Acoustic absorption evaluation of high-modulus puncture resistance composites made by recycled selvages

Abstract

Glass fabric and Kevlar fabric reinforced recycled nonwovens were produced to form G-Ply and K-Ply high-modulus composites. Three types of composites — non-hot-pressed (N-), hot-pressed (H-) and laminated (L-) composites, were respectively combined by various plies, including non-treated G-Ply or K-Ply, hot-pressed G-Ply or K-Ply, and laminated-together G-Ply or K-Ply. Their acoustic absorption and puncture resistance were respectively discussed in terms of plies number, fabric interlayer, ply sequence and backed air thickness. It is found that the maximum acoustic absorption coefficient occurs at lower frequencies as increase of layers. The K-Ply N-composite shows higher porous acoustic absorption coefficient but lower puncture resistance as compared to G-Ply at more than three layers. The backed air improves acoustic absorption of N-composite almost at entire frequency and H-, L-composites at certain vibrating frequency. For combinations of one K-Ply and four G-Ply, the K-Ply sequence has a significant effect on acoustic absorption of N-composite and L-composite.

Keywords

needle-punched nonwoven high-modulus fabric recycled selvages acoustic absorption puncture resistance

Introduction

As economy and industrialization develop, noise pollution has become the third pollution following air pollution and water pollution. Noise produces many harmful effects to the human nervous system, such as sleeplessness, exhaustion and hypomnesis. Furthermore, digestive problems can also be caused by long-term noise. In addition, some dangerous signals are difficult to perceive and thus cause accidents due to the shielding effect of noise.¹ The use of acoustic-absorbing materials is one of the present effective noise controls in automobiles, the manufacturing environment, building compartments and equipments. In addition, flexible compartments often suffer from penetration due to sharp weapons, such as sharp spikes, broken glass and needles especially in violent places, such as prisons, police stations and mechanical assembling workshops.

Acoustic-absorbing materials include porous acoustic-absorbing materials and resonance acoustic-absorbing materials in accordance with absorption mechanisms.² The common porous absorbing materials comprise glass wool, foam, mineral fiber and their related composites. Even though such materials have excellent acoustic absorption, they bring about environment pollution and then harm to human health. Moreover, these materials have a high acoustic absorption coefficient at high frequencies, but a low coefficient at low and medium frequencies;³ their mechanical properties alone are not enough to defend against noise pollution. Resonance absorbing materials show a high absorption coefficient around the vibrating frequency; thus, they are appropriate for absorbing noises at low and medium frequencies.³ Therefore, it is clear that the independent porous or resonance absorbing materials have no ability for wide-frequency noise absorption. In view of their puncture resistance property, composite materials are a good choice for improvement of both acoustic absorption and puncture resistance.

In 1995, Narang⁴ conducted a numerical model of acoustic absorption related to fiber volume fraction for predicting the porous absorbing property. Afterwards, many researches were devoted to improving porous acoustic absorption at low and medium frequencies. Lou et al.⁵ and Tai et al.⁶ successfully improved acoustic absorption by increasing fiber density and thickness at low and medium frequencies. Lin et al.⁷^–¹⁰ prepared nonwoven/polyurethane (PU) foam, nonwoven/PU foam/thermoplastic polyurethane (TPU) grid and nonwoven/TPU film, and found that resonance between nonwoven and PU foam or TPU film or grid, and between PU foam and TPU grid improves the acoustic absorption at low and medium frequencies; in addition, they found that multiple needle-punched nonwovens inserted with polypropylene (PP) selvages improves the acoustic absorption. In 2009, Ersoy and Küçük¹¹ showed that using backing with single cotton cloth, the acoustic absorption of PET/PP nonwoven and tea-leaf-fiber were both enhanced at frequencies of 500–4500 Hz and 500–6300 Hz, respectively. The effect of layer sequence on acoustic absorption was discussed by Nazire et al.¹²

