EP0194860B1

EP0194860B1 - Heat-sensitive transferring recording medium

Info

Publication number: EP0194860B1
Application number: EP19860301743
Authority: EP
Inventors: Seiji Ueyama; Hiroyasu Onoe
Original assignee: General Co Ltd
Current assignee: General Co Ltd
Priority date: 1985-03-12
Filing date: 1986-03-11
Publication date: 1992-05-20
Anticipated expiration: 2006-03-11
Also published as: EP0194860A3; EP0194860A2; DE3685351D1

Description

This invention relates to a thermal transfer recording material for use in thermal transfer recording devices such as thermal facsimile machines, thermal printers and the like.
Thermal recording systems which are of the non-impact type have recently attracted attention since they are free of noise and can be easily handled.
Indeed, conventional thermal recording systems are free of noise and require neither development nor fixation of the images they produce but the resulting record is liable to falsification and of poor durability.
One particular thermal transfer recording material proposed for overcoming these drawbacks includes a fusible ink layer on a substrate. The ink layer is contacted with ordinary receiving paper (recording paper) followed by heating with a thermal head through the substrate to melt the ink layer resulting in transfer of the heated portion to the receiving paper.
Such a procedure can result in good printed characters when the receiving paper has a high surface smoothness but when the smoothness is low, for example, the Bekk smoothness value is not higher than 50 sec., the ink layer contacts only some portions of the paper but there is no contact in other portions, because of the uneven surface. This results in a low transfer efficiency, formation of voids, and lack of image sharpness. In addition, since the ink has a high fluidity, the ink penetrates into the receiving paper so that the density of the printed characters is low and good printed images cannot be obtained.
Conventional ink manufacturing methods use a hot melt type coating material or an organic solvent type coating material and the content of resin components is at most 20% by weight. In the case of hot melt ink, the greater the content of resin component, the higher is the melt viscosity and coating is not possible.
In the case of organic solvent type inks, it is difficult to dissolve or disperse waxes, in the solvents and difficult to remove the organic solvent from the resulting wax solution or dispersion during drying.
US-A-4453839 describes a laminated thermal transfer medium which includes a resistive substrate layer top coated successively with aluminium, a release layer and an ink layer and bottom coated with graphite. That medium is used as a thermal printing ribbon having a lift-off correction capability, in that the ink layer is selected so as to adhere selectively to the paper being printed during the printing operation as a result of the release layer being heated to an elevated temperature, and to adhere selectively to the release layer upon that layer being heated to a lower temperature during lift-off correction.
In contrast, the thermal transfer recording material according to the invention does not embody a lift-off correction capability; indeed, it seeks to provide a system for forming a positive and reliable bond between the characters and the paper or other surface onto which the characters are transferred.
According to the present invention, there is now provided a thermal transfer recording material which comprises a substrate layer having coated thereon a thermal release layer, a superposed thermal transfer ink layer having a melt viscosity higher than that of the thermal release layer and containing a coloring material dispersed in a heat fusible binder, and a surface cohesive layer coated on the thermal transfer ink layer and having a melt viscosity intermediate that of the thermal release layer and that of the thermal transfer ink layer.
The substrate layer of the materials of the present invention may consist of paper of, for example, less than 20 µm thick, such as glassine paper, condenser paper and the like, or a heat resistant film of, for example, less than 10 µm thick, such as polyester, polyimide, nylon, polypropylene and the like. Such plastics films from 2 to 10µm thick are preferred.
The substrate layer is more preferably composed of a plastics film provided with a heat resistant protective layer on its surface opposite that coated with the thermal release layer, thermal transfer layer, and surface cohesive layer.
The heat resistant protective layer for the substrate may be of higher fatty acids, fluorocarbon polymers, silicone resins or the like.
In order to obtain printed images of high density containing few voids on paper of low smoothness, it is necessary to make the transfer layer in block form and for the thermal transfer ink layer to be of high melt viscosity so as to reduce penetration of the ink into the paper. Transfer is then effected not as a point but as a plane.
When the thermal transfer ink layer has a high melt viscosity and is coated directly on the substrate, release of the transfer ink layer may become difficult. It is then necessary to facilitate release of the transfer ink layer from the substrate. This is achieved in accordance with the invention by coating on the substrate a thermal release layer. The melt viscosity of the thermal transfer ink layer can then be even greater so as to reduce penetration of the ink into the paper and effect transfer of ink in block form.
The thermal release layer is preferably such that it can be easily melted when heated and has a low melt viscosity. To this end, the thermal release layer suitably comprises, for example, 50 to 100 parts by weight of wax, 0 to 30 parts by weight of binder, and 0 to 50 parts by weight of coloring agent or pigment.
According to the invention, the thermal transfer ink layer has a melt viscosity higher than that of the thermal release layer.
Since the viscosity of the thermal transfer ink layer is high, there is little penetration of the ink into the receiving paper and printed characters of high density can be obtained. Even if the receiving paper is of less smoothness, the thermal transfer recording material can give a clear void-free record of high density at a high transfer efficiency.
According to the invention, the thermal transfer recording material comprises a surface cohesive layer coated on the thermal transfer ink layer, the surface cohesive layer having a melt viscosity intermediate that of the thermal release layer and that of the thermal transfer ink layer.
It is preferred that the thermal release layer is easily melted when heated and has a low melt viscosity. Also, it is preferred that the thermal transfer ink layer has a high melt viscosity. Further the surface cohesive layer preferably becomes cohesive when heated and its melt viscosity is between that of the thermal release layer and that of the thermal transfer ink layer.
In this context, the thermal transfer ink layer preferably comprises, for example, 0 to 30 parts by weight of wax, 20 to 50 parts by weight of binder and 40 to 80 parts by weight of pigment, and the surface cohesive layer preferably comprises, for example, 0 to 50 parts by weight of wax, 50 to 80 parts by weight of binder and 0 to 50 parts by weight of pigment. Also, the thickness of the thermal release layer is preferably 1 to 4 µm, that of the thermal transfer ink layer is preferably 1 to 8 µm and that of the surface cohesive layer is preferably 1 to 8 µm.
The thermal transfer recording material according to the invention suitably has the following constitution: the thermal release layer is mainly composed of a low melting wax having a low melt viscosity; the thermal transfer ink layer is mainly composed of a pigment which hardly melts and has a weak film-shapability; and the surface cohesive layer is mainly composed of a binder which becomes cohesive when heated by a thermal printing head.
A thermal transfer recording material according to the present invention therefore includes a substrate on which there is coated a layer for facilitating the release of an ink layer (i.e. a thermal release layer), a layer which has only slight fluidity (i.e. a thermal transfer ink layer), and a surface layer which is cohesive and can adhere to a receiving paper (i.e. a surface cohesive layer). This three-layer heat-sensitive transfer recording material acts in such a manner that the ink layer (color layer and cohesive layer) is released from the substrate by heating with a thermal head and the cohesion of the cohesive layer permits complete transfer of the ink layer to a receiving paper. Thus, printed void-free images of high transfer efficiency can be produced.
Also, since the color layer does not become substantially fluid by heating, printed images of high density can be produced.
The coloring agents contained in the thermal release layer may be dyes or coloring pigments; the pigments contained in the thermal transfer ink layer and optionally contained in the surface cohesive layer may be coloring pigments or extender pigments.

