Ultra-porous Flexible PET/Aerogel Blanket for Sound ... - Springer Link

Fibers and Polymers 2009, Vol.10, No.5, 731-737

DOI 10.1007/s12221-010-0731-3

Ultra-porous Flexible PET/Aerogel Blanket for Sound Absorption and Thermal Insulation Kyung Wha Oh*, Duk Ki Kim1, and Seong Hun Kim1

Department of Home Economics Education, Chung-Ang University, Seoul 156-756, Korea Department of Fiber and Polymer Engineering, Hanyang University, Seoul 133-791, Korea

1

(Received April 5, 2009; Revised July 16, 2009; Accepted July 26, 2009)

Abstract: Ultra porous and flexible PET/Aerogel blankets were prepared at ambient pressure, and their acoustic and thermal insulation properties were characterized. Two methods were selected for the preparation of PET/Aerogel blanket. Method I was a direct gelation of silica on PET. PET non-woven fabric was dipped and swelled in TEOS/ethanol mixture, and pH of reaction media was controlled to 2.5 using HCl to promote hydrolysis. After acid hydrolysis, pH was controlled to 7,8,9, and 10 with NH4OH for the condensation. Method II was by the dipping of PET non-woven fabric in the dispersion of Silica hydrogel. The gelation process was same with Method I. However, PET fabric was not dipped in reaction media. After the hydrogel was dispersed and aged in EtOH for 24 hrs, then, PET non-woven fabric was dipped in the dispersion of hydrogel/ EtOH for 24 hrs. The surface modification was carried out in TMCS/n-hexane solution, then the blanket was washed with nhexane and dried at room temperature to prevent the shrinkage. The silica areogels synthesized in optimum conditions exhibit porous network structure. Silica aerogel of highly homogeneous and smallest spherical particle clusters with pores was prepared by gelation process at pH 7. When direct gelation of silica was performed in PET nonwoven matrix (Method I), silica aerogel clusters were formed efficiently surrounding PET fibers forming network structure. The existence of a great amount of silica aerogel of more homogeneous and smaller size in the cell wall material has positive effect on the sound absorption and thermal insulation. Keywords: Aerogel, PET, Sound absorption, Thermal insulation, Hybrid commercial development [4,5]. Recently, the main methods adopted for ambient pressure drying including network strengthen [7-9], solvent exchange/surface modification [10] of wet gels. The latter involves end-capping of hydroxyl groups on the silica surface with chlorosilanes to prevent condensation reactions during drying step [4]. Surface silation under appropriate conditions yields particles with low density as well as quite stable surface hydrophobicity However, the current drawbacks of aerogel are its high production cost, brittleness, and instability toward atmospheric moisture. Once their mechanical properties are improved and the production costs reduced, aerogels can become the insulators of the future. For industrial, commercial, and residential application, a flexible, ultra-porous aerogel blanket is demanded to deliver thermal and sound absorption performance in an easy to handle and environmentally safe product [11-13]. Introducing nonwoven matrix into a silica aerogel network is expected to add mechanical strength to the overall composite. Therefore, the present work aims to produce a flexible and mechanically strengthen hybrid by embedding aerogel in nonwoven fiber matrix which can be dried at ambient pressure, and stable under atmospheric conditions. Two methods were selected for the preparation of PET/ Aerogel blanket; one is a direct gelation of silica on PET nonwoven and the other method is by the dipping of PET nonwoven in the dispersion of Silica hydrogel. The synthesis of aerogel consisted of a two step process - acid hydrolysis of tetraethoxysilane (TEOS) followed by basic condensation

Introduction

With a rise in the standard of living, the enhancement of sound quality of residential building is being gradually important. Harmonious system of adequate interior sound environment with maximum sound facilities accordant with the purpose of architectural buildings like music halls, multipurpose halls, and conference rooms when planning are being demanded [1,2]. Conventionally used sound absorbents for building interior such as glass wool, rock wool, urethane foam, and styrofoam have caused some problems in health, insulation, environment, and flammability [3,4]. Thus, environmentally friendly materials of high flame resistance and high insulation as well as good sound absorption properties are needed. Silica aerogel are unique porous materials containing of more than 90 % air and less than 10 % solid silica in the form of highly cross-linked network structure. Due to their large surface area, very low density, low sound velocity, inflammability, and very low thermal conductivity, silica aerogel have found increased attention in various field such as thermal insulation, flame retardant, sound damping, and drug delivery system [5,6]. The use of aerogels could lead to significant weight reductions. Conventionally silica aerogels are prepared by super critical drying of wet gels to avoid capillary stress [6], but it is so energy intensive and high cost of batch mode hindering *Corresponding author: [email protected] 731

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and solvent exchange/surface modification. The performance of prepared PET/Aerogel blanket by two different methods was evaluated in terms of sound absorption, thermal insulation.

