Ultra-porous flexible PET/Aerogel blanket for sound absorption and thermal insulation

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

731

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, Korea1Department of Fiber and Polymer Engineering, Hanyang University, Seoul 133-791, Korea

(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 thermalinsulation properties were characterized. Two methods were selected for the preparation of PET/Aerogel blanket. Method Iwas a direct gelation of silica on PET. PET non-woven fabric was dipped and swelled in TEOS/ethanol mixture, and pH ofreaction media was controlled to 2.5 using HCl to promote hydrolysis. After acid hydrolysis, pH was controlled to 7,8,9, and10 with NH4OH for the condensation. Method II was by the dipping of PET non-woven fabric in the dispersion of Silicahydrogel. The gelation process was same with Method I. However, PET fabric was not dipped in reaction media. After thehydrogel 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 n-hexane and dried at room temperature to prevent the shrinkage. The silica areogels synthesized in optimum conditions exhibitporous network structure. Silica aerogel of highly homogeneous and smallest spherical particle clusters with pores was pre-pared by gelation process at pH 7. When direct gelation of silica was performed in PET nonwoven matrix (Method I), silicaaerogel clusters were formed efficiently surrounding PET fibers forming network structure. The existence of a great amountof silica aerogel of more homogeneous and smaller size in the cell wall material has positive effect on the sound absorptionand thermal insulation.

Keywords: Aerogel, PET, Sound absorption, Thermal insulation, Hybrid

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, multi-

purpose 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

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*Corresponding author: [email protected]

DOI 10.1007/s12221-010-0731-3

732 Fibers and Polymers 2009, Vol.10, No.5 Kyung Wha Oh et al.

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/cm3

was purchased from Mirae Trading Co..

Gelation of Silica Hydrogel in PET Non-woven Matrix


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 cm3,

and the volume of mixture was approximately 600 ml

(TEOS:EtOH:H2O=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/H2O mixture

(EtOH:H2O=2:1 (mole ratio)) was poured to the bath and pH

of media controlled to 7, 8, 9, and 10 with NH4OH for the

condensation. This two step gelation was described as

follows. After aging for 24 hrs following the addition of

NH4OH, the reaction media was exchanged with a large

amount of ethanol for aging of the silica and washing of

unreacted monomer and water.

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 oC for 24 hrs. The hydrogel blanket

was dipped in TMCS/n-hexane solution, and the surface

modification was carried out at 50oC for 24 hrs (Figure 1).

After modification, the blanket was washed with n-hexane

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/n-

hexane 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 Frequency-

dependence 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 NH4OH. 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-1 are due to the

hydroxyl group at the end of gel network or residual water

molecules. The peaks at 1090 and 460 cm-1

by the Si-O-Si

vibration means the successful chemical gelation by

Figure 1. Surface modification of silica.

Flexible PET/Aerogel Blanket Fibers and Polymers 2009, Vol.10, No.5 733

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

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


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

Figure 2. FT-IR spectra of Silica powder extracted from sample

SA1.

Figure 3. Contact angle of (a) hydrogel and (b) aerogel extracted from PET/aerogel blanket gelized from different pH.


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 4. Particle size distribution of silica aerogels extracted from

PET/aerogel blankets; (a) SA1 (b) SA2 (c) SA3, and (d) SA4.

Figure 5. SEM images of aerogel surface.

Figure 6. SEM images of PET/silica aerogel blanket fracture.

Table 1. Density of PET/silica aerogel blanket

Mean (S.D.)

Sample Add-on (%) Density (g/cm3)

SA0 0.0 (0.00) 0.037 (0.0004)

SA1 390.2 (4.95) 0.184 (0.0019)

SA2 362.2 (6.34) 0.174 (0.0024)

SA3 278.2 (7.14) 0.142 (0.0027)

SA4 250.1 (6.02) 0.132 (0.0023)

SA5 96.4 (5.81) 0.074 (0.0022)

SA6 68.1 (9.95) 0.063 (0.0037)

SA7 82.1 (9.29) 0.068 (0.0035)

SA8 40.1 (6.65) 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

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

Figure 8. Sound absorption coefficient of PET/aerogel blanket by

different methods at high frequency region; (a) Method I and (b)

Method II.

Figure 7. Sound absorption coefficient of PET/aerogel blanket

prepared by different methods at low frequency region; (a) Method

I and (b) Method II.


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].

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-2007-313-

C00836).

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Table 2. 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.


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Ultra-porous flexible PET/Aerogel blanket for sound absorption and thermal insulation

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