WO2004069428A1

WO2004069428A1 - Method of fast curing water-borne paint coatings

Info

Publication number: WO2004069428A1
Application number: PCT/AU2004/000122
Authority: WO
Inventors: Victoria Chapman; James Viatos; David Peter Buxton; David Frank Christian
Original assignee: Bluescope Steel Limited
Priority date: 2003-02-04
Filing date: 2004-02-03
Publication date: 2004-08-19
Also published as: CN1761533A; JP2006516472A; KR20050095892A; AU2003900491A0; MY136993A; NZ541838A

Abstract

A method of curing a water-borne paint that has been applied as a liquid onto a substrate and forms a paint coating on the substrate is disclosed. The method includes heating the coated substrate to and holding it at a temperature that is below (but preferably no more than 5-10° below) the boiling point of water in the paint and evaporating an amount of the water in the paint from the paint so that there are substantially no surface defects in the form of blisters in the resultant paint coating that are caused by water in the paint boiling during a subsequent curing step (also known as “solvent boil” or “water boil”). The method also includes heating the substrate in a subsequent curing step to a higher temperature than the evaporation temperature of the preceding step and curing the paint. Figure 1 is a flow chart illustrating a production line from a metal coating and thereafter a water-borne paint coating on a steel strip. Steel strip is uncoiled from a coiler (3) and continuously fed through a metal coating section (5), a paint applicator section (7), and a curing section (9). The curing section containing a spaced apart induction evaporation oven (11) and induction curing oven (13).

Description

METHOD OF FAST CURING WATER-BORNE PAINT COATINGS

The present invention relates to forming a coating of a water-borne paint on a substrate.

The invention relates particularly, although by no means exclusively, to a method of forming a thin ornamental and/or protective coating of a water-borne paint on a substrate that is in the form of a metal strip,

The term ^wwater-borne paint" is understood herein to mean a paint that includes (i) water that acts as a dispersant or carrier liquid (ii) polymeric material (thermosetting and thermoplastic) , such as polymeric film forming material, dispersed and/or dissolved in the water, (iii) a pigment or pigments dispersed in the water and/or the polymeric material; and (iv) optionally additives, eg wetting, dispersion and antimicrobial agents.

The term "thin" as used herein is understood to mean a coating thickness of up to 60 micron.

Typically, the invention is applicable to the production of painted metallic (including steel, aluminium and other non-ferrous metals and alloys) strip, particularly painted metal coated steel strip, that is suitable to be used as the starting material in the production of building cladding sheets and other steel metal products for the building industry, appliance cabinets, vehicle bodies and many other sheet metal products.

Ornamental and protective paint coatings are conventionally applied to metal coated steel strip, such as galvanised or ZINCALUME (Registered Trade Mark) coated coiled stock, by coating the strip with solvent-based and water-based paint compositions by means of a liquid paint applicator such as a roll coater or a curtain σoater. Typically, the paint includes polymeric film-forming materials, pigments and inert fillers dispersed and/or dissolved in a solvent or water. The coated strip is transferred from the liquid paint applicator station to an oven, such as a hot air convection oven, an induction oven or an infra-red oven, and the strip is heated to cure the paint. Typically, the oven heats the coated strip to a curing temperature and holds the coated strip at that temperature for a predetermined period of time.

It is important that ovens be capable of heating coated strip quickly so that the curing step does not limit production rate on paint lines.

It is also important that large (and therefore expensive) ovens that allow curing over a longer time frame and thereby slow production rates are not required.

It has been found that an inherent limitation of water-borne paints is that the paints cannot be applied at film thicknesses typical of topcoats and cured quickly, say, in oven dwell times of less than 20 seconds, preferably less than 15 seconds, without introducing quality issues. Specifically, quick curing of such paints results in surface defects in the form of blisters in the resultant coating that are caused by water in the paints boiling during the curing process . These defects are generally referred to as "water boil or solvent boil" and are hereinafter referred to as "solve t boil**.

An object of the present invention is to provide a method of curing water-borne paint that enables water- borne paints to be cured quickly.

