Chapter 1

Inkjet Inks

Inkjet recording has many advantages; for example, recording can be carried out at high speed, there is little noise, coloring is easy, high resolution can be achieved and recording on plain paper can be carried out (1).

As a result of these advantages, equipment and facilities employing this recording method have become remarkably widespread. Regarding the ink used in this recording method, an aqueous ink is the most commonly used in terms of safety and odor. In the inkjet recording method, images are formed by ejecting thousands of droplets of ink per second.

Inkjet inks are well known and are typically liquid compositions comprised of a carrier liquid, colorants such as dyes or pigments, and optional additives such as thickeners and preservatives to obtain the desired properties (2). Different types of colorants may be used for inks, for example, simple color pigments and water-soluble dyes. There are monographs on the chemistry of inkjet inks (3).

Further, the state of the art in high precision traditional printing methods as well as recently emerging techniques have been reviewed (4). Micro- and nanoprinting techniques have found a number of applications in electronics, biotechnology, and material synthesis or patterning.

1.1 History of Inkjet Printing

Concise comments on the history of inkjet printing have been given (5, 6). Actually, the idea of inkjet printing traces back to the 19th century. In 1878 Lord Rayleigh studied the breakup of droplets when a pressure wave was applied (7). However, it was only in 1960 that Richard G. Sweet fabricated printed equipment based on these previous discovered principles (8). In addition, it was found that the droplets could be charged when passing a nozzle connected with an electrode.

By mounting a drop generator on a movable carriage that can scan across the paper the horizontal positioning of the drops can be achieved. Such an embodiment was fabricated by the A.B. Dick Company as the world's first inkjet printer (5).

The early products were known as continuous inkjet printers because they relied on a steady stream of droplets. Only a small fraction of these drops are needed for printing and the rest of the drops are deflected away from the paper into a gutter, where the ink is collected and possibly recirculated back to a reservoir.

The later developed thermal inkjet technology has the advantage of being able to position a drop on demand. Ink drops are not emitted continuously but only when needed for printing. This property eliminates the need for additional systems to capture and recirculate the wasted ink (5).

Early patents on modern printing technology were awarded in the 1980th (9,10), and patents are still being awarded today (11). The leading companies in this technology were and still are Canon and Hewlett-Packard (12-14). Concomitantly, literature dealing with compositions for printing inks also appeared (15-18).

In 1990, a piezoelectric inkjet printing principle was introduced by Epson (5,6,19). However, earlier patents with regard to piezoelectric elements can be found (20):

An ink jet print head for projecting droplets of ink on demand includes a pressurization chamber including at least one wall defining a vibratory plate. A nozzle is open to the pressurization chamber and defines a fluid passage through which ink is ejected. A piezoelectric element is operatively coupled to the vibratory plate which is selectively energized to vibrate the wall thereby changing the volume of the pressurization chamber to eject ink through the nozzle. A vibratory system is defined by the piezoelectric element and the vibratory plate.

Actually, until now, there is a remarkably large and still growing number of patents in the field of printers and printing inks.

1.2 Image Forming Methods

Various methods are known for forming an image on a recording medium such as paper based on an image data signal (21):

• Electrophotographic methods,
• Sublimation-type thermal transfer methods,
• Melt-type thermal transfer method, and
• Inkjet recording methods.

The electrophotographic method requires a process of forming an electrostatic latent image on a photoreceptor drum by charging and light exposure. Therefore, the system becomes complicated, resulting in increased costs of production (21).

The thermal transfer method can be conducted by an inexpensive apparatus, but requires the use of ink ribbons, which causes an increase in running costs and the generation of waste.

In the inkjet recording method, an image is directly formed by ejecting ink only to the regions of a support, e.g., paper, which should become the regions of the image. Therefore the ink is used efficiently, which results in reduced costs. Moreover, inkjet recording apparatuses are not noisy (21).

