Claims:1. A process for determining quality of paper, said process comprising the steps of:
a) obtaining a paper;
b) contacting at least one ink with the paper to obtain an ink mark;
c) determining penetration depth of the at least one ink through the paper;
d) determining a predicted value based on the penetration depth and at least one other parameter; and
e) analysing the predicted value to determine quality of paper.
2. The process for determining quality of paper as claimed in claim 1, wherein determining the penetration of the at least one ink through paper is carried out by measuring fluorescence of the ink mark by confocal microscopy.
3. The process for determining quality of paper as claimed in claim 2, wherein measuring fluorescence of the ink mark by confocal microscopy is carried out by exciting the ink mark at a wavelength in the range of 450-650 nm.
4. The process for determining quality of paper as claimed in claim 3, wherein measuring fluorescence of the ink mark by confocal microscopy is carried out by exciting the ink mark at 633 nm.
5. The process for determining quality of paper as claimed in claim 3, wherein measuring fluorescence of the ink mark by confocal microscopy is carried out by exciting the ink mark at 543 nm.
6. The process for determining quality of paper as claimed in claim 3, wherein measuring fluorescence of the ink mark by confocal microscopy is carried out by exciting the ink mark at 594 nm.
7. The process for determining quality of paper as claimed in claim 1, wherein the penetration depth is in the range of 0 – 150 µm.
8. The process for determining quality of paper as claimed in claim 1, wherein determining penetration depth of the at least one ink through the paper is carried out within a time period in the range of 5-15 minutes.
9. The process for determining quality of paper as claimed in claim 1, wherein the at least one parameter is selected from the group consisting of consumer panel rating, visual rating, Cobb test results, Pelikan test results, and combinations thereof.
10. The process for determining quality of paper as claimed in claim 1, wherein the at least one ink has an excitation wavelength range in the range of 450 – 650 nm.
11. The process for determining quality of paper as claimed in claim 1, wherein the paper can be aged or new paper.
12. The process for determining quality of paper as claimed in claim 1, wherein obtaining an ink mark is carried out by hand writing or by employing a universal testing machine.
13. The process for determining quality of paper as claimed in claim 1, wherein the process does not comprise additional processing steps selected from the group consisting of UV-curing, washing, spraying of at least one ink, drying, and combinations thereof.
14. The process for determining quality of paper as claimed in claim 1, wherein the paper has a GSM in the range of 34-120.
, Description:[002] The art of paper making has been practiced for centuries, with the earliest reports dating back to Qing dynasty (China, 18th Century). The modern methods of paper making have evolved into a high-throughput machine-intensive practice. It has been reported that over 71 million tonnes of paper and cardboard were consumed in the year 2015 in US alone (www.statistica.com/statistics/240565/consumption-volume-of-paper-and-paperboard-in-selected-countries/). The art of making paper and ink are interrelated. The placement of the ink onto the paper surface results in the absorption of the ink onto the cellulose fibers and the quality and thickness of paper can affect the absorption. Also, ink can behave differently on hydrophobic and hydrophilic surfaces. The mobility of the ink is minimal after drying of the solvent in the ink, as ink remains attached to the cellulose fibers by bonding. Paper having a typical thickness in the range of 50-120 µm are used for day to day usage.
[003] Quality of printing is not merely dependent on the thickness, but heavily dependent on paper quality. An associated aspect is ink strikethrough. It is defined as the visibility of the print of ink on the reverse side of paper depending upon dye and pigment. This causes a lot of problems in acceptance of the document. However, a number of problems are associated with grading paper. For e.g. the variation in ink spread with aging of the paper.
[004] There are few methods to evaluate the strikethrough such as Pelikan test and Cobb test. In the Pelikan test, the samples are dropped into a trough of ink and evaluated for the time taken for uptake of 50% ink by weight. It is generally accepted that 50% absorption would occur within 30 minutes in a good quality paper. However, these tests are subjective and does not give an accurate picture of strikethrough. On the other hand, the Cobb’s test being a water-based test, its relevance to organic solvent-based inks is low. Furthermore, the discrepancy introduced by aging of paper cannot be identified by these tests.
