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From THE MAGAZINE OF FANTASY AND SCIENCE FICTION, June 1993.
F&SF, Box 56, Cornwall CT 06753 $26/yr USA $31/yr other
F&SF Science Column #7:
SUPERGLUE
This is the Golden Age of Glue.
For thousands of years, humanity got by with natural glues like
pitch, resin, wax, and blood; products of hoof and hide and treesap
and tar. But during the past century, and especially during the past
thirty years, there has been a silent revolution in adhesion.
This stealthy yet steady technological improvement has been
difficult to fully comprehend, for glue is a humble stuff, and the
better it works, the harder it is to notice. Nevertheless, much of the
basic character of our everyday environment is now due to advanced
adhesion chemistry.
Many popular artifacts from the pre-glue epoch look clunky
and almost Victorian today. These creations relied on bolts, nuts,
rivets, pins, staples, nails, screws, stitches, straps, bevels, knobs, and
bent flaps of tin. No more. The popular demand for consumer
objects ever lighter, smaller, cheaper, faster and sleeker has led to
great changes in the design of everyday things.
Glue determines much of the difference between our
grandparent's shoes, with their sturdy leather soles, elaborate
stitching, and cobbler's nails, and the eerie-looking modern jogging-
shoe with its laminated plastic soles, fabric uppers and sleek foam
inlays. Glue also makes much of the difference between the big
family radio cabinet of the 1940s and the sleek black hand-sized
clamshell of a modern Sony Walkman.
Glue holds this very magazine together. And if you happen to
be reading this article off a computer (as you well may), then you
are even more indebted to glue; modern microelectronic assembly
would be impossible without it.
Glue dominates the modern packaging industry. Glue also has
a strong presence in automobiles, aerospace, electronics, dentistry,
medicine, and household appliances of all kinds. Glue infiltrates
grocery bags, envelopes, books, magazines, labels, paper cups, and
cardboard boxes; there are five different kinds of glue in a common
filtered cigarette. Glue lurks invisibly in the structure of our
shelters, in ceramic tiling, carpets, counter tops, gutters, wall siding,
ceiling panels and floor linoleum. It's in furniture, cooking utensils,
and cosmetics. This galaxy of applications doesn't even count the
vast modern spooling mileage of adhesive tapes: package tape,
industrial tape, surgical tape, masking tape, electrical tape, duct tape,
plumbing tape, and much, much more.
Glue is a major industrial industry and has been growing at
twice the rate of GNP for many years, as adhesives leak and stick
into areas formerly dominated by other fasteners. Glues also create
new markets all their own, such as Post-it Notes (first premiered in
April 1980, and now omnipresent in over 350 varieties).
The global glue industry is estimated to produce about twelve
billion pounds of adhesives every year. Adhesion is a $13 billion
market in which every major national economy has a stake. The
adhesives industry has its own specialty magazines, such as
Adhesives Age andSAMPE Journal; its own trade groups, like the
Adhesives Manufacturers Association, The Adhesion Society, and the
Adhesives and Sealant Council; and its own seminars, workshops and
technical conferences. Adhesives corporations like 3M, National
Starch, Eastman Kodak, Sumitomo, and Henkel are among the world's
most potent technical industries.
Given all this, it's amazing how little is definitively known
about how glue actually works -- the actual science of adhesion.
There are quite good industrial rules-of-thumb for creating glues;
industrial technicians can now combine all kinds of arcane
ingredients to design glues with well-defined specifications:
qualities such as shear strength, green strength, tack, electrical
conductivity, transparency, and impact resistance. But when it
comes to actually describing why glue is sticky, it's a different
matter, and a far from simple one.
A good glue has low surface tension; it spreads rapidly and
thoroughly, so that it will wet the entire surface of the substrate.
Good wetting is a key to strong adhesive bonds; bad wetting leads
to problems like "starved joints," and crannies full of trapped air,
moisture, or other atmospheric contaminants, which can weaken the
bond.
