I am the undisputed champion of gluing things at the Carter house. Whenever something breaks, out pops the epoxy, or other special purpose adhesive, to do the job. In fact, I actually like gluing things together. I feel it is a challenge to get things back together so that you can barely see the fracture line. I don't know what I would do without my clear epoxy!
Two Methods of Sticking
Adhesives are fairly interesting in a way. Have you ever stopped to think how they work? Different glues work in different ways. Some of the simplest glues, like old fashioned white glue, yellow glue and basic protein glues, work by flowing into the tiny open spaces of the object being glued. This is why white glues and such don't stick very well to smooth surfaces.
The water in the glue dries and the hardened glue is tightly knitted into the wood, paper or other material. If it is a simple water soluble glue, the addition of water (re-hydration) at a later date softens the glue and allows you to unstick the objects. This is why steaming an envelope works. Now, don't think I go around opening people's mail ... I just saw this done on TV during a murder mystery.
The other method things stick together is an actual chemical reaction or bonding. Orbiting electrons in one compound react and grab onto the molecules of another object. This type of bond can be tenacious. Epoxies are good examples of chemical bonding glues.
Because of the different types of bonding possibilities, you might be able to see why one singular glue is not a universal product. In other words, an epoxy that can bond two pieces of glass or metal together will probably do a horrible job of bonding two pieces of wood. This is why you really have to match the glue to the job.
Wood & Paper Adhesives
Old fashioned white school glue and the basic yellow glues incorporate a water based polyvinyl acetate resin as the glue. This happens to be the same thing that is used in many low end latex paints. Remember, paint is nothing more than colored glue.
The white and yellow glues are excellent for wood and wood type products like paper and cardboard. These glues can penetrate well into the wood fibers to create a bond. This penetration capability can also be a nightmare if you intend to stain the wood.
If you try to wipe off white or yellow glue that squeezes from a joint you can have real problems. First, the water from the rag opens the pores of the wood. Then you dilute the glue slightly so it can penetrate even deeper into the wood. Even though you get the glue off the surface, it penetrates into the grain. When it dries, this glue blocks any stain from penetrating into the wood. The result is a light area next to richly stained wood.
The solution is to let glue sit on the wood. Once it dries, carefully chisel it off or sand the area.
The New Glues
Perhaps the most exciting thing in glues - not much happens in this arena when you stop and think about it - is the new urethane glues. These glues can fill voids, they bubble and foam, they can be extremely waterproof, and exceedingly strong.
The urethane glues are great for wood workers. These glues do not soak deeply into the wood pores like the white and yellow glues.
These new glues are excellent at bonding different materials. You can use them to bond glass, wood, metal and plastic to one another.
These are specialized glues in my opinion. They are coarse in the sense that they dry very thick. They are wonderful if you need to fill a gap. Some are formulated to stick to wet or frozen lumber if you can believe that!
The downside to these products is their relatively lower strength. They often test out at around 600 pounds per square inch. That is quite strong but only about 15 percent of the other glues.
When you use construction adhesive together with nails and screws you can create a structure with exceptional strength.
Another nice thing about construction adhesives is that they are formulated to be somewhat elastic. If the lumber moves that is glued, the bond will not break. Think of these products as extremely sticky caulks. Chemically, that is exactly what they are.
In closing, to get the best performance from the product you choose, be sure to read the label. Pick a glue that will work for you. Always test it first before you use it.
Stopping Adhesive Failures
When I was doing the research for this column, I spoke with some of the top adhesive research chemists in America. I asked them about adhesive / glue failures. I was interested in trying to minimize failures when people like you and I use glues. The answers were almost identical from all of the chemists:
- Apply the glue to a clean, dust free surface.
- Apply it evenly to both surfaces.
- Read the instructions on the bottle / jar BEFORE you have a failure, not after the glue doesn't work.
None of the above answers surprised me. Remember, glue sticks to whatever it comes into contact with. So, it you apply glue to dust, the dust will stick well. But dust isn't "glued" to the wood or the other surface you are working with. Even application is key as well. Too much glue or not enough will create a weak joint.