JP2002240191A - Bottom panel of microwave oven and precoated steel panel therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a precoated steel panel enhanced in roller attaching ring abrasion resistance and suitable for the bottom panel of an electronic oven by improving a resin itself without adding a lubricant. SOLUTION: A topcoating film, of which the elongation at 20 deg.C is not less than 20% and the storage modulus E' at 20-150 deg.C is not less than 2×101 N/mm2, is formed on the surface of a steel panel. As the steel panel, it is preferable to use one to which chromium-free coating pretreatment and/or chromium-free primer is applied. Further, as the steel panel, it is preferable to use one which is obtained by providing a non-chromium type chemical forming treatment layer on the surface of a Zn type, Al type of An-Al type plated steel panel and further providing an undercoating film layer. A corrosion inhibitor (A) is added to a coating film resin, and a polyphosphate (B) is further added to the coating film in such an amount that the combined addition amount of the corrosion inhibitor (A) and the polyphosphate (B) becomes 5-150 pts.mass per 100 pts.mass of the resin, on the chemical forming treatment layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a precoated steel sheet used for a microwave oven bottom plate and to a microwave oven bottom plate using the same.

[0002]

2. Description of the Related Art As a coated steel sheet applied to various home appliances, appliances, parts, interior materials, sandwich panels, exteriors, etc., a coated steel sheet manufactured by a pre-coating method in which a coil is coated and baked by a continuous coating equipment. It has been widely used because it is more productive and economical than the post-coated steel sheet that is painted after processing. Above all, it is widely used for home electric appliances. In particular, almost 100% pre-coated steel sheet is currently used for a microwave oven outer panel such as a single-function microwave oven.

[0003] On the other hand, most of the inner boxes of microwave ovens still use a post-coated steel plate which is coated with a plated material after processing. The reason behind the delay in using pre-coated steel sheets is that the inner box is in an environment where the coating film deteriorates and discolors easily, such as heat and steam during cooking, food, and seasonings, compared to the outer box. Therefore, it is desirable not to use Cr, which is an environmental load substance, and that the coating film is easily worn by a roller mounting ring during cooking.

[0004]

In particular, the bottom plate of a microwave oven may exceed 100 ° C. during heating and cooking, so that it is required to have a roller mounting ring abrasion resistance at high temperatures. In order to improve the wear resistance, it is effective to include a lubricant in the coating film. However, with a polyolefin-based lubricant commonly used for precoated steel sheets, at high temperatures exceeding 100 ° C., the lubricant in the coating film melts, and the original lubricating performance cannot be obtained. Lubricant (PTFE) that is effective even at 100 ° C or higher
Are expensive as compared with polyolefin-based ones, and have disadvantages such as lowering the gloss of the coating film. The present invention has been devised to solve such a problem. By improving the resin itself, the present invention provides a pre-coated steel sheet that is suitable for a microwave oven bottom plate by increasing the resistance of a roller mounting ring to wear. Aim.

[0005]

The precoated steel sheet for a microwave oven bottom plate and the microwave oven bottom plate according to the present invention have a coating film elongation rate of 20 ° C.
A coating film having a storage elastic modulus E 'of 0% or more at 20 to 150 ° C. of 2 × 10 ¹ N / mm ² or more is formed. It is preferable to use a steel sheet that has been subjected to a chrome-free coating pretreatment and / or a chrome-free primer.

[0006] Further, as a steel sheet, a non-chromium-based chemical conversion treatment layer containing one or more fluoroacids is coated on the surface of a Zn-based or Al-based or Zn-Al-based plated steel sheet in an amount of 0.5 to 500 mg with a fluorine adhesion amount. / M ² and / or a total metal deposition amount of 0.1 to 500 mg / m ^2, and a corrosion inhibitor obtained by bonding calcium ions for corrosion inhibition to porous silica particles by ion exchange ( A) is used in an amount of 2 to 5 with respect to 100 parts by mass of the resin component of the coating film.
0 parts by mass, and the polyphosphate (B) was further added to the coating resin, and the mixing ratio of A and B was A / B = 60/4 by mass ratio.
0-5 / 95, and undercoating added so that the combined amount of the corrosion inhibitor (A) and the polypinate (B) is 5 to 150 parts by mass or less based on 100 parts by mass of the resin. It is preferable to use one having a film layer.

