Weathering sealants

Aircraft Sealants. Polysulfides have been used for sealing fuel tanks and aircraft stmctural components since the 1940s. There are stringent requirements for these sealants. They must have outstanding resistance to fuels and exceUent adhesion to many different materials such as various metal aUoys and protective coatings used in aircraft constmction. The sealants must also perform in extremely variable weather conditions (10). The polysulfides have an exceUent performance record. 

Wea.ther lbillty. One of the more destmctive elements is exposure to sunlight specifically, ultraviolet (uv) light. AH sealants are affected by weathering, but there is much difference in the effect of weathering on different sealants. Most sHicones are stable with respect to uv exposure. Urethanes and polysulfides show effects of uv exposure, but can be formulated with uv absorbers to provide reasonable lifetimes in most appHcations. However, there are exceptions in aH classes of sealants and specifiers must be carehil to look for test data that has proven a specific sealant s durabHity. The source of the test data is also important data from an independent testing laboratory is generaHy apt to be more reHable. 

The hydrogenated materials have a higher maximum service temperature than the conventional SBS materials and, because of the absence of double bonds in the chain, better weathering properties. Known as SEBs these materials now find use in adhesives, sealants and roofing membrane compounds. These also find use in highly filled automotive acoustic barriers and in medical, sports and leisure applications. 

Sealants for insulated glass windows are based on butyl polymers. These sealants have excellent weathering and ageing, low-moisture vapour transmission and are non-fogging [66]. 

Three different commercial formulations of silicone sealants from Dow Corning was used in the NSF sponsored studies. They were DC-790, DC-995, and DC-983, in the order of increasing modulus. Dumbbell test coupons (samples) were prepared as per the ASTM standards. Some test coupons were maintained at ambient conditions as control and the rest were subjected to simulated weathering. The weathered coupons were removed from the test layout at regular intervals of time and were tested for any changes in crosslink density due to exposure. 

Some hydrocarbons may form gel clusters or layers with some sealant fluids therefore, providing for cold weather heating and/or cleaning of the unit is necessary. 

The weather conditions also added headaches. In winter the temperature sometimes gets below -50°F (-45°C) and may remain at -30°F (-35°C) for sustained periods. It is so cold that certain buildings need air locks to prevent excessive heat losses when people enter or leave. Many ordinary construction materials cannot withstand the temperature. The usual mastic sealing compound has to be replaced by an expensive silicone sealant. In addition to this the region has high winds and is earthquake-prone. 2... 

It is mainly used in the industrial production of rigid polyurethane foams. Many other uses are in the fields of coatings, adhesives, sealants and elastomers such as paints, adhesives, weather-resistant sealing materials and foohvear. There is use also in the production of particle board (bonding of wood) and mould cores for the foundry industry (European Union, 1999). 

Of great practical interest are sealants and compounds from low-molecular silicone elastomers, which in the presence of such solidifying agents as alkoxyaminosilanes (ADE-3, AHM-9), acetoxyorganosilanes, etc. solidify at room temperature in contact with humidify in air. They have good moisture and weather resistance, endure high and low tempera-... 

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