Method for coating a metal strip

A method for extrusion coating both sides of a metal strip. A metal strip, such as aluminum alloy strip, is moved through a pre-conditioner, two extrusion dies, a post heater and a cooling system. Both sides of the strip are coated with thin coatings of polyester material. The coated metal strip is useful for containers, such as cans and can ends, as well as for automobiles, appliances, aerospace, construction and electrical devices.

This invention relates to a method and apparatus for applying a polymer 
coating on a strip of metal and, in particular, to a method of coating 
both sides of an aluminum strip with thermoplastic resins from extruders 
or extrusion dies which are positioned on opposite sides of the strip. The 
product of this invention is a strip of metal, such as aluminum, which has 
thin polymer coatings on both sides thereof and which is particularly well 
suited for use in packaging applications such as can ends and can bodies. 
BACKGROUND OF THE INVENTION 
It is known to coat metal sheet or strip with thermoplastic resin on one or 
both sides to improve the corrosion resistance, formability, appearance or 
other properties of the material. The coating can be applied by a variety 
of processes such as roll coating, reverse roll coating, spraying, 
electrocoating, powder coating, and lamination. The coated strip may be 
used for applications such as in cans and can ends, foil pouches, lidding 
stock, appliances, electrical devices, construction, aerospace or 
automotive body sheet. 
U.S. Pat. No. 5,093,208 to Heyes et al. discloses a method for forming a 
laminated metal sheet in which a precast thermoplastic polyester film is 
pressed against one or both surfaces of a metal sheet to adhere the film 
to the sheet in a non-crystalline form. The uncoated sheet of metal is 
heated to a temperature above the melting point of the polyester film and 
the film is applied to the sheet under pressure to form a laminate 
material. The laminate material is then heated to above the melting point 
of the film to improve the bond of the plastic film to the metal and is 
quenched rapidly to a temperature below the glass transition point of the 
polyester to form a non-crystalline polyester. The quenching is done by 
passing the laminate through a curtain of water. 
European Patent Application 0067060 in the name of Taiyo Steel Ltd. 
discloses a method of producing a coated metal plate by directly 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 into an 
independent film. The thickness of the film can be less than 50 microns 
and preferably down to 35 to 5 microns. The patent states that since the 
step of forming an independent film is omitted, the cost of producing the 
coated metal is reduced. Suitable thermoplastic resins used for coating of 
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 
mono-layer or multilayers of the same or different resins. The patent 
application discloses applying the resin on only one side of the metal 
strip. 
An improved process is desired for applying a thin polymer coating on both 
sides of a metal strip suitable for use in applications such as packaging. 
A process is desired for producing tight adhesion or welding of the 
polymer to the strip so that the polymer will not delaminate during 
subsequent forming of the strip or use of the products produced from the 
strip. 
SUMMARY OF THE INVENTION 
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. 
Accordingly, an object of this invention is to provide a method of adhering 
polyester resin on both sides of a metal strip to produce an improved 
strip at reduced cost. 
The above and other objects and advantages of this invention will be more 
fully understood and appreciated with reference to the following 
description and the drawings attached hereto.

DESCRIPTION OF THE INVENTION 
The drawings appended hereto illustrate systems for coating both sides of a 
strip of metal as it travels from an rewind coil to a second coil on which 
the metal is wound after it has been coated. Referring in particular to 
FIG. 1, a strip 10 of aluminum alloy is unwound from coil 12, moves around 
tension rollers 14, travels vertically upward over a roll 16 and then 
downward from roll 16 through the coating apparatus. A back-up roll 18 may 
be used to maintain the metal strip 10 in a flat condition as it moves 
over support roll 16. 
As the strip 10 moves downwardly from roll 16, it is first heated by heater 
20 to a temperature close to the melting point of the polyester to be 
applied thereto. In the embodiment illustrated in FIG. 1, the heater is an 
induction heater, but other heaters or preconditions such as flame 
treatment, infrared, plasma and/or corona discharge may also be employed 
either singularly or in combination. Flame heaters can be used in tandem 
(one on each side) or on one side only to enhance performance (improved 
bonding as well as heating). The coil 12 may also be used, which is still 
hot from the prior processing, such as rolling or heat treatment, to 
minimize or even eliminate the need for heating by heater 20. A typical 
temperature to which the metal is heated is in the range of about 
121.degree.-260.degree. C. (250.degree.-500.degree. F.) depending on the 
particular polymer that is to be applied to the strip. 
