Manufacture of graded porosity foams: simulation of local ultrasonic pressure and comparison with experimental results

2016-09-30T14:17:21Z (GMT) by Carmen Torres Jonathan R. Corney
The manufacture of polymeric solid foams with an engineered distribution of mechanical properties has been possible by irradiating ultrasound on a viscoelastic reacting mixture. Structures with a heterogeneous pore size distribution offer great advantages when compared to homogeneous distributions in many applications that require strength with minimal amount of material (e.g. airplane wings). However, manufacturing solutions lag well behind the demand of these components. Sonication has been recently demonstrated as a potential technique that can support these materials fabrication processes. The mechanism involves bubble growth in a polymeric melt undergoing foaming that is influenced by the ultrasonic environment (i.e. sound pressure, frequency and exposure time). Once the foam solidifies, the final porosity distribution within the solid reflects the sonication conditions. In order to obtain sophisticated distributions of porosity and porosity gradients, fine control on the acoustic pressure field has to be achieved. This paper presents an attempt to correlate acoustic pressure to porosity gradation by comparison of simulated acoustic field and engineered porosity analysed on experimental polyurethane foams. COMSOL Multiphysics™ has been used to recreate the process in the irradiation chamber; and the acoustic fields, both in the environment and the reaction vessel, have been simulated and validated. Results from this study will allow the optimisation of the manufacturing process of functionally tailored materials with the sonication method.