METHOD AND APPARATUS FOR COATING A METAL STRIP AND PRODUCT THEREOF Description of the Invention This invention relates to a method and apparatus for applying a polymeric coating on a metal strip and in particular to a method for coating both sides of a strip. of aluminum with thermoplastic resins of extrusion apparatuses and extrusion nozzles which are positioned to deposit the thermoplastic resin on the opposite sides of the strip. The product of this invention is a strip of metal, such as aluminum, which has thin polimeric coatings on both sides thereof and which has many applications, but is particularly suitable for use in packaging applications such as ends of cans and can bodies. The coating of sheets or strips of metal with thermoplastic resins on one or both sides to improve the corrosion resistance, formability and appearance or other properties of the material is well known. The coating can be applied by a variety of processes such as rolcoating, reverse rolcoating, spraying or spraying, electrocoating, powder coating and laminating. The coated strip can be used for applications such as cans and ends of cans, thin film bags, lids material, appliances, electrical devices, construction, automotive or aerospace body sheets. U.S. Patent No. 5,093,208 issued to Heyes discloses a method for forming a laminated metal sheet in which a prefabricated thermoplastic polyester film is pressed against one or both REF: 25758 surfaces of a metal sheet to adhere the film to the sheet in a non-crystalline form. The uncoated metal sheet is heated to a temperature higher than the melting temperature of the polyester film and the film is applied to the sheet under pressure to form a laminate. Then the laminate is heated to a temperature higher than the melting temperature of the film, to improve the adhesion of the plastic film to the metal and it is quickly cooled to a temperature lower than the glass transition temperature of the polyester to form a polyester not crystalline. Abrupt cooling is done by passing the sheet through a curtain of water. European patent application 0,067,060 in the name of Tauyo Steel Ltd., describes a method for producing a coated metal plate by directly extruding (or extruding) a thermoplastic resin onto the heated surface of the plate. According to that patent application, molten resin is applied directly from the extrusion die to the metal plate without forming the resin in a separate film. The thickness of the film can be less than 50 microns and preferably 35 to 5 microns. The patent application states that because the step of forming an independent film is omitted, the cost of producing the coated metal is reduced. Suitable thermoplastic resins used for coating metal surfaces include polyolefins, acrylic resins, polyesters, polyamides, polyvinylchlorides and many other resins as listed in the published patent application. The resin can be coated either as a monolayer or multiple layers of the same resin or different resins. The patent application describes the application of the ream on only one side of the metal strip.
An improved process is desired to apply a thin polymeric coating on both sides of a metal strip suitable for use in applications such as packaging. A process is desired to produce strong adhesion or welding of the polymer to the strip, such that the polymer does not de-laminate during the subsequent formation of the strip or the use of the products resulting from the strip. This invention provides a method for coating both sides of a metal strip with thin thermoplastic polymer resin to form a coated strip suitable for use in packaging and other applications. t Correspondingly, an object of this invention is to provide an improved method for adhering the polyester resin on both sides of a metal strip. The foregoing and other objects and advantages of this invention will be more fully understood and appreciated with reference to the following description and the drawings appended hereto. Figure 1 is a schematic side elevational view of one embodiment of a system of this invention. Figure 2 is a schematic side elevational view of a portion of another embodiment of this invention. Figures 3 and 4 are schematic side elevation views of the additional embodiments of this invention. Figure 5 is a partial cross-sectional view of the strip and the extrusion nozzles of Figure 4, rather enlarged to show the application of the resin to the strip.
Figures 6 to 14 are schematic side elevation views of the additional embodiments of this invention. The drawings appended hereto illustrate systems for coating both sides of a metal strip as it travels from a first coil to a second coil onto which the metal is rolled after it is coated. With particular reference to Figure 1, an aluminum alloy strip 10 is unwound from the spool 12, moved around the tension rollers 14, travels vertically upwards on a roller 16 and then downwards from the roller 16 through the coating apparatus. A back-up cylinder 18 can be used to hold the metal strip 10 in a flat condition as it moves over the support roll 16. As the strip 10 moves downwardly from the roll 16, it is first heated by the heating element 20 at a temperature close to or greater than the melting temperature of the polymer to be applied thereto. In the embodiment illustrated in Figure 1, the heating element is an induction heating element, but other heating elements or preconditioners, such as flame, infrared, plasma and / or corona discharge can also be employed either individually or in combination. The heating elements to the flame can be used in tandem (one on each side) or on one side only to improve performance (to improve adhesion as well as heating). The coil 12 can also be used, which is still hot from the pre-processing, such as rolling or heat treatment, to minimize or even eliminate the need for heating by the heating element 20. A typical temperature at which the metal is heated, before the application of the thermoplastic matenal, is in the range of approximately 121-260 ° C (250 ° -500 ° F) depending on a variety of factors, mainly of the particular polymer that to be applied to the strip Two separate extrusion coating systems 21 and 31 are provided to apply thin strips of thermoplastic polymer, such as polyester resin to the two surfaces of the heated tape Systems 21, 31 are arranged just below the induction heating element 20 Extrusion coating systems 21, 31 each include an extrusion apparatus for feeding a molten polymeric extrusion product through a laminar nozzle 22, 32 having a narrow outlet slit to produce a ribbon thin extrusion product 24, 34, which is passed through a stack of three rollers Alternatively, an extrusion apparatus n can feed both extrusion nozzles via transfer tubes or other multiple The first rollers 26, 36 of the systems 21, 31 are clamping and stretching rollers which are maintained at a temperature which will promote adhesion and clamping of the product polymer extrudate to the polished roller surface A classical temperature for this purpose is in the range of approximately 120 to 180 ° C (248 ° F-356 ° F), depending on the ream used The surface speed of the rollers 26, 36 is substantially faster than the speed of the extrudate leaving the nozzle 22, 32, to thereby stretch the polymer to a reduced thickness. The common speed ratios of the drawing speed at the extrusion product rates