جستجوی مقالات مرتبط با کلیدواژه « Polyethylene » در نشریات گروه « مهندسی شیمی، نفت و پلیمر »

Today, one of the most widely used thermoplastic polymers in the world is polyethylene, which is used in various industries. Polyethylene has found many applications due to its unique properties such as low cost of production and very good processability. However, the weak mechanical properties of this polymer compared to metals have led to the use of additives such as various nanoparticles to improve it in most cases. For this reason, in recent years the development of various nanocomposites based on polyethylene has become important. Silica is the common nanoparticles in the field of polymer nanocomposites, which the special features such as easy access, low toxicity, and high surface area, and molecular stability have made this nanoparticle a suitable option for improvement the physical and mechanical properties of polymers. Also, in some cases, in order to solve the problem of the improper dispersion and agglomeration of these nanoparticles, the surface modification of these mineral materials by different methods has been the focus of researchers. In the recent studies, the use of pure and surface-modified silica nanoparticles in thermoplastic matrix, including polyethylene has been reported to improvement the physical and mechanical properties of the polymer. In this paper, the effect of pure silica nanoparticle and its surface-modified types on the crystallinity of nanocomposite based on various types of polyethylene has been reviewed.

In recent decades, the microstructure of polymers and their molecular structure engineering are of great interest due to their tremendous effects on the final properties of polymers. With the increasing demand of polymers, introducing new types of them by changing the microstructure and as a result improving the properties has become particularly important. Polyethylene is a commercial and widely used resin in the industry, which has a diverse molecular structure and consequently, various viscoelastic, mechanical, thermal, and processing properties. Investigating the effect of microstructure on the viscoelastic response of polyethylenes can provide a very good prediction of the processing behavior and related issues; which makes it possible for engineers to achieve the desired goal by spending less time and energy. In general, the presence of long chain branches in the structure of polyethylene improves the melt strength and elasticity. So far, various methods have been presented to control the molecular architecture of different polyethylenes, the most important of which are polymerization and the use of specific catalysts, high-energy radiation, and reactive melt modification. In this article, the microstructure effect of linear- and branched-polyethylenes on the rheological properties and the research performed in this field are reviewed.

Various industries’ waste, including polymer and waste sludge, can be released into the environment and cause irreparable damage. Therefore, their reuse in different fields is very important. This study focuses on the simultaneous use of waste polyethylene (PE), oily waste sludge (WS) resulting from the oil refining process, and micronized styrene-butadiene-styrene to improve the quality of base bitumen (PG58-22). Using the integrated variance optimal mixture design (I-OMD) method, which is used for the first time to optimize bitumen modification conditions, is the second goal of this project. The percentage composition of independent variables, including PE, SBS, and WS, were optimized based on several responses such as penetration (Pen), softening point (SP), ductility (Duct), retained penetration after short-term ageing (rPen), and change of mass after short term ageing (CM). Under the optimal conditions obtained for the modified bitumen, the rheological properties were studied. In the optimal conditions obtained using this method, the values of the independent factors PE, SBS, and WS were 3, 5, and 4% (wt/wt), respectively. Also, in the above optimal conditions, the response values of Pen, SP, Duct, rPen, and CM were 51 dmm, 64 °C, 27 cm, 83%, and 0.07%, respectively. In addition, under the above conditions, the rheological parameters of modified bitumen obtained from the bending beam rheometer and dynamic shear rheometer showed the characteristics of PG64-22 bitumen.

Polyethylene is one of the most widely used polymers in the food packaging industry. Due to high structural diversity, low cost, easy processability, and flexibility in the design and architecture of chains and their impact on the control of polymer engineering properties, different grades of polyethylene have made them one of the most important options in the manufacture of polymer food packaging. The outdoor high consumption of polyethylene has caused the process of stabilizing it against UV radiation as the main factor of degradation in the outdoor to increase its usage time. This indicates the need to increase the stability of this polymer against photodegradation. Generally, there are three ways to improve the stability of polymers against photodegradation: blocking or preventing the radiation from coming into contact with the polymer (mainly ultraviolet), using wave absorbing additives, and using additives that deactivate photodegradation reactions or the intermediate materials in the polymer. In the past decades, many advances have been made in the use of steric hindered amine compounds and their derivatives to increase the photostabilization of polymers. These compounds were generally introduced in four different categories to improve the optical stability performance of polymers by various mechanisms such as reducing and eliminating hydroperoxides, intermediate ions, quenching the excited state, and preventing the occurrence of photoreactions of polymers against UV radiation. In the following, after introducing these materials and their effectiveness mechanisms, various methods to improve the compatibility of amine stabilizers with polyethylene matrix were reviewed.

