One type of tack mat, which is known as a"peel-off mat", consists of a stack of sheets of paper, each of which has an adhesive coated upper surface. As people step on the mat, dust and other contaminants are retained on the sticky surface of the mat. After a while, as the contaminants build up, the effectiveness of the mat is reduced. The top sheet of paper is then peeled away to reveal a fresh adhesive coated surface. These mats are not washable and must be replaced when all the sheets of paper have been exhausted.

Also known is a type of flooring designed for use in clean rooms, which is sold by Dycem Limited under the trade marks CLEAN ZONE and PROTECTAMAT. These products have a dust retaining surface made of a highly plasticised polyvinyl chloride (PVC) composition. The plasticiser migrates through the PVC composition and as a result the dust retaining surface is always coated with a thin layer of plasticiser, giving it a tacky surface. The product can be washed, using detergent and water, to remove the surface layer of plasticiser and any entrapped contaminants. After washing, the tacky surface is quickly restored by migration of plasticiser through the PVC composition. A highly plasticised material and floor coverings made of that material are described in British Patents 1399191, 1475366, 2025319 and 2144139, the contents of which are incorporated by reference herein.

Although the flooring compositions described above are normally used to cover the entire floor area of a clean room, smaller pieces have also been used as loose laid mats, which can be placed outside the entrances to clean rooms. The advantage of these loose laid mats over

the peel-off mats described above is that if they are regularly cleaned, they can be used for several years, thereby providing protection much more cheaply than a peel-off mat. The disadvantages are that the mat can present a trip hazard, it is not very stable and can move around in use, and regular cleaning, which is essential to maintain effectiveness, is difficult.

It is an object of the present invention to provide a method of manufacturing a mat that mitigates at least some of the aforementioned disadvantages.

According to the present invention there is provided a method of manufacturing a mat, in which a top layer including a sheet of a highly-plasticised PVC material is bonded to a backing layer of uncured rubber by pressing the top layer and the backing layer together while heating the mat to a temperature sufficient to soften and cure the rubber material.

This bonding process produces a strong bond between the highly-plasticised PVC material and the rubber backing layer. The rubber backing layer supports and adds stability to the mat, making it lie flat on the floor without moving. It also reduces the potential trip hazard.

The provision of a rubber backing makes it possible to wash the mat using a commercial washing machine, making it much easier to remove contaminants and restore the effectiveness of the mat.

The highly plasticised PVC material is bonded to the rubber backing layer by means of heat and pressure, for example in a heated press. This cures the rubber, at the same time bonding it to the highly plasticised PVC material. A similar process is used for making dust control mats with a tufted pile, as described in our European patent application, Publication No: EPO 847 327, the contents of which are incorporated by reference herein.

There are, however, a number of complications associated with the use of this process to manufacture a tack mat. First, when making rubber backed dust control mats it is normal to cover the laid-up mat, before it is placed in the heated press, with a release sheet made of PTFE coated woven glass, to prevent the heated rubber from sticking to the press.

Similarly, a release sheet is preferably used when making a rubber-backed tack mat, to prevent it from sticking to the press. The textured surface of the release sheet also helps gas to escape from between the press and the PVC layer.

One result of using a woven release sheet is that it produces a woven fabric imprint in the surface of the mat. Whilst this is desirable in the rubber border of a tufted pile mat as it improves the mat's appearance, it is undesirable in a tack mat, which requires a very smooth surface to ensure that contaminants are completely removed from the feet of people stepping on the mat.

One aspect of the present invention is therefore concerned with the problem of making a tack mat that has a rubber backing layer and a very smooth top surface.

Another complication associated with use of the above-mentioned process to manufacture a rubber backed tack mat is that it is very difficult to prevent air bubbles being trapped between the highly plasticised PVC material and the rubber backing. The inventors have found that this problem may be partially alleviated by using a highly plasticised PVC layer that has already been bonded to a backing layer of woven polyester. However, this is not sufficient on its own to eliminate the problem.

A further aspect of the invention is therefore concerned with a method of manufacturing rubber backed tack mats, without forming air bubbles between the highly plasticised PVC material and the rubber backing.

According to a preferred embodiment of the invention, the top layer is perforated prior to the bonding process. The perforations may have a diameter in range 1-5mm, preferably approximately 2.4mm. The average separation of the perforations may be in the range 5- 50mm, and is preferably approximately 17. 5mm.

