NL2016657B1

NL2016657B1 - Digital printing apparatus and digital printing process

Info

Publication number: NL2016657B1
Application number: NL2016657A
Authority: NL
Inventors: Erik Dries Deprez Lode; Jozef Johan Op De Beeck Werner; Jeroom Maria Van Gaens Wouter
Original assignee: Xeikon Mfg Nv
Priority date: 2016-04-22
Filing date: 2016-04-22
Publication date: 2017-11-15
Also published as: NL2016657A; WO2017182553A1

Abstract

Digital printing process and apparatus. The process comprises developing portions of liquid toner comprising toner particles and a substantially non-polar carrier liquid; transferring the developed portions of liquid toner to a first surface of a recording medium and fusing toner particles transferred to the recording medium to produce a printed image on the recording medium; coating or laminating the printed image with carrier liquid barrier material to form a carrier liquid barrier layer, wherein the carrier liquid barrier layer is substantially impermeable to the substantially non-polar carrier liquid of the liquid toner, and wherein there is non-polar carrier liquid present in the recording medium when the printed image is coated or laminated with the carrier liquid barrier material; wherein the second surface of the recording medium is also provided with a carrier liquid barrier layer; and wherein the steps of transferring and coating or laminating the printed image occur in-line.

Description

Digital printing apparatus and digital printing process Field of the invention

The present invention relates to a digital printing apparatus and a digital printing process using liquid toner.

Background

Liquid toners are known in the art and usually comprise at least carrier liquid and marking particles.

For example, earlier European patent application publication no. 2713210 describes a liquid toner (also called a liquid developer dispersion) for use in a digital printing apparatus comprising a nonvolatile carrier liquid, a marking particle and a dispersing compound or a combination of dispersing compounds.

In this earlier European patent application, the carrier liquid is said to be any suitable non-volatile liquid such as a silicone fluid, a hydrocarbon liquid, a vegetable oil, or any combinations thereof.

In addition, in EP2713210, the marking particles are said to comprise coloured particles (i.e. a pigment) and a binder resin. It is stated in this earlier application that the marking particles may be formed by extruding the binder resin and the pigment particles, and a polyester resin is said to preferably be used as binder resin.

It is also stated in EP2713210 that the marking particles are chargeable imaging particles (or have chargeable locations) which allows the marking particles to develop a static electric charge, enabling them to be transported between different components of a printing system by applying a suitable electric field.

An apparatus and a process for digital printing using liquid toner are described in the earlier US patent application published under no. 2009/0052948. This US application describes a digital printing apparatus that uses liquid toner comprising chargeable imaging particles and a carrier liquid.

In US 2009/0052948, the apparatus is said to comprise an imaging member adapted to sustain a pattern of electric charge forming a latent image on its surface, a development member arranged to receive a quantity of liquid toner from a reservoir and to develop the latent image by transferring a portion of the quantity of liquid toner onto the imaging member in accordance with the pattern, and depositing means arranged to deposit the transferred portion (i.e. the developed image) onto a printing substrate.

An electrostatic printing process using liquid toner can be used to print on a variety of different recording mediums. For example, it is known to use such a digital printing process to print on paper, and polymeric films. However, if there is some carrier liquid remaining on the recording medium after printing, this may cause problems.

For example, if liquid toner is used to print on packaging material, carrier liquid may migrate from the printed surface of the material to the other surface. This may mean that, when the packaging is assembled, the product contained within the packaging comes into contact with a portion of the residual carrier liquid. This may cause deterioration of the product and/or be harmful. For example, if the product is a food product, the migration of the carrier liquid to the internal surface of the packaging and further into the food product could have health implications.

This migration of mineral oil into food products is a known problem when using paper or cardboard packaging coming from recycled paper and cardboard waste containing mineral oils.

This is especially so when the source of paper waste contains newspapers as these are typically printed with mineral oil based printing inks.

The transfer of carrier liquid into food products occurs by surface-to-surface migration, i.e. from direct contact between the inner surface of the packaging and the food product. The transfer also occurs by gas-phase migration which involves the evaporation of volatile (mineral oil) components and the subsequent deposition of the evaporated components onto the packaged food product.

Several studies have been carried out to determine how much carrier liquid is transferred from packaging. Each of these studies detected appreciable amounts of mineral oil residues in the recycled paper and cardboard and also in the packed food products. For example, a recent study by Foodwatch, where 120 food products packed in paper or cardboard were tested, revealed that the amount of mineral oils contained in the packaging varied from less than 10 mg per kilo of the packaging material up to approximately 1250 mg/kg.

To overcome problems associated with residual carrier liquid, known digital printing processes comprise a means for removing substantially all of the carrier liquid. For example, digital printing systems may comprise a mechanical means such as a roller or suction means for removing the remaining carrier liquid. Alternatively, a printing system may get rid of the residual carrier liquid using a drying means to assist with the evaporation of the carrier liquid. In some processes the use of volatile carrier liquids is preferred so that the carrier liquid can already be removed before the image is transferred to the substrate, for example, by using a means for evaporating excess carrier liquid from the final roller used to transfer the image to the substrate. Moreover, when using such volatile carrier liquids, further carrier liquid can evaporate from the printed substrate after the printing process.

However, it has been found that the step of removing the residual carrier liquid is often ineffective as much of the residual carrier liquid still remains on the printed substrate. This is especially true if a substantially non-volatile liquid is used as the carrier liquid. However, the use of a nonvolatile liquid as the carrier liquid has a number of advantages. For example, liquid toners based on substantially non-volatile carrier liquids are less prone to gas-phase migration. Additionally, the use of volatile carrier liquids is also sometimes considered to be undesirable due to the larger quantities of volatile organic compounds (VOC) released by such liquids.

Summary

It is therefore an object of the present invention to overcome problems associated with known liquid toner digital printing processes.

