UCRGJC-117641 PREPRINT

New Organic Aerogels Based upon a Phenolic-Furfural Reaction

L. W. Hrubesh RECEIVED

This paper was prepared for presentation at the International Symposium on Aerogels #4

Berkeley, CA September 18-21,1994

September 1994

This ha preprint of a paper intended for publication in a jownalorproceedings. Since changes may be made before pubfiation, this preprint is made available with the understanding that it will not be cited ar reproduced without the permission of the author.

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NEW ORGANIC AEROGELS BASED UPON A PHENOLIC-FURFURAL REACTION

R.W. Pekala and C.T. Alviso Chemistry & Materials Science Department Lawrence Livermore National Laboratory

Livermore, CA 94550

X. Lu, J. GrOl3, and J. Fricke Physikalisches Institut der Universitat Wurzburg

Am Hubland D-97074 Wurzburg, Germany

ABSTRACT

The aqueous polycondensation of (1) resorcinol with formaldehyde and (2) melamine with formaldehyde are two proven synthetic routes for the formation of organic aerogels. Recently, we have discovered a new type of organic aerogel based upon a phenolic- furfural (PF) reaction. This sol-gel polymerization has a major advantage over past approaches since it can be conducted in alcohol (e.g., 1-propanol), thereby eliminating the need for a solvent exchange step prior to supercri tical drying from carbon dioxide. The resultant aerogels are dark brown in color and can be converted to a carbonized version upon pyrolysis in an inert atmosphere. BET surface areas of 350-600 m2/g have been measured, and transmission electron microscopy reveals an interconnected structure of irregularly-shaped particles or platelets with -1 0 nm dimensions. Thermal conductivities as low as 0.015 W/m-K have been recorded for PF aerogels under ambient conditians. This paper describes the chemistry-structure-property relationships of these new materials in detail.

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INTRODUCTION

The hydrolysis and condensation of metal alkoxides has been the traditional synthetic route for the formation of inorganic aerogels (e.g., silica, titania, alumina) [ 1-31. In 1987, Pekala patented the first organic aerogel based upon the aqueous polycondensation of resorcinol with formaldehyde [4]. Because these aerogels are composed of a highly crosslinked aromatic polymer, they can also be pyrolyzed in an inert atmosphere to form carbon aerogels [5-71.

Transmission electron microscopy reveals that organic aerogels have a similar nanosmcture to silica aerogels. A major advantage of organic aerogels is their low 2 (atomic number) composition. Resorcinol-formaldehyde aerogels are even better thermal insulators than silica aerogels when measured under ambient conditions. A record low thermal conductivity value of 0.012 W/m-K was obtained at a density of 160 kg/m3 [8]. Carbon aerogels are of particular interest because they are the first electrically conductive aerogels to be synthesized. The low resistivity of the aerogel network and the large surface areas per unit volume are being exploited in supercapacitors that have both high power and energy densities 19-12]. These data demonstrate the importance of controlling both the structure and composition of aerogels, and it provides motivation to develop new organic aerogel systems.

The major requirements for an organic aerogel reaction include the presence of a multifunctional monomer and the ability to achieve a high crosslink density. In this paper, we discuss a new organic aerogel based upon a phenolic-furfural (PF) reaction. Unlike previous organic sol-gel reactions, this one is conducted in alcohol using an acid catalyst, thereby eliminating the need for a solvent exchange step prior to supercritical drying, The thermal, acoustic, and mechanical properties of these materials are evaluated as a function of aerogel density.

EXPERIMENTAL

PF gels were prepared from a commercially available polymer solution (FurCarb UP520; QO Chemicals, Inc., West Lafayette, IN). This solution is composed of approximately a 5050 mixture of a phenolic novolak resin dissolved in furfural. The Furcarb UP520 was

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diluted with 1-propanol and 10 phr catalyst (a mixture of aromatic acid chlorides; 42001; QO Chemicals, Inc., West Lafayette, IN) was added. Solutions prepared with different amounts of diluent were then poured into glass vials, sealed, and cured for 7 days at 85 OC. A small amount of syneresis was observed during the cure cycle, allowing the gels to be easily removed.

