SILICA-BASED OPACIFYING AGENTS AND METHODS OF MANUFACTURING AND USE THEREOF

Abstract

Improved silica-based opacifying agents are described. Opacifying agents are useful in water-based coating compositions to provide exceptional properties to a wood-based substrate. The films resulting from silica-based opacifying agents on a wood substrate unexpectedly provide improved chemical resistance and / or film clarity to the wood substrate surface. Methods for preparing and using silica-based opacifying agents are also described.

Description

SILICA-BASED PACIFYING AGENTS AND METHODS OF MANUFACTURING AND USE THEREOF FIELD OF INVENTION The present invention relates to improved silica-based opacifying agents. In one aspect, the present invention relates to improved silica-based opacifying agents for aqueous coating systems. In another aspect, the invention relates to silica-based opacifying agents comprising composite silica particles, aqueous coating compositions containing the silica-based opacifying agents, and methods for manufacturing and using the compositions. BACKGROUND OF THE INVENTION Silica-based opacifying agents are widely used in coating and paint formulations to reduce the gloss of coated films. In solvent-based coatings or 100% solids UV-curing formulations, high levels of silica are required for effective opacification and gloss reduction. On the other hand, high concentrations of hydrophilic silica can alter the rheological properties of solvent-based lacquers and frequently lead to adhesion and dispensability problems. To address these issues, pzzAnn / Lznz / E / YiAi Ref. 325070 problems, typically the above technique is aimed at treating the surface of particulate silica to make it hydrophobic and therefore more compatible with solvent systems and organic products in formulations. For this purpose, silicas coated with wax and / or polymer have frequently been used. U.S. Patent No. US6039798 describes a wax-coated silica opacifying agent in which the silica is an amorphous silica having a pore volume of at least 1.5 cm3 / g, preferably at least 1.8 cm3 / g. The wax coating is present in the range of 6% to 15% by weight (wt%) of the opacifying agent and comprises a synthetic polyethylene wax. Patent No. EP0759959 describes a wax-coated silica opacifying agent characterized in that the silica is an amorphous silica having a pore size distribution in which 90% of the pores have a diameter greater than 15 nanometers, and less than 20% of the pore volume is in pores having a pore diameter between 10 and 30 nanometers, the wax coating being present in the range of approximately 2% to approximately 15% by weight of the opacifying agent and comprising a hard microcrystalline wax, a plasticizing microcrystalline wax, a synthetic polyethylene wax, or a mixture thereof. U.S. Patent No. 20050065268 describes a pzzAnn / Lznz / E / YiAi silica opacifying agent comprising particulate amorphous silica in which the silica particles have been treated with a hydrophilic polyolefin wax. U.S. Patent No. 6921781 describes the coating of at least a portion of a surface of at least one silica particle with at least one wax, wherein the coating is carried out in at least one gas at a temperature higher than the melting point of the wax and lower than the decomposition temperature of the wax. The wax content is defined as 2-15% by weight of the silica weight. U.S. Patent No. 7303624 describes that a structurally coated silica can be prepared by spraying and mixing a fumed silica with water and a coating agent in a suitable mixing container, then by grinding and then conditioning the product. U.S. Patent No. 8926748 describes an opacifying agent useful for the preparation of matte coatings comprising inorganic oxide particles and wax coated on the inorganic oxide particles, wherein the wax has a crystallinity of approximately 50% or more and the wax is present in an amount that varies from 15% by weight to 30% by weight based on the total weight of the opacifying agent. Patent No. WO 1999051692 describes an invention relating to an opacifying agent based on silicon dioxide pzzRnn / Lznz / E / YiAi, the silicon dioxide particles having a particle size of 2.5 to 20 pm and a moisture content of 0 to 65% by weight, based on the opacifying agent, and are coated with 0.2 to 10% by weight of a ureaurethane derivative or a mixture of urea-urethane derivatives. Currently, solvent-based coating compositions are undesirable due to environmental concerns and health and safety issues. Government regulations have driven the reduction and elimination of volatile organic compounds (VOCs) in paint and coating formulations, and the use of substantially more water-based coatings is being promoted. Various types of opacifying agents have been used in aqueous formulations, including silica opacifying agents, organic opacifying agents, and mixtures of the two. Silica-based opacifying agents such as ACEMATT® TS100 and SYLOID® C807 have excellent opacifying efficacy and film clarity in aqueous formulations, but they tend to have weaker chemical and weather resistance, frequently clearing or becoming cloudy when exposed to chemicals or changing weather conditions. Silica-based opacifying agents also tend to have poor thermal stress resistance when subjected to rapid temperature changes.While not intending to impose any theory, an adhesive failure at the opacifying agent-latex interface results in a crack that is an effective light scatterer. Similarly, adhesive failure caused by stresses due to film swelling (later shrinkage) during wetting and drying, exacerbated by particle shrinkage of the opacifying agents during drying, may be the cause of these disadvantages with silica opacifying agents in aqueous coating formulations. All of these disadvantages are undesirable in coating applications on wood substrates. In addition, pure organic opacifying agents were used. For example, urea-formaldehyde resin-based opacifying agents such as DEUTERON® MK and CERAFLOUR® 920 are known. However, both of these opacifying agents have environmental problems because they can potentially release residual starting material, toxic formaldehyde. A modified micronized polyethylene-based opacifying agent, CERAFLOUR® 929, is also available. However, this product has poor opacifying efficacy compared to silica-based opacifying agents in coating formulations. Organic opacifying agents are also known to have worse film clarity compared to pure silica-based opacifying agents.This is likely due to the fact that silica-based opacifying agents have a refractive index close to that of other components (e.g., binders) in the coated film, whereas the difference in refractive index between organic-based opacifying agents and such components is greater. Furthermore, organic opacifying agents are typically more difficult to produce and also more expensive. The mixture of silica-based and organic opacifying agents has also been used in water-based systems to balance the required coating film properties. However, this creates additional complexity in already complicated paint or coating formulation systems. Consequently, there remains a need for simple solutions to develop opacifying agents that (i) are suitable for use in aqueous coating systems, and (ii) provide clear coating films with improved chemical resistance and / or improved film clarity in combination with good opacifying efficacy. SUMMARY OF THE INVENTION The present invention addresses the aforementioned need in the art by discovering improved silica-based opacifying agents that provide good opacifying efficacy in combination with improved chemical resistance in a coated film resulting from aqueous or water-based coating compositions. Unexpectedly, it has been discovered that incorporating composite silica / wax particles having a relatively low total porosity (e.g., less than 0.2 cubic centimeters per gram (cc / g)) and a relatively low final particle surface area (e.g., less than 10 square meters per gram (m² / g)) as opacifying agents in aqueous coating compositions provides improved chemical resistance in films formed from the dried coating composition, particularly when applied to a wood substrate.Advantageously, the aqueous compositions of the present invention also provide exceptional enhanced film clarity properties to the surface of a wood substrate compared to prior aqueous coated compositions. Accordingly, the present invention provides silica-based opacifying agents comprising composite silica particles comprising: (a) porous silica particles; and (b) one or more waxes in a sufficient amount to at least partially fill the pores of the porous silica particles such that the composite silica particles have (i) a total pore volume of less than 0.2 cc / g as determined by the Barrett pzzRnn / Lznz / E / YiAi method Joyner-Halenda (BJH) and (ii) a final particle surface area less than 10 square meters per gram (m2 / g) as determined by the Brunauer Emmet Teller (BET) nitrogen adsorption method. In some desired embodiments, the silica-based opacifying agent comprises composite silica particles wherein the porous silica particles have an initial pore volume of approximately 0.2 cc / ga to approximately 2.5 cc / g as determined by the BJH method and an initial particle surface area of ​​approximately 100 m² / ga to approximately 1000 m² / g as determined by the BET nitrogen adsorption method. The composite silica particles are coated or treated with one or more waxes such that the final composite silica particles have (i) a total pore volume less than 0.2 cc / g as determined by the BJH method, and (ii) a final particle surface area less than 10 m² / g as determined by the BET nitrogen adsorption method. The composite silica particles typically comprise from approximately 15.0 percent by weight (wt%) to approximately 65.0% by weight, based on a total weight of the composite silica particles, of one or more waxes at least partially within the pores of the porous silica particles. The present invention also provides aqueous coating compositions or formulations comprising the improved silica-based opacifying agents of the invention, which compositions, after drying, provide dry, transparent coating films that have improved chemical resistance and film clarity. For the purposes of the invention, the term "transparent coating film" is used herein to indicate a film that is transparent or substantially transparent, such that the natural grain of a wood substrate is substantially visible to the human eye when applied to at least one surface of a wood substrate.The coating compositions of the present invention provide films with improved chemical resistance and / or film clarity compared to known coatings / films that do not contain the silica particle opacifying agents of the present invention. The present invention further relates to methods for manufacturing opacifying agents for composite silica particles and a method for preparing aqueous coating compositions comprising the opacifying agents of the invention. The present invention also relates to methods for coating a substrate with the aqueous coating compositions mentioned herein. In a preferred embodiment, the substrate is a pzzRnn / Lznz / E / YiAi wood substrate. In other embodiments, the method of using the composite silica particle opacifying agents described herein comprises a method for improving the chemical resistance and / or film clarity, or any combination thereof, of a substrate, particularly a wood substrate, wherein the method comprises incorporating the composite silica particle opacifying agents described herein into an aqueous coating composition prior to applying the coating composition to the substrate. Unexpectedly, the coating compositions described herein provide improved