Glue has always suffered from a slight image problem. It is seen by some as a kind of liquid bandage – a temporary fix that’s a half-step above chewing gum on the ladder of shoddy repair products. But experienced DIYers know that glue is essential to furniture, plywood, engineered lumber and much more – glue even holds high-tech airplanes together.
The first glues featured natural ingredients such as Rice, wheat and animal hides. The glues found in today’s workshops are considerably more complex, featuring synthetic ingredients such as aliphatic resin, di-isocyanate or urea formaldehyde. Whether new or old, each type of glue has its benefits and its drawbacks. To help you make smart adhesive choices, we’ll compare and contrast a few of the most common types of glue.
How glue works
Glue is dispensed as a liquid and hardens into a solid, forming a mechanical bond with the surfaces it contacts and a cohesive bond with its own molecules. The transformation from liquid to solid can occur through one or more of three processes: evaporation, chemical reaction and thermal conversion.
Traditional glues rely on evaporation. The solids that form the glue bond are suspended in liquid that evaporates after the glue is applied. Evaporative glues include white glue, regular yellow wood glue and organic adhesives such as hide glue, fish glue and wheat or rice paste.
Some glues, including polyurethane glue, cyanoacrylate and epoxy, undergo chemical changes that cause them to harden and bind together. With two-part glues (such as epoxy), the chemical reactions occur when the parts are combined. In other cases (polyurethane and cyanoacrylate), the glue reaction is triggered by ambient moisture or by the absence of air.
Other glues, such as hot-melt glue, rely on thermal conversion. A solid that is heated to liquid form and then reverts to a solid when it cools undergoes this conversion. In the case of glue, the heated liquid is applied to the workpiece, and it bonds to the surfaces as it cools and reshapes itself to fill the area of application.
Exterior-rated carpenter’s glue (Type I and II), resorcinol and urea formaldehyde glue (plastic resin glue) are common glues that undergo both evaporation and a chemical reaction in the process of hardening. Because it gives off heat as it hardens, epoxy goes through both a chemical and a thermal reaction.
Points of comparison
When choosing a type of glue for a specific application, consider these properties:
- Strength (shear resistance) — Almost all of the glues discussed in this article have a rated shear resistance of 2,000 to 4,000 psi, which is greater than most wood species. The exceptions are hobby-grade hot-melt glue (500 to 1,000 psi) and some epoxies that can develop strength of 8,000 psi or higher.
- Tack — This is the amount of initial stickiness a glue exhibits. High-tack glues such as hot-melt and hot hide glue provide an advantage when joining two parts that cannot be clamped (for example, at a miter joint in crown molding). Low-tack glues, including most yellow wood glue and plastic resin glue, come in handy for veneering and complex glue-ups.
- Shrinkage — Evaporative glues shrink the most. In some cases, half or more of the wet glue volume is liquid that evaporates as the glue cures. If you use proper clamping practices, shrinkage is not a problem. But where a strong bond is required and a gap exists, use a low-shrinkage glue such as polyurethane hot-melt glue or epoxy (see “Gap-Filling Glues,” below).
- Appearance — Glue is seldom used as a decorative element. High-clarity versions of several types are available, these include hide glue, cyanoacrylate, epoxy and hot-melt glue. Because it dries to a dark purple color, resorcinol leaves a pronounced glue line.
- Moisture resistance — Type I and Type II yellow glues are formulated for use in exterior and wet areas. Polyurethane glue and epoxy have excellent moisture resistance. Resorcinol is a consumer-grade glue that’s rated for constant submersion (hence its popularity as a marine adhesive). Hide glue, white glue and standard yellow wood glue are poor choices for exterior projects.
- Safety — Low toxicity and low odor are two of the main reasons white glue and yellow wood glue are so popular. On the other end of the spectrum, epoxy, resorcinol and plastic resin glue require special handling and safety equipment. Always wear the recommended safety gear, follow all ventilation guidelines and dispose of old product properly. Glue manufacturers are required to file a detailed Material Safety Data Sheet (MSDS) for every product. You can find a MSDS on the Internet or request one from the manufacturer.
Consider these additional properties as they relate to specific gluing tasks:
- Heat resistance
- Shelf life (how long the product remains usable in its original container -- about one year for most premixed glues)
- Viscosity (this can be altered on most glue types with solvents, extenders and other additives)
- Solids percentage (the ratio of solid resins to liquid)
- Pot life (how long the glue stays workable once it is prepared and exposed to the air)
- Working time (the amount of time the work may be adjusted before the glue sets up)
- Open assembly time (time between application and joining of parts)
- Closed assembly time (time between joining of parts and clamping)
- Creep (a condition where mechanical pressure from wood forces causes a glue joint to slip, allowing the wood parts to shift and become misaligned).
