Material science

Torbjörn Löfqvist, Jerker Delsing ii Dedicated to the memory of Bernard Schlecht who taught me to climb and inspired me to study iv Abstract For the paper and pulp industry in Sweden and Finland to remain competitive against countries with lower overheads, they have to constantly strive to improve the quality and the efficiency of the manufacturing processes. One of the ways of doing this is to introduce sensors that will provide valuable online feedback on the characteristics of the pulp so that adjustments can be made to optimise the manufacturing process. The measurement method proposed in this thesis is based on ultrasound, since it is rapid, inexpensive, nondestructive and non-intrusive. Thus could be done online. Since ultrasound propagation and attenuation depends on the material properties through which is propagates, it has the potential to provide measurements of material properties such as pulp fibre density and elasticity.

PACS: 430Fn 430Hq 430Jr 43.35Cg 43.30Es a b s t r a c t An investigation of a new method for measuring fibre material properties from ultrasonic attenuation in a dilute suspension of synthetic fibres of uniform geometry is presented. The method is based on inversely solving an ultrasound scattering and absorption model of suspended fibres in water for the material properties of the fibres. Experimental results were obtained from three suspensions of nylon 66 fibres each with different fibre diameters. A forward solution to the model with reference material values is compared to experimental data to verify the model's behaviour. Estimates of the shear and Young's modulus, the compressional wave velocity, Poisson's ratio and loss tangent from nylon 66 fibres are compared to data available from other sources. Experimental data confirms that the model successfully predicts that the resonance features in the frequency response of the attenuation are a function of diameter. Consistent estimated values for the compressional wave velocity and the Poisson's ratio were found to be difficult to obtain but in combination gave values of shear modulus within previously reported values and with low sensitivity to noise. Young's modulus was underestimated by 54% but was consistent and had low sensitivity to noise. The underestimation is believed to be caused by the assumption of isotropic material used in the model. Additional tests on isotropic fibre would confirm this. Further analysis of the model sensitivity and the reasons for the resonance features are required.

PACS: 430Fn 430Hq 430Jr 43.35Cg 43.30Es a b s t r a c t An investigation of a new method for measuring fibre material properties from ultrasonic attenuation in a dilute suspension of synthetic fibres of uniform geometry is presented. The method is based on inversely solving an ultrasound scattering and absorption model of suspended fibres in water for the material properties of the fibres. Experimental results were obtained from three suspensions of nylon 66 fibres each with different fibre diameters. A forward solution to the model with reference material values is compared to experimental data to verify the model's behaviour. Estimates of the shear and Young's modulus, the compressional wave velocity, Poisson's ratio and loss tangent from nylon 66 fibres are compared to data available from other sources. Experimental data confirms that the model successfully predicts that the resonance features in the frequency response of the attenuation are a function of diameter. Consistent estimated values for the compressional wave velocity and the Poisson's ratio were found to be difficult to obtain but in combination gave values of shear modulus within previously reported values and with low sensitivity to noise. Young's modulus was underestimated by 54% but was consistent and had low sensitivity to noise. The underestimation is believed to be caused by the assumption of isotropic material used in the model. Additional tests on isotropic fibre would confirm this. Further analysis of the model sensitivity and the reasons for the resonance features are required.

The presented study concerns the application of the pulse-echo ultrasound measurement technique in determining the elastic properties of fibres suspended in water. The two kinds of fibre materials are investigated, nylon 6/6 fibres and softwood pulp fibres. The fibre mass fraction was 0.5% for nylon and ranges from 0% up to 1% for softwood pulp. The ultrasonic measurements are performed in the frequency range of 2-11 MHz. It is shown that the velocity dispersion of the ultrasound is small for each suspension sample. In obtaining the fibres longitudinal Young's modulus two methods are used, one based on phase velocity and one based on acoustic attenuation. It is found that both methods gives reasonable estimates of the longitudinal Young's modulus for nylon 6/6. For pulp fibres the determined Young's modulus is overestimated in comparison with earlier findings.

