COMMON POLYMERS

In Table 3.7, some common polymers and their structures are listed. They are also identified by their recycling code, which can often be found on plastic products.

Of note are HDPE and LDPE, which are very common plastics coded differently, but chemically apparently identical. HDPE and LDPE are made of the same monomer, but their larger scale structures are different. HDPE has very straight chains of polyethylene, and these stack on each other neatly to make a harder, denser, higher melting-point plastic. Polyethylene in LDPE tends to have branches, and this leads to the strands stacking relatively poorly. This makes LDPE softer, less dense, and lower melting relative to HDPE.

POLYMER CONCERNS

A major concern surrounding polymers is the potential for environmental contamination from use of plastics. This can occur either from leeching of unreacted monomers into the environment from the plastic, or from the breakdown of the plastic from years of exposure to UV light from the sun or other environmental conditions.

For instance, a major controversy surrounds the use of polycarbonate plastics in drinking bottles. When a polymerization reaction occurs, not all of the monomers react, and some end up trapped in the plastic, unreacted. One of the monomers, bisphenol A, was shown to have a biological effect, as it mimics the functions of certain hormones in the body. This could potentially affect the health of people who used the water bottles, as, over time, small amounts of the bisphenol A would leech into the water, and would end up being consumed by the owner. Alternative plastics with different feedstocks had to be developed to bypass this problem.

PVC faces similar potential concerns. Polyvinylchloride itself is a carcinogen, and if burned releases caustic HCl gas. Furthermore, as PVC ages, it has the potential to enter the biosphere as a potentially toxic microplastic. These concerns often lead to regulation, and have to be carefully considered when using or disposing of PVC.

Table 3.7. Common polymers

Name

Code

Structure

Example Uses

Polyethylene ter- ephthalate (PETE)

Soda bottles, sails, polyester

High-density polyethylene (HDPE)

Grocery bags, milk jugs, plastic lumber

Polyvinyl chloride

(PVC or V)

Pipes, shower curtains, cleaning agent bottles

Low-density polyethylene (LDPE)

Plastic bags, 6- pack rings, tubing

Polypropylene

(PP)

Ropes, packaging materials, food containers

Polystyrene

(PS)

Insulation, plastic utensils

polytetrafluoro-

ethylene

(Teflon)

Cookware, low friction surfaces

Kevlar

Body armor

Note: Brackets denote the segmers of the shown polymer.

*Code 7 classification is for “other” polymers, anything that does not fall into the other categories. Some examples of other polymers are given.

 
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