AU2002301628B2

AU2002301628B2 - Process for the continuous coating of a strip by a fluid film of crosslinkable polymer

Info

Publication number: AU2002301628B2
Application number: AU2002301628A
Authority: AU
Inventors: Frederic Jenny; Thierry Soas; Richard Verhalle
Original assignee: USINOR SA
Current assignee: ArcelorMittal France SA
Priority date: 2001-10-29
Filing date: 2002-10-24
Publication date: 2007-08-16
Anticipated expiration: 2022-10-24
Also published as: JP2003164796A; FR2831470A1; EP1306138A2; EP1306138A3

Description

AUSTRALIA

Patents Act COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority Related Art: Name of Applicant: Usinor Actual Inventor(s): Frederic Jenny, Thierry Soas, Richard Verhalle Address for Service and Correspondence: PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA Invention Title: PROCESS FOR THE CONTINUOUS COATING OF A STRIP BY A FLUID FILM OF CROSSLINKABLE POLYMER Our Ref: 680367 POF Code: 288070/288070 The following statement is a full description of this invention, including the best method of performing it known to applicant(s): -1eooeq SPROCESS FOR THE CONTINUOUS COATING OF A STRIP BY A FLUID FILM OF CROSSLINKABLE POLYMER C' The present invention relates to a process for the continuous 5 coating of a strip by a fluid film of crosslinkable polymer. The present invention 00 oO N applies to crosslinkable polymers which have to be cured by heat treatment, by

INO

irradiation (UV radiation, electron beam, and the like), after their application to C' the strip or by any other appropriate process.

SA reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was, in Australia, known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

The deposition of crosslinkable organic coatings, such as paints or varnishes, for example, is used in particular to protect metal strips against corrosion or alternatively to cover them with paint. For both these usages, it is important for the film obtained to exhibit a constant thickness and an excellent surface condition devoid of craters, whether to guarantee protection against corrosion or for the strip to have an attractive appearance and a uniform colour.

Attempts have therefore been made to develop techniques which make it possible to obtain strips coated with a thin film of crosslinkable polymer having both a constant thickness and the best possible surface condition.

A particularly widely used technique for depositing coatings based on crosslinkable polymers devoid of solvent or of diluent on a strip in forward progression is powder coating. This technique consists in applying a layer of polymer in the powder form to the strip, in then melting the polymer in order to respread it, and curing it for a few minutes at a temperature conventionally between 1800C and 200C, so as to form a film.

However, powder coating does not make it possible to produce thin organic coatings of uniform thickness at high rates and at reasonable costs. The coating obtained by powder coating is never perfectly P: ie\20MO 1629- etypd paqs 15. 7 71 1oc 2 smooth and always exhibits an "orange peel" appearance.

This is why, to produce thin and uniform organic coatings over a strip in forward progression at high rates and at reasonable costs, while employing polymers with a formulation close to those of the standard powder polymers applied according to the strip powder coating process mentioned above, that is to say polymers which are solid or very viscous at ambient temperature and which have a softening temperature greater than ambient temperature, i.e. greater than 50 0 C, it is preferable to use the process described below.

This process consists in forming a film of molten crosslinkable polymer by forced flow through a nozzle fed with molten crosslinkable polymer, either in an extruder or in a bulkmelter, onto an applicator roll, preferably with a deformable surface, in contact with the surface of the strip to be coated. The transfer of the film formed by forced flow onto the strip in forward progression is carried out by friction of the applicator roll and of the strip, the forward progression of the two surfaces occuring in opposite directions. The term used is then "reverse transfer".

The curing of these polymers is carried out under conditions different from those of the standard powder polymers; the curing temperature is higher, of the order of 230 to 270 0 C, and the curing time is much shorter, of the order of 10 to 60 s.

This reverse transfer coating process does not make it possible to employ standard powder polymers, the viscosity of which is too high for it to be possible to extrude them without risk of crosslinking under good conditions, that is to say with a satisfactory flow rate and an acceptable pressure drop. Formulations comprising crosslinkable polymers which are devoid of solvent or of diluent and which have the distinguishing feature of exhibiting a temperature for the beginning of crosslinking which is higher than that of powder polymers, as well as a lower viscosity, have therefore been developed for this type of process. However, despite the use of this type of crosslinkable polymer, the reverse transfer Sprocess remains difficult to employ as it is necessary all the same to heat the _polymer in order to reduce its viscosity before forming a film thereof by forced C' flow through the nozzle. If the polymer is not heated enough, its viscosity does not decrease sufficiently and this prevents it from flowing with a satisfactory 00 Ci flow rate and an acceptable pressure drop. On the other hand, if the polymer is heated too much, it begins to crosslink before the curing stage, which prevents C' the polymer film from being correctly formed and which risks blocking the C)coating tools.

c 10 This process exhibits the disadvantage of requiring very precise adjustment of the temperatures in the bulkmelter or extruder and in the nozzle.

