IES20050132A2 - Method and apparatus for coating a substrate with a hot flowable viscous adhesive - Google Patents

Abstract

A method and apparatus for applying a coating of a hot flowable adhesive to one surface of a thin flexible substrate.The adhesive is heated to above the degradation temperature of the substrate, so as to coat the substrate. The adhesive is applied while the opposite surface of the substrate is in intimate contact with a heat sink, the heat sink being cooled to a temperature substantially below the degradation temperature such that heat is conducted away from the substrate so that the substrate does not reach its degradation temperature and is consequently undamaged. <Figure 1>

Description

This invention relates to a method and apparatus for coating a thin flexible substrate with a hot flowable viscous adhesive. The invention is particularly, but not exclusively, applicable to coating a hot-melt adhesive onto a thin sheet of PVC to be used as a veneer (also known as a PVC foil) in the manufacture of items of furniture such as kitchen doors, drawer fronts, pelmets and the like.

Current methods in the manufacture of MDF-based kitchen furniture and fittings generally comprise the following steps : 1. Manufacturers purchase sheets of MDF and machine them to the shape required (e.g. doors, panels, etc.) . 2. The MDF doors or panels are sprayed with a twopack heat activated adhesive. This is a timeconsuming process, since the components react on contact forming a sticky surface which requires several hours to properly dry. It is also prone to quality control issues, since the MDF may not be sprayed evenly, e.g. due to bad workmanship. If an area of the MDF, particularly at an edge, is uncoated, then the veneer is likely to come away from the substrate within a short period of use.

INT rr'ki.AU H/oo £/>9, I /o* IE 05 0 1 3 2 3. A thin flexible PVC material is purchased, with different patterns for different wood effects. The PVC is not coated with adhesive. The PVC is in rolls approximately 1.5m in width and 150-250m in length, and has a thickness of about 300450 microns. 4. The PVC is pulled off a roll in sheets of 3m length onto a special press, and laid on top of the MDF doors that have been sprayed with adhesive. The press then produces a vacuum and heats the assembly up to 120°C, which activates the adhesive on the MDF doors and allows the PVC to stick and form a permanent bond.

In the above procedure, the MDF is sprayed with the adhesive, set aside for drying of several hours, and the PVC bonded to it. This is particularly timeconsuming and costly. There are important reasons why the industry has had to use this process rather than some faster adhesive application process.

One of the main limitations is the need for an adhesive that will meet certain safety standards. A furniture panel mounted above a cooker must be stable at quite high temperatures. British Standard BS 6222 Part 3 (and equivalent standards in other systems) specifies that veneer or foil coated panels must be able to withstand elevated temperatures over a period of time without exhibiting reduced adhesion between the foil and the panel. One therefore needs an adhesive that sets solid at temperatures below about 70-90 degrees IE 0 5 0 ί 32 centigrade. Two pack adhesives which react to form a solid adhesive at room temperature are therefore particularly useful because the components may be liquid at room temperature before meeting, at which point the reaction occurs.

Meltable adhesives, such as are used in hot flow processes, must have a melting point above the test temperatures. Such adhesives are typically extremely viscous at lower temperatures and need to be heated to 200 degrees or so to allow them to be used in liquid application processes. Such adhesives cannot be readily sprayed because of the high temperatures involved. Hot melt application processes are unsuitable for typical furniture panels because of the need to coat irregular surfaces, corners, routed features, protrusions, and the like.

A better solution would be to coat the PVC with the adhesive, which would then allow the direct application of the PVC to the MDF without the spraying and handling involved in coating the MDF.

The normal application of an adhesive to PVC is through use of a release-liner system. However, it is difficult to find a satisfactory adhesive that can be applied with a release liner technique at a temperature below that at which PVC foil will degrade. In any event, use of this method results in greater effort (in applying and subsequently removing the liner), and greater cost due to the extra materials involved.

IE 0 5 0 1 32 Directly coating the PVC with the adhesive (e.g. using a hot melt technique) faces the problem that the hotmelt adhesive is very viscous and must be heated to about 205-210°C for it to be liquefied sufficiently for application. If PVC comes into contact with a substance at this temperature, the PVC instantly begins to shrivel up and melt (e.g. like a foil-coated plastic snack packet - such as a packet of potato crisps or chips - placed in an oven or on a fire).

