Khanna A.S. (Ed.) High-Performance Organic Coatings: Selection, application and evaluation
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Organic coatings for marine and shipping applications 367
16.6 Conclusions and future trends
This chapter describes the mechanisms of corrosion and fouling protection
as well as failure mechanisms of coating systems in marine and shipping
applications. The ideal status is a truly durably protected structure, while
costs are minimized over the lifetime. In order to achieve this situation
proper design of protected structures is needed, while maintenance and
eventual repair remain important.
Environmental issues nowadays rightfully require new approaches in
coating technology, but they also make market introduction of new coatings
cumbersome. Assessment of the performance of coatings under develop-
especially important for the development of new coatings. Two approaches
can be followed in principle: acceleration of the degradation process by
enhancing the aggressiveness of the medium, with a risk of changing the
degradation mechanism leading to false conclusions, or enhancing the sen-
sitivity of the test method under normal conditions, ensuring the mecha-
nism remains unchanged. The second approach is the better one, but
requires much R&D to cover the various degradation mechanisms. The
16.9 Instrument for drag resistance measurements.
ment and in practical conditions is vital, but sometimes extremely difficult.
Accelerated testing remains a field with a huge potential for improvement,
© 2008, Woodhead Publishing Limited
368 High-performance organic coatings
authors expect that the development of sensitive testing methods will be
the major tool for design of new coating applications together with funda-
mental studies on the adhesion of coatings to metallic substrates at the
molecular level.
16.7 References
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TU Delft, 1995
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Organic coatings for marine and shipping applications 369
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(1993) 1923
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Coatings 39 (2000) 67
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Stability 72 (2001) 115
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and Stability 75 (2002) 193
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53. J. Maxted, J. Corros. Sci. Eng. 2 (1999) paper 15
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45 (2002) 405
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corrosion protection, Jotun Technical paper (2004) 1–12
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the stability of coatings, in: European Federation of Corrosion Publications,
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Atlanta, GA
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Coatings Europe, July 1996
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Boca Raton, FL
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G.M. Ferrari, A. Iversen, F. Zou, D. Thierry, N. Dowling, S. Morel, C. Houpert,
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372
17
Organic coil coating of metal
M DE SOUZA, University of Campinas, Brazil
17.1 Introduction
Coil coating is a metal anticorrosive protection technology developed to
promote the use of steel and aluminium in the most varied environmental
conditions such as roof and wall elements in the building industry, casings
for household appliances and internal parts or bodywork structures for the
automotive industry.
The process was started in the middle of the twentieth century in North
America by Joseph Hunter who designed his line to assist local manufac-
turers of blinds (Drufke, 2006). With this process high performance liquid
coils, like pre-painting, in a range of colours, cured in seconds and recoiled.
The main metal substrates used in the coil coating process are galvanised
steel, cold-rolled steel, electrogalvanised steel and aluminium. In addition,
coil coated metal can be formed without any loss in surface quality or
appearance. Usually, coil coated products have been most dominant in the
building market. However, the use of this technology has increased and coil
coated products also appear in our homes and workplace as casings for
domestic appliances such as microwaves, refrigerators and washing
machines. Characteristics such as the reproducible manufacturing process,
lower cost and reduced environmental impact have contributed to increase
use of the coil coating process (Seija, 2007).
process will be explained, together with the procedures involved in it and
some information about the development of coil coating. The different
applications and end uses of coil coatings will be shown. Lastly, coil coating
tendencies and progress will be explored in order to exemplify and rein-
force the importance of the coil coating process nowadays.
In this chapter the coil coating technology will be explored. At first the
coatings could be applied to flat metal sheets or strips packaged in rolls or
© 2008, Woodhead Publishing Limited
Organic coil coating of metal 373
17.2 Understanding the coil coating process
Metal coil surfacing, known as coil coating, is a continuous process by which
or coils. The metal substrate is delivered in coil form from the rolling mills.