According to a first aspect of the invention there is provided a digital printing process comprising: (i) developing portions of a liquid toner comprising toner particles and a substantially nonpolar carrier liquid; (ii) transferring the developed portions of liquid toner to a first surface of a recording medium and fusing toner particles transferred to the recording medium to produce a printed image on the recording medium, wherein at least a part of the substantially nonpolar carrier liquid transferred to the recording medium is absorbed by the recording medium; (iii) coating or laminating the printed image with carrier liquid barrier material to form a carrier liquid barrier layer, wherein the carrier liquid barrier layer is substantially impermeable to the substantially non-polar carrier liquid of the liquid toner, and wherein there is non-polar carrier liquid present in the recording medium when the printed image is coated or laminated with the carrier liquid barrier material; wherein the recording medium comprises a second surface opposite to the first surface, wherein the second surface of the recording medium is also provided with a carrier liquid barrier layer; and wherein the steps of (ii) transferring and (iii) coating or laminating the printed image occur in-line.

The printed image should be coated or laminated with a substantially transparent carrier liquid barrier layer so that the printed image is visible below this layer.

As is clear from the above description of the first aspect of the invention, the carrier liquid barrier layer may be formed by coating a surface of the recording medium with a carrier liquid barrier material or by laminating a carrier liquid barrier material onto the recording medium.

The amount of carrier liquid contained in the liquid toner used in the digital printing process depends on the solid content of the toner, i.e. on the percentage weight (weight %) of the toner particles,. For example, the initial liquid toner may comprise 25% solid content. If the solid content of the liquid toner is 25%, this leaves 75% carrier liquid in the initial liquid toner.

However, during the printing process, liquid toner may be transferred to components of the printing apparatus. For example, if the printing process comprises roll-to-roll transfer, carrier liquid may be left on the ‘donor’ roller(s). Therefore, at the time of transferring the image to the recording medium, the liquid toner may comprise approximately 40 to 60% solid content.

In some embodiments of the invention, at least 2 gsm (grams per square metre) of carrier liquid (applied in the form of liquid toner) may be transferred to a surface of a recording medium. For example, between 2 and 8 gsm of carrier liquid may be transferred to a surface of a recording medium.

In some embodiments of the printing process, approximately 1.5 to 2 gsm of carrier liquid may be transferred to the recording medium for each colour of liquid toner. In such embodiments, each colour of liquid toner may be applied to the recording medium by a separate roller of the printing system. In these embodiments, a layer of toner will be applied across the recording medium for each colour. Therefore, in areas where no image is printed on the recording medium, carrier liquid is still transferred to the recording medium, resulting in the application of approximately 0.5 gsm of carrier liquid per printed colour even in unprinted areas.

Therefore, to summarise, each colour developer station may apply between 0.5 gsm and 2 gsm of carrier liquid to a surface of a recording medium. The exact amount of carrier liquid transferred by each colour developer station at least partially depends on whether an image is printed in that specific colour or not.

In embodiments of the printing process in which 0.5 to 2 gsm of carrier liquid is transferred to a surface of a recording medium for each colour of liquid toner used in the printing process, if the image is printed using four (different) colours, between 2 and 8 gsm of carrier liquid may be applied to the surface of the recording medium during the printing process.

In other embodiments, fewer or more than four colour developer stations may be used, and this may result in different amounts of carrier liquid being applied onto a surface of the recording medium.

In some embodiments of the digital printing process, in the step (ii) of transferring the developed portions of liquid toner to a first surface of a recording medium to produce a printed image on the recording medium, at least 2 gsm of carrier liquid is transferred to the recording medium.

For example, 2 gsm of carrier liquid may be applied to each side of the recording medium. In some embodiments, between 2 and 8 gsm of carrier liquid may be applied to each side.

In embodiments of the digital printing process, a recording medium with a weight between 50 gsm and 400 gsm may be used.

In some embodiments of the digital printing process, at least 50 percent of the carrier liquid transferred to the recording medium may be present in the recording medium after the step of coating the printed image with a carrier liquid barrier material. In the calculations discussed below, it has been assumed that 80 percent of the carrier liquid transferred to the recording medium is present in the recording medium after the coating step.

In one example, the printing process may use a 400 gsm cardboard substrate as the recording medium. If a 400 gsm cardboard substrate is used and 1.6 gsm (80% of 2 gsm carrier liquid which is applied) of carrier liquid is absorbed by the first surface of the recording medium, the carrier liquid will be present in amounts of 4 grams per kilogram of the recording medium (i.e. 4 g/kg).

Alternatively, a 50 gsm paper substrate could be used. If 6.4 gsm (80% of 8 gsm carrier liquid which is applied) of carrier liquid is absorbed by the first surface of the recording medium, the carrier liquid will be present in amounts of 128 grams per kilogram of the recording medium (i.e. 128 g/kg).

In another embodiment, the printing process may print on both surfaces of the recording medium, in essence a duplex printing process. If a 50 gsm paper substrate is used and 6.4 gsm (80% of 8 gsm carrier liquid which is applied) of carrier liquid is absorbed by both the first and the second surface of the recording medium, the carrier liquid will be present in amounts of 256 grams per kilogram of the recording medium (i.e. 256 g/kg).

As discussed above, in some embodiments, not all of the carrier may be absorbed by the recording medium. For example, some of the transferred carrier liquid may be removed from the recording medium after printing (this is discussed in more detail below). If the digital printing process uses a 400 gsm substrate, and if only 30 percent (instead of the assumed 80%) of the, for example, 2 gsm applied carrier liquid is still present on the recording medium after the coating step, each kilogram of the recording medium will contain 1.5 grams of carrier liquid.

In some embodiments of the digital printing process, after step (iii) of the process the printed and coated/laminated recording medium is rewound. Alternatively, the printed and coated/ laminated recording medium is cut into sheets and stacked.

