Environmental etch performance, and scratch and mar of automotive clearcoats

Environmental etch damage to automotive coatings, and scratch and mar of these coatings are an important element of customer satisfaction as well as a significant warranty repair consideration for automotive companies. The conditions that result in environmental etch are examined and a laboratory test proposed. Data from this test are compared to automotive hoods exposed in Florida. The performance of various crosslinking chemistries is discussed and the requirements for improved environmental etch are outlined. Scratch and mar performance of these systems is also reviewed. We have found that coatings respond to physical stress by elastic recovery, by plastic flow and by brittle fracture. Classifying types of damage in this way is important for understanding the chemistry needed for improved scratch and mar of coatings.     

On the complexity of approximating k-set packing

Given a k-uniform hypergraph, the Maximum k -Set Packing problem is to find the maximum disjoint set of edges. We prove that this problem cannot be efficiently approximated to within a factor of unless P = NP. This improves the previous hardness of approximation factor of by Trevisan. This result extends to the problem of k-Dimensional-Matching.     

Robotic deposition of 3d nanocomposite and ceramic fiber architectures via UV curable colloidal inks

A novel approach for producing predetermined, complex 3d ceramic architectures by robotic deposition where UV radiation is used for solidification is presented. Homogeneous, highly loaded, solvent free colloidal inks with controlled viscoelastic properties are achieved by proper selection of monomers and surfactants. Room temperature deposition of complex 3d fiber networks having filaments in the 100 μm range is demonstrated for Al₂O₃ and hydroxyapatite model particles for structural and biomedical applications. Solidification of the structures by UV radiation allows additional shaping of the structures by post-printing processes such as cutting, folding and bonding. 2d and 3d architectures with high aspect ratios retain their shape and transform to macroscopic ceramics after thermal debinding and sintering procedures. Sintered alumina fiber networks functionalized with a 3-5 μm layer of TiO₂ nanoparticles exhibit photocatalytic activity for the decomposition of formaldehyde as a similar weight of loose powder, indicating possible applications in catalytic reactors prototypes.     

Chemically induced porogen decomposition in premembranes for porogen derived membranes

Studies of chemically induced porogen decomposition in Porogen Derived Membranes show that substantially complete decomposition of porogen molecules, molecularly dispersed in the polymeric matrix, is attainable. Several hours are required for decomposition by reagents penetrating into the premembrane at ambient. Decomposition rate is determined by characteristics of polymer, porogen, and reagent used, by polymer/porogen weight ratio, by reagent concentration, and by temperature. The concept of decomposition by an internal catalyst was also validated. In addition to direct relevance to the new method of membrane formation, the study provides better understanding of phenomena of general interest, such as transport of small molecules through changing polymeric matrices. © 1992 John Wiley & Sons, Inc.     

Corrosion behaviour of high pressure die-cast and semi-solid cast AZ91D alloys

The microstructure and the corrosion behaviour of high pressure die-cast and semi-solid cast AZ91D magnesium alloys have been investigated. Semi-solid processing leads to a structure with large rounded grains of a solid solution of magnesium (α phase) whereas die-cast alloys are more homogeneous. Electrochemical measurements, particularly with impedance spectroscopy, have shown that the semi-solid cast alloy possesses a corrosion rate at least 35% below that of the die-cast alloy. This can be explained with considering the differences between the composition of the primary α phase and the volume fraction of the β phase, in the two types of alloy.     

The fluorescence decay of human serum albumin and its subfractions

Human serum albumin does not decay monoexponentially although it contains a single tryptophan residue per molecule. The molecular population is thus heterogeneous with respect to the tryptophan emission. The separated monomeric and dimeric molecules of this protein, as well as various fractions isolated by the procedures of Foster and his coworkers, exhibit deviations from monoexponential decay which are comparable to those of the unfractionated protein; thus, the heterogeneity in molecular population of human serum albumin persists in the various fractions. By comparing the fluorescence decay data of this protein in the presence of thyroxine with the corresponding quenching data it was found that the fluorescence of the protein does not respond uniformly to the binding for all protein molecules. Qualitatively similar behavior was found for bovine serum albumin. In view of the above, binding studies followed by fluorescence should be viewed as averages over a heterogeneous population of the molecules of the serum albumin.     

Multidimensional urban segregation: toward a neural network measure

We introduce a multidimensional, neural network approach to reveal and measure urban segregation phenomena, based on the self-organizing map algorithm (SOM). The multidimensionality of SOM allows one to apprehend a large number of variables simultaneously, defined on census blocks or other types of statistical blocks, and to perform clustering along them. Levels of segregation are then measured through correlations between distances on the neural network and distances on the actual geographical map. Further, the stochasticity of SOM enables one to quantify levels of heterogeneity across census blocks. We illustrate this new method on data available for the city of Paris.

     

Premembranes for porogen-derived membranes and thermal decomposition of porogens

Various t-butyl and di-t-butyl esters, N-boc and di-N-boc amines, metal salts of N-boc amino acid, and glycerol–formic acid esters are potential porogens for porogen-derived membranes. Introduction of more than 20 such compounds into polysulfone or poly(vinyl chloride) films was studied. Transparency, IR spectra, changes in glass transition temperature, and differential scanning calorimetry were used to differentiate between molecular dispersion and formation of micro phases. The results show that for many of the studied additives more than 10% by weight can be practically molecularly dispersed in the polymers. The upper limit of porogen concentration in premembranes depends on characteristics of the polymer and of the porogen, on casting solvent composition, and on drying procedure. Metal salts of N-boc amino acids were found to be suitable heat-decomposable porogens. Their decomposition in premembranes proceeds in two stages, in the first of which 2 C₄H₈ + 2CO₂ are released per salt molecule. The metal complex formed goes through decarboxylation at a higher temperature. A heating procedure for completion of the first decomposition stage and for avoiding the second was determined.