What Kind of Glue?

If you are making something, you probably need to stick stuff together. Chances are, you will use an adhesive or glue of some type. Humans have been using glue for thousands of years, going back to caveman times. Over the years, we have developed an amazing variety of glues, which we use for nearly every purpose.

Why are adhesives sticky? Adhesives are made of molecules, combinations of atoms that may attract other groups of atoms. It’s sort of like static electricity, but more permanent. Other types of adhesives flow and mix on a surface, and are so thick (or viscous) that they hold things together. Think about how honey makes your fingers stick together if you get some on them.

Some glues are sticky polymers dissolved in water or another liquid, like common white glue. As the polymers dry, the liquid evaporates and the solid, sticky adhesive stays behind.

Other glues help things stick because of a chemical reaction. Silicone and cyanoacrylate glues, like Krazy Glue and Gorilla Glue, react with water vapor in the air and harden through a chemical reaction.

Because water helps them harden, cyanoacrylates can be used to close wounds without stitches. Be sure to read the other article in this issue of Celebrating Chemistry about wound adhesives!

Epoxy glues have two parts that start the chemical reaction when mixed together. Special epoxy glues used by dentists harden when exposed to ultraviolet light. The bonds made by epoxy glues are waterproof and strong. There are so many differences in the kinds of materials you may want to stick together that we have practically zillions of types to choose from.

Here are some common kinds of glue you may have used.

• Elmer’s School Glue — This glue is not runny, and is washable, safe, and nontoxic, so it’s good for everyone to use and is easy to clean up. It works for joining lots of materials for school crafts projects.
• School Paste — This comes in a tub with a brush attached to the lid. It is very useful for making posters or papier-mâché models.
• Glue Stick — Has the advantage of being very easy to use for joining paper, as it can be applied directly to the pieces to be joined.
• Wood Glue — Is used for making furniture and cabinets. It has to be strong and resistant to moisture and humidity. The glue has to be good at filling in gaps and imperfections in the wood surface.
• Spray Glue — Comes in aerosol cans and can be used for gluing big projects, like mounting a poster or photo enlargement.
• Fabric Glue — The small fibers in fabric need a special glue. Fabric glue can be used to attach patches or repair seams.
• Hot Glue — Uses a special ‘heat gun’ to melt a stick of glue. The glue melts, and when it cools it makes a strong, flexible joint. It is very useful for arts and crafts.

And here are a couple of uncommon glues.

• Hide Glue — Has been used to make cellos and violins for hundreds of years and is still used today. It comes from animal skins, bones, tendons, and other tissues, similar to gelatin. It is applied warm and forms a strong bond when set.
• Dental Glue — If you have braces, you know they were glued onto your teeth using a special orthodontic glue. This glue is a special resin that hardens when exposed to a special light source. Dental glues are non-toxic and unaffected by the saliva and moisture in a mouth.
• Shellfish Glue — Mussels, barnacles, and oysters attach to rocks using an adhesive protein they make themselves. Now scientists have created a manmade version of this natural glue that can be applied and cured under water. It could be used for repairing boats or securing piers or buoys. They may also be used for surgery.

No matter how big or small, chemistry has the right glue for the job! Chemists work in the lab to develop glues and adhesives before they are produced in a factory and then sold in the store. See if you can spot at least 6 places in the illustration below where you can use glues, tapes, or adhesives around your home. Can you think of other situations where the types of glue mentioned in this article could be used? You might find adhesives in surprising places!

Dr. Alex Madonik is a Chemistry Instructor at Peralta Community College in Oakland, CA.