The perforations allow gas to escape from between the top layer and the backing layer during the bonding process, so avoiding the formation of bubbles and ensuring a good bond between the two layers.

Advantageously, at least the top layer of the mat is covered during the bonding process by a release sheet. The release sheet preferably comprises a sheet of PTFE-coated woven glass. The release sheet prevents the mat from sticking to the press and helps gases to escape.

The weave of the release sheet is preferably finer than 10 ends/cm. This creates a very smooth upper mat surface, which is important for efficient contaminant removal. The backing layer of the mat may also be covered during the bonding process by a release sheet.

Advantageously, after the bonding process has been completed, the mat is allowed to cool to a temperature of less than 50 °C and preferably less than 40 °C, before the release sheets are removed, to avoid damaging the surface of the mat.

After the bonding process has been completed, the mat is preferably kept substantially flat until it has cooled substantially to room temperature.

After the bonding process has been completed, the mat may be pressed for a second time in a heated press, to provide a substantially smooth upper surface and remove any surface markings from the mat.

The top layer may include a supporting layer of woven fabric, for example of woven polyester. This helps to improve bonding between the top layer and the backing layer.

The mat may be heated in a press at a temperature in the range 150-200 ° C, and preferably approximately 165°C. The mat may be heated for a time in the range 10-35 mins, and preferably approximately 25 mins.

The mat may be heated in a flat bed press. Alternatively, the mat may be pressed between heated rollers.

The highly-plasticised PVC material may include 15%-45% by weight of PVC and 55%- 85% by weight of a plasticiser.

The backing layer may have a thickness in the range 1-3 mm, and preferably approximately 1.8 mm. Advantageously, the backing layer has tapered edges, which significantly reduce the trip hazard. The top layer may have a thickness in the range 1-3 mm and preferably approximately 2 mm.

Various embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic side section of a finished tack mat; and Figure 2 is a schematic side section of a tack mat laid up in a press.

The tack mat 2 includes a top layer 4 comprising a sheet of highly plasticised PVC material that is bonded to and supported by a sheet of woven polyester. The top layer 4 is bonded to a backing layer 6 of natural or synthetic rubber. The rubber backing layer 6 extends outwards beyond the top layer 4, forming a peripheral border 8. The edges of the border 8 are tapered to reduce the risk of creating a trip hazard.

A process for making a tack mat will now be described with reference to Figure 2, which is a schematic exploded cross-section of a tack mat as laid up in a press during manufacture. The press includes a heated metal platen 10 that is mounted above an inflatable diaphragm 12. In the case of a press having a fixed opening aperture, the platen 10 and the inflatable diaphragm 12 are both fixed in position. Alternatively, one or both of the platen and the diaphragm may be movable relative to the other, to provide a press with a variable opening aperture.

The lay up consists of the following items placed in order on the inflatable diaphragm 12: a bottom release sheet 14 of PTFE coated woven glass, a sheet of uncured nitrile rubber 16 that forms the backing layer 6, a sheet 18 of highly plasticised PVC material on a supporting sheet 20 of woven polyester (these sheets 18,20 forming the top layer 4 of the finished mat) and a top release sheet 22 of PTFE coated woven glass fabric.

The highly plasticised PVC material 18 preferably comprises from 15-45% of polyvinyl chloride and from 55-85% by weight of a plasticiser. Suitable highly plasticised PVC materials are described in GB 1399191, GB 1475366 and GB 2025319. The material may have a thickness of 1-3 mm, our preferred thickness being 2 mm (although thinner materials may also be suitable).

The supporting layer of woven polyester may for example be that sold under the trade name ARVEX MFT 351, which has the following specification: Fibre: multifilament polyester/multifilament polyester Weave: Plain Weight: 260 gsm Finish: Loom state Warp threads : 13/cm Weft threads : 11/cm Tensile warp: 4200 N/5cm Tensile weft: 1800 N/5cm Thickness : 0.54 mm The top layer 4 comprising the sheet 18 of highly plasticised PVC material and the supporting sheet 20 of woven polyester is perforated prior to being laid up in the press. For example, we perforate the material with 3/32" (2.4mm) diameter pins on a 17. 5mm triangular pitch. The perforations (which are not shown in the drawing) allow air to escape from between the top layer 4 and the rubber backing layer 6 during the pressing operation.

