Is Vacuum Enough Pressure for Urea Formaldehyde Gluing?

      A woodworker asks if he can effectively set pre-catalyzed plastic resin glues such as wettable urea-formaldehyde powder using a vacuum bag for clamping. October 27, 2009

We've used various types of glues for veneering cabinet door and drawer faces in small vacuum bags (everyone has their favorite glue type), but I'm intending to try a pre-cat PR glue for the first time. Most things I read about veneering in vacuum bags say you need to develop a pressure of about 22" of Hg for most applications. 22" has always worked well for us. However, the pre-cat PR glue instructions say softwoods and machined hardwoods can be glued at low pressure - 50 psi. Doing a quick conversion, 50 psi equals roughly 101" of Hg. For gluing a non-backed veneer, of say maple, to a substrate of plywood, is the typical vacuum bag pressure of 22" of Hg enough for a successful bond using pre-cat PR glue?

Forum Responses
(Adhesive Forum)
From contributor E:
Is PR phenol resorcinol or phenolic resin? Do you want the glueup to achieve higher water and heat resistance? Do crosslinked PVA properties still not reach the requirement?

From contributor J:
Setting aside the requirements of this particular glue, it is physically impossible for a vacuum press on planet earth to develop 50 psi. All a vac press does is get the air inside the bag out of the way so that normal atmospheric pressure bears on the bag's contents. Atmospheric pressure is something less than 15 psi, depending on elevation and weather. That's the most you can get out of a vacuum press in even an imaginary, theoretically perfect environment, unless you put that vacuum press inside a giant, artificially pressurized chamber.

From Jeff Pitcher, forum technical advisor:
Contributor J is correct. You can't exceed one atmosphere of pressure with a vacuum bag press. If you multiply the mercury level reading by .4912 it will give you your psi. With a bag press you need to have your pressure as high as possible. That said, I'm not sure which adhesive you refer to when you mention a "pre-cat PR."

From the original questioner:
Sorry for not being more specific. By PR glue, I'm referring to plastic resin glue, aka urea-formaldehyde glue. The pre-catalyzed versions have you add water to reconstitute, in contrast to the resin plus powder systems.

In response to contributor E, I was giving this adhesive a try not for the advantages of increased heat and moisture resistance, but for its increased stability (harder) and lower initial moisture content. Thanks for the responses so far.

From Jeff Pitcher, forum technical advisor:
Thanks. A bag press is perfect for use with powdered resin. Just be sure to leave your piece in the bag for plenty of time and make sure your temperature is above 70 F.

From contributor B:
I use Unibond 800 regularly for bent laminations with bag pressure at 19" hg. Works perfect. Speed up the cure with electric blankets and you can cycle parts much quicker. I am averaging about 125 deg under the blankets and get hard enough cures to pull parts in a couple hours.

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