Novel Kaolin Pigment for High Solids Ink Jet Coating

Ink jet is a non-impact digital printing method. In recent years, there has been a massive increase in the use of ink jet printers for both home and office. Their low cost along with the rise of color printing are primary factors contributing to this accelerated growth. Digital photography is one of the most important driving forces behind color printing in homes. Because of lightweight and compact heads, the ink jet method also has potential for integration in-line with other printing methods. It is expected that ink jet will continue to develop into an increasingly versatile color printing method in future.1

Ink jet printing requires special paper for achieving high quality images due to the nature of the inks used and the design of the print head. Most of these inks are anionic and are more than 90% water and water soluble solvent.2 Inks are jetted from a series of very small orifices, each approximately 10-70 ?m in diameter, to specified positions on a media to create an image. Multi-purpose plain paper is unsuitable for good quality ink jet printing since it causes numerous problems such as feathering, wicking, color bleeding, low color density, strike-through, cockle/curl, etc. Consequently, ink jet papers are commonly coated with special ink receptive layers formulated to provide good print quality and adequate ink drying/absorption.

Paper for ink jet printing can be classified into several categories: (a) standard bond-uncoated, (b) surface sized plain, (c) silica coated matte, and (d) coated photo quality (glossy and matte). Print quality improves and the cost per sheet increases rapidly as we move from (a) to (d). There is a large quality gap between (b) and (c). There is room for a grade that is better than multipurpose paper and approaching that of the silica coated grade, yet that could be manufactured at a lower cost.3

Amorphous silica such as silica gel is the most commonly used pigment for the matte grade ink jet coating applications. The high surface area, porous silica pigment provides high porosity coatings for quick absorption of ink solvent and rapid ink drying time. However, silica gel is expensive and can only be made down at very low solids. For example, most silica gels can be made down at only 15-18% solids, which will invariably result in low coating solids.

Most of the conventional inorganic pigments such as kaolin clays and carbonates have a relatively low surface area and yield low porosity coatings. There is a new pigment, which is radically modified from a fine particle kaolin clay. This new pigment has an ability to replace a significant amount of silica (50 to 75%) in coating formulations while offering improved benefits such as higher coating solids, coat weight control, lower ozone fading, ease of handling and, most importantly, lower pigment and drying cost.

Experimental

Pigment characteristics: Pigment samples were characterized for particle size, surface area, and particle morphology. Particle size was measured using sedimentation and laser diffraction methods. Surface area was measured using nitrogen adsorption. The particle morphology was studied using scanning electron microscopy.

Pigment dispersion: The novel kaolin based ink jet pigment was dispersed at high solids (58-62%) using either anionic or cationic dispersant. The cationic dispersant was added in two stages for proper dispersion and to prevent time dependent gelling. Initially, 1.25% (active basis) poly-DADMAC calculated for a target solids (e.g., 60%) was added to the make down water. The dry ink jet clay was then added to the water at low mixer speed to completely wet out the clay. High-intensity mixing followed. Additional 0.35% cationic dispersant was added after the high intensity mixing to prevent time dependent gelling.

Silica gel dispersion: Silica gel was dispersed separately in water at 18% solids.

The cationically dispersed ink jet kaolin/silica gel blends were prepared by adding the kaolin slurry to water dispersed silica gel. Two percent poly-DADMAC based on the amount of silica was added to the pre-dispersed kaolin slurry before adding silica gel. Thus, the amount of poly-DADMAC to be used at this stage would vary depending upon the desired ink jet kaolin/silica gel blend ratio. The solids of the individual components and the blend ratio would determine the final solids of the blends. For example, at 62% novel kaolin and 18% silica gel solids, the 75:25 and 50:50 blends would have 38.5% and 28% solids respectively.

Coating preparation: Coating colors were prepared using cationically dispersed novel ink jet pigment and silica gel at 75:25, 50:50 and 0:100 ratios. To the pigment blend slurries, an appropriate amount of either polyvinyl alcohol (PVOH) or vinyl acetate ethylene (VAE) copolymer latex binder was added. Lastly, an appropriate amount of additional poly-DADMAC was added as a dye fixing agent.

Coating porosity by mercury intrusion: Pore volume and pore size (diameter) distributions of the coatings were determined by means of mercury intrusion to ~ 2.067x10^sup 5^ kPa (~30,000 psi) using the Autopore 9215. The coatings were applied onto an impervious polyester film with a basis weight of 138 g/m^sup 2^. In the mercury intrusion method, the volume of mercury intruded as a function of applied pressure is measured. The pore radius, r, is related to the applied pressure as follows:4

r = -2 γ cosθ / P

Where γ is the surface tension and θ is the contact angle of mercury. A contact angle of 130? and a surface tension of 485 dyne/cm were used for this study.

