GB2073163A - Coating material - Google Patents

Abstract

A coating material is formed mainly of thin leaves of vermiculite, having an aspect ratio of at least 5. A coating film for reflecting infrared rays is formed of vermiculite containing 5 to 30% by weight of iron. Alternatively the material may be used as an additive in resin-based paints.

Description

SPECIFICATION Paint additive This invention relates to clay mineral for use as a paint additive, which may be employed to provide a paint film, a pigment, covering material, ultraviolet light absorbing material, visible infrared light reflecting material of the like.

There are known flaky mica particles as clay mineral which may be employed as an additive to paints. Since mica has a lamellar structure, a paint film containing mica has a laminated structure which contributes to an improvement in the properties of the paint film such as resistance to moisture and light.

The paint additive according to the present invention supersedes the above-mentioned mica particles. This paint additive, due to its having a higher aspect ratio than mica and its high surface activity, is useful as an agent for increasing the resistance to moisture and light of a paint film, and as a pigment or the like.

Accordingly, one object of the present invention is to provide a novel inorganic paint additive having a high aspect ratio and high surface activity, at low cost and with ease of manufacture.

Another object of the invention is to provide a paint additive having excellent resistances to weather and heat, as well as low transmittance to ultraviolet light.

A further object of the invention is to provide a paint additive having a metallic luster and a high reflectivity for infrared rays depending on the composition or vermiculite to be employed.

Still further object of the invention is to provide on the surface of a radiant heat insulating material a novel coating film having a high reflectivity for infrared rays as well as excellent resistances to oxidation, corrosion and heat.

Accordingly, the present invention provides a paint additive comprising clay mineral composed mainly of thin leaves of vermiculite, said leaves having an aspect ratio of at least 5.

According to a further aspect, the invention provides a radiant heat insulating material comprising a substrate having a coating film thereon for reflecting infrared rays, said coating film being formed by thin leaves of vermiculite arranged in layers, said vermiculite containing 5 to 30% by weight of iron.

Vermiculite to be employed in the present invention is a flaky mineral comprising flakes or leaves of aluminium silicate, in which alumunium, iron and the like are substituted at random for silicon atoms in the tetrahedral positions. Its typical chemical formula is as follows.

fE0.0,9 } (Mg3) [Si,Al]4 O,, (OH)2.nH20 in which E stands for an exchangeable cation. Vermiculite is different from mica and montmorillonite which are also flaky minerals, in chemical constitution (particularly the Al203/SiO2 ratio), the state of ions between layers, the mode of ion substitution in layers, the state of water between layers, cation exchange capacity degree of swelling with water or organic matter, flaking phenomenon between layers by heating, and the like.

It is known that vermiculite exfoliates in the form of an accordion when heated at a temperature of about 1 0000 C. Vermiculite may also be exfoliated by chemical treatment in the presence of aqueous hydrogen peroxide or the like. Exfoliated vermiculite is used as an aggregate for lightweight concrete, a soil improver, soundproofing material or heat insulating material, or for other purposes. The thin leaves of vermiculite of which the paint additive of this invention is mainly composed can be easily obtained by crushing such exfoliated vermiculite.

The thin leaves of vermiculite are laminar pieces composed mainly of aluminium silicate. The laminar pieces are formed by the separation of aluminium silicate layers defining the substrate of vermiculite, which is caused by evaporation of water from among the layers of vermiculite. The thin leaves consist of one layer to several thousand layers of aluminium silicate, and it has a thickness not exceeding 1 OO,u and an aspect ratio of at least 5.

Crushing may be performed by a mixer, a ball mill, a vibration mill, or mortar and pestle, either in a dry system or a wet system. The thin leaves obtained by crushing by a mixer in a wet condition, as compared with other methods, have a comparatively high aspect ratio and large particle size. From chemically exfoliated vermiculite, the thin leaves having a higher aspect ratio will be obtained.

The paint additive of this invention preferably comprises of thin leaves of vermiculite as hereinabove described. This paint additive may be used as a pigment owing to its colour, oil absorption and high covering power. Further, this paint additive has excellent heat resistance such that it withstands a temperature of 8000 C, and has a low transmittance to ultraviolet light. The paint additive has a metallic lustre and a high reflectivity for infrared rays, depending on the kind of vermiculite. Since the paint additive has a high surface activity and a high aspect ratio, a paint film having a sufficient strength can be formed even by 100% vermiculite not containing any resin at all.Since vermiculite itself forms a slight bond (hydrogen bonds) to organic matter, it is more advantageously employed to increase weather resistance of organic paints, as compared with mica.

