A. I. Isayev

Novel Ultrasonic Technology for Devulcanization of Waste Rubbers

Abstract A novel patented process and several reactors have been developed for devulcanization of waste rubbers. The technology is based on the use of the high power ultrasonics. The ultrasonic waves of certain levels in the presence of pressure and heat rapidly break up the three-dimensional network in crosslinked rubbers. The devulcanized rubber can be reprocessed, shaped and revulcanized in much the same way as a virgin rubber. The first laboratory reactor has been scaled up to pilot-plant level by the National Feedscrew and Machining, Inc. Various devulcanization experiments were carried out with model styrene-butadiene rubber (SBR) and with ground rubber tire (GRT). Curing behavior, Theological properties, and structural characteristics of rubbers devulcanized at various processing conditions were studied, as well as mechanical properties of revulcanized rubber samples. A possible mechanism of the devulcanization is discussed. The performed measurements indicate that the rubbers are partially devulca...

Toward a viscoelastic modelling of the injection molding of polymers

SummaryBy using theLeonov viscoelastic constitutive equation, an idealized problem has been solved for onedimensional, unsteady, non-isothermal flow of polymer between two parallel plates and the subsequent non-isothermal relaxation following cessation of flow. Numerical results are presented for the time dependence of the pressure gradient, the gapwise distribution of linear velocity, shear rate, shear stress and normalstress differences, together with the components of birefringence in different planes. Comparison of the present predictions for the pressure gradient with results based upon an “inelastic” model indicate close agreement whereas the corresponding predictions for normal-stress differences are found to be markedly different from those for the “inelastic” case.The model is applied to the injection-molding process which is treated in terms of a filling and a cooling stage. Final results are given in terms of the distribution of residual stresses and associated birefringences in the molded part, as influenced by the rheological and thermal properties of the polymer and the processing conditions. The theoretical predictions are compared with birefringence measurements in the literature. Reasonable agreement is obtained for the position and value of maximum birefringence in the 1–2 plane although the birefringence predictions in the 1–3 and 2–3 planes are found to be markedly smaller than the measured values. The present theory indicates that, for a given polymer, the main factors affecting residual stresses and birefringence are melt temperature and flow rate, both of which should be held at the highest permissible levels.ZusammenfassungDas idealisierte Problem einer eindimensionalen, instationären, nicht-isothermen Strömung eines Polymeren zwischen zwei parallelen Platten sowie das der nichtisothermen Relaxation, die auf das Anhalten dieser Strömung folgt, wird mit Hilfe der viskoelastischen Stoffgleichung vonLeonov gelöst. Numerische Ergebnisse werden für die Zeitabhängigkeit der folgenden Größen gegeben: des Druckgradienten, der Verteilung der linearen Geschwindigkeit, der Schergeschwindigkeit, der Schubspannung, der Normalspannungsdifferenzen sowie der Komponenten der Doppelbrechung in verschiedenen Ebenen. Die hier vorliegenden Voraussagen sind bezüglich des Druckgradienten in guter Ubereinstimmung mit denen, die auf dem “inelastischen“ Modell beruhen, unterscheiden sich von diesen aber wesentlich bezüglich der Normalspannungsdifferenzen.Das Modell wird auf den Spritzgußprozeß angewandt. Dieser wird als zweistufiger Prozeß, bestehend aus einer Abfüll- und einer Kühlstufe, behandelt. Numerische Ergebnisse werden für die Verteilung der Restspannungen und der assoziierten Doppelbrechung im Formteil gegeben, so wie sie durch die rheologischen und thermischen Eigenschaften des Polymeren und der Prozeßbedingungen beeinflußt werden. Die theoretischen Voraussagen für die Doppelbrechung werden mit Meßergebnissen aus der Literatur verglichen. Gute Übereinstimmung wird für die Lage und den Wert der maximalen Doppelbrechung in der 1–2 Ebene erzielt, während die Voraussagen für die Werte der Doppelbrechung in den 1–3 und 2–3 Ebenen wesentlich kleiner als die gemessenen Werte ausfallen. Die vorliegende Theorie zeigt an, daß für ein gegebenes Polymer die Schmelzentemperatur und die Einspritzgeschwindigkeit als Hauptfaktoren zu werten sind, die die Restspannungen und die Doppelbrechung beeinflussen. Diese sollen auf dem höchstzulässigen Stand gehalten werden.

