Bruce Sterling - Essays. FSF Columns

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