Thermal transfer medium

Abstract: There is provided by the present invention a thermal transfer medium (20) which employs reactive components that crosslink when heated during image transfer to provide images with high scratch and smear resistance. The reactive components comprise an encapsulated liquid epoxy resin and a crosslinker which remain separate while incorporated within a thermal transfer layer (24) until exposed to a thermal print head (30).

Description: The present invention relates to thermal transfer printing wherein images are formed on a receiving substrate by heating extremely precise areas of a print ribbon with thin film resistors. This heating of the localized area causes transfer of ink or other sensible material from the ribbon to the receiving substrate. The sensible material is typically a pigment or dye which can be detected optically or magnetically. Thermal transfer printing has displaced impact printing in many applications due to advantages such as the relatively low noise levels which are attained during the printing operation. Thermal transfer printing is widely used in special applications such as in the printing of machine readable bar codes and magnetic alpha-numeric characters. The thermal transfer process provides great flexibility in generating images and allows for broad variations in style, size and colour of the printed image. There are some limitations on the applications for thermal transfer printing. For example, the properties of the thermal transfer formulation which permit transfer from a carrier to a receiving substrate can place limitations on the permanency of the printed matter. Printed matter from conventional processes can smear or smudge, especially when subjected to a subsequent sorting operation. Additionally, where the surface of a receiving substrate is subject to scratching, the problem is compounded. This smearing can make character recognition such as optical character recognition or magnetic ink character recognition difficult and sometimes impossible. In extreme cases, smearing can make it difficult to read bar codes. JP-A-60212389 discloses a thermal transfer medium comprising a thermally, transversable ink layer, said layer comprising a heat-fusible binder, a hard crosslinkable resin compound and a coloring agent. It is an object of the present invention to provide a thermal transfer medium which forms scratch and smear resistant images. According to the present invention there is provided a thermal transfer medium comprising a flexible substrate and a thermal transfer layer, characterized in that said thermal transfer layer has a softening point below 200°C and comprises a thermoplastic resin binder which is solid at ambient temperature and has a softening point below 200°C, an epoxy resin which is liquid at ambient temperature and reactive at ambient temperature, a crosslinker which crosslinks the epoxy resin at ambient temperature once activated and has a softening point below 200°C, and a sensible material, wherein the epoxy resin is encapsulated within a microcapsule which ruptures and releases the epoxy resin at a temperature from 50°C to 200°C under the pressure of a thermal print head. It should be understood that the images obtained from thermal transfer layers in accordance with the present invention contain high molecular weight crosslinked epoxy resin. As a result these images show high smear and scratch resistance. Also, it is not essential to carry out a baking step after transfer in order to achieve smear and scratch resistance. One embodiment of the present invention will now be described by way of example with reference to the accompanying drawings in which:- Fig. 1 illustrates a thermal transfer medium of the present invention; Fig. 2 illustrates a thermal transfer medium of the present invention after thermal transfer to a substrate; and Fig. 3 illustrates a thermal transfer medium of the present invention in a printing operation wherein thermal transfer is taking place. Thermal transfer medium 20, as illustrated in Fig. 1, is a preferred embodiment of this invention and comprises substrate 22 of a flexible material which is preferably a thin smooth paper or plastic-like material and a thermal transfer layer 24. Tissue type paper materials such as 30-40 gauge capacitor tissue, manufactured by Glatz and polyester-type plastic materials such as 14-35 gauge polyester film manufactured by Dupont under the trademark Mylar® are suitable. Polyethylene naphthalate films, polyethylene terephthalate films, polyamide films such as nylon, polyolefin films such as polypropylene film, cellulose films such