Material Marvels: History of the food can

Materials World magazine
,
29 Oct 2019

Invention of the food can changed the way we lived and ate forever, and preservation helped provide greater food security and underpinned industrial, exploration and military advances. Ceri Jones plots the evolution and future of the can.

The food can represents exploration and endurance – armies, navies, merchants, explorers seeking new land, and migrants to the new world all depended on the ability to preserve not only foods but also vital nutrients. At a more fundamental level, canning perishable goods like milk, meat and vegetables, helped improve the health of people living in large cities and areas a long way from fresh produce.

But alongside necessity, the humble food can spurred hundreds of years of technology innovations to refine this immensely important package.

Invention of the first ever food can was credited to Frenchman Nicholas Appert in 1809. Appert spent 14 years developing a food preservation method, which eventually won 12,000 francs in a competition launched by Napoleon Bonaparte, who needed a way to feed the French Army and Navy during his war campaigns.

However, the can was incidental and Appert won the prize for discovering a method of food sterilisation. He developed a process of first cooking food to kill the bacteria, before packing it into glass jars with stoppers to seal out air.

Needing to compete in military prowess, British inventors took the technique and replaced the easily breakable glass with tin. British man Peter Durand received the first patent for a tin can in 1810, and later migrated to the USA where he invented the first tin-plated can in 1818. This much-improved version consisted of iron coated in tin to prevent corrosion and rust, finally soldered shut.

‘The earliest cans were laboriously made by hand. Iron was pounded into sheets and dipped into molten tin. The resulting tinplate was then soaked in brine baths, creating a hot and odorous atmosphere. Using considerable skill and muscle, artisans cut the sheets into rectangular bodies and round ends. The body pieces were bent around a cylindrical mould and the seams and ends were soldered in place. One of the ends was made with a circular hole through which the food would be stuffed. Once filled, the holes were plugged with a soldered metal cap. This soldering process sometimes left a bit of soot mixed in with the can's contents,’ said the Can Manufacturer’s Institute (CMI), USA.

Packing in innovation

Over the mid-1800s to the early-1900s, use of cans extended to more varied products, including tobacco and cigarettes, aerosols, condensed milk, corned beef and toothpaste. Then in 1938, the introduction of a ginger ale by Cliquot Club started a renewed period of development. This first canned soft drink was beset by packaging design problems and when Pepsi-Cola released the first carbonated soft drink in 1948, it required a completely new design. Thicker metal kept the high-pressure cans safe but the high levels of acidity compromised taste.

Over the decades, ‘the carbonic, phosphoric and citric acids in soft drinks presented a risk for rapid corrosion of exposed tin and iron in the can. To solve the problem, organic coatings were used to line the inside of cans making them heavier and more encasing’, CMI explained.

Popularity of tinplate was challenged by introduction of an aluminium can in 1963. Manufacturers rapidly switched to aluminium as they could make cans more easily out of only two pieces – body and closure – and the surface was better for advertising and printing to appeal to a modern audience.

‘The aluminium can was easily integrated into the package market because of its ductility, its support of carbonated pressure, its lighter weight and its resistance to corrosion. But perhaps the most critical element in the aluminium can's success was its recycling value. Aluminium can recycling excelled economically in its competition with steel because of the efficiencies aluminium cans realised by using recycled materials instead of costly and non-renewable virgin aluminium ore. Steel did not achieve similar economies in the recycling process,’ said CMI.

What’s inside that counts

To prevent acids in food and drinks from reacting with the metal interiors and degrading, very thin coatings were used as a barrier, generally oleophilic materials commonly used to protect boats and large structures. Over time, the linings progressed to include polyvinylchloride and epoxies. Then in the mid-1950s, the accidental discovery of a tough epoxy resin brought about the use of Bisphenol A (BPA), which quickly became the preferred material of can lining due to its ease of use and stable performance.

By 20 years ago, BPA use had expanded to 80% of metal cans, with the remaining 20% being made up of a combination of vinyl, acrylic and polyester coatings. Therefore, in 1992 when Dr David Feldman released a study that suggested BPA had unusual oestrogen-like properties, the industry faced a significant problem.

