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Silicone Adhesives General Description Silicone adhesives are typically supplied as one-part systems that range in viscosity from self-leveling liquids to non-slumping pastes. They cure to soft thermoset elastomers with excellent property retention over a wide temperature range. Silicones have good adhesion to many substrates, but are limited in their utility as structural adhesives by their low cohesive strength. Silicone adhesives are typically cured via reaction with ambient humidity, although formulations are also available which can be cured by heat, mixing of two components, or exposure to ultraviolet light. Since the cure of moisture-curing silicones is dependent on moisture diffusing through the silicone matrix, the cure rate is strongly affected by the ambient relative humidity and the maximum depth of cure is limited to 0.375 to 0.500". At 50% RH, moisture cure silicones will cure to a tack-free surface in 5 to 60 minutes, depending on the type used. Complete cure through thick sections of silicone can take up to 72 hours. It should be noted that adhesive strength may continue to develop for 1 to 2 weeks after the silicone has been applied. This occurs because the reaction between the reactive groups on the silicone polymer and the reactive groups on the substrate surface is slower than the crosslinking reaction of the silicone groups with themselves. Moisture curing silicones are categorized by the by-product given off as they react with moisture. For example, acetoxy cure silicones give off acetic acid. Alkoxy cure silicones give off alcohols (typically methanol or ethanol), and oxime curing silicones evolve methyl ethyl ketoxime. Acetoxy cure silicones are known for their ability to cure rapidly and develop good adhesion to many substrates. Their largest limitation is the potential for the by-product acetic acid to promote corrosion. Alkoxy cure silicones, on the other hand, do not have this limitation because the alcohol by-products are noncorrosive. This makes them well suited for electronic and medical applications where acetic acid could be a problem. Unfortunately, alkoxy cure silicones typically have lower adhesion and take longer to cure than acetoxy cure silicones. Oxime cure silicones offer cure speeds and adhesion that rival, and in some cases surpass, that of acetoxy cure silicones. In addition, the oxime they evolve will not corrode ferric substrates, although it can stain copper or brass. Consequently, oxime silicones have found widespread use in automotive gasketing applications. The chief limitation of all moisture curing silicones is the difficulty associated with accelerating the cure rate. This concern was addressed through the development of UV cure silicones. Ultraviolet light curing silicones generally also have a secondary moisture cure mechanism to insure that any silicone which is not irradiated with ultraviolet light will still cure. Upon exposure to ultraviolet light of the proper wavelength and intensity, they will form a tack-free surface and cure to a polymer with up to 80% of its ultimate physical strength in less than a minute. Initial adhesion can be good, but because ultimate bond strength is dependent on the moisture cure portion of the silicone, full bond strength can take 1 to 2 weeks to develop. Silicones with a secondary acetoxy cure show good bond strength while those with a secondary alkoxy cure are lower. Chemistry Silicone formulations are available which can be cured through moisture, heat, mixing two components, and exposure to ultraviolet light. The silicones used for adhesives are typically the one-part moisture curing and UV curing silicones. All silicones have a chemical backbone made up of silicone to oxygen bonds, known as siloxane bonds. It is the high energy of this bond that gives silicones their unique high temperature performance properties. Advantages • One-part systems available • Solvent-free • Room temperature cure • Excellent adhesion to many substrates • Extremely flexible • UV curing and two-part formulations available Disadvantages • Low cohesive strength • Moisture cure systems have limited depth of cure • Swelled by non-polar solvents The Loctite® Design Guide for Bonding Plastics, Volume 6 11PDF Image | Design Guide for Bonding Plastics Volume 6 LT-2197
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