ABSTRACT USE OF ULTRASOUND TO PREDICT INK-JET PRINT QUALITY. By Neeraj Sharma

Transcription

1 ABSTRACT USE OF ULTRASOUND TO PREDICT INK-JET PRINT QUALITY By Neeraj Sharma Quantifying water absorption and spreading on paper is possible with conventional tests such as HST, contact angle etc. but the time scale of these methods is much larger than that relevant to ink-jet printing [1]. The intensity of sound waves passing through the sheet of paper changes with the wetting of the sheet [2,3]. This response of ultrasound to wetting of paper can be measured on a time scale relevant to ink-jet printing. In this work the efforts are made to study the correlation between ultrasound response to wetting of papers and the print quality of those papers. Papers are also characterized for water absorption and spreading characteristics using conventional test methods. Papers were studied in two categories: photographic papers and office copy papers. Photographic quality papers were found having some correlation between print quality parameters and ultrasound response to wetting while office copy papers did not show any correlations.

2 USE OF ULTRASOUND TO PREDICT INK-JET PRINT QUALITY A Thesis Submitted to the Faculty at Miami University in partial fulfillment of the requirement for the degree of Master of Science Department of Paper Science and Engineering By Neeraj Sharma Miami University Oxford, Ohio 2005 Advisor Dr. M D Sikora Reader Dr. D W Coffin

3 Table of Contents CHAPTER I INTRODUCTION Introduction Problem Statement. 2 CHAPTER II BACKGROUND Inkjet printing Ink jet print quality parameters Dye fixation in ink-jet printing Wetting of fibrous surface Paper parameters affecting ink jet print quality Role of pore morphology Degree of sizing of paper Paper roughness Ultrasound transmission through the paper during wetting Illustration of Penetration Dynamic Analyzer (PDA) CHAPTER III EXPERIMENTAL PROGRAM 24 CHAPTER IV RESULTS AND DISCUSSION Paper characterization Surface characterization Paper characterization for sizing 38 ii

4 1.3 Characterization for surface energy Acoustic response to photographic quality papers and office copy papers Trials with PDA Final PDA runs for office copy papers and photographic quality papers Results obtained form print quality measurements Data analysis Data analysis, using data obtained from conventional tests Data analysis, using data over a 0.9 second time span Data analysis after normalizing the data obtained CHAPTER V SUMMARY OF RESULTS CHAPTER VI CONCLUSIONS 88 CHAPTER VII RECOMMENDATIONS REFERENCES 91 APPENDICES 93 iii

5 CHAPTER I INTRODUCTION 1. Introduction In the last few years, the popularity of ink-jet printing has grown dramatically for use with desktop computing systems. It has been realized that ink-jet printing places much greater demands on paper than laser printing and high speed copying [4]. In the later processes ink used melts on the surface to print paper, while in inkjet printing, the ink used is often water based. The printing process in inkjet printing involves controlled penetration as well as spreading of ink into paper, placing greater demands on paper. It has been generally accepted that the colorants used in printing should be deposited in a way that they remain near the paper surface to maximize color density and contrast while minimizing show through. In ink-jet printing it takes a fraction of a second for a droplet to print the surface. A 98 % correlation can be calculated between the dot size at 120 ms and the final dot size after a drying time of several hours [1]. The waterbased ink actually depends on penetration and absorption for its drying mechanism. Some evaporation of water takes place, but this drying mechanism is often very slow [1]. Therefore initial penetration behavior becomes a very important contributor to the print quality. There is need for characterization of papers on the basis of initial wetting behavior. A number of techniques like Cobb, HST etc. exist to characterize wetting in paper, but these cannot differentiate papers on the basis of initial wetting, which is of the order of only a few milli-seconds. Using ultrasound to explore initial wetting of paper seems to give results which are encouraging as intensity of ultrasound passing through 1

6 paper changes remarkably with wetting [2,3]. This change in the intensity of ultrasound transmission can be recorded at a time scale of milli-seconds, which is appropriate for inkjet printing. When sound waves are transmitted through a sheet of paper being wetted with water, the intensity of waves reaching the receiver changes with time, based upon the wetting characteristics of the specimen [2,3]. Different response curves can be obtained for different grades of paper depending on the rate of penetration by water. In this research, ultrasonic penetration response curves have been generated for different ink-jet printable papers. Papers tested were office copy papers and photographic quality papers, because they represent opposite ends of the print quality spectrum for inkjet printable papers. The objective of this research was to compare the results for office copy papers and photographic quality papers for inkjet performance by the printing industry standards of print density and dot area. The final challenge was to explore possible correlation between the ultrasonic response curves and standard print quality criteria. 2. Problem Statement The purpose of this research was to determine whether time-dependent acoustic response curves obtained during Z- directional wetting of a paper can permit predictable correlation to ink jet print quality parameters. 2

7 The objectives of this research were: 1. Quantify the differences between photographic quality and office copy papers for ink jet performance by the printing industry standards of print density and dot gain. 2. Determine if response to acoustic propagation can be used to predict ink jet paper print quality measured by print density and dot area. 3

8 CHAPTER II BACKGROUND 1. Ink jet printing Ink jet printing is a radical departure from conventional printing and reprographic technologies. It relies on the ejection of uniformly shaped droplets of dye solutions that form an image on paper. Typical dot size produced on the paper by the ink droplet ranges from 300 µm to 600 µm, depending upon the properties of the substrate [1]. Ink jet can be continuous, where drops are selected electrostatically for imaging, or of the drop-ondemand type, where drops are produced only when required, thus obviating the need for electrostatic deflection [5]. In ink jet printing, similar to thermal transfer printers, small drops of liquid ink are dropped on the paper surface. The flow properties of the ink-jet ink are crucial to performance if drop formation is to be correct. For some continuous ink-jet (CIJ) systems this flow can be as much as one million droplets per second although 64,000 per second are common [6]. Drying, stability and adhesion are also important. Two other properties of concern are surface tension and conductivity (for continuous drop printers). All of the early inkjet inks were dye-based but this gives problems with the durability of the print. Dye based systems are generally of poor light fastness and water resistance. Many of the continuous jet inks have now gone to pigmented formulations to get opacity and greater durability but they don t offer vibrant colors that dye can. 4

