Recent Progress in Ink Jet Technologies II Copyright 1999, IS&T Magenta Dyes for Ink Jet Applications Wolfgang Bauer, Dieter Baumgart and Walter Zöller Hoechst Aktiengesellschaft, Frankfurt, Germany Abstract Dyes versus Pigments Within the competing non-impact printing technologies, The innovations which are necessary to meet the specific ink jet has emerged as one of the leading digital printing requirements for colorants in ink jet systems are reflected technologies due to its capability to print black and color on by a high level of patent applications. The main topic has many surfaces. been directed toward tailoring water soluble dyes for ink jet Significant improvements have been achieved in this applications5 and ink formulations to improve print quality, field including the development of dye and pigment based for example less feathering and reduced black-color and inks. For recent multicolor industrial ink jet applications color-color bleeding. Recently, pigment dispersions exhib- such as near-photographic color printing and wide format iting increased stability due to specific resin additives have color graphics, there is a growing demand for light fast and been claimed and carbon black based inks have been intro- water fast dyes. duced into commercial printers2 showing advantages in Developing water soluble magenta dyes showing the water and light fastness as compared to water soluble black desired brightness and hue combined with good fading and food, acid or direct dyes.6 water resistance has proved a difficult target. However, considerable progress has been achieved in In this paper, a survey of known magenta dyes is pre- developing water soluble dyes with improved water fast- sented including spectroscopic features, aggregation behav- ness due to differential solubility.1,7,8 Recently, the use of iour and fastness properties. In addition, novel polycyclic dyes versus pigments has been reviewed for various ink jet metal-free magenta dyes are presented which are derived applications.9,10 from pigments, vat dyes and solvent dyes. Accordingly, the choice of dye or pigment depends on Optical and light stability characteristics of the novel how dye properties such as color gamut, transparency, col- dyes are compared with commonly used magenta dyes. or strength and ease of use are ranked against pigment at- tributes like light fastness, insolubility and crystallinity. So Introduction far, only carbon black based inks have appeared on the printer market but no trichromat based on color pigments. In contrast to electrophotography (laser printing, photo- Obviously, the development of pigmented ink formulations copying) and thermography, ink jet printing is a true prima- with stable particle sizes < 100 nm to prevent light scatter- ry non-impact process. Liquid ink droplets passing through ing has been a difficult target. fine nozzles are electronically directed to surfaces includ- Significant parameters for the primary colors yellow, ing paper, plastics, metals, ceramics and textiles.1,2 magenta and cyan used in the subtractive color system are Due to considerable progress in hardware and ink peak wavelength, narrow and symmetrical absorption technology, ink jet has proven to be the first technology curves and the absence of secondary absorptions.11 that achieved a high level of color quality at a reasonable Dyes being present in the monomolecular state satisfy price. these requirements best. However, many dyes show anom- There are two basic forms of ink jet systems, continu- alous spectroscopic behaviour at higher concentrations and ous and drop-on-demand, which can be subdivided further. do not appear to obey Beer’s law. This effect is ascribed to Classical ink jet applications are monochrome and color of- the formation of dimers, trimers and higher aggregates of fice printers, and industrial printers for marking, address- dye molecules. In most cases aggregation results in broader ing and coding. Recently, attractive multicolor industrial absorption curves and in a decrease of brightness. The pu- ink jet applications have appeared including wide format rity of the reflected light is reduced which is also true for printing, color graphics, color proofing3 and photographic scattering of light from pigment particles. imaging.4 Useful relationships between light fastness of colored Within the different components—hardware, sub- substrates and the physical and chemical structures of col- strate, ink—comprising a reliable ink jet system, inks play orants are well established.12 Accordingly, besides the in- a decisive role. Basically there are three types of inks-aque- herent photostability of the colorant, the degree of ous, solvent and hot melt-suited for the different printing aggregation of the