Low Density Polyethylene Production Method
Jul 05, 2023| 1. The polymerization method of low-density polyethylene
According to the polymerization method, low-density polyethylene can be divided into high-pressure method and low-pressure method. According to the type of reactor, it can be divided into kettle method and tube method. Using ethylene as raw material, sent to the reactor, under the action of the initiator, the polymerization reaction is carried out by high-pressure compression, and the material coming out of the reactor is removed by the separator after the unreacted ethylene, and then granulated by melt extrusion, dried, blended, and sent to packaging.
2. The LDPE and LLDPE
Both LDPE and LLDPE have good rheological or melt flowability. LLDPE has less shear sensitivity because it has a narrow molecular weight distribution and short branched chains. During shearing processes (e.g. extrusion), LLDPE maintains a greater viscosity and is therefore more difficult to process than LDPE with the same melting index. In extrusion, LLDPE's lower shear sensitivity enables faster stress relaxation of polymer molecular chains, and thus less sensitivity of physical properties to changes in blowing ratio.
In melt extension, LLDPE generally has a lower viscosity at various strain rates. That is, it will not produce strain hardening when stretched like LDPE. As the deformation rate of polyethylene increases, LDPE shows an alarming increase in viscosity, which is caused by molecular chain entanglement.
This phenomenon is not observed in LLDPE, where the lack of long branched chains makes the polymer not tangled. This property is extremely important for film applications, as LLDPE films are easier to make thinner films while maintaining their strength and toughness. The rheology of LLDPE can be summarized as "rigid on shear" and "soft on extension". When replacing LDPE with LLDPE, film extrusion equipment and conditions must be modified. The high viscosity of LLDPE requires more power in the extruder and provides higher melt temperature and pressure. The die gap spacing must be widened to avoid reduced yield due to high back pressure and melt breakage.