ASTM D6556-21

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D6556 − 19a D6556 − 21
Standard Test Method for
Carbon Black—Total and External Surface Area by Nitrogen
Adsorption
This standard is issued under the fixed designation D6556; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method covers the determination of the total surface area by the Brunauer, Emmett, and Teller (B.E.T. NSA) theory
of multilayer gas adsorption behavior using multipoint determinations and the external surface area based on the statistical
thickness surface area method.
1.2 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of
regulatory limitations prior to use. (The minimum safety equipment should include protective gloves, sturdy eye and face
protection).
1.4 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D1799 Practice for Carbon Black—Sampling Packaged Shipments
D1900 Practice for Carbon Black—Sampling Bulk Shipments
D3765 Test Method for Carbon Black—CTAB (Cetyltrimethylammonium Bromide) Surface Area (Withdrawn 2007)
D4483 Practice for Evaluating Precision for Test Method Standards in the Rubber and Carbon Black Manufacturing Industries
3. Summary of Test Method
3.1 The total and external surface areas are measured by evaluating the amount of nitrogen adsorbed, at liquid nitrogen
temperature, by a carbon black at several partial pressures of nitrogen. The adsorption data is used to calculate the NSA and STSA
values.
4. Significance and Use
4.1 This test method is used to measure the total and external surface area of carbon blacks based on multipoint nitrogen
This test method is under the jurisdiction of ASTM Committee D24 on Carbon Black and is the direct responsibility of Subcommittee D24.21 on Carbon Black Surface
Area and Related Properties.
Current edition approved Dec. 15, 2019May 1, 2021. Published January 2020July 2021. Originally approved in 2000. Last previous edition approved in 2019 as
D6556 – 19.D6556 – 19a. DOI: 10.1520/D6556-19A.10.1520/D6556-21.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
The last approved version of this historical standard is referenced on www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6556 − 21
adsorption. The NSA measurement is based on the B.E.T. theory and it includes the total surface area, inclusive of micropores,
pore diameters less than 2 nm (20 Å). The external surface area, based on the statistical thickness method (STSA), is defined as
the specific surface area that is accessible to rubber.
4.2 CTAB Surface Area (formerly Test Method D3765) has been withdrawn. The CTAB value may be estimated from the STSA
value using Eq 1. The equation is based on a linear regression of the STSA and CTAB measured values of the SRB 5 standards.
CTAB 5 STSA*1.017012.6434 (1)
5. Apparatus
5.1 Multipoint Static-Volumetric Gas Adsorption Apparatus, with Dewar flasks and all other accessories required for operation.
5.2 Sample Cells, that when attached to the adsorption apparatus, will maintain isolation of the sample from the atmosphere
–5 3
equivalent to a helium leak rate of <10 cm /min, per atmosphere of pressure difference.
5.3 Balance, Analytical, with 0.1 mg sensitivity.
5.4 Heating Mantle or Equivalent, capable of maintaining a temperature of 300 6 10°C.
5.5 Oven, Gravity Convection, capable of maintaining a temperature of 125 6 10°C.
6. Reagents
6.1 Liquid Nitrogen, 98 % or higher purity.
6.2 Ultra-High Purity Nitrogen Gas, cylinder or other source of prepurified nitrogen gas.
6.3 Ultra-High Purity Helium Gas, cylinder or other source of prepurified helium gas.
7. Sampling
7.1 Samples may be taken in accordance with Practices D1799 and D1900.
8. Sample Preparation Procedure
8.1 Dry a portion of carbon black at 125°C for 1 h. If the carbon black is known to be substantially free of moisture, or subsequent
preparation steps are known to be adequate for moisture removal, then this step may be omitted.
8.2 Condition an empty sample cell for a minimum of 10 min at the same conditions intended for degassing the sample. Weigh
the empty sample cell to the nearest 0.1 mg and record the mass.
8.3 Weigh approximately 0.4 g of the carbon black into the sample cell.
NOTE 1—For carbon black powder samples, add enough carbon black to give a depth of approximately 2 in. in straight wall sample tubes, or
approximately 0.4 g for bulb-type sample cells.
8.4 Flow Degassing:
8.4.1 Open the gas control valve and insert the delivery tube into the sample tube, through the sample bed, and allow purging with
nitrogen for a minimum of 1 min.
