Isotactic polypropylene is a polyolefin in which every methyl group lies on the same side of the chain, and the regular arrangement allows the chains to crystallise. Commercial grades contain a few percent of chain defects that interrupt the regular sequence and lower both the crystallinity and the melting point [1]. The polymer crystallises in several polymorphs, the monoclinic alpha form being the most common, while the hexagonal beta form appears in the presence of a specific nucleating agent and gives a higher toughness [2]. Adding a nucleating agent raises the crystallisation temperature and shortens the cycle time, and a clarifying agent reduces the size of the spherulites enough to make the part transparent [3]. The impact strength falls with temperature and with the crystallinity, and a rubbery phase is usually copolymerised or blended in to recover it [4]. Because stereoregularity is set by the catalyst, the properties of a grade cannot be reproduced without the same catalyst system [5].
- tacticity
- ziegler-natta catalyst
- monoclinic alpha form
- beta nucleating agent
- spherulite
- clarifying agent
- crystallinity
Polymer crystallization and properties·Polymers and Plastics·Materials Science·Physical Sciences
References
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