Common glue types
Because glue is so versatile, categorizing types by function is not always helpful. A woodworking glue can be useful for other applications, just as a glue formulated for nonwood applications may be the best choice for a special woodworking situation. Here’s how common types of glue compare.
PVA and aliphatic resin — Household white glue and yellow wood glue account for most glue consumption. White glue is typically called PVA glue (polyvinyl acetate), and yellow wood glue is referred to as aliphatic resin glue. Yellow glue tends to have more solids are greater heat and moisture resistance than white glue, as well as better initial tack and shorter clamping time. The longer open assembly time of white glues is and advantage for jobs that need more working time.
White (aka PVA): Bonds paper, cloth, wood, pottery.
Yellow (aka aliphatic resin): Used for woodworking and carpentry, types I and II are waterproof.
White glue (most famously, Elmer’s Glue-All) has a host of household uses for bonding paper, wood, fabrics and other porous materials. It can be used to size walls and fabrics as well. It dries to a clearer color than yellow glue, and it is cheaper. Do not substitute “school glue” for white PVA, it is not nearly as strong.
Although any type of adhesive may be rated as Type I (waterproof), Type II (water resistant) or Type III (not water resistant), the ratings are applied most often to aliphatic resin glues. Regular Type III yellow wood glue is suited for interior use only. Type II wood glue undergoes a chemical reaction that enhances cross-linking among the polymer strands, resulting in an exterior-rated bond. Type I glues must withstand a more rigorous battery of boiling and heat tests to earn a waterproof rating.
Polyurethane glue — A relative newcomer to the DIY market, polyurethane glue is a versatile, ready-to-use adhesive. Suitable for interior or exterior use, poly glues can bond wood, masonry, metal and other surfaces that are impossible to glue with white or yellow wood glue.
Polyurethane glue requires moisture. In most cases, the ambient water in the air and the workpiece are sufficient to trigger the required chemical reaction in the glue. But in dry conditions or when bonding nonporous workpieces, you should moisten the work before applying the glue.
Polyurethane Glue: Wood and nonwood bonding, exterior and interior.
If allowed to expand, polyurethane glue will foam as it dries. It can also cause skin discoloration, and it is relatively expensive. Nevertheless, during the last 10 years polyurethane glue has won many dedicated converts.
Epoxy — Developed largely for marine use, two-part epoxy has found its way into the home and shop, where new uses are being developed all the time. When the resin (Part A) and the hardener (Part B) are combined, epoxy is almost 100 percent solids. This gives it a structural advantage over most other glue types because the bonds created by the cross-linking resinous polymers are very strong.
Epoxy is probably the most effective gap-filling glue. It bonds to many surfaces, and it can be painted on as a wood sealer or shaped and sanded if it’s used as a wood filler.
Epoxy: bonding, sealing, gap-filling, nonwood joinery, hardware reinforcement, Quick-Set epoxy sets up faster.
Large quantities of epoxy normally are sold in canisters that are sized to match the mixing ratios, which can be anywhere from 1-to-1 (resin to hardener) to 4- or 5-to-1. In general, a higher ratio of resin to hardener yields a stronger bond.
When mixed with a filling agent, epoxy can be used as a sandable wood filler. Other filling agents create higher strength but are not as easy to sand.
Coat post ends with epoxy to protect the wood from moisture before setting the post in concrete or in the ground.
Smaller quantities of epoxy come in single-use syringes that blend and dispense the product as you depress the plunger. Epoxy also comes in tubes of putty; the resin is wrapped around the hardener like pigs in blankets so you can break off chunks and knead them into a hard-setting filler material.
Standard marine-type epoxy is slow-curing, taking four hours or more to set up. Fast-curing versions marketed as five-minute epoxy or 3.5-minute epoxy are excellent for quick repairs, but at 2,000 psi their shear resistance yields as little as one-quarter the strength of standard epoxy.
Cyanoacrylate glue — Known by woodworkers and model builders as CA glue, most people recognize this adhesive by the brand names Super Glue and Krazy Glue. For minor household repairs, such as fixing a broken coffee mug, CA glue can’t be beat. In the shop, woodturners may use CA glue to mount bowl blanks to a waste block. It can also attach small and nonmetal parts with no clamping required.
Cyanoacrylate Glue (aka CA, Super Glue, Krazy Glue): Fast bond on ceramics, plastic, metal, wood, glass and rubber.
If you consider the cost by the ounce, CA glue is expensive, but only small amounts are needed. The shelf life is usually six months or less (another reason it comes in such small tubes). Most CA glue is moisture-cured, so you may need to moisten the surfaces very lightly before applying the glue.
Cyanoacrylate glue is perfect for making quick but permanent repairs to broken household items. Most brands are effective on a wide range of both wood and nonwood materials.
