Ac

SVD

h

•

•

•

•

•

a

ARRAA

KROWTW

1

dPVr

0h

Colloids and Surfaces A: Physicochem. Eng. Aspects 445 (2014) 128– 134

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

jo ur nal ho me p ag e: www.elsev ier .com/ locate /co lsur fa

dvanced thermal insulation and absorption properties of recycledellulose aerogels

on T. Nguyen1, Jingduo Feng1, Shao Kai Ng, Janet P.W. Wong,incent B.C. Tan, Hai M. Duong ∗

epartment of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore

i g h l i g h t s

For the first time, paper waste can beconverted into a green cellulose aero-gel.The material has high water/oilabsorption capacities of 18–20 timesof its weight.Up to 99.8% of the liquid is recoveredsimply by squeezing the aerogel.The material shows low thermal con-ductivities of 0.029–0.032 Wm−1K−1.The aerogel shows good flexibilityand mechanical property.

g r a p h i c a l a b s t r a c t

r t i c l e i n f o

rticle history:eceived 19 August 2013eceived in revised form 1 January 2014ccepted 15 January 2014vailable online 24 January 2014

a b s t r a c t

A cost effective and scalable recipe for fabricating biodegradable cellulose aerogels from paper waste hasbeen realized. The green aerogel is macroporous and has extremely low density and thermal conductiv-ity: 0.04 g cm−3 and 0.029–0.032 Wm−1K−1, respectively. It is highly absorbent, absorbing 18–20 timesits weight in liquid. Up to 99.8% of the liquid is recovered simply by squeezing the aerogel. The fabri-cation can be optimized for absorbing polar (water) or non-polar liquids (oil). Coating the aerogel with

eywords:ecycled cellulose aerogelil spillater absorption

hermal insulationater repellent.

methyltrimethoxysilane improves its hydrophobicity without affecting its absorbency. Mechanically, theaerogel is flexible yet strong making a wide range of applications possible.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Oil spill is one of the most serious accidents that have beenestroying our ecosystem, for example, the explosion of a British

etroleum (BP) drilling rig in the Gulf of Mexico and the Exxonaldez incident in Alaska [1,2]. It is preferred that oil can beemoved completely from water for recovery and environment

∗ Corresponding author. Tel.: +65 97699600.E-mail address: mpedhm@nus.edu.sg (H.M. Duong).

1 Contributed equally to this work.

927-7757/$ – see front matter © 2014 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.colsurfa.2014.01.015

protection. Current methods (booms and skimmers, dispersants,and burning) are not so effective to achieve this objective andexpensive. Besides these methods, sorbents are considered as themost rapid and effective technique for oil spill cleaning. Sorbents foroil removal can be classified as inorganic mineral, synthetic organic,and natural organic materials. However, high cost, low sorptioncapacity, and low decomposition after usage are main drawbacksof this technique [1–10]. Therefore, it is necessary to invent novel

renewable, sustainable sorbents with low cost and high oil sorptioncapability.

Superabsorbent polymers are materials that have the ability toabsorb and retain large volumes of water. As a result, they are used

S.T. Nguyen et al. / Colloids and Surfaces A: Physic

incbsa

fttbwctmt

shecmtertp

arrCdckiht(omsmrwtf[bss

Tensile and compression tests: The tests are carried out on an

Fig. 1. Global aerogel market forecast (Source: BBC research).

n water-absorbing applications such as baby diapers and feminineapkins [11–15]. They can swell in water to form rubbery gels thatan be up to 99 wt% water. However, the absorbed water can onlye removed by drying [16]. As a consequence, there is a demand toynthesize new absorbent materials with high absorption capacitynd easy water removal.

High energy consumption, climate changes, and the exhaust ofossil fuels require more sustainable and energy efficient construc-ion solutions. To meet the demand of improved energy efficiency,he thermal insulation of building has an important role. To achieveetter thermal insulation with current insulation materials, thickeralls are expected [17–20]. However, this will increase the building

ost and affect several issues like floor area, architectural restric-ions, etc. As a result, it is essential to develop cheap insulation

aterials with low thermal conductivity for construction applica-ions.

With globalization and population boom, increase in paper con-umption has resulted in large amount of paper-related waste. Itas been found that 25–40% of municipal solid waste generatedach year worldwide is paper-related [21]. The huge mass of wasteauses forest destruction, difficulty for disposal as well as environ-ental pollution. Therefore, it is important to recycle or convert

his enormous waste into useful products. There have been severalfforts in solving this problem. For instance, 63% of paper waste wasecycled in the USA in 2010 [22]. Paper waste has also been inves-igated as a raw material for the production of bioethanol, polymerrecursors, particleboard, etc. [21,23,24].

