Preparation and sound absorption properties of multi-layer composites formed by waste polyphenylene sulfide filters

Abstract

Composite materials with multi-layer structure were prepared by combining polyurethane film and waste polyphenylene sulfide filters by hot melting, which could be used as sound-absorbing materials to contribute to the noise control and reutilization of industrial waste. The structure, mechanical property, pore size distribution and stability of waste polyphenylene sulfide filters were investigated and could directly affect the quality of recycled products. The effects of layer numbers, layering sequence and polyurethane film thickness on sound absorption and insulation properties were also analyzed using an impedance tube absorption test system. The results showed that multi-layer materials had a better sound absorption effect than single-layer polyphenylene sulfide filters at low frequency. The sound absorption coefficients of the single-layer sample was only 0.081 at 1000 Hz, while the sound absorption coefficients of the five-layer sample could reach 0.26. The sound absorption properties of multi-layer materials were slightly affected by layering sequence, but polyurethane film thickness had a significant effect on sound absorption properties. The α of the SUS1 and USU1 samples were 0.151 and 0.170, while the α of the SUS3 and USU3 samples were only 0.076 and 0.056, respectively. Meanwhile, the sound insulation properties of multi-layer materials could be significantly influenced by the polyurethane film position. The sound transmission loss of the SUSUSU3 sample was just 32.83 dB, whereas that of the USUSU3 sample could reach 47.94 dB. Moreover, when polyurethane film was located on the sound facing side of multi-layer materials, the increase in polyurethane film thickness could cause the average sound reduction index of the USUSU sample to increase from 28.76 dB to 47.94 dB.

Keywords

Sound absorption polyphenylene sulfide composite materials waste filters sound insulation

The air pollutants discharged by industry are mainly characterized by high-temperature dust. The main control measures for air pollutants are emission reduction, and the emission should also be strictly filtered. Currently, the bag-type dust removal system is much more effective and economical than other dust removal systems, and has been widely used in the high-temperature flue gas filtration of waste incineration plants, steel plants, cement plants and coal-fired power plants. Filter bags are the key material of the bag dust removal system, and their performance determines the dust emission concentration and energy consumption.^1–5 Polyphenylene sulfide (PPS) fiber is one of the most widely used thermoplastic fibers in the field of high-temperature flue gas filtration due to its good mechanical properties, outstanding thermal stability and excellent flame retardance. The filter bags formed by PPS needle-punched nonwovens have also become the preferred filter materials for high-temperature flue gas filtration.^6–10

With the upgrade of flue gas emission standards, the quantity demand for PPS filter bags will greatly increase, and the replacement of waste PPS filter bags will also greatly increase. The design service life of PPS filters is usually 24∼48 months based on the difference in preparation technology and operating conditions. Due to the application of pulse dust-cleaning technology, the PPS filter bags will not be changed during the designed service life to reduce the interference with normal production, unless only a few of the filter bags are damaged to affect the filtering effect. China consumes more than 60 million filter bags every year, which is equivalent to 200 million m³ filter material and more than 100,000 t fibers. More than 50% of the filter bags are made of PPS fibers. Therefore, the amount of waste PPS filter bags is huge every year, and recycling PPS filter bag is also urgent.

However, most of the waste PPS filter bags are currently disposed of by incineration or landfill.^11–13 A large quantity of smoke, acid or alkaline gas and carcinogens can be produced during the incineration process of waste PPS filter bags, which can cause serious harm to air quality. Moreover, the dust particles, acid and alkaline substances and carcinogens contained in the waste PPS filter bags can also cause pollution of soil and groundwater and occupy plenty of land through the landfill treatment.¹⁴ These treatment methods not only waste resources but also cause serious damage to the environment. The most important problem is that PPS fibers cannot be recycled. For commercial PPS staple fibers, the price is 140,000∼180,000 yuan/t, while the price of PPS filament can reach 200,000∼300,000 yuan/t. Therefore, the price of new PPS needle-punched nonwovens, which are the raw materials of filter bags, can reach 50∼80 yuan/m² due to the difference in gram weight. However, the recovery price of waste PPS filter bags composed of pure PPS fibers is only 15∼20 yuan/t, and the waste PPS filter bags composed of PPS fibers and other fibers are hardly recycled and even require payment for disposal. This is due to the difficulty of recycling PPS filter bags and the immature dust treatment technology. Therefore, it is of great social significance and economic value to recycle waste PPS fibers and filter bags.

There are a few studies on the recycling of waste PPS fiber and filter bags. Currently, the commonly used recycling methods can be roughly divided into chemical recovery methods and physical recovery methods.^15–19 The chemical recovery methods refer to the PPS macromolecular chains being dissolved or decomposed using a suitable organic solvent. The precipitates of high polymer or polymer monomer are then separated and reused.^15,19 However, the harsh reaction conditions, complex process and high recovery cost limit the application of the chemical recovery method, hence the chemical recovery methods are still in the laboratory stage.

Physical recovery methods refer to waste PPS filter bags being generated into the initial state and reusing them by mechanical processing. Currently, they can be divided into mechanical pulverization, melt reprocessing, fiber disassembly and direct utilization.^14,16–18 Mechanical pulverization and melt reprocessing methods have been partially applied because of their simple process and low cost. However, these methods have strict requirements on the dust removal process, which can directly affect the quality of recycled products. Moreover, the dust removal process can produce a large amount of sewage and sludge which requires further treatment. The recovery products also have low value and a narrow application field, and the structure and performance of PPS fibers and nonwovens cannot be used. In contrast, fiber disassembly and direct utilization methods have low requirements on dust removal, and the structure and performance of PPS fibers and nonwovens can be fully utilized. However, the application fields are still limited due to the morphology of recycled products.

