Electrostatic Atomization Of Liquids

Abstract

The process of atomizing liquids having conductivity in 10.sup.-.sup.6 to 10.sup.-.sup.3 ohms - centimeters range using either a negative or a positive electrostatic charge for the purpose of producing the aerosol which is deposited. In this process an ambient atmosphere is placed around the atomizing electrodes of an electric field with gases and vapors having a higher breakdown that air which vapors include short chained aliphatic compounds containing halogens and inorganic halogens. As a result, both negative and positive charges can be used in atomizing with liquids in the indicated conductivity range.

Description

The electrostatic atomization of liquids is already known in connection with various lacquer spraying techniques. Among others there are those in which lacquers are sprayed, without the use of compressed air, simply by means of the atomizing effect of powerful inhomogeneous electric fields, and are deposited on conductive surfaces. In electrostatic lacquer spraying on a commercial scale, one wishes to accomplish the atomization and deposition of large quantities of lacquer economically. The ranges of sizes of the droplets and the sign of their electric charge are of secondary importance. It is noteworthy that not all combinations of pigments and solvents can be sprayed electrically and certain conditions have to be fulfilled in respect of the electrical properties of the lacquer. These include primarily the range of electrical conductivity which generally has an upper limit of 10.sup. .sup.-6 Ohm.sup..sup.-1 cm..sup. .sup.-1.

The electrostatic atomization of dye solutions and color pigment dispersions for developing latent electrical images by the electrophotographic process is described in German Pat. Specifications Nos. 1,164,829, and 1,172,955. When electrically charged dye aerosols are used for the development of images in this way, the size of the droplets and nature of charge of the droplets are of decisive importance. It is thus necessary to provide special electrode arrangements, predetermined conductivity regions for relatively high electrical conductivity of the liquids, and predetermined electrical field distributions and field densities. This process operates within a critical field of atomizability in that the coloring liquid used in the negatively charged aerosols must have a conductivity above 10.sup..sup.- 5 Ohm.sup..sup.-1 cm..sup. .sup.-1. Positive atomization also becomes difficult in this conductive region because interfering corona discharges then easily occur at the atomization electrodes, and may severely inhibit or even prevent the atomization.

The object of this invention is to provide a means for atomizing liquids which have electrical conductivity higher than 10.sup. .sup.-5 Ohm.sup..sup.-1 cm..sup..sup.-1, by the use of both negative and positive electrostatic charges. Another object of the invention is the improvement of the operational safety of electrostatic atomization apparatuses.

A process has now been found in which liquids having conductivities of up to 10.sup..sup.-3 Ohm.sup. .sup.-1 cm..sup. .sup.-1 or more and preferably conductivities between 10.sup.6 and 10.sup..sup.-3 Ohm.sup..sup.-1 cm..sup..sup.-1 , can be negatively or positively atomized using simple electrodes at normal pressure, and which moreover has high operational safety. The process according to the invention is characterized in that the liquids are electrostatically sprayed by atomizing electrodes which are surrounded by an atmosphere of a gas which has a higher electric breakdown voltage than air at an atmospheric pressure of 760 mm. Hg. The breakdown voltage measured with plane electrodes at a distance of 1 cm. should preferably be at least 35 kv./cm. Reference is made to Handbook, Landolt-Bornstein, Vol. IV, part 3, p. 107 stating the breakdown value at 760 torr, 20.degree. C. and 11g. H.sub.2 0 per cubic meter of air between plane electrodes at a distance of 1 cm. is 31.0 kv./cm.

According to a preferred embodiment of the invention, the process comprises supplying to the air surrounding the atomization electrodes, gases or vapors which have a higher electric breakdown potential than air. Such gases preferably contain halogen atoms in their molecules, in particular chlorine or fluorine, and are short-chained aliphatic compounds having preferably not more than three C-atoms, in particular the so-called refrigerants which are used as spray propellants namely CC1.sub.3 F, C.sub.2 C1.sub.2 F.sub.4, C.sub.2 C1.sub.3 F.sub.3, CC1.sub.2 F.sub.2, CC1F.sub.3 , etc. Low boiling inorganic halogen compounds are also especially suitable in particular fluorine compounds such as SF.sub.6. An example of a suitable vapor with high electric breakdown potential is CC1.sub.4. The formation of a corona discharge at the atomization electrodes is largely prevented in the presence of such gases of vapor so that a powerful electric field which does not vary with time can develop for use in the atomization of the liquid.

The concentration of the additional gas or vapor in the atmospheric air surrounding the atomization electrodes may vary within wide limits. It is determined by the desired breakdown potential of the gas atmosphere surrounding the atomization electrode. The required breakdown potential may depend on the electrical properties of the liquid which is to be atomized. The concentration of the gas added further depends, within certain limits, on the degree of moisture in the air of course, and on the breakdown potential of the gas or vapor itself. Depending on the conditions, it is usually sufficient to add quantities of about 5 to 50 percent in order to achieve the breakdown potentials stated above. In general, the desired conditions are achieved by concentrations of about 20 to 50 percent. There is no upper limit to the concentrations added since the process can, of course, be carried out particularly well in an atmosphere of the pure gas or vapor of high breakdown potential. On the other hand the concentrations may be limited by economic considerations. The average expert will find no difficulty in determining the optimum proportion by volume of gas to add for any particular atomization process.