Experimental Materials

Tetraethoxysilane (TEOS) and Trimethylchlorosilane (TMCS) were purchased from Sigma-Aldrich Chemical Co. and Tokyo Chemical Industry Co., respectively. PET non-woven fabric with thickness of 5 mm and density of 0.037 g/cm was purchased from Mirae Trading Co..

Figure 1.

Surface modification of silica.

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Gelation of Silica Hydrogel in PET Non-woven Matrix

Two methods were selected for the preparation of PET/ Aerogel blanket. First method was a direct gelation of silica on PET (Method I). PET non-woven fabric was dipped and swelled in TEOS/ethanol mixture, and distilled water was introduced. The volume of PET non-woven was 190 cm , and the volume of mixture was approximately 600 ml (TEOS:EtOH:H O=1:3:1 (mole ratio)). To promote hydrolysis, pH of reaction media was controlled to 2.5 using HCl. After agitation for 10 minutes, 500 ml of EtOH/H O mixture (EtOH:H O=2:1 (mole ratio)) was poured to the bath and pH of media controlled to 7, 8, 9, and 10 with NH OH for the condensation. This two step gelation was described as follows. After aging for 24 hrs following the addition of NH OH, the reaction media was exchanged with a large amount of ethanol for aging of the silica and washing of unreacted monomer and water. 3

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Second method was by the dipping of PET non-woven fabric in the dispersion of Silica hydrogel (Method II). The gelation process was same with Method I, whereas, PET fabric was not dipped in reaction media. After the silica hydrogel formed in hard cluster, ultrasonication was used to break it. The hydrogel particles were dispersed and aged in EtOH for 24 hrs. Then, PET non-woven fabric was dipped in the dispersion for 24 hrs.

Surface Modification of Silica

Each PET/silica hydrogel blanket was dipped in n-hexane to remove EtOH at 50 C for 24 hrs. The hydrogel blanket was dipped in TMCS/n-hexane solution, and the surface modification was carried out at 50 C for 24 hrs (Figure 1). After modification, the blanket was washed with n-hexane o

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and dried at room temperature to prevent the shrinkage. PET/silica aerogel blankets with pH 7, 8, 9, and 10 prepared by Method I was designated SA1, SA2, SA3, and SA4, and The blankets with pH 7, 8, 9, and 10 prepared by Method II was designated SA5, SA6, SA7, and SA8. Control PET nonwoven without aerogel was designated as SA0.

Characterization of Hydrogel and Aerogel Particles and PET/Aerogel Blanket

To ensure the gelation of silica and modification of hydrogel to aerogel, each particle extracted from the blankets after gelation, and after modification with TMCS/nhexane solution was characterized using FT-IR spectroscopy. The structure of each particle was characterized with wide angle x-ray diffraction (WAXD). The particle size with different pH in gelation was compared. To observe hydrophilicity of each powder, contact angle of water on pellet of each powder. The surface of each powder was observed using scanning electron microscopy. Sound absorption and Frequencydependence of each blanket was characterized using (ISO 10534-1:1996). Three measurements were taken and averaged. The aerogel formation in PET non-woven was observed using Scanning electron microscopy. Thermal insulation property was measured by hot plate method (KS K 0560 A). Five measurements were taken and averaged for density and thermal insulation values.

Results and Discussion Properties of Silica Aerogel

The aerogels used for this experiment were prepared by two step gelation process using tetra ethoxysilane(TEOS). In order to control the basic condensation after acid hydrolysis, pH of gelation was varied from 7 to 10 with NH OH. And to ensure the gelation of silica and modification of hydrogel to aerogel, silica aerogels extracted from the prepared PET/ Aerogel blankets was characterized using FT-IR spectroscopy. FT-IR spectra of silica hydrogel and aerogel were shown in Figure 2. The peaks at 1630 and 3430 cm are due to the hydroxyl group at the end of gel network or residual water molecules. The peaks at 1090 and 460 cm by the Si-O-Si vibration means the successful chemical gelation by 4

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Flexible PET/Aerogel Blanket

Figure 2.

SA1.

FT-IR spectra of Silica powder extracted from sample

covalent bonding between TEOS. However, on the spectrum of aerogel, peaks at 1260, 806, and 2964 results from methyl group with sp3 bonding. The surface modification of silica hydrogel to aerogel could be confirmed by the observation of these peaks. The coupling of hydroxyl group with TCMS also results in the decrease of hydrophilicity. Hydrophilicity of each particle was characterized using contact angle measurement. Since hydrogel and aerogel particles had very low density, they were pelletized to disc form through the high pressure molding. Figure 3 shows the images of water droplet on each pellet and contact angles between water and gel pellet surface. The pH in gelation media affects contact angle obviously. In the case of hydrogel, all the pellet specimens showed excellent hydrophilicity compared to the pellets of aerogel. This is caused by the effect of hydroxyl group at the end of gel surface. As the pH

Figure 3.

