Dynamic Interactions of Pigment-Based Inks on Chemically Modified

Article pubs.acs.org/IECR

Dynamic Interactions of Pigment-Based Inks on Chemically Modified Papers and Their Influence on Inkjet Print Quality Sónia C. L. Sousa,† António de O. Mendes,† Paulo T. Fiadeiro,‡ and Ana M. M. Ramos*,† Textile and Paper Materials Unit, University of Beira Interior, Rua Marquês D’Á vila e Bolama, 6201-001 Covilhã, Portugal Remote Sensing Unit, University of Beira Interior, Rua Marquês D’Á vila e Bolama, 6201-001 Covilhã, Portugal

† ‡

ABSTRACT: Research on paper surface modification, paper properties, paper−ink interactions, and their influence on inkjet print quality are currently subjects of great interest. In this study, dynamic interactions between aqueous pigment black ink and modified paper and their effect on print quality are discussed. Formulations of starch and blends of starch and poly(diallyldimethylammonium) chloride or polyoxyethylene (100) stearyl ether were used on base paper. Modified papers were printed on two printers. The paper performance is discussed in terms of surface energy, dispersive and polar components, and air permeability given the ink spreading and absorption. Results showed that paper modified with a blend of starch and poly(diallyldimethylammonium) chloride improves inkjet print quality. This modified paper showed higher black print density and lower line width, raggedness, intercolor bleed, and dot gain because of the spreading−absorption balance arising from the low surface energy and polar component.

1. INTRODUCTION Inkjet printing has become a constant presence on consumer desktops because of its low cost, reliability, quickness, and convenience for printing digital files.1 Usually, it is considered that an inkjet system consists of three elements: printing technology, ink properties, and paper characteristics, each of them contributing to the final print quality. The printing technology affects drop formation and jet stability of the ink. The most important ink properties are viscosity, surface tension, and nature of the colorants.2−4 The paper characteristics, such as surface topography and porosity, surface energy, and chemical groups present at the paper surface determine the ink setting and adhesion.4−8 The surface sizing, coating, and/or calendering are processes used to modify paper properties and improve print quality. Surface sizing involves the application of a formulation on the paper surface, typically composed of starch and a hydrophobizing agent, with a dry solids content ranging from 2 to 15 wt %.9,10 Nevertheless, polymers such as modified starches, polyvinyl alcohol, sodium alginate, carboxymethyl cellulose, polyurethane, styrene-acrylate, and styrenemaleic anhydride copolymers have also been used.5,11−15 Sizing formulations including polyoxometalates, polymers based on glycol ethers, magnesium chloride, or quaternary agents derived from fatty acids are also reported in the literature.6−8,14 Most published studies concerning paper surface treatments are focused on the coating contribution in the inkjet print quality.16−27 Regarding the surface sizing, a few studies focusing this subject have been published.4−8,11,14 However, this matter is particularly important because office papers are uncoated but surface sized. The dynamic interactions between ink and paper such as drop impact, spreading, and penetration affect inkjet print quality. According to Kannangara and Shen,28 the impact spreading process sets up the initial conditions for the subsequent ink wicking and penetration into the paper surface. Although absorption and spreading of the inkjet ink on © 2014 American Chemical Society

uncoated papers are associated with the print quality, studies regarding the relationship between them are limited and incomplete.5,8 Several studies have been carried out concerning the spreading and/or absorption processes of liquid drops on porous solid surfaces. However, the obtained results are not correlated with print quality.28−39 In these studies, high-speed cameras were used to capture images of the ink drop on the substrate surface, under side, and/or top view over time. Simultaneously, studies of the ink penetration depth were also carried out during the last ten years.40−49 The most common methods for determining the penetration depth of the ink (rotogravure, offset, or inkjet) into paper are based on crosssection analysis, differing in sample preparation, image acquisition, sample size, and resolution.41−44,46,47 It is also referenced in literature the delamination method of the printed paper.45,49 Confocal laser scanning microscopy was also applied to determine the three-dimensional distribution of the black and magenta dye-based inkjet inks in printed paper samples by detection of fluorescent agents added to the inks.40 A nondestructive method referenced in the literature consists of estimating the ink penetration depth from the spectral reflectance based on the Kubelka−Munk theory.44,48 According to Yang et al.,41,44 the internal sizing of the paper decreases the penetration depth of the dye-based inks. In particular, this author found remarkable impacts of ink penetration on optical density, which cause reduction of color saturation and color shift.44 Contrary to these results, Pauler et al.48 found that for dye-based inks the internal sizing of the paper has no impact on color reproduction but does reduce ink penetration into the paper. However, for pigment-based inks, the modification of Received: Revised: Accepted: Published: 4660

