Studies on Rheological Behavior and Structure Development of High‐Density Polyethylene in the Presence of Ultrasonic Oscillations During Extrusion

The effects of ultrasonic oscillations on properties and structure of extruded high‐density polyethylene (HDPE) were studied. The experimental results show that ultrasonic oscillations can improve the surface appearance of the HDPE extrudates; increase the productivity of the HDPE extrudates; and decrease the die pressure, melt viscosity, and flow activation energy of the HDPE. The processing properties of the HDPE improve greatly in the presence of ultrasonic oscillations. Linear viscoelastic properties tests show that dynamic shear viscosity and zero shear viscosity decrease in the presence of ultrasonic oscillations. Ultrasonic oscillations can improve crystal perfection and thermal stability of HDPE. At appropriate ultrasound intensity, ultrasonic oscillations could also increase the mechanical strength of extruded HDPE. The gel permeation chromatography (GPC) results show that at high ultrasound intensity and low rotation speed of extrusion, ultrasonic oscillations causes chain scission of HDPE, which result in a decrease of molecular weight and an increase of melt flow index.     

MODELING OF THE INDUSTRIAL PROCESS OF PEROXIDE-INITIATED POLYPROPYLENE (HOMOPOLYMERS) CONTROLLED DEGRADATION

The degradation of polypropylene (PP) by means of peroxides in the extruder is a common procedure for increasing the number of product grades in the PP industry. We provide a semiempirical equation for the kinetics of the degradation process of homopolymers; the equation relates the change in melt flow index with the amount of peroxide employed, which is of value for the laboratory and the industrial scales. Properties of the degraded products, such as molecular weight, molecular weight distribution, flexural modulus, and impact energy, also were correlated with the melt flow index of the original product and with its level of degradation making use of empirical equations. These equations can predict the properties of the homopolymer PP degraded with peroxide within a range of good enough accuracy.     

Ultrasonic degradation of butadiene, styrene and their copolymers

Ultrasonic degradation of commercially important polymers, styrene-butadiene (SBR) rubber, acrylonitrile-butadiene (NBR) rubber, styrene-acrylonitrile (SAN), polybutadiene rubber and polystyrene were investigated. The molecular weight distributions were measured using gel permeation chromatography (GPC). A model based on continuous distribution kinetics approach was used to study the time evolution of molecular weight distribution for these polymers during degradation. The effect of solvent properties and ultrasound intensity on the degradation of SBR rubber was investigated using different pure solvents and mixed solvents of varying volatility and different ultrasonic intensities.     

Ultrasonic effects on the degradation kinetics,preliminary characterization and antioxidant activities of polysaccharides from Phellinus linteus mycelia

In this study, a high-molecular-weight polysaccharide PL-N isolated from the alkaline extract of Phellinus linteus mycelia was degraded by ultrasound. Results showed that ultrasound treatment at different ultrasonic intensities decreased the intrinsic viscosity and molecular weight of PL-N, as well as narrowed the molecular weight distribution. A larger reduction in intrinsic viscosity and molecular weight was caused by a higher ultrasonic intensity. The degradation kinetics model was fitted to (1/M_t − 1/M₀) = k·t, and the reaction rate constant (k) increased with increasing ultrasonic intensity. Ultrasound degradation did not change the primary structure of PL-N, and scanning electron microscopy analysis indicated that the morphology of the original PL-N was different from that of degraded PL-N fractions. Antioxidant activity assays in vitro indicated that the degraded PL-N fraction with low molecular weight had stronger hydroxyl radical scavenging capacity and higher TEAC and FRAP values.     

Power density modulated ultrasonic degradation of perfluoroalkyl substances with and without sparging Argon

The power density modulates the dynamics of the chemical reactions during the ultrasonic breakdown of organic compounds. We evaluated the ultrasonic degradation of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) at various power densities (30 W/L–262 W/L) with and without sparging Argon. We observed pseudo-first-order degradation kinetics at an initial PFASs concentration of 100 nM over a range of power density. The rate kinetics of degradation shows a non-linear increase with an increase in power density. We proposed a four-parameter logistic regression (4PLR) equation that empirically fits the degradation rate kinetics with the power density. The 4PLR equation predicts that the maximum achievable half-life of PFOA and PFOS sonochemical degradation are 1 and 10 min under a given set of experimental conditions. The high bulk-water temperature (i.e., 30 °C) of the aqueous sample helps increase the degradation rate of PFOA and PFOS. The addition of oxidants such as iodate and chlorate help enhance PFOA degradation in an argon environment at an ultrasonic frequency of 575 kHz.     

