페놀수지를 결합재로 한 탄소섬유 복합재료의 활성화 온도에 따른

표면 및 흡착특성의 변화

박 수 진, 김 기 동, 이 재 락

한국화학연구소 화학소재연구부

Influence of Activation Temperature on the Surface and Adsorption Property Changes of Carbon Fiber-reinforced Phenol Matrix Composites

Soo-Jin PARK, Ki-Dong KIM, and Jae-Rock LEE

Advanced Materials Division, Korea Research Institute of Chemical Technology,

P.O. Box 107, Yusong, Taejon 305-600, Korea

Introduction

Carbon fibers are expected to be in the increasing demand for composite materials in automobile, housing, aerospace, and leisure industries due to high strength and high elasticity. These excellent mechanical properties of carbon fibers used for the composite materials necessitate a good interfacial adhesion between fiber and matrix [1-3]. In recent year, activated carbon fibers (ACFs) have drawn great attention and widely used for purification and separation in many fields, due to its high adsorption characteristics. The adsorption properties of ACFs contain micropores in a significant proportion, and these pores are largely depended on the manufacturing processes. It is well known that activation process is essential to high performance ACFs, and the processing conditions are greatly considered by physical (or thermal) and chemical activations depending on the nature of the raw materials used [4-7].

Recently, the development of porous carbon monolithic composites for on-board water recovery units include applications in both military and recreational vehicles and removal of volatile organic compounds (VOCs) with high flow rates and low VOC concentrations. However, details of the composite manufacturing method and the activation procedure have not been fully known yet. It was claimed that the activated carbon fiber composites (ACFCs) possess an open internal structure which presents low resistance to the flow of fluids, and allows direct access of contacting fluid to surface of composites. Consequently, they offer a potential solution to the problems of removing VOCs diluted in large volume in air. Also, the composites are used in catalytic support and other industrial applications requiring high strength and corrosion resistance at severe real conditions [8-10].

In this work, we investigate the influence of activation temperature on surface and adsorption structures, such as specific surface area and porosity by burn-off, and on surface properties, such as pH, acid-base values, and pressure drop of the ACFCs prepared from the PAN-based carbon fabrics and phenolic resin binder.

Experimental

(1) Sample preparation

The composites used for preparing ACFCs were manufactured via molding process with oxidized PAN-based carbon fabric (plain type) and phenol resin (resole type) at a ratio of 7:3 by weight. After cured the composites by following process, as seen in Fig. 1, they were carbonized in an inert environment at 1,000℃ for 2 hs and then activated by CO2-gas at a heating rate of 5℃.min-1 to each temperature of 700, 800, 900 and 1,000℃ for 2 hs. The CO2 flow rate was maintained at 30 ml.min-1 by means of a metering valve and measured using calibrated rotameters. The composites were denoted as ACFC-700, ACFC-800, ACFC-900 and ACFC-1,000 to the activation temperatures.

<Fig. 1.> Curing process of ACFCs.

(2) Surface area and pore structure

Specific surface area and pore structure of ACFCs studied were measured by nitrogen gas adsorption at 77K using ASAP 2400 instrument from BET-method [11]. Prior to, the samples were dried in vacuum at 100℃ for 24 hs, then were degassed for 6 hs at 473K to obtain a residual pressure of less than 10-3 torr. A BET plot was constructed and the apparent BET surface area was extracted from the linear region in the range of relative pressure (P/P0) was 0.05~0.25. The amount adsorbed was converted to liquid volume by assuming a molar volume of liquid nitrogen of 35 cm3.mol-1.

(3) Surface properties

The surface pH of ACFCs was measured according to the ASTM D-3838. About 1.0 g of each sample was added to 20 ml distilled buffer solution (pH 7.0 at 25℃), and the mixture was shaken for 12 hs. Then the pH was measured after filtration with a nylon membrane filter (0.45 □ in diameter).

