Aging-resistant iron oxide coloring agent for resins

Abstract: An iron oxide coloring agent suitable for use in a resin, which agent comprises plate-like hematite particles having a coating of amorphous silicon dioxide or hydrated alumina, said coating containing silicon dioxide fine particles or hydrated alumina fine particles, said fine particles having an average particle diameter of from 5 to 100 nm (50 to 1,000 Å).

Description: The present invention relates to an iron oxide coloring agent suitable for use in a resin, to a process for producing such an agent and to shaped articles of a resin containing the agent. Resins having chlorine atoms, especially chlorine-containing thermoplastic resins, are used as molding materials. For example, polyvinyl chloride, chlorinated polyethylene, chlorinated polypropylene and polyvinylidene chloride are well known. Among these, polyvinyl chloride is widely used. The phenomena of aging of chlorine-containing resins will now be explained citing a polyvinyl chloride representative of these resins. In polyvinyl chloride, even if it is not colored with an iron oxide pigment, the polyvinyl chloride is decomposed and degenerated by heat-treatment during processing or light exposed to after it is molded into products, thereby causing the phenomena of aging. Such phenomena of aging are caused by reason that a part of chlorine bonding in a polyvinyl chloride is decomposed at 100 to 200°C by heat or light action, thereby generating hydrogen chloride and forming a polyene structure having a double bonding, and further the hydrogen chloride generated secondarily acts on the resin itself, thereby cutting a part of C-C bonding of a polymer or producing a cross-linking as a chain reaction. It is generally known that in order to suppress such aging, a neutralizer such as a lead compound and metallic soap for neutralizing the hydrogen chloride generated by decomposition or a resin stabilizer such as an organic tin compound and an epoxy compound which is effective in suppressing the generation of a double bonding, is added to a polyvinyl chloride powder for an ordinary thermoforming material even when it is not colored by an iron oxide pigment. As coloring agents for polyvinyl chloride, inorganic pigments which are excellent in the light resistance and heat resistance, namely, iron oxide pigments such as ferric oxide pigments are conventionally used widely. However, since metals such as Mn, Fe, Co, Ni, Cu and Zn have an accelerating action of the dehydrochlorination, as well known, in the case of mixing an iron oxide pigment with a polyvinyl chloride as a coloring agent, the aging of the resin is greatly accelerated. Various methods of improving the aging resistance of resins containing iron oxide pigments have been studied. For example, a method of improving aging resistance by forming a continuous fine silica film on the surfaces of the particles of an iron oxide pigment (Japanese Patent Publication No. 54-7292 (1979)), a method of improving aging resistance by coating the surfaces of the particles of an iron oxide pigment with glassy Na and/or K-Si (Japanese Patent Publication No. 53-11537 (1978)) and a method of improving aging resistance by depositing SiO₂ on the surfaces of the particles of an iron oxide pigment and further depositing aluminum hydroxide on the deposited SiO₂ (Japanese Patent Application Laid-Open (KOKAI) No. 53-36538 (1978)) are known. An iron oxide coloring agent imparting excellent aging resistance is now in demand, but the above-described known iron oxide pigments cannot be said to impart a sufficient aging resistance, as will be shown in the later-described Comparative Examples. In a first aspect of the present invention, there is provided an iron oxide coloring agent suitable for use in a resin, which agent comprises plate-like hematite particles having a coating of amorphous silicon dioxide or hydrated alumina, said coating containing silicon dioxide fine particles or hydrated alumina fine particles, said fine particles having an average particle diameter of from 5 to 100 nm (50 to 1,000 Å. This agent imparts excellent aging resistance. In a second aspect of the present invention, there is provided a process for producing such an agent, which process comprises adding colloidal silicon dioxide fine particles or hydrated alumina fine particles, and a silicate or an aluminate, to a suspension containing plate-like hematite particles and adjusting the pH of the resultant suspension to from 5.5 to 9.0. In a third aspect of the invention, there are provided shaped articles of a resin containing the iron oxide coloring agent of the invention. In the present invention, the