HRP930109A2 - Laminated metal sheet - Google Patents

Description

The field of technology

This invention relates to a laminated sheet.

Technical problem

The application of polymeric materials to sheet metal, such as metal tape, involves a well-known and well-documented technique. The obtained laminates have numerous applications, such as, for example, the production of container bodies and lids for food and beverage containers, as well as for containers with aerosols.

Polyester coatings are often used to coat sheet metal in order to achieve good corrosion resistance when it comes to sheet metal. It is usual to try coating the sheet with polyester resin that has a crystalline and oriented structure, because such polymer films show low permeability of oxygen, water and steam. However, it has been found that it is not easy to achieve adhesion of such a crystalline, biaxially oriented polyester film to the sheet.

State of the art

One of the solutions to this problem is to apply biaxially oriented polyester to the sheet by applying such production conditions that require heating the sheet to high temperatures, in order to melt at least one part of the biaxially oriented polyester. Such proposals are found, for example, in GB 759876 and GB 2123746. However, while this solution has certain advantages, it relies on very strict control of the temperature of the sheet during application, and on laminating rollers made of very resistant material, as they require very high temperatures for lamination of biaxially oriented polyethylene terephthalate film on sheet metal.

An alternative solution is to provide an intermediate layer of adhesive between the layer of crystalline polyester and the sheet, to which this layer should adhere. This type of solution to the problem is described, for example, in GB 2164899, which provides for the application of an adhesive epoxy resin to a sheet, in order to apply a film of polyester resin. This allows the larnination process to be carried out at lower temperatures, but creates a coating that forms relatively poorly in deep drawing processes and represents a relatively expensive way to solve the aforementioned difficulties.

Similarly, GB 1501353 describes a mixture of polyester, α-olefin copolymer and epoxy compound to form adhesives suitable for use in laminating thermoplastic resins to metal. GB 2055687 describes a laminate formed by hot bonding a film of biaxially oriented polyester film to a sheet using an adhesive layer, the adhesive layer being made of a polymer blend containing high melting point polyester and low melting point polyester. Polyester blends may contain polyolefin resin.

None of the mentioned patents consider the problems that arise with deep-drawn containers made of polyester-metal laminate.

Common adhesives can be used to connect the sheet and the film. For example, isocyanate-based adhesives can be applied to polyester film or sheet before the two are brought into contact in a laminating bath.

In practice, if one wants to make laminate containers according to GB 2123746 and 2164899, usually by deep drawing a disc of 189 mm diameter into a container of 100 mm height and 65 mm diameter (this size is widely used in the canning industry), serious damage to the polyester coating is observed, leading to significant losses of the coating metal cover. As a consequence, the durability of the corresponding container, when a food product is packaged in it, is significantly reduced, so that the container is unacceptable from a commercial point of view. Damage to the coating in deep drawing is a consequence of the forming process in which the laminar oriented film is stressed beyond the elongation limit, which causes breakage of the oriented polyester. This phenomenon has a detrimental effect on the oriented polyester coatings described in GB 2123746 and 2164899, as well as on those produced and laminated by other processes where no attention has been paid to the ability to extract the coatings.

Lamination of polyester to metal is also described in GB 1566422. This patent specification states that the laminates described herein can be applied for deep drawing. However, the polyesters used to form the laminates in GB 1566422 are polyesters of a very specific class because they have an intrinsic viscosity of 1.2 to 1.8 and a laminar film crystallinity of less than or equal to 30%. This specific grade of polyester has an intrinsic viscosity range such that it excludes common commercially available polyethylene terephthalate homopolymer materials, such as biaxially oriented polyethylene terephthalate materials that are commonly considered materials of choice for sheet lamination when good corrosion resistance is desired, such as to achieve low permeability of oxygen, water and steam of such biaxially oriented polyesters.

Description of the solution to the technical problem with practical examples

This invention aims to provide a process for the production of polymer/metal laminates from common, commercially available biaxially oriented polyester materials without the application of high temperatures to achieve thermal lamination of polyester to sheet, with a better laminate, which has properties that make it suitable for forming a can or container deep extraction procedures.