The PF gels are dark brown in color, making it difficult to evaluate the gel structure via light scattering. After removal from the glass vials, the gels were placed directly into a pressure vessel (Polaron@, Watford, UK). The pressure vessel was filled With liquified carbon dioxide which was completely exchanged over several days for the alcohol present in the pores of the PF gels. The pressure vessel was then heated above the critical point of carbon dioxide (Tc = 31OC; Pc = 7.4 MPa). After slowly venting the pressure vessel, the PF gels were removed and further characterized.

PF aerogels are composed of a highly crosslinked polymer mamx, and they can be pyrolyzed in an inert atmosphere to produce carbon aerogels. Pyrolysis was conducted in a 3-zone Lindberg furnace (Model #54657-S) under nitrogen flow using the following ramp cycle: 22 OC + 250 OC in 2 hrs., held at 250 "C for 4 hrs., 250 + 1050 OC in 9.5 hrs., and held at 1050 "C for 4 hrs. The furnace was then allowed to cool under its own thermal mass back to room temperature in - 16 hours. The carbon aerogels are black in color. Pyrolysis leads to volumetric shrinkage of - 70% with an accompanying mass loss of - 45%.

A variety of characterization techniques were used to establish the chemistry-structure- property relationships of PF aerogels and their carbonized derivatives. Particle size and surface area were evaluated with transmission electron microscopy and gas adsorption techniques as previously described [ 13,141. The elastic properties of the aerogels were probed with the ultrasonic pulse-echo technique [ 151. One ultrasonic transducer transmitted sound waves, another one acted as receiver. The acoustic coupling of the transducers to the specimens was achieved without coupling agent by applying a small mechanical load. Due to the reIativeIy high acoustic attenuation, only frequencies less than 200 lcHz were used. Thermal conductivity measurements were performed using a hot-wire device as described elsewhere [ 161.

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RESULTS

The sol-gel polymerization of a phenolic novolak with furfural is depicted in Figure 1. The phenolic novolak is a low molecular weight polymer derived from the acid catalyzed reaction of a molar excess of phenol with formaldehyde. This multifunctional oligomer must be reacted with additional aldehyde, in this case furfural, to form a thermoset resin or crosslinked gel.

Transmission electron micrographs of a PF aerogel and its carbonized derivative are shown in Figure 2. The TEMs of both aerogels reveal interconnected particles with irregular shapes, unlike previous organic aerogels which have distinct spherical particles or interconnected fibers. The characteristic particle size for PF aerogels is approximately 10 nm. During pyrolysis, PF aerogels undergo -45% mass loss and volumetric shrinkage of -70%. Figure 2 reveals that the particle size of the resultant carbon aerogel appears to be slightly larger than its PF precursor. A similar result was obtained from the pyrolysis of resorcinol-formaldehyde aerogels synthesized at R/C=50, whereas a reduction in particle size was observed for aerogels synthesized at higher R/C ratios [ 13,171. The change in particle size upon pyrolysis likely depends upon whether the precursor aerogel has a "polymeric" or "colloidal" structure. BET surface area for the PF aerogels is 385 k 16 m2/g over a density range of 100-250 kg/m3, while the carbon aerogels exhibit surface areas of 512 & 40 m2/g over a density range of 300-450 kg/m3.

Figure 3 shows the thermal conductivities of PF aerogels as a function of gas pressure for densities p = 128,141, and 188 kg/m3. At room temperature, the total thermal conductivities of PF aerogels in air are between 0.015 and O:Oi7 W/m-K and between 0.0045 and 0.0065 W/m-K after evacuation. The total thermal conductivity in air shows a minimum at a density of about 200 kg/m3, whereas a monotonic increase with density occurs after evacuation. From the difference of the thermal conductivities before and after evacuation, the gaseous conduction Xg is obtained. The gaseous conduction ranges from 0.009 to 0.012 W/m-K, far below the value for non-convecting air (i.e., 0.026 W/m- K) and providing direct evidence that these new porous materials are aerogels. As with other aerogel compositions, Xg becomes negligible below 50 mbar.

Infrared absorption measurements were also performed on PF aerogels. Using Rosseland mean averaging, the temperature dependent specific extinction e(T) was derived. A value of approximately 50 m2/kg was obtained, similar to the value for resorcinol-

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formaldehyde aerogels. Using this value and subtracting the radiative contribution to the heat transfer, the solid thermal conductivities hs of PF aerogels are shown as a function of density in Figure 4.

