Accelerating the degradation of polyolefins through additives and blending

Polyolefins are popular because of their low price, useful properties, broad supply chains, and mature processing facilities, but they do not easily degrade in the natural environment, and hence, the development of degradable polyolefins has attracted increasing interest. Oxidative degradation and blending with natural polymers can accelerate the degradation of polyolefins in natural environments. In this article, we review the research and developments in the acceleration of the degradation of polyolefin blends and composites, including both the fundamental science, such as the degradation mechanisms and characterizations, and application techniques, such as the processing conditions and formulations. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40750.     

Solid-state shear pulverization of post-industrial ultra-high molecular weight polyethylene: Particle morphology and molecular structure modifications toward conventional mechanical recycling

Ultra-high molecular weight polyethylene (UHMWPE) is one of the most prominent high-performance thermoplastics for biomedical, leisure, and coating applications. Large-scale recycling of UHMWPE is extremely difficult due to the high melt viscosity of the material as well as its exceptional chemical resistance and impact strength. There is a need for a commercially scalable methodology that can process the waste feedstock for mechanical recycling while sustaining the outstanding physical properties of the material. Solid-state shear pulverization (SSSP) is a continuous, twin-screw extruder-based processing technique in which the low-temperature application of shear and compressive forces impart changes in structure at different length scales to overcome the challenges of difficult-to-recycle polymers. This paper investigates the use of SSSP in mechanically recycling post-industrial scrap UHMWPE (rUHMWPE) material from a local ski and snowboard manufacturer. The SSSP-processed particles are flat, micron-scale flakes with enhanced surface area, which can sinter very quickly when compression molded. The molded rUHMWPE samples in turn exhibit enhanced ductility and toughness compared to the as-received scrap material, based on the tunable mechanochemical modification of the ethylene chains.     

Fabrication of super‐ductile PP/LDPE blended parts with a chemical blowing agent

The mechanical blending of polypropylene (PP) and low density polyethylene (LDPE) is an economical and simple method for producing new polymeric materials for specific applications. However, the reduction in strain‐at‐break of the blend is one of its main shortcomings. In this study, PP/LDPE foamed parts were fabricated by conventional injection molding (CIM) with azodicarbonamide as a chemical blowing agent (CBA) and tested for tensile properties at two test speeds. Also, the fracture surfaces of the parts were investigated by scanning electron microscopy (SEM). In addition, to investigate the underlying mechanism of the super‐ductility, the tested samples were carefully analyzed and compared, and further characterized by differential scanning calorimetry and SEM. The results suggest that fabricating PP/LDPE super‐ductile parts using CIM with a CBA is feasible. The results also indicate that there is a close relationship between the mechanical properties and morphological structures, which are deeply influenced by the dosage of CBA, the PP/LDPE ratio, and the packing parameters. Furthermore, compared to conventional injection molded solid parts, the ductility of the foamed parts can be dramatically improved by the formation of microfibrils in the PP phase, which come into being under certain processing conditions. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44101.     

The effect of high temperature annealing process on crystallization process of polypropylene,mechanical properties,and surface quality of plastic parts

This work studied the effects and action mechanism of high‐temperature annealing process parameters, such as annealing temperature, annealing duration and cooling speed, on the microstructural evolution of polypropylene (PP) on different thickness layers, the surface quality, and mechanical properties of PP plastic parts. The results show that when the PP plastic parts are annealed at slightly higher than 100°C, the resin on the surface and internal layers of plastic parts just generates the relaxation and rearrangement at the molecular level. Only at an enough high annealing temperature, the secondary crystallization and phase transformation process can be observed. The crystallinity of all annealed samples is higher than that of unannealed samples, but the crystallinity is decreased with the increase of cooling speed after annealing duration, and the annealing duration exceeding 60 min almost has no effect on the crystallinity. The microstructural change of PP on the internal layer of plastic parts is weaker than that on the surface layer. The surface hardness of the plastic parts mainly depends on the crystallinity of the surface layer, whereas the surface roughness of the plastic parts depends on not only the crystallinity, but also the space conformation of molecular chains and the residual stress. With the change of annealing process parameters, the tensile and impact strengths of plastic parts show a non‐monotonic change law. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42773.     

