EP0260853A2

EP0260853A2 - Spraying process

Info

Publication number: EP0260853A2
Application number: EP87307875A
Authority: EP
Inventors: Robert Arthur Head; Timothy James Noakes
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1986-09-15
Filing date: 1987-09-07
Publication date: 1988-03-23
Also published as: EP0260853A3; ZA876791B; AU7830687A; IE872402L; JPS63119864A; GB8622144D0

Abstract

A process for the discontinuous spray application of a fluid comprising supplying such a fluid to a sprayhead (12), applying a controllably variable electrical field to the fluid such that at controlled field intensities the fluid can move from the sprayhead under the influence of the field to form a spray, effecting relative movement of the sprayhead (12) and a target (2), and switching the spray on and off, characterised in that the field is applied by means of constant potentials applied to the sprayhead (12) and a charged field adjusting electrode (20) and spraying only occurs when the target (2) is a desired distance from the sprayhead, and apparatus therefor.

Description

This invention relates to apparatus and a process to the application of inter alia adhesives and surface coatings, in particular reactive adhesives and coatings by electrostatic spraying, and to production lines utilising such application methods.
In known electrostatic spraying liquid introduced via a charged spray nozzle into a relatively intense electric field produces charged fibres or droplets which tend to move towards earth and these may be collected on a suitable substrate which may be a field electrode or a target other than the field electrode.
From UK 1,569,707 it is also known to place an earthed 'field intensifying electrode' closely adjacent to the sprayhead, which may advantageously be used to increase the field intensity at the sprayhead totally independently of any (generally earthed) target of the spray and in otherwise conventional electrostatic spraying of non-reactive materials, and in particular to encourage spray formation at lower than conventional sprayhead operating potentials.
The industrial application of adhesives and surface coatings, for example, in production lines for packaging, parts assembly (such as shoes and aero and boat frames) or surgical dressings and drapes, generally requires accuracy and economy of application, which necessitates contact application methods.
Additionally, known production lines need relatively high throughput rates for economic operation, requiring fast-set/cure adhesives or coatings, which for energy economy preferably set and/or cure at room temperature. Such necessary fluid systems often react too rapidly to be contact applied. The applied material is generally a single fluid precursor of an adhesive or surface coating which reacts with itself, after external activation, but the problem is even more acute for mutually reacting multicomponent systems which are so sprayed, where reaction starts on mixing.
Electrostatic spraying can combine the accuracy of contact application with the speed of spray application. This method is especially useful for rapidly reacting room temperature systems, especially where such systems react too rapidly to be applied by contact methods.
However, accuracy and economy of application on a high speed piecework production line requires rapid switching of the spray on and off.
Further, in some other applications of spraying the supply of targets to the spraying zone is totally random with long time intervals between targets, yet reasonable accuracy of and thrift in application is desirable (eg the spraying of insect or human intruders). Again, this requires rapid switching of the spray, but also that switching is actuated by arrival of the target and preferably not by contact with the target.
The present invention is accordingly concerned with a requirement to switch an electrostatic spray on and off fairly rapidly. This could be achieved at low rates by controlling the supply of liquid or by switching the high voltage generator. However, both possibilities would present problems if the switching transition were required to be very rapid, say of the order of milliseconds. High voltage generators, for example, usually generate a high voltage from rapidly switching a current in the primary of a step up transformer. The high voltage pulses which result in the secondary are rectified and smoothed to produce a direct output. Since the output is smoothed, there is a relatively long time constant in the output circuit so that the output would not respond to switching the primary circuit to give a rapid transition between on and off. Switching the high voltage output would produce its own problems, and in any case the spray head may well have sufficient capacitance not to respond well to rapid switching of the high voltage, whether that is achieved by switching the primary or the secondary circuit of the generator.
We have now found a solution to such problems, and accordingly the present invention in a first aspect provides a process for the discontinuous spray application of a fluid (which may be an adhesive or a surface coating or a precursor thereof) comprising supplying such a fluid to a sprayhead, applying a controllably variable electrical field to the fluid such that at controlled field intensities the fluid can move from the sprayhead under the influence of the field to form a spray, effecting relative movement of the sprayhead and a target, and switching the spray on and off, characterised in that the field is applied by means of constant potentials applied to the sprayhead and a charged field adjusting electrode and spraying only occurs when the target is a desired distance from the sprayhead.
The potential on the sprayhead and the position of, and the potential on, the electrode is so adjusted that when the target is far distant from the electrode the intensity at the sprayhead of the field between sprayhead and electrode is so low that spraying is suppressed, but on decreasing the sprayhead to target distance, the target affects the electrical field to the extent that at a pre-set position of the target the intensity at the sprayhead of the field between the sprayhead, the (generally earth-potential) target and the field adjusting electrode is sufficient to permit spraying, and the target is sprayed. The pre-set trigger position is generally in the spray 'line of fire', and most, if not all, of the spray will be attracted to impinge on the target; in fact spraying parameters may be controlled as described hereinafter so that essentially all the spray impinges on only a part of the substrate if desired. In practice, even with non- or semi-conductive substrates, (for example cellulosic, plastics or siliceous substrates) the use of a conventional target (field) electrode behind such substrates is not strictly necessary, unless strict control of the spray pattern is desired.
Advantageously, the field intensity for spray production at the sprayhead may be enhanced and the potentials therefor decreased (ceteris paribus) by providing the sprayhead with an edge from which the fluid can form a spray. This is described in further detail hereinafter.
Suitable process parameters are described in detail hereinafter in relation to apparatus for the process.
The fluid may often be an adhesive or surface coating or a precursor thereof. Such a fluid precursor may be any composition which is capable of transforming into the final coating in one or more of the following ways:
It may comprise a solution, dispersion and/or suspension which can undergo solvent loss, a solidifiable melt, and/or one or more components which will react, generally by polymerisation or polycondensation, for example by free radical, ionic or group transfer polymerisation.
If a single fluid precursor, eg of an adhesive or surface coating, the precursor will react with itself after leaving the sprayhead (generally after external activation) to form the adhesive or surface coating. Multiple precursor components will react or continue to react mutually on mixing on or after leaving the sprayhead(s).
Fluids of interest are more fully discussed hereinafter.
Where a single fluid is sprayed in accordance with the present invention only a single sprayhead with a single outlet is necessary. When a plurality of fluids (for example two) is sprayed, this may be achieved either by

a) multiple single sprayheads arranged so that the eg two fluids are mutually contacted on a target surface, or (for adhesives) on separate target surfaces which are subsequently contacted together, or
b) a single sprayhead with multiple feeds either to a single outlet (eg from a mixing chamber) to deliver a single fluid to the outlet, or to multiple outlets to give pre-flight, point-of-flight or in-flight contact of the fluids.

