Frequently Asked Questions about TAPPI Standards, Methods, and TIPs
- What is the difference between the opaque TAPPI Dirt Chart and the transparent TAPPI Dirt Chart?
- Does TAPPI recommend suppliers of testing equipment or calibration standards for TAPPI Test Methods?
- What are the different brightness methods for pulp, paper, and paperboard?
- What is the relationship between Gurley stiffness units and Taber stiffness units?
- What is the difference between bending resistance, bending moment, Taber stiffness and bending stiffness?
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The opaque TAPPI Dirt Estimation Chart is a photographic print of the original chart developed for use with TAPPI T 213 “Dirt in Pulp” and T 437 “Dirt in Paper and Paperboard.” The basis for these two tests is to determine the numerical or visual estimation of dirt in paper, paperboard, or pulp in terms of equivalent black area (EBA). EBA of a dirt speck is defined as the area of a round black spot on a white background on the Dirt Chart which makes the same visual impression on its background as does the dirt speck on the particular background in which it is embedded. Reflected light is the proper basis of comparison. Only the TAPPI Dirt Estimation Chart, printed on the opaque, white background can properly be used in the tests. Photocopies, transparencies, plastic-covered cards, or printed reproductions of the chart (including the reproductions of the charts printed in the Test Methods for informational purposes) will not give equivalent results and must not be used in order to run the tests as specified in the Test Methods. The official Dirt Estimation Chart for use with T 213 and T 437 must be ordered and purchased separately from the Test Method.
Another method, T 537 “Dirt Count in Paper and Paperboard (Optical Character Recognition – OCR),” was developed in 1981 and is intended for the numerical estimation of cleanliness for OCR purposes of paper and paperboard in terms of the frequency of dirt, specks, or marks. This method may be used in applications where the number of specks per unit area rather than the equivalent black area is required. In this method, each dirt speck is counted individually regardless of size, shape, or color. Reflected light is also the basis for this method, so the TAPPI Dirt Estimation Chart is used, although the information on the chart relates to its use in T 437 and should be ignored when used with T 537. As with the use of the chart with T 213 and T 437, photocopies, transparencies, plastic-covered cards, or printed reproductions of the chart (including the reproductions of the charts printed in the Test Method for informational purposes) will not give equivalent results and must not be used in order to run the tests as specified in T 537. The official Dirt Estimation Chart for use with T 537 must be ordered and purchased separately from the Test Method.
Over the years, a transparent version of the TAPPI Dirt Estimation Chart was produced by request from industry segments that wanted only to have a size estimation method for determining size of spots, defects, or inclusions in paper or other industrial materials such as textiles or plastics. Using the transparent chart for T 213, T 437, or T 537 is NOT acceptable, because those methods are based on reflected, not transmitted, light. In 1996, a method, T 564 “Transparent Chart for the Estimation of Defect Size,” was developed for use with a transparent version of the Dirt Chart. The transparent Size Estimation Chart (ordered and purchased separately from the TAPPI Test Method) should only be used in accordance with the procedure described in T 564. The transparent chart has also been adopted for use with an International Standard, ISO 5350-3 “Pulps – Estimation of Dirt and Shives. Part 3 – Inspection by Reflected Light.”
Does TAPPI recommend suppliers of testing equipment or calibration standards for TAPPI Test Methods?
TAPPI does not endorse or certify any supplier’s products or services. However, TAPPI does maintain a list of test equipment suppliers and a list of reference material suppliers. Accompanying these two lists are two other lists – the Supplier Directory, which provides the contact information for vendors listed on either of the lists and the Test Labs list. Companies are listed on the Supplier List when they notify TAPPI that they provide some or all of the equipment or materials needed to run the test. Companies may be listed on the Reference Materials List when they claim to provide calibration services or reference materials for specific TAPPI Test Methods, and they complete a form indicating that they adhere to the general criteria for acceptance as a provider of calibration services or reference material as defined in TAPPI T 1211 "Acceptance Procedures for Calibration Laboratories."
Often one will find references to "GE Brightness," which is a directional brightness measurement utilizing essentially parallel beams of light to illuminate the paper surface at an angle of 45°. This brightness measurement is most common in the United States, and is the one described in TAPPI T 452 "Brightness of Pulp, Paper, and Paperboard (Directional Reflectance at 457 nm)."
In Canada, Europe, and South America, Elrepho or ISO brightness is more commonly used. ISO Standards 2469 "Paper, board, and pulps – Measurement of diffuse reflectance factor," 2470 "Paper and Board – Measurement of Diffuse Blue Reflectance Factor (ISO Brightness)" and 3688 "Pulps -- Measurement of Diffuse Blue Reflectance Factor (ISO Brightness)" are the appropriate standards for this measurement.
Diffuse brightness may also be measured using TAPPI T 525 "Diffuse brightness of paper, paperboard and pulp (d/0)" and TAPPI T 527 "Color of paper and paperboard (d/0, C/2)," which uses an integrating sphere to provide diffuse illumination and perpendicular observation geometry. The illumination from the lamps strikes the inner wall of a sphere, which is coated with a high reflectance white material, and multiple reflections from this surface diffuse the light before it strikes the sample. The reflected light is viewed by a photocell positioned to view the sample perpendicularly.
