Multi-fiber needle-punched nonwoven composites: Effects of heat treatment on sound absorption performance

Abstract

Nonwovens have been increasingly used in car interiors for noise reduction. Most of these nonwovens are subjected to thermal treatments to give the nonwovens their final three-dimensional forms. Therefore, it became crucial to investigate the effects of thermal treatment on sound absorption characteristics of nonwovens. In this study, the effects of the material and treatment parameters on airflow resistivity and normal-incidence sound absorption coefficient of thermally treated three-layered nonwoven composites have been studied. The material parameters included fiber size and porosity. The treatment factors included the temperature and duration. The thermally treated three-layered nonwoven composites are classified into three types based on the material content and fiber blend. Sandwich structures consisting of polylactide/hemp/polylactide and polypropylene/glassfiber/polypropylene layers were called LHL and PGP, respectively. The sample which consisted of three layers of an intimate blend of polypropylene-glassfiber was named as PGI. Both temperature and duration of thermal treatment have been found to affect air flow resistivity and sound absorption. An increase in air flow resistivity and a decrease in sound absorption have been detected with heat treatment. A similarity has been observed between the thermal behaviors of PGP and PGI, which included the same thermoplastic polymer fiber. Variation in air flow resistivity of sandwich structure nonwoven composites increased with the increase in temperature, which was not observed in the intimate blend ones. The air flow resistivity of heat-treated nonwovens followed a steeper trend compared to unheated nonwovens per change in material parameters. In terms of treatment parameters, the difference between the thermal treatment and the melting point of the thermoplastics constituent of the nonwoven composite was found to be a significant factor on sound absorption. This effect of treatment temperature on sound absorption changed with treatment duration. The sound absorptive characteristic of the nonwoven composites in terms of sound frequency underwent a change with thermal treatment due to the structural changes with exposure to high temperature.

Keywords

Sound absorption heat treatment composite biodegradable

Introduction

Increasing consumer demand for better driving comfort pushes the automotive industry for better and more effective noise control measures [1,2]. The enhanced noise reduction positively affects the potential customers’ quality perception of the vehicle and thus renders the manufacturer advantageous in terms of competitiveness. Among several methods of noise control, sound absorption comes as a viable means [3]. Sound absorption in vehicle is achieved by attaching sound absorbers, which are porous materials, to various auto parts including floor-coverings, package trays, headliners, trunk liners and door panels. Automotive noise control has conventionally been achieved by sound absorbers of glassfiber and foam materials [2,3]. However, these materials pose some risks for human health and the environment [2]. Thus, bio-fiber-based nonwovens emerge as viable alternatives, which are increasingly appraised by media and research communities [4].

Bio-fibers are advantageous in terms of lower weight, economical price, good acoustic performance, and better safety with shatterproof characteristic [5]. Nonwovens, similarly, offer some inherent advantages, such as the economical price of the raw materials, efficient thermo-processing, lower specific weight and their ability to be recycled [3]. Nevertheless, there is limited research and modeling effort devoted to understanding the noise control performance of nonwovens [2,3,6].

Among such studies, Na and Cho [1] investigated the effects of plasma treatment on the sound absorption and pore structure of thermal bonded nonwovens. The nonwovens were made from regular polyester, hollow polyester, jute and kenaf fibers with low-density polyester fibers. They found that the plasma treatment increased the sound absorption of the hollow polyester and kenaf nonwovens while decreasing that of jute nonwovens.

Jayaraman [6] investigated effects of some material, production, and post-treatment methods on the sound absorption of needled nonwovens. He detected that fiber fineness, optimum web layer sequencing, and flame-retardant treatment influenced the noise reduction performance positively. He found that the presence of either an air gap behind the material or a film on the surface of the nonwoven fabric to increase sound absorption in low- to mid-frequency range. He reported that webs formed by air-laying provided better noise control compared to those produced by carding.

