JPH0330848A - Method and apparatus for electrostatic spray of liquid - Google Patents

Abstract

PURPOSE: To increase flow rate while maintaining a desirable particle size by combining and installing side by side a pair of atomizing edges and field- intensifying electrodes of a spraying head equipped with a plural number of the said field-intensifying electrodes. CONSTITUTION: Atomizing edges are set in such a manner that they are separated by the distance (a) and that the closest portions on nozzle assemblies (10 and 11) relative to the surfaces of combined electrodes (26 to 28) form the atomizing edges by themselves (12, 13). High voltage is applied to each of nozzle electrodes (22) and low voltage relative to earth is applied to each of cores of the electrodes (26 to 28) so as to generate an electric potential difference between the nozzle electrodes (22) and the electrodes (26 to 28). Liquid reaching the atomizing edges is exposed to an electric field of high strength generated by the applied electric potential difference and becomes a series of liquid particles to be broken up into atomized particles. High supply amount is thus able to be maintained for its median particle size.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method and apparatus for electrostatic spraying of liquids. In particular, the present invention provides a method for supplying liquid along a surface to the edge of that surface (hereinafter referred to as the "atomizing edge") and to supplying the liquid along the atomizing edge and the field-enhancing electrode (often at the atomizing edge). (described as a field-enhancing electrode spaced apart from The present invention relates to a method and an apparatus using a spray head that sprays the atomized edge.

PRIOR ART Such a method and apparatus is disclosed in British Patent No. 1,589,707.

Conventional electrostatic spraying systems are usually used for large-scale spraying, for example paint spraying in the automotive industry. Such electrostatic spraying systems are broadly divided into two types: the first type first atomizes the liquid to be sprayed and then charges the formed spray, and the second type uses high voltage A spinning element, such as a disk or bell, which is maintained at a constant temperature and atomizes the liquid primarily by gravity. Generally, simply such an atomization system will deliver the required amount of liquid with appropriate atomization characteristics (eg, particle size) for film formation.

Problems to be Solved by the Invention The drawbacks of such known systems are primarily that they produce polydisperse spray particles containing many fine particles of less than 10 μm. Second, the charging of the spray is 100% or less. As a result, fine particles large enough to be inhaled escape into the air, causing air pollution. Another cause of T8 staining in such known systems is that their transfer efficiency tends to be low, for example on the order of 60%.

These drawbacks are particularly important in European Patent No. 186983
This problem has been substantially solved by the spray device disclosed in FIGS. 7 to 7 and the descriptions associated with these figures. The device disclosed in the above-mentioned European patent uses a nearly monodisperse spray (in the case of droplets, the median particle size of the volume
dialleter) and number median particle size (number
(mediandiameter)) and the particle size can be controlled to reduce the risk of inhalation. Furthermore, the complete charging of the spray and the relative improvement in transport efficiency further reduce the risk of inhalation and air pollution. However, a problem with the known atomizing devices mentioned here is that they can only operate at relatively low flow rates, which are too low for large-scale atomization such as paint atomization.

Various solutions have been proposed to increase the flow rate while maintaining the desired small particle size.

The solution proposed in EP 193,348 is to flow a gas into the region of the electric field and set the direction and velocity of the gas flow to remove charged droplets of liquid from the region of the electric field, thereby reducing the electric field. This reduces the formation of space charges that affect the size of the space charge.

Another solution is proposed in EP 186,983. This solution consists of constructing the field-enhancing electrode as a conductive or semi-conductive core, which core has a dielectric strength sufficiently high to prevent the generation of sparks between the electrode and the spray head. It is sheathed with a material that has a volume resistivity low enough to conduct charge collected on the surface to a conductive or semiconductive core. It has been found that the use of such electrodes in particular allows relatively high potential differences to be applied between the spray head and the field-enhancing electrode without destructive spark generation. Accordingly, the higher the potential difference, the greater the allowable flow rate for a given particle size, and by using such electrodes it is possible to increase the flow rate while maintaining the desired particle size.

The purpose of the present invention is to maintain the desired particle size and increase the flow rate.

The present invention is based on the very surprising discovery that two atomizing edges and their associated field-enhancing electrodes can be brought into close proximity to each other without substantially uncontrollable electrostatic interference between them. It is something.

