In my Ph.D. research, I need to carry out an etching of glass samples in HF acid. I use an ultrasonic bath for the experiment, but to avoid corrosion of steel I put a small plastic (polypropylene) jar with 5% HF with glass samples into the ultrasound bath full of water. Recently, my colleague told me that the plastic jar absorbs most of the ultrasonic energy and my etching process goes the way it would be without the bath at all. So, first of all, is it true that a polypropylene beaker absorbs ultrasound? What kind of beaker should I use instead which wouldn't be affected by HF acid?
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Polymers, like other materials as well, attenuate / absorb acoustic waves. Beside reflection (echo, similar to active sonar), this renders ultrasound suitable for non-destructive inspections. The extent of absorbing ultrasound depends on several factors (e.g., temperature, density/crystallinity of the sample, deployed frequency) and may be expressed as attenuation and by acoustic impedance.
The following table is part of a lecture about medical diagnostic applications of ultrasound:
(source, slide 5.)
A more detailed listing, specifically addressing polymers, may be found here. From data like
|---------------------------+----------+---------+-----------+-------------|
| Plastic material | Velocity | Density | Impedance | Attenuation |
| | [m/s] | [g/qcm] | | [dB/cm] |
|---------------------------+----------+---------+-----------+-------------|
| Polyethylene, low density | 1950 | 0.92 | 1.79 | 2.4 |
| Polypropylene, Profax | 2740 | 0.88 | 2.40 | 5.1 |
| Polypropylene, white | 2660 | 0.89 | 2.36 | 18.2 |
|---------------------------+----------+---------+-----------+-------------|
some differences in polymers of same name, but different brand (and degree of polymerisation, etc.) become apparent. (Interestingly, Teflon/PTFE is not mentioned in their listing, yet according to this study possess a much higher acoustical impedance than PP.)
Hence, beside chemical resistance against HF, you likely have to test the efficiency of the ultrasound waves passing through the walls of your beaker anyway.
answered Jul 5, 2017 at 21:56
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$\begingroup$ Buttonwood, how should I test the efficiency of the ultrasound waves passing through the walls of the beaker? $\endgroup$ Commented Jul 6, 2017 at 9:16
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$\begingroup$ Fasten the flat bottom beakers immersed in the bath with a clamp with some space just above the resonators to prevent moving / tilting (alternatively, for some baths, there are baskets with holding springs already installed, e.g. tovatech.com/blog/1504/ultrasonic-cleaner/…). Add sample (or sample on sample mount) and HF into a test beaker in question, start the etching for x min; then test with an other sample and new HF solution in a different beaker for the same time, power and soln. temperature; compare the results. $\endgroup$ Commented Jul 6, 2017 at 11:05
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$\begingroup$ I though it is possible to measure the power of ultrasound inside and outside of the beaker like in optical spectroscopy. $\endgroup$ Commented Jul 6, 2017 at 12:06
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$\begingroup$ So far I did not witnessed an org.chem. lab with experimental access to record the numbers of dB(A). This is the reason for the empirical approach "with beaker (material) A it works better, than with B or C". However if your institution has know-how and equipment to record the acoustic spectra, go for it. An other meta reference found (materials.springer.com/lb/docs/sm_nlb_978-0-387-69002-5_60) focusses more on polymers deployed in a medical context; wich may be not applicable here (different frequencies then the ones used in a bath). $\endgroup$ Commented Jul 6, 2017 at 12:24