Over the past few decades, the direct utilization method of waste PPS filter bags has not been thoroughly studied due to the primitive form of nonwovens, which limits the forms and application field of recycled products. The studies about the morphology and properties of waste PPS filter bags have also not been concerned. However, the PPS fibers in the filter bags are of flexible entanglement to form a three-dimensional stereoscopic structure. A large number of small interconnected internal voids can be formed inside the waste PPS filter bags. Therefore, waste PPS filter bags can be attributed to the typical porous materials. Meanwhile, porous materials can be used as efficient sound-absorbing materials because they contain many interconnected small voids and pathways which can be connected with the outside through the surface. Therefore, the flowing air repeatedly rubs against the hole wall to deplete sound energy to achieve a sound-absorbing effect by thermal loss and viscous loss.^20–23 Moreover, recycled conventional nonwovens have been used as ecological alternatives in the automotive industry due to the excellent sound absorption at medium and high frequencies. Therefore, it can be predicted that waste PPS filter bags can be directly used to prepare porous sound-absorbing materials due to their three-dimensional porous structure.

In this paper, the structure and properties of waste PPS filter bags are investigated to evaluate the recovery availability by the direct utilization method. Then the feasibility of waste PPS filter bags as porous sound-absorbing materials is first explored in this research. Therefore, the sound absorption property of multi-layer sound-absorbing composites composed of waste PPS filter bags and polyurethane film is also respectively investigated. The effects of the layering sequence, polyurethane film thickness and layering numbers on sound absorption coefficients (SACs) and sound transmission loss (STL) are researched.

Experimental details

Materials

Waste PPS filter bags (PPS fibers + polytetrafluoroethylene (PTFE) ground fabric, thickness of 1.59 mm, and areal density of 780.5 g/m²) were supplied from Anhui Yuanchen Environmental Protection Technology Co., Ltd. (Anhui, China). The waste PPS filter bags are replaced after serving in a coal-fired plant for 36 months. Fusible interlining of nonwoven fabrics (copolyamide PA, 18 g/m², melting point 110∼125°C) were obtained from Guangzhou Baiyu Textile Co., Ltd. (Guangdong, China). Polyurethane film (thickness 0.5, 0.8 and 1.2 mm) was purchased from Dongguan Jinda Plastic Electronic Co., Ltd. (Guangdong, China). Sodium hydroxide (AR) was purchased from Sinopharm Chemical Reagent Co., Ltd.

Preparation of multi-layer sound-absorbing composite materials

First, the sewing thread of waste PPS filter bags was removed and changed from cylinder to plane, and the samples were soaked in a 4% sodium hydroxide solution for 4 h to remove the dust and neutralize acid dew. Then, the samples were placed in an ultrasonic cleaner for 1 h, and dried in an electric blast oven at 80°C for 12 h. Moreover, two kinds of multi-layer porous sound-absorbing composites were prepared.

The multi-layer sound-absorbing composite materials composed of pure waste PPS filter bags were manufactured as follows. The waste PPS filter bags after alkali washing and fusible interlining were compounded at high temperature and pressure free in the hot blast oven. The hot blast temperature was set at 125°C, and the time was set at 2 min. The thickness of fusible interlining of nonwoven fabrics (<0.01 mm) was ignored. The multi-layer sound-absorbing composite materials composed of waste PPS filter bags and polyurethane films with different thickness were also compounded at high temperature and pressure free in the hot blast oven. The hot blast temperature was set at 150°C, and the time was set at 3 min. Finally, all samples were cooled in the air. The details of multi-layer sound-absorbing composite materials are listed in Table 1.

Table 1.

The details of multi-layer porous sound-absorbing composite materials

Samples	Layer number	First layer material/thickness (mm)	Second layer material/thickness (mm)	Third layer material/thickness (mm)	Fourth layer material/thickness (mm)	Fifth layer material/thickness (mm)
S	Single layer	PPS/1.49
SS	Two layers	PPS/1.49	PPS/1.49
SSS	Three layers	PPS/1.49	PPS/1.49	PPS/1.49
SUS1		PPS/1.49	PU/0.5	PPS/1.49
SUS2		PPS/1.49	PU/0.8	PPS/1.49
SUS3		PPS/1.49	PU/1.2	PPS/1.49
USU1		PU/0.5	PPS/1.49	PU/0.5
USU2		PU/0.8	PPS/1.49	PU/0.8
USU3		PU/1.2	PPS/1.49	PU/1.2
SSSS	Four layers	PPS/1.49	PPS/1.49	PPS/1.49	PPS/1.49
SSSSS	Five layers	PPS/1.49	PPS/1.49	PPS/1.49	PPS/1.49	PPS/1.49
SUSUS1		PPS/1.49	PU/0.5	PPS/1.49	PU/0.5	PPS/1.49
SUSUS2		PPS/1.49	PU/0.8	PPS/1.49	PU/0.8	PPS/1.49
SUSUS3		PPS/1.49	PU/1.2	PPS/1.49	PU/1.2	PPS/1.49
USUSU1		PU/0.5	PPS/	PU/0.5	PPS/1.49	PU/0.5
USUSU2		PU/0.8	PPS/	PU/0.8	PPS/1.49	PU/0.8
USUSU3		PU/1.2	PPS/	PU/1.2	PPS/1.49	PU/1.2

PPS: polyphenylene sulfide; PU: polyurethane.

In the later study of this paper, waste PPS filter bags are called PPS for short, fusible interlining of nonwoven fabrics (copolyamide PA) is called cPA for short, and polyurethane films is called PU for short. In addition, the first layer of multi-layer sound-absorbing composites represents the side facing the sound source. Schematic diagrams of multi-layer sound-absorbing composites are shown in Figure 1.

Figure 1.

Schematic diagrams of multi-layer porous sound-absorbing composites.

