Properties of Polyethylene and Polystyrene

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 chemical resistance, tighter cell structure, more puncture and tear resistance[2[.

Polystyrene

Polystyrene is a type of polymer with thermoplastic properties produced from the petroleum-derived monomer, styrene. In solid form, polystyrene is a colorless and rigid plastic. However, this material may also be returned to a liquid state by heating and used again for molding or extrusion[4].

Aim
The aim is to analyze the data to determine the Young’s modulus of the samples using Stress-Strain graph. Method
After analyzing the data that was given, both stress and strain were calculated for all the samples by using these formulas: σ=FA ε=δL
σ = normal stress on a plane perpendicular to the longitudinal axis of the specimen P = applied load
A = original cross sectional area
ε = normal strain in the longitudinal direction
δ = change in the specimen’s gage length
L = original gage length

All the values were given in the data sheet. By putting all the values into the formula, gives us different values of Stress and Strain for different samples. The Young’s Modulus of the samples can be determined by taking the linear part of the curves and calculating their gradient.

Results

Graphs

By using the graphs above, the gradient can be calculated for all the samples.

Samples 1

* Thin: 608.107 MPa
* Thick: 643.771 MPa
* Polystyrene: 1071.952 MPa
Samples 2
* Thin: 398.776 MPa
* Thick: 400.000 MPa
* Polystyrene: 1118.478 MPa
Average
* Thin: 503.442MPa
* Thick: 521.885MPa
* Polystyrene: 1095.215MPa

The actual values for Young’s Modulus:
* LDPE- 200-400 MPa[5]
* HDPE- 600-1400 MPa[6]
* LLDPE-250-700 MPa[7]
* Polystyrene- 3.0-3.5 GPa[8]

Discussion

After calculating all the values for different samples, we have determined the values of Young’s modulus. We have certained the values, which аre close to the actual values, apart from the Polystyrene. This may be occured because of the human or system error in reading or putting the data. The rest of the values аre noticeably close the actual values. Therefore, there is an assumption that the samples were HDPE, because there was a significant apply of force for a little extension.

Reference:

[1] http://www.wisegeek.org/what-is-polyethylene.htm

[2] http://wikipedia.org/wiki/Polyethylene

[3] Lester H. Gabriel, Ph.D., P.E., History and Physical Chemistry of HDPE, chapter 1 [4] http://www.wisegeek.org/what-is-polystyrene.htm
[5] http://www.matbase.com/material/polymers/commodity/ldpe/properties [6] http://www.matbase.com/material/polymers/commodity/hdpe/properties [7] http://www.matbase.com/material/polymers/commodity/lldpe/properties [8] http://www.engineeringtoolbox.com/young-modulus-d_417.html