This invention concerns the application of plastic to substrates, more particularly the application of plastics materials to ground substrates such as concrete and tarmacadam for road markings, playground and other decorative, sporting and/or promotional purposes, and for the provision of non-slip and non-skid surfacing e.g. to stadium steps

Conventionally, plastics materials, which may be coloured and/or contain, or have applied to them on laying, grit slip and/or skid protection and/or retroreflective and or luminescent particles for improved visibility or for special effects, are applied using gas torches to melt the plastic and cause it to adhere to the substrate.

The use of the gas torches is a skilled affair, requiring judgement as to the temperature to which the plastic is brought and must be done with care and attention to get the required adhesion to the substrate without burning the plastic. Moreover, if anti-slip/skid grit is comprised in the plastic at manufacture, the heat can cause the plastic to soften so that the grit sinks below the surface, and more must be added, again at just the right temperature so that it will not also sink in. Retroreflective microbeads require application at a different temperature if they are not also to be swallowed up by the plastic and rendered ineffective.

The gas torched, moreover, give rise to local atmospheric pollution, and work areas, especially in schools, must be cleared to avoid risk of injury. The present invention provides better methods for the application of plastics materials, particularly in the circumstances described above, reducing hazard and pollution, and permitting much better control of the application and a correspondingly better finished product. The invention comprises a method for applying plastic to substrates comprising placing the plastic on the substrate and applying microwave heating to cause the plastic to soften and adhere.

Gas torch heating heats the top surface of the plastic, the heat conducting down through the plastic to the loser surface, so that the top surface must be heated to a higher temperature, or significantly longer, or both, than the lower surface. While is it important that the lower surface is heated sufficiently to achieve bonding to the substrate, it is also important that the top surface is not overheated, which can affect, inter alia, its appearance and the behaviour of anti-skid/anti-slip, luminescent and retroreflective inclusions.

Microwave heating, however, and especially with the plastics materials used for application to roads and like surfaces, which are around 2 - 6 mm thick, will heat the material uniformly throughout its depth. And microwave heating can be controlled so as to achieve a desired temperature and hold it for as long as may be necessary. The overwhelming majority of microwave applications, including microwave cooking, involve heating water. The optimum frequency for this is 2.54 GHz. The plastics materials most often used for application to road and like surfaces are found to absorb microwaves at a frequency of 2.54 GHz very well. This means that essentially standard microwave equipment such as is used in the majority of applications will serve also for present purposes.

Other frequencies, however, may be found to work as well or even better, and different frequencies may work better with differed thermoplastic materials. It may well be found, also, that, especially where the material comprises a blend of plastics materials, it may be heated with more than one microwave frequency, either at the same time or sequentially.

It may also be the case that the material may differ in its constitution throughout its depth, so that the ground-contacting layer may be affected more by the frequency used or may be affected by a frequency different from the frequency that affects the top surface. Thus the material may first be cause to adhere to the substrate, then the top surface may be heated so as to accept grit for anti-slip/anti-skid purposes and/or luminescent and/or retroreflective particles. The application of microwave heating may be controlled in accordance with a measure of its effect. The temperature to which the plastics material is raised, may, for example, be measured, as by an air temperature gauge, on the assumption that the air temperature close to the surface is an indication of the surface temperature, or by an infra-red thermometer, or otherwise as may be convenient. Desirably, the temperature

measurement can be used in a control arrangement, for example in a feedback loop cutting off power supply to a microwave generator when a predetermined temperature is attained, or reducing power proportionally to the difference between a desired and an actual temperature. The microwave power may be controlled, or the frequency. The microwave heating may be applied by moving a microwave head over the plastics material, microwave power being supplied to the head from a microwave generator such as a cavity magnetron through a waveguide. He head may be fashioned somewhat as a lawnmower or carpet sweeper with a handle by which it may be moved over the plastics material.

