Coating composition and coated metal plate, metal container and metal closure coated with the coating composition

Abstract

The present invention relates to a polyester coating composition used for forming a coating film of a coated metal plate, a metal container, a metal closure, etc. The coating composition has a crosslinking agent and a curing catalyst incorporated into a mixed polyester resin comprising a mixture of a polyester resin (A) having an acid value of 2 to 50 mg KOH/g and a glass transition temperature (Tg) of 35 to 100 C. and a polyester resin (B) having an acid value of 0 to 50 mg KOH/g and a glass transition temperature (Tg) of 20 to 25 C. Thus, a coating composition excellent in resistance to embrittlement over time, corrosion resistance, scrape resistance, retort resistance, and blocking resistance can be provided.

Claims

1. A coating composition consisting essentially of a crosslinking agent, a curing catalyst and a mixed polyester resin, the mixed polyester resin consisting essentially of a mixture of a polyester resin (A) and a polyester resin (B) at a weight ratio (A):(B)=98:2 to 50:50, the polyester resin (A) is a polyester resin provided with an acid value resulting from a ring-opening addition reaction of a compound having one or more carboxylic acid anhydride structures in a molecule, and when a total content of carboxylic acid anhydrides to be used for the ring-opening addition reaction is 100 mol %, 10 mol % or more of the carboxylic acid anhydrides are compounds having two or more carboxylic acid anhydride groups in a molecule, wherein the polyester resin (A) has an acid value of 2 to 45 mg KOH/g and a glass transition temperature (Tg) of 35 to 90 C., and the polyester resin (B) has an acid value of 2 to 40 mg KOH/g and a glass transition temperature (Tg) of -20 to 25 C., the crosslinking agent is a resol type phenolic resin and/or an amino resin, and is incorporated in an amount of 1 to 30 parts by weight relative to 100 parts by weight of a resin solid content of the mixed polyester resin, the curing catalyst is incorporated in an amount of 0.1 to 3.0 parts by weight relative to 100 parts by weight of a total resin solids content, the polyester resin (A) has a number average molecular weight of 8,000 to 21,000, the polyester resin (B) has a number average molecular weight of 5,000 to 25,000, the mixed polyester resin has a glass transition temperature (Tgmix) calculated from the following Equation (1) of 40 to 80 C.:
1/Tgmix=Wa/Tga+Wb/Tgb . . . (1) where Tgmix represents the glass transition temperature (K) of the mixed polyester resin, Tga represents the glass transition temperature (K) of the polyester resin (A), Tgb represents the glass transition temperature (K) of the polyester resin (B), Wa represents a weight fraction of the polyester resin (A), and Wb represents a weight fraction of the polyester resin (B), the polyester resin (A) comprises a carboxylic acid component selected from the group consisting of aromatic dicarboxylic acids, aliphatic dicarboxylic acids, unsaturated dicarboxylic acids, alicyclic dicarboxylic acids, trivalent or higher valent carboxylic acids and monocarboxylic acids and an alcohol component selected from the group consisting of aliphatic glycols, ether glycols, alicyclic polyalcohols and trivalent or higher valent polyalcohols, the polyester resin (B) comprises a carboxylic acid component selected from the group consisting of aromatic dicarboxylic acids, aliphatic dicarboxylic acids, unsaturated dicarboxylic acids, alicyclic dicarboxylic acids, trivalent or higher valent carboxylic acids and monocarboxylic acids and an alcohol component selected from the group consisting of aliphatic glycols, ether glycols, alicyclic polyalcohols and trivalent or higher valent polyalcohols, and the polyesters (A) and (B) are non-crystalline polyesters.

2. A coated metal plate coated with the coating composition according to claim 1.

3. The coated metal plate according to claim 2 having the coating composition coil baked.

4. A metal container having a coating film formed thereon, the coating film comprising the coating composition according to claim 1.

