EP0439231B1

EP0439231B1 - A polymeric matrix for use on a receptor sheet for thermal transfer recording

Info

Publication number: EP0439231B1
Application number: EP19910200112
Authority: EP
Inventors: David Atherton; Kang Sun
Original assignee: Arkwright Inc
Current assignee: Arkwright Inc
Priority date: 1990-01-25
Filing date: 1991-01-21
Publication date: 1997-06-18
Anticipated expiration: 2011-01-21
Also published as: EP0439231A2; DE69126553D1; DE69126553T2; JPH07223381A; CA2034847A1; EP0439231A3

Description

Background and Field of the Invention

This invention relates to an assemblage for use in a thermal transfer recording process and consisting of a receptor sheet and a donor member comprising an ink donor layer. More specifically the invention relates to a polymeric matrix for use on a receptor sheet utilized in such assemblage.
One of the more important non-impact printing technologies is the thermal transfer printing process. It has several advantages over traditional mechanical impact printing, such as high resolution, low noise level and high speed. However, a thermal transfer printer requires a printing medium tailored for its specific process.
The thermal transfer printing process involves three components: a thermal print head, a thermal transfer ribbon consisting of a foundation and a heat-sensitive ink donor layer applied thereon, and an ink receptor sheet. The inked side of the thermal transfer ribbon is placed in contact with the ink receptor sheet, and heat from the thermal print head is applied to the backside of the thermal transfer ribbon. The heat is conducted through the plastic or paper ribbon and locally raises the ink temperature above its softening point. The softened ink partially wets the ink receptor sheet, transfers to it and re-solidifies.
A wide variety of different types of thermal transfer ink receptor media have been proposed heretofore. For example, Japanese Patent 63-237,989 describes an ink-receptor sheet containing two aromatic polyamide layers having different roughness; Japanese Patent 64-072,662 describes a two-layer ink-receptor sheet comprising a metal oxide layer and an adhesion layer; Japanese Patent 64-072,663 describes an ink-receptor sheet consisting of a sponge urethane or a foam styrene; and Japanese Patent 63-69,685 describes an ink-receptor sheet containing an aluminum silicate and a polymer binder. Some patents describe wax-containing and wax compatible ink receptor layers. Examples of such patents include Japanese Patents 59-229,394, 60-49,997, 60-174,695, 60-154,096, 64-072,664, 63-237,988, 63-170,087 and European Patent Publication 228,835. Some patents describe ink receptor layers coated on an opaque substrate such as paper. Examples of such patents include Japanese Patents 60-49,997, 60-54,891, 59-229,394, 63-17,079, 61-139,487, 60-56,594, 63-237,986, 62-173,293, 63-77,780, and 59-194,888.
Despite the substantial prior art, none have achieved the image quality required by the end user. Density is frequently low and half tones are poorly rendered, resulting in inadequate tonal quality.
We have now devised a polymeric matrix which is particularly suitable as an ink receptor medium for thermal transfer recording.

Summary of the Invention

An object of the present invention is to provide a polymeric matrix for use in thermal transfer printing which is capable of producing recorded images having improved image density and resolution.
A key feature of this invention is to formulate a polymeric matrix containing at least one hard polymeric element and one soft polymeric element. The polymeric matrix may also contain a pigment, a surface active agent and a conductive agent and other additives.
According to a first embodiment of the present invention, an assemblage for use in a thermal transfer recording process is provided, consisting of a receptor sheet and a donor member comprising an ink donor layer, said receptor sheet comprising a polymeric matrix comprised of at least one hard element and at least one soft element, said hard element being a polymeric material with a softening temperature of at least about 5°C higher than that of said ink donor layer and said soft element being a polymeric material with a softening temperature of at least about 5°C lower than that of said ink donor layer.
In a further embodiment, a method of forming an image on a receptor sheet is provided, wherein image forming material from the ink donor layer of a donor sheet is transferred to the image receptive layer of the receptor sheet which comprises a polymeric matrix comprised of at least one hard element and at least one soft element, said hard and soft elements being polymeric materials having softening temperatures which fulfil the above set condition with respect to the softening temperature of the ink donor layer.

