Sound absorption and compression properties of perpendicular-laid nonwovens

Abstract

This study presents an investigation into the sound absorption behavior and compression properties of perpendicular-laid nonwovens. Seven types of perpendicular-laid nonwovens produced by vibrating and rotating perpendicular lappers were selected. Nonwovens with varying thickness and areal density were prepared by the heat-pressing method to investigate the effect of structural parameters such as thickness and areal density on sound absorption ability. Measurements of sound absorption properties were carried out with a Brüel and Kjær measuring instrument. The effect of manufacturing techniques on sound absorption performance and compression properties was investigated. The effect of porosity on sound absorption ability was studied. The influence of density and fiber orientation angle on compression properties was analyzed. The results show that samples prepared by vibrating perpendicular lapper exhibit better compression properties, whereas there is no significant influence of two manufacturing techniques on sound absorption performance. The increase of areal density results in improvement in the sound absorption ability. The increase of thickness can improve the sound absorption coefficient in the low-frequency range, but decrease of the coefficient occurred in the high-frequency range. A quadratic relationship between porosity and sound absorption ability has been found. The results also show that compressional resistance has a strong relation with density – the correlation coefficient is 0.95, indicating that the compressional resistance is directly proportional to the density of perpendicular-laid nonwovens. The results indicate that the perpendicular-laid nonwovens with higher initial fiber orientation angle have better compression properties.

Keywords

compression property fiber orientation nonwovens porosity sound absorption

Perpendicular-laid nonwovens, which have a typical high-loft nonwoven structure, are widely used for thermal and acoustic comfort in the automobile industry. Due to the majority of fibers being oriented in the vertical plane, perpendicular-laid nonwovens exhibit high resistance to compression and excellent elastic recovery after repeated loading.¹ Vibrating perpendicular lapper (STRUTO) and rotating perpendicular lapper (WAVEMAKER) are mainly used to lay the feeding web in the vertical form.^1–2

Acoustic, mechanical, and thermal properties of perpendicular-laid nonwovens have been studied by researchers.^3–10 Tascan and colleagues^3–4 investigated the effect of fiber geometry and fabric density on acoustic behavior of perpendicular-laid nonwoven fabrics. They stated that fabrics made from 3 denier fibers are better sound absorbers and insulators than those made from 15 denier fibers; their results indicated that the perpendicular-laid nonwoven fabrics made from 4DG and trilobal fibers show better sound insulation results than nonwoven fabrics made from round fibers. In their research, they also stated that perpendicular-laid nonwoven fabrics with higher fabric densities exhibit better sound insulation abilities than those fabrics with lower densities. Kalinova⁵ evaluated the sound absorption coefficient of perpendicular-laid and longitudinal-laid nonwoven fabrics, and concluded that perpendicular-laid nonwoven fabric has a better sound absorption performance than a longitudinal-laid nonwoven fabric. Yang and colleagues⁶ investigated the sound absorption behavior and thermal properties (thermal resistance and thermal conductivity) of perpendicular-laid nonwovens and stated that sound absorption of perpendicular-laid nonwovens strongly correlated with their thermal resistance. The acoustic behavior and air permeability of perpendicular-laid nonwoven fabric have been studied by Yang and colleagues,⁷ who reported that the sound absorption ability is inversely proportional to air permeability, and stated that air permeability can be used as a criterion of sound absorption behavior of perpendicular-laid nonwovens. Kang and colleagues⁸ investigated the relationship between the fiber orientation distribution and mechanical properties of perpendicular-laid nonwovens prepared by air and mechanical folding systems; they stated that with the increase of web density, the compressional resistance of air folding nonwoven increases as well while that of mechanical folding material decreases. Their results also showed that air folding nonwovens have lower strain at maximum stress than mechanical folding nonwovens. The compressional behavior of perpendicular-laid nonwovens was introduced by Parikh and colleagues.^9–10 They compared the compressional resistance of perpendicular-laid and cross-laid nonwovens, and concluded that perpendicular-laid nonwovens have higher compressional resistance and better recovery properties than cross-laid nonwovens.