In the digital printing process described above, the steps of transferring and coating the printed image occurs in-line. If the printing and coating steps take place in-line, this means that the printing and coating steps form stages of a continuous process and no winding and/or stacking of the recording medium takes place between the printing and coating processes. Coating the printing image in-line is advantageous as it prevents “set-off’ occurring during the rewinding and/or stacking process, i.e. in-line coating prevents migration or transfer of the carrier liquid from the printed image to the opposite surface of a recording medium though contact between the two opposing surfaces during the rewinding and/or stacking process.

For example, if the printed material is stacked after printing, and if the formation of the printed image and coating the printed image did not take place in-line, carrier liquid remaining on the printed surface would be transferred to a surface of a second sheet of material which is adjacent to the printed material in the stack. Similarly, if the printed material is wound after printing, carrier liquid would be transferred to the overlying layer of the wound material.

In some embodiments, substantially the entire first surface of the recording medium is coated with the carrier liquid barrier material. Preferably, the whole surface of the recording medium is coated with the carrier liquid barrier material. In embodiments where the printed image makes up only a portion of the first surface, both the printed image and the non-image areas are coated with the carrier liquid barrier material.

If the printing process involves printing on a single side of the recording medium, i.e. a simplex printing process, carrier liquid used in the printing process may migrate through the recording medium, from the first surface on which the required pattern is printed to the unprinted second surface. In order to prevent the carrier liquid from being transferred from this second surface of the substrate, this second surface is also provided with a carrier liquid barrier layer.

In a simplex printing process, the carrier liquid barrier layer is preferably formed on the second surface of the recording medium before an image is printed on the first surface of the recording medium.

The recording medium may be supplied to the above mentioned digital printing process with a coating of carrier liquid barrier material already applied to its second surface. In such embodiments, the step of coating the second surface does not form part of the digital printing process. Instead, the formation of the carrier liquid barrier layer on the second surface before printing can be done in a separate unit. This may allow for a more flexible, efficient and/or fast process for coating the second surface of the recorded medium with the carrier liquid barrier material. Alternatively, the recording medium may be purchased from the supplier already coated with a carrier liquid barrier layer on its second surface.

In other embodiments, the digital printing process may further comprise a step in which the second surface of the recording medium is coated or laminated with a carrier liquid barrier material to form the carrier liquid barrier layer. In some embodiments, coating/ laminating the second surface of the recording medium with barrier material may occur in or after step (iii) of the digital printing process. If both the first and second surfaces of the recording medium are coated with the barrier material in step (iii), the coating may occur simultaneously on both sides, or the barrier layer may be formed on the second surface of the recording medium after it has been formed on the first surface and vice-versa.

In other embodiments of the digital printing process, the second surface of the recording medium may be coated or laminated with the carrier liquid barrier material before step (i) of the digital printing process. In such embodiments, the step of coating/ laminating the second surface occurs before an image is formed on the recording medium.

In embodiments of the printing process, coating the printed image on the first surface of the recording medium takes place in-line with the step of transferring the printed image on the substrate. If winding or stacking of the printed material was to occur before the carrier liquid barrier layer is formed on this printed image, carrier liquid may be transferred from the printed surface to the opposite side of the recording medium, i.e. so called “set-off’. If a carrier liquid barrier layer has already been formed on this second side of the recording medium, carrier liquid may be transferred to the outer surface of this barrier layer. This would mean that when the recording medium is used as a packaging material, for example, the product contained within this packaging may still be contaminated by the carrier liquid.

The above described digital printing process may also be used for double-sided printing. In such a duplex printing process, a desired pattern is printed on both the first and second surfaces of the recording medium. Therefore, in the digital printing process of the present invention, after printing, the printed images on both the first and second surfaces are coated with a barrier layer. This prevents any carrier liquid from being transferred from either printed surface. The coating on both surfaces occurs in-line with the printing step.

The carrier liquid barrier layer may be formed from any material that is substantially impermeable to a non-polar carrier liquid. In this disclosure “substantially impermeable” is used to describe a barrier layer that limits the migration of the substantially non-polar carrier liquid through the layer of material, so that it does not weaken the adhesive strength of Scotch® Magic™ tape applied to the barrier layer by more than 0.6 N when compared to the adhesive strength of the Scotch® Magic™ tape applied to the same barrier layer provided on an unprinted recording medium (i.e. a recording medium that does not contain carrier liquid). Preferably, this means that the migration is limited to 10 ppm or 10 mg/1 kg of food or 10 mg/6 dm2 (based on the European definition that a cube of 1 dm3, and thus a surface area of 6 dm2, holds 1 kg of food). More preferably the migration is limited to 1 ppm.

Suitable coating compositions for the carrier liquid barrier layer include aqueous dispersions of polymers or co-polymers such as poly(meth)acrylate, styrene acrylate, polyamide, polyester, polyurethane, polyureas, polyether, silicone and mixtures thereof. In order to form the carrier liquid barrier layer, these compositions are applied onto the recording medium and then dried to substantially remove the water and create a substantially impermeable barrier layer. Specific examples of suitable coating compositions include Epotal® and Ecovio® from BASF, and Beckopox®, Daotan®, Ucecoat® and Ucecryl® from Allnex.

Other suitable coating compositions for forming the carrier liquid barrier layer include radiation-curable coating mixtures comprising monomers and/or oligomers and/or polymers based on (poly)(meth)acrylates, polyethers, polyesters, polyamides, polyurethanes, polyureas, silicones and co-polymers thereof. After application onto the recording medium, these coatings are cured, for instance in the presence of a photo-initiator, to create cross-linking and a substantially impermeable barrier layer. Examples of suitable materials for a radiation-curable coating mixture include Ebecryl®, Esacure® and UceCoat® from Allnex.

