An efficient heuristic for reliability design optimization problems

This paper develops an efficient heuristic to solve two typical combinatorial optimization problems frequently met when designing highly reliable systems. The first one is the redundancy allocation problem (RAP) of series-parallel binary-state systems. The design goal of the RAP is to select the optimal combination of elements and redundancy levels to maximize system reliability subject to the system budget and to the system weight. The second problem is the expansion-scheduling problem (ESP) of multi-state series-parallel systems. In this problem, the study period is divided into several stages. At each stage, the demand is represented as a piecewise cumulative load curve. During the system lifetime, the demand can increase and the total productivity may become insufficient to assume the demand. To increase the total system productivity, elements are added to the existing system. The objective in the ESP is to minimize the sum of costs of the investments over the study period while satisfying availability constraints at each stage. The heuristic approach developed to solve the RAP and the ESP is based on a combination of space partitioning, genetic algorithms (GA) and tabu search (TS). After dividing the search space into a set of disjoint subsets, this approach uses GA to select the subspaces, and applies TS to each selected subspace. Numerical results for the test problems from previous research are reported and compared. The results show the advantages of the proposed approach for solving both problems.     

High‐solid‐content emulsions. V. Applications of miniemulsions to high solids and viscosity control

The use of miniemulsions of styrene and butyl acrylate to make high‐solid‐content, low‐viscosity latices was investigated. Products were obtained with solid concentrations greater than 70 wt % and viscosities as low as 350 mPa s at shear rates of 20 s^?1. This was possible with monomodal and bimodal latices. In addition, the compartmentalization of the reactive species in the miniemulsion systems was exploited to produce latices with high solid concentrations, low viscosities, and bimodal molecular weight distributions. This demonstrated a potentially interesting application for this sort of material. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 745–752, 2005     

Core–Shell Polymer Adhesive for Aluminized Coatings: From Improved Barrier Properties to Commercial Formulation

The development and formulation of core–shell latex‐based adhesives to improve the adhesion of aluminium to poly(ethylene terephthalate) (PET) films and enhance the permeability of the final laminate to oxygen and water is described. All particles have a soft acrylic component (the shell in core–shell particles) to improve adhesion, and occasionally a hydrophobic core to enhance the permeability. The performance of the different latexes is compared to that of a commercially available reference material. The coatings formed by core–shell nanoparticles present a lower permeability to water than the particles synthesized in the absence of the seed, while the permeability to oxygen is found to be mainly related to the correct deposition of the metallic layer. In terms of the industrial formulation, it is found that a limited amount of a wetting agent (WA) is needed to ensure the correct spreading of the latex onto the PET substrate because of the low levels of surfactant used for the latex production. This compound has a positive effect on the adhesive strength of the coating. A curing agent also improves barrier properties, but the optimal level of this compound is dependent on the degree of crosslinker used in the base latex.     

Continuous tubular reactors for latex production: Conventional emulsion and miniemulsion polymerizations

An investigation of the use of continuous tubular reactors for the production of polymeric latices is presented. In an initial step, it is demonstrated that tubular reactors can be used to produce latices by conventional emulsion polymerization at solids contents of up to 30% by volume. Under these conditions, the kinetics are identical to those found in batch reactors (as expected), and stable operating conditions are obtained, usually after one to two residence times. The use of a miniemulsion in a tubular reactor was also investigated, and it is demonstrated that in this case, solids contents of at least 60% can be obtained under stable operating conditions. It is therefore proposed that difficulties in operating tubular reactors with conventional emulsion polymerization arise essentially from the presence of monomer droplets in the early stages of the reaction. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2195–2207, 2004     

An Improved Proportional Integral Estimator of the Stator Resistance for a Direct Torque Controlled Induction Motor

In this paper, an improved PI （proportional integral） stator resistance estimation for a DTC （direct torque controlled） induction motor is proposed. This estimation method is based on an on-line stator resistance correction regarding the variations of the stator current estimation error. In fact, the input variable of the P1 estimator is the stator current estimation error. The main idea is to tune accurately the stator resistance value relatively to the evolution of the stator current estimation error gradient to avoid the drive instability and ensure the tracking of the actual value of the stator resistance. But there is an unavoidable steady state error between the filtered stator current modulus and its estimated value from the dq model of the machine which is due to pseudo random commutations of the inverter switches. This may deteriorate the performance of the proposed fuzzy stator resistance estimator. An offset has been introduced in order to overcome this problem, for different speed command values and load torques. Simulation results show that the proposed estimator was able to successfully track the actual value of the stator resistance lbr different operating conditions.     

