1,720,992 research outputs found
NR sulphur vulcanization: Interaction study between TBBS and DPG by means of a combined experimental rheometer and meta-model best fitting strategy
No full textThe paper aims at studying the possible interaction between two different accelerators (DPG and TBBS) in the chemical kinetic of Natural Rubber (NR) vulcanized with sulphur. Two different numerical meta-models, which belong to the family of the so-called response surfaces RS are compared. The first is linear against TBBS and DPG and therefore well reproduces no interaction between the accelerators, whereas the latter is quadratic with bilinear term. The numerical meta-models are calibrated on the experimental data obtained by using the same mixture with several DPG and TBBS concentrations, varying the curing temperature in the range 150-180°C and obtaining rheometer curves with a step of 10°C. In order to study any possible interaction between the two accelerators - and eventually evaluating its engineering relevance - rheometer data are normalized by means of the well-known Sun and Isayev normalization approach and two output parameters are assumed as meaningful to have an insight into the possible interaction, namely time at maximum torque and reversion percentage. Both RSs are deduced from standard best fitting of experimental data available. It is found that, generally, there is a sort of interaction between TBBS and DPG, but that the error introduced making use of a linear model (no interaction) is generally lower than 10%, i.e. fully acceptable from an engineering standpoint
Comprehensive kinetic numerical model for NR and high-cis poly-butadiene rubber blends
No full textA simple but robust kinetic mathematical model to predict the mechanical/thermal behaviour of NR and high-cis polybutadiene rubber blends is presented. The benchmark blend is a 70% NR with 30% high-cis polybutadiene blend vulcanized in presence of sulphur at 1 phr and single accelerants TBSS or DPG at concentrations equal to 1-0 and 0-1 respectively. Experimental rheometer curves are at disposal for both the NR-PB blend and single NR/PB rubbers at 170°C and 180°C. Rheometer curves at 150°C are utilized as reversion free references to normalize experimental data. The numerical model is based on a modification of Han's kinetic approach, where a linear interaction between NR and PB is accounted for. The determination of kinetic constants is possible by means of a constrained minimization approach that uses Sequential Quadratic Programming and fits through standard least-squares normalized rheometer curves. The procedure is benchmarked on 1-1-0 and 1-0-1 concentrations of S-TBBS-DPG at 170°C and 180°C. Quite good agreement is found against experimental data, with values of kinetic constants addressing a reduction of vulcanization rate with respect to pure NR and a beneficial reversion decrease due to PB contribution
Kinetic model for S-TBBS-DPG NR vulcanization: Extrapolation from S-TBBS and S-DPG experimental data
No full textA robust extrapolation model requiring few input parameters to predict the kinetic constants characterizing the curing behavior of NR vulcanized in presence of sulfur and two accelerants (TBSS and DPG) at different concentrations is discussed. The numerical model based on the reproduction of rheometer curves by means of the well-known Han's kinetic model, which describes with kinetic base the most important NR vulcanization phases, namely curing initiation, formation of matured crosslinked polymer and reversion. The derived mathematical model is a closed form exponential function depending on only three kinetic constants. The procedure proposed is a two-step one. In the first step, kinetic constants of NR in the presence of single activators (i.e. either only with S and TBBS or S and DPG) are estimated by means of an interactive trial and error optimization software (GURU) that proceeds in approximating more and more strictly normalized experimental rheometer curves with Han's function. Four different concentrations of S and TBSS (or S and DPG) are assumed as calibration points. In the second step, from the results obtained previously, kinetic constants for NR with any S-TBBS-DPG concentration of technical relevance are deduced by means of standard mathematical extrapolation. The procedure is benchmarked on 16 different S-TBBS-DPG concentrations at two temperatures (150°C and 180°C), for which both experimental data are available and kinetic constants are previously derived with GURU. Quite good agreement is found, meaning that the approach may be useful for practical purposes, because expensive and cumbersome experimental investigations can be avoided
A Dataset and a Convolutional Model for Iconography Classification in Paintings