Detailed Description of the Invention

A receptor sheet, in general, comprises a backing, or base film, and an image receptive layer.
The base film may be comprised of various materials, and numerous suitable backings are known in the art and commercially available.
Examples of materials suitable for use as base substrates or backings include paper, polyesters, polysulfones, polycarbonates, poly(vinylchloride), poly(vinylacetate), polyolefins, polystyrenes and cellulose esters. Specific examples include polypropylene, cellulose acetate, cellulose acetate butyrate, and, most preferably, polyethylene terephthalate. These backing materials can be transparent or opaque depending upon the ultimate use of the final product.
The image receptor layer in the assemblage according to the invention, is comprised of a polymeric matrix, optionally in combination with a pigment, a surface active agent and/or a conductive (anti-static) agent. The polymeric matrix of the invention importantly comprises at least one hard element and at least one soft element as defined above.
The polymeric matrix should have the following characteristics:

1. At least one element in the polymeric matrix has a softening temperature that is at least 5°C lower than that of the ink donor layer, i.e. a soft element.
2. At least one element in the polymeric matrix has a softening temperature that is at least 5°C higher than that of the ink donor layer, i.e. a hard element.
3. It can be coated on plastics or paper.
4. It is stable to light and has good heat stability.
5. It is an optically uniform, non-tacky and smooth film.

