NL2015472B1

NL2015472B1 - Digital printing process, digital printing system and recording medium.

Info

Publication number: NL2015472B1
Application number: NL2015472A
Authority: NL
Inventors: Jeroom Maria Van Gaens Wouter; Erik Dries Deprez Lode; Jozef Johan Op De Beeck Werner
Original assignee: Xeikon Mfg Nv
Priority date: 2015-09-21
Filing date: 2015-09-21
Publication date: 2017-04-19

Abstract

The digital printing process comprises the steps of developing portions of a liquid toner comprising toner particles and a substantially non-polar carrier liquid, and transferring of the developed portions of the liquid toner to a recording medium. Thereafter, the developed portions of the liquid toner are fused to obtain fused portions and remaining carrier liquid. Herein, the recording medium comprises a base substrate layer and at least one carrier liquid absorbing coating layer made from a material which is different from the material of the base substrate layer, and wherein at least a part of the remaining carrier liquid is absorbed in the carrier liquid absorbing coating layer.

Description

Digital printing process, digital printing system and recording medium

FIELD OF THE INVENTION

The field of the invention relates to a digital printing process, a digital printing system and an electrophotography recording medium for printing with a liquid toner comprising toner particles and a substantially non-polar carrier liquid.

BACKGROUND OF THE INVENTION

Digital printing processes using liquid toner are known per se, for instance from EP 2713210. The liquid toner comprises toner particles and a substantially non-polar carrier liquid. The toner particles or marking particles comprise coloured particles (also called ink particles or pigment) and a binder resin, which is a polymer, that embeds the ink particles and other optional compounds like wax, plasticizers or other additives. The binder resin is sufficiently incompatible with the non-polar carrier liquid, so that it will not be plasticized by the carrier liquid, even above room temperature. The resin has preferably a high transparency, provides good colour developing properties and has a high fixing property on the substrate. Preferably the binder resin is a polyester resin, but also other polymers like polyols and functionalised styrene acrylates are suitable for the printing process. At a surface of the toner particles dispersing agent is present to stabilise the dispersion of the toner particles in the non-polar carrier liquid. The liquid toner is provided on a development member, as a liquid toner layer, and is charged. It is then transferred via an imaging member to define a desired imaging pattern, and optionally via one or more further members, to a recording medium. Fusing is carried out, resulting in release of the carrier liquid from the transferred liquid toner layer and conversion of the toner particles into a continuous toner film, still having the imaging pattern earlier defined. The carrier liquid is then removed mechanically, i.e. in a liquid phase by a mechanical means such as a roller or by suction or the like rather than by evaporation. Such digital printing processes are preferably carried out at high-speed, which is for instance at least 0.5 m/s (the speed indicating the propagation speed of the recording medium). The presence of a carrier liquid phase helps the speed of the printing process since the transfer of toner particles is done in an electrophoretic way. A consequence of this way of printing is the presence of carrier liquid also in the non-printing areas (i.e. the areas surrounding the image areas) together with an amount of interstitial carrier liquid which resides in between the toner particles in the image areas and which is at least partially liberated during fusing.

The amount of carrier liquid which remains present on the recording medium after fusing is therefore relatively large. It has been found that this presence may hinder a good adherence of the fused portions to the recording medium, especially for synthetic non absorbing recording media.

Further, for labels where the carrier liquid can migrate through the bulk of the recording medium, it has been found that the carrier liquid may interact with a glue layer present on the back side of the label, hereby reducing the gluing power. Also, problems have been encountered with the mechanical removal of carrier liquid, resulting in a printed image that remains more or less wet. Such a situation is unpractical in use, and thus undesired. Moreover, it may lead to blurring of the image during subsequent transportation or unwanted setoff.

SUMMARY OF THE INVENTION

It is therefore an object of embodiments of the invention to provide an improved digital printing process allowing obtaining good print results on a variety of recording mediums, when printing with a liquid toner comprising toner particles and a substantially non-polar carrier liquid. More in particular is it desirable to obtain a recording medium with a printed image with sufficient printing quality, which adheres well to the fused liquid toner of the printed image, and which is sufficiently dry. Further objects relate to the provision of an improved recording medium and an improved digital printing system.

According to a first aspect there is provided a digital printing process comprising: developing portions of a liquid toner comprising toner particles and a substantially non-polar carrier liquid; transferring of the developed portions of the liquid toner to a recording medium; and fusing said developed portions of the liquid toner to obtain fused portions and remaining carrier liquid. The recording medium comprises a base substrate layer and at least one carrier liquid absorbing coating layer made from a material which is different from the material of the base substrate layer. At least a part of the remaining carrier liquid is absorbed in the carrier liquid absorbing coating layer.

According to a second aspect there is provided a liquid toner electrophotography recording medium comprising a base substrate layer and at least one substantially non-polar carrier liquid absorbing coating layer. The base substrate layer may be more or less carrier liquid absorbing, up to the level where it substantially does not absorb carrier liquid. In the former case the coating layer may be used to prevent the carrier liquid from penetrating through the absorbing base substrate layer. This can be useful when the printed matter is used e.g. in (food) packaging areas or in applications where no migration above the migration limit of molecules of carrier liquid are tolerated.

According to a third aspect there is provided a digital printing system comprising: a coating station configured for coating a base substrate layer with at least one carrier liquid absorbing coating layer in order to obtain a coated recording medium; and a printing station configured for developing portions of a liquid toner comprising toner particles and a substantially non-polar carrier liquid, for transferring the developed portions to the at least one carrier liquid absorbing coating layer of the recording medium, and for fusing the developed portions of the liquid toner to obtain fused portions and remaining carrier liquid at least partially absorbed in the at least one carrier liquid absorbing coating layer.