Characterization

The morphology and structure of waste and new PPS filter bags were first measured to evaluate the performance. An S-4800 scanning electron microscope (Hitachi Co. Ltd., Japan) was used to observe the surface and internal structure of PPS filter bags. The thickness and surface density of PPS filter bags were measured according to GB/T24218.1-2009²⁴ and GB/T24218.2-2009.²⁵ The tensile properties and bursting properties of PPS filter bags were measured according to GB/T6719-2009.²⁶ The thermo-stability of PPS filter bags was characterized using a Q500 thermogravimetric analyzer (TA Instruments Co., Ltd., New Castle, DE, USA) from 30°C to 800°C with a heating rate of 10°C/min. The whole test was conducted in a N₂ atmosphere of 50 mL/min. The change of functional groups in PPS filter bags was evaluated by using a Nicolet-10 Fourier transform infrared spectrometer (Thermo Fisher Scientific Co. Ltd., Waltham, MA, USA). The test ranged from 4000 cm⁻¹ to 400 cm⁻¹. The pore size distribution and average pore size of PPS filter bags were measured by using a PMI iPORE 1100 capillary flow porometer according to ASTM F316-03²⁷ (Porous Materials Inc., Ithaca, NY, USA).

The sound absorption properties of samples were measured by using a SW422/SW477 impedance tube absorption test system (BASWA Co., Ltd., China) based on the transfer function method according to GB/T 18696.2-2002²⁸ and GB/T 18696.1-2004.²⁹ The samples were cut into round discs with two diameters of 100 and 30 mm using stainless steel molds, respectively. Figure 2 presents the schematic diagrams of the impedance tube absorption test system. The test was carried out in the condition of atmospheric temperature 25°C and relative humidity 65%. The effective range of sound testing frequency was 63∼6300 Hz. The average SACs ( $α$ ) and average sound reduction index ( $\bar{R}$ ) of samples were calculated according to the following equations:³⁰

α = \frac{α_{125} + α_{250} + α_{500} + α_{1000} + α_{2000} + α_{4000}}{6}

(1)

\bar{R} = \frac{\begin{matrix} R_{100} + R_{125} + R_{160} + R_{200} {+ R}_{250} {+ R}_{315} {+ R}_{400} \\ {+ R}_{500} {+ R}_{630} {+ R}_{800} {+ R}_{1000} {+ R}_{1250} {+ R}_{1600} \\ {+ {+ R}_{2000} {+ R}_{2250} {+ R}_{3150} {+ R}_{4000} + R}_{5000} \end{matrix}}{18}

(2)

where α₁₂₅, α₂₅₀, α₅₀₀, α₁₀₀₀, α₂₀₀₀ and α₄₀₀₀ were the SACs of samples at the frequencies of 125, 250, 500, 1000 and 2000 Hz. R₁₀₀, R₁₂₅, R₁₆₀, R₂₀₀, R₂₅₀, R₃₁₅, R₄₀₀, R₅₀₀, R₆₃₀, R₈₀₀, R₁₀₀₀, R₁₂₅₀, R₁₆₀₀, R₂₀₀₀, R₂₂₅₀, R₃₁₅₀, R₄₀₀₀ and R₅₀₀₀ were the STLs of samples at the frequencies of 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2250, 3150, 4000 and 5000 Hz.

Figure 2.

Schematic diagrams of the impedance tube absorption test system: (a) the experimental device for measurement of the sound absorption and (b) the experimental device for measurement of the sound insulation.

Results and discussion

Morphology of PPS filter

Figure 3 shows the morphology of waste and new PPS filter bags after alkali cleaning. From Figure 3(a) and (b), it can be observed that waste PPS filter bags turn yellow but there is no obvious damage. From Figure 3(c), it can be seen that part of the dust particles still adheres to the surface fibers of PPS, while the surface fiber of the new PPS filter is relatively smooth and clean. Moreover, PPS filter materials are usually treated by emulsion impregnation to improve the oxidation resistance. Therefore, a film can be formed on the surface of new PPS filter fibers, and a bit of the particles can be observed from Figure 3(e). This phenomenon indicates that dust particles are difficult to be completely cleaned during the process of alkali washing, and the fiber surface of waste PPS filters become rougher. From Figure 3(d), it can be observed that dust particles adhering to the internal fibers of PPS are greatly reduced. There are still a lot of interconnected small voids and pathways inside the PPS which are not blocked by dust particles. Furthermore, there is no obvious damage and fracture on the fiber surface of PPS, and the fibers display a relatively intact state. Therefore, there are reasons to believe that waste PPS filter bags after alkali washing can be used to manufacture multi-layer porous sound-absorbing materials.

Figure 3.

Photographs of waste polyphenylene sulfide (PPS) filter (a), new PPS filter (b) and scanning electron microscope (SEM) images of waste and new PPS filter bags after alkali cleaning; (c) surface of waste PPS filter; (d) interior of waste PPS filter; (e) surface of new PPS filter and (f) interior of new PPS filter.

Physical properties of PPS filter

The thickness, areal density and pore size distribution of waste and new PPS filter after alkali cleaning are shown in Figure 4. Compared with new PPS filter, the areal density of PPS increases substantially and the thickness decreases. The thickness of PPS decreases from 1.59 mm to 1.49 mm after alkali cleaning, which indicates that the surface filter cake and part of the internal dust particles can be removed by alkali cleaning. However, the thickness still decreases by 17.2% (1.80 mm to 1.49 mm) compared to new PPS filter, as shown in Figure 4(a). This is because the combining effects of repeated air pulse soot cleaning, high temperature and oxidation in the using process of PPS filter bags can make their original fluffy structure become much tighter.^3,5,6 In addition, the influence of dust accumulation on the surface of the filter can also reduce the thickness of waste PPS filter.

Figure 4.

Thickness, areal density and pore size distribution of waste and new polyphenylene sulfide (PPS) filter after alkali cleaning: (a) thickness and areal density; (b) pore size distribution of waste PPS filter and (b) pore size distribution of new PPS filter.

The areal density of waste PPS filter after alkali cleaning decreases from 780.5 g/m² to 661.1 g/m², which indicates that most of the dust particles adhering to PPS can be removed by alkali cleaning, whereas the areal density of waste PPS filter after alkali washing is still higher than that of new PPS filter bags, increasing by 12.2% (661.1 to 589.2 g/m²). This indicates that a small amount of dust particles inside waste PPS filter bags are difficult to remove by alkali cleaning. Meanwhile, PPS macro molecules can combine with a large amount of oxygen elements due to the oxygen of PPS filter in the using process, which also results in the increase of areal density.