Aerogels are dried gels with high porosities, large surface areas,nd extremely light weights [17,25–28]. Cellulose is an ideal mate-ial for medical, cosmetic, and several bioapplications due to itsenewable, biocompatible, and biodegradable properties [9,29].ellulose fibers from paper waste, which are cheap, widely abun-ant with high flexibility and good mechanical properties can beombined with aerogel structure to form a new porous materialnown as cellulose aerogel with high porosity, high sorption capac-ty, low thermal conductivity, and good flexibility. This materialas great potentials for water absorption, oil spill cleaning, andhermal insulation applications. According to BBC research [30]Fig. 1), the total global aerogel market will grow at a very high ratef 19.3% from 2012 to 2017 and reach global revenues of $332.2illion by 2017. Especially, the thermal and acoustic insulation

ector accounted for 82.3% of all revenues in 2012 and shows theost potential growth with a five-year compound annual growth

ate of 20.2% from 2012 to 2017. Sales of aerogels for this sectorere generated primarily by their utilization in industrial insula-

ion. There have been some works on cellulose aerogels preparedrom high-quality cellulose, wood powder, or bacterial cellulose26,29,31–33]. However, to the best of our knowledge, there have

een rare studies on recycled cellulose aerogels. Therefore, in ourtudy, we carried out investigations on green aerogels synthe-ized from recycled cellulose fibers for water/oil sorption, water

ochem. Eng. Aspects 445 (2014) 128– 134 129

repellency, and thermal insulation. This work will open a new chap-ter in recycling paper waste into useful and valuable products.

2. Experimental

2.1. Development of hydrophilic recycled cellulose aerogels

Recycled cellulose fibers (2 wt%) is dispersed into sodiumhydroxide/urea solutions (1.9 wt%/10 wt%) by sonicating for 6 min.Thereafter, the solution is placed in refrigerator for more than24 h to allow gelation of the solution. After the solution has beenfrozen, it is then thawed at room temperature and then followedby immersing into ethanol (99 vol%) for coagulation. The specimenthickness is controlled at 1 cm with a diameter of 3.5 cm using abeaker as a mold. After coagulation, solvent exchange is carried outby immersing the gel in de-ionized (DI) water for two days. Thesample is then frozen in a freezer at −18 ◦C for 12 h. After that,freeze drying is carried out for two days with a ScanVac CoolSafe95-15 Pro freeze dryer (Denmark) to obtain the desired aerogel.

2.2. Development of hydrophobic recycled cellulose aerogels

The aerogels in Section 2.1 are used to develop hydrophobicrecycled cellulose aerogels. For hydrophobic coating, two differentcoating methods: physical and chemical, are used. In the physi-cal coating method, a commercial water repellent spray (ReviveX®Nubuck) is used to spray onto the dried aerogel from a distanceof 15 cm and then left to dry for one day at room temperature.In the second method, the chemical method, the recycled celluloseaerogel is placed in a big glass bottle. A small open glass vial contain-ing methyltrimethoxysilane (MTMS) is added into the glass bottle.Then, the glass bottle is capped and heated in an oven at 70 ◦C for 2 hfor the silanation reaction. Thereafter, the coated sample is placedin a vacuum oven to remove the excess coating reagent until thepressure reaches 0.03 mbar. MTMS is chosen as the coating agentbecause it is cheap, commonly used for fabrication of hydrophobicand oleophilic aerogels; and the coating process is simple [34–37].

2.3. Characterization of the developed cellulose aerogels

To characterize the material, several techniques includingfield-emission scanning electron microscopy (FE-SEM), thermo-gravimetric analysis (TGA), thermal conductivity measurement,compression test, and water absorption test are carried out.

Field-emission scanning electron microscopy: Sample is kept ina dry cabinet prior to FE-SEM. The sample is then coated with athin gold layer using sputtering. A Hitachi S4300 scanning electronmicroscope (Japan) operated at 1/5 kV is used to capture structuralimages of the aerogels.

Thermogravimetric analysis: TGA is carried out to determine theweight loss in relation to temperature with a Shimadzu DTG60H(Japan). The specimen is heated up to 150 ◦C for 1 h to ensure thatthe adsorbed water in the specimen is removed. Then, the specimenis heated to 1000 ◦C at a rate of 5 ◦C min−1 in air.

