WO2018184911A1 - Process for producing a polymer coated metal substrate and a metal strip substrate provided with a polymer coating - Google Patents

Abstract

This invention relates to a process for producing a polymer coated metal substrate and a metal strip substrate provided with a polymer coating.

Description

PROCESS FOR PRODUCING A POLYMER COATED METAL SUBSTRATE AND A METAL STRIP SUBSTRATE PROVIDED WITH A POLYMER COATING.

This invention relates to a process for producing a polymer coated metal substrate and a metal strip substrate provided with a polymer coating.

In the packaging industry the use of polymer-coated substrates is becoming more and more common in the production of cans. The polymer coated substrate can be produced by extruding a molten polymer film directly onto the metallic substrate or by producing a thermoplastic polymer film that is subsequently laminated, as a solid film, onto a metallic substrate in an integrated or separate lamination process step.

Extrusion coating of a metal substrate is disclosed in for instance EP0067060 and US5919517. Several problems are associated with this technique, in particular the tendency of a molten polymer web to decrease in width after leaving the die ('neck-in'), thickening of the film edges, waviness of film edges, which result in unstable processing behaviour and unsatisfactory properties of the coated product.

An improvement of the above direct extrusion coating process involves the use of an intermediate roll to support the molten web while leaving the die, and to stabilise the extruded film, remove its thick edges, etc. The extruded film is then brought into contact with the metal substrate without intermediate winding and/or unwinding of the film (see, for instance, EP1019248).

In spite of the above improvements, certain intrinsic limitations of the extrusion coating process still remain. Most importantly, the speed of the extrusion coating process is limited. While the substrate can be moved, without any problems, at very high speeds, up to several hundreds of meters per minute, extruding a molten polymer film from a flat die is limited by various factors. Of particular importance is the phenomenon of melt fracture due to critical shear stress at the wall of the die. When multi-layered films are extruded, which is common in polymer-coated metal substrates (see EP0067060 and US5919517), other phenomena such as interfacial flow instability will occur (W. J. Schrenk et al., Polym. Eng. Sci. 18 (1978) 620). This will lead to non-uniform layer thickness, waviness and fold-over of the layer interface, intermixing of layers, and irregular surface structure and appearance.

Also from economic perspective, the extrusion coating process is not preferred. In order to have an efficient process, the coating operation has to be performed in a continuous, reel-to-reel process. Handling of the metal substrate at high speed in such a process requires a processing line equipped with multiple uncoilers, recoilers, cutters, bridles, looper towers, welding machine and the like, requiring a very high capital investment. Process interruption, product change-over and maintenance stops associated with the extrusion process also stop the metal processing line and severely limit its utilisation rate. Therefore running an efficient and high-speed coating operation based on an extrusion process is very difficult.

Many of the above problems can be overcome by using a film lamination process rather than an extrusion coating process. In a film-laminating process, a pre-manufactured polymer film is supplied, unwound and passed through a pair of laminating rolls together with the strip of metal substrate and optionally a second film, simultaneously laminated to the opposite side of the strip. This process can be conducted efficiently, at high speed with maximised utilisation of the metal processing line. The quality of the polymer film can be checked prior to the lamination stage, thus avoiding the manufacture of defective product due to insufficient film quality. Well-known examples of film lamination process are based on the use of biaxially stretched polyester films (see, for instance, EP0312303, EP0312304 Bl and EP901899).

A separate film lamination process allows higher processing speeds, but processability and success of the lamination step is strongly dependent on the mechanical and physical properties of the film. The extruded films from thermoplastic polyesters such as PET are mechanically very weak and cannot be used in a film lamination process at a commercially viable speed. Moreover, cast polyester films are susceptible to 'physical aging', which tends to further degrade the mechanical properties and handling characteristics when the cast film is stored. For these reasons polyester films for laminating onto metallic substrates are usually biaxially drawn to achieve the necessary level of mechanical strength and stability.