[005] The existing methods for establishing quality of paper are not only time consuming, but also inaccurate. Hence, there is a need for a reproducible method for grading paper quality, which can even differentiate between new and aged paper.
SUMMARY OF THE INVENTION
[006] In an aspect of the present invention, there is provided a process for determining quality of paper, said process comprising the steps of: a) obtaining a paper; b) contacting at least one ink with the paper to obtain an ink mark; c) determining penetration depth of the at least one ink through the paper; d) determining a predicted value based on the penetration depth and at least one other parameter; and e) analysing the predicted value to determine quality of paper.
[007] These and other features, aspects, and advantages of the present subject matter will be better understood with reference to the following description and appended claims. This summary is provided to introduce a selection of concepts in a simplified form. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.
BRIEF DESCRIPTION OF THE DRAWINGS
[008] The detailed description is described with reference to the accompanying figures. The same numbers are used throughout the drawings to reference like features and components.
[009] Figure 1 depicts the confocal laser scanning microscopy (CLSM) imaging of plain paper with 10x objective and 633nm excitation laser, in accordance with an implementation of the present subject matter.
[0010] Figure 2 depicts the CLSM imaging of ink line drawn on paper with 10x objective and 633nm excitation laser, in accordance with an implementation of the present subject matter.
[0011] Figure 3 depicts gallery view (Z stacking) of ink penetration (shown in red) in paper sample, in accordance with an implementation of the present subject matter.
[0012] Figure 4 depicts 3D view of the ink penetration through the paper sample, in accordance with an implementation of the present subject matter.
[0013] Figure 5 depicts CLSM profilometric analysis of the paper samples, in accordance with an implementation of the present subject matter.
[0014] Figure 6a depicts the line drawn on test paper by using UTM machine and ILD tester that is taken for CLSM imaging, in accordance with an implementation of the present subject matter.
[0015] Figure 6b depicts the ink strike through visible on the reverse side of the paper of Figure 6a, in accordance with an implementation of the present subject matter.

DETAILED DESCRIPTION OF THE INVENTION
[0016] Those skilled in the art will be aware that the present disclosure is subject to variations and modifications other than those specifically described. It is to be understood that the present disclosure includes all such variations and modifications. The disclosure also includes all such steps, features, compositions, and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of any or more of such steps or features.
Definitions
[0017] For convenience, before further description of the present disclosure, certain terms employed in the specification, and examples are delineated here. These definitions should be read in the light of the remainder of the disclosure and understood as by a person of skill in the art. The terms used herein have the meanings recognized and known to those of skill in the art, however, for convenience and completeness, particular terms and their meanings are set forth below.
[0018] The articles “a”, “an” and “the” are used to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article.
[0019] The terms “comprise” and “comprising” are used in the inclusive, open sense, meaning that additional elements may be included. It is not intended to be construed as “consists of only”.
[0020] Throughout this specification, unless the context requires otherwise the word “comprise”, and variations such as “comprises” and “comprising”, will be understood to imply the inclusion of a stated element or step or group of element or steps but not the exclusion of any other element or step or group of element or steps.
[0021] The term “including” is used to mean “including but not limited to”. “Including” and “including but not limited to” are used interchangeably.
[0022] The term GSM refers to the substance weight of paper, relating to an area of paper that remains constant, irrespective of sheet size, expressed as grams per square metre. In an embodiment of the present disclosure, there is provided a process for determining quality of paper as described herein, wherein the paper has a GSM in the range of 34-120.
[0023] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the disclosure, the preferred methods, and materials are now described. All publications mentioned herein are incorporated herein by reference.
[0024] The present disclosure is not to be limited in scope by the specific embodiments described herein, which are intended for the purposes of exemplification only. Functionally-equivalent products, compositions, and methods are clearly within the scope of the disclosure, as described herein.
