All about gluing
Five links to a good glue joint

Adhesive is just one out of five links in the chain that makes for a successful woodworking glue joint.

First of two parts

Following quite a bit of discussion and email about gluing, rather than answer all the individual questions, here is a primer about gluing that will be helpful to everyone.

Nature does a darn good job gluing individual cells together into a strong piece of wood. We then cut the wood apart and re-glue it into shapes that we prefer. Of all the commercial woodworking adhesives that are used to put wood back together, all are 1-12 times stronger or more than the dry wood itself. So, we should never have a glue line failure, unless something went wrong.

First off, what’s the difference between glue and adhesive? Today, within our industry, they are the same.

Brief history
There are records of a glue being used around 200,000 BC. Prior to WW II, most adhesives were animal based. That is, they used a natural occurring long molecule for the adhesive. 

Some of the glues were blood glues, hide glues and milk (casein) glues. After WW II, there was a rapid switch to petrochemical adhesives.

The white colored glue we know as Elmer’s glue was invented in 1947. It used Elmer the Bull (Elsie the Cow’s husband) as its mascot, but there were no animal products in it. 

The industry started using large amounts of this product within a few years after it was invented.

Anatomy of a glue joint
To help us understand a glue joint, we use an analogy comparing the joint to the links of a chain, as illustrated below. In this illustration, links #4 and #5 are the strength of the wood’s surface; links #2 and #3 are the mechanical and chemical bond between the wood and adhesive; and link #1 is the strength of the adhesive itself. 

The weakest link determines the overall strength of the joint. Let’s look at each of these links in more detail.

Link 1: adhesive choices
Although the latest adhesives are more expensive than the old-time favorite PVAs, the glue cost in a piece of furniture is still minuscule, so cost should not be a factor when choosing an adhesive. 

Rather look for other characteristics that you need in your product. Always be aware of any ventilation requirements. Within each group there are many varieties with many different properties.

PVA — For more than 50 years, we’ve used polyvinyl acetate (PVA), an adhesive that is carried by water and is called an emulsion adhesive. 

It still is popular today. In some cases, the water in the adhesive can cause issues. Many PVAs are softened and lose their power if heated or exposed to water. 

Other PVAs chemically grab hold of the wood (cross linking) and form a bond that is not affected much by heat or moisture. 

There are many PVA adhesives; a popular one is Titebond. 

With any PVA, read the label or CIS to make sure that no fumes or other issues could be an issue in your operation. 

Indeed, some PVAs did. have very small amounts of formaldehyde. Generally, PVA adhesive works best if the glue-line thickness is 0.002” to 0.006”.

PUR — In recent decades, polyurethane or PUR glues became popular. These do not have water in them, so shrinking or swelling due to the adhesive were no longer issues. 

Some PURs are hot melts, and others are applied like a PVA but without added water. 
Moisture can be a catalyst. Hot melts can cure by cooling and then a further reaction occurs afterward. 
One popular PUR at the hardware store is the original Gorilla glue. 

PURs are generally able to fill gaps and maintain their strength when exposed to water and heat.

EPOXY — This is a two part adhesive, one part resin and one part catalyst. They are mixed together in a precise amount. 

Once mixed, the adhesive begins to generate its own heat that makes the adhesive change chemically and also grab hold of the wood. 

A thin layer of epoxy will not generate enough heat to cure well, so epoxy joints always need to be thick to be strong. 

Mixing and then handling of the mixture requires care and speed, so epoxy may not be the best adhesive for many operations. 

Excessive pressure reduces the epoxy in the joint so much that heat is not generated and therefore it is a sure way to develop a weak joint. 

Epoxy is expensive and clean-up can be difficult.

EPI — Emulsion Polymer Isocyanate is a newer adhesive to our industry. It is a two-component system with an emulsion component and an isocyanate functional cross-linking component. The glue line is cold curing, has high flexibility, low creep, contains no formaldehyde and gives excellent water resistance. 

It is especially suited for laminated beams, but does work well with furniture and cabinet manufacturing.

COMPOSITE PRODUCTS ADHESIVES — Particleboard, plywood and similar composite products are glued tougher with adhesives. Some of the adhesives have been melamine formaldehyde (MF), urea formaldehyde (UF) phenol formaldehyde (F). 

The amount of adhesive used is a key factor in the composite’s strength. Most of the adhesives for composites in the past were canalized by formaldehyde. 

Pressure from Europe resulted in newer adhesives that do not contain formaldehyde. Occasionally, tests show that imported products from some parts of Asia still use these older adhesives, which can be an issue when used in the U.S. 

In furniture and cabinets, we would seldom use these adhesives, but composite products might have used them.

A catalyst is a chemical that facilitates a chemical reaction and then after the reaction is complete (the adhesive hardens), the catalyst is released. Formaldehyde was a popular catalyst, but the release afterwards created an unhealthy condition in the air for humans. Some adhesives, like epoxy have the catalyst and the resin in separate containers. The two parts are mixed together, and the adhesive begins to work. Heat often helps the reaction go quickly. In fact, epoxy generates its own heat; otherwise, heat must be added during curing.

In the next part of this discussion we will consider the four other links in the glue joint chain. 


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About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.