Although the biaxial stretching process generally results in films of high quality and homogeneity, the biaxial stretching process is a complex operation, requiring large-scale and complex equipment with high capital investment and high operational cost. Also, due to the scale of the process, biaxial film stretching lines do not easily allow changes in e.g. the polymer film composition and are inflexible in terms of product change-over. Hence, a simpler process was proposed in EP2701905, which uses a cast film that is stretched in the longitudinal direction (also called machine direction) only. This greatly simplifies the film manufacture process and product flexibility while still producing a film that is suitable for the high-speed film laminating process outlined above.

During casting of a single- or multi-layer film, there will always be imperfections in the film that are oriented along the casting direction. The most commonly known defects are variations in film thickness across the width of the film. Also commonly known are variations in thickness of individual sub-layers in multi-layer films. These also lead to imperfect products (e.g. colour inhomogeneity if a colour is added to only one sub-layer) even when the total film thickness is constant across the film width. However, stretching in the longitudinal direction does not resolve, and may even amplify, machine direction propagated imperfections, such as die lines, flow lines, and other film defects related to flow instabilities in the casting process. Hence, there is still opportunity to improve current processes for manufacturing polymer-coated metal substrate of high quality in an economical and technically feasible manner.

It is an objective of this invention to improve the film lamination process by reducing machine direction propagated imperfections in the polymer film to be laminated.

It is also an objective of this invention to provide an alternative process for producing a polymer film for producing polymer-coated metallic substrates.

It is also an objective of this invention to provide a process for producing a polymer coated substrate which is excellent in coating homogeneity, homogeneous coating appearance, colour and can forming behaviour.

It is also an objective of this invention to provide a process for producing a polymer coated substrate at high speed.

It is also an objective of this invention to provide a process for producing a polymer coated substrate at high speed requiring a lower capital expenditure.

One or more of these objectives are reached by a process for producing a polymer- coated metal substrate comprising the steps of:

providing a metal strip as a substrate;

providing a polymer film for coating onto the substrate;

optionally providing an adhesion layer for promoting the adhesion between the substrate and the polymer film;

wherein the polymer film consisting of one or more layers is produced by:

melting one or more suitable mixtures of polymer granules in one or more extruders;

passing the one or more molten polymers through one or more dies or calendars to form a cast polymer film consisting of the one or more layers;

cooling the cast polymer film to form a solid polymer film;

optionally trimming the edges of the extruded polymer film;

reducing the thickness of the solid polymer film by stretching the solid polymer film in a stretching unit by exerting a stretching force only in the transverse direction (TD);

subjecting the TD-stretched polymer film to a heat-setting step;

optionally trimming the edges of the TD-stretched polymer film;

optionally coiling the TD-stretched polymer film and uncoiling the TD-stretched polymer film;

followed by:

laminating the TD-stretched polymer film onto the substrate to produce a polymer-coated substrate;

post-heating the polymer-coated substrate to significantly reduce (or remove) the molecular orientation and crystallinity of the polymer film; cooling, preferably fast cooling, of the post-heated polymer-coated substrate, optionally coiling the cooled post-heated polymer-coated substrate

The Transverse Direction or Cross-machine Direction stretching process (TD stretching) naturally smoothens out machine direction propagated imperfections, such as die lines, flow lines, and other film defects related to flow instabilities in the casting process. In the stretching process, thicker parts will stretch a little bit further than thinner parts as the film will attempt to reach a constant stress over its entire width. As a result, all regions will reach the same final thickness after stretching. TD stretching will increase the width of the film and decrease its thickness, while the film length will remain essentially constant. For example, in an ideal stretching process with TD-stretch ratio of 4.0, a cast film of 0.50 m in width and 100 μιτι in thickness, subjected to this process, will be converted into a TD- stretched film of about 2.00 m wide and 25 μιτι in thickness. Therefore, the cast film width and thickness need to be chosen with the desired properties and dimensions of the TD- stretched film in mind. In the case of polyester films, which is a preferred embodiment of the present invention, cast films are usually non-oriented and essentially in an amorphous state. A suitable temperature at which such amorphous films can be stretched is around the glass transition temperature (Tg). A suitable range for practical purposes in the invention is that the stretching temperature is between Tg - 30 and Tg + 30 °C, preferably between Tg - 20 and Tg + 20 °C. The Tg of the cast film can be conveniently determined by various methods, such as dynamic mechanical thermal analysis (DMTA) or differential scanning calorimetry (DSC) as described in ISO 11357-2 :2013 : Determination of glass transition temperature.

