Construction sealants weathering

Weathering of construction sealants is an issue of importance not only to sealant formulators, manufacturers, and specifiers, but also to contractors and users. Sealants can fail due to weather exposure and aging. This type of failure is often characterized by discoloration and crazing and/or stifihess of the sealant surface and is a result of the individual or... 

MalikP l used dynamic mechanical analysis to study the effect of accelerated weathering on the storage and loss moduli of commercially available polyurethane (PU) and silicone construction sealants. Three multicomponent PU and one-component silicone sealants were used in the study. The sealants were exposed to 8 hours of UV exposure at 65°C alternating with 4 hours of condensation at 50°C for 600 and 1000 hours. The moduli G, G tan 5, and the dynamic viscosity (77) were measured in the temperature range of 25° to 125°C. The temperature was increased to 125°C, held for 2 hours and cooled to 25°C. 

Weathering (9). Free films of commercially available silicone and polyurethane construction sealants were placed back-to-back in a frame where the films were separated with an aluminum divider. All of the free films were loaded into a xenon-arc Weather-o-meter (Atlas Electric Devices Company) for weathering. One series of films was thus exposed to light and water while the other was exposed to water only. The total time of exposure was 8000 hours. 

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... 

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

Sealants, for construction joints retention of movement capability high tear strength good recovery weathering resistance range of colours... 

The silicone polymer backbone is composed of Si-O-Si bonds. This bond is very strong and stable with a bond energy of 87 Kcal/ mole. The polymer can tolerate 250°C to 300°C without decomposing.— The fully compounded silicone sealant, when cured to a rubber, can withstand 200°C for sustained periods of time with no special additives and even higher temperatures with polymer modifications and/or heat stability additives.— The Sl-O-Si molecular structure is also transparent to U.V., so silicone sealants are virtually unaffected by weather. Samples of silicone sealants used in exterior construction applications have been tested after 20 years of actual performance. These samples exhibited essentially no change in physical properties or adhesion during that time period. 

Silicone adhesives cure without the application of heat or pressure to form permanently flexible silicone rubber. The rubber remains flexible despite the exposure to high or low temperatures, weather, moisture, oxygen, ozone, or UV radiation. This makes them useful for joining and sealing joints in which considerable movement can be expected, such as intermediate layers between plastics and other materials of construction (e.g., acryUc glazing). Several types of silicone adhesives/sealants are available, including one-part and two-part systems. 

Room temperature vulcanizing (RTV) systems with outstanding resistance to weathering together with good adhesion to a wide variety of substrates form the basis of families of coatings, sealants and adhesives which find much use in exposed applications particularly in the construction Industry and this is one of the most Important applications of silicones today. 

Among sealants produced in the United States, 80% are used in construction. Due to their excellent resistance to hydraulic and aircraft fuels, weather, water, water chemical treatment, UV hght, the fields of polysulfide oligomer appUcations and their end uses are... 

ASTM C 1589 [406] Standard Practice for Outdoor Weathering of Construction Seals and Sealants... 

Silicone adhesives/sealants have good resistance to water and weathering (UV and chemical attack). This, coupled with their high flexibility and good gap-filling makes them suitable for many applications in the construction, marine and vehicle assembly industries. 

Sealants and caulks are used to fill joints, gaps and cavities between two or more similar or dissimilar substrates. Sealants and caulks seal these discontinuities in structures for economy, convenience, and functional necessity. Their purpose is to isolate and control conditions, such as water and weather, to optimize the functioning of the structure being sealed. Today the number of applications for sealants and caulks in construction, industrial and consumer markets is growing. These materials are required to seal and adhere to the appropriate mating surfaces over a wide range of temperatures, environmental stress and joint movement conditions. These surfaces to be sealed include a wide variety of glass, concrete, masonry, wood, steel, aluminum, and plastic sub-... 

RTV silicone adhesive is a rubberlike polymer called polydimethsiloxane. RTV stands for room temperature vulcanizing, or simply a rubber that cures at room temperature. Silicone rubber adhesives are made using a complicated process that turns elemental silicon metal made from sand (silica) into a rubbery polymer. When cured, silicone rubber adhesives/sealants have excellent resistance to heat (500—600 °F) and moisture that makes them exceptionally suited for outdoor weathering applications such as sealant and caulking compounds in the construction industry. Because of its exceptional properties, silicone adhesive has been used in some exotic applications such as the soles of the boots worn by the first astronauts to walk on the moon. Silicone adhesive/sealants are used to seal windows, doors, and portholes on the space shuttle and many satellite missiles. A special silicone adhesive is used to bond the heat shield tiles on space shuttles. 

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