In addition to the above characteristics, a commercially useful receptor sheet should also feed reliably through thermal transfer printers. Those receptor sheets to be used for transparencies must also be transparent to light and handleable under conditions typically encountered during use of overhead transparencies.
The polymer matrix can be a copolymer, a combination of homopolymers, a combination of copolymers, or a combination of homopolymers and copolymers.
Copolymers employed in the polymeric matrix according to invention are preferably block copolymers and graft copolymers. However, random copolymers, comb copolymers and star copolymers can be used as long as the monomeric elements in the molecular chains give the above-mentioned characteristics.
The hard elements have softening temperatures that are at least 5°C higher than that of the ink donor layer. The presence of hard elements ensures that desired image resolution, density gradation and handling properties can be achieved. The hard elements are preferably methyl methacrylate, ethyl methacrylate, acrylonitrile, methacrylonitrile, vinyl alcohol, ethylene terephthalate, vinyl butyl ether, carbonate, vinyl chloride, melamine formaldehyde, acrylic acid, unsaturated esters, cellulose esters, epoxy, styrene, phenol formaldehyde, and urethane, among others. The hard elements can also be obtained by crosslinking soft elements having crosslinkable functional groups.
The soft elements have softening temperatures that are at least 5°C lower than that of the ink donor layer. The soft elements give desired image density, adhesion and uniformity. The soft elements are usually structurally similar to the softening agent or binders employed in the ink donor layer. These elements include methyl acrylate, ethyl acrylate, butyl acrylate, ethylene, propylene, butadiene, isobutene, cellulose acetate, n-hexyl methacrylate, vinyl acetate, cellulose ethers, petroleum resins, styrene-butadiene, ethylene-acrylic acid, vinyl acetate-acrylic acid, styrene-butyl acrylate, caprolactam, cellulose nitrate, oxymethylene, vinyl methyl ether, vinyl ethyl ether, vinyl butyl ether, siloxane, urethane, vinylidine chloride, ethylene adipate, hexamethylene adipamide, n-propylmethacrylate, n-butyl methacrylate, n-hexyl methacrylate, n-octyl methacrylate, and tetramethylene sebacate, among others.
Various combinations of hard and soft elements are possible, but consistent within the important requirement of the invention that the polymeric matrix comprise at least one hard element and at least one soft element. As defined above, the hard element should have a Ts of at least about 5°C higher than that of the ink donor layer, and the soft element should have a Ts of at least about 5°C lower than that of the ink donor layer. Preferably, the hard element should have a Ts of from 10 to 80°C, most preferably from 20 to 50°C (especially 40°C) above the Ts of the donor layer, and the soft element should have a Ts of from 10 to 80°C, most preferably from 20 to 50°C (especially 30°C) lower than the Ts of the donor layer.
The amount of the hard and soft elements can vary, balancing the desired characteristics of handling and resolution. If the polymeric matrix is too hard, then the product will exhibit poor transfer from the donor sheet. On the other hand, a sheet which is too soft will exhibit too much transfer. Generally, therefore, the matrix should contain from 5 to 80% soft elements. Evaluation of a product for determining a proper balance of hard and soft elements can be done by a half tone test. A product produced according to the invention provides regular shaped, well defined dot transfer which gives excellent half tones. Such an improved product is particularly attained by using soft elements which are wax compatible or wettable, having chemical affinity to the binder of the donor sheet, but hard elements which are not wax compatible and do not have such chemical affinity.
The polymeric matrix of the invention can be comprised of various combinations of the above described hard and soft elements, so long as the matrix contains at least one hard and at least one soft element. Softening temperature (oftentimes interchangeable with the glass transition temperature, Tg), as used herein, means Vicat softening temperature determined in accordance with ASTM D1525 (1982) for polymers with no sharp melting point, or, for polymers which do exhibit a sharp melting point, the melting point itself. This Vicat method has a measurement accuracy of ± 4°C.
An alternative method for determining the Ts is the differential scanning calorimetry (DSC) measurement made using a DSC thermal analyzer, such as the DuPont 910. Under this method, measurements of heat flow versus temperature are made on heating in the range of -100°C to 200°C at a heating rate of 20°C/min, while the sample chamber is purged with dry N₂. For amorphous polymers, the softening temperature is defined as the temperature where the initial base line deflects. For highly crystallized polymers, the softening temperature is the melting point itself. This DSC method has a measurement accuracy of ± 4°C.
Either measurement method can be used, but for highly crosslinked polymers, the VICAT method is more sensitive and appropriate. However, whichever method is selected, that method should consistently be used as the method for all of the determinations of softening temperatures.
As noted above, the base film, in addition to the polymeric matrix, may contain a surface active agent, a conductive agent, and/or a pigment and a pretreat on the base substrate to enhance adhesion.
A preferred example of the polymeric matrix of this invention contains a copolymer of ethylene-styrene-butylacrylate-acrylic acid, a copolymer of methyl methacrylate-butyl methacrylate, a homopolymer of methyl methacrylate, a surface active agent and pigments.
The surface active agent, such as wetting agent, dispersing agent, defoaming agent and anti-foaming agent, etc., may be incorporated into the polymer matrix to improve the surface properties and the coatability. Both hydrocarbon type and fluorochemical type surface active agent can be used.
Preferable examples of conductive (anti-static) agents used in the invention include sulfonated polystyrene, poly(dimethyl diallyl ammonium chloride), copolymers of dimethyl diallyl ammonium chloride and diacetone acrylamide, quaternary acrylics, copolymer of dimethylammoniumchloride and N-methyl acrylamide, quaternary cellulose acetate and other conductive materials.
The surface resistivity of the receiving sheet is usually about 1 x 10⁷ - 1 x 10¹⁴ ohms/sq. at 50% relative humidity and 20°C.
Pigments useful in the invention are those which are per se known in the art for thermal transfer printing, including, for example, calcium carbonate, titanium oxide, Kaolin, aluminium hydroxide, polyolefin particulates such as polyethylene, polypropylene or polytetrafluoroethylene, silica etc. These and other organic and inorganic pigments can be used to modify the surface properties off the medium and particularly offer increased recoatability, abrasion resistance, slip, and anti-blocking characteristics. Most preferably the incorporation of the pigment in the polymer matrix gives a Sheffield surface smoothness of from about 5 to 100. According to a preferred embodiment of the invention, a polymer matrix for a transparent ink receptive sheet comprises about 0.05 to about 10% pigment by weight of the dried coating for a transparent sheet and a polymeric matrix for opaque ink receptive sheet comprises about 0% to about 70% pigment by weight of the dried coating.
The coating solution, which is used for the formation of the polymer coating on the polymeric base film substrate or paper, generally is an aqueous solution, but organic solvents such as methanol, ethanol, cellosolve solvent, etc., can be employed in combination with water, if desired. An organic solvent soluble formulation may also be devised with performs like the aqueous based systems.
An additional coalescing agent may also be used in the receptor sheet to improve leveling, scrub resistance, gloss, adhesion, and enamel holdout. Various useful coalescing agents are known in the art and comprise high-boiling solvents such as butyl cellosolve acetate, hexylene glycol, ethylene glycol and propylene glycol monomethyl ether.
Any of a number of coating methods may be employed to prepare the receptor sheets according to the invention, such as roller coating, wire-bar coating, dip-coating, air-knife coating, spray coating, curtain coating, doctor coating, gravure coating, reverse roll coating, stretch-flow coating, bead coating or extrusion coating. The matrix layer is preferably coated to a thickness of about 0.05 to 0.50 mil, to produce a dry coat weight of about 0.05 to 4.0 g/M².
The polymeric matrix of this invention can be applied to one or both sides of the supporting base film substrate. In those products with a coating on both sides, the polymeric matrices on the sides of the supporting substrate need not necessarily be identical. The coat weight of a dried coating is preferably about 0.05 to about 4 grams per square meter of coating, although workable coatings may be achieved with lesser or greater coat weights.
As noted above in a thermal transfer printing process, printing is accomplished by the application of heat from the thermal print head to the thermal transfer ribbon or donor sheet which softens the ink and transfers it to the receptive sheet. Such donor sheets comprise a backing or base layer with a coating layer of donor material. Various donor sheets known in the art are useful in the present invention, including for example those described in U.S. Patents 4,474,744; 4,572,624; 4,463,034 and 4,315,643. One useful commercially available sheet is that sold by Cal Comp for Model 5602 Color Master Plotter.
Other commercially available donor sheets are those sold by Seiko and Versatec. As examples, the following commercially available donor sheets have the following softening temperatures (Ts):

