Synthesis and Characterization of Aramid-Honeycomb Reinforced ...

Article

From Fragile to Resilient Insulation: Synthesis and Characterization of Aramid-Honeycomb Reinforced Silica Aerogel Composite Materials Marina Schwan *, Matthias Rößler, Barbara Milow and Lorenz Ratke Received: 27 October 2015; Accepted: 9 December 2015; Published: 22 December 2015 Academic Editors: Francoise Quignard and Nathalie Tanchoux Institute of Materials Research, German Aerospace Center, Linder Hoehe, 51170 Cologne, Germany; [email protected] (M.R.); [email protected] (B.M.); [email protected] (L.R.) * Correspondence: [email protected]; Tel.: +49-2203-601-3427; Fax: +49-2203-696480

Abstract: The production of a new composite material embedding aramid honeycomb materials into nano-porous silica aerogels is studied. Our aim is to improve the poor mechanical strength of silica aerogels by aramid honeycombs without losing the amazing properties of the aerogels like little density and low thermal conductivity. The composite materials were prepared using two formulations of silica aerogels in combination with aramid honeycomb materials of different cell sizes. The silica aerogels are prepared using silicon alkoxides methyltrimethoxysilane and tetraethylorthosilicate as precursors in a two-step acid–base sol–gel process. Shortly in advance of the gelation point, the aramid honeycombs were fluted by the sol, gelation occurred and, after the aging process, the gel bodies were supercritically dried. The properties of the received composite materials are satisfying. Even the thermal conductivities and the densities are a bit higher than for pure aerogels. Most importantly, the mechanical strength is improved by a factor of 2.3 compared to aramid honeycomb materials and by a factor of 10 compared to the two silica aerogels themselves. The composite materials have a good prospective to be used as an impressive insulation material. Keywords: fluffy silica aerogels; flexible silica aerogels; honeycomb-composite; thermal insulating; mechanical properties

1. Introduction Nowadays, incombustible and non-toxic thermal insulation for buildings, industrial applications as well as for the automotive sector are more important than ever. A moderate consumption of energy is highly required to save the natural resources of fossil fuels, reducing the CO2 foot print and protecting the world’s climate [1]. Materials like expanded polystyrene, polyurethane foams or mineral wool are well established and already possess a low thermal conductivity and a low density too. However, these materials are only suitable for a limited range of applications. The applicability depends on the thermal stability according to the operating conditions like temperature or mechanical loading range. In addition, a long-term flame resistance, preventing the production of toxic decomposition products is not given too. Even the manufacturing of some is a toxic and energy intensive process. Aerogels might close this gap. Research on silica aerogels and its thermal properties has already been started by Kistler [2] in the early thirties’ of the last century. They are easy to be prepared by a two-step acid-base sol-gel process followed by a suitable almost shrinkage-free drying procedure. Aerogels and aerogel based materials have a unique combination of physical properties [3]. The morphology of the silica aerogels—with its open-porous nanostructure built of small interconnected particles and small pore sizes (~10 nm) leading to an immense high porosity of up to 99.9%—is Gels 2016, 2, 1; doi:10.3390/gels2010001

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responsible for the low density (~100 kg¨ m´3 ), low thermal conductivity (0.03–0.01 W¨ (m¨ K)´1 ), huge internal surface (~1000 m2 ¨ g´1 ), and low mechanical strength (~50–500 kPa). Especially the impressive low thermal conductivities of silica aerogels down to 0.01 W¨ (m¨ K)´1 are promising for countless new insulation applications in aeronautics, mobility or the building industry [4]. One key factor that hinders the industrial utilization of aerogels is their fragility, with low Young’s moduli and their characteristic brittleness. These prevent their usage in many fields of application and therefore industrial fabrication of monolithic aerogel for instance in the form of tiles is still not on the way. Reducing the rigidity and brittleness of aerogels is more than necessary. Several attempts to chemically modify the microstructure have been carried out and lead to soft and mechanically flexible silica aerogels [5–7]. For example, Maleki et al. proposed polymer-reinforced silica aerogels with compression strength from 11 to 400 kPa and thermal conductivity of 0.039–0.093 W¨ (m¨ K)´1 [6]. A second focus to improve the mechanical strength of silica aerogels is to fabricate aerogel composite materials. This has been presented by Liao et al. in 2012 and Mazraeh-shahi et al. in 2014 [8,9]. The combination of aerogels and binders is patented as a successful insulating material with suitable mechanical properties [10], as well as aerogel composites with polyurethane foams [11,12]. One other way to prevent the disintegration of silica aerogels was suggested by Capadona et al. The authors could significantly increase the stiffness and strength by crosslinking with isocyanate [13]. Several methods to improve elastic properties are summarized by Meador [14]. In general, a polymer coating on the skeletal nanostructure makes aerogels mechanically stronger [14]. To improve the mechanical properties of silica aerogels, other groups suggest a variety of other approaches, such as incorporation of tungsten disulfide nanotubes [15], chemical vapor deposition treatment with hexamethyldisilazane [16], or polymer-reinforcement allowing ambient drying of silica aerogels [17]. However, organic resorcinol-formaldehyde aerogels and their pyrolized form—carbon aerogels possess also high stiffness. Chen et al. investigated a combination of phenolic resin and carbon aerogels. They could increase flexural strength by 18.4% and impact strength by 101% [18]. In this paper, we present a new composite material combining chemically modified—fluffy and low-flexible—silica aerogels with aramid honeycombs for reinforcement. Beside the two types of modified silica aerogels, the honeycomb materials and corresponding honeycomb composite materials were synthesized and characterized. A comparison of the materials is performed with respect to their mechanical and thermal properties as well as their morphology. Their suitability as a light weight super insulating material is proven. 2. Results and Discussion 2.1. Appearance and Properties of the Primary Materials, Aerogels and Honeycomb Materials In our study, two different types of aerogels were synthesized and characterized. Due to different precursors we used, the aerogels produced exhibit different haptic and mechanical properties. They both are elastic but with different degrees of deformability. The first type is a methyltrimethoxysilane (MTMS) based silica aerogel, which is highly flexible. It is reversible deformable like a marshmallow. This aerogel, we call in the present study SA1 (silica aerogel 1). Due to its high flexibility, we also call it super-flexible. The second type is an methyltrimethoxysilane[3-(2,3-Epoxypropoxy)-propyl]-trimethoxysilan (MTMS-GPTMS) based aerogel having lower degree of flexibility. This type is rubber-like and more brittle. We name this aerogel SA2 (silica aerogel 2) or low-flexible by reason of its reduced deformability. The pure SA1 aerogel, shown in the Figure 1 is plain white and extremely fluffy. When slightly blowing over its surface, aerogel powder comes off and one can hardly feel any counterforce when compressing it by hand. Shrinkage after supercritical drying was below 5%, and is neglected in further considerations. It shows low density (0.037 g¨ cm´3 ) and low thermal conductivity (0.034 W¨ (m¨ K)´1 ), as given in Table 1. Due to the high flexibility, its compressive modulus is only