It is commonly expected by those skilled in the art that as the atomizing edges are brought closer together to effectively form a single atomizing head, the interaction of the electric fields formed by potentials of the same polarity at the two atomizing edges increases. It was thought that the atomization capacity would be significantly impaired due to this, so that it would be almost the same if just one atomization edge was used.

[Means for Solving the Problems] According to the present invention, a liquid electrostatic spraying device includes a pair of atomizing edges extending side by side, a liquid supply device for each atomizing edge,
a plurality of field-enhancing electrodes extending along the length of the atomizing edge, a pair of field-enhancing electrodes being associated with each atomizing edge;
comprising an atomizing head, one on each side and in front of the atomizing edge, having a power supply for applying a potential difference between the atomizing edge and the field-enhancing electrode, and comprising: a) the atomizing edge; b) the spacing between each of the field-enhancing electrodes and its associated atomizing edge is greater than or equal to 3 mm, preferably ICl11; c) the atomizing edge and the electric field The measurable potential difference between the conductors supplying the potential difference applied between the reinforcing electrode is 1 to 3 KV/no+ per unit length of the field-enhancing electrode spacing from the atomizing edge.
It is characterized by being in the range of

The atomizing head thus comprises two atomizing edges, which substantially absorb the target moving relative to the atomizing head within a short period of time so as to be equivalent to one atomization in terms of paint atomization. can be placed close to each other so that they can spray the same area. However, for a given particle size, these two atomizing edges deliver about twice as many particles per unit area per unit time as a single atomizing edge.

[The operating parameters of the working device are 80P viscosity and 2×10'Ω
For liquids with resistivity of CH, the shoe length of the spray head (
median particle size of 100 μm at a flow rate of 20 cc/s per m)
may be selected such that spraying not exceeding .

If the atomization edge spacing is greater than about 300 a+m, the spray coating will not be completely applied, resulting in patchiness or flaking. The actual spacing is 2011 between the atomization edges.
It must not be less than I11.

It has been found that the longer the atomization edges, the greater the distance between them can be. For example, for atomizing edges with a length of the order of 150 mo+, the spacing between the atomizing edges is preferably in the range 20-100 mm, and for atomizing edges with a length of the order of 80 h+s+ , the spacing between the atomization edges is preferably 100-300I
This is the range of II11.

The spacing between each electrode and its associated atomizing edge is approximately 3
! If it is less than 11, the liquid can adhere to the electrode and form liquid bridges that adversely affect atomization. If the spacing between each electrode and its associated atomizing edge is much greater than about ICIm, potential gradients are lost and contamination problems can occur; these problems can only be solved by reducing the operating voltage. Just make it very expensive. Ideally, the potential gradient between each electrode and its associated atomizing edge should be as large as possible without leading to breakdown or contamination.

Advantageously, the present invention makes use of the armored electrode structure disclosed in EP 186,983, which advantageously allows relatively high levels of potential gradients within the range of 1 to 3 KV/n+a+ mentioned above without dielectric breakdown. It can be achieved.

If the atomization edge spacing is relatively large, it is necessary to use four field-enhancing electrodes. However, if the atomization edges are relatively closely spaced, three field-enhancing electrodes are used, with the central field-enhancing electrode being common to both atomization edges. To vary the spray signature or pattern, the potential of two adjacent electrodes when four field-enhancing electrodes are used, or the potential of the central electrode when three field-enhancing electrodes are used, is The value can be changed with respect to the potential of the opposite electrode.

Advantageously, in practice the potential with respect to ground at the atomization edge is made as high as possible. The higher the potential, the more compressed the footprint between the two atomization edges will be, and therefore the higher the spray density in this region. This compression effect also increases with increasing length of the atomizing edge. This increase in spray density was found to be very beneficial in forming a good spray coverage.

To provide flexibility in the use of the device, the power supply is preferably adjustable to provide an adjustable potential to the atomizing edge and the field-enhancing electrode. Preferably, it is also possible to supply each of the field-enhancing electrodes with a different potential.

[Example] The invention will now be further explained by way of example with reference to the accompanying drawings.

