Carst And Walker
Aluminium cans

The secret life of the aluminium can, a feat of engineering

When was the last time you paused between sips of your favourite soda and wondered about that can in your hand? If you’re like most people, the answer is likely never. But that seemingly unremarkable object is actually a marvel of modern manufacturing. It is, in fact, a glorious thing….

A FEW years ago, I finagled my way into Can School, a small industry-only event hosted annually by the Ball Corporation, the world’s largest canmaker. There, in a conference room just north of Denver, engineers chatted about “improved pour rates” and “recloseability” and the “opening performance” of cans. One guy handed me a business card that said “Can Whisperer.” Another wore a shirt that said “Can Solo.”

It was a scene of intense devotion, and as such, it was only fitting that the first thing I learned there was that manufacturing aluminum cans is so challenging, and requires such a vast amount of study, design, and precise machining, that many consider cans the most engineered products in the world.

If you drink beer, or soda, or juice, or sports drinks, or if you have ever preserved fruits or vegetables in glass jars, the name Ball probably sounds familiar. The people of the world go through 180 billion aluminum beverage cans a year; enough to build dozens of towers to the moon. Ball makes about a quarter of them.

Yet even with that much practice, making perfect 12-ounce cans remains a battle. Throughout the process, the aluminum behaves begrudgingly. It tries to jam the machines. Once filled, it wants to interact with the product inside and change its taste.

But mostly, cans yearn to corrode (thereby leaking onto other cans, and causing more corrosion). Rust, it turns out, is a can’s number one enemy—and a can’s only defense is an invisible epoxy shield, just microns thick. (Without that shield, a can of Coke would corrode in three days.) At Can School, I got a hint of what goes into that coating.

Weighing this corrosion

Before coating the insides of their cans, Ball needs to know how corrosive the beverage within will be. The epoxy coating, after all, costs about a half penny per can and Ball doesn’t want to waste it. Also, some beverages are so corrosive that no amount of coating will protect their cans. (Roughly one in seven new energy drinks are too corrosive to put in cans.)

Ball is not in the business of sending cans out into the world to be slaughtered by overaggressive liquids. The coating must perform. Otherwise, cans explode, and legal costs climb.

Empirically, Ball has figured out how to determine the corrosivity of potential products. Corrosion-wise, sodium benzoate is bad. Copper is bad. Sugar is good. It absorbs carbon dioxide, decreasing the pressure within a can, and it also inhibits other corrosion reactions, because sugar tends to deposit onto pores in the coating.

Thus, Diet Coke underperforms regular Coke on at least two counts. Citric acid and phosphoric acid are equally bad. Red #40 is pretty bad, and high chlorides are really bad, and together, they’re really, really bad. (Ball has developed a carefully-guarded formula quantifying all of this corrosion badness, but it was not displayed on any graph at Can School.)

Ball doesn’t manufacture cans by the billions until after the beverage and the coating and the interaction between the two have been examined. After the company’s engineers examine package-product interaction chemically, they double-check with their tongues. For these studies, there’s the flavour room.

During Can School, I got a tour from a tall, thin Ball engineer named Ed Laperle.

Flavour testers at Ball learn to detect parts per million, then parts per billion, and eventually, parts per trillion. If they can’t taste something, nobody can. The one exception may be cats. On account of felines’ extreme organoleptic capacities, wet cat food is packaged in cans with “particularly low levels of taint”.

Ball’s flavour testers spend much of their time drinking beer, because beer, says Laperle, is very susceptible to “flavour scalping” if it comes into contact with too much coating.

The cost alone persuades Ball; the smell just rubs it in. Beer, Laperle explained, is actually so mild that the can does not require a coating. He called beer a “nice oxygen scavenger,” describing how proteins in beer consume dissolved oxygen, keep it from accessing and corroding the aluminum. (It’s the same for orange juice, in which vitamin C consumes oxygen—which is why canners were able to package it so long ago.)

It turns out that cans were made for beer, and beer was made for cans. In fact, the only reason beer cans have a coating at all is so that the carbon dioxide doesn’t escape at once. The coating smoothes out the surface of the metal, so that the gas has no microbumps from which to propagate.

Plastics, Plastics, Plastics

The formulas for coatings are proprietary, but we know the epoxies must be affordable, sprayable, curable, strong, flexible, and sticky.

They’re also very specific: for nearly every variety of food or beverage, there’s a coating that’s been devised just for it. Coatings for tomatoes must be stain resistant, those for fish must resist sulfur, and those for fruits and pickles must resist acids. There’s one coating for tomatoes, one for beans, one for potatoes, and another for corn, peas, fish, and shrimp.

For beer, engineers might use a coating that contains cyclodextrin, a donut-shaped carbohydrate that traps bad-tasting molecules. Chocolate, especially sensitive to adulteration, requires its own coating. Those for meats must contain a lubricious wax, called a meat release agent, so that the meat slides right out.

Extra challenging are fruits and vegetables including beets, currants, and plums, which contain the red pigment anthocyanin, because they’re some of the most corrosive.

Rust: The Longest WarThe top of the corrosive list, though, belongs to rhubarb. It, alone among foods, requires three layers of lacquer, and even with that much protection, still boasts a shorter shelf-life than its peers.

All told, there are over fifteen thousand coatings, and though most of them serve inside food cans, many of them perform their work inside beverage containers….. Read the full article

Excerpt from RUST: THE LONGEST WAR. Copyright © 2014 by Jonathan Waldman. Printed by permission of Simon & Schuster, Inc.

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