Study On Preparation Of High Melt Strength Polypropylene Through Reactive Extrusion Process Induced By Ultraviolet Irradiation

Common polypropylene (PP) has attractive advantages over other thermoplastics, such as polyethylene and PVC, due to its good comprehensive properties. However, PP exhibits weak melt strength (MS) and no strain-hardening behavior in the melt-state due to its linear architecture, which results in difficulties in the process of thermoforming, blow molding, and extrusion foaming. Thus High Melt Strength Polypropylene (HMSPP) has been studied long time ago. The MS drawback of common linear PP could be overcome by several ways. One method is to form long chain branching on common PP macromolecules using an irradiation process. The other option is to form a minute quantity of crosslinking entanglement onto common PP chains by using organic peroxide and crosslinking reagent. In addition, blending PP with other polymers has been considered as a potential method to improve its MS.This paper has presented a new reactive extrusion process for the production of long-chain branched polypropylene by employing a photoinitiator (benzophonene, BP) and an ultrviolet (UV) lamp to produce radicals in polypropylene matrix. A multifunctional monomer was used to enhance and control the recombination reaction during extrusion to form a crosslinking or long-chain branched structure. This method is simple, low cost and efficient to industial application.The results show that degradation is the dominant reaction of this reactive extrusion process only added photoinitiator to initiate. The melt strength of this production is very low due to its short branched chain. We must employ a multifunctional monomer, such as TMPTA, to capture the PP- radicals to form a long-chain branched structure. Without oxygen, when the irradiation temperature is 210℃, screw speed is 200rpm, feeding rate is 60rpm, BP=0.10wt%, TMPTA=2.0wt%, MS of the production reaches the maxium, 7.8cN, which is 3 times as high as raw PP. Furthermore, the mechanical strength of modified PPs is almost invariant while the processing temperature range becomes much wider than before. The modified PPs prepared in oxygen atmosphere keeps its high MS and has higher melt index but narrow processing temperature range. Meanwhile, we derived some well-foamed sheet by chemical foaming extrusion when the die temperature was at 180℃, screw speed was 50rpm, AC=0.5wt%. Its die swell ratio is up to 4 and the bubble diameter is about 200μm.