range from about 5 1 to 40 1 The resin of the extrusion apparatus is usually around 0 127 - 0 635 mm (0 005 - 0 025 inches) thick and is stretched to a reduced thickness of approximately 0.0076 - 0.038 mm (0.0003 - 0.0015 inches) in thickness. The second rollers 28, 38 are colder than the first rollers and are designed to polish and cool the extrusion product by rolling contact between the rollers and the extrusion product. The second rollers 28, 38 also transfer the extrusion product to the third rollers which are the applicator rollers. The third rollers 30, 40 can be loaded under tension when using springs, hydraulic, pneumatic or similar components and preferably have resilient outer surfaces (such as elastomers resistant to high temperatures or roller covers, for prng the extrusion products semi-cooled against the heated metal strip or strip 10. The third rollers 30, 40 of the two extrusion assemblies hold the opposite sides of the strip 10 against the prre or force they exert on each other, such that the semi-cooled extrusion products 24, 34 can be prd against the strip under the prre of such third rollers 30, 40. The metal coated strip 11 continues on its vertical downward travel further or through a second heating element 42 which uniformly heats the metal or plastic or metal and plastic, especially at the interface between them at a temperature to be consumed to the adhesion of the polymer to the metal strip, to substantially reduce or otherwise detrimentally affect the desired properties of the metal strip or the plastics coating thereon. The desired temperature will depend in particular on the particular polymeric material which is applied as a coating, but is in the range of about 200 ° C to 260 ° C (392 ° F - 500 ° F). The second heating element 42 is preferably a heating element of the induction type, which is well known in the art. Alternatively, the heating element 42 could be a convection oven or an infrared heating element. After leaving the second heating element 42 and while continuing in a vertical downward direction, the coated strip 11 is rapidly cooled, such as by a spraying (or spraying) of water 44, a curtain of water or other means of appropriate cooling. Such cooling should decrease the temperature of the composite structure at a temperature sufficiently low to allow bending of the coated strip around the rollers without detrimentally affecting the coating or the metal. In a preferred method of coating, an aluminum alloy, such as alloy 3004, can be coated with polyester resin, the composite structure preferably being cooled to a temperature lthan about 40 ° C (104 ° F) before it is contact with roll 48. In such a preferred embodiment, the cooling is fast enough that the polyester coating thereon solidifies in a substantially non-crystalline form. The rate of cooling to accomplish this will depend on the polyester. The cooling rate can be controlled by controlling the temperature and volumetric flow velocity of the cooling water against the coated strip. In the embodiment illustrated in Figure 1, the coated strip is moved through a bath 46, such as a water bath and around the rollers 48 and 50 on the opposite ends of the bath before the coating dries. The water bath completes the cooling proc
From the water bath 46, the coated strip 11 preferably moves vertically upwardly through a drying system 52 to remove residual moisture from the strip before rewinding it. The drying system 52 may typically comprise air blowers. hot air Then the composite strip moves on the rollers 54, 56 and 58 and on a rewinder 60 The system may include accumulators, not shown, to compensate for roller changes or reel changes and may also include means for leveling or flattening the metal after it has been coated The system also preferably includes cut-outs or deburring, not shown, for trimming the edges of the coated metal strip 11 or for removing any polymer extending beyond the metal edges. The deburring machines can be placed in vain points along the path of the strip, such as immediately after the polymeric resin is applied to the strip, after the spray cooler or after the drying system The aluminum strip that is coated by this invention may be of a variety of alloys and tempers, depending on the use that is to be made of the strip Some aluminum alloys Suitable common for forming ends of cans and can bodies can include alloys of Aluminum Association 5042, 5182 and 3004 in intermediate to hard tempers, which include temples H-14, H-19 and H-39, among others. of metal is usually 0 1778 - 0 356 mm (0 007 to 0 014 inches) in thickness In accordance with this invention, a vapedad of thermoplastic polymers, such as polyester, can be used to coat an aluminum strip which is designed for use in packaging, such as cans or ends of cans. A preferred polyester resin is a high melt viscosity resin (HMV) of the type that has hitherto been used for coating baking metal trays, packing metal sheets for liquids and packing thermally sealable metal sheets. The copolymer resin of HMV PT8307 SELAR®, sold by E.l. Du Pont de Nemours Company is an example of a high performance polyester resin suitable for use in this invention. Such a copolymer can also be combined with other thermoplastic polyesters such as bottle-grade polyesters having intrinsic viscosities of about 0.71 IV and above. For example, a combination of the copolymer of HMV PT8307, SELAR® with PET T89 sold by Hoescht-Celanese can provide improved performance for the coated aluminum strip according to this invention for use in the manufacture of products such as can ends. drinks. Other thermoplastic polymers suitable for use in this application include polypropylene, polyethylene, polyamides (nylon), polyimides, polycarbonates, and polyvinyl chloride (PVC) among others. Figure 2 shows a portion of an alternative embodiment of a system for carrying out the present invention. In this system, the metal strip 70 is coated on both sides as the strip moves preferably vertically upward instead of vertically downward as in Figure 1. The metal strip 70 moves around a feeder roller 72 and vertically upwards of that roller through a preheater 74, such as an induction heating system. The strip is then moved through an optional flame treatment apparatus 76 and between the opposing extrusion systems 78, 80 to coat both sides of the strip. The flame treatment apparatus improves the receptivity of the strip to adhesion by the resin coating. The extrusion coating systems 78, 80 in Figure 2 are similar to that of Figure 1, except that the systems 78, 80 include only two rolls instead of three rolls as in Figure 1. The surface speed of the rolls Clamping and stretching 82, 84 is several times faster than the exit velocity of the polymer from the extrusion nozzles 90, 92, for stretching and thinning the extrusion product as in the system of Figure 1. The rollers 86, 88 which are colder that the rollers 82, 84 receive the extrusion product from the rollers 82, 84 and apply it to the strip 70. After the strip 70 has been coated on both sides, the strip continues to move vertically upward to an insulated chamber 90. which contains a cooling roller and to flip plates 90 to cool the strip and redirect it vertically upwards. The chamber 94 is preferably insulated for accurate temperature control of the strip as it moves over the cooling roller and for flipping plates 