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2016, 1, 0001 3Gels kPa. The SEM image in the Figure 2 shows the microstructure of an SA1 sample with large particles Gels 2016, 1, 0001 and pore sizes. For flexible aerogels, large pores are responsible for the reversible deformation of the Table 1. Properties of produced aerogels and honeycombs. network [19]. Table 1. Properties of produced aerogels and honeycombs. Mean Envelope Thermal Skeletal Compressive Porosity Table 1. Properties of produced aerogels and honeycombs. MeanSize Envelope Thermal Skeletal Pore Density Material Conductivity Density a (MPa) Compressive Porosity Modulus (%) −1) Pore(nm) Size Density Material Conductivity Density (g·cm−3) (W·(m·K) (g·cm−3) a (MPa) Modulus (%) Mean Envelope Thermal −1) −3) Skeletal −3) (nm) (g·cm (W·(m·K) (g·cm Porosity Compressive Super-flexible Pore Size242 Density Material Conductivity 0.034 0.037 Density1.38 (%) 97.3 Modulus a0.003 (MPa) Super-flexible ´3 ´3 ´1 aerogel SA1 (nm)242 (g¨ cm 1.38 ) (g¨ cm 0.037 ) (W¨ (m¨ K) ) 0.034 0.003 97.3 aerogel SA1 Low-flexible Super-flexible 0.038 0.074 Low-flexible 0.034 0.003 0.037 0.092 1.38 1.41 97.3 93.5 242 113 aerogel SA2SA1 aerogel 0.038 0.074 0.092 1.41 93.5 113 aerogel SA2 Aramid Low-flexible 0.038 0.074in-plane 0.092 1.41 93.5 113 0.030 Aramid aerogel SA2 honeycomb 0.060 b 0.029 c 0.030 in-plane b c 10.7 out-of-plane honeycomb 0.060 0.029 Aramid C1-3.2-29 0.030 in-plane 10.7 out-of-plane 0.029 c honeycomb 0.060 b C1-3.2-29 10.7 out-of-plane Aramid C1-3.2-29 0.086 in-plane Aramid 0.092 b honeycomb 0.07 b 0.086 in-plane Aramid b b 12.2in-plane out-of-plane 0.092 honeycomb 0.07 0.086 b b d A10-92-5.2 honeycomb 12.2 out-of-plane 0.07 0.092 d 12.2 out-of-plane A10-92-5.2 A10-92-5.2 d Aramid Aramid 0.035 in-plane Aramid c 0.035 in-plane honeycomb 0.024 0.08bb b 0.035 in-plane c c honeycomb 0.08 11.3 out-of-plane 0.0240.024 - - honeycomb 0.08 e 11.3 out-of-plane 11.3 out-of-plane C1-6.4-24 C1-6.4-24e e C1-6.4-24 a Due to the fact that aerogels are isotropic, the compression tests on pure aerogels were done only in aa Due to the fact that aerogels are isotropic, the compression tests on pure aerogels were done only in one Due to the fact that aerogels are isotropic, the compression tests on pure aerogels were done only in b from data sheetc [20]; c from data sheet d the value in [20] is given for cell size b from d [21]; e the 4.8 one direction; b data d the direction; sheet value invalue [20] isingiven cell size fromsheet data[20]; sheet from [20]; cdata from data[21]; sheetthe [21]; [20] for is given for4.8 cellmm; size 4.8 one direction; e value in [20] is given for cell size 6.3 mm. mm; the value in [20] is given for cell size 6.3 mm. mm; e the value in [20] is given for cell size 6.3 mm.

Figure1.1. Super-flexible Super-flexible silica Figure aerogel SA1. Figure silica aerogel aerogelSA1. SA1.

Figure 2. Microstructure of SA1 (magnification of image 1k× and of inset 5k×). Figure and of of inset inset5k×). 5kˆ). Figure2.2.Microstructure Microstructureof ofSA1 SA1(magnification (magnificationof of image image 1kˆ 1k× and

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To characterize the porous structure of an aerogel the porosity and mean pore size were calculated. The porosity was determined by using of Equation (1) from envelope ρenv. and skeletal ρskel . Gels densities. 2016, 1, 0001 ˙ ˆ ρ (1) Φ “ 1 ´ env. ¨ 100% ρskel. To characterize the porous structure of an aerogel the porosity and mean pore size were calculated. The porosity was determined by using of Equation (1) from envelope ρenv. and skeletal ρskel. The specific pore volume v pore is given by densities. 1 1 vpore “ ρ . ´ (2) ρskel. Φ = 1 −ρenv. ∙ 100% (1) ρ .

and can usedpore to calculate average pore size dav using [22] The be specific volume vthe pore is given by dav =“

4¨ vpore − .SBET

.

(3) (2)

