AQUEOUS COATING COMPOSITION AND COATED METAL SUBSTRATE

Abstract

An aqueous coating composition and a coated metal sheet having a coating formed of the aqueous coating composition. The aqueous coating composition includes a base resin of a polyester resin having an acid value of not lower than 10 mgKOH/g and lower than 70 mgKOH/g and a glass transition temperature of not lower than 35° C., and the composition contains β-hydroxyalkylamide compound as a curing agent. The equivalent ratio of the hydroxyl group of the β-hydroxyalkylamide compound to the carboxyl group of the polyester resin (OH group/COOH group molar ratio) is in the range of 0.8 to 3.0.

Claims

1. An aqueous coating composition containing a polyester resin as a base resin having an acid value of not less than 10 mgKOH/g and less than 70 mgKOH/g and a glass transition temperature of not lower than 35° C., and a β-hydroxyalkylamide compound as a curing agent, wherein the β-hydroxyalkylamide compound contains a hydroxyl group at an equivalent ratio (OH group/COOH group molar ratio) in a range of 0.8 to 3.0 to a carboxyl group of the polyester resin.

2. The aqueous coating composition according to claim 1, wherein the polyester resin is either an acrylic unmodified polyester resin or an acrylic modified polyester resin having a content (modified amount) of an acrylic resin component of less than 10% by mass.

3. The aqueous coating composition according to claim 1, wherein the β-hydroxyalkylamide compound has a solid content blend amount in a range of 1 to 20 parts by mass relative to 100 parts by mass of a solid content of the polyester resin.

4. The aqueous coating composition according to claim 1, wherein 100 to 80 mol % of polyvalent carboxylic acid component constituting the polyester resin is an aromatic dicarboxylic acid, and 0 to 20 mol % is an aliphatic dicarboxylic acid.

5. The aqueous coating composition according to claim 4, wherein the molar rate of the aliphatic dicarboxylic acid having more than 6 carbon atoms is less than 10 mol % in the polyvalent carboxylic acid component constituting the polyester resin.

6. A coated metal substrate having a coating formed of the aqueous coating composition according to claim 1.

7. The coated metal substrate according to claim 6, wherein the coating has a thickness of not less than 2 μm and less than 15 μm.

Description

EXAMPLES

[0079] Hereinafter, the present invention will be specified below with reference to Examples. In the Examples, the term “part” indicates “part by mass”.

[0080] The respective measurement items for the polyester resins A-L are described below. All of the polyester resins A-L are acrylic unmodified polyester resins.

(Measurement of Number Average Molecular Weight)

[0081] Solid polyester resins were measured using gel permeation chromatography (GPC) based on a calibration curve of standard polystyrene.

(Measurement of Glass Transition Temperature)

[0082] Solid polyester resins were measured using a differential scanning calorimeter (DSC).

(Measurement of Acid Value)

[0083] 1.0 g of polyester resin was dissolved in 10 ml of chloroform, which was titrated with a 0.1 N KOH-ethanol solution to determine its resin acid value (mgKOH/g). Here, phenolphthalein was used as the indicator. A solvent such as tetrahydrofuran was used for a polyester resin not dissoluble in the chloroform.

(Measurement of Monomer Composition)

[0084] 30 mg of solid polyester resin was dissolved in 0.6 ml of deuterated chloroform, which was subjected to a 1H-NMR measurement, and the monomer composition ratio was determined from the peak intensity. The composition ratio was determined except for trace components (less than 1 mol % relative to all of the monomer components).

[Preparation of Coated Metal Substrate (Coated Metal Sheet)]

[0085] Each of the aqueous coating compositions in Examples, Comparative Examples and Reference Examples was prepared by coating a metal sheet by use of a bar coater so as to have a thickness of 9 μm after baking, and baked at 250° C. for 60 seconds. The metal sheet used here was a phosphate chromate-based surface-treatment aluminum sheet (3104 alloy, thickness: 0.28 mm, weight of chromium in the surface-treatment film: 20 mg/m.sup.2).

