Method for Coating Inorganic Compounds in Powdered Form

This invention relates to an improved process for preparing composite materials by coating a powder of an inorganic substance with a polymer.

Various methods for coating powdery inorganic materials with organic polymers by means of in situ polymerization have heretofore been proposed. For example, U.S. Pat. No. 3,330,693 discloses a method for coating the surface of iron oxide powder with polyethylene in an aqueous medium, and British Pat. No. 1,156,653 discloses a method for coating the surface of red iron oxide, indigo red or titanium oxide powder with a copolymer of methyl methacrylate and methacrylic acid in petroleum ether. Further, Japanese Pat. No. 302,461 discloses a method for preparing compositions comprising a pigment and a polymeric substance by polymerizing a monomer in the presence of various kinds of pigments with the aid of a catalytic system.

In each of these known methods, however, the polymerization of the monomer proceeds predominantly in the reaction medium rather than on the surface of the particles of the inorganic compound and, therefore, the amount of the polymer actually coated on the particles is very small and the bulk of the polymer does not participate in the coating. In short, each of these known methods is defective in that the ratio of the coated polymer to the total of the available polymer is very low.

It is, therefore, a primary object of this invention to provide an effective method for coating the surface of powdery inorganic substances with a polymeric material.

Another object of this invention is to provide a method for coating a powdery transition metal compound with a polymer in which a vinyl monomer is polymerized selectively on the surface of particles of the transition metal compound by utilizing the transition metal compound per se as one component of the polymerization catalyst system.

A further object of this invention is to provide an effective method for coating the surface of a powdery inorganic substance with a polymer in which particles of an inorganic substance which are capable of participating in the polymerization catalyst system are associated with particles of the transition metal compound.

Other and further objects, features and advantages of this invention will appear more fully from the following description.

The term "transition metal compound" as used herein means a compound selected from the class consisting of metals of the group ranging from Sc having an atomic number of 21 to Zn having an atomic number of 30, metals of the groups ranging from Y having an atomic number of 39 to Cd having an atomic number of 48 and metals of the group ranging from La having an atomic number of 57 to Hg having an atomic number of 80.

The term "vinyl monomer" as used herein means a monomer that can be vinyl-polymerized in the presence of a radical polymerization initiator.

In accordance with this invention, a polymer-coated powder of a transition metal compound is obtained by contacting a transition metal compound in powdered form with a vinyl monomer in an aqueous medium containing hydrogen sulfite ions.

Oxides, hydroxides, nitrides, carbides, silicides, sulfides, carbonates, sulfates, halides, double salts and complex salts of metals selected from the group consisting of silver, copper, zinc, titanium, zirconium, niobium, vanadium, chromium, tungsten, manganese, nickel and cobalt, which are insoluble or hardly soluble and stable in an aqueous medium, are especially preferred as the transition metal compound in this invention. As specific examples of these especially preferred transition metal compounds, there can be mentioned oxides, such as silver oxide, chromium oxide, cobalt oxide, zinc oxide, tungsten dioxide, copper oxide, titanium oxide, niobium oxide, vanadium pentoxide, zirconium oxide, manganese oxide and nickel oxide; the corresponding sulfides, carbides, nitrides and silicides; composite oxides, such as cobalt titanate and nickel vanadate; hydroxides, such as cupric hydroxide, nickel hydroxide and cobalt hydroxide; sulfates, such as cobalt sulfate, copper sulfate, manganese sulfate, nickel sulfate and zinc sulfate; and the corresponding carbonates and halides.

