The story be­hind sticky rub­ber


Learn the sci­ence be­hind sticky rub­ber.

WHAT’S THE RIGHT shoe for bal­anc­ing on minis­cule gran­ite nubs in Yosemite? Pol­ished lime­stone in Ri­fle? Plas­tic at your lo­cal gym? Ev­ery year, Climb­ing’s gear testers spend hun­dreds of hours pars­ing the dif­fer­ences be­tween each rock shoe to find the best kicks for each dis­ci­pline. But long be­fore our testers crammed their grubby climber toes into the new­est shoes, chemists and en­gi­neers were de­vel­op­ing the rub­ber com­pounds that make climb­ing shoes the one thing they need to be: sticky.

“Rub­ber” refers broadly to a class of tough, elas­tic ma­te­ri­als so ubiq­ui­tous it is dif­fi­cult to de­fine. About 40 per­cent of the rub­ber we use is la­tex, made nat­u­rally by rub­ber trees. On an atomic level, la­tex and other rub­bers are made up of long strands of car­bon atoms dec­o­rated with hy­dro­gen atoms. Each string is a sin­gle mol­e­cule that’s 40,000 to 50,000 car­bons long.

Each chain is floppy on its own, but to­gether they are strong along the axis, like a sec­tion of rope. They tan­gle and stick to one an­other to form flex­i­ble, sticky rub­ber that can bend with your foot and mold it­self around sand­stone grains, mon­zonite crys­tals, or the pat­terned plas­tic of gym holds. The more con­tact it makes with a sur­face, the more fric­tion it gen­er­ates, keep­ing your foot where it needs to stick.

Rub­ber has an­other trick, too: It’s called strain-in­duced crys­tal­liza­tion. Put enough pres­sure on nat­u­ral rub­ber—say shift­ing most of your weight onto a shal­low toe­hold—and the rub­ber changes from a tan­gle of strands to a highly or­dered crys­tal form. That gives rub­ber ex­tra strength and dura­bil­ity, ex­plains Matthew Yang, a pro­fes­sor at Fer­ris State Univer­sity in Michi­gan, one of the last ded­i­cated U.S. pro­grams train­ing rub­ber sci­en­tists.

The first ev­i­dence of la­tex use comes from the Olmec so­ci­ety that in­hab­ited the Ama­zon jun­gle about 3,000 years ago. The Olmec ex­tracted the white, sticky la­tex sap from rub­ber trees and used it to wa­ter­proof boots and cloth­ing. Euro­peans saw the sub­stance for the first time in the early 1700s. Joseph Pri­est­ley, a Bri­tish chemist, de­scribed one of its first uses in the West: rub­bing away er­rant pen­cil marks, hence the word “rub­ber.” A Bri­tish ex­plorer smug­gled tens of thou­sands of rub­ber seeds out of Brazil, which banned their ex­port to pro­tect its monopoly. Saplings ended up in In­dia and South­east Asia, where most rub­ber now comes from.

Th­ese days, nat­u­ral rub­ber is a won­der, but it isn’t per­fect. It’s too gummy when hot and too brit­tle when cold. Those long hy­dro­car­bon chains slip too much at high tem­per­a­tures and not enough at low ones. The first big im­prove­ment was vul­can­iza­tion, a process de­vel­oped in the 1800s that hard­ens the rub­ber with sul­fur at a high tem­per­a­ture. Vul­can­ized la­tex stays strong and flex­i­ble at a range of

tem­per­a­tures and keeps its elas­tic­ity after re­peated stretch­ing. This dis­cov­ery gave us rub­ber tires, con­veyer belts, and shoe soles.

Most rub­bers used now are syn­thetic, made from fos­sil fu­els with ad­di­tives by chemists. Car­bon black is a filler that makes rub­ber stiffer, stronger, and more durable. Other ad­di­tives speed up the man­u­fac­tur­ing process or give a splash of color.