5. A metal closure having a coating film formed thereon, the coating film comprising the coating composition according to claim 1.

Description

EXAMPLES

(1) The present invention will be described more specifically by reference to examples, in which parts refer to parts by weight.

(2) Respective items were measured by the following methods:

(3) (1) Number Average Molecular Weight of Polyester Resin

(4) Measured by gel permeation chromatography (GPC) using the calibration curve of standard polystyrene.

(5) (2) Glass Transition Temperature

(6) Measured at a temperature increase rate of 20 C./min using a differential scanning calorimeter (DSC).

(7) (3) Acid Value

(8) 0.2 g of polyester was dissolved in 20 ml of chloroform, titrated with a 0.1N KOH-ethanol solution to obtain the acid value of the resin (mgKOH/g). Phenolphthalein was used as an indicator.

(9) (4) Preparation of Test Coated Plate

(10) The coating composition of each of the Examples and Comparative Examples was coated on a #5182 aluminum sheet with a thickness of 0.28 mm using a bar coater to form a film (dry film weight 100 mg/100 cm.sup.2), and the film was baked by a coil oven. The baking conditions were a peak metal temperature (PMT) of 260 C. for the aluminum sheet, and a total baking time (TOT) of 28 seconds. The resulting coated plates were each subjected to various tests according to the test methods explained below.

(11) (4)-1 Resistance to Embrittlement Over Time

(12) Resistance to embrittlement over time was evaluated in terms of initial processability versus processability over time.

(13) Processability:

(14) The test coated plate was cut into a size of 35 cm such that the side of the aluminum sheet in its rolling direction would be the long side of the resulting test piece. The test piece was then folded in parallel to its short side such that its coated surface would face outward. Two aluminum sheets 0.28 mm thick were interposed inside the fold in an atmosphere of 25 C., and subjected to impact bending with the use of a seam folding type Du Pont impact tester. An iron weight weighing 3.5 kg and having a flat contact surface for impact bending was dropped from a height of 12.5 cm. A 2 cm width portion at the bent leading end was brought into contact with a sponge dipped in a 1% aqueous solution of sodium chloride, a voltage of 6.2 V was applied, and the current value (mA) of the 2 cm portion was measured 4 seconds later.

(15) Initial processability: Evaluated within 8 hours after preparation of the test coated plate.

(16) Processability over time: The test coated plate was placed in a thermostatic chamber at 40 C. within 8 hours after preparation, stored there for 1 month, and then evaluated as was initial processability.

(17) Evaluation criteria were as follows:

(18) (Excellent): Less than 0.5 mA.

(19) (Good): 0.5 mA to less than 1.0 mA.

(20) (Fair): 1.0 mA to less than 3 mA.

(21) X (Poor): 3 mA or more.

(22) (4)-2 Corrosion Resistance

(23) The coated plate having convexities formed by the Du Pont impact tester (impact bar inches, weight of 300 g was dropped from the height of 12.5 cm) was soaked in a contents-simulating liquid (citric acid/ethanol/water-5/10/85) at 40 C. for 7 days, and the degree of corrosion was visually evaluated.

(24) : No corrosion.

(25) : Little corrosion.

(26) : Slight corrosion.

(27) X: Marked corrosion.

(28) (4)-3 Retort Resistance

(29) The test coated plate was soaked in water, treated at 125 C. for 30 minutes in an autoclave, and then evaluated for blushing.

(30) Blushing: Evaluated visually by observing the state of blushing of the coating film.

(31) : No blushing.

(32) : Little blushing.

(33) : Slight blushing.

(34) X: Marked blushing.

(35) (4)-4 Blocking Resistance

(36) The coated surfaces of the coated plates each cut to a size of 88 cm were superposed on each other. The coated plates in this superposed state were placed under a pressure of 0.36 MPa at a temperature of 50 C. for 2 minutes by use of a thermocompression press, and then cooled to room temperature. Then, the degree of blocking between the respective coated surfaces was evaluated by peeling the coated plates from each other, beginning at their ends.