The head may be on wheels or rollers, or may be supported by an air cushion in the manner of a hovercraft. The wheels or rollers may be located outwith the compass of the microwave head, so that they do not run over freshly molten, as yet unsolidified plastics material.

The head may be moved manually or by a motorised arrangement, which may be steerable and guided automatically as by a computer program. Such latter arrangement may well be appropriate for a larger scale installation such for example as a school playground game or sports layout. Thermoplastic materials may be applied as a step surfacing to stadium steps for anti-slip provision, and, while a movable head may well serve this purpose, it may be found more convenient to provide a step-sized microwave head that can just be moved up or down a staircase without the need for any movement over the individual steps. To cater for different widths of step, the head may be extendable telescopically according to the side- to-side and/or front-to-back measurement of the step or the size of the step surfacing, which may be smaller than the size of the step.

The head may comprise a microwave shield, which may comprise a carapace of metal mesh, which will prevent leakage of microwave energy, yet allow visual inspection of the plastics material being treated.

The volume of 4 mm thick plastics material under a 200 X 100 mm head is 80,000 cubic millimetres, or 80 cubic centimetres. Assuming that the material temperature has to be raised through 100°C, and assuming very roughly that the heat input required is the same as for water, then it will require 0.01 kWh of electrical energy. A 1000mm x 400mm step would require twenty times that, or 0.2 kWh. On that basis, 1000 steps would need 200 kWh, which, at a cost of 15p/kWh is about £30. This compares favourably with an estimated cost of £600 for gas for torches.

Clearly mains power can be used, if available. Otherwise, a portable generator or a battery with an inverter can provide the power.

The invention also comprises apparatus for applying plastic to substrates comprising means for applying microwave heating to plastic placed on a substrate and to cause the plastic to soften and adhere to the substrate.

The apparatus may comprise a head adapted for movement over the substrate through which microwave power is supplied. The head may be movable on wheels or rollers or may be supported on an air cushion, hovercraft-fashion, and may comprise a carapace that shields workers from microwave radiation, yet permits visual inspection of the plastic.

Sensor means may be incorporated which sense, inter alia, the temperature of the plastic, and such sensor means may be incorporated in a control arrangement controlling the microwave power

Methods and apparatus for applying plastic to substrates according to the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 shows the conventional way of applying plastics materials to substrates for road marking, playground games and like applications;

Figure 2 shows one method for applying plastics materials according to the

invention; Figure 3 is a side elevation that shows another method, particularly applicable to applying an anti-slip surface to a step;

Figure 4 is a front elevation of Figure 3;

Figure 5 is a front elevation of one embodiment of apparatus;

Figure 6 is a plan view of the embodiment of Figure 5;

Figure 7 is a plan view of a telescopically extendable apparatus; and

Figure 8 is a diagrammatic view of apparatus including grit and bead dispensing equipment and a control arrangement. Figure 1 depicts the prior art method and equipment for applying thermoplastic material surfacing to roads, school playgrounds, front-of-building logos and the like, in which the plastics material 11, supplied for example by Preformed Markings Limited of West Byfleet in Surrey and others, is bonded to a concrete or tarmacadam ground surface 12 by application of heat from a torch 13, supplied with butane or like gas from a bottle 14, played over its surface.

This must be done carefully and with a degree of skill and experience, to get the temperature of the plastic just right. Too hot, and the plastic surface will be damaged, not hot enough and the plastic to surface bond will be inadequate.

The plastic material often comes preloaded with grit as an anti-slip, anti-skid provision. The grit is usually of glass or flint, so as not to affect the colour of the material, which is, of course, primarily important in laying down children's playground markings. However, heating the surface of the plastic usually melts is so that the grit sinks beneath, where it is ineffective. More grit is often applied while the surface is still hot, so that it sinks into, but not below, the surface.

The use of naked flame is prohibited in many locations, notably on the London

Underground, where there is, however, a need for providing non-slip surfacing and warning notices on platform edges, and resort is had to painting, which is far less effective as an ant-slip provision, and which wears quickly with even moderate footfall.

