WO1996034151A1

WO1996034151A1 - Particle distributor

Info

Publication number: WO1996034151A1
Application number: PCT/GB1996/000974
Authority: WO
Inventors: Michael Philip Ridley
Original assignee: R.S. Clare & Co. Limited
Priority date: 1995-04-28
Filing date: 1996-04-25
Publication date: 1996-10-31
Also published as: AU5403996A; GB9508627D0

Abstract

A device for discharging particles onto a receiving surface comprises a launching surface (40, 47), particle supply means (10, 31) for dispensing particles towards the launching surface at a controlled rate, and means (46, 58) for supplying a flow of gas across the launching surface, wherein, in use particles are accelerated by the flow of gas and thereby projected onto the receiving surface.

Description

TITLE: Particle Distributor

DESCRIPTION

The present invention relates to a device and a method for embedding particles in a surface. The said device and method are particularly, but not exclusively, suitable for embedding reflective particles in a highway marking.

In order to improve the retroreflectivity of paint or thermoplastic markings on highways, it is known to embed glass particles or spheres in the markings. The particles or spheres are distributed over, and embedded in, the marking after it is laid but before it solidifies.

This distribution and embedment of glass particles or spheres on thermoplastic or paint highway surface markings generally utilises a compressed air operated ^Mbead gun", wherein glass microspheres of a range of diameters typically up to 3.00mm are drawn from a containing reservoir in a flow of compressed air at pressures between 3bar and 5bar. The air flow may be heated to temperatures up to 300°C, but more commonly distribution takes place at ambient temperature.

The flow of air borne glass microspheres is directed towards the marking by a nozzle and impinges on the hot molten surface of the thermoplastic marking or painted surface immediately after its placement.

By this crude technique a random distribution and embedding of the microspheres takes place. Very little control of the distribution process is possible using this known technique.

Alternatively, in small scale operations, distribution is sometimes carried out using hand sieves or gravity ^,,seed^M distributors as used in agriculture. The present art lacks refinement of control in the depth of embedding and in the density of distribution of microspheres.

In addition, the application of microspheres by the above described methods results in a considerable wastage of microspheres which are not captured by the molten or viscous receiving surface.

In practice, wastage of up to 35% of microspheres takes place when using a compressed air bead gun and attempting to achieve the commonly specified bonded spread of 450/500gm/m².

The optimum surface temperature of thermoplastic surface markings at the instant of capture of the microspheres is generally within the range 165/190°C, dependent on the physical characteristics of the thermoplastic selected. Deviation from the optimum value can prevent the particles from being properly embedded in the surface.

It is often observed that embedment of microspheres to depths of only one third of their diameters takes place, resulting in early traffic wear and displacement. However, when the surface marking is too hot, partial microsphere submergence often takes place, which reduces initial retroreflectivity.

One reason why microspheres are often too shallowly embedded is that the powerful air flow, used in conventional bead guns chills the surface of the thermoplastic marking, thereby hardening it.

Another reason why optimal depth of embedment is often not achieved by the known methods is that smaller microspheres accelerate more quickly in the air flow than larger microspheres (due to their higher ratio of surface area to mass) and so reach the surface of the marking before the larger microspheres. Now, in the mixtures of the microspheres typically used for road marking, smaller microspheres greatly outnumber the larger ones. The applicant has found that when a typical mixture is distributed to provide a surface density of microspheres (by mass) of 450gm./m², the resultant number density of particles with diameters circa 0.1mm or below 0.18mm sieve size is approximately 1400/cm² or more, while the number density of particles with diameters circa 1mm. is only 0.5/cm². Thus, the smaller microspheres effectively cover the receiving surface, preventing the later deposited larger particles from being properly embedded. This consequence of known deposition techniques is particularly disadvantageous because the larger microspheres are most important in determining the retroreflectivity of the marking.

