WO1990014235A1

WO1990014235A1 - Heat-sensitive record material

Info

Publication number: WO1990014235A1
Application number: PCT/GB1990/000776
Authority: WO
Inventors: Eric Thomson; Elizabeth Mckay Park; Kathleen Anne Hamilton
Original assignee: Smith & Mclaurin Limited
Priority date: 1989-05-18
Filing date: 1990-05-18
Publication date: 1990-11-29
Also published as: GB8911419D0

Abstract

A heat-sensitive record material comprising a substrate having on at least one surface thereof a heat sensitive-layer containing colour forming reagents capable of reacting to produce a colour upon the application of heat, which heat sensitive layer bearing surface also carries a stabiliser comprising a moiety of general formula (I), wherein each of R1 and R2 which may be the same or different is one of hydrogen, hydroxyl, a halide, lower alkyl, lower alkoxy, benzyloxy or substituted benzyloxy wherein the substituent is a lower alkyl or alkoxy group or a halide or hydroxyl; Y is a sulphur or oxygen and M is a divalent metal.

Description

Heat-Sensitive Record Material

This invention relates to heat sensitive record

materials. It is particularly, but not exclusively, concerned with the use of paper substrates, which are coated with thermally-activated chemicals containing colour formers and developers in which an image may be created by the intimate localised application of heat to cause a chemical reaction in the coating. Such thermographic materials are used in a variety of ways for the purposes of displaying and storing information especially in telecommunication systems where fast acting image formation compounds are desirable. These

materials are currently in great demand for use in computer systems, telefacsimile receivers, photocopiers,

electrocardiographs, cash registers, automatic ticket vending machines, labelling machines and the like because they do not require consumable materials such as inks or ribbons etc.

required by other printing systems.

The colour formers and developers and uses thereof have been the subject of extensive research for over twenty years and many patents exist in this area. Thus, for information on this subject, one could refer to the following British patents and published applications :- 1 053 905, 1 135 540, 1 294 430,

1 560 086, 1 600 781, 2 064 801 A, 2 031 177 A,

2 051 391 A, 2 074 335 A and 2 122 363 B.

One problem which still presents difficulty in this field is the sensitivity of the recording material to its

environment and handling. Thus. the record material may be damaged by contact with solvents, plasticisers, moisture in the air, exposure to sunlight, etc. Thus the permanence of a recorded image is not certain and deterioration in image quality is unfortunately frequently observed. This is due in part to the fact that the image-forming reaction is a

reversible reaction. This reversibility presents two image retention problems. The first is a time dependent one whereby gradual fading of image is apparent without any obvious physical or chemical damage necessarily having been suffered by the thermographic material. The second is the loss of image due to some accidental abuse of the thermographic material such as by spillage of chemicals on the image

carrying, surface. It will be appreciated that solution of the former stability problem may, but does not necessarily, lead to solution of the latter which in many respects is the more difficult to solve.

Efforts have been made to overcome the aforementioned problems by applying a protective top coat over the heat sensitive coating to provide a physical barrier to reagents or environmental factors which would tend to weaken or destroy the recorded image. Thus the use as a top coating of an aqueous emulsion of a film-forming resin able to withstand chemical attack is known, as is the fixing of a preformed chemically resistant film to the recording material with an appropriate adhesive. One could refer to US Patent

Nos 4 370 370, 4 388 362 and 4 484 204 for disclosure of protected thermographic papers. However it would be desirable if use of such a protective coating could be avoided. Certain problems have been found with protective barrier coatings and include for example the tendency of the product to blister or curl and the coating method is slower so that to achieve an acceptable throughput of product it would be necessary to increase the drying temperature which can lead to premature activation of the colour forming reagents in the coating material. Furthermore where a pre-formed protective film is fixed to the heat-sensitive record material, the solvent remaining in the film may also produce an undesirable colour- forming reaction. Thus -those in the art have sought other means of providing a protective layer over the heat-sensitive record material. Such proposals include providing a final coating step in which a top coat is formed from a resin layer which is formed from at least one oligomer having at least one ethylenically unsaturated bond in the molecule, the said resin layer being cured by an electron beam being passed over the resin-coated heat-sensitive record material. It is considered inconvenient to require use of such apparatus in coating equipment and it also increases the cost thereof.