According to the present invention there is provided a method of curing a water-borne paint that has been applied as a liquid onto a substrate and forms a paint coating on the substrate, the method including the steps of:

(a) heating the coated substrate to and holding it at a temperature that is below the boiling point of water in the paint and evaporating an amount of the water in the paint from the paint so that there is substantially no solvent boil (as described herein) of the paint coating on the substrate^* after a subsequent curing step; and

.(b) heating the substrate in a subsequent curing step to a higher temperature than the evaporation temperature of step (a) and curing the paint.

The applicant has found surprisingly that the above-described 2-stage curing method can produce a substrate, such as a metal coated steel strip, that has a paint coating with minimal solvent boil in a very short time period and that the method is a viable option for use on existing paint lines known to the applicant without adversely affecting production rates and at a reasonable capital cost. The 2-stage curing method is also a viable option as part of a paint line retrofit to metal coating lines that do not include paint lines, and this is a very important application of the present invention. In particular, the 2-stage curing method does not require substantial space for equipment, and this is an important consideration in relation to retrofitting to es∑isting paint lines and metal coating lines.

The term "cure" as used herein is understood to mean cross-linking of thermosetting polymeric material in paint and drying thermoplastic polymeric material.

The term "boiling point of water in the paint" is understood herein to mean the lowest boiling point liquid in the paint. Due to boiling point depression by slight amounts of solvent in paint, the "boiling point" is likely to be that of a solvent/water azeotrope, not of pure water.

In general terms, the method of the present invention can achieve very fast cures of water-borne coatings without significant solvent boil by heating the coating rapidly, for example with induction or infra-red heating, in 2 stages and preferably with a temperature hold zone between the 2 stage .

The purpose of the temperature hold sone, which should be maintained a little below the boiling point of water (ie mostly <100°C at 1 atmosphere pressure) , is to facilitate separation of the processes of evaporation and boiling. By maintaining the thin wet waterborne films just below the boiling point of water, the release of the majority of the water is fast but controlled by the process of evaporation only in this hold section. With most of the water released in this way, by the end of the hold zone the coating can then be ramped quickly through the boiling point of water to the desired peak cure temperature .

Preferably step (a) includes evaporating a substantial amount of the water in the paint.

The term "substantial amount of the water" is understood herein to mean at least 50% by weight of the water in the paint.

Preferably step (a) evaporates at least 60% by weight of the water in the paint.

Preferably step (a) includes holding the temperature at the evaporation temperature for less than 5 seconds .

Preferably ste (a) includes holding the temperature at the evaporation temperature for 1-5 seconds .

Preferably the evaporation temperature is as close to the boiling point of water in the paint (as defined herein) as possible.

Typically, the evaporation temperature is selected to be at least 5°C lower than the boiling point of water in the paint - for line operation reasons to avoid boiling the water in the paint.

More typically, the evaporation temperature is between 5 and 10°C lower than the boiling point of water in the paint.

Preferably step (a) includes heating the coated substrate to the evaporation temperature from a lower starting temperature.

Preferably step (a) includes ramping the temperature up to the evaporation temperature from the starting temperature in less than 2 seconds.

More preferably step (a) includes ramping the temperature up to the evaporation temperature from the starting temperature in 0.5-1.5 seconds.

Preferably step (a) includes supplying moving hot air to facilitate evaporation of water in the paint.

Preferably step (b) includes heating the substrate to the higher temperature in less than 6 seconds . More preferably step (b) includes heating the substrate to the higher temperature in less than 4 seconds .

It is preferred particularly that step (b) includes heating the substrate to the higher temperature in less than 2 seconds.

Preferably step (b) includes heating the substrate from the evaporation temperature to a peak metal temperature of 180-260°C.

More preferably step (b) includes heating the substrate from the evaporation temperature to a peak metal temperature of 190-260°C.

It is preferred particularly that step (b) includes heating the substrate from the evaporation temperature to a peak metal temperature of 210-260°C.

Preferably the paint should include as high a solids loading as possible.

Typically, the paint includes 25-50% solids by volume (polymeric material and pigment) and the balance liquid, predominantly water.

Preferably the method includes heating the substrate in evaporation stage (a) and cure stage (b) for less than 10 seconds.

More preferably the method includes heating the substrate in stages (a) and (b) for less than 8 seconds.

