JPH0847670A - Method for continuously coating metallic strip with aqueous organic resin coating - Google Patents

Abstract

PURPOSE:To form a coating film excellent in appearance with short-time and low-temp. baking at the time of continuously coating a metallic strip with an aqueous org. resin coating. CONSTITUTION:The water-base org. resin coating with the resin component consisting of a thermoplastic resin having <70 deg.C glass transition temp. is kept at <=24 deg.C and applied with a roller on one or both sides of a metallic strip preferably kept at <=35 deg.C at 5g/m<2>, based on the dry resin, then the hot air at 80-300 deg.C is blown against the coated surface at a speed of 15-80m/sec from plural slit nozzles set in the width direction of the strip, and the coating is baked and dried at the maximum strip temp. of 35-90 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for continuously coating a metal strip with an aqueous organic resin paint. More specifically, the present invention is a continuous coating of a water-based organic resin paint for imparting corrosion resistance, weather resistance, lubricity, designability (coloring), etc. to a metal surface, by smoothing by baking at a low temperature and / or a short time. The present invention relates to a continuous coating method for metal strips, which enables stable formation of various resin coating films. The method of the present invention is
It is also suitable for continuous coating on metal strips with a thickness of 2.0 mm or more.

[0002]

2. Description of the Related Art Various metal plates [for example, steel plates, aluminum plates,
Apply organic resin paint to stainless steel plate, zinc plate, copper plate, etc., as well as to these metal plates that have been surface treated (eg, Zn-based plating, Zn-based plating + chromate treatment), and apply organic resin paint to the surface. Forming an organic resin coating film to impart corrosion resistance, weather resistance, lubricity, and designability has been widely practiced conventionally.

As an organic resin paint, improvement of working environment,
In order to save resources and protect the global environment, the organic solvent-based paints that have been widely used in the past are being changed to water-based paints that do not contain organic solvents. As the water-based organic resin coating, there is an aqueous coating using a water-soluble resin, but an emulsion coating in which organic resin fine particles are dispersed in water is generally used from the viewpoint of coating film performance. As the organic resin used for the water-based paint, both thermoplastic resin and thermoplastic resin are used.

The coated steel sheets coated with a water-based organic resin coating that have been proposed so far include, for example, the following. (1) Steel sheet mainly composed of urethane-modified polyolefin resin, coated with resin coating containing fluororesin particles and silica particles (Japanese Patent Laid-Open No. 3-17189), (2) Mainly composed of acrylated epoxy resin, fluororesin Steel sheet coated with resin coating containing particles and silica particles (JP-A-3-96337), (3) zinc-based or aluminum-based coated steel sheet coated with a resin composite (mixture) having a glass transition temperature of 70 ° C or higher ( (JP-A-2-43040), (4) A steel sheet in which an acrylic resin paint containing chromic acid and silica particles is coated on a chromate coating film (JP-A-61-227179). It is described that in these coated steel sheets, the resin coating film improves the corrosion resistance, weather resistance, fingerprint resistance, lubricity, alkali resistance and the like.

In the case of large-volume / high-speed processing, the coating of a metal plate is a continuous process in which the metal plate is run in the form of a strip (strip) and the metal strip is passed through a coating zone and a baking zone in sequence. It is done by painting method. A roll coater is widely used for this continuous coating method.

As shown in FIGS. 1 and 2, the coating by the roll coater is performed by transferring the paint contained in the paint pan 4 from the coating roll 6 to the metal strip 1 via the pickup roll 5. Reference numeral 7 is a backup roll provided as necessary. The arrangement of roll coaters for double-sided coating includes the S-type roll coater shown in FIG. 1 and the I-type roll coater shown in FIG.

In both the S type and the I type, a reverse roll system in which the coating roll 6 is rotated in the direction opposite to the traveling direction of the metal strip 1 and a coating roll 6 in the same direction as the traveling direction of the metal strip 1 are used. There are two types of natural rolls that rotate in the direction of. In high-speed production with high line speed, the reverse roll coating is more stable than the natural roll coating in that the coating thickness is uniform in the width direction of the metal strip, and it appears parallel to the traveling direction of the metal strip. It is known that there is an advantage that streak-like defects called roping, roll eyes, etc. are less likely to occur.

[0008]

However, there are the following problems in the conventional coating of a water-based organic resin coating on a metal plate, particularly continuous coating by roll coating.