Suitable additives can also be incorporated into the coating composition to increase gloss level, wettability etc. Additives may additionally or alternatively be added to act as anti-blocking agents, defoaming agents, rheology modifiers etc. Such additives can include PCC, GCC, kaolin, organoclays, waxes, nonionic/anionic surfactants such as ammonia salts and sulphonic/phosphoric acid esters, silicon based polymers, fluoro-modifred polymers, fatty acid esters, etc.

In yet another embodiment, the carrier liquid barrier layer is composed of a synthetic polymer film material, which can be applied to the printed surface and/or the (unprinted) second surface by lamination. In some embodiments, the lamination may take place by extrusion, for example, by co-extrusion. For example, the recording medium may comprise a layer of PET (polyethylene terephthalate) polymer, where this PET layer is co-extruded with a polyamide polymer, wherein the polyamide layer forms the carrier liquid barrier layer.

Examples of suitable synthetic polymer film materials which can be laminated onto the recording medium to act as carrier liquid barrier layer, are Ultramid® and EcoVio® supplied by BASF and Walki®-Most supplied by Walki.

In some embodiments, the carrier liquid barrier layers on the first and second surfaces of the recording medium are made from the same material. In other embodiments, the carrier liquid barrier layers on the two surfaces have different compositions.

The recording medium may be any material capable of absorbing the non-polar carrier liquid of a liquid toner. The recording medium may be any material capable of receiving a layer or layers of liquid toner, whereby the liquid toner marking particles adhere to the surface of the recording medium during fusing. Preferably, the recording medium is capable of absorbing a substantial part of the substantially non-polar carrier liquid of a liquid toner.

In some embodiments, the recording medium may comprise paper or cardboard (such as paperboard or corrugated fibre board), and synthetic polymer film materials.

The recording medium may comprise multiple layers. For example, the recording medium may comprise a base substrate layer coated with an absorbent layer. The base substrate layer may comprise a synthetic polymer material, paper and cardboard such as paperboard or corrugated fibre board.

In another embodiment, the recording medium may be composed of an absorbent layer applied on a synthetic polymer base substrate layer that already acts as a carrier liquid barrier layer. In this multi-layer recording medium, the absorbent layer may contain a block copolymer with a first polymer block for adhesion to the base substrate layer of the recording medium, and a second polymer block based on olefins, alkenes, alkadienes or acrylic acid derivatives compatible with the carrier liquid. The absorbent layer may be formed by applying an aqueous emulsion of a block copolymer to the base layer and then drying this layer or by co-extruding the absorbent material with the barrier film material. The absorbent layer may be configured to absorb more than 3 ml of carrier liquid per square meter of recording medium. The absorbent layer may also be configured to absorb more than 90 % of the carrier liquid transferred to the recording medium.

In other embodiments, the recording medium may comprise both a layer of absorbent material and a polymeric film, wherein a separate carrier liquid barrier layer is applied to the polymeric film. Whether or not a separate carrier liquid barrier layer is required depends on the migration/barrier properties of the polymeric film. If the polymeric film does not act as a barrier for the carrier liquid, a separate carrier liquid barrier layer will be required.

In some embodiments, the recording medium may be a part of an adhesive label, said label comprising an adhesive layer arranged on the carrier liquid barrier layer on the second surface of the recording medium. The recording medium may be the “facestock” of the adhesive label, wherein this facestock may comprise paper, synthetic polymer etc.

It was surprisingly found by the applicants that another problem may be caused by the presence of carrier liquid in a recording medium if the recording medium forms part of an adhesive label. In the case of self-adhesive labels, carrier liquid that is absorbed in the printing process may migrate through the label facestock from the printed surface to the other surface and into the adhesive layer. It has been found that the carrier liquid may interact with this adhesive, thereby reducing the ability of the adhesive layer to adhere to a surface.

For example, the adhesive strength of a 155 gsm label material sold under the tradename 'HGW Premium S2000N-BG40BR IMP' with an adhesive layer on the backside has been found to be negatively affected by the substantially non-polar carrier liquids Cl810 and 2-ethylhexyl stearate.

Adhesive labels comprise facestock and an adhesive layer affixed to the facestock, where the adhesive layer can adhere the facestock to a substrate. In a first type of label, the facestock may be a layer of a pulp-based material such as paper. When this first type of label is used as the recording medium in a digital printing process, the carrier liquid may migrate through the paper to the adhesive layer. The migrated carrier liquid may then interact with the adhesive layer present on the backside of the label, reducing the gluing power. In order to prevent this, the present invention may provide a carrier liquid barrier layer between the facestock material and the adhesive layer.

In a second type of label, the facestock may comprise a layer of a synthetic polymer. Some types of synthetic polymer used in labels, such as normal or casted polyethylene, polypropylene or polyethylene terephthalate, have very low absorption capacity for non-polar liquids or absorb the non-polar liquid so slowly that the polymer is not compatible with high speed printing conditions. Therefore, in such embodiments of the second type of label, the facestock comprises an absorbent layer on the synthetic polymer. In terms of the present invention, if the synthetic polymer of the label forms a barrier for the substantially non-polar carrier liquid, the layer of synthetic polymer is considered to be a carrier liquid barrier layer. The absorbent layer of the label would form the recording medium of the present invention.

In some embodiments of the digital printing process, an adhesive label material may be formed by applying an adhesive material to the outer surface of the carrier liquid barrier layer on the second surface of the recording medium after step (iii) of the process.

In alternative embodiments, the recording medium may be supplied to the digital printing process already with a carrier liquid barrier layer on its second surface and an adhesive material on the outer surface of this barrier layer. In such embodiments, a backing layer to protect the adhesive may be provided on the outer surface of the adhesive.

The above described digital printing process may also further comprise, before coating the printed image with a carrier liquid barrier material, mechanically removing a portion of the transferred carrier liquid from the recording medium. In such embodiments, the carrier liquid may be removed, for example, by means of rollers, by means of blowing off the carrier liquid, or by means of suction.