protection to a given wood substrate, compared to known liquid coating compositions as measured using a colorimeter (e.g., a Spectro-Guide 45 / 0 portable colorimeter). The present invention is further directed to substrates coated with aqueous compositions containing the composite silica particle opacifying agents described herein. In some illustrative embodiments, the substrate comprises a wood substrate coated with aqueous compositions containing the composite silica particle opacifying agents described herein. These and other features and advantages of the present pzzAnn / Lznz / E / YiAi invention will become evident after a review of the following detailed description of the described embodiments and appended claims. DETAILED DESCRIPTION OF THE INVENTION To promote understanding of the principles of the present invention, descriptions of specific embodiments of the invention follow, and specific language is used to describe those embodiments. However, it is understood that no limitation of the scope of the invention is intended by using specific language. Alterations, further modifications, and other applications of the principles of the present invention described herein are contemplated as those that would normally occur to a person skilled in the art to which the invention belongs. It should be noted that, as used in the present description and in the appended claims, the singular forms a, and, and the include plural referents, unless the context clearly indicates otherwise. Thus, for example, a reference to an oxide includes a plurality of such oxides, and a reference to oxide includes a reference to one or more oxides and their equivalents known to those skilled in the art, etc. Approximately, when modifying, for example, the amount of an ingredient in a coated particle and / or composition, concentrations, volumes, process temperatures, process times, recoveries or yields, flow rates, and similar values ​​and ranges thereof, used in the description of the methods, refers to the variation in numerical quantity that may occur, for example, through typical measurement and handling procedures; through unintentional error in these procedures; through differences in the ingredients used to carry out the methods; and similar proximate considerations. The term approximately also encompasses quantities that differ due to the aging of a formulation with a particular initial concentration or mixture, and quantities that differ due to blending or processing of a formulation with a particular initial concentration or mixture.Although modified by the term approximately, the claims appended to this include equivalents. As used herein, the term crystalline refers to a solid material whose constituent atoms, molecules, or ions are arranged in an ordered pattern extending in all three directions, which can be measured by X-ray diffraction or differential scanning calorimetry. As used herein, the term amorphous refers to a solid material whose constituent atoms, molecules, or ions are arranged in a random, non-ordered pattern extending in all three directions, which can be determined by X-ray diffraction or differential scanning calorimetry. As used in this description, the term BET particle surface area is defined as a particle surface area as measured by the Brunauer-Emmet-Teller (BET) nitrogen adsorption method. As used in the present description, the phrase total pore volume refers to the average pore volume of a plurality of particles (i.e. any particles, porous silica particles, or composite silica particles, respectively) as determined using Barrett-Joyner-Halenda (BJH) nitrogen porosimetry as described in DIN 66134. As used in the present description, the phrase mean pore volume refers to a pore volume size (V50, which is a pore volume distribution of various particles with 50 percent by volume of the pore volume of particles that is less than this number and 50 percent by volume of the pore volume of particles that is greater than this number in size) measured by Barrett-Joyner-Halenda (BJH) nitrogen porosimetry as described in DIN 66134. As used in the present description, the phrase particle size refers to the mean particle size (D50, which is a volume distribution of various particles with 50 percent by volume of particles that are smaller than this number, and 50 percent by volume of particles that are larger than this number in size), measured by dynamic light scattering when the particles are soaked in water or an organic solvent such as acetone or ethanol. For the purposes of this invention, the gloss values ​​mentioned herein were measured when the aqueous coating compositions were applied to a flat, smooth, non-penetrating black paper and allowed to dry for at least four (4) days at room temperature. All chemical resistance and film clarity values ​​were also based on measurements on the black papers. Similar improvements were found on other substrates, including wood. The present invention relates to improved silica-based opacifying agents that provide enhanced chemical resistance in films resulting from the application of an aqueous coating composition to a substrate, preferably a wood substrate. Typically, the improved silica-based opacifying agents comprise composite silica particles having low porosity, i.e., a total pore volume of less than 0.2 cc / g as determined by the BJH method, and a final particle surface area of ​​less than 10 m² / g as determined by the BET nitrogen adsorption method. The amount of one or more waxes in the final composite silica particles will vary depending on the initial porosity of the porous silica used to prepare the composite silica particles.Typically, one or more waxes are present in a sufficient amount to produce a composite silica particle that has from approximately 15.0% by weight to approximately 65.0% by weight, based on a total weight of the composite silica particles. In a desired embodiment of the invention, the opacifying agent comprises composite silica particles comprising porous silica particles treated with one or more waxes to provide composite silica particles having (i) a total pore volume of approximately 0.00 cc / ga to less than 0.20 cc / g, as determined by the BJH method, (ii) a final particle surface area of ​​approximately 0 m² / ga to less than 10.0 m² / g, as determined by the BET nitrogen adsorption method, and (iii) approximately 15.0 wt% to approximately 65.0 wt%, based on the total weight of the composite silica particles, of one or more waxes at least partially within the pores of the porous silica particles. Typically, the porous silica particles used to form the composite silica particles in this modality have an initial pore volume of approximately 0.20 cc / ga.50 cc / g, as determined by the BJH method. In another desired embodiment, the silica-based pzzRnn / Lznz / E / YiAi opacifying agent comprises composite silica particles prepared from porous silica particles having an initial pore volume of approximately 1.7 cc / g to approximately 2.2 cc / g as determined by the BJH method, coated or treated with one or more waxes in such a manner that the final composite silica particles have (i) a total pore volume less than 0.2 cc / g as determined by the BJH method, and (ii) a final particle surface area less than 10 m2 / g as determined by the BET nitrogen adsorption method. In this desired embodiment, the composite silica particles typically comprise from approximately 55.0% by weight to approximately 65.0% by weight, based on a total weight of the composite silica particles, of one or more waxes at least partially within the pores of the porous silica particles. In another desired embodiment, the silica-based opacifying agent comprises composite silica particles prepared from porous silica particles having an initial pore volume of approximately 1.2 cc / g to approximately 1.7 cc / g as determined by the BJH method, coated or treated with one or more waxes in such a manner that the final composite silica particles have (i) a total pore volume less than 0.2 cc / g as determined by the BJH method, and (ii) a final particle surface area less than 10 m² / g as determined by the pzzAnn / Lznz / E / YiAi nitrogen adsorption method. In this desired embodiment, the composite silica particles typically comprise from approximately 45.0% by weight to approximately 55.0% by weight, based on a total weight of the composite silica particles, of one or more waxes at least partially within the pores of the porous silica particles. In another desired embodiment, the silica-based opacifying agent comprises composite silica particles prepared from porous silica particles having an initial pore volume of approximately 1.0 cc / g to approximately 1.2 cc / g as determined by the BJH method, coated or treated with one or more waxes such that the final composite silica particles have (i) a total pore volume of less than 0.2 cc / g as determined by the BJH method, and (ii) a final particle surface area of ​​less than 10 m² / g as determined by the BET nitrogen adsorption method. In this desired embodiment, the composite silica particles typically comprise from approximately 35.0 wt% to approximately 65.0 wt%, based on the total weight of the composite silica particles, of one or more waxes at least partially within the pores of the porous silica particles. In another desired embodiment, the silica-based opacifying agent comprises composite silica particles prepared from porous silica particles having an initial pore volume pzzAnn / Lznz / E / YiAi of approximately 0.6 cc / g to approximately 1.0 cc / g as determined by the BJH method, and are coated or treated with one or more waxes in such a manner that the final composite silica particles have (i) a total pore volume less than 0.2 cc / g as determined by the BJH method, and (ii) a final particle surface area less than 10 m2 / g as determined by the BET nitrogen adsorption method. In this desired embodiment, the composite silica particles typically comprise from approximately 25.0% by weight to approximately 35.0% by weight, based on a total weight of the composite silica particles, of one or more waxes at least partially within the pores of the porous silica particles. In another desired embodiment, the silica-based opacifying agent comprises composite silica particles prepared from porous silica particles having an initial pore volume of approximately 0.3 cc / g to approximately 0.6 cc / g as determined by the BJH method, coated or treated with one or more waxes such that the final composite silica particles have (i) a total pore volume of less than 0.2 cc / g as determined by the BJH method, and (ii) a final particle surface area of ​​less than 10 m² / g as determined by the BET nitrogen adsorption method. In this desired embodiment, the composite silica particles typically comprise approximately pzzRnn / Lznz / E / YiAi 15.0% by weight to approximately 25.0% by weight, based on a total weight of the composite silica particles, of one or more waxes at least partially within the pores of the porous silica particles. To form composite silica particles, porous silica particles are treated with one or more waxes in a sufficient amount to provide composite silica particles that have (i) the desired low porosity, i.e., a total pore volume less than 0.20 cc / g as determined by the BJH method, and (ii) the desired low final particle surface area, i.e., less than 10.0 m² / g, as determined by the BET nitrogen adsorption method. Typically, the composite silica particles comprise from approximately 15.0 wt% to approximately 65.0 wt%, based on the total weight of the composite silica particles, as described earlier in this description.Typically, one or more waxes are coated onto the porous silica particles in any quantity sufficient