There are some instances where a final barrier coat may still be required even with very stable image-forming materials. An example of this would be where the final product has to be printed by means of a process utilising UV stabilised inks and solvent based inks. Nevertheless, the stabiliser obtainable according to the invention remains useful even in such cases to improve image stability.

An object of this invention is to provide an improved heat-sensitive record material, especially for use as a label, and method of forming same by which problems relating to stability apparent in the prior art are obviated or mitigated. In particular this invention seeks to provide a means of stabilising a thermographic material against chemical attack.

Accordingly this invention provides a heat-sensitive record material comprising a substrate having on at least one surface thereof a heat sensitive-layer containing colour forming reagents capable of reacting to produce a colour upon the application of heat characterised in that said heat sensitive layer bearing surface also carries a stabiliser comprising a moiety of the general formula I

wherein each of R₁ and R₂ which may be the same or different is one of hydrogen, hydroxyl, a halide, lower alkyl, lower alkoxy, benzyloxy or substituted benzyloxy wherein the

substituent is a lower alkyl or alkoxy group or a halide or hydroxyl; Y is a sulphur or oxygen and M is a divalent metal.

Preferably whenever R₁ is not hydrogen, R₂ represents a hydrogen and vice versa. However it will be appreciated that one of said substituents may be a small group other than hydrogen whilst the other is a larger group. It has been found preferable to have R₂ as hydrogen or a small group such as methyl or methoxy whilst R₁ is one of the above identified substituents.

Therefore R₁ may be a C₁-C₄ alkyl such as methyl or t- butyl, C₁-C₄ alkoxy such as methoxy or ethoxy, benzyloxy, or substituted benzyloxy such as 4-methylbenzyloxy or 4-chloro benzyloxy for example or chloro substituent whilst R₂ is hydrogen.

Preferably R₁ is benzyloxy and R₂ is hydrogen, whilst Y may be either oxygen or sulphur.

The metal is preferably Zn or Cd in order to obtain optimum image versus background contrast. Zinc is the most preferred metal, and the chloride or oxide thereof are most suitable for reaction to obtain the stabiliser of the

invention.

Methods for obtaining certain compounds of formula I where Y is sulphur are known and particularly described in Japanese Publication No. 62-145059 (ASASHI Chemical Company - JAPAN), wherein the use thereof is described as a sensitiser to improve the reactivity of bisphenol A, as a developer, with leuco dyes.

Surprisingly these compounds and others according to the invention are now found to improve the stability of the colour forming layer to such an extent that it is now found

unnecessary to apply a barrier top coat.

The stabiliser may be introduced with a general system binder or encapsulated by a material preventing its release until thermal activation. It may be present in a precoat mix or applied in a final topcoat mix. The amounts thereof are not critical but vary depending on the coating composition selected. Generally it will be sufficient to add from about 0.5 to about 5 moles, preferably from 1 to 3.5 moles, of stabiliser per mole of dye.

Typically it will be most convenient to prepare a dye dispersion containing dye, binder, water, usual additives separately from a developer dispersion containing phenolic developer, binder, water and additives. The particle size aimed for by grinding will normally be in the range of about 0.5 to 10 microns, preferably 0.5 to 3 microns. The

stabilising effect is obtained even if the chemical resistance stabiliser is added at different stages in the preparation process but it is preferably added in a final mixing stage of coating preparation. Thus the stabiliser may be ground with components of the developer dispersion or separately and mixed therewith, or ground separately and applied as a precoat to the intended substrate. It may also be ground separately with its own binder if desired and added to the final coating mix i.e. dye/developer mix. The attached drawing illustrates examples of this process schematically.