Although the sound absorption performance of perpendicular-laid nonwovens has been studied, there is limited research focusing on the effect of manufacturing techniques, thickness, and areal density on the sound absorption performance of perpendicular-laid nonwoven fabric. Besides, it is essential to understand the effect of fiber orientation on the compression property of perpendicular-laid nonwoven fabric. Thus, the major objective of this study is to understand the effect of manufacturing techniques and structure parameters on sound absorption and compression properties. Seven types of perpendicular-laid nonwovens were selected. Thinner perpendicular-laid nonwoven samples with varying thicknesses were formed using the heat-pressing method. The sound absorption property of samples was measured using a Brüel and Kjær impedance tube.

Experiment

Material

Five perpendicular-laid nonwoven fabrics prepared by vibrating perpendicular lapper at the Technical University of Liberec, Czech Republic, as well as two types of commercially available nonwoven fabrics which were separately made by vibrating perpendicular lapper and rotating perpendicular lapper were selected to carry out this study.

The vibrating perpendicular lapper (STRUTO) is illustrated in Figure 1. The carded web is fed onto the conveyor belt and a reciprocating forming comb pulls the carded web toward the hold-back roller to form a fold. The fold is pulled off the comb by a system of needles placed on a reciprocating compressing bar and pushed to the fiber layer, which is created and moved between the conveyor belt and a wire grid. The fiber layer is bonded by melt-bonding fibers present in the fiber blend when it passes through the thermobonding chamber.
Figure 1.
Vibrating perpendicular lapper

The rotating perpendicular lapper (WAVEMAKER) is shown in Figure 2. Carded web is brought to the working roller. The teeth of the working roller form the carded web into folds, creating a fiber layer between the conveyor belt and wire grid. Again, the fiber layer is bonded in the thermobonding chamber.¹
Figure 2.
Rotating perpendicular lapper.

From Figures 1 and 2, it can be seen that both STRUTO and WAVEMAKER technologies can create nonwovens with vertically oriented fibers. This vertical orientation provides good mechanical performance, especially compression properties. Moreover, due to the thermal bonded structure and high initial thickness of the perpendicular-laid nonwovens, varied thicknesses of STRUTO and WAVEMAKER nonwovens can be obtained through thermal treatment. Based on these characteristics, the perpendicular-laid nonwovens can be used in many automotive functions for sound and thermal insulation, such as under the bonnet, door panels, headliners, A–B–C pillars, and the luggage compartment.

In this study, sample A was prepared by rotating perpendicular; samples B, C, D, E, F, and G were produced by vibrating perpendicular lapper. In order to produce nonwoven samples with different thicknesses, the heat-pressing method was applied. Samples A, B, and C were compressed under 600 Pa pressure at 130℃ for 5 min; thickness gauges were applied to obtain samples at certain thicknesses. The characteristics of the nonwoven specimens are listed in Table 1. The perpendicular-laid nonwoven samples were made by different types of polyester fibers. Samples A, B, C, A1, B1, C1, C2, C3, C4, and C5 have the same fiber content, while samples D, E, F, and G have a different fiber content. The details of five types of fiber i, ii, iii, iv, and v are illustrated in Table 2. The sheath part of fibers iii and v are low-melting polyethylene terephthalate (PET).
Table 1.
Characteristics of perpendicular-laid nonwovens

Samples Fiber content WA of diameter (µm) WA of fineness (dtex) Manufacturing techniques Thickness (mm) Areal density (g/m²) Porosity (%) Bulk density (kg/m³) Airflow resistivity (Pa·s/m²)

A 30% i 45% ii 25% iii 20.96 3.08 WAVEMAKER 24.09 507.5 97.60 21.07 5757.05

A1 STRUTO 20.76 507.5 97.21 24.45 7319.58

B STRUTO 28.36 478.3 98.08 16.87 4828.55

B1 STRUTO 20.32 478.3 97.31 23.54 7498.51

C STRUTO 27.48 465.2 98.07 16.93 4108.94

C1 STRUTO 23.87 465.2 97.78 19.49 5337.58

C2 STRUTO 20.69 465.2 97.44 22.48 7029.82

C3 STRUTO 16.85 465.2 96.85 27.61 10,181.53

C4 STRUTO 13.31 465.2 96.01 34.95 12,868.69

C5 STRUTO 10.43 465.2 94.91 44.60 20,474.6

D 70% iv 30% v 23.21 5.35 STRUTO 20.82 335.7 98.68 16.12 2208.3

E STRUTO 19.85 317.5 98.69 15.99 2031.52

F STRUTO 20.12 198.6 99.19 9.87 1024.4

G STRUTO 20.66 259.3 98.97 12.55 1727.8

WA is weighted average; WAVEMAKER is rotating perpendicular lapper; STRUTO is vibrating perpendicular lapper.