amount of amorphous silicon dioxide or hydrated alumina containing silicon dioxide fine particles or hydrated alumina fine particles having an average particle diameter of 50 to 1,000 Å and covering the surfaces of plate-like hematite particles is generally 0.001 to 0.1 g, preferably 0.005 to 0.05 g, of Si calculated as SiO₂ and generally 0.001 to 0.1 g, preferably 0.005 to 0.05 g, of A1 calculated as A1(OH)₃ per m²/g of the specific surface area of the hematite particles. If this amount is less than 0.001 g, the improvement of the aging resistance is insufficient. On the other hand, if it exceeds 0.1 g, the orientation property of the particles is inferior and use of such a large amount of Si and A1 is economically disadvantageous. The content of silicon dioxide fine particles or hydrated alumina fine particles in the coating material according to the present invention is typically 5 to 95 wt%, preferably 10 to 90 wt%. If it is less than 5 wt%, the orientation property of the particles is inferior. If it exceeds 95%, the aging resistance is inferior. The plate-like hematite particles used in the present invention are coated with amorphous silicon dioxide or hydrated alumina containing silicon dioxide fine particles or hydrated alumina fine particles haying an average particle diameter of 50 to 1,000 Å, preferably 100 to 500 Å by adding colloidal silicon dioxide fine particles or hydrated alumina fine particles and a silicate or an aluminate to a suspension containing plate-like hematite particles and adjusting the pH of the suspension. As the starting material plate-like hematite particles, hematite particles typically having an average particle diameter of 0.1 to 100 µm, a thickness of 0.005 to 15 µm, a plate ratio (diameter of plate surface/thickness of particle) of 2 : 1 to 500 : 1 and a specific surface area of 0.05 to 60 m²/g and obtained naturally or by synthesis are usable. Plate-like hematite particles having an average particle diameter of 0.5 to 5.0 µm, a thickness of 50 to 500 Å, a plate ratio of 50 : 1 to 500 : 1 and a specific surface area of 5 to 30 m²/g are preferable because they can sufficiently exhibit a shielding effect to light or heat due to their large plate ratio. As the silicon dioxide fine particles, commercially available colloidal silica and the like may be used. The silicon dioxide fine particles can thus be derived from colliodal silica. As the hydrated alumina fine particles, commercially available alumina sol and the like may be used. The hydrated alumina fine particles can be derived from alumina sol or aluminium hydroxide. As the silicate, sodium silicate and potassium silicate are usable. The silicate reacts with an acidic aqueous solution and deposits on the surfaces of the plate-like hematite particles as amorphous silicon dioxide. As the aluminate, sodium aluminate, aluminum chloride, aluminum sulfate, etc. are usable. The aluminate is neutralized and deposits on the surfaces of the plate-like hematite particle as amorphous hydrated alumina. The hydrated alumina may have any structure such as a gibbsite structure and a boehmite structure, and also includes aluminum hydroxide in the form of gel or the like. After the resultant mixed suspension is stirred, sulfuric acid, nitric acid or the like is added thereto so as to adjust the pH of the stirred suspension to the range of 5.5 to 9.0. Thus, the surfaces of the hematite particles are coated with amorphous silicon dioxide or hydrated aluminum containing silicon dioxide fine particles or hydrated alumina fine particles having an average particle diameter of 50 to 1,000 Å. The reason why an iron oxide coloring pigment which imparts excellent aging resistance is obtained in accordance with the present invention is that the surfaces of plate-like hematite particles are coated with amorphous silicon dioxide or hydrated aluminum containing silicon dioxide fine particles or hydrated alumina fine particles having an average particle diameter of 50 to 1,000 Å. As shown in the later-described Examples and Comparative Examples, the aging resistance is inferior both in the case of coating the surfaces of plate-like hematite particles only with silicon dioxide fine particles or hydrated alumina fine particles and in the case of coating the surfaces of plate-like hematite particles only with amorphous silicon dioxide or hydrated aluminum. The present inventors consider the reason why an iron coloring pigment which imparts excellent aging resistance is obtained in accordance with the present invention to be as follows. The silicon dioxide fine particles or hydrated alumina fine particles having an average particle diameter of 50 to 1,000 Å separate the