In addition, it does not make it possible to obtain coatings exhibiting a surface condition which is always satisfactory.

The aim of the present invention is therefore to overcome or ameliorate at least one or more of the disadvantages of the processes of the prior art.

According to the present invention there is provided a process for the continuous coating of at least one face of at least one strip by a mono- or multi-layer fluid film of crosslinkable polymer devoid of solvent or of diluent exhibiting a temperature for the beginning of crosslinking Tc and with a softening temperature of greater than 50'C, wherein the process includes the following stages: the strip is made to progress forward continuously over at least one support, a fluid film of a blend of the said polymer with a temporary additive, in the molten state, is deposited on the strip, the said film being obtained by melting and then forced flow of the said polymer/temporary additive blend, the temperature for formation of the said film TF being less than the temperature for the beginning of crosslinking less 200C and the said temporary additive exhibiting a boiling point between the temperature for formation of the said film TF less 300C and the temperature for the beginning of crosslinking To.

P; 200202 3. CZ-Cyp py. 25.1O00 ho SThe present invention provides a process for the continuous coating of strips Sby a crosslinkable polymer devoid of solvent or of diluent and with a softening q temperature of greater than 500C which makes it possible to avoid any risk of an untimely beginning of crosslinking of the polymer before the formation of oo N the film and to obtain a thin film of crosslinked polymer of uniform thickness

IND

exhibiting a surface condition devoid of craters.

q The inventors have demonstrated that the addition of this temporary additive greatly reduces the viscosity of the polymer. Consequently, it becomes c 10 possible to increase the difference between the temperature for formation of the crosslinkable polymer film and the temperature for the beginning of crosslinking of the polymer by decreasing the temperature for formation of the polymer film, which makes it possible to reduce the risk of crosslinking before formation of the film. In an entirely unexpected way, the inventors also found that, by adding, to the crosslinkable polymer, a temporary additive with a boiling point between the temperature for formation of the polymer film less 300C and the temperature for the beginning of the crosslinking of the polymer, the formation of craters in the crosslinked polymer film is avoided.

The temporary additive acts not only as temporary plasticizer of the polymer but also improves the surface appearance of the film.

The process according to the invention can also exhibit the following characteristics: the temporary additive exhibits a boiling point B.p. between the temperature for formation of the said film TF plus or minus 200C, the temporary additive exhibits a boiling point B.p. between 700C and 1550C, preferably between 900C and 1200C, P \il; \2002 30 162 l, ;-le 25 37 .doc 5 the temporary additive is added in liquid form to said polymer, the temporary additive is chosen from ethanol, water, butyl acetate, amyl acetate and cyclohexanone, alone or as mixtures, the temporary additive is added to the polymer in a proportion of between 1 and 5% by weight of the crosslinkable polymer, preferably between 2 and 4%, the temporary additive is incorporated beforehand in the polymer, the polymer/temporary additive blend is conditioned in the form of solid particles and then the said blend is melted, either in an extruder or in a keg, the temporary additive is incorporated in the polymer and the polymer/temporary additive blend is conditioned directly in the solid form in a bulkmelter, a blend of the polymer, in the form of solid particles, with the temporary additive, in the liquid form, is prepared beforehand and then this blend is introduced into at least one hopper of an extruder, the temporary additive is added to the polymer in an extruder and the polymer/temporary additive blend is melted in the extruder at a temperature TF at least 10 0 C below the temperature for formation Tfo of the film of polymer not blended with a temporary additive, the temporary additive is added to the polymer by injection into the extruder, the temporary additive is added to the polymer in a region for mixing the molten polymer, the temporary additive is added to a bulkmelter and the polymer/temporary additive blend is melted in the bulkmelter at a temperature TF at least 0 C lower than the temperature for formation Tfo of the film of polymer not blended with a temporary additive, the temporary additive is added by injection into a region of the bulkmelter where the polymer is melted, the temporary additive is added in the inlet 6 region of a system for heating homogenization of the molten polymer and the film of the polymer/temporary additive blend is formed by forced flow in a nozzle at a temperature TF at least 100C lower than the temperature for formation Tfo of the film of polymer not blended with a temporary additive, the temporary additive is added by injection under pressure, the fluid film of polymer in the molten state is deposited on the strip by forced flow of the said polymer in the molten state onto an applicator roll with a deformable surface, the said applicator roll being driven in rotation in the direction of forward progression of the strip, and then the said film is transferred from the applicator roll to the strip by compressing it between the support of the strip and the applicator roll, the fluid film of polymer in the molten state is deposited on the strip by forced flow of the said polymer in the molten state onto an applicator roll with a deformable surface, the said applicator roll being driven in rotation in the opposite direction to the direction of forward progression of the strip, and then the said film is transferred from the applicator roll onto the strip by compressing the support of the strip and the applicator roll, the fluid film of polymer in the molten state is deposited on the strip by forced flow of the said polymer in the molten state onto a pinch roll situated upstream of the applicator roll, the said pinch roll being driven in rotation in the opposite direction to the direction of rotation of the applicator roll, the fluid film of polymer in the molten state is deposited on the strip by forced flow of the said polymer in the molten state onto a pinch roll situated upstream of the applicator roll, the said pinch roll being driven in rotation in the same direction as the direction of rotation of the applicator roll, between the formation by forced flow onto the 7 applicator or pinch roll of the film and the transfer of this film from this applicator or pinch roll onto the strip, the crosslinkable polymer is thermally conditioned in order to lower the viscosity of this polymer, the fluid film of polymer in the molten state is deposited by forced flow of the said polymer directly onto the strip, the strip in forward progression is preheated to a temperature between the temperature for formation TF of the film of polymer blended with the temporary additive and the temperature Tc for the beginning of crosslinking, the thickness of the coating obtained by this process is between 3 and 100 pm, preferably between and 50 pm, the strip is a metal strip.

