APPLICATIONS: High temperature siloxane elastomers which can perform from -50°C to +300°C

IN A NUTSHELL: A new type of high temperature resistant elastomers for applications under a variety of extreme service conditions ranging from deep-sea under-water environments to outer space has been developed. This unique family of materials, referred to as exactly alternating silarylene-siloxane, SARSOX, polymers, includes a variety of different structures that can be precisely designed to satisfy a large number of extreme and unusual requirements. Some of these may include retention of useful mechanical and/or electrical properties during extended exposures to an unprecedented range of operating temperatures from -70°C to 400°C, pronounced inertness to a variety of chemical agents and resistance to solvents, very low surface energies and non-stick character and so on. These materials are best tailor-made for every desired application and may be presently obtained in quantities of hundreds of grams.

 
THE PROBLEM The rapid advancement of modern technology increasingly demands high temperature resistant elastomers for a wide variety of engineering applications under extreme service conditions, particularly for aerospace, defense, energy and computer industries. Such materials are generally required to have long-term thermal, thermo-oxidative and hydrolytic stability at and above 300-350°C and maintain pronounced flexibility to well below ambient temperatures (i.e., -30°C and below). They also often need to retain useful mechanical and electrical properties over as wide as possible range of operating temperatures, show inertness toward chemically aggressive environments and resistance to various solvents, and be economical and processable by usual industry-used methods. Their potential fields of applications range from components in high flying jet planes and space vehicles to the fabrication of metal articles where retention of elasticity is required upon short term contacts with hot molds followed by quick cooling to room temperature or below, from computer chips production where resist layers are needed that can withstand temperatures used in the baking cycles to high-temperature integral fuel tank sealants which need to operate between -50°C and +350°C without swelling on contact with jet fuels, from gas separation processes which require polymers resistant to changes in temperature and pH to ablative materials, adhesives, protective and insulating coatings, gaskets, sealants and O-rings, stationary phases for high temperature gas chromatography and so on. However, although the needs are great, successful solutions are still not available since traditional carbon-based rubbers cannot withstand longer exposures to 150-200°C and above, while commercial silicone-based products start decomposing below 300°C if even traces of ionic substances are present in the environment.

THE STATE OF TECHNOLOGY To develop successful high temperature resistant elastomers, many investigations of various types of new materials have been carried out in the last decades. Among these, the most successful ones were focused on modified siloxane polymers in order to take advantage of their well-known combination of excellent low temperature elasticity and high temperature stability. In fact, polyalkylsiloxanes, such as polydimethylsiloxane, -[Si(CH₃)₂-O]_n-, the base polymer of the present-day silicone industry, and polydiethylsiloxane, -[Si(CH₃CH₂)₂-O]_n-, have the lowest glass temperatures known to polymer science of -125°C and -140°C, respectively (note that a rubber becomes elastic at about 20°C above its glass temperature, so the lower the glass temperature the lower the service temperature at which this happens), and at the same time can withstand by about 100-150°C higher upper use temperatures than any carbon-based elastomer.

Therefore it seemed promising that new siloxane-based polymers can be designed with better thermal and thermo-oxidative properties but with retained low temperature elasticity. As a consequence, many such polymers have been developed and tested, including metallosiloxanes, polysilazanes, phenylsilsesquioxanes, polyaryloxysilanes, polysilarylenes, poly(carborane-siloxanes) and poly(silalkarylene-siloxanes). However, the most success has been achieved with silarylene-siloxane, SARSOX, polymers in which a fraction of oxygen atoms in the polysiloxane -(SiR₂-O)_n- main chains is replaced by thermally stable aromatic units, Ar, in relative amounts in which they do not substantially disrupt the low temperature flexibility of the Si-O chains while at the same time prevent their traditional cyclization degradation mechanisms from occurring thus increasing their thermal and termo-oxidative stability.

OUR NOVELTY We found that the best combinations of low temperature elasticity (the glass temperature) and high temperature stability (both purely thermal and thermo-oxidative) in SARSOX polymers are obtained from exactly alternating silarylene and siloxane structures in which every third oxygen atom in the parent polysiloxane chain is replaced with an aromatic group (such as p- or m-phenylene, diphenyl-ether or alike) to give -[SiR₂-Ar-SiR₂-O-SiR₂-O]_n- type chains. Depending on the aromatic unit and/or side groups used, such polymers have glass temperatures ranging from -70°C to -30°C and retain their useful mechanical properties for extended periods of time to well above 300-350°C. They can be made by a variety of different synthetic approaches via step-growth polymerization strategies where the choice of particular preparation strategy predetermines the resulting polymer molecular weight which may range from 20,000 to above 500,000. They can be made with different Ar units or their combinations, and with different side groups, R, including methyl, phenyl, trifluoropropyl, hydrido, vinyl, etc. Depending on which side groups are used, they can be crosslinked (i.e., vulcanized) into useful elastomers using traditional silicone-crosslinking processes including both room temperature vulcanization (RTV) based on hydrosilylation chemistry and high temperature curing with free radicali nitiators based on hydrogen abstraction. The crosslinking density of such elastomers (and hence their mechanical properties) can be easily dialed-in by selecting the relative content of the crosslinkable groups incorporated into the precursor polymer which, in turn, can be easily controlled during the synthesis of the latter. Furthermore, SARSOX elastomers can be processed by traditional processing techniques of rubber industry, filled with a variety of compatible fillers, including both extending and reinforcing ones, to achieve excellent mechanical properties at relatively low filler concentrations, and by adding appropriate antioxidants, they can be made even more stable to thermo-oxidative processes. Also, by introducing fluorocarbon units into the side groups, they can be made long-term solvent resistant and with even lower surface energy (i.e., increased non-stick properties) than with traditional methyl groups. At present, these unique polymers clearly represent materials of choice for high temperature resistant elastomers that can be used in terrestrial applications from equator to polar circles, as well as under extreme conditions encountered in deep-sea and space environments.