1,721,287 research outputs found
Direct and closed form analytical model for the prediction of reaction kinetic of EPDM accelerated sulphur vulcanization
No full textA novel direct model with kinetic base for the prediction of the final vulcanization level of EPDM cured with sulphur is presented. The model bases on a preliminary characterization of rubber through standard rheometer tests and allows an accurate prediction of the crosslinking degree at both successive curing times and different controlled temperatures. Both the case of indefinite increase of the torque and reversion can be handled. The approach proposed bases on a previously presented exponential model, where a calibration of three kinetic constants at fixed temperature by means of non-linear least square fitting was required. Here the exponential model is superseded and kinetic constants are evaluated through simple closed form formulas.
The applicability of the approach is immediate and makes the model extremely appealing when fast and reliable estimates of crosslinking density of cured EPDM are required. To show the capabilities of the approach proposed, a comprehensive comparison with both available experimental data and results obtained numerically with the exponential model for real compounds at different temperatures is finally provided
Kinetic Finite Element Model to optimize sulfur vulcanization: application to extruded EPDM weather-strips
No full textA numerical two-phase approach based on experimental
scorch curve data fitting, to predict the optimal exposure
time and cure temperature of extruded thick items is applied for the study of a real weather-strip. In the first phase, an existing single equation kinetic model is used to predict the crosslinking density under sulfur vulcanization at variable temperatures. The model requires the calibration of only three kinetic constants. The variation with respect to temperature of such parameters is then evaluated by means of two experimental cure curves performed at two different temperatures. In the second phase, kinetic reaction parameters are implemented in finite element software, to perform thermal analyses on an extruded weatherstrip.
Once evaluated the final mechanical properties of the item point by point, a set of compression tests is numerically simulated, assuming that the rubber behaves as a Mooney–Rivlin material under the large deformations. Elastic properties of the item are evaluated as a function of the vulcanization degree evaluated in the second phase. It is found that suboptimal vulcanizations result into lower elastic moduli and hence great deformability, sometimes incompatible with real scale engineering applications
Genetic algorithm for the optimization of rubber insulated high voltage power cables production lines
No full textThe optimization of vulcanized EPM/EPDM mechanical properties must be a fundamental task for electric cables producers. Insulator mechanical properties depend on the vulcanization degree of the rubber, which is obtained by manufacturers by means of a number of different peroxides acting as reticulation inducers. The industrial production process consists in the extrusion coating of the conductor with elastomeric compounds, and in the successive vulcanization of the insulator rubber through the continuous vulcanization tube, a pressurized high temperature tube filled with nitrogen (the high temperature is a condition necessary to activate the cross-linking agent). Finally, water and/or air are used to cool the insulated cable at ambient temperature. Changes of process variables associated with the vulcanization tube can cause considerable changes in output mechanical properties of the elastomers.
In this framework, in the present paper a novel genetic algorithm with zooming and elitist strategy is used for the determination of optimal production line parameters to use in order to maximize rubber output mechanical properties. Nitrogen temperature and rubber exposition time are assumed as production parameters to optimize, whereas EPM/EPDM final tensile strength is considered as objective function.
The GA approach proposed exploits a specifically crafted zooming strategy, consisting in the subdivision of the population at each iteration into two sub-groups, depending on individual's grade of fitness (elitist strategy). Different genetic procedures are applied to the sub-groups, consisting of both two typologies of admissible mutations for the elite sub-population and mutation and reproduction for the remaining individuals. In order to improve algorithm convergence, a user-defined population percentage, depending on individual's fitness, is replaced with new phenotypes at the end of each iteration, enforcing in this way the chromosomes renewal.
In order to test the reliability of the method proposed, a technical meaningful case study consisting of a high voltage cable used by Italian Railways and cross-linked with a number of different peroxides is discussed. Numerical results show that particular care has to be used by practitioners in the choice of nitrogen temperature and exposition time of production lines to obtain high quality products
Optimization of extrusion production lines for EPDM rubber vulcanized with sulphur: A two-phase model based on Finite Elements and kinetic second order differential equation.
