Glass manufacturing – Processes – Self-supporting particle making
Reexamination Certificate
2001-08-22
2003-02-11
Colaianni, Michael (Department: 1731)
Glass manufacturing
Processes
Self-supporting particle making
C065S021400, C065S017200, C065S030110, C423S038000, C423S042000, C423S046000
Reexamination Certificate
active
06516633
ABSTRACT:
FIELD OF INVENTION
This invention is directed to a process for production of colloidal photochromic glass nanoparticles, that is, unagglomerated particles of ca. 100 nm or less in diameter.
BACKGROUND OF INVENTION
Nanotechnology, defined broadly as the manufacturing and application of nanometer-sized materials (e.g. nanoparticles), is experiencing unprecedented research and development, largely due to the unique and valuable properties of such materials. In particular, the application of nanoparticles to macro-sized materials can add novel properties to the macro-sized material, without alteration of desired physical properties inherent to the macro-material. These synergies do not exist when two macro-materials are combined.
One method for the controlled syntheses of numerous types of nanoparticles is “microemulsion-mediated” synthesis. This technique consists of combining differing amounts of two immiscible liquids and a surfactant or surfactant mixture. A surfactant is an ambiphilic molecule containing at least two segments, each of which is soluble in one of the two immiscible liquid phases. Surfactants act by decreasing the surface tension between the two phases, resulting in a dispersion of droplets called “micelles”, between 2 and 50 nm in size, with the lesser volume of liquid (the disperse phase) within the other liquid (the continuous phase). Most commonly, the continuous phase is water and the dispersed phase is a hydrocarbon oil. A dispersion of water in oil is called a reverse micelle dispersion.
The reverse micelles function as “nano-reactors” for reactions of molecules that dissolve exclusively in the aqueous phase. Brownian motion causes the micelles to continuously collide, coalesce, and break apart, resulting in constant exchange of the contents of the micelles. The addition of a reverse micelle dispersion or aqueous solution of reactant A to a reverse micelle dispersion of reactant B will cause reaction between A and B to afford product C as micelles containing the reactants collide and coalesce.
While highly controllable, the size of the nanoparticles produced is not directly related to micelle size and is not easily correlated to the concentration of reactants. An important factor governing particle size is the ratio R of the rate of nucleation (r
n
) to the rate of growth or agglomeration (r
g
), where R=r
n
/r
g
. A large ratio provides many, very small particles, whereas a smaller ratio results in fewer, larger particles. These competing factors can be empirically controlled by variation of reactant concentration and microemulsion compositional parameters (stirring speed, surfactant choice, amount of surfactant used, and temperature). Shah has reviewed the state of the art in this area. (Pillai et al.,
Adv. In Coll. and Interface Sci
., 55,241 (1995)).
These techniques have been employed to produce mono-disperse inorganic nanoparticles, which can be purified on an industrial scale through cross-flow membrane filtration (U.S. Pat. No. 5,879,715). Mono-disperse glass nanoparticles have also been produced by addition of basic aqueous solutions to a reverse micelle emulsion containing silica sol and other inorganic glass components (U.S. Pat. No. 5,837,025). The basic solution causes precipitation of glass nanoparticles within the micelles. The resulting nanoparticles are isolated by temperature-induced phase separation, washing, and centrifugation.
Photochromic glass absorbs visible light (darkens) upon exposure to actinic radiation, e.g. ultraviolet light, and ceases absorption (fades) when the actinic radiation is removed. This reversible behavior is ascribed to nanocrystalline silver halide particles dispersed in the glass. Reversible ionization of the silver halide causes the photochromic effect. It is noteworthy that the silver halide particles must be isolated for reversibility; this is the function of the glass. In many embodiments, small amounts of copper(I) salts are also embedded in the glass; the copper sensitizes the silver halide particles to UV light.
In typical production, the silver halide is included in the glass melt and the melt is shaped into the desired form and cooled. Often, the shaped product is not photochromic and must be heated to a temperature between the annealing point and the softening point of the glass, typically between 500° and 900° C. This heating allows for a phase separation of the silver halide within the glass. The resulting nanocrystals need to be 5 nm or greater in size to exhibit photochromic behavior. It would be desirable to develop a process of forming nanoparticles of silver halide and embedding them in glass. Such a process would avoid the extra costs associated with the annealing process.
At present there are no methods described in the literature for preparing photochromic glass nanoparticles.
SUMMARY OF THE INVENTION
The present invention is directed to microemulsion techniques for rapidly preparing photochromic glass nanoparticles and to the photochromic glass nanoparticles so prepared.
More particularly, the method of the invention comprises the combination of two microemulsions, one microemulsion containing a water-soluble silver salt and glass precursors and the other microemulsion containing a halide salt and an initiator for glass formation, which process rapidly yields glass-encased silver halide particles. This invention gives nanometer-sized (preferably about 100 nm or less in diameter) unagglomerated silver halide particles embedded in glass, thus providing photochromic glass nanoparticles without further annealing, or at most mild annealing. Preparation of these nanoparticles can be at ambient or slightly elevated temperature. These nanoparticles would be valuable as added components to any macro-material that one might wish to have photochromic properties. The particles would impart photochromism while not affecting the physical properties of the material. In one embodiment of the invention, the photochromic glass nanoparticles comprise reactive sites on their surface to allow attachment of the nanoparticles to macro-materials.
This invention is further directed to fibers, yarns, fabrics (which may be woven, knitted, stitch-bonded or nonwoven), other textiles, or finished goods (encompassed collectively herein under the terms “textiles” or “webs”) treated with the photochromic glass nanoparticles. Such textiles and webs exhibit the property of reversible, light-induced darkening.
DETAILED DESCRIPTION OF THE INVENTION
The method of the present invention comprises two parallel but non-interactive reactions, namely the formation of silver halide and the formation of glass. The reactions occur coincidentally in the same reverse micelle reactors. The reactions are initiated by combining two distinct precursor microemulsions. Each precursor microemulsion is chemically stable, as it contains only one or some of the components for each reaction. The components for formation of a silver halide nanoparticle are a water-soluble silver salt and a halide salt. The components for formation of glass are a glass composition comprising glass precursors and an initiator of glass formation. It is apparent that the precursor microemulsions must be constructed such that the reactive components are kept apart until mixing, but no other restriction on the microemulsion composition is implied. Thus, one preferred embodiment consists of a first precursor microemulsion containing a soluble silver salt and the glass-forming components, and a second precursor microemulsion containing a halide salt and an initiator of glass formation.
The precursor microemulsions are prepared by methods known in the art (see, for example, U.S. Pat. No. 5,837,025). The particulars of the preparation can be determined by one of skill in the art without undue experimentation.
Those skilled in the art will recognize that the rates of the two reactions are different. Insoluble silver halide particles form almost instantaneously when silver salts and halide salts are combined, whereas glass synthesis occurs at a slower rate. In the pres
Erskine Lael L.
Millward Dan B.
Soane David S.
Colaianni Michael
Larson Jacqueline S.
Nano-Tex, LLC
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