As the vinyl monomer to be used in this invention, there can be mentioned, for example, unsaturated carboxylic acids, such as acrylic acid and methacrylic acid; salts and esters of these unsaturated carboxylic acids; vinyl esters of fatty acids, such as vinyl acetate, vinyl propionate, vinyl laurate and vinyl stearate; dienes, such as chloroprene, butadiene and isoprene; olefins, such as 1-butene and 1-pentene; halogenated olefins, such as vinyl chloride and vinylidene chloride; styrenes, such as styrene, .alpha.-methylstyrene and pentachlorostyrene; heterocyclic vinyl compounds, such as 2-vinylpyridine and 2-vinylthiophene; vinyl ketones, such as methyl vinyl ketone and ethyl vinyl ketone; vinyl ethers, such as methyl vinyl ether and ethyl vinyl ether; and crosslinkable vinyl compounds, such as divinylbenzene and N-methylol acrylamide. These vinyl monomers can be used alone or in the form of mixtures of two or more. These vinyl monomers are generally used in amounts of 0.1 to 20 parts by weight per 100 parts by weight of the transition metal compound, but larger amounts can be used is desired.

In particular, the vinyl monomer can be an alkyl ester of acrylic acid or methacrylic acid, styrene, or a mixture of styrene with other comonomers.

In this invention, it is indispensable that hydrogen sulfite ions should be present in an aqueous medium to be used for the polymerization. This can be accomplished by incorporating in the polymerization medium a hydrogen sulfite ion-supplying substance, such as sulfur dioxide, an aqueous solution of sulfurous acid, a hydrogen sulfite and a combination of a sulfite and an acid. A preferred hydrogen sulfite ion-supplying substance is sulfur dioxide or an aqueous solution of sulfurous acid. The amount of the hydrogen sulfite-supplying substance is generally within a range of 0.01 to 100 parts by weight, calculated as sulfur dioxide, per 100 parts by weight of the vinyl monomer. It is possible to use larger amounts, but no particular advantage is attained.

In this invention, the polymerization is carried out in an aqueous medium. As the aqueous medium, water or a mixture of water and an alcohol is preferably employed. The polymerization can be conducted according to any solution polymerization, suspension polymerization and emulsion polymerization procedures.

The order of addition of the components is not particularly critical in the process of this invention, but generally a transition metal compound and a vinyl monomer are mixed in advance, and a hydrogen sulfite ion-supplying substance is added to initiate the polymerization. In this case, the reaction can be accelerated by heating, if desired. The polymerization generally lasts for 0.5 to 6 hours.

In the process of this invention, the transition metal compound per se forms a radical polymerization initiator together with hydrogen sulfite ions and polymerization of the vinyl monomer is advanced by virtue of the so-formed radical polymerization initiator. Therefore, the polymer is formed predominantly on surfaces of particles of the transition metal compound and, hence, transition metal compound particles closely and tightly coated with the polymer can be obtained.

In case the polymerization initiator of the present invention is used together with a known radical polymerization initiator, e.g., organic peroxide, polymerization of the monomer proceeds predominantly in the reaction medium rather than on the surface of the inorganic compound particles and, consequently, the amount of the polymer actually coated on the particles is very small and the bulk of the polymer does not participate in the coating.

In accordance with another embodiment of this invention, a hydrogen sulfite ion-supplying substance and a transition metal or a compound thereof are added to a mixture comprising an inert inorganic material itself substantially free of transition metal and a vinyl monomer to cause polymerization of the vinyl monomer, whereby the inert inorganic material can be coated with the resulting polymer.

The term "inert inorgaic material" is intended to mean that this material is substantially free of transition metal or any compound of such a metal in the sense that the material either does not contain a transition metal at all or in so small an amount as to be immaterial. Therefore, when such inert inorganic material is present with hydrogen sulfite ions, that mixture has no activity whatsoever to initiate polymerization of the vinyl monomer or, if it has such activity, it is so low as to give an insignificantly low polymerization rate. As examples of such inert inorganic material, there can be mentioned quartz sand, gypsum, calcium sulfite, calcium carbonate, silica gel, zeolite, aluminum and mixtures thereof.