9 Ink jet printing creates colors with halftone methods based on four different basic colors, which are cyan, magenta, yellow and black. Dots could partly overlap, but there is no color mixing possibility. Due to very fine dots ink jet printers allow photo realistic results especially with specially coated paper. 2. Ink jet print quality parameters Print quality is traditionally defined mainly by two parameters: 1) print density, 2) dot gain. Print density is a measure of the reflection density of the image, and the higher this value, the higher the perceived quality of the image. Print density is defined as, D = log , where R is the % reflection. Density of an image can be measured by using a R reflection Spectrodensitometer. Dot gain is a measure of the fractional increase in area of the single ink jet drop after it strikes the paper surface [7]. A large value of this quantity is undesirable for print quality as it reflects excessive ink spreading after contacting the surface. This results in a fuzzy image. It is measured using the single pixels in the print pattern with a microscope interfaced to an image analysis apparatus [7]. It can also be calculated by using a spectrodensitometer. A spectrodensitometer calculates dot size using either the Murray-Davies formula or the Yule-Nielson formula [8]. Murray-Davies simply calculates dot by comparing the density of tint minus paper with the density of the solid minus paper. 5

10 The Yule-Nielson formula is similar to Murray-Davies except that it allows compensating for the amount of light that is absorbed or trapped when a dot measurement is taken. This is done by first dividing the densities of the paper and solid by an n factor value that is based on the properties of the substrate material. The Murray-Davies formula for calculating the dot is: ( Dt ) 1 10 Apparent Dot Area = ( Ds ) 1 10 *100 Where: D t = Density of tint minus density of paper D s = Density of solid minus density of paper The Yule-Nielson formula for calculating Dot is Apparent Dot Area = ( Dt ) / n ( Ds ) / n *100 Where: D t = Density of tint minus density of paper D s = Density of solid minus density of paper n = n factor 3. Dye fixation in ink jet printing Typical Home Ink-Jet Printing is unique among important paper printing processes in the amount and mobility of the applied inks. Offset, gravure, flexo, and newsprint inks are applied in layers about 1-5 µm in thickness for each color. Before drying, ink-jet ink layers may be 15 µm or more in thickness for each color [9]. Offset, gravure, and flexo inks have binders to hold the colorants in position; most aqueous inkjet inks have none. Inks containing binder increase markedly in viscosity when small 6

11 amounts of solvent are removed; ink-jet inks remain mobile even when most of their solvent volume is removed [9]. Most ink-jet color inks are dilute solutions of one or more acid dyes. The solvents, which account for up to 98% of the ink weight, are blends of water and various organic materials, typically high-boiling-point alcohols [9]. For good printed appearance, the dyes should be fixed on the paper s outer surface with only enough lateral spreading from the position of drop impact to merge adjacent drops in solid colors. In fact, proper dye fixation is the key to several components of ink-jet print quality, including (a) high optical density or intensity of the color, (b) high resolution or sharpness of ink boundaries, (c) low image print-through to the back side of the paper, and (d) high resistance of the image to smearing when wetted [9]. 4. Wetting of fibrous surface Wetting of fiber is a displacement of a fiber-air interface with a fiber-liquid interface. Wetting of a fibrous assembly is a complex process. Various wetting mechanisms, such as spreading, immersion, adhesion, and capillary penetration, may operate simultaneously. The forces in equilibrium at a solid liquid boundary are commonly described by the Young-Dupre equation [10,11]: γ SV - γ SL = γ LV cosθ..(2.1) 7

12 Where γ denotes an interfacial tension; the subscripts S, L, and V denote solid, liquid, and vapor surfaces, respectively; and θ is the equilibrium contact angle. 5. Paper parameters affecting ink jet print quality Parameters which govern the ink jet print quality, include paper surface properties like porosity, roughness, and degree of sizing [1] Role of pore morphology: Experiments on a range of pore networks illustrate the critical role played by morphology and fluid properties in determining the physics of fluid penetration [12]. The kinetics of capillary penetration of wetting liquids into porous media is of particular interest due to its applications in the paper industry and in printing technologies. Conventional inks set while passing through a printing press by capillary transport within the pores on the surface of the paper. As the ink vehicle is drawn from the newly transferred ink film into the pores of either the paper coating or the surface of an uncoated paper, resin particles concentrate in the ink and its effective shear viscosity rises dramatically; typically, by greater than three orders of magnitude [12]. It is well known that a high concentration of micro pores in the surface of paper gives the most desirable ink jet printing results. Rather than pore sizes, sharp discontinuities in the walls of pores that are common in paper and coatings may determine the rate of penetration [12]. 8

13 Washburn theory is commonly used to model the penetration of liquids into porous materials where the rate of penetration is a function of the balance between surface tension forces and viscous drag. The Washburn equation is (13): dl/dt = rσ lv cosθ/4µl. (2.2) Where, r = capillary radius l = depth of penetration t = penetration time µ = viscosity of liquid σ lv = surface tension of the medium θ = angle of contact between paper and liquid The above-specified Washburn equation has its limitations. In this equation interfacial contact angle is assumed to be constant and the pore morphology is reduced to an equivalent cylindrical pore [4,12,14]. This is a gross oversimplification of the true morphology of most porous materials. For example, a paper product in reality is a material made up of a solid cellulose fiber matrix in many cases coated with a consolidated mass of pigment and binder. It has been noted before that important differences exist between penetration of a liquid into a capillary and penetration into a more complex porous medium. In the latter case it was noted that penetration would be dependent on convergence and divergence of pore geometry, on the presence of discontinuities and the continual adsorption of vapor mediating the liquid flow through the porous medium [12]. 9