[22] of about 242 nm. and can be used to calculate averageand pore size dav using The SA1 exhibits high porositythe of 97.3% average pore size The low-flexible silica aerogel (SA2), shown ∙in Figure 3, is much stiffer in contrast to the SA1. = (3) Being white as well, with a haptic like an eraser or rubber, it is still slightly flexible and compressible. Though significantly more of robust the super-flexible The being SA1 exhibits high porosity 97.3%than and average pore size of aerogels, about 242 the nm. SA2 aerogel is still easy breakableThe intolow-flexible pieces. Its silica structure consists of small, interconnected particles porestointhe theSA1. range of aerogel (SA2), shown in Figure 3, is much stiffer inand contrast 0.5–1Being µm as shown in Figure Compared to SA1 SA2it aerogels, it appears a finer structure. white as well, with a 4. haptic like an eraser or and rubber, is still slightly flexible with and compressible. Though being more thanthat the super-flexible aerogels, the SA2two aerogel is still easy than The calculation of significantly average pore sizerobust confirms the pores of SA2 are almost times smaller ´ 3 breakable into pieces. Its structure consists of small, interconnected particles and pores in the range SA1. The low-flexible aerogel exhibits higher envelope density (0.092 g¨ cm ) caused by the higher 0.5–1 μm as shown in Figure Compared to SA1 and SA2 W¨ aerogels, a finer solidof concentration. A slightly higher4.thermal conductivity (0.038 (m¨ K)it´1appears ) and anwith almost 25 times structure. The calculation of average pore size confirms that the pores of SA2 are almost two times higher compressive modulus compared to SA1 is measured consequentially, as given in Table 1. The smaller than SA1. The low-flexible aerogel exhibits higher envelope density (0.092 g·cm−3) caused by porosity of SA2 is slightly lower (93.5%). the higher solid concentration. A slightly higher thermal conductivity (0.038 W·(m·K)−1) and an The envelope density of aramidmodulus honeycombs of type C1, shown inconsequentially, Figure 5, is slightly almost 25 times higher compressive compared to SA1 is measured as givenlower compared to the synthesized aerogels, SA1 and SA2. The envelope density of honeycombs type in Table 1. The porosity of SA2 is slightly lower (93.5%). A10-92-5.2 is similar SA2 aerogels and much higher thanC1, of SA1. The envelopetodensity of aramid honeycombs of type shown in Figure 5, is slightly lower compared to the synthesized is aerogels, SA2. The envelope densityheat of honeycombs Their thermal conductivity almost SA1 two and times higher, due to higher transfer via type the solid ´ A10-92-5.2 is similar to SA2 aerogels and much higher than of SA1. and gaseous phases. The aramid fibers form dense walls, with a density of 1.44 g¨ cm 3 being Theirthan thermal almostSince two times higher, due to higher transfer via theis solid much higher thatconductivity of aerogelsis [23]. the heat transfer via theheat solid backbone directly −3 being much and gaseous phases. The aramid fibers form dense walls, with a density of 1.44 g·cm proportional to the density of backbone material, the thermal conductivity of aramid is higher. The higher than that of aerogels [23]. Since the heat transfer via the solid backbone is directly proportional heat transfer via gaseous phase depends, amongst other parameters, on the pore dimension. In the to the density of backbone material, the thermal conductivity of aramid is higher. The heat transfer cells of 3.2–6.4 mm, the diffusive and convective heat transfer is predominant and leads also to a high via gaseous phase depends, amongst other parameters, on the pore dimension. In the cells of 3.2–6.4 thermal of cells aerogel should decrease thealso heattotransfer and lead to mm,conductivity the diffusive [4]. and Filling convective heat with transfer is predominant and leads a high thermal betterconductivity insulating materials. [4]. Filling of cells with aerogel should decrease the heat transfer and lead to better As expected, the stiffness of aramid honeycomb material is significantly higher than that of insulating materials. As expected, the stiffness of aramid honeycomb material is releasing significantly higherthey than spring that of back aerogels. The honeycomb materials are highly resilient. After a load, aerogels. are highlyA10-92-5.2 resilient. After releasing a load, density they spring to to their initialThe sizehoneycomb and shape.materials The honeycomb shows the highest andback the highest their initial size andinshape. The honeycomb A10-92-5.2 shows the highest density look and the highest compressive modulus both directions. The properties of C1 type honeycombs similar. compressive modulus in both directions. The properties of C1 type honeycombs look similar.

Figure3.3.Low-flexible Low-flexible silica Figure silicaaerogel aerogelSA2. SA2.

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Figure 4. Microstructure Microstructure ofSA2 SA2(magnification (magnification of of image image 10k× and ofof inset 24k×). Figure 4. SA2 (magnification and of inset 24k×). Figure 4. Microstructure ofof image10k× 10kˆ and inset 24kˆ). Figure 4. Microstructure of SA2 (magnification of image 10k× and of inset 24k×).

Figure 5.5. C1-6.4-24 coveredwith withphenolic phenolic resin. Figure 5.Aramid Aramidhoneycomb honeycomb C1-6.4-24 C1-6.4-24 covered covered with phenolic resin. Figure Aramid honeycomb resin. Figure 5. Aramid honeycomb C1-6.4-24 covered with phenolic resin.

2.2. Appearance and Propertiesofof ofthe theAerogel-Honeycomb-Composite Aerogel-Honeycomb-Composite Materials Materials 2.2.2.2. Appearance and Properties Materials Appearance and Properties the Aerogel-Honeycomb-Composite 2.2. Appearance and Properties of the Aerogel-Honeycomb-Composite Materials SA1 aerogel The composite materials, depicted in Figures Figures 666 and and 7,7containing containing show sound composite materials, depicted show sound TheThe composite materials, depicted in Figures and7, containingSA1 SA1aerogel aerogel show sound adhesion on the honeycomb and surround it thoroughly without any cracks. Some small pores are adhesion on thehoneycomb honeycomb and surround without anyany cracks. Some small pores are Theoncomposite materials, depicted in itFigures 6 and 7, containing SA1 aerogel show sound adhesion the and surround itthoroughly thoroughly without cracks. Some small pores visible on on the surface caused bysurround formation of air air bubbles bubbles during sol-gel synthesis. These holes visible the surface caused by formation of during sol-gel synthesis. These holes adhesion on the honeycomb and it thoroughly without any cracks. Some small pores are visible on the surface caused by formation of air bubbles during sol-gel synthesis. These are holes (encircled) could negatively affect the composite andbubbles cause aa weakening. weakening. Figures and 99These depict the the (encircled) could negatively affect the composite and Figures 88 and visible oncould the surface caused by the formation of air sol-gel synthesis. (encircled) negatively affect composite andcause cause during a weakening. Figures 8 depict and 9holes depict compositescould with low-flexible low-flexible SA2the aerogels. Without any defects defects inside the the material, the samples composites with SA2 aerogels. Without any inside material, samples affect composite and cause weakening. Figures 8 andthe 9 depict the the(encircled) composites withnegatively low-flexible SA2 aerogels. Without anyadefects inside the material, the samples exhibit good good adhesion. adhesion. The The firm firm contact contact between between aerogels aerogels and and honeycomb honeycomb material material was was additionally additionally exhibit composites with low-flexible SA2 aerogels. Without any defects inside the material, the samples exhibit good adhesion. The firm contact between honeycomb was additionally approved by SEM. SEM. Figures Figures 10 and and 11 show show both aerogels materials:and honeycombs andmaterial aerogels. One can see see approved by 10 11 both materials: honeycombs aerogels. can exhibit good adhesion. The10 firm contact between aerogels and honeycomband material wasOne additionally approved by SEM. Figures and 11 show both materials: honeycombs and aerogels. One can aramid fibers covered with aerogel particles. This confirms a continuous, firm contact between the see aramid fibers covered with 10 aerogel particles. Thismaterials: confirms ahoneycombs continuous, and firmaerogels. contact between the approved by SEM. Figures and 11 show both One can seethe aramid fibers covered with aerogel particles. This confirms a continuous, firm contact between two materials. materials. two aramid fibers covered with aerogel particles. This confirms a continuous, firm contact between the two materials. two materials.

