Low density sound absorbing polyurethane foam via cell openness manipulation

Abstract

Noise, vibration, and harshness (NVH) characteristics are one of the important factors which determine the high performance luxury vehicle. As NVH performance increases, customers sensitivity to noise and the quality and satisfaction on the car interior comfort increases. Thus, the choice of excellent sound absorbing material is necessary. Polyurethane foams (PUFs) are widely used as sound absorbers in interior parts of automobiles as well as in other applications in acoustics. The sound absorbing characteristics of acoustical material such as PUF, mostly in open or semi-open cell structure, are majorly dependent on its microstructural change with a variation of frequency. Cell structure of the PUF can be affected by the ratio between polyurethane resin premix (polyol, cross-linking agent, blowing agent, catalyst) and isocyanate. Furthermore, sound absorption performance of porous media is well-known to be influenced by density and thickness of the foam depending on each different frequency range. In an attempt to satisfy the fabrication of low density sound absorbing semi-open cell PUF, cell openness manipulation method is applied by adding a chemically reactive cell opening agent, polyethylene glycol 2000 (PEG 2000) into the polyol mixture. Experimentally, a number of pores in PUF were increased by 0, 3 (15％ cell openness), and 6 (22％ cell openness) wt％ of PEG 2000 assuming that cell openness of the PUFs is dependent on the content of PEG 2000. The cell morphologies of the foams were examined using a scanning electron microscopy (SEM). The sound absorption coefficients of each sample were measured by a two-microphone B&K impedance tube. For the comparison of both the experimental and the numerical simulation results, a multiscale modeling involving poroacoustics parameters based on Johnson-Champoux-Allard (JCA) model was developed. This modeling method was used to obtain the sound absorption coefficient of each periodic unit cell (PUC) with four different cell openness (15, 25, 50, and 100％), which are assumed to be the imitation of real fabricated PUF cell structures in ideal conditions. Further quantitative acoustical analyses e.g. a root mean square (RMS) value, a noise reduction coefficient (NRC), and 1/3 octave band spectrogram of the PUFs were conducted. The equivalent comparison between experimental (3 wt％ addition of PEG 2000) and simulation results (15％ cell openness of PUC) had the best sound absorption performance of PUF (a density of 40 kg/m3, 500 m, and thickness of 2 cm). These two results reveal a new potential replacement for conventional PUF used in cars which density (80 kg/m3) doubles compared to our fabricated sample and which will surpass not only the sound absorption performance, but even improve fuel efficiency by lowering the weight of PUF in the auto industry.