Patent Application
20200207074
This disclosure relates generally to systems for separating solvents and other fluids, such as water, from an ink droplet without using evaporation and methods of operating the same.
Inkjet printers function by ejecting small droplets (typically on the order of 1-10 picoliters) of ink in a directed fashion onto media underneath a print head. Contact of the ink droplets onto the paper forms picture elements that collectively constitute a printed image. In general, darker or lighter areas of an image require more or less ink, respectively, per unit area of the paper.
However, the use of water-based inks results in penetration of a large amount of water into the paper, or other print substrate. This creates a need for additional drying in order to enable fast printing speeds, causes undesired deformation of the paper, and places challenges on print quality due to lateral spreading of the ink. Non-aqueous solvents, which can have a much lower latent heat of vaporization than water, are not a viable alternative due to added operating costs and safety concerns arising from the production of large amounts of flammable vapors that also present health risks if inhaled. Slow-evaporating non-aqueous solvents have many of the same issues as with water. Removing a significant fraction of the water content, or other solvent(s), from ink droplets between the time they are produced (e.g., by jetting) and the time they impact the paper would mitigate or avoid these issues.
Embodiments described herein are directed to an apparatus. The apparatus comprises a solvent permeable transfer substrate having a first surface and a second surface opposite the first surface. An ejector is configured to eject a droplet comprising at least one solvent onto the first surface of the transfer substrate. A reservoir comprises a draw solution and is configured to place the draw solution in contact with the second surface of the transfer substrate, and a print substrate is configured to contact a portion of the first surface of the transfer substrate.
Other embodiments are directed to a system. The system comprises a solvent permeable transfer substrate having a first surface and a second surface opposite the first surface. An ejector is configured to eject a droplet having a first osmotic pressure and comprising at least one solvent and at least one other component onto the first surface of the transfer substrate. A reservoir comprises a draw solution having a second osmotic pressure, higher than the first osmotic pressure, and the reservoir is configured to place the draw solution in contact with the second surface of the transfer substrate. A separator is coupled to the reservoir and is configured to separate the draw solution from the at least one solvent, and a print substrate is configured to contact a portion of the first surface of the transfer substrate.
Further embodiments are directed to a method. The method includes placing a droplet comprising at least one solvent onto a first surface of a solvent permeable transfer substrate. At least a portion of the solvent from the droplet is transported across the transfer substrate to a second surface of the transfer substrate, where the second surface opposes the first surface. The method further includes transferring the at least one other component and solvent remaining on the first surface of the transfer substrate to a print substrate.
The above summary is not intended to describe each disclosed embodiment or every implementation of the present disclosure. The figures and the detailed description below more particularly exemplify illustrative embodiments.
The discussion below refers to the following figures, wherein the same reference number may be used to identify the similar/same component in multiple figures. However, the use of a number to refer to a component in a given figure is not intended to limit the component in another figure labeled with the same number. The figures are not necessarily to scale.
Inkjet printing with water-based inks results in penetration of a large amount of water into the paper, or print substrate, which causes undesirable deformation of the paper and possible degradation of print quality. Some systems for in-flight drying of ink droplets have been proposed to counteract the above-described issues with ink solvents such as water. For high-speed commercial printing, dryers are employed to rapidly remove water from the printed paper. Inks that utilize non-aqueous solvents with low vapor pressures (i.e., that have low rates of evaporation) also face the same challenges. To date, dryers have focused on evaporative removal of solvent, whether through air movement or by employing heaters.
However, the high jetting speed (about 5 m/s) and short distance between the print head and paper (about 1 mm) result in very short flight times (about 100-200 us) during which solvent from the ink droplets must be removed. For example, in order to remove 90% of the water from a 10 pL (picoliter) droplet of aqueous ink within 200 us, a volumetric power density of 10 MW/mL is required. This assumes that all the input energy is used to overcome the latent heat of vaporization of water and is not radiated to the environment.
