Color Reproduction Consistency and Capability of Tree-free Copy Paper

The life cycle of print starts with paper choices – specifying environmentally preferable paper products can reduce the effect that printing has on the planet. Over the past two centuries, wood is the primary raw material in paper manufacturing. However, wood-based paper carries a significant “ecological shadow” of energy consumption, bleaching chemicals, and water used in its production. In its 2010 report, United Nations Environment Program (UNEP) identified pulp and paper industry as one of the largest direct contributors to human toxicity. The substances from paper and paperboard mills that contribute most to human toxicity impact are mercury (II) ion, beryllium, and hydrogen fluoride. Motivated by legislation, consumer pressure, and the desire to become more efficient, the pulp and paper industry in the United States has invested in new technologies and processes that reduce its environmental impact. Tree-free fiber used in production is one way to minimize or eliminate the environmental impacts. This paper studied sustainable development and use of tree-free copy paper for the laser printer. The color reproduction capability and process capability of tree-free paper were evaluated in terms of optical density, print contrast, and color gamut. Introduction Tree-Free Paper is made without the use of tree fiber. There are a variety of alternative tree-free fibers that can be sourced to make paper and reduce the demand on forests. Basically, tree-free paper can be divided into two main categories: organic tree-free paper and nonorganic tree-free paper1, 2, 3, 4. Organic tree-free paper uses fibers derived from plant sources such as residues from agricultural crops, or plants grown specifically for papermaking. Nonorganic tree-free paper is usually made of plastic polymers or minerals. Tree-free fibers have advantages of producing paper with fewer chemicals, less energy, and less water than wood, offering farmers alternative crop options, promoting biodiversity by relieving pressures of deforestation, and taking advantage of readily available and underused fibers. However, the development of these materials for widespread consumer use has not yet occurred1,5. So far, the applications of tree-free paper are focused on stationery and office use. Several kenaf and hemp products mixed with recycled paper fibers and tree-free papers manufactured from agricultural residues (such as coffee, mango, lemon, and banana) are used to produce quality stationery, which add different elements to design. These products have made it to market, but none have been a big success so far. Sugar cane bagasse, on the other hand, has made some inroads in the North American office paper market. It biodegrades faster than woodbased paper, and can be recycled with paper made from trees. Experimental In order to study the color reproduction and process capability of tree-free copy paper, three commercially available tree-free paperssugarcane copy paper A, B, and C were selected and tested, with a wood-based copy paper as reference. Table 1 shows characteristics of tested tree-free copy papers. Like wood-based copy paper, tree-free copy papers use optical brightener agent (OBA) to bring up the desired brightness. Table 1: characteristics of tested tree-free copy papers Paper Paper Weight Brightness OBA Paper White L* a* b* Wood-based 20# 92 Y 95.53 1.94 -6.54 Sugarcane A 20# 93 Y 92.64 4.3 -10.05 Sugarcane B 22# 92 Y 93.17 3.95 -10.25 Sugarcane C 20# 92 Y 93.94 2.25 -7.46 The color reproduction capability of tree-free paper was evaluated in terms of optical density, print contrast and color gamut. A Xerox DocuColor 250 laser printer with toner-based inks (profiled as a CMYK device) was used in the study. Fifty samples of each substrate were collected and measured with an X-Rite i1iO spectrophotometer. ICC profiles were generated for the digital printers by using ProfileMaker 5.0.10. ICC profiles were then loaded into CHROMiX ColorThink Pro 3 software and the gamut volumes of the ICC profiles were determined. The optical densities and print contrast of tested tree-free papers were measured using an X-Rite 530 SpectroDensitometer. The color reproduction consistency and capability of tree-free papers were discussed. One of indices used to measures process capability is Cp index. It is defined as the ratio of the designated specification range to the individual paper