Fluid sprinkling – spraying – and diffusing – Electrostatic type – Induction charging
Patent
1996-05-14
1998-07-14
Kashnikow, Andres
Fluid sprinkling, spraying, and diffusing
Electrostatic type
Induction charging
239290, 239697, 361227, 361228, B05B 500
Patent
active
057791627
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the electrostatic spraying of liquids by the application of a high voltage to liquid emerging at the outlet of a nozzle whereby an electric field is developed which is effective to draw the liquid into a ligament which is of smaller diameter than the nozzle outlet and breaks up to produce a spray. Devices for effecting electrostatic spraying in this manner are disclosed in our prior EP-A-441501 and 501725.
Although such devices are suitable for spraying liquids of varying resistivities and viscosities, some liquids are less amenable than others to spraying by means of electrostatic devices of this type, especially when there is a requirement for the production of divergent sprays with droplets having a narrow size distribution and with a volume mean diameter (VMD) of 100 microns or less at flow rates up to 4 cc/min or higher. A liquid having a resistivity of the order of 5.times.10.sup.6 ohm.multidot.cm and a viscosity of the order of 1 Poise is representative of such liquids which are less amenable to spraying when the spray is to comply with these requirements on droplet size and flow rate. Resistivities and viscosities of this magnitude are typical for paint formulations.
An important parameter governing the VMD of the spray droplets is the potential applied to the liquid emerging at the nozzle outlet. The higher the potential applied, the greater the acceleration of the ligament away from the nozzle and the smaller the diameter of the resulting ligament. However, for liquids having a resistivity of the order of 5.times.10.sup.6 ohm.multidot.cm, as the applied potential increases, spurious spraying effects arise which are probably attributable at least in part to corona discharge taking place as the electric field in the vicinity of the nozzle outlet intensifies. The nature of these effects can vary from one nozzle to another but, in general, the spray becomes poorly divergent and polydisperse and is wholly unsatisfactory for many spraying applications, particularly the coating of paints onto substrates.
Flow rates of the order of 4 cc/min or higher can be achieved by providing for forced feed of the liquid to the nozzle outlet (as opposed to a passive feed such as gravity feed or a capillary action as disclosed in for instance EP-A-120633). Forced feed can be achieved in various ways, for instance by means of a propellant gas as disclosed in EP-A-441501 in which a so-called barrier pack is used, or by means of user-applied pressure as disclosed in EP-A-482814.
From EP-A-441501, it is known to provide a focusing shroud of electrically insulating material adjacent the nozzle outlet in order to permit focusing of the spray. From EP-A-501725, it also known to provide a shroud component of electrically insulating material encircling the nozzle with the aim of modifying the potential gradient in the immediate vicinity of the nozzle outlet so as to facilitate the spraying of liquids having resistivities lower than 1.times.10.sup.6 ohm.multidot.cm. In both cases, during spraying a voltage is established on the shroud component which is of the same polarity as the voltage produced at the nozzle outlet, that voltage being established as a result of charge collecting on the shroud in the course of the spraying operation.
It is also known from prior U.S. Pat. No. 4,854,506 to provide an electrostatic spraying device in which an electrode is mounted adjacent to the spraying nozzle and in which an electrical potential is applied to that electrode so as to develop an intense electrical field between the liquid emerging at the nozzle and the electrode. The electrode comprises a core of conductive or semiconducting material sheathed in a material of semi-insulating material having a volume resistivity of 5.times.10.sup.11 to 5.times.10.sup.13 ohm cm and a dielectric strength greater than 15 kV/mm for the purpose of allowing a higher potential to be maintained between the nozzle and the electrode. The potential applied to the electrode may be of the same polarity as the potential applied to the
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Green Michael Leslie
Jefferies Andrew
Noakes Timothy James
Prendergast Maurice Joseph
Daniels, III John F.
Evans Robin O.
Imperial Chemical Industries plc
Kashnikow Andres
Sheehan John M.
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