1. Understanding the Basic Structure

An ASTM D2000 specification is a sequence of letters and numbers that define the type, class, and various physical properties of the material. The code generally follows this standardized format:

M 1 BC 407 B14 Z1 Z2

M (Optional Prefix)

The letter M is an optional prefix that indicates that the properties are given in SI (metric) units. If the “M” is absent, the properties are specified in English (pound-inch) units.

Title       units

M           Metrics (MPa, ∘C, mm3)

No          Imperial (psi, ∘F, in3)

2. Define Type and Class

The first three characters after the optional prefix “M” define the Type and Class of the O-ring material, which relate to its resistance to thermal aging and swelling in oil.

First Digit: Grade Number

The first digit, known as the Grade Number (often 1), specifies the level of testing requirements. Grade 1 means that only the basic requirements of the standard need to be met. Higher digits (e.g., 2, 3, 4, etc.) indicate that additional, specific tests—often listed later in the code—are required, usually for more demanding applications.

First Letter: Type

The first letter denotes the Elastomer Type, which is based on the material’s resistance to thermal aging. This is determined by the maximum temperature the elastomer can withstand while retaining at least 50% of its initial tensile strength and elongation after 70 hours of aging in an air oven.

Letter (Type)       Test Temperature (Limit)               Common Polymers (Examples)

A             70∘C (158∘F)       Natural Rubber (NR)

B             100∘C (212∘F)    SBR, Nitrile Rubber (NBR)

C             125∘C (257∘F)    EPDM, Chloroprene (CR)

D            150∘C (302∘F)    Special NBR, Fluoroelastomer (FKM)

E             175∘C (347∘F)    Silicone (VMQ)

F             200∘C (392∘F)    Silicone (VMQ), FKM

G            225∘C (437∘F)    FK extension

H            250∘C (482∘F)    FK extension

Second Letter: Class

The second letter indicates the Class of the elastomer, which is defined by its resistance to swelling in oil. This is measured by the maximum percentage change in volume after being immersed in the reference oil ASTM No. 3 for 70 hours at the temperature specified by the Type.

Letter (Class)      Max Volume Swelling (%)

A             No requirements

B             140

C             120

D            100

E             80

F             60

G            40

H            30

J              20

K             10

3. Define Hardness and Tensile Strength

The next three numbers specify two fundamental mechanical properties of rubber.

First Number: Durometer Hardness (4)

This single number represents the nominal hardness of the toric cord, measured on the Shore A scale. To obtain the value in degrees Shore A, simply multiply this number by 10.

Figure    Nominal Hardness (Shore A)

3             30 ±5

4             40 ±5

5             50 ±5

6             60 ±5

7             70 ±5

8             80 ±5

Last Two Digits: Minimum Tensile Strength

The last two digits (here, 07) indicate the minimum tensile strength of the material in MPa (Megapascals) or in psi (pounds per square inch), depending on the presence of the prefix ‘M’.

If ‘M’ is present (Metric Units): The value is in MPa. (Ex: 07 MPa equals 7 MPa)

If ‘M’ is absent (English Units): The value is in psi×100. (Ex: 07 ×100 psi equals 700 psi)

Analysis of example M 1 BC 407:

The material is Type B (Max 100∘C) and Class C (Max 120% swelling), with a hardness of 40 Shore A (±5) and a minimum tensile strength of 7 MPa.

4. Specific Property Requirements (eg B14, EA14, EF11, EO31, etc)

After the basic properties (Type, Class, Hardness, Tensile), the specification may include codes for additional test requirements. These codes consist of a letter and two numbers.

Letter: Physical Property Tested

The letter indicates the nature of the physical property test required.

Letters   Property Tested

A             Aging (Tensile Strength/Elongation)

B             Permanent compression set

C             Ozone Resistance

D            Dynamic flexibility

E             Fluid Resistance (EH: Oils, EC: Fuels, EA: Water)

F             Low temperature resistance

G            Tear resistance

H            Resistance to flexion

K             Abrasion resistance

Z             Special requirements (see Section 5)

Two Figures: Conditions and Limits of the Test

The two numbers following the letter specify the exact conditions under which the test is to be performed (temperature, time, medium) and the maximum acceptable result. ASTM D2000 provides detailed tables for decoding these number pairs.

Analysis of example B14:

B means Compression Set test.

14 is a specific code which, according to the standard, indicates the test conditions (for example, 22 hours to 100∘C) and the maximum value of permanent deformation permitted.

5. Special Requirements (Z1 Z2)

Any requirement not covered by the standard codes (A to K) is designated by one or more codes beginning with the letter Z.

Z1, Z2, Z3, etc.: Each ‘Z’ code indicates a special requirement which must be described in detail in a table or note accompanying the O-ring specification.

Example: A code could be M 1 BC 407 B14 Z1

Z1 could mean: “Ozone Resistance: 100 hours without cracking at 50 pphm à 40∘C”

The Z code allows the buyer and supplier to agree on specific properties that are critical to the end application, but do not fit within the ASTM D2000 test categories.