EP1892115A1

EP1892115A1 - Method of forming a phase change ink image on a self-laminating recording medium

Info

Publication number: EP1892115A1
Application number: EP07113964A
Authority: EP
Inventors: Guido G. Willems; Johannes P.J.C. Rooijakkers; Rudolf A.H.M. Sturme
Original assignee: Oce Technologies BV
Current assignee: Canon Production Printing Netherlands BV
Priority date: 2006-08-22
Filing date: 2007-08-07
Publication date: 2008-02-27
Anticipated expiration: 2027-08-07
Also published as: EP1892115B1

Abstract

The present invention pertains to a method of creating an image on a recording medium (2) comprising a support (15, 16) having thereon a porous fusible layer (17), using an ink composition (20) that is solid at room temperature and liquid at elevated temperature comprising the steps of generating droplets of the ink composition with an ink jet print head (4), transferring the droplets of the ink composition to the surface of the fusible layer (17), thermally treating the recording medium such that the ink transferred to fusible layer passes into the medium away from the surface of the fusible layer whilst the fusible layer remains substantially unfused, and when the ink has passed into the medium, treating the recording medium to fuse the fusible layer to become a protective overcoat (17'). The invention also pertains to a system for creating an image on a recording medium.

Description

The invention pertains to a method of creating an image on a recording medium with an ink composition that is solid at room temperature and liquid at elevated temperature. The invention also pertains to a system for creating an image on a recording medium with said ink composition.
A method of creating an image on a recording medium with an ink composition that is solid at room temperature and liquid at elevated temperature (also called "hot melt" or "phase change ink") is known from US 6,497,940 . This patent describes a recording medium for the ink composition which has improved clarity, improved resistance to surface scratching and improved ink adhesion. The recording medium comprises a polyethylene terephthalate support coated with a lower receptor layer comprising 82-97 wt.% of silica and 3-18 wt.% of PVA, polyvinylpyrrolidone, polyacrylamide, methylcellulose or gelatin. An optional upper layer comprises 32-70 wt.% of a matrix polymer, 15-52 wt.% of inorganic particulate material and 5-53 wt.% of soft polymer mixture. The term 'soft polymer mixture' describes a polymer or mixture of polymers that soften during the image transfer step of the printing. The softening allows the ink composition and the upper layer to become chemically or physically mated for mechanical durability reasons. The soft polymer matrix must be sufficiently soft to allow the ink composition and the coating to become intimately interrelated and yet rigid enough to avoid scratching and sticking with adjoining films.
Although suitable to obtain adequate mechanical durability, a disadvantage of this known method is that the print quality deteriorates when the printed medium is subjected to thermal load, for example by storing the printed medium at 35°C for several weeks or months. It is for example noticed that the gloss level decreases substantially, and that local artefacts arise in the printed images.
The object of the present invention is to provide an improved method of obtaining images of a hot melt ink that overcomes or at least mitigates the disadvantages of the known method. To this end an ink jet printing method has been devised, using a recording medium comprising a support having thereon a porous fusible layer, comprising the steps of generating droplets of the ink composition with an ink jet print head, transferring the droplets of the ink composition to the surface of the fusible layer, thermally treating the recording medium such that the ink transferred to fusible layer passes into the medium away from the surface of the fusible layer whilst the fusible layer remains substantially unfused, and when the ink has passed into the medium, treating the recording medium to fuse the fusible layer to become a protective overcoat.
With this method, in contrast with the prior art method, the ink, due to the thermal treatment, passes into the medium such that an upper layer arises that is substantially free of ink, i.e. substantially free of at least the meltable vehicle components of the ink (which typically constitute 90-99% of the hot melt ink). Then, this upper layer is fused (optionally together with other parts of the medium), which can be done by any known physical or chemical method that allows the porous layer to become a fused layer, to become a protective overcoat. This way a continuous film (i.e. a film wherein the pores or interstices have disappeared for the major part) is created that substantially shuts off interaction between the environment and the ink trapped in the medium. It appears that, next to a very good mechanical durability, a very durable high gloss can be achieved with the method according to the present invention, even under circumstances of relatively high thermal load.