8.4.2 Place a heating mantle or other source of heat around the sample cell and degas the sample at 300 6 10°C for ⁄2 h or longer
to ensure that all traces of moisture condensing in the top of the tube are absent. The minimum degassing time that gives a stable
surface area (that is, a surface area that does not increase with additional degassing) shall be the minimum time used for degassing.
Failure to do so will lead to underreporting of the NSA and STSA values for the sample under test.
D6556 − 21
NOTE 2—For carbon blacks at their moisture pickup equilibrium, like standard reference blacks that have been exposed to atmospheric conditions, longer
degassing times likely will be required in order to achieve stable results. It has been found that for SRB-8B, SRB-8C, and SRB-9C, it is necessary to
extend the degassing time to at least 60 min in order to reliably obtain the target NSA and STSA values. This may be true for other high structure and
high porosity carbon blacks. Again, failure to fully degas the carbon black will result in underreporting of NSA and STSA values.
8.4.3 Once the typical degassing times have been determined, future samples can be degassed on the basis of time alone, if desired,
allowing a reasonable margin of excess time. Some samples will be found to require less than ⁄2 h, especially if moisture exposure
has been minimal. In these cases, the minimum time that gives a stable surface area shall be the minimum time used for degassing.
It will be necessary to determine these minimum times for each grade of carbon black.
8.4.4 After degassing, the sample tube may be moved directly to the analyzer. Otherwise, remove the sample tube from the heat
source and continue the flow of purging gas until it is ready for analysis.
8.4.5 Go directly to Section 9 and continue the remaining steps of the procedure.
8.5 Vacuum Degassing:
8.5.1 With the apparatus at atmospheric pressure, place the sample cell containing the carbon black onto the degassing apparatus.
8.5.2 Begin the degassing procedure as appropriate for the apparatus.
8.5.3 Place a heating mantle or other source of heat around the sample cell and degas the sample at 300 6 10°C for ⁄2 h or longer
as required to obtain and hold a pressure less than 1.4 Pa (10 μm Hg).
NOTE 3—Attention! One-half hour vacuum degassing may be inadequate for some grades and may result in statistically different results to flow degassing.
8.5.4 Once the typical degassing times have been determined, future samples can be degassed on the basis of time alone, if desired,
allowing a reasonable margin of excess time. Some samples will be found to require less than ⁄2 h, especially if moisture exposure
has been minimal. In these cases, the minimum time that gives a stable surface area shall be the minimum time used for degassing
(see Note 2). It will be necessary to determine these minimum times for each grade of carbon black.
8.5.5 Go directly to Section 9 and continue the remaining steps of the procedure.
9. Measurement Procedure
9.1 Refer to the user’s manual or specific instructions for the multipoint gas adsorption analyzer to be used, and become
thoroughly familiar with the procedures. There are numerous instruments available that offer a variety of saturation vapor pressure
(P ) measurement options and Dewar sizes. Follow all manufacturer’s specific recommendations for free space and P
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measurement options to obtain reliable NSA and STSA values.
9.2 Fill the Dewar with liquid nitrogen (LN2) and allow it to reach temperature equilibrium. The Dewar shall be refilled
approximately once per hour or between each analysis. The time required to reach equilibrium is dependent upon Dewar size and
quality. Consult the manufacturer’s recommendations for Dewar equilibration times and conditions.
9.2.1 Small Dewar (<1 L)—Fill and cover the Dewar for a minimum of 2 h prior to use. The Dewar should be cleaned and dried
at the end of each day.
9.2.2 Large Dewar (>1 L)—Fill and cover the Dewar for a minimum of 16 h prior to use, unless continuous P measurements are
o
employed. For continuous P , use a 2-h Dewar equilibration. Once equilibration is reached, a large Dewar can maintain this
o
equilibration for several days if kept filled and covered. The cleaning frequency is left to the discretion of the operator, but is not
to exceed once per week. Consult the manufacturer’s recommendations for Dewar cleaning.
9.3 Following is a list of P measurement options, listed in decreasing order of reliability of P values. Follow all manufacturer’s
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recommendations for determination of P values that are to be used for NSA and STSA calculations.
o
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9.3.1 Continuous P (measurement at each relative pressure point)—This method is considered the best practice. When available
o
with the instrument and software package, it is recommended to use this option.
9.3.2 Single P Per Analysis—Although this value can be measured before, during, or after the analysis, a P value measured at
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the end of the analysis is preferred, since STSA is calculated from the last data points acquired and is significantly influenced by
P values. This method requires that a P value be determined prior to initiating any measurements to ensure equilibrium of the
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Dewar as described in 9.4. Subsequently, a new P value is measured for each analysis, which is used for calculating NSA/STSA
o
values.