(1) The conventional water-based organic resin paint requires a relatively high metal surface temperature of about 100 ° C. or higher in the baking and drying process. When this temperature is around 100 ° C,
The performance (corrosion resistance, adhesion, etc.) of the formed resin coating film is easily affected by the baking conditions, and if the temperature is significantly lower than 100 ° C, the coating film performance deteriorates.

Therefore, in order to secure this metal surface temperature, in the case of baking and drying in a hot air oven, heating for a long time is performed.
(Reduction of production line speed or extension of baking furnace) is required, or expensive baking equipment such as induction heating furnace and infrared furnace is required. These lead to lower efficiency and higher equipment costs.

Further, particularly in the case of a thick metal plate having a plate thickness of 2 mm or more, the surface temperature of the metal does not rise sufficiently in the hot blast stove due to the dissipation of heat to the inside, and the fusion of emulsion particles and the crosslinking of the resin (functional group) (Bonding reaction of) becomes insufficient and the performance of the coating film tends to deteriorate.

(2) In order to lower the baking temperature so that baking can be carried out even in a short baking oven, a paint having a relatively low glass transition temperature (hereinafter sometimes referred to as Tg) of the resin component should be used. It is advantageous to use. However, in water-based paints with a relatively low Tg of resin component, fusion and cross-linking curing may be too fast, and leveling immediately after roll coating (smoothness of the coating film surface due to paint flow on the surface) may not occur sufficiently. . As a result, the streak-like defects described above tend to remain in the coating film after baking. This defect not only causes problems in appearance, but also causes microscopic deterioration in uniformity of the coating film, which causes variations in performance.

This defect can be suppressed or eliminated by adjusting the viscosity of the paint, the roughness of the roll, the peripheral speed of the roll, and the distance between the rolls (the distance between the pickup roll and the coating roll).
Depending on the resin component, the range of appropriate conditions is narrow, which leads to a reduction in coating efficiency, surface quality of coated metal plates, and yield.

(3) In the roll coating of an aqueous paint having a relatively low Tg of a resin component, there is a problem in the stability of the paint in the paint pan and on the coating roll, which hinders continuous coating. Specifically, the paint may solidify or film over time in the paint pan, resulting in an insufficient film thickness or obstructing the circulation of the paint, causing overflow to the outside of the paint pan. In addition, the physical properties of the paint in the paint pan may change.

Furthermore, the high temperature around the roll coater,
The temperature of the coating roll may rise due to friction between the coating roll and the metal strip, and the coating material may solidify on the surface of the coating roll. In this case, it is necessary to perform maintenance of the coating roll by polishing the coating roll to remove the solidified coating or to replace the coating roll, which causes problems such as increased labor and generation of defective products.

An object of the present invention is to provide a continuous coating method for metal strips which does not have the above-mentioned problems using an aqueous organic resin coating material. A more specific object of the present invention is to provide a continuous coating method capable of obtaining good coating performance even at a low temperature and / or a short time even on a thick metal plate, and further to provide a resin component. The present invention is to provide a continuous coating method capable of producing a coated metal sheet having a beautiful appearance in which the problems (2) and (3) are solved by using an aqueous organic resin coating having a relatively low Tg. .

[0017]

Means for Solving the Problems As a result of repeating various investigation experiments, the inventors of the present invention performed coating and baking under specific conditions using a thermoplastic resin having Tg of less than 70 ° C. The inventors have found that even a metal plate having a thick coating material can obtain good coating performance by baking at a low temperature and / or for a short time, and arrived at the present invention.

The present invention is a method for continuously coating a water-based organic resin paint on a metal strip, wherein one or both sides of the metal strip is made of a thermoplastic resin having a glass transition temperature of less than 70 ° C. After applying a water-based organic resin paint with a roll coater at a paint temperature of 24 ° C or less and a dry resin adhesion amount of 5 g / m ² or less, at a plate surface position from a plurality of slit nozzles installed in the plate width direction. Temperature 80 ~ 300 ℃, wind speed 15 ~ 80
It is a continuous coating method characterized by performing baking and drying at a maximum plate temperature (sometimes abbreviated as PMT) of 35 to 90 ° C. by blowing hot air of m / sec onto the coated surface.
In a preferred embodiment, the temperature of the metal strip on the roll coater entrance side is
Keep below 35 ° C.