More preferably the method includes heating the substrate in stages (a) and (b) for less than 6 seconds. Preferably the method includes passing the coated substrate continuously through an evaporation oven and carrying out evaporation stage (a) in the evaporation oven and thereafter passing the coated substrate through a separate curing oven and curing the paint in the curing oven.

More preferably the method includes heating the coated substrate to the evaporation temperature in the evaporation oven and allowing evaporation to continue during the period of time that the coated substrate travels from the evaporation oven to the curing oven.

Preferably the spacing between the ovens and the rate of movement of the substrate between the ovens is selected so that there is sufficient time at the evaporation temperature to achieve the required amount of evaporation.

Preferably the substrate is a steel strip that has a coating of zinc or zinc/aluminium alloy on the strip.

According to the present invention there is also provided a method of forming a coating of a paint on a substrate that includes the steps of;

(a) applying a water-borne paint as a liquid onto a substrate and forming a paint coating on the substrate; and

(b) curing the paint in accordance with the method described above and producing a dry paint coating on the substrate.

According to the present invention there is also provided a method of forming a coating of a paint on a substrate that includes the steps of:

(a) forming a coating of a metal on the substrate;

(b) applying a water-borne paint as a liquid onto the metal coated substrate and forming a paint coating on the substrate; and

(c) curing the paint in accordance with the method described above and producing a dry paint coating on the substrate.

Preferably the dry paint coating thickness is less than 25 microns.

More preferably the dry paint coating thickness is less than 20 microns.

More preferably the dry paint coating thickness is less than 15 microns .

It is preferred particularly that the dry paint coating thickness be less than 12 microns.

According to the present invention there is provided a metallic (including steel, aluminium and other non-ferrous metals and alloys) strip, that is suitable for use as a starting material in the production of building cladding sheets and other steel metal products for the building industry having a paint coating of a water borne paint cured by the above-described method.

According to the present invention there is also provided a paint line for forming a paint coating of a predetermined dry paint coating thickness on metal coated strip, the paint line including: (a) a means for applying a water-borne paint as a liquid onto a substrate and forming a paint coating on metal coated strip; and

(b) a means for curing the paint in accordance with the method described above and producing a dry paint coating on the metal coated strip.

The present invention is described further by way of example with reference to the accompanying drawings of which:

Figure 1 is a flow chart illustrating a production line for forming a metal coating and thereafter a water borne-paint coating on steel strip; and

Figure 2 is a temperature/time plot for a preferred embodiment of a 2-stage method of curing painted metal coated steel strip in accordance with the present invention.

The present invention is described below in relation to Figure 1 in the context of an important application of the invention as part of a paint line for forming a paint coating of a predetermined dry paint coating thickness on metal coated steel strip. Whilst this is an important application of the invention, it ^'is noted that it is not the only application.

With reference to Figure 1, steel strip is uncoiled from a coiler 3 and fed continuously through a metal coating section 5, a paint applicator section 7, and a curing section 9 to produce painted metal coated steel strip.

The metal coating section 5 may be of any suitable configuration to form a coating of zinc or aluminium/zinc alloy on the exposed surfaces of the steel strip.

By way of example, the steel strip may be coated by a hot dip coating method that involves passing strip through one or more heat treatment furnaces and thereafter into and through a bath of molten coating metal held in a coating pot. Within the bath the strip passes around one or more sink rolls and is taken upwardly out of the bath. After leaving the coating bath the strip passes through a coating thickness station, such as a gas knife or gas wiping station at which its coated surfaces are subjected to jets of wiping gas to control the thickness of the coating.

The paint applicator section 7 may be of any suitable configuration for applying a water-borne paint in a liquid form onto at least one of the surfaces of the steel strip.

By way of example, the paint applicator 7 may include one or more liquid paint applicators, such as roll coaters or curtain coaters that can form a uniform, preselected thickness, wet coating of paint on the strip.

In a preferred embodiment of the present invention, the curing section 9 includes two spaced apart induction ovens 11, 13 that are capable of heating the painted metal coated steel strip from the paint applicator 7 in accordance with the temperature/time profile shown in Figure 2 to produce a dry paint coating having a preselected thickness.

