Propylene–ethylene random copolymers: Comonomer effects on crystallinity and application properties

Abstract

For understanding the correlation chain from the polymer structure to the final application properties of propylene-ethylene random copolymers, four commercial grades with different ethylene content (0 -8 mol %) but identical molecular weight distributions were investigated. Structural investigations concerning the comonomer distribution, using two different techniques (temperature rising elution fractionation and stepwise isothermal segregation), showed an increase of inhomogeneity with the total comonomer content, which was reflected for temperature rising elution fractionation in a widening of the elution temperature range. Crystallinity and crystallization speed studies confirmed the reduced overall crystallinity and increase of ␥-modification content as reported previously, but they also showed the reason for these effects in the distinctive decrease of the spherulitic growth speed. Good correlations of all these parameters to the mechanical performance of the four materials were obtained.

Flow-induced crystallization of propylene/ethylene random copolymers

“…At high heating rates the melting behavior of their fractions was unimodal. A slightly lower melting temperature is observed compared to the values of Gahleitner et al [29], which is probably due to the use of a different DSC device and/or a different batch of material. The amount of crystallinity of the samples compares well.…”

“…All four grades have a weight average molecular mass M w * 310 kg mol -1 and a polydispersity M w /M n * 3.4 [29]. Molecular and other physical properties are listed in Table 1.…”

“…Copolymerization of polypropylene with a small amount of ethylene is an appropriate way to improve both the optical and certain mechanical properties (low temperature brittleness) [29][30][31]. First, the four grades used are characterized using differential scanning calorimetry (DSC) and rheometry.…”

Sterilization of propylene/ethylene random copolymers: Annealing effects on crystalline structure and transparency as influenced by polymer structure and nucleation

“…Figure 5 gives the partial enthalpies for the primary (high T m , depending on C 2 content) and secondary (low T m , 131°C) crystallization fractions in the films from the B-series (conventional catalyst, limited randomness). The qualitative correlation of the latter part to the haze increase in Figure 3 is quite striking -at the highest comonomer content the chain irregularities obviously even reduce the capacity for post-crystallization, in line with the overall crystallization speed reduction found for such materials before [1]. A possible explanation for the limited correlation between haze and crystallinity changes for material series A mentioned above could be the changing relation between primary and secondary crystals with changing comonomer content.…”

“…Apart from homopolymers, ethylene-propylene (EP) random copolymers are the most important materials for this segment, taking advantage of their remarkable see-through performance. The good optical quality is a result of their slower crystallization speed compared to standard isotactic PP (iPP) resulting in less crystallinity and a finer crystal structure [1]. A growing fraction of this segment is subjected to pasteurization or sterilization processes, like in medical applications or food packaging.…”