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Manufacturing and acoustical characterization of electrospun PolyVinylPyrrolidone (PVP) Blankets

Antonio Sorrentino1

University of Naples Federico II, Naples, Italy, 80125

In this work, new light soundproofing fibrous materials were obtained through electrospinning technique. The sound absorbers were obtained in the form of blankets of thin disks of 10 cm diameter, characterized by fibres with a diameter in order of 2 µm. Different activities involved for acoustical characterization of specimens, in particular to the evaluation of the sound absorption coefficient and flow resistance. Sound absorption coefficient was measured by a Kundt’s tube; for the evaluation of flow resistance a prototypal facility was designed and developed using a theoretical model proposed by Uno Ingard. The acquisition system of this prototype was programmed and controlled in Labview.The acoustic properties of electrospun woven mats are compared to the traditional soundproofing materials ones. The effect of assembling PVP and traditional soundproofing glass wool on sound absorption is also shown. The experimental results suggest that sound energy dissipation occurs for a mixing of fibres friction absorption and a resonance phenomena of the sound wave with the natural frequency of vibration of the disks pile (acting as a membrane).

Nomenclature α = sound absorption coefficient R = flow resistance W/V = weight by volume ΔP = air pressure difference across a specimen v = airflow velocity through the specimen, in m/s C = correction factor M = piston mass g = gravitational acceleration S = airflow exposed surface

A = piston section L = piston covered distance t = piston falling time ϕ = inclination angle

I.   Introduction ircraft requires continuous improvements in the cabin comfort to compete in the global market. As a consequence airplanes manufacturers are requested to design and produce aircraft offering high standards of comfort. In

parallel, work on weight and production cost reductions is providing new materials and manufacturing solutions that are very attractive from the structural point of view.

Over structural requirements, human comfort inside such environments also plays an important role in their acceptance, and Airliners have paid great attention - over the past twenty years - to noise and vibration, as well as the air quality aspects, all addressing the human sensations of comfort1. This is mainly due to crew member compliance issues linked to longer trips and to passenger expectations, viewing comfort as one of the main aircraft quality indicators. ______________________________________ 1Graduate student, Departmento of Industrial Engineering, via Claudio 21, 80125, Napoli

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Aircraft noise is generally divided into two sources: external and internal noise sources.Internal noise field, affecting passenger comfort and crew performance, is mainly due to intense Boundary Layer Noise, high engine noise levels transmitted into the aircraft via the structure and/or air as well as very annoying noise of the internal systems (air-conditioning, ventilation and ducts).

For what concern the external sources, indeed, the main components contributions are the propellers, the wings and the Low Pressure Compressors (LPCs)2-3. Overall aircraft interior noise is combination of mentioned components that, with various degrees and different effects, penetrate into the aircraft cabin. The sources and paths of airborne and structure-borne noise generating the interior noise level of an aircraft cabin are illustrated in Fig. 1.

Engine noise is highly dependent on propulsion type4. A turbofan, in fact, emits noise from two main sources: jet noise (produced by a high velocity jet exhausted from the core of the engine) and fan loading noise (that is the result of the pressure disturbance). A turboprop, instead, generates noise by periodic sources as rotating engine parts, rotating props and by vortices shed from the propellers. Jet noise spectrum is dominated by high frequency broad band noise; turboprop noise, instead, is dominated by a few low frequency tones.

In Figure 2 the general trend of the Sound Pressure Level (SPL) of a turboprop aircraft is reported5. Noise attenuation methods involve active and passive controls. In active noise control, the reduction of the internal sound field is obtained by installing many microphones and speakers within the room to cancel the noise; other methods involve modification of acoustic transmission properties by installing actuators and sensors near the source of the vibration6.

Passive noise control includes structural modifications or damping augmentation by using natural carpets or tuned panels to absorb vibrations at certain frequencies7-8.Porous materials can be classified as a passive means of noise control. The efficiency of passive noise control is greatest at higher frequencies, perhaps in the region of 1000 - 3000 Hz9, however they are still capable of providing significant noise reduction (i.e. greater than 10 dB) above 125 Hz.

Porous materials such as polymer foams, or fibre insulation, are capable of absorbing sound by converting the energy present in a sound wave into heat energy. The conversion process depends upon the geometry of the porous material as well as the frequency in question10.Many modern cellular foams are created from polymeric materials by the use of chemical blowing agents11 and it is possible to vary the size and geometry of their cellular construction. The alteration of the size and cell type of foams affects their acoustic properties, as well as combining layers of different kinds. Interior noise can be treated by placing the

engines in order to minimize the noise directly radiated to the cabin and also by providing insulating material over the

Figure 1. Main interior noise sources and paths.

