For questions relating to the structures, properties, production methods and potential hazards of polymer materials commonly known as plastics
Plastic, as a noun, is defined as "any one of a wide range of synthetic or semi-synthetic organic solid compounds which are moldable". The word was originally an adjective, from Latin and Greek roots for "moldable". Another definition commonly describing modern plastics is "a class of synthetic organic resins", a "resin" in turn generally defined as an organic substance originating as a thick liquid that hardens into a translucent solid, for instance, amber (a natural resin of fossilized tree sap).
The vast majority of plastics are organic "polymers"; long chains of repeating linear or aromatic compounds known as monomers. The exact method and bonding of the monomers differs between plastics, as do the general properties of the resulting resin.
Plastics are typically separated into two categories; thermoset resins and thermoplastics. The primary difference is in their behavior when heated; thermoset resins, once hardened (typically by the evaporation of a solvent), remain solid and rigid even under high heat, while thermoplastics will melt and can be remolded when heated. Common examples of plastics of both types include:
- Phenol formaldehyde resin - used commonly in gaming devices, such as billiard balls, dice, dominoes etc. Its density and hardness make it desirable over lighter and softer thermoplastics which can deform with use.
- Epoxy resin - typically used as a resin suspension for other materials, such as paper/cloth (forming Micarta), carbon fiber weave (forming carbon fiber composite), and silicates (forming fiberglass, in turn a common substrate for the production of printed circuit boards)
- Melamine resin - formed from polymerization of melamine (cyanamide trimer) and formaldehyde, this lightweight thermoset is commonly used in dinnerware as a more durable substitute for clay ceramics, as well as being a key component in Formica, some laminate flooring, and other chemically-resistant hard surfaces.
- Poly-ethylene terephthalate (PET) - known as polyester when spun into fibers, it is also seen in various vacuum-molded products such as food packaging, cold drink cups, blister and bubble packaging, soda bottles, etc. Its use with food is slightly controversial as the phthalate compounds used as a plasticizer can leach out when the plastic is heated, and certain of these compounds are known to disrupt human hormones.
- Polycarbonate - optically clear and rigid, it is commonly used in applications requiring impact and pressure resistance, such as in safety eyewear, headlight enclosures, waterproof camera housings, etc. Most formulations use Bisphenol-A as the monomer, joined by ester linkages, and when the plastic degrades with age, heat, and contact with water, it can release this chemical, which is also a known endocrine disruptor. As such, most food-based uses for BPA polycarbonate plastics, especially use by infants, have been replaced with other formulations.
- Poly-vinyl chloride (PVC) - A versatile plastic produced from the vinyl chloride monomer, the "raw" polymer is very brittle and degrades easily, so it is almost always combined with additional stabilizers and plasticizers such as phthalates. Its use in several applications, particularly in food packaging and water/wastewater handling, has raised controversy due to the unstable nature of the polymer-phthalate matrix, leading to release of the endocrine-disrupting phthalates into the food or water.
- Polyethylene - A common "food-grade" plastic, this polymer is naturally flexible without the use of phthalate plasticizers, and so is favored for applications involving food storage, especially those requiring heating in the packaging. Two variants are commonly seen; high-density, forming a more rigid plastic commonly used for milk jugs and condiment bottles, and low-density, often milky in appearance and used for squeeze bottles.
- Polypropylene - Another common food-grade plastic that doesn't require a plasticiser, this plastic is commonly seen in reusable food storage containers, in bags for prepared fresh vegetables and snack foods, and for protective covers for paper products.
- Polystyrene - In rigid form, it's transparent (but can be colored) and somewhat brittle, and is commonly used for CD "jewel cases" and small moldable parts such as for scale models and some toys and gaming pieces. In foamed form, it's used for disposable food packaging, packing peanuts, etc. Because it doesn't biodegrade and was originally produced using CFCs, the use of polystyrene foam in disposable containers is highly controversial, and at one point it had been largely abandoned in favor of other options. It remains popular in a non-CFC formulation for packaging hot foods and foods that would dissolve or soak through paper products, like raw meat.