Inventors have found that a porous fusible layer, i.e. a layer having pores or interstices permitting the passage of fluid ink, in itself is not sufficient to arrive at the result that is obtainable with the present invention. It is essential that the recording medium is thermally treated such that the ink passes into the medium away from the surface of the fusible layer. Otherwise, ink can remain on the surface and thus an ink free upper layer cannot be readily provided. However, the thermal treatment should be such that the fusible layer remains substantially unfused while the ink passes into the medium. This is to prevent the too early amalgamation of the ink and the fusible layer as is known from the prior art. The thermal treatment in itself is not restricted to any type of treatment but can be for example radiation treatment, contact treatment, conductive treatment etc. This treatment can take place before the ink is transferred to the surface of the recording medium (pre-heating of the medium), at the same time, afterwards, or even a combination of these methods can be used, as long as it leads to a temperature rise of (parts of) the medium that allow passing of the hot melt ink into the medium whilst preventing substantial fusing of the fusible layer.
In the method according to the present invention, the treatment to fuse the porous layer should not be performed until the ink has passed into the medium to create an ink free upper layer. It is noted however that the treatment to fuse the porous layer and the thermal treatment to allow the ink to pass into the medium could for example be accomplished by a one-step thermal treatment, for example by passing the printed recording medium through one single pair of heated rollers. By applying such heated rollers namely, the viscosity of the ink droplets that reside on the surface of the porous layer can be forced to decrease to a value of typically about 10 mPa.s when heated above the melting point of the ink (for example, heating to 120°C for an ink with a melting point of 90°C). Although such a high temperature might also be sufficient to fuse a fusible layer consisting out of a polyester polymer with a glass-transition temperature of for example 65°C, the time scale for the migration of the fluid ink into the medium is typically much (10 to 1000 times) shorter than the time scale for the fusing of the fusible (polymer) layer. Thus, what appears to be one single thermal treatment at first glance, can, under certain circumstances and with the right choice of materials in effect be regarded as a first treatment enabling ink to pass into the medium to provide an ink free upper layer, followed by a treatment to fuse the fusible layer, i.e. at least the ink free upper part of that layer, to form a closed, protective overcoat.
Ink compositions that are solid at room temperature and liquid at an elevated temperature are commonly known. Examples of such an ink compositions are given in EP 1 067 157 and publication number 497067 in the September 2005 Volume of Research Disclosure (published by Emsworth Design Inc., Park Avenue South, New York, USA). The exemplified inks are composed of mixture of a crystalline base material, an amorphous binder and a gelling agent.
In order to print the ink composition by way of an ink jet head, the ink composition needs to be in the fluid state. This requires the use of an ink jet head that can operate at temperatures above the melting temperature of the ink, typically at temperatures between 90 and 140°C. To this end print heads can be used that comprise heated pressure chambers filled with the fluid ink and connected to nozzles, wherein the pressure required to fire a droplet is generated by firing a transducer, such as for example a piezo-electric transducer.
The recording medium as used in the method according to the present invention comprises a support for the porous fusible layer. This support can be opaque, translucent or transparent. There may be used, for example, plain papers, resin coated paper, various plastics including a polyester resin such as poly(ethylene-terephtalate), poly(ethylene-naphtalate), a polycarbonate resin, a fluorine resin such as poly(tetrafluoro ethylene), metal foil, vinyl, fabric, laminated or coextruded supports. Inkjet papers that can be used as a support in the meaning of the present invention are also described in the proceedings of the TAPPI Coating Conference of 2002, that is, in the paper by Hyun-Kook Lee, Margaret Joyce, Paul Flemming and John Cameron of the Western Michigan University (see for example Table 1 of this paper).