Figure 4(b) and (c) show the pore size distribution of waste and new PPS filter after alkali cleaning. It can be observed that the average pore size and maximum pore size of PPS is much lower than that of new PPS filter. This is because the high-pressure dusting and repeated air pulse soot cleaning of filter in the using process make the structure of PPS become dense.^3,5–7 Moreover, a small amount of dust particles still attaches to the inside of PPS which can also cause the decrease of the pore size.

Tensile and bursting properties of PPS filter

Figure 5(a) shows the tensile strength and elongation at break of PPS in the lateral and longitudinal direction. It can be observed that the longitudinal tensile strength and lateral tensile strength of PPS exhibit a downward trend compared with that of new PPS filter. The lateral tensile strength of PPS is down by 18.0% from 1254.7 N to 1028.3 N, while the longitudinal tensile strength just falls by 4.4% from 931.0 N to 890.4 N. This phenomenon indicates that the decrease in tensile strength in PPS filter can be attributed to the erosion of dust particles, the oxidative cleavage of PPS macromolecules and the repeated air pulse soot cleaning in the using process.^3,5,31–33 However, it is obvious that the decline range of tensile strength in PPS is limited, and PPS still retains a high tensile strength.

Figure 5.

Tensile strength, elongation at break (a) and bursting strength (b) of waste and new polyphenylene sulfide (PPS) filter.

Meanwhile, the lateral elongation at break of PPS decreases, while the longitudinal elongation at break is higher than that of new PPS filter. After long-term use, the lateral elongation at break of PPS decreases from 45.2% to 29.1%, whereas the longitudinal elongation at break increases from 7.6% to 9.9%. The decrease in lateral elongation at break can be attributed to the increasing crystallinity of PPS fibers, and the oxidative cleavage and cross-linking of PPS macromolecules after the long-term use of PPS filter at high temperature.^32–34 The increase of longitudinal elongation at break may be related to the structural changes of the PTFE ground fabric.

The relational graph of bursting strength and deformation of PPS filter bags is shown in Figure 5(b). As shown in Figure 5(b), it can be seen that the bursting strength of PPS is much lower than that of new PPS filter, down by 35.1%. Meanwhile, the deformation of PPS at the maximum bursting strength is also reduced by 35.1% compared to new PPS filter. This is also due to the influence of high temperature, high air pressure and air oxidation in the using process of PPS filter. The internal structure of PPS filter is practically damaged, and the surface of PPS generates some micro holes. Therefore, PPS is easier to break under the bursting pressure. Meanwhile, the interior of PPS becomes tight due to the repeated air pulse soot cleaning and oxidation in the using process. The oxidative cleavage and cross-linking of some PPS macromolecules can also result in a decrease in PPS filter toughness.

Thermal stability and functional groups change of PPS filter

Figure 6(a) shows the thermogravimetric analysis (TGA) curves of PPS filter bags under a nitrogen atmosphere. It can be found that both waste PPS filter and new PPS filter exhibit a one-step thermal degradation process over the entire test temperature range, but the thermal stability of waste PPS filter decreased compared with that of new PPS filter. PPS and new PPS filter both show a slight mass loss before 496.4°C, and the mass loss of PPS is higher than that of new PPS filter. This part of mass loss can be attributed to the absorbed water and anti-oxidation emulsion. Moreover, some small molecules can be generated by the oxidative cleavage of PPS macromolecules in the using process of PPS filter.^10,35–37 The small molecules can be further decomposed and volatilized before 496.4°C, which leads to a higher mass loss than new PPS filter. When the test temperature reaches 496.4°C, the mass loss of two samples both increase rapidly, and the thermal decomposition rate is also accelerated. However, the thermal decomposition rate of PPS is higher than that of new PPS filter. When the test temperature reaches 576.3°C, the thermal decomposition rates of two samples decrease and the residual mass fraction is substantially stable.

Figure 6.

Thermogravimetric analysis (TGA) curves and Fourier transform infrared (FTIR) spectrogram of waste and new polyphenylene sulfide (PPS) filter.

The residual mass fraction of new PPS filter is 41.3%, which is significantly higher than that of PPS (23.2%). This because partial macromolecule chains are broken, molecular weight decreases, and some small molecules are generated due to the high-temperature oxidation in the using process.^34,36 During the thermal decomposition, the thermal stability of broken macromolecule chains and small molecules is lower than that of the original PPS macromolecule chains. Therefore, these substances first degrade to generate gas and small molecule decomposition products, which can further promote the degradation of PPS macromolecules. For new PPS filter, the macromolecular chains can be cross-linked in the process of thermal decomposition, and the formed products are difficult to decompose. Therefore, the residual mass fraction of PPS is lower than that of new PPS filter.

The changes of chemical functional groups are measured using attenuated total reflection (ATR)–Fourier transform infrared (FTIR). The FTIR spectrum of PPS and new PPS filter after alkali cleaning is shown in Figure 6(b). The 1569, 1467, 1382, 1084, 1007 and 807 cm⁻¹ are the characteristic peaks of PPS macromolecules, which can be found both from the FTIR spectrums of PPS and new PPS filter.^34,38 The peaks at 1569, 1467, and 1382 cm⁻¹ belong to the stretching vibration absorption peaks of the benzene ring skeleton. The peak at 1084 cm⁻¹ can be attributed to the stretching vibration absorption peak of the C-S bond. The peak at 1007 cm⁻¹ belongs to the stretching vibration peak of benzene ring C=CH in-plane. The peak at 807 cm⁻¹ is the characteristic peak of the ρ-disubstituted benzene ring. Moreover, new peaks at 1201, 1148 and 629 cm⁻¹ can also be observed from the FTIR spectrum of PPS. The peaks at 1201 cm⁻¹ and 1148 cm⁻¹ can be attributed to the asymmetric stretching vibration and symmetric stretching vibration of the S=O bond in sulfuryl, respectively. The peak at 629 cm⁻¹ can be attributed to the stretching vibration of the C-S bond in the phenylsulfonyl group. Therefore, it can be indicated that PPS macromolecules in waste filter bags are seriously oxidized in the using process.