Understanding the degradation of MEA in PEMFC: Definition of structural markers and comparison between laboratory and on‐site ageing

Different accelerated tests in 12 fuel cells stack were performed in laboratory, namely on/off, back‐up, and base‐load regimes. In parallel, membrane electrode assemblies (MEA) were integrated in two “on‐site” systems for GSM relay application. One of them was dedicated to base‐load power applications while the second fuel cells coupled with photovoltaic panels operated in semibase load mode. To investigate the influence of the power profiles on MEA degradation, over 80 CCB MEAs (5 layers) were studied at different scales using ex situ characterizations such as tensile tests, TGA‐MS, DMTA, and SEM. A series of complementary microstructural ageing markers were thereby identified. The isolated influence of dry‐wet cycling on MEA properties was also established after passive hydro‐thermal (HT) ageing performed continuously for 10 months in the laboratory. The changes of each marker as a function of HT ageing time permitted to define a temporal benchmark. Based on these indicators, the main changes occurred in the MEA properties appear after a 5 months dry‐humid cycling (up to about 1800 cycles). The trends observed were useful to compare and estimate the degree of degradation of each ageing tests. Thus, the accelerated tests performed in laboratory for at least 500 h in stack did not reveal systematic MEA modifications. On the contrary, the 1500 h “on‐site” system operation results in some MEA degradations which origins are discussed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Surface degradation and hydrophobic recovery of polyolefins treated by air corona and nitrogen atmospheric pressure glow discharge

The surface degradation and production of low molecular weight oxidized materials (LMWOM) on biaxially oriented polypropylene (BOPP) and low‐density polyethylene (LDPE) films was investigated and compared for two different dielectric barrier discharge (DBD) treatment types, namely air corona and nitrogen atmospheric pressure glow discharge (N₂ APGD). Contact angle measurements, X‐ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) analyses were performed in conjunction with rinsing the treated films in water. It is shown that N₂ APGD treatments of both polyolefins result in much less surface degradation, therefore, allowing for a significantly higher degree of functionalization and better wettability. Hydrophobic recovery of the treated films has also been studied by monitoring their surface energy (γ_s) over a period of time extending up to several months after treatment. Following both surface modification techniques, the treated polyolefin films were both found to undergo hydrophobic recovery; however, for N₂ APGD modified surfaces, γ_s ceases to decrease after a few days and attains a higher stable value than in the case of air corona treated films. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1291–1303, 2004     

Evaluation of processing effects in injection‐molded amorphous and crystalline thermoplastics using an excimer laser

The ablation behavior of amorphous [polystyrene (PS), polycarbonate (PC)] and crystalline [PET, glass‐filled poly(butylene terephthalate) (PBT)] polymers by 248‐nm KrF excimer laser irradiation were investigated for different injection‐molding conditions, namely, injection flow rate, injection pressure, and mold temperature, as a possible method for evaluating processing effects in the specimens. For this purpose, dumbbell‐shaped samples were injection‐molded under different sets of processing conditions, and weight loss measurements were carried out for the different injection‐molding conditions. Some of the crystalline (PET) samples were annealed at different annealing times and temperatures. For PET, the weight loss decreased with increasing mold temperature and remained insensitive to injection flow rate. Annealing time and temperature significantly reduced weight loss in PET. For PBT, the weight loss due to laser ablation decreased with increasing material packing due to pressure, and it also showed some sensitivity to flow rate variation. The major effect was seen with glass‐filled PBT samples. The weight loss decreased drastically with increasing glass fiber content. Laser ablation allowed us to observe process‐induced fiber orientation by scanning electron microscopy in PBT samples. For PS and PC, the weight loss increased with increasing injection flow rate and mold temperature and decreased with increasing injection pressure. The position near the gate showed higher ablation than the position at the end for all the conditions. A decrease in the material orientation with injection speed and mold temperature led to an increase in the weight loss, whereas an increase in the injection pressure, and consequently orientation, led to a lower weight loss for PS and PC. Higher residual stress samples showed higher weight losses. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 2006     

