JP4924357B2 - Laminated steel sheet for container materials - Google Patents

Description

  The present invention is suitable for can bodies (general cans) such as 18L cans, pail cans, drum cans, etc., and is suitable for heat resistance, post-processing corrosion resistance, versatility (from acidic contents to alkaline contents). The present invention relates to a resin-laminated steel sheet excellent in suitability for a wide range of applications.

  In general can applications other than beverage cans (particularly in the field of large cans), attempts have been made to produce highly corrosion-resistant cans using various laminated steel plates (resin-coated steel plates). In general can applications, unlike beverage cans, the contents to be filled are diverse, such as chemicals, surfactants, paints, foods and oils, and the properties of the contents are diverse from acidic to alkaline. Polyester resins such as polyethylene terephthalate that are generally used as a resin for laminate cans are difficult to apply to alkaline contents because the film is hydrolyzed. Olefin resins such as polypropylene and polyethylene are well known as resins having corrosion resistance to a wide range of contents from acid to alkali. As a can material using such an olefin resin, Patent Document 1 discloses a polyethylene laminated steel sheet, and Patent Document 2 discloses a polypropylene laminated steel sheet.

However, as a result of examining the characteristics of these conventional olefin-based resin laminated steel sheets for general cans in detail, it has been found that there are the following problems.
Polypropylene resin is excellent in heat resistance as an olefin resin, but when bending is applied, cracks may occur near the apex of the bending. Some of these cracks are very fine, and even if they are not detected at the time of processing, there are cases where they become a problem after filling the contents. For example, when it is in contact with highly penetrating contents such as a surfactant for a long time, the liquid penetrates into minute cracks and corrodes the base, so it cannot withstand long-term storage. There is a drawback. Conventionally, as a method of improving the processing crack resistance of polypropylene resin, a method of optimizing the crystallinity by adjusting the cooling rate after resin lamination has been proposed. However, according to a study by the present inventors, since the polypropylene resin has a high crystallization speed, in the application where coating printing is performed on the outer surface of the laminated steel sheet, crystallization proceeds by heating the coating printing, and the above method It was found difficult to obtain a sufficient effect.

  Further, as another method for improving the processing crack resistance of polypropylene resin, a method of randomly copolymerizing α-olefin such as ethylene with polypropylene at a maximum of about 10% has been proposed.

  On the other hand, polyethylene resin has good processing crack resistance, but when polyethylene resin is laminated on the inner surface of the can, the melting point of the resin itself is as low as around 120 ° C. It is unavoidable that it is fused to the plate conveying equipment or thermally deformed (occurrence of contact traces on the laminate resin surface).

  Patent Document 3 proposes a laminated steel sheet in which a polypropylene resin and a polyethylene resin are combined. This document discloses a laminated steel sheet having a three-layer structure in which a polypropylene resin layer and a polyethylene resin layer are adhered to each other with a mixed layer of polypropylene resin and polyethylene resin. However, since polypropylene resin and polyethylene resin generally have poor adhesion, peeling is likely to occur between resin layers, and when internal pressure is applied to the can body after canning, delamination occurs at the tightening portion of the can lid and body plate, Airtightness leaks from there and it is difficult to obtain a desired pressure strength, so that it is not suitable for practical use as a can.

  Patent Document 4 solves the problem of delamination by block copolymerizing propylene, which is a monomer of polypropylene and polyethylene, and ethylene. And this trial says that the processing crack resistance and heat resistance can be satisfied to some extent.

However, as a problem common to these propylene-ethylene copolymers, there is a problem that the coating components are peeled off or eluted little by little when they come into contact with a laminate roll, a sheet passing roll, or other equipment. The amount of peeling (or the amount of elution) is extremely small and does not affect the performance of the product itself, but dirt is deposited on the equipment on the contact side. For this reason, it is necessary to clean the equipment regularly. And it is difficult to remove the adhered dirt, increasing the operation load.
JP-A-53-141786 Japanese Patent No. 2733589 Japanese Patent Application Laid-Open No. 64-82931 JP 2003-285394 A

  An object of the present invention is a laminated steel plate suitable for can bodies and lids for general cans including large cans such as 18L cans and pail cans. An object of the present invention is to provide a laminated steel sheet for a container material which is excellent in heat resistance, post-processing corrosion resistance, and versatility (suitability for use from acidic contents to alkaline contents) without depositing film components on rolls and other equipment.

