# ASTM D6556-17

Designation: D6556 − 17Standard Test Method forCarbon Black—Total and External Surface Area by NitrogenAdsorption1This standard is issued under the fixed designation D6556; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (´) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers the determination of the totalsurface area by the Brunauer, Emmett, and Teller (B.E.T. NSA)theory of multilayer gas adsorption behavior using multipointdeterminations and the external surface area based on thestatistical thickness surface area method.1.2 The values stated in SI units are to be regarded as thestandard. The values given in parentheses are for informationonly.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety, health, and environmental practices and deter-mine the applicability of regulatory limitations prior to use.(The minimum safety equipment should include protectivegloves, sturdy eye and face protection).1.4 This international standard was developed in accor-dance with internationally recognized principles on standard-ization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recom-mendations issued by the World Trade Organization TechnicalBarriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:2D1799 Practice for Carbon Black—Sampling PackagedShipmentsD1900 Practice for Carbon Black—Sampling Bulk Ship-mentsD3765 Test Method for Carbon Black—CTAB (Cetyltrim-ethylammonium Bromide) Surface Area (Withdrawn2007)3D4483 Practice for Evaluating Precision for Test MethodStandards in the Rubber and Carbon Black ManufacturingIndustries3. Summary of Test Method3.1 The total and external surface areas are measured byevaluating the amount of nitrogen adsorbed, at liquid nitrogentemperature, by a carbon black at several partial pressures ofnitrogen. The adsorption data is used to calculate the NSA andSTSA values.4. Significance and Use4.1 This test method is used to measure the total andexternal surface area of carbon blacks based on multipointnitrogen adsorption. The NSA measurement is based on theB.E.T. theory and it includes the total surface area, inclusive ofmicropores, pore diameters less than 2 nm (20 Å). The externalsurface area, based on the statistical thickness method (STSA),is defined as the specific surface area that is accessible torubber.4.2 CTAB Surface Area (formerly Test Method D3765) hasbeen withdrawn. The CTAB value may be estimated from theSTSA value using Eq 1. The equation is based on a linearregression of the STSAand CTAB measured values of the SRB5 standards.CTAB 5 STSA*1.017012.6434 (1)5. Apparatus5.1 Multipoint Static-Volumetric GasAdsorptionApparatus,with Dewar flasks and all other accessories required foroperation.5.2 Sample Cells, that when attached to the adsorptionapparatus, will maintain isolation of the sample from theatmosphere equivalent to a helium leak rate of 1 L)—Fill and cover the Dewar for aminimum of 16 h prior to use, unless continuous Pomeasure-ments are employed. For continuous Po, use a 2-h Dewarequilibration. Once equilibration is reached, a large Dewar canmaintain this equilibration for several days if kept filled andcovered. The cleaning frequency is left to the discretion of theoperator, but is not to exceed once per week.9.3 Following is a list of Pomeasurement options:9.3.1 Continuous Po(measurement at each relative pressurepoint)—This method is considered the best practice. Whenavailable with the instrument and software package, it isrecommended to use this option.9.3.2 Single PoPer Analysis—Although this value can bemeasured before, during, or after the run, a Povalue measuredat the end of the analysis is preferred, since STSAis calculatedD6556 − 172from the last data points acquired and is significantly influ-enced by Povalues. This method requires that a Povalue bedetermined prior to initiating any measurements to ensureequilibrium of the Dewar as described in 9.4. Subsequently, anew Povalue is measured for each run, which is used forcalculating NSA/STSA values.9.3.3 Daily Po—This method is used when evidence of astable Dewar is present and no changes in atmospheric pressuregreater than 0.13 kPa (1 mm Hg) occur.9.3.4 Calculated Po—This method calculates a Povalue bymeasuring atmospheric pressure and adding a value between1.3 and 2.6 kPa (10 and 20 mm Hg). The operator isresponsible for determining the constant used in their labora-tory; however, 2.0 kPa (15 mm Hg) is most commonly used.9.4 With the exception of continuous Pomeasurements, it isrecommended that the Povalue be determined prior to initiat-ing NSA/STSAanalyses.APovalue of 1.3 to 2.6 kPa (10 to 20mm Hg) above atmospheric pressure and two consecutive Povalues that differ by no more that 0.13 kPa (1 mm Hg) over a10-min time period are indications of a stable Dewar. Experi-ence will teach the operator about expected differences in Poand atmospheric