to partially or substantially fill the pores of the porous silica particles to provide the desired low porosity in the composite silica particles, but not so much that the one or more waxes fill the pores of the porous silica particles and begin to accumulate along the outer surface of the resulting composite silica / wax particles. Suitable porous silica particles useful in the preparation of the composite particle opacifying agents of the present invention include, but are not limited to, silica gel, precipitated silica, pyrogenic silica, and colloidal silica. Suitable porous silica particles also include, but are not limited to, ordered mesoporous silica particles prepared via an organic template (e.g., a surfactant) during silica particle formation, followed by high-temperature treatment to burn off the organic elements. Particularly preferred porous silica particles comprise silica gel or precipitated silica particles. The preferred porous silica particles having low porosity useful in the present invention include, but are not limited to, silica gels or precipitated silicas having BJH pore volumes less than or equal to 2.5 cc / g, and in some embodiments, from approximately 0.2 cc / g to approximately 2.5 cc / g. Commercially available porous silica particles useful in the present invention include particles available from WR Grace (Columbra, MD) under the trade name SYLOID®, such as SYLOID® C807 silica gel particles, SYLOID® C809 silica gel particles, SYLOID® MX106 precipitated silica particles, SYLOBLOC® silica particles, and DARACLAR® silica particles. In a preferred embodiment, the porous silica particles used to form the opacifying agents of the present invention comprise porous silica particles having a purity of at least approximately 93.0 wt% SiO2, or at least approximately 93.5 wt% SiO2, at least approximately 94.0 wt% SiO2, at least approximately 95.0 wt% SiO2, at least approximately 96.0 wt% SiO2, at least approximately 97.0 wt% SiO2, or at least approximately 98.0 wt% SiO2 up to 100 wt% SiO2 based on the total weight of the porous silica particle. The porous silica particles used to form the opacifying agents of the present invention can have a variety of different symmetrical, asymmetrical, or irregular shapes, including chain, rod, or ribbon forms. The porous silica particles can have different structures, including amorphous or crystalline structures, etc. In a preferred embodiment, the porous silica particles are amorphous. The porous silica particles can include mixtures of particles comprising different compositions, sizes, shapes, or physical structures, or they can be the same except for different surface treatments. The porosity of the porous silica particles can be intraparticle or interparticle, in cases where smaller particles agglomerate to form larger particles. The porous silica particles used to form the composite silica particles typically have an initial particle surface area of ​​approximately 100 m2 / ga to approximately 1000 m2 / g, more typically, approximately 200 m2 / ga to approximately 800 m2 / g, as determined by the BET nitrogen adsorption method, whereas the composite silica particles used to form the opacifying agents of the present invention have a final BET particle surface area of ​​0 m2 / g to less than 10.0 m2 / g (or any interval of values ​​between 0 m2 / g to less than 10.0 m2 / g, in increments of 0.1 m2 / g, e.g., from approximately 0.1 m2 / ga to approximately 9.9 m2 / g). In some embodiments, the composite silica particles have a final BET particle surface area of ​​approximately 1.0 m2 / g up to approximately 9.0 m2 / g (or any interval of values ​​between 1.0 m2 / g and 9.0 m2 / g, in increments of 0.1 m2 / g, e.g.)., from approximately 1.0 m2 / g to approximately 8.9 m2 / g). In other embodiments, the composite silica particles used to form the opacifying agents of the present invention have a BET particle surface area of ​​approximately 2.0 m2 / g to approximately 8.0 m2 / g. The porosity of the composite silica particles used to form the opacifying agent of the present invention is low, i.e., it has a total pore volume of less than 0.2 cc / g, as determined by the BJH method. In one embodiment, the total pore volume of the composite silica particles is less than 0.19 cc / g. In a preferred embodiment, the composite particles have a total pore volume of approximately 0.001 to approximately 0.19 cc / g. In a more preferred embodiment, the total pore volume of the composite silica particles ranges from approximately 0.01 to approximately 0.05 cc / g. The opacifying agents of the present invention typically have an average particle size of approximately 1.0 micron (µm) to approximately 50 µm (or any range of values ​​between and including 1.0 µm up to approximately 50 µm, in increments of 0.1 µm, e.g., from approximately 39.2 µm to approximately 49.1 µm). However, it should be understood that the opacifying agents of the present invention may have any average particle size depending on the use of the particles. In some embodiments, the opacifying agents of the present invention have an average particle size of approximately 2.0 µm to approximately 20.0 µm. In some embodiments, the opacifying agents of the present invention have an average particle size of approximately 3.0 µm to approximately 15.0 µm. The one or more waxes may comprise, but are not limited to, a hydrocarbon wax (i.e., a wax comprising relatively long alkyl chains, e.g., alkyl chains having 20 or more carbon atoms therein, with or without one or more diverse functional groups such as fatty acids, long-chain primary and secondary alcohols, unsaturated bonds, aromatics, amides, ketones, and aldehydes), a paraffin wax (i.e., 20-40 carbon atoms without additional functional groups), a polyethylene wax, a polypropylene wax, a vegetable wax such as carnauba wax (i.e., Brazil wax), an animal wax such as beeswax, or any combination thereof. Commercially available waxes suitable for use in the present invention include, but are not limited to, waxes available from Mitsui Chemicals, LLC (Osaka, Japan) under the trade names Hi-WAX™ or EXCEREX™ waxes, waxes available from Honeywell Performance Additives (Morristown, New Jersey) under the trade names RHEOLUB® waxes; and waxes available from TH.C.TROMM GmbH (Cologne, Germany) under the trade names POLARWACHS® waxes. In some embodiments, the opacifying agent comprises composite silica particles comprising porous silica particles with a polyethylene wax, a polypropylene wax, or a combination thereof. In some desired embodiments, the composite particles comprise a polyethylene wax having an average molecular weight of at least 2000. This relatively high molecular weight polyethylene wax is commercially available from TH.C.TROMM GmbH (Cologne, Germany) under the trade name POLARWACHS® wax. The composite silica particles useful in the present invention can be prepared by contacting porous silica particles with one or more waxes in a manner sufficient to provide composite particles having (i) a total pore volume of approximately 0.00 cc / ga to less than 0.20 cc / g (more typically, from approximately 0.001 cc / ga to approximately 0.19 cc / g), as determined by the BJH method, (ii) a final particle surface area of ​​approximately 0 m² / ga to less than 10.0 m² / g (more typically, from approximately 0.1 m² / ga to less than approximately 9.9 m² / g), as determined by the BET nitrogen adsorption method, and (iii) from approximately 15.0 wt% to approximately 65.0 wt% (more typically, from approximately 22.0 wt% to approximately 63.0 wt%).8% by weight), based on the total weight of the composite silica particles, of one or more waxes at least partially within the pores of the porous silica particles. Any conventional method may be used to contact the porous silica particles with one or more waxes to provide the composite silica particles. In some embodiments, the contact step may be a wet process. The wet contact process step may comprise dissolving or dispersing the one or more waxes in a solvent to form a solvent mixture; incorporating the porous silica particles into the solvent mixture; and removing or evaporating the solvent from the solvent mixture to form composite silica particles. The composite silica particles can then undergo size reduction. Any known method for reducing particle size can be used, including, but not limited to, a grinding stage such as a ball mill or a crushing stage with a mortar and pestle. In one embodiment, the composite silica particles undergo a size reduction stage, where the average particle size of the composite silica particles is reduced to a first average particle size of less than approximately 500 microns (µm). Once reduced in size, the coated silica particles are desirably heat-treated at an elevated temperature for a period of time. Typically, the elevated temperature is from approximately 90 °C to approximately 140 °C (or any range of values ​​between 90 °C and 140 °C, in increments of 1.0 °C, for example, from approximately 91.0 °C to approximately 102.0 °C).0 °C). Typically, the heat treatment time period varies from approximately 1.0 hour (h) to approximately 4.0 h (or any interval of values ​​between pzzRnn / Lznz / E / YiAi .0 h up to and including 4.0 h, in increments of 1.0 minute, e.g., from approximately 1.0 h and 9 minutes to approximately 2.0 h and 5 minutes). In an illustrative heat treatment step for forming composite silica particles of the present invention, the elevated temperature of the heat treatment step varies from approximately 100 °C to approximately 130 °C, and the heat treatment time period varies from approximately 1.0 h to approximately 1.5 h. After any optional heat treatment stage, the heat-treated composite silica particles are allowed to cool. Once cooled, the heat-treated composite silica particles may be further reduced in size to result in a final particle size smaller than approximately 100.0 pm (or any range of values ​​from approximately 1.0 pm up to and including 100.0 pm, in increments of 1.0 pm, for example, from approximately 4.0 pm to approximately 6.7 pm). As described above, any known method for particle size reduction may be used. In one illustrative embodiment, a milling stage may be used to result in composite silica particles having a final particle size smaller than approximately 45.0 pm. In other illustrative embodiments, the contact step may not involve any solvent and, therefore, be a dry process. In one embodiment, the dry process may comprise melting one or more waxes to form a molten liquid and incorporating the porous silica particles into the molten liquid, followed by particle size reduction. In still other embodiments, the dry process may comprise simultaneously contacting and mixing (a) the one or more waxes and (b) the porous silica particles in a conventional mixer such as a ribbon mixer, a Henschel mixer, a twin-screw extruder, a fluid energy mill (FEM), or a high-temperature micronizing jet mill (i.e., a temperature that melts any wax if necessary). In these embodiments, the particle size reduction and heating steps are combined, and further particle size reduction may or may not be required. Coating compositions The opacifying agents of the present invention are useful for preparing coating compositions comprising aqueous suspensions or dispersions of the silica particle opacifying agents described herein. In a preferred embodiment, the coating composition is an aqueous or water-based coating composition, which generally produces a transparent coating film upon drying on a substrate. When the substrate is wood, the transparent coating film allows the natural color and grain structure of the wood, such as teak, cherry, oak, walnut, mahogany, and rosewood, to be visible