[0025] As mentioned previously, the existing methods of determining quality of paper suffer from various drawbacks. To overcome these problems, a novel method to analyze the quality of paper through ink penetration levels is described herein-below. The basis of this method is auto-fluorescence property of the ink. The auto-fluorescence is traced using confocal laser scanning microscope’s (CLSM) moving stage in Z-direction from the point of fluorescence detection to the point of fluorescence disappearance. The method was found to be very robust and could measure small differences in the strikethrough and thus could be a valuable commercial tool in segregating grades of paper.
[0026] In an embodiment of the present disclosure, there is provided a process for determining quality of paper, said process comprising the steps of: a) obtaining a paper; b) contacting at least one ink with the paper to obtain an ink mark; c) determining penetration depth of the at least one ink through the paper; d) determining a predicted value based on the penetration depth and at least one other parameter; and e) analysing the predicted value to determine quality of paper.
[0027] In an embodiment of the present disclosure, there is provided a process for determining quality of paper as described herein, wherein determining the penetration of the at least one ink through paper is carried out by measuring fluorescence of the ink mark by confocal microscopy.
[0028] In an embodiment of the present disclosure, there is provided a process for determining quality of paper, said process comprising the steps of: a) obtaining a paper; b) contacting at least one ink with the paper to obtain an ink mark; c) determining penetration depth of the at least one ink through the paper; d) determining a predicted value based on the penetration depth and at least one other parameter; and e) analysing the predicted value to determine quality of paper, wherein determining the penetration of the at least one ink through paper is carried out by measuring fluorescence of the ink mark by confocal microscopy.
[0029] In an embodiment of the present disclosure, there is provided a process for determining quality of paper as described herein, wherein measuring fluorescence of the ink mark by confocal microscopy is carried out by exciting the ink mark at a wavelength in the range of 450-650 nm. In another embodiment of the present disclosure, exciting the ink mark is at a wavelength in the range of 480-640 nm. In another embodiment of the present disclosure, exciting the ink mark is at a wavelength in the range of 500-650 nm.
[0030] In an embodiment of the present disclosure, there is provided a process for determining quality of paper, said process comprising the steps of: a) obtaining a paper; b) contacting at least one ink with the paper to obtain an ink mark; c) determining penetration of the at least one ink through paper is carried out by measuring fluorescence of the ink mark by confocal microscopy by exciting the ink mark at a wavelength in the range of 450-650 nm to obtain penetration depth; d) determining a predicted value based on the penetration depth and at least one other parameter; and e) analysing the predicted value to determine quality of paper.
[0031] In an embodiment of the present disclosure, there is provided a process for determining quality of paper as described herein, wherein measuring fluorescence of the ink mark by confocal microscopy is carried out by exciting the ink mark at 633 nm.
[0032] In an embodiment of the present disclosure, there is provided a process for determining quality of paper as described herein, wherein measuring fluorescence of the ink mark by confocal microscopy is carried out by exciting the ink mark at 543 nm.
[0033] In an embodiment of the present disclosure, there is provided a process for determining quality of paper as described herein, wherein measuring fluorescence of the ink mark by confocal microscopy is carried out by exciting the ink mark at 594 nm.
[0034] In an embodiment of the present disclosure, there is provided a process for determining quality of paper as described herein, wherein the penetration depth is in the range of 0 – 150 µm. In another embodiment of the present disclosure, the penetration depth is in the range of 10 – 140 µm. In another embodiment of the present disclosure, the penetration depth is in the range of 30 – 120 µm. In another embodiment of the present disclosure, the penetration depth is in the range of 1 – 150 µm. In another embodiment of the present disclosure, the penetration depth is in the range of 10 – 50 µm. In another embodiment of the present disclosure, the penetration depth is in the range of 100 – 150 µm.
[0035] In an embodiment of the present disclosure, there is provided a process for determining quality of paper, said process comprising the steps of: a) obtaining a paper; b) contacting at least one ink with the paper to obtain an ink mark; c) determining penetration depth of the at least one ink through the paper; d) determining a predicted value based on the penetration depth and at least one other parameter; and e) analysing the predicted value to determine quality of paper, wherein the penetration depth is in the range of 0 – 150 µm.
