WO1998036852A1

WO1998036852A1 - Curing of coatings

Info

Publication number: WO1998036852A1
Application number: PCT/GB1998/000564
Authority: WO
Inventors: Alan Peach; Peter Anthony Greenaway
Original assignee: The Aerogen Company Limited
Priority date: 1997-02-22
Filing date: 1998-02-23
Publication date: 1998-08-27
Also published as: GB9703731D0

Abstract

A coating (12), particularly a silicone release coating, applied to a base layer (10), e.g. a moving web of paper, is cured by applying direct heat from a flame (214). The curing occurs almost instantaneously across the base layer (10) and moisture loss from the base layer is reduced to such an extent that a subsequent re-humidifying step can be omitted. The invention is particularly useful in the manufacture of laminated, self-adhesive products (10, 12, 14, 16) such as 'peel and stick' labels.

Description

DESCRIPTION CURING OF COATINGS

The present invention relates to a method and apparatus for curing a coating applied to a substrate and in particular, but not exclusively, for curing a release coating, e.g. a silicone release coating, applied to a continuous moving substrate.

The use of self-adhesive labels, tapes, vinyl films, envelope closures and the like has grown considerably during recent years and its growth is expected to continue for the foreseeable future.

The most significant feature of self-adhesive products is that they will adhere strongly to a wide variety of surfaces (e.g. paper, board, plastic films, metals and glass) by the application of only a light finger pressure or smoothing action and without the need for any remoistening before application. Self-adhesive products are often referred to as "peel and stick" in contrast to the remoistened products which are commonly referred to as "lick and stick".

This characteristic of self-adhesive products is provided by the use of a thin coating of a so-called "pressure sensitive adhesive" on one side of the label or tape. As a consequence of this pressure sensitivity the adhesive must be protected from accidental contact with other surfaces during its manufacture, storage, transport and while going through other subsequent processes such as printing, die-cutting, slitting and automatic application devices.

This protection is typically provided by a silicone coated paper or plastic film, often referred to as the release liner or backing. Just prior to the final end use of the label or tape, the liner is peeled away and discarded. Self-adhesive labels are manufactured by die-cutting the label shape in a self- adhesive laminate and peeling away the waste "skeleton" .

A typical self-adhesive laminate is shown in Fig. 1 , which is a greatly enlarged cross-section (not to scale) of a known laminate. The laminate comprises a base layer of a liner 10 (which may be paper or film). A silicone coating 12 is applied to one surface of the liner 10 at an application rate of typically lg/m² and a pressure sensitive adhesive layer 14 is applied on top of the silicone coating at an application rate of typically 20g/m². Finally, the label material 16 is positioned on top of the pressure sensitive adhesive.

In the production of, for example, self-adhesive labels, the release liner is used as the carrier for the adhesive coating. The majority of self-adhesive laminates used for label production is manufactured on fully integrated equipment, often referred to as an "in-line" coating and laminating machine.

The silicone coating can be applied as an emulsion, a solvent solution or as a 100% active fluid (solvent-less) with the consistency of a motor oil. In order for the silicone to provide a benign, non-stick coating which protects but which does not affect the performance of the adhesive it must be hardened or "cured" on the paper or plastic film surface.

The typical manufacturing process for a self-adhesive label is illustrated in Fig. 2. A continuous web W of uncoated liner 10 (of paper or film) is fed to a silicone coating machine 100 where the silicone coating is applied. The coated paper is then passed to a hot air curing oven 102 in order to cure the silicone coating. Typically the dwell time of the paper in the oven is a matter of seconds (typically 2 to 5 seconds) . Although the silicone coating thickness is only typically 1 micron (10^"6 m) the relatively short dwell time in the hot air curing oven 102 is insufficient to raise its temperature to the curing point. Instead, the heat required to raise the coating temperature comes from the liner by conduction. Typically, by the time the liner temperature has reached 90 °C the silicone coating thereon has been cured.

If a paper liner is used, one unwanted side effect of passing it through the hot air curing oven 102 is that the liner will loose some of its moisture of equilibrium (approximately 6%). This results in curling of the paper at the edges and cockling across the paper surface. These defects must be reversed by rehumidifying the paper by passing the liner with the cured silicone coating thereon through a rehumidifying unit 104. The web 10 is then rewound and stored until required.

When it is desired to apply adhesive to the coated paper the wound web is unwound and is passed (in a process separate from the application of silicone coating) through an adhesive coating machine 106 for applying the pressure sensitive adhesive 14, and thence to a drying oven 108 for drying the adhesive and a further rehumidifying unit 110. The web is then rewound again. When the labels are to be applied, the wound web is unwound and fed (in a separate process) to a unit 112 for applying the label material 16. The formed laminate L is then rewound and passed to further processing stages such as slitting, to label printing and converting processes.