According to the first aspect of this invention, a process with simultaneous lamination of a polymer/metal/polymer laminate is provided, wherein the process includes

simultaneous lamination on each of the main sheet surfaces of a composite polyester film (A) consisting of an inner layer (Al) of usually non-crystalline polyester whose softening point (Ts-Al) is below 200°C and melting point (Tm-Al) above 150° C, but below 250°C, and the outer layer (A2) of biaxially oriented linear polyester whose crystallinity is greater than 30% and whose melting point (Tm-A2) is above 250°C, whereby the sheet is heated to a temperature T1 above the softening temperature (Ts-Al), or, even better, above the melting point (Tm-Al) of the polyester from the inner layer (Al), in order to cause the softening or, even better, the melting of the inner layers (Al) and their adhesion to the sheet, but below temperature (Tm-A2) at which the outer layer (A2) will melt when it comes into contact with the metal strip at temperature T1 and

reheating the resulting laminate to a temperature T2 sufficient to cause the polymer films (Al) to react with the sheet, but so that the outer surface of the outer layer (A2) remains below the temperature (Tm-A2).

According to another aspect of this invention, a laminated sheet is provided on both main surfaces of which polymer films are applied, wherein the polymer films are applied to the sheet by simultaneous thermal lamination, and the polymer film applied to each main surface of the sheet has a composite polyester film (A) consisting from the inner layer (Al) of usually non-crystalline linear polyester, whose softening point is below 200°C and melting point above 150°C, and below 250°C and from the outer layer (A2) of biaxially oriented linear polyester, whose melting point is above 250°C .

Preferably, each composite polyester film (A) is a film produced by coextrusion.

The lamination process, according to this invention, consists of several stages. In the first stage, the metal is preheated to a temperature T1 above the softening temperature (Ts-Al) of the layer polyester (Al), or, even better, above its melting point (Tm-Al), but below the temperature (Tm-A2) at which the outer surface of the outer layer (A2) melts when it comes into contact with the metal strip at temperature T1. This temperature T1 is usually from 120 to 260°C, and best from 200 to 250°C.

In the second stage, the films and metal are brought together into a laminating bath, where a close and uniform contact is established without bending. In this step, the contact layers are the inner layer (Al) of the amorphous polyester, the metal and, on the opposite side of the metal, the inner layer (Al) of the second polyester film (A).

In the third step, the resulting laminate is heated again, preferably by induction heating of the metal core, to a temperature of 250°C, this temperature being chosen so that the outer surface of the outer layer (2) of the biaxially oriented polyester is kept below the melting point (Tm-A2). of biaxially oriented polyester in order to regulate melting in the biaxially oriented layer (A2) of the polyester film (A).

Although the outer surface of the polyester film (A) is maintained below its melting point, but with the metal core above the melting point of said polyester, there is a rapid interaction between the metal and each of the inner polyester layers (Al). To achieve this interaction, the laminate is held above approx. 250°C for 1 to 30 seconds, then allowed to cool rapidly to a temperature above 200°C and then cooled rapidly and evenly with water. By allowing the polyester layers to cool to 200°C or lower before cooling, the danger of bubbles forming in the polyester films is minimized. Also, the laminate can cool before the polyester can recrystallize to any extent. The rate of crystallization of polyethylene terephthalate is maximal at temperatures of approx. 160 to 180°C. It is understandable that it is recommended to cool molded laminates with polyethylene terephthalate coatings at a temperature of approx. 200°C.

We discovered that if it is ensured that the outer surfaces of the biaxially oriented films (A) remain below their melting points, a sufficient part of the excellent properties of the biaxially oriented polyester film can be retained, i.e. polyethylene terephthalate. We also found that the residual properties of the non-crystalline (melted) and biaxially oriented (non-melted) polyester in the A2 outer layer regulate the coating's tensile properties and formability in deep drawing processes. The amount of biaxial orientation retained is inversely proportional to the deep drawing properties and should be controlled to meet the required forming requirements.