After stretching, it is necessary to heat-set the film. The film is heated to a suitable temperature for a brief period of time, while being held under tension. This treatment will affect (further) crystallisation of the film while being kept under tension. The stretching process itself, which is conducted at around Tg, may already introduce some crystallisation. The time and temperature conditions of heat setting should be sufficient to affect crystallisation of the film and to reduce the thermal shrinkage of the film. The heat-setting step needs to further increase the degree of crystallinity and thus needs to be conducted at a substantially higher temperature than Tg. For crystallisable polymers, the temperature where crystallisation is fastest is normally about half way between Tg and the melting point (Tm). We may consider this Tmid = Tg + (Tm - Tg)/2 to be the minimum temperature for heat-setting. To enable a short treatment time, higher temperatures, up to Tmid + 60°C, may be employed. A well-known polyester, suitable to be used in the present invention, is polyethylene terephthalate or PET with Tg = 80°C and Tm = 260°C. Optimum heat-setting temperatures are therefore in the range 170 - 230°C. In order to limit the size of the equipment, in particular the length of the heat-set oven, the heat-set time should be short, preferably 2 seconds or less, preferably 1 second or less. Typical conditions are 1 second at 210°C.

The heat-setting step is necessary in order to limit the amount of thermal shrinkage in the width direction of the TD-stretched film. If shrinkage in the width direction of the film would occur during laminating, both the thickness and the width of the film would change prior to laminating, resulting in a defective laminated product. Whereas shrinkage of the film in longitudinal direction can be easily suppressed by applying tension to the film in the machine direction, this method cannot be applied to suppress shrinkage in the transverse direction of the film. Therefore, the TD-stretched film should have intrinsically low shrinkage properties and for this, heat-setting is considered to be a necessary step.

The TD-stretched polymer film can be produced in a two-step process (film casting and stretching done separately) or in an integrated process (film casting combined with stretching). In the two-step process the solid polymer film is preferably wound onto a reel or coil prior to being fed to the stretching unit. There may be a considerable amount of time between the production of the solid polymer film and the stretching step in the two-step process. The alternative is that the solid polymer film is fed directly into the stretching unit to produce the TD-stretched polymer film.

In the film casting process, a molten polymer film consisting of one or more layers is produced by melting a suitable mixture of polymers, e.g. in the form of granules, in one or more extruders and passing the molten polymer through an extrusion die, usually a flat die. The molten polymer film is solidified, e.g. by casting it onto a cooled roll, or in case of a calendar process, between two or more rolls. The film is then essentially amorphous and non-oriented. When the film contains more than one layer, obtainable for instance by co- extrusion, one of the outer layers will function as a so-called adhesion layer, having a composition such that it will create a better bond to the metal substrate than the other layers. After casting, thick edges of the film resulting from 'neck in' can optionally be trimmed off. Any trimmed-off material may be fed back into one of the extruders, optionally after intermediate reprocessing, to limit material losses and to optimise cost efficiency. The cast and trimmed film is either coiled onto a reel or passed on directly to the stretching unit. It is also possible to slit the solid polymer film into a plurality of narrower films, optionally coiled and uncoiled, and wherein the narrower films are subsequently stretched in the transverse direction and subjected to a heat-setting step, optionally trimmed, and optionally coiled.

In the stretching process, the solid polymer film is fed through an appropriate stretching unit. The TD-stretching process can be performed by different methods, which are well-known in the art. The technology dates back from the time when biaxial stretching of polymer films was developed (see, for instance, US2779684). The most common method uses a so-called tenter frame. The film is directed between parallel rows of tenter clips. The tenter clips grasp the edges of the film and move outward to stretch the film transversely. Typically, a TD stretching unit is divided into four zones, including a preheating zone, a stretching zone, a heat-setting zone and a cooling zone. The first zone or pre-heating zone is represented by the entry of the stretching unit until the transverse stretching begins. The second zone is the section between the beginning and the end of the transverse stretching. The third zone is the heat-setting zone where the film is subjected to an elevated temperature, as described above, while the film is held under transverse tension. The fourth zone is usually open to the environment and serves to cool down the film. Many variations of the classical tenter frame process have been described, for instance in EP0016653, US3816584 and US7229271, and references therein. For our present invention, the method employed to conduct the transverse stretching is not restricted.

