While consulting for a small consumer-electronics startup about how to make their product greener (see previous posts on choosing good plastics and metal cases), I did some research on adhesives: how they affect recyclability, and whether they can be used well.
The company asking me for advice had started out planning to make their device (a small electronic widget roughly the size of an iPod Nano) entirely out of plastic, but later decided to make it partly from extruded aluminum, partly from plastic. It looked like that meant they had to use adhesives to hold everything together, since there just isn't enough room in the tiny body for strong snap-fits. This makes recycling harder, for two reasons: first, because now the device is harder to disassemble into its component materials; and second, because now the component materials will have some gunk on them (adhesive residue) that cause problems in recycling the metal or plastic. So how can this be done well? It's hard, but here are some possible options.
Don't Be Too Strong
Fixing the disassembly problem is fairly simple: you just use weak enough glue (or a small enough amount of strong glue) that whoever disassembles the device can just pop the parts out by hand, overwhelming the strength of the glue. This way, disassembly doesn't take significantly longer than it would with snap-fit parts. If the glue is too strong, you have to pry things out with a tool, or (if the glue is stronger than the plastic itself) you have to release the glue somehow. Most glues can be released by dissolving them in nasty solvents like acetone, or burning them off in a furnace, but some glues dissolve in water, and other glues melt at low enough temperatures that your plastic parts won't be affected. The advantage of dissolving or melting your adhesives is that then they can be removed from your parts, avoiding the problem of getting gunk in the recycling furnaces.
You say, if the glue is weak, doesn't that defeat the purpose? But that's where smart design comes in: you can design the interface of the two parts so that during actual use of the device, the glue joint is always pushed on from the outside, pushing the two parts tighter together instead of trying to separate them, while during disassembly (when the electronic guts of the device are already removed) it can be pushed on from the inside. However, the weak glue solution still leaves the problem of glue-gunk on parts that you want to recycle.
Recycling With Glue
Aluminum parts with a little glue-gunk on them are still recyclable, because the gunk will get burned off during the re-melting process; however, this requires the recycler to have special emission controls on their smelter in order to avoid air pollution, and can introduce some impurities in the metal. Many smelters in the US and Asia have these emission controls, but even those that do will pay a higher price for clean scrap. I talked with five different scrap brokers (companies who take your scrap and sell it to smelters; you can't really go directly to the smelter yourself unless you're a big company with huge amounts of scrap), and they said contradictory things. One said that having even a tiny amount of glue reduced the scrap value from 40¢ per pound to 15¢ per pound; another said it reduced the value from 70¢/lb. to 60¢/lb; two other brokers said that small amounts of glue didn't really matter, and didn't reduce the value at all.
Plastic parts with glue-gunk on them are a much worse proposition. All four places I talked to said that even a little glue, even if it's easily removable, cuts your value in half (from 40¢/lb. to 20¢/lb., say), and bad glue or a lot of glue cuts your scrap value to almost worthless (maybe 5¢/lb.) This isn't because of emissions concerns like aluminum, it's because the melted glue mixes in with the plastic and affects the plastic's mechanical and molding properties (glue molecules get in the way of polymer-chain bonding). This is in the worst-case scenario. The best case scenario is that, if you use reversible glue, the recycler will clean off the glue (with water or heat or both) and then that scrap will be as good as other clean scrap. This takes time and processing, though, which is why your resale value gets cut in half or worse. And you can only use certain adhesives that are reversible--epoxies (basically any two-part mix) are right out. So how do you find reversible glues? Well, turns out that's pretty easy.
Water Soluble Glue
Cyanoacrylate (a.k.a. "superglue", "krazy glue", and various other brand names) is water-soluble. A Loctite rep told me, "If you superglue your eye shut, a warm compress will unglue it in a day or so." Presumably, then, a tub of boiling water would get cyanoacrylate off in a couple hours. (A plastic part, that is, not your eye.) By the way, Loctite is a very good company environmentally (for a glue/chemical company), I recommend them. Cyanoacrylate is probably your best bet, because it's strong, so you don't have to use very much, and it sets rapidly, which is good in manufacturing.
Common Elmer's glue ("white glue") and some wood glues are water-soluble. White glue is not feasible for much manufacturing because it takes minutes or hours to set instead of seconds; it usually is not a very strong adhesive, and can also shrink when setting, causing parts to move or bend. But there are also higher-performance kinds that have stronger adhesion and faster setting times; many wood glues are stronger than the wood they glue (though they're still not very useful for plastic or metal).