type process range, for optical density, print contrast, and color gamut parameters. Cp index is calculated as (upper specification limit lower specification limit)/(6*Sigma). In other words, this ratio expresses the proportion of the range of the normal curve for each paper type that falls within that specification limits. For this study, a relative specification range was determined based on data for the selected paper types and used to calculate the Cp indices, as described below. Color-related Attributes Table 2 lists color-related attributes for the wood-based and sugarcane paper samples from the laser printer. Color density and print contrast values are shown for yellow (Y), magenta (M), cyan (C), and black (K). The average optical density measurements of tested tree-free copy paper are lower than those of wood-based copy paper. Although the wood-based copy paper yielded higher average optical densities, it tended to have larger color reproduction variability. The wood-based copy paper had higher average print contrast, with the exception of black. The sugarcane 8 ©2012 Society for Imaging Science and Technology C copy paper had lower print contrast with larger variability. It also shows that the wood-based copy paper produced a wider color gamut with smaller color reproduction variability, while sugarcane B copy paper having larger color reproduction variability. Table 2: Color-related attributes of tested copy papers Wood-based Sugarcane A Sugarcane B Sugarcane C Mean S.D. Mean S.D. Mean S.D. Mean S.D. Optical Density Y 0.85 0.02 0.84 0.01 0.84 0.01 0.82 0.02 M 1.10 0.02 1.06 0.02 1.08 0.02 1.05 0.02 C 1.22 0.03 1.17 0.02 1.20 0.02 1.16 0.02 K 1.61 0.03 1.58 0.03 1.60 0.04 1.58 0.06 Print Contrast Y 19.09 2.28 18.29 1.56 18.69 1.53 16.48 2.51 M 32.11 2.38 30.57 1.67 30.97 2.29 29.04 1.95 C 25.25 1.70 24.29 1.11 23.54 1.30 22.42 1.69 K 39.69 1.86 41.13 1.23 40.91 1.89 40.47 2.41 Color Gamut 336,358 3,712 312,351 3,414 308,103 10,248 312,096 5,365 Note: S.D. represents Standard Deviation (Sigma). Figure 1 illustrates the color gamut comparison for the woodbased and sugarcane copy papers. Note the black projection line represents the color gamut of the wood-based paper reference. The color gamut of wood-based copy paper is larger, especially in the yellow regions. (a) Sugarcane A (true color) v.s. woodbased (wireframe) (b) Sugarcane B (true color) v.s. woodbased (wireframe) (c) Sugarcane C (true color) v.s. woodbased (wireframe) Figure 1: Color gamut comparison for the copy paper Microscope images of tested copy papers (black line) are shown in Figure 2, at 40X magnifications. It shows that woodbased copy paper tended to produce a smoother, sharper edge. (a) Wood-based copy paper (b) Sugarcane A copy paper (c) Sugarcane B copy paper (d) Sugarcane C copy paper Figure 2: Microscope images (@40X magnification) One-way ANOVA Analysis One-way Analysis of Variance (ANOVA) statistical procedure was employed to determine whether the differences in optical density, print contrast, and color gamut of tested copy paper were significant. The significant level (α) was set at 0.05 for all tests. Table 3 to Table 6 present One-way ANOVA tests on the optical density difference among the tested copy papers for yellow, magenta, cyan, and black, respectively. It shows that the significant value of p is 0.000 < 0.05 (α) for observed optical densities yellow, magenta, and cyan (with p = 0.001 for black), that is, at least one pair of the mean optical density values is significantly different at 0.05 levels. The 95% confidence intervals of measurements are also exhibited in the lower part of tables. It shows that Sugarcane B copy paper and wood-based copy paper have similar optical density values for yellow (as their 95% confidence intervals of measurements are overlap with each other). Sugarcane A and C copy papers have similar optical density values for black. Table 3: One-way ANOVA test on the optical density of yellow Source DF SS MS F P Factor 3 0.016390 0.005463 25.83 0.000 Error 196 0.041450 0.000211 Total 199 0.057840 Individual 95% CIs For Mean Based on Pooled StDev Level N Mean StDev -------+---------+---------+---------+-Wood-based 50 0.84600 0.01552 (-----*----) Sugarcane A 50 0.83600 0.01400 (----*----) Sugarcane B 50 0.84320 0.01115 (----*----) Sugarcane C 50 0.82260 0.01688 (----*----) -------+---------+---------+---------+-0.8240 0.8320 0.8400 0.8480 Table 4: One-way ANOVA test on the optical density