Influence of sea polymer removal on sound absorption behavior of islands-in-the-sea spunbonded nonwovens

Abstract

This work presents the results of efforts focused on the development of relatively lightweight and fibrous acoustic webs. For this objective, nonwoven webs that contain bicomponent filaments with islands-in-the-sea cross sections were produced by spunbonding, which involves the extrusion of sea and island polymer melts through dies, cooling and attenuating the bicomponent filaments by high-velocity air streams. Nylon 6 and polyethylene were used as the island and sea polymers, respectively. Webs were hydroentangled with high-pressure water jets prior to the dissolving process to obtain fiber entanglement. Sea polymer was removed from the spunbonded nonwovens by using a reflux dissolution setup. Weight, thickness, air permeability, pore size and sound absorption coefficients of the nonwoven samples were measured before and after the sea polymer removal. Results demonstrated that sea polymer removal led to further bicomponent filament fibrillation, which affected sound absorption positively. The structure with the higher number of island fibers had better acoustical properties. Lightweight and fibrous acoustic nonwovens can be obtained with the method given in this study.

Keywords

nonwoven spunbond sound absorption acoustic islands-in-the-sea

Nonwovens are complex fibrous and porous structures, and often have very low solidity. They are inherently lightweight due to pores filled with air. The pores are also in the form of a bundle of capillaries. Nonwovens are known to have useful acoustical properties and are used as sound absorbers because of their complex fiber network geometry, bulk and low density.^1,2 These structures inherently have interconnected pores with tortuous paths.² Effects of structural parameters on sound absorption behaviors of nonwovens have been widely investigated in sound absorption-related literature of nonwovens.^1–8 Sound absorption in nonwovens occurs due to frictional resistance between fibers and air trapped in the pores that dissipates acoustic energy during sound wave propagation.^1,3,4 In bulky and thick nonwovens, sound absorption is generally higher^1,3–5 due to longer tortuous paths that offer more frictional resistance while sound waves pass through. The structures having higher numbers of fibers per unit volume and a large fiber surface area also offers more frictional resistance, which leads to higher sound absorption performance.⁵ Fiber shape is another important parameter, since there is the possibility to increase the total fiber surface area.⁶ Sound waves interact with a larger solid surface during propagation that increases sound absorption.

To form structures with more fibers per unit volume, one would need to resort to the use of thinner fibers. The spunbonding process, with bicomponent filament technology coupled with a fiber-splitting process, allows the production of submicro- and microfiber-based nonwovens. Segmented-pie or islands-in-the-sea cross sections are often chosen to produce bicomponent filaments. Fiber diameters ranging from 0.3–5 µm can be obtained after a suitable post-process that releases the finer fibers.^9,10 Producing thin fibers with islands-in-the-sea cross sections is feasible since the number of islands in a circular bicomponent filament is limitless theoretically and the sea component that surrounds island fibers makes the whole bicomponent filament more spinnable.¹¹ Island fibers of the resulting nonwoven can be split by mechanical forces. For instance, high-speed hydroentangling water jets can be used for this purpose.^2,11,12 Durany et al.¹² and Anantharamaiah et al.¹¹ demonstrated that hydroentangled islands-in-the-sea spunbond structures can form superior structures in terms of strength, durability and porosity. It was also shown that islands-in-the-sea fibers can be fibrillated/fractured by hydroentangling to deliver micro- and nanodenier fibers with tremendously high surface areas. Fibrous structures having high fiber surface areas offer possible solutions for some applications like filtration and sound absorption. Recently, Yeom and Pourdeyhimi¹³ investigated the aerosol filtration characteristics of these structures and found that the hydroentangling process improved the aerosol filtering efficiency by increasing the electrostatic charging on the web. Acoustical absorptive properties of hydroentangled spunbonded nonwovens made from islands-in-the-sea bicomponent filaments were reported by Suvari et al.² Nonwovens produced from higher numbers of island fibers absorbed more sound energy according to the results.²

Another method that can release island fibers from bicomponent filaments is the dissolution of the sea component. After removing the sea polymer from the bicomponent filament by dissolving, nonwovens only contain thin island fibers, have higher total fiber surface areas and also become lighter.⁹ Fedorova and Pourdeyhimi⁹ used a dissolution technique for the spunbonded webs and explored the idea that the islands-in-the-sea configuration is a promising technique for high-speed and high-throughput production of strong and lightweight nonwovens comprised of micro- and nanofibers.

