Material properties of the cell walls in nanofibri

Authors: Prashanth Srinivasa Artem Kulachenko Filip Karlberg

Publish Date: 2016/12/26

Volume: 24, Issue: 2, Pages: 519-533

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Abstract

The mechanical properties of the nanofibrillar cellulose foam depend on the microstructure of the foam and on the constituent solid properties The latter are hard to extract experimentally due to difficulties in performing the experiments on the microscale The aim of this work is to provide methodology for doing it indirectly using extracted geometry of the microstructure Xray computed tomography scans are used to reconstruct the microstructure of a nanofibrillar cellulose foam sample By varying the levels of thresholding structure of differing porosities of the same foam structure are obtained and their macroscopic properties of the uniaxial compression are computed by finite element simulations A power law relation equivalent to classical foam scaling laws are fit to the data obtained from simulation at different relative densities for the same structure The relation thus obtained is used to determine the cell wall material properties viz elastic modulus and yield strength by extrapolating it to the experimental porosity and using the measured response at this porosity The simulations also provide qualitative insights into the nature of irreversible deformations not only corroborating the experimental results but also providing possible explanation to the mechanisms responsible for crushable behaviour of the nanofibrillar cellulose foams in compressionNano fibrillar cellulose NFC foams are a class of low density cellular materials These were pioneered by Pääkkö et al 2008 in their work on NFC aerogels We have in an earlier paper Srinivasa et al 2015 remarked on the distinction between NFC aerogels and NFC foams and shall continue to reference it by the latter name Since cellular materials play a preeminent role in many engineering applications the development of a biodegradable alternative to polymeric and inorganic cellular materials naturally becomes interesting and valuable The mechanical properties of these materials have been the subject of many articles Svagan et al 2008 Sehaqui et al 2010 Ali and Gibson 2013 Martoïa et al 2016 Most of these works have concentrated on the role of chemical compositions in altering the microstructure and thus to their effect on the macroscopic or bulk properties Some of these works have also alluded to the cell wall material properties of the foam under consideration by using the classical foam scaling laws Gibson et al 1982 Gibson and Ashby 1982 1988 to extrapolate to unit relative density Sehaqui et al 2010 Gordeyeva et al 2016 or through micro indentation tests carried out on NFC thin films Ali and Gibson 2013 To the best of our knowledge these are the only two methods by which the cell wall material properties have been estimated for NFC foam materials until nowThe properties of the constituent solid material also referred to as the cell wall properties is of importance in evaluating the effect of the raw materials and the changes brought by various chemical treatments prior or during manufacturing Isolating the solid material and carrying out physical experiments to determine the material properties is a formidable task owing to the length scales involved Alternative methods include carrying out tests on structures made from material identical to those of the cell wall material An example of which is the indentation tests carried out on NFC thin films Ali and Gibson 2013 These are however not always representative of the actual material in the cell walls since what the cell walls experience during foam manufacturing differ to those in preparing NFC films In this context it is our aim to explore the possibility of estimating the cell wall material properties based on the direct finite element simulations on the structure that is reconstructed from tomography scansReconstructed tomography scans of various cellular structures have been used before in simulations for varying purposes ranging from studying the microstructural deformation mechanisms to inverse estimation of cell wall properties through parametric sweep Maire et al 2003 Caty et al 2008 Jeon et al 2009 2010 Natesaiyer et al 2015 One of the primary concerns in reconstructing cellular/porous structures from their tomography scans is the question of segmentation or thresholding Usually the value of the threshold required to get a clear microstructure does not necessarily correlate with the value required to match the porosity Burdened with the same circumstance we utilise the thresholding value as a parameter and reconstruct structures at differing levels of thresholding We use the computed results from the series of simulations conducted on different porosity levels and the experimental results for the given porosity to arrive at the cell wall material properties using the scaling laws and extrapolation based on themThere exist several methods to produce NFC foams each having their own merits For small scale production the methods of freeze drying Aulin et al 2010b Tchang Cervin et al 2012 supercritical carbon dioxide Sehaqui et al 2011 drying are well suited whereas when scaling up is desired Cervin et al 2013 have demonstrated that cellulose foams with improved mechanical properties can be obtained by drying aqueous foams stabilised with surfacemodified NFC The primary focus of this work is to demonstrate a method to obtain cell wall properties from tomography simulations and to use these simulations as a verification case for further large scale numerical simulations Thus we shall restrict ourselves to foams that were previously characterised experimentally The foams used in this study were the same as those characterised experimentally in the study by Srinivasa et al 2015 and the detailed description of the preparation procedure is found hereThe NFC foam used in this study was prepared at Innventia from anionic NFC obtained from carboxymethylation pretreatment of the fibres Pääkkö et al 2007 Wågberg et al 2008 After the carboxymethylation treatment the fibres were treated with NaHCO3 solution to increase the separation of fibres into nanofibrils After being washed with deionized water and drained the resulting dispersion was homogenised at 1700 bar using a highpressure homogeniser The NFC dispersion thus obtained was put in a mould and the mould was dipped into liquid nitrogen Once frozen the mould was vacuum 10−5 MPa dried for 5 days Two separate porosities namely 9896 and 9813 were obtained These porosities were computed based on the percentage by weight of the cellulose in the CNF gel that was used to prepare the foams which is a common method used with these materials Chen et al 2004 Diddens et al 2008 Srinivasa et al 2015 Since the porosity of the foams can be influenced by humidity the porosities of the dry foams were also computed at two different conditions Firstly foam samples were dried at 110 °C in an oven They were then weighed on a chemical balance and their dimensions measured using digital calipers At a relative humidity of 23 and temperature of 245 °C the porosity of the two dried foams were calculated to be 9899 ± 002 and 9819 ± 006 respectively The samples were then allowed to rest in a climate controlled chamber maintained at 40 relative humidity and temperature of 245 °C for 1 day that is the conditions the foams were scanned at Subsequently they were weighed and the porosities were calculated to be 9885 ± 008 and 9806 ± 001 respectivelyXray computed microtomography or Xray tomography is a nondestructive method available to image the microstructure of porous and other materials It relies on the usage of Xray beams to scan a 3D object at various orientations and obtaining a stack of images also called “slices” that correspond to the 2D planes of the 3D object These images are then used to reconstruct the spatial density distribution thereby providing the image of the 3D structure The tomography scans in this work were carried out at the 4DImaging lab Division of Solid Mechanics Lund University The instrument used was a Zeiss XRadia XRM 520 Xray tomograph The Xray tube voltage was maintained at 80 kV and images of resolution 600 and 3039 nm were obtained The tomography scans were obtained as a stack of grayscale images which contain 16bit unsigned data The resolution in this case refers to the size of an individual voxel in 3dimensions Thus each voxel is of dimension 600 nm × 600 nm × 600 nm The tomography scans were carried out at a relative humidity of 40 at a temperature of 28 °CThe stack of images is processed as a whole and a series of image processing techniques are applied to reconstruct the foam structure from the scans To begin with a process called as histogram equalization is applied to the stack of images This is done to alter the histogram of intensities into a desired shape In general the intensity values of the pixels/voxels in the image are mapped on such that the output has a more uniform distribution of intensities Fisher et al 2003 This has the effect of stretching the dynamic range and making details easier to see It makes the histogram of the image to be better distributed thereby providing better contrast

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