Print quality measurement: An in-house developed print target was printed on Canon 8200, HP 990cxi, and Epson C82 printers and print quality was assessed using a densitometer, machine vision system (for image analysis), and visual rating.

Several ink jet print quality attributes such as horizontal and vertical line quality, positive text quality of 8 point Times Roman text, and inter-color bleed between black and yellow, blue and red and red and green were measured using the ImageXpert system, an automated, fully integrated machine vision-based image quality measurement system.

A high magnification 3-CCD color camera was used to analyze the samples. The camera resolution was approximately 5.6 microns per pixel. The use of a 3-CCD camera provides full resolution images for each color plane. The color channel that provided the best contrast was activated to capture images.5,6,7

Ozone "dark" fading: The ozone fading of solid cyan (C), magenta (M), yellow (Y), and black (K) prints was studied as a function of time. The coated sheets were prepared using a vinyl acetate ethylene copolymer binder system and printed on Canon 8200 and HP 990cxi printers. The printed paper samples were exposed to ozone in a chamber for 3, 10, 15, 22, 36 and 48 hours at 1 ppm concentration, 55% relative humidity, and 23?C. The ozone flow rate was 150 L/min with an internal circulating fan on for homogeneous distribution of ozone. The ozone was generated through a bank of UV lamps, controlled through a closed loop system containing the lamps and a Horiba Model APOA-360 Ozone monitor. The CMYK color densities were measured using Gretag Macbeth Spectrolino before and after exposure to ozone. The results for the duplicate samples (five readings per sample) were averaged, and the difference was reported as density % loss (ΔD) from a 1.0 initial density.

Results and Discussion

Pigment properties: The novel kaolin based pigment used in this study is produced specifically for ink jet applications by radically modifying a fine particle kaolin clay. The pigment particle size, surface area, and mercury pore volume are presented in Table 1.

The surface area of the novel ink jet clay pigment is twice that of the fine particle feed clay, but the particle size is much coarser than the feed. An initial analysis of the surface area and particle size data appears to be contradictory. However, scanning electron micrographs show that the novel ink jet pigment particles have unique and highly aggregated morphology and each aggregate is composed of relatively small primary particles. The novel kaolin pigment mimics the general morphology of the silica gel, although the surface area of silica gel is much higher than that of the kaolin pigment (Table 1). The high surface area of silica gel is contributed by its internal porosity.

Pigment dispersion and suspension stability: Normally, the pigments for conventional paper coating are anionically dispersed. However, most water-based ink jet inks are anionic. Therefore, a cationic coating is required to fix the ink dye molecules on the surface. The coating can be made cationic by either using a cationically dispersed pigment or adding a cationic dye-fixative to the coating prepared from an anionically dispersed pigment. However, an anionic dispersion is not compatible with the cationic dye fixative in the coating and will lead to flocculation or thickening of the coating color. This problem can be avoided by first cationically dispersing the pigment.

After testing several types of cationic polymers, a low molecular weight poly-DADMAC was found to effectively disperse the novel kaolin pigment. The solids, pH, and viscosity data of the cationic and anionic dispersions are given in Table 2.

The novel kaolin yields relatively good viscosities and high solids with anionic and cationic dispersions compared with the silica gel slurry, which can be made down only at low solids (18% or lower). Because of the large particle (aggregate) size and a very low Brookfield viscosity, the anionic slurry is not stable and particles tend to settle within a short time. An addition of medium viscosity carboxymethyl cellulose (CMC) at 0.15% would effectively stabilize the suspension over 30 days.

The cationic polymer-based dispersion tends to give a higher Brookfield viscosity than the anionic polymer and is stable for at least 30 days. Thus, the poly-DADMAC not only serves as an effective cationic dispersant but also as an effective suspending agent.

Coating preparation: The coating color formulations and the properties for 75:25, 50:50 blends of novel ink jet kaolin/silica gel and 100% silica gel with two different binders are presented in Table 3. The data show that a coating color can be prepared at as high solids as 39%, depending upon the silica level.

Coating porosity: Pore size distributions using mercury intrusion were measured and the total intrusion volumes are presented in Table 4. In general, all of the coatings for both vinyl acetate/ethylene and polyvinyl alcohol binder systems have two pore sizes, one large and one small. The larger pore size for vinyl acetate/ethylene is centered on 0.13 ?m, 0.75 ?m 1.13 ?m, and 1.5 ?m for 100% novel kaolin, 75:25, 50:50, and 100% silica gel respectively. The smaller pores for both binder systems are relatively fine and are under 0.06 ?m.