[0007]

[Function] (Finish coating) The present inventors investigated the abrasion resistance of the coating by the roller mounting ring, not the concept of symptomatic treatment by adding a lubricant, but the fundamental improvement of the resin itself. . It is considered that the abrasion of the roller mounting ring and the coating film is caused by the fact that the roller sinks into the coating film due to the weight of the dish or dish when the roller passes over the coating film, and the rolling resistance increases. Therefore, as a result of investigating the characteristics and abrasion properties of various coatings to suppress sinking,
It was found that this could be suppressed by adjusting the storage modulus.

[0008] More specifically, from room temperature (20 ° C) to 150 ° C
In the temperature range of 2 × 10 ¹
By using a coating film having an N / mm ² or more, the abrasion resistance of the roller mounting ring could be improved. The storage elastic modulus E 'mentioned here can be determined, for example, by applying a low frequency (for example, 10 Hz) forced vibration to a film sample and measuring stress and strain. Assuming that the length of the sample is L, the deformation is ΔL, the cross-sectional area is S, the stress is F, and the phase difference between stress and strain is δ, the storage elastic modulus E ′ can be expressed by the following equation (1). E ′ = (FL / S △ L) cos δ (1) Such storage elastic modulus E ′ can be measured by a measuring device such as Vibron or rheograph.

It is considered that the reason for this is that when the storage elastic modulus of the coating film is high, that is, when the displacement with respect to the load is small, the sinking when passing through the roller is small, so that the rolling wear between the coating film and the roller is reduced. Storage modulus E 'of coating film
Is 2 × 10 ¹ N / mm ² or more, it is possible to suppress the sinking of the roller and improve the wear resistance. At room temperature, many resins have a storage elastic modulus E ′ of 2 ×
It is not less than 10 ¹ N / mm ² , but decreases at high temperatures.
When a microwave oven is used, the temperature may be as high as about 150 ° C., and the storage elastic modulus E ′ in the range of 20 to 150 ° C. is 2 ×
It is necessary to be 10 ¹ N / mm ² or more.

[0010] Further, for use in a microwave oven, 0T workability such as caulking or hemming is required. For this reason, the film elongation needs to be at least 20% or more. As a top coat, the film elongation at room temperature (20 ° C) is 2
There is no particular limitation as long as the storage elastic modulus E ′ of the coating film is 2 × 10 ¹ N / mm ² or more in a temperature range of 0% or more and normal temperature (20 ° C.) to 150 ° C. The storage modulus E ′ is 2 × 10 ¹ N /
To mm ² or more, the molecular weight of the resin, the molecular weight between crosslinks,
Tg (glass transition point) and whether the resin is linear or branched, can be adjusted by the type and amount of a curing agent, surface concentration, and the like.

For example, if a blanched melamine resin or a methylated melamine resin is added as a cross-linking agent to a branched high molecular weight polyester resin having a molecular weight of about 10,000 or more in an excess of several to several tens of percent over the reaction equivalent and baked. Good. Alternatively, for example, a molecular weight of about 10,000
Butylated melamine resin or a mixture of butylated melamine resin and methylated melamine resin is used as a crosslinking agent in the above-mentioned linear high-molecular polyester resin at 10% based on the reaction equivalent.
A curing agent added in excess of 数 10% may be concentrated on the surface. In addition, various pigments and additives can be appropriately added, and there is no particular limitation. The film thickness may be about 5 to 20 μm, similarly to a normal precoated steel sheet.

(Chrome-free coating pretreatment) The chromium-free coating pretreatment includes tannic acid treatment, phosphate treatment,
Known treatments such as a silane coupling treatment and a silica-based pretreatment can be used. Among them, one or two fluoroacids
Preferably, the non-chromium-based chemical conversion treatment layer containing at least one species is provided so as to have a fluorine deposition amount of 0.5 to 500 mg / m ² and / or a total metal deposition amount of 0.1 to 500 mg / m ^2. It becomes a precoated steel sheet which is more excellent in corrosion resistance, moisture resistance and food stain resistance.

In particular, H ₂ TiF ₆ , H ₂ ZrF ₆ , H ₂ HfF ₆ ,
Non-chromium chemical conversion treatment containing any one or more of fluoroacids such as H ₂ SiF ₆ , H ₂ GeF ₆ , H ₂ SnF ₆ , HBF _4, etc. is performed at a fluorine adhesion amount of 0.5 to 500 mg / m ^2.
It has been found that by performing the chemical conversion treatment so that the corrosion resistance becomes significantly higher, the corrosion resistance is dramatically improved.

