4. Summary
3.6 Effect of polyelectrolytes addition on shrinkage of the NFC gel
The shrinkage of the NFC gel in terms of its diameter after freeze drying is shown in Figs. 4-30 and 4-31. The addition polyelectrolyte by 1% shrunk the NFC more compared to NFC without a polyelectrolyte. From the aspect of addition amount, the shrinkage of NFC gel was increased up to around 1%
and then decreased over this dosage. Because freeze drying minimized the effect of the meniscus of the water, the shrinkage have resulted due to a change of a substructure, which is flocculation of NFC. Flocculation of NFC increased the shrinkage because the attraction of each nanofibrils caused the decrease from the volume of the NFC gel. However a higher addition amount reduced the shrinkage of the NFC mat because of electrostatic repulsion.
Excessive addition amount decreased the shrinkage below that of an untreated NFC suspension.
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Fig. 4-30. Drying shrinkage of NFC suspension with addition of PEI.
Fig. 4-31. Drying shrinkage of NFC suspension with addition of poly-DADMAC and CPAM.
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4. Summary
The effects of a polyelectrolyte on the mechanical properties of NFC were evaluated. The zeta potential of NFC with a polyelectrolyte and addition amount was measured and the electrostatic interaction between nanofibrils was determined from the results. The viscosity and the hysteresis index indicated that structure type and the molecular weight of the polyelectrolytes determined the reformation of the network structure after it was deformed by an increase in the shear rate. The dewatering ability of the NFC suspensions in a diluted state (0.02%) and in a highly associated state (1.5%) was evaluated.
The flocculation behavior of NFC with polyelectrolyte influenced the dewatering ability. A higher molecular weight of a polyelectrolyte showed mostly better dewatering with a proper addition amount which was close to the isoelectric point. However, over dosages of the NFC suspension decreased the dewatering ability. Polyelectrolyte with a high molecular weight and high charge density promoted the decrease of dewatering ability. The PSS decreased the dewatering ability. Not only the electrostatic interaction, but also steric forces acted between the polyelectrolyte and NFC. The mechanical properties, including rheological properties of NFC were affected by polyelectrolytes. The addition amount and the molecular weight of polyelectrolyte were both important in the mechanical properties of NFC in suspension and mat state. This indicates that the flocculation of NFC affected by the polyelectrolyte at the molecular level. The NFC suspension was influenced by the polyelectrolyte in terms of the network strength, the strain at break, and the tensile breaking stress.
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Chapter 5.
Overall conclusions and future work
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NFC has great potential as a new material in cosmetics, automobiles, pharmaceutical and medical science, electrical devices, papermaking industry, and other applications. NFC has a high aspect ratio and hydrophilic characteristics that leads to unique rheological properties and network forming ability from even low consistency. NFC suspension may have several processes such as transferring, dewatering and drying to use as the shape of sheet or foam. Therefore, the network properties and mechanical properties of NFC need investigation and better understanding. This study was carried out to investigate the fundamental properties of nanofibrillated cellulose and the possibility of changing the network properties by polyelectolyte. Diverse evaluations in nanofibrillation of cellulose pulp fiber were performed. The network properties were evaluated by dynamic and tensile measurements. The effects of nanofibillation, increase in solids content of NFC, and polyelectrolyte on the network properties were investigated.
NFC was prepared and characterized. The morphology, water retention value, sedimentation concentration, dewatering property and rheological properties were investigated. The degree of nanofibrillation from pulp fiber can be determined by an evaluation of those characteristics. In this study, no further change by a mechanical treatment (grinding) of pulp fiber was noted after than 15 - 20 passes. However, it may be more important that the characteristics of NFC should be considered depending on the level of demand for further applications. The dewatering abilities of pulp fiber and NFC were compared. It was found that the initial consistency of NFC was significantly affected by its specific resistance to filtration. NFC showed a higher specific resistance to filtration by tenfold compared to pulp fiber at a consistency of 2%. This indicates that a conventional dewatering process
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during papermaking is highly limited in the case of NFC.