Table 2.
Fiber specifications⁶

Fiber code Types of PET Diameter (µm) Fineness (dtex) Staple length (mm) Ratio of core and sheath

i Hollow PET 28.64 4.92 70.00 –

ii PET 15.25 1.76 50.00 –

iii Bicomponent PET 22.02 3.24 50.00 3:1

iv PET 26.91 6.70 57.00 –

v Bicomponent PET 14.58 2.20 38.00 3:1

PET: polyethylene terephthalate.

Samples A1, B1, C1, C2, C3, C4, and C5 were obtained by the heat-pressing method based on nonwoven A, B, and C. The content percentage of samples is based on weight. Fabric thicknesses were measured with an Alambeta device (SENSORA, Liberec, Czech Republic). Fabric areal density was determined according to ISO 9073-1:1989. Sample porosities were determined according to ASTM C830-00: Standard test methods for apparent porosity, liquid absorption, apparent specific gravity, and bulk density of refractory shapes by vacuum pressure.¹¹ The equation for determination of porosity is as follows:
$h = (1 - \frac{ρ_{w}}{ρ_{f}}) \times 100 %$
(1)
where h is porosity, $ρ_{w}$ is fabric density, and $ρ_{f}$ is weighted average absolute density of the fibers. The voids in hollow fibers are not included in the porosity, since open pores that carry airflow with the ambient air are of interest in terms of sound absorption and the closed pores are less efficient than open pores in sound absorption.^12–15 The airflow resistivity of porous materials indicates some of their structural properties. It was used to establish correlations between the structure of porous materials and their sound absorption properties. In the present study, airflow resistivity was measured by using an AFD300 Acoustic Flow device (The Gesellschaft für Akustikforschung Dresden mbH, Dresden, Germany) according to ASTM C522-03. Cross-sectional microscopic images of samples are shown in Figure 3.
Figure 3.
Cross-sectional microscopic pictures of perpendicular-laid nonwovens.

Sound absorption testing

Acoustic properties of materials can be tested by reverberant chamber methods, steady-state methods, and impedance tube methods. The impedance tube method was used to obtain normal incidence sound absorption coefficient (SAC) in this study. The sound absorption of perpendicular-laid nonwovens was determined according to ASTM E1050: Standard test method for impedance and absorption of acoustical properties using a tube, two microphones and a digital frequency analysis system, which was developed to determine the absorption ability of materials for normal incidence sound waves.¹⁶ A Brüel and Kjær measurement system containing Type 4206 Impedance Tubes, PULSE Analyzer Type 3560, and Type 7758 Material Test Software (BRüEL & KJÆR, Nærum, Denmark) was used for testing, with the frequency range from 50 Hz to 6400 Hz. A large tube (100 mm in diameter) and a small tube (29 mm in diameter) were used for measuring the sound absorption in the low-frequency range (50–1600 Hz) and high-frequency range (500–6400 Hz) respectively.¹⁷ A sketch of the impedance tube method measurement system is shown in Figure 4. A loudspeaker is mounted at one end of the impedance tube and the tested sample is placed at the another end. The loudspeaker generates broadband, stationary random sound waves. These incident plane sound waves propagate in the tube and hit the sample surface. The reflected sound wave signals are picked up and compared to the incident sound waves.¹⁸ Perpendicular-laid samples were sampled by using a die cutter. Five samples were measured for each nonwoven fabric.
Figure 4.
Impedance tube measurement system.