Thermal conductivity measurement: Thermal conductivities ofsamples are measured at room temperature using a C-Therm TCiThermal Conductivity Analyzer (C-Therm Technologies, Canada).The sensor of the equipment is put on a stable and flat table withthe sensor head facing upwards. The sample is placed directly onthe top of the sensor with a loaded weight to make sure a goodsurface contact between the sample and the sensor.

Instron 5500 microtester (USA). The sample size for the compres-sion test is 38 mm (diameter) × 11 mm (thickness). The sample sizefor the tensile test is 32.9 × 9.54 × 2.54 mm with a gauge length of

130 S.T. Nguyen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 445 (2014) 128– 134

ose ae

4l

sTnwiwTsatc

eto

sibaif

3

f[pptpcr

TD

mV

Fig. 2. (a) Recycled cellulose fibers. (b) Recycled cellul

mm. During the mechanical test, the specimen is subjected to aoading of 1 mm min−1 for both tensile and compression.

Water absorption test: Water absorption capability of aerogelamples is investigated in DI water using a modified ASTM D570-98.he dry sample dimensions are 38 mm (diameter) × 11 mm (thick-ess). The dry sample is weighed and immersed in 800 ml of DIater for a certain time. After the immersion, the wet sample

s lifted up at a rate of 200 mm min−1 with a dip coater. Excessater on the surface of the sample is removed with filter paper.

he wet sample is weighed, measured its dimensions, dried orqueezed and weighed again. The test is repeated three times withn immersion time of 2 h. To dry the aerogel, it takes seven days forhe absorbed water to evaporate naturally under the atmosphereonditions.

Oil absorption test: The test is similar to the water absorption testxcept that the excess oil is allowed to drain for 30 s after lifting uphe wet sample. A motor oil with a viscosity of 130 cP and a cookingil with a viscosity of 55 cP are used in the test.

Water contact angle test: The test is carried out for the coatedamples on a VCA Optima goniometer (AST Products Inc., USA) tonvestigate their water repellency. Water is being dispensed, dropy drop, using the syringe control of the machine. This is repeatedt different positions of the sample and an average is taken. The tests also carried out for coated samples that are left in the atmosphereor several days.

. Results and discussion

In literature, most of research on cellulose aerogel startedrom high-quality cellulose, bacterial cellulose, or wood powder26,29,31–33]. In our study, recycled cellulose fibers (Fig. 2a) fromaper waste were chosen as raw materials. The aerogels were pre-ared using a sodium hydroxide/urea method [31,38,39] due to

he low cost and convenience of the technique. To dry the sam-les without destroying their structures, freeze drying method wasarried out. In this technique, the samples are frozen and the sur-ounding pressure is reduced to allow the frozen water in the

able 1ata of water absorption test with various absorbance times.

md (g) mw (g) ms (g) mu mr

(a) Two-hour water absorbance tests of the cellulose aerogels (absorbedFirst absorption 0.7778 16.1996 0.6279 19.8 0.9Second absorption 0.6279 10.4870 0.6142 15.7 0.9Third absorption 0.6142 9.5602 0.6239 14.6 0.9(b) Twenty-minute water absorbance tests of the cellulose aerogels (abFirst absorption 0.9690 8.1315 0.9840 7.4 0.9Second absorption 4.6447 0.9765 3.8 0.9Third absorption 4.8530 0.9712 4.0 0.9

d, weight of initial sample; mw, weight of wet sample; ms, weight of dried/squeezed samd, volume of initial sample; Vw, volume of wet sample; Vs, volume of naturally dried or s

rogel. (c) FE-SEM image of recycled cellulose aerogel.

samples to sublimate directly from the solid phase to the gas phase,which creates minimal force on the pore walls of the aerogels, pre-venting the porous structure from collapsing. As shown in Fig. 2band a light and porous aerogel was formed after the freeze dryingstep. FE-SEM was used to investigate the morphology of the cellu-lose aerogel prepared from recycled cellulose fibers. Fig. 2c showsan image of the internal structure of the material. It can be seenthat the aerogel has an open porous network structure of uniformfibers (about 8 �m wide), indicating that recycled cellulose fiberssuccessfully self-assembled via hydrogen bonding to form a three-dimensional porous network. The width of the recycled cellulosefibers is much larger than that of nanocellulose fibers (2–100 nm)[29,40–42], but consistent with data for recycled cellulose fibers inthe literature [43–45]. From Fig. 2c, it can be observed that poresize of the aerogel is in the range of 40–200 �m, indicating themacroporous property of the material. The sample has a density of0.040 g cm−3 calculated from the weight and volume of the aerogel(Table 1). With a cellulose fiber density of 1.5 g cm−3 [46], the poros-ity of the aerogel sample is 94.8%. This value is lower than that ofcellulose aerogels made from nanocellulose fibers [26,29,31], prob-ably due to the macroporous structure of the recycled celluloseaerogel compared to the nanoporous network of the nanocelluloseaerogels [26,29].

