Enclosure for painting and a method of enforcing evaporation from a coating on a panel surface

A coating of paint on a panel is dried by directing a narrow jet of air from a supply, preferably from an air mover, held at a predetermined distance from the panel towards one edge region of the panel, the jet being substantially narrower, when it reaches the panel edge region, than the length of the panel edge and the jet being inclined to the plane of the panel such that the air from the jet is entrained by the panel in a spreading laminar flow across the panel surface from that edge region to all the other edges thereof. This induces such laminar flow over substantially the whole surface and replaces vapor-laden air closely adjacent the surface with fresh air to accelerate drying. In a painting booth for cars several such air movers use part of the bulk air flow in the booth.

FIELD OF THE INVENTION 
This invention relates to the accelerated evaporation of water or other 
solvent from a coating on the surface of a panel, and is particularly 
useful for accelerating the drying of intermediate and final coats of 
water borne coatings for example during the re-painting of road vehicles. 
It also concerns a booth or other enclosure for the painting or 
re-painting or coating of motor vehicles and the like. 
DESCRIPTION OF THE PRIOR ART 
Before the advent of water-borne vehicle paints in the 1970's, all paint 
for vehicles was solvent-based, and was applied as a primer, then a base 
coat and then a top coat. The solvent generally evaporated rapidly between 
coats without the need for excess temperature. 
Paints conventionally used in decorating motor vehicles are solvent-borne 
and are formulated to be applied by spraying. A spray paint is designed to 
have low viscosity at its point of atomisation, so that it atomises easily 
and to have high viscosity at the target, for example the vehicle body or 
body panel to prevent sagging. In solvent-borne paints this viscosity 
change is achieved by evaporation of solvent while the paint spray is in 
flight between the spray gun and the target. 
When water-borne paints were first introduced into the motor industry in 
the early 1970's, they were designed to function on spraying in the same 
way as their solvent based counterparts, that is to change viscosity in 
flight through solvent (in this case water) evaporation between the gun 
and the target. However, as compared with the organic liquids employed as 
carrier vehicles in solvent-borne paints, water has certain unique 
properties. First, unlike organic solvents it is present in the atmosphere 
and variations in its partial pressure (that is its relative ambient 
humidity) alter from day to day the rate at which it will evaporate. 
Second, its latent heat of vaporisation is high and therefore more energy 
is required per unit mass to evaporate water as compared with organic 
solvent. In consequence, these first introduced water-borne paints had to 
be sprayed in carefully controlled air-conditioned environments. They were 
never really technically satisfactory and this led to them having to be 
withdrawn. The first truly effective water-borne painting system for motor 
vehicles is that described in EP-B-38127 and comprises a water-borne base 
coat-clear coat system. 
Base coat clear coat systems were again introduced into the motor industry 
in the early 1970's in order to improve the appearance of the top coat or 
outer-most coat on the finished vehicle, especially for metallic effect 
paints. The top coat is responsible for the gloss and colour of the 
vehicle as well as for protecting the vehicle against weathering, 
scratches, stone chipping and related damage to its surface. In a 
conventional one-coat top coat the top coat paint has to provide all these 
features. A base coat-clear coat system consists of two different paints. 
The base coat, which is applied first is highly pigmented and provides the 
colour and appearance (especially the metallic effect) only, whereas the 
gloss and stability to weathering abrasion and stone chipping comes from 
the clear coat. 
EP-B-38127 referred to above relies on a water-borne base coat and it 
overcomes the problem of the viscosity change required in a spray paint in 
a revolutionary way. The paints are formulated so as to be thixotropic or 
pseudoplastic and so relatively little or no evaporation of water is 
required in flight to ensure the high quality spray performance called for 
in car painting. 
The consequence of this is that the paint film can sometimes contain 
relatively large levels of water. When the painting step is taking place 
during vehicle production, this presents little or no difficulty. The base 
coat resin system is sufficiently robust to allow wet-on-wet application 
of clear coat, that is the clear coat can be applied over the base coat 
after the base coat has been given very little time to dry. The whole of 
the top coat film is subsequently baked at a high-temperature which drives 
off any water and cures the film. 