Such a high volumetric power density is difficult to achieve. Laser pulses can deliver a large amount of energy to a focused area in a small amount of time, but the uneven absorption of laser energy results in droplet expansion and fragmentation. A 10 MW/mL calculated power density has been reported using a dielectric (microwave) heating system incorporated into a microfluidic cell, but the cell required fluorocarbon oil as a carrier medium to transport the water droplets. Because individual water droplets were surrounded by liquid of a different phase, no evaporation was possible. Moreover, the absorbed power density was found to be much lower at around 0.01 MW/mL.
Both laser pulsing and dielectric heating require expensive equipment that may outweigh any benefits arising from reduced drying requirements. Alternatively, the time between droplet production and impact could be substantially increased, thereby lowering the power requirements for ink evaporation. This is commonly achieved through use of an intermediate transfer surface, which can ensure that droplet registration is maintained until final transfer to a print substrate. Even then, evaporative removal of ink solvents presents challenges. The high temperatures required for speedy ink drying are detrimental to ink ejectors, which necessitates the use of heat shields or an extended length transfer belt for thermal management. Evaporated solvent from the ink can also recondense and collect on cooler surfaces before it has a chance to be vented, which is highly undesirable.
Alternative approaches, described in various embodiments herein, are directed to systems and methods for non-evaporative removal of solvents from inks utilizing an osmotic process. For example, ink droplets are jetted onto an intermediate transfer substrate surface. The transfer substrate is a membrane such as a forward osmosis membrane which a first surface receiving the droplets and a second, opposite facing surface in contact with a draw solution (e.g., having high osmotic pressure, such as a brine; having a high affinity for water, such as a concentrated aqueous solution of glycerol; or being an organic solvent, such as ethanol). The osmotic pressure of any particular ink and draw solution pair is defined herein with respect to a pure solvent which comprises the largest solvent component of the droplet. Small molecule solvents (typically having a molecular weight lower than 200 grams/mole, including water) are transported across the membrane surface through osmosis, thereby reducing the ink volume on the intermediate transfer substrate surface. The transfer substrate could comprise other types of semi-permeable membranes, which can also transport solvents, such as reverse osmosis membranes, microporous membranes, and ion exchange membranes. The ink is subsequently transferred to a print substrate after the droplet dries to a predetermined degree.
Turning to
Because printer inks include a pigment or dye in a mixture of different solvents, the rate of solvent transport would not be consistent over time. For example, the rate would slow over time due to the inorganic particles/dissolved materials/solutes holding onto solvent (e.g., water) as the volume of water in an ink droplet decreases. However, since the presence of other organic compounds, which can be thought of as solutes in water that themselves affect the osmotic pressure of the ink, will hinder the transport of water from the ink into aqueous draw solution, an organic solvent may, in some embodiments, be a more effective (e.g., increased flux) draw solution for aqueous printer inks.
In addition, the rate of solvent transport will be quickly retarded if the transported solvent is allowed to accumulate on the second surface of the membrane. Thus, the draw solution is circulated during operation using an agitator. While on a smaller scale this may be accomplished by agitating the draw solution with magnetic stirrers and/or pumps, commercial printing processes may require larger-scale agitator configurations.
In certain embodiments, the draw solution is further agitated by a second plurality of rollers 214 positioned within the draw solution reservoir 226. The second plurality of rollers 214 contact the draw solution 206 and bring a portion of the draw solution 206 in contact with the second surface 230 of the membrane 204, while also removing transported solvents from the immediate vicinity of the second surface 230 of the membrane 204. Agitation also serves to control concentration gradients within the draw solution. Alternatively, the draw solution 206 can be pumped in such a way that it comes in contact with, and flows relative to, the second surface 230 of the membrane 204. The flow distribution of draw solution 206 can be controlled by appropriate channels or fins, in order to reduce or minimize spatial variations in the removal of transported water away from the second surface 230 of the membrane 204.
