GB2039465A - Process for the manufacture of a printed polyethylene sheet - Google Patents

Abstract

The invention relates to the manufacture of a printed polyethylene sheet which is very suitable for use as an exercise mat or play mat. According to the invention, a flexible sheet of polyethylene which is provided with print marks on one or both sides is subjected to a pretreatment, on the surface provided with print marks, in order to render the said surface receptive to adhesion, after which a transparent layer of polyethylene is extruded over the treated surface. Preferably, the extruded layer exhibits a matt surface after cooling.

Description

SPECIFICATION Process for the manufacture of a printed polyethylene sheet The invention relates to a process for the manufacture of a printed polyethylene sheet, wherein a flexible polyethylene sheet is printed in patterns with an ink.

What is particularly concerned is the manufacture of a polyethylene sheet which is suitable for use as an exercise mat or play mat, e.g. as commercially available under the name of "Twister" game mat.

The polyethylene sheet is produced from high pressure polyethylene by means of an extrusion process and possesses a glossy surface. Low pressure polyethylene, which after extrusion has a matt surface, cannot be used because the sheet obtainable with this material is, due to its greater brittleness, unsuitable for use as a play mat or exercise mat.

A suitable extrusion process is the so-called slit extrusion process, whereby the fluid heated polyethylene is forced through a slit-shaped orifice and is then cooled. A very suitable extrusion process is, above all, blown film extrusion, wherein a tube is extruded, which, before being cooled, is inflated, and is then cooled and cut open.

On the face which is to be printed, the polyethylene sheet is subjected to an oxidation treatment such as flame treatment or treatment with electric sparks (corona discharge), in order to render the surface receptive to the application of ink, after which ink is applied, in the desired pattern, to the surface which has thus been modified, in accordance with a printing process which is in itself known, such as the screen printing process.

In the Twister game mat the print pattern is formed by coloured circular print marks which are arranged colour-wise in parallel strips over the polyethylene surface.

The Twister game mat manufactured in accordance with the known method has the disadvantage that the print marks wear off very rapidly. Above all when the game mat is used on a hard base, such as paving, the footwear of the players exerts a strong friction force above all on the printed areas of the sheet, and results in rapid wear of the print marks. A further disadvantage is that the glossy surface of the sheet can give rise to troublesome light reflections.

Furthermore, the choice of printing ink is limited, in that the printing ink employed, present on the surface of the sheet, must not contain any harmful or toxic constituents.

Attempts have been made to reduce the rapid wear of the printed marks by manufacturing a Twister game mat based on a PVC sheet. Though some improvements can be found, due to the fact that the print marks have penetrated somewhat deeper in the case of the PVC sheet, the improvement achieved is still insufficient. Furthermore, PVC is a less attractive material in that, because of the plasticiser used therein, the PVC sheet produced has an unpleasant odour. In addition, PVC is an environmentally undesirable product which decomposes under the influence of moisture and light, eliminating HC1.

The present invention aims to provide a process for the manufacture of a printed polyethylene sheet which is very appropriate for use as a play mat and does not exhibit the abovementioned disadvantages.

According to the present invention, this object is achieved by a process of the abovementioned type which is characterised in that the printed surface is subjected to a pretreatment which is in itself known in order to render the entire surface receptive to adhesion and in that thereafter a transparent layer of polyethylene is extruded over the pretreated surface.

The pretreatment is a process which is in itself known, and whereby the surface is activated and rendered suitable for adhesion of the layer of polyethylene extruded over the surface, such as, for example, a flame oxidation process or an oxidation process with the aid of electric sparks, the so-called corona discharge.

The pretreatment must be carried over the entire surface, and thus also over the parts of the surface provided with print marks, since otherwise ruckling will occur in the area of the print marks, which, above all when folding up or rolling up the flexible sheet, can lead to delamination.

It should be noted that Netherlands Patent Specification 143,168 describes a process for the manufacture of a serving tray, wherein a printed polyethylene sheet is provided with a protective polyethylene layer. This is an expensive discontinuous process wherein the printed plastic sheet is placed in a cavity of a mould, which is subsequently filled with fluid polyethylene. In contrast to the process according to the present invention, the sheet provided with the printing ink is not pretreated over the entire surface, so that in the area of the print marks good adhesion to the protective polyethylene layer is not achieved.

In view of the fact that according to the known process a stiff and non-foldable serving tray is produced, the poor adhesion mentioned would give few problems.