Type Color Ts (°C)

Cal Comp Blue 40

Cal Comp Red 40

Cal Comp Yellow 35-40

Seiko Blue 29

Seiko Red 26

Seiko Yellow 28

Versatec Blue 40

Versatec Red 45

Versatec Yellow 45

In such known donor sheets, the backing or base layer is generally a paper or plastic film, such as laminated, synthetic, or glasine paper; or polyester, polypropylene, polystyrene or polyethylene films.
The transfer layer is comprised of a coloring agent, in combination with a binder and softening agent. Various known compositions can be used in the invention by first ascertaining the softening point of the donor sheet, and then appropriately selecting at least one hard element and one soft element for use in the receptive sheet.
The following examples are provided to more specifically describe the invention.

Examples

A polyethylene terephthalate film used as a light-transmissive substrate was coated on its surface with the following Polymer Matrices A, B and C by means of a Meyer rod coater so as to have a dried film thickness of 2 µm, followed by drying at 120°C for 1 minute. These products were then used with a donor sheet having a Ts of about 40°C.

Polymer Matrix A
76 RES 7800 (50.0%)¹	24.0 parts
Rhoplex AC-73 (46.5%)²	15.0 parts
Rhoplex B-85 (38.0%)²	9.0 parts
Versa-TL 125 (6%)³	3.3 parts
Surfynol 104 Surfactant⁴	0.2 parts
Water	48.0 parts

1. 76 RES 7800 - A copolymer of vinyl acetate - acrylic acid sold by Unocal Chemical Division, Unocal Corporation (Ts = 22°C)
2. Rhoplex AC-73 - A copolymer of methyl methacrylate-butyl methacrylate (TS = 23°C) and Rhoplex B-85 - A poly(methyl methacrylate) (Ts = 82°C) sold by Rohm & Haas Company.
3. Versa-TL 125 sold by National Starch & Chemical Corporation
4. Surfynol 104 surfactant sold by Air Products & Chemicals, Inc.