Two separate extrusion systems 21 and 31 are provided for applying thin 
webs of polyester resin to the two surfaces of the heated web. The systems 
21, 31 are disposed just below the induction heater 20. The extrusion 
systems 21, 31 each include an extruder 22, 32 for delivering a molten 
polyester extrudate through a sheet die having a narrow exit slit to 
produce a thin web of extrudate 24, 34 which is passed through a three 
roll stack. Alternatively, one extruder may feed both extrusion dies via 
transfer pipes or other manifolding. 
The first rolls 26, 36 of the systems 21, 31 are pinning and drawing rolls 
which are maintained at a temperature which will promote sticking or 
clinging of the polyester extrudate to the polished surface of the roll. A 
typical temperature for this purpose is in the range of about 120.degree. 
to 180.degree. C. (248.degree.-356.degree. F.), depending on the resin 
being used. The surface speed of the rolls 26, 36 is substantially faster 
than the speed of the extrudate coming out of the die 22, 32, thus drawing 
the polyester to a reduced thickness. Typical speed ratios of drawing 
velocity to extrudate velocities range from about 5:1 to 40:1. The resin 
from the extruder is typically approximately 0.127-0.635 mm (0.005-0.025 
inches) thick and is drawn to a reduced thickness of approximately 
0.0076-0.038 mm (0.0003-0.0015 inches) thick. 
The second rolls 28, 38 are cooler than the first rolls and are designed to 
polish and cool the extrudate by rolling contact between the rolls and the 
extrudate. The second rolls 28, 38 also transfer the extrudate to the 
third rolls which are the applicator rolls. The third rolls 30, 40 may be 
tension loaded using springs, pneumatics, or the like and preferably have 
resilient (such as high temperature resistant elastomers) surfaces to 
press the semi-cooled extrudates against the heated metal web or strip 10. 
The third rolls 30, 40 of the two extrusion sets are preferably 
diametrically opposed and support opposite sides of the strip 10 against 
the pressure or force of each other so that the semi-cooled extrudates 24, 
34 can be pressed against the strip under the pressure of such third rolls 
30, 40. 
The coated strip of metal 11 continues its vertical downward travel past or 
through a second heater 42 which uniformly heats the metal to a 
temperature that will consummate bonding of the polyester to the metal 
strip without substantially reducing or otherwise deleteriously affecting 
the desired properties of the metal strip or the plastic coating thereon. 
The desired temperature will depend on the particular polyester material 
which is being applied as a coating but is somewhere in the range of 
approximately 200.degree. to 260.degree. C. (392-500.degree. F.). The 
second heater 42 is preferably an induction type heater, which is well 
known in the art. Alternatively, the heater 42 could be a convection oven 
or an infrared heater. 
Upon exit from the second heater 42, and while continuing in a vertical 
downwardly direction, the coated strip 11 is rapidly cooled as by a water 
spray 44, a water curtain, or other suitable cooling means. Such cooling 
must lower the temperature of the composite structure to a low enough 
temperature to allow turning the coated strip around rollers without 
deleteriously affecting the coating or the metal. In a preferred method of 
coating an aluminum alloy can sheet with polyester resin, the composite 
structure is preferably cooled to below approximately 40.degree. C. 
(104.degree. F.) before it contacts roller 48. In such a preferred 
embodiment, cooling is fast enough that the polyester coating on it is 
solidified in a substantially non-crystalline form. The speed of cooling 
to accomplish this will depend on the polyester. The rate of cooling can 
be controlled by controlling the temperature and volume rate of flow of 
the cooling water against the coated strip. 
In the embodiment illustrated in FIG. 1, the coated strip moves through a 
water bath 46 and around rollers 48 and 50 on opposite ends of the bath 
before the coating is dried. The water bath completes the cooling process. 
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. The drying system 52 may typically comprise warm 
air blowers. The composite strip next moves over rollers 54, 56 and 58 and 
onto a rewinder 60. The system may include accumulators, not shown, to 
accommodate roll changes and may also include means for leveling the 
material. The system also preferably includes trimmers, not shown, for 
trimming the edges of the coated metal web 11 to remove any polymer that 
extends past the edges of the metal. The trimmers may be located at 
various points along the path of the strip such as immediately after the 
polyester 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 which is to be made of the strip. 