The sound absorption results obtained by impedance tube are plotted against frequency between 50 Hz and 6400 Hz. In order to numerically investigate the effect of perpendicular-laid nonwoven fabric structure properties on the sound absorption ability, the noise reduction coefficient (NRC) and average value of SAC ( $\bar{α}$ ) of all the nonwovens were calculated. The NRC has been calculated as the average value of measured values for 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz, which provides a decent and simple quantification of how well the porous material will absorb the noise. The $\bar{α}$ is the average of the SAC for the whole sound absorption coefficient measurement range. The NRCs of perpendicular nonwovens were calculated using the following equation:
$NRC = \frac{α_{250 Hz} + α_{500 Hz} + α_{1000 Hz} + α_{2000 Hz}}{4}$
(2)

During the impedance tube measurement process, the SAC was measured at even numbered frequencies in the range of 2 Hz to 6400 Hz. The values of SAC are not accurate under 50 Hz, so the $\bar{α}$ was calculated from 50 Hz to 6400 Hz. The equation for calculating $\bar{α}$ is as follows:
$\bar{α} = \frac{\sum_{i = 25}^{n} α (f_{2 i})}{n - 24}$
(3)
where n is the total number of measurements in the test range and is equal to 3200.⁶ The NRC and computed $\bar{α}$ values for the perpendicular-laid nonwovens are listed in Table 3.
Table 3.
$\bar{α}$ and NRC of perpendicular-laid nonwovens

Sample codes $\bar{α}$
NRC

Mean value SD Mean value SD

A 0.580 0.009 0.254 0.003

A1 0.581 0.007 0.239 0.004

B 0.588 0.007 0.281 0.003

B1 0.545 0.012 0.228 0.005

C 0.523 0.006 0.255 0.005

C1 0.544 0.006 0.242 0.002

C2 0.521 0.005 0.216 0.003

C3 0.528 0.008 0.191 0.003

C4 0.433 0.015 0.147 0.005

C5 0.444 0.028 0.136 0.006

D 0.315 0.028 0.142 0.010

E 0.290 0.017 0.129 0.006

F 0.201 0.007 0.092 0.004

G 0.242 0.014 0.110 0.005

$\bar{α}$ , average value of sound absorption coefficient, NRC: noise reduction coefficient.

Compressional resistance testing

The compression energy and compression load of perpendicular-laid nonwovens were carried out by using a universal testing machine (TIRATEST 2300, TEMPOS, Opava, Czech Republic). Circular perpendicular-laid nonwoven samples of diameter 10 cm were prepared. The compression tests were conducted at a velocity of 10 mm/min according to the ASTM D575-91 (Standard test methods for rubber properties in compression). All the nonwoven specimens were compressed up to a deformation of 90% of the initial thickness in an atmospheric condition of 20℃ and 65% relative humidity. Five tests were carried out for each sample. The fiber orientation angle at different compression stages was analyzed.

Results and discussion

Sound absorption properties

The normal incidence SAC of perpendicular-laid nonwovens was determined as a function of the sound frequency. The SAC indicates how much of the sound is absorbed in the material. The SAC data in the low-frequency range (50–1600 Hz) and high-frequency range (500–6400 Hz) were obtained by using Type 4206 large and small tubes, respectively. Later, the measurement data from large and small tubes were combined to form the curves for the frequency range between 50–6400 Hz. In this section, SAC, $\bar{α}$ , and NRC of perpendicular-laid nonwoven samples are presented and compared against the manufacturing techniques and physical parameters. The NRC and $\bar{α}$ of samples are computed according to equations (2) and (3); the values are listed in Table 3. Both the NRC and $\bar{α}$ are dimensionless numbers.

Samples produced by different manufacturing techniques were measured for sound absorption performance, and the results are shown in Figure 5. Sample A was produced by rotating perpendicular lapper (WAVEMAKER), and samples B and C were prepared by vibrating perpendicular lapper (STRUTO). Samples A1, B1, and C2 were obtained from samples A, B, and C through the heat-pressing method. It is found that the SAC for all the samples rises sharply with the increasing frequency, but the curves flatten after around 3500 Hz. From Figure 5(a) it can be seen that sample B exhibits the highest SAC in the low-frequency band, but after 3500 Hz sample A shows better sound absorption abilities. Meanwhile, sample C shows the lowest SAC after 1000 Hz compared to samples B and A. In Figure 5(b), sample A1 exhibits the best sound absorption ability while sample C2 shows the lowest SAC. It is found that samples with higher areal density have better sound absorption performance, and samples with lower areal density are weaker absorbers. The reason for this phenomenon can be sample thickness and areal density differences.^19–20 Nonwoven thickness is a very important factor determining the sound absorption ability. Generally, the increase of thickness results in an increase of SAC at the low-frequency range. Moreover, the sound absorption of fibrous material involves viscous losses, which convert acoustic energy into heat as the sound wave travels through the interconnected pores of fibers of the material. Thus, for high areal density samples there are more fibers involved in the viscous losses and more acoustic energy is dissipated in the form of heat energy.²⁰ In the case of similar thickness, the increase of fabric areal density leads to an increase in sound absorption performance. Based on this analysis, by comparing SAC of samples A, B, C, A1, B1, and C2, it is hard to conclude that samples produced by rotating perpendicular lapper have better sound absorption performance.
Figure 5.
SAC of samples produced by different manufacturing techniques: (a) SAC of original samples; (b) SAC of samples prepared by the heat-pressing method from original samples. The manufacturing techniques, thicknesses, and areal densities of samples are listed following the sample codes. WAVEMAKER is the rotating perpendicular lapper, STRUTO is the vibrating perpendicular lapper.

