FR2652011A1 - METHOD FOR APPLYING LIQUID COATING PRODUCT AND INSTALLATION IMPLEMENTING SAID METHOD - Google Patents

Abstract

Control of the dryness of the sprayed coating product as a function of the ambient temperature and humidity at the spray site. According to the invention, mixing means (33) are provided for adding at least one diluent, via feed means (34), to a raw coating product via feed means (16), the flow of diluent being determined by measuring the ambient temperature and / or the hygrometry, by means of temperature (40) and / or hygrometry (41) sensors.

Description

The invention relates to a method for applying a product of

  liquid coating, typically a paint or varnish, on objects to be covered. It relates more particularly to improvement to better control the degree of dryness of the coating product when it reaches, in the sprayed state, on an object to be covered. The invention also relates to an installation

implementing this method.

  It is known that the quality and the appearance of the coating product film deposited on an object, such as, for example, an automobile body, depend in particular on the degree of dryness of the coating product in the pulverized state. More specifically, the term "degree of dryness" refers to the viscosity of the droplets of sprayed coating product, when they reach the object to be covered after their journey through the air. If the particles are too liquid, it is possible to obtain castings on the object before the desired thickness is reached, and / or mottling if it is a metallic effect paint. If they are too viscous, they bind poorly to each other and the surface of the film is "tense", taking a look called "orange peel" or a dusty appearance. The parameters which influence the degree of dryness as defined above are, in particular: - the temperature of the object - the temperature of the drops of coating product at the end of their path in the air - the dilution of the product coating prior to spraying - the relative partial pressure of vapors in the atmosphere in the vicinity of the spraying - the volatility of the diluents of said coating product. The invention follows from the following findings and analysis. The temperature of the objects is generally conditioned by the treatments they undergo before the application of the coating product. If the mass of such an object is important, especially if it is a car body, the control of this temperature may require the bringing into play of powers

important thermal.

  On the other hand, it has been found that the temperature of the droplets of coating product on their arrival on the object depends practically only on the ambient temperature in the vicinity of the object being treated (that is to say concretely, the temperature inside the projection booth in which the object passes). Indeed, the heat capacity of the droplets is too low for their initial temperature, that is to say the temperature of the coating product before spraying, can play a determining role. Moreover, the jets of spray air and / or conformation of the spray jet, even if the sprayer is rotating, cause too much ambient air so that their own temperature has a significant influence. Controlling the ambient temperature requires the use of very large thermal powers because of the permanent ventilation required in the projection booth,

  although some of this power is sometimes recycled.

  It has been proposed to modify the dilution of the coating product to control droplet viscosity.

upon their arrival on the object.

  However, in known installations, this dosage is made for a relatively long period of time, at least for a day or more depending on the expected climatic conditions. If the said conditions do not correspond to reality or if they vary rapidly, the coating product becomes unsuitable and the conditions

application are degraded.

  The invention provides an improvement to act more effectively on the dilution at least and / or

  preferably, on the volatility of the coating product.

  In this spirit, the invention relates to a method for applying liquid coating product, by spraying, on objects to be coated, in which a selected quantity of a diluent is added to such a raw coating product, characterized in that it consists in adjusting the proportion of at least one such diluent according to at least one climatic condition prevailing in the vicinity of

objects being processed.

  In practice, said climatic conditions are those that prevail in at least one application booth in which transit objects to be covered. Among said climatic conditions, the temperature and / or the relative humidity are more particularly those which are taken into account. "Raw coating material" is understood to mean such a product which is relatively too viscous in relation to what is suitable for its use under the best conditions, that is to say requiring to be further diluted whatever the circumstances. and in particular whatever the ambient temperature and the relative humidity at the place of spraying. If we add a single diluent, we can lower

  the viscosity of the coating product to a desired value.

