Shore A Scale

Commercial products have hardnesses from 28 on the Shore A scale (which is very soft) to 45 on the Shore D scale (almost leathery). Specific gravities usually range from 0.9 to 1.20 some products intended for soundproofing have specific gravities as high as 1.95. Properties of representative grades are given in Table 9. 

Hardness measured on a Shore durometer. In addition to the commonly used Shore A scale there are several others O, OO, B, C and D, designed to open out either the soft or the hard end of the A scale. 

Figure 4.138 displays examples of compression sets after 22 h at 100°C, which are rather harsh conditions, for PP/NBR-V grades with hardnesses on the Shore A scale. Note ... 

These materials are based on a partially crosslinked, chlorinated olefin interpolymer alloy. MPRs are marketed in several series according to cost, performance and processability. Hardnesses are in the Shore A 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. 

Morgans et al61 carried out comprehensive trials to compare the IRHD and Shore micro tests concluding that the IRHD was better for very small test pieces but the Shore better for bent test pieces. They also note the fact that the Shore test is not totally non-destructive. This work was continued to include the normal IRHD and Shore A scales and to consider curved surfaces62. The results are a good illustration of the differences in measured hardness that can be found between different instruments with variation of test piece geometry. 

Substitute for Conventional Vulcanized Rubbers, For this application, the products are processed by techniques and equipment developed for conventional thermoplastics, ie, injection molding, extrusion, etc. The S—B—S and S—EB—S polymers are preferred (small amounts of S—EP—S are also used). To obtain a satisfactory balance of properties, they must be compounded with oils, fillers, or other polymers compounding reduces costs. Compounding ingredients and their effects on properties are given in Table 8. Oils with high aromatic content should be avoided because they plasticize the polystyrene domains. Polystyrene is often used as an ingredient in S—B—S-based compounds it makes the products harder and improves their processibility. In S—EB—S-based compounds, crystalline polyolefins such as polypropylene and polyethylene are preferred. Some work has been reported on blends of liquid polysiloxanes with S—EB—S block copolymers. The products are primarily intended for medical and pharmaceutical-type applications and hardnesses as low as 5 on the Shore A scale have been reported (53). 

Durometer hardness is measured on tests specimens that meet specific standards for shape and thickness. Durometer hardness is usually measured using the Shore A scale, which measures relative hardness on a scale of 0 to 100 units. Most rubber components for medical use are found in the 35-60 range with 40-50 typical for rubber vial stoppers. Durometer Hardness may be measured on some actual components if they have a sufficiently large flat surface and thickness, i.e., 28-32 mm IV stoppers. 

A general rule is that natural rubbers have better mechanical properties than the synthetic rubbers but the latter have better corrosion resistance. Natural rubbers are superior in certain applications such as with wet chlorine and hydrochloric acid. Natural rubber-based ebonite provides good resistance for such application at higher temperatures up to 90 °C. Corrosion resistance increases with increasing hardness, from a range of 60 on the Shore A scale to 80 on the Shore D scale. Higher proportions of sulfur increase the hardness range in the Shore D scale. 

Natural rubbers, such as pure gum with a durometer hardness of 40 on the Shore A scale, display surprising long life in sliding abrasion. Such linings are used in sand and slurry handling pumps and pipings where particles are in fine size and no grit is present. 

Where resistance to chipping and tearing is required, pure rubber must be compounded with carbon black to give it a tough composition and hardness of 60-65 on the Shore A scale. Such linings are used in chutes, hoppers, bunkers, dampers, etc. 

Since the flow causes sliding abrasion at a low angle of incidence in the piping, the rubber which can be suggested for this application is a low durometer hardness (40 on the Shore A scale) natural rubber. For lower flow rates at ranges of 1,200 litres per minute up to 4,000 litres per minute, a hard rubber or a semi-ebonite rubber can be used since the abrasive wear of the finely meshed particles will be negligible. 

Iron ore with a solid content of 25-30% in water is being handled in these feed launders. A feed launder is a mild steel, rubber lined chute through which ore slurry is conveyed to a concentrator tank. The velocity of the slurry is 0.66 m/s. As a result, 6 mm thick soft natural rubber compound of hardness 40 on the Shore A scale is well suited for this application. 

The materials handled in the bins are 20% solid by weight and have a flow of 2.9 tons of water per hour and 178 litres/minute of pulp. The size of the material is 100 to over 325 BS specification mesh. Maximum acid concentration is 20% hydrochloric acid. The temperature in the system is 60 °C. A 6 mm thick natural rubber lining of shore hardness 50 on the Shore A scale is used to protect the equipment against the abrasive environment as well as the corrosion effects of hydrochloric acid. 

For wet grinding copper ore in the ball mills, at ambient temperature, a 6 mm thick natural rubber compound of 45-50 on the Shore A scale is used successfully as an abrasion resistant layer. 

The Shore A scale, or as it is sometimes called, durometer hardness, enjoys considerable success in North America and in particular is widely accepted by the automotive industry the scale is an integral part of the classification system for elastomeric materials used in automotive applications (ASTM D2000). Shore hardness has also become the industry standard for the rapidly growing thermoplastic rubber sector, where advantage can be taken of the complementary Shore D scale for harder grades and for rubber-modified plastics. Seven Shore scales are described in ASTM D2240, while Shore A and D hardness are also detailed in the international standard for pocket hardness meters. ISO 7619 (BS903. Part A57). The latter also specifies a pocket meter based on the IRHD scale. 

Correlations are frequently sought between the IRHD and Shore A scales [3], There is approximate numerical equivalence for vulcanized natural rubber and competitive synthetic rubbers, but there can be significant differences for some elastomers. In some product and material specifications both scales are included as acceptable alternatives, but this practice is not widely pursued. 

FIGURE 1.18 Relations between shear modulus G and indentation hardness —, Shore A Scale —, International Rubber Hardness Scale (from Tobolsky and Mark, 1971). 

Figure 15.8 shows the evolution of the parameter of surface hardness type Shore A, depending on the concentration of nanoparticles for vulcanized NR and magnetic nanocomposites (NR/NZF). Dots represent the experimental data and the line represents the theoretical fit. Measurements were made using a typical laboratorial durometer in Shore A scale. The assay was performed in quintuplicate and measurement time equal to Is, in accordance with the international standard ISO 868 2003. The hardness type Shore A was determined from Equation (15.1) ... 

TPR n. Registered trade name of Uniroyal Inc. (Washington, DC) for a family of thermoplastic rubbers based mainly on ethylene and propylene. Grades range in hardness (Shore A scale) from 65 to 90. Processable by the usual thermoplastics methods, these materials have the properties of vulcanized rubber. 

Percentage of 4HB monomer (by NMR) Durometer hardness (shore A scale) Glass transition temperature, Tgi C)... 

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