Wikipedia defines glue as "Glue is a sticky material (usually a liquid) that can stick two or more things together. Glue can be made from plant or animal parts, or it can be made from oil-based chemicals. ". I other words, glue is sticky stuff.
There are different types of glue that can be distinguished by the way they are activated
- Solvent based
- Melting solvent
- Resin with catalyst
This type of glue is made from a bonding agent dissolved in a solvent. This solvent can be water or some other chemical compound. As the solvent evaporates the binding agent dries and sticks the items together. The type of bonding agent defines what materials can be glued together. This type of glue is used mostly for porous materials like paper, cardboard, wood or leather.
This type of glue uses the material it want to stick together as a bonding agent. It dissolves the material and then evaporates binding the two parts together. This type of glue is used to glue plastics together. A commonly used melting solvent is acetone to glue ABS parts together. The acetone melts the ABS and makes the parts blend together. Once evaporated the bond is very strong as both parts have become one.
Glue reacting with a catalyst
Some chemical glues only get hard when they can react with another chemical compound. That compound is not consumed in the reaction, it just triggers the hardening. This is called a catalyst. A typical example of a catalyst glue is superglue. This glue gardens when it gets in contact with water. The reaction goes even faster when salts are added. That's what the kicker is for superglue : a special salt dissolved in water. Breathing on superglue makes it react faster too
Resin with hardner
A resin is also called a polymer. It's a very long chain of atoms forming strands. In its liquid state the strands in the resin are relatively short but once the hardner is added the strands start sticking together forming ultra long molecules that can consist of millions of atoms. These very long strands also form bonds between each other making it impossible for them to move. this way the resin becomes hard. A typical glue that uses this method is epoxy. It comes in two parts ; the resin and the hardner. You typically use 50 to 60 percent of resin to create the glue. The rest is hardner
This type of glue is becoming incredibly popular and is more commonly knows as hot glue. I personally refer to it as hot snot. It is made from a themoplastic that is heated up to the point is becomes a liquid. When it is applied to a material it either created a mechanical bond by taking the shape of the surface of the material or it creates chemical bridges with the surface (hydroxide bonds). A lesser know application of this type of glue is PVB foil in car windshields. Two very thin glass panes are pressed together with a sheet of PVB between them. This sandwich is then heated to the melting point of the PVB foil so it can created OH bonds with the glass. Once cooled, the two glass panes are stuck together so when the glass breaks it doesn't create dangrous chards.