14 5.2. Degree of sizing of paper: Sizing may be defined as the degree of resistance to absorbance and respective wettability of paper or paperboard in the presence of liquids such as water, dilute alkaline solutions and acids, inks, printing inks, coatings, oils and fats etc. It is known that sizing of paper reduces the extent of ink spreading and the depth of ink penetration. In ink jet printing, excessive spreading of inks on the surface of the paper, measured as dot gain, is responsible for the increase of grayness of the printed image. Excessive penetration of the ink into paper reduces the color density of the print image and, in severe cases, causes print through. To minimize these above stated phenomena, which cause inferior print quality, it is desirable to provide high degree of sizing to the paper. But excessive sizing of paper causes delay in drying during printing. So there should be a balance between two. A benchmarking study by Hamilton, et al. in 1990 revealed that several of the most obvious sheet physical properties appeared to have no correlation with a subjective ranking of ink jet image quality, including smoothness, ash level, Parker Print Surf, and porosity [7]. However the liquid penetration rate as measured by the Bristow wheel test did correlate strongly with a subjective measure of print quality, as did a static contact angle test [7]. This result suggested that ink jet print quality was dependent on the level of sizing present in the sheet. The work by Hamilton, et al. confirmed the general wettability dependence of ink jet printing. In this work, sheets sized with increasing amounts of alkenyl succinic anhydride (ASA) displayed a corresponding increase in print quality. It is also supported by work done by Barker et al. [15]. Interestingly, in the 10

15 benchmarking study, measurements of HST sizing didn t correlate well with print quality. This suggests that the relationship of fiber and/or sheet wettability to print quality might be complex. 5.3 Paper roughness It is well known that the absorption properties of paper have a distinct effect on the drying and spreading of aqueous inks, which affects the ink jet print quality. The importance of paper roughness is also considered very important as it affects the final size and the shape of the dot [1]. It is noticed that the diameter of the dot is smaller for a smoother substrate [1]. The roundness of a dot is also higher for a smoother substrate [1]. 6. Ultrasound transmission across the paper during wetting When sound waves are transmitted through paper, a part of the energy is reflected back, some is absorbed, some is scattered and the rest is transmitted. So the intensity of the waves reaching the receiver end is a cumulative function of these phenomena. The amounts of these phenomena vary substantially with the change in the surface and structural properties of paper as the wetting of paper proceeds [2]. Ultrasonic reflection may be described in equation 2.3. ρ= ( ) 2 Z1 Z 2 ( Z + Z ) (2.3) ρ = reflection coefficient of the ultrasound Z 1 = ultrasound impedance of the liquid involved (g/cm 2.s) and Z 2 = ultrasound impedance of the dry surface of the sample involved (g/cm 2.s). 11

16 The ultrasound impedance for water and air is well known; Z (air) = 45 g/cm 2 s, Z (water) = 150,000 g/cm 2 s. For wood it ranges between 3000 g/cm 2 s and 6000 g/cm 2 s depending upon the wood species. An ultrasound wave going through water to an air film has a reflection coefficient of After initial wetting, Z for the second layer changes to a higher value since the air is replaced by water. Therefore, during the period of changing ultrasound impedance, the received signal is increasing since the reflection coefficient is decreasing. For example, reflection coefficient for water and wood is only about When sound waves are going from water to wood, only about 25 % of the signal is reflected back explaining the higher intensity at the receiver as compared to water-air system. During the penetration process the wetting front of the liquid moves into the material. As the surface becomes increasingly wetted the liquid moves further into the material. During this dynamic process, the ultrasound absorption is continuously changing. The gradual decline in the intensity of a beam of ultrasonic radiation with distance traveled through media may be attributed to absorption due to viscous losses and approximated by [2]: I = I 0.e -(4π.ν 2.η/ ρ 3 0c 0 ).x. (2.4) I = ultrasound intensity I 0 = ultrasound intensity for x = 0 x = distance moved by ultrasound in the medium 12

17 ν = ultrasound frequency η = viscosity of the material ρ = material density c 0 = ultrasound speed in the material The application of this theory to practice results in an increasing signal during the penetration process following the initial wetting as in e -(4π.ν 2.η/ ρ 3 0c 0 ).x, say e -(y).x, y for air is 16,340.3, which explains high absorption of ultrasound intensity by the air film in the starting, resulting in a low signal at the receiving end. Later this film is replaced by water, which has y value only So now absorption is much less as compared to that by air in the starting, resulting in increased signal at the receiving end. As the liquid front penetrates the paper, the signal is further modified by the scattering of the signal from the entrained air in the specimen, even after the specimen becomes saturated. A mathematical expression of the scattering phenomenon is shown in equation 2.5 [2]. I sca = I 0 ω 4 0 R 6 (1 + 3/2cosθ) 2 /9c 4 r 2.(2.5) I sca = Scattering intensity I 0 = transmitter intensity ω 0 = ultrasound frequency R = radius of the scattering center 13

18 c = ultrasound speed r = length between receiver and scattering center θ = angle between ultrasound beam and receiver This expression is after Rayleigh. One condition for the Rayleigh scattering is the size of the scattering center (R) should be much smaller than wavelength of the ultrasound (at least one tenth fraction). The air bubble diameter in paper should be similar to the pore size in average paper of ~ 5 to 50 µm [2]. The ultrasound speed in water is 1,483 m/s. By using 2 MHz sound waves, wavelength of ultrasound will be µm (1,483 µm in case of 1 MHz sound waves), which is much more than ten times of the bubble diameter (R ~ 5 to 50 µm). Therefore, the Rayleigh scattering expression appears to apply. Another reason for the loss of ultrasound signal after initial wetting is the loss of stiffness. When the pore fills with water, this allows contact between water and the fiber walls or adsorption. Absorption, then diffusion of liquid into the walls of the fibers reduces the strength of the fibers and breaks the inter-fiber hydrogen bonds which lowers the stiffness of the fibers, and the stiffness of the paper mesh, as well. This loss of elasticity increases the losses related to wave propagation in the sample [16]. 14

19 7. Illustration of Penetration Dynamic Analyzer (PDA) The Penetration-Dynamic Analyzer (PDA) from EMTEC Electronic is an analytical instrument designed for investigating the dynamics of the penetration of samples of solid materials, such as papers, by liquids. Data acquisition commences with initial contact, and results are displayed in the form of curves of intensity of signals reaching the receiving end of the PDA. There are several cases where these curves may be interpreted as degree of penetration. An illustration of the instrument is provided in Figure 1. Figure 1. EMTEC Penetration Dynamic Analyzer (PDA) [17] The analytical technique employed is based on recording the change of intensity of ultrasonic signals transmitted by solid samples while one of their faces is in contact with a liquid, which penetrates into the solid with time. The solid sample to be investigated is mounted on a plastic sample holder using double-side adhesive tape, which both provides that the exposed surface of the sample 15