As the transition metal to be incorporated with the inert inorganic material, there can be used, for example, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium and niobium. These transition metals can be used in the forms of such compounds as oxides, hydroxides and various salts. It is also possible to use these transition metals in the form of rocks, slag, ore, red mud, alloys and the like. The transition metal or compound is added in an amount of at least 0.01% by weight, preferably at least 0.5% by weight, as calculated as the metal, based on the inert inorganic material. There is no upper limit of the amount of the transition metal or compound and it may be used in much larger amounts, but in order to obtain a product composed mainly of the inert inorganic material, it is preferred that the amount of the transition metal or compound be less than 50% by weight.

In this invention, the order of addition of the components is not critical and any optional order can be adopted, but it is advantageous that the inert inorganic material is treated in advance with a transition metal or its compound and a vinyl monomer and a hydrogen sulfite ion-supplying substances are then added thereto to effect polymerization. In the case of silica gel, for example, it is possible to conduct the impregnation treatment with use of a solution of iron chloride.

In accordance with this invention, a degree of polymerization of 80% of higher can be obtained by conducting the polymerization for such a short time as 1 to 5 hours, and the coating of the inert inorganic material with a polymer can be accomplished very smoothly.

The polymer-coated inorganic material obtained according to this invention can be molded as is or after incorporation with a suitable filler or pigment. Therefore, it can be used effectively as a molding material. Moreover, it can be used broadly in various fields as a filler for plastics, a glaze for ceramics, a material for the preparation of catalysts, a material for preparing refractories and the like.

This invention will now be illustrated in further detail by reference to the following examples which are not intended to limit the scope of this invention.

A 100-ml capacity three-neck flask, immersed in a water tank maintained at 50.degree.C, was charged with 100 ml of water, 10.0 g of an oxide of a size not larger than 100 mesh and identified in Table 1 and 2.0 g of a monomer indicated in Table 1, and 0.3 g of sulfurous acid gas was blown into the mixture over a period of 20 minutes under agitation to effect the reaction. After completion of the gas blowing, the reaction mixture was further agitated for 1 hour to advance the reaction. Then, the resulting slurry was filtered and the recovered solid was dried at 50.degree.C under reduced pressure until its weight was stabilized. The results are shown in Table 1. When the thus-obtained dried products were observed under an electron microscope, it was found that in each product the polymeric material effectively covered the particle surfaces.

A 100-ml capacity, three-neck flask immersed in a water tank maintained at 50.degree.C was charged with 100 ml of water, 10.0 g of a powdery oxide of a size not larger than 100 mesh and identified in Table 2 and 2.0 g of methyl methacrylate, and 3.0 ml of a 1.7 N aqueous solution of sulfurous acid was added under agitation. The reaction was conducted for 2 hours, and the resulting slurry product was filtered. The recovered solid was dried at 50.degree.C under reduced pressure until no further weight change occurred. The results are are shown in Table 2. When the thus obtained dried products were observed under an electron microscope, it was found that in each product the polymer effectively covered the particle surfaces.

A 100-ml capacity three-neck flask immersed in a water tank maintained at 50.degree.C was charged with 40 ml of water, 5.0 g of powdery nickel hydroxide of a size not larger than 100 mesh and 5.4 g of methyl methacrylate, and 3.46 g of sulfurous acid gas was blown to the mixture under agitation over a period of 30 minutes to effect the reaction. The reaction mixture was further agitated for 1 hour and 30 minutes. The resulting slurry product was filtered, and dried at 50.degree.C under reduced pressure until no further weight change occurred, whereby 9.09 g of a solid product was obtained. The polymer content in the resulting solid product was found to be 53.5% by weight.

Preparation of a polymer-coated powder of a transition metal compound was conducted in the same manner as in Example 3. More specifically, a 100-ml capacity, three-neck flask immersed in a water tank maintained at 50.degree.C was charged with 100 ml of water, 10.0 g of a powdery hydroxide of a size not larger than 100 mesh and identified in Table 3 and 2.0 g of methyl methacrylate, and 0.5 g of sulfurous acid gas was blown into the mixture under agitation over a period of 15 minutes. Then, the reaction mixture was further agitated for 1 hour and 45 minutes. The resulting slurry product was filtered and dried at 50.degree.C under reduced pressure until no further weight change occurred. The results are shown in Table 3. When the thus obtained dried products were observed under an electron microscope, it was found that in each product the polymer effectively covered the hydroxide surface.