(Flavor Sorption Test)

[0086] A test piece having a size of 2.5×5 cm (surface area of the coating: 12.5 cm.sup.2) was prepared from each of the coated metal sheets obtained in Examples 1-14, Comparative Examples 1-3 and Reference Examples 1-4. As a model flavor test solution, a 5% aqueous ethanol solution containing 2 ppm of limonene was prepared. The model flavor test solution was placed in a glass bottle with a packing (DURAN bottle), and the test piece was immersed in the solution. The bottle was sealed and stored at 30° C. for two weeks. The test piece was then taken out from the glass bottle, and washed with water. After removing the water droplets, the test piece was immersed in 50 ml of diethyl ether, sealed, and stored at room temperature for 24 hours. The extract was concentrated in a concentrator and subjected to GC/MS (gas chromatography-mass spectrometry). From the component peaks derived from limonene obtained from the GC/MS, the sorption amount was determined by a calibration curve, and the ratio of the sorption amount to the limonene charge amount was determined as the limonene sorption rate (%) from the following Equation (2). The results are shown in Table 1 and Table 2.

Limonene sorption rate (%)=limonene sorption amount/limonene charge amount×100  (2)

[0087] The evaluation criteria are as follows.

⊚: Limonene sorption rate is less than 5%
◯: Limonene sorption rate is 5% or more and less than 15%
Δ: Limonene sorption rate is 15% or more and less than 25%
X: Limonene sorption rate is 25% or more

(Retort Whitening Resistance)

[0088] Each of the coated metal sheets obtained in Examples 4, 5 and 15-26, Comparative Examples 4-11, and Reference Examples 5 and 6 was placed in an upright state inside a stainless cup, and ion-exchanged water was poured into this cup to a height half the coated metal sheet. This was placed in an autoclave and subjected to a retort treatment at 125° C. for 30 minutes. After the treatment, the coated metal sheet was taken out and air-dried, and then, the whitened state (whitening) of the portion of the coating, i.e., the portion immersed in water was visually evaluated. The results are shown in Tables 1, 3, and 4.

[0089] The evaluation criteria are as follows.

◯: Not whitened
Δ: Slightly whitened
X: Significantly whitened

(Curability)

[0090] The curability of the coated metal sheet was evaluated by MEK extraction rate. Specifically, a test piece of 5 cm×5 cm was cut out from a coated metal sheet. After measuring the mass (W1), the test piece was immersed in 200 ml of boiling MEK (methyl ethyl ketone) for 1 hour under 80° C. reflux so as to conduct a one-hour MEK extraction at the boiling point. After the extraction, the test piece was washed and dried, and then, its mass after extraction (W2) was measured. Further, the coating was delaminated by a decomposition method using concentrated sulfuric acid, washed and dried, and the mass (W3) of the test piece was measured. The MEK extraction rate (% by mass) of the coating of the coated metal sheet can be determined by the following Equation (3).

MEK extraction rate (%)=100×(W1−W2)/(W1−W3)  (3)

[0091] Evaluation criteria of the coated metal sheets obtained in Examples 1-14, Comparative Examples 1-3, and Reference Examples 1-4 are as follows. The results are shown in Tables 1 and 2.

⊚: Less than 10%
◯: 10% or more and less than 20%
Δ: 20% or more and less than 30%
X: 30% or more

[0092] Evaluation criteria of the coated metal sheets obtained in Examples 15-26, Comparative Examples 4-11 and Reference Examples 5 and 6 are as follows. The results are shown in Tables 3 and 4.

◯: Less than 20%
Δ: 20% or more and less than 30%
X: 30% or more

(Workability)

[0093] A coated metal sheet was cut out to make a test piece of 3.5×3 cm such that the long side thereof would correspond to the direction for rolling the aluminum sheet. This test piece was bent parallel to the short sides such that the coated surface would make the outer surface. Two aluminum sheets having the same thickness as that of the coated metal sheet were sandwiched as spacers inside the bent test piece. Later, a bending process was carried out under an atmosphere of 25° C. by dropping a weight of 3 kg from the height of 40 cm. The bent tip portion (2 cm width) was brought into contact with a sponge soaked in a 1% aqueous sodium chloride solution, energized at a voltage of 6.3 V for 4 seconds, and the current value (mA) after 4 seconds was measured.

