Milk cartons are water tight paper containers used for packaging milk for retail distribution. One of the most common supermarket items, and found in nearly every home, the milk carton is nonetheless a precision product, manufactured according to exacting standards.
Up until recent times, milk was not usually available as a retail item. Once milk is removed from the cow, it spoils quickly in heat, and is vulnerable to contamination. Until this century, the most economical and hygienic way to store milk was to leave it in the animal. In Europe, a town cow keeper would bring his or her cow directly to the doorstep of the customer, and milk the animal there into a household container. In some places, milk was sold from a shop next door to the cow stall. In either case, the milk could not be safely stored for anything but a small amount of time. A large metal milk container was developed in Europe between 1860 and 1870. Called a churn, the lidded metal container could hold about 21.12 gal (801) of milk. Milk in churns was shipped by rail from farming areas into towns, where the demand for milk was high. Milk in metal churns was also dispensed door to door. Instead of the cow keeper bringing the cow, now the milk was ladled out of the churn into a smaller household bucket or can. The glass milk bottle was invented in 1884. This offered convenience to milk consumers, since the sterilized bottles could be kept sealed until needed. Milk that was pasteurized (quickly heated to above boiling, then cooled) was resistant to bacterial contamination and spoilage for several days. Bottled milk became prevalent across the United States and Europe through World War II, though glass containers are rarely seen now.
The first paper milk carton was introduced in 1933. Wax was applied to the paper, to make it waterproof. In 1940, polyethylene was introduced as the waterproofing material. Refillable glass bottles reigned for a long time after milk cartons were introduced, but by 1968, over 70% of milk packaged in the United States went into paper cartons.
The manufacture of milk cartons is actually a two-step process, at two different locations. The carton manufacturer cuts and prints the carton, which is shipped in a "knocked down" or flattened form to the milk packager. The packager completes the process by forming, filling, and sealing the carton.
Milk containers are made from paperboard coated with a waterproof plastic, generally polyethylene. The wood pulp that is used to make paperboard for milk cartons is a blend of softwood and hardwood. Softwood is usually a type of pine, though the actual trees used vary depending on the location of the paper mill. Softwood produces long wood fibers that provide strength to the paperboard. Hardwood comes from deciduous trees such as oaks. Hardwood has shorter fibers that make for a better printing surface. Pulp for milk carton board is usually 60% hardwood and 40% soft.
Several other chemicals are used to make milk cartons. One is oxygenated chlorine, which bleaches the wood pulp. Other chemicals specific to each manufacturer are added to the paper to add strength. Chemical pigments in the ink are used for the printing process as well.
The refined pulp flows into the headbox of the Fourdrinier machine. In the headbox, a mixture of water and pulp is spread across a continually moving screen. The water drains away below through the openings in the screen, leaving a mat of damp wood fiber. The mat is drawn through huge rollers that squeeze out additional water. Next, the paperboard is dried, by passing it over steam-heated cylinders.
Manufacturers make quality checks at every step along the manufacturing process. The pulp must be inspected to make sure it is the proper color and density, and has the desired fiber characteristics. As the pulp is a blend of long and short fibers, from soft and hard-wood trees, batches may differ according to the kind and proportion of trees used. The paperboard must pass numerous quality checks, for different reasons. The Federal Drug Administration (FDA) requires that milk cartons meet strict standards for hygiene and safety. For instance, the FDA must approve any chemicals added to the paperboard and the manufacturer must be able to prove it is meeting its regulated requirements. The width, thickness, and fiber mix of the paperboard is continually monitored by instruments attached to the paper-making machine, and the board is checked for contaminants as well. At the dairy or milk processing plant, forming and filling of the cartons is done under exacting standards for hygiene and safety.
The manufacturing process for milk cartons is extremely efficient, and there is very little waste. However, most used cartons are thrown in the trash and end up in landfills. It is possible to recycle them, though, if the appropriate recycling facilities exist. A milk carton recycler collects empty cartons from large users such as schools and hospitals. Then the recycler shreds the cartons, sanitizes them, and ties the shreds into bales. A pulp mill buys the bales from the recycler. At the mill, the polyethylene coating is separated from the paper, and strained off for re-use by a plastics manufacturer. The shredded cartons are then reprocessed into pulp, and can be used to make high grades of printing and writing paper.
Milk carton manufacturing has not changed dramatically for many years, because the process is already highly streamlined and efficient. An increasingly popular modification to the tradition gable-topped carton is the addition of a plastic pour spout, but this requires only minor changes in the manufacturing process. As milk consumption falls in the United States, future changes might be in the graphic design of the cartons, as dairies compete harder for customers. Because the gable-topped cartons are very cost-effective to manufacture, packagers are searching for other products that can be sold in them. However, the polyethylene coating for milk cartons is not appropriate for every liquid. For instance, wine and motor oil have different characteristics than milk, and so need different waterproof barriers. Chemists and design engineers are currently researching new plastic coatings, so that other liquids besides milk can use paper cartons.
"Milk Carton Recap." Packaging Digest (August 1994): 36-37.
"Milk Carton Recycling Does Everybody Good!" Science Activities (Winter 1994): 5.
— Angela Woodward