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PVA Glue

I really like veneering with PVA because it's fast, easy and reliable at lower temperatures. The most important aspect is not too use too much glue. While filled and slow set PVA's are an option, I prefer plain old Titebond. While I use Titebond II, some prefer the harder setting original Titebond (I've never tried it).

What finally motivated this post was two lines from a recent email I received from Edward Ferri at qualityvak.com:

"Typical Yellow & White Glues : 68f recommended temperature.
VAK-Bond 2000 : 65f minimum recommended temperature. "

Plastic resin glues require a chemical reaction to cure properly. That reaction typically requires a minimum temperature of 70-75 F. If VAK-Bond 2000 can produce a reliable bond at 65 F, that's great.

My issue is with the statement that 68 F is recommended when using PVA's (a source request went unanswered). PVA glues set when they dry out, when the surrounding materials (little to zero environmental influence) absorb moisture from glue. Temperature only has an effect when it is too low for the remaining adhesive particles to bond properly. That temperature is typically 50-55 F and is known as chalk temperature (white/chalky looking glue line).

What you should know about veneering and laminating glues

The first glue related email I got from Edward is also a page on his site with the above title. While he makes some good points, others are just plain wrong. I wrote the following some time ago and it appears to be PVA-centric. it probably wasn't "finished", but I am going to post it as is/was.

"[PVA] glues cure by the water moving away from the joint and when it reaches the edge of your substrate it will immediately vaporize in the vacuum. This is noticeable in the vacuum bag as you will see the glue squeeze out bubbling, this is the moisture from the glue."

Even plain water will not vaporize at room temperature (72F) unless the vacuum pressure is over 29 in-HG (not something you'll see/achieve w/ a vacuum press). Even if the moisture were to vaporise, it wouldn't go anywhere, only its state (gas/liquid/solid) would change. The glue sets because the moisture in the glue is absorbed by surrounding materials. The initial/temporary bubbling of glue squeeze out is air being evacuated.

"Using a substrate like MDF, which is porous, will give a shorter dry time in the press as compared to using birchboard which is not very porous, thus a much longer drying time in the press.   This is particularly important if you are doing something like a curved jamb made from a hardwood (oak, maple, etc.)."

For the most part, set time is determined by glue line thickness and the proximity of absorbent materials. Using an MDF cover sheet, the set times for veneer over Birch and veneer over MDF are be pretty close. I imagine curved, multiple ply, layups can take noticeably longer because there's more glue (moisture to absorb) and the tolerances/fit between the pieces is not as tight.

"With a PVA, it takes a long time for the moisture to migrate to the edge of the laminates and vaporize, unless the substrate is something like MDF which absorbs the moisture."

If the substrate, veneer, and cover sheet(s) cannot absorb enough moisture, the glue won't set. There is no/minimal air exchange and nowhere else for the moisture to go. While there is typically a bit of air around the edges, that air will only absorb a minuscule amount of moisture (increased RH). There is nothing special about the edges of the layup. Vacuum is not some magical sponge that sucks moisture from the glue (more later).

"I had a customer who purchased a system to make curved jambs and he used yellow glue. It took days to cure in the vacuum bag."

See above, too much glue and not enough absorbent material (or the moisture content of the material was too high). That said, a rigid glue is generally preferred for solid wood layups with any significant bend.

"The water based PVA glues also have the same problem about a water base finish softening the glue at a seam or the holes in a burl."

From what I've heard, this is not an issue. If it worries you, use a water resistant PVA. I use Titebond II and water based finishes for just about everything (including veneering).

"Another issue with PVA glues is cupping of a panel. This can occur with other glues; however, most of the phone calls I received were with PVA's. The problem is due to moisture on one side that has not fully evaporated and within 15 minutes of removing a panel from the bag, it starts to cup. The immediate solution is to return it to the bag and press for a longer time. The next pressing should be left in the VAK bag for a longer period of time to prevent this from happening again. Another solution is to veneer both sides at the same time, thus you have balance. This is a good veneering practice as veneering the back side provides for long term balance against uneven moisture absorption."

Moisture causes bow, glue/veneer shrinkage can cause cupping. If you are veneering one side at a time (perfectly acceptable), the veneered side will come out of the press flat or slightly bowed. Any bow will go away as the layup dries. If you do not veneer the other side of the panel, the veneered side will have a tendency to cup as the glue (shrinkage) and veneer (high > low moisture content) fully cures/dries. Press time will have no affect on this. Balanced panels, no matter what materials are used, are required to insure the stability of the panel.

"Normally PVA's have a shorter cure time in the bag, than other glues, which can be from a half hour to several hours. In general, cure time depends on several factors: type of glue, temperature of the shop, size of the work piece and the substrate material. We discussed the substrate material (porous being better and therefore faster curing for PVA glues), the temperature of the shop (putting an electric blanket over the VAK bag can get the temperature up to 100 and cut the drying time in half or less. I have dried wood in the bag using this method.). Temperature also affects the open time. If it's 90 in the shop you normally need to ; apply the glue, assemble the pieces and get in the VAK bag and under pressure in less than 5 minutes. A urea or epoxy will give you more time." "

PVA's are only affected by moisture, or lack thereof. An electric blanket will have no effect on PVA set time. It's always a good idea to get PVA layups into the press in as short of a time as possible, less than 5 minutes

"With PVA's a small 15" x 24" panel will dry faster than a 4' x 8' sheet. I have had several calls from uses who took a 4x8 sheet out of the bag in an hour or so, just as they did with smaller cabinet doors. The 4x8 panel started to blister in the center after it was out of the bag for 15 minutes. The reason being, that the moisture in the center of a 4x8 sheet takes much longer to migrate to the edge and evaporate that it does on a 15" x 24" cabinet door."

There are only two reasons why larger panels would take longer to set than smaller ones, thicker glue spread and/or less absorbent substrate (and/or cover sheet). Other reasons for blistering could be too little glue, too much open time/drying, or too little pressure (thick cover sheet or stacking) in that area.

Most wood and wood products are hygroscopic, they absorb (/release) and distribute moisture until they are in equilibrium with the surrounding environment. The path of least resistance for the moisture in the glue is the dryer materials surrounding it. It doesn't matter if the surrounding materials are MDF or Lignum vitae, or if the glue-up is in a vacuum or in open circulating air, most of the moisture in the glue is absorbed by, and (when removed from the vacuum) passes through, the surrounding materials.

"Using the correct glue for the job is paramount for success. A customer, in the north, built a curved staircase for a job in California. It shipped by truck and when the staircase was unloaded, the laminates were peeling apart. Why? Yellow glue was used and the truck went thru the Arizona desert during the summer. The temperature inside truck gets well over 100 and the glue softened. This coupled with the fact that the laminates were under tension caused the delamination."

While I agree that that a plastic resin glue would have been a better choice, this borders on FUD (fear uncertainty and doubt). According to Titebond specifications, extended exposure to 150 degree temperature reduces the bond strength from 3,750 psi to 1,750 psi. I've done numerous tests, including toasting veneered samples until they turned brown, and had no delaminations. There would have to be some pretty significant stresses, and poorly fitting pieces, for a PVA glue line to be the point of failure.
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