No full textA numerical two-phase approach, based on experimental curometer charts and aimed at predicting the optimal production line parameters (exposition time and cure temperature) for extruded thick rubber items cured with accelerated sulphur is presented.
In the first phase, a simple kinetic model based on the actual reticulation reactions occurring during sulphur curing is utilized to fit experimental curometer data. The model is able to predict the degree of crosslinking at successive curing times and at different controlled temperatures and it requires the calibration of only three kinetic constants. The variation of such parameters with temperature is then evaluated by means of three experimental cure curves performed at three different temperatures. Both the case of indefinite increase of the torque and reversion can be handled.
In the second phase, considering the same rubber compound of step one, kinetic reaction parameters are implemented in a Finite Element (FE) software, specifically developed to perform thermal analyses on complex 2D geometries. As an example, an extruded cylindrical thick EPDM item is considered and meshed through four-noded isoparametric plane elements. Several FE simulations are repeated by changing exposition time tc and external cure temperature Tn, to evaluate for each (tc,Tn) couple the corresponding mechanical properties of the item at the end of the thermal treatment. An alternating tangent approach (AT) is used to drastically reduce the computational efforts required to converge to the optimal solution associated with the maximization of the average tensile strength
Numerical Model for the Prediction of Final Mechanical Properties of EPDM Vulcanized with Peroxides. Part II: Results.
No full textIn the present Part II, the kinetic numerical model discussed in detail in Part I, is applied to the huge amount of experimental data collected by the authors. Rheometer curves are fitted numerically for three different curing temperatures, for all the curing agents investigated at a concentration equal to 150%, which have proved to be the most adequate to obtain optimal crosslinking of the EPDM under consideration. The comparisons with experimental data at three different temperatures allow estimating partial kinetic constants
of the model as a function of the absolute temperature, with a direct comparison with simplified formulas provided by the peroxide producers. The application of the numerical kinetic model to a large variety of real scale examples allows a direct validation of the capabilities of the approach proposed, as well as an evaluation of the most suitable production parameters to use in practice (curing time and temperature, concentration and typology of the curing agents) in order to maximize rubber output mechanical properties and crosslink homogeneity, without the need to perform costly experimental campaign
A new simple numerical model based on experimental scorch curve data fitting for the interpretation of sulphur vulcanization
No full textSulphur was the first agent used to vulcanize commercial elastomers (e.g. natural rubber) and allows meaningful cost reductions during the industrial process (production cost ratio between peroxides and accelerated sulphur is around 5).
Therefore, accelerated sulphur vulcanization is the most popular technique for the production of polydiene and EPDM elastomers items. At present, crosslinking mechanisms are not analytically known in detail, therefore reticulation kinetic has to be deduced from mechanical properties obtained during standardized tests, as for instance the oscillating disc rheometer. In the present paper, we propose a numerical model to fit experimental rheometer data based on a simple composite three functions curve, able to describe the increase of the viscosity at successive curing times at different controlled temperature to use during the production of thick items vulcanized with sulphur. It is believed that rheometer curve is able to give an indirect information on the rubber reticulation kinetic at different temperatures, to use in a successive step to establish simplified analytical kinetic formulas to adopt in the accelerated sulphur vulcanization of polydiene and EPDM elastomers. In the model, it is necessary to collect rheometer curves at different specimen temperatures, because vulcanization in industrial practice occurs at variable temperatures during curing, with considerable differences from the core to boundary of the item. Once that rheometer curves are suitably collected in a database, they are used to predict the optimal vulcanization of real items industrially produced. Finally, a so called alternating tangent approach (AT) is implemented to determine optimal input parameters (curing external temperature Tn and rubber exposition time t) to use in the production process. Output mechanical property (objective function) to optimize is represented by the average tensile strength of the item. A meaningful example of engineering interest, consisting of a thick 2D EPDM cylinder is illustrated to validate the model proposed
