Polyethylene (PE) is a light and versatile synthetic resin. This compound is made from the polymerization of ethylene. Polyethylene is an important family member of polyolefin resins. Also, it is the world’s most widely used plastic, being made into products ranging from shopping bags and clear food wrap to automobile fuel tanks and detergent bottles. In addition, it can be spun or slit into synthetic fibres or modified to take on the elastic properties of the rubber.
Chemical Composition and Molecular Structure
Ethylene, having the chemical formula as C2H4, is a gaseous hydrocarbon, which is commonly produced by ethane cracking, which, in turn, is a primary constituent of natural gas or can be distilled from petroleum. Essentially, ethylene molecules are composed of two methylene units - CH2, together with a double bond between the carbon atoms, where the structure is represented using the formula CH2=CH2. Under the polymerization catalyst influence, the double bond is broken, and the resultant extra single bond can be used to link to a carbon atom in the other ethylene molecule. Therefore, made into the repeating unit of a large, polymeric (multiple-unit) molecule and the ethylene, which has the chemical structure as given below:
The secret to polyethylene properties is the simplest structure, which is replicated thousands of times in a single molecule. Big, chain-like molecules with hydrogen atoms bound to a carbon backbone may be made in branched or linear shapes. Branched versions are called linear low density polyethylene (LLDPE) or low density polyethylene (LDPE); linear versions are called ultrahigh-molecular-weight polyethylene (UHMWPE) and high density polyethylene (HDPE).
The basic polyethylene composition is modified by including the other elements or chemical groups, as in the chlorosulfonated and chlorinated polyethylene cases. Additionally, ethylene is copolymerized with other monomers such as propylene or vinyl acetate to produce a number of ethylene copolymers.
Major Polyethylene Compounds
Low Density Polyethylene
LDPE can be prepared from the gaseous ethylene under more high pressures (up to around 50,000 pounds per square inch or around 350 megapascals) and high temperatures (up to 350 °C) in the presence of oxide initiators. These processes yield a polymer-type structure with both short and long branches.
Since the branches prevent the polyethylene molecules from closely packing together in stiff, hard, and crystalline arrangements, LDPE is given as a very flexible material. Its melting point is given as nearly 110 °C. Principal uses of this compound are in packaging film, grocery and trash bags, agricultural mulch, cable and wire insulation, toys, housewares, and squeeze bottles. #4 is the LDPE’s plastic (polyethylene plastic) recycling code.
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Linear Low Density Polyethylene
Structurally, LLDPE is the same as LDPE. It is produced by copolymerization the ethylene with 1-butene and fewer amounts of 1-octene and 1-hexene, using metallocene or Ziegler-Natta catalysts. The resulting structure has a linear backbone, but uniform, short branches that keep the polymer chains from packed together tightly, similar to LDPE's longer branches.
Overall, LLDPE contains the same properties as LDPE and competes for similar markets. The major advantages of LLDPE are given as the polymerization conditions are less energy-intensive and that the properties of the polymer can be altered by differentiating the amount and type of its chemical ingredients. #4 is LLDPE’s plastic (polyethylene plastic) recycling code.
High Density Polyethylene
HDPE is made at low pressures and temperatures, using the metallocene catalysts and Ziegler-Natta or activated chromium oxide (which is called Phillips catalyst). In its structure, the lack of branches allows the polymer chains to be closely packed together by resulting in a dense, highly crystalline material of moderate and high strength stiffness.
It can withstand prolonged exposure to 120 °C because it has a melting point greater than 20 °C and is higher than LDPE. The products are blow-moulded bottles for household cleaners and milk, blow-extruded grocery bags, agricultural mulch, and construction film, and injection-moulded pails, appliance housings, toys, and caps. #2 is HDPE’s plastic recycling code.
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Ultra High-Molecular-Weight Polyethylene
Ultrahigh-molecular-weight variants of linear polyethylene are available, with molecular weights ranging from 3,000,000 to 6,000,000 atomic units, compared to 500,000 atomic units for HDPE. These polymers are spun into fibres and, after that, stretched, or drawn, into a highly crystalline state by resulting in tensile strength and high stiffness several times that of steel. Yarns, which are made from these fibres, are woven into bulletproof vests.
Ethylene is copolymerized with many other compounds. Also, ethylene-vinyl acetate copolymer (EVA), for suppose, is produced by the copolymerization of vinyl acetate and ethylene under pressure, using the free-radical catalysts. Several various grades are manufactured, with the vinyl acetate content differing from 5 to 50% by weight. The copolymers of EVA are more permeable to the gases and moisture than that of polyethylene, but they are more transparent and very less crystalline, and they represent better grease and oil resistance. Principal uses of this compound are adhesives, packaging film, tubing, toys, wire coatings, gaskets, carpet backing, and drum liners.