The longitudinal sound velocities and elastic constants cl1 for PF and carbon aerogels are shown in Figure 5. The inset shows the scaling exponent for these pmperties with respect to aerogel density.

DISCUSSION

PF aerogels have two major advantages over previous organic aerogels: (1) the crosslinked gels can be processed more quickly because the solvent exchange step has been eliminated and (2) the starting monomer/oligomer solution is relatively inexpensive (Le., $1.80/kg). TEM and BET surface area data for PF aerogels and their carbonized derivatives are similar to other aerogels, except that the interconnected particles have irregular shapes which mimic flat platelets in many cases.

In order to be classified as an aerogel, a porous material must have both a solid matrix and pore phase with characteristic dimensions less than 100 nm. The thermal transport through such materials can be described as

where hs is the solid conductivity, hg is the gaseous conductivity, and hr is the radiative contribution. PF aerogels that are 1 cm thick provide enough IR absorption to be considered optically thick. As shown in Figure 4, the solid conductivity of PF aerogels is

hs a p a , where a= 2.0

in the density range 100-500 kg/m3. The scaling exponent for the solid thermal conductivity is larger than the one for resorcinol-fomialdehyde aerogels. Figure 6 shows the gaseous conductivity at ambient conditions, derived by subtracting the radiative and solid conductivity contributions from htotai, for PF and RF aerogels as a function of density. The PF aerogels have a larger average pore size than RF aerogels at any given density. Nevertheless, the pore size is still small enough to suppress gaseous conduction and eliminate convection.

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From acoustic measurements, the elastic constant c 11 was derived according to

C l l = + p 131

where CL is the longitudinal sound velocity and p is the bulk density. Figure 5 shows a log-log plot of both the elastic constant and longitudinal sound velocity versus aerogel density. The scaling exponent for the elastic constant of PF aerogels is higher than RF aerogels, whereas the exponents for the carbonized derivatives of each type of aerogel are identical [17]. The higher exponent of the PF aerogels is similar to acid catalyzed silica and melamine-formaldehyde aerogels, and it may indicate similarities in the sol-gel pathway as proposed previously [lS].

The scaling exponent of the longitudinal sound velocities for both PF and carbon aerogels is similar to other inorganic and organic aerogels [ 191. In Figure 7, the elastic constant for these new carbon aerogels is compared to a series of samples derived from resorcinol- formaldhyde aerogels. Within the error bars of the data, these new carbon aerogels are identical to previous carbon aerogels derived from resorcinol-formaldehyde at an R/C ratio of 200. The elastic moduli were determined with an accuracy of approximately 10%. This error is smaller than typical variations of the elastic moduli at a given density. Small structural differences are probably responsible for this variation. The new PF- derived carbon aerogels, however, show elastic moduli that do not systematically deviate from those of RF-derived carbon aerogels.

SUMMARY A new type of organic aerogel has been developed from the reaction of a phenolic novolak with furfural. The resultant PF aerogels can also be pyrolyzed in an inert atmosphere to form carbon aerogels. Thermal conductivities as low as 0.015 W/m-K have been measured under ambient conditions. Acoustic data also confirm that these new materials are aerogels. Future research will be directed at attempting to change the PF aerogel structure using different polymerization conditions,

ACKNOWLEDGMENT The Universitat Wiirzburg authors would like to thank the German BMFT'LBonn for support. Aerogel synthesis was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract #W-7405-ENG-48

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with financial support from the Office of Basic Energy Sciences, Division of Advanced Energy Projects.

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REFERENCES [I]