The characteristics and advantages of the present invention will become more clearly apparent during the description which will follow, given by way of non-limiting example, with reference to the appended figures, in which: Figure 1 is a diagrammatic view in cross section of a plant for the continuous coating of a strip according to the invention, Figure 2 is a diagrammatic view in cross section of an alternative form of the plant of Figure i.

The invention applies not only to strips of metal, such as, for example, steel, aluminium or an aluminium alloy, but also of glass, of plastic, of plywood or of any other suitable material, and the strip may have been painted beforehand or covered beforehand with a coating on one or both faces.

The crosslinkable polymer devoid of solvent or of diluent may be crosslinkable thermally or alternatively by irradiation (UV or electron beam). For example, mention will be made, by way of indication, of thermosetting compositions based on hydroxylated polyesters and on blocked isocyanates, and reference will be made to Application WO 95/21706 for a general description of products of this type.

The polymers crosslinkable by radiation can be radical or cationic systems, c indeed even hybrid systems.

The polymer can comprise fillers, pigments or additives, such as oO N catalysts, plasticizers, stabilizers or any other suitable additive.

In the context of the present invention, the term "temporary additive" 0 M is understood to mean an additive which is added to the crosslinkable polymer O before the formation of the polymer film and which is not, or not to any S 10 significant extent, re-encountered in the composition of the crosslinked polymer film. This temporary additive is neither a solvent nor a diluent of the crosslinkable polymer.

The polymers used in this process have softening temperatures, temperatures for the beginning of flow and temperatures for the beginning of crosslinking which are different.

Generally, the temperature for the beginning of crosslinking is the temperature from which an increase in the viscosity of greater than 10% in less than 15 min is observed. The polymers used in the context of the present invention are such that their softening temperature is greater than 500C, which means that they are solid or very viscous at ambient temperature.

To continuously coat at least one face of at least one strip by a mono- or multilayer fluid film of crosslinkable polymer, the strip progresses forward continuously over at least one support and a fluid film of the said polymer in the molten state, obtained by melting and then forced flow of the polymer, the temperature for formation Tfo of this film being lower than the temperature for the beginning of crosslinking Tc of the polymer by at least is deposited on the strip.

According to the invention, a temporary P. \U.er\Ji\2OO23U:626--e jped paq (25 70) doc 9 additive, the boiling point B.p. of which is between the temperature for formation TF of the film of polymer blended with the temporary additive less 30 0 C and the temperature for the beginning of crosslinking of the polymer is added to the crosslinkable polymer before the formation of the film. As the crosslinkable polymer does not dissolve in the temporary additive, a homogeneous blend is not formed. This temporary additive acts as temporary plasticizer of the polymer by rendering it less viscous, which makes it possible to melt the polymer/temporary additive blend in the extruder or in the bulkmelter at a temperature TF at least 100C lower than the temperature for formation Tfo of the film of polymer not blended with a temporary additive. It is thus possible to form the fluid film of polymer in the molten state on the applicator roll by forced flow of the polymer through the nozzle at a temperature TF at least 10°C lower than the temperature for formation Tfo of the film of polymer not blended with a temporary additive. Consequently, the difference between the temperature for the beginning of crosslinking Tc of the polymer and the temperature for formation TF of the said film of polymer blended with a temporary additive is increased by more than 20 0 C and any risk of crosslinking of the polymer before forming the polymer film is thus avoided.