pressure in their laboratory.NOTE 4—A minimum wait time of 10 min is recommended between Pomeasurements, as immersing the Pocell into the LN2 disrupts thetemperature equilibration. Pomeasurements taken at short intervals willresult in erroneously high and unstable values.9.5 Determine the free space of the sample cell by measure-ment with helium or by calculation using an assumed carbonblack density of 1.9 g/cm3.9.6 Obtain a minimum of five data points evenly spaced inthe 0.1 to 0.5 relative pressure (P/Po) range. For some treadcarbon blacks, particularly N100 and N200 series, it is neces-sary to measure two additional data points, 0.05 and 0.075, inorder to increase the accuracy of the NSAmeasurement.Adatapoint consists of the relative pressure of equilibrium and thetotal amount of nitrogen gas adsorbed by the sample at thatrelative pressure.9.7 Determine the mass of the cell with dry sample to thenearest 0.1 mg. This may be done before or after measuringnitrogen adsorption. Avoid inconsistent use of helium, as abuoyancy error of 1 mg/cm3of cell volume can occur. As analternative, the carbon black mass may be determined directlyby pouring it from the sample cell into a tared weighing pan,taking care to remove all of the carbon black.10. Calculation10.1 Most automated instruments will perform the follow-ing computations at the completion of the analysis. The usermust verify that the internal computations conform to thefollowing method.10.2 Sample Mass:sample mass ~dried! 5 ~mass of cell1sample! 2 ~mass of cell! (2)Record masses to nearest 0.1 mg.10.3 Volume of Nitrogen Adsorbed:10.3.1 Calculate total volume of nitrogen adsorbed per gramof sample to the nearest 0.0001 cm3/g as follows:Va5Volume of Nitrogen for each dosing in cm3sample mass in g(3)10.4 Nitrogen Surface Area:10.4.1 Determine the nitrogen surface area (NSA) using aB.E.T. plot from the Brunauer, Emmett, and Teller4equation asfollows:PVa~Po2 P!51VmC1C 2 1VmC3PPo(4)where:P = manometer pressure in kPa,Po= saturation vapor pressure of nitrogen in kPa,Vm= volume of nitrogen per gram that covers one mono-molecular layer in standard cm3/g, andC = B.E.T. constant. Its numerical value depends on theheat of adsorption of the monomolecular layer.10.4.2 Plot P/Poon the X-axis versusPVa~Po2P!on theY-axis, for data sets having P/Poin the range of 0.05 to 0.30(linear region of B.E.T. equation).10.4.3 The data points (three or more) that give the beststraight line are used to calculate the slope and y-intercept. Theslope and y-intercept are used to calculate the surface area. Forexamples of how to select the proper relative pressure range,see Table 1.4Brunuaer, Emmett, and Teller, Journal of the American Chemical Society, Vol.60 , 1938, p. 309.TABLE 1 Example of NSA Data AnalysisN121ARaw Data CalculationP/PoVol. Ads.,cm3/gRel. Press.RangeCorrelationCoefficientNSA,m2/g0.0500 26.716 . . . . . . . . .0.1000 29.753 . . . . . . . . .0.1500 32.313 0.05–0.15 0.999981 123.90.2000 34.692 0.05–0.20 0.999992 124.00.2500 37.110 0.05–0.25 0.999990 123.60.3000 39.641 0.05–0.30 0.999935 122.8N326BRaw Data CalculationP/PoVol. Ads.,cm3/gRel. Press.RangeCorrelationCoefficientNSA,m2/g0.0500 16.675 . . . . . . . . .0.1000 18.318 . . . . . . . . .0.1500 19.859 0.05–0.15 0.999960 75.60.2000 21.426 0.05–0.20 0.999948 76.30.2500 23.035 0.05–0.25 0.999964 76.60.3000 24.751 0.05–0.30 0.999979 76.6N683BRaw Data CalculationP/PoVol. Ads.,cm3/gRel. Press.RangeCorrelationCoefficientNSA,m2/g0.0500 8.194 . . . . . . . . .0.1000 9.113 . . . . . . . . .0.1500 9.945 0.05–0.15 0.999939 38.20.2000 10.739 0.05–0.20 0.999950 38.50.2500 11.543 0.05–0.25 0.999972 38.60.3000 12.364 0.05–0.30 0.999973 38.4AThe most accurate NSA is measured between 0.05 and 0.20 relative pressure.BThe most accurate NSA is measured between 0.05 and 0.30 relative pressure.D6556 − 17310.4.4 As an alternative, the interpretation of the properrelative pressure can generally be simplified by specifying thefollowing pressure ranges for the various carbon black types:BET RangeN300 and Carcass Grades 0.1–0.3N100 and N200 0.05–0.2Carbon Blacks 130 m2/g 0.05–0.1It is the responsibility of the operator to assure that theseguidelines are appropriate for their samples.10.4.5 A B.E.T. plot that yields a negative y-intercept couldbe indicative of the presence of micropores (2 nm diameter),but other factors can produce a negative y-intercept. Thesurface area is calculated from three or more points within thepressure range that yields the highest correlation coefficientand a positive y-intercept.10.4.6 Calculate the nitrogen surface area to the nearest 0.1m2/g as follows:Surface area ~m2/g! 5 Vm 34.35 (5)where:Vm =1B1MB = Y-axis intercept, 610–5,M = slope of the straight line, 610–5, and4.35 = area occupied by 1 cm3of nitrogen =~6.02 31023!