[0019]

[Action]

(Water-based organic resin coating) The water-based organic resin coating used in the coating method of the present invention is an emulsion type coating (dispersed resin particle size is about
In addition to 0.1 to 1 μm), a water-dispersed paint (colloidal dispersion) having a smaller particle diameter than this, a water-soluble resin paint, or a composite emulsion paint in which an emulsion paint is mixed with another water-based paint. Emulsion type paints or complex emulsion type paints are preferred.

In the present invention, in order to enable low temperature baking, an aqueous organic resin paint whose resin component is a thermoplastic resin is used. When a water-based paint composed of a thermosetting resin is used, a special cross-linking agent is required in order to apply the baking conditions (PMT ≦ 90 ° C.) of the present invention, and there is a problem in stability of the paint. A thermosetting resin having a cross-linking temperature of 90 ° C or lower has not been put into practical use at present. Resin component in paint (1
Or two or more) are selected so that the Tg is less than 70 ° C.

When the Tg is 70 ° C. or higher (eg, in the case of a long-chain polymer compound, etc.), under the baking conditions of the present invention, the resin film formation and the appearance and performance of the coating film after baking are insufficient. Becomes For example, for acrylic polymer resin with Tg of 70 ° C or higher,
Even if an organic solvent or low Tg polymer resin is included as an auxiliary agent for low-temperature film formation, if the Tg of the entire resin is 70 ° C or higher, the appearance of the coating film after baking will be whitened or small pinholes will appear. Easily occurs, and the coating film performance tends to vary. Although it depends on the type of resin used, the Tg
Is preferably in the range of -10 to 60 ° C, particularly 0 to 50 ° C.

The aqueous organic resin coating material used in the present invention is not particularly limited as long as it is a thermoplastic resin having a resin component Tg of less than 70 ° C. The resin component of the paint may be selected from one or two or more of the known thermoplastic hydrophilic resins that have been conventionally used for water-based organic resin paints so that the Tg is less than 70 ° C. Examples of suitable emulsion-type thermoplastic hydrophilic resins are shown below, but are not limited thereto.

(1) Acrylic polymer emulsion: For example, acrylic monomers such as methyl acrylate, ethyl acrylate, isopropyl methacrylate, acrylonitrile, acrylamide, methacrylamide, acrylic acid ester, methacrylic acid ester, and alkoxyalkyl (meth) acrylate, Or these polymers or copolymers as a resin skeleton, as a monomer that contributes to the hydrophilicity and adhesion to the steel plate substrate to the skeleton, ethylenically unsaturated carboxylic acid monomer (acrylic acid, methacrylic acid maleic acid, Crotonic acid, etc.), alkoxymethylene (meth) acrylamide, acid phosphooxyalkyl (meth) acrylate, etc., or a combination of two or more thereof, and an emulsion having a Tg of less than 70 ° C. The skeletal resin is made of styrene, vinyl acetate, vinyl ester, vinyl chloride,
It may be copolymerized with a vinyl compound such as ethylene.

(2) Cr-containing acrylic polymer emulsion: For example, chromic acid (chromic anhydride), chromate salt (ammonium dichromate salt, sodium salt, potassium salt, barium salt) are added to the above acrylic polymer emulsion. A resin composition in which corrosion resistance, alkali resistance, etc. are improved by adding (strontium salt, etc.) to combine Cr with the resin and by including Cr ^{6+ in} the formed coating film. Furthermore, Ni, Co, Zn, Mn,
Heavy metal ions such as Fe may be added.

(3) Urethane-based polymer emulsion: For example, alkyd-modified or acrylic-modified with a hydrophilic group introduced,
Epoxy-modified urethane resin emulsion. (4) Polyester polymer emulsion: For example, an acrylic-modified, urethane-modified, epoxy-modified, alkyd-modified, or silicone-modified polyester resin emulsion having a hydrophilic group introduced therein.

(5) Epoxy polymer emulsion: For example, emulsion of di- or polyglycidyl ether of aliphatic polyhydric alcohol, dicarboxylic acid diglycidyl ester, nitrogen, heterocyclic ring-containing epoxy compound, acryl-modified epoxy resin and the like.

The water-based organic resin coating composition may contain various additives in addition to the resin component, pigment and solvent (water or a mixed solvent of water and alcohol). For example, water-dispersible resin (colloidal dispersion) or water-soluble resin that is the same as or different from the base resin in the paint for the purpose of improving the adhesion of the resulting paint film to the substrate, improving the corrosion resistance of the paint film, and improving the weather resistance. Resin, chromate, colloidal silica or the like may be added.

