Photocatalytic coating and its manufacturing method, and coated material exhibiting photocatalytic function obtained by applying coating

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Technical field to which the invention belongs]

The present invention relates to a titanium oxide-based photocatalytic paint having excellent photocatalytic activity and transparency, a method for producing the same, and a coating material having a photocatalytic function obtained by applying the paint.

【0002】

[Conventional technology]

As a method for obtaining this type of photocatalytic thin film, as described in Japanese Patent Application Laid-Open No. 7-100378, a photocatalytic thin film is obtained by coating a substrate with a titania sol prepared from titanium alkoxide and alcohol amines and then firing the film. There are known methods of forming. Further, a method for producing a two-layer coating type coating film in which an undercoat film is provided between the substrate and the titanium oxide layer is also known in order to protect the substrate and improve the adhesion to the titanium oxide layer.

【0003】

Of these, the method of firing using titania sol requires a firing process at a high temperature, which not only may reduce the transparency of the obtained photocatalyst thin film, but also requires a firing furnace, which is costly. There was a problem. Further, the two-layer coating type that can be treated at a low temperature requires two coatings and drying, which increases the number of treatment steps and cannot be said to be a simple method. In this two-layer coating type, in order to fully bring out the activity of the photocatalyst, the content of titanium oxide must be 80% by weight or more, so that not only the transparency of the film formed is not sufficiently obtained, but also the substrate. There is a problem that a stable film cannot be formed because sufficient adhesion with the film cannot be obtained.

【0004】

As a measure to solve the above problems, the applicant applied for ultrafine particle anatase type titanium oxide having an average particle size of 0.01 to 0.1 μm of primary particles, an organic solvent, β-diketone, a titanate-based or aluminate-based coupling agent, and silica sol. We proposed a photocatalytic paint (Japanese Patent Laid-Open No. 10-195341). By using the photocatalytic coating material shown in this publication, a photocatalytic coating film having excellent transparency, catalytic activity, and coating film strength can be formed.

【0005】

[Problems to be Solved by the Invention]

However, by forming a coating film using the photocatalytic coating material shown in Japanese Patent Application Laid-Open No. 10-195341, a coating film having higher transparency, photocatalytic activity, and film strength than conventional photocatalytic thin films can be obtained. However, with the expansion of applications of photocatalysts, the development of photocatalytic paints capable of forming a coating film having higher transparency is required.

【0006】

An object of the present invention is to provide a photocatalytic coating material having excellent transparency, hardness and decomposition performance, a method for producing the same, and a coating material having a photocatalytic function obtained by applying the coating material.

【0007】

[Means for solving problems]

The invention according to claim 1 is an improvement of a photocatalytic coating material containing anatase-type titanium oxide powder, a mixed alcohol, a β-diketone, a titanate-based coupling agent, and a silica sol. Its characteristic composition is that the anatase-type titanium oxide powder has a primary particle size of 0.01 to 0.03 μm and an apparent specific gravity of 50 to 200 g / L, and the primary particles and the primary particles are aggregated or agglomerated. It is a photocatalytic paint consisting of. In the invention according to claim 1, by dispersing the anatase-type titanium oxide powder having such physical properties in the coating material, a photocatalytic coating material having excellent transparency, hardness and decomposition performance can be obtained.

【0008】

The invention according to claim 2 is the invention according to claim 1, wherein the anatase-type titanium oxide powder has a BET specific surface area of 30 to 100 m.²It is / g, and is a photocatalytic paint in which the primary particles and the particles of the secondary particles in which the primary particles are aggregated or agglomerated are dispersed in the paint at an interval of 0.01 to 0.3 μm. The invention according to claim 3 is an invention according to claim 1 or 2, which is a photocatalytic paint having a titanium oxide powder content of 0.5 to 20% by weight. The invention according to claim 4 is an invention according to any one of claims 1 to 3, wherein the content of β-diketone is 0.5 to 10% by weight with respect to the titanium oxide powder. The invention according to claim 5 is an invention according to any one of claims 1 to 4, wherein the amount of the titanate-based coupling agent added is 0.1 to 5% by weight based on the titanium oxide powder. The invention according to claim 6 is an invention according to any one of claims 1 to 5, wherein the mixed alcohol is a photocatalytic paint which is a mixed solution of 4 to 15% by weight of methyl alcohol and 96 to 85% by weight of ethyl alcohol. is there. The invention according to claim 7 is an invention according to any one of claims 1 to 6, wherein the silica sol is a hydrolyzate or a partial hydrolyzate of ethyl silicate.