In puncture resistance studies, fibers used have high shear strength, high impact strength and high modulus, such as ultrahigh molecular weight polyethylene (UHMWPE), p-aramid, polybutylece terephthalate (PBT) and poly-p-phenylenebenzobisthiazole (PBO). Many studies emphasize Kevlar fiber and its products, and the puncture resistance property is discussed in relation to number of layers, thickness, density, modulus and structure.¹³^–¹⁶ In addition, by means of thermoplastic-impregnate, it was found that the puncture resistance of aramid fabric significantly improved due to windowing reduction in the fabric.¹⁷

In this paper, we intended to improve acoustic properties in wide-range frequencies; high-modulus fabric-reinforced nonwovens were prepared by the needle-punch process to meet the demand for puncture resistance by reduced windowing. Afterwards, three types of composites were prepared to review the effects of hot-pressing and interlaminar mobilization on acoustic absorption and puncture resistance. In addition, parameters including the number of layers, the fabric interlayer, ply sequence and backed air gap were studied as related to these two properties.

Experimental details

Experimental materials

Kevlar fibers taken from recycled unidirectional selvages (DuPont Company, America) had a length of 50–60 mm. The selvages are mainly composed of 2820 Denier (D) K129 multifilaments. Nylon 6 staple fibers with tenacity of 10 g/d (fineness: 6 Denier, length: 64 mm) were provided by Taiwan Chemical Fiber Co., Ltd, Taiwan. Sheath-core low-T_m polyester fibers (Huvis Chemical Fiber Co., South Korea) were had a fineness of 4 D and length of 51 mm; their sheath was low-melting-point polyester with a melting point of 110°C, and their core was general polyester with a melting point of 265°C. The low-T_m polyester fibers formed spot-bonded nonwoven fabrics with higher strength and softer texture. The 0.31-mm-thick KN2600N1 glass fabric (Jinsor-Tech Industrial Corp., Taiwan), composed of 1100 Denier glass fibers, had a weight of 328 g/m² and density of 34 ends × 26 picks/inch. The 17 ends × 17 picks/inch 0.31-mm-thick EK10 Kevlar fabric composed of 1500 Denier Kevlar fibers with a weight of 227 g/m² was supplied by Formosa Taffeta Co., Ltd, Taiwan.

Composite preparation

Nonwoven fabrics with a weight of 150 ± 20 g/m² were manufactured at the needle-punching density of 100 needles/cm². They were composed of 20 wt% Kevlar fibers, 50 wt% nylon 6 fibers and 30 wt% low-T_m polyester fibers. Afterwards, high-modulus glass fabric and Kevlar fabric were respectively inserted between double nonwoven fabrics, forming glass-fabric-reinforced ply (G-Ply) and Kevlar-fabric-reinforced ply (K-Ply) at the needle-punching density of 100 needles/cm², as shown in Figures 1(a) and (b).

Different layers (from one layer to five layers) of G-Ply and K-Ply were used to form three types of composites – the N-composite, H-composite and L-composite. The N-composite was directly composed of different layers of non-treated G-Ply and K-Ply during testing. The H-composite was composed of different layers of G-ply or K-ply, respectively, hot-pressed by a twin-roller hot-presser (0.5 m/min velocity, 1.5 mm distance, 160°C) during testing. The L-composite was performed by different layers of G-ply and K-ply laminated together using a flat hot-presser (5 min, 10 mm distance). The structural parameters of single G-Ply and K-Ply in different types of composites are given in Table 1.

Table 1.

Parameters of single-layer glass-fabric-reinforced ply (G-Ply) and Kevlar-fabric-reinforced ply (K-Ply) in three types of composites

Composites	Single-layer thickness (mm)	Area density (g/m²)	Volume density (kg/m³)
G-Ply	2.53 ± 0.10	656 ± 12	259.39 ± 8.50
Hot-pressed G-Ply	2.06 ± 0.08	661 ± 12	320.8 ± 12.51
Laminated G-Ply	2.02 ± 0.10	660 ± 11	324.28 ± 13.09
K-Ply	2.42 ± 0.15	568 ± 10	244.58 ± 13.25
Hot-pressed K-Ply	2.01 ± 0.07	560 ± 15	274.67 ± 6.76
Laminated K-Ply	2.08 ± 0.06	562 ± 15	287.9 ± 8.48