Accordingly, (for the spraying of a single fluid from a single outlet) in one aspect the present invention provides an apparatus for the spraying of a fluid (in particular an adhesive or surface coating or a precursor thereof) comprising a sprayhead with a channel for the fluid communicating with an outlet and means for subjecting the fluid to an intense electrical field such that the fluid is capable of moving from the sprayhead to form a spray under the influence of the field, the means including means for applying a first potential with respect to a target to the fluid and a field adjusting electrode mounted spaced from the sprayhead with means to apply a second potential with respect to the target to that electrode, the field adjusting electrode being dimensioned and arranged and the means for applying the first and second potentials permitting adjustment of those potentials such that spraying only occurs when the target is a desired distance from the sprayhead.
The apparatus is particularly suitable for fast production lines. Accordingly in a favoured embodiment the apparatus also comprises means to move the target with respect to the sprayhead.
As noted hereinbefore in respect of the process of the present invention the sprayhead desirably has an edge from which the fluid can form a spray.
The sprayhead may have any of the forms conventionally used for the atomisation of a fluid in an electrostatic field. Thus at the end of the feed channel the sprayhead may have an outlet in the form of a nozzle orifice (generally circular), an annular slot (for example formed by a circular aperture with a concentric core) or a rectangular slot. Such slots may be straight, or curved convexly or concavely, along their length. All these apertures are preferably of capillary cross dimensions, eg 0.05 to 0.5 mm, to assist atomisation. Similarly, as noted above, it is preferred that the apertures are near or defined by sharp edges, which may additionally be toothed, so that the field intensity at the sprayhead surface is advantageously enhanced, and hence the atomisation is enhanced, and the first potential for spraying reduced. This may conveniently be achieved by having a slot outlet at or near the apex of a sphenoidal (ie wedge-shaped) outlet edge, or a circular orifice or annular slot outlet at or near the apex of a conical outlet face, of the sprayhead. Suitable bevelled faces of the edge and of the cone are as further described for the inner components of corresponding sprayheads for a plurality of fluids, and in the description of a specific embodiment of the apparatus, hereinafter.
A favoured sprayhead comprises an open elongate channel, and an outlet for the fluid which is one longitudinal lip of the channel, such that in use fluid passed (eg pumped) along the channel overflows, and moves from, the lip under the influence of the applied electric field to form a spray. Again, to enhance atomisation it is preferred that the lip is near or defined by a sharp, preferably toothed, edge. Suitable such sprayheads are further described with regard to a specific embodiment of the apparatus hereinafter.
The present invention avoids the need to interrupt rapidly the potential applied to the sprayhead in order to switch the spraying on and off. However, it is also desirable that it avoids the need to interrupt or vary rapidly the supply of fluid to the sprayhead. The supply rate may be held constant and the problem of oversupply in the absence of spraying minimised by maximising as far as is feasible the rate of movement of a succession of targets from a rest position to a 'trigger' position, ie to minimise the non-spray time. This is of course a feature of the optional means to effect target movement recited hereinbefore.
Such oversupply, however, is mainly a problem only when the excess fluid drips from the sprayhead. This can be minimised, where the sprayhead has an edge, by so shaping the sprayhead at or near the edge that it tends to accumulate a bead of fluid when there is no spraying between successive targets. For example, the sprayhead may have a groove in an external or internal surface over which the fluid is capable of flowing, where the bead can accumulate and be discharged in subsequent spraying. The groove may conveniently run alongside and equidistant from the edge itself.
Oversupply itself may be minimised in the foregoing sprayheads by providing means for continuous circulation of the fluid through the sprayhead in competition with, but balanced by routine trial against, fluid sprayed via the outlet. For example, the channel in the foregoing sprayheads may communicate between the outlet and a part of a circulation loop eg a gallery, or in the case of the open channel sprayhead may be part of such a circulation loop. The latter type of sprayhead is preferred for this reason.
Recirculation is of course generally unsuitable for a fluid which is a mixture of mutually reacting components.
Where the speed of supply interruption (eg pump shutdown) is not critical, for example during a prolonged interruption in a succession of targets, the supply may be interrupted and conveniently such interruption may be automated. For example, the current in the means for applying the first potential (eg the low voltage input of a high voltage generator) may be sensed and eg a pump automatically shut down when a prolonged absence of current is sensed, indicating a prolonged cessation of spraying.
As noted hereinbefore, a plurality of fluids may be sprayed from a single sprayhead (eg the components of a multicomponent adhesive or surface coating precursor), each fluid being fed to a separate outlet, the outlets being arranged such that good mixing of the two or more components is achieved on or whilst moving from the sprayhead. Again a sharp, optionally toothed edge, defining or near the outlet apertures is preferred, since it advantageously enhances the field intensity at the sprayhead surface.
In all other respects the apparatus for spraying a plurality of fluids from a multiple outlet sprayhead is the same as that for spraying a single fluid from a single outlet sprayhead. Such an apparatus will now be defined and the sprayheads described before a description of common features of both apparatus.
Accordingly, in another aspect, the present invention provides an apparatus for the spraying of a plurality of fluids (in particular the precursor components of a multicomponent adhesive or surface coating) comprising a sprayhead with a plurality of channels for said fluids each communicating with an outlet, and means for subjecting the fluids at the outlets to an intense electrical field, such that the fluids are capable of moving from the sprayhead under the influence of the field to form a spray, the outlets being so disposed that the fluids mix on, or whilst moving from, the sprayhead, in which apparatus the means include means for applying a first potential with respect to a target to at least one of the fluids and a field adjusting electrode mounted spaced from to the sprayhead with means to apply a second potential (with respect to the target) to that electrode, the field adjusting electrode being so dimensioned and arranged, and the means for applying the first and second potentials permitting adjustment of those potentials such that spraying only occurs when the target is a desired distance from the sprayhead.
When a plurality of fluids is sprayed from such a single sprayhead with multiple feeds, these feeds may conveniently end in closely adjacent orifices or slots (for example parallel rectangular slots) or a closely adjacent coaxial circular orifice and annular slot(s) or similarly coaxial annular slots. Mixing of the components occurs at the point of flight, and/or during flight.
As for single sprayheads, sharp, optionally toothed, edges are preferred, and may be advantageous.
Where the feeds end in closely adjacent parallel slots, the feeds are conveniently in the form of channels between parallel plates.
Good mixing of two components is achieved where such a sprayhead comprises four such plates, two inner and two outer, defining three channels. Suitably, the plates are symmetrical about the central channel, the edges of the plates by the outlet slots are chamfered towards the central outlet, and the central outlet is located downstream of the two outer outlets, to give a broadly sphenoidal (ie wedge-shaped) outlet edge of the sprayhead.
Suitably the included angle of the chamfers or bevels on the inner plates is less than the included angle of the bevels on the outer plates, being respectively preferably in the ranges 10 to 60° and 80 to 150°.
