GB2252838A - Heat- or pressure-sensitive recording medium - Google Patents

Abstract

A heat- or pressure-sensitive recording medium comprises a microporous plastics film (1) and a contrasting underlayer (3). Heat or impact on the microporous film causes it to change from an opaque to a clear state revealing the contrasting underlayer. The medium may be used to reveal impact e.g. of a golf ball on the head of a golf club to which it is attached, or it may be used to detect temperature rises above a characteristic limit at which the film (1) becomes transparent. The underlayer (3) is preferably a coloured adhesive which is adhered to a support (5) which is coated with a pressure-sensitive adhesive (7) and a release layer (9) which is peeled off before attachment to the head of a golf club. Film (1) may be covered with a protective layer. <IMAGE>

Description

RECORDING MEDIUM This invention relates to a recording medium and to its use for recording information as a result of the application of pressure (including mechanical impact) or heat.

This invention is based on the discovery that a relatively thin normally opaque film of microporous plastics material can become transparent under the action of heat or pressure separately to reveal a differently coloured underlayer present to provide contrast, and that a film of this kind can record impact upon it, writing upon it by means of a stylus, typewriter, impact printer, pen or the like, or can reveal when it has been heated to an activation temperature at which the microporous film becomes molten and its internal microporosity is destroyed.

The invention therefore provides a recording medium that can be marked by heat or pressure comprising a normally opaque microporous plastics film and a contrasting layer on one face of the film, an opposite face of the film revealing the contrasting layer when it becomes transparent, for example because the porosity of the film is destroyed, for example by heat or pressure.

In one form of the recording medium the microporous film forms one external face of the medium. In this form the medium exhibits the best sensitivity to mechanical impact, and it can be used for example as a suitably outlined patch for adhesion to a face of a golf club to record the impact of a ball on the golf club. The recording patch above described has the advantage that it can provide a permanent record of the impact of a golf ball on the face of a golf club, and that if the club is repeatedly struck while the patch is present on the face of the golf club, the golfer can derive from the coincidence of successive areas of colouration an indication of how consistently he is hitting the ball.

The diameter of the imprint gives an indication as to how hard he has struck the ball and he can also derive this information from the degree of contrast obtained in the resulting coloured area of the patch where there is a gradation based on the force with which the ball has been struck.

Some forms of the patch can be made relatively insensitive to appearance; of colour as a result of accidental impact which facilitates their reuse in the manner aforesaid. In an alternative form of the ,.

recording medium, the microporous film is protected on the face remote from the colour layer by a transparent overlayer. This form of the recording medium lends itself best to alternative applications where sensitivity to mechanical impact is less of a consideration or is positively undesirable. For example, in this form the recording medium can be used as a writing surface and is particularly advantageous in outdoor uses where the recording medium can be made in the form where it is resistant to dirt, water or grease. Thus the recording medium can be used in factory environments, or in outdoor environments where rain is a problem or even under water.

The invention will now be described, by way of example only, with reference to the accompanying drawings in which; figure 1 is a view in section of a small portion of a laminated film for attachment to the face of a golf club to indicate the impact of a golf ball on the golf club; and Figure 2 is a diagram illustrating how the film of figure 1 is made.

In figure 1 a laminated film for attachment to the golf club comprises a microporous top layer 1 which is normally opaque and has a white appearance but becomes transparent when it is struck by a golf ball. The layer 1 is preferably a highly porous very thin membrane having a fibrillar internal structure. It may, for example, be of about 60% perocity and in thickness about 12 pm. The blind face of the microporous film 1 is attached by means of a coloured adhesive 3 to a support layer 5 of inextensible but flexible material for example Melinex film. The other face of the support layer 5 has a further pressure sensitive adhesive layer 7 which has the property, when the laminated film is attached to the face of a golf club, of not competitively adhering to the face of golf club even after the laminated film has recorded the impact of a golf ball.The pressure sensitive adhesive layer 7 is covered by a release layer 9 e.g. of siliconised paper.

The material used to make the normally opaque microporous plastics film is preferably polyethylene on grounds of ease of conversion into a microporous film having the required combination of properties, availability and cost. Other materials from which the microporous film can be made include cellulose nitrate or acetate, polypropylene, polyvinyl chloride, ethylene tetrafluoroethylene, polyvinylidene fluoride and polytetrafluoro ethylene. Porous films made from these polymers can change from an opaque to a clear state on the application of pressure or heat. However, it is important to provide sufficient contrast between the opaque and clear forms of the film, and an appropriate level of force required to achieve the change of state. The factors which determine these properties are the thickness of the film, the porosity of the film and the internal pore structure.In a recording medium that is to change state by melting, the melting point of the microporous film is the significant quantity and the other factors are less important.