1.3 Commercial Printing

The inkjet technique is applied to both office printers and household printers. Furthermore it is increasingly being applied in the field of commercial printing (22).

In the commercial printing field, printed sheets are required to have an appearance similar to that of printed sheets obtained by using general printing paper, rather than paper that has a surface that completely blocks penetration of ink solvent into the base paper, e.g., a photograph.

However, when a solvent absorption layer of a recording medium has a thickness of 20-30 µm the surface gloss, texture and stiffness are limited.

Therefore, the application of inkjet techniques to commercial printing has been limited to posters and forms for which the restrictions on surface gloss, texture, and stiffness are tolerable.

1.4 Nozzle Design

The stability of liquid jets and the influence of nozzle design have been assessed (23). A major task of a nozzle is the efficient conversion of potential energy to kinetic energy. This is best achieved by a sudden, smooth contraction of the flow area from the supply line to the desired nozzle diameter.

The best angle of convergence seems to be uncertain. The aspect ratio of the nozzle is highly dependent on the initial jet velocity profile and the subsequent jet surface shape. Rounding and polishing of the internal surfaces of the nozzle seem to be of importance for optimal performance (23).

1.5 Classification of Inks

The inks used in the various inkjet printers can be classified as either dye-based or pigment-based (24). A dye is a colorant which is dissolved or dispersed in the carrier medium. A pigment is a colorant that is insoluble in the carrier medium, but is dispersed or suspended in the form of small particles, often stabilized against flocculation and settling by the use of dispersing agents.

1.6 Thermal Inkjet

Thermal inkjet printheads produce ink droplets from the thermal vaporization of the ink solvent (25). In the inkjet process, a resistor is rapidly heated to produce a vapor bubble, which subsequently ejects a droplet from the orifice.

This process is extremely efficient and reproducible. Modern thermal inkjet printheads for industrial graphics applications are capable of generating uniform drops of 4 p l or smaller in volume at frequencies of 36 k Hz or greater. Typical commercial thermal inkjet devices are specifically designed to vaporize water or solvents that have physical properties close to those of water, i.e., high boiling point, large heat capacity, and low molecular weight.

Nearly all of the commercial inks available for thermal inkjet systems are water-based, so they contain more than 50% water. Such aqueous inks have one or more drawbacks such as long ink dry times or poor adhesion to semiporous or nonporous substrates.

Inks with attractive performance characteristics, such as short dry times, long decap times and good adhesion when using a thermal inkjet system, have been developed (25).

These compositions contain volatile organic solvents, humectants, binder resins, and dyes. The solvents are low molecular alcohols, e.g., ethanol or methanol, and ketones, e.g., methyl ethyl ketone.

A thermal inkjet ink composition has one or more attractive features, such as short unassisted dry times of printed alphanumeric or graphic images, long decap times, good adhesion to semiporous and nonporous substrates, and safety or material compatibility with one or more components of a thermal inkjet printer (25). The decap time is the time that printer nozzles can be uncovered and idle before they will become ineffective and need to be cleaned.

1.7 Photographic Printing

The recording media used in photographic printing are (26):

• Glossy plain paper obtained by laminating an ink receiving layer and then a glossy layer on base paper as a substrate,
• A photo-like paper obtained by laminating a recording layer laminated with an ink receiving layer and a glossy layer,
• A recording layer serving as a glossy layer ink receiving layer on a resin film or resin-coated paper as a substrate.

As the demand for a recording medium having high gloss and high-quality texture has increased in recent years, photo-like paper using a substrate having enhanced smoothness has become mainstream.

A glossy layer is normally formed by coating an aqueous dispersion solution containing inorganic microparticles such as colloidal silica or alumina sol, and a hydrophilic resin serving as a binder of the inorganic microparticles onto a substrate to impart the recording layer with a function of a void forming agent capable of penetrating and absorbing an ink, and photo-like gloss. As hydrophilic resin, poly(vinyl alcohol) is mainly used.

In a glossy plain...