Chemical treatment of wood fibers to enhance the sound absorption coefficient of flexible polyurethane composite foams

Accepted Manuscript Chemical treatment of wood fibers to enhance the sound absorption coefficient of flexible polyurethane composite foams Hyeon Choe, Giwook Sung, Jung Hyeun Kim PII:

S0266-3538(17)32804-X

DOI:

10.1016/j.compscitech.2017.12.024

Reference:

CSTE 7012

To appear in:

Composites Science and Technology

Received Date: 26 June 2017 Revised Date:

7 November 2017

Accepted Date: 23 December 2017

Please cite this article as: Choe H, Sung G, Kim JH, Chemical treatment of wood fibers to enhance the sound absorption coefficient of flexible polyurethane composite foams, Composites Science and Technology (2018), doi: 10.1016/j.compscitech.2017.12.024. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Chemical treatment of wood fibers to enhance the sound absorption coefficient of flexible polyurethane composite foams

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Hyeon Choe, Giwook Sung, and Jung Hyeun Kim*

Department of Chemical Engineering, University of Seoul,

*

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163 Seoulsiripdae-ro, Dongdaemun-gu, Seoul 02504, South Korea

Corresponding author. Tel.: +82 2 6490 2369; fax: +82 2 6490 2364.

E-mail: [email protected]

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ACCEPTED MANUSCRIPT Abstract Polyurethane foams are commonly used as sound absorption materials in the automobile industry because their well-defined structure allows for effective absorption of sound waves

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through air friction and the structural vibration of cell walls. In this study, chemically treated wood fibers were incorporated into the polyurethane foams to improve their sound absorption coefficient by enhancing the compatibility between the wood fibers and the polyurethane matrix. The open porosity of the composite foams was strongly dependent on the chemical

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treatments of wood fibers as well as the wood fiber contents in the composites, and it was

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related to the air-flow resistivity and tortuosity of the foam materials. Model calculations revealed that high air-flow resistivity led to a high sound absorption coefficient, which agreed well with experimental observations. Therefore, for achieving high sound absorption performance in composite foams, no more than the optimum amount of a coupling agent must be used to improve the interfacial compatibility between the wood fibers and the

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polyurethane matrix. The use of excessive amount of the silane coupling agent led to

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Keywords

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intramolecular agglomeration, thus negatively affecting the sound absorption behavior.

Polyurethane foam; sound absorption; wood fiber; chemical treatment

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ACCEPTED MANUSCRIPT 1. Introduction Noise pollution is a serious issue encountered in driving automobiles, and thus, the development of sound absorption materials has attracted considerable attention in the

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automobile industry to provide comfortable driving conditions for passengers. While driving vehicles, passengers experience uncomfortable noise generated from two different mechanisms: airborne noise (500-8000 Hz) due to collisions of air molecules with the car and structure-borne noise (30-500 Hz) due to mechanical vibrations [1]. Both types of noise

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eventually reach passengers in the surrounding air through air molecular vibrations [2].

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Therefore, it is important to fabricate appropriate materials that can effectively absorb noise by air friction in porous cavities and damping motions on the material body before transferring noise to passengers [3].

Generally, sound waves can be dissipated by panel absorbers and Helmholtz resonators in

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the low-frequency region (< 500 Hz) and by porous materials in the middle- and highfrequency regions (> 500 Hz). Typical porous absorbers are fibrous (such as glass wool, rock wool, and fiberglass) and foam materials [4]. Amongst foam materials, polyurethane (PU)

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foam is commonly used in the automobile industry because of its light weight, high sound absorption, and ease of production [5]. In recent studies, the sound absorption efficiency of

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PU foams was greatly enhanced by modulating the basic formulation (catalyst, cross-linking agent, surfactant, blowing agent, isocyanate) [6-11]. In addition, because of their economical and eco-friendly benefits, various types of natural fillers have also been employed in manufacturing PU foams as sound absorbing materials, including bamboo leaves and stems [12], rice hulls [13], and cotton and wool fibers [14]. When adding fillers to PU foams, the interfacial compatibility between the filler surface and the PU matrix is a key parameter for 3

ACCEPTED MANUSCRIPT improving the physical properties of filler–PU composite foams. For example, NaOH and silane treatments for wood fibers significantly improved the mechanical properties of wood– plastic composites [15].