~16.2 310220!224006.02 × 1023= Avogadro’s number,16.2 × 10–20= area of nitrogen molecule in m2, and22400 = number of cm3occupied by one mole of gasat STP.10.5 Statistical Thickness Surface Area:10.5.1 Determine the STSA5of the black using a plot of thevolume of nitrogen gas adsorbed per gram of sample at STP(Va) versus the statistical layer thickness (t).10.5.2 Prepare the Va-t plot by plotting t (nm) on the X axisversus Va(dm3/kg at STP) on the Y axis, for data sets havingP/Poequally spaced in the range of 0.2 to 0.5.where:t = statistical layer thickness of carbon black =0.088(P/Po)2+ 0.645 (P/Po) + 0.298NOTE 5—The carbon black thickness model was developed using anN762 carbon black in the P/Po range of 0.2 to 0.5. T values calculatedoutside of this range are invalid and will result in erroneous STSAvalues.10.5.3 Determine the slope of the Va-t plot using standardlinear regression.10.5.4 Calculate the STSA to the nearest 0.1 m2/g asfollows:STSA 5 M 315.47 (6)where:M = slope of the Va-t plot, and15.47 = a constant for the conversion of nitrogen gas toliquid volume, and conversion of units to m2/g.10.5.5 STSAis based on a thickness model developed usingan N762 carbon black. This carbon black was chosen becauseof its low surface area and low structure level. This universalmodel does not perfectly apply to all carbon blacks;consequently, while it is theoretically impossible for externalsurface area to be higher than total surface area, in practicethere are instances where STSA is higher than NSA. Foranalyses that yield STSA values that are higher than NSA, themeasured STSA value should be reported.11. Report11.1 Report the following information:11.1.1 Proper sample identification,11.1.2 Number of data points and relative pressures used toobtain both NSA and STSA,11.1.3 The sample mass to the nearest 0.1 mg, and11.1.4 The NSAor STSA, or both, of the sample reported tothe nearest 0.1 m2/g.12. Precision and Bias12.1 These precision statements have been prepared inaccordance with Practice D4483. Refer to this practice forterminology and other statistical details.12.2 An Interlaboratory precision program (ITP) informa-tion was conducted as detailed in Table 2. Both repeatability5Magee, R. W., Rubber Chemistry and Technology, Vol. 68, No. 4, 1995, p. 590.TABLE 2 SRB8 ITP InformationNumber of Labs (M/H/L)SRB8 Material Grade Producer Test PeriodD6556NSAD6556STSASRB-8A N326 Continental March 2008 58 (0/1/0) 54 (1/2/0)SRB-8A2 N326 Continental March 2013 67 (1/2/0) 63 (1/2/0)SRB-8B N134 Cabot June 2009 65 (0/3/1) 62 (2/2/2)SRB-8B2 N134 Cabot March/April 2014 11 (1/1/0) 15 (1/1/0)SRB-8C HS Tread Columbian September 2010 63 (1/2/0) 58 (2/3/0)SRB-8D LS Carcass Cabot March 2009 64 (1/2/2) 60 (1/2/0)SRB-8E N660 Orion September 2008 54 (1/1/1) 51 (1/1/0)SRB-8F N683 Orion March 2010 67 (2/2/0) 64 (1/1/0)SRB-8F2 N683 Orion March 2015 30 (0/1/0) 44 (2/1/0)SRB-8GAN990 Cancarb Last half of 1996 N/A N/AASRB-8G was produced and approved in the last half of 1996 as SRB-5G and has continued to be included in the current SRB sets since that time. At the time it wasproduced and approved it was D24’s practice to only publish the within-laboratory standard deviation, Sr, and associated limits.The between-laboratory standard deviation,SR, was never published and since the data is no longer available it is not possible to calculate or publish the SR values and corresponding limits. The SRB G materialwas only tested for NSA, STSA, and OAN per the test method version available in 1996.D6556 − 174and reproducibility represent short-term (daily) testing condi-tions. The testing was performed using two operators in eachlaboratory performing the test once on each of two days (totalof four tests). A test result is the value obtained from a singledetermination. Acceptable difference values were not mea-sured. The between operator component of variation is in-cluded in the calculated values for r and R.12.3 Nitrogen Surface Area (NSA):12.3.1 A Type 1 interlaboratory precision program wasconducted. Both repeatability and reproducibility representshort-term (daily) testing conditions. The testing was per-formed using two operators in each laboratory performing thetest once on each material on each of two days (total of fourtests). The number of participating laboratories is listed inTable 2.12.3.2 The results of the precision calculations for this testare given in Table 3. The materials are arranged in ascending“mean level” order.12.3.3 Repeatability—The pooled relative repeatability, (r),of the NSAtest has been established as 1.1 %.Any other valuein Table 3 may be used as an estimate of repeatability, asappropriate. The difference between two single test results (ordeterminations) found on identical test material under therepeatability conditions prescribed for this test will exceed therepeatability on an average of not more than once in 20 casesin the normal and correct operation of