WO1996000619A1

WO1996000619A1 - Applying coating compositions to substrates

Info

Publication number: WO1996000619A1
Application number: PCT/EP1995/002442
Authority: WO
Inventors: Stephen Holt; A. H. A. Bergmans; J. M. Jaspers
Original assignee: Redland Technologies Limited
Priority date: 1994-06-28
Filing date: 1995-06-23
Publication date: 1996-01-11
Also published as: GB9412944D0; GB2290730A; AU2924195A

Abstract

The invention provides apparatus (10) for spraying a coating composition onto a substrate comprising means for forming a curtain of the coating composition and means for atomising the formed curtain. The curtain of coating composition is formed by allowing it to flow over a transfer surface (14, 43, 53). Preferably the transfer surface (43) has upstand portions (44) thereon, to ensure that the coating composition flows over the transfer surface (43) in a uniform manner. Alternatively, the transfer surface (53) is convex. The curtain of the coating composition is atomised using an air knife which preferably follow a fan-shaped or diverging path before it contacts the curtain of coating composition, so that the stream of air supplied is evenly distributed across the whole width of the curtain.

Description

Applying Coating Compositions To Substrates

The present invention relates to a novel method and apparatus for applying a coating composition onto a substrate.

Virtually all products manufactured today have at least one coating composition applied to their outer surface in order to, for example, give the product a desired colour or surface finish or to protect the surface by making it durable, waterproof or stain resistant.

It is not surprising, therefore, that a great number of coating materials are available for these applications; and each of the coatings are designed for specific chemical and physical characteristics to meet the requirements of the particular product. Hereinafter the 'product' will be referred to as the 'substrate'.

The coating may be applied to the substrate by conventional dipping or brushing techniques, or it may be applied by spraying using a spray gun which may or may not be air assisted.

Air assisted spray guns are the apparatus of choice in industries such as the metal, automotive and furniture coating industries. They consist of a central nozzle through which the coating composition is discharged from a pressurised source. Compressed air flows through an annular orifice surrounding the nozzle and acts to atomise the coating and to direct the droplets of the coating material, thus formed, onto the substrate to be coated. The advantages of air assisted spraying systems are that the degree of atomisation is high and that fairly wide nozzle outlets can be used to guard against blockage when more viscous coating materials are used. However, a uniform layer of coating on the substrate can only be achieved with over spraying. Also, the use of compressed air results in high energy use.

Airless spray systems use hydraulic pressure to force the coating through a specially shaped nozzle. The increase in the velocity through the nozzle leads to atomisation of the coating. The main advantage of this system is that there is a reduction in over spray as compared with air assisted spray systems. However, as the viscosity of the coating compositions increases then atomisation will only occur if the pressure also increases. There is also a tendency for the nozzle to block when thicker compositions are used. Furthermore, the surface finish and the adhesion of the coating to the substrate are often found to be poor.

Spinning disc and spinning cup systems are used in the food and pharmaceutical industries for a wide range of materials from low viscosity oils to coatings with a high solids content. These systems are also used in the construction materials industry for applying glaze to clay tiles and resin to wood chips to form particle boards.

Both of these systems operate on the principle that a jet of liquid will break into small droplets if a certain speed of rotation is exceeded. This critical speed is related to the surface tension and viscosity of the coating composition being applied.

A spinning disc applicator generally consists of a flat or conical disc which is arranged such that its spinning axis is almost perpendicular to the substrate conveyor. The coating composition is fed to the centre of the disc and it travels to the periphery where it is thrown off by tangential forces in the form of droplets. Although the applicator is relatively simple in construction and there is no problem with blockage when a viscous or even a solid coating composition is used, the main disadvantage is that only a small proportion (typically 20%) of the sprayed coating composition actually contacts the substrate to be coated. This means that, firstly, significant over spraying is needed to ensure an even coating and secondly, if the system is to be efficient, the remaining material has to be collected and recycled.