An ejector 324 jets ink droplets 302 onto an external surface of the membrane 304. The solvent transport then proceeds as described above through osmosis. The draw solution 306 is flowed through the tube thereby removing solvents from the ink droplets 302 away from the back, internal side of the membrane 304, through the openings 308. The tube 305 is rotated in conjunction with the membrane 304 (e.g., via an axle through an open center portion of the tube 310, or through frictional contact with a different roller), as indicated by arrow 312, thereby moving the droplet 302 away from the ejector 324 and towards eventual transfer to a print substrate. The rotation serves to agitate the draw solution 306 within the tube; however, additional agitating elements may be included within the draw solution reservoir. Agitation also serves to control concentration gradients within the draw solution. For example, in certain embodiments, a concentration gradient along the angular dimension of the tubular draw solution reservoir may be preferred over a concentration gradient in the lengthwise dimension.
In certain embodiments, the draw solution is recycled or disposed of. If the draw solution is inexpensive (e.g., aqueous sodium chloride), and the components of the ink that are transported can be safely released to the environment, simple disposal of the draw solution may suffice. However, if the draw solution or transported ink components are expensive or hazardous, the draw solution can be regenerated, i.e., purified, e.g., through distillation. The distillation process is similar to thermal processes for water desalination. For example, ethanol is more volatile than many components in aqueous inks. If ethanol is used as a draw solution, it can easily be separated from the transported ink components, which themselves could be reused in a new batch of aqueous ink. If a solution of a nonvolatile inorganic or organic material is used as the draw solvent, distillation could remove the comparatively more volatile draw solution solvent as well as water and solvents that have been absorbed from the ink, thereby reconcentrating the draw solution. Reusing the solvents absorbed from the ink could enable significant cost savings with respect to the cost of the ink.
Using aqueous ink and ethanol as the draw solution as one example, the stream 410 may include ethanol, water, glycerol, and other organic solvents originally present in the ink such as 2-pyrrolidinone. The purified stream 416 then comprises mostly ethanol and is recycled back to the draw solution reservoir 418. The recycling of the fluid can also support agitation of the draw solution 406 within the reservoir 418. The remaining at least one stream 414 may include the less volatile components of stream 410 such as water, glycerol, 2-pyrrolidinone, and other organic solvents. These components may be disposed, separated further, and/or recycled for further ink production. Using aqueous ink and a concentrated aqueous glycerol solution as another example, the stream 410 may include water, glycerol, and other organic solvents originally present in the ink such as 2-pyrrolidinone. The purified stream 416 then contains a lowered water content and the remaining at least one other stream 414 comprises mostly water.
Processes for separating solvents from an ink are further described in connection with
The net effect of the non-evaporative ink drying system is to remove solvents (such as water) from inkjet inks without having to work against their low volatility and high latent heat of vaporization. Preliminary experiments on water and aqueous inkjet inks, discussed below, show that the rate of solvent transport is sufficiently fast for these systems to be reasonably sized and that subsequent transfer onto a print substrate is not problematic. Incorporation of a non-evaporative ink drying system into a printer would allow for large energy savings while removing the need for complex thermal management that is otherwise necessary when using powerful thermal dryers. Any energy inputs would be limited to an optional regeneration of the draw solution, which itself could be separately located from the printer at a separate facility. Solvents removed from the ink could be reused to make more ink, which would lead to lower environmental impact and increased cost savings.
Preliminary validation of the osmotic ink drying was completed using a cellulose triacetate (CTA) forward osmosis membrane purchased from FTSH2O. With a draw solution of 4 M sodium chloride solution in water, a 10 μL water droplet sitting on the surface of the membrane has a roughly hemispherical shape and is completely transported in about four minutes at room temperature. This translates to a water flux of 17 L/m2h. At this flux, complete water transport will occur within 2.4 seconds for a 10 pL water droplet.
In place of the concentrated salt solution (i.e., brine), an organic solvent that contains little water can also serve as a good draw solution for water transport, even when the donating fluid contains other solvents or solutes which would be expected to increase in concentration as more water is transported. With absolute ethanol as the draw solution, a 10 μL water droplet is completely transported across the membrane in about three minutes.
Simple qualitative experiments were performed with two different ink compositions (A and B). Composition A is a mixture of solvents that closely resembles a commercial aqueous pigmented ink (Collins PWK 1223), but which omits any pigment and dissolved latex to facilitate experimental observation. Composition B is a pigmented black ink (Impika A0011533 HD2) that contains carbon black as the pigment. The compositions of the inks are further detailed below in Table 1A (composition A) and Table 1B (composition B).