To investigate the water absorption capability of the recycledcellulose aerogel, the aerogel samples were subjected to threecycles of water absorption tests using a dip coater for 2 h (Fig. 3).The size and weight of the sample were measured before and aftereach test. The wet sample was then dried at room temperaturefor seven days. Approximate 19.8, 15.7, and 14.6 times of its dryweight of water were absorbed in the first, second, and third tests,respectively, as can be seen in Table 1a. The absorbance capacitiesof the developed cellulose aerogels are 5X higher than those of sandand saw dust and also almost equal to those of commercial poly-

mer sorbents [47–51]. Although no cross-linkers were used in thesynthesis, the aerogel preserved its shape after being immersed inwater for 2 h, indicating that the material has a stable structure dueto the cellulose–cellulose hydrogen bonding.

Vd (mm3) Vw (mm3) Vs (mm3) rV1 rV2

water in the aerogels was removed by a natural evaporation process)99 20,676 18,879 14,743 0.91 0.7199 12,566 11,341 0.61 0.5599 12,566 10,752 0.61 0.52sorbed water in the aerogels was removed by a squeezing process)98 12,475 15,080 4247 1.20 0.3498 5283 3232 0.42 0.2699 5114 3232 0.41 0.26

ple; mu, water uptake content; mr, water content removed after drying/squeezing;queezed sample; rV1, volume ratio Vw/Vd; rV2, volume ratio Vs/Vd.

S.T. Nguyen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 445 (2014) 128– 134 131

F r. (b)

S . (f) W

tatsb(os1wowp

ig. 3. Water absorption test with squeezing: (a) Absorption test with a dip coatequeezed sample after first test. (e) Squeezed sample in water after first absorption

For further demonstrating the flexibility and absorbance rate ofhe compressed cellulose aerogels (in Table 1b), we used a celluloseerogel absorbing 7.4 times of its dry weight of water for 20 min inhe first test. Normally, absorbed water can only be removed fromuperabsorbent polymers by drying. In contrast, water absorbedy the cellulose aerogel is removed easily by simply squeezing itFig. 3d). The mass measurements in Table 1 show that almost allf the absorbed water (99.8%) was removed from the compressedample (mr = 0.998). The volume of the compressed sample is about/3 of that of the original sample. When the compressed sample

as placed back into water (Fig. 3e), it recovered to almost its

riginal shape within 30 s. The wet sample was removed from theater after 20 min only. There is some shrinkage of the wet sam-le (Fig. 3f) compared to the one in Fig. 3b, possibly due to partial

Fig. 4. (a) Flexibility of the aerogel. (b) A 200-g load on th

Dry aerogel sample before the test. (c) Wet sample after first absorption test. (d)et sample after second absorption. (g) Wet sample after third absorption.

collapse of pores within the aerogel during squeezing. The volumeratio of the wet sample after the second 20-min water absorptiontest and the original dry sample is only 0.42. In the second test, thesample absorbed 3.8 times of its dry weight of water. Again, mostof the absorbed water can be removed (mr = 0.998). The volumeof the wet sample after the third 20-min water absorbance test issimilar to that of the wet sample after the second test indicating nofurther shrinkage of the aerogel (Fig. 3g). The amount of absorbedwater in the third test is almost the same as that in the second test.A mr value of 0.999 for the third water absorption test again indi-

cates that nearly all the absorbed water was eliminated simply bysqueezing the aerogel.

For practical applications, flexibility of the recycled celluloseaerogel is very important. As displayed in Fig. 4a, the aerogel can be

e aerogel. (c) Tensile curve. (d) Compressive curve.

132 S.T. Nguyen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 445 (2014) 128– 134

urem

esrpbfi

Foa

Fig. 5. (a) A setup of thermal conductivity meas

asily and repeatedly bent 180◦ without damaging the shape of theample. The flexibility of the recycled cellulose aerogel is compa-

able to that of nanocellulose aerogels [29]. A qualitative test waserformed for the sample to investigate its mechanical strengthy loading a 200-g weight on the sample for 1 h, 5 h, one day, andve days (Fig. 4b). It was seen that no shape change of the aerogel

ig. 6. (a) >SEM image of the sample coated with the commercial water repellent agent. (bf the sample coated with the commercial water repellent agent. (d) Water contact angleverage water contact angle.

ent. (b) TGA result of recycled cellulose aerogel.

was found after the test durations. For further understanding, themechanical property of the material, tensile and compression tests

were performed with an Instron 5500 microtester. The results inFig. 4c and d show that the yield and tensile strengths of the aerogelare ca. 1080 and 1470 N m−2, respectively, with a Young’s modulusof 11 kPa.

