John N. Hay

Recent developments in the chemistry of cyanate esters

During the last decade, aromatic cyanate esters (CE) have become established as a new and unique class of high-performance thermosetting resins for use as prepreg matrices in both the electronic and aerospace industries. The basic properties of CE resins, such as low moisture absorption, excellent electrical properties and good flammability characteristics, make them attractive composite matrices and differentiate them from standard epoxy resins (epoxies) and bismaleimides. Although they are relative newcomers to the composites industry, CE resins are enjoying unprecedented success for certain applications. Several major space and radome manufacturers have qualified CE resins despite the extensive database on epoxies and the inherently conservative nature of industry. Increasing demands on the materials used in these areas have stimulated the use of CE resins over other more conventional polymers. The aim of this review is to bring to the readers attention the more recent developments in the science of cyanate esters.

Recent technological developments in cyanate ester resins

Although they are relative newcomers to the composites industry, cyanate ester (CE) resins are enjoying unprecedented success in certain applications. Several major space and radome manufacturers have qualified CE resins despite the extensive database on epoxies and the inherently conservative nature of the industry. Increasing demands on the materials used in these areas have stimulated the use of CE resins over other more conventional polymers. The aim of this review is to bring to the reader’s attention the more recent developments in the processing, toughening, properties and applications of cyanate esters, an exciting, versatile and important class of resins.

Synthesis of sub-200 nm silsesquioxane particles using a modified Stöber sol–gel route

A range of silsesquioxane nanoparticles has been synthesised by a modified Stober method. An aqueous sodium silicate solution was used as a seed for further growth of methyl-, ethyl-, phenyl- and vinyl-modified silica spheres. Particles with diameters in the range 60–180 nm were prepared by hydrolysis and condensation reactions of organotrimethoxysilane precursors in a mixture of water, ammonia and ethanol at room temperature. The materials prepared in this study have relatively low specific surface areas and average pore diameters typical of mesoporous solids. The results suggest that the mechanism for the formation of the particles is primarily aggregation and monomer addition.

Stability of various metalloporphyrin catalysts during hydrogen peroxide epoxidation of alkene

Abstract The ability of certain Fe and Mn metalloporphyrins to catalyse the H 2 O 2 epoxidation of cyclooctene is analysed. The ‘efficiency’ of epoxidation is dissected into contributions due to inherent catalyst epoxidation ability, catalyst H 2 O 2 dismutation ability, and catalyst stability towards the oxidant. It is shown that catalyst stability is a major factor contributing to apparent catalyst ability. The preparation of a sol–gel encapsulated metalloporphyrin is reported and it is shown to exhibit reduced rate of epoxidation, but a much enhanced stability.

Preparation of Inorganic Oxides via a Non-Hydrolytic Sol-Gel Route

Inorganic oxides have been synthesized successfully under mild reaction conditions using a solvent-free non-hydrolytic sol-gel process based on the condensation reaction of “metal” chlorides with oxygen donors such as alkoxides, aldehydes and ethers. Iron(III) chloride was found to be an effective catalyst for the reactions. The order of reactivity of the halides was generally titanium > aluminium > silicon, but in some cases reaction was halted by premature gelation of intermediate species. Silica, alumina and titania were all prepared and characterized by various methods. Calcination studies on the silicas showed these materials to be amorphous up to at least 700°C, but devitrification occurred at 1000°C. Crystallization was studied by X-ray powder diffraction.

Synthesis of Organic-Inorganic Hybrid Particles by Sol-Gel Chemistry

The synthesis of ORganically MOdified SILica (ORMOSIL) particles has been carried out using both the hydrolytic and non-hydrolytic sol-gel routes. The hybrid (nano)composites are organically modified with an alkyl or aryl group covalently bonded to silicon. Hybrids have been synthesised in an aqueous sol-gel process by a modified Stöber route, producing spherical nanoparticles with diameters in the range 50–300 nm. The size of the particles can be controlled by control of certain reaction parameters. Smaller ormosil nanoparticles can be synthesised by a base-catalysed emulsion polymerisation route, by varying the type and concentration of surfactant and precursor feed rate. In this case, particles in the size range 3.5–10 nm can be obtained. Hybrids have been synthesised from hyperbranched polyesters by encapsulation in a silica matrix using the hydrolytic sol-gel route. Optimisation of the reaction conditions allows the hybrids to be produced as isolated sub-micron spherical particles.Ormosil particles have also been synthesised using the non-hydrolytic sol-gel route, which may lead to products of different morphologies because of the different polarity of the reaction medium. Different reaction conditions were studied in order to optimise the size and shape of the particles, including choice of solvent, use of surfactants and addition of polystyrene. Dimethylsulfoxide acts as a novel oxygen donor for the catalyst-free formation of colourless silsesquioxanes.

Review Environmentally friendly coatings using carbon dioxide as the carrier medium

The use of supercritical or liquid carbon dioxide as a medium for delivering coating systems is attracting much interest because of concerns over the environmental effects of volatile organic compound (VOC) emissions from conventional coatings. Significant reductions in VOC emissions can be achieved by replacing some or all of the organic solvent by CO2 in spray coatings. Technical and commercial benefits are also claimed for these systems, including improved coating efficiency and operating cost savings. In this review, the range of current and potential applications achievable using CO2-based coatings is discussed. In addition to spray coatings onto a variety of substrate surfaces, CO2 processes can be used to produce controlled particle size powders for use in powder coatings and also for the coating of preformed particles such as metal powders and pharmaceuticals for controlled release in drug delivery. Use of CO2 in spin coating and microlithography offers the potential for significant waste reduction. Specific substrates where use of CO2 can be beneficial include the treatment of building stone and wood treatment. CO2 can aid surface impregnation of substrates because of its high diffusivity and the potential for substrate swelling.

A versatile route to organically-modified silicas and porous silicas via the non-hydrolytic sol–gel process

The non-hydrolytic sol–gel route has been used to provide a versatile synthesis of organically-modified silicas (ormosils), via a rapid, facile reaction in the absence of any solvent. The ormosils are synthesised by reaction of silicon halides with alkoxides, where one or both of the reactants contains the organic substituent. The products are organic–inorganic hybrid nanocomposite materials, where the organic modification is an alkyl, aryl or vinyl group covalently bonded to silicon. The ormosils have low surface areas, but may be converted to high surface area, largely microporous silicas by thermal degradation of the organic component.

Evidence for parallel destructive, and competitive epoxidation and dismutation pathways in metalloporphyrin-catalysed alkene oxidation by hydrogen peroxide

Abstract A kinetic and product yield analysis of the F20TPPFeCl-catalysed H2O2 epoxidation of cyclooctene has shown that decomposition is not via either the oxoperferryl intermediate (F20TPP+ )FeIVO or the oxoferryl intermediate F20TPPFeIVO, but appears to involve direct oxidation of the porphyrin in parallel with the catalytic epoxidation cycle. This is in contrast to the reaction of F20TPPFeCl in the absence of alkene where decomposition is via F20TPPFeIVO.

Toughening of Epoxy Resins by Polyimides Synthesized from Bisanilines

A range of thermoplastic polyimides has been synthesized and used to modify a DGEBA-DDS epoxy thermoset. The influence of polyimide end-group functionality and particle size has been examined. Increases in fracture toughness of up to three times that of the neat resin have been achieved, together with no loss of modulus. Thermal capability is little affected across the range of modifying polyimides. The influence of copoly(imide–imide)s has also been studied. Morphological examination has revealed a range of structures in the blends, including some which are unusual. A particulate toughening mechanism is proposed for one high fracture toughness system.