Devulcanization of waste tire rubber by powerful ultrasound

Ground rubber tire (GRT) is devulcanized in a continuous process where the rubber crumb is conveyed by a screw extruder to a thin gap between a stationary die and a vibrating horn. Ultrasound amplitude, gap thickness and the duration of the treatment are varied, and their effect on the rubber properties is studied. The degree of devulcanization is characterized by the crosslink density and gel fraction of the devulcanized rubber. These characteristics are found to correlate uniquely with each other over a wide range of the treatment conditions. The results specific energy of ultrasound consumed per unit mass of the rubber is found to be the operating parameter correlating with the properties of the devulcanized rubber. The mechanical properties of thr re-vulcanized samples reach a maximum with an increasing degree of devulcanization, and then drop as a result of excessive degradation. Even without optimization of cure recipes, tensile strength and ultimate elongation of re-vulcanized tire rubber as high as 10.5 MPa and 250u, respectively, are achieved

Residual Stress Development in the Injection Molding of Polymers

Abstract Injection molding is one of the most widely employed methods for the fabri- cating of polymer articles, being characterized by high production rates and accurately dimensioned products. The process includes the flow of polymer melt through a runner system and gates followed by injection into a cold mold, packing under high pressure, and subsequent cooling to solidification. Accordingly, during the injection-molding process the polymer undergoes simultaneous mechanical and therma! influences while in fluid, rubbery, and glassy states. Such effects introduce residual stresses and strains into the final product [1,2], resulting in highly anisotropic mechanical behavior [3–9] and warpage and shrinkage [10–13]. Thus, understanding the factors governing the residual-stress development during molding is of great importance.

Ultrasound devulcanization of sulfur vulcanized SBR : Crosslink density and molecular mobility

Abstract Sulfur-cured SBR vulcanizates are devulcanized by high power ultrasonic irradiation in a continuous process under various processing conditions. Crosslink density and gel fraction of devulcanized rubber are measured. The unique dependence of crosslink density on gel fraction of devulcanized samples obtained at various processing conditions is established. DSC studies indicate a significant increase in the glass transition temperature of the devulcanized rubber having crosslink density lower than that of the original vulcanizates. Solid state NMR studies confirm that these changes in Tg are due to a decrease of molecular mobility as measured by a decrease in spin-spin relaxation time. Ultrasonic devulcanization is accompanied by a significant increase in xcis-trans isomerization. Chemical changes explaining the observed phenomena are discussed.

Nonisothermal Vulcanization of Rubber Compounds

Abstract In the present paper, progress has been made in developing a model for nonisothermal vulcanization of rubber compounds. The model is presently based on differential scanning calorimetry (DSC) measurements of heat evolved during vulcanization. The model parameters are determined from several isothermal DSC scans including measurements of induction time and rate of vulcanization. This model has conveniently been employed for a calculation of the temperature distribution during vulcanization of a rubber slab based on a one-dimensional heat-conduction equation with a heat-generation term. The simulation allowed us to predict the dynamics of the development of the state of cure distribution in the rubber slabs. Predicted results from the vulcanization model have been verified with experimental data on induction time and rate of vulcanization from nonisothermal DSC scans and with measurements of the state of cure distribution in rubber slabs cured under different vulcanization conditions between two he...

BLENDS OF A THERMOTROPIC LIQUID-CRYSTAL POLYESTER WITH POLY(PHENYLENE SULPHIDE)

Abstract Blends of a thermotropic liquid-crystalline polymer (LCP), based on a copolyester of hydroxynaphthoic acid and hydroxybenzoic acid, with an engineering thermoplastic, poly(phenylene sulphide) (PPS), were prepared using a co-rotating twin-screw extruder. The blends were subsequently injection moulded to study their mechanical, dynamic, impact and morphological characteristics. Rheological studies have indicated a higher shear viscosity of the LCP as compared to PPS, with the shear viscosity of some of the blends being higher than that of the LCP. Dynamic and thermal studies have indicated overlapping temperature transitions of the pure components. Therefore, based on these measurements it is not possible to make a conclusion about the compatibility of the blends. Thermogravimetric studies have indicated the poor thermal stability of the blends when compared to the pure materials. The tensile and impact properties of these blends were found to increase with the addition of LCP. However, the pure LCP exhibits the highest properties. The morphology of the blends has indicated their incompatibility and the formation of LCP droplets in the matrix of PPS at nearly all concentrations of LCP, with the formation of the skin-core morphology being clearly visible at LCP content greater than 25%.

