Reichelt Chemietechnik Magazine
Two Key Characteristics for Hoses and Hose Technology
Who doesn’t use them somewhere — hoses — the versatile helpers used in countless applications. They safely transport gases, liquids, and even flowable solids exactly where they’re needed. Hoses are used in many areas: fuel hoses deliver petrol from the tank to the engine, chemical hoses move hazardous substances securely from A to B without endangering people or the environment, and heating gas could hardly be supplied to a household storage tank without gas-tight hoses. Pressure-resistant hoses are essential for transmitting force in pneumatics and hydraulics, as well as in medical technology.
Just as varied as the applications are the different hose materials available. Most hoses today are made from elastomers. Depending on the application, hoses must meet a range of demands regarding their stability and resistance to different fluids. They may also need to be pressure-resistant, heat-resistant, and UV-resistant.
To evaluate the specific properties of hoses, characteristics such as elasticity, temperature-dependent flexibility, static and dynamic strength, hardness, dimensional stability, and deformability are crucial.
Plastics and Hardness
This hardness generally refers to the mechanical resistance of a material against the penetration of a harder object. In contrast, strength refers to resistance against deformation or fracture.
Both are material properties that describe the suitability and wear resistance of plastics and other materials. In testing, the two properties cannot be completely separated, as hardness measurements are always influenced by the material’s strength.
In hose manufacturing, material characteristics are significantly influenced by the choice of polymer, the degree of cross-linking, and the type of fillers used.
These factors allow the specific performance features of a polymer to be defined and optimized for different applications.
Why Hardness Testing Matters
Testing in plastics provides crucial information about deformation behaviour and mechanical durability, which in turn helps assess the quality of hose materials. It can also serve as an indicator of the sealing performance of an elastomer, much like how hardness is considered when selecting sealing rings.
Shore Hardness
The Shore hardness test is a simple and effective method for assessing materials and a straightforward way to measure the hardness of elastomers and soft plastics. In hose technology, this measurement is a useful indicator of material quality.
Combined with other material properties, such as density, hardness values can give good insight into a material’s characteristics. However, on their own, these measurements offer only limited information.
Technical purchasers and users alike can use the hardness scale as a practical guideline for evaluating whether a hose material is suitable for their specific needs. Such measurements can also indicate the degree of aging in a hose during service. As plastics age, their mechanical properties change — hardness testing delivers fast, easy-to-interpret results in such cases.
The Testing Method
In 1915, American engineer Albert Ferdinand Shore (1876–1936) defined the penetration depth of a test pin as a hardness metric for elastomers and rubber-like plastics and developed the first measurement method.
Measurement of hardness is performed using a device known as a durometer. This durometer uses a small, hardened steel pin (the indenter) pressed into the material with a defined spring force.
The indenter tapers to a conical point, so that the spring force is transferred directly through the tip onto the material. The penetration depth is measured on a scale from 0 to 100 Shore units, where 2.5 mm corresponds to 0 Shore and 0 mm corresponds to 100 Shore.
Shore Hardness A is used for testing soft elastomers, such as acrylonitrile-butadiene rubber (also known as Nitrile Butadiene Rubber, NBR). A test weight of 1 kg is applied to the material for 15 seconds. The indenter used for this test has a flat tip with a diameter of 0.79 mm and a tip angle of 35°.
Shore Hardness D is used for testing tough elastomers and thermoplastics, such as polyethylene (PE) or polyamides (PA). In this case, a test weight of 5 kg is applied for 15 seconds using a needle-shaped indenter. The indenter has a rounded, ball-like tip with a diameter of 0.2 mm and a tip angle of 30°.
How Shore Hardness is Measured
Concrete guidelines for performing Shore hardness measurements and calibrating testing equipment are defined in the ISO 7619-1 standard. Each measurement must be taken on a test specimen with a material thickness of at least 6 mm.
Additionally, each specimen must be measured at least five times, with each test point spaced at least 6 mm apart. To ensure comparable results when testing hoses, measurements must be taken on flat, parallel surfaces.
The measured hardness value can decrease by several points with increasing dwell time. For this reason, the test duration defined in the standard — 3 seconds for vulcanized rubbers and 15 seconds for thermoplastic elastomers (TPE) — must be observed precisely. The Shore hardness value is recorded according to ISO 7619-1 as follows: “65 SHORE A 3 s”. This indicates a measured Shore A hardness of 65 with a test duration of 3 seconds.
Since temperature also plays a significant role in the hardness testing of plastics, the specimen must be stored under testing conditions for at least 1 hour at a measurement temperature of 23 ± 2 °C before testing. Because hardness testing is performed on the surface of the material, the result is only partially representative of the overall material quality.
Additional standards such as ISO 868 (for harder plastics and hard rubber) and the American standard ASTM D2240-00 also exist, but they specify slightly different test conditions and ways of reporting results.
Comparing Shore A and Shore D
The Shore A method is used for testing soft polymers. For example, synthetic rubbers such as NBR typically have a Shore A hardness between 50 and 70. Harder polymers like polyethylene (PE) have a Shore A hardness of over 90. When measured using the Shore D method, PE shows a nominal hardness between 40 and 65, which is significantly lower than its Shore A value. Despite the different test instruments and procedures, there is a nonlinear correlation between Shore A and Shore D values, as described by K. Trobisch.1 The two scales can therefore be converted into one another. According to this relationship, 50 Shore A corresponds approximately to 10 Shore D, and 75 Shore A roughly to 20 Shore D.
1 K. Trobisch: "Über den Zusammenhang zwischen Shore A- und Shore D-Härte". In: Magazine "Kautschuk, Gummi, Kunststoffe" 34 , Nr. 5 (1989), p. 347-349