For example, an acrylic water-soluble resin or urethane water-soluble resin having a lower Tg (30 ° C. or less) and a smaller particle size may be added to the acrylic emulsion. When adding colloidal silica, a silane coupling agent may be added together.

Further, in order to make it possible to omit oiling and degreasing at the time of pressing, as a lubricant for keeping the formed coating film lubricity, paraffin, a polyolefin wax, a fatty acid lubricant (stearic acid , Palmitic acid, lauric acid, etc.), fluoropolymers (polytetrafluoroethylene, polyvinylidene fluoride, etc.), metallic soaps
1 of (calcium stearate, lead stearate, etc.)
It is also possible to add one kind or two or more kinds to the paint.
Further, other additives commonly used in paints, for example, one kind or two or more kinds of a dispersant, a stabilizer, a reaction accelerator and the like can be appropriately blended.

The viscosity of the paint (resin liquid) is 0.003 to 0.04 Pa, although it depends on the capacity of the baking oven and the production line speed.
-The range of S (20 ° C) is preferable. The viscosity of the paint can be adjusted by the molecular weight and concentration of the resin, the type and amount of the additive, and the like. If it is less than 0.003 Pa ・ S, a wind pattern may be generated by the hot air during baking until the resin dries, while if it exceeds 0.04 Pa ・ S, sufficient leveling does not occur during baking, and the roll is coated during roll coating. The eyes (striation) remain as they are, and the microscopic smoothness of the resin coating film surface is poor. More preferable paint viscosity is 0.005 ~
It is 0.03Pa ・ S (20 ℃).

(Coating Substrate) The coating method of the present invention can be applied to continuous coating of any metal strip. Examples of applicable strip-shaped substrates include steel plates, plated steel plates, stainless steel plates, non-ferrous metal plates (aluminum plates, copper plates, etc.). Among them, as a plated steel sheet for improving corrosion resistance, for example, Zn-based (molten Zn, electric Zn, vapor deposition Zn
Plating), Zn-Al series (5% Al-Zn, 55% Al-Zn plating, etc.), Zn-Fe series (electrical Zn-Fe alloy, alloyed hot-dip Zn plated steel sheet, etc.). Also, as a pretreatment for painting, chromate treatment (reaction type, coating type, electrolytic type, etc.)
Or a phosphate treatment (Zn phosphate, Ca phosphate, Fe phosphate treatment, etc.) or the like.

(Coating process) The above-mentioned water-based organic resin coating material is continuously coated (that is, roll coating) on one side or both sides of the metal strip by a roll coater. The amount of adhesion at the time of application is 5 g / m ² or less in terms of dry film adhesion. If the adhered amount is larger than this, the baking will be insufficient or it will take a long time under the baking conditions of the present invention. The lower limit of the coating thickness is not particularly specified, but in order to ensure sufficient coating film performance, it is preferable that the dry coating film adhesion amount is 0.5 g / m ² or more. A more preferable adhesion amount is 0.5 to 3.0 g / m ² .

Roll coating is performed at a coating temperature of 24 ° C. or lower.
The paint temperature in the present invention means the temperature in the paint pan. Therefore, if necessary, the paint pan is cooled to ensure this paint temperature. When the coating temperature exceeds 24 ° C, the physical properties of the coating change over time, and film coating due to emulsion solidification on the coating surface in the coating pan, coarsening due to agglomeration of particles inside, and gelation occur. As a result, the coating film performance deteriorates, the circulation of the paint becomes difficult, and the coating operation does not proceed smoothly. Further, when the coating temperature is high, roll winding easily occurs due to solidification of the coating on the surface of the coating roll.

The coating material can be cooled by, for example, installing a constant temperature type cooling device in the coating material circulation system. The coating temperature is not particularly limited as long as it is 24 ° C. or lower, and it is usually in the range of 5 to 24 ° C., although it varies depending on the resin type. However, considering the cooling cost to keep the temperature low, 15
It is economical within the range of -24 ° C.

The temperature of the metal strip on the entrance side of the roll coater (that is, when it enters the roll coater) (hereinafter, also referred to as the plate temperature or the plate entrance temperature) is the stability of the resin liquid and the coating,
It is preferable to keep the temperature lower than about 40 ° C because it affects the appearance of the coating.