Embodiments of the invention are based inter alia on the insight that the recording mediums may be sufficiently dry after a liquid electrophotography process, by adding a specific coating layer to the recording medium. This specific coating layer is a carrier liquid absorbing coating layer which is able to absorb substantially non-polar carrier liquid that remains on the recording medium. Furthermore, the printing quality is good. This is surprising, since absorption of carrier liquid in the coating layer presupposes affinity with the non-polar carrier liquid, whereas the toner particles are typically based on a more polar polyester binder resin. Hence, it was expected that any coating layer that may absorb carrier liquid, would lead to issues with the adhesion of the fused toner portions to the recording medium. It is believed by the inventors, without desiring to be bound thereto, that this surprising result is obtained in that the coating layer may absorb carrier liquid rather than merely being a suitably adsorption site thereto.

In an exemplary embodiment the substantially non-polar carrier liquid absorbing layer is configured to absorb in the digital printing process a substantially non-volatile carrier liquid as defined hereinafter. This class of substantially non-polar, substantially non-volatile carrier liquids has beneficial properties for use in digital electrophotography printing.

In a first embodiment, the carrier liquid absorbing coating layer comprises two or more different domains with different affinities for the carrier liquid and the toner. For instance, one domain has a larger affinity for the carrier liquid, which will be named the apolar section. The at least one other domain is more polar and has a greater affinity for the resin of the toner particles. The domains may have sizes in the area of in the range of nm2 up to square microns (pin2) or even up to macroscopical areas such as mm2. The different domains may be present in the layer as different materials. Alternatively, the different domains may be formed as copolymers. The copolymers could for instance be block copolymers, graft polymers, polymers based on different pre-polymers that are mutually cross-linked. Farger domain sizes may be achieved, for instance by formation of microstructures, such as the formation of crystallites, or physical segregation of the domains (while chemically connected), or selective binding of one domain onto the base substrate layer, while the other domain is oriented away from the base substrate layer. The inventors have found good adhesion and excellent printing qualities for an image printed in a digital printing process based on toner particles in a substantially non-polar carrier liquid, wherein the recording medium contained a carrier liquid absorbing coating layer having an apolar section and a polar section integrated in a block copolymer and wherein the base substrate layer was a non-absorbing base substrate layer (e.g. polypropylene foil). In one specific implementation, the carrier liquid absorbing coating layer was coated from an organic solvent, such as for instance toluene.

In a further embodiment, the carrier liquid absorbing coating layer contains a dispersing agent with polar groups. The dispersing agent is for instance a water compatible dispersing agent. More preferably, the carrier liquid absorbing coating layer is applied as an emulsion, and particularly a water-based (aqueous) emulsion containing the dispersing agent and one or more polymers able to form different domains. Tests herewith were positive. It is believed that the presence of a water compatible dispersing agent may have created a polar surface with high enough surface energy to create a good adhesion between the toner resin, for instance a polyester material, and the carrier liquid absorbing coating layer .

In an exemplary implementation thereof, the dispersing agent of the coating layer is an anionic emulsifier such as sodium, potassium and ammonium salts of higher fatty acids, and sulphonated derviates of aliphatic, arylaliphatic or naphtenic compounds, for instance sodium lauryl sulphate, sodium dodecyl benzene sulfonate, sodium dioctyl sulfosuccinate. In another implementation, the dispersing agent is a quaternary salt such as acetyl dimethyl benzyl ammonium chloride and hexadecyl trimethyl ammonium bromide. In a further implementation, the dispersing agent is an amphoteric emulsifier such as alkylamino or alkylimino propionic acid. In again another implementation, the dispersing agent is based on a head group of ethylene oxide units, such as polyoxyethylenated alkylphenols, such as nonylphenyl polyoxyethylene glycol and octylphenyl polyoxyethylene glycol, polyoxyethylenated straight-chain alcohols and polyoxyethylenated polyoxypropylene glycols (i.e. block copolymers formed from ethylene oxide and propylene oxide). In again a further implementation, the dispersing agent is a so-called hyperdispersant having a polar anchoring group, for instance based on a polyamine or polyimine and substantially non-polar stabilizing groups, for instance based on olefins or fatty acid polymers. A polymeric dispersing agent is preferred. It is believed that such dispersing agent may contribute to the binding of the liquid toner to the underlying substrate.

In again a further embodiment, the carrier liquid absorbing coating layer is applied in accordance with a predetermined pattern, such that portions of the base surface layer remain exposed. The predetermined pattern is such that the carrier liquid absorbing coating layer effectively forms discrete oil absorbing domains on the base substrate layer. Such an application is for instance achieved by control of the contact angle of the coating composition on the base substrate layer. Alternatively or additionally, use can be made of non-continuous application techniques, including for instance printing, or spraying in a manner to form separate droplets. Dispersing agents in the coating layer, for instance charged dispersing agents may contribute thereto.

In an exemplary embodiment, the carrier-liquid absorbing coating layer has an absorption capacity which is higher than 3 ml/m2 for substantially non-polar carrier liquids, preferably between 4 and 12 ml/m2. This is deemed a suitable absorption capacity that allows on the one hand an adequate absorption and on the other hand an adequately printed image, without problems such as a height variation of the surface of the recording medium. It is observed that the actual absorption in the absorbing coating layer may depend on a variety of parameters, including the number of colors printed on the recording medium (i.e. the number of transfer steps to the recording medium), the transferred image (particularly the coverage of the recording medium, i.e. 1, 10 or up to 40% or more), and/or any settings of the printing process.

Preferably, but not limited hereto, the substantially non-polar carrier liquid is absorbed to its full capacity in the carrier liquid absorbing coating layer upon heating. More preferably, the absorption up to the full absorption capacity is achieved by heating to a temperature of at least 50 °C and even more preferred of at least 60 °C during a predetermined period. For instance, this heating may occur during the fusing treatment. This heating is understood to accelerate diffusion of the carrier liquid into the absorption layer and to allow that the molecules of the carrier liquid and the absorption layer adopt an energetically advantageous molecular configuration with mutual physical bonding.

More particularly, this absorption capacity is specified for substantially non-polar carrier liquids chosen from the group of mineral oils, low or high viscosity liquid paraffins, isoparaffinic hydrocarbons, internal or terminal alkenes and polyenes, fatty acid glycerides, fatty acid esters or vegetable oils or combinations thereof. Typical commercially available carrier liquids are parrafin oils (Isopar and Exxsol) from Exxon, white mineral oils from Sonneborn Inc and vegetable oils from Oleon. The term ‘substantially non-polar’ is used in the context of the application to encompass entirely non-polar materials such as alkanes and non-polar materials that are slightly more polar than alkanes, such as fatty acid based material that include a carboxyl-group.

