EP0354122B1 - Multiple use recording materials for thermal transfer - Google Patents

Abstract

The invention relates to a recording material using multipass thermal transfer, comprising a bottom support coated with at least one layer of a thermofusible ink which has a melting temperature in the range from 50 to 90<o>C and comprising at least one colouring substance and a thermofusible carrier for this colouring substance, characterised in that the ink contains from 15 to 50%, relative to the weight of the ink, of at least one thermal transfer polymer resin mixed homogeneously with the other constituents of the ink, having a softening point of between 60 and 130<o>C, a tensile strength of less than 8N/mm<2> at 20<o>C, an elongation of between 0.04 and 6 m/m, a melt viscosity of less than 5 Pa s at 200<o>C and an adhesiveness to the bottom support such that, at the temperature of thermal transfer, the force needed to detach the ink from the said support is greater than the force needed to break the internal cohesion of the ink.

Description

The present invention relates to new recording materials by thermal transfer such as sheets, films, tapes, which can be used several times.

The process of recording information, of whatever nature, by thermal transfer is widely used because of the numerous advantages which it presents (in particular simplicity and low cost of the apparatus used, stability of the recordings ). However, the users of this process consider that its operating cost can be lowered by the reuse of recording materials (sheets, films, tapes) several times in succession. Indeed, during the use of thermal recording materials of the usual type comprising a common hot-melt ink, all of the ink carried by a small part of the support of the material is transferred from said material to the receiving material. It therefore becomes impossible to reuse the recording material a second time, for a new recording operation and to fully use the ink still carried by the support after a first thermal transfer if it is desired to maintain an optical density. uniform information transferred. This single use of the recording material increases the cost of implementing thermal transfer recording and does not make it possible to improve the autonomy of the cassettes of ribbons or the like. We therefore sought to overcome these drawbacks by developing recording materials ensuring a gradual thermal transfer of the hot-melt ink. Various solutions have been proposed for this purpose. Thus, heat transfer materials have been described in which the hot-melt ink impregnates a continuous porous structure or in the form of porous particles, cf. : Japanese patent application no. 54/68253; French patent application No. 85 09729 published under No. 2,566,328. Heat transfer materials have also been described, the hot-melt ink layer of which contains fine organic particles (phenolic resins, epoxy resins) or mineral particles (metal oxides , metallic powders, molecular sieves, diatomaceous earth) porous or not forming a barrier layer slowing the transfer of hot-melt ink. It has also been proposed to slow down the transfer of the hot-melt ink from its support to the material intended to receive the recording by placing between the recording material and the ink layer, a layer of an adhesive compound. , cf. for example: published Japanese applications No. 60 / 54,893; 60 / 255.490; 60 / 54,894; 61 / 255.895. Despite the advantage they present, these different techniques have the disadvantage of complicating the manufacture of recording materials, so that the industry is always looking for a simple means allowing the gradual delivery of an ink heat transfer from a medium so that the recording material can be reused repeatedly and resulting in each use to a recording with good optical density. The present invention specifically relates to a new solution to the problem of reusable heat transfer materials designated below, for convenience, multi-pass recording materials.

A first objective of the present invention lies in the development of simple multi-pass recording materials.

A second objective of the present invention lies in increasing the number of possible re-uses of the multi-pass recording material.

A third objective of the present invention lies in improving the optical density of recordings resulting from the use of multi-pass recording materials.

More specifically, the subject of the present invention is a multi-pass thermal transfer recording material comprising a base support coated with at least one layer of a hot-melt ink having a melting temperature in the range of 50 to 90 ° C. and comprising at least one coloring substance and a hot-melt vehicle for this coloring substance, characterized in that the ink comprises from 15 to 50%, relative to the weight of the ink, of at least one polymeric thermal transfer resin, homogeneously mixed with the other constituents of the ink, having a softening point of between 60 and 130 ° C, a tensile strength of less than 8N / mm² at 20 ° C, an elongation of between 0.04 and 6 m / m, a viscosity in the molten state of less than 5 Pa.s at 200 ° C., and an adhesion to the base support such that, at the heat transfer temperature, the force necessary to peel off the ink from said support is superior ure at the force necessary to break the internal cohesion of the ink.

Thereafter and for reasons of convenience, the polymeric thermal transfer resin ensuring the gradual transfer of the ink will be designated by the expression "transfer resin".

