HARDNESS SCALES USED FOR URETHANES
The hardness of urethanes can easily be measured with an inexpensive instrument called a durometer. However, hardness is not a good indicator of performance. It cannot be relied upon as a specification by itself. First, let's discuss the commonly used hardness scales. We will then proceed to show you that hardness is not a sufficient indicator of quality or performance.
WHY HARDNESS MEASUREMENT ALONE IS A POOR QUALITY INDICATOR:
The principle urethane systems we work with day-to-day are derived from eight basic chemical structures. They are:
TDI ETHER/POLYOL
TDI ESTER/POLYOL
TDI ESTER/DIAMINE
PPDI ESTER/POLYOL
TDI ETHER/DIAMINE
MDI ETHER/POLYOL
MDI ESTER/POLYOL
PPDI ETHER/POLYOL
COMPOUNDS OF THE SAME HARDNESS ARE NOT THE SAME
The urethane systems above can make compounds in the same hardness range, but with drastically different physical and engineering properties. That's why we stress that hardness alone is not an adequate specification for any rubber, but particularly for urethanes.
Observe the difference between a TDI Ether System and an MDI Ester at the same hardness:
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BOTH MATERIALS ARE THE SAME HARDNESS, Durometer A95
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TDI ETHER (Material A)
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MDI ESTER (Material B)
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100% Modulus, psi (Mpa)
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2170 (15.0)
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1900 (13.1)
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200% Modulus, psi
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3130 (21.6)
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2220 (15.3)
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300% Modulus, psi
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5230 (36.0)
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2960 (20.4)
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Tensile Strength, psi
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5850 (40.0)
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7220 (50.0)
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Elongation Break, %
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320
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450
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Die C Tear Strength, pli (kN/m)
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500 (87.0)
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780 (136.0)
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D-470 Split Tear Strength, pli (kN/m)
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150 (26.0)
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176 (31.0)
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It's obvious from the data that the two elastomers are not the same, even though the hardness is the same. We have ways to identify (with spectrographic and chemical analysis) the structure of elastomers at the same hardness, but the process is costly and time consuming. Actually, they are both excellent materials but with widely different engineering properties. One will perform better in some applications and the other will perform well also, but in different applications.
The data in the above table was produced in laboratory tensile tests. For most industrial applications, we need to know how they behave in dynamic situations and how they will endure the application's environment. Very few urethane applications would require a part to be strained in tension 100%. Most applications operate in a tensile strain range of 10-30% or less. The table (above) shows only the first strain cycle. The cycled stress-strain test (below) shows a big difference between the two. After 10-20 strain cycles, equilibrium is achieved and performance is reproducible. The different behavior tells us that an application as a shock mount or die spring would benefit from the consistent spring rate and ability to handle reasonable loads of the TDI-Ether (Material A).
On the other hand, a chain sprocket with relatively low loads might benefit from the more forgiving flexibility of the MDI Ester (Material B) if there was a mismatch of mating surfaces.
NEXT: What's Important To Specify?
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Hardness Scales. Note: the durometer "A" scale is used for the softer urethanes. The durometer "D" scale is used for the harder urethane compounds (above 95 A durometer).
Stress-strain cycling of two different 95 A durometer urethanes.
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