In a preferred embodiment, the carrier liquid has volatility below 25%, preferably below 15% and more preferably below 10% according to the volatility test described in the example section. This typically corresponds with an average boiling point well above 200 °C, preferably above 250 °C and even more preferably above 270 °C. A carrier liquid with a volatility below 25% is herein called a substantially non-volatile carrier liquid, and a carrier liquid with a volatility above 25% is herein called a volatile carrier liquid. The use of volatile carrier liquids may result in stripes in the print direction and thus in an unacceptable image quality. These printing errors are deemed due to evaporation during the printing process (i.e. at low temperatures) thereby causing contamination of corona wires and possibly scraper failure on the various members like development and intermediate members in the printing apparatus. Additionally low volatile carriers are not desired due to higher emissions of volatile organic compounds (VOC) as well.

In an exemplary embodiment the at least one carrier liquid absorbing coating layer is configured to absorb more than 90% of the carrier liquid absorbed in the recording medium. In other words, by adding the at least one carrier liquid absorbing coating layer the amount of the carrier liquid that is taken up in the base substrate layer can be significantly reduced or avoided. This is particularly relevant for base substrate layers comprising oil absorbing materials, such as paper based materials.

In one exemplary embodiment, the substantially non-polar carrier liquid absorbing coating layer is a top layer, i.e. it is a layer that is present at the surface of the electrophotography recording medium and is in direct contact with the liquid toner as of transfer to the recording medium. This presence as a top layer is deemed beneficial to enable absorption of the carrier liquid at a sufficiently high speed as desired in the foreseen liquid toner printing applications. Also, it is found that the presence of the carrier liquid absorbing coating layer as a top layer is beneficial so as to enable expansion of the layer upon absorption. More particularly, it has been surprisingly found that the absorption of the substantially non-polar carrier liquid in the carrier liquid absorbing coating layer does not result in any visible non-planarity of the recording medium, or at least not in a significant increase in non-planarity.

In an alternative embodiment, the carrier liquid absorbing coating layer is covered by a further layer, for instance to enhance adhesion of liquid toner, to improve scratch resistance and/or to reduce the tackiness of the coating layer. This further layer can for instance be a porous layer, a membrane and/or a patterned layer, which can be further cross-linked. Since these effects originate from a specific surface free energy and topology of a film, this further layer may contain surfactants such as a fluorinated surfactant, glycerol stearate, inorganic fillers such as talc, kaolin, calcium carbonate, silicon dioxide and/or organic polymer fillers.

In an exemplary embodiment the carrier liquid absorbing coating layer contains a polymer material. Good results have been obtained with copolymers, such as a block copolymer or a graft polymer. However, homopolymers made of monomers with suitable groups, or blends of homopolymers are not excluded. A polymer material is deemed beneficial as it is capable of forming a coating, particularly a homogeneous coating, in which the carrier liquid absorbing functionality is integrated. Furthermore, polymer materials have sufficient elasticity to expand upon absorbing carrier liquid without creating non-planarities such as bubbles.

In an exemplary embodiment, the carrier liquid absorbing coating layer comprises a block copolymer comprising a first polymer block for adhesion to the base substrate layer. A suitable adhesion block is for instance based on polystyrene. The block copolymers may be diblock, triblock and multiblock copolymers.The second polymer block is for instance based on unsaturated polymers such as polyolefins, polyalkenes, polyalkadienes or polyacrylic acid derivatives or saturated versions thereof. The block copolymers may further be provided with modifiers that are blended into the block copolymer and/or grafted thereon. For example, slightly polar monomers such as styrene can be added to the second polymer block to enhance the affinity for more polar carrier liquids, e.g. vegetable oils.

One example is a styrene-butadiene block copolymer. The synthesis hereof is known per se and typically involves anionic polymerisation, for instance initiated by lithium alkyls in cycloaliphatic media as solvent, giving rise to a diblock copolymer. A difunctional or trifunctional coupling agent or the addition of styrene gives triblock copolymers, either linear block copolymers or radial block copolymers. The first block, such as based on styrene, and more generally a vinyl arene is a thermoplastic polymer block, and even more generally a polymerized monoalkenyl aromatic hydrocarbon block, for instance with a weight average molecular weight of about 4,000 to 115,000. The second block is for instance based on a diene monomer, for instance a conjugated diene monomer and may be at least partially hydrogenated. It for instance has a weight-average molecular weight of 20,000 to 450,000. The lowest number of the total polymer should at least be well above 2,000 to avoid any additional migration of this polymer. The first block suitably constitutes 2-40 wt% of the block copolymer. Further formulations may be prepared for optimization of the oil absorbance and adhesion properties, for instance by blending various polymers with different hydrogenation degrees. One such polymer is known from US4,880,878 that is included herein by reference.

Specific examples, without limiting the scope of this application, are styrene-butadiene block copolymers, styrene-isoprene block copolymers, styrene-ethylene/butylene block copolymers, styrene-ethylene/propylene block copolymers, styrene-acrylic acid block copolymers, styrene-acrylate block copolymers styrene-butadiene-styrene block copolymers, styrene-isoprene-styrene block copolymers, styrene-ethylene/butylene-styrene block copolymers, styrene -ethylene/propylene-styrene block copolymers, styrene-acrylic acid-styrene block copolymers, styrene-acrylate-styrene block copolymers and mixtures thereof. Optionally the copolymer can be grafted with polar grafts to enhance the adhesion to the base substrate layer and/or to enhance compatibility with polar solvents, as exemplified by maleic anhydride grafts.

The recording medium having a base substrate layer and one or two carrier liquid absorbing coating layers containing one or more block copolymers is suitably manufactured by applying the at least one carrier liquid absorbing coating layer on the base substrate layer. Suitable coating techniques include spraying, printing, dipping, co-extrusion and the like. In an exemplary embodiment, the block copolymer is applied onto the base substrate layer in the form of an emulsion, and more particularly in an aqueous emulsion. The application of the coating layer in the form of an aqueous emulsion enables the in situ coating of a base substrate layer, particularly by avoiding the generation of organic vapours. Furthermore, good results have been obtained for the oil absorption as well as the toner adhesion.