(37) : No blocking.

(38) : Slight blocking.

(39) X: Marked blocking.

(40) (4)-5 Scrape Resistance

(41) The slide tester HEIDON-14DR (produced by Shinto Scientific Co., Ltd.) was used. A ball bearing under a load was slid over the surface of the coating film, and the number of times the ball bearing was slid until the coating film was scraped to expose the aluminum substrate was counted for evaluation. The test conditions were a stainless steel ball with a diameter of 10 mm, a sliding speed of 6000 mm/min, a sliding distance of 10 mm, a load of 1 kg, and a coated plate temperature of 25 C.

(42) : 1,000 times or more.

(43) : 500 times to less than 1,000 times.

(44) : 100 times to less than 500 times.

(45) X: Less than 100 times.

(46) (5) Preparation of Closure

(47) A 206 diameter shell (a closure before mounting of an opening tab is referred to as a shell) having a depth, of a radius portion of a reinforced annular groove from the center panel, of 2.5 mm and a radius of curvature, of the radius portion, of 0.50 mm was formed from the aforementioned coated plate by a shell molding machine. The shell was scored (score remaining thickness 95 m), riveted, and mounted with an opening tab, from the closure outer side, to prepare a closure.

(48) (5)-1 Resistance of Closure to Embrittlement Over Time

(49) Processability over time: The coated plate after 1 month of storage at 40 C. was formed into a shell. The shell was coated with a compound, then stored for 2 weeks at 40 C., and then subjected to end formation steps (scoring, mounting of tab, etc.).

(50) The closure obtained as above was evaluated in the following manner:

(51) Five of the coated closures immediately after coating (within 8 hours at RT) and those after 1 month of storage (40 C.) were measured for current value, and the difference between the averages of the measured values was used for evaluation. The measurement conditions were energization at a voltage of 6.2 V for 4 seconds, followed by the measurement of the current values.

(52) : Within 0.2 mA (no increase in value).

(53) : Within 0.5 mA (no increase in value).

(54) : Within 1.0 mA (no increase in value).

(55) X: Increase in value.

(56) (5)-2 Corrosion Resistance of Closure

(57) A 350 g can formed by drawing and ironing a tin-free steel material having both surfaces laminated with PET was cold packed with Coca Cola (liquid temperature: 5 C., amount of air inside the can: 2 ml or less) under the following conditions: gas volume of 3.75, and amount of contents 345 g. Then, the packed can was warmed (40 C., 10 minutes). The so obtained sample was stored in an inverted posture (the closure facing downward) in a thermostatic chamber at 37 C. for 6 months. Then, the closure was opened, and the inner surface of the closure was observed. The observation was made by a stereoscopic microscope and a metallurgical microscope, and the presence of corrosion spots on, and the presence of perforations in, the inner surface of the closure were examined (n=5 for each item).

(58) : No corrosion or little corrosion (except score and rivet).

(59) X: Perforations or possibility of perforations.

(60) (5)-3 Retort Resistance of Closure

(61) A 350 g can formed by drawing and ironing a tin-free steel material laminated on both surfaces with PET was charged with 340 g of water. Liquid nitrogen was added dropwise so as to make the internal pressure 0.1 kgf/cm.sup.2, and then the can was seamed with an aluminum closure. The seamed can was subjected to stationary retort treatment with steam at 125 C. for 30 minutes, and cooled such that the internal liquid temperature fell to lower than 40 C. within 10 minutes after completion of heating.

(62) The closure was then removed, and the state of blushing on the inner surface of the closure was evaluated (n=3 for each condition).

(63) : No blushing.

(64) : Little blushing.

(65) : Slight blushing.

(66) X: Marked blushing.