96. The roller 96 preferably has an outer jacket diameter of at least about 0.9 m (three feet). The large diameter of the roller minimizes the efforts of the metal due to the effects of curvature. The temperature of the roller 96 and the strip 71 is controlled by the fluid 91 in an annular chamber 93 between the outer cover 97 and the inner cover 95 of the roller. The annular chamber 93 is preferably filled but not capacity, to minimize the effects of inertia (provides continuous damping) and allow speed control and tracking. The composite coated strip 71 moves vertically downward from the plate and plate tumbling roller 96, through a post-heater 98 which heats the composite strip to a temperature of about 204-260 ° C (400-500 ° C). F) for improving the adhesion of the polymer, such as polyester resin, to the strip as in the embodiment of Figure 1. The heating element 98 can be a conventional induction heating element, convection oven or infrared heating element. . The composite strip 71 is moved from the heating element 98, through the cooling or cooling means not shown, to a second cooling roller and to flip plates 99 and from that roller to a rewinder roller not shown. Roller 99 is similar in design and dimensions to roll 96 described above. Figure 3 is a schematic of another embodiment of this invention, in which the clean laminate material 100, at room temperature, is unwound from an unwinder 102 and fed upward on a set of draw rolls 104 consisting of the roller 103 and an optional backup roller 105 at the top of the processing stack. Accumulators, not shown, may be included to compensate for changes of the reel in the unwinder 102. From the set of stretch rollers 104, the belt 100 travels in a vertical and downward direction and is preferably tilted approximately 30-45. degrees of the vertical. Such inclination facilitates the downstream extrusion coating and the arrangement of the machinery. The ribbon 100 passes through a preheater 106, where an induction field is generated to uniformly heat the metal to a temperature that will improve the downstream "stripping" resistance of the adhered polymer to the strip, without substantially reducing or otherwise detrimentally affect the desired properties of the metal. As used herein, "peel" resistance means that the polymer adheres to the metal strip with sufficient retention force that the polymer will not delaminate from the strip during subsequent processing. The desired temperature should be in the range of about 204 ° C-260 ° C (400-500) and preferably about 215-246 ° C (425 ° F-475 ° F) when the polyester is applied. The preheated ribbon 100 continues in a downward inclined direction and passes through an optional flame treatment apparatus 108. The flame treatment apparatus can reduce the surface of the preheated metal to remove, minimize or improve the oxides and by this reduce the adhesion of a polymer which is subsequently applied to it. Next, the heated and treated tape 100 enters the first of two extrusion coating stations. An extrusion apparatus, not shown, plasticizes a PET polymer or other thermoplastic resin and feeds it through a laminar nozzle 110 which is positioned either vertically or obliquely from the vertical and which has a narrow outlet slit. The slit is adjusted to produce a back pressure to the extrusion apparatus which allows the extension of an extrusion product 112 to a width at least as wide as the width of the strip 100. The slit may have a width less than the width of the extrusion. strip 100, depending on various factors such as the nature and thickness of the polymeric resin, the relative speeds of the extrusion apparatus and the metal strip and the shape of the nozzle, the shape of the film of the extrusion apparatus, among others factors. The extrusion product 112 is stretched to a stack of rollers 114 to reduce its thickness to the final thickness for application to the belt. The stretch thickness ratio should be about 10-25 1, depending on the extruded polymer The two-roll stack 114 is arranged in such a way that a plane through the central line of the rollers is inclined about 30 degrees from the horizontal Roller 116 for flipping plates or "inside" preferably has a resilient surface made of elastomer resistant to high temperatures and it is cooled internally and / or externally to minimize the detection of the elastomer. The pressure roller 118 or outside is made of chrome-plated, polished steel and is preferably maintained at a temperature of less than about 66 ° C or 150 ° F (for the polyester) which is lower than the "adhesion" temperature of the molten polymer which applies pressure from the polymer tubing as it is applied to the strip mat This improves the adhesion of the polymer to the metal 100, as well as improves the surface appearance The surface velocity of the rolls 116, 118 is approximately 10 times faster than the exit velocity of the extrusion product of the extrusion die 110. , so as to stretch the polymer on the tape 100 to its desired thickness in a range of about 0.00660 mm to 0.02032 mm (0-3.0 mils) and preferably about 0.0106 mm (04 mils). Two-roll stacking 114 covers the first side of the tape 100 with proper "peel" resistance to prevent separation of the polymer from the metal during subsequent processing. The one-sided tape 101 comes out of the stack 114 and turns approximately 60 degrees (as a result of the preferred positioning of the second extrusion station) on the roller 116 coated with elastomer, to tilt the cin ta down 30-45 degrees from the vertical (approximately 60 degrees from the entry position to the first stacking). The one-sided, pre-heated coated tape 101 continues in an inclined direction 30-45 degrees and downwards, it can pass through a second optional (and possibly larger) flame or other type of reinforcing heating element 120, in where the surface of the preheated metal is treated to eliminate / minimize the oxides on the second surface and improve the adhesion of the polymer, as well as providing the necessary "reinforcement" of temperature to achieve optimum adhesion conditions. Then the preheated and pretreated tape 101 enters the second of the two extrusion coating stations to coat the opposite side of the tape that was coated by the first coating station. The performance requirements of the extrusion apparatus, the arrangement and the process for the second extrusion apparatus are identical to the first extrusion apparatus. The molten extruded product 122 of the extrusion die 124 is passed to the inter roller space of a stack of two rollers 126 having an array in which a plane through the center lines of the rollers 128, 130 is inclined approximately 30-45 degrees from the horizontal (45-60 degrees from the position of the center line of the first stack 114). The geometries, arrangement, performance and functions of the rolls 128, 130 are identical to that of the first stack 114. The second side of the preheated tape 101 is coated with the extrusion product 122 to produce a "peel" resistance appropriate, as described above for the first side. Next, the tape 104 coated on both sides leaves the stack 126 and is rotated at about 45-90 degrees on the rubber or rubber coated roll to obtain a preferred post-casting for the induction bonding heating element 132 at about 30. -45 degrees from the vertical in a downward direction. The now coated tape 103 