【0009】

By adopting the configuration according to claims 2 to 7, a photocatalytic coating material having further excellent transparency, hardness and decomposition performance can be obtained.

【0010】

The invention according to claim 8 is characterized in that anatase-type titanium oxide powder is uniformly dispersed in a mixed alcohol as a solvent in the presence of a β-diketone and a titanate-based coupling agent, and then a silica sol is mixed. This is a method for producing a photocatalytic paint. The invention according to claim 9 is the invention according to claim 8, wherein the anatase-type titanium oxide powder hydrolyzes titanium tetrachloride in a high-temperature gas phase to rapidly cool the reaction product. The method.

【0011】

The invention according to claim 10 is characterized in that the photocatalytic paint according to any one of claims 1 to 7 or the photocatalytic paint obtained by the production method according to claim 8 or 9 is applied to the surface of a base material to form the photocatalytic paint. It is a coating material having a photocatalytic function. The invention according to claim 11 is the invention according to claim 10, which is a coating material having an inorganic base layer on the surface of the base material and a photocatalyst film formed from a photocatalytic paint on the base layer. The invention according to claim 12 is an invention according to claim 10 or 11, wherein the base material is a group consisting of glass, plastic, metal, wood, ceramic including tile, cement, concrete, stone, fiber, paper and leather. It is a coating material that is a more selected material. The invention according to claim 13 is the invention according to claim 11, which is a coating material in which the inorganic base layer is made of silica or alumina.

【0012】

The invention according to claim 14 is a processed stone product whose surface is coated with the coating material according to any one of claims 10 to 13. The invention according to claim 15 is a wall material whose surface is coated with the coating material according to any one of claims 10 to 13. The invention according to claim 16 is a glass whose surface is coated with the coating material according to any one of claims 10 to 13.

【0013】

BEST MODE FOR CARRYING OUT THE INVENTION

Next, an embodiment of the present invention will be described. The present inventors are characterized in that the primary particle size is 0.01 to 0.03 μm and the apparent specific gravity thereof is 50 to 200 g / L, and the primary particles and the primary particles are agglomerated or agglomerated secondary particles. Anatase-type titanium oxide powder is uniformly dispersed in a mixed alcohol as an organic solvent in the presence of β-diketone and titanate-based coupling agent, and then silica sol is mixed with this dispersion to obtain a paint that is more suitable. The present invention has been obtained based on the finding that a coating film having high transparency and excellent hardness and decomposition performance can be formed.

【0014】

Anatase-type titanium oxide powder consisting of primary particles with a primary particle size of 0.01 to 0.03 μm and an apparent specific gravity of 50 to 200 g / L and secondary particles in which primary particles are aggregated or agglomerated is a mixed alcohol and β- In a liquid consisting of a diketone and a titanate-based coupling agent, it is uniformly and highly dispersed to a state close to that of primary particles. It is difficult to obtain titanium oxide powder having a primary particle size of less than 0.01 μm, and if it exceeds 0.03 μm, the transparency of the photocatalytic paint decreases. If the apparent specific gravity is less than 50 g / L, it cannot be uniformly dispersed, and if the apparent specific gravity exceeds 200 g / L, a problem occurs in the transparency of the coating film. The ratio of primary particles to secondary particles is preferably 1.0 to 3.0 for primary particles / secondary particles in terms of number ratio. When the number of primary particles / secondary particles is less than the lower limit value, that is, when the number of secondary particles is larger than that of the primary particles, the transparency tends to decrease, and when the upper limit value is exceeded, production is difficult.

【0015】

By uniformly mixing an appropriate amount of silica sol with this dispersion, a photocatalytic paint with excellent transparency, hardness and decomposition performance can be obtained in which anatase-type titanium oxide powder is uniformly and highly dispersed in the paint to a state extremely close to primary particles. Be done.

