Abstract:
A process to prepare a stable dispersion of nanoparticles and non-aqueous media. A polymeric dispersant is combined with non-aqueous media to form a mixture. Nanoparticles are then added to the mixture.

Description:
FIELD  
         [0001]    The present invention relates to the preparation of stable dispersions of substantially spherical nanocrystalline metal oxides in non-aqueous media. Stable dispersions of substantially spherical nanocrystalline metal oxides in non-aqueous media would be of use as a component of transparent coatings on surfaces to yield unique properties such as abrasion resistance, radiation absorption, and catalytic function. Stable non-aqueous dispersions may also function as abrasive or polishing fluids, thermal fluids, catalytic additives, electro-rheological fluids, etc. Such dispersions could also act as a convenient means of transporting well-dispersed nanocrystalline metal oxides to a point of application.  
         BACKGROUND  
         [0002]    Conventionally, stable colloidal dispersions of metal oxides in non-aqueous media are prepared using long-chain carboxylic acids (fatty acids) or diesters of phosphoric acid in non-aqueous solvents (See, U.S. Pat. No. 6,093,223 (Lemaire, et al.), U.S. Pat. No. 6,136,048 (Rhodia Chimie) and U.S. Pat. No. 6,210,451 (Rhone-Poulenc Chimie)). Such dispersions comprise agglomerates of crystallites, which are nanometer sized, and exhibit rapid settling of the metal oxide particles. Another method used to stabilize metal oxides in non-aqueous media has been to prepare hydrocarbon-soluble coordination compounds such as ceric 2,4-hexandionate or other acetylacetonate derivatives (See, U.S. Pat. Nos. 4,036,605 and 4,211,535 (Hartle), U.S. Pat. No. 5,716,547 (Rhone Poulenc Chimie)). Such coordination compounds may in certain instances yield stable dispersions, but also substantially alter the nature of the nanocrystalline oxide.  
           [0003]    Based on conventional methodology for the dispersion of metal oxide particles in non-aqueous media an attempt was made to disperse substantially spherical nanocrystalline metal oxides using surfactants well known in the art. However, when conventional surfactants were employed in a manner and at concentrations expected to result in stable dispersions of substantially spherical nanocrystalline metal oxide particles none of the materials or methodologies described in the prior art yielded stable dispersions. Instead, attempts to prepare substantially stable dispersions of substantially spherical nanocrystalline metal oxides in non-aqueous media using conventional surfactants lead to either rapid settling of the metal oxide particles, or agglomeration followed by rapid settling.  
           [0004]    Surprisingly, polymeric dispersants, comprised of polymeric chains (molecules with repeating backbone units) and featuring one or more anchor groups, were found to be very effective at yielding substantially stable dispersions of substantially spherical nanocrystalline metal oxides in non-aqueous media. Dispersion stability is enhanced if the polymeric dispersant is essentially soluble in the non-aqueous media.  
         SUMMARY  
         [0005]    Polymeric dispersants with one or more anchor groups and polymeric chains were very effective at yielding substantially stable dispersions of substantially spherical nanocrystalline metal oxides in non-aqueous media. Dispersion stability was enhanced if the polymeric dispersant was soluble in the non-aqueous media.  
           [0006]    In one example the invention comprises a process to prepare a stable dispersion of nanoparticles and non-aqueous media. The process includes combining a polymeric dispersant with the non-aqueous media to form a mixture and adding nanoparticles to the mixture.  
         DETAILED DESCRIPTION  
         [0007]    A detailed discussion of exemplary embodiments of the invention is presented herein, for illustrative purposes.  
           [0008]    The dispersability of substantially spherical metal oxides was evaluated in non-aqueous media using a variety of pigment dispersants, surfactants, wetting agents, coupling agents, etc. (referred to collectively as dispersants). The non-aqueous media is selected from a group comprising polar hydrocarbons, non-polar hydrocarbons, alcohols, and silicones. The evaluated dispersants had the following characteristics:  
           [0009]    Molecular size varied from high molecular weight polymers to low molecular weight coupling agents;  
           [0010]    Anchoring groups were selected from a group comprising acidic, basic, and neutral; and  
           [0011]    Ionic character was selected from a group comprising cationic, anionic, and neutral.  
           [0012]    The one criterion required for each of the dispersants was that it be soluble in the non-aqueous media.  
           [0013]    The dispersion of substantially spherical nanocrystalline metal oxide or mixed metal oxide (referred to collectively as “oxides”) in non-aqueous media was evaluated by the following criterion:  