In order to investigate the effect of areal density on sound absorption performance of perpendicular-laid nonwovens, seven types of nonwoven samples with similar thicknesses are compared in Figures 6 and 7. It is found that the SAC of samples D, E, F, and G sharply increases with the increase of frequency at the whole measured band, while the curves of samples A1, B1, and C1 no longer sharply increase after 3500 Hz. Also, the sound absorption performance shows a similar trend for the samples’ areal density. This means that the sound absorption ability of similar-thickness perpendicular-laid nonwovens increases with an increase of the sample’s areal density or bulk density. As analyzed earlier, in the case of similar thickness, the increase of fabric areal density leads to an increase in sound absorption performance. This phenomenon can be seen in Figure 7; both NRC and average value of SAC increase with the increase of areal density. It is also observed that the NRC and average value of SAC have a strong correlation with areal density: the coefficients of determination (R²) are 0.987 and 0.990, respectively. It can be concluded that higher areal density gives better sound absorption ability for perpendicular-laid nonwovens.
Figure 6.
Sound absorption coefficient of nonwovens with different areal densities.

Figure 7.
Effect of areal density on sound absorption performance.

Samples C1, C2, C3, C4, and C5 were made from sample C through the heat-pressing method. As the material gets thicker, the sound absorption in the low-frequency range increases.^6, ^21–22 Sound absorption performance of samples with the same areal density but different thicknesses are shown in Figure 8. It can be seen that a sample’s sound absorption ability in low-frequency bands decreased with the reduction of the sample’s thickness, whereas the SAC increases at high-frequency bands with the reduction in thickness. This can be explained by resonance. The resonance phenomena occurs toward the low-frequency band for thicker samples.²³ It is also found that the peak values of SAC increase and shift toward the higher-frequency side with the reduction in sample thickness. But the effect of decreasing thickness on SAC peak values is limited, since the peak values no longer increase after the thickness reaches a critical value. In addition, samples C4 and C5, with thicknesses less than 16.85 mm, show a significant decrease of SAC in the 50–4500 Hz range.
Figure 8.
Sound absorption coefficient of samples with a different thickness.

The effect of thickness on the sound absorption performance is presented in Figure 9. The coefficient of determination (R²) between the NRC and thickness is 0.970, and this value is 0.073 for the average value of SAC and thickness, indicating that the NRC of perpendicular-laid nonwovens has a very strong correlation with thickness, while there is an insignificant relationship between average value of SAC and thickness. As described above, increased thickness results in SAC increases at low-frequency bands and SAC reductions at high-frequency bands. NRC was defined as a material’s sound absorption ability at low frequencies, and the average value of SAC was considered to be the sound absorption ability in the whole measurement frequency range. This may be the reason for this phenomenon.
Figure 9.
Effect of thickness on sound absorption performance.

Porosity has a strong influence on sound absorption performance of fibrous materials. Figure 10 illustrates the effect of porosity on sound absorption performance of perpendicular-laid nonwovens. It can be seen that the average value of SAC increases with increases in porosity. The average value of SAC reached peak values between 97% and 98% porosity, but the average value of SAC sharply decreases after 98% porosity. Porosity is inversely proportional to perpendicular-laid nonwoven specific airflow resistance.⁶ Lower porosity means higher specific airflow resistance, which means fiber movement rarely occurs when sound waves pass through the materials.²⁴ High porosity results in fewer fibers being involved in the viscous losses, which will decrease the sound absorption performance of the materials. The quadratic correlation between porosity and average value of SAC was calculated and is shown in Figure 10. As shown in Figure 10, porosity has a quadratic relation with the average value of SAC, with coefficient of determination (R²) being 0.81017.
Figure 10.
Effect of porosity on sound absorption performance.

