The Science Behind a Soda Can

28th March 2018

Chemicals, Science

The science behind a soda can is more interesting than you'd think

You’ve probably heard many horror stories about your soda of choice. But could the seemingly unremarkable and greatly overlooked soda can be worse for you than the beverage inside?

Acidity of Fizzy Drinks

Soda cans have a lot going on beneath their aluminium surface. Lining their interior is a protective shield that prevents the soda inside from reacting with the metal.  

Fizzy drinks are generally corrosive substances, not only for our teeth but for metals as well. This is because, alongside carbonated water and lots of sugar, soda contains acid.

The most commercially popular drink, Coke, is one of the most acidic drinks on the shelf. With a pH of 2.5, Coke contains 2 ingredients that are common in nearly all soda drinks: phosphoric acid and citric acid.

While the presence of these acids is masked by the addition of sugar and other preservatives, Coke – and many fizzy drinks – still has a very acidic pH reading, especially when you consider that battery acid is not far off with a pH of 1.

Although a tasty beverage, the acidity of soda is bad for a couple of reasons. The most obvious is its impact on our bodies. The least obvious is its impact on the soda can.

CAN You Believe It?

Acids react with and corrode various metals, including the aluminium used in a soda can. If a typical fizzy drink was contained in an aluminium can without a protective shield, it could eventually eat through the material.

This is because of phosphoric and citric acid. Soda can manufacturers, such as Ball, have determined these substances to have high corrosion levels which they must compensate for.

To prevent any accidents, a protective coating is added to the inside of the soda can. Typically a polymer plastic lining, this coating shields the aluminium from the soda and prevents them from reacting together.

The soda can protects against the acidity of fizzy drinks

Most fizzy drinks contain phosphoric acid and citric acid, giving them an average pH of 2.5. This means that soda can be corrosive to the aluminium can, and has the potential to eat through it if a protective liner isn’t present. Every soda can has a hidden liner inside to prevent the beverage from reacting with the metal.

Soda Can Linings

While the resin used in these protective linings is generally polymer or epoxy, there are many different materials that are sometimes used instead:

  • Polyethylene
  • Polypropylene
  • Polyester
  • Styrene
  • Acrylic
  • Vinyl

There are several factors that must also be considered when manufacturing these crucial liners. It’s not only important for them to be affordable, they must also be:

  • Curable so that they can harden
  • Strong and stable so that they don’t tear or rupture while the can is being handled
  • Flexible in order to fit the shapes of different cans
  • Sprayable for easy application
  • Sticky so that they keep close to the walls of the can and don’t separate
  • Sustainable with a long shelf-life

Protective liners are not restricted to the soda can. They are an important part of nearly every canned food product.

So far, the lining inside a soda can doesn’t seem to have any downfalls. So why does it have the potential to be more harmful to us than the drink itself?

Bisphenol-A (BPA)

The dangers associated with a soda can all come down to one compound: bisphenol-A.

Commonly known as BPA, this organic synthetic compound constitutes approximately 80% of epoxy/polymer resins used in soda can and canned food liners. While this makes a tenacious coating, BPA has a notorious reputation.

BPA has the ability to mimic natural hormones in the body. In this way, it has been classified as an endocrine disruptor. It enters the body by transferring into the soda, which we then drink.

This means that BPA actually interferes with our hormonal system by altering or breaking down the production of hormones.

Every soda can contains traces of BPA

The protective liner inside soda cans contains BPA. While this is a dangerous compound that can disrupt hormone production, its dose in canned food and drink is nowhere near high enough to be considered harmful.

BPA and the Endocrine System

Hormones are an integral part of our biology. They are produced and stored in the endocrine system, a chemical messenger system that includes (but is not limited to):

  • Pituitary gland
  • Thyroid gland
  • Parathyroid gland
  • Adrenal glands
  • Pancreas
  • Ovaries and testicles

By circulating hormones through the body, the endocrine system affects nearly every organ and cell. It is responsible for a range of important processes, including:

  • Hair growth
  • Reproduction
  • Cognitive performance
  • Injury response
  • Sensory perception
  • Cell division
  • Metabolic rate

By disrupting this vital system, BPA has the potential to cause problems with fertility, development and overall health. It has even be linked to various cancers, and in 2012 was banned from being used in baby bottles, sippy cups and any packaging used by infants.

Should You Be Worried?

At this point, you’re probably scared to death of that can of Coke or Fanta in your fridge. Well, fear not! While BPA is a dangerous chemical, its toxicity to the body all depends on a dose-response relationship.

A dose-response relationship outlines that the danger of a substance – be that BPA, theobromine or even sugar – runs parallel to the dose we are exposed to.

In the case of BPA, this means that its presence in food and drink packaging doesn’t necessarily mean you’re going be harmed by it. Everything in moderation, right?

In fact, the exposure limit of BPA – the point after which it becomes dangerous – is nearly impossible to reach given the small amount of BPA present in can linings (4.5 parts per trillion).

An article from 2017 has explained that a person weighing an average of 70kg would have to eat over 64 cans of green beans a day in order pass BPA’s exposure limit. In terms of drinking soda, you would have to consume 8,000 cans per day.

The science behind a soda can

To become vulnerable to the harmful effects of BPA, you would need to drink approximately 8,000 cans of soda a day.

At-Home Experiment

Next time you have an empty soda can and some spare sodium hydroxide solution, give the experiment below a try to see the liner for yourself:

  1. Take an empty soda can and wash it out with some water
  2. Using sandpaper, scratch off the paint from the middle of the can. This will allow the aluminium to dissolve easier later on
  3. Fill the can with water and find something – like a hook – to attach to the tab
  4. Use this hook to submerge ¾ of the can in a 3 molar solution of sodium hydroxide (a strong base). Alternatively, 6M sodium hydroxide can also be used for faster results
  5. Leave the can for 24 hours in 3M solution or 5-10 minutes in 6M solution
  6. Over time, the strong base will dissolve the aluminium, revealing the plastic lining

To see how the experiment is performed, you can watch it here.

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