SYNTHESIS AND APPLICATION OF NANOPARTICLES BY A HIGH GRAVITY METHOD

摘要

Fast chemical reactions involved in nanomaterials synthesis, polymerization, special chemicals production, reactive absorption, etc., are often difficult to control in terms of product quality, process efficiency and production consistency.After a theoretical analysis on such processes based on chemical reaction engineering fundamentals, an idea to intensify micromixing (mixing on the molecular scale) and mass transfer and therefore to control the process ideally was proposed.By experimental investigations of mass transfer and micromixing characteristics in the Rotating Packed Bed (RPB, or "HIGEE" device), we achieved unique intense micromixing. This led us to the invention of using RPB as a reactor for the fabrication of nanoparticles (Chen et al., 2000).RPB consists mainly of a rotating packed rotator inside a stationary casing. The high gravity environment created by the RPB, which could be orders of magnitude larger than gravity, causes aqueous reactants going through the packing to spread or split into micro or nano droplets, threads or thin films, thus markedly intensifying mass transfer and micromixing to the extent of 1 to 3 orders of magnitude larger than that in a conventional packed bed.In 1994, the first RPB reactor was designed to synthesize nanoparticles of CaCO3 through multiphase reaction between Ca(OH)2 slurry and CO2 gas, and nanoparticles of 15～30nm in mean size and with very uniform particle size distribution was obtained. In 1997, a pilot-scale RPB reactor was successfully set up for operation, and in 2000, the first commercial-scale RPB reactor for synthesis of such nanoparticles came into operation in China, establishing a milestone in the use of RPB as a reactor for the fabrication of nanomaterials (Chen et al., 2002).Since then, the high gravity method has been employed for the synthesis of inorganic and organic nanoparticles via gas-liquid, liquid-liquid, and gas-liquid-solid multiphase reactions, e.g. inorganic nanoparticles like nanosized CaCO3, TiO2,SiO2, ZnO, Al2O3, ZnS, BaTiO3, BaCO3, SrCO3, Al(OH)3 and Mg(OH)2 flame retardants, and organic nano-pharmaceuticals including benzoic acid, salbutamol sulfate and cephradine. This technology received extensive attention in the field of nanomaterials fabrication and application. Dudukovic et al. commented, "The first large-scale application of RPB as a reactor occurred in China in production of nano CaCO3 by HGRP (high gravity reactive precipitation)of carbon dioxide and lime. Uniformly small particles were made in the RPB due to achievement of a sharp supersaturation interface and very short liquid residence times in the device." (Dudukovic et al., 2002). Date et al. said, "HGRP represents a second generation of strategies for nanosizing of hydrophobic drugs. In our opinion, among various methodologies described eariier, supercritical anti-solvent enhanced mass transfer method and HGRP method has potential to become technologies of the future owing to their simplicity, ease of scale-up and nanosizing efficiency" (Date et al., 2004).As-synthesized nano CaCO3 was employed as a template to synthesize silica hollow spheres (SHS) with mesostructured walls. Characterizations indicated that the obtained SHS had an average diameter of about 40 nm with a surface haviors of Brilliant Blue F (BB), which was used as a model drug. Loaded inside the inner core and on the surfaces of SHS,BB was released slowly into a bulk solution for as long as 1 140min as compared to only 10min for the normal SiO2nanoparticles, thus exhibiting a typical sustained release pattern without any burst effect. In addition, higher BET value of the carrier, lower pH value and lower temperature prolonged BB release from SHS, while stirring speed indicated little influence on the release behavior, showing the promising future of SHS in controlled drug delivery (Li et al., 2004).Nano-CaCO3 synthesized by the high gravity method was also employed as a filler to improve the performance of organic materials. By adding CaCO3 nanoparticles into polypropylene-ethylene copolymer (PPE) matrix, the toughness of the matrix was substantially increased. At a nanosized CaCO3 content of 12 phr (parts per hundred PPE resin by weight),matrix. In the nanosized CaCO3/PPE/SBS (styrene-butadiene-styrene) system, the rubbery phase and filler phase were independently dispersed in the PPE matrix. As a result of the addition of nanosized CaCO3, the viscosity of PPE matrix significantly increased. The increased shear force during compounding continuously broke down SBS particles, resulting in the reduction of the SBS particle size and improving the dispersion of SBS in the polymer matrix. Thus the toughening effect of SBS on matrix was improved. Simultaneously, the existence of SBS provided the matrix with good intrinsic toughness, satisfying the condition that nanosized inorganic particles of CaCO3 efficiently toughened the polymer matrix,thus fully exhibiting the synergistic toughening function of nanosized CaCO3 and SBS on PPE matrix (Chen et al., 2004).As-prepared nano-CaCO3 was blended with TiO2 and other additives to prepare complex master batches for use in the coloring of polypropylene. It was found that the obtained nano-CaCO3 is an excellent pigment dispersant, which can partially replace TiO2 pigments for polypropylene resin coloring. Nano-CaCO3 can prompt the dispersion of TiO2 in polymer matrix, boosting the whiteness of the materials without a negative effect on the UV absorbency of the materials (Guo et al.,2004). Studies on the mechanical properties of nano-CaCO3 toughened epoxy resin composite indicated that impact strength and flexural modulus of the composite improved remarkably when 6wt.% of nano-CaCO3 was added. Surface treatment of nano-CaCO3 by titanate coupling agents significantly improved the dispersibility of nano-CaCO3 in such a high viscous matrix (Li et al, 2005).