  This simple solution gives good results when the atmospheric conditions vary little and when the variations of dilution are not too important. Indeed, a lowering of the viscosity affects the fineness of spraying if the spraying conditions are not changed simultaneously. It has been found, for example, that the increase in the dilution (which lowers the viscosity and therefore refines the spray) facilitates the evaporation of the diluents during the air path and that the change in viscosity of the droplets to their

  arrival on the object could be less important than expected.

  According to another important characteristic of the invention, the volatility of the coating product is primarily affected before it is sprayed by adding to said raw coating product, under the conditions stated above, at least two diluents of different volatilities and by adjusting proportions

  said diluents according to said climatic conditions.

  The two diluents may for example be added in variable proportion ranging from 0 to 100% for each of them. Diluents are preferably chosen such that their dilution capacity and that of their mixture do not differ too much so that the viscosity of the final coating product, after their addition, depends relatively little on the proportions of the mixture. According to a variant of the process, it is also possible to modify the proportions and the flow rate simultaneously.

total diluents added.

  For example, it is possible to mix adjacent diluents such as xylene and toluene, or as metylisoamyl ketone and metylisobutyl ketone or butyl alcohol and isopropyl alcohol. These "couples" of solvents have indeed sufficiently different volatilities to adjust the overall volatility of the mixture without the variations in the proportions of said diluents not significantly affect the viscosity of said final coating product before spraying. It is also possible to mix diluents of different families provided that they are compatible, in order to extend, if necessary, the

Volatility range of the mixture.

  In the case of an electrostatic projection, the various constituents of the mixture are chosen so as not to modify in a harmful way, in particular to lower the

resistivity of the final product.

  The principle of the invention as defined above can be implemented, depending on the circumstances, in

  different locations of a projection installation.

  Thus, if the installation comprises only a projection booth, or a small number of booths in which substantially the same conditions of temperature and relative humidity prevail, it is sufficient to add the diluents to the outlet of a single tank enclosing said raw coating product. For example, temperature and hygrometry sensors may be placed in a suitably chosen location to deliver signals representative of the temperature and humidity.

  ambient humidity inside the cabin or cabins.

  They can for example be placed in one of the cabins.

  These sensors can then be integrated with a servo system controlling in particular the respective flow rates.

  both diluents and the raw coating product.

  The invention also relates to a spray application installation of liquid coating product on objects to be coated, comprising at least one coating product projector and mixing means connected to supply said projector, characterized in that said mixing means are connected, on the one hand, to feed means for at least one raw coating product and, on the other hand, to feed means for at least one diluent, by means of dosing means for said diluent , driven, in particular, by at least one sensor of a climatic condition prevailing in the vicinity of said objects being processed. In practice, it will be possible to use at least one temperature sensor and / or one hygrometric sensor. For a more complex installation, comprising several cabins installed in locations where the temperature and / or humidity can be significantly different (more or less far from a drying oven, near the north or south facade of the building, .. .) The number of dilution equipment with specific temperature and humidity sensors can be multiplied. It is possible, for example, to provide such cabin equipment with a corresponding servocontrol system, each cabin receiving at least one raw coating product coming from a common tank and at least one diluent having the characteristics defined above. In the case of a quick-change product installation, connections with a corresponding number of tanks, established by closed-loop circulation circuits feeding product change units, may be provided.

  associated with the different cabins, according to a known arrangement.

  The invention will be better understood and other advantages thereof will appear more clearly in the light of the

  description which will follow, given only as

  for example, and with reference to the accompanying drawings in which: - Figures 1 and 2 are graphs illustrating the evolution of the volatility of the coating product as a function of climatic conditions for different types of coating products; FIG. 3 is a block diagram of a rapid change product coating plant, embodying the invention, and FIG. 4 is a graph illustrating a relationship between the proportions of diluents employed and volatility

of the final mixture.