Dynamic Rheological Behavior of HDPE/UHMWPE Blends

The rheological behaviors of high-density polyethylene (HDPE)/ultra-high molecular weight polyethylene (UHMWPE) blends prepared by melt blending and solution blending were studied. The results showed that the rheological parameters (G′, G ^″, and η*) of both types of blends increased gradually with increasing fraction of UHMWPE, while the tanδ decreased correspondingly. Comparing blends with the same UHMWPE content, all G′, G ^″, and η* values of solution blends were higher, and the tanδ of the solution blends were remarkably lower than those of the melt blends. Combined with the scanning electron microscopy (SEM) observations, it was proved that, because of its very high melt viscosity, the UHMWPE chain is difficult to diffuse and be distributed well in the HDPE matrix by melt blending, resulting in a two-phase-like morphology. On the other hand, the blends prepared by the solution blending showed a homogeneous distribution of UHMWPE in the HDPE matrix. In addition, the state of aggregation of the UHMWPE in the HDPE matrix can be distinguished well by time–temperature superposition (TTS) curves; i.e., the two-phase-like morphology in the melt blends can be detected by the failure of TTS in the high-frequency range, which cannot be reflected by Cole–Cole plots and Han curves.     

Studies on HDPE functionalized by ultraviolet irradiation and interfacial interaction of HDPE/STC blend

Compared with ultraviolet irradiation in air, high density polyethylene (HDPE) could be very quickly functionalized by ultraviolet irradiation in ozone atmosphere, introducing oxygen-containing groups such as C=O and C-O onto the molecular chains of HDPE. After ultraviolet irradiation in ozone atmosphere, the molecular weight of HDPE decreased, its distribution became wider, the melt index (MI) increased, and the water contact angle decreased. After irradiating for a short time in ozone atmosphere, the interfacial interaction between the irradiated HDPE and sericite-tridymite-cristobalite (STC) particles is improved. The yield and notched impact strength of the HDPE/STC (60/40) blend with 10 min-irradiated HDPE are increased from 25.5 MPa and 61 J/m for the nonirradiated HDPE/STC (60/40) blend to 30.2 MPa and 360 J/m for the irradiated blend, respectively.     

超声光栅与平面透射光栅衍射图样的比较研究

分析了超声光栅的形成机理,给出了超声光栅衍射图样光强分布的解析表示,通过理论分析和数值模拟对超声光栅与平面透射光栅的衍射图样进行了比较研究.结果表明其衍射主极大满足类似的光栅方程,但衍射条纹的强度分布不同;都能产生缺级现象,但规律不同;超声光栅是一种动态光栅,各级衍射谱线的频率不同.     

Study on mechanism of chitosan degradation with hydrodynamic cavitation

Hydrodynamic cavitation is an effective method for chitosan degradation, of which the mechanism directly determines the molecular weight distribution of degradation products. In this study, based on the Monte Carlo simulation and experimental results, the mechanism of chitosan degradation with hydrodynamic cavitation and molecular weight distribution of products were analyzed. The results showed that the algorithm established in the simulation could effectively analyze degradation mechanism and the factors that influenced degradation mechanism and molecular weight distribution of products. The degradation with hydrodynamic cavitation was caused by chemical and mechanical effects, of which the former dominated the degradation process. The outlet and inlet angles and throat length of the cavitator had major and minor influences on the degradation pattern, respectively. The chemical effect led to random cuts resulting in wide distribution of the products, while the mechanical effect led to central cuts resulting in narrow distribution of the products. With more central cuts, the slide-shaped molecular weight distribution curve of degradation products was gradually transferred into a bell-shaped curve. These results provide instructions for researches on the molecular weight distribution of chitosan products degraded with hydrodynamic cavitation.     

Ultrasonic depolymerization of aqueous carboxymethylcellulose

Prolonged exposure of solutions of macromolecules to high-energy ultrasonic waves produces a permanent reduction in viscosity. However, the exact mechanism by which degradation occurs is still open to discussion. According to this study hydrodynamic forces played the primary role in degradation process. This study showed that there is an optimal carboxymethylcellulose (CMC) concentration to the most efficient degradation. Ultrasound degraded preferentially large CMC molecules and cleavage took place roughly at the centre of the CMC molecules. Degradation of CMC did not proceed below a certain molecular mass. During ultrasonic degradation the molecular mass distribution narrowed. For any polymer degradation process to become acceptable to industry, it is important to be able to specify the sonication conditions to produce a particular relative molecular mass distribution.     