or substantially visible to the eye, characteristics that can be highly valued in applications such as furniture and wood carving. Coating compositions may comprise the composite silica particle opacifying agents described above, in addition to various other ingredients used in coating compositions. Examples of other ingredients that may be present in the compositions include, but are not limited to, an aqueous film-forming binder resin, such as a self-crosslinking modified acrylic copolymer emulsion or a NEOCRYL® XK12 acrylic latex binder (available from Royal DSM, Heerlen, The Netherlands), and a coalescing solvent such as dipropylene glycol n-butyl ether (DOWANOL™ PDnB). The composition may or may not contain color pigments such as organic pigments or white titanium dioxide inorganic pigments, provided that, where the substrate is wood, the natural wood grain remains substantially visible to the eye.When the composition contains a color pigment, a dispersant can also be included in the formulation. The remainder of the composition is typically water. Other diluents may also be included besides water, such as, but not limited to, aliphatics, aromatics, alcohols, ketones, turpentine, petroleum distillate, esters, glycol ethers, low molecular weight synthetic resins, and the like. Environmentally friendly diluents, such as water, are preferred. Various other additives may also be included in the compositions, including, but not limited to, additives to modify surface tension, improve flow properties, enhance the appearance of the finish, increase the open time of the paint, improve pigment stability, impart antifreeze properties, control foaming, control dissolution, etc. Additional additives that may be included in the compositions include, but are not limited to, surfactants, catalysts, thickeners, stabilizers, emulsifiers, texturizers, adhesion promoters, UV stabilizers, gloss eliminators, biocides to combat bacterial growth, and the like.The oil can be included as a rheological agent, a gloss modifier, and a protective agent that will reduce coating damage that would otherwise occur as a result of formation processes and degrading elements in the service environment of the coated materials. The coating compositions of the present invention typically comprise (I) from approximately 1.0% by weight to approximately 50.0% by weight (or any range of values ​​between 1.0% by weight and including 50.0% by weight, in increments of 0.1% by weight, for example, from approximately 1.3% to approximately 4.8% by weight) of the composite silica particle opacifying agents, and (II) from approximately 99.0% by weight to approximately 50.0% by weight (or any range of values ​​between 99.0% by weight and including 50.0% by weight, in increments of 0.1% by weight, for example, from approximately 0.3% to approximately 44.8% by weight) of one or more additional components, with both weight percentages of components (I) and (II) based on a total weight of the coating composition. In some desired embodiments, the coating compositions of the present invention comprise (I) from about 2.0 wt% to about 8.0 wt% (or any range of values ​​between 2.0 wt% and including 8.0 wt%, in increments of 0.1 wt%, for example, from about 2.3 to about 4.8 wt%) of the composite silica particle opacifying agents, and (II) from about 98.0 wt% to about 92.0 wt% (or any range of values ​​between 98.0 wt% and 8.0 wt%) of one or more additional components, with both weight percentages of components (I) and (II) based on a total weight of the coating composition. Uses The present invention is further directed to the use of composite silica particle opacifying agents in various coating applications / processes. When used as an opacifying agent in coating compositions, the composite silica particles described herein provide improved chemical resistance in addition to other desirable properties such as improved thermal stress resistance, improved weather resistance, improved film clarity, or any combination thereof in the final coating. In a preferred embodiment, the silica particle opacifying agents of the invention are useful in methods for improving the chemical resistance of a film resulting from an aqueous coating composition applied to a substrate. In another embodiment, the silica particle opacifying agents of the invention are useful in methods for simultaneously improving the chemical resistance and film clarity of a transparent cut film resulting from an aqueous film coating composition applied to a substrate. In a particularly preferred embodiment, the substrate is a wood substrate. In a desired embodiment, a pzzRnn / Lznz / E / YiAi wood substrate is treated with an aqueous coating composition thereof, wherein the coating composition comprises the composite silica particle coating agents of the invention on a surface of the wood substrate. Other substrates that can be coated with the coating compositions according to the present invention include, but are not limited to, leather, plastics (e.g., vinyl), metal (e.g., coil) or metal alloys, cement or concrete, or other industrial finishes. Generally, the method for using an opacifying agent in a coating composition according to the invention comprises incorporating the composite silica particle opacifying agents of the invention into a coating composition, preferably an aqueous coating composition, before applying the coating composition to the substrate. The typical incorporation step includes mixing or dispersing the composite silica particle opacifying agents into the formulation. The method for applying the coating composition to a substrate includes brushing, rolling, air spraying, lowering the water level, or other possible methods. As further described in the examples below, incorporating the silica particle opacifying agent of the present invention into an aqueous coating composition and subsequently applying the coating composition onto a substrate with drying, provides coated films with improved chemical resistance alone or in combination with improved film clarity compared to known coatings / films that do not contain silica particle opacifying agents of the present invention. For example, in some embodiments, a coating composition comprising silica particle opacifying agents composed in a sufficient quantity to achieve a 60° gloss value between 5.0 and 25.0 units, more preferably between 10.0 and 15.0 units (e.g., 12.5 units), in a dry film, results in a film, preferably a transparent coating film, on a substrate, wherein the film exhibits a 24 h AL* water damage of less than 6.0 units (or any range of values ​​less than 6.0 units, in increments of 0.1 units, e.g., from about 0.0 units to about 1.5 units) as measured using a Spectro-Guide 45 / 0 portable colorimeter and the method described in the examples below. In some embodiments, a coating composition comprising the silica particle opacifying agents of the invention in a sufficient amount to achieve a brightness value of 60° between 5.0 and 25.0 pzzRnn / Lznz / E / YiAi units, more preferably between 10.0 and 15.0 units (e.g., 12.5 units), in a dry film, results in a film, preferably a transparent coating film, on a substrate, wherein the film exhibits a 24 h AL* 50 / 50 water / ethanol damage of less than 6.5 units (or any range of values ​​less than 6.5 units, in increments of 0.1 units, e.g., from about 0.0 units to about 4.4 units) as measured using a Spectro-Guide 45 / 0 portable colorimeter and the method described in the examples below. In some embodiments, a coating composition comprising the silica particle opacifying agents of the invention in a sufficient amount to achieve a 60° gloss value between 5.0 and 25.0 units, more preferably between 10.0 and 15.0 units (e.g., 12.5 units), in a dry film, results in a film, preferably a transparent coating film, on a substrate, and the film exhibits a 1 h AL* 50 / 50 water / ethanol damage of less than 4.0 units (or any range of values ​​less than 4.0 units, in increments of 0.1 units, e.g., from about 0.0 units to about 0.2 units) as measured using a Spectro-Guide 45 / 0 portable colorimeter and the method described in the examples below. In some embodiments, a pzzRnn / Lznz / E / YiAi coating composition comprising the composite silica particle opacifying agents described herein in a sufficient quantity to achieve a 60° gloss value between 5.0 and 25.0 units, more preferably between 10.0 and 15.0 units (e.g., 12.5 units), in a dry film, results in a film, preferably a transparent coating film, on a substrate, and the film exhibits a 4 h AL* 50 / 50 water / ethanol damage of less than 4.5 units (or any range of values ​​less than 4.5 units, in increments of 0.1 units, e.g., from about 0.0 units to about 0.5 units) as measured using a Spectro-Guide 45 / 0 portable colorimeter and the method described in the examples below. In some desired modalities, films having one or more of the above improvements in water and / or chemical resistance also exhibit an AL* film clarity of less than 7.0 units (or any range of values ​​less than 7.0 units, in increments of 0.1 units, e.g., from approximately 1.2 units to approximately 5.0 units) as measured using a Spectro-Guide 45 / 0 handheld colorimeter and the method described in the examples below. In some desired embodiments, a coating composition comprising the composite silica particle opacifying agents described herein in a sufficient quantity to achieve a 60° gloss value between 5.0 and 25.0 units, more preferably between 10.0 and 15.0 units (e.g., 12.5 units), in a dry, transparent coating film applied to a substrate, forms a film exhibiting one or more or all of: (a) a 24 h AL* water damage of less than 6.0 units (or any range of values ​​less than 6.0 units, in increments of 0.1 units, e.g., from approximately 0.0 units to approximately 1.5 units) as measured using a Spectro-Guide 45 / 0 portable colorimeter and the method described in the examples below, (b) a 24 h AL* water / 50 / ethanol damage of less than that 6.5 units (or any interval of values ​​less than 6.5 units, in increments of 0.