In use one component fluid is run down the central channel, and the other is run down the two outer channels.
A three-plate two-channel sprayhead may also be used, with the central plate having two opposing bevels as described above for the two inner plates, and the outer plates are chamfered as described above for the ouer plates; thus the sprayhead is also generally sphenoidal.
The edges of the plates may be straight, or convexly or concavely curved, along their length.
Similarly, where for two components the feeds end in a closely adjacent circular orifice and a coaxial annular slot, these feeds are conveniently in the form of a central tube bore and a channel defined by that tube and a coaxial outer tube interior. Again, good mixing is achieved when the outlet ends of the tubes are symmetrical about the common tube axis and chamfered towards the central outlet and the central outlet is downstream of the outer outlet, to give a broadly conical outlet face of the sprayhead.
Suitable and preferred included angles of the bevelled edges of the tubes are as described for the analogous plates hereinbefore.
In use one fluid component is run down the central channel, and the other is run down the outer channel.
A single liquid may also be sprayed by any such foregoing multiple outlet arrangement, either by supplying the same liquid to all outlets, or by not supplying liquid to one or two outlets as appropriate.
Oversupply and dripping in the absence of spraying may be minimised for multiple fluids as described for corresponding single outlet sprayheads hereinbefore; if recirculation is employed for mutually reactive multicomponent fluids, these should of course be kept separate.
All the foregoing sphenoidal or conical sprayheads whether with single or multiple outlets are also described hereinafter with reference to specific embodiments of the present invention, although neither the foregoing or later description is to be taken as limiting the scope of the present invention. Any conformation of sprayhead capable of producing a spray which is capable of being regulated in conjunction with a field adjusting electrode and a target in accordance with the present invention may be used in the apparatus of the present invention, and to produce spray droplets (eg of adhesives and surface coatings precursors) in the process of the present invention.
The orientation of any of the above sprayheads in use is in general not critical.
In all the foregoing sprayheads (single and multiple) suitable means for subjecting the fluids at the outlet(s) to an electrical field include a chargeable conducting or semi-conducting (eg metal) electrode in the sprayhead in contact in use with at least one of the fluids (and preferably both or all) either at the outlet(s) (for example a conducting or semi-conducting sprayhead outlet surface) or a short distance upstream thereof (for example as a buried electrode within a non-conducting sprayhead).
It is often convenient for such a charging electrode to be at the location(s) where the fluid(s) leave the sprayhead, for example at an edge, if present. If the electrode is spaced from the location(s), spraying will not take place at all if that spacing is above a ceiling value which varies inversely with the resistivity of the fluid(s) being sprayed. This ceiling value may be readily determined by routine trial. This limitation is less of a problem where the electrical path to the spraying location is of relatively large cross-section, eg in the open-channel 'bath' sprayhead described hereinbefore.
In all the foregoing apparatus any appropriate method of producing and means of applying the desired first potential applied via the charging electrode may be used, for example transformed and rectified mains supplies or a Van der Graaf or other high voltage generator.
Conventional insulation precautions must of course be taken for all points at other than earth potential.
The field adjusting electrode may be placed in any position relative to the sprayhead and may be of any nature and have any conformation compatible with its function. Within these wide limits, the electrode is often placed generally downstream of the emerging spray often to be roughly equidistant or further from the sprayhead than the general trigger position of the target.
Preferably the electrode is adjustably mounted to permit ready variation in relative positions of the sprayhead, electrode and target trigger position.
Typical sprayhead-electrode distances (and hence sprayhead-target trigger distances) lie in the range of 10 to 100 mm for targets of the order of size of domestic cartons or components thereof, and may be for example 15 to 50 mm. Larger distances, and hence necessarily higher spraying potentials, may of course be used or may be necessary for larger targets. However, it must be appreciated that at greater distances the risk of other objects in or coming into the sprayhead environment becoming unwanted competing targets for spraying increases.
Amongst suitable forms, the electrode may be or be part of an extended shield at or beyond the target trigger distance from the sprayhead and arranged to allow the target to move to the trigger position. For example, it may be or may be part of a flat or convexly or concavely curved or dished plate; and it may eg have a slot or other opening through which the target can protrude to the trigger position, it may be resiliently deformable or pivoted against a bias so that it can be pushed aside by the moving target and resile or be biased to its quenching position as the sprayed target moves on, or it may be continuous and fixed and the apparatus then so arranged that in use the target is moved between the sprayhead and electrode to the trigger position. However, on all the foregoing any shield-like structure is not essential.
The electrode itself may be of a conductive or semi-conductive material, eg a metal. In such cases, where the electrode is part of a larger integer as described above it may be mounted on a semi-conductive or insulative material, eg a plastic. Thus for example when it forms part of a shield plate with a slot, it may conveniently be in the form of two metal strips running along the edge of the slot in a plastics plate.
It is often desirable that the electrode conforms to the shape of the sprayhead outlet or edge insofar as compatible with movement of the target, since this tends to favour even field intensities over the outlet or edge and to ensure full quenching. Thus the electrode may be a single flat plate, bar, strip or edge or twin parallel bars, strips or edges by a straight edge or slot, or a flat or dished disc, or an annulus or torus by a circular nozzle or annular slot.
The electrode may conform correspondingly for a convexly or concavely curved slot.
As noted hereinbefore multiple single-output sprayheads may be arranged to deliver multiple fluids to the same or different target surfaces (eg the same or opposite faces of a flap). If desired the same arrangement may be used for multiple deliveries of the same fluid, or the same arrangement of multiple-outlet sprayheads may be used to deliver the same multi-component fluid.
In all such cases the sprayheads may be so arranged that each and/or the charging electrode in each is a field adjusting electrode for at least one other sprayhead. In the simplest case, of two sprayheads and/or charging electrodes (hereinafter 'sprayheads') acting as mutual field adjusting electrodes, it is preferred (as above) that they conform mutually eg two identical or mirror-image, straight-edge sprayheads will be arranged as mutual mirror-images with the edges parallel.
For all the foregoing conductive or semi-conductive field adjusting electrodes suitable means for applying the second potential to the electrode include transformed and rectified mains supplies or a Van der Graaf or other high voltage generator.
Less favourably, insulative electrodes of the foregoing conformations may also be used. Such an electrode may be charged before mounting in the apparatus, or the second potential may be applied by mounting and spraying it in the present apparatus until it acquires sufficient second potential from the spray to quench the spray in the absence of a target. However, in the long term the electrode charge will tend to leak away and cannot readily be restored. This will tend to give rise to a deteriorating switching action.
Similarly, a permanently charged electrode surface may be provided by an electrode made of an insulating material with permanent macro-scale charge separation (an electret material).