The temperature in the post-lamination zone can vary due to the regulation of properties, especially formability, which is necessary for polyester coatings. This regulation can be achieved to a satisfactory value if induction heating is used for subsequent heating of the laminate behind the laminating bath. It is convenient to use pyrometers to identify the temperature of polyesters, for example a single frequency device operating at 7.8 microns, where polyesters have optimal radiant power. Alternatively, devices that detect the change from biaxial orientation to crystal-oriented or amorphous polyester can be used to detect the conditions of the polyester film (eg, X-ray diffractometer).

The exact temperature T1 to which the sheet must be heated during lamination depends on the thickness of the films that are applied, as well as on the chemical nature of said films. Temperatures from 140 to 270°C are suitable for coextruded biaxially oriented polyethylene terephthalate.

The temperature T2, which should be applied to reheat the larninate behind the laminating bath, is usually in the range of 250 to 270°C. The exact temperature to be applied depends on the dwell time before the laminate cools and the relative masses of the coatings on the metal. Temperatures above 270°C lead to complete melting of polyethylene terephthalate films, leading to loss of biaxial orientation and its associated properties. The temperature at the lower values of this limit must be determined in order to achieve a satisfactory strength of the connection between the sheet and the polymer films connected to it, and for a short time during which the laminate is heated to the required temperature. Commercial processes usually require a retention time of approx. 2 seconds.

The metal part, on which polymer films are applied, is usually in the form of a metal strip made of steel or aluminum or their alloys, which are further used in the packaging industry.

Thickness ranges are typically 0.05 mm to 0.4 mm for steel and 0.02 mm to 0.4 mm for aluminum.

The steel may be tin-plated, preferably passivated with the usual chrome treatment, or alternatively, nickel or zinc electroplated, black iron sheet or phosphated black iron sheet, which is usually chromated after phosphating.

A suitable steel finish is electrolytic chromium plating steel (ECCS) with a double layer of metallic chromium and chromium oxide. In such steels, the amounts of metallic chromium and chromium oxide can vary widely. Usually, the content of metallic chromium is in the range from 0.1 to 0.20 g/m2, and the content of chromium oxide from 0.005 to 0.05 g/m2. ECCS is usually carried out from precipitation systems containing sulfur- or fluorine-based catalysts.

A composite polyester film (A) is applied to each surface of the sheet or strip, which is conveniently produced by co-extrusion and orientation prior to application to the sheet or metal strip. Composite polyester film (A) consists of a thinner inner layer (A), mostly non-crystalline polyester, whose softening temperature is below 200°C, and melting point around 150°C, but below 250°C, and a thicker outer layer (A2), which is a biaxially oriented linear polyester, the crystallinity of which is greater than 30%, and the melting point above 250°C.

The highly crystalline outer layer (A2) is polyethylene terephthalate. It is preferable that the inner layer (Al) be a linear copolyester, for example some amorphous copolymer of approximately 80% mole % ethylene terephthalate and 20% ethylene isophthalate. Copolyesters of terephthalic acid and two alcohols, eg ethylene glycol and cyclohexane-dimethanol, are also suitable for use as an inner layer.

(Al) .

Typically, the biaxially oriented polyester in the outer layer (A2) has a crystallinity greater than 30%, preferably 40 to 50%.

The crystallinity of the polyester resin can be determined by X-ray diffraction, as described in GB 1566422, or by density measurement and using the following relationships:

Vc = (P - Pa) (Pc - Pa)-1

where

Vc is the crystallinity of the volume fraction,

P is the density of the sample,

Pa De density of amorphous material,

Pc is the density of the crystalline material.

P can be measured in a density column using a mixture of zinc chloride and water, or n-heptane and carbon tetrachloride.

A biaxially oriented polyester film, which is used as an outer layer, can have a certain range of orientation and can be formed by drawing an amorphous extruded polymer in the longitudinal direction at temperatures above the glass transition temperature of that polymer by a factor of 2.2 to 3.8 and similarly in the transverse direction by a factor of 2.2 to 4.2, typically 2.2 x 2.2 to 3.0 x 3.0. When the applied coatings are to be used for deep drawn metal vessels, it is better to limit the orientation to draw by a factor of about 2.5 in both longitudinal and transverse directions.

The heating temperature is usually between 200 and 220°C, or, even better, from 210 to 220°C. Lower heating temperatures usually result in an increased tendency of film orientation to shrink upon reheating.