Figure 6. 6. Super-flexible Super-flexible silica silica aerogel aerogel (SA1) (SA1) with with aramid aramid honeycombs honeycombs A10-92-5.2. A10-92-5.2. Figure

of 15 15 55(SA1) of Figure6.6.Super-flexible Super-flexiblesilica silicaaerogel aerogel (SA1) with Figure with aramid aramidhoneycombs honeycombsA10-92-5.2. A10-92-5.2.

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Figure 7. Super-flexible silica aerogel (SA1) with aramid honeycombs C1-3.2-29. Figure 7. Super-flexible silica aerogel (SA1) with aramid honeycombs C1-3.2-29. C1-3.2-29. Figure 7. Super-flexible silica aerogel (SA1) with aramid honeycombs C1-3.2-29.

Figure 8. Low-flexible silica aerogel (SA2) with aramid honeycombs C1-3.2-29. Figure C1-3.2-29. Figure 8. 8. Low-flexible Low-flexible silica silica aerogel aerogel (SA2) (SA2) with with aramid aramid honeycombs honeycombs C1-3.2-29. C1-3.2-29.

Figure 9. Low-flexible silica aerogel (SA2) with aramid honeycombs C1-6.4-24. Figure 9. Low-flexible silica aerogel (SA2) with aramid honeycombs C1-6.4-24. Figure (SA2) with with aramid aramid honeycombs honeycombs C1-6.4-24. C1-6.4-24. Figure 9. 9. Low-flexible Low-flexible silica silica aerogel aerogel (SA2)

Figure 10. SEM image (magnification 208×) of a cut-off cross-section of the SA1 Figure 10. SEM (magnification aa cut-off of SA1 Figure imageimage (magnification 208ˆ) of 208×) a cut-offof ofcross-section the SA1 aerogel-honeycomb SEM image (magnification ofcross-section cut-off cross-section of the the SA1 Figure 10.10.SEM aerogel-honeycomb composite, showing 208×) aramid-fibers, covered with good adhering aerogel-honeycomb composite, showing aramid-fibers, covered with good adhering composite, showing aramid-fibers, covered witharamid-fibers, good adhering aerogel particles. aerogel-honeycomb composite, showing covered with good adhering aerogel particles. aerogel particles. aerogel particles.

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Figure 11. 11. Adhesion Adhesion of of SA2 SA2 aerogel aerogel particles particles on on aramid aramid fiber fiber (magnification (magnification4.0kˆ). 4.0k×). Figure

2.3. Thermal Properties 2.3. Thermal Properties As already mentioned, heat in solids is transferred via different mechanisms [24–27]. The As already mentioned, heat in solids is transferred via different mechanisms [24–27]. The composites with higher density should transfer heat faster through their solid network. The density composites with higher density should transfer heat faster through their solid network. The density of the composites produced depends on two parameters. First, SA2 aerogels have higher densities, of the composites produced depends on two parameters. First, SA2 aerogels have higher densities, therefore the composites using SA2 aerogel will have a higher density too. Second, the higher the therefore the composites using SA2 aerogel will have a higher density too. Second, the higher the volume fraction of the honeycomb, the higher the envelope density. Composites with honeycomb volume fraction of the honeycomb, the higher the envelope density. Composites with honeycomb type C1-3.2-29 (with the smallest cell size) consist of the highest amount of aramid (volume fraction: type C1-3.2-29 (with the smallest cell size) consist of the highest amount of aramid (volume fraction: 7.3 vol.-%) as shown in the Table 2. On the other hand, the higher the volume fraction of the aerogel, 7.3 vol.-%) as shown in the Table 2. On the other hand, the higher the volume fraction of the aerogel, the lower the thermal conductivity of composites should be. the lower the thermal conductivity of composites should be. We calculated the theoretical thermal conductivity with the rule of mixture We calculated the theoretical thermal conductivity with the rule of mixture = ∙ ∙ (4) λeff “ λAerogel ¨ ΦAerogel ` λHoneycomb ¨ ΦHoneycomb (4) where λ denotes the thermal conductivities and Φ the volume fraction of components. The results where λ denotes the thermal conductivities and Φ the volume fraction of components. The results are given in the Table 2. are given in the Table 2. Table 2. Properties of produced honeycomb. Table 2. Properties of produced honeycomb.

Honeyco Envelope Aerogel Theoretical Thermal Measured Thermal mb Theoretical Thermal Envelope Measured Thermal Density Sample (volume Conductivity λeff Conductivity Aerogel Honeycomb Conductivity λ (volume Sample Density Conductivity eff −3) %)(volume %) (volume %)(g·cm(g¨ (W·(m·K)−1´1 ) (W·(m·K)−1) cm´3 ) (W¨ (m¨ K)´1 ) (W¨ (m¨ K) ) %) SA1 C1-3.2-29 92.7 92.7 7.3 0.036 0.0380.038 SA1 C1-3.2-29 7.3 0.069 0.069 0.036 SA1 A10-92-5.2 95.0 5.0 0.062 0.036 0.039 SA1 A10-925.0 0.062 0.073 0.036 SA1 C1-6.4-24 95.0 96.2 3.8 0.036 0.0360.039 5.2 SA2 C1-3.2-29 92.7 7.3 0.091 0.040 0.044 SA1 C1-6.4-24 3.8 0.073 0.092 0.036 SA2 A10-92-5.2 96.2 95.0 5.0 0.040 0.0440.036 SA2 C1-6.4-24 92.7 96.2 3.8 0.040 0.0390.044 SA2 C1-3.2-29 7.3 0.091 0.091 0.040 SA2 A10-9295.0 5.0 0.092 0.040 0.044 5.2 Because of the honeycomb structure, the volume fraction of the filling is over 90%. It SA2depends C1-6.4-24only on 96.2 3.8 0.091 0.040used. As expected, the0.039 the pores dimension of the honeycomb materials thermal Because ofcalculated the honeycomb structure, volume of the filling over 90%. It different depends conductivities for composites arethe equal, evenfraction with different aerogelisamounts and onlysizes on of the dimension of the honeycomb materials Asaffects expected, the thermal cell thepores honeycomb materials. Higher conductivity of SA2 used. aerogels the conductivity of conductivities calculated for composites are equal, even with different aerogel amounts and different composites, which is slightly higher. cell sizes of the honeycomb Higher conductivity of SA2measured aerogels for affects the conductivity Compared to theoreticalmaterials. values, the thermal conductivities several samples are of composites, which is slightly higher. higher. The differences are negligible and average about 2%–5%. In general, the conductivity in Compared to theoretical the thermal for several samples comparison to the honeycombvalues, itself (0.06–0.08 W¨conductivities (m¨ K)´1 ) wasmeasured substantially decreased. Withare a higher.cell The differences are negligible and average about 2%–5%. general, the conductivity in larger diameter, the volume and therefore the mass fraction of theInaerogel increase, which results comparison to the honeycomb itself (0.06–0.08 W·(m·K)−1) was substantially decreased. With a larger cell diameter, the volume and therefore the mass fraction of the aerogel increase, which results in a 7 of 15

