B. Volkert, K. Hettrich, S. Fischer
Fraunhofer Institute for Applied Polymer Research IAP,
Keywords: nanocellulose, high-pressure homogenization, pre-treatments, barrier properties
Summary:Novel spherical nano-scaled cellulose particles have been prepared by high-pressure homogenizing of four different types of pre-treated cellulose samples with MICROFLUIDIZER™ processor (MF) in aqueous media. One possibility of pre-treatment is a decrystallization step realized by dissolving and regenerating cellulose from a melt NMMNO*H2O solvent system. Nanocellulose was obtained by a subsequent high-pressure mechanical treatment of the precipitate in aqueous dispersion. Decrystallization was also realized by grinding cellulose in a planetary mill. Ground cellulose was subsequently dispersed with high-speed stirrer Ultra-Turrax™ (UT) and high-pressure homogenizer. The amorphous intermediates were characterized by means of WAXS, Raman spectroscopy and DP determination. By another way the preparation of nano-scaled cellulose was conducted by hydrolysis and following mechanical treatment of hydrolyzed cellulose with Ultra-Turrax™ and MICROFLUIDIZER™. A further alternative was given by the mechanical treatment of aqueous dispersions of low substituted cellulose derivatives. For example methyl cellulose, carboxymethyl cellulose and oxidized cellulose gave nano-scaled materials with interesting properties. In order to obtain information about cellulose particle sizes, UT and MF treated dispersions were characterized by means of static and dynamic light scattering (DLS), ultra-centrifugation and scanning electron microscopy (SEM). Rheomechanical measurements revealed the viscoelastic properties and gel-like structure of the materials as well as time- and shear-dependent effects like thixotropy and pseudoplasticity (structural viscosity). In conjunction with potential applications film forming properties and temperature dependent behaviour (e.g. viscosity) of the materials were investigated. Additionally, the barrier properties against oxygen and moisture of nanocellulose coated polyester foils were examined. Selected samples of nano-cellulosic dispersions were dried via lyophilizsation, via spray drying, and solvent exchange. The dried products were characterized in terms of particle morphology (SEM) and porosity (mercury porosimetry and volumetric gas adsoprtion (BET)) in dependence on the drying method. Re-dispersed samples were compared with starting dispersions by means of SEM, DLS and rheometry.