NL2010807C2

NL2010807C2 - Liquid developer dispersion with reduced caking.

Info

Publication number: NL2010807C2
Application number: NL2010807A
Authority: NL
Inventors: Lode Erik Dries Deprez; Werner Jozef Johan Beeck; Wim Libaers; Dirk Marcel Constant Gijsbrechts; William Constant Waterschoot
Original assignee: Xeikon Ip B V
Priority date: 2013-05-15
Filing date: 2013-05-15
Publication date: 2014-11-24

Description

Liquid developer dispersion with reduced caking

This invention is related to a liquid developer dispersion, to a printing process using the liquid developer dispersion and a method for the reduction of caking of a liquid developer dispersion.

Liquid developer dispersions are known in the art and are used for electrophotography and developing electrostatic latent images formed on a latent image carrying member in a digital printing process. An example of such printing apparatus is provided in US patent with publication number 7,995,953, the content of which is incorporated into this application in its entirety by reference. Liquid developer dispersions, also called liquid toners, are usually made of a carrier liquid and marking particles. The marking particles are dispersed in the carrier liquid. The liquid developer dispersion comprises also dispersing agents to assist in evenly dispersing the marking particles in the carrier liquid.

A digital printing apparatus using liquid developer dispersion is often provided with means to collect excess liquid developer dispersion. Excess liquid developer dispersion is liquid developer dispersion that remains on a member and is not transferred to the substrate during the printing process. The collected excess liquid developer dispersion can be recycled and can be reused. A problem of the known liquid developer dispersions is that the excess liquid developer dispersion remaining on the member has a tendency to show caking. Caking is the formation of lumps in the dispersion resulting in a liquid where there is no uniform dispersion of the marking particles. Caking often results in an increase of viscosity of the liquid. Caking in liquid developer dispersion is the process where marking particles are no longer evenly dispersed in the carrier liquid , and wherein at least some of the marking particles no longer act as individual particles. Excess liquid developer dispersion that shows caking cannot be used for printing as such and needs to be treated first in order to re-obtain a homogeneously dispersed liquid toner which has similar conductivity and viscosity properties as the starting liquid developer dispersion. It is thought that caking is the result of marking particles that come so close into each others neighborhood on the developing member, so that they start to feel each others presence and start interacting with each other. Caking can also be the result of injecting charge on the liquid developer dispersion and applying high shearing forces which are typically present when a thin layer of liquid developer dispersion passes through a very narrow gap between two members of the printing apparatus.

It is an object of the invention to provide a liquid developer dispersion, where caking, especially caking occurring in excess liquid developer dispersion, is prevented or reduced, so that the excess liquid developer dispersion can be reused as such, or can be reused by applying only a small number of steps required for obtaining a homogenously dispersed liquid developer dispersion.

This object, amongst other objects, is met at least partially, if not completely by a liquid developer dispersion as defined in claim 1.

Especially, this object, amongst other objects, is met at least partially, by a liquid developer dispersion comprising a dispersing agent, a carrier liquid, marking particles and spacer particles.

The inventors surprisingly found that a liquid developer dispersion comprising spacer particles according to the invention shows a reduced amount of caking. Especially, the caking of excess liquid developer dispersion on a member during the printing process is reduced. Without being bound to a theory, the inventors believe that the spacer particles lay between marking particles, and so prevent cohesion of the marking particles and prevent or reduce the occurrence of caking, even during the printing process. The marking particles in the liquid developer dispersion are surrounded by dispersing agents. When caking occurs, it seems that the dispersing agents loose their function. Although the dispersing agents still reside on the marking particles, the marking particles tend to clit together upon which caking occurs. Due to the presence of spacer particles it becomes more difficult for the marking particles to come into each others neighborhood. The spacer particles are able to prevent or reduce caking occurrence of the liquid developer dispersion, and in particular the excess liquid developer dispersion. This is illustrated in figure 1 where the spacer particles 3 are represented by a spheres, the marking particles 1 by flattened particles and the dispersing agents 2 by small spheres with a coil. The diameter or the maximum size of the spacer, is larger than the length of a dispersing agent residing on the marking particle so that a space particle can lay in between two marking particles without being hindered by the dispersing agents that are bound to the marking particles.