20 will remain flat and smooth during analytical runs and that the liquid will contact one face of the sample only. Sample holder used may be grooved or flat. Grooved sample holder provides passage for air to pass through the rear face of the sample (Figure 2) while the flat one causes air to trap in the sample, making scattering centers. Sample holder Tape Water level Grooves Grooves Grooved cylinder Sample holder Figure 2. Depiction of sample holder and grooved pressing roll When using the grooved sample holder, first double sided tape is adhered to the sample holder in a way so that the bottom side of the grooves is totally covered by the tape while the top portion is left uncovered by the tape as shown in Figure 2. Now the tape is pressed into the grooves by using the grooved roll shown in Figure 2. Now the paper to be tested is affixed to the tape applying only marginal pressure so that the paper remains flat. Now there are spaces between paper and tape in the grooved areas where air can escape when forced out of paper by the penetrating water front. This sample holder, 16

21 complete with sample, is then immersed in a measuring cell filled with the liquid to be involved. This immersion procedure takes around 10 ms or more, the time elapsed from the instant the center of the sample crosses the surface of the liquid and the instant the sample holder reaches the end of its travel. Once the sample holder has been fully inserted into the cell and the sample is totally immersed in the liquid, an analytical run is initiated Figure 3. The experimental arrangement of PDA [17] and the sample is irradiated with ultrasonic signals incident roughly normal to its surface. Ultrasonic radiation transmitted by the sample and received at the detector is converted into analytical data and transmitted to a PC. The durations of analytical runs are freely user-selectable, and may range from milli seconds to minutes. 17

22 Figure 4. Stages in the sample-immersion procedures [17] Water, most other liquids, and the plastic of the sample holder transmit ultrasonic signals virtually unattenuated over short distance. For example, an ultrasonic wave of 1 MHz frequency passing through water will loose half of its intensity over a distance of 20 m [17]. However, dry paper strongly absorbs and scatters the ultrasonic signals. It is shown in figure 5. Figure 5a. shows the attenuation of signal through water and dry paper. There is negligible attenuation in signal intensity through water while it is much more significant through dry paper. Figure 5b. shows the effect of wetting on attenuation. As the wet paper attenuates the ultrasound intensity less than the dry paper, intensity at the receiver increases with increased portion of paper being wetted. 18

23 Figure 5a. Attenuation in water and dry paper 19

24 Figure 5b. Attenuation in water and partially wet paper 20

25 As this wetting is controlled by the rate of penetration and the surface geometry of paper, change in intensity at the receiver end will dependent on these parameters as shown in Figures 6 and 7 below. Figure 6 explains the effect of degree of sizing on the ultrasound attenuation for samples having grooved backing, which allows trapped air to escape from rear faces. Figure 6. Effect of degree of sizing on ultrasound attenuation In sized paper a layer of air associated with paper surface is thin as compared to that in case of unsized paper, this explains higher value of intensity at the receiver in case of sized paper in the beginning. As the time lapses, sized paper resists the wetting therefore rate of increase in intensity at receiver is moderate. In case of unsized paper, as soon as water contacts paper, it wets the paper very rapidly causing steep gain in the intensity at the receiver. 21

26 Figure 7 explains the relative intensity at the receiver for samples consisting of different pore size, for the experiments employing grooved sample holders that leave the rear faces of samples exposed to allow air to escape through their rear faces. Figure 7. Effect of porosity on ultrasound attenuation There is significant difference in time when intensity at the receiver starts increasing. In case of low porosity paper as there is small pore volume, it will take more time to remove air film through paper in compare to high porosity paper. Same reason is responsible for the lower rate of increase in intensity in case of low porosity paper, as rate of wetting of paper by water is lower in case of low porosity paper. 22

27 Figure 8 shows an illustration of outputs generated by PDA. Figure 8. Data generated during the PDA run on a general office copy paper In Figure 8, TP1 is the point of time when the wetting starts, G1 is the ultrasound intensity gradient of progressing curvature, max1 is the time when the intensity reaches to maximum, L is the difference in intensity between the point of initial intensity (at s) and the point of maximum intensity (at s), G2 is the ultrasound intensity gradient of receding curvature. As this thesis is more concentrated to initial wetting, the main parameters important for this work are G1, L1, TP1 and max1. 23

28 CHAPTER III EXPERIMENTAL PROGRAM The experimental program was conducted in three phases. Papers were divided in to two groups. One group consisted of office copy papers and the other group consisted of photographic quality papers. Nine different brand name papers were chosen for each group. In the first phase all the papers (nine in each group of office copy papers and photographic quality papers ) were first characterized for surface characteristics. After analyzing the data obtained from surface measurements, five papers were selected from each group. These selected five papers from each group were characterized for wetting and penetration. In the second phase, all the five papers from each group were tested for acoustic response. In the third and final phase, print quality measurements were taken for these selected papers. The experimental work was started with following papers: Office copy papers: (i) (ii) (iii) (iv) (v) (vi) (vii) BOISE Copy Paper (O.1) GREAT WHITE MultiUse 20 Paper (O.2) IBM Multi-Purpose Paper (O.3) Office DEPOT Multi Purpose (O.4) XEROX BUSINESS MULTIPURPOSE PAPER (O.5) GP GeoCycle (O.6) Georgia-Pacific Copier Paper (O.7) 24

29 (viii) (ix) HammerMill Copy Plus (O.8) WILLCOPY multi-purpose paper (O.9) Photographic quality Papers (i) (ii) (iii) (iv) (v) (vi) (vii) (viii) (ix) Kodak PREMIUM Picture Paper (P.1) hp premium photo paper glossy (P.2) Jet PRINTPHOTO PREMIUM PHOTO PAPER (P.3) RoyalBrites Glossy photo (P.4) IBM Heavyweight Ultra-Glossy Photo paper(p.5) EPSON PROFESSIONAL MEDIA PHOTO PAPER (P.6) IBM Digital Photo Ink Jet Paper (P.7) Jet PRINTPHOTO MULTI-PROJECT PHOTO PAPER (P.8) Jet PRINTPHOTO PROFESSIONAL PHOTO PAPER (P.9) 25

