Coating processes – Direct application of electrical – magnetic – wave – or... – Pretreatment of substrate or post-treatment of coated substrate
Reexamination Certificate
2003-01-21
2004-04-06
Pianalto, Bernard (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Pretreatment of substrate or post-treatment of coated substrate
C427S346000, C427S376100, C427S424000, C427S427000, C427S600000
Reexamination Certificate
active
06716490
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for making an enameled steel sheet. In particular, the present invention relates to a method for making an enameled steel sheet having a flat (smooth) surface that is suitable for marker boards, black boards, tunnel interior materials, building interior and exterior boards, and the like.
2. Description of the Related Art
Enameled steel sheets having enamel layers on their surfaces exhibit high corrosion resistance, enhanced chemical resistance, high weathering resistance, and striking appearance and have prolonged life. The enameled steel sheets are superior to coated steel sheets in hardness, weathering resistance, efficiency of washing, heat resistance, and inflammability. On the other hand, a disadvantage of the enameled steel sheets is formation of surface irregularity, which looks like the skin of an orange and is called “orange peel”. Surface flatness is particularly essential for enameled steel sheets for marker boards. “Orange peel” would devalue sales functions of the enameled steel sheets.
In general, a process for making enameled steel sheets includes the steps of coating enamel slurry (hereinafter merely referred to as slurry) on a surface of a base sheet and firing the sheet to form an enamel layer on the sheet. It is believed that one reason for the formation of “orange peel” is coating of the slurry by spraying or the like, although its mechanism is still unclear. Since droplets formed on the surface of the steel sheet by spraying are deposited as it is on the surface, the surface after the spraying of the slurry also has irregularity. This irregularity would probably remains as “orange peel” after firing.
A possible solution not for forming “orange peel” is use of a roll coater or a knife coater that can form a coating layer with a flat surface. Unfortunately, coating with a coater, which involves high operation cost, is unsuitable for manufacturing of enameled products. Furthermore, slurry used in a coater process should have higher viscosity than slurry used in a spraying process; hence, a large quantity of organic thickener such as carboxymethylcellulose (CMC) should be compounded to increase the viscosity. However, such an organic thickener results in deterioration of enamel characteristics of the fired product, such as blackish enamel color and increased bubble defects in the enamel layer.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for making an enameled steel sheet that has a flat surface without “orange peel”, and is superior in enamel characteristics such as hardness, weathering resistance, efficiency of washing, heat resistance, inflammability, corrosion resistance, chemical resistance, and appearance, at low cost.
The inventors have discovered that an enameled steel sheet with a flat surface is not obtainable unless the surface of the substrate coated with slurry is flat. The inventors have also discovered that a slurry having a low static surface tension is effective for formation of a flat slurry layer at an initial stage of coating because of enhanced wettability of the slurry to the substrate. Furthermore, the inventors have discovered that a low-viscosity slurry has enhanced wettability to the substrate and thus readily forms a flat slurry layer by coating.
According to a first aspect of the present invention, a method for making an enameled steel sheet includes the steps of spraying a slurry to form a slurry layer onto a surface of a substrate and firing the slurry layer, wherein the slurry has a static surface tension of 50 dyne/cm or less and an apparent viscosity of 500 mPa·s that is measured with a model E viscometer at a rotation of 100 rpm. The slurry may contain a surfactant for controlling the static surface tension. Preferably, the slurry has a specific gravity of at least 1.3.
As a result of further investigation, the present inventors have discovered that the surface unevenness or irregularity of the slurry layer coated on the substrate can be removed by vibration of the substrate when the slurry is applied or when the slurry is still fluid and that the fired substrate also has a significantly flat enamel surface.
According to a second aspect of the present invention, a method for making an enameled steel sheet includes a step of spraying a slurry for forming an enamel layer onto a surface of a substrate, wherein the substrate is vibrated when the slurry is applied or when the slurry applied is still fluid. Preferably, the substrate is vibrated by an acceleration of vibration of at least 1 G wherein G is gravitational constant.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The method according to the present invention will now be described in detail. In the specification, “the method” indicates one that is common to both the first and second aspects, unless otherwise specifically stated.
The method for making an enameled steel sheet according to the present invention includes coating slurry on a surface of a substrate and firing the coating layer to form an enamel layer.
In the present invention, the substrate may be composed of any material. Examples of the substrate include plain steel sheets such as SPP, SPCC, SPHC, and SS; stainless steel sheets such as ferritic stainless steel sheets (SUS430 series) and austenitic stainless steel sheets (SUS304 series); metal sheets such as ferritic heat resisting steel sheets (SUH409 series) and aluminum sheets; galvanized steel sheets such as aluminum-plated steel sheets, aluminum/zinc-plated steel sheets, and zinc/iron-plated steel sheets; and enameled sheets thereof. Herein, SPP, SPCC, SPHC, and SS, SUS430, SUS304, and SUH409 are referred to in Japanese Industrial Standard (JIS) G3133, G3141, G3131, G3101, G4304, G4305 and G4312.
Pretreatment for the substrate is not necessarily required. However, degreasing treatment is effective in improvement in wettability of slurry. The substrate may be subjected to any surface treatment such as plating for improving the adhesiveness between the steel sheet of the substrate and the enamel layer. Examples of surface treatment include chromating, phosphating, plating with nickel, cobalt, molybdenum, or manganese, and alkaline etching. For improving adhesiveness between the steel sheet substrate and the enamel layer, nickel plating or alkaline etching is particularly effective in acceleration of reaction of the steel sheet substrate with the enamel layer.
The slurry coated on the surface of the substrate may be prepared by any conventional process. The slurry is generally prepared as follows: Frit (powder) composed of feldspar, soda ash, boric acid, and silica sand, color pigments such as titanium oxide, zinc oxide, iron oxide, and cobalt oxide, additives for improving dispersibility such as clay, potassium chloride, and nitrate salts are compounded with a dispersion medium such as water and are pulverized in, for example, a ball mill.
In the present invention, the slurry is preferably applied to the substrate by a spraying process in view of workability and process cost. Among spraying processes that are preferably used are a plain spraying process and a rotary spraying process. As in an electrostatic coating process, an electrostatic voltage may be applied to the slurry during the spraying process, the substrate being grounded, so that the substrate attracts a larger amount of slurry by electrostatic force. This process improves the utilization rate. In the spraying process, the spraying pressure is preferably in the range of 0.1 MPa to less than 1.0 MPa.
Independent portions of the first aspect and the second aspect according to the present invention will now be described. In the first aspect, the slurry preferably has a static surface tension of 50 dyne/cm or less and more preferably 35 dyne/cm or less. Such slurry enhances wettability to the substrate, forms a flat surface at an initial coating state, and thus form a flat enamel surface. A static surface tension exceeding 50 dyne/cm impairs wettability, resulting in many s
Fukushima Yasumasa
Hayashi Kazuhiro
Noma Shinichi
Taguchi Masao
Takagi Masato
Kawasaki Steel Metal Products & Engineering Inc.
Pianalto Bernard
Piper Rudnick LLP
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