FIELD OF THE INVENTION
The present invention relates to a method of curing a water-borne paint coating.
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 "water-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, e.g. 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.
BACKGROUND AND PRIOR ART
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 coater. 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"
STATEMENT OF INVENTION
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.
OBJECTIVES OF THE INVENTION
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
DETAILED DESCRIPTION
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 existing 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 stages.
The purpose of the temperature hold zone, 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 step (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 AS 1580.481.1.9 (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 A 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 boll
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 dtt 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
details.
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-S1 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 exemplified:
Hold Temperature Effect (the higher the hold temperature, but below 100eC, 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:
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 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.
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 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
(Table Removed)
Appendix 2 - Group A Examples: Effect of Varying Time at Hold Temperature mainiv & other Effects
Part A - Summary of Test Conditions
(Table Removed)
Appendix 2 - Group A Examples: Effect of Varying Time at Hold Temperature mainly & other Effects
Part B - Results
(Table Removed)

We Claim:
1. A method of curing a water-borne paint applied on a metal strip and forms coated
substrate, the method comprising the steps of:
(a) heating the coated substrate to a temperature at least 5°C below the boiling point of water in the paint and holding at the said temperature for less than 5 seconds (as herein described) and evaporating at least 50% by weight of the water in the paint; and
(b) heating the substrate in a subsequent curing step to a peak metal temperature of 180-260°C for the time as herein described and curing the paint.

2. The method as claimed in claim 1, wherein at least 60% by weight of the water in the paint is evaporated.
3. The method as claimed in any one of the preceding claims wherein the temperature in step (a) is held for 1-5 seconds.
4. The method as claimed in any one of the preceding claims wherein the temperature of step (a) is between 5 and 10°C lower than the boiling point of water in the paint.
5. The method as claimed in any one of the preceding claims wherein the coated substrate is heated to the temperature of step (a) in less than 2 seconds.
6. The method as claimed in any one of the preceding claims wherein the coated substrate is heated to the temperature of step (a) in 0.5-1.5 seconds.
7. The method as claimed in any one of the preceding claims wherein moving hot air in step (a) is supplied to facilitate evaporation of water in the paint.
8. The method as claimed in any one of the preceding claims wherein the substrate is heated to the higher temperature of step (b) in less than 6 seconds.
9. The method as claimed in any one of the preceding claims wherein the substrate is heated to the higher temperature of step (b) in less than 4 seconds.

10. The method as claimed in any one of the preceding claims wherein the substrate is heated to the higher temperature of step (b) in less than 2 seconds.
11. The method as claimed in any one of the preceding claims wherein the substrate of step (b) is heated to a peak metal temperature of 190-260°C.
12. The method as claimed in any one of the preceding claims wherein the paint is 25-50% solids by volume as herein described and the balance liquid as herein described.
13. The method as claimed in claim 1, wherein the substrate in stages (a) and (b)is heated for less 10 seconds.
14. The method as claimed in claim 13, wherein the substrate is heated in stages (a) and (b) for less than 8 seconds.

14. The method as claimed in claim 13 wherein the substrate is heated in stages (a) and (b) for less than 6 seconds.
15. The method as claimed in any one of the preceding claims wherein the coated substrate is passed continuously through an evaporation oven and evaporation stage (a) is carried out in the evaporation oven and thereafter the coated substrate is passed through a separate curing oven and the paint is cured in the curing oven.
16. A method of curing a water-borne paint applied on a metal strip and forms coated substrate, substantially as herein described with reference to the foregoing examples and accompanying drawings.