Study On Maleimide Grafted Polypropylene With High Fluidity And Its Application

Polypropylene (PP) has high melt viscosity and low polarity. It is difficult for PP melt to infiltrate into the glass fiber strands and envlop every filaments and adhere to glass strongly when long glass fiber reinforced PP pellets are produced by melt impregnating process. Therefore, it is necessary to modify PP to improve its fluidity and polarity.In this thesis, PP was melt-grafted with maleimides: N-phenyl maleimide (N-PMI) and diphenylmethyene bismaleimide (BMI) on a twin screw Haake Rheometer at 230℃. The influences of monomers and initiators on the melt flowing rate (MFR) and mechanical properties of modified PP were investigated. The modified PP was characterized with infrared spectrum (FT-IR), the crystalline structure of the modified PP was analyzed with differential scanning calorimetry (DSC), and the changes of thermal stabilities of PP after melt grafting were studied with thermal gravimetric analysis (TGA).The results of FT-IR showed that the monomers N-PMI and BMI could grafted on PP with and without DTBP, the grafting ratio increased with increasing the amount of monomers. The results of DSC showed that the fusion temperature reduced and the crystallinity decreased by the addition of monomers. TGA revealed that the onset decomposition temperature (Tonset) of the modified PP increased with the increase of monomer content, which meant that the thermal stabilities of modified PP were improved by increasing N-PMI and BMI content.The results showed that the MFR of PP increased initially with increasing the amount of N-PMI and BMI and subsequently decreased, the MFR reached the maximum value at the contents of N-PMI and BMI 1 phr and 0.5 phr, respectively. The MFR of modified PP increased rapidly with increasing amount of initiator di-tert-butyl peroxide (DTBP). Also, the impact strength of PP increased initially with the increase of N-PMI and BMI content and then reduced, but the flexural strength, flexural modulus and tensile strength had no obvious changes. The impact strength of BMI modified PP decreased significantly by the addition of DTBP, but had no obvious change with increasing amount of DTBP,and the impact strength of modified PP with high fluidity kept at around 25J/m after modification. The flexural strength, flexural modulus and tensile strength of BMI modified PP had no obvious change with the increase of BMI and DTBP content.Based on the modification of PP by polar monomers, the phase transition plasticizer—phthalimide (PTI) was introduced into the compounds in order to further improve the fluidity. The influences of N-PMI, BMI and PTI on the MFR, impact strength, the heat distortion temperature (HDT), and the cooperativeness between N-PMI, BMI and PTI were studied by using orthogonal method. The influence of PTI on the MFR and mechanical properties of the modified PP was studied. The phase structure of modified PP was analyzed by using DSC and X-ray diffraction (XRD); and the change of the thermal stabilities of PP after modification was studied by TGA.The results of orthogonal experiments showed that N-PMI, BMI and PTI had significant influence on the MFR and HDT of the modified PP, but there was no copperativeness between monomers. On the other hand, these monomers had certain effect on the impact strength of modified PP and some cooperativeness between them.The results of DSC and XRD showed that PTI had little effect on the MFR of PP and could dissolve in maleimide modified PP melt and enhance the MFR of the melt remarkably. It would precipitate and recrystallize after molding, and therefore, improved the HDT and mechanical properties. At 1 phr N-PMI, 0.5 phr BMI and 1 phr PTI, the MFR of the modified PP could be as 3 times high as that of the original PP while the HDT increased 12 oC. The results of TGA showed that the thermal stabilities of the modified PP increased obviously with increasing amount of PTI.Modified PP with high fluidity was reinforced with glass fiber, and the reinforcing efficiency of short fiber and long fiber was compared. The influence of injection process and the length of glass fiber on the mechanical properties of long glass fiber reinforced PP (LGFR-PP) was studied, and the fractured surface was observed by scanning electronic microscope (SEM).The mechanical properties of LGFR-PP prepared by self-developed melting impregnating equipment were obviously superior to the PP compounds reinforced by short glass fiber. The flexural strength of the LGFR-PP was as high as 180.9MPa, the impact strength was 239.5J/m, and the tensile strength was 112.0Pa. The stiffness and strength did not change much as molding parameters changed. However, as screw speed increased, the length of glass fiber decreased while the impact strength of resultant LGFR-PP firstly decreased and subsequently increased. SEM revealed that the interfacial adhesion between PP matrix and glass fiber of modified PP and LGFR-PP prepared by melting impregnation was much better than the others.