           [0014]    Dispersion appearance and viscosity—The lower the dispersion viscosity at a given nanocrystalline metal oxide concentration, the more effective the dispersant.  
           [0015]    Solvated Particle Size—The smaller the mean particle size measured for solvated nanocrystalline metal oxides in dispersion, the more effective the dispersant. SPS was measured by dynamic light scattering (DLS) of the dispersed particles and reported as the mean volume-weighted diameter of the solvated particle. The solvated particle diameter is approximately 3 to 5 times more than the discrete particle diameter for a substantially spherical nanocrystalline metal oxide, depending on the metal oxide—non-aqueous media pair.  
           [0016]    Dispersion Stability—The greater the stability of a dispersion of nanocrystalline metal oxide the more effective the dispersant.  
           [0017]    The study evaluated substantially spherical nanocrystalline metal oxide concentrations in the non-aqueous media from 0.001-wt % to 60-wt % and dispersant concentration with respect to metal oxide from 0.5-wt % to 40-wt %. Dispersions were prepared by high-shear mixing techniques such as rotor-stator methods, ultrasonic methods, and other methods known to those skilled in the art.  
           [0018]    Specifically, the dispersability of substantially spherical nanocrystalline metal oxides into alcohols was evaluated. More specifically, the evaluated alcohol was ethanol (EtOH). The substantially spherical nanocrystalline metal oxides tested were selected from a group comprising aluminum oxide, antimony tin oxide (ATO), cerium oxide, and zinc oxide. The most effective dispersant type for the substantially spherical nanocrystalline metal oxides was polyvinylpyrolidone with a MW of 9700—this is a polymeric material containing multiple basic anchor groups.  
           [0019]    Specifically, the dispersability of substantially spherical nanocrystalline metal oxides into non-polar hydrocarbons was evaluated. More specifically, the evaluated non-polar hydrocarbon was heptane. The substantially spherical nanocrystalline metal oxides tested were selected from a group comprising aluminum oxide, antimony tin oxide (ATO), cerium oxide, iron oxide, indium tin oxide (ITO), and zinc oxide. The most effective dispersants feature two specific properties: (1) molecular weight greater than 1,000 and (2) one or more anchor groups exhibiting either acidic or basic character. Substantially spherical nanocrystalline metal oxides have both acid and base sites on their surface, and the effectiveness of these dispersants results from a strong affinity of the acid/basic anchor group for the surface sites. In addition, the polymeric chains associated with the dispersants are particularly effective at providing the steric repulsion necessary to prevent aggregation in the non-polar hydrocarbon.  
           [0020]    Specifically, the dispersability of substantially spherical nanocrystalline metal oxides into polar hydrocarbons was evaluated. More specifically, the evaluated polar hydrocarbons were selected from the group consisting of propylmethoxyacetate (PMA), methyl ethyl ketone (MEK), and iso-propyl alcohol (IPA). The substantially spherical nanocrystalline metal oxides tested were selected from a group comprising aluminum oxide, antimony tin oxide (ATO), cerium oxide, and zinc oxide. For a given metal oxide, the better dispersant for a given polar hydrocarbon varied due to dispersant solubility in the tested polar hydrocarbon. But in general, the most effective dispersants feature two specific properties: (1) molecular weight greater than 1,000 and (2) multiple basic anchoring groups.  
           [0021]    In general, a stable dispersion of substantially spherical nanocrystalline metal oxides and non-aqueous media is formed using (1) polymeric dispersants having molecular weight greater than 1000, and (1) one or more acidic or basic anchoring groups that interact with the metal oxide surface. In general, both homopolymers and copolymers can be effective dispersants for nanocrystalline metal oxides provided the following requirements are met: (1) molecular weight greater than 1000, (2) one or more achor groups with acidic or basic character, and (3) soluble in the non-aqueous media. However, certain homopolymer and copolymer dispersants may be rendered ineffective, even if the above listed requirements are met, due to:  
           [0022]    Anchor groups are sterically hindered or inaccessible with respect to the metal oxide surface and are not able to efficiently interact to provide efficient particle dispersion in the non-aqueous media, and/or;  
           [0023]    The acidic or basic character of the anchor group is of a chemical type that does not form an interaction with the metal oxide surface of sufficient strength to provide efficient particle dispersion in the non-aqueous media.  
           [0024]    Theory not withstanding, there may exist a complex relationship between dispersant molecular weight and dispersion stability, making it is very difficult to generalize as to this relationship. 
       
