Manufacture of heterogeneous thermoplastic foams is of great interest to automotive, transportation, and building industries, owing to their capability of damping low frequency noise. Unfortunately, most of the fabrication methods reported in literature either use cumbersome batch processes which have long processing times and produce small sized samples, or produce foams that are not appropriate for sound absorption applications.

In this context, this work aimed at manufacturing of thermoplastic acoustic foams with double porosity and graded porosity by processes that can potentially be scaled up for mass production. To this end, three different strategies were proposed and examined in this thesis. In the first approach, bimodal porous foams were developed by foaming a low melt strength polymer in presence of additives, such as sodium bicarbonate and wollastonite that can increase the population of small cells. Double porosity foams were also developed by foaming a base polymer after blending it with another low melt strength polymer to introduce large cells into its fine cellular structure. A third approach was utilized to produce graded porous foams by applying a temperature gradient to induce varying pre-foaming during compression molding followed by oven expansion.

Experimental results showed that foams containing a combination of small cells and large cells were developed by simultaneous addition of wollastonite and sodium bicarbonate. Compared with homogeneous sample, sound absorption coefficient of the sample prepared by 1% wollastonite and 1% sodium bicarbonate increased from 0.11 to 0.53 at 250Hz, from 0.21 to 0.72 at 500Hz, and from 0.20 to 0.61 at 1000Hz. Cells larger than 2mm were developed within a finer cellular structure by addition of a low melt strength EVA grade to a foamable base polymer. Addition of 5%wt. of the polymer increased sound absorption coefficient from 0.11 to 0.42 at 250 Hz, from 0.20 to 0.56 at 500Hz, and from 0.20 to 0.55 at 1000Hz. The porosity gradient developed across the foam sample increased when the temperature gradient increased, and a porosity difference of 9.1%was developed when imposing a temperature gradient of 60°C for 4 min. For these conditions, sound absorption coefficient increased from 0.12 to 0.33 at 250Hz, from 0.26 to 0.59 at 500Hz and from 0.25 to 0.62 at 1000Hz.