OPACIFIERS IN COMPOSITES

[sushantpatel_doc]

1. A composite opacifier for coatings comprising:

A. thermoplastic polymer microspheres; and

B. particulate inorganic opacifier component, a material portion thereof having a particle size of from about 200 to about 2000 millimicrons in diameter;

C. said particulate opacifier component comprising from about 20 to 97 weight per cent of the mixture and being adhered to and embedded in the surface of said microspheres: and

D. said composite opacifier having a composite specific gravity of from about 0.1 to about 2.8 gm/cc.

2. A composite opacifier of claim 1 having a composite specific gravity of from about 0.15 to 1.5 gm/cc.

3. The composite opacifier of claim 1 wherein a material proportion of said particulate opacifier component has a blend of particle sizes within said range.

4. The composite opacifier of claim 1 wherein said particulate opacifier component has a particle size of from about 400 to 800 millimicrons.

5. The composite opacifier of claim 4 wherein a material proportion of said particulate opacifier component has a blend of particle sizes within said range.

6. A composite opacifier for coatings, said opacifier being prepared by the process comprising:

A. mixing a particulate inorganic coating opacifier component with expandable thermoplastic resin microspheres, said opacifier component comprising from about 20 to 97 weight per cent of the mixture, with a material proportion of said opacifier component having a particle size of from about 200 to about 2000 millimicrons;

B. heating the mixture of said opacifier component and said microspheres under conditions of time, temperature and pressure to cause said opacifier component to be embedded in and adhered to the surface of said microspheres;

C. thermally expanding said composite to attain a composite specific gravity of from about 0.1 to about 2.8 gm/cc;

D. recovering said opacifier for incorporation into coating formulations.

Description:

2. TECHNICAL FIELD

The technical field of the present invention is coatings and paints and to new and improved opacifiers affording enhanced hiding power and of opacifier materials having a specific gravity controllable to values significantly lower than typical in the prior art and often substantially the same as the coating vehicle and thus having little tendency to settle or float or otherwise separate from the coating formulation.

SUMMARY OF THE INVENTION

The present invention in specific terms, relates to composite materials for use as opacifiers in a wide variety of coatings, and diverse types of coatings, paints, and the like.

More particularly, the present invention relates to a composite of dry, expanded thermoplastic microspheres having adhered to or embedded in the surfaces thereof inorganic opacifier materials, such as titanium dioxide, zinc oxide, calcium carbonate, talc, and the like.

Still more particularly, the invention relates to such composites where the proportions of the microsphere component and the opacifiers, and the degree of expansion of the microspheres can be balanced to afford a specific gravity of the composite material as necessary to prevent floating or settling in the continuous phase of the paint vehicle, i.e., generally within the broad range of about 0.1 to 2.8 gm/cc, and preferably about 0.15 to 1.5 gm/cc.

PRIOR ART

A wide diversity of opacifiers are known to paint and coating technology, and the present invention can be employed, with suitable adjustments for the specific characteristics of the materials selected, with any of them.

As those of ordinary skill in the art will well understand, the present invention will be described in relation to the most commonly employed of such materials. While this is not intended to exclude the employment of still other opacifier materials, it is believed that the full nature of the present invention and the parameters which should guide the art in its use will be most conveniently and fully understood in relation to such materials. These include, as mentioned above, such materials as titanium dioxide, in both anatase and rutile forms, zinc oxide, calcium carbonate, talc, and where appropriate to the discussion, other related materials.

The opacifiers of the prior art have specific gravities which are generally quite high, ranging from about 1.75 up to as much as about 4.5. As those of ordinary skill in the art are well aware, these materials have a decided tendency to separate from the medium and settle, often as a hard settlement.

It is also common, in order to achieve the opacities desired, to employ the opacifiers in quite substantial proportions, most often in excess of the “critical pigment volume” required in order to achieve a degree of porosity which produces a pigment or opacifier air interface which enhances the opacity of the coating when dry. Such porosity, however, can lead to infiltration of environmental liquids, i.e. water from rain or the like, which operates to displace air from the pores, wet the pigment, thus reducing the effectiveness of the opacifier and greatly reducing the durability of the coating.

