Patent Application
20080055380
In the detailed description of the preferred embodiment of the invention presented below, reference is made to the accompanying drawings, in which:
A printed image is formed using an ink having electrically charged marking particles. Although ink such as typical ink jet inks including pigment particles can be used, so long as the other physical requirements of the inks as described herein are met, it is preferable that the ink have polymeric particles. Although clear polymeric particles can be used if desired, it is generally preferable to use polymeric particles including a dye, pigment, or other colorant. In this description, the term “marking particles” shall include said polymeric particles whether or not they have a colorant.
The ink is deposited in an image-wise fashion using appropriate ink jet deposition methods such as a continuous ink jet stream or drop-on-demand technology onto an intermediate.
Ink jet droplet pattern 3 is subjected to post-deposition processing by radio frequency drying unit 5, the processing changing properties of the ink droplets of pattern 3.
The post-deposition treatment reduces the size of the fluid droplets and changes their rheological properties. For example, the post-deposition treatment may increase a viscosity of the droplets in pattern 3.
In the embodiment of
In embodiments where the post-deposition treatment includes heating transfer surface 4, should be cooled to a temperature compatible with the type of medium 9 being printed upon before it comes into contact with medium 9. In the embodiment of
The post-deposition treatment of the droplets of pattern 3 facilitates droplet transfer while preserving dot integrity. Dot integrity is preserved when the shape (i.e. the outline of a dot on the surface of medium 9) is preserved and is consistent from dot to dot. Dots that are deformed from a geometric shape of the nozzles and the transferring surface, or droplets that have coalesced, therefore represent a loss in dot integrity.
Belt 4′ is cleaned by a pre-cleaning unit 11, which removes any remaining ink in preparation for the deposition of more droplets by nozzle array 2. If it is necessary or desirable to control the affinity of the surface of the continuous belt 4′ for the fluid droplets being deposited on it, pre-cleaning unit 11 may clean transfer surface 4 using a liquid hydrophobic cleansing agent, which may be sprayed on or wiped on.
According to this invention, an improved mechanism for ink jet imaging via an intermediate member is provided. Here an ink jet image is concentrated using radio frequency energy, provided by device 5 on an intermediate (belt 4′) prior to transfer to paper. Radio frequency (RF) drying operates over a low range of frequencies such as less than 300 MHz (13.56, 27.12, and 40.68 MHz), compared to microwave drying (915 and 2450 MHz).
Radio frequency concentration -of the ink offers advantages over microwave drying. RF drying is more discriminating toward water and therefore minimizes heating of typical intermediate materials made of plastics or rubbers. Excess heating of the intermediate member materials can lead to distortion of the intermediate member, and this can lead to artifacts such as paper cockle, image smear and/or misregistration, for example, during transfer.
The general principle of RF drying is to expose the ink image sample to an alternating electrical field at appropriate frequency. Polar molecules within the sample respond to the field by rotating. This rotation leads to friction and heating within the sample. If the materials also contain ionic species, these will also move relative to the field. Collisions of these particles with other species lead to collisions and heating. The susceptibility of a material to be polarized by the field is described by the materials dielectric permittivity, ε*. The dielectric permittivity is defined by the following equation:
ε*=ε′−iε″
where ε′ is the dielectric constant and ε″ is the dielectric loss factor.
The power per unit volume absorbed and converted into heat of any material is given by:
W=2πωE 2ε″
where ω is the frequency and E is the electric field strength. The dielectric loss factor is frequency dependent and a function of the moisture and ionic conductivity of the material. The frequency chosen depends on the characteristic of the materials to be dried. For rapid heat-up, conditions are chosen to maximize the dielectric loss factor. For heat sensitive materials, a frequency is chosen to minimize the dielectric loss factor of the dry material. The penetration depth of the energy waves is inversely proportional to frequency.
Referring again to
To prepare a suitable ink, 85.02 g of a cyan pigment dispersion containing 12.35 wt. % active Pigment Blue 15:3 pigmnent, 0.3 g of biocide Proxel® GXL from Avecia Inc. of Wilmington, Del. (17.0% active), 0.24 g of surfactant EnviroGem® AD01 from Air Products of, 0.9 g of foam suppressant DAPRO® DF 1492 from Elementis Specialties of Hightstown, N.J., and 10.44 g of a polyurethane binder polymer (28.73% active) were added together with distilled water so that the final weight of the ink was 300.0 g. The final ink contained 3.5 wt. % CYAN Pigment. 0.017 wt. % Proxel® GLX, 0.08 wt. % EnviroGem® AD01, 0.3 wt. % DAPRO′ DF 1492, 1.0 wt. % polyurethane binder polymer, and 95.103 wt. % water. The solution was stirred for several hours at 300 RPM using a Lightn™ A310 Axial Flow Impeller from Lightnin of Rochester, N.Y., and then filtered through a 1.0 μm Profile 11 polypropylene filter.
