Mariano Pracella

Spherulite nucleation in blends of isotactic polypropylene with high-density polyethylene

Abstract Primary nucleation of spherulites in blends of isotactic polypropylene (iPP) with high-density polyethylene (HDPE) has been investigated by means of differential scanning calorimetry and optical microscopy. The number of iPP nuclei in blends crystallized isothermally at temperatures greater than 127°C decreases with increasing HDPE concentration to a much greater extent than follows from the decreasing amount of iPP, whereas in blends crystallized below 127°C this number increases strongly. The shapes of the spherulite size distributions indicate that athermal (heterogeneous) nucleation is disturbed. Experiments with nucleating agents demonstrate that heterogeneous nuclei migrate acrossinterphase boundaries from iPP melt to HDPE melt during mixing due to the difference in interfacial energy between the nuclei and the molten components of the blend. At temperatures below 127°C the HDPE crystals growing in the blend cause additional heterogeneous nucleation of iPP spherulites. This nucleating activity of HDPE crystals is responsible for the increase in nucleation density of the blend in spite of migration.

Compatibilization and properties of poly(ethylene terephthalate)/polyethylene blends based on recycled materials

The morphology and physical/mechanical properties of non-compatibilized and compatibilized blends of poly(ethylene terephthalate) (PET) and polyethylene (HDPE) obtained from virgin and post-consumer packaging materials were investigated. The blend compatibilization was carried out by melt-mixing in the presence of various polyolefins functionalized with reactive groups (HDPE-g-MA, EPR-g- MA, E-AA, E-GMA, SEBS-g-MA). The effect of the type and concentration of compatibilizer, as well as the mixing conditions on the phase morphology, crystallization behavior, chemical interactions and melt rheology of the blends was then examined by means of electron microscopy (SEM), scanning calorimetry (DSC), FTIR and NMR spectroscopy, and capillary rheometry. The results pointed out that ethy- lene-co-glycidyl methacrylate copolymer (E-GMA) displayed a higher compatibilizing efficiency giving rise to a neat improvement of phase dispersion and interfacial adhesion in the blends, as compared to the other compatibilizers examined. This was accounted for by a high reactivity of epoxy groups with the chain end-groups of PET during meltmixing and the effect of in situ formed graft copolymer on the interfacial properties (reduction of coalescence, interpenetration with homopolymer phases). Tensile mechanical tests showed that elongation at break of recycled PET/HDPE (75/25) blends was markedly enhanced upon addition of E- GMA amount lower than 5 wt.-%.

Characterization of scrap poly(ethylene terephthalate)

Abstract The goals of the investigation were: to indicate the methods of characterization of recycled polymers, to show general tendencies in properties deterioration and characterize recyclates on Central Europe and European Community markets. The properties and composition of scrap poly(ethylene terephthalate) from several sources were characterized by: TGA, DSC, FTIR, tensile properties, dynamic viscosity, intrinsic viscosity and thermo-oxidative stability. We found that all PET regrinds contained admixture of other polymers (0.1–5 wt%). The presence of more than 50 ppm PVC makes PET worthless for advanced application as film forming, because it catalyzes the hydrolysis and reduces the strength of material. Although the individual flakes of recycled PET show almost unchanged molecular characteristics and properties, the processed regrinds always exhibit worse properties. Partial restoration of recycled PET properties can be achieved by careful working, removing the dust fraction and by proper drying. The difference between studied PETs results from different applied recycling procedure. Admixtures of polymers without compatibilizer always deteriorate tensile properties. Various levels of stability of polymer viscous flow during film and tape extrusion were observed, depending on composition of recycled PET from various sources. Microgels were observed in all samples during film extrusion.

Spherulite nucleation in polypropylene blends with low density polyethylene

Abstract Primary nucleation of spherulites in blends of isotactic polypropylene (iPP) with low density polyethylene (LDPE) was investigated by means of differential scanning calorimetry and optical microscopy. The number of iPP spherulites in the blend decreases with increasing LDPE concentration to a much greater extent than follows from the decreasing amount of iPP. The shapes of spherulite size distributions indicate that athermal (heterogeneous) primary nucleation is inhibited. The density of primary nucleation in the blends decreases strongly with increasing mixing time. The same effect was observed in the blends with the nucleating agent which was added to iPP or LDPE. These experiments demonstrate that heterogenoeus nuclei migrate across interphase boundaries from the iPP melt to the LDPE melt during the mixing process. It is suggested that the interfacial energy difference between the nuclei and the molten components of the blend is responsible for the migration of nuclei.

Study of blends of nylon 6 with EVOH and carboxyl-modified EVOH and a preliminary approach to films for packaging applications

Blends of nylon 6 (Ny6) with ethylene-co-vinyl alcohol (EVOH) and EVOH modified with the introduction of carboxyl groups (EVOH–COOH) have been studied by Fourier transform infrared spectroscopy, scanning electron microscopy, differential scanning calorimetry, and dynamic-mechanical thermal analysis. The thermal and thermomechanical analyses of the blends show that the melting, crystallization, and relaxational behavior are affected by the blend composition and the presence of carboxyl groups on the EVOH chains. Nevertheless, microscopic and thermal investigations demonstrate the biphasic nature of the two-blend systems. Selective solvent extraction of the EVOH or EVOH–COOH phase in their blends and Fourier transform infrared analysis of the residual products indicates the occurrence of ionic linkages between the amino groups of the polyamide and the carboxyl groups of the modified EVOH, whereas specific interactions are evidenced for Ny6/EVOH blends. Tests performed on extruded Ny6/EVOH films show that the addition of EVOH effectively reduces the gas permeability of Nylon, whereas the addition of small amounts of EVOH–COOH helps to control and stabilize melt rheology.