) SEM image of the sample coated with MTMS. (c) Water contact angle measurement measurement of the sample coated with MTMS. (e) Effect of exposure time on the

S.T. Nguyen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 445 (2014) 128– 134 133

F MTMo test.

ecoeliiabotmcTtcaAdc

tt2trlas

arbfNlFlMWnuhpii

ig. 7. (a) >Motor oil absorption test of MTMS-coated recycled aerogel sample. (b)f MTMS-coated sample. (d) MTMS-coated sample after the cooking oil absorption

Greenhouse effect has been warming up the earth and threat-ning human life. It was found that CO2 emission from buildingsontributed more than 30% of greenhouse gas emission in devel-ped countries in 2005 [17]. Buildings also accounted for 39% ofnergy consumption in this year [52]. Improving thermal insu-ation of buildings is one of the most effective solutions for thessue. Therefore, there have been many efforts in developing newnsulation materials [20]. Silica aerogels have been investigateds insulation materials for buildings [17]. However, they are veryrittle. A flexible aerogel-based insulation material has been devel-ped by Aspen Aerogels (the USA), but it is much more expensivehan conventional insulation materials [17]. To investigate the ther-

al insulation ability of our recycled cellulose aerogel, a thermalonductivity measurement was carried out with a C-Therm TCihermal Conductivity Analyzer System (Fig. 5a). The measuredhermal conductivity of the material is 0.032 Wm−1K−1, which isomparable to those of good insulation materials such as silicaerogel (0.026 Wm−1K−1), wool (0.03–0.04 Wm−1K−1), and Aspenerogels products (0.021 Wm−1K−1) [53,54]. This low thermal con-uctivity value and the low cost of paper waste make the recycledellulose aerogel promising for thermal insulation applications.

To evaluate the thermal stability of the cellulose aerogel, a TGAest was performed for the sample in air (Fig. 5b). It can be seenhat there is a weight loss of 23% in the temperature range of5–230 ◦C due to the removal of absorbed water and some urearace left in the sample. Then, a weight loss of 42% occurs in theange of 230–330 ◦C due to the degradation and burning of the cel-ulose aerogel structure. There is small drop of the sample weightt 550–630 ◦C possibly due to the oxidation of some stable localtructures of the aerogel.

As discussed above, recycled cellulose aerogel is promising forpplying on building walls as an effective thermal insulation mate-ial. On the other hand, if the material is repellent to water, it cane applied onto the exterior sides of the walls to protect buildingsrom moisture attack. Therefore, a water repellent agent (ReviveX®ubuck) or MTMS was applied on the surface of the recycled cel-

ulose aerogel via a physical or chemical method. As shown inig. 6a, the surface of the aerogel is covered by the water repel-ent polymer and all the pores are fully covered. In contrast, the

TMS-functionalized sample still has a porous structure (Fig. 6b).ater contact angle measurements were performed for the origi-

al uncoated sample and the coated samples (Fig. 6c and d). For thencoated sample, water is easily absorbed by the aerogel due to the

ydrophilic nature of cellulose. As can be seen in Fig. 6c and d, thehysically coated sample has a water contact angle of 130.7◦, which

s smaller than that of the chemically coated sample (135.2◦). Thisndicates that the MTMS coating is more water repellent than the

S-coated sample after the motor oil absorption test. (c) Cooking oil absorption test

coating with the commercial agent. The samples were then exposedin air and sunlight for several days and their water contact angleswere measured during the exposure time (Fig. 6e). It can be seenthat both of the two samples show little changes in water contactangle, indicating their excellent water repellent durability.

When the material is coated with MTMS, it will becomehydrophobic and oleophilic and therefore, has a good affinity tooil. The MTMS-coated recycled cellulose aerogel was used for theoil absorption test (Fig. 7). It shows strong affinities to both of themotor and cooking oils with high oil uptake contents of 18 and17.6 g g−1, respectively.