30 Procedure Phase 1. Characterization of papers 1.1 Paper surface characterization by Sheffield smoothness: All the nine papers in each group of office copy papers and photographic quality papers were evaluated for Sheffield smoothness first. Sheffield SMOOTHCHEK was used for the measurements. Papers were conditioned first according to TAPPI specifications. Fifty samples per paper were run for testing. Three replicates per sample were measured to ensure high level of correctness of measurements Parker Print-Surf (PPS) roughness: PPS roughness measurements were taken for both office copy papers and photographic quality papers. MESSMER BUCHEL Parker Print-Surf instrument was used for the measurements. First papers were conditioned according to TAPPI specifications. Three replicates per sheet were tested for office copy papers while for photographic grade papers four replicated per sheet were tested because of less number of sheets available for measurements in this group Parker print-surf (PPS) air permeance: Parker Print-Surf (PPS) air permeance tests were conducted using MESSMER BUCHEL Parker Print-Surf tester. First samples were conditioned according to TAPPI specifications. Fifty sheets were tested for office copy papers. Four replicates were tested for each sheet of paper. 26

31 Air permeance measurements for photographic quality papers didn t yield any results as photographic quality papers didn t allow air to pass through. 1.2 Characterization of papers for sizing Hercules sizing test Nine papers from each group were tested for Hercules sizing test (HST) for 80 % reflectance value using 10 % formic acid ink. HST testing was done for photographic quality papers but except P.7, none of the papers yielded any results. Tests were conducted using ink having formic acid concentration up to 40 % Cobb size test: Selected five (5) papers from each group were tested for sizing measuring Cobb values. Tests were conducted according TAPPI specifications. In the office copy paper group, some papers could not be tested as water was passing through the sheet. In the photographic quality paper group, some papers could not be tested as the coating was coming out with the blotting paper, used to remove the free water from the sheet Bristow wheel test Selected five (5) papers from each group were tested for Bristow wheel track length at constant speed of 3.47 mm/s using water-based ink. 27

32 1.3 Characterization of papers for surface energy by Contact angle measurements Equilibrium contact angle tests were conducted on selected papers from each group. KRUSS Goniometer was used for the measurements. Ten samples were tested for every paper. Three replicates were tested for every sheet of paper. Phase 2. Ultrasound transmission during penetration: Selected papers from each group were evaluated for acoustic response during penetration with water. Emtec PenetrationDynamicAnalyzer (PDA) was used for the measurements. Grooved sample holder was used for the experiments, which provides passage for air to pass through the rear face of the sample when the air present in the sample is pushed by penetrating water front. The flat sample holder causes air to trap in the sample, making scattering centers. First experiments were conducted with 1 MHz ultrasound frequency and ultrasound signal was observed through 10 mm diameter sample area. It was observed that there was high degree of variation with in replicates. Some experiments were conducted to observe the variability with 1 MHz ultrasound frequency and 35 mm sample diameter, 2 MHz ultrasound frequency and 35 mm sample diameter and 2 MHz ultrasound frequency and 10 mm sample diameter. It was observed that there was least variability with in a paper when ultrasound signal of 1 MHz was transmitted through a 35 mm diameter sample area. Keeping these conditions, ten samples for each paper were 28

33 tested for ultrasound transmission during penetration. Five replicates were tested for each sample paper. Phase 3. Print quality measurements: In the third and final phase selected papers from each group were tested for dot area and print density measurements using a X-Rite Spectrodensitometer. Print patterns for measurements of dot area and print density were made of rectangles of black and twenty five percent grey shades as shown in Figure 9, using Microsoft PowerPoint program. Figure 9. Patterns used to measure the % dot area and print density Papers were printed on a Hewlett Packard deskjet 920cvr color printer. This is a drop-on-demand type ink jet printer. Reason for preferring drop-on-demand ink jet printer over continuous ink jet (CIJ) printer was that during printing with CIJ printer, ink drops fall on the paper with high speed [1]. This high kinetic energy of the CIJ droplets ensures an immediate, pressurized contact with the paper surface which obviates the possibility of wetting delay. When taking the measurements of ultrasound signal during 29

34 wetting of paper, wetting delay takes place as there is no extra pressure on the surface of paper. The wetting of paper with drop-on-demand printer simulates better with wetting of paper during ultrasound measurements as wetting delay happens in both of these cases. Ten samples for each paper were printed with above specified print pattern. Samples were conditioned after printing according to TAPPI specifications before testing. 3.1 Dot area measurements: Density of dot was measured for 25% grey printed area using Yule-Nielson formula. According to Yule-Nielson formula, Apparent Dot Area = ( Dt ) / n ( Ds ) / n *100 Where: D = Density of tint minus density of paper t D = Density of solid minus density of paper s n = n factor A n factor value 1.7 was used for photographic quality papers and 2.65 for office copy papers, following the specifications from X-Rite. As it was discussed earlier that this n factor is used to compensate for the amount of light that is absorbed or trapped when a dot measurement is taken. The higher n value for office copy paper corresponds to the higher amount of light absorbed in this case as office copy papers have higher surface roughness than photographic quality papers. Measurements were taken on ten sheets for every grade of paper. Five measurements for dot area were conducted for every sheet of paper. 30

35 3.2 Print density measurements: Print density was measured using the density minus paper function in the instrument. With this function, first instrument reads the density of paper, and then it reads the density of black printed area and provides the difference of density of black area and density of paper. Ten samples per paper were tested with five replicates for every sheet of paper. 31

36 CHAPTER IV RESULTS AND DISCUSSION 1. Paper characterization 1.1 surface characterizations First all the papers from both groups of office copy papers and photographic quality papers were characterized on the basis of standard surface characteristics. 1.1.a Smoothness Table 1 and 2 show the results obtained from Sheffield smoothness measurements for office copy papers and photographic quality papers respectively. Table 1 Sheffield smoothness values for office copy papers Paper Sheffield smoothness, SU St dev. O O O O O O O O O Avg s =

37 Table 2 Sheffield smoothness values for photographic quality papers Paper Sheffield smoothness, SU St dev. P P P P P P P P P Avg s = 26.9 Figure 10 shows the difference in Sheffield smoothness for office copy papers and photographic quality papers Sheffield smoothness, SU O.1 O.2 O.3 O.4 O.5 O.6 O.7 O.8 O.9 P.1 P.2 P.3 P.4 P.5 P.6 P.7 P.8 P.9 Office Photo Paper Figure 10. Sheffield smoothness values with 99% confidence intervals for all the papers 33