The measured retroreflectivity of the completed surface marking achieved using this known method is commonly within the range of 100/200 racd/m² (milli candela per metre²) but random results, difficult to repeat, up to 300mcd/m² have been reported. At the lower end, results of only 80/100mcd/m² are often achieved for initial retroreflectivity when attempting to achieve specified values in excess of 100mcd/m². It is an object of the present invention to provide a method and device for consistently producing a marking of improved retroreflectivity. More specifically, it is intended to provide a method and device to permit improved control over the density of distribution of reflective particles and over the depth to which they are embedded in a marking.

A further object of the present invention is to overcome the above described problem of prior capture of smaller particles preventing proper embedment of larger particles.

Still a further object of the present device is to reduce wastage of microspheres during their embedment in the marking.

It has been found that the retroreflectivity (that is, the proportion of incident light reflected back to its source) of road markings depends critically on the depth of embedment of microspheres in a marking.

Theoretical studies carried out by the applicant show that for typical driving conditions, in which the angle of incidence of light from vehicle headlamps is small (of the order of 1 degree), the optimal depth of embedment is substantially 0.6 diameter, and deviation from this optimal value dramatically reduces the retroreflectivity contributed by a microsphere. This optimal value applies for glass microspheres of refractive index 1.5 - 1.55, which is typical of the glass currently used in microspheres. These theoretical results have been confirmed by experiment.

Improved microspheres are available using glass of refractive index in the range 1.9 - 2.1, and for these a different and still more tightly controlled distribution and depth of embedment are required.

The present invention is therefore adapted to produce a marking of increased retroreflectivity by controlling the distribution and embedment of the microspheres.

In accordance with one aspect of the present, there is provided a device for discharging particles onto a receiving surface, comprising a launching surface, particle supply means adapted to dispense particles towards the launching surface at a controlled rate, and gas supply means adapted to supply a flow of gas across the launching surface, wherein, in use, particles are accelerated by the flow of gas and thereby projected onto the receiving surface.

In this way, selective, controlled distribution of microspheres is provided for.

Preferably, the means for providing a flow of gas comprise a fan or blower preferably having an impeller with one or more blades. In this case, the flow of gas comprises air, which may be heated. It is preferred that the gas supply means are disposed to direct gas into a pressure chamber. The pressure chamber is provided with a gas flow outlet, and may be further provided with a pressure control outlet. Preferably, the pressure control outlet is provided with a pressure control valve. Gas flow may also be controlled by speed of rotation of the impeller or by variation of the pitch of the impeller blade(s).

Preferably, heating means are provided for heating the flow of gas. Where a fan or blower is used, the heating means and the fan or blower may be combined.

In a particularly preferred embodiment of the present invention, the launch surface is formed as part of a shaped channel. Alternatively, the launch surface may be a concave surface of a tube or duct. The launch surface may comprise individual segments, preferably overlapping each other, and which are preferably movable relative to each other to expand or reduce the area of the launch surface. In that way it may be possible to vary the spread of discharged particles.

It is preferred that the launch surface is inclined. Preferably, the particles traverse the inclined launch surface in an upward direction.

The supply means preferably comprises an Archimedean screw or a bladed impeller, whose axis may be horizontal, vertical or inclined. Alternatively, the supply means may comprise a vessel for containing particles provided, or in flow connection, with a dispensing aperture of predetermined size through which particles are dispensed by gravity.

In accordance with a further aspect of the invention, there is provided a distribution apparatus comprising a vehicle fitted with a device for discharging particles in accordance with the present invention.

The vehicle may, in accordance with the present invention, be a manually movable trolley or cart, or a motorised vehicle. Preferably, the distribution apparatus further comprises means for measuring the temperature of a receiving surface over which the apparatus is moved. Operational parameters of the device for discharging particles may then be controlled in dependence upon the said temperature.

In a particularly preferred embodiment of the present invention, control means are provided whereby the rate of supply of particles by the supply means is controlled in dependence upon speed of travel of the vehicle. This may be achieved, in accordance with a preferred embodiment of the present invention, by providing the vehicle with a wheel which is disposed to contact the receiving surface, in use, and which is connected to the supply means by a transmission. Alternatively, the control means may be provided with a speed transducer for providing a signal for controlling the supply means in dependence upon the measured speed of travel of the vehicle. In an alternative embodiment in accordance with the present invention, the rate of supply of particles is manually preset or manually controlled.