of magenta Source DF SS MS F P Factor 3 0.058212 0.019404 43.41 0.000 Error 196 0.087620 0.000447 Total 199 0.145832 Individual 95% CIs For Mean Based on Pooled StDev Level N Mean StDev ---+---------+---------+---------+-----Wood-based 50 1.0980 0.0238 (---*---) Sugarcane A 50 1.0644 0.0206 (---*---) Sugarcane B 50 1.0770 0.0221 (---*---) Sugarcane C 50 1.0518 0.0176 (---*---) ---+---------+---------+---------+-----1.050 1.065 1.080 1.095 NIP 28 and Digital Fabrication 2012 9 Table 5: One-way ANOVA test on the optical density of cyan Source DF SS MS F P Factor 3 0.089698 0.029899 55.15 0.000 Error 196 0.106252 0.000542 Total 199 0.195950 Individual 95% CIs For Mean Based on Pooled StDev Level N Mean StDev --+---------+---------+---------+------Wood-based 50 1.2160 0.0280 (--*--) Sugarcane A 50 1.1738 0.0223 (--*--) Sugarcane B 50 1.1986 0.0170 (--*---) Sugarcane C 50 1.1616 0.0244 (--*--) --+---------+---------+---------+------1.160 1.180 1.200 1.220 Table 6: One-way ANOVA test on the optical density of black Source DF SS MS F P Factor 3 0.02691 0.00897 5.39 0.001 Error 196 0.32598 0.00166 Total 199 0.35289 Individual 95% CIs For Mean Based on Pooled StDev Level N Mean StDev ----+---------+---------+---------+----Wood-based 50 1.6084 0.0377 (------*-------) Sugarcane A 50 1.5798 0.0277 (------*-------) Sugarcane B 50 1.5980 0.0348 (------*-------) Sugarcane C 50 1.5828 0.0570 (------*-------) ----+---------+---------+---------+----1.575 1.590 1.605 1.620 Tables 7, 8, 9 and 10 display One-way ANOVA tests on the print contrast difference among the tested copy papers. It shows that the significant value of p is 0.000 < 0.05 (α) for observed print contrast yellow, magenta, and cyan (with p = 0.001 for black), in other words, at least one pair of the mean print contrast values is significantly different at 0.05 levels. According to 95% confidence intervals of measurements, wood-based and sugarcane A & B copy papers have similar print contrast values for yellow. The average print contrast value of sugarcane A copy paper is close to that of sugarcane B copy paper. It also shows that sugarcane copy papers have similar print contrast values for black. Table 7: One-way ANOVA test on the print contrast of yellow Source DF SS MS F P Factor 3 198.25 66.08 16.27 0.000 Error 196 795.85 4.06 Total 199 994.10 Individual 95% CIs For Mean Based on Pooled StDev Level N Mean StDev -+---------+---------+---------+-------Wood-based 50 19.087 2.276 (-----*----) Sugarcane A 50 18.285 1.563 (-----*----) Sugarcane B 50 18.690 1.526 (-----*-----) Sugarcane C 50 16.483 2.508 (-----*----) -+---------+---------+---------+-------16.0 17.0 18.0 19.0 Table 8: One-way ANOVA test on the print contrast of magenta Source DF SS MS F P Factor 3 241.61 80.54 18.38 0.000 Error 196 858.65 4.38 Total 199 1100.26 Individual 95% CIs For Mean Based on Pooled StDev Level N Mean StDev ---+---------+---------+---------+-----Wood-based 50 32.106 2.383 (----*---) Sugarcane A 50 30.576 1.675 (----*----) Sugarcane B 50 30.973 2.287 (----*----) Sugarcane C 50 29.036 1.952 (----*----) ---+---------+---------+---------+-----28.8 30.0 31.2 32.4 Table 9: One-way ANOVA test on the print contrast of cyan Source DF SS MS F P Factor 3 214.35 71.45 33.04 0.000 Error 196 423.88 2.16 Total 199 638.24 Individual 95% CIs For Mean Based on Pooled StDev Level N Mean StDev +---------+---------+---------+--------Wood-based 50 25.253 1.699 (----*---) Sugarcane A 50 24.288 1.110 (---*---) Sugarcane B 50 23.537 1.299 (---*---) Sugarcane C 50 22.425 1.687 (---*---) +---------+---------+---------+--------22.0 23.0 24.0 25.0 Table 10: One-way ANOVA test on the print contrast of black Source DF SS MS F P Factor 3 60.37 20.12 5.61 0.001 Error 196 702.84 3.59 Total 199 763.21 Individual 95% CIs For Mean Based on Pooled StDev Level N Mean StDev -+---------+---------+---------+-------Wood-based 50 39.693 1.859 (-------*-------) Sugarcane A 50 41.127 1.228 (-------*------) Sugarcane B 50 40.914 1.890 (------*-------) Sugarcane C 50 40.465 2.409 (------*-------) -+---------+---------+---------+-------39.20 39.90 40.60 41.30 One-way ANOVA test on the color gamut difference among the tested copy papers was shown in Table 11. It shows that at least one pair of the mean color gamut values is significantly different at 0.05 levels (the significant value of p is 0.000 < 0.05 (α)). Based upon 95% confidence intervals of measurements, the color gamut of wood-based copy paper is significantly