【0016】

Anatase-type titanium oxide powder has a BET specific surface area of 30 to 100 m.²It is preferably / g. BET specific surface area 30-100m²The reason for setting / g is that sufficient catalytic activity cannot be obtained below the lower limit value, and dispersibility decreases when the upper limit value is exceeded. It is preferable that the primary particles and the particles of the secondary particles in which the primary particles are aggregated or agglomerated are dispersed in the coating material at an interval of 0.01 to 0.3 μm. By leaving the titanium oxide particles at intervals of 0.01 to 0.3 μm in this way, the transparency of the coating film does not decrease. The distance between the titanium oxide particles is preferably 0.02 to 0.15 μm. By applying this photocatalytic paint on the base material, it is possible to apply it thinly and uniformly, and the adhesion between the titanium oxide particles and the base material due to the action of the β-diketone and the titanate-based coupling agent and the silica sol. Can be improved and a stable photocatalyst thin film can be formed.

【0017】

The content of titanium oxide powder is 0.5 to 20% by weight. It is preferably contained in a proportion of 1.0 to 10.0% by weight. If the content of the titanium oxide powder is less than 0.5% by weight, sufficient catalytic activity cannot be obtained, and if it exceeds 20% by weight, the dispersibility of titanium oxide is lowered, and the haze of the formed photocatalytic thin film is deteriorated.

【0018】

In β-diketone, polar functional groups (ketone groups) act on titanium oxide fine powder and polar groups (hydroxyl groups and oxygen groups) existing on the surface of the base material and condense during baking to form titanium oxide powder. It is considered that the closest packing occurred, and the powder and the powder and the substrate were bonded to each other and acted as a film-forming agent to improve the adhesion. Examples of β-diketone include 2,4-pentanedione, 3-methyl-2,4-pentanedione, 3-isopropyl-2,4-pentanedione, 2,2-dimethyl-3,5-hexanedione and the like. Be done. The β-diketone is contained in a proportion of 0.5 to 10% by weight based on the titanium oxide powder. It is preferably contained in a proportion of 1.0 to 5.0% by weight. If the content of β-diketone is less than 0.5% by weight, sufficient dispersibility cannot be obtained, and if it exceeds 10.0% by weight, the dispersibility is not further improved.

【0019】

The coupling agent acts as a low haze agent. By adding a coupling agent, haze is reduced (transparency is improved) because secondary agglutination groups are not formed in the film structure, uniform closest packing and surface smoothing accuracy are further improved. Guessed. Examples of the coupling agent include a titanate-based coupling agent containing a dialkylpyrophosphate group or a dialkylphosphite group as shown in the following formulas (1) to (5), and one or more of them may be used. Can be used.

【0020】

[Chemical 1]

<img file="JP2003253156A_D0001.tif" />【0021】

[Chemical 2]

<img file="JP2003253156A_D0002.tif" />【0022】

[Chemical 3]

<img file="JP2003253156A_D0003.tif" />【0023】

[Chemical 4]

<img file="JP2003253156A_D0004.tif" />【0024】

[Chemical 5]

<img file="JP2003253156A_D0005.tif" />【0025】

The titanate-based coupling agent is contained in a ratio of 0.1 to 5% by weight with respect to the titanium oxide powder. It is preferably contained in a proportion of 0.5 to 2.0% by weight. If the content of the coupling agent is less than 0.1% by weight, the effect of dispersibility and haze reduction cannot be obtained, and if it exceeds 5.0% by weight, the haze reduction and dispersibility are not further improved.

【0026】

As the mixed alcohol used as the solvent, a mixed solution composed of methyl alcohol and ethyl alcohol is suitable. The content ratio of this mixed alcohol is 4 to 15% by weight for methyl alcohol and 96 to 85% by weight for ethyl alcohol. When the content ratio of methyl alcohol is less than 4% by weight, an extremely transparent photocatalytic thin film cannot be obtained, and even if it exceeds 15% by weight, no further effect can be obtained. The amount of the solvent contained in the photocatalytic paint is not particularly limited as long as a viscosity suitable for coating can be obtained.

【0027】

Further, the uniform mixing action of the silica sol does not lower the transparency, sufficient catalytic activity can be obtained, and the action of the silica sol further improves the adhesion to the base material. Examples of the silica sol include a hydrolyzate of ethyl silicate or a partially hydrolyzate.