It is preferred that the distribution apparatus further comprises marking means for laying a surface marking. In particular, the marking means may be adapted to lay a thermoplastic or paint layer on a road surface.

According to still another aspect of the present invention, there is provided a method of selectively distributing particles over a surface, comprising the steps of dispensing particles at a controlled rate, accelerating the dispensed particles to a velocity dependent on their size, and projecting the particles at a predetermined angle to the horizontal.

Preferably, the particles are accelerated using a flow of gas.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:-

Figure 1 is a cross section through a first embodiment of an apparatus in accordance with the present invention;

Figure 2 is an enlarged cross section of a channel and associated components of the apparatus shown in Figure 1; Figure 3 is a section through a supply means in accordance with a further embodiment of the present invention;

Figure 4 is a plan view of yet a further embodiment of an apparatus according to the invention; Figure 5 is an elevation on line BB of Figure 4;

Figure 6 is an elevation on line CC of Figure 4; and

Figure 7 is a plan view of detail of the apparatus of Figure 4.

Figure 1 shows a first embodiment of the present invention. Briefly, glass microspheres are dropped at a controlled rate from an Archimedean screw conveyor 10 and are guided through an outlet slot 31 towards a shaped channel 40, which comprises an upwardly inclined ramp portion 47. A flow of air over the ramp portion 47 is provided by a fan or blower 58 via a pressure chamber

46. The flow of air propels the microspheres across the ramp portion 47, thereby launching the microspheres at a controlled angle and velocity, so that the microspheres travel a controlled horizontal distance before impinging on a receiving surface 34.

The specific embodiment shown in Figure 1 will now be described in more detail.

Reflective glass microspheres of differing sizes are contained within a hopper bottomed vessel 2 which, according to the present embodiment, comprises a frusto conical lower surface 4 which permits discharge of the microspheres through an aperture 6 in its lowest part.

The aperture 6 in the hopper bottomed vessel communicates with one end of a connecting pipe 8, the other end of which leads into an upper portion of a helical cavity in an Archimedean screw conveyor 10. The hopper bottomed vessel 2 is above the screw conveyor, so that microspheres are fed from the hopper bottomed vessel through the connecting pipe 8 to the screw conveyor 10 by gravity. The screw conveyor comprises a screw element 12 which, according to the present embodiment of the invention, is rotatable about a vertical axis and is enclosed within a cylinder 14. The screw element 12 is preferably resiliently deformable to minimise damage to the microspheres.

As the screw element rotates, microspheres are transported axially downwardly along the screw conveyor 10 to be discharged through a discharge opening 16 at the bottom of the cylinder 14. The discharge opening 16 is provided with a closure plate 18, which is upwardly biassed by a spring 20 so that when the closure plate 18 is not forced open by downwardly moving microspheres 19, it contacts a bottom surface of the cylinder 14, closing the discharge opening 16. In this way it is ensured that microspheres are discharged only when the screw element 12 is rotated.

The screw conveyor 10 projects through an upper horizontal wall 22 into a cuboid cavity 24 defined by the upper horizontal wall 22, a vertical outer wall 26, two side walls 28 (which, in Figure 1, are parallel to the plane of the paper), a vertical wall 30, and an internal dividing wall 32. The bottom of the cuboid cavity 24 is open towards a receiving surface 34 towards which the microspheres are to be projected.

The microspheres are discharged by the screw conveyor 10 into an upper region of the cuboid cavity 24. A wedge shaped hopper 36, defined by an inclined plate 38 and the dividing wall 32, is disposed within the cuboid cavity 24 below the screw conveyor 10 to catch the discharged microspheres. At its lowest point, the wedge shaped hopper 36 has an outlet slot 31, defined between the dividing wall 32 and a lower edge of the inclined plate 38, through which the microspheres fall. The microspheres then fall towards a channel 40 of precise geometric shape. In cross section, as Figure 2 most clearly shows, the channel 40 of the present embodiment has a vertically extending portion 42 which is joined to a vertical sub-wall 44 of a pressure chamber 46, to be described below. A lower portion of the channel 40 is upwardly concave, being semicircularly curved towards an upwardly inclined ramp portion 47.