In motor vehicle re-spray, the position is a little different. A re-sprayed 
vehicle cannot be subjected to baking at the temperatures used on a 
vehicle production line. Damage would be caused to temperature sensitive 
and meltable components. Hence it is desirable to be able to remove rather 
more water from the base coat. 
Many techniques have been devised for drying and baking motor vehicles 
painted with solvent-borne paint. Superficially many of these techniques 
might seem to be directly applicable to the drying of water-borne paints 
after mere routine modification. However, such is the difference in 
behaviour as between water-borne paints and solvent-borne paints that the 
outcome of apparently minor modifications on the behaviour of a 
water-borne system is often not at all clear. With solvent-based paints, 
the problem of removing solvent from painted vehicles has been addressed 
primarily by proposing a substantial bulk air flow through the booth 
containing the vehicle. For example in U.S. Pat. No. 1,606,442 (1926), a 
solvent-based coating is dried in an air-warmed and specially humidified 
booth. The coating is then hardened by cooling in a bulk air-flow. 
Blowing air at water-based coatings tends to cause the formation of a skin 
on the outer surface which then severely limits proper loss of water from 
within the film. This has adverse consequences on the appearance of film, 
since shrinkage of the film can be uneven and flake control in metallic or 
mica flake containing films deteriorates. 
A further disadvantage of air-blowing systems has been the disturbance of 
dust from adjacent surfaces, which contaminates the coating. 
It is of course known, e.g. from FR-A-2029314, to heat a car chassis to a 
high temperature such as 200.degree. C. during the manufacturing process, 
in a hot-air blown kiln, to cure a base coating, and indeed infra-red 
radiative heating has been proposed for accelerating secondary coatings 
preparatory to a top coating. Heating in this way is not only expensive 
for a motor vehicle re-spray process but also of course, impractical when 
considering drying an assembled vehicle. There is therefore a demand for a 
method of accelerating the drying of such a coating, or indeed of any 
other coating on a panel, which is energy efficient and which reduces the 
"flash off" time to acceptable levels, without increasing the risk of dust 
contamination inherent with the application of non-aqueous solvent-based 
coatings. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention provides a method of forcing evaporation 
of a solvent such as water from a coating on a predefined surface of a 
panel by directing a jet of air from an air supply held at a predetermined 
distance from the panel towards one edge region of the panel, the jet 
being substantially narrower, when it reaches the panel edge region, than 
the length of the panel edge and the jet being inclined to the plane of 
the panel such that the air from the jet is entrained by the panel in a 
spreading, predominantly laminar flow across the panel surface over that 
edge region and from that edge region to all the other edges thereof, 
thereby inducing such laminar flow over substantially the whole surface 
and replacing vapor-laden air closely adjacent the surface with fresh air 
to accelerate drying. The use of an essentially local air supply allows 
the position and direction of the air jet to be controlled so as to 
optimise the drying effect of the air, and so as to avoid disturbing any 
dust which may be present on adjacent surfaces. While the flow velocity of 
the air jet may be 1 to 2 ms.sup.-1 as it reaches and travels along the 
panel surface, there is no need to increase the usual flow rate of drying 
air which may be moving in bulk elsewhere, e.g. from ceiling to floor in a 
booth. This also avoids dust disturbance. 
We have found that this method is particularly energy-efficient, and that 
it is surprisingly effective in drying panels such as vehicle doors and 
bonnets. 
The invention could also be beneficial in forced evaporation from thick 
films such as the thick water-borne primer coatings already mentioned, 
provided that the trapping of water or other solvent can be overcome. 
Acceleration of evaporation can be further improved, in situations where 
the minimising of energy consumption is not so critical, by the 
application of thermal energy, either by pre-heating the air which is to 
form the jet of air, or by using radiative heat sources such as IR panels 
directed at the surface of the panel to be dried. 
