Iran’s petrochemical industry was established in late 1950s. Availability of enormous hydrocarbon reserves and also having an exemplary domestic market, the industry rapidly developed in the ensuing years. Since its inception, the industry has travelled a long and challenging road from its humble origins to become a significant source in the global petrochemical market. The genesis of petrochemical industry in Iran dates back 1964, when the National Petrochemical Company (NPC) was set up to plan for the development of this high-potential industry.
Jam Empire Trading (JET) Company is a prominent manufacturer, supplier and exporter of several petrochemical products in the Middle East. Having had an extended existence in international markets, Jam Empire Trading (JET) Company is able to present its services in all aspects of this business, from consultation to exporting. Polyethylene is one of the Jam Empire Trading (JET)’s specialties which makes us distinguished from our rivals, as we are able to find the most proper product for our customers among plethora of products promptly.
What is Polyethylene?
Polyethylene is a polymer formed from ethylene (C2H4), which is a gas having a molecular weight of 28. The generic chemical formula for polyethylene is –(C2H4)n–, where n is the degree of polymerization. A schematic of the chemical structures for ethylene and polyethylene is shown in Figure 1.
For UHMWPE (Ultra-high-molecular-weight polyethylene), the molecular chain can consist of as many as 200,000 ethylene repeat units. Put another way, the molecular chain of UHMWPE contains up to 400,000 carbon atoms. There are several kinds of polyethylene (LDPE, LLDPE, HDPE, UHMWPE), which are synthesized with different molecular weights and chain architectures.
LDPE and LLDPE refer to low-density polyethylene and linear low-density polyethylene, respectively. These polyethylenes generally have branched and linear chain architectures, respectively, each with a molecular weight of typically less than 50,000 g/mol.
Figure 1. Schematic of the chemical structures of ethylene and polyethylene.
HDPE is a linear polymer with a molecular weight of up to 200,000 g/mol. UHMWPE, in comparison, has a viscosity average molecular weight of up to 6 million g/mol. In fact, the molecular weight is so ultra-high that it cannot be measured directly by conventional means and must instead be inferred by its intrinsic viscosity.
Table 1. Typical Average Physical Properties of HDPE, UHMWPE
*Testing conducted at 23°C.
Table 1 summarizes the physical and mechanical properties of HDPE and UHMWPE. As shown in Table 1, UHMWPE has a higher ultimate strength and impact strength than HDPE. Perhaps more relevant from a clinical perspective, UHMWPE is significantly more abrasion resistant and wear resistant than HDPE. The following wear data for UHMWPE and HDPE was collected using a contemporary, multidirectional hip simulator. Based on hip simulator data, shown in Figure 2, the volumetric wear rate for HDPE is 4.3 times greater than that of UHMWPE.
Figure 2. Comparison of wear rates of HDPE and UHMWPE in a multidirectional hip simulator.
Different Grades of Polyethylene
Many types of polyethylene exist, all having essentially the same backbone of covalently linked carbon atoms with pendant hydrogens; variations arise chiefly from branches that modify the nature of the material. There are many types of branches, ranging from simple alkyl groups to acid and ester functionalities. To a lesser extent, variations arise from defects in the polymer backbone; these consist principally of vinyl groups, which are often associated with chain ends. In the solid state, branches and other defects in the regular chain structure limit a sample’s crystallinity level. Chains that have few defects have a higher degree of resin will increase as the degree of crystallinity than those that have many.
Figure 3. Chemical structure of pure polyethylene.
As the packing of crystalline regions is better than that of non-crystalline regions, the overall density of a polyethylene Generally, the higher the concentration of branches, the lower the density of the solid. The principal classes of polyethylene are illustrated schematically in Figure 4 and 5.
a. High Density Polyethylene. High density polyethylene (HDPE) is chemically the closest in structure to pure polyethylene. It consists primarily of unbranched molecules with very few flaws to mar its linearity. The general form of high density polyethylene is shown in Figure 4a. With an extremely low level of defects to hinder organization, a high degree of crystallinity can be achieved, resulting in resins that have a high density (relative to other types of polyethylene). Some resins of this type are copolymerized with a very small concentration of 1-alkenes in order to reduce the crystallinity level slightly. High density polyethylene resins typically have densities falling in the range of approximately 0.94–0.97 g/cm3. Due to its very low level of branching, high density polyethylene is sometimes referred to as linear polyethylene (LPE).