The application of coating compositions by an electrostatic process has been used by various industries over the last few years. Although liquids can be broken down into small particles by electrostatic forces, such atomisation is not efficient and is rarely used as a method by itself. More often, the coating composition is first atomised using one of the coating systems described above and then the formed droplets are electrostatically charged as they leave the spray gun or the rotating surface. As they disperse, they are attracted to the nearest earth, i.e. the substrate to be coated. The system is advantageous in that most of the atomised coating composition lands on the substrate to be coated. Thus little if any over spray is necessary to obtain a uniform coating. Also, the wrap around phenomenon exhibited by electrostatically charged particles means that coating composition can be applied on areas not directly in the line of the spray gun. This occurs because the charged particles of coating are repelled by each other and are pushed around the sides of any object. However, the major disadvantage of using this system is the high cost of the equipment.

Curtain coating is used in industries such as the sheet metal furniture and carpet industries, for coating flat substrates with a continuous paint or lacquer film. The coating composition is metered through a slot to form a continuous curtain of material. The substrates to be coated pass through the curtain and the coating thickness is determined by the flow rate of the coating composition and the speed at which the substrates to be coated are conveyed. The main difficulties with this system are found to be:- a) the inability to achieve good coatings at high speeds, - 4 -

b) the fact that poor coating of vertical surfaces is experienced, and c) the shadow effects which are caused when profiled substrates are coated.

Roller coating is used to coat metal cans, sheet and strip and to add a coating to paper. The operating principle of this method is to meter the coating composition onto an applicator roller from a metering roller or reservoir. The applicator roller then transfers the coating composition to the substrate in a continuous painting operation. This system is fairly efficient when coating substantially flat substrates, however, it is difficult to achieve good coverage of profiled surfaces.

Finally, there is a particular type of brush applicator which is widely used for coating products such as concrete roof tiles with a slurry coating composition. A modified version of this type of apparatus is described in US 4743471. In summary, this technique operates by, firstly, having a layer of slurry coating composition applied along the length of a rotating transfer roller; this roller then transfers the applied slurry coating composition onto a transfer plate and the slurry is then allowed to flow over the surface of the transfer plate onto a rotating cylindrical brush. The substrate to be coated is conveyed beneath the cylindrical brush and as the brush rotates, the slurry coating composition is slung or sprayed onto the substrate. The main advantages of this system are its simple construction and its ability to be used with many profiled substrates. However, the disadvantages include the production of non-smooth slurry coatings and the fact that non-uniform coatings are formed on substrates with complex profiles.

The aim of the present invention, therefore, is to provide a novel method and apparatus for spraying a coating composition onto a substrate which will give a uniform coating on either flat or profiled substrates and which can be used with coating compositions of high and low viscosity without the risk of blockage. Furthermore, the invention seeks to provide apparatus which gives a coating with a surface texture which can be varied as required. For example, it is desirous that the apparatus according to the present invention can be used to produce a coating with either a smooth texture or one which resembles orange peel. In addition the present invention should provide apparatus which has a low capital cost and which is cost effective to maintain.

According to the present invention there is provided apparatus for spraying a coating composition onto a substrate comprising means for forming a curtain of the coating composition and means for atomising the formed curtain of the coating composition.

The term 'curtain' is to be interpreted as meaning the form in which the coating composition appears when a stream of composition falls freely under gravity.

The curtain of the coating composition may be atomised by a stream of compressed air and preferably this stream of air is supplied as an evenly distributed knife edge of compressed air. The stream of air can be caused to flow substantially in one direction but preferably it is caused to flow along a diverging or fan-shaped path.

The curtain of the coating composition may be formed by any conventional means but preferably the means comprises a generally smooth transfer surface which has at least one edge portion; wherein, in use, a supply of the coating composition is caused to flow over the generally smooth transfer surface and then to flow over the at least one edge portion such that the coating composition falls from the said edge portion as a curtain. The generally smooth transfer surface is preferably provided by a transfer plate. It is advantageous to control the flow of the coating composition over the generally smooth transfer surface so that an even curtain of the coating composition is formed. The required control may be conveniently achieved by either providing a transfer surface which is at least partially convex in profile or by providing upstand portions on a transfer surface which is substantially flat in profile. Upstand portions may also be provided on a convex portion of the transfer surface.

It is also advantageous to obtain the desired rate of flow of the coating composition over the transfer surface by optimizing the radius of curvature of the convex portion of the transfer surface in relation to the viscosity of the coating composition.

