Preparation And Properties Of The Foamable Polypropylene Resin Based On The Grafted Styrene

Polypropylene (PP) foam has many outstanding advantages on the usability, production cost and environmental protection, and therefore, it is considered as one of the most promising alternatives for other thermoplastic foams. However, the low melt strength, weak sag resistances and no strain hardening of general PP make it difficult to achieve PP foams with high performance, which limits its application in the foaming field. Therefore, it is especially important to develop an approach to prepare the foamable PP resin. In this thesis, the foamable PP resin was prepared via grafting reaction and simultaneous addition of silicone oil or sorbitol nucleating agent through reactive extrusion process.Firstly, the foamable PP resins (PP-St) were prepared by using benzoyl peroxide (BPO) as initiator and styrene (St) as grafting monomer through the extrusion process. The results of Fourier transform infrared spectroscopy (FTIR), high temperature gel permeation chromatography (HT-GPC) and melt flow rate (MFR) measurements showed that St was successfully grafted onto isotactic PP (iPP). The rheological data of PP-St, such as obvious shear thinning in η*-co at low frequency, deviating from the terminal behavior in G’-co at low frequency and obvious strain hardening for extensional viscosity, proved that the long chain branches and micro-crosslinking structures were formed in PP-St. The optimum contents of BPO and St were 0.5 wt% and 1.0 wt%, respectively, and correspondingly, the MFR, Mw and the relative grafting degree of St of PP-St were 0.54 g/lOmin,641 Kg/mol and 0.086, respectively.Secondly, the crystallization behavior, crystalline forms and foaming properties of PP-St were investigated. The results of differential scanning calorimetry (DSC), X-ray diffraction (XRD) and polarizing microscope (POM) showed that the existence of long chain branches and micro-crosslinking structures improved the crystallization temperature, shorten the crystallization time and decreased the spherulite size of PP-St. In addition, the existence of long chain branches and micro-crosslinking structures suppressed the formation of β-crystal, but favored the formation of y-crystal, and the kγ was independent of BPO and St contents, remaining at about 2%. The foaming properties of PP-St were studied via a batch foaming process using CO2 as the foaming agent. The results showed that the cell size in PP-St foams became more uniform with the increase of BPO and St contents. For PP-St with better foaming properties, whose suitable foaming temperatures and foaming pressures were 150～170℃ and 10～16 MPa, respectively. When the foaming temperature was 155℃ and the foaming pressure was 12 MPa, the expansion ratio of PP-St reached the maximum (37 times), and the corresponding cell density and average cell diameter were 8.6 × 107 cell/cm3 and 63 μm, respectively.Thirdly, the foamable PP resins (PP-St-ASO) were prepared by using St and acrylic silicone oil (ASO) as the grafting monomers. The results of FTIR, MFR and melt strength showed that St and ASO were successfully grafted onto iPP and St promoted the grafting reaction of ASO. In addition, the grafting reaction of St played the key role in increasing the melt strength of PP-St-ASO. The foaming properties of PP-St-ASO were also investigated by the batch foaming method. The results showed that ASO, with high CO2 solubility and low surface tension, could promote the cell nucleation in the foaming process, thereby leading to the formation of PP-St-ASO foam with small cell size, high cell density and expansion ratio. When the addition of St and ASO were both 1.0 wt%, the melt strength of PP-St-ASO was 0.77 N, and the cell density, average cell diameter and expansion ratio of the corresponding foam were 1.3 × 108 cell/cm3, 68 μm and 46 times, respectively.Fourthly, the foamable PP resins (PP-St/PDMS) were prepared by using St as the grafting monomer and simultaneously mixing with polydimethylsiloxane (PDMS) as the cell nucleating agent. The results of FTIR and MFR showed that PDMS almost had no effect on the grafting reaction of St. The CO2 solubility of PP-St was higher than that of neat iPP and PP-St/PDMS, indicating that the grafting reaction of St improved the CO2 solubility of PP-St, but mixing with PDMS decreased the CO2 solubility of PP-St/PDMS. Compared with PP-St foam, PP-St/PDMS foams exhibited higher cell density, smaller cell size and narrower cell size distribution, especially for PDMS with lower viscosity. Because PDMS with low viscosity was easier to disperse and form small domains in PP-St/PDMS matrix during the extrusion process, which subsequently act as the cell nucleation sites in the foaming process. When the kinematic viscosity of PDMS was 50 cs, the optimum content was 2.0 wt%, and the cell density, average cell diameter and expansion ratio of the corresponding PP-St/PDMS foam were 3.6 × 108 cell/cm3,42 μm and 39 times, respectively.Finally, the foamable PP resins (PP-St/S20) were prepared by using St as the grafting monomer and simultaneous addition of 1,3:2,4-bis(p-methylbenzylidene) sorbitol (S20), and the foaming properties were investigated. The results showed that with the increase of St contents, there was no significant chang in the cell density, average cell diameter and expansion ratio of PP-St/S20, but the cell size distribution became more uniform and the cell wall became more integral. When the addition of St and S20 were both 0.3 wt%, the cell density, average cell diameter and expansion ratio of PP-St/S20 foam were 1.2 × 108 cell/cm3,63 μm and 41 times, respectively. Compared to the foaming performance of PP/S20, PP-St and PP-St/S20, PP-St/S20 foam had higher cell density and more intact cell wall, indicating that the grafting reaction of St and the mixing with S20 had synergistic effect on cell nucleation and protecting cell structure.