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?
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.
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?
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.