The present invention relates, broadly/ to metal-coated microspheres. More particularly, the present invention relates to hollow microspheres having exposed metallic particles bound thereto by a ther osetting adhesive material.

BACKGROUND OF THE INVENTION

As general background information, the following are mentioned. U.S. Patent No. 4,137,367 (1979) discloses phyllosilicate minerals which are superficially etched under specific conditions with dilute acid to remove an outer octahedral layer so as to preserve structural integrity. _N . The acid etch is said to expose silanol groups which are receptive to subsequent condensation with an organo-silane in a suitable solvent under mild conditions.

Also known are cellular glass pellet cores that have been bonded to and coated with fly ash particles. Such pellets are rather large, on the order of 0.5 to 20 mm and are described in U.S. Patent No. 4,143,202.

Various coupling agents composed of organo- functional silanes plus an amine silicate and treatments therewith for reinforcing fibers are also known. Such an amine silicate component has a degree of polymerization less than 1000. It is said in U.S. Patent No. 3,649,320 (1976) that the formulation enables better control over the spatial

arrangement of the coupling agent about the surface of the reinforcement material.

Efforts to improve the compatibility of organic polymers and resins with pre-heated coal fly ash are disclosed in U.S. Patent No. 4,336,284

(1982). These efforts include partially covering coal fly ash with an essentially hydrophobic mono- molecular partial coating of a chemical agent, the thickness being less than 100°A. In the past, efforts to dissipate and control static electricity build-up necessitated the use of carbon powder fillers in composite materials. These composite materials, laden with carbon, were used to prevent static electricity build-up in hospitals and, for example, computer centers. Disadvantageously, however, such composites exhibited poor physical properties.

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SUMMARY OF THE INVENTION

The present invention pertains to a product and a process for producing metal-coated hollow microspheres. The process comprises vigorously admixing hollow microspheres with an adhesive binder to coat the hollow microspheres, adding metal flakes to the thus coated hollow microspheres, slowly and uniformly heating the microsphere-binder-metal intermediate product to a temperature of up to about 350 ^β F, thereafter intermittently admixing or tumbling (i.e. agitating) the heated hollow- microsphere-binder-metal intermediate product in the absence of further heating so as to cure the binder whereby the resulting product, metal-coated hollow microspheres, is obtained, and subsequently recovering the product.

An object of the present invention is to provide a simple process for preparing metal-coated hollow microspheres.

Yet another object of the present invention is to prepare metal-coated hollow microspheres having an exposed metal surface(s).

Still another object of the present invention is to provide metal-coated hollow microspheres suitable for a wide variety of end-use applications including dissipative ingredients, marine coatings and/or paints, glassy concrete, EMI shielding, and RFI shielding.

Yet a further object of the present invention is to provide a process that obviates the need for acid-pretreatment of hollow microspheres prior to use.

Other objects, features, and characteristics of the present invention, as well as the method and operation thereof, will become more apparent upon consideration of the following description and the appended claims all of which form a part of this specification.

DETAILED DESCRIPTION OF THE INVENTION

The process of the present invention comprises, generally, the following combination of steps. A selected quantity of hollow microspheres is vigorously admixed and blended with an adhesive, preferably a thermosetting adhesive, until the hollow microspheres are wet-out, i.e. coated with the adhesive binder. Next, the desired metal, in the form of flakes, is slowly added to the wet-out hollow microspheres thereby providing metal-coated hollow microspheres having exposed metallic

surfaces. Subsequently, the metal flakes are permanently bonded to the hollow microspheres by curing the binder which includes slowly and uniformly heating the microspheres to a maximum temperature of about 350 ^β F. Following this curing procedure, the metal coated hollow microspheres are intermittently mixed or intermittently tumbled, in the absence of any further heating, until the metal- coated hollow microspheres are dry. The resulting metal-coated hollow microspheres are then recovered.

