Ultrasonic decrosslinking of crosslinked high‐density polyethylene: Effect of screw design

The effect of screw design on decrosslinking of the crosslinked high‐density polyethylene (XHDPE) by means of ultrasonic twin‐screw extruder with two screw configurations is investigated. Die pressure and ultrasonic power consumption during extrusion are recorded. Swelling characteristics, rheological properties, thermal analysis, scanning electron microscopy, and tensile properties are used to investigate the structure–property relationship of decrosslinked XHDPE. It is found that the screw configuration with conveying elements and reverse conveying elements (decrosslinking screws) is an effective means to reduce the gel fraction and crosslink density of decrosslinked XHDPE and significantly improve its processibility. Rheological properties of decrosslinked XHDPE are correlated with structural changes occurring during ultrasonic decrosslinking. The presence of the highly branched sol in decrosslinked XHDPE is revealed through measurements of the activation energy for flow. Comparison of morphologies of the lamellar structure of HDPE, XHDPE, and decrosslinked XHDPE reveals that the presence of the crosslink network inhibits the lamella growth. Significant improvements in the mechanical performance of decrosslinked XHDPE are obtained by using decrosslinking screws. The molecular structure and morphology of the lamellar structure of decrosslinked XHDPE are used to explain the processing–solid‐state property relationship. The measured results on the gel fraction and crosslink density are compared with those of numerical simulations. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40680.     

Role of charged defects in the photoconductivity of Se95As5 chalcogenide glassy semiconductor with the EuF3 impurity

The energy spectrum of local states associated with charged defects D ^? and D ⁺ playing a significant role in carrier generation and recombination in the chalcogenide glassy semiconductor system Se₉₅As₅ containing EuF₃ impurities is proposed based on the study of the temperature dependence of the dark conductivity and steady-state photoconductivity, current-luminance characteristic, and the spectral distribution of the photocurrent. It is shown that EuF₃ impurities nonmonotonically change the concentrations of these states. Low concentrations form chemical compounds with selenium and arsenic due to the chemical activity of the rare-earth element and fluorine ions, which result in a decrease in the concentration of initial intrinsic defects. High concentrations, according to the charged-defect model, lead to a decrease in the concentration of D ⁺ centers and an increase in the concentration of D ^? centers due to the presence of Eu³⁺ ions. Some parameters of the charged-defect model are estimated, in particular the effective correlation energy U _eff (0.6 eV) and the polaron relaxation energy (W ⁺ = 0.4 eV, W ^? = 0.45 eV).     

Role of samarium atoms in the formation of the structure of As-Se-S chalcogenide vitreous semiconductors

The Raman spectra of films of samarium-doped As-Se-S three-component chalcogenide vitreous semiconductors are studied. It is shown that the spectra are constituted by four bands in the frequency ranges 10–100, 195–290, 290–404, and 420–507 cm^?1, which strongly vary with doping. The structural units in As-Se-S and their possible changes upon doping are determined. The results obtained are accounted for by the specific distribution of samarium atoms in the sample and by their chemical activity.     

Novel ultrasonic process for in‐situ copolymer formation and compatibilization of immiscible polymers

In‐situ copolymer formation and compatibilization of immiscible polymers in blends were discovered to occur during ultrasonic‐assisted extrusion in the melt state under high pressures. Residence times were of the order of seconds. The mechanical performance of plastic/rubber and rubber/rubber blends subjected to ultrasonic treatment was significantly enhanced compared to those of identical blends not subjected to ultrasonic treatment and having similar phase morphologies. The appearance of a high molecular weight tail in the rubber/rubber blends, stabilization of phase morphology in the melt state, the presence of a transition interface nanolayer and a decrease in the amount of extractable components in the plastic/rubber blends after ultrasonic‐assisted extrusion point towards the occurrence of in‐situ copolymer formation at the interfaces.     

Effects of the presence of water on ultrasonic devulcanization of polydimethylsiloxane

The effects of the presence of water on ultrasonic devulcanization of 30 phr silica‐filled polydimethylsiloxane (PMDS) were investigated at increasing feed rates and different die gap sizes. The results showed that the initial die entrance pressure without ultrasound for wet rubber was higher than in the case without water and then it decreased monotonously with applying ultrasound. The die pressure for wet rubber decreased significantly even at low ultrasonic amplitude, while that for dry rubber changed little at low amplitude. The power consumption at an amplitude of 10 μm, where devulcanization was most effectively achieved, was lower for wet rubber even though the pressure was lower. The crosslink density and gel fraction after the devulcanization of wet rubber were lower than those of dry rubber, indicating that the presence of water facilitates the devulcanization process under the same devulcanization conditions. The good mechanical properties of recycled silica‐filled PDMS were obtained at higher feed rates and at lower ultrasound amplitudes, which are directly related to the economics of a recycling process. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2630–2638, 2003     

