Formation of Double Melting Peak of Poly(propylene-co-ethylene-co-1-butene) During Pre-Expansion Process for Production of Expanded Polypropylene

초록

A new additional melting peak is formed on the DSC thermogram as a result of pre-expansion of poly(propylene-coethylene- co-1-butene) (terpolymer). The new melting peak appears at ca. 152oC which is about 12oC higher than the tail end temperature (offset temperature) of the pristine terpolymer to exhibit a double melting peak. The origin of the new melting peak is traced to melting and recrystallization of less perfect crystals of the terpolymer at the preexpansion temperature which situates usually in-between the onset and the offset temperature of the melting peak, contrary to the common presumption that the less perfect crystals are melted leaving the more perfect crystals intact behind during the pre-expansion step, and that the peak at ca. 152oC comes from melting of the more perfect crystals. The higher was the temperature to which the terpolymer was heated above the offset temperature, the longer time of annealing was required at the pre-expansion temperature for the new peak to appear. Once the terpolymer was completely melted by heating it to 175oC, at least 24 hours of the annealing was needed to show up the new melting peak, indicating that crystallization with homogeneous nucleation takes place much more slowly than that with self-nucleation.