Effect of the glass-transition temperature of latexes on drying-stress development of latex films and inkjet coating layers

Coating pigment and binders

A gel-type micro-silica (Dongyang Chem. Co., Seoul, Korea) was used as a pigment. This micro-silica is highly hydrophilic because of the silanol groups on the surface and has an average particle diameter of 7 μm and numerous micropores. The specific surface area and oil absorbency of the silica pigment are 300m2/g and 300cm3/100g, respectively. Figure 1 shows the appearance of a silica pigment coating layer by scanning electron micrography (SEM).

Figure 1

SEM image of silica coating layer consisted of polyvinyl alcohol (PVOH 35 pph) and polyvinyl acetate (PVAc; 35 pph) binders with polydiallyldimethylammonium chloride (poly-DADMAC; 15 pph) as cationic polymer.

Latex binders

To investigate the effect of latex binders on film formation and drying-stress development, conventional polyvinyl acetate (PVAc) latex (Samji Corp., Chungju, Korea), which are being used as a binder for inkjet grades, and three types of S/A latices (SA-18, SA-40 and SA-48; polymerised in the laboratory) were used. The Tg values, determined using a differential scanning calorimeter (DSC250, TA Instrument, US), of the three S/A latices were 17.6 °C, 39.7 °C, and 47.6 °C, respectively. The particle sizes and charge densities of the latices were 174–190 nm and −125 to −168 µeq/g, respectively (Table 1). To determine the particle size and the charge density, a Zetasizer (nano-ZS, Malvern, U.K.) and a charge analyser (Mütec PCD03, Germany) were used, respectively. The low shear viscosities and water retention values were also determined using a Brookfield viscometer (DV-2, U.S.A.) and Åbo Akademi gravimetric water retention meter (ÅA-GWR, Kaltec Scientific, U.S.A.), respectively.

The low shear viscosities and water retention values of the coating colours prepared using the S/A latices were lower than those of coating colours prepared using PVAc as a binder (Table 1).

Drying-stress measurement

The drying stress was evaluated using a cantilever deflection method, which involves measuring the deflection of a film-coated substrate. Deflection was measured in terms of beam deflection using a laser and a position-sensing detector (PSD) (Figure 2). More detailed information on the stress measurement apparatus can be found in the literature (Kim et al. 2009, 2010). Briefly, the shrinkage of the wet coating layer during the drying process leads to the development of tensile stress, which deflects the substrate. A microscope cover-glass (thickness, 150 μm; dimensions, 50 × 24 mm2, Marienfeld, Lauda-Königshofen, Germany) that was gold-coated on the reverse was used as the substrate to enhance the laser reflection. The cover glass was cut into 50 × 6 mm2 and used as a coating substrate. The latex suspension or coating colour was applied to the substrate with a doctor blade (Kipae ENT, Suwon, Korea) in dimensions of 45 × 6 mm2. The coating layer placed in an environmental chamber was dried at a constant temperature of 25 °C ± 1 °C and a relative humidity of 10 % ± 2 %. In some experiments, higher drying temperature was employed. In this case, hot air stream at the controlled temperature was introduced to the environmental chamber. The thickness of the wet coating layer was approximately 150 μm, and that of the dried coating layer was 75 ± 5 μm.

The substrate deflection was converted to drying stress using the Corcoran equation (Corcoran 1969, Boerman and Perera 1998, Kim et al. 2009, Oh et al. 2019) given by Equation (1).

where D, E, t, L and ν are deflection, elastic modulus, thickness, coating length, and Poisson ratio, respectively. Subscript s and c designate the substrate and coating film after drying, respectively. The second term of Equation (1) describes stress relief, which can be neglected if the substrate thickness is much greater than the thickness of the coating layer (i. e., ts≫tc) or the elastic modulus of the substrate is much greater than the elastic modulus of the coating layer (i. e., Es≫Ec).

Figure 2

Stress measurement technique with beam-deflection method using a laser and position-sensing detector (PSD).

