Wood Glue Selection for Woodworking

In this story Jim Hingst explains the differences among the various wood glues used for woodworking. These include PVA adhesives, polyurethane adhesives, epoxy adhesives, animal hide glues and cyanoacrylate adhesives. 

Selecting the right glue for your woodworking project by and large follows the Pareto Principle. For 80% of these projects, carpenter’s yellow glue is the best choice. For the remaining 20% of applications, there is a wide range of options.           

Understanding the physical properties of the different glues will help you select the right product for your woodworking applications. Because there are so many different types of glues and adhesives, I will limit my examination to those products that are most widely used, which will include PVA adhesives, polyurethane adhesives, epoxy adhesives, animal hide glues and cyanoacrylate adhesives.

PVA Adhesives

The most common glues that you will find in any woodworking shop are the PVA wood glues. These consist of the white glues, such as Elmer’s Glue, and yellow carpenter’s glues.  PVA stands for Polyvinyl Acetate, which is the name of the resin. Within this glue classification, several varieties are available. All of these types are very easy to use. You just spread it onto the surfaces to be bonded with a brush or roller applicator; clamp it; and wait for it to bond. The major differences among the varieties include bonding strength and the open working time.

Time-SavingTip: After gluing up a panel, many woodworkers like to keep the clamps on for three to four hours. That can tie up a workbench. For this reason, schedule your glue ups for the end of the day.  It won’t hurt keeping the clamps on overnight.

Polyurethane Glues

Polyurethane glues, such as Gorilla Glue, became popular after they were first introduced, in a large part because they were touted as the strongest glue on the planet. But are they? In my opinion, that’s a stretch. For edge gluing boards to make a panel, a polyurethane glue probably is no stronger than a carpenter’s yellow glue, such as Titebond or Titebond II.

Polyurethanes, however, excel in certain applications compared to yellow glue. One advantage is that these glues cure when the wood has moisture in it. In fact, these glues require moisture for curing. After curing, polyurethane also forms a moisture resistant bond.

Another advantage is that if you are gluing end grain to end grain, polyurethane outperforms PVA. That’s because yellow glue is readily absorbed by the end grain, leaving too little glue at the joint to form a strong bond. Polyurethane, on the other hand, creates a foam, much of which stays at the surface of the wood. That’s great, but how often will you glue wood at the end grain?

Frankly, for edge gluing it really doesn’t matter whether you are using polyurethane glue or a PVA yellow glue. Both types of glue are stronger than the wood itself. That means that if a joint fails, the wood will break before the glue will.

For most woodworking applications, yellow glue is all you need, if you have tight fitting joints. If you have gaps between two boards, polyurethane won’t help. While polyurethane will foam and fill a gap, the dried foam does not have much strength. The only real solution is to plane the wood to eliminate the gaps or use a two part epoxy glue. One nice feature of polyurethane glue is that the open time is about 30 minutes, which is important if you are laminating several boards together.

The major disadvantage in using a polyurethane glue is it is a pain in the neck to work with.  Make sure that you wear latex gloves when working with the glue or it will stain you skin black. In addition, once you apply it, you better keep your eye on it. Depending on how much you moisten the wood, the foaming adhesive can get out of control, leaving you with a messy to clean up. While I use polyurethane glue for some specialized applications, other glues are often better alternatives.

Polyurethane glues undoubtedly have positive features compared to yellow glue. They provide better water resistance. In most cases, they provide for more aesthetically appealing results when staining. Another advantage of the polyurethane glues is that they are easy to sand. Because the foaming of polyurethane glues is messy, manufacturers have been reformulating the glues so they foam less.

Unlike PVA glues, wait for the glue to dry before cleaning up the squeeze out. Generally, all it takes to clean up the excess glue is to scrape it off with a putty knife.

Tip: Only buy small containers of polyurethane. Often after opening a bottle for one application the remainder  hardens and it wasted. If you have any glue remaining after opening the bottle,  squeeze out the excess air in the container and store it upside down. This prevents the glue from hardening.