continues in an inclined and downward direction and passes through a second heating element 132, preferably an induction heating element, to uniformly heat the metal / plastic inferium at a temperature that will consume an adhesion from the plastic to the metal tape without substantially reducing or otherwise detrimentally affecting the desired properties of the metal or plastic. The temperature is about 204-228X (400-550 ° F) and preferably about 215-246 ° C (425-475 ° F). After leaving the induction heating element 132 and while continuing in an inclined and downward direction, the spray nozzles 134 (or other appropriate devices) cool the composite structure to a temperature low enough to allow it to swerve around the roll 136 without detrimentally affecting the end-use performance requirements of the composite material. The semi-cooled compound 103 is rotated and passed through a horizontal water bath 138 to consummate the cooling process. A drying system 140 is used after the compound 103 leaves the bath 138 to remove residual moisture after rewinding. The leveling or flattening is carried out to eliminate the stresses produced by turning or bending the metal strip 100 on the rollers. Then, the entire material 103 is rewound by the rewinder 142. Accumulators, not shown, can be used to compensate for roller changes and reel changes on the rewinder 142. Figures 4 and 5 illustrate a further embodiment of this invention, in wherein the metal strip 150 is moved vertically upwards during the coating process and in which the extrusion nozzles 152, 154 apply the molten resin directly against the opposite sides of the strip. The system of figure 4 includes an unwinder 156 from which the strip 150 travels upwards through an induction preheater 158 and then between two extrusion nozzles 152, 154. The nozzles 152, 154 are fed by conventional extrusion apparatuses. , not shown. Figure 5 is an enlarged view of the nozzles 152, 154 as they apply the extrusion product 160, 162 directly to the metal strip 150. The orifices of the nozzle are positioned close to the strip, such that the force of the extrusion product that comes out of the nozzles is applied against the strip. The nozzles are positioned within approximately 5 to 20 mm of the strip and preferably less than 10 mm from the strip. The metal strip 150 travels approximately 10-20 times faster than the extrusion product that exits the nozzles 152, 154, so that the extrusion product is stretched and reduced in thickness by pulling the strip on the product. of extrusion. The extrusion product can be in the range of 0.0127 to
0. 0508 mm (0.0005 - 0.002 inches) thick on each surface of the strip. The nozzles 152, 154 are preferably directed opposite each other on the opposite faces of the strip 150, so that the pressure of the extrusion product of the opposite sides of the strip will center the strip between the nozzles. The molten polymer strikes the surface of the metal strip almost immediately after the extrudate exits the nozzles, such that the polymer does not cools or narrows before it is applied to the strip. This helps ensure the application of uniform coatings of the resin on both sides of the strip. From the extrusion nozzles 152, 154, the coated strip 151 is preferably moved through an induction-type post heater 164, which heats the composite strip to a temperature higher than the melting temperature of the polyester resin for Improve the adhesion of the resin to the strip. Then the composite strip is rapidly cooled by means not shown and travels on rollers 166 and 168 to a rewinder 170. Figures 6 to 14 show alternative embodiments of this invention for coating both sides of the metal strip, such as aluminum , steel, copper, metal laminates or the like. All these embodiments include means for preheating the metal strip, first and second extrusion coating apparatuses that include application nozzles and rollers, means for post-heating the strip after it has been coated on both sides and means for cooling the strip. The systems may optionally include means for reheating the strip between the first and second coating apparatuses. All systems include an extrusion apparatus or apparatuses for feeding the extruded polymer product to the nozzles. Each of the first and second extrusion coating apparatuses in the systems includes a casting roller, which is brought into contact with the strip of the polymer extrusion product to press it against the metal strip and a backing roll which holds the strip metal and provides a space between rollers to press the strip metal and the polymer tape together to adhere the polymer to the face of the strip. The systems may optionally include a support roll for one or both backing rolls, to hold the backing roll and help it to cool down. The preheater, reheater and postheater in these systems can be in a variety of forms, such as induction, flame, infrared, radiant, electric, fossil fuel, convection, heating rolls or any combination of such devices . The strip can also be preheated in the form of a coil or preprocessing the strip either to supplement or replace a preheating device. A preferred form of the heating element is an TFX® induction heating element which is available from Davy McKee (Poole) Ltd., of Poole, England. The nozzles in these systems are positioned within approximately 10.2 - 30.5 cm (4 - 12 inches) and more preferably approximately 15.2 - 20.3 cm (6 - 8 inches) (depending on the sizes of the nozzle and roller), from the space between nozzles between each pair or rollers. The extruded polymer tapes are preferably contacted with the metal strip and the casting roller substantially simultaneously in the space between rollers or are brought into contact with the metal strip just ahead of the roll space. Alternatively, the extruded tapes can be brought into contact with the casting roller at a few degrees of rotation before entering the space between rollers. Such contact of the casting roller before the space between rollers should not be more than a few degrees of rotation of the roller, such as approximately 0-25 °., to minimize the cooling of the polymer before the polymer comes into contact with the metal strip in the space between rollers. The extruded ribbons of the polymer can be about 0 127-0254 mm (0.005 to 0.030 inches) thick and are preferably pulled down by the metal strip and the rollers to reduce the thickness of the ribbons. The draw ratio may be in the range of about 1: 1 to 200: 1 and more preferably about 10: 1 to 40: 1. As used herein, draw ratio means the ratio of the thickness of the tape as It is extruded to the thickness of the tape as it is applied to the strip metal. The draw or draw ratio is generally determined by the difference between the extrusion ratio of the nozzles and the speed of the metal strip being coated. For example, a draw ratio of 20: 1 generally means that the strip moves approximately 20 times faster than the speed of the tape as it exits the nozzle orifice. The techniques for stretching and thinning extruded polymer tapes are well known in the art. For some systems, it may be desirable to provide complementary means ahead of the roller pairs to hold or apply the extruded tapes against the face of the metal strip. The complementary fastening means may include air blades, electrostatic devices and vacuum fastening means among others. The tapes can be