AU2002328372B2 - Method and device for continuously coating a strip with a fluid film having a predetermined thickness and made from a crosslinkable polymer that is free from solvent and diluent - Google Patents

Abstract

The invention relates to a method of continuously coating a strip (B) with a fluid film having a pre-determined thickness E which is made from a crosslinkable polymer that is free from solvent and diluent, the softening temperature thereof being greater than 50 DEG C. The inventive method is characterised in that: the strip (B) is fed continuously over at least one support (1) at a controlled speed; at least one fluid coat of said polymer in molten state, which is obtained by means of fusion and subsequently the forced flow of the polymer, is deposited on part of the width of the strip (B), said coat having an average thickness E' that is greater than the pre-determined thickness E and the coat formation temperature being lower than the polmyer cross-linking start temperature; the molten polymer is distributed over the entire width of the strip (B) using a smoothing device with a deformable surface (4) which forms a continually-renewed reserve of molten polymer over the entire width of said strip (B); and a film of uniform thickness E is formed by shearing the molten polymer originating from said reserve between the strip (B) and the smoothing device with the deformable surface (4). The invention also relates to a coating device that is used to carry out said method.

Description

WO 03/008109 PCT/FR02/02329 Method and device for continuously coating a strip with a fluid film having a predetermined thickness and made from a crosslinkable polymer that is free from solvent or diluent The present invention relates to a method and to a device for continuously coating a strip with a fluid film having a uniform predetermined thickness and made from a crosslinkable polymer that is free from solvent or diluent, the softening temperature of which is above 2 C. This type of polymer exhibits the feature of having to be cured by heat treatment or by irradiation (UV radiation, electron beam, etc.) after it has been applied.

Crosslinkable organic coatings, such as paints or varnishes for example, are deposited in order among other things among other things to protect metal strips from corrosion, or else to cover them with paint. For both these usages, it is important for the film obtained to have everywhere a constant thickness, whether to ensure corrosion protection of the strip or to give it a uniform color. Attempts have therefore been made to develop techniques for obtaining strips coated with a thin film of crosslinkable polymer having the most uniform possible thickness.

One technology used in particular for depositing coatings based on thermoplastic polymers on a running strip is extrusion through a calibration device such as a T-shaped die, which is used to form a self-supporting sheet of molten polymer, which is firstly stretched beneath the die and then pressed against the running strip. To press the molten coating film onto the substrate, either a lamination operation may be carried out using a chilled roll (extrusion-lamination) or a coating operation by means of a contactless device (extrusion-coating) such as, for example, a partial vacuum, or the application of an electrostatic field.

2 Attempts were then been made to develop similar methods for depositing, by melt coating, crosslinkable organic coatings. For example, PCT patent WO 95/21706 (DSM discloses coating with a roll or with an air die in order to transfer a melt-extruded thermosetting coating film prestretched in air 5 to 20 times so as to deposit a thin thickness onto a preheated metal strip.

However, the functional precursors used in crosslinkable coatings are low-molecular-weight prepolymers that have a very low melt viscosity.

Consequently, it is very difficult to produce a selfsupporting free film, sized by a die and capable of withstanding a high stretch ratio in order to form a uniform coating of very small thickness (around 5 to jtm) on a running strip. It has been found that the coatings obtained vary greatly in thickness, precluding this technology from being used to produce thin uniform coatings.

Another technique, better suited to the melt application of a crosslinkable organic coating, consists in forming a premetered polymer film, by forcing it to flow through a heated die placed almost in contact with the strip to be covered. According to this direct extrusion-coating technology, the thinning of the material is obtained by shear between the exit plane of the die and the plane of the strip. However, this direct coating technique is well suited only when the substrates have a very regular surface finish or are flexible (plastic films, paper or board) and supported by a deformable roller, in which case the uniform distribution of the molten material, obtained upon leaving the die, is maintained in the sizing gap for the final coating formed by the space between the die and the running strip. Patent US 5 962 075 (Avery Dennison) discloses its use for producing multilayer coatings. Patent application WO 94/01224 (Courtaulds Coatings) discloses compositions of thermosetting systems for thin coatings on metals, which can be applied by this technique.