In an alternative embodiment, the block copolymer is applied onto the base substrate layer from an organic solvent. In again an alternative embodiment, use is made of co-extrusion for the application of the carrier liquid absorbing coating layer on the base substrate layer. This is deemed particularly suitable, when the base substrate layer contains a synthetic polymer material. Coextrusion is deemed advantageous, as it can avoid the use of a solvent or dispersing agent that needs to be removed again, typically in a drying step.

In an exemplary embodiment the fusing comprises non-contact fusing and/or contact fusing the developed portions of the liquid toner. Suitable non-contact fusing techniques use hot air, IR, UV, EB and ultrasonic curing or other known methods of image fusing. Contact fusing may be performed by compression between heated rollers or pressure rollers only. It is observed that the fusing step in exemplary embodiments does not necessarily occur after transfer and that the mechanical removal of carrier liquid does not necessarily occurs after fusing. Particularly in one embodiment, the fusing may be carried out as a two-step process comprising a non-contact fusing step and a contact fusing step. In an exemplary embodiment the mechanical removal occurs at least partially in a step between the non-contact fusing and the contact fusing. In a further implementation, the mechanical removal may be carried out in two steps, for instance, before and/or during and/or after the non-contact fusing.

In another exemplary embodiment the coalescence could take place on a heated belt and both the carrier liquid and the toner film can be transferred to a substrate containing this absorbing layer so that the non volatile carrier liquid is absorbed and the toner film is transferred in an adhesive way to said substrate.

In an exemplary embodiment, the digital printing process is a multicolour printing process, wherein fused toner portions in for instance five different colours (white, cyan, magenta, yellow, black) are provided onto the recording medium. For each colour, a separate transfer step is carried out. Such multicolour printing process increases the amount of carrier liquid remaining on the recording medium, since carrier liquid will remain after each transfer step in both printed and non-printed areas and also with each amount of toner particles which is transferred. Therefore embodiments of the invention will significantly improve the printing result.

In an exemplary embodiment, the electrophotography recording medium comprises a base substrate layer of a synthetic polymer, such as normal or casted polyethylene, polypropylene or polyethylene terephthalate. Such plastics, which are for instance used in labels, have very low oil absorption capacity by themselves or do it in such a slow way that it is not compatible with the high speed printing conditions described earlier. The addition of the coating layer of the invention strongly improves their behaviour and, surprisingly, enables the use of digital printing by means of liquid toner, particularly in a substantially fast printing process without evaporation of carrier liquid.The degree of absorption may be to such an extent that the material feels dry and can be coated or laminated afterwards.

In another exemplary embodiment, the electrophotography recording medium comprises a base substrate layer of paper, cardboard such as paperboard or corrugated fibre board. Such a cardboard substrate layer may have an oil absorption capacity which is temporary, i.e. the carrier liquid may be absorbed but is easily desorbed again in a later stage, particularly to an underlying medium that has an affinity for carrier liquid. In these types of substrate layers, the carrier liquid-absorbing coating layer significantly reduces or inhibits absorption of carrier liquid into the base substrate layer. As a consequence, undesired contamination of the base substrate layer and eventually unwanted subsequent migration towards food or pharmaceutical materials can be prevented.

In yet another exemplary embodiment, the electrophotography recording medium is a label material, said label material comprising a glue layer arranged at one side of the base substrate layer and the at least one substantially non-polar carrier liquid absorbing coating layer arranged at the other side of the base substrate layer. By absorbing the carrier liquid in the carrier liquid absorbing coating layer, it is avoided that carrier liquid modifies the adherence properties of the glue layer in case the substrate is more or less permeable for the carrier liquid.

Thus, in one preferred embodiment, the recording medium comprises a carrier liquid absorbing coating layer that is configured for permanently absorbing the carrier liquid. Such permanent absorption is understood to be absorption with the formation of physical bonds to the material of the carrier liquid absorbing coating layer, particularly the apolar sections therein. It is to be distinguished from absorption into pores of a porous material substantially without interaction. It is observed that such substantially complete absorption of the carrier liquid is also desired and in some applications even mandatory, if the base substrate layer is acting as perfect functional barrier preventing the migration from the printed side to the food contact side. When the printed side touches the food contact side - for instance when rewinded or cut and stacked - migration from the printed layer to the food contact side can still occur. This is generally known as setoff.

In an exemplary embodiment, the electrophotography recording medium is provided with a carrier liquid-absorbing coating layer on both sides of the base substrate layer. Such a recording medium is suitable for enablement of double sided printing, particularly duplex printing with twice the advantages described above.

In an exemplary embodiment, a printing system is provided with a coating station for in line deposition or coating of the carrier liquid absorbing coating layer prior to the printing process. The coating station may comprise spraying heads for the deposition of a composition comprising a carrier liquid absorbing material, such as the above mentioned block copolymer. The composition is suitably an aqueous composition and is more suitably an emulsion. Rather than spraying heads, alternative coating means may be used, such as for instance an anilox roller, screen printing tool or dip coating. In one further embodiment, a drying tool may be present so as to ensure that the recording medium is dried to a desired humidity level. The coating station, with the optional drying tool may be integrated into the printing system, but may be a separate station in an alternative embodiment.

According to a third aspect, the invention provides a liquid toner electrophotography recording medium comprising a base substrate layer and at least one substantially non-polar carrier liquid absorbing polymer.

According to a fourth aspect, the invention provides a digital printing process, comprising the development of portions of a liquid toner comprising toner particles and a substantially non-polar carrier liquid and the transfer of the developed portions of the liquid toner to a recording medium, wherein the recording medium comprises a base substrate layer and at least one substantially nonpolar carrier liquid absorbing polymer, and wherein the carrier liquid is absorbed by the at least one substantially non-polar carrier liquid absorbing polymer.