The invention also provides a booth or other enclosure for the painting or 
re-painting of panelled articles such as motor vehicles, having an air 
inlet and an air outlet for the bulk movement of drying air over a painted 
article standing in the booth; and characterised by at least one supplier 
of air at a flow velocity substantially greater than that of the bulk 
movement, means for holding the supplier a predetermined position and 
orientation, in use, in relation to a panel of the painted article which 
is to be dried, such as to direct a jet of drying air towards one edge 
region of the panel, the air supplier being so shaped, and the flow 
velocity being such, that the jet is substantially narrower, when it 
reaches the panel edge region, than the length of the panel edge, and the 
air supplier being positioned such that the jet is inclined to the plane 
of the panel and the air from the jet is entrained by the panel in a 
spreading, predominantly laminar flow across the panel surface over that 
edge region and from that edge region to all the other edges thereof, 
thereby inducing such laminar flow over substantially the whole surface 
and replacing vapor-laden air closely adjacent the surface with fresh air 
to accelerate evaporation. 
The preferred form of air supplier is of the "air mover" type, i.e. one 
which is arranged to entrain a portion of the bulk flow of air from the 
enclosure's inlet so as to increase the volumetric rate of flow; thus the 
air supplier combines the pressurised air with the bulk air flow to 
generate a directional outflow at the greater flow velocity. 
Conveniently, the air supply is positioned at the correct predetermined 
distance and inclination by adjusting a supporting frame. 
In order that the invention may be better understood, two embodiments will 
now be described, by way of example only, with reference to the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In these examples, a thin water borne base coating on a vehicle panel is 
dried using a relatively fast moving air stream adjacent to the coated 
panel. This disturbs the air close to the panel which contains high 
moisture levels and continually replaces it with drier air. The air 
temperature may be higher than that of the surrounding air, or the system 
may be used in conjunction with infrared heating, so as to replace the 
latent heat of evaporation. 
A preferred example of drying apparatus embodying the invention is shown in 
FIGS. 1 and 2. A re-painting booth 1 is of conventional design with a 
filtered air inlet 3 in the ceiling and a grid 4 in the central region of 
the floor for extracting moisture-laden air. A car 2, with panels which 
will have been coated with paint sprayed in the booth 1, stands over the 
grid 4. There is a bulk flow of air generally downwards, as shown by 
arrows in FIG. 2, typically at 0.5 ms.sup.-1. A pressurised air supply 9 
of conventional construction has an outlet for paint-spraying (not shown). 
Twelve air suppliers in the form of cylindrical air movers 7 (available 
commercially) are positioned adjustably, in four "zones" of three, just 
below the bulk air inlet 3 and within its periphery, at least 0.5 m from 
the outer edges of the filters. Each air mover 7 is of known construction, 
having an annular strip outlet, on the axis of the cylinder, for air 
supplied under pressure. The strip outlet is shaped such that the air is 
entrained along an inner wall of a hollow body of generally cylindrical 
shape, so that the air is made to flow axially in an annulus. This flow 
drags or entrains slower-moving bulk air in a cylinder from a low pressure 
inlet region, so as to generate a cylindrical outward flow generally along 
the axis. The flow is at a substantially greater velocity than the 0.5 
ms.sup.-1 velocity of the bulk flow, such that when it reaches a target 
panel on the car 2, after a slight divergence and slowing, it will have a 
velocity of between 1 and 2 ms.sup.-1, as measured parallel to the panel 
surface and 0.5 to 1 cm from the surface. 
The air movers 7 are fixed to two supply pipes 5 arranged parallel to one 
another lengthwise of the car 2 and grid 4. Each supply pipe 5 is 
supported for rotation about its axis by three spaced angle brackets 6 
secured to the inlet 3. On each supply pipe 5, the six air movers are 
mutually parallel (although an air mover at each end can be inclined 
inwardly, to assist drying of end panels), grouped into two zones of 
three, on corresponding halves of the pipe. A manual lever 8 connected to 
the pipe 5 allows the air movers 7 to be angled appropriately. An air line 
92,93,94,95 leads from an air supply control box 91 to each zone of three 
air movers 7 by way of a channel within the supply pipe 5. 
