Study On The Crystallization Behaviors And Flame-Retardant Properties Of Thermoplastic Polyolefins

Thermoplastic elastomers（TPEs） are a new class of materials that behave like thermoset rubbers, possessing the advantages of processibility and convertibility via thermoplastic techniques. Such excellent performances result in the rapid development of TPEs. Thermoplastic polyolefins（TPOs） are an important kind of TPEs, and widely used in electric wires and cables, household electrical appliances, automobile industry, and medical equipments. Polymers undergo complex thermodynamic conditions and flow fields during processing, which lead to the changes in the rheological properties and aggregation structures of materials. For TPOs, the mechanical properties are greatly influenced by the internal crystallization behaviors. Thus, to build relationships between micro-structures and macro-performances of TPOs become an important research subject. However, due to the inherently high flammability, the usage of TPOs is limited. With the improvement of requirements in flame-retardant and environmental performances of TPOs in recent years, more and more researches have been focused on the high-efficiency halogen-free flame retardant TPOs.In this work, the crystallization behaviors of polypropylene（PP）/elastomer blends under quiescent and shear conditions were observed by rheological and thermodynamic methods. The relationships between crystallization behaviors and viscoelastic properties, and the influence of the molecular structures and the components variation on the crystallization behaviors were discussed. Theoretical models were also used to analyze the crystallization behaviors of TPOs. The flame-retardant olefin block copolymers（OBC）, a new class of block TPOs, with hologen-free intumescent flame retardants were also investigated. The contents and resultes of this work are shown as follows:（1） The compatibilization by OBC in the blends of PP/ethylene-propylene-diene terpolymer（EPDM） and the phase morphology of the ternary blends were investigated by rheology, scanning electron microscopy（SEM）, differential scanning calorimetry（DSC） and wide-angle X-ray diffraction（WAXD） measurements. In the temperature range of 150 to 200 oC, OBC was shown to have a better compatibility with PP than EPDM. Compared to the decrease of interfacial tension, the changes caused by OBC of viscosity ratio in the blends took the dominant position in controlling the final morphology of the blend. The existence of simultaneous crystallization behavior between PP and OBC was also found. Moreover, OBC had a nucleation effect for the forming of β-crystals of PP and this effect was not proportional to the OBC content. With the existence of OBC, the PP/EPDM blends exhibited enhanced mechanical properties, including Young’s modulus, tensile strength and elongation at break.（2） Isothermal and non-isothermal crystallization behaviors of the blends of long chain branched polypropylene（LCB PP） and poly（ethylene-co-octene）（PEOc） were studied at different weight ratios under quiescent and shear flow using polarized optical microscope（POM）, DSC, and rheological measurements. Experimental results showed that the crystallization of LCB PP/PEOc blend was significantly accelerated due to the existence of long chain branches（LCBs）, the blend being able to rapidly crystallize even at 146 °C. The addition of PEOc that acted as a nucleating agent was also increased the crystallization rate of LCB PP. However, the crystallization rate of LCB PP was reduced when the PEOc concentration was more than 60 wt%, showing a retarded crystallization growth mechanism. The morphology of the binary blend was changed from a sea-island structure to a co-continuous phase structure when the PEOc concentration was increased from 40 wt% to 60 wt%. Palierne Model was used to predict the dynamic viscoelastic behaviors and caculate the interfacial tension of the blends. Possible crystallization mechanisms for both LCB PP/PEOc and iPP/PEOc blends were proposed.（3） The non-isothermal crystallization behaviors of self-made LCB PP, dynamically vulcanized LCB PP/ EPDM blends at various weight ratios were investigated by DSC, rheology and POM. It was found that the nucleation and the crystallization kinetics of PP can be induced and improved by LCBs, and the pentaerythritol triacrylate（PETA） on the backbones of PP took the dominant place in the induction mechanism. The influence of EDPM on the crystallization combines the effects of enhancement and retardation, depending on its content in the blends. The LCB PP/EPDM blends exhibited a different crystallization behavior in comparison with that of linear PP/EPDM blends. Possible crystallization mechanisms for LCB PP/EPDM and PP/EPDM blends were proposed.（4） Rao model can be used to predict the non-isothermal crystallization of mono-component polymer. However, this model ignores the effect of interphase boundary to the crystallization process. For the blends system, this effect should be taken into consideration. In view of this, a modified Rao model was proposed by using the second law of thermodynamics. The modified Rao model can predict the crystallization for LCB PP and TPO blends, such as PP/ PEOc and LCB PP/ PEOc physical blends, and dynamically vulcanized LCB PP/ EPDM blends very precisely. The parameters used in the modified terms had specific physical significances which can be used to quantificationally evaluate the effect of heterogeneous nucleation to the PP crystallization behavior. This modified model provided a theoretical foundation for the study on the non-isothermal crystallization process for the TPO blends.（5） A series of halogen-free intumescent flame retardants（IFR） based on home-made melamine phosphate（MP） and pentaerythritol（PER） system（MPPER） including lanthanum oxide（La₂O₃）, basic nickel carbonate（NiCO₃·2Ni（OH）₂·4H₂O）, polyamid 12（PA12）, and expandable graphite（EG） compounding with MPPER, were adopted for flame retarding olefin block copolymers（OBC）. Flame-retardant effects and thermal stabilities of OBC-IFR composites were investigated by limiting oxygen index（LOI）, vertical burning test（UL-94） and thermogravimetry analysis（TGA） along with the analysis of morphological structures of the char residue by SEM. The mechanical properties and the flame-retardant mechanism of the final composited materials have been also discussed. The loading of suitable amount of IFR used in the present work can improve effectively the flame retardancy of the OBC. PA12 in OBC/MPPER system, can enhance the flame retardancy and tensile strength of the materials concurrently. The materials could pass the UL-94 V-0 test when the loading of PA12 was 6.4 wt% and MPPER was 33.6 wt%. Both NiCO₃·2Ni（OH）₂·4H₂O and La₂O₃ had a strong catalytic action in improving the carbonaceous formation of MPPER system and enhanced the flame retradacny of OBC/MPPER composites significantly even if the loading of them was as low as 0.5 phr, which only caused a less decrease in the mechanical property. While the OBC/EG system could pass the UL-94 V-0 test when the loading of EG was 40 wt%, the addition of EG at the amount of 15-18 wt% to MPPER system lowered the total loading of flame retardants in OBC to 30 wt% at the same flame-retardant level.