Some typical aluminum alloys suitable to be forming can ends and can 
bodies include Aluminum Association alloys 5182 and 3004 in the H-19 or 
H-39 tempers. The metal strip is typically 0.1778-0.356 mm (0.007 to 0.014 
inches) thick. 
In accordance with this invention, a variety of thermoplastic polyester 
resins can be used to coat an aluminum strip which is designed for use in 
packaging such as cans or can ends. A preferred polyester resin is a high 
melt viscosity (HMV) resin of the type that has heretofor been used to 
coat ovenable metal trays, liquid foil packaging and heat sealable foil 
packaging. Selar.RTM. PT8307 HMV copolymer resin sold by E. I. Du Pont de 
Nemours company is an example of a high performance polyester resin 
suitable for use in this invention. Such copolymer can also be blended 
with other thermoplastic polyesters. For example, a blend of Selar.RTM. 
PT8307 HMV copolymer with T89 PET sold by Hoescht-Celanese may provide 
improved performance for aluminum strip coated in accordance with this 
invention for use in making products such as ends for beverage cans. Other 
thermoplastic resins suitable for use in this application include 
polypropylene, polyethylene, polyamides (nylon), polycarbonates and 
polyvinyl chloride (PVC). 
FIG. 2 shows a portion of an alternative embodiment of a system for 
practice of the present invention. In this system, the metal strip 70 is 
coated on both sides as the strip moves vertically upwardly instead of 
vertically downwardly as in FIG. 1. The metal strip 70 moves around an 
infeed roll 72 and vertically upwardly from that roll through a pre-heater 
74 such as an induction heating system. The strip then moves through a 
flame treater 76 and between the opposed extrusion systems 78, 80 for 
coating both sides of the strip. The flame treater enhances the 
receptivity of the strip to bonding by the resin coating. 
The extrusion systems 78, 80 in FIG. 2 are similar to that of FIG. 1 except 
that the systems 78, 80 each include only two rolls rather than three 
rolls as in FIG. 1. The surface speed of the pinning and drawing rolls 82, 
84 is several times faster than the exit speed of the extruder dies 90, 92 
so as to draw and thin the extrudate as in the system of FIG. 1. Rolls 86, 
88, which are cooler than rolls 82, 84, receive the extrudate from rolls 
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 upwardly into an insulated chamber 94 which contains a 
cooling and a turning roll 96 for cooling the strip and redirecting it 
vertically downwardly. The chamber 94 is preferably insulated for accurate 
temperature control of the strip as it moves over the cooling and turning 
roll 96. The roll 96 preferably has a diameter of at least approximately 
three feet. The roll's large diameter minimizes stressing of the metal due 
to curvature effects. The temperature of roll 96 and strip 71 is 
controlled by fluid 91 in an annular chamber 93 between the roll's outer 
shell 97 and an inner shell 95. The annular chamber 93 is not filled to 
capacity so as to minimize the inertia effects (provides viscous damping) 
and enable speed control and tracking. 
The composite coated strip 71 moves vertically downwardly from the turning 
and cooling roll 96 through a post heater 98 which heats the composite 
strip to approximately 204.degree.-260.degree. C. (400.degree.-500.degree. 
F.) to enhance bonding of the polyester resin to the strip as in 
embodiment of FIG. 1. The heater 98 may be a conventional induction 
heater, convection oven or infrared heater. The composite strip 71 moves 
from the heater 98 to a second cooling and turning roll 99 and from that 
roll to a rewind not shown. Roll 99 is similar in design and dimensions to 
roll 96 described above. 
FIG. 3 is a schematic of another embodiment of this invention in which 
cleaned, room temperature, conditioned sheet stock 100 is unwound from an 
unwinder 102 and fed upwards over a draw roll set 104 consisting of roll 
103 and back-up roll 105 at the top of the processing stack. Accumulators, 
not shown, may be included to accommodate coil changes on the unwinder 
102. 