Compression molding is commonly used to produce large automotive components. It is a simple and cost-efficient process with low amount of waste [7]. Compression molding converts planar nonwoven fabrics into three-dimensional components. These make it necessary to study the effects of compression molding on the performance of such sound absorbers.

The process has two constituents: compression and heating [2]. Effect of compression on sound absorption of porous materials has been the subject of very limited research efforts [8,9]. Nevertheless, the authors were not able to find any in-depth studies on the influence of the heat treatment on sound absorption performance of nonwovens. The heating constituent is investigated in this study, as compression has been studied in a previous work [8].

In this paper, air flow resistivity and sound absorption properties of heat-treated three-layered multi-fiber nonwoven composites which included polypropylene (PP), glassfiber, polylactide (PLA), and hemp have been studied. The effects of material and treatment factors on noise control performance of thermally processed nonwoven composites have been investigated.

Materials and methods

Materials

Four different types of fibers were used in nonwoven production: polypropylene (PP), polylactide (PLA), glassfiber, and hemp. PP fibers, PLA fibers, and cardable glassfibers were provided by Nonwovens Cooperative Research Center (NC, USA), Fiber Innovation Technology, Inc (TN, USA), and AGY (SC, USA), respectively. Field-retted and BioFibeRefinery™ processed hemp fibers were purchased from Stemergy (Canada, Ontario). The properties of the fibers are given in Table 1.

Table 1.

Fiber parameters.

Fiber	Structure	Void ratio (%)	Fiber radius × 10^–6 (m)		Fiber density × 10^–3 (kg·m^–3)	Apparent fiber density × 10^–3 (kg·m^–3)
Fiber	Structure	Void ratio (%)	Mean	σ	Fiber density × 10^–3 (kg·m^–3)	Apparent fiber density × 10^–3 (kg·m^–3)
PLA	Hollow	18	30.9	3.1	1.24	1.02
PP	Solid	N.A.	31.7	1.3	0.91	0.91
Glass fiber^a	Solid	N.A.	9.00	0.74	2.50	2.50
Glass fiber^b	Solid	N.A.	10.9	0.82	2.50	2.50
Hemp	Multifibrillar	N.A.	42	38	1.45	1.45

Glass fiber in layered PP/glass fiber/PP blend.

In intimate polypropylene–glass fiber blend.

Sample preparation

Fibers were opened by a Truetzschler® Opener. All fiber types were opened in mono-fiber form except for one sample set which included 33% glassfiber and 66% PP fiber blend.

Webs of fibers were formed by air laying in a Truetzschler® Tuft Feeder Scanfeed with a target basis weight of 330 gsm (gr·m⁻²). The produced webs are shown in Table 2. The formed webs were pre-needled in an NSC Asselin™ Pre-needler.

Table 2.

Fiber webs produced by Truetzschler Tuft Feeder Scanfeed.

Web numbers	Fiber composition of webs	Blend ratios (%)
1	Hemp	100
2	PLA	100
3	PP	100
4	Glass fiber	100
5	Glass fiber/PP	33/67

Three layers of webs were stacked to form LHL (PLA/Hemp/PLA), PGP (PP/Glassfiber/PP), and PGI (PP-Glassfiber intimate blend) nonwoven structures as shown in Table 3. The three-layered webs were needled by using an NSC Asselin needle-punch loom, which was set at 100 cmċmin⁻¹ speed, 100-cm width, with needles on both sides, 228 strokesċcm⁻¹, 175 punchesċcm⁻² penetration density, 3 mm penetration depth. Groz-Beckert 15 × 17 × 40 × 3 needles were used. The target basis weight was 1000 gsm (gr·m⁻²).

Table 3.

Layering of fiber webs for needle-punching.