Structure and physical property relationships in processed polybutene‐1

The effect of SCORIM was investigated on three grades of polybutene‐1 and one grade of ethylene–butene‐1 copolymer. The methods and processing conditions used for injection molding and the properties of the moldings are reported. Phase transformations and their relationship with mechanical properties are discussed in detail. Both, conventional and shear‐controlled orientation injection molding (SCORIM) were employed to produce moldings. SCORIM is based on the application of specific macroscopic shears to a solidifying melt. The multiple shear action enhances molecular alignment. The moldings were investigated by performing mechanical tests, fractographic analysis, differential scanning calorimetry studies, wide‐angle X‐ray diffraction, polarized light microscopy, and atomic force microscopy. The application of SCORIM improves the mechanical performance. Molecular orientation results in the formation of shish‐kebab morphology. One grade of polybutene‐1 exhibited a greater than fivefold increase in Young's modulus. The application of high cavity pressures favored the formation of the stable Form I' in polybutene‐1. The formation of Form I' led to a decrease in crystallinity and mechanical properties. However, this loss was by far smaller than the gain obtained via the formation of shish‐kebab morphology. The relationship between mechanical properties and micromorphologies of the investigated materials is explained. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 814–824, 2003     

Methods of long chain branching detection in PE by triple‐detector gel permeation chromatography

Long chain branching (LCB) in polyethylene is one of the key microstructures that controls processing and final properties. Gel permeation chromatography (GPC) with viscometer (IV) and/or light scattering (LS) has been intensely used to quantify LCB. The widespread method to quantify LCB from GPC with IV or LS is the method of LCB frequency (LCBf) based on the Zimm–Stockmayer (ZS) random branching model. In this work, the conventional approach was compared with the recently developed method, called gpcBR. The comparison of the sensitivity of both methods is made on highly branched polymer, that is, various grades of commercial LDPE and also on polymer with very low level of LCB, that is, a commercial HDPE with no LCB, converted into several branched test samples of gradually increasing LCB by multiple extrusion. Finally, the linkages of LCB quantities from both methods to the rheological data and processing properties are illustrated. The new gpcBR index can access lower LCB level and shows obviously better relationship with both rheological data and processing properties than LCBf from the conventional ZS model. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42222.     

Molecular dynamics simulation of diffusion behavior of cyclohexane in natural rubber during reclamation

Reclaiming oil is a common additive involved in many physical rubber reclamation processes. In this work, molecular dynamics simulation is used to investigate the diffusion of oil in natural rubber (NR) under different temperatures and pressures. The structures of polyisoprene and cyclohexane molecules are constructed. The diffusion coefficients (Ds) of cyclohexane molecules in polyisoprene under different temperatures and pressures are calculated. The free volumes are also obtained to better understand the change in microstructure. The diffusion experiments of oil in NR samples under different temperatures and pressures are also conducted. The simulation results show that D increases as the temperature rises, especially at temperatures above 400 K, while the pressure does not affect D significantly. The fractional free volume calculated from the free volume and occupied volume exhibits a similar trend to that of D. Furthermore, the calculated Ds agree well with the experimental results. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40347.     

Production of non‐crosslinked thermoplastic foams with a controlled density and a wide range of cellular structures

A novel foaming route, with respect to existing industrial foaming processes, called “Improved Compression Molding” (ICM), which allows producing non‐crosslinked thermoplastic foams in a wide density range, is described in this work. This process is different from others because it is possible to control independently density and cellular structure and therefore, tailored cellular polymers can be produced. To understand the process, a collection of polypropylene foams, with relative densities ranging from 0.3 to 0.6 were produced. The influence of foaming parameters, on foams microstructure and mechanical response was analyzed. Results revealed that for similar densities, foams with different open cell content and cell size can be achieved. In addition, it was proved that mechanical behavior strongly depends on the degree of interconnectivity of the cells. The analysis of the relative mechanical properties allowed determining the influence of microstructure on mechanical behavior as well as quantifying the efficiency of the foaming process to produce light‐weight stiff materials. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42324.     

