Lumber Moisture Content and Glue Failures

When you cross-cut and rip lumber at too high a moisture content, then quickly glue up panels (such as stair treads), glue failures may soon occur. February 5, 2007

It has rained in our location for the last 6 days. Our average MC has gone from 7-8 to 8-9.5. We are in a coastal area, so 9% is not unacceptable, but rather high. Today we had about 40 glue failures! We check MC in the wood and it came back 13%, and our adhesive (PVA EVA co-polymer, not a PUR) is not recommended for applications over 11% MC. We checked all the other lumber in the plant, and it was averaging 9% except for one bundle that was averaging 13-14%. The problem is we have traced 200 panels glued from a previous bundle from same vendor and they are holding together, but the MC is 13-14%. We notified the vendor and they checked their stock and averaged 8-9%. I don't think we can let these panels go. I am worried about not only glue joint failures, but what we can expect from the panels when they acclimate.

Forum Responses
(Adhesive Forum)
From Professor Gene Wengert, forum technical advisor:
It is easy to check a panel or two. Get a small plastic room and put a little heat in it. Twenty degrees F of heat above room temperature for you will achieve under 6% MC and if there is any problem, it will show up in a few hours.

I suspect that you and your supplier are using different moisture meters. Are you using a pinless? If so, the rain in the past few days will cause the surface to be wetter and the pinless will overreact to this moisture and give you a high reading.

Can you provide more detail about the glue failures? Are they at the end of the panels? Are the surfaces that you are gluing freshly prepared (not more than one hour old)? How are you clamping, etc.?

I am sure that if you called in an expert to view your operation, the problem(s) would be obvious. Maybe your glue salesperson can help. You might benefit from reading appropriate chapters in the book "The Wood Doctor's RX" that is available from FDM Magazine's book store.

From contributor J:
Gene, our moisture readings were correct; the vendor came to us that day and checked, and his readings were actually higher (we both use a pin type Delmhurst). Some of his rough stock from the same batch was reading upwards of 16-18%. The bottom line was his importer gave him some stuff that was probably only air dried and no one including us checked until it was too late.

The material was prepped freshly on a straight line rip, with glue line blade. On exotic species (this one being Jatoba), we always glue within 4 hours. I know 20 min is preferable, but it just doesn't always happen that way. We strictly manufacture stair treads and edge glued panels and have probably seen 5-10 failures on this particular species out of countless of thousands per year. This leads me to believe that 40 failures in one sequence with consideration to the MC readings we got would have to be moisture related, but we are fairly new to this, so you would probably know better than I would.

The failed joints, I am told, were on the ends of the panels. We would simply drop the panel on the ground and one or more of the joints would come completely apart, leaving a smooth clean surface with no wood grain tearout.

I would greatly appreciate talking with you further on where we could find somebody to actually inspect our process and offer any critical advice on what we could be doing to improve our process. We glue over 32 different species and have only learned from trial and error what seems to work and what doesn't, and at times on some isolated species we are having joint failures up to 4-6 months after our product is installed and finished. Any advice would be greatly appreciated.

From Professor Gene Wengert, forum technical advisor:
Are the pins insulated? If not, you will read the wettest spot, even if it just the surface. When you rip wetter material and the air is not real humid, then in a few minutes the ends will lose moisture through the end grain and begin to shrink. Within 2 hours, the ends will be too narrow and will not mate... That is, there will be a gap over 0.006 inches. Perhaps you are already aware that the MC of the wood must be very close to the EMC of the air.

A second concern is any stress in the wood.

A third concern is any oils that exude from the wood. These oily woods must be cleaned before they can be glued. The heat of the rip saw brings oils to the freshly prepared surface. Oiliness varies in the same species from one lot of wood to another.

I will discuss technical consultants with you privately.