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In this study, chemically treated wood fibers were used to fabricate wood-fiber–PU composite foams to improve their sound absorption efficiency. First, the effect of wood fibers in the composite foams was determined by examining the sound absorption coefficient with

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various wood fiber contents. Second, a fixed amount of wood fibers was used for the composite foams after sequential chemical treatments with NaOH and a silane coupling agent.

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Then, the optimum amount of coupling agent to provide the highest sound absorption coefficient was determined. The chemically treated wood fibers were examined using Fourier transform infrared (FTIR) spectroscopy to evaluate qualitative changes in the wood fiber surfaces. In order to better understand the relationships between the sound absorption

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coefficient and the physical properties of composite foams, model calculations for sound absorption coefficient were also performed by adjusting material parameters such as air-flow resistivity, tortuosity, and solid porosity. In addition, the cellular morphology (i.e., pores and

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cavities, open porosity) and thermomechanical properties were also measured to concretely

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interpret the sound absorption results.

2. Experimental 2.1. Materials

Wood fibers (ABOCEL® C100) were obtained from JRS to fabricate the PU composite foams, and their particle size and bulk density were 31-100 µm and 172 g/L, respectively.

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ACCEPTED MANUSCRIPT The fibers were dried in a convection oven for 24 h at 80 °C prior to use. Sodium hydroxide (Samchun Pure Chemical), and γ-aminopropyltriethoxysilane (APTES, 99%, Sigma-Aldrich) were used to chemically treat the wood fibers. For the silane hydrolysis reaction, deionized

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water (resistivity≥18 MΩ), ethanol (95%, Samchun Pure Chemical), and acetic acid glycol (Duksan Pure Chemicals) were used. The acetic acid glycol was used to adjust the pH of the silane hydrolysis solution. Polyether polyol (KE-810, KPX Chemical, OH value: 28±2, Mw:

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6000 g/mol, fav=3) and isocyanate (KW 5029/1C-B, BASF, %NCO: 35±0.5) were used to fabricate flexible PU foams. KW 5029/1C-B consists of 78% 4,4'-methylene bis(phenyl

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isocyanate), 5% benzene 1,1-methylene bis(4-isocyanato) homopolymer, and 17% toluene diisocyanate. NE-1070 (N,N'-dimethylaminopropylurea, Air Products and Chemicals) and NE-210

(N,N,N'-trimethylhydroxyethyl-bis-(aminoethyl)ether

and

bis-

dimethylaminopropylurea, Air Products and Chemicals) were used as gelling and blowing

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catalysts, respectively. A silicone surfactant (L-3002, Momentive) was used to obtain a wellscattered cell structure. Diethanolamine (DEA, Sigma-Aldrich) was used as a chainextending component. Carbon dioxide gas was generated by a blowing reaction between

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isocyanate functional groups and water molecules.

2.2. Chemical treatments for wood fibers For the NaOH treatment, a 5 wt% NaOH solution was prepared with deionized water, and the wood fibers were dispersed in the NaOH solution at room temperature under vigorous stirring for 30 min. The treated wood fibers were filtered and washed with deionized water until the pH of the washed solution became neutral. Subsequently, the washed fibers were 5

ACCEPTED MANUSCRIPT dried in a convection oven at 60 °C for 24 h. Some of the NaOH-treated wood fibers were subsequently silanized. For hydrolysis reactions of the APTES molecules, 1-5 wt% APTES was mixed with 95%

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ethanol and then stirred for 1 h at pH 4.5 for a rapid hydrolysis reaction. Next, the NaOHtreated wood fibers were dispersed in the hydrolyzed silane solution to silanize the wood fiber surfaces for 2 h at room temperature. The silanized wood fibers were filtered, rinsed

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thoroughly with ethanol, and dried in the convection oven for 24 h at 60 °C.

2.3. Synthesis

A one-shot method was used to fabricate the PU composite foams. For the one-shot polymerization, a polyol system composed of mutually non-reactive components such as

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polyol, catalysts, crosslinking agents, blowing agents, and a surfactant was first prepared in a 400 mL paper cup according to the experimental formulations shown in Table 1. Each polyol mixture was stirred for 10 min at 1,700 rpm using a mechanical stirrer, and the wood fibers

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were homogeneously dispersed during this mixing time. With this uniformly mixed polyol system, isocyanate components were subsequently added to the polyol systems with equal