This paint additive has further advantages such as low cost of material and ease of manufacture.

Gold colour pigment, for example, is obtained by burning gold vermiculite and crushing the resultant exfoliated vermiculite by a mixer in a wet condition. By finely crushing the same exfoliated gold vermiculite by a ball mill, there is obtained a brown pigment. Various other pigments are obtained from white vermiculite by formation of a compound (between layers in the thin leaves) of vermiculite and the organic compound, or by absorption of the organic compound onto the thin leaves of vermiculite. Also, inorganic matter such as SnO2 may be absorbed onto the thin leaves of vermiculite for the purpose of increasing its reflectivity for infrared rays. Paint may be obtained merely by use of the paint additive of the invention, which is dispersed in a suitable solvent such as water.This paint is applied with a brush or through spraying, and then dried to evaporate the solvent, whereby a paint film formed of the paint additive alone can be obtained. This paint film can withstand a temperature of about 8000C because the film doesn't include any organic constituent.

The paint additive of this invention may be added to a paint containing a vehicle constituent in the same manner as in a known pigment. In this case, either a vehicle of a baking finish type such as phenol resin or melamine resin or a vehicle of a non-baking finish type such as lacquer may be used. Usually, about 1 to 40 parts by volume of the paint additive of the invention is added per 100 parts by volume of paint. However, this may be varied depending on the purpose.

The thin leaves of vermiculite containing 5 to 30% by weight of iron has a function of reflecting infrared rays. Therefore, if such vermiculite dispersed in water or the like is applied to the surface of a heat insulating material such as a refractory brick or a refractory wall, excellent radiant heat insulating material can be obtained.

Known methods for heat insulating include coating of the surface of a refractory brick with silicon carbide and evaporation of metals onto glass fibres. It is generally required that coating for the purpose of heat insulation should give the substrate to be coated such properties as high reflectivity for infrared rays, and resistances to heat, oxidation, corrosion and weather, by easy coating at a low cost and with strong bonding of the film onto the substrate. However, silicon carbide in the conventional method only gives reflectivity for infrared rays as low as 13% (at 3000C) to 4% (at 1 200"C), and metals can provide no sufficient resistances to oxidation and corrosion.

The paint additive of the invention consisting essentially of thin leaves of vermiculite containing 5 to 30% by weight of iron is effectively applied to the surface of insulating material such as a refractory brick or wall to provide a radiant heat insulating material with a coating film for reflecting infrared rays.

Since vermiculite to be employed in the present invention is a clay mineral, it provides a novel coating film having excellent resistances to oxidation and corrosion, as well as heat resistance, and a comparatively excellent reflectivity for infrared rays.

The coating film for reflecting infrared rays is easily obtained by stacking the thin leaves of vermiculite in layers on a heat insulating material to a thickness of about 10 to 1 00y. Since vermiculite has high surface activity and a high aspect ratio, the coating film having a sufficient strength can be obtained even from 100% vermiculite.

Heat insulating materials to which the coating film of the invention may be applied include refractory bricks or diatomaceous earth, fireclay bricks, heat insulating material of cellular glass, a furnace wall made of silica, alumina castable, or the like, and a furnace tube made of silicon nitride.

Thus, the radiant heat insulating material having a coating film thereon according to the present invention is made of inorganic matters alone, and possesses resistances to heat, oxidation and corrosion. Therefore, it may be used similarly to conventional refractory bricks and furnace walls. In addition, the coating film of the invention can prevent the radiant heat to a certain degree by virtue of its excellent reflecting effect for infrared rays The aspect ratio (X) referred to in this specification means a value defined by the ratio of the particle size (A) to the thickness (T) of thin leaves of vermiculite: X=A/T Having generaily described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not presented in a limiting sense.

EXAMPLE 1 Vermiculite produced in China (having a particle size of about 5 to 8 mm) was heated at 1.0000C for two minutes, whereby gold exfoliated vermiculite was obtained.

Ten grams of this exfoliated vermiculite, placed in one litre of water, was crushed for 10 minutes by a mixer having steel stirring blades each having a diameter of 55 mm and operating at a rotating speed of 5,000 rpm. The crushed material was then filtered and dried to prepare the paint additive of this example. By observation with a scanning electron microscope, it was confirmed that this paint additive was composed mainly of thin leaves of vermiculite having a thickness of 0.1 to 1 00no a particle size of 1 to 1 ,000,u and an aspect ratio of 10 to 10,000.