    
    
     EXAMPLE 1  
     Zinc Oxide Dispersions in EtOH Using a Polymeric Dispersant  
       [0025]    A dispersion of substantially spherical nanocrystalline zinc oxide in ethanol was prepared by combining 4.00 g of zinc oxide powder with a solution comprised of 0.20 g of polyvinylpyrolidone (PVP) K-15 (ISP Corporation) dissolved in 6.00 g of ethanol. The mixture was subjected to ultrasonic vibration for 30 minutes to yield a stable dispersion of the zinc oxide in ethanol.  
         [0026]    The solvated particle size of ZnO nanoparticles was determined by DLS. For substantially spherical nanocrystalline zinc oxide—EtOH dispersion, made with PVP K-15, a mean volume-weighted solvated diameter of 320 nm was measured indicating no particle aggregation or flocculation.  
                                     TABLE 1                           Substantially Spherical Nanocrystalline Zinc Oxide in EtOH            Dispersant   Type   Viscosity   SPS, nm   Stability               Polyvinylpyrolidone   MW = 9700,   Low   320   Stable           basic anchor       Polyvinylpyrolidone   MW = 66,800,   Low   340   Stable           basic anchor                  
 
       Comparative Example 1  
     Zinc Oxide Dispersions in Ethanol Using Low Molecular Weight Dispersants  
       [0027]    Dispersions of zinc oxide in ethanol were prepared by mixing 3.00 g of zinc oxide with 7.00 g of ethanol containing 0.30 g of the dispersants (surfactants, wetting agents, coupling agents, etc) listed in the table below. In one case, no dispersant was used. The resulting mixtures were subjected to ultrasonic vibration for 30 minutes. Compared to the polymeric dispersants listed in Example 1, none of the low molecular weight dispersants in Table 2 resulted in a stable dispersion of the nanocrystalline zinc oxide particles as evidenced by either rapid particle settling, flocculation, or gelling of the mixture.  
                                                           TABLE 2                           Substantially Spherical Nanocrystalline Zinc Oxide in EtOH                        SPS,           Dispersant   Type   Viscosity   nm   Stability                    E 335   70% PVP, 30%   Very High   NA   Rapid           polyvinylacetate           settling       Solsperse   Polymeric alkoxylate   Very High   NA   Flocculation       20000       Hydropalat   Nonionic and ionic   Very High   NA   Flocculation       3216   surfactant mixture       KR-55   Titanate coupling agent   Very High   NA   Flocculation       LICA 38   Titanate coupling agent   Very High   NA   Rapid                       settling                  
 
       EXAMPLE 2  
     Cerium Oxide Dispersions in Heptane Using a Polymeric Dispersant  
       [0028]    Dispersions of nanocrystalline cerium oxide in heptane were prepared by blending 3.33 g of cerium oxide powder with 5.35 g of heptane and 40 wt % of the polymeric dispersants included in Table 3 with respect to cerium oxide (with the exception of 13 wt % for Solsperse 17000). The mixtures were subjected to ultrasonic vibration for 30 minutes, and each resulted in stable dispersions of the cerium oxide nanoparticles in heptane. The resulting mean particle diameter measured for each of the cerium oxide dispersions with the polymeric dispersants is also included in Table 3, with the results indicating a high degree of dispersion and no particle aggregation or flocculation.  
                                     TABLE 3                           Substantially Spherical Nanocrystalline Cerium Oxide       Dispersions in Heptane                        SPS,           Dispersant   Type   Viscosity   nm   Stability               Solsperse 17000   Basic polyamide/   Low   280   Stable           polyester       Ganex V-216   Polyvinylpyrolidone/   Low   320   Stable           poly-C16-olefin       Ganex V-220   Polyvinylpyrolidone/   Low   340   Stable           poly-C16-olefin       Solsperse 3000   Acidic polymer   Low   340   Stable                  
 