The dividing wall 32 separates the pressure chamber 46 from the cuboid cavity 24. A lower portion of the dividing wall 32 overlaps with the vertically extending portion 42 of the channel, thereby defining a slot 48 which is in flow connection with the pressure chamber 46.

The bottom edge 50 of the dividing wall 32 is adjacent the upwardly concave face of the channel 40, being separated therefrom by a distance which, according to the present embodiment, is substantially the same as the width of the aforementioned slot 48. In the present embodiment, this distance is 3mm. The said bottom edge 50 of the dividing wall 32 is curved to provide for smooth airflow through the slot 48. The pressure chamber 46 of the present embodiment is a substantially enclosed cavity defined by a second vertical outer wall 52, an upper horizontal wall 54, a lower horizontal wall 56, the two side walls 28 and, as previously described, the dividing wall 32 and the vertical sub-wall 44.

The upper horizontal wall 54 is penetrated by an aperture through which air is driven into the pressure chamber 46 by a low pressure fan blower 58 which is mounted on the upper surface of the upper horizontal wall 54. The temperature of the airflow from the blower 58 is controlled by an electrical heating element within the blower within the range ambient to 150°C, depending upon weather conditions and the type and temperature of any thermally sensitive surface marking in use. Higher air temperatures may be used, depending on the type of road marking in use. The pressure of air within the pressure chamber 46, and thus the rate of flow of air through the slot 48, may be controlled by varying the fan speed of the blower 58. Alternatively, this pressure may be controlled by means of a valve which permits discharge of air from the pressure chamber 46. In the illustrated embodiment, such a valve is provided in the form of a plate valve comprising a valve plate 60 which is pivotally mounted on one of the side walls 28 and is movable to cover or uncover an aperture 61 which penetrates the said side wall 28 and so connects with the pressure chamber 46. A more sophisticated valve system could be incorporated to allow a specified pressure to be set and automatically maintained. In the present embodiment, the pressure in the pressure chamber can be varied from 0.29mbar (3mm- watergauge) upwards. The pressure is adjusted in accordance with factors including the planned maximum rate of discharge of microspheres and the sizes of the microspheres. The present embodiment produces a maximum pressure of 3.06mbar (32mm. watergauge), with the plate valve fully closed. The available range of pressures is sufficient for the sizes of particles and the rates of distribution used for surface marking. The present embodiment produces an air velocity through the slot 48 of approximately 7 metres per second at a pressure of 0.29mbar (3mm-watergauge) which may be increased by partially closing the sliding plate valve.

When a microsphere falls into the lower portion of the channel, it meets the flow of air leaving the pressure chamber 46 through the slot 48, and consequently is accelerated up the ramp portion 47 of the channel and is projected on a trajectory which depends on the angle of inclination of the ramp portion 47 to the horizontal, and on the initial velocity achieved by the particle. The microspheres thus travel a controlled horizontal distance before impinging on the receiving surface 34, in which they are thereby embedded.

The width of the stream of air borne spheres can be varied by adjustment of two vertical side plates 62, which are hingedly mounted on the side walls 28 within the cuboid cavity 24 to permit variation in width of distribution of spheres to suit changes in width of line marking. One advantage of the present invention arises from the dynamics of motion of the microspheres, which will now be considered.

The glass microspheres most commonly used for surface application comprise a mixture of different diameters, typically as follows.

SIEVE SIZE fnuin % RETAINED AVERAGE % RETAINED 0.850 0/5 2.6

0.600 5/20 13.1

0.300 30/75 55.3

0.180 10/30 21.1 below 0.180 0/15 7.9

100.0 For special purposes, for instance to enhance retroreflectivity under wet weather conditions, microspheres up to 4mm diameter are also used (the present invention is equally well suited to dispensing these larger particles, although some minor changes to the geometry of the channel 40 and the associated components, shown in Figure 2, may be required). Consider the dynamics of a microsphere of diameter D in a flow of compressed air applying a force f per mm²:-

Total force F on the projected area of each microsphere, F=fιrD² 4

The density m of the microsphere results in total mass

6 The resulting acceleration, a, of a microsphere is therefore:- a-=F

3 X f = 1.5f

M _ 2D m mD ie, the acceleration is inversely proportional to the diameter of the microsphere.