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NCO indices and further mixed for 10 s at 6,000 rpm. After mixing, each mixture was loaded into an aluminum mold (230 mm × 230 mm × 30 mm) and kept at 60 °C for 20 min to complete the reaction. Then, the composite foams were removed from the mold and further dried in a convection oven at 60 °C for 30 min. The foams were stored under ambient condition with a relative humidity of 50±10% before being cut into samples for subsequent measurements. 6

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2.4. Characterizations In order to examine the effects of chemical treatments on the surface characteristics of

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wood fibers, FTIR spectroscopy (Frontier, PerkinElmer) was performed. The instrument was equipped with an attenuated total reflection accessory. The FTIR spectra of the untreated and treated wood fibers were obtained from 8 scans and a resolution of 4 cm-1. Scanning electron

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microscopy (SEM, SNE-3000M, SEC, at 15 kV) was used to examine the surface

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morphologies of the fibers and the cell structures of the PU composite foams. For the image analysis, Image Pro Plus software (Media Cybernetic) was used to analyze the cavity and pore sizes in the SEM images, and the relative ratios of different types of pores (open, partially open, and closed) were also determined. Fifteen images of each PU composite foam were analyzed. Dynamic mechanical analysis (DMA, Q800, TA Instruments) was used to

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analyze the damping properties of the PU foams under compressive deformation mode of at a frequency of 1 Hz and a strain amplitude of 40 µm. The temperature was varied from −80 °C to +20 °C with a heating rate 5 °C/min. For the DMA experiments, disk-shaped samples were

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cut to a thickness of 8 mm and diameter of 40 mm. To measure the sound absorption

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coefficient, two impedance tube devices were used for low (SW420, BSWA) and high (SW470, BSWA) frequencies, with two 1/4 inch microphones (MPA416, BSWA). For the sound absorption measurements, the thickness of all samples was 20 mm, while the diameters were 100 mm for low frequencies (63-1600 Hz) and 30 mm for high frequencies (1000-6300 Hz). The results of low and high frequencies were combined using VA-Lab software (BSWA) for single-range plots.

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3. Results and discussion 3.1. Optimization of the wood fiber content in PU composite foams

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For the fabrication of composite materials, the types and contents of fillers are crucial for obtaining the optimum performance for their final applications. In this work, wood-fiber–PU composite foams were manufactured to improve the sound absorption efficiency of no-filler

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foams. As a first step, the optimum amount of wood fibers in the PU composite foams was

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determined by considering the sound absorption coefficient as well as the porous morphology.

3.1.1. Morphology

Porous morphology is strongly related to the acoustic behavior of foam materials because

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sound wave propagations are directly influenced by the friction of air molecules and cell walls. Fig. 1 shows a typical image of cavities and pores in wood-fiber–PU composite foams, including a schematic diagram of a cavity with various pore types. Due to the rapid

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generation of CO2 gases from intermediate carbamic acids in the early stage of reactions, the CO2 molecules inflate cavities until they meet other cavities. Then, pores are created when

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the thinned cavity walls cannot endure the pressure emerging from both sides. At this stage, the types of pores are strongly dependent on the cavity wall thickness and drainage flow rate. Generally, in the case of thin cavity walls, open pores are dominantly produced due to the low wall strength to the cavity pressures and due to the high drainage flow rate. However, if the cavity walls become thicker, they tend to solidify at a reduced drainage flow rate before forming fully open pores, and thus, partially open pores are predominant. In addition, if the 8

ACCEPTED MANUSCRIPT gelling reaction completes before the cavity walls are broken, the pores remain closed

[16].

Average cavity and pore sizes were analyzed from about 100 cavities and 400 pores for all samples, and the open porosity was also calculated using the following equation [17]:

where

=( ,

+

, and

/2)/(

+

+

)

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(1)

are the numbers of open, partially open, and closed pores,

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respectively. Table 2 summarizes the cavity and pore sizes, and open porosity for various raw wood fiber (RWF) contents in the PU composite foams. As the RWF contents increased, the

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sizes of the pores and cavities remained nearly unchanged. However, the open porosity decreased with the increasing RWF content up to 1 wt%, but it increased beyond this RWF content. With less than 1 wt% RWF, the wood fiber played a crucial role in increasing the matrix viscosity, thus leading to the formation of thick cavity walls and reducing the drainage

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flow rate. Therefore, the open porosity decreased with the increasing RWF content up to this limit. With higher RWF contents, the interfacial failures between the RWF surfaces and the PU matrix became significant, and thus, the open pore ratio increased. The resulting open

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porosity was closely related to the sound absorption coefficient because sound waves can

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experience various collisions through the open porous flow paths.