The Figure 2 profile is a profile that is applicable for dry paint coating thicknesses up to and including 12 microns . Specifically, the as-painted metal coated strip is heated in the upstream oven 11 for a period of time of 0.60 seconds from a starting temperature Ti to an evaporation temperature T₂ that is at least 5°C lower than the boiling point of water in the paint. The strip exiting the evaporation oven 11 travels to the downstream oven 13 in a period of 3.23 seconds and during this period remains substantially at the evaporation temperature T₂. The strip is heated to a peak metal temperature T₃ of 210°C in the downstream curing oven and is held at that temperature to allow curing of the thermosetting polymeric material in the paint. The residence time of the strip in the curing oven is 2.13 second.

The applicant has found that during the heating period in the evaporation oven 11 and during the subsequent "hold" period between the ovens 11, 13 there is sufficient evaporation of water from the paint to at least substantially avoid solvent boil of the paint coating in the curing oven 13.

In overall terms, the above-described temperature/time profile enables the production of high quality painted metal coated steel strip in a surprisingly short overall heating time period.

The present invention is described further with reference to the following Examples .

Laboratory studies were conducted to simulate the impact of the variables that have an influence on the ability to minimise solvent boil.

These studies were conducted using a resistance heater to simulate rapid curing of waterborne coatings on sheet metal panels. Steel based test panels of dimensions 300mm X 125mm X 0.42mm and coated with an AZ150 class ZINCALUME^® metal coating were painted and cured through different temperature-time cycles using a welded thermocouple on each panel to control and monitor the test cure cycles .

Cured films were examined for any solvent boil present using a system originally designed for rating blistering in paints after weathering tests, found in the Australian Standard AS1580.481.1.9 (1991). This standard rates the density and size of blisters as per the tables reproduced in Appendix 1. This table also contains the equivalent but different ratings system found in ASTM D714-87 as a cross reference. From these rating tables coatings with AS ratings of 0 or 1-S1 were taken to be actually or practically free from solvent boil, and coatings given any other rating taken to have different degrees of solvent boil.

Because of the number of variables that can impact on defining regions of solvent boil/no solvent boil the examples given herein are in groupings where some of the impacting variables were fixed as described in the sections below.

Group A Examples

These examples produced 10 +/- 1, and 12 +/- 1 micron dft coatings; and the variables tested were time at hold temperature and other effects; with no hot air assistance. See appendix 2 (Parts -£-, and B) for details.

From Appendix 2 the following effects are exemplified:

Hold Time Effect (longer is better for solvent boil prevention) For Paint B at 12 microns dry film thickness (dft) , going from panels 12 and 13 with a 3.0s hold time and panel 16 with a 3.5s hold time, to panel 10 with a 2.5s hold time, gave the transition from no solvent boil to boil.

Volume Solids Effect (higher is better for solvent boil prevention)

The lower volume solids paint C gave solvent boil under conditions for panel 5, where under the same conditions the other two higher volume solids paints A and

B did not show solvent boil, ie for panels 3 and 4. In a similar way, panel 17 with paint C showed solvent boil whereas paint B did not show solvent boil for panel 16.

Time to Hold Temperature Effect (longer is slightly better for solvent boil prevention)

The greater time to hold temperature showed a small beneficial effect, comparing panel 9 with panel 6, where both were coated with paint A.

Film Thickness Effect (lower thickness is better for solvent boil prevention)

Comparing panels 17 and 18, using paint C with the same cure cycle, panel 18 with approximately 1 micron lower dft showed no solvent boil whereas panel 17 did show solvent boil.

Comparison to Straight Ramp Temperature-time Profile

For equivalent paints and dft's panels 19 and 20 using a 6s straight ramp to 210°C gave significant solvent boil, whereas panels 16 and 18 with the 6s staged curing profile showed no solvent boil. Group B Examples

These examples produced 8-12 micron dft films, and investigated varying moving hot air over strip parameters and some other effects. In this set of results straight ramp cures of coatings were conducted so that improvements could be demonstrated over known significant solvent boil failure regions by applying moving hot air over painted strip. See Appendix 3 ( Parts A and B) for

Qθϋ lXS •

From Appendix 3 the following effects are exemplified:

Moving Hot Air over painted Strip Effect (Hot air assistance reduces disposition towards solvent boil)

With each test panel in this table there was a reduction in the severity of coating solvent boil when moving hot air was applied over the coating being cured, relative to the controls without moving hot air assistance. For example, panels 4-7 gave solvent boil ratings of 3-Sl to 4-S2, well below the control's 5-S2 for panel 3 at the same dft.