Most of the studies on islands-in-the-sea webs have primarily focused on optimizing their mechanical and bonding strengths, and the formation of durable nonwoven fabrics.^{9,11,12,14,15} The aim of this study is to form lightweight and thin fibrous structures by removing the sea polymer and investigate the sound absorption performance of the islands-in-the-sea nonwovens with varying numbers of island fibers.

Materials

All the nonwoven webs made from islands-in-the-sea bicomponent filaments used in this study were produced at the Nonwovens Institute’s Partners’ pilot facilities located at North Carolina State University. Nylon-6 (PA6) and polyethylene (PE) were used as the island and sea polymers, respectively. Basic properties of the polymers used in the bicomponent filaments are summarized in Table 1. Filaments in the webs contain weight ratio of 75% island and 25% sea polymer.

Table 1.

Properties of polymers used

Polymer	Trade name	Supplier	Melting temperatureT_m (^oC)	Density(g/cm³)
PA6	Ultramid BS 700	BASF	220	1.14
PE	ASPUN 6811A	Dow Chemical Co.	125	0.94

PA6: nylon-6; PE: polyethylene.

Methods

Fabric formation

Webs that contain bicomponent filaments with island-in-the-sea cross sections were produced by spunbonding, which involves the extrusion of sea and island polymer melts through dies, cooling and attenuating the bicomponent filaments by high-velocity air streams. Spunbonded webs with island counts of 1, 19, 37 and 108 were produced (Table 2) in order to establish the influence of the number of island fibers on sound absorption performance after the dissolution process. In Figure 1, a scanning electron micrograph of the 108-island bicomponent filament cross sections of the spunbonded web is given. As may be seen in Figure 1, sea polymer (PE) surrounds thin circular fibers (island fibers) in a bicomponent filament. The basis weights of the spunbonded bicomponent nonwoven webs prior to the post-processes were kept at ∼ 100 g/m².

Table 2.

Description of the islands-in-the-sea webs produced

Number of islands	Polymer		Polymer ratio (%)		Specific energy given (MJ/kg)
Number of islands	Island	Sea	Island	Sea	Specific energy given (MJ/kg)
1	PA6	PE	75	25	67.5
19	PA6	PE	75	25	67.5
37	PA6	PE	75	25	67.5
108	PA6	PE	75	25	67.5

PA6: nylon-6; PE: polyethylene.

Figure 1.

Cross sections of bicomponent filaments having 108 island fibers.

The structural integrity of the webs should be obtained by a bonding process before removal of the sea polymer.⁹ Thermal bonding is not suitable due to the possible loss of bonding points after sea polymer removal. On the other hand, structures after hydroentangling have the ability to withstand the post-processing steps required for dissolving of the sea from the islands-in-the-sea nonwovens.⁹ For this reason, the spunbonded webs were hydroentangled with high-speed water jets prior to the dissolving process to obtain fiber entanglement and create consolidation. Note that hydroentangling also contributes to bicomponent filament fibrillation.¹¹ The 108-islands spunbonded web was subjected to different levels of hydroentangling energy in order to obtain different levels of consolidation and fiber fibrillation. The 108-islands spunbonded web was passed 1–5 times through the hydroentangling unit, transferring 22.5 MJ/kg energy at each passage (Table 3). Webs with island counts of 1, 19 and 37 were subjected to the 67.5 MJ/kg (three passes) total specific hydroentangling energy, which was found to be the optimal energy level for fibrillation and consolidation of the webs in the literature.^2,12

Table 3.