According to a fifth aspect, the invention relates to the use of such a liquid toner electrophotography recording medium for a digital printing process.

It has been surprisingly found by the inventors that the addition of a substantially non-polar carrier liquid absorbing polymer is very effective for use in digital printing processes using liquid toner. Particularly, such a polymer material may have a significantly large absorption capacity and may permanently absorb the carrier liquid, therewith limiting migration to the other side of the recording medium (i.e. the side opposite the side on which the printing is being performed). This is e.g. advantageous when the recording medium is e.g. a package, for instance a food package or a pharmaceuticals package.

In a preferred embodiment, the polymer is elastic, at least after absorbing, so that it can reshape without forming bubbles that would be visible for a reader of the printed image.

In an exemplary embodiment, the at least one substantially non-polar carrier liquid absorbing polymer is applied as a layer onto the base substrate layer. In an alternative embodiment, the at least one substantially non-polar carrier liquid absorbing polymer is integrated into the base substrate layer. A suitable technique for doing so is for instance extrusion or co-extrusion with a synthetic polymer material constituting at least partially the base substrate layer. In case of a carrier liquid absorbing base substrate layer, like paper or cardboard, the polymer material may be added as an ingredient to the bulk base substrate material. Other application techniques are not excluded, such as integrating the at least one carrier liquid absorbing polymer in the form of fibres or semicrystalline domains upon manufacturing of the base substrate layer, or integrating the at least one carrier liquid absorbing polymer in a paper based material during manufacturing of the base substrate layer.

According to an exemplary embodiment there is provided a liquid toner electrophotography recording medium, comprising a base substrate material and at least one substantially non-polar carrier liquid absorbing polymer which is different from the base substrate material. This polymer may be present as a coating layer but may also be integrated or incorporated in a base substrate layer comprising the base substrate material.

According to an exemplary embodiment, the at least one carrier liquid absorbing polymer comprises a block copolymer with a first polymer block for adhesion to the base substrate material and a second polymer block based on olefins, alkenes, alkadienes or acrylic acid derivatives. In a further implementation, the second polymer block is based on monomers chosen from the group of butadiene, isoprene, acrylic acid and derivatives thereof and mixtures thereof. In again a further implementation, the first polymer block is polystyrene. Most suitably, the block copolymer is chosen from a diblock, triblock and multiblock copolymer.

According to an exemplary embodiment, the at least one carrier liquid absorbing coating polymer is applied to the base substrate material by applying an aqueous emulsion of a block copolymer with a first polymer block for adhesion to the base substrate material and a second polymer block based on olefins, alkenes, alkadienes or acrylic acid derivatives, and subsequent drying.

According to an exemplary embodiment, the liquid toner electrophotography recording medium is obtainable by extrusion of a synthetic polymer and the at least one substantially non-polar carrier liquid absorbing polymer.

According to an exemplary embodiment the at least one carrier liquid absorbing coating polymer is configured and combined with the base substrate material such as to absorb more than 3 ml of carrier liquid per square meter of recording medium.

According to an exemplary embodiment the base substrate material is a label material, said label material comprising an adhesive layer arranged at one side of a base substrate layer comprising the base substrate material, wherein the at least one carrier liquid absorbing polymer material is arranged at the other side of the base substrate layer or incorporated in the base substrate layer.

According to an exemplary embodiment the at least one carrier liquid absorbing polymer material is arranged as a first carrier liquid-absorbing layer on a first side of a base substrate layer comprising the base substrate material, and as a second carrier liquid absorbing layer on a second side of the base substrate layer.

According to an exemplary embodiment the base substrate material comprises any one of the following: a synthetic polymer material, paper, cardboard such as paperboard or corrugated fibre board.

Further embodiments and implementations as described above for the first and second aspect are also applicable to the third, fourth and fifth aspect of the invention.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying drawings are used to illustrate presently preferred non-limiting exemplary embodiments of the present invention. The above and other advantages of the features and objects of the invention will become more apparent and the invention will be better understood from the following detailed description when read in conjunction with the accompanying drawings, in which:

Fig. 1 is a schematic view illustrating a first embodiment of the invention;

Fig. 2 is a schematic view illustrating an exemplary embodiment of an electrography recording medium; and

Fig. 3 is a schematic view illustrating an exemplary embodiment of an electrography recording medium.

DESCRIPTION OF ILLUSTRATED EMBODIMENTS

Figure 1 illustrates diagrammatically a first embodiment of a digital printing apparatus of the invention, for setting out in more detail the overall process of the invention. The apparatus shown in figure 1 comprises a reservoir 100, a feed member 120, a toner member 130, an imaging member 140, an intermediate member 150 and a transfer member 160. A recording medium 199 is transported between intermediate member 150 and transfer member 160. The development member 130, the imaging member 140 and the intermediate member 150 are shown to be provided with a removal device 133, 146, 153, e.g. a scraper, but this could also be a scraper in combination with a loosening roller. Without loss of generality, the aforementioned members are illustrated and described as rollers, but the skilled person understands that they can be implemented differently, e.g. as belts.

In operation, an amount of liquid toner dispersion, initially stored in a liquid toner dispersion reservoir 100, is applied via a feed member 120, to a development member 130, an imaging member 140, and an optional intermediate member 150, and finally to a recording medium 199. The development member 130, imaging member 140, and intermediate member 150 all transfer part of the liquid toner dispersion 100 adhering to their surface to their successor; the part of the liquid toner dispersion 100 that remains present on the member’s surface, i.e. the excess liquid toner dispersion, which remains after selective, imagewise transfer, is removed after the transfer stage by appropriate means.