After completing the stretching, the film edges may be trimmed to ensure proper winding and further processing of the drawn film. At this stage, the amount of material that needs to be trimmed off is usually very small. This trimmed-off material can be recycled. After stretching and the optional trimming the film is coiled on a reel. Between stretching and coiling, one or more of defect inspection, gauge measurement, surface treatment (corona, flame, spraying of (liquid) additives or agents, etc.) and/or slitting into multiple widths may be performed. When the casting and stretching is done at multiple widths of the final end product, the relative fraction of the material trimmed off and potentially lost is substantially smaller, thus resulting in a higher yield.

A disadvantage of the non-oriented solid polymer film is that it is mechanically weak and possibly brittle. However, the inventors found that it can be processed excellently in the transverse stretching process because the film is still relatively thick at that stage of the process. Also, the above-mentioned physical aging process does not severely limit the processability of the cast film, provided that it is sufficiently thick. The inventors found that a suitable minimum thickness of the solid polymer film prior to stretching is in the order of 50 micron. The stretched films have a thickness that corresponds to the desired final thickness of the polymer coating on the metal substrate. In other words, laminating the stretched film onto the metal substrate produces directly the desired coating thickness. Typically, the thickness of the stretched film is between 5 and 50 microns.

In the process according to the invention it is essential that the cast film is stretched in the transverse direction only and not stretched in the longitudinal direction. As a result of the transverse stretching process, machine direction propagated imperfections originating from the film casting process are eliminated, which is one of the main benefits of the present invention. The TD-stretched polymer film may be highly crystalline and/or oriented. Therefore the metal substrate, after laminating with the film, is heated to a post heat temperature designed to remove all orientation and crystallinity present in the coating. A subsequent rapid cooling step creates a polymer coated metal strip with a highly amorphous (i.e. largely non-crystalline) and essentially non-oriented polymer coating. This material is suited to create a very good formable material, with excellent adhesion and barrier properties, and thus very suitable for making e.g. deep drawn cans. Essential is the high speed with which this process can be used. Only technical limitations and control issues limit the speed at which the laminating line can be run. The inventors found that the process can be excellently performed at line speeds of from 400 to 700 m/min. Higher speeds of up to 1200 m/min are currently being considered.

The inventors found that polymer films, such as polyester films, which are perfectly suitable for lamination onto metallic substrates at high speed, can be produced by the inventive process. The polymer film is cast at a relatively high thickness and subsequently drawn and oriented in the transverse direction only. By the TD-stretching process, the film becomes wider and thinner, and the desired final thickness of the polymer coating film is thereby achieved. Also, if the drawing process is conducted under the proper conditions, the film will achieve a high degree of homogeneity, high mechanical strength, low shrinkage and good handling characteristics for high speed lamination, and is freed from physical aging, thus allowing virtually unlimited storage of the TD-stretched film prior to lamination. To improve the surface properties of the metal strip, it is possible to add a surface treatment of the steel and/or the film prior to the entry in the lamination nip. Examples are ozone generators, corona treatment or flame treatment. These additional treatments are not essential, but give an improved performance if needed.

The metal strip substrate is preferably preheated prior to laminating the TD-stretched polymer film onto the substrate to produce the polymer-coated substrate. If the substrate is preheated, then it is preferably preheated to a temperature between the glass transition temperature and the melting temperature of the TD-stretched polymer film or films, or in case of a multi-layer film, at least to a temperature between the glass transition temperature and the melting temperature of the layer of the TD-stretched polymer film or films that will be adhered to the substrate (the adhesion layer).

