From the first part, we know what is polyethylene and how it appeared in our world. Now we focus on its features because they are what makes polyethylene the star.
Main types of polyethylene available on the market
The most common types of polyethylene (PE) are high density polyethylene (HDPE), medium density polyethylene (MDPE) and low density polyethylene (LDPE).
Yes, we can recognize them by their density. We also need to know that there are more types of PE and each type has different properties, which depend mostly on the crystal structure and the molecular weight.
The density is ≥0.941 g/cm3. This type has a low degree of branching so it has low intermolecular forces and tensile strength.
Milk jugs, detergent bottles, butter tubs, garbage containers and water pipes are the mean applications. One third of all the toys are done from HDPE. In 2007 the global HDPE consumption reached a volume of more than 30 million tons.
The density range is 0.926–0.940 g/cm3. MDPE has good shock resistance properties. It is also less notch-sensitive than HDPE, its stress cracking resistance is better than HDPE.
MDPE find applications in gas pipes, packaging film, carrier bags and screw closures.
Density range is 0.910–0.940 g/cm3. LDPE has a high degree of short and long chain branching, which means LDPE doesn’t create the crystal structure and gives unique and desirable flow properties when melted.
However, it has less strong intermolecular forces as well as lower tensile strength. LDPE is used for inflexible containers and flexible plastic film applications. In 2009 the global LDPE market had a volume of circa €15.9 billion.
Another very specific type of PE is cross-linked polyethylene (PEX or XLPE). This example is specially interesting because thanks to cross-link bonds, the thermoplastic material becomes a thermoset. The high-temperature properties are improved. The flow is reduced but the chemical resistance is enhanced.
That means new interesting features and applications. PEX is used in some potable-water plumbing systems, artificial joints, dental application, canoes and kayaks, power cable insulation, and many, many more.
From the above list of applications you can already guess some features of the polymer. So, polyethylene is flexible, waxy, and transparent. It is also thermoplastic and non-polar with high molecular weight hydrocarbons.
PE molecule is symmetric and because of that it is partially crystalline. As we know higher crystallinity increases the density. This influences on the mechanical properties (low strength, hardness rigidity and friction, high impact strength) and the chemical stability (excellent chemical resistance on strong acids, base and salts).
The melting point of MDPE, HDPE is in the range 120-180oC, but the LDPE is 105 – 115oC. Generally PE has good resistance on a low temperature.
How to recognize polyethylene?
In order to recognize PE you can do a flame test. The PE burns slowly with a blue flame with a yellow tip and it smells like paraffin. But remember, be careful as this is a test with fire.
Price of PE
Price of PE depends on the type and can vary a lot depending on the oil situation on the market. Below you can see some websites where you can forecast or find current prices.
http://www.plastech.pl/ceny-tworzyw (the situation on polish market)
Price forcasting for 2016:
Next time we will talk about Polypropylene.
- Mechanical Properties of Polyethylene H CareyE. F. SchulzG. J. Dienes