Number of hydroentangling passages and corresponding specific energies given to the 108-islands web

Number of islands	Hydroentanglement Number of passages	Specific energy given (MJ/kg)
108	1	22.5
108	2	45.0
108	3	67.5
108	4	90.0
108	5	112.5

Hydroentangled webs were treated with a solvent, xylene (Sigma-Aldrich), in a reflux setup to remove the sea polymer (PE) from the fibrous structure. The dissolution method (Figure 2) used to remove the sea polymer is carried out in the following sequence: flat-bottomed flask with 250 ml xylene is placed on a hot plate at 100℃ surface temperature. The head/joint of the flask is sealed with a PTFE joint sleeve to establish a leak-proof connection. Then flask is fitted to the reflux system, which allows liquid circulation at the outer wall of the gas-outlet spiral to drop the temperature of the vapor. A sufficient temperature drop leads condensation. Xylene in the flask is heated up to its boiling point of 137℃. After removing the flask, the nonwoven specimen is put into the flask with hot xylene and then flask is fitted to reflux system again. Heat evaporates xylene during dissolution but the mass of xylene remains the same because liquid drops come back to the flask by condensation. After 45 minutes, the nonwoven sample is taken out from the xylene and put into hot pure water to clean the remaining xylene and sea polymer from the fabric surface. The sample is also cleaned in beaker with cold pure water. Finally, the nonwoven sample is placed in a hot oven at a temperature of 60℃ for 15 minutes for drying.

Figure 2.

Schematic diagram of the dissolution process.

The quantity of the dissolved polymer can be calculated by measuring the weights of the samples before and after the dissolution process. It is expected that the nonwoven will be 25% lighter after removal of the sea polymer.

Testing methods

The impedance tube method was used in order to determine sound absorption coefficients of the nonwovens in this work. The main approach of the method is to generate a specific sound wave and measure the effect of the material’s presence on the sound wave propagation. The most important advantage of this method and instrument is the accommodation of small samples. The instrument that was used to assess the sound absorption coefficient during the experiments is the BSWA Impedance Tube Kit Type SW 260 with the small tube setup. In this setup, the small sound-receiving (impedance) tube is mounted facing the open end of the large emitting tube with a loudspeaker, and the small sample holder is placed at the far end of the small tube. As a sound source, the loudspeaker generates broadband, stationary sound waves in the range of 1–6.1 kHz. The impedance tube is designed to generate plane waves that propagate in the tube toward the sample and interact with it.

Round specimens of 30-mm diameter were sampled from nonwovens by using a die cutter. Sound absorption coefficients of eight specimens of each type of nonwoven were measured with 2.5 mm air spaces behind the specimens, in accordance with ASTM (American Society for Testing and Materials) E 1050-08.

Fabric mass per unit area was determined according to ASTM D 3776-07, where eight specimens of 100 cm² were taken from fabrics by a sample cutter and weights were measured with an analytical balance. Thickness was measured by using a thickness gauge according to the ASTM D 5729-97 conditions.

The air permeability tests were performed using an SDL Atlas M021A Air Permeability Test device as per ASMT D 737-04. Pressure drop and test area were chosen as 125 Pa and 20 cm², respectively. Eight samples from each spunbonded nonwoven were tested for accurate air permeability results. Average results were calculated. For the calculation of the airflow resistivity $(R_{o})$ in $(Pa . s / m^{2})$ , equation (1) was used

R_{o} = \frac{Δ P}{A_{p} \times 0.00508 \times l}

(1)

In equation (1), A_p is the air permeability (cfm), l is the thickness of nonwoven (m) and

Δ P

is the pressure drop, which is equal to 125 Pa according to the standard test method of ASTM D 737-04.