Also, after the final quenching step following the post heat step additional heat treatments can be applied with which the physical structure of the coating (e.g., crystal I in ity) can be further modified. Examples for such a treatment are flame treatment, corona treatment, infrared heaters, lasers or hot air furnaces. This treatment can further improve the barrier properties of the film at the expense of some of the formability of the polymer. However, for some particular applications this loss of formability may be justified.

The time between the coiling of the TD-stretched film and the lamination onto the metal strip can vary between almost immediately after stretching and coiling, or even inline without intermediate coiling, to very long. The stability of the sufficiently TD-stretched film is such that the film can be stored for up to 5 years or longer. However it is preferable to process the film within 6 months, and even more preferably within 1 month. Lamination of the TD-stretched film onto the metal substrate is preferably accomplished in a process separate from the TD-stretched film production because of the vulnerability for disturbance in the chain of high speed processes.

In an embodiment the draw ratio of the solid polymer film is between 2 and 12, preferably at least 3 and/or at most 6. These values are optimised from a technical and commercial point of view.

Lamination can be performed in the same line where a metallic coating on the strip is applied, for example a tinplating line. It could also be done in a stand-alone, independent lamination line. The lamination onto the substrate is done using pressing rolls. It can be done on one side or two sides, depending on the application of the coated metal strip. Always a pair of rolls is used for pressing the film against the metal. When coating both sides of the substrate with the TD-stretched film, it can be done simultaneously or in two steps.

The polymer films are laminated to the steel strip by a process schematically shown in Figure 1. Figure 2 shows a preferred embodiment of such a process. The metal strip (1) is passed through a first heating device (2) where the temperature of the metal strip is raised to a value suitable for lamination, Tl. A film coil (3a) and (not needed for single side lamination) another film coil (3b) are unwound and passed, together with the pre-heated metal strip, through a pair of laminating rollers (4a, 4b). The laminated product (5) is passed through a second heating device, the post-heat device (6), to remove the orientation and crystallinity within the chosen residence time in the post heat section. The gas atmosphere in the second heating device can be air, or a protective or inert atmosphere such as nitrogen atmosphere containing less than 1000 ppm of oxygen. After the post- heat device, the laminated product is rapidly cooled by a quenching device (7, not shown) such as a tank filled with cold water.

The method of pre-heating the metal strip in the first heating device is not particularly limited and may include passing the strip over heated rolls, conductive heating, inductive heating, radiative heating etc. The method of post-heating the laminated product in the second heating device is preferably a contactless method, such as heating in a hot gas environment or inductive heating.

To achieve a good bond between the metal and polymer film, two adhesion techniques can be used. The first technique involves the use of a (liquid) adhesion promoter or primer. The adhesive layer is applied in e.g. liquid form for example by dipping, spraying or roll coating to either the metal substrate or to the polymer film. The second method is known as heat seal lamination. The metal is heated to a temperature which results in softening of the layer of the film which is brought in contact with the metal. This layer is known as the adhesion side or, when a multi-layer film is used, the adhesion layer. The required substrate preheat temperature depends on the polymer to be laminated upon the substrate. For amorphous polymers the temperature is at least 50°C above the Tg. For (semi-)crystalline polymers, the substrate preheat temperature is between 10 to 50°C below the melting point of the highest melting polymer in the adhesion layer. The exact temperature used is determined using for example viscosity data of the polymers used, the line speed, the lamination pressure, the modulus of the film, the roughness of both the film and the metallic strip, etc. and is a matter of routine experimentation. The preheat temperature is chosen such that the adhesion layer will completely cover the roughness of the metal strip, where the outside of the film, touching the laminating rolls should not exceed the sticking temperatures of the film on the lamination rolls to prevent sticking of the film to these rolls. Preferably the lamination pressure in the laminating step to laminate the TD-stretched film onto the substrate is between 0.1 MPa and 10 MPa, preferably at least 0.5 and/or at most 2.5 MPa.

