Study On Mechanical And Thermal Conductivity Propreties Of Melt-stretched Polyethylene Nanocomposite Films

Polyethylene has been widely used in many fields of daily life and industry due to its light weight,chemical stability,low cost and easy processing.Pushing the mechanical property and thermal conductivity limits of polyethylene is highly desirable for expanding applications of this general plastic to some special occasions like collision protection and thermal management materials,which has attracted extensive attention from industry and academia.To this end,many efforts have been devoted to improving the performances of polyethylene by various approaches,such as mixing,blending and in-situ polymerization.In this thesis,aiming at the enhancement of mechanical and thermal conductivity properties,we have developed a new processing strategy of melt stretching to prepare the high-performance of polyethylene-based material.The close relationship between macro-properties and micro-structure has been well established,which has guiding significance for achieving polyethylene products with high performance.The main research contents are as follows:(1)Compared to polyethylene fibers,it still remains a challenge for fabricating large-scale film products with superb mechanical properties.Herein,high density polyethylene(HDPE)films reinforced with CNTs nanofiller were prepared via an ingenious high-speed melt stretching strategy.The effects of filler content and stretching ratio on the mechanical properties of the films were mainly studied.As the filler content was 3 wt% and the stretching ratio was 80,the resulting nanocomposite films showed the highest tensile strength of 147 MPa in reported HDPE-based composites,while the Young’s modulus kept simultaneously a high level of 2249 MPa.Through the characterization of the micro-structure of films,the mechanical enhancement is mainly attributed to the strong synergistic effect of filler and processing field on the structural formation and evolution,which is involved in the following three aspects:(1)forming the densely distributed shish-kebab superstructures,(2)making a nearly perfect orientation of shish-kebab crystal and(3)reinforcing the physical connectivity of shish-kebab crystal network.The construction of these characteristic microstructures greatly improves the load transfer efficiency and underlies a significant enhancement on the mechanical performance.Furthermore,the nanocomposite films displayed an excellent erosion resistance,ensuring their durability in harsh using environments.Due to the strong nanocomposite films fabricated directly by melt processing,the current work is of guiding significance in engineering practice and lights a feasible path towards expanding applications of general plastics to some special occasions like collision protection and structural materials.(2)Polyethylene composites have attracted increasing interest as thermal management materials owing to their ease of processing and potential low costs.However,most polyethylene composites have only modest thermal conductivities even at high contents of additives,resulting in high costs and reduced mechanical properties.The main reason is that the filler is difficult to disperse and orient well in the matrix.In this work,highly thermally conductive HDPE/GNP/CNTs nanocomposite films were prepared via a facile melt stretching strategy.It was shown that melt stretching with stretching ratio of 12 could induce the high orientation of GNP filler,while the CNTs(3 wt%)played a bridging role between GNP.As a result,the anisotropic index and thermal conductivity of the film filled with 20 wt% GNP and 3 wt% CNTs were as high as 1000% and 7.6 W/(m K),respectively.Furthermore,compared to the samples with randomly dispersed GNP,both the elongation at break and the tensile strength of meltstretched film were improved a lot at the same filler content.To understand the process of heat transfer,the finite element simulation was carried out.It was found that the oriented GNP could allow the heat transfer more readily along the in-plane direction of film.Therefore,the melt stretching has been demonstrated to be a simple and effective method to fabricate highly thermally conductive and mechanically strong polyethylene composites.