Typically, in use the sprayhead will be at a first potential with respect to the target, and the electrode (which may be a second sprayhead as hereinbefore described) will be at a second potential which is the same as, or is of the same polarity as, but less than, the first potential. When spraying a target at earth potential, in general each potential will be in the range of ±20 to 50 KV. Where the field adjusting electrode is not a sprayhead, the first potential will often be ±25 to 50 KV and the second potential may then be ±20 to 40 KV. Where the electrode is a second sprayhead both potentials will often be the same.
Within the foregoing range of parameters, the desired trigger position of the target (such that the fluid only moves from the sprayhead under the field when the target is at that position) may be controlled by adjusting the position and dimensions of the field adjusting electrode with respect to the sprayhead and the target, and the first and second potentials. To some extent the effect of adjusting these parameters to achieve the desired effect is a matter of routine trial.
Atomisation and/or effective spray deposition in the present process may in some instances be assisted by a gas stream with (or less usually across) the issuing spray. In some cases this stream may have further advantages, for example, if heated, as a thermal initiator, or for bearing a dispersion of a catalyst or a catalytic initiator and/or promoter, for some free-radical curing adhesives and surface coatings.
Under the effect of the electrostatic field, the spray droplets (eg of adhesives and surface coatings precursors) produced in accordance with the present invention tend to produce layers on the target which are essentially pin-hole free and without occluded air pockets. This is especially useful both for achieving a desirable adhesive layer which is of good adhesion to maximum target surface area and free from blemish, and for producing thick, pin-hole free barrier coatings, eg anti-corrosion and insulation coatings.
The process of the present invention is versatile in that adhesives films as thin as about 1 micron may be applied rapidly, as well as thick surface coatings (such as high build coatings) in excess of 1 mm thick. Similarly, the enabled used of rapid-reacting systems also facilitates the production of high build coatings without sag.
The process of the present invention may be used in application to a wide variety of substrates (including those which may be so treated by contact or conventional spraying methods) to the extent they are compatible with any given adhesive or surface coating. As noted hereinbefore the process is especially useful in connection with continuous piecework production lines especially for the versatile application of thin adhesives films or thick or thin surface coatings to piece targets. Examples of target substrates include the following,
packaging, eg paper, board, wood, and plastics film and sheet;
sheeting, (including sheeting for processing to packaging, laminates, shoe soles, aero skins and boat hulls, and such processed items), eg paper, board, plastics, metals.
textiles, (as pieces, in particular precursor components of garments)
structural members, (including shoe uppers, stiffeners and frames for packaging and for laminate or uniform-composition surfaces eg for aero and boat frames) eg plastics, metal and board.
It will be apparent that the sprayhead(s) and the target may be stationary relative to one another during the spraying, or (any of) the sprayhead(s) and the target may move relative to each other during the spraying. Any such movement may be continuous or discontinuous and for only part or all or in the absence of the spraying process.
As noted hereinbefore the process is especially useful where such movement is continuous during and in the absence of the spraying process.
It is often convenient to move the target past (a) stationary sprayhead(s), especially where the movement is continuous, although it may be useful for example to move each piece on a production line to a spraying position, and thereafter to move (a) sprayhead(s) for application to different parts of the piece, especially for application of an adhesives precursor in a specific pattern to (large) sheeting for further processing.
Whether apparatus is used in which a single sprayhead applies a single fluid or a multiple-feed sprayhead applies several fluids, several such apparatus may be used synchronously on different parts of the same target, or on different targets. It will be apparent that a plurality of single sprayhead apparatus may be arranged in this way to effect mixing of a plurality of fluids on the target by a process comprising sequential spraying of single fluids to the same area, ie whilst moving a given area from one spray to another, or the case of adhesives by separately spraying two target surfaces which are subsequently contacted together.
However, we have found that mixing in flight generally gives far more satisfactory mixing of the components leading to more homogenous cure and thus better quality adhesion and surface coating.
The apparatus may also comprise one or more target (field) electrodes to further influence the spray pattern.
Accordingly in a further aspect, the invention provides a process of the present invention characterised in that the distribution of the electrostatic field is controllably modified by at least one target electrode.
The electrode will generally be discrete and move with the target, and may also enhance its 'triggering' effect, and, as described further below, may be used at such a potential that it effectively becomes the target and enables the use of sprayheads and field adjusting electrodes at lower potentials with respect to target substrate (earth) potentials.
The shape and location of and relative potential on the target electrode(s) all affect the spray pattern. A variety of patterns may be achieved in this way, and accordingly these parameters may vary between wide limits, depending as they do on the same parameters inter alia as do the field potential and spraying distance (q.v.), and suitable values may similarly be readily determined by routine trial.
In one of it simplest forms a target electrode may be a conventional conductive target (field) electrode conveniently moving with, and behind, the target, and at the same (eg earth) potential as the target or (more usually) at a greater potential difference with respect to the sprayhead than is the target. Sprayhead-target electrode potential differences may suitably be ±25 to 40 KV (as for sprayhead-target differences given hereinbefore). However, the corresponding target-substrate target electrode may vary between wide limits, but for example 0 to 10 KV, so that sprayhead- target substrate-target electrode potentials may vary widely, but for example respectively, ±15 to 40 - 0 - ± 10 to 0 KV, with a corresponding optional reduction in field adjusting electrode potential to earth. This may be useful in reducing problems of discharge to earth associated with potentials at the higher end of the given ranges.
Such an electrode will tend to focus the spray into a pattern on the substrate which tends to conform to the shape of the electrode. Relatively complex deposition patterns may be achieved. Thus such a target electrode or each in a pattern of several such target electrodes may be in any suitable shape, and the pattern of electrodes may be so spread apart and positioned, that the distribution of the electrostatic field is modified by the charged electrode(s) to achieve the desired deposition pattern.
In some cases such electrodes will tend to further accelerate the spray droplets towards the target, and may enhance adhesion of the product coat to the target. The target electrode may be run off an inductive or resistive tap off the sprayhead supply, or vice versa, or off a separate supply, and may be provided with means to hold it at a single potential or to vary and hold it at different potentials at will.
Target electrodes may also be in a relatively complex pattern, parts of which can be controllably activated to vary the deposition pattern as required or according to varying relative size or shape of the target in the present apparatus. This may be achieved by switching appropriate electrodes by contacting sensors with a template or the surface to be treated. Such a method is particularly useful for complex and variable deposition patterns eg on shoe bodies, aero and boat frames or in 'printing' by spraying.