The route that a liquid developer dispersion follows in a digital printing process can be summarized as follows. A more detailed overview of the process can be found in the patent application with application number EP 12 175 762.9, the content of which is incorporated into this application in its entirety by this reference. An amount of liquid developer dispersion, comprising spacer particles, is stored in a reservoir and applied on a supply member, transferred to a development member, subsequently transferred to an imaging member, and finally transferred, optionally via an intermediate member, to a substrate. Without loss of generality, the members are in most cases rollers. The part of the liquid toner that remains present on a member's surface following contact with its successor is called the excess liquid developer dispersion. This excess developer dispersion can be removed and collected using appropriate means after transfer of the liquid developer dispersion to its successor has occurred. The liquid developer dispersion according to the invention, that remains on a member after a transfer stage i.e. the excess liquid developer dispersion, shows a reduced amount of caking compared with liquid developer dispersion without spacer particles.

The spacer particles are preferably inert, and do not react to form any intermolecular bond with the other compounds in the liquid developer dispersion. The spacer particles remain free in the liquid developer dispersion or excess liquid developer dispersion. With "free" is meant that besides usual weak interactions between the spacer particles and the other compounds of the liquid developer dispersion, the particles are unbound.

Furthermore, in one embodiment the spacer particles are dispersed in the liquid developer dispersion. In one embodiment, the liquid developer dispersion comprising the spacer particles is a dispersed composition. In one embodiment, the spacer particles do not influence the dispersion of the liquid developer dispersion. In one embodiment, the spacer particles do not cause flocculation of the liquid developer dispersion.

The liquid developer dispersion according to this invention comprises marking particles, dispersing agent, carrier liquid and spacer particles. It may also comprise further compounds such as compounds for controlling the viscosity or charge control agent (CCA), wax, plasticizers, and other additives.

In an embodiment of the invention, a concentration of "liquid developer dispersion" refers to a concentration wherein the liquid developer dispersion has a solid content so that it can be used as such in a digital printing process. In other words, the liquid developer dispersion according to the invention has a solid content that is at working strength and does not require a dilution. On the contrary, "concentrated liquid developer dispersion" has a solid content which is too high to be used for printing and is generally diluted with carrier liquid to obtain a liquid developer dispersion which is ready to be used in a digital printing process. A typical solid content of a liquid developer dispersion is a solid content of between 10 to 30 wt%, such as a solid content of 25 wt%.

In yet another embodiment of this invention, the marking particles comprise colored particles (also called ink particles or pigment) and a binder resin, which is a polymer, preferably a transparent polymer, embedding the ink particles and other optional compounds like wax, plasticizer or other additives.

In yet another embodiment of this invention, the carrier liquid can be any suitable liquid as is known in the art, and may be silicone fluids, hydrocarbon liquids and vegetable oils, or any combination thereof.

In yet another embodiment of this invention, the dispersing agent is able to create a dispersion of marking particles in the carrier liquid.

In one embodiment according to the invention, the spacer particle is a silica particle. Preferably, the silica particle is a colloidal silica particle. In one embodiment, the silica particle is hydrophobic. In another embodiment, the silica particle is dispersed or solved in oil and forms a colloidal silica or a colloidal hydrophobic silica. In another embodiment, the silica particle is silicium oxide, preferably hydrophobic silicium oxide. Preferably, the spacer particle is selected from the group consisting of fumed silica such as aerosil RX50, aerosil R972, aerosil RY50, aerosil NAX50; aerosil NA50, Aerosil 50 from Evonik Industries and Cabosil P28 and P30 from Cabot Corporation; aluminium dioxide and titanium dioxide such as Aeroxide T805 and Aeroxide C805 from Evonik Industries, or a mixture thereof .

In one embodiment, the spacer particle is a combination of more than one kind of spacer particle.

Preferably, the spacer particles do not interfere substantially with the digital printing process. Preferably, the liquid developer dispersion with the spacer particles, according to the invention, do not influence significantly the maximum speed of the printing process. Advantageously, the liquid developer dispersion is adapted to be used in a high speed digital printing equipment with a print speed of more than 50 cm/s. Furthermore, it is preferred that the spacer particle has no significant influence on the conductivity of the carrier liquid and on the chargeability of the marking particles. Surprisingly, the inventors have observed that spacer particles according to embodiments of the invention do not significantly interfere with the printing process and there is no significant influence on the electrophotographic properties of the liquid developer dispersion.

In another embodiment according to the invention, the spacer particle has a particle size between 5 and 200 nm, preferably between 30 and 100 nm, and for example about 50 nm. To realize the most efficient reduction in caking, the diameter of the spacer particle is preferably larger than the length of the dispersing agent.

In yet another embodiment, the spacer particles have a concentration in the liquid developer dispersion that is sufficient to reduce caking.