Usually the inner layer (Al) should be continuous and have a thickness of 2 to 3 microns. It is preferable that the ratio of the thickness of the outer polyester layer (A2) to the inner polyester layer (Al) be between 12 to 3, whereby the total thickness of the combined layers should be from 12 to 25 microns.

If necessary, one or more polyester layers may contain an inorganic anti-blocking agent, such as synthetic silica having an average particle size of 0.5 to 5 microns.

Also, if necessary, the outer polyester layer (A2) can usually contain 0.5 to 10% by weight. of polyester used in the layer (Al). This material can be produced by adding co-extruded film to a PET extruder when making the film. The presence of this additional polyester improves the formability of the polyester film.

Also, if necessary, the outer polyester layer (A2) can be colored with common pigments, such as titanium dioxide.

The basic function of the inner polyester layer (Al) is to adhere to the metal surface at temperatures below the melting point of the outer crystalline polyester layer (A2).

This is important so that this layer can retain its amorphous structure after orientation and heating of the film, if the inner layer is to be bonded below its melting point.

A particularly advantageous aspect of this invention is that before the film is applied the temperature of the metal can be raised to the range of 200 to 250°C, the use of biaxially oriented polyethylene terephthalate films with an orientation induced by pulling in both directions with a factor of approx. 2.2 x 2.2 to 3.0 x 3.0, as well as the application of induction heating to reheat the laminate after film application and regulate the degree of melting in the biaxially oriented part of the coating (A2).

During this specification, internal viscosities were measured at 25°C in o-chlorophenol solutions with a concentration of 5 g/l.

The following examples are provided to further illustrate the present invention.

Examples 1 and 2 (comparative)

Laminates of biaxially oriented polyethylene terephthalate film applied to each side of the ECCS metal strip were prepared in accordance with GB 2123746. Details of the materials used to prepare the laminates are listed in Table 1. The properties of these laminates when used for the production of cans, or lids for cans, are shown are in table 3 (examples 6 and 7).

Examples 3 to 5

Polymer/metal/polymer laminates are prepared by the process carried out on the device shown schematically in Figures 9 or 10 of the attached drawings. Sheet M is preheated to the appropriate temperature T1 using heater 1.

The temperature T1 is usually in the range of 120 to 260°C. The polyester films A are fed from the feed rollers 2 and 4 and applied to the opposite sides of the preheated sheet and between the laminating rollers 6, 8, the radius of which is usually 100 - 400 mm. Lamination is usually done using pliers with a power of 200 - 400 N/m between the lamination rollers.

In the laminating pliers, a tight and uniform contact without wrinkles is established between the sheet and the polymer films. Behind the laminating rollers, the resulting laminate is heated again, preferably using an induction heater 10, to temperature T2, at which the polymer films (A) react and become connected to the sheet. The temperature T2 is usually in the range of 250 to 270°C. The metal/polymer laminate is held at temperature T2 or below T2 (typically above 200°C) for a short time, usually no longer than 2 seconds, and then rapidly and uniformly cooled with water to a temperature below the glass transition temperature of the polyester films (A) . Cooling can be done in any of the standard ways, but is usually done by passing the laminate through a tank of water, as shown in Figure 9, or by passing the laminate through a curtain of cooling water, shown in Figure 10.

In principle, the procedure shown in Figure 9 is better, where the lamination is done vertically. The vertical movement of the metal strip through the lamination stage allows for a faster cooling rate and gives better results and more even cooling.

Figure 9 also schematically shows a diagram of a typical temperature profile that may be encountered in the process shown in Figure 9.

Table 1 shows the types of polymers that are laminated to the metal strip, as well as the thickness of each layer.

Examples 1 and 2 are provided for comparison. These. laminates were obtained by the process described in GB 2123746.

Laminates of the types described in Examples 3 to 5 were obtained by the process listed in Table 2.

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Labels with table 1

Metal: 0.21 mm thick ECCS for lids and 0.18 mm thick for deep drawing cans.