The air supply control box 91 includes a pressure gauge and a valve for 
each zone. Usually, only one zone is used at any time, and the pressure is 
limited to 2 bar (30 p.s.i.) to give a flow rate of 425 liters (15 cubic 
feet) per minute. A flow restrictor is preferably provided, upstream of 
the valves, so that even if all four zones are active, the flow rate does 
not exceed 850 liters (30 cubic feet) per minute. These requirements are 
entirely compatible with conventional air supplies for painting booths, 
e.g. for two spray guns and airfed masks. The air flow from each air mover 
proceeds downwardly, substantially independently of its neighbouring air 
movers, to reach the edge of the panel, or panel portion, to which it is 
directed. When it reaches the panel edge its width is still substantially 
less than, for example 10-20% the length of that edge of the panel. If the 
panel is a typical car panel and is say 2 m below the air mover, the jet 
will typically have diverged to a width of about 10-20 cm as it impinges 
upon the panel. As it reaches the panel it is deflected by the panel, but 
is then "attached" by the panel surface and made to flow in a generally 
laminar curtain parallel to the panel, spreading out, along the panel edge 
and from that edge to other edges so as to reach the entire periphery of 
the panel. The phenomenon of attachment is believed to result in part from 
the Coanda effect. The laminar flow originating from the air mover will 
also tend to entrain more air from the bulk air flow reaching the panel. 
Examples of this air flow are shown schematically in FIG. 2. 
With the benefit of air extraction from beneath the car 2, drying air is 
drawn around the panels facing partly or wholly downwards, so these panels 
can also be dried using the principles of the invention. 
The air movers must be positioned and angled carefully to obtain fully the 
benefits described; this is explained in greater detail below. 
While the booth is described as a painting booth, it should be appreciated 
that the booth could be used solely for drying, if required. 
We have found that power consumption for the air movers is 1.8-3.6 kW for 
one zone, 3.0-4.8 kW for two zones, and less than 6 kW for all four zones. 
The air movers need not be cylindrical, and in the example which follows 
they are flat having an alongate outlet. The principle of causing a 
laminar, divergent flow over the panel is, however, the same. Moreover, 
this type of air mover is also available commercially. 
As shown in FIGS. 3 and 4, a motor vehicle whose panels have been sprayed 
with a water borne coating is resting on the floor of a booth. The booth 
is ventilated in a conventional manner, with moisture laden air being 
extracted from the floor region. 
Pressurised air is delivered in a fan-shaped, narrow jet 11, from an air 
outlet 10 at each appropriate position, or from the same air outlet which 
is moved from position to position. The or each outlet 10 is supported 
adjustably on a support frame, of which examples are shown in FIGS. 5 and 
6 and are described in greater detail below. 
The air outlet 10, known already as a "strip air mover", produces a broad, 
flat band of air 11, diverging only slightly, which is directed as a jet 
to a portion of one edge region of the panel. Thus one air outlet is 
disposed adjacent the front hinge of the door panel 20 so as to distribute 
air over the generally rectangular major portion of the door panel. 
Another position for the air outlet, as shown, in order to distribute air 
over half of the bonnet 21, is a short distance above and to the front of 
the headlight. In both examples, the angle of inclination of the principal 
axis of the air jet 11 relative to the plane of the panel is approximately 
45.degree., and within the range 20.degree.-80.degree. in any event. We 
have found that for more elongate panels, the outlet 10 should be inclined 
at a shallow angle, such as 20.degree.-30.degree., to the plane of the 
panel, and arranged to direct the air at the shorter dimension, i.e. the 
width of the panel, so that the air has sufficient forward velocity 
parallel to the panel surface to reach the far edge of the surface. 
The distance of the air outlet 10 from the nearest part of the panel 
surface should be about 50 cm to 60 cm or about 2 feet: any nearer, and 
the smooth flow is disturbed with the result that the jet fails to reach 
the far edges of the panel with a smooth laminar flow. Any further than 
this from the panel and the jet (in this particular example) would expand 
dimensionally and volumetrically too far to enable it still to achieve the 
desired result. 
