Process and apparatus for producing laminated materials

A process and apparatus for producing laminated materials comprising a metal sheet laminated on one or both sides to a polymer film or films. The process comprises forming a laminate by a combination of heat and pressure followed by reheating. A heat zone is provided for heating and/or cooling the laminate after the reheat stage and prior to quenching so that the laminate enters the quenching stage at a substantially constant temperature, irrespective of line speed.

This invention relates to a process and apparatus for producing laminated 
materials comprising a metal sheet laminated on one or both sides to a 
polymer film or films. 
U.S. Pat. No. 3,679,513 describes a process for applying polyethylene films 
to a steel substrate during a pre-determined heating cycle. The steel 
substrate is coated with chromium/chromium oxide and preheated so as to 
soften the polyethylene films which are applied to either side of the 
substrate between nip rolls. The resultant laminate is then reheated and 
temperature is maintained for from 5 to 240 seconds. Rapid cooling then 
takes place firstly by forced air to lower the temperature of the laminate 
to below the softening point of the polyethylene films, and finally by a 
water spray. In situations where fast line speeds are not of commercial 
importance, U.S. Pat. No. 3,679,513 suggests cooling in still air. 
Both the rapid and the still air cooling methods of U.S. Pat. No. 3,679,513 
lead to the polymeric film becoming at least partially crystalline. In 
particular, it has been found that forced air cooling below approximately 
180.degree. C. or spray quenching from a higher temperature leads to 
"spotting" of coating. These cooling methods cool only where the air or 
spray strikes the coating so that only local cooling occurs and a mottled, 
partially crystalline coating results. Crystallinity is particularly to be 
avoided when laminated materials are to be shaped into articles such as 
cans or can ends. In the case of a polypropylene film, "crystallinity" 
means .alpha.-form (rather than smectic) crystallinity. 
EP-B-0312309 seeks to overcome the problems of U.S. Pat. No. 5,679,513 and 
describes a process for laminating polypropylene film to a metal 
substrate. The resultant laminate is reheated to a temperature above the 
melting point of polypropylene and then uniformly and rapidly quenched by 
flooding the laminate with continuously cooled water which travels on the 
laminate to a collecting tank. Such rapid and uniform cooling seeks to 
prevent the polypropylene from having an x-form crystallinity and indeed 
provides a substantially smectic form of polypropylene. This form is less 
susceptible to deformations when shaped. 
A problem which arises on start-up of lamination processes when line speed 
is slow is that recrystallisation or, in the case of polypropylene, the 
formation of an x-form crystallinity can take place as cooling occurs 
after leaving the reheat zone prior to quenching. This recrystallisation 
is also exhibited when cooled in still air as suggested in U.S. Pat. No. 
3,679,513. 
This invention therefore seeks to provide a process and apparatus in which 
partial crystallinity and development of discontinuities are avoided. 
EP-A-0402004 describes a process for laminating aluminium and polyester in 
which the laminate is cooled by forced air after a reheat stage and prior 
to rapid quenching in water. In order to ensure the desired polyester 
structure in the laminate, the laminate should enter the water at a 
specific temperature T3. This temperature will, however, vary according to 
the line speed of the lamination process. 
According to the present invention, there is provided a process for 
producing a polymer/metal laminate comprising the steps of: 
(a) heating a metal strip in a first heater to a first temperature T1; 
(b) feeding to a lamination nip both a strip of polymer film and the metal 
strip at said temperature T1, which is above the initial adhesion point of 
the polymer film, to cause intimate contact of the film with the metal 
strip; 
(c) reheating the resultant laminate in second heater to a second 
temperature T2 to cause the film to interact with and bond to the metal 
strip; 
(d) passing the laminate through a heat/cool zone to heat and/or cool the 
laminate across its width, according to the line speed, to an exit 
temperature T3; 
(e) allowing bonding to develop between the film and the metal strip in 
said heat/cool zone; and 
(f) quenching the laminate rapidly and uniformly, the laminate entering 
this quenching stage at the temperature T3, irrespective of line speed. 
Preferably, bonding is allowed to develop for at least one second while 
heating and/or cooling the laminate across its width. The heating and/or 
cooling stage thus ensures that the laminate enters the quenching stage at 
substantially the same temperature T.sub.3, irrespective of the line 
speed. 