In order to carry out the process according to the invention, it is only necessary that the immediate surroundings of the atomization electrode should have a breakdown potential within the range indicated above. The atmosphere of high breakdown potential need only be maintained up to a distance of not more than about 5 cm. from the atomization electrode. This makes it relatively easy to construct suitable apparatus for the process of the invention since the usual additional apparatuses which are required for electrostatic atomization, in particular electrostatic atomization with the use of counterelectrodes or receiver electrodes for the development of latent electrostatic images by means of dye aerosols can be chosen just as freely as in the case of normal electrostatic atomization in air.

This process can be applied to all electrostatic atomization but is particularly advantageous in the case of atomization of dye liquids for electrophotographic image development. By using suitable dye solutions and dispersions of high conductivity, it is possible to use both negative and positive development processes on the photoconductive layers which normally consist of a mixture of zinc oxide and binder. Since the size of the droplets from the electrostatic dye aerosol decreases with increasing conductivity of the liquid, it is possible to achieve by these means higher optical resolution in the development of the image.

Compounds which have a low chlorine content and high fluorine content, such as CC1.sub.2 F.sub.2, are particularly suitable for use as aliphatic chlorine-fluorine compounds, owing to their low toxity and general noninflammability. The use of sulfur hexafluoride which is also nontoxic is particularly advantageous owing to its particularly high breakdown potential.

A suitable atomization electrode for lacquering any metal articles by means of electrostatic atomization is illustrated diagrammatically in FIG. 1 of the accompanying drawings. The tube 1 constitutes the electrode and preferably has sharp edges at the top and from which the liquid is atomized. This rim may be funnel-shaped, for example, and arranged as a surface of revolution about the longitudinal axis of the tube. The electrode tube 1 is surrounded by a tube 2, made for example of plastic, through which the additional high-breakdown gas is introduced. Outer tube 2 has an open annular orifice 3, at the spraying end of the electrode, through which the gas of higher breakdown voltage escapes and circulates in the immediate vicinity of the electrode rim.

A preferred means of application of the process of the invention for the development of electrostatic images is illustrated diagrammatically in the FIG. 2 of the attached drawings. The electrophotographic layer 4 which carries the electrical image is attached by a support to a grounded metal plate 5. A wire sieve 7, stretched in a metal frame 6 and connected to a source of voltage is arranged in front of the photoconductive layer. The frame is attached to a plastic casing 8 which contains the additional gas of high electric breakdown potential. The desired concentration of additional gas in the electrode chamber is controlled by the influx rate of the gas entering the plastic casing through the aperture 9. Excess gas can escape through the aperture 10. The atomization electrode 11, which is fed from outside with colored developer liquid in known manner enters the plastic casing through this aperture. The nature of the electrode itself is not important and can be of any known design.

Example 1

A metal article is coated electrostatically with a dye pigment from a dispersion which has an electrical conductivity of a 5 .times.10.sup..sup.-5 Ohm.sup..sup.-1 cm..sup..sup.- 1. The metal article is connected, for this purpose, to ground. Lacquering is carried out by means of an atomization electrode of the type shown in FIG. 1. In order to carry out the operation, the atomization electrode 1 has a potential of -50 kv. (with respect to ground) applied to it from a high-voltage source. Sulfur hexafluoride is used as the additional gas and is introduced through the plastic tube 2 which surrounds the electrode. Atomization is extraordinarily uniform, and safe in operation even at high voltages. This makes it possible to atomize relatively large quantities of liquid per unit time.

Example 2

A photoconductive layer consisting of zinc oxide as the photoconductive substance, and an insulating silicone resin (e.g. a polyphenylmethyl resin) as binder is applied to a conductive support. The photoconductive layer is negatively charged in the usual manner and exposed through a negative original to form an image. An electrostatic charge image is obtained. Development is carried out by means of liquid aerosol using an apparatus of the type shown in FIG. 2. A potential of -200 v. is applied to the wire sieve 7. The atomization electrode 11 is connected to a high-voltage source and receives a potential of -35 k. Dichlorodifluoromethane is used as the additional gas and is introduced through the aperture 6. The supply of this gas is controlled in such a manner that its concentration is about 20 to 50 percent. The development liquid has the following composition: 30 percent concentrated Astra-new fuchsine (Schultz-Farbstofftabellen, 7 th Edition No. 782) 70 percent benzyl alcohol.

The conductivity of the dye carrying liquid is 3 .times.10.sup..sup.-4 Ohm.sup..sup.- 1 cm..sup..sup.-1. The spraying time is about 10 seconds. A positive image of the negative original is obtained.

Claims

We claim:

1. In the process of electrostatically atomizing coloring liquids having an electrical conductivity in the range of 10.sup..sup.- 5 Ohm.sup. 1 cm..sup..sup.- 1 to 10.sup..sup.-3 ohm.sup..sup.- 1 cm..sup..sup.- 1 by an electrostatic voltage, the improvement according to which the coloring liquid having an electrical conductivity in said range is atomized by the field of an electrostatic charge by applying a negative or a positive charge For atomization of the liquid in an atmosphere between electrodes with the atmosphere within at least 5 cm. of the atomizing electrodes being of a vapor or of a gas selected from the group consisting of sulfur hexafluoride, dichlorodifluoromethane, trichlorofluoromethane, chlorotrifluoromethane, trichlorotrifluoroethane and dichlorotetrafluoroethane, in an amount of 5 to 50 percent by volume, whereby the atomization is effectuatable by either sign of the potential of the electrostatic atomizing electrodes.