Today the appearance of food packaging is very important because it has an advertising aspect and helps to sell the product better. In many cases, instead of relying on creating a beautiful color to give the appearance of a polymer package, the consumer wants the packaged product to be clearly visible in order to ensure its health and quality. Some polymers are highly transparent, however, when water molecules condense on the surface, small water droplets form. In transparent films and structures, water droplets act as a scatterer for incident light on the surface. This phenomenon, which is called fogging, makes the film appear opaque. Droplets are formed when the surface tension of the polymer is lower than the surface tension of water, and as a result, the water remains in the droplet form instead of forming a continuous layer. By increasing the critical surface tension of the polymer surface, anti-fog additives allow water molecules to spread on the surface and form a continuous layer of water that does not scatter light. Therefore, it does not interfere with the transparency of the film surface. In this article, the phenomenon of fogging, the production of polyethylene films with anti-fog capability, the types of anti-fog additives and the methods of mixing these materials with polymer have been reviewed, among others. Also, the methods of evaluating the anti-fog properties of polyethylene films were introduced.

In recent years, the use of polymers in the packaging industry has increased significantly due to their excellent advantages over conventional materials. In the meantime, polyethylenes (PE) have become one of the most important options in the manufacture of polymer food packaging, due to high structural diversity, low cost, easy processability, and flexibility in the design and chain architecture and their effect on the control of polymer engineering properties. The aim of this article is to introduce the heat sealing process of polymer films with an emphasis on polyethylene polymer as well as methods and mechanisms related to this process. In the heat-sealing process study, the diffusion phenomenon of tangled chains through the interface and the effect of chain structure on improving the adhesion strength and increasing the heat-sealing capability of polyethylene films were studied. Understanding the factors affecting the chain diffusion phenomenon, by controlling these factors, the strength of the interface during the heat-sealing process can be increased, which leads to improvement the adhesion of the two polymers to each other. Also, different methods of modifying the structure of polyethylene chains and placing different side groups on the chain were introduced. Hyper branching in the polymerization stage, hyper branching using high-energy rays, and hyper branching in the reactive melt mixing state using free radical generators are the three main methods for changing the structure of polyethylene chains. Each of these methods has its advantages and limitations, and different methods are used according to the desired properties of the polymer.

Hypothesis: 

The use of plasma is widely used as a method to change polymer surfaces. The use of atmospheric cold plasma has more advantages than other plasma, laser and X-ray methods. This method is simple and it uses inexpensive equipment. Considering the many uses of polyethylene in industry, it can be effective to investigate its changes against cold plasma.

A dielectric barrier discharge (DBD) plasma under atmospheric pressure was used to increase the hydrophobicity of low-density polyethylene (LDPE). After studying the optical emission spectrum (OES) of the produced plasma, its effects on surface and depth changes including surface morphology, chemical composition and polymer crystal structure were studied through scanning electron microscopy (SEM), attenuated total reflectance-Fourier transform infrared spectroscopy (FTIR), X-ray diffractometry (XRD) and positron lifetime spectroscopy (PALS). Moreover, the contact angle analysis (CA) was used to examine the changes in the hydrophobicity of the polymer. 