as triacetate film and polycarbonate films are also suitable. The substrates should have high tensile strength to provide ease in handling and coating and preferably provide these properties at minimum thickness and low heat resistance to prolong the life of heating elements within thermal print heads. The thickness is preferably 3 to 50 µm. If desired, the substrate or base film may be provided with a backcoating on the surface opposite the thermal transfer layer. Thermal transfer layer 24 has a softening point below 200°C, preferably below 150°C and most preferably from 50°C to 80°C. Softening temperatures within this range enable the thermal transfer medium to be used in conventional thermal transfer printers, which typically have print heads which operate at temperatures in the range of 100°C to 250°C, more typically, temperatures in the range of 100°C to 150°C. The term "softening point" as used herein, refers to the temperature at which a solid material becomes malleable and flowable. The thermal transfer layer comprises a thermoplastic resin matrix which is solid at ambient temperature, an encapsulated epoxy resin which is liquid at ambient temperature, a crosslinker for the epoxy resin, and a sensible material. The epoxy resin and crosslinker are selected so as to be reactive at ambient temperature, which will facilitate rapid reaction when the components are melt mixed from a printing operation. The crosslinker is preferably solid at ambient temperature so that it may be easily isolated from the encapsulated epoxy resin within the thermal transfer layer. If a solid, the crosslinker must have a softening point below 200°C, preferably below 150°C and most preferably 50°C to 80°C. It is contemplated that liquid crosslinkers can be dispersed in the thermoplastic resin matrix with or without encapsulation. The thermoplastic resin which forms the matrix also has a softening temperature below 200°C, preferably below 150°C, and most preferably in the range 50°C to 80°C, consistent with the softening temperature requirements of the thermal transfer layer described above. Such softening temperatures allow the crosslinker to mix with the liquid epoxy resin when heated at temperatures in the range of 100°C to 250°C, such as by a conventional thermal print head, allowing the crosslinking reaction to proceed. Where the thermoplastic resin matrix and/or crosslinker have a softening point above 100°C, consideration must be given to employ a print head with an operating temperature sufficiently high for mixing of the epoxy resin and crosslinker to occur. The thermal transfer layer may be processed using a solvent, which can be aqueous or organic, with a boiling point below 200 C. The solvent need only solubilize the thermoplastic resin matrix during processing. Preferred solvents include ester solvents and mineral spirits. These solvents may suspend either the crosslinker or encapsulated epoxy resin to form a separate phase to ensure shelf stability. The solvent should not solubilize the encapsulating material which surrounds the liquid epoxy resin. Examples of suitable thermoplastic resins are polyvinyl chloride, polyvinyl acetate, vinyl chloride-vinyl acetate copolymers, polyethylene, polypropylene, polyacetal, ethylenevinyl acetate copolymers, ethylene alkyl (meth)acrylate copolymers, ethylene-ethyl acetate copolymer, polystyrene, styrene copolymers, polyamide, ethylcellulose, epoxy resin, xylene resin, ketone resin, petroleum resin, rosin or its derivatives, terpene resin, polyurethane resin, polyvinyl butyryl, synthetic rubber such as styrene-butadiene rubber, nitrile rubber, acrylic rubber and ethylene-propylene rubber. Also suitable are polyvinyl alcohol, ethylene alkyl (meth)acrylate copolymers, styrene-alkyl (meth) acrylate copolymer, saturated polyesters and the like. It is recognized that mixtures of the above-identified resins can be used. In the viewpoint of transfer sensitivity, it is desirable for the thermoplastic resins to have a low softening temperature. From the viewpoint of image integrity, it is desirable for these resins to have a high softening temperature. The thermoplastic resin is preferably used in an amount of about 5 to 15 percent by weight, particularly 10 percent by weight based on the weight of total dry ingredients of the coating formulation which forms the thermal transfer layer. The preferred epoxy resins suitable for use in this invention have at least two oxirane groups, so as to provide significant increases in molecular weight. It is also preferable for the epoxy resins to have hydroxy side groups where the crosslinker used will react with these