PhD ThesisImpact resistant materials (IRMs) are widely used in the automotive and packaging
industry. Their main purpose is the protection of the transported occupants or
goods. Cellular materials as well as structures combine lightness with large
deformation under load. The energy absorption mechanism is provided by limiting
the peak load and ensuring the elastic deformation of the IRMs. Polymeric foams
are largely used as IRMs due to their cellular structure. Prediction of the foam
properties in terms of Young’s Modulus (Elastic Modulus) and the onset of Plateau
Region can be related to the foam density and the mechanical properties of the bulk
material (Gibson and Ashby model). The structure of the foam is only partly
accounted for in the Gibson and Ashby model in terms of material density.
However, it is possible to produce cellular materials with the same density but very
different internal architectures. This cannot easily be exploited in conventional
polymer foams but the processing of High Internal Phase Emulsion (PolyHIPE) and
its polymerisation route to produce PolyHIPE Polymers (PHPs) can produce
materials with very different structures. Experiments have revealed that the PHPs
properties are dictated by their detailed structure. Elastic PHPs with: 1) varying
ratio of polymerizable oil phase with respect to aqueous phase and 2) varying
mixing time/energy input were produced and tested by mechanical compression at
different temperatures and strain rates. The elastic modulus increases with a
quadratic law as a function of the polymerizable oil phase content of the HIPE when
the mixing time is the same, as predicted by the model. The Specific Absorption
Energy (SAE), represented by the area under the stress-strain curve, increases in a
similar way.
Increasing mixing time on HIPE has the effect of modifying the cellular structure.
Smaller pores and narrower distribution of pores are observed. Such features are
consistent for any set of PHPs densities and represent a design tool when some
specific mechanical characteristics are prescribed.
The assessment of process-structure-properties relationships was performed by
combining the mechanical response of the various PHPs with the imaging of their
structure by Scanning Electron Microscopy. The properties of PHPs were
benchmarked with reference to two commercially available products. One material
is characterised by a porous structure with a relatively high Young’s Modulus while
the other by a non-porous and composite-like solid structure with lower elastic
modulus. The properties of the PHPs can be engineered to shift from a foam-like
material to a composite-like through the processing parameters which in turn
modify the material porous structure. The temperature has very limited effect on
the PHPs material unlike for the reference commercial materials.
The enhancement of properties (increasing Elastic Modulus and SAE) induced by
changing the processing route are remarkable for such a class of porous materials.
When plotted on a Modulus-Density chart, the PHPs fill an existing material-chart
gap, representing a new class of materials and opening new possibilities as IRMs.Try & Lilly Lt