A 100-ml capacity, three-neck flask immersed in a water tank maintained at 50.degree.C was charged with 100 ml of methanol, 10.0 g of a water-soluble or water-insoluble inorganic salt of a size not larger than 100 mesh and identified in Table 4 and 2.0 g of methyl methacrylate, and 5.0 ml of a 1.7 N aqueous solution of sulfurous acid was added to the mixture under agitation. In this state, the reaction was conducted for 24 hours. The resulting slurry product was filtered and dried at 50.degree.C under reduced pressure until no further weight change occurred. The results are shown in Table 4. When the thus obtained dried products were observed under electron microscope, it was found that in each product the polymer effectively covered the surfaces of the inorganic salt particles.

3-Liter capacity, three-neck flasks immersed in a water tank maintained at 50.degree.C were respectively charged with 400 g each of the following inert inorganic materials - calcium sulfite (less than 150 mesh), sea sand having a quartz content higher than 99.5% (less than 200 mesh), aluminum oxide (less than 200 mesh) and gypsum dihydrate (100 mesh), and 5 g of a transition metal or one of the following transition metal compounds -oxide, copper powder (100 mesh) zircon sand (less than 100 mesh) and nickel oxide, plus a prescribed amount of a vinyl monomer indicated in Table 5 and 1.6 Kg of water. Then, nitrogen was blown into the mixture for 15 minutes and 100 ml of a 1 N aqueous solution of sulfurous acid was added to the mixture. The polymerization was conducted in the thusformed nitrogen atmosphere under agitation. When 2 hours or 4 hours had passed from addition of the sulfurous acid aqueous solution, 1 g of hydroquinone was added to the reaction mixture, and the product was recovered by filtration, washed sufficiently with water and vacuum-dried at 130.degree.C. Data of the degree of polymerization in the resultant product are shown in Table 5.

For comparison, the polymerization was carried out without employing the transition metal or a compound thereof and the results are also shown in Table 5.

Each of the products was compression molded at 180.degree.C under 200 Kg/cm.sup.2 and the flexural strength of the molded article was determined to obtain the values shown in Table 5.

200 g of glass fiber powder (200 mesh), 200, g of aluminum oxide (.gamma.-Al.sub.2 O.sub.3 having a purity higher than 99.9%), 5 g of red iron oxide, 80 g of methyl methacrylate and 1.6 Kg of water were mixed, and 100 ml of a 1 N aqueous solution of sulfurous acid was added to the mixture. Under the same conditions described in Example 6, the polymerization was carried out at 50.degree.C under agitation for 2 hours to obtain a product of a good quality coating in which the degree of polymerization was 92% and the polymer content was 15.4% by weight.

The flexural strength of a molded product obtained by compression molding of the above-coated product at 200.degree.C under 200 Kg/cm.sup.2 was 212 Kg/cm.sup.2.

A 3-liter capacity, three-neck flask immersed in a water tank maintained at 50.degree.C as charged with 1.6 Kg of water, 200 g of powdery gypsum dihydrate, 200 g of powdery calcium sulfite, 5.0 g of powdery cobalt oxide and 80 g of methyl methacrylate, and 5.0 g of sulfurous acid gas was blown into the mixture under agitation over a period of 30 minutes. Then, the reaction mixture was further agitated for 11/2 hours to advance the reaction. The resulting slurry product was filtered, washed sufficiently with water and dried at 100.degree.C under reduced pressure to obtain 450.2 g of a dry solid in which the polymer content was 14.7% by weight. In the resultant product, the coating condition was very good.