Therefore, in an air stream bearing microspheres the acceleration imparted to, for example, a 0.2mm sphere will be approximately 5 x the acceleration imparted to a 1mm sphere.

Hence, the length of the trajectory of a microsphere (ie the horizontal distance it travels before reaching the receiving surface) varies inversely with the diameter of the microsphere. Figure 1 shows five trajectories of microspheres, the longest of these, labelled 64, being the trajectory of a 0.2mm sphere and the shortest, labelled 66, the trajectory of a 2mm sphere.

The following lengths of trajectory for microspheres of a range of diameters in common usage have been experimentally determined, using a pressure in the pressure chamber of 0.76mbar (8mm watergauge). Diameter of Average length of microsphere trajectory

2.0mm 40mm

1.0mm 62mm

0.6/0/8mm 75mm 0.5mm 110mm

<0.3/0.2mm 140mm

In use, the illustrated device is mounted on a vehicle or trolley and moved forward in the direction shown by an arrow in Figure l, at a speed v over a surface bearing a marking. In a typical embodiment, means for depositing the marking are mounted on the same vehicle and precede the device.

The separation caused by the difference in the trajectories of microspheres of different sizes causes the larger microspheres to reach the molten thermoplastic or paint receiving layer 34 closer to the channel 40 than the smaller microspheres.

This separation, in conjunction with the movement of the dispensing equipment, causes a continuous uniform spread of microspheres on to the receiving surface in which spheres of larger size are embedded before the smaller spheres, thus avoiding the above described problem of impaired embedment due to prior capture of smaller spheres.

In order that a consistent surface density of microspheres is applied, the rate of delivery of microspheres may, according to the present invention, be related to the speed v of travel of the device over the surface marking.

In particular, according to the present embodiment it is provided that the rate of delivery of microspheres is directly proportional to the speed v at which the device travels over the marking. This is achieved by relating the rate of rotation of the Archimedean screw element 12 to the speed v. In one specific embodiment (not illustrated) the screw element 12 is driven via a transmission by a wheel bearing on the ground over which the device is passing, so that the rate of rotation of the screw, and consequently the rate of delivery of microspheres, is directly proportional to the speed v.

Alternatively, in another specific embodiment, a velocity transducer or other means for measuring the speed of travel of the device over the ground is provided and the screw element 12 is driven at a rate which is related to the measured speed.

A consistent surface density of microspheres may also be achieved by maintaining certain operating parameters at predetermined levels. For example, the vehicle may be moved forward at a steady speed (6 km/hour is typical) while the rate of distribution of microspheres is maintained at a corresponding, predetermined level. Similarly, the air pressure in the pressure chamber is normally preset at a level appropriate to the mixture of microspheres in use.

An alternative means for supplying microspheres at a controlled rate to the wedge shaped hopper 36 is illustrated in Figure 3. As in the first described embodiment, an Archimedean screw conveyor is used to regulate the rate of supply of microspheres, and the microspheres are gravity fed to the screw conveyor via a connecting pipe 68 from a reservoir.

However, in this embodiment the screw element 70 of the screw conveyor rotates about a horizontal axis and is enclosed within a cylinder 72 whose longitudinal axis is horizontal. Both ends of the cylinder 72 are closed. Exit holes 74 are provided in an upper surface of the cylinder 72 through which the microspheres are ejected from the screw conveyor. The exit holes 74 are distributed along the length of the cylinder 72, so that falling microspheres are distributed more evenly over the inclined plate 38 defining the wedge shaped hopper 36. In this way, an improved, even flow of microspheres through the outlet slot 31 is provided. The present invention is not limited to embodiments comprising an Archimedean screw or other mechanised supply means. In a particularly simple embodiment, (not illustrated) the Archimedean screw is dispensed with, and particles are fed from a storage vessel to the channel 40 by gravity, the rate of particle feed being controlled by an aperture of predetermined size and shape through which the particles pass to reach the channel.

