Abhesion

Sometimes we want to have abhesion, i.e. deliberately poor adhesion. We now have the intellectual tools to plan for abhesion.

Water effects

Surface energy effects are relatively small effect except for water effects in pure surface energy cases. If we deliberately go from 42 to 32 dyne/cm that's not a big effect. And some siloxanes show release because of low dissipation, not low surface energy. However, there is a classic example (again from Kendall in his Sticky Universe book) of an epoxy that sticks quite well to a glass surface - until a drop of water is placed at the edge. If s designates the surface and c the coating, water gains γ(cosθs+cosθc) if the coating separates; θs and θc are contact angles of surface and coating and γ=72. This gives a maximum of 72*2 if both are fully wetted = 144mJ/m2. More typically θs~0° and θc~50° giving 72*1.64= 118mJ/m2. Separation loses γsc – typically 50mJ/m2. So water loves to separate two hydrophilic surfaces. If the glass is treated with a silane that lowers its surface energy the overall adhesion is lower, but the water no longer drives along the interface, so in this case less is more in terms of stability to water!

A sharp interface

It is relatively easy to have a "sharp" interface by either having crystalline materials (un-corona-treated PE) or by ensuring no intermingling or entanglement via incompatibility (for polymers use: Hansen Solubility Parameters to maximise the "Distance" between them) and/or from making sure there's insufficient temperature/time for intermingling to take place.

No chemical entanglements

This is similar to the previous one - just have too few chemical bonds and/or make sure that they are not coupled into some sort of entangled network

Too much chemical entanglement

Conversely, make sure there is so much cross-linking that the system is brittle, ensuring little dissipation and therefore "only" chemical bonds which are in the range of 1J/m².

Prepare the surface badly

Surface junk through under-cleaning or from over-zealous corona/plasma will give poor adhesion. Roughening is nearly always useless at best (the extra surface area provides negligible extra adhesion) and can often be positively harmful if the adhesive cannot flow into the structure.

Arrange the mechanics badly

If you are involved in structural adhesion then focussing all the forces onto the weakest spot will ensure bad adhesion.

Use the wrong temperature/time

An adhesive which is perfect at 40° or tested at 1Hz might be perfectly useless at 0° or 100Hz. tTS/WLF should help you get the correct (bad!) combination of factors for abhesion.