Silicones/Silicone adhesives applications

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. 

Silicone adhesives are generally applied in a liquid and uncured state. It is therefore the physical and chemical properties of the polymers, or more precisely of the polymer formulation, that guide the various processes leading to the formation of the cured silicone network. The choice of the cure system can be guided by a variety of parameters that includes cure time and temperature, rheological properties in relation with the application process, substrates, the environment the adhesive joints will be subjected to and its subsequent durability, and of course, cost. 

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. 

Pure PDMS networks are mechanically weak and do not satisfy the adhesive and cohesive requirements needed for most applications in which the silicone adhesive joint is subjected to various stresses. For crosslinked silicones to become high performing adhesives, they need to be strengthened. 

Fillers can also be used to promote or enhance the thermal stability of the silicone adhesive. Normal silicone systems can withstand exposure to temperatures of 200 C for long hours without degradation. However, in some applications the silicone must withstand exposure to temperatures of 280 C. This can be achieved by adding thermal stabilizers to the adhesive formulations. These are mainly composed of metal oxides such as iron oxide and cerium oxide, copper organic complexes, or carbon black. The mechanisms by which the thermal stabilization occurs are discussed in terms of radical chemistry. 

Among all the low energy interactions, London dispersion forces are considered as the main contributors to the physical adsorption mechanism. They are ubiquitous and their range of interaction is in the order 2 molecular diameters. For this reason, this mechanism is always operative and effective only in the topmost surface layers of a material. It is this low level of adhesion energy combined with the viscoelastic properties of the silicone matrix that has been exploited in silicone release coatings and in silicone molds used to release 3-dimensional objects. However, most adhesive applications require much higher energies of adhesion and other mechanisms need to be involved. 

We have attempted to relate the basics of silicone chemistry to applications where adhesion is an important property. These applications cover a vast industrial arena that does not make a review of this sort easy. Instead, we focused on the fundamental aspects of silicone physics and chemistry and related them to adhesion and adherence properties. We have attempted to use a logical structure to help the reader understand silicone adhesion. Adhesion and cohesion have been considered as they both determine the ultimate performance of an adhesive joint. 

This type of material is commonly used in the production of semiconductor devices.57 The silica layer is used as a starting layer for integrated circuit (IC) build-up. IC layer materials range from single crystals and doped polycrystalline silicon, silicon nitride, thermally-grown oxide to vapour deposited or sputtered metal or metal silicide layers. Structural adhesion of the various layers is obtained by the application of organosilanes, such as AEAPTS, APTS and GPTS. 

Chemical synthesis methods are not only used for the synthesis of chemical compounds, but also for materials. Examples are silicones, polymers, adhesives and ceramic powders [1]. But if one compares the potential of chemical synthesis with the number of materials produced by these routes, one has to say that the potential of chemiced synthesis is only used to an extremely small extent. The focus of chemical research still is mainly directed to "new chemistry", generating new compounds. So far, sol-gel chemistry as an interesting route for material synthesis never has come to a real breakthrough in application and even is rarely used in silicon chemistry, although a large methodical overlap exists [2,3]. 

An example of an application of these special adhesives is their contribution to the construction of liquid gas tanks and tanker gas transport. Worldwide transport of this fuel would not be feasible from an ecological point of view without this adhesive technology, since the construction of large-scale tankers was first made possible by development and use of suitable insulation and bonding applications. The adhesives had to yield high-strength bonds, for instance at temperatures down to —160°C. This specification was met by use of these specially modified adhesives with an interpolation of PUR and silicone adhesives (MS adhesives), see details in Sect. 3.6. 

The rheological properties of adhesives and sealants are important in many applications. When these products must be pumped or applied through automated equipment, the flow characteristics at pertinent shear rates are critical. Sophisticated rheological measurements can be performed to predict performance. The rheology of silicone adhesives and sealants can be tailored through adjustment of polymer viscosity, filler loading, and incorporation of various additives. 

Electrical-andjor thermal-conducting adhesives [1,9]. The epoxies and acrylates described above are filled with metal powders to get electrical-conducting adhesives. For special applications polyimide and silicone adhesives are used also. Since the metallic particles must touch each other inside the resins to reach a sufficient level of conductivity, a metal content of 70 to 80 wt % is necessary. Silver is the metal generally used, since specific resistances of the filled adhesives down to about 10 " S2cm can be achieved (metallic silver has a specific resistance of 1.6 x 10 S2cm). Using other metals, such as copper or nickel, the accessible electrical conductivity is too small. On the other side, copper-filled resins show good thermal conductivity and are therefore used for such... 

MAJOR APPLICATIONS Antifoam fluids, lubricants, protective gels, and elastomers and sealants in applications exposed to hydrocarbon fuels and oils and organic solvents in the automotive and aerospace industries. Longer fluorocarbon side-chain fluorosilicones are available with developing use as release coatings for silicone-based adhesives. 

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