Multiporous open-cell poly(vinyl formal) foams for sound absorption

A series of multiporous open-cell poly(vinyl formal) (PVF) foams were obtained by crosslinking poly(vinyl alcohol) (PVA) with different contents of formaldehyde in aqueous solution. Water did not only act as the solvent of PVA, but also as the pore-forming agent during the acetalization process. With the increasing acetalization degree, the multiporous PVF foams gradually separated out from water. And the higher the acetalization degree reached, the bigger the volume shrinkage observed. PVF foams with different pore sizes were obtained by changing the formaldehyde dosage. According to the results of FTIR spectra, the intensity ratio of O-H and C-H stretching mode (nu(s)(O-H)/nu(s)(C-H)) could reveal the acetalization degrees of PVF foams. The thermal properties and morphologies were investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and scanning electron microscope (SEM). The PVF foams could be used as sound absorbing materials because of the open-cell multiporous structures. The sound absorption coefficient (a) of the PVF foam largely increased with the decreasing pore size, especially in the frequency range of 800-2500 Hz. And the highest a value of 0.98 was obtained at 2000 Hz for PVF-3.0.