Continuous ultrasonic process for in situ compatibilization of polypropylene/natural rubber blends

The immiscible polypropylene (PP)/natural rubber (NR) blends of various concentrations were prepared by using a twin-screw extruder. The prepared blends were passed through the reactor where they were ultrasonically treated by an extrusion process. Mechanical properties and rheology of the obtained blends were studied, along with morphology by using the scanning electron microscopy and the atomic force microscopy (AFM). Mechanical properties of the treated blends were found to improve significantly in comparison with those of untreated blends. Under most treatment conditions, no significant differences in the viscosity of the treated and untreated blends were observed. The AFM studies revealed the development of interfacial layers, interfacial roughening and improved interfacial adhesion between PP and NR phases in the blends subjected to ultrasonic treatment. At the same time weak adhesion and delamination at the interface were found in the untreated blends. The improved interfacial adhesion, morphology and mechanical properties are believed to be due to the formation of in situ copolymer at the interface of two immiscible polymers caused by an ultrasonic treatment without the use of any chemicals.

Transient shear flow behavior of polymeric fluids according to the Leonov model

SummaryThe viscoelastic behavior of polymeric systems based upon the Leonov model has been examined for (i) stress growth and relaxation with intermittent shear flow, (ii) stress relaxation after a step in the shear strain and (iii) elastic recovery after shear flow. A large number of modes have been conveniently incorporated through the determination of the model parameters from conventional rheological data by using an effective least-square procedure. With a sufficient number of modes, the predictions are in very good agreement with corresponding experiments in literature, including the recent data for cases (i) and (ii) obtained by optical methods.The present theory agrees also with the Lodge-Meissner relation (τ11 −τ22)/τ12 =γ0 in a step-shear experiment. In general, the Leonov model leads to results which, in these test cases, are comparable to those from Wagners theory. It is, however, considerably less difficult to apply, thus offering the possibility of analysing flow problems of practical interest.ZusammenfassungDas viskoelastische Verhalten von Polymer-Systemen wurde auf der Basis des Leonov-Modells für die folgenden Fälle untersucht: (i) Spannungsaufbau und Relaxation bei unterbrochener Scherströmung, (ii) abklingende Spannung nach einem Sprung in der Scherdeformation und (iii) elastische Erholung nach voraufgegangener Scherung. Eine große Zahl von Relaxations-Elementen konnte einbezogen werden, wobei die Modellparameter von herkömmlichen rheologischen Daten mit Hilfe der Methode der kleinsten Quadrate ermittelt wurden. Bei einer genügend großen Zahl von Relaxations-Elementen stimmen die gewonnenen Vorhersagen sehr gut mit experimentellen Daten aus der Literatur überein. Dies gilt vor allem auch für solche Daten, die mit Hilfe von optischen Methoden für die Fälle (i) und (ii) ermittelt worden sind.Die hier vorgelegte Theorie bestätigt ebenfalls die Lodge-Meissner-Beziehung (τ11 −τ22)/τ12 =γ0 bei einer sprunghaften Änderung der Schergeschwindigkeit. Durchweg führt das Leonov-Modell in diesen Testfällen zu Ergebnissen, die mit Wagners Theorie vergleichbar sind. Leonovs Modell ist jedoch weitaus einfacher anzuwenden und bietet somit die Möglichkeit, auch praktisch bedeutsame Strömungsprobleme zu analysieren.

Continuous ultrasonic devulcanization of unfilled NR vulcanizates

Sulfur-cured unfilled natural rubber (NR) is successfully devulcanized in a continuous extrusion process under the application of high-power ultrasonic energy. The die characteristics and ultrasonic power consumption are measured. A unique correlation is found between the crosslink density and gel fraction of the devulcanized NR. This correlation is independent of the processing parameters, such as barrel temperature, die gap, flow rate, and amplitude of ultrasound. However, these parameters do influence the degree of devulcanization. In most cases, the degree of devulcanization is found to pass through a maximum at an intermediate level of ultrasonic energy. It is hypothesized that simultaneous breakup and reformation of crosslinks occur during the devulcanization of NR, with the relative contribution of each being determined by the process parameters. The cure curves and mechanical properties of the revulcanized NR are studied. The mechanical properties are found to depend on the revulcanization recipe. On optimizing it, tensile strength as high as 14.2 MPa is achieved, which is about 70% of that of the virgin NR vulcanizate. Ultimate elongation as high as 670% is obtained, which is the same as that of the virgin NR vulcanizate. Such stress–strain behavior is an indication that the devulcanized NR maintains the strain-induced crystallization characteristics inherent to the virgin NR vulcanizates.