1. Field of the Invention
This invention relates to the electrostatic spraying of liquids in such a way that the liquid is initially projected from a spray head in the form of a ligament which thereafter breaks up into droplets under the influence of Coulombic forces to produce an atomised spray. Electrostatic spraying of this type is well known and is described in for example our prior British Patent No. 1569707.
2. Description of the Related Art
In conventional ligament mode spraying, it is widely recognised that liquid resistivity is vitally important to securing satisfactory atomisation and that aqueous and other liquids which have relatively low resistivities become more and more unsuitable for use in ligament mode spraying as resistivity reduces below 1.times.10.sup.7 ohm cm.
Although not limited thereto, the present invention is particularly concerned with the spraying of relatively low resistivity liquids such as aqueous, alcohol and aqueous/alcohol based liquids commonly used in personal care products such as deodorants, anti-perspirants, scents and hair sprays. In the past, many such products have been marketed as aerosol products in which a propellant is used to cause atomisation of the liquid into fine droplets typically less than 50 microns in diameter.
However, because of the currently perceived environmental problems associated with the propellants conventionally used in aerosols, attention has turned to alternative methods of dispensing personal care liquids. Electrostatic spraying offers one alternative approach, but, where the ingredient to be dispensed is combined with an aqueous and/or alcohol carrier (or other relatively low resistivity liquid), current wisdom suggests that, with practical flow rates (typically several cc/min), such carriers will not allow dispensing of the product as droplets with a size range comparable to that attainable with aerosol sprays.
EP-A-152446 discloses a device for the electrostatic spraying of aqueous liquids and explains that, for reasons not completely understood, satisfactory atomisation of aqueous formulations can only be achieved at flow rates that are undesirably low for many purposes and ligamentary formation is not obtained with aqueous liquids. EP-A-152446 proposes the use of a corona discharge needle electrode assembly in the vicinity of a sprayhead including a narrow metal tube having a diameter of 400 microns, the arrangement being such that the electrode assembly is symmetrically disposed about the emerging liquid and produces ions which bombard the liquid so that the liquid assumes a stable ligamentary form. It is stated that the illustrated embodiment produces droplets having a volume median diameter of 10 to 50 microns. For personal care products and like products for domestic use, it is considered undesirable to locate an assembly of needle electrodes in the vicinity of the outlet of the device both from an aesthetic standpoint and also in terms of the risk of potential electrostatic shock.