The present inventors have studied to solve the above-described problems of the prior art. As a result, the following knowledge was obtained.

  In order to impart versatility (suitability for use from acidic contents to alkaline contents), olefin resins such as polyethylene and polypropylene are desirable. However, polyethylene has good corrosion resistance after processing, but has insufficient heat resistance and causes problems such as fusion to a contact object at high temperatures such as paint baking. On the other hand, polypropylene has good heat resistance but is inferior in corrosion resistance after processing. When bending is performed on polypropylene, cracks may occur in the vicinity of the apex of the bent portion. This crack is the cause of corrosion resistance deterioration. The cause of cracks in the processed part is due to high-speed deformation in the vicinity of the apex during bending. In expanding and overhanging processes, the amount of elongation is not large in the vicinity of the apex of the bent part, but it is extended in the bending direction instantaneously, and if the resin film does not correspond to this high-speed deformation, cracks occur. . That is, a polypropylene single layer does not support high-speed deformation.

  In order to improve the crack resistance of the processed part, an ethylene component may be contained mainly with a propylene component. That is, in accordance with the provisions of the present invention, a copolymer of propylene and ethylene, preferably a compound obtained by compounding ethylene-propylene rubber (hereinafter sometimes referred to as EPR) using polypropylene as a matrix is preferably used. However, when the ethylene ratio is high, the heat resistance is inferior, so it is necessary to keep it at an appropriate amount. Therefore, the content of the ethylene component has a compatible range from the viewpoint of post-processing corrosion resistance and heat resistance. This compatible range is the specified range of the present invention, and the content thereof is 3 to 30 mol%. Below the lower limit, the effect of containing an ethylene component is low, and the effect of improving corrosion resistance after processing is small. On the other hand, if the upper limit is exceeded, the ethylene component becomes excessive and the heat resistance deteriorates.

  The thickness of the propylene-ethylene copolymer is 15 to 200 μm. If it is less than 15 μm, it is difficult to form a film and the production cost becomes high, and it becomes difficult to relax the high-speed deformability of an upper polypropylene layer, which will be described later, and as a whole, the corrosion resistance after processing deteriorates. On the other hand, when the upper limit of 200 μm is exceeded, even if the film thickness is increased, the container material characteristics do not increase, but the manufacturing cost increases.

  Next, in order to solve the problem of depositing film components on the sheet-passing rolls and the like, laminated steel plates of various propylene-ethylene copolymers were produced, and the components deposited on the sheet-passing rolls were analyzed. As a result, in any copolymer, the deposition component was a mixture of polyethylene and polypropylene. From this result, it was found that when the propylene-ethylene copolymer is exposed on the surface layer, the constituent components tend to fall off.

  As a result of further detailed analysis, it was found that the ethylene / propylene ratio of the deposit tended to be higher than the average ethylene / propylene ratio of the entire film. That is, it is presumed that the ethylene-rich portion is easily dropped off.

  Based on this analysis fact, the inventors considered a film form in which the film component is difficult to fall off. Such a drop-off phenomenon is not observed in the case of a polyester film containing neither an ethylene component nor a propylene component. Therefore, it is expected that the polyester resin is improved when the polyester resin is arranged in the uppermost layer. However, laminating polyester and olefin is not only difficult to form a film, but because polyester has the property of causing hydrolysis under alkaline conditions, the versatility of the contents is impaired.

  Therefore, the inventors have found that a polypropylene single layer is disposed on the upper layer as a result of further investigation. And when the polypropylene single layer was arrange | positioned to the upper layer, the deposition to a threading roll etc. was eliminated. However, there are cases in which the properties of the lower layer are hindered simply by arranging them.

  That is, as described above, the lower layer propylene-ethylene copolymer is rich in high speed deformability, but the upper layer polypropylene single layer is inferior in high speed deformability. As a result, there are cases where the high-speed deformability, that is, the corrosion resistance after processing is inferior as a whole. On the other hand, according to the study by the inventors, it was found that the degree of inhibition of high-speed deformability can be ignored if the thickness of the upper layer is 5 μm or less. Therefore, in the present invention, the thickness of the upper layer is set to 5 μm or less. Moreover, since it is difficult to produce a layer having a thickness of less than 1 μm in a technically stable manner, the lower limit is set to 1 μm or more.

