Use of heat-resistant coating compositions for coating metal substrates

Abstract: This invention provides an improved water based coating composition. The said coating composition comprises a light mineral such as expanded perlite, dilute sodium metasilicate solution, and a metal oxide. The coating composition of the invention has one or more of the following qualities when cured: ability to withstand burning; rust resistant; non-poisonous; odourless; water resistant; non-conductive; heat resistant; acid resistant; alkaline resistant; and ability to integrate with cement.

Description: This invention relates to the use of water-based coating compositions for coating metal substrates. Steel has wide usage in the industrial sector. The greatest drawback of steel is its tendency to rust easily. Steel conducts electricity and heat, and the difference between its contracted state when cooled and expanded state when heated is very great. Though there are various types of rust-proof paints in the market, they generally emit an unpleasant odour. Moreover, the aromatic hydrocarbons used are poisonous, and the period of rust-resistance provided is short. Though iron is non-inflammable, the paint used to coat iron is flammable, and can thus be a fire hazard. An alternative to coating is to use a chroming process as a means of rust-proofing materials, for example using chromed zinc, aluminium, tin, or magnesium. Chromed iron, though rust-proof, still conducts heat, and is not acid resistant. It cannot withstand burning, and becomes rusty after a period of time. Thus all of the above mentioned materials have defects which greatly limit their use as iron coating materials. DE-A-1796230 describes coatings for metal surfaces. The coats are prepared from ceramic dispersions comprising nepheline or nepheline-syenite, water and a metal oxide of the metal it is desired to protect. US-A-4277355 describes insulative fire-proof water based coating compositions comprising amongst others, granular, porous thermoinsulating inorganic filler selected from lava rock, cinder slag and mixtures thereof. The present invention involves use of an improved water-based coating composition, having the following qualities upon dry heating at high temperatures:- 1. ability to withstand burning; 2. rust resistant; 3. non-poisonous; 4. odourless; 5. water resistant; 6. non-conductive; 7. heat resistant; 8. acid resistant; and 9. alkali resistant. The coating composition can withstand submergence in cold water after burning at high temperatures, without any explosion, formation of cracks or decolorization. The coating composition has the ability to integrate with cement for building purposes, especially for protection against earthquakes. The coating composition can be used with other paints to provide additional protection. The coating composition, when glazed, becomes smooth and glossy like a ceramic coated iron surface, thereby widening its usage outside the industrial sector. The coating compositions are suitable for use in the heavy industrial sector, including use in the heat and pipe lines in chemical factories, steam ships, large oil tanks, vehicles and trains, owing to their multi-functional ability to withstand submergence upon heating, and to integrate with cement. The coating composition has the appearance of a ceramic coating when glazed. The features of the coating composition used according to the invention, and its corresponding advantageous effects with reference to prior art,will now be enumerated and described:- 1. Non-poisonous The raw materials used in formulating the coating composition are minerals. Poisonous raw materials like asbestos and lead compounds, arsenic compounds and cyanide are not used, and neither are petrochemical materials. Thus during burning and oxidation of the coating composition, no poisonous substances will be emitted. Thus the present-invention employs an environmentally clean coating material for industrial use. 2. Odourless In general, the process of painting for purposes of coating as practised in the prior art causes the emission of an unpleasant odour. If the area to be painted is wide, and proper ventilation is not available, the odour could cause suffocation. Even when an odourless paint is used, it still emits an odour when burnt. Further, the usage of red lead compounds as colouring is not merely odorous.but also poisonous. . The coating composition used in the present invention is completely odour free, even when burnt at high temperatures. 