Friction And Wear Properties Of The In Situ Polymerized Nano-Al₂O₃/PE Composites

In this thesis, methylaluminoxane (MAO) and MAO/nano-Al₂O₃ were prepared by controlled hydrolysis of trimethylaluminum (TMA) with nano-Al₂O₃ as water carrier. In presence of MAO and MAO/nano-Al₂O₃, series of nano-Al₂O₃/PE composites were prepared via in situ polymerization of ethylene by using bis(cyclopentadienyl) zirconium dichloride (Cp₂ZrCl₂) as catalyst precursor. The structure and performance of the obtained nanoAl₂O₃/PE composites were characterized and examined. The influences of nano-Al₂O₃ content on the physical and mechanical properties of nanoAl₂O₃/PE composites were studied.1. The influences of surface hydroxyl contents of nano-Al₂O₃, H2O/TMA and H2O/nano-Al₂O₃ ratios on the yield and product composition were studied. The results show the total MAO productivity can reach up to 100%. The proportions of free MAO were found increasing with the increase of H2O/nano-Al₂O₃ ratio. MAO content of the obtained nano-Al₂O₃/MAO composites increased with the increase of H2O/TMA ratio and surface hydroxyl contents of nano-Al₂O₃, decreased with the H2O/nano-Al₂O₃ ratio increasing. Series of MAO/nano-Al₂O₃ composites with different MAO content were prepared with H2O/TMA ratio at 0.85. Free MAO was also prepared by using Al₂(SO₄)₃·18H₂O as water source for comparison.2. Ethylene polymerizations were performed in toluene with Cp₂ZrCl₂ as catalyst precursor and free MAO and nano-Al₂O₃/MAO composites as activators. The results show free MAO prepared with H₂O/nano-Al₂O₃ mixtures is as active as that obtained with Al₂(SO₄)₃·18H₂O. Nano-Al₂O₃/MAO composites prepared with H2O/TMA ratio below 1.04 were proved inactive for ethylene polymerization. The results show MAO of the nano-Al₂O₃/MAO composites is as active as that of free MAO and thus nano-Al₂O₃/MAO composites can be used as co-catalyst alone. In situ polymerizations of ethylene were preformed by using nano-Al₂O₃/MAO composites as co-catalyst alone or else adding extra free MAO. The nano-Al₂O₃ content of nanoAl₂O₃/PE composites can be easily controlled by adjusting Cp₂ZrCl₂ concentration, MAO content in nano-Al₂O₃/MAO composites, as well as ethylene pressure and polymerization time. Series of nano-Al₂O₃/PE composites with different nano-Al₂O₃ content (2.1%, 3.6%, 6.0%, 13.4%, 21.9% and 31.1%) were prepared at same polymerization condition (temperature 65oC, ethylene 1.0Mpa, polymerization time 1hr).3. The influences of nano-Al₂O₃ contents on hardness, tensile strength, and friction & wear resistance of the nano-Al₂O₃/PE composites were determined and studied. The dispersion state of nano-Al₂O₃ particles in PE matrix, morphology of the worn scar, micro-feature and element component of the worn surface were determined by means of TEM, SEM, 3D and EDS. The results show the hardness of the nanoAl₂O₃/PE composites increased with the nanoAl₂O₃ content increasing. Broken elongation and tensile strength of nanoAl₂O₃/PE composites reached their maximum at nano-Al₂O₃ contents of 3.6wt.% and 6.0wt.% respectively. Frictional coefficients reached its minimum at nano-Al₂O₃ content between 3.6 to 6.0wt.%. Best wear resistance was found at nano-Al₂O₃ content around 6.0wt.%. The dominant wear mode of the obtained nanoAl₂O₃/PE composites is fatigue wear, compared to the adhesive wear and abrasive wear of pure PE.