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in aGels decrease thermal conductivity composite material. Theresults best results could be achieved 2016, 1, 0001 decrease ofofthermal conductivity for for the the composite material. The best could be achieved with withSA1 SA1aerogel aerogeland andhoneycombs honeycombsC1-6.4-24. C1-6.4-24. Here, the thermal conductivity could be successfully Here, the thermal conductivity could be successfully decrease conductivity for the composite material. The best results could be achieved with reduced by of 40 percent in in comparison totothe material itself. reduced bythermal 40 percent comparison thehoneycomb honeycomb material itself. SA1 aerogel and honeycombs C1-6.4-24. Here, the thermal conductivity could be successfully by 40Properties percent in comparison to the honeycomb material itself. 2.4.reduced Mechanical 2.4. Mechanical Properties

Important requirements are aa sufficient sufficientstiffness, stiffness,a asuitable suitable loading Important requirementsfor forinsulating insulating materials materials are loading 2.4. Mechanical Properties capacity and, addition,a acertain certainflexibility. flexibility. Flexible Flexible insulation a perfect contact capacity and, in in addition, insulationcan canguarantee guarantee a perfect contact Important requirements insulating materials are asosufficient stiffness, afluids suitable between insulated surfacefor and the insulating that no air canloading flow between thethe insulated surface and the insulatingmaterial, material, so that no or airother or other fluids caninflow capacity and, in addition, a certain flexibility. Flexible insulation can guarantee a perfect contact between. in-between. The extremely soft flexible aerogels satisfy this requirement if they are reinforced e.g., by between the insulated surfaceaerogels and thesatisfy insulating so that no air other fluids can inThe honeycomb extremely soft flexible this material, requirement if they areorreinforced e.g., byflow flexible flexible materials. between. honeycomb materials. The mechanical of thesatisfy synthesized compositesif were tested in-plane andbyout-of-plane The extremely soft properties flexible aerogels this requirement they tested are reinforced e.g., flexible The mechanical properties of the synthesized composites were in-plane and out-of-plane as shown in Figure 12. honeycomb as shown inmaterials. Figure 12. The mechanical properties of the synthesized composites were tested in-plane and out-of-plane as shown in Figure 12.

Figure Schematicrepresentation representation of conducted Figure 12.12. Schematic conductedcompression compressiontests. tests. Figure 12. Schematic representation of conducted compression tests. The effect of the aerogel filling of the honeycombs on the mechanical properties will be discussed

The effect of the aerogel filling of the honeycombs on the mechanical properties will be discussed on the example of the C1-3.2-29 honeycomb material. Figures 13–16 display the load-displacement on the example of C1-3.2-29 honeycomb material.onFigures 13–16 display thewill load-displacement effectrepresentative of the the aerogel filling ofof theC1-3.2-29. honeycombs the mechanical properties beSA1 discussed dataThe of four samples Each figure compares the pure aerogel or SA2 data ofthe four representative samples of C1-3.2-29. EachFigures figure compares the the pure aerogel SA1 or SA2 on example of the C1-3.2-29 honeycomb material. 13–16 display load-displacement as references, the empty honeycombs and the composite material. as references, the empty honeycombs the composite material. data of four representative samples C1-3.2-29. Each compares the80%. pureAfter aerogel SA1 or SA2of The soft and super-flexible SA1ofand silica aerogel wasfigure compressed up to reaching 40% as references, the empty honeycombs and the composite material. The soft anda first super-flexible aerogel was compressed 80%.followed After reaching compression, small crackSA1 was silica observed. As shown in Figure 13, up theytowere by several40% The soft and super-flexible SA1 silica aerogel was compressed up to 80%. After reaching 40% ofto by of compression, a first small crack was observed. As shown in Figure 13, they were other cracks. They indicate irreversible deformation of the material. Further compressionfollowed leads compression, a first small crack was observed. As shown in Figure 13, they were followed by several several other cracks. They indicate irreversible deformation of the material. Further compression densification of the porous structure. The strain increased rapidly and reached 0.015 MPa at other cracks. They indicate irreversible deformation of the material.rapidly Furtherand compression leadsMPa to at leads to compression. densification of the porous structure. The strain increased reached 0.015 80% densification of the porous structure. The strain increased rapidly and reached 0.015 MPa at 80% compression. 80% compression.

in-plane

Stress [MPa] Stress [MPa]

0.020

in-plane

0.020 0.015 0.015 0.010

Honeycomb C1-3.2-29 HoneycombComposite C1-3.2-29

0.010 0.005

Composite SA1 silica aerogel

0.005 0.000 0

SA1 silica aerogel 20 40

0.000 0

20

60

Compression [%] 40 60

80 80

100 100

Compression [%]

Figure 13. Compression curves of SA1 with C1-3.2-29 in-plane.

8 ofof15SA1 with C1-3.2-29 in-plane. Figure 13. Compression curves Figure 13. Compression curves of SA1 with C1-3.2-29 in-plane.

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The stress-compression curve of empty honeycomb in-plane shows three regions. regions. The first The The stress-compression stress-compression curve curve of of empty honeycomb in-plane shows three regions. The The first first region isischaracterized by a aconstant slope with rising stress. ThisThis slope waswas usedused to determine the region characterized by constant slope with rising stress. slope to determine region is characterized by a constant slope with rising stress. This slope was used to determine the compressive modulus. After reaching a maximum, the region of elastic deformation ends and a the compressive modulus. After reaching a maximum, region elastic deformation ends and compressive modulus. After reaching a maximum, thethe region of of elastic deformation ends and a plateau is is reached, aplateau plateau reached,which whichindicates indicatesthe thesecond secondregion. region.Further Furtherdeformations deformationsin inthe thestructure structure are are is reached, which indicates the second region. Further deformations in the structure are reflected in the long plateau, which extends up to 70%. Many cracks are characteristic for the reflected Manysmall small reflected in in the the long long plateau, plateau, which which extends extends up up to to 70%. 70%. Many small cracks cracks are are characteristic characteristic for for the the deformations of the walls. Finally, when the cell walls touch each other, densification starts and the deformations of the walls. Finally, when the cell walls touch each other, densification starts and the deformations of the walls. Finally, when the cell walls touch each other, densification starts and the stress rises [28]. stress stress rises rises [28]. [28].