It was found that the temperature T.sub.3 of the laminated material 
immediately before quenching was critical to ensuring that the polymer 
film, i.e. the coating, remained amorphous or, in the case of 
polypropylene, in substantially smectic form. Particular temperatures 
T.sub.3 are required according to type of polymer film and preferably this 
temperature has a tolerance of .+-.15.degree. C., irrespective of line 
speed. 
Fine control of temperature T.sub.3 is preferably achieved by heating the 
laminate at low line speeds and cooling the laminate at higher line 
speeds. Heating may be by arrays of infra-red, hot air or induction 
heaters and cooling may be by one or more fans which blow air through 
cooling means comprising nozzles, slits, or arrays of holes. Both heaters 
and cooling means preferably extend across at least the whole width of the 
laminate and may be provided on both sides of the laminate. Heaters and 
coolers on both sides of the laminate are preferred where polymer films 
are applied to both sides. 
Advantageously, heating and/or cooling is by a column of heaters and 
cooling means, alternating along the column. These alternating columns 
have been found to be particularly advantageous in ensuring precise 
temperature conditions T.sub.3 prior to quenching. 
The heaters are preferably positioned between 20 mm and 100 mm from the 
laminate but may be moved towards or away from the laminate ouside this 
range as desired. A particular preferred position of the heaters from the 
laminate is 50 mm. The cooling nozzles etc are typically positioned closer 
to the laminate than the heaters, for example about 10 mm closer than the 
heaters. 
In a particularly preferred embodiment,the process further comprises 
monitoring the temperature T.sub.3 and adjusting power supplied to the 
heaters and/or fans so as to maintain temperature T.sub.3 within a 30 
degree acceptable range (ie. .+-.15.degree. C.), irrespective of line 
speed. 
In another embodiment, the process may also include recirculating warm air. 
Recirculating may also include further heating of air to be blown onto the 
laminate. Alternatively, cooling may comprise blowing cool, fresh air or 
warming fresh air and blowing this warmed air onto the laminate. Usually, 
an exhaust fan is provided which expels air which is not to be recycled. 
According to another aspect of the present invention, there is provided an 
apparatus for producing a laminated material comprising a polymer film 
bonded to a metal substrate, said apparatus comprising: 
a first heater for heating a metal strip to a first temperature T1; 
means for feeding to a lamination nip both a strip of polymer film and the 
metal strip at said temperature T1, which is above the initial adhesion 
point of the polymer film, to cause intimate contact of the film with the 
metal strip; 
a second heater for reheating the resultant laminate to a second 
temperature T2 to cause the film to interact with and bond to the metal 
strip; 
a heat/cool zone comprising heaters and coolers disposed adjacent to the 
laminate; 
sensing means for monitoring the temperature T3 across the width of the 
laminate as the laminate exits the heat/cool zone; and 
means for quenching the laminate rapidly and uniformly, the laminate 
entering the quenching means at the third temperature T3. 
Preferably, the apparatus includes means for feeding the metal strip and 
two strips of polymer film to the lamination nip. 
The heaters may be infra-red heaters or hot air or induction heaters, for 
example, and the coolers may be one or more fans in conjunction with 
nozzles, slits, or arrays holes across the width of the laminate. The 
heaters and nozzles are usually disposed alternately in a column, 
preferably with the nozzles nearer to the laminate than the heaters. 
The sensing means preferably comprises at least one pyrometer, for 
monitoring the temperature T.sub.3 and adjusting power supplied to the 
heaters and one or more fans. There may be an array of pyrometers, or 
other sensors, across the width of the laminate. The third temperature 
T.sub.3 thus remains substantially constant irrespective of line speed. 
In another preferred embodiment, the apparatus may include a recirculation 
damper. By adjusting the damper position, fresh or recirculated air may be 
blown through the nozzles, or a combination of both fresh and recirculated 
air.

