Importance of surface science and fundamental studies in heterogeneous catalysis

At Union Carbide Corporation, surface science and fundamental studies have played extremely important roles in the discovery, development, and diagnosis of several valuable commercial and developmental catalyst systems. Prior to the late '60s, it was very common among scientists and engineers to refer to catalysis, particularly heterogeneous catalysis, as either an “art” or “magic”. Primary reasons behind these labels were our inability to understand the performance of “poor” and “good” catalysts nominally having the same bulk composition. With the development of surface science in the early '70s, heterogeneous catalysis field was relatively easy picking for diagnosis and improved understanding of many of these poorly understood catalyst systems. In fact, there were many “sad” stories across the chemical industry of catalysts that prematurely deactivated or essentially died and there was no known cause or relationship of performance with observable physical–chemical properties. In all such instances, the bulk characterization techniques failed to identify or uncover the cause or causes of such activity decline. However, through the use of surface science and fundamental characterizations, three such “sad stories” turned into “success stories” at Union Carbide. In addition, it will also be shown that the early use of surface science and fundamental studies led to the discovery, development and enhanced understanding of several catalyst systems. Many of the early surface science techniques along with the newly developed techniques continue to and will play a very important role in the future development of next generation catalysts and catalytic processes for the industrial use and environmental protection. This revised version was published online in July 2006 with corrections to the Cover Date.     

Investigation of interactions between silicones and stratum corneum lipids

Ingredients of topically applied skin care formulations have not only positive effects on the appearance of human skin but can also disturb the Stratum corneum (SC) lipid barrier. In the present study, the influence of silicones (PDMS), as often used cosmetic ingredients, on the microstructure of SC lipids was investigated. For this purpose the interactions of four different PDMS with excised human SC were examined first using differential scanning calorimetry (DSC) and wide angle X-ray diffraction for physical characterization. Because the physical properties of human stratum corneum strongly depend on the lipid composition, showing inter-and intra-individual differences, the interactions with an in vitro model lipid system containing SC fatty acids were also studied, using polarized light microscopy, transmission electron microscopy, small angle X-ray diffraction and DSC. The results revealed that the investigated PDMS do not change either the microstructure of excised human SC or the biphasic lamellar/inverse hexagonal structure of the in vitro model. We concluded that PDMS will not cause any side-effects when topically applied and that our simplified in vitro model could be helpful for estimating interactions between cosmetic ingredients and other topically applied substances and the skin barrier at an early moment of formulation development.     

膨胀型硅丙乳液防火涂料

本文介绍了以硅丙乳液为成膜物 ,添加成炭剂、阻燃剂、发泡剂制成的膨胀型防火涂料的组成、生产方法、性能及特点     

Polar silicones: structure-dielectric properties relationship

A series of polar silicones was synthesized in order to compare their dielectric properties. Different substituents with high dipole moment (epoxy, pyridyl, aldehyde, cyano-, nitroazobenzene) were attached by hydrosilylation to a poly(dimethyl-methylhydro)siloxane. Thiol-ene addition on a dimethyl-methylvinyl siloxane copolymer with similar composition was also used for chemical modifications with chloro- or carboxy- derivatives. This approach allowed comparison of properties with emphasis on dielectric behavior measured in liquid state, as a preliminary step in design and preparation of materials suitable for dielectric elastomers. Although a relatively low content of polar groups was used (8%), permittivity values of 5.4 and even 7.4 were achieved (at 10 kHz), either due to the large dipole moment or to the presence of important amounts of moisture. The water sorption capacity of the polar silicones was investigated by dynamic vapor sorption, while structural parameters of model molecules were calculated, in order to correlate the dielectric properties with the polarity/hydrophilicity of the substituents to the silicone chain. A combined effect of the calculated dipole moment, molar polarizability, molar volume, and the measured water sorption capacity on dielectric permittivity was observed.     

Vinyl ester resin modified with silicone‐based additives: II. Flammability properties

Silicone‐based additives have been used as fire retardants for thermoplastics, presenting the advantages of improving processing and impact resistance of the polymers. In this work we used three different silicone‐based additives as modifiers of a thermoset based on a vinyl ester resin. The additives are fine powders made up of about 50 wt % ultra high molecular weight polydimethylsiloxane and 50 wt % silica. The differences between them are the functional groups inserted on the additives and the size and size distribution of the particles. The additives were dispersed in resin containing 35 wt % of styrene. For curing the mixture a conventional catalyst and initiator were used and the reaction was carried out in two ways, differing in the curing temperature, the post curing temperature, and the time, and in the addition of dimethylaniline (DMA) as a promoter of the polyaddition reaction. The samples were characterized by thermogravimetric analyses and swelling experiments. The fire retardances of the samples were evaluated by the determination of the flash‐ignition, self‐ignition, and pyrolysis temperatures (ASTM D1919–91a), and of the oxygen index (ASTM D‐2863–91). The results obtained showed that the silicone‐based additives and the methods used in the preparation of the modified resin influence the flash‐ignition, self‐ignition, and pyrolysis temperatures, but not the oxygen index. Samples cured by different methods present different network characteristics, which influence their thermal decomposition. The volatile species produced by thermal decomposition may be a combination of inert and active species. The network structure may influence only the inert fraction of the volatiles, not the combustibles. These volatile inert species (smoke‐black, water vapor, carbon dioxide, etc.) probably dilute the combustibles in the solid and in the gaseous phase, increasing the flash‐ignition temperature of the samples. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 644–649, 2006     