Figure 2 to 8 illustrate the inventive method and apparatus for applying the material. As shown in Figure 2, with the material 11 placed on the surface 12, microwave heating is applied from a microwave head 15 supplied with microwave energy from a microwave generator 16 which is, in turn, supplied with electric power from a diesel or like generator 17, although, of course, if mains electricity is to hand, that could equally well be used. The microwave head 15 is in a chassis 18 on wheels or rollers 19 and provided with a handle 21 by which it may be moved over the plastics material 12. A widely available microwave generator working at a frequency of 2.54 GHz has been found to heat the types of plastics materials commonly used for surface markings very well. As that is also the frequency at which water strongly absorbs energy, the head 15 could be used, if necessary, to dry the surface 12 before applying the plastics material 11.

This apparatus is adapted to bond plastics materials to substrates for road markings, playground markings, such for example as may be seen at wwwprojectplaygrounds.co.uk, in which plastic shapes of different colours are laid adjacent each other and fused together. It may also serve for the application of anti-slip provision on steps, especially in sports and other stadiums, where on at least some steps promotional logos from stadium sponsors may be incorporated as described in GB2483750, and will avoid problems of naked flames adjacent plastic seating. However, Figures 3 and 4 illustrate an arrangement in which a microwave head 35 is adapted to cover essentially the entire top surface of a step 32 and bond the material 11, to the surface or, in the case of a matrix with an applique, bond the materials to the surface and to each other. This, without the need to traverse the microwave head 35 over the surface, where space may be restricted.

A diesel or like generator 17 can sit on the next step up, or down, as may be convenient. The microwave head 15 will be essentially similar to that shown in Figure 2, but different in plan, clearly, and comprise a microwave generator 16 connected the generator 17. The microwave generator 16 has an upper handle 16a by which it may be lifted from step to step and traversed over thee step.

Figures 5 and 6 illustrate a microwave head 11 in more detail. It comprises a microwave generator 16 carried on a chassis 51 mounted on wheels or castors 52, with a metal mesh carapace 15 to protect against leakage of microwave radiation. Microwave energy passes from generator 16 through a waveguide 53 to an antenna 54 which radiates if on to the plastics material 12. Figure 7 shows a width-adjustable microwave head for treatment of different widths of step without having to traverse over the step. The antenna 54 and carapace 15 are telescopically adjustable widthwise.

Figure 8 illustrates a composite apparatus comprising a microwave head 11 comprising a microwave generator 16 with an antenna 54 and carapace 15, as in earlier embodiments, supplied with electrical power from an electricity generator 17 through a control unit 81. Plastics material temperature, sensed by a temperature sensor 82, is used by the control unit 81 to regulate the supply of electrical energy to the microwave generator 16 so as to cut off or reduce the supply when the temperature is at or near the desired softening temperature.

The measure temperature is also used to control the supply of grit to the plastic surface from a grit dispenser 82 and of retroreflective beads from a bead dispenser 83. Because microwaves heat the material essentially uniformly throughout the thickness of the material, the temperature of the ground/material interface will be essentially the same as the temperature of the top surface of the material, and it is not necessary to heat the top surface to a higher temperature than the optimum substrate attachment temperature in order for the heat to conduct through the material.

It may be, however, that the use of microwave heating gives rise to further improvements, particularly in regard to material formulation. As the microwave heating can be effective throughout the depth of the material, the material could itself comprise different layers, including a ground interface layer that can melt at a lower temperature than the upper layer, which may then be manufacturer-provided with grit and/or retroreflective and/or luminescent particles

It may also be the case that the material may have more than one layer or even differ continuously in its constitution throughout its depth, so that the ground-contacting layer may be affected more by the frequency used or may be affected by a frequency different from the frequency that affects the top surface. Thus the material may first be caused to adhere to the substrate, then the top surface may be heated so as to accept grit for anti- slip/anti-skid purposes and/or luminescent and/or retroreflective particles.