More particularly, the process for preparing metal-coated hollow microspheres according to the present invention comprises: (a) vigorously admixing a major quantity of hollow microspheres with about 3 to about 6 weight percent (based on the weight of the final product) of a thermosetting binder adhesive until the hollow microspheres are wet-out; (b) slowly adding metal flakes having an average size of 6-10 microns to the thus wet-out hollow microspheres from step (a) until the wetrout hollow microspheres are fully coated with the metal flakes; (c) binding the metal flakes to the wet-out hollow microspheres by slowly applying heat to raise the temperature of the metal coated-hollow microspheres from step (b) to a temperature between about 220°F and about 240°F; and (d) intermittently agitating the metal-coated hollow microspheres from step (c) in the absence of any further heating until the resultant metal-coated hollow microspheres are dry.

During the initial admixing of the hollow microspheres with the binder, heat can be applied to raise the temperature of the mixture in the range of about 120°F to about 180°F, and preferably between

140°F and 160°F. The present process is essentially a solventless one.

In the above-described process, the adhesive binder initially introduced into a mixing vessel which has already been charged with a quantity of cenospheres, i.e. microspheres. Suitable techniques for applying the binder to the microspheres include the spraying or misting methods as well as directly pouring the binder onto the microspheres. The adhesive can thus be introduced into the vessel in the form of a mist, liquid or vapor. During this application, however, the microspheres and adhesive binder should be agitated so as to insure proper coating of the microspheres. Next, a desired quantity of metal flakes is added, preferably slowly, to the mixing "vessel. The metal .flake addition continues until the microspheres, previously coated with the uncured adhesive binder, are fully covered by metal flakes. The metal flakes tend to stick to the uncured adhesive binder. An acceptable and suitable metal flake coating is readily determined by visual inspection. For more critical end-use applications, more control may be required and in such cases inspection of periodic samples, for example, under a 40 power microscope is an exemplary control technique.

Representative end use applications of the products of the present invention include:

(a) use in composite materials to control static electricity in critical applications such as in operating rooms and aircraft;

(b) use in shielding layers in microcircuitry, such as printed circuit boards; and

(c) use in molding wherein either solid or flexible substrates require an outer layer having electrically conductive properties for radio frequency shielding. Suitable molding processes include injection molding or powder-in-mold-coating techniques.

The hollow microspheres suitable for use in the present invention include a wide variety of commercial-grade microsphere products. Generally, the hollow microspheres have an average particle size ranging from about 60 microns up to about 180 microns. The hollow microspheres can, of course, have larger diameters, but generally the average diameter falls within the above-stated range. More particularly, the hollow microspheres have an average particle size diameter ranging between 100 microns and 180 microns and still more particularly from 100 to 150 microns. More advantageously, the microspheres have a narrow distribution of average particle sizes. The size of the hollow microspheres employed in the present process, from an average diameter particle size perspective, will affect the weight percent of the metal flake employed in the present process. The larger the hollow microspheres are, the greater is the quantity of metal flakes required.

Advantageously, hollow fly ash microspheres are employed in the present process to produce the present products. Such hollow microspheres exhibit high compressive strengths and thus can withstand considerable amounts of shear generated in intensive

mixing. An exemplary fly ash hollow microsphere, suitable for use herein, is described in Table 1, below.

TABLE I

Chemical Analysis of Typical Fly

Ash Hollow Microspheres

Ingredient % by Weight

Silica (as Si0 ₂ ) 55.0-66.0

Alumina (as A1 ₂ 0 ₃ ) 25.0-30.0 Iron Oxides (as Fe ₂ 0 ₂ ) 4.0-10.0

Calcium (as CaO) 0.2-0.6

Magnesium (as MgO) 1.0-2.0

Alkalai (as Na ₂ 0, K ₂ 0) 0.5-4.0

«•_

The hollow microspheres are essentially dry; that is, preferably they are substantially water-free prior to use in the present invention. In the process of the present invention, the microspheres are admixed with about 3 to about 6 weight percent of a binder adhesive, based on the weight of the final product. The binder adhesive can also be used in a lesser amount ranging from about 3 to about 4 weight percent, again based on the weight of the final product.