In FIG. 1 it can be seen that the apparatus comprises a first roll 10 over 
which a metal strip 15 is passed and second and third rolls 20, 30, over 
each of which a polymeric film strip 25, 35 respectively, is passed. Pinch 
rolls 40, 45 bring the metal strip 15 and polymeric film strips 25, 35 
together and quenching apparatus 50 immerses the resultant laminate 60 in 
a copious flood of cooling liquid in accordance with EP-B-0319309. 
A preheater 100 is located between roll 10 and pinch rolls 49, 45 and 
serves to preheat the metal strip 15 to a temperature T.sub.1 above the 
initial adhesion point of the polymeric films before laminating at the 
pinch rolls 40,45. A second heater 110 reheats the laminate 60 to a 
temperature T.sub.2 higher than the preheat temperature T.sub.1, Heat/cool 
zone 120 is located between heater 110 and the quenching apparatus 50 and 
serves to heat and/or cool the laminate 60, according to the line speed 
and laminate thickness, to ensure that the laminate 60 is at temperature 
T.sub.3 immediately prior to quenching, irrespective of line speed. The 
length of zone 120 is such that the laminate 60 will take at least one 
second to pass through the zone at maximum line speed. 
The quenching apparatus 50 comprises a reservoir 70 for containing a 
cooling liquid 75, a pump 80 to draw liquid from the reservoir 70, a heat 
exchanger 85 to cool liquid delivered by the pump and a trough 90 which 
receives cooled liquid from the heat exchanger 85. The laminate 60 passes 
through trough 90 and is entirely flooded edge to edge with cooled liquid. 
Alternatively, a preferred form of quenching apparatus comprises a pair of 
horizontal distibutor bars between which the laminate passes, generally 
vertically, and cooling liquid impinges at a small angle to the laminate 
from both sides, thus flooding the laminate with a continuously renewed 
supply of cooled liquid. 
Heat/cool zone 120 is shown in more detail in FIG. 2 and comprises a zone 
200 through which the laminate 60 passes prior to quenching, which 
consists of a column of nozzles or slits 210 and heaters 220 on each side 
of the laminate. The columns are offset from each other and each row 
alternates nozzle-heater- nozzle etc, with a nozzle at each column end, so 
that a nozzle on one column opposes a heater on the other column, except 
at the ends. A pyrometer 230 monitors the temperature T.sub.3 of the 
laminate as it leaves the zone 200. 
Power supplied to the heaters 220 and variable speed fans 243 is controlled 
by automatic adjustment of power supplied to each of these in response to 
the pyrometer output. The fans 240 are variable speed so that the amount 
of fresh air drawn into the zone through air inlets 250 can be adjusted. 
In the embodiment of FIG. 2, dampers 260 are provided which enable air to 
be recirculated by fans 240. Recirculated air passes from each fan 240 
past thermocouple 270, which adjusts both the damper position and preheat 
heater 280, and then past the preheat heater 280 to the nozzle 220. An 
exhaust 290 allows fumes and heat build-up to be removed from the zone. 
Generally, the maximum width of the laminate 60 will be one meter with both 
heaters and nozzles extending beyond the edge of the laminate. The heaters 
will usually extend by at least 60 mm beyond each edge and the nozzles by 
at least 65 mm. It will be appreciated these sizes may vary according to 
the laminate width, availability of specific heaters and nozzles and width 
of metal/polymers available. 
The distance of the laminate strip from the heaters is variable. Typically, 
this distance will be 50 mm and the nozzles are usually about 10 mm 
forward of the heaters. Variation in these distances is achieved by moving 
the heaters and/or nozzles on a frame towards or away from the strip. 
Limits on these distances are given both by the physical constraints of 
the zone assembly and by heat transfer requirements. A range of from 20 mm 
to 100 mm for the heater distance from the laminate would generally be 
acceptable. 
Plow of air through the heat zone is shown in FIG. 3. Fresh air is drawn in 
through air inlets 250 by circulation fans 240. Thermocouple 270 monitors 
the temperature of the air and dampers 260 and preheat heaters 280 are 
adjusted accordingly. The air then enters inner chamber 300 from which it 
passes through nozzles 210 onto the laminate 60 as it travels through the 
zone 120. 