The charging of the toner on the development member 130 is done by charging device 131. This charging device 131 can be a corona or a biased roll. By charging the toner the liquid toner dispersion splits into an inner layer at the surface adjacent of the development member 130 and an outer layer. The inner layer is richer in toner particles and the outer layer is richer in carrier liquid. The transition between these two layers may be gradual. Figure 1 further shows a discharging corona 132 that is provided downstream of the area of the rotational contact between the toner roller 130 and the imaging roller 140. The discharging corona 132 is suitable for modifying or removing the charge in the dispersion. The removed material is preferably recycled with or without adjustment into liquid toner reservoir 100. A sensitive step in the printing process is the fusing of the liquid toner. This fusing is to result in coalescence of the toner particles and adherence to the recording medium. Typically use is made of a heat treatment that takes place shortly before, during or more preferably shortly after the transfer of the portions of liquid toner to the recording medium. The term ‘coalescence’ refers herein to the process wherein toner particles melt together and form a film or continuous phase that adheres well to the recording medium and that is separated from any carrier liquid. Suitably, the carrier liquid is thereafter removed in a separate step, for instance by means of rollers, by means of blowing off the carrier liquid, by means of suction. Suitably, this process occurs at “high speed”, for instance 50 cm/s or up to 3 m/s or more, so as to enable high-speed printing.

In the exemplary embodiment of figure 1, the fusing is carried out by means of a fusing unit 670, e.g. a non contact fusing unit, e.g. an IR fusing unit, optionally in combination with a contact fusing unit. The non-contact coalescence results in the formation of a film that is adhered to the recording medium 199 and a further liberation of the carrier liquid. The optional contact fusing (not illustrated) may remove the carrier liquid created during the coalenscence, enhance the adhesion and improve gloss of the film.

Further, use may be made of a liquid removal unit that further removes an amount of carrier liquid from the recording medium. This is for instance desired so as to enable that carrier liquid released from the liquid toner towards the exposed surface can be removed efficiently, particularly at a high printing speed. Furthermore, a liquid removal unit is deemed advantageous for specific type of materials and layers, particularly in case the carrier liquid absorption at room temperature is substantially slower compared to elevated temperatures. An advantage of further carrier liquid removal is that this carrier liquid also may be recycled and reused within the machine. The liquid removal unit may be embodied as a roller that is in rotational contact with the recording medium after fusing, or with an outer layer of the liquid toner dispersion transferred to the recording medium, wherein an electric field is applied to cause the charged toner particles to be close to the surface of the recording medium prior to the fusing process. The liquid removal unit in the form of a roller, as mentioned before, may be provided upstream of the contact fusing unit 670. This may increase the efficiency of the non-contact coalescence.

In another embodiment, a non-contact fusing unit 670 may precede the liquid removal unit. This order has the advantage that the liquid removal may be more efficient and easy. The heat generated by the fusing unit will induce film formation and adherence to the recording medium, so that no electric field may be needed for the carrier liquid removal.

In yet another embodiment, a liquid removal unit may be inserted between a non-contact fusing unit and a contact fusing unit.

It will be understood that for duplex and multicolour printing several such units are typically available.

Figure 2 shows an illustrative cross-sectional view of an electrophotography recording medium 199. The recording medium 199 has a first side 201 and a second side 202. It comprises a base substrate layer 200, with a first carrier-liquid absorbing coating layer 211 on the first side 201 and a second carrier liquid absorbing coating layer 212 on the second side. The base substrate layer 200 comprises in one embodiment a synthetic polymer, more particularly a polyolefin or other engineering (co)polymer such as polyethylene terephthalate, rather than a natural polymer such as cellulose. In alternative embodiments, use is made of paperboard and of cellulose based materials. Any blends as known per se in the art of paper manufacturing may be used alternatively. Furthermore, the base substrate layer 200 may be prepared to contain one of more coatings or impregnated materials as known per se, for instance based on calcium carbonate, which is for instance used to reduce the cellulose content of traditional paper.

Figure 3 shows an illustrative cross-sectional view of an electrophotography recording medium 199 in the form of a label material. The recording medium 199 has a first side 301 and a second side 302. It comprises a base substrate layer 300, with a first carrier-liquid absorbing coating layer 311 on the first side 301 and a glue layer 320 on the second side. The base substrate layer 300 comprises in one embodiment a synthetic polymer layer. In alternative embodiments, use is made of paperboard and of cellulose based materials. More generally the base substrate layer 300 may be made of any suitable label material. EXAMPLES Test methods

Viscosity test

The viscosity of the carrier liquid is measured with a Haake Rheostress RS600 operated in shear rate sweep from 0.1 to 3000 1/s at 25 °C.

The instrument is equipped with a cone/plate geometry type C60/10 and the gap is set to 0.052 mm. Volatility test

To evaluate the degree of volatility of the different carrier liquids the following test procedure was used. A disposable aluminum cup filled with approximately 7.5 grams of carrier liquid, was put in a heating cell for 2 hours. The infrared heater of the heating cell ensured a constant temperature of 80 °C. During this 2 hours the mass loss was recorded by a balance within the heating cell. A grating located at the top side of the heating cell inhibited condensation of the carrier liquid after evaporation.

The volatility (V) is expressed as a percentage of the weight loss of the different carrier liquids. A value above 15% indicates that optimal printing conditions cannot be guaranteed and that scraper failure and/or corona wire contamination is likely to occur. V = weight loss/original weight x 100%.

Oil absorption test A container was subsequently charged with 10 weight equivalents of carrier liquid and 1 weight equivalent of oil absorbing polymer. After 1 hour, the (swollen) polymer was removed and the remaining carrier liquid was weighed. It should be noted that for polymers only available as water based emulsions, a drying step was performed overnight in an oven at 75°C to ensure that all moisture is removed. The oil absorbing capacity of the polymers was ranked as follows: 1: >95% absorption of carrier liquid: preferred 2: > 50% absorption of carrier liquid: acceptable 3: < 50% absorption of carrier liquid: unacceptable

Adhesive strength test

This test is used to check the adhesive strength of the glue layer of a label material after applying a certain amount of oil on the label material.

