|Inhibitors can slow the rate of cure, as displayed by comparison of uninhibited silicone foam (left) to inhibited silicone foam (right).
Curable silicone systems can be challenging to use because each is formulated with specific amounts of catalysts, crosslinkers, polymers and inhibitors that can be sensitive to different processes, additives, and environmental conditions. Specifically, catalysts and inhibitors have significant influence over how fast a silicone cures. Catalysts are the components that incite curing; inhibitors are chemical entities that either temporarily or irreversibly prevent curing. Whether an inhibitor stalls or permanently stops a silicone from curing depends on the catalyst utilized. Understanding the effects of different inhibitors on the catalyst utilized is key to working with curable silicone systems so as to fully benefit from their unique and useful properties.
Cure and Inhibition
Silicones are typically cured using tin catalyzed condensation-cure systems and platinum catalyzed addition-cure systems. These systems are termed "condensation" or "addition" because of the chemical reactions that constitute their curing processes. In condensation-cure systems, hydroxyl groups of condensation-cure silicones condense together, causing the silicone to experience a weight loss after cure. Addition-cure systems involve the direct addition of the hydride functional crosslinker to the vinyl functional polymer, forming an ethylene bridge crosslink.
Because this mechanism involves no leaving group, unlike that of tin-catalyzed silicones, these systems experience very low shrinkage, namely, less than 1 percent compared to the 3-6 percent undergone by tin systems. Also unlike tin-catalyzed silicones, which cure at room temperature, platinum systems are often cured quickly with heat, although they can also be formulated to cure at low temperatures and room temperature. The fast cure time and lack of volatile byproducts often make platinum systems preferable to tin systems, but the higher sensitivity of platinum systems to inhibition is often perceived as a downside.
Especially for platinum catalyzed addition-cure silicones, the effects of inhibition can be detrimental to the cure as well as to the physical properties. Not all inhibition is bad, however. Cure inhibitors are often added by the manufacturer to prolong pot life (working time) and delay curing time. For example, tin-catalyzed condensation cure adhesives are inhibited by Isopropyl alcohol, a cleaning solvent commonly used in medical device manufacturing. Once the alcohol is removed or has evaporated the cure proceeds.
While some catalysts are difficult to inhibit, such as tin, the platinum catalyst used in addition-cure silicones is more susceptible not only to welcomed inhibition but also to what is referred to as poisoning. Poisons of platinum can temporarily and sometimes permanently halt or slow the vulcanization of addition-cure adhesives by isolating any platinum contained in the material; their effects can range from a slight surface tack to a complete failure to cure, depending on loading level.
When working with platinum catalyzed silicones, materials that poison the platinum must be avoided during processing for the material to cure correctly. Most notably, organotin, organosulfur, and some amine containing compounds may permanently inhibit cure. Sulfur is perhaps the most common poison of platinum, so avoiding sulfur-cured materials such as sulfur cured rubber, wood, latex, neoprene, Buna N, and natural rubber is a good rule of thumb.
Sometimes it is not known whether materials used in the material preparation and/or assembly processes contain poisons of platinum. Any number of things can contact a platinum catalyzed silicone before it has fully cured, and some applications or processing parameters may even require silicones to be in contact with devices prior to full cure. In these cases, measures can still be taken to prevent potential cure inhibition. When working with platinum catalyzed silicone adhesives, for instance, it may be beneficial (or even necessary) to heat the substrate to which the adhesive will be applied in order to bake out any volatile poisons contained there. Moreover, if unsure of the suitability of a material, curing a small amount of silicone in contact with the questionable material can be an effective way to evaluate potential poisoning effects.
A platinum catalyst allows silicones to be formulated to provide a wide range of physical properties, adjustable work times and reaction rates if cured in the right conditions. Because poisons can cause unwanted inhibition that, once begun, is out of the manufacturer's control, eliminating these from the material preparation and assembly processes is of the utmost importance in order to maintain mechanical integrity and achieve the maximum level of performance in any given application.
Contact: NuSil Technology LLC, 1050 Cindy Lane, Carpinteria, CA 93013 800-424-9300 or 805-684-8780 fax: 805-566-9905 Web: http://www.nusil.com