Polymerization of epoxy resins initiated by metal complexes

BACKGROUND: Processing parameters and material properties of epoxy resins can be vastly influenced by choice of curing agent. In this work, metal complexes were investigated as initiators for anionic and cationic epoxide polymerization. Systems for thermally induced and electron beam‐induced curing are described. RESULTS: Zinc or cobalt imidazole complexes of the type [M(imidazole)₂(anion)₂] are efficient initiators for anionic polymerization of glycidyl‐based epoxy resins. The complexes can be employed to prepare tailored resin systems ranging from fast curing systems at slightly elevated temperatures to systems with very high thermal latencies curable at temperatures far above 150 °C. Silver complexes [Ag(L)_n]SbF₆ (L = crown ether or alkene) are highly efficient initiators for cationic curing and low initiator contents of around 1% are sufficient to reach high degrees of crosslinking. The complexes are excellent initiators for both thermally induced and electron beam‐induced polymerizations. CONCLUSION: Metal complexes are powerful initiators for the homopolymerization of epoxy resins and can be designed not only for anionic and cationic polymerization but also for thermal and radiation curing. Based on this study and additional work, a library can be compiled which allows retrieval of optimized metal–ligand–anion combinations and adjustment of the initiators to the respective processing and material demands. Copyright © 2009 Society of Chemical Industry     

Forschungsgesellschaft Computer Automation

SciencenewsNeugrünoung

Forschungsgesellschaft Computer Automation     

Multiple-relaxation-time lattice Boltzmann models in three dimensions

This article provides a concise exposition of the multiple-relaxation-time lattice Boltzmann equation, with examples of 15-velocity and 19-velocity models in three dimensions. Simulation of a diagonally lid-driven cavity flow in three dimensions at Re = 500 and 2000 is performed. The results clearly demonstrate the superior numerical stability of the multiple-relaxation-time lattice Boltzmann equation over the popular lattice Bhatnagar-Gross-Krook equation.     

Corrosion and permanent fracture resistance of coated and conventional orthodontic wires

The corrosion processes are presumed to have negative consequences on biocompatibility, aesthetic appearance and the frictional behavior between the bracket and the guiding arch during orthodontic treatment. A group of new guiding arches are the coated orthodontic wires. The present in-vitro study investigated the corrosion behavior and permanent fracture resistance of eight coated wires of different dimensions. Five superelastic nickel titanium (NiTi) wires (Titanol® Low Force River Finish Gold and Gold 2: Forestadent Corp.; Titanol® Superelastic tooth colored: Forestadent Corp.; Bioforce Sentalloy longuard: GAC Corp.; NiTi Imagination: GAC Corp.), two -titanium-wires (TMA Low Friction longuard: Ormco Corp.; TMA Low Friction longuard Purple: Ormco Corp.) and one steel wire (Stainless Steel Imagination: GAC Corp.) were selected. For comparison reasons three uncoated arch wires (Rematitan® Lite Dimple: Dentaurum Corp.; Titanol® Low Force River Finish: Forestadent Corp.; Bioforce Sentalloy: GAC Corp.) were included in the investigation. Surface modifications were made of teflon, polyethylene and by ion implantation. The corrosion processes have been carried out by the use of a specialized electrochemical cell. In a second experimental series the wires were exposed to mechanical stresses. Finally, all wires were examined in a scanning electron microscope. The results indicated that teflon coating prevented the corrosion of the wires. As expected, the -titanium wires did not corrode either. The other wires showed rupture potentials between 187 mV and 602 mV (NHE). After mechanical stress testing the wires could be subdivided into three groups. In the first group no differences could be recognized, the second group showed changes in their crystallographic structure and in the last group the teflon coating was peeled off from the surface of the wires.     

Sparse on-line gaussian processes

We develop an approach for sparse representations of gaussian process (GP) models (which are Bayesian types of kernel machines) in order to overcome their limitations for large data sets. The method is based on a combination of a Bayesian on-line algorithm, together with a sequential construction of a relevant subsample of the data that fully specifies the prediction of the GP model. By using an appealing parameterization and projection techniques in a reproducing kernel Hilbert space, recursions for the effective parameters and a sparse gaussian approximation of the posterior process are obtained. This allows for both a propagation of predictions and Bayesian error measures. The significance and robustness of our approach are demonstrated on a variety of experiments.     