groups. At least a portion of the epoxy resins used have two or more oxirane groups. The preferred resins include the low molecular weight epoxy novolak resins obtained by reacting epichlorohydrin with liquid phenol/formaldehyde resin or liquid cresol/formaldehyde resin. These resins are generally B-stage resins in a partial state of cure which have multiple epoxide groups. Preferred epoxy resins also include low molecular weight polyglycidyl ether polymers obtained by reaction of epichlorohydrin with a liquid polyhydroxy monomer. These polymers are generally linear and have terminal epoxide groups. Low molecular weight polymers with aliphatic backbones are typically suitable if liquid at ambient temperature. These include those polyglycidyl ethers obtained by reaction of epichlorohydrin with 1,4-butanediol or trimethylol propane. The preferred epoxy resins discussed above are suitably reactive at ambient temperature. The epoxy resins most preferred are typically highly reactive so that the printed image will be scratch and smear resistant immediately. The liquid epoxy resins are encapsulated within microcapsules which are stable within the thermal transfer layer at ambient temperature and rupture and release the epoxy resin upon the application of heat and pressure from a thermal print head, preferably at temperatures in the range of 50°C to 200°C. The microcapsules are preferably of a size below 75 µm. The composition of the microcapsule shell, i.e., the encapsulating material, can vary widely from natural to synthetic materials which have a softening point below 200°C. To ensure compatibility with the matrix resin, the same resin can be used for the shell. Preferably, the encapsulating material is a thermoplastic resin with a softening point below 150°C, most preferably 50°C to 80°C. An example of a suitable encapsulated epoxy resin is available from ND Industries, Inc. under the tradename ND Microspheres®. Crosslinkers or hardeners suitable for use in this invention are those conventionally used to cure epoxy resins which satisfy the melting/softening point requirements discussed above. Liquid crosslinkers can be used and may be encapsulated with a material that will rupture consistent with the microcapsules that contain the epoxy resin. Preferred crosslinkers remain active at ambient temperature once the reaction is initiated. Suitable crosslinkers will react with the epoxide groups, hydroxyl groups or both. To improve shelf stability of the thermal transfer medium, it is preferable for solid crosslinkers to have an activation temperature in the range of 60°C to 100°C. Crosslinkers with activation temperatures above 100°C can be used, provided the activation temperature is below the operating temperature of the print head to be used. Examples of suitable crosslinkers are polyamines which include prepolymers or oligomers of an amine (diamine), with or without another monomer, having at least two primary or secondary groups. These prepolymers/oligomers are often referred to as modified-amines. If solid, they must meet the melting point/softening point requirements. Examples of suitable modified amines are sold under the tradename Epi-cure P101 and Ancamine 2014FG sold by Shell Chemical Co. and Air Products, respectively. Other suitable crosslinkers include carboxylic acid functional polyester resins, phenol-formaldehyde resins and amino-formaldehyde resins. Included with the phenol-formaldehyde resins are resols and phenol-novolak resins. Another component of the thermal transfer layer is a sensible material which is capable of being sensed visually, by optical means, by magnetic means, by electroconductive means or by photoelectric means. The sensible material is typically a colouring agent such as a dye or pigment or magnetic particles. Any colouring agent used in conventional ink ribbons is suitable, including carbon black and a variety of organic and inorganic colouring pigments and dyes, examples of which include phthalocyanine dyes, fluorescent naphthalimide dyes and others such as cadmium, primrose, chrome yellow, ultra marine blue, titanium dioxide, zinc oxide, iron oxide, cobalt oxide, nickel oxide, etc. In the case of the magnetic thermal printing, the thermal transfer coating includes a magnetic pigment or particles for use in imaging or in coating operations to enable optical, human or machine reading of the characters. The magnetic thermal transfer ribbon provides the advantages of thermal printing while encoding or imaging the substrate with a magnetic signal inducible ink. The sensible material is typically used in an amount from about 5 to 50 parts by weight