After being coated with MTMS, the material shows a thermalconductivity value of 0.029 Wm−1K−1, which is lower than thethermal conductivity (0.032 Wm−1K−1) of the uncoated sample,indicating an improvement of thermal insulation property due tothe MTMS coating. As stated above, the total global aerogel mar-ket will grow at a very high rate of 19.3% from 2012 to 2017 withmore than 80% of aerogels used for thermal and acoustic insulation.Therefore, with high thermal insulation property, our recycled cel-lulose aerogel is extremely promising for the industrial insulationmarket.

4. Conclusions

In summary, for the first time, paper waste can be success-fully converted into a green cellulose aerogel with high waterand oil absorption capacities, good thermal insulation, and waterrepellent properties. The material was prepared via a simplealkaline/urea and freeze drying method. Field-emission scanningelectron microscopy revealed the macroporous structure of thematerial. Water and oil absorption tests showed that the uncoatedand the coated aerogels have high liquid absorption capacitiesof 18–20 times their own weights and can be easily reused bya simple squeezing to remove the absorbed liquid. The aerogelshowed good flexibility and mechanical property. Thermal conduc-tivities of 0.029–0.032 Wm−1K−1 were found for the green aerogelconfirming that it is an excellent alternative material for thermalinsulation applications. With a hydrophobic coating, the materialdisplayed good water repellent property and stability.

Acknowledgements

The authors deeply acknowledge Singapore National Environ-ment Agency–Environment Technology Research Program (7thRFP) (R-265-000-450-490) for the financial support for the project.We would also like to thank Insul-Dek Engineering Pte. Ltd.

1 Physic

(&(

R

[

[

[

[

[

[

[

[

[

[

[

[

[ .

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[[

[

[

[

[

[

34 S.T. Nguyen et al. / Colloids and Surfaces A:

Singapore) for giving us recycled cellulose fibers, ITS Science Medical Pte. Ltd. (Singapore) and C-Therm Technologies Ltd.

Canada) for help in thermal conductivity measurement.

eferences

[1] C. Cojocaru, M. Macoveanu, I. Cretescu, Peat-based sorbents for the removal ofoil spills from water surface: Application of artificial neural network modeling,Colloid. Surface. A 384 (2011) 675–684.

[2] X. Yuan, T.C.M. Chung, Novel solution to oil spill recovery: using thermodegrad-able polyolefin oil superabsorbent polymer (oil–SAP), Energ. Fuel. 26 (2012)4896–4902.

[3] K.C. Payne, C.D. Jackson, C.E. Aizpurua, O.J. Rojas, M.A. Hubbe, Oil spills abate-ment: factors affecting oil uptake by cellulosic fibers, Environ. Sci. Technol. 46(2012) 7725–7730.

[4] M. Radetic, V. Ilic, D. Radojevic, R. Miladinovic, D. Jocic, P. Jovancic, Efficiencyof recycled wool-based nonwoven material for the removal of oils from water,Chemosphere 70 (2008) 525–530.

[5] C. Teas, S. Kalligeros, F. Zanikos, S. Stournas, E. Lois, G. Anastopoulos, Inves-tigation of the effectiveness of absorbent materials in oil spills clean up,Desalination 140 (2001) 259–264.

[6] D. Wang, E. McLaughlin, R. Pfeffer, Y.S. Lin, Adsorption of oils from pure liquidand oil–water emulsion on hydrophobic silica aerogels, Sep. Purif. Technol. 99(2012) 28–35.

[7] D. Wang, T. Silbaugh, R. Pfeffer, Y.S. Lin, Removal of emulsified oil from waterby inverse fluidization of hydrophobic aerogels, Powder Technol. 203 (2010)298–309.

[8] Q. Zhu, Q. Pan, F. Liu, Facile removal and collection of oils from water surfacesthrough superhydrophobic and superoleophilic sponges, J. Phys. Chem. C 115(2011) 17464–17470.

[9] J.T. Korhonen, M. Kettunen, R.H.A. Ras, O. Ikkala, Hydrophobic nanocelluloseaerogels as floating, sustainable, reusable, and recyclable oil absorbents, ACSAppl. Mater. Interfac. 3 (2011) 1813–1816.

10] J. Wang, Y. Zheng, A. Wang, Superhydrophobic kapok fiber oil-absorbent:Preparation and high oil absorbency, Chem. Eng. J. 213 (2012) 1–7.

11] M. Elliott, Superabsorbent Polymers, http://chimianet.zefat.ac.il/download/Super-absorbant polymers.pdf, BASF, 2013.