From the draw roll set 104, the web 100 travels in a vertical and downward 
direction, and preferably slanted about 30 degrees from the vertical. Such 
slant facilitates downstream extrusion coating and machinery arrangement. 
The web 100 passes through a pre-heater 106, wherein an induction field is 
generated to uniformly heat the metal to a temperature that will enhance 
downstream "green peel" strength of the bonded polyester to the strip 
without substantially reducing or otherwise deleteriously affecting the 
desired metal properties. As used herein, "green peel" strength means that 
the polyester is adhered to the metal strip with sufficient holding power 
that the polyester will not delaminate from the strip during subsequent 
processing. The desired temperature should be in the range of 
approximately 204.degree.-260.degree. C. (400.degree.-500.degree. F.), and 
preferably approximately 215.degree.-246.degree. C. 
(425.degree.-475.degree. F.). 
The pre-heated web 100 continues in a downwardly slanted direction and 
passes through a flame surface treater 108. The flame treater may reduce 
the surface of the pre-heated metal to eliminate, minimize or enhance 
oxides, and thereby enhance adhesion of a polymer which is subsequently 
applied to it. 
The heated and treated web 100 next enters the first of two extrusion 
coating stations. An extruder, not shown, melt-plasticizes a PET polymer 
or other thermoplastic resin and delivers it through a sheet die 110 which 
is positioned obliquely from vertical and which has a narrow exit slit. 
The slit is set to produce a back-pressure to the extruder that enables 
spreading of an extrudate 112 to a width greater than the width of strip 
100. The slit may have a width less than the width of the strip 100 
depending on several factors such as the nature and thickness of the 
polymer resin, the relative speeds of the extruder and metal strip and the 
shape of the die among other factors. The extrudate 112 is drawn into a 
roll stack 114 to reduce its thickness to the final thickness for 
application to the web. The draw thickness ratio should be approximately 
10-25:1, depending on the extruded polymer. 
The two-roll stack 114 is disposed such that a plane through the centerline 
of the rolls is slanted approximately 30 degrees from horizontal. The 
"inside" or turning roll 116 is rubber-coated and has a surface 
temperature of approximately 205.degree. C. (401.degree. F.) which 
maintains the pre-heat temperature of the web 100 as it goes through the 
rolls. 
The outside or pressure roll 118 is chrome steel, polished, and maintained 
at a temperature in a range of approximately 150.degree.-20.degree. C. 
(302.degree.-392.degree. F.) which below the "stickiness" point of the 
molten polymer which applies line pressure to the polymer as it is laid 
down. This enhances adhesion of the polymer to the metal 100 as well as 
improves surface appearance. The surface speed of the rolls 116, 118 is 
approximately 10 times faster than the extrudate's exit speed from the 
extrusion die 110, thus drawing the polymer onto the web 100 to its 
desired thickness in a range of approximately 0.00762 mm to 0.02032 mm 
(0.3-0.8 mils) and preferably about 0.01016 mm (0.4 mils). The two-roll 
stack 114 coats the first side of the web 100 with adequate "green peel" 
strength to avoid separation of the polymer from the metal during the 
subsequent processing. 
The single-side coated web 101 next exits the stack 114 and turns 
approximately 60 degrees (as a result of the preferred positioning of the 
second extrusion station) over the rubber coated roll 116 to slant the web 
downward 30 degrees from vertical (approximately 60 degrees from the entry 
position into the first stack). The pre-heated and single-side coated web 
101 continues in a 30-degree slanted and downward direction, passes 
through a second (and possibly larger) flame treater 120, wherein the 
surface of the pre-heated metal is treated to eliminate/minimize oxides on 
the second surface and enhance adhesion of the polymer, as well as to 
provide any needed temperature "boost" to achieve optimum bonding 
conditions. 
The pre-heated and pre-treated web 101 next enters the second of the two 
extrusion coating stations to coat the opposite side of the web than was 
coated by the first coating station. The extruder performance 
requirements, arrangement, and process for the second extruder are 
identical to the first extruder. The melted extrudate 122 from extrusion 
die 124 is passed into the nip of a two-roll stack 126 having an 
arrangement in which a plane through the centerlines of the rolls 128, 130 
is inclined approximately 30 degrees from the horizontal (60 degrees from 
the centerline position 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 
pre-heated web 101 is coated with extrudate 122 to produce adequate "green 
peel" strength, as described above for the first side. The double-side 
coated web 103 next exits the stack 126 and is preferably turned 
approximately 60 degrees over the rubber coated roll to achieve a 
preferred positioning for the second induction bonding heater 132 at 
approximately 30 degrees from vertical in a downward direction. 