Chain Degradation under Low-Intensity Sonication of Polymer Solutions in the Presence of Filler: Mechanism of Ultrasonic Degradation of Flexible Chain Macromolecules

Routine dispersion of fillers in polymer solutions in a usual ultrasonic cleaning bath has been shown to lead to chain degradation. It is the filler presence, only, that has been demonstrated to provoke chain degradation under low-intensity sonication. A critical analysis of the literature concerned with the effects arising from propagation of acoustic waves in a liquid and an experimental study of ultrasonic degradation of polymers in solution were carried out. Based on these results, the mechanisms of chain degradation were discussed. Our previously proposed universal mechanism of chain degradation in inhomogeneous hydrodynamic fields has been shown to explain the basic facts repeatedly confirmed over the years of studying the ultrasonic degradation: (i) the existence of a limiting molecular weight such that macromolecules with lower molecular weights are not subject to degradation and (ii) the dependences of degradation rate on polymer molecular weight, polymer concentration, and temperature.     

Thermorheology and Crystallization Behaviors of Polyethylenes: Effect of Molecular Attributes

Correlations between polyethylenes of different compositions and branching architectures and the temperature dependence of their viscoelastic behavior as well as the dependence of the nonisothermal crystallization behaviors on the cooling rate were described. To analyze the thermorheological behavior of the various classical polyethylenes, a method proposed by van Gurp and Palmen was utilized and the classical high-pressure low-density polyethylene (LDPE) was found to be thermorheologically complex, while for high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE), thermorheological simplicity was observed. The Avrami and Mo methods were applied to describe the nonisothermal crystallization kinetics of the different PEs for various cooling rate. The values of the kinetic parameter F(T), kinetic crystallization rate constant (Z_c), and half-time of crystallization (t_1/2) indicated that long-chain branching (LCB) had the role of being a heterogeneous nucleating agent and accelerated the crystallization of polyethylene. Moreover, an HDPE sample of both high molecular weight (M_w) and molecular weight distribution (MWD) had a different crystallization rate dependence from the other samples at various corresponding cooling rates. The crystallization activation energy for nonisothermal crystallization of different PEs was determined using the Kissinger method and showed that the presence of LCB as well as high M_w can increase the crystallization activation energy of polyethylene.     

Co-crystallization of Blends of High-density Polyethylene with Linear Low-density Polyethylene: An Investigation with Successive Self-nucleation and Annealing (SSA) Technique

Two kinds of polyethylenes, high-density polyethylene (HDPE) with few chain branches and short-chain branched linear low-density polyethylene (LLDPE) with a relatively larger average molecular weight, were melt blended together in various mass ratios based on consideration of their practical applications. After identifying the good compatibility of the blends, their crystallization behaviors were studied by the successive self-nucleation and annealing (SSA) technique. The SSA analysis showed that not merely the number of melting fractions in the SSA curves changed with the blend composition, but also the content of the first two melting fractions at high temperature of SSA curves showed a positive deviation and a negative deviation with the blend composition, respectively. These phenomena, as well as the interesting appearance of a stepped increase of the lamellar thickness of each fraction with the highest temperature in each sample, indicated that co-crystallization occurred between HDPE and LLDPE. The results from wide-angle X-ray diffraction (WAXD) supported the conjecture obtained by the SSA analysis.     

Crystal Morphology of Water-Assisted Injection Molded High-Density Polyethylene with Two Different Molecular Weights

The crystal morphology of water-assisted injection molded (WAIM) parts of high-density polyethylene (HDPE) with two different molecular weights was investigated. The results showed that for the WAIM parts of HDPE with higher molecular weight, oriented lamellar structures formed in the outer layer, whereas spherulites formed in the core and inner layers, at positions both near the water inlet and near the end of the water channel. However, for the WAIM parts of HDPE with lower molecular weight, spherulites formed in all three layers at a position near the water inlet, whereas oriented lamellar structures formed in the outer layer and banded spherulites were dominant in both core and inner layers at a position near the end of the water channel. The crystal morphology development was interpreted with the aid of stress and temperature fields within the mold cavity under melt filling and high-pressure water penetration during the WAIM.     

