열에너지 저장을 위한 고밀도 폴리에틸렌의 특성에 관한 연구

김재우, 최호석

대전광역시 유성구 궁동 220,

충남대학교 공과대학 화학공학과

A Study on Characteristics of High Density Polyethylene for Thermal Energy Storage

Jae-Woo Kim, Ho-Suk Choi

Department of Chemical Engineering, Chungnam National University, 220 Kung-dong, Yusong-gu, Taejon 305-764, Korea

Introduction

Radio frequency plasma treatments may be employed to modify the surface properties of polymer materials. A pure plasma is known to contain both positive and negative ions, atoms and metastables of atomic, which have been used to modify polymer surface properties such as crosslinking, wettability and adhesion without change of bulk property[1-4].

In this study, we focus on the surface crosslinking of HDPE particles. Particle is difficult to be handled in the plasma treatment because of large surface area per the unit mass of the particles. The untouched regions of the particles or powder surface are never modified. In the previous studies, Anand et al.(1981) modified polyethylene particles in a fluorine plasma which was induced in a vibrating fluidized bed[5]. Masuoka et al.(1984) used a rotary kiln as a plasma reactor to coat powders with polymer film[6]. Generally the fluidized bed type reactor is favorable to treat powders because of the uniformity in the bed and the ability of continuous powder handling.

HDPE is presently one of the most widely used out of all synthetic polymers. HDPE has a melting point of 120∼135℃ and latent heat of fusion is as high as 180∼210J/g. It has been reported that the latent heat of HDPE is suitable for its use as a thermal energy storage material. Three method of controlled chemical and physical crosslinking of HDPE have been reported: (1) peroxide-initiated, free radical crosslinking, (2) vinyl triethoxy silane grafting and crosslinking, and (3) electron beam crosslinking[7]. In all of these cases, the crosslinks eliminate the melt flow(high temperature form stability) but they decrease crystallinity and heat of fusion. All three method of crosslinking have been reported to result in 20∼30% losses of the initial heat of fusion[7-8]. Therefore, plasma technique was developed in this study.

Experimental

A schematic diagram of the plasma treatment apparatus used in this study is shown in Fig. 1. Gas flow rate is regulated and measured with a micro needle valve and regulator. The reduced pressure in the reactor is maintained by means of a mechanical rotary vacuum pump(Woosung vacuum Co., 200□/min) and diffusion pump(KODIVAC Co., 360□/sec). Pressure is measured with TC gauge(Varian Co., 801, Analog type). Entrained particles from the reactor are captured by a trap. An inductively coupled electrode(4mm-OD copper tube, 2 tune) for glow discharge at 13.56MHz frequency was placed at a distance of 30mm from gas inlet and was connected to an auto matching network and an rf power generator(YOUNGSIN Engineering Co., 0∼300W). To prevent overheating of the electrode and diffusion pump, cooling system(mechanical fan) is supplied to electrode and diffusion pump. To mix the particles, magnetic stirrer(TOPS Co. MS-300, R.P.M : max 1000) is used. The reactor is made of 90mm ID × 110mm-high Pyrex glass column.

The HDPE particles(density: 0.956g/□, M.I.: 0.04g/10min, YUZEX Co., HDPE 8800) of 20g are loaded into the reactor and gas in the reactor is evacuated by rotary(R.P) and diffusion pump(D.P) to 10mTorr. After the reactor is evacuated to base pressure, argon gas(purity: 99.999%) is admitted and allowed to flow for approximately 10min so that the pressure in the reactor becomes 60, 100, 200mTorr. HDPE particles are rotated inside the reactor at rotation speed of 600rpm with magnetic stirrer. Then, power is adjusted to start glow discharge. After the desired treatment time, plasma is turn off, and the samples are kept in the reactor for more than 20min at the given gas flow rate. After plasma treatment, the characteristics of the HDPE samples are measured. The experimental variables and their ranges for plasma surface crosslinking of HDPE particles in modified plasma reactor are summarized in Table 1.

Result and discussion

Pressure, rf power, treatment time and type of gas are parameter which are known to influence the surface modification of polymer[Anand et al., 1981]. The importance of these parameters are reconfirmed here for the polymer materials. The effect of these parameters are showed here in terms of various analysis methods. In this study, surface properties of argon plasma-treated HDPE are compared to plasma-untreated HDPE.

