibre and/or cellulose nanofibrils reinforced foams using synchrotron X-ray microtomography. Moreover it would be desirable to identify and analyse the mechanism of deformation determining the stiffness and strength of reinforced foams. Preforms based on polymer and wood fibres will be prepared using a commingling technique developed by INNVENTIA in Sweden. These will be placed into a closed, heated and pressurized, vessel where supercritical carbon dioxide (CO2) will be used to foam the composite preforms. Different processing parameters (e.g. gas saturation temperature and pressure, depressurization and cooling rate, etc.) will be varied to control cell sizes and distributions and allow nucleation and growth of pores while maintaining the position of the reinforcement. The microstructure, of compounds and foams, will be determined using X-ray microtomography at beamline ID19 at the European Synchrotron Radiation Facility (ESRF) and studied by computerized image analysis at Grenoble INP-Pagora, in Grenoble, France. It is also possible to preform in situ mechanical testing in the tomograph to follow microstructural changes during deformation. The tomograph gives three-dimensional images of the samples with high resolution. Quantitative morphological data can be extracted from these images e.g. porosity, cell wall thickness, cell size, fibre orientation distribution, etc. These data will be very useful to understand and link to the mechanical properties of reinforced foams which will be experimentally determined. The idea is to study the influence of processing parameters on microstructure and in the end on the mechanical properties of cellulose based cellular composites.

Activities:
The approach can be outlined as follows:
• Read relevant literature
• Wet commingling and compression moulding to compound (INNVENTIA)
• Supercritical CO2 foaming (EPFL)
• Compression testing of foams (EPFL)
• Synchrotron X-ray microtomography (ESRF) combined with in situ compression testing
• Collect and analyse mechanical data and morphology of the compound before foaming and after of the corresponding reinforced foam
• Image analysis to extract useful microstructural data
• Understand deformation mechanics in reinforced foams, build a simple model to account for reinforcing phase and predict and compare with existing experimental data
• Parametric study to determine potential of reinforced foams (effect of fibre fraction, aspect ration, orientation, etc.
• Report writing (in English)
• Poster and oral presentation at the participating institutions

For a duration of 20 weeks with start in February 2010, the project will be carried out first at EPFL in Lausanne, Switzerland (material preparation, foaming) then at Grenoble INP-Pagora in Grenoble, France (morphology characterisation, mechanical testing and image analysis).

Prerequisites:
A strong background in material science, polymer and composite processing technology, is advisable. An interest in material mechanics and image analysis is welcome. The candidate should preferably be a last-year student in a suitable M.Sc. programme in Engineering.

Contacts:
EPFL, http://ltc.epfl.ch
Cristian Neagu, +41788222425, cristian.neagu@epfl.ch