Over the range of hot air temperatures and speeds examined the relative importance of hot air speed vs hot air temperature was not discerned.

Film Thickness Effect (Lower film thickness is better for solvent boil prevention/reduction)

In most cases the lower film thickness coatings showed significantly lower solvent boil ratings, eg panel 18 versus 19, and panel 22 versus 23. Group C Examples

These examples produced 12 +/- 1 um dft coatings with fixed cure cycle times; and the variables tested were different hold temperatures and different peak cure temperatures (and therefore ramp rates going from hold temperature to peak cure temperature) ; with no hot air assistance. See Appendix 4 for details.

From Appendix 4 the following effects are exemp1ified;

Hold Temperature Effect (the higher the hold temperature, but below 100°C, the greater the minimisation/prevention of solvent boil)

It can be seen that with a hold temperature of 80°C solvent boil is starting to arise even at the lower ramp rates, whereas it is absent at 90 or 95°C, apart from where the ramp rate was just under 100°C°C/s (96.7°C/s).

Ramp Rate Effect (the slower the transition through the boiling point of water the greater the minimisation/prevention of solvent boil)

If all other cure condition factors are kept constant there is a threshold ramp rate above which solvent boil begins to occur. For the set of cure conditions used in the table and for the 95°C hold temperature the threshold is somewhere between 83.3 and 96.7°C/s.

Group D Examples

These examples produced 15+/- 1 um dft films with fixed cure cycle times; and the variables tested were different hold temperatures and different peak cure temperatures (and therefore ramp rates going from hold temperature to peak cure temperature) ; with no hot air assistance. See Appendix 5 for details.

From .Appendix 5 the following effects are exemplified, which are very similar to those for Group D examples, only being for thicker films:

It can be seen that with hold temperatures of 75 or 80°C solvent boil is starting to arise even at the lower ramp rates, whereas at 90 or 95°C higher ramp rates are required before solvent boil is observed.

If all other cure condition factors are kept constant there is a threshold ramp rate above which solvent boil begins to occur. For the set of cure conditions used in the table and for the 95°C hold temperature the threshold is somewhere between 56.7 and 70° C/s - this is a significantly lower threshold rate than for the same example for 12 micron films given above in the Group C example discussions.

Many modifications may be made to the preferred embodiment of the present invention described above without departing from the spirit and scope of the present invention. Appendix 1 - Rating System used for Degree of Solvent Boil

Notes:

(1 ) The rating system used is taken from the standards below used to assess the size and density of paint blistering from weathered paint films:

. AS1580.481.1.9 (1991 ) - "Coatings - Exposed to Weathering - Degree of Blistering" . The rating system from this standard was the one used in the results tables . ASTM D714-87 (Re-approved 1994) - "Evaluating Degree of Blistering of Paints" . This rating system is given in the tables below as a cross reference for individuals more familiar with the

ASTM ratings

Table 1: Rating of Paint films for DENSITY of Blisters

Appendix 2 - Group A Exam les: Effect of Varvinq Time at Hold Temperature mainly & other Effects

Part A - Summary of Test Conditions

Notes for table:

(1 ) Conditions that were fixed or varied for film preparations:

CO c

CD CO

m

CO

I m m 00

73 i c-

m r σ>

(2) Three paints used here were:

73 O .Paint A = grey colour with Volume Solids of 42.5% .Paint B = grey colour with Volume Solids of 42.5%

C .Paint C = grey colour with Volume Solids of 35%

(3) Solvent Boil ratings in the results table use the scale in Appendix 1

Appendix 2 - Group A Examples: Effect of Varvinq Time at Hold Temperature mainly & other Effects Part B - Results

(1 ) Paint dry film thicknesses were 10 +/- 1 microns or 12 +/- 1 microns. However when slightly greater accuracy was required more data points were taken with a more accurate result recorded in the table