completely emptied on the metal strip or they can be emptied to a wider extent than the metal and then cut out to remove the excess coating. For most applications, the casting roll is preferably a hard metal roll having a coating of chromium, chromium oxide, aluminum oxide or other hard metal roll surface thereon. Such roller surfaces can be polished or textured. The casting roller is preferably cooled to a temperature lower than the adhesion or softening temperature of the polymer, such that the polymer will not adhere to the roll. The backing roller for most applications preferably has a resilient outer surface portion made of silicone rubber, polyurethane, chlorotrifluoroethylene polymers such as VITON® O KEL-F®, tetrafluoroethylene fluorocarbon polymers such as TEFLON® or other synthetic rubber or elastomeric material resistant to high temperatures or combinations of such materials. VITON®, KEL-F® and TEFLON® are trademarks of E.l. Du Pont de Nemours Company. The outer surface of such an elastomeric material preferably has a Durometer hardness of about 78-85 Shore A. For some applications, it may be desirable to have a hard surface such as elastomers of TEFLON®, VITON® O KEL-F® on a more resilient material such as natural or synthetic rubber to provide a hard wear surface and proper compressibility. The casting roller and the backing roller should have relatively smooth surfaces in a range of approximately 2 - 20 root mean square (rms). For some applications, the casting roller may alternatively have a hard surface of synthetic rubber resistant to high temperatures, as described for the backing roll. The casting roller and the backing roller are pressed against the strip metal and the polymeric tape as the strip and the belt travel through the gap between rollers to thereby adhere the strip to the strip. The compression of the rollers between each other presses the metal strip against the resilient material on the backing roller and helps to ensure that the polymeric tape is pressed against the metal strip to the full extent of the space between rollers, without gaps in the Contact. The force through the space between rollers can vary slightly due to the misalignment of the rollers or small variations in the thickness of the strip and roller finishes among other things, but must not have inadequate roll strength gaps. Compression of the rollers together presses the elastomeric material onto the backing and / or casting rollers to produce a contact band in the space between rollers along the length of the rollers, which is believed to compensate for some errors in the rollers. the misalignment of the rollers out of the smoothness of the metal strip and provides a more even distribution of the strength of the polymeric tape (s) against the metal strip for better uniformity of the coating and the accession. Apparatus for providing the force to press the rollers together and adjust or adjust the force are well known in the art and include pneumatic and hydraulic cylinders, jacks and screws which act on the rollers. The polymeric coatings applied by this invention can be any of a variety of resins as described above with respect to Figure 1. The resins consist essentially preferably of 100% polymer with little or no solvent therein that can be volatilized. The same resins or different resins may be applied on opposite sides of the strip and one or both of the coatings may contain a pigment or other additive. The strip metal is preferably an intermediate to a hardened aluminum alloy having a thickness of about 0.1778-0.356 mm (0.007 to 0.014 inches) as described with respect to Figure 1, but may also consist of other metals such as steel or copper or laminates. The strip preferably is pre-treated and pre-treated, such as by anodization or conversion coating (preferably without chromium) or surface corrugation to improve the performance and improve the adhesion of the polymeric coatings to the strip. For example, the aluminum strip can be cleaned and treated with titanium or zirconium phosphate treatments, silicate treatment or BETZ METCHEM® conversion coatings. BETZ METCHEM® is a registered trademark of Betz Laboratories, Inc., of Horsham, Pennsylvania. The strip can also be pre-coated on one or both sides with organic coatings or finishes to improve the adhesion of the polymer to the strip. In the operation of these systems, the metal strip is moved through the system at speeds in the range of approximately 90-450 meters per minute (mpm) (300-1500 feet per minute (fpm)) and preferably approximately 180 - 360 mpm (600 - 1200 fpm). Higher speeds obviously increase productivity and also reduce the period of time (residence time) during which the metal is at elevated temperatures. Shorter residence times are sometimes preferred to minimize the reduction in metal properties. Referring now to Figure 6, it is illustrated that the coating system includes a roller 172 on which the metal strip 174 travels to be fed through a preheating device 173, such as an induction heating element which heat the strip to a temperature in the range of approximately 121 ° C - 288 ° C (250 - 500 ° F) depending on the metal and tempering of the strip, the desired properties of the strip after coating and the polymers to be applied Inter alia. For an aluminum strip to be coated by polyester resin for use of the coated strip in packaging applications, a more preferred heating range is approximately 204 ° C-288 ° C (400 to 550 ° F). The preheat temperature, also as the reheat and postheat temperatures, should not be so high as to detrimentally affect the desired properties of the metal strip or the polymeric coatings on the strip. The preheated strip 174 is sequentially coated on the opposite faces by two extrusion nozzles 176, 178 and two pairs of rollers 180, 182 and 184, 186. One or two extrusion apparatuses, not shown, feed the molten polymer resin to the extrusion nozzles 176, 178. The resins can have a temperature in the range of about 177 ° C to 343 ° C (350 ° F-650 ° F) as it is fed to the nozzles 176, 178 and the nozzles are preferably preheated such as by electrical resistance means to maintain the resin at the desired temperature. Extrusion nozzles 176, 178 have narrow, elongated mouth holes therein, corresponding approximately in length to the width of strip 174 which is coated, which is approximately 25.4 cm - 215.9 cm (10 - 85 inches) or more. The length of the orifice of the nozzle is preferably at least as wide or wider than the width of the strip 174, such that the strip of the polymer extruded from each nozzle will completely cover the strip. The orifices of the nozzle are long and narrow in order to extrude or extrude the thin ribbons. The holes of the nozzle can be up to 0.762 mm (0.030 inches) and preferably are in a range of about 0.127 mm -0.381 mm (0.005 to 0.015 inches). The nozzles are generally conventional nozzles and are available from a variety of suppliers. The nozzles 176 and 178 extrude thin strips 188 and 190 which are applied against the opposite sides of the strip 174 by the roller pairs 180, 182 and 184, 186. In the first pair of rollers, the roller 182 is the roller The recess which is brought into contact with the ribbon 188 of the polymer exiting the nozzle 176 and the roller 180 is a backing roller which holds the strip 174 against the casting roller. As stated above, the