Suppressing the skin-core structure of injection-molded isotactic polypropylene via combination of an in situ microfibrillar network and an interfacial compatibilizer

Lightweight and High Impact Toughness PP/PET/POE Composite Foams Fabricated by In Situ Nanofibrillation and Microcellular Injection Molding

Polypropylene (PP) has become the most promising and candidate material for fabricating lightweight products. Microcellular injection molding (MIM) is a cost-effective technology for manufacturing porous plastic products. However, it is still challenging to fabricate high-performance PP microcellular components. Herein, we reported an efficient strategy to produce lightweight and high impact toughness foamed PP/polyethylene terephthalate (PET)/polyolefin-based elastomer (POE) components by combining in situ fibrillation (INF) and MIM technologies. First, the INF composite was prepared by integrating twin-screw compounding with melt spinning. SEM analysis showed PET nanofibrils with a diameter of 258 nm were achieved and distributed uniformly in the PP due to the POE's inducing elaboration effect. Rheological and DSC analysis demonstrated PET nanofibrils pronouncedly improved PP's viscoelasticity and crystal nucleation rate, respectively. Compared with PP foam, INF composite foam showed more stretched cells in the skin layer and refined spherical cells in the core layer. Due to the synergistic toughening effect of PET nanofibrils and POE elastic particles, the impact strength of INF composite foams was 295.3% higher than that of PP foam and 191.2% higher than that of melt-blended PP/PET foam. The results gathered in this study reveal potential applications for PP based INF composite foams in the manufacturing of lightweight automotive products with enhanced impact properties.

The Feasibility of Using the MFC Concept to Upcycle Mixed Recycled Plastics

Several mixed recycled plastics, namely, mixed bilayer polypropylene/poly (ethylene terephthalate) (PP/PET) film, mixed polyolefins (MPO) and talc-filled PP were selected for this study and used as matrices for the preparation of microfibrillar composites (MFCs) with PET as reinforcement fibres. MFCs with recycled matrices were successfully prepared by a three-step processing (extrusion—cold drawing—injection moulding), although significant difficulties in processing were observed. Contrary to previous results with virgin PP, no outstanding mechanical properties were achieved; they showed little or almost no improvement compared to the properties of unreinforced recycled plastics. SEM characterisation showed a high level of PET fibre coalescence present in the MFC made from recycled PP/PET film, while in the other MFCs, a large heterogeneity of the microstructure was identified. Despite these disappointing results, the MFC concept remains an interesting approach for the upcycling of mixed polymer waste. However, the current study shows that the approach requires further in-depth investigations which consider various factors such as viscosity, heterogeneity, the presence of different additives and levels of degradation.

Relationship between the Processing, Structure, and Properties of Microfibrillar Composites

The relationship between processing, morphology, and properties of polymeric materials has been the subject of numerous studies of academic and industrial research. Finding an answer to this question might result in guidelines on how to design polymeric materials. Microfibrillar composites (MFCs) are an interesting class of polymer-polymer composites. The advantage of the MFC concept lies in developing in situ microfibrils by which a perfect homogeneous distribution of the reinforcement in the matrix can be achieved. Their potentially excellent mechanical properties are strongly dependent on the aspect ratio of the fibrils, which is developed through a three-stage production process: melt blending, fibrillation, and isotropization. During melt blending, the polymers undergo different morphological changes, such as a breakup and coalescence of the droplets, which play a crucial role in defining the microstructure. During processing, various parameters may affect the morphology of the MFCs, which must be taken into account. Besides the processing parameters, the microstructure of the composite is dependent on the composition ratio of the blend and viscosity of the components, as well as the dispersion and distribution of the microfibrils. The objective here is to outline this importance and bring together an overview of the processing-structure-property relationship for MFCs.