In addition, during the melting of the crosslinkable polymer, during the formation by forced flow of the polymer in the molten state and before the polymer film crosslinks, the temporary additive produces significant foaming of the polymer. The degassing of the polymer is thus facilitated before the crosslinking of the polymer and a crosslinked polymer film devoid of craters is obtained.

10 The inventors have demonstrated that, when the boiling point B.p. of the temporary additive is equal to or greater than the temperature for the beginning of crosslinking of the polymer, the additive does not have the time to act and it generates numerous craters by breaking the already crosslinked polymer film.

The boiling -point B.p. of the temporary additive is preferably equal to the temperature for formation TF of the film of polymer blended with a temporary additive plus or minus 20 0 C. This is because, when the boiling point B.p. of the temporary additive is lower than the temperature for formation TF of the film of polymer blended with a temporary additive less 0 C, the additive has a tendency to evaporate in the extruder or in the bulkmelter. The temporary additive still acts as temporary plasticizer of the polymer but loses its effect on the surface appearance of the polymer film.

The boiling point B.p. of the temporary additive is chosen as a function of the crosslinkable polymer according to the invention with which it is desired to form a film by forced flow. It is between and 155 0 C, preferably between 90 and 120 0

C.

The temporary additive is advantageously chosen from ethanol 780C), water 1000C), butyl acetate 126 0 amyl acetate 1490C) and cyclohexanone 1550C), alone or as a mixture.

More preferably, the temporary additive selected is water.

The temporary additive is added, preferably in liquid form, in a proportion of between 1 and 5% by weight of the crosslinkable polymer, preferably between 2 and 4%.

If less than 1% of temporary additive is added to the polymer, no effect of plasticization of the polymer and no significant improvement in the surface appearance of the crosslinked film are observed.

If more than 5% of temporary additive is added to the polymer, the polymer is saturated with temporary 11 additive and a portion of the additive is not dispersed and remains in the form of droplets.

A plant for the coating, by reverse transfer, of a strip B in forward progression by a fluid film of crosslinkable polymer, comprising means for melting the polymer, means for formation of the polymer film and means for reverse transfer of the film onto the strip B, have been represented in Figure 1.

The means for melting the polymer are composed of an extruder 1 comprising a hopper la, into which the polymer is introduced in the form of solid particles, and means for heating and mixing the polymer composed of an endless screw lb which forces the molten polymer through a die. The extruder can also comprise a device for regulating the flow rate of the molten polymer (not represented), such as a pump, which is positioned between the outlet of the extruder and the means for formation of the polymer film. The feeding of the molten polymer as far as the means for formation of the film is carried out, for example, via a heating flexible coupling 2. The heating flexible coupling 2 can advantageously be equipped with a system for heating homogenization 10 of the molten polymer, such as, for example, a static mixer.

The means for formation of the polymer film comprise a nozzle 3 equipped with a slot resting against the deformable surface of an applicator roll 4.

The molten polymer film flows in a forced manner through the slot of the nozzle 3 onto the applicator roll 4. The slot of the nozzle 3 is provided with means for adjusting the position of the edges of the slot of the nozzle with respect to the surface of the applicator roll 4. As the nozzle 3 is parallel to the applicator roll 4, a film of uniform thickness is formed on the latter.

The means for reverse transfer of the polymer film are composed of the applicator roll 4 and of a roll for supporting 5 the strip B which are driven in rotation in opposite directions. The applicator roll 4 12 is in contact with the surface of the strip B, which is resting on the support roll 5 and is driven in forward progression according to the arrow F by driving means (not represented). The applicator roll 4 is generally deformable and is formed of a core of steel coated with a layer of elastomer, whereas the support roll 5 is generally not deformable.

The coating plant according to the invention can also comprise preheating means 6, such as an induction oven, which make it possible to bring the strip B to a temperature between the temperature for formation TF of the film of polymer blended with the temporary additive and the temperature Tc for the beginning of crosslinking of the polymer, so as to improve the quality of the adhesion between the polymer and the strip B.

According to a first embodiment of the invention, a blend of the polymer, in the form of solid particles, with the temporary additive, in the liquid form, is prepared in a first stage and, in a second stage, this blend is introduced into the hopper la of the extruder i.