MOST SNEAK­ERS YOU BUY use stan­dard, mass-pro­duced rub­ber for­mu­las. Com­pa­nies like Vi­bram, a man­u­fac­turer that pro­duces much of the rub­ber found on climb­ing shoes, try to thrive in the niches. Vi­bram makes soles for smoke­jumpers who para­chute into wild­fires and for sol­diers op­er­at­ing in both scorch­ing Mid­dle Eastern deserts and above the Arc­tic Cir­cle. They make rub­ber for climb­ing-shoe com­pa­nies like Scarpa and La Sportiva. Climb­ing-shoe rub­bers XS Edge and XS Grip 2 are two of Vi­bram’s best-sell­ing com­pounds.

So what ex­actly goes into your fa­vorite shoe rub­ber? Sole mak­ers usu­ally won’t say be­cause the spe­cific for­mu­las are as closely guarded as the recipe for Coca-Cola. At a min­i­mum, climb­ing-shoe soles con­tain fillers, like car­bon and clay, and the rub­ber it­self is not vul­can­ized as much as other rub­bers. This keeps them soft and sticky. One key to a good rub­ber is con­trol­ling—and be­ing able to re­peat—the chem­i­cal re­ac­tions be­tween in­gre­di­ents when the soles are made.

The gen­eral idea be­hind “sticky rub­ber” is that it’s soft so it forms around the small­est of div­ots and bumps, cre­at­ing more sur­face con­tact be­tween rub­ber and rock and thus of­fer­ing greater fric­tion. Sticky rub­ber de­forms in this man­ner hun­dreds of times on any given climb­ing day and bounces back to the orig­i­nal shape, but the soft­ness also means it will wear out faster than the rub­ber on hik­ing boots or sneak­ers.

Five Ten’s high-fric­tion of­fer­ing Mi6 is softer than the clas­sic C4 rub­ber, and thus great for over­hangs where you need max­i­mum pur­chase with a small amount of rub­ber con­tact. C4 is made to stick with­out giv­ing too much. On small edges where a lot of your weight is on your feet, C4 soles won’t de­form and slide out from un­der you.

Vi­bram has a small plant in Quabag, Mas­sachusetts, a ru­ral town where the fac­tory feels like its beat­ing heart. Some of Vi­bram’s chemists are based here, oth­ers in Mi­lan, Italy. Vi­bram USA’s VP of In­no­va­tion and Op­er­a­tions, Chris Favreau, and chemist Weilin Peng are two peo­ple well-versed in the com­plex world of rub­ber, from how to make a new rub­ber com­pound that’s a lit­tle bit harder to some­thing that is de­signed to smear well.

Es­sen­tially, ev­ery sole com­pound is a mix­ture of dif­fer­ent kinds of rub­ber: la­tex, bu­ta­di­ene, chloro­prene, and oth­ers. To make a rub­ber that would hold its edge better—some­thing a lit­tle harder—they might start by adding more bu­ta­di­ene. But tweak­ing that one fac­tor might throw off some other as­pect of the rub­ber, some­thing as fun­da­men­tal as how long the soles take to man­u­fac­ture.

“It ends up be­ing a lot of art and sci­ence to­gether to cre­ate a rub­ber for­mu­la­tion,” says Favreau.

Very few new com­pounds are wholly in­vented and cre­ated from scratch. Most are vari­a­tions on ex­ist­ing for­mu­las, op­ti­mized for this ap­pli­ca­tion or that pref­er­ence. Favreau and Peng both rely on the rub­ber chemist’s bi­ble, The Van­der­bilt Rub­ber Handbook, a tech­ni­cal man­ual about rub­ber com­pound­ing and pro­cess­ing now in its 14th edi­tion. But their process isn’t some­thing they can fully ex­plain to some­one who isn’t in the busi­ness.