different from that of sugarcane copy paper. The average color gamut value of sugarcane A copy paper is close to that of sugarcane C copy paper. Table 11: One-way ANOVA test on the color gamut Sourc e DF SS MS F P Factor 3 2496764462 4 832254820 8 209.06 0.000 Error 196 7802567760 39809019 Total 199 3277021238 5 Individual 95% CIs For Mean Based on Pooled StDev Level N Mean StDev -------+---------+---------+---------+-Wood-based 50 336358 3712 (-*--) Sugarcane A 50 312351 3414 (-*--) Sugarcane B 50 308103 10248 (-*-) Sugarcane C 50 312096 5365 (-*-) -------+---------+---------+---------+-312000 320000 328000 336000 Capability Analyses The tools within the Minitab 16.0 software used to analyze the consistency for optical density and color gamut measurements are individual control chart (I chart), moving range charts (MR chart), and capability analysis. Individual control chart (I chart) and moving range charts (MR chart) were used to remove the outlier data. The capability analysis tool was used to calculate Cp index for each paper type. In order to do the capability analysis, lower specification limit (LSL) and upper specification limit (USL) are required input parameters. However, due to lack of historical parameters of LSL and USL for color-related attributes of paper, relative specification limits were determined using test data. After eliminating all outlier points, revised Sigma (the process standard deviation) was calculated for each paper type and 10 ©2012 Society for Imaging Science and Technology the average Sigma was computed from the Sigmas of wood-based and sugarcane papers. The relative LSL and USL (Tables 12) were obtained by subtracting and adding the appropriate average 3*Sigma value from each individual paper type mean, respectively. Table 12: The LSL and USL for each attribute Wood-based Sugarcane A Sugarcane B Sugarcane C LSL USL LSL USL LSL USL LSL USL Optical Density Y 0.80 0.89 0.79 0.88 0.80 0.89 0.78 0.87 M 1.03 1.16 1.00 1.13 1.01 1.14 0.99 1.12 C 1.15 1.28 1.11 1.24 1.14 1.26 1.10 1.22 K 1.49 1.73 1.46 1.70 1.48 1.72 1.47 1.70 Print Contrast Y 12.54 25.65 11.73 24.84 12.20 25.31 9.93 23.04 M 25.51 38.71 23.98 37.18 24.37 37.57 22.34 35.54 C 21.01 29.49 20.05 28.53 19.29 27.78 18.25 26.74 K 34.58 44.80 36.01 46.22 35.80 46.02 35.35 45.57 Color Gamut 322,602 350,114 298,595 326,107 294,533 322,045 298,340 325,852 Using LSL and USL values in Tables 12, the relative Cp indices were calculated. Results for color attributes are shown in Table 13. A higher Cp index indicates more capable or more consistent results from the printing process. As shown in Table 13, the sugarcane B had the largest relative Cp index for optical densities yellow (Cp = 1.39) and cyan (Cp = 1.83). The Sugarcane A copy paper had the largest relative Cp for the print contrast cyan (Cp = 1.13), black (Cp = 1.80), and color gamut (Cp = 1.65). Overall, sugarcane A was the most capable copy paper for delivering consistent results in print contrast and color gamut. The sugarcane C copy paper, on the other hand, was the least capable paper for delivering consistent results in optical density and print contrast, with exception of magenta. Table 13: The relative PCR (Cp value) for the tested copy papers Cp value Copy Paper Sugarcane A Sugarcane B Sugarcane C Optical Density Y 1.07 1.02 1.39 0.73 M 0.91 0.99 0.93 1.23 C 0.83 1.07 1.83 0.76 K 1.20 1.26 1.20 0.65 Print Contrast Y 0.88 1.23 1.39 0.75 M 0.84 1.15 0.85 1.30 C 0.84 1.13 1.07 1.01 K 1.08 1.80 0.93 0.68 Color Gamut 1.06 1.65 0.54 0.84 Conclusions Achieving uniformity of printing and obtaining good color reproduction performance are crucial in the print production. This study investigated the copy paper application of sugarcane alternatives. It was found that, sugarcane A copy paper was competitive with wood-based copy paper in terms of color reproduction consistency. Although wood-based copy paper yielded higher optical density, print contrast, and color gamut, sugarcane A was the most capable copy paper for delivering consistent results in color-related attributes. The sugarcane C copy paper, on the other hand, was the least capable paper for delivering consistent results. Users can choose sugarcane A copy paper as alternative when consistency is the highest priority. Acknowledgements The author thanks the Appalachian State University Research Council (URC) grant for support for this work, and Sustainable Development Program for sugarcane paper donations.