02 Jan Aerospace Polymer Sealants
Where people have, since the industrial revolution, used metals, nature uses polymers. Almost all biological systems are built of polymers which not only perform mechanical functions (like wood, bone, cartilage, leather) but also contain and regulate chemical reactions (leaf, veins, cells). People use these natural polymers, of course, and have done so for thousands of years. But it is only in this century that they have learned how to make polymers of their own. Early efforts (bakelite, celluloid, formaldehyde plastics) were floppy and not very strong; it is still a characteristic of most simple synthetic polymers that their stiffness (for a given section) is much less than that of metal or, indeed, of wood or bone. That is because wood and bone are composites: they are really made up of stiff fibres or particles, embedded in a matrix of simple polymer.
People have learned how to make composites too: the industries which make high-performance glass, carbon, or Kevlar-fibre reinforced polymers (GFRP, CFRP, KFRP) enjoy a faster growth rate (over 10% per year) than almost any other branch of materials production. These new materials are stiff, strong and light. Though expensive, they are finding increasing use in aerospace, transport and sporting goods.
Aerospace advanced materials and how to seal or join them.
The new polymers are as exciting as the new composites. By crystallising, or by cross-linking, or by orienting the chains, new polymers are being made which are as stiff as aluminium; they will quickly find their way into production. The new processing methods can impart resistance to heat as well as to mechanical deformation, opening up new ranges of application for polymers which have already penetrated heavily into a market which used to be dominated by metals. No designer can afford to neglect the opportunities now offered by polymers and composites.
But it is a mistake to imagine that metal components can simply be replaced by components of these newer materials without rethinking the design. Polymers are less stiff, less strong and less tough than most metals, so the new component requires careful redesign. Composites, it is true, are stiff and strong. But they are often very anisotropic, and because they are bound by polymers, their properties can change radically with a small change in temperature. Proper design with polymers requires a good understanding of their properties and where they come from.
Fuel Tank Sealants, Corrosion Inhibitive Sealants, Windshield and Canopy Sealants, High Temperature Sealants, Firewall Sealants, Electrically Conductive Sealants, Low Adhesion Sealants for Access Doors, Fast Cure Sealants for Flight Line Repairs and Jointing Compounds all make use of the emerging technology of aerospace polymer sealants, polysuphide sealants and polyurethane sealants for the following advantages they offer… higher performance, resistance to higher temperatures, better application properties, rapid cure, low specific gravity, and better health and safety characteristics, chromate free sealants which also inhibit corrosion.
Polysulphide sealants use one of the older polymer technologies for the manufacture of high-performance sealants. Despite a decline in popularity, they continue to be used in many parts of the world.
Chemically, polysulfides are sulfur-containing liquid polymers. Others are modified polysulfides (polythioethers). They are normally cured using manganese dioxide, lead oxide or a dichromate. Polysulfide sealants can be formulated as one- or two-component products.
Polysulphide sealants are mainly used for the manufacture of insulating glass windows. Because of their excellent resistance to jet fuel, they are also used for expansion joint sealants in aircraft construction and in various aerospace applications, including sealants for aircraft fuel tanks and windshields. Electrically conductive sealants are often based on polysulfides.
Polyurethane sealants use an organic compound with particularly good moisture- and corrosion-resistance characteristics. Thus, polyurethane sealant is useful in both industrial and commercial applications. Additionally, polyurethane is often used as a heavy-duty adhesive, as well as a coating.