Study on the Composite Structure of SAP and Microfibrillated Cellulose. (Part II). Study on the Dispersion Stability of the MFC or the BC in a Solvent Mixture of Water and Organic Solvent. (1999)

By uniformly dispersing microfibrillated cellulose (the “MFC”) or bacterial cellulose (the “BC”) in a solvent mixture of water and hydrophilic organic solvent, a dispersion system of low concentration and high viscosity can be obtained. If super absorbent polymer particles (the “SAP”) are dispersed in this system, the SAP can be dispersed stably at a high concentration by virtue of such viscosity effect. The authors have found that, by forming a sheet from such a dispersion slurry and removing the solvent component therefrom, a SAP/MFC composite or a SAP/BC composite can be obtained efficiently where the MFC or the BC acts as the bonding agent. The MFC or the BC retains water stably in its microfibril structure when it is dispersed in water. That is to say, the MFC or the BC exhibits a similar behavior as it is in a so-called hydrating condition. Their hydrating condition, however, may vary very much depending on the kinds or amounts of organic solvents added. This report summarizes the results of the dispersing concentrations of the MFC or the BC with respect to the dispersion stability of the MFC or the BC in the solvent systems of water and organic solvents. The conclusions obtained are as follows : (1) Both of the S-MFC and the BC can be dispersed stably in organic solvents and water mixture systems where the SAP does not swell or coagulate. As an indication of the dispersion stability, the volumes of phase separation were measured.(2) Three kinds of fine fibers in the form of microfibrillated fibers (the “MF”), namely, the S-MFC, diluted BC and refined BC, were compared in terms of the dispersion stability, and found in the order of refined BC, the BC and the S-MFC. Thus, it was found that different dispersion stabilities were given different of the MF. This was probably because of different hydration capabilities, that is to say, different viscosities or amounts of water retained when they are dispersed in water.(3) The MF shows a very high dependence of dispersion stability on its concentration in water dispersion systems or in organic solvents/water mixture systems.(4) The dispersion stabilities of solvent systems were investigated when the ratios of organic solvent to water varied. It was consequently found that there existed a critical stability concentration value (Y value), beyond which the dispersion was stable. The Y value differs with the kinds of the MF. Specifically, the Y value of the S-MFC was around 0.3%, that of the BC was around 0.25%, and the value of refined BC was around 0.05%(5) As it was considered that increasing dispersion stability by the addition of an organic solvent was because of the viscosity of the system, changes in viscosity with three kinds of the MF, namely, the S-MFC, the BC and the refined BC, dispersed in an ethanol/water system, which is a standard condition, were investigated. The viscosity of any one of the these three MFs was higher as dispersed in an ethanol/water system than in a water dispersion system. It was confirmed that the maximum viscosity existed at around 50/50 of the ethanol/water ratio. From these results it was concluded that improved dispersion stability in solvent mixture systems was attributable to the increase in viscosity of the system.(6) If propylene glycol (the “PG”) is used as the organic solvent, the viscosity of the PG and the interaction of MF as dispersed in a solvent mixture so act in concert as to increase the viscosity of the MF dispersed system as the PG concentration increases so that the dispersion gets to be more stabilized. From this fact, the PG/water =70/30 system together with the ethanol/water =60/40 system was selected as a standard dispersion medium to be considered in future studies.