When the plate temperature immediately before the roll coater exceeds 35 ° C., the surface temperature of the coating roll rises due to friction between the coating roll and the metal strip plate as the coating work progresses, and the coating material on the surface of the coating roll is removed. It may solidify and the paint may wind around the coating roll. This wrapping is likely to occur when the paint temperature in the paint pan is relatively high (eg, 15-24 ° C). If the winding is light, the wound portion will be uneven coating, but if the winding is bad, the drive roll of the coating roll will trip and the coating will not be performed.
However, if the plate temperature is kept at 35 ° C or lower, the surface temperature of the coating roll due to the friction is kept lower than the solidification temperature of the resin even if the coating temperature is relatively high at 15 to 24 ° C, and the wrapping phenomenon occurs. Can be completely prevented. Therefore, the paint temperature is 15-24 ℃
When the temperature is high, it is preferable to control the plate temperature to 35 ° C or lower. If the paint temperature is lower than 15 ℃, the plate temperature
Wrapping is unlikely to occur even if the temperature exceeds 35 ° C (eg 40 ° C).

The method of controlling the plate entrance temperature is not particularly limited, but for example, if there is a cleaning step before roll coating, the plate temperature can be kept low by the water temperature of the cleaning water. The lower limit of the entry temperature of the plate is not particularly limited, but it is costly to keep this temperature too low, so the preferred entry temperature is 20 to 35 ° C.

Roll coating can be carried out using any conventional roll coater. For double-sided coating, both the S-type roll coater shown in FIG. 1 and the I-type roll coater shown in the figure can be used. The metal strip 1 is conveyed in the direction of the arrow, during which the paint is applied by the roll coater 2, and the paint is dried in the baking and drying oven 3 to form a coating film. The coating method may be either a reverse roll method or a natural roll method. Further, a spray / roll squeezing method can also be used in the coating step, and this is also included in the roll coating of the present invention.

(Baking process) The metal strip coated with the roll is
Next, hot air is blown onto the coated surface from a large number of slit nozzles arranged in the width direction of the plate to heat the coated surface for baking and drying. The time from application to heating with hot air is not particularly limited. As the baking / drying oven, for example, a conventional catenary-type or floater-type oven-type hot-air baking / drying oven or a simple dryer can be used.

FIG. 3 is an explanatory view showing an example of a hot air baking and drying oven equipped with a large number of slit nozzles. The baking / drying furnace 3 has a large number of slit nozzles 11 (six upper and lower in the illustrated example) for blowing hot air from above and below the metal strip 1, and the hot air passes from a hot air supply pipe 12 to a distributor 13 to each slit nozzle. After being supplied to the metal strip 11 and sprayed onto the metal strip 1 at a high speed, it is discharged from the exhaust pipe 14. Each slit nozzle 11
Is attached to the distributor 13 such that the slit longitudinal direction faces the plate width direction (direction perpendicular to the traveling direction of the strip plate). In the illustrated example, in order to suppress the blowing of hot air to the coating device side, the slit nozzles from the inlet side up to the second slit nozzle are installed so as to be inclined toward the outlet side, but the orientation of the slit nozzles is not particularly limited. .

In the present invention, the temperature and velocity of the hot air at the plate surface position are within the ranges of 80 to 300 ° C. and 15 to 80 m / sec. That is, the conventional hot air baking and drying oven (the wind speed is usually 15 m
(less than / sec), increase the wind speed. By baking and drying at this high wind speed, leveling before film formation is promoted, and good coating performance can be obtained even when the hot air temperature is lowered to 80 ° C. The temperature and velocity of the hot air at the plate surface position can be measured at the band passing position without the metal band plate.

When the hot air temperature is lower than 80 ° C., it takes time to evaporate (evaporate) water during film formation, and the coating appearance is deteriorated such as whitening of the resin in consideration of the emulsion film formation speed. On the other hand, if the hot air temperature exceeds 300 ° C, the resin will form a film during the initial heating in the oven, resulting in insufficient leveling of the resin, cracks (unevenness defects) or pinholes in the coating film, and the coating appearance also remains. to degrade. The hot air temperature is preferably 80 to 250 ° C, more preferably 100 to 250 ° C.

When the air velocity of the hot air is less than 15 m / sec, the leveling promoting effect is lowered, and further, the evaporation rate of water is lowered, and the solidification rate of the coating film is also lowered. On the other hand, the wind speed is 80
If it exceeds m / sec, wind ripples will occur, and on the contrary, the coating appearance will deteriorate. The wind speed is preferably 20 to 80 m / sec, more preferably 30 to 80 m / sec. FIG. 4 illustrates the relationship between the temperature and speed of the hot air and the coating appearance.