Morphology and mechanical properties of PP/HMWPE blends

Mechanical properties and morphology of blends of polypropylene (PP) with high molecular weight polyethylene (HMWPE) prepared by coprecipitation from xylene solution are investigated. Compared to blends of PP with commercial high-density polyethylene (HDPE), the mechanical properties of the blends of PP/HMWPE are much superior to those of PP/HDPE blends. Not only is the tensile strength stronger, but also the elongation at break is much higher than that of the PP/HDPE blends of the same composition. These differences increase with increasing HMWPE and HDPE content. Scanning electron microscopy of the fracture surface resulting from the tensile tests shows that the compatibility in PP/ HMWPE blends is much better than that in PP/HDPE blends. This is most likely attributable to the enhanced chain entanglement of HMWPE with the PP in the amorphous phase due to the lower crystallinity, owing to the high molecular weight of the HMWPE, and a much more flexible chain. The thermal behavior and spherulite morphology of both blends are also investigated.     

Sonochemical degradation for toxic halogenated organic compounds.

This paper describes the degradation of p-chlorophenol using three different ultrasonic devices. The dissipated power in the reaction matrix was measured based on calorimetric method. The study showed that hydrogen peroxide can improve the sonochemical reaction and gases dissolved in reaction matrix can affect the process to a small extent. The reaction mechanism and kinetics of degradation were also investigated.     

Kinetics of the ultrasonic degradation of poly (alkyl methacrylates)

The influence of the alkyl group substituents on the ultrasonic degradation of poly (alkyl methacrylate)s, namely poly (methyl methacrylate) (PMMA), poly (ethyl methacrylate) (PEMA) and poly (butyl methacrylate) (PBMA) was studied. The rate coefficient increased with an increase in the number of carbon atoms in the alkyl group: thus the order of degradation was PBMA>PEMA>PMMA. This was attributed to the scission of the main chain, which increases with the length of the side chain. The ultrasonic degradation of PBMA was investigated in various solvents, at different temperatures and at different ultrasound intensities. The degradation rate coefficients increased logarithmically with the decrease in vapor pressure and increased linearly with an increase in viscosity of the solvent and ultrasound intensity. The effect of three different initiators, benzoyl peroxide (BPO), dicumyl peroxide (DCP) and azo-bisisobutyronitrile (AIBN) on the ultrasonic degradation of PBMA was also studied. The degradation of the polymer decreased in the presence of the initiator. A continuous distribution model was developed for the radical mechanism involved in degradation and was used to determine the degradation rate coefficients of PBMA in presence of initiator. The model indicated that the degradation rate coefficient of the interaction of the PBMA radical with the initiator is independent of the dissociation constant of the initiator.     

Optimisation of 20 kHz sonoreactor geometry on the basis of numerical simulation of local ultrasonic intensity and qualitative comparison with experimental results

The intensity distribution of the ultrasonic energy is, after the frequency, the most significant parameter to characterize ultrasonic fields in any sonochemical experiment. Whereas in the case of low intensity ultrasound the measurement of intensity and its distribution is well solved, in the case of high intensity (when cavitation takes place) the measurement is much more complicated. That is why the predicting the acoustic pressure distribution within the cell is desirable. A numerical solution of the wave equation gave the distribution of intensity within the cell. The calculations together with experimental verification have shown that the whole reactor behaves like a resonator and the energy distribution depends strongly on its shape. The agreement between computational simulations and experiments allowed optimisation of the shape of the sonochemical reactor. The optimal geometry resulted in a strong increase in intensity along a large part of the cell. The advantages of such optimised geometry are (i) the ultrasonic power necessary for obtaining cavitation is low; (ii) low power delivered to the system results in only weak heating, consequently, no cooling is necessary and (iii) the "active volume" is large, i.e. the fraction of the reactor volume with high intensity is large and is not limited to a vicinity close to the horn tip.     

The influence of frequency on the mechanical and radical effects for the ultrasonic degradation of dextranes

The ratio of mechanical and radical effects for the ultrasonic degradation of dextranes in aqueous solutions was studied in dependence of frequency and molecular weight of the dextranes. For low ultrasound frequency (35 kHz) a stronger increase of the polymer degradation with increasing molecular weight was found as expected on the basis of the radicals present. This is due to the mechanical effects of ultrasound. Applying higher frequencies (>500 kHz) only radical reactions are responsible for the degradation. Below a molecular weight limit of 40000 the mechanical effects vanish.     

Mechanisms of ultrasonic modulation of multiply scattered incoherent light based on diffusion theory

An analytic equation interpreting the intensity of ultrasound-modulated scattering light is derived,based on diffusion theory and previous explanations of the intensity modulation mechanism.Furthermore,an experiment of ultrasonic modulation of incoherent light in a scattering medium is developed.This analytical model agrees well with experimental results,which confirms the validity of the proposed intensity modulation mechanism.The model supplements the existing research on the ultrasonic modulation mechanism of scattering light.