(2) Solvent Boil Blister ratings that were considered failures were shaded grey in the table ie:

XX = appreciable coating solvent boil

I = no coating solvent boil

Appendix 3 - Group B Exam les: Effect of Varvinq Moving Hot Air over Strip Parameters mainly and other effects

Part A - Summary of Test Conditions

Notes for table:

CO c (1 ) Conditions that were fixed or varied for film preparations:

CD CO

m

CO

I m m t o

73 c i- m r σ>

73

O

C (2) One paints used here was:

.Paint B = grey colour with Volume Solids of 42.5%

(3) Solvent Boil ratings in the results table use the scale in Appendix 1

Appendix 3

Par

Panel Strai Number

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26 3.0s from 25 to210degC 215 21.9 8.9

Notes for Table:

Appendix 4 - Group C Examples: Effect of Varying Different Hold

Temperatures & Peak Cure Temperatures/Final Ramp Rates

Notes:

(1 ) This page maps out the film blister results for films of the following 3 paints applied to give 12+/-1 micron dft:

.Paint D = Grey colour with volume solids of 41 %, mainly used in the table of results, and marked (D) .Paint E = Light Blue colour with volume solids of 43%, and marked (E) in table .Paint F = Light Blue colour with volume solids of 50%, and marked (F) in table

(2) The generalised bake cycle used was: 0 - 0.6s, 25 degC - Initial Hold T

0.6 - 3.6s, Initial Hold T - slightly air cooled Hold T

3.6 - 5.10s Slightly air cooled Hold T - PMT (degC)

2s air cooling

10s force air cooling

.with the actual hold temperatures and Peak Metal Temperatures (PMT's) attained given in the table

(3) Solvent Boil ratings in the results table use the scale in Appendix 1

(4) Solvent Boil Blister ratings that were considered failures were shaded grey in the table ie:

= appreciable coating solvent boil = no coating solvent boil

The table below is identical to the one above but instead of showing the solvent boil blister ratings shows the ramp rates in the final cure cycle stage in degCts, as a ready reference:

Appendix 5 - Group D Examples: Effect of Varying Different Hold

Temperatures & Peak Cure Temperatures/Final Ramp Rates

Notes:

(1 ) This page maps out the film blister results for films of the following 3 paints applied to give 15+/-1 micron dft:

.Paint D = Grey colour with volume solids of 41 %,mainly used in the table of results.and marked (D) .Paint E = Light Blue colour with volume-solids of 43%, and marked (E) in table .Paint F = Light Blue colour with volume solids of 50%, and marked (F) in table

(2) The generalised bake cycle used was: 0 - 0.6s, 25 degC - Initial Hold T

0.6 - 3.6s, Initial Hold T - slightly air cooled Hold T

3.6 - 5.10s Slightly air cooled Hold T - PMT (degC)

2s air cooling

10s force air cooling

(3) Solvent Boil ratings in the results table use the scale in Appendix 1

> i. lX, .t! > = appreciable coating solvent boil = no coating solvent boil

The table below is identical to the one above but instead of showing the solvent boil blister ratings shows the ramp rates in the final cure cycle stage in degC/s, as a ready reference:

Claims

CLAIMS :

1. A method of curing a water-borne paint that has been applied as a liquid onto a substrate and forms a paint coating on the substrate, the method including the steps of:

(a) heating the coated substrate to and holding it at a temperature that is below the boiling point of water in the paint and evaporating an amount of the water in the paint from the paint so that there is substantially no solvent boil (as described herein) of the paint coating on the substrate after a subsequent curing step; and

(b) heating the substrate in a subsequent curing step to a higher temperature than the evaporation temperature of step (a) and curing the paint.

2. The method defined in claim 1 wherein step (a) includes evaporating a substantial amount of the water in the paint.

3. The method defined in claim 1 wherein step (a) includes evaporating a at least 50% by weight of the water in the paint.

4. The method defined in claim 1 wherein step (a) includes evaporating at least 60% by weight of the water in the pain .

5. The method defined in any one of the preceding claims wherein step (a) includes holding the temperature at the evaporation temperature for less than 5 seconds.