October 24, 2013 February 17, 2014 March 5, 2014 March 5, 2014 dx.doi.org/10.1021/ie403595f | Ind. Eng. Chem. Res. 2014, 53, 4660−4668

Industrial & Engineering Chemistry Research

Article

Figure 1. Chemical structure of (a) starch (S), (b) poly(diallyldimethylammonium) chloride (PD), and (c) polyoxyethylene (100) stearyl ether (POE).

the paper surface for reduction of the ink penetration has a positive effect on color reproduction. This work focuses on dynamic interactions (spreading and absorption) of black pigment-based inkjet inks on chemically modified papers and their influence on inkjet print quality parameters. The impact of chemical modifications on paper properties is also evaluated. The study of ink−paper interactions includes the analysis of the dynamic interaction of the ink drops on the paper surface and subsequent assessment of ink penetration depth into the paper structure. The present study concerns the understanding of which paper properties affect the absorption and spreading processes and, simultaneously, how they influence the print parameters.

was 20 m/min. Modified papers were dried on an IR dryer for 2 min. After this time, the temperature reached 100 °C. Papers were then conditioned at 23 °C and 50% relative humidity before characterization, printing, and evaluation of the ink− paper interactions. The modified papers were not calendered. 2.2. Characterization of the Chemically Modified Papers. The formulation pick-up was determined by the average basis weight increase after paper surface modification. Basis weight was obtained according to ISO 536:1995. Thickness was determined according to ISO 534:2011 using an Adamel Lhomargy model MI 20 micrometer. Bekk smoothness was measured on the Messmer Büchel model K533, in accordance with ISO 5627:1995. Surface roughness parameters Ra (arithmetic average height), Rq (root-meansquare roughness), Rp (maximum peak height), and Rv (maximum valley depth), were acquired using an Altisurf 500 optical profilometer coupled with the PaperMap 4.0 software. Each reported value corresponds to the mean of four 2D-profile analyses with 30 mm length and a resolution of 0.5 μm. To determine Sdr parameter (interfacial area ratio), 3D-profile analyses were carried out in the same measurement system. For each sample, a square area of 4.8 × 4.8 mm2 was scanned using a spatial resolution of 2 μm. Bendtsen air permeability was assessed following ISO 5636-3:1992, using the Andersson & Sorensen smoothness and porosity tester. The porosity of the samples was determined using a mercury porosimeter Micromeritics AutoPore IV 9500 series. The maximum applied pressure of mercury was 30 000 psia, corresponding to pores of about 0.006 μm diameter. Contact angle measurements were carried out on an OCAH 200 DataPhysics instrument by sessile drop method using deionized water, methylene iodide (Aldrich, 99% purity), and ethylene glycol (Merck, > 99.5% purity) as test probe liquids to determine the surface energy parameters of each paper sample, using the Owens, Wendt, Rabel, and Kaelble approach.50−52 Surface tension of the used liquids can be found in the literature.53 2.3. Ink−Paper Interactions. The ink−paper interactions assessment comprises two phases. The first concerns the analysis of the ink drop behavior on the paper surface. The second is devoted to the examination of the ink penetration depth into the paper structure. To accomplish these objectives, two optical systems were implemented by our research team.54−56 These systems have been used to carry out experimental assays on the modified paper using black pigment-based aqueous inks Lexmark 34-XL and the HP C9412A. An ink drop of approximately 0.5 μL is ejected from the needle of a syringe that falls on the paper surface. The drop impact has an average speed of 0.67 m/s. Three high-speed image detectors are used to capture simultaneously three different views of the ink drop on the paper surface. These

