Synthesis of Polypropylene/Poly(ethylene-co-propylene) In-Reactor Alloys by Periodic Switching Polymerization Process—Effects of Gas Phase Polymerization Time on Polymer Properties
2013
Mehtarani, Reza | Fu, Zhisheng | Fan, Zhiqiang | Tu, Songtao | Feng, Lian-Fang
A series of polypropylene/poly(ethylene-co-propylene) in-reactor alloys was prepared by a periodic switching polymerization process (PSPP) in which the monomer feed was periodically switched between pure propylene and an ethylene/propylene mixture using a commercial TiCl₄/MgCl₂/phthalate–Al(C₂H₅)₃/DCPDMS Ziegler–Natta catalyst. In this work, the influence of the total time of gas phase PSPP (tgₐₛ) on the composition, morphology, and properties of the alloys were investigated. The rate of copolymer production was almost constant in the gas phase PSPP reaction for as long as 2 h, implying that the diffusion barrier does not significantly increase with tgₐₛ in the gas phase PSPP. By fractionating the PP/EPR alloys to three fractions, random ethylene/propylene copolymer (EPR), segmented ethylene/propylene copolymer (EPS), and isotactic polypropylene (iPP), it was found that the EPR and EPS content of the alloys increased with increasing tgₐₛ. The chain structure of the fractions also changed with an increase in tgₐₛ. An increase in tgₐₛ led to a decrease in the content of [PPP] and [EEE] triads in EPS fraction, but the influence of tgₐₛ on the sequence distributions of EPR fractions can be ignored. The average size of the dispersed phase domains or EPR domains increased first and then leveled off with increase in tgₐₛ. The number of EPR domains increased with an increase in tgₐₛ. Toughness of the alloy was improved by prolonging PSPP. The impact strength at low temperature (−20 °C) was more sensitive to the EPR content than that at room temperature. The flexural modulus was sensitive to the EPS content. The toughness–stiffness balance of the alloy is significantly influenced by both tgₐₛ and the switching frequency of PSPP.
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