Here is my help in clarifying some of the great points that guys have brought up in this thread. I would leave the sub-atomic or sub-orbital discussion for the quantum chemistry forum! Focusing on the “functional groups” might be more helpful. All materials, or rather compounds, are made up from the 115 different elements on the periodical table, these being oxygen, hydrogen, iron, aluminum, and etc. These elements based on their outer orbital configuration, once again going to skip this, will want to combine with other elements to form a more stable “functional group”. For example, one oxygen is actually not stable, so when it comes in contact with another oxygen, it bonds to it making a different oxygen to oxygen compound. This new oxygen bonded to oxygen (known as O2) compound is more stable and is really what we all require when breathing air into our lungs. Now here is another example, oxygen can also bond to two hydrogens, once again to make a more stable compound and this hydrogen-oxygen-hydrogen compound is known as water (H2O). Thus, the outer orbital configuration plays a role into what elements can bond with other elements, but my focus is the “functional group” which is made from all these different combinations of elements. Example of a “functional group”, if I take my water compound (hydrogen-oxygen-hydrogen) and replace one of the hydrogens with another compound (as long as it is stable) I can then make other compounds that have this oxygen-hydrogen group and thus, this oxygen-hydrogen group becomes a “functional group”. And now, this new compound with the oxygen-hydrogen “functional group” can mix with other materials with this same “functional group” such as water. These examples show compounds that are made from the elements that are connected by covalent bonds, covalent bonds are strong because they form and create a more stable compound. However, there is other types of bonds as well, ionic, van der wall, and hydrogen bonding. These exist and play an important part in creating compounds but do not form as strong of a bond as the covalent bond. Take a look at hydrogen bonding, this exists when the oxygen-hydrogen “functional group” interacts with another oxygen-hydrogen “functional group”, the two hydrogens create a bond, again not as strong as the covalent bond but still it is termed a bond. Here is an example of hydrogen bonding, when you fill a coffee mug full of water, you can slowly fill it until the water actually is above the rim and the water can rise above the rim and not spill out, this is because hydrogen bonding is occurring between each water molecule holding them together. Hydrogen bonding basically occurs due to the similarity of certain “functional groups” within compounds that are covalently bonded from the various elements.
This is where the “Water Break” test becomes helpful when bonding materials, when water spreads out on your bonding surface, it means that the water is mixing with the surface compounds (otherwise known as wetting out the surface). If the water beads up, then the water is not compatible with the bonding surface just like the oil and water story, they just don’t mix. But then, we all know that we can get oil and water to mix if we add some soap. Soap is made up of two “functional groups”, one of the “functional groups” mixes with water and the other “functional group” mixes with the oil, thus it is termed bifunctional. Industry will use emulsifier and coupling agent as terms to describe this bifunctional configuration. Now we can bring in “sizing”, fiberglass requires some kind of sizing in order to be fully wet out by the resin. Untreated fiberglass does not have the correct “functional group” on its surface to allow complete wet out from epoxy resins. Therefore, fiberglass is treated with a coupling agent (again, this is similar to your soap, bifunctional) and the most common one used is A1100. When purchasing fiberglass for epoxy, phenolic, or unsaturated polyester resin applications, A1100 is what you would want. A1100 is an amino-silane, the amino “functional group” will provide compatibility with the resin and the silane “functional group” is compatible with the fiberglass surface.
Now to the epoxy resins, when epoxies cure (this is known as crosslinking) some of the covalent bonds in the epoxy resin are broken and are then re-formed with the hardener creating new “functional groups”. This new “functional group” is typically an oxygen-hydrogen “functional group” and this group is very good at forming the hydrogen bond, like the water in the coffee mug example. This is the bond that makes epoxies so good as an adhesive. For example, a properly prepared piece of aluminum for an aircraft application will have a good uniform oxygen-hydrogen “functional group” on the aluminum surface and when an epoxy adhesive is properly used to bond two pieces of this aircraft aluminum, the oxygen-hydrogen “functional group” from the aluminum and the oxygen-hydrogen “functional group” from the epoxy will form this hydrogen bonding network. The same thing occurs with the A1100 sized/coated fiberglass. Thus, epoxies make good adhesives as long as their functionality is similar to the bonding surface functionality.