Coatings for inkjet printing papers affect printing quality and stability

Most ink-jet printing inks are made with dyes using water and other high-boiling hydrophilic solvents as dye carriers. The chemical properties and formulation components of inkjet printing inks not only determine the ink droplet ejection performance and the security of the printing system, but also affect the quality of the printed image. The physical properties of inkjet-receiving matrices are key to achieving the ultimate inkjet performance. Therefore, gloss, air permeability, opacity, surface energy, dimensional stability, strength, whiteness, and thickness are all important parameters that determine print quality, and are also factors that manufacturers need to consider when manufacturing ink-jet printing paper. Porous inked coatings will provide fast drying performance, but generally have poorer image quality than non-porous media that require longer drying times.

The uncoated paper surface is rough and has hydrophilic fibers that promote the diffusion and penetration of ink droplets, resulting in low quality images or textual content. Therefore, surface sizing is often required to improve the density and bleeding of pigments. . The surface coating of the coated paper improves the surface structure and performance of the base paper, and can provide a better ink receiving layer, thereby achieving high printing quality and image stability.

The most common inorganic pigments currently used for ink coating are sio2, kaolin, al2o3 and caco3. With the development of nanotechnology, very fine particles can be produced. Inkjet coatings containing nano-sized particles have received great attention from raw material suppliers and paper manufacturers to achieve bright, image-like print quality.

This study investigated three pigment particle types, colloidal sio2, sintered sio2, and smoked sio2, and their application in inkjet coating formulations.

1 raw materials and methods

1.1 Raw materials

Colloidal sio2 and smoke method sio2 both use commercial samples, colloidal sio2 uses nalco n2329, and smoke method sio2 uses cabot 019. Sintered sio2 is an experimental nalco sample. All tests used celvol 203 polyvinyl alcohol (pva) as an adhesive.

The base paper used for the coating test contains no sizing agent. The physical properties were: quantitative 156.9 g/m2, smoothness 5.26 μm, brightness 92.5%, gloss (75°) 12.0.

1.2 Coating research

1.2.1 Preparation
The paint formulation used had a pigment to adhesive ratio of 80/20. The coating was prepared using the following methods: using a variety of mayer bars and an air-drying or cylindrical laboratory coater (clc) operating at 3000 r/min and drying with an infrared dryer. The amount of coating was determined on a specific area of ​​the sample using a labwave 9000 microwave moisture analyzer.

1.2.2 Calendering
Calendering was performed using a hot/soft nip calender at about 3.10 mPa (450 psi) and 80°C, and each paper was calendered through 3 nips prior to performing gloss measurements.

1.2.3 Determination of Gloss
Gloss was measured using a gardner multi-angle gloss meter at a 75° angle. Measure 10 points on each sheet, calculate the average, and report the results.

1.3 Printing Research
Samples were printed on canons 450 and epson stylus color 900 (thermal and piezo type, respectively) printers. All samples had the same coating weight (16~18g/m2).

1.3.1 Determination

The experimental glossiness of the printed gloss, the ink density, and the four colors of blue, red, yellow, and black (cmyk) were measured. Calculate the average of at least 5 data points and report the results. The instrument used is as follows: 60° printing gloss - gardner nova gloss meter
Printing density - x-rite 408

Experimental value - data color spectroflash

1.3.2 Image Analysis

Samples were printed on the epson pro 5000 and hp design jet 20ps presses. Images were taken with a ccd camera and analyzed with the image pro-plus software.

1.3.3 Coating Porosity Study

Coatings were prepared on mellinex 534 non-porous stents. Optically determine the coating thickness and calculate the porosity of the coating.

2 Results and Discussion

Porous ink-jet paints are mainly composed of an insoluble inorganic pigment particle and some adhesives. It may also contain some special additives to enhance the coating performance. There are more and more reports on new materials used for porous, high-gloss inking coatings.

In this paper, three different SiO2 particles, such as colloidal sio2, sintering sio2, and smoke method sio2, were studied and their effects on various coating properties and properties were investigated. The average particle sizes of the three sio2 particles were 95 nm, 278 nm, and 318 nm, respectively.

2.1 Effect on coating formulation

The basic formulation of the paint is polyvinyl alcohol as an adhesive, and the ratio of pigment to adhesive is 80/20. The formulation properties of the coating are shown in Table 1. Colloidal sio2 and sintered sio2 have the potential to achieve high solids content at lower brookfield viscosities.

2.2 Effect on Coating Structure

The final coating structure is changed by changing the particle size of the particles. It is expected that the gloss of the coating will be inversely proportional to the particle size, and the gloss of the coating will decrease as the particle size increases. Calendering can be used to improve gloss and produce high gloss ink jet printing paper. Figure 1 shows the gloss value of each particle before and after calendering in a hot/soft nip. As can be seen from FIG. 1, the sintered SiO2 has the highest gloss measurement. Its value is obviously higher than that of smoke method sio2 of the same particle size, and it further increases after calendering.

Inspection of the coated surface with a scanning electron microscope (sem) was also expected. The surface image is always consistent with the measurement of gloss. The colloidal sio2 based coated image showed minor cracks.

2.3 Effect on Printing Performance

The performance of an ink jet pigment is good for its printed image. The test pattern was printed on coated and calendered paper. Figure 2 shows the ink density values ​​for the four colors blue, red, yellow, and black (cmyb). The performance of all three sio2 particles was compared. It can be seen from Fig. 2 that the concentration of black pigment inks for colloidal sio2 and sintered sio2 is slightly higher.

In addition to cationic mordants, these pigments showed poor water resistance in the tests. Particle structure and pH have no effect on water resistance, which is controlled by the surface chemistry of sio2. The addition of a cationic polymer, such as polydadmac, improves the water resistance of the three particles, however, colloidal sio2 and sintered sio2 particles reduce their surface gloss. The study of printing characteristics by image analysis technology further illustrates the differences among the three sio2 particles. Table 2 shows the image analysis results of the epson printer.

From Table 2, it can be seen that the area of ​​the dot from top to bottom is significantly reduced. The smaller area is better, and the print resolution can be improved without losing the ink density. The test results for the dot roundness values ​​show that the sintered SiO2 coating has a more uniform coating structure. In contrast, the smoke method sio2 has a lower dot area due to its higher porosity, but irregular smoke particles produce higher dot roundness values ​​and thus uneven coating.

Figure 3 shows the porosity of the three sio2 particles. The porosity of colloidal sio2 and sintered sio2 is lower than that of sio2. This data is consistent with the observed dot area results.

The porosity of the colloidal sio2 based coating is created by the interstitial porosity created by the spherical particles during the filling process. The sintered SiO2 with intercellular particle porosity is expected to have more porosity, but its porosity is only about 10% higher than that of the colloidal SiO2. This may be due to the degradation of some of the sintered sio2 as a result of its colloidal raw materials.

Smoke method sio2 showed the highest coating porosity. The final image quality requires a balance between coating porosity and coating uniformity. Although the print quality of colloidal sio2 and sintered sio2 coatings is acceptable, their lower porosity limits their effectiveness.

3 Conclusion

Colloidal sio2 and sintered sio2 particles have more advantages in the rheology, percentage of solids, gloss, ink concentration, and dot roundness of the coating formulation than smoke sio2. However, its coating exhibits a lower porosity. In contrast, smoke sio2 with a rigid, irregular shape can produce more porous layers. However, it also reduces the gloss and the solids content of the coating formulation. In order to obtain a low-cost, high-gloss ink-jet paint, all factors must be balanced.

Reprinted from: "International Paper"