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A Method of Producing Polyethylene Wax Having a Low Polydispersity Index

Keun Ho Choi

Department of Chemical Engineering, Hanbat National University, Daejeon 305-719, Korea (Received 10 January 2001; accepted 19 September 2001)

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Abstract−In this study, to solve the problems of previous techniques, a new thermal degradation method was used for pro- ducing polyethylene wax having a low polydispersity index. The thermal degradation method was characterized by putting the preheating stage before the reaction stage. At the preheating stage, to increase the thermal conductivity of raw material, the raw material was heated up to a certain temperature(250-370^oC), which was lower than the reaction temperature(380-400^oC), and was remained at that temperature for a certain time. Polyethylene wax with a low polydispersity index(less than 3.90) could be produced within 5.40 hr using both the thermal degradation method and an external-loop reactor in which specially designed impellers were installed for more effective heat transfer. The effect of the temperature of preheating stage on the polydispersity of product, the viscosity of medium and the manufacturing time was negligible. An increase in the number of preheating stage only marginally affected the polydispersity index of product. When the reaction temperature increased, the viscosity of medium was more rapidly decreased with time and, hence, the reaction time was decreased. The viscosity of medium was decreased with increasing the reaction time.

Key words: Polyethylene Wax, Thermal Degradation, Stepped Heating Method, Polydispersity Index

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†To whom correspondence should be addressed.

E-mail: khchoi@hyunam.tnut.ac.kr

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3. Top connection 11. N₂ line

4. Riser 12. Motor

5. Bottom connection 13. Downcomer

6. Valve 4 14. Thermocouple

7. Valve 1 15. Impeller

8. Vent line 16. Valve 3

Fig. 2. Details of impeller.

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Fig. 4. Variation of the medium and wall temperatures with time(opera- tion without preheating step).

Fig. 5. Variation of the medium and wall temperatures with time(oper- ation with preheating stage).

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Table 1. Operation conditions and physical properties of the products

L Co. A-C 6A Run 1 Run 2 Run 3 Run 4 Run 5 Run 6 Run 7 Run 8

T_r(^oC) 390 - 390 390 390 380 390 400 390 390

T_s(^oC) - - - 310 250-370

(7 steps)

330 330 330 270 370

Viscosity(cP) 293 372 344 367 288 274 322 295 316 342

M_n(g mol⁻¹) 2139 2548 2508 4481 4312 5941 4918 4983 5214 5243

M_w(g mol⁻¹) 18683 11248 23138 9232 15773 21426 17821 15158 17814 18759

M_w/M_n(-) 8.74 4.41 9.23 3.83 3.66 3.61 3.62 3.04 3.42 3.58

Manufacturing time including the reaction step(hr) 8 - 2.17 3.15 9.33 5.36 3.10 2.28 3.28 3.17 Fig. 8. Effect of the number of preheating stage on the viscosity.

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M_n : number average of molecular weight [g/mol]

M_w : weight average of molecular weight [g/mol]

T : temperature [^oC]

t : time [min]

T_m : medium temperature [^oC]

T_p : preheating stage temperature [^oC]

T_r : reaction temperature [^oC]

t_r : reaction time [min]

T_w : wall temperature [^oC]

^^ò

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