[0036] In an embodiment of the present disclosure, there is provided a process for determining quality of paper as described herein, wherein determining penetration depth of the at least one ink through the paper is carried out within a time period in the range of 5-15 minutes. In another embodiment of the present disclosure, determining penetration depth of the at least one ink through the paper is carried out within a time period in the range of 7-13 minutes. In another embodiment of the present disclosure, determining penetration depth of the at least one ink through the paper is carried out within a time period in the range of 5-13 minutes. In another embodiment of the present disclosure, determining penetration depth of the at least one ink through the paper is carried out within a time period in the range of 7-15 minutes.
[0037] In an embodiment of the present disclosure, there is provided a process for determining quality of paper, said process comprising the steps of: a) obtaining a paper; b) contacting at least one ink with the paper to obtain an ink mark; c) determining penetration depth of the at least one ink through the paper; d) determining a predicted value based on the penetration depth and at least one other parameter; and e) analysing the predicted value to determine quality of paper, wherein determining penetration depth of the at least one ink through the paper is carried out within a time period in the range of 5-15 minutes.
[0038] In an embodiment of the present disclosure, there is provided a process for determining quality of paper as described herein, wherein the at least one parameter is selected from the group consisting of consumer panel rating, visual rating, Cobb test results, Pelikan test results, and combinations thereof. In another embodiment of the present disclosure, the at least one parameter is selected from the combination of consumer panel rating, Cobb test results and Pelikan test results.
[0039] In an embodiment of the present disclosure, there is provided a process for determining quality of paper as described herein, wherein a) the penetration depth in the range of 0.85-1.00 times the thickness of the paper and cobb test results greater than 22 g/m2 is assigned a predicted value ‘bad’; b) penetration depth in the range of 0.80-0.85 times the thickness of the paper and cobb test results in the range of 18 – 22 g/m2 is assigned a predicted value ‘medium’; and c) penetration depth in the range of 0.01-0.80 times the thickness of the paper and cobb test results less than 18 g/m2 is assigned a predicted value ‘good’.
[0040] In an embodiment of the present disclosure, there is provided a process for determining quality of paper as described herein, wherein a) the penetration depth in the range of 0.85-1.00 times the thickness of the paper and pelikan test results less than 35 min is assigned a predicted value ‘bad’; b) penetration depth in the range of 0.80-0.85 times the thickness of the paper and pelikan test results in the range of 35 – 50 min is assigned a predicted value ‘medium’; and c) penetration depth in the range of 0.01-0.80 times the thickness of the paper and pelikan test results greater than 50 min is assigned a predicted value ‘good’.
[0041] In an embodiment of the present disclosure, there is provided a process for determining quality of paper as described herein, wherein a) the penetration depth in the range of 0.85-1.00 times the thickness of the paper, cobb test results greater than 22 g/m2 and pelikan test results less than 35 min is assigned a predicted value ‘bad’; b) penetration depth in the range of 0.80-0.85 times the thickness of the paper, cobb test results in the range of 18 – 22 g/m2 and pelikan test results in the range of 35 – 50 min is assigned a predicted value ‘medium’; and c) penetration depth in the range of 0.01-0.80 times the thickness of the paper, cobb test results less than 18 g/m2 and pelikan test results greater than 50 min is assigned a predicted value ‘good’.
[0042] In an embodiment of the present disclosure, there is provided a process for determining quality of paper, said process comprising the steps of: a) obtaining a paper; b) contacting at least one ink with the paper to obtain an ink mark; c) determining penetration depth of the at least one ink through the paper; d) determining a predicted value based on the penetration depth and at least one other parameter; and e) analysing the predicted value to determine quality of paper, wherein the at least one parameter is selected from the group consisting of consumer panel rating, visual rating, Cobb test results, Pelikan test results, and combinations thereof.