Transferring noncharring polyolefins to charring polymers with the presence of Mo/Mg/Ni/O catalysts and the application in flame retardancy

This article presents a novel investigation on the Mo/Mg/Ni/O catalysts (Nmm‐cats) with which the noncharring polyolefins are basically transferred to the charring polymers under forced flaming conditions and the flame retardancy of polyolefins is improved dramatically. The results of model charring experiments show that when appropriate Mo/Mg/Ni molar ratio is adopted and only 3% Nmm‐cats is blended, the Nmm‐cats belong to high‐efficient charring catalysts that can deposit 56 and 64 wt % of volatile products in‐situ from linear low‐density polyethylene (LLDPE) and polypropylene (PP), respectively. The charring properties are characterized by scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy; and the char‐forming mechanisms are analyzed by wide‐angle X‐ray diffraction experiment. The improvement in flammability properties for LLDPE and PP is demonstrated by using a cone calorimeter. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Image analysis of flame behavior for polyolefins and polystyrene in vertical flame test

Although the UL-94 vertical flame test is often used to evaluate the flammability of polymers based on ignition time and combustion duration parameters, a significant amount of information regarding the time course of combustion is difficult to analyze in detail. Herein, image analysis of the time course of the upper and lower end heights, total area, and color division of flame was performed for polyolefins and polystyrene with different molecular weights in the vertical flame test. The combustion process was classified into two stages: before and after the first drip. In the first stage, the upward spread velocity, oscillation, and color of flame were analyzed. It was assumed that the difference in the fuel production rates or thermal decomposition products depends on the molecular structure. In the second stage, the melt flowability, flame position, and combustion continuity differed vastly depending on the molecular structure or molecular weight.     

Thermal and fire degradation of recycled and polluted polypropylene‐based materials

In this study, the effect of processing cycles and two pollutants (engine oil (HM) and ethylene glycol (EG)) on the thermal and rheological properties of polypropylene‐based materials (108MF97 and 7510) has been studied. It was investigated if polymers coming from bumper face bar could keep their properties and can be reused after recycling. The different results demonstrate that the two polymers that were polluted and recycled do not show any decrease of their intrinsic properties. Moreover, for one of the two polymers (108MF97), the presence of engine oil enables to increase the thermal stability and reaction to fire. Finally, it appears that the reuse of such polymers is possible. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Gas permeability of thin dense films from polymer blend of thermoplastic elastomer and polyolefin

Stretched thin films composed of a thermoplastic elastomer, a polystyrene‐block‐poly(ethylene butylene)‐block‐polystyrene triblock copolymer (SEBS), and polyolefins, poly(ethylene‐co‐ethylacrylate) and poly(ethylene‐co‐propylene), were obtained by blow‐molding, uniaxial stretching, and cooling to room temperature and the gas permeability of the stretched films was investigated. When the as‐blown annealed film was subjected to uniaxial stretching in the machine direction, P_O2 and P_N2 increased with an increase in the stretching ratio K and approached a constant value at high stretching ratios. In addition, P_O2/P_N2 decreased gradually with K and approached a value of 2.95–3.0. The reason for this unique gas permeation behavior is that the molecular mobility of poly(ethylene butylene) chains in a direction normal to the film increases and reaches an equilibrium state at around K = 4.5. The change in gas permeability of the stretched films can be explained using a deformation model for the SEBS matrix. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39386.     

