Properties of Silicone Adhesives

Thermal and thermomechanical properties. Silicones, as a class, are rated among the highest temperature stable polymers. They can withstand temperatures of200°C, almost continuously, without degradation of physical or electrical properties and have been used at temperatures as high as 300°C. Because of their high thermal stabilities, they are used as adhesives and encapsulants for electronic modules that are expected to perform in extreme temperature environments, such as near automotive engines and in deep-well sensors. Because of their low moduli of elasticity, silicones also fare well at very low temperatures. They are rated for continuous use at -80°C, but may be used at even lower temperatures. 

Associated with their low moduli and elastomeric properties is their ability to dissipate stresses and to act as stress buffers between harder, less flexible materials such as epoxies. Because of their stress-dissipating properties, soft silicones are often used to bond or encapsulate fragile components such as glass diodes, laser diodes, and electro-optical devices. However, the CTEs of elastomeric silicones are generally high (about 200 

Electrical properties. The electrical properties, especially for the high-purity semiconductor-grade silicones, are excellent even at temperature extremes of-80°C and 200°C and in high humidity. Silicone adhesives are available as electrically conductive and as electrically insulative types. Typical electrical properties for moisture- and heat-cured silicone adhesives are given in Table 3.12. 

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Table 3.12 Thermal and electrical properties of silicone adhesives ...

Table 3.12. Thermal and Electrical Properties of Silicone Adhesives ... 

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. 

In silicone adhesives used to bond structural glazing assemblies, the silicone network is made of very long PDMS chains and is filled with silica that improves the elastomeric properties of the adhesive. The strength of such an adhesive is strongly enhanced through various mechanisms of energy absorption. 

Even silicone-based materials cannot fulfill all requirements in diverse applications. One property of silicone materials is their high gas and moisture permeability, which is disadvantageous in, e.g., edge sealing of insulation gls s, where a barrier for water and gas has to be provided. Hydrocarbon polymer-based materials like poly-isobutene possess these desirable properties. However, they lack the ready curability and adhesion of RTV silicone materials. Preferably the advantages of the hydrocarbon base polymer and the silane condensation reaction should be combined. 

Many of the unique properties of silicone oils are associated with the surface effects of dimethylsiloxanes, eg, imparting water repellency to fabrics, antifoaming agents, release liners for adhesive labels, and a variety of polishes and waxes (343). Dimethylsilicone oils can spread onto many solid and liquid surfaces to form films of molecular dimensions (344,345). This phenomenon is gready affected by even small changes in the chemical structure of siloxane in the siloxane polymer. Increasing the size of the alkyl substituent from methyl to ethyl dramatically reduces the film-forming ability of the polymer (346). The phenyl-substituted silicones are spread onto water or solid surfaces more slowly than PDMS (347). 

Pressure-Sensitive Adhesives. Silicone PSAs are used primarily in specialty tape applications that require the superior properties of silicones, including resistance to harsh chemical environments and temperature extremes (398,399). Silicone PSAs are also used in applications requiring long service life, electrical insulation, and protection from moisture. Another distinctive advantage of silicone PSAs is their ability to wet low surface energy tape substrates such as PTFE. 

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. 

Reliability of the electrical properties of silicone-based isotropic adhesives has been the major difficulty to overcome and has essentially prevented commercialization. Another problem associated with silicones is that the addition polymerization reaction of silicones must be carefully controlled to prevent cure inhibition from various common chemical contaminants such as amines and sulfides. Other concerns include low-molecular-weight silicone polymer migration onto wirebond pads and very high GTE. There has been some activity in the development of hybrid resins that contain silicone blocks as comonomer with epoxies such that the epoxy processing can be maintained with the added stress reduction property of the silicones [52]. 

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