The acid and base values on sample surfaces were determined by Boehm's titration method [12]. According to the Boehm method, the acidic group content was determined by neutralization with some bases of increasing strength, while the number of basic groups was estimated by neutralization with 0.1 N HCl solution. Known portions of each sample (1.0 g) were added to 100 ml of 0.1 N NaOH solution and then shaken mechanically for 48 hs. After filtration of the mixture, a moiety of the supernatant liquid was then back-titrated with 0.1 N HCl solution, being used 0.01 M phenolphthalein solution as indicator. Likewise, the base value was determined by converse titration.

(4) Pressure drop

The air flow resistance properties of ACFCs were measured to the ASTM F-778. This method offers procedures sufficient to determine the gas flow characteristics of flat specimens of media used in the filtration process. Insert a sample in the test cell (d = 65 mm), and measured its rated air flow resistance using a test apparatus and instrument procedure established.

Results and Discussion

The surface properties and adsorption characteristics of ACFCs are listed in Table 1. As a result, it shows that the surface natures are amphoteric with slight basic properties, and the specific surface area and porous structure are well-developed with increasing the activation temperature. That is probably due to the carbon surface-oxygen complexes formed by more stable and developed basic surfaces, which are electron donor structures. Also, the changes of surface structure on ACFCs can be attributed to an activated diffusion effect associated with a slow diffusion of adsorbate molecules through very narrow pore entrances or constrictions.

<Fig. 2.> Isothermal N2-gas adsorption of ACFCs studied.

The adsorption isotherms of N2-gas at liquid nitrogen temperature are shown in Fig. 2. All of the isotherms have two characteristic regions; one below the relative pressure (P/P0≤0.05), where a substantial increase of the adsorbate uptake is observed with increasing P/P0. This is an indicative of microporosity in all samples. While, at P/P0 values larger than about 0.05, very little gas adsorbed with increased P/P0 indicated that minimal meso- or macropore structure in the ACFCs studied.

<Fig. 3.> Pore size distribution of ACFCs with activation temperature.

Fig. 3, shows the development of pore structure for ACFCs studied with activation temperature. As increasing the activation temperature, the average pore diameter of samples decreases, as expected. Apparently, the PAN fibers are not reactive to CO2 at 700℃, but are consumed readily with increasing the activation temperature. The increase in both specific surface area and total pore volume, combined with a very slight decrease in pore diameter, suggest that CO2 activation did not etch the fibers. The large number of micropores in ACFCs seems to be reasonable because the fibril structure of PAN fibers is conductive to large pore volumes.

Fig. 4 provides the comparative pressure drop data of ACFCs with air flow velocity. As an experimental result, the pressure drop shows a trend to increase with the increasing the air velocity, and there is some differences in the performance of each sample, which can be found that pressure drop is systematically depended on activation temperature. In the activation temperatures studied, the pore structures of ACFCs are well-developed with increasing the burn-off. From which, the ACFC-700 shows higher pressure drop data than the others. However, the ACFCs studied in this work have an appropriate filtering property which may be resulted in below 3.0 mmH2O at 200 cm3.min-1 for the air cleaning filter media on automotive engine, building, plants, etc [13].

<Fig. 4.> Pressure drop of ACFCs with air flow velocity.

Table 1. Surface and adsorption properties of ACFCs

with different activation temperatures

Terms Samples	pH	Acid value (mequiv.g-1)	Base value (mequiv.g-1)	SBET (m2.g-1)	VT (cm3.g-1)	DP (Å)
ACFC-700 ACFC-800 ACFC-900 ACFC-1000	6.8 6.9 7.2 7.4	180 135 80 65	90 70 55 50	340 482 576 548	0.26 0.29 0.34 0.30	28.5 24.2 17.6 19.2

Conclusions

In this work, the ACFCs showed different specific surface area, porosity and surface property, depending on activation temperature. The evolution of pore size, pore volume and specific surface area could be easily observed in increasing the burn-off with activation temperature. Among them, the ACFC-900 produced the well-developed pore structure from adsorption characteristics. Also, the activation temperature influenced the surface nature of ACFCs, when the temperature was less than 800℃, the surface was developed in acidic properties, but it was base at higher than 900℃.

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