The metal coil is placed at the beginning of the line, and then unrolled at
a constant speed, passing through various pre-treatment and coating proc-
esses before being recoiled. Figure 17.1 presents a diagram to give a better
understanding of the process. In some installations, the wet section contains
an electrogalvanising operation. Most coil coating lines have accumulators
at the entry and exit that permit continuous metal strip movement through
the coating process while a new coating is mounted at the entry and/or a
full coil removed at the exit. Some important aspects that could be observed
in a coil line are that production gives minimal pollution with little waste
product, productivity is improved with reduced processing time, wastage of
raw materials is decreased, and bottlenecks are substantially reduced or
a week, and are more than capable of coating over 4.5 × 10
6
kg of steel or
2.2 × 10
6
kg of aluminium per week. In addition, the process complies with
the most demanding conditions of pollution control regulations (Drufke,
2006).
Coil coating in general is dependent on several factors, such as the nature
of the substrate, the properties of the coating materials, the speed of appli-
cation, the roll pressure and the temperature and humidity in the applica-
tion area. A fundamental principle of coil coated material is that the
material travels through the various process sections in a continuous manner
without ever stopping or changing speed. As a result, consistent properties
are obtained in the coated material (Drufke, 2005b).
One of the advantages of the coil coating process is the cleaning and
position, the cleaning and treating are done thoroughly. This is a unique
Uncoiling and splicing
Accumulator
Cleaning and pre-treatment
Primer unit and
cure oven
Laminating and
recoiling
Coating unit
and oven
Accumulator
totally eliminated. Coil lines typically run 24 hours a day, five to seven days
protective or decorative organic coatings are applied to flat metals in rolls
treating section. Because the metal is in a flat form and in a suspended
17.1 Coil coating flow diagram.
© 2008, Woodhead Publishing Limited
374 High-performance organic coatings
quality of the coil coating operation and is a central reason why coil coated
products perform in such a superior manner (Froehlich, 1998).
from one installation to another, the operations generally follow a set model
of common steps or processes: uncoiling the bare metal, cleaning, treating,
painting, curing, cooling and rewinding the same coil for shipment or further
process. Some of these steps will be explained in detail in sequence.
17.2.1 Cleaning and pre-treatment
sequent organic coating. The metal moves through a sequence of tanks that
serve to both clean and treat the surface by chemical conversion. Generally
a freshwater rinse tank follows each cleaning tank, which is usually equipped
with a high pressure pump feeding a series of spray bars that clean both
sides of the metal. Variable solutions are used in these tanks: alkaline and
acid solutions, electrolytic solutions, sometimes steam and mechanical
brushing rolls within the tanks contribute to the cleaning. The chemical
treatments of the metal surface start after cleaning. Chemical treatments
are useful to inhibit rust and to promote coating adhesion to the metal
surface. One of several different chemical formulas is used in this step, each
being specially designed for the particular metal substrate, application tech-
nique and planned end use (Froehlich, 1998). Nevertheless some of these
chemical conversion coatings use hexavalent chromium, which is strictly
regulated due to the toxicity of its compounds and suspected carcinogenic-
ity. Hence coil coaters, chemical suppliers and researchers are working on
eliminating chromium from their process and waste streams (Duarte et al.,
2005; Drufke, 2005b). In this respect, many different new treatments have
been proposed, for example pigments based on phosphates, zinc com-
2000).
17.2.2 Coating application
There are several general classes of coil coating lines; the choice of system
depends on a combination of life expectancy and commercial considera-
of the coil line, types of coatings being used and types of metal being coated.
In almost every case, however, the coater is equipped with a vessel, a steel
or ceramic pickup roll and a rubber-covered coating roll. The coating is
circulated in the paint vessel. A rotating pickup roll is partially immersed
in the coating and rotates in a counter-clockwise direction. The coating is
Although the physical configurations of coil coating lines are different
These procedures are the first on any coil coating line. The purpose of this
section is to create an oil-free surface that enables better fixation of a sub-
tions. There are a number of coater designs depending on the configuration
pounds or barrier coatings without inhibitive pigments ( Deflorian et al.,
© 2008, Woodhead Publishing Limited
Organic coil coating of metal 375
then transferred onto the coating roll which is rotating in the reverse direc-
tion to the roll coating, from where it is applied to the substrate. By adjust-
thickness can be controlled (Graziano, 2002). To promote coating curing,
the coated strip is heated in an oven at a temperature in the range 300–
500°C. The dwell time of the coil coating is from 15 to 60 seconds and the
upper limit for the metal temperature is 250°C, known as the Pick Metal
Temperature (TMP), above which organic material might oxidise (Korhonen
et al., 2003).