Polymer Matrix B
Rhoplex AC-73 (46.5%)	8.47 parts
Rhoplex B-85 (38.0%)	3.50 parts
Rhoplex HA-16 (45.5%)	30.15 parts
Shamrock S-395²	0.06 parts
Ammonium Hydroxide	0.37 parts
Syntran AX 9-253 (40.0%)³	12.99 parts
Syloid 169⁴	0.06 parts
Surfynol 104⁵	0.07 parts
Cellosolve Solvent	1.07 parts
Water	43.25 parts

1. Rhoplex HA-16 - copolymer of butyl methacrylate and isobutyl methacrylate (Ts = 18°C) sold by Rohm & Haas Company.
2. Shamrock S-395 sold by Shamrock Chemicals Corporation.
3. Syntran AX 9-253 - a copolymer of ethylene-styrene-butyl acrylate-acrylic acid (Ts = 32°C for soft element and 92°C for hard element) sold by Interpolymer Corporation
4. Syloid 169 sold by W.R. Grace & Company
5. Surfynol 104 sold by Air Products & Chemicals, Inc.

Polymer Matrix C
Rhoplex B-88 (42.5%)¹	27.27 parts
Syntran AX 9-253 (40.0%)	45.45 parts
Water	22.73 parts
Cellosolve Solvent	0.045 parts

1. Rhoplex B-88 - a modified poly(methyl methacrylate) (Ts = 62°C) sold by Rohm & Haas Company

Polymer Matrix D
Syntran AX 9-253 (40%)	60 parts
Water	40 parts

For comparison purposes the following two Comparative receptor sheet Examples were prepared.

Comparative Example 1

Airflex 410¹ (55%) 50 parts

Water 50 parts

1. Airflex 410 - a copolymer of vinyl acetate and ethylene (Ts = < 0°C) sold by Air Products and Chemicals, Inc.

Comparative Example 2
CMC 12M8¹	1 part
Water	99 parts

1. CMC 12M8 - sodium carboxymethylcellulose (Ts = > 80°C) sold by Hercules.

The receptor sheet of Comparative Example 1 is too soft and tacky to be useful. It also exhibits too much transfer from the donor sheet. On the other hand, the receptor sheet of Comparative Example 2 is too hard and exhibits poor image transfer.
The invention being thus described, it will be obvious that the same may be varied in many ways.

