What do you do if you can no longer use tools that have become virtually essential because they are dangerous? The catalyst that polymer chemists use for the specific reaction theyre working with is often so well established, tailored and optimized for the job, that taking it away presents a major problem. Faced with upgraded toxicity status of the organotin compounds they use, silicone sealant and some polyurethane product manufacturers have faced just such a predicament. Consequently, they have been forced to move to new catalysts. Yet, while some have changed to marginally less toxic organotins, in Europe and Japan others have now been able to develop completely different catalyst systems altogether. Tin (IV) compounds are Lewis acids that can help polymerization by catalysing condensation reactions, with dibutyltin dilaurate (DBTL) the most commonly used for this purpose1. DBTL has long been widely exploited in room temperature vulcanizable one-part (RTV-1) and two-part (RTV-2) silicone adhesives and sealants, as well as polyurethanes. More recently, however, dibutyltin and dioctyltin compounds have come to be considered as Carcinogenic, Mutagenic or Toxic to Reproduction (CMR) category 2. This means that they should be regarded as having one or more of these properties in humans, and in March 2010 the EU imposed official limits on their use 2. From January 1 2012 products for supply to the general public will not be able to contain dibutyltin compounds above levels equivalent to more than 0.1 per cent of tin by weight. RTV-1 and RTV-2 silicones have a reprieve until January 1 2015, but its clear that formulators now face the challenge of operating without a catalyst that they rely upon. The 2015 extension is due in part to the recognition that, compared to DBTL, the most commonly considered alternative catalysts do not provide satisfactory performance in silicones. For example, although titanium-based catalysts are widely used in RTV-1 compositions, they have slower kinetics than tin-based catalysts and cannot be used in RTV-2 compositions due to gelling problems. Other catalysts promoted as replacements based on zinc, zirconium or aluminum are far from being as effective as DBTL, with their adhesive properties being diminished.
One approach that is already being used that does hold some promise is the inclusion of amine catalysts3. Adding them helps improve how effectively existing catalysts promote curing, and in some cases amines like DBU INSERT INTO [lzx].[dbo].[tb_new]([id],[type],[title],[source],[personal],[image],[contents],[time],[number]) VALUES (1,8-diazabicyclo[5.4.0]undec-7-ene) can be used as catalysts in isolation. However, when these products are cured, the amine "bleeds out" onto the surface and can stain objects it comes into contact with, and their adhesive properties and water resistance can be poor. Japanese researchers have avoided these problems by using amine catalysts in silicone adhesives and sealants with appropriate combinations and amounts of silane coupling agent adhesion promoters. Altogether the epoxy- and amino-functionalized silanes amount to between 3.5 and 4 per cent of the formulation by weight. Compositions with epoxy:aminosilane ratios between 2:1 and 1:2 avoid amine catalyst bleedout and provide good adhesion. Though DBU or 1,4,5,6-tetrahydropyrimidine can be used as catalysts, the scientists prefer 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD).
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