Polyethylene (PE) presents a large class of polymers with an exponentially growing consumption worldwide. Although possessing a relatively simple chemical formula of the repeating CHx units, the chain length, the type and length of the branching, and density, offer a wide scope of possible PE grades, each suitable for different types of commercial application. Since the 1950s, several types of ethylene polymerisation catalysts and polymerisation processes have been developed. Linear low density polyethylene (LLDPE) is one of the most versatile types of PE, being used for the production of thin films, bags, toys, containers, pipes, covers and flexible tubing. The production of LLDPE involves the co-polymerisation of ethylene with α-olefins using Phillips, Ziegler-Natta or metallocene-type catalysts in gas, slurry or solution phase processes.
In this PhD dissertation, the triethylaluminium-induced in-situethylene oligomerisation properties of Cr/Ti/SiO2 Phillips-type catalysts have been studied in order to offer an alternative route for the incorporation of short-chain branching inside the PE, without the need of an additional α-olefin feed, multiple reactors or catalysts.
The production of PE with “reverse” co-monomer incorporation without any externally added co-monomer via the in-situ oligomerisation of ethylene opens the possibility for the industrial production of this specific type of LLDPE using a single catalyst. The practical advantages are numerous. First of all, the elimination of a separate co-monomer feed decreases the price of raw materials. The costs of the purification, transport and chances of reactor poisoning are thus also decreased. The catalyst can be utilised within the existing Phillips polymerisation lines, while the whole process in order to produce PE with reverse SCBD can be performed within a single-reactor system in contrast to dual-reactor systems. Furthermore, the compatibility issues of the catalysts used in tandem catalysis could be also avoided.
This was achieved through the integration of multiple analytical techniques, i.e. STXM, UV-Vis-NIR DRS, DRIFTS, GC, GC-MS, NMR, SEM-EDX and XRD; and the development and use of new operando setups. Ultimately, the characterisation methods and setups developed for the research of this catalytic system can be applicable for other studies involving solid catalysts, especially in the case of very sensitive materials when a highly controlled reaction atmosphere or when the minimal sample manipulation is required.
The presented findings open a new era of Phillips-type polymerisation catalysis, while the developed characterisation methods offer a unique approach to study other catalytic systems in great detail.