It is particularly preferable for the transfer surface with a convex portion to be provided with a flat portion adjacent the side edge portion of said plate.

Alternatively, for certain substrates, e.g.. profiled substrates, it may be advantageous to control the flow of the coating so that either a pulsed or a discontinuous curtain of the coating composition is formed.

The atomised coating composition is preferably directed onto the substrate to be coated by using a further stream of compressed air to blow the atomised coating onto the substrate. This further stream of compressed air is also advantageous in that it controls the amount of fine mist formed during the atomisation of the coating composition.

In an alternative embodiment of the apparatus according to the present invention, the transfer surface is associated, on its under surface, with a series of radial grooves which act to constrain the stream of compressed air to take up a fan-shape configuration prior to atomisation of the coating composition. The provision of the radial grooves improves the atomisation of the coating and also decreases the noise generated when the apparatus is in operation.

According to the invention there is further provided a method of spraying a coating composition onto a substrate which comprises the steps of:-

a) forming a curtain of the coating composition; b) atomising the curtain of the coating composition formed in step a); and c) positioning the substrate to be coated in the path of the atomised coating composition formed in step b).

Preferably, the above method uses a curtain of coating composition which is atomised using a stream of compressed air. It is further preferable that a knife edge of compressed air is used and particularly preferable that the compressed air is caused to flow along a diverging or fan-shape path.

Three embodiments of the invention are illustrated by way of example in the accompanying drawings, in which;

Figure 1 shows a front perspective view of the spraying apparatus according to a first embodiment of the invention;

Figure 2 shows a side section view through A-A of the spraying apparatus shown in Figure 1 ;

Figure 3 shows a front perspective view of the spraying apparatus according to a second embodiment of the invention; Figure 4 shows a front perspective view of the spraying apparatus according to a third embodiment of the invention;

Figure 5 shows a side section through B-B of the spraying apparatus shown in Figure 4; and

Figure 6 shows a plan section through C-C of the spraying apparatus shown in Figure 5.

The spraying apparatus 10, shown in Figure 1 , has an upper transfer plate 12 which is trapezoidal in shape and which has a substantially flat upper surface 14. The shortest side 16 of the transfer plate 12, together with the two sides 18 and 20 which are adjacent to the side 16, are enclosed by wall portions 22, 24 and 26 respectively. The upper transfer plate 12 also has a free side 28 extending between the sides 18 and 20 and opposite the side 16 thereof. The plate 12 is inclined downwardly from the wall portion 22 at an angle of approximately 45°, as shown in Figure 2.

A coating inlet pipe 30 is located above and adjacent to the wall portion 22 and is positioned between the two side wall portions 24 and 26 such that one end 32 of the coating inlet pipe 30 is just clear of the upper surface 14 of the upper transfer plate 12. The end 32 of the coating inlet pipe 30 is shaped in the form of an elongate slot 34.

The elongate slot 34 is directed towards and is oriented parallel with the side 28 of the upper transfer plate 12.

The apparatus also has an air passage 36, shown more clearly in Figure 2, which air passage 36 is defined by the upper transfer plate 12 and a lower plate 13 of similar geometry. The air passage 36 is linked via an inlet pipe 37 to a supply of compressed air. The air passage 36 terminates at its downstream end in an opening 38 defined by the free sides 28 and 39 respectively of the plates 12 and 13.

When the apparatus according to this first embodiment of the invention is operated, a continuous stream of compressed air is supplied through the air passage 36 in the direction towards the opening 38, i.e.. in the direction of the arrow shown in said passage in Figure 2. As this compressed air exits the air passage 36 it forms a knife edge of air at the opening 38. A continuous stream of the coating composition is then supplied via the coating inlet pipe 30 and out of its elongate slot 34 onto the upper surface 14 of the upper transfer plate 12. The coating composition is allowed to flow over the upper surface 14 towards the free side 28 of the upper transfer plate 12, whereupon, the coating composition falls over the side 28 in the form of a curtain (not shown). The said curtain falls in front of the slot shaped opening 38 of the air passage 36 and is atomised by the knife edge of compressed air which is formed at the opening 38. The atomised coating composition then falls onto the substrate to be coated which is oriented to lie in the path of the atomised coating composition.