Web number	Web code	Layer 1	Layer 2	Layer 3
1	LHL	PLA	Hemp	PLA
2	PGP	PP	Glass fiber	PP
3	PGI	Glass fiber/PP	Glass fiber/PP	Glass fiber/PP

A Pasadena Hydraulics Inc. (P.H.I) model hydraulic press was used for heat treatment. Different levels of temperature were applied to samples for different durations. The selected temperatures were 125℃, 145℃, 165℃, and 185℃. This selection was based primarily on the temperatures a sound absorber is supposed to withstand during compression molding process. The nonwoven composites were designed for car interior applications; so, the selected temperatures exceed the possible temperature exposure, whereas the temperatures in various parts of vehicles can reach extreme values such as 550℃ for the surface of an exhaust manifold of a passenger vehicle driving uphill [10]. Thermal treatment conditions for PGP, PGI, and LHL are shown in Table 4. Due to limited material source, a reduced experimental set up was applied to LHL fabrics.

Table 4.

Thermal treatment conditions for LHL, PGP, and PGI.

Fabric	Temperature (℃)	Duration (min)
Fabric	Temperature (℃)	7.5	15	30	45
LHL	125	X	X
	145	X	X
	165	X	X
	185	X	X
PGP	125	X	X	X	X
	145	X	X	X	X
	165	X	X	X	X
	185	X	X	X	X
PGI	125	X	X	X	X
	145	X	X	X	X
	165	X	X	X	X
	185	X	X	X	X

Characterization

Fibers’ diameters, fabrics’ mass per unit area, thickness, porosity, air flow resistivity values, and sound absorption coefficients were measured. Samples were subjected to conditioning at 20℃ and 65% relative humidity for at least 24 hours prior to characterization processes. Conditioning of heat-treated samples was performed after they had cooled.

Fiber diameter measurements were carried out according to ASTM D 1577-07 standard test methods for Linear Density of textile fibers [11]. The average fiber diameters were obtained by fiber diameter measurements on the scanning electron microscope (SEM) images. A Hitachi S-3200 N SEM at the Analytical Instrumentation Facility of North Carolina State University was used. A 4Pi EDS/Digital Imaging system was used to acquire SEM digital images and line scans. At least 30 specimens of hemp fibers and 10 specimens of manmade fibers were measured.

Fabric mass per unit area values were obtained in accordance with ASTM D 3776-07 standard test method for mass per unit area (weight) of fabric [12]. Thickness measurements were taken from each sample using an AMES thickness gauge with a pressure level of 4.14 kPa according to ASTM D 5729-97 standard test method for Thickness of Nonwoven Fabrics [13].

Nonwovens’ porosity values were determined according to ASTM C 830-00 Standard test methods for apparent porosity, liquid absorption, apparent specific gravity, and bulk density of refractory shapes by vacuum pressure [14] using equation (1):

h = 1 - \frac{ρ_{w}}{ρ_{f}},

(1)

where h is porosity, ρ_w is fabric density, and ρ_f is the density of the fiber. PLA fibers, in the current study, had a hollow structure, while all the other fibers were solid. As only the pores that carry air flow in connection with the ambient air are of interest in terms of sound absorption, the voids in PLA fibers were not included in the porosity [7,8]. In other words, PLA fibers were assumed to act like solid fibers for sound absorption. Thus, the fiber density was corrected by a factor of void ratio subtracted from unity (1–0.18) in porosity calculation.

Air flow resistivity of nonwoven webs was measured according to ASTM D 737-04 standard test method for air permeability of textile fabrics [15]. The Frazier air permeability tester (Frazier Precision Instrument Company Inc.) was used. The Frazier differential pressure air permeability instrument gave the rate of the flow of air in cubic feet per square foot of sample area per minute, the Frazier Number, at a differential pressure of 0.5 inches of water [16]. These units have been converted to air flow resistivity, r₀, in Pa·s·m⁻² as shown in equation (2), where l is the thickness of the fabric in meters (equation derived from Frazier [16] and ASTM D 737-04 [15]):

r_{o} = \frac{0.5 \times 249}{Frazier_Number \times 0.00508 \times l} .