(a) 1 h AL* 50 / 50 water / ethanol damage of less than 4.0 units (or any range of values ​​less than 4.0 units, in increments of 0.1 units, for example, from approximately 0.0 units to approximately 1.8 units) as measured using a Spectro-Guide 45 / 0 portable colorimeter and the method described in the examples below, (b) 4 h AL* 50 / 50 water / ethanol damage of less than 4.5 units (or any range of values ​​less than 4.5 units, in increments of 0.1 units, for example, from approximately 0.0 units to approximately 0.5 units) as measured using a Spectro-Guide 45 / 0 portable colorimeter and the method described in the examples below, (c) 4 h AL* 50 / 50 water / ethanol damage of less than 4.5 units (or any range of values ​​less than 4.5 units, in increments of 0.1 units, for example, from approximately 0.0 units to approximately 0.5 units) as measured using a portable colorimeter Spectro-Guide 45 / 0 and the method described in the examples below and (e) a film clarity AL* less than 7.0 units (or any range of values ​​less than 7.0 units, in increments of 0.1 units, for example, from approximately 1.2 units to approximately 5.0 units) as measured using a Spectro-Guide 45 / 0 portable colorimeter and the method described in the examples below. In other desired embodiments, a coating composition comprising the composite silica particle opacifying agents described herein in a quantity sufficient to achieve a 60° gloss value between 5.0 and 25.0 units, more preferably between 10.0 and 15.0 units (e.g., 12.5 units), in a dry, transparent coating film applied to a substrate, forms a film exhibiting one or more or all of: (a) a 24 h AL* water damage of less than 4.1 units (or any range of values ​​less than 4.1 units, in increments of 0.1 units, e.g., from approximately 0.0 units to approximately 0.2 units) as measured using a Spectro-Guide 45 / 0 portable colorimeter and the method described in the examples below, (b) a 24 h AL* water / 50 / ethanol damage of less than 4.0 units (or any range of values ​​less than 4.0 units, in increments of pzzAnn / Lznz / E / YiAi (a) 0.1 units, for example, from approximately 0.0 units to approximately 0.2 units) as measured using a Spectro-Guide 45 / 0 handheld colorimeter and the method described in the examples below, (c) 1 h AL* 50 / 50 water / ethanol damage of less than 3.0 units (or any range of values ​​less than 3.0 units, in increments of 0.1 units, for example, from approximately 0.0 units to approximately 1.8 units) as measured using a Spectro-Guide 45 / 0 handheld colorimeter and the method described in the examples below, (d) 4 h AL* 50 / 50 water / ethanol damage of less than 4.2 units (or any range of values ​​less than 4.2 units, in increments of 0.1 units, for example, from approximately 0.0 units to approximately 0.5 units) as measured using a Spectro-Guide 45 / 0 handheld colorimeter and the method described in the examples below and (e) a film clarity AL* less than 6.5 units (or any interval of values ​​less than 6.5 units, in increments of 0.1 units, for example, from approximately 1.2 units to approximately 5.0 units) as measured using a Spectro-Guide 45 / 0 portable colorimeter and the method described in the examples below. In still other desired embodiments, a coating composition comprising the composite silica particle opacifying agents described herein in a quantity sufficient to achieve a 60° brightness pzzAnn / Lznz / E / YiAi value between 5.0 and 25.0 units, more preferably between 10.0 and 15.0 units (e.g., 12.5 units), in a dry transparent coating film applied to a substrate, forms a film exhibiting one or more or all of (a) a 24 h AL* water damage of less than 1.5 units (or any range of values ​​less than 1.5 units, in increments of 0.1 units, e.g., from approximately 0.0 units to approximately 0.2 units) as measured using a Spectro-Guide 45 / 0 portable colorimeter and the method described in the examples below, (b) a 50 / 50 water / ethanol damage of 24 h AL* less than 1.0 units (or any interval of values ​​less than 1.0 units, in increments of 0.(a) 1 h AL* 50 / 50 water / ethanol damage less than 1.0 unit (or any range of values ​​less than 1.0 unit, in increments of 0.1 unit, for example, from approximately 0.0 units to approximately 0.2 units) as measured using a Spectro-Guide 45 / 0 handheld colorimeter and the method described in the examples below, (b) 4 h AL* 50 / 50 water / ethanol damage less than 1.0 unit (or any range of values ​​less than 1.0 unit, in increments of 0.1 units, for example, from approximately 0.0 units to approximately 0.5 units) as measured using a Spectro-Guide 45 / 0 handheld colorimeter and the method described in the examples below, (c) 4 h AL* 50 / 50 water / ethanol damage less than 1.0 unit (or any range of values ​​less than 1.0 unit, in increments of 0.1 units, for example, from approximately 0.0 units to approximately 0.5 units) as measured using a handheld colorimeter Spectro-Guide 45 / 0 and the method described in the examples below and (e) a film clarity AL* less than 6.1 unit (or any interval of values ​​less than 6.1 units, in increments of 0.1 units, for example, from approximately 1.2 units to approximately 5.0 units) as measured using a Spectro-Guide 45 / 0 portable colorimeter and the method described in the examples below. Without being limited to any particular theory, it is hypothesized that the improved chemical resistance properties attributable to the improved composite silica particle opacifying agents, and the resulting compositions and films, may be due to one or more of the following factors: 1) the coated wax at least partially fills the pores of the porous silica particles and provides a reduction in particle shrinkage during solvent exposure and drying; 2) the coated wax leads to improved adhesion between the opacifying particle and the latex; 3) the coated wax has the ability to better flow and fill cracks as they form;4) the coated wax leads to reduced stress at the latex particle interface due to the softening of the latex in the region surrounding the particle, 5) diffusion of the wax into at least some of the film pores, thereby reducing the penetration of water, ethanol or other solvent into the film, and 6) the coated wax at least partially fills the pores of the porous silica particles but is not in excess to further coat the outer surface of the porous silica particles. It should be understood that although the composite silica particle opacifying agents, methods, and uses described above are described as comprising one or more components or steps, the composite silica particles, methods, and uses described above may comprise, consist of, or essentially consist of any of the components or steps of the composite silica particles, methods, and uses described above. Accordingly, where the present invention, or a portion thereof, is described by an open term such as "comprising," it should readily be understood that (unless otherwise indicated) the description of the present invention, or a portion thereof, should also be construed as describing the present invention, or a portion thereof, by using the terms "essentially consisting of" or "consisting of," or variations thereof as described below. As used herein, the terms comprise, comprising, include, having, containing, pzzRnn / Lznz / E / YiAi characterized by, or any other variation thereof, are intended to encompass a non-exclusive inclusion, subject to any limitations explicitly stated otherwise, of the mentioned components. For example, a composite silica particle, method, and / or use comprising a list of elements (e.g., components or steps) is not necessarily limited to only those elements (or components or steps) but may include other elements (or components or steps) not expressly listed or inherent in the composite silica particle, method, and / or use. As used herein, the transitional phrases "consists of" and "consisting of" exclude any unspecified element, step, or component. For example, "consists of" or "consisting of" used in a claim would limit the claim to the components, materials, or steps specifically mentioned in the claim, except for impurities commonly associated with them (i.e., impurities within a given component). When the phrase "consists of" or "consisting of" appears in a clause of the body of a claim, rather than immediately following the preamble, the phrase "consists of" or "consisting of" is limited only to the elements (or components or steps) set forth in that clause; other elements (or components) are not excluded by the claim as a whole. As used herein, the transitional phrases "essentially consists of" and "essentially consists of" are used to define composite silica particles, methods, and / or uses that include materials, steps, features, components, or elements, in addition to those literally described, provided that such additional materials, steps, features, components, or elements do not materially affect the basic and novel feature(s) of the claimed invention. The term "essentially consists of" occupies a middle ground between "comprising" and "consisting of." The present invention is further illustrated by the following examples, which should not be interpreted in any way as imposing limitations on its scope. On the contrary, it should be clearly understood that various other embodiments, modifications, and equivalents thereof may be used, which, after reading the description herein, may be suggested to those skilled in the art without departing from the spirit of the present invention and / or the scope of the appended claims. Examples The following examples describe (i) the processes according to the present invention for preparing composite silica particles, and (ii) the evaluation of the composite silica particles in coating compositions. The procedures for level reduction and brightness (opaque efficiency), the methods for measuring film clarity and chemical resistance used in the examples below were as follows: Level descent procedure and level descent cards Level draws were performed using a Gardner Company laboratory coiled wire rod with a No. 40 wire gauge. With this wire gauge, the wet film thickness was approximately 100 µm. After drying, the dry film thickness was approximately 30-35 µm. The level draw plates used were 219 x 286 mm² flat black papers from Leneta Company, Inc. (Mahwah, NJ). The procedure for each level draw was as follows: 1. In a clean, dust-free room, an empty level-lowering plate was placed on a vacuum support. 2. Using a pipette, approximately 2-5 ml of a well-mixed coating composition sample was placed onto and near the top of a sample sheet. 3. The ends of the level-draw rod were immediately grasped. Using the thumbs of both hands to prevent the rod from bending or flexing away from the sample, the level-draw rod was lowered through the liquid reservoir, diffusing and measuring the fluid through the sample sheet. After a predetermined level draw was achieved, the level-draw rod was immersed in a cleaning tray. 4. After the level drop, the level drop samples were left at room temperature for at least four days to allow complete drying of the coated layer. 5. After the level drop coated plate dried, chemical resistance, film clarity, opacifying effectiveness and cold controls were carried out using the following procedures. Gloss (opaque effectiveness). Testing and measurement methods for film clarity and chemical resistance: A Micro-TRI-Gloss portable gloss meter (BYK-Gardner USA, Columbra, MD) was used to read the film gloss. Gloss values ​​were measured and recorded at 60° intervals. A Spectro-Guide 45 / 0 portable colorimeter (also from BYK Gardner) was used to monitor chemical damage and film clarity. L* values ​​were obtained by taking colorimeter readings on a given coated film. On the black background card, the non-opaque stock solution (from Example pzzRnn / Lznz / E / YiAi) 6) gave an L* value of approximately 7.9. The addition of an opacifying agent (e.g., composite silica particles of the present invention or commercially available comparative opacifying agents) to the stock solution made the film whiter (i.e., resulting in a higher L* value) and the film clarity matte was defined as the difference between the new L* value and the L* value of the film formed out of the stock solution containing no opacifying agent. The chemical resistance testing methods used were in accordance with the specifications of European Standard EN 12720 / DIM 68861-1. Resistance to deionized water and a 50 / 50 ethanol-water solution was tested. The test was performed as follows: 1. Circles (1 inch in diameter) were cut from Fisherbrand filter paper. 2. The circles were soaked in water or in a 50 / 50 ethanol / water solution for 30 seconds. 3. Each soaked circle was placed on a dry level-drop card, and then covered with a weighing plate to prevent evaporation. 4. After a certain amount of time (i.e., 24 hours for the water test, and 1 hour and 4 hours for the 50 / 50 ethanol in water test), the weighing plate and paper were removed. pzzRnn / Lznz / E / YiAi 5. A white mark formed in the contact area over time, and after the entire night, L* values ​​were measured using the Spectro-Guide 45 / 0 colorimeter. 