Both ink formulations were tested on the forward osmosis membrane with either 4 M sodium chloride or absolute ethanol as the draw solution. The results of the observed behavior of various fluid droplets (water, Composition A, and Composition B) with respect to various draw solutions (none, 4 M sodium chloride, and ethanol) on the FTSH2O forward osmosis membrane are compiled in Table 2 below.
While no transport of Composition A could be visually observed after ten minutes, Composition B showed signs of drying at the edges of the drop after about two minutes when using ethanol as the draw solution. Thus, not every ink is compatible with the forward osmosis membrane. Even though about 80% of Compositions A and B are shared components, there is a large difference in their transport across the forward osmosis membrane. Therefore, care should be taken to identify any compounds which could react with, or deactivate, the membrane. The observed behavior of the different ink compositions under different conditions was based on visual observation because it is difficult to estimate the volume of a liquid droplet solely from its appearance. Thus, the results of Table 2 are qualitative, not quantitative. However, it was clear whether or not solvent transport occurred for each set of conditions.
For quantitative analysis, a bench top test setup was constructed as shown in
The quantitative results of solvent flux measurements for a variety of draw solutions, listed in order of increasing strength/flux, are summarized in Table 3 below.
Using the two strongest solvents above and the quantitative test setup, the transport of various inks was again tested. The results of solvent flux measurements for a variety of ink compositions with respect to these two draw solutions are summarized in Table 4 below.
Again, differing ink compositions have different transport rates using the same draw solution. However, with respect to the Impika HF008R13243, the stronger draw solution, 40% LiCl, had faster time to dry that was consistent with the increased drying time with respect to the water data provided in Table 3. Cabot Cab-o-jet 300 and DyStar Jettex SDP Black are black pigment dispersions that are typically mixed with other cosolvents to form an ink formulation. They have comparatively higher transport rates because of the absence of any cosolvents which have their own contributions to the osmotic pressure of the ink.
After a sufficient volume of solvent is removed from the ink droplet (e.g., typically 10-90%), the ink is transferred from the first surface of the transfer substrate to a print substrate. As discussed above, the transfer substrate, i.e., membrane, used herein was cellulose triacetate (CTA). It is a material that has been optimized for water transport by being nonporous and surprisingly hydrophobic. This may be seen
These properties enable ink that has been jetted onto the membrane surface to be easily transferred to paper (or other print substrates) with little retention on the membrane. This was demonstrated by directly printing 5 mm×5 mm squares of black ink (Impika HD2 A0011533) onto a CTA membrane using a Dimatix 2800 printer. The squares were then immediately pressed onto paper at a pressure of about 10 psi. Most, but not all, of the ink was transferred. This is shown in
However, the membrane was observed to dry out and become more brittle after about five minutes if printing were to be done onto a membrane that had just been blotted dry. Thus, printing was done with a very small volume of water trapped between the membrane and an underlying plastic sheet substrate to keep the membrane hydrated indefinitely as long as liquid was present under it. The ink was also observed to form droplets instead of spreading evenly on the membrane surface. This may be due to the surface energy of the membrane being too low or the surface not being perfectly flat due to the woven polymer reinforcement under it.
As set forth above, various embodiments directed to non-evaporative drying of ink can be implemented to improve printing processes. The process and system can remove volatile components such as water without evaporation or vapor condensation issues. Without the thermal components and management, non-evaporative separation consumes less energy, does not require thermal protection for other equipment, and the increased droplet drying speed enables small printing device sizes.
Unless otherwise indicated, all numbers expressing feature sizes, amounts, and physical properties used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings disclosed herein. The use of numerical ranges by endpoints includes all numbers within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5) and any range within that range.
The foregoing description has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. It is also not intended to limit the embodiments to aqueous inks or inks that contain water. Many modifications and variations are possible in light of the above teachings. Any or all features of the disclosed embodiments can be applied individually or in any combination and are not meant to be limiting, but purely illustrative. It is intended that the scope of the invention be limited not with this detailed description, but rather, determined by the claims appended hereto.