(2)

Normal incident sound absorption coefficients (NAC) of samples were obtained according to ASTM E 1050-07 standard test method for impedance and absorption of acoustical materials using a tube, two microphones and a digital frequency analysis system [17]. A minimum of three specimens from each sample were tested. A Bruel and Kjaer Pulse™ acoustic material testing system was used at Carcoustics Tech Center. A 29-mm-diameter tube was used at the frequency range of 500 Hz–5 kHz. Normal-incidence sound absorption coefficient was calculated according to the following equation:

α_{n} = 1 - | Я |^{2},

(3)

where α _n is normal incidence sound absorption coefficient (NAC), and Я is reflection coefficient.

For the statistical data analysis, the SAS 8.2 system was used. Two sets of analyses were carried out according to the dependent variables: namely air flow resistivity and normal-incidence sound absorption coefficient (NAC). For each dependent variable, two analysis sets have been conducted: one for investigating the effect of material parameters and the other for the investigation of treatment parameters on the dependent variables. A general linear model has been adopted.

Results and discussion

The effects of material and treatment factors on normal-incidence sound absorption (NAC) performance of the heated nonwovens were mathematically modeled in this research. The assumptions and evidence for the assumptions can be found in Yilmaz et al. [8] and [18]. The mathematical models that were derived by the authors are presented in the following sections. In the generated models, the physical units of all terms, either the variables or the coefficients, are compatible within the correlations.

Effect of heat treatment on air flow resistivity

There is a close relationship between air flow resistance and sound absorption. Air flow resistivity, r₀, is air flow resistance divided by the surface area and the thickness of porous material. Air flow resistivity has been used by a number of researchers to model sound absorption [2,8,18 –21].

Air flow resistivity and structure parameter information of heat-treated PGP, PGI, and LHL fabrics are given in Table 5. Among the collected data, the air flow resistivity values of only 125℃ and 145℃ were used. The air flow resistance figures obtained for 165℃ and 185℃ were measured to be even lower than those of 125℃ and 145℃. The fabrics that were treated at 165℃ and 185℃ retained more rigid structure and lost their flexibility and porosity; therefore, a higher air flow resistivity had been expected. The low values might be due to increased leakage between the rigid “fabrics” and the sample holder which was designed for textile materials. Therefore, the data for 165℃ and 185℃ were eliminated from the model. As seen from Table 5, heat treatment increased air flow resistivity, as expected. Interestingly, the highest increase was observed in LHL fabrics.

Table 5.

Air flow resistivity and structure parameter information of heat treated PP/glass fiber/PP layered fabrics (PGP), PP–glass fiber intimate blend fabrics (PGI), and PLA/hemp/PLA layered fabrics (LHL).

Fabric	Basis weight (kg·m^–2)		Thickness (mm)		Porosity (1 – ρ _w /ρ _f )		Massivity (ρ _w /ρ _f )		Airflow resistivity (10³Pa·s/m²)
Fabric	Mean	σ	Mean	σ	Mean	σ	Mean	σ	Mean	σ
PGP
Control	1.57	0.13	12.4	0.27	0.90	0.01	0.10	0.01	47.8	4.2
125℃	1.86	0.26	10.6	0.72	0.83	0.01	0.17	0.01	93.5	13.4
145℃	1.53	0.16	9.61	0.40	0.84	0.01	0.16	0.01	86.5	11.9
165℃	2.10	0.32	6.81	1.01	0.69	0.05	0.31	0.05	…	…
185℃	1.63	0.25	6.39	0.59	0.75	0.03	0.25	0.03	…	…
PGI
Control	1.49	0.14	13.1	0.64	0.91	0.01	0.09	0.01	46.1	3.93
125℃	1.29	0.14	10.0	0.30	0.88	0.01	0.12	0.01	46.3	7.66
145℃	1.69	0.18	9.86	0.33	0.83	0.01	0.17	0.01	79.5	9.69
165℃	2.01	0.21	7.08	1.03	0.71	0.07	0.29	0.07	…	…
185℃	2.36	0.36	7.17	0.65	0.67	0.03	0.33	0.03	…	…
LHL
Control	1.32	0.08	12.7	0.29	0.91	0.01	0.09	0.01	25.0	3.7
125℃	1.48	0.16	8.56	0.82	0.86	0.01	0.14	0.01	56.1	6.23
145℃	1.67	0.18	7.55	0.56	0.83	0.02	0.17	0.02	92.4	24.8
165℃	2.02	0.22	4.89	0.33	0.63	0.01	0.37	0.01	…	…
185℃	1.58	0.18	3.90	0.26	0.62	0.02	0.38	0.02	…	…