Figure 3 shows a spraying apparatus 40 which is a modified version of the spraying apparatus 10 shown in Figures 1 and 2. The form of the spraying apparatus 40 and its operation when the apparatus is in use, is equivalent to that of the apparatus 10 in every respect except that it has an upper transfer plate 42 which has an upper surface 43 with upstand portions 44 disposed thereon. The said upstand portions 44 protrude above the plane of the upper surface 43. The reason for these upstand portions 44 is to ensure that a uniform spray of the coating composition is produced. Generally, this is difficult to achieve due to a tendency for more of the coating to accumulate at the centre 47 of the free side 46 of the upper surface 43 than at the outside edges 48 and 49. The upstand portions 44 are provided to direct the coating composition over substantially the whole of the upper surface 43 thereby preventing the central accumulation and thus facilitating an even flow of the coating over the upper transfer plate 42. The result of this is that the coating composition flows as a uniform curtain over the free side 46, and this, in turn, gives a uniform atomised spray.

Figure 4 shows a spraying apparatus 50 which is a further modification of the spraying apparatus shown in Figures 1 and 2. The apparatus 50 has a transfer plate 52 which is trapezoidal in shape and which has an upper surface 53 (shown in Figure 5). The shortest side 54 of the transfer plate 52, together with the two sides 56 and 58 which are adjacent to the side 54, are bounded by wall portions 60, 62 and 64 respectively. The transfer plate 52 also has a free side 66 extending between sides 56 and 58 and opposite the side 54 thereof. The coating composition flows over the free side 66 when the apparatus is in use. The transfer plate 52 is inclined downwardly from wall portion 60 at an angle of approximately 45° as shown in Figure 5.

The upper surface 53 of the transfer plate 52 has a first portion 68 which is convex in profile and a second portion 70 which is substantially flat. The convex portion 68 of the transfer plate 52 directs the flow of the coating composition over substantially the whole of the surface 53 and prevents the accumulation of the coating composition towards the centre of the surface 53, in a similar manner to that described above in relation to the modified version of the apparatus shown in Figure 3. The flat second portion 70 is located adjacent the free side 66 and is provided to ensure that the coating composition flows over the free side 66 in a uniform manner. The apparatus shown in Figure 4 also has an air knife 72 for atomising the curtain of the coating composition when it runs over the free side 66. The air knife 72 (see Figures 5 and 6) is located on the underside of the transfer plate 52 and comprises a compressed air inlet pipe 74, a plenum chamber 76 and a "land" portion 78. The latter portion 78 is provided with a plurality of radial grooves 80 (shown in Figure 6) which cause the compressed air to flow along a diverging or fan-shaped path immediately prior its exit from the air knife via the opening 82.

As described above with reference to Figures 1 and 2, a coating inlet pipe 86 is located adjacent to the wall portion 60 and is positioned between the two side wall portions 62 and 64. The end 88 of the coating inlet pipe 86 is just clear of the upper surface 53 of the transfer plate 52.

When the apparatus as illustrated in Figures 4,5 and 6 is in operation, a continuous stream of compressed air is supplied to the air knife 72 via the inlet pipe 74 and into the plenum chamber 76 towards the direction of the opening 82. As the compressed air exits the air knife 72 it is caused to form a fan-shaped knife edge of air at the opening 82 by the radial grooves 80 provided on the "land" portion 78. A continuous stream of the coating composition is then supplied via the coating inlet pipe 86 and out of its end 88 onto the upper surface 53 of the transfer plate 52. The coating composition is allowed to flow over the upper surface 53 towards the free side 66 of the transfer plate 52, whereupon, the coating composition falls over the side 66 in the form of a curtain. The said curtain falls in front of the opening 82 of the air knife 72 and is atomised by the knife edge of compressed air which is formed at the opening 82. The atomised coating composition then falls onto the substrate to be coated which is oriented to lie in the path of the atomised coating composition. - 12 -

It will be appreciated that the invention can be performed using any suitable apparatus and it is not limited to that shown in the specific embodiments described herein. For example, the transfer surface may comprise the outer surface of a roller. Also, the transfer plate 12, 42, and 52 need not be trapezoidal in shape and the upper surface of the transfer plate 12, 42, 52 may be inclined downwardly at any angle to the wall portion 22, 64. Furthermore, the coating inlet pipe 30 and 86 need not be located above the transfer plate; the coating composition may be supplied using any suitable means.

