Study On The Preparation And Properties Of UHMWPE/Polypropylene Composites

Polypropylene?PP?is a general synthetic resin,which has excellent chemical resistance,heat resistance,non-toxic,easy processing and other excellent comprehensive performances.It is widely used in the field of equipment manufacturing,automobile industry,molds and household appliances.However,polypropylene has some shortcomings,such as poor toughness,low temperature brittleness,poor aging resistance,low impact strength and high shrinkage rate,which limits the application of polypropylene as engineering material.Therefore,it is necessary to carry out modification treatment to strengthen and toughen polypropylene for widening its using fields.Ultra-high molecular weight polyethylene?UHMWPE?is a new type of thermoplastic engineering plastic with linear structure and excellent comprehensive properties.It is unparalleled compared with other engineering plastics in terms of wear resistance,corrosion resistance,impact resistance and self-lubricating properties.It is considered as an ideal high-performance plastic that can be widely used in various fields instead of steel.Therefore,UHMWPE was selected as a reinforcing material in the hope of improving the mechanical properties of polypropylene after mixing with polypropylene.However,UHMWPE has rather high molecular weight,highly entangled molecular chains and poor melt flow,making it difficult to achieve homogeneous mixture with polypropylene at submicron levels.In this thesis,UHMWPE was pre-treated with processing aids?high-boiling mineral oil?,and melt-blended with PP to prepare UHMWPE/PP composites.The effects of swelling time and temperature on the swelling ratio of UHMWPE and processing methods on the extrusion of composite materials were investigated.The mechanical properties,rheological properties,crystallization properties and thermal properties of the composites were also studied.The main contents and conclusions are summarized as follows:1.Preparation of UHMWPE jelly is an important factor in the preparation of homogeneous mixing UHMWPE/PP composites.The preparation of UHMWPE jelly is influenced by temperature and time,and the swelling effect of UHMWPE is judged by the optimum swelling temperature and swelling ratio.The results show that the swelling ratio of UHMWPE first increases and then decreases with the increase of temperature,and the optimum swelling temperature is about 136?.With the increase of the swelling time,the swelling ratio of UHMWPE increased gradually.After 20 minutes,the swelling ratio did not change and tended to be stable.2.The UHMWPE jelly and PP were melt-blended to study the effects of single and twin-screw extrusion temperature,screw speed and UHMWPE content on the preparation and mechanical properties of the UHMWPE/PP composites.The analysis results show that the extrusion mixing material was stable and the blending effect was best when the extrusion temperature and the screw speed are 220?and 250 r/min,respectively.The mechanical properties of the composite prepared by single screw extrusion decreased obviously,but through the twin-screw extrusion enhanced the impact strength and tensile strength of the composites.The impact strength and tensile strength of the composites increased with the increase of UHMWPE content.When the content reaches 5%,the impact strength and tensile strength increased by 22%and 8%,respectively.3.The rheological properties of UHMWPE/PP blends melt were investigated by capillary rheometer.The melt flow of the composites belongs to shear thinning type and the flow type does not change.With the addition of UHMWPE,the apparent viscosity and shear stress of PP decreased.In addition,the non-Newtonian index decreased and the activation energy of viscous flow decreased.Compared with pure PP,the melt viscosity of the blended with UHMWPE of 5%decreased by 12.3 MPa and the shear stress decreased from 59.81kPa to 37.94 kPa at a shear rate of 1400 s^-1.The viscous flow activation energy decreased by1.98 kJ/mol.4.The results of DSC show that there are two melting endotherms in the system,and two kinds of crystals in the composite material.When crystallized,the two kinds of crystals competed and interfered with each other,resulting in a decrease of crystal refinement.As the UHMWPE content increases,the crystallinity of the composites decreases gradually.With the introduction of UHMWP,the crystallization rate of the composite increases,the crystallization time decreases,while the Avrami index does not change significantly.In addition,the results of Kissinger equation show that the crystallization activation energy of composites was 16.34 k J/mol lower than pure PP,when the content of UHMWPE in composites is 1 wt%.