Messages Archive 2010

Joint life ( how long will glue last)

Bill Tindall
There have been some discussions about how long various glues will last till the glue degrades and the joint fails. No doubt this fretting has left some wondering if the stuff they are assembling with modern glue will last as well as stuff assembled with hide glue. It will take another few hundred years to answer this question with certainty but apparently there is not yet any cause for concern(see below) if you are using glue of similar chemical composition to Titebond. If you are using cyanoacrylate adhesive there is ample cause for worry upon which I can elaborate if there is interest.

I met a retired chemist from Franklin glue company and he has been receptive to answering questions. He also sent me a DVD with loads of technical data on glue and gluing, the content of which will no doubt be dismissed by those unwilling to believe anything useful in woodworking has been invented in the last 100 years. However, I have found it informative reading. I asked him about what is know about the service life of polyvinyl acetate based glue (Titebond and the like). I have copied his answer below:

The product sold as Titebond Original Wood Glue has been marketed since the early 1950's, and we are not aware of any tendency for bonds made with that product to change or degrade over time. While some types of glues can physically or chemically degrade over time, particularly under exposure to moisture or heat, the most common cause of change in most bonds is a result of the dynamic nature of wood, perhaps coupled with extremes of heat or moisture.

As you know, all wood changes dimension as it changes in moisture content, and that dimension change can create stress in an assembly. Small changes, and those that occur slowly, like seasonal changes in moisture, cause little stress, and rarely affect a well made bond, but much larger stresses can occur when a glued project is exposed to large or rapid changes in moisture. It is those large stress levels that are prone to eventually affect a joint, with that change more likely to occur if the original quality of the joint is somewhat suspect. In the case of glues like our Liquid Hide Glue, the moisture itself also softens or weakens the glue, so a joint is likely to fail if it is exposed to extended periods of high humidity or if it ever actually gets wet. For the rest of the Titebond family, changes in a bond are more likely to occur due to exposure to excessive heat than due to high humidity or a little dampness. In that regard, the glue is
somewhat softened by the heat, with a product like Titebond Original only about half as strong at 150F as it is at room temperature. While the heat does not directly damage the glue (so the glue would be just as strong after it returned to room temperature), the heat does tend to rapidly dry the wood, and the shrinkage associated with that rapid drying can sometimes create enough stress to distort the glue and damage the bond while the glue is warmer and weaker.

Ultimately, the ability to avoid excessive heat, and to finish a piece in such a way as to reasonably control the swings in moisture to which it will be exposed is the best way to assure that the integrity of the bonds will not be compromised by their age.

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