The role of opacifiers in coating and paint technology are generally well understood by practitioners in the art. The opacifiers of the present invention are well behaved specimens within the spectrum of familiar and well known materials, and will give those of ordinary skill in the art little difficulty once the central properties and parameters which distinguish the materials from the conventions of the known materials are clearly defined and understood.

As the art well understands, it is ordinarily the difference in refractive indices between the coating binder and the opacifier material which dictates the hiding power of a particular coating. That is, the greater the difference in refractive indices at each occurrence of interface between the coating binder and the opacifier material, the greater the hiding power of the coating. Coating binders have refractive indices which typically are in the vicinity of about 1.5 or 1.6. Opacifiers are most often materials having refractive indices greater than about 1.8, and are more effective, generally, as the refractive index increases. Titanium dioxides, having refractive indices greater than 2, are among the most effective opacifiers in general use, and are often preferred for that reason.

It has long been known that air, having a refractive index of 1.0, makes a superior interface with opacifiers, and that making paints and coatings porous by exceeding the “critical pigment volume” or “CPV”, loading opacifiers at the surface, and the entrainment of air in the coating formulation can all enhance the hiding power. This technique, by creating an interface of air and opacifier, is known to be quite effective, but in some contexts results in the compromise of other properties of the coating formulation or the resultant coating.

The inclusion of air also enhances opacity through the air-binder interface, since the binder has refractive index which is materially different from that of air. This attribute of such systems is lesser than the air-opacifier interface effect.

The behavior of binders, opacifiers, and the inclusion of air through one or more of the techniques known to the art are all well known, and are in fact well quantifiable through the application of the Lorentz-Lorenz equations and the Fresnel equations. Both diffraction and dispersive effects are accounted for through these techniques. Through the application of the techniques known to the art, hiding power of a particular coating formulation can be predicted quantitatively with considerable reliability.

The materials referred to herein as expanded or expandable thermoplastic microspheres are most often the materials described in Morehouse, U.S. Pat. No. 3,615,972, and like materials. These materials are per se known in the art, and do not as such form a part of the present invention. While such materials are disclosed in a substantial number of prior art teachings, the Morehouse patent cited above is the most complete description of the materials and their formation, and is hence the most relevant and material such teaching in relation to the present invention.

The use of opacifiers, as discussed herein, is a vast, well documented practice in the art. The use of such materials to render coatings opaque is not per se a part of the present inventio

        

[sushantpatel_doc]

FILLERS
Industrial composites use glass, kevlar, graphite, mica, wood, hollow glass spheres, or the like to modify resin. Dental composites require materials that match tooth color and translucency so an optical index of 1.5 is required. Materials such as strontium glass, barium glass, quartz, borosilicate glass, ceramic, silica, prepolymerized resin, or the like are used.

Fillers are placed in dental composites to reduce shrinkage upon curing. Physical properties of composite are improved by fillers, however, composite characteristics change based on filler material, surface, size, load, shape, surface modifiers, optical index, filler load and size distribution.

Fillers are classified by material, shape and size. Fillers are irregular or spherical in shape depending on the mode of manufacture. Spherical particles are easier to incorporate into a resin mix and to fill more space leaving less resin. One size spherical particle occupies a certain space. Adding smaller particles fills the space between the larger particles to take up more space. There is less resin remaining and therefore, less shrinkage on curing the more size particles used in proper distribution.

One micron is a critical filler size. Fillers greater than one micron are visible to the human eye. As resin matrix around filler particles wears, the filler becomes prominent and visible so the composite surface looks rough. Fillers less than one micron do not produce a rough appearing surface with aging. Fillers greater than one micron are referred to as macrofills and fillers less than one micron are referred to as microfills. A new classification of filler is the nano particles. The nano particles fill between all other particles to further reduce shrinkage. A mixture of different particle sizes is referred to as a hybrid.

Distribution of filler particles can be uniform or distributed over a bell curve so a microfill composite might contain many particles greater than one micron but the predominance of particles are one micron or less.

        

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