Comparison of model estimates and real data points of untreated PGP, PGI, and LHL fabrics and ones treated at 125℃ or 145℃ are shown in Figure 1, where r₀ is air flow resistivity in mks rayl/m or Pa·s·m⁻², ρ_w is the density of web, and ρ_f is the weighted average density of the fiber in kgċm⁻³, and ã is the weighted root mean square fiber diameter of the fabric in meters. Here, the model generated by Yilmaz et al. [8] for the compressed nonwoven composites was fitted to the heat-treated samples in the current research. Similar to the compressed samples, the airflow resistivity is closely related to the massivity, which equals to unity minus porosity, given as the density of the web divided by the density of the constituent fiber. As seen in the figure, while the trends of PGP and PGI are close to each other, LHL is following a different trend. This might be due to the fact that PGP and PGI are composed of the same fiber mix, polypropylene, and glassfiber; whereas LHL is composed of PLA and hemp fibers, which behave differently in different temperatures, especially the thermoplastic polymers: polypropylene and PLA.

Figure 1.

Comparison of statistical model estimates vs. actual values for air flow resistivity values of heat-treated PGP, PGI, and LHL.

The steeper slope of heat-treated LHL was removed from the model, and the remaining data points of heated fabrics were compared to the model established before [2] which included the untreated, compressed, or alkalized, single- and multi-fiber fabrics. The air flow resistivity of the unheated fabrics can be predicted by the massivity and the fiber size to high precision. The higher the massivity and the lower the fiber size, the higher is the air flow resistivity. The graph including the unheated fabrics from [2] and thermal treated fabrics from current research is shown in Figure 2. As seen from the figure, the heated fabrics follow a different trend compared to the unheated fabrics. This might be due to the structural changes in the polymeric materials with heat. Under heat, shrinkage has been observed in fabrics: both the thickness and the planar dimensions decreased. The steeper slope gives evidence that the dimensional change does not explain all of the difference, as it is already included in the model. There might be changes in the fiber shape or size due to relaxation.

Figure 2.

Comparison of air flow resistivity behaviors of the heated vs. unheated fabrics. The heated fabrics do not include LHL.

The air flow resistivity behaviors of all fabrics treated at different temperatures are compared in Figure 3. It is seen that as the temperature increased, the variation in PGP and LHL also increased, whereas this is not observed in PGI. The reason may be that the individual layers in the sandwich structures (PGP and LHL) were composed of different fibers that acted differently under heat, whereas all layers in PGI include the same fiber mix. For PGP and PGI, the slope at 145℃ is steeper than it is for unheated samples. For LHL, the high variation makes it impossible to obtain a trend.

Figure 3.

Comparison air flow resistivity behaviors of the fabrics heated at different temperatures in ℃.

Effect of heat treatment on NAC

In Figure 4, the average NAC values of heat-treated fabrics are shown. In general, heat treatment led to a decrease in the sound absorption coefficient values, as expected. After heat treatment, not only the thickness, which is a very important parameter for sound absorption, decreased; but also the fabric shrank in the planar dimensions. This made the fabric even denser. At higher temperatures, the fabrics lost their bulky fibrous form and turned into rigid materials which reflected sound rather than absorbing it. For PGP and PGI fabrics, the data points at 165℃ and 185℃, and for LHL, the ones at 185℃ follow a different trend from those of untreated or for lower temperatures. The reason for this should be that these temperatures are higher than melting point of PP and PLA, respectively. At and above the melting point, these polymers melted so the fabric became more rigid and less porous once cooled and the rigid structure behaved differently than the former flexible textile structure.