This invention is made | formed based on the above knowledge, and has the following characteristics.
[1] A propylene-ethylene copolymer having a thickness of 15 to 200 μm is formed on at least one surface of the surface-treated steel sheet, and a polypropylene resin layer having a thickness of 1 to 5 μm is further formed on the propylene-ethylene copolymer. A laminated steel sheet for container materials, wherein the ethylene component is 3 to 30 mol% when the total of the ethylene component and the propylene component is 100 mol%.
[2] The laminated steel sheet for container material according to [1], wherein the propylene-ethylene copolymer is a polypropylene-based matrix and a compound of ethylene-propylene rubber.
[3] The laminated steel sheet for container materials according to any one of [1] or [2], wherein an adhesive layer is provided below the propylene-ethylene copolymer.

According to the present invention, no film components are deposited on the passing plate roll, laminating roll, and other equipment when passing, and heat resistance, post-processing corrosion resistance, versatility (application suitability from acidic contents to alkaline contents) Is obtained.
And since the laminated steel plate of this invention has the above characteristics, it is suitable for the can body part and cover part of general can uses including large cans, such as an 18L can and a pail can.

  Hereinafter, the details of the present invention and the reasons for limitation will be described.

The surface-treated steel sheet that is the substrate of the laminated steel sheet of the present invention is not particularly limited. For example, a tin steel plate and an electrolytic chromate-treated steel plate (hereinafter referred to as tin-free steel: TFS) may be mentioned. Of these, electrolytic chromate-treated steel sheets (tin-free steel: TFS) are desirable from the viewpoint of economic efficiency and ensuring adhesion between resins.
As the steel sheet to which electrolytic chromate is applied, any steel sheet that is usually used for this type of surface-treated steel sheet can be used. For example, a normal low carbon cold-rolled steel sheet having a thickness of 0.1 to 0.5 mm, low carbon Al killed steel plates or the like are used, and a chromium metal hydrated oxide is formed on the metal chromium layer from below by electrolytic chromate treatment on these steel plates. The amount of chromium deposited on the metal chromium layer is preferably 40 to 200 mg / m ² per side. When the adhesion amount is less than 40 mg / m ^{2, the} coating with the surface treatment layer is damaged when an impact is applied, and corrosion is likely to proceed. Even if the adhesion amount exceeds 200 mg / m ² , there is no problem in terms of performance, but it is not preferable from an economical viewpoint. A more preferable range is 80 to 150 mg / m ² . Moreover, the adhesion amount of the chromium hydrated oxide layer is preferably 3 to 25 mg / m ^{2 in} terms of metal chromium per side. When the adhesion amount is less than 3 mg / m ² , the metal chromium layer is not uniformly covered with the chromium oxide, the exposed area of the metal chromium layer becomes large, the adhesion with the resin layer is impaired, and the resin layer is wrinkled Since corrosion is likely to proceed and corrosion resistance is lowered, it is not preferable. On the other hand, if it exceeds 25 mg / m ² , the surface color tone of TFS is deteriorated because the chromium oxide layer is too thick.

  The laminated steel sheet of the present invention has a propylene-ethylene copolymer having a thickness of 15 to 200 μm on at least one surface of the surface-treated steel sheet (the steel sheet surface on the inner surface side of the can), and further has an upper layer of 1 to 5 μm. It has a polypropylene resin layer. And the said propylene-ethylene copolymer is 3-30 mol% of ethylene components, when the sum total of an ethylene component and a propylene component is 100 mol%. Furthermore, it is preferable to use the propylene-ethylene copolymer having polypropylene as a parent phase and a compound of ethylene-propylene rubber. It is also possible to have an adhesive layer in the lower layer of the propylene-ethylene copolymer.

