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Business Technology

 

Weatherability and Ink Jet Inks By Dr. Ray A. Work-III

 

Introduction

 

Since the introduction of the first, high resolution, wide-format inkjet graphics color printer by Encad in 1992 and its subsequent promotion as a graphics printer by Lasermaster, print providers have been attempting to use these printers for an increasing number of graphics applications. Sometimes they were successful but frequently they were disappointed with the results. Often these inkjet graphic prints suffered from inadequate light fastness and/or water fastness. The color in the graphic images would fade. Even worse the cyan, yellow, magenta and black dyes would fade at different rates causing very objectionable color shifts. When exposed to moisture the dyes would run. Initially the colorants used in the inks in these printers were based on water-soluble dyes developed for the office market. While these inks were satisfactory for office use, they proved inadequate in many wide-format applications, particularly for those applications where the graphic is exposed to direct sunlight indoors or when used outdoors.

In an attempt to sell inkjet inks and special inkjet media into this attractive market, both after market supplies providers and printer manufacturers began offering products, which claimed to be new and improved ultraviolet (UV) inks and waterproof media. More often than not the claims made for these products were exaggerated. At best only modest improvements were actually made. True outdoor light fastness, like that achieved with electrostatic systems or traditional screen printing, had not been achieved. Water fastness without complete encapsulation was not available either. This frustrated many print providers who bought the products and sold prints only to have them returned by dissatisfied customers. As a result of these shortcomings, many print providers have shied away from using inkjet printing for more demanding applications even though their customers preferred the larger color gamut and high resolution available only with inkjet.

Beginning in 1996, products offering significant improvements in both ink and media chemistries have been introduced. Now authentic claims can be made for outdoor weatherable ink/media systems. While still not as durable as the most durable electrostatic and screen printing products, they provide up to two years light fastness and weatherability when used as recommended. Today, several major manufacturers provide written warranties incorporating matched inks, media and over lamination films. This durability has been accomplished with little or no compromise in color gamut compared to dye-based ink systems used today. This has been done while maintaining the printer reliability required for production printing of defect free prints. These new pigmented inks and their matching media are gaining an increasing share of the inkjet consumables market. They finally give the print provider a viable inkjet option. This report addresses some of the history, reviews the current state of the art in pigment inkjet inks in high resolution printing and provides a forecast of future product enhancement.

 

Light Fastness

Although many claims have been made regarding dye-based inkjet ink light fastness, moderate to long term exposure to direct sunlight requires the use of pigmented inkjet inks to achieve good results. The dye stuff molecules used in water-, oil- or solvent-based dye inks and those of pigment-based inks may be very similar. The big difference between the two is the much larger number of molecules that make up the pigment particle verses a single molecule for the soluble dye. This redundancy gives the pigment particles many color contributors to the color one sees. This is a major reason pigments are used in paints, screen printing inks and electrostatic toners and dyes are not. When one molecule in a pigment particle absorbs UV rays and its color is destroyed, additional molecules remain to contribute to the color. This redundancy is not present with the individual dye molecules present in dye-based inks. When the dye molecules' color is destroyed, no color remains.

If pigments are so much better; why do people offer dye-based inks? The answer is simple: it is very difficult to produce good pigmented inkjet inks. The production of high quality pigmented inks requires very sophisticated technologies that most ink manufacturers do not possess. Dye-based inkjet inks are much easier to produce. In addition, not until recently have pigmented inks and matched media been able to offer large color gamut offered by dye-based inks and matching media.

Some ink producers have incorporated UV absorbing additives into dye-based inks to improve light fastness. UV absorbing components have been added to coating formulations on inkjet media to shield the dyes from the UV rays. UV absorbing, over lamination films provide improvements in the light fastness of dye-based inkjet printed graphics. While all of these can provide a modest improvement in light fastness, they prove inadequate for true outdoor durability since the individual dye molecules provide inadequate inherent light stability. Recently ink formulators have selected dyes that are inherently more light fast. These new dyes have aided in improving the performance of dye-based inks, making them suitable for long term indoor applications and short term outdoor applications. In published tests by an independent laboratory, Hewlett Packard claims that their new photo printer inks will survive up to 10 years indoor light stability. This approaches traditional photographic print light stability. Some companies, like Ilford, have created dye ink and media combinations that also provide improved light fastness since the dyes and special ingredients in the media coatings interact to reduce the effectiveness of the UV light at destroying the color of the dye molecules.

Special media requirements, however, invariably lead to poor media selection and increased cost. Some companies claim their prints will be archival. This is a favorite term for photographic companies in explaining the life of a photographic print. Archival is a term that refers to fade resistance in the dark. This is not a measure of light fastness in indoor or outdoor display applications. The consensus today is that pigments must be used for outdoor applications where color fastness is required for periods in excess of a few weeks and I believe it will remain so for the foreseeable future.

