Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Reexamination Certificate
2001-12-13
2003-10-14
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
C428S402210, C428S407000, C427S213340, C424S408000, C525S242000, C525S301000, C523S122000, C523S205000, C521S056000, C521S140000, C264S004700
Reexamination Certificate
active
06632531
ABSTRACT:
The present invention relates to a method of preparing porous particles using a fugitive substance, and to gas-filled porous particles and their aqueous dispersions.
Air-filled porous particles are valued for their ability to opacify otherwise transparent coatings. This ability to opacify is of particular value when a coating is being applied to a surface of a substrate to hide markings on and coloration of that surface. It is thought that such opacification is achieved in much the same way pigments such as titanium dioxide (TiO
2
) opacify polymeric coatings into which they are incorporated. Like small particles of TiO
2
dispersed in a coating, the air-filled voids scatter light. The scattering of visible light, observable as opacity, is particularly intense when the scattering center, in this case the air-filled pore, is between about 50 nanometers and 1 micron in diameter. The term “hiding power” is used herein to refer to the ability of a given coating to hide the substrate surface to which it is applied. An increase in opacity for a given coating will be observed as an increase in hiding power.
When porous particles are dispersed throughout a dry polymeric coating, the pore or pores in each particle typically contain air. Because the air filled pores behave like TiO
2
, porous particles can completely, or partially replace pigments like TiO
2
in coating compositions for which opacity is a requirement. Replacement of inorganic pigments, such as TiO
2
, with air has obvious financial and environmental advantages associated with it. As a result, porous particles have been commercially successful as materials that provide hiding power to coatings.
Porous particles are typically produced as aqueous dispersions of particles, the pores of which are filled with water. These aqueous dispersions of particles are incorporated into coating compositions, including a variety of paints. Because the refractive index difference between water and polymer is typically much less than that between air and polymer, or other gases and polymer, the coating composition does not achieve full opacity during application to a substrate and formation of a coating until that coating has dried to the extent that the water in the pores has been replaced with air. It is an unfortunate reality that, although the ultimate “dry hiding power” imparted to a coating by porous particles may be such that it would be fully acceptable to a customer, the lower “wet hiding power” observable during application of the coating may dissuade the customer from proceeding to completion. In this way, for example, a coating composition that replaces all or a portion of TiO
2
with cheaper, more environmentally desirable air is often rejected as an option based not upon actual performance of the coating once dry, but rather based upon anxiety experienced by the customer during application.
It is, therefore, desirable to prepare aqueous dispersions of porous particles having pores filled with air, or other gaseous material. In that way, wet hiding power similar to the ultimate dry hiding power of a given coating composition can be achieved.
U.S. Pat. No. 5,225,279 discloses a method of forming porous particles by first dispersing droplets of monomer and other hydrophobic substances in water, followed by polymerization. The polymer formed during the polymerization bears acidic groups such as those of carboxylic acid monomers, and phase separation after polymerization is facilitated by neutralization of those acidic groups with base such as, for example, ammonia. The neutralization converts the carboxylic acid groups, most of which are not ionized, into carboxylate salts that are fully ionized. By making the polymer more ionic, and therefore more hydrophilic, in this way, the polymer is rendered less soluble in the hydrophobic component of the droplet, and phase separation occurs readily. This technique, disclosed in detail in U.S. Pat. No. 5,225,279, has the further advantage that the more ionic, more hydrophilic polymer bearing, for example, ammonium carboxylate groups will tend to move to the droplet/water interface, increasing the propensity to form a continuous shell around a single pore containing the more hydrophobic phase. Similarly, polymers bearing salt moieties may be formed by neutralization of basic groups on those polymers. For example, amine functional polymers may be neutralized with hydrochloric acid or acetic acid to form more hydrophilic ionic polymers. Because dispersion of the droplets prior to polymerization is accomplished exclusively by mechanical means, the particle size distribution of both droplets and porous particles is broad. When a volatile solvent is included in the droplets as a hydrophobic substance, polymerization and neutralization produces porous particles which may be separated from the aqueous phase and dried, creating air-filled particles having a broad particle size distribution.
U.S. Pat. No. 5,976,405 discloses porous particles having particle sizes in the range of 0.150 micron to 15 microns, and very narrow particle size distributions that derive from using a low molecular weight seed polymer, itself having a very narrow particle size distribution. This method does not require neutralization after polymerization, although neutralization may be a preferred option in some cases. When a volatile solvent is included in the droplets as a hydrophobic substance, polymerization and phase separation produces porous particles which may be separated from the aqueous phase and dried, creating air-filled particles having a very narrow particle size distribution. Unfortunately, neither U.S. Pat. No. 5,976,405 nor U.S. Pat. No. 5,225,279 disclose air-filled particles dispersed in water.
Displacement by air of a solvent contained in a pore, while a particle is still dispersed in water, is extremely difficult or impossible. Attempts to remove low boiling solvents (i.e., solvents having a normal boiling point of 30° C. to 70° C.) from the pores of porous particles having continuous shells is very difficult when those particles are dispersed in water, even at reduced pressure and elevated temperature. At reduced pressure, the amount of gas (e.g., nitrogen or air) available to replace the solvent is very limited and the diffusion of the gas through the aqueous phase slows the replacement process further. As a result, solvent escaping from the pore is not easily replaced with gas. The partial vacuum thus created in the pore may then bring about collapse of the particle, with concomitant change in particle shape, and loss of the desired porosity. If, on the other hand, elevated temperatures are used to encourage evaporation of the solvent, it is often the case that the required temperature for efficient evaporation is at least as high as the softening temperature of the polymeric phase, with the result that the desired porous structure is lost.
We have, surprisingly, found that it is possible to prepare aqueous dispersions of porous particles having a polymeric phase filled with at least one gaseous substance, for example, air. The method of preparation of these porous particles involves the use of a fugitive substance during polymerization. The fugitive substance can then be removed smoothly and continuously at temperatures as low as 1° C. above the freezing point of the aqueous phase, allowing even porous particles having a polymeric phase which softens at least 5° C. above the freezing point of the aqueous phase to be freed of the fugitive substance without loss of porosity, and without loss of shape.
Polymeric particles having pores filled with gas may be prepared according to the method of the present invention in a wide range of particle sizes (PS) spanning 0.015 microns to at least 250 microns, and particle size distributions (PSDs) including broad, narrow, very narrow, monodisperse, bimodal, and multimodal.
The present invention relates to a method of preparing an aqueous dispersion of a plurality of porous particles, wherein said porous particles comprise at least one polymeric phase and a pore f
Hemenway Carl P.
Kiliman Leszek
Rohm and Haas Company
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