5.3.1.1 FIBER TYPE

A wide variety of fibers are used in biocomposite sound absorbers, including con­ventional synthetic fibers, conventional plant fibers, exotic plant fibers, animal fi­bers, reclaimed fibers of chemical and natural origin and engineered compostable fibers like poly (lactic acid), as given in Table 5.2. The characteristics of the con­stituent fibers also have an important effect on sound absorption.

The effect of fiber type on sound absorption is hard to detect as it is often ac­companied by differences in fiber size and shape as seen in Figs. 5.5 and 5.6. Fiber type determines the relationship between the fiber size and flow resistivity.14 The following relationships have been reported:

where r0,g, r0,p, and r0,w represent flow resistivity values of glassfiber, polyester14 and wool46 webs in mks rayl/m, respectively; ц stands for viscosity of air which is 1.84×10-5 kg m1 s1, h denominates porosity, a is fiber radius in m, and p represents density of the fiber mat in kgm-3.

Fiber/material Production method Investigated parameters Measured parameters Frequency range (Hz) Thickness (mm) Fiber diam­ eter (ціп) Max. NAC Publication PP, PLA, glass — fiber, hemp Air laying, needle- punching, thermal treatment Heat treatment NAC, air flow permeability 500-5000 3.90-13.1 9-42 0.99 Yilmaz et al.8 PES, PP, cotton, wool, jute, rice straw, sawdust, jute, Needle-punching Thickness, cover plate, air gap, composition NAC 100-6300 2.53-22.6 N. S. 0.99 Seddeq et al.56 PP, PLA, glass — fiber, hemp Air laying, needle — punching Compression NAC, air flow permeability 500-5000 7.91-13.1 9-42 0.99 Yilmaz et al.9 PP, hemp Air laying, needle- punching, alkaliza­tion Alkalization NAC, air flow permeability 500-6400 10.61-12.53 32M2 0.99 Yilmaz et al.20 PP, Bamboo strips Laying stacking com­pression molding Blend ratio, thick­ness, fiber type, fiber orientation NAC, Noise Reduction Coefficient (NR) 0-3000 1.16-10.12 75 0.80 Huda et al.48 Coir fiber, wood particle debris, phenolic resin Needle-punching, resin bonding Blend ratio, needle-punching, fiber placement NAC, Noise Reduction Coefficient (NR) 125M000 N. S N. S 0.99 Yao et al.57 PU binder, pine sawdust, recycled rubber Resin bonding Blending ratio, thickness, mate­rial type, NAC 50-10,000 20-40 1-4* mm 0.92 Borlea et al.26 PP, PLA, glass — fiber, hemp Air laying, needle — punching Porosity, fiber type and size, layer sequence NAC, air flow permeability 500-6400 11.45-12.68 9-42 0.99 Yilmaz et al.58

Fiber/material Production method Investigated parameters Measured parameters Frequency range (Hz) Thickness (mm) Fiber diam­ eter (Mm) Max. NAC Publication PES, formalde­hyde, recycled PS, woodchip, furnace slag, municipal waste, power plant ash Granulation Resin bonding Material type 10-3,150 2.5-10* mm 1-2* mm 0.91 Bratu et al.51 Flax tow Grinding, washing, microwave, molding Grinding, micro — wave treating, molding, thick­ness NAC 100^1000 2-10 N. S 0.82 El Hajj et al.54 Recycled pulp, luffa fibers, yam waste Wet laying, cold pressing Blend ratio, mate­rial type NAC 500^1800 N. S N. S 0.13 Karademir et al.52 PP, Jute, PES Carding, needle — punching Material density, number of layers Sound insula­tion N/S 2.6-51 8.7 N/A Sengupta59 Jute, bamboo, banana, jute Carding, needle — punching Fiber type NAC 100-1600 49-6.4 N. S 0.20 Thilagivath21 PP, hemp, rapeseed straw, beech and flax Extrusion granulat­ing, compression molding Fiber type NAC 1000 -6500 N/S N. S 0.32 Markiewicz et al.49 PP, mechani­cally split corn husks, jute Spunbonding, mold­ing Fiber type, blend ratio, NAC 300-3000 3.2 1.3 0.42 Huda and Yang60

N/A: Not applicable, N. S: Not stated, *: granule diameter, PES: polyester, PP: polypropylene, PU: polyurethane, PS: polystyrene, PLA: poly lactic acid.