(67) (5)-4 Scrape Resistance of Closure

(68) Continuous processing was carried out for 50 of the samples at each level by a conversion press, and the state of the coating film of the side wall of the rivet was visually observed. For damage to the coating film, the presence of metal exposure was judged by dipping the sample in a 20% aqueous solution of copper sulfate for 1 minute and examining Cu precipitation.

(69) : No damage to coating film.

(70) : Superficial damage (no metal exposure).

(71) X: Scrape occurred (metal exposed).

(72) [Synthesis of Polyester Resin (A)]

Synthesis Example

Polyester Resin (A)-a

(73) 446 parts of dimethylterephthalic acid, 2.2 parts of trimellitic acid, 57 parts of ethylene glycol, 246 parts of propylene glycol, 66 parts of 1,4-cyclohexanedimethanol, and 0.2 parts of titanium tetrabutoxide were charged into a 3 L four-necked flask to construct a polymerization system. The system was gradually heated to 220 C. over 4 hours to perform ester interchange. The pressure was reduced over 30 minutes to 10 mm Hg for initial polymerization, and the temperature was increased to 250 C. Under these conditions, postpolymerization was performed for 90 minutes at 1 mm Hg or lower. When the target molecular weight was reached, the contents were cooled to 220 C. in a nitrogen atmosphere. Then, 20 parts of ethylene glycol bistrimellitate dianhydride and 5 parts of trimellitic anhydride were successively charged into the system, and stirring was continued at 200 to 230 C. for 1 hour under a nitrogen atmosphere. The contents were then withdrawn to yield polyester resin (A)-a of the present invention. The Tg, acid value and number average molecular weight of this resin are shown in Table 1.

(74) [Synthesis Example of Polyester Resin (B)]

Synthesis Example

Polyester Resin (B)-a

(75) 106 parts of terephthalic acid, 225 parts of isophthalic acid, 4 parts of trimellitic anhydride, 143 parts of 2-methyl-1,3-propanediol, 86 parts of 1,4-butanediol, 92 parts of 1,4-cyclohexanedimethanol, and 0.13 parts of titanium tetrabutoxide were charged into a 2 L four-necked flask, and the temperature was gradually raised to 220 C. over 4 hours to distill off water and carry out esterification. After a predetermined amount of water was distilled off, the pressure was reduced over 30 minutes to 10 mmHg to perform initial polymerization, and the temperature was raised to 250 C. Under these conditions, postpolymerization was performed for 50 minutes at 1 mm Hg or lower. Then, the polymerization under reduced pressure was terminated, and the resulting resin was withdrawn to obtain polyester resin (B)-a of the present invention. The Tg, acid value, and number average molecular weight of the resin are shown in Table 2.

(76) Similarly, polyester resins (A)-b to (A)-i having the acid values and the Tgs shown in Table 1 and Table 2 were prepared, except that the carboxylic anhydride was used as an indispensable material, its amount was changed, and the types or amounts of the other monomers were changed. Also similarly, polyester resins (B)-b to (B)-g having the acid values and the Tgs shown in Table 1 and Table 2 were prepared, except that the types or amounts of the monomers were changed.

(77) TABLE-US-00001 TABLE 1 Polyester resin (A) Examples Comparative Examples (A)-a (A)-b (A)-c (A)-d (A)-e (A)-f (A)-g (A)-h (A)-i Tg ( C.) 90 40 55 45 50 102 30 53 43 Acid value 16 14 45 5 15 14 18 61 1 (mgKOH/g) Number average 15,000 20,000 8,000 15,000 17,000 18,000 16,000 6,000 22,000 molecular weight

(78) TABLE-US-00002 TABLE 2 Polyester resin (B) Examples Comparative Examples (B)-a (B)-b (B)-c (B)-d (B)-e (B)-f (B)-g Tg ( C.) 22 17 5 7 5 30 3 Acid value 15 10 40 1 15 10 60 (mgKOH/g) Number average 13,000 10,000 8,000 18,000 13,000 10,000 5,000 molecular weight
[Preparation of Coating Composition]

