초임계 이산화탄소를 이용한 입자 형태의 축합중합체의 제조

이경남, 이해준, 김중현

연세대학교 화학공학과 나노입자 공정 연구실

Dispersed Condensation Polymerization Process in Supercritical CO2

Kyung-Nam Lee, Hae-Joon Lee, Jung-Hyun Kim

Nanosphere Process & Technology Laboratory, Dept. of Chem. Eng.,

Yonsei Univ., 134 Shinchon-dong, Sudaemoon-ku, Seoul 120-749, Korea

Introduction

Organic aerogels have continuous porosity, an ultrafine cell/pore size (≤ 50 nm), high surface area (400∼1000 m2/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. We have found new process to circumvent the energy-saving concerns about the synthetic route for organic aerogels by preparing these technologically important organic aerogels in one-step. Specially, we have synthesized a new type of organic aerogel based upon a P/F reaction.

The dissemination of VOCs and aqueous-waste streams into our environment has prompted considerable worldwide concern, and environmental issues are now of paramount interest to the chemical industry. To survive, the chemical industry must be able to confirm to more environmentally sound practices in the manufacture and processing of products. Herein, we describe the use of supercritical CO2 as a continuous phase for the manufacture (and processing) of organic aerogels. Specially, we have developed a sol-emulsion-

gel polymerization process in which CO2-soluble stabilizers, molecularly engineered to be interfacially active in supercritical CO2, are used to form a stable organic gel microspheres consisting of a dispersion phase in a supercritical CO2 continuous phase. This procedure allows for the synthesis of organic aerogles in the form of micrometer-sized microspheres with high surface area by one-step process. The formation of organic aerogel microspheres offers an attractive 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. Microspheres are useful in packed bed reactors in which connective flow through the bed can occur with relative ease while maintain-

ing 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.

This proceeding focuses the design of one-step process for producing P/F aerogel microspheres in supercritical CO2 by sol-emulsion-gel polymerization which are more suitable for handling and which can be used in a variety of applications.

Theory

Scheme 1 shows a schematic diagram of the principal reactions for phenolic novolac with furfural under conditions used in this study. P/F aerogels have two major advantages over previous organic aerogels: (a) The P/F mixtures have the ability to be formed directly in an organic solvent (i.e. alcohol) that is compatible with supercritical CO2. Therefore the synthetic process of P/F aerogel is simpler than that of other organic aerogels because the acetone exchange prior to supercritical drying has been eliminated and (b) the starting monomer/oligomer solution is relatively inexpensive. The primary differences in the process for producing new P/F aerogel micropsheres and the prior process for producing R/F aerogel microspheres were shown in scheme 2.

Experimental

In a typical sol-emulsion-gel polymerization reaction, phenolic novolac (4.5 g), furfural (4.5 g), p-toluenesulfonic acid (0.9 g), heptyl alcohol (23.3 g), and PDMS (polydimethylsiloxane, 6.7 g) were put into a 50 ml high-pressure vessel. The cell was purged with nitrogen for 10 min and then filled with CO2. The cell was heated to 80 ℃ and the pressure was increased to 1200 psi. The polymerization was continued at these conditions for 24 h. At the end of the polymerization, the cell was cooled to 40 ℃. But the pressure was increased to 2000 psi by the addition of more CO2 (conducted by pressure generator). The gel was exchanged into supercritical CO2, after which it was supercritically dried. The overall experimental proceeded by one-step process. The solubility of alcohol in supercritical CO2 varies with temperature and pressure of the system. As a result, the solubility of alcohol in supercritical CO2 was so low that the polymerization was conducted heterogeneously with high temperature and low pressure. But the solubility was so high that the solvent exchange was enabled by addition of CO2 with low temperature and high pressure. As process condition varied, P/F aerogel microspheres were prepared and continuously alcohol in the P/F aerogel microsphere was exchanged with supercritical CO2 in one-step. The overall experimental procedure was shown in scheme 3.

Specific surface area of P/F aerogel microspheres was evaluated by BET from nitrogen adsorption isotherms at 78 K.

Results and Discussion

The sol-emulsion-gel technique was successfully used to prepare micro-sized P/F aerogel microspheres. In this system, P/F aerogel microspheres were prepared by one-step process contrary to conventional 2 stage method, in which P/F solution reacted to form gel in supercritical CO2 and the gel was exchanged into supercritical CO2 in one batch reactor. SEM micrographs of P/F aerogel microspheres are presented in Fig. 1. The sizes of these microspheres ranged from submicrometer to a few mcirometers. The aerogel microspheres sizes were controlled by the composition of the system and the dispersion conditions-stirring rate, the contents and type of stabilizer, the type of catalyst.

SEM micrographs of internal morphology (Fig. 2) and surface area showed that the P/F aerogel microspheres synthesized in supercritical CO2 was identical to the other organic aerogel made in VOCs. BET indicated that the P/F aerogel microspheres made in supercritical CO2 had a surface area of 450∼700 m2/g. Similar results could be obtained for a variety of other multifunctional organic monomers.

Scheme 1. P/F gelation mechanisms: (a) addition, (b), (c) condensation,

and (d) disproportionation termination

Scheme 2. Synthetic process for organic aerogel microspheres:

(a) existing process for R/F aerogel microspheres,

(b) new one-step process for P/F aerogel microspheres

Scheme 3. Overall experimental procedure

for P/F aerogel microspheres

Fig. 1. SEM micrograph of P/F

aerogel microspheres

Fig. 2. SEM micrograph of

internal morphology

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