An Application of Extreme Value Theory for Measuring Financial Risk

Assessing the probability of rare and extreme events is an important issue in the risk management of financial portfolios. Extreme value theory provides the solid fundamentals needed for the statistical modelling of such events and the computation of extreme risk measures. The focus of the paper is on the use of extreme value theory to compute tail risk measures and the related confidence intervals, applying it to several major stock market indices.     

Basic investigations on debris cooling

An experimental investigation of boiling phenomena in inductively heated particle beds has been performed. The major aim of these experiments is to provide data for validating numerical codes used in reactor safety. The experiments can be divided in three parts: boiling experiments, dryout experiments and quenching experiments. In boiling experiments, the pressure gradients have been measured along the bed height for different flow modes, different heat inputs and different system pressures. In dryout experiments, the minimum heat input has been determined for which the particle bed starts to superheat significantly above the saturation temperature. The final test series deals with the cool down behaviour of strongly superheated particles by flooding them with cold water. The initial temperatures ranged from 200 up to 900 °C in top-quenching experiments and from 230 up to 450 °C in bottom-quenching experiments. All experiments were performed with pre-oxidised stainless steel balls of 6 and 3 mm diameter in a cylindrical crucible. The bed height was 640 mm and the bed diameter was 125 mm for boiling and dryout experiments, respectively 150 mm for quenching experiments. The experimental results are compared with various available dryout models.     

Validation and application of the WABE code: Investigations of constitutive laws and 2D effects on debris coolability

The WABE-2D model aims at the problem of coolability of degraded core material during a severe accident in a light water reactor (LWR) and describes the transient boil-off and quenching behavior of debris beds. It is being developed in the frame of the KESS code system, which is considered to describe the processes of core heatup, melting, degradation and relocation to the lower plenum as well as the subsequent behavior. The models developed in this frame are being integrated in the German system code ATHLET-CD.An emphasis of the present contribution lies on multidimensional aspects of the cooling behavior. From multidimensional features a significant improvement of overall coolability is expected compared to what is concluded based on classical one-dimensional analyses. Such analyses – also mainly oriented at top cooling conditions – additionally miss the expected importance of interfacial drag which should support coolability in co-current flow situations due to bottom flooding. The latter situation plays a role in the multidimensional behavior expected under realistic conditions. Thus, a further emphasis in the present contribution lies on the constitutive drag laws and their effects in such configurations.Calculations comparing top and bottom flooding and the influence of interfacial friction are presented. An explanation for effects observed in related experiments at Forschungszentrum Karlsruhe is provided based on this influence. The significant increase of dryout heat flux with water inflow from below, driven by a lateral water column, is reproduced and understood. Enhanced cooling due to this and in general by lateral inflow is also demonstrated for reactor scenarios, considering particulate debris in the lower head of the reactor pressure vessel (RPV) of a LWR or in a deep water pool in the reactor cavity of a boiling water reactor (BWR). Cooling by steam flow through local dry zones can establish under lateral water supply to regions below and yield a further extension of coolability. Quenching of hot material is also analyzed. Finally, cases with loss of coolability, dry zone formation and melting are considered, especially in the perspective to analyze melt pool formation in the lower head of the RPV and the history of thermal interaction with the lower head wall. The latter will determine failure possibilities and modes of the RPV.     

Simulation of melt jet breakup and debris bed formation in water pools with IKEJET/IKEMIX

The objective of the development of the code system KESS is simulating the processes of core melting, relocation of core material to the lower head of the reactor pressure vessel (RPV) and its further heatup, modelling of fission product release and coolability of the core material. In the scope of the code development, IKEJET and IKEMIX were designed as key models for the breakup of a molten jet falling into a water pool, cooling of fragments and the formation of particulate debris beds. Calculations were performed with these codes, simulating FARO corium quenching experiments at saturated (L-28) and subcooled (L-31) conditions, as well as PREMIX experiments, e.g. PM-16. With the assumption of a reduced interfacial friction between water and steam as compared to usually applied laws, the melt breakup, energy release from the melt and pressurisation of the vessel observed in the experiments are reproduced with a reasonable accuracy. The model is further applied to reactor conditions, calculating the relocation of a mass of corium of 30 t into the lower plenum, its fragmentation and the formation of a particle bed.