  The most complete implementation of the invention results from the observation that for some products the volatility of the diluents of the product to be sprayed must decrease with the temperature of the projection zone and increase with its moisture. , which can be represented by the graph of Figure 1 which, as a first approximation, assumes linear relations. In this graph, T is the temperature, H is the hygrometry and V the volatility. At point A representing a hot and dry atmosphere (near an oven, for example), there is a low VA volatility. At point B representing a hot and humid environment (summer storm, for example) or point C representing a cold and dry atmosphere (winter, for example), corresponds to average volatility. At point D representing a cold and humid atmosphere (in the spring, by

  example) corresponds to a high volatility VD.

  With other coating products, on the contrary, the volatility must decrease with humidity, which is reflected in the graph of Figure 2. This difference in behavior can be attributed to the greater or lesser

Affinity of the product for water.

  The partial pressures of water vapor and diluents and the evaporation rates being practically independent of one another, this effect of moisture can be explained by the fact that the heat of evaporation of the diluents, of about 150 calories per gram, can lower the temperature of the drops of coating product, for example about twenty degrees for drops of 15 microns radius and specific heat of about 0.6 calories per gram and per degree, losing 10% of their mass. This lowering of the temperature of their surface and their immediate environment can

  cause condensation of part of the water vapor.

  Indeed, in an atmosphere at 25 C and 80% relative humidity, that is to say with a partial pressure of water vapor of 25 millibars, such a drop of 20 lowers the saturation vapor pressure to about 9 millibars, much lower value than the partial pressure. The supersaturation thus appearing, would cause the rapid condensation of water vapor. Even assuming that only 10% of the heat absorbed by the evaporation of the diluents serves to condense water vapor, the rest lowering the temperature of the ambient air by radiation, conduction, fan convection ... can calculate that each drop can receive 0.25% of its mass in water. The ratio, in the spray, between the coating material mass and the moving air mass (mostly entrained ambient air) is of the order of 10%, it is found that the mass of water thus condensate, which represents 0.025% of the moving air mass, is small compared to the condensable vapor since, at the partial pressure of 25 millibars, a mass of water vapor corresponds to 1.5% of the air mass; 'at the saturation vapor pressure corresponds only a mass

  of 0.5% of the same air mass.

  It can also be shown that, although the mere thermal agitation of the water vapor molecules largely allows for condensing such a small fraction of the water available on the drops of product during their journey between the projector and the object to be covered. the presence of the electric field, atmospheric ions and electric charges on the drops, in the case of an electrostatic projection, can accelerate this condensation and even cause around the ions, the birth of slightly charged drops of water, which are deposited on the object as

drops of product.

  - If the coating product is hydrophobic, for example because the diluents or solvents are hydrocarbons, this water, because of its high surface tension could either be rejected on the surface o, hindering the evaporation of diluents still present ( most of the coating products contain at least 50%) it would cause pouring of product remained too liquid, either remain trapped and cause pores or pitting during its evaporation, after that of diluents due to its relatively low volatility. In this case, the use of volatile diluents could only lower the temperature of the

  drops of coating product and cause the phenomenon.

  If the coating product is hydrophilic, for example because it contains polar diluents or solvents, such as alcohols, the use of volatile diluents

  would not present this risk and may be appropriate.

  In the case of a humid atmosphere, it therefore seems advantageous to use polar diluents, provided they do not contain too much water, to correct

  the viscosity of the coating product.

  It follows from the considerations developed above that in the context of the implementation of the method according to the invention, it is advantageous to choose diluents with degrees of polarity that are larger or smaller depending on the

ambient humidity.