Crystallization and Microstructure Evolution of Microinjection Molded Isotactic Polypropylene with the Assistance of Poly(Ethylene Terephthalate)

In this work, a series of isotactic polypropylene/poly(ethylene terephthalate) (iPP/PET) samples were prepared by microinjection molding (μIM) and mini-injection molding (IM). The properties of the samples were investigated in detail by differential scanning calorimetry (DSC), Wide-Angle X-ray Diffraction (WAXD), Polarized light microscope (PLM) and scanning electron microscopy (SEM). Results showed that the difference in thermomechanical history between both processing methods leads to the formation of different microstructures in corresponding iPP/PET moldings. For example, the dispersed spherical PET phase deforms and emerges into continuous in-situ microfibrils due to the intensive shearing flow field and temperature field in μIM. Additionally, the incorporation of PET facilitates both the laminar branching and the reservation of oriented molecular chains, thereby leading to forming a typical hybrid structure (i.e., fan-shaped β-crystals and transcrystalline). Furthermore, more compact and higher degrees of oriented structure can be obtained via increasing the content of PET. Such hybrid structure leads to a remarkable enhancement of mechanical property in terms of μIM samples.

The Effect of Injection Molding Temperature on the Morphology and Mechanical Properties of PP/PET Blends and Microfibrillar Composites

Within this research the effect of injection molding temperature on polypropylene (PP)/poly(ethylene terephthalate) (PET) blends and microfibrillar composites was investigated. Injection molding blends (IMBs) and microfibrillar composites (MFCs) of PP/PET have been prepared in a weight ratio 70/30. The samples were processed at three different injection molding temperatures (T_im) (210, 230, 280 °C) and subjected to extensive characterization. The observations from the fracture surfaces of MFCs showed that PET fibers can be achieved by three step processing. The results indicated that T_im has a big influence on morphology of IMBs and MFCs. With increasing the T_im, distinctive variations in particle and fiber diameters were noticed. The differences in mechanical performances were obtained by flexural and impact tests. Establishing relationships between the processing parameters, properties, and morphology of composites is of key importance for the valorization of MFC polymers.

Evolution of unique nano-cylindrical structure in poly(styrene-b-isoprene-b-styrene) prepared under "dynamic packing injection moulding"

This work reports the evolution of ordered nano-cylindrical structures in a thermoplastic elastomer, poly(styrene-b-isoprene-b-styrene) (SIS), utilizing a newly designed processing technique, so-called "dynamic-packing injection moulding". In this injection moulding technique, controlled oscillating shears with different shear cessation times under constant pressure were applied on the moulded samples during cooling. It was found that these additional controlled oscillating shears resulted in a change of orientation in skin-core structures in these samples, compared with corresponding "reference" samples processed via traditional injection moulding (without controlled oscillating shears). For the "reference" samples, a highly oriented PS cylindrical structure combined with relatively weak lateral ordering was observed in their skin layers, whereas the lateral ordering of the PS nano-cylinders gradually disappeared when entering the core region. On the other hand, for the SIS samples obtained via "dynamic-packing injection moulding", the orientation of the PS nano-cylinders in the skin layers was similar to the case of the "reference" sample due to their extremely fast cooling rate. However, the lateral ordering of these cylinders had been extended to the core region. With an increase in the cessation time, the lateral ordering of the PS nano-cylinders was further improved and finally resulted in hexagonal lateral packing along the flow direction in the mould. Furthermore, a mixture of parallel/perpendicular orientation of the cylinders relative to the flow direction was found, particularly when the cessation time was short (such as 3 s). We speculated that this specific perpendicular orientation was a transient state for development of a final parallel orientation aligned with the flow direction with increasing cessation time, accompanied by a further enhancement of the nano-cylindrical parallel orientation. This study could provide a better understanding of the shear and relaxation effects on the structural evolution of this class of thermoplastic elastomers, enhancing supramolecular ordered cylindrical orientation in the core region, and paving a way to tune the nano-structures of block copolymers via this new processing technique to achieve desired properties.

Unusual hierarchical structures of micro-injection molded isotactic polypropylene in presence of an in situ microfibrillar network and a β-nucleating agent

The morphological development of iPP in presence of an in situ microfibrillar network and a β-nucleating agent under micro-injection molding.

New understanding of the hierarchical distribution of isotactic polypropylene blends formed by microinjection-molded poly(ethylene terephthalate) and β-nucleating agent

Hierarchical distribution of β-crystals in microinjection molded poly(ethylene terephthalate)/β-nucleating agent-nucleated isotactic polypropylene blends.

Suppressing the skin–core structure in injection-molded HDPE parts via the combination of pre-shear and UHMWPE

Skin–core structure of a injection-molded high density polyethylene (HDPE) part is largely relieved due to the synergetic effects of pre-shear and UHMWPE, leading to a remarkable increase of tensile strength.