Type D durometer

The weight, thickness, and hardness of each coupon was measured before and after designated periods of exposure to determine the type and rate of physical and mechanical change. The hardness was measured with a Type D Durometer Instrument in accordance with ASTM D2240, and visual observations were made to identify changes in color or form. In addition, scanning electron micrographs of specific coupons were used to further identify the type and rate of surface attack. 

Figure 1 (a) Indcmer for T pc A duromcter. (h) Indenler for Type D durometer. 

The hardness, as determined with the type D Durometer, is a measure of resistance to indentation by a pointed indenter. 

Hardness - Hardness, as applied to rubber products, is the relative resistance of the surface to indentation under specified conditions. Hardness of rubber is usually measured with a small spring-loaded hardness gauge known as a durometer (ASTM D-2240). The durometer may be handheld or mounted on a stand. The measurement is made by pressing the indentor against the sample and reading the scale, which is calibrated in arbitrary units ranging from 0 (soft) to 100 (hard). A Type A durometer is used for most soft rubber products there is also a Type D durometer for hard rubber and plastic-like materials. On the A scale, a gum rubber band would measure around 40, a tire tread 60, and a shoe sole 80. 

There are several ways of measuring indentation, but they differ only in the type of equipment used. Basically, they all measure the size of indentation produced by a hardened steel or diamond tool under a defined pressure. A durometer is an instrument for measuring hardness by pressing a needlelike instrument into the specimen. Durometers are available in several scales for measuring relatively hard, brittle materials to soft elastomers. The two types appropriate for most cured adhesives and sealants are the Shore Type A and Shore Type D. ASTM C 661 offers a method for measuring indentation hardness of elastomeric-type sealants. 

The two most common types of durometers used for plastics are the Shore Type A and Shore Type D. They differ in the spring force and the geometry of the indentor, as shown in Figure 3.45. Due to creep, readings should be taken after a fixed time interval, often chosen as 10 sec. Typical hardness values of some of the common plastics measured by different test methods are shown in Table 3.4. 

Two types of durometers are most commonly used— Type A and Type D. The basic difference between the two types is the shape and dimension of the indentor. The hardness numbers derived from either scale are just numbers without any units. The Type A durometer is used with relatively soft material and Type D is used with slightly harder material. A commercially available durometer hardness measuring instrument is shown in Figure 2-62. 

Impact strength is usually only determined for plastics and hard elastomer compounds (i.e., those in the D durometer hardness range), where the impact resistance of the material in actual service is important. Soft elastomer compounds will bend, not break, on impact unless they have been cooled below their brittleness temperature. Several types of impact testers and test methods are available. Basically, they all measure the energy required to break a test specimen in a single sharp blow, and can be performed over a temperature range from -70 to +120°C (-94 to +248°F). The most widely used tests are Izod Impact and Charpy Impact both procedures are given in ASTM D-256. 

Durometer hardness testing determines the resistance of the sample to penetration by an indentor, which is a spring-loaded point initially protruding 1 mm from a flat surface. The profile of the indentor and the elastic constant of the spring to which it is attached define the type of durometer. In the case of polyethylene, durometer Type D is commonly used. When the flat surface is brought into firm contact with a block of polymer (minimum thickness 6.35 mm), the depth to which the indentor penetrates the specimen is inversely proportional to its hardness. After a fixed interval of time (commonly 10 secs), the hardness, in arbitrary units, is read directly from the scale. 

For many years there was no move to produce an international standard for durometers but one was eventually published in 1986. ISO 7619 is now in two parts44,45, separating a meter calibrated in IRHD from the others. Part 1 now covers the Shore A and D type meters, a meter designated AO for soft materials and a micro Shore type meter designated AM. The Shore A scale corresponds approximately to the IRHD scale and the D scale can conveniently be used for hard rubbers above about 90 Shore A. The AO meter is suitable for rubbers less than 20 Shore A, whilst the AM meter covers the normal Shore A range. As expected from its name, the meter in Part 2 of the standard covers the IRHD range. 

Durometers are also covered in ASTM D224049 which specifies no less than eleven scales, A, B, C, D, DO, E, M, O, OO, OOO and OOO-S plus R referring to a particular foot geometry. A, D, E and M correspond to the ISO types A, D, AO and M, a significant difference being that ASTM has not... 

The hardness measured with the small pocket type of hardness meter is associated with the durometer tests. Although there is not an international standard [1] that covers durometers, there is one for plastics, ISO R868 [10], which was used [9]. The two durometers specified are the Shore A and D which are also described in ASTM D2240 [11] where they are intended to cover both rubbers and plastics. The A scale corresponds approximately to the IRHD (International Rubber Hardness Degrees) scale and the D scale can be conveniently be used for hard rubbers above about 90 IRHD. 

Since most silicone adhesives and sealants are elastomeric in nature, their physical property testing often parallels classical rubber testing approaches. Common tests include durometer, tensile strength, elongation, and modulus. Several methods are available for the measurement of rubber properties, but the most commonly used are the American Society for Testing and Materials (ASTM) D-412, Test Method for Rubber Properties in Tension, and the ASTM C-661, Standard Test for Indentation Hardness of Elastomeric-Type Sealants by Means of a Durometer. These properties vary widely with the product... 

The resistance of rubber to deformation is known as hardness and is measured by an instrument known as a durometer, made by the Shore Instrument Company. There are several Shore instruments, namely, A, B, C, D, and O. Each is designed to give different readings from soft sponge up to ebonite-type materials. For elastomeric materials Shore A is the commonly used scale with 0 being soft and 100 hard. By comparison a rubber band would have a reading of approximately 35 and a rubber tire tread 70. 

Tear resistance was measured at 22°C using a trouser tear specimen described in ASTM D 470-82. The crosshead speed was 500 mm/min. The data reported are averages of results for six specimens. A hand-held Shore Durometer, hardness type "A-2" was used to determine the hardness of the samples, according to ASTM D 2240-75. Specimens were rectangular, 25 mm in length by 13 mm wide. Four plies of sample were used to achieve a thickness of 6.4 mm. Three readings were taken, 6 mm apart on the surface of the four plies. Hardness values measured on the blocks tended to be as much as five units lower. 

Shore Hardness - Indentation hardness of a material as determined by the depth of an indentation made with an indentor of the Shore-type durometer. The scale reading on this durometer is from 0, corresponding to 0.100" depth, to 100 for zero depth. The Shore A indenter has a sharp point, is spring-loaded to 822 gf, and is used for softer plastics. The Shore D indenter has a blunt point, is spring-loaded to 10 Ibf, and is used for harder plastics. Also c WsA. Shore A, Shore D.. 

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