Characterization of polypropylene–polyethylene blends by temperature rising elution and crystallization analysis fractionation

The introduction of single-site catalysts in the polyolefins industry opens new routes to design resins with improved performance through multicatalyst-multireactor processes. Physical combination of various polyolefin types in a secondary extrusion process is also a common practice to achieve new products with improved properties. The new resins have complex structures, especially in terms of composition distribution, and their characterization is not always an easy task. Techniques like temperature rising elution fractionation (TREF) or crystallization analysis fractionation (CRYSTAF) are currently used to characterize the composition distribution of these resins. It has been shown that certain combinations of polyolefins may result in equivocal results if only TREF or CRYSTAF is used separately for their characterization

Gas-phase polymerization of ethylene using supported metallocene catalysts: Study of polymerization conditions

The influence of polymerization conditions on the gas-phase polymerization of ethylene with a supported metallocene catalyst (Cp2ZrCl2/SiO2) was investigated. The experiments were carried out in a 1.0-L semi-batch autoclave reactor and in a 0.6-L glass reactor, at pressures up to 100 psi and temperatures up to 57degreesC. Anchor and helicoidal agitators were used. Trimethylaluminium (TMA) and methylaluminoxane (MAO) were used as cocatalysts. Hydrogen was used as the chain-transfer agent. The polymer products were characterized by gel permeation chromatography (GPC) and scanning electron microscopy (SEM). Effects of pre-contact time of the catalyst and cocatalyst, catalyst injection configuration, agitator design, hydrogen volume, polymerization temperature and pressure were studied. The heterogeneity of polymer particles inside different zones of the reactor was also verified. The results indicated that several of these factors were crucial to obtain well-controlled gas-phase polymerization. SEM analyses showed that irregular growth of polymer within the shells often formed polymer particles of irregular shape. Flory's distribution was used to describe the polymerization mechanism for each site type. Interactions between catalyst sites and support surface are considered to broaden these distributions. Several Flory's distributions (4 to 7 different site types) were required to describe the broad and sometimes multimodal molecular weight distributions. Mass and heat transfer resistances may also be important in further broadening these distributions.203131895190