Tip: No glue will work well if the wood is contaminated. If you are working with an oily wood, such as cedar, wipe the surface down with acetone to remove any of the oils. Failure to clean the oily contaminant can result in failure of the bond.

 

Epoxy Adhesive

For extremely demanding applications, two part, marine grade epoxy glues are usually the best choice. These glues form a very strong bond and stick to just about anything. That can be a big advantage if you are bonding two different types of materials, such as wood and metal or wood and plastic.

The epoxy glues are also waterproof, solvent resistant and chemical resistant. With respect to service temperature range, these glues withstand extreme heat and cold, which make them great for outdoor applications. What’s more, epoxy glues can fill gaps. Most of the other glues cannot make that claim. One perceived disadvantage of epoxy glue is its cost. In defense of these glues, just remember that you get what you pay for.

The only real downside, in my opinion, is that these two part glues are more difficult to work with when compared to carpenter’s glues. You cannot use these glues straight from the bottle. First, you must thoroughly mix the resin and the hardener in the correct proportions. Some epoxies must be mixed with a thickener (or filler) appropriate for you specific application. Then you must wait for the components to chemically react before use. Glued parts take longer to bond. What's more, it takes about a week before full adhesion is achieved.

Working with epoxy glues requires a slight change in your fabrication process. Ratios of resin and hardener will vary from one manufacturer to another. Be sure to read the manufacturer’s technical bulletins prior to use. Follow the instructions to the letter. Failure to mix the resin and hardener in the correct proportions can result in failure of the bond.  

Safety Note: When working with epoxies, wear latex gloves to prevent skin contact.

When edge clamping boards, you just need to snug them together with just enough pressure to hold parts together.  Any additional pressure can squeeze out too much glue and actually weaken the bond. You can clean any glue that squeezes between the boards with  a putty knife and wiping the surface with denatured alcohol. Cleaning up the glue when it is soft is preferable to having to sand hardened glue.

The joint line will be noticeably thicker than when working with other glues. This poses no structural problem. Typically, glues are stronger than the fibers of the wood. In fact, compared to white and yellow glues, epoxy glues are more flexible and will bend under pressure. More brittle glues, with less flexibility and impact resistance, often break under the same pressure and stresses.

Test, Don’t Guess. As a word of caution, if you are bonding two different types of materials it is best to test the bond strength of the dissimilar materials prior to fabrication. While epoxy glues stick to just about anything, there are some low energy plastics, such as polyethylene, that it will not stick to. 

Cyanoacrylate Glue 

Cyanoacrylate glue, which is marketed under names such as Crazy Glue and Super Glue, was one of chemistry’s happy accidents. During World War II British scientists were trying to develop an acrylic for applications such as fighter aircraft canopies. They failed in developing a new acrylic. Instead, they ended up with this goop that stuck to everything and was difficult to work with. These scientists also failed because they did not recognize its potential as an adhesive. Their idea was scrapped. 

It wasn’t until the 1950s that the project was revisited. By 1958 the glue was finally commercialized. In some respects, their new product was a super glue. Many crafters used the quick setting cyanoacrylate glue for model building and  simple household repairs. In the medical field, the glue was used as an alternative to sutures. 

Although cyanoacrylate glue forms a very quick, strong, water resistant bond to  many materials, including wood, stone, ceramics and metal, it is not a miracle adhesive for woodworkers. In fact, its uses are rather limited. Here’s why. While cyanoacrylate glues have tremendous tensile strength, they are extremely brittle and exhibit very poor shear strength. Subjected to extreme movement, flexing or impact, the bond can easily break. 

In addition, cyanoacrylate glues do not work well on oily woods because the oil repels the water, which prevents the adhesive from wetting the surface. Adhesion to acidic woods, such as oak and mahogany, can also be problematic. 