Inkjet coating and analysis

Inkjet base paper produced by Hansol Paper Co., Ltd. (Cheonan, Korea) with a basis weight of 94 ± 2 g/m2 was used for coating. The brightness and opacity of the base paper were 88.0 % and 89.9 %, respectively. Coating colour was prepared using silica pigment and 35 pph of PVOH and 35 pph of a latex as binders. Poly-DADMAC (15 pph) was used as a cationic additive and the final solids of the coating colour was 15 %. An Maiyoh laboratory sheet coater (Mitsui Electric, Chiba, Japan) was used for the production of coated paper. A double coating was applied to obtain a coat weight of 8–9g/m2, and then dried at 130 °C. The coated paper was calendered twice with a linear pressure of 112 kN/m at 40 °C using a soft nip calender. After calendering, the paper was conditioned at 23 °C and 50 % RH for more than 24 h, and then brightness (ISO 2470), opacity (ISO 2471), and gloss (ISO 8254-1) were tested using an Elrepho spectrophotometer (L&W, Sweden) and a gloss meter (L&W, Sweden).

Effect of the Tg of binder

Tg is one of the most important variables affecting the drying-stress development and the formation of the film structure of polymer latices (Hahn et al. 1986, Linné et al. 1988, Juhué et al. 1993, Kim and Winnik 1995, Pekcan and Canpolat 1996, Laudone et al. 2003). Tg is material-specific, but it also changes depending on the dispersing medium or solvent and on the drying rate (Routh and Russel 1998). In general, a polymer chain having a high Tg tends to shrink during drying, thereby developing drying stress (Tirumkudulu and Russel 2004, Martinez and Lewis 2002b).

Figure 3A shows the drying stress of a latex film, SA40, with a Tg of 39.7 °C, when it was dried at 25 °C and at a relative humidity of 10 %. Figure 3C shows images of the film in the drying process and surface cracks that developed. As seen here, drying stress develops from the initial stage of drying because drying does not proceed uniformly throughout the sample but from the edge of the film (Sheetz 1965, Chevalier et al. 1992, Mazur and Plazek 1994, Winnik and Feng 1996, Routh and Russel 1998). Latex particles consolidate into a form of close packing at the edge or drying front but remain saturated with water. Continued evaporation from this packed region draws fluid from the bulk solution. This horizontal flux of fluid brings particles into the packed region, propagating the drying front of close-packed particles through the film. When the inner part of the film still has some liquid phase, it provides liquid water to the packed particles at the dried edge, which prevents cracking of the dried edge. However, from the moment when no more liquid phase is supplied to the dried edge, cracks start to form; this is indicated by the point in the graph where the stress decreases vertically, i. e. stress relaxation occurs instantaneously by cracking of the dried film. The maximum drying stress depends on the particle size, the surface tension of the liquid phase, and the capillary pressure (Tirumkudulu and Russel 2004, Laudone et al. 2003, 2004, Wedin et al. 2005).

Figure 3

Drying stress and crack in film formation of SA18 and SA40 latex coatings. The images of drying films for SA40 at stages 1 to 5 are shown along with the schematics of the particle arrangements at three locations: a, b, and c. Micrographs show the difference in cracks in SA18 and SA40 latices after the completion of drying.

Figure 3A also shows the drying stress determined for the latex with lower Tg. As shown here, the low Tg latex SA18 showed much lower drying stress than the high Tg latex SA40, indicating that the low-Tg latex has much greater deformability. Laudone et al. (2003) have shown that a high Tg latex produced greater drying stress than a low Tg latex, and they attributed this to the deformability of the low Tg latex. SA40 exhibited abrupt stress relaxation, which resulted in the severe cracks as shown in Figure 3B. In contrast, SA18 showed a dimple-type crack at the centre of the dried film.

Figure 4 depicts the changes occurring when two latices having different Tg values are applied on a substrate and dried. As the milky latex emulsion on the slide dries, it becomes transparent as the latex particles become packed, and a drying front appears. When the Tg of the latex is lower than the drying temperature, the latex forms a film because of the internal diffusion of polymer chains between the particles. However, at the final stage of drying, a highly concentrated drying stress develops at the centre of the film that dries last, which makes a dimple at the centre of the film with a few large cracks. In contrast, when the Tg of the latex is higher than the drying temperature, many cracks develop at the dry front of the film.