This method makes it possible to produce very thin layers (thickness 5 Pm) and very high uniformity pm) provided that the polymer melt feed rate and the run speed of the strip, the variations of which would result in thickness fluctuations of the coating over time, are kept perfectly constant.

However, when it is desired to implement this die coating technology in order to apply a crosslinkable coating to a relatively rigid substrate which has flatness defects or thickness heterogeneities of comparable, or even greater, amplitude than the thickness of the coating to be applied, it is impossible to carry out highprecision direct coating by this method. This is because the thickness of the coating is controlled by the virtual gap formed by the separation between the downstream face of the die and the running substrate. The thickness heterogeneities of the substrate therefore cause local variations in this gap, which result in instantaneous thickness fluctuations in the coating.

Furthermore, the running strip is not always perfectly aligned against the coating die, thereby possibly resulting in poor coverage of the strip, which will only be partially coated.

The above discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia before the priority date of each claim of this application.

It would therefore be desirable to remedy the drawbacks of the methods described above by providing a method for continuously coating strips which makes it possible to apply a smooth uniform film of crosslinkable polymer of predetermined thickness over the entire width of this strip, irrespective of W:JulieAndrevAPats\PCT-FR02-02329 Speci.doc irregularities in thickness or in width or any defects in flatness or positioning of the strip.

A method for continuously coating a strip with a fluid film having a predetermined thickness and made from a crosslinkable polymer that is free from solvent or diluent, the softening temperature of which is above 50 0

C,

wherein: the said strip is being made to run continuously over at least one support at a controlled speed; at least one fluid sheet of said polymer in the melt state, obtained by melting followed by forced flow of the polymer, is deposited over a portion of the width of the said strip, said sheet having a mean thickness greater than said predetermined thickness, the temperature at which this said sheet is formed being below the temperature at which the said polymer starts to crosslink; the said molten polymer is being distributed over the entire width of the said strip by means of a smoothing device with a deformable surface in the form of a flexible doctor blade which forms a continuously replenished reserve of molten polymer over the entire width of said strip and a film of uniform thickness film of said predetermined thickness is formed by shearing the said molten polymer coming from of said reserve, between the said strip and the said flexible doctor blade.

Unlike in the case of an application carried out without a smoothing device, the presence of the reserve of material firstly makes it possible to compensate for the fluctuations in the metering gap resulting from short-period distortions of the strip. Next, the reserve makes it possible to deliver a uniform layer of material over the entire width of the substrate by metering it through a constant gap formed by the flexible doctor blade and the strip. Good uniformity of distribution of the material results from the fact that local thickness or flatness variations of the strip are compensated for by the flexibility of the doctor blade.

Furthermore, a film of smooth appearance is obtained thanks to the reserve formed between the doctor blade and the strip, which makes it possible for the W:Uulie Andrew\Pats\PCT-FR02-02329 Speci.doc molten material to flow with no instability between the strip and the smoothing device.

Finally, premetering the material by forced flow makes it possible for the total material flow rate to be easily controlled, allowing the predetermined mean thickness E of the coating to be achieved. By adjusting the total flow rate of the fluid material according to the width and run speed of the strip, instantaneous control of the coating thickness is achieved, which thickness may furthermore be kept constant by a servocontrol system without intervention by an operator.

The method according to the invention therefore makes it possible to obtain a smooth film of uniform thickness, covering the entire width of the strip, whatever the irregularities in this strip and the way in which it is positioned on this support.