3. Water Resistant This is a feature of great importance in painting materials. Oils and emulsions which are not water resistant result in peeling off, rusting, blackening or the growth of algae, moss or fungi. The coating composition used according to the present invention, after dry heating, is highly water resistant, and, even when left submerged in water for up to 18 months, no peeling or rust formation is observed. This is because the coating composition attaches firmly to any surface, and has strong resistance to water. The coating composition, when left in wet areas shows no evidence of blackening, or of algal, moss or fungal growth. If the coating composition is added together with paints, glaze or cement, a non-rust period of 5 to 10 years is possible. 4. Non-conductivity The coating composition used according to the invention is non-conductive, owing to the non-conductive nature of the materials used in the formulation of the said coating composition. Non- conductivity is an important feature in the industrial sector, especially in oil tank vehicles which need to be resistant to electricity and static electricity in order to reduce the risk of fire. 5. Heat Resistant The obvious defects of iron and steel is their ability to rust, and to conduct heat. In order to remedy this shortcoming, it is advisable to use coatings which are not only rust proof, but which also have excellent heat resistant qualities. These functions are fulfilled by the coating composition used according to the present invention.The white coloured coating composition functions best for heat resistance. Experiments were carried out to illustrate this using two flower pots, in both of which were placed thermometers. One was covered with a zinc-iron plate coated in white, and the other with an uncoated zinc-iron plate. The flower pots were then placed in the sun for one hour, after which temperatures were read. The thermometer covered with the white-coated plate showed a temperature of 36°C, while the one covered with the zinc iron plate showed a temperature of 51°C, giving a difference in temperature of 15°C. If placed under sunlight for 20 minutes, the increase in temperature is between 32 - 39°c. While the temperature of the white coated plate remains constant at 36°C, the temperature of the zinc iron plate continues increasing with continuous heating. In view of this, it is concluded that the white coating composition used according to the invention are better heat-resistant materials. Even a thin layer coated onto a thick layer of ceramic provides better heat resistance as compared to a thick layer of ceramic. This was tested using an electric ceramic container. When the water in the container was at boiling point, the cover of the ceramic container was too hot to be touched, whilst the cover of the white coated container could still be touched. The heat resistant quality of the coating composition used according to the invention has wide industrial application. Using the said coating composition on the interiors of buses, trains, cars and ships provides heat resistance, and reduces the requirement for air conditioning. The ability to resist heat is particularly useful in car oil tanks. A test of the evaporation process of petrol was conducted. It was found that petrol at 40 - 50°C evaporation point resulted in a loss of 10%. If a car is parked under the sun for a period of time, the temperature of its oil tank would exceed 40°C. As such the loss could be 10% per period of parking. This results in wastage and unnecessary petrol expenditure. However, should the car oil tank be painted white, its temperature would not exceed 36°C regardless of the length of time it is in the sun. Remaining at a constant temperature below 40°C is a definite saving with regard to petrol loss. Heat resistance is also needed in large oil tanks, and oil and gas pipelines, to maintain reasonable temperatures. 6. Acid Resistant The coating composition used according to the invention, upon submergence in concentrated sulphuric or nitric acid for ten days remains unchanged, thus proving its high resistance to acids. However, the coating composition does not exhibit equal resistance to hydrochloric acid. The chlorine component of hydrochloric acid (HCl) reacts chemically with the sodium component of the coating composition to form the salt sodium chloride (NaCl). When submerged in 37% HCl, the coating remains unchanged for two days, after which the surface of the coating cracks, but continues to remain undissolved. The industrial usage of nitric and sulphuric acids is very wide, especially in chemical factories which comprise one third of the industrial sector. The contact of iron goods with acids causes corrosion and rust formation. However, coating these goods with the coating composition used according to the invention protects them from acid corrosion. This is also true for chromed steel materials. 