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
References
- Chen, J.; Chang, Y.; Isothermal crystallization kinetics and morphology development of isotactic polypropylene blends with atactic polypropylene. Journal of Applied Polymer Science 2007, 103, 1093-1104, 10.1002/app.25354.
- Hamad, F.G.; Colby, R.H.; Milner, S.T.; Transition in Crystal Morphology for Flow-Induced Crystallization of Isotactic Polypropylene. Macromolecules 2016, 49, 5561-5575, 10.1021/acs.macromol.6b00303.
- Speranza, V.; Salomone, R.; Pantani, R.; Effects of Pressure and Cooling Rates on Crystallization Behavior and Morphology of Isotactic Polypropylene. Crystals 2023, 13, 922, 10.3390/cryst13060922.
- Chen, J.H.; Tsai, F.C.; Nien, Y.H.; Yeh, P.H.; Isothermal crystallization of isotactic polypropylene blended with low molecular weight atactic polypropylene. Part I. Thermal properties and morphology development. Polymer 2005, 46, 5680-5688, 10.1016/j.polymer.2005.03.107.
- De Carvalho, B.; Bretas, R.E.S.; Quiescent crystallization kinetics and morphology of isotactic polypropylene resins for injection molding. I. Isothermal crystallization. Journal of Applied Polymer Science 1998, 68, 1159-1176, 10.1002/(sici)1097-4628(19980516)68:7<1159::aid-app13>3.0.co;2-t.