Sound transmission loss of hierarchically porous composites produced by hydrogel templating and viscous trapping techniques
We have developed two different methods for fabrication of hierarchically porous composites which are environmentally friendly, inexpensive and give a large amount of control over the composite microstructure. The hydrogel bead templating method involved introducing a slurry of hydrogel beads as templates into a gypsum slurry that, upon drying, left pores reflecting their size. The overall porosity reflected the volume percentage of hydrogel bead slurry used. Using mixtures of large and small hydrogel beads in controlled volume ratios as templates, we produced hierarchically porous gypsum composites that had tailorable microstructures at the same overall porosity. The viscous trapping method involved utilisation of an aqueous solution of a thickening agent, methylcellulose, during the setting process of an aqueous gypsum slurry. The methylcellulose solution traps the hydrated gypsum particles in solution and stops their sedimentation as the continuous gypsum network forms, allowing formation of an expanded microstructure. This method allows a good degree of control over the porosity which is directly controlled by the volume percentage of methylcellulose solution used. The mechanical strength of the porous composites decreased as the porosity increased. The composites with smaller pores had increased compressional strength and Young's modulus compared to the ones produced with large pores, at constant porosity. The hierarchically porous gypsum composites showed an intermediate Young's modulus and an increased compressional strength. We also studied the sound transmission loss of these hierarchically porous composites. We found that the ones produced by the viscous trapping method had a lower sound transmission loss over the frequency range investigated as the overall porosity was increased. We demonstrated the effect of the composite pore size at a constant porosity on the sound transmission loss. Our experiments showed that porous composites with large pores showed increased sound transmission loss at lower sound frequencies compared to those with small pores. As the sound frequency increased, the difference between their STL spectra decreased and at the higher frequency range (>2420 Hz) the composites with smaller pores began to perform better. The hierarchically porous composite had an intermediate STL spectrum, suggesting a way of tailoring the hierarchically porous structure at constant porosity to achieve desired sound insulating properties at certain frequencies.