A PMI Capillary Flow Porometer (model CFP-1100-AX) was used to measure the pore size of spunbonded nonwovens. The wetting liquid ‘Galwick’, with a surface tension of 15.9 dynes/cm, was used to fill the pores of the specimens. In this test method, air pressure is applied in small incremental steps perpendicular to the saturated nonwoven to gradually force out the wetting liquid. Larger pores require a lower pressure, while liquid in the smallest pore requires higher pressures. Pore size is detected by sensing flow through the pore.¹⁶

Nonwoven cross sections were examined by using scanning electron microscopy (SEM). Scanning electron micrographs were obtained on a Tescan VEGA3 SBU microscope. Prior to scanning, the specimens were coated with a thin layer (4.4 nm) of AuPd using a Cressington 108 Sputter Coater.

The solid volume fraction (SVF) is the ratio of solid material volume to the total volume of the fabric and can be calculated according to equation (2).⁷ $ρ_{fabric}$ is the fabric bulk density and $ρ_{fiber}$ is the fiber density in equation (2). The weighted average fiber density $\hat{ρ}$ (see equation (3)) can be used in equation (2) for the calculation of the SVF values of the multi-component materials

SVF = \frac{ρ_{fabric}}{ρ_{fiber}}

(2)

\hat{ρ} = \frac{ρ_{i} ρ_{j}}{γ_{i} ρ_{j} + γ_{j} ρ_{i}}

(3)

In equation (3),

ρ_{i}

is the density in g/cm³ and

γ_{i}

is the weight percentage of the fiber in the nonwoven. The subscript “j” stands for the other fiber/component in equation (3).

Details of the test methods are given in Table 4. All the samples were conditioned at 65% ± 2% relative humidity and a temperature of 21 ± 1℃ before each test for at least 24 hours.

Table 4.

Tests performed on spunbonded nonwovens

Characterization	Test method	Instrument
Mass per unit area	ASTM D 3776-07	Sample cutter, analytical balance
Thickness	ASTM D 5729-97	Thickness gauge
Air permeability	ASTM D 737-04	SDL Atlas M021A air permeability tester
Pore size	PMI method	PMI CFP-1100-AX capillary flow porometer
Sound absorption coefficient	ASTM E 1050-08	BSWA impedance tube
Cross section images	N/A	Tescan VEGA3 SEM

ASTM: American Society for Testing and Materials.

Results and discussion

Effect of sea polymer removal on sound absorption performance

The spunbonded web with 108 islands was subjected to different levels of hydroentangling energy in order to obtain different levels of consolidation and bicomponent filament fibrillation. The 108-islands web was passed one to five times through the hydroentangling unit. The amount of hydroentangling energy transferred to the web for each pass was 22.5 MJ/kg. Afterwards, nonwoven specimens with one (22.5 MJ/kg) to five passes (112.5 MJ/kg) were treated in the solvent to remove the sea polymer. The percentage ratio of the removed polymer was calculated by measuring weights of the specimens before and after the dissolution procedure. The results are given in Figure 3. It should be noted that a 25% weight reduction corresponds to full sea polymer dissolution.

Figure 3.

Weight change after the sea polymer removal.

It seems that most of the sea polymer (average 21.62%) was removed from the fibrous structure of the 108-island samples according to the weight-loss results in Figure 3. An ANOVA (analysis of variance) F test for independent groups was applied for the removed polymer percentages with a significance level (α) of 0.05. According to the statistical test results, at least one of the groups tested differed from the other groups. An LSD (least significant difference) test, which makes direct comparisons between individual groups, was run afterwards (α = 0.05). The LSD test result indicates that differences between removed polymer percentages at 22.5, 45 and 67.5 MJ/kg were statistically insignificant. On the other hand, removed polymer percentages at 90 and 112.5 MJ/kg differed from those at 22.5, 45 and 67.5 MJ/kg statistically. It appears that higher hydroentangling energy levels cause higher fiber entanglement, which probably reduces the percentage of the removed polymer.