The fusible layer may comprise e.g. fusible polymeric particles. The particles may have any size provided that when constituted as a layer, they provide for pores or interstices that allow the fluid ink to pass into the medium and create an ink free upper layer, and that they upon a fusing treatment will coalesce to form a continuous overcoat film. Typical particle size are in the range of 1-10 micrometers.
It is noted that US 6,497,480 describes an ink jet recording element comprising a support having, in sequence, a porous ink-retaining layer and a fusible porous ink-transporting layer comprising fusible polymeric particles and a film-forming hydrophobic binder. US 6,811 , 253 describes an ink jet printing method, comprising printing onto a receiving medium having an ink receiving layer and an upper protective layer and heating the printed image to form a stable image-protecting coating. The upper protective layer comprises particulate polymeric beads having a film-forming temperature of 100-120 °C with a hydrophilic binder. This invention conveniently protects ink jet images.
The technical disclosure of these patents however is restricted to the use of water or solvent based inks. These inks are completely different from hot melt inks, not only in their intrinsic properties, but also in their behaviour when printed on media. In particular, these inks do not suffer from the problem of gloss deterioration when printed on dedicated ink jet gloss media. Next to that, these inks are fluid at room temperature and thus readily migrate through the fusible ink transporting layer into the ink receiving layer. Indeed, both references remain silent about a thermal treatment enabling the ink to pass into the recording medium. Thus, it is clear that the method according to the present invention is not known from or obviated by the disclosure of these references.
In an embodiment of the present invention the recording medium that is used has a support comprising a base layer and thereon an ink receiving layer. The base layer may be any suitable layer for providing adequate mechanical strength or other property if needed (depending on the application). The ink receiving layer is designed to absorb the ink that passes into the medium. Such an absorptive layer can be constituted in many forms, as long as the interaction with the ink is such that this ink can be absorbed, either partly or completely, in this layer. In a further embodiment the ink receiving layer is a micro-porous layer. Such a layer comprises pores or interstices with a cross-section around the (sub-)micrometer range. Such a layer can contain organic or inorganic particles and typically has a thickness of about 1 µm to about 50 µm. Examples of organic particles that may be used include acrylic resins, styrenic resins, cellulose derivatives, polyvinyl resins, ethylene-allyl copolymers and polycondensation polymers. Examples of inorganic particles that can be used in the ink receiving layer include silica, alumina, titianium dioxide, clay, calcium carbonate, barium sulfate or zinc oxide. The micro-porous ink receiving layer may comprise from about 20% to about 100% of particles and from about 0% to about 80% of polymeric binder, preferably from about 80% to about 95% of polymeric particles and from about 20% to 5% of polymeric binder. The polymeric binder may be a hydrophilic or hydrophobic polymer (depending i.a. on the type of ink for which the receiving medium is designed), such as poly(vinyl alcohol), poly(vinylpyrrolidone), gelatin, cellulose ethers, poly(oxazolines), poly(vinylacetamides), partially hydrolyzed poly(vinyl acetate/vinyl alcohol), poly(acrylic acid), poly(acrylamide), poly(alkylene oxide), sufonated or phosphared polyesters and polystyrenes, casein, zein, albumin, chitin, chitosan, dextran, pectin, collagen derivatives, collodian, agar-agar, arrowroot, guar, carrageenan, tragacanth, xanthan, rhamsan and the like.