Examples of the binders, waxes, and coloring agents which may be used in the thermal transfer recording material of the present invention are set out in the Table below.

Table

Wax	Paraffin wax
	Microcrystalline wax
	Carnauba wax
	Shellac wax
	Montan wax
	Higher fatty acids
	Higher fatty acid amides
	Higher alcohols
	Metallic soaps
Binder	Polyvinyl acetate
	Polyvinyl chloride
	Polyvinyl butyral
	Polyethylene
	Polyamides
	Hydroxyethylcellulose
	Methylcellulose
	Nitrocellulose
	Polystyrenes
	Polyesters
	Polyacrylates
	Vinyl chloride-vinyl acetate copolymers
	Ethylene-vinyl acetate copolymers
	Ethylene-organic acid copolymers
	Vinyl chloride-vinylidene chloride copolymers
Coloring agent and pigment	Coloring pigments such as carbon black, iron oxide, Prussian blue, titanium oxide, lake red, and the like;
	Dyes such as basic dyes, neozapon dyes and the like;
	Extender pigments such as calcium carbonate, clay, talc and the like.

The thermal transfer recording materials of the invention can be produced by conventional procedures. For example, the constituent components for the coating compositions for the thermal release layer, for the thermal transfer ink layer, and for the surface cohesive layer may be dispersed and mixed in heated ball mills or attrition mills or dispersed in solvents or water, and then successively applied to a substrate by hot melt coating, solvent coating or aqueous coating.
When a heat resistant protective layer is to be provided on the substrate, the ingredients for the heat resistant protective layer may be dispersed in and mixed with a solvent and applied by solvent coating to the opposite surface of the substrate before the other layers are applied.
The following Examples illustrate the invention. Parts and percentages are by weight.

Comparative Example 1

Paraffin wax	40 parts
Carnauba wax	30 parts
Ethylene-vinyl acetate (90:10) copolymer	10 parts
Carbon black	20 parts

An ink composed of the above ingredients was applied to a 6 µm polyester film in a thickness of 4 µm by means of hot melt coating.