Pigments should be chosen as well for indoor applications where prints are expected to remain colorfast and to maintain color balance for many years. Oil paintings, interior walls, wall coverings and even upholstered furniture use pigmented paints or inks and remain vivid for many years. Pigments should also be chosen for indoor prints that are exposed to direct or indirect sunlight for extended periods of time. We have found that well designed commercially available pigmented inkjet inks printed on compatible media can deliver more than 10 times the fade resistance in accelerated, simulated sunlight tests compared to commercially available dye based inks that we were able to obtain and test side by side. Today, well-respected companies, like 3M, Encad and W. H. Brady, offer up to 24-month warranties on outdoor signage produced with high color gamut, pigmented inks printed on specified substrates that are over laminated with matching lamination films.

In the future, using pigments similar to those used in long term outdoor warranted electrostatic toners and screen printing inks, light fast inks and robust media giving up to five to seven years outdoor weatherability will be available. In these inkjet inks, as with those inks and toners used in electrostatic and screen printing applications, this stability will be achieved with some reduction in color gamut. With properly prepared inks optimized in particle size and on media optimized for the inks, a larger gamut compared to these traditional printing technologies will be achieved at the same level of light fastness since much cleaner materials are used in inkjet technologies.

 

Pigmented Inkjet Inks

In June 1996 Encad began shipping their Graphic Outdoor inks for the Novajet and Novajet Pro family of wide format inkjet printers. These inks represent the first generation of pigmented water-based color inkjet inks for high resolution, wide format inkjet printing for these thermal inkjet printers provided through a printer manufacturer. Aqueous pigmented inks were also announced in 1996 and shipped in 1997 by Mimaki and Roland Digital and their private label customers like Raster Graphics. A number of other announcements have been made for water-based inks by Colorspan (formerly Lasermaster), Sihl and Graphic Utilities for thermal inkjet printers. Products from Fuji Film (and their distribution partner W. H. Brady) and Mutoh of Japan have utilized piezo inkjet technology with water-based pigmented inks. Idanit, an Israeli company, introduced a volatile solvent-based pigmented inkjet printer capable of very high speeds that will print directly on vinyl. Nur Advanced Technologies (now Nur Macroprinters) offers its Blue Board, a follow-on product to the Scitex Outboard and Wide Board Products. They offer high speed, lower resolution continuous flow inkjet printing in super wide format printers using volatile solvent based pigmented inks.

In 1997 many new pigmented ink products have been announced for nearly every form of inkjet technology. Raster Graphics and Xerox have introduced pigmented oil-based inks for their wide-format printers using piezo inkjet printheads. Vutek offers a high resolution, high speed, super wide inkjet printer based on piezo printhead technology using volatile solvent-based pigmented inks which print directly on vinyl for outdoor applications. Eastman Kodak announced in 1996 -and is now shipping -color pigmented water-based inkjet inks for the Novajet Pro E printers they purchase from Encad and private label. They claim these inks will give one year outdoor light fastness with a wide color gamut on their matched media. Colorspan also began shipping pigmented inkjet inks for their thermal inkjet printers. In the fall, Encad introduced pigmented color inks for their Pro E series of inkjet printers and Hewlett Packard introduced durable pigmented water-based inks for their Designjet 2000 and 2500 wide-format graphics printers. These HP printers are the first example of water-based pigmented thermal inkjet inks printing at true 600 x 600 dpi to produce photo realistic graphics output offered by an Original Equipment Manufacturer (OEM).

 

Color Gamut

A common belief was that, in order to use light fast pigments in process color inkjet graphics printing, the user must sacrifice significant color gamut and accept dull graphics as with electrostatic printing. This is not necessarily the case. The color and light fastness that is achieved in a print from a color inkjet printer is dependent upon a number of very important factors the colorant is but one of them. Pigments and dyes both require careful selection. The inks, media and software must be matched to achieve optimum performance. Trade-offs in properties must always be made to meet the end use needs.

First, consider the choice of colorant. Dyes were the first choice since they are much easier to formulate into inkjet inks. In inks developed in the early 90's the primary consideration was color brightness on plain paper. These dyes were notoriously poor for light fastness with serious fading experienced in even a few hours of exposure to direct sunlight. The dyes were chosen for their bright colors and function in a printhead. This was the right choice for office presentations and to highlight color on bar charts and pie charts. Light fastness only posed a problem when someone decided to put a print on the window or on a wall for a long period of time. That was not the intended application. With the development of wide-format color printing in 1992, the printers that were developed used the same inks and printheads used in office printers since they were the only ones available. Very quickly the fading performance of wide-format graphics became a major product deficiency in the eyes of the print provider who was trying to sell prints, signs or art produced on these printers.