In an embodiment the porous fusible layer comprises a thermoplastic material. This has the advantage that a thermal treatment can be used to fuse the layer to from the continuous protective overcoat. The material constituting the fusible layer can in principle be any thermoplastic material composed from a thermoplastic compound only, or from a combination of a thermoplastic compound with for example a film forming polymer or other additives (in any ratio), depending on the specific application of the recorded medium. For example, if the recorded medium will be subjected to temperatures above 80°C, it is recommended that a thermoplastic material is being used having a glass transition temperature above 80°C, in particular above 90°C.
Examples of suitable thermoplastic compounds include polyesters, polyesteramides, polyethylenes and polyurethanes.
In another embodiment the fusible layer is provided on the support via a screen printing technique. Surprisingly it has been seen that by using a screen printing technique, which in itself is commonly known, in a very simple and convenient way a porosity can be provided that is adequate to allow fluid hot melt ink to pass into the recording medium, while still being "dense" enough to allow adequate fusing under moderate loads, such as for example a moderate temperature and/or pressure rise. Other techniques, such as rod coating of resins, or resin dispersions in aqueous or non-aqueous solvents, optionally in addition with a mechanical method to force pores or interstices in the fusible layer, can provide for the same result but are less practical in use.
In another embodiment the ink composition comprises a meltable ink vehicle consisting at least of a material that can be obtained in a crystalline phase at 25 °C. Preferably, the crystalline material of the ink vehicle and the material constituting the fusible layer are substantially incompatible at 25°C, i.e. both materials do not spontaneously mix at molecular level at this temperature. This way, the durability is further improved.
In another embodiment the fusible layer comprises a material having a glass transition temperature higher than 50°C. With this glass transition temperature it can be prevented that the recording media will stick together when a stack of media is exposed to a temperature of up to approximately 50°C. On the other hand, it is preferable that the glass transition temperature is lower than 70°C to enable fusing of the layer by applying a moderate temperature rise.
The invention also pertains to a system for creating an image on a recording medium comprising a support having thereon a porous fusible layer, the system comprising an ink jet print head adjusted to jet droplets of an ink composition that is solid at room temperature and liquid at an elevated temperature, and transfer these droplets to the surface of the fusible layer, the system optionally comprising an intermediate member for temporarily accepting the droplets of the ink composition before the said transfer to the fusible layer, and further comprising a thermal treating element for thermally treating the recording medium and a control arrangement to control the thermal treating element such that the ink transferred to fusible layer passes into the medium away from the surface of the fusible layer whilst the fusible layer remains substantially unfused, and a transporting means for transporting the recording medium from an ink transfer position to a fuse position adjacent a fuse means capable of treating the recording medium to fuse the fusible layer to become a protective overcoat. The control arrangement can be any piece of hardware that is designed to control the thermal treatment of the recording medium (for example in the form of an ASIC). However, the control arrangement does not essentially need to be one single piece of hardware, it may be distributed over the system. Moreover, this arrangement can in a smaller or larger part be provided in the form of software running on one or more processors (generic and/or programmable), which software is used to control the corresponding hardware. All kinds of control arrangements can be designed to be suitable, as long as they are capable of adequately controlling the thermal treatment of the recording medium in line with the present invention.
The optional intermediate member may be a belt or a drum, optionally having an elastomeric top layer such as for example an intermediate medium known from US 2003/0234841 A1 (see fig. 1, element 14) or US 6,905,203 (see fig. 1, element 1). When using such an intermediate medium, the entire image can be printed on the intermediate member prior to being transferred to the recording medium. Using an intermediate member has the advantage that the process is more reliable, especially with respect to blocking of the nozzles by paper dust, and less dependent on variations in the type of recording medium.
The invention will further be explained by means of the following non-limitative illustrative figures and examples, wherein:

Figure 1: is a schematic representation of an inkjet printer comprising multiple printheads
Figure 2: is a schematic representation of an embodiment of the method according to the present invention
Figure 3: is a schematic representation of an ink jet printer and fusing station.

Example 1 describes a process of making a recording medium for use in the method according to the invention.
Example 2 describes a result that can be achieved with the method according to the invention.

Figure 1

Figure 1 is a diagram showing an inkjet printer. According to this embodiment, the printer comprises a roller 1 used to support a recording medium 2, and to transport it along the carriage 3. This carriage 3 comprises a carrier 5 to which four print heads 4a, 4b, 4c and 4d have been fitted. Each print head 4a, 4b, 4c, 4d contains its own colour, in this case cyan (C), magenta (M), yellow (Y) and black (K) respectively. The print heads 4a, 4b, 4c, 4d are heated using heating elements 9, which have been fitted to the rear of each print head 4a, 4b, 4c, 4d and to the carrier 5. The temperature of the print heads 4a, 4b, 4c, 4d is maintained at the correct level by application of a central control unit 10 (controller). The roller 1 may rotate around its own axis as indicated by arrow A. In this manner, the receiving medium may be moved in the sub-scanning direction (often referred to as the X direction) relative to the carrier 5, and therefore also relative to the print heads 4a, 4b, 4c, 4d. The carriage 3 may be moved in reciprocation using suitable drive mechanisms (not shown) in a direction indicated by double arrow B, parallel to roller 1. To this end, the carrier 5 is moved across the guide rods 6 and 7. This direction is generally referred to as the main scanning direction or Y direction. In this manner, the receiving medium 2 may be fully scanned by the print heads 4a, 4b, 4c, 4d. According to the embodiment as shown in this figure, each print head 4a, 4b, 4c, 4d comprises a number of internal ink chambers (not shown), each with its own exit opening (nozzle) 8. The nozzles 8 in this embodiment form one row per print head perpendicular to the axis of roller 1 (i.e. the row extends in the sub-scanning direction). In a practical embodiment of an inkjet printer, the number of ink chambers per print head will be many times greater and the nozzles 8 will be arranged over two or more rows. Each ink chamber comprises a piezo-electric converter (not shown) that may generate a pressure wave in the ink chamber so that an ink drop is ejected from the nozzle of the associated chamber in the direction of the receiving medium 2. The converters may be actuated image-wise via an associated electrical drive circuit (not shown) by application of the central control unit 10. In this manner, an image made up of ink drops may be formed on receiving medium 2. If a receiving medium 2 is printed using such a printer where ink drops are ejected from ink chambers, this receiving medium 2, or some of it, is (imaginarily) divided into fixed locations that form a regular field of pixel rows and pixel columns. According to one embodiment, the pixel rows are perpendicular to the pixel columns. The individual locations thus produced may each be provided with one or more ink drops. The number of locations per unit of length in the directions parallel to the pixel rows and pixel columns is referred to as the resolution of the printed image, for example indicated as 400x600 d.p.i. ("dots per inch"). By actuating a row of print head nozzles 8 of the inkjet printer image-wise when it is moved relative to the receiving medium 2 as the carrier 5 moves, an image, or some of it, made up of ink drops is formed on the receiving medium 2, or at least in a strip as wide as the length of the nozzle row.
Roller 1 is internally provided with a radiation heater 12, which heater is configured to heat the receiving medium 2 by heating the roller circumference which gives off its heat to the medium that contacts the roller 1. This way, the recording medium can be treated to allow the hot melt ink to pass into the medium immediately after the corresponding ink droplets have hit the surface of the recording medium. The heater 12 is controlled by a piece of control software that is incorporated in controller 10. In this embodiment, the software makes use of a memory which comprises numerous combinations of inks that can be used in the printer and recording media that can be used according to the present invention. Each combination is linked to a dedicated setting of the temperature of the roller, which setting is the parameter to be directly controlled in the embodied system. That is, the temperature of the roller surface is measured using a contact sensor (not shown) and compared to the pre-programmed temperature setting. Depending on the difference between the pre-programmed temperature and the measured temperature, the heater is controlled to make this difference as small as necessary.

Figure 2

Figure 2 is a schematical representation of the interaction of phase change ink with a recording medium according to an embodiment of the present invention. Figure 2A shows the recording medium comprising a base layer 15 and an ink receiving layer 16 (which together form a support in the sense of the present invention) and a fusible layer 17. The base layer in this case is a plain paper substrate. The ink receiving layer is a commonly used formulation of 90 % by weight of silica nanoparticles in the range of 300 to 30 nanometers in 10 % by weight of polyvinyl alcohol. The high weight fraction of particulate material with respect to the binder fraction results in a highly porous layer that has good ink retaining properties.
The fusible layer is a film of thermoplastic polymer particles (polyesther resin; Mn = 900, Tg = 55°C). These particles have an average size between 2 to 5 µm and are rod-coated on the support starting with an aqueous dispersion of the particles (10 weight % of resin in water) to provide a film of about 20µm in thickness. On top of the fusible layer 17 lies a layer of solid phase change ink droplets (20) that has been jetted onto the surface of the recording medium.
Figure 2B represents the situation after the thermal treatment of the recording medium. The ink droplets that resided on top of the fusible layer have passed into the recording medium, in this case into the ink receiving layer 16, away from the surface of the fusible layer which remained substantially unfused. In this particular case, after the ink has passed into the medium, the complete fusible layer is substantially free of ink. It is noted however that the present invention as claimed in the appended claims also compasses that only an upper layer of the fusible layer has become substantially free of ink.
Figure 2C depicts the situation after a fusing treatment of the recording medium. The fusible layer has been fused to form a closed, protective overcoat 17'. In tsi case the complete layer 17 has been fused to become layer 17'. It should be understood however that the present invention as claimed in the appended claims also compasses that only the upper ink free layer of the fusible layer is fused to become a protective overcoat.

