| Polypropylene (PP) is one of the conventional thermoplastics with a number of desirable basic properties. However, the commercially available linear iPP is ill-suited for melt-state processing operations such as foaming, thermoforming and blow molding, because of its lower melt strength. Currently, it has great demand for high melt strength PP (HMSPP) on the market, but there are no domestic industrial products supplied. Therefore, the studies on the PP melt strength modification have an important application prospect. The purpose of this research is to improve the melt strength of commercially linear PP, and develope a novel and feasible method for preparing HMSPP by one-step reactive extrusion.Firstly, HMSPP was successfully prepared by melt grafting reaction in the presence of macromonomer vinyl polydimethylsiloxane (VS), co-monomer styrene (St) and initiator benzoyl peroxide (BPO) via one step reactive extrusion, and had no gel. The results of FTIR, MFR, GPC, ICP-MS and rheology analysis showed that St and VS were grafted onto the PP backbones successfully, and St not only can effectively control the degradation of PP, but also can react with VS to form branched structure, because when the concentration of VS and BPO was constant, the grafting degree of St increased with the increase of the concentration of St, and the grafting degree of VS increased, too. Dynamic shear rheological tests showed that the elastic response of HMSPP at low frequencies was significantly enhanced in comparison with that of iPP, such as higher G’and η*at lower frequency, appearing a platform in tanδ～ω polt and deviating from linear iPP in G’～G" plot. Meanwhile, HMSPP also appeared strain harding behaviour when the tensile rate was ls-1, and the fracture strain value increased to more than3. The optimum conditions for preparing HMSPP was as follow:when the concentration of VS, St and initiator BPO was1wt%,1wt%and0.5wt%of isotactic polypropylene (iPP), reapectively, the melt strength of PP-g-VS/St was the highest, up to0.29N, which was12times higher than that of iPP, and the Mw, MFR and grafting degree of VS was up to384Kg/mol,0.55g/10min and0.027%, respectively.Secondly, the crystallization behavior and crystal morphology HMSPP was studied by differential scanning calorimetry thermal (DSC), the polarization microscopy (POM), and wide angle X-ray diffraction (WAXD) analysis methods, and isothermal and non-isothermal crystallization kinetics was described using Avramie quation, Hoffman theory, Mo equation and Kissger equation, respectively. The results showed that the crystallization temperature, melting temperature and crystallinity of HMSPP increased with the increase of melt strength from0.03IN to0.29N. For the highest melt strength sample PP-g-VS/St, the crystallization temperature increased by14℃, melting temperature increased by2℃, and the crystallinity increased by about6%than that of iPP. In the isothermal crystallization process, the Avrami index of HMSPP decreased to2with the increase of melt strength, which indicated that branching structures of HMSPP acted as a heterogeneous nucleating agent. In the non-isothermal crystallization process, the presence of branching structure accelerated nucleation of HMSPP at the early stage, and then hindered the growth of crystal at the later stage. Meanwhile, the fold surface free energy of HMSPP decreased, and the non-isothermal crystallization activation energy of HMSPP increased gradually with the increase of melt strength. In the crystal morphology, the presence of branching structures made the crystal size of HMSPP decreased, and the number of crystal increased.Thirdly, The foaming ability of HMSPP was investigated by using supercritical carbon dioxide as the blowing agent, and the effects of foaming temperature and foaming pressure on the cell structure of PP-g-VS/St foam were studied to find the optimal foaming process. The results showed that grafting monomer has a significant effect on foaming ability of HMSPP. Since the monomer St can control the degration during the reactive extrusion and the monomer VS can increse the solubility of CO2, the HMSPP prepared by grafting VS and St had the best foaming ability, such as higher cell density, smaller and more uniform cell size. In the other hand, the melt stength will affect the foaming ability of HMSPP when the grafting monomer was the same. Compared with the cell structure of PP-g-VS/Stl, PP-g-VS/St2and PP-g-VS/St, whose melt strength was0.031,0.23and0.29N respectively, it can be found that the open cell disappeared gradually. PP-g-VS/St had an excellent foaming properties, and the suitable foaming temperature range was from145to165℃, and the suitable foaming pressure interval was from18to14MPa. When the foaming pressure was14MPa and the foaming temperature was160℃, the maximum expansion ratio of PP-g-VS/St was up to66times, and the cell density, the cell diameter and the foam apparent density was5.8×107cell/cm3,139.8μm and0.0133g/cm3, respectively.Fourthly, the effect of the injection molding process on the crystal structure of HMSPP was investigated. The results showed that HMSPP forms α and β crystal in the injection molding conditions, and the cooling rate, packing pressure, melt temperature and mold temperature were not the key factors for the formation of the β crystal of HMSPP, while the shearing and high cooling rate in the injection molding process was the necessary condition for the generation of β crystal of HMSPP. When a rheometer was used to simulate the effect of shearing on the crystal of PP-g-VS/St, it can be found that the β crystal content of HMSPP increased at first and then leveled off with increasing shearing rate and shearing time, and increased with the increase of shearing temperature from170to230℃. The β crystal content of PP-g-VS/St was up to35.6%when the shearing rate, shearing time, shearing temperature and cooling rate was70s-1,60s,230℃and280℃/min, respectively, which was almost equal to that of injection molded sample. The mechanical properties of HMSPP increased with the melt strength from0.031N to0.29N, the increased range of PP-g-VS/St was the highest, whose tensile strength, flexural modulus and impact strength increased by28.6%,40.0%and466.7%respectively compared to iPP. After annealing, the β crystal of injection molding sample of PP-g-VS/St was converted into β crystal, and the impact strength of PP-g-VS/St decreased, but it was still higher than that of iPP, which indicated that the excellent impact strength of HMSPP was caused by branching structures and the β crystal together.Finally, due to PP-g-VS/St can form β crystal during the injection process, the effects of PP-g-VS/St used as polymeric β crystal nucleating agent on the crystallization temperature, crystal structure, crystal morphology and mechanical performance of iPP were investigated. The results showed that PP-g-VS/St can effectively induce β-iPP, and increase the crystallization rate, crystallization temperature, rigidity and toughness of iPP. When the addition content of PP-g-VS/St was50wt%, the relative content of β crystal of nucleated iPP reached32.8%, the crystallization temperature increase by about10℃. When the addition content of PP-g-VS/St was70wt%, the impact strength, flexural modulus and tensile strength increased by260.0%,54.7%and17.2%at room temperature, respectively. |
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