7. Alkali, Resistant Testing was done wherein the coating composition used according to the invention was submerged in soap solution for 15 days. No change was detected, thus proving the alkali; resistant quality of the coating composition. 8. Rust Resistant Tests were conducted whereby the coating composition used according to the invention was submerged in salt water for over 15 days. The results yielded no change in the coating material. If the density is insufficient , or the layers at the edge are damaged, then the defective or damaged part of the coated iron was found to be rusty. well-coated iron was found to withstand submergence for over a month. This exhibits the rust resistant quality of the coating composition used according to the invention. If the said composition is coated onto cement and submerged in salt water for over six months, the coating was found to remain clear. The coating composition, when used in addition with paints, cement or glaze, forms excellent rust resistant materials with the potential ability to withstand rust for 5 to 10 years. 9.Ability to withstand burning The coating composition used according to the invention has the ability to withstand continuous burning. Submergence in cold water subsequent to burning does not cause cracking or decolorization of the coating composition. The coating compositions of red and white type have undergone successful experiments, whereby repeated burning for 20 - 30 minutes was carried out. Subsequently, the coatings were submerged in cold water. The coating on the thinner iron plate was burnt red without bending. When placed in water, there was no cracking or change of colour. The coating on the thicker iron plate when placed in water bubbled but did not peel off, and did not show a change of colour. It is thus concluded that in spite of the sudden decrease in temperature from a few hundred degrees to about ten degrees, and the great difference between the contracted state when cooled and the expanded state when heated, the coating composition used according to the invention exhibited no cracking or change in colour. Commonly used materials like chromed metals and stainless steel cannot withstand burning without cracking or changing form. The white and red type coating compositions, when coated onto car exhaust pipes, have remained intact without peeling off for periods of up to a year. The only change observed was a slight discoloration of the white colour. However, the red coloured coating remained unchanged. The ability to withstand burning for long periods of time shows the structural strength of these coatings. When coated onto gas stoves, the outer layer remains sound and good in the central support area sustaining continuous burning. No damage was observed, even after a period of one year. The ability of the coating composition used according to the invention to withstand burning can be put to use in the coating of heat and steam pipes which by nature are continuously heated or burnt. The coatings can also be used as fire-proof materials. The coated foundation irons of steel structures, bridges and factories, including the inner fixtures, when completely burnt by fire,were found to remain undamaged, and could be re-used, thereby reducing loss due to fire. The coating composition is particularly useful in steam ships, airport hangars, harbours and godowns. The coating composition, when painted on large oil tanks, is fire resistant, as the protective layer of the iron is of ceramic composition and is heat proof. During a fire, the coatings prevent instant burning and explosions, thus allowing time for emergency measures to be taken. 10. Ability to integrate with cement. The coating material used according to the invention, which is of mineral composition, can be integrated with cement in buildings as an earthquake resistance measure, owing to the ability of said coating composition to withstand burning, combined with its qualities of acid, alkali, water and rust resistance. The composition is useful for coating foundation irons. Recently there came into use a black fluid coated onto foundation irons for rust resistance purposes. As the said fluid is a petrochemical product, it can age easily. The coating material used according to the invention, which is of mineral composition, can match cement and can be integrated with cement for long periods of time. Such an integration is earthquake resistant, as the coated foundation irons integrated with cement remain upright after an earthquake. This reduces the risk of collapse. The