dye on the substrate belongs to the most technologies. Solvent dyes are mainly used in inks for con- important factors.13 tinuous printers and phase-change devices whereas the Clear electron micrographs of aggregates of direct most common inks for drop-on-demand office printers, dyes in cellulose films have been obtained showing a direct both piezo and thermal, are aqueous based. Consequently, relationship between light fastness and aggregation. Dyes colorants for aqueous ink jet inks are water soluble dyes se- of low light fastness did not reveal any particles in the fibre lected from food, acid, direct and reactive dyes. while those with high light stability formed clearly detectable particles up to 1 υm. Correspondingly, the light RecentRecentRecent Progress Progress Progress in Ink in in JetInk Ink TechnologiesJet Jet Technologies Technologies II II II 474 474 Chapter Chapter Chapter 6, 6, Materials6, Ink Ink and and Media andMedia Media Recent Progress in Ink Jet Technologies II Copyright 1999, IS&T fastness of most pigments is related to the particle size A color change is observed towards duller shades because the surface area decreases as its particle size which has been explained by the formation of sheet like ag- increases.12 gregates.14 Magenta Dyes, State of the Art Magenta dyes exhibiting a combination of optimal bright- NH O ness and hue, high light fastness, high water and rub fast- 2 ness and good solubility are proving extremely difficult to N HN 1,5 develop. N Using xanthene dyes such as CI Acid Red 52 1, ink jet O S + prints are obtained showing excellent brightness in accor- O Na dance with narrow absorption curves. However, 1 exhibits OH O very poor light fastness, rated 2 according to the blue wool + scale (1 to 8), and low water fastness. Na O S O O + N O N 3 Cu O O O O O + S Na S O O O N N + O Na O S O S O O O O S 1 + O Na O O S + O O O Na O S NH O R O 4 N NH O O O S O + O S S Na O O O O S O O O + + Na Na N + N OH 2 (R = OSO Na) Na 3 N N O The water fastness of 1 can be improved by introduc- O + Na S NH ing carboxyl groups, however the light fastness remains O poor.7 O Monoazo dyes derived from H - Acid of the type CI Acid Red 249 or CI Reactive Red 180 2 are used commer- 5 cially exhibiting enhanced light fastness and a combination of high brightness and moderate water fastness.5 In addition to numerous patent applications directed to Dyes of type 2 mainly exist in the hydrazone form magenta ink jet dyes, mainly by modifying dyes of type 2, which has been related to moderate light fastness.14 3 and 4, magenta disazo dyes based on J-Acid derivatives In contrast to 2, monoazo dyes of the type CI Acid Red have been proposed for ink jet printing.15 37 3 exist exclusively in the azo form 14 and show in- Disazo dyes like CI Direct Red 81 5 are not suited as creased light fastness as compared to hydrazone dyes 2, primary colors for ink jet printing due to secondary absorp- however at the expense of brightness. tions.5 Light fastness is further improved in copper complex In Table 1 spectroscopic properties and light and water dyes such as CI Reactive Red 23 4. fastness characteristics of dyes 1 to 4 are summarized. RecentRecentRecent Progress Progress Progress in Ink in in JetInk Ink TechnologiesJet Jet Technologies Technologies II II II 475 475 Chapter Chapter Chapter 6, 6, Materials6, Ink Ink and and Media andMedia Media Recent Progress in Ink Jet Technologies II Copyright 1999, IS&T Table 1. Properties of Magenta Dyes 1 to 4 50000 λmax (nm) light water 1 2 3 4 (ε) fastness fastness dE 40000 Dye 30000 7.31 E-6 M 7.31 E-5 M 1 565 (89300) 2 30% 44.3 7.31 E-4 M a mola mol 20000 2 544 (31300) 4 65% 10.7 520 (30500) 3 513 (23500) 5 70% 8.1 10000 4 524 (20000) 6 50% 4.8 0 350 400 450 500 550 600 650 700 750 Figure 2. Variation of absorption spectrum of CI Direct Red 81 with concentration 1) 10-5 moles/ l in water (pH 11.5) 2) blue wool scale; 2.5% ink in water/ diethyleneglycol Apparently, in planar dyes with extended conjugation, 3) 1 min water immersion fastness dye aggregation already occurs at low concentrations. 4) 96 h xenon test Self-association of CI Reactive Red 180 has been monitored spectrophotometrically at dye concentrations of 10-3 to 10-5 moles/l. The longer wavelength at 544 nm, Results and Discussion most marked in dilute solutions, is typical of the monomer, while the band at 520 nm corresponds to the dimer.16 At Dye Aggregation higher concentrations further self-association can occur. According to Figure 1, CI Acid Red 52 1 shows a sharp However, rather than progressive association into trimers absorption band centered at 565 nm (ε = 90000) at low con- and tetramers, clustering to form larger aggregates usually centration of 10-6 to 10-5 moles/l with a shoulder at 531 nm.