6. Chemical damage (inversely related to chemical resistance) was defined as the difference between the L* value of the white mark (i.e., the highest reading of at least three readings) and the surrounding undamaged background of the film. The percentage change was also calculated. Formation of a stock solution for testing coating compositions The components listed in Table 1 below were combined as described below to form a stock solution for testing coating compositions as described below. pzzAnn / Lznz / E / YiAi Table 1. Stock solution for testing coating compositions Raw Material Supplier Weight (g) Comments NEOCRYL® KX12 DSM 77.43 Acrylic emulsion Deionized water 11.07 PDnB from DOWANOL™ Dow Chemical 8.85 Coalescing BYK® 024 BYK 0.55 Defoamer SURFYNOL®104E Air products 1.11 Wetting and defoaming RHEOLATE® 299 Elementis 0.22 Rheolate BYK® 346 BYK 0.77 77.43 grams (g) of NEOCRYL® KX12 and 5.53 g of deionized water were mixed in a first container. 8.85 g of DOWANOL™ DPnB and 5.54 g of deionized water were mixed in a second container. The contents of the second container were then slowly poured into the first container. The mixture was dispersed at 1500 rpm for 15 minutes using a Gardner Company DISPERMAT® disperser (Pompano Beach, Florida) with a 30 mm wide blade. 0.55 g of BYK® 024, 1.11 g of SURFYNOL® 104E, and 0.22 g of RHEOLATE® 299 were added to the mixture in the first container. The mixture was then dispersed at 2500 rpm for 10 minutes using the DISPERMAT® disperser. 0.77 g of BYK® 346 was added to the mixture in the first container. The mixture was then dispersed at 1000 rpm for 5 minutes using the DISPERMAT® disperser. The resulting mixture was then used as a stock solution, which can be stored for up to 1 month. Formation of coating compositions comprising an opacifying agent and the stock solution The coating compositions comprising an opacifying agent (e.g., silica particles of the present invention or comparable commercially available opacifying agents) and the stock solution were prepared as follows. The objective of using an opacifying agent was to reduce the gloss of the coated film and obtain a target gloss range of 10.0 to 15.0, or approximately 12.5, at 60°. To achieve this gloss level, the required amounts of opacifying agents varied for different samples and were determined from a separate loading study. After a specified amount of opacifying agent was added to a given amount of the stock solution formed in Example 3 above, the resulting mixture was dispersed at 2500 rpm for 30 min using the DISPERMAT® disperser and then allowed to settle overnight at room temperature. The level drops to test each coating composition were performed on the second day (i.e., the day after making a given coating composition) using the level drop procedure described above. Nitrogen pore volume and BET surface area measurements of composite particle samples The nitrogen pore volumes of the composite silica particles, as described in the examples below, were measured using an Autosorb® 1Q analyzer, available from Quantachrome Instrument (Boynton Beach, EL). Degassing for each sample was performed at 65 °C (i.e., below the wax melting temperature of approximately 80 °C) for 4 hours. Nitrogen desorption and adsorption isotherms were measured at 77 K with a nitrogen pressure increase from 0.01% atm to 0.998% atm, and subsequently with a decrease from 0.998% atm to 0.025% atm, respectively. Pore volumes were calculated using the AsiQwin™ software, version 5.0, based on BJH theory. See, for example, Barrett et al., The Determination of Pore Volume and Area Distributions in Porous Substances. I. Computations from Nitrogen Isotherms, J. Am. Chem. Soc., 1951, 73 (1), pp.373-380, and the BET surface area were also calculated based on the Brunauer-Emmet Teller method (Brunauer et al., Adsorption of Gases in Multimolecular Layers. J. Am. Chem. Soc. 1938, 60 (2): 309-319). This material is incorporated in full by reference herein. The measured pore volumes and BET surface areas for the composite silica particles are summarized in Tables 2 and 4. General method for the formation of composite silica / wax particles One to ten grams of polyethylene were dissolved in 60–100 ml of toluene with heating. Ten grams of porous silica particles were mixed with the wax solution. The mixture was left in a crystallization dish in a well-ventilated fume hood overnight to allow all the solvent to evaporate. The dried residue was ground with a mortar and pestle to allow all particles to pass through a 500 µm sieve. The sieved particles were then heated at 120 °C for one hour. After drying, the composite silica particles were cooled to room temperature, and the particle size was further reduced using an analytical mill to allow the composite silica particles to pass through a 45 µm (325 mesh) sieve. The sieved composite silica particles were suitable for use as is, directly, for example, in a paint formulation. Examples 1-8: To evaluate the effectiveness of opacifying agents on silica / wax composite particles at wax levels greater than 50% by weight, based on the total weight of the silica / wax composite particles, samples were prepared using the procedure described above. Each sample was subsequently evaluated for chemical resistance and chemical damage. In Examples 1-8, porous silica gel particles with an initial pore volume of approximately 1.87 cc / g and a mean particle size of approximately 9 µm were treated with varying amounts of waxes (from 50 wt% to 75 wt%) and loaded into the stock solution described above. Table 2 below lists the results for these samples. pzzRnn / Lznz / E / YiAi Table 2. Samples with the highest amount of waxes Example No. Wax levels (%) Average particle size (pm) BET surface area of ​​compounds (m2 / g) Pore volume of compounds (cc / g) 1 50 6.04 42 0.30 2 52.5 5.97 32.4 0.22 3 55 5.84 21.5 0.14 4 58 5.84 13.9 0.12 5 60 6.05 5.4 0.03 6 65 40.1 (difficult to sieve) 0.6 0.00 7 70 Difficult to sieve 0 0.00 8 75 Difficult to sieve 0 0.00 As shown in Table 2, starting with a silica pore volume of 1.87 cc / g, there is a gradual reduction in total pore volume and final BET surface area with increasing wax levels. At approximately 60% wax, the pore volume is close to zero, while the final BET falls below 10.0 m² / g. Above 60 wt%, sample preparation became problematic, as shown in Table 2. At 65 wt% wax, the particle size of the product was quite large at 40.0 pm, and at 70 wt% and above, the composite silica particles could not be sieved.Although not limited to any one theory, it is believed that an extra amount of wax, after the initial pores have been completely filled (pore volume very close to zero), will remain outside the composite silica particles and these waxes would behave like a sticky glue to hold all the composite silica particles together. The evaluation results of the samples in Examples 1-6 were listed in Table 3. pzzRnn / Lznz / E / YiAi Table 3. Evaluation results for Examples 1-6 Example no. Average particle size (pm) Wax levels (%) Filler (%) 60° gloss Chemical damage (AL*) Film clarity Water (24 h) Ethanol / water (1 h) Ethanol / water (4 h) 1 6.04 50 6.19 12.5 1.28 3.6 4.0 5.5 2 5.97 52.5 6.35 12.5 0.95 3.2 3.5 5.6 3 5.84 55 6.71 12.5 0.60 1.4 1.9 5.8 4 6.07 58 5.96 12.5 0.72 1.2 1.5 6.1 5 6.05 60 6.64 12.5 0.02 0.0 0.0 6.1 6 40.1 65 6.58 12.5 0.73 0.4 1.5 8.5 As shown, with higher wax content and reduced pore volume levels for the opacifying agents of the composite wax / silica particles, chemical damage decreases. At approximately 60 wt% wax with a near-zero pore volume, Example 5 shows essentially no damage. For the 65 wt% wax sample (Example 6), the chemical damage was worse than in Example 5, and the film clarity was also significantly worse. Comparative example 9: For Comparative Example 9, the commercially available pure wax-based opacifying agent CERAFLOUR® 929 was used. The chemical damage data for this comparative example are shown below in Table 4. Table 4. Comparison of pzzAnn / Lznz / E / YiAi silica / wax composite particles with commercial pure wax product Example No. Comments Particle Size (pm) Filler (%) 60° Gloss Chemical Damage (AL*) Film Clarity Water (24 h) Ethanol / Water (1 h) Ethanol / Water (4 h) 5 60% Silica / Wax 5.60 7.05 12.5 0.2 0.0 0.0 6.0 9 CERAFLOUR® 929 7.70 7.11 12.5 0.0 0.0 0.0 6.8 As shown, 60 wt% silica particles composed of silica / wax showed similar performance to pure wax-based opacifying agents, with the 60 wt% silica particle composite opacifying agent having advantages over the commercially available opacifying agent (e.g., a lower wax content is required to achieve 12.5 brightness, and better film clarity (i.e., lower AL* value)). Examples 10-13 In Examples 10-13, porous silica particles having a lower initial pore volume were selected as starting materials for wax coating and performance comparison. In Example 10, porous silica gel particles were used with an initial pore volume of approximately 1.14 cc / g and an average particle size of approximately 7.0 pm. In Example 11, the porous silica particles from Example 10 were used as the starting material, and these particles were coated with 30 wt% polyethylene (PE) wax. In Example 12, the porous silica particles from Example 10 were used as the starting material, and these particles were coated with 40 wt% PE wax. In Example 13, the porous silica particles from Example 10 were used as the starting material, and these particles were coated with 50 wt% PE wax. The following Table 5 lists the physical properties and evaluation results for these examples. pzzAnn / Lznz / E / YiAi Table 5. Low volume porous silica particles pzzRnn / Lznz / E / YiAi and their wax-coated samples Example No. Wax levels (% by weight) Average particle size (pm) Measured BET (m2 / g) Measured PV (cc / g) Loading (%) Film gloss AL* (water, 24 h) AL* (ethanol / water, 24 h) 10 0 8.02 306 1.14 3.36 12.5 7.4 15.2 11 30 5.44 98 0.41 5.65 12.5 3.9 6.9 12 40 6.05 56 0.27 5.74 12.5 4.1 6.5 13 50 6.12 5.7 0.03 7.13 12.5 0.4 0.3 Although the invention has been described in a limited number of embodiments, these specific embodiments are not intended to limit the scope of the invention as otherwise described and claimed herein. It may be evident to those skilled in the art after reviewing the illustrative embodiments herein that further modifications, equivalents, and variations are possible. All parts and percentages in the examples, as well as throughout the rest of the description, are by weight, unless otherwise specified.Furthermore, any range of numbers cited in the description or claims, such as those representing a particular set of properties, units of measurement, conditions, physical states, or percentages, is intended to be incorporated, literally and expressly, into the present description, by reference or otherwise, any number within that range, including any subset of numbers within any cited range. For example, when describing a numerical range with a lower limit, RL, and an upper limit, Ru, any number R that falls within the range is specifically described. In particular, the following numbers R within the range are specifically described: R = RL + k(Ru - RL), where k is a variable that varies from 1% to 100% in 1% increments, e.g., k is 1%, 2%, 3%, 4%, 5%, ... 50%, 51%, 52%, ... 95%, 96%, 97%, 98%, 99%, or 100%.Furthermore, any numerical interval represented by two values ​​of R, as calculated above, is further specifically described. Any modifications to the invention, other than those shown and described herein, will be apparent to those skilled in the art from the foregoing description and the accompanying figures. It is intended that such modifications fall within the scope of the appended claims. All publications cited herein are incorporated in full by reference. It is hereby stated that, as of this date, the best method known to the applicant for putting the aforementioned invention into practice is the one that is clear from the present description of the invention.