Homogenisation devices for the production of miniemulsions

The use of static mixers and a high-intensity homogeniser (type rotor-stator) to generate polymerisable miniemulsions was investigated. The results showed that it is possible to make stable, polymerisable dispersions of droplets with an average particle size of the same order as those obtained using ultrasonification in the case of the rotor-stator, and slightly larger with very simple static mixers. Homogenisation times are on the order of 1-4 h when a previously identified stabilisation system is used to form monomer droplets Ouzineb, K., et al., [2004. Stabilization and compartmentalization in miniemulsion polymerization. Journal of Applied Polymer Science 91, 115-124]. An order of magnitude energy balance shows that static mixers can be used to generate droplets using significantly less energy than is needed for the rotor-stator or ultrasonification. Results also show that the two techniques presented here can also be used to generate relatively concentrated polymerisable dispersions.     

Tabu search for the redundancy allocation problem of homogenous series–parallel multi-state systems

This paper develops an efficient tabu search (TS) heuristic to solve the redundancy allocation problem for multi-state series–parallel systems. The system has a range of performance levels from perfect functioning to complete failure. Identical redundant elements are included in order to achieve a desirable level of availability. The elements of the system are characterized by their cost, performance and availability. These elements are chosen from a list of products available in the market. System availability is defined as the ability to satisfy consumer demand, which is represented as a piecewise cumulative load curve. A universal generating function technique is applied to evaluate system availability. The proposed TS heuristic determines the minimal cost system configuration under availability constraints. An originality of our approach is that it proceeds by dividing the search space into a set of disjoint subsets, and then by applying TS to each subset. The design problem, solved in this study, has been previously analyzed using genetic algorithms (GAs). Numerical results for the test problems from previous research are reported, and larger test problems are randomly generated. Comparisons show that the proposed TS out-performs GA solutions, in terms of both the solution quality and the execution time.     

Synergistic effect of talc/calcined kaolin binary fillers on rigid PVC: Improved properties of PVC composites

This study aims to develop poly(vinyl chloride) (PVC) composites prepared by melt mixing by using a combination of talc and calcined kaolin as filler in order to improve the disadvantages of rigid PVC, which is widely used in industry, such as poor mechanical properties and low thermal stability. In addition, PVC/talc and PVC/calcined kaolin composites were also examined for comparison. The calcined kaolin is modified with urea to increase the surface area (chemical treatment), while the surface of both fillers is coated with stearic acid for good compatibility with PVC (mechanical treatment). Scanning electron microscopy micrographs showed homogeneous distribution of mechanically processed fillers. Attenuated total reflectance–Fourier-transform infrared spectroscopy analysis revealed successful coating of the fillers with stearic acid. Also, new bands were detected in the spectrum of urea-treated calcined kaolin, showing an effective chemical treatment. It has been observed that the treatment of fillers improves the mechanical properties of PVC. Thermogravimetric results showed that delaminated calcined kaolin increased the thermal stability of PVC composites. The results of this study proved that binary filler composites combine good properties of both (synergistic effect) and good filler dispersion can be obtained by using stearic acid and urea.     

Study of compartmentalization in the polymerization of miniemulsions of styrene and butyl methacrylate

Different surfactant and costabilization systems were studied for the miniemulsion polymerizations of styrene (St) and of butyl methacrylate (BMA). It was found that the combination of sodium dodecyl sulfate, Triton X‐405, and octadecyl acrylate yielded excellent results. With this system all of the droplets initially present in the reactor polymerized. This stabilization system was then used in the study of compartmentalization of the monomer droplets in miniemulsion polymerization. Blends of BMA and St miniemulsions were prepared separately and polymerized together to study mass transfer in this system. It was observed that no matter is exchanged between the different phases. This compartmentalization was also demonstrated and exploited by producing a bimodal molecular weight distributions latex, achieved with a semicontinuous process. The lower MWD was created by adding a second miniemulsion containing a transfer agent. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 115–124, 2004