1) Polymerization Process
The slurry or gas phase processes for the production of HDPE or LLDPE operating with low bulk density polymers require reactors with large volumes in order to obtain the necessary residence time. Particularly, in the gas phase reactors, the presence of fines with low bulk density causes problems. In fact, due to the friction of the polymer particles present in the reactor, the fines are especially prone to the formation of electrostatic charges and tend to deposit and adhere to the metallic walls. These stagnant deposits do not allow the exchange of the reaction heat and become hot spots, which can form layers of agglomerates containing eventually melt polymer. After a period of time chunks of agglomerates can fall down and plug the product discharge system. The described effects are enhanced when the reaction is carried out with a highly active catalyst.
In U.S. Pat. No. 5,410,002 a summary of patents on the above described phenomena is presented.
Therefore, it is crucial for the process that the catalyst used enables a total control of the polymer morphology resulting in product grains without fines, with high bulk density and good flowing properties.
Another very important aspect is that the catalyst must have a slow decaying time to permit the use of reactors in series. This arrangement makes possible the obtainment of bimodal products in addition to the reduction of the total volume of the reaction for the same production basis. Moreover, process operations with a different condition of reaction in at least two reactors in series, makes possible the use of catalysts with higher particle size thereby minimizing the formation of electrostatic charges.
2) Polymer Structure
The catalyst properties are fundamental to the polymer structure, mainly with respect to the molecular weight distribution, comonomer insertion in the polymeric chain and soluble content.
Each application used to achieve a final product from high density polyethylene (HDPE) or linear low density polyethylene (LLDPE) requires a specific polymeric structure.
To obtain a film with improved optical and mechanical properties and avoid blocking problems, a polymer having a narrow molecular weight distribution (MFR<27), is required. A large quantity of LLDPE applications requires products with a density=0.918 and a MI=0.7, and in most cases a xylene soluble content<10% is desired. Indeed, when the xylene soluble content of the polymer is present in a high concentration (>10%) and with a low molecular weight, this soluble content tends to migrate to the film surface causing blocking, in addition to the unsatisfactory optical properties (low gloss and high haze).
3) Ziegler-Natta Polyethylene Catalyst
Due to the strong competition existent in the polyethylene market, the catalyst production cost is a fundamental component. Therefore it is mandatory that the catalyst for producing polyethylene be manufactured by a simple route, from low cost raw material, without generating gaseous, liquid or solid effluents which are hard to treat.
U.S. Pat. No. 5,188,997 describes a synthesis process for Ziegler-Natta catalysts from silica and magnesium chloride alcoholate. The results reported demonstrate that this catalyst produces a polymer with low bulk density (0.23 to 0.30 g/ml) and with an intermediate molecular weight distribution (MFR 30.0 to 37.8).
U.S. Pat. No. 5,585,317 describes the synthesis of a catalyst supported on a magnesium chloride based carrier. The reported examples relates to the production of polymers having good morphology, characterized by the absence of fines and by the high bulk density which is, for LLDPE, produced between 0.32 and 0.40 g/ml and for HDPE, between 0.33 and 0.438 g/ml.
However, in the case of LLDPE production, the polymer obtained presents an undesired comonomer distribution in its chain, evidenced by the high xylene soluble content at different polymer densities. As an example, a polyethylene with 0.919 g/ml of polymer density has a xylene soluble content of 12.5% by weight.