3.1.2. Sound absorption coefficient (α) The sound absorption coefficient α is defined as the ratio of the sound energy absorbed by a sample to the incident sound energy as a function of frequency. Generally, sound energy can be absorbed by absorbent materials through thermal dissipation mechanisms. Therefore, 9

ACCEPTED MANUSCRIPT material factors such as air-flow resistivity, tortuosity, and porosity are key parameters in improving α. To better understand the effects of each parameter on α, model calculations with analytical expressions were performed by referring to the work of Fahy and Thompson

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[2]. Table 3 summarizes the analytical equations used in the calculations, and Fig. 2 shows the effects of air-flow resistivity (r), tortuosity (s), and solid porosity (ϕ) on α. By increasing the air-flow resistivity by 50%, α increased by about 20% at the peak values. Increased air-

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flow resistivity indicates that more interconnected pores are blocked, and thus, more sound energy can be lost through the internal pores. In the case of a 50% tortuosity increase, the

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peak location moves to the lower-frequency region (≅ higher-wavelength condition). Tortuosity is the ratio between the actual distance air molecules pass through and the foam thickness. Tortuosity mainly affects the location of the quarter-wavelength point (λ/4, for maximum energy dissipation), where the path length in porous materials coincides with the

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quarter wavelength of the incident sound energy [4, 18]. Therefore, the increased quarterwavelength point at high tortuosity shifted the sound absorption peak to the lower-frequency region. In addition, the 20% increment in the solid porosity slightly changed α for the entire

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frequency range due to the increased air friction with sound waves.

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Fig. 3 shows the α values of the RWF–PU composite foams as a function of the RWF content. As shown in Fig. 3a, α tended to increase with the increasing RWF content up to 1 wt%, but it started to decrease after this point. This tendency can also be related to the open porosity (Table 2) and air-flow resistivity (Fig. 2). Therefore, the maximum α at 1 wt% RWF content was achieved with the lowest open porosity and the highest air-flow resistivity. Fig. 3b shows the frequency values at the peak locations of α as a function of the RWF content. 10

ACCEPTED MANUSCRIPT The lowest frequency was obtained at 1 wt% RWF content, indicating that the highest tortuosity was achieved at this RWF content, as shown in Fig. 2, which demonstrates the tortuosity effect. Considering the changes in α with the RWF contents, 1 wt% of wood fibers

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was used for subsequent experimental investigations with chemical treatments.

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3.2. PU composite foams with chemically treated wood fibers (NWF, AWF)

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3.2.1. Chemical treatments

In manufacturing polymer composites, good interfacial adhesion between the polymer matrix and filler surface plays a crucial role in achieving outstanding physical properties [15]. First, the wood fibers were treated with NaOH (hereafter denoted by NWF) to increase the

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surface area, thereby increasing the number of possible coupling reaction sites, and to promote mechanical interlocking between the fiber and matrix [19]. Wood fibers mainly consist of cellulose, hemicellulose, and lignin, with cellulose as the major component.

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Cellulose is strongly resistant to NaOH chemical attack due to its glucose structure, which forms crystallites through hydrogen bonding, but hemicellulose is soluble in NaOH solutions

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because of its amorphous structure and various monomers such as xylose, arabinose, mannose, and uronic acid [20]. In addition, lignin has an aromatic ring and hydroxyl groups, and it is also soluble in NaOH solutions [21]. Therefore, hemicellulose and lignin could be partially dissolved out by the NaOH treatment. As a result, NWF generally exhibited a decreased absorbance spectrum, as shown in Fig. 4. In the following chemical treatment step, hydroxyl functionalities introduced by the 11

ACCEPTED MANUSCRIPT APTES hydrolysis could combine with wood fiber surfaces by condensation reactions with the hydroxyl groups of the wood fibers. Thus, APTES-treated wood fiber (AWF) has amino groups connected to the silane structure. After the APTES treatment, the wood fiber surfaces

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were also examined with FTIR spectroscopy, and Fig. 4 reveals the increments in the absorbance intensities at 3400-3200 cm-1, 1650 cm-1, and 1590 cm-1 from amine groups and at 1150-950 cm-1, and 896 cm-1 from Si−O bonds. Therefore, the wood fiber surfaces are

