Analyzing particle growth and morphology evolution of polyethylene based on electrostatic separation

doi:10.11949/0438-1157.20211838

Abstract

Abstract:

Analyzing the morphology evolution of polyolefin particles during the growth process is crucial for understanding the polymerization mechanism and regulating product properties. The complex particle growth of polyolefins leads to a large morphology difference and an extremely wide range of particle size distribution. However, existing studies focus on the process of primary polyolefins growing from the fragmentation of homogeneous catalysts, but lack of systematic studies on the subsequent growth and morphology evolution of primary polyolefins with different morphologies. Besides, a method of batch sorting polyolefin with different morphologies is needed to support statistical analysis of morphology evolution. An electrostatic-morphology co-separation method for polyolefin particles was developed based on the morphology-dependence of same-material particles with the same size. Through this method, batch sorting of polyolefin particles with similar sizes but different morphologies was achieved and the morphology evolution during polyethylene particle growth was investigated. The results show that there is a general phenomenon of morphology deterioration during polyethylene particle growth. As the particle size increases, the particle morphology gradually deviates from the sphere. The coupling analysis of particle size, morphology and crystallinity indicates two possible modes of particle growth as well as morphology deterioration: the particle fragmentation caused by excessive crystallization rate, and the replication of catalyst morphology. The method and results in this paper provide important support for the study of polyolefin morphology and the development of high-performance polyolefin catalysts.

Key words: polyethylene, electrostatics, morphology, fluidized-bed, polymerization, powder technology

摘要：

解析聚烯烃颗粒生长过程的形貌演变规律对认识聚合反应机理和调控产品性能至关重要。然而，聚烯烃复杂的颗粒生长行为导致极宽的粒径分布和较大的形貌差异，现有研究关注形貌均一的催化剂破碎生长成初级聚烯烃的过程，缺少对形貌各异的初级聚烯烃后续生长和形貌演化的系统研究。此外，亟需一种能够批量分选不同形貌聚烯烃的手段，支撑聚烯烃颗粒生长形貌的统计解析。基于同质同粒径颗粒摩擦荷电的形貌依赖性，开发了聚烯烃颗粒静电-形貌协同分选技术，实现了尺寸相近的不同形貌聚烯烃颗粒的批量分选，并基于此考察了聚乙烯颗粒生长过程中形貌的分化与演变规律。结果显示，聚乙烯颗粒生长过程中存在普遍的形貌劣化现象，随着粒径增大，颗粒形貌逐渐偏离标准球形；颗粒粒径、形貌、结晶度等的耦合解析表明聚乙烯颗粒存在两种可能的颗粒生长模式和形貌劣化路径：结晶速率过快导致的颗粒破碎和催化剂形貌复制效应导致的形貌劣化。研究方法和结果可为聚烯烃形貌研究和开发高性能聚烯烃催化剂提供重要支撑。

关键词: 聚乙烯, 静电, 形貌, 流化床, 聚合, 粉体技术

CLC Number:

TQ 325.12

Shiyi GE, Yao YANG, Zhengliang HUANG, Jingyuan SUN, Jingdai WANG, Yongrong YANG. Analyzing particle growth and morphology evolution of polyethylene based on electrostatic separation[J]. CIESC Journal, 2022, 73(4): 1585-1596.

葛世轶, 杨遥, 黄正梁, 孙婧元, 王靖岱, 阳永荣. 基于静电分选解析聚乙烯颗粒生长与形貌演变[J]. 化工学报, 2022, 73(4): 1585-1596.

Figures/Tables 14

Fig.1 Schematic diagram of fluidized bed and electrostatic separator

Table 1 Properties of polyethylene particles

颗粒	商业牌号	类型	催化剂	密度/(kg/m³)	熔融指数/(g/10 min)
PE-A	QHM32	HDPE	Metallocene	937	0.6
PE-B	QHM22	HDPE	Metallocene	937	0.6
PE-C	DMG1820	LLDPE	Ziegler-Natta	918	2.0
PE-D	DGDB2480	HDPE	Chromium	945	0.46

Fig.2 SEM images and circularity of positively and negatively charged PE particles of 600—850 μm

Table 2 Circularity and corresponding projection images

Circularity	Typical images
0.93—0.94
0.92—0.93
0.91—0.92
0.90—0.91
0.89—0.90
0.88—0.89

Fig.3 Circularity of PE particles with various narrow PSDs and SEM images of positively and negatively charged particles by electrostatic separation

Fig.4 Median diameter and median circularity of four kinds of PE particles with various narrow PSDs

Fig.5 Melting enthalpy of positively and negatively charged particles with various narrow PSDs for four grades of PE

Fig.6 Schematic diagram of nascent PE particle fragmentation

Fig.7 Force-displacement curves of PE particles with different morphology

Fig.8 Elasticity modulus of PE-A particles measured by AFM

Fig.9 Raman spectra of rough PE-A particles (r/r0=1.0)

Fig.10 Surface crystallinity of Raman spectra for PE-A particle profiles

Fig.11 Schematic diagram of particle growth pattern of polyethylene

Fig.12 SEM images of PE-E particles and its catalyst

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