Study On The Melt Extrusion Process Of UHMWPE And Its Morphology And Properties Under Elongational Flow

Ultra high molecular weight polyethylene(UHMWPE) is a comprehensive high performance engineering thermoplastic with excellent properties,such as impact resistance, wear resistance, self-lubricating, low temperature resistance, chemical inertness, biocompatibility, resistance to stress cracking resistance, non-toxic health and other properties. UHMWPE is widely used in biomedical, microelectronics, materials transportation, food processing machinery, chemical machinery, textile machinery and other application fields. However, exceedingly large molecular weight and very long molecule chains makes it has a high degree of physical entanglements, leading to extremely high melt viscosity and poor melt flowability. In addition, UHMWPE melt has a very low critical shear rate, resulting in the extremely difficult of extruding and moulding. It cannot be processed and shaped directly by the conventional extrusion or injection equipment, which has led to greatly limitations of applications and promotion. The materials are easy to slip in the feeding section of screw extruder and cause the plug phenomenon during extrusion processing, which has led to a very low productivity. In current actual production process, additives must be added in order to reduce the extrusion resistance, however, the production efficiency is still very low. Obviously, the conventional extrusion processing method and equipment cannot solve the processing problem of UHMWPE effectively.In this article, it is the first to apply the new polymer processing method and technology based on elongational rheology to the processing of UHMWPE resin. The eccentric rotor plasticizing and conveying system, which dominated by the volume elongational deformation, is used to process UHMWPE resin and study the volume pulse extrusion process and mechanism under the elongational deformation. In addition, the melting processing and mechanism of UHMWPE under the elongational deformation was discussed by investigating the materials morphology at different locations in the eccentric rotor system.By using the eccentric rotor extruder, without any additives, pure UHMWPE resin was plasticized and extruded efficiently. For UHMWPE resin with molecular weight of 4800000, the extrusion yield is about 48kg/h at the rotational speed of 125 r/min, reaching the production efficiency of ordinary PE, which has effectively solve the problem of low productivity for UHMWPE processing.Pure UHMWPE with molecular weight of 4800000 can still get good plasticizing at high rotational speed and the plasticizing effect is not affected by molecule weight of UHMWPE resin. This indicates that melting and plasticizing in eccentric rotor extruder is a fast and efficient process. The results show that the volume elongational deformation effect and rolling effect dramatically makes a continuous change for the volume of UHMWPE resin, which have made the unmelted material in the “core” portion of the block close to the heat source and enhanced the heat and mass transfer markedly, resulted in promoting the melting and plasticizing process. The in-situ fiberizing phenomenon under volume elongational deformation can also improve the heat and mass transfer during extrusion processing.The extrusion processing process under elongational flow is well to maintain the molecular weight of UHMWPE. Compared to the nascent UHMWPE rensin, the rheological properties, thermal properties and mechanical properties of extrudates via eccentric rotor extruder have little difference, indicating the little difference of molecular weight of UHMWPE resin.UHMWPE melt easily occurs melt fracture and deformation phenomenon when subjected to complex effects of elongation or shear or deformation recovery and so on. Due to the poor mobility and long relaxation time, the fractured melt will take a long time to reform a continuous melt.The use of additives or the second component polymers with smaller molecular weight have exacerbated the melt fracture phenomenon during extrusion processing, and it is difficult to carry continuous molding even at a very low rotational speed. Because the extrusion process via eccentric rotor extruder is short and efficient processing process and it only has a short residence time for UHMWPE melt to reply. For the reason that the ordinary extrusion die does not apply to the extrusion of UHMWPE via eccentric rotor extruder and a special extrusion die should be designed according to the melt characteristic of UHMWPE.