The method according to the invention may also have the following features, individually or in combination: at least one fluid sheet of molten polymer is deposited directly on the strip by forced flow of said molten polymer; at least one fluid sheet of molten polymer is deposited on the strip, by forced flow of said molten polymer, over a transfer roll with a deformable surface, said transfer roll being rotated in the direction in which the strip runs, and then said sheet is transferred from the transfer roll. onto the strip by compressing it between the support and the transfer roll; several fluid sheets, which may be separate, are formed simultaneously by forced flow of molten polymer, said sheets being distributed over the width of the strip and having a mean thickness greater than said predetermined thickness, the temperatures at which these sheets are formed being below the temperature at which the polymer starts to crosslink, the molten polymer is then distributed over the entire width of the strip by means of a flexible doctor blade which forms a continuously replenished reserve of molten polymer over the entire width of said strip, and a continuous film of uniform thickness is formed by shearing the molten polymer coming from said reserve, between the strip and the smoothing device with a deformable surface; W:Uulie\AndrevPats.PCT-FRO2-02329 Spedidoc the strip is preheated to a temperature greater than or equal to the softening temperature of said polymer prior to deposition of the fluid sheet or sheets of molten polymer; the sheet or sheets of molten polymer are formed by extrusion.

According to a second aspect, the present invention provides a device for continuously coating a strip with a fluid film having a predetermined thickness and made of a crosslinkable polymer that is free from solvent or diluent, the softening temperature of which is above 500C, including: means for continuously driving the said strip; means for supporting the said strip; means for melting said crosslinkable polymer; means for forming, by forced flow, at least one fluid sheet of said molten polymer; and means for smoothing the sheet of molten polymer deposited on the running strip in order to obtain a uniform film of said predetermined thickness, said means for smoothing including a flexible doctor blade.

The features and advantages of the present invention will become more clearly apparent over the course of the following description, which is given by way of nonlimiting example, with reference to the appended figure in which: figure 1 is a schematic cross-sectional view of a first embodiment of a device according to the invention.

W:Julie\Andrew~Pats\PCT-FR02.02329 Speci.doc The aim of the invention is to continuously coat a strip with a fluid film of predetermined thickness E made of a crosslinkable polymer that is free from solvent or diluent. This predetermined thickness may, preferably, be between 2 and 100 pm and even more preferably be between 5 and 15 pm. It is preferably less than the thickness of the strip.

The thickness of the strip is, for example, between 0.1 and 4 mm. The material of the strip may be a metal, such as steel, aluminium or an aluminium alloy, or else glass or plywood, for example, and the strip may have been painted or covered beforehand with a coating on one or both sides.

W:\Julie\Andrew\Pats\PCT-FR02.02329 Sped.doc 8 The crosslinkable polymer which is free from solvent or diluent may be thermally crosslinkable, or else crosslinkable by irradiation (UV or electron beam). By way of indication, mention may be made of thermosetting compositions based on hydroxylated polyesters and blocked isocyanates, and reference may be made to application WO 95/21706 for a general description of products of this type, these being well known to those skilled in the art. The radiation-crosslinkable polymers may be radical or cationic systems, or even hybrid systems.

The polymer may, in the usual manner, contain fillers, pigments and additives, such as catalysts, plasticizers, stabilizers or any other known additive.

For the purposes of the present invention, the temperature at which crosslinking starts means the temperature above which an increase in the viscosity of greater than 10% in less than 15 minutes is observed.

The polymers used within the context of the present invention are such that their softening temperature is above 502C, which means that they are solids or are highly viscous at room temperature.

Figure 1 shows a strip B bearing on a support roll 1 made of steel coated with an elastomer and driven so as to run in the direction of the arrow F by drive means (not shown).

The device according to the invention also includes preheating means 2, such as an induction oven, allowing the strip B to be raised to a temperature greater than or equal to the temperature of the molten polymer film, so as to improve the quality of the adhesion between the polymer and the strip and to make it easier for the product to be spread out and smoothed by lowering its viscosity while it passes beneath the smoothing means.

