Enhanced PE by Heterogeneous Catalyst Design for Disentangled UHMWPE

Ultra-high molecular weight polyethylene (PE) has properties exceeding current PE in terms of for example abrasion resistance, low coefficient of friction and high impact strength even at low temperatures. The requirements to such UHMWPE are twofold; firstly, very high molecular weights are required; 10- 50 time higher compared to current PE grades; and secondly, the polymer chains shall be disentangled. The latter contrasts with the current commodity PE grades where polymer chains are always entangled. Disentangled polymer chains are a key requirement for obtaining the enhanced properties characteristic of UHMWPE. One challenge is to develop a method for a strictly controlled polymerization of ethylene in the presence of a suitable catalyst. The objective is to achieve extremely high molecular weight while keeping the newly formed long polymer chains separated so that they will crystallise as they form, without having the time to reorganise and intertwine with other chains. The candidate has developed a novel, homogenous polymerization process and has screened the capability of a work horse catalyst and a range of development catalysts with production of polymer samples for further investigation. The process is strongly dependent on kinetics and process conditions in which polymerization reaction takes place and in particular requires unconventionally low temperature and low pressure. Such homogenous polymerization is however a challenging process due to ease of reactor fouling which hampers development work and with concomitant risk of plant shutdown in full scale. Therefore, a novel heterogeneous catalyst was designed and successfully synthesized. Further, a new UHMWPE polymerization process was developed and successfully implemented in a 2-liter pilot reactor. The new method enabled testing under a wide range of polymerization conditions including at high temperatures, resulting in advantageous high catalyst activity yet with no fouling. The experimental design was employed for efficient and systematic investigation of the impact of process conditions on the resulting molecular weight and on the entanglement density. The developed predictive models for polymerization showed high validity. The second challenge is related to the characterisation of the degree of polymer chain disentanglement. Any of the conventional scientific and industrial methods require sample preparation by various techniques such as dissolution, melting and, optionally, use of shear forces that inevitably, disturbs any inherent disentanglement in the virgin polymer sample and direct observation of the degree of disentanglement cannot be done. Test methods and mathematical models were developed for melt rheology and solid-state NMR analyses of polymer chain entanglement with minimum or no prior sample preparation. Entanglement density of the investigated polymer samples varied considerably and this was, unexpectedly, found to be independent of molecular weight.

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Polyethylene (LDPE, LLDPE, HDPE) are well developed commodity polymers that are key enablers for the transition towards a more resource efficient and circular plastics economy. Their versatility is demonstrated by their application in packaging, building and construction, automotive, Electrical & Electronic, Agriculture, Household, Leisure and Sports, and other market segments. Ultra High Molecular Weight PE (UHMWPE) is an engineered polymer, with superior properties due to the very high molecular weights (10 – 50 times higher than for conventional HDPE). The excellent mechanical properties, such as high abrasion resistance, low coefficient of friction and high impact strength even at very low temperatures, open up new application areas. UHMWPE is currently used in medical implants, lightweight strong fibres, tapes for ballistic applications, ropes for replacement of steel cables and other demanding applications. UHMWPE is a semi-crystalline polymer that is currently produced in entangled form. This results in very high melt viscosity, and processing in conventional polymer conversion equipment is impossible. Disentangled UHMWPE, on the other hand, has lower viscosity and melt processing in conventional industrial plastics conversion equipment is possible. Disentangled UHMWPE is today made in small volumes in an environmentally unfavourable, post reactor process using toxic solvents (e.g. decalin, xylene). The project will develop state-of-the art heterogeneous catalysts without using common silica supports for the polymerisation of ethylene to UHMWPE. The catalysts will be fully characterised, and their ability to produce UHMWPE in an industrial bench scale reactor will be demonstrated. Ultimately, the project will demonstrate the in situ production of disentangled UHMWPE in a heterogeneously catalysed ethylene polymerisation process. The disentanglement will be demonstrated by industrially relevant characterisation and testing.