Figure 11.
Effect of airflow resistivity on sound absorption performance.

Airflow resistivity is a measure of how easily air can enter a porous material and the resistance to that airflow passing through a structure. The airflow resistivity of a porous material is one of its most important defining characteristics. Once the airflow resistivity is known, a series of empirical models can be used to find the characteristic impedance and wavenumber, and thus to obtain the surface impedance and SAC. But the modeling process of determination of impedance and SAC through airflow resistivity is tedious. In order to investigate the influence of airflow resistivity on sound absorption performance in a simple way, the correlation between them is illustrated in Figure 11. It is found that below 6000 Pa·s/m², increases in airflow resistivity also cause increases in sound absorption ability. The highest value of both average value of SAC and NRC appeared at the range between 5000 and 7000 Pa·s/m² of airflow resistivity. After that, the sound absorption ability shows a decreasing trend with increasing airflow resistivity. This phenomenon is completely compatible with Zent and Long’s research.¹⁹

Compression property

The resultant compression for perpendicular-laid nonwovens is compression of the nonwoven fabric structure constructed by fibers and compression of constituent fibers themselves. In the case of perpendicular-laid nonwovens, the compression of the fibers themselves may be performed prior to the compression of the structure due to the weak bending rigidity of fibers. Fiber compression may follow the collapse of the overall structure.⁸

The compression properties of all the perpendicular-laid nonwovens are given in Table 4 and shown in Figure 12. The absorbed energy was computed by multiplying load pressure and thickness reduction of the samples. Sample C5 exhibits the highest compressional resistance at 50% and 90% thickness reduction, while sample C3 absorbs the highest energy during the compression test. In addition, sample F has the lowest compressional resistance and energy absorption of compression.
Table 4.
Compression properties of perpendicular-laid nonwovens

Sample codes 50% thickness reduction
90% thickness reduction
90% thickness reduction

Pressure (kPa) Pressure (kPa) Absorbed energy (10^–3 J)

A 1.909 5.038 321.888

A1 2.693 11.199 432.159

B 2.124 7.614 451.673

B1 3.005 11.067 469.267

C 4.298 11.954 815.708

C1 5.816 17.268 988.175

C2 7.954 20.234 1111.066

C3 10.96 31.864 1277.924

C4 12.643 35.423 1129.402

C5 15.003 50.568 1132.771

D 3.279 6.075 413.628

E 2.685 4.905 331.906

F 0.889 2.605 130.661

G 1.413 3.574 208.086

Figure 12.
Compression properties of perpendicular-laid nonwovens.

The compression curves of samples prepared by vibrating and rotating perpendicular lappers are compared in Figure 13. It can be seen that nonwovens (samples B and B1) produced by vibrating perpendicular lapper exhibit better compressional resistance compared to nonwovens (samples A and A1) prepared by rotating perpendicular lapper. From Tables 2 and 4, it can also be seen that samples B and B1 absorb more energy during compression measurement, although samples A and A1 have higher densities. This phenomenon can be explained by the different fiber orientations in these samples. Samples B and B1 have higher fiber orientation angles compared to samples A and A1 in Figure 3. As stated earlier, the nonwoven structure compression is performed prior to the compression of fibers themselves. Samples with higher initial fiber orientation angles require greater amounts of energy at the compression stage. Thus, it can be concluded that perpendicular-laid nonwovens made by STRUTO have better compression properties than WAVEMAKER nonwovens.
Figure 13.
Compression pressure of samples produced by different manufacturing techniques.

The compression properties of samples treated by the heat-pressing method are shown in Figure 14. It is observed that sample C exhibits the lowest compressional resistance. The compressional resistance increases with decreasing thickness. It is also found that the pressure sharply rises with greater thickness reduction during compression measurement of samples C3, C4, and C5, while the compressional resistance curves of samples C, C1, and C2 remain relatively flat before 75% thickness reduction.
Figure 14.
Compression curves of samples treated by heat-pressing.