Their work is about the end user’s pri­or­i­ties, of­ten think­ing through the process out loud. For a climb­ing shoe, grip on rock is para­mount. Favreau and Peng can ex­ert in­cred­i­ble con­trol over a vari­able like grip. They can make a rub­ber that sticks and slides, or one that sticks and then breaks cleanly. Dura­bil­ity would be far­ther down the list for a high-per­for­mance rock shoe.

Then there are man­u­fac­tur­ing con­cerns. The softer rub­ber that wraps over your toes for techy toe hooks and heel-toe cams needs to stay at­tached to the stiffer rub­ber un­der­foot and to the leather up­per—and both rub­bers have to play well with the glue.

And of course there’s cost. The Vi­bram guys laugh about a com­pound they made re­cently—“best in the world,” says Favreau—that would have more than dou­bled the price of their cus­tomer’s prod­uct. That one didn’t make it out of the lab.

Rub­ber chemists have equip­ment that prods, pokes, squeezes, and stretches the com­pounds, and some of the labs have small climb­ing walls to get a ba­sic idea about how a shoe will per­form. They try to dial in all those el­e­ments in the lab, but noth­ing goes to mar­ket be­fore testers get their hands on—or rather feet into—it.

HIGH UP ON A WALL some­where, two-time USA sport-climb­ing champ Carlo Traversi tests new rub­ber for­mu­las for his spon­sor Five Ten. He’s wear­ing a dif­fer­ent com­pound on each foot, not know­ing which is which. The lab sent him boxes of num­bered pro­to­types. He’ll wear them for a few weeks, first in the gym, then push­ing their lim­its out­doors to see what they can do.

“It takes a long time to feel out and learn to ap­pre­ci­ate a new rub­ber com­pound,” says Traversi. “It’s a re­la­tion­ship based on trust.”

One of Traversi’s shoes is per­form­ing about as well as he ex­pected. It might even be a com­pound he’s worn be­fore. But the other one keeps sur­pris­ing him, stick­ing where he didn’t think it would. He tests it fur­ther on dif­fer­ent holds and rock types, try­ing to find its lim­its. Th­ese are the kinds of details he’ll re­port back to Five Ten’s sci­en­tists. He takes notes after ev­ery ses­sion with a pro­to­type, com­par­ing the shoe to pre­vi­ous ver­sions they’ve sent him and de­tail­ing any big pos­i­tives or neg­a­tives he’s no­ticed. The sci­en­tists will mull over his notes, tin­ker with their com­pounds, and send him a new shoe to put through the paces. He’s just one of hun­dreds of testers around the world.

Traversi meets with chemists and shoe de­sign­ers a cou­ple times a year at Five Ten head­quar­ters in Red­lands, Cal­i­for­nia. In be­tween th­ese meet­ings, he’s email­ing, call­ing, and send­ing pic­tures as he tests the shoes they send him. He says the de­sign­ers usu­ally keep him in the dark about what they’re do­ing. The less he knows, the more ob­jec­tive he can be.

“Some­times, I get them telling me, ‘Hey, you should go test it more on this stuff,’ or, ‘Hmm, that’s in­ter­est­ing,’” he says. They might ask him to test a shoe more on one sur­face or an­other, try­ing to push the bound­aries. And some­times, Traversi says, he can find cre­ative ways to use new rub­bers that maybe the de­sign­ers didn’t en­vi­sion. “That can be a re­ally good way of de­sign­ing later mod­els,” he says. Mak­ing a new rub­ber com­pound—from de­sign to test­ing to store shelves—can take years. Five Ten rub­ber spe­cial­ist Ja­son Jack­man ex­plains how in this way, the process is a bit like climb­ing it­self.

“There’s a lot of work and a lot of in­cre­men­tal steps be­tween the idea and the fi­nal re­sult,” Jack­man says. “You have small, tan­gi­ble wins that even­tu­ally equal a gi­ant win.”


Newspapers in English

Newspapers from USA

© PressReader. All rights reserved.