“Plastics” ≈ polymers (macromolecules). From Greek plastikos = “able to be molded.” Available as sheet, film, rod, tubing.

Advantages: low density, corrosion & chemical resistance, low electrical/thermal conductivity, noise reduction, color/transparency choice, low cost, easy complex shaping.

Disadvantages: low strength & stiffness, high thermal expansion, low service temperature (up to ~350°C).

Structure & Polymerization

Monomer = basic unit (“mer” = part); poly = many.
Polymerization (catalyst, heat, pressure) links monomers into repeating chains.
Molecular weight = sum of mer weights (10,000–10,000,000). Degree of polymerization (DP) = polymer MW ÷ mer MW.
Higher DP → higher tensile/impact strength and viscosity (harder to shape → higher cost).
Ethylene chains by DP: 1 = gas, 6 = liquid, 35 = grease, 140 = wax, 1350 = hard plastic.

Chain Types

Linear (not literally straight).
Branched — entanglement hinders motion.
Cross-linked — thermosets; ↑hardness/strength/stiffness/brittleness with cross-linking.
Network — highly cross-linked.

Crystallinity

Cooling rate sets the degree of crystallinity. Linear polymers can be highly crystalline; highly branched cannot.
↑Crystallinity → stiffer, harder, less ductile, denser, less rubbery, more solvent/heat-resistant.
Fully amorphous polymers can be transparent (e.g. acrylics).

Glass-Transition Temperature

Amorphous polymers change across a narrow $T_g$ range: hard/rigid/glassy below, rubbery/leathery above.

Thermoplastics vs Thermosets

Thermoplastics: heating above $T_g$ /melt weakens secondary bonds → easy to reshape; cooling restores them — reversible. ~2 orders of magnitude less stiff than metals. ↑T → ↓strength/ $E$ , ↑toughness. Conductivity can be raised by doping.

Thermosets: 3-D cross-linked (cured) into one giant molecule; curing is irreversible; no sharp $T_g$ . Better mechanical/thermal/chemical properties, electrical resistance, dimensional stability.

Additives

Plasticizers — flexibility/softness (↓ $T_g$ ).
Carbon black — UV protection.
Antioxidants — resist oxidation.
Colorants — organic dyes / inorganic pigments.
Lubricants — reduce friction, prevent mold sticking.
Flame retardants — Cl, Br, P (Teflon doesn’t burn).
Fillers — wood flour, silica, cellulose fibers, glass (lower cost).

Thermosets in Practice

Epoxies: high mechanical/electrical properties, strong adhesives; fiber-reinforced → structural.
Polyesters: good chemical/electrical properties; glass/carbon reinforced → boats, chairs, auto bodies, pools.

Biodegradability & Recycling

Biodegradable = microbes degrade it without toxic by-products (agricultural waste, vegetable oils).
Recycling: thermoplastics remelt & reform (recycling-symbol triangle with polymer number).

Elastomers (Rubbers)

“Elastic mer” — amorphous, low $T_g$ , large recoverable elastic deformation.
Once cross-linked (vulcanization with sulfur) they can’t be reshaped (a tire is one giant molecule — not recyclable). Convert mechanical energy to heat → vibration damping.
Natural rubber — tires, heels, engine mounts. Synthetic — better heat/gasoline/chemical resistance. Silicones — highest service T (~315°C). Polyurethane — excellent abrasion/cut/tear resistance.