casting roller 182 is preferably a hard metal roll and the backing roll 180 preferably has an outer roller surface or resilient cover, such as an outer layer of silicone rubber thereon. Both of the rollers 180, 182 are preferably cooled by a cooling fluid such as water, which is circulated through them. The casting roll is preferably cooled to a temperature below 66 ° C (150 ° F) in such a manner that the polymer tape will not adhere thereto. The backing roller 180 is preferably cooled internally and / or externally to minimize heat damage to the resilient layer on the roller. A support roller 181 can optionally be provided to support the backing roller 180 and to help cool it. As shown, the rollers 180, 182 can be positioned parallel with their axes side by side in a substantially horizontal plane, such that strip metal 174 and polymeric tape 188 can be fed down into the inter-roll space between the rollers. rollers and outwards through the bottom of the space between rollers. The strip 174 can follow the outer surface of the backing roll about an arc of 0 to 120 ° from the roll before the strip separates from the surface of the roll to travel to the reheater 192. The polymeric tape 188 over the strip metal 174 preferably has minimal contact with the casting roller 182 in order to minimize possible adhesion or adverse effects on the belt by the roller. This contact minimization is especially applicable for polyester resins, because more contact and more cooling of the resin by the casting roller is desirable for polypropylene resins (see Figure 14). The rollers 180, 182 are pressed together with a force of approximately 9.0-53.7 kg per cm (50-300 pounds per linear inch (pli)), preferably approximately 21.5-32.2 kg per cm (120-180 pli) and more preferably about 26.9 Kg / cm (150 pli) along the length of the space between rollers. This force causes the resilient compressible outer portion of the backing roll 180 to be deformed or slightly driven to, ensuring that there are no gaps in the force of the rollers against the metal strip through the entire length of the space between rollers and providing a measure of forgetting or compensating for the misalignment of the rollers or out of smoothness of the sheet material. But this force does not reduce the caliber of the polymer or materials. As stated above, this compression of the compressible layer on the backing roller 180 produces a narrow contact band between the rolls 180, 182 and the strip 174 in the space between rolls. Depending on the amount of force pressing the rolls together and the resilience of the support roll 180, among other factors, a typical contact band can be approximately 0.64-2.54 cm (1 inch) wide and usually around 1.9 cm (3? inches) wide. After the strip 174 has been coated on one side, it can optionally be reheated, such as for example with an induction heating element 192 or the like. The strip can be reheated to a temperature in the range of about 120 ° C - 288X (250 ° F - 550 ° F), depending on the polymer being applied and more preferably at about 204 ° C - 288 ° C (400 ° C) F to 55 ° F) for polyester coatings. For some applications and some polymers, it may not be necessary to reheat strip 174 before it is coated on its opposite face. From the reheater 192, the strip 174 travels to the second extrusion nozzle 178 and the pair of rollers 184, 186 and the optional cooling roller 187 so that a second polymeric strip 190 is applied to the opposite side of the strip of the one coated by the first ribbon 188. The distance from the exit of the first space between rollers to the second space between rollers is preferably kept short to control the heat loss of the metal as it travels between the two spaces between rollers. The second nozzle 178 and the second pair of rollers 184, 186 are similar to the first nozzle 176 and the pair of rollers 180, 182, except that the rollers are inverted, the second drain roller is on the opposite side of the strip of the first draining roller, the axes of the rollers are in a different plane and the second nozzle 178 is in a different orientation. In order for the strip 174 to pass in a substantially straight line through the second roll space, the plane through the axes of the rolls 184, 186 is substantially perpendicular to the piano of the strip moving through the space between rollers and at an angle to the vertical. The strip 174 therefore has minimal contact with the rollers 184, 186 except for the narrow contact band produced by the resilient deformation of the resilient material of the outer portion on the backing roller 186. As stated above, it is believed that this minimization contact of the rollers 184, 186 against the polymer on the strip 174 is useful for improving the quality and performance of the final coated product for some polyester resins. For other polymers such as polypropylene, a substantial roll wrap and cooling of the polymer is preferred before the coated strip is separated from the roll. As with the first set of rollers, the second set of rollers 184, 186 must be pressed against the metal strip 174 and the polymeric tape with sufficient force to ensure that the polymeric belt 190 is pressed tightly against the strip throughout the entire length of the belt. width of the space between rollers. The force between the second roller assembly 184, 186 should be in the range of about 9.0-53.7 kg / cm (50-300 pli) and preferably about 21.5-32.2 kg / cm (120-180 pli). After the strip 174 has been coated on both sides or faces with the polymeric bands 188, 190, the fully coated strip travels through a post-heater 194 and through a system for cooling the coated strip. Although it is not essential for the invention, it is desirable to minimize the contact of the coated strip by rollers or other mechanical devices between the coating rollers 184, 186 and when the polymer has been solidified by cooling. For example, it is desirable that the strip 174 travel in a substantially straight line from the rollers 184, 186 through a post-heater 194 and through means, not shown, to at least partially cool the strip to at least a lower temperature than the melting temperature of the polymeric coatings on the strip. In this way, contact with the polymer on the strip with the rollers or the like is prevented before the polymer solidifies and the coatings are not likely to be adversely affected by the rollers or the like. The postheater 194 is preferably an induction heating element, an infrared heating element, a convection oven or a combination of two or all three which can rapidly heat the resin on the belt to at least about the softening and drying temperature. preference greater than the melting temperature of the polymers. It is important that such heating is not so high as to significantly affect the properties of the metal in the strip or the polymer coatings on the strip. The heating of the polymers at a temperature of about their melting temperatures may be desirable to cause the polymers to flow and thereby restore any defects and / or align any disuniformity of the coatings on the strip. After the strip has been post-heated, it is rapidly cooled to solidify the coating in a substantially non-crystalline form. It may be desirable to first partially cool the strip with air or another gas at a temperature lower than the melting temperature of the polymer and then to cool the