High speed inkjet - the next offset?

By Ralph Frisk, Vesa Kukkamo and Dan Varney

BRUSSELS, Feb. 27, 2011 (RISI) - New MCC (modified calcium carbonate) coating pigments are helping papermakers develop innovative and cost-efficient paper products for the rapidly changing publishing industry. The benefits of these new pigments include:

• Superior cost-performance ratio
• High solids content
• Excellent rheology
• Excellent runnability (MSP, blade, curtain coater)
• Simple formulation
• Low binder demand
• Improved paper surface properties
• Excellent black & white and color print quality with dye or pigment-based inks.

With today's marketers demanding more relevant, variable and personalized marketing messages, today's industrial inkjet printing technology has become the main contender to meet this growing demand. Some industry experts are predicting that inkjet coated grades will continue to realize exceptionally high growth over the next 10 years and that inkjet will continue to steal more and more share from the offset market. Some are even predicting that inkjet grades will achieve the same volume levels as traditional offset grades within the next 10 years.

Figure 1 - Porosity of pigment tablets

However, until recently, inkjet was seen as a small niche market for SOHO desktop printers, where improved office papers were stated to be "good enough". The paper industry has now realized that high speed inkjet is a huge new business opportunity as these paper volumes start to grow. The opportunities for reformulating paper coatings, in particular, at lower binder levels than traditional offest, allow a new base point for margins to be established.

With shorter run rates, electronic collation, versioning, operational simplicity and lower cost
of entry, inkjet has proven itself as a profitable vehicle to move forward in a challenged printing industry. As a result, the new media marketers and printers are now looking for better substrates to meet these needs.

Pore size distribution of pigment tablets (Mercury intrusion)

At the forefront of inkjet pigment development is a family of high solids modified calcium carbonates (MCC) slurries for MSP, blade and curtain coater applications. These application developments have focused on pigments that provide excellent coater runnability, and ease of operation in a simple but efficient coating color formulation while targeting the technical end-use performance demanded by the new print technologies.

When compared with the current treated or pigmented paper grades, these novel MCC pigments provide improved full color process printability and significantly improved runnability. This is mainly due to coating color solids of 50-60 %, which eliminates the runnability challenges often linked to low solids coating pigments with a high degree of coating color volume modification.

This concept opens up the opportunity for improved transactional prints and new potential, at lower costs, for full color process transpromotional print jobs.

Figure 3 - Viscosity as a function of cationic additive addition

Inkjet demands

Omya has helped develop this technology with its Omyajet^® 5000 product line, which has been shown to absorb relatively high loads of ink solvent (mainly water) rapidly, while simultaneously fixing the dye or immobilizing the ink pigment at or near the paper surface to avoid ink bleed and ink wicking and to generate maximum print density.

To meet the demand for anionic dye fixation, papermakers typically apply a cationic surface treatment onto an absorbing base sheet. However, as printing technologies have been developed towards full color printing, the image quality of such treated inkjet papers has been shown to be insufficient for high value marketing messages, i.e. transpromotional and direct mail markets. As a consequence, an increased demand for new, coated substrates is developing.

The new MCC coating pigments for these applications have been engineered to provide significantly higher amounts of ultrafine pores for maximizing the capillary absorption as well as enough permeability to keep the access for fine pores open. Traditional coating pigments typically create too dense of a coating layer that lacks both the necessary capillaries and coating permeability. On the other hand, typical high SSA (specific, surface area) inkjet pigments such as silica or special PCC show extremely high capillarity and large permeability.

Figure 4 - Image quality

The Omyajet MCC series has been designed to fulfill the need for control between these two extremes and combine their positive features to meet the technical requirements of future substrates for high speed, high color quality inkjet printing.

This MCC technology allows inkjet papermakers to create a custom pore structure for the pigment so that the nano-sized internal pores provide the needed capillarity absorption while the permeability has been optimized by means of optimal particle size distribution.

Figures 1 and 2 illustrate the pore structure of formed pigment tablets of three pigments.

Omyajet^® 5000 is not just tailor-made and optimized in terms of liquid absorption, but also in terms of rheology. Traditionally, inkjet pigments with high surface area lead to poor coating rheology and high binder demand. This is mainly due to low solids content and high modification degree of the pigment. Due to their high solids content and excellent water retention, these pigments can be applied easily using all typical coating devices (MSP, blade or curtain coater). Furthermore, the amount of binder can be reduced below the level of coated offset paper, which provides a considerable economic advantage.

Table 1 summarizes the coating color formulations and properties using the above-mentioned three pigment types. Rub resistance indicates the surface strength, where values below 0.5 are generally showing acceptable surface strengths. It can be observed that even with a significantly higher binder amount, the high SSA PCC product is lacking in respect to surface strength, whereas Omyajet^® 5000 shows acceptable surface strength with only three parts of binder. Application tests have shown that all typical natural (PVOH, starch) and/or synthetic binders (PVAc latex etc.) can be used together with Omyajet 5000.

Table 1 - Color formulations containing standard GCC, inkjet MCC, high surface area PCC, respectively

Cationic additives are a very important functional part of the coating color if the surface is to be printed with anionic dye-based ink. New technology HS inkjet machines are typically operating with such inks; hence the dye fixation is achieved by using cationic polymers in the coating color. Omyajet^® 5000 is dispersed anionically or cationically, nonetheless, the use of anionically dispersed pigment together with a cationic additive in the coating color does not require any special equipment, and a typical mixer in a coating color kitchen is able to overcome the viscosity peak as seen in Fig. 3.

The increase in viscosity is a result of flocculation of negative and positive charged components. After a certain dose level of cationic polymer the system is cationicly stabilized and the viscosity drops back near to its original level. In this application study a newly developed cationic polymer from Coatex (TOPSPERSE^® 67 K) was used.

It is recommended to use 5 pts of this polymer on 100 pts of dry pigment to secure the smooth runnability and optimal image quality.

Figure 5 - Paper and print properties of tested substrates

Commercial trial

A commercial HS inkjet printing trial (Océ JetStream 1000) was performed to visualize the benefits of Omyajet^® 5000. A highly modified reference commercial silica coated inkjet paper was used. Offset and Omyajet^® 5000 coated papers were produced in pilot scale, using a blade coating device and formulations shown in Table 1, the applied coat weight was 10 g/m² per side.

The printed images clearly show the poor performance of offset paper, which is due to a lack of optimal pore structure and cationic dye fixative in the coating color. This leads to uneven ink absorption and poor optical density. On the other hand, the commercial silica coated paper and Omyajet^® 5000 coated paper show superior print quality with minor differences. However, when looking at the paper results in Fig. 5, it is clear that these two papers have very different properties. As shown earlier, the modification of Omyajet^® 5000 is significantly different compared with conventional inkjet coating pigments (silica, special PCC...). There are clear benefits in terms of application, but also in terms of paper properties.

In conclusion, current coated inkjet papers seem to be over-engineered for new technology high-speed inkjet presses. Therefore, the highly engineered MCC (modified calcium carbonate) pigments show clear benefits for paper producers delivering a unique combination of both faster speeds and exceptional color quality at a lower cost.

Ralph Frisk, global product manager, Paper Filling & Specialties; Vesa Kukkamo, manager product performance, Applied Technology Services, Omya International, Switzerland; Dan Varney, paper laboratory manager, Omya International, Proctor, VT

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