9.3.3 Daily P —This method is used when evidence of a stable Dewar is present and no changes in atmospheric pressure greater
o
than 0.13 kPa (1 mm Hg) occur.
9.3.4 Calculated P —This method calculates a P value by measuring atmospheric pressure and adding a value between 1.3 and
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2.6 kPa (10 and 20 mm Hg). The operator is responsible for determining the constant used in their laboratory; however, 2.0 kPa
(15 mm Hg) is most commonly used.
9.4 With the exception of continuous P measurements, it is recommended that the P value be determined prior to initiating
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NSA/STSA analyses. A P value of 1.3 to 2.6 kPa (10 to 20 mm Hg) above atmospheric pressure and two consecutive P values
o o
that differ by no more that 0.13 kPa (1 mm Hg) over a 10-min time period are indications of a stable Dewar. Experience will teach
the operator about expected differences in P and atmospheric pressure in their laboratory.
o
NOTE 4—A minimum wait time of 10 min is recommended between P measurements, as immersing the P cell into the LN2 disrupts the temperature
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equilibration. P measurements taken at short intervals will result in erroneously high and unstable values.
o
9.5 Determine the free space of the sample cell by measurement with helium or by calculation using an assumed carbon black
density of 1.9 g/cm . For calculated sample free space, the free space of the empty sample cells must be known, generally from
an analysis with empty sample cells. The free space for each cell must be determined for some cell designs. Follow all
manufacturer’s recommendations for obtaining optimum free space values, whether measured directly over the sample, or
TABLE 1 Example of NSA Data Analysis
A
N121
Raw Data Calculation
Vol. Ads., Rel. Press. Correlation NSA,
3 2
P/Po cm /g Range Coefficient m /g
0.0500 26.716 . . . . . . . . .
0.1000 29.753 . . . . . . . . .
0.1500 32.313 0.05–0.15 0.999981 123.9
0.2000 34.692 0.05–0.20 0.999992 124.0
0.2500 37.110 0.05–0.25 0.999990 123.6
0.3000 39.641 0.05–0.30 0.999935 122.8
B
N326
Raw Data Calculation
Vol. Ads., Rel. Press. Correlation NSA,
3 2
P/Po cm /g Range Coefficient m /g
0.0500 16.675 . . . . . . . . .
0.1000 18.318 . . . . . . . . .
0.1500 19.859 0.05–0.15 0.999960 75.6
0.2000 21.426 0.05–0.20 0.999948 76.3
0.2500 23.035 0.05–0.25 0.999964 76.6
0.3000 24.751 0.05–0.30 0.999979 76.6
B
N683
Raw Data Calculation
Vol. Ads., Rel. Press. Correlation NSA,
3 2
P/Po cm /g Range Coefficient m /g
0.0500 8.194 . . . . . . . . .
0.1000 9.113 . . . . . . . . .
0.1500 9.945 0.05–0.15 0.999939 38.2
0.2000 10.739 0.05–0.20 0.999950 38.5
0.2500 11.543 0.05–0.25 0.999972 38.6
0.3000 12.364 0.05–0.30 0.999973 38.4
A
The most accurate NSA is measured between 0.05 and 0.20 relative pressure.
B
The most accurate NSA is measured between 0.05 and 0.30 relative pressure.
D6556 − 21
calculated using the free space volume of the sample cells, along with the sample mass and density. Follow all manufacturer’s
recommendations for maintaining stable, and perhaps minimized, free space values, for example, by using filler rods.
9.6 Obtain a minimum of five data points evenly spaced in the 0.1 to 0.5 relative pressure (P/P ) range. For some tread carbon
o
blacks, particularly N100 and N200 series, it is necessary to measure two additional data points, 0.05 and 0.075, in order to increase
the accuracy of the NSA measurement. A data point consists of the relative pressure of equilibrium and the total amount of nitrogen
gas adsorbed by the sample at that relative pressure.
9.7 Determine the mass of the cell with dry sample to the nearest 0.1 mg. This may be done before or after measuring nitrogen
adsorption. Avoid inconsistent use of helium, particularly when backfilling at the end of degassing or an analysis, as a buoyancy
error of 1 mg/cm of cell volume can occur. As an alternative, the carbon black mass may be determined direct
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