Polyester A: In Examples 3 to 5, the polyester layer (Al) was a non-crystalline (i.e., amorphous) polyester, which is a copolymer of ethylene terephthalate (approx. 80 mol%) and ethylene isophthalate {approx. 20% mole). The softening temperature of polyester is approx. 140°C, and the melting point of polyester was 210°C. The internal viscosity of polyester was from 0.6 to 0.7.

Biaxial PET (I): Represents biaxially oriented polyethylene terephthalate, which has an orientation of approx. 3.2 x 3.2, crystallinity approx. 50% and melting point approx. 260°C.

Biaxial PET (II): Represents biaxially oriented polyethylene terephthalate, which has an orientation of approx. 2.5 x 2.5 crystallinity approx. 45% and melting point approx. 260°C.

Biaxial PET (III): Represents biaxially oriented polyethylene terephthalate containing 5% 80:20% mole copolymer of ethylene terephthalate and ethylene isophthalate. The polymer has an orientation of approx. 3.2 x 3.2, crystallinity approx. 50% and melting point approx. 260°C.

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The metal/polymer laminates of Examples 6 through 15 were formed into various parts for containers and lids, such as deep drawn and redrawn cans and ends of aerosol or beverage containers. Illustrations of the shapes of common products obtainable from the metal-polymer laminates of the present invention are shown in Figures 2 through 8 of the accompanying drawings, which show openable and non-openable can lids drawn and redrawn and partially reinforced can walls, easy-open beverage can lids, aerosol containers, aerosol cones and domes.

The properties of the laminates of Examples 6 to 15, after forming into products, such as drawn and redrawn cans or lids of aerosol or beverage cans, were verified by subjecting the products to various tests. The results are shown in Table 3.

Behavior during formatting

Polyethylene terephthalate coatings are controlled on laminates made of cans for food products (drawn and re-drawn cans with a diameter of 65 mm and a height of 100 mm), and the end parts of cans for aerosols and drinks. The inspection was carried out visually by immersion for 2 minutes in acidified copper, and then checking the copper deposits on the surfaces of the exposed metal. The results are presented in two columns titled "Shaping" in Table 3.

Behavior during retorting

Cans made of laminate were filled with a solution of citric acid (0.65%), sodium chloride (1.0%) and malic acid (0.42%) at pH 4.3, the upper end was connected to the can and retorted for 1 hour at 121°C. The can was cooled, opened and the state of the polymer film was examined. The results are shown in the column titled "Retorting" in Table 3.

Relative XRD peak height (see Table 2)

Biaxially oriented films or laminates are placed in an X-ray diffractor. The count rate was measured when a flat sample was exposed to a beam of monochromatic X-rays using a detector. The sample and the detector rotate harmoniously, in relation to the beam, maintaining such a geometry that the angle between the sample and the beam (G) and the detector and the beam remains in the ratio 1:2, as in the case of diffraction imaging of normal powder. This arrangement creates information on flat parallel surfaces of the sample.

In the case of biaxially oriented polyethylene terephthalates, the plane (1,0,0) gives a high counting rate at θ = 13°, but in amorphous PET this peak is absent. The peak height ratio at θ = 13 for the laminate and the original film is related to the amount of residual orientation. The relative height of the XRD peak (Table 2) represents the ratio of the count rate for the laminated PET coating to the corresponding free film at 9 = 13.

The results are listed in Table 3 and show that laminates according to the present invention can be used on

satisfactory method for both shallow drawn parts and deep drawn parts (see examples 8 to 15) .

Laminates prepared under the more favorable conditions of the process of this invention are given in Examples 10, 12 and 14 and can easily be formed from them into shallow drawing parts of acceptable properties, as well as deep drawing parts which show no loss of corrosion protection. In contrast, the laminated polyesters according to GB 2133746, shown in examples 6 and 7, fracture if the laminate is drawn deeper. Polyester according to GB 2123746 has a limited elongation at break and this is usually exceeded when forming deep drawn cans with the result that the polyester coating cracks and the corrosion protection of the sheet is lost.

Examples 10, 12 and 14 in Tables 2 and 3 show more favorable production conditions in accordance with this invention. A comparison of the properties of the laminates obtained in these examples with the properties of the laminates obtained in examples 9 and 13 shows that the application of pre-lamination metal temperatures of 150°C can cause poor adhesion of the polyester film to the sheet after forming and retorting. The favorable metal temperature before applying the film to the sheet is 200-250°C, below the melting point of PET in the outer layer (A2).