2. EXPERIMENTAL SECTION 2.1. Chemical Modification of the Paper Surface. A base paper with 76.3 g/m2 basis weight, produced from bleached Eucalyptus globulus kraft pulp and internally sized with alkenyl succinic anhydride (ASA) was used for surface modification. The following chemicals were used for the formulations: native maize starch (supplied by the industry) with amylose/amylopectin ratio of 0.35 and a molecular weight of 3 × 108 g/mol, high molecular weight (4−5 × 105 g/mol) poly(diallyldimethylammonium) chloride (Aldrich), and polyoxyethylene (100) stearyl ether (Brij S 100) (Sigma, molecular weight 4670 g/mol). The chemical structures of these compounds are shown in Figure 1. Native maize starch was enzymatically converted before application, while the other chemicals were used as received. The enzymatic conversion of starch was carried out according to the following procedure: 60 g (o.d.) of starch granules were added to water (240 mL at 60 °C) under mechanical stirring. Then, 27 mg of α-amylase was added, and the mixture was heated to 80 °C and left at this temperature for 10 min (always under stirring). The enzymatic conversion was stopped by the addition of 10 mL of a ZnSO4 solution (30 g/L). The obtained suspension was heated to 90 °C and left stirring at this temperature for 15 min. This enzymatic conversion of starch resulted in the decrease of its molecular weight to 3.4 × 106 g/ mol. Three formulations were prepared with 12 wt % solids content; one of them composed only of starch (S) and the other two composed of a blend 50:50 (% w/w) of starch with poly(diallyldimethylammonium) chloride (PD/S) and starch with polyoxyethylene (100) stearyl ether (POE/S). These formulations were applied on one side of the base paper using a laboratory reverse roll coater (Mathis Type RRC-BW) at room temperature. The distance between the dip roll and the applicator roll was 125 μm. The pressure between the applicator and transport rolls was 3.5 bar, and the roll speed 4661

dx.doi.org/10.1021/ie403595f | Ind. Eng. Chem. Res. 2014, 53, 4660−4668


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Table 1. Pick-up, Structural, and Surface Properties of Modified Papers property

S

PD/S

POE/S

pick-up (g/m2) thickness (μm) Bekk smoothness (s) Ra (μm) Rq (μm) Rp (μm) Rv (μm) air permeability (μm/Pa s) porosity (%) Dmean pore (μm)

3.5 ± 0.22 107.2 ± 1.33 14.3 ± 1.33 2.7 ± 0.11 3.5 ± 0.13 8.5 ± 0.36 11.1 ± 0.62 2.2 ± 0.50 58.6 ± 0.3 1.11 ± 0.45

4.3 ± 0.25 110.0 ± 1.91 14.1 ± 1.15 2.7 ± 0.05 3.4 ± 0.05 8.3 ± 0.13 10.2 ± 0.59 2.5 ± 0.24 60.2 ± 0.4 1.61 ± 0.14

1.6 ± 0.30 109.2 ± 1.21 16.1 ± 1.14 2.8 ± 0.05 3.5 ± 0.07 9.4 ± 0.45 10.8 ± 0.21 7.8 ± 0.21 60.3 ± 0.1 1.36 ± 0.02

4 by the least-squares method using the Solver add-in from Microsoft Excel.

image detectors record an image every 4.65 ms during a time period of 10 s. Two of the views, which are perpendicular to each other, are lateral views that coincide with the machine and cross directions of the paper. The third view corresponds to the top view of the paper surface. The images are then processed using a Matlab application software in order to calculate the contact angles, drop volume, spreading, and radial wicking areas of the ink drop. The results presented in this work correspond to the average taken from four experimental assays. To study the ink penetration depth into the paper structure, the paper area where the ink drop was deposited was crosssectioned using a rotary microtome and zoom images were captured by an image detector fitted with a proper zoom lens. For this purpose the paper sample was embedded in paraffin and after solidification was placed in the sample holder of the microtome. The cross-section images were used to evaluate the form of ink distribution below the paper surface and also to determine the ink penetration depth. In addition to this information, the data acquired during the ink drop interaction on the paper surface, namely, the drop volume and radial wicking area, were also used. The ink penetration depth calculation follows the procedure described by Lundberg et al.,39 with a few exceptions. This procedure consists of several steps, described below. The volume of absorbed ink over time Vabsorbed was calculated according to eq 1. Vabsorbed = Vinitial − Vabove − Vevap