A wide variety of adhesives and surface coatings may be applied in accordance with the present invention (including application to substrates mentioned hereinbefore). These are generally sprayed as room temperature liquids which are, or are solutions, dispersions or suspensions of, reactive polymers or polymer precursors as convenient. Any solvent or vehicle may be inert or it may react with the reactive component(s) of the sprayed fluid.
Generally, the low shear rate viscosity at the spraying temperature of the sprayed fluid(s) should be less than 40 poise, for example between 0.5 to 5 poise although it is a surprising feature of the present invention that fluids with viscosities in excess of 5 poise (and even in excess of 30 poise) may be satisfactorily sprayed to give good coats. Similarly, fluids of a wide range of resitivities may be sprayed, for example (10⁶ to 10¹⁰) ohm cm.
The spraying temperature of the fluid(s) may be any compatible with the fluid(s) or its/their reaction rate. Temperatures from 0 to 80°C for example 15 to 50°C may be used.
Among multi- (generally two-) component systems which may be applied as separate fluids from multiple single sprayheads or as a single fluid from a single multiple-feed sprayhead are adhesives and surface coatings, which include barrier coatings (eg anti-corrosion coatings for ferrous metals), high-build coatings and mould gel coatings for injection or cold press moulding and inks and paints, the last named especially where target electrodes can be controllably activated to vary the deposition pattern as required.
One class of materials includes both adhesives and surface coatings based on known polymers or polymer precursors which undergo reaction after leaving the sprayhead(s) or their feeds to a sprayhead, typically by free radical, ionic or group transfer polymerisation or by condensation. Surface coatings of this type include barrier, high-building and mould gel coatings as mentioned hereinbefore.
This class of adhesives and surface coatings includes acrylics (which are favoured rapid-reaction materials typically with setting/curing/ gelling times at ambient temperatures of less than 30 sec and in some cases less than 10 sec), unsaturated polyester-olefinic/acrylic copolymers, polyurethanes and polyureas, heterocycles which include epoxides and lactams, and thiolenes.
The above-mentioned sub-classes of materials include the following:
Acrylics:
'Vinylic acids': ie monomers and oligomers (including co-oligomers) of unsaturated carboxylic acids, which thus include acrylic, methacrylic and itaconic acids.
Acrylic esters: monomers and oligomers (including co-oligomers) of mono- and polyols esterified with vinylic acids. These include:
esters of alkoxy- and aryloxy-alkanols, eg ethoxy- and phenoxy-ethanol;
mono- and poly- esters of diols, eg ethylene, propylene and hexylene glycols;
mono- and poly- esters of mono- and poly-alkoxylated diols and triols, eg the foregoing ethoxylated with 1-3 oxide residues.
Polyester acrylics: polyesters with or without olefinic unsaturation in the polymer backbone, typically M Wt 200-5000 with terminal vinylic acid ester units capping terminal hydroxy groups. These include the following polyester backbones: straight chain alkanediol-alkanedioic acid esters, eg adipic or terephthalic - ethylene/propylene/hexylene glycol/Bisphenol A diol polyesters; hydroxyalkanoic acid oligomers, eg caprolactone and hydroxybutyric acid oligomers; and polycarbonates, eg biphenylene polycarbonates.
Urethane acrylics: polymer backbones having terminal isocyanate groups capped with hydroxy-substituted vinylic acid ester units, eg hydroxy-ethyl/-propyl/-hexyl vinylic acid esters. These include the following polymer backbones:
di- or tri- functional aliphatic (including alicyclic) and/or aromatic isocyanates, biurets or isocyanurates, or similar oligomers of such isocyanates, eg bi/tris(isocyanato-phenyl/cyclohexyl) methane.
polyurethanes from the above with diols, eg ethylene/propylene/hexylene glycol(s); polyalkoxylated derivatives thereof; and/or polyester diols as for polyester acrylics above.
Any of the foregoing may be copolymerised with hydroxyamines or amines to give a ureido-substituted backbone.
Epoxy acrylics: polyether backbones with terminal vinylic acid ester groups as for polyester acrylics above. These include polymer backbones based on Bisphenol A at least bis-alkoxylated (eg ethoxylated).
Functionalised acrylics: eg copolymers of vinylic monomer species, with hydroxy-substituted vinylic acid ester groups, (eg hydroxy- ethyl/-propyl/-hexyl vinylic acid esters) with vinyl acid ester units capping the hydroxy groups or with epoxy groups. Suitable monomer species include:
unsaturated esters in which the unsaturation may be in the acid or alcohol component, eg vinylic acid esters, also including crotonates, such as methyl/butyl acrylate/methacrylate, and vinyl/alkyl esters, such as vinyl acetate; and vinylic acids themselves.
The backbone may contain other monomer species such as vinyl halides or vinylidene dihalides.
All the foregoing acrylics may suitably be free-radical polymerised using a conventional peroxide (eg benzoyl peroxide) initiator with an amine accelerator/promoter conveniently by feeding (acrylic + initiator) and (acrylic + promoter) from two sprayheads/feeds, or by thermally activating the spray of (acrylic + initiator) in the spray or on the substrate. Group transfer polymerisation using a conventional silicon-based catalyst (eg Me₃SiCN) with a fluoride ion activator may also be used, conveniently by separate (acrylic + catalyst) and (acrylic + activator) feeds. In all the aforementioned acrylics any solvent or vehicle for the monomer or oligomer may be inert, or it may itself contain vinylic or acrylic functions and so be capable of reaction with the dissolved, dispersed or suspended oligomer. Solvents of the latter type include N-vinylpyrrolidone, vinyl acetate and 'vinylic acid' esters of mono- and polyols such as tetrahydrofuranol and ethylene glycol.
Unsaturated polyesters - olefinic/acrylic copolymers:
Copolymers of polyesters having olefinic unsaturation generally in acid residues/with olefins or vinylic acids or esters. Apt polyester components include straight chain alkenediol-alkenedioic acid esters. Suitable acids include maleic, fumaric, itaconic and mesaconic acids. Suitable glycols include ethylene/propylene/hexylene glycols and alkoxylated derivatives thereof and of Bisphenol A. Such polyesters will typically also include saturated comonomers, eg saturated acids, such as phthalic, sebacic and/or adipic acids; and hydroxyalkenoic acid oligomers.
Suitable comonomers include those monomer species described for functionalised acrylics above, and styrene and methylated styrenes.
Polymerisation may be conveniently effected as described for acrylics above by feeding initiator and promoter from separate sprayheads or feeds with any combination of comonomers therewith.
Polyurethane/Polyureas:
Polyisocyanates with polyols and/or (poly)olamines as described for urethane acrylics above, and polyamine analogues of these polyols. These react to give polyurethanes or their polyurea analogues.
Polymerisation of the polyurethane precursors may be effected by feeding polyisocyanate from sprayhead or feed and polyol from another. The necessary catalyst, eg a tertiary amine or a tin compound such as dibutyltin dilaurate may be fed from either sprayhead or feed. Polymerisation of the polyurea precursors may be effected analogously without the need for a catalyst.
Heterocycles (including epoxides):
Heterocycles such as: epoxides, eg epoxyalkylated Bisphenol A derivatives (eg epoxyethylated), glycidyl esters and cycloalkene epoxides; thioepoxides, eg thio analogues of the above; lactones, eg caprolactone; lactams, eg caprolactam; and oxazolines.
Ring-opening and polymerisation of the heterocycle fed from one sprayhead or feed may be catalysed by eg an amine or a Lewis or Bronsted acid (eg BF₃ H⁺) fed from the other.
Thiolenes:
Unsaturated (olefinic) materials such as described for acrylics and unsaturated polyesters, etc, above, reacting with eg (poly)thiols. Virtually any such sulphur species is suitable.
Free radical polymerisation may be effected by feeding a peroxide initiator from one sprayhead or feed and an amine promoter from another, with olefin and/or thiol, etc, from each sprayhead or feed. Free radical polymerisation may be effected by feeding a peroxide initiator from one sprayhead or feed and an amine promoter from another, with olefin and/or thiol, etc, from each sprayhead or feed.
Single component systems which may be applied from single sprayheads include adhesives, and surface coatings which include barrier, high build and mould gel coatings, inks and paints.
Such single component systems include any material

