In order to understand the problem that wide-spread plastics use poses for the environment, and what we can do about it, it is important to understand exactly what plastics are to begin with: what they are made of, how they are made, and why.
A material can be called a plastic if it satisfies three conditions: it's main ingredient must be a polymer material, it must be fluid at some point during processing (usually processed using heat), and it must be solid in its final form. Plastics can be made up of many different kinds of polymer, and can be processed in many different ways, but as long as they satisfy these three conditions, they are bona fide plastics.
The main ingredient of any plastic is a polymer, a type of molecule that takes the form of a long chain. The word polymer comes from two Greek words, poly meaning many and mer meaning parts. So, as the name implies, polymers are made of many parts, called monomers or monomeric units, that are chained together.
Polymers can come in different shapes. For example, microwaveable food containers and Dacron carpets are made of linear polymers. Soft and flexible shampoo bottle and milk jugs are generally made using branched polymers. Car tires and bowling balls, on the other hand, are composed of cross-linked polymers. All of these polymer types are long and flexible molecules, so they can wind together and tangle like spaghetti on a plate.
Some polymers are synthetically produced, such as nylon and polyester, while others can be found in nature: silk, hair, natural rubber and even starch are examples of polymers. In principle, any of these polymers could be used to produce plastics; in practice, however, over 90% of all plastics are made from just five polymers, all of which are synthetic.
The initial unprocessed mass of polymer, called resin, is processed into different shapes using a variety of methods, including: extrusion, injection molding, compression molding, transfer molding and casting. Different processing techniques result in the wide variety of forms that plastic can take: ranging from thin films and elastic sheets, to resiliant panels and hard, solid three-dimensional shapes.
During this process they are also often combined with plasticizers and other additives, such as coloring, to increase their strength or flexibility, or to improve their appearance.
The pure polymer resin by itself may not always have the properties needed in the final product: it may be strong but too brittle, flexible but too elastic, or flexible and elastic but just plain ugly. Just like the polymer material itself, additives come in different varieties: some can be found in the environment, while others are manufactured. The amounts and types of additives used in manufacturing plastics are another factor that influence how environmentally-friendly they are.