Comparative Example 2

A paraffin wax (m.p. 65°C) was applied to a 6 µm polyester film in a thickness of 1 µm by hot melt coating.

To the surface of the resulting paraffin wax layer was applied an ink composed of the following ingredients:

	Amount (parts)	Solid content (%)
Ethylene-vinyl acetate (90:10) copolymer emulsion (solid content, 45%)	60	69.2
Carnauba emulsion (solid content, 20%)	20	15.4
Carbon black dispersion (solid content 30%)	20	15.4

by Mayer bar coating and dried to form a heat-sensitive transfer ink layer of 4 µm thick.

Comparative Example 3

Paraffin wax was applied to a 6 µm polyester film in a thickness of 1 µm by a hot melt coating method to form a heat-sensitive release layer.
To the surface of the heat-sensitive release layer was applied a composition composed of the above components by a Mayer bar method, followed by drying to form a heat-sensitive transfer ink layer.

Comparative Example 4

Paraffin wax	70 parts
Ethylene-vinyl acetate (90:10) copolymer	10 parts
Carbon black	20 parts

A composition composed of the above components was applied to a 6 µm polyester film in a thickness of 2 µm by a hot melt coating method to form a heat-sensitive release layer.

To the surface of the heat-sensitive release layer was applied a composition composed of the following components by a Mayer bar method and dried to form a heat-sensitive transfer ink layer 3 µm thick.

	Component (parts)	Solid matter(%)
Ethylene-vinyl acetate (90:10) emulsion (solid matter 45%)	160	(68.6)
Carnauba emulsion (solid matter 30%)	50	(14.3)
Carbon black dispersion (solid matter 30%)	60	(17)

Example 1

To a 6 µm polyester film were successively applied the following layers:

Heat-sensitive release layer:

Paraffin wax was applied in a thickness of 1 µm by hot melt coating.
The above components were applied by solvent coating and dried. Thickness was 2 µm.
The above components were applied by solvent coating and dried. Thickness was 2 µm.

Example 2

To a 6 µm polyester film were successively applied the following layers:

The above components were applied by hot melt coating in a thickness of 2 µm.

Coloring agent layer:
	Component (parts)	Solid matter(%)
Ethylene-vinyl acetate (90:10) copolymer emulsion (solid matter 45%)	50	30.6
Zinc stearate dispersion (solid matter 30%)	50	20.4
Carbon black dispersion (solid matter 30%)	120	49.0

The above components were applied by solvent coating in a thickness of 2 µm.

Heat-sensitive cohesive layer:
	Component (parts)	Solid matter(%)
Ethylene-vinyl acetate (90:10) copolymer emulsion (solid matter 45%)	100	71.4
Carnauba emulsion (solid matter 30%)	30	14.3
Carbon black dispersion (solid matter 30%)	30	14.3

The above components were applied by solvent coating and dried. The thickness was 2 µm.

Test method

The heat-sensitive transfer recording materials prepared above were tested by means of a heat-sensitive printer (cycle, 1.2 m/sec.; applied pulse width, 0.9 m sec., power, 0.5 W/Dot) with a receiving paper (Bekk test, 16 sec; Hammer Mill Bond paper) (JIS P8119).
The heat-sensitive transfer recording material prepared in the Comparative Examples gave many voids and low density images while those prepared in Examples 1 and 2 gave good printed characters of few voids and high density.

Claims

A thermal transfer recording material for the thermal printing of sharp transfer images, comprising a substrate layer having coated thereon a thermal release layer, a superposed thermal transfer ink layer having a melt viscosity higher than that of the thermal release layer and containing a coloring material dispersed in a heat fusible binder, and a surface cohesive layer coated on the thermal transfer ink layer and having a melt viscosity intermediate that of the thermal release layer and that of the thermal transfer ink layer.
A thermal transfer recording material according to claim 1, wherein the surface cohesive layer comprises 0 to 50 parts by weight wax, 50 to 80 parts by weight binder and 0 to 50 parts by weight coloring agent.
A thermal transfer recording material according to claim 1 or claim 2, wherein the thermal transfer ink layer comprises 0 to 30 parts by weight wax, 20 to 50 parts by weight binder and 40 to 80 parts by weight coloring agent.
A thermal transfer recording material according to any one of claims 1 to 3, wherein the thermal release layer comprises 50 to 100 parts by weight wax, 0 to 30 parts by weight binder and 0 to 50 parts by weight coloring agent.
A thermal transfer recording material according to any one of claims 1 to 4, wherein the substrate is a plastics film coated on the side opposite the thermal release layer with a heat resistive protection layer.