Have you ever wondered why some plastic bags are stronger than others? Why bags from clothing stores at the mall tend to be strong and thick, whereas those from your local grocery store are flimsy and thin? And then there are those wispy, clear bags you get back with your dry cleaning. There are many types of plastic bags, all designed for different purposes.
The plastics we use today have come a long way since Alexander Parkes first introduced them to the world at London's Great International Exhibition in 1862. Parkes's material was an organic derivative of cellulose that could be molded when heated and then maintained its shape upon cooling.
But it wasn't until 1909 that the word "plastic" was coined. Leo H. Baekeland used the term to describe a new class of materials that included "bakelite," a substance he created from coal tar. Bakelite was used to produce many items, including telephones, cameras, and even ashtrays. Although a key component in these items, plastics did not become really popular until after World War I, when petroleum, a more easily processed substance than coal, became readily available. Petroleum and natural gas are the primary sources of the key ingredients in plastic.
Plastics are composed of polymers--large molecules consisting of repeating units called monomers. In the case of plastic bags, the repeating units are ethylene, or ethene. When ethylene molecules are polymerized to form polyethylene, they form long chains of carbon atoms in which each carbon also is bonded to two hydrogen atoms.
Many kinds of polyethylene can be made from ethylene. Plastic bags typically are made from one of three basic types: high-density polyethylene (HDPE), low-density polyethylene (LDPE), or linear low-density polyethylene (LLDPE). Those thick, glossy shopping bags from the mall are LLDPE, while grocery bags are HDPE, and garment bags from the dry cleaner are LDPE. The major difference between these three materials is the degree of branching of the polymer chain. HDPE and LLDPE are composed of linear, unbranched chains, while LDPE chains are branched.
Branching can influence a number of physical properties including tensile strength and crystallinity. The more branched a molecule is, the lower is its tensile strength and crystallinity. That's why garment bags from the dry cleaner are so weak and flimsy. They are made from highly branched LDPE.
Another difference among these types of plastics is the method in which they are made. HDPE and LLDPE are made by Ziegler-Natta vinyl polymerization, a method that uses a transition-metal catalyst, like TiCl3, to initiate polymerization. Karl Ziegler and Giulio Natta received the 1963 Nobel Prize in Chemistry for developing this method.