Sound absorption and sound insulation properties of PPS filter

The SAC and STL of samples were measured by the impedance tube absorption test system. The SAC and STL of samples are shown in Figures 7 to 16, and average SACs (α), average STL ( $\bar{R}$ ) and thickness of samples are shown in Table 2. The effects of the layer numbers, layering sequence and thickness of PU are also discussed.

Table 2.

Sound absorption and sound insulation property of multi-layer materials

Samples	α	$\bar{R}$ (dB)	Thickness (mm)	Areal density (kg/m²)	Samples	α	$\bar{R}$ (dB)	Thickness (mm)	Areal density (kg/m²)
S	0.094	6.03	1.49	0.66	USU1	0.170	20.87	3.38	1.92
SS	0.130	13.53	3.01	1.35	USU2	0.093	23.02	3.87	2.56
SSS	0.184	16.02	4.67	2.01	USU3	0.056	25.67	4.23	3.23
SSSS	0.193	18.67	5.91	2.65	SUSUS1	0.122	30.45	7.54	3.17
SSSSS	0.204	22.01	7.33	3.39	SUSUS2	0.129	30.32	8.08	3.91
SUS1	0.151	25.33	4.76	1.83	SUSUS3	0.051	32.83	8.52	4.77
SUS2	0.134	21.00	5.01	2.18	USUSU1	0.087	28.76	5.82	3.29
SUS3	0.076	25.33	5.22	2.64	USUSU2	0.131	30.32	6.87	4.35
					USUSU3	0.064	47.94	7.52	5.22

Effect of layer numbers on sound absorption property

The effect of layer numbers of PPS on SACs is shown in Figures 7 and 8. It is evident that PPS and PU films possess completely different sound absorption properties. PPS has a bad sound absorption effect at the low frequency range of 63–1000 Hz, and exhibits a good sound absorption effect at the high frequency above 2500 Hz. However, PU with different thickness all show good sound absorption effects at the low frequency range of 600–1000 Hz, while exhibiting a bad sound absorption effect at high frequency above 3000 Hz. The SACs of U1, U2 and U3 can reach 0.61, 0.64 and 0.44 at 800 Hz, respectively.

Figure 7.

The sound absorption coefficients of polyurethane (PU) films and waste polyphenylene sulfide (PPS) filter: (a) low frequency and (b) high frequency.

Figure 8.

The effect of layer numbers of polyphenylene sulfide (PPS) filter on sound absorption coefficients: (a) low frequency and (b) high frequency.

Moreover, it can be seen that the layer numbers of PPS have a significant influence on SAC of samples from Figure 8. With the increase of layer numbers, the SAC also exhibit the escalating trend in the low frequency. When the layer number reaches five, the SAC of sample SSSSS can be 0.26 at 1000 Hz, whereas that of S is just 0.081. However, the SAC of samples increases first and then decreases with the increase of layer numbers in the high frequency. When the frequency is above 2500 Hz, the SAC value of sample SSS begins to exceed that of samples SSSS and SSSSS, and the sample SSS has a SAC value of 0.55 at 6300 Hz. This phenomenon can be attributed to the increase in areal density and decrease of porosity. ³⁹ As shown in Table 2 and Figure 8(a), it can be seen that the SAC values increase with the increase in areal density in the low frequency. However, the samples become dense and the porosity decreases when the areal density increases to a certain degree, which results in the air flow resistance being greater than the optimal air flow resistance.^22,40 Hence, the sound absorption effect of samples decreases in the high frequency.

Effect of layering sequence on sound absorption property

The SAC of three-layer and five-layer samples with different layering sequences are shown in Figure 9. It can be seen that the sound absorption of samples is slightly affected by the layering sequence. As shown in Figure 7, PU exhibits a good sound absorption effect, and PPS has a bad sound absorption effect at the low frequency range. However, the three-layer samples SUS1 and USU1 both show good sound absorption at the low frequency range of 250–630 Hz and exhibit extremely bad sound absorption at the high frequency. This indicates that PU plays a dominant role in the sound absorption effect of muti-layer composite materials. The SAC value of samples SUS1 and USU1 is even lower than that of single layer S at the frequencies above 2500 Hz and 1250 Hz, respectively. In contrast, the SAC value of samples SUS1 and USU1 can reach 0.53 and 0.46 at 400 Hz, respectively. This can be attributed to the fact that PU and PPS may constitute a resonant cavity structure which generates sound absorption peaks at 400 Hz, but greatly reduces sound absorption at the medium and high frequency.^41–43 Moreover, when incident sound waves hit the surface of PU, PU can vibrate because soft PU is sensitive to the sound pressure variation due to the sound dampening properties. Therefore, the sound absorption effect of PPS porous materials backed by PU is better than that of PPS backed by the rigid wall. The vibration of PU can also create more scattering and energy loss of sound waves when sound waves transmit on the surface of PU from the porous materials.⁴⁴

Figure 9.

The effect of layering sequence on sound absorption coefficients of three-layer and five-layer samples: (a) and (c) low frequency; (b) and (d) high frequency.

From Figure 9(c) and (d), it can be also seen that five-layer samples SUSUS1 and USUSU1 also show a good sound absorption effect at the low frequency range of 250–500 Hz and exhibit an extremely bad sound absorption effect at the high frequency. The SAC value of samples SUSUS1 and USUSU1 can reach 0.38 and 0.34 at 315 Hz, respectively, whereas the SAC value is also lower than that of single layer S at the medium and high frequencies. This can also be attributed to the resonant cavity structure composed of soft PU and PPS porous materials. Moreover, it can also be seen that three-layer and five-layer samples composed of PU and PPS all exhibit a better sound absorption effect at low frequency than samples composed of pure PPS, which indicates that PU has a significant influence on the sound absorption effect at low frequencies of multi-layer composite materials.