Referring to Figures 4 to 7 of the accompanying drawings, apparatus 100 for distributing glass beads 102 onto a thermoplastic or paint road marking 104 comprises a bead supply (not shown) which supplies the beads via a pipe 106 to a horizontal dispensing cylinder 108. The cylinder 108 is mounted to vertical support plate 110 and contains a four blade rotatable impeller 112. The cylinder 108 has bead outlet ports 114 at intervals along its length. The ports are generally horizontal slots positioned above the axis of rotation of the impeller.

Also mounted to the support plate 100 is an air inlet tube 116 supplied from a blower (not shown) such as of the type described for the other illustrated embodiments. The tube has a first horizontal section before bending downwards to a vertical section which is connected to a horizontal air projection tube 118. The air projection tube 118 is flexible and has a bracket 120 at each end connected by a screw member 1 22, whereby the brackets 120 can be drawn towards or moved away from each other to alter the degree of curvature of the tube 118. Along its length the tube 118 has a series of horizontal slots 124 each associated with its own ramp segment 126. Each ramp segment 126 is flared and overlaps with adjacent ramp segments when the tube 118 is straight. The ramp segments 126 extend upwards generally at an angle of about 28° to the horizontal.

The tube 118 is arranged below the dispensing cylinder 108 so that glass beads fall from the cylinder 108 onto the ramp segments where air from the blower exits the tube 118 to discharge the beads up the ramp segments to fall onto the road marking 104.

The apparatus 100 further includes a horizontal cross bar 128 at the opposite ends of which are mounted side deflector plates 130. The plates 130 can be slid inwards or outwards to match the width of the road marking as illustrated in Figure 6 in order to confine lateral spread of the glass beads.

The curvature of the tube 118 may be altered to adjust the spread of glass beads being discharged onto the road marking. For a wider spread, the curvature of the tube is increased by drawing the brackets 120 towards each other and conversely for a narrower spread the brackets are moved apart by the screw member 122.

Having set up the width of spread of glass beads and the lateral confinement as described above to suit the width of road marking onto which the glass beads are to be discharged the apparatus is operated substantially in accordance with the other illustrated embodiments. The apparatus is moved in the direction indicated by arrow Y following a road marking device. Larger, heavier glass beads are only discharged a relatively short distance compared to smaller, lighter glass beads so that a substantially uniform spread of beads is achieved. Furthermore, the larger beads are embedded before the smaller beads reducing the effect of the problem of impaired embedment experienced with prior art equipment.

While the above described embodiments of the present invention have all been adapted to distribute reflective microspheres on road markings, the invention has applications in other situations where a mixture of particles needs to be selectively distributed. One such application is in the manufacture of reflective plastics laminates, such as materials used for high visibility road signals, in which reflecting microparticles are enclosed in layers of plastics. The particle distributor in accordance with the present invention may be adapted for use in distributing the microparticles onto the said plastics layers during the production process. In this process, in contrast to the above described embodiments, the workpiece may be drawn past a stationary particle distributor in accordance with the invention, under factory conditions.

The present invention offers several advantages over the known apparatus and method for distributing reflective particles onto a marking.

By controlling the rate of delivery of particles, wastage is minimised and it is possible to ensure that a consistent surface density of particles is applied.

It is possible, using the present invention, to measure the temperature of the surface layer on which particles are being deposited and to set operating parameters, such as the temperature of airflow and the temperature of the marking immediately prior to laying, accordingly. This makes possible improvements in the control of the depth of embedment of the microspheres.

The problem associated with known bead guns of chilling of the surface of a hot thermoplastic surface marking by a flow of compressed air directed towards the marking is not encountered when the present invention is used.

