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Beautiful Images in Print8th Installment

Beautiful Images in Print8th Installment

In this column we explore how viscosity measurement plays an active role in our daily lives. The subject of this particular column is ‘ink’. If we think of a product closely associated with ink, the ballpoint pen may come to immediate mind. We may also understand how the ink used in ballpoint pens, including control of ink viscosity, have significantly affected the design and writing performance of these writing instruments.
In this column, we shall also examine a product which has become very familiar to all of us in recent years as a component of our home personal computer system - the inkjet printer. Inkjet printers employ precise control of ink to create photographic quality, high resolution, vivid color image prints. Viscosity is key in the manufacture of the ink used in these printers.

Writing performance of the ballpoint pen - a brief history

We may gain a better appreciation of the nature and characteristics of ink by understanding how a ballpoint pen works. Ballpoint pens have become an integral, taken-for-granted, part of our lives. We are all familiar with the sensory experience of writing with ballpoint pens and can thus relate to the writing performance of these products.
Ballpoint pens may be broadly grouped according to type, writing performance, and ink composition. Although the idea for ballpoint pens already existed at the end of the 19th century, it was Ladislas Biro, a Hungarian newspaper editor, who developed a working, practical pen in 1943. The inks of this period were high viscosity oils which contained dissolved dyes. Although writing with these ballpoint pens was notably smoother than with metal-nibbed pens, the ink used had a tendency to harden quickly, left impressions on the reverse side of the paper and was susceptible to water smears.
These oil-based inks were followed by the development of water-based inks. The low viscosity characteristics of water-based inks however, presented other challenges such as leakage problems. Pentel was the first company to introduce a commercially viable product - the Ball Pentel (first domestic sales in 1972) - which used water-based inks. This water-based ink ball point pen employed a tip made of a resin and the writing ‘feel’ achieved by this pen offered a heretofore new and favorable experience in writing. Older readers of this column may probably remember their pleasant surprise when writing with the Ball Pentel for the first time.
Control of smudging and leakage was the focus of further pen design efforts. Among the various design approaches taken emerged the water-based gel ink ballpoint pen. First developed (in the world) by the Sakura Color Products Corp. in 1984 and named the “Ball Sign” pen, the product skillfully employed the viscosity characteristics (thixotropy) of ink. While the ink (gel) remained inactive in the pen, stable, high viscosity characteristics similar to oil-based inks were maintained. As the ball in the pen tip was rotated during the writing movement however, shear was applied and ink viscosity fell to the consistency of water-based inks, providing a smooth writing experience. In addition, as the ink permeated the paper, it reverted to its smear-resistant gel state.
As an anecdote, the evolution of ink began with the invention of the Gutenberg type press printer. In the Middle Ages, transcriptions to sheets of sheepskin were with ink which were mixtures of wax or soot and gum resin, water, and, in some cases, metallic powder. Such inks were not particularly suitable for type presses and this led to further development of new types of inks. The ink utilized by Gutenberg seems to have been a mixture of various types oils, mercury sulfide, and other ingredients.
In later years, the advent of the fountain pen again spurred new progress in the development of writing inks.

Inkjet printers

It can be said that performance improvements achieved in home-use inkjet printers can be related to the historical innovation in ballpoint pens. The quality sought in prints of inkjet printers include good color and good environmental resistance (against water smudges, color fading due to sunlight and gases, etc.). There are also demanding technical requirements - such as uniform ink drop formation. Indeed, one may wonder about the type of ink most suitable for inkjet printers. To gain an insight on this issue, it is important to understand how inkjet printers work.
Inkjet printers ‘print’ by spraying fine droplets of ink at high speed onto paper. The spot created by the particle on the paper is called a ‘dot’. Characters and images are formed by the linking and banding together of these fine dots. Instead of transferring inked images on printing plates to paper by the direct contact impact method of conventional printing, the distinguishing feature of inkjet printers is the non-contact, spray method of ink transfer of an image to paper by means of tiny nozzles incorporated in the printer’s ink supply heads. There are two main technologies employed by inkjet printers to form droplets. One employs tiny electrical charges which deforms a metal plate to force tiny amounts of ink out of the nozzle (piezo method). The other method employs heat which vaporizes the ink to create bubbles that expand to push the ink out of the nozzle (bubble jet, thermal jet).
The quality of the printer is defined by the accuracy of the spray units described above and the properties of the ink and paper (plain paper, coated paper, printing paper, etc.). With regard to print quality, there have been remarkable improvements in ink performance in recent years. Vivid prints are now obtainable regardless of the kind of paper selected. Most inks used in home inkjet printers are dye-containing, water-based inks and the stability obtainable in the spraying of such inks is a factor in the fine colors achieved. On the other hand however, there are drawbacks with such inks as printed results are susceptible to wetness and are prone to smudging, for example. Consequently there have been further innovations in coloring agents which utilize pigments. Pigments, instead of permeating paper, form colors by affixing themselves onto the surface of the paper. This type of ink provides better resistance to wetness-caused smudging.

The difference between dyes and pigments

Inks are composed of solvent, coloring agents, and additives. They are classified according to the different types of solvent and coloring agents used and are applicable whether the products are ballpoint pens or printers. Simply, inks which use oil solvents are called oil-based inks and inks which use solvents of water are called water-based inks. These inks are further classified according to differences in coloring agent − dye-containing, water-based inks; pigmented, oil-based inks, etc.
How do dyes and pigments differ? Both dyes and pigments function as coloring substances. Dyes are of unimolecular structure and are dissolved in solvents such as water or oil to form color images when absorbed by the paper. Pigments however do not dissolve in solvents, but instead remain suspended in the solvent as particles Color is provided by the particles themselves. These structural differences underlie the differences in ink characteristics.
Dyes dissolved in solvents provide superior color quality and tonal reproduction. But as such inks are absorbed by paper, they are apt to smear when wet. They are also affected by sunlight and other environmental factors and tend to fade. In contrast, the stable molecular structure of pigments is ‘fixed’ to the printed material, enabling them to withstand the effects of sunlight and water with good resistance to smudging even when water permeates into the paper. Based on these descriptions one may conclude that pigments are superior to dyes as coloring agents. However as previous mentioned, dye-containing inks are the most prevalent inks used in home printers. Why is this? The answer is that dye-containing inks are superior in terms of uniformity and are able to form fine-sized water droplets which are ideal for the sprayer head mechanisms of inkjet printers.
Pigment inks, which are solvents with suspended particles of pigment, have poor stability and have other disadvantages such as their tendency to clog spray nozzles.
This is where viscosity control comes into play and the requirements are demanding. The lower the viscosity, the smoother the spraying characteristics. On the other hand, dispersion stabilizers are indispensable for better printing stability when pigment inks are used, but the use of such additives raises viscosity levels. It has thus become a major task and goal of manufacturers of inks and printing equipment to reconcile such conflicting factors and issues.

Our next column will be devoted to a summary of these discussions.

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