Examining the paper bag making process reveals intricate practices that took time and ingenuity to master. Understanding the process involves learning about the history of paper bags, logging practices, pulp manufacturing and commercial bag making machine. Since many bags today are recycled several times, it's important to look at this process as well.
According to a fact sheet from The Ohio State University, the Chinese used sheets of mulberry bark for wrapping food, around the first or second century B.C. Paper bags made from wood pulp, as we know them today, came about in the industrial revolution. As explained by the Smithsonian Institution's History Wired website, in 1879 Margaret Knight received a patent for a folding and pasting machine that revolutionized paper bag making. Today's paper bag making machine operates on the same principles as this original.
The most common source of pulp for paper bags is wood. According to the University of Wisconsin Madison, in 1997 about 87 percent of roundwood used in the U.S. was for industrial purposes, while 13 percent was for fuel purposes. Out of the 87 percent, 36 percent was for pulp-based products, which is primarily paper. Softwoods, such as pine and spruce, are common species used for industrial paper making.
Making Pulp and Paper
Making pulp means separating plant cells and mixing them with water. Industrial digesters use chemicals, heat and pressure to separate cellulose fibers by dissolving at least some of the bonding lignin. Sodium hydroxide and sodium sulfide are the most common chemicals used for dissolving bonds, according to Santa Ana College. This is in contrast to using sulfites, which are acidic. Washing the fibers removes additives and the bonding lignin from the mixture. A University of Penn State website explains how pulp is converted to paper using forming or fabric screens and removing moisture with heat and pressure. Paper making is a water-intensive process and sulfurs released from mills contribute to pollution problems such as acid rain. "Kraft" paper is the term describing the strong, rough paper used for making paper bags.
Today's Paper Bag Making Machines
Final processing into paper bags is entirely automated . Looking at an unfolded paper bag, we see it's a three-dimensional rectangle. The complexity comes in the folds, which form the bottom of the sack. This folding is what Margaret Knight's original bag making machine accomplished, allowing for mass production of the bags. Today's machines operate under the same basic folding principles, with some advances that allow for the bags to stand up on their own and be self-unfolding. Once the machine folds the bag, it pastes it together at the bottom.
Recycling of used paper bags reduces the number of trees harvested and is less energy-intensive than making pulp from lumber. Laminating, dyeing and other processes that smooth paper bags make them uneconomical to recycle. Collecting of used paper bags from drop-off points is an economical and less energy-intensive way of making recycled paper bags.