a) which does not react in or after flight, eg solutions of pressure sensitive adhesives, or
b) reacts in or after flight, ie any system where reaction is externally initiated at the sprayhead, in flight or on the substrate.

Within category a) pressure sensitive adhesives include conventional acrylic adhesives of this type based on polymers and copolymers of hydroxyalkyl vinylic acid esters.
Within category b) the external initiation may be radiation curing, ie by radiation such as microwave, uv, visible, ir, electron beam or sonic, or chemical such as in anaerobic adhesives and (eg nitrogen stoving) coatings or by treating the fluid in flight with a catalyst, or catalytic initiator and/or promoter, eg dispersed in a gas mass or stream in contact with the spray.
Radiation curing systems are essentially free-radical curing systems, which may be initiated in or after flight. All the free-radical adhesives/coatings materials described above for multi-component systems may be used, ie acrylics, unsaturated polyester-olefin/acrylics and thiolenes. Radiation curing systems also include monomer or oligomer precursors of the acrylic pressure sensitive adhesives described above; these are generally cured on the substrate.
The single sprayed fluid will contain all the desired monomers and/or oligomers and a conventional catalyst/initiator which itself is initiated by the relevant radiation. For example, suitable uv curing catalysts include benzophenone-amine systems.
Anaerobic adhesives and coatings are generally cured on the substrate.
All the foregoing adhesives (single or multi-component) may also contain conventional thermoplastic components, eg tackifying resins, and surfactive slip agents to promote 'wetting'.
Surface coatings, which include inter alia barrier coatings and mould gel coatings for injection or cold press moulding, may also contain conventional surfactive agents and adhesion promoters insofar as compatible with the function of the coating.
Preferred adhesives and surface coatings, the precursors of which may be sprayed as single or multiple fluids in accordance with the present invention, include materials amongst the favoured acrylics such as adhesives based on acrylic esters, for example monomers and oligomers (including co-oligomers) of mono- and polyols esterified with vinylic acids (as hereinbefore defined). These include: esters of alkanols, eg of 2-ethylhexanol, n-butanol, isobornyl alcohol, mono- and di-esters of diols eg ethanediol, esters of alkoxy- and aryloxyalkanols, eg ethoxyethanol, and mono- and polyesters of polyalkoxylated diols and triols, eg mono- and polyethoxylated neopentylene glycol diacrylate; and urethane acrylates, as hereinbefore defined.
A group of such materials of interest consists of pressure-sensitive adhesives.
One embodiment of the invention, given by way of example only, will now be described with reference to the accompanying drawings, in which:

