Automotive Paints.pdf

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10
Automotive paints
D
A
Ansdell
10.1
Introduction
The demands and performance required for automotive coatings are considerable.
There is a need for ‘body’ protection, such as anti-corrosion and stone-chip resis-
tance, and for a durable and appealing finish. Products also have to be appropriate
to mass-production conditions, and in this respect, must be robust, flexible, and eco-
nomic to use.
Vehicle construction has gone through significant changes since its inception at
the turn
of
the century. A t the present time the substrate is generally of mild steel
but may also contain other alloys and include plastic components; the shape is
inevitably complex and certain parts of the vehicle are almost inaccessible and dif-
ficult to paint. Production rates are high, e.g. 45 units/hour, and this requires process
and material technology to meet the limitations this imposes.
Vehicle production (passenger cars and commercial vehicles) throughout the
world in 1995 was
47.6
million utilizing
of
the order of 900 million litres of paint. In
relation to these figures, vehicle production in Western Europe was
12.6
million.
requiring 254 million litres of paint.
These figures are broken down and represented in Table 10.1 and diagrammati-
cally (refer Figs. 10.1 and
10.2).
The paint products used are principally primers and surfacers (fillers), designated
the undercoating system, and the finish or topcoat. In a modern painting system the
relative use is broadly in percentage terms, primer
:
surfacer
:
topcoat,
30:
20: 50. In
different parts of the world there is often variation in product technology, particu-
larly in topcoats, and this can have a significant influence on paint performance,
specifications and details of the process.
The basic objectives of the painting process are to protect and decorate. In order
to achieve this the process is broken down into a number of different component
parts. These parts, or ‘layers’, are applied in a specific order and although the func-
tion of each ‘layer’ is specific
it
relates very closely to the others to provide the
desired balance of properties.
412
Paint and surface coatings
Table 10.1
-
Western European vehicle production
1995 (millions)
Passenger
cars
United Kingdom
France
Germany
Italy
Spain
Other
1.376
2.836
3.753
1.117
1.506
0.748
11.336
Commercial
vehicles
0.193
0.319
0.237
0.150
0.250
0.138
-
1.287
Total
1.569
3.155
3.990
1.267
1.756
0.886
12.623
The component parts
of
the painting system are:
Metal pretreatment
Primer
Surfacer (filler)
Finish
Protection
Decoration
This fundamental process may be divided into three basic systems which may be
classified as follows:
spray priming system;
dip priming system;
electropriming system (currently most widely used).
The reason for this classification
is
that the method of priming is the major differ-
ence between them; the subsequent process is virtually identical. All these three
systems are used over suitable pretreatments.
Spray application is used for surfacers and topcoats. There are a variety of dif-
ferent types of spraying systems, e.g. air atomized (hand and/or automatic) and elec-
trostatic methods. These are described in more detail later.
10.1.1 Spray priming system
This comprises:
Zinc-rich primer: internal sections only.
Primer surfacer: 40-50 ym.
Finish: 45-55 ym.
The zinc-rich primer is applied
to
the internal surfaces of body components before
welding
of
the body is undertaken.
This
is to give corrosion protection to such sur-
faces which are otherwise inaccessible for painting in following operations.
The primer surfacer provides a measure
of
corrosion protection to the outer skins
of the phosphated body.
This system was used before the advent
of
dipping primers but is now only found
where low volume production does not justify the cost or plant to dip or electro-
prime bodies.
Automotive paints
413
Fig.
10.1
-Vehicle
production
1995.
10.1.2
Dip
priming
system
Anti-corrosive dipping primer:
12-18
pm.
Primer surfacer:
40-50
pm.
Finish:
45-55
pm.
This system was widely used on mass-production lines before the introduction
of electropriming in the early
1960s.
In addition to its normal function the primer
414
Paint and surface coatings
Fig. 10.2
-
Paint
volumes 1995.
surfacer needed to provide a measure of corrosion protection since the underbody
dip primer only covers the lower sections of the unpainted car body.
Now more or less obsolete, this process still finds limited use where low volume
production or other factors such as cost prevail.
10.1.3 Electropriming system
Electrocoat (anodic or cathodic): 18-25 pm.
Surfacer: 35-40
pm.
Anti-chip coating: 50-100pm.
Finish: 45-55 pm.
Electropriming, predominantly cathodic, is currently the accepted standard in mass-
production plants.
It
is an efficient, relatively simple operation with a high degree
Automotive paints
415
of automation. Introduced in the early 1960s with anodic technology, subsequently
replaced by cathodic technology, it has set new standards in processing and corro-
sion protection.
Inclusion
of
an anti-chip coating to protect vulnerable areas has grown consid-
erably over the past ten years. It simply reinforces the surfacer's resistance to stone-
chipping and forms a highly important part of the overall painting system.
In the processes outlined above all coatings need stoving; this is required to
achieve very high levels of performance and to facilitate processing in a convey-
orized production line. Stovings vary from as high as
180"C
for cathodic primers to
as low as 80°C for repair finishes.
A typical modern paint process/plant is shown in Fig. 10.3 in schematic form.
10.1.4 Performance
Individual manufacturers have their own performance specifications and, for
example, while they may vary on the type of finish and local plant/processing con-
ditions, there are basic aspects which are common to all:
e
a
e
e
a
a
e
e
e
m
a
e
e
Appearance: gloss and distinction of image.
Durability: film integrity, colour and gloss retention, and free from blistering and
corrosion.
Mechanical properties and stone-chip resistance.
Substrate: acceptable performance over a variety
of
substrates including
plastics.
Adhesion.
Workability: amenable to modern methods of application.
Ability to meet environmental
requirementshegulations:product
and process.
Corrosion and humidity performance.
Petrol and solvent resistance.
Etch resistance.
General chemical resistance, e.g. acid and alkali resistance.
Hardness and mar resistance.
Repair properties.
The products required to achieve these properties are described later. They require
extremely sophisticated technology and a lengthy process involving various
methods of application and stoving. Costs in energy, labour, space, and capital are
considerable and future processes will have to take other factors into account, such
as anti-pollution, while still continuing to improve performance standards. For
example, new developments have
to
back up improved warranties from car pro-
ducers such as a six year anti-corrosion guarantee and, with certain manufacturers.
up to three year's overall coating performance.
10.2
Pretreatment
The pretreatment process has three purposes:
To
remove the mill and pressing oils ingrained in the steel and any other tem-
porary protective coatings.

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