The slit nozzle has a slit length equal to or larger than the plate width and is attached horizontally to the band plate in the plate width direction so that hot air can be uniformly blown to the entire width of the metal band plate.

Slit width of slit nozzle, pitch (slit installation interval in the longitudinal direction), distance from metal strip plate surface,
The number of installations is set so that the maximum attainable plate temperature (PMT) of the metal strip heated by the blowing of the hot air is a predetermined temperature within the range of 35 to 90 ° C. Slit width is 0.5-10 mm
Especially, the range of 3 to 7 mm is preferable, and the pitch is 200 to 1000 mm, especially from the drying speed and the coating appearance (leveling).
A range of 300 to 1000 mm is preferred. The distance between the slit nozzle outlet (hot air outlet) and the surface of the metal strip is 50 to 300 m.
It is preferably m, particularly in the range of 100 to 250 mm.
The number of slit nozzles (the number of slit nozzles per side when provided on both sides of the plate) depends on the length of the oven, dryer, etc., but it is generally desirable to set at 4 or more, especially 6 to
A range of 20 is preferable. Since a conventional hot air baking drying furnace is generally equipped with a large number of slit nozzles of 20 or more, the number of slit nozzles can be greatly reduced according to the present invention, and the baking length of the baking drying oven can be significantly shortened. be able to. The passage time (baking time) in the baking / drying oven is generally in the range of 5 to 30 seconds, preferably 5 to 20 seconds.

In the step of baking the metal strip, as described above, the low temperature baking of PMT of 35 to 90 ° C, preferably 40 to 80 ° C is carried out. PMT is selected according to the resin type and the oven length of the baking and drying oven to obtain good coating performance and coating appearance. The temperature in the baking and drying oven is generally 1
The temperature is 00 to 300 ° C, preferably 100 to 250 ° C.

[0047]

EXAMPLES The present invention will be specifically described below with reference to examples. In the examples, the temperature of the paint was maintained by a constant temperature type cooling device of the paint circulation system. Further, the distance between the slit nozzle outlet and the surface of the metal strip in the baking / drying furnace is
It was 200 mm in all the examples.

Example 1 A double-sided hot-dip galvanized steel strip having a plate width of 1000 mm, a plate thickness of 1.6 mm, and a coating weight of 60 g / m ² (per surface) has a chromium coating amount of 30 mg / m ² (per surface). ) The coating type chromate treatment was applied (spray treatment → baking at 80 ° C for 10 seconds). Then, after cooling the temperature of the steel strip by air cooling, an S-type roll coater (natural roll system) was used for this steel strip, and the urethane-modified acrylic aqueous emulsion resin paint (Tg of the resin component was 40 ° C. Colloidal silica (containing colloidal silica) was continuously applied to both sides in a film thickness of 1 g / m ² of coating amount. The steel strip temperature on the inlet side of the roll coater was 33 ° C. Slit width 5mm following the steel strip coming out of the roll coater
The coating film was baked and dried by passing through a catenary type baking and drying furnace (hot air drying oven) having a total furnace length of 15 m, which was equipped with 15 slit nozzles at a pitch of 800 mm each at the top and bottom. Since the line speed of the steel strip was 80 m / min, the baking and drying time in the baking and drying furnace is about 11 seconds. Table 1 summarizes the paint temperature during coating, the hot air conditions in the baking and drying oven, and the maximum plate temperature (PMT).

The coating appearance obtained by visually observing the coated steel strip obtained after baking and drying was observed by SEM (scanning electron microscope) of the coating film surface.
Adhesion of the coating film was evaluated by the following criteria by observing the coating film formation state and by the coating film peeling area ratio in the cellophane tape peeling test of the bent portion after 2T bending. The results are also shown in Table 1 together with the adhesion amount of the coating film after baking and drying.

Evaluation of coating appearance ○: Good appearance ×: Occurrence of stripe defects (striation) Evaluation of coating film formation condition ○: No cracking (unevenness), no pinholes △: Cracking, small pinholes or insufficient leveling ×: Wacking , Generation of pinholes, adhesion ○: Peeling area ratio 0% △: Peeling area ratio 1 to 50% ×: Peeling area ratio over 50%

[0051]

[Table 1]

For comparison, when the same coating system (that is, urethane-modified acrylic water-based emulsion resin coating material) adjusted so that Tg was 100 ° C. was applied in the same manner, the PMT was 120 ° C. If the hot air temperature is not increased so as to obtain the coating results comparable to those of the examples of the present invention in Table 1 (the appearance, the coating film forming state, and the adhesion are all good).