In some embodiments of the process, at least 50 percent of the transferred carrier liquid is present in the recording medium after the step of coating the printed image with a carrier liquid barrier material. For example, at least 60 percent of the transferred carrier liquid is absorbed by the recording medium and may, therefore, be present in the recording medium after the step of coating the printed image with a carrier liquid barrier material. In such embodiments, at least 80 percent, at least 90 percent, or at least 95 percent of the carrier liquid transferred to the recording medium may be present in the recording medium after the step of coating the printed image with a carrier liquid barrier material. In some embodiments, substantially all of the carrier liquid transferred to the recording medium may present in the recording medium after the step of coating the printed image with a carrier liquid barrier material.

In some embodiments the digital printing process may further comprise drying and/or UV curing or electron-beam (EB) curing the carrier liquid barrier material after it has been applied to the recording medium to form the substantially impermeable carrier liquid barrier layer.

According to a second aspect of the invention there is provided digital printing apparatus comprising: (a) 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 a first surface of a recording medium; and for fusing toner particles transferred to the recording medium to produce a printed image on the recording medium, wherein at least a part of the substantially non-polar carrier liquid transferred to the recording medium is absorbed by the recording medium; and (b) a coating or laminating device in-line with the printing station, wherein the coating or laminating device is configured to coat or laminate the printed image on the recording medium with carrier liquid barrier material to form a carrier liquid barrier layer, wherein the carrier liquid barrier layer is substantially impermeable to the non-polar carrier liquid, and wherein there is non-polar carrier liquid present in the recording medium when the printed image is coated or laminated with the carrier liquid barrier material.

As with the digital printing process described above, the recording medium used in the above mentioned apparatus may be any material capable of receiving a layer or layers of liquid toner, whereby the liquid toner marking particles adhere to the surface of the recording medium during fusing. Preferably the recording medium is capable of absorbing a substantial part of the substantially non-polar carrier liquid of a liquid toner. For example, the recording medium may comprise paper or cardboard (such as paperboard or corrugated fibre board), and synthetic polymer film materials. Further information regarding suitable recording mediums for use with this apparatus is given above in connection with the digital printing process. The features described above in relation to the digital printing process regarding suitable recording medium also apply to this second aspect of the invention.

In some embodiments, the recording medium used with the apparatus comprises a second surface opposite to the first surface of the recording medium, wherein the second surface is provided with a carrier liquid barrier layer. For example, the recording medium may be a label material, the label having an adhesive layer arranged on the carrier liquid barrier layer on the second surface of the recording medium.

In embodiments of the apparatus, the coating or lamination device may also be configured to provide the second surface of the recording medium with carrier liquid barrier material to form a carrier liquid barrier layer on the second surface.

The carrier liquid barrier layers on the first and second surfaces of the recording medium may be formed from the same material. Alternatively, the carrier liquid barrier layers on the two surfaces may have different compositions.

As described in relation to the digital printing processes, suitable coating compositions for forming the carrier liquid barrier layer include aqueous dispersions of poly(meth)acrylate, styrene acrylate, polyamide, polyester, polyurethane, polyureas, polyether, silicone and mixtures thereof, for example, Epotal® and Ecovio® from BASF, and Beckopox®, Daotan®, Ucecoat® and Ucecryl® from Allnex.

Radiation-curable coating mixtures comprising monomers and/or oligomers and/or polymers based on (poly)(meth)acrylates, polyethers, polyesters, polyamides, polyurethanes, polyureas, silicones and co-polymers thereof may also be used to form the carrier liquid. Examples of suitable materials for a radiation-curable coating mixture include Ebecryl®, Esacure® and UceCoat® from Allnex.

Additives to change properties of the coating layer such as PCC, GCC, kaolin, organoclays, waxes, nonionic/anionic surfactants such as ammonia salts and sulphonic/phosphoric acid esters, silicon based polymers, fluoro-modified polymers, fatty acid esters can also be incorporated into the coating composition.

In some embodiments of the process, at least 50 percent of the transferred carrier liquid is present in the recording medium after the step of coating the printed image with a carrier liquid barrier material.

The printing station of the digital printing apparatus may comprise a mechanical removal means for mechanically removing a portion of the transferred carrier liquid from the recording medium.

In some embodiments the digital printing apparatus may further comprise a drying device configured to dry the carrier liquid barrier material after it has been applied to the recording medium to form the carrier liquid barrier layer. The drying device may form part of the coating device of the apparatus. Alternatively, the drying device may be provided in-line with the coating device.

In some embodiments the digital printing apparatus may further comprise a curing device configured to cure the carrier liquid barrier material after it has been applied to the recording medium to form the carrier liquid barrier layer. The curing device may form part of the coating device of the apparatus. Alternatively, the curing device may be provided in-line with the coating device. Suitable sources for curing the curable coating composition include sources of actinic radiation, like UV lamps and UV LEDs or particle beams, like an electron beam.

The above mentioned digital printing process and apparatus are said to be used with a substantially non-polar carrier liquid. Such substantially non-polar carrier liquids may be 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, vegetable oils, alkyl carbonates and combinations thereof. Typical commercially available carrier liquids include parrafin oils (Isopar and Exxsol) from Exxon, white mineral oils from Sonneborn Inc, vegetable oil(s) (and vegetable oil derivatives) from Oleon and alkyl carbonates from Evonik. 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 and alkyl carbonates that include a carboxyl-group.

The features described above in relation to the digital printing process regarding suitable carrier liquids, for example, the weight of carrier liquid that may be transferred to the recording medium, also apply to this second aspect of the invention.