Claims

An assemblage for use in a thermal transfer recording process and consisting of a receptor sheet and a donor member comprising an ink donor layer, said receptor sheet comprising a polymeric matrix comprised of at least one hard element and at least one soft element, said hard element being a polymeric material with a softening temperature of at least about 5°C higher than that of said ink donor layer and said soft element being a polymeric material with a softening temperature of at least about 5°C lower than that of said ink donor layer.
The assemblage according to claim 1, wherein said polymeric matrix is coated on a base substrate.
The assemblage according to claim 1, wherein said soft and hard elements are homopolymers.
The assemblage according to claim 1, wherein said soft and hard elements are copolymers.
The assemblage according to claim 1, wherein said soft and hard elements are the combinations of homopolymers and copolymers.
The assemblage according to claim 1, wherein the polymeric matrix further contains at least one member selected from the group consisting of a pigment, a surface active agent and a conductive agent.
The assemblage according to claim 1, wherein said soft element is at least one polymer selected from methyl acrylate, ethyl acrylate, butyl acrylate, ethylene, butadiene, isobutene, cellulose acetate, vinyl acetate, cellulose ethers, petroleum resin, styrene-butadiene and siloxane.
The assemblage according to claim 1, wherein said hard element is at least one polymer selected from mehylmethacrylate, ethyl methacrylate, vinyl alcohol, vinyl chloride, melamine formaldehyde, acrylic acid, unsaturated esters, styrene, phenol formaldehyde and urethane.
The assemblage according to claim 6, which further contains a hydrocarbon surface active agent and a fluorochemical surface active agent.
The assemblage according to claim 6, wherein said conductive agent is selected from the group consisting of sulfonated polystyrene resin, copolymer of dimethyl diallyl ammonium chloride and diacetone acrylamide, poly(dimethyl diallyl ammonium chloride), quaternary acrylics, copolymer of dimethyl ammonium chloride and N-methyl acrylamide and quaternary cellulose acetate.
The assemblage according to claim 6, wherein said pigment is a polyolefin pigment.
The assemblage according to claim 11, wherein said pigment is polyethylene, polypropylene or polytetrafluororoethylene.
The assemblage according to claim 6, wherein the pigment is silica.
The assemblage according to claim 6, wherein said conductive agents in the polymeric matrix give a surface resistivity of from about 1 x 10⁷ to 1 x 10¹⁴ ohms/sq. at 50% relative humidity and 20°C.
The assemblage according to claim 6, wherein said pigment incorporated in the polymer matrix give a Sheffield surface smoothness of from about 5 to about 100.
The asemblage according to claim 1, wherein said hard and soft elements have softening temperatures of from 20 to 50°C higher or lower, respectively, than that of the donor layer.
An assemblage for use in a thermal transfer recording process and consisting of a receptor sheet and a donor member comprising an ink donor layer, said receptor sheet comprising:
a base substrate; and a polymeric matrix coated on said base substrate, said polymeric matrix being comprised of a pigment, a surface active agent, a conductive agent and at least one hard element and at least one soft element, said hard element being a polymeric material with a softening temperature of from about 10 to about 80°C higher than that of said donor layer, and said soft element being a polymeric material with a softening temperature of about 10 to about 80°C lower than that of said donor layer.
The assemblage according to claim 17, wherein said soft element is at least one polymer selected from methyl acrylate, ethyl acrylate, butyl acrylate, ethylene, butadiene, isobutene, cellulose acetate, vinyl acetate, cellulose ethers, petroleum resin, styrene-butadiene and siloxane.
The assemblage according to claim 17, wherein said hard element is at least one polymer selected from methylmethacrylate, ethyl methacrylate, vinyl alcohol, vinyl chloride, melamine formaldehyde, acrylic acid, unsaturated esters, styrene, phenol formaldehyde and urethane.
The assemblage according to claim 18, wherein said hard element is at least one polymer selected from methylmethacrylate, ethyl methacrylate, vinyl alcohol, vinyl chloride, melamine formaldehyde, acrylic acid, unsaturated esters, styrene, phenol formaldehyde and urethane.
A method of forming an image on a receptor sheet which comprises the steps of:
a) providing a receptor sheet having an image receptive layer: and

b) transferring image forming material from the ink donor layer of a donor sheet in an imagewise manner to said image receptive layer of said receptor sheet,
wherein said image receptive layer comprises a polymeric matrix comprised of at least one hard element and at least one soft element, said hard element being a polymeric material with a softening temperature of at least about 5°C higher than that of said ink donor layer and said soft element being a polymeric material with a softening temperature lower of at least about 5°C lower than that of said ink donor layer.
The method of claim 21, wherein said transfer of image-forming material is effected by heat and pressure.
The method of claim 22, wherein said hard and soft elements have softening temperatures of from 20 to 50°C higher or lower, respectively, than that of the donor layer.
The method of claim 22, wherein said soft element is at least one polymer selected from methyl acrylate, ethyl acrylate, butyl acrylate, ethylene, butadiene, isobutene, cellulose acetate, vinyl acetate, cellulose ethers, petroleum resin, styrene-butadiene and siloxane.
The method of claim 22, wherein said hard element is at least one polymer selected from vinyl chloride, melamine formaldehyde, acrylic acid, unsaturated esters, styrene, phenol formaldehyde and urethane.