According to another embodiment of the invention, the temporary additive is added by injection into one of the regions of the extruder 1, preferably in a mixing region of the extrusion screw ib, so as to obtain better dispersion of the additive in the polymer.

The polymer/temporary additive blend can thus be melted in the extruder 1 at a temperature TF at least 10 0 C lower than the temperature for formation Tfo of the film of polymer not blended with a temporary additive.

According to another embodiment of the invention, the temporary additive is added by injection into the system for heating homogenization 10 of the molten polymer, preferably at the inlet of the system in order to promote the dispersion of the temporary additive in the polymer. In this way, the film of the polymer/temporary additive blend is formed by forced 13 flow into the nozzle 3 at a temperature TF at least 0 C lower than the temperature of formation Tfo of the film of polymer not blended with a temporary additive.

When the polymer/temporary additive blend has been melted in the extruder 1 and the fluid film of polymer in the molten state had been formed by forced flow through the nozzle 3, the polymer film is transferred from the applicator roll onto the strip B by compressing the support roll 5 of the strip B and the applicator roll 4, the applicator roll 4 being driven in rotation in the opposite direction to the direction of forward progression of the strip B.

The coated strip BR subsequently passes through crosslinking means 7 which can be composed, for example, of an induction or convection oven, when the polymer is thermally crosslinkable, or alternatively of UV lamps or of electron beams, when it is photocrosslinkable. In the latter case, the effect of the radiation is advantageously combined with that of the temperature to accelerate the curing of the polymer.

In order to facilitate the transfer of the polymer film formed by forced flow from the applicator roll 4 to the strip B, the polymer can advantageously be thermally conditioned in order to lower its viscosity to a value lower than the viscosity measured under the conditions of forced flow. The means for conditioning the polymer film (not represented) are formed by an internal heating system of the applicator roll and/or by an external heating system, such as, for example, infrared lamps.

According to another embodiment of the invention, the applicator roll 4 is driven in rotation in the direction of forward progression of the strip B and, in this case, the polymer film formed on the applicator roll 4 by forced flow is transferred from the applicator roll 4 onto the strip B by compressing it between the roll for support 5 of the strip B and the applicator roll 4. This is the direct transfer coating process.

14 According to another embodiment of the invention, the plant can comprise a pinch roll (not represented) situated upstream of the applicator roll 4 and, in this case, the slot of the nozzle 3 is resting against the surface of the pinch roll. The fluid film of polymer in the molten state is formed on the pinch roll. The film is transferred onto the applicator roll 4, the surface of the pinch roll progressing forward in contact with the surface of the applicator roll 4 either in the opposite direction or in the same direction. The film is then transferred from the applicator roll 4 onto the strip B, either by direct transfer or by reverse transfer.

According to another embodiment of the invention, the plant does not comprise an applicator roll 4. The slot of the nozzle 3 then rests directly against the surface of the strip B and the film formed by forced flow through the nozzle is deposited directly on the strip B.

The plant of Figure 2 differs from the preceding plant in that the means for melting the polymer are composed of a bulkmelter 8 comprising a keg 8a and a heating plate 8b equipped with a pump 9 which makes it possible to control the flow rate of molten polymer.

As in the plant of Figure 1, the feeding of the molten polymer as far as the nozzle 3 is carried out, for example, via a heating flexible coupling 2. The heating flexible coupling 2 can advantageously be equipped with a system for heating homogenization 10 of the molten polymer, such as, for example, a static mixer.

According to an embodiment of the invention in the case of the plant 2, the temporary additive is added by injection into a region of the bulkmelter where the polymer is melted. In this way, the polymer/temporary additive blend is melted in the bulkmelter 8 at a temperature TF at least 10 0 C lower than the temperature for formation Tfo of the film of 15 polymer not blended with a temporary additive.

According to another embodiment of the invention, the temporary additive is added by injection into the system for heating homogenization 10 of the molten polymer, preferably at the inlet of the system in order to promote the dispersion of the temporary additive in the polymer. In this way, the film of the polymer/temporary additive blend is formed by forced flow into a nozzle 3 at a temperature TF at least 10 0

C

lower than the temperature for formation Tfo of the film of polymer not blended with a temporary additive.

According to an embodiment of the invention common to the plants of Figures 1 and 2, the crosslinkable polymer is employed in the form of solid particles, in which polymer the temporary additive has been incorporated beforehand. In order to obtain the crosslinkable polymer in the solid form already incorporating the temporary additive, a blend comprising the polymer, in the form of solid particles, with the additive, in the liquid form, is prepared, this blend is melted, for example in an extruder, and a sheet thereof is formed, which sheet is allowed to cool. After complete cooling of the sheet, it is milled, so as to obtain the polymer in the form of solid particles incorporating the temporary additive.