Figure 4.

Average normal-incidence sound absorption coefficient of fabrics treated at different temperatures. (a) PGP, (b) PGI, and (c) LHL.

The model below was generated with the variables of frequency, air flow resistivity, and thickness. In addition to air flow resistivity, the frequency of sound and the thickness of the porous material substantially affect sound absorption. Though the relationship is not linear, as the frequency and the thickness increases, the absorption performance of the porous material increases [2]. Fabrics treated at 165℃ or 185℃ were excluded from the model, as the air flow resistivity measures for those temperatures had been eliminated. A very high coefficient of determination, R² value, of 0.98 was achieved.

The comparison of the model estimates and the real data points are given in Figure 5. There, y-axis gives the real NAC values and x-axis is the model prediction which is a function of frequency, air flow resistivity, and thickness. As seen from the figure, all three fabrics agreed well with the model:

α_{n} = \sin (- 5.09 \times 10^{- 1} + Of + ⪻_{0} + Ql),

(4)

where α _n is the normal-incidence sound absorption coefficient (NAC), f is frequency in Hertz, r_o is air flow resistivity in mks rayl/m, l is fabric thickness in meters, O is 2.53 × 10⁻⁴Hz⁻¹, P is 1.39 × 10⁻⁶kg⁻¹ċm³ċs, and Q is 3.70 × 10¹m⁻¹. Here, O, P and Q are coefficients which were given units in order to establish consistency of units in the formula.

Figure 5.

Comparison of statistical model estimates vs. actual values for NAC of heat-treated fabrics. X-axis is a function of frequency, thickness, and air flow resistivity (equation (4)).

Among heat-treatment parameters, temperature and duration have been found to be the significant factors. To reflect the difference in the structure of the starting materials, fiber diameter, a’, and fiber density, ρ_f, were included in the model, as shown in the equation (5). To be consistent with the normalization of air flow resistivity in the previous study on the effect of compression on sound absorption [8], sin⁻¹(α_n) was normalized by area weight [gsm × 10⁻³]^1.6. All treatment temperatures were included in the model. A reasonable R² of 0.85 was achieved.

As seen in equation (5), the temperature term took place as the “difference between treatment temperature and the melting point of the thermoplastic fiber of the nonwoven composite of interest” rather than “its absolute value”. The difference between the treatment temperature and the melting point of the polymer has been as very important factor in terms of NAC, as it is represented by three terms in equation (5). The cross term t × ΔT evidences that the effect of temperature on NAC changes with treatment duration. This might be due to the fact that the change in the polymer morphology is dependent on the duration of exposure to high temperature. The other cross-term f × ΔT explains that the sound absorption behavior of the nonwoven composite in terms of sound frequency underwent a change with heat treatment, which might also predicate on the structural change of the material due to exposure to high temperature.

Figure 6 gives the comparison of model estimates and real data points. There, y-axis gives the real NAC values and x-axis is the model prediction. As seen from Figure 6, all of the three fabrics agreed well with the model, with LHL giving slightly lower values. Figure 6 shows that fabrics treated at 165℃ tended to give lower values. The reason for this might be the shrinkage of thermoplastic layers at this temperature, which was not observed for the other temperatures. The data points of fabrics treated at 165℃ were excluded as shown in Figure 7. This leads to a 2-point increase in R² to reach 0.87. In Figures 6 and 7, y-axes are a function of normalized NAC and x-axes a function of frequency, temperature, duration, fiber diameter, fiber density, and area mass of the fabric.