Full text linkIconography in art is the discipline that studies the visual content of artworks to determine their motifs and themes and to characterize the way these are represented. It is a subject of active research for a variety of purposes, including the interpretation of meaning, the investigation of the origin and diffusion in time and space of representations, and the study of influences across artists and artworks. With the proliferation of digital archives of art images, the possibility arises of applying Computer Vision techniques to the analysis of art images at an unprecedented scale, which may support iconography research and education. In this article, we introduce a novel paintings dataset for iconography classification and present the quantitative and qualitative results of applying a Convolutional Neural Network (CNN) classifier to the recognition of the iconography of artworks. The proposed classifier achieves good performances (71.17% Precision, 70.89% Recall, 70.25% F1-Score, and 72.73% Average Precision) in the task of identifying saints in Christian religious paintings, a task made difficult by the presence of classes with very similar visual features. Qualitative analysis of the results shows that the CNN focuses on the traditional iconic motifs that characterize the representation of each saint and exploits such hints to attain correct identification. The ultimate goal of our work is to enable the automatic extraction, decomposition, and comparison of iconography elements to support iconographic studies and automatic artwork annotation
An integrated kinetic-Fe vulcanization model to predict the optimal curing of thick rubber pads for applications in seismic isolation
No full textThick rubber pads for cheap isolation of low rise buildings require an optimal vulcanization along the thickness. The present paper is aimed at presenting an integrated three-step approach for the optimization of curing in presence of items exhibiting considerable thickness. It requires a preliminary standard experimental characterization in a rheometer, conceived to tune a simple kinetic steady-state model finally nested into a Finite Element FE heat transmission code, which allows to simulate the curing process in full 3D heat transmission problems. The kinetic model is a phenomenological approach based on 3 kinetic constants, aimed at predicting the initial curing rate, maximum crosslinking and reversion. Kinetic constants are deduced fitting normalized experimental rheometer curves. FE transient curing computations are carried out on a thick rubber pad, used for the production of unbonded low cost seismic isolation devices. The entire 3D geometry is discretized with a refined mesh, in order to realistically reproduce the vulcanization occurring in an industrial mold. From simulation results, interesting design considerations can be drawn, allowing to establish time and temperature of curing as a function of the rubber thickness and chemical composition
Quasi-analytical kinetic model for natural rubber and polybutadiene rubber blends
No full textA very simple kinetic model for natural rubber (NR) and polybutadiene (PB) blends is presented. The model is characterized by a completely uncoupled curing between NR and PB, NR being modeled with a primary vulcanization and a subsequent de-vulcanization and PB only by a simple first order model of vulcanization. The assumptions made are roughly in agreement with the actual experimental behavior of the constituent materials in a rheometer chamber, where PB exhibits a quite stable behavior even at high curing temperatures and long vulcanization times. As a result of the simplifications assumed into the curing model adopted, the numerical approach uses only on three kinetic constants, two for NR and one for PB. Such assumptions allow for a quite straightforward determination of the kinetic constants by means of a simple semi-analytical approach. The reliability of the procedure proposed is benchmarked on some 70% NR- 30% PB blends with two different accelerants (N-terbutyl, 2-benzothiazylsulfenamide TBBS and N,N-diphenylguanidine DPG) in different concentrations tested experimentally on a standard rheometer chamber at 170 and 180 °C. Quite good match is found between numerical predictions and normalized rheometer curves, with a clear practical impact into the Finite Element FE modelling of vulcanization of real items
Comparative numerical study on the optimal vulcanization of rubber compounds through traditional curing and microwaves