【0028】

Next, a method for producing the photocatalytic paint will be described. First, its primary particle size is 0.01 to 0.03 μm, its apparent specific gravity is 50 to 200 g / L, and its BET specific surface area is 30 to 100 m.²Prepare a predetermined amount of anatase-type titanium oxide powder having physical properties within the range of / g and consisting of primary particles and secondary particles in which primary particles are aggregated or agglomerated. This anatase-type titanium oxide powder is obtained by hydrolyzing titanium tetrachloride in a high-temperature gas phase and rapidly cooling the reaction product. Next, this titanium oxide powder is mixed with a liquid consisting of a mixed alcohol, β-diketone and a titanate-based coupling agent, and is uniformly dispersed in a paint shaker for a predetermined time with a predetermined amount of zirconia beads, for example. Next, the photocatalytic coating material of the present invention can be produced by adding an appropriate amount of a silica sol solution having a predetermined concentration to this dispersion and uniformly mixing them. By applying this photocatalytic paint to a predetermined substrate surface with a spin coater, for example, and drying it, a coating material having a photocatalytic coating film on the surface can be obtained.

【0029】

It is also possible to form an inorganic base layer on the surface of the base material, apply a photocatalytic paint on the base layer, and dry the base layer to form a photocatalyst film to obtain a coating material having a photocatalytic function. The coating method on the substrate using the photocatalytic coating material of the present invention can be applied by a spin coating method, a dipping method, a spray method or the like, but the coating method is not particularly limited.

【0030】

The material used for the substrate of the present invention is selected from the group consisting of glass, plastic, metal, wood, ceramics including tiles, cement, concrete, stone, fiber, paper and leather. As glass, indoor environment purification (contamination substance decomposition) glass such as fluorescent lamps and windows, water purification glass such as water tanks and living cages, antifogging glass for cars, antifouling glass for CRT, LCD screens, windows, mirrors, eyeglasses, etc. , Antifouling of cameras and optical equipment, anti-glass lenses, etc. Plastics include AV equipment, computers, mice, keyboards, remote controls, floppy (registered trademark) disks, and other equipment and their peripheral products, car interiors, furniture, kitchens, baths, and household items used in washrooms, etc. There are antifouling, antibacterial, antifloppy plastics, etc. used by. Examples of the metal include antifouling, antibacterial, antifungal stainless steel, antifouling, and antibacterial treated doorknobs used for worktables such as clothes drying tables, clothes drying rods, kitchens, laboratories, washing areas, ventilation fans, and the like. Wood is used for antifouling furniture, antibacterial amusement facilities in parks, and the like. Building materials such as ceramics, cement, concrete, and stone, including tiles, include antifouling treated exterior wall materials, roofs, flooring, and other interior wall materials that have indoor environmental purification (pollutant decomposition) properties, antifouling, antibacterial, and antifungal. There are various processed interior products. As paper, it can be used for antibacterial treated stationery and the like. Examples of fibers such as films include transparent antibacterial films for food packaging, transparent ethylene gas decomposition films for vegetable preservation, environmental and water purification films, and the like. As described above, various base materials have antifouling, environmental purification, antibacterial, and antifungal effects. Therefore, as long as the conditions can be irradiated with ultraviolet rays emitted from sunlight, fluorescent lamps, etc., there are many other than those illustrated. It can be used for various purposes. Examples of the inorganic base layer include silica and alumina. The processed stone product, wall material or glass whose surface is coated with the coating material of the present invention is excellent in transparency and hardness and exhibits high decomposition performance.

【0031】

[Example]

Next, examples of the present invention will be described in detail together with comparative examples. &lt;Example 1&gt; 40 g of a mixed alcohol of 4.7% by weight of methyl alcohol and 95.3% by weight of ethyl alcohol in an organic solvent, 0.25 g of 2,4-pentanedione in β-diketone, and a titanate-based cup represented by the above chemical formula (1). Ring agent 0.25 g, primary particle size 0.021 μm, apparent specific weight 130 g / L, BET specific surface area 50 m²10 g of / g anatase-type titanium oxide was mixed and dispersed with 100 g of anatase beads in a paint shaker for 16 hours. 11 g of a 10 wt% silica sol solution was mixed with this dispersion to prepare a photocatalytic paint. The prepared photocatalytic paint was applied to a glass substrate with a spin coater and dried at 125 ° C. for 1 hour to obtain a coating material in which a photocatalytic thin film was formed on the surface of the glass substrate. When the morphology of the titanium oxide powder of the obtained photocatalytic paint was measured with a transmission electron microscope, it was confirmed that the particles were dispersed in the paint at intervals of 0.02 to 0.3 μm.