38 Figure 10 shows the Sheffield smoothness values for all the papers belonging to both groups with 99 % confidence intervals. This illustrates that smoothness values differ for different paper grades with in a group for both groups but as a whole, office copy papers have higher roughness than photographic quality papers. Average smoothness values for both groups of papers are shown as office and photo respectively in Figure 10. T-test conducted for the S h e f f i e l d s m o o t h n e s s for these two groups of papers yielded t = > tc (tc = 2.58 for 99 % confidence). This T-test analysis confirms that these two groups of papers can be distinguished with 99 % confidence on the basis of Sheffield smoothness. As the standard deviation from Sheffield smoothness values was very high for both groups of papers, it was decided to conduct Parker Print-Surf (PPS) roughness test. PPS roughness measurements were taken for both office copy papers and photographic quality papers. Table 3 and 4 show the results obtained from the experimental work conducted to measure the PPS roughness for office copy papers and photographic quality papers respectively: 34

39 Table 3 Parker Print-Surf Roughness for office copy papers Paper Parker print-surf roughness, microns St dev. O O O O O O O O O Avg s = 0.27 Table 4 Parker Print-Surf Roughness for photographic papers Paper Parker print-surf roughness, microns St dev. P P P P P P P P P Avg s = 0.47 When comparing results obtained for PPS roughness for office copy papers and photographic quality papers, Office copy papers show higher roughness values than photographic papers (6.22 microns for office copy papers as compared to only 1.38 microns for photo papers). Figure 11 shows the range of Parker print-surf smoothness for each group of papers. 35

40 O.1 O.2 O.3 O.4 O.5 O.6 O.7 O.8 O.9 P.1 P.2 P.3 P.4 P.5 P.6 P.7 P.8 Parker print-surf smoothness, microns P.9 Office Photo Paper Figure 11. PPS smoothness for both groups of papers with 99 % confidence Figures 11 show that photographic quality papers have very low roughness as compared to office copy papers. T-test conducted for PPS roughness for these two groups of papers yielded t = > tc (tc = 2.58 for 99 % confidence). This T-test analysis confirms that these two groups of papers can be distinguished with 99 % confidence on the basis of PPS roughness. 1.1.b Air permeability All the papers were characterized for air permeability using Parker Print-Surf tester. PPS air permeability tests conducted for photographic quality papers didn t produce any results as these papers didn t allow air to pass through at all. This indicates that photo quality papers have very little air permeability or no air permeability at all. 36

41 Following are the results obtained from PPS air permeability test for office copy papers: Table 5 Parker Print-Surf air permeability results for office copy papers Paper PPS air permeability, Seconds St dev. O O O O O O O O O Avg s = 2.52 Figure 12 shows the range of Parker print-surf air permeability for each group of papers with 99% confidence intervals Air Permeability, Sec O.1 O.2 O.3 O.4 O.5 O.6 O.7 O.8 O.9 Office Paper Figure 12. PPS air permeability for office copy papers with 99 % confidence 37

42 Based upon results obtained from PPS air permeability test it can be concluded that photographic quality papers and office copy papers can be differentiated based upon the results obtained from PPS air permeability test. Photographic quality papers have no air permeability while office copy papers have air permeability ranging from 7 seconds to 15 seconds (Figure 12). 1.2 Paper characterization for sizing All the papers belonging to both the groups were characterized for response to penetration with water. 1.2.a Hercules sizing testing (HST) was conducted on all the papers from each group to characterize on the basis of water penetration. Table 6 shows the results obtained from HST testing for office copy papers. Table 6 Table showing HST values for office copy papers Paper HST, s St dev. O O O O O O O O O Avg St dev

43 99% confidence intervals. Figure 13 shows the range of HST values for each group of papers with HST, Sec O.1 O.2 O.3 O.4 O.5 O.6 O.7 O.8 O.9 Office Paper Figure 13. HST value for office copy papers with 99 % confidence HST testing was done for photographic quality papers too but except P.7 none of the paper yielded HST values. Testing was run for photographic quality papers for up to 5000 seconds and with 40 % formic acid concentration in test ink. It indicates that the photographic quality papers were very hard sized or had a penetration barrier that also stopped air. Average HST value for P.7 paper was seconds with standard deviation value of The average HST value for office copy papers was seconds with a standard deviation of

44 From HST results it can be concluded that office copy papers and photographic quality papers can be differentiated on the basis of HST values. Photographic quality papers exhibit high HST values (750 s and above) while HST values for office copy papers are much lower (less than 250 s). 1.2.b Cobb testing was done on selected five papers from each group. Cobb results could not be produced for some of the papers from office copy paper group as water was passing to areas outside the sample holder, which would cause higher apparent Cobb value than the actual value for those papers. The reason for some photographic papers for not producing the Cobb results was that when these papers were pressed against roll after placing a blotting paper to remove excess water, the coating of paper was partially lost to the blotting paper. This would result in apparent low Cobb value that the actual one. To avoid this problem, photographic papers were tested for Cobb using only 25 second time period. Still only some of these papers yielded Cobb results. Table 7 shows the Cobb results obtained for all the papers. Table 7 Cobb test results for all the papers Office copy papers Photographic quality papers Paper Cobb, g/m 2 St dev. Paper Cobb, g/m 2 St dev. O P O P O Avg. = s = 2.47 Avg. = 50.7 s = 6.93 Figure 14 shows the range of Cobb values for each group of papers with 99% confidence intervals. 40

45 Cobb, g/m O.2 O.3 O.5 P.1 P.3 Office Photo Paper Figure 14. Cobb values for both groups of papers with 99% confidence Figure 14 illustrates that the Cobb values are on the higher side for photographic quality papers. Last two columns in Figure 14 specified as office and photo illustrates the difference in Cobb values between two groups Figure 14 show that the photographic quality papers have a higher Cobb value than the office copy papers. But Cobb value is not a relevant parameter for comparing office copy papers and photographic quality papers for sizing as the basis weight and thickness of photographic quality papers was much higher than that of office copy papers. Another reason of this difference is that photographic papers have a thick layer of coating which can retain much higher amount of water during Cobb test by increasing the 41