d(t )fit = kt a

In eq 4, a and k are constants that depend on the ink−paper combination. 2.4. Characterization of the Inks. The surface tension of the used inks was evaluated by the pendant drop method using the OCAH 200 DataPhysics instrument. The apparent viscosity of the inks was assessed using a Hakke R150 rheometer with a cone/plate geometry. The assays were carried out varying the shear stress from 0 to 40 Pa for 180 s, obtaining as a response the shear rate, which ranged from 0 to 6500 s−1. The apparent viscosity was determined in the range of 3000 at 6500 s−1, where the two variables are directly proportional. 2.5. Print Evaluation. A printing target composed of solid areas, lines, and dots was drawn using desktop publishing software (Adobe InDesign CS). The solid area was printed with pure black (C = 0, M = 0, Y = 0, K = 100) for evaluation of the black print density. Black line and black/yellow lines on yellow/ black background were drawn with widths of 350 and 500 μm, respectively, for line quality parameters evaluation. Regarding the evaluation of the dot quality, perfectly circular black dots of 400 μm diameters were drawn. The target was printed on the modified papers using two inkjet printers (Lexmark X-8350 and HP Photosmart Pro B8850), both with pigment-based black inks. The base paper without surface modification (reference paper, RP) was also printed in order to evaluate the impact of paper chemical modification on inkjet print quality. The print conditions were set to “plain paper” and “best print quality”. Black print density measurements were carried out with an AvaMouse hand-held reflection Spectrophotometer. The analysis of line and dot quality parameters was performed with a QEA, personal image analysis system (PIAS-II), whose measurement principle is based on ISO 13660 standard. Horizontal black line width, raggedness (a measure of the contour irregularity), and blur (characterizing the distinctness of the transition from line to background) were measured. Intercolor bleed (a measure of the spreading between two adjacent colors) was calculated from the black and yellow lines width. In addition, black dot gain (increase of the dot size) and circularity (deviation of the dot shape relative to a perfect circle) were also measured. Dot gain, in percent, corresponds to the difference between measured and theoretical diameters (400 μm).

(1)

where Vinitial is the initial ink drop volume, Vabove the volume of the ink drop on the paper surface, and Vevap the evaporated volume. It was assumed that Vevap is negligible compared to volume change due to the absorption over the time scale in question. On the basis of the assumption that the volume of ink absorbed has a paraboloid shape, the ink penetration depth d(t) was calculated from eq 2, whereas eq 3 was used when the ink distribution has a cylindrical shape. V (t )absorbed =

πR2(t ) d (t ) 2

V (t )absorbed = πR2(t )d(t )

(4)

(2) (3)

In eq 2 and 3, R(t) represents the radial wicking radius through time, considering that area has a circular shape. The calculated d(t) values were corrected with the porosity to take into account the fact that beyond the pores there are all the paper constituents. To evaluate the time-dependency of the ink penetration, the calculated values for the ink penetration depth were fitted to eq

3. RESULTS AND DISCUSSION 3.1. Characterization of the Papers. The results of the paper characterization concerning the thickness, roughness 4662



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POE, an ethoxylated (100) fatty alcohol, is a nonionic surfactant; thus, the POE/S formulation produces papers with higher surface energy and significant polar component when compared with the papers modified with the PD/S formulation. 3.2. Ink−Paper Interactions. On ink−paper interaction studies, ink drop area and radial wicking area are normalized with the projected ink drop area A0 associated to the initial volume Vi, assuming that it is approximately spherical before the impact. Similarly, the volume change is also normalized using the initial volume. This normalization eliminates the effects of drop size due to the differences between initial volumes of the different performed assays. The papers exhibit similar trends concerning dynamic contact angles for both black inks; however, each paper shows a differentiated behavior between them (Figure 3). The POE/S-modified paper reveals an initial sharp decrease of the contact angle due to the ink spreading and strong absorption (Figures 3−6). Initially, the ink drops spread as the region directly beneath the ink drop becomes saturated. For long time periods, the saturation region extends beyond the edge of the ink drop and the suction overpowers the spreading, causing drop retraction. For this reason, after reaching a maximum value, the ink drop area shows a decreasing trend over time (Figure 6). Furthermore, the radial wicking area increases continuously over time (Figure 5). For the S-modified paper, the contact angle at short times is determined by the ink drop spreading, whereas at long times it is determined by the ink absorption (Figures 3−6). In turn, the PD/S-modified paper presents a slight decrease in contact angle due to the low absorption into paper bulk (Figures 3 and 4). This paper does not present wicking or spreading of the ink drop (Figures 5 and 6). Figure 7 shows the ink drop images acquired at specified times, which illustrate the differences in the interaction of the black ink drops on the studied papers. Images of the cross-section area of a single ink drop deposited onto the PD/S-modified paper are depicted in Figure 8. The PD/S-paper and the Lexmark ink are the only combination where the ink does not penetrate the overall thickness of the paper, staying closer to the surface. For this combination, the ink profile appears cylindrical, similar to what happens for the remaining ink−paper combinations, except for the combinations S-paper−Lexmark ink and PD/S-paper−HP Photosmart ink, where the ink distribution inside the samples shows a paraboloid shape. In the analyzed paper samples, the filter cake of ink pigment found by Desie et al.33 does not appear because these papers have pore diameters larger than