Regitex, a Japanese company, specifically formulates their glues to be eco-sensitive. They have a whole line of water-soluble latex glues for different applications (envelopes, food containers, medical tape, shoes, and general use; one kind is even cured by UV light instead of drying, which is very fast.)
Hide glue (yes, glue made from animal hide) is another water-soluble glue which can bond metal, plastic, glass, and cloth, although it's mostly used for wood. Sometimes it requires a bit of vinegar in the water to help dissolve it, but vinegar is a mild chemical. Hide glue is archival (three-hundred-year-old violins are made with it), and it requires just a thin coating, which is nearly transparent, so it should not interfere with aesthetics. However, like Elmer's glue, it may take an hour to set, and it shrinks when setting. You can get it in liquid form or do-it-yourself granular form.
There are various other specialty glues that are water soluble: AquaBond makes temporary high performance adhesives for semiconductor processing; Mosaic Mercantile makes it for mosaic tiling; Dongguan Jelly makes it for fabric; 3M even uses water soluble adhesive in some tape for masking printed circuit boards during soldering.
All this begs the question of what do you do with the water you used to dissolve the glue? Some of these glues might be sufficiently natural/biodegradable to dump down your sewer line without guilt (a couple claim to be), but I would expect all of these chemicals to require wastewater treatment. The environmental and financial costs of this should be factored into your decision about whether you can sensibly use adhesives.
Heat-reversible glues, from the companies I talked to, would be fine for metal parts, but they don't help with recycling plastic. They don't disappear under heat, they just lose their stick; so on plastic they melt into the mix and gunk up the polymer. There may be ones that vaporize or drip off entirely before the plastic melts, but I didn't find one on the market. That said, if you can un-stick the glue and then scrape or wash it off, it could still be an option.
The trusty hot glue that you use for making models and crafts is heat-reversible. It softens at 75°C, where you can sort of pull things apart, and melts at 130°C, where it doesn't hold anything in place anymore; this is much cooler than most plastics melt, so on a disassembly line, you might be able to place the devices in an oven that melted the glue until it dripped off or got sprayed off. This would be much more hassle and energy use than dealing with water-soluble glues, however. And hot glue's melting point is so low it would melt on a car dashboard in an Arizona summer; not the best choice for a portable electronic device.
There are also higher-performance heat-reversible glues being investigated in R&D labs. Active Disassembly Research ltd. in the UK doesn't sell glue retail, but they develop heat-reversible glues, latches, hooks, screws, springs, and other fasteners in an impressive variety. Check out their video gallery (no direct link); they're one of the main companies making devices which actively disassemble themselves. In the lab, researchers at Sandia National Labs have made heat-reversible tape, and scientists at Germany's Fraunhofer Institute have made a heat-reversible glue with nano-scale ferrite particles that, when exposed to an oscillating magnetic field, jitter around to heat up the glue without heating up much else. Neither of these are commercial products yet, but they hopefully will be within the next several years.
While some heat-reversible glues basically become sludge after removal, it's at least theoretically possible to reuse other ones. Hot glue is a thermoplastic, and as long as it is not contaminated with dust, dirt, or other residue, it should stick as well the second time as it did the first. However, I personally have not had good luck trying to reuse hot glue, and I would expect other heat-reversible glues to be equally tricky. If they're becoming a waste stream, keep this in mind when deciding whether you can sensibly use adhesives.
Generally speaking, it's best to avoid adhesives in a product in order to ensure easy recyclability, but that doesn't have to be the end of the story. There are no doubt situations where using a reversible glue actually makes disassembly faster and easier than snap-apart fastening would, without too much cleanup required for recycling. In the case of the company I was advising, however (and probably for most companies), once they saw the obstacles that adhesives introduce, they decided it was best to avoid them altogether. Snap fits, interference fits, screws, and the like are simpler and cheaper routes to making products easily disassembled and easily recycled.
I work for a small progressive foundation and we're in need of replacing our carpeting. Six years ago they renovated the place, found that the glue that would be used to hold down the carpet liner/underlayment was toxic, and told the contractor to leave the liner out, and so now our carpet forms substantial dunes, like the Sahara. Do you know anything about greener carpet or carpet-liner adhesives?
Could a snap-together design substitute for glue?