coated integrated foundation irons may remain unchanged for 10 - 100 years. As such, the coating composition used according to the invention is beneficial in building engineering. Accordingly, the present invention provides use of a water-based coating composition comprising suspension of (a) a heat-resistant metal oxide and (b) expanded pearlite, both in finely divided form, and having a particle size in the range of 250 to 350 micrometres, in an aqueous solution of an alkali metal silicate for coating metal substrates. The materials used in formulating the water-based coating composition are preferably as follows: 1. Expanded Perlite A. Chemical Composition SiO 2 73% Al 2 O 3 14.9% Na 2 O 3 3.15% K 2 O 4.9% MgO 1.54% CaO 1.11% Fe 2 O 3 1.07% H 2 O (105%) 0.60% Loss on Ignition 1.29% B. Physical Properties Colour White pH 7.0 Specific Gravity 2.3 Specific Heat 0.2 Bulk Density 100-200 kg/m 3 Refractive Index 1.5 Melting Point 1260°C Solubility Insoluble in acid and alkali Light capacity Used in wide temperature zone Odourless and Kondo'sonous Strong chemical stability Heat resistant and retains heat Non-corrosive and non-inflammable The proportion of expanded pearlite may be 8 to 20% by weight of the composition. 2. Sodium Metasilicate A. Chemical Composition (a) Na 2 SiO 3 (b) Na 2 SiO 3 .9H 2 O (c) Na 2 Si 4 O 9 B. Physical Properties Melting Point (a) 1018°C (b) 48°C Soluble in water and alkali Not soluble in ethyl alcohol and acids. Na 2 SiO 3 .9H 2 O in drum form with suitable water concentration is preferred. Each drum has a preferable weight of 330 kg, inclusive of weight of drum. Na 2 SiO 3 is also suitable for use. The proportion of alkali metal silicate may be 8 to 30% by weight of the composition (calculated as solids). The metal oxide may be an iron oxide or titanium oxide, such as Fe 2 O 3 or TiO 2 . The metal oxide is typically 10 to 30% by weight of the composition. A. Chemical Composition (a) Ti 2 O 3 (b) TiO 2 (c) TiO 3 B. Physical Properties The colour of (a) and (c) is black. The yellow coloured compound is not used, instead of which is used compound (b) which is white. The Specific Gravity of (b) is 3.95, and the melting point is 1550°C. It is soluble in concentrated sulphuric acid, but insoluble in water. 4. Ferric Oxide Red, Iron Oxide Red A. Chemical Composition Fe 2 O 3 B. Physical Properties Specific Gravity 5.12 - 5.24 Melting Point 1548°C Soluble in acid but not in water. In general, titanium oxide of the formula TiO 2 or Ti 2 O 3 are used, as these two compounds are in powder form. Other oxides or colouring materials can be used in the same proportion. Whatever metal oxide is used, the particle size is preferably in the range of 250 to 350 micrometres. Perlite powder and sodium metasilicate are preferably used in combination with the aforementioned oxides. The preferred formulation of the coating composition according to the invention will now be disclosed. 1. Expanded Perlite The perlite is processed through burning and is subsequently crushed into a powder, with a particle size preferably in the range of 250 to 350 micrometres and having a low specific gravity of between 0.24 to 0.34. It is preferably not more than 80 kg per cubic meter. 2. Sodium Metasilicate Dilute sodium metasilicate is used. Each drum has a standard weight of 330 kg. It is then diluted two-fold in water to form a solution. The proportion of metal oxide (a), expanded pearlite (b) and alkali metal silicate are in the weight ratios 4:3:16-17. 3. Preferred metal oxide components (a) Titanium Oxides (white), preferably mixed with the expanded perlite, and the diluted sodium metasilicate solutions in the weight ratio of 4:3:16. (b) Ferric Oxide (red), preferably mixed with the expanded perlite and the diluted sodium metasilicate solution in the weight ratio of 4:3:17. The weights are obtained by calculation. Thus 3 kg expanded perlite powder may be used with 4 kg ferric oxide powder and a further 17 kg dilute sodium metasilicate. The above components are mixed well in order to produce the preferred formulation shown above. 4. The invention also provides a method of producing a coating on an iron or other metal substrate by forming on the substrate a layer of the water-based coating composition and curing the layer by drying. The substrate may be a clean non-chromed steel sheet. The layer may be dried and cured at ambient temperature for at least 24 hours. The layer may be cured by heating for, e.g. about half an hour in a flame, to the temperature of up to about 800°C, preferably 500-800°C. Typically, the layer is dried and cured by gradual heating up to 800°C over a period of 20 to 30 minutes. The heating time depends on the thickness of the iron, a thin sheet requiring only 10 minutes of heating. The sheet can be put to use upon cooling. To avoid blackening by charcoal flames, the fuel used for heating should preferably be gas or electricity. In general, rust and oil contents have to be removed prior to recoating. Coating by dipping can be used. After dipping, the substrate should preferably be wind-dried to keep off additional heat which might result in bubble-formation. This method produces a better end-product. 