Figure 4

Schematic diagram of the drying process of latex films with different Tg values.

Effect of drying temperature

Drying temperature plays an important role in the development of drying stress. Generally, the drying stress decreases with the increase in drying temperature because this increases the relaxation of polymer chains (Francis et al. 2002). The difference in the thermal expansion coefficients of the coating material and the substrate, which causes thermal stress in the system (Winnik and Feng 1996, Martinez and Lewis 2002a, 2002b), also decreases with the increase of drying temperature because of the increased elongation of the coating layer.

Figure 5 shows the effect of drying temperature on the drying stress for SA40. In this sample, the maximum drying stress decreased with an increase in drying temperature, which also gave lower final drying stresses. Cracks were observed on all of the three films, including when the drying temperature was higher than the Tg of the latex. This indicated that the thermal stresses developed due to the difference in the thermal expansion coefficients of the coating and substrate.

Figure 5

Stress development of SA-2 latex coating dried at three temperatures.

When the drying temperature was lower than the Tg of the latex, cracks occurred at the drying front as drying proceeded (Figure 6). Conversely, when the drying temperature, i. e. 80 °C, was higher than the Tg of the latex, cracks that were generated at the drying front disappeared and left a clear film with a few minor cracks and softer outlines. If the drying temperature is high enough, all of the cracks will disappear because of the interdiffusion of polymer chains in the film.

Figure 6

Confocal laser scanning microscope (CLSM) images of SA40 latex dried at three temperatures.

Effect of pigment

Coating pigments are the principal component in pigment coatings and have a profound effect on the structure of the coating layer and the drying stress. In general, the pigment volume concentration (PVC) in a pigment coating colour is higher than the critical pigment volume concentration (CPVC), which is defined as the point at which there is sufficient binder to provide a completely absorbed layer on the pigment surface as well as within the interstitial spaces between the pigment particles in a close-packed system (Lepoutre 1989).

In general, the drying stress of the coating solution increases with increasing PVC. It has been shown that the drying stress increases rapidly as the PVC approaches the CPVC (Perera and Eynde 1984). This is because when the PVC approaches the CPVC, most polymer chains become adsorbed and immobilised on the pigment surface, thereby increasing the stress of the dried coating layer. However, when the PVC is equal to or higher than the CPVC, cracks may occur between pigments, which reduces the drying stress (Laudone et al. 2003). Even though the drying temperature is lower than the Tg of the binder, the drying stress generally increases as the PVC increases (Perera 2004).

Figure 7 shows the drying stress of a silica coating during drying at 25 °C and 105 °C. The drying stress patterns obtained at 25 °C and 105 °C were similar. However, both the maximum and final drying stresses were lower at a drying temperature of 105 °C than at a drying temperature of 25 °C. The drying stress increased as the silica particles were packed in the initial stage of drying, and from the moment when menisci started to form between pigment particles, the drying stress increased rapidly and reached a maximum because of the capillary force generated by the menisci (Laudone et al. 2004). Then, the drying stress decreased rapidly at the moment when the cracks formed. Because the water in the micropores in the silica particles generates capillary pressure and further contracts the coating layer, the drying stress tends to increase slightly even after cracking. In the case of silica without micropores, the drying stress does not increase further after crack formation (Martinez and Lewis 2002a).

Figure 8 shows the drying stress for the silica coatings containing PVAc as a binder. The PVCs of the coating were 0 %, 10 %, 60 %, and 100 %, and the coatings were dried at 25 °C. The CPVC of the silica was 14.3 %. The coatings with PVCs of 0 % and 10 % formed continuous films without cracks. Addition of silica to the PVAc latex increased the maximum and final drying stresses. The highest maximum and final drying stresses were obtained when the pigment content was 10 %, whereas the drying stress decreased when the pigment content exceeded the CPVC. Figure 9 shows the drying stress for the coatings containing PVAc binder dried at 105 °C. It can be seen that high-temperature drying did not result in an abrupt change in the drying stress, indicating that the thermal relaxation of the drying stress had occurred. When the PVC is higher than the CPVC, the pigment plays the dominant role in the drying stress development, whereas when the PVC is lower than the CPVC, the binder plays the dominant role.