parameters, and Bendtsen air permeability and porosity properties are shown in Table 1. In the same table are also listed the formulation pick-up values, which range from 1.6 to 4.3 g/m2 depending on the formulation viscosity because their solids content and application conditions were kept constant. Although the starch pick-up is higher than POE/S pick-up, the thickness of the paper modified only with starch is lower than that of the POE/S modified paper. This result suggests that the starch formulation may penetrate into the paper structure causing slightly lower values of air permeability, porosity, and mean pore diameter. Concerning roughness, minor differences are observed, either by Bekk smoothness or by topography parameters, showing that POE/S paper is rather rougher than the others. The influence of topography on the contact angle measurements is usually taken into account by the Wenzel equation using the Sdr parameter obtained from 3D topographical measurements. In this study, the Sdr values are very similar for all papers surface (11.6 and 11.7%), and for this reason the contact angles were not corrected because no significant differences were noticed. Figure 2 shows the effects of the paper surface

Figure 2. Surface energy parameters of the chemically modified papers.

modification on the surface energy and their dispersive and polar components. The PD/S-modified paper exhibits surface energy and polar component lower than those of the S- and POE/S-modified papers. Starch is a highly hydrophilic polymer because of its hydroxyl groups,57,58 which explains the higher surface energy and polar component of the S-modified paper when compared with PD/S-paper. The formulation with starch and PD, the latter being a linear polymer with a hydrophilic charged quaternary ammonium group in each chain unit,59,60 reduces the surface free energy and its polar component. The

Figure 3. Contact angle evolution of the Lexmark (left panel) and HP Photosmart (right panel) ink drops on the modified papers. 4663



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Figure 4. Normalized ink drop volume as a function of time for the Lexmark (left panel) and HP Photosmart (right panel) inks on the modified papers.

Figure 5. Normalized radial wicking area of ink drops as a function of time for the Lexmark (left panel) and HP Photosmart (right panel) inks on the modified papers surface.

Figure 6. Normalized area of ink drops as a function of time for the Lexmark (left panel) and HP Photosmart (right panel) inks on the modified papers surface.

the pigments (about 1 μm and 100 nm, respectively), which allows their migration. Regarding the behavior of both inks, some differences are visible arising from the different surface tension and viscosity. The surface tension of the Lexmark and the HP Photosmart inks are 45.6 ± 0.3 and 36.8 ± 0.3 mN/m, respectively. The apparent viscosities are 5.9 ± 0.1 and 6.4 ± 0.1 mPa s for the Lexmark and HP Photosmart inks, respectively. On the three papers, the ink spreading area for the Lexmark is higher than that for the HP Photosmart, both at the beginning and at the end of the process (Figure 6). These results show that the slightly higher viscosity of HP Photosmart ink attenuates its spread. Thus, it is assumed that the effect of viscosity is greater than that of the surface tension on the spreading process.

However, from the analysis of Figure 6, it can be observed that the difference in spreading area between the two inks in the beginning of the process is higher than that when it reaches a maximum value, except for PD/S-modified paper. However, when spreading occurs in papers with a high polar component, the importance of surface tension is greater than that of the viscosity. Because the roughness is similar between the samples, it is believed that this does not influence whatsoever the differences recorded in the spreading process. The absorption process is more marked for the HP Photosmart ink. This statement results from the greater variation of volume and higher wicking of the ink drop over time (Figures 4 and 5). In addition, the ink penetration for HP Photosmart is higher than that for the Lexmark ink, except for 4664



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Table 2. Constants a and k Arising from Fitting to Calculated Penetration Depth for the Lexmark and HP Photosmart Black Inks Lexmark X-8350

HP Photosmart Pro B8850 a

paper

a

k (μm/s )

a

k (μm/sa)

