Polyethylene is a manmade, synthetic material classified as a polyolefin in the collective group known as plastics. Polyethylene (abbreviated PE) is sometimes referred to as polyethene, polythene and less commonly as polymethylene.
The plastic polyethylene is often correlated with its most commonly used product variants: high density (HDPE), low density (LDPE) and cross-linked polyethylene (XLPE; PEX); while other types are also produced. The characteristics and classification of the different polyethylene types vary based on the specifics of PE manufacture, its density rating and degree of carbon-to-carbon branching.
At its atomic level, polyethylene is a chain of repeating, single monomer units that are linked together through a synthesis reaction termed polymerization. In PE, the repeating subunit is ethylene, a small carbon compound represented in formula as H2C=CH2 or C2H4. When properly reacted, ethylene molecules will form chemical bonds one after the other to produce polyethylene, which is by definition a nonpolar, saturated hydrocarbon polymer chain with high molecular weight.
The resulting PE polymer can be further classified as either a thermoplastic or thermoset plastic material. Thermoplastics are thermo-softening materials capable of remelting, reuse recycling, and account for most PE types. While thermoset plastics offer certain performance differences, they do not melt on heating but burn, and include the cross linked polyethylene type. The resulting polyethylene specs and resin type are fully dependent on how the polymer is manufactured.
How is Polyethylene Made
Polyethylene belongs to a class of polymer materials that are made from petroleum hydrocarbon refinery by-products. Ethylene gas in particular is used to make polyethylene and belongs to the naptha group of natural oil derived substances along with propene (propylene; C3H6), butene (C4H8), benzene and xylene, amongst others. Ethylene is a secondary product generated from the natural oil or gas cracking process that is used to produce modern combustion fuels. Polypropylene, Polystyrene, Polyvinyl Chloride (PVC) and Nylon also belong to this category.
For polyethylene production, the ethylene gas starting material is reacted under particular temperatures, pressures and available catalysts that together induce polymerization of the ethylene compound. Polyethylene synthesis temperatures range from 158°F (70°C) to 572°F (300°C); common pressures range from 9.8 atm (144 psi) to 296 atm (4350 psi); and the Ziegler-Natta catalyst, so named after initial PE researchers, is the most frequently used catalyst.
Depending on the details of the polymerization reaction, the result will be polyethylene with a specific degree of molecular branching, density, and linking that will place the resulting product in one of PE’s various classifications. The standard, most used polyethylene resin classifications are:
- Low density (LDPE)
- Linear low density (LLDPE)
- Medium density (MDPE)
- High density (HDPE)
- Ultra high molecular weight (UHMWPE)
- Ultra low molecular weight (ULMWPE)
- Cross-linked polyethylene (XLPE; PEX)
- Chlorinated polyethylene (CPE)
The different types of PE will vary in their chemical and physical properties as well as any potential additives included during manufacture. Polyethylene additives can modify PE appearance by adding colorants, enhance its malleability through plasticizers, provide UV and weather protection with antioxidants, and increase its resistance to fire and microbial growth, to name a few examples. Additives are frequently included during manufacture to further modify PE’s characteristics and often to match the specific needs of an individual application.
Important Polyethylene Characteristics & Properties
The specifications and properties of polyethylene depend entirely on how the resin is processed and what additives if any have been included. Details such as density, specific gravity, molecular weight, temperature maximums and minimums, chemical sensitivity, physical durability and its total amorphous state versus crystalline state all vary on the degree of polyethylene branching and its specific processing.
When polyethylene is compared among plastics and other material types in general, PE is reported as durable, resilient against impacts, tears and drops, and resistant to chemical corrosion, rust and weathering. When fabricated with the additive carbon black or titanium dioxide, PE also becomes resistant to sunlight UV degradation and long term suitable for outdoor use.
In terms of chemical strength, polyethylene has marked chemical compatibility and can be significantly more resistant than other materials, even metals, depending on the chemical to be handled.
The maximum service temperature is dependent on the specific PE strain as well as additives and design specifications. HDPE is commonly rated to a sustained service temp of 100°F, with maximum spikes to 120°F – 130°F. XLPE is rated to a sustained service temp of 100°F, with maximum spikes to 130°F – 140°F. LDPE max temperature ratings are around 110°F. Note these values are for continuous storage and operation temperatures, not the materials’ melting temperatures.
Additional characteristics for PE include it is lightweight, maneuverable, moldable, and with a fairly low production cost that provides cost effectiveness in terms of purchase price, installation and service life.
What is Polyethylene Used For
Polyethylene is one of the world’s most used plastic material types in terms of market abundance, production weight, and diversity among resulting products. PE resins are used in blow molding, rotational molding or push method extrusion processes to produce the wide variety of PE products across industries and applications. Agriculture, chemical processing, consumer products, healthcare and pharmaceutics, plumbing, commodity handling, logistics and storage are a few example industries that make use of polyethylene.
Polyethylene is used to produce various household items such as utensils, eat ware, appliances, electronics, toys and a variety of other individual, everyday items. It is used in the fabrication of storage containers and tanks of all shapes, sizes and with intended commodities from water, chemicals, food ingredients, syrups, oils, greases, to fuels.
Frequent chemical types used with PE include strong acids such as hydrochloric acid, strong caustics such as sodium hydroxide, and potential oxidizers such as sodium hypochlorite (bleach).
Polyethylene is often used to manufacture pipes and tubing that are widely used for fluid transfer, pumps and delivery systems. Other common examples of PE use include healthcare equipment, electrical components and wiring sheaths, as well as structural materials used in construction.
The wide use and extensive versatility of polyethylene can be contributed to its unique physical and chemical properties as well as its capability to be modified and/or developed into its different characteristic strains.
Takeaway | What is Polyethylene
Polyethylene is a fairly recent implementation in the field of material science and polymer engineering, as are all plastics. Polyethylene further categorizes into thermoplastics such as HDPE, LDPE and thermosets such as XLPE/PEX. PE is a long carbon chain polymer made of ethylene refined from natural oils and gas resources.
Among the different plastic types, polyethylene is one of the world’s most manufactured, fabricated and engineered plastics next to polypropylene. PE is commonly used in the production of many modern-day items as well as applications that range from manufacturing to commodity handling.
As a product material option, polyethylene is a durable, comparatively low cost and corrosion resistant synthetic material that offers significant compatibility within chemical scenarios to water use to food grade operations.
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