【0032】

&lt;Example 2&gt; Example 1 except that 150 g of a mixed alcohol of 10.2% by weight of methyl alcohol and 89.8% by weight of ethyl alcohol was used as an organic solvent, and 0.1 g of a titanate-based coupling agent represented by the above chemical formula (1) was used. A photocatalyst coating material was prepared in the same manner, and a photocatalyst thin film was formed on the surface of the glass substrate by the same method as in Example 1. When the morphology of the titanium oxide powder of the obtained photocatalytic paint was measured with a transmission electron microscope, it was confirmed that the particles were dispersed in the paint at intervals of 0.01 to 0.15 μm. &lt;Example 3&gt; Example 1 except that 150 g of a mixed alcohol of 4.7% by weight methyl alcohol and 95.3% by weight of ethyl alcohol was used as an organic solvent, and 0.1 g of a titanate-based coupling agent represented by the above chemical formula (2) was used. A photocatalyst coating material was prepared in the same manner, and a photocatalyst thin film was formed on the surface of the glass substrate by the same method as in Example 1. When the morphology of the titanium oxide powder of the obtained photocatalytic paint was measured with a transmission electron microscope, it was confirmed that the particles were dispersed in the paint at intervals of 0.02 to 0.15 μm. &lt;Example 4&gt; 40 g of a mixed alcohol of 10.2% by weight of methyl alcohol and 89.8% by weight of ethyl alcohol in an organic solvent, 0.5 g of 3-methyl-2,4-pentanedione in β-diketone, shown in the above chemical formula (3). A photocatalytic coating material was prepared in the same manner as in Example 1 except that 0.5 g of the titanate-based coupling agent was used, and a photocatalytic thin film was formed on the surface of the glass substrate by the same method as in Example 1. When the morphology of the titanium oxide powder of the obtained photocatalytic paint was measured with a transmission electron microscope, it was confirmed that the particles were dispersed in the paint at intervals of 0.05 to 0.20 μm.

【0033】

&lt;Example 5&gt; 150 g of a mixed alcohol of 10.2% by weight of methyl alcohol and 89.8% by weight of ethyl alcohol in an organic solvent, 0.5 g of 3-methyl-2,4-pentandione in β-diketone, shown in the above chemical formula (4). A photocatalytic coating material was prepared in the same manner as in Example 1 except that 0.25 g of the titanate-based coupling agent and 43 g of a 10 wt% silica sol solution were used, and a photocatalytic thin film was formed on the surface of the glass substrate by the same method as in Example 1. .. When the morphology of the titanium oxide powder of the obtained photocatalytic paint was measured with a transmission electron microscope, it was confirmed that the particles were dispersed in the paint at intervals of 0.05 to 0.10 μm. &lt;Example 6&gt; 40 g of a mixed alcohol of 4.7% by weight methyl alcohol and 95.3% by weight of ethyl alcohol in an organic solvent, 1.0 g of 3-isopropyl-2,4-pentandione in β-diketone, shown in the above chemical formula (5). A photocatalyst coating material was prepared in the same manner as in Example 1 except that 0.4 g of a titanate-based coupling agent and 43 g of a 10 wt% silica sol solution were used, and a photocatalytic thin film was formed on the surface of a glass substrate by the same method as in Example 1. .. When the form of the titanium oxide powder of the obtained photocatalytic paint was measured with a transmission electron microscope, it was confirmed that the particles were dispersed in the paint at intervals of 0.05 to 0.25 μm.

【0034】

&lt;Comparative Example 1&gt; A photocatalytic coating material was prepared in the same manner as in Example 1 except that 40 g of ethyl alcohol and 0.1 g of the titanate-based coupling agent represented by the above chemical formula (1) were used as the organic solvent, and the same as in Example 1. A photocatalyst thin film was formed on the surface of the glass substrate by the above method. When the morphology of the titanium oxide powder of the obtained photocatalytic paint was measured with a transmission electron microscope, it was confirmed that the particles were dispersed in the paint at intervals of 0.1 to 0.4 μm. &lt;Comparative Example 2&gt; A photocatalytic paint was prepared in the same manner as in Example 1 except that 40 g of ethyl alcohol and 0.4 g of the titanate-based coupling agent represented by the above chemical formula (2) were used as an organic solvent and β-diketone was not added. , A photocatalyst thin film was formed on the surface of the glass substrate by the same method as in Example 1. When the morphology of the titanium oxide powder of the obtained photocatalytic paint was measured with a transmission electron microscope, it was confirmed that the particles were dispersed in the paint at intervals of 0.2 to 0.5 μm.