46 viscosity. All these factors provide photographic quality papers much more water retaining capacity than office copy papers. From results obtained from preliminary tests, it was decided to select five papers from each group. Two criteria of choosing papers were surface characteristics and resistance to water penetration. As the tests run for water penetration did not give comprehensive results (conventional sizing tests like Cobb and HST did not yield results for some papers), it was decided to choose papers based upon the surface characteristics. Papers were selected based upon Parker print-surf smoothness values. Criteria for the selection of papers were to cover the entire range of papers in both the groups (Figure 9). Papers O.1 through O.5 were selected from office copy paper group and papers P.1 through P.5 were selected from photographic quality paper group. From photographic quality paper group, P.6 was also selected because it had the maximum PPS roughness values in the group but was later dropped as the results obtained from PDA run for this paper were totally different from results obtained from PDA runs on all the other papers. This behavior of P.6 is shown and explained in later section where the results obtained from PDA run are discussed. 1.2.c Bristow wheel test was conducted on all the five (5) selected papers from each group to see the relationship between print quality parameters and Bristow wheel track length. Bristow wheel test could not be run on photographic quality papers as during the run coating from paper was mixing with the ink used in test which was causing uneven distribution of ink on paper surface as well as it was obstructing the movement of wheel. 42

47 Table 8 Bristow wheel test results for office copy papers Paper Bristow wheel track length, cm St dev. O O O O O Avg s = 2.54 Table 8 shows that the Bristow wheel track length for office copy papers ranges from 4.76 cm to cm with an average value of 8.81 cm. Figure 15 shows the graphical representation of Bristow wheel track length with 99 % confidence intervals Bristow wheel track length, cm O.1 O.2 O.3 O.4 O.5 Office Paper Figure 15. Bristow wheel track length for office copy papers with 99% confidence 43

48 Figure 15 illustrates the area covered by ink on different papers for the same amount of ink. This value corresponds to degree of sizing of paper. A heavily sized paper will have a larger area covered as there is less absorption and there is more ink available on the surface of paper. Correlation between Bristow wheel track length and print quality parameters is discussed in the data analysis section. 1.3 Characterization for surface energy Contact angle testing was done on selected papers using Kruss Goniometer. Table 9 shows the equilibrium contact angle values for photographic quality papers and office copy papers. Table 9 Contact angle values for all the selected papers Office copy papers Photographic quality papers Paper Contact angle St dev. Paper Contact angle St dev. O P O P O P O P O P Avg. = s = 2.50 Avg. = s =

49 Contact angle O.1 O.2 O.3 O.4 O.5 P.1 P.2 P.3 P.4 P.5 Office Photo Paper Figure 16. Contact angle values for both groups of papers with 99 % confidence Figure 16 illustrates that the contact angle values for office copy papers are higher than that for photographic quality papers. For office papers there is not much difference for contact angle with in the group but for photographic quality papers contact angle varies with in the group. Figures 16 show that the Contact angle value is much higher for office copy papers than that for photographic quality. T-test conducted for the Contact angle for these two groups of papers yielded t = > tc (tc = 2.90 for 99 % confidence). This T- test analysis confirms that these two groups of papers can be distinguished with 99 % confidence on the basis of Contact angle. 45

50 Correlation between contact angle data and print quality parameters is discussed in data analysis section. 2. Acoustic response to photographic quality papers and office copy papers Acoustic response curves were obtained for all the selected papers in both categories with the use of Penetration Dynamic Analyzer (PDA). Sample holder used for the experiments was a grooved one because we are simulating the wetting of paper in printer, which allows air to pass through. Time for the run was kept at a maximum of thirty (30) seconds for all the samples, as our goal is to observe the initial wetting important to the ink-jet printing process. Along with the shape of curves, computer software is also capable of providing following parameters for each curve, which are as follows: i. Time when the wetting starts ii. iii. iv. Time when the intensity reaches a maximum G 1, Slope of progressing curvature G 2, Slope of receding curvature v. L, difference between ultrasonic intensity at the time of maximum and the time when wetting starts. G and L are associated with initial penetration. Higher value of G indicates that there is higher rate of water absorption by the paper and L indicates the total amount of water absorbed by the sheet of paper in that time period. 46

51 Ten sheets for each of paper were tested. Five replicates were run for every sheet of paper. Other than running the papers some other experiments were conducted to see the effect of various changes on ultrasound propagation. 2.1 Trials with PDA First a number of independent test parameters were evaluated to see the response of ultrasound propagation to test condition modifications. These test conditions involved changing water used for the tests from distilled to de-ionized measuring the ultrasound propagation through 35 mm diameter area and 10 mm diameter area using the flat sample holder and the grooved sample holder Trials were run to see the variability in ultrasonic attenuation during wetting of paper when experiments were performed with distilled water and de-ionized water. O.3 office copy paper was used for the trials. The results obtained for desired parameters from above described experiments are summarized in Table10. 47

52 Table 10 Parameters values obtained from tests with PDA on O.3 office copy paper for distilled and de-ionized water With distilled water With de-ionized water L Grad L Grad Avg St. dev C.V Table 10 illustrates that the variance in L is on higher side for experiments conducted with de-ionized water while variance in gradient is less in that case (C.V.). As the gradient of ultrasound signal was assumed as more important variable of two, so it was decided to conduct tests using de-ionized water to keep the variability low during replications PDA has two options of measuring ultrasound propagation either through 10 mm diameter measurement area or through 35 mm diameter measurement area. Trials were conducted with both of these options using O.3 office copy paper. The objective here too was to see the variability of ultrasound attenuation during replication of samples. Figures 17 and 18 show the results obtained from experiments. 48

53 Figure 17. Plots obtained from PDA run with 10 mm diameter option Figure 18. Plots obtained from PDA run with 35 mm diameter option Comparing Figures 17 and 18 it is concluded that PDA runs with 35 mm option are more consistent than runs with 10 mm option. It is also in accordance with expected outcome. 49