One of the problems with the known method and apparatus for forming self-adhesive laminates is that the hot air curing oven 102 is large (typically about 5m long and 3.0m wide), is very expensive (typically in excess of £250,000 at 1996 prices) and has a high energy demand. Moreover, if the liner layer 10 is paper then, as mentioned above, a rehumidifying unit 104 is required following the curing of the silicone release layer. The rehumidifying units are large, expensive and very often form the rate determining step of the whole production operation since paper loses water much more quickly than it reabsorbs it.

It is an aim of the present invention to overcome completely or reduce the impact of one or more of the above disadvantages with the known apparatus and method.

In accordance with a first aspect of the present invention, a method of curing a release coating applied to a substrate comprises applying direct heat from a flame and displacing the substrate and the flame relative to one another.

Preferably, combustion products from the flame contact the coating. For example, the flame may impinge directly on the surface of the coating in order to cure the coating and to bond it onto the surface of the substrate.

The precise fuel/air mixture, temperature of the flame and its degree of impingement on the coating result in very rapid curing of the coating. If a paper substrate is used, the very rapid curing results in only a minimal loss of paper moisture.

It is believed that, when a flame contacts the coating, the coating is cured as a result of being enveloped in a hot plasma liberated from the combustion of fuel gases. It is believed that the hot plasma combines heat energy and free radicals which attach themselves to the structure of the coating, thereby producing a cross-linking cure.

The flame may be substantially fixed in position and the substrate may be moved past the flame.

The present invention is particularly applicable to continuous curing of a coating on a moving web of liner as part of the manufacturing process for a self-adhesive laminate. In such cases, curing of the coating effectively occurs almost instantaneously across the width of the web, in contrast to the prior art arrangements in which the web has a dwell time of several seconds through the hot air curing oven. This results in the apparatus for carrying out the method of the present invention being small and compact, in contrast to the hot air curing ovens of the prior art.

In order to reduce moisture loss from the substrate, its temperature is preferably kept to a minimum by cooling the substrate during curing. If the substrate is in the form of a continuous web, the back of the paper web can be passed over a cooled (e.g. liquid-cooled) roller or drum positioned directly beneath the flame impingement area.

Direct heat from a flame may be applied at a plurality of spaced-apart locations of the coated substrate. The locations where heat is applied may be spaced apart in the direction of travel of the substrate and/or transversely thereto. Direct heat may be applied across substantially the whole width of the substrate.

The present invention also includes a method of producing a coated, continuous, moving substrate, comprising the steps of applying an uncured coating to the substrate and curing the coating in accordance with the first aspect of the present invention.

The present invention also includes a method of producing a laminate comprising producing a coated, continuous, moving substrate in accordance with the present invention and applying a layer onto the cured coating. The layer may comprise adhesive and a further layer (e.g. which will form a label or a plurality of labels) may be applied onto the adhesive layer.

In accordance with a second aspect of the present invention, an apparatus for curing a coating applied to a substrate comprises a burner positioned adjacent to the substrate such that the coated substrate is heated directly by a flame from the burner and means for displacing the substrate and burner relative to one another.

Preferably, the burner is positioned such that combustion products from the flame contact the coating. For example, the burner may be arranged such that the burner flame impinges directly on the coating to be cured.

There may be means for cooling the substrate as the coated substrate is heated.

The burner may be substantially fixed in position and the substrate may be moved past the burner.

In one arrangement, the substrate is in the form of a web which passes over a roller or drum positioned directly beneath the flame impingement area. Preferably, the roller or drum is cooled to prevent excessive heating of the substrate. Conveniently, the roller may be liquid-cooled.

There may be a plurality of burners and/or burner parts which are arranged such that the coated substrate is heated directly by a flame at a plurality of spaced- apart locations. The locations may be spaced apart in the direction of travel of the substrate and/or transversely thereto. The burners may be arranged to apply a flame across substantially the whole width of the substrate.

There may also be means for conveying a substrate in the form of a continuous web. The present invention also includes an apparatus for producing a coated, continuous, moving substrate, comprising means for applying an uncured coating to the substrate and an apparatus for curing the substrate in accordance with the second aspect of the present invention.

The present invention also includes an apparatus for producing a laminate, comprising apparatus for producing a coated, continuous, moving substrate in accordance with the present invention and means for applying a layer onto the cured coating. There may be means for applying a layer of adhesive and means for applying a further layer (e.g. which will form a label or plurality of labels) onto the adhesive layer.