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of gelation media increased to 10, the contact angle of hydrogel increased. This may be due to the size and morphology of particles forming gel network. More hydrophilic polymeric hyrogel can be produced by slow condensation process under weak base or neutral pH condition. On the contrary, contact angle of water on aerogel decreased with the increase of pH. More hydrophobic aerogel can be produced with more hydrophilic hydrogel after surface modification. This hydrophilicity drop of aerogel according to the increase of pH shows opposite tendency with the case of hydrogel. This phenomenon can be explained through particle size analysis. Figure 5 shows the SEM morphology of aerogels obtained using EtOH/TMCS/n-Hexane solution for modification of the hydrogels. The synthesized silica areogels exhibit porous network structure. As shown in Figures 4 and 5, particle size of silica aerogel prepared by condensation process under pH 7 (SA1) is smaller and more homogeneous. SA1 shows homogeneous spherical particle clusters with pores. Whereas, SA2~SA4 obtained by condensation at higher pH shows aggregation without porous structure. Their particle size was significantly increased. This is probably due to the formation of colloid gel or gelatin-like sedimentation under fast condensation process. The size of arerogel and surface area are expected to affect their network structure formed in PET nonwoven matrix.

Preparation of PET/Silica Aerogel Blanket

Two methods were selected for the preparation of PET/ Aerogel blanket. Method I was conducted by a direct gelation of silica within PET matrix. Method II was conducted by dipping of PET non-woven fabric in the dispersion of silica hydrogel. Figure 6 shows the SEM

Contact angle of (a) hydrogel and (b) aerogel extracted from PET/aerogel blanket gelized from different pH.


Particle size distribution of silica aerogels extracted from PET/aerogel blankets; (a) SA1 (b) SA2 (c) SA3, and (d) SA4. Figure 4.

Kyung Wha Oh et al.

Figure 5.

Table 1.

morphology of PET/silica hydrogel blanket obtained by two different methods. When direct gelation of silica was conducted in PET nonwoven matrix (Method I), silica aerogel clusters were formed by surrounding PET fibers forming network structure. However, when PET non-woven fabric was dipped in the dispersion of hydrogel particles, it seemed difficult to produce gel network after surface modification under ambient drying. But granular silica aerogels were deposited in the PET nonwoven matrix. In this case small particle sedimentation occurred on the surface of PET fiber and adhered to the PET matrix via Van deer Waals’s forces. As pH of condensation bath increased, the amount of silica aerogel formed in PET nonwovens decreased due to

Figure 6.

SEM images of PET/silica aerogel blanket fracture.

SEM images of aerogel surface.

Density of PET/silica aerogel blanket

Sample SA0 SA1 SA2 SA3 SA4 SA5 SA6 SA7 SA8

Add-on (%) 0.0 (0.00) 390.2 (4.95) 362.2 (6.34) 278.2 (7.14) 250.1 (6.02) 96.4 (5.81) 68.1 (9.95) 82.1 (9.29) 40.1 (6.65)

Mean (S.D.) Density (g/cm3) 0.037 (0.0004) 0.184 (0.0019) 0.174 (0.0024) 0.142 (0.0027) 0.132 (0.0023) 0.074 (0.0022) 0.063 (0.0037) 0.068 (0.0035) 0.053 (0.0026)



difficulty in polymeric gel formation. And the smaller particles produced at low pH can be diffused much easily into the PET nonwoven matrix. The density of PET/silica aerogel blanket prepared is shown in Table 1.

Acoustic and Thermal Properties of PET/Silica Aerogel Blanket

Monolithic aerogels are well known for their low density and low sound velocity [14]. Despite their low density, their rigidity is too high and they behave as rigid solids and exhibit consequently a high reflection coefficient, at low acoustical frequency [14]. However, hybrid PET/aerogel blanket prepared in this work provided better sound absorption. Sound absorption coefficient was measured by ISO 10534-1:1996 (Determination of sound absorption coefficient and impedance in impedance tubes, Part 1: Method using standing wave ratio). Prepared hybrid PET/ silica aerogel blanket was placed at the one side of impedance tube and incident sound wave passed through

Sound absorption coefficient of PET/aerogel blanket prepared by different methods at low frequency region; (a) Method I and (b) Method II. Figure 7.