These fluoroacids are dissolved in water, activate the plating surface by the hydrofluoric acid etching effect, bring about the effect of improving the adhesion, and at the same time, improve the adhesion.
It is presumed that the metal components in the fluoroacid, such as, for example, precipitate on the plating surface and form a corrosion-resistant film. This alone does not provide as much corrosion protection as the chromate film, but in combination with the undercoat paint of the present invention, calcium ions, which are rust-preventive ions supplied from the corrosion inhibitor contained in the undercoat film, cause Zn on the plating surface to At the same time as bonding with the like, form a stable compound barrier layer such as fluoride ion and calcium fluoride in the chemical conversion treatment film,
It is assumed that the chemical conversion coating is further strengthened and the rust prevention effect is improved. Among the fluoroacids, H ₂ TiF
₆ indicates the best effect.

The amount of the chemical conversion coating film is 0.1 in terms of fluorine.
5 to 500 mg / m ² are suitable. If the amount is less than 0.5 mg / m ² , the rust prevention effect is insufficient, and if the amount is more than 500 mg / m ² , the effect is saturated. In the chemical conversion coating, at least fluoroacid is 0.5 to 500 mg / fluorine equivalent.
It is sufficient if m ^{2 is} adhered. In addition to fluoroacid, T
i, Zr, Mo, Si, Al, Hf, Ge, Sn and other metal oxides, hydroxides, phosphates, carbonates, etc. alone,
Or mixed, dissolved or dispersed, water-soluble or water-dispersible polymer, silane coupling agent,
A titanium coupling agent, an antifoaming agent, a surfactant, and the like commonly used for a chemical conversion treatment film may be appropriately added. The amount of addition is preferably 0.1 to 500 mg / m ² in terms of total metal. As in the above-mentioned fluorine conversion, if the amount is less than 0.1 mg / m ² , the rust prevention effect is insufficient, and if the amount is more than 500 mg / m ² , the effect is saturated. Known coating methods such as roll coating, spraying, and dipping are sufficient.

(Chromium-free primer) As a chromium-free primer, a corrosion inhibitor comprising cations such as calcium, zinc, cobalt, lead, strontium and barium bonded to porous silica particles in a coating film by ion exchange, silane cup It is preferable to use a steel sheet on which an undercoating film is formed using a known non-chromic and rust-preventive pigment such as a ring agent and a phosphate compound.

In particular, the corrosion inhibitor (A), which is obtained by binding calcium ions for corrosion inhibition to porous silica particles by ion exchange, is used in an amount of 2 to 100 parts by mass of the resin component of the coating film.
50 parts by mass were further added, and the polyphosphate (B) was further added to the coating resin, and the mixing ratio of A and B was A / B = 60/4 by mass ratio.
0-5 / 95, and undercoating added so that the combined amount of the corrosion inhibitor (A) and the polypinate (B) is 5 to 150 parts by mass or less based on 100 parts by mass of the resin. By providing the film, a precoated steel sheet having excellent corrosion resistance, moisture resistance, and food contamination resistance is obtained.

The rust-preventing effect of the corrosion inhibitor in which calcium ions are bonded to porous silica particles by ion exchange is because the calcium ions bonded to the silica particles are easily eluted by the water that has entered the coating film. The corrosion resistance is not persistent. However, when the polyphosphate is added, it is considered that ionic bond such as chelate bond is formed on the surface of the silica particles to suppress elution of calcium ions.
In addition, since the polyphosphate has a pH buffering action, acidification by corrosive ions such as hydrogen ions is weakened, and elution of calcium ions may be reduced.

As the polyphosphate, aluminum polyphosphates such as aluminum pyrophosphate, aluminum metaphosphate and aluminum dihydrogen triphosphate exhibit an excellent calcium ion suppressing effect. In particular, aluminum dihydrogen tripolyphosphate exhibits the best effect.
The polyphosphate may be treated by adding zinc, magnesium, titanium, calcium or other compounds, or a silane coupling agent, silicon oil or the like, or coating, in order to enhance corrosion resistance and storage stability. .

