ABSTRACT:Grafting of polyethylenes is an important method used in the modification of polyolefins with functional groups. Lately, the use of extruders as polymerization reactors has increased considerably. However, knowledge of the details of the reaction in the extruder is still limited. In this investigation, the grafting of various commercial polyethylenes, high-density polyethylenes (HDPEs), and linear low-density polyethylenes (LLDPEs) with diethyl maleate (DEM) was carried out in two corotating twinscrew extruders with different screw configurations and extrusion conditions. Two initiators at different concentrations were used. It was found that when the initiator level was raised in the LLDPEs the grafting degree increased and the molecular weight distribution of the grafted LLDPE2 did not show appreciable differences when they were compared to the virgin resin. On the other hand, the terminal vinyl group concentrations decreased at the expense of increasing the trans unsaturation concentrations. This last result is consistent with the formation of long-chain branching. Additionally, the weight-average molecular weight of grafted high-density polyethylene (HDPE1-g-DEM) decreased. The grafting efficiencies were consistent with the attained residence times and also with the kinetics of the decomposition of the peroxides.

In the present work, the effectiveness of using a styrene/ ethylene-butylene/styrene rubber grafted with maleic anhydride (MA) as toughening material in ternary blends with two polyamide-6 nanocomposites or PP with different molecular weight as matrix phases was evaluated in terms of SEM and TEM morphology, WAXD, capillary rheometry, and tensile properties determination. Two metallocene polyethylenes and two polypropylenes were used as the dispersed phase in these ternary blends, and the effect of the comonomer contents in the former was also evaluated. Results showed that the blends prepared with metallocene polyethylenes are tougher than those prepared with PP as the dispersed phase and no significant changes in the Young's modulus or tensile strength were observed when changing the type of metallocene polyethylenes used. When a blend of SEBS/SEBS-g-MA instead of SEBS-g-MA alone was used as a compatibilizing agent in the ternary PA-6 nanocomposite blends, a major decrease in the Young's modulus and tensile strength and a higher toughness were observed. The formation of PA-OMMT particle clusters seems to improve the toughness/stiffness balance. These results are related to the degree of clay dispersion in the polyamide and to the type of morphology developed in the different blends. POLYM. ENG. SCI., 46:1111-1120, 2006.

The preparation of polypropylene (PP) nanocomposites was studied using clay and three types of modified PP (m-PP) as compatibilizers: diethyl maleate grafted PP (PP-g-DEM), maleic anhydride grafted PP (PP-g-MA), and PP grafted with carbamyl maleamic acid (PP-g-UMA). The clay was made organophylic by an acid treatment with octadecylamine. PP functionalization and blending were carried out in an internal mixer. Blends of PP containing 20 and 40 wt% each of the modified PP and 5 wt% of organophilic clay (IMt), in each case, were prepared. Samples were characterized by transmission electron microscopy (TEM), differential scanning calorimetry (DSC), optical microscopy, and mechanical testing. The presence of tactoid, intercalated and exfoliated structures was observed by TEM in all the samples containing clay and modified PP, which also showed improved mechanical properties with tensile modulus as much as three times that of PP. Melting temperature did not vary significantly with the addition of clay. However, because of the clay's nucleating effect, an increase in the crystallization temperature was observed, accompanied by a slight decrease in the degree of crystallinity. The best results were obtained when PP-g-MA was used as the compatibilizer; intermediate results were obtained with the use of PP-g-UMA, followed by the results obtained when PP-g-DEM was used. Property enhancements were obtained when a higher percentage of modified PP was employed. POLYM. COMPOS., 27:451-460, 2006.

In this work we present a n experimental study of shear and apparent elongational behavior of linear low-density (LLDPE) and low-density (LDPE) polyethylene blends by means of capillary rheometry. The characterization of these rheological properties is crucial in the design of a blend that combines the ease of processing of LDPE with the mechanical advantages of the LLDPE. TWO different low-density polyethylenes and one common linear low-density polyethylene were used to prepare the blends. The results obtained indicate a strong sensitivity of the rheology of the blend to changes in the molecular weight of the LDPE employed. For the higher molecular weight LDPE, the shear viscosity of the blend was essentially equal to that of the LDPE homopolymer up to a concentration of 25% of LLDPE, whereas the apparent extensional viscosity was appreciably lower. For the lower molecular weight LDPE, the same trend was obtained regarding the shear viscosity, but in this case the apparent extensional viscosity of the blend was somewhat higher than that of the LDPE homopolymer.

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Grafting of preformed polymers is an important method for preparing polymers with functional groups. Lately, the use of extruders as polymerization reactors has increased considerably in industry. However, the knowledge of the total reaction process is still limited. Grafting of diethylmaleate on linear low‐density polyethylenes (LLDPEs) was carried out in solution and in a corotating twin screw extruder. The effects of initiator and diethylmaleate concentrations, temperature, and reaction time on graft content and on crosslinking were investigated. The functionalization reaction was also conducted in the presence of an electron donor material to minimize the amount of crosslinked products in the extruder. The grafted products were characterized by means of FTIR and the thermal behavior of LLDPEs and that of its grafted products was determined.

Poly(lactic acid) (PLA) nanocomposite ternary blends based on unmodified sepiolite were prepared by melt blending using a corotating twin-screw extruder. Two grafted polymers were used as compatibilizer agents, in an effort to increase the PLA tensile toughness. The influence of incorporating a low-cost commodity low-density polyethylene, as dispersed phase to the composites on thermal degradation, and rheological and tensile properties was studied. The morphology of the blends and composites was determined through transmission and scanning electron microscopy techniques. Results showed that the compatibilized blends prepared without clay have higher thermal degradation susceptibility and tensile toughness than those prepared with sepiolite and significant changes in complex viscosity and melt elasticity values were observed between them. The nanocomposite blends exhibited similar thermal degradation, lower tensile strength, and Young's modulus values and increased elongation at break and tensile toughness, complex viscosity, and storage modulus compared with those of the nanocomposite of PLA. These results are related to the clay dispersion, to the type of morphology of the different blends, to the localization of the sepiolite in the different phases, the thermomechanical degradation of the PLA matrix phase during melt blending and the grafting degree of the compatibilizers used. POLYM. ENG. SCI., 52:988-