Preparation Of Long Chain Branched Polypropylene And Its Properties Study

Due to the poor melt strength, ordinary linear isotactic polypropylene(iPP) usually exhibits uneven container wall thickness when thermal forming, curled edges and contraction during extrusion coating or calendaring, and cell collapse in the process of extrusion foaming. The introduction of long chain branched(LCB) structures into ordinary linear PP can effectively solve these problems. Long chain branched polypropylene(LCBPP) has become more and more significant in many industrial applications. Here, one kind of LCBPP was fabricated via molten radical branching reaction, using a torque rheometer as reactor. The results are described as follow:(1) The influence of processing craft and material ratio on the structure and performance of modified PP was investigated by altering the reaction temperature, categories of initiators, monomers and radical regulating agents, as well as their dosages. The results indicated that higher temperature can promote crosslinking and degradation and hinder branching reaction, while too low temperature would make deficient melting of PP, the appropriate temperature was about 180℃. Initiators with different half-lives made diverse influences on the branching reaction of PP. Dicumyl peroxide(DCP) and 2,5-Dimethyl-2,5-di(tert-butylperoxy) hexane peroxide(DHBP) had a better synergy with the radical regulating agents, used to control the radical activity, thus the reactive process could be regulated more easily. Benzoyl peroxide(BPO) was unfit for the branching reaction of PP as an initiator for its short half-life. A high initiator concentration could lead to PP’s degradation, thus making high MFR and low melt strength. In our experiments, the best dosage of DCP and DHBP were 0.05 phr and 0.1phr, respectively. Dithiocarbamate with different substituent groups and central atoms made various regulating effects on the activity of radical in the system. Zinc dimethyldithiocarbamate(ZDMC) had a better regulating effect on thereaction owing to its smaller substituent group and higher activity, but its dosage must be controlled in a proper range to match with the corresponding initiator. Acrylate monomers with different functionality degrees can make diverse impacts on the branching frequency and the lengths of branching chains. The branching level of branching chains improved with the increase of functionality degrees, and the “multiplicity” became more obvious. The sag resistance can be enhanced by increasing the monomer dosage, while excess monomers would make more gels.(2) Depending upon the torque-time curve, some samples with different reaction times were chosen to analyze the influence of the reaction time on the structures and properties of modified PPs. The results showed that the architecture of PP samples gradually varied from a linear structure to branching structure as the reaction times went by, with apparent variations in rheological parameters such as storage modulus(G′), complex viscosity(η*), and loss angle(δ) at the same time. When the reaction time reached to the second torque peak, rheological performances of modified PP became the best: the terminal slope of G′ decreased from 1.38 to 0.44 and the value of η*ω=0.01rad·s-1 increased from 15039.8 Pa·s to 60338.4 Pa·s. In addition, a gel-like platform appeared in the δ-ω curve. The Cole-Cole plots of modified PPs deviated from the semicircle, indicating that the introduction of LCB could change the relaxation mechanism of PP. With the reaction progress, the architecture of PP gradually transformed from linear structure to branching structure, and on the time corresponding to the second torque peak, a kind of “sparse and long” branches can be formed and in the following reaction course, the degradation of PP backbone, as well as branching chains occurred, finally formed “dense and short” branches.(3) The performances in extrusion foaming and macroscopic properties of modified PP were investigated. The texting results showed that stronger die swelling in modified PP when compared with linear raw PP. The foamed samples of modified PP exhibited larger average width(51.78mm) and thickness(12.32mm), higher foaming ratio and lower density(0.084g·cm-3) than plain PP foamed specimen. In the process of foaming, no gas escaped from the modified PP surface and no ruptured cell was observed on the sample surface. The cell morphology displayed a regular shape offaveolate. In comparison with plain PP, the tensile strength, bending strength and flexural modulus of modified PP were improved 18.2%, 77.3% and 78.7%, respectively. Furthermore, a increase of 10℃ in vicat softening temperature was discovered. The DMA results revealed that the molecular movements of modified PP have a more sensitive dependence on temperature than linear PP, while nearly no change was observed in its glass transition temperature.