After the metal substrate has been prepared in a suitable way, the TD-stretched polymer film is brought in contact with the strip using laminating rolls. These rolls are pressed onto the metal strip to generate a good bond. The laminating rolls are usually covered with a relatively thick elastomeric coating which create a laminating nip when pressed together, and which can adopt to thickness variation or thickness profile ("strip crown') of the metal substrate and coating film without damaging either coating or substrate. Also, the rolls can be cooled, at least on the outside, but optionally also on the inside of the roll. The roll diameter, thickness of the elastomeric covering and applied roll pressure may be configured and optimised in such a way that an appropriate laminating nip is created, thus allowing sufficient time for the metal substrate and polymer film to establish a good bond. The tension in the polymer film should be carefully controlled to ensure good web tracking and avoid laminating defects such as creases, folds and wrinkles.

The lamination pressure in the laminating step, e.g. in the lamination nip between two laminating rolls, is preferably between 0.1 MPa and 10 MPa. Higher values will result in excessive wear of the lamination rolls, lower pressure will result in insufficient adhesion between the coating and the metal and in an increased risk of air entrapment. Preferably the lamination pressure is at least 0.5 MPa and/or at most 2.5 MPa.

After the nip, the coated strip is optionally cooled using e.g. cold air, in order to impart sufficient rigidity, strength and/or toughness for further handling of the semifinished product and to allow contact with additional rolls which may be present in the lamination process (deflector rolls etc.).

After cooling, the essential post heating step is applied . The temperature setting of the post heat is defined by the polymer properties. The TD-stretched film is highly oriented and, if crystallisable polymers are used, highly crystalline. The post heat temperature is chosen such that that the orientation and crystallinity is removed within the chosen residence time in the post heat section. The residence time is preferably at least 0.1 and preferably at most 10 seconds, or preferably at most 5 seconds. For polycondensates, such as polyesters or polyamides, the post heat temperature is preferably between Tm and Tm + 50°C. For non-crystallisable vinyl polymers, such as polystyrene or polyacrylate, the post heat temperature is preferably between Tg + 50°C and Tg + 150°C and for crystallisable polyaddition polymers, such as polyolefins, the post heat temperature is preferably between Tm + 50°C and Tm + 150°C. The post-heating step may be conducted in an inert or non-oxidising gas atmosphere (see co-pending EP16163683.2). Although it is preferable that all orientation and crystallinity is removed, a small amount of crystallinity and/or orientation is allowable. However, this must not exceed more than 50 %, or preferably more than 20% of the crystallinity and/or orientation which existed prior to the post-heat. A method for measuring crystallinity by X-ray diffraction is given in GN 1566422, page 5 line 31-50. Alternatively the crystallinity can be determined from density measurements as described in EP0312304, page 2, line 27-37. Crystallinity can also be determined by differential scanning calorimetry (DSC), e.g. using a Mettler Toledo DSC821e calorimeter operated at a sample heating rate of 10°C/min.

The hot metal coated strip is cooled very rapidly after exiting the post heat section. This is preferably done in a cold water bath, but could also be done with cooled rolls or cold gasses, as long as the cooling rate of the polymer film is at least 100°C/s, more preferably at least 400°C/s.

The width of the laminating film and the width of the metal substrate may be adjusted to each other by various means, in order to achieve the desired laminated end product. For instance, the film may be wider than the metal substrate, and the excess width is trimmed off just before the laminating rolls. Also, the wider film may be laminated to the metal substrate, and trimming off the excess width, together with a small amount of metal strip after laminating, to obtain a metal strip covered with the polymer film over its entire width. The film may also be somewhat narrower than the metal strip, leaving a small bare edge of uncoated metal at the side of the strip. By these various methods, it is possible to produce films in a limited number of fixed widths, thus greatly enhancing productivity and efficiency, while still offering a wide range of finished product widths.

Although the polymer film produced according to the invention can be used for applications other than cans or containers, it is particularly suitable for those applications where properties like adhesion, barrier properties and formability are essential. This makes it very suitable for the production of cans and containers. However the film may also be used in the production of laminated metal substrates for building materials, furniture or materials for transport applications (automotive, aerospace, etc.).