12] K. Kabiri, H. Omidian, M.J. Zohuriaan-Mehr, S. Doroudiani, Superabsorbenthydrogel composites and nanocomposites: A review, Polym. Compos. 32 (2011)277–289.

13] K. Kabiri, M.J. Zohuriaan-Mehr, Superabsorbent hydrogel composites, Polym.Adv. Technol. 14 (2003) 438–444.

14] K. Kabiri, M.J. Zohuriaan-Mehr, Porous superabsorbent hydrogel composites:Synthesis, morphology and swelling rate, Macromol. Mater. Eng. 289 (2004)653–661.

15] M.J. Zohuriaan-Mehr, K. Kabiri, Superabsorbent polymer materials: A review,Iran. Polym. J. (English Edition) 17 (2008) 451–477.

16] W. Zhang, Y. Zhang, C. Lu, Y. Deng, Aerogels from crosslinked cellulosenano/micro-fibrils and their fast shape recovery property in water, J. Mater.Chem. 22 (2012) 11642–11650.

17] R. Baetens, B.P. Jelle, A. Gustavsen, Aerogel insulation for building applications:A state-of-the-art review, Energ. Buildings 43 (2011) 761–769.

18] A. Briga-Sá, D. Nascimento, N. Teixeira, J. Pinto, F. Caldeira, H. Varum, A. Paiva,Textile waste as an alternative thermal insulation building material solution,Constr. Build. Mater. 38 (2013) 155–160.

19] C. Buratti, E. Moretti, Glazing systems with silica aerogel for energy savings inbuildings, Appl. Energ. 98 (2012) 396–403.

20] B.P. Jelle, Traditional, state-of-the-art and future thermal building insula-tion materials and solutions–properties, requirements and possibilities, Energ.Buildings 43 (2011) 2549–2563.

21] A. Nourbakhsh, A. Ashori, Particleboard made from waste paper treated withmaleic anhydride, Waste Manage. Res. 28 (2010) 51–55.

22] Paper Recycling, http://www.epa.gov/osw/conserve/materials/paper/basic infohtm, 2013.

23] D. Mishra, S. Vijay Kumar, Value-based polymer precursors from paper wasteand its application in polyurethane foams, J. Cell. Plast. 46 (2010) 15–30.

24] L. Wang, R. Templer, R.J. Murphy, High-solids loading enzymatic hydrolysis ofwaste papers for biofuel production, Appl. Energ. 99 (2012) 23–31.

25] M.A. Aegerter, N. Leventis, M.M. Koebel, Aerogels Handbook, Springer,New York, USA, 2011.

26] N.T. Cervin, C. Aulin, P.T. Larsson, L. Wågberg, Ultra porous nanocellulose aero-gels as separation medium for mixtures of oil/water liquids, Cellulose 19 (2012)401–410.

[

[

ochem. Eng. Aspects 445 (2014) 128– 134

27] F. Liebner, A. Potthast, T. Rosenau, E. Haimer, M. Wendland, Cellulose aero-gels: Highly porous, ultra-lightweight materials, Holzforschung 62 (2008)129–135.

28] R.T. Olsson, M.A.S. Azizi Samir, G. Salazar-Alvarez, L. Belova, V. Ström, L.A.Berglund, O. Ikkala, J. Nogués, U.W. Gedde, Making flexible magnetic aerogelsand stiff magnetic nanopaper using cellulose nanofibrils as templates, NatureNanotechnol. 5 (2010) 584–588.

29] W. Chen, H. Yu, Q. Li, Y. Liu, J. Li, Ultralight and highly flexible aerogels withlong cellulose I nanofibers, Soft Matter 7 (2011) 10360–10368.

30] Thermal and Acoustic Insulation Sector of Aerogels To Grow To $283.8 Mil-lion in 2017, http://www.bccresearch.com/pressroom/report/code/AVM052C,2013.

31] J. Cai, S. Kimura, M. Wada, S. Kuga, L. Zhang, Cellulose aerogels from aqueousalkali hydroxide-urea solution, ChemSusChem 1 (2008) 149–154.

32] F. Liebner, E. Haimer, M. Wendland, M.A. Neouze, K. Schlufter, P. Miethe, T.Heinze, A. Potthast, T. Rosenau, Aerogels from unaltered bacterial cellulose:Application of scCO2 drying for the preparation of shaped, ultra-lightweightcellulosic aerogels, Macromol. Biosci. 10 (2010) 349–352.

33] Z. Wang, S. Liu, Y. Matsumoto, S. Kuga, Cellulose gel and aerogel from LiCl/DMSOsolution, Cellulose 19 (2012) 393–399.