[0043] In an embodiment of the present disclosure, there is provided a process for determining quality of paper as described herein, wherein the at least one ink has an excitation wavelength range in the range of 450 – 650 nm. In another embodiment of the present disclosure, the at least one ink has an excitation wavelength range in the range of 480 – 640 nm. In another embodiment of the present disclosure, the at least one ink has an excitation wavelength range in the range of 500 – 650 nm.
[0044] In an embodiment of the present disclosure, there is provided a process for determining quality of paper, said process comprising the steps of: a) obtaining a paper; b) contacting at least one ink with the paper to obtain an ink mark; c) determining penetration depth of the at least one ink through the paper; d) determining a predicted value based on the penetration depth and at least one other parameter; and e) analysing the predicted value to determine quality of paper, wherein the at least one ink has an excitation wavelength range in the range of 450 – 650 nm.
[0045] In an embodiment of the present disclosure, there is provided a process for determining quality of paper as described herein, wherein the paper can be aged or new paper. In another embodiment of the present disclosure, the paper is aged. In another embodiment of the present disclosure, the paper is new.
[0046] In an embodiment of the present disclosure, there is provided a process for determining quality of paper, said process comprising the steps of: a) obtaining a paper; b) contacting at least one ink with the paper to obtain an ink mark; c) determining penetration depth of the at least one ink through the paper; d) determining a predicted value based on the penetration depth and at least one other parameter; and e) analysing the predicted value to determine quality of paper, wherein the paper can be aged or new paper.
[0047] In an embodiment of the present disclosure, there is provided a process for determining quality of paper as described herein, wherein obtaining an ink mark is carried out by hand writing or by employing a universal testing machine. In another embodiment of the present disclosure, obtaining an ink mark is carried out by hand writing. In another embodiment of the present disclosure, obtaining an ink mark is carried out by employing a universal testing machine.
[0048] In an embodiment of the present disclosure, there is provided a process for determining quality of paper, said process comprising the steps of: a) obtaining a paper; b) contacting at least one ink with the paper to obtain an ink mark; c) determining penetration depth of the at least one ink through the paper; d) determining a predicted value based on the penetration depth and at least one other parameter; and e) analysing the predicted value to determine quality of paper, wherein obtaining an ink mark is carried out by hand writing or by employing a universal testing machine.
[0049] In an embodiment of the present disclosure, there is provided a process for determining quality of paper as described herein, wherein the process does not comprise additional processing steps selected from the group consisting of UV-curing, washing, spraying of at least one ink, drying, and combinations thereof.
[0050] In an embodiment of the present disclosure, there is provided a process for determining quality of paper as described herein, wherein the paper has a GSM in the range of 34-120. In another embodiment of the present disclosure, the paper has a GSM in the range of 40-110. In another embodiment of the present disclosure, the paper has a GSM in the range of 34-100. In another embodiment of the present disclosure, the paper has a GSM in the range of 34-50. In another embodiment of the present disclosure, the paper has a GSM in the range of 50-120. In another embodiment of the present disclosure, the paper has a GSM in the range of 80-120.
[0051] In an embodiment of the present disclosure, there is provided a process for determining quality of paper, said process comprising the steps of: a) obtaining a paper; b) contacting at least one ink with the paper to obtain an ink mark; c) determining penetration depth of the at least one ink through the paper; d) determining a predicted value based on the penetration depth and at least one other parameter; and e) analysing the predicted value to determine quality of paper, wherein the paper has a GSM in the range of 34-120.
[0052] In an embodiment of the present disclosure, there is provided a process for determining quality of paper, said process comprising the steps of: a) obtaining a paper; b) contacting at least one ink with the paper to obtain an ink mark; c) determining penetration depth of the at least one ink through the paper; d) determining a predicted value based on the penetration depth and at least one other parameter; and e) analysing the predicted value to determine quality of paper, wherein determining penetration depth of the at least one ink through the paper is carried out within a time period in the range of 5-15 minutes, and the paper has a GSM in the range of 34-120.
