Colour reactive transfer material

Abstract

 A transfer material comprising a carrier sheet, one face of which is provided with a coating of a composition comprising a colourless colour former and a waxy material in which the colour former is at least substantially soluble, an oily or fatty substance forming a transfer agent or mobiliser for causing transfer of the colour former from the carrier sheet to the face of a second or acceptor sheet caused to abut there-against. By the application of mechanical pressure, the carrier sheet is pressed against such adjacent face, which latter is provided with an electron-absorbing substance, so as to form a coloured marking. The transfer agent or mobiliser is wholly or partially encapsulated in micro-capsules, the encapsulated transfer agent being provided on either one of, or both, said coated faces. Hence, at worst, the transfer agent is only minimally absorbed by the carrier sheet.

Description

[0001] The present invention relates to colour reactive transfer materials of the type which comprises a carrier sheet coated on its rear face and containing a substantially colourless colour former which is adapted to react, under mechanical pressure with a so-called colour acceptor which forms part of the front face coating of an adjacent second sheet located therebelow so as to form a coloured image of the latter. The rear face coating of the first sheet further comprises a waxy base layer which is solid at room temperature and oily and/or fatty substances, which are known as transfer agents or mobilisers.

[0002] In conventional transfer materials of this type, the colour former is generally dissolved in oil and encapsulated in micro-capsules. By so doing, during storage and handling undesired reactions of the colourless colour former with the colour acceptor on the abutting sheet are prevented.

[0003] Micro-capsules are, however, somewhat expensive to prepare and use. In order, therefore, to avoid the use of such micro-capsules, it is already known to use colour formers which are soluble in the wax components of the composition applied to the rear face layer of the carrier sheet, but which are insoluble in the oily or greasy components thereof. This is disclosed in German Offenlegungsschrift No. P 2747899.5. This proposal effectively prevents undesired colour reactions from occurring. However, in colour transfer materials of this type, there is a tendency for the oily components (the transfer agents or mobilisers) to be absorbed into the carrier sheet due to the limited oil retention capacity of the wax components. This, in turn, delays the transfer caused by the application of pressure to the backing layer and hence the colour reaction between the colour former and the acceptor.

[0004] The present invention seeks to provide a transfer material of this general type in which the disadvantages mentioned hereinbefore are minimized.

[0005] In accordance with the present invention, there is provided a colour reactive transfer materialcomprising a sheet or web of carrier material, one face of said sheet or web being provided with a coating comprising a colourless colour former which is capable, by the application of mechanical pressure thereto, of forming a coloured marking on the face of a second or acceptor sheet or web caused to abut thereagainst the second sheet or web having, on said abutting face, a surface coating of a colour acceptor including an electron-accepting material or substance, the coating on the carrier material further comprising a waxy material which is solid at room temperature and in which the colour former is at least substantially soluble, a transfer agent being provided for causing the transfer of the colour former from the backing sheet or web to the second sheet or web characterised in that the transfer agent is at least partially encapsulated in pressure-sensitive micro-capsules, the encapsulated transfer agent being provided on either one of, or both, said coated faces. By so doing, the migration of the transfer agent or mobiliser into the first or carrier sheet is at least substantially reduced compared with known systems and may even be completely prevented. The number of micro-capsules required is much lower than in conventional transfer materials in which both the colour former and the mobiliser are enclosed in micro-capsules.

[0006] Because of the separation of the colour former from the transfer agent or mobiliser achieved by using a transfer material in accordance with the present invention, the transfer agent or mobiliser may, if desired, also constitute a solvent for the colour former. In such.a case, an acceleration of the reaction speed occurs and/or if desired, a reduction in the number of micro-capsules required per unit area or both of these effects may be -produced.

[0007] If, as is one possibility according to the present invention, the encapsulated transfer agent is provided in the coating of the second or acceptor sheet, a further improvement of the speed of reaction between the colour reactants after the pressure has been applied and a prevention of migration of the transfer agent into the base material (usually paper), can be achieved. This is because part of the transfer agent is isolated from the rest of the transfer agent. In such a case, the portion of the encapsulated transfer agent in the second sheet is prevented from migrating into the base material and the separation of the encapsulated transfer agent from the colour former makes it possible to use_an organic solvent for the colour former and/or the colour former-containing waxy material as a constituent of the transfer agent. This will again result in an increased reaction rate between the colour reactants or in a further reduction of the number of micro-capsules required per unit area, or both of these effects may be produced.

[0008] We have found that if the coating of the second or acceptor sheet contains 5 - 40% by weight, preferably 10 - 15% by weight, of transfer agent encapsulated in micro-capsules and/or organic solvent clearly improved transfer properties compared with known wax-based transfer materials are achieved.

[0009] The micro-capsules are relatively small, that is to say, they desirably have a size of from 4 to 8 microns. They preferably comprise resins such as polyacrylates or polyamides. The capsules may be produced from an aqueous medium and, in such a case, may be made ready for use in powder form by, for example, spray drying.

[0010] The invention will be further described, by way of illustration only, with reference to the following Examples.

Example 1.

[0011] A rear face coating or backing layer for the first or carrier sheet is prepared in accordance with the present invention having the following composition by weight :-

[0012] The colour former concentrate has the following composition by weight:-

[0013] The colour former concentrate is heated to 110⁰ C, and then cooled with stirring. The concentrate is then mixed with the remaining constituents of the composition at 90° C and is applied, in a molten state at 90° C in a quantity of from 2 to 12g/m² onto a paper sheet having a weight of 50 g/m². This operation may be effected by way of a hot melt pressure printer.