Polymer-metal laminates made using this process may be used for cans or containers, more preferably formed cans made using deep drawing and/or stretching and/or wall ironing. The TD-stretched polymer films and polymer coated substrates that can be produced by the process according to the invention are preferably based on polycondensates, such as polyesters, co-polyesters (including PET, PBT, polyethylene furanoate (PEF), poly(lactic acid) (PLA)) or polyamides, polyolefins, elastomers, non-crystallisable vinyl polymers, such as polystyrene, polyacrylate, PVC or PVDC, crystallisable polyaddition polymers, such as polyolefins, or any other polymer that can be formed in a film by extrusion. The polymer coating may consist of one or more layers. Preferably the TD-stretched film comprises or consists of polyethylene terephthalate, IPA-modified polyethylene terephthalate, CHDM- modified polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polyethylene furanoate, poly (lactic acid) or copolymers or blends thereof.

The process according to the invention has particular advantages when producing polymer films which consist of essentially linear, thermoplastic polymers produced by polycondensation reactions (polyesters, polyamides, polycarbonates, polyimides etc.). This structure limits the speed at which these polymers can be extruded and therefore extrusion coating for these polymers is limited to low speeds. For polyolefins such as PE and PP, the maximum possible extrusion speeds are much higher due to their molecular architecture (high molecular weight, short-chain branching, long-chain branching, etc.). Extrusion and extrusion coating at > 600 m/min is known for polyolefins.

In an embodiment the one or more of the layers of the TD-stretched film contains colour additives such as inorganic pigments or organic dyes, and/or wherein the TD- stretched film is non-transparent or opaque, and/or wherein the TD-stretched film is white. Because of the TD-stretching, any local differences in pigmentation as a result of the casting of the polymer film will have been removed by the TD-stretching.

The metallic substrate can be an uncoated metal such as steel or aluminium or aluminium alloys or a metallic-coated metal such as tinplate or galvanised steel, and may contain an additional conversion layer or passivation layer to further enhance the product performance and/or promote adhesion between the metal and the polymer coating. This additional conversion layer or passivation layer can e.g. be based on chromium oxide, chromium/chromium oxide, titanium oxide, zirconium oxide, phosphates.

The present invention is aimed to produce polymer-coated materials at high productivity with relatively low capital expenditure (compact unit operations), relatively low fixed costs whilst maintaining the variable costs (high line speed) and flexible production logistics (integrated drawing or not, variable storage time possible, easy polymer changeover). Enabling high line speed is one of the key benefits of this invention but it will also work at lower line speeds. The process according to the invention provides excellent polymer- coated metals, which can be produced at extremely high speeds having excellent properties to produce a can from the material. Also, the process can be operated using compact unit operations and allows high flexibility in product composition and production logistics. The invention will now be explained further by means of the following non-limiting figures and examples.

EXAMPLE

A cast film of poly (ethylene terephthalate) was produced by melting the polymer granules in a lab-scale extruder, passing the molten polymer through a flat die and cooling the extruded polymer film on a chilled casting roll. The as-produced film had a thickness of 400 μιτι and contained clearly visible surface irregularities in the film casting direction.

Stretching of the film was done using a Bruckner Karo IV lab-scale biaxial stretching machine. Samples measuring 9 x 9 cm were cut from the cast film and placed in the stretching machine. The samples were stretched at various ratios as specified in Table 1 at a stretching temperature of 80°C, followed by heat-setting at 200°C during 1 second while keeping the sample under tension. When the samples were stretched in the machine direction (machine direction stretching or MD-stretching), the initially present surface irregularities of the cast film persisted in the stretched sample. When the samples were stretched in the transverse direction (transverse direction stretching or TD-stretching), the initially present surface irregularities were removed and a stretched sample with perfectly homogeneous structure was obtained.

Table 1. Stretching experiments

FIGURES

The invention is further described in the following, non-limiting figures. The invention is further described in the following, non-limiting figures. Figure 1 shows schematically (from left to right) the steps to arrive to a polymer coated material based on a metal strip and (in this case) two polymer films by lamination. Fig. 2 shows a preferred embodiment of this process for double-sided lamination of polymer films on a steel strip. The polymer film prior to lamination are heat-set TD-stretched polymer film. Obviously, if only one heat- set TD-stretched polymer film would be provided, then the substrate would be coated on one side only. Figure 3 shows a schematic sequence of the various steps, including some optional (dashed) ones. Finally, in figure 4, the difference between TD-stretching, MD- stretching and biaxial stretching is clarified.

