To understand what resins (or plastics, take your pick for a term) are, it's helpful to be clear that they're not just one thing.
"One way to think about resins is to think about metals," says Kingsbury. "They are lead and gold and aluminum and iron. They all fall under the category of metals, but they have different properties. They're mined differently and have different cost structures. Plastic is the same." Translated into the world of resins, that means that there are many different types under the umbrella term of plastics, including polyethylene terephthalate (PET) -- one of the most common kinds, often used in water bottles -- and high density polyethylene (HDPE), which comprises milk bottles.
Of course, the question really isn't "What products are resins in?" It's "What are they not in?" As mentioned, resins are found in all sorts of packaging, as well as in airplanes. But that's just the beginning. There are lots of resins used in plenty of different products, and here are a few examples: Polyurethane resins are used to make furniture foam and footwear; alkyd resins are used in paint; and formaldehyde resins are used in wood glues [source: O'Lochlainn].
Although it's important to know there are a variety of resins, there are some broad generalizations that can be made about how they're all produced. For the most part, resins are derived from natural gas and oil -- think of gas and oil as the feedstock or building blocks of most plastic (though there are some exceptions, which you'll learn about on the last page of this article). To take that a step further, on a basic molecular level, resins are simply links of monomers derived from oil and natural gas. So, when you hear the term polymer used interchangeably with resins or plastics, that's because it's perhaps a more precise description of the end product of the process used to produce resins: Polymers are made of many monomers.
An example will probably help. Polyethylene, the most common plastic in the world, is made up of many monomers called ethylene. In order to get polyethylene, a process takes place that breaks a double carbon bond in ethylene gas and joins it to another ethylene [source: Kingsbury]. And when you string together enough ethylene monomers, you not only have polyethylene, you actually have a solid, which can be shaped to form a container like a milk bottle.
Even better, though, is to think of it the way Bill Carroll describes it. Carroll, who is chairman of the board of the American Chemistry Council, says it's best to think of monomers as small, one-inch pieces of yarn. If a few of them are piled in front of you, it's easy to pick them apart strand by strand. But if you have a pile of much longer pieces of yarn all entangled and intermingled, it's going to be much more difficult to pull them apart. In other words, the tangled mess of yarn is equivalent to resins, plastics or polymers. "This is how you should understand where the strength of polymers and plastics comes from," Carroll says.