agglomeration of the plate surfaces of the plate-like hematite particles into discrete particles and a continuous uniform and dense coating film of amorphous silicon dioxide or hydrated alumina is formed on the surfaces of the discrete particles with the silicon dioxide fine particles or the hydrated alumina fine particles as connecting points. The continuous coating film sufficiently suppresses the dehydrochlorination which may be caused by the plate-like hematite particles themselves, and the discrete plate-like hematite particles are oriented in the direction of the plane of the resin to a high degree, so that they fully display the shielding effect to light and heat. In this way, the iron oxide coloring pigment can sufficiently suppress the influence of light and heat on the resin. The coated hematite particles as an iron oxide coloring agent according to the present invention have an average plate surface diameter of 0.1 to 100 µm, preferably 0.5 to 5.0 µm, a thickness of 0.005 to 15 µm, preferably 50 to 500 Å and a plate ratio (diameter of plate surface/thickness of particle) of 2 : 1 to 500 : 1, preferably 50 : 1 to 500 : 1. In an iron oxide coloring agent of the present invention, the time in which it is resistant to color change due to aging at 130°C is not less than 50 hours, preferably, not less than 60 hours, more preferably not less than 65 hours, and the orientation degree is not less than 28, preferably not less than 30. An iron oxide coloring agent of the present invention is suitable as an iron oxide coloring agent for aging-resistant resins due to imparting excellent aging resistance. [EXAMPLES] The present invention will be explained in the following with reference to the following Examples and Comparative Examples. In each of the following Examples and Comparative Examples, the average particle diameter is expressed by the value obtained from the measurement of an electron micrograph, and the thickness and the plate ratio are expressed by the value obtained from the specific surface area measured by a BET method and the average particle diameter obtained in the above-described way. The aging resistance is expressed by the time required for the color of a test piece for a heat resistance test to change into black. The test piece was produced by weighting out 1 g of a mixture which was obtained by mixing 1 g of an iron oxide coloring agent and 0.6 g of dioctyl phthalate, mixing the mixture with 50 g of a polyvinyl chloride compound having the following composition, kneading the resultant mixture by a hot roll and molding the kneaded mixture. The thus-obtained test piece was placed in a Geer oven at 130°C for observation. 〈Composition of the Polyvinyl Chloride Compound〉 Polyvinyl chloride 103 EP (produced by Japanese Zeon Co., Ltd.) 100 g Dioctyl phthalate 50 g Calcium stearate 0.5 g Zinc stearate 0.5 g The orientation degree is expressed by the ratio of the intensity I (006) of the X-ray peak of hematite in the face 006 to the intensity I (110) of the X-ray peak of hematite in the face 110 obtained by X-ray diffraction of the test piece. The larger the value is, the more excellent is the orientation degree. Example 1 100 g of plate-like hematite particles having an average diameter of 3.0 µm, a thickness of 0.025 µm, a plate ratio (diameter of plate surface/thickness of particle) of 120 and a BET specific surface area of 15.5 m²/g, 120 g of a colloidal silica Snowtex ST-20 (produced by Nissan Chemicals Industries, Ltd., SiO₂: 20 wt%) and 3.15 g of sodium aluminate (the A l content corresponds to 3 wt% calculated as aluminum hydroxide) were added to 10 l of water and mixed with each other. Thereafter, sodium hydroxide was added to the mixture to obtain a mixed suspension of pH 10.7. After the suspension was stirred and mixed, sulfuric acid was added thereto to adjust the pH of the suspension to 7.0, thereby obtaining plate-like hematite particles wherein amorphous hydrated aluminum containing SiO₂ fine particles was deposited onto the surfaces thereof. (The coating process with this pH adjustment will be referred to as "Process A" hereinafter.) The suspension containing the precipitated particles was filtered, washed with water and dried by an ordinary method. According to fluorescent X-ray spectroscopy, the amount of Si existent on the surfaces of the particles was 22 wt% calculated as SiO₂ based on the plate-like iron oxide and the amount of aluminum was 2.8 wt% calculated as A l (OH)₃ based on the plate-like iron oxide. As a result of observation through an electron microscope, the particles were proved to be discrete. The results of examination of the test piece produced from this particle powder are shown in Table 2. Example 2 100 g of plate-like hematite particles having an average diameter of 2.8 µm, a thickness of 0.026 µm, a plate ratio of 108 and a BET specific surface area of 14.9 m²/g and 58.6 g of water-glass (produced by Tokuyama Soda Co., Ltd., Si content corresponds to 29.0 wt% calculated as SiO₂) were added to 10 l of water and mixed with each other. Thereafter, sulfuric acid was added to the mixture to adjust the pH of the mixture to 3. 