No full textThe main problem in the industrial production process of thick EPM/EPDM elements 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 insufficient to permit the utilization of the items in several applications where it is required a certain level of performance. Possible ways to improve rubber output mechanical properties include a careful calibration of exposition time and curing temperature in traditional heating or a vulcanization through innovative techniques, such as microwaves. In the present paper, a comprehensive numerical model able to give predictions on the optimized final mechanical properties of vulcanized 2D and 3D thick rubber items is presented and applied to a meaningful example of engineering interest. A detailed comparative numerical study is finally presented in order to establish pros and cons of traditional vulcanization vs microwaves curing. © 2012 American Institute of Physics
COMBINED NUMERICAL, FINITE ELEMENT AND EXPERIMENTAL-OPTIMIZATION APPROACH IN THE PRODUCTION PROCESS OF MEDIUM-VOLTAGE, RUBBER-INSULATED ELECTRIC CABLES VULCANIZED WITH STEAM WATER. PART 1: DSC AND RHEOMETER EXPERIMENTAL RESULTS
Full text linkThe standard industrial process to produce medium-voltage electric cables based on EPDM consists of cross-linking by peroxides with high-temperature steam (pressurized water vapor). Suboptimal material cross-linking is usually due to a decrease of the temperature along the vulcanization pipe. Temperature variations are connected to variations in steam pressure into pipe system. A combined numerical and experimental approach to optimize the production process of medium-voltage, rubber-insulated electric cables vulcanized with steam water is presented. The numerical part of this process is based on the use of finite elements and an optimization genetic algorithm (GA) and will be presented in Part 2. In Part 1, attention focuses on the experimental investigation. In particular, the final cross-linking degree is experimentally obtained by means of differential scanning calorimetry (DSC) determination of nondecomposed peroxide from the external layer to the core of the cable insulation. The final task is to minimize the difference between numerically predicted and experimentally determined cross-linking degree using a steam-temperature profile along the pipe to explain the variations. A preliminary evaluation of kinetic-reaction constants of rubber cured with peroxides is provided with the support of a comprehensive experimental investigation of the curing process by means of standard rheometer characterizations done at different curing temperatures. An existing mathematical, kinetic model is applied to the experimentally determined rheometer curves, allowing the determination of partial-reaction kinetic constants used in the finite-element computations
Practical designs and seismic performances of residential masonry building isolated with fiber reinforced elastomeric isolators
No full textSince the seismic performance of masonry is considerably low, the masonry structure is not recommended for regions with high seismic risks. However, it is so far the preferred structure for low-class housing, particularly in developing countries. Comprehensive numerical and experimental studies are carried out to evaluate the possibility to build low-rise masonry housing with a suitable isolation system. Low-cost seismic rubber isolators reinforced with fiber are examined to product an innovative isolation device. To reduce the cost, rubber materials considered are recycled compounds derived from manufacturing waste. The utilization of fiber instead of steel can reduce the weight and allows the engineer to implement unbonded application of base isolators. Some technological details for masonry building are proposed to assure that the isolation system works well. In order to evaluate the seismic performance of such isolation system, non-linear static and dynamic analyses are performed through numerical models in a commercial finite element code. The results indicate excellent performances of low-cost isolation system so that this innovative residential masonry building is feasible for real application particularly in developing countries
Interactive GUI software for natural rubber vulcanization degree numerical prediction
No full textA graphical user interface software called GURU suitable to fit rheometer curves in Natural Rubber (NR) sulphur vulcanization is proposed. Experimental data are loaded using Excel (experimental output comes from a moving die rheometer registration), normalized and fitted with a numerical model that follows the general scheme proposed by Han. Han,s chemical model translates into mathematics by means of a first order ODE system, admitting a closed form solution for the crosslinking density. Three kinetic constants characterize the model and they must be found in such a way to minimize the absolute error between normalized experimental data and numerical predictions. GURU works to minimize the error by means of a trial and error procedure handled interactively by means of sliders, assigning a value for each kinetic constant and a visual comparison between numerical and experimental curves. An experimental case of technical relevance is shown as benchmark
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