Because composites that are too thick could bring inconvenient and uncomfortable results, the composites at most have five layers. Moreover, only single K-Ply was stacked for five-layer composites in view of the lower cost. The sequence of single K-Ply changed in five-layer composites, forming 1K/4G, 1G/1K/3G, 2 G/1K/2G, 3G/1K/1G and 4G/1K composites. “G” represents G-ply, “K” represents K-ply, and the digit preceding G or K refers to the number of plies. Figure 1(c) shows the structural diagram of the 4G/1K composite.

Testing methods

Acoustic absorption property

According to ASTM E1050-10, the acoustic absorption coefficient was tested by a 40-mm-diameter two-microphone impedance tube, as shown in Figure 2, at a frequency range of 128–4000 Hz at a relative humidity of 65 ± 2% and room temperature of 20 ± 1%. Sound waves were incident and then reflected by the rigid wall. The normal acoustic absorption coefficient α was defined as

α = \frac{E_{a}}{E_{i}} = \frac{E_{i} - E_{r}}{E_{i}}

(1)

where E_a is absorbing acoustic energy, E_i is incident acoustic energy and E_r is reflected acoustic energy; greater α represents better acoustic absorption.

Puncture test

Based on ASTM F1342-05, the puncture test was conducted using an Instron 5566 universal tester. Probe A (2.03-mm diameter, rounded tip radius of 2.03 mm and conical angle of 26°) was fixed on the load cell and driven at a constant rate of 508 mm/min. The samples were in the size of 100 mm × 100 mm and were placed between two circular plates each with a 10 mm-diameter hole in the center. Five specimens were replicated for definitive puncture resistance and its standard deviation.

Results and discussion

The stereomicroscope observations of high-modulus composites

The N-composite was composed of non-treated G-Ply, including a glass fabric interlayer and nonwovens, as shown in Figure 3. After needle-punching, the vertical fiber tuft was formed to bond the fabric and nonwoven webs. Coupled with hot-pressing, the fiber web was spot-bonded to reinforce the nonwovens, and thus influenced the acoustic and puncture resistance property. Therefore, H- and L-composites were prepared in contrast to the N-composite. The H-composite, composed of three layers of hot-pressed G-Ply, is shown in Figure 4. The thermobonding points due to low-T_m polyester are successfully observed in puncture damage, as displayed in Figure 5.

Acoustic absorption property of the high-modulus composite

Effect of number of layers on acoustic absorption

Figure 6 shows the acoustic absorptions of N-composites from a single layer to five layers of G-Ply. In almost the entire frequency range, the absorption coefficient firstly increases to the maximum at lower frequencies and then changes to be minor (at least two layers) at higher frequencies, which shows a similar sound-absorbing property to porous sound-absorbing materials.^6,11,12,18^–²¹ That gradual increase of absorption coefficient is attributed to the fact that the sound pressure makes the air in the composite nonwovens vibrate, which then results in frictional resistance between the nonwovens, and an air gap when the sound strikes the face of the nonwovens. To overcome this friction resistance, the incident sound energy is consumed as heat is dissipated in the surrounding air.²⁰ Moreover, with the addition of G-Ply (at least two layers), the maximum absorption coefficient is produced at a lower frequency; however, the maximum absorption coefficient becomes lower. For five-layer G-Ply, its maximum acoustic absorption coefficient decreases to 0.491. This is because as the number of layers increases, the amount of glass fabric grows accordingly. Thus, more high-frequency acoustic waves are reflected back and, simultaneously, the acoustic energy elsewhere is conversely heightened in the closed impedance tube. Subsequently, the absorption coefficient at high frequency, that is, where the maximum coefficient occurs, falls significantly with the addition of G-Plies.