Reactive compatibilization of polyolefin/PET blends by melt grafting with glycidyl methacrylate

The melt free radical grafting of glycidyl methacrylate (GMA) onto high-density polyethylene (HDPE) was carried out in Brabender internal mixer. The GMA content of the grafted HDPE (HDPE-g-GMA) was determined through FTIR by means of a calibration curve. The influence of reaction procedure, radical initiator concentration and addition of a co-monomer (styrene) on the grafting efficiency was examined. Blends of poly(ethylene terephthalate) (PET) with HDPE and HDPE-g-GMA (75/25 w/w) were prepared by melt mixing in internal mixer. The morphology of the blends was then analysed by SEM microscopy. PET/HDPE-g-GMA blends displayed improved phase dispersion and interfacial adhesion as compared to unfanctionalized PET/HDPE blend.

Thermal characterization of liquid crystal polyesters based on mesogenic aromatic triad units

Abstract The isothermal crystallization and melting processes have been investigated in liquid crystal polyesters based on mesogenic bis(4-carboxyphenyl) terephthalate units and flexible alkylene spacers. The kinetic and thermodynamic parameters for the nucleation and growth of crystals from the smectic phase are presented and discussed with reference to the polymer chain structure.

Synthesis of PP–LCP graft copolymers and their compatibilizing activity for PP/LCP blends

The aim of this work was the synthesis of new graft copolymers consisting of polypropylene (PP) backbones and liquid crystalline polymer (LCP) branches, to be used as compatibilizing agents for PP/LCP blends. The PP-g-LCP copolymers have been prepared by polycondensation of the monomers of a semiflexible liquid crystalline polyester (SBH 1 : 1 : 2), that is, sebacic acid (S), 4,4′-dihydroxybiphenyl (B), and 4-hydroxybenzoic acid (H) in the mole ratio of 1 : 1 : 2, carried out in the presence of appropriate amounts of a commercial acrylic-acid-functionalized polypropylene (PPAA). The polycondensation products, referred to as COPP50 and COPP70, having a calculated PPAA concentration of 50 and 70 wt %, respectively, have been fractionated with boiling toluene and xylene, and the soluble and insoluble fractions have been characterized by Fourier transform infrared and nuclear magnetic resonance spectroscopy, scanning electron microscopy (SEM), differential scanning calorimetry, and X-ray diffraction. All analytical characterizations have concordantly shown that the products are formed by intricate mixtures of unreacted PPAA and SBH together with PP-g-SBH copolymers of different composition. Exploratory experiments carried out by adding small amounts of COPP50 or COPP70 into binary mixtures of isotactic polypropylene (iPP) and SBH while blending have demonstrated that this practice leads to an appreciable improvement of the dispersion of the minor LCP phase, as well as to an increase of the crystallization rate of iPP.

Thermal stability of blends of nylon 6 with polyolefins that contain acrylic acid

The objective of the present work was to study the effect of interfacial interactions on the thermal stability of blends of Nylon 6 with: polypropylene (iPP), polypropylene grafted with 6% by weight acrylic acid (iPP-AA), and a random copolymer of ethylene and acrylic acid with 8% by weight acrylic acid (PE-AA). Thermogravimetrical analysis (TGA) was performed on all the samples and both qualitative and quantitative analysis were made in order to evaluate the thermal stability of the materials under investigation. The results indicate that when there are strong intermolecular interactions between the polymers to be mixed (and therefore strong interfacial activity), the thermal stability in the blends can be increased with respect to the values expected in view of a simple additive rule of mixing and synergistic effects can be produced.

Property tuning of poly(lactic acid)/cellulose bio-composites through blending with modified ethylene-vinyl acetate copolymer.

The effect of addition of an ethylene-vinyl acetate copolymer modified with glycidyl methacrylate (EVA-GMA) on the structure and properties of poly(lactic acid) (PLA) composites with cellulose micro fibres (CF) was investigated. Binary (PLA/CF) and ternary (PLA/EVA-GMA/CF) composites obtained by melt mixing in Brabender mixer were analysed by SEM, POM, WAXS, DSC, TGA and tensile tests. The miscibility and morphology of PLA/EVA-GMA blends were first examined as a function of composition: a large rise of PLA spherulite growth rate in the blends was discovered with increasing the EVA-GMA content (0-30 wt%) in the isothermal crystallization both from the melt and the solid state. PLA/EVA-GMA/CF ternary composites displayed improved adhesion and dispersion of fibres into the matrix as compared to PLA/CF system. Marked changes of thermodynamic and tensile parameters, as elastic modulus, strength and elongation at break were observed for the composites, depending on blend composition, polymer miscibility and fibre-matrix chemical interactions at the interface.