  The installation shown in FIG. 3 is more particularly of the type comprising dilution means associated specifically with each cabin. For simplicity, there is shown only one cabin 11 in which is installed at least one coating product projector 12 (paint and / or varnish) here of the electrostatic type. The cabin is crossed by a conveyor 13 carrying the objects to be painted 14, in this case automobile bodies. The cabin 11 is further equipped with a product change unit 16, conventional, comprising a set of controlled valves communicating with a collector. There are valves 17a, 17b, 17c connected to different raw coating product distribution circuits, a controlled valve 18 connected to a cleaning product dispensing circuit 19 and a controlled valve 20 connected to a dispensing circuit. compressed air 21. The raw coating product distribution circuits 22a, 22b, 22c are conventionally closed-loop circulation circuits fed by respective reservoirs 23a, 23b, 23c, grouped together at a plant location sometimes very far from the projection booths. These tanks and circulation circuits are connected to the product change units of all cabins. The outputs of the above-mentioned controlled valves are all connected to a first collector 24 whose output is connected to a geared pump 25, driven by a motor 26. The output of the pump 25 is connected to a flowmeter 27, of

  structure similar to that of the gear pump.

  All the equipment described above is known per se and constitutes means of supplying such a raw coating product for the spotlight (s) 12. The valves 17, 18 and 20 are controlled by a computer 30 which manages all the operating sequences of the installation. It receives in particular, as data, the signal delivered by the flowmeter 27 and a signal representative of the position of the object in the cabin, for example prepared from one or

  several sensors 32 associated with the conveyor 13.

  According to the invention, the installation is completed by mixing means 33 inserted upstream of the projector 12 and connected, on the one hand, to the output of the raw coating product feed means and, on the other hand, supply means for at least two diluents of different volatilities 34. More specifically, the mixing means comprise a second collector 35 connected to the output of the flowmeter 27 and a static mixer 35a connected to the outlet of the collector 35. The means supply 34 comprise two similar branches 34a, 34b in which arrive respectively the two diluents, under pressure. Each branch comprises, in cascade, a controlled pressure regulator 37a or 37b, a venturi flowmeter 38a or 38b, and a controlled valve 36a or 36b, respectively. The output of each valve is connected to the second collector 35. The elements contained in these branches form, in connection with the computer 30, means for dosing the two diluents. The valves and pressure regulators are controlled by the computer 30 which, as data, reviews the signals produced by the flowmeters 38. The flow rates of the two diluents are thus controlled in each corresponding branch via the control of the pressure regulators. , so as to achieve the desired dosage. On the other hand, the raw coating product flow rate is controlled via the control of the gear pump 25. Alternatively, the branches 34a and 34b could be equipped with

  gear and volumetric sensors.

  The output of the static mixer 35a is connected to the

  projector 12 via a valve 38 controlled by the computer 30.

  In addition, a temperature sensor 40 and a hygrometry sensor 41 are installed at a location for measuring ambient temperature and humidity at room temperature.

  neighborhood of objects being processed.

  In the example described, the sensors are installed inside the cabin 11 and their outputs are connected to data inputs of the computer 30. The latter is programmed to adjust the ratio of the flows in the two branches 34a, 34b, in particular according to the information delivered by the sensors 40 and 41. Finally, a temperature compensated viscometer 42 can be inserted upstream of the projector 12, here between the valve 38 and the mixer 35a. The signal delivered by this viscometer is also applied as data to the computer 30, to continuously monitor the viscosity of the final coating product, just before its spraying, so that it is carried out in

good conditions.

  If the installation comprises several cabins, it is advantageous to provide mixing means, raw product supply means, diluent supply means and aforementioned dosing means, specific to

each cabin.

  Similarly, it is possible to provide a temperature sensor and a hygrometry sensor for each cabin, preferably installed in it. However, if a group of cabins can be considered as always operating at the same temperature and at the same hygrometry, it is possible to envisage using only one pair of sensors for this group, by

  example, placed in one of them.

