Abstract
Foam-forming has in the past predominantly been used to create two-dimensional sheet-like fibrous materials. Allowing the foam to drain freely and decay under gravity, rather than applying a vacuum to remove it rapidly, we can produce lightweight three-dimensional fibrous structures from cellulose fibres, of potential use for thermal and acoustic insulation. muCT scanning of the fibrous materials enable us to determine both void size distributions and also distributions of fibre orientations. Through image analysis and uniaxial compression testing, we find that the orientation of the fibres, rather than the size of the voids, determine the compressive strength of the material. The fibrous samples display a layering of the fibres perpendicular to the direction of drainage of the precursor liquid foam. This leads to an anisotropy of the compressive behaviour of the samples. Varying the initial liquid fraction of the foam allows for tuning of the compressive strength. We show an increase in over seven times can be achieved for samples of the same density (13 kg.m-3).Graphic abstract
Highlights
There is an increased drive to replace lightweight products derived from petrochemicals, such as expanded polystyrene, polyurethane and phenolic foam with new, sustainable and environmentally friendly materials
How does the average long time bubble radius of 0.4mm in the fibre-foam dispersion compare with the average void size in the foam-formed dried material? In order to investigate this we carried out a void size analysis using X-ray data of two fibrous samples created from fibre-foam dispersions with initial liquid fractions of i=0.25 and i=0.50
Image analysis of CT scan data revealed that the average void size of the fibrous samples was 50% larger than the average bubble size in the precursor foam possibly due to bubble coalescence
Summary
There is an increased drive to replace lightweight products derived from petrochemicals, such as expanded polystyrene, polyurethane and phenolic foam with new, sustainable and environmentally friendly materials. As the foam slowly decays as a consequence of drainage and evaporation, it leaves behind a low density bulky structure in the form of a network of fibres. Unlike in the recent work by [7] neither the wet foam fibre dispersions nor the dried sample are compressed. This results in samples of density 13 kg.m-3 compared with densities of about 40 kg.m-3 by [7]. While Pöhler et al use X-ray tomography mainly to illustrate inhomogeneities in the sample we use the technique to characterise and quantify the internal structure of our fibre networks. X-ray tomography confirms the role that foam drainage has for fibre alignment and how this in turn determines mechanical stiffness of the dried fibrous material. This, together with the identification of the average void size, is important for the tailoring of these materials for particular applications
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