[0094] Evaluation criteria of the coated metal sheets obtained in Examples 1-14, Comparative Examples 1-3 and Reference Examples 1-4 are as follows. The results are shown in Tables 1 and 2.

⊚: Less than 1 mA
◯: 1 mA or more and less than 3 mA
Δ: 3 mA or more and less than 10 mA
X: 10 mA or more

[0095] Evaluation criteria of the coated metal sheets obtained in Examples 15-26, Comparative Examples 4-11 and Reference Examples 5 and 6 are as follows. The results are shown in Tables 3 and 4.

◯: Less than 1 mA
Δ: 1 mA or more and less than 3 mA
X: 3 mA or more

(Preparation of Aqueous Coating Composition)

Example 1

[0096] The polyester resin as the base resin was polyester resin-A (acid value: 23 mgKOH/g, Tg: 80° C., Mn=8,000, monomer composition: terephthalic acid component/ethylene glycol component/propylene glycol component=50/10/40 mol %). The β-hydroxyalkylamide compound used as the curing agent was N,N,N′,N′-tetrakis(2-hydroxypropyl)adipoamide (CAS: 57843-53-5, hydroxyl value (theoretical value): 596 mgKOH/g, and expressed as “β-hydroxyalkylamide” in the Table). 333 parts (solid content: 100 parts) of an aqueous dispersion liquid of the polyester resin-A (resin solid content concentration: 30% by mass, isopropyl alcohol concentration: 18% by mass) and 16.7 parts (solid content: 5 parts) of the aqueous solution of the β-hydroxyalkylamide compound (solid content concentration: 30% by mass) that had been adjusted using ion-exchanged water were introduced into a glass container and stirred for 10 minutes to obtain an aqueous coating composition having a solid content concentration of 30% by mass and a solid content blend ratio of polyester resin/curing agent of 100/5 (mass ratio).

Examples 2-5

[0097] Aqueous coating compositions were prepared as shown in Table 1 in the same manner as in Example 1 except that the polyester resin was replaced by polyester resin-B (Tg: 67° C., Mn=9,000, acid value: 18 mgKOH/g, monomer composition: terephthalic acid component/isophthalic acid component/ethylene glycol component/neopentyl glycol component=36/14/24/26 mol %), polyester resin-C (Tg: 40° C., Mn=9,000, acid value: 17 mgKOH/g, monomer composition: terephthalic acid component/isophthalic acid component/adipic acid component/ethylene glycol component/neopentyl glycol component=28/15/7/25/25 mol %), polyester resin-E (acid value: 50 mgKOH/g, Tg: 46° C., Mn=3,000) or polyester resin-F (acid value: 58 mgKOH/g, Tg: 70° C., Mn: 3,000).

Example 6

[0098] An aqueous coating composition was prepared in the same manner as in Example 1 except that the polyester resin used here was a mixture (Tg.sub.mix: 65.7° C., AV.sub.mix: 21.8 mgKOH/g) of polyester resin-A and polyester resin-G (Tg: −25° C., Mn=17,000, acid value: 11 mgKOH/g, monomer composition: terephthalic acid component/isophthalic acid component/sebacic acid component/1,4-butanediol component=14/17/19/50 mol %) mixed at a mass ratio of 90:10.

Examples 7-14, Comparative Example 1

[0099] Aqueous coating compositions were prepared in the same manner as in Example 6 except that the polyester resins or the solid content blend ratios were changed as shown in Table 1. The polyester resins other than the aforementioned ones were: polyester resin-H (Tg: 8° C., Mn=19,000, acid value: 8 mgKOH/g, monomer composition: terephthalic acid component/isophthalic acid component/sebacic acid component/ethylene glycol component/neopentyl glycol component=30/5/15/22/28 mol %); polyester resin-I (Tg: 20° C., Mn=17,000, acid value: 8 mgKOH/g, monomer composition: terephthalic acid component/isophthalic acid component/sebacic acid component/ethylene glycol component/neopentyl glycol component=31/7/12/30/20 mol %); polyester resin-J (acid value: 5 mgKOH/g, Tg: 52° C., Mn=17,000, monomer composition: terephthalic acid component/isophthalic acid component/adipic acid component/ethylene glycol component/neopentyl glycol component=23/23/4/24/26 mol %), and polyester resin-D (acid value: 36 mg KOH/g, Tg: 80° C., Mn=5,000).