6. The method defined in any one of the preceding claims wherein step (a) includes holding the temperature at the evaporation temperature for 1-5 seconds.

7. The method defined in any one of the preceding claims wherein the evaporation temperature of step (a) is as close to the boiling point of water in the paint as possible.

8. The method defined in any one of the preceding claims wherein the evaporation temperature of step (a) is selected to be at least 5°C lower than the boiling point of water in the paint.

9. The method defined in any one of the preceding claims wherein the evaporation temperature of step (a) is selected to be between 5 and 10°C lower than the boiling point of water in the paint.

10. The method defined in any one of the preceding claims wherein step (a) includes heating the coated substrate to the evaporation temperature from a lower starting temperature.

11. The method defined in any one of the preceding claims wherein step (a) includes ramping the temperature up to the evaporation temperature from the starting temperature in less than 2 seconds.

12. The method defined in any one of the preceding claims wherein step (a) includes ramping the temperature up to the evaporation temperature from the starting temperature in 0.5-1.5 seconds.

13. The method defined in any one of the preceding claims wherein step (a) includes supplying moving hot air to facilitate evaporation of water in the paint.

14. The method defined in any one of the preceding claims wherein step (b) includes heating the substrate to the higher temperature in less than 6 seconds.

15. The method defined in any one of the preceding claims wherein step (b) includes heating the substrate to the higher temperature in less than 4 seconds.

16. The method defined in any one of the preceding claims wherein step (b) includes heating the substrate to the higher temperature in less than 2 seconds .

17. The method defined in any one of the preceding claims wherein step (b) includes heating the substrate from the evaporation temperature to a peak metal temperature of 180-260°C.

18. The method defined in any one of the preceding claims wherein step (b) includes heating the substrate from the evaporation temperature to a peak metal temperature of 190-260°C.

19. The method defined in any one of the preceding claims wherein the paint includes 25-50% solids by volume (polymeric material and pigment) and the balance liquid, predominantly water.

20. The method defined in any one of the preceding claims includes heating the substrate in the evaporation stage (a) and the cure stage (b) for less than 10 seconds,

21 o The method defined in claim 20 includes heating the substrate in stages (a) and (b) for less than 8 seconds .

22. The method defined in claim 20 includes heating the substrate in stages (a) and (b) for less than 6 seconds .

23. The method defined in any one of the preceding claims includes passing the coated substrate continuously through an evaporation oven and carrying out evaporation stage (a) in the evaporation oven and thereafter passing the coated substrate through a separate curing oven and curing the paint in the curing oven.

24. The method defined in claim 20 includes heating the coated substrate to the evaporation temperature in the evaporation oven and allowing evaporation to continue during the period of time that the coated substrate travels from the evaporation oven to the curing oven.

25. The method defined in claim 20 wherein the spacing between the ovens and the rate of movement of the substrate between the ovens is selected so that there is sufficient time at the evaporation temperature to achieve the required amount of evaporation.

26. The method defined in any one of the preceding claims wherein the substrate is a steel strip that has a coating of zinc or zinc/aluminium alloy on the strip.

27. A method of forming a coating of a paint on a substrate that includes the steps of:

(a) applying a water-borne paint as a liquid onto a substrate and forming a paint coating on the substrate? and

(b) curing the paint in accordance with the method defined in any one of the preceding claims and producing a dry paint coating on the substrate.

28. A method of forming a coating of a paint on a substrate that includes the steps of: (a) forming a coating of a metal on the substrate;

(c) curing the paint in accordance with the method defined in any one of claims 1 to 26 and producing a dry paint coating on the substrate.

29. A metallic strip that is suitable for use as a starting material in the production of building cladding sheets and other steel metal products for the building industry having a paint coating of a water-borne paint cured by the method defined in any one of claims 1 to 26.

30. A paint line for forming a paint coating of a predetermined dry paint coating thickness on metal coated strip, the paint line including:

(a) a means for applying a water-borne paint as a liquid onto a metal coated strip and forming a paint coating on the strip; and

(b) a means for curing the paint in accordance with the method defined in any one of claims 1 to 25 and producing a dry paint coating on the metal coated strip.