During the following decades, multiple research projects confirmed BPA as an endocrine activator, and many cited the potential danger of its use. However, investigations by multiple independent health organisations and governments have all agreed that it would be unlikely for a person to ingest enough of the substance to cause any harm.

The European Food Safety Authority stated that, ‘BPA poses no health risk to consumers of any age group – including unborn children, infants and adolescents – at current exposure levels’. But still, Canada listed it as a toxic substance in 2010, the US Food and Drug Administration banned its use for baby bottles and formula in 2012, and in 2017, the European Chemicals Agency concluded that BPA should be listed as a substance of very high concern.

Covering a problem

In recent years, material innovation has come about due to sustained public fear causing companies all along the food and beverages supply chain to disassociate their products from BPA. USA-based packaging coatings firm Sherwin-Williams (formerly Valspar) set out to develop a new lining material that would match the performance of BPA.

The challenge then was how to find a material that would match BPA’s characteristics. ‘Properties like corrosion resistance, pack resistance, pack stability, the ability to allow high-speed manufacturing at around 2,000 cans a minute being produced – all of these were developed over a 50-year time span. And every time we challenged BPA epoxy to do something, it was always able to rise to the occasion by formulation,’ Sherwin-Williams VP Compliance and Technology Marketing, Thomas Mallen, told Materials World.   

‘We and the other coatings manufacturers were constantly hitting our heads against the inherent limits of other materials, such as polyester, vinyls, and acrylics, to reach the performance level that people had grown accustomed to. So we said, in the world of bisphenol, of which there are thousands, is there at least one that is not endocrine active and that can make a good epoxy? With those conditions, we started searching,’ he added.

Mallen’s team embarked on a comprehensive materials testing process and hired two staff toxicologists to help develop a new material, including engagement with retailers and manufacturers from the start, in preparation for extensive analysis by public health authorities.

‘As there was not a commercial option available, we were going to have to develop the supply chain upstream of us to be able to get it manufactured. The manufacturing group would have to be ready and able to handle new raw materials they had not used before, or materials in a different state of matter,’ Mallen said.

He explained that, in a conventional epoxy manufacturing plant, there are two components – the BPA-based resin and Bisphenol A itself. One is a liquid and the other is a flaked solid. They have to come together in the reactor with a catalyst under heat in a closed vessel, but the solid matter is more difficult to measure precisely, so it was necessary to invest millions of dollars in new factory manufacturing equipment.

Safe and sealed

The company finally developed a suitable substance called tetramethyl bisphenol F. ‘This bisphenol is too complex to get into the receptor sites in order to trigger any kind of endocrine activity, but it's not so complex that you cannot make a good polymer out of it, in the order of micrometres.

The coating essentially converts itself into carbon dioxide and water. So the metal recyclability is unaffected and the coating is easily removed. In tests, it is either matching or exceeding BPA, which we didn't think was possible,’ Mallen said.

‘In terms of the environmental impact, it is very low. Now we are trying to make epoxies formulated to be applied to other end uses, such as food cans, aluminium impact bottles, and extruded bottles.

Sherwin-Williams’ vision for the coating spans beverage cans, foods, aluminium bags etc., to serve the steel and aluminium markets. Reaching these markets has required several reformulations suitable for spray and coil coating applications, as each piece is coated differently, for instance, closures and jar lids are applied differently than sheet metal.

‘It is applied on a coil of aluminium, and then the aluminium rolled back up, the big coil 10,000lb coil is sent to an end maker and they punch out the ends. Then the ends are brought together with the cans when they are sealed. Those are the two big technology areas where we call it V70. The spray coating would be called the 7EQ38. And the encoding would be called the 70QO5 and they are both water-based coatings.

Sherwin-Williams commercialised its BPA alternative in the USA in 2017, and is currently working with European regulatory authorities to bring it to the EU. Work is ongoing, but, according to Mallen, such safe, reliable and high-performance epoxies are the unexpected but crucial next stage in the development of modern food canning.