Strips with an identical cross section were cut out of all test samples. Subsequently, every strip was glued onto the stainless steel test bank 2 hours after its preparation. Finally, the test strip was pulled from the test bank using the ASTM D3330 AF test method. The adhesive strength was ranked according to the force necessary to pull the strip from the test bank, relatively versus the reference label (label without application of oil): 1: 100%: preferred 2: >75%: acceptable 3: <75%: unacceptable

Toner adhesion test

The adhesion of the toner is measured by a tape test. Liquid toner with a solid content of 40 wt% is applied on a substrate by means of a 10 pm coating bar. In the following step, the toner is fused by putting the sample on a 90 °C hotplate for 20 seconds. Finally, a tape type Scotch Magic tape with a width of 19 mm and length of 150 mm is put on the fused image and removed slowly under an angle of 90 to 150°. The tape is visually inspected and the adhesion is ranked as follows: 1: no toner left on the tape: preferred 2: small deposition of toner on the tape: still acceptable 3: clear deposition of toner on the tape: unacceptable

Image quality test A printing test was performed with liquid developer dispersions LD1 to LD5 in a printer having a setup as illustrated in figure 1. A printing test was performed for 1 hour. Before and after the printing run a reference print is made to check the presence of stripes. The print before the starting of the print run needs to be stripe free.

If no scraper failure is observed, printed sample are visually inspected and the image quality is ranked as follows: 1: no stripes: preferred 2: limited amount of stripes: still acceptable 3: large amount of stripes or scraper failure: unacceptable

Example 1

Several polymers were screened regarding their carrier liquid absorbing capacity. The different polymers and carrier liquids are summarised in table 1. Isopar L was purchased from Exxon, Lytol from Sonneborn, Cl8 internal olefin from Ineos, isopropyl laurate and 2-ethylhexyl stearate from Oleon.

The results of the oil absorption test are summarised in Table 2.

Table 2: results from oil absorption tests

From the results in table 2 it is clear that only the oil absorbing polymers based on block polymers (OPM 3-5) have sufficient oil absorbing capacity.

Example 2

The reference sample for the adhesive strength test is a 155 gsm label material (tradename 'HGW Premium S2000N-BG40BR IMP’) with an adhesive layer on the backside. To test the influence of carrier liquid absorption on the adhesive strength, 8 mL of different carrier liquids (LIQ3 & LIQ5) were applied with a bar coater on the uncoated reference sample and the reference sample on which 6 gsm of styrene-isoprene-styrene triblock copolymer (0PM5) was applied from a 60 wt% aqueous emulsion with a bar coater.

The results of the oil adhesive strength test are summarised in table 3.

Table 3: results from adhesive strength test

From the results in table 3 it is clear that the oil absorption layer on top of the label material leads to a much smaller decrease in adhesive strength compared to the uncoated label material (compare Ex 3-4 with Comp 1).

Example 3

The reference material for the toner adhesion and image quality test is a 46 gsm polypropylene white label (tradename 'UPM PP White TC60 RP37 HD70 White’). Different oil absorbing polymers were applied as aqueous emulsions with a bar coater followed by drying for 1 hour at 40 °C.

Several liquid developer dispersions are prepared and comprise a marking particle, a carrier liquid and a dispersing agent.

The ingredients used to prepare the marking particles are summarised in table 4. The marking particles (MARI) are prepared by kneading the ingredients as mentioned in table 1 at a temperature of 100 to 120 °C for 45 minutes. This mixture is cooled down and milled down to 10 pm by a fluidized bed mill.

Table 4: ingredients of marking particles

(1) measured according to ASTM D3418

The carrier liquids used to prepare the liquid developer dispersions are those of Table 1.

Table 5: other ingredients of liquid developers

(2) the weight (grams) of dispersing agent that is needed to neutralize 1 mol of acid

Liquid developer dispersions were prepared with the ingredients mentioned in table 6. A predispersion of the ingredients is made and stirred for 10 min at room temperature. The predispersion is than brought into the liquid milling device. The liquid developer dispersion is milled down to a dv50 of 1.8 to 2.5pm using a bead mill. The milling was done until the desired particle size, viscosity and conductivity was obtained.

Table 6: liquid developer dispersion composition

The results of the toner adhesion strength test and image quality test are summarised in Table 7.

Table 7 - results of toner adhesion test and image quality test

From the results in table 7 it is clear that all liquid developers give good toner adhesion on a polypropylene (PP) label, given that a coating is applied (compare Ex 1-5 and Comp 1). LD1 gives good toner adhesion on an uncoated PP label, but during the printing test, scraper failure occurred after 30 min and the test was stopped (Comp 2). While changing the oil absorbing polymer does not have an effect on the toner adhesion (Ex 1 and 6), decreasing the coating weight from 6 to 2 g/m2 leads to a slight decrease in toner adhesion, which is still acceptable (compare Ex 1 and 7).

Claims

A digital printing process comprising: - developing parts of a liquid toner comprising toner particles and a substantially non-polar carrier fluid; - transferring the developed parts of the liquid toner to an information carrier; and melting the developed portions of the liquid toner to obtain molten portions and residual carrier fluid; wherein the information carrier comprises a base substrate layer and at least one carrier fluid-absorbing coating layer made of a material different from the material of the base substrate layer, and wherein at least a portion of the remaining carrier fluid is absorbed into the carrier fluid-absorbing coating layer.

The printing process according to claim 1, further comprising mechanically removing residual carrier fluid from the information carrier.

The printing process according to claim 1 or 2, wherein the at least one carrier liquid absorbing coating layer is configured to absorb more than 3 ml carrier liquid per square meter of information carrier.

The printing process according to claim 1 or 2, wherein the at least one carrier fluid-absorbing coating layer is configured to absorb more than 90% of the carrier fluid present on the information carrier.

The printing process according to any of the preceding claims, wherein the information carrier is a label material, which label material comprises a line layer applied on one side of the base substrate layer and the at least one carrier liquid-absorbing coating layer applied on the other side of the base substrate layer .

The printing process according to any of claims 1-4, wherein the at least one carrier liquid-absorbing coating layer comprises a first carrier liquid-absorbing coating layer on a first side of the base substrate layer and a second carrier liquid-absorbing coating layer on a second side of the base substrate layer, and wherein the printing process comprises transferring liquid toner to the first and second side of the information carrier.

The printing process according to any of the preceding claims, wherein the base substrate layer comprises one of the following: a synthetic polymeric material layer, paper, cardboard such as folding cardboard or corrugated cardboard.

The printing process according to any of the preceding claims, wherein the melting comprises heating the developed portions of the liquid toner.