9 Along the generatrix defining the contact between the strip B and the roll 1, there is an extruder (not shown) that includes means for melting the polymer and a die 3 provided with an extrusion slot through which the sheet of molten polymer is forced to flow out onto the strip B. The extruder may also include a flow rate regulating member (not shown) such as a metering pump, which is placed between the extruder and the die 3.

The molten material is fed into the extruder via, for example, a coupled hose. The extruder may be replaced with a melter or a heating drum pump fitted with a screw for pressurizing the liquid or with a gear pump allowing the flow rate of material to be controlled.

The means of forming the sheet by forced flow here consist of the die 3 pressing against the surface of the strip B and provided with means for adjusting the position of the edges of its extrusion slot relative to the surface of the strip B. The die 3 is positioned by means of cylinders or adjustably positioned supports so as to bring it almost into contact with the strip, parallel to the generatrix of the support roll 1.

The slot of the die 3 may have a length less than the width of the strip B in order to form a polymer sheet not covering the entire width of the strip B, but it may also have a length greater than or equal to this width. The slot of the die 3 may furthermore have a length that can be adjusted by displacement of an insert. Another way of adjusting the coating consists in using a die having elements that are juxtaposed but fed separately. This embodiment makes it possible to use the adjustment of the material flow rate in the lateral elements either to vary the width of the sheet formed or to modulate its thickness at the external elements.

10 It is also possible to use a slot die whose downstream face is formed by a perfectly straight metal bar, parallel or slightly inclined to the plane of the strip. It will be possible to adjust the exit profile of the die by machining and to mechanically set its inclination relative to the plane of the strip so as to obtain a perfectly smooth appearance of the coating.

It will also be possible to place, facing the exit of the die, a rotary bar (Meier bar) built into the die assembly and made to undergo a slight rotational movement, allowing the material to be distributed on contact with the strip. This rotary bar therefore constitutes an element that can easily changed, making it possible to avoid remachining the lips of the die should it be knocked or damaged. Furthermore, its rotary movement makes it easier to remove particles or aggregates liable to remain trapped in the application gap.

The thickness profile of the molten polymer sheet may be set differently between the edges and the rest of the body of the die 3. In the central part of the die 3, the material is therefore fed uniformly, whereas at the ends of the die 3 a slight overthickness of material is generated so as to supply, in a controlled manner, the reserve of molten material formed all the way out to the edges of the strip B. This controlled overthickness of the material at the two ends of the die 3 may be easily achieved by varying, for example, the aperture profile of the lips of the die 3. This oversupply of material therefore compensates for the transverse flow of the material in the reserve.

However, it is also possible to feed the entire die 3 uniformly.

It will also be possible to increase the number of dies, these being distributed over the width of the strip B, or else to use a die of adjustable width.

11 The coating device then includes means for smoothing the sheet of molten polymer, these being in the form of a flexible doctor blade 4 supported by a heating element (not shown). Its angle of inclination is set so as to stabilize the reserve of material, which takes the form of a bead and is self-regulated by the following mechanism: if because of an external perturbation too large an amount of material builds up in the bead, the latter will have a tendency to lift the doctor blade and be absorbed, allowing more material to pass through. If the amount of material in the bead decreases, retention of the material decreases and the bead grows and then stabilizes.

The smoothing blade may be produced as a suspension spring, by superimposing metal or plastic blades of various stiffnesses and various thicknesses. Depending on the heat losses arising in the smoothing operation, it may or may not be necessary to preheat the smoothing device, at least in its most rigid part.

The spreading and smoothing of the molten material caused by the metering and smoothing bead are easier the lower the viscosity of the material therein or the higher the temperature of the strip B (which largely controls the temperature of the material in the bead).

Thus, to help to lower the viscosity of the material in the bead as far as possible, the strip B may be preheated to a temperature very much above the temperature at which the polymer starts to crosslink.