Comparative Examples 2 and 3

[0100] Aqueous coating compositions were prepared in the same manner as in Example 1 except that the polyester resins used here or the solid content blend ratios were changed as shown in Table 1. The polyester resins were polyester resin-K (acid value: 8 mgKOH/g, Tg: 65° C., Mn=13,000, monomer composition: terephthalic acid component/isophthalic acid component/ethylene glycol component/neopentyl glycol component=26/24/21/29 mol %) and polyester resin-L (acid value: 74 mgKOH/g, Tg: 80° C., Mn=3,000).

Reference Examples 1 and 2

[0101] Aqueous coating compositions were prepared in the same manner as in Example 1 except that the polyester resin used here was polyester resin-M, which was an acrylic modified polyester resin modified with an acrylic resin (Tg: 60° C., acid value: 15 mgKOH/g, content of acrylic component: 70% by mass), or polyester resin-N (Tg: 50° C., acid value: 30 mgKOH/g, content of acrylic component: 40% by mass).

Reference Example 3

[0102] An aqueous coating composition was prepared in the same manner as in Example 1 except that the polyester resin used here was a mixture (Tg.sub.mix: 37.4° C.) of polyester resin-A and polyester resin-O (acid value: less than 3 mgKOH/g, Tg: 4° C., Mn=8,000) mixed at a mass ratio of 50:50.

Reference Example 4

[0103] An aqueous coating composition (solid content concentration: 30% by mass, solid content blend ratio: polyester resin/curing agent/curing catalyst=100/10/1) was prepared by using a mixture of polyester resin-A and polyester resin-G mixed at a mass ratio of 80:20 (AV.sub.mix: 20.6 mgKOH/g), a curing agent of a meta cresol-based resol-type phenol resin obtained by butyl etherification of a methylol group (rate of etherified methylol group: 90 mol %, Mn=1,600), and a curing catalyst of dodecylbenzenesulfonic acid (amine neutralized product). For the dodecylbenzenesulfonic acid, “dodecylbenzenesulfonic acid (soft type) (mixture)” manufactured by Tokyo Chemical Industry Co., Ltd. was used.

Examples 15-26, Comparative Examples 4-11

[0104] An aqueous coating composition was prepared in the same manner as in Example 1, except that the type of polyester resin or the solid content blend ratio was changed as shown in Tables 3 and 4. The polyester resin other than the polyester resins A-O was polyester resin-P (acrylic unmodified polyester resin, acid value: 31 mgKOH/g, Tg: 38° C., Mn=6,000).

Reference Example 5

[0105] An aqueous coating composition was prepared in the same manner as in Example 1, except that the polyester resin used here was polyester resin-N, which is an acrylic-modified polyester resin modified with an acrylic resin (Tg: 50° C., acid value: 30 mgKOH/g, content of acrylic component: 40% by mass).

Reference Example 6

[0106] An aqueous coating composition was prepared in the same manner as in Example 1, except that the polyester resin used here was a mixture (Tg.sub.mix: 39° C.) of polyester resin-P and polyester resin-Q (acid value: less than 3 mgKOH/g, Tg: 40° C., Mn: 20,000) mixed at a mass ratio of 50:50.

[0107] Tables 1 and 2 indicate items for the aqueous coating compositions (type of polyester resin, type of curing agent, solid content blend ratio, molar ratio of polyvalent carboxylic acid component constituting polyester resin, and functional group equivalent ratio) and evaluation results for flavor sorption resistance, curability and workability, and retort whitening resistance (this is involved only in Examples 4 and 5). Here, Table 1 refers to Examples 1-14, and Table 2 refers to Comparative Examples 1-3 and Reference Examples 1-4.