(79) Predetermined amounts of the polyester resin (A), the polyester resin (B) and a solvent were charged into a flask, heated, and thoroughly stirred at 100 C. or lower until being dissolved. After complete dissolution, the resulting polyester resin solution was cooled to room temperature and, with thorough stirring, a predetermined crosslinking agent and a predetermined curing catalyst were added to prepare a coating composition. The crosslinking agents used were Sumilite Resin PR-53893A (m-cresol resol type phenolic resin) of Sumitomo Durez Co., Ltd. and Mycoat 506 (butylated melamine resin) of Nihon Cytec Industries Inc. The curing catalyst used was Cycat 602 (amine-blocked dodecylbenzenesulfonic acid) of Nihon Cytec Industries Inc. Coated metal plates and coated metal closures were produced using the respective coating compositions, and evaluated. The results are shown in Table 3 and Table 4.

(80) TABLE-US-00003 TABLE 3 Number average Tg Acid value molecular Examples ( C.) (mgKOH/g) weight 1 2 3 4 5 6 7 8 9 10 Polyester (A)-a 90 16 15,000 90 resin (A) (A)-b 40 14 20,000 90 (A)-c 55 45 8,000 90 (A)-d 45 5 15,000 90 (A)-e 50 15 17,000 90 90 90 90 90 98 (A)-f 102 14 18,000 (A)-g 30 18 16,000 (A)-h 53 61 6,000 (A)-i 43 1 22,000 Polyester (B)-a 22 15 13,000 10 resin (B) (B)-b 17 10 10,000 10 (B)-c 5 40 8,000 10 (B)-d 7 1 18,000 10 (B)-e 5 15 13,000 10 10 10 10 10 2 (B)-f 30 10 10,000 (B)-g 3 60 5,000 Tgmix( C.) 80 36 49 40 47 42 45 45 45 49 Crosslinking Sumilite Resin (PR-53893A) 10 10 10 10 10 10 10 10 10 10 agent Mycoat 506 Curing catalyst Cycat 602 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 Number average Tg Acid value molecular Examples ( C.) (mgKOH/g) weight 11 12 13 14 15 16 17 18 19 20 Polyester (A)-a 90 16 15,000 resin (A) (A)-b 40 14 20,000 (A)-c 55 45 8,000 (A)-d 45 5 15,000 (A)-e 50 15 17,000 80 70 50 90 90 90 90 90 90 90 (A)-f 102 14 18,000 (A)-g 30 18 16,000 (A)-h 53 61 6,000 (A)-i 43 1 22,000 Polyester (B)-a 22 15 13,000 50 resin (B) (B)-b 17 10 10,000 (B)-c 5 40 8,000 (B)-d 7 1 18,000 (B)-e 5 15 13,000 20 30 10 10 10 10 10 10 10 (B)-f 30 10 10,000 (B)-g 3 60 5,000 Tgmix( C.) 40 35 35 45 45 45 45 45 45 45 Crosslinking Sumilite Resin (PR-53893A) 10 10 10 1 30 10 15 0 10 10 agent Mycoat 506 5 15 30 Curing catalyst Cycat 602 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.1 3.0 Examples 1 2 3 4 5 6 7 8 9 10 Evaluation Resistance to embrittlement of coated over time (initial plate processability) Resistance to embrittlement over time (processability over time) Corrosion resistance Retort resistance Blocking resistance Scrape resistance Evaluation Resistance to embrittlement of closure over time (processability over time) Corrosion resistance Retort resistance Scrape resistance Examples 11 12 13 14 15 16 17 18 19 20 Evaluation Resistance to embrittlement of coated over time (initial plate processability) Resistance to embrittlement over time (processability over time) Corrosion resistance Retort resistance Blocking resistance Scrape resistance Evaluation Resistance to embrittlement of closure over time (processability over time) Corrosion resistance Retort resistance Scrape resistance