0.8 0.8

out-of-plane out-of-plane

20 20

0

40 40

0.4 0.4

60 60

80 80

Honeycomb HoneycombC1-3.2-29 C1-3.2-29

0.2 0.2

Composite Composite SA1 SA1silica silicaaerogel aerogel

0.0 0.0

00

20 20

40 60 40 60 Compression Compression[%] [%]

80 80

Figure Figure14. 14.Compression Compressioncurves curvesof ofSA1 SA1with withC1-3.2-29 C1-3.2-29out-of-plane. out-of-plane. Figure 14. Compression curves of SA1 with C1-3.2-29 out-of-plane.

0.2 0.2 In-plane In-plane

Stress[MPa] [MPa] Stress

Stress[MPa] [MPa] Stress

0.6 0.6

0.05 0.05 0.04 0.04 0.03 0.03 0.02 0.02 0.01 0.01 0.00 0.000

0.1 0.1 SA2 silica aerogel SA2 silica aerogel

Composite Composite Honeycomb C1-3.2-29 Honeycomb C1-3.2-29 0.0 0.0

0 0

20 20

40 60 40 60 Compression [%] Compression [%]

80 80

Figure 15. Compression of SA2 with C1-3.2-29 in-plane. Figure Compressioncurves curves Figure 15. 15. Compression curves of of SA2 SA2 with with C1-3.2-29 C1-3.2-29 in-plane. in-plane.

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100 100

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Stress [MPa]

1.0

out-of-plane

Honeycomb C1-3.2-29

0.5

Composite

SA2 silica aerogel

0.0 0

20

40

60

80

100

Compression [%]

Figure 16. Compression curves of SA2 with C1-3.2-29 out-of-plane. Figure 16. Compression curves of SA2 with C1-3.2-29 out-of-plane.

In contrast, the curve of the composite exhibits a higher slope in the first region. The filling of In contrast, the curve of the composite exhibits a higher slope in the first region. The filling of honeycomb cells increases the deformation resistance, even if the filling material is very soft as, in our honeycomb cells increases the deformation resistance, even if the filling material is very soft as, in case, the SA1 aerogel. The compressive modulus of the honeycomb material is increased by factor 2.3 our case, the SA1 aerogel. The compressive modulus of the honeycomb material is increased by factor and compared to the pure aerogel by a factor of ten. The roughness of the curve in the plateau region 2.3 and compared to the pure aerogel by a factor of ten. The roughness of the curve in the plateau indicates several large cracks. They reflect a rupture inside the composite material. region indicates several large cracks. They reflect a rupture inside the composite material. Nevertheless, some difficulties occurred during testing. As loading progressed, the samples Nevertheless, some difficulties occurred during testing. As loading progressed, the samples bent bent slightly so that perfect uniaxial load could not be reached. Thicker samples could help to avoid slightly so that perfect uniaxial load could not be reached. Thicker samples could help to avoid this problem. this problem. The stress-compression curves out-of-plane show another progression in the Figure 14. The The stress-compression curves out-of-plane show another progression in the Figure 14. The loading capacity in that direction is much higher. A first deformation seen as a bending of the stiff loading capacity in that direction is much higher. A first deformation seen as a bending of the stiff walls is observed after reaching 0.4 MPa. Then, the resistance becomes weaker and the walls start walls is observed after reaching 0.4 MPa. Then, the resistance becomes weaker and the walls start bending at several positions. After 80% compression, irreversible densification of the honeycomb bending material takes place. at several positions. After 80% compression, irreversible densification of the honeycomb material The curve of the composite material looks quite similar. The delayed rise of the curve for the takes place. out-of-plane measurements is caused by protruding aerogel, which could not be cut perfectly without The curve of the composite material looks quite similar. The delayed rise of the curve for the damaging the composites structure, resulting in an offset. For the honeycomb material, the nearly out-of-plane measurements is caused by protruding aerogel, which could not be cut perfectly without linear behavior of the first part, which can be found for all samples, indicates elastic behavior over a damaging the composites structure, resulting in an offset. For the honeycomb material, the nearly large range of deformation. The linear relation between stress and compression ends with a maximum linear behavior of the first part, which can be found for all samples, indicates elastic behavior over a followed by a region of almost constant stress. In out-of-plane, the compressive modulus decreased large range of deformation. The linear relation between stress and compression ends with a by 5%. The weakening of the composite could be caused by defects in the material. As shown in maximum followed by a region of almost constant stress. In out-of-plane, the compressive modulus Figure 7, air bubbles were formed between cell walls and aerogel, so that a continuous contact is not decreased by 5%. The weakening of the composite could be caused by defects in the material. As given in the composite. One can assume that, under loading, the cracks will start at these positions. shown in Figure 7, air bubbles were formed between cell walls and aerogel, so that a continuous The compression test of SA2 aerogel is shown in Figure 15. A short linear region at the beginning, contact is not given in the composite. One can assume that, under loading, the cracks will start at where the compression was determined, is followed by two jumps in the curve. Compared to SA1, we these positions. can see a steeper slope in the Hookean region, which speaks for a higher compressive modulus. When The compression test of SA2 aerogel is shown in Figure 15. A short linear region at the beginning, the compression of 30% is exceeded, irreversible deformations occur in the aerogel material. After where the compression was determined, is followed by two jumps in the curve. Compared to SA1, first fracturing of pore walls, which is reflected in the jumps, the stress starts to rise. The aerogel loses we can see a steeper slope in the Hookean region, which speaks for a higher compressive modulus. porosity and becomes a compact material. The compressive modulus of the composite material is four When the compression of 30% is exceeded, irreversible deformations occur in the aerogel material. times higher than that of the empty honeycomb material itself. Under in-plane loading, several cracks After first fracturing of pore walls, which is reflected in the jumps, the stress starts to rise. The aerogel along the walls arise. As seen in Figure 17, the contact between the two materials of the composite loses porosity and becomes a compact material. The compressive modulus of the composite material under uniaxial load is the weakest point. To improve the strength of adhesion, a pre-treatment of the is four times higher than that of the empty honeycomb material itself. Under in-plane loading, surface of the honeycomb with e.g., surfactants, should be performed. several cracks along the walls arise. As seen in Figure 17, the contact between the two materials of

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11 of 15 under uniaxial load is the weakest point. To improve the strength of adhesion, a pre-treatment of the surface of the honeycomb with e.g., surfactants, should be performed.

Figure Compression test Figure 17. 17. Compression test of of SA2 SA2 with with C1-3.2-29 C1-3.2-29 honeycombs honeycombs in-plane. in-plane. Several Several cracks cracks and and holes holes arose arose during during compression. compression.

