Ugoretz: Glue

Archeologists tell us that people have been using glue to stick things together for many thousands of years; birch bark tar for spear points as long ago as 200,000 years, plant gum and ochre in Africa in 70,000 BC, a collagen based substance in the middle east about 8,000 BC and a form of hide glue in Egyptian furniture around 2,000 BC.

Improved animal based adhesives were made by ancient craftspeople who learned to clean and boil skins, sinew or hooves in alkaline solution to yield collagen, a fibrous protein. This could be dried, pulverized, and mixed with water to produce a sticky gel which solidifies when cooled. Hide and other animal glues continue to be used today. Fish glue (isinglass or ichthocol) is used in bookmaking and bonding wood to other materials. Rabbit skin glue is quite strong, useful for gilding, traditional woodworking and painting techniques.

Hide glue is still the choice for reproduction of antiques, repair of musical instruments or furniture and veneering. It has the useful qualities of neutral pH, reversibility with heat, rigidity, strength and adhering to itself. Stradivarius is said to have used a secret formula, partly responsible for the quality of his violins. Hide glue is not water resistant, requires heating and some time to prepare and can spoil in the glue pot. When urea is added, hide glue remains liquid and stable at room temperature, available as Old Brown Glue® or Titebond Liquid Hide®.

Synthetic adhesives, first developed in the 1930s and used in industry during WW II, became commercially available in the late 40s. There are two categories; reactive, which harden in a chemical reaction (cyanoacrylates and urethanes – exposure to moisture, epoxies – on mixing of two components) and non-reactive, which harden with solvent evaporation (polyvinyl acetate or PVA and contact cement).

PVA glues dominate the market for general woodworking. The original PVA product was Elmer's Glue-All®, introduced by Borden (yes, the dairy company) in 1947. Borden became a maker of adhesives during WW II using casein, a byproduct of milk and the main component of cheese. Casein glue was used in aircraft construction. There are now many brands and types of PVA glue which differ in water resistance and, to a lesser degree, in strength and working time. They are all subject to creep under sustained stress. Common brands of PVA glue are Titebond, Elmer's and Gorilla.

ANSI (American National Standards Organization) classifies glue according to water resistance: Type I – waterproof, Type II – water resistant, Type III – no water resistance. Type I glues have longer working time and is more expensive.  This is confusing because Titebond® names their PVA products “Original” which is not water resistant, “Titebond II” – which is water resistant and “Titebond III” – which is waterproof.

Urea-formaldehyde glue (Weldwood Plastic Resin®, Unibond 800®) is one of several resin (originally, any sticky, mineral-rich secretion from plants or trees – now, synthetic polymers with similar properties) based adhesives. They all have good strength, long working time, rigidity and durability. They are useful for bentwood lamination and veneering. Precautions for release of formaldehyde vapor are required Weldwood comes in powder form and is activated by mixing with water. Unibond 800 is a liquid, activated by addition of a hardener and very water resistant. Variants are resorcinol-formaldehyde; water and solvent proof, approved for wooden aircraft construction and  phenol-formaldehyde, used in plywood manufacture.

Cyanoacrylate glue has an interesting history. It was synthesized in 1942 in an effort to make clear plastic gun sights. But it stuck to nearly everything. The inventor, Harry Coover, noted this but did not pursue its potential as an adhesive. In 1947, when working at Eastman Kodak, his colleague, Fred Joyner, saw CA instantly bond two glass prisms. In 1958 Eastman Kodak marketed CA adhesive as Eastman #910, later Super Glue®.

CA glues polymerize on contact with hydroxyl ions in water, forming a strong, durable plastic mesh. Wood chemistry delays polymerization but bonding is enhanced if surfaces are moistened. CA has no gap filling ability and will not bond to itself or to cured PVA. If surfaces are not smooth and flat, joints will fail. Gel versions work better with wood because they are less prone to wicking. CA is best suited for small parts and repairs. It will instantly bond to skin and is very dangerous on eye contact. Irritating, sensitizing fumes are also to be avoided.

Epoxy glue requires mixing of resin and hardener and is available in formulations with different setting rates. Epoxy has considerable gap filling ability and, in fact, produces weaker bonds in very tight joints. It bonds well to other adhesives, with the exception of PVA glue. These properties makes epoxy practical for joint repair. An article on this subject can be found at;

http://www.sawmillcreek.org/showthread.php?21822-Are-Your-Glue-Joints-Repairable

Polyurethane glue (Excel®, Gorilla Glue®), the most recent new adhesive, was introduced in the U.S. in 2003, marketed as very strong, waterproof, all purpose glue. Although able to bond a wide variety of materials, it has a property that limit its use in woodworking. When it cures it expands, producing a tough foam. With very thin application and tight clamping, strong joints can be made. However, without tight, well clamped joints, the foam pushes surfaces apart, producing a weak joint and leaking out at the edges making clean up a problem because it is resistant to solvents. The manufacturer of Gorilla glue maintains that it produces stronger joints than PVA glue when used on end grain.

Fine Woodworking (August, 2007 #192) published a test of the strength of joints made with six different adhesives. Each glue was used to make bridle joints in maple, oak and ipe', with tight, snug or loose fit. A Type I PVA glue, Titebond III®, had the greatest average breaking strength of the six samples. Polyurethane glue failed in all loose fitting joints. A summary of the results of this test is printed at the end of this article.

Glue joints can, and will if you are not careful, eventually fail for many reasons. The most important are: starved joint – due to insufficient adhesive, excessive clamping pressure, surfaces not freshly sanded or planed, preventing penetration of adhesive into wood fibers or wicking into endgrain; premature cure – due to exceeding the working time of the adhesive; failure to cure and bond – due to over age glue, cold, excessive moisture, bad chemistry, contaminated or oily surfaces; mechanical weakness – due to poor clamping, loose fit, rough or non-flat surfaces or inadequate surface area.

Two valuable references that include many aspects of gluing not covered in this article. A comprehensive discussion of the science and application of adhesives used in woodworking is published in Adhesive Bonding of Wood  USDA Bulletin #1512. http://www.woodcenter.org/docs/tb1512.pdf

Chapter 11 of Bruce Hoadley's excellent book, Understanding Wood, explains the elements of successfully joining wood with glue.

From Fine Woodworking  August, 2007

By Dick Ugoretz (originally published on January 2016)

^{Editor's Note: Light edits have been made to remove broken web links.}