Studies related to sound absorption properties of biocomposites include struc­tures made up of different fibers. Surface properties of fibers and their cross-sections also play an important role. Accordingly, Nick et al.45 found greater absorption for cotton-polypropylene (PP) blend fibrous material for automotive applications com­pared to the flax-polypropylene and hemp-polypropylene blends. This was probably due to the inherent superior fineness of cotton fibers as compared to flax and hemp.

Jayaraman et al.47 examined the effect of kenaf fiber inclusion, which is a natural bast fiber, on the absorption of sound in fibrous absorbers. The addition of kenaf had a negative effect on the noise reduction performance compared to polyester and re­claimed polyester fibers, however, this effect is less pronounced in high frequencies. This negative effect may also be due to natural coarseness of kenaf fiber compared to synthetic fibers.

Parikh et al.22 developed composites in various weight ratios of natural and syn­thetic fibers including kenaf, jute, waste cotton, and flax with recycled polyester and off-quality polypropylene and compared to absorbers of conventional fibers, that is, 70% polyester and 30% polypropylene. They reported that each of the natural fibers contributed to noise reduction because of their absorptive properties in comparison with the conventional material. Furthermore, adding a soft cotton underpad was found to greatly enhance the sound absorption properties of the nonwoven floor coverings.

Huda et al.48 produced unconsolidated light-weight (0.312 g/cm3) composites by laying fine bamboo strips on a PP web and by a subsequent compression molding process. They reported better mechanical and noise reduction capabilities for the mentioned composites compared to jute-based composites.

Markiewicz et al.49 produced composites including PP and lignocellulosic fillers and measured their sound absorption performance in the 1000-6500 Hz frequency range. They reported the hemp filler addition allowed for significant increase in noise reduction over 3000 Hz, whereas rapeseed straw, beech and flax filler added to PP suppressed sound in the 3000-4000 Hz range.

Brencis et al.50 presented a research study with an aim to develop a sound ab­sorber from gypsum foam reinforced by fibrous hemp. They claimed that the gyp­sum, Gypsies rock, a local resource in Latvia, can have performance characteristics comparable to other state-of-the-art thermal and sound insulation materials. Addi­tionally, gypsum poses an important fire-resistance characteristic. Fragility, which is the disadvantage of the gypsum material, claimed to be avoided with the use of plant fibers, such as hemp, as a reinforcement element.

Bratu et al.51 studied composite materials including pellets from plastic bottles, sawdust, and ash from plant and sterile municipal wastes in a polymer type organic matrix in different blend ratios. The effects of the blend ratio and the type of the waste material on the sound absorption performance were investigated. They re­ported that use of sawdust and woodchips were advantageous in terms of noise reduction compared to the other recycled materials.

Karademir et al.52 prepared biocomposites through a wet laying process from recycled corrugated boards with addition of 30% yarn waste and 15% luffa fibers. They found that the addition of luffa fibers and yarn waste led to an increase in sound absorption together with an increase in air permeability at the expense of tensile strength.

Among the very few examples of biocomposites containing materials of ani­mal origin, Huda and Yang examined the sound absorption performance of ground chicken quill based PP composite and compared it with jute-based PP composites.53 They reported that the chicken quill based composites resulted in better sound ab­sorption performance in 500-2200 Hz frequency range as shown in Fig. 5.7.

ё 0.30 —

tL 0.20

0.10

0. 00

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2

Frequency (kHz)

FIGURE 5.7 Sound absorption of PP-chicken quill composites compared to PP — jute mats at different thicknesses and blend ratios (From Huda, S.; Yang, Y Composites Science and Technology, 2008.53 With Permission from Elsevier.).