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

Fig. 1 is a cross-section (not to scale) of a self-adhesive label, which can be formed with the present invention;

Fig. 2 is a schematic illustration of a conventional method and apparatus for producing self-adhesive labels;

Fig. 3 is a schematic illustration of an apparatus for forming a self-adhesive laminate, in accordance with the present invention;

Fig. 4 is a perspective view illustrating a first embodiment of apparatus for curing a release coating, in accordance with the present invention;

Fig. 5 is a side view of a second embodiment of apparatus for curing a release coating, in accordance with the present invention; and

Fig. 6 is a side view of a third embodiment of apparatus for curing a release coating, in accordance with the present invention.

Referring to Fig. 3, a method of producing a self-adhesive laminate comprises many of the steps of the known apparatus as illustrated in Fig. 2, and the same components and steps will be identified with the same reference numerals where applicable.

A web of uncoated liner 10 is passed through a silicone coating machine 100. However, instead of the hot air curing oven 102 and re- hydrating unit 104 of Fig. 2, the coated web passes through a flame curing apparatus 200, which will be described in more detail hereinafter with reference -to Fig. 4.

The silicone coating is cured in the direct flame curing apparatus 200 and thereafter passes through the adhesive coating machine 106, the drying oven for the adhesive 108, the rehumidifying unit 110 and the application of the label layer at step 112 to form the complete laminate.

Alternatively, as indicated in dotted lines in Fig. 3, the web may be rewound after the flame curing step 200 and/or after the re-humidifying step 110, and unwound prior to the respective next step, in a similar manner to the prior art as described above with reference to Fig. 2.

Referring to Fig. 4, one apparatus 200 for curing the release coating comprises a drum or roller 210 over which the coated web 10 passes. The rotational axis of the drum 210 is aligned perpendicularly to the direction of the travel of the web and the drum 210 is rotatably mounted in a bath 212 containing cooling water. Excess cooling water is removed from the surface of the drum by means of a squeegee-type member 214.

A longitudinal gas/air burner 212 or a plurality of burners, dependent on the web speed, is arranged with its axis parallel to the rotational axis of the drum 210, i.e. transversely to the direction of movement of the web or across the width of the web 10. The burner 212 has a plurality of downwardly facing flame apertures or parts and is positioned such that the combustion products from the burner, preferably the flames 214 from the burner, impinge directly upon the uncured coated surface of the web 10.

As the web 10 emerges from the silicone coating machine 100, it is guided around a portion of the periphery of the drum 210 by means of an input guide roller 216 and an exit guide roller 218. As the web 210 passes over the uppermost portion of the drum 210 the silicone release coating is heated directly by the combustion products (in this case the flames 214) emerging from the burner 212. The application of the hot flame to the silicone release coating cures it very rapidly and the web 10 with the cured coating on the upper surface thereof thereafter passes to the adhesive coating machine 106.

A second embodiment of apparatus for curing a release coating is illustrated in Fig. 5. The apparatus comprises an internally water-cooled drum or roller 230 over which the coated web 10 passes. Several longitudinal gas/ air burners 212 are arranged with their axes parallel to the rotational axis of the drum 230, i.e. across the width of the web 10 and are spaced apart in the direction of movement of the web. The burners 212 have a plurality of downwardly facing flame apertures and are positioned such that the combustion products from the burner, preferably the flames 214 from the burner, impinge directly upon the uncured surface of the web 10.

As the web 10 emerges from the silicone coating machine it is guided around a portion of the drum periphery by means of an input guide roller 232 and an exit guide roller 234. As the web passes the burners 212 the silicone coating is heated directly by the combustion products (in this case the flames 214) emerging from the burners, causing the coating to cure rapidly. The web then passes to the adhesive coating machine.

The third embodiment, shown in Fig. 6, is very similar to the second embodiment and uses the same internally water-cooled drum 230. In this particular embodiment, only two longitudinal burners 212 are arranged across the web. However, by the use of three guide rollers 236,238,240 the web is guided round a greater portion of the of the periphery of the drum 230, for a greater cooling of the web 10.

In each of the embodiments, it will be noted that at the point of application of the flames 214 to the release coating on the web, the undersurface of the web is in contact with the water-cooled drum 210 or 230. Thus, the heat transfer to the web 10 is minimised. This is particularly important in the case of paper webs, in which cooling of the web prevents loss of a significant amount of moisture from the web and thus removes the need for a rehydrating step immediately after the curing step, in contrast to the prior art.