The now-coated web 103 continues in a slanted and downward direction and 
passes through a second heater 132, preferably an induction heater, to 
uniformly heat the metal to a temperature that will consummate a bond of 
the plastic to the metal web without substantially reducing or otherwise 
deleteriously affecting the desired metal properties or the plastic. The 
temperature is preferably approximately 215.degree. C. (425.degree. F.). 
Upon exit from the second induction heater 132, and while continuing in a 
slanted and downward direction, spray nozzles 134 (or other suitable 
devices) cool the composite structure to a temperature low enough to allow 
turning around roller 136 without deleteriously affecting the metal's 
ultimate end-use performance requirements. The semi-cooled composite 103 
is turned and passed through a horizontal water bath 138 to complete the 
cooling process. 
A drying system 140 is used after the composite 103 leaves the bath 138 to 
remove residual moisture before rewinding. Leveling, which is well known 
in the art, is performed to remove stresses produced by the turning or 
bending of the metal strip 100 over the rolls. The completed material 103 
is then rewound by rewinder 142. Accumulators, not shown, can be used to 
accommodate roll changes on the rewinder 142. 
FIGS. 4 and 5 illustrate a further embodiment of this invention in which 
the metal strip 150 is moved vertically upwardly during the coating 
process and in which the extrusion dies 152, 154 apply the molten resin 
directly against the opposite sides of the strip. The system of FIG. 4 
includes an unwinder 156 from which strip 150 travels upwardly through an 
induction pre-heater 158, and then between two extrusion dies 152, 154. 
The dies 152, 154 are fed by conventional extruders not shown. 
FIG. 5 is a greatly enlarged showing of the dies 152, 154 as they apply 
extrudate 160, 162 directly to the metal strip 150. The die orifices are 
positioned close to the strip so that the force of the extrudate issuing 
from the dies is applied against the strip. The dies are positioned within 
about 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 
does the extrudate issuing from the dies 152, 154 so the extrudate is 
drawn and reduced in thickness by pull of the strip on the extrudate. The 
extrudate may be in the range of 0.0127 to 0.0508 mm (0.0005-0.002 inches) 
thick on each surface of the strip. 
The dies 152, 154 are preferably directly opposed to each other on opposite 
faces of the strip 150 so the pressure of the extrudate from opposite 
sides of the strip will center the strip between the dies. The molten 
polymer impinges upon the surface of the metal strip almost immediately 
after the extrudate exits the dies, so the polymer does not cool or 
neck-in before it is applied to the strip. This helps to ensure the 
application of uniform coatings of the resin on both faces of the strip. 
From the extrusion dies 152, 154, the coated strip 151 preferably moves 
through an induction type post heater 164 which heats the composite strip 
to above the glass transition temperature (approximately 
204.degree.-260.degree. C.) of the polyester resin to enhance bonding of 
the resin to the strip. The composite strip is then quickly cooled by 
means not shown and travels over rolls 166 and 168 to a recoiler 170. 
Aluminum strip which has been coated in accordance with this invention has 
many advantages over strip that has been coated or laminated in accordance 
with prior art methods. One important advantage is that the coating is 
welded or tightly adhered to both sides of the metal substrate and is much 
less likely to peel or delaminate when the strip is formed into products 
such as cans, can ends, or decorative trim for automobiles or appliances. 
The strip can also be produced at less cost than prior art strip because 
this invention eliminates secondary processes of forming, rolling and 
unrolling of films that are laminated to the strip by some prior art 
techniques. 
It is therefore seen that this invention provides an improved process for 
coating both sides of a metal strip with thermoplastic coatings and to an 
improved strip which has been so formed. While some alternative modes for 
practicing the invention have been described, it will be apparent that the 
appended claims are intended to cover all modes and embodiments which fall 
within the spirit of the invention. For example, the metal strip could be 
moved horizontally as it is being coated or the metal strip could be steel 
or an alloy thereof instead of an aluminum alloy.