The multipass thermal transfer ink which has been developed contains, like the usual inks of single pass thermal transfer materials, at least one coloring substance of chromatic colors such as blue, red and yellow or at least one coloring substance black and a hot-melt vehicle of said substance, that is to say a compound or a mixture of compounds compatible (s) with the dye (s) and melting at a temperature between 50 and 150 ° C. These single pass inks generally have a transfer rate greater than 80%. In the present application, the rate of transfer is defined by the ratio of the quantity of ink transferred from the thermal transfer material on the receiving material to the quantity of ink present on the transfer material before the last transfer reported at 100.

• avoir une très bonne adhérence sur le support de base (film polyester par exemple),
• avoir une très bonne fusibilité (température de fusion comprise entre 50 et 90°C),
• avoir une force de cohésion interne de l'encre au moment du transfert (lors de la séparation du matériau d'enregistrement d'avec le support-récepteur) qui soit suffisamment faible pour permettre une rupture au sein de la couche d'encre et non à l'interface avec le support de base.

To obtain a multi-pass material, an ink has been developed whose transfer rate is between 5 and 60%. For this, the ink must meet the following conditions:

• have very good adhesion to the base support (polyester film for example),
• have very good fusibility (melting temperature between 50 and 90 ° C),
• have an internal cohesive force of the ink at the time of transfer (when separating the recording material from the receiving medium) which is sufficiently weak to allow a rupture within the ink layer and not at the interface with the basic support.

The coloring substances which are suitable for the preparation of the ink of the thermal transfer materials in accordance with the invention are those usually used. The term "coloring substances" denotes organosoluble or water-soluble dyes or pigments. They can be inorganic or organic, of natural or synthetic origin. Dyes such as those mentioned in DE-A-35 20 308 can therefore be used; EP-A-0063000; DE-A-36 06 710. Among the black dyes, mention may be made, for specific purposes: carbon black; dyes sold under the commercial brands: NOIR CERES by the company BAYER; NOIR NEOPRENE by BASF; NOIR AU GRAS by the company CIBA-GEIGY; magnetic iron oxide such as those sold under the brand name BAYFERROX by the company BAYER. In the case of black dyes, it has been found that the combination of one or more organic dyes with magnetic iron oxide leads to excellent optical densities of the recording resulting from the thermal transfer and a good conservation of the impressions. The quality of the latter is particularly suitable for character recognition by optical or magnetic reading.

As a vehicle for coloring substances, use is made of one or more organic compounds, melting between 50 and 150 ° C., such as those used in the usual manner in thermal transfer inks. They can be natural or synthetic products. As such, there may be mentioned, without limitation: vegetable waxes such as carnauba wax, candellila wax; animal waxes such as lanolin, beeswax; mineral waxes such as montane waxes; synthetic waxes such as paraffin; microcrystalline waxes; fatty acids highly condensed in carbon, their esters and their amides such as: stearic acid, palmitic acid, stearamide, palmitamide; alkali salts of fatty acids; polyols such as polyethylene glycol, sorbitol, polypropylene glycol; polyol ethers such as polyethylene glycol and lanolin ethers; alcohols with high carbon condensation (palmitic, stearyl, cetyl alcohols); polymers such as polyvinyl esters (polyvinyl acetate), ethylene / vinyl acetate copolymers.

• une bonne adhérence sur le support de base,
• une température de fusion comprise entre 60 et 130°C, de préférence entre 70 et 100°C,
• une résistance à la traction inférieure à 8N/mm² à 20°C d'après la norme DIN 53455,
• une viscosité à l'état fondu inférieure à 5 Pa.s à 200°C.

The transfer resin, or the mixture of transfer resins, is the key ingredient of the inks usable in the multi-pass recording materials of the invention and must have the following properties:

• good adhesion to the base support,
• a melting temperature between 60 and 130 ° C, preferably between 70 and 100 ° C,
• a tensile strength of less than 8N / mm² at 20 ° C according to DIN 53455,
• a melt viscosity of less than 5 Pa.s at 200 ° C.

The transfer resin (or the mixture of transfer resins) is chosen so as to ensure, for each of the first ten uses (or passes) of the thermal transfer support, an ink transfer rate of between 5% and 60% . Without limitation, mention may be made of polyamide resins such as those sold under the trade names EURELON by the company SCHERING; terpene resins such as those sold under the trademark DERTOLENE-DERTOPHENE or DERTENATE by the company DRT or rosins such as those sold under the trademark STAYBELITE by the company HERCULES.

