Different treatments may be applied to biocomposite according to the effect required. Examples of some chemical treatments include alkalization, enzymatic treatments, flame-retardant and antimicrobial agent applications. Physical treatments include compression and thermal treatments.

Among physical treatments, compression and thermal treatment play a very im­portant role as they form the basis of molding process, which is a must for most of the noise control composites. Compression of a fibrous mat deteriorates its sound absorption properties according to Castagnede et al.81 For a given homogeneous po­rous layer, compression is followed by a decrease in terms of porosity and thickness, and at the same time by an increase of tortuosity and resistivity. Jayaraman et al.,47 Yilmaz et al.,4 and Yilmaz et al.58 also found a decrease in sound absorption with compression. The finding of Yilmaz et al.20 is presented in Fig. 5.12.

Yilmaz et al.4,8 investigated the effects of thermal treatment on noise control ca­pability. Yilmaz et al.8 treated three-layered sandwich structures of PLA/Hemp/PLA at temperature points of 125, 145, 165 and 185°C. An increase in air flow resistivity and a decrease in NAC were found. The decrease in NAC reached a substantial ex­tent when the melting point of the constituent thermoplastic fiber, PLA, is reached as seen in Fig. 5.13. Yilmaz et al.4 investigated the effects of thermal treatment on noise reduction performance of three-layered PP/Hemp/PP sandwich structures at 150 and 185°C (see Fig. 5.14). They reported a slight increase in NAC for the lower frequency range for 150°C treated composite and a drastic decrease in NAC for the biocomposite treated at 185°C as shown in Fig. 5.12. Similar to the find­ing of Yilmaz et al.8, temperature exceeding the melting point of the thermoplastic component was reported to be very deteriorating for sound absorption performance. The slight increase in the lower region which is experienced for the 150°C treated sample might be due to increase airflow resistance. Accordingly, by fine-tuning the parameters of thermal treatment, the noise reduction capability of the biocomposite may be preserved or it can even be enhanced.

Among chemical treatments, Jayaraman et al.47 studied the effect of fire retar — dancy treatment on sound absorption. They found that the treatment had a positive impact on the sound absorption of kenaf fibers. Yilmaz et al.9 applied alkalization treatment on PP/hemp/PP to detect the effect of the treatment on sound absorption. As known, the alkalization process partially removes lignin, pectin and hemicel — luloses present in the fibers and leads to a separation between the fibrils of the natural technical fibers. This results in finer fibers and a rougher surface, which might enhance sound attenuation. However, alkalization did not increase the sound absorption coefficient as expected. In contrary, the NAC values decreased with the decrease in the mass of the material due to loss of hemp fiber constituents and pos­sibly the distortion of the pores during the wet treatment as given in Fig. 5.15.

Among wet treatments, Huda and Yang55 extracted fibers from cornhusks by al­kalization and further treated a part of them with cellulose and xylanase enzymes to produce finer fibers with the elimination of extracellulosic materials. They blended cornhusk fibers with PP fibers then formed a web by carding and thermally bonded the web to produce a composite structure. They obtained better sound absorption from composites consisting enzyme treated cornhusk fibers compared to those that were untreated as shown in Fig. 5.16.

5.4 CONCLUDING REMARKS

Fibrous materials act as noise control elements in a wide range of applications as they present a cost-effective, light-weight, and environmentally friendly alterna­tive to conventional materials. Fibrous materials should be designed as composite structures to address the demands in terms of esthetics and performance character­istics such as moldability, durability and enhanced noise reduction capacity. De­signing of an effective composite sound absorbing noise control element requires a good understanding of sound propagation in fibrous materials. This chapter has presented an overview of natural fiber based biocomposites as rigid porous sound absorbers. Sound absorption mechanisms that take place during sound wave propa­gation through fibrous media have been given. An overview of empirical models which may be applicable to explain sound propagation in biocomposites has been explained. Based on the models, factors that affect sound absorption behavior of biocomposites have been investigated referring to previous studies.

Most of the research and modeling effort for understanding the sound absorption characteristics of fibrous materials have been devoted to conventional petro-based materials. More study is needed with sound absorbers of biocomposites, which of­fer a safer production and service life with effective utilization of scarce resources.

Natural fibers, engineered environmentally friendly polymers, and recycled ma­terials can be given as examples of “green” materials to be used during production of noise absorber biocomposites based on previous studies. There is especially very limited knowledge pertaining to the acoustical properties of biocomposites consisting of engineered biopolymers or agricultural byproduct materials. Future research ef­forts devoted to the understanding of these materials may enhance our understand­ing of sustainable materials as noise reduction elements.

KEYWORDS

Biocomposites

Fiber reinforced Composites

Noise Control

Noise Pollution

Sound Absorption Materials

Sound Absorption Mechanism