Note: Descriptions are shown in the official language in which they were submitted.
` ` 1059~66
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COATING PROC~SS
l~lis invention relates to a coating process for hollow
metal articles that is particularly useful in the coating of
vehicle bodies.
It has heretofore been proposed to coat hollow metal
articles with an electrically insulating coating on the outer
sùrfaces of the article and then with an electrophoretic
coating on the inner surfaces; the advantages of such a coating
method being that the insulating coating ensures a cosmetically
acceptable coating having the necessary corrosion protection on
the visible outer surfaces and limits the consumption of
electrophoretic coating material, whilst the electrophoretic
coating provides the normally necessary corrosion protection
within internal sections that are otherwise difficult to coat.
Whilst such a method of coating has advantages with
regard to the coated article, the process has disadvantages
in production because the need to apply two separate coatings
necessitates the use of additional space-consuming plant to
separately apply and stove each of the coatings.
Moreover the need for separate stoving stations inherently
means that considerable energy is eonsumed in the several stoving
operations, and this disadvantage is accentuated by the pre~sent
general need to provide plant for treating the fumes generated
during each stoving operation.
Thus the plant necessary for the production application
of such a process would be both space consuming and expensive
in installation and energy consuming in operation.
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According to one aspect of this invention a process for
coating a hollow metal article includes
a) preparation of the surface of the article to be
coated;
b) spraying a water-borne primer/surfacer onto
preselected areas of the outer surface of the
article;
c) drying the article without curing, or without fully
curing, the primer/surfacer coating material
d) electrophoretically coating the remaining surfaces
of the article with a primer;
e) baking the article such as to at least partially
cure both the electrophoretically deposited primer
coating and the primer/surfacer coating;
f) applying atop coat on at least those surfaces of
the article that will be visually apparent; and
g) stoving the article such as to cure the top coat
and fully cure any previously non-fully cured
coating.
Thus the process of invention provides the advantage
that as the primer/surfacer only requires partial curing before
the electrophoretic coating step, the drying may be effected at
a relatively low temperature, with a consequential potential
saving in energy consumption as compared with a full bake, and
the fumes generated at this stage do not normally require
incineration or other treatment thereby providing a further
potential saving in energy consumption.
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Prel`erabl)r, tllerefore~ the process includes baking the
article to part cure the primer/surfacer coating at a low
temperature such as to avoid a need for incineration of the
fumes generated during baking.
Furthermore it has been found that certain primer/surfacers
can be applied satisfac-torily to a surface that is not completely
dry. Hence~ according to another aspect of the invention a
process for coating articles includes the steps of
a) surface preparation terminating in a water rinse,
b) applying a primer/surfacer onto the sur~ace of the
article in a "damp dry" state.
In the art and in this specification the expression
"damp dr~" means that the surface is free of surface water
without being in a completely dry state such as is achieved
by baking. Such a damp dry state may be produced after a
water rinse by high pressure air blowing of the surface or by a
low pressure warm air blow.
Thus, the conventional heated bake step which usually
preceeds the application of paint following a water rinse is
eliminated with a consequential potential saving in tho space
requirements and the cost of produotion plant 7 and a consequentia]
potential saving in energy conswllption.
It should be appreciated that the above aspect of the
invention may be combined such as to provide a process for
coating hollow metal articles which includes
a) surface preparation te~minating with a water rinse;
b) spraying a water borne primer/surfacer onto preselected
outer surfaces of the article in a damp dry state by
spray means;
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c) drying the article without curing, or without fully
curing, the primer/surfacer coating material;
d) electrophoretically coating the remaining surfaces
of the article with a primer;
e) baking the article such as to at least partially
cure both -the electrophoretically deposited primer
coating and the primer/surfacer coating;
f) applying a top coat on at least those surfaces of ~;
the article that will be visually apparent 9 and
g) stoving the article such as to cure the top coat
and fully cure any previously non-fully cured coating.
In the art it has, in the past, been found impracticable
to apply a primer/surfacer to an article of complicated shape,
such as a vehicle body, by fully automatic means that include
spray guns that have to track the surface of the article.
For example in the case of vehicle bodies it has been found t~
that when automatic spray guns are used it is still necessary
for certain areas of the automatically sprayed coating to be
"reinforced" by further manual controlled spraying.
However, this invention can eliminate such a need for
manual applied reinforcemont and allow the fully automatic
application of primer/surfacer. This is because, by using a
primer/surfacer that is compatible with the electrophoretically
deposited primer coating in terms of the required appearance
and paint performance of the finished article, the
electrophoretically deposited primer coating will serve to
"reinforce" areas -that were inadequately covered with
automatically sprayed primer/surfacer.
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Moreover, if malfunctions in the automatic spraying
installation should cause defects, such as for example "runs"
and "sags" in the sprayed primer/sur*acer coating~ the defects
can normally be removed by wiping witll an appropriate solvent
and suitable wiper after partial curing of the sprayed
primer/surfacer because even areas that are overwiped such
a~ to leave bare substrate or only a very thin coating
of primer/surfacer will subsequently be coated with
electrophoretically applied primer.
Furthermore~ in the prior art when automatic spraying
of surfacer has followed electrophoretic priming~ it has been
found that a subsequent top coat may not adhere satisfactorily
in the marginal re~ions of the area covered by the sprayed
surfacer because, in these regions, the surfacer tends to
remain as discrete droplets.- Consequently it has in some such
cases been necessary for the droplets of surfacer to be rubbed
off in these marginal regions to ensure satis~aclory
adhesion and appearance of a subsequent top coat. In the
present invention it has been found that the electrophoretic
primer coats around~ and in some cases over, the droplets
of sprayed primer/surfacer and the top coat normally
adheres satisfactorily without any need ~or the above described
manual preparation.