Figure 7 displays the acoustic-absorbing property of H-composites with different layers of hot-pressed G-Ply at frequencies from 128 to 4000 Hz. From a single layer to three layers, the acoustic absorption coefficient steadily rises up at entire frequency range; nevertheless, a relatively wider absorption peak occurs at about 1000 Hz at four or five layers. Increasing from a single layer to three layers, the absorption coefficient becomes higher over the entire frequency range; however, from four layers to five layers, the absorption peak occurs at lower frequencies.

Figure 8 reveals the acoustic absorption coefficient of L-composites with different layers. At a single layer or two layers, the acoustic absorption profiles present the same as that of fluffy porous sound-absorbing materials. When increasing to four or five layers, the absorption coefficient yields a peak before reaching an almost plateau. This is due to a combination of sound-absorbing effects of porous sound-absorbing material and panel resonance.

As shown in Figures 6 –8, it is found that the layer increase (more than one layer), namely thickness increase, results in a higher absorption coefficient at low and medium frequencies but a lower coefficient at high frequency, irrespective of types of composites. That is because the low-frequency and medium-frequency sound waves have a longer wavelength, and generate diffraction attenuation when encountering with composite obstacles. Due to this, the acoustic energy would be decreased more after thickening composites, which results in a higher absorption coefficient. However, sound waves at high frequency are easily reflected on the surface of glass fiber. Thus, more sound energy is accumulated in the impedance tube owing to the multilayer glass fabric, and the absorption coefficient reduces.

Comparing N-, H- and L-composites, the N-composite presents a higher maximum absorption coefficient than H-, and L-composites at the same number of layers; however, at low frequency and high frequency, the H-composite has better acoustic-absorbing performance than N- and L-composites at four and five layers. In general, the maximum absorption coefficient occurs for the two-layer N-composite, as high as 0.77, and with four layers the absorption coefficient of the H-composite is above 0.5 at 1000 Hz. In addition, the H-composite has better acoustic-absorbing property than the L-composite with the same number of layers. Because separated inter-plies vibrate more intensely than integrated plies, the H-composite consumes more acoustic energy than the L-composite.

Effect of fabric interlayer on acoustic absorption

Figure 9 shows acoustic absorption coefficients of N-composites with multilayer K-Ply. With an increasing number of layers, the maximum acoustic absorption coefficient occurs at lower frequencies. For five layers of K-Ply, the absorption coefficient reaches about 0.7 at 1000 Hz, higher than the same layers of G-Ply, which is due to higher flow resistivity resulting from lower fiber density, based on the equation as follows:²²

r_{0} d^{2} ρ^{- 1.5} = C

(2)

where r₀ is flow resistivity, d is fiber diameter, ρ is fiber density and C is a constant.

In addition, all maximum acoustic absorption coefficients reach above 0.7 for different numbers of layers of K-Ply. With additional layers, the maximum absorption shows a minor decrease, showing that multilayer Kevlar fabric only reflects few acoustic waves.

Figure 10 shows comparative acoustic absorption coefficients of the N-composite, H-composite and L-composite, respectively, composed of five-layer K-Ply. It is found that the N-composite has a higher absorption coefficient than the other two composites when the frequency is below 800 Hz and above 1000 Hz. However, in the range from 800 to 1000 Hz, the H-composite absorption is a little better than that of the N-composite, showing vibration between hot-pressed plies. In addition, the L-composite acoustic property is worst of all. This is due to the hot-pressing effect. The low-melting polyester after hot-pressing bonds the fibers in nonwovens, blocking the air way and thus reflecting the acoustic energy on to the face.

Effect of ply sequence on acoustic absorption

By comparative study of Figures 6 –8 and Figures 9 and 10, it is found that the same layer of K-Ply shows better acoustic absorption than that of G-Ply. Hence, due to the higher cost of K-Ply, only a single layer of K-Ply was inserted between four layers of G-Ply. Moreover, lower-density K-Ply sequences produce various transmitting paths of sound waves, impacting the acoustic property of the composites.