The rubber backing layer 6 is preferably made of nitrile rubber and advantageously has a thickness in the range of lmm-3mm, preferably about 1. 8mm (we use a thickness of 0. 070" = 1. 78mm). The dimensions of the rubber backing layer may typically comprise a width of 850mm and a length of 1500mm, although other sizes are of course possible. The dimensions of the top layer 4 are slightly less, to provide a rubber border 8 around the periphery of the mat that is about 20-30mm wide.

The release sheets 14,22 cover the laid up mat to prevent the rubber backing sheet 16 and the highly plasticised PVC layer 18 from sticking to the press. The textured surface of the release sheet also allows gas to escape from between the release sheet and the PVC layer.

Being made of a woven material, the release sheets tend to impart a woven fabric imprint onto the exposed surfaces of the mat. This imprint reflects the fineness of the weave, which is by determined by the diameter of the glass filaments, the construction of the warp and weft threads, the type of weave and the weight of the Teflon coat. In order to keep the top

surface of the mat as smooth as possible, the upper release sheet 22 is made of a very finely woven glass fabric, for example that sold by CS-Interglas Ltd of Dorset, England and identified as Alpha Maritex Teflon TC 217, which has the following specification: Fabric Yarn : warp EC5-22x2x3 weft EC5-22x2x3 Thread count: warp ends 11. 0/cm weft ends 11. 5/cm Tensile strength: warp 700 N/cm weft 720 N/cm Weight 319 g/m2 Weave plain Thickness 0. 4mm 10% Coated fabric Coating PTFE grey both sides Weight 575 g/m2 Thickness 0.43 mm Tensile strength warp 540 N/cm weft 590 N/cm Temperature resistance-36 to 260°C In the yarn specification EC5-22x2x3 the letter E indicates the filaments are of glass, C indicates continuous filaments, and 5 indicates mat the diameter of the filaments is 5 microns. The number 22 indicates 22 tex (i. e. the weight of 1000m of the yarn is 22g). A 22 tex yarn is made up of 34 filaments of 5 microns diameter. The term x2x3 indicates that two of these threads are twisted together and three of the two ply threads are twisted together to produce the yarn for the warp and weft yarns.

The press is heated to a temperature of about 170°C. The laid up tack mat is positioned between the platen 10 and the diaphragm 12, for example on a conveyor belt or tray, and the diaphragm is then inflated, for example to a pressure of 301bs psi, pressing the laid up tack mat against the heated platen, for cycle time of 10-35 minutes, typically about 25 minutes. This cures the rubber and creates a strong bond between the top layer 4 and the

backing layer 6. The edges of the rubber backing layer also become slightly tapered as the heated rubber flows outwards.

During the curing process, any gas trapped between the top layer 4 and the rubber backing layer 6 escapes through the perforations, thereby creating a good bond between the two layers. The imperfections formed in the top layer by the perforations are removed during curing as the PVC material softens and flows.

After completion of the curing process, the mat is removed from the oven and is allowed to cool to a temperature of less than 50°C, preferably about 40°C, to allow the surface of the mat to cool and harden. This usually takes about 3 minutes. The release sheets 14,22 can then be removed.

After the release sheets 14,22 have been removed, the mat is allowed to cool further to room temperature. The mat is kept flat during this time, as if it is rolled or folded it will retain a shape memory and will not lie flat during use. Finally, the rubber borders 8 are trimmed and the mat is prepared for dispatch.

An alternative manufacturing method comprises manufacturing a mat substantially as described above, although in this case the very fine weave top release sheet 22 may be replaced by a normal release sheet with a coarser weave. After the mat has been removed from the press and the release sheets (or at least the top release sheet) have been removed, the finished mat is pressed between smooth rollers, which are heated to a temperature of approximately 100°C. This reheats and softens the PVC material and removes the imprint left by the release sheet, leaving a very smooth surface.

For an even smoother surface, the two methods may be combined, in which case a mat that already has a very smooth surface owing to use of a fine weave top release sheet is made even smoother by pressing it between heated rollers.

Various modifications of the invention are possible. For example, simple logos and messages may be created in the top surface of the mat by cutting out and inserting shapes of different colours (in a similar manner to marquetry). The mat may also be provided with a reinforced border edge by moulding the mat within a fiame, for example as described in our European patent application, Publication No: 0 847 327.