|John Bailey has been the Quality Systems Manager at Continental Carbon since 1991. A member of D24 since 1988, he has been chairman of Subcommittee D24.61 on Sampling and Statistics for more than 10 years. He has also held other chairman and secretary positions in D24.
Jeffery Melsom is lab manager at Michelin’s Sandy Springs, S.C., plant. He is active on ASTM Committees D11 on Rubber and D24 on Carbon Black and serves on the ASTM board of directors. He has held officer positions in three D24 subcommittees over the years and is the editor of D24’s biannual newsletter, The Carbon Aggregate.
Laboratory Proficiency Rating System for Carbon Black
As a part of developing any ASTM test method, there is a need to determine a precision and bias statement. ASTM International has mandated in its standards style manual, Form and Style for ASTM Standards, that each standard developed contain a P&B statement. In many instances the development of the P&B statement is delayed when the committee tries to bring together all the means to develop the statements. This delay can be as short as the time between committee meetings or as long as years, depending on the committee and the use of the standard. Primarily at issue is the availability of a viable and homogeneous reference material, distribution of the reference samples, a sufficient number of participating laboratories, timing when a laboratory may have resources available to conduct the testing, and costs to manage and process the round robin data.
Pulling all of this together can be challenging, but technical Committee D24 on Carbon Black has accomplished this by instituting its own Laboratory Proficiency Rating System.
ASTM International Committee D24 faces challenges similar to those of many other technical committees when it comes to improving test method proficiency and preparing mandatory precision and bias statements. The members of D24 (producers, users, and instrument manufacturers) face similar challenges in maintaining testing proficiency in their laboratories and knowing how well their laboratories perform compared to others in conducting those tests.
Many carbon black producers have had in-house programs to measure testing proficiency within their laboratories. Some of these programs included a few customers because most customers did not have the resources to conduct a program on their own. Regardless of how well-run these in-house programs may have been, they still reached only a limited number of laboratories and provided little, if any, insight on how any one program compared to all the similar programs.
The time and effort to organize and participate in an interlaboratory testing program (ITP), commonly referred to as a “round robin,” discouraged many potential participants. Most saw the ITP as a lot of work with little tangible benefit to the participants. Usually, fewer than 10 laboratories would agree to participate in the ITP.
Another challenge faced by D24 was the fact that there were no absolute standard values for carbon black test method results for use in calibrating test equipment. Over the years the committee has developed standard reference blacks (SRBs) and industry reference blacks (IRBs) to be used to validate testing accuracy. Consensus target values and acceptable test limits are determined for each SRB or IRB through an interlaboratory testing program. The typical useful life of SRB carbon blacks is about five years before they are depleted, making it necessary to organize an ITP every five years. It commonly takes two years to select a candidate material, and produce, test, and validate it through the ITP. Some carbon black properties, most notably iodine number (tested to ASTM D 1510, Test Method for Carbon BlackIodine Adsorption Number), are known to change with time due to aging. This makes it desirable to validate the target and acceptable limits during the life of the SRB. Of course, such a validation requires organizing another ITP.
The concept of D24 organizing and running a standing ITP for testing proficiency was discussed between 1994 and 1995. A number of conditions that the ITP would need to satisfy were outlined. A task group was formed to study the proposal and report at the next meeting. A proposal was presented by John Bailey, of Continental Carbon Company, chairman of Subcommittee D24.61 on Statistics, on how D24 could organize a standing ITP to address the challenges of improving testing proficiency, preparing precision and bias statements, validating SRB and IRB materials, and benefiting the participating laboratories by giving them feedback on their testing proficiency. Continental Carbon offered to champion the program and organize the ITP under the jurisdiction of Subcommittee D24.61 on Sampling and Statistics, provide and distribute the initial material, and prepare the reports to go back to the participants. With the exception of providing and distributing the materials, Continental Carbon has continued to provide this support to the industry. D24 members were willing to proceed with a limited ITP. The program would become known as the Laboratory Proficiency Rating System (LPRS).