The initial surface preparation may include the normal
cleaning and chemical conversion coating, such as a zinc or
iron phosphate in the case of ferrous substrates, and preferably,
particularly in the case of vehicle bodies, includes a spray-
dip-spray sequence in which each zone of the chemical processing
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plant contains botll a spray and a dip stage which rnay for
convenience be accommodated in the same part of a process
tunnel. In the case of ~ehicle boclias, the dip level should
usually be arranged to ensure that all maior structural box
members of the vehicle body that are susceptible to corrosion
are immersed thus giving a more controlled chemical conditioning
of the box members than can be accommodated by spray only.
Secondary sprays should also usually be arranged at the exit
of eacll 20ne to ensure removal of any residues and scum
which may be left on the external surfaces as the vehicle
eaves the dip tank.
In the prior art the surface preparation of mixed-metal
vehicle bodies before coating by electrodeposition has been
critical and special procedures or phosphating solutions have
been necessary in order to ensure a consistent finished
appearance. For example such bodies may typically have a
main structure which is formed from steel and have non-stressed
parts, such as doors and boot lids, which are formed from
aluminium. In the prior art such aluminium parts have been
suitably prepared before the parts were fixed to the main body
and the subsequent surface perparatiorl solutions have been
chosen to be effective on the steel parts but ineffective
on the alwninium parts, alternatively special phosphates have
been used which effectively pretreat both the steel and aluminium
parts.
In practising this invention, the visually significant
parts of such mixed-metal bodies are coated with sprayed
primer/surfacer before electrophoretic painting, and thus the
need for different pretreatment for the different metals or
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a subsequent top coat may not adhere satisfactorily in the
marginal regions of the area covered by the sprayed surfacer
because, in these regions, the surfacer tends to remain as
discrete droplets. Consequently it has in some such cases been
necessary for the droplets of surfacer to be rubbed off in these
marginal regions to ensure satisfactory adhesion and appearance
of a subsequent top coat. In the present invention it has been
found that the electrophoretic primer coats around, and in some
cases over, the droplets of sprayed primer/surfacer and the top
coat normally adheres satisfactorily without any need for the
above described manual preparation.
The initial surface preparation may include the normal
cleaning and chemical conversion coating, such as a zinc or iron
phosphate in the case of ferrous substrates, and preferably, par-
ticularly in the case of vehicle bodies, includes a spray-dip-
spray sequence in which each zone of the chemical processing plant
contains both a spray and a dip stage which may for convenience be
accommodated in the same part of a process tunnel. In the case of
vehicle bodies, the dip level should usually be arranged to ensure
that all major structural box members of the vehicle body that are
susceptible to corrosion are immersed thus giving a more controlled
chemical conditioning of the box members than can be accommodated by
spray only. Secondary sprays should also usually be arranged at the
exit of each zone to ensure removal of any residues and scum which may
be left on the external surfaces as the vehicle leaves the dip tank.
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to ensure any subsequent solvent wipe operation will not
result in tacky sealer being spread over the surface of the
~` article.
The final top coating may comprise a conventional liquid
paint or a powd3r coating which is conveniently applied
electrostatically in kno~n manner.
The final stoving may comprise a conventional stoving
operation at a tempera-ture and for a time suitable for the
top coat that has been ussd.
In the case of vehicle bodies the electrophoretic stoving
plant should normally be provided with full incineration
~ facilities to reduce the fumes caused by stoving degradation
s products given off from the primer/surfacer and the
electrophorctic primer during any flash-off. Usually
the heat generated during incineration may be recovered and
used, for example, in the pretreatment process, or the
primer/surfacer drying zone or the stoving of the top coat.
Thu~s the process provides yet another potential saving in
~j energy consumption.
The invention will now be illustrated by the following
; examples.
EXAMPLE 1
A mild steel door for an automotive vehicle was cleaned
and pretreated with a proprietory zinc phosphate solution,
rinsed with demineralised water and had excess water removed
by air jets. A water-borne epoxy primer/surfacer was sprayed
onto the outer panel of the door at a film thickness of
approximately 37-40 micrometers.
After 10 mimltes flash-off at ambient temperatures and
drying at 110 C for 10 minutes such that the pr~mer/surfacer
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was not fuliy cured, the door was to-tally lmmersed in an
epoxy electroprimer and electroprimed. Over the whole of' the
primer/sur~acer-electroprimer inter~ace there was a film
thickness o~ at least 25 micrometers. The primed cloor
was then stoved for 20 minutes a-t 160C (eff`ective metal
temperature) such that -the sprayed primer/surfacer and the
electroprimer is f`ully cured. Then the door was sprayed
with a pigrnented alkyd/melamine automotive enamel to give
a total ~ilm thlckness o~ 55-70 micrometers and stoved for
20 minutes at 150 C (ef~f~ective metal temperature). The door
was considered to have an acceptable visual appearance and
an acceptable performance as assessed by conventional
accelerated testing methods.
EXAMPLE 2
The coating process of` Example 1 was repeated using an
aluminium door that was pretreated in a proprietory oxychromate
bath. Once again the coated door was acceptable.
EXAMPLE 3
A mild steel door was coated as in Example 1 except
that -the top coat was a pigmented acry]ic powder that was
stoved f`or 15 minutes at 180 C (ef~f'octive metal temperature).
Again the coated door was acceptable.
