PFFC - February 2008 - (Page 14)

PROCESS MANAGEMENT �� Drying—Part 1: Liquid Inks �� I By David Argent Contributing Editor �� Process improvement expert David Argent has 30+ years of experience in process analysis with�particular �emphasis on ink and coating design and performance. Contact him at 636-391-8180; djvargent@sbcglobal.net. 14 FEBRUARY 2008 | �� n the next three columns, we will cover the �� Solvent-Based Inks basic mechanisms of ink drying. This month we look at inks drying primarily by evapora�� tion and absorption as related to gravure and �� flexo printing. �� 1 | Solvent-Based Inks | The fast solvent release and strong adhesion of solvent inks make them ide�� ally suited to printing on nonabsorbent surfaces, and they are the primary ink type for this market segment. On absorbent substrates solvent-based inks dry by a combination of penetration of the ink �� into the substrate and evaporation of the ink solvent from the ink film. Normally, there is no chem�� ical interaction with the substrate. Solvent inks are �� very resoluble, and for this reason, they are often �� favored in gravure printing of paper and board due to lower tendency to plug the image cells. �� 2 | Water-Based Inks | These inks are based on �� a combination of acidic resin held in solution by �� amine and water-borne resin dispersions known as �� emulsions. Solution resins aid print quality, and the emulsion is used to provide resistance properties in �� Water-Based Inks �� the dried ink. During drying, several mechanisms �� are happening at the same time. On absorbent �� surfaces such as paper and board, the ink com�� ponents will start to penetrate into the substrate. �� Evaporation of water and amine takes place at the �� same time, often aided by dryers on the press. �� Loss of the liquid part of the binder due to �� evaporation and penetration has several effects. The �� ink starts to gel. The acid resin starts to precipitate due to amine removal. The rate of drying depends �� on the absorbency of the substrate. Acid substrate Water-based flexo inks ideally are suited for print�� components can help set the ink resin by neutraling on paper and board and dominate this segment. �� izing the amine and precipitating the soluble ink Water-based inks do not dry or adhere as well to resin. On absorbent stocks the combination of pre- nonabsorbent surfaces. The primary reason is that �� cipitation and coalescence produces a drying rate there is no absorbency factor to assist the drying. �� often as fast as that of solvent-based inks. So�� these substrates are printed more frequently with �� Formulation of water-based inks requires a balsolvent-based inks. �� ance of ingredients that are often in conflict. A Due to�� loss of volatile components in liquid inks more fugitive amine assists drying speed, but print during drying, the dry ink film always is substantially �� quality suffers. Loss of amine in low-coverage thinner than the wet ink film. Factors that in�� decks leads to ink bodying and print decrease drying speed include the use of more �� fects, but monitoring and maintaining volatile ink components, increased web �� pH in the specified range will keep temperature, and higher air velocity the ink in balance. in the dryers. Not so obvious is that �� In many cases, such as corrustronger �� inks dry more quickly due �� gated box printing, the presses are to metering of a thinner ink film to MORE ON DRYING achieve a specific color. not equipped with dryers. Due to LIQUID INKS ��month this column will expenetration of the binder, there is a Next Part 2 of 2 positive mechanical anchor of ink to plore some of these factors in a comsubstrate, providing strong adhesion. puter simulation. �� COMING NEXT MONTH �� WWW.PFFC-ONLINE.COM http://WWW.PFFC-ONLINE.COM

Table of Contents Feed for the Digital Edition of PFFC - February 2008

PFFC - February 2008 Contents First Glance Calendar of Events From the Editor Web Lines Process Management News Clips Narrow Web & Label Reporter Narrow Web & Label What’s New Products Carton & Box Reporter Offset Printing A Triple Ply: Coating & Laminating Global Market Is Strong for Vacuum-Coated Products Melamine Innovation: A New Choice for Barrier Coatings How To Optimize Your Coating Solution’s Properties What’s New Products Contract Services Directory Custom Marketplace What’s New Products Services Directory Classified Marketplace Advertisers Index Experience Speaks

PFFC - February 2008

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