Figure 7

Drying stress of silica coating at the drying temperatures of 25 °C and 105 °C.

Figure 8

Drying stress of silica-PVAc suspension during drying at 25 °C.

Figure 9

Drying stress of silica-PVAc suspension during drying at 105 °C.

Figure 10 shows the drying stress of coating colours containing silica and an SA40 latex binder with a Tg of 39.7 °C, which was dried at 25 °C. Because the Tg of this latex was higher than the drying temperature, the latex film cracked when no silica pigment was present in the coating. The largest final stress was obtained when the coating contained 10 % silica. Figure 11 shows the drying stress obtained for the coating colours containing SA40 binder and dried at 105 °C. The maximum and final drying stresses of samples dried at 105 °C were substantially lower than those of samples dried at 25 °C. When the PVC was lower than the CPVC, the cracked film became a smooth surface at 105 °C because the cracked latex formed a continuous film in the coating layer. However, cracks still remained after drying if the PVC was greater than the CPVC, because the amount of latex present was insufficient to fill all of the gaps between pigment particles. It is interesting to see that the maximum drying stresses for SA40 coating at 105 °C was similar to those of PVAc coating, even though the Tg of SA40 was 51.7 °C higher than PVAc. This may be attributed to the particle size difference of these two latices. Laudone et al. (2004) have shown that smaller particles tend to give low stress during drying.

Figure 10

Drying stress of silica and SA40 suspension dried at 25 °C.

Figure 11

Normalisation of drying stress of silica and SA40 suspension during 105 °C drying.

Figure 12 shows the drying stress obtained for the silica coatings containing SA48 binder and dried at 25 °C. As in the case for SA40, when the PVC was lower or higher than the CPVC, the drying stress curves were similar to those of the latex and pigment curves, respectively. When the drying temperature was 25 °C, cracks were observed in all of the coatings. However, both the final and maximum drying stresses were lower than those of SA40 coatings. In general, when the Tg of latex is low, the drying stress decreases. In contrast, the higher the Tg, the more microcracks are generated, and as these form, stress generation and relaxation occur simultaneously, which contributes to the overall decrease in the drying stress. The drying stress curve in Figure 12 rather gradual increases in drying stress in the beginning while the slope for SA40 coatings are rather abrupt indicating the concurrent occurrence of cracks in SA48 coatings. When the Tg was higher than the drying temperature, numerous cracks formed, and this contributed to relaxing of the drying stress of the film.

Figure 12

Drying stress of silica and SA48 suspension during drying at 25 °C.

Figure 13

Maximum and final drying stresses of silica and SA48 suspension during drying at 105 °C.

Figure 13 shows the drying stress curves of silica coatings containing SA48 binder with a Tg of 47.6 °C and dried at 105 °C. In general, when the drying temperature is much higher than the Tg of latex, a continuous film forms. However, in the silica coating prepared using SA48 as a binder, the SA48 latex film formed several cracks even when dried at 105 °C. It was interesting to see that the silica coating with 10 % PVC still cracked during drying; this shows that the difference in the Tg of SA40 and SA48 latices led to a large difference in their respective film-forming properties.

Properties of inkjet paper

Table 2 shows the properties of the coated papers. No difference in brightness was observed irrespective of the binder type used. PVAc latex showed advantages in picking resistance and bleeding, whereas SA40 and SA48 latices gave a slightly higher opacity and gloss values than PVAc latex. Overall, it appeared that S/A latex can be used in inkjet coating binder. A more detailed investigation of the print quality of inkjet papers prepared using S/A latices is necessary, and should explore print density, water fastness, roundness of printing dots, and coating structure.