5. Upon heating, the coating material undergoes a chemical change and is cured. The coating solidifies and becomes firm and resistant to water and acids. The above product, having undergone the heat of combustion, produces a ceramic coating compound integrated with the iron. With the additional coating of paint, cement or glaze on the surface of the upper layer, its anti-rust capacity is greatly increased. The layer may subsequently be calcined. The product theoretically needs heat energy to produce the chemical reactions and chemical decomposition which expedite the formation of the resistant effect against water and acids. When the fire spraying method cannot be used, or when the coated materials are fixed and immovable, a dry heated coating can be applied, provided that it is thinly coated. The method of submergence coating cannot be used, as this may thicken the coating, resulting in difficulty in drying by sunlight or wind-drying. During coating, the material has to be protected from rain. Even when dried, a lapse of 24 hours is necessary to ensure resistance against water. This is because the resistance only becomes effective after completion of the chemical decomposition process. It thus requires some time. When dried in this manner, the resistance initially shown against water and acids is not as good as when the coating is dried through burning and heating. After a lapse of time, during which the chemical reaction or decomposition process has been completed, the effect of resistance obtained after the coating has been thoroughly dried is the same as the effect when the coating is dried through burning and heating. Inner walls and iron can generally be coated in a manner similar to painting. White coating material, asbestos, can be used on heat pipes, steam pipes and other such items. A heat reducing substitute also does not need to be dried by heating. The red coating materials, after exposure to sun for 2 - 3 hours, become water resistant. The white coatings, which have better heat resistant effect, are slower, and thus require exposure to sunshine for a period of 5 - 6 hours in order to be effectively water-resistant. When the coating materials are used outdoors, care has to be taken to protect the materials from rain when they are still wet, so as to avoid fading of the colour. If the coatings are being used as undercoatings for rust resistance, spray coating or painting can be done after the coatings are thoroughly dry. The merits of the coating composition according to the invention were evaluated by SIRIM through a series of tests, the results of which are illustrated in the following pages:- Samples used were red and white coatings, coated onto (1) iron plate. Type of Test Requirement Results 1. Water Resistance Test (After 18 hours, soaking) a) Red coating on iron plate i) Immediately after removal from the Water No crinkling, blistering, cracking and peeling No obvious defects on the coating. The immersion water turned slight cloudy. ii). After 2 hours Variation of gloss, cloudiness, whitening, degree of discoloration not larger than control sample No abnormality The coating film turned powdery b) White Coating on iron plate i). Immediately after removal from water No crinkling, blistering, cracking and peeling. Complied ii) After 2 hours Variation of gloss, cloudiness, whitening, degree of discoloration not larger than control sample. No abnomality Complied Type of Test Requirement Results 2. Acid Resistance to (Conc. Sulphuric acid 95-97%) (After 24 hours, soaking) a) Red coating on iron plate No blistering, cracking, peeling, pinhole and Complied i) Immediately after removal from the acid softening, coloration, and turbidity (Solution) ii). After 2 hours Variation of gloss, cloudiness, whitening, degree of discoloration not larger than control sample. No abnormality Complied b) White Coating on iron plate i). Immediately after removal from the acid No blistering, cracking, peeling, pinhole and softening, coloration and turbidity (solution). Complied ii) After 2 hours Variation of gloss, cloudiness, whitening, degree of discoloration not larger than control sample. No