Inkjet technology is currently experiencing a development boost, driven not only by printer advances, but also by the materials used. This affects both SOHO (small office, home office) and industrial inkjet printing applications. Current developments are mostly in digital photograph printing, which is concerned with generating pictures that can match traditional silver halide photographs in terms of permanence and appearance, and with wide and ultra-wide format printing, which is booming particularly in the Far East through, for example, advertising on areas such as building walls. Inkjet prints offer the advantage of low investment costs for small print runs, but the PVC substrates employed for them need to have adequate lightfastness and weatherability. Furthermore, industrial inkjet printers are getting faster all the time and will soon catch up with traditional offset printing techniques in some areas. This means that the inks have to meet tough specifications on viscosity and drying.
It was against this background that the 74 conference participants from 10 countries gathered to listen to and discuss 16 high-level technical papers on the latest developments in materials of relevance to the inkjet sector. Although the various papers cannot be presented here in detail, two keynote papers provided an excellent insight into current technical problems and attempted solutions.
A World of CompromisesDr. Christine Halik (Ciba) opened the conference by describing the current state of inkjet materials which, as she explained, is marked by a "world full of compromises":
- Colorants that offer greater brilliance tend to show lower lightfastness.
- Nanoporous papers have been developed that dry faster, but they are more susceptible to gas fading (decomposition of color primarily due to ozone).
- Pigment or colorant? This is not an easy question to answer: pigments have superior colorfastness (hence their use outdoors) while colorants have greater brilliance (use in photo printing).
- In pigment inks, smaller (nano) particles lead to higher colour strength, but this comes at the cost of weatherability.
- There are two ways to coat paper for inkjet photograph printing: The aforementioned nanoporous paper coatings (usually containing polyvinyl alcohols filled with nano-silica) and swelling, pure polymer coatings (gelatine or acrylates and polyurethanes). The former fail to provide good protection for colorants, and gas fading is a problem. The latter offer protection, but dry very slowly because of their non-porous nature.
- The water resistance of high-gloss photographic prints must at least extend to fingerprints. Mordants are commonly used for this in paper coatings, but they impair lightfastness.
- UV-curing inkjet inks "are still a niche market, but are about to take off," according to Halik. These systems are forecast to grow at double-digit rates up until the end of the decade. The primary applications are believed to lie in nonporous substrates. Full UV cure is still a problem with porous substrates. These formulations, which do not contain any solvents as they are 100% systems, require extremely low viscosity for processing reasons, and this is why the ratio of oligomers/monomers has to be kept very low. But this is precisely the cause of extensive inhibition by oxygen of UV curing - highly effective photoinitiators are a must in these formulations.
Fine Details of Inkjet Paper ResearchDr. Richard Hann (ICI Imagedata) discussed the subject of inkjet paper coatings in more detail in the second keynote paper. Current developments in this area include:
- Attempts to use antioxidants to minimize gas fading in nanoporous paper coatings. The problem here is that this only delays colorant degradation.
- Attempts to incorporate porosity into resins, either through selective polymer incompatibility or by blending solventborne and solvent-less systems, to combine the advantages of swelling microporous paper coatings.
- Improved polymer-swelling coatings employ acrylamides for better lightfastness, and cationic acrylics for better water resistance. The drying rate is "dramatically improved" by tailor-made molecular weight distributions.
- As far as reducing the cost of inkjet photographic papers is concerned - this is currently a limiting factor - attempts are being made to utilize the absorption of the cellulose paper substrate as a means of economizing on complex absorption layers. The substrate is currently not used for ink absorption as it is generally sealed with a dense PE layer. While structures of this kind do exist, they are so thin that the photographs do not look like traditional ones and there are problems with waviness during printing. Solutions currently being investigated include controlled solvent absorption by means of gels in the paper formulation.
Broad Range of Discussion TopicsAnother firm item on the conference agenda was the discussion groups formed to answer expert questions submitted by participants in advance. These issues ranged from the oft-problematic patent situation, to prognostications about the future of individual inkjet segments, through to specific questions on paper coating.
The Conference Proceedings for the ECC "The Power of Inkjet Materials" are still available at a price of EUR 350,- from Vincentz Network, Amanda Beyer, phone +49 511 9910-212, fax +49 511 9910-279, e-mail email@example.com.
The ECC "The Power of Inkjet Materials II" is scheduled to be held on Dec. 2-3, 2004, again in Berlin, Germany. For more information, click www.coatings.de/ecc.cfm.