of the total dry ingredients for the coating formulation which provides the thermal transfer layer. To enhance the activity of the crosslinker, an accelerator may be incorporated in the thermal transfer layer. Examples include tertiary amines and TGIC (triglycidylisocyanurate). The accelerators may be liquid or solid at ambient temperature with a softening point below 200°C. The accelerator preferably functions at a temperature in the range of 20°C to 250°C to accelerate the crosslinking reaction. The epoxy resin preferably comprises from 30 to 65 percent by weight based on the total weight of the thermal transfer layer, excluding solvent. The crosslinker preferably comprises 5 to 25 percent by weight of the thermal transfer layer, based on the total weight, excluding solvent. The thermal transfer layer does not require the use of conventional waxes and plasticizers typically used in thermal transfer media, but their use is not excluded from the thermal transfer media of this invention. The thermal transfer layer may contain conventional additives typically used in conventional thermal transfer media to aid in processing and performance of the thermal transfer layer. These include flexibilizers such as oil, weatherability improvers such as UV light absorbers, scratch and abrasion improvers such as polytetrafluoroethylene and micronized polyethylene and fillers. Amounts of up to 45 percent by weight total additives, based on the total weight of the thermal transfer layer, excluding solvent, are suitable. The thermal transfer layer can be obtained by preparing a coating formulation and applying it to a substrate by conventional coating techniques such as a Meyer Rod or like wire-round doctor bar set up on a typical solvent coating machine to provide the desired coating thickness which equates to a coating weight preferably between 1.9 to 4.3 g/m 2 . A temperature of approximately 38°C to 66°C, preferably below 49°C, is maintained during the entire coating process. After the coating formulation is applied to the substrate, preferably 3 to 50 µm thick, the substrate is passed through a dryer at an elevated temperature to ensure drying and adherence of the coating 24 onto the substrate 22 in making the transfer ribbon 20, but without rupturing the microcapsules. The thermal transfer layer can be fully transferred onto a receiving substrate such as paper or synthetic resin at a temperature in the range of 75°C to 200°C. The reaction between the liquid epoxy resin and crosslinker is instantaneous and preferably proceeds at ambient temperature until complete. The coating formulation can be based on aqueous or organic solvents such as ester solvents and mineral spirits with a boiling point below 200°C, preferably in the range of 150°C to 190°C and preferably contains solids in an amount in the range of about 10 to 50 percent by weight. Most preferably, the coating formulation contains about 30 percent solids. To prepare a suitable coating formulation which forms the thermal transfer layer, the thermoplastic binder is typically dissolved in a solvent. Once dissolved, the polymer solution is agitated and the remaining reactive components (the encapsulated epoxy resin and crosslinker) are dispersed therein. The mixture is transferred to an attritor and the sensible material is added thereto and agitated for about 2 hours at a temperature less than the activation temperature for the crosslinker and less than the temperature at which the microcapsules rupture. Where the ND Microspheres® and Epicure® P101 crosslinker are based, the temperature is maintained below 49°C. The thermal transfer ribbon provides the advantages of the thermal printing. When the thermal transfer layer is exposed to the heating elements (thin film resistors) of the thermal print head, the thermoplastic resin binder and crosslinker melt mix and the microcapsules containing epoxy resin rupture. Reaction commences rapidly and the thermal transfer layer is transferred from the ribbon to the receiving substrate to produce a precisely defined image on the document. Fig. 2 illustrates image 32 on receiving substrate 28 following transfer from thermal transfer layer 24 of thermal transfer medium 20. Once initiated, the reaction can proceed at room temperature. Fig. 3 shows use of thermal transfer medium 20 in a printing operation. More particularly, Fig. 3 shows the heating of thermal transfer medium 20 by print head 30 where rupture of the microspheres and mixing of the crosslinker and epoxy resin takes place during transfer of thermal transfer layer 24 onto receiving substrate 28. The heat from the print head 30 softens