Elastomeric silicone sealants

Elastomeric silicone sealants first appeared in the early 1950 s. The first of these, patented by J. F. Hyde of Dow Corning Corporation, was a two-part system consisting of an acid-ended polysiloxane and polysilicate.— With this system, materials which cured at room temperature were possible and the term "RTV" (Room Temperature Vulcanizing) was coined. These two-part systems required premixing in appropriate ratios before application and curing. 

The first silicone sealants were developed in the early 1940 s. These early sealants resembled putties. They contained only a filler and a polydlmethylslloxane fluid they did not cure to an elastomeric solid. Nevertheless, they offered one main advantage over organic sealants painting was not required to prevent hardening and weathering. 

Since most silicone adhesives and sealants are elastomeric in nature, their physical property testing often parallels classical rubber testing approaches. Common tests include durometer, tensile strength, elongation, and modulus. Several methods are available for the measurement of rubber properties, but the most commonly used are the American Society for Testing and Materials (ASTM) D-412, Test Method for Rubber Properties in Tension, and the ASTM C-661, Standard Test for Indentation Hardness of Elastomeric-Type Sealants by Means of a Durometer. These properties vary widely with the product... 

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. 

The adhesion-in-peel test was used to determine peel strength and adhesion characteristics of a cured-in-place silicone elastomeric joint sealant on aluminium substrates. The sealant used was Dow Corning Type 3145 RTV Adhesive Sealant. The results showed that the silicone sealant had poor adhesive bonding to the untreated aluminium. Plasma polymerisation of hexamethyl-disiloxane(HMDS) onto the aluminium was shown to move the locus of adhesive failure to between the plasma film and the silicone. Plasma polymerisation of HMDS... 

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. 

Silicone (Beers, 1977 Elias, 1986 Prane, 1990) - Silicone sealants became commercially available in the late 1950 s. Since that time they have found widespread use in many markets including the construction, industrial, automotive, and consmner due to their unique eombination of properties and versatility. These include exeellent UV, ozone, ehemical and water resistance excellent elastomeric properties (extension, compression, high recovery) over a wide temperature range (-85 to 400°F/-65 to 200°C) ease of application (even... 

Although the primary function of sealants is to seal, adhesion promoters are often added, which allows them to adhere to the adjoining base materials. It is therefore sometimes difficult to distinguish between an adhesive and a sealant. For example, structural silicone adhesives are used in the building construction industry owing to their sealing, adhesive, elastomeric properties, and their resistance to harsh environmental conditions [67,70,77]. 

SEALANTS. Any organic substance that is soft enough to pour or extrude and is capable of subsequent hardening to fonn a permanent bond with the substrate. Most sealants are synthetic polymers (silicones, urethanes, acrylics, polychloroprene) that are semisolid before application and later become elastomeric. 

The introduction of different types of reactive groups on the polydimethylsiloxane backbone enables crosslinking reactions to take place. The silicones can be applied as coatings on a variety of substrates, or as sealants. Elastomeric products such as seals, tubing, or other shaped products can be made by extrusion or molding processes. 

Cure time testing tends to be somewhat subjective, but again there are methods available, such as ASTM C-679, Tack-Free Time of Elastomeric Sealants. Tack-free time is the curing time required for the product to develop a skin that is not damaged when subjected to application and removal of a plastic film. It is important in all cases to determine the cure characteristics of the product in actual working conditions. Since most silicones cure by reaction with moisture in the air, the sensitivity of cure time to humidity should be determined. Surface cure rates can usually be tailored to meet application requirements. 

Product technologies that have high adhesion include elastomeric adhesives and sealants (see Silicone adhesion and Rubber-based adhesives). Their adhesive strength reflects both adhesion and cohesion properties (see Peel tests and Rheological theory). In each of these product technologies, the unique physico-chemical properties of silicones, in general, and in adhesion, in particular, originate from the atomic and chemical bond structures of the PDMS backbone. 

Within solvent release type thermosetting sealants, there are Neoprene, hutadiene-styrene, chlorosulfonated polyethylene, EPDM, and silicones. Solvent release types constitute the largest variety of sealants and are composed of three parts (1) the liquid portion of the compound which is the basic non-volatile polymer/elastomeric vehicle, (2) the pigment component, and a (3) solvent or thinner component used to ease the process and to control the thickness. The sealant is cured and its required viscosity is controlled by the evaporation of solvent. 

Polysulfide sealants, although facing strong challenges from silicones and polyurethanes, continue to be the dominant segment of the elastomeric sealant market (Table 1) [2],... 

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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