Preparation and properties of carbon nanotube/binary‐polymer composites with a double‐segregated structure

A carbon nanotube (CNT)/poly(methyl methacrylate) (PMMA)/ultrahigh molecular weight polyethylene (UHMWPE) composite containing a double‐segregated structure was formalized by means of a facile mechanical mixing technology. In the composite, the CNTs were decorated on the surfaces of PMMA granules, and the CNTs decorated granules formed the continuous segregated conducting layers at the interfaces between UHMWPE particles. Morphology observations confirmed the formation of a specific double‐segregated CNT conductive network, resulting in an ultralow percolation threshold of ～0.2 wt %. The double‐segregated composite containing only 0.8 wt % CNT loading exhibited a high electrical conductivity of ～0.2 S m^?1 and efficient electromagnetic shielding effectiveness of ～19.6 dB, respectively. The thermal conductivity, temperature‐resistivity behaviors, and mechanical properties of the double‐segregated composites were also studied. This work provided a novel conductive network structure to attain a high‐performance conducting polymer composite at low filler loadings. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39789.     

A comparative study of ethylene and propylene polymerization over titanium–magnesium catalysts of different composition

New data on the molecular weight characteristics of polypropylene (PP) and polyethylene (PE) were obtained from the polymerization over supported titanium–magnesium catalysts differing in their compositions (presence and absence of internal and external donors). Internal and external donors were found to affect the molecular weight of polymers in a different manner for ethylene and propylene polymerization. The introduction of the internal donor increases the molecular weight of PP and does not affect the molecular weight of PE. The effect of external donor introduced to catalytic system on the polymer molecular weight depends on catalyst composition: for a catalyst without internal donor, the introduction of the external donor increases the molecular weight of PP and does not affect that of PE. In the case of catalyst with the internal donor, the introduction of the external donor increases the molecular weight of PP and substantially decreases that of PE. The data on polymerization degree of the polymers produced under conditions when chain transfer with hydrogen was the dominant reaction were used to calculate the values for ethylene polymerization over the catalysts of different composition. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40658.     

Effect of the melt flow index and melt flow rate on the thermal degradation kinetics of commercial polyolefins

In this study, several polyolefins, including different grades of polypropylene (PP), high‐density polyethylene, linear low‐density polyethylene, and low‐density polyethylene, were tested by thermogravimetric analysis (TGA), and the relationships of their melt flow index (MFI) and melt flow ratio (MFR) values to the thermogravimetry (TG) curves, differential thermogravimetry (DTG) curves, and activation energy of thermal degradation were investigated. Kinetic evaluations were performed by Friedman and Kissinger analysis methods, and the apparent activation energy values for the overall degradation of different grades of polyethylenes (PEs) and PPs were determined. We found that for the samples with lower MFIs, the thermograms shifted to higher temperatures. Meanwhile, a higher activation energy was needed for their thermal degradation. Also, for samples with higher values of MFR, as a means of molecular weight distribution, a lower activation energy was needed for their thermal degradation, and their TGA thermograms shifted to lower temperatures. The breadth of the DTG curves depended on the MFR in the PEs, although MFR had little effect on the DTG curves in the studied PP grades. Among all of the samples studied, the injection‐molding grades with medium MFIs and low MFRs degraded at higher temperatures and showed better thermal stability. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2012     

Influence of recycling and contamination on structure and transport properties of polypropylene

In this work the effect of three processing cycles on the physical properties of polypropylene (PP) was studied and related to the changes in the structure (molecular weight, molecular weight distribution, morphology) occurring during recycling. In order to simulate both the use and the recycling process of PP in the laboratory, PP pellets were contaminated with three model substances and submitted to three cycles of processing by injection molding. The bars for testing were produced from virgin and recycled polymers. The amount of degradation occurring during the reprocessing was estimated by means of viscosity and gel permeation chromatography measurements. Differential scanning calorimetry and scanning electron microscopy analyses were performed to investigate the crystallization behavior and the morphology of virgin and recycled PP. In order to obtain information on the structural organization and mobility of the amorphous phase, a dynamic mechanical thermal analysis and measurements of the transport properties were carried out. The results obtained were attributed to the different structural organization of the bars in relation to the number of processing cycles and contamination. Part of the work was focused on the recyclability of PP for food packaging applications, considering the residual contamination and migrational behavior after repeated processing cycles. In particular, the relationships existing between reprocessing, the residual amounts of contaminants, and the migrational behavior were investigated. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1768–1778, 2003