The coated strip BR passes through crosslinking means which may consist, for example, of an induction oven, if the polymer is thermally crosslinkable, or UV lamps or electron beams, if it is photocrosslinkable. In the latter case, it is advantageous to combine the radiation effect with the temperature effect in order to accelerate the curing of the product. One of the advantages of the method according to the invention lies in the fact that the coating film obtained is very taut and bubble-free, thereby allowing it to be introduced, without cooling and without additional preheating, into the thermalor radiation-curing device.

In an alternative embodiment, there is provided a device similar to that in figure 1 but making it possible to deposit two sheets of molten polymer that are initially separate, and therefore comprising means for forming two sheets by forced flow, these being distributed over the width of the strip B. These means comprise two identical dies (not shown) provided with extrusion slots through which the sheets of molten polymer are forced to flow onto the strip B. The device may also include two flowrate regulating members placed upstream of the dies.

The application of a fluid film of predetermined thickness E to the strip B, obtained from two separate sheets of polymer of thickness E' greater than the predetermined thickness E, in order to obtain a coated strip BR is carried out in the following manner.

The strip B runs, for example at a speed of 30 m/min, supported by the support roll 1. Optionally, the strip is preheated, preferably to a temperature greater than or equal to the softening temperature of the crosslinkable polymer, by passing it through the preheating means 2.

At the same time, the crosslinkable polymer is melted in each extruder and two sheets of polymer are formed on the strip B by forcing them to flow through respective dies, the temperatures at which the sheets form being below the crosslinking temperature of the polymer. These two sheets are separate and cover part of the width of the strip B. Their thickness E' is greater than the predetermined thickness E. The amount of material deposited on the strip B may be very simply controlled by slaving the extrusion rate to the run speed of the strip B.

W:Uulie Andrew\Pats\PCT-FRO2-02329 Specidoc The strip coated with these two sheets then passes under the doctor blade 4, which forces the excess material, with respect to E, to flow away transversely relative to the strip B, thus forming a reserve of molten material taking the form of a bead. The polymer is thus distributed over the entire width of the strip B, the two sheets thus being merged.

The film obtained after passing under said doctor blade 4 has a uniform thickness E and a smooth appearance with no discontinuities.

The operation of such a system therefore makes it possible to produce, very simply, a modular assembly that can be easily adapted to the variations in width of the strip to be coated.

In a further alternative embodiment, there is provided a device similar to that of figure 1 but furthermore including a transfer roll with a deformable surface (not shown), rotating in a direction which corresponds to the run direction of the strip B. The transfer roll is disposed between the strip B and the die 3. Firstly, a sheet of molten polymer is formed on this roll by extrusion-coating. The sheet is then applied to the strip B by transfer, in "direct" mode, of the sheet deposited beforehand on the transfer roll.

This method of implementation has the advantage of allowing uniform coating with a film of very small thickness, less than 20 pm, which would be very difficult to obtain without this roll. This is because, if the sheet is extruded directly onto the strip B and the die is placed at a distance of less than 20 pm from the strip, there is a high risk of damaging the die if the strip exhibits irregularities of this amount. The addition of a transfer roll therefore makes it possible to achieve very small thicknesses without any risk of damaging the device.

W:Uulie\AndrevwPats\PCT-FR02-02329 Sped.doc The description given above of various embodiments of the invention is in no way limiting. Thus, it will be possible, for example, to implement the method according to the invention so as to coat, whether simultaneously or not, each side of the running strip.

W:\Jlie AndrewXPatsPCT-FRO2-O2329 Spectdoc

Claims (14)

1. A method for continuously coating a strip with a fluid film having a predetermined thickness and made from a crosslinkable polymer that is free from solvent or diluent, the softening temperature of which is above 50 0 C, wherein: said strip being made to run continuously over at least one support at a controlled speed; at least one fluid sheet of said polymer in the melt state, obtained by melting followed by forced flow of the polymer, is deposited over a portion of the width of said strip, said sheet having a mean thickness greater than said predetermined thickness, the temperature at which said sheet is formed being below the temperature at which said polymer starts to crosslink; said molten polymer being distributed over the entire width of said strip by means of a smoothing device in the form of a flexible doctor blade which forms a continuously replenished reserve of molten polymer over the entire width of said strip; and a uniform thickness film of said predetermined thickness is formed by shearing said molten polymer of said reserve, between said strip and said flexible doctor blade.