The amount of the polyphosphate to be added to the resin coating is determined by the ratio of the corrosion inhibitor (A) to the polyphosphate (B) in mass ratio of A
/ B = 60/40 to 5/95, and the total amount of the corrosion inhibitor (A) and the polypinate (B) is adjusted to the resin component 10
The amount is preferably from 5 to 150 parts by mass with respect to 0 parts by mass. If the amount of the polyphosphate (B) is less than 60/40, the effect of suppressing elution of calcium ions is reduced, and wet blisters are easily generated in the coating film. If it is more than 5/95, the corrosion inhibitor is insufficient. Therefore, the corrosion resistance of the coating film is reduced. Since the amount of calcium ions bonded to the corrosion inhibitor is generally about 3 to 40% of the particles, the use of a corrosion inhibitor having a large amount of calcium ions requires a high proportion of polyphosphate to be added. The addition ratio may be adjusted as appropriate, such as lowering the ratio. When the total amount of the corrosion inhibitor and the polyphosphate is less than 5 parts by mass, the corrosion resistance of the coating film decreases, and when the total amount exceeds 150 parts by mass, the pigment concentration becomes too high, and the processability and adhesion of the coating film are increased. Is reduced.

(Substrate, Others) Examples of the steel sheet used as the substrate of the precoated steel sheet of the present invention include hot-dip galvanizing, alloyed hot-dip galvanizing, 5% aluminum-zinc plating, 6% aluminum-3% magnesium- A galvanized steel sheet such as a galvanized sheet or a zinc-aluminum-based plated steel sheet such as a 55% aluminum-zinc plated sheet is preferable. Before performing the chemical conversion treatment, the base steel sheet may be subjected to alkali degreasing or surface conditioning treatment. Alkaline degreasing and surface conditioning may be those commonly used in the production of pre-coated steel sheets. For example, the surface conditioning is nickel or cobalt deposition type acidic surface conditioning, so that the adhesion amount becomes 1 to 50 mg / m ^2. Is preferred.

The resin system used for the undercoat film may be a known system such as a polyester system, a polymer polyester system, an epoxy system, an epoxy-modified polyester system, an epoxy-modified polymer polyester system, and a polyethersulfone system. Various additives such as pigments such as titanium oxide, calcium carbonate, and silica can be added to these undercoat paints in addition to the rust inhibitor. The molecular weight, the glass transition temperature, the amount of the pigment to be added, and the like of the coating resin may be appropriately adjusted depending on the application of the coated steel sheet. For example, in order to enhance workability, it is preferable to use an undercoat resin having a high elongation (about 50% or more) or a low glass transition temperature (about 40 ° C. or less).

The undercoat film thickness is 1
About 15 μm is sufficient. The product configuration may be three coats provided with an intermediate coating layer or one coat. Painting·
The baking method may be a known method such as roll coating in a continuous coating facility. When this precoated steel sheet is processed into a microwave oven bottom plate, it is processed by press working, caulking, or bending in the same manner as a normal precoated steel sheet.

[0024]

EXAMPLE A hot-dip galvanized steel sheet having a thickness of 0.5 mm and a coating amount of 45 g / m ^{2 on} one side was subjected to an alkali degreasing treatment, and then a chromate treatment was performed at a coating amount of 40 mg / m ² (shown as Chromate in Table 2). Alternatively, a chromium-free coating pretreatment consisting of fluorotitanic acid, colloidal silica, zirconium hydroxide, and a water-soluble resin is performed by adding titanium + zirconium in an amount of 1
0 mg / m ² , and 40 mg / m ^{2 in} terms of fluorine adhesion amount (in Table 2,
(Displayed as non-chromium), dried at 100 ° C., and subjected to pre-coating treatment.

An epoxy-modified high-molecular polyester-based primer obtained by adding 30 parts by mass of strontium chromate to 100 parts by mass of the resin (indicated as chromium-based in Table 2), or dihydrogen tripolyphosphate to calcium ion-exchanged silica A thermosetting epoxy-modified polymer polyester resin paint (indicated as non-chromium in Table 2) obtained by adding 30 parts by mass of 100 parts by mass of a resin in which aluminum was blended in a ratio of 25/75 was dried. And paint so that the maximum temperature is 215 ° C.
Bake for 0 seconds. In addition, Table 1 below shows (A) to (E)
Is applied so that the dry film thickness is 15 μm, and the maximum attained plate temperature is 230 ° C.
And baked for 40 seconds.