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believed to be well modified after the NaOH and APTES treatments. Table 4 summarizes the specific band positions for various bonds (OH, CH, COOH, C=O, aromatic ring) from

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cellulose, hemicellulose, and lignin components as well as for the APTES coupling agent. Fig. 5 shows images of the wood fiber surfaces before and after surface treatments. The images were clearly different after the NaOH treatment, as shown in Fig. 5b, and the wood fiber surfaces exhibited many voids, which was possibly due to the depletion of the

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hemicellulose and lignin portions. Tomyangkul et al. also reported that treating bagasse and oil palm fibers with NaOH produced porous structures by dissolving out hemicellulose and lignin, respectively [22]. Lin et al. showed increments in the pore sizes and distributions of

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Solka-Floc fibers due to NaOH treatments [23]. Therefore, NWF surfaces are believed to be advantageously compatible with the PU matrix due to their potential for mechanical

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interlocking. In addition, the increased surface area after the NaOH treatments provides higher chances of coupling reactions with the hydroxyl groups of the hydrolyzed APTES. Thus, the wood fiber surfaces were sequentially modified by chemical treatments in manufacturing the PU composite foams. SEM images showing interfacial adhesive characteristic between wood-fiber and PU matrix are demonstrated in Fig. 6, and the AWF reveals better adhesive image with PU matrix than the RWF case due to the improved 12

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3.2.2. Morphology The effects of the chemical treatments on the foam morphology, pore and cavity sizes, and open porosity were analyzed using SEM images. Table 5 shows the results of the wood-

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fiber–PU composite foams including RWF, NWF, and AWF at 1 wt%. The average pore and cavity sizes were similar at 1 wt% wood fiber content, regardless of the chemical treatments,

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but the open porosity revealed a decreasing tendency with additional chemical treatments. First, compared with the no-filler case, the addition of wood fiber decreased the open porosity, which could be due to the increased matrix viscosity and reduced drainage flow rate of the thick-walled foam. Then, chemical treatments with NaOH and APTES further reduced the

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open porosity, which could be attributed to the increased compatibility between the PU matrix and the treated fiber surfaces. At the lowest open porosity value exhibited by AWF2, the highest air-flow resistivity and tortuosity in the composite foams were expected. However,

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the open porosity began to increase again beyond AWF2 (2 wt% APTES used in 95% ethanol), which could be due to the high possibility of intramolecular agglomerations of

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APTES through hydrogen bonding between the hydroxyl and amine groups on the wood fiber surfaces [15]. Therefore, an excess amount of APTES when treating wood fibers might negatively affect compatibilization. The effect of chemical treatments on the relative numbers of pore types is again shown in Fig. 7 for the RWF, NWF, and AWF2 (lowest open porosity) cases. The open pore ratio decreased with further treatments, but the partially open and closed pore ratios increased due 13

ACCEPTED MANUSCRIPT to the additional chemical treatments. The decreasing tendency of the open pore ratio can be considered similar to the effect of the open porosity result because it is closely related to the relative number of open pores. The lowest open pore ratio (the highest partially open and

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closed pore ratios) with AWF2 leads to higher air-flow resistivity and tortuosity in the composite foams, which can ultimately enhance the sound absorption behavior.

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3.2.3. Sound absorption coefficient (α)

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In order to demonstrate the effect of the chemical treatments on α, Fig. 8 plots the highest values of α from the PU composite foams with chemically treated wood fibers. The sound absorption coefficient increased with additional treatments in the order RWF, NWF, AWF2 point, but past AWF2, the peak α began to decrease. This phenomenon is exactly analogous

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to the open porosity results, as demonstrated in Table 5, because the sound absorption is directly influenced by the air-flow resistivity and tortuosity, which are closely dependent on the open porosity of foam materials. In APTES treatments, excess APTES played a negative

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role in compatibilizing the wood fiber surfaces and the PU matrix by forming intramolecular hydrogen bonds. Therefore, it is important to apply the optimum amount of coupling agent

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when treating wood fiber for manufacturing wood-fiber–PU composite foams to improve their sound absorption.