Table 5 summarizes the mass per unit area and thickness data for all samples. The mass per unit area of the nonwoven samples with different passes (hydroentangling energies) decreased following the sea polymer removal. Note that the mass per unit area of the samples increased slightly after each hydroentangling passage in both cases. The consolidation appears to lead to a reduction in width and an increment in mass per unit area of the nonwoven.² These basic web properties are used to calculate the solidity of the webs.

Table 5.

Properties of 108-island samples before and after the sea polymer removal

Hydroentangling energy (MJ/kg)	Mass per unit area (g/m²)				Thickness (mm)
Hydroentangling energy (MJ/kg)	Before	SD	After	SD	Before	SD	After	SD
22.5	111.9	1.8	89.2	2.4	0.49	0.02	0.44	0.02
45.0	130.2	1.5	104.6	2.6	0.47	0.01	0.45	0.01
67.5	137.8	3.8	109.9	4.3	0.49	0.03	0.48	0.03
90.0	145.0	5.0	118.8	2.6	0.51	0.02	0.51	0.02
112.5	159.0	4.3	127.2	4.9	0.56	0.02	0.56	0.02

Figure 4 shows the SVF results. The amount of solid volume in nonwoven structures decreased due to removed sea polymer (Figure 4). An ANOVA F test for independent groups was applied for the SVF results with a significance level (α) of 0.05. Note that, in both cases, the fabrics processed greater than 22.5 MJ/kg are not affected significantly by hydroentangling energy according to the statistical test results.

Figure 4.

Solid volume fraction change after sea removal.

Dent¹⁷ published an interesting work dealing with the roles of the structural parameters on air permeability. There is a functional relationship between the airflow resistivity and SVF of nonwovens according to the study. Nonwovens with a close SVF should also have close airflow resistivity if they contain fibers of the same size.¹⁷ Since the nonwovens hydroentangled greater than 22.5 MJ/kg have the same SVF statistically, they should have close airflow resistivity if they contain fibers of the same size according to Dent’s¹⁷ study. However, the amount of free thin island fibers should vary in nonwovens processed at different hydroentangling energy levels. A higher number of free island fibers can be expected since more bicomponent filaments can be broken/split at higher hydroentangling energy levels. According to the results in Figure 5, airflow resistivity increases with increased hydroentangling energy. The reason for the increment should be the effect of a higher number of free thin island fibers. Note that the airflow resistivity increment appears to be minimal beyond three passes or an energy of 67.5 MJ/kg for the dissolved nonwoven samples.

Figure 5.

Airflow resistivity results of 108-island nonwovens before and after the sea polymer removal.

Despite the reduction in SVF after sea polymer removal, airflow resistivity increased until three passes. This indicates that bicomponent filaments were further fibrillated during sea polymer removal, especially for the nonwovens with one and two passes.

The results for the pore sizes are reported in Table 6. The mean flow pore diameters given in Table 6 are a measure of the size of majority of the pores.¹⁶ As expected,² the mean flow pore diameter decreases with increasing hydroentangling energy.

Table 6.

Pore size of the 108-island nonwovens before and after sea polymer removal

Mean flow pore diameter (µm)	Hydroentangling energy (MJ/kg)
Mean flow pore diameter (µm)	22.5	45	67.5	90	112.5
Before	12.85	6.97	5.18	4.87	4.38
After	8.84	4.90	3.78	3.44	3.37

Mean flow pore diameter decreases after sea polymer removal as well. This again indicates that further fibrillation occurs during dissolution. More free island fibers appear to constitute smaller pores. However, note that pore diameter decrement is larger until three passes or a specific energy of 67.5 MJ/kg, indicating that dissolution is more effective for the relatively lower hydroentangling energies in terms of fibrillation.

Figure 6 shows cross section SEM images of the 108-island nonwoven at one pass before (Figure 6(a)) and after (Figure 6(b)) the sea polymer removal. Figure 6(a) demonstrates that one pass of hydroentangling is not sufficient for significant fibrillation. After sea polymer removal (for one pass), free island fibers can be seen in Figure 6(b). However, note that island fibers in the same bicomponent filament appear to be close to each other (Figure 6(b)).