Samples B1, D, E, F, and G, with similar thicknesses, were prepared by vibrating perpendicular lapper. These samples were chosen to investigate the effect of density on the compression property. From Figure 15 it can be seen that compressional resistance increases with increasing density. The compressional resistance of perpendicular-laid nonwovens has a strong correlation with density, with a correlation coefficient of 0.954, which means the perpendicular-laid nonwovens with higher density usually exhibit better compression properties.
Figure 15.
Effect of density on the compression property.

The cross-sectional microscopic images of samples A, B, and C in different compression states are shown in Figure 16. ImageJ software was used to obtain fiber orientation angles based on cross-sectional microscopic pictures of nonwovens. Each average orientation angle was calculated from 10 measurements. Figure 17 describes the effect of thickness reduction on the fiber orientation angle. From these two graphs, it is found that the fiber orientation angle decreases with the increase of thickness reduction. Also, samples B and C have higher initial fiber orientation angles compared to sample A; The initial fiber orientation angle of samples A, B, and C are 56.07°, 87.26°, and 79.09°, respectively. Apparently, the majority of fibers are oriented toward the same direction as thickness reduction increases. Shearing deformation happens in the compression process, as can be seen in Figure 16. At the beginning, the compression is applied along the fiber axis for perpendicular-laid nonwovens. Some fibers stay in stable equilibrium, and others fail by buckling because of the irregularity of fiber length and non-uniformity of fiber position in the nonwoven structure. With increasing compression load, the fibers yield to higher stress by buckling.
Figure 16.
Fiber orientation of perpendicular-laid nonwovens under different compression state.

Figure 17.
Effect of thickness reduction on fiber orientation angle.

The effect of compression load on fiber orientation angle is shown in Figure 18. It can be seen that the fiber orientation angle sharply decreases with increases in compression load, but the slopes flatten at the end of the compression cycle. It is also observed that samples B and C have higher orientation angles compared to sample A under the same compression load. This indicates that perpendicular-laid nonwovens with higher fiber orientation angles exhibit better compression resistance.
Figure 18.
Effect of compression load on fiber orientation angle.

Conclusion

Seven types of perpendicular-laid nonwoven fabrics were produced by two different manufacturing techniques: vibrating and rotating perpendicular lappers. The heat-pressing method was employed to form samples with varying thicknesses and areal densities. Sound absorption properties were measured by Brüel and Kjær Type 4206 Impedance Tube. TIRATEST 2300 was used for compression measurements. The following conclusions can be drawn from this study:
Two different manufacturing techniques, vibration and rotating perpendicular lapper, have insignificant influence on sound absorption performance of nonwoven fabrics. However, samples prepared by vibrating perpendicular lapper exhibit better compression properties compared to samples prepared by the other technique.

Both thickness and areal density have significant influence on sound absorption performance of perpendicular-laid nonwovens. The increase of areal density results in improvement of sound absorption for nonwovens. Whereas the increase of thickness can significantly improve SAC in the low-frequency range, the SAC will decrease in the high-frequency range.

For perpendicular-laid nonwoven fabrics, there is a quadratic relationship between porosity and sound absorption ability, with coefficient of determination being 0.81. Regardless of thickness and density, samples exhibit the best sound absorption performance with airflow resistivity around 6000 Pa·s/m².

The compressional resistance of perpendicular-laid nonwovens has a strong correlation with density; nonwovens with higher density usually exhibit better compression properties.

Shearing deformation occurs during the compression process in perpendicular-laid nonwovens. Fiber orientation angle decreases with greater thickness reduction. Samples with higher fiber orientation angles exhibit better compression properties.

