Multicomponent sealant

Polyurethane sealants are available as in both single-component and multicomponent formulations and in each case a variety of performance... 

Silicone adhesives and sealants were introduced approximately 40 years ago and many of the silicones used in the early days are still performing. Products are available in a variety of forms, from pastelike materials to flowable adhesives. Both single- and multicomponent versions are available, with several different cure chemistries. Most of the silicones of commerce are based on polydimethylsiloxane (PDMS) polymers. Other siloxane polymers may be used when resistance to ultrahigh temperature, ultralow temperature, or solvents is required. 

Silicones are available in one- and multicomponent forms. The one-component types are commercially the most important and will be the focus of most of this discussion. These products, which generally cure by reaction with atmospheric moisture, are called RTV (room-temperature vulcanizing) sealants or adhesives. The surface cure rate of these products is a function of the cure system, but the rate of cure in depth depends on the ability to transmit water vapor through the mass of sealant. Silicones are highly permeable to moisture vapor, and generally the one-component types cure at a rate of about 0.3 cm/day. Due to this high vapor permeability, the one-component silicones typically cure faster than do their nonsilicone counterparts. 

The multicomponent products generally do not rely on moisture penetration for cure. Their chief attribute is fast cure in very deep sections. Thus many industrial production lines that demand fast cure use a two-component sealant (including the use of silicone encapsulants for electrical components). Cure of these two-part systems can be accelerated further by additional catalyst or exposure to elevated temperatures. 

It was concluded that the technique used was a powerful iheologi-cal technique because it provided quantitative evidence that, as indicated by the xenon and QUV tests, there are significant differences between urethanes (A), (B), and (C) sealants. Also, at least one silieone sealant was much stiffer below -35°C (-31°F) than the polyurethanes studied. One ofthe four multicomponent urethane sealants showed similar performance to silicone sealants under severe test condition. Finally, the authors eoncluded that the current test incorporated in ASTM C 920 Speeifieation for Elastomeric Joint Sealants does not identify the critical performance differences between sealants and must be updated to accomplish this critical task. 

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. 

Bulk Materials. These are single or multi-component sealants and caulks sold in units from 1.5 to 50 gallons, to be applied by knife or filled into caulking cartridges (see Fig. 2) or mechanical pumping equipment. Multicomponent materials (two or more components) are typically mixed by powered drill mixer (see Fig. 3) or meter mix equipment. 

Test for staining and color change of single or multicomponent joint sealants... 

While the basic chemistry and physical behavior of linear polysiloxane polymers is well studied and, generally speaking, well understood, simple unmodified polysiloxanes are rarely encountered in real world applications. The majority of commercial silicone products are complex multicomponent formulations which take the form of elastomeric rubbers, adhesives, sealants, coatings, fluids or gels. Silicone elastomers are chemically crosslinked networks of linear polysiloxanes which exhibit poor mechanical properties in their native, unmodified state [1]. In order to obtain the desired combinations of mechanical, physical and chemical properties for a specific application, commercial high-performance silicone elastomers... 

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