It is also to be appreciated that the coating composition used in the performance of the invention may be a liquid of any viscosity or it may comprise solid particles in, for example, granular or powder form.

It will be further appreciated that, depending upon the particular substrate to be coated, more than one set of apparatus according to the invention may be used, either simultaneously or in series.

The radial grooves which direct the flow of compressed air may alternatively be formed on the underside of the transfer plate or they may comprise any suitable means to disperse the air flow so that it is distributed evenly across substantially the whole of the free side 66.

Claims

1. Apparatus for spraying a coating composition onto a substrate comprising means for forming a curtain of the coating composition and means for atomising the formed curtain of coating composition.

2. Apparatus for spraying a coating composition onto a substrate according to Claim 1 wherein the curtain of the coating composition is atomised by a stream of compressed air.

3. Apparatus for spraying a coating composition onto a substrate according to Claim 2 comprising means which, in use, provides an evenly distributed knife edge of compressed air.

4. Apparatus for spraying a coating composition onto a substrate according to Claims 2 or 3 comprising means which, in use, causes the stream of compressed air to flow along a diverging or fan-shaped path.

5. Apparatus for spraying a coating composition onto a substrate according to any of Claims 1 to 4 wherein the means for forming the curtain of the coating composition comprises a generally smooth transfer surface which has at least one edge portion, wherein, in use, a supply of coating composition is caused to flow over the generally smooth transfer surface and then to flow over the at least one edge portion such that the coating composition falls from the said edge portion as a curtain.

6. Apparatus for spraying a coating composition onto a substrate according to Claim 5 wherein the generally smooth transfer surface is provided by a transfer plate.

7. Apparatus for spraying a coating composition onto a substrate according to Claims 5 or 6 further comprising means to control the flow of the coating composition over the generally smooth transfer surface to give an even curtain of the coating composition.

8. Apparatus for spraying a coating composition onto a substrate according to Claim 7 wherein the means to control the flow of the coating composition comprises upstand portions on the generally smooth transfer surface which upstand portions protrude above the plane of the generally smooth transfer surface.

9. Apparatus for spraying a coating composition onto a substrate according to Claim 7 wherein the means to facilitate even flow of the coating composition comprises a transfer plate which is at least partially convex in profile.

10. Apparatus for spraying a coating composition onto a substrate according to any preceding Claim comprising a further stream of compressed air to direct the atomised coating composition onto the substrate to be coated.

11. A method of spraying a coating composition onto a substrate comprising the steps of:-

a) forming a curtain of the coating composition;

b) atomising the curtain of the coating composition formed in step a); and c) positioning the substrate to be coated in the path of the atomised coating composition formed in step b).

12. A method of spraying a coating composition onto a substrate according to Claim 11 comprising the steps of:-

a) forming a curtain of coating composition; b) atomising the curtain of the coating composition formed in step a) using a stream of compressed air; and c) positioning the substrate to be coated in the path of the atomised coating composition formed in step b).

13 A method of spraying a coating composition onto a substrate according to Claim 11 or 12 comprising the steps of:- a) forming a curtain of coating composition; b) atomising the curtain of the coating composition formed in step a) using an evenly distributed knife edge of compressed air; and c) positioning the substrate to be coated in the path of the atomised coating composition formed in step b).

14. A method of spraying a coating composition onto a substrate according to any of Claims 11 to 13 comprising the steps of:- a) forming a curtain of the coating composition; b) atomising the curtain of the coating composition formed in step a) using a stream of compressed air which has been caused to flow along a diverging or fan-shaped path; and c) positioning the substrate to be coated in the path of the atomised coating composition formed in step b).

15. A method of spraying a coating composition onto a substrate using the apparatus of any one of Claims 1 to 10.

16. Apparatus for spraying a coating composition onto a substrate as hereinbefore described and as illustrated in Figures 1 , 2, 3, 4, 5 and 6.