       Comparative Example 2  
     Cerium Oxide Dispersions in Heptane Using Low Molecular Weight Dispersants  
       [0029]    Mixtures of nanocrystalline cerium oxide in heptane were prepared by blending 3.33 g of cerium oxide powder with 5.35 g of heptane and the polymeric dispersants in Table 4 at 40-wt % with respect to cerium oxide. The mixtures were subjected to ultrasonic vibration for 30 minutes. Compared to the stable dispersions achieved with polymeric dispersants in Example 2, none of the low molecular weight dispersants in Table 4 produced stable dispersions of cerium oxide in heptane.  
                                     TABLE 4                           Substantially Spherical Nanocrystalline Cerium Oxide       Dispersions in Heptane                        SPS,           Dispersant   Type   Viscosity   nm   Stability               None       Very High   NA   Flocculation       Stearic Acid   Fatty acid   High   392   Rapid                       Settling       Lorama D100   Fatty acid ester   High   883   Rapid                       Settling       K-Sperse 131   Alkylnaphthanlene-   Very High   NA   Flocculation           sulfonicacid salt       Emphos PS-21A   Phosphate ester   High   608   Rapid                       Settling       Silwet L77   Silicone polymer   Very High   NA   Flocculation       Stearamide   Fatty amide   Very High   NA   Flocculation                  
 
       EXAMPLE 3  
     Aluminum Oxide Dispersions in PMA Using Polymeric Dispersants  
       [0030]    Dispersions of nanocrystalline aluminum oxide in propylmethoxyacetate (PMA) were prepared by blending 4.00 g of aluminum oxide powder with 5.60 g of PMA containing 0.40 g of the polymeric dispersants listed in Table 5. The mixtures were subjected to ultrasonic vibration for 30 minutes, to yield stable dispersions of the aluminum oxide nanoparticles in PMA. The resulting mean particle diameter measured for the two aluminum oxide dispersions with the polymeric dispersants is also included in Table 5 demonstrating the high degree of dispersion and stability.  
                                     TABLE 5                           Substantially Spherical Nanocrystalline Aluminum Oxide       Dispersions in PMA                        SPS,           Dispersant   Type   Viscosity   nm   Stability               Solsperse 24000   Basic polymer   Low   120   Stable       Solsperse 32000   Basic polyamide/   Low   130   Stable           polyester       Paraloid B-99N   Polymethylmethacrylate   Low   140   Stable       Disperbyk 111   Acidic copolymer   Low   130   Stable       Disperbyk 163   Block copolymer   Low   130   Stable                  
 
       Comparative Example 3  
     Aluminum Oxide Dispersions in PMA Using Low Molecular Weight Dispersants  
       [0031]    Dispersions of nanocrystalline aluminum oxide in propylmethoxyacetate (PMA) were prepared by blending 2.00 g of aluminum oxide powder with 7.600 g of PMA containing 0.40 g of each of the low molecular weight dispersants listed in Table 6. The mixtures were subjected to ultrasonic vibration for 30 minutes. Compared to the aluminum oxide dispersions of Specific Example 3 prepared with polymeric dispersants, the low molecular weight dispersants in Table 6 did not produce stable dispersions of aluminum oxide in PMA.  
                                                           TABLE 6                           Aluminum Oxide Dispersions in PMA                        SPS,           Dispersant   Type   Viscosity   nm   Stability                    Lorama D100   Fatty acid ester   Very High   NA   Rapid                       Settling       K-Sperse 131   Alkylnaphthanlene-   Very High   NA   Flocculation           sulfonicacid salt       Emphos PS-21A   Phosphate ester   Very High   NA   Rapid                       Settling       Ser-Ad FA 196   Anionic Surfactant   Very High   NA   Rapid                       Settling                  
 
         [0032]    The preceding embodiments are illustrative of the practice of the invention. It is to be understood, however, that other expedients known to those skilled in the art, or disclosed herein, may be employed without departing from the spirit of the invention or the scope of the appended claims. Although preferred embodiments have been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions and the like can be made without departing from the spirit of the invention and these are therefore considered to be within the scope of the invention as defined in the following claims.