Research Of High Melt Strength Polypropylene

Polypropylene (PP) is a general plastic with various fine properties. However, due to its linear chain structure and appropinquity of softening point and melting point, its melt strength and melt viscosity harshly drops with the temperature rising beyond its melting point, which results in the difficulties in thermoforming, extrusion coating, and extrusion foaming. Because of its particular molecular structure, superior properties and huge application potentials, the study on High Melt Strength Polypropylene (HMSPP) has been a field of growing interest for years.The purpose of this work was to obtain long chain branching HMSPP and to investigate the technique of its production process. The hinges of this study included the formation of longer side chains in order to acquire higher melt strength and the control of degradation of iPP and cross-linking side reactions, which may lead to the formation of gel.In this study, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane and tert-Butylperoxy 2-ethylhexyl carbonate were used as initiators, while three types of acrylates with various number of functional groups, namely epoxypropyl methacrylate (GMA), 1,6-Hexanediol diacrylate (HDDA), and pentaerythritol triacrylate(PETA) were used as grafts. PP/LDPE blend was melt grafted by these acrylate grafts in a twin-screw extruder to yield blend with higher melt strength.The effects of initiator type, graft type, as well as LDPE dosage on the blend's properties were investigated. Methods including FTIR, DSC, GPC and extensional viscosity test were used to test the resulted grafting products in terms of molecular structure and other characters.The results indicated that tert-Butylperoxy 2-ethylhexyl carbonate was a better initiator, while both HDDA and PETA helped to improve the blend's melt strength. The dosage of initiator and graft also strongly affected the blend's melt strength and melt flow rate (MFR). Lack of initiator resulted in self-polymerization of the grafts and, therefore, the formation of oligomers, which would bring down the melt strengthand increase the MFR. On the other hand, excess initiator had similar effects due to the increasing of PP degradation. Generally, the melt strength would grow with the increasing dosage of grafts to a certain degree, then start to drop because excess grafts would self-polymerize to form oligomers. Adding of LDPE causes the melt strength to grow but the stretching and bending strength to decrease.The IR spectrogram proved that the acrylate grafts had been grafted onto the PP chains. Higher melt strength was always accompanied with higher stretching viscosity and strain gauging during stretching, both of which indicated the existence of branching structure. DSC results showed that higher Tc and Tm coexisted with higher melt strength, while molecular weight distribution almost remain unchanged as indicated by the GPC test.The affecting factors of the graft reaction were also investigated. It was found that process condition of 190℃, 100rpm was optimal for the graft reaction. The increase in dosage of monomer and initiating agent could both improve the graft ratio.Degradation of PP and formation of gelatin were also studied. It was found that adding of styrene and reducing the initiator dosage could effectively minimize the degradation of PP, while gelation occurred when too much LDPE were used. In general, gelation could be avoided by limiting the graft dosage to no more than 1 portion.