Netherlands Patent Application 112,046 describes a process for covering a film, which is provided with inscriptions, with a transparent thermoplastic film.

According to this process, the films are placed between polished metal plates and are subsequently combined by means of heat and pressure. Here again, an expensive discontinuous process is involved. Furthermore, the sheet provided with inscrpitions is not pretreated, in contrast to the method according to the invention.

According to the invention a layer of polyethylene is extruded over the printed pretreated surface. An appropriate process is the slit extrusion process mentioned earlier, whereby a printed and pretreated polyethylene sheet of great length is continuously provided with a layer of polyethylene. The laminate obtained, which is produced continuously in web form, is cut, after cooling, to the desired size. The laminar product, which has been cut off or cut out, as a rule has an area of from one to several m2 and can possess any of a variety of shapes, such as circular, oval or rectangular. The laminar product has a low thickness, e.g. of about 0.1 to 0.3 mm, the total thick ness of the polyethylene sheet being from 0.1 to 0.3 mm and the thickness of the extruded layer of polyethylene being from 0.01 to 0.05 mm.

In an advantageous embodiment of the process according to the invention, a polyethylene sheet which is only slightly transparent, if at all, is, on both surfaces, printed with ink, pretreated and provided with an extruded transparent layer of polyethylene.

This achieves the advantage that the product obtained, when used as a play mat, can be used on both sides. It should be noted that the known Twister game mat is only provided with print marks on one side. In the known mat, the surface not provided with print marks serves as the base surface and in doing so suffers such friction that two-sided printing is out of the question.

In a further preferred embodiment of the process according to the invention, a transparent layer of polyethylene which after cooling exhibits a matt su rface is extruded over the pretreated surface or surfaces.

A suitable layer of polyethylene having a maat surface is obtained by extruding low-pressure polyethylene, having a specific gravity of 0.95 to 0.97 and a crystallinityof65to80%, over the treated surface. Surprisingly, it has been found that, though the extruded layer is stiffer than a similar layer of highpressure polyethylene which has a glossy surface, an excellent protective layer for the print marks is nevertheless obtained. Here, it is very probable that the low thickness of the layer and the good adhesion have an advantageous effect.

It is also possible to employ a commercially available high-pressure polyethylene which has been specially developed for producing a matt surface. An appropriate product is that known by the tradename Petrothene. The amount required is low so that the possible higher price of such a special product hardly increases the cost.

The invention is explained in more detail with the aid of the illustrative embodiment which follows.

Illustrative embodiment: A polyethylene sheet, provided with print marks on one side, and having a great length, a width of 134 cm and a thickness of 0.14 mm, is pretreated by means of a corona discharge and is then passed under the extrusion orifice of an extruder having a screw of 420 cm length and 15 cm diameter. The extrusion orifice has a width of 130 cm. A highpressure polyethylene, having a specific gravity of 0.924 and a crystallinity of 45 to 55%, is present in the extruder and is extruded as a thin film, of thickness 0.02 mm, at an extruder temperature of 320"C, over the surface of the polyethylene sheet which has been provided with print marks and has been pretreated.

The resulting laminar product of great length is cut to the desired size after the extruded film of polyethylene has sufficiently cooled (to 25"C) in air.

A sample exposed to practical use shows that when the sheet obtained according to the process is used as an exercise mat no damage of the print marks was visible yet after one week, whilst when using the polyethylene Twister game mat produced in accordance with the known method, such great damage to the print marks occurred within one week that further use appeared no longer possible.

Claims (6)

1. Process for the ma nufactu re of a printed polyethylene sheet, wherein a flexible polyethylene sheet is printed in patterns with an ink, characterised in that the printed surface is subjected to a pretreatment which is in itself known in order to render the entire surface receptive to adhesion and in that thereafter a transparent layer of polyethylene is extruded over the pretreated surface.

2. Process according to Claim 1, characterised in that a polyethylene sheet which is only slightly transparent, if at all, is, on both surfaces, printed with ink, pretreated and provided with an extruded transparent layer of polyethylene.

3. Process according to Claim 1 or2, characterised in that a transparent layer of polyethylene which after cooling exhibits a matt surface is extruded over the pretreated surface or surfaces.

4. Process according to claim 3, characterised in that the polyethylene extruded is a low-pressure polyethylene having a specific gravity of 0.95 to 0.97 and a crystallinity of 65 to 80%.

5. Process according to claim 1, substantially as described in the foregoing description of an illustrative embodiment.

6. A printed polyethylene sheet manufactured by a process according to any preceding claim.