Chemically cured sealant

All chemically curing sealants employ catalysts, and silicone sealants are no exception. 

A typical construction involves a combination system employing a butyl mastic as the barrier sealant between the metal spacer and glass and a back-up, chemically curing sealant serving as an adhesive to hold the unit together. An insulated glass mastic formulation is shown in Table 14. Vistanex LM low molecular weight PIB is combined with a platy talc filler... 

D 5893 Specification for Cold Applied, Single Component, Chemically Curing Silicone Joint Sealant for Portland Cement Concrete Pavements... 

Two-component materials often use water as the chemical curing agent, by using wet fillers or by adding water to one of the components (SWRI, 1995). A fungicide can be incorporated in sealants used for moist environments (i.e. sanitary sealants). 

ASTM D 5893/D 5893M. 2010. Standard specification for cold applied, single component, chemically curing silicone joint sealant for Portland cement concrete pavements. West Conshohocken, PA ASTM International. 

Cold applied single component, chemically curing, silicone sealant may also be used for crack filling, complying to CEN EN 14188-2 (2004), ASTM D 5893 (2010) or other specifications. 

The elastomeric type sealants are chemically curing systems based on a limited variety of polymers. The best known of these being polysulfide, silicone (see Silicones structures), Polyurethane and the modified silicones, which are urethane prepolymers with reactive silane end groups. All these are available as single- or two-part systems. 

Two-part sealants, such as the chemically curing two-part polysulfide, silicone, epoxy or urethane resilient sealants, are applied on-site by mixing two parts the polymer base part and the catalyst together and within the pot-life of hardening, which is usually one hour, the sealant is obtained. Chemically curing thermosets have much greater service lives than the others, and they usually need adhesion additives in order to achieve a proper bond to a surface. 

Polyurethane is the fastest growing chemical family in the adhesives and sealants industry, as polyurethanes enjoy a very versatile chemistry One or two components, chemical curing, humidity curing or PUR reactive hotmelts, flexible, elastomeric or rigid adhesives, sealants, and potting compounds. All kinds of adhesives and sealants may be formulated with numerous different PU, polyols and isocyanate raw materials. 

Catalysts and Retarders (Elias, 1986 Petersen, 1986 Regan, 1986) -Catalysts (accelerators) speed up the chemical curing reaction without being consumed themselves. Retarders (inhibitors) slow down such reactions. They are always used in conjunction with a curing agent which is itself consumed in the polymerization process. The catalysts or retarders in the adhesive or sealant are usually used at very low concentrations 0.1 weight percent. 

A chemical property of silicones is the possibility of building reactivity on the polymer [1,32,33]. This allows the building of cured silicone networks of controlled molecular architectures with specific adhesion properties while maintaining the inherent physical properties of the PDMS chains. The combination of the unique bulk characteristics of the silicone networks, the surface properties of the PDMS segments, and the specificity and controllability of the reactive groups, produces unique materials useful as adhesives, protective encapsulants, coatings and sealants. 

There are many applications for silicone adhesives, sealants, or coatings where the condensation curing systems are not suitable. This is because they are relatively slow to cure, they require moisture to cure that can itself be in some cases uncontrollable, and they evolve by-products that cause shrinkage. Adhesives needed in automotive, electronics, microelectronics, micro electromechanical systems, avionic, and other hi-tech applications are usually confined to vei7 small volumes, which can make access to moisture difficult. Also, their proximity to very sensitive mechanical or electronic components requires a system that does not evolve reactive chemicals. 

Silicone They have excellent heat resistance up to 260°C (500°F), chemical resistance, good electricals, compatible with human body tissues, etc. and a high cost. There are the room temperature vulcanizing (RTV) types that cure and cross-link at ambient temperatures, catalyzed by moisture in the air. It is a good sealant and excellent for making flexible molds for casting. It is widely used for human implants. 

Room-temperature-vulcanizing silicone rubber (General Electric and Dow Corning) is available at hardware stores and is very useful as an adhesive and sealant. Atmospheric moisture is necessary to effect the cure, so broad areas of impermeable materials should not be cemented with these preparations. The uncured material evolves acetic acid, and the cured material appears to lose some weight in high vacuum but if used with moderation, it can be considered a satisfactory vacuum sealant for most chemical vacuum systems. 

Aliphatic amines Convenience Room temperature cure, fast elevated-temperature cure Low viscosity Low formulation cost Moderate chemical resistance Critical mix ratios Strong skin irritant High vapor pressure Short working life, exothermic Poor bond strength above 80°C Rigid, poor peel and impact properties Adhesives and sealants Casting and encapsulation Coatings... 

Epoxy-based primers are commonly used in the aerospace and automotive industries. These primers have good chemical resistance and provide corrosion resistance to aluminum and other common metals. Primer base resins, curing agents, and additives are much like adhesive or sealant formulations except for the addition of solvents or low-viscosity resins to provide a high degree of flow. 

The rate of reversion, or hydrolytic instability, depends on the chemical structure of the base polymer, its degree of crosslinking, and the permeability of the adhesive or sealant. Certain chemical linkages such as ester, urethane, amide, and urea can be hydrolyzed. The rate of attack is fastest for ester-based linkages. Ester linkages are present in certain types of polyurethanes and anhydride cured epoxies. Generally, amine cured epoxies offer better hydrolytic stability than anhydride cured types. 

Certain chemical linkages are susceptible to hydrolytic attack and, if present in an adhesive or sealant, are potential sites for irreversible reaction with water that has diffused into thejoint. Such hydrolytic (chemical) degradation causes a permanent reduction in the cured physical properties. The functional groups present in the chains are hydrolyzed, resulting in both chain breaking and loss of crosslinking. 

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