Propylene-ethylene copolymer having a thickness of 15 to 200 [mu] m At least one surface of the surface-treated steel sheet of the present invention has a propylene-ethylene copolymer. And when the sum total of an ethylene component and a propylene component shall be 100 mol%, ethylene component shall be 3-30 mol%. The reason for selecting the copolymer system of propylene and ethylene is to achieve both heat resistance and post-processing corrosion resistance as described above.
In addition, the lower limit of the ethylene component was determined to be 3 mol% or more because the corrosion resistance after processing deteriorates when the ethylene component was less than this, and the upper limit was determined to be 30 mol% or less. This is because when it increases, the heat resistance deteriorates.
The lower limit of the thickness is due to an increase in manufacturing cost, difficulty in relieving high-speed deformability of the upper polypropylene layer, and deterioration in post-processing corrosion resistance as a whole. The upper limit of the thickness is because, even if the film thickness is further increased, the manufacturing cost increases even though the container material characteristics do not increase.

1-5 μm polypropylene resin layer As described above, the outermost layer has a polypropylene resin layer for the purpose of solving the problem of depositing film components on rolls and the like. As described above, the thickness is set to 1 to 5 μm in consideration of the manufacturing cost, the relaxation of the high-speed deformability of the lower polypropylene layer, and the like. The upper limit of the thickness is a thickness that does not hinder the corrosion resistance after processing, and the lower limit of the thickness is the lower limit thickness that enables industrially accurate film formation.

  The propylene-ethylene copolymer and the resin of the upper polypropylene layer preferably have a melt flow rate (MFR JIS K6758) of 0.5 to 20 g / 10 min. When the melt flow rate is less than 0.5 g / 10 min, when a film is formed or when direct extrusion lamination is performed, the motor load of the extruder increases and productivity decreases. If the MFR is too large, the surface roughness becomes small and the anti-blocking property decreases, so that it is preferably 20 g / 10 min or less.

Adhesive layer In the present invention, it is also possible to have an adhesive layer in the lower layer of the propylene-ethylene copolymer. The adhesive layer may be appropriately selected according to the specification application. For example, an adhesive polypropylene resin mixed with an appropriate amount of an adhesive polyethylene resin is preferably used. Specifically, a mixture of 2 to 10 μm thick adhesive polyethylene and adhesive polypropylene containing 8 to 20% adhesive polyethylene is suitable.

As a specific composition of the adhesive layer, the adhesive polypropylene is a homopolymer of propylene, or a block or random copolymer of propylene and α-olefin. Examples of the α-olefin in the latter case include ethylene, 1-butene, 1-hexene, 1-heptene, 1-octene, 4-methyl-1-pentene, etc., and use one or more of these. Can do.
The melt flow rate (MFR JIS K6758) of the adhesive polypropylene is preferably 0.5 to 20 g / 10 min.
The adhesive polypropylene preferably has an adhesive property (thermal adhesive property) by introducing an unsaturated carboxylic acid and / or a derivative thereof into a polypropylene resin. As the unsaturated carboxylic acid or derivative thereof used for this acid modification, unsaturated carboxylic acid or derivative thereof such as maleic acid, acrylic acid, fumaric acid, tetrahydrophthalic acid, itaconic acid, citraconic acid, crotonic acid, nadic acid, For example, amides, imides, anhydrides, esters, acid halides and the like can be used, and one or more of these can be used, but maleic anhydride is generally used. Graft polymerization is common as a method for introducing these unsaturated carboxylic acids and / or derivatives thereof into polypropylene. In particular, graft polymerization in which maleic anhydride is 0.01 to 5% by weight is preferable.

