用负载型Z-N催化剂制备形貌规则的空心球形聚乙烯粒子

Preparation of Regular Polyethylene Hollow Spheres with Supported Z-N Catalysts

采用一种含二酯类内给电子体的氯化镁负载Ziegler-Natta催化剂和三异丁基铝（TIBA）助催化剂,在同一反应器中经常压丙烯预聚和加压下的乙烯与少量丙烯淤浆共聚合两步反应,高效地合成了外径为0.4~3 mm的中空球形聚乙烯粒子,其球形粒子的外表面和内部空腔的内表面均较为平滑,壳层厚度均匀.聚合反应的催化剂效率最高达到7.49 kg/g Ti·h·MPa（6.5 kg PE/g Cat.）.研究了助催化剂种类、助催化剂浓度（Al/Ti摩尔比）和乙烯聚合阶段的丙烯共单体添加量对聚合物粒子形貌的影响,发现采用三乙基铝（TEA）或TEA/TIBA混合物为助催化剂时,虽然聚合物粒子仍呈中空球形形态,但其内部空腔的形状极不规则.以TIBA为助催化剂时,在一定范围内提高Al/Ti比和丙烯添加量均可显著提高催化效率,聚合反应动力学曲线由衰减型变为增长型,但聚合物粒子的中空形貌得以保持.对比丙烯预聚产物的粒子形貌发现,助催化剂含TEA时所得预聚物粒子结构较松散,内部有较多孔隙和裂纹,而改为TIBA后预聚物粒子内部相对致密.

Polyethylene hollow spheres with diameters of 0. 4- 3 mm were synthesized by a two-step slurry polymerization in a single reactor with a spherical Mg Cl2-supported Ziegler-Natta catalyst activated by triisobutylaluminum（ TIBA）,in which the first step was prepolymerization with propylene under 0. 1 MPa and the second step was ethylene copolymerization with small amount of propylene under 0. 6 MPa at 65 ℃. The hollow spheres have rather smooth outer and inner surfaces and relatively uniform thickness of the shell layer.Under optimized conditions,the catalyst efficiency of the polymerization reached about 7. 49 kg/g Ti·h·MPa（ or 6. 5 kg PE/g Cat.）. The effects of amount of propylene comonomer,cocatalyst concentration（ Al/Ti molar ratio） and type of cocatalyst on the particle morphology and polymerization kinetics were investigated. With increasing the Al/Ti molar ratio and the amount of added propylene,the catalytic activity was sharply increased and the particle morphology was further improved,meanwhile the kinetic curve changed from decay type to build-up type. When TEA or TEA/TIBA mixtures were used as the cocatalyst,the inner surface of the central void became rather irregular,though the whole particle was still spherical. The effects of cocatalyst on particle morphology were explained according to a diffusion controlled morphogenesis model based on the morphology of prepolymer particles.