Rosestela Perera

Polypropylene/wood flour composites: treatments and properties

In this research, the variations produced on the mechanical, morphological and thermal properties and on the melt index (MI) of a composite of polypropylene and wood flour (WF) by the modification of the filler were studied. The filler was treated with sodium hydroxide at different immersion times and with vinil-tris-(2-metoxietoxi)-silane. Polypropylenes functionalized with maleic anhydride (maleated polypropylene, MAPP) were also used as compatibilizers. All the treatments showed the same tendency to slightly increase tensile modulus and tensile strength of the composites, but they did not affect their MFI. Morphological studies showed that the MAPP and silane improve the polymer–WF adhesion and the dispersion of the particles, while the alkaline treatment only improves the dispersion. The silane-modified samples and the use of MAPP produced samples with lower water absorption than those of untreated WF composites. The addition of the filler and the treatments carried out on it caused an increase in the crystallization temperature.

Grafting of polyethylenes by reactive extrusion. I. Influence on the molecular structure

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 twin-screw 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.

Comparative analysis of reactive extrusion of LDPE and LLDPE

Abstract Grafting of polyethylenes is an important method used in the modification of polyolefins with functional groups. Nevertheless, few systematic studies have been conducted regarding this type of reactions and the interdependent variables are not yet accurately established. In this investigation, the grafting of two commercial polyethylenes, low-density polyethylene and linear low-density polyethylene with diethyl maleate was carried out in a corotating twin-screw extruder at different extrusion conditions. In order to establish the interdependence of the extrusion conditions employed, a combined experimental and simplified one-dimensional, non-isothermal model was implemented in the study of flow in the elements of the screw-extruder. The effects of the grafting degree and the processing conditions employed in the preparation of the functionalized materials on their shear and elongational viscosities at high shear rates, and on their thermal and tensile properties were studied.

Compatibilization of Polyolefin/Hydroxyapatite Composites Using Grafted Polymers

The objective of the present work is to evaluate the compatibilization of composites of polyolefins (high-density polyethylene and polypropylene) with 30 wt.% hydroxyapatite of nanoscale particle size, employing grafted polymers to improve the interaction between phases and hence, the composites properties. The functionalization of a high-density polyethylene and a polypropylene was performed in solution, employing acrylic acid as the grafting monomer and dicumyl peroxide as the initiator. The grafting degree was determined through volumetric titration. The nanoscale hydroxyapatite was synthesized through a precipitation method using calcium hydroxide and ammonium phosphate. Polyolefin composites were melt-blended in an internal mixer, varying the amount of compatibilizing agent at 4, 6, 8, and 10 phr. The grafting degrees obtained were 0.72 and 0.38 wt% for the high-density polyethylene and polypropylene, respectively. On the other hand, the melt flow rate values of the composites slightly decreased as the grafted polymer contents increased. The crystallinity degree also decreased in the presence of grafted polymers. The incorporation of the compatibilizing agent favored the dispersion and distribution of the hydroxyapatite particles. The Youngs modulus and the tensile stress slightly increased in all composites. The values of activation energy decreased with the addition of the grafted polymers, which is attributed to the less thermally stable functionalized polymer, which is prone to thermal degradation at relatively lower temperatures due to the presence of a large content of carboxylic groups. Nonetheless, the activation energy increased with the amount of functionalized polymer incorporated into the composites due to the interaction between components at the polymer/filler interface.

Optimal Conditioning for the Preparation of HDPE-HA Composites in an Internal Mixer

ABSTRACT In the last years research and development of new materials and methods for bone repairing and substitution has been a priority in the biomedicine field. HDPE/Hydroxyapatite (HA) composites have been previously used successfully for such purposes. In this study, HDPE/HA composites prepared in an internal mixer were characterized. Mixing conditions, such as temperature and mixing rate, were optimized. Results showed that high mixing rates and lower temperatures improved the filler dispersion in the polymeric matrix. At optimal conditions composites with 10, 20, 30 pph of HA were prepared and their rheological, mechanical, thermal and thermodegradative properties were determined to establish the optimum filler content. Inclusion of HA particles decreased the melt flow index. The incorporation of HA also increased the Youngs Modulus and strength at break and decreased the elongation at break due to elasticity loss. Thermal properties and initial degradation temperature were not affected with the inclusion of the filler but an enhancement in the activation energy was observed. A titanate coupling agent (Lica 12) was added to composites to improve the interfacial interactions. Lica 12 did not produce any variation in the mechanical and rheological behavior of composites. A slight decrease in composite crystallinity was detected for composites coupled with 0.5, 0.7 and 1.0% Lica 12. The thermal stability increased with the addition of the coupling agent.

CHEMICAL MODIFICATION OF STYRENE/ETHYLENE-BUTHYLENE/STYRENE COPOLYMERS

The chemical modification of two thermoplastic elastomer styrene/ethylene-buthylene/styrene copolymers (SEBS) with diethylmaleate (DEM) as functionalizing agent was carried out in a corotating twin screw extruder and in an internal mixer. The residence time distribution of the extruder was studied due to its great influence on the grafting chemical reaction, the degree of functionalization, and the viscoelastic properties of the obtained modified polymers. The influence of copolymer melt viscosities, and DEM and initiator concentration ratio on the grafting degrees was studied as well. In this case, an internal mixer was used. Optimal processing conditions were established to obtain grafted polymers characterized by good processability and an absence of crosslinking. The grafting degree achieved in the internal mixer was the same for both copolymers with different viscosities and it increased as the DEM and initiator concentrations and concentration ratios were increased.

Thermal and Tensile Properties of PP Nanocomposite Blends

Polypropylene/polyamide-6 and polypropylene/metallocene polyethylene blends containing 2.5 phr of organophilic modified montmorillonite were prepared in a twin-screw extruder followed by injection molding. In order to compare, blends without layered clay were also made. Styreneethylene- butylene-styrene copolymer and polypropylene grafted with anhydride maleic were used as compatibilizers in the ternary blends and in the PP nanocomposite preparation, respectively. The presence of tactoids, intercalated and exfoliated structures was observed by TEM in some of the samples containing layered clay and modified PP materials. Results showed that the compatibilized blends prepared without clay are tougher than those prepared with the nanocomposite of PP as the matrix phase and no significant changes in tensile moduli were observed between them. However, the binary blend with a nanocomposite of PP as matrix and metallocene polyethylene exhibited better tensile toughness and lower tensile modulus, than those prepared with a nanocomposite of PP and polyamide-6 as dispersed phase. These results are related to the degree of clay dispersion in the PP and to the type of morphology developed in the different blends. Differential scanning calorimetry (DSC) showed that blends with a finer and homogeneously dispersed morphology determined by SEM, the PA component exhibited fractionated crystallization exotherms in the temperature range of 159-185°C. Also, nucleation of the PP component by PA phase and/or the layered clay was observed in the blends with PA as dispersed phase.