The adhesive polyethylene to be mixed with the adhesive polypropylene-based resin is an ethylene homopolymer, or a block or random copolymer of ethylene and α-olefin, but the latter is required to ensure adhesion with the upper layer. These copolymers are preferred. When the polyethylene resin is a block or random copolymer of ethylene and α-olefin, the amount of α-olefin giving a side chain is preferably 1 to 25 mol%. If the amount of α-olefin is less than 1 mol%, the adhesion to the upper polypropylene-based resin layer decreases, whereas if it exceeds 25 mol%, the adhesiveness at room temperature increases and film formation becomes difficult. Examples of the α-olefin include propylene, 1-butene, 1-hexene, 1-heptene, 1-octene, and 4-methyl-1-pentene, and one or more of these can be used. A particularly preferable copolymer of the polyethylene resin is a random copolymer.
The melt flow rate (MFR ASTM D1238) of the above-mentioned adhesive polyethylene is desirably 0.5 to 50 g / 10 min from the viewpoint of film forming properties.
The adhesive polyethylene preferably has an adhesive property (thermal adhesive property) by introducing an unsaturated carboxylic acid and / or a derivative thereof into a polyethylene resin. As the unsaturated carboxylic acid or derivative thereof used for this acid modification, unsaturated carboxylic acid or derivative thereof such as maleic acid, acrylic acid, fumaric acid, tetrahydrophthalic acid, itaconic acid, citraconic acid, crotonic acid, nadic acid, For example, amides, imides, anhydrides, esters, acid halides and the like can be used, and one or more of these can be used. Maleic anhydride, methyl acrylate, methyl methacrylate, etc. are generally used. Is. Among these, from the viewpoint of corrosion resistance, it is preferable to use maleic anhydride alone or a mixture of maleic anhydride and one or more of other unsaturated carboxylic acids.

  Further, glycidyl methacrylate, vinyl acetate, methyl acrylate, and ionomer may be used alone or in admixture of two or more. Examples of methods for introducing these unsaturated carboxylic acids and / or derivatives thereof into polyethylene include graft polymerization, random polymerization, and block polymerization. In particular, graft polymerization in which maleic anhydride is 0.01 to 5% by weight is preferable.

  The adhesion between the resin layer and the laminated steel plate may be an extrusion method in which the resin layer is directly formed on the steel plate or a thermocompression bonding method in which the resin layer is once formed into a film and thermocompression bonded.

  Further, in the resin layer defined in the present invention, an appropriate amount of a heat stabilizer, an antioxidant, a weathering stabilizer, an antistatic agent, a pigment, a dye, or the like may be blended as long as the effects of the present invention are not impaired. However, it is desirable that a low melting point and easily soluble compound in an aqueous solution or a low melting point compound such as a phenolic antioxidant should not be blended as much as possible.

Example of the present invention by film thermocompression bonding
A cold rolled steel sheet having a thickness of 0.32 mm, which is usually used for general cans, such as 18L cans, is subjected to electrolytic degreasing and pickling by a conventional method, and then subjected to electrolytic chromate treatment by a known method, and the adhesion amount is 100 mg / m. ^An electrolytic chromate treatment layer composed of a chromium hydrated oxide layer having an adhesion amount in terms of metal chromium of 8 mg / m ² was formed on the upper layer of the metal chromium layer.
The surface-treated steel sheet obtained as described above was heated to the melting point of the adhesive layer of the resin film to 250 ° C., the resin film was laminated on one surface of the steel sheet, and then rapidly cooled with water within 2 seconds to produce a laminated steel sheet. The details of the resin layer and the resin used are shown in Table 1. Further, after the above lamination, clear coating was performed on the surface corresponding to the outer surface of the can at a temperature not higher than the melting point of the upper resin. As the adhesive layer, a mixture of adhesive polypropylene and adhesive polyethylene was used. Adhesive polyethylene was obtained by copolymerizing 5 mol% of α-olefin, and the mixing ratio of adhesive polyethylene was 15 mol%, film thickness Used was 5 μm.

Invention Example by Extrusion Method Cold-rolled steel sheet having a thickness of 0.32 mm similar to that of the invention example of the above-described film thermocompression bonding method is subjected to electrolytic degreasing and pickling by a conventional method, followed by electrolytic chromate treatment by a known method, and adhesion the amount and the metallic chromium layer of 100 mg / m ^2, the adhesion amount of reckoned as metal chromium thereon was formed an electrolytic chromate treatment layer made of hydrated chromium oxide layer of 8 mg / m ^2. Next, preheating was performed at about 140 ° C., and a molten resin was applied by a T-die extrusion method (extruder method), followed by rapid cooling with water within 2 seconds to produce a laminated steel plate. The details of the resin layer and the resin used are shown in Table 1. Further, after the above lamination, clear coating was performed on the surface corresponding to the outer surface of the can at a temperature not higher than the melting point of the upper resin. As the adhesive layer, a mixture of adhesive polypropylene and adhesive polyethylene was used. Adhesive polyethylene was obtained by copolymerizing 5 mol% of α-olefin, and the mixing ratio of adhesive polyethylene was 15 mol%, film thickness Used was 5 μm.

