ISO 48 Rubber, Vulcanized or Thermoplastic - Determination of Hardness

The “ISO 48 Rubber, vulcanized or thermoplastic – Determination of hardness” standard, published by the International Organization for Standardization (ISO), consists of a number of sub-sections published under the same title:

• The standard “ISO 48-1 Part 1: Introduction and guidance” describes a procedure for determining the hardness of vulcanized and thermoplastic rubbers. It is intended to provide an understanding of the importance of hardness as a material property and to assist in selecting an appropriate test method.

For the purposes of this standard, the following terms and definitions apply:

• International rubber hardness ratings (IRHD): refers to the hardness scale chosen so that “0” represents the hardness of a material with a zero Young’s modulus and “100” represents the hardness of a material with an infinite Young’s modulus. The following conditions are met over most of the normal hardness range:
  • An international rubber hardness grade always represents approximately the same proportional difference in Young's modulus.
  • For highly elastic rubbers, the IRHD and Shore A scales are comparable.
• Standard hardness refers to the hardness obtained using the procedures described in ISO 48-2 on test pieces of standard thickness and not less than the specified minimum lateral dimensions, in international rubber hardness grades.
• Apparent hardness refers to the hardness obtained in international rubber hardness grades using the procedures described in the ISO 48-2 standard on test pieces of non-standard dimensions.

When applied to rubbers, the term hardness refers to a measure of hardness obtained from an indentation test. An indenter is pressed into the rubber under a specified force and the resulting indentation is measured. Unlike some methods for other materials, indentation is measured by the applied load. In most tests, the indenter is surrounded by a foot resting on the test piece under a specified force. In dead load tests, the indentation measured is the difference between the indentation caused by a small initial force and the indentation caused by a larger final force.

• The standard “ISO 48-2 Part 2: Hardness between 10 IRHD and 100 IRHD” describes four methods for determining the hardness of vulcanized or thermoplastic rubbers on flat surfaces (standard hardness methods) and four methods for determining the apparent hardness of curved surfaces (apparent hardness methods).

Hardness is expressed in international rubber hardness degrees (IRHD). The methods cover the hardness range from 10 IRHD to 100 IRHD. These methods differ primarily in the diameter of the indented ball and the magnitude of the indentation force, which are selected to suit the particular application. The range of applicability of each method is shown in the figure given in the standard.

This standard specifies four methods for determining standard hardness:

• Method N (normal test): Suitable for rubbers with hardness in the range of 35 IRHD to 85 IRHD, but can also be used for hardness in the range of 30 IRHD to 95 IRHD.
• Method H (high hardness test): Suitable for rubbers with hardness in the range of 85 IRHD to 100 IRHD.
• Method L (low hardness test): Suitable for rubbers with hardness in the range of 10 IRHD to 35 IRHD.
• Method M (microtesting): Essentially a scaled-up version of the regular test method N, allowing thinner and smaller test pieces to be tested. Suitable for rubbers with hardnesses in the range of 35 IRHD to 85 IRHD, but can also be used for rubbers with hardnesses in the range of 30 IRHD to 95 IRHD.

This standard also describes four methods for determining the apparent hardness of inclined surfaces, namely CN, CH, CL and CM. These methods are modifications of methods N, H, L and M respectively and are used when the rubber surface being tested is inclined. In this case, there are two possibilities:

• The test piece or product being tested is large enough to rest on the hardness tester
• The test piece or product is small enough that both the test piece and the instrument rest on a common support.

In principle, the hardness test consists of measuring the difference between the depths of indentation into the rubber by a ball under a small contact force and a large total force. From this difference, the hardness is obtained in terms of very low rubber hardness degrees (VLRH), which are read directly as very low rubber hardness degrees, calculated from the tables and inserted into the indentation measuring device, using the table given in the standard or graphs or a scale based on this table.

• The standard “ISO 48-4 Part 4: Indentation hardness by the durometer method (Shore hardness)” describes a method for determining the indentation hardness (Shore hardness) of vulcanized or thermoplastic rubber using hardness meters having the following scales:
  • A scale for rubbers in the normal hardness range
  • D scale for rubbers in the high hardness range
  • AO scale for low hardness rubbers and cellular rubbers
  • AM scale for thin rubber test pieces in the normal hardness range.

In principle, with this test, an indentation of specified dimensions is pressed into a test piece under a specified load and the depth of the indentation is measured. This indentation is converted into a hardness value by means of a specified relationship.

• The standard “ISO 48-5 Part 5: Indentation hardness by IRHD pocket measurement method” describes a method for determining the indentation hardness of vulcanized or thermoplastic rubber by means of a pocket hardness meter calibrated in IRHD. In principle, this standard specifies a method for determining the indentation hardness of vulcanized or thermoplastic rubber by means of a pocket hardness meter calibrated in IRHD. The use of such meters is primarily for control purposes and not for specification purposes. It is possible to increase the accuracy by fixing the pocket hardness meter to a support.
• The standard “ISO 48-6 Part 6: Apparent hardness of rubber-lined cylinders by the IRHD method” describes a method for determining the apparent hardness of vulcanized or thermoplastic rubber cylinder covers, expressed in international rubber hardness degrees (IRHD). This method is similar in principle to the methods used for determining rubber hardness in ISO 48-2. It essentially consists of measuring the penetration depth of a spherical indenter operating under a specified force. The apparatus used is similar to that described in ISO 48-2 and has a base specifically designed for application to cylinder covers and similar curved surfaces. In some cylinders, there may be significant variations in the thickness of the rubber on the cylinder surface, which may affect the measured apparent hardness.
• The standard "ISO 48-7 Part 7: Apparent hardness of rubber-lined cylinders by the Shore-type hardness meter method" describes a method for determining the apparent hardness of vulcanized or thermoplastic rubber cylinder covers, expressed as Shore hardness, for measurements where only moderate accuracy is required. The method and apparatus used are essentially those described in ISO 48-4, in which case the measurements are made on the inclined surface of the conditioned rubber-lined cylinder instead of on a flat test piece. Shore type A and type D instruments are specified, the latter being used for measurements on cylinders with high hardness. In some cylinders there may be a significant variation in the thickness of the rubber on the cylinder surface, which may affect the measured apparent hardness.
• The standard “ISO 48-8 Part 8: Apparent hardness of rubber-coated cylinders by the Pusey and Jones method” describes a method for determining the apparent hardness of vulcanized or thermoplastic rubber cylinder covers, expressed as the Pusey and Jones indentation value. The Pusey and Jones plastometer is used to measure the depth of indentation made by an indenter into the surface of the rubber under a given force. The indentation value is not the same as the value measured by the International Rubber Hardness Test Method ISO 48-2, because in this method the rubber immediately adjacent to the indentation is already compressed. The Pusey and Jones indentation value is an inverse measure of hardness, i.e. the harder the rubber, the lower the Pusey and Jones indentation value.
• The standard “ISO 48-9 Part 9: Calibration and verification of hardness testing instruments” describes the procedures for the calibration and verification of A, D, AO and AM type hardness testers, IRHD pocket hardness testers, IRHD dead load instruments and dead load instruments using the very low rubber hardness scale.
• The standard “ISO 48-10 Determination of hardness” is still under development. This standard will replace the standard “ISO 3387 Rubber - Determination of crystallization effects by hardness measurements”.

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