α n = \sin [(- S + Uf + V Δ T - Wt Δ T - Yf Δ T + \frac{Z}{a 2 ρ_{f}^{1.6}}) (gsm \times 10 - 3) 1.6],

(5)

where t is duration in minutes, ΔT is the difference between the treatment temperature and the melting point of the thermoplastic polymer in the fabric in Kelvin, gsm is the area mass of the fabric in 10³kgċm⁻², S is 9.30 × 10⁻²kg^−4.8ċm^9.6, U is 8.50 × 10⁻⁵Hz⁻¹ċkg^−4.8ċm^9.6, V is 1.73 × 10⁻¹°K⁻¹ċkg^−4.8ċm^9.6, W is 4.3 × 10⁻²min⁻¹°K⁻¹ċkg^−4.8ċm^9.6, Y is 6.57 × 10⁻⁴°K⁻¹ċHerz⁻¹ċkg^−4.8ċm^9.6, and Z is 2.59 × 10⁶kg^−3.2ċm^−10.8. Here, similar to equation 4; S, V, W, Y, and Z are coefficients that were given units in order to establish consistency of units in the formula. The boundaries suggested for validity are a basis weight of 1.29 to 2.36 kgċm⁻², fiber diameter of 16.3 × 10⁻⁶ to 32.8 × 10⁻⁶ m, a fabric thickness of 3.9 × 10⁻⁴ to 1.31 × 10⁻³ m, porosity of 0.62 to 0.91, and a frequency range of 500 Hz–5 kHz.

Figure 6.

Comparison of statistical model estimates (equation (5)) vs. actual normalized values for NAC of heated fabrics. X-axis is a function of frequency, temperature, duration, fiber diameter, fiber density, and area mass. Fabric types are shown separately.

Figure 7.

Comparison of statistical model estimates (equation (5)) vs. actual values for NAC of heated fabrics. Fabrics treated at 165℃ are excluded. X-axis is a function of frequency, temperature, duration, fiber diameter, fiber density, and area mass (gsm).

Conclusion

The effects of the material and treatment parameters on airflow resistivity and normal-incidence sound absorption coefficient of thermally treated three-layered nonwoven composites have been studied. The material parameters included fiber size and porosity, and the treatment factors included the temperature and duration for both treatments. The thermally treated three-layered nonwoven composites can be classified into three types based on the material content and fiber blend. LHL and PGP were sandwich structures consisting of PLA/hemp/PLA, PP/glassfiber/PP layers, respectively. PGI consisted of three layers of an intimate blend of PP-glass fiber. Both temperature and duration of thermal treatment have been found to affect air flow resistivity and sound absorption. An increase in air flow resistivity and a decrease in absorption have been found with heat treatment. A similarity has been observed between the thermal behaviors of PGP and PGI which included the same thermoplastic polymer fiber. Variation in air flow resistivity of sandwich structure nonwoven composites increased with the increase in temperature, which was not observed in the intimate blend ones. This was attributed to the difference in the behaviors of individual layers of different polymer fibers against temperature. The air flow resistivity of heat-treated nonwovens followed a steeper trend compared to unheated nonwovens per change in material parameters. In terms of treatment parameters, the difference between the thermal treatment and the melting point of the thermoplastics constituent of the nonwoven composite was found to be a significant factor on sound absorption. This effect of treatment temperature on sound absorption changed with treatment duration. The sound absorptive characteristic of the nonwoven composites in terms of sound frequency underwent a change with thermal treatment due to the structural changes with exposure to high temperature.

Footnotes

Acknowledgements

Authors would like to thank Mr. Michael Hodge from Fiber Innovations, Inc., and Ms. Amy Shuttleworth Vining from AGY for donating PLA and glassfibers, respectively, Dr. Behnam Pourdeyhimi of Nonwovens Cooperative Research Center for the nonwoven production, Mrs. Carrie Knoebe Houghston for her valuable help in statistical programming, and Carcoustics Tech Center for the sound absorption measurements. The authors would like to thank also the Turkish Council of Higher Education for granting the primary author a scholarship for her doctoral research and education at North Carolina State University.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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