  The operation is as follows. The computer 30 receives from a production management center signals characterizing the operations to be performed on the body that enters the cabin 11. It selects the product then, based on the information transmitted by the sensors 32 connected to the conveyor, it regulates the flow of raw coating material needed to paint this or that part of the bodywork on the move. This is done mainly by controlling the flow rate of the pump 25. At the same time, according to the temperature and hygrometry information provided by the sensors 40 and 41, it determines the ratio of the flow rates of the two diluents of different volatilities, added to the raw coating material, in the mixing means 33. The ratio of the diluent flow rates determines the degree of dryness of the coating product on the coated object. Of course, the flow rate of diluent in each branch 34a, 34b depends on the flow rate of the raw coating product controlled by the pump 25. It is the ratio of these two diluent flow rates that is maintained at a given value for a temperature and a temperature. hygrometry data. The signal provided by the viscometer may optionally be used to correct the total flow rate of the diluents, without changing their ratio, if the viscosity of the product of

  final coating does not match the expected value.

  Among the data stored in the computer are, for each product, the viscosity of the diluted product

  as well as, on the one hand, the volatility-temperature-

  hygrometry deduced for example from the graph of Figure 1 and secondly a relationship illustrated by the graph of Figure 4 between the proportions of the two diluents D% (abscissa) and the volatility V of the final mixture obtained. This last relation is not always one-to-one and can present a positive or negative optimum as the graph shows. It is therefore necessary to decide (for example for economic reasons or reasons of stability of the enslavement) of a criterion of choice. Finally, the global function linking temperature and humidity to the flow rate of each auxiliary diluent can generally only be approached by the

  calculated and corrected, point by point, experimentally.

Claims (12)

  A method for applying liquid coating product, by spraying, on objects to be coated, in which (35) a selected quantity of a diluent is added to such a raw coating product, characterized in that it consists adjusting the proportion of at least one such diluent (341 based on at least one climatic condition (40, 41)   prevailing in the vicinity of objects being processed.   2. Process according to claim 1, characterized in that   that such a climatic condition is the temperature (40).   3- A method according to claim 1 or 2, characterized in that such a climatic condition is the hygrometry of the atmosphere (41).   4- Method according to one of the preceding claims   characterized in that at least two diluents are added to said raw coating product and the relative proportions are varied according to the climatic condition (s). - Process according to claim 4, characterized in that the diluents are chosen so that their mixture has substantially the same dilution power whatever the proportions of this mixture.   6. Method according to one of the preceding claims,   characterized in that the degree of polarity of the diluent (s)   is chosen according to the hygrometry of the atmosphere.   7. Application installation by spraying liquid coating product onto objects to be coated, comprising at least one coating product projector (12) and mixing means connected to feed said projector, characterized in that said mixing means (33) ) are connected, on the one hand, to means for supplying at least one raw coating product (10) and, on the other hand, to means for supplying at least one diluent (34) with intermediate metering means of said diluent, controlled in particular by at least one sensor (40, 41) of a climatic condition prevailing in the vicinity of said objects being processed.   8- Installation according to claim 7, characterized in that it comprises a temperature sensor (40) and / or a hygrometric sensor (41) for driving said dosing means.   9- Installation according to claim 7 or 8, characterized in that it comprises means for supplying at least two diluents of volatilities and / or polarities different (34a, 34b).   - Installation according to one of claims 7 to 9   of the type comprising a plurality of projection booths (11), characterized in that it comprises mixing means (33), feeding means (16, 34) and means for   dosage (30, 37, 38), specific for each cabin.   11- Installation according to one of claims 8 to   , of the type comprising a plurality of projection booths (11), characterized in that it comprises a temperature sensor (40) and / or a hygrometric sensor (41) mentioned above for each   cabin, preferably installed inside it.   12- Installation according to one of claims 7 to   11, characterized in that said metering means (34) comprise means for adjusting the respective flow rates (37, 38) of said diluents, converging towards said means mixers.   13- Installation according to claim 7, characterized in that said dosing means constitute a closed-loop control system of the flow of or diluents.   14- Installation according to claim 13, characterized in that said servo system is   responsive to the flow of said raw coating product.   - Installation according to one of claims 7 to   14, characterized in that the output of said mixing means is connected to said projector by means of a viscometer (42) and that the latter is connected to said dosing means.