There are different types with varying ranges of crystallinity and density. The density of polyolefins is the lowest among all polymers and is divided into three ranges for polyethylene: the low density region from 0.91 to 0.925 g, the light polyethylene region (LDPE), the medium density region (MDPE) from 0.926 Up to 0.94 g / cm3 and the density range from 0.94 to 0.965 g / cm3 which is labeled as heavy polyethylene (HDPE).
Heavy Polyethylene (HDPE)
The name HDPE is used if the main chain is polyethylene and has few or no short side branches. HDPE is a white, opaque solid that forms brittle films. These polymers are very crystalline (70-80%) and their melting point is about 130 degrees Celsius. Density is usually used as a measure of the percentage of crystals of alkyls such as alpha-olefins (α) including 1-butene, 1-hexene and 1-octene as a comonomer during polymerization. In this case, short branches form along the main polymer chain. These small branches reduce the crystallinity and hardness of the polymer. Due to its high crystallinity, heavy polyethylene has the highest stiffness and the lowest permeability among other types of polyethylene. Due to these two properties, this material is used in the manufacture of food and chemical storage containers. Low permeability, high rigidity and good resistance to corrosion have led to the use of this material in the manufacture of pipes for water, sewage and natural gas. The good tensile strength of heavy polyethylene makes it suitable for short-term films.
Light polyethylene (LDPE)
LDPE polymerization is done by free radical method, at high pressure of 122-303 MPa and at temperature of 130-135 degrees Celsius. The polymerization time is very short and takes 15 seconds to 20 minutes, depending on the type of reactor used. The usual method of making it creates branches and produces chains with long and short branches. Side chains prevent crystallization. Due to this, the percentage of crystallinity in LDPE is low. Its density is between 0.915-0.930 g / cm3 and the crystallinity percentage is about 40 to 50%. Long branches on LDPE molecules are expected to modify its flow properties. The presence of these branches widens the molecular weight distribution
The presence of these branches determines the degree of crystallinity and the transfer temperature, and affects the crystallographic parameters such as the size of the single cells, the adjustment angle, and the size of the crystals. The short branches in LDPE are mostly normal butyl, but ethyl and possibly butyl and hexyl are formed to a lesser extent by radical attack from the back (biting). Long strands of tens or hundreds of carbon atoms in the structure of these polymers are formed by intermolecular chain transfer. LDPE has good branching structure, high melt strength and lower viscosity due to its branching structure, which makes it suitable for blowing film production. In fact, more than half of the consumption of lightweight polyethylene is allocated to this process.
Linear Lightweight Polyethylene (LLDPE)
Linear Lightweight Polyethylene (LLDPE) has revolutionized the polyethylene industry. In fact, this type of polyethylene is competitive with other polyethylenes in terms of properties. LLDPE is the toughest type of polyethylene and has excellent impact and tear strength. However, HDPE has a lower toughness than other types of polyethylenes and is more sensitive to cracking than other types. In addition, the amount of LLDPE warping is lower than other types.
LLDPE is produced at 3.5 MPa and 100 ° C by Ziegler catalyst or Philips catalyst with a variety of intermediate metals in liquid phase and gas phase slurry reactors and no long branches are seen in the structure. From a chemical point of view, LLDPE is composed of ethylene copolymerizing with α-olefins containing 3 to 12 carbon atoms, and the amount of α-olefin comonomer in LLDPE is about 5 to 10% by weight. The α-olefins are attached to the main polyethylene chain in the form of short strands, each with different properties. The most commonly used communomers in industry are propene, 1-butene, 4-methyl, 1-pentene, 1-hexene and 1-octene.
Precise control of the reaction conditions and the quality of the catalyst affect the amount of branching and crystallization. Also, the distribution of comonomer units in LLDPE chains depends on the type of catalyst used. For example, metallocenes produce a uniform distribution of branches, while Ziegler-Nata catalysts produce a wide distribution. On the other hand, the type of polymerization process, chain length as well as the amount of α-olefin used in the polymer structure affect its properties.
Butene is the most widely used and oldest consumable commonomer, and recent advances have led to the production of LLDPE with the 1-hexene and 1-octone commonomers. Many major companies around the world now use 1-hexane to produce LLDPE. 1-Hexene, due to being higher than 1-butene, further reduces the crystallinity in LLDPE. This has led to optimal processability as well as better physical and mechanical properties. Naturally, reducing the crystallinity causes the melt and solid to penetrate better together.
This at low temperatures causes good formation and strongly affects the impact properties of the polymer. This is especially important in rotational molding processes due to the better mixing of the melt by the phenomenon of penetration without applying pressure.
Polyethylene with very high molecular weight
High molecular weight polyethylene (UHMWPE) has a molecular weight of 4 to 6 million. These polymers are mainly produced with metallocene catalysts. Due to the fact that they have very long molecules, they do not flow much due to cutting and they can not be processed by conventional methods for polyethylene.
Very hard molecules move and if they are subjected to shear stress, they break and the wear and impact properties of the final product are reduced. The two most common shaping methods are piston extrusion drilling and powder compression molding. In these methods, the powder particles are compressed, melted, and penetrated by pressure and heat.
These polymers have excellent abrasion resistance, high impact and chemical resistance and low coefficient of friction. Excellent mechanical properties, ease of machining and low cost have made it possible to replace metals in some applications. In addition, their gel-processed process produces fibers whose mechanical strength is comparable to that of steel.
