Core Classification: Thermoplastics vs. Thermosets
Plastics' response to heat defines their fundamental group, which dictates reusability and processing:
| Category | Key Trait | Structure | Recyclability |
|---|---|---|---|
| Thermoplastics | Soften when heated, harden when cooled; reprocessable | Linear/branched polymer chains, no permanent cross - links | High; can be melted and remolded multiple times |
| Thermosets | Once cured (heated/chemically treated), cannot be remelted or reshaped | Permanent cross - linked molecular networks | Low; irreversible structure after setting |
Common Thermoplastics (Recycling Codes #1 - #7)
Thermoplastics dominate daily life and industrial use, with the 7 - code system aiding recycling and identification:
PET/PETE (Polyethylene Terephthalate, #1): A clear, rigid thermoplastic with good barrier properties and strength. Widely used for beverage bottles, food jars, and polyester textiles (fleece). It is one of the most recycled plastics, often turned into new bottles or fiberfill.
HDPE (High - Density Polyethylene, #2): Semi - crystalline, tough, and chemical - resistant, with a linear structure. Used for milk jugs, detergent bottles, grocery bags, and pipes. Highly recyclable into plastic lumber or new containers.
PVC (Polyvinyl Chloride, #3): Versatile; rigid (unplasticized, uPVC) for pipes/window frames, or flexible (plasticized) for cables, flooring, and inflatables. Limited curbside recycling due to potential toxic emissions when processed.
LDPE (Low - Density Polyethylene, #4): Branched structure gives flexibility and ductility. Common in plastic bags, squeeze bottles, and shrink wrap. Recyclable but less often collected curbside.
PP (Polypropylene, #5): Semi - crystalline, heat - resistant (melting point ~160°C), and fatigue - resistant. Used for yogurt cups, bottle caps, automotive parts, and food containers. Recyclable into similar products.
PS (Polystyrene, #6): Rigid (for CD cases, cutlery) or foamed (Styrofoam, for packaging/insulation). Lightweight but low recyclability due to low density and contamination issues.
Other (#7): A catch - all for plastics like polycarbonate (PC, for safety glass/eyeglasses), ABS (Acrylonitrile Butadiene Styrene, for electronics housings/car parts), and bioplastics (e.g., PLA, made from corn starch for 3D printing/food packaging).
Key Thermosets
Thermosets excel in heat and chemical resistance for durable applications:
Bakelite (Phenol - Formaldehyde): The first synthetic plastic. Rigid, heat - resistant, and an electrical insulator, used for electrical switches and appliance handles.
Epoxy Resins: Strong adhesion and chemical resistance. Used in adhesives, coatings, and composite materials (e.g., carbon fiber parts).
Melamine - Formaldehyde: Heat - and scratch - resistant, ideal for kitchenware (plates, bowls) and laminate surfaces.
Polyester Resins: Often paired with fiberglass for boat hulls, bathtubs, and construction composites.
Special & Engineering Plastics
Engineering Thermoplastics: Offer enhanced mechanical properties for structural use. Examples include polyamide (nylon, for gears), polycarbonate (PC, for safety shields), and PEEK (Polyether Ether Ketone, for aerospace/medical implants).
Bioplastics: Derived from renewable sources (starch, sugarcane). PLA (Polylactic Acid) is common for 3D printing and disposable cutlery; PHA (Polyhydroxyalkanoates) is biodegradable in soil/water.
Quick Takeaway
Thermoplastics are the most common and recyclable; thermosets are permanent and heat - resistant; engineering/special plastics serve high - performance needs. Understanding these types helps with proper use, recycling, and material selection.