Glue Joint Movement — Causes and Cures
Question (WOODWEB Member) :
From the original questioner:
So what is the right glue?
From Contributor O:
I believe this is 100% due to the glue. We use Urea Resin (dry resin mixed with water, or liquid resin mixed with catalyst) glue for panels like yours and have no problems. Once or twice some new guy tried to squeeze some Titebond-ed panels by, and the glue lines gave him away.
From Jeff Pitcher, forum technical advisor:
The right glue depends on a number of variables. What do you expect from your glue line - water resistance, heat resistance? Does your panel need to qualify for LEED or CARB2? How long can you wait for the glue to cure? How are you applying the glue? How are you clamping your panel? In general, any industrial quality PVA glue will produce a perfectly acceptable panel. Raised (or sunken) glue lines indicate a moisture issue. They usually appear when a panel has been machined too quickly after being glued. As I've mentioned before in this forum PVA glue does not creep. Once it has fully cured it will not move. However, if the pieces being glued are under stress (as in the case of a curved lamination) creep can occur in the form of springback. Some movement of the curve will be experienced. I will add that Contributor O is correct in suggesting the use of a UF resin. Not so much to avoid creep but because of the high heat and water resistance it brings to the construction. There are drawbacks to this type of adhesive. Namely, it takes a long time to cure at room temperature and it won't meet LEED or CARB requirements. It is used extensively by many butcher block manufacturers.
From contributor A:
I concur with Contributor O 100%. We have had a few threads over the years on WOODWEB about this very phenomenon. It typically comes up in the finishing forum, when someone can't sand off the glue line no matter the number of attempts. The problem is often referred to as creep in the woodworking crowd. Creep is technically a material science word referring to an extreme amount of stretch that occurs just before it yields. Saran wrap is a great example. Most materials do not creep much. Glues are either soft or hard when cured. They are all different.
Hard to Soft:
These are all obvious if you spray paint over them. We stopped using Titebond 2 for anything after we figured this out. The last five years we have only used Titebond 1 for all interior work. Exterior was only epoxy until they made Titebond 3. Poly is occasionally used for bonding dissimilar materials (never for wood/wood). Titebond 2 and 3 are rated for counters, however I would not use them because of the creep issue.
From contributor L:
I tested on hard maple using a moderate curved laminations setup between a ridged outer form and a slightly springy inner form: Original Titebond, very slight creep. Urea Formaldehyde: no creep, epoxy West Systems - lots of creep. White glue, Elmer's - tons of creep. In this case creep refers to the ability of the glue to keep the laminations from moving one against the other. Hard maple and the bent shape cause considerable continuous stress on the glue lines! Next, each part was mortised through the glue lines. All except the urea did fine. The urea shows signs of brittle glue line near the mortise. On table tops, original Titebond did fine as long as the glue joints were well fit, with no glue filling gaps. The excess glue was removed as soon as possible. The parts then had to dry long enough for the excess water absorbed next to the glue line to come to equilibrium with the surrounding wood, at least 24 hours for 4/4 and three times that for 8 quarter. We've made conference tables using Original Titebond, a high gloss finish and no issues with glue lines showing. We don't use Titebond 2 and only use 3 for exterior semi-protected work. Work that will be fully exposed uses resorcinol. Urethane glues have been a disappointment in both their strength and the mess involved. Just my experience doing non-scientific testing and 30 plus years of making mistakes and trying to learn from them.
From Gene Wengert, forum technical advisor:
One of the reasons that this discussion has various answers is that there are different events going on. With a counter top, the clamps are used to squeeze out the excess glue (aiming for 0.002 to 0.006" joint thickness). With the large number of clamps being used in the original situation, there is a lot of pressure on the adhesive, even after it cures somewhat. Once the clamps are removed, the pressure on the adhesive drops to close to zero (unless a hollow-ground joint was used with more adhesive in the center (thickness-wise) than on the faces. In any case, assuming that the adhesive cured properly, forces on the joint will occur only if the wood moves. The reason that wood moves is due to shrinkage or swelling which is only caused by moisture changes. When sanding wood with a cured joint, the heat will cause the adhesive to expand, especially PVA adhesives and especially thicker than 0.005" glue lines. The cross-linking PVAs are subject to this movement. The best results were when we mixed a urea/PVA blend. When finishing a wood piece with a glue joint, we can also have the finish affect the glue and cause the adhesive to flow. This has been observed in many real-life manufacturing situations. When a dried panel with joints is subjected to 80% RH, the glue joint will seem to grow out of the joint, especially with a joint thicker than 0.005". This is a practical test result and not theory.
Note that achieve a thin glue joint thickness requires the joint to be tight-fitting. This can only be achieved if the MC of the wood is equal to the EMC of the air. As this seldom happens, then the alternative is to joint the surfaces to be glued and then glue them within a few minutes. In fact, we did some practical tests in one plant where we glued immediately and then also glued the same wood 24 hours later. The difference was obvious. After 24 hours, the wood had moved enough so that the joint was ok in some spots, but not in others. It was too wide in some spots, in other words.
When using a water-based finish, the water will cause the surface fiber to swell and put pressure on the glue in the joint and cause the joint to develop a ridge, no matter how good the sanding job was. Again, this is seen in practice oftentimes. Now, look at a different situation: If we glue curved laminations together, we will have pressure on the glue joint all the time, as the laminations want to return to their original shape. With this continuous and fairly high pressure, the joint itself may creep - creep being the plastic deformation of a joint under load. All PVA adhesives do have a tendency to creep (using this definition). The wood itself also is plastic and will creep quite a bit too.
So, back to the original problem. As this occurred in just one case, I do wonder if the wood MC was a bit high (cherry is often dried a bit higher in MC at some operations) and the EMC was a bit low (winter time). Further, the joints were made flat and true on one day, but were not glued until a day or two later, giving us a thick joint in spots. Then, perhaps the joint was sanded with dull sandpaper or a lot of pressure to develop heating at the surface. Maybe the countertop was also exposed to a high humidity. Finally, perhaps the finish used reacted with the adhesive causing the glue to expand. Does any of this sound like a possibility? That is, something that was different about this top compared to the other tops? PVA’s do move under pressure (or creep), but they also move for other reasons which we would technically not call creep.
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