[0108] Tables 3 and 4 indicate items for the aqueous coating compositions (type of polyester resin, type of curing agent, solid content blend ratio, and functional group equivalent ratio) and evaluation results for retort whitening resistance, curability and workability. Here, Table 3 refers to Examples 15-26, and Table 4 refers to Comparative Examples 4-11 and Reference Examples 5 and 6.

TABLE-US-00001 TABLE 1 Tg Acid value Examples (° C.) (mgKOH/g) Mn 1 2 3 4 5 6 7 Aqueous coating Polyester A 80 23 8,000 100 90 80 composition resin B 67 18 9,000 100 blend ratio C 40 17 9,000 100 D 80 36 5,000 E 46 50 3,000 100 F 70 58 3,000 100 G −25 11 17,000 10 20 H 8 8 19,000 I 20 8 17,000 J 52 5 17,000 Curing agent β-hydroxyalkylamide 5 5 5 13 15 5 5 Polyester resin Tg.sub.mix (° C.) — — — — — 65.7 52.4 properties AV.sub.mix (mgKOH/g) — — — — — 21.8 20.6 Aromatic dicarboxylic acid:aliphatic 100:0 100:0 86:14 92:8 100:0 96.6:3.4 93.1:6.9 dicarboxylic acid (molar ratio) Rate of aliphatic dicarboxylic acid having more 0 0 0 8 0 3.4 6.9 than 6 carbon atoms (mol %) Functional group equivalent 1.30 1.66 1.75 1.55 1.49 1.37 1.45 ratio (OH group/COOH group molar ratio) Evaluation Flavor sorption ⊚ ⊚ ◯ ◯ ⊚ ⊚ ◯ Curability ⊚ ◯ ◯ ⊚ ◯ ⊚ ◯ Workability ⊚ ⊚ ⊚ ◯ ◯ ⊚ ⊚ Retort whitening resistance — — — ◯ Δ — — Tg Acid value Examples (° C.) (mgKOH/g) Mn 8 9 10 11 12 13 14 Aqueous coating Polyester A 80 23 8,000 70 70 80 60 40 50 composition resin B 67 18 9,000 blend ratio C 40 17 9,000 D 80 36 5,000 80 E 46 50 3,000 F 70 58 3,000 G −25 11 17,000 30 20 H 8 8 19,000 30 I 20 8 17,000 20 40 60 J 52 5 17,000 50 Curing agent β-hydroxyalkylamide 5 5 5 5 5 5 5 Polyester resin Tg.sub.mix (° C.) 40.2 54.8 65.6 52.3 40 65.4 52.4 properties AV.sub.mix (mgKOH/g) 19.4 18.5 20 17 14 14 31 Aromatic dicarboxylic acid:aliphatic 89.5:10.5 91.5:8.5 95.3:4.7 90.6:9.4 85.8:14.2 96.4:3.6 92.4:7.6 dicarboxylic acid (molar ratio) Rate of aliphatic dicarboxylic acid having more 10.5 8.5 4.7 9.4 14.2 0 7.6 than 6 carbon atoms (mol %) Functional group equivalent 1.54 1.61 1.49 1.75 2.13 2.13 0.96 ratio (OH group/COOH group molar ratio) Evaluation Flavor sorption Δ ◯ ⊚ ◯ Δ ⊚ ◯ Curability ◯ ◯ ⊚ ◯ ◯ Δ ⊚ Workability ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ Retort whitening resistance — — — — — — —

