When dealing with the curing process of thick items, the main problem in industrial practice is constituted by the different temperatures which undergo internal (cooler) and external regions. Indeed, while internal layers remain essentially under-vulcanized, external coating is always over-vulcanized, resulting in an overall average tensile strength of the item insufficient to permit its utilization in several applications where it is required a threshold level of performance. This work presents a comprehensive numerical model to optimize mechanical properties of thick rubber vulcanized items, comprising medium and high voltage electric cables and 3D devices. Several vulcanization systems are considered, including peroxides and accelerated sulphur. In the case of peroxides, both a genetic algorithm (GA) with zooming and elitist strategy and an alternating tangent (AT) approach are adopted to determine the optimal final mechanical properties (tensile strength) of 2D and 3D rubber items. The use of a mixture of peroxides is also considered, demonstrating that ad hoc mixtures may help in reducing the curing time and/or in increasing the optimal tensile strength in both core and skin of thick devices. For sulphur vulcanization, a mathematical kinetic model is presented to accurately predict the crosslinking density of vulcanized rubber. In the first phase, a simple kinetic model based on the actual reticulation reactions occurring during sulphur curing is proposed. The model is conceived to fit experimental RPM 2000 data, and is able to predict the crosslinking degree, changing both curing times and vulcanization temperatures. The model requires a parametric calibration by means of only three kinetic constants. The temperature variation of such parameters is then estimated by means of three experimental RPM 2000 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, kinetic reaction parameters are implemented in a Finite Element 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 changing both exposition time tc and external curing temperature Tc, to evaluate for each (tc,Tc) couple the corresponding mechanical properties of the item at the end of the thermal treatment. The same AT approach used for peroxide curing again helps to drastically reduce the computational effort required to converge to the optimal solution associated with the maximization of the average tensile strength. From simulations results, it is shown how the model, basing on the actual reactions occurring in practice, allows to estimate the overall degree of vulcanization of real manufactured items.

A comprehensive numerical model for the interpretation of crosslinking with peroxides and sulphur: chemical mechanisms and optimal vulcanization of real items

MILANI, GABRIELE;
2011-01-01

Abstract

When dealing with the curing process of thick items, the main problem in industrial practice is constituted by the different temperatures which undergo internal (cooler) and external regions. Indeed, while internal layers remain essentially under-vulcanized, external coating is always over-vulcanized, resulting in an overall average tensile strength of the item insufficient to permit its utilization in several applications where it is required a threshold level of performance. This work presents a comprehensive numerical model to optimize mechanical properties of thick rubber vulcanized items, comprising medium and high voltage electric cables and 3D devices. Several vulcanization systems are considered, including peroxides and accelerated sulphur. In the case of peroxides, both a genetic algorithm (GA) with zooming and elitist strategy and an alternating tangent (AT) approach are adopted to determine the optimal final mechanical properties (tensile strength) of 2D and 3D rubber items. The use of a mixture of peroxides is also considered, demonstrating that ad hoc mixtures may help in reducing the curing time and/or in increasing the optimal tensile strength in both core and skin of thick devices. For sulphur vulcanization, a mathematical kinetic model is presented to accurately predict the crosslinking density of vulcanized rubber. In the first phase, a simple kinetic model based on the actual reticulation reactions occurring during sulphur curing is proposed. The model is conceived to fit experimental RPM 2000 data, and is able to predict the crosslinking degree, changing both curing times and vulcanization temperatures. The model requires a parametric calibration by means of only three kinetic constants. The temperature variation of such parameters is then estimated by means of three experimental RPM 2000 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, kinetic reaction parameters are implemented in a Finite Element 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 changing both exposition time tc and external curing temperature Tc, to evaluate for each (tc,Tc) couple the corresponding mechanical properties of the item at the end of the thermal treatment. The same AT approach used for peroxide curing again helps to drastically reduce the computational effort required to converge to the optimal solution associated with the maximization of the average tensile strength. From simulations results, it is shown how the model, basing on the actual reactions occurring in practice, allows to estimate the overall degree of vulcanization of real manufactured items.
2011
vulcanization; kinetic model; peroxides; accelerated sulfur; crosslinking; mathematical model
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11311/608391
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