Based on the data from FTIR and XRD analyses, it was found that plasma irradiation for 180 s affects the depth of a few nanometers of the polyethylene surface and does not cause significant changes in the chemical bonds and crystal structure of the polymer. In other words, plasma radiation can be used for nanometer-scale modification of the surface. On the other hand, the SEM images indicate that the plasma radiation changes the primary flat surface of the polymer into a porous surface. The results of CA analysis, while confirming this issue, show an increase in the hydrophobicity of the polymer after plasma irradiation. The results of PALS spectroscopy also reveal that at micron depth due to the sudden rise in temperature during plasma irradiation, the free volume of the material increases as a result of pore merging.

For more than sixty years, thousands of patents and articles have been published annually on various manufacturing methods and comparing the properties and performance of manufactured polyethylenes. High-density polyethylene (HDPE) is one of the most widely used polyethylene in different grades, including pipes, films, fibers, bottles, etc., which about 50 million tons of this product is produced in slurry processes worldwide. Several technologies have been presented for the production of polyethylene in industrial plants. Each of these technologies has special operating conditions and type of catalyst and produces a specific product grade. High density polyethylene can be produced by gas phase or slurry. In this article, the processes and catalysts for the production of polyethylene in the slurry phase are reviewed.  These processes include Hostalen slurry process by Basell, CX slurry process by Mitsui, MarTECH process by Phillips, Innovene S process by Inoes, and Bostar process by Borealis. Hostalen and CX technology are similar in general terms and the main process, but they have differences that are discussed in this article. Commercialized catalysts used in the polyethylene production process, including TH series from Basell, RZ and PZ series from Mitsui, and BCH and BCE series from Sinopec, are also investigated.

Pressure vessels are one of the most widely used and high-risk equipment in various industries. Pressure vessels are divided into four types based on material and structure. Weakness in design, construction and production, service and especially incorrect choice of materials are the causes of pressure vessel failure. With the introduction of composites into the world of these vessels, many design weaknesses and their mechanical properties have been improved. By using high density polymers such as polyethylene, the weight of the pressure vessel is reduced. Type IV pressure vessels with the simultaneous use of composite structure and polymer liner are able to reduce weight by up to one-fifth and double the useful life compared to the type I of vessels. The advantages of the fourth type of pressure vessels are corrosion resistance, no fatigue in consecutive loads, lightness and portability, high operating pressure (700 bar) and lack of complexity of the production method, and their disadvantage is the high production cost due to the use of carbon fibers. These pressure vessels are used to store alternative fuels such as hydrogen and portable individual oxygen capsules. In this article, the pressure vessels types are briefly reviewed and the fourth type is compared with other types.

This study was conducted to evaluate the influence of gamma irradiation doses and packaging materials on the shelf life of edible button mushroom )Agaricus bisporus). To do so, the fresh mushrooms were packed in two different packages: (polyethylene and nanocomposite) and immediately irradiated (0, 1, 2 and 3 kGy) then stored (2–3°C). The effect of these parameters on qualitative characteristics of mushrooms (weight loss, cap diameter, pH, total soluble solids, firmness, and whiteness) during storage (1, 5, 10, 15 and 20 days after irradiation) was evaluated. Our findings showed that the mutual interaction of irradiation dose and packaging material, also irradiation dose and storage time were significant (p < 0.01) in all measured properties except firmness. It can be concluded that using proper doses of irradiation (1-2 kGy) and appropriate packaging material (nanocomposite) could significantly increase the shelf life of edible button mushrooms for 20 days.

P olyolefins (POs) are one of the most widely used classes of polymeric materials withlow flammability. Today, the use of halogen-free retardants (HFFRs) for polyolefinsis important due to the environmental problems of halogen additives. Halogen-free flameretardants are divided into several categories, which the most important include phosphorusbasedtypes, minerals, nitrogen, silicon, boron, and intumescent systems. In this article,the operation mechanism and performance of these additives as the flame retardants forPOs-based systems are reviewed. The results show that in order to achieve the desiredflame resistance properties for polyolefins, due to the relatively low efficiency of mosthalogen-free flame retardant compounds, large amounts are used in the formulation,resulting in a significant decrease of thier mechanical properties. To overcome this problem,combinations of halogen-free flame retardants with synergistic effect should be used forPOs. Based on the studies, it has been found that intumescent systems are the most effectiveadditives, along with most of the halogen-free additives to achieve the synergistic effectin creating flame resistance for polyolefins. This synergistic effect is created by the solidphase mechanism and the strengthening of the protective coating in this phase due to thepresence of a intumescent systems.