a portion of the thermal transfer layer and ruptures the capsules resulting in mixed portion 40. Reaction of the epoxy resin and crosslinker in mixed portion 40 results in image 32. The images obtained from the thermal transfer layers of the present invention show high smear and scratch resistance. Also, these images can be obtained using conventional thermal transfer printers; i.e. excessively high print head energies are not required. Example 1 A coating formulation with the components within Table 1 was prepared by dissolving the EVA binder in solvent and adding the epoxy and modified polyamine while under agitation. The mixture was transferred to an attritor with a cooling jacket. The attritor was started and carbon black added, ensuring that the temperature of contents of the vessel did not exceed 49°C. The mixture was ground for two hours at 200-250 rpm. Use %Dry Dry(grams) Wet(grams) Mineral spirits Solvent NA NA 450.0 Ethylene vinyl acetate (EVA) 1 Binder 10.0 15.0 15.0 Encapsulated Epoxy Resin 2 Epoxy 65.0 97.5 97.5 Modified polyamine 3 Hardener 10.0 15.0 15.0 Carbon black 4 Pigment 15.0 22.5 22.5 The coating formulation is applied to polyester terephthalate (PET) film with coat weights in the range of 1.9 to 4.0 g/m 2 with conventional equipment. MATERIALS Chemical Name Trade Name Manufacturer City State 1 Ethylene vinyl acetate (EVA) Escorene MV02514 Exxon Chemical Co. Houston TX 2 Encapsulated epoxy ND Microspheres ND Industries, Inc. Troy MI 3 Modified polyamine Epicure P101 Shell Chemical Co. Houston TX 4 Carbon black Raven 1255 Columbian Chemicals Co. Atlanta GA

Claims: A thermal transfer medium (20) comprising a flexible substrate (22) and a thermal transfer layer (24), characterized in that said thermal transfer layer has a softening point below 200°C and comprises a thermoplastic resin binder which is solid at ambient temperature and has a softening point below 200°C, an epoxy resin which is liquid at ambient temperature and reactive at ambient temperature, a crosslinker which crosslinks the epoxy resin at ambient temperature once activated and has a softening point below 200°C, and a sensible material, wherein the epoxy resin is encapsulated within a microcapsule which ruptures and releases the epoxy resin at a temperature from 50°C to 200°C under the pressure of a thermal print head (30). A thermal transfer medium according to claim 1, characterized in that said thermal transfer medium (20) is processed with a solvent with a boiling point below 200 C which solubilizes the thermoplastic binder without solubilizing the microcapsules. A thermal transfer medium according to claim 1 or claim 2, characterized in that said thermal transfer layer (24) and crosslinker have a softening point in the range of 50°C to 80°C. A thermal transfer medium according to any one of the preceding claims, characterized in that said crosslinker is an amine hardener, and said thermal transfer layer (24) contains from 30 to 65 weight % microcapsules with encapsulated epoxy resin and 5 to 25 percent by weight amine hardener, based on the total weight of the thermal transfer layer. A thermal transfer medium according to any one of the preceding claims, characterized in that said substrate (22) is a polyethylene terephthalate film and said thermal transfer layer (24) has a coating weight of 1.9 to 4.3 g/m 2 . A thermal transfer medium according to any one of the preceding claims, characterized in that the crosslinker is activated to initiate polymerization with the liquid epoxy resin at temperatures in the range of 60°C to 100°C. A thermal transfer medium according to any one of the preceding claims, characterized in that said thermal transfer layer (24) comprises encapsulated liquid epoxy resin within microcapsules of a size less than 75 µm. A thermal transfer medium according to any one of the preceding claims, characterized in that said thermal transfer layer (24) comprises more than one crosslinker which is active at ambient temperature. A thermal transfer medium according to any one of the preceding claims, characterized in that the liquid epoxy resin is a low molecular weight polyglycidyl ether obtained by reaction of epichlorohydrin with a liquid polyhydroxy monomer. A thermal transfer medium according to any one of the preceding claims, characterized by an accelerator for the crosslinking reaction between the epoxy resin and the crosslinker.

Inventors: LORENZ MICHAEL A, ROSENBAUM JOHN C, ROTH JOSEPH D, LORENZ, MICHAEL A., ROSENBAUM, JOHN C., ROTH, JOSEPH D.
Applicants: NCR INT INC, NCR INTERNATIONAL INC.
Application year: 1997
Main classification: B41M5
IPC codes: B41J2, B41M5