Claims

Process for producing a polymer-coated metal substrate comprising the steps of:

- providing a metal strip as a substrate;

- providing a polymer film for coating onto the substrate;

- optionally providing an adhesion layer for promoting the adhesion between the substrate and the polymer film;

wherein the polymer film consisting of one or more layers is produced by:

- melting one or more suitable mixtures of polymer granules in one or more extruders;

- passing the one or more molten polymers through one or more dies or calendars to form a cast polymer film consisting of the one or more layers;

- cooling the cast polymer film to form a solid polymer film;

- optionally trimming the edges of the extruded polymer film;

- reducing the thickness of the solid polymer film by stretching the solid polymer film in a stretching unit by exerting a stretching force only in the transverse direction (TD);

- subjecting the TD-stretched polymer film to a heat-setting step;

- optionally trimming the edges of the TD-stretched polymer film;

- optionally coiling the TD-stretched polymer film and uncoiling the TD-stretched polymer film;

followed by:

- laminating the TD-stretched polymer film onto the substrate to produce a polymer- coated substrate;

- post-heating the polymer-coated substrate to significantly reduce (or remove) the molecular orientation and crystallinity of the polymer film;

- cooling, preferably fast cooling, of the post-heated polymer-coated substrate.

- optionally coiling the cooled post-heated polymer-coated substrate

Process according to claim 1 wherein the solid polymer film is wound onto a reel prior to being fed to the stretching unit.

Process according to claim 1 wherein the solid polymer film is fed directly to the stretching unit.

Process according to any one of the preceding claims wherein the solid polymer film is slit into a plurality of narrower films, optionally coiled and uncoiled, and wherein the narrower films are subsequently stretched in the transverse direction and subjected to a heat-setting step, optionally trimmed, and optionally coiled.

Process according to any one of the preceding claims wherein the TD-stretched and heat-set solid polymer film is slit into narrower films.

Process according to any one of the preceding claims wherein the substrate is preheated prior to laminating the TD-stretched polymer film onto the substrate to produce the polymer-coated substrate.

Process according to claim 6 wherein the substrate is preheated prior to laminating the TD-stretched polymer film onto the substrate to a temperature between the glass transition temperature and the melting temperature of the TD-stretched polymer film or films.

Process according to any one of the preceding claims wherein draw ratio is between 2 and 12, preferably at least 3 and/or at most 6.

Process according to any one of the preceding claims wherein the lamination pressure in the laminating step is between 0.1 MPa and 10 MPa, preferably at least 0.5 and/or at most 2.5 MPa.

0. Process according to any one of the preceding claims wherein the trimmed-off material resulting from the trimming of the edges of the extruded polymer film and/or the TD-stretched polymer film is fed back into one or more of the extruders after intermediate reprocessing of the trimmed-off material or immediately after trimming.

1. Process according to any one of the preceding claims wherein the TD-stretched film comprises or consists of one or more of a polymer from the group of polymers consisting of:

- polycondensates, such as polyesters, co-polyesters or polyamides

- non-crystallisable vinyl polymers, such as polystyrene, polyacrylate, PVC or PVDC or

- crystallisable polyaddition polymers, such as polyolefins.

2. Process according to any one of the preceding claims wherein the TD-stretched film comprises or consists of polyethylene terephthalate, IPA-modified polyethylene terephthalate, CHDM-modified polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polyethylene furanoate, poly (lactic acid) or copolymers or blends thereof.

13. Process according to any one of the preceding claims wherein the one or more of the layers of the TD-stretched film contains colour additives such as inorganic pigments or organic dyes, and/or wherein the TD-stretched film is non-transparent or opaque, and/or wherein the TD-stretched film is white.

14. Process according to any one of the preceding claims wherein TD-stretched polymer film is laminated onto both surfaces of the substrate to produce a double-sided polymer-coated substrate.

15. Polymer-coated substrate obtainable by the process according to any one of claims 1 to 14, and/or can or container produced from the polymer-coated substrate obtainable by the process according to any one of claims 1 to 14.