Adhesives and Woodworking

PVA vs. Urea Resin in Panel Manufacturing

Jeff Pitcher - Wednesday, February 06, 2008
A high percentage of panels are glued with urea formaldehyde resin and because it is a thermosetting, non-reversible adhesive it has no practical limitation on heat resistance.  In other words, urea formaldehyde resin glue lines will not soften at any temperature.  A small proportion of your panels are glued with polyvinyl acetate adhesives (PVA) and this glue line will begin to soften at around 160F with danger of bond failure at 200F plus or minus 10 degrees.  This is normal for all PVA whether cross linking or not, and there really is no way this can be significantly increased.  However, it should be understood that glueline softening is also a function of time and glue line depth as well as temperature. In a finishing operation the time under a high temperature of perhaps 300F,  probably will not be long enough to significantly soften a PVA glue line, especially given that your panels will have 3/4 inch of glue line and the depth of heat penetration will be quite small even with exposure of a minute or two.
 

Veneer Bleed Through Prevention

Jeff Pitcher - Tuesday, February 05, 2008
Occasionally, I receive calls from manufacturers experiencing "bleed through" when laying up veneers.  Bleed through is the result of moisture in the adhesive carrying glue solids through the pores of the veneer to the surface.  This usually results in finishing problems.  Bleed through normally occurs when using porous species such as oak but can occur with any veneer.  The major cause is a heavy glue spread.  By minimizing the spread you can lessen the bleed through.  Additionally, allowing some open time after the adhesive has been applied to the substrate will result in less bleed through.  Finally, the use of a "filled" adhesive can help prevent bleed through by clogging the pores of the wood.
 

Lesson Learned when Demonstrating Contact Cement

Jeff Pitcher - Thursday, September 27, 2007
I recently shot a demonstration video to show the proper application of contact cement.  During the first take I was somewhat distracted by the filming and I only applied cursory pressure with a J-roller.  At the "moment of truth" when I tried to lift the laminate it separated easily from the substrate.  Somewhat taken aback, I re-shot the video and concentrated on doing everything properly.  I paid close attention to applying plenty of pressure with the J-roller.  As expected, the laminate was impossible to remove in one piece.  The moral of this story is to be sure to apply plenty of pressure when using contact cement.  It may save you from a re-work!
 

Temperature vs. Adhesive

Jeff Pitcher - Monday, September 17, 2007
With Fall quickly approaching it's time to start looking at the effect of temperature on the gluing process.  PVA glues must be used above freezing.  In fact, many of them have a minimum use temperature above 45 degrees F.  This is especially true of crosslinking PVAs.  The biggest mistake we see is when workers forget that the wood they're gluing needs to be at the right temperature as well as the ambient temperature of the area where the gluing is done.  An easy way to tell if temperature is affecting your glue line is to examine it for "chalking".  If the temperature is too low for the adhesive being used the glue line will look like chalk and will have little or no strength.

Temperature also plays a significant role in the amount of curing time needed when using urea resins.  The cooler the temperature the longer the clamping time necessary.  At temperatures much below 70 degrees F the user risks "dry out" rather than a cure.  "Dry out" occurs because the moisture in the glue line has evaporated before a chemical reaction could take place.  This results in a weak bond.

Finally, it's very important to pay attention to the ambient temperature when using pressurized canisters of contact cement.  At temperatures below 60-65 degrees F the rubber in the adhesive begins to stiffen causing irregular spray patterns and general difficulty in spraying.   Be sure to keep the canister off of any concrete floors and store them in a warm area.
 

Tolex Shelf Life

Jeff Pitcher - Tuesday, September 11, 2007
tolex shelf lifeA Customer with some older Tolex adhesive requested information on shelf life and how to tell if it is still good. Here are some thoughts on this:

While the rated shelf life of the Tolex glue is 1 year, realistically, it can often be used (if stored correctly) for longer time periods than that. Doing a test bond with some scrap material provides the answer we need. First, check if the adhesive is still in a uniform solution. If the adhesive is lumpy or inconsistent, the adhesive solids will not go correctly "back into solution" with agitation. Consequently, if the adhesive is separated or lumpy it is no longer usable. Second, if it is still consistent, the next step is to do the test bond. The fastest way to test is to use the traditional "two sided" adhesive method rather than the alternative Tolex single sided application method. Spread the adhesive on a test piece of Tolex and plywood (or MDF or particle board). Allow the water to flash off for 20-40 minutes until dry. Put the substrates together and smooth out. If you get good initial grab and the Tolex sticks well, then long term performance will be fine as well. If you find that you need to purchase more product instead, Antique Electronic Supply offers quart containers that may be a better size for you. 480-820-5411 is their number and www.tubesandmore.com is their web address.
 

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