Then, 50 grams of the paint additive thus obtained was dispersed in one litre of water to prepare a paint not containing any vehicle therein. This paint was applied with a brush to the entire surface of a commercially available porous refractory brick, and dried at 800C for 5 hours to evaporate water, whereby a paint film was formed on the entire surface of the brick. The film thus obtained had a gold metallic lustre and a reflectivity for infrared rays (wave lengths of 2 to 50#) as high as 30%.

A heat retaining property of the refractory brick having this paint film was examined. First, the paint additive according to this example was applied to the entire surfaces of four refractory bricks each measuring 230 mm by 114 mm by 65 mm and four refractory bricks each measuring 230 mm by 65 mm by 65 mm to form a paint film thereon. Then, a heater was prepared by winding a nicrome resistance wire on the outer circumference of an alumina tube having a length of 200 mm and the outside diameter of 35 mm. This heater was placed straight between a pair of refractory bricks each having a concave portion of a semi-circle shape in cross section for receiving this heater, whereby a centre portion of an electric furnace measuring 230 mm (in length) by 114 mm (in width) by 114 mm (in depth) was prepared.The bottom portion of the electric furnace was formed of two conventional refractory bricks disposed side by side and each measuring 230 mm by 114 mm by 65 mm, on which the centre portion of the electric furnace was placed. Four refractory bricks (230 mm x 114 m x 65 mm) with the paint film were placed in contact with four sides of the centre portion of the electric furnace and the other four refractory bricks (230 m x 65 mm x 65 mm) also with the paint film were placed at the remaining four corners, for the purpose of heat retaining. One conventional refractory brick measuring 230 mm by 114 mm by 65 mm was placed on top of the furnace as a lid.

The electric power needed to keep the centre portion of the electric furnace at 1 ,0000C was compared between two cases, i.e. one employing conventional refractory bricks as heat retaining material to be placed around the periphery of the electric furnace and the other employing refractory bricks provided with the paint film on their entire surface according to this example. The test results revealed that the latter case employing the paint film of this example could save about 15% of electric power.

The light reflecting effect of the coating film was also examined in this example by use of vermiculite produced in Japan (having a particle size of about 5 to 8 mm and containing 23% by weight of iron).

For this purpose, the paint was prepared by quite the same method as described above from thin leaves of vermiculite having a thickness of 0.1 to 100ss, a particle size of 1 to 1 ,000 and an aspect ratio of 10 to 10,000. The paint was applied to the entire surface of a porous refractory brick to form a coating film having a thickness of about for reflecting infrared rays.

Figure 1 shows a perspective view of the radiant heat insulating material, partly taken away, in which reference numeral 1 shows the heat insulating material and 2 shows the coating film for reflecting infrared rays.

The coating film obtained had a gold metallic lustre and a reflectivity for infrared rays (having wavelengths of 2 to 50y) as high as 30%. Refractory bricks each having such coating film on their entire surface were placed around the centre portion of the electric furnace in the same manner as described above, and it was confirmed that the electric power needed to keep the centre portion of the electric furnace at 1 ,0000C was saved by about 15% as compared with the conventional refractory bricks.

Further, for heating the centre portion of the furnace from room temperature (1 90C) to 1 ,0000C, the conventional brick needed the electric power of 740 Whr, while the bricks having the coating film of.

this example only needed 600 Whr, thus saving much electric power.

When the centre portion of the furnace was kept at 1 ,0000C, the coating film was heated to 6000C and kept for 10 hours. However, the coating film did not lose its metallic lustre nor its light reflecting effect.

Refractory bricks employed in this example had a bulk specific gravity of 0.55 and a coefficient of thermal conductivity of 0.16 Kcal/m.hr. C at 3500C.

EXAMPLE 2 Fifty grams of the exfoliated vermiculite as in EXAMPLE 1 and 300 grams of water were put in an alumina ball mill having an internal volume of 800 cm3 and having fifteen alumina balls of 10 mm in diameter. They were crushed at a rotating speed of 1 00 rpm for 24 hours. Then, water was removed by using a Buchner funnel, followed by drying at 1 200C for 22 hours, whereby a paint additive of this example was obtained. This paint additive composed mainly of thin leaves of vermiculite having a thickness of 0.1 to 1 , a particle size of 10 to 1 00M and an aspect ratio of about 10 to 100. The properties of the paint additive as a pigment are shown in TABLE 1.

Pigments formed of this paint additive were mixed with other materials with contents shown in TABLE 2 to obtain three kinds of paints A, B and C. These paints were applied to mild steel plates by the method shown in TABLE 2, and baked at 1 400C for 30 minutes to obtain brown paint films. The properties of these paint films are shown in TABLE 2. These paint films were tested for 1,000 hours by use of weatherometer under the following conditions: temperature: 500C, humidity: 80% raining time: 18 min, raining interval: 120 min.

