Abstract
The polymerization of ethylene to Ultra-High Molecular Weight Polyethylene (UHMWPE) in certain reaction conditions allows synthesis of nascent powders with a considerably lower amount of entanglements: the material obtained is of great interest from both academic and industrial viewpoints. From an academic point of view, it is interesting to follow the evolution of the metastable melt state with the progressive entanglements formation. Industrially, it is valuable to have a solvent-free processing route for the production of high modulus, high strength tapes. Since the polymer synthesis is performed in the presence of a solvent, it is interesting to investigate the influence that the reaction medium can have on the catalyst activity, resultant molecular characteristics, and polymer morphology at the macroscopic as wells as microscopic level. In this paper, we present the effect that two typical polymerization solvents, toluene and heptane, and mixtures of them, have on the catalytic performance and on the polymer properties. The observations are that an unexpected increase of catalyst activity, accompanied by a significant improvement in mechanical properties, is found when using a carefully chosen mixture of solvents. A tentative explanation is given on the basis of the presented results.
Highlights
The extremely high melt viscosity of Ultra-High Molecular Weight Polyethylene (UHMWPE) makes conventional processing techniques, including screw extrusion and injection moulding, difficult to accomplish
In the first set (PE1–5) we have tested different concentrations of catalyst, in the second set (PE6–12) we have performed reactions with different solvents and mixtures of them, and in the third set (PE 13–15) we have explored the influence of upscaling the process from a 1 l to a 10 l reactor
Our hypothesis to justify this result is that the very high activity of this catalytic system makes the amount of ethylene present in solution at monomer partial pressure of ∼1 bar too low to comply with the fast consumption when the catalyst concentration is higher than 20 μM, leading to a process limited by the ethylene uptake rate
Summary
The extremely high melt viscosity of UHMWPE makes conventional processing techniques, including screw extrusion and injection moulding, difficult to accomplish. In spite of its good performance in short-term applications, it has been reported that wear, creep, and fatigue fracture are usually observed in long-term applications that can lead to serious consequences: for example, debris generated at articulating surfaces of UHMWPE prostheses is transported to the hard and soft tissues surrounding the joint, causing chronic inflammatory reactions and bone desorption [3] For this reason, continuous efforts are made to understand the structure-property relationship of this material. It is possible to increase the rate of crystallization compared to polymerization; by using an homogeneous catalytic system at low concentration of catalyst (dilute solutions) and monomer (low monomer pressure), it is possible to have chains growing far from each other and crystallizing independently, leading to a less entangled polymer This simple concept allows synthesis of disentangled polymer directly from polymerization, with no need for further treatments with large quantities of solvents, and the material obtained can be drawn and even compression-moulded at temperatures below its melting temperature. In the present article we wish to report a new insight arising from the use of different solvent and solvent mixtures in the polymerization reaction
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