Effect of the thickness of PU films on sound absorption property

Figures 10 and 11 show the SACs of three-layer and five-layer samples with different thicknesses of PU. From Figure 10, it can be observed that the maximum SAC shifts to the higher frequency with the increase in PU thickness for three-layer samples. The α of SUS and USU three-layer samples also decreases with the increase in PU thickness, as shown in Table 2. The softness of PU is negatively correlated with the thickness, so the effect of the resonant cavity can be weakened by increasing the thickness.⁴⁴ The scattering and energy loss of sound waves caused by the vibration of PU can also be weakened due to the increasing rigidity. These changes may improve the sound absorption effect at the medium frequency, and decrease the sound absorption at the high frequency. The sound absorption effect of the samples at the low frequency can also be further improved by decreasing the PU thickness. The same phenomenon can also be observed for SUSUS and USUSU five-layer samples from Figure 11. The maximum SAC shifts to the higher frequency with the increase in PU thickness for five-layer samples. From Table 2, it can be seen the α of SUSUS and USUSU five-layer samples also decreases with the increase in PU thickness. Moreover, more molten PU can enter the internal cavity of PPS in the process of preparation, which can also lead to the decrease of porosity and the sound absorption effect of multi-layer materials.

Figure 10.

The effect of the thickness of polyurethane films on sound absorption coefficients of three-layer samples: (a) and (c) low frequency; (b) and (d) high frequency.

Figure 11.

The effect of the thickness of polyurethane films on sound absorption coefficients of five-layer samples: (a) and (c) low frequency; (b) and (d) high frequency.

Effect of layer numbers on sound insulation property

Figures 12 and 13 show the effect of the layer numbers of PPS on sound insulation property. It is evident that PPS and PU also possess completely different sound insulation properties. PPS has bad sound insulation in the whole frequency range. However, PU with different thicknesses all show good sound insulation in the whole frequency range, and the STL of PU increases with the increase in thickness. Moreover, the layer numbers of PPS can significantly influence the STL of samples, as shown in Figure 13. The sound insulation effect of multi-layer samples increases with the increase in layer numbers of PPS. This phenomenon can be attributed to the sound insulation effect being positively associated with the thickness and areal density of materials composed by one ingredient.⁴⁰ Moreover, the fiber packing density of samples can also be calculated,³⁹ and the fiber packing density of samples exhibits an increasing trend with the increase in layer numbers. The fiber packing density of S is 0.326, and that of SSSSS can reach 0.340. Therefore, the increasing fiber packing density can also lead to the increase in the sound insulation effect.

Figure 12.

The sound transmission loss of polyurethane films and waste polyphenylene sulfide (PPS) filter: (a) low frequency and (b) high frequency.

Figure 13.

The effect of layer numbers of polyphenylene sulfide (PPS) filter on sound transmission loss: (a) low frequency and (b) high frequency.

Effect of layering sequence on sound insulation property

The STL of three-layer and five-layer samples with different layering sequences are shown in Figure 14. It can be seen that the sound insulation effect of samples is significantly affected by the layering sequence. The sound insulation effect of multi-layer samples composed of PPS and PU is much higher than that of samples composed of pure PPS. The $\bar{R}$ of SUS1 and USU1 can reach 25.33 and 20.87 dB, respectively, while the $\bar{R}$ of SSS is just 16.02 dB. There is no significant difference in the STL of three-layer and five-layer samples with different layering sequences below 800 Hz, as shown in Figure 14(a) and (c). However, the STL of three-layer and five-layer samples composed of PPS and PU is higher than that of samples composed of pure PPS above 800 Hz, as shown in Figure 14(b) and (d). Multi-layer materials composed of pure PPS just possess a porous structure which can effectively absorb sound but reflects sound weakly. Therefore, the sound energy through the materials is higher than that of multi-layer materials composed of PPS and PU because PU can both absorb and reflect sound due to the damping performance.

Figure 14.

The effect of the layering sequence on sound transmission loss of three-layer and five-layer samples: (a) and (c) low frequency; (b) and (d) high frequency.

For multi-layer materials composed of PPS and PU, it can be found that the sound insulation effect of samples in which PPS is located on the sound facing side is much better than that of samples in which PU is located on the sound facing side. This phenomenon can be attributed to two points. First, the thickness of SUS1 and SUSUS1 is higher than that of USU1 and USUSU1, respectively, which can improve the sound insulation effect. Second, when PPS is on the sound facing side, the improvement of the sound absorption effect caused by the resonant cavity structure composed of PPS and PU is higher than that of PU located at the sound facing side.⁴⁵ However, it needs to be pointed out that the differences are shrinking with the increase in the layer numbers of multi-layer materials.

Effect of the thickness of PU films on sound insulation property

Figures 15 and 16 show the STL of three-layer and five-layer samples with different thicknesses of PU. For SUS-structure materials, it can be observed that the sound insulation effect first decreases and then increases with the increase in PU thickness, as shown in Figure 15 and Table 2. However, the sound insulation effect increases with the increase in PU thickness for USU-structure materials. Moreover, the gap in transmission loss between SUS-structure and USU-structure materials has also been narrowing due to the increase in PU thickness. The $\bar{R}$ of SUS1 and USU1 is 25.33 and 20.87 dB with a large gap, while the $\bar{R}$ of SUS3 and USU3 reaches 25.33 and 25.67 dB with a tiny difference. This phenomenon can be attributed to two main points. The sound absorption effect of the resonant cavity structure composed of PPS and PU is related to the PU thickness due to the difference in rigidity. Next, the reflection, scattering and energy loss of sound waves caused by the vibration of PU is also related to the PU thickness which can influence the damping property.⁴⁶ It can be observed that the PU thickness has little effect on the sound insulation effect of SUSUS-structure materials in the whole frequency range from Figure 16 and Table 2. The $\bar{R}$ of SUSUS1 is 30.45 dB, meanwhile the $\bar{R}$ of SUSUS3 is just 32.83 dB with a slight increase. However, the PU thickness is positively associated with the sound insulation effect of USUSU-structure materials. The $\bar{R}$ of USUSU1 is 28.76 dB, meanwhile the $\bar{R}$ of USUSU3 can reach 47.94 dB with a significant increase. Therefore, the $\bar{R}$ increases with the increase in PU thickness for multi-layer materials in which PU is located on the sound facing side as shown in Table 2.