Fig. 9a shows a plot of α for the wood-fiber–PU composite foams over the entire frequency range. With the increasing chemical treatment steps, the peak values of the α curves moved not only to a higher position but also into a lower-frequency region from the reference case. As shown in the inset of Fig. 9a, adapted from model calculations (Fig. 2), the 14

ACCEPTED MANUSCRIPT peak α value increased with the increasing air-flow resistivity, and it moved to the lowerfrequency region with increasing tortuosity. The higher air-flow resistivity makes the sound wave remain longer in the foam and collide with more air molecules, thus increasing the

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sound absorption. In addition, the increased path length of sound waves at a higher tortuosity leads to a higher possibility of energy dissipation with a longer quarter-wavelength condition, thus shifting the peak to the low-frequency region. The improved sound absorption

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coefficients of the wood fiber-PU composite foams revealed higher values than our previous results containing other types of fillers in PU composite foams [24]. Therefore, the improved

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interfacial compatibility between the wood fiber surfaces and the PU matrix led to a low open porosity in the wood-fiber–PU composite foams, which induced a high sound absorption property with a high air-flow resistivity at a low open porosity. In the study of Verdejo et al. [25], the acoustic activity is defined by normalizing the integrated area of the sound

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absorption coefficients by the frequency range. Fig. 9b shows the result of the acoustic activity of the present study, and it reveals the same trend with the sound absorption coefficients. Furthermore, our results (highest value of 0.76) showed similar or slightly

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improved value in acoustic activity than their results (highest value of 0.74).

3.2.4. Thermomechanical properties In addition to the sound absorption due to the friction of air molecular collisions with sound waves through the porous medium, sound energy can also be dissipated as heat energy by the vibrational damping motion of the cell walls. The damping potential of materials is generally a measure of how much kinetic energy can be lost by material deformations 15

ACCEPTED MANUSCRIPT through molecular rearrangements. Fig. 10 shows the thermomechanical analysis for the storage modulus (E') and loss modulus (E''). E′ and E″ increased with further surface treatments. The E′ increment indicated that the material strength increased due to the

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enhanced surface compatibility between the wood fiber surfaces and the PU matrix, and this high E′ value is also very important for withstanding high-load conditions with long-term use [24]. In addition, E″ is directly related to the energy dissipation through polymer chain

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movements due to incoming energy. As shown in Fig. 10b, E″ increased with the increasing chemical treatments steps from RWF, NWF, to AWF2, and thus, sound absorption due to

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damping effects through molecular movements also improved with the increasing interfacial adhesive strength (see Fig. 9a). In other similar studies, tan δ (E″/E′) can also be used to analyze sound absorption ability of damping materials. Fig. 10c shows the tan δ by comparing with E″ in Fig. 10b, and it shows the same trend with E″ results for various wood

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fiber cases. Therefore, the thermomechanical property is very useful for understanding the sound absorption efficiency achieved by the molecular rearrangements of polymer chains,

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4. Conclusions

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referring to both the loss modulus of foam materials and the open porosity (shown in Table 5).

Wood-fiber–PU composite foams were fabricated including raw and NaOH- and APTEStreated wood fibers. For the fabrication of wood-fiber–PU composite foams, interfacial compatibility between the wood fiber surfaces and the PU matrix is a crucial factor for improving the physical properties of the foam. Chemical treatment of the wood fibers changed the relative ratios of open, partially open, and closed pores, and thus, the open 16

ACCEPTED MANUSCRIPT porosity decreased with the increasing number of chemical treatments. As the open porosity decreased, air-flow resistivity and tortuosity increased, which resulted in longer paths for air to pass through in the porous medium. Higher sound absorption was achieved at the higher

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air-flow resistivity. The peak values of α indicated that the optimum content of wood fiber in the wood-fiber–PU composite foams was 1 wt%. For treating the wood fibers with the APTES coupling agent, an excess of APTES led to a negative effect on the interfacial compatibilization due to intramolecular hydrogen bonding. Therefore, using the optimum

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concentration of this coupling agent in chemical treatments is necessary to improve the α

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values. The thermomechanical analysis showed that E′ and E″ increased with the additional chemical treatments, which also indicated the high potential of these foams for long-term under high-load conditions and high energy dissipation through the molecular rearrangements

Acknowledgement

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of polymer chains due to incoming sound energy.

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This research was partially supported by X-mind Corps program of National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning

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(NRF-2017H1D8A1030582).