Typical dyes usable for the purposes of labels include the following:

Leuco bases of triphenylmethane dyes, such as;

3,3-bis(p-dimethylaminophenyl)-phthalide,

3,3-bis(p-dimethylaminophenyl)-6-dimethylamino-phthalide,

3,3-bis(p-dimethylaminophenyl)-6-diethylamino-phthalide,

3 ,3-bis(p-dimethylaminophenyl)-6-chloro-phthalide,

Leuco bases of fluoran dyes, such as;

3-cyclohexylamino-6-chlorofluoran;

3-(N,N-diethylamino)-5-methyl-7-(N,N-dibenzyl- amino)fluoran;

3-dimethylamino-5,7-dimethylfluoran;

3-diethylamino-7-methylfluoran;

3-diethylamino-7-t-butyIfluoran;

Fluoran dyes, such as

3-diethylamino-7-chlorofluoran;

3-diethylamino-6-methyl-7-chlorofluoran;

3-pyrrolidino-6-methyl-7-anilinofluoran;

3-dimethylamino-6-methyl-7-anilinofluoran;

2-(N-(3'-trifluoromethylphenyl)amino)-6-diethylamino- fluoran;

3-diethylamino-7-cyclohexylaminofluoran;

2-(3,6-bis(diethylamino)-9-(o-chloroanilino)xanthyl- benzoic acid lactam;

3-dimethylamino-6-methyl-7-p-butylanilinofluoran;

Lactone dyes, such as;

3-(2'-hydroxy-4'-dimethylaminophenyl)-3-(2'-methoxy-5'- chlorophenyl)phthalide;

3-(2'-hydroxy-4'-dimethylaminophenyl)-3-(2'-methoxy-5'- nitrophenyl)phthalide;

3-(2'-hydroxy-4'-dimethylaminophenyl)-3-(2'-methoxy-5'- methylphenyl)phthalide;

3-(2'-methoxy-4'-dimethylaminophenyl)-3-(2'-hydroxy-4-' chloro-5'-methylphenyl)phthalide.

The usual acids can be used, such as:

boric acid, oxalic acid, maleic acid, tartaric acid, citric acid succinic acid, benzoic acid, stearic acid, gallic acid, salicylic acid, 1-hydroxy-2-naphthoic acid, o-hydroxybenzoic acid, m-hydroxybenzoic acid, 2-hydroxy-p-toluic acid and various phenols such as bisphenol A, bisphenol S, benzyl parahydroxybenzoate, bis(3-allyl-4-hydroxyphenyl sulphone), 4- hydroxy-4-isopropoxy-diphenyl sulphone, 4,4-thiodiphenol, bis- (4-hydroxy-3-methylphenylsulphide) or methyl-3,5-dichloro-4- hydroxybenzoate, etc. as is known in the art.

Likewise usual binders may be used including polyvinyl alcohol, methoxy cellulose, hydroxyethyl cellulose,

carboxymethyl cellulose, etc. and fillers such as finely powdered calcium carbonate, magnesium carbonate, alumina, silica etc. can be included in the thermographic coating.

Thus any of the known thermographic coating compositions can be used and modified in accordance with the invention by inclusion therein, as stabilisers, the materials described hereinbefore with reference to formula I.

Use of stabilisers including a moiety of general formula I wherein R₁ is benzyloxy, R₂ is hydrogen, Y is sulphur and M is zinc, or wherein R₁ is benzyloxy, R₂ is hydrogen, Y is oxygen and M is zinc as chemical stabilisers for thermographic coatings, especially for labels, particularly where Y = o, has been found particularly effective in accordance with the invention. As discussed above these stabilisers may be freely added at any appropriate point in preparation of the coating composition or applied as precoat or top coat to a

thermographic coating. An enhanced effect in localising the effect of the stabiliser to thermally activated regions of the thermographic coated substrate is obtainable by grinding the stabiliser and mixing with an encapsulating medium before adding it into the coating composition. The encapsulating medium may be melted before the ground stabiliser is added thereto or it may be a material ground intimately with the stabiliser to achieve the same encapsulating effect. The encapsulating material may comprise a waxy substance such as a fatty acid amide such as stearamide optionally with a binder.

Therefore according to the present invention there is also provided a coating composition containing a stabiliser as described above, encapsulated in a material allowing intimate contact of the stabiliser with the colour-forming materials only following thermal activation of the coating composition.