Because the instrument geometry of T 452 is different from that of T 525, ISO 2469, ISO 2470, and ISO 3688, there is no simple relationship between the two brightness scales. Instruments employing different geometries cannot be expected to agree with each other unless the sample being measured embodies ideal optical properties. The randomness of the disagreement between the different instruments makes it impossible to derive a correction equation or table to achieve correlation between the directional and diffuse brightness values.
A good resource with more detailed technical information on brightness measurements is Technical Bulletin No. 101 "Diffuse vs. Directional Brightness Measurement," published by Technidyne Corporation (100 Quality Avenue, New Albany, IN 47150, or on the web at www.technidyne.com, or email to firstname.lastname@example.org.
Up until the late 1960’s there was little information available about the precision statement for the Taber stiffness tester. In June 1969, an article was published in TAPPI Journal which summarized interlaboratory evaluation involving 24 Taber stiffness testers in 15 laboratories (Verseput, H.W., “Precision of the Taber Stiffness Test,” Tappi 52 (6):1136 (1969). He further continued his study to develop an empirical correlation between the Taber and Gurley stiffness testers, based upon 183 paired tests on paperboard in a single laboratory. The Taber 100 and 500 g-cm scales were used.
The following paragraphs in quotes and Equations 1 and 2 are excerpts from that article:
"Although the instruments differ considerably in construction and operation, they appear to measure the same property of the specimen, and thus should correlate reasonably well, subject to the limitations found by the Institute of Paper Chemistry. These include the likelihood of error in Gurley results on specimens cut shorter than 2 ½ inches, and the error introduced by the sample weight in both instruments. In paperboard work, however, the latter is inconsequential."
“The task committee was provided with the results from 183 pairs of Taber-Gurley comparisons. A simple least-squares regression was performed, resulting in two lines so nearly congruent that they are shown as one in Fig. 2, which also shows the 95% confidence limits. The correlation coefficient of 0.985, and the regression equations are:
G = 233 + 68.35 T (1)
T = 0.01419 G – 0.935 (2)
where G = Gurley stiffness value, and T = Taber stiffness value.”
- The range of the correlation tests between Taber and Gurley were from 20 – 150 g-cm units on the Taber. This roughly corresponds with 2,000 to 10,000 mgf Gurley stiffness units.
- In TAPPI Test Method T 543 (Gurley), the working range is described as 1.39 to 56,888 Gurley units.
- In TAPPI Test Method T 489 (Taber), the maximum working range is described as 5000 g-cm. Further, paragraph reference is made to a modification of the Taber instrument for measurements in the 0 – 10 Taber stiffness range.
Please note that in TAPPI T 543, there is a reference to Equation 2, above. One should be cautioned that this study was based on paperboard samples, which fall within a narrow working range of these instruments. The following tables show examples of the calculated differences between Equations 1 and 2:
|Taber Value||Conversion to Gurley using Equation 1|
|10 *||916.5 *|
|300 *||20,738 *|
|806.3 *||55,344 *|
|Gurley Value||Conversion to Taber using Equation 2|
|1.39 * lowest possible||-0.915 (not meaningful)|
|56,888 * highest possible||806.3 *|
* designates conversion examples beyond the range of the study
The above examples of Taber-to-Gurley and Gurley-to-Taber conversions will help the user assess the suitability of the use of these equations for their purposes. The techniques used to provide linear regressions (Equation 1 and Equation 2) did not give us equations that were mathematically equal. At the low end of the experimental data (20 Taber units), the error between the two equations is 21.77 Taber / 20 Taber = 1.0885, or almost 9% different, in the 1600 Gurley range. At the high end, the error is less, being about 1.5 %. These errors are solely due to the selection of which regression equation one chooses to use; it does not take into account any instrumental or other grade-specific differences that may occur in testing products that are different from those studied. Keep in mind that this study was probably performed using only one Gurley stiffness tester.
In summary, one might state that there is a reasonable correlation between Gurley and Taber stiffness for paperboard grades in the range from 20 to 150 g-cm (Taber), which approximately corresponds to a range of 2000 to 10,000 Gurley.
What is the difference between bending resistance, bending moment, Taber stiffness and bending stiffness?
- Bending resistance is the force or bending moment needed to deflect a test piece under specified conditions.
- Bending moment = Taber stiffness (see TAPPI T 566 and T 489)
- Bending stiffness is a paper property independent of specimen size, deflection angle, etc. Often the term "bending stiffness" is used in the industry instead of the correct term "bending resistance."
- Bending moment (Taber) = bending force (L&W-type instrument) X bending length.
- The theoretical relationship between Taber and L&W, according to ISO 2493 (taking into account that L&W has 50 mm bending length and Taber (model 150-B) has 51.8:
- Bending resistance Taber (mN) = Taber scale reading (gf-cm) × 2.03