Figure 1 is a schematic side view of apparatus of the invention, for spraying adhesive onto the flaps of a cardboard carton, eg for soap powder;
Figure 2 is a schematic section on arrows A-A i Figure 1; and
Figure 3 to 5 show in more detail schematic sections through sprayheads suitable for use in the apparatus of Figures 1 and 2. The sprayheads of Figures 4 and 5 (also depicted in Figures 1 and 2) are suitable for two component adhesives precursors which would cure on mixing in a conventional sprayhead to the extent of being unusable. The sprayheads of Figures 3 and 6 are suitable for single component adhesives precursors which are cured after leaving the sprayhead, or (atypically) for premixed multi-component adhesives precursors which do not cure to a deleterious extent in the sprayhead.
Figure 6 is a schematic section (similar to Figure 2) of part of apparatus of the invention, differing from Figure 2 only in the form and position of the sprayhead and field adjusting electrode.

The apparatus illustrated in Figures 1 and 2 is intended for spraying a multi-component fluid adhesive precursor onto a target flap 2 of a cardboard carton 4, eg for soap powder. If fluid is sprayed onto both sides of the flap 2, the cartons may be closed by folding in flaps 6 followed by the flap 2, to which the fluid has been applied, and finally flap 8. A series of such cartons 4, with the flaps 2 sticking straight up, are transported by a conveyor 10, separated by aluminium spacers through an apparatus in accordance with the present invention which comprises two sprayheads 12,12 so arranged that in use they can deliver the fluid to the two opposite faces of the flap 2, and with channels 24a and 24b (in Figures 4 and 5) and means for supplying a first potential to at least one component of the fluid, here fluid-contacting surfaces 18,18 (in Figures 4 and 5) connected to a first high voltage terminal of a high voltage generator; (not shown). The apparatus further comprises a field adjusting electrode 20, here two parallel metal strips mounted spaced from the sprayheads 12,12 with means to apply a second potential to this electrode, here a second high-voltage terminal of a high-voltage generator (not shown). The two parts of electrode 20 are mounted on opposite edges of a slot through an insulative (polypropylene) plate 22, through which slot the flap 2 passes in use. The electrode 20 is dimensioned and arranged, and in use the first and second potentials are adjusted, such that the electric field established at the sprayhead is insufficient to cause spraying, but spraying takes place when the target flap 2 passes between the two parts of electrode 20 to a preset trigger position. Both sides of the flap 2 are thus sprayed with the fluids until the flap 2 moves out from between the electrodes and ceases to actuate the spraying.
It will be seen that the target flap 2 itself effectively switches the spray on and off. High rates of switching, for example spraying rates of 5 to 10 targets per second can be achieved using this embodiment of the invention.
The conveyor 10 is a means for moving the target 2 relative to the sprayheads 12,11 and is part of this favoured embodiment of the invention.
Referring now to various integers of the apparatus of Figures 1 and 2:
The spray-generating electrical field intensity of the sprayheads 12,12 is enhanced by the edges 14,14 on the sprayheads 12,12 adjacent to the outlets 24a and 24b in each sprayhead 12, from which outlets the fluids to be sprayed are delivered to the spraying edge 14.
The width of the spray produced by the sprayhead 12 is rather too narrow to cover all of the flap 2 at one time. Since it is required to apply adhesive to all of the flap 2, each sprayhead 12 extends along the path of travel of the flap 2 and is angled as illustrated (see Figure 1) so that one end of the sprayhead 12 sprays the bottom of the flap 2 and the other end sprays the top of the flap 2.
The sprayheads 12,12 and the two parts of the electrode 20 are respectively identical and mounted as mutual mirror images about a vertical plane of symmetry through the slot. The sprayhead edges 14,14 are parallel. In use the same first potential is put on the sprayheads 12,12 and the same second potential on the two halves of the electrode 20. Each sprayhead 12 is thus quenched equally by the electrode 20, and/or by the other sprayhead 12. If the distance between sprayheads 12,12 is not too great (as described hereinbefore) the electrode 20 and plate 22 may be omitted, and each sprayhead 12 used as an electrode to quench the other. (In each case it will be seen that the or each electrode 20 or 12 conforms generally to the sprayheads 12,12 or the other sprayhead 12 or the edge(s) 14 thereof.)
In use sprayhead 12 - electrode 20 distances of 15 to 30 mm may be used with a first potential of ± 30 KV, a second potential of ± 25 KV, and an earth potential target 2.
Referring to Figure 3, the sprayhead 12 shown in section (which may be used in the apparatus of Figures 1 and 2) has a gallery 21 which may be supplied in use via a supply duct 23 with a single component adhesives precursor or a premixed multi-component adhesives precursor which does not cure so fast as to be unsprayable.
The single component may be a liquid which is converted to an adhesive after spraying as hereinbefore described eg by exposure to ultra violet radiation. The gallery 2 distributes adhesive to a slot 24 at the centre of a spraying edge 14. (Although the slot, naturally, has two sides the electrostatic effect is that of one edge.)
Near the exit from the slot 24 at the spraying edge 14, is positioned a strip of conducting or semi-conducting material, over the surface 18 of which, the adhesive passes on its way to the spraying edge 14.
The conducting or semi-conducting electrode surface 18 is connected to a high voltage output terminal of a high voltage generator (not shown) as described hereinbefore.
Other forms of sprayhead 12 (which are those illustrated in the apparatus of the present invention in Figures 1 and 2) are illustrated in Figures 4 and 5. In these sphenoidal sprayheads 12 each of two components of an adhesives precursor may be supplied through separate supply slots 24a and 24b, upstream of a spraying edge 14, to flow over the exterior sprayhead surfaces 44 to meet only at the edge 14 and mix on spraying from the edge.
The three-piece sprayhead illustrated in Figure 5 has conducting or semi-conducting surfaces 18 upstream of the edge 14 similar to Figure 3. In the Figure 5 arrangement, however, there are two surfaces 18, one in each of the slots 24a and 24b. In another arrangement (not illustrated) the Figure 5 nozzle could be modified to include a third, centre slot, all as described hereinbefore. In this case it is possible to supply a conducting or semi-conducting surface 18 in the centre slot.
The sprayhead illustrated in Figure 4 differs in that the edge 14 is provided as a part of the conducting or semi-conducting surfaces 18, at the tip of the sprayhead.
In all the sprayheads 12 of Figures 2 to 5, the sphenoidal spraying edge may be replaced by a conical spraying face with corresponding changes in the outlet slots as hereinbefore described.
Referring to Figure 6. This shows part of an apparatus broadly similar to that depicted in Figure 2 for spraying a flap 2, but differing mainly in that it is arranged to spray only one face 32 of the flap 2 from a single 'bath' sprayhead 12 arranged level with a field adjusting electrode 20. The sprayhead 12 shown in section has an elongate channel 21 through the length of which in use a single component adhesives precursor (of the type described briefly in relation to Figure 3) may be circulated by pump (not shown) via a supply duct 23 and a drainage duct 25. One lip of the channel 21 forms a transversely protruding, elongate, and toothed spraying edge 14 and the channel 21 arranged as depicted in Figure 6 so that fluid in the channel will tend to overflow the edge 14.
Not far from the spraying edge 14 is positioned a longitudinally extending conductive wire charging electrode 18 in contact with the fluid in the channel 21 in use.
Mounted level, and running parallel, with the spraying edge 14 (and spaced from it) is a cylindrical bar field adjusting electrode 20.
All other components of the apparatus (not shown) and operating parameters are as described in relation to Figure 1.
The apparatus may be used, and functions, essentially as described in relation to Figure 1. A second potential applied to the electrode 20 quenches the spraying of a fluid (circulated through the sprayhead 12) from the edge 14 at a first potential applied via the charge electrode 18 until the flap 2 passes between the sprayhead 12 and electrode 20, and the face 32 of the flap 2 is sprayed with the fluid adhesives precursor. Conveniently the fluid is a radiation (uv) curing precursor for a pressure-sensitive adhesive. Further downstream along the conveyor 10 transporting the cardboard carton 4 (neither shown) the apparatus will be provided with a uv lamp 34 (not shown) arranged to cure the adhesives precursor on the flap face 32.
In use sprayhead 12 - electrode 20 distances of 15 to 30 mm may be used with a first potential of ±40 KV, a second potential of ±35 KV and an earth potential target 2. A sprayhead target distance of 4 to 10 mm, and a sprayhead-electrode distance of 15 to 25 mm are suitable.
As depicted, the sprayhead 12 and edge 14 are level along their lengths, and will typically not spray all the flap 32, but only deposit a 1 to 2 mm wide line on the flap 32. The sprayhead 12 may however be inclined along its length, analogously to the inclination of the sprayheads 23, 23 in Figures 1 and 2. The feasible degree of tilt will vary with the fluid viscosity inter alia, but with suitably thick fluids an inclination of 10 mm or more (eg in a 150 to 300 mm channel) may be effected to give a corresponding 10 mm plus swathe of fluid on the face 32 of the flap 2.
If desired, a corresponding sprayhead 12ʹ and electrode 20ʹ may be arranged just downstream of the sprayhead 12 and electrode 20 to spray the other face 33 of the flap 2, with a uv lamp 34 or pair of lamps 34, 34ʹ arranged to cure the adhesives precursor on both flap faces 32, 33.
Again, if desired, two opposed mutual mirror image sprayheads 12, 12 may be arranged with a 'slotted shield' field adjusting electrode 20, all as in Figures 1 and 2 to spray both faces 32, 33 of the flap simultaneously. The electrode 29 may also be omitted as in Figures 1 and 2.

