The linear polyethylene is obtained by the most complex and specific method by using polymerization with special metallocene catalysts. Linear low-density polyethylene is quite stable to rupture, impact and puncture, has a low density and high plasticity. The thin film is made of LHPPE as it saves material and reduces the effect on the environment. Although this polymer is suitable for many applications, it is most frequently used in the manufacture of packaging films because of its good transparency, flexibility and solidity. Almost the entire volume of LHPPE consumed in Russia is supplied for the production of films (stretch film produced by blowing method and on cast-lines, multi-layer shrink film and lamination film). Meanwhile, the linear polyethylene in Russia practically is never used for the production of garbage bags and packages. In the near future, with the appearance of more technologically sophisticated equipment, the LHPPE would more and more displace the HPPE both from the single layer applications and from the production of multilayer films.
The properties of linear low-density polyethylene are medium between the properties of the LDPE and the properties of HDPE. But in comparison with HPPE, the HDPE has a more homogeneous distribution of polymer groups by the molecular weight. The basic advantages of linear low-density polyethylene are high chemical stability, high performance characteristics both at relatively high and low temperatures, high resistance to cracking, improved resistance to puncture.
The linear polyethylene has the highest physical and chemical properties.
LLDPE has the highest values of tensile strength and break lengthening. Quite high melting temperature makes it possible to use the melt polyethylene for packaging hot food. Due to the presence of a large number of short lateral protrusions, at the sliding deformation without creating the considerable internal stresses the LHPPE is characterized by the excellent melt elasticity. That allows to obtain quite a thin film of 6 to 25 microns. However, due to its high crystallinity the LLDPE is less transparent than the other types of polyethylene. Special optical agents are injected into the LHPPE in order to obtain a more transparent polyethylene.
The physical and chemical properties of linear high-pressure polyethylene (LHPPE)
|Tensile yield point (50 mm/min), MPa||10-19|
|Modulus of elasticity at tension (1 mm/min), MPa||200-550|
|Relative tensile elongation (50 mm/min), more than %||50|
|Melting temperature, С||120-130|
|Solubility, above °С||120|