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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 cra
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 ca
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