【0035】

&lt;Comparative Example 3&gt; Using 40 g of ethyl alcohol as an organic solvent, 0.5 g of 2,4-pentanedione as β-diketone, and 43 g of a 10 wt% silica sol solution, the same procedure as in Example 1 except that a titanate coupling agent is not added. A photocatalyst coating material was prepared, and a photocatalyst thin film was formed on the surface of the glass substrate in the same manner as in Example 1. When the morphology of the titanium oxide powder of the obtained photocatalytic paint was measured with a transmission electron microscope, it was confirmed that the particles were dispersed in the paint at intervals of 0.2 to 0.5 μm. &lt;Comparative Example 4&gt; Titanium oxide with a primary particle size of 0.04 μm, an apparent specific gravity of 250 g / L, and a BET specific surface area of 25 m.²A photocatalytic coating material was prepared in the same manner as in Example 1 except that 10 g of anatase-type titanium oxide of / g was used, and a photocatalytic thin film was formed on the glass substrate surface in the same manner as in Example 1. When the form of the titanium oxide powder of the obtained photocatalytic paint was measured with a transmission electron microscope, it was confirmed that the particles were dispersed in the paint at intervals of 0.2 to 0.4 μm.

【0036】

&lt;Comparative Test and Evaluation&gt; The haze, pencil hardness and photocatalytic activity of the photocatalytic thin films of the coating materials obtained in Examples 1 to 6 and Comparative Examples 1 to 4 were measured. A haze computer HGM-3D manufactured by Suga Test Instruments Co., Ltd. was used for haze measurement. For the photocatalytic activity, the removal rate obtained by the procedure shown below was used as an index of the photocatalytic activity. First, a glass substrate coated with a photocatalytic thin film was placed in a 1 L glass (Pyrex (registered trademark)) container and sealed. Next, 350 ppm (initial concentration) of acetaldehyde was introduced into the container. Next, irradiate the container with an irradiation amount of 1.2 mW / cm.²It was irradiated for 2 hours with the ultraviolet lamp of. The acetaldehyde concentration inside the container after irradiation was measured with a gas detector tube (manufactured by Gastec), and the removal rate was determined based on the formula shown below.

【0037】

Removal rate [%] = [(Initial concentration-Concentration after light irradiation) ÷ Initial concentration] × 100 Table 1 shows the measurement results of the photocatalytic thin films of the coating materials obtained in Examples 1 to 6 and Comparative Examples 1 to 4, respectively.

【0038】

[table 1]

<img file="JP2003253156A_D0006.tif" />【0039】

As is clear from Table 1, in Comparative Examples 1 to 3 in which ethyl alcohol was used as the organic solvent, the haze was high and the transparency was inferior, the hardness with a pencil was as soft as H to 3H, and the acetaldehyde removal rate was 75 to 90%. It shows a low number. Further, in Comparative Example 4 using titanium oxide whose physical properties are outside the range of the present invention, the haze value was high, and both the pencil hardness and the acetaldehyde removal rate were low. On the other hand, in Examples 1 to 6, the haze was extremely low, the hardness with a pencil was as hard as 3H to 4H, and the acetaldehyde removal rate was as high as 85 to 95%.

【0040】

[Effect of the invention]

As described above, according to the present invention, the primary particle size is 0.01 to 0.03 μm and the apparent specific gravity thereof is 50 to 200 g / L, and the primary particle and the primary particle are aggregated or agglomerated. Anatase-type titanium oxide powder consisting of β-diketone and titanate-based coupling agent is uniformly dispersed in mixed alcohol as a solvent, and then silica sol is mixed to make anatase-type titanium oxide powder extremely primary. A photocatalyst coating material having excellent transparency, hardness and decomposition performance can be obtained, which is uniform and highly dispersed to a state close to particles.