Claims

1. An opacifying agent for aqueous coating compositions characterized in that it comprises composite silica particles, the composite silica particles comprising: (a) porous silica particles; and (b) one or more waxes in a sufficient amount to at least partially fill pores of the porous silica particle such that the composite silica particles have a total pore volume of less than 0.20 cc / g as determined by the Barrett-Joyner-Halenda (BJH) method and a final particle surface area of ​​less than 10.0 square meters per gram (m2 / g) as determined by the Brunauer Emmet Teller (BET) nitrogen adsorption method.

2. The opacifying agent according to claim 1, characterized in that the porous silica particles have an initial pore volume of approximately 0.20 cc / ga to approximately 2.50 cc / g, as determined by the Barrett-Joyner-Halenda (BJH) method.

3. The opacifying agent according to claim 1 or 2, characterized in that the porous silica particles pzzAnn / Lznz / E / YiAi have an initial particle surface area of ​​approximately 100 m2 / ga to approximately 1000 m2 / g as determined by the BET nitrogen adsorption method, wherein the porous silica particles preferably have an initial particle surface area of ​​approximately 200 m2 / ga to approximately 800 m2 / g as determined by the BET nitrogen adsorption method.

4. The opacifying agent according to any one of claims 1 to 3, characterized in that the composite silica particles comprise from approximately 15.0% by weight to less than approximately 65.0% by weight of one or more waxes, based on the total weight of the composite silica particles.

5. The opacifying agent according to any one of claims 1-4, characterized in that a) the porous silica particles have an initial pore volume of approximately 1.70 cc / g to approximately 2.20 cc / g as determined by the Barrett-Joyner-Halenda (BJH) method, wherein the composite silica particles preferably comprise from approximately 55.0 wt% to approximately 65.0 wt% of one or more waxes, based on the total weight of the composite silica particles; b) the porous silica particles have an initial pore volume of approximately 1.20 cc / g to approximately 1.70 cc / g as determined by the Barrett-Joyner-Halenda (BJH) method, wherein the composite silica particles preferably comprise from approximately 45.0 wt% to approximately 55.0 wt% of one or more waxes, based on the total weight of the composite silica particles.0% by weight of one or more waxes, based on a total weight of the composite silica particles; oc) wherein the porous silica particles have an initial pore volume of approximately 1.00 cc / ga to approximately 1.20 cc / g as determined by the Barrett-Joyner-Halenda (BJH) method, wherein the composite silica particles preferably comprise from approximately 35.0% by weight to approximately 45.0% by weight of one or more waxes, based on a total weight of the composite silica particles; od) wherein the porous silica particles have an initial pore volume of approximately 0.60 cc / ga to approximately 1.00 cc / g as determined by the Barrett-Joyner-Halenda (BJH) method, wherein the composite silica particles preferably comprise from approximately 25.0% by weight to approximately 35.0% by weight of one or more waxes, based on a total weight of the composite silica particles; e) wherein the porous silica particles have an initial pore volume of approximately 0.30 cc / g to approximately 0.60 cc / g as determined by the Barrett-Joyner-Halenda (BJH) method, wherein the composite silica particles preferably comprise from approximately 15.0% by weight to approximately 25.0% by weight of one or more waxes, based on a total weight of the composite silica particles.

