Phenolic-Furfural 에어로겔 입자의 단일공정 합성

이경남, 서용준, 이해준, 김중현

연세대학교 화학공학과 고분자 소재 연구실

Synthesis of Phenolic-Furfural Aerogel Microspheres

by One-Step Process

Kyung-Nam Lee, Yong-Joon Suh, Hae-Joon Lee, and Jung-Hyun Kim

Polymer Materials Laboratory, Department of Chemical Engineering,

Yonsei University, 134 Shinchon-Dong, Seodaemoon-Ku, Seoul 120-749, Korea

Introduction

The aqueous polycondensation of resorcinol with formaldehyde (RF) and melamine with formaldehyde (MF) are two proven synthetic routes for the formation of organic aerogels. A new type of organic aerogel based upon a phenolic-furfural (PF) reaction was recently discovered.

The ultrastructure and properties of organic aerogels are analogous to their inorganic aerogels. In general, these materials have continuous porosity, an ultrafine pore size (≤50 □), high surface area (400-1000 □/g), and a solid matrix composed of interconnected colloidal-like particles or polymeric chains with characteristic diameters of 10 □. This ultrastructure and the low Z (atomic number) composition are responsible for the unique optical, thermal, electrical, and acoustic properties of organic aerogels. For example, RF aerogels have thermal conductivities lower than silica aerogels.

In spite of many advantage of these silica aerogel systems, the presence of silicon (Z=14) in the composition, however, often limits its effectiveness for many applications, such as in high energy physics or as parts for initial confinement fusion targets and the like, where a low number for Z (atomic number) is preferred. Pure organic foams or aerogels, consisting of mostly carbon (Z=6), and hydrogen (Z=1) with some oxygen (Z=8), low density aerogels of ultra-fine pore size could be produced.

The formation of organic aerogel microspheres offers an attractive alternative to monolith and granule production. It has been recognized that the formation of organic aerogel foam in small spheres would greatly expand the use of these materials, as well as reduce the process time and equipment costs in applications where they would be appropriate. For example, for energy storage applications, the microspheres have the advantage of allowing greater mechanical flexibility of a composite electrode, or may, using an appropriate binder, be used in a single cell, low voltage "jelly roll" cell, similar to conventional "AA", "C" or "D" cells, but with lighter weight. Microspheres are also useful in packed bed reactors in which connective flow through the bed can occur with relative ease while maintaining a very high surface area packing. In addition, the microspheres may be utilized in air filtration and medical applications, as well as in energy storage applications, such as in a double layer capacitor. Additionally, a need exists for the processing of such materials in way that results in a very fine powder that is easily flowable and easily transferable from one container to another.

In the sol-dispersion-gel method, oil droplets containing PF solution are dispersed and stabilized in supercritical CO2 by addition of an appropriate stabilizer. The choice of the stabilizer to be used in an dispersion is determined by stabilizer's anchor-soluble balance (ASB) number and the composition of the dispersion. The dispersion droplets serve as microreactors during gelation.

The PF mixtures have the ability to be formed directly in an organic solvent (i. e. alcohol) that is compatible with supercritical CO2. Therefore, the need for solvent exchange prior to supercritical drying is eliminated. Therefore, the synthetic process of PF aerogel is simpler than that of other organic aerogels.

In this proceeding, we focus on a one-step process for producing PF aerogel microspheres which are much more suitable for handling and which can be used in a variety of applications.

Theory

The sol-dispersion-gel polymerization of a phenolic novolac with furfural is depicted in Fig. 1. The phenolic novolac is a low molecular weight polymer derived from the acid-catalyst 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.

Experimental

In the sol-dispersion-gel process, phenolic novolac (Kangnam Chemical Co.), furfural (Acros Organics, 99 %), p-toluenesulfonic acid (Duksan Pure Chemical Co.), heptyl alcohol (Junsei Chemical Co.), and PDMS (polydimethylsiloxane, Dow Corning Co.) were used as-received.

Phenolic and furfural were mixed, and p-toluenesulfonic acid was added as an acid catalyst. Heptyl alcohol was added in order to adjust the overall reactant weight concentration. The mixture was poured into a steel vessel, sealed and polymerized at 80 ℃, 1200 psi for 24 hrs. The basic recipe of sol-dispersion-gel reaction is given in Table 1.

Phenolic-furfural-heptyl alcohol mixtures were dispersed in supercritical CO2. The solubility of heptyl alcohol in supercritical CO2 varies with temperature and pressure of the system. As process condition varied, PF gel microsphere was prepared and continuously heptyl alcohol in the PF gel microsphere was exchanged with supercritical CO2. The reactants were heated to 80 ℃ to form the gel. The gel was exchanged into supercritical CO2 in 40 ℃, after which it was supercritically dried. The overall experimental proceeded by one-step process.

Optical microscopy (Nikon, SMZ-2T) and scanning electron microscopy (SEM, Jeol 1100) were used to examine the average particle size and particle size distribution of PF aerogel microspheres.

Aerogel density was determined by weighing samples of known dimensions. Specific surface area was evaluated by BET from nitrogen adsorption      isortherms at 78 K.

Results and Discussion

The sol-dispersion-gel technique was successfully used to prepare micro-sized PF aerogel microspheres. Phenolic-furfural 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. In this system, PF aerogel microspheres were prepared by one-step process contrary to conventioncal 2-stage method, in which PF solution reacted to form gel in supercritical CO2 and the gel was exchanged into supercritical CO2 in one batch reactor. SEM micrographs of PF aerogel microspheres are presented in Fig. 2. The sizes of these microspheres ranged from submicrometer to a few hundred micrometers. In general, the aerogel microsphere sizes were controlled by the composition of the system and the dispersion conditions- the stirring rate, the contents and type of stabilizer, the type of catalyst. SEM and BET surface area data for PF aerogels are similar to other aerogels.

Fig.. A schematic diagram of a phenolic novolac oligomer with furfural. Crosslinking occurs between aromatic rings through the aldehyde group on furfural.

Fig. 2. SEM of PF aerogel microspheres.

Table . Basic recipe of PF aerogel microspheres

	Component	Amount
Dispersion Phase	Phenolic-novolac	9.09/13.6/18.18 g
	Furfural	9.09/13.6/18.18 g1
	p-toluenesulfonic acid	1.82/2.8/3.64 g2
	Hepyl alcohol	80/70/60 g
Continuous Phase	PDMS	10 g3
	Supercritical CO2	390 ml

1:Phenolic/furfural weight ratio = 1

2:Phenolic/Catalyst weight ratio = 5

3:PDMS/reactants weight ratio = 10 %

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