partially cooled strip with water sprays or a water bath. It is believed that partial cooling of the strip with air minimizes the possible adverse effects that the water could have on the polymer that is still molten. As used herein, "rapid cooling" means that the polymer coatings are cooled rapidly after the coated strip leaves the postheater as the strip is traveling at approximately 91-457 meters per minute (300-1500 feet per minute). ) and preferably approximately 182-366 meters per minute (600-1200 feet per minute) The quench or quench unit is positioned within a few meters, such as approximately 1 5 - 15 meters (5 - 50 feet) from the post-heater, in such a way that the polymeric coatings are preferably solidified in a time of less than about 10 seconds and more preferably less than about one second after the coated strip leaves the heater post. After the strip has cooled, it can be further processed. , such as by cutting the edges, cutting, leveling, winding in a coil or processing into products such as ends of cans or can bodies being rewound or not. Figure 7 shows an alternative system similar to the system shown in Figure 6, except that the upper pair of rollers 202, 204 is positioned with the axes of the rollers arranged in a plane which is perpendicular to the strip 196 passing through. of the space between rollers. Folding rollers, not shown, can be added to assist cooling rolls 202, 208 and 212. In this system, there is minimal contact of the strip against the backing roller, so that there is less heat transfer from the strip to the backing roller and less thermal damage to the resilient outer portion of the backing roller 202. This also means less cooling of the metal strip, which may prevent the need to reinforce or reheat the strip before it is coated on the reverse side. If the slow reheat is desired, the coated strip on one side has its direction tacked by the roller 208 and is passed through a reinforcing heating element 210. The reverse side of the strip is then coated by the nozzle 216 and the rollers 212, 214. Then the fully coated strip is post-heated and cooled off in a manner similar to that described above with reference to Figure 6. Figure 8 illustrates an alternative embodiment of this invention, which is similar to that of Figure 6, except that the rollers 218 220 in the lower coating station are arranged horizontally side by side with their axes in a substantially horizontal plane and the strip 222 follows the backing roller 210 for approximately 90 ° of rotation, after which the strip travels to the postheating and cooling, not shown. Figure 8 also shows by dotted lines several alternative paths for the strip 222 to travel after leaving the space between rollers in the lower emptying station. Figure 9 illustrates another embodiment of this invention with a substantially vertical travel direction of the metal strip 230 through the preheater 231, a first set of rollers 232, a reinforcing heating element 234 and a second set of rollers 236. From the second roller assembly, the doubly coated strip travels through a post-heater, not shown and a cooling system not shown. If space permits, the postheater is preferably positioned vertically in line under the two cooling rollers and the strip is preferably cooled to a temperature lower than the melting temperature of the polymer before it is contacted by a roller to flip veneers. Such cooling to a temperature lower than the melting temperature of the polymer can be by cooling with air, after which the strip can be turned to pass through a sudden cooling by fluid, such as a sudden cooling by water. Figure 10 shows another embodiment of this invention in which the metal strip 240 travels substantially horizontally between the two extrusion nozzles 242, 244 and the pairs of rollers. In this system, strip 240 is rotated around a backing roll 246 which forms a gap between rolls with a casting roll 248. Extrusion die 242 extrudes a thin polymer ribbon 250 above the roll space to be stretched and reduced in thickness before it is pressed against, and adhered to, the strip. The dump roll 248 and the back roll 246 are cooled as in the systems previously described. From the backing roll 246, the strip 240 is moved, horizontally through an optional reinforcing heating element 252 and then around another backing roll 254 which forms a gap between the rolls with the emptying roller 256. The nozzle Extrusion 244 extrudes a second polymer ribbon 258, which is stretched to reduce its thickness and pressed against the strip 240 in the space between the rollers 254, 256. Both rollers 254, 256 are preferably cooled as in FIG. first pair of rollers. Then the doubly coated strip travels through a postheater 260 and then a cooling / off system 262 to result in the final product which can be rewound to a coil 264 or further processed. Figure 11 shows another embodiment for coating the metal strip 264, in which the strip travels on a roll to flip sheets 265 and through a preheater 266 and in which the polymeric tape 268, 270 is applied substantially simultaneously on the opposite sides of the strip. The extrusion nozzles 272, 274 in this system extrude the polymeric blanks 268, 270 which are stretched by the drain rollers 276, 278 to the space between rollers and pressed against opposite sides of the metal strip At least one, if not both, of the draining rollers 276, 278 preferably has a compressible outer layer such as TEFLON® elastomer. , VITON®, KEL-F® in order to ensure a continuous adhesion force across the entire length of the space between rollers. The system preferably includes cooling rollers 280, 282 to help cool the emptying rollers 276, 278 and prolong the life of the emptying rollers. The system further includes a postheater 284 and cooling means such as water sprinklers 286 similar to those shown in the previous figures. A further embodiment of the invention is shown in Figure 12, in which the strip material, such as an aluminum strip 290, travels on a roller for flipping sheets 292, through a preheater 294 and between a top pouring roller. 300 and an upper backing roller 302 to be coated on a first side by a polymeric belt 298 which is extruded from an upper nozzle 296. The casting roller is preferably a polished steel roller and the backing roller preferably an external compressible layer on it. A top cooling roller 303 is preferably included to prolong the life of the compressible material on the backing roller 302. From the space between the rolls 300, 302, the coated strip on one side preferably travels through a heating element 304 to reheat or reinforce the temperature of the strip for coating the reverse side by a second polymeric tape 312 which is extruded by the extrusion nozzle 310. A lower pouring roller 308 and the backing roller 306 press the tape 312 against the strip to adhere to the strip. Lower pouring roll 308 is preferably polished steel and lower backing roll 306 has a compressible outer surface such as TEFLON®, VITON® O KEL-F® elastomer. A lower cooling roller 314 may optionally be included, such as with the upper roller assembly. After application of the second polymeric tape, the coated strip on both sides is preferably post-heated to a temperature higher than the melting temperature of the polymer (s) by a heating element 316 and then cooled rapidly, such as by water sprinklers 318. Still a further embodiment of this invention is shown in, Figure 13, in which the material of the metal strip 320 is sequentially coated on the opposite sides as the strip travels in a general path in the form of "S" through the system. In this system, the metal strip 320 travels on a roller to flip sheets 322, through a preheater 324 and between a drain roller 330 and a backing roller 332 so that a first polymeric tape 324 of the nozzle 328 is adhered to the side of the strip. A cooling roller 348 is preferably included to prolong the life of the compressible material on the reinforcing roll. In this system, the location of the reloading roller 330 and the backing roller 332 is such that the metal strip 320 is partially wrapped around the rollers for approximately 45-90 ° of rotation of the rollers, depending on the direction travel of the strip with respect to a plane through the axes of the two rollers.