Dye manufacturers worked hard to develop new dyes for inks which gave better light fastness while maintaining the color brilliance which inkjet customers were accustomed to achieving in their prints. Gradual progress was made. As the applications for the output turned from pie charts to natural images the dye ink gamut was reduced as well. No longer were the very bright fluorescent colors of the office ink acceptable for natural images. Metamerism, the changing of the color balance seen with changes in spectral illumination of the print, became more important so the fluorescence became objectionable. With the development of more light fast fluorescence-free dyes, the gamut was reduced somewhat as the ink formulator rebalanced the trade-offs to better match the new end user needs. Concurrently, the pigmented ink designers were choosing pigments, dispersants and dispersion processes that allowed the production of well-functioning inks. This technology provided inks that would not flocculate (clump together) and settle out while providing the transparent colors needed for high gamut process color printing.

Just as important as the inks are the substrates. With both dye and pigment inks, the ink once printed on the substrate must give optimum gamut, gloss and gloss uniformity. Initially when pigmented inks were introduced optimized media were not available. This resulted in inadequate performance for some users and supported the belief that pigments gave dull colors. Overcoming this problem has required significant investment in time and money, however, today media is available with optimum performance. Both dye and pigment ink/media systems are available that give very large color gamut. In general it is fair to say that the gamut generally achieved by optimized inkjet prints is superior to e-stat and four-color offset as well as screen printing where process color is used. Pigment color gamut in well-formulated inks on optimum media is essentially equivalent to the best gamut the light fast dye based inks and ink/media systems available today.

 

Print Reliability

Pigments or dyes, high gamut or low gamut, matter only when the ink and printer work well together to provide a reliable printing system. Many industry experts think that pigments will plug the orifice (or nozzle) of the inkjet print head like clumps of sand because pigments are particles. The fact is that a well made pigmented ink can be every bit as reliable as a dye-based ink. Properly designed pigmented inks are more reliable than many of the dye-based inks commonly available on the market today. The keys to reliability are clean inks, free of contaminates which can crystallize out when solvent evaporation occurs, and stable dispersions which will not destabilize and plug the orifice or settle out. Very small particle-sized pigments in the inks are required so that they will remain suspended and not settle. Finally the inks must be properly formulated for the intended printer and media. The typical size of the pigment particles DuPont provides in inks supplied to its customers average about 0.1 to 0.2 microns in diameter. This compares to an inkjet orifice diameter of more than 100 to 200 times that size. Plugging of an orifice by pigment particles of this size will not occur unless the particles are destabilized and flocculate (clump together).

Thermal inkjet technology, like that practiced by HP and Encad, heat the ink that is immediately in contact with the resister, forming a bubble that drives the drop of ink out of the orifice. A thin film of ink will reach a temperature of more than 300 degrees centigrade, potentially burning materials in the ink onto the resister. Inks must be formulated which do not build up a coating of material on the resister. If this burning on or kogation occurs, it will insulate the ink from the resister and disrupt the bubble formation process. The result will be reduced ink drop velocity, reduced drop size, poor drop formation and eventually failure to eject a drop. The drop ejection process in thermal inkjet is a violent event. In a few microseconds a super heated steam bubble is produced and then collapses. This thermal and physical shock will destabilize traditional dispersions produced from commonly available dispersing agents. Well-designed pigmented ink dispersions will provide necessary stability against this shock and freedom from kogation and can provide a robust performance solution.

 

Media and Lamination Films

In order to achieve acceptable outdoor performance, the inks, the media and the over lamination films must be designed to perform well together. Initially inks were developed with office paper applications in mind. The first inkjet inks were not water fast. In 1990 steps were taken to improve the water fastness of the black office inks but even today the color dye-based inks offered for many office printers give little or no water fastness on plain paper. Outdoor performance requires a much higher level of water fastness as well as protection from pollution and rain, ice and snow.

To achieve water fastness and outdoor weatherability, media suppliers have developed many papers, films, and other substrates, that aid in creating the overall durability needed. As many as 60 different companies provide inkjet media and over lamination films to the marketplace today. Many claims have been made about the outdoor durability of these materials. Most of these claims have been exaggerated. It is difficult for the consumer to know which claims are true and which are not. Several companies offer performance warranties on the materials that they supply when used together with specified inks. Several of the companies are large respected suppliers to the signage market. It is generally safer to source from those companies, which have a reputation for providing quality products, and which stand behind their performance.