[0053] In an embodiment of the present disclosure, there is provided a process for determining quality of paper, said process comprising the steps of: a) obtaining a paper; b) contacting at least one ink with the paper to obtain an ink mark; c) determining penetration depth of the at least one ink through the paper; d) determining a predicted value based on the penetration depth and at least one other parameter; and e) analysing the predicted value to determine quality of paper, wherein the penetration depth is in the range of 0 – 150 µm, and the paper has a GSM in the range of 34-120.
[0054] In an embodiment of the present disclosure, there is provided a process for determining quality of paper, said process comprising the steps of: a) obtaining a paper; b) contacting at least one ink with the paper to obtain an ink mark; c) determining the penetration of the at least one ink through paper is carried out by measuring fluorescence of the ink mark by confocal microscopy by exciting the ink mark at a wavelength in the range of 450-650 nm to obtain penetration depth; d) determining a predicted value based on the penetration depth and at least one other parameter; and e) analysing the predicted value to determine quality of paper, wherein the paper has a GSM in the range of 34-120.

EXAMPLES
[0055] The disclosure will now be illustrated with working examples, which is intended to illustrate the working of disclosure and not intended to take restrictively to imply any limitations on the scope of the present disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice of the disclosed methods and compositions, the exemplary methods, devices and materials are described herein. It is to be understood that this disclosure is not limited to particular methods, and experimental conditions described, as such methods and conditions may apply.
[0056] A new process of quantitatively and accurately measuring strikethrough in paper has been developed using Confocal Laser Scanning Microscopy (CLSM). It can be used to differentiate different grades (quality) of paper samples. The new method is faster, reliable, reproducible and most importantly non-subjective compared to the conventional method (Pelikan test).

Material and methods
Pelikan test
[0057] Pelikan ink test is indicated in terms of ink penetration time. The materials required for the test are Pelikan ink 4001 vessel, test samples size 8x8 cm and stop watch.
[0058] Procedure- The ink is filled into the vessel and filling height should be approx. 5 mm. Then the samples are prepared and dropped onto the ink and the time is measured until 50% of the surface is penetrated by the ink. The contact with fingers to the sample is to be minimized because of fat transfer, which can result in wrong conclusions. Photographs of the penetration behavior gave good hint to judge the sizing performance and uniformity. For comparing different papers this method has to be performed in one session. Normally a good-sized sheet doesn’t show 50% ink strike through before 30 min. Even 60 min of ink swimming time are observed for hard sized paper.
Cobb test
[0059] Cobb test indicates water absorptiveness (Cobb value), the mass of water absorbed in a specific time by 1 square meter (10.76 square feet) of paper, board or corrugated fiber board under specified conditions. The equipment required is Cobb tester.
[0060] Procedure- 12.5 × 12.5 cm sizes samples were cut and weighed to note initial weight of samples. The weighed specimens were laid on the metal plate covered with a rubber mat. 100 ml of water was poured in the ring as rapidly as possible thus giving a head of 1 ± 0.1 cm; and the stopwatch was started immediately. After 45 seconds, the water was removed and the wetted side of the specimen were placed on a sheet of blotting paper. A second sheet of blotting paper was placed on top of the specimen and excess water was removed by moving hand roller over samples. Final weight of sample was noted.
Calculation: Cobb value = (Final weight of sample– Initial weight) ? 100
Confocal Laser Scanning Microscope (CLSM) – for measuring strike through
[0061] This method was applied to measure the ink penetration with a sensitivity in micron level. This quantitative analysis is applicable to grading the paper (varying GSM) and pen (like gel, fountain pen and ball pen etc.). The following were the apparatus and machines used for preparation of samples and measurement of ink penetration.
[0062] Universal testing machine (UTM)- Draw the line on paper with constant load and angle. Apply constant load for fountain pen 100g load / ball pen 150g load and angle 70°.