Cyanoacrylate glues are actually two part synthetic adhesives. The small tube that you buy in the store is comprised of the resin, the hardener and a weak acid, which  inhibits the chemical reaction between the resin and hardener. Exposed to moisture, the acid dissolves and the chemical reaction begins. That’s why you need to wet the surfaces prior to adhering them. Once the polymerization process begins, small molecules bond together to form bigger molecules. The internal strength of the molecules holding together is called “cohesion”. In this process, the glue wets out onto the substrate and mechanically bonds to it. This is called “adhesion”.

Safety Note: You probably have heard stories about people gluing body parts together. While some stories might be urban myth, many are true. When working with cyanoacrylate glue, wear latex gloves to prevent gluing your fingers together. Gloves also will prevent allergic reactions and skin irritation. Work in a well ventilated area to prevent inhalation of fumes that could irritate your lungs. In using cyanoacrylate glue do not touch your eyes. It is also advisable to wear safety glasses.

Animal Hide Glue

Animal glues, such as hide glues, have been used for thousands of years. This type of glue uses an animal protein called collagen as its binding agent. Collagen is pretty tough stuff, because is the protein used to make up the connective tissue in bones, skin, ligaments and tendons. You can make this type of glue easily enough, if you are so inclined. All you need to do is cook animal hides, or bones of horns in a big caldron. After you slow cook it for hours, a gelatin eventually forms, which is subsequently dried. To convert  the dried gelatinous material into glue, you add water and cook it again at a low temperature.  Hide glues have been used in fine furniture making for hundreds of years.

Scale and Measuring Cups. An accurate scale comes in handy for weighing the glue, which is typically measured by dry weight. See the photo below. Water, on the other hand, is measured by volume using measuring cups.

Glue pot. For controlled heating of the animal hide glue, invest in an electric glue pot. With the heat set at 140ºF (60ºC), the chances of burning the glue are reduced. Working with a glue pot is much more convenient and accurate than using a double-boiler and a candy thermometer. Electric glue pots are manufactured in a number of sizes. Finding a distributor that sells them may take some searching and numerous phone calls. I ordered mine from Quill, Hair and Ferrule in South Carolina.

After mixing the glue, put it in the glue pot. Let the pellets of glue soak for about a half of an hour before turning the heat on. Using a kitchen timer will help you keep track of the time. 

As the glue pellets absorb the moisture, the mixture will take on the appearance of oatmeal. See the photo below.

If you are heating the glue using a double boiler, carefully monitor the temperature. Do not heat the glue directly on the burner.

Gradually increase the heat until you reach 140° F. Occasionally stir the glue, to prevent burning. You will need to maintain a temperature between 140° and 150°F.

Keep a tight lid on the pot. The glue pot does not come with a cover. This means that you will have to improvise in creating a lid. You can use a piece of Sintra or glass as a cover, but a ceramic plate works even better. The lid will prevent the water in the glue mixture from evaporating. This accomplishes two things:

• First, by preventing the water in the glue from evaporating, the mixture will not thicken.

• Secondly, and more importantly, animal glue stinks to high heaven and permeates the work environment. A lid helps contain the smell.

 

About Jim Hingst: After fourteen years as Business Development Manager at RTape, Jim Hingst retired. He was involved in many facets of the company’s business, including marketing, sales, product development and technical service.

Hingst began his career 42 years ago in the graphic arts field creating and producing advertising and promotional materials for a large test equipment manufacturer.  Working for offset printers, large format screen printers, vinyl film manufacturers, and application tape companies, his experience included estimating, production planning, purchasing and production art, as well as sales and marketing. In his capacity as a salesman, Hingst was recognized with numerous sales achievement awards.

Drawing on his experience in production and as graphics installation subcontractor, Hingst provided the industry with practical advice, publishing more than 150 articles for  publications, such as  Signs Canada, SignCraft,  Signs of the Times, Screen Printing, Sign and Digital Graphics and  Sign Builder Illustrated. He also posted more than 325 stories on his blog (hingstssignpost.blogspot.com). In 2007 Hingst’s book, Vinyl Sign Techniques, was published.

Slides of Jim's Artwork

© 2015 Jim Hingst