Part Two--Case Studies
Case 11--Hate Crime
When sound waves hit a surface they can be reflected, transmitted or absorbed/dissipated.[201] Sound wave dissipation can occur in the form of heat energy resulting from the friction between air flowing in and/or out of the cells, as well as in the form of kinetic energy associated with the stretching, bending or buckling of the cells due to the sound pressure.[202] The efficiency of sound absorbers can be assessed by the sound absorption coefficient. The sound absorption coefficient values of the PUFs prepared at frequencies ranging from 125 to 4000 Hz are plotted in Figure 6. 7. Care should be taken that the lines are only a guide to the eye as measurements were taken at specific frequency values.
Figure 6. 7 - Sound absorption coefficient values of CG100-P0, CG75-P25, PUF-CG50-P50 PUF-CG25-P75 and PUF-CG0-P100 measured in a standing wave apparatus (normal incidence)
From the results presented in Figure 6. 7, it can be seen that the sound absorption coefficient values globally increases with the frequency for all the PUFs.
PUF-CG100-POL0, has higher sound absorption coefficient values which is nearly 1.0 at 4000 Hz. This is due to its better ability to damp the sound wave energy as a result of the smaller percentage of open cells and more regular pore size and structure. This type of cellular structure is associated with increased airflow resistivity which improves sound absorption in the whole range of frequencies.[275,279,280] As mentioned before, the textural properties of this foam may be associated with the presence of fatty acids and methyl esters in CG which act as co-surfactants. Moreover, the role of these compounds as plasticizers may also confer better damping properties.[202]
In turn, the coffee grounds derived foam presents slightly higher sound absorption coefficient values at low frequencies (125–250 Hz). In this range the larger average cell size and higher open cell content may be responsible for its slightly higher sound absorption coefficient values.[204] Yet, as the frequency increases the sound absorption efficiency of this sample is not as good as that of the foam prepared using CG only. This is mainly due to the higher percentage of open cells, irregularity of the cellular structure, as well as the rigidity of the polymer matrix conferred by the lignocellulosic moieties, and the extent of crosslinking. On the one hand the higher porosity and lower density offers less resistance to sound waves as the path is reduced hence, dissipation of sound energy is reduced.
Additionally, the rigidity of the polymer matrix prevents vibration of the cell walls and struts, thus energy dissipation is not very efficient at least until high enough frequencies are applied.
In other words the conversion of sound energy into kinetic energy, is more difficult due to the lower capability of the cells structure to stretch, band and buckle.[281] All together, these characteristics seem to limit the damping capacity of this foam. However, at frequencies of 4000 Hz the sound absorption coefficient value also reach an interesting value (0.8) indicating that when the energy associated with the sound wave is high enough, the chain segments between crosslinking points gain mobility and are able to absorb the sound energy.
In fact, the rigidity associated with the lignocellulosic moieties cannot be the sole reason for the viscoelastic properties of POL derived foams as it is known that this type of polyol can afford foams with good damping properties using distinct formulations.[51] Nevertheless, in the present study, the combined effect of the rigid lignocellulosic compounds content and crosslinking extent is so significant that addition of 25 wt% of POL already reduced the damping ability of PUF-CG75-POL25.
Interestingly, at medium frequencies (250 and 500 Hz), the highest value of sound absorption (0.53) was achieved by the foam prepared using a 50/50 mixture of CG and coffee grounds polyol. This suggests a balance between the cell structure and mechanical properties of the CG50-POL50 in this frequency range. However, the performance of the PUF-CG25-POL75 cannot be explained at medium frequency, but it can be speculated that it may derive from limited miscibility CG and POL, and subsequent effect on the pore structure.
In general, for low frequencies, the pore size, structure and density of foams prepared seem to play a major role. In turn, at higher frequencies, the viscoelastic properties of the polymer network seem to be determinant unless high enough energy is involved to overcome the energy barrier associated with the mobility of stiffer moieties. Indeed, as the POL content increased, the stiffness of the ensuing foams increased compromising their ability to damp the sound wave energy up to 2000 Hz. Giwook Sung et al. have studied the effect of isocyanate content on the sound absorption coefficient of PUFs and concluded that in high frequency region, the sound absorption coefficient generally decreased with increasing the polymeric MDI content due to the decreased damping effect.[279] Similarly, the use of higher molar mass isocyanates and rigid isocyanates also led to a reduction of the sound absorption efficiency as a result of increased stiffness of the PU matrix at higher frequencies.[275,279,280,282] Curiously, in another study, addition of nanofillers to flexible PUFs yielded higher absorption coefficients even at higher frequencies. This was attributed
to the increase of the number of partially opened pores and the enhancing motion of the nanofillers.[280]
In order to assess the potential use of our foams in sound insulation with other foams reported in the literature, the noise reduction coefficients (NRC) were calculate and the values obtained are presented in Figure 6. 8.
Figure 6. 8 - Noise reduction coefficient (NRC) of samples obtained at frequencies 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz
As it can be conclude from Figure 6. 8 the NRC values of the foams prepared using polyols derived from renewable resources are generally similar to those reported for other materials used as sound absorbers (0.3-0.5) [201,283–285] and in some cases higher, indicating that these rigid foams have potential as sound absorptive materials. Furthermore, judicious blending of these polyols and formulations allows broadening the range of frequencies where rigid PUFs can be efficiently used as sound absorbers.
6.5. Conclusions
In the present study, CG and a liquefied coffee ground derived polyol were used to produce ecofriendly thermally stable sound absorbers PUFs. The properties of the foams indicated that the presence of lignocellulosic material on the coffee ground derived polyol increased the cell sizes and open cells content which increased the sound absorption coefficient values of the foams at low frequencies. On the other hand, the lower stiffness of the CG foams improved the sound absorption coefficient values of the foams at higher frequencies. In the middle frequencies range, the combination of the porous structure and mechanical properties of the foams prepared using a 50/50 mixture of polyols, present higher capability of sound absorption. From this work the suitability of CG and/or POL derived PUFs as sound absorbing materials has been proven even though formulation adjustments are still required in order to promote the formation of a more open and interconnected pore structure in order to increase the sound absorbing properties of foams richer in POL. Finally, from thermal conductivity and TGA measurements, it was observed that these foams are good thermal insulation materials and present thermal stability well beyond ambient temperatures.