The spray head shown in FIG. 1 has two straight, substantially parallel nozzle assemblies 101U,
．． The edges 12,13 of 11 each form a spray edge. Each nozzle assembly 10, 11 consists of two plate members 14, 15 arranged opposite each other. The opposing surfaces abut in a portion 1B of their surfaces and are spaced apart in the remaining portion to provide a liquid flow slot 17 extending over the entire length of the plate member 14.15 and leading from a passage 18 to the atomizing edge 12.13. is forming. One of the opposing surfaces extends from the other opposing surface to form a raised lip 20 which extends from the outlet of the slot 17 to the atomizing edge 12.13. Plate member 14.15
The edges are beveled as shown. Edge 12.13
may be tooth-shaped or straight. The passage 18 consists of a passage formed in the surface of the plate member 15 and extending in the longitudinal direction.

Each passageway 18 is connected via a tube 19 to a liquid source (not shown). Alternatively, the edges of the root plate members 14.15 can be aligned as shown in FIGS. 2-4.

Plate members 14.15 are comprised of an insulating material and are connected to each nozzle assembly 10.11 so that a potential difference can be applied to the liquid supply.
is provided with an electrode 22. Each electrode 22 is positioned adjacent the atomizing edge 12.13 and is disposed on an opposing surface of the plate member 15. Each electrode 22 is connected to a high voltage generator via an insulated conductor 23. Atomization edge 12.13
The actual potential at electrode 2 through the liquid to be sprayed is
2 and the atomization edge 12.13.

The resistivity of liquid paint is on the order of 107ΩCrA,
Also, when the applied voltage between them is 40 to 80 KV, about 1
0% potential drop may be tolerated.

What is essentially required is that the electric field at the atomization edge be sufficient to form the required saliva-based spray. In place of the electrode 22, a plate member 14,15 of each nozzle assembly
One or both of the two can be made of conductive or semi-conductive material and can be connected to a high voltage generator.

The nozzle assembly 10.11 is supported on a suitable frame 25,
This frame 25 also has three linear electrodes 26, 27, 28
(herein referred to as field-enhancing electrodes, but also often referred to as field-adjusting electrodes). These electrodes extend substantially parallel to each other and parallel to the atomizing edge 12.13, electrode 26.27 being located on the outside of the atomizing edge 12.13 and electrode 28 being located on the outside of the atomizing edge 12.13. It extends midway between 12 and 13. The electrodes 26-28 are arranged in front of the atomizing edge 12.13 in the atomizing direction. Therefore the counter electrode 26.2
8 is combined with the atomizing edge 12 and also has a counter electrode 27,28
is combined with the atomizing edge 13.

Each of the electrodes 26-28 consists of a core 30 of conductive material, such as carbon, and a sheath 31 of semi-insulating material, such as soda glass. The resistivity of the exterior 31 is 101'Ωcff
It is of the order of l. These electrodes are covered by European Patent No. 186
It is of the type disclosed in detail in No. 983.

The cores 30 of these electrodes may be held at ground potential.

Alternatively, these cores may be applied with a voltage relative to ground that creates the desired potential difference between the atomizing edge 12.13 and the electrodes 26-28.

In FIG. 1, the atomization edges are shown separated by a distance a. The closest location on the nozzle assembly 10,ll to the surface of the combined electrodes 26-28 is the atomizing edge 12.13. The spacing between the atomization edge and the surface of the electrodes 26-28 is designated b in FIG.

In one experimental example of paint spraying using the apparatus shown in Figure 1,
The value of a was 36ffllll and the spacing was approximately IC1i. Please note that the drawings are not drawn to exact scale. The atomizing edge 12.13 may be of any desired length, in this example 150 Ili.

The effect of varying the length of the atomization edge will be discussed later. The width of each slot 17 was selected to take into account the viscosity of the liquid to be atomized and to provide the desired flow rate from each nozzle with a conventional vessel pressure head. In this example, a liquid with a viscosity of 8 cP is configured to deliver a nozzle of at least 10 cc/sec per unit length (m) of the atomizing head, with a total liquid delivery of at least 10 cc/sec per unit length (m) of the atomizing head. The amount was made to be at least 20 cc. This flow rate can advantageously be adjusted to different values.

In use of the apparatus, a high voltage relative to ground is applied to each of the nozzle electrodes 22, and a low voltage relative to ground is applied to each of the cores of electrodes 26-28, as specified below. A potential difference was created between the electrodes 26 to 28. The flow rate of the liquid paint supplied from the container was also adjusted to the desired value. The liquid reaching the atomizing edges is exposed to a high intensity electric field generated by the applied potential difference, which causes the liquid to become a series of liquid systems extending from each of the atomizing edges and the atomizing edges. At a distance from part 12.13 it broke up into atomized particles as detailed in GB 1569707 and EP 186983.

