8 Intro Organic & Plastics
8.6 Big 6 Plastics
8.7 LDPE vs HDPE
8.8 Other Plastics
8.11 Nature's Polymers
8.42 Learning Outcomes
There are a lot of different plastics all around us. But there are 6 major plastics that all have the capability of being recycled in some way. Here I show you the name, recycle symbol, abbreviation, monomer, and polymer structure formulas. Two of them are chemically identical (the polyethylenes), and three more are just substitutions on polyethylene. That leaves just one somewhat different one - PET. Memorize them and know their properties and typical uses.
Pretty much the most basic polymer of all, polyethylene. It is made via radical initiated reaction of the monomer ethylene (H2C=CH2). LDPE is considerably branched which is why is is softer, weaker, and tends to find uses where more flexibility is needed and not strength.
properties: A fairly soft polymer with a high amount of flexibility. Resistant to both acids and bases. Can be manufactured to be fairly strong - used as a container for many liquids. Small gas molecules diffuse through LDPE fairly easily.
uses: paper or plastic? Yes, LDPE IS that plastic - plastic grocery bags. Various thin films and sheeting. Bubble wrap used in packaging (and popping). Those little green army men toys. Wiring insulation.
Just like LDPE, HDPE is made via radical initiated reaction of the monomer ethylene (H2C=CH2) - except that a special catalyst is used to make it almost completely straight chain with very little branching. The lack of branching leads to a much more crystalline like substance which gives it more strength and a higher melting point.
properties: Matches all the same chemical properties because it is the same substance. However, it is far more rigid and much stronger than LDPE due to its straight chain nature.
uses: Half and whole gallon containers for milk, juice, other liquids where strength is important. Instant rollout fencing for quick containment. Various cleaning buckets/containers. Tyvek (trademark of DuPont) is a breathable plastic material/fabric used for all sorts of purposes. Lots of use during COVID-19 times as PPE - those bunny-suits that professionals use for protection. Tyvek is also used as a housewrap on new construction - allows water vapor out and keeps out water liquid. Many packaging envelopes are made from Tyvek as well as printable wristbands used for shows and festivals.
PVC comes from the monomer vinyl chloride (H2C=CH-Cl) which is really just chloroethylene, one hydrogen is substituted with a chlorine.
properties: Can be either rigid or quite pliable depending on the processing and whether plasticizers* are used. Nice clear polymer with a nice shine. Often made with pigments and fillers for color and strength. Resistant to oils, acids, and bases.
uses: Plumbing pipes! All that white pipe you see on new home builds and remodeling - that's PVC. Credit cards and other plastic cards (IDs). Shower curtains, "rubber" boots, various hoses and tubing.
Now we substitute a methyl group (-CH3) for one of the H's on ethylene to get propylene (H2C=CH-CH3) as our monomer unit.
properties: That methyl group helps with adding even more oil resistance. Also a bit higher toughness than the PEs - has a higher melting point. Very opaque and is often pigmented.
uses: Microwavable containers. Bottle caps. Containers for more fatty/oily foods like yogurt, margarine, cream + shampoo bottles and other cosmetic containers. Also a fiber used in tough fabrics for carpeting and furniture upholstery.
And for our last of this type, we substitute a phenyl group (-C6H5) for one of the H's on ethylene to get styrene (H2C=CH-C6H5) as our monomer unit.
properties: Can be very clear/transparent. Pretty good gas impermeability. Can be hard and somewhat brittle but can also be whipped/foamed to softer less rigid products. Very soluble into many organic solvents - acetone really dissolves this stuff like crazy.
uses: Certain types of food wraps. As a hard clear plastic: Old CD cases/ audio cassette cases. Transparent cups, disposable champagne glasses. As a soft foam: it's styrofoam! disposable coffee cups - also many beverage cups from fast food restaurants. To go containers from restaurants. Egg cartons and packing peanuts. Cheap ice chests/coolers.
PET is the oddball in the group in that it is a copolymer which means there are two monomer units and they add to each other in an alternating fashion. The first monomer is ethylene glycol which is like ethane except two adjacent H's are swapped out for hydroxyl groups (HO-CH2-CH2-OH). Also note there is no double bond - the reactive parts are the hydroxyl groups. The second monomer is terephthalic acid (HOOC-C6H4-COOH). This is a benzene ring with two carboxylic acid groups stuck on it in opposite positions (aka para- positions)
properties: Very transparent and very strong. Super high resistance to acids and bases + zero diffusion of gases.
uses: Soft-drink/soda bottles with the screw top. All kinds of clear and tough food containers - mayonnaise, warehouse sized nuts, and peanut butter containers along with many others. As a fiber used in various clothing products (polyester), fleece type fabrics, carpets, and fiber-fill insulation.
Almost all polymers can benefit from the addition of a plasticizer. A plasticizer is a chemical compound that is added to a polymer so that the polymer's physical properties are altered to be softer and far more pliable/flexible. Think PVC pipes... hard and rigid (very little if any plasticizer) vs a shower curtain (also PVC) which is very soft and flexible. Plasticizers are also what contribute to that "new car smell" of brand new cars. Plasticizers by themselves tend to have fairly high vapor pressures which leads to their escape from the plastic into the air and you smell them - a "plastic-y" smell. More on the specifics of these later (if I get around to writing it 🙂 ).