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Aust. J. Basic Appl. Sci., 3 (2009) 4610-4617. [19] X. Li, L.G. Tabil, S. Panigrahi, Chemical treatments of natural fiber for use in natural fiber-reinforced composites: a review, J. Polym. Environ., 15 (2007) 25-33. [20] F.M. Gírio, C. Fonseca, F. Carvalheiro, L.C. Duarte, S. Marques, R. Bogel-Łukasik, Hemicelluloses for fuel ethanol: a review, Bioresour. Technol., 101 (2010) 4775-4800. [21] M.J. John, S. Thomas, Biofibres and biocomposites, Carbohydr. Polym., 71 (2008) 343364. [22] S. Tomyangkul, P. Pongmuksuwan, W. Harnnarongchai, K. Chaochanchaikul, Enhancing sound absorption properties of open-cell natural rubber foams with treated bagasse and oil palm fibers, J. Reinf. Plast. Compos., 35 (2016) 688-697. [23] K. Lin, M. Ladisch, M. Voloch, J. Patterson, C. Noller, Effect of pretreatments and fermentation on pore size in cellulosic materials, Biotechnol. Bioeng., 27 (1985) 14271433. [24] G. Sung, J.H. Kim, Influence of filler surface characteristics on morphological, physical, acoustic properties of polyurethane composite foams filled with inorganic fillers, Compos. Sci. Technol., 146 (2017) 147-154. [25] R. Verdejo, R. Stämpfli, M. Alvarez-Lainez, S. Mourad, M. Rodriguez-Perez, P. Brühwiler, M. Shaffer, Enhanced acoustic damping in flexible polyurethane foams filled with carbon nanotubes, Compos. Sci. Technol., 69 (2009) 1564-1569.

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[26] M. schwanninger, J.C. Rodrigues, H. Pereira, B. Hinterstoisser, Effects of short-time vibratory ball milling on the shape of FT-IR spectra of wood and cellulose, Vib. Spectrosc., 36 (2004) 23-40. [27] K.K. Pandey, A study of chemical structure of soft and hardwood and wood polymers by FTIR spectroscopy, J. Appl. Polym. Sci, 71 (1999) 1969-1975. [28] J.P. Matinlinna, S. Avreva, L.V.J. Lassila, P.K. Vallittu, Characterization of siloxane films on titanium substrate derived from three aminosilanes, Surf. Interface Anal., 36 (2004) 1314-1322. [29] M. Gueye, T. Gries, C. Noel, S. Bulou, P. Choquet, E. Lecoq, S. Migot, T. Belmonte, Dissociation of 3-aminopropyltriethoxysilane in Ar-N2 afterglow: application to nanoparticles synthesis., Int. Symp. Plasma Chem., 22 (2015). http://www.ispcconference.org/ispcproc/ispc22/P-III-6-33.pdf (accessed 06.16.17). [30] J.G. Gwon, S.Y. Lee, G.H. Doh, J.H. Kim., Characterization of chemically modified wood fibers using FTIR spectroscopy for biocomposites, J. Appl. Polym. Sci., 116 (2010) 3212-3219. [31] D.E. Leyden, J.B. Atwater, Hydrolysis and condensation of alkoxysilanes investigated by internal reflection FTIR spectroscopy, J. Adhes. Sci. Technol., 5 (1991) 815-829.

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ACCEPTED MANUSCRIPT Table 1. Formulation details for manufacturing wood fiber–PU composite foams. Material

NWF AWF

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0.60

4.00

1.32

0 (0)

0.34 (0.2)

1.02 (0.6)

1.71 (1.0)

2.40 (1.4)

NaOH treated wood fiber at 1 wt%

APTES treated wood fiber at 1 wt% 61.38

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1.44

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Isocyanate** (KW 5029/1C-B) * Raw wood fiber (RWF, not treated)

100

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Alcohol (KE-810) Gelling catalyst (NE-1070) Blowing catalyst (NE-210) Chain extender (DEA) Blowing agent (H2O) Surfactant (L-3002) RWF* (Arbocell C100) (wt%)

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Polyol system

Content (g)

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3.10 (1.8)

ACCEPTED MANUSCRIPT Table 2. Sizes of pores and cavities in raw wood fiber–PU composite foams. *The open porosity was calculated based on the ratio of open, partially open, and closed pores ( , , and , respectively). Cavity size (µm) 731 ± 165 772 ± 272 755 ± 230 747 ± 224 753 ± 238 760 ± 242

45 /46 /47

Open Porosity*

0.626/0.206/0.168 0.611/0.221/0.168 0.593/0.238/0.169 0.583/0.247/0.170 0.592/0.240/0.168 0.598/0.237/0.165