The thickness of the microporous plastics film is preferably in the range 7 to 100 pm, preferably about 12 um when it is required to change state by pressure. A thick film requires more force to become clear than a thin film, whereas a thin film may not have the required opacity and may reveal the underlying coloured layer and provide insufficient contrast. Films of less than 7 um thickness will not normally provide the required contrast whereas films whose thickness is greater than 100 pm may be too insensitive to be of practical value.

The porosity of the film is preferably 30 to 70% by volume measured by the density of the film compared to the bulk density of the material from which it is made.

The mean pore size is advantageously in the range from 0.1 to 2 pm and the films must have a fribrillar or reticulated internal structure to give the required quality of opacity. The porosity in itself does not give a sufficient indication of opacity. For example, a porous film of polypropylene having a porosity of 38% and a mean pore size of 0.02 pm such as is sold under the trade name CELGARD 2400 has a columnar pore structure and does not exhibit the required opacity. By "columnar" there is implied a structure in which the pores extend more or less straight through the bulk of the film from one face to the other to provide continuous pathways therebetween.

The purpose of the coloured layer is to enhance the contrast between the opaque and the clear forms of the microporous film and it may comprise a coating 5 to 20 um thick of a coloured water-based polymer resin. The colouring agent may be a water-soluble dye or a pigment dissolved or dispersed in the resin. The use of a water-based polymer resin rather than a solvent based resin means that the coating becomes deposited on the surface of the microporous film without penetrating into its pores to any significant degree. Suitable coating resins include vinyl acetate emulsions such as those sold under the trade names EMULTEX 592 and REVACRYL 396 (Harlow Chemicals, UK). Suitable water soluble dyes are available from Hoechst under the trade names COLANYL or FLEXONYL.The coloured layer may also be a pre-formed coloured film or layer attached to one face of the microporous film by means of a clean adhesive. The film may be of coloured paper or coloured plastics.

The adhesive layer can be applied to the microporous film by reverse roller coating as indicated below. The microporous film having a coloured layer on one face is a relatively fragile structure and it is most conveniently adhered to a support layer which may be of a polyester film, metal foil or paper etc. Where a polyester film is used, its thickness may be 25 to 100 jim, conveniently about 75 jim. Where the recording medium is to be adhered to an article for impact recording e.g. when used in relation to golf clubs, the support layer is coated with a pressure sensitive adhesive preferably a water-based adhesive such as the vinyl acetate adhesive sold under the trade name REVACRYL 396. This latter adhesive has the advantage that it can be removed easily from the face of the golf club without leaving a deposit behind.The adhesive coated face of the support layer is protected by a suitable readily removable protective material such as siliconised paper, siliconised plastic or a metal foil.

When used as a writing medium the microporous plastics film (layer 1 in fig 1) is overcoated with a suitable protective layer 5 (not shown) to 75 Mm thick, the overcoating being of a transparent plastic material such as polyester film (Melinex). The provision of a protective film as aforesaid brings about a reduction in contrast and an increase in force required to convert the film from an opaque to a clear state compared to a similar film without the protective layer.

The following preparative example and examples describes a method for manufacturing a porous polyethylene film and the use of that film to make recording media according to the invention.

PREPARATIVE EXAMPLE A medium density polyethylene film was formed from an extrudable formulation having the following composition: polyethylene (Sclair 8405, Du Pont) 5.5kg lithium carbonate (6-8 ym particle size) 4.5kg lithium stearate powder 47g The above formulation was charged into a twin screw 0 extruder and extruded at about 150 C as a rod which was then granulated. The granules were subjected to a second extrusion step in which a flat film was formed which was stretched in line to form a flat film, the flat film passing over a roller at 1100C at which stretching was initiated, stretching downstream from that roller being carried out at room temperature at a draw ratio of 14:3.

The resulting film was then passed through a bath of dilute nitric acid to remove the filler, washed with water, further washed with organic solvents to remove the water and dried using a hot air dryer. It had a thickness of 12 microns after extraction and a porosity of 38% and a pore size of about 0.17 microns. The film was generally opaque and had a milky white appearance.

