Tim Lamb wrote:
In message , The Medway
Handyman writes
Ian Stirling wrote:
wrote:
Need a bit if help choosing the best kind of araldite to use!
If I'm correct the standard 5-min isn't the strongest. Does it follow
the slower drying ones (eg 30 min) set to stronger bonds?
All else being equal, yes.
Generally the longer the setting time the stronger the bond. A single part
polyurethane might be easier, like Gorilla Glue.
You can accelerate Araldite curing time by applying a modest level of
heat. What it does to bond strength is beyond my knowledge.
AIUI.
Epoxies are complex beasts.
Basically, the faster the epoxy, the more the reaction does not go to
completion, but sticks part way there.
This means there is less reacted epoxy molecules as a proportion of the
whole, and the ends of the molecules which would normally join together
instead get immobilised by the molecules that are set around them.
Once it's 'set' (say 10 times the setting time) not much happens.
Until you raise it above a certain temperature for the first time.
Then the strength drops a bit as the ends of the unbound molecules whip
around due to thermal vibrations, and reform so that more of them are
joined up.
When it cools again, it's stronger - though less flexible than before.
Heating again to the same temperature as before does nothing.
Setting at elevated temperatures does nothing bad - apart from losing
flexibility.
Then there is the whole issue of differences in the coefficient of
expansion of the epoxy and the substrate - if you cure it at elevated
temperatures, you're going to lock in some stress, which may weaken the
joint.
Then there is heat generated while setting, and the fact that it can
get hot enough to damage the setting epoxy.