10 g of alumina sol AS-520 (produced by Nissan Chemicals Industries, Ltd., solid content: 20%, boehmite structure) was added to the mixture and the resultant mixture was stirred and mixed. A sodium hydroxide solution was then added to the mixture to adjust the pH thereof to 7, thereby obtaining plate-like hematite particles wherein amorphous silicon dioxide containing boehmite fine particles was deposited onto the surfaces thereof. (The coating process with this pH adjustment will be referred to as "Process B" hereinafter.) The suspension containing the precipitated particles was filtered, washed with water and dried by an ordinary method. According to fluorescent X-ray spectroscopy, the amount of Si existent on the surfaces of the particles was 16 wt% calculated as SiO₂ based on the plate-like iron oxide and the amount of amorphous aluminum hydroxide was 1.8 wt% based on the plate-like iron oxide. Examples 3 to 7, Comparative Examples 1 to 7 Iron oxide coloring agents were obtained in the same method as in Example 1 except for varying the kind of iron oxide particles, the kind and the amount of silicon dioxide fine particles or hydrated alumina fine particles, the kind and the amount of water-soluble silicate or aluminate and the coating method. The main producing conditions thereof are shown in Table 1 and the results of examinations of the test pieces produced therefrom are shown in Table 2. In the coating process in Example 4, after sulfuric acid was added to an aqueous suspension containing the plate-like hematite particles so as to adjust the pH to 3.0, colloidal silica Snowtex ST-0 (produced by Nissan Chemicals Industries, Ltd., SiO₂: 20 wt%) and aluminum sulfate were added to and mixed with the suspension. Thereafter, sodium hydroxide was added to the mixture to obtain a mixed suspension of pH 7.0. (The coating process with this pH adjustment will be referred to as "Process C" hereinafter.)

Claims: An iron oxide coloring agent suitable for use in a resin, which agent comprises plate-like hematite particles having a coating of amorphous silicon dioxide or hydrated alumina, said coating containing silicon dioxide fine particles or hydrated alumina fine particles, said fine particles having an average particle diameter of from 5 to 100 nm (50 to 1,000 Å). An agent according to claim 1, wherein from 0.005 to 0.05 g of Si, calculated as SiO₂, is present on the surface of the hematite particles per m² of the specific surface area of the hematite particles. An agent according to claim 1 or 2, wherein from 0.005 to 0.05g of Al, calculated as Al(OH)₃, is present on the surface of the hematite particles per m² of the specific surface area of the hematite particles. An agent according to any one of the preceding claims, wherein the amount of silicon dioxide fine particles or hydrated alumina fine particles in the coating is from 10 to 90 wt%. An agent according to any one of the preceding claims, wherein the plate-like hematite particles have an average particle diameter of from 0.1 to 100 µm, a thickness of from 0.005 to 15 µm, and a plate ratio (diameter of plate surface/thickness of particle) of from 2 : 1 to 500 : 1. A process for producing an agent as claimed in any one of the preceding claims, which process comprises adding colloidal silicon dioxide fine particles or hydrated alumina fine particles, and a silicate or an aluminate, to a suspension containing plate-like hematite particles and adjusting the pH of the resultant suspension to from 5.5 to 9.0. A process according to claim 6, wherein the silicon dioxide fine particles are derived from colloidal silica and the silicate is sodium silicate or potassium silicate. A process according to claim 6 or 7, wherein the hydrated alumina fine particles are derived from alumina sol or aluminium hydroxide. A process according to claim 6, 7 or 8, wherein the aluminate is sodium aluminate, aluminium chloride or aluminium sulfate. Shaped articles of a resin containing an iron oxide coloring agent as claimed in any one of claims 1 to 5.

Inventors: KATAMOTO TSUTOMU, KATAMOTO, TSUTOMU
Applicants: TODA KOGYO CORP, TODA KOGYO CORP.
Application year: 1991
Main classification: C09C1
IPC codes: C08L101, C08K9, C08L27, C09C1, C08K3