Example 2 An I-type roll coater (reverse roll system) was used on an electrogalvanized steel sheet having a plate width of 1000 mm, a plate thickness of 3.2 mm, and a coating weight of 20 g / m ² (per surface). An acrylic water-based emulsion resin paint containing a resin component having a Tg of 20 ° C. and containing strontium chromate and polyethylene wax (lubricant) was continuously applied to both sides in an amount of 2 g / m ² . The steel strip temperature at the roll coater entrance side is 33
° C. Following the steel strip from the roll coater, a slit nozzle with a slit width of 3 mm is placed up and down at a pitch of 500 mm, 8 each
The product was passed through a catenary-type hot air drying oven with a total furnace length of 5 m, which was equipped with a book, to be baked and dried. Since the line speed of the steel strip was 50 m / min, the baking and drying time in the baking and drying furnace is 6 seconds. Table 2 shows the paint temperature at the time of application, hot air conditions and PMT in the baking and drying oven, and the test results.

[0054]

[Table 2]

(Example 3) The amount of Cr deposited on both surfaces of a stainless steel plate having a plate width of 1200 mm and a plate thickness of 2.0 mm in the same manner as in Example 1.
A coating type chromate treatment of 30 mg / m ² was performed. After air cooling, a polyester aqueous emulsion resin coating having a resin component Tg of 10 ° C. was continuously applied to both sides using an I type roll coater (natural roll method), aiming at an adhesion amount of 1 g / m ² . The steel strip temperature on the inlet side of the roll coater was 33 ° C. This steel strip was subsequently passed through the same hot-air drying oven used in Example 2 for baking and drying. Since the line speed of the steel strip was 60 m / min, the baking and drying time in the baking and drying furnace was 5 seconds. Table 3 shows the paint temperature at the time of application, hot air conditions and PMT in the baking and drying oven, and the test results.

[0056]

[Table 3]

(Example 4) The amount of Cr deposited on both sides of an aluminum plate having a plate width of 1200 mm and a plate thickness of 4.0 mm in the same manner as in Example 1.
A coating type chromate treatment of 10 mg / m ² was performed. After cooling with air, an epoxy type aqueous emulsion resin coating material having a resin component Tg of 70 ° C. was continuously applied to both sides using an I type roll coater (reverse roll type), aiming at an adhesion amount of 1 g / m ² .
The temperature of the steel strip on the inlet side of the roll coater was 31 ° C. This steel strip is followed by a slit nozzle with a slit width of 5 mm.
Baking and drying were carried out by passing through a floater type hot air drying oven with a total length of 10 m, which was equipped with 10 pieces each at 700 mm above and below. Since the line speed of the steel strip was 100 m / min, the baking and drying time in the baking and drying furnace is 6 seconds. Table 4 shows the coating temperature at the time of coating, the hot air condition and PMT in the baking and drying oven, and the test results.

[0058]

[Table 4]

For comparison, when the same coating system adjusted to have Tg of 90 ° C. was used for the same coating, the hot air temperature was not raised until PMT was 120 ° C.
It was not possible to obtain coating results comparable to those of the invention examples in Table 4.

(Embodiment 5) An acrylic styrene emulsion resin having a resin component Tg of 30 ° C. was used on both sides of a cold-rolled steel plate having a plate width of 1000 mm and a plate thickness of 1.0 mm by using an S-type roll coater (reverse roll system). A paint (containing calcium stearate as a lubricant) was continuously applied on both sides with an aim of an adhesion amount of 2 g / m ² . The steel strip temperature on the inlet side of the roll coater was 33 ° C.
This steel strip was subsequently passed through a hot air drying oven similar to that used in Example 4 for baking and drying. Since the line speed of the steel strip was 80 m / min, the baking and drying time in the baking and drying furnace was 7.5 seconds. Paint temperature during application,
Table 5 shows the hot air conditions and PMT in the baking and drying oven, and the test results.