But it is not enough just to wet a surface thoroughly; if that
were the case, then water would be a glue. Liquid glue changes
form; it cures, creating a solid interface between surfaces that
becomes a permanent bond.
The exact nature of that bond is pretty much anybody's guess.
There are no less than four major physico-chemical theories about
what makes things stick: mechanical theory, adsorption theory,
electrostatic theory and diffusion theory. Perhaps molecular strands
of glue become physically tangled and hooked around irregularities
in the surface, seeping into microscopic pores and cracks. Or, glue
molecules may be attracted by covalent bonds, or acid-base
interactions, or exotic van der Waals forces and London dispersion
forces, which have to do with arcane dipolar resonances between
magnetically imbalanced molecules. Diffusion theorists favor the
idea that glue actually blends into the top few hundred molecules of
the contact surface.
Different glues and different substrates have very different
chemical constituents. It's likely that all of these processes may have
something to do with the nature of what we call "stickiness" -- that
everybody's right, only in different ways and under different
circumstances.
In 1989 the National Science Foundation formally established
the Center for Polymeric Adhesives and Composites. This Center's
charter is to establish "a coherent philosophy and systematic
methodology for the creation of new and advanced polymeric
adhesives" -- in other words, to bring genuine detailed scientific
understanding to a process hitherto dominated by industrial rules of
thumb. The Center has been inventing new adhesion test methods
involving vacuum ovens, interferometers, and infrared microscopes,
and is establishing computer models of the adhesion process. The
Center's corporate sponsors -- Amoco, Boeing, DuPont, Exxon,
Hoechst Celanese, IBM, Monsanto, Philips, and Shell, to name a few of
them -- are wishing them all the best.
We can study the basics of glue through examining one typical
candidate. Let's examine one well-known superstar of modern
adhesion: that wondrous and well-nigh legendary substance known
as "superglue." Superglue, which also travels under the aliases of
SuperBonder, Permabond, Pronto, Black Max, Alpha Ace, Krazy Glue
and (in Mexico) Kola Loka, is known to chemists as cyanoacrylate
(C5H5NO2).
Cyanoacrylate was first discovered in 1942 in a search for
materials to make clear plastic gunsights for the second world war.
The American researchers quickly rejected cyanoacrylate because
the wretched stuff stuck to everything and made a horrible mess. In
1951, cyanoacrylate was rediscovered by Eastman Kodak researchers
Harry Coover and Fred Joyner, who ruined a perfectly useful
refractometer with it -- and then recognized its true potential.
Cyanoacrylate became known as Eastman compound #910. Eastman
910 first captured the popular imagination in 1958, when Dr Coover
appeared on the "I've Got a Secret" TV game show and lifted host
Gary Moore off the floor with a single drop of the stuff.
This stunt still makes very good television and cyanoacrylate
now has a yearly commercial market of $325 million.
Cyanoacrylate is an especially lovely and appealing glue,
because it is (relatively) nontoxic, very fast-acting, extremely strong,
needs no other mixer or catalyst, sticks with a gentle touch, and does
not require any fancy industrial gizmos such as ovens, presses, vices,
clamps, or autoclaves. Actually, cyanoacrylate does require a
chemical trigger to cause it to set, but with amazing convenience, that
trigger is the hydroxyl ions in common water. And under natural
atmospheric conditions, a thin layer of water is naturally present on
almost any surface one might want to glue.
Cyanoacrylate is a "thermosetting adhesive," which means that
(unlike sealing wax, pitch, and other "hot melt" adhesives) it cannot
be heated and softened repeatedly. As it cures and sets,
cyanoacrylate becomes permanently crosslinked, forming a tough
and permanent polymer plastic.
In its natural state in its native Superglue tube from the
convenience store, a molecule of cyanoacrylate looks something like
this:
CN
/
CH2=C
\
COOR
The R is a variable (an "alkyl group") which slightly changes
the character of the molecule; cyanoacrylate is commercially
available in ethyl, methyl, isopropyl, allyl, butyl, isobutyl,
methoxyethyl, and ethoxyethyl cyanoacrylate esters. These
chemical variants have slightly different setting properties and
degrees of gooiness.