In the manufacturing process of paper the mass fraction and material properties of the fibres in the pulp suspension are important for the quality of the finished product. This study presents two different methods of pulp characterisation. The first is based on phase velocity, which we use to investigate the composition of the pulp. Here a method is presented where the optimal number of circular shifts within the sampling window of the signal is determined which gives, in a weakly dispersive medium, a continuous phase spectrum and minimizes the likelihood of discontinuities within the bandwidth. Hence, the ambiguity in phase unwrapping is avoided. The results from phase velocity measurements show that the phase velocity weakly increases with increasing amount of fines in the suspension. The dispersion is caused by the fibres and it correlates with fibre mass fraction. The second method is based on attenuation and is used to characterise the wood fibres. The results of the attenuation experiments show that it is possible to inversely calculate wood fibre properties by fitting the model to the experimental data, if the fibre diameter distribution is known. However, the accuracy of these calculation is difficult to determined and more work in this area is required.

Use of switchable ionic liquid (SIL) pulp offers an efficient and greener technology to produce nanofibers via ultrafine grinding. In this study, we demonstrate that SIL pulp opens up a mechanically efficient route to the nanofibrillation of wood pulp, thus providing both a low cost and chemically benign route to the production of cellulose nanofibers. The degree of fibrillation during the process was evaluated by viscosity and optical microscopy of SIL treated, bleached SIL treated and a reference pulp. Furthermore, films were prepared from the fibrillated material for characterization and tensile testing. It was observed that substantially improved mechanical properties were attained as a result of the grinding process, thus signifying nanofibrillation. Both SIL treated and bleached SIL treated pulps were fibrillated into nanofibers with fiber diameters below 15 nm thus forming networks of hydrophilic nature with an intact crystalline structure. Notably, it was found that the SIL pulp could be fibrillated more efficiently than traditional pulp since nanofibers could be produced with more than 30% less energy when compared to the reference pulp. Additionally, bleaching reduced the energy demand by further 16%. The study demonstrated that this switchable ionic liquid treatment has considerable potential in the commercial production of nanofibers due to the increased efficiency in fibrillation.

Biobased nanocomposite sheets of cellulose nanofibres (CNF) and cellulose acetate butyrate (CAB) were prepared using resin impregnation. Porous nanofibre networks together with a low viscosity thermoplastic resin were the key elements in the processing. SEM images of the network before the impregnation showed high porosity and after the impregnation indicated impregnated fibre network. A significant improvement in the visible light transmittance was observed for the nanocomposite compared to the nanofibre network, which is explained on the filling of the pores with a transparent matrix. The tensile tests showed an increase of 364% and 145% for stiffness and strength respectively for nanocomposites with 60 wt.% CNF when compared to CAB. Dynamic mechanical properties showed a good interaction between the CAB and cellulose nanofibres. These results show that CAB impregnated cellulose nanofibre networks are promising biocomposite that could be used in applications where transparency and good mechanical properties are of interest.

Nanocellulose is often being regarded as the next generation renewable reinforcement for the production of high performance biocomposites. This feature article reviews the various nanocellulose reinforced polymer composites reported in literature and discusses the potential of nanocellulose as reinforcement for the production of renewable high performance polymer nanocomposites. The theoretical and experimentally determined tensile properties of nanocellulose are also reviewed. In addition to this, the reinforcing ability of BC and NFC is juxtaposed. In order to analyse the various cellulose-reinforced polymer nanocomposites reported in literature, Cox-Krenchel and rule-of-mixture models have been used to elucidate the potential of nanocellulose in composite applications. There may be potential for improvement since the tensile modulus and strength of most cellulose nanocomposites reported in literature scale linearly with the tensile modulus and strength of the cellulose nanopaper structures. Better dispersion of individual cellulose nanofibres in the polymer matrix may improve composite properties.

Bio-based continuous fibers were prepared by melt spinning cellulose acetate butyrate (CAB), cellulose nanocrystals (CNC) and triethyl citrate. A CNC organo-gel dispersion technique was used and the prepared materials (2 and 10 wt% CNC) were melt spun using a twin-screw micro-compounder and drawn to a ratio of 1.5. The microscopy studies showed that the addition of CNC in CAB resulted in defect-free and smooth fiber surfaces. An addition of 10 wt% CNC enhanced the storage modulus and increased the tensile strength and Young's modulus. Fiber drawing improved the mechanical properties further. In addition, a micromechanical model of the composite material was used to estimate the stiffness and showed that theoretical values were exceeded for the lower concentration of CNC but not reached for the higher concentration. In conclusion, this dispersion technique combined with melt spinning can be used to produce all-cellulose nanocomposites fibers and that both the increase in CNC volume fraction and the fiber drawing increased the mechanical performance.

