ASTM D4678-11

Reference materials are vitally important in product and specification testing, in research and development work, in technical service work, and in quality control operations in the rubber and carbon black industries. They are especially valuable for referee purposes.

Categories, Classes, and Types of Reference Materials (RM):

Reference materials are divided into two categories:

Industry Reference Materials (IRM)—Materials that have been prepared according to a specified production process to generate a uniform lot; the parameters that define the quality of the lot are evaluated by a specified measurement program.

Common-Source Reference Materials (CRM)—Materials that have been prepared to be as uniform as possible but do not have established property (parameter) values; the knowledge of a common or single source is sufficient for certain less critical applications.

Industry reference materials (IRMs) are divided into additional classes and types according to the method of evaluating the lot parameters and according to the production process for generating the lot material. These are explained more fully (refer to Annex A3 and Annex A4 for more details on the discussion in Section 3).

The following lot parameters are important for reference material use:

Accepted Reference Value (AR Value)—An average IRM property or parameter value established by way of a specified test program.

Test Lot Limits (TL Limits)—These are limits defined as ±3 times the standard deviation of individual IRM test results across the entire lot for the property or parameter(s) that defines lot quality; the measurements are conducted in the laboratory of the organization producing the IRM.

Although the limits as defined in 3.2.3.2 are given in terms of ±3 times the standard deviation, the rejection of individual portions of the lot as being outlier or non-typical portions in assessing the homogeneity of the lot is done on the basis of ±2 times the appropriate standard deviation, that is, on the basis of a 95 % confidence interval. See Annex A3 and Annex A4 for more information and the evaluation procedures.

All IRMs have an AR value and TL limits; however the AR value may be obtained in one of two ways to produce one of two classes of AR values:

Global AR Value—This AR value is obtained from an interlaboratory test program where the word “global” indicates an average value across many laboratories.

Local AR Value—This is an AR value obtained in one laboratory or at one location, usually the laboratory responsible for preparation of the homogeneous lot.

An additional parameter is of importance for IRMs that have a global AR value:

Between-Laboratory Limits (BL)—The group of laboratories that conduct interlaboratory testing to establish an AR-value are not equivalent to a system or population typical of industrial production operations that use the usual ±3 standard deviation limits. Such production operations are systems that have been purged of all assignable causes of variation and are in a state of ‘statistical control’ with only random variations that cannot be removed. Thus, the recommended limits on all IRMs are the ±2 standard deviation limits that pertain to a 95 % confidence level. If for serious reasons that can be totally justified, ±3 standard deviation limits are required, these may be used provided that full and complete documentation is supplied to justify the limits.

The homogeneity or uniformity of the lot, which determines the magnitude of the TL limits, may be designated as one of two different levels of uniformity. The key factor that determines the level of uniformity is the capability of blending the IRM portions or parts that constitute the lot, to ensure a high degree of uniformity from the blending process. IRMs that cannot be blended will have an extra residual amount of variation (portion to portion) that lowers the level of uniformity.

Uniformity Level 1 (UL-1)—This is the most uniform or highest level of homogeneity that can be attained by the use of a specified test for measuring the parameter that defines lot quality; it is obtained by the use of a blended material and is referred to as a Type B (B = blended) IRM.

Uniformity Level 2 (UL-2)—This is the lesser degree of uniformity that is attained by the use ofa specified test for measuring the parameter that defines lot quality; it is normally obtained for non-blended materials and is referred to as a Type NB (not blended) IRM.

IRMs have a number of use applications in the technical areas, as cited in 3.1.

Single Laboratory Self Evaluation—The IRM may be used in a given laboratory (or with a given test system) to compare the test results within the laboratory to the accepted reference value for the IRM. An IRM can also be used for internal statistical quality control (SQC) operations.

Multi-Laboratory Evaluation—The IRM may be used between two or more laboratories to determine if the test systems in the laboratories are operating within selected control limits.

One or more IRMs may be used in the preparation of compounds to be used for evaluating non-reference materials in compound testing and performance.

Reference liquid IRMs may be used for immersion testing of various candidate or other reference compounds. Such immersion testing is important due to the deleterious influences of immersion liquids on rubber compounds.

IRMs may also be used to eliminate interlaboratory testing variation known as “test bias:” a difference between two (or more) laboratories that is essentially constant between the laboratories for a given test property level, irrespective of the time of the test comparisons. In such applications a differential test measurement value, (IRM − experimental material), becomes a corrected test result; this corrected value is used as the measure of performance rather than the “as-measured” test value on the experimental material of interest.

Average values play an important role in various operations and decisions in this practice. For this practice, “average” is defined as the arithmetic mean.

The various characteristics of IRMs and CRMs (categories, classes, types) are listed in summary form in Table 1.

This practice and the IRM program it describes is being developed to replace a standardization program conducted by the National Institute of Standards and Technology (NIST) that was begun in 1948 and is being phased out. The standard materials developed by the NIST program are referred to as Standard Reference Materials or SRM.

To provide for some continuity in the “phase-outphase-in” operation, Appendix X3 lists the rubber and compounding materials still carried by NIST on an interim basis. Appendix X3 will be retained in this practice until such time as NIST materials are no longer used.

It is not feasible to write into this practice all the necessary specifications, modes of preparation, sampling, and testing protocols, for the wide variety of materials that will eventually become IRM. Therefore this practice is published to give general guidelines for IRMs.

A permanent IRM Steering Committee within Subcommittee D11.20 shall be constituted by Subcommittee D11.90 in consultation with Committee D24 to assist in the utilization of this practice and to make technical and, where required, policy decisions regarding the preparation and administration of IRM. The IRM Steering Committee shall have members of both Committees D11 and D24.

TABLE 1 Categories of Reference Materials^A

^A AR value = accepted reference value.

TL limits = test lot limits.

Global = AR value obtained from an interlaboratory test program.

Local = AR value obtained from one laboratory.

Type-B = IRM that has been blended to ensure high uniformity.

Type-NB = IRM that cannot be blended.

UL-1 and UL-2 = levels of uniformity in the IRM lot; UL-1 is higher uniformity than UL-2.

See Annex A3 and Annex A4 for more information.