• point de ramollissement d'environ 95°C (norme DIN 52011),
• température de fusion d'environ 73°C (mesurée sur une courbe d'analyse thermique différentielle obtenue avec le thermo-système METTLER FP800, cellule FP81),
• viscosité à l'état fondu de 1 à 1,5 Pa.s à 160°C,
• résistance à la traction d'environ 3,5 N/mm² à 20°C,
• allongement à 20°C d'environ 0,7 m/m (norme DIN 53455).

The transfer resin which is most suitable in the invention is an adhesive polyamide resin of flexible type having the following characteristics:

• softening point of around 95 ° C (standard DIN 52011),
• melting temperature of approximately 73 ° C (measured on a differential thermal analysis curve obtained with the METTLER FP800 thermosystem, FP81 cell),
• melt viscosity of 1 to 1.5 Pa.s at 160 ° C,
• tensile strength of around 3.5 N / mm² at 20 ° C,
• extension at 20 ° C of about 0.7 m / m (standard DIN 53455).

Such a resin is sold under the trademark EURELON-2095 by the company SCHERING.

The amount of polymer resin used to ensure a gradual transfer of the ink with each pass depends on the nature of the support and the composition of the latter. In general, this amount represents from 15 to 50% by weight and preferably from 20 to 40% of the total of the transferable composition.

It is possible, without departing from the scope of the present invention, to combine two or more of two transfer resins.

As the support for the ink layer, those used usually are used. They may be films, sheets or ribbons of film-forming polymers such as linear polyesters and, in particular, polyterephthalates of diols (for example ethylene glycol); polyamides (polyhexamethyleneadipamide, polycaprolactam); polyethylene; polypropylene; polycarbonates; cellulose derivatives (cellulose esters, paper). Polyterephthalates are particularly suitable. The dorsal face of the support, that is to say the face opposite to that carrying the ink, can be provided with the usual coatings intended to provide them with good thermal resistance and / or good machinability and / or antistatic properties. To this end, mention may be made of back coatings made of polysiloxanes or polyurethanes.

The thickness of the ink layer containing the polymer resin is determined so as to ensure sufficient optical density for recording after at least six passes of the heat transfer medium. In general, it is between 4 and 35 μm and preferably between 4 and 20 μm. It is possible, without departing from the scope of the present invention, to place on the ink layer according to the invention, an ink layer containing less than 5% by weight of transfer resin and, preferably, does not not containing it, that is to say a layer of ink transferred more than 80% on the first pass. To promote rupture within the hot-melt assembly, the viscosity in the molten state of the second layer of ink must be lower than that of the first layer containing the transfer resin and its melting temperature must not exceed that of the first layer of ink. This layer can have a thickness of between 2 μm and 10 μm.

The ink of the recording materials according to the invention may comprise, in addition to the transfer resin, other polymer resins used in the inks single or multi-pass such as for example polyvinyl acetates, ethylene / vinyl acetate copolymers (EVAC) and the usual additives (for example: plasticizers).

The multi-pass materials in accordance with the present invention can be obtained by the usual methods of coating film-forming polymer supports, or by a solution of the ink composition in one or more organic solvents: ketones (methyl ethyl ketone), aliphatic hydrocarbons (hexane ), cycloaliphatic or aromatic (toluene), alcohols (propanol, isopropanol) or, preferably, by a molten composition.

The inks in solution are obtained by dissolving the soluble substances in the chosen solvent (s) and then, if necessary, dispersing the insoluble substances (pigments for example) by means of a turbine or a ball mixer.

The melted inks are prepared by melting the meltable ingredients by heating between 80 and 180 ° C. then addition of the coloring substance (s) with stirring using a turbine or a blender. marbles.

The following nonlimiting examples illustrate the invention and show how it can be put into practice.

In these examples, the optical density was measured by reflection on transferred solids, using a densitometer of the trade mark MACBETH TR-927.

EXAMPLE 1 to 11

In the following examples, tapes were prepared by coating a film of polyethylene terephthalate 6 μm thick comprising a backing layer based on a polysiloxane resin. Depending on the composition of the ink, the support was coated with the fade or with a solution.

The conditions of adhesion, fusibility and low internal cohesion necessary for multi-pass and not single-pass operation have been optimized by adding to the transfer resin additional products such as natural or synthetic waxes, paraffins, fatty acids or other products with a low melting point. known in traditional single pass inks. In particular those cited in Table 1 below.

