Adhesion of silicone sealants

Gutowski, V.S., Russel, L. and Cerra, A. (19 ) New tests for adhesion of silicone sealants, in ASTM Special Technical Publication 1200, American Society for Testing and Materials, Philadelphia, PA, pp. 87-104. 

Figure 8.53. Effect of fumed silica surface area on peel adhesion of silicone sealant. [Data from Cochrane H, Lin C-S, Rubber World, 1985.]...

The role played by the various ingredients in the composition of sealant, and in particular on the durability of adhesion has been discussed recently [77]. Inert plasticizers, such as trimethylsilyl-endblocked-PDMS, are typically added to silicone sealant compositions in order to adjust the rheology of the uncured sealant. They result in a reduction of the modulus and hardness of the cured sealant. Differences in the durability of silicone sealants are found to be due to differences in their cure chemistry, and more specifically to the nature and... 

The analogous large-volume series ZSK MEGAvolume with 8.7 Nm/cm3, a diameter ratio of 1.8, and screw speeds of up to 1800 rpm is used for compounds with a high solid matter content, such as those often found in the chemical/food/pharmaceutical industries. The maximum speeds of up to 1800 rpm are used mainly for low-viscosity products, such as pressure-sensitive adhesives and silicon sealants because they provide better elastomer and filler dispersion. Other processes often require large volume only and operate at very moderate speeds of 100 to 600 rpm to protect the product or for direct extrusion into films, sheets, or profiles. 

The use of silicone sealant for insulating glass (double glazing) gives excellent adhesion to glass which is unaffected by UV radiation but which must be used with a waterproof sealant such as polyisobutylene to reduce moisture vapour transmission (MVT). The use of silicone sealant alone will result in water condensation inside the unit in a relatively short period of time and would permit water vapour to be absorbed into the unit and eventually exhaust the desiccant used in the space. The use of small beads of polyisobutylene as a primary seal (Figure 6.3) reduces the MVT to a very low number, thus correcting the one deficiency of silicone (Panek and Cook, 1984). Sealants for SSG systems may be applied on... 

With these tools, it has been possible to develop a whole range of silicone sealants with cure kinetics designed for many different requirements (Fig. 11). The acetoxy sealants are fast curing, and thus well adapted to glazing applications. The oxime sealants are better suited for adhesion on plastics. For applications where a longer tooling time is required, such as expansion seals or perimeter joints, Alkoxy sealants have been developed with a longer skin-over time. 

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. 

Chapter 4 Chemistry of Silicone Sealants and Adhesives, by Dr Andreas Wolf, a scientist at Dow Coming. This is a very important and comprehensive chapter (180 pages), which explains, with many exclusive technical and scientific information, the chemistry of silicones, their technical characteristics, the formulation of silicone sealants, their applications in all industries, and mainly in constmction of course, including a very large bibliography. 

The silicone adhesive family often provides the desired properties for many applications where a seal is required. These products are suitable where there are severe movements between substrates caused by changes in temperature, pressure and vibration. However, aggressive chemicals such as jet fuel, strong acids or alkalis, organic chemicals, can cause swelling and debonding of silicone sealants and thus affect the permeability and integrity of the seal. 

The chemistry of silicone sealants is much the same as that discussed above for silicone pressure-sensitive and mbber-based adhesives, with the exception of the MQ taddfier addition. The basic polymer for silicone sealants is that based upon poly (dimethyl siloxane)diol. This material can be cured by any one of a number of polyfunctional curing agents, all of which are based upon tetrachlorosilane. Thus, such materials as methyl triacetoxysilane or methyl trimethoxysilane can be used in conjunction with a poly(dimethyl siloxane)diol to yield a room... 

The polyether silicone displays adhesion similar to that of silicone sealants but they have other properties of interest. For example, the paintability of polyether silicones is much better than that of silicones because of the polyether content. The property in which this material falls short in comparison to standard silicones is temperature resistance. 

The surface of the substrate, the silicone/substrate interface, and the bulk properties of silicones all play significant and influential roles that affect practical adhesion and performance of the silicone. The design of silicone adhesives, sealants, coatings, encapsulants or any products where adhesion property is needed requires the development chemist to have a thorough understanding of both silicone chemistry and adhesion phenomena. 

Previous reviews on silicones in relation to adhesion have dealt with specific technologies such as adhesives, sealants, and coupling agents [12-17]. This review attempts to address the fundamental properties of silicones and to relate them to various aspects of adhesion technologies. The perspective taken in this review is from the point of view of a newcomer in the field of adhesion and silicones. 

This chapter first reviews the general structures and properties of silicone polymers. It goes on to describe the crosslinking chemistry and the properties of the crosslinked networks. The promotion of both adhesive and cohesive strength is then discussed. The build up of adhesion and the loss of adhesive strength are explained in the light of the fundamental theories of adhesion. The final section of the chapter illustrates the use of silicones in various adhesion applications and leads to the design of specific adhesive and sealant products. 

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. 

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]. 

The surface energy of silicones, the liquid nature of the silicone polymers, the mechanical properties of the filled networks, the relative insensitivity to temperature variations from well below zero to very high, and the inherent or added reactivity towards specific substrates, are among the properties that have contributed to the success of silicone materials as adhesives, sealants, coatings, encapsulants, etc. 

Silicone adhesives are a generic class of materials encompassing sealants, encapsulants, and PSAs. Sealants and encapsulants were briefly discussed along with other silicone rubbers. Fundamental aspects of adhesion technology in silicones are discussed in a recent chapter by Parbhoo et al.4is Silicone sealants and adhesives are also discussed in a couple of recent publications.436-438... 

Acrylics. There are two principal classes of acrylic sealants latex acrylics and solvent-release actylics. High molecular weight latex acrylic polymers are prepared by emulsion polymerization of alkyl esters of acrylic acid, The emulsion polymers are compounded inlo sealants by adding fillers, plasticizers, freeze-thaw stabilizers, thickeners, and adhesion promoters. As is true of the silicone lalex sealants, die acrylic latex sealants are easy to apply and clean with water. 

At present industry widely manufactures glue sealant Elastosil based on low-molecular silicone elastomers. Besides elastomers, the compositions of these sealants include fillers, plasticisers, solidifiers and adhesive components in some cases pigments are used to give them a colour. 

Polyurethane sealant formulations use TDI or MDI prepolymers made from polyether polyols. The sealants contain 30—50% of the prepolymer the remainder consists of pigments, fillers, plasticizers, adhesion promoters, and other additives. The curing of the sealant is conducted with atmospheric moisture. One-component windshield sealants utilize diethyl malonate-blocked MDI prepolymers (46). Several polyurethane hybrid systems, containing epoxies, silicones, or polysulfide, are also used. 

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