This polymer incorporating the temporary additive is subsequently processed, that is to say that it is melted, either in the extruder 1 or in the bulkmelter 8, so as to form a film thereof by forced flow through the nozzle 3.

In the specific case of a plant 2, the polymer/temporary additive blend can be conditioned directly in the solid form in the keg 8a. To do this, the crosslinkable polymer/temporary additive blend is melted directly, for example by extrusion, in a keg 8a without passing through the intermediate stage of formation of solid particles of crosslinkable polymer incorporating the temporary additive. The blend is then allowed to cool, so as to obtain a solid block of the Spolymer/temporary additive blend in the keg 8a which can be used directly to form a film by forced flow according to the invention.

According to a preferred form of the invention which is common to both plants, the temporary additive is injected under pressure. To this end, use 00 Nis made, for example, of an injection needle fed via a high pressure piston pump.

O

The fluidizing effect of the temporary additive in the polymer can 0 have other advantages: S 10 the viscosity of conventional powder polymers may be sufficiently lowered for it to be possible to form a fluid film of polymer in the molten state by forced flow onto the strip B in forward progression; thin and uniform organic films can thus be obtained at reasonable costs while avoiding having recourse to polymer formulations especially developed for this type of coating process and which are more expensive.

The inventors have demonstrated that the torque for transfer between the applicator roll 4 and the strip B decreases when a temporary additive is added to the polymer; the applicator roll 4 is therefore less stressed and the duration of its use substantially increases.

The inventors have also demonstrated that, at constant temperature, the flow rate of molten polymer in the extruder 1 increases; it is thus possible to reduce the temperature for extrusion of the polymer while retaining good flow of the polymer.

The extruder 1 and nozzle 3 are easier and faster to clean; thus, when it is desired to change the type of polymer or to change the colour of the polymer, the output of the coating line is reduced to a lesser extent.

The description which has just been given of various embodiments is in no way limiting. Thus, it is possible, for example, to employ the process according to the invention for coating, simultaneously or otherwise, each of the faces of the strip B in forward P: Ur~j;:-\Q-030 69- tyedp~ge, (25 IO 0) do- 17 progression.

The aim of the following examples is to illustrate the invention.

Example 1: The aim of this example is to illustrate the plasticizing effect of a temporary additive according to the invention when it is added to a thermosetting crosslinkable polymer.

By way of example, the composition of the thermosetting crosslinkable polymer is as follows: Hydroxylated polyester resin: Uretdione curing agent: TiO 2 pigment (for a white appearance of the final coating): 33% Additives (spreading agent, catalysts, and the like): 2% This composition was developed to be applied to a strip in forward progression according to the reverse transfer coating process.

The characteristics of this polymer are, for example: Temperature 110 120 130 140 150 Viscosity (Pa-s) 450 250 130 65 Number-average molar mass: Mn 4 000 Weight-average molar mass: Mw 14 000 g/mol Temperature for the beginning of crosslinking Tc: 160°C A face of a steel strip was coated with a layer of this thermosetting crosslinkable polymer by employing the reverse transfer coating process as illustrated in Fig. 1. To this end, the polymer was extruded and a fluid film of the polymer was formed on the applicator roll by forced flow of the molten 18 polymer through the nozzle at a temperature Tfo of 130 0

C

(measured in the nozzle) Subsequently, 2% of water 100 0 C) and 2% of butyl acetate 126 0 C) respectively were added to the thermosetting crosslinkable polymer as temporary additive. Then, as above, the polymer/temporary additive blend was extruded and a fluid film of the polymer/temporary additive blend was formed by forced flow of the blend onto the applicator roll at a temperature TF of 130 0 C (measured in the nozzle).

During the application of the coating to the steel strip, the following five variables were measured: the torque of the extruder, which represents the power absorbed by the work of shearing the molten polymer in the extruder and consequently represents the viscosity of the polymer. The more fluid the polymer (low viscosity), the less strain on the extruder in moving the polymer forward and the lower the torque of the extruder, the pressure in the extruder, which is the pressure of the molten polymer at the outlet of the extruder, the pressure in the nozzle, which is the pressure of the molten polymer measured after the formation of the film by forced flow and immediately before the application of this film on the applicator roll, the torque for transfer from the applicator roll with respect to the steel strip, which represents the effort necessary to detach the film of molten polymer from the applicator roll and to transfer it onto the strip, the flow rate of the molten polymer, a film of which is formed by forced flow.