34] G. Hayase, K. Kanamori, K. Nakanishi, New flexible aerogels and xero-gels derived from methyltrimethoxysilane/dimethyldimethoxysilane co-precursors, J. Mater. Chem. 21 (2011) 17077–17079.

35] M.S. Kavale, D.B. Mahadik, V.G. Parale, A.V. Rao, P.B. Wagh, S.C. Gupta,Methyltrimethoxysilane based flexible silica aerogels for oil absorption appli-cations, AIP Conf. Proc. 1447 (2012) 1283–1284.

36] A. Venkateswara Rao, S.D. Bhagat, H. Hirashima, G.M. Pajonk, Synthesis of flex-ible silica aerogels using methyltrimethoxysilane (MTMS) precursor, J. ColloidInterf. Sci. 300 (2006) 279–285.

37] A. Venkateswara Rao, N.D. Hegde, H. Hirashima, Absorption and desorption oforganic liquids in elastic superhydrophobic silica aerogels, J. Colloid Interf. Sci.305 (2007) 124–132.

38] C. Chang, L. Zhang, Cellulose-based hydrogels: Present status and applicationprospects, Carbohydr. Polym. 84 (2011) 40–53.

39] N. Isobe, S. Kimura, M. Wada, S. Kuga, Mechanism of cellulose gelation fromaqueous alkali-urea solution, Carbohydr. Polym. 89 (2012) 1298–1300.

40] H. Jin, Y. Nishiyama, M. Wada, S. Kuga, Nanofibrillar cellulose aerogels, Colloid.Surface. A 240 (2004) 63–67.

41] H. Jin, M. Pääkkö, J. Netral, L.A. Berglund, C. Neagu, P.E. Bourban, M. Ankerfors, T.Lindström, O. Ikkala, Effects of different drying methods on textural propertiesof nanocellulose aerogels, in: Proceedings of 17th International Conference onComposite Materials; 2009 July 27-31, Edinburgh, Scotland, 2009.

42] D. Klemm, F. Kramer, S. Moritz, T. Lindström, M. Ankerfors, D. Gray, A. Dorris,Nanocelluloses: A new family of nature-based materials, Angew. Chem. Int.Edit. 50 (2011) 5438–5466.

43] R. Bhardwaj, A.K. Mohanty, L.T. Drzal, F. Pourboghrat, M. Misra, Renewableresource-based green composites from recycled cellulose fiber and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) bioplastic, Biomacromolecules 7(2006) 2044–2051.

44] M.S. Huda, L.T. Drzal, M. Misra, A.K. Mohanty, K. Williams, D.F. Mielewski, Astudy on biocomposites from recycled newspaper fiber and poly(lactic acid),Ind. Eng. Chem. Res. 44 (2005) 5593–5601.

45] I.M. Low, J. Somers, I.J. Kho, B.A. Latelle, Fabrication and properties ofrecycled cellulose fibre-reinforced epoxy composites, Compos. Interf. 16 (2009)659–669.

46] S.M. Lee, International Encyclopedia of Composites, VCH, New York, 1991.47] U.S. EPA, Sorbents, http://www.epa.gov/oem/content/learning/sorbents.htm,

2013.48] M.O. Adebajo, R.L. Frost, J.T. Kloprogge, O. Carmody, S. Kokot, Porous materials

for oil spill cleanup: a review of synthesis and absorbing properties, J. PorousMater. 10 (2003) 159–170.

49] A. Bayat, S.F. Aghamiri, A. Moheb, G.R. Vakili-Nezhaad, Oil spill cleanupfrom sea water by sorbent materials, Chem. Eng. Technol. 28 (2005) 1525–1528.

50] H.M. Choi, R.M. Cloud, Natural sorbents in oil spill cleanup, Environ. Sci. Technol.26 (1992) 772–776.

51] R. Wahi, L.A. Chuah, T.S.Y. Choong, Z. Ngaini, M.M. Nourouzi, Oil removal fromaqueous state by natural fibrous sorbent: An overview, Sep. Purif. Technol. 113(2013) 51–63.

52] U.S. Environmental Protection Agency, Buildings and their Impact on the Envi-

ronment: A Statistical Summary, in: U.E.P. Agency (Ed.), 2009.

53] S. Sequeira, D.V. Evtuguin, I. Portugal, Preparation and properties of cellu-lose/silica hybrid composites, Polym. Compos. 30 (2009) 1275–1282.

54] Pyrogel XT-E, http://www.aerogel.com/products/pdf/Pyrogel XT-E DS.pdf,2013.