The adhesive binder employed in the present process is preferably a thermosetting type adhesive. More particularly, the binder comprises an organo-functional silane having organo-reactive radical functional groups that can be polymerized at

elevated temperatures, along with a reactive diluent. The diluent tends to extend the silane and also co-polymerizes with it.

The inorganic moiety of the silane molecule attaches to the microsphere at the lower temperatures described in the admixing step, and is covalently bonded thereto by a hydrolysis reaction. During the curing step, the organic moiety of the silane molecule co-polymerizes and cross-links with the reactive diluent to form a thermosetting polymer which binds the metal flakes to the hollow microspheres.

An exemplary organo-functional silane product is, for example, 3[2 (vinyl benzylamino) ethylamino] propyltrimethoxy silane. suitable reactive copolymerizable constituents include, for example, various lactones such as gamma- butyrolactone.

The metal flakes employed in the present invention are very small sized. The flakes should have as low an average flake size as feasible. The larger the average flake size, the more difficult it is to provide a smooth finish with a paint or other coating incorporating such hollow microspheres. Also, metal flake to microsphere bonding is inconsistent at large particle sizes. More particularly, the average size of the metal flakes can range, for example, from about 2 microns up to and including about 10 microns. Preferably, the average size of metal flakes ranges from about 6 microns to about 10 microns. Advantageously this latter range results in an aesthetically pleasing product suitable for desired end-use applications. Representative metals employed in flake form in the process of the present invention of include, for

example, zinc, aluminum, silver, copper, stainless steel, platinum and gold.

Typically, the metal flakes are vigorously blended with the microspheres coated with binder adhesive in an amount ranging from about 15 to about 30 weight percent of the weight of the adhesive binder coated hollow microspheres. Particularly, and more preferably, the metal flakes are added in an amount ranging from about 17 weight percent to about 25 weight percent. Most advantageously, the metal flakes are added in an amount of about 18 to about 22 weight percent. This latter weight percent range provides most advantageous results when the hollow microspheres have an average particle size average of about 165-170 microns. Excessive metal flakes can be easily removed at this stage or during subsequent work-up of the final product;

During the curing step, the temperature is preferably raised and maintained lower than about 350°F and advantageously lower than 300°F. More particularly, the temperature is subsequently uniformly raised within several minutes until, by visual observation or by other means, it is apparent that the thermosetting binding adhesive has commenced curing. Typically, after the temperature has been slowly raised up to about 220°F to about 240°F, the binder will begin curing within a matter of a few minutes. In large production runs, this step can be thermostatically controlled in conjunction with suitable timing mechanisms.

The heating step of the present invention is critical. Excessive heating or an excessive rate of applying heat leads to improperly cured products and defects which result, for example, from the difference of coefficient of expansion for the metal

flakes and that of the hollow microspheres. Thus, excessive heat expands. the metal flakes breaking them loose from the hollow microsphere during step (c). After the binder begins curing, the product is very carefully, but intermittently, tumbled or admixed on a cyclic basis. The intermittent tumbling can occur off and on for several minutes or longer. For example, in a blender, mixer, or other similar conventional apparatus, the products being cured are left in a (quiescent state for a few minutes and then admixed or tumbled for a very brief period of time, generally about one-half of minute, followed by a quiescent state. This intermittent cycling or admixing/tumbling can occur about 15-20 times during this phase of the present invention. During this phase various by-products such as, for example, the water of hydration or methyl alcohol are removed. In addition, intermittent admixing or tumbling insures that the binder adhesive properly cures while, at the same time, the metal flakes are not split off from the microspheres.

The product of the present invention has excellent physical properties and unexpectedly can replace up to about 10 times its weight of plain metal in conventional applications. In addition, a dissipative coating containing such a product has advantageous properties.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be

limited to the disclosed embodiments. On the contrary, the present invention covers various modifications and equivalent arrangements included within the spirit and scope of the appended claims.