[0061]

[Table 5]

(Example 6) A S-type roll coater (natural roll) was applied to both sides of a molten 55% Al-Zn alloy-plated steel sheet having a sheet width of 1000 mm, a sheet thickness of 2.0 mm, and a coating weight of 80 g / m ² (per surface). Method), an acrylic water-based emulsion resin paint (containing strontium chromate and urethane water-soluble resin) having a Tg of 40 ° C. as a resin component was continuously applied to both sides at an adhesion amount of 2 g / m ² . The temperature of the steel strip on the inlet side of the roll coater is 32.
° C. This furnace strip is followed by a slit furnace with a slit width of 3 mm and a pitch of 1000 mm, each with 16 vertical slits.
It was passed through a 20 m catenary type hot air drying oven to be baked and dried. Since the line speed of the steel strip was 80 m / min, the baking and drying time in the baking and drying furnace is 15 seconds. The paint temperature at the time of application and the hot air conditions in the baking and drying oven
Table 6 shows the PMT and the test results.

[0063]

[Table 6]

For comparison, when the same coating system adjusted to have Tg of 100 ° C. was used for the same coating, the hot air temperature was not raised so that the PMT was 130 ° C. It was not possible to obtain a coating result comparable to Example 6 of the present invention.

Example 7 Example 1 was repeated, except that the steel strip temperature (entry plate temperature) on the coating roll entrance side and the temperature in the paint pan were changed as shown in Table 7. The test results are also shown in Table 7. As can be seen from Table 7, when the paint pan temperature is relatively high at 15 to 24 ° C, the paint will be wrapped around the coating roll unless the entry plate temperature is 35 ° C or lower.
A coated steel strip with good coating performance could not be obtained. However, by lowering the paint pan temperature below 15 ° C, wrapping did not occur even when the entry plate temperature exceeded 35 ° C, and it was possible to avoid a significant deterioration in coating film performance.

[0066]

[Table 7]

[0067]

EFFECTS OF THE INVENTION According to the continuous coating method of the present invention, it is possible to impart functions such as corrosion resistance, lubricity, weather resistance, and designability to a metal strip by using a water-based organic resin coating material that has little adverse effect on the environment. Conventional water-based organic resin coating requires baking and drying at a relatively high temperature and / or for a long time.
According to the present invention, a continuous coating method of a thin film on a metal strip plate has been established which can form a coating film excellent in coating appearance and adhesion even if it is baked at a low temperature for a short time.

Specifically, an aqueous paint comprising a low Tg resin component is used, and the low Tg resin component is controlled by controlling the paint temperature and the coating amount during roll coating and the heating conditions during baking and drying to specific conditions. The resin film-fusion and water evaporation are generated at a speed commensurate with the above, and sufficient leveling is possible. As a result, it is possible to obtain a coated metal strip having a good coating appearance and adhesion without streak defects, fine cracks, pinholes, etc. even when baked at low temperature for a short time.

Further, since the maximum reached plate temperature may be as low as 35 to 90 ° C., which is lower than the conventional one, the plate thickness of 2.0 mm which was difficult in the past
It is possible to continuously coat the above thick material with the water-based organic resin paint. Furthermore, since the oven length of the baking and drying oven is short,
It can also be applied when the length of the baking oven is limited due to locational conditions.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of an S-type roll coater that can be used in a coating process of the present invention.

FIG. 2 is an explanatory view of an I-type roll coater usable in the coating process of the present invention.

FIG. 3 is an explanatory view of a baking / drying furnace equipped with a slit nozzle, which is used in the baking process of the present invention.

FIG. 4 is an explanatory diagram showing the relationship between the temperature and velocity of hot air during baking by hot air heating and the coating appearance.

[Explanation of symbols] 1: Metal strip 2: Roll coater 3:
Baking and drying oven 4: Paint pan 5: Pickup roll 6:
Coating roll 7: Backup roll 11:
Slit nozzle 12: Hot air supply pipe 13: Distributor 14:
Exhaust pipe

Claims (2)

[Claims] 1. A method for continuously coating a water-based organic resin coating on a metal strip, which comprises a thermoplastic resin having a glass transition temperature of less than 70 ° C. as a resin component on one or both sides of the metal strip. Was applied with a roll coater at a paint temperature of 24 ° C. or less to a dry resin adhesion amount of 5 g / m ² or less, and then from a plurality of slit nozzles installed in the plate width direction,
The maximum reached plate temperature is 35 ~ by blowing hot air with a temperature of 80 ~ 300 ℃ at a plate surface position and a wind speed of 15 ~ 80 m / sec
A continuous coating method characterized by baking and drying at 90 ° C. 2. The coating temperature is 15 to 24 ° C., and the temperature of the metal strip on the roll coater entrance side is 35 ° C. or lower.
Continuous coating method described.