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Ask Dr. Dave

January 1, 2010
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Adhesive and sealant troubleshooting tips from Dr. Dave Dunn.

Question: I am using a mercaptan hardener to cure a fairly conventional 5-minute gel epoxy adhesive. However, I would like to lower the viscosity of the mercaptan from around 12000 cps to less than 1000 for my adhesive, and also get a slightly more flexible and soft system. What can you recommend for this?

Answer: You need to dilute the mercaptans with either a non-reactive epoxy diluent or, preferably, a diluent that will react with the system on curing. Aromatic hydroxyl-containing additives are usually effective in this sort of system - look at diluting with nonylphenol or benzyl alcohol. Additions of 5-10% of the additive should provide the reduction in viscosity you desire, but be sure to check the properties of the final adhesive to ensure that you do not lose key properties such as tensile strength.  If the system becomes too soft, you may have to adjust by adding something like an epoxy novolac to the epoxy side to get more crosslinking. It is sometimes better to dilute the epoxy side of the formulation with a reactive diluent so properties can be maintained.

Question: I am looking for a temporary flexible adhesive to hold some steel parts together, but I want to break the bond before some final finishing operations. What can I use?

Answer: There are several low-strength adhesive systems that might be suitable. Many hot melts, for example, will provide decent bond strengths but can then be broken apart with a sharp impact, or by heating. A pressure-sensitive adhesive or double-sided tape might work. An interesting alternative might be to use a solvent-based neoprene cement - the sort of adhesive that is used to bond wheel nuts to wheels in auto racing. Neoprene cement is a fairly strong adhesive, but when the wheels are put on the car, the torque from the wheel wrench breaks the bond instantly.

Any views or opinions expressed in this column are those of the author and do not represent those of Adhesives & Sealants Industry, its staff, Editorial Advisory Board or BNP Media.

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