Samples	Fiber content	WA of diameter (µm)	WA of fineness (dtex)	Manufacturing techniques	Thickness (mm)	Areal density (g/m²)	Porosity (%)	Bulk density (kg/m³)	Airflow resistivity (Pa·s/m²)
A	30% i 45% ii 25% iii	20.96	3.08	WAVEMAKER	24.09	507.5	97.60	21.07	5757.05
A1	STRUTO	20.76	507.5	97.21	24.45	7319.58
B	STRUTO	28.36	478.3	98.08	16.87	4828.55
B1	STRUTO	20.32	478.3	97.31	23.54	7498.51
C	STRUTO	27.48	465.2	98.07	16.93	4108.94
C1	STRUTO	23.87	465.2	97.78	19.49	5337.58
C2	STRUTO	20.69	465.2	97.44	22.48	7029.82
C3	STRUTO	16.85	465.2	96.85	27.61	10,181.53
C4	STRUTO	13.31	465.2	96.01	34.95	12,868.69
C5	STRUTO	10.43	465.2	94.91	44.60	20,474.6
D	70% iv 30% v	23.21	5.35	STRUTO	20.82	335.7	98.68	16.12	2208.3
E	STRUTO	19.85	317.5	98.69	15.99	2031.52
F	STRUTO	20.12	198.6	99.19	9.87	1024.4
G	STRUTO	20.66	259.3	98.97	12.55	1727.8

Fiber code	Types of PET	Diameter (µm)	Fineness (dtex)	Staple length (mm)	Ratio of core and sheath
i	Hollow PET	28.64	4.92	70.00	–
ii	PET	15.25	1.76	50.00	–
iii	Bicomponent PET	22.02	3.24	50.00	3:1
iv	PET	26.91	6.70	57.00	–
v	Bicomponent PET	14.58	2.20	38.00	3:1

Sample codes	$\bar{α}$	NRC
A	0.580	0.009	0.254	0.003
A1	0.581	0.007	0.239	0.004
B	0.588	0.007	0.281	0.003
B1	0.545	0.012	0.228	0.005
C	0.523	0.006	0.255	0.005
C1	0.544	0.006	0.242	0.002
C2	0.521	0.005	0.216	0.003
C3	0.528	0.008	0.191	0.003
C4	0.433	0.015	0.147	0.005
C5	0.444	0.028	0.136	0.006
D	0.315	0.028	0.142	0.010
E	0.290	0.017	0.129	0.006
F	0.201	0.007	0.092	0.004
G	0.242	0.014	0.110	0.005

Sample codes	50% thickness reduction	90% thickness reduction	90% thickness reduction
A	1.909	5.038	321.888
A1	2.693	11.199	432.159
B	2.124	7.614	451.673
B1	3.005	11.067	469.267
C	4.298	11.954	815.708
C1	5.816	17.268	988.175
C2	7.954	20.234	1111.066
C3	10.96	31.864	1277.924
C4	12.643	35.423	1129.402
C5	15.003	50.568	1132.771
D	3.279	6.075	413.628
E	2.685	4.905	331.906
F	0.889	2.605	130.661
G	1.413	3.574	208.086

Footnotes

Declaration of conflicting interests

The authors declare 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 research project of the Student Grant Competition of Technical University of Liberec no. 21197, granted by the Ministry of Education Youth and Sports of the Czech Republic

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Sample codes	$\bar{α}$		NRC
Sample codes	Mean value	SD	Mean value	SD
A	0.580	0.009	0.254	0.003
A1	0.581	0.007	0.239	0.004
B	0.588	0.007	0.281	0.003
B1	0.545	0.012	0.228	0.005
C	0.523	0.006	0.255	0.005
C1	0.544	0.006	0.242	0.002
C2	0.521	0.005	0.216	0.003
C3	0.528	0.008	0.191	0.003
C4	0.433	0.015	0.147	0.005
C5	0.444	0.028	0.136	0.006
D	0.315	0.028	0.142	0.010
E	0.290	0.017	0.129	0.006
F	0.201	0.007	0.092	0.004
G	0.242	0.014	0.110	0.005

Sample codes	50% thickness reduction	90% thickness reduction	90% thickness reduction
Sample codes	Pressure (kPa)	Pressure (kPa)	Absorbed energy (10^–3 J)
A	1.909	5.038	321.888
A1	2.693	11.199	432.159
B	2.124	7.614	451.673
B1	3.005	11.067	469.267
C	4.298	11.954	815.708
C1	5.816	17.268	988.175
C2	7.954	20.234	1111.066
C3	10.96	31.864	1277.924
C4	12.643	35.423	1129.402
C5	15.003	50.568	1132.771
D	3.279	6.075	413.628
E	2.685	4.905	331.906
F	0.889	2.605	130.661
G	1.413	3.574	208.086