Figure 2. Sound Pressure Level of a turboprop aircraft5.

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entire surface of the aircraft and passenger compartments. Installing the appropriate sound absorbing material in the walls of the fuselage is one of the simplest ways to gain noise reduction.

Recently the authors12 demonstrated that soundproofing blankets could be produced through electrospinning; they have sound absorption coefficient near unity at very low frequencies that can be tuned by changing the sample mass.

In this paper the acoustic properties of electrospun woven mats produced from more concentrated PVP solutions are reported and compared to traditional soundproofing materials. The effect of assembling together PVP and traditional soundproofing materials on sound absorption and flow resistance is also shown.The test specimens are described in Section 2. An overview of the experimental investigation is provided in Section 3; hence, the achieved results are summarised in Section 4. Some concluding remarks are given in Section 5.

II.   Materials and manufacturing Traditional sound absorption materials include foams, fibres, membranes, perforated panels, etc. These materials

have good noise reduction abilities at the high frequencies, but exhibit insufficient sound absorption properties in the low and medium frequency range (250-2000 Hz) in which human sensitivity to noise is fairly high. Therefore materials with excellent noise reduction properties in the low and medium frequency range are highly desirable for acoustical purposes. The specimens investigated in this paper are made of Polyvinylpyrrolidone (PVP) with molecular weight of 1.3x106 g/mol dissolved in ethanol; they have a density of 0.789 g/ml and purity > 99.8 % purchased by Sigma-Aldrich and used without any additional purification or processing. Several specimens, called δ series, were manufactured by electrospinning 15% w/v PVP solution in ethanol at a flow rate of 0.200 mL/min under an applying electrical potential of 20 kV over a fixed collection distance of 39 cm at room temperature of 23 ±2° and humidity (45 ±10%) (Fig. 3). Electrospun nanocomposite fibres are lightweight, dimensionally stable, porous, flexible, and can absorb sound waves at high, medium, and low frequencies.

Figures 4 and 5 show respectively an investigated specimen and its Scanning with Electron Microscope (SEM).

Some characteristics of the manufactured and investigated specimens are reported in Table 1.

Figure 3. Sketch illustrating set-up of electro-spinning technique.

Figure 4. Investigated specimen. Figure 5. SEM image of PVP specimen.

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The diameter of the traditional acoustical fibres is in the range of micrometers (5-100 µm); electrospun fibres may

instead have diameters ranging from 10 µm to 1 µm. Electrospun mats have special characteristics such as large specific surface area, high porosity, flexibility, and extremely low weight which are helpful in obtaining preferable acoustical damping performance.

III.   Experimental investigations The acoustical properties of PVP blankets have been investigated with particular focus at the influence of different

chemical configurations on the sound absorption coefficient α and the evaluation of flow resistance R.Alfa was measured with a Kundt’s tube, while the flow resistance was quantified with an experimental prototype designed and developed using a theoretical model proposed by Uno Ingard19. The acquisition system of this prototype was programmed and controlled in Labview.

A.   Sound absorption coefficient An acoustic impedance tube (Fig.6) was used to test material samples for normal incidence sound absorption

according to regulations13-14. The two-microphone transfer-function method of the Bruel & Kjaer impedance tube was used to determine the acoustical properties of the electrospun fibres at different frequencies. In particular a measurement of the acoustic absorption coefficient in the frequency range of 200 - 1600 Hz was possible thanks to the dimensions of the tube having a diameter of 100mm and a length of 2m.

The experimental set-up consists of a loud speaker that produces sound waves which travels along the pipe up to specimen; these waves are partially absorbed by the material and partially reflected by it. If all the sound waves are reflected (100 %) then the amplitude of reflected wave pattern will be the same as that of the incident one. Whilst, if the material absorbs sound, the reflected wave will have different amplitude compared to that of the incident one.

In an impedance tube, the sound pressure is measured using two microphone probes at two fixed locations and then the transfer function between the two measurements is calculated. This technique leads to determine the sound acoustic absorption coefficient, the complex reflection coefficient and the normal acoustic impedance of the material.

B.   Flow resistance The experimental set-up (Fig.7) used to measure the flow resistance was a prototype designed and developed at Department of Industrial Engineering of Naples. The apparatus enables to estimate absorbed sound energy of blankets and in particular to test the coupling between airflow and the material. It is possible, differently from standard test methods20, to generate airflow without any pumps. Moreover it is possible to measure the pressure drop across the specimen without microphones.

