Crystallization Behavior And Kinetics Of Polypropylene Catalloys

Polypropylene catalloys prepared by in-reactor blending technique with a spherical Ti system catalyst is a novel modified polypropylene (PP) material which exhibits excellent mechanical strength and impact properties. It was documented that PP-c was actually a ternary semicrystalline/amorphous polymer blends consisting of propylene homopolymer, ethylene-propylene random copolymer and ethylene-propylene block copolymer with different PE and PP segmental length. Because structure and morphology of PP-c are strongly dependent on the crystallization behavior within polymer processing, to obtain better properties, it is essential for us to understand the crystallization behavior and kinetic course of PP-c. In this thesis, structures, morphology, melting behavior, crystallization kinetics of various PP-c with different compositions were investigated as well as the effects of various factors. Especially, theoretic studies on nonisothermal crystallization kinetics were emphasized.It was found that the crystalline structures of PP-cats change with variations of the crystallization conditions and composition under non-isothermal crystallization. The crystalline phase might consist of α-PP, β-PP and PE crystals when PEP60 crystallized at cooling rates faster than 5 °C/min, whereas there is α-PP and PE crystals in PEP60 samples. PP-c, except PEP60, produced only a-PP crystal when they crystallized at 2 ℃/min. Both α-PP and β-PP could be found in PP-c with increasing of cooling rate. The content of β-PP in PP-c increased with the increase of EP copolymer content, but the overall crystallinity decreased. These results indicated the existence of EP copolymer went against the crystallization of propylene homopolymer and subsequently resulted in the depression of the crystallinity ability of PP-c. WAXD measurements revealed that both the increases of the content of EP copolymer and cooling rate are in favor of the crystal growth along the direction of b axis in PP-c. It is suggested that the formation of β-PP here results from the chain orientation of PP to some extent which could be induced by the differences between propylene homopolymer and ethylene-propylene block copolymer and liquid-solidphase-separation.For PEP60 containing the highest content of EP copolymer, the crystalline phase consisted of or-PP, /?-PP and PE crystals in isothermal crystallization, while there was only a-PP crystal in PEP20 which contained less EP copolymer. The coexistence of a-crystal and /^-crystal of PP were found in other PP-c samples crystallized isothermally and the content of yS-PP depended remarkably on crystallization temperatures. The experimental results showed that the thermal-treatment at 220 °C greatly increased the /^-crystal content in various PP-c samples and induced the existence of /^-crystal in PEP20 samples. Furthermore, it was found that the PP-c samples crystallized isothermally at 124 °C hold the most content of /?-PP. With the increase of EP copolymer, the decrease of crystal perfection and the increase of nucleation density of PP-cats system could be obviously observed. Compared with those being treated thermally at 200 °C, the PEP20 samples crystallized isothermally at 124 °C after being treated thermally at 220 °C for 30 minutes exhibited smaller crystalline size and clear morphology of /?-spherulite, Suggesting that thermal-treatment at 220 °C for PP-c resulted in the changes of microstructure of PP-c systems and EP copolymers were partially rejected by crystalline structure during crystallization process.Different from the fact accepted by researchers over the past several decades that K(T) depended only on the crystallization temperature (7) and consequently the non-isothermal crystallization kinetics parameters could be obtained by plottingln[-ln(l-X(7))] versus m/^ at a given T, we found that the cooling function (K) in Ozawa model was a binary function of crystallization temperature (7) and the cooling rate (A) through theoretic analysis and experimental method. Hence, a new parameter, crystallization transition temperature (Tz) representing the change of crystallization mechanism around it was proposed. For all constant cooling rates, the relative crystallinity corresponding to Tz always equals to 63.21 %. A novel solution to Ozawa model was proposed and non-isothermal crystallization data of pure PP was applied. The results indicated that the crystallization processed with the same mechanismabove Tz under cooling rates of 1 ~ 80 °C/min and Avrami exponent is almost equal to 3, indicating sporadic nucleation and three-dimensional growth. However, the crystallization processed with instantaneous nucleation and three-dimensional growth above Tz under cooling rates of 0.2 ~ 0.5 °C/min. It is suggested the change of crystallization mechanism at Tz resulted from the impingement of spherulites. The result is disagreeing with the previous reports in which the crystallization mechanism varying with crystallization temperature was proposed.Furthermore, isothermal crystallization kinetics of PP-c was studied. Similar to pure PP, the isothermal crystallization kinetics of PP-c in early stage could be described by Avrami model and crystallization processed with instantaneous nucleation and three-dimensional growth. It was found that the overall crystallization rate depended remarkably on the content of EP content in PP-c. With the increase of EP content in the PP-c the crystallization half-time tl/2 decreased obviously. The spherulite growth rates of PP and PP-c were measured and they decreased with the increase of EP content in the PP-c. It is attributed to the higher viscosity of PP-c melt obstructing the movement of propylene chain. Through morphology observation, it is found that the nucleation density of PP-c is rather higher than pure PP. The higher overall crystallization rate of PP-c should be attributed to the nucleation effect of EP content.It is noting that the non-isothermal crystallization kinetics of PP-c couldn't be described by conventional Ozawa plotting. However, the crystallization mechanism of PP-c could be satisfactorily explained using new method proposed by us and the crystallization processed with instantaneous nucleation and three-dimensional growth. All crystallization onset temperature (To), crystallization end temperature (7S) and crystallization transition temperature (Tz) of PP-c were higher than pure PP. In addition, To, Ts and Tz of PP-c increased with the increase of the EP content. These results indicated the stronger nucleation ability of PP-c than pure PP. The Lauritzen-Hoffman analysis for spherulite growth rate data of PP and PP-c showed that there existed a transition of regime from II into III in PP, PEP20, PEP30 and PEP 40, where regime III means that the rate of lateral spreading is considerably slowerthan the nucleation rate and regime II means that the above two rates are comparable. However, the crystallization of PEP50 and PEP60 always processed in regime II and no transition of regime from II into III was observed. Both calculated K% and cre for PP-c increased with the increase of EP content indicated that the existence of EP copolymer was in favor of nucleation of PP-c melt and went against regularly folding of the molecule chain. Taking the higher viscosity of PP-c into account, we believed that the enhancement of viscosity played an important role in the shift of transition temperature of regime from II to III. The activation energy of non-isothermal crystallization for PP-c was evaluated through Friedman method. The resulting data showed that the activation energy of PP-c is higher than pure PP, indicating the higher crystallization ability of PP-c.