Accordingly, the invention provides a method of applying a coating of a hot flowable adhesive to one surface of a thin flexible substrate, the adhesive being above the degradation temperature of the substrate, wherein the adhesive is applied while the opposite surface of the substrate is in intimate contact with a heat sink cooled to a temperature substantially below the degradation temperature such that heat is conducted sufficiently rapidly away from the substrate that the latter does not reach its degradation temperature.

By degradation temperature we mean the substrate temperature above which the character or quality of the substrate would significantly deteriorate. This is about 150°C for the thin flexible PVC sheeting typically used in furniture manufacture as described above and used in the preferred embodiment.

Preferably the substrate is conveyed past the heat sink as a continuous length from a supply roll to a take-up ΙΕ ο 5 0 1 3 2 roll, and the adhesive is applied to the substrate as it passes the heat sink.

Most preferably the heat sink comprises at least one cooled roller around which the substrate passes.

The invention further provides a method of veneering an item of furniture, comprising the steps of coating a thin flexible substrate with a hot melt adhesive by the method specified above, drying the adhesive, applying the substrate to the item of furniture, and applying heat and pressure to the substrate to activate the adhesive and cause the substrate to conform to and bond to the item.

The invention further provides a slot die for coating a substrate with a hot flowable viscous adhesive, the die comprising top and bottom elongated substantially parallel die members separated by a shim, the shim defining an adhesive exit slot between longitudinal forward edges of the die members and preventing exit of the adhesive from between longitudinal rear edges of the die members, wherein the forward edge of the bottom die member extends beyond the forward edge of the top die member to form a narrow ledge on the bottom die member, whereby in use a viscous adhesive extruded from the slot beyond the forward edge of the top die member is received on the ledge before being picked up by a substrate moving past the forward edge of the bottom die member.

IE 05 0 132 In general, use of a wide extrusion slot die means that it is harder to control the coating weight of the substance to be applied. The ledge created between the forward edges of the die members allows for a more even coating weight of the adhesive to be applied, and can be altered depending on how far the forward edge of the bottom die extends beyond the forward edge of the top die .

The invention further provides a slot die for coating a substrate with a hot flowable viscous adhesive, the die comprising top and bottom elongated substantially parallel die members separated by a shim, the shim defining an adhesive exit slot between longitudinal forward edges of the die members and preventing exit of the adhesive from between longitudinal rear edges of the die members, wherein the shim comprises a flat elongated back portion and a pair of flat arms extending forwardly from each end of the back portion to define an elongated generally U-shaped flat planar body which confines the adhesive on three sides and is open towards the adhesive exit slot, wherein the inside edges of the arms diverge away from the back portion such that the distance between the free ends of the arms is greater than the distance between the points at which the arms meet the forward edge of the back portion, whereby the length of the exit slot is greater than the length of the forward edge of the back portion between the arms.

Use of conventional shims in extrusion slot dies can prevent the even spread of the coating substance to the IE 0 5 0 ί 3 2 ends of the die. The shim in the present application has tapered arms edges, which allow for an even flow of the substance to be applied along the whole length of the slot die. In preferred embodiments the arms taper to a point at the forward corner of the die.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is coating a adhesive .

Fig. 2 is apparatus a schematic side view of an apparatus for thin flexible PVC substrate with a hot-melt an exploded view of a slot die used in the of Fig. 1.

Fig. 3 is a top view of the slot die of Fig. 2, omitting the top die member.

Fig. 4 is a cross-section through the slot die taken on the line IV-IV of Fig. 3.

Fig. 5 is an enlarged view of the adhesive exit slot of the slot die of Fig. 2.

Referring initially to Fig. 1 of the drawings, the apparatus comprises PVC supply and take-up rolls 10 and 12 respectively, rotatable about substantially parallel axes. Initially the roll 10 is full and the roll 12 is empty. The PVC on the full roll 10 is a continuous flexible sheet 150-250m long, and is about 1.5m wide IE 0 5 0 ί 32 and, depending on the application, 300-450 microns thick. At this stage it is not coated with adhesive.

In use the PVC sheet 20 is conveyed continuously from the supply roll 10 to the take-up roll 12 along a path defined by guide rollers 14 and refrigerated rollers 16, 18.