Today there are water-based pigmented inks, such as DuPont provides through HP and Encad. There are oil-based inks, like those offered by Olympus (and their private label customers like Xerox) and Raster Graphics Piezo Print 5000, for piezo printheads that dry by soaking into the substrate. With these inks the oil remains there indefinitely. There are volatile solvent inks, like those offered by Idanit, Vutek and Nur, which produce organic chemical vapors when they dry. There are phase-change inks, such as those offered by Tektronix and Colorspan (in their Displaymaker Express Printer), that freeze on the substrate. Each of these approaches has advantages and disadvantages and each requires design variations in over lamination films, laminating conditions and/or media to achieve optimum performance.

Inkjet media designed for water-based inks have water absorbing coatings. If these coatings are not sealed, moisture may diffuse into the coating and cause delamination of the graphic., These water-based ink coatings also tend to retain the moisture from the ink after printing. When melt adhesive-type lamination films are used at temperatures above the boiling point of water, vapor bubbles may form in the laminated graphic. If the vapor bubbles are then removed by pricking holes in the over lamination film, the resulting hole may allow moisture penetration from the weather to cause delamination of the graphic.

For oil-based inks which dry by penetration, lamination may also be problematic since the oil may interfere with the lamination film adhesive resulting in delamination. Since the oil-based inks penetrate the media, for paper-like media they may fill voids in the paper causing the paper to become transparent, like vellum. This reduces the reflectivity of the media, more where there is more ink and less where there is less ink. This results in a smaller color gamut in high ink areas causing the images to appear more pastel in color. Since these oils do not evaporate they will diffuse over time throughout the media.

With phase change inks the melted wax-like ink is frozen onto the surface of the media. Lamination film with hot melt adhesives must be heated to form a bond to the graphic. That heat may melt the ink on the media. This will destroy the graphic. When pressure sensitive over lamination films are used on phase change ink prints, the adhesive may not adhere to high ink coverage areas due to the waxy nature of the ink. Delamination may occur.

 

State-of-the-Art

An inkjet printing system is a combination of multiple components. Today there are matched media and ink sets which are warranted for up to two years outdoor weatherability but there are no universal inks, media and lamination films that will work together in every combination. It's Buyer Beware" for claims of water fastness and outdoor weatherability with no over lamination or other protection. Just because a printed media can remain submersed in water without losing its image doesn't mean it will stand up to pouring rain, winds, high humidity, UV-fading and even dirt and pollution for a significant period of time without over lamination. Some media are available that will weather acceptably without over lamination for short periods of time with certain inks. Look at the warranties closely. One should rub the wet image to determine if it is indeed durable enough for specific uses. Beware of claims that appear too good to he true. They probably are.

 

Trends for the Future

Science dictates that the ink offerings in the wide format inkjet printing world will converge to pigments in demanding applications. Comparable performance in end user pricing, excellent color gamut and image quality with long term durability will make the dye-based inks obsolete. Many solvent-based, oil-based and wax-based inks will be replaced with water-based paint-like inks which are more substrate independent with enhanced physical durability while providing freedom from objectionable organic solvent retention or emission. DuPont believes significant improvements in performance in outdoor inkjet printed graphics will be made over the next decade. Through those improvements along with higher resolution, higher speed and more four-color process printing, inkjet will replace electrostatic printing and many applications where screen printing and vinyl cutting are used today. Inkjet will command a very large part of all outdoor printed graphics.

 

By Dr. Ray A. Work, III

 

Dr. Ray A. Work, III had been with DuPont for more than 25 years and has held research, research management, business management, and market development positions. Dr. Work has been awarded a BS Degree in Chemistry from Auburn University Auburn, Alabama and a PhD in Physical Inorganic Chemistry from Louisiana State University, New Orleans. After a post-doctoral research fellowship at the University of Hawaii, Honolulu, he joined DuPont in 1972. He has been awarded 12 U.S. patents, has published more than 30 technical papers and is a frequent speaker at conferences, symposia and technology workshops on the subject of ink jet inks worldwide.

Editors Note: While DuPont Ink jet Enterprise is a member of DPI, the opinions expressed in this article are those of the author and do not necessarily reflect the official view of DPI. Note: While DuPont Ink jet Enterprise is a member of DPI, the opinions expressed in this article are those of the author and do not necessarily reflect the official view of DPI. Note: While DuPont Ink jet Enterprise is a member of DPI, the opinions expressed in this article are those of the author and do not necessarily reflect the official view of DPI. Editors Note: While DuPont Ink jet Enterprise is a member of DPI, the opinions expressed in this article are those of the author and do not necessarily reflect the official view of DPI.