The labels produced in accordance with the invention were subjected to accelerated tests in order to represent typical environmental hazards which are known to have detrimental effects on prior art heat-sensitive record materials.

The formulations tested will now be briefly described. For the purposes of the tests two preferred stabilisers preselected from those of general formula I were used;

stabiliser A of the formula (I) wherein R₁ is benzyloxy, R₂ is hydrogen, Y is sulphur and M is zinc; and

stabiliser B of the formula (I) wherein R₁ is benzyloxy, R₂ is hydrogen, Y is oxygen and M is zinc. These stabilisers were added to coating compositions made up as follows

Composition 1 contains leuco dye, a blend of phenolic developers, binders, sensitiser with other usual additives.

Composition 2 contains leuco dye, a single phenolic developer, binders, sensitiser and other usual additives.

Composition 3 contains leuco dye, a blend of phenolic developers, binders, other usual additives in a thermal maincoat whilst the stabiliser is contained in a topcoat which also comprises a binder and typical additives.

Composition 4 contains leuco dye, a single phenolic developer, binders, sensitiser and other usual additives in a thermal maincoat whilst the stabiliser is contained in a precoat also comprising binder, sensitiser and additives.

Eight formulations were made up from these Compositions (C) as follows:

Formulation l comprising C 1 with stabiliser A;

Formulation 2 comprising C 1 with stabiliser B;

Formulation 3 comprising C 2 with stabiliser A;

Formulation 4 comprising C 2 with stabiliser B;

Formulation 5 comprising C 3 with stabiliser A; Formulation 6 comprising C 3 with stabiliser B;

Formulation 7 comprising C 4 with stabiliser A;

Formulation 8 comprising C 4 with stabiliser B;

These formulations (F) were applied to smooth standard white label-quality paper as a uniform coating on one side of the paper and air dried to give a dried coating weight in the region of 5 - 12 g.m^-2 (where a precoat was applied this was in the region of 2 - 5 g.m^-2; top coat when applied was in the region of 2 - 5 g.m^-2). The dynamic colouring sensitivity of the coating was measured using a Macbeth densitometer RD923 after thermal imaging treatment of the paper using a typical thermal printing head (maximum image density 1.2 W/dot). The results are shown in Table I below and the performance of stabiliser A versus stabiliser B can be compared.

Environmental Stability Tests

In the tests identified below and referred to by number in the subsequent tables each test sample was a label

comprising a smooth white paper substrate coated with the formulations described above. The labels were subjected to treatment, at room temperature unless otherwise indicated, with a variety of solvents, agents and environmental

conditions which would be typical of those to which a label in normal use might be subjected in order to determine the environmental resistance thereof. These are as follows:

a) alcohol - immersion in 50/50 MeOH/water for 1 hour b) vinegar - immersion in 2% acetic acid for 1 hour c) NaCl - immersion in saturated NaCl solution for 1 hour

d) water - immersion in tap water for 15 hours e) oil - smeared with unsaturated vegetable oil and left for 15 hours at 40°C

f) plasticiser - 3 layers of PVC cling film above and below sample for 15 hours at 4 psi, at 40°C

g) light - UV light meter for 1 hour

h) moisture - 93% relative humidity, 10 days at 40°C i) temperature - 50°C for 1 hour, dry

j) motor oil - smeared with motor oil and left for

15 hours at 60°C

Each sample was subjected to image testing using the standard thermal head printing equipment mentioned above to apply a similar image to each sample and the image density was first recorded immediately and again after each of the said tests. In each case the image density was calculated as a percentage retention of the original i.e.

Image density after testing

Image density before testing x 100

Table II shows the results obtained with reference to background (B.G.).

From this it can be seen that a generally high level of resistance to chemical attack and adverse environmental conditions is achieved using the stabilisers of this

invention.

The effect of changing the other components of the coating composition was considered. Therefore further tests using different colour formers (currently commercially

available dyes) were carried out under identical conditions with type B stabiliser compositions. Thus prepared thermal paper was activated on a Hobart 18VP printer and the resulting image densities were measured in a Macbeth Densitometer RD923. Thereafter the same chemical and environmental stability tests as described above were carried out on these formulations 9 to 12.