Every paint film retained its original surface condition and colour, and showed excellent weather resistance.

TABLE 1

Item Value Note oil absorption 29.5 kneading method specific gravity 2.1 g/cc by pycnometer withstanding temp. 800 C destruction of vermiculite crystal transmittance to ultraviolet light to 50% (4.000-3.200 A) virulence none alkali resistance # excellent acid resistance # fairly good colour H: 9.3R, V: 3.5 Munsell system C: 5.0 of colour TABLE 2

kind: A B C vermiculite powder 30% 20% 35% (pigment) alkyl resin (60% 35% 31% 33% Content nonvolatile component) (weight %) amino resin (50% 18% 16% 17% nonvolatile component) xylene 17% 35% 15% coating method brush spray brush Treating method baking condition 140 C 30 min thickness of film 100# 30# 100# gloss 93% 93% 91% (mirror reflectivity : 60 C) impact test 45 cm 50 cm 43 cm Film (DuPont 1/4R 500 g) characteristics pencil hardness 2H 2H 2H boiling water excellent excellent excellent (resistance 15 min) alkali resistance " " " (60%NaOH, 20 C, 24hr) acid resistance " " " (10%H2SO4, 20 C, 24hr) EXAMPLE 3 Vermiculite produced in Kenya (having a diameter of 4 to 6 mm) was heated at 1 ,0000C for two minutes, whereby white exfoliated vermiculite was obtained. This exfoliated vermiculite was crushed by the same method as in EXAMPLE 2 to obtain milk-white powder of vermiculite.Fifty grams of this powder was dispersed in 0.5 litre of water, to which about 1 g of diaminostilbene was added per 100 grams of vermiculite powder as as to allow diaminostilbene to be absorbed on thin leaves or between layers in thin leaves of vermiculite. A brown pigment was obtained by drying in the same manner as in EXAMPLE 2. This pigment had a thickness of 0.1 to 1 Fit, a particle size of 10 to 100,u and an aspect ratio of 10 to 1,000. Other properties of this pigment are shown in TABLE 3.

TABLE 3

Item Value | Note oil absorption 30.2 kneading method specif ic gravity 1.9 g/ce by pycnometer withstanding temp. 200"C decomposition of diaminosti Ibene transmittance to ultraviolet light to 30% (4,000 - 3,200 A) virulence none alkali resistance excellent acid resistance fairly good colour H: 9.6R, V: 4.0 Munsell system C: 4.5 of colour This pigment was mixed with other materials shown at B in TABLE 2, with contents shown, to obtain a paint. This paint was applied to a mild steel plate with a brush, followed by baking at 1 300C for 20 minutes. The paint film thus formed had a thickness of about 100y, a gloss of 94% (mirror reflectivity: 60 ), DuPont impact value of 41 cm (1/4R 500 g) and pencil hardness of 2H, and had excellent resistances to boiling water (for 1 5 min.), alkali and acid.

Claims (13)

1. A paint additive comprising clay mineral composed mainly of thin leaves of vermiculite, said leaves having an aspect ratio of at least 5.

2. A paint additive according to Claim 1, wherein said leaves have a thickness below 1 ooh.

3. A paint additive according to Claim 1 or 2, wherein said vermiculite includes 5 to 30% by weight of iron.

4. A paint additive according to Claim 1, 2 or 3, wherein said clay mineral is composed entirely of vermiculite apart from insignificant impurities.

5. A paint additive according to any preceding claim wherein said paint additive includes an organic compound absorbed on the surface of said thin leaves of vermiculite.

6. A paint additive according to any one of Claims 1 to 4, wherein said paint additive includes an inorganic compound absorbed on the surface of said thin leaves of vermiculite.

7. A paint additive according to Claim 6, wherein said inorganic compound is SnO2.

8. A radiant heat insulating material comprising a substrate having a coating film thereon for reflecting infrared rays, said coating film being formed by thin leaves of vermiculite arranged in layers, said vermiculite containing 5 to 30% by weight of iron.

9. A radiant heat insulating material according to Claim 8, wherein said coating film has a thickness of about 10 to 100ju.

10. A radiant heat insulating material according to Claim 8 or 9, wherein said coating film is formed entirely of vermiculite, except for insignificant impurities.

11. A radiant heat insulating material according to Claim 8, 9 or 10, wherein said substrate is a brick or a furnace wall made of one material selected from the group consisting of diatomaceous earth, fireclay, cellular glass, silica, alumina and silicon nitride.

12. A paint additive substantially as herein described.

13. A radiant heat insulating material substantially as herein described.