Thermal and mechanical properties of cross-linked and uncross-linked linear low-density polyethylene–wax blends

Particle morphology, structure and properties of nascent ultra-high molecular weight polyethylene

The effects of particle morphology on the structure and swelling/dissolution and rheological properties of nascent ultra-high molecular weight polyethylene (UHMWPE) in liquid paraffin (LP) were elaborately explored in this article. Nascent UHMWPE with different particle morphologies was prepared via pre-polymerization technique and direct polymerization. The melting temperature and crystallinity of UHMWPE resins with different particle morphologies were compared, and a schematic diagram was proposed to illustrate the mechanism of UHMWPE particle growth synthesized by pre-polymerization method and direct polymerization. The polymer globules in the nascent UHMWPE prepared by using pre-polymerization technique are densely packed and a positive correlation between the particle size and the viscosity-averaged molecular weight can be observed. The split phenomenon of particles and the fluctuation in the viscosity of UHMWPE/LP system prepared by direct polymerization can be observed at a low heating rate and there is no correlation between particle size and viscosity-averaged molecular weight.

Comprehensive Analysis of Oxidized Waxes by Solvent and Thermal Gradient Interaction Chromatography and Two-Dimensional Liquid Chromatography

This report addresses the comprehensive analysis of oxidized/functionalized polyethylene waxes according to chemical composition and molar mass by selective chromatographic methods. For the first time, tailored high-temperature interaction chromatography in solvent gradient (HT-SGIC) and thermal gradient (HT-TGIC) modes are used for the chemical composition separation of these materials. Separation protocols are developed using three model wax samples with different degrees of oxidation. For the chromatographic separations polar silica gel is used as the stationary phase. Solvent gradients of decane and cyclohexanone are used in HT-SGIC at 110 °C to separate the bulk waxes into several heterogeneous fractions according to polarity and the type of functionality. Column temperature and gradient manipulation are shown to influence chromatographic resolution and retention. The HT-SGIC investigations are complemented by HT-TGIC separations where a solvent mixture of decane and cyclohexanone is used as the mobile phase in isocratic mode. It is shown that HT-SGIC and HT-TGIC provide different types of separation, however, both are predominantly based on differences in functionality. To provide comprehensive information on chemical composition (functionality) and molar mass, HT-SGIC and HT-TGIC are coupled to HT-SEC, using ortho-dichlorobenzene as the second dimension mobile phase. Clear differences between oxidized and nonoxidized waxes are detected in HT-2D-LC providing comprehensive information on the molecular heterogeneity of these materials.

Supercritical CO₂ Foaming of Radiation Cross-Linked Isotactic Polypropylene in the Presence of TAIC

Since the maximum foaming temperature window is only about 4 °C for supercritical CO₂ (scCO₂) foaming of pristine polypropylene, it is important to raise the melt strength of polypropylene in order to more easily achieve scCO₂ foaming. In this work, radiation cross-linked isotactic polypropylene, assisted by the addition of a polyfunctional monomer (triallylisocyanurate, TAIC), was employed in the scCO₂ foaming process in order to understand the benefits of radiation cross-linking. Due to significantly enhanced melt strength and the decreased degree of crystallinity caused by cross-linking, the scCO₂ foaming behavior of polypropylene was dramatically changed. The cell size distribution, cell diameter, cell density, volume expansion ratio, and foaming rate of radiation-cross-linked polypropylene under different foaming conditions were analyzed and compared. It was found that radiation cross-linking favors the foamability and formation of well-defined cell structures. The optimal absorbed dose with the addition of 2 wt % TAIC was 30 kGy. Additionally, the foaming temperature window was expanded to about 8 °C, making the handling of scCO₂ foaming of isotactic polypropylene much easier.

Lipid functionalized biopolymers: A review

Lipids are the main source of energy and widely used for various applications. In this review, the modification of lipids by using them in combination with other biomaterials like natural and synthetic polymers is elaborated. These new blends have characteristic features of both polymers and are characterized by different techniques (NMR, DSC, TGA, IR and Raman spectroscopy etc.) to understand their structure, properties and functional behavior. Lipids are hydrophobic, have anti-oxidant and anti-bacterial properties and thus impart hydrophobicity and flexibility to the polymers. While the polymers, on the other hand, make the lipids tougher. Properties of few polymers such as starch, polyethylene protein and chitosan that have brittleness, low combustion rate and hydrophobicity, are improved by incorporation of lipids ultimately increased their flexibility, combustion rate and hydrophobicity respectively. This review article is also focused on emerging fields for the applications of these composite materials. The most notable application of composite materials are in the field of paint industry.

Effect of Hot Water and Water-carrying Agent on the Properties of Silane-water Crosslinked Linear Low Density Polyethylene

The influence of hot water and the water-carrying agent aluminum potassium sulfate dodecahydrate (alum) on the mechanical and thermal properties of the silane-water crosslinked linear low density polyethylene (LLDPE) was investigated. LLDPE was melt-grafted with various content of vinyl trimethoxysilane (VTMS) using dicumyl peroxide (DCP) as initiator. The results show that the contribution of hot water to gel content is poor, about 2 to 16%, compared with that of the silane grafted LLDPE. Is feasible to achieve silane-water crosslinking reaction with alum for self-crosslinked LLDPE; furthermore, the self-crosslinked LLDPE has lower crosslink density (ν). It was found that alum could not only conduct silane-water crosslinking reaction effectively, but also improve the mechanical properties, especially the elongation behavior. The heat of fusion (ΔHm), the melting temperature (Tm), and the degree of crystallinity (Xc) of the hot water crosslinked LLDPE decreased with the advancement of VTMS. The Avrami exponent n of self-crosslinked sample was larger than that of hot water crosslinked sample. This means that the alum could contribute to the growth of crystallization.

MANUFACTURING AND CHARACTERIZATION OF CARBON NANOTUBE/POLYETHYLENE COMPOSITES

The present study describes a method to fabricate polymer matrix nanocomposites by reinforcing multi-walled carbon nanotubes through an extrusion process. Linear low density polyethylene (LLDPE) powder and multi-walled carbon nanotubes (CNTs) are first dry mixed and extruded in the form of filaments by a single screw extrusion process. After extrusion, the filament is partially cooled by chilled air, dried, and continuously wound in a spool. The filaments are then laid in roving, stacked in a unidirectional fashion, and consolidated in a compression molding machine to come up with laminated composites. Thermo gravimetric analysis (TGA) has been performed to compare the thermal stability of as-fabricated composites with the neat polymer. The TGA result shows that the extruded composites are thermally more stable than their neat counterparts. The crystalline nature of CNTs and of as-fabricated composites were identified by X-ray diffraction (XRD) studies. The XRD results indicate that the nanocomposite materials are more crystalline than the neat systems, and the differential scanning calorimetry studies also confirmed the same trend. The scanning electron microscopy result showed that the sizes of extruded neat and nanophased filaments were about 117 and 73 μm, respectively. Tensile coupons from the consolidated panels were then extracted both in longitudinal (0°) and in transverse (90°) directions and tested in a Minimat Tester. It was found that with the addition of 2% by weight of CNTs in LLDPE, the tensile strength and modulus of the composite has increased by about 34 and 38%, respectively. The (0°) and (90°) coupons have also demonstrated that there are directional effects in the tensile response, which is believed to have been caused by the alignment of CNTs during the extrusion process. It is our understanding that such improvement in properties is because of the increase in crystallinity of the polymer due to CNT infusion, and also due to the alignment of CNTs in the extrusion direction in the nanocomposites. Details of the fabrication procedures, synthesis of composites, and mechanical testing are included in the paper.