In one embodiment, the spacer particles have a concentration of between 0.8 wt% and 28 wt%, preferably between 1.2 wt% and 20 wt% and most preferably between 2 wt% and 12 wt% with regard to the weight of the marking particles in the liquid developer dispersion.

According to another aspect, the invention relates to a method for preparing a liquid developer dispersion comprising the addition of spacer particles to a dispersion of a carrier liquid, dispersing agent and marking particles.

In one embodiment according to this aspect, the spacer particles are added to concentrated liquid developer dispersion, whereupon the concentrated liquid developer dispersion is diluted with carrier liquid to obtain liquid developer dispersion with a solid content at working strength.

According to another embodiment the spacer particles are added to the dry marking particles, whereupon the dispersion is prepared using the mix of spacer and marking particles .

The technical effects and advantages of the various embodiments of the method of reducing caking of collected excess developer dispersion according to the present invention correspond mutatis mutandis to those described above in connection with developer dispersion according to the invention.

In another aspect, the invention is related to the use of a liquid developer dispersion according to invention in a digital printing process.

The technical effects and advantages of the various embodiments of the use of the liquid developer dispersion in a digital printing process according to the present invention correspond mutatis mutandis to those described above in connection with developer dispersion according to the invention.

The above generally describes the invention but to assist with understanding, reference will now be made to the figures and non-limiting examples which shows embodiments of the invention.

Figure 1 is a representation of a liquid developer dispersion according to an embodiment of the invention;

Figure 2 is a schematic diagram of a system according to a first embodiment of the present invention; and

Figure 1 illustrates schematically a liquid developer dispersion according to the invention. The spacer particles 3 are represented by spheres. The dispersing agents 2 are represented by small spheres with a coil. Marking particles 1 are represented by flattened particles. The area represents liquid in which the spacer particles, dispersing agents and/or marking particles reside.

Figure 2 schematically illustrates the application of an amount of liquid developer dispersion 100, initially stored in a liquid developer dispersion reservoir 110, also called main reservoir, via a toner supply member 120, a development member 130, an imaging member 140, and an optional intermediate member 150, to a substrate 199.

Without loss of generality, the aforementioned members are all illustrated and described as rollers. The development member 130, imaging member 140, and intermediate member 150 all transfer part of the liguid developer dispersion 100 adhering to their surface to their successor; the part of the liquid developer dispersion 100 that remains present on the member's surface, thus the excess liquid developer dispersion, is removed after the transfer stage by appropriate means. These means are schematically illustrated as respective removal means 133, 146, 153.

The excess liquid developer dispersion removed by scraper 133 is added to a reservoir 180. Optionally the excess liquid developer dispersion removed by scrapers 146 and 153 may be added to the same reservoir 180.

Alternatively there may be provided separate reservoirs/lines for the collection of excess liquid developer dispersion removed by scrapers 146 and 153.

Optionally dispersing composition and/or carrier liquid may be added to the excess liquid developer dispersion before being removed, see reference numeral 154.

Optionally the collected excess liquid developer dispersion 186 is mixed using a suitable mixing means 181. Optionally, the collected excess liquid developer dispersion is brought to a solid content (SC) at a level suitable for reintroduction in the main reservoir 110. Carrier liquid optionally comprising spacer particles may be added to the excess liquid developer dispersion. To that end there are provided carrier liquid adding means 183 and toner adding means 184. The solid content is measured using a solid content sensor 185 in the line between the second reservoir 180 and the main reservoir 110, and there is provided a controller 182 to control the amount of carrier liquid and the amount of toner concentrate to be added based on the measured solid content.

Example

Effect of spacer particles on conductivity and caking level of the liquid developer dispersion

Test system

As there are no standard measurement tools for caking as encountered in a liquid toner electrophotographic process, a test system was built consisting of a simplified version of the apparatus described in Figure 2. The system comprises a liquid developer dispersion reservoir, a toner supply roller, a development roller, a thickening corona for toner compaction on the development roller and a polyurethane scraper blade to remove excess developer from the development roller. As the test system has no photoconductor, all toner applied to the development member is considered excess toner. Hence this test system makes it possible to perform worst-case caking experiments.

The test system can be run at various levels of thickening corona intensity and roller speed (typical surface speed = 60 cm/s). The caking level is evaluated by visual inspection of the toner on the scraper blade and ranked as follows: 0 refers to no caking, meaning that a very good result is obtained.

1, 2, or 3 refers to caking levels where small acceptable amounts of caking occurs, where 1 refers to a good result, 2 refers to a moderate result, and 3 refers to a result that is just acceptable.