0.729 0.721 0.712 0.706 0.712 0.717

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Pore size (µm) 187 ± 97 212 ± 104 203 ± 99 200 ± 92 206 ± 93 208 ± 101

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Parameters

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Table 3. Summary of modeling parameters and analytical expressions for calculations. Expressions

4 ( ) cos /( ′

( )

′

cos + 1

"

+ #$

′

cos

"

)

′

( ′ /, Non-dimensional characteristic specific acoustic impedance

(′

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- . " /(&/ " ) − ,. /(0/ &/ " ) Complex wavenumber [rad m−1]

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′

Sound absorption coefficient ′ − & '()*+ Non-dimensional normal specific acoustic impedance

ω

1⁄"

(1 − /3)1⁄" Non-dimensional complex wavenumber ω0/ s/rφ Non-dimensional frequency 2πf Circular frequency [rad s−1]

θ: incident angle [rad], *: foam thickness [m], f: ordinary frequency [Hz], r: air-flow resistivity [kg s−1 m−2], s: tortuosity, φ: solid porosity, ρ0: air density [kg m−3], c0: speed of sound in air [m s−1]

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ACCEPTED MANUSCRIPT Table 4. FTIR peak position for wood fiber and APTES coupling agent. Band Position (cm-1) 3630 − 3030 2955 − 2837 Wood 1730 − 1725 Fiber 1640 1605 − 1593 1120 − 985 3400 − 3200 1650 APTES 1590 1150 − 950 896

Reference

[26, 27]

[28] [29] [28] [30] [31]

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Assignment O-H asymmetric stretching C-H stretching Carboxylic groups in hemicellulose C=O bonds in hemicelluloses Aromatic ring skeletal vibrations in lignin C-O bond NH2 stretching vibration NH asymmetric bending NH bending vibration Si-O-Si asymmetric stretching and /or Si-O-C band Si-OH stretch

Table 5. Sizes of cavities and pores in chemical treated wood fiber–PU composite foams.

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Wood Fiber Types Pore size (µm) Cavity size (µm) Open Porosity None 187 ± 97 731 ± 165 0.729 RWF 200 ± 92 747 ± 224 0.706 NWF 198 ± 91 742 ± 223 0.680 1 188 ± 100 743 ± 227 0.671 2 185 ± 90 736 ± 166 0.651 AWF* 3 191 ± 90 739 ± 179 0.661 4 194 ± 101 745 ± 199 0.671 5 199 ± 106 762 ± 208 0.673 * AWFx means that NWF was treated at the x wt% APTES in 95% ethanol.

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Fig. 1. Typical SEM image showing cavities and pores in wood fiber-PU composite foams. The yellow dashed circle represents cavity area.

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Fig. 2. Model calculations for sound absorption coefficient considering air flow resistivity (r), tortuosity (s), and solid porosity (ϕ). For reference, r = 1.2×104, s = 1.6, ϕ = 0.8; 50% increment of flow resistivity, r = 1.8×104; 50% increment of tortuosity, s = 2.4; 20% increment of solid porosity, ϕ = 0.96

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Fig. 3. Sound absorption coefficient (α) as a function of wood fiber contents in the PU composite foams for the peak values (a) and peak frequencies (b).

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Fig. 4. FTIR spectra of RWF, NWF, and AWF for the wave numbers of (a) 3800 to 2600 cm−1, (b) 1820 to 1520 cm−1, and (c) 1220 to 780 cm−1. In AWF measurement, 2 wt% APTES was treated with NWF in 95% ethanol.

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Fig. 5. SEM images of RWF (a), NWF (b), and AWF (c). For AWF measurement, 2 wt% APTES was treated with NWF in 95% ethanol. All images have the same magnification.

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Fig. 6. SEM images of the PU composite foams with RWF (a) and AWF (b).

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Fig. 7. Relative ratios of open, partial open, and closed pores in the wood fiber-PU composite foams at 1 wt% wood fiber content.

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Fig. 8. Peak values from the sound absorption coefficients of wood fiber-PU composite foams at 1 wt % wood fiber content.

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Fig. 9. Sound absorption coefficient (a) and acoustic activity (b) of wood fiber-PU composite foams including various types of wood fibers at 1 wt%. The error bars were calculated from 5 different samples.

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Fig. 10. Thermomechanical analysis for E' (a), E'' (b), and tan δ (c) with wood fiber-PU foams as a function of temperature.