Interface evolution and penetration behavior during two‐component transfer molding. Part II: Simulation and experiment

Simulation and experimental study of the pressure‐controlled sequential sandwich transfer molding of two SBR rubber compounds under isothermal condition have been carried out to obtain a two‐layered sandwich structure. One SBR compound, which is intended for the skin material, is first laid up in the cavity. Then, another SBR compound, intended for the core material, is transferred to penetrate into the skin material and to push the lay‐up to fully fill the cavity, resulting in an encapsulated skin/core sandwich structure. Two cases involving different material combinations with different viscosity ratios have been studied. The rheological interaction of the skin/core components and its effect on the penetration behavior and interface shape have been investigated. The influence of processing conditions, such as the volume fraction transferred and pressure, is discussed. The penetration and encapsulation behavior, and the interface development are found to be significantly affected by the rheological properties of the compounds and the volume fraction transferred. However, at a constant volume fraction transferred, the pressure imposed during transfer molding is found to have a little effect on the interface development. These experimental findings are in good agreement with the present predictions based on the model and simulation proposed in Part I of this study. Polym. Eng. Sci. 44:697–713, 2004. © 2004 Society of Plastics Engineers.     

Improved properties of blends of ultrasonically treated unfilled polyurethane rubber

The aim of this investigation is to improve the properties of the polyurethane rubber that has been devulcanized using the coaxial ultrasonic reactor. After devulcanization of the cured rubber under varying gap sizes and amplitudes, blends of the devulcanized and virgin rubber at different ratios are prepared. Rheological and mechanical properties, hardness as well as gel fraction and crosslink density of the vulcanized blends have been investigated. Results show that compared to the vulcanizates of the devulcanized samples, the vulcanizates of the blends show a remarkable improvement in properties, which are sometimes comparable to or even better than the virgin vulcanizate. Blends have also prepared using different proportions of ground samples and virgin rubber, and a comparison of properties between these blends and the blends of the devulcanized rubber has been carried out. The results show that compared to the ground samples, the blends of the devulcanized samples are easier to mix and exhibit enhanced and more uniform properties. Polym. Eng. Sci. 44:794–804, 2004. © 2004 Society of Plastics Engineers.     

Recycling of unfilled polyurethane rubber using high‐power ultrasound

This investigation deals with the recycling of polyurethane rubber by the application of high‐power ultrasound in a continuous ultrasonic coaxial reactor. The cured rubber has been devulcanized at various feed rates and various gap sizes and then revulcanized again with certain adjustments in the curing recipe. The die pressure and the total power consumption have been recorded as a function of the processing conditions. The rheological and mechanical properties, hardness, gel fraction, and crosslink density of the original, devulcanized, and revulcanized samples have been measured and compared in an attempt to determine the optimum condition for devulcanization. Gel permeation chromatography (GPC) has been carried out with the sol part of the devulcanized samples to study the devulcanization and degradation. The results show that at low flow rates and narrow gaps, the material is degraded very quickly and, therefore, exhibits very poor mechanical properties. However, increasing the feed rate results in an improvement of the mechanical properties. Measured values of the crosslink densities and gel fractions indicate the processing conditions under which greater devulcanization and degradation of the samples take place. The lower molecular weights of the sol, extracted from the devulcanized samples, obtained in the GPC experiments in comparison with polyurethane gum indicate a breakdown of the polymeric chains as a result of devulcanization. The devulcanized samples show a higher activation energy of viscous flow, possibly because of the formation of branched structures. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 980–989, 2003     

Effect of ultrasonic extrusion of star styrene-butadiene rubber on properties of carbon black- and silica-filled compounds and vulcanizates

Extrusion of star styrene-butadiene rubber (SBR) without and with ultrasonic treatment at amplitudes 3.5, 5, 7.5, and 10 μm was carried out. The molecular structure of untreated and treated star SBR was determined. Significant reduction of die pressure was observed during ultrasonic treatment due to the thixotropic and degradation effects. Ultrasonic treatment of star SBR at 3.5 μm created molecules of higher molecular weight via long-chain branching without gel formation. Ultrasonic treatment of star SBR at 5 μm created a small amount of gel. At high ultrasonic amplitudes more gel was generated hindering mixing of star SBR with silica. Extruded star SBR was compounded with carbon black and precipitated silica, with and without silane. It was found that the long-chain branching induced by ultrasonic treatment improved the rubber–filler interaction in precipitated silica without silane, as confirmed by the increase of bound rubber content. The filler–filler interaction was reduced in silica compounds without silane, as indicated by study of Payne effect. The significantly improved rubber–filler interaction and reduced filler–filler interaction led to an increase of the modulus at 100% elongation and tensile strength of SBR/silica vulcanizates. Extensive comparisons were made with earlier study on ultrasonic treatment of linear SBR. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47451.