According to a third aspect of the invention, there is provided a digital printing process using a liquid toner comprising toner particles and a substantially non-polar carrier liquid, wherein the digital printing process comprises: (i) developing portions of a liquid toner comprising toner particles and a substantially nonpolar carrier liquid; (ii) transferring the developed portions of liquid toner to a first surface of a recording medium to produce a printed image on the recording medium, wherein at least a part of the substantially non-polar carrier liquid is absorbed in the recording medium; (iii) coating or laminating a second surface of the recording medium with carrier liquid barrier material to form a carrier liquid barrier layer, wherein the carrier liquid barrier layer is substantially impermeable to the substantially non-polar carrier liquid of the liquid toner; (iv) applying an adhesive material to the carrier liquid barrier layer to form an adhesive label material; and (v) fixing a releasable protective backing to the exposed surface of the adhesive material; such that when the printed image is coated or laminated with the carrier liquid barrier material there is non-polar carrier liquid present in the recording medium.

In some embodiments of this third aspect, the protective backing may be formed from a material that forms a carrier liquid barrier layer.

In this third aspect of the invention, the steps of (ii) transferring the printed image, (iii) coating or laminating with a carrier liquid barrier material and/or (iv) applying the adhesive may occur inline.

Any feature described in relation to the process of the first aspect of the invention that is also relevant to the third aspect of the invention may also form part of the digital printing process of the third aspect of the invention. For example, in the third aspect of the invention, the recording medium of the digital printing process may comprise paper or cardboard, such as paperboard or corrugated fibre board. In addition, the process may comprise mechanically removing a portion of the transferred carrier liquid from the recording medium.

In some embodiments, at least 50 percent of the transferred carrier liquid is present in the recording medium after the step of coating or laminating the printed image with carrier liquid barrier material. In some embodiments, at least 2 gsm of carrier liquid is transferred to the recording medium in step (ii) of the process.

According to a fourth aspect of the invention, there is provided an adhesive label material comprising a first layer of a pulp-based material such as paper or paperboard, a second layer of a carrier liquid barrier material applied to a surface of the first layer, and an adhesive material applied to the exposed surface of the second layer. In this fourth aspect of the invention, the pulp-based material forming the first layer will be configured to absorb a non-polar carrier liquid.

In some embodiments of this fourth aspect of the invention, the adhesive label material further comprises a releasable protective backing fixed to the exposed surface of the adhesive material.

In some embodiments, the protective backing may be formed from a material that forms a carrier liquid barrier layer.

Volatility test

To evaluate the degree of volatility of different carrier liquids, the following test procedure is used:

Approximately 7.5 grams of carrier liquid are added to a disposable aluminum cup. This cup is then put in an infrared heating cell (Mettler Toledo LJ16) for 2 hours where a constant temperature of 80 °C is applied.

The infrared heating cell comprises an opening in the form of a grate in the top surface of the heating cell. This grate allows evaporated carrier liquid to escape the heating cell, thereby inhibiting the condensation of the carrier liquid on the surface of the heating cell after it has been evaporated.

The mass of carrier liquid remaining in the cup after 2 hours is measured by a balance provided within the heating cell. This weight measurement allows the mass of carrier liquid that has evaporated within the 2 hour period to be determined.

The weight loss value can then be used to calculate the percentage loss of liquid from the cup over the 2 hour period: V {%) = weight loss/original weight x 100.

The resultant percentage value V gives an indication of the volatility of the liquid.

In this document, a carrier liquid with a V value below 25% is termed “a substantially non-volatile carrier liquid”. Similarly, if the V value is above 25%, the carrier liquid is called “a volatile carrier liquid”.

It has been determined that if the V value of the carrier liquid is above 25%, this indicates that optimal printing conditions cannot be guaranteed during the digital printing process. For example, the use of volatile carrier liquids may result in stripes in the print direction and thus in an unacceptable image quality. Such printing errors are thought to be caused by evaporation of the carrier liquid during the printing process as the evaporated carrier liquid is thought to contaminate components of the printing apparatus.

Therefore, the carrier liquid used in the liquid toners of the digital printing process and apparatus of the present invention may have a V value below 25%, preferably below 15% and more preferably below 10% according to the above described volatility test. Such V values typically correspond with liquids with an average boiling point above 200 °C. Preferably the average boiling point of the non-polar carrier liquid is above 250 °C, more preferably above 270 °C.

Volatility values for some carrier liquids suitable for use in the present invention are given in the table below:

Isopar L was purchased from Exxon, Lytol from Sonneborn, Cl8 internal olefin from Ineos, and 2-ethylhexyl stearate from Oleon.

Barrier test

To evaluate the barrier properties of the applied coatings, a peel test was performed. The two coatings listed in Table 1 were tested by these peel tests and the results of the peel tests are given in Table 2 below.

Table 1:

In the peel tests, either Sappi Galerie Fine paper with a weight of 90 gsm, or UPM Digi Finesse Gloss paper with a weight of 115 gsm, was used as the recording medium.

For the samples comprising the non-curable Daotan® coating, the following coating method was used: A first side of the recording medium was coated with the Daotan® compound using a wire rod on an automated wire coater (Braive instruments, type 4340M6 SP). The thickness of this coating was varied by using multiple rods with a different wire radius (type Elcometer 4360/4361). The coating was subsequently dried for 2 minutes at 100°C in a vented oven.

For the samples comprising the curable UVOPV coating, the following coating method was used: The UVOPV coating was applied using a wire rod on an automated wire coater (Braive instruments, type 4340M6 SP). Subsequently, the coating was then cured in a UV curing step comprising applying a Fusion Lighthammer system equipped with an iron doped mercury bulb of 180 W/cm. The samples were cured at 12.5 cm/s at max power output (180 W/cm).

After the carrier liquid barrier layer had been formed on the recording medium, a substantially nonpolar carrier liquid was applied to the non-coated side of the recording medium using a wire rod on an automated wire coater (Braive instruments, type 4340M6 SP). Either 2-ethylhexyl stearate or C18IO was used as the substantially non-polar carrier liquid. In these peel tests, 10 gsm of a substantially non-polar carrier liquid was applied to the recording medium.

In comparative tests 1 and 2 (Comp 1 and Comp 2 in table 1), the non-polar carrier liquid was applied in the same way to an uncoated recording medium.

