1. Field of the Invention
The present invention relates to an inkjet recording ink, an ink cartridge, an inkjet recording method, an inkjet recording apparatus and an ink recorded matter.
2. Description of the Related Art
In recent years, an inkjet recording method has become more widely used since it is an image forming method which is performed through a simpler process than in other recording methods, which easily forms full-color images and which can form high-definite images even with a simple apparatus
The inkjet recording method is applied to an image forming apparatus where a small amount of ink is made to jet through pressure given by bubbles generated with heat or by a piezoelectric element or electrostatic force whereby the ink is attached onto recording media such as paper and then quickly dried (or penetrated into the recording media) to obtain images. The inkjet recording method has been finding its applications in printers or printing applications for industrial use as well as personal use.
In particular, demand for industrial use has recently been increased, which has been requiring responsiveness to high-speed printing or various recording media. There has also been a need of an inkjet printer equipped with a line head for responding to increase in printing speed.
Furthermore, increased demand has recently arisen for aqueous ink from the environmental or safety aspect. However, such aqueous ink depends greatly on recording media and raises various problems in images. In particular, these problems are significant when unsmooth paper is used as recording media.
The aqueous ink requires time until it is dried. In addition, the aqueous ink is highly compatible to paper and penetrates it to a higher extent. Especially for non-coated, relatively unsmooth paper, the colorant penetrates paper to reach the inside thereof and as a result the formed image is decreased in density of the colorant. This is a problem that is not involved by a solvent ink.
In order to increase the drying speed of ink on recording media to respond to the printing speed that is recently increasing more and more, a penetrating agent is added to an ink for water serving as a solvent to permeate recording media, thereby achieving quick drying.
However, incorporation of a penetrating agent increases penetration property of not only water but also colorant, leading to a considerable decrease in image density unlike inks used in paints or ballpoint pens.
In view of this, in one proposed method for increasing the image density of prints, acidic carbon black is oxidized with hypochlorous acid to control the ratio of the amount of functional groups on the surface to the specific surface area of the carbon black (see Japanese Patent Application Laid-Open (JP-A) No. 2000-319572).
However, the treatment with hypochlorous acid in the above proposal is performed in a wet process, requiring dehydration and/or washing leading to cost elevation. In addition, salts and other substances remain even after washing, and the obtained ink has not shown satisfactory storage stability when used for an inkjet process. Furthermore, it has not been satisfactory in terms of image density also when unsmooth paper was used.
Similarly, defining the amount of functional groups on the surface, the specific surface area, and other properties of carbon black is proposed in order to improve dispersion stability (see JP-A No. 2004-224955). This proposal, however, includes performing wet oxidation with peroxodisulfates and thus for the same reasons has not been satisfactory in storage stability or image density.
Also, there has been proposed use of furnace carbon black having the defined DBP oil absorption amount and acid groups (see JP-A No. 10-324818).
However, in order for furnace carbon black to have the above defined DBP oil absorption amount, the specific surface area of furnace carbon black used has to be large. In this case, even when the number of acid groups is increased by oxidation, the number of effective acid groups actually present on the carbon black surface is small. It has not been satisfactory in storage stability or image density when used for ink.
Also, there has been proposed an ink using oxidized carbon black whose volatile content is adjusted to 25% or higher (see JP-A No. 2001-164148). In this proposal, such a high volatile content makes carbon black more wet to water, resulting in improved water dispersibility. However, an increased amount of impurities during oxidation degrades the stability of the obtained ink. In addition, the carbon black becomes too high in compatibility to water, and the pigment tends to penetrate paper together with water, making it impossible to attain satisfactory image density.
Also, there has been proposed used of carbon black having the defined volatile content and the CTAB surface area/iodine value (see JP-A No. 11-349849). This proposal, however, is difficult to attain both satisfactory image density and satisfactory storage stability.
Also, there has been proposed an inkjet recording ink using a naphthalene sulfonic acid-formalin condensate as a dispersing agent for carbon black (see JP-A Nos. 2009-067907 and 2009-149815). These proposals, however, improve water dispersibility but make the ink too highly compatible to water. As a result, the pigment tends to penetrate paper more together with water, not achieving satisfactory image density.
Also, there has been an aqueous ink set containing two inks of different colors, one of which contains a water-dispersible colorant containing a surfactant dispersing agent (see JP-A No. 2008-260926). This proposal improves bleeding at the boundary between the two colors, but is not satisfactory with respect to image density.
Also, there has been proposed a recording liquid containing: carbon black having a DBP oil absorption amount of 140 mL/100 g or more and a pH of 6 or higher; and a water-soluble polymer compound containing a carboxylic acid and having an acid value of 250 mgKOH/g or more as a free acid (see JP-A No. 2000-290554). In addition, there has been proposed a recording liquid containing, as carbon black, a modified carbon black the surface of which has been coated with a metal oxide at least partially (see Japanese Patent (JP-B) No. 4389348). Furthermore, there has been proposed a recording liquid containing carbon black having a DBP oil absorption amount of 140 mL/100 g or more, a volatile content of 4% lower, a pH of 7 to 14, and a BET specific surface area of 100 m2/g or more (see JP-B No. 3907263). These proposals, however, cannot be said that they are satisfactory in terms of image density.
Meanwhile, the diameter of nozzles tends to be smaller in order to achieve high quality images and high speed processes. However, inkjet recording inks containing a pigment easily involves aggregation of its solid content as a result of evaporation of water, and thus ensuring their ejection stability is desired.
For example, there has been proposed an aqueous ink composition containing a pigment, water, an organic solvent, a resin and a surface tension adjuster for ink compositions (see JP-A No. 2011-68838). In this proposal, the organic solvent is a compound represented by General Formula: R1OCH2CH2CONR2R3 (where R1 is a C1 to C8 alkyl group, R2 and R3 are each independently a hydrogen atom, a C1 to C6 alkyl group, or a C1 to C6 alkyl group containing an ether bond) and the amount of the organic solvent is 10% by mass to 49% by mass relative to the total amount of water and the organic solvent.
However, the above proposal cannot simultaneously achieve satisfactory levels of image density on plain paper, storage stability, ejection stability, adhesion resistance and drying property.