Figure 6.

Cross section images of one-pass 108-island nonwoven before (left) and after (right) the sea polymer removal.

Figure 7 shows cross section and surface SEM images of the 108-island nonwoven at three passes before (Figure 7(a)) and after (Figure 7(b) and (c)) the sea polymer removal. Although the degree of fibrillation is difficult to quantify, qualitatively, three passages of hydroentangling resulted in significant fibrillation according to the SEM image in Figure 7(a). After dissolution of the three-pass nonwoven, free island fibers without sea polymer can be seen in Figure 7(b) (cross section) and Figure 7(c) (nonwoven surface). Sound absorption results of the 108 islands nonwovens with different passes before and after the sea removal are given in Figures 8 to 12.

Figure 7.

SEM images of three-pass 108-island nonwoven before (a) and after (b and c) the sea polymer removal. Sound absorption results of the 108-island nonwovens with different passes before and after the sea removal are given in Figures 8 to 12.

The Student’s t-test was applied for the sound absorption results given in Figure 8 with a significance level (α) of 0.05. Probability (p) values were calculated for the sound absorption results between 1 and 6.1 kHz with 2.7 Hz intervals. Test results indicated that sound absorption differences after the frequency of 2.57 kHz are statistically significant for the data given in Figure 8. For one pass, sound absorption increases after the sea polymer removal for most of the frequency range (Figure 8), although the solidity (or packing density) of the fabric drops (see Figure 4). It seems that further fibrillation occurred after sea polymer removal. Released thin island fibers resulted in higher sound energy absorption for the nonwoven with one pass.

Figure 8.

Sound absorption coefficients of the 108-island nonwovens for one pass.

After sea polymer removal for two passes, sound absorption behavior remains nearly the same with a slight increase in higher frequency region (Figure 9). Although the solid volume decreased after the dissolution process, the resulting free island fibers positively affected the sound absorption behavior of the nonwoven with two passes.

Figure 9.

Sound absorption coefficients of the 108-island nonwovens for two passes.

The Student’s t-Test was applied for the sound absorption data given in Figures 10 and 11 to figure out if the experimental results obtained before and after sea polymer removal were significantly different. Since the sound absorption coefficients were measured from 1 to 6.1 kHz with 2.7 Hz intervals, p values were calculated for the sound absorption results at each frequency. According to the statistical test results (α = 0.05), sound absorption differences after 1.70 and 1.82 kHz are statistically significant for the data given in Figures 10 and 11, respectively.

Figure 10.

Sound absorption coefficients of the 108-island nonwovens for three passes.

Figure 11.

Sound absorption coefficients of the 108-island nonwovens for four passes.

After dissolution of the thee-, four- and five-times hydroentangled nonwovens, sound absorption curves shifted to a higher frequency range due to an SVF drop (Figures 10 to 12). If samples have similar thickness, a reduction in SVF affects in a way that causes sound absorption to increase in the higher frequencies at the expense of that in the lower frequencies.^2,8 However, note that for the three passes, the sound absorption curve change can be regarded as small than that of four and five passes. It seems that dissolution still releases some thin island fibers of the three-pass nonwoven, which probably prevents a larger change due to an SVF drop.

Figure 12.

Sound absorption coefficients of the 108-island nonwovens for five passes.

Sound absorption results of the dissolved nonwovens with varying hydroentangling passages are given together in Figure 13 for comparison purposes. Clearly, the structure at three passes is superior to those at one and two passes. The difference between three, four or five passes is smaller. It seems that dissolving the nonwovens with higher amounts of hydroentangling energies (after three passages) does not lead to further significant fibrillation. Prior to the dissolving of the web, 67.5 MJ/kg (three passes) total specific hydroentangling energy is considered optimal energy level for consolidation and fibrillation. This hydroentangling energy level can also be considered optimal with respect to sound absorption performance of the dissolved islands-in-the-sea spunbonded nonwovens.

Figure 13.

