Visual Examination - Print Characteristics (2)

Medium Class and Structure (cont.)

I. Paper Media
Paper is the traditional substrate for impact printing and was thus the first and most logical choice as a medium for the beginning developments in non-impact printing. It is still the primary choice substrate material, although it is often just one of many ingredients that make up a layered structure. As in the conventional printing industry, there are two main directions of paper research for digital applications: that of developing the paper for the printing technology, and that of adapting the technology to the paper. As digital printing has spread, so has the need for diversifying ink jet paper become apparent. Paper mills that wanted to participate in the new boom for non-impact papers, specifically that of ink jet paper, have had to adjust to accommodate the special needs this paper dictates in its production.

Ink jet technology is the most paper sensitive among non-impact printing processes. Paper needs at least an internal and a surface sizing in order to render adequate results in ink jet printing. Increasingly,⁷ paper is being coated with special ink receptor coatings which either modify or completely cover the cellulose fiber mesh that makes up the paper to achieve the best print results. Certain parameters, such as smoothness/roughness, dimensional stability, stiffness, absorption, mechanical strength, pH, and optical appearance govern the manufacturing process of paper. The bleaching and pulping processes as well as the content ratio of softwood and hardwood are also being modified to result in an ideal paper. There can be no one ideal paper for ink jet, however, as the technologies, combinations of ink, media, and print heads, and not to forget saleability of the product dictate distinct paper characteristics.

Paper without Surface Sizing
Bond paper (Figure 2) is specifically manufactured for use in desktop printers, photocopy machines, and for general office applications. It must therefore be widely available in mass quantities and at a low price, factors that usually do not allow for high-end permanence considerations. It is produced in many variations, among them bleached and unbleached, recycled, and colored, but the vast majority of all bond paper is manufactured as white, standard cut, individual sheets. It was recognized early in ink jet development that unsized paper allowed the liquid ink arriving at its surface to wick along the fibers. In the paper manufacturing process, there is a distinction between internal and surface sizing.

Figure 2. Plain bond paper8

Figure 3. Cross-section of ink jet on IBM Multipurpose Paper, 62.5 x magn., transmitted light. The liquid ink has penetrated into the paper interior by wicking along the fibers. Figure 4. Cross-section of Alps Microdry (Thermal Transfer) on bond paper, 62.5 x magn., transmitted light. The pigments have been fused to the paper surface.

Internal sizing will increase the wet strength of a paper by strengthening the bonds between individual fibers. Surface sizing is added to modify the characteristics of the paper surface by sealing the surface fibers. Typical surface sizers are starches and gelatin. In addition, latex and a whole spectrum of combinations and synthetic substances can be used. The sealing of the fibers leads to a more uniform paper surface which is less susceptible to abrasion, an effect which can be enhanced by the process of calendering. The effect is that of further sealing and evening out the paper surface which visually results in a higher gloss. Further substances, termed additives or fillers, modify color, brightness, strength, opacity, and other characteristics of the paper.

"Plain" bond paper is the typical paper utilized in photocopy machines, electrophotographic (laser) printers, and office environments. It is not recommended for use in liquid ink jet printing by the industry, as it is not specifically adapted to receiving liquid ink in a way that would render good image quality. Although the often incorporated calcium carbonate will absorb some water, the paper is always apt to cockle due to the amount of liquid applied to the medium in a typical ink jet printing process. This paper can, however, lead to good results with phase change ink jet or electrostatic processes.

Surface Sized Ink Jet Paper
This uncoated medium can be considered a compromise between so-called plain paper and specially coated paper, as it integrates features of the receptor coating of the latter to the external sizing of the former (Figure 5). The paper is additionally surface-sized with starches, poly(vinyl alcohol), or a combination of partly hydrophobic polymers such as alkenyl succinic anhydride (ASA) and ketene dimers (AKD), which, apart from increasing the smoothness of the paper surface by filling in voids, have the function of a binder that will sustain the colorant at the paper surface and decrease ink wicking along the paper fibers. Another way to decrease wicking and increase dot sharpness is to add pigments such as calcium carbonate (CaCO₃) or titanium dioxide (TiO₂) to the external sizing, which tend to break up the random flow of ink along the paper fibers and also make the paper whiter.⁹

Figure 5. Surface sized paper for ink jet applications

Figure 6. Cross-section of ink jet (IRIS) on Somerset Rough White (500g) paper, 25 x magn., transmitted light. The liquid ink has remained on the surface of the thick paper due to the surface size designed specifically for ink jet aplications.

"Archival" Papers
Papers that are more stable, or more resistant to the typical signs of paper deterioration, yellowing and embrittlement, are often termed "archival". The use of this word has been discussed extensively in recent times, and various definitions have surfaced. Independent of the terminology, and apart from environmental influences, two integral factors largely determine the integral stability of a paper: the source, quality and processing of the fibers, and the choice of sizing.

High quality, high rag content papers tend to be internally sized with synthetic alkaline substances that enhance their durability by offering an alkaline reserve. In addition, the paper can be loaded, or contain further buffer substances. The most common of these is calcium carbonate, CaCO₃, which is used to raise the pH of a paper. The use of further modification additives is more restricted among high quality papers than in bond paper manufacturing. High quality paper is often surface sized with either starch or gelatin.

Typical papers of this category are 100% rag fine art papers of differing weight and often with a rough surface texture . These papers usually feature a special surface sizing for ink jet applications that binds the ink, but does not alter the surface characteristics of the paper.

Notes
⁷ Nancy Hopper, "Digital Printing Phenomenon," TAPPI: New Printing Technologies: Symposium Proceedings, (Atlanta, GA: TAPPI Press, 1996), p. 31. back
⁸ The following illustration designs are adapted from those used in James Reilly, Care and Identification of 19th-Century Photographic Prints, (KODAK Publication No. G-2S, 1986), p. 53. back
⁹ David Lee, Wayne Fass, Alan Winslow, "Engineering an Ink Jet Paper - What's Involved?" IS&T: Recent Progress in Ink Jet Technologies, ed. Rezanka and Eschbach, (Springfield, VA: IS&T, 1996), p. 252. back