Referring to FIGS. 2-4, the various spray patterns obtained by applying various voltages to electrodes 26-28 in this example are shown.

In each of FIGS. 2-4, the voltage at the nozzle electrode 22 was 40 KV. In FIG. 2, the voltage across all field enhancement electrodes 26-28 was 15 KV.

Under these conditions, a spray pattern in which two sprays slightly overlapped was obtained as shown in the figure.

In FIG. 3, the voltage at the outer electrode 26,27 is L5
KV, and the voltage at the central electrode 28 was II KV. In this case, a spray pattern in which two sprays largely overlapped was obtained as shown in the figure.

In FIG. 4, the voltage at the outer electrodes 26,27 is 15
KV, and the voltage at the inner electrode 28 was 19 KV. In the spray pattern obtained in this case, the two sprays were spaced apart as shown. It is therefore seen that there is no significant interference between the sprays and that, in fact, the spray pattern can be easily controlled by varying the voltage. The spray pattern is also centered on the central electrode 28.
The same can be modified by changing the spacing to be different for the .

In another example shown schematically in FIG.
．． The distance a between 13 was 20 cm. This value was large, and it was necessary to use two central electrodes, indicated by 28a and 28b, in order to correspond with other drawings. Spacing is 18
It was 0a+Im. The length of each atomization edge was 60C11.

The voltage at electrode 22 was 80 KV and the voltage at field enhancement electrodes 26-28 was 40 KV. The target surface at ground potential is the atomization edge 12, I3 to 3
It was at a distance of 0 cm and moved at a speed equal to 1 m in 19 seconds. The liquid paint, a reddish-gray surface protection material used in the automotive industry, had a resistivity of 4XlO''Ω/C1 and a viscosity of about 3 poise.

Also, the flow rate of liquid paint is approximately 100cc/cm through each nozzle.
It was a minute.

The coating thus obtained was found to have good gloss, low speckles, and good flatness without stripping. The thickness of the coating was approximately 34-60 μm.

Reference is now made to Figures 6a and 6b, which illustrate comparative examples of spray patterns obtained using the apparatus of Figures 2 and 5. The length and spacing a of the atomizing heads referred to in FIGS. 2 and 5 are the same as those in FIGS. Ba and 6.
In figure b it is shown proportionally. The spray pattern is also shown as a contour, and the closer the contours are to each other, the higher the density of the spray pattern. It can be seen that with the relatively long atomization edges 12,13 of FIG. 5, the edge effect or edge spreading is less pronounced and the spray pattern is relatively compressed between the atomization edges. This results in a relatively dense spray, which is very advantageous for atomizing paints. A similar compression effect can be achieved by increasing the voltage at electrode 22 relative to ground.

[Effects of the Invention] As explained above, the paired nozzle structure of the present invention has the following effects:
In addition to providing a relatively high feed rate for a given median particle size as described above, it also allows the target to be fed with one of the two components from one nozzle and the other from the other nozzle. It can also be used in two component systems where the two components need to be supplied substantially simultaneously.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing one embodiment of the spray head according to the present invention, and FIGS. 2 to 4 show various types of spray head and electric field enhancing electrode, respectively. Figure 5 is a schematic diagram showing a modification of the spray head shown in Figure 1 and the spray pattern obtained using it; Figure 6a; FIG. 6b shows the effect obtained by using atomizing heads of different lengths, FIG. 6a shows the spray pattern for the relatively short atomizing head of FIG. 2, and FIG. 6b shows the spray pattern as shown in FIG. FIG. 3 is a diagram showing a spray pattern for a relatively long atomization head. Figure 11: Nozzle assembly [3; atomization edge 15: plate members 27, 28, 28a, 28b: field-enhancing electrodes 36
mm Ft'g, 4°~, 6a. Fig, 6b.

