On Sunday, September 17, 2017 at 2:25:42 PM UTC-7, Mad Roger wrote:
On Sun, 17 Sep 2017 16:14:09 -0400,
Frank wrote:
[about glue]

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.

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.

Oh, it's happening. The SURFACES of wood, plastic, cloth have material-air interfaces,
with some kind of loose chemical links (that would hold the material together, if you hadn't
run out of material when you got to the surface...) which is loosely holding on to
(maybe) oxygen or another interface layer of random stuff.

The glue first has to come into closer contact than that oxygen molecule, i.e.
it has to wet the surface and displace the O2. It has to flow into contact,
and either repel or dissolve the contaminants already present. Afterward,
it has to solidify into something with tensile strength, or shear strength, enough
to transfer force through the glue to the surface it bonds to.

Something like tin/lead solder joining metsls is EXACTLY covalent (metallic) bonding,
but with more complex materials, there's just a lot of different kinds of bonds
involved, Good glues, like soap, have hydrophobic and hydrophilic and all sorts
of bondable molecular sites, but unlike soap, they also harden or stiffen after some
time due to cooling, chemical restructuring, loss of solvent (vehicle).