6. The opacifying agent according to any of claims 1 to 5, characterized in that the composite silica particles have a total pore volume less than or equal to 0.19 cc / g, preferably 0.001 cc / g approximately 0.19 cc / g, more preferably 0.01 cc / g approximately 0.05 cc / g, as determined by the BJH method.

7. The opacifying agent according to any of claims 1 to 6, characterized in that the composite silica particles have a final particle surface area of ​​approximately 1.0 m2 / ga to approximately 9.0 m2 / g, preferably 2.0 m2 / ga to approximately 8.0 m2 / g, as determined by the BET nitrogen adsorption method.

8. The opacifying agent according to any of claims 1 to 7, characterized in that the porous silica particles comprise silica gel, precipitated silica, or pyrogenic silica particles.

9. The opacifying agent according to any of claims 1 to 8, characterized in that the pzzAnn / Lznz / E / YiAi porous silica particles have an average particle size of approximately 1.0 micron (pm) to approximately 50.0 pm.

10. The opacifying agent according to any of claims 1 to 9, characterized in that the one or more waxes comprise a hydrocarbon wax, a paraffin wax, a polyethylene wax, a polypropylene wax, a vegetable wax, an animal wax, or any combination thereof, wherein the one or more waxes preferably comprise a polyethylene wax, a polypropylene wax, or a combination thereof, more preferably comprising a polyethylene wax having an average molecular weight of at least 2000.

11. The opacifying agent according to any of claims 1 to 10, characterized in that the composite silica particles are free-flowing particles.

12. The opacifying agent according to any one of claims 1 to 11, characterized in that when incorporated into a coating composition and applied onto a flat, smooth, non-penetrating black paper, it enables the coating composition to form a transparent coating film having a 60° gloss value between approximately 5.0 and approximately 25.0, preferably a 60° gloss value of approximately 12.5, as measured using a Micro-pzzRnn / Lznz / E / YiAi TRI-Gloss portable gloss meter, wherein a) the transparent coating film exhibits a 24 h AL* water damage of less than 6.0 units, preferably less than approximately 4.1 units, more preferably less than approximately 1.5 units, and even more preferably less than approximately 0.5 units, as measured using a Spectro-Guide 45 / 0 portable colorimeter, and / ob) the clear coating film preferably exhibits a 24 h AL* 50 / 50 water / ethanol damage of less than 6.5 units, more preferably less than 4.0 units, even more preferably less than 0.5 units, even more preferably less than 4.0 units, as measured using a Spectro-Guide 45 / 0 portable colorimeter, and / oc) the clear coating film preferably exhibits a 1 h AL* 50 / 50 water / ethanol damage of less than 4.0 units, more preferably less than 1.0 units, even more preferably less than 1.0 units, even more preferably less than 0.5 units, as measured using a Spectro-Guide 45 / 0 portable colorimeter, and / od) the clear coating film preferably exhibits a 4 h AL* 50 / 50 water / ethanol damage of less than 4.5 units, with a higher preference less than 1.0 units, even with a higher preference less than 0.5 units, as measured using a Spectro-Guide 45 / 0 portable colorimeter, and / or e) the clear coating film exhibits a film clarity AL* less than 7.0 units, with higher preference less than 6.5 units, even with higher preference less than 5.0 units, as measured using a Spectro-Guide 45 / 0 portable colorimeter.

13. The opacifying agent according to any of claims 1 to 12, characterized in that when incorporated into a coating composition and applied to a substrate, it enables the coating composition to exhibit a film clarity AL* of less than 7.0 units, preferably less than 6.5 units, more preferably less than 5.0 units, as measured using a Spectro-Guide 45 / 0 portable colorimeter.

14. A method for preparing the opacifying agent according to any of claims 1 to 13, characterized in that it comprises: contacting porous silica particles with one or more waxes to form composite silica particles having a total pore volume less than 0.20 cc / q as determined by the BJH method and a final particle surface area less than 10.0 square meters per qramo (m2 / g) as determined by the Brunauer Emmet Teller (BET) nitrogen adsorption method.

15. The method according to claim 14, characterized in that the contact step comprises: a) dissolving or dispersing one or more waxes in a solvent to form a suspension or dispersion; incorporating the porous silica particles into the suspension or dispersion; and removing the solvent from the suspension or dispersion to form the composite silica particles; or wherein the contact step comprises: melting one or more waxes to form a molten liquid; and incorporating the porous silica particles into the molten liquid to form the composite silica particles;or wherein the contact step comprises: simultaneously mixing or grinding (a) one or more waxes and (b) the porous silica particles with heat to form the composite silica particles, and / or) reducing the particle size of the composite silica particles to obtain composite silica particles having a final particle size less than 100 microns (pm), preferably further comprising: forming composite silica particles having a first particle size less than approximately 500 microns (pm); heat-treating the composite silica particles at an elevated temperature for a heat-treating time period; allowing the heat-treated composite silica particles to cool;and grinding the heat-treated composite silica particles to obtain a final particle size smaller than approximately 100 µm, wherein the composite silica particles are preferably heat-treated at an elevated temperature ranging from approximately 90 °C to approximately 140 °C, and the heat treatment time ranges from approximately 1.0 hour (h) to approximately 4.0 h, wherein more preferably the elevated temperature ranges from approximately 100 °C to approximately 130 °C, and the heat treatment time ranges from approximately 1.0 h to approximately 1.5 h.; 16. The method according to claim 15, characterized in that the reduction step results in composite silica particles having a final particle size less than approximately 45.0 pm.

17. A coating composition characterized in that it comprises the opacifying agent according to any of claims 1 to 16.

18. The coating composition according to claim 17, characterized in that a) it comprises an aqueous composition, and / ob) it preferably comprises up to approximately 50% by weight of the opacifying agent, more preferably from approximately 2.0% by weight to approximately 8.0% by weight, based on the total weight of the coating composition, and / oc) wherein the coating composition, when applied to a flat, smooth, non-penetrating black paper, forms a film having a 60° gloss value of approximately 5.0 to approximately 25.0, preferably approximately 12.5, as measured using a Micro-TRI-Gloss portable gloss meter, and / od) wherein the film according to claim 18 c) preferably exhibits a 24 h AL* water damage of less than 6.0 units, more preferably less than approximately 4.1 units, and even more preferably less than approximately 1.5 units, even more preferably less than approximately 0.5 units, as measured using a Spectro-Guide 45 / 0 portable colorimeter, and / oe) wherein the film according to claim 18 c) od) preferably exhibits a 24 h AL* 50 / 50 aqua / ethanol damage of less than 6.5 units, more preferably less than 4.0 units, even more preferably less than 0.5 units, as measured using a Spectro-Guide 45 / 0 portable colorimeter, and / of) wherein the film according to claim 18 c) od) exhibits a 1 h AL* 50 / 50 aqua / ethanol damage of less than 4.0 units, more preferably less than 1.0 units, even more preferably less than 0.5 units, as measured using a Spectro-Guide 45 / 0 portable colorimeter, and / og) in wherein the film according to claim 18 c) od) preferably exhibits a 50 / 50 water / ethanol damage of 4 h AL* less than 4.5 units, more preferably less than 1.0 units, even more preferably less than 0.5 units, as measured using a Spectro-Guide 45 / 0 portable colorimeter, and / or) wherein the film according to claim 18 c) od) preferably exhibits a film clarity AL* less than 7.0 units, more preferably less than 6.5 units, even more preferably less than 5.0 units, as measured using a SpectroGuide 45 / 0 portable colorimeter.

19. A substrate coated with the coating composition according to claim 18, characterized in that it preferably comprises a wood substrate.

20. A method for improving the chemical resistance of an aqueous composition applied to a wood substrate, characterized in that it comprises: incorporating the opacifying agent according to any of claims 1 to 13 into the coating composition; applying the coating composition to at least one surface of a wood substrate to form a coating; and drying the coating to form a film on at least one surface of the wood substrate, wherein the film is preferably a transparent coating film.