Polypropylene Modified By Supercritical Reactive Extrusion

The reactive extrusion of isotactic polypropylene (iPP) grafting maleic anhydride (PP-g-MAH) in the presence of supercritical carbon dioxide (scCO₂) was investigated. Because of the mild condition and well swollen performance in iPP melt, scCO₂ was imported into the extruder system to reduce process temperature as well as enhance mass transfer. It was found that the temperature of reactive sections of twin screw extruder could be successfully reduced from 190℃to 160℃after supercritical fluid's importing. Consequently, effectively suppressing main chain degradation could be observed because of obtained PP-g-MAH with relative higher molecular weight and narrower distribution. The experimental results indicated increase in both grafting degree of resultant PP-g-MAH and grafting efficiency of reactive extrusion process. Especially, the grafting efficiency could be close to 85% while low levels of MAH and peroxide initiator were employed. Based on the multi-cage effects of monomer and free radical, the effect of scCO₂ on melt grafting system was explained. Furthermore, the different feeding modes resulted in significant difference in grafting degree and intrinsic viscosity. It has been shown we can get products of specified characters by adjusting operating conditions. Judging from the micro-structure and crystallization, the effect of scCO₂ is to adjust the ratio of various structures, which makes it different from the regular process.Basing on the above results, the combination of MAH's melt grafting and preparation of long chain branched PP (LCBPP) was studied, to produce LCBPP in one-step using extruder. Meanwhile, the importing of scCO₂ can enhance coupling of free radical grafting and radical reaction, effectively reduce process temperature, save the energy and control the degradation. Because the system is complex, contenting PP, MAH, initiator and EDA, the ratio of component and residence time are crucial. When MAH wt%=4%, the viscosity of product obviously rises, but when MAH wt%=2%, the viscosity rises without scCO₂ and doesn't change with scCO₂. Referring to EDA, the viscosity always rises in this region of NH₂/MAH=1-2,. Through rheological test, it is found that there is obvious chains entangle after EDA's addition, and the phase angle changes comparing with PP-g-MAH. Through the DSC test, it has been found that the structure is complex and the grafting product coexists with LCBPP, so there is no definite rule of melting point, crystal point and crystal temperature, on the other hand, the extrusion stability needs further optimizing via equipment's adjustment.The rising of PP's melting strength makes it possible to prevent melt dripping during combustion. While in the field of the resin's flame retardance research, intumescent flame retardant (IFR) based on polyphosphate (APP) becomes the hotspot as its identity of low smoke, corrodibility, and toxicity for flame-retardant polymeric materials. There are mainly two ways to modify APP: increase the degree of polymerization (DP) and capsulation. In this research, APP is dispersed and covered by melamine (MEL) to reduce solubility and granular size. After the reaction under high temperature and high shear rate, the granular size and distribution of both IFR with high DP (IFR_H) and IFR with low DP (IFR_L) were changed. Especially, the size of IFR_L becomes 1/9 of original ones with narrower distribution. Its solubility becomes 1/8 of the ones without modification.. Meanwhile, the TGA test is conducted to find that after reaction, the former two decompose stages isn't in evidence any more, instead of smooth and sustaining release of phosphate acid. And MEL's cover of IFR_L have smooth surface instead of rough surface of the ones without modification. Through X-defraction test, it is found that IFR_L changes from pure I -type to mixture of I & II -type, which is preferable for the flame retardance of polymeric materials.