A while back I looked at lubricants for the home shop, with an eye to the physics and chemistry behind lubrication. Talking about how to keep parts moving got me thinking about the other side of the equation – what’s the science behind sticking stuff together? Home shops have a lot of applications for adhesives, so it probably pays to know how they work so you can choose the right glue for the job. We’ll also take a look at a couple of broad classes of adhesives that are handy to have around the home shop.
The Science of Sticky
So how do adhesives work anyway? It turns out to be one of those “we’ve got some ideas, but nobody’s sure” things. Our biggest clues come from looking at the chemical structure of adhesives, and as usual it’s helpful to look to nature for first principles. From the mucilage or latex secreted by some plants to trap insects to the boiled connective tissues of horses sent to the proverbial glue factory, natural adhesives are all macromolecules of some sort. The long protein chains of hide glue, the polysaccharides secreted by plants and animals in need of a little stickiness, or even the hydrocarbon chains and terpene resins that early humans learned to gather from tarpits and pine trees to glue ax heads to hafts – all sticky stuff is composed of long molecules.
Whether natural or synthetic, polymer adhesives have a lot of sites to interact with each other and with the substrates they’re sticking to. The degree to which these molecules stick to each other is called their
adhesivenesscohesiveness, while how they stick to something else is called their cohesivenessadhesiveness. Adhesiveness has a lot to do with molecules being attracted to each other thanks to van der Waals interactions, which is the total force between molecules based on their electrostatic charges and polarization moments. The forces are weak individually, but macromolecules offer lots of places for the force to act, contributing to both adhesive and cohesive properties. Additionally, macromolecules are good at penetrating into the pores of the substrate, contributing greatly to the adhesive properties by mechanically locking the adhesive to substrate.
Adhesives can be roughly organized into two broad categories based on how they cure. Non-reactive adhesives cure by non-chemical transformations, such as evaporation of a solvent or by cooling. Reactive adhesives undergo some sort of chemical change, generally polymerization, during the curing process. To be useful, both reactive and non-reactive adhesives need to be prevented from curing until they’re applied. Non-reactive adhesives are pretty simple to manage – a sealed container to keep solvents from evaporating, or keeping a stick of hot glue at its melting point. Reactive adhesives can be a little harder to control, though, and might take measures as extreme as making the adhesive a two-part formulation that won’t react until mixed, or keeping light-cured resins in the dark.
Now that we know a little about how adhesives work, let’s look at a few glues that you might want to keep around the shop.
Those of a certain vintage will remember when “Krazy Glue” burst onto the mass-market scene in the early 70s with cheesy commercials about how a single drop was “strong enough to keep this man suspended in midair.” Cyanoacrylate glues have come a long way since those days, and while results are not always equal to the hype, CA glues are a great tool to have around the shop. A reactive adhesive based on the rapid polymerization of methyl-2-cyanoacrylate, the polymerization reaction is catalyzed by water; even water vapor in the air will do it, which is why CA glues go off so fast once the tube is opened. Even a factory sealed tube is only good for about a year, so it’s best to write the date of the package and rotate your stock. You can extend the life of a tube by keeping it in the freezer, though, since the polymerization reaction slows way down with decreased temperature.
CA glues are good for a lot of things, but not every material makes a good substrate. Very smooth surfaces such as glass are poor candidates for regular CA glues. Among the best substrates for CA glues are human tissues, with the skin on fingers and eyelids being particularly vulnerable. Acetone is your friend here, at least for the fingers; you’ll want to get to a doctor for the eyelids. You’ll also want to keep CA glues off of cotton and wool; the MSDS for some glues warn of a strongly exothermic reaction that can cause spontaneous combustion, but as it turns out, this usually doesn’t happen. Still, better safe than combusted.
One neat trick is using CA glues as a filler by combining them with a powdered substrate. Baking soda is the classic example – the mixture forms a hard but workable material that fills gaps and cracks. Woodworkers also mix CA with fine sawdust to fill defects in veneers and fine furniture.
Not generally thought of as adhesives, threadlockers are nonetheless just that – adhesives engineered to glue metals together. Threadlockers are one-part reactive adhesives similar to CA, but generally based on monomers of methacrylate. The adhesive cures when exposed to the electrochemically active and oxygen-free environment deep inside the threads of fasteners. As it polymerizes, the threadlocker works into the gaps in and between the metals and increases the friction needed to loosen the connection.
Threadlockers come in various strengths and with different formulations for filling wide gaps in worn threads or for wicking into preassembled fasteners. For less reactive metals like stainless steel, or for fasteners coated with cadmium or oxide compounds, a primer may be necessary to get the polymerization reaction started.
Everyone knows epoxy – it’s the stuff that’ll stick metal to metal or pot components for a waterproof application. Epoxides are three-atom rings with oxygen at the vertex. The bonds are highly strained, so epoxides are very reactive. Resins containing epoxides readily polymerize with themselves to form cross-linked structures with properties very similar to plastics. Most epoxies are two-part adhesives consisting of a resin and a hardener, most commonly available as a double-barrel syringe that mixes the two components in the correct proportion immediately prior to application.
Epoxy adhesives can be engineered to perform almost any job, and some are classified as structural adhesives that hold together things like buildings, boats, and airplanes. In electronics we see epoxies all the time, with epoxy resin and glass fibers being the main components of FR-4 circuit boards.
3D printers often require a little adhesive assistance to get prints to stick to the bed. Advice abounds in online forums and around the local hackerspace as grizzled 3D-printing vets swear their method is the best and all others are inferior. A common suggestion is to use hairspray, which after all is just an air-cured single-component aerosol-dispensed adhesive for hair. This works, but the method has its drawbacks and is not without risk. A safer alternative is plain old white glue or wood glue, which is poly-vinyl acetate (PVA) adhesive. PVA is a non-reactive adhesive that cures as the water in the solution evaporates. For 3D printing, a dilute solution of PVA brushed onto the heated bed of the printer will keep ABS and PLA prints stuck down until the bed cools, at which point they can be pried off.
Finally, no discussion of shop adhesives would be complete without the go-to glue of hackers and crafters alike – the hot glue gun. Great for temporary bonds, wire management, reinforcing connections, and general potting needs, hot glues are non-reactive thermoplastic adhesives that cure by cooling. Most glue sticks are composed of the copolymer ethylene-vinyl acetate (EVA) which cures quickly after being dispensed. Hot glue has some advantages over other non-reactive glues – for example, it doesn’t change volume as it cures since it doesn’t lose solvent to evaporation. It’s not great for heat-sensitive substrates, of course, or for applications that are going to be subjected to high temperatures, but for a lot of simple jobs it works fine.
That’s it for our quick summary of our favorite adhesives, but we’re sure that we’ve missed out on some special-purpose gems. Are there any adhesives that you just can’t live without? Tell us tales of how you’ve gotten out of sticky situations.