The apparatus consists of:

•   plexiglass tube •   piston •   specimens holder •   AQ system

Table 1. Specimen characteristics.

Figure 6. Kundt’s tube by which have been tested the sound absorption coefficient of PVP specimens.

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The airflow used to characterize the specimens is generated by the piston falling into the plexiglass tube (internal diameter = 120mm and length = 55cm); at lower end is located the testing blanket into a wood holder. Two photodetectors, controlled with NI-myRIO and programmed in LabView, measure the piston falling time by which is possible to calculate R.

Specimen made by fibres having a small diameter have a higher flow resistance than the specimens with few fibres having large diameters. When sound enters in a porous material, its amplitude is decreased by friction as the waves try to move through the tortuous passages21. This friction quantity which can be expressed by resistance of the material to airflow is called specific flow resistance and is defined as:

𝑅 =  𝛥𝑃𝑣

(1)

where, R = specific flow resistance, in (N·s)/m3 = 1 rayl ΔP = air pressure difference across a specimen, in Pa v = airflow velocity through the specimen, in m/s The theoretical model proposed by Uno Ingard and used in this work for measuring R, is shown below:

𝑅 =  𝐶𝑀𝑔𝑐𝑜𝑠𝜙𝑆

𝐿𝐴1𝑡

(2)

where, R = specific flow resistance, in (N·s)/m3

C = correction factor for the loss of pressure estimation, C=1 M = piston mass, in kg g = gravitational acceleration, in m/s2 S = airflow exposed surface, in m2

A = piston section, in m2 L = piston covered distance, in m t = falling time, in s ϕ = inclination angle, in degrees

Figure 8. Sketch illustrating flow resistance set-up.

Figure 7. Flow resistance tube.

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the Eq. (2) allows to measure the specific flow resistance just using falling time variable t.

IV.   Results

A.   Sound absorption coefficient of PVP blankets In this section the sound absorption coefficients, measured for different configurations of specimens in the

frequency range 200-1600 Hz, are reported. Before discussing the obtained results, it is worth reminding15-17 that sound energy can be absorbed according to the following mechanisms:

1. Friction between the fibres of a porous material or in the voids of a non-fibrous material (a dissipative absorber);

2. Energy dissipation when the sound wave is resonant with the natural frequency of vibration of a membrane; 3. Energy dissipation of a tuned cavity absorber based on the principle of a Helmholtz resonator. Figure 9 shows the changes in sound absorption coefficients of specimens δ for frequency range 200-1600 Hz at

increasing number of layers (and hence of the sample mass too). From the results, it is evident that the samples show, at a specific frequency (depending on the specimen mass), a high absorption coefficient value, close to 0,9. However the samples appear to have a good acoustical absorbing behaviour in the overall frequency range till 1600 Hz.

1.   Comparison of the sound absorption coefficient of PVP blankets with traditional materials

In order to appreciate the good results carried out from the tests, the sound absorption coefficient of the PVP blanket is compared with the one carried out for some investigated traditional soundproofing materials: glass wool fibres, polyester and aerogel (Fig.10).

Glass wool is a porous material obtained from synthetic fibres that are commonly used for thermal insulation and sound absorption because of its high performance and low cost. Its diffuse-field sound absorption coefficient is very high at mid-high frequencies. On the other hand, it has several cons: it can be harmful for human health if its fibres are inhaled, causing skin irritation. Hence such materials must be adequately overlaid if directly exposed to the air.

Figura 9. Sound absorption coefficient of test specimen at different mass. Figure 9. Sound absorption coefficient at different mass.

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Moreover they can pulverize because of vibrations and are not resistant to water, oil and chemical agents and this makes unwise their application on absorbing noise barriers.

Glass wool fibres Polyester Aerogel

The second tested samples were sound absorbing blankets of recycled polyester (PET) fibres. As the natural fibres, polyester blankets respect both the environment and the human health and they have good thermal and sound insulation which reduces noise pollution and prevents heat loss.

Finally the third test sample is Spaceloft®, a commercial aerogel reinforced by PET fibres and glass wool. Aerogels are the lightest solid materials containing in their volume approximately 98.2% air18. They are excellent

thermal insulator because they almost nullify two of the three methods of heat transfer (convection, conduction, and radiation); they are good conductive insulators because they are composed almost entirely of gas, and gases are very poor heat conductors. Finally they are good convective inhibitors because air cannot circulate through the lattice. As aerogels have high porosity and high specific surface area, sound waves are strongly absorbed and attenuated and for this reason aerogels are also used for sound insulation.

A comparison of the sound absorption coefficient in the frequency range 200-1600 Hz of the aforementioned materials with the PVP blanket, is reported in Figure 11.