Claims (1)

[Claims] 1. A plurality of atomizing edges (12, 13) and the atomizing edges (
12, 13) and in front of the atomizing edges (12, 13).
3) an atomizing head with a plurality of longitudinally extending paired field-enhancing electrodes (26, 27, 28, 28a, 28b), with atomizing edges (12, 13) and field-enhancing electrodes (26, 27, 28, 28a, 28b); 26, 27, 28, 28a, 28b) and a power supply for applying a potential difference between the field-enhancing electrodes (26, 28b) in combination with the pair of atomizing edges (12, 13). , 27, 28; or 26, 27;
28a and 28b) extend side by side, and a) atomization edge (
12, 13) between (a) ranges from 20 to 300 mm; b) field-enhancing electrodes (26, 27, 28, 28
a, 28b) and its associated atomizing edge (12
, 13) is 3 mm or more, and c) the atomization edge (12, 13) and the electric field enhancement electrode (26, 2
7, 28, 28a, 28b) between the conductors providing a potential difference applied between the atomizing edge (12
, 13) to field-enhancing electrodes (26, 27, 28, 28a
, 28b) per unit length of the interval between 1 and 3 KV/mm. Liquid according to claim 1, characterized in that two or three field-enhancing electrodes (26, 27, 28) are provided, one of which is associated with an atomizing edge (12, 13). electrostatic spray device. 3. According to claim 1 or 2, the length of the atomizing edges (12, 13) is of the order of 150 mm and the spacing between the atomizing edges (12, 13) is in the range from 20 to 100 mm. electrostatic spraying device for liquids. 4. According to claim 1 or 2, the length of the atomizing edges (12, 13) is of the order of 600 mm and the spacing between the atomizing edges (12, 13) is in the range 100 to 300 mm. electrostatic spraying device for liquids. 5. Electric field enhancement electrodes (26, 27, 28, 28a, 28b
5. An electrostatic spraying device for liquids according to any one of claims 1 to 4, each of which comprises a core (30) of electrically conductive material and a sheath (31) of semi-insulating material. 6. The power supply unit applies a first potential with respect to ground to the atomizing edges (12, 13) and the field enhancing electrodes (26, 27, 28).
, 28a, 28b) are connected to apply a second potential that is lower with respect to ground. 7. Device for electrostatic spraying of liquids according to claim 6, wherein the power supply device is capable of applying an adjustable potential to the atomizing edges (12, 13) and the field-enhancing electrodes (28, 27, 28, 28a, 28b). . 8. The power supply device has electric field enhancing electrodes (26, 27, 28, 2
8. The liquid electrostatic spraying device according to claim 7, wherein different potentials can be applied to each of 8a and 28b). 20 cc per unit length (m) of the spray head for a liquid with a viscosity of 9.8 cP and a resistivity of 2 x 10^8 Ωcm
1. The operating parameters are selected such that atomization with a median particle size not exceeding 100 μm is achieved at a flow rate of /sec/.
9. The liquid electrostatic spraying device according to any one of items 1 to 8. 10. A pair of atomizing edges are arranged side by side, a liquid supply device is coupled to each atomizing edge, and a number of electric field enhancing electrodes are arranged along the longitudinal direction of the atomizing edge, and the electric field enhancing electrodes are arranged side by side. a pair is assembled with each atomizing edge and placed on each side of the atomizing edge and in front of it, and between the atomizing edge and the field-enhancing electrode, a) a spacing between the atomizing edges; is in the range of 20 to 300 mm, b) the spacing between each of the field-enhancing electrodes and its associated atomizing edge is 3 mm or more, and c) the distance between the atomizing edge and the electric field-enhancing electrode is The power supply is connected to apply a measurable potential difference between the conductors that provides a potential difference in the range of 1 to 3 KV/mm per unit length of the field-enhancing electrode spacing from the atomizing edge, and the field-enhancing A method for electrostatic atomization of liquids, characterized in that a target is placed at a distance in front of an electrode, and the liquid to be atomized is supplied to the atomization edge to electrostatically atomize the target. 11. The voltage applied to the atomization edge is of the order of 80 KV, the voltage applied to the field enhancement electrode is of the order of 40 KV, and the target is at ground potential.
A method of electrostatic spraying of a liquid as described in . 12. According to claim 10 or 11, the potential applied to the one or more field-enhancing electrodes arranged between the two atomizing edges is different from the potential applied to the field-enhancing electrodes outside the two atomizing edges. Method of electrostatic spraying of liquids as described.