Since I only wear glasses late at night when I take out my contacts, for the past couple of years I’ve been making do with an old pair of frames broken in two places: the nose piece and the right earpiece. I must have glued them back together a dozen times, but the repairs never last.
Most glues hold just long enough for me to put the glasses back on your nose and start walking, then let go. Although this offers me an opportunity to improve my hand-eye coordination as I attempt to catch the separately descending pieces, it is an otherwise unrewarding experience that I would happily forego.
So I decided to delve into the world of glues, knowing that whether I found a way to fix my glasses or not, I’d at least end up with a column.
A glue (or, for those who don’t like four-letter words, an adhesive) is, simply, any substance capable of holding two surfaces together in a strong bond. Adhesives can be classified in various ways: natural or synthetic; rigid or flexible; or thermoplastic and thermosetting. (The former soften when heated; the latter require heat to set properly.)
We normally talk about glues “drying,” but a better term is “curing,” because although some glues do harden as water or other solvents evaporate from them, some harden through a chemical reaction called polymerization (changing from a liquid to a solid), and some harden through cooling.
It’s impossible to talk about glues in generic terms, because, as Consumer Reports magazine found out when they decided to test glues, there are more than 15,000 glues on the market today, of more than 100 distinct chemical types. Of course, some types are more common than others.
Take the familiar white glue. This is one glue that really does cure by drying; it’s water-based. That’s why it only works well on porous surfaces such as wood: the water carries the adhesive into the pores in the two surfaces being glued, and then evaporates, hardening the glue and binding the two surfaces together. But that high water content is also why white glue tends to make paper wrinkle and why you don’t want to use it glue your boat together: water dissolved it once, and it can do so again. Since white glue cures by losing part of its substance through evaporation, it also tends to shrink as it dries.
Epoxies are another common type of glue. They’re made from synthetic resins. (Resins are compounds that can form thin, continuous films.) Epoxy resins are probably the strongest adhesives in current use, and they’ll stick to just about anything. They’re also resistant to solvents, changes in temperature, and even electricity. They’re an example of a glue that cures through chemical reaction, which means they don’t shrink as they harden.
Normally, epoxies are formed by mixing two substances (epichlorohydrin and Bisphenol A, if you must know, but I doubt you’re any the wiser for it–I’m certainly not), which react to produce a long polymer chain–the kind of long, stringy molecule that I talked about in my recent column on plastics. This is not, however, why most commercial epoxy adhesives come in two containers, the contents of which must be mixed (carefully). The epoxy has actually already been made and is in one container; the other contains a hardener. Getting the proportions wrong can greatly affect the adhesive’s strength. This inconvenience is probably why, even though epoxy is such a great adhesive, it’s not what most people reach for first around the household.
If epoxies are the strongest adhesive around, you may well ask, what about the so-called “super glues?”
More properly known as cyanocrylates, these one-drop wonder glues certainly work well for certain applications. They cure through a very fast chemical reaction–so fast that some newer types actually have additives to make them bond less quickly. This quick bond makes them good for gluing hard-to-clamp objects. It also makes them good for gluing your fingers to your screwdriver, your workbench or your nose. Fortunately, acetone (nail-polish remover) will dissolve the bond.
Contact and plastic cements both harden by the evaporation of a solvent (which is why they stink so much and require lots of ventilation when you’re using them). Contact cement sticks best to itself: you brush some onto the two surfaces you want to stick together, let it dry a bit, and then touch the two surfaces together. Trouble is, you have to be very precise when you do so, because contact cement bonds instantly.
Plastic cements are best typified by the little tubes of glue every kid who has ever built a model airplane has used. In the case of model airplanes, they glue not only by hardening as their solvent evaporates, but by dissolving some of the model’s plastic, literally melting and fusing the two pieces being glued.
Thermoplastic glue is also known as hot glue because it comes in a stick that has to be melted in a glue gun before it can be applied. It’s really just a modern variation on one of the first adhesives ever used: hot wax. (Hot wax, by the way, is still used as an adhesive in printers’ shops for laying out print jobs. It’s great for applying paper to paper, because although it sticks well enough to keep things in place, it doesn’t stick so hard that you can’t change your mind later.) Like hot wax, thermoplastic glue flows easily when hot, then sets very quickly as it cools. (Thermoplastic glue binds things a lot more strongly than hot wax, though!)
These are just a few common examples of glues. As I said, there are at least a hundred chemical types, most of which I haven’t even touched on: silicone rubber, for instance, or urethane. And the search for new glues is ongoing.
For the last few years scientists have been studying the adhesive produced by mussels that glues the creatures to rocks so strongly that even crashing surf can’t knock them off. None of the current commercial adhesives really work well in extremely wet environments, so obviously nature knows something we don’t. If research on this glue pans out, it could be used, for example, to hold artificial hips in place or fix broken bones without the need for a cast. (The human body, at 75 percent water, qualifies as an “extremely wet environment.”) It could also make hull repairs to ships easier–they could be repaired at sea, without having to be dragged into dry dock.
Researchers are looking at other glues that could be used medically, in place of sutures and staples, especially in places where tissue damage must be minimized. European surgeons, for example, are already using a natural adhesive made of blood-clotting agents.
So this sticky branch of science marches onward, but there’s still no reliable way to glue glasses frames back together. (According to Consumer Reports, that’s because skin oils contaminate the joint. The best glue they could come up with for the job was a catalyzed acrylic glue called Devcon Plastic Welder, and the glasses still broke again after a month.) Last week I gave up and bought new frames.
Like the cure for the common cold, a glue for gimpy glasses remains an as-yet unachievable dream.