abnormality Complied Type of Test Requirement Results 3. Acid Resistance to concentrated Nitric Acid 65%. (After 24 hours, soaking) a) Red coating on iron plate No blistering, cracking, peeling, pinhole and Complied i) Immediately after removal from the acid softening, coloration, and turbidity (Solution) ii). After 2 hours Variation of gloss, cloudiness, whitening, degree of discoloration not larger than control sample. No abnormality Complied b) White Coating on iron plate i). Immediately after removal from the acid No blistering, cracking, peeling, pinhole and softening, coloration and turbidity (solution). Complied ii) After 2 hours Variation of gloss, cloudiness, whitening, degree of discoloration not larger than control sample. No abnormality Complied Type of Test Result 6. Thermal Isolation Test a) Inside temperature of the flower pot after being exposed under the sun for 1 hour. . Covered with white coated metal panel 38.5 °C . Covered withuncoated metal panel. 41.0 °C b) Temperature of the metal surface after being exposed under the sun for 1 hour. . White coated metal panel 40.0 °C . Uncoated metal panel 46.0 °C Type of Test Result 5. High heat resistance test High heat resistance test (after 20 minutes exposure to flame from a bunsen burner and dipping immediately into cold water) a) Red coating on iron plate No cracking, wrinkling, melting, abnormality and other coating film defects b) White coating on iron plate No cracking, wringkling, melting, abnormality and other coating film defects Type of Test Requirement Result 4. Salt Spray Test (after 96 hours, soaking) a) Red coating on iron plate plate No blisting, softening sign of corrosion and other sign of deterioration. Complied b) White Coating on iron plate No blistering, softening sign of corrosion and other sign of deterioration. Complied Type of Test Time Requirement Observation Result 2.a) Alkali Resistance (Red Sample) 24 hours No blistering, Cracking, peeling, pinhole and softening, coloration and turbidity (solution) Immediate completion Complied with the requirement. Variation of gloss, cloudiness, whitening, degree of discoloration not larger than control sample. No abnormality. After 2 hours Complied with the requirement. b) Alkali Resistance (White Sample) 24 hours No blistering, cracking, peeling, pinhole and softening, coloration and turbidity (solution) Immediate Completion Complied with the requirement. Variation of gloss, cloudiness, whitening, degree of discoloration not larger than control sample. No abnormality After 2 hours Complied with the requirement.

Claims: Use of a water-based coating composition comprising a suspension of (a) a heat-resistant metal oxide and (b) expanded perlite, both in finely divided form and having a particle size in the range of 250 to 350 micrometres, in an aqueous solution of an alkali metal silicate for coating metal substrates. Use according to claim 1 in which the metal oxide (a) is an iron oxide or a titanium oxide, having a particle size in the range of 250 to 350 micrometres. Use according to claim 2 in which the metal oxide (a) is Fe 2 O 3 or TiO 2 . Use according to any one of claims 1 to 3 in which the proportion of the said metal oxide (a) is 10 to 30% by weight of the composition. Use according to any one of claims 1 to 4 in which the expanded perlite has an apparent specific gravity in the range 0.24 to 0.34. Use according to any one of claims 1 to 5 in which the proportion of expanded perlite is 8 to 20% by weight of the composition. Use according to any one of claims 1 to 6 in which the alkali metal silicate is sodium metasilicate. Use according to any one of claims 1 to 7 in which the proportion of the alkali metal silicate is 8 to 30% by weight of the composition (calculated as solids). Use according to any one of claims 1 to 8 in which the proportions of metal oxide (a), expanded perlite (b) and alkali metal silicate are in the weight ratios of 4:3:16-17. A method of producing a coating on an iron or other metal substrate, which comprises forming on said substrate a layer of a composition as defined in any one of claims 1 to 9 and curing the said layer by drying the said layer. A method according to claim 10 in which the said layer is dried and cured at ambient temperature for at least 24 hours. A method according to claim 10 in which the said layer is dried and cured by gradual heating up to 800°C over a period of 20 to 30 minutes. A method according to claim 10 in which the said layer is subsequently calcined.

Inventors: YIU MOK, YIU, MOK
Applicants: MAGNUMAS COATINGS SDN BHD
Application year: 1993
Main classification: C09D1
IPC codes: C09D1, C09D5, C23C30, C04B20, C04B28