The inks A to K, the composition of which in percentages by weight is given in Table I below, were deposited on the support by the molten process for ink A and in the form of a solution for inks B to K. In the latter In this case, the coated supports were dried in a dryer between 50 and 150 ° C. Eleven films coated with a 14 µm ink layer were obtained which were transformed into thermal transfer ribbons.

The ribbons thus obtained were subjected to transfer tests. To this end, the same text was reproduced six consecutive times with each sample of tape, then the density of the text reproduced after each pass was measured. The transfer was carried out on a 50-second satin-finish receiving paper measured on a BEKK-31 E device, under the transfer conditions applied to single-pass ribbons (temperature between 60 and 70 ° C). The results are shown in Table II.

- acide stéarique

: 7,7%

- paraffine

: 19,1%

- cire de carnauba

: 23,6%

- oxyde de fer magnétique

: 41,9%

- noir de carbone

: 7,7%

For comparison, an ink A ′ was prepared containing the same constituents as ink A with the exception of the polyamide and terpene resins. This ink has the following weight composition:

- stearic acid

: 7.7%

- paraffin

: 19.1%

- carnauba wax

: 23.6%

- magnetic iron oxide

: 41.9%

- carbon black

: 7.7%

The ribbons obtained using this ink (thickness of the layer 14 μm) led to the following optical densities after each of the six passages in a heat transfer machine.

EXAMPLE 12

• une couche d'encre A de 9µm
• une couche d'encre d'épaisseur 5 µm, ne comportant pas de résine de transfert et dont la composition pondérale est la suivante :

. acide stéarique

: 8%

. EVAC

: 9%

. paraffine

: 38%

. oxyde de fer magnétique

: 26%

. NOIR CERES

: 19%

A thermal transfer ribbon comprising two layers was prepared:

• a 9µm layer of ink A
• a layer of ink of thickness 5 μm, not comprising transfer resin and whose composition by weight is as follows:

. stearic acid

: 8%

. EVAC

: 9%

. paraffin

: 38%

. magnetic iron oxide

: 26%

. BLACK CERES

: 19%

The first layer was melted and the second in solution.

The optical densities measured after each of the six passes of the same ribbon sample in a thermal transfer machine are as follows:

Claims (9)

1. Material for recording using multipass heat-transfer, comprising a base carrier coated with at least one layer of a thermofusible ink which has a melting temperature within the range from 50 to 90°C and comprising at least one colouring substance and a thermofusible vehicle for this colouring substance, characterized in that the ink comprises from 15 to 50%, relative to the weight of the ink, of at least one heat-transfer polymer resin mixed homogeneously with the other constituents of the ink, having a softening point of between 60 and 130°C (DIN method 52011), a tensile strength lower than 8 N/mm² at 20°C (DIN method 53455), an elongation of between 0.04 and 6 m/m (DIN method 53455), a melt viscosity lower than 5 Pa.s at 200°C, and an adhesiveness to the base carrier such that, at the heat transfer temperature, the force needed to separate the ink from the said carrier is greater than the force needed to break the internal cohesion of the ink.
2. Recording material according to Claim 1, characterized in that the ink comprises from 20 to 40% by weight of the said heat transfer polymer resin.
3. Recording material according to Claim 1 or 2, characterized in that the transfer resin comprises a polyamide resin.
4. Recording material according to Claim 1 or 2, characterized in that the transfer resin comprises a terpene resin.
5. Recording material according to Claim 1 or 2, characterized in that the transfer resin is a mixture of a polyamide resin and of a terpene resin.
6. Recording material according to Claim 3, characterized in that the polyamide resin has a softening point of approximately 95°C, a melting temperature of approximately 73°C, a melt viscosity of 1-1.5 Pa.s at 160°C, a tensile strength of approximately 3.5 N/mm² at 20°C and an elongation of approximately 0.7 m/m at 20°C.
7. Recording material according to any one of Claims 1 to 6, characterized in that it has a degree of transfer of 5 to 60% at each of the passes.
8. Recording material according to any one of Claims 1 to 7, characterized in that the thickness of the ink layer is comprised between 4 and 35 µm.
9. Recording material according to any one of Claims 1 to 8, characterized in that it comprises a second layer of ink free from transfer resin, whose melt viscosity is lower than that of the ink of the first layer and whose melting temperature does not exceed that of the first layer.