C1-6.4-24

Honeycomb type

A0-92-5.2

C1-3.2-29

Compressive modulus [MPa]

compression curves curves of of the composite composite materials materials containing SA2 show similar The out-of-plane compression behavior as as the the ones ones with with SA1 SA1 aerogel. aerogel. The The stiffness stiffness of of these these composites composites is is improved improved by by 13%. 13%. behavior The compression compression curves curves of of all other composites produced, produced, combining combining the two types of aerogel SA2 with with various various honeycomb honeycomb materials, materials, look look similar similar to to the the given given examples. examples. The results of SA1 and SA2 compression tests tests are are summarized summarized in in Figure Figure 18. 18. In all cases, the in-plane compressive moduli of the compression the composite composite materials materials are are increased. increased. Due to higher stiffness of SA2 aerogel, the corresponding composite materials materials are are stiffer stiffer too. too. The composite The highest highest values values are are reached with the medium cell size A10-92-5.2 honeycomb honeycomb from from HEXEL HEXELr®. Honeycombs of type C1-6.4-24 A10-92-5.2 Honeycombs of type C1-6.4-24 with with bigger cell size and Gels 2016, 1, 0001 therefore highest highest aerogel aerogel amount amount showed showed the the lowest loweststiffness. stiffness. therefore The results of the out-of-plane compression tests are similar. The highest improvement is carried 18 honeycomb material with a cell size of 6.4 mm. No increase is reached for both out using A10-92-5.2 Honeycombs in-plane SA1 composite in-plane SA2 composite in-plane out-of-plane SA1 composite out-of-plane SA2 composite out-of-plane types of aerogel. In general,Honeycombs weakening of a honeycomb material is caused by the high capability of 16 moisture absorption of aramid [29]. The cells of the honeycomb materials consist of aramid fibers, which take up humidity by capillary forces. For honeycomb materials with cell size of 3.2 mm, the 14 13.58 moisture absorption is 1.3% and, for 4.8 mm, it is 1.7% [20]. Therefore, to avoid moisture absorption, 13.34 12.11 all honeycomb 12 materials of aramid are covered by phenolic resins. As soon as the honeycombs are 11.3 cut, the protecting layer is broken and moisture can be12.2 absorbed via the cut surfaces. 10.7 To summarize aramid honeycomb 10 the thermal and mechanical properties of the silica-aerogel10.48 10.2 composite materials, we point out that the lowest thermal conductivity could be reached with SA1 8.46 aerogels and C1-6.4-24 honeycomb materials. In contrast, the same composite possesses poor mechanical properties. The highest improvement in terms 0.14 of stiffness could be achieved with SA2 and SA1 aerogels and A10-92-5.2 type honeycomb materials. The honeycombs A10-92-5.2 possess the 0.12 0.11 highest strength 0.1(0.09 MPa in-plane and 12.2 MPa out-of-plane) compared with other honeycomb 0.09 materials. It can be expected that the combining of A10-92-5.2 with aerogels would lead to highest 0.07 0.07an important role. values. On the other side, the ratio of both materials in the composite plays also The hybrid with the largest cell size (C1-6.4-24) contains a high amount of soft aerogel, which 0.03strength. Obviously, the cell size of A10-92-5.2 represents 0.03 the best ratio of reduces the compressive 0.03 combined materials. 0.0 4 6

Figure Figure 18. 18. Compressive moduli of of empty empty and and field field honeycombs honeycombs in in two twodirections. directions. Since Since we we could could observe observe defects defects in in the the manufactured manufactured materials materials (air (air bubbles, bubbles, holes), holes), these these weak weak points points should should be be prevented. prevented. Research Research along along this this line lineisiscurrently currentlyin inprogress. progress.

11 of 15 3. Conclusions Two different silica aerogels, filled in the cells of aramid honeycomb structures of different cell dimensions, are synthesized and investigated. Different types of aerogels: super-flexible SA1 and low-flexible SA2 are successfully combined with the honeycomb material and suitable adherence and

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The results of the out-of-plane compression tests are similar. The highest improvement is carried out using A10-92-5.2 honeycomb material with a cell size of 6.4 mm. No increase is reached for both types of aerogel. In general, weakening of a honeycomb material is caused by the high capability of moisture absorption of aramid [29]. The cells of the honeycomb materials consist of aramid fibers, which take up humidity by capillary forces. For honeycomb materials with cell size of 3.2 mm, the moisture absorption is 1.3% and, for 4.8 mm, it is 1.7% [20]. Therefore, to avoid moisture absorption, all honeycomb materials of aramid are covered by phenolic resins. As soon as the honeycombs are cut, the protecting layer is broken and moisture can be absorbed via the cut surfaces. To summarize the thermal and mechanical properties of the silica-aerogel aramid honeycomb composite materials, we point out that the lowest thermal conductivity could be reached with SA1 aerogels and C1-6.4-24 honeycomb materials. In contrast, the same composite possesses poor mechanical properties. The highest improvement in terms of stiffness could be achieved with SA2 and SA1 aerogels and A10-92-5.2 type honeycomb materials. The honeycombs A10-92-5.2 possess the highest strength (0.09 MPa in-plane and 12.2 MPa out-of-plane) compared with other honeycomb materials. It can be expected that the combining of A10-92-5.2 with aerogels would lead to highest values. On the other side, the ratio of both materials in the composite plays also an important role. The hybrid with the largest cell size (C1-6.4-24) contains a high amount of soft aerogel, which reduces the compressive strength. Obviously, the cell size of A10-92-5.2 represents the best ratio of combined materials. 3. Conclusions Two different silica aerogels, filled in the cells of aramid honeycomb structures of different cell dimensions, are synthesized and investigated. Different types of aerogels: super-flexible SA1 and low-flexible SA2 are successfully combined with the honeycomb material and suitable adherence and thermal conductivities of 0.036–0.044 W¨ (m¨ K)´1 are achieved. Both thermal conductivity and compressive modulus depend on cell dimension of honeycombs. High aerogel volume fractions lead to the highest decrease of thermal conductivity but not to an improvement of the mechanical properties. The mechanical properties on the other hand are remarkably increased, compared to pure aerogels, and represented by stress-strain curves generated from uniaxial compression tests. The huge differences in mechanical properties are caused by the different microstructures of the aerogels as observed in SEM. These results complement other experimental advances in the investigation of aerogel-honeycomb-composite material and provide a better understanding of the interaction of the aerogels tested both in synthesis, under uniaxial loading and with respect to thermal conductivity. Depending on the intended application, a careful choice of the utilized honeycomb material will give the opportunity to tailor the composite materials characteristics. It was demonstrated that aerogel-honeycomb-composite materials have the potential to enable new practical applications for silica aerogel insulation via diminishing the aerogels limiting fragility. Finally, a non-toxic, non-fuming, flame retarding light insulation material with sufficient contact between the two composite materials has been presented, which shows drastically improved mechanical properties in contrast to pure aerogel while maintaining low thermal conductivity. 4. Experimental Section 4.1. Materials for Synthesis Methyltrimethoxysilane (MTMS) 95% was purchased from Sigma-Aldrich. Tetraethyl orthosilicate (TEOS) > 99%, [3-(2,3-Epoxypropoxy)-propyl]-trimethoxysilan (GPTMS) ě 97%, ammonia (NH4 OH) 28%–30%, and hydrochloric acid 10´4 M from Merck. The surfactant N-(Hexadecyl)trimethyl-ammonium chloride (CTAC) 96% and diethylentriamine (DETA) 99% were supplied by Alfa Aesar. Solvents ethanol 96% and methanol 98.5% were purchased by Walter CMP