b. Low Density Polyethylene. Low density polyethylene (LDPE) is so named because such polymers contain substantial concentrations of branches that hinder the crystallization process, resulting in relatively low densities. The branches primarily consist of ethyl and butyl groups together with some long-chain branches. A simplified representation of the structure of low density polyethylene is shown in Figure 4b. Due to the nature of the high pressure polymerization process by which low density polyethylene is produced, the ethyl and butyl branches are frequently clustered together, separated by lengthy runs of unbranched backbone. Long-chain branches occur at random intervals along the length of the main chain. The long-chain branches can themselves in turn be branched. The numerous branches characteristic of low density polyethylene molecules inhibit their ability to crystallize, reducing resin density relative to high density polyethylene. Low density polyethylene resins typically have densities falling in the range of approximately 0.90–0.94 g/cm3.
c. Linear Low Density Polyethylene. Linear low density polyethylene (LLDPE) resins consist of molecules with linear polyethylene backbones to which are attached short alkyl groups at random intervals. These materials are produced by the copolymerization of ethylene with 1-alkenes. The general structure of linear low density polyethylene resins is shown schematically in Figure 4c. The branches most commonly encountered are ethyl, butyl, or hexyl groups but can be a variety of other alkyl groups, both linear and branched. A typical average separation of branches along the main chain is 25–100 carbon atoms. Linear low density polyethylene resins may also contain small levels of long-chain branching, but there is not the same degree of branching complexity as is found in low density polyethylene. Chemically these resins can be thought of as a compromise between linear polyethylene and low density polyethylene, hence the name. The branches hinder crystallization to some extent, reducing density relative to high density polyethylene. The result is a density range of approximately 0.90–0.94 g/cm3.
d. Very Low Density Polyethylene. Very low density polyethylene (VLDPE)—also known as ultralow density polyethylene (ULDPE)—is a specialized form of linear low density polyethylene that has a much higher concentration of short-chain branches. The general structure of very low density polyethylene is shown in Figure 5d. A typical separation of branches would fall in the range of 7–25 backbone carbon atoms. The high level of branching inhibits crystallization very effectively, resulting in a material that is predominantly noncrystalline. The high levels of disorder are reflected in the very low densities, which fall in the range of 0.86–0.90 g/cm3.
e. Ethylene-Vinyl Ester Copolymers. By far the most commonly encountered ethylene-vinyl ester copolymer is ethylene-vinyl acetate (EVA). These copolymers are made by the same high pressure process as low density polyethylene and therefore contain both short- and long-chain branches in addition to acetate groups. The general structure of ethylene-vinyl acetate resins is shown schematically in Figure 5e (in which ‘‘VA’’ indicates an acetate group). The acetate groups interact with one another via dispersive forces, tending to cluster. The inclusion of polar groups endows such copolymers with greater chemical reactivity than high density, low density, or linear low density polyethylene. The acetate branches hinder crystallization in proportion to their incorporation level; at low levels these copolymers have physical properties similar to those of low density polyethylene, but at high levels of incorporation they are elastomeric. Due to the incorporation of oxygen, ethylene-vinyl acetate copolymers exhibit higher densities at a given crystallinity level than polyethylene resins comprising only carbon and hydrogen.
Figure 4. Schematic representations of the different classes of polyethylene. (a) High density polyethylene; (b) low density polyethylene; (c) linear low density polyethylene.
f. Ionomers. Ionomers are copolymers of ethylene and acrylic acids that have been neutralized (wholly or partially) to form metal salts. The copolymerization of these molecules takes place under conditions similar to those under which low density polyethylene is made; thus, in addition to polar groups, ionomers contain all the branches normally associated with low density polyethylene. The neutralized acid functionalities from adjacent chains interact with the associated metal cations to form clusters that bind neighboring chains together. A two-dimensional representation of an ionomer cluster is shown in Figure 6. The complex branching structure of ionomers and the existence of polar clusters drastically reduce their ability to crystallize. Despite their low levels of crystallinity, the density of ionomers is normally the highest of all polyethylenes due to the relatively high atomic weight of the oxygen and metal atoms in the ionic clusters.