[0026]

The storage elastic modulus E 'is as follows: film thickness 20 μm, width 5
Using a free coating film of mm, the frequency was measured at 10 Hz using a rheograph solid manufactured by Toyo Seiki. This is shown in Table 2.
A sample was prepared with the combination of and the following performance was evaluated. The evaluation method of each characteristic is shown below. (1) Abrasion of roller mounting ring Wearing a cooking dish with 6 kg of weight on a commercially available roller mounting ring for a microwave oven, place at 20 ° C., 100 ° C., 150
After rotating at 240 ° C. for 240 hours, the abrasion state of the coating film was evaluated according to the following evaluation criteria. <Appearance of coating film> ；: No abnormality. Δ: The top coating film was worn and the gloss was remarkably shrunk. ×: The top coat was worn, and the undercoat was exposed.

(2) Workability The presence or absence of cracks in the coating film in the 10 ° 0T bent portion was evaluated using a 20-fold loupe. <Processed part coating crack> ○: No abnormality. Δ: Cracking of the coating film was observed in a part (less than 10% of the whole). ×: Cracking of the coating film was observed in 10% or more of the whole. The evaluation results are shown in Table 2.

When the storage elastic modulus E 'and the coating film elongation are within the ranges defined in the claims, the roller mounting ring has excellent wear resistance and excellent workability. On the other hand, in Test Nos. 7, 8 and 9 in which the storage elastic modulus E ′ was smaller than the specified value, the roller mounting ring abrasion resistance was not sufficient. Further, in Test No. 9, which also falls outside the range in terms of the coating film elongation, the workability was not sufficient.

(3) Food stain resistance Coffee and Worcester sauce are dropped on the surface of the coated film,
After heating at 0 ° C. × 1 hour, the surface was washed with water and the appearance change was observed. <Appearance of coating film> A: Almost no dirt or stain. ；: Some stains and stains are observed. Δ: Medium dirt and stains are observed. X: remarkable stain and peeling of the coating film are observed.

[0031]

[0032]

As described above, according to the present invention, by using a resin having a large coating film elastic modulus and a large coating film elongation ratio as a resin itself, without using a lubricant or the like, It was possible to obtain a precoated steel sheet suitable for a microwave oven bottom plate with improved resistance to roller mounting ring wear and good workability.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) B05D 7/14 B05D 7/14 J 7/24 303 7/24 303A F-term (Reference) 4D075 AE03 CA02 DA06 DB02 DB02 DC19 EA05 EB01 EB16 EB35 EB45 EB56 EB60 EC02 EC03 EC15 EC54 4F100 AA04C AA04H AA05B AA20C AA20H AB03A AB10A AB18A AK01C AK01D BA02 BA04 BA10A BA10D CA14C CA30C CC00C CC00D DE01C E00B DEC

Claims (4)

[Claims] 1. The steel sheet has a coating film elongation at 20 ° C. of 2
A precoated steel sheet for a bottom plate of a microwave oven, wherein an overcoat film having a storage elastic modulus E ′ of 0% or more and 20 to 150 ° C. of 2 × 10 ¹ N / mm ² or more is formed. 2. The precoated steel sheet for a microwave oven bottom plate according to claim 1, wherein the steel sheet has been subjected to a chrome-free coating pretreatment and / or a chromium-free primer. 3. The steel sheet is made of Zn-based, Al-based or Z-based steel.
On the surface of the n-Al-based plated steel sheet, a non-chromium-based chemical conversion treatment layer containing one or more kinds of fluoroacids is coated with 0.5 to 500 mg / m ^{2 in} terms of fluorine adhesion and / or 0.1 in total metal adhesion. ５００500 mg / m ² ,
Further, 2 to 50 parts by weight of a corrosion inhibitor (A) obtained by binding calcium ions for corrosion inhibition to porous silica particles by ion exchange is added to 100 parts by weight of the resin component of the coating film. The polyphosphate (B) is used as the membrane resin, and the mixing ratio of A and B is A / B = 60/40 to 5/95 by mass ratio, and the corrosion inhibitor (A) and the polypinate (B) are used.
Is 5 to 1 with respect to 100 parts by mass of the resin.
The precoated steel sheet for a bottom plate of a microwave oven according to claim 1 or 2, wherein the precoated steel sheet has a base coat layer added so as to be 50 parts by mass or less. 4. A microwave oven bottom plate using the precoated steel sheet according to claim 1.