Example 15 shows that if the conditions (i.e. temperature) during reheating are such that they lead to complete melting of the polyester coating, the coating becomes dark during retorting and appears unacceptable.

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Claims (17)

1. Process for the production of polymer/metal/-polymer laminates by simultaneous lamination, characterized by the fact that it includes i) simultaneous lamination on each of the main sheet surfaces of a composite polyester film (A) consisting of an inner layer (Al), usually a non-crystalline polyester whose softening temperature (Ts-Al) is below 200°C and melting point (Tm-Al) above 150°C, but below 250°C, and the outer layer (A2) of biaxially oriented linear polyester, whose crystallinity is greater than 30% and whose melting point (Tm-A2) is above 250°C, whereby the sheet is heated to a temperature T1 above the softening temperature (Ts-Al), or, even better, above the melting point (Tm-Al) of the polyester from the inner layer (Al), in order to cause the softening or, even better, the melting of the inner layers (Al) and their adhesion to sheet, but below the temperature (Tm-A2), at which the outer layer (A2) will melt when it comes into contact with the metal strip at temperature T1 and ii) reheating the resulting laminate to a temperature T2, sufficient to cause the polymer films (Al) to react with the sheet, but so that the outer surface of the outer layer (A2) remains below the temperature (Tm-A2). 2. The method according to claim 1, characterized in that the temperature T1 is from 120 to 260°C, or even better from 200 to 250°C. 3. The method according to claim 1 or 2, characterized in that the laminate is reheated using induction heaters. 4. The process according to any of the preceding claims, characterized in that the temperature T2 is from 250 to 270°C. 5. The method according to claim 4, characterized in that the laminate is heated to a temperature of 250 to 270°C, and then held for at least 1 second at 200°C before cooling. 6. The method according to claim 5, characterized in that the laminate is heated to a temperature of 250°C, and then held for 2 seconds above 240°C before cooling. 7. The method according to claim 5 or 6, characterized in that the laminate is cooled quickly and evenly, preferably by immersion in a tank with water or line cooling with water. 8. The method according to any of the previous claims, characterized in that the polyester of the inner layer (Al) is a copolymer of ethylene terephthalate and ethylene isophthalate, or a copolymer obtained from terephthalic acid and two alcohols, usually ethylene glycol and cyclohexane-dimethanol. 9. The method according to claim 8, characterized in that the molar ratio of ethylene terephthalate and ethylene isophthalate is 80:20. 10. The method according to any of the preceding claims, characterized in that the outer layer (A2) is biaxially oriented polyethylene terephthalate. 11. The method according to claim 10, characterized in that the polyester film of the outer layer (A2) has a crystallinity of 40 to 50%. 12. The method according to any of the previous claims, characterized in that the sheet is steel, electrolytically coated with chromium with a double layer of metallic chromium and chromium oxide. 13. Laminated sheet, characterized in that it contains a polymer film applied to both main surfaces, wherein the polymer films are applied to the sheet by simultaneous thermal lamination, and the polymeric film applied to each main surface of the sheet is a composite polyester film (A), which consists from the inner layer (Al), usually from non-crystalline linear polyester, whose softening temperature is below 200°C and melting point above 150°C, and below 250°C, and from the outer layer (A2), biaxially oriented polyester, whose melting point is above 250°C. 14. Laminated sheet according to claim 13, characterized in that the polyester of the inner layer (Al) is a copolymer of ethylene terephthalate and ethylene isophthalate, or a copolymer obtained from terephthalic acid and two alcohols, usually ethylene glycol and cyclohexane-dimethanol. 15. Laminated sheet according to claim 14, characterized in that the molar ratio of ethylene terephthalate and ethylene isophthalate is 80:20. 16. Laminated sheet according to claim 13, 14 or 15, characterized in that the outer layer (A2) is biaxially oriented polyethylene terephthalate. 17. A laminated sheet according to any one of claims 13 to 16, characterized in that the sheet is actually steel, electrolytically coated with chromium with a double layer of metallic chromium and chromium oxide.