Vinyl ester resin modified with silicone‐based additives. I. Mechanical properties

Silicone‐based additives have been used as fire retardants for thermoplastics and present the advantages of improving the processing and impact resistance of the polymers. In this study, we used three different silicone‐based additives as modifiers of a vinyl ester resin. The additives were fine powders made up of about 50 wt % polydimethylsiloxane and 50 wt % silica. The differences among them were the functional groups inserted in the polymer chains and the size and size distribution of the particles. The additives were dispersed in resin containing 35 wt % styrene. To cure the mixture, a conventional catalyst and initiator were used, and the reaction was carried out in three ways, which differed in the curing temperature, postcuring temperature, time, and addition of dimethylaniline (DMA) as a promoter of the polyaddition reaction. Dynamic mechanical analysis showed that the phase behavior of the resulting composites depended strongly on the curing conditions. The flexural modulus of composites containing 5 wt % additive was lower than that for the cured resin. The impact resistance of the composites also depended on the curing conditions but not on the composition or size of the particle of the additive. The fracture morphologies of specimens subjected to impact resistance tests were different for samples cured in the presence or in the absence of DMA, which suggested that it influenced the mechanism of network formation. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Cross-linking of hexenyl silicone polymers via the olefin metathesis reaction

The olefin metathesis reaction was explored as a novel method to produce cross-linked silicone polymers. Although this reaction is well-known for non-silicone-containing monomers, there are no examples of this reaction in the presence of siloxanes. We have discovered a catalyst system for the reaction of silicone substrates comprised of a 1:4 molar ratio of WCl₆/Sn(CH₃)₄. No metathesis is seen with the traditional ratio of catalyst and cocatalyst. In a model system, 1-hexenylmethylbis(trimethylsiloxy)silane underwent self-metathesis in greater than 75% yield. The catalyst system also promoted metathesis of a hexenyl containing silicone polymer.     

Poly(dimethylsiloxane) coatings for controlled drug release. II. Mechanism of the crosslinking reaction in emulsion

Earlier work focused on the crosslinking of hydroxyl‐terminated poly(dimethylsiloxane) (PDMS) particles in a stable latex suitable for spraying onto drug tablets, and established the conditions for eliminating the usual toxic catalysts that would be unacceptable in such pharmaceutical coating materials. Use of these coatings for controlling the rate of release of a drug, however, requires a better understanding of their properties and thus clarification of the mechanism through which the crosslinking occurs. The present study approaches this goal by documenting the effects of anionic, cationic, and nonionic surfactants at various concentrations, and in acidic, neutral, or basic media. FTIR spectroscopy was used to monitor the transportation of crosslinker from the water phase into the hydrophobic PDMS phase. The results suggest a possible mechanism for the crosslinking in sufficient detail to be used to optimize the coatings for drug‐release applications. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2186–2194, 2004     

Bamboo is a suitable template for polymerizations

In situ hydrosilation reactions and ring‐opening metathesis polymerizations were demonstrated to occur in bulk samples of bamboo. Bamboo was infused with silicone precursors or cyclodiene monomers with the assistance of supercritical CO₂ and subsequently crosslinked or polymerized, respectively. Bending stiffness, energy release rate, and specific fire resistance properties of the bamboo‐polymer composites were measured and compared with unmodified bamboo. Bamboo‐silicone composites showed an increase in fire resistance and a slight increase in mechanical properties. The bamboo‐poly(alkenamer) composites showed significant increases in mechanical properties. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Synthesis,characterization, and properties of novel organic/inorganic epoxy hybrids containing nitrogen/silicon via the sol–gel method

Organic–inorganic hybrids were prepared with a diglycidyl ether of bisphenol A (DGEBA) type epoxy and silane‐modified isocyanuric acid triglycidyl ester via the sol–gel process. The DGEBA‐type epoxy was modified by a coupling agent to improve the compatibility of the organic and inorganic phases. The sol–gel technique was used successfully to incorporate silicon and nitrogen into the network of hybrids, increasing the thermal stability. Fourier transform infrared and ²⁹Si‐NMR were used to characterize the structures of the hybrids. The results revealed that trisubstituted siloxane bonds (T 3 ) was the major environment forming a network structure. The morphology of the ceramer was examined with scanning electron microscopy, Si mapping, and transmission electron microscopy. The particle sizes were less than 100 nm. The hybrids were nanocomposites. The ultraviolet–visible spectra of the epoxy hybrid showed no obvious absorbance over a range of 400–800 nm. This phenomenon revealed that the hybrids were transparent. The thermogravimetric analysis revealed that the char yields of the hybrids increased with the contents of the inorganic components. The integral procedure decomposition temperatures of the hybrids were higher than that of the pure epoxy. The thermal stability of the hybrids increased with the contents of the inorganic components. The inorganic components could improve the thermal stability of the pure epoxy. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007