Figure 5. Schematic representations of the different classes of polyethylene. (d) very low density polyethylene; (e) ethylene-vinyl acetate copolymer; (f) cross-linked polyethylene.
g. Cross-Linked Polyethylene. Cross-linked polyethylene (XLPE) consists of polyethylene that has been chemically modified to covalently link adjacent chains. A schematic representation of cross-linked polyethylene is shown
Figure 6. Schematic representation of an ionomer cluster.
in Figure 5f. Cross-links may comprise either direct carbon–carbon bonds or bridging species such as siloxanes. Cross-links occur at random intervals along chains; the concentration can vary widely, from an average of only one per several thousand carbon atoms to one per few dozen carbon atoms. The effect of crosslinking is to create a gel-like network of interconnected chains. The network is essentially insoluble, although it can be swollen by various organic solvents. This is in direct contrast to the non-cross-linked varieties of polyethylene that are soluble in appropriate solvents at high temperature. Cross-links greatly hinder crystallization, limiting the free movement of chains required to organize into crystallites. Thus the density of a cross-linked polyethylene is lower than that of the polyethylene resin on which it is based.
Polyethylene is made in a polymerization reaction by building long molecular chains comprised of ethylene monomers, mostly by using catalysts. The type and nature of the catalysts are of great influence on the polymerization. As catalysts became more efficient, the polyethylene products became purer and more versatile and the production process became simpler and more efficient. Polyethylene (PE) is a family of resins made from the polymerization of ethylene gas. It is produced either in radical polymerization reactions or in catalytic polymerization reactions. Most PE molecules contain "branches" in their chains which are formed spontaneously in case of radical polymerization or deliberately by copolymerization of ethylene with α−olefins in case of catalytic polymerization. PE resins are classified according to their density which partly depends on the type of branching.
A large number of production processes exist for PE with some general similarities. But the processes are evolving continuously. So the specifics can be significantly different and the following descriptions and graphic displays should be, therefore, considered exemplarily only with no direct relation to existing plant or process designs.
Generic polymerization process
Similarities between the processes follow a generic olefin polymerization process scheme as shown in Fig 7 (from left):
Feedstock materials and additives must be purified and catalyst material must be prepared. And - in case of a high pressure process - the gas must be compressed in several stages.
- Polymerization takes place either in the gas phase (fluidized bed or stirred reactor), the liquid phase (slurry or solution), or in a high pressure environment. Polymerization is the heart of the processes. On any one unit, only one of the three processes is used.
- Polymer particles are then separated from still existing monomers and diluents, pelletized, dried and dispatched.
- Monomers and diluents are recovered and fed again to the process.
Figure 7. Generic Polyethylene (olefin) polymerization process, simplified
In gas-phase polymerization (Fig 8, left) the ethylene is contacted with solid catalyst material intimately dispersed in an agitated bed of dry polymer powder. Two different methods are used to carry out this reaction
- In the fluidized-bed process the monomer flows through a perforated distribution plate at the reactor bottom and rapid gas circulation ensures fluidization and heat removal. Unreacted polymer is separated from the polymer particles at the top of the reactor and recycled. Fluidized-bed plants are able to produce either LLDPE or HDPE and are free of constraints from viscosity (solution process) or solubility (slurry process). A modification uses a second reactor connected in series to perform copolymerization.
- The stirred-bed process uses a horizontal or vertical reactor with compartments, in which the bed of polymer particles is agitated by mixing blades.
The gas-phase polymerization technology is economical and flexible and can accommodate a large variety of catalysts. It is by far the most common process in modern ethylene production plants. Some processes are listed in Table 2.