Preferably, after receiving the first coating, the metal strip 320 is made to turn in the direction, by means of a roll to turn plates 336 and travels through a heating element 338 to reinforce the temperature of the strip, followed by the application of a coating to the opposite surface of the strip by the reject roller 334, the backing roller 346, the cooling roller 348 and the extrusion nozzle 340 which extrudes the polymeric tape 342 to the space between rollers. The location of the rollers 344, 346 with respect to the travel direction of the strip 320 is similar to the upper coating station, such that the strip is also partially wrapped around the rollers as the strip moves to through the lower coating station. From the lower coating station, the strip 320 with the coating on both sides is preferably post-heated at a temperature higher than the melting temperature of the polymer or polymers and then rapidly cooled to solidify the polymers on the strip. Figure 14 shows a further embodiment of the invention, which is particularly suitable for applying polypropylene coatings on both sides of an aluminum strip material. In this system, the metal strip 352 travels on a roll to flip sheets 354, down to about 30-60 ° with respect to the vertical through a preheater 356 and to the space between rolls between a backing roll 358 and a casting / cooling roll 360 so that a polymeric tape 364 of nozzle 362 is applied to the strip. In this system, strip metal 352 is partially wrapped around casting / cooling roller 360 so that the roller cools the polymer on the strip, to ensure that the polymer and especially a polypropylene material, is detached from the roll. and stay on the metal strip. A unwinding roll 370 can be used to cause the strip 352 to follow the casting / cooling roll 360 as shown. The pour / cool roll is preferably internally cooled and has a relatively large diameter such as about 0.91-1.83 meters (3-6 feet) in diameter to sufficiently cool the strip and the polymer thereon. The backup roller preferably has an outer layer compressible thereon and is preferably cooled internally and / or externally. From the upper coating station, the strip 352 travels on a roller to flip sheets 372, through a heating element 374 so that a second coating is applied by a lower coating station which is essentially the same as the station. of top coating. The lower coating station includes an extrusion die 382, a backup roll 376, a drain / cool roll 378 and a unwind roll 384 for applying a polymeric tape, such as a polypropylene to the metal strip. Then, preferably, the double-coated strip of the unwinding roller 384 is post-heated and rapidly cooled as with the other systems described herein. For some applications of this invention, the coatings may be different polymers on opposite sides of the metal strip and may have different thicknesses. For example, the coating on one surface may be a combination of a high viscosity polyester in the molten state and a bottle-grade polyester and the coating on the other surface may be a polyethylene or vinyl resin. The coating on one or both surfaces may also include a pigment or coloring material thereon. The metal of the coating strip according to this invention, for use in packaging applications, such as the use in the manufacture of can bodies or can ends, requires that the coatings adhere strongly to the metal strip. The use of the strip for packaging applications also requires that the surfaces of the coatings be smooth and shiny. Surfaces must have a minimum of irregularities in them, such as embossing or surface defects. The mechanical properties of the metal, such as tensile strength, yield strength, elongation, formability and corrosion resistance are also preferably maximized. The coatings must also be flexible, such that they do not crack or delaminate when the strip is processed to the final product such as a can body, can end or other products. Coatings for packaging applications are also quite thin, such as approximately 0.00127 cm (2 mils) in thickness and should be substantially uniform in thickness. In the practice of this invention, the path of the metal strip through the pairs of rollers, the post-heating apparatus and the cooling / quenching system plays an important role in the quality of the coated strip. In particular, it is desirable to minimize the contact of the coatings on the strip with the rolls before the coatings are cooled to a temperature at least less than the melting temperature and possibly the softening temperature of the polymers in the coatings. For some systems, it is desirable to air-cool the coated strip, after the post-heating, to a temperature lower than the melting temperature of the polymers before quenching with water. This cooling with air minimizes the possible adverse effects on the coating by water in cooling. The aluminum strip which has been coated according to this invention has many advantages over the strip that has been coated or laminated according to the methods of the invention. previous technique An important advantage is that the coating is strongly adhered or bonded on both sides of the metal substrate and does not detach or dislodge when the strip is formed into products such as stretched or stretched and iron lined can bodies, can ends or Decorative ornaments for cars and artifacts. The strip can also be produced at less cost than the prior art strip, because this invention eliminates the secondary processes of forming, laminating and delamination of the films that are laminated to the strip by the previous techniques. Consequently it is seen that this invention provides an improved continuous process for coating both sides of a strip of metal with thermoplastic coatings and also an improved strip which has been formed in that way. While alternative embodiments have been devised to carry out the invention, it will be evident that the appended claims are intended to cover all modes and modalities falling within the spirit of the invention. For example, the coating or coatings on the metal strip may be polished insofar as the coating is close to its melting temperature or to a higher temperature, by means of an encounter roller on which the coated strip is passed from after post-heating and before the coatings cool. Another alternative processing will be apparent in view of the description contained herein. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, the content of the following is claimed as property.