Matt, thixotropic paint formulation

Details Matt, thixotropic paint formulation Matt, thixotropic paint formulation

2002-03-08

2004-04-13

Niland, Patrick D. (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Processes of preparing a desired or intentional composition...

C523S205000, C523S209000, C523S212000, C523S214000, C523S216000, C524S492000, C524S493000, C524S494000, C524S556000, C524S590000, C524S599000, C524S601000, C524S612000, C524S847000

Reexamination Certificate

active

06720368

ABSTRACT:

FIELD OF THE INVENTION
This invention pertains to a matt, thixotropic paint formulation and a method of producing such a paint formulation.
BACKGROUND OF THE INVENTION
Metal oxides, particularly silica, have been incorporated into certain surface coating materials in order to achieve the impression of “mattness.” A wet film applied to a substrate is initially held flat by the forces of surface tension, thereby resulting in a glossy surface. As the film dries and cures, the increasing viscoelasticity associated with the sol-gel transition hinders the movement of particles into the film, and the surface deforms to accommodate the matting agent particles. This roughness is maintained in the solidified film, which then is characterized by a matt finish. While the matting effect attributable to certain metal oxides, especially porous silicas, produces a desirable surface appearance for some applications, a paint formulation desirably possesses additional properties, such as a certain degree of thixotropy, in order to enhance its utility for a variety of applications.
Thixotropy is a characteristic of a paint formulation that relates to its viscosity. Indeed, as a paint formulation becomes more thixotropic, the sagging behavior of the paint becomes less apparent. For certain applications, a highly thixotropic paint is desired so as to minimize dripping, running, and sagging during application. Prior attempts at improving the thixotropic properties of a paint formulation have used a number of additives, such as modified resins and metal oxides into the formulation.
Silica, an inorganic material having silicon dioxide (SiO
2
) as a basic structural unit, is useful in a wide variety of commercial applications. Silica exists in a variety of molecular forms, which include, for example, monomers, dimers, oligomers, cyclic forms, and polymers. In addition, silica can be amorphous, crystalline, hydrated, solvated, or dry, and can exist in a variety of particulate and aggregated states.
Amorphous silica can be formed by molecular precipitation, for example, by cooling a supersaturated solution, concentrating an undersaturated solution, or by careful hydrolysis of a solution of a labile silica precursor, such as a SiCl
4
, esters of silica, Si(OR)
4
, and the like, to provide a supersaturated solution of Si(OH)
4
, from which precipitates amorphous silica.
Pyrogenic, or “fumed silica,” which typically has a particle size from about 2-20 nm, is formed from the vapor phase. For example, silica (usually sand) can be vaporized at about 2000° C. and cooled to form anhydrous amorphous silica particles. Alternatively, silica can be sublimed at about 1500° C. in the presence of a reducing agent (e.g., coke) to form SiO, which can be oxidized to form particulate silica. Other methods of producing fumed silica include, for example, oxidation of SiCl
4
at high temperatures or burning SiCl
4
in the presence of methane or hydrogen.
Silica solutions exhibit polymerization behavior, resulting in the increase of Si—O—Si bonds and decrease of Si—OH bonds. In an aqueous medium, amorphous silica dissolves (and/or depolymerizes), forming Si(OH)
4
, which undergoes polymerization to form discrete particles with internal Si—O—Si bonds and external Si—OH bonds on the particle surface. Under certain conditions, the polymeric silica particles thus formed will further associate to give chains and networks comprising the individual particles.
Generally, under neutral or alkaline conditions (pH 7 or greater), the particles tend to grow in size and decrease in number, whereas under acidic conditions (pH<7), the particles have a greater tendency to aggregate to form clusters, and eventually three-dimensional networks. Salts can be present to reduce the electrostatic repulsion between particles, so that aggregation of particles will be more likely to occur under neutral or alkaline conditions.
The term “sol” refers to a dispersion of discrete, colloidal particles, for example, of amorphous silica in an aqueous medium. If the sol is stable, a sol does not gel or settle even after several years of storage, and may contain up to about 50 wt. % silica and particle sizes up to 300 nm, although particles larger than about 70 nm settle slowly depending on the viscosity of the medium. A silica sol can be formed, for example, by growing particles to a certain size in a weakly alkaline solution, or by addition of dilute acid to a solution of sodium silicate (e.g., Na
2
SiO
3
) with rapid mixing, until the pH drops to about 8-10, followed by removal of Na
+
(e.g., by ion-exchange resin or electrodialysis). Silica sols, depending upon the type of silica, the particle size, and the nature of the particles, can be destabilized to form gels under mildly acidic to strongly acidic conditions.
The term “gel” refers to a coherent, rigid, continuous three-dimensional network of colloidal particles. Silica gels can be produced by the aggregation of colloidal silica particles (typically under acidic conditions when neutralizing salts are absent) to form a three dimensional gel microstructure. Whether a gel will form under a particular set of conditions, however, can depend on the silica properties, such as, for example, particle size and the nature of the particle surface. The term “hydrogel” refers to a gel in which the pores (spaces within the gel microstructure) are filled with water. Similarly, the term “alcogel” refers to a gel in which the pores are filled with an alcohol. When a gel is dried to form a xerogel, evaporation can result in a substantial collapse of the gel, giving a relatively high density collapsed powder. In contrast, when a gel is dried by means in which the gel microstructure is substantially preserved (e.g., supercritical drying as described in U.S. Pat. No. 3,652,214), a low density xerogel, known as an “aerogel” is formed. Silica aerogels may have very unusual and highly desirable properties such as, for example, optical transparency, extremely low density, and unprecedented low thermal conductivity. See Herrmann et al.,
Journal of Non
-
Crystalline Solids,
186, 380-387 (1995).
Synthetic silicas, such as the micronized silica xerogels and precipitates used as matting agents, typically are hydrophilic, owing to the silanol groups present on the surface of the silica particles. It is well-known that such silicas provide little, if any, thixotropy to the paint and can even cause other additives used for thixotropy to lose their effect.
Attempts have been made to produce a silica matting agent which has a limited deleterious effect on the thixotropy of a composition. For example, Aldcroft,
Polymers Paint Colour J.
184, 423-425 (Sep. 7, 1994), describes modifying the structure of a silica gel to make an effective matting agent but which perturbs the thixotropy little. In addition, U.S. Pat. No. 5,221,337 (Lüers et al.) discloses that the modification of silica with an organic polyol compound can lead to a matting agent which has practically no negative effect on the thixotropy of a paint formulation. This approach, however, remains problematic, as the addition of both a thixotropic additive and a matting agent can lead to undesirable interactions in a paint formulation.
Thus, a need remains for a paint formulation that exhibits both an improved matting and thixotropic effect when applied to various substrates. The invention seeks to provide such a paint formulation. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.
BRIEF SUMMARY OF THE INVENTION
The invention is predicated, at least in part, on the surprising discovery that a silica-based matting agent can be utilized which contributes thixotropy to a paint formulation, and does not spoil the effect of other additives used to optimize the paint formulation. The invention provides a matt, thixotropic paint formulation comprising a resin system and a matting agent. The matting agent comprises a hydrophobic metal oxide, which has a surface moiety (i.e.,

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