As has been described, the present invention provides for embedment of larger particles before smaller ones, further reducing wastage, (by reducing the number of particles which are not sufficiently deeply embedded to be retained) and permitting the particles to be embedded to a predetermined optimal depth. When the particles distributed are reflective microspheres, the rectroreflectivity of the marking is significantly improved.

Claims

1. A device for discharging particles onto a receiving surface, comprising a launching surface, particle supply means for dispensing particles towards the launching surface at a controlled rate, and means for supplying a flow of gas across the launching surface, wherein, in use, particles are accelerated by the flow of gas and thereby projected onto the receiving surface.

2. A device as claimed in claim 1, wherein the means for providing a flow of gas comprises a fan or blower.

3. A device as claimed in claim 2, wherein the fan or blower has an impeller with a blade or blades.

4. A device as claimed in claim 1, 2 or 3, further comprising means for heating the flow of gas.

5. A device as claimed in claim 4, wherein the heating means is combined with the fan or blower.

6. A device as claimed in any one of claims 1 to 4, wherein the gas supply means are disposed to direct gas into a pressure chamber having a gas flow outlet.

7. A device as claimed in claim 6, wherein the pressure chamber has a pressure control outlet.

8. A device as claimed in claim 7, wherein the pressure control outlet has a pressure control valve.

9. A device as claimed in any one of claims 1 to 8, wherein the launch surface is formed as part a shaped channel.

10. A device as claimed in any one of claims 1 to 9, wherein the launch surface is a concave surface of a tube or duct.

11. A device as claimed in claims 9 or 10, wherein the launch surface comprises overlapping segments.

12. A device as claimed in claim 11, wherein the overlapping segments are movable relative to each other to expand the area of the launch surface.

13. A device as claimed in any one of claims 1 to 12, wherein the launch surface is inclined.

14. A device as claimed in claim 13, wherein the launch surface is arranged for the particles to traverse same in an upward direction.

15. A device as claimed in any one of claims 1 to 14, wherein the particle supply means comprises an Archimedean screw or bladed impeller.

16. A device as claimed in any one of claims 1 to 14, wherein the supply means comprises a vessel for containing particles provided, or in flow connection, with a dispensing aperture of predetermined size through which particles are disposed by gravity.

17. A distribution apparatus comprising a vehicle fitted with a device for discharging particles onto a receiving surface as defined in any one of claims 1 to 16 .

18. Apparatus as claimed in claim 17, further comprising means for measuring the temperature of a receiving surface over which the apparatus is moved.

19. Apparatus as claimed in claim 18, wherein operational parameters of the device for discharging particles are controllable in dependence upon the temperature measured.

20. Apparatus as claimed in claim 17, 18 or 19, further comprising means for controlling the rate of supply of particles by the supply means in dependence upon speed of travel of the vehicle.

21. Apparatus as claimed in claim 20, wherein the vehicle has a wheel which is disposed to contact the receiving surface, in use, and which is connected to the supply means by a transmission.

22. Apparatus as claimed in claim 20, wherein the means for controlling the rate of supply of particles includes a speed transducer for providing a control signal to the supply means in dependence upon the speed of travel of the vehicle.

23. Apparatus as claimed in any one of claims 17 to 22, further comprising marking means for laying a surface marking.

24. Apparatus as claimed in claim 23, wherein the marking means is adapted to lay a thermoplastic or paint layer on a road surface.

25. A method of selectively distributing particles over a surface comprising the steps of dispensing particles at a controlled rate, accelerating the dispensed particles to a velocity dependent on their size, and projecting the particles at a predetermined angle to the horizontal.

26. A method as claimed in claim 25, wherein the particles are accelerated using a flow of gas.

27. A device for discharging particles substantially as hereinbefore described with reference to and as illustrated in any of Figures 1 and 2, 3 or 4 to 7 of the accompanying drawings.

28. A distribution apparatus substantially as hereinbefore described with reference to and as illustrated in any of Figures 1 and 2, 3 or 4 to 7 of the accompanying drawings.

29. A method of selectively distributing particles over a surface substantially as hereinbefore described with reference to any of Figures 1 and 2, 3 or 4 to 7 of the accompanying drawings.