We have found that with careful positioning of the air outlet in relation 
to the panel it is possible to cause the air jet to become entrained by 
the panel surface and to spread over the surface with a laminar flow 
across the panel surface. Surprisingly, the flow of air is still 
substantial and reasonably uniform even at the far corners of the panel. 
Whilst there is no adverse effect on the quality of the coating if some 
portions of the panel are dried more quickly than others, the energy 
efficiency of the system is clearly optimised by the present arrangement 
which delivers a steady flow surprisingly uniformly over the panel. 
The degree to which the drying process can be accelerated in this way 
depends to some extent on the humidity of the atmosphere. A typical period 
for unassisted drying, i.e. a typical flash-off time for one coat, is 10 
to 30 minutes. With the air jet this can be reduced to about 5 minutes. 
This can if necessary be reduced further to about 1 or 2 minutes with the 
use of heat energy, typically using 3 kW to 6 kW power for each air 
outlet. 
Thermal energy may be applied by preheating the air from a compressor, in a 
conventional manner. Alternatively, or in addition, thermal energy may be 
applied by radiation for example from one or more IR heating panels 13 
(FIG. 3). 
In this example, the air is supplied under pressure of 2 bar (30 psi) from 
a compressor. This input pressure is restricted to 2 bar (30 psi) by a 
pressure limiter, and the minimum height of the air outlet is kept to 60 
cm from the floor of the booth, in order to minimise the problem of dust 
disturbance. Clearly, the jets should never be directed towards any 
surface which may collect dust. 
In this example, the dimension of the air outlet is 7.5 cm long by 
approximately 100-125 microns wide; the air consumption rate is 
approximately 425 liters per minute or 15 cfm (cubic feet per minute) at 2 
bar (30 psi); the velocity of air as it moves over the panel surface is 
between 1 and 2 meters per second and the area of coverage of the panel is 
approximately half a square meter. 
The support frame shown in FIG. 5 consists of a wheeled trolley 40 on which 
is pivoted a horizontal support arm 41, pivotal as shown by arrow 33. The 
support arm 41 is joined to two horizontal extensions 12 to form a T 
structure. The arm extensions 12 are pivotable about a horizontal axis as 
shown by arrow 34. Each arm extension 12 is linked telescopically, as 
shown by arrows 32, to a further extension piece connected to an air 
outlet 10. The connection to the air outlet 10 also allows for pivotal 
adjustment, as shown by arrows 30, about a horizontal axis; each air 
outlet 10 is also pivotable about the axis of the support arms 12, as 
shown by arrows 31. 
An alternative arrangement for the support frame is shown in FIG. 6. A 
single high level aluminium rail 50, approximately 20 cm by 5 cm in 
section, for example mounted on the wall of the booth, supports a sliding 
bracket 60, for horizontal sliding motion as shown by arrow 51. A support 
arm 61 is mounted by means of a universal joint on the arm 60, allowing 
pivotal movement about two perpendicular axes, as shown by arrows 62 and 
63. The remaining components of the support frame are the same as those 
described above with reference to FIG. 5. 
The support frame of FIG. 5 is removable from the panels being dried by 
means of the wheeled trolley. The support frame of FIG. 6 is retractable, 
either manually or automatically, along the rail to another part of the 
booth. 
Although the invention has been illustrated by a method of accelerating the 
drying of a water borne coating, it is clearly applicable to other types 
of coating. Moreover, the invention is capable of use with panels of a 
wide variety of shapes: it works best with flat panels, but satisfactory 
results can still be achieved with less regular configurations. The 
important feature of the invention is that the air jet is entrained by the 
panel and that the flow across the panel surface is mainly laminar, and 
non turbulent. 
The booth could incorporate a differential in the rates of bulk air flow 
from different regions of the ceiling, e.g. rather faster flow in a 
peripheral region, but even then the flow rate would be less than that of 
the air from the air movers (or other air suppliers).