Sound absorption coefficients of the 108-island nonwovens after dissolution.

Sound absorption of dissolved nonwovens with different numbers of island fibers

Spunbonded nonwovens made from islands-in-the-sea bicomponent filaments with island counts of 1, 19, 37 and 108 were passed three times through the hydroentangling unit corresponding to a specific energy of 67.5 MJ/kg. The nonwoven specimens were treated in the solvent to remove the sea polymer. Percentage ratios of the removed polymer are given in Figure 14. Mass per unit area and thickness data before and after the dissolution are given in Table 7.

Figure 14.

Weight change of the nonwovens with varying island counts after the sea polymer removal.

Table 7.

Basic properties of the nonwovens with different numbers of islands

Number of islands	Mass per unit area (g/m²)				Thickness (mm)
Number of islands	Before	SD	After	SD	Before	SD	After	SD
1	111.1	3.6	89.3	2.2	0.56	0.01	0.66	0.06
19	134.9	5.1	107.6	3.3	0.47	0.02	0.47	0.03
37	135.9	5.3	111.2	5.3	0.46	0.03	0.46	0.02
108	137.8	3.8	109.9	4.3	0.49	0.03	0.48	0.03

Weights of the nonwoven samples decreased with an average value of 21.35% after the dissolution (Figure 14). SVF values decreased after sea polymer removal as well (Figure 15). Except for the nonwoven with one island, the mass per unit area and SVF values of the samples with different numbers of islands are close to each other, as can be seen from Table 7 and Figure 15.

Figure 15.

Solid volume fraction change of the nonwovens with varying island counts after sea removal.

Airflow resistivity results of the nonwovens before and after sea polymer removal are shown in Figure 16. In both cases, airflow resistivity increases as the number of island fibers increases. More thin fibers cause more resistance to air flow as expected. After sea removal, airflow resistivity decreased due to SVF drop.

Figure 16.

Airflow resistivity results of nonwovens with varying island counts.

As expected, an increase in island count resulted in smaller pores (see Table 8). For the nonwovens with 1 and 19 islands, removal of the sea polymer created larger pores. On the other hand, the higher number of free fibers due to sea removal constituted smaller pores for the 37- and 108-island nonwovens. This is further demonstrated by the SEM images of the nonwovens with 1 and 37 islands (see Figures 17 and 18).

Table 8.

Pore sizes of the islands-in-the-sea nonwovens before and after sea polymer removal

Mean flow pore diameter (µm)	Number of islands in the sea
Mean flow pore diameter (µm)	1	19	37	108
Before	17.29	6.41	6.31	5.18
After	37.57	7.04	5.71	3.78

Figure 17.

Cross section images of one-island nonwoven before (a) and after (b) sea polymer removal.

Figure 18.

Cross section images of 37-island nonwoven before (a) and after (b) sea polymer removal.

Sea polymer can be seen in Figure 17(a) before the dissolution. Due to sea polymer removal, pores become larger in the structure, as may be seen in Figure 17(b).

Bicomponent islands-in-the-sea filaments with 37 islands before the dissolution are seen in Figure 18(a). It seems that the higher number of free island fibers constituted smaller pores after sea polymer removal, according to Figure 18(b).

Previously, Suvari et al.² reported that hydroentangled webs with higher island counts result in higher sound absorption. A similar result was found in our study for the dissolved nonwovens with different numbers of island fibers. As seen in Figure 19, the sound absorption coefficient increases with the number of island fibers. It seems smaller pores and thinner fibrous structures of 108-island nonwovens lead to more sound energy dissipation. Since the 108-island nonwoven structure has more fibers per unit volume and consequently has a large fiber surface area, fibers interact with oscillating air trapped in the pores and offer more frictional resistance, which leads to higher sound absorption performance.

Figure 19.

Sound absorption coefficients of the nonwovens with varying island counts after sea removal.