2. The method as claimed in claim 1, wherein at least one fluid sheet of said molten polymer is deposited directly on said strip by forced flow of said molten polymer.

3. The method as claimed in claim 1 wherein at least one fluid sheet of said molten polymer is deposited on said strip, by forced flow of said molten polymer, over a transfer roll with a deformable surface, said transfer roll being rotated in the direction in which said strip runs, and then said sheet is transferred from said transfer roll onto said strip by compressing it between said support and said transfer roll.

4. The method as claimed in any one of claims 1 to 3 wherein several fluid sheets are formed simultaneously by forced flow of molten polymer, said sheets WA\Tresse\Pats%2002328372 SpedAmend. 16-02-04.doc being distributed over the width of the strip and said mean thickness being greater than said predetermined thickness, the temperatures at which said sheets are formed being below the temperature at which said polymer starts to crosslink, said molten polymer then being distributed over the entire width of said strip by means of said flexible doctor blade which forms a continuously replenished reserve of molten polymer over the entire width of said strip and a continuous film of said uniform thickness is formed by shearing said molten polymer coming from said reserve, between said strip and said flexible doctor blade.

The method as claimed in any one of claims 1 to 4, wherein said sheet or sheets of molten polymer are formed by extrusion.

6. The method as claimed in any one of claims 1 to 5, wherein said strip is preheated to a temperature greater than or equal to the softening temperature of said polymer prior to deposition of said fluid sheet or sheets of molten polymer.

7. The method as claimed in any one of claims 1 to 6, wherein said strip is simultaneously coated on each of its sides.

8. The method as claimed in any one of claims 1 to 7, wherein said strip is a metal strip.

9. A device for continuously coating a strip with a fluid film having a predetermined thickness and made of a crosslinkable polymer that is free from solvent or diluent, the softening temperature of which is above 500C, including: means for continuously driving said strip; means for supporting said strip; means for melting said crosslinkable polymer; means for forming, by forced flow, at least one fluid sheet of said molten polymer; and W:\Tresse\Pats\2002328372 SpeciAmend. 16-02-04.doc means for smoothing the sheet of molten polymer deposited on the running strip in order to obtain a uniform film of said predetermined thickness, said means for smoothing including a flexible doctor blade.

10. The device as claimed in claim 9 further including a transfer roll with a deformable surface, which transfer roll may be rotated in the direction in which said strip runs and is intended to transfer said at least one fluid sheet of molten polymer onto said strip.

11. The device as claimed in claim 9 or 10, including means for forming, by forced flow, several sheets of molten polymer.

12. The device as claimed in any one of claims 9 to 11 wherein said means for forming, by forced flow, at least one fluid sheet of said molten polymer includes an extruder provided with a die having an extrusion slot and, placed between the extruder and the die, a flow rate regulating member.

13. The device as claimed in claim 12 wherein said means for forming, by forced flow, at least one fluid sheet of said molten polymer are formed by said die which bears against the strip and is provided with means for adjusting the position of the edges of said extrusion slot of said die relative to said running strip.

14. A method for continuously coating a strip substantially as hereinbefore described, with reference to the embodiments illustrated in Figure 1. A device for continuously coating a strip substantially as hereinbefore described with reference to the embodiment illustrated in Figure 1. DATED: 26 May 2004 PHILLIPS ORMONDE FITZPATRICK Attorneys for: USINOR W:\Tresse\Pats\2002328372 SpeciAmend. 16-02-04.doc