The printing process according to any of the preceding claims, wherein the melting comprises non-contact melting and / or contact melting of the developed parts of the liquid toner.

The printing process according to any of the preceding claims, wherein, during the transfer of the developed parts, the information carrier is moved at a speed of at least 0.5 m / s.

The printing process according to any one of the preceding claims, wherein the melting is carried out after the transfer.

The printing process according to any of the preceding claims, wherein the carrier liquid-absorbing coating layer comprises a substantially non-polar carrier liquid-absorbing polymer material, preferably in the form of a block of copolymer material.

A liquid toner electrophotography information carrier comprising a base substrate layer and at least one substantially non-polar carrier liquid-absorbing coating layer.

The liquid toner electrophotography information carrier according to claim 13, wherein the at least one carrier liquid-absorbing coating layer is configured to absorb more than three ml carrier liquid per square meter of information carrier.

The liquid toner electrophotography information carrier according to claim 13 or 14, wherein the information carrier is a label material, which label material comprises an adhesive layer applied to one side of the base substrate layer, the at least one carrier liquid-absorbing coating layer being applied to the other side of the base substrate layer.

The liquid toner electrophotography information carrier according to any one of claims 13-15, wherein the at least one carrier liquid-absorbing coating layer is a first carrier liquid-absorbing coating layer on a first side of the base substrate layer and a second carrier liquid-absorbing coating layer on a second side of the base substrate layer, includes.

The liquid toner electrophotography information carrier according to any of claims 13-16, wherein the base substrate layer comprises at least one of the following: a synthetic polymer material, paper, cardboard such as folding cardboard or corrugated cardboard.

The liquid toner electrophotography information carrier according to any of claims 13-17, wherein the carrier liquid-absorbing coating layer comprises a block copolymer with a first polymer block for bonding the base substrate layer and a second polymer block based on olefins, olefins, alkadienes or acrylic acid derivatives.

The liquid toner electrophotography information carrier according to claim 18, wherein the second polymer block is based on monomers selected from the group of butadiene, isoprene acrylic acid, and derivatives thereof and mixtures thereof.

The liquid toner electrophotography information carrier according to claim 18 or 19, wherein the first polymer block is polystyrene.

The liquid toner electrophotography information carrier according to any of claims 17-20, wherein the block copolymer is selected from a diblock, triblock and multi-block copolymer.

The liquid toner electrophotography information carrier according to any of claims 13-21, wherein the non-polar carrier liquid-absorbing coating layer is obtainable by applying an aqueous emulsion of a block copolymer with a first polymer block for adhering to the base substrate layer and a second polymer block on based on olefins, olefins, alkadienes or acrylic acid derivatives, and by subsequently drying them.

The liquid toner electrophotography information carrier according to any of claims 13-21, wherein the base substrate layer and the carrier liquid-absorbing coating layer are provided as a laminate obtainable by co-extrusion of a synthetic polymer and a carrier liquid-absorbing polymer material.

A digital printing system, in particular for carrying out a printing process according to any one of claims 1-12, comprising: a coating station which is arranged for coating a base substrate layer with at least one carrier-absorbing coating layer for obtaining a coated information carrier ; a printing station adapted to develop portions of a liquid toner comprising toner particles and substantially non-polar carrier fluid; for transferring the developed parts to the at least one carrier-absorbing coating layer of the information carrier; and for melting the developed portions of the liquid toner to obtain molten portions and residual carrier fluid that is at least partially absorbed in the at least one carrier fluid-absorbing coating layer.

The digital printing system of claim 24, wherein the printing station comprises a contact fusing unit configured for contact melting the developed parts and / or a non-contact fusing unit configured for non-contact melting the developed parts.

Digital printing system according to claim 24 or 25, wherein the printing station comprises mechanical removal means for removing at least a part of the remaining carrier fluid.

A liquid toner electrophotography information carrier comprising a base substrate material and at least one non-polar carrier liquid-absorbing polymer.

The liquid toner electrophotography information carrier according to claim 27, wherein the at least one substantially non-polar carrier liquid-absorbing polymer is a block copolymer with a first polymer block for adhering to the base substrate material and a second polymer block based on olefins, olefins, alkadienes or acrylic acid derivatives , includes.

The liquid toner electrophotography information carrier according to claim 27 or 28, wherein the at least one substantially non-polar carrier liquid-absorbing polymer is present as a coating layer on the base substrate material.

The liquid toner electrophotography information carrier according to claim 27 or 28, wherein the at least one substantially non-polar carrier liquid-absorbing polymer is integrated into the base substrate material.

The liquid toner electrophotography information carrier according to any of claims 27-30, wherein the at least one substantially non-polar carrier liquid-absorbing polymer is combined with the base substrate material by an extrusion or co-extrusion process.

The liquid toner electrophotography information carrier according to any of claims 27-31, wherein the at least one carrier liquid-absorbing coating material is configured and arranged such that more than 3 ml of carrier liquid per square meter of information carrier is absorbed.

The liquid toner electrophotography information carrier according to any of claims 27-32, wherein the base substrate material is a label material, which label material comprises an adhesive layer applied to one side of a base substrate layer comprising base substrate material, wherein the at least one carrier fluid-absorbing polymer material is arranged on the other side of the base substrate layer or is included in the base substrate layer.

The liquid toner electrophotography information carrier according to any of claims 27-33, wherein the at least one carrier liquid-absorbing coating material is applied as a first one carrier liquid-absorbing layer on a first side of a basic substrate layer comprising the basic substrate material, and as a second one carrier liquid absorbent layer on a second side of the base substrate layer.

The liquid toner electrophotography information carrier according to any of claims 27-34, wherein the base substrate material comprises one of the following: a synthetic polymer material, paper, cardboard such as folding cardboard or corrugated cardboard.

Use of a liquid toner electrophotography information carrier according to any of claims 27-35 for a digital printing process comprising: - developing parts of a liquid toner comprising toner particles and a substantially non-polar carrier liquid; - transferring the developed portions of the liquid toner to the liquid toner electrophotography information carrier; and - melting the developed portions of the liquid toner to obtain molten portions and residual carrier fluid.