WHAT'S THAT STUFF?

November 16, 1998

Volume 76, Number 46

CENEAR 76 46 1-56

ISSN 0009-2347 Ink I nks are found in almost every aspect of human activity. We read newspapers, magazines, and books on a daily basis. We post lists on our refrigerators, jot things down on our calenders, and leave sticky notes for our coworkers and friends. We make piles of photocopies and print lots of pages from our computers. We buy myriad packaged consumer products printed with ink. We use stamps and money, again printed with ink. And if you delve into finding out more about ink, you will learn as I did that people use a lot of ink writing about ink--although I calculate that it took only microliters to print the page you are reading. More about that later. What is ink? I already had a general idea what ink is: It is an organic or inorganic pigment or dye dissolved or suspended in a solvent--essentially the same as paint. I confirmed that fact after digging around a bit, and then learned a whole lot more. I found out that the first inks were fruit or vegetable juices; protective secretions from cephalopods such as squid, cuttlefish, and octopus; blood from some types of shellfish; and tannin from galls, nuts, or bark from trees. The first man-made ink appeared in Egypt about 4,500 years ago and was made from animal or vegetable charcoal (lampblack) mixed with glue. Today's inks are divided into two classes: printing inks and writing inks. Printing inks are further broken down into two subclasses: ink for conventional printing, in which a mechanical plate comes in contact with or transfers an image to the paper or object being printed on; and ink for digital nonimpact printing, which includes ink-jet and electrophotographic technologies. Color printing inks are made primarily with linseed oil, soybean oil, or a heavy petroleum distillate as the solvent (called the vehicle) combined with organic pigments. The pigments are made up of salts of multiring nitrogen-containing compounds (dyes), such as yellow lake, peacock blue, phthalocyanine green, and diarylide orange. Inorganic pigments also are used in printing inks to a lesser extent. Some examples are chrome green (Cr 2 O 3 ), Prussian blue (Fe 4 [Fe(CN) 6 ] 3 ), cadmium yellow (CdS), and molybdate orange (a mix of lead chromate, molybdate, and sulfate). Black ink is made using carbon black. And white pigments, such as titanium dioxide, are used either by themselves or to adjust characteristics of color inks. Inks also contain additives such as waxes, lubricants, surfactants, and drying agents to aid printing and to impart any desired special characteristics. Printing ink is a $10 billion global industry. The Census Bureau tracks about 250 printing ink companies in the U.S., which in 1997 produced 2.2 billion lb of ink with sales of $4 billion. Older style writing inks, such as in fountain pens, use a fluid water-based dye system. But in the 1950s, when ballpoint pens became fashionable, the writing ink industry shifted to pastelike oil-based dye systems. The thick consistency allows capillary action to keep the ink flowing well, and the inks generally are nonsmearing and quicker drying than water-based systems. Dyes tend to be preferred over pigments for writing inks because pigments can't be dispersed minutely enough and tend to clog the pen tip. And water-based dye or pigment systems are still used for markers, highlighters, and rollerball pens. A few pen manufacturers, such as Bic (which sells about 3 million pens per day), make their own ink, but most pen manufacturers buy their ink. That sums up the gobs of information I found on inks. But I still had one burning question: How come ink from the daily paper sometimes smudges off onto your fingers? I called the Washington Post to find out. Inks dry by different processes, explained Hugh J. Price, the paper's director of production planning. Linseed oil inks dry by air oxidation, which solidifies the vehicle. Inks with alcohol- or petroleum-based solvents dry by evaporation, usually assisted by heating the paper. Newspapers are generally printed with a mineral oil ink at a very fast rate--several thousand feet per minute. Because newsprint is not heated, that allows little time for the ink to air-dry, Price noted. Instead, the ink is absorbed by the inner fibers of the sheet of paper and remains there a bit damp during most of the transient life of the paper--the vehicle doesn't completely evaporate. So when you handle the paper some of the ink can rub off onto your fingers. The amount depends on how fresh the newspaper is. Ink on the pages of books, magazines, newspaper inserts, and catalogs doesn't smudge off, Price added, because they are usually completely dried during the print run and are printed on a different type of paper. As my ink odyssey was winding down, I had a chance to see ink in action during a trip to Brown Printing Co. in Wasesca, Minn., which prints C&EN. I got to see pages of C&EN rolling off the end of one of the presses. The four colors of ink used--black, cyan, magenta, and yellow--are pumped into different units of a press. The ink sits in a "fountain" where it is picked up and transferred via the etched printing plate to the paper. I couldn't help sticking my finger into one of the fountains to check out the ink. Although I expected the ink to be viscous, I was still surprised at how thick it was when I looked at my fingertip covered with cyan goo. That made me curious as to how much ink it takes to print C&EN. With data from the press run I had been watching, I calculated that an average size C&EN issue of 80 pages requires a total of only about 68 gal of ink to print just more than 150,000 copies. That works out to about 3,500 gal of ink for an entire year of C&EN. That's about 20 L per page--a bargain. So what does the future hold for ink? Could ink someday become passé? The advent of personal computers, personal electronics, and the Internet may one day replace libraries full of printed books and periodicals with electronic products. For example, electrophoretic inks that currently are being commercialized can be corrected, edited, or updated if needed by momentarily applying an electric field. And electronic books (e-books) with digital displays, something similar to a palmtop computer, may be in vogue in a few years. But the great paperless society hasn't begun to show itself yet--people simply like paper too much. And as long as there's paper, then there must be ink. Steve Ritter For more information on ink, please see these Web sites. National Association of Printing Ink Manufacturers Printing Industries of America Sun Chemical Writing Instrument Manufacturers Association Chemical & Engineering News

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