A two-coat paint system is normally used. Lines for this are called
tandem lines and can apply a primer and a top coat in a single pass (Drufke,
2005b). The primer is usually applied directly onto the pre-treated metal
surface and will probably contain anti-corrosive pigments to protect the
metal substrate from corrosion and also to prevent the undercutting of
paint, safeguarding the durability of the product and overall quality.
system that maximises the performance of the product for special end-use
applications. Primers are used on galvanised and cold-rolled steel for exte-
ium. Epoxy esters are mainly used for more demanding applications
system’s weathering and mechanical performance. Figure 17.2 illustrates
35 microns for a primer and 15 to 200 microns for a topcoat, and is deter-
usage.
17.2.3 Curing
The curing section of coil coating lines is an important step during the
process. Although the ovens may look the same from the outside, there are
important differences that have a substantial effect on the actual process
of removing solvents and curing the coating. The industry usually utilises a
Organic topcoat
Primer
Pre-treatment
Metallic coating
Metallic substrate
17.2 Coil coated metal cross-sectioned.
ing the pressure between the transfer roll and the coating roll, the film
Various primers can be selected to help create an industry- specific coating
rior exposure or when water-borne finishes are applied to steel or alumin-
(Paul, 1996). The second coat of finishing paint provides the colour
and appearance of the final coated system, and plays a large part in the
a coil coated metal. The dry film thickness of the coatings is usually 5 to
mined by the quality of the paint chosen and the pre-coated metal’s final
© 2008, Woodhead Publishing Limited
376 High-performance organic coatings
natural gas convection oven with control of solvent concentration in order
ate incineration technology to ensure that solvent emissions are destroyed
before being emitted to the atmosphere (Froehlich, 1998). Some coatings
ovens become necessary; these kinds of ovens have increased line speeds
because of better energy transfers. In the case of water-based coatings,
from reduced maintenance costs.
The degree of cure of the system is of fundamental importance, since it
trast, degradation of the coating can also occur for overcured systems,
which leads to problems of brittleness (Perruchot et al., 2003).
17.2.4 Quality control
The pre-painting process is designed to produce highly uniform and perfect
quality coatings. The coatings are precisely applied with special equipment
dures such as cutting, punching, parting, perforating, bending and sketch
to be executed with the organic coating already applied to the metal surface.
To avoid damaging the paint, the tools used need to be in very good condi-
tion and extreme care must be taken during the processing work (Seija,
the coatings are strongly bonded to the metal and frequently are applied
to both sides of the metal and could be available in various degrees of gloss,
and end-user.
In order to ensure the best results, numerous quality control tests are
performed during the coil coating process. Usually, samples are removed
at various stages of manufacture, and the factory mixing sheets are usually
paint (Morgans, 1990). First of all, before coating, the physical performance
of the paints and the metal substrates is tested. Properties like colour, vis-
cosity and gloss are investigated. The uniformity of viscosity is essential for
modern production where continuity of supply is involved. During the
entire coating process the surface treatment is tested and adjusted; this
curing temperature and production line speed are controlled and measured
(Seija, 2007).
to improved energy efficiency. The ovens are also equipped with appropri-
electrical and gas- fired IR ovens could be used, once these ovens benefit
Undercured systems lead to soft, weak films with poor durability. In con-
that provides a high-quality finish, which permits various processing proce-
2007). Manufacturers using coil coating metals will find consistent colour,
texture, thickness and performance in their finished products. Moreover,
must conform to exacting specifications agreed to by the supplier, coater
designed to show the results on intermediate and final tests on a batch of
procedure ensures a high-quality finish. Likewise, the paint thickness,
require a very high curing temperature and high impingement flotation
influences the mechanical and adhesive properties of the resulting coating.
colour, texture, flexibility and combinations of them. In addition, products
© 2008, Woodhead Publishing Limited