The invention is not restricted to the details of the foregoing embodiments. For example the embodiments show that the burner (or burners) are stationary and the web moves past the burner. However, it would be possible instead for the web to be stationary, or moved intermittently, and for the burner (or burners) to be moved over the stationary web or stationary portion of the web. Also, although the longitudinal burners are described and illustrated as having a plurality of apertures or parts forming several separate flames it would be possible for the burner to have an elongate aperture, preferably extending across the width of the substrate, for producing an elongate flame "sheet".

Claims

1. A method of curing a coating applied to a substrate, comprising applying direct heat from a flame and displacing the substrate and the flame relative to- one another.

2. A method as claimed in claim 1, wherein combustion products from the flame contact the coating.

3. A method as claimed in claim 2, wherein the flame impinges directly on the coating.

4. A method as claimed in any of claims 1 to 3, wherein the substrate is cooled during application of the direct heat from the flame.

5. A method as claimed in any of claims 1 to 4, wherein direct heat from a flame is applied at a plurality of spaced-apart locations.

6. A method as claimed in any of the preceding claims, wherein the flame is substantially fixed in position and the substrate is moved past the flame.

7. A method as claimed in claim 5, wherein the substrate comprises a continuous, moving web.

8. A method as claimed in claim 7, wherein the direct heat from the flame is applied as the web passes over a roller or drum.

9. A method as claimed in claim 8, wherein the roller is cooled.

10. A method as claimed in any of claims 7 to 9, wherein direct heat from a flame is applied at a plurality of locations spaced-apart in the direction of travel of the web.

11. A method as claimed in any of claims 7 to 10, wherein direct heat from a flame is applied at a plurality of locations spaced apart in a direction transverse to the direction of travel of the web.

12. A method as claimed in claim 11 , wherein direct heat from a flame is applied across substantially the whole width of the web.

13. A method as claimed in any of claims 1 to 12, wherein the substrate comprises paper.

14. A method as claimed in any of claims 1 to 13, wherein the coating comprises a release coating.

15. A method as claimed in claim 14, wherein the release coating comprises a silicone release coating.

16: A method of producing a coated, continuous, moving substrate, comprising the steps of applying an uncured coating to the substrate and curing the coating in accordance with a method as claimed in any of claims 1 to 15.

17. A method of producing a laminate comprising producing a coated substrate in accordance with a method as claimed in claim 16, and applying a layer onto the cured coating.

18. A method as claimed in claim 17, comprising applying a layer of adhesive onto the coating.

19. A method as claimed in claim 18, comprising applying a further layer onto the layer of adhesive.

20. A method as claimed in claim 19, wherein the further layer comprises a label or a plurality of labels.

21. Apparatus for curing a coating applied to a substrate, comprising a burner positioned adjacent to the substrate such that the coated substrate is heated directly by a flame from the burner and means for displacing the substrate and burner relative to one another.

22. Apparatus as claimed in claim 21, wherein the burner is positioned such that the combustion products from the flame contact the coating.

23. Apparatus as claimed in claim 22, wherein the flame impinges directly on the coating.

24. Apparatus as claimed in any of claims 21 to 23, further comprising means for cooling the substrate as the coated substrate is heated.

25. Apparatus as claimed in any of claims 21 to 24, wherein the burner is substantially fixed in position and the substrate is moved past the burner.

26. Apparatus as claimed in claim 25, wherein the substrate cooling means comprises a roller or drum over which the substrate passes as the coated substrate is heated.

27. Apparatus as claimed in claim 26, wherein the roller or drum is cooled.

28. Apparatus as claimed in claim 27, wherein the roller or drum is liquid-cooled.

29. Apparatus as claimed in any of claims 21 to 28, comprising a plurality of burners or burner parts which are arranged such that the coated substrate is heated directly by a flame at a plurality of spaced-apart locations.

30. Apparatus as claimed in claim 28, wherein the locations are spaced apart in the direction of travel of the substrate.

31. Apparatus as claimed in claim 30, wherein the locations are spaced apart in a direction transverse to the direction of travel of the substrate.

32. Apparatus as claimed in claim 31 , comprising means for producing a flame or a plurality of flames across the width of the substrate.

33. Apparatus as claimed in any of claims 21 to 32, comprising conveying means for conveying a substrate in the form of a continuous web.

34. Apparatus for producing a coated, continuous, moving substrate, comprising means for applying an uncured coating to the substrate and apparatus for curing the coating as claimed in any of claims 21 to 33.

35. Apparatus for producing a laminate, comprising apparatus for producing a coated, continuous, moving substrate as claimed in claim 34 and means for applying a layer onto the coating.

36. Apparatus as claimed in claim 35, comprising means for applying a layer of adhesive onto the coating.

37. Apparatus as claimed in claim 36, comprising means for applying a further layer onto the layer of adhesive.