1. Field of the Invention
The present invention relates to an ink jet recording ink, and to an ink jet recording method, an ink jet recording head, and an ink jet recording apparatus using the ink jet recording ink. In particular, the present invention relates to a technology suitable for ink jet recording of a thermal system, and among others, a technology effective when applied to a line type recording head.
2. Description of the Related Art
Ink jet recording apparatus have advantages in their low noise, low running cost, easiness of downsizing, easiness of small-lot full-color printing, and the like, and are currently widely applied not only to a printer but also to a copying machine, a printing machine, and the like. The ink jet recording apparatus are widely used for home use, for business use, and for industrial use.
As a recording system for the ink jet recording apparatus, a thermal system and a piezoelectric system are known. Among others, the thermal system, in which ink is ejected by generating an air bubble in ink in a nozzle through heating, has advantages in a relatively simple head structure, high printing speed, high density of print pixels, and the like. Further, a structure of an ink jet recording head (also referred to as a recording head or simply referred to as a head) has also been progressed. In addition to a related-art serial system, in which printing is carried out by horizontally reciprocating the head while a sheet is fed, a line system is becoming adopted, in which a long head corresponding to a width of a sheet is used and printing is carried out under a state in which only the sheet is fed without moving the head. In the line system, a sheet is fed under a fixed long head so that printing can be carried out in a stroke. Thus, the line system is suitable for high-speed printing and is becoming adopted in the fields of large-format printing and printers for industrial use, in which high image quality and high-speed printing are required.
In the ink jet recording method of the thermal system, water evaporates from the ink in the nozzle when heated to increase the viscosity of the ink, which may cause nozzle clogging and sticking of the ink to the head. Thus, there is a problem in that an image defect is liable to be caused due to ejection failure of the ink, dot misalignment, or the like. On the other hand, in the above-mentioned serial system, the inside of the nozzle is always kept in a fresh state by operation of ejecting, toward a cap, ink in the nozzle that is thickened by evaporation (preparatory ejection) every time a scan ends. However, in the case of the line system, the head is fixed, and thus, recovery operation such as the preparatory ejection that is carried out in the serial system cannot be carried out. When nozzle clogging or ink sticking is caused in a head of the line system, it is necessary to stop the printing and recover the state of the head so that ink is normally ejected from a nozzle array. In the case of the head of the line system, when the recovery operation of the head is frequently carried out, the printing needs to be stopped every time the recovery operation is carried out, which reduces productivity. Further, when the ink firmly sticks, even a head of the serial system cannot easily recover by cleaning operation such as preparatory ejection, and thus, it is demanded that a head to which ink sticks be able to recover a normal condition by simple cleaning operation.
In view of the circumstances described above, particularly in ink jet recording using a head of the line system (hereinafter also referred to as a line type head), in order to stably print a satisfactory image without an image defect such as ejection failure or dot misalignment, and without reduction in productivity, ink jet recording ink to be used is required not to cause the image defect due to nozzle clogging even when the ink is exposed to the air.
On the other hand, as a method of inhibiting the nozzle clogging with the ink jet recording ink, use of ink containing no water, such as oil-based ink or solid ink, is conceived. However, from the viewpoint of safety, influence on the environment, and energy saving, aqueous ink is more demanded. Therefore, in the aqueous ink jet recording ink, to inhibit the nozzle clogging, various kinds of proposals have been made thus far as described below.
As the aqueous ink jet recording ink that suppresses clogging of an ejection orifice of a nozzle, for example, an ink characterized by containing ethylene urea and a self-dispersed pigment having a phosphonate group with a modified surface as a functional group has been proposed (Japanese Patent Application Laid-Open No. 2012-31350). Japanese Patent Application Laid-Open No. 2012-31350 describes that the ink jet recording ink having the above-mentioned structure achieves both of the reliability that retention of the ink in a cap for covering the recording head is inhibited and thus the sticking recovery property of the ink in the recording head is excellent and the image characteristic that an image excellent in image density and bleeding resistance can be recorded.
Further, as an aqueous ink jet recording ink that suppresses clogging of an ejection orifice, a recording liquid (ink) characterized by having a water activity (Aw) of 0.70 to 0.90 has been proposed (Japanese Patent Application Laid-Open No. S61-157566). Japanese Patent Application Laid-Open No. S61-157566 describes that, in the above-mentioned structure, a solid content is less liable to be precipitated out of the ink and the ink is less liable to change the physical properties thereof during storage of the ink, and that nozzle clogging is less liable to occur.
Further, an aqueous ink jet recording ink has been proposed, which contains a water-soluble compound exhibiting a hydrophilicity-hydrophobicity coefficient of 0.26 or more as determined by the water activity value thereof, and has defined therein the total amount of an anionic functional group of a self-dispersed pigment and an amount of cesium ions contained in the ink (Japanese Patent Application Laid-Open No. 2011-195826). Japanese Patent Application Laid-Open No. 2011-195826 describes that, in the above-mentioned structure, while the ink is promptly permeated in plain paper, an obtained image has a high optical density, and further, nozzle clogging is inhibited.
However, detailed studies conducted by the inventors of the present invention have revealed that, in the technologies described in Japanese Patent Application Laid-Open Nos. 2012-31350, S61-157566, and 2011-195826, particularly when the line type head carries out printing at high speed in the ink jet recording of the thermal system, evaporation of ink in the vicinity of the nozzle may proceed excessively to cause nozzle clogging with ink. As described above, when an image defect such as ink ejection failure or dot misalignment is caused due to clogging with ink, in the line type head, it is necessary to stop the printing and recover the state of the head so that ink is normally ejected from a nozzle array, and thus, inhibition of clogging with ink is particularly an important subject. Further, demand for higher image quality in ink jet recording in recent years reduces the size of an ink droplet to be ejected so that the opening area of a nozzle tends to be reduced. In terms of this point as well, inhibition of nozzle clogging is an important subject to be addressed. Still further, there has been a demand to develop ink with excellent resolubility and sticking recovery property, which can inhibit sticking thereof to a head and, even when sticking thereof to the head is caused, allows the head to promptly recover a normal condition by simple recovery operation irrespective of the structure of the head.
The above-mentioned problem of clogging with ink is more significant when pigment ink using a pigment as a coloring material is used compared with a case of ink using a dye as a coloring material. Particularly when a resin-dispersed pigment in which a pigment is dispersed in aqueous ink by a polymeric dispersant is used as a coloring material, clogging with ink tends to occur. Particularly in the ink jet recording ink of the thermal system, a block copolymer including a hydrophobic block and a hydrophilic block is hitherto used as a polymeric dispersant for dispersing a pigment, and by using such a block copolymer for dispersing a pigment, ink ejection stability and storage stability are attained. On the other hand, along with development of such ink for industrial use, mainly from the viewpoint of cost efficiency, studies on a polymeric dispersant to be used for dispersing a pigment have been conducted and a random copolymer is becoming available instead of a block copolymer, which is difficult to design and synthesize. However, inhibition of clogging with ink in the case where an ink in which a resin-dispersed pigment using a random copolymer as a polymeric dispersant is used as a coloring material is used in the ink jet recording of the thermal system, in particular, in the case of producing a record with the line type head, has not been fully studied partly because such ink has not been used. The inventors of the present invention have reached an acknowledgement that development of ink jet recording ink capable of stably and effectively inhibiting clogging therewith particularly in the above-mentioned case is urgently required.