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and VWR International, respectively. Deionized water was used for synthesis. Carbon dioxide 4.5 (purity ě 99.995%) for supercritical drying was purchased by Praxair, Germany. Sealable polypropylene containers of 60 mL for gelation (with screw top) and 400 mL containers for washing (press-on lid) were purchased from VWR, Germany. The chemicals were used as received. The honeycombs with properties given in Table 3 were purchased by Hexcel and Schütz Industry Services. Table 3. Tabular listing of the aerospace qualified honeycomb used [20,21]. Manufacturer

Utilized Honeycomb

Cell Size (mm)

Height (mm)

Schütz Industry Services Schütz Industry Services Hexcel

C1-3.2-29 C1-6.4-24 A10-92-5.2

3.2 6.4 5.2

10 10 10

4.2. Synthesis of Aerogels-Honeycomb Composites The aramid honeycombs were first cut to rectangular samples of 35 ˆ 35 mm and 10 mm height. They were placed on the bottom of sealable polypropylene containers. For this study, two types of silica aerogel were synthesized. We used several precursors and alkaline catalysts to achieve different mechanical properties of aerogels. Synthesis of MTMS based silica aerogel: for the synthesis of MTMS based silica aerogel, the molar ratios of MTMS:Methanol:CTAC:NH4 OH:HCl were set to 1:35:4:4:4. In the synthesis, the precursor MTMS and methanol as solvent were mixed with the surfactant CTAC in a beaker at room temperature and stirred 5 min with a cross magnetic stirring bar. Then, 0.0001 M HCl solution was added to start the hydrolysis. The mixture is then stirred for 3 h with the same stirring velocity while being covered with aluminum foil. After 3 h, ammonia as an alkaline catalyst was added to start the condensation reaction and stirred for a few seconds and transferred into polypropylene beakers of 52 mm diameter with 35 mm ˆ 35 mm honeycomb samples inside. The honeycombs were completely covered with the solution. These beakers are then transferred into an oven for 3 days for gelation and aging at 50 ˝ C. Synthesis of MTMS-GPTMS based silica aerogel: The same procedure is used for the low-flexible aerogel SA2, but with a molar ratios of MTMS:GPTMS:Methanol:CTAC:DETA:HCl of 1:0.25:30:0.071:0.125:30 with added GPTMS and alkaline DETA instead of NH4 OH. The synthesis of this aerogel is based on the work of Aravind et al. [30]. After aging, the gels were cooled down to the room temperature and were transferred in an ethanol bath to remove the residual chemicals and to exchange water with ethanol. The ethanol was refreshed twice a day and six times in total, which ensures that water in the samples is exchanged with ethanol, which is soluble in supercritical carbon dioxide used in the final supercritical drying process. The supercritical drying was carried out for 32 h and with CO2 in an autoclave at 46 ˝ C and 97 bars with a mass flow rate of 14 kg¨ h´1 . The degassing rate was adjusted to 0.1 bars per minute. Finally, cylindrically shaped aerogel-honeycomb-composite samples are obtained, which then are carefully cut into 35 mm ˆ 35 mm quadratic samples with height of 10 mm. 4.3. Characterization Since the aim of this work is improvement of thermal and mechanical properties, the analysis was focused on these two aspects. The thermal conductivity was measured via Transient Plane Source (TPS) method using a Hot Disk Thermal Constants Analyzer TPS2500 (HotDisk, Göteborg, Sweden) [31,32]. The measurements were done between at 23.6–27.9 ˝ C and 1003–1015 bar atmospheric pressure, with humidity ranging between 43.8% and 67.9%. The compression tests were done at ambient conditions with help of a compression machine (Latzke, Wiehl-Marienhagen Germany) and load cells of 5000 N, with 1 mm¨ min´1 speed of compression. Since standard

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testing methods for aerogels do not exist, the compression tests were based on recommendation of ISO 844:2014. The samples were compressed up to 50% of their original length. These data was enriched with complementing data of envelope density and microstructure, which characterize the aerogels themselves. The envelope density of the samples was measured pycnometrically with a GeoPyc 1360 (Micromeritics, Norcross, GA, USA). The skeletal density was measured with AccuPyc (Micromeritics, Norcross, GA, USA). Surface area of aerogels was determined by nitrogen adsorption-desorption method BET (TriStarII, Micromeritics, Norcross, GA, USA).The microstructure of the aerogels and composites, especially the bonding between the aramid honeycomb and the aerogel, was studied with the help of SEM (Merlin, Carl Zeiss SMT, Oberkochen, Germany), using the detector for secondary electrons. Acknowledgments: The authors would like to acknowledge Aravind Parakkulam Ramaswamy for great support with experiments. We also thank Matthias Kolbe from Institute of Materials Physics in Space for his support with SEM. Author Contributions: Dr Barbara Milow had the original idea for the study and, with all co-authors carried out the design. Prof Lorenz Ratke was responsible for recruitment and follow-up of study participants. Matthias Rößler was responsible for data cleaning, and carried out the analyses. Marina Schwan drafted the manuscript, which was revised by all authors. All authors read and approved the final manuscript. Conflicts of Interest: The authors declare that they have no conflict of interest. The corresponding author collects the conflict of interest disclosure forms from all authors. The manuscript has not been submitted to other journal for simultaneous consideration. The manuscript has not been published previously. No data have been fabricated or manipulated (including images) to support our conclusions. Consent to submit has been received explicitly from all co-authors before the work will be submitted. Authors whose names appear on the submission have contributed sufficiently to the scientific work and therefore share collective responsibility and accountability for the results.

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