Figure 3

Figure 3 is a schematic representation of a system for creating an image according to the present invention. On the left, the inkjet printer as elaborately described with reference to figure 1 is depicted, by showing its most important components, i.e. the internally heated roller 1 for transporting the recording medium 2, and one of the printing heads 4. This system further comprises a fuse roller arrangement that consists of heated roller 36 and back roller 35 which are slightly pressed together (typical line pressure 250 Newton per meter) by adequate pressure providing means (not shown). Both rollers are provided with compliant surface layers, in this example soft silicone elastomer layers (20 ShA hardness). The transport roller 1 is capable of transporting the printed recording medium 2 from the ink transfer position 30 to fuse position 40. The rollers are heated to a temperature of about 140 °C which in this case (chosen resin for the fusible layer is a polyester resin with a Tg of 55°C) is adequate to heat at least an upper layer of the fusible layer such that it fuses into a continuous protective overcoat.

Example 1

Example 1 describes a process of making a recording medium for use in the method according to the invention. A resin is chosen, typically having a Tg (glass transition temperature) between 50 and 70°C. As an example, the polyester resin exemplified as resin number 3 in table 2 of international patent application PCT/EP2006/062614 is chosen. A 10 % resin dispersion (10 weight percent of resin) in water is created such that the resin particles have an average size below 1 micron (which can be easily verified with a light scattering particle size measurement apparatus). The viscosity of this dispersion is in almost the same as that of the water itself and the dispersion has a milk-like appearance.
A clean screen that is commonly used in a screen printer is placed on top of a suitable substrate, in this case Kodak Instant Dry Glossy photopaper, 190 g. It is ensured that there is good contact between the screen and the substrate by supporting the substrate and putting adequate pressure on the framework of the screen. The screen in this case is made out of mono-filaments of an inert fabric, which filaments are 40 micron wide leaving openings of 20 x 20 micrometers.
The resin dispersion is applied on the side of the screen facing away from the substrate, making sure that it is not able to wet the substrate yet. With a rubber squegee the dispersion is applied over the whole screen in one movement. It is important that the squegee does not run dry because this will cause imperfections in the coated layer. Any surplus dispersion is removed from the screen because this can interact with the substrate causing cockling or too thick layers of coating. After the coating step, the screen and substrate are left in place to allow drying. This can be done under ambient conditions. After drying the screen is removed. This method results in a recording medium having on the substrate as a support, a fusible layer consisting of dots of the polyester resin of about 40 x 40 micrometers wide and a height of about 15 to 20 micrometers in a regular pattern and a open space of a few micron in between the dots. The thickness of the dots can be varied, for example by changing the amount of resin in the dispersion.
Other coating techniques for obtaining a fusible layer can also be successfully applied. For example, it is recognised that rod-coating techniques or cast coating techniques, both when starting from aqueous or non-aqueous media as a carrier for the fusible material, can be applied to obtain a recording media for use in the present invention.
The invention can also be applied to parts of recording media. For example, if an overcoat is only desired for a certain smaller image of a complete image (for example, a company name above a letter, which name should appear in high gloss), a medium could be made having the fusible layer only at the location corresponding to the company name.

Example 2

In this example a result is described that can be achieved with the method according to the invention. With the screen print method described here-above ("example 1 ") four recording media are produced. For the fusible layer, four different kinds of resin are chosen, namely resin number 3 in table 2 of international patent application PCT/EP2006/062614 , resin number 1 in the same table of that patent application, a polyester-amide resin having a M_n (number averaged moleculair weight) of 900 and a Tg of 65°C, and a semi-crystalline polyolefine resin (available from Dow Chemical Corporation under the trade name "Engage").
These four materials are each printed with three different hot melt inks of a constitution as given in table 1. Each of these ink compositions contains a crystalline material (K), an amorphous material (A) and a gelling agent (G). The ink compositions are provided with a dye (KI), Macrolex Rot (Bayer) in the case of the ink compositions I and III, Orasol Blau (Ciba-Geigy) in the case of composition II. In addition, the three ink compositions are provided with wetting agent (V) BYK 307 (Byk Chemie). The chemical formulae of the crystalline base material (K), the amorphous binder (A) and the gelling agent (G) are given in tables 2, 3a and 1 respectively of EP 1 067 157 .

