Role of Crystallization on Polyolefin Interfaces: An Improved Outlook for Polyolefin Blends

Abstract

Polyolefins including linear low density polyethylene (LLDPE), high density polyethylene (HDPE), and isotactic polypropylene (iPP) account for nearly 2/3 of the worldwide plastics market. With wide-ranging applications, often short term in nature such as packaging, recycling of polyolefins is becoming increasingly important in developing a sustainable worldwide plastics market. However, it is difficult to separate polyolefins in mixed recycle streams; it would be advantageous to melt blend them, but their immiscibility leads to blends with poor properties. Here we demonstrate the role of synthetic history (i.e., site specific metallocene vs heterogeneous Ziegler−Natta catalyzed) on the oligomer content of HDPE, LLDPE, and iPP and its influence on adhesion between PE and iPP. Using a range of polymers and processing conditions, we identify four classes of such interfaces with a wide range of interfacial adhesion strengths (G IC ): excess oligomer (G IC < 30 N/m), easy chain pullout (G IC ≅ 100 N/m), kinetically trapped entanglements (G IC ≅ 600 N/m), and crystallization across the interface (G IC > 1200 N/m). Using molecular weight distribution data, we identified a critical oligomer content where the interfacial failure mechanism transitions from cohesive failure (G IC > 1200 N/m) to adhesive failure (G IC ≅ 100 N/m). Transmission electron microscopy (TEM) and atomic force microscopy (AFM) highlight distinct interfacial semicrystalline morphologies for each class of polyolefin interface which are defined by molecular parameters and processing conditions. Polyolefin blends were compression molded to highlight the role of interfacial strength in blends formed from mixed polyolefin streams; weak interfaces resulting from excess oligomer buildup yielded brittle failure while superior interfacial adhesion resulted in ductile blend failure.

C−H Functionalization of Commodity Polymers

“…[181] Theaddition of acompatibilizer, which is typically ab lock or graft copolymer, before melt processing improves the interfacial adhesion of the two phases and leads to more homogeneous materials. [182,183] TheC ÀHf unctionalization of polymers holds considerable potential for the synthesis of otherwise difficult to access graft or block copolymers.M ethods that install initiators for the ensuing polymerization, such as hydroxylation of branched polyolefins and subsequent grafting of aliphatic polyesters (Figure 9), [97] demonstrates recent examples of these opportunities.…”

“…The compatibilization of polymer mixtures commonly isolated from recycling streams can lead to physical properties that exceed those of the pristine materials themselves, as demonstrated by a recent case study of compatibilizing post‐industrial iPP/polyethylene terephthalate blends . The addition of a compatibilizer, which is typically a block or graft copolymer, before melt processing improves the interfacial adhesion of the two phases and leads to more homogeneous materials . The C−H functionalization of polymers holds considerable potential for the synthesis of otherwise difficult to access graft or block copolymers.…”

Review on modification strategies of polyethylene/polypropylene immiscible thermoplastic polymer blends for enhancing their mechanical behavior

“…This also contributes in tackling the aforementioned PE and PP recycling problem. A good example is the study carried out by Jordan et al 21 who analyzed how the interfacial adhesion of PE/PP bilayer changes when employing different catalyzed PEs and PPs (heterogeneous ZN vs. site-specific M), as well as different processing conditions, such as cooling rate of the processed PE/PP bilayers. These factors affect the crystallization of the polymers and the alignments of their chains at the PE/PP bilayer interface, having a great impact on the interfacial adhesion.…”

C‐H‐Funktionalisierung von Standardpolymeren

“…Die Kompatibilisierung von Polymergemischen, die gewöhnlich aus Recyclingströmen abgetrennt werden, kann physikalische Eigenschaften zur Folge haben, die jene der einzelnen Materialien übertreffen, wie mit einer aktuellen Fallstudie zur Kompatibilisierung von in der Produktion anfallenden iPP/Polyethylenterephthalat‐Gemischen demonstriert wurde . Die vor der Schmelzverarbeitung erfolgende Zugabe eines Kompatibilisierungsmittels, das üblicherweise ein Block‐ oder Pfropfcopolymer ist, verbessert die Grenzflächenadhäsion der beiden Phasen und führt zu homogeneren Materialien . Die C‐H‐Funktionalisierung von Polymeren birgt ein erhebliches Potenzial für die Synthese von Pfropf‐ oder Blockcopolymeren, die ansonsten schwer zugänglich sind.…”