CASE STUDIES: Acoustic Vibrations

VISCOELASTIC CHARACTERIZATION OF THERMOSET AND THERMOPLASTIC RUBBERS
PELabs was retained by a leading developer of telecommunications systems to characterize the viscoelastic properties of three elastomers of similar Shore A hardnesses. The materials were being considered for an acoustic vibration-isolator for a small electronic device. Originally the designers specified a thermoplastic rubber (TPR), but the offshore molder couldn't mold the part on a production basis. He suggested a thermoset rubber of the same hardness and provided samples of the isolator in EPDM and silicone rubbers. The engineer who had worked through the vibration analysis expressed concern that the viscoelastic properties might differ, even though the hardness values were close.

Obtaining the needed data turned out to be a prime example of the important role experience plays when dealing with sophisticated software. The raw data were generated on an instrument supplied with a data-analysis software package. Blindly passing the data through the software analysis generated physically unreasonable results… the software's algorithm could not handle this particular set of data. In response, PELabs engineers developed a straight-forward methodology that allowed these data to be successfully analyzed using an ordinary spreadsheet program. The result was excellent sets of viscoelastic master curves that could be used by the PELabs customer in their acoustic vibration analysis.

When the data analysis software doesn't produce acceptable viscoelastic master curves, it is easy for one to throw up his hands and declare defeat. It is important to remember that the analysis, called time-temperature superposition, rests on a sound experimental and theoretical foundation. Sometimes it is necessary to be creative in analyzing a particular set of data. PELabs engineers have successfully developed master curves for over 600 plastic materials during the past 30 years.

In the case at hand, the raw data was imported into a simple spreadsheet and manually analyzed. Since PELabs engineers know that the ratio of the viscous modulus, E", to the elastic modulus, E', shifts horizontally regardless of the need for a vertical shift in E' and E", these data were shifted first. The ratio, Tan δ, was plotted in the spreadsheet and shifted to form a Tan δ master curve.

With the horizontal shift values in hand, the elastic modulus, E', was shifted. The results of this treatment is shown in Figure 1. Clearly, having the Figure 1 plot provides the remainder of the analysis. The vertical distance between each constant temperature segment are the incremental values of the required vertical shift for an E' master curve. The resulting master curves for all three elastomers are shown in Figure 2, from which, given the shift factors a_T and b_T, E' can be calculated at any time and temperature.

Figure 1: Elastic Modulus, E', data shifted horizontally using the shift values from Tan δ shifting

Figure 2: E' Master curves for EPDM, TPR and Silicone elastomers

Figure 2 makes clear the validity of the concerns expressed by the design engineer. The three elastomers have dramatically differing modulus-frequency responses. This is shown in Table A where the Shore A hardness and modulus are compared. For this comparison the E' values at 100 Hz were taken from Figure 2 for all three elastomers.

The result of this case study was another satisfied PELabs customer armed with good viscoelastic data for evaluating the effect of a material change. For PELabs it is another confirmation of the importance of high quality viscoelastic data. We are thankful to our customers who give us the opportunity to serve their needs.

*For a full presentation of this analysis, see DMA Viscoelastic Analysis of Two Thermoset and One Thermoplastic Rubber in the RESOURCES section of this website.

**The principle of Time-Temperature Superposition is explained in the article, Designing for Stress Relaxation, in the Resources section of this website.

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