I would like to thank my supervisor Torbjörn Löfqvist, for his comments and support. I would also like to thank Jerker Delsing for reminding me of the big picture. Thanks also to my friends and colleagues at CSEE for their support and patient nodding as I rant on about waves, boundary conditions and about the fact that I've done it wrong-oh no its all okay again etc, etc. I would especially like to thank Kristina Berglund, who has shown amazing flare at correcting my English and has patiently gone through version after version of this thesis and most importantly come up with the elastic band analogy. Appreciation also goes the Forskningskola för Kvinnor, for giving me the confidence to forge ahead and for the laughing therapy. Finally my special thanks goes to my husband, Erik, without whose support I could not have done this. And my Mum and Dad for the proofreading and comments.

The hypotheses that reactive uncured, thermoset bicomponent fibres can be prepared and mixed with reinforcing fibres and ultimately used in preparation of a composite was tested and is described. It is thought that such fibres have the two potential advantages: (1) to enable manufacturing with pai1icle doped resins e.g. nanocomposites which add functionality to composites and (2) increased efficiency of structural composite manufacturing by increasing the level of automation. The structure of the thermoset fibres comprises of a sheath of thermoplastic and a core of uncured the1moset resin. Once manufactured, the fibres were wound with a reinforced fibre onto a plate, consolidated and cured. The resulting composite was examined and compared to other composites made with the same manufacturing method from commercially available materials. The results show that a laminate can be produced using these reactive bicomponent fibres. The resin system successfully impregnates the reinforcing carbon fibres and that the thermoplastic separates from the epoxy resin system during consolidation. In comparison to reference material, the bicomponent laminate shows promising characteristics. However, the processes developed are currently on a lab-scale and considerable improvement of various bicomponent fibre properties, such as the strength, are required before the technology can be used on a larger scale.

This review addresses the recent developments of the processing of cellulose nanocomposites, focusing on the most used techniques, including solution casting, melt-processing of thermoplastic cellulose nanocomposites and resin impregnation of cellulose nanopapers using thermoset resins. Important techniques, such as partially dissolved cellulose nanocomposites, nanocomposite foams reinforced with nanocellulose, as well as long continuous fibers or filaments, are also addressed. It is shown how the research on cellulose nanocomposites has rapidly increased during the last 10 years, and manufacturing techniques have been developed from simple casting to these more sophisticated methods. To produce cellulose nanocomposites for commercial use, the processing of these materials must be developed from laboratory to industrially viable methods.

Cellulosic nanomaterials are potential candidates in different areas, especially in water treatment. In the current work, palm fruit stalks cellulose nanofibers (CNF), TEMPO-oxidized CNF (OCNF), and activated carbon (AC) were used to make thin film membranes for removal of E. coli bacteria from water. Two types of layered membranes were produced: a single layer setup of crosslinked CNF and a two-layer setup of AC/OCNF (bottom) and crosslinked CNF (up) on hardened filter paper. The prepared membranes were evaluated regarding their microstructure and layers thickness using scanning electron microscopy (SEM). Water flux and rejection of E. coli bacteria was tested using dead end stirred cells at 1 MPa pressure. Thickness of the cosslinked CNF layer in both types of membranes was about 0.75 micron. The results showed that exchanging water by isopropyl alcohol before drying increased porosity of membranes, and thus resulted in increasing pure water flux and flux of bacteria suspension. The two-layer AC/OCNF/CNF membrane had much higher water flux than the single layer CNF due to higher porosity seen on the surface of the former. Both types of membranes showed high capability of removing E. coli bacteria (rejection~96-99%) with slightly higher efficiency for the AC/OCNF/CNF membrane than CNF membrane. AC/OCNF/CNF membrane also showed resistance against growth of E. coli and S. aureus bacteria on the upper CNF surface while the single layer CNF membrane did not show resistance against growth of the aforementioned bacteria.

An in situ polymerization method is developed and utilized to achieve superior dispersion of cellulose nanocrystals in hydrophobic polymer matrices, resulting in highly reinforced bio-nanocomposites with a very low fraction of cellulose nanocrystals.

Surfaces of carrot nanofibre coatings were modified by a gliding arc in atmospheric pressure air. The treatment strengthened wetting of deionized water and glycerol, increased an oxygen content, CO and C=O, and moderately roughened the surfaces. In the perspective of composite materials, these changes to the nanofibres can potentially improve their processability when they are to be impregnated with a polymeric matrix. However, longer exposure to the gliding arc reduced oxidation and roughness of the surface, and thus there exists an optimum condition to achieve good wetting to solvents.

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