[0014] The transfer sheet thus prepared may then be used in conjunction with a second sheet having a front face coating containing attapulgit or reactive phenol resins. For use in form sets comprising a plurality of sheets, the intermediate sheet or sheets are coated on both their front and rear faces with the appropriate materials.

Example 2

[0015] A rear face coating or backing layer for the first sheet having the following composition by weight :

[0016] The colour former concentrate has the following composition by weight

[0017] The constituents of the colour former concentrate are mixed in a cold state, then heated to 130° C with stirring. This temperature is maintained for ten minutes and the mixture is then rapidly cooled with vigorous stirring. It is then mixed with the other constituents of the composition at 90⁰ C and may be applied as described in Example 1.

Example 3

[0018] A composition having the following composition by weight is prepared:-

[0019] The crystal violet lactone is dissolved in the mixture of waxes at 120⁰ C with vigorous stirring. The mixture is cooled to 80° C and the remaining constituents of the composition added thereto. The completed composition is applied in a molten state by way of the application of pressure or by brush application on a sheet or web-like carrier material.

Example 4

[0020] A composition having the following composition by weight, is prepared :-

[0021] The wax is heated in a sealed container to a temperature of 90° C, and the container is then connected to a vacuum pump to produce a reduced pressure therein of 30 mm Hg. The aqueous micro-capsule dispersion is added slowly whilst stirring is effected. Stirring and evacuation of the container are continued until the mixture is free from water. The vacuum is then removed from the container and the remaining constituents are added, whilst stirring is effected and at 90° C, until the mixture is homogenous.

Example 5

[0022] A rear face coating or backing layer composition having the following composition by weight is prepared:-

[0023] The waxes, oil and calcium carbonate are mixed, heated to 110⁰ C with stirring for approximately 30 minutes. The colour former concentrate is then added thereto

[0024] The colour former concentrate has the following composition by weight :-

[0025] The colour former concentrate is heated to about 110°C and cooled whilst stirring. Subsequently the back face coating is applied, in a molten state at 90°C in an amount of 2 - 12 g/m to a paper sheet having a weight of about 50 g/m².

[0026] The materials, with the exception of the micro-capsules and the latex are ground for 45 minutes in an attrition mill. The micro-capsules and the latex are then added thereto and stirred for 5 minutes without grinding. The pH of the coating is adjusted to 10 with sodium hydroxide.

[0027] This latter coating is applied to the appropriate face of the acceptor sheet with an air brush in the amount which leaves about 7 g/m² of solid coating after the water has been evaporated.

Example 6

[0028] An alternative coating for the acceptor sheet for use with the backing layer composition of the carrier sheet set forth in Example 5 has the following composition by weight :-

[0029] This coating is prepared and applied as described in Example 5.

[0030] It will be readily apparent that although no specific examples have been given, the transfer agent encapsulated in micro-capsules may be distributed partially on the carrier sheet and partially in the acceptor sheet .

Claims

1. A colour reactive transfer material comprising a sheet or web of carrier material, one face of said sheet or web being provided with a coating comprising a colourless colour former which is capable, by the application of mechanical pressure thereto, of forming a coloured marking on the face of a second or acceptor sheet or web caused to abut there- against the second sheet or web having, on said abutting face, a surface coating of a colour acceptor including an electron-accepting material or substance, the coating on the carrier material further comprising a waxy material which is solid at room temperature and in which the colour former is at least substantially soluble, a transfer agent being provided for causing the transfer of the colour former from the backing sheet or web to the second sheet or web characterised in that the transfer agent is at least partially encapsulated in pressure-sensitive micro-capsules, the encapsulated transfer agent being provided on either one of, or both, said coated faces.

2. A transfer material as claimed in claim 1, characterised in that the transfer agent is selected from the group consisting of mineral oils, vegetable oils, animal fat oils, halocarbons, hydrogenated terphenyl, polyglycols and mixtures thereof.

3. A transfer material as claimed in claim 1 or 2, characterised in that the colour former is at least partially soluble in the transfer agent.

4. A transfer material as claimed in claim 1 or 2, characterised in that the colour former is predominantly insoluble in the transfer agent and the transfer agent is present in the form of a dispersion in the waxy material.

5. A transfer material as claimed in claim 4, characterised in that the encapsulated transfer agent comprises a mixture of oily and/or fatty substances, which mixture comprises at least one substance in which the colour former is soluble and at least one substance in which the colour former is insoluble.

6. A transfer material as claimed in any one of claims 1 to 5, characterised in that the waxy material is selected from the group consisting of paraffins, micro-paraffin waxes, ester waxes, partially saponified ester waxes, amide waxes and oxazolin wax.

7. A transfer material as claimed in any preceding claim, characterised in that the composition comprises, by weight 40% to 70% of waxy material, not more than 25% pigment filler materials, 10% to 45% transfer material of which 10% to 25% of the total weight composition is encapsulated in micro-capsules, the remainder of the composition being the colour former.

8. A transfer material as claimed in any preceding claim, characterised in that the composition is coated on the carrier material in an amount of from 2 to 12 g/m2.

9. A transfer material as claimed in any preceding claim, characterised in that the carrier material is provided with a coating of the composition containing the colour former on one face and is provided on its opposed face with a layer of a colour developer containing electron accepting materials or substances.

10. A transfer material as claimed in any preceding claim, characterised in that micro-capsule encapsulated transfer agent is provided on said acceptor sheet, and constitutes 5% to 40%, optionally 10% to 15%, by weight of the acceptor coating provided on said acceptor sheet.

11. A transfer material as claimed in claim 10, characterised in that said encapsulated transfer agent additionally comprises an organic solvent capable of dissolving said colour former and/or said waxy material.