Metal oxide sols and process for making the same

Colloid systems and wetting agents; subcombinations thereof; pro – Continuous liquid or supercritical phase: colloid systems;... – Aqueous continuous liquid phase and discontinuous phase...

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C423SDIG001, C516S033000, C516S088000, C516S093000, C524S397000, C524S398000, C524S399000, C526S318200, C556S055000

Utility Patent

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06169119

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to metal oxide sols and processes for making the same. More particularly, the invention relates to metal oxide sols and processes for making the same involving the use of a multifunctional compound containing a chelating group.
BACKGROUND OF THE INVENTION
It is known to incorporate additives into base substances, such as polymers, ceramics etc., as a means of enhancing the properties of the base substance or as a means of imparting new properties to the base substance. For instance, it is known to add filler compounds to resins in order to improve the physical properties of the resin.
As a specific example, curable electrical component coatings include inorganic additives to achieve the desired surface electrical stress endurance. Such inorganic additive materials include alumina, silica and fumed metal oxide particulate additives and other non-transparent materials. Many inorganic additives are by nature resistive to high temperature processing, both in production and in use, and they are resistive to oxidative degradation. Most inorganic additives, however, due to their compositional and physical makeup, require the use of high shear mixing when incorporated into a polymer to achieve a uniform, homogeneous composition. High shear mixing inherently creates voids in the resulting coating due to the entrapment of air in the protective coating mixture. The presence of voids in the cured coating allows corona generation which attacks the underlying substrate and degrades the coating itself under electrical stress when in use. Accordingly, it is desired to provide an additive which does not require high shear mixing and/or which does not lead to the presence of voids in a base substance in which it is incorporated.
In photocurable resin systems, using non-transparent material additives with photocure processing techniques results in non-uniform curing, as the light energy curing agent may unevenly penetrate the curable resin, due to particle blockage and scattering, thus curing some resin segments and not curing others. Another problem caused by the same uneven, non-uniform penetration of the various additives is the premature cure of the resin. When using a photo initiated curing process, it is generally necessary to have particles of less than 0.2 microns. Particles in excess of 0.2 microns are capable of scattering light, thus potentially resulting in uneven curing. Commercially available particulate fillers which require high speed mixing to maintain homogeneity tend to agglomerate causing regions of higher particle concentration and regions of lower concentration. This can lead to accelerated oxidation in the particle-poor regions. Accordingly, it is desired to provide an additive which does not agglomerate, which is small in size, transparent in nature and/or capable of uniform distribution.
One problem with using metal oxide particulate material in a liquid substance is the propensity for precipitation of the material from a solution over time, thus limiting the shelf life of the solution. For example, the use of commercially available fumed alumina or silica results in precipitation of the particulate metal oxide material after about one week in storage. Since fumed alumina or silica is of high viscosity, increased amounts of solvent are needed to attain a coatable composition. Accordingly, it is desired to provide an additive which does not precipitate from solution and/or has a desirable viscosity.
U.S. Pat. No. 4,760,296 generally relates to the inclusion of organosilicates or organoaluminates as the organo-metallic material of choice to achieve improved electrical stress endurance of an epoxy resin system. The '296 patent also relates to organoaluminates such as aluminum acetylacetonate and aluminum di-sec-butoxide acetoacetic ester chelate, which can be used to produce clear resins. However, the organoaluminum compounds of the '296 patent are not suitable for a variety of resin systems. This is because they tend to (1) plasticize the cured articles, (2) generate nonuniform distribution of the additives in the cured articles, and/or (3) bleach out with aging. The same three disadvantages are associated with using fumed aluminum oxide in resin systems. Using fumed aluminum oxide also involves the disadvantages that a clear solution cannot be formed and that the viscosity is undesirably high, further contributing to the creation of voids in the resulting coating thus rendering the coating susceptible to corona attack.
These problems are minimized by using the metal oxide sols of the present invention, which can be low viscosity materials and which can lead to the creation of transparent resins susceptible to photocuring processes.
SUMMARY OF THE INVENTION
In one embodiment, the present invention relates to metal oxide sols comprising a liquid and a polycondensation product of about 2 to about 5,000 monomers of a partially hydrolyzed chelated metal oxide precursor.
In another embodiment, the present invention relates to a process for making a metal oxide sol comprising contacting a metal oxide precursor with a multifunctional compound in a liquid to provide a chelated metal oxide precursor; contacting the chelated metal oxide precursor with a hydrolyzing agent to provide partially hydrolyzed chelated metal oxide precursor monomers; and permitting the partially hydrolyzed chelated metal oxide precursor monomers to polycondense thereby forming a metal oxide sol.
In yet another embodiment, the present invention relates to a metal oxide sol made by a process comprising contacting a metal oxide precursor with a multifunctional compound to provide a chelated metal oxide precursor; hydrolyzing the chelated metal oxide precursor to provide partially hydrolyzed chelated metal oxide precursor monomers; and permitting the partially hydrolyzed chelated metal oxide precursor monomers to polycondense thereby forming a metal oxide sol.
One advantage of the metal oxide sols is that they are relatively stable. This leads to a long shelf life. Another advantage is that the open functionalities of the condensed partially hydrolyzed chelated metal oxide precursors of the metal oxide sols have the capability of reacting with a base substance in which it is incorporated.
Another advantage in plastics made with the inventive metal oxide sols is that corona resistance of a polymer substance made with the metal oxide sols is markedly increased because of the presence of the condensed partially hydrolyzed chelated metal oxide precursors. In this connection, the metal oxide sols are not polymer fillers in the traditional sense (merely dispersed in the polymer); instead, the metal oxide sols are incorporated into the polymer network on a molecular level.
DETAILED DESCRIPTION OF THE INVENTION
The invention relates to metal oxide sols and processes for making the metal oxide sols. A “sol”, as the term is used herein, refers to a composition comprising a liquid colloidal dispersion containing a liquid phase and a dispersed phase. The liquid phase of the liquid colloidal dispersion may be aqueous and/or organic, and in particular, may be at least one of water and organic liquids such as alcohols, glycols and other protic organic solvents. Organic solvents include methanol, ethanol, propanol, isopropanol, sec-butanol, t-butanol, methoxyethanol, ethoxyethoxyethanol, ethylene glycol and propylene glycol. The liquid phase may also be a liquid or partially liquid substance to which a metal oxide sol can be added such as resin monomers. For example, in the case where it is desired to incorporate metal oxide sols into a curable resin, the liquid phase of the metal oxide sols may be constituted by curable resin monomers in liquid form.
The dispersed phase of the liquid colloidal dispersion comprises condensed partially hydrolyzed chelated metal oxide precursors. The condensed partially hydrolyzed chelated metal oxide precursors are micro-clusters which generally have an amorphous shape, although in some embodiments a somewhat symmetrical shape is obtained.

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