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from the other side of tube, then sound absorption was measured by receiving sound by microphone. In general, the acoustic propagation in aerogels depends on aerogel density, its size and texture, and morphology of pores, etc. [3,4,15]. In this study, the effect of silica aerogel size and content in hybrid PET/silica aerogel blanket produced under different pH condition of condensation and matrix embedding method, on the acoustic properties were investigated at constant thickness (5 mm). As shown in Table 1, the density of hybrid PET/silica aerogel blanket increased with increasing add-on. In both methods, the higher density of hybrid PET/silica aerogel blanket is obtained at pH 7 during condensation process. Figures 7 and 8 show typical frequency-dependence of sound absorption of PET/silica aerogel blanket at low and high frequency ranges. Sound absorption coefficient values of PET/silica aerogel blanket prepared by both method I and II are quite low and constant (below 0.1) at low frequencies below 1000 Hz. At frequencies above 1000 Hz, sound

Sound absorption coefficient of PET/aerogel blanket by different methods at high frequency region; (a) Method I and (b) Method II. Figure 8.


Kyung Wha Oh et al.

absorption coefficient of PET/aerogel blanket prepared by method I increased steadily. Especially SA1 sample shows good sound absorption property at higher frequency range. This feature is in agreement with other porous sound absorption materials [14,16,17]. Compared with control PET nonwoven material, PET/silica aerogel blanket prepared under optimum condition has apparent higher absorb peak at high frequencies. It proves that silica aerogel added in PET nonwoven has great advantage for sound absorption. It could be interpreted by more wave energy absorbed by interface between silica particles and PET matrix. The sound absorption property are related to the vibration energy absorption of materials [3,11,12]. When the sound wave is incident upon the hybrid surface, the air among pores begins to vibrate and lead the cell wall material to vibrate too. The existence of a great amount of silica aerogel of more homogeneous and smaller size in the cell wall material has positive effect on the energy absorption. Moreover, higher absorption at higher frequency ranges is very important for sound damping. Since the most sensitive range of human acoustic sense is from 2500 to 5000 Hz, and air-borne noise is mostly contained in medium and high frequency ranges of 500~ 8000 Hz [17]. Therefore, PET/silica aerogel blanket prepared at optimum condition can be considered as a good sound damping material. Table 2 shows thermal insulation of PET/silica aerogel blanket. As compared to control PET nonwoven, PET/silica aerogel blanket provides high thermal insulation. The thermal insulation values of PET/silica aerogel blanket increased with increasing silica aerogel content. Heat energy can be transferred by conduction, convection, radiation. The flow of heat can be delayed by addressing one or more of these mechanisms and is dependent on the physical properties of the material employed to do this. Aerogels are good thermal insulators because they almost nullify the three methods of heat transfer (convection, conduction and radiation). They are good conductive insulators because they are composed almost entirely of an air (more than 95 %)

which is very poor heat conductors. It has lower thermal conductivity than air. They are also good convective inhibitors because air cannot circulate through the lattice. In addition, aerogel is a good insulator because it absorbs the infrared radiation and does not let infrared radiation from a heated material pass through at standard temperatures [3]. Therefore, by replacing air with aerogel in open structure of nonwoven material, thermal insulation property increased. These values are significantly higher than commercially available thermal insulation material of similar thickness [18].

Thermal insulation values of PET/silica aerogel blanket Mean (S.D.) Sample Thermal insulation (%) SA0 61.8 (1.48) SA1 90.0 (1.74) SA2 87.5 (2.37) SA3 80.8 (3.18) SA4 80.1 (2.85) SA5 70.7 (1.69) SA6 65.8 (1.42) SA7 66.4 (0.98) SA8 64.7 (1.12) *Air temperature: 22.5 oC.

References

Conclusion Ultra porous and flexible PET/aerogel blankets were prepared at ambient pressure, and their acoustic and thermal insulation properties were characterized. Two methods were selected for the preparation of PET/aerogel blanket; one is a direct gelation of silica on PET nonwoven and the other method is by the dipping of PET nonwoven in the dispersion of silica hydrogel by varying pH condition during gelation process. The synthesized silica areogels produced in optimum condition exhibit porous network structure. Silica aerogel of highly homogeneous and smallest spherical particle clusters with pores was prepared by gelation process under pH7. When direct gelation of silica was performed in PET nonwoven matrix (Method I), silica aerogel clusters were formed surrounding PET fibers forming network structure, and the higher density of hybrid PET/silica aerogel blanket was obtained at pH 7 in gelation. The existence of a great amount of silica aerogel of more homogeneous and smaller size in the cell wall material has positive effect on the sound absorption and thermal insulation.

Acknowlogement This work was supported by the Korea Research Foundation Grant funded by the Korean Government (KRF-2007-313C00836).

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