Shaoyun Guo

Surface modification of magnesium hydroxide and its application in flame retardant polypropylene composites

In this article, titanate and zinc stearate modified superfine magnesium hydroxide [Mg(OH)2] was filled into polypropylene (PP) as a flame retardant (FR). The structure and morphologies of untreated and treated Mg(OH)2 particles were characterized by Fourier transform infrared (FTIR), wide-angle X-ray diffraction (WAXD), and scanning electron microscope (SEM). PP/Mg(OH)2 (1:1) composites were also prepared in co-rotating twin-screw extruder, and the effects of treatment agents on the rheological behavior, mechanical properties, and flame retardancy of PP/Mg(OH)2 composites were studied. The results from FTIR and WAXD show that treatment agents are adsorbed onto the surface of Mg(OH)2 particles. The complex viscosity (η*) values of the composites decrease with the addition of various treatment agents. Surface treatment agent could significantly improve tensile and impact strength of PP/Mg(OH)2 composites due to its enhanced interfacial adhesion between Mg(OH)2 particles and the PP matrix. According to limiting oxygen index (LOI), titanate treated magnesium hydroxide (MH) greatly enhanced flame retardancy of PP/Mg(OH)2 composites.

Electrical Properties of Polypropylene-Based Composites Controlled by Multilayered Distribution of Conductive Particles

Materials consisting of alternating layers of pure polypropylene (PP) and carbon black filled polypropylene (PPCB) were fabricated in this work. The electrical behaviors of the multilayered composites were investigated from two directions: (1) Parallel to interfaces. The confined layer space allowed for a more compact connection between CB particles, while the conductive pathways tended to be broken up with increasing number of layers leading to a distinct enhancement of the electrical resistivity due to the separation of insulated PP layers. (2) Vertical to interfaces. The alternating assemblies of insulated and conductive layers like a parallel-plate capacitor made the electrical conductivity become frequency dependent. Following the layer multiplication process, the dielectric permittivity was significantly enhanced due to the accumulation of electrical charges at interfaces. Thus, as a microwave was incident on the dielectric medium, the interfacial polarization made the main contribution to inherent dissipation of microwave energy, so that the absorbing peak became strengthened when the material had more layers. Furthermore, the layer interfaces in the multilayered system were also effective to inhibit the propagation of cracks in the stretching process, leading to a larger elongation at the break than that of the PP/CB conventional system, which provided a potential route to fabricate electrical materials with optimal mechanical properties.

Morphological evolution and toughening mechanism of polypropylene and polypropylene/poly(ethylene-co-octene) alternating multilayered materials with enhanced low-temperature toughness

In this paper, polypropylene (PP) and polypropylene/poly(ethylene-co-octene) blends (PP/POE) were fabricated into alternating multilayered materials to improve the low-temperature toughness of PP efficiently compared with conventional PP/POE blends. POM, SEM, micro-FTIR and part-impact test were performed to characterize and investigate the alternating multilayered microstructure and its relationship with mechanical properties. The results showed that the unique alternating multilayered microstructure could generate a distinctive distribution of POE, resulting in the great change in both macro- and micro-morphology of the materials. Most interestingly, the morphological evolution of the dispersed POE phase before and after the impact showed that a brittle–ductile transition (BDT) layer was formed at the interlayer interface between the adjacent PP layer and the PP/POE layer during the impact process, which was the main reason for the great improvement of the low-temperature toughness. Moreover, the rigidity of alternating multilayered materials was maintained very well because of the existence of the rigid PP layer, indicating that the alternating multilayered microstructure was very helpful to maintain a good balance between toughness and rigidity.

Tunable Shape Memory Performances via Multilayer Assembly of Thermoplastic Polyurethane and Polycaprolactone

Shape memory materials containing alternating layers of thermoplastic polyurethane (TPU) and polycaprolactone (PCL) were fabricated through layer-multiplying extrusion. As a type of special co-continuous morphology, the multilayer structure had stable and well-defined continuous layer spaces and could be controlled by changing the number of layers. Compared with conventional polymer blends, the multilayer-assembled system with the same compositions had higher shape-fixing and -recovery ratios that could be further improved by increasing the number of layers. By analyzing from a viscoelastic model, the deformation energy preserved in elastic TPU layers would be balanced by adjacent PCL layers through interfacial shearing effect so that each component in the multilayer structure was capable of endowing the maximum contribution to both of the shape-fixing and -recovery stages. Besides, the influence of the hardness of TPU layers and the morphology of PCL layers were respectively concerned as well. Results revealed that choosing low-hardness TPU or replacing neat PCL layers by TPU/PCL blend with co-continuous morphology were beneficial to achieving outstanding shape memory performances.

Molecular insights into hydrogen bonds in polyurethane/hindered phenol hybrids: evolution and relationship with damping properties

In this study, a polyurethane/hindered phenol system was prepared as a melt in order to study the yet unclear mechanism of the formation of hydrogen bonds (H-bonds) in analogous systems. The evolution of intermolecular H-bonds between ester carbonyl/phenolic hydroxyl groups and urethane carbonyl/phenolic hydroxyl groups was detected, for the first time, by infrared analysis. Subsequent dynamic mechanical analysis combined with thermal analysis showed the fluctuation of the glass transition temperatures and the damping properties of the hybrids. From X-ray diffraction analysis the existence was observed of only amorphous hindered phenol in the polyurethane, further molecular dynamic simulation, based on an amorphous cell, characterized the number of H-bonds, the H-bond predominant binding energy and the fractional free volume in a quantitative manner. It was observed that the variation of the simulation data was in accordance with the fluctuation change of the damping properties, thus a relationship was established between the evolution of the H-bonds and the damping properties.

