Polymers & Elastomers
Polymers make up roughly 8–10% of a modern car by weight, but that percentage is growing as engineers replace metal brackets, housings, and structural components with engineered plastics. In aerospace, high-performance polymers serve as matrix materials for composites and as seals and bushings in extreme environments.
Thermoplastics vs. Thermosets
This is the most fundamental distinction in polymer engineering:
Thermoplastics — Long polymer chains held together by weak intermolecular forces (van der Waals, hydrogen bonds). Can be melted and re-formed repeatedly. Recyclable. Processed by injection molding, extrusion, blow molding, thermoforming. Thermosets — Polymer chains are chemically cross-linked during curing. Once set, they cannot be re-melted — they char and decompose. Stronger and more temperature-resistant than thermoplastics, but not recyclable by melting. Processed by compression molding, RTM, autoclave curing.| Property | Thermoplastic | Thermoset |
|---|---|---|
| Recyclable by melting? | Yes | No |
| Temperature resistance | Lower | Higher |
| Toughness | Generally higher | Generally more brittle |
| Processing | Fast (injection molding) | Slower (curing required) |
| Examples | PP, ABS, PA, PC, PEEK | Epoxy, polyurethane, vinyl ester |
Key Automotive Thermoplastics
Polypropylene (PP)
The most-used polymer in cars — ~20% of all automotive plastics by weight. Cheap, light (0.9 g/cm³), chemical resistant, easily recyclable.
Applications: Bumper fascias, interior door trim, instrument panels, battery cases (including EV battery housings), HVAC ducts, fender liners. Properties: σu ~30 MPa, E ~1.5 GPa, Tg ~-10°C (brittle at low temperatures unless modified with rubber particles).ABS (Acrylonitrile Butadiene Styrene)
Good impact resistance, surface finish, and dimensional stability. Takes paint and plating well.
Applications: Interior trim panels, pillar covers, grilles, mirror housings, center console components.Nylon / Polyamide (PA6, PA66)
The engineering plastic workhorse for under-hood components. High strength, stiffness, and temperature resistance, especially when glass-fiber reinforced (PA66-GF30 or PA66-GF50).
Applications: Intake manifolds, radiator end tanks, engine covers, oil pans, cable ties, gear teeth. Properties (PA66-GF30): σu ~185 MPa, E ~9 GPa, continuous use temp ~120°C, Tg ~70°C.POM (Polyoxymethylene / Acetal / Delrin)
Very low friction, excellent dimensional stability, resistance to moisture. The "gear and bearing plastic."
Applications: Fuel system components, window regulator gears, door lock mechanisms, clips, fasteners.Polycarbonate (PC)
Transparent, extremely impact-resistant (250× stronger than glass by weight). UV-stable with coatings.
Applications: Headlamp lenses, sunroofs, instrument cluster covers, EV charging port covers.PBT (Polybutylene Terephthalate)
Fast crystallization — ideal for high-volume injection molding. Good electrical properties and chemical resistance.
Applications: Electrical connectors, coil bobbins, mirror housings, sensor housings.PEEK (Polyether Ether Ketone)
The premium high-performance thermoplastic. Continuous use up to 250°C, excellent chemical resistance, inherently flame-retardant. Very expensive (~70–100 USD/kg).
Applications: Aerospace bushings and seals (replacing bronze), oil & gas downhole components, medical implants, wire insulation. Properties: σu ~100 MPa, E ~4.5 GPa, Tg ~143°C, Tm ~343°C.Key Thermosets
Epoxy
The dominant matrix resin for aerospace composites (CFRP). High strength, good adhesion, low shrinkage, excellent chemical resistance. Tg typically 120–180°C depending on formulation.
Applications: CFRP matrix (prepreg tape and fabric), structural adhesives (crash-bonded automotive joints), electronic encapsulation, tooling.Polyurethane (PU)
Extremely versatile — can be rigid foam, flexible foam, elastomer, coating, or adhesive depending on formulation.
Applications: Seat cushions and headrests (flexible foam), insulation panels (rigid foam), steering wheel skins, RIM bumper fascias, clearcoat paints.Vinyl Ester
Better chemical resistance and toughness than polyester at moderate cost. Used where epoxy is too expensive.
Applications: Chemical storage tanks, marine hulls, corrosion-resistant pipes, wind turbine blades (some designs).BMC/SMC (Sheet/Bulk Molding Compound)
Chopped glass fiber in polyester or vinyl ester resin. Compression molded into rigid panels and structural parts.
Applications: Corvette body panels (since 1953), truck hood and fender panels, headlamp reflectors, electrical enclosures.Elastomers (Rubbers)
Elastomers are cross-linked polymers that can undergo large reversible deformations (hundreds of percent strain). They're essential for sealing, damping, and flexible connections.
| Type | Key Property | Application |
|---|---|---|
| NR (Natural Rubber) | High resilience, tear resistance | Tire treads, engine mounts |
| SBR (Styrene-Butadiene) | Wear resistance, low cost | Tire compounds (70% of all tires) |
| EPDM | Weather/ozone/UV resistance | Door seals, window seals, weatherstripping, coolant hoses |
| NBR (Nitrile) | Oil/fuel resistance | Fuel hoses, O-rings, gaskets |
| Silicone | Wide temperature range (-60 to +230°C) | Gaskets, spark plug boots, high-temp seals |
| FKM / Viton | Chemical & high-temp resistance | Fuel injector O-rings, transmission seals, chemical process seals |
| CR (Neoprene) | Moderate oil/weather resistance | V-belts, CV joint boots, adhesives |
Glass Transition Temperature (Tg)
Every polymer has a Tg — the temperature below which it's hard and glassy, above which it's soft and rubbery (for amorphous regions). This is arguably the most important single number for polymer design:
- Below Tg: Polymer is rigid, brittle, glassy
- Above Tg: Polymer becomes flexible, rubbery, loses stiffness
| Polymer | Tg (°C) |
|---|---|
| PP | ~ -10 |
| HDPE | ~ -120 |
| PET | ~75 |
| PC | ~147 |
| PA66 | ~70 |
| PEEK | ~143 |
| Epoxy (typical) | ~120–180 |
Industry Trends
Bio-based polymers: PA11 (from castor oil) used in automotive fuel lines. PLA exploring structural uses. Ford uses soy-based foam in seats. Recycled content: EU targets 25% recycled plastic in new vehicles. PP and PA are the easiest to recycle mechanically. Metal replacement: Glass-fiber-reinforced PA66 replacing die-cast aluminum for brackets, pedal assemblies, and structural mounts. Weight savings of 30–50%. Self-healing polymers: Research-stage materials with microcapsules that release healing agents when cracked. Potential for composite repair.Key Takeaways
- Thermoplastics can be re-melted and recycled; thermosets are permanently cross-linked and stronger at temperature
- PP dominates automotive interiors, PA66 dominates under-hood, PEEK dominates high-performance aerospace
- Epoxy is the standard composite matrix for aerospace CFRP
- Elastomer selection is driven by the fluid environment: EPDM for weather, NBR for oil, FKM for fuel, silicone for temperature
- Tg is the critical design parameter — it sets the upper (structural) or lower (elastomeric) temperature limit for a polymer