The effect of treatment time(min) on the gel content(%) for argon plasma-treated HDPE particles is shown in Fig. 2. It is obvious that gel content(%) for the plasma-treated HDPE increases with increasing treatment time due to the formation of crosslinked backbone structure at the surface layer, but levels-off with further treatment time. Under the rf power of 50W, gel content of plasma-treated HDPE increases from 0 to 0.7% during the time interval of 0 to 20min. This figure is indicated that in argon plasma(under 100mTorr, 200W), gel content percent of plasma-treated HDPE rapidly increases from 0 to 1.2% as reaction proceeds(0 to 10min), and level off after 10min. The total degree of gel content is also a function of the plasma treatment time.

The effect of the rf power supplied(50∼200W) to plasma on gel content(%) is shown in Fig. 3. This figure represents the degree of surface crosslinking of HDPE particle treated by argon plasma at a given condition. As can be seen, gel content of HDPE particle gradually increases with increasing rf power in reactor.

Fig. 4. shows the comparison of DSC diagrams for the plasma-untreated HDPE particle and plasma-treated HDPE particle to 100W rf power for 1hr(at 100mTorr). The melting point of plasma treated and untreated HDPE particles measured with DSC were 136.13℃(treated) and 134.51℃(untreated). From this result, melting point of plasma treated HDPE surface layer than plasma untreated HDPE increased about 1.62℃, but the shape of plasma treated HDPE were almost the same as those of plasma untreated HDPE. It means that the crystallinity of the HDPE was not changed with the crosslinking to 100W rf power, so endotherm peaks for phase change from crystalline to amorphous showed the same shape both in plasma treated HDPE and untreated HDPE. Since the thickness of crosslinked layer is very thin only at surface layer, the influence of the surface crosslinking on the bulk thermal property is not important.

References

1.	Clark, D. T., Dilks, A. J., Polym. Chem. Edn., 17, 957 (1997).

2.	Hudis, M., "Surface Crosslinking of Polyethylene using a Hydrogen Glow     Discharge", J. Appl. Polym. Sci., 16, 2397 (1972).

3.	Inagaki, N. S., Tasaka and Abe, H., "Surface Modification of Polyethylene     Powder Using Plasma Reactor with Fluidized Bed", J. Appl. Polym. Sci.,      46, 595 (1992).

4.	Park, S. H., Kim, S. D., "Plasma Surface Treatment of HDPE Powder in a    Fluidized Bed Reactor", Polymer Bulletin, 33, 249 (1994).

5.	Anand, M., Cohen, R. E. and Baddour, R. F., "Surface Modification of Low    Density Polyethylene in a Fluorine Gas Plasma", Polymer, 22, 361 (1981).

6.	Masuoka, T., Hirasa, O. and Suda, Y., "Plasma Surface Treatment            Apparatus for Powdery Materials", Bull. Res. Inst. Polym. Text.(Jpn.), 144,    73 (1984).

7.	Salyer, I. O., Davision, J. E., "Thermal Energy Storage in Crosslinked         Pellets of High Density Polyethylene for Home Heating and Cooling via       Off-Peak Electric Power Utilization", J. Appl. Polym. Sci., 28, 2903 (1983).

8.	Boneh, H., Abraham, M. and Narkis, M., "Thermal Energy Storage in         Columns Packed with Crosslinked Polyethylene Tubes", Polym. Eng. Sci.,     24, 1227 (1984).

9.	ASTM D2765-95. Standard Test Method for Determination of Gel Content     and Swell Ratio of Crosslinked Ethylene Plastics. Philadelphi : American      Society for Testing and Materials.

Table 1. Experimental conditions and ranges

Gas HDPE particle, [mm] pressure, [mTorr] Treatment time, [min] RF power, [w] Magnetic stirrer speed, [rpm]	Argon(99.999% purity) 3 60, 100, 200, 300 0, 1, 2, 3, 4, 5, 10, 20, 30, 40 0, 50, 100, 150, 200 0, 600

Fig. 1. The experimental apparatus for plasma treatment.

Fig. 2. Effect of treatment time on gel content(%) of argon plasma treated HDPE particle.

Fig. 4. DSC diagrams of HDPE particle: (a) argon plasma treated (100mTorr, 100W, 1hr), (b) argon plasma untreated.

Fig. 3. Effect of gas pressure on gel content(%) of argon plasma treated HDPE particle.