The effects of the solids content of a NFC mat on the network properties were investigated with the rheological and tensile properties. It was difficult to adjust the solids content of NFC. In this study, however, the solids content of NFC could be adjusted up to 20% by controlling the dewatering time using pressurized dewatering equipment. The solids content more than 20% could be gained by controlling the drying time at room temperature. The rheological properties were evaluated in strain sweep mode and frequency sweep mode.
In the case of NFC suspension, the yield stress had a power law relationship with the solids content up to 10%. In the case of NFC mat, the tensile properties were evaluated at high solids content, more than 10%.
The yield stress had a power law relationship with the solids content over a wide range from 0.5% to 99% independent of the measurement method.
Elastic modulus, tensile breaking stress and strain at break are evaluated. A power law relationship with a wide range of solids content was evaluated. The exponent was close to 3 that indicated the flocculation behavior of nanofibrils were similar to pulp fiber, although the aspect ratio is much higher and the dimension is much smaller than pulp fiber. However, NFC formed the network structure from 0.2% which was 2 times lower than that of pulp fiber, and the network strength is 5 - 20 times higher than pulp fiber with solids content. The elastic modulus and tensile breaking stress increased exponentially as a function of solids content. The wet strain at break of pulp fiber decreased gradually. However, the NFC increased until 50% of the solids content. The nanofibrillation of pulp fiber increased the tensile breaking
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stress and strain at break within all solids content tested. NFC mat of 25% has a similar tensile breaking stress to pulp fiber mat of 50%. When the degree of nanofibrillation is high, the strain at break increased more rapidly. Over 50%
of the solids content, and strain at break decreased greatly. Therefore, the increase in the ability of maintaining the network structure even at higher deformation of NFC mat may be the reason. However, the solids content affected the ability of standing from deformation. Above 50% of the solids content, the rigidity increased significantly, which may be due the increase of hydrogen bonded areas.
The effects of a polyelectrolyte on the network properties of NFC were evaluated. An addition of polyelectrolyte changed the surface charge of NFC and determined the flocculation behavior. The viscosity and the hysteresis index indicated that the structure and the molecular weight of the polyelectrolytes determined the reformation of the network structure after it was deformed by the increase in the shear strain rate. The dewatering ability of the NFC suspensions in a non-associated state and in a highly associated state was evaluated. A higher molecular weight of polyelectrolyte commonly showed a better dewatering ability at proper addition amount which was closed to isoelectric point. However, overdosage on the NFC suspension decreased the dewatering ability and a high molecular weight and a high charge density promoted the decrease of the dewatering ability. An anionic polyelectrolyte decreased the dewatering ability. Not only the electrostatic interaction, but also steric forces acted between polyelectrolyte and NFC. The mechanical properties, including the rheological properties of NFC, were
164
affected by polyelectrolytes. The addition amount and the molecular weight were both important in mechanical properties. The flocculation of NFC influenced the network strength by polyelectrolyte, even though the molecular weight was not great. The tensile breaking stress was more distinguishable with addition of polyelectrolyte in awet mat.
The network properties of NFC were determined by the substructure of the network. The substructure could be controlled with the type of polyelectrolyte. Because NFC also has colloidal characteristics, not only electrostatic interaction but also steric forces influenced the network properties, in particular, the NFC suspension was already flocculated at a consistency of 1.5% when the polyelectrolyte added. Therefore, controlling the substructure of NFC can be a method for controlling whole network properties of NFC. To utilize NFC requires an understanding of the relationship between NFC and water. Due to the NFC network which forms in the water from the preparation process, the removal of the water needs to be performed strategically. Not only by controlling network properties of NFC, but also the effective dewatering process such as continuous operation is required. It was expected that this study would be a fundamental research for utilization of NFC.
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