S PD/S POE/S

0.48 0.60 0.26

48.4 14.7 92.6

0.55 0.91 0.29

30.3 14.1 89.0

(areas where the pigments remain) is more visible than the vehicle. However, this latter may spread and penetrate the paper bulk deeper than the pigment. Moreover, in some cases, the maximum penetration depth calculated is higher than the paper thickness. This may be due to the radial wicking of the vehicle over time, which was not taken into account. Regarding the fitting parameters shown in Table 2, the k values have different dimensions so they cannot be compared. Concerning the constant a, the values range from 0.26 to 0.60 for the Lexmark ink and from 0.29 to 0.91 for the HP Photosmart ink. According to Lundberg et al.,39 the constant a is roughly 0.5 for several paper grades, i.e., fairly close to timedependency of liquid penetration given by the Lucas− Washburn equation. However, in the present study, the results are rather different between them. The PD/S-modified paper exhibits the highest value especially for HP Photosmart ink, suggesting that the ink penetration depth changes linearly with time. The S-modified paper is the only one in which the timedependency of the ink penetration depth is quite near to that given by the Lucas-Washburn equation. Moreover, the lowest values of the constant a are given by the POE/S-modified paper, showing that the ink penetration depth varies too quickly in the initial stage, becoming slower for longer times. These results suggest that air permeability seems to have an important effect on the ink penetration because the S and POE/Smodified papers have similar surface energy parameters and distinct time-dependency of penetration. Surface energy is also important, as S and PD/S-modified papers have similar air permeability but different surface chemistry and show different time-dependency of penetration. Concerning ink spreading and absorption, the behavior of the samples is determined simultaneously by surface energy and permeability. The S and POE/S-modified papers have high surface energy and elevated polar component, but the low permeability of the S-modified paper results in lower absorption and higher spreading of the ink compared to the POE/S-

Figure 7. Images of an ink drop on paper surface at time 0 and 10 s, under two views (MD, machine direction; TV, top view). Left columns: Lexmark black ink. Right columns: HP Photosmart black ink.

Figure 8. Cross-sectional images of the ink penetration into the PD/Smodified paper. Left panel: Lexmark black ink. Right panel: HP Photosmart black ink.

the POE/S-modified paper where differences are not visible because of the strong penetration. Unlike the spreading process, the absorption is governed by the surface tension because inks with lower surface tension are more absorbed. The ink penetration depth over time was calculated from eq 2 or 3 depending on the distribution of the ink inside the paper. This selection was based on the cross-sectional images of the paper after drop deposition. The penetration depth fitting and the constants a and k are displayed in Figure 9 and Table 2, respectively. It appears that the calculated ink penetration depth is higher than that observed on the cross-sectional images. This happens because in the images the black spot

Figure 9. Fitting of the calculated penetration depth for the Lexmark (left panel) and HP Photosmart (right panel) ink on the modified papers. 4665



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spreading, i.e., raggedness and circularity follow the same trends as line width or dot gain. It is noteworthy that the results for line and dot print quality parameters obtained on both printers cannot be compared because the differences are a result not only of the ink properties but also of the operating mode of the printers, namely, resolution, size, and deposition of the ink drops. When the results of the ink−paper dynamic interactions are compared, evaluated by monitoring the ink drop behavior on the papers surface and the results of the print quality, it is possible to conclude that the study of the ink drop spreading and absorption can predict print quality parameters such as black print density, line width, intercolor bleed, and dot gain. Papers with higher ink spreading like the S-modified papers exhibit worse performance for the aforementioned line and dot quality parameters. Papers that present greater ink absorption exhibit lower black print density because of the entrainment of the pigment particles together with the ink vehicle into the paper bulk. Even though the volume of the ink drops used in the study of ink−paper interaction is considerably higher than that used by inkjet printers, the obtained results showed that the optical systems employed are valuable tools to predict inkjet printing quality.

modified paper. Thus, the spreading is comparatively favored to the absorption when we have, simultaneously, papers with low permeability, high surface energy, and elevated polar component. The PD/S paper has minor surface energy and polar component, which limits ink absorption and spreading. 3.3. Print Quality. Concerning black print density (Figure 10), the modified papers printed on the Lexmark printer

Figure 10. Black print density for the papers printed on Lexmark X8350 and HP Photosmart Pro B8850 printers.

