Printing techniques can be broadly categorized into two groups: analog and digital. Common analog techniques are offset lithography, flexographic, gravure and screen printing. Inkjet and electrophotographic printing are the most prevalent digital technologies. Digital printing has an advantage over its analog counterpart in that printed output can be digitally altered, meaning that every printed page can be different. To change the printed output of an analog printer, a new set of imaging plates or stencils must be produced. Digital printing methods are more cost effective at low run lengths (number of pages), whereas at large page counts analog printing is more economical. Print quality is another vector of comparison between printing methods. Analog prints often have superior image quality and typically operate at higher printing speeds than digital printing methods.
Since the mid-1980s electrophotographic (EP) printing, commonly known as laser printing, has been a popular choice among consumers who demand high quality, professional looking printed communications. State-of-the-art commercial EP printers now have image quality that rivals lithographic offset printers, although printing speed and cost still favor analog printers for many print jobs.
Liquid electrophotographic (LEP) printing is a variant of EP printing that has superior image quality and the advantage of being compatible with a broad substrate gamut (coated and uncoated paper, plastic sheet, cardboard, folded cartons, shrink wrap and labels, for example). LEP ink uses a dielectric carrier fluid and pigmented resin as colorant particles. Electrophoretic attraction of charged ink particles to a laser exposed photoconductor forms the image, which is transferred to a heated blanket prior to final transfer to the substrate. High quality output can be achieved at print speeds consistent with many commercial printing requirements. However, analog offset printing is still favored for higher volume printing jobs.
Inkjet printers are now common and affordable and allow one to obtain photographic quality albeit at low printing speed. They are used in home printing, office printing and more recently, in commercial printing. Key advantages for inkjet technology in the commercial printing market are that printing width can be easily scaled and high print speeds are possible. Challenges facing traditional inkjet technology include a limited substrate gamut and high energy cost for removal of carrier fluid from water-based inks. Carrier fluid removal can limit print speed and areal coverage.
Inkjet inks typically include pigment particles dispersed in a carrier medium. It is a goal to have the particles remain dispersed in the carrier medium for extended periods of time. Significant sedimentation can result in poor inkjet print head performance by clogging print head nozzles or fluid channels. Also, it is a goal to have ink particles smaller than a maximum size to ensure proper inkjet print head performance. Particles above a maximum size may clog print head nozzles or fluid channels, or may adversely affect drop trajectory, leading to poor image quality.
One method for reducing sedimentation in ink compositions involves the addition of dispersants to the ink composition. Three major categories of dispersants are ionic (anionic and cationic), non-ionic and amphoteric. Stabilization of ink dispersions may be achieved with ionic dispersants, but the dispersants may give rise to highly charged compositions that are not suitable for at least some types of printing that require minimal or no charge.
Certain embodiments have other features that are one of in addition to and in lieu of the features illustrated in the above-referenced figures. These and other features are detailed below with reference to the preceding drawings.