Figures 11 –13 show comparative acoustic absorption coefficients of N-composites, H-composites and L-composites with different K-Ply sequences. As shown in Figure 11, the 1K/4G, herein single K-Ply on the surface, displays the optimum acoustic absorption over the entire testing frequency. With a different K-Ply sequence, the absorption coefficients of the H-composite reveal almost the same at below 2250 Hz, as shown in Figure 12; however, the 1K/4G H-composite shows higher acoustic absorption than the other at above 2250 Hz. Nevertheless, it is interesting that the 3G/1K/1G L-composite has the optimum acoustic absorption, as shown in Figure 13. When K-Ply lies in the second and the fourth places, the acoustic absorption coefficients firstly increase and then plateau; however, when K-Ply is located at the first, third and fifth places, their absorption of the L-composite shows a sharp peak before reaching a plateau. That indicates that at uneven K-Ply, the vibration due to composites themselves consumes the sound energy as heat; the L-composite at even K-Ply produces boundary layer losses to offset sound energy due to the relative acoustic speed between air molecular and porous composites. Generally, the 1K/4G N- and H-composites and 3G/1K/1G L-composites have the maximum acoustic absorption property.

It is found from Figure 13 that the absorption coefficients of 10 mm-thickness 4G/1K and 3G/1K/1G L-composites is only most about 0.25 at 1000 Hz, meaning that only 25% of acoustic energy is being absorbed. That demonstrates that a 10 mm -thickness L-composite is insufficient to improve the acoustic-absorbing property at medium frequency. This is due to a sole porous absorbing structure. When the thickness of 4G/1K and 3G/1K/1G L-composites both increase to 20 mm, their maximum absorption coefficient reaches up to 0.614 at 880 Hz and 0.635 at 992 Hz; at a thickness of 30 mm, maximum absorption occurs at 0.584 at 880 Hz and 0.617 at 1024 Hz, as shown in Figure 14. This indicates that the effect of the thickness increase on the absorption coefficient achieves the maximum at up to 20 mm thickness. In addition, a sharp peak is found on the absorption profiles, which shows that the panel resonance has become the main absorption mechanism.

Effect of backed air thickness on acoustic absorption

Figures 15 –17 show acoustic absorption coefficients of the 1K/4G N-composite, H-composite and L-composite with 10, 20 and 30 mm-thick backed air gaps between the composites and the rigid wall. As shown in Figure 15, the acoustic absorption property improves over all frequencies when the backed air gap is thickened from 10 to 30 mm, whereas the backed air gap is confined to acoustic absorption when increasing from 30 to 40 mm.

When backing with a 30 mm-thick air gap, the maximum absorption coefficient reaches 0.77 at high frequency and above 0.5 from 500 Hz, increasing by an absorption coefficient of about 0.1 at high frequency and 0.2 at 500 Hz, compared to that with non-backed air. This is because sound waves are reflected back and forth between the composites and rigid wall, damping the sound energy and then increasing additional acoustic absorption.

Figure 16 shows the acoustic absorption of the 1K/4G H-composite with a backed air gap. The acoustic absorption profiles show a sharp peaks at 648 Hz (10 mm thick), 676 Hz (20 mm thick) and 600 Hz (30 mm thick), which characterize the resonance panel vibrating at certain frequencies when sound waves are incident on the surface of composites.²⁰ Moreover, the 1K/4G H-composite with a 30 mm thick air gap has the maximum absorption coefficient of 0.71 at 600 Hz. From Figure 17, the maximum absorption coefficient of the 1K/4G L-composite is lower than that of H-composite shown in Figure 16, which reveals a weaker panel resonance when transmitting the sound waves in the composites. In addition, it is found that after being backed with the air gap, the 3G/1K/1G L-composite acoustic property displays a panel resonance characteristic rather than the porous sound-absorbing feature shown in Figure 13, which is indicated in Figure 18.

Above all, the air gap thickness increase is conductive to improving the acoustic-absorbing property over almost the entire frequency for the porous acoustic-absorbing N-composite. However, the increase in the backed air gap H-composite only improves the absorption coefficient at the vibrating frequency. However, for the L-composite, increasing the backed air gap shows almost no improvement. Moreover, the increase of backed air thickness is amount to the increase of composites thickness, compared with Figure 11 and Figures 17 and 18.

