Properties of Polyethylene and Polystyrene

Topics: Polyethylene, Plastic, Young's modulus Pages: 4 (759 words) Published: March 3, 2013
Background Theory.


Polyethyene is a type of polymer that is used in our daily life. Polyethylene(abbreviated PE) is a thermoplastic polymer, meaning that is can be melted to a liquid state and remolded as it returns to a solid state. PE is chemically synthesized from ethylene, a compound that is usually made from petroleum or natural gas[1]. The main mechanical properties of PE are the ultimate tensile strength and the modulus of elasticity.

There are different types of Polyethylene, which differ from each other by the density and branching, but the main types of PE are :

* LDPE (Low Density Polyethylene)
* HDPE (High Density Polyethylene)
* LLDPE (Linear-Low Density Polyethylene)
* MDPE (Medium Density Polyethylene)

Low Density Polyethylene (LDPE), has a density range of 0.910–0.940 g/cm3 . LDPE has a high degree of short and long chain branching, which means that the chains do not pack into the crystal structure as well. It has, therefore, less strong intermolecular forces as the instantaneous-dipole induced-dipole attraction is less. This results in a lower tensile strength and increased ductility [2].

High Denisty Polyethylene (HDPE), has a density range of 0.941-0.965 g/cm3. Is a thermoplastic material composed of carbon and hydrogen atoms joined together forming high molecular weight products[3]. HDPE has a low degree of branching and thus stronger intermolecular forces and tensile strength[2]. It is the least stretchy type of PE. Linear-Low Density Polyethylene (LLDPE), has a density range of 0.915–0.925 g/cm3. Has higher tensile strength than LDPE.  Exhibits higher impact and puncture resistance than LDPE.  Lower thickness (gauge) films can be blown compared to LDPE, with better environmental stress cracking resistance compared to LDPE[2].

Medium Density Polyethylene, has a density range of 0.926–0.940 g/cm3. MDPE is rarely used for flexible plastic sheeting. Has more strength than LDPE, a little more...
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