(81) TABLE-US-00004 TABLE 4 Number average Tg Acid value molecular Comparative Examples ( C.) (mgKOH/g) weight 1 2 3 4 5 6 7 Polyester (A)-a 90 16 15,000 resin (A) (A)-b 40 14 20,000 (A)-c 55 45 8,000 (A)-d 45 5 15,000 (A)-e 50 15 17,000 80 90 90 (A)-f 102 14 18,000 90 (A)-g 30 18 16,000 90 10 (A)-h 53 61 6,000 90 (A)-i 43 1 22,000 90 Polyester (B)-a 22 15 13,000 resin (B) (B)-b 17 10 10,000 (B)-c 5 40 8,000 (B)-d 7 1 18,000 (B)-e 5 15 13,000 10 10 10 10 (B)-f 30 10 10,000 20 (B)-g 3 60 5,000 10 Tgmix( C.) 89 27 47 39 30 44 48 Crosslinking Sumilite Resin (PR-53893A) 10 10 10 10 10 10 10 agent Mycoat 506 Curing catalyst Cycat 602 1.0 1.0 1.0 1.0 1.0 1.0 1.0 Number average Tg Acid value molecular Comparative Examples ( C.) (mgKOH/g) weight 8 9 10 11 12 13 14 Polyester (A)-a 90 16 15,000 resin (A) (A)-b 40 14 20,000 100 (A)-c 55 45 8,000 (A)-d 45 5 15,000 (A)-e 50 15 17,000 40 90 90 90 90 (A)-f 102 14 18,000 (A)-g 30 18 16,000 (A)-h 53 61 6,000 (A)-i 43 1 22,000 Polyester (B)-a 22 15 13,000 100 resin (B) (B)-b 17 10 10,000 (B)-c 5 40 8,000 (B)-d 7 1 18,000 (B)-e 5 15 13,000 60 10 10 10 10 (B)-f 30 10 10,000 (B)-g 3 60 5,000 Tgmix( C.) 21 45 45 45 45 Crosslinking Sumilite Resin (PR-53893A) 10 35 0 10 10 10 10 agent Mycoat 506 Curing catalyst Cycat 602 1.0 1.0 1.0 0.0 4.0 1.0 1.0 Comparative Examples 1 2 3 4 5 6 7 Evaluation Resistance to embrittlement of coated over time (initial plate processability) Resistance to embrittlement X over time (processability over time) Corrosion resistance Retort resistance Blocking resistance X X Scrape resistance Evaluation Resistance to embrittlement X of closure over time (processability over time) Corrosion resistance X X X Retort resistance Scrape resistance Comparative Examples 8 9 10 11 12 13 14 Evaluation Resistance to embrittlement of coated over time (initial plate processability) Resistance to embrittlement X over time (processability over time) Corrosion resistance X X Retort resistance X Blocking resistance X Scrape resistance X Evaluation Resistance to embrittlement of closure over time (processability over time) Corrosion resistance X X X Retort resistance X Scrape resistance X

INDUSTRIAL APPLICABILITY

(82) The coating composition of the present invention is excellent in all of coating film performance characteristics such as processability, resistance to embrittlement over time, corrosion resistance, scrape resistance, blocking resistance, and retort resistance. In particular, the coating composition can be used preferably for a coated metal plate which is subjected to baking in a coil oven where baking is performed at a high temperature in a short time to cause marked embrittlement over time.

(83) Furthermore, the coated metal plate having a coating film from the coating composition of the present invention is excellent in processability, resistance to embrittlement over time, scrape resistance, and blocking resistance, and can be used preferably for the formation of a metal container or a metal closure which involves harsh processing.

(84) Besides, the metal container or metal closure of the present invention has excellent corrosion resistance, retort resistance, and flavor properties, and thus can be used preferably for a metal container for storing corrosive contents or contents requiring retort sterilization.