Compositions: coating or plastic – Coating or plastic compositions – Marking
Reexamination Certificate
2001-04-06
2004-07-06
Bell, Mark L. (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Marking
C106S031600, C106S031650
Reexamination Certificate
active
06758888
ABSTRACT:
This invention relates to an emulsion ink for stencil printing, particularly an emulsion ink for stencil printing excellent in ink fixability, high in printing density and less declining in printing density even after printing many sheets.
The heat sensitive stencil printing system recently allows high grade prints to be obtained readily and quickly since the heat sensitive stencil making technique in which a thermal printing head is used as a perforation device with digitized image processing has been adopted. Therefore, the system has been increasingly noticed as convenient information processing terminals.
The stencil printing system has a feature of good and simple operability that requires no skilled operator, compared with offset, gravure and letterpress printing systems. In other words, the stencil printing system is advantageous in that no skill is required for operation and that no special work such as machine washing is required after completion of use. Thus, the stencil printing system does not require any specialist for operation, but is very suitable for printing a relatively small number of sheets.
Furthermore, in the heat sensitive stencil making technique using a thermal printing head, size of perforations in a stencil has been standardized, thereby facilitating control of transfer amount of ink, compared with the conventional stencil making methods in which infrared light or xenon flash was used as a heat energy source. Therefore, the possibility that such problems as set-off and blurring occur due to excessive ink transfer has decreased, and the image grade has improved dramatically.
As described above, the stencil printing system is advantageous in that printing speed is high similarly to the offset, gravure and letterpress printing systems, and also that it is easy to operate and can be installed as an office machine like PPC and ink jet printers. So, the stencil printing system has expanded its use rapidly.
For enhancing the image grade, it is proposed to improve fixability of ink onto paper and achieve a higher density of images by adding an oil-soluble dye or a mixture of an oil-soluble dye and a pigment, as a colorant, to the oil phase of a water-in-oil (W/O) emulsion ink (JP-A-06-9912), to produce a toned ink free from color separation by adding a water-soluble dye to the water phase (JP-A-05-117565), to produce an ink high in image density and good in fixability by adding 5.0 to 12 wt % of a pigment to the ink (JP-A-09-328645), to prevent set-off by letting an emulsion ink contain solid particles or waxes of, for example, a polyamide or polystyrene with an average particle size of 8 to 30 &mgr;m (JP-A-06-116525), to improve fixability on paper and storage stability by keeping an average particle size of pigments dispersed in the oil phase in a range of 0.15 to 0.4 &mgr;m (JP-A-07-179799), and so on.
These proposals are intended to improve the printing grade, but if the pigment content is increased to raise the printing density, it often happens that pigment aggregation occurs in the water-in-oil (W/O) emulsion ink. If this phenomenon occurs, it often happens that the color expected from the corresponding pigment content is not obtained, thereby causing the printing density to decline.
Furthermore, the pigment aggregation in a water-in-oil (W/O) emulsion ink depends upon dispersion state of pigments. Pigments poor in dispersibility such as phthalocyanine pigments including phthalocyanine blue and phthalocyanine green and dioxazine pigments are large in average particle size and contain numerous non-uniform coarse particles. If such pigments are used, the pigment aggregation is apt to occur in the aforementioned water-in-oil (W/O) emulsion ink.
If the pigment aggregation grows to form large lumps, it can happen that perforations of a stencil sheet are clogged. With recent increase in resolution of thermal heads, pigments are required to be still smaller than 30 (&mgr;m) in particle size. It is, therefore, increasingly necessary to prevent the pigment aggregation as far as possible for preventing the clogging of the stencil sheet, while ensuring the ink fixability and keeping the storage stability.
For improving the storage stability, it is proposed to obtain an ink excellent in storage stability and low in temperature dependence by treating a water-in-oil (W/O) emulsion ink at a high shear rate using, for example, stirring blades or rotor stator in the production of the ink (JP-A-06-192606), to achieve the fixability on paper and the storage stability by keeping the particle size of the emulsion of the water phase at 10 &mgr;m or less and the average particle size of the pigment dispersed in the oil phase at 0.4 &mgr;m or less (JP-A-07-179799), and to effectively decrease the rise of ink viscosity during storage at high temperature by keeping the sum of the water phase particles content and the carbon content of a water-in-oil type emulsion ink at 72 wt % or less based on the total weight of the ink (JP-A-09-328645). However, if the particle size of the emulsion is kept too small for making the emulsion stable, it in turn can happen that the pigment aggregation occurs.
On the other hand, for water-in-oil (W/O) emulsion inks for stencil printing, it has been a general practice to keep the ratio of the water phase weight to the oil phase weight high (many proposals are presented, for example, in JP-A-61-255967, JP-A-04-132777, JP-A-04-288375, JP-A-05-93161, JP-A-06-33007, JP-A-06-107998, JP-A-07-150091, JP-A-10-245516, etc.). However, if the weight ratio of the water phase is raised, it can also happen that the pigment aggregation occurs.
In an emulsion containing a pigment in the oil phase, it seems that the pigment aggregation cannot be prevented even in reference to the ratio by weight of water phase/oil phase. In view of this, the object of this invention is to effectively prevent the pigment aggregation in reference to another indicator.
To achieve the above object, the inventors have studied intensively the physical properties of emulsion inks in reference to various indicators, and as a result, have found that if the ratio of the volume of the water phase to the volume of the liquid components of the oil phase (i.e., volume of water phase/volume of liquid component of oil phase) is referred to as an indicator, it is possible to produce an emulsion ink for stencil printing, which is excellent in printing grade such as printing density and fixability but does not cause perforated stencil sheets to be clogged with coarse pigment particles or aggregated pigments even after many sheets have been printed. Thus, this invention has been completed.
This invention provides an emulsion ink for stencil printing, which is a water-in-oil (W/O) emulsion ink having an oil phase and a water phase, characterized in that the ratio by volume of the water phase to the liquid components of the oil phase (namely, volume of water phase/volume of liquid components of oil phase) is in a range of 1.0 to 3.5 at 23° C.
In the water-in-oil (W/O) emulsion ink used for stencil printing, the addition amount of the water phase has been generally set in a higher proportion. The reason is that it contributes to improvements of various properties by way of keeping the viscosity change due to the ink temperature change small, raising the ink penetration rate, enhancing the structural viscosity of the ink, preventing the ink from flowing out of the printing machine, and lowering the spinnability of the ink when the printing drum is separated from a printing medium. Therefore, most of the conventional inks have high water contents.
However, it was found that if the water content is made higher than a certain level in a water-in-oil (W/O) emulsion containing a pigment in the oil phase thereof, the pigment aggregation occurs. In general, the state of the maximum water content corresponds to the closest packing in terms of volume. However, the closest packing includes hexagonal closest packing and cubic closest packing, and it is actually unknown which of the closest packing patterns occur
Ogawa Hiroyuki
Okuda Sadanao
Yamada Hiromichi
Bell Mark L.
Faison Veronica F.
Fitch Even Tabin & Flannery
Riso Kagaku Corporation
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