A rubber durometer is a measure of the hardness of rubber. The typical scale for softer rubber is Shore A with a range of 0 to 100, with 0 being the softest and 100 being the hardest. Rubber durometer is a simple consideration for vibration dampening because people consider hardness as a measure of the ability of rubber to absorb shock.
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Rubber with a higher durometer will typically be less able to absorb shock than rubber with a lower durometer. The reason is that rubber with a higher durometer is more rigid. Therefore, selecting an appropriate durometer for the application is vital when choosing rubber for vibration dampening.
A better understanding of rubber-dampening properties can be gained by looking at the viscoelastic properties of a given compound. The different polymer types provide a better level of dampening based on the polymer structure and chemical chains. During dampening, there is a component of how much energy is stored in a polymer (elastic) and how much energy is dissipated as heat (viscous). Dynamic Mechanical Analysis (DMA) is a standard method for measuring this. A rubber compound undergoes oscillating stress while the resulting strain is recorded. The ratio of dynamic stress to dynamic strain is E* or complex modulus which can be resolved into the storage modulus (E&#;) and the loss modulus (E&#;). E&#; is the ability of a material to store energy and is related to the stiffness of the rubber. E&#; is the ability to dissipate heat due to the molecular motions.
Glass Transition (Tg) is vital to dampening rubber because rubber stiffens at cold temperatures and softens at higher temperatures. Thus changes in surrounding air temperatures can change the performance of a rubber dampener based on the polymer.
The following are some tips for choosing the correct rubber durometer for vibration dampening:
Consider the type of vibration that will be absorbed
Consider the frequency of the vibration
Consider the amplitude of the vibration
Consider the environment in which the rubber will be used
Load force for the displacement of the rubber
Once you have considered these factors, you can select the suitable rubber durometer for your application.
This paper is a simplified look at different rubber polymers and the change in durometers over a given temperature range.
Durometer at a Given Temperature
We tested six compounds that are typically used for dampening applications. All durometers are Shore A scale.
06SL7AP &#; 70 Silicone &#; Peroxide Cured
01VT7EE &#; 75 FKM ( 67% Fluorine) &#; Bisphenol cured
18EP7AP &#; 70 EPDM &#; Peroxide cured
27BN7AP &#; 70 NBR &#; Sulfur Cured
03BU7AP &#; 70 Butyl &#; Sulfur Cured
08SL6ML &#; 70 Liquid Silicone Rubber (LSR) &#; Platinum cured
Table 1: Gives the durometer reading instantaneous after 30 min exposure to temperature.
Durometer (Shore A) at Temperature &#;C Compound D 0 23 50 10006SL7AP
VMQ
69
68
66
63
01VT7EE
FKM
84
79
75
72
18EP7AP
EPDM
74
71
67
63
27BN7AP
NBR
74
70
65
64
03BU7AP
IIR
81
75
69
63
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08SL6ML
VMQ
60
60
57
58
Table 2: Durometer reading after exposure to 50°C, then allowed to be at room temperature, and durometer recorded at given minutes after exposure.
Durometer (Shore A) After 50&#;C exposure x Minutes Compound D 1 5 10 2006SL7AP
VMQ
67
66
66
67
01VT7EE
FKM
76
76
77
78
18EP7AP
EPDM
67
67
68
68
27BN7AP
NBR
66
66
67
68
03BU7AP
IIR
70
72
74
75
08SL6ML
VMQ
58
58
59
59
Conclusions
This simplified testing does show that the durometer changes in a given temperature range. FKM rubber will harden even at relatively lower temperatures, like 0°C. You can see the durometer starts to increase from 23°C down to 0°C. Butyl rubber typically had a good low-temperature, its performance showed the same. It was interesting to see the drop in durometer at elevated temperatures for NBR and EP compounds. Silicones have a broad operating temperature range which is shown. The LSR-type silicone does seem to perform the best, probably due to the better efficiency of the platinum cure than the peroxide cure.
Ref:
Ucar H, Basdogan I. Dynamic characterization and modeling of rubber shock absorbers: A comprehensive case study. Journal of Low-Frequency Noise, Vibration, and Active Control. ;37(3):509-518. doi:10./
V G Geethamma, R Asaletha, Nandakumar Kalarikkal and Sabu Thomas. Vibrations and Sound Damping in Polymers. Indian Academy of Sciences.
Rubber Durometer Change Over Temperature for Dampening
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