The first ITP was conducted in September 1995 using an N234 carbon black. Samples of approximately 2 kg were prepared and packaged by Continental Carbon, shipped to over 45 individual labs and afterward all the results returned to Continental Carbon for data processing, statistical analysis, and reporting. During this initial round, data was collected for six tests: iodine number (D 1510), nitrogen surface area (NSA) (D 4820, Test Methods for Carbon Black-Surface Area by Multipoint B.E.T. Nitrogen Adsorption (withdrawn, replaced by D 6556)), CTAB surface area (D 3765, Test Method for Carbon BlackCTAB (Cetyltrimethylammonium Bromide) Surface Area), n-dibutyl phthalate (D 2414, Test Method for Carbon BlackOil Absorption Number (OAN)), CDBP (D 3493, Test Method for Carbon BlackOil Absorption Number of Compressed Sample (COAN)), and tint (D 3265, Test Method for Carbon BlackTint Strength). Thirty-six laboratories returned data in the first ITP. The first data file was designed to handle seven tests and 50 participating laboratories.
The results of this first LPRS were so well received that the program more than doubled in size through the addition of seven more tests and expanding the available laboratory slots to 75. A second LPRS was conducted in March 1996 (using N650), adding tests for ash (D 1506, Test Methods for Carbon Black-Ash Content), sieve residue (D 1514, Test Method for Carbon BlackSieve Residue), toluene discoloration (D 1618, Test Method for Carbon Black ExtractablesTransmittance of Toluene Extract), sulfur content (D 1619, Test Methods for Carbon BlackSulfur Content), pellet hardness maximum and average (D 3313, Test Method for Carbon BlackIndividual Pellet Hardness, or D 5230, Test Method for Carbon BlackAutomated Individual Pellet Hardness), and statistical thickness surface area (D 5816, Test Methods for Carbon Black-External Surface Area by Multipoint Nitrogen Adsorption (Withdrawn 2000, replaced by D 6556). Fifty-seven laboratories participated in this LPRS.
To date, 17 LPRS test rounds have been conducted covering a wide range of materials. The continued success of this program is a testament to the willingness of D24 members to put aside competitive differences and work together to improve testing in the carbon black industry.
Due to a declining use of the test, the CTAB surface area test has been dropped from the program. The other 12 tests have become base tests and data has been collected for them on all the test rounds. The former CTAB slot and the one vacant slot have been dedicated to collecting data on a variety of tests for the purpose of preparing precision and bias statements. LPRS data has been used to prepare new precision and bias statements for 12 base test methods and mass strength (D 1937, Test Method for Carbon Black, Pelleted Mass Strength) and pH (D 1512, Test Methods for Carbon Black pH Value). Data is currently being collected for fines and attrition (D 1508, Test Method for Carbon Black, Pelleted Fines and Attrition).
In addition to preparing precision and bias statements, LPRS data has been used to validate the uniformity of certain SRB and IRB materials, publish new target and acceptable range values for the SRBs, document the equivalency of results for paraffin oil and n-dibutyl phthalate use in D 2414 when testing ASTM designated carbon blacks, and investigate SRB candidate materials.
Prior to LPRS, the laboratories participating in an ITP were usually from North America, if not exclusively in the United States. To date, laboratories from 23 countries, 21 of which are not in North America, have participated in LPRS. Every continent and region of the world is represented, making this a truly international program. In some cases the precision and bias data coming from LPRS shows wider limits than were previously published. Due to the much larger number of laboratories participating and the international representation of those laboratories, it is generally felt that the precision and bias numbers more accurately reflect the state of testing proficiency in the carbon black industry.
For the future, a proposal was presented at the June 2005 D24 meeting to add a new report for those companies that have multiple laboratories participating. The report would better help them identify those laboratories and areas where testing improvement is needed. A test round has not been conducted since this proposal, so there has not been an opportunity to see how useful this report may be, but the concept was well received by D24 members.
During the last LPRS (March 2005), 55 laboratories participated from 15 companies representing 18 countries. Today samples are provided either by one of the carbon black manufacturers in North America (and shipped to Balentine Enterprises for repackaging and distribution) or samples are selected from the ASTM-approved inventory of SRBs or IRBs.
The longevity of the LPRS program and the various uses of the data demonstrates its value and that it has met or exceeded the expectations of the original proposal. The fact that it is conducted entirely within the technical committee makes it a unique program and a source of pride and satisfaction for D24’s members. //