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SJ. Teichner, G.A. Nicolaon, M.A. Vicarini, and G.E.E. Gardes, Adv. Coll. Interf. Sci., 5,245 (1976). Aerogels, J. Fricke, ed., (New Yo* Springer-Verlag, 1986). H.D. Gesser and P.C. Goswami, Chem. Rev., 89,765 (1989). "Low Density, Resorcinol-Formaldehyde Aerogels," U.S. patent #4,873,218, Issued October 10, 1989; U.S. patent #4,997,8O4, Issued March 5, 1991. RW. Pekala and EM. Kong, ACS Poiym. Prpts., a( l), 221 (1989). RW. Pekala and C.T. Alviso, in Novel Forms of Carbon, C.L. Renschler, J.J. Pouch, and D.M Cox, eds., Mater. Res. Soc. Symp. Proc. m,3 (1992). R.W. Pekala, S.T. Mayer, J.L. Kaschmitter, and F.M. Kong, in Proc. of Inrl Symposium on Advances in Sol-Gel Processing and Applications, Chicago, IL, August 24-28; 1993, in press. X. Lu, M.C. Arduini-Schuster, 3. Kuhn, 0. Nilsson, J. Fricke, and R.W. Pekala, Science, - 255,971 (1992). S.T. Mayer, R.W. Pekala, and J.L. Kaschmitter, J. Electrochem. Soc., 140(2), 446 (1993). S.T. Mayer, J.L. Kaschmitter, and R.W. Pekala, Proc. of l83rd Electochem. SOC. Mtg., Honolulu, HI, May 16-21, 1993. R.W. Pekala, S.T. Mayer, J.L. Kaschmitter, and F.M. Kong, Proc. of Int'l Symposium on Advances in Sol-Gel Processing and Applications, Chicago, IL, August 24-28, 1993, in press. R.W. Pekala, S.T. Mayer, J.F. Poco, and J.L. Kaschmitter, in Novel Forms of Carbon II , C.L. Renschler, J.J. Pouch, and D.M Cox, eds., Materials Research Society Symp. Proc., in press, S.S. Hulsey, C.T. Alviso, F.M. Kong, and R.W. Pekala, in Novel F o m of Carbon, C.L. Renschler, J.J. Pouch, and D.M. Cox, eds., MRS Symp. Proc. 2212, 53 (1992). R.W. Pekala and F.M. Kong, J. de Physique, Coll. Suppl. %(4), (24-33, (1989). J. Gross, J. Fricke, R.W. Pekala, and L.W. Hrubesh, Phys. Rev. B,45(22), 12 774

[8]

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(1992). X. Lu, R. Caps, -J. Fricke, R.W. Pekala, C.T. Alviso, and L.W. Hrubesh, in this issue. R.W. Pekala, C.T. Alviso, and J.D. LeMay, J. Non-Cryst. Solids, u , 6 7 (1990).

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[17]

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[18] R.W. Pekala, L.W. Hrubesh,T.M. Tillotson, C.T. Alviso, J.F. Poco, and J.D. LeMay, in Mechanical Properties of Porous and Cellular Materials, L. Gibson, D. Green, and K. Sieradski, eds., Materials Research Society Symp. Roc. 241, 197 (1991). J. GroP and J. Fricke, J. Non-Cryst. Solids, 145,217 (1992). [l?]

Figure 1.

FIGURE CAPTIONS

A schematic diagram of a phenolic novolak oligomer with furfural in the Furcarb UP520 resin solution. Crosslinking occurs between aromatic rings though the aldehyde group on furfural.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Transmission electron micrograph of (a) phenolic-furfural aerogel and (b) its carbonized derivative. The aerogels have bulk densities of 156 and 335 kg/m3, respectively.

Variation of total thermal conductivity of PF aerogel monoliths as a function of gas pressure for different bulk densities. All measurements were performed at room temperature, 0 = 128 kg/m3, A = 141 kg/m3, [7= 188 kg/m3.

A plot of the solid conductivity As as a function of bulk density for various PF (V ) and RF (0 ) aerogels. All RF aerogels were prepared at an R/C ratio of 200.

A log-log plot of the elastic modulus cl1 and longitudinal sound velocity CL for PF aerogels as a function of their bulk density. The numbers in the legend indicate the scaling exponents (i.e., slopes of solid lines).

Figure 6. A plot of the gaseous conductivity hg as a function of bulk density for various PF (V ) and RF (0 ) aerogels. All RF aerogels were prepared at an R/C ratio of 200.

Figure 7. A log-log plot of the elastic modulus cl1 for carbon aerogels as a function of their bulk density. Carbon aerogels derived from resorcinol-

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formaldehyde were prepared at an R/C ratio of 200. All PF aerogels were pyrolyzed at 1050 "C whereas the RF aerogels were pyrolyzed at 1050, 1500,1800, and 2100 "C.

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Technical Information Department Lawrence Livermore National Laboratory University of California Livermore, California 94551

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