Example

An apparatus as described in relation to Figure 6 was set up, comprising a motorised conveyor to transport empty cardboard carton targets past the sprayhead, to be sprayed, and subsequently past a UV lamp in a UV oven, to cure the spray deposited on a flap on each target. The sprayhead channel was 150 mm long and one lip, nearest the target spraying position bore a toothed edge.
The apparatus was used to spray the target flaps with a liquid precursor to a pressure-sensitive adhesive.
The liquid comprised a polymer of n-butyl acrylate dissolved 60% w/w in 2-ethyl hexyl acrylate and hydroxy ethyl acrylate, with about 4% of a conventional photoiniiator (Iragacure 651) added. Its viscosity was about 10 Poise, and its resistivity about 10 ⁷ ohm.cm.
The liquid was pumped continuously from a reservoir through the sprayhead channel by a motorised gear pump, unsprayed liquid being returned to the reservoir via an outlet from the sprayhead channel.
Each target (at earth potential) was driven continually past the sprayhead edge at about 1m/sec. The sprayhead potential was set at about 40 KV and that of the field adjusting electrode at about 35 KV (both with respect to earth).
The distances between the edge and the target, and between the target and the electrode, were about 5 and 15 mm respectively.
A deposit of adhesive precursor was sprayed onto the target flap to form a strip about 50 microns thick, and 4 mm wide, the spray starting and stopping as described above solely due to the presence or absence of the target near the sprayhead.
The deposit was cured by exposure to the UV light, its passage through the oven tunnel taking about 1 second. Each carton was removed, and the flap bearing the adhesive was bent down and pressed onto the corresponding part of the carton body, whereupon a secure adhesive bond was formed.

Claims

1. A process for the discontinuous spray application of a fluid comprising supplying such a fluid to a sprayhead, applying a controllably variable electrical field to the fluid such that at controlled field intensities the fluid can move from the sprayhead under the influence of the field to form a spray, effecting relative movement of the sprayhead and a target, and switching the spray on and off, characterised in that the field is applied by means of constant potentials applied to the sprayhead and a charged field adjusting electrode and spraying only occurs when the target is a desired distance from the sprayhead.

2. A process according to claim 1 wherein the fluid is one of a plurality of fluids supplied to separate outlets in the sprayhead.

3. A process according claim 1 wherein the sprayhead has an optionally toothed edge from which the fluid can form a spray.

4. A process according to claim 1 wherein the fluid is supplied continuously to the sprayhead.

5. An apparatus for the spraying of a fluid comprising a sprayhead with a channel for the fluid communicating with an outlet and means for subjecting the fluid to an intense electrical field such that the fluid is capable of moving from the sprayhead to form a spray under the influence of the field, the means including means for applying a first potential with respect to a target to the fluid and a field adjusting electrode mounted spaced from the sprayhead with means to apply a second potential with respect to the target to that electrode, the field adjusting electrode being dimensioned and arranged and the means for applying the first and second potentials permitting adjustment of those potentials such that spraying only occurs when the target is a desired distance from the sprayhead.

6. An apparatus according to claim 5, wherein the sprayhead comprises an open elongate channel, and an outlet for the fluid which is one longitudinal lip of the channel, such that in use fluid passed along the channel overflows and moves from, the lip under the influence of the applied electric field to form a spray.

7. An apparatus according to claim 5, wherein the sprayhead has means for continuous circulation of the fluid through the sprayhead during an interrupted spraying process.

8. An apparatus according to claim 5, wherein the sprayhead has an optionally toothed edge from which the fluid can form a spray.

9. An apparatus for spraying a plurality of fluids, comprising a sprayhead with a plurality of channels for said fluids each communicating with an outlet, and means for subjecting the fluids at the outlets to an intense electrical field, such that the fluids are capable of moving from the sprayhead under the influence of the field to form a spray, the outlets being so disposed that the fluids mix on, or whilst moving from, the sprayhead, in which apparatus the means include means for applying a first potential with respect to a target to at least one of the fluids and a field adjusting electrode mounted spaced from the sprayhead with means to apply a second potential (with respect to the target) to that electrode, the field adjusting electrode being so dimensioned and arranged, and the means for applying the first and second potentials permitting adjustment of those potentials such that spraying only occurs when the target is a desired distance from the sprayhead.

10. An apparatus according to claim 5 or 9 which also comprises means to move the target with respect to the sprayhead.