[0063] Rudi Hutt universal testing method- The method was adopted using the universal testing machine. The test paper is placed in position by opening the door of the instrument. Known weights are placed in the sleeves along with the writing refill and locked it with locking pin. The bar holding the refill along with load are pressed towards the paper until the bar gets magnetically locked. The instrument is switched on to initiate drawing on the paper. Once completed the bar automatically gets released. The same is continued with the required samples and the system closed by turning off the instrument. Parameters for measurement are listed below-
Writing angle : 70°
Writing speed : 4.5 m/min
Paper speed : 50mm/min
Point load : 100g/150g for gel/ball pen respectively
[0064] Indentation load deflection (ILD) tester- Draw the line on paper with constant load. Apply constant load for fountain pen 100g load / ball pen 150g load and constant angle 75°.
[0065] Confocal laser scanning microscopy (CLSM)- The test sample with the line was imaged using LSM 710 confocal microscope from Carl Zeiss. Auto-fluorescence property of the ink was used as marker to study the penetration through different grades of paper.
[0066] Procedure- The test samples were conditioned at ambient condition based on TAPPI / ISO standard. A line was drawn on test sample by using UTM machine or ILD tester with required pen. 1cmx1cm area of sample with the line was taken and placed in between clean glass slide and cover slip, sealed and taken for imaging. Prepared sample slide was imaged using LSM 710 confocal microscope from Carl Zeiss. Auto-fluorescence property of the ink was used as marker to study the penetration through different grades of paper. The limits of ink penetration were fixed by moving the stage in Z-direction (Z stacking) from the starting point of ink’s auto fluorescence to the farthest point where the auto fluorescence of the ink just disappears. Triplicate of images were collected per sample using similar conditions. Analysis of images was performed using ZEN 2010 software.
CLSM test parameters:
Objective: 10x Plan-Apochromat
Excitation Wavelength: 633nm (2% Laser power)
Emission Wavelength: 638 – 747nm
Pinhole Size: 41µm
Digital Zoom: 0.6
Scanning Area: 1414.22µm x 1414.22µm
Scanning Pixel Format: 512 x 512 pixels (Frame mode)
Z-Step: 1µm
[0067] The contrast between line drawn using ILD tester and UTM is depicted in Figure 6a. The ink strike through is faintly visible on the reverse side of the same paper in Figure 6b.
Example 1
Establishing working protocol for grading quality of paper
[0068] In order to accurately determine extent of ink penetration, CLSM based measurement of plain paper was carried out to rule out fluorescence of paper in the selected wavelength range (Figure 1). Auto- fluorescence of ink upon application on paper without quenching of the auto-fluorescence signal can be observed in Figure 2.
[0069] Taking this auto-fluorescence of the ink as cue, the measurement of penetration of the ink through the paper using Z-stacking with 1µm optical sectioning with CLSM was carried out. This provided the exact depth (or penetration) of the ink through various paper samples (Figures 3 and 4). The penetration levels of ink in microns were taken as the measure to grade the paper quality. Lesser the ink penetration better the quality of paper.
[0070] Conventionally, rate of ink adsorption (water/solvent) is measured by Pelikan or Cobb test. Rudi Hutt method was confirmed to using the Universal testing machine (UTM) were adopted as standard methods of writing to avoid the variation in writing angle and pressure. The strikethrough values in various paper samples and the effects of writing angle, pressure, different inks and different mode of writing was obtained using above confocal imaging approach and compared with consumer panel test (CPT) results. The results of the study have been presented in Table 1. Furthermore, the results obtained using Pelikan and Cobb test for the same set of paper samples were also tabulated. It was found that the results trends were similar to the confocal analysis results (Table 1). It is known that the time taken for Pelikan test is very high (typical time per sample- 1 hour) whereas, the time taken for CLSM and Cobb test are similar (typical time per sample- 10 min). But, the drawback with Cobb’s test is that the test is only used for water absorptivity testing not for solvent absorption.