Ultrasound Assisted Development of Structure and Properties of Polyamide 6/Montmorillonite Nanocomposites

In this paper, polyamide 6/montmorillonite nanocomposites (PA6CNs) were prepared via conventional and an ultrasonic extrusion technology developed in our laboratory. The structure and morphology of montmorillonite dispersed in conventional and ultrasonicated PA6CNs were studied by x‐ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The experimental results gained by XRD, differential scanning calorimetry (DSC), and polarizing optical microscopy (POM) showed that the dispersed morphology of montmorillonite plays an important role in the crystalline form, nucleation rate, crystallization temperature, crystallinity, and spherulite size of PA6 crystals. The ductility of conventional PA6CNs decreases with the addition of montmorillonite because of the presence of large, stacked montmorillonite particles. Compared with the conventional PA6CNs, the elongation at break and impact strength of the ultrasonicated PA6CN increase greatly due to the improved dispersion of montmorillonite and decreased size of spherulites, while also showing the higher yield strength. Thermogravimetric analysis (TGA) revealed a decrease in thermal stability of conventional PA6CNs, with the introduction of ultrasound further accelerating thermal decomposition. A possible explanation for the observed decrease in polymer thermal stability on ultrasonic treatment is provided.

Combustion Characteristics of Polypropylene/Magnesium Hydroxide/Expandable Graphite Composites

In this article, the limiting oxygen index (LOI), UL-94 test, and cone calorimeter test (CCT) were used to study the combustion behaviors of polypropylene (PP)/magnesium hydroxide (MH)/expandable graphite (EG) composites and the synergistic flame-retardant effects of EG with modified MH on PP. The residues from the composites were characterized by X-ray photoelectron spectroscopy (XPS). The experimental results showed that the particle size of EG had a great effect on the flammability of the PP/MH/EG composites. The data obtained from the CCT indicated that the heat release rate (HRR), the effective heat of combustion (EHC), and the mass loss rate (MLR) of PP/MH/EG composites decreased markedly with decreasing particle size of EG. XPS data indicated that EG could efficiently improve the char layer structure and flame retardancy of the composites.

Flammability and Thermal Oxidative Degradation Kinetics of Magnesium Hydroxide and Expandable Graphite Flame Retarded Polypropylene Composites

The flammability and the thermal oxidative degradation kinetics of expandable graphite (EG) with magnesium hydroxide (MH) in flame‐retardant polypropylene (PP) composites were studied by limiting oxygen index (LOI), UL‐94 test, and thermogravimetric analysis (TGA). The results show that EG is a good synergist for improving the flame retardancy of PP/MH composite and the effect is enhanced with decreasing EG particle size. The Kissinger method and Flynn‐Wall‐Ozawa method were used to determine the apparent activation energy (E) for degradation of PP and flame retarded PP composites. The data obtained from the TGA curve indicate that EG markedly increases the thermal degradation temperature of PP/MH composites and improves the thermal stability of the composites. The kinetic results show that the values of E for degradation of flame retarded PP composites is much higher than that of neat PP, especially PP/MH composites with suitable amount of EG, which indicates that the flame retardants used in this work have a great effect on the mechanisms of pyrolysis and combustion of PP.

Molecular insights into the damping mechanism of poly(vinyl acetate)/hindered phenol hybrids by a combination of experiment and molecular dynamics simulation

The fundamental mechanism of the improved damping properties of poly(vinyl acetate) (PVAc), contributed by the introduction of hindered phenols, was systematically elucidated by two-dimensional infrared (2D IR) spectroscopy, dynamic mechanical analysis (DMA), differential scanning calorimeter (DSC), X-ray diffraction (XRD) and molecular dynamics (MD) simulation. The 2D IR results revealed the evolution of hydrogen bonds (H-bonds) from intermolecular H-bonds to H-bond networks of PVAc/hindered phenols. Note that subsequent DMA results revealed that the damping properties of PVAc exhibited two different degrees of improvement due to the addition of hindered phenol. Moreover, DSC results showed that all hybrids were miscible, as concentration fluctuations changed irregularly. In accordance with the XRD observation of only amorphous hindered phenols existing in the PVAc matrix, further MD simulation, based on an amorphous cell, characterized the number of H-bonds, the binding energy and the fractional free volume (FFV) of the hybrids. It was observed that the variation tendency of the simulation data was in accordance with the experimental results. Therefore, the damping mechanism of PVAc/hindered phenol hybrids was proposed through a detailed analysis on the synergistic effect of the number of intermolecular H-bonds and the binding energy between PVAc and the hindered phenol, as well as the FFV or dynamic heterogeneity.

The Kinetic Studies of Elimination of HCl During Thermal Decomposition of PVC in the Presence of Transition Metal Oxides

Abstract: The effects of nine kinds of transition metal oxides on the elimination kinetics of hydrogen chloride (HCl) during thermal decomposition of polyvinyl chloride (PVC) at 210°C were investigated using online measurement of conductivity. It was found that the presence of TiO2, V2O5, Cr2O3, and MoO3 obviously decreases the elimination rate of HCl and the amount of HCl released by a factor of 10–20 times, indicating that they have a strong inhibiting effect on the release of HCl. The presence of MnO2, ZnO, CuO, Co2O3, and Fe2O3 promotes the elimination of HCl during thermal decomposition of PVC. The maximum elimination rate of HCl in PVC/ZnO, especially, is ca. 10 times as high as that of PVC. In this work, the LnLn kinetic model, Ln(1 − α) = − ktn, describes the kinetics of elimination of HCl during thermal decomposition of PVC. The elimination rate of HCl during thermal decomposition of PVC increases with the rise of the apparent order of reaction, n, in this model n value well describes the elimination kinetics of HCl during thermal decomposition of PVC.