Garrett - Glue Joints Last Installment
Bob Smalser, Seabeck, WA
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This Western Red Cedar heartwood test layup, that included resorcinol, West System epoxy, liquid polyurethane, 3M 5200, and PL Premium polyurethane construction adhesive was pulled from the pond water after 6 months of submersion, cleaned, and a sample removed from one end and resoaked while the large end was allowed to air dry.
There were no differences in adhesion between the sample allowed to dry and the one that remained soaked.
Destructive testing showed that all the products tested continued to bond strongly after 6 months of submersion. The 3M 5200 poly sealant and PL Premium poly construction adhesive, when applied without clamping pressure, lost adhesion in saturated cedar�but with a clamping pressure suitable for resorcinol, their bonds remained strong.
The �saturated� cedar test layup stabilized at only 24% EMC when corrected for species and temperature. I expected a bit higher reading, but this is consistent with the 30% moisture content of cedar heartwood in the log and the 21% of submerged bilge planking protected by paint. Cedar logs do sink, however, and remain sunk long after their heavier sapwood has rotted away. So this test layup goes back in the water to see if more time will provide a higher reading. I also intend to do another layup of White Oak, which I suspect will rise to over 30% EMC with saturation.
General Notes on Glues and Goos
Resorcinol: The marine standard. If you can get 70 degrees F or higher for an overnight cure and consistent and high clamping pressure with no gaps, you won�t go wrong using it. Likes wood at 10-15% EMC, according to Navy tests. Long open time. Repairable with epoxy. Ugly red glue line.
Marine Epoxy: The repair and restoration standard. Bonds well to a wide variety of materials, and usable in almost all flexibility and temperature conditions. Needs no clamping pressure, only contact�fills gaps well. Likes wood below 12% EMC. Repairable with itself, joints can often be broken apart for repair with using heat. Clear glue line and can be dyed to match the wood. Controllable open time with different hardeners. Slightly permeable to water vapor and there are reports of failures in fully saturated wood and with White Oak. Very sensitive to UV, requiring protection.
3M 5200: A rubbery, polyurethane sealant in various colors with adhesive properties sometimes used as a glue. Fails as a glue under water saturation without high clamping pressure, and without the proper strength testing I couldn�t do here, it�s not recommended as a stand-alone marine glue. Repairable with epoxy.
Liquid Polyurethane: Gorilla Glue, Elmer�s Probond, Elmer�s Ultimate, and others. Versatile in temperature and bonding wet wood with moderate open time, these glues aren�t rated for below waterline use but initial use shows potential as a marine glue. Likes high clamping pressure and fits similar to resorcinol�it won�t fill gaps. Will successfully glue green wood at 30% EMC. Repairable with epoxy. Noticeable, yellow-brown glue lines.
PL Premium Construction Adhesive: This polyurethane goo shows promise as a marine glue with further testing and use. Works like 3M 5200 but cures and behaves like liquid poly. Appears to bond well to everything epoxy does, and more where epoxy and liquid poly won�t, perhaps because of a higher isocyanate content�it bonds to difficult surfaces only cyanoacrylate super glues will bond to. The only general-use glue I�ve found that will bond difficult aliphatic-contaminated surfaces. Appears flexible to temperature and moisture content with gap-filling ability, but as a construction adhesive, its open time is shorter than liquid poly. Appeared to like high clamping pressure, and unlike other glues, wouldn�t bond at all without at least some. Repairable with itself and epoxy. Glue line as in liquid poly.
Urea Formaldehyde Plastic Resin Glue: Weldwood, DAP and others. The old interior furniture standard, and in older marine applications that required well-blended glue lines. Still preferred by many, as it is a no-creep glue easily repaired using epoxy. Long open time, it needs tight fits and 65 degrees F or higher for an overnight cure�it doesn�t fill gaps. Best glue line among them all and moderate water resistance still make it useful for interior marine brightwork applications. A relatively brittle glue and UV sensitive, it requires protection�.but its brittleness is an aid to repairability, as joints can be broken apart for repair. An inexpensive powder with a short, one-year shelf life.
The Titebond Family of Aliphatics: Convenient. No mixing, just squeeze. Short open times, fast tack, and short clamping times. Fast, and an acceptable long-grain layup glue�in heated, commercial shops, I�ve had rough-cut Titebond panel layups in and out of the clamps and thru the planer inside of an hour. Flexible in temperature and to a lesser extent in moisture content, but the bottled glue can freeze in unheated shops. A flexible glue, it has been reported to creep under load, sometimes several years after the joint was made. The latest �Titebond III� appears to be a stronger glue than its two predecessors. Difficult glues to repair, as they won�t stick to themselves and no other glues will except cyanoacrylates, which are too brittle for general use. Epoxy and fabric aren�t bonding to aliphatic glue lines in marine strip construction, compounding repair difficulties. While not definitive, the new PL Premium appears to bond well to Titebond III residue and is worth pursuing by those repairing old white and yellow aliphatic joints.