Table 1- Results of various tests performed on paper samples (before aging)
Sl. No. Pelikan test results in min Thickness of paper in µm Cobb test results in g/m2 Ink Penetration (CLSM) Visual rating CPT
Hand written (HW) Universal testing machine (UTM)
1 55 75 18.3 64 60 Good 3
2 52 75 19.9 66 61 Good 1
3 45 73 18.4 64 65 Good 2
4 40 72 17.5 65 64 Medium 4
5 35 72 19.1 69 71 Bad 5

[0071] The Table 1 above lists samples of paper with varying degree of ink penetration (strike through). As can be seen, the Pelikan and Cobb test results provide similar trend as the penetration observed by CLSM. In addition, visual rating and consumer panel test results are also provided for comparison.
Example 2
Testing of aged paper
[0072] Testing was carried out in order to test whether aging of paper had any effect on the strikethrough measurements. The same set of samples used for strike through measurements earlier (depicted in Table 1) were used. Accelerated aging was carried out by storing the samples for 72 hours after manufacture at 105 ?. The results of the study have been presented in Table 2. Variations in ink penetration due to aging process was observed.
Table 2- Results of various tests performed on paper samples (after aging)
Sl. No. Pelikan test results in min Thickness of paper in µm Cobb test results in g/m2 Ink Penetration (CLSM) in µm Visual rating CPT
Hand written (HW) Universal testing machine (UTM)
1 55 75 18.3 66 63 Good 3
2 52 75 19.9 73 70 Good 1
3 45 73 18.4 67 64 Good 2
4 40 72 17.5 72 73 Medium 4
5 35 72 19.1 71 74 Bad 5

[0073] As can be observed from Tables 1 and 2, that Cobb test and Pelikan test are unable in providing an accurate grading for aged paper. However, CLSM is able to accurately determine ink penetration which is in sync with the visual rating and CPT.
Example 3
Roughness measurements
[0074] Profilometric analysis of these paper surfaces to check if any correlations exist with respect to ink strike through with the smoothness/roughness of surfaces was carried out (Figure 5). Interestingly the results using CLSM showed no significant differences in the surface smoothness/roughness parameters. Differences in the mesh work of the fibers before and after aging of paper were not observed. However, as mentioned in Example 2, clear differences in ink penetration studies were observed with aged paper using CLSM.
[0075] Hence, it may be concluded that the newly developed method using CLSM could simulate measurement of strikethrough in paper samples that are comparable to conventional approaches like Pelikan test. Also, it can differentiate the paper samples that are similar in surface profiles (such as aged paper).
Example 4
Grading paper
[0076] On comparison of Tables 1 and 2, it is clear that CLSM is sensitive in providing an accurate estimate of ink penetration. Furthermore, the various parameters including visual rating, consumer panel test results (CPT), Cobb test result and Pelikan test results were combined to establish a grading scheme that can determine quality of paper accurately. Table 3 below depicts the grading scheme.
Table 3-
Sl. No. Grade Thickness of paper in µm Ink penetration (CLSM) in µm
Pelikan test results in min Cobb test results in g/m2
1 Bad 75 > 65 <35 >22
2 Medium 75 >60 to <65 >35 to <50 >18 to <22
3 Good 75 < 60 >50 <18

[0077] The grading scheme is provided by a process comprising the steps of: a) obtaining a paper; b) contacting at least one ink with the paper to obtain an ink mark; c) determining penetration depth of the at least one ink through the paper; d) determining a predicted value based on the penetration depth and at least one other parameter; and e) analysing the predicted value to determine quality of paper.
[0078] The above-mentioned steps provide a convenient method for grading. Herein, the various parameters pertaining to the steps of determining penetration depth and the criteria for determination of predicted values based on the various listed parameters are mentioned in the Table 3 above.
[0079] The samples were graded as good, medium and bad based on the various listed parameters in Table 3 above. Thus, providing a robust and convenient method that is able to detect minor variations in paper quality.
Advantages of the present disclosure:
[0080] The present disclosure reveals a process for determining quality of paper based on measurement of ink strike through. The method is robust, reproducible and convenient to carry out. Additionally, it is applicable on all types of paper and wide range of inks. The highlight of the method lies in its ability to discern minute differences in quality of paper including identifying quality variations between new and aged paper.