Figure 2 (courtesy of DeFelsko)ĭigital durometers with separate (remote) probes (Figure 2) are becoming increasingly popular. While the temperature and humidity data is required to be reported by the ASTM standard, there is no correction of the hardness values based on the prevailing ambient conditions. Since temperature and humidity can influence the hardness value, the surface temperature of the coated surface and the relative humidity of the surrounding air should be measured and recorded prior to testing. Any inherent surface roughness may produce erroneous hardness values. The surface of the coating to be tested should be clean and smooth. This resistance to indention is displayed on the gage dial or digital display as a hardness value. Naturally, a cured, hardened coating will provide resistance to the indentor under the force of the applied load. The durometer contains a calibrated spring that is used to apply perpendicular force to the indentor. A small cone-shaped indentor protrudes from the pressor foot (the base of the tester). Figure 1Ī Shore D Durometer is a small hand-held device (Figure 1) that is used to measure the indention hardness of various materials like hard rubber, plastics, soft metals, and epoxies and other coating materials. According to the ASTM standard, Durometer hardness values less than 20 and greater than 90 are not considered reliable and it suggests not recording readings below 20 and above 90 for either the Shore A or Shore D instruments. For softer, thick film coating materials, a Shore A durometer may be more useful since it has a lower spring force. This article describes the use of a Shore D Durometer, since many of the thick film, chemically resistant coatings fall into the hardness range that a Shore D durometer can accurately measure. This standard covers twelve types of measurement devices (Types A, B, C, D, DO, O, OO, OOO, OOO-S and R). The minimum acceptable hardness value is often established by the manufacturer of the coating.ĭurometer hardness testing is performed according to the procedure described in ASTM D2240, Standard Test Method for Rubber Property – Durometer Hardness. The project specification should indicate the minimum acceptable hardness value prior to placing the coating system into service. Note that there is no correlation between the hardness testing methods described herein and there is no pass/fail criterion indicated in the respective ASTM standard test methods. The hardness of thick film coatings (the method indicates “thick film” is considered a minimum of 6 mm or 240 mils) is typically measured using an indentor-type tester that measures the resistance to indentation under a specific spring force load, while thin film coatings (a reference to thickness is not included the method) are frequently assessed for the degree of hardness using the pencil hardness test. The selection of the test method is frequently dictated by the thickness of the coating materials, although variations in the thicknesses tested are permitted if useful information can be obtained. There are two common test methods described in this article: the indentor-type tester (Durometer) and pencil hardness, which is more of a sheer hardness test. An uncured coating that remains soft can sustain damage while in service for example, backfilling a ditch too quickly and damaging a newly-applied pipe coating that has not achieved its full cure, which can adversely impact pipeline integrity When comparing hardness values listed on a coating manufacturer’s product data sheet, the information is frequently considered an indication of its degree of cure and its inherent performance characteristics. The balance of hardness with other final film properties is determined by the end use of the product. The absolute hardness of a coating is not always the ultimate goal of the formulation, and an increase in hardness can be accompanied by brittleness or a decrease in the flexibility of the coating. While hardness testing is frequently used to assess the degree of cure, they can also be used to measure hardness properties formulated into coatings. The hardness of a coating material is a relative property that may be interpreted in a variety of ways by different industries that use coating/lining materials.
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