2. LITERATURE REVIEW
2.1 Types of Polyethylene
The most common polyolefin is polyethylene. Polyethylene has the basic structure of -[CH2CH2]- (shown in Figure 2.1) which consists of a long chain of repeated ethylene units. Even though polyethylene has this simple structure, there are several families of polyethylene with different branching structures. Since polyethylene is a partially crystalline solid, different branching structures affect the crystallinity and density.
Figure 2.1: Molecular structure of Polyethylene
Commercially, polyethylene is generally classified as high density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and very low density polyethylene (VLDPE); table 2.1 shows the comparison of physical properties between HDPE and LDPE. If the polymer backbone is linear and contains no or few short chain branches (as shown in Figure 2.2a), the term HDPE is used. HDPE is a white opaque solid that is rigid and forms films that are crispy to the touch. This polymer is highly crystalline (70-80%) and has a melting point of up to 135 °C. As a measure of crystallinity, density is often used having a value between 0.96-0.97 g/cm3 for HDPE.
To reduce crystallinity, alkyl substituents such as α-olefins (1-butene, 1-hexene, 1-octene, etc.) are introduced as comonomers during the polymerization to incorporate short chain branches (SCB) along the backbone. These alkyl SCBs are concentrated in the amorphous regions of the copolymer where they are mobile and their effect is to reduce stiffness.
This polyethylene is called LLDPE and is shown in Figure 2.2b. LLDPE has branching of almost uniform length to form random copolymers and sometimes “blocky” copolymers. However, the highest concentration of branches is generally found on the shorter chains. This type of polyethylene has a crystallinity of around 40-50% and has a lower melting point of 105-115°C. Depending on the density and comonomer content,...