Table 1. Hot melt ink formulations

	Ink I		ink II		ink III
K	58.3	HMDI-MEG	67.6	CYCLO-2T	66.8	HMDI-MEG
A	38.1	DITRIM-50CHI	28.8	GLYPOCHI	28.6	PBPA-BuP
G
	3	Gel-23	2	Gel-1	4	Gel-23
KI	0.5	Macrolex Rot	1.5	Orasol Blau	0.5	Macrolex Rot
V	0.1	BYK 307	0.1	BYK 307	0.1	BYK 307

The inks are jetted at a temperature of 130°C to the recording media which are kept at a temperature of 35°C. Here-after, the recording media are transferred to an oven which is kept at 90°C. After 30 seconds the inks have passed into the media, and they are transferred to an oven kept at 110°C. Here they stay for 90 seconds which is sufficient to fuse an ink free upper layer of the recording media. Then they are brought to ambient conditions.
To test the durability under thermal load, the printed and now "sealed" recording media are brought over to an oven at a temperature of 45°C. The image quality is inspected every 24 hours. A first relevant feature of the image quality is the gloss level (which can be measured for example using a Hunter 75° glossmeter according to TAPPI procedure T 480 OM-92). Another feature is the appearance of white spots, possibly a crystallisation effect of the crystalline material in the ink vehicle. It appears that during the first few days a slight deterioration of the gloss level occurs (typically from 70 to 60%). However, this decrease in gloss can hardly be noticed with the naked human eye. After that, the gloss level remains substantially constant, up to even 800 hours of thermal load. No white spots can be noticed wit the naked human eye.

Claims

A method of creating an image on a recording medium (2) comprising a support (15, 16) having thereon a porous fusible layer (17), using an ink composition (20) that is solid at room temperature and liquid at elevated temperature comprising the steps of:
- generating droplets of the ink composition with an ink jet print head (4),

- transferring the droplets of the ink composition to the surface of the fusible layer (17),

- thermally treating the recording medium such that the ink transferred to fusible layer passes into the medium away from the surface of the fusible layer whilst the fusible layer remains substantially unfused,

- when the ink has passed into the medium, treating the recording medium to fuse the fusible layer to become a protective overcoat (17').
A method according to claim one characterised in that the recording medium that is used has a support comprising a base layer and thereon an ink receiving layer.
A method according to claim 2, wherein the ink receiving layer is a micro-porous layer.
A method according to any of the preceding claims characterised in that the fusible layer comprises a thermoplastic material.
A method according to any of the preceding claims characterised in that the fusible layer is provided on the support via a screen printing technique.
A method according to any of the preceding claims characterised in that ink composition that is used comprises a meltable ink vehicle consisting at least of a material that can be obtained in a crystalline phase at 25 °C.
A method according to claim 6 characterised in that the crystalline material of the ink vehicle and the material constituting the fusible layer are substantially incompatible at 25°C.
A method according to any of the preceding claims, characterised in that the fusible layer comprises a material having a glass transition temperature higher than 50 °C.
A system for creating an image on a recording medium (2) comprising a support (15, 16) having thereon a porous fusible layer (17), the system comprising an ink jet print head (4) adjusted to jet droplets of an ink composition (20) that is solid at room temperature and liquid at an elevated temperature, and transfer these droplets to the surface of the fusible layer, the system optionally comprising an intermediate member for temporarily accepting the droplets of the ink composition before the said transfer to the fusible layer, and further comprising a thermal treating element (12) for thermally treating the recording medium (2) and a control arrangement (10) to control the thermal treating element such that the ink transferred to fusible layer passes into the medium away from the surface of the fusible layer whilst the fusible layer remains substantially unfused, and a transporting means (1) for transporting the recording medium (2) from an ink transfer position (30) to a fuse position (40) adjacent a fuse means (35, 36) capable of treating the recording medium to fuse the fusible layer (17) to become a protective overcoat (17').