Why is Wood Glue So Strong

Why is Wood Glue So Strong?

We’ve written about wood glues in a past post, and if you want to read up a little bit on wood glues, you’ll find that post here.  You’ll find some basic information about wood glue’s constituent chemical composition that generally identifies how it works.

You have PVAs (polyvinyl acetate) like Elmer’s Glue, polyurethane glues like Gorilla Glue, and aliphatic resin glues like Titebond.  Each has its own qualities, but the one thing they all share is the strength of the bond they can create between pieces of wood.

It’s the “why” of that strength we want to consider today.  Glues today are very strong, stronger, even than the wood they join. They are also effective adhesives with other materials too, but we’ll focus our attention on wood.

What is Wood Glue Made Of?

Three of the key ingredients in the various choices of adhesives, already mentioned, are:

  • Polyvinyl acetate, also known as school glue, carpenter’s glue, white glue, is an effective adhesive, is a rubbery polymer, and the main constituent of Elmer’s Glue.  It’s widely available and used with such porous materials as wood, paper (remember using it in school?), and cloth.
  • A strong and durable glue ingredient and versatile enough to use with wood, plastic, metal, glass, and even concrete.  Gorilla Glue is a popular polyurethane-based glue product.
  • Aliphatic Resin. A synthetic adhesive with that familiar yellow color and creamy consistency we associate with wood glue.  Its ultimate strength is similar to that of PVA, as is its chemical composition, but it is modified for added strength and waterproof quality.

Each has its own strengths and best uses, but all of them will form a strong glue joint when bonding pieces of wood together.  They are not to be confused with the use of epoxy, though.

Epoxies are two-part adhesives formed when epoxy resin is mixed with a hardener.  It’s a good gap-filler, and you’ve likely seen videos of colored epoxies being used to fill gaps in wood for unique tabletops.  In those uses, though, it’s the gap-filler properties of epoxies being put to use, rather than its adhesive strength.

It’s also more expensive to produce than other resins.  The raw products are pricey, and the production process is complicated and has a small margin for error.

Wood Glue Shear Strength

Wood glue really does hold, and in most cases, will be stronger than the wood it bonds. Most wood glues will withstand a hold of about 3500 psi (pounds per square inch). 

1) Is wood glue strong enough on its own?

When an application of wood glue is spread in an even coating on the surfaces of the wood being joined, the bond formed will be stronger than the wood.  However, wood is not especially strong, and this is why other fasteners such as nails, screws, and joints are also used.

2) Is wood glue stronger than screws?

Yes.  The glue is used along the entire span of the wood’s surface, while screws add strength only at the point of their use. 

3) Is wood glue stronger than nails?

Screws are stronger than nails, and glue is stronger than screws, so the answer is yes.  Wood glue is stronger than nails for the same reason.

4) Other strength enhancements

To maximize the strength of wood glue, two additional measures are necessary – pressure and time.

Clamps used to press the wood pieces together while the glue is drying ensures an even strength along the span of the bond.  Use a damp cloth to remove excess glue after the clamps have been applied and tightened; and, use a chisel or sandpaper to remove any hardened glue that might seep out of the seam. 

Allowing sufficient time for the glue to dry gives it time to be absorbed into the wood during the drying process.  This absorption into the wood enhances the strength of the bond, as the glue and the wood become one.

In preparing for this piece, we found a story about one avid woodworking enthusiast who saves the end cuts from his glue-ups.  He then breaks them to inspect for himself the strength of the bond that develops.  He also uses this practice to instruct his apprentices and the customers of his furniture pieces.

When the glue is allowed to dry sufficiently, his glued-up pieces always break somewhere along the wood and not at the bond.  Apprentices get the point, and customers purchase the furniture with confidence.

Is Wood Glue Really Stronger than Wood?

Is Wood Glue Really Stronger than Wood

All of this leads to the final question:  Is wood glue really stronger than the wood it binds?  The answer is yes, and the reason is simple to understand. 

The glue has its own inherent strength.  During the application and drying process, wood glue is absorbed into the wood, becoming a part of it.  When you add its intrinsic strength to whatever the strength of the wood may be, it’s simple math:  one plus one is two, and two is greater than one. 

The strength of the glue plus the strength of the wood is greater than the strength of the wood alone.  It’s all math.

The end result makes the effort of gluing, and the time it takes to dry well worth it for the quality of the project, whether it’s a tabletop, a desktop, or some other fine piece of furniture. 

We’re fans of Gorilla Glue for many uses joining many materials.  It’s versatile and strong and holds its bond well. 

But, for joining woods, we prefer either a PVA or an aliphatic resin.  Each is absorbed well into the woods being joined, and each has an inherent strength that adds to a long bond.  One is white, the other yellow and creamy, but they each do the job well.

Be sure to keep wood glue on hand in your woodworking shop, as it is essential to virtually every project.