TABLE-US-00002 TABLE 2 Tg Acid value Comparative Examples Reference Examples (° C.) (mgKOH/g) M1) n 1 2 3 1 2 3 4 Aqueous coating Polyester A 80 23 8,000 60 50 80 composition resin G −25 11 17,000 40 blend ratio K 65 8 13,000 100 L 80 74 3,000 100 M 60 15 — 100 N 50 30 — 100 O 4 <3 20,000 50 Curing β-hydroxyalkylamide 5 5 5 5 5 5 agent resol-type phenolresion 10 Polyester Tg.sub.mix (° C.) 28.9 — — — — 37.4 52.4 resin AV.sub.mix (mgKOH/g) 18.2 — — — — — 20.6 properties Aromatic dicarboxylic acid:aliphatic dicarboxylic 85.9:14.1 100:0 100:0 — — — 93.1:6.9 acid (molar ratio) Rate of aliphatic dicarboxylic acid having more 14.1 0 0 — — — 6.9 than 6 carbon atoms (mol %) Functional group equivalent 1.64 3.73 0.40 1.99 0.99 — — ratio (OH group/COOH grup molar ratio) Evaluation Flavor sorption X ⊚ ⊚ ⊚ ◯ Unevaluated Unevaluated Curability Δ X ◯ Δ ◯ X ◯ Workability ⊚ ⊚ X X X Unevaluated X

TABLE-US-00003 TABLE 3 Tg Acid value Examples (° C.) (mgKOH/g) Mn 15 16 17 18 19 20 21 Aqueous coating Polyester C 40 17 9,000 100 100 100 100 100 100 composition resin B 67 18 9,000 100 blend ratio A 80 23 8,000 P 38 31 6,000 D 80 36 5,000 Curing agent β-hydroxyalkylamide 3 4.1 4.6 6 6.5 8 5 Functional group equivalent 1.05 1.44 1.61 2.10 2.28 2.80 1.66 ratio (OH group/COOH group molar ratio) Evaluation Retort whitening resistance ◯ ◯ ◯ ◯ Δ Δ ◯ Curability Δ ◯ ◯ ◯ ◯ Δ ◯ Workability ◯ ◯ ◯ ◯ ◯ ◯ ◯ Tg Acid value Examples (° C.) (mgKOH/g) Mn 22 23 24 25 26 Aqueous coating Polyester C 40 17 9,000 composition resin B 67 18 9,000 blend ratio A 80 23 8,000 100 100 P 38 31 6,000 100 100 D 80 36 5,000 100 Curing agent β-hydroxyalkylamide 6 8 8 12 9 Functional group equivalent 1.55 2.07 1.54 2.31 1.49 ratio (OH group/COOH group molar ratio) Evaluation Retort whitening resistance ◯ ◯ ◯ Δ ◯ Curability ◯ ◯ ◯ ◯ ◯ Workability ◯ ◯ ◯ ◯ ◯

TABLE-US-00004 TABLE 4 Tg Acid value Comparative Examples (° C.) (mgKOH/g) Mn 4 5 6 7 8 9 Aqueous coating Polyester C 40 17 9,000 100 100 100 composition resin A 80 23 8,000 100 blend ratio P 38 31 6,000 100 E 46 50 3,000 100 J 52 5 17,000 L 80 74 3,000 Q 40 <3 20,000 N 50 30 — Curing agent β-hydroxyalkylamide 1 2 10 12 3 4 Functional group equivalent 0.35 0.70 3.51 3.11 0.58 0.48 ratio (OH group/COOH group molar ratio) Evaluation Retort whitening resistance X X X X X X Curability X X X ◯ X X Workability X X Δ ◯ X X Tg Acid value Comparative Examples Reference Examples (° C.) (mgKOH/g) Mn 10 11 5 6 Aqueous coating Polyester C 40 17 9,000 composition resin A 80 23 8,000 blend ratio P 38 31 6,000 50 E 46 50 3,000 J 52 5 17,000 100 L 80 74 3,000 100 Q 40 <3 20,000 50 N 50 30 — 100 Curing agent β-hydroxyalkylamide 1.3 15 8 5 Functional group equivalent 1.55 1.19 1.59 — ratio (OH group/COOH group molar ratio) Evaluation Retort whitening resistance X Δ Δ X Curability X ◯ ◯ Unevaluated Workability ◯ X X Unevaluated

INDUSTRIAL APPLICABILITY

[0109] The aqueous coating composition of the present invention is capable of forming a coating excellent in coating properties such as curability and workability, which is also particularly excellent in flavor sorption resistance and retort whitening resistance, so that the composition can be suitably used to produce a metal can to be filled with a beverage or the like containing an aroma component or a content that requires retort sterilization, or a metal lid, for instance.