Molecular structure and general properties
Polyethylene consists of hydrocarbon chains with the most basic component being the ethylene molecule, consisting of 2 carbon and 4 hydrogen atoms. When ethylene molecules are combined together in straight or branched chains, polyethylene is formed. This process involves splitting the double bond between the 2 carbon atoms and creating a free radical to join to the next ethylene molecule. The macromolecules are not covalently joined, but are held together in a crystalline structure through intermolecular forces. The lower the number of side branches, the lower the crystallinity and hence the higher the density as can be observed in the differing properties for differing types of polyethylene.
Polyethylene is weather resistant but can become brittle when exposed to sunlight for extended periods of time. This limitation can be overcome through the addition of UV stabilizers. It can be ignited and will continue to burn after the ignition source is removed with a yellow tipped blue flame, which will cause the plastic to drip. The surface properties of polyethylene prevent it from being stuck together or imprinted without pretreatment. Polyethylene can be transparent, milky-opaque, or opaque, depending on the grade of material, the thickness of the product, and the presence of additives.
Classifications of polyethylene
Low-Density Polyethylene (LDPE) is a very flexible material with unique flow properties that makes it particularly suitable for shopping bags and other plastic film applications. LDPE has high ductility but low tensile strength, which is evident in the real world by its propensity to stretch when strained.
Linear Low-Density Polyethylene (LLDPE) is very similar to LDPE, but offers added advantages. Specifically, the properties of LLDPE can be altered by adjusting the formula constituents, and the overall production process for LLDPE is typically less energy-intensive than LDPE.
High-Density Polyethylene (HDPE) is a robust, moderately stiff plastic with a highly crystalline structure. It is frequently used in plastic for milk cartons, laundry detergent, garbage bins, and cutting boards.
Ultrahigh Molecular Weight Polyethylene (UHMW) is an extremely dense version of polyethylene, with molecular weights typically an order of magnitude greater than HDPE. It can be spun into threads with tensile strengths many times greater than steel and is frequently incorporated into bulletproof vests and other high-performance equipment.
Polyethylene is the most common form of thermoplastic used in consumer products, and has a wide variety of applications. The following are some of the more common uses:
Food packaging material: Because polyethylene is so resistant to water absorption and chemical damage, it is certified as safe to use for food packaging applications.
Medical tubing: The lack of porosity of polyethylene makes it an ideal material to use for catheters and other medical product due to the resistance to contamination.
Buckets, bottles, and bins: Due to the hardness of plastic and its durability under stress, it is suited to consumer products.
Bullet proof vests: UHMWPE fibers have high tensile strengths, yet are very light—making them ideal for safety applications.
High-tensile cables: They are light, therefore easy to install, but strong, so are useful in complex industrial applications.
|Items|| LDPE ||HDPE|| UHMWPE|
|Water absorption 24HR (%)||<0.01||0||0|
|Tensile strength|| 1800-2200||4600||3100|
|Notched impact strength||No break ||3||No break|
Coefficient of linear thermal expansion (x10^-5 in/in/deg F)
Max continued use temperature (def F)
Approx. melting temperature (deg F)
Dielectric strength (V/mil) short time, 1/8″ thick
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