On Sun, 17 Sep 2017 16:14:09 -0400,
Frank wrote:

There are a lot of forces at work but mostly chemical, both covalent and
ionic.

Mechanical is also important as glue might penetrate when it is liquid.

The mechanical properties of the glue are also important particularly
when bonding materials with considerably different mechanical properties.

Not a simple subject and I worked in the area in R&D for several years.

Let's take the three most common types of household glue first:

1. Elmer's Glue (for example, wood to wood or paper to wood)
2. Crazy Glue (for example, plastic to plastic or steel to steel)
3. Shoe Goo (for example cloth to leather or rubber to cloth)

There's just no way that these three glues are "ionic" bonding with the
wood, plastic, or cloth respectively. Just no way. There is no "outer
layer" of electrons being shared in these cases.

Likewise with covalent bonding. It's just not happening.

I can't explain why though - but it's just not that type of bonding that is
going on. What type of bonding "is" going on, I don't know though.

I suspect nobody knows, but that's why I asked - which is to find someone
who knows what kind of bonding force is happening when two items are glued
together.