These five variables are correlated with the viscosity of the polymer. This is because, when the viscosity of the polymer decreases, the torques and pressures decrease while the polymer flow rate 19 increases.

These five variables have been listed in Table I.

Table I POLYMER Extruder P P Transfer Flow torque extruder nozzle torque rate outlet (bar) (daN.m) (kg/h) (bar) Without 50 130 20 20 temporary additive With 2% of 30 80 10 15 water With 2% of 35 95 13 17 13.5 butyl acetate On reading this table, it is found that the temporary additive indeed acts as temporary plasticizer of the polymer since it lowers the viscosity of the polymer/temporary additive blend, which is reflected by a decrease in the torque in the extruder.

By adding 2% of temporary additive to the polymer, it was possible to subsequently extrude the polymer/temporary additive blend and to form a film of the blend by forced flow at a temperature from 20 to 0 C lower than the temperature for formation of the polymer without additive Tfo, i.e. at a temperature TF of between 100 and 110 0

C.

In addition, when the temporary additive chosen is water, it could be observed that the strip of steel coated with the polymer film exhibits a very attractive surface appearance devoid of craters.

Example 2: The aim of this example is to show that, by virtue of the invention, which makes it possible to extrude or to melt a polymer at a temperature further Saway from the temperature for the beginning of crosslinking under conditions which ensure correct flow of the molten polymer, it is possible to extrude standard powder polymers, that is to say polymers with a higher viscosity and qn a lower temperature for the beginning of crosslinking than those of the polymer described above.

00 By way of example, the composition of a standard powder polymer IN is as follows: r Carboxylated polyester resin: TGIC (triglycidyl isocyanurate) C curing agent: TiO 2 pigment (for a white appearance of the final coating): 33% Additives (spreading agent, catalysts, and the like): 2% The characteristics of this thermosetting polymer are, for example: Temperature (oC) 110 120 130 140 Viscosity (Pas) 5 000 2 000 750 300 Number-average molar mass: Mn 4 000 Weight-average molar mass: Mw 19 000 g/mol Temperature for the beginning of crosslinking Tc: 140°C The viscosity of this polymer is too high to be able to be extruded at a temperature lower than its temperature for the beginning of crosslinking less On the other hand, by adding 2% of water to this polymer, the viscosity falls sufficiently for the polymer/temporary additive blend to be able to be extruded under good flow conditions, so as to form a film by forced flow through the nozzle at a temperature TF of 1150C.

Throughout the description and claims of this specification the word "comprise" and variations of that word, such as "comprises" and "comprising", are not intended to exclude other additives or components or integers.

P:\Use,\Jul :,\2002312e- p (2b doc

Claims

1. A process for the continuous coating of at least one face of at least q one strip by a mono- or multi-layer fluid film of crosslinkable polymer devoid of solvent or of diluent exhibiting a temperature for the beginning of crosslinking o00 c To and with a softening temperature of greater than 500C, wherein the process INO includes the following stages: C the strip is made to progress forward continuously over at least Sone support, c 10 a fluid film of a blend of the said polymer with a temporary additive, in the molten state, is deposited on the strip, the said film being obtained by melting and then forced flow of the said polymer/temporary additive blend, the temperature for formation of the said film TF being less than the temperature for the beginning of crosslinking less 20'C and the said temporary additive exhibiting a boiling point between the temperature for formation of the said film TF less 300C and the temperature for the beginning of crosslinking T_.

2. A coating process according to Claim 1, wherein said temporary additive exhibits a boiling point between the temperature for formation of the said film TF plus or minus 200C.

3. A coating process according to Claim 1 or 2, wherein the temporary additive exhibits a boiling point between 700C and 1550C.

4. A coating process according to claim 3, wherein the temporary additive exhibits a boiling point between 900C and 1200C.

5. A coating process according to any one of Claims 1 to 4, wherein the temporary additive is added in liquid form to the said polymer.

6. A coating process according to any one of Claims 1 to 5, wherein the temporary additive is chosen from ethanol, water, butyl acetate, amyl acetate and cyclohexanone, alone or as mixtures.

7. A coating process according to any one of Claims 1 to 6, wherein the temporary additive is added to the polymer in a proportion of between 1% and 5% by weight of the crosslinkable polymer. P.NU'-\-le\ZOOZ3Ol629--ety pages 1(Z5.707).OC

8. A coating process according to claim 7, wherein the temporary additive is added to the polymer in a proportion of between 2% and 4% by weight of the crosslinkable polymer.

