Elastomers

Definition

Elastomers are rubber-like polymers that combine high, reversible deformability with low stiffness. They typically sustain large strains (often >100% and up to several hundred percent) and return to their original shape when the load is removed. This behavior arises from long-chain macromolecules arranged in a lightly crosslinked network (thermoset rubbers) or a microphase-separated morphology with soft and hard segments (thermoplastic elastomers, TPEs). Elastomers are viscoelastic: they store elastic energy yet dissipate some as heat (damping). For useful elasticity, their glass transition temperature (Tg) is below the service temperature.

Key properties

  • Mechanical: low Young’s modulus; high elongation; resilience; tunable hardness (typically Shore A to Shore D); good tear and abrasion resistance (compound dependent).
  • Dynamic behavior: viscoelastic damping (hysteresis), with energy dissipation useful for vibration/shock isolation but potentially causing heat build-up under high cyclic loads.
  • Thermal: wide temperature capability varies by chemistry (e.g., silicones remain flexible at very low temperatures and tolerate high heat; fluorinated elastomers handle elevated temperatures; EPDM performs well in hot water/steam). Compression set and stress relaxation increase with temperature.
  • Chemical and environmental resistance: highly formulation dependent. Nonpolar rubbers (e.g., NR, SBR) swell in oils/fuels; NBR/HNBR resist oils; EPDM resists water, steam, and many polar fluids; fluorinated elastomers withstand fuels, solvents, and many chemicals. Ozone/UV/weathering resistance is excellent for EPDM and silicones.
  • Electrical: generally good insulators; antistatic/ESD formulations are available.
  • Permeation: elastomers are relatively permeable to gases and small molecules; barrier performance can be improved with fillers, coatings, or specific chemistries.
  • Density: typically ~0.9–1.4 g/cm³, aiding lightweight designs.

Types and families (examples)

  • General-purpose thermoset rubbers: NR (natural rubber), SBR (styrene-butadiene), BR (butadiene), EPDM (ethylene-propylene-diene), CR (chloroprene).
  • Oil/fuel/chemical-resistant: NBR (nitrile), HNBR (hydrogenated nitrile), ACM (polyacrylate), AEM (ethylene-acrylate), FKM/FPM (fluoroelastomers), FFKM (perfluoroelastomers).
  • High/low-temperature specialties: VMQ (silicone), FVMQ (fluorosilicone).
  • Thermoplastic elastomers (TPEs): TPE-S (SBS/SEBS), TPE-O (polyolefin blends), TPE-V (TPV, dynamically vulcanized EPDM/PP), TPU (thermoplastic polyurethane), TPE-E (COPE), TPE-A (TPA).

Benefits and common applications

  • Elastic sealing and compliance: conforms to mating surfaces and maintains seals under thermal cycling, vibration, and tolerance variation (gaskets, O-rings, shaft seals, weatherstrips).
  • Vibration, shock, and noise control: viscoelastic damping in mounts, bushings, isolators, tires, and acoustic seals.
  • Flexibility and fatigue resistance: durable in hoses, bellows, diaphragms, belts, cable jackets, and protective boots.
  • Environmental durability (grade dependent): options for resistance to oils, fuels, coolants, chemicals, ozone/UV, and temperature extremes.
  • Process versatility: supports molding, extrusion, overmolding, foaming, coating, and additive manufacturing for prototypes and some production parts.

Representative industries and parts

  • Automotive and mobility: tires; seals and gaskets; hoses and tubing; NVH components; wire and cable jacketing; interior soft-touch parts.
  • Electronics and electrical: connector seals; cable insulation and strain relief; gasketing for enclosures; potting/encapsulation.
  • Industrial and construction: hoses, belts, expansion joints, weatherseals, vibration isolators.
  • Medical and consumer: tubing, seals, wearables, grips, footwear midsoles/outsoles (grade- and regulatory-dependent).

Processing and compounding

  • Compounding: base polymer blended with fillers (e.g., carbon black, silica), plasticizers/oils, curatives/accelerators, coupling agents, antioxidants, antiozonants, and stabilizers to tailor hardness, modulus, damping, abrasion, permeability, and aging.
  • Thermoset rubbers: mixed on mills or internal mixers, shaped by compression/transfer/injection molding or extrusion/calendering, then crosslinked (vulcanized) using sulfur, peroxides, metal oxides, or addition-cure systems (e.g., silicones). Post-cure may be used to optimize properties or reduce volatiles.
  • Thermoplastic elastomers (TPEs): processed via conventional thermoplastic methods (injection molding, extrusion, blow molding) with no chemical cure; suitable for reprocessing, recycling of sprues/runners, and multi-material 2K overmolding onto rigid plastics or metals.
  • Secondary operations: bonding to substrates (primers/adhesives, co-molding), surface texturing/coatings, foaming (closed- or open-cell), and in some cases additive manufacturing (e.g., TPU via FFF/SLS) for prototyping or low-volume parts.

Design and selection notes

  • Match chemistry to environment: choose elastomer family for fluids, temperature, ozone/UV, and radiation exposure (e.g., EPDM for water/glycol, NBR/HNBR for oils, FKM/FFKM for fuels/solvents, silicone for wide temperature and electrical insulation).
  • Account for time/temperature effects: viscoelastic behavior leads to creep, stress relaxation, and compression set; specify appropriate hardness, crosslink density, and cure system.
  • Geometry matters: seal performance depends on shape factor, squeeze, and groove design; consider tolerances and thermal expansion mismatches.
  • Dynamic service: manage heat build-up and fatigue under cyclic loads; filler type and crosslinking influence hysteresis and durability.
  • Permeation and outgassing: consider barriers, coatings, or alternate chemistries where gas or solvent transmission and emissions are critical.
  • Standards and testing: common references include ISO 1629 (nomenclature), ASTM D2000 (line callouts/specifications), ASTM D2240 (hardness), ASTM D412 (tensile/elongation), ASTM D395 (compression set), ASTM D471 (fluid resistance), and relevant regulatory standards (e.g., UL, FDA/USP, REACH/RoHS) by application.
  • Sustainability: TPEs enable easier recycling; options include bio-based rubbers, reclaim/crumb incorporation, and emerging devulcanization and circularity routes.

Related terms

Rubber; rubbery polymer; vulcanizate; thermoset rubber; thermoplastic elastomer (TPE); thermoplastic vulcanizate (TPV); elastomeric foam; gasket; O-ring.

Sector-specific example: electric vehicles (EVs)

  • Battery systems: foam-in-place and profile gaskets for pack lids and module interfaces; low-outgassing, dielectric, and flame-retardant grades; vent membranes; coolants-compatible hose materials.
  • E-powertrain and electronics: oil/dielectric-fluid-resistant shaft seals; high-temperature seals in motors/inverters/gearboxes; cable and connector seals for high ingress protection.
  • NVH and body: mounts, bushings, acoustic seals to attenuate motor whine and road noise; weatherstrips for aerodynamic and acoustic sealing.
  • Integration and lightweighting: TPE overmolding for integrated seals and strain-relief features, reducing part count and assembly complexity.

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