Figure 15.

The effect of the thickness of polyurethane films on the sound transmission loss of three-layer samples: (a) and (c) low frequency; (b) and (d) high frequency.

Figure 16.

The effect of the thickness of polyurethane films on sound transmission loss of five-layer samples: (a) and (c) low frequency; (b) and (d) high frequency.

Conclusions

The alkali cleaning cannot completely remove the dust particles adhering to the waste PPS filters, and the PPS fibers in waste filters still maintain a relatively intact state. The thickness and pore size of waste PPS filters show a significant decline compared with new PPS filters. The tensile and bursting properties of waste PPS filters also decrease compared with new PPS filters due to the oxidative breaking and cross-linking of PPS macromolecules, but the tensile strength can still maintain 80% of the new PPS filters. Moreover, the thermal stability of waste PPS filters also decreases compared with new PPS filters, which can be attributed to the oxidative cleavage of PPS macromolecules.

Waste PPS filter possesses bad sound absorption at the low frequency range below 1000 Hz and good sound absorption at the high frequency range above 1000 Hz. The sound absorption property of multi-layer materials only composed of waste PPS filter exhibits an escalating trend with the increase in layer numbers at the low frequency, while it increases first and then decreases at the high frequency. However, the sound insulation property increases with the increase in layer numbers in the whole frequency range. The $\bar{R}$ of sample S is only 6.03 dB, while the $\bar{R}$ of sample SSSSS can reach 22.01 dB.

Moreover, multi-layer materials composed of waste PPS filters and PU film show good sound absorption at the low frequency and exhibit extremely bad sound absorption at the high frequency, which significantly changes the sound absorption characteristics of waste PPS filters. Furthermore, the layering sequence has little effect on the sound absorption property of multi-layer materials, but the thickness of PU film has a significant effect on the sound absorption property. The maximum SAC shifts to the higher frequency and the α of multi-layer materials decreases by increasing the thickness of PU film. Meanwhile, the PU film position change can significantly influence the sound insulation property of multi-layer materials. When the PU film is located on the sound facing side, the thickness of PU film has contributed to the improvement of the sound insulation property of multi-layer materials. With the increase in PU film thickness, the $\bar{R}$ of USU and USUSU multi-layer materials significantly increases. The $\bar{R}$ of USU multi-layer materials increases from 20.87 dB to 25.67 dB, and the $\bar{R}$ of USUSU multi-layer materials also increases from 28.76 dB to 47.94 dB, when the thickness in PU film increases from 0.5 mm to 1.2 mm.

Footnotes

Declaration of conflicting interests

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

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship and/or publication of this article: This research was supported by the National Natural Science Foundation of China (52203114), the Outstanding Top Talent Cultivation Project of Anhui Province (gxgwfx2022018), the Open Project Program of Fujian Key Laboratory of Novel Functional Textile Fibers and Materials (FKLTFM2101), the Anhui Province International Cooperation Research Center of Textile Structure Composites (2021ACTC06) and the Yong and Middle-aged Top Talent Project of Anhui Polytechnic University.

ORCID iD

Huizhen Ke

References

Fukui

Ichiba

Rozy

MIF

, et al. Effects of NO₂ gas concentration on the degradation of polyphenylene sulfide non-woven bag filter at high temperature. Adv Powder Technol 2021; 32: 3278–3287.

Heo

Eom

, et al. High-performance bag filter with a super-hydrophobic microporous polytetrafluoroethylene layer fabricated by air-assisted electrospraying. Sci Total Environ 2021; 783: 147043.

Zhang

Liu

Hsu

, et al. Nanofiber gas filters with high-temperature stability for efficient PM2.5 removal from the pollution sources. Nano Lett 2016; 16: 3642–3649.

Zhou

Wang

, et al. Application and research of dry-type filtration dust collection technology in large tunnel construction. Adv Powder Technol 2017; 28: 3213–3221.

Mukhopadhyay

7-Composites nonwovens in filters: theory. In: Dipayan Das and Behnam Pourdeyhimi (eds) Composite Non-Woven Materials. Cambridge: Woodhead Publishing, 2014, pp. 120–163.

Rozy

MIF

Ito

Une

, et al. A continuous-flow exposure method to determine degradation of polyphenylene sulfide non-woven bag-filter media by NO₂ gas at high temperature. Adv Powder Technol 2019, 30: 2881–2889.

Liu

Duan

, et al. Performance and reaction mechanism for low-temperature NO_x catalytic synergistic HgO oxidation of catalytic polyphenylene sulfide filter materials. Asia-Pac J Chem Eng 2020; 15: e2403.

Shi

Lin

, et al. A robust superhydrophobic PPS-PTFE/SiO₂ composite coating on AZ31 Mg alloy with excellent wear and corrosion resistance properties. J Alloy Compd 2017; 721: 157–163.

Zhou

Wang

, et al. Polyphenylene sulfide fabric with enhanced oxidation resistance and hydrophobicity through polybenzoxazine surface coating for emission control in harsh environment. J Hazard Mater 2022; 432: 128735.

10.

Lian

Ren

Han

, et al. Kinetics and evolved gas analysis of the thermo-oxidative decomposition for neat PPS fiber and nano Ti-SiO₂ modified PPS fiber. J Mol Struct 2019; 1196: 734–746.

11.

Wang

Jiang

Liu

Life problem analysis on PPS filter application of bag dusters in coal-fired power plants. Adv Mater Res 2011; 236–238: 2464–2470.

12.

Tanthapanichakoon

Furuuchi

Nitta

, et al. Degradation of semi-crystalline PPS bag-filter materials by NO and O₂ at high temperature. Polym Degrad Stabil 2006; 91: 1637–1644.

13.

Overview of recycling of waste filter bags in coal-fired power plants. Text Sci Res 2019; 11: 74–76.

14.