Some of the samples coated with a barrier layer were subsequently exposed to a heat treatment procedure. This heat treatment procedure comprised putting the samples in a vented oven at a temperature of 100°C for 1 minute.

For all of the samples, a strip of Scotch® Magic™ tape (25 mm wide) was applied to the coating formed on the recording medium (in accordance with Finat test method 21) and the samples were left in an open atmosphere for 12h.

The Scotch® Magic™ tape was then pulled from the substrate using a peel tester (FP-2250, TA Instruments) according to the ASTM D3330-AF test method. The force required to pull the tape from the substrate is indicative of the amount of non-polar carrier liquid that migrated through the barrier and weakened the adhesive of the scotch magic tape. The force value (in Newton) of the coated recording medium without carrier liquid is defined as A, the force value (in Newton) of the coated recording medium with carrier liquid is defined as B. Typical force values range from 5-8 Newton.

The barrier value (BV) is then defined as follows:

BV = A-B

The smaller the barrier value (BV), the more non-polar carrier liquid that is blocked by the coating layer.

The barrier property of the different coating formulations and weights was ranked as follows: 1: BV < 0.6 Newton: preferred 2: BV > 0.6 Newton: unacceptable

Table 2:

From the results in Table 2 it is clear that a barrier coating is necessary to avoid a reduction in peel strength in the barrier test (compare Exl-6 with Compl-2). It can be also seen from these results that some coatings can be suitable for fatty acid esters as well as hydrocarbon carrier liquids (see Exl-4), and that different types of paper can be used (compare Exl&4 with Ex 6&5 resp.).

Brief description of 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 electrophotography recording medium after printing and coating;

Fig. 3 is a schematic view illustrating a second exemplary embodiment of an electrophotography recording medium after printing and coating;

Fig. 4 is a schematic view illustrating another exemplary embodiment of an electrophotography recording medium after printing and coating; and

Fig. 5 is a schematic view illustrating a further exemplary embodiment of an electrophotography recording medium after printing and coating.

Description of exemplary 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 printing station with a reservoir 100, a feed member 120, a development member 130, an imaging member 140, an intermediate member 150, a transfer member 160, and a fusing unit 170. 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 each shown to be provided with a removal device 133, 146, 153, for example, a scraper. This removal device could, for example, 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 the appropriate removal device 133,146, 153.

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, for example. Charging the toner causes the liquid toner dispersion to split 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 of the composition) by adding it back into liquid toner reservoir 100.

Fusing is carried out by means of a fusing unit 170, for example, a non contact fusing unit such as an IR fusing unit. Optionally, the non-contact fusing unit may be used in combination with a contact fusing unit. The non-contact fusing unit causes coalescence of the toner particles, resulting in the formation of a film that adheres to the recording medium 199 and (further) liberation of 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. 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.

It will be understood that for duplex and/or multicolour printing several such fusing units are typically available. A portion of the carrier liquid may be removed after fusing by a mechanical removal means (not shown), 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. However, a substantial portion of carrier liquid remains absorbed in the recording medium 199. The removed carrier liquid may be recycled and reused within the machine.

In the apparatus of figure 1, the printing station is followed in-line by a coating or lamination device 200 configured to coat or laminate the printed image on the recording medium 199 with a carrier liquid barrier material to form a carrier liquid barrier layer, wherein the carrier liquid barrier layer is substantially impermeable to the non-polar carrier liquid.

Figure 2 shows an illustrative cross-sectional view of an electrophotography recording medium after printing and coating or lamination. The recording medium 199 has a first surface 201 and a second surface 202 opposite to the first surface 201. On the first surface 201 a number of printed portions P are present, and carrier liquid O is absorbed in the recording medium 199. The printed portions P are covered by a first carrier liquid barrier layer 211. The second surface 202 of the recording medium 199 is provided with a second carrier liquid barrier layer 212. Therefore, in the recording medium of figure 2, the carrier liquid is contained between the two carrier liquid barrier layers 211, 212. If the printed and coated recording medium is used as a packaging material, migration of the carrier liquid into the packaged product is prevented.

Figure 3 shows an illustrative cross-sectional view of an electrophotography recording medium after duplex-printing and coating or lamination. The recording medium 199 has a first surface 201 and a second surface 202 opposite to the first surface 201. On the first and second surface 201, 202 a number of printed portions P are present, and carrier liquid O is absorbed in the recording medium 199. The printed portions P on the first surface 201 are covered by a first carrier liquid barrier layer 211 and by a second carrier liquid barrier layer 212 on the second surface 202. As with the recording medium of figure 2, the non-polar carrier liquid is trapped between two carrier liquid barrier layers 211, 212.

Figure 4 shows an illustrative cross-sectional view of an electrophotography recording medium in the form of a label after printing and coating or lamination. The recording medium 199 has a first surface 201 and a second surface 202 opposite to the first surface 201. On the first surface 201 a number of printed portions P are present, and carrier liquid O is absorbed in the recording medium 199. The printed portions P are covered by a first carrier liquid barrier layer 211 at the first surface 201. The second surface 202 of the recording medium 199 is provided with a second carrier liquid barrier layer 212, and with an adhesive outer layer 213. This arrangement helps to avoid the migration of carrier liquid into the adhesive layer 213.

Adhesive labels such as that shown in Figure 4 are often provided with a backing layer (not shown) on the adhesive surface 213. This backing layer protects the adhesive surface before use. When a user wants to apply the adhesive label to a surface, they need to peel of the backing layer to expose the adhesive surface 213. The backing layer may be made of any material that readily peels off the adhesive, for example, a siliconized material.

Figure 5 shows another illustrative cross-sectional view of an electrophotography recording medium after printing and coating or lamination. In this figure, the recording medium 220 is an absorbent layer formed on a polymeric film material 230. This film material is substantially impermeable to the carrier liquid.