9. A coating process according to any one of Claims 1 to 8, wherein the temporary additive is incorporated beforehand in the polymer, the 00 C polymer/temporary additive blend is conditioned in the form of solid particles and then the said blend is melted, either in an extruder or in a bulkmelter.

10. A coating process according to any one of Claims 1 to 8, wherein the temporary additive is incorporated in the polymer and the (N 10 polymer/temporary additive blend is conditioned directly in solid form in a keg.

11. A coating process according to any one of Claims 1 to 8, wherein a blend of the polymer, in the form of solid particles, with the temporary additive, in liquid form, is prepared beforehand and then this blend is introduced into at least one hopper of an extruder.

12. A coating process according to any one of Claims 1 to 8, wherein the temporary additive is added to the polymer in an extruder and the polymer/temporary additive blend is melted in the extruder at a temperature TF at least 100C lower than the temperature for formation Tfo of the film of polymer not blended with a temporary additive.

13. A coating process according to Claim 12, wherein the temporary additive is added to the polymer by injection into the extruder.

14. A coating process according to Claim 13, wherein the temporary additive is added to the polymer in a region for mixing the molten polymer.

A coating process according to any one of Claims 1 to 8, wherein the temporary additive is added to a bulkmelter and the polymer/temporary additive blend is melted in the bulkmelter at a temperature TF at least lower than the temperature for formation TfO of the film of polymer not blended with a temporary additive.

16. A coating process according to Claim 15, wherein the temporary additive is added by injection into a region of the bulkmelter where the polymer is melted.

17. A coating process according to any one of Claims 1 to 8, wherein the temporary additive is added in the inlet region of a system for heating F Wulie\2002301628-typedn pagq,25..07).doc homogenization of the molten polymer and the film of the polymer/temporary additive blend is formed by forced flow in a nozzle at a temperature TF at least 0 C lower than the temperature for formation Tfo of the film of polymer not q blended with a temporary additive.

18. A coating process according to any one of Claims 12 to 17, wherein 00oO C the temporary additive is added by injection under pressure.

19. A coating process according to any one of Claims 1 to 18, wherein Mq the fluid film of polymer in the molten state is deposited on the strip by forced tN Sflow of the said polymer in the molten state onto an applicator roll with a C 10 deformable surface, the said applicator roll being driven in rotation in the direction of forward progression of the strip, and then the said film is transferred from the applicator roll onto the strip by compressing it between the support of the strip and the applicator roll.

A coating process according to any one of Claims 1 to 18, wherein the fluid film of polymer in the molten state is deposited on the strip by forced flow of the said polymer in the molten state onto an applicator roll with a deformable surface, the said applicator roll being driven in rotation in the opposite direction to the direction of forward progression of the strip, and then the said film is transferred from the applicator roll onto the strip by compressing the support of the strip and the applicator roll.

21. A coating process according to Claim 19 or 20, wherein the fluid film of polymer in the molten state is deposited on the strip by forced flow of the said polymer in the molten state onto a pinch roll situated upstream of the applicator roll, the said pinch roll being driven in rotation in the opposite direction to the direction of rotation of the applicator roll.

22. A coating process according to Claim 19 or 20, wherein the fluid film of polymer in the molten state is deposited on the strip by forced flow of the said polymer in the molten state onto a pinch roll situated upstream of the applicator roll, the said pinch roll being driven in rotation in the same direction as the direction of rotation of the applicator roll.

23. A coating process according to any one of Claims 18 to 22, wherein, between the formation by forced flow onto the applicator roll or pinch roll of the film and the transfer of this film from this applicator roll or pinch roll 2020:626-,ellyFd pg9-i25.7.O7, dcc 24 0 onto the strip, the crosslinkable polymer is thermally conditioned in order to lower the viscosity of this polymer.

24. A coating process according to any one of Claims 1 to 18, wherein q the fluid film of polymer in the molten state is deposited by forced flow of the said polymer directly onto the strip. 00O C

25. A coating process according to any one of Claims 1 to 24, wherein the strip in forward progression is preheated to a temperature between the ¢q temperature for formation TF of the film of polymer blended with the temporary Sadditive and the temperature T, for the beginning of crosslinking. S 10

26. A coating process according to any one of Claims 1 to 25, wherein the thickness of the coating obtained by the said process is between 3pm and 100pm.

27. A coating process according to claim 26, wherein the thickness of the coating obtained by the said process is between 5pm and

28. A coating process according to any one of Claims 1 to 27, wherein the strip is a metal strip.

29. A coating process according to any one of the embodiments substantially as herein exemplified. P \Jul ,.\002301628- lyp~d paqe25 1 911 do.