Conclusions

The removal of the sea polymer from spunbonded nonwovens that contain bicomponent filaments with islands-in-the-sea cross sections is mostly achieved by the dissolution process. After removing the sea polymer, spunbonded nonwovens contain only thin island fibers and also get lighter. Sea polymer removal leads to further bicomponent filament fibrillation, especially for the spunbonded webs processed at lower hydroentangling energies (one and two passes). Island fibers released due to fibrillation have a positive influence on sound absorption behavior. It was also shown that sound absorption coefficients of the dissolved structures increase by the number of island fibers. Our results indicate that dissolution method given in this study can be used to form lightweight and fibrous sound absorptive spunbonded nonwovens. These structures have the potential to be used in cars in place of high volume, fibrous bulky sound absorbers.

Footnotes

Acknowledgements

Fabrics were produced at the Nonwovens Institute’s Partners’ pilot facilities located at North Carolina State University. Their support is gratefully acknowledged.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by The Scientific and Technological Research Council of Turkey (TUBITAK): Grant No. 215M340.

References

Barron

. Industrial noise control and acoustics, New York: Marcel Dekker, 2003.

Suvari

Ulcay

Maze

et al.

Acoustical absorptive properties of spunbonded nonwovens made from islands-in-the-sea bicomponent filaments. J Text Inst 2013; 104: 438–445.

Aso

Kinoshita

. Sound absorption characteristics of fiber assemblies. J Text Mach Soc Japan 1964; 10: 209–217.

Genis

Kostyleva

Kostylev

. Sound-absorbing properties of fibrous materials prepared by the aerodynamic method. Fibre Chem 1990; 21: 389–392.

Suvari

Ulcay

Pourdeyhimi

. Sound absorption analysis of thermally bonded high-loft nonwovens. Text Res J 2016; 86: 837–847.

Tascan

Vaughn

. Effects of fiber denier, fiber cross-sectional shape and fabric density on acoustical behavior of vertically lapped nonwoven fabrics. J Eng Fiber Fabr 2008; 3: 32–38.

Russell

. Handbook of nonwovens, Cambridge: Woodhead Publishing Limited, 2007.

Aso

Kinoshita

. Absorption of sound wave by fabrics. J Text Mach Soc Japan 1963; 9: 32–39.

Fedorova

Pourdeyhimi

. High strength nylon micro- and nanofiber based nonwovens via spunbonding. J Appl Polym Sci 2007; 104: 3434–3442.

10.

Hollowell

Anantharamaiah

Pourdeyhimi

. Hybrid mixed media nonwovens composed of macrofibers and microfibers. Part I: Three-layer segmented pie configuration. J Text Inst 2013; 104: 972–979.

11.

Anantharamaiah

Verenich

Pourdeyhimi

. Durable nonwoven fabrics via fracturing bicomponent islands-in-the-sea filaments. J Eng Fabr Fibers 2008; 3: 1–9.

12.

Durany

Anantharamaiah

Pourdeyhimi

. High surface area nonwovens via fibrillating spunbonded nonwovens comprising islands-in-the-sea bicomponent filaments: Structure–process–property relationships. J Mater Sci 2009; 44: 5926–5934.

13.

Yeom

Pourdeyhimi

. Aerosol filtration properties of PA6/PE islands-in-the-sea bicomponent spunbond web fibrillated by high-pressure water jets. J Mater Sci 2011; 46: 5761–5767.

14.

Fedorova

Verenich

Pourdeyhimi

. Strength optimization of thermally bonded spunbond nonwovens. J Eng Fiber Fabr 2007; 2: 38–48.

15.

Pourdeyhimi B, Fedorova NV and Sharp SR. Lightweight high-tensile, high-tear strength bicomponent nonwoven fabrics. Patent US20060223405A1, USA, 2006.

16.

Jena

Gupta

. Liquid extrusion techniques for pore structure evaluation of nonwovens. Int Nonwovens J 2003; 12: 45–53.

17.

Dent

. The air-permeability of non-woven fabrics. J Text Inst 1976; 67: 220–224.