Rezipe i rimm del Porta : La letteratura in dialetto milanese dal Rajberti al Tessa e oltre
No full textDifferential model accounting for reversion for EPDM vulcanized with peroxides
No full textOne of the main drawbacks of EPM/EPDM rubber vulcanization by peroxides is the lack of selectivity, which leads to a number of side reactions. The reaction mechanisms at the base of peroxides crosslinking are generally known and include the formation of alkyl and allyl (in the EPDM case) macro-radicals through H-abstraction from the macromolecular chains and the combination of these macro-radicals, which macroscopically is known with the term “vulcanization”. In the paper, a simple but
effective mathematical model having kinetic base, to predict the vulcanization degree of rubber vulcanized with peroxides, is presented. The approach takes contemporarily into consideration, albeit within a simplified scheme, the actual reactions occurring
during peroxidic curing, namely initiation, H-abstraction, combination and addition, and supersedes the simplified approach used in practice, which assumes for peroxidic curing a single first order reaction. After a suitable re-arrangement of the first order system of differential equations obtained from the actual kinetic system adopted, a single second order non-linear differential equation is obtained and numerically solved by means of a Runge–Kutta approach. Kinetic parameters to set are evaluated by means of a standard least squares procedure where target data are represented by experimental values available, i.e. normalized rheometer curves or percentage crosslink density experimentally evaluated by means of more sophisticated procedures. In order to have an insight into the reliability of the numerical approach proposed, two cases of technical interest are investigated in detail: the first is an EPDM crosslinked with two different peroxides, whereas the second is a compound with high level of unsaturation, showing reversion at medium-high vulcanization temperature (175◦C)
Numerical Model for the Prediction of Final Mechanical Properties of EPDM Vulcanized with Peroxides. Part I: Basis of the Numerical Model and Experimental Campaign
No full textIn the paper, a simple but effective mathematical model having kinetic base, to predict the vulcanization degree of rubber vulcanized with peroxides, is presented. The approach takes contemporarily into consideration, albeit within a simplified scheme, the actual reactions occurring during peroxidic curing, namely initiation, H-abstraction, combination and addition, and supersedes the simplified approach used in practice, which assumes for peroxidic curing a single first order reaction. After a suitable re-arrangement of the first order system of differential equations obtained from the actual kinetic system adopted, a single second order non-linear differential equation is obtained and numerically solved by means of a Runge-Kutta approach. Kinetic parameters to set are evaluated by means of a standard least squares procedure where target data are represented by experimental values available, i.e. normalized rheometer curves. In order to assess numerical results, a wide experimental campaign is conducted, varying curing agents, vulcanization temperatures and concentration of the peroxides. Both rheometer curves to compare with those provided by the numerical model and stretch-strain curves of the vulcanized samples are obtained, to have a precise insight into the most suitable parameters to use during curing
Numerical model for the interpretation of sulfur vulcanization through the oscillating disk cure meter test.
No full textSulfur was the first agent used to vulcanize the first commercial elastomer, i.e. natural rubber (NR). At present, accelerated sulfur vulcanization is suitable not only for natural rubber (NR) and its synthetic counterpart (IR), but also for other synthetic rubbers such as polybutadiene rubber (BR), styrene-butadiene rubber (SBR), nitrile rubber (NBR), butyl rubber (IIR) and ethylene-propylene-diene rubber (EPDM). Even though the sulfur vulcanization of natural rubber was discovered more than 150 years ago, its mechanism is still not completely understood. In fact, one of the fundamental issues to establish is whether the predominant reaction pathway is via ionic or free-radical intermediates or both and which is the influence of the number and variety of the ingredients used in the vulcanization recipe in terms of network properties. In this contribution, a numerical model useful to correlate reaction kinetic parameters and experimental oscillating disk cure meter (ASTM test D 2084) curves, fitted by means of a simple mathematical model, is presented. The numerical approach is applied to systems with marching behavior, plateau level and reversion. Combining the present mathematical model with all analytical data for single recipes, it is possible to estimate in advance the optimal vulcanization temperature of real items, also in relation with their dimensions
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