In figure 5, the absorbent layer 220 has a first surface 221 and a second surface 222 opposite to the first surface 221. A number of printed portions P are present on the first surface 221, and carrier liquid O is absorbed into the body of the absorbent layer 220. The printed portions P are covered by a first carrier liquid barrier layer 211. As the absorbent layer is formed directly on the polymeric film material 230, the second surface 222 of the absorbent layer 220 is in direct contact with the film 230. This film 230 acts as a second carrier liquid barrier layer. In this way, the carrier liquid O is trapped between two carrier liquid barrier layers 211, 230. Therefore, if the printed and coated or laminated recording medium of figure 5 is used as a packaging material, no carrier liquid can contaminate the packaged product.

In other embodiments, the polymeric film may not act as a barrier for the carrier liquid. Therefore, in these embodiments, a separate carrier liquid barrier layer must be applied to the outside surface of the polymeric film.

Claims

A digital printing process comprising: (i) developing portions of a liquid toner comprising toner particles and a substantially non-polar carrier fluid; (ii) transferring the developed parts of liquid toner to a first surface of a recording medium and fusing toner particles transferred to the recording medium to produce a printed image on the recording medium, wherein at least a portion of the substantially non-polar carrier fluid transferred to the recording medium is absorbed by the recording medium, (iii) coating or laminating the printed image with a carrier fluid barrier material to form a carrier fluid barrier layer, the carrier fluid barrier layer being substantially impervious to the substantially apolar carrier fluid of the liquid toner, and wherein an apolar carrier fluid is present in the recording medium when the printed image is coated or laminated with the carrier fluid barrier material; wherein the recording medium comprises a second surface opposite the first surface, the second surface of the recording medium also being provided with a carrier liquid barrier layer; and wherein the steps (ii) of transferring and (iii) of coating or laminating the printed image are performed in-line.

The digital printing process according to claim 1, wherein the second surface of the recording medium is coated or laminated with the carrier liquid barrier material prior to step (i) of the digital printing process.

The digital printing process according to claim 1, wherein the coating or laminating of the second surface of the recording medium with carrier liquid barrier material occurs during or after step (iii) of the digital printing process.

The digital printing process according to any of the preceding claims, wherein after step (iii) of the digital printing process, the recording medium is rolled up or cut into sheets and stacked.

The digital printing process according to any of the preceding claims, wherein the recording medium is part of an adhesive label, which label comprises an adhesive layer which is applied to the carrier liquid barrier layer on the second surface of the recording medium.

The digital printing process according to any one of the preceding claims, wherein the recording medium comprises paper or cardboard, such as board paper or corrugated fiber board.

The digital printing process according to any of the preceding claims, wherein the recording medium comprises at least one synthetic polymer film.

The digital printing process according to any of the preceding claims, wherein the carrier liquid barrier layer on the first and / or second surface of the recording medium comprises at least one synthetic polymer layer.

The digital printing process according to any of the preceding claims, further comprising, prior to coating or laminating the printed image with carrier liquid barrier material, the mechanical removal of a portion of the transferred carrier fluid from the recording medium.

The digital printing process according to any of the preceding claims, wherein at least 50% of the transferred carrier fluid is present in the recording medium after coating or laminating the printed image with carrier fluid barrier material.

The digital printing process according to any of the preceding claims, wherein in step (ii) for transferring developed portions of liquid toner to a first surface of a recording medium for producing a printed image on the recording medium, at least 2 g / m2 carrier fluid is transferred to the recording medium.

The digital printing process according to claim 11, wherein 2 to 8 g / m2 carrier fluid is transferred to the recording medium.

A digital printing device comprising: (a) a printing station that is adapted to develop portions of a liquid toner comprising toner particles and a substantially non-polar carrier fluid; for transferring the developed portions to a first surface of a recording medium; and fusing toner particles transferred to the recording medium to produce a printed image on the recording medium, wherein at least a portion of the substantially non-polar carrier fluid transferred to the recording medium is absorbed; and (b) a coating or laminating device in-line with the printing station, wherein the coating or laminating device is adapted to coat or laminate the printed image on the recording medium with a carrier fluid barrier material to form a carrier fluid barrier layer, the carrier fluid barrier layer being substantially impermeable is for the non-polar carrier fluid, and wherein an non-polar carrier fluid is present in the recording medium when the printed image is coated or laminated with the carrier fluid barrier material.

The digital printing device according to claim 13, wherein the recording medium comprises paper or cardboard, such as paperboard or corrugated fiber board.

The digital printing device according to claim 13 or 14, wherein the recording medium comprises at least one synthetic polymer film.

The digital printing device according to any of claims 13-15, wherein the carrier liquid barrier layer on the first and / or second face of the recording medium comprises at least one synthetic polymer film.

The digital printing device of claim 13 or 16, wherein at least 50 percent of the substantially non-polar carrier fluid transferred to the recording medium is absorbed by the recording medium.

The digital printing device according to any of claims 13-17, wherein at least 2 g / m2 of the substantially non-polar carrier fluid is transferred to the recording medium.

The digital printing device according to any of claims 13-18, wherein 2-8 g / m2 of the substantially non-polar carrier fluid is transferred to the recording medium.

The digital printing device according to any of claims 13-19, wherein the recording medium comprises a second surface opposite the first surface of the recording medium, wherein the second surface is provided with a carrier liquid barrier layer.

The digital printing device of claim 20, wherein the recording medium is part of an adhesive label, which label comprises an adhesive layer applied to the carrier liquid barrier layer on the second surface of the recording medium.

The digital printing device of claim 20 or 21, wherein the coating or laminating device is also configured to coat or laminate the second surface of the recording medium with carrier fluid barrier material to form a carrier fluid barrier layer on the second surface.

The digital printing device according to any of claims 13-20, wherein the printing station comprises a mechanical removal means for the mechanical removal of carrier fluid from the recording medium.