Overall, the pore size and pore volume increases with increasing silica gel content in the formulation. However, the pore volume of the 75:25 and 50:50 novel kaolin/silica gel blends is much higher than the weighted averages of the 100% novel kaolin and silica gel, the pore volume of 50:50 closely matching that of 100% silica gel. This explains the rapid ink drying of the blend coatings with good ink jet print quality. Despite the much higher surface area and pore volume (powder/pigment) of silica gel compared with the novel kaolin, the higher than expected pore volume makes these blend coatings very attractive. The 100% silica gel coating requires a very high level of binder (65 parts) to avoid dusting. The binder demand decreases sharply for the blends and the coating can be formulated at a higher pigment volume concentration than the silica gel alone. For example, a binder level as low as nine parts for the 75:25 blend and 15 parts for the 50:50 blend imparts sufficient strength to the coating. The lower binder demand and the unique morphology of the novel kaolin contribute to the higher than expected pore volume of the blend coatings.

Print quality: Table 5 data show that the optical density of the coated sheets either improves or remains the same with silica gel level in the coating depending upon the printer and the ink type, but is much improved over the multipurpose base paper.

The cyan density on the Canon printer for the 50:50 novel kaolin/silica gel blend is comparable to the 100% silica gel coating while magenta, yellow, and black are lower. On the HP printer, black density for the blends is comparable to 100% silica gel while other color densities are lower. Overall, the Epson C82 densities for the blends are comparable to the 100% silica gel coatings.

The visual inter-color bleed and sharpness ratings show that the 50:50 blend is equivalent to the 100% silica gel while the 75:25 blend is marginally poorer for Canon 8200 and HP 990cxi printers. These attributes are comparable for all the coatings on Epson C82 printer.

Three attributes of ink jet print quality - line, text and inter-color bleed, on two different printers (Canon 8200 and HP 990cxi) - were measured using a machine vision system.

The horizontal and vertical line quality data for 75:25 and 50:50 novel kaolin/silica gel and 100% silica gel are presented in Tables 6-7. The line width and the raggedness for each sample set are within a very small range indicating that the line quality is similar on all the samples from one printer.

Text quality was assessed using the letter "g" printed with 8 point font and the number of parts, average gray intensity, area, and perimeter length were measured, Table 8. The number of parts is expected to be one; if not, it implies the character is broken and the other attributes cannot be measured.

Inter-color bleed was measured between black and yellow, blue and red, and red and green for one of the finest lines in the print target. The difference in line width between the printed line surrounded by unprinted paper and the width of the same line printed with a solid area was considered as an indicator of the extent of inter-color bleed. The inter-color bleed does vary from sample to sample as well as printer to printer. Also, the extent of bleed for a set of colors varies from sample to sample without any trend with silica gel level in the formulation. In general, the inter-color bleed for all samples is low indicating that as much as 75% silica gel can be substituted with the novel kaolin in the formulation without significantly affecting the inter-color bleed performance.

Ozone fading: There are several environmental factors that affect the permanence of ink jet prints. Ozone is one of the principal contributors of so called dark fading or gas fading and has been the subject of several recent studies.8,9,10 The dye-based ink jet inks are more prone to ozone fading than pigment based inks. The degree of fading would depend upon the ozone concentration in the environment and time of exposure. The microporous coating (pigmented coating) is also more vulnerable to ozone fading than natural or synthetic polymers/organic coatings that swell in contact with water or inks. The vulnerability of microporous coating to ozone fading has been attributed to easy access of gaseous molecules to the dye molecules in the porous coating while the dye molecules are encapsulated and protected in the swellable coating. Therefore, it was important to determine whether a coating with a lower porosity such as the novel kaolin (or the blends with silica gel) would be less vulnerable to ozone fading compared with a coating with 100% silica gel.

Conclusions

Novel kaolin based pigment having high surface area has been developed for ink jet matte coating applications. Its unique morphology allows high solids dispersion with either anionic or cationic dispersant, yet much better viscosity than silica based pigment slurries.

The high pigment solids permit high coating solids with different levels of silica pigment in the coating. The high solids coatings with good rheology in turn are expected to run at commercial speeds with various application methods such as metering size press, blade, or rod. The higher coating solids is also expected to significantly reduce the energy requirement for drying.

Despite its much lower surface area compared with silica gel, the novel kaolin pigment with silica blend coatings have higher coating pore volume than estimated from the weighted averages of either the 100% novel kaolin or 100% silica gel coating.

The line quality, text quality, and inter-color bleed of the novel kaolin/silica gel blends closely match that of the 100% silica gel. In most cases, however, the optical density improves with silica gel level in the coating.

An intermediate to high-quality coated ink jet paper can be produced using the new kaolin-based pigment depending upon the level of silica gel in the formulation.