Solid adhesive to be applied to the PVC sheet 20 is added to a melt tank 22 where the adhesive is heated to melt it to form a viscous liquid. In this embodiment the adhesive is a polyamide adhesive and is heated to a temperature of approximately 205°C, which reduces the viscosity of the molten adhesive to allow it to be pumped into and extruded from a die. The hot viscous adhesive is pumped along a supply line 24 to an adhesive applicator 26 including a slot die 28, Figs. 2-5, where the adhesive 30 (Fig. 5) is applied as a thin coating 32 continuously across the width of the moving PVC sheet 20. The speed of the PVC sheet is typically 10-50 meters/min and the adhesive applied with a coating weight in the range 40-75g per square meter. The slot die 28 is heated to about 200cC to maintain the temperature of the adhesive prior to application to the PVC, and is biased against the moving PVC sheet with a pressure of about 3 bar.

At the point at which the adhesive is applied to the exposed surface of the thin PVC sheet 20, the opposite surface of the sheet is in intimate contact with the refrigerated roller 16. The roller 16 is a hollow steel cylinder which contains 401trs of water and antifreeze at -5°C, the roller 16 being maintained at IE 0 5 0 1 32 this temperature by continuously pumping chilled water through it via standard rotary bearings. The roller 16 therefore acts as a relatively massive heat sink for the thin PVC sheet 20, so that the heat of the adhesive 30 and die 28 is rapidly conducted away from the PVC sheet by the roller. The pressure of the slot die 28 on the sheet 20 ensures that the PVC is in uniform intimate contact with the roller 16 across its full width.

As a result, the PVC sheet 20 (which is already cold since it was brought into contact with the roller 16 prior to reaching the die 28) does not have time to reach its degradation temperature before the PVC has moved away from the heated die 28 and the adhesive coating 32 has cooled to below the degradation temperature. In this embodiment a second refrigerated roller 18 is placed immediately downstream of the roller 16. The roller 18 is constructed and cooled the same as the roller 16. The roller 18 continues to cool the adhesive and ensure that the temperature of the PVC does not rise above the degradation temperature. The outer surface of the roller 16 may be covered with a thin layer of rubber to allow a certain amount of give when applying the adhesive. If desired the PVC sheet can be pre-cooled by a third refrigerated roller (not shown) located upstream of the roller 16.

By the time the PVC sheet 20 has left the chilled rollers 16, 18 the adhesive coating is dry and so the coated PVC can be directly wound into the take-up roll ΙΕ ο50 1 32 for storage for further use, without requiring a release liner.

If necessary, by swapping the now full take-up roll 12 with the empty supply roll 10, the adhesive-coated PVC sheet 20 can be passed a second time through the apparatus to increase the thickness of the adhesive coating. This might be necessary where the desired final thickness of adhesive, if applied in one pass at a given speed of the PVC sheet 20, would be too thick for sufficiently rapid cooling by the heat sinks 16, 18 to avoid degradation of the PVC sheet.

The adhesive-coated PVC sheet (or other veneer material) is now cut from the roll 12 in appropriatesized lengths for application to MDF furniture panels such as doors, drawer fronts, etc. This is done using a conventional press, the PVC being laid on top of the furniture panel, adhesive side down. The press then produces a vacuum and heats the assembly up to 120°C, which re-activates the adhesive on the PVC and allows the PVC to conform to and adhere to the MDF panel and form a permanent bond.

Referring now to Figs. 2-5, the slot die 28 comprises top and bottom elongated substantially parallel die members 40, 42 respectively. The die members 40, 42 are separated by a shim 44, the three components 40, 42, 44 being secured tightly together by bolts, not shown (the bolt holes 46 are shown in Fig. 3, but are not otherwise seen in the drawings). The die members ΙΕ ο 5 0 1 32 40, 42 are made of high tensile stainless steel while the shim 44 is made of brass or Teflon.

By maintaining the die members 40, 42 slightly spaced apart with the shim 44, a slot 48 is defined between the longitudinal forward edges 50, 52 of the die members 40, 42. However, the forward edge 52 of the bottom die member 42 extends beyond the forward edge 50 of the top die member 40 to form a narrow ledge 54 on the bottom die member 42 projecting forwardly of the upper die member 40.