Wang

Zhu

Yuan

, et al. A new recycling strategy for preparing flame retardants from polyphenylene sulfide waste textiles. Compos Commun 2021; 27: 100852.

15.

Okubo

Sugeno

Tagaya

Chemical recycling of poly(p-phenylene sulfide) in high temperature fluids. Polym Degrad Stabil 2015; 111: 109–113.

16.

Zhang

TJ.

Exploration on poly (phenylene sulfide) waste recycling. Plastics 2016; 45: 44–46.

17.

Wei

Zhang

MC.

Main sources, recycling and reuse methods of waste bags. Techn Text 2018; 36: 29–32.

18.

Zeng

XF.

Recycling of waste filter bags in coal-fired power plants. Energ Conser Environ Prot 2017; 9: 71–73.

19.

Kim

Lee

, et al. Rapid biodegradation of polyphenylene sulfide plastic beads by Pseudomonas sp. Sci Total Environ 2020; 720: 137616.

20.

Ozkal

Callioglu

FC.

Effect of nanofiber spinning duration on the sound absorption capacity of nonwovens produced from recycled polyethylene terephthalate fibers. Appl Acoust 2020; 169: 107458.

21.

Liu

Tang

Deng

ZM.

Sound absorption properties for multi-layer of composite materials using nonwoven fabrics with kapok. J Ind Text 2022; 51: 1601–1615.

22.

Zhang

Gong

, et al. Fiber-based flexible composite with dual-gradient structure for sound insulation. Compos Part B-Eng 2020; 198: 108166.

23.

Lyu

Wang

, et al. Sound absorption, thermal, and flame retardant properties of nonwoven wall cloth with waste fibers. J Eng Fiber Fabr 2020; 15: 1–11.

24.

GB/T 24218.1-2009: Textile-Test methods for nonwovens. Part 1: Determination of mass per unit area, China National Textile and Apparel Council. Beijing: Standards Press of China, 2009.

25.

GB/T 24218.2-2009: Textile-Test methods for nonwovens. Part 2: Determination of thickness, China National Textile and Apparel Council. Beijing: Standards Press of China, 2009.

26.

GB/T 6719- 2009: Specifications for bag house, China National Textile and Apparel Council. Beijing: Standards Press of China, 2009.

27.

ASTM F316-03: Standard Test Methods for Pore Size Characteristics of Membrane Filters by Bubble Point and Mean Flow Pore Test. West Conshohocken: ASTM International Headquarters, 2011.

28.

GB/T 18696.2-2002: Acoustics. Determination of sound absorption coefficient and impedance in impedance tubes. Part 2: Transfer function method, China National Textile and Apparel Council. Beijing: Standards Press of China, 2002.

29.

GB/T 18696.1-2004: Acoustics-Determination of sound absorption coefficient and impedance in impedance tubes. Part 1: Method using standing wave ratio, China National Textile and Apparel Council. Beijing: Standards Press of China, 2004.

30.

Taban

Khavanin

Ohadi

, et al. Study on the acoustic characteristics of natural date palm fibres: experimental and theoretical approaches. Build Environ 2019; 161: 106274.

31.

Guo

Yang

, et al. Synergistic improvement of thermal conductivities of polyphenylene sulfide composites filled with boron nitride hybrid fillers. Compos Part A-Appl S 2017; 95: 267–273.

32.

Yang

Xiang

, et al. Competition of memory effect and thermal degradation on the crystallization behavior of polyphenylene sulfide. Polymer 2022; 262: 125427.

33.

You

Bai

ZH.

Study on UV resistance of polyphenylene sulfide modified TiO₂ by phenylsiloxane. Acta Polym Sin 2022; 53: 673–682.

34.

Xing

Z Z

Deng

B Y

2018 Enhanced oxidation resistance of polyphenylene sulfide composites based on montmorillonite modified by benzimidazolium salt, Polymers2018; 10: 83.

35.

Lian

Dai

Zhang

, et al. Enhancing the resistance against oxidation of polyphenylene sulphide fiber via incorporation of nanoTiO₂-SiO₂ and its mechanistic analysis. Polym Degrad Stabil 2016; 129: 77–86.

36.

Deng

Song

, et al. Thermal aging effects on the mechanical behavior of glass fiber-reinforced polyphenylene sulfide composites. Polymers 2022; 14: 1275.

37.

Hou

Gao

, et al. Enhanced photo-stability polyphenylene sulfide fiber via incorporation of multi-walled carbon nanotubes using exciton quenching. Compos Part A-Appl S 2020; 129: 105716.

38.

Kim

Lee

Roh

, et al. Effect of covalent functionalization of MWCNTs on the thermal properties and non-isothermal crystallization behaviors of PPS composites. Polymers 2017; 9: 460.

39.

Das

Neckář

3-Structure of composite nonwovens. In: Dipayan Das and Behnam Pourdeyhimi (eds) Composite Non-Woven Materials. Cambridge: Woodhead Publishing, 2014, pp. 30–57.

40.

Wang

Liu

, et al. Experimental study on effects of laminated structure on acoustic insulation property of composites. Adv Mater Res 2011; 284–286: 53–57.

41.

ang

Fuchs

HV.

Predicting the absorption of open weave textiles and micro-perforated membranes backed by an air space. J Sound Vib 1999; 220: 905–920.

42.

Kim

Park

JH.

Double resonance porous structure backed by air cavity for low frequency sound absorption improvement. Compos Struct 2018; 183: 545–549.

43.

Selamet

Lee

, et al. Helmholtz resonator lined with absorbing material. J Acoust Soc Am 2005; 117: 725–733.

44.

Gaulon

Pierre

Derec

, et al. Acoustic absorption of solid foams with thin membranes. Appl Phys Lett 2018; 112: 261904.

45.

Wang

, et al. Sound absorption performance of porous metal fiber materials with different structures. Appl Acoust 2019; 145: 431–438.

46.

Rabbi

Bahrambeygi

Shoushtari

, et al. Manufacturing of PAN or PU nanofiber layers/PET nonwoven composite as highly effective sound absorbers. Adv Polym Technol 2014; 33: 1–8.