Puncture resistance property of the high-modulus composite

Effect of number of layers on puncture resistance

Figure 19 shows puncture resistances of the N-composite, H-composite and L-composite with layers of G-Ply from one to five layers. With the addition of layers, the three composites yield a gradual increase in puncture resistances. The N-composite reveals an almost steady puncture resistance at four layers of G-Ply. Moreover, the puncture resistance of the N-composite is higher than that of the other composites. The five layers the G-Ply N-composite exhibit the highest puncture resistance, reaching a mean value of 354.28 N, which is due to the thickness increase.

Effect of fabric interlayer on puncture resistance

Figure 20 shows puncture resistances of N-composites, H-composites and L-composites from one to five layers of K-Ply. As K-Ply increases, the puncture resistances of the three different composites are improved; furthermore, from three layers, it has a slight increase for the N-composite. In addition, the N-composite presents the highest puncture resistance compared to the others. However, it also found that K-Ply N-composite exhibits lower puncture resistance than the G-Ply N-composite when exceeding three layers, as shown by comparing Figures 19 and 20. This is due to the lower density of the Kevlar fabric interlayer producing a smaller friction and pressure effect on probes for resisting against puncture energy.²³

Effect of ply sequence on puncture resistance

The Kevlar fabric interlayer has high modulus and high shear strength, thus K-Ply passivates the testing probe and further improves the puncture resistance of composites. The K-Ply sequence impacts the passivation effect to probes.

Figure 21 shows puncture resistances of five-layer N-composites, H-composites and L-composites with different K-Ply sequences. In comparison, the puncture resistances of the H-composite and L-composite are lower than those of the N-composite. This is due to finer low-T_m polyester fiber and thinner composites after hot-pressing, which expanded the inter-fiber space and thus decreased puncture resistance.¹⁷ With different single K-Ply sequences, the puncture resistance of the N-composite and H-composite displays no significant difference among them. However, the 1K/4G L-composite has the maximum puncture resistance. This means that the K-Ply surface plays a significant passivation role to probes for higher-modulus composites. Obviously, the standard deviation of puncture resistance of the N-composite is larger than that of the other composites, which is attributed to fiber slippage when suffering from puncture force.

Conclusions

Recycled Kevlar nonwoven reinforced with Kevlar fabric and glass fabric was produced for use as acoustic absorption composites, coupled with excellent puncture resistance. The non-hot-pressed composites have porous sound-absorbing character, and friction resistance to resist against probes in puncture penetration; the interlaminar mobilization produces a panel resonance character for hot-pressed K-Ply and G-Ply, and interlayer shear strength, in particular for hot-pressed K-Ply.

With increasing of G-Ply and K-Ply layers, the maximum absorption coefficient moves to the lower frequencies, and the puncture resistances are improved. Moreover, the 5K N-composite has better acoustic absorption as compared with the 5G composite, reaching about 0.7 at 1000 Hz. The four-layer G-Ply N-composite exhibits higher puncture resistance, reaching a mean value of 341.17 N. With a different K-Ply sequence, the 1K/4G N-composite shows the best acoustic absorption coefficient of 0.68 at 1120 Hz; the 3G/1K/1G N-composite has the optimum puncture resistance of 391.005 N. After backing with a 30 mm-thick air gap, the maximum absorption coefficient of the 1K/4G N-composite reaches 0.77 at high frequency, and it exceeds 0.5 from a frequency of 500 Hz; that of the 1K/4G H-composite achieves 0.71 at 600 Hz frequency. According to this study, suitable structures and compositions of composites can be chosen to satisfy the demand for acoustic absorption and puncture resistance in materials. In the following study, the additional nonwoven fabrics will be combined in the resulting composites, which is intended to improve the acoustic property and puncture resistance after hot-pressing.

Footnotes

Funding

The authors are grateful to the Laboratory of Fiber Application and Manufacturing, Feng Chia University, for providing research materials, laboratory equipment and financial support (NSC100-2621-M-166-001).

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