Romeu A. Pereira

Thermal behavior of gamma-irradiated recycled polyethylene blends

The interest on recycled materials from post-consumer polymers present in discarded commercial packaging has gained increasing attention. The development of new engineering materials based on degraded polymers is an interesting possibility. Blends of low cost plastics such as polyolefins, polystyrene, poly(vinyl chloride), discarded in urban waste are of particular interest. The Instituto de Macromoleculas had developed a two-step process for recovering plastic residues which allows to obtain materials with good properties. One of these materials may be used as plastic lumber and consists of a recycled 75/25 LDPE/HDPE partially degraded blend. The exposure of this blend to low doses of 60Co gamma radiation improves its mechanical properties due to partial crosslinking. Differential scanning calorimetry, gel permeation chromatography, infrared spectroscopy and X-ray diffraction have been used to investigate the effects of gamma irradiation on the thermal behavior of polymer material. The melting and crystallization temperatures were found to decrease as the radiation dose increases, showing a tendency to stabilization at higher values. The correlation of the degradation process, melting heat and molecular weight distribution is discussed.

Morphology and thermal properties of polyethylenes made by metallocene and Ziegler-Natta catalysts

Abstract Morphology and thermal behavior of high density polyethylene and ethylene-octene copolymers prepared with classical Ziegler-Natta (zHDPE) and metallocene catalysts (mHDPE and mLLDPE) are comparatively discussed. Wide angle X-ray diffraction (WAXD) data at room temperature showed that mHDPE crystals are more perfect with smaller unit cell dimensions and have higher crystalline density and larger crystallite sizes when compared to zHDPEs. In the case of copolymers, the unit cell dimensions did not change significatively when the number of branches was increased. Average interplanar chain distances in the amorphous phase increased with increasing octene content in the copolymer, showing a marked influence on the packing of the amorphous phase and confirming that the branches are concentrated in this phase. The existence of second phase in the ethylene-octene copolymers prepared with metallocenes was identified by calorimetry. The existence of this phase is time-dependent and was interpreted as the result of hexyl branches ordering in the amorphous phase. A morphological model based on Vile three-phase model, WAXD results and calculations involving empirical equations for fold and extended chain lamellae crystals was proposed to explain the time-dependent thermal behavior of these copolymers.

Recycled car bumpers' impact resistance investigated by wide‐angle X‐ray diffraction

The recycled polyolefinic product from discarded standard car bumpers collected from Rio de Janeiro suburb shops was characterized by mechanical and wide-angle X-ray scattering (WAXS) methods. We found that the recycled plastic mixture is composed mainly of polypropylene (PP), containing ethylene-propylene-diene (EPDM) terpolymers and a minor proportion of high-density polyethylene (HDPE), and is highly resistant to impact. The results were compared with the corresponding data obtained from binary and ternary blends of virgin PP, EPDM, and HDPE.

Thermal Behavior of Poly(ethylene terephthalate) Crystalline and Amorphous Phases by Wide Angle X‐ray Scattering

Structural changes in the crystalline and amorphous phases of semicrystalline, compression molded, isotropic poly(ethylene terephthalate (PET) samples obtained from bottle grade resin, when it was heated from room temperature to temperatures near the crystalline melting temperature, were investigated by wide angle X‐ray scattering (WAXS). Semicrystalline PET samples were obtained by thermally treating initially amorphous samples at 120°C and 200°C for 24 h, and WAXS curves were taken in intervals of approximately 20°C up to a temperature near Tm. For each temperature, crystallographic data were obtained by using Winplotr and Celref programs to perform fittings of the WAXS curves and to calculate related parameters like unit cell dimensions, density, and volume, as well as crystallite sizes in directions, degree of crystallinity (X c), and distance between chains in the amorphous phase. The crystallite sizes in the and directions slightly increased, indicating a growth in the crystal lateral dimensions, while in the direction practically no variation was observed. The unit cell volume had a discrete increase due to thermal expansion, and its density decreased. The X c had a significant decrease, changing from 34% to 14% in the sample treated at 120°C, and from 34% to 21% in the sample treated at 200°C. The X c decrease with increasing temperature was accomplished by the increase in the intensity and symmetry of reflections and crystal rearrangement due to melting/recrystallization processes of crystals formed at lower annealing temperatures. The increase in temperature leads to variation in the two amorphous halos between 0 and 35°. While the distance between chains remains constant and the area increases for the first halo (located at approximately 17.5°), the distance between chains and the area of the second halo (which is located at about 24.5°) decreases. This second halo was associated with paracrystallinity.

