Composition Dependence of Flow-Induced Crystallization in High-Density Polyethylene/Isotactic Polypropylene Blends.

Area of Science:

• Materials Science
• Polymer Science
• Chemical Engineering

Background:

• Polyolefins like high-density polyethylene (HDPE) and isotactic polypropylene (iPP) dominate the plastics market.
• Recycling challenges arise from poor mechanical properties of co-mingled HDPE-iPP blends due to low interfacial adhesion.
• Brittle behavior in mixed recyclates stems from phase separated domains detaching under stress.

Purpose of the Study:

• To investigate the impact of shear on crystallization kinetics and rheological properties of HDPE-iPP blends.
• To understand how shear influences the morphology and mechanical behavior of mixed polyolefin recyclates.
• To identify strategies for improving the processability and properties of recycled HDPE-iPP mixtures.

Main Methods:

• Utilized differential scanning calorimetry (DSC) to analyze thermal properties.
• Employed rheo-Raman spectroscopy, polarized optical microscopy, and scanning electron microscopy (SEM) for structural and morphological analysis.
• Conducted experiments under both quiescent and shear conditions to compare crystallization behavior.

Main Results:

• Quiescent blends showed decreased crystallization temperature when isotactic polypropylene (iPP) formed the droplet phase, consistent with fractionated crystallization.
• Shear induced elongated domains and unexpectedly enhanced flow-induced crystallization (FIC) in all iPP-containing blends compared to pure iPP.
• Flow-induced crystallization effectively counteracted the negative effects of fractionated crystallization, suggesting shear-induced nucleation in droplet phases.

Conclusions:

• Shear significantly enhances the crystallization kinetics of HDPE-iPP blends, improving their processability.
• The observed enhancement in flow-induced crystallization is attributed to microflow fields within deforming iPP domains.
• These findings offer a pathway to improve the mechanical properties and recyclability of mixed polyolefin waste streams.