Production of polyethylene (PE) of a relatively high molecular weight with improved properties and acceptable processability has been allocated to many research efforts. In the slurry polymerization, immobilization of homogeneous metallocene catalyst on a nano-sized support leads to improved mechanical and thermal properties in addition to controlled morphology and appropriate particle size distribution of product. Specific surface area of support particles can be an effective parameter affecting the immobilization process of catalyst and product properties. In this research the main purpose was to produce PE/nanosilica nanocomposite, using an in-situ polymerization technique, in a disentangled state.

A metallocene catalyst, such as zirconocene dichloride (Cp2ZrCl2), was immobilized on the surface of modified nano-fumed silica particles. Three grades of nano-fumed silica having specific surface areas of 380, 200 and 50 m2/g were used. First the surface of thermally pretreated nanosilica was chemically modified using methylaluminoxane. Then, by adding the catalyst, Cp2ZrCl2, immobilization reaction and activation of the catalyst were performed simultaneously. Finally ethylene polymerization was conducted using the prepared catalyst under the atmospheric pressure of monomer at 30°C.

The maximum polymerization yield was related to the heterogenized catalyst on nanosilica with a specific surface area of 200 m2/g. The results of solid state drawability and buildup of modulus in time sweep rheometry exhibited that the synthesized polyethylene is in the less entangled state. Reducing the concentration and density of the active sites on the heterogenized catalyst resulted in the reduced number of chain entanglements. Tensile test results showed that nanocomposite samples possess better mechanical properties compared to the pure polyethylene, an indication of appropriate distribution of silica nanoparticles into the polyethylene matrix which was evidenced using SEM images.

Membranes are thin layers that act as selective barriers to separate the components of materials and control the mass transfer between different phases. Therefore, their two main functions include: component selectivity and the permeability characteristics. Membranes according to their material sources are classified into four categories referred as polymeric, ceramic, metal and liquid types. Among these, polymeric membranes are the most favorable materials because of their availability in various chemical structures, their optimum physical properties and lower prices. Polymeric membranes are produced by four main methods known as solution casting, stretching, template leaching and track etching. Among them, stretching technique is inexpensive and because no solvent is used in this process, stretching method has lower environmental impacts. Among different polymers, polyolefines such as polyethylene (PE) and polypropylene (PP), due to their lower price, semi-crystallinity, good mechanical properties, chemical stability and easy process ability, are more appropriate for manufacturing of polymeric membranes by stretching method. Porous membranes are used in applications such as battery separators and drug delivery devices for controlling the permeation rate of chemical components. In this paper, we present some useful information on the above subject by reviewing the related published works in this area.

Polyethylene is degraded by exposure to heat, shear rate and oxygen in melting process. In order to achieve the polyethylene melt stability against thermal oxidation phenomena, the use of stabilizers is a key factor for the preservation of physical and chemical properties. In recent years, various natural stabilizers during the melting process of polyethylene have been of interest to the researchers, since during storage of food in packaging containers produced with synthetic stabilizers, these additives can migrate to food and endanger human health. Some natural stabilizers are phenolic compounds in plant sources that have higher resistance to synthetic stabilizers at high temperatures and have higher melt stability. In this paper, the effect of several natural stabilizers on the polyethylene melt stability is investigated. Stability performance is determined by measuring rheological properties, thermal oxidation stability and polymer color. The results of the flow index measurement of polyethylene containing these natural stabilizers show that the viscosity changes are insignificant during the melt process, but the synthetic stabilizers undergo changes in their molten flow index and it is degraded during the melt process. On the other hand, the evaluation of yellowness index shows that the natural stabilizer affects the color of polyethylene. However, despite this slight weakness of natural stabilizers, it can be used to prevent heat damage in cases where the color of the product is a secondary issue. As a result, natural stabilizers can be a good alternative to synthetic stabilizers during the melting process of polyethylene.