While early candy wrappers were made mostly from cellophane, modern wrappers are made of a polypropylene film. Second only to polyethylene in popularity, polypropylene production began in the 1950s and has gradually increased in usage, making the material among the strongest, most versatile and cost-effective commercially used polymers.
Polypropylene has an operating temperature of 255 degrees F, which is ideal for its use in the packaging of prepared foods that require long-term, unrefrigerated, hermetical sealing. Unlike its more practical thermoplastic counterparts, polypropylene is low-temperature sensitive and is not as easily heat-sealable. Therefore, for food-product casings like candy, polypropylene must be produced using biaxial orientation; polypropylene produced in this way is commonly referred to as BOPP (biaxially oriented polypropylene.)
Production of polypropylene begins with a method called slot extrusion, which forces the molten plastic through a straight slot, then onto a conveyor belt to a “chill roll” to remove heat. Heat removal is facilitated by a “water bath” method, as well as air cooling. The film is then reheated using constant-temperature rolls to crystallize the polypropylene.
The process of biaxially stretching polypropylene is a favored method of production because it increases the material’s stiffness and its resistance to moisture accrued through oil, grease, water vapor and oxidation. In addition to the moisture barrier that it creates, BOPP also contributes to the strength of the polypropylene at low temperatures. All of these qualities contribute to BOPP as the wrapper of choice for most candies and prepared foods. Biaxial orientation then stretches the film in two directions; the machine and transverse. The film is then heated until it becomes more malleable and is mechanically stretched to an extent of 300 to 400 percent. Common finished gauges, as found in candy wrappers, are about 0.001 inch.
Candy Wrappers: Then and Now
Although cellophane receives most of the notoriety when recalling candy wrappers, it was a material known as glassine, in 1917, which became the first to accommodate printing. The modern finished films of polypropylene are printed using flexographic printing.
The manufacturing of polypropylene has taken a turn toward more modern “green” production techniques. Hydrocarbons such as coal and fuel are slowly being phased-out in favor of more environmentally favorable, renewable sources of energy. The latest, most innovative fuel alternative in the manufacture of polypropylene is Brazilian ethanol. This sugar cane-based ethanol has made polypropylene virtually indistinguishable from its fuel-based derivative.
References and ResourcesMaier, Clive and Calafut, Teresa. "Polypropylene: The Definitive User's Guide and Databook"
McGraw-Hill's Access Science Encyclopedia of Science and Technology Online" Aaron L. Brody. "Food Manufacturing"
Robertson, Gordon. "Food Packaging: Principles and Practices"