The Production of Linear Low Density Polyethylene (LLDPE)

Linear low density polyethylene (LLDPE) was first produced in 1977 when Union Carbide announced the development of the Unipol Process, which uses a gas-phase process and transition metal catalysts. LLDPE has a linear molecular structure similar to that of high density polyethylene (HDPE) and a density capability equal to that of low density polyethylene (LDPE). The production of LLDPE incurs significantly lower capital costs since the process requires low pressures. In the low pressure gas-phase process, the polymerization occurs in a gaseous state. Ethylene and co-monomers such as alpha-olefins, ranging from propylene to 1-octene can be used. The effect of these co-monomers is to control the product density. Commerciall y however, 1-butene and 1-hexene are typically used. The gas phase process requires no diluent or solvent, which greatly simplifies the amount of equipment required in the process, because there is no need to recover and recycle a diluent or solvent. Raw material purification is also simplified since the ethylene and co-monomers are the only reactor feeds that need purification from trace amounts of catalyst poisons.

Below is an actual fluidized bed reactor:

Copyright, Texaco, 1990

The gaseous ethylene and co-monomer are fed continuously into the fluidized bed reactor. The transition metal catalysts are also added directly to the reactor. The reaction pressure ranges from 100-300 psig, and the reaction temperature is generally around 100 degrees Celsius. The gaseous stream is circulated through the cooler by a blower to remove the high heat of polymerization. The monomer has a residence time of about 3-5 hours with typical conversions as high as 97%. The polymer uses the surface of the catalyst grain as a solid-surface to grow on. The reaction takes place just below the melting point of the polymer. Hence the cooling loop is essential to maintain temperatures low enough such that the polymer does not melt and coagulate in the reactor. The particles grow to about 1000 mm and are removed through a product discharge system, utilizing a gas-lock chamber. The monomer is easily purged after the gas-lock chamber. The purged polyethylene is in granular form and can be re-extruded and pelletized. Any additives or stabilizers can be added to the polymer during this primary extrusion process.

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