4 refers to a caking level that is not acceptable.

5 refers to severe caking.

Conductivity measurement

The conductivity is measured with a device as described in the patent application with number EP1120647 filed in the name of the applicant, where the device has an electric field strength of 1.25x10s V/m.

Liquid developer dispersion A liquid developer dispersion comprising a marking particle, a carrier liquid and a dispersing agent is prepared. The ingredients used to prepare the marking particles and the liquid developer dispersions are summarized in table 1.

(1) measured according to ASTM D3418 (2) the amount of dispersing agent that is needed to neutralize 1 mol of acid

Table 2 shows the composition of the marking particles. The marking particles are prepared by kneading the ingredients of table 2 at a temperature of 100 to 120°C for 45 minutes. This mixture is cooled down and milled down to obtain particles with a size of aboutlO pm using a fluidized bed mill.

Afterwards a liquid developer dispersion having the composition as indicated in table 3 is prepared. A predispersion of the ingredients is made and stirred for 10 min at room temperature. The predispersion is brought into a liquid milling device. The liquid developer dispersion is milled down to a dv50 of 1.5 to 2.5 pm with a bead mill. The milling was done until the desired particle size, viscosity and conductivity was obtained.

Results

The inventors performed a printing test using a diluted liquid developer LD1, having a solid content (SC) of 25% in the test system as described above. Spacer particles were added to the diluted liquid developer dispersion. Several concentrations, and types of spacer particles were tested as depicted in table 4. Table 4 shows the caking of the excess liquid developer dispersion comprising the spacer particles using the test system as described above.

Table 4 shows that liquid developer dispersion comprising spacer particles reduces caking. Furthermore, it can be observed that the toner conductivity does not change significantly, indicating that the electrophotographic properties are not adversely affected by the presence of the spacer particles.

Claims

Liquid development dispersion comprising: - marking particles in a carrier liquid, which marking particles comprise colored particles and a binder resin, - dispersing agent for dispersing the marking particles, in particular present on the marking particles, and - free spacer particles dispersed in the development dispersion.

The liquid development dispersion of claim 1, wherein the spacer particles are colloidal particles, preferably hydrophobic colloidal particles.

The liquid development dispersion of claim 1, wherein the spacer particles are silica particles and preferably hydrophobic silicon dioxide particles.

The liquid development dispersion according to any of claims 1 to 3, wherein the spacer particles are selected from the group consisting of fumed silica, aluminum dioxide, titanium dioxide or a mixture thereof.

The liquid development dispersion of any one of claims 1 to 4, wherein the spacer particles have a particle size between 30 and 100 nm, preferably between 40 and 60 nm and more preferably about 50 nm.

Liquid development dispersion according to any of claims 1 to 5, wherein the spacer particles have a concentration between 0.8% by weight and 28% by weight, preferably between 1.2% by weight and 20% by weight and more preferably between 2% by weight and 12% by weight relative to the weight of the marking particles in the liquid development dispersion.

The liquid development dispersion of any one of the preceding claims, wherein the dispersant is on the marker particles, and wherein a spacer particle has a larger size than a length of a dispersant molecule on a marker particle.

A method for preparing a liquid development dispersion according to any one of claims 1 to 7, comprising the step of adding spacer particles, marking particles and dispersants to a carrier liquid.

The method of claim 8, wherein the spacer particles are silica particles.

The method of claim 8 or 9, wherein the spacer particles are hydrophobic and are preferably hydrophobic silica particles.

The method of any one of claims 8 to 10, wherein the spacer particles are selected from the group consisting of fumed silica, aluminum dioxide, titanium dioxide or a mixture thereof.

The method of claim one of claims 8 to 11, wherein the spacer particles have a particle size between 20 and 100 nm, preferably between 40 and 60 nm, and more preferably about 50 nm.

A method according to any one of claims 8 to 12, wherein the spacer particles are administered in a concentration of between 0.8% by weight and 28% by weight, preferably between 1.2% by weight and 20% by weight and more preferably between 2% by weight % and 12% by weight relative to the weight of the marking particles in the liquid development dispersion.

Use of a liquid development dispersion according to any one of claims 1 to 7 in an image-forming device, preferably a digital printing device.

A printing process in which liquid development dispersion is used, in which process excess liquid development dispersion remains on a member and is not transferred to a substrate, which excess liquid development dispersion is collected for reuse, characterized in that the development dispersion according to any of claims 1-7 is used to prevent or limit caking of the excessive liquid development dispersion.