Figure 8. PE production principles: gas-phase, high-pressure, liquid-phase (from left)
In liquid-phase processes (slurry or suspension, Fig. 2, right) catalyst and polymer particles are suspended in an inert solvent, typically a light or heavy hydrocarbon. Super-critical slurry polymeriza-tion processes use supercritical propane as diluent. Slurry processes run in loop reactors with the solvent circulating, stirred tank reactors with a high boiling solvent or a “liquid pool“ in which polymerization takes place in a boiling light solvent. A variety of catalysts can be used in these processes. Processes in solution require, as their last step, the stripping of the solvent. Supercritical polymerization in the slurry loop provides advantages (e.g. higher productivity, improved product properties) over subcritical polymerization. Advanced processes combine a loop reactor with one or two gase-phase reactors, placed in series, where the second stage of the reaction takes place in the gas-phase reactors. For bimodal polymers, lower molecular weights are formed in the loop reactor, while high molecular weights are formed in the gasphase reactor. Some processes are listed in Table 2.
Table 2. Common PE production processes
High Pressure Processes
In high pressure processes (Fig. 2, center) autoclave or tubular reactors (pressure in excess of 3,000 bar) are used, but the processes are similar, comprising compression, polymerization, pelletizing, and dispatch as major steps. Fresh ethylene enters the reactor and is mixed with the low pressure recycle. After further compression the mixture enters the reactor for polymerization. Oxygen or peroxide may be used as initiators. A tubular reactor typically consists of several hundred meters of jacketed highpressure tubing arranged as a series of straight sections connected by 180° bends. High pressure processes can produce LLDPE homopolymers and vinyl acetate copoymers in addition to the normal range of LDPEs. Some processes are listed in Table 2.
Polyethylene (PE) is the most widely used variety of synthetic resins in China. It is mainly used to make high-frequency insulating materials such as film, container, pipe, monofilament, wire, cable and many other daily necessities.
With the development of petrochemical industry, polyethylene production has been rapid development with production accounts for about 1/4 of total plastic production. The rapid expansion China's national economy had created a favorable environment for the development of synthetic resin industry. Polyethylene (PE) industry is expected to grow at a faster rate.
Within the period from 2008 to 2011, new projects in the Asia-Pacific region are mainly located in China, India and South Korea and China. They will continue to be a source of power. China is becoming the world's largest exporter of PE films and bags which supplies to North America, Western Europe and Japan in large quantities. In addition to the industry on the film, woven bags, pipe, cable materials, hollow containers, turnover boxes and other products will lead to strong demand for polyethylene consumption growth. Hence, China's polyethylene production capacity is expected to grow rapidly like before. At present, China's polyethylene industry production and development has the following major characteristics:
- Rapid increase in production capacity and production;
- The supply gap is large, the dependence on imported goods has not changed;
- Strong consumer demand in conjunction with the rapid development of the future;
- Remains a problem as the industry is becoming increasingly competitive
- LLDPE gradually finds its way to the LDPE market share
PE is a promising synthetic material with great physical and chemical properties. PE has high degree of mechanical properties and excellent combination of good dielectric properties. In addition, the molding process is good and prices is low. They are particularly important in following aspects:
- Electrical insulation: Due to its high stability, moisture resistance and high dielectric properties, it is an excellent material in the making of insulation material in electrical, non-electrical engineering and many other relevant aspects.
- Material to resist chemical agents: Can be used for chemical structure materials such as pipes, anti-corrosion lining and so on.
- Packaging: Polyethylene sheet has low density, soft, water impermeable, high tear strength and chemical resistance. These characteristics are necessary for packaging materials hence polyethylene film has a high market value in the packaging industry and is gradually replacing celluloid.
- Polyethylene after radiation treatment: a. is hard to deform; b. will not produce environmental stress cracking; c. strong elasticity; d. excellent electrical insulation and solvent resistance; e. high temperature resistance; f. low power factor. Hence the greater performance after radiation puts it into a wider range of uses. For examples, insulating materials for capacitors and transformers and higher temperature parts in aircraft. However, the cross-linking reaction of polyethylene during radiation results in difficulty to undergo subsequent processing.
In addition to the above-mentioned purposes, there are many uses of polyethylene, such as various medical equipment, spraying metal, wood, fabric and other materials. High-density polyethylene can be used as rubber