A cyan ink was prepared similar to Example 1 except that 170.04 g of the cyan pigment dispersion, and 20.88 g of polyurethane binder was used such that the final ink contained 2.0 wt. % polyurethane polymer and 7.0 wt. % cyan pigment of the total ink. Total solids equaled 11.5 wt. %, with the balance containing water.
Example 3 of a suitable ink was prepared similar to Example 1 except that 30.0 g of Glycerol humectant was added such that the final ink contained 10 wt. % glycerol of the total ink.
Inks for examples 4-6 were prepared similar to Examples 1-3 except the binder used was sulphonated polyester (SP) ionomer Eastman AQ-55® by Eastman Chemical Company of Kingsport, Tenn.
Inks for Examples 7-9 were prepared similar to Examples 1-3 except the binder used was PLURONIC® L-44 triblock (TB) copolymer binder.
Example 10 of a suitable ink was prepared similar to Example 9 except that that 5 g potassium chloride were added such that the final ink contained 5 wt. % potassium chloride of the total ink.
Ink median particle size was measured by light scattering using the Microtrac® UPA150 by Microtrac of Austin, Tex., at 25° C. Ink conductivity was measured with an Orion® Model 550 PH/conductivity meter by Thermo Electron Corporation of Waltham, Mass., at 25° C. Ink static surface tension was measured with a Krüss® digital tensiometer by Krüss GmbH of Hamburg, Germany, at 25° C. Ink viscosity was measured with an Anton Paar® viscometer by Anton Paar GmbH of Graz, Austria at 25° C.
Prior to concentrating (i.e., drying), the ink was uniformly applied to a polyimide sheet with a wire wound coating rod. With a known coated area and measured increase in weight of the inked substrate, the mass laydown before drying could be calculated. A comparison of the mass laydown after drying to before drying was then used to calculate the percentage of water removed. Typical mass laydowns before drying were 1.9 mg/cm2 over a coated area of 10.0×19.0 mm. The RF equipment used for drying was the Macrowave™ heating system from the Radio Frequency Co. of Millis, Mass. This 30 kilowatt unit operated at 40.68±0.05 kHz. The inked substrate was conveyed on a belt. The RF drying circuit consisted of a series of electrodes in contact with the belt (intermediate member 4). The distance from the first to last electrode was 2.44 meters. The web was needed to transport the sample. However, in a printing application, a polyimide belt or other suitable intermediate member would be in direct contact with the electrodes.
The scaling factor for RF drying is the time the sample is in contact with the RF electrical circuit. Each ink was run under two different RF exposures: 1.0 and 1.5 seconds. Since the total distance of the electrodes is 2.44 meters this corresponds to processing speeds of 2.44 and 1.63 meters/second. The results are listed in Table 2 below and plotted in
The effect of increased exposure is evident by the increase in the percentage of water removed. At either exposure all of the inks were concentrated to a significant degree. The lowest degree of water removal was 43% with several inks reaching 100% removal.
The polyurethane binder has the largest ionic character with 33% of the polymer containing carboxylic groups. These inks had the highest conductivities and RF drying removed the most water. The sulphonated polyester and triblock binders with 9% and 0% ionic character were less active.
The conductivity of the solutions was a strong predictor of water removal and showed that conduction is a significant heating mechanism. This is most evident at the low exposure condition where there is a larger range in the water removal data. The addition of a monovalent salt dramatically increases the conductivity resulting in complete water removal even at low exposure times and triblock binders. Values greater than 100% were achieved at the longer exposure leading to the conclusion that some of the glycerol was volatized under this condition. Monovalent salts have the advantage over divalent or trivalent salts that they can be used to increase conductivity without destabilizing the ink pigment.
The effect of glycerol on water removal was very dependent on the binder polymer used. The most significant effect was seen with the sulphonated polyester binder. The mechanism of this interaction is not clear.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