Plastic resin glue — Packed as a dry powder resembling wheat germ, plastic resin glue is also known as urea formaldehyde glue, urea resin glue, UF glue and urac. Today it is used mostly in commercial cabinetmaking operations and by plywood manufacturers.
Plastic Resin Glue (aka urea formaldehyde, urac, UF): Veneering, laminating, commercial bonding of plywood, particleboard, MDF.
A big benefit to urea formaldehyde glue is that it sets quickly when heat is introduced. DAP/Weldwood’s Plastic Resin Glue requires clamping for 13 hours at 70 degrees, but only 30 minutes at 100 degrees. Users gain a long open time for glue-up, but they can eliminate most of the clamping time by heating the glued product in a low-temperature kiln.
Resorcinol — This two-part glue is used in marine applications and for bent laminations and veneer work. It is rated for full water submersion. One resorcinol component (the catalyst) is a powder, and the other (the resin) is a liquid. The product functions like epoxy, but because the liquid component contains solvents, it shrinks when it dries. Like the closely related plastic resin glue, resorcinol predates modern epoxies, which compete successfully with both types of glue.
Resorcinol: Boat, marine and other submerged applications.
Hot-melt glue — This is the most prevalent adhesive that relies on thermal conversion. Glue-gun sticks sold at hardware stores and craft shops are useful for model making or temporarily tacking parts of a jig together, but they are too weak to be of much use in permanent wood joinery.
Delivered at a temperature of 140 to 160 degrees, hobby-grade hot glue has a shear resistance of about 500 psi. Higher-strength glue sticks for standard glue guns are available. The best place to find them is on the Internet, where you can even find glow-in-the-dark glue sticks.
Hot-Melt Glue: Hobby and craft work, polyurethane hot glue used for gap-filling, clamp-free woodworking joinery.
Hide glue — This venerable old adhesive has become something of a relic. Made from the skin and bones of rendered animals, hide glue offers only one noteworthy benefit over other glue types: reversibility. Because the glue is liquefied with heat, it can be reheated to a near-liquid state so wood joints can be easily disassembled without damage. For repairing musical instruments and some kinds of antique restoration, this can be an advantage.
Many glues claim to be gap-filling adhesives that can create a strong bond even in a loose joint. Although most will fill the gap with dry glue, only a limited number of glue types can make a long-lasting joint. These include two-part epoxy and polyurethane hot-melt glue.
Polyester hot-melt glue goes on hotter and dries stronger than regular hot-melt, making it a good choice for gap-filling applications, such as holding a broken or too-small tenon in a mortise.
Epoxy and hot-melt glue are effective at gap filling because they are 100 percent solids when applied. That means the hardening and adhesion occur without a loss of volume. When an evaporative glue such as PVA dries, the liquid disappears from the joint. If the joint is under pressure from a clamp, the space occupied by the liquid will fill in with solids under compression. But if there is no pressure, as in the case of a loose joint, the liquid will be replaced with air, causing the dried glue to have a spongy texture that weakens the adhesion. (This is one of the many reasons it is important to clamp glued joints, even if they look and feel perfectly fitted.)
Some chemically reactive glues, such as polyurethane glue, contain 100 percent solids but are less effective as gap fillers because they give off carbon dioxide, which forms bubbles in the dried glue if no clamping pressure is applied. Hobby-type hot-melt glue will fill the gap with a solid mass, but it doesn’t have enough cohesive strength for joinery.
Hide Glue: Antique and musical instrument repair
Hide glue comes in granular (often called pearl) form or as a powder. The granular product is more common, which is unfortunate because it has one drawback that neither powder nor liquid hide glue suffers from: It stinks. Liquid hide glue is also derived from animal hides, but it is a liquid at room temperature and cures through evaporation only. It is moisture-soluble for good reversibility, and it has good rigidity to resist creep.
Hide glue pearls, granules or powder are mixed with water and heated to about 140 degrees. The glue bond is strong but easily reversed.
Although it’s a good idea to have a few types of glue on hand for emergency repairs (see “Advice for the Newer Gluer,” below), the short shelf life of most products precludes stockpiling. So when you make a shopping list for your next project, give as much consideration to the glue you use as the building materials you select.
ADVICE FOR THE NEWER GLUER
You can tackle almost any gluing job around your house and shop with a fairly small selection of glues. An entry-level kit should contain yellow wood glue for wood projects, polyurethane glue for nonwood gluing and smaller wood tasks, and cyanoacrylate for small household repairs.
When shopping for glue, buy small containers. Most glues have a shelf life of one year or less, and smaller containers will result in less waste. Write the date of purchase on every new glue container.
Store glue in a relatively warm environment so it won’t freeze. Some types can withstand a few freeze/thaw cycles, but it’s better not to risk it. And always read the container for instructions on safety and proper usage.