Comparative Example The comparative example was carried out in the same manner as in the above-described example of the present invention except that the resin layer and the resin having the upper layer of the surface-treated steel sheet were used with a resin having a composition outside the scope of the present invention. Details of the resin layer and the resin used are shown in Table 2.

  Performance evaluation was performed by the method shown below with respect to the laminated steel sheets of the present invention examples and comparative examples obtained as described above.

(1) Heat resistance (Fusibility of the laminate surface to a metal holder during paint printing)
A cylindrical tube having a diameter of 50 mm and a weight of 200 g was placed on the laminate surface of the resin-coated steel plate. In this state, after holding at 150 ° C. for 30 minutes, it was cooled and examined for the presence or absence of contact traces on the cylindrical body.

(2) After processing, the corrosion-resistant laminated steel sheet was punched to 48mmφ, and this sample was slightly drawn to a height of 2mm with a punch and die set with a shoulder radius of 1.7mmR and a diameter of 25mm. The produced sample becomes a sample shaped like a hat made of a cylindrical portion along the punch and the remaining flat plate portion. The shoulder portion of the cylindrical portion of the hat-shaped outer surface side (convex side) sample is substantially bent. This sample was immersed in a neutral detergent (trade name: Rypon F) at 38 ° C. for 2 days, taken out, washed with water and dried. Next, an appropriate amount of an electrolytic solution (1% NaCl + surfactant) was poured into the petri dish, and the sample was placed on the petri dish so that the lower side was the convex side, so that only the shoulder and upper surface of the sample were immersed. In this state, a voltage of 6.2 V was applied between the base steel plate of the sample and the electrolytic solution (the sample was a cathode), and the current value after 4 seconds after voltage loading was measured. If the current value was less than 10 mA, it was rated as “◯”, and if it was 10 mA or more, it was marked as “X”.

(3) Deposit evaluation Each steel plate was produced continuously for 10000 m, and the state of the resin deposited on the sheet-passing roll was inspected. The case where no deposit was observed was marked with ○, and the case where it was recognized was marked with ×.

  The results obtained are shown in Table 3.

In the example of the present invention, all the characteristics of heat resistance, post-processing corrosion resistance, and deposit evaluation were excellent.
On the other hand, in Comparative Example 1, since the propylene layer of the upper resin was too thick, the high-speed deformation relaxation effect of the lower polypropylene layer was not sufficiently effective, and the post-processing corrosion resistance was not obtained. In Comparative Example 2, since the ratio of the ethylene component in the polypropylene-based layer was too low, the corrosion resistance after processing was insufficient.
Since the comparative example 3 had an ethylene ratio too high, it resulted in inferior heat resistance. In Comparative Examples 4 and 5, the polypropylene layer was a random copolymer type or an EPR compound type, and the ratio of the ethylene component was too low, so the corrosion resistance after processing was insufficient. Comparative Example 6 was a comparative example laminated by an extrusion method, and the corrosion resistance after processing was insufficient because the ratio of the ethylene component was too low. In Comparative Example 7, since the polypropylene-based layer was too thin, the corrosion resistance after processing was inferior. Comparative Examples 8 and 9 are examples where the polypropylene-based layer is not ethylene-propylene copolymer, and the polypropylene single layer has poor post-processing corrosion resistance, and the polyethylene single layer has poor heat resistance. It was. Comparative Examples 10 to 12 are comparative examples in the case where there was no upper polypropylene layer, and the deposit evaluation was poor.

Claims (3)

  The surface-treated steel sheet has a propylene-ethylene copolymer having a thickness of 15 to 200 μm on at least one surface, and further has a polypropylene resin layer of 1 to 5 μm on the upper layer. The propylene-ethylene copolymer has an ethylene component and A laminated steel sheet for container materials, wherein the ethylene component is 3 to 30 mol% when the total of propylene components is 100 mol%.   2. The laminated steel sheet for container materials according to claim 1, wherein the propylene-ethylene copolymer is made of polypropylene as a matrix and compounded with ethylene-propylene rubber.   3. The laminated steel sheet for container materials according to claim 1, wherein an adhesive layer is provided in a lower layer of the propylene-ethylene copolymer.