As expected, the trend of the sound absorption coefficient curve of the glass wool fibres and of the polyester increase with frequency reaching high values of α over 1000 Hz. The curve representing the aerogel, indeed, has a behaviour as bell-shape with αmax around 0,6, that is not very high. Finally the curve representing the PVP blankets (δ

Figure 10. Soundproofing materials.

Figure 11. Sound absorption coefficient of several test specimen at fixed mass (12 g).

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series) shows a double behaviour having the maximum α at a frequency (which depend on the mass) and an average value close to 0,6-0,7 for all the other frequencies. All these specimens have been tested for fixed mass (12 g).

2.   Sound absorption synergistic effects In this section, sound absorption measurements for sample composites, obtained combining PVP with Glass Wool

fibres, are given in order to prove a synergistic effect. The composites, having a total mass of 24 g, have been realized and measured for different configurations: Glass Wool/PVP (combo-delta forward), PVP/Glass Wool (combo-delta back) and PVP/Glass Wool/PVP (sandwich).

Figure 12 shows the sound absorption coefficient of PVP blankets (12g and 24g) and Wool Glass sample (12g) and of the three aforementioned configurations of composites.From the results it can be noted that the different allocation of the samples in the composites provides different behaviours and trends of the sound absorption coefficient curve; it shows a peak in the low frequency range, followed by a slight decrease of the sound absorption value and a finally increase steadily up to a maximum value of 0.6-0.7 at 1600 Hz.

B.   Flow resistance of PVP blankets The estimation of flow resistance regarded different PVP specimens, in particular the different blankets differ for

the thickness (mass) and hence for the number of layers. The flow resistance tested specimens have a thickness range from 35mm till 48mm.

The Figure 13 shows the results of measurements: the direct correlation between thickness and flow resistance for the same material. Moreover the increasing of one this two values (or both) corresponds to an increasing of piston falling time too.

The specimens have flow resistance values in a range 1600 - 2000 (N·s)/m3.

Figure 12. Sound absorption coefficient of Wool glass/PVP composites.

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1.   Comparison of the flow resistance coefficient of PVP blankets with traditional materials

The achieved results for δ blankets were compared to some traditional soundproofing materials: glass wool fibres, polyester and kenaf. At first, glass wool was compared to PVP blankets for the same thickness (Fig.14). The PVP blankets have a density of 45 kg/m3, instead the glass wool shows a density of 27 kg/m3. The ratio between the flow resistance of δ series and glass wool was about 3:1; increasing the thickness the trend looks the same.

Figure 13. Flow resistance values of several PVP specimens with different thicknesses

Figure 14. Flow resistance of PVP and Glass wool specimens with different thicknesses

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The same performance was also confirmed comparing the PVP blankets to the glass wool for the same mass (12g) and different thickness. In Figure 15 are shown the results of this comparison between δ series and traditional materials.

Figure 15. Flow resistance of several test specimen at fixed mass (12 g).

V.   Conclusion Ultra porous and flexible PVP blankets were prepared at ambient pressure by electrospinning process, and their

acoustic properties were characterized. The measurement technique adopted to evaluate the sound acoustic absorption has been the classical two-microphone transfer-function method; the flow resistance was estimated with a prototypal apparatus using a Uno Ingard method. Measurements were carried out for specimens with a surface of 10 cm and variables thicknesses, hence mass. Preliminary experimental results showed very interesting features of such materials, particularly enhancing the absorptions coefficients without increasing the weight significantly. These results make this material suitable for industrial applications where the comfort is required. Furthermore, comparing the sound absorption coefficient of these PVP samples with traditional acoustical material it was evident that PVP electrospun samples exhibit higher sound absorption coefficient at all frequency range (200-1600 Hz), with a peak close to 1 at a specific frequency (low frequency range) depending on the mass sample and an average value (close to 0.5-0.6) in the medium-high frequency range. When assembled together with glass wool a synergistic effect is envisaged. The results about flow resistance seems consistent with sound absorptions tests, the δ series shows flow resistance values in a ratio of 3:1 (or more) in the comparison with some traditional materials. Future works will be focused on mixing the investigated PVP blankets with other materials in order to improve the sound absorption properties in a wider frequency range and to increase properties as insolubility and flame retardancy.

Acknowledgments This work has been developed during my mater’s degree thesis internship at University of Naples Federico II, it

was a partnership between Department of Industrial Engineering and Department of Chemical, Materials and Production Engineering. It has been possible thanks to the professionalism and passion of all the people that I met on this journey and that gave me the opportunity to improve my skills.

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