The laminate 60 travels through the center of an outer chamber 310 from 
which air can be removed to exhaust 290 by means of a variable speed 
exhaust fan (not shown). The air to be extracted can travel from either 
end of the columns of heaters and nozzles as shown by the double arrows in 
FIG. 3. 
If the damper 260 is moved to the position shown in FIG. 2, warm air will 
be recirculated by fan 240 and its temperature monitored by thermocouple 
270. Thermocouple 270 can be used to switch preheater 280 and/or to adjust 
the setting of damper 260. 
The laminate travels from top to bottom as indicated in FIGS. 2 and 3 and 
its temperature T.sub.3 is monitored by pyrometer 230 as it leaves the 
zone. The temperature T.sub.3 which is required will vary according to the 
polymer films used but will be maintained substantially constant at both 
start up speed and as the line speed is increased to maximum. 
The time taken for the laminate to pass through the heat/cool zone is 
generally at least 1 second at maximum line speed but a major requirement 
of the zone is that it can cope with a wide range of line speeds of 8 
m/min to 80 m/min or more. 8 m/min is a typical start-up speed and at such 
a slow line speed it is imperative that the laminate be heated to avoid 
recrystallisation of the coating(s) or the return of a polypropylene film 
from smectic to .alpha.-form and to ensure that T.sub.3 is controlled to 
.+-.15.degree. C. of its required value. 
As the line speed increases following start-up procedure, the amount of 
heat input required will decrease until it becomes necessary for power 
input to the heaters 220 to be reduced and/or the damper 260 to be 
adjusted to allow fresh cool air to be drawn by fan into heat zone 120 to 
cool the laminate. 
The thickness of the laminate will also dictate the heat and/or cooling 
required to achieve desired temperature T.sub.3 within a tolerance of 
+/-15.degree. C. 
It can thus be seen that a number of variables are involved in the 
heat/cool zone, particular settings being given in the examples which 
follow. 
EXAMPLE 1. 
A 0.2 mm gauge steel strip was preheated and 15 .mu.m PET film applied in 
the nip rolls to one side of the strip and a 40 .mu.m polypropylene film 
to the other side so that initial adhesion of the films to the strip 
occurred. The resultant laminate was reheated to a temperature T.sub.2 of 
255.degree. C., this being the maximum temperature reached by the laminate 
during the lamination process. The laminate then entered the heat/cool 
zone and the heaters and/or fans were adjusted so that the laminate first 
contacted the quenching water at a temperature T.sub.3 of 215.degree. C. 
The length of the heat/cool zone was such that the time taken to travel 
from the exit of the reheat to the quenching stage was 2 seconds at 
maximum line speed. 
At start-up speed of 8 m/min, full heating of about 30 to 40 kW was applied 
in the heat/cool zone, once the strip and film were moving. The fans were 
switched on at minimum so as to avoid the effects of inertia when cooling 
became required as the line speed increased. The exhaust fan was also on 
minimum to avoid over-cooling of the laminate by exhaust air at low line 
speed. 
As the line speed was increased, T.sub.3 was monitored and heating reduced 
automatically until little or no heating was required and the fan speed 
was then increased as necessary to provide cooling to ensure T.sub.3 
remained constant. The exhaust fan speed was also increased to expel 
exhaust gases more quickly. 
Fine control of the heaters and fans ensured that temperature T.sub.3 
remained at about 215.degree. C. for the laminate of this example for line 
speeds of from 8 m/min to 80 m/min. 
EXAMPLE 2 
A 0.17 mm gauge steel strip was preheated and 20 .mu.m polypropylene film 
applied in the nip rolls to one side of the strip and a 40 .mu.m 
polypropylene film to the other side so that initial adhesion of the films 
to the strip occurred. The resultant lamminate was reheated to a 
temperature T.sub.2 of 230.degree. C. In the heat/cool zone, the heaters 
and/or fans were adjusted automatically so that the laminate first 
contacted the quenching water at a temperature T.sub.3 of 200.degree. C. 
For this example, full heating of 20 to 30 kW was applied at start-up speed 
of 8 m/min and reduced as the speed increased. Heating and/or cooling was 
adjusted automatically as in example 1 so as to ensure that T.sub.3 
remained at about 200.degree. C. for line speeds of from 8 m/min to 80 
m/min.