The Age of Plastics: A Chemical Introduction
I just want to say one word to you— just one word.
Are you listening?
Yes I am.
Exactly how do you mean?
There’s a great future in plastics. Think about it. Will you think about it?
Yes, I will.
Enough said. That’s a deal. - The Graduate (1967)
Ubiquity & Mystery
When it comes to plastics, there are typically two narratives that dominate the conversation: one of ubiquity and one of mystery. Hopefully, this blog post will eradicate the latter. This comical 1967 scene from The Graduate takes place after plastics had already penetrated many different industries like the textile industry. The commercialization of plastics occurred much earlier during World War II when natural sources of latex, wool, silk, and other materials were cut off, making the use of synthetics critical. During this time period, we saw the use of nylon, acrylic, neoprene, SBR, polyethylene, and many more polymers take the place of natural materials that were no longer available. Yet this scene capitalizes on the overwhelming feeling that plastics’ potential is exceedingly endless.
Conveniently for this blog series focused on food packaging, in 1967, the year the graduate’s family friend shares his insight into the shifting world of materials, plastics were beginning to make serious ripples in the food packaging industry as plastic bottles were being developed. At that time two leading package producers, Continental Can Co. and Owens-Illinois, Inc. established substantial R&D efforts aimed at developing PETE bottle technology. Since then PETE, which stands for Polyethylene Terephthalate, has become the most commonly used plastic for single-use beverage bottles.
Before getting into a serious discussion about plastics, one must address the mystery of plastic. A general uneasiness associated with plastics is the idea that they are materials not found in nature and created in lab somewhere kind of like Frankenstein. For the most part, these thoughts do not manifest themselves into anything more than jokes and afterthoughts. However, I believe that the veil over plastics and their manufacturing should be removed for all consumers to see and unders
tand. Let’s begin with a simple math equation.
Monomers + Polymerization = Plastics!
Plastics all share two traits:
1) The components: plastics are all made up of small organic molecules, which contain carbon as well as other substances, called monomers—these molecules are unique because they have the ability to join together to create long chains
2) The chemical reaction: plastics, once the monomer is chosen, all undergo a chemical reaction called polymerization that transforms the singular monomers into long chains called polymers. The mass of polymers that results is known as resin—think of a plate of spaghetti. Product manufacturers buy plastic in the form of resin.
- Manufacturers then mix with the resin a complex blend of materials known collectively as additives. Some examples of additives include flame retardants, light stabilizers, and pigments.
- This resin and additives mixture is then melted and molded into the desired plastic shapes.
A cool note on resin: the recycling number codes found on most plastics identify the type of resin used to create the plastic and are helpful to recycling facilities when sorting the plastics. All the plastics of the same resin are melted down again and sold as raw materials to other product manufacturers. These codes are important for recycling facilities as well as for consumers to distinguish between contentious plastics and safer ones. For example, the Environmental Working Group warns against reusing plastics with a #7 or #3 recycling code on the bottom. The plastic containers I reuse to carry my sandwiches for lunch everyday are made of #5 plastic, which is one of the safer plastics to look for.
Bio-Based Building Blocks
Although natural polymers exist in the world like DNA, silk, leather, cattle horns, tortoise shells, and tree saps, today plastics are typically created using synthetic polymers. Plastics’ association with the word synthetic, which tends to have bad connotations, is a trait of plastics that can make people uncomfortable. However, synthetic simply refers to synthesis, which means that "something is put together". Synthetic materials are made of building blocks that are put together in factories. The building blocks that manufacturers use today to create plastics are most commonly made up of fossil fuels such as petroleum and natural gas. However, it was not always this way. Cellophane, one of the earliest forms of plastics used with food, penetrated the food packaging industry in the 1950’s. The name Cellophane, which is derived from the word “cellulose” and the French word for transparent “diaphane”, speaks to the material’s and plastics’ bio-based origins.
Cellulose is an interesting material; as referred to in A History of Food Packaging, cellulose is also used to create paper, paperboard, and cloth. Cellulose is made up of chain-like molecules found in plant cell walls and vegetable fibers that act as a protective barrier. Cellulose is a natural polymer! However, it acts as the building block for cellophane and undergoes a chemical reaction to become a synthetic polymer. To make cellophane, an alkaline solution of cellulose fibers (usually wood or cotton) is squeezed through a narrow slit into an acid bath. The acid regenerates the cellulose, forming a film. The process of creating cellophane illustrates the two important requirements to create plastic: the chain-like molecules that make up the plastic’s structure and the chemical modification of an organic material.
Let’s look at PETE again by breaking down its name—polyethylene terephthalate. PETE’s name is sometimes written as poly(ethylene terephthalate). Ethylene terephthalate is PETE’s monomer or otherwise known as building block. Ethylene refers to a colorless, sweet liquid, which comes from refined petroleum and natural gas. Poly means many—just as polymer means many singular units. So polyethylene terephthalate is the polymer for PETE. This polymer is then processed in different ways and mixed with different additives to yield a variety of products. For example, PETE can be turned into fiber strands to become polyester or molded into bottles to become beverage or shampoo containers. The synthetic polymers act as the DNA for plastics, so, yes, your polyester shirt and the soda bottle in your hand are related. Now that we addressed the mystery of plastics, we can begin a serious discussion on plastics.
The next blog in this food packaging series will discuss the pro’s and con’s of plastics in the modern age.
Lisa Nash, Green Seal