54 As the measurement area increases, there is less variability expected due to averaging of local variations Effect of sample holder and tape on ultrasound Experiments were conducted to see the effect of sample holder and tape on the absolute value of ultrasound intensity reaching the receiver. In every experiment sample holder was taken out after some time so that the intensity of ultrasound signal after that time was same for all the experiments. Figure 19 illustrates the response of ultrasound to all of those trials. Sample holder (flat backing)/35 mm option Sample holder (grooved backing)/10 mm option Sample holder (grooved backing)/35 mm option Sample holder (grooved backing)/35 mm option/ tape without air % Intensity Sample holder (grooved backing)/35 mm option/ tape with air Time, Sec Figure 19. Plots obtained from PDA runs with only sample holder and tape Figure 19 shows that sample holder having flat backing absorbs less amount of signal than the sample holder with grooved backing. Change in diameter, ultrasonic signal is measured through, doesn t make any effect and the intensity of signal is same in both cases. When air present in the grooves in sample holder was not extricated (sample 50

55 holder (grooved)/with air), than the intensity of signal was found more than in the experiment with air extricated (sample holder(grooved)/tape without air). 2.2 Final PDA runs for office copy papers and photographic quality papers To reduce the variability it was decided that ten samples be run for every paper grade. Five replicates were run for every sheet of paper to minimize the variability due to the selection area of sheet. This makes fifty replicates for every paper. It was also observed that there was less variation if machine direction was kept horizontal during runs. So in final runs following points were considered: Keep machine direction in horizontal position Run papers with 1 MHz, 35 mm diameter option with de-ionized water PDA runs for office copy papers It was decided to run the PDA for 30 seconds for all the papers to observe the effect of wetting on ultrasound response over a broad range of time. Figure 20 shows the results obtained from experiments run with PDA for O.1 office copy paper. Figure 20. Plots obtained from PDA run for O.1 office copy paper 51

56 It can be seen from Figure 20 that all the papers follow the same trend. First there is steep increase in ultrasound intensity received by the receiver. The peak intensity is followed by the gradual decay in intensity signal received with time. The steep increase in intensity received by the receiver can be explained on the fact that during the initial stage of wetting, water replaces the air voids which causes the increase in the signal intensity during this phase as illustrated in Figure 4b. Once the paper is saturated with water, the gradual decay in signal intensity might be due to the gradual decrease in fiberfiber bonding as earlier discussed in literature [15]. Another reason for the gradual decay in ultrasound intensity may be the dominant role of air pockets within the sample, which grow in diameter as the water front pushes the distributed air. This effect is explained in equation 2.5. The mean plot obtained for O.1 papers is shown in Figure 21. Figure 21. Mean PDA plot for O.1 office copy paper 52

57 Figure 21 shows the mean plot obtained from Figure 20. This plot represents the mean response of ultrasound to wetting of paper with time for O.1 papers. Experiments to examine the ultrasound response during wetting were also run for other office copy papers. These papers also exhibited the same kind of response as shown for O.1 office copy papers. Figure 22 shows the mean plots of ultrasound response to wetting for all the five papers belonging to office copy papers category. Figure 22. Mean plots for office copy papers Figure 22 shows the mean ultrasound response to wetting with time for all five papers belonging to office copy paper category. All the papers belonging to this group follow the same trend as O.1 papers. 53

58 2.2.2 PDA runs for photographic quality papers: This section discusses the ultrasound response to wetting for photographic quality papers, obtained from the experiments done with PDA. papers. Figure 23 shows the ultrasound response to wetting for P.1 photographic quality Figure 23. Plots obtained from PDA run for P.1 photographic quality paper It can be seen from Figure 23 that all the papers belonging to P.1 group follow the same trend for the ultrasonic response during wetting. Signal intensity in increasing gradually earlier with time and after some time it approaches a constant value. This paper differs in ultrasound response from office copy papers in that in case of all office copy papers the ultrasound intensity received by the receiver increases sharply during the initial phase of wetting followed by gradual decrease while in this photographic quality paper during the initial phase of wetting there is only gradual increase in ultrasound 54

59 signal intensity. The slower increase in signal in present case can be explained on the fact that photographic quality papers are much more resistant to water penetration as compared to office copy papers as shown by paper sizing results (HST and Cobb values). The ultrasound signal changes slowly with time during wetting of these papers due to slower penetration by water. Due to the slow water penetration these papers do not attain a maximum intensity in the experimental frame of time (30 seconds). Figure 24 shows the mean plot obtained for P.1 photographic papers. Figure 24. Mean PDA plot for P.1 photographic quality paper Figure 24 shows the mean plot obtained from Figure 23. This plot represents the mean response of ultrasound to wetting of paper with time for P.1 papers. 55

60 Papers belonging to other photographic papers were also tested for ultrasound response to wetting in the same way as P.1 photographic papers. Figure 25 shows the mean ultrasound responses with wetting during time for P.1 through P.5 photographic quality papers. Figure 25. Mean plots obtained from PDA run for photographic quality papers It can be seen in Figure 25 that all the P.1 through P.5 photographic quality papers follow the same trend for the ultrasound response during wetting. Papers belonging to P.6 group were tested for ultrasound response during wetting in the same way as other groups of papers. Figure 26 shows the ultrasound response with wetting during time for this group of papers. 56

61 Figure 26. Plots obtained from PDA run for P.6 photographic quality paper Figures 26 shows that the response of P.6 to ultrasonic attenuation is totally different from other papers as this paper shows a decrease in received signal initially while all the other papers in each group show an increase in received signal when the wetting starts. This different behavior for P. 6 may be due to the role of scattering during penetration. As the roughness of this paper is much more than roughness of other papers in the photographic quality paper group (Table 4), high roughness will cause more air to be associated with paper during initial wetting. This will cause formation of air pockets in paper, which will scatter ultrasonic signal. This will cause a decrease in received ultrasonic signal to the receiver end during initial wetting. After initial wetting, as the air will move out of paper gradually, received ultrasonic signal will increase as shown for other photographic grades of papers. 57

62 Figure 27 shows the mean plot obtained from Figure 26. This plot represents the mean response of ultrasound to wetting of paper with time for P.6 papers. Figure 27. Mean PDA plot for P.6 photographic quality paper As the response of P.6 photographic papers was unexpectedly different from that for other photographic grade papers, this paper was not included in analysis relating ink jet print quality to PDA generated parameters PDA data in tabular form No maximum for photographic quality papers could be found during the time span of 30 seconds as there was continuous increase in the ultrasound intensity reaching the receiver during this time period. This kind of response was consistent with resistance to liquid 58