exhibited values higher than those of the reference paper, whereas the modified papers printed on the HP Photosmart printer showed values of black print density either higher or slightly lower than those of RP. The PD/S-modified paper exhibits superior black print density, followed by the S-modified paper and, last, by the POE/S-modified paper. This sequence for print density corresponds inversely to the absorption behavior of the modified papers, i.e., minor ink absorption originates higher print density. It should be noted that the standard deviation obtained for the S-modified papers on both printers is higher than that of the other papers because of the nonuniform ink coverage. Table 3 shows the effect of chemical modification of the paper surface on the print quality parameters. It can be noticed that the paper modified with starch shows poorer quality even when compared with the reference paper. In contrast, the PD/ S-modified paper gave the best print quality, except for the blur when it is printed on the Lexmark. The results for the modified papers demonstrated that a lower surface energy with a slight polar component is favorable for black ink restraint on the paper surface, preventing it from spreading and therefore improving the dot and line print quality. Sousa et al.8 had reported a positive effect of low surface energy of the paper on line and dot quality parameters. From the data in Table 3, it is also possible to observe a relationship between spreading and uniformity of the ink

4. CONCLUSION In this work, the paper surface was chemically modified and the results showed that the used chemicals produce papers with slight differences in air permeability and distinct surface energy parameters. The PD/S-modified paper presents lower surface energy and slight polar component, limiting spreading and absorption of the ink drops. Consequently, higher black print density and good print quality of lines and dots were achieved. It was found that in the S-modified paper the spreading of the ink is favored compared to the absorption, resulting in poor print quality of the line and dot. This happens because of the combination of low permeability, high surface energy, and high polar component. The evaluation of the dynamic behavior of the ink drop on the paper surface can predict print quality parameters, such as black print density, line width, intercolor bleed, and dot gain. The study of ink−paper dynamic interactions demonstrates that the spreading of Lexmark ink is superior to that observed for HP Photosmart ink. The higher viscosity of the latter ink creates resistance to spreading that is greater than that resulting from the higher surface tension of the Lexmark ink. However, the ink absorption seems to be more influenced by surface tension than by viscosity because the absorption is more pronounced for low surface tension ink (HP Photosmart).

Table 3. Line Width, Raggedness, Blur, Intercolor Bleed, Dot Gain, and Circularity for the Analyzed Papers printer Lexmark X-8350

HP Photosmart Pro B8850

paper

width (μm)

RP S PD/S POE/S RP S PD/S POE/S

469 ± 1 475 ± 8 440 ± 2 456 ± 3 404 ± 1 408 ± 15 382 ± 7 394 ± 1

raggedness (μm) 16.00 19.75 13.50 14.25 13.30 14.25 9.00 9.25

± ± ± ± ± ± ± ±

1.00 1.27 0.50 1.46 1.06 2.85 1.12 0.35 4666

blur (μm) 180.50 167.75 170.50 167.00 161.00 171.00 155.00 161.00

± ± ± ± ± ± ± ±

6.40 6.49 5.41 0.50 4.47 5.59 3.64 4.47

ICB 43.40 44.00 36.75 37.63 51.40 49.13 38.50 42.88

± ± ± ± ± ± ± ±

dot gain (%) 2.13 2.37 2.06 0.40 1.59 0.95 0.56 0.73

18.50 19.85 12.55 15.90 15.80 24.10 12.95 17.00

± ± ± ± ± ± ± ±

0.17 0.12 0.06 0.04 0.17 0.13 0.11 0.19

dot circularity 1.97 1.78 1.72 1.73 1.81 1.91 1.76 1.81

± ± ± ± ± ± ± ±

0.15 0.07 0.16 0.17 0.17 0.31 0.03 0.11



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It was verified that time-dependency of ink penetration is affected by surface energy of the paper, but in papers with high surface energy and polarity, the air permeability also plays an important role. The overall results showed that paper surface modification with the PD/S formulation presents an interesting solution for improving inkjet print quality.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +351275319885. Fax: +351275319730. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was carried out under the project PADIS (FCOMP01-0202-FEDER-05348), which was cofinanced by the European Union through FEDER (Fundo Europeu de Desenvolvimento Regional) in the extent of QREN (Quadro de Referência Estratégico Nacional 2007-2013) through COMPETE (Programa Operacional Factores de Competitividade). The authors acknowledge RAIZ - Institute of Forest and Paper Research for providing starch and base paper. We also appreciate the access to laboratory reverse roll coater and paper testing facilities.

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REFERENCES

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