Have you ever tried to open a can without a can opener? For nearly 50 years people relied on their creativity to open the molded tinplate. It took that long for the handy can opener to be invented after the patent application for cans had been filed.
Today, people can access the contents of a can in an instant using small can openers attached to the lid or the side. And the content of cans could not be more different—children's toys, food, or shoe polish. The thin metal shell has proven itself in numerous applications. It is light, virtually indestructible, and heat resistant. On top of this, it fulfills its real task of protecting the product from outside influences, such as in transportation or during loading and unloading. Special DYNAPOL® polyesters from Evonik keep the tinplate corrosion-free. These polyesters are used as main binders in respective can coatings.
A typical tinplate can often contains several coatings—up to three on the outside and at least one on the inside. The exterior coatings are for decorative purposes and protect the can from rust and the color print from getting scratched. The interior coating in the can prevents the acid in food attacking the metal and metal ions leaching into the food.
The difficulty in formulating the coatings has to do with the way the cans are manufactured. First, the coating is applied on a flat tinplate or a sheet of aluminum and burned in. Then the body and lid of the can are manufactured from the coated sheets. The coating must be suitably flexible or it will tear and flake, and it must also offer adequate rigidity and protection. The polyester resins from Evonik get this balancing act just right and provide both—maximum protection and high flexibility. In high-quality cans, each of the three exterior coatings therefore contains DYNAPOL® binders.
Protection outside and in
An entirely different dimension of protection takes effect in the can's interior. Since most foods contain ingredients such as acetic acid or lactic acid, oils or salts, the can also needs to be protected on the inside. Otherwise, the food acids may attack the metal, allowing metal ions to leach into the food. Many interior can coatings contain epoxy resins with the building block bisphenol- A (BPA), which has long been suspected of being harmful to health. For this reason, scientists and environmental protection organizations are increasingly calling for the use of BPA-free coatings. "With DYNAPOL® we offer suitable alternative solutions for protecting the interiors of tin cans," says Dr. Thorsten Brand from Evonik. "Our aim is to deliver sustainable products that meet customers' requirements." This also applies when the contents are particularly aggressive, for example sauerkraut, tuna or particularly spicy chili. "Our products are entirely odorless and tasteless," Brand adds.
DYNAPOL® protects food in tinplate cans without compromising taste. After the filling process, the can is subjected to the next stress test: sterilization. To prolong the shelf life of food, the cans have to enter the sterilization chamber at temperatures of 130 degrees centigrade and under high pressure. It is only when the coatings have withstood this step unscathed that the round tinplate is ready for sale.
An over 200-year-old patent
How well the coatings stand up in everyday life—in storage, transportation, and on supermarket shelves—is something that can only be seen after many months. This is taken into account in the development of new coating systems through corresponding long-term testing.
Incidentally, it was none other than Napoleon who in his time called for a competition for inventions to make foods last for particularly long periods. The winner proposed the sterilization process. The industrial breakthrough in food conservation through sterilization happened some time later with the development of the tin can, which was patented in 1810. Can openers were not invented until half a century later.