Effect of di-n-butyl ether on the synthesis of Ziegler–Natta catalyst for propylene polymerization

Abstract The influence of di-n-butyl ether (DBE) on the characteristics of Ziegler–Natta catalysts evaluated in propylene polymerization was investigated. The catalysts were synthesized by the reduction of TiCl 4 in toluene solution with AlEt 3 · DBE complex at various DBE/TiCl 4 mole ratios. The effect of DBE on the catalyst crystalline forms, determined by X-ray diffraction, was examined. The results show that a suitable amount of DBE leads to the formation of δ -TiCl 3 with high activity and enhanced stereoselectivity.

WAXS study on orthorhombic crystalline phases in hot-drawn high density polyethylene

Abstract The temperature dependence in the range 23–135°C of hot-drawn HDPE plates maintained free from contraction was investigated by WAXS and DSC. A specially designed cylindrical chamber was used for the WAXS patterns. The results confirm the coexistence of two populations of orthorhombic crystals which have different lattice and thermal behaviour. The crystal lattice parameter a shows expansion, while b exhibits expansion up to 90°C, followed by contraction as the temperature is raised to melting. On the other hand, the c parameter is practically constant.

Orthorhombic-hexagonal Phase Transition in High Density Polyethylene Crystals

Abstract The crystal structure of drawn samples of high density polyethylene was investigated by using wide-angle X-ray scattering (WAXS) with CoKα and MoKa radiations. HDPE films with fiber-like morphology and constrained to a constant length on heating were studied in the range from room temperature to 158[ddot]C. Orthorhombic-hexagonal phase transition, which was previously reported only for UHMWPE fibers, was also proved to exist in moderate molecular weight HDPE drawn films. Differently from the orthorhombic phase, which melts at 129[ddot]C, the hexagonal phase formed melts at 158[ddot]C. A new hexagonal reflection with interplanar distance 2.139(7) Å, indexed as (101), is reported.

Characterization of Ziegler-Natta catalysts based on TiCl3 synthesized by different methods*

SummaryCatalysts based on TiCl3, modified by ethers, were prepared by different methods. Using di-n-butyl ether (DBE) as an internal base, complexes were formed in one-step reaction, either with TiCl4 and diethylaluminum chloride (DEAC) in iso-octane solution (System A) or with triethylaluminum (TEA) in toluene solution (System B). Diisoamyl ether (DIAE) and tetrahydrofuran (THF) were used as internal bases for the β-TiCl3 treatment during a three-step synthesis (System C). DIAE was also complexed with DEAC in a one-step synthesis in toluene solution (System D) or in iso-octane solution (System E). While the use of DIAE in the three-step synthesis has mainly led to the formation of γ-TiCl3 (System C), the use of DIAE in the one-step synthesis led to a highly active and stereospecific β-TiCl3 (Systems D and E). The type of solvent employed in systems A and B had a strong influence on the properties of the catalyst. Possible causes for the formation of different crystalline structures of TiCl3 are discussed by comparing the different routes of the TiCl3 synthesis.

Highly active and stereospecific catalyst based on β-TiCl3 for propylene polymerization

Abstract A method was developed to prepare a crystalline structure of β-TiCl 3 (brown) using an electron-donor compound as an internal Lewis base, namely, diisoamyl ether (DIAE). β-TiCl 3 combined with triethylaluminium (TEA) resulted in highly active and stereospecific catalyst systems close to those based on violet TiCl 3 (α-, γ- or δ-TiCl 3 ) for propylene polymerization. The influence of solvent and amount of Lewis base employed in the catalyst synthesis on the catalyst properties and structure was investigated. The catalyst structure was examined by X-ray diffraction.

Influence of the method of synthesis on the properties of TiCl3

Abstract The role of ethers in the synthesis of a highly active and stereospecific catalyst for propylene polymerization has been investigated. Diisoamyl ether (DIAE) and tetrahydrofuran (THF) were employed as electron-donors in the β-TiCl 3 treatment during the three-step synthesis. Dibutyl ether (DBE) was used as a first internal base complexed with TiCl 4 and Et 2 ClAl (DEAC) in iso-octane solution in the one-step synthesis. While the use of DIAE and THF in the three-step synthesis has mainly led to the formation of λ-TiCl 3 , the use of DBE in the one-step synthesis has led to the formation of a highly active and stereospecific δ-TiCl 3 . The effects of ethyl benzoate, tributyl phosphate and diisobutyl phthalate as second internal bases on the crystalline modification, activity and stereospecificity of TiCl 3 have also been studied. Possible causes for the formation of different TiCl 3 crystalline structures are discussed through comparison of both routes of TiCl 3 synthesis.