Thus it can be recognised that merely changing the colour former has no significant effect on the chemical resistance of the image formed using the stabilised composition.

Thereafter the effect of introducing the stabiliser in different forms and at different times during coating mix make-up was investigated. Simultaneously the effect of changing the developer was investigated. It was found that there was no significant difference when a different developer was used.

In the Tables V and VI below the results of testing of five coating formulations is reported.

In Formulation 13 stabiliser B is added to Composition 1 described above. In Formulation 14, based on Composition 1, Stabiliser B is encapsulated in a phenolic developer/binder mix. In Formulation 15, based on Composition 2, Stabiliser B is encapsulated using stearamide/binder. In Formulation 16, based on Composition 4, stabiliser B is applied in a precoat to the substrate. In Formulation 17 based on Composition 2, Stabiliser B is added as the last component in the final mix. Formulation 18 contains no stabiliser.

Formulations 13 to 18 were subjected to the same standard tests as for previous formulations and the results shown in Table VI below.

The stabilisers A and B are obtainable from commercial sources but could be obtainable, in the case of type A

stabilisers by a method involving the reaction of an

appropriately substituted benzoic acid halide with hydrogen sulphide and/or an alkali metal salt of hydrogen sulphide and then reacting the product with a multivalent metal salt, for example the sulphate, chloride, nitrate or acetate of zinc or cadmium, in water or a lower alcohol, or polyhydric alcohol as reaction medium. The reaction temperature is not critical but it is usual to carry this out at 0 - 100°C and reaction is completed within about 24 hours or less.

Stabilisers of type B may be obtainable by a method involving the reaction of phenol with p-chloromethylbenzoic acid in a medium containing aqueous strong base to form the

corresponding p-phenoxymethylbenzoate which upon treatment with aqueous hydrochloric acid produces the free acid which may then be reacted with zinc carbonate for example to produce Stabiliser B.

The following stabilisers have also been found useful for solving the problems addressed herein:

The following compounds of similar organic structure but lacking the metal component have been tried but found to be less successful for the present purpose which suggests the importance of the presence of the metal in the material of the stabiliser of this invention.

In all of the examples, including the comparative examples the thermographic coating was applied in a manner known per se. Determination of success was measured by the tests mentioned hereinbefore.

Claims

1. A heat-sensitive record material comprising a substrate having on at least one surface thereof a heat sensitive-layer containing colour forming reagents capable of reacting to produce a colour upon the application of heat characterised in that said heat sensitive layer bearing surface also carries a stabiliser comprising a moiety of the general formula I

2. A heat sensitive record material according to claim 1 characterised in that R₁ is a C₁-C₄ alkyl, C₁-C₄ alkoxy, benzyloxy, or substituted benzyloxy, or chloro substituent whilst R₂ is hydrogen.

3. A heat sensitive record material according to claim 2 characterised in that R₁ is 4-methylbenzyloxy-, 4-chloro- benzyloxy-, or benzyloxy-.

4. A heat sensitive record material according to any one of claims 1 to 3 characterised in that the metal is Zn or Cd.

5. A heat sensitive record material according to any one of claims 1 to 4 characterised in that Y is oxygen.

6. A heat sensitive record material according to any one of claims 1 to 5 characterised in that the stabiliser is present in an amount of from about 0.5 to about 5 moles per mole of dye in the colour forming reagents.

7. A heat sensitive record material according to any one of the preceding claims characterised in that the stabiliser is ground to a particle size of from 0.5 to 10 microns and added to a coating composition in a final mixing stage.

8. A heat sensitive record material according to any one of the preceding claims characterised in that the stabiliser is encapsulated by a material preventing its release until thermal activation of the of the coating composition.

9. A heat sensitive record material according to claim 9 characterised in that the encapsulating material is a

stearamide.

10. A heat sensitive record material according to any one of claims 1 to 7 characterised in that the stabiliser is present in a precoat mix or applied in a final topcoat mix.