After setting or "ionic polymerization," however, Superglue
looks something like this:
CN CN CN
| | |
- CH2C -(CH2C)-(CH2C)- (etc. etc. etc)
| | |
COOR COOR COOR
The single cyanoacrylate "monomer" joins up like a series of
plastic popper-beads, becoming a long chain. Within the thickening
liquid glue, these growing chains whip about through Brownian
motion, a process technically known as "reptation," named after the
crawling of snakes. As the reptating molecules thrash, then wriggle,
then finally merely twitch, the once- thin and viscous liquid becomes
a tough mass of fossilized, interpenetrating plastic molecular
spaghetti.
And it is strong. Even pure cyanoacrylate can lift a ton with a
single square-inch bond, and one advanced elastomer-modified '80s
mix, "Black Max" from Loctite Corporation, can go up to 3,100 pounds.
This is enough strength to rip the surface right off most substrates.
Unless it's made of chrome steel, the object you're gluing will likely
give up the ghost well before a properly anchored layer of Superglue
will.
Superglue quickly found industrial uses in automotive trim,
phonograph needle cartridges, video cassettes, transformer
laminations, circuit boards, and sporting goods. But early superglues
had definite drawbacks. The stuff dispersed so easily that it
sometimes precipitated as vapor, forming a white film on surfaces
where it wasn't needed; this is known as "blooming." Though
extremely strong under tension, superglue was not very good at
sudden lateral shocks or "shear forces," which could cause the glue-
bond to snap. Moisture weakened it, especially on metal-to-metal
bonds, and prolonged exposure to heat would cook all the strength
out of it.
The stuff also coagulated inside the tube with annoying speed,
turning into a useless and frustrating plastic lump that no amount of
squeezing of pinpoking could budge -- until the tube burst and and
the thin slippery gush cemented one's fingers, hair, and desk in a
mummified membrane that only acetone could cut.
Today, however, through a quiet process of incremental
improvement, superglue has become more potent and more useful
than ever. Modern superglues are packaged with stabilizers and
thickeners and catalysts and gels, improving heat capacity, reducing
brittleness, improving resistance to damp and acids and alkalis.
Today the wicked stuff is basically getting into everything.
Including people. In Europe, superglue is routinely used in
surgery, actually gluing human flesh and viscera to replace sutures
and hemostats. And Superglue is quite an old hand at attaching fake
fingernails -- a practice that has sometimes had grisly consequences
when the tiny clear superglue bottle is mistaken for a bottle of
eyedrops. (I haven't the heart to detail the consequences of this
mishap, but if you're not squeamish you might try consulting The
Journal of the American Medical Association, May 2, 1990 v263 n17
p2301).
Superglue is potent and almost magical stuff, the champion of
popular glues and, in its own quiet way, something of an historical
advent. There is something pleasantly marvelous, almost Arabian
Nights-like, about a drop of liquid that can lift a ton; and yet one can
buy the stuff anywhere today, and it's cheap. There are many urban
legends about terrible things done with superglue; car-doors locked
forever, parking meters welded into useless lumps, and various tales
of sexual vengeance that are little better than elaborate dirty jokes.
There are also persistent rumors of real-life superglue muggings, in
which victims are attached spreadeagled to cars or plate-glass
windows, while their glue-wielding assailants rifle their pockets at
leisure and then stroll off, leaving the victim helplessly immobilized.
While superglue crime is hard to document, there is no
question about its real-life use for law enforcement. The detection
of fingerprints has been revolutionized with special kits of fuming
ethyl-gel cyanoacrylate. The fumes from a ripped-open foil packet of
chemically smoking superglue will settle and cure on the skin oils
left in human fingerprints, turning the smear into a visible solid
object. Thanks to superglue, the lightest touch on a weapon can
become a lump of plastic guilt, cementing the perpetrator to his
crime in a permanent bond.
And surely it would be simple justice if the world's first
convicted superglue mugger were apprehended in just this way.