A highly elongated oval trough 56 is set into and extends along the length of the upper surface of the bottom die member 42, and one or more adhesive entrance ports 58 (only one shown) communicate with the trough 56 from the rear of the die member 42. In use, adhesive is pumped into the trough 56 via the port(s) for distribution along the length of the die.

The shim 44 comprises a flat elongated back portion 44A and a pair of flat arms 44B extending forwardly from each end of the back portion to define an elongated generally U-shaped flat planar body which confines the adhesive on three sides and is open towards the slot 48. Thus the shim 44 prevents the exit of adhesive 30 from the ends and between the longitudinal rear edges of the die members 40, 42 and ensures that the adhesive exits only through the front slot 48. The inside edges 44C of the arms 44B diverge away from the back portion 44A at about 45° such that the distance between the free ends of the arms is greater than the distance ΙΕ ο 5 0 1 3 2 between the points at which the arms meet the forward edge of the back portion. Thus the length of the exit slot 48 is greater than the length of the forward edge of the back portion 44A between the arms 44B, allowing the slot 48 to extend substantially the full length of the die members 40, 42.

The length of the die members 40, 42 is approximately 1.5m (i.e. about the same as the width of the PVC sheet 20) and the depth of the adhesive exit slot 48, as determined by the thickness of the shim 44, ranges from 0.25mm to 2mm depending on the viscosity of the adhesive used. For example, a slot depth of 1mm would typically be needed for an adhesive having a viscosity of 44,000 millipascals. The width of the ledge 54, i.e. the amount by which the edge 52 projects beyond the edge 50, is about 0.25mm. In use the viscous adhesive extruded from the slot 48 beyond the forward edge 50 of the top die member 40 is received on the ledge 54 where it is picked up by the PVC sheet 20 moving past the forward edge 52 of the bottom die member 42. In order to minimise the area of die 28 in contact or near to the PVC sheet 20 the forward edges 50, 52 of the top and bottom die members 40, 42 project forwardly slightly relative to the main body of the die members .

The weight of the adhesive coating 32 on the PVC sheet depends upon the adhesive viscosity, the shim thickness, the rate of pumping the adhesive 30 to the slot die 28, and the speed of travel· of the PVC sheet . The latter two variables, as well as the ΙΕ ο 5 Ο 1 3 2 temperature of the adhesive and die, can be set by an operator control panel 34. The depth of the slot 48 can be set by using a shim 44 of the desired thickness.

The apparatus can be used to coat other thin flexible sheet material with other hot-melt adhesives. In all cases, however, the relevant parameters are selected so that the particular sheet material being coated does not reach its degradation temperature before the sheet material has moved away from the heated die and the adhesive has cooled to below the degradation temperature .

The invention is not limited to the embodiment described herein which may be modified or varied without departing from the scope of the invention.

IE 0 5 0 1 32 Fig. 1 ο ΙΕ ο 5 Ο 1 3 2 44Β 44Β Fig. 4 Fig. 5 ΙΕ ο 5 Ο 1 32

Claims (3)

1. The following revised Claims were filed on 3 F<1 March 2006 ΙΕ ο 5 0 1 32 slot beyond the forward edge of the top die member is received on the ledge before being picked up by a substrate moving past the forward edge of the bottom die member.

2. 4. A slot die for coating a substrate with a hot flowable viscous adhesive, the die comprising top and bottom elongated substantially parallel die members separated by a shim, the shim defining an adhesive exit slot between longitudinal forward edges of the die members and preventing exit of the adhesive from between longitudinal rear edges of the die members, wherein the shim comprises a flat elongated back portion and a pair of flat arms extending forwardly from each end of the back portion to define an elongated generally U-shaped flat planar body which confines the adhesive on three sides and is open towards the adhesive exit slot, wherein the inside edges of the arms diverge away from the back portion such that the distance between the free ends of the arms is greater than the distance between the points at which the arms meet the forward edge of the back portion, whereby the length of the exit slot is greater than the length of the forward edge of the back portion between the arms.

3. 5. A method of applying a coating of a hot flexible adhesive to one surface of a thin flexible substrate as substantially described in the description, with reference to the drawings.