EXAMPLE 1 A recording medium was prepared from the above mentioned porous film using apparatus as shown in figure 2 of the attached drawing which diagramatically illustrates apparatus for coating the microporous film with a coloured adhesive layer and then attaching a support layer to the adhesive layer. Microporous polyethylene film from a supply roll 10 travels to a reverse roll coater 12 where a coating of predetermined thickness is applied from a tray containing a coloured water-based latex adhesive. The coated film passes in the direction of arrow 13 through a drying tunnel 14 and thence to lamination rolls 16 where its adhesive coated face has attached to it a support of Melinex film 18 from a supply roll 20 the direction of travel being indicated by arrow 19.The lamination rolls 16 are heated to typically about 500C, and the film passes in the direction of arrow 23 from them to chill rolls 21 and then to a take up roll 22. For application of a pressure sensitive adhesive followed by a siliconised paper protective layer the same procedure as described with reference to figure 2 is followed. That is to say the recording medium and support layer is positioned in place of the supply roll 10, the exposed face of the Melinex support layer is coated with a water-based vinyl acetate adhesive (REVACRYL 396) which is selected because it can be readily removed from a golf club face without leaving a sticky deposit behind. The clear adhesive is applied in the reverse roll coater 12 and the film is passed through the drying tunnel 14 and then siliconised paper from roll 20 is attached to the adhesive coated face using lamination rolls 16 as before.The film is passed over the chill rolls 21 and taken up on the take up roll 22.

If it is desired to use the film directly to form impact recording patches for golf clubs or other similar equipment, the film from the take up roll is passed e.g.

to a stamping device where it is stamped out in the required shape.

EXAMPLE 2 Instead of applying the adhesive and the siliconised paper backing, the face of the recording medium at which the microporous plastics film 1 (fig 1) is exposed is coated with a clear adhesive using a reverse roll coater 12 as aforesaid, and the film is passed through the drying tunnel 14. A protective film of Melinex 12 jim thick is applied over the micrporous plastics film 1 (fig 1) to provide a protective coating, again using lamination rolls 16 followed by chill rolls. The resulting protective film is taken up on the take up roll 22 and provides a writing medium. It may be cut to an appropriate size e.g. into individual sheets. The film described above may also be used as a thermochromic device i.e. a device for recording a temperature excursion above a predetermined limit temperature determined by the melting temperature of the microporous film, in which case the film can be cut into small test patches or continuous strips.

Claims (25)

1. A recording medium that can be marked by heat or pressure comprising a normally opaque microporous plastics film and a coloured layer at one face of the film, an opposite face of the film revealing the coloured layer when the porosity of the film is destroyed by heat or pressure.

2. A medium according to claim 1, wherein the microporous film forms an external face of the medium.

3. A medium according to claim 1, wherein the microporous film is protected by a transparent covering layer.

4. A recording medium according to any preceding claim, wherein the colouring layer comprises a dye or pigment dispersed in a resin.

5. A medium according to claim 4, wherein the coloured layer is attached to one face of the film.

6. A medium according to claim 4 or 5, wherein the resin is a hot melt resin.

7. A recording medium according to claim 4, wherein the resin is a pressure sensitive adhesive.

8. A recording medium according to claim 7, wherein the dye and the pressure sensitive adhesive are water soluble.

9. A recording medium according to any of claims 1 to 3, wherein the coloured layer is a film attached to one face of the microporous film by means of a clear adhesive.

10. A recording medium according to any preceding claim, wherein the microporous plastics film is of polyethylene.

11. A recording medium according to any of claims 1 to 9, wherein the plastics film is of cellulose nitrate or acetate, polypropylene, polyvinyl chloride, ethylene tetrafluoroethylene copolymer, polyvinylidine fluoride or polytetrafluoroethylene.

12. A recording medium according to any preceding claim, wherein the microporous is film is 7 to 100 jim thick.

13. A recording medium according to any of claims 1 to 11 wherein the microporous film is 12 to 20 jim thick.

14. A recording medium according to any preceding claim, wherein the microporous film has a porosity of 30 to 70% by volume measured by density.

15. A recording medium according to claim 3, wherein the microporous film is coated with a layer 5 to 20 um thick of a water based resin together with a dye or pigment.

16. A recording medium according to any preceding claim, wherein the face of the microporous film to which the coloured layer is attached carries a pressure sensitive adhesive, and a support layer is adhered to the adhesive face of the medium.

17. A recording medium according to claim 16, wherein the support layer is a polyester film.

18. A recording medium according to claim 17, wherein the thickness of the polyester film is 25 to 100 jim.

19. A recording medium according to claim 16, 17 or 18, wherein a face of the support film remote from the microporous film is coated with an adhesive, the adhesive being further covered by a release coated paper or other removable protective layer.

20. A recording medium according to claim 3, wherein the transparent overlayer is of a polyester film.

21. A recording medium according to claim 3 or 20, wherein the overlayer is 5 to 75 pm thick.

22. A recording medium according to